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RADIATION CURING AND GRAFTING
OF CHARGE TRANSFER COMPLEXES
By
Elvis Zilic
2008
Thesis submitted to the University of Western Sydney is in fulfilment of the requirements
for admission to the Doctor of Philosophy.
Declaration
I declare that this thesis is my own work, and has not been submitted in any form for
another degree or diploma at any university or other institution of tertiary education.
Information derived from published or unpublished works of others has been
acknowledged in the text, and a list of references is given.
Elvis Zilic
ii
Acknowledgements
I would like to thank my supervisors Dr Gary Dennis and Professor Jack Garnett for their
continued support, advice and assistance during the PhD years.
The technical staff at the University of Western Sydney, Rydalmere are thanked for their
assistance in providing necessary equipment and directions.
I thank all fellow postgraduates and academics for the support and friendships throughout
the years.
Finally would like to thank my family for their encouragement to further my studies at
university.
iii
ABSTRACT
Charge transfer (CT) complexes have been used in a number of radiation polymerisation
processes including grafting and curing. The complexes studied include donor (D)
monomers like vinyl ethers and vinyl acetate (VA) with acceptor (A) monomers such as
maleic anhydride (MA). Both UV and EB have been utilised as radiation sources. The
complexes are directly grafted to these substrates in the presence of radiation. The
complexes yield novel copolymers when radiation cured with concurrent grafting
improving the properties of the finished product. The term cure grafting has been
proposed for this concurrent grafting process.
Studies in basic photografting work to complement the cure grafting have been proposed.
The role of solvent in grafting is discussed, particularly the effect of aromatics in
photografting to naturally occurring trunk polymers like wool and cellulose. The effect
of the double bond molar ratio (DBMR) of the DA components in grafting is examined.
The ultraviolet (UV) conditions for gel formation during photografting, hence the
importance of homopolymer yields in these processes is reported. A plausible
mechanism to explain the results from this photografting work is proposed. The
significance of these photografting studies in the related field of curing, especially in UV
and ionising radiation (EB) systems, is discussed.
EB curing and cure grafting of charge transfer (CT) monomer complexes is investigated.
The EB results are compared with UV curing and cure grafting of the same complexes.
The work has been extended to include EB/UV curing and cure grafting of thiol-ene
iv
systems. The significance of these results in the potential commercial application of these
complexes is discussed.
Variables affecting the UV/EB curing and cure grafting of thiol-enes on cellulose have
been studied. These include effect of varying the type of olefin, increasing the
functionality of the thiol, use of acrylate monomers and oligomers in hybrid systems,
altering the surface structure of the cellulose and finally the role of air in these processes
particularly with EB. Photopolymerisation of the thiol-enes in bulk has also been
investigated.
The thesis content is based on the published work of 14 research papers over the course
of the project.
v
List of Abbreviations
AA Acrylic acid
ACN Acrylonitrile
AEEMA 2-(1-Acetoxyethoxy)ethyl methacrylate
AUA Alipthatic urethane acrylate
BBP Benzoyl biphenyl
BDDVE 1,4-Butanediol divinyl ether
BEHMA Bis (2-ethylhexyl) maleate
CHMI Cylcohexane maleimide
CHVE 1,4-Cyclohexane dimethanol divinyl ether
CT Charge Transfer
CTC Charge Transfer Complex
DA Donor-Acceptor
DAMA Diallyl maleate
DBMA Dibutyl maleate
DBMR Double bond molar ratio
DEMA Diethyl maleate
DMF Dimethyl formamide
DMMA Dimethyl maleate
DMPA 2,2-Dimethoxy 2-phenyl acetophenone
DMSO Dimethyl sulfoxide
DSC Differential Scanning Calorimetry
DVE-3 Triethylene glycol divinyl ether
EA Ethyl acrylate
EB Electron Beam
vi
EDA Electron Donor Acceptor
EGDVE Ethylene glycol divinyl ether
EMI Ethyl maleimide
EPA Epoxy acrylate
EPR Electron paramagnetic resonance
FRPI Free radical photoinitiator
FTIR Fourier Transform Infra-red Spectroscopy
HBVE Hydroxy butyl vinyl ether
HCl Hydrochloric acid
HDDA Hexanediol Diacrylate
HEMA Hydroxy ethyl methyl acrylate
LED Light Emitting Diode
MA Maleic anhydride
MAc Maleic acid
MBMA Mono-butyl maleate
MEHMA Mono-2-ethylhexyl maleate
MI Maleimide
MMA Methyl methacrylate
MMI Methyl maleimide
NMR Nuclear Magnetic Resonance
NVP N-Vinyl pyrrolidone
PE Polyester
PEA Polyester acrylate
PI Photoinitiator
PMI Phenyl maleimide
vii
PMS Para-methoxy styrene
PP Polypropylene
PTP Pentaerithritol trimethylolpropane
RH Relative humidity
RTIR Real Time Infra-red Spectroscopy
SEM Scanning electron microscopy
ST Styrene
TAT 1,3,5-Triazine-2,4,6-(1H, 3H, 5H)-trione
TEG Triethylene glycol
THF Tetrahydrofuran
TMPTA Trimethyl propane triacrylate
TMPTEA Trimethylol propane triethoxy acrylate
TMPTVE Trimethylol propane trivinyl ether
TPGDA Tripropylene glycol diacrylate
TTP Trimethylolpropane tris (3-mercaptopropionate)
UA Urethane acrylate
UV Ultraviolet visible
VA Vinyl acetate
VE Vinyl ether
viii
List of Publications
1. Garnett, J.L., Ng, L.-T., Viengkhou, V., Zilic, E. F., Polymer.International, Volume
48, pp. 1016-1026, 1999.
2. Garnett, J.L., Ng, L.-T., Viengkhou, V., Hennessy, I., Shah, N.H., Zilic, E.F.,
Proc.RadTech Europe, Berlin, pp. 677, 1999.
3. Garnett, J.L., Ng, L.-T., Viengkhou, Hennessy, I., V., Zilic, E.F., Radiation Physics
and Chemistry, Volume 57, Number 3, March 2000, pp. 355-359(5)
4. Garnett, J.L. and Zilic, E.F., Proc RadTech Europe 2001, Basle, 2001, pp. 233.
5. Garnett, J.L., Ng, L-T., Nguyen, D., Swami, S. and Zilic, E.F., Radiation Physics and
Chemistry, Volume 63, Number 3, March 2002, pp. 459-463.
6. Garnett, J.L., Ng, L.-T., Nguyen, D. and Zilic, E.F. Radiation Physics and Chemistry,
Volume 62, Number 1, July 2001, pp. 89-98.
7. Dennis, G. R., Garnett, J.L. and Zilic, E.F. Proc. RadTech, North America,
Indianapolis, USA, pp.1002 (2002).
8. Dennis, G. R., Garnett, J.L. and Zilic, E.F. Proc ACUN – 4 “Composite Systems –
Macrocomposites, Microcomposites, Nanocomposites”, Australia, 21-25 July 2002.
9. Dennis, G. R., Garnett, J.L. and Zilic, E.F. Radcure Coatings and Inks: Materials and
Markets, 24-25 June 2002, Manchester, UK.
10. Dennis, G.R., Garnett, J.L., Zilic, E.F., Radiation Physics and Chemistry, Volume
67, Number 3, June 2003, pp. 391-395.
ix
11. Dennis, G. R., Garnett, J.L. and Zilic, E.F., Radiation Physics and Chemistry,
Volume 71, Issues 1-2 , September-October 2004, pp. 217-221.
14. Dennis, G. R., Garnett, J.L. and Zilic, E.F., Proc. RadTech Asia 2003, Yokahama,
Japan, 2003, pp. 254.
15. Dennis, G. R., Garnett, J.L. and Zilic, E.F. Proc. RadTech International North
America, 2004.
16. Dennis GR, Garnett JL, McKean G, Zilic EF. Curing and cure grafting using novel
UV-V lamps - Comparison with EB systems. Technical Conference Proceedings -
Radcure Coatings & Inks Conference Economy & Performance, 21 - 22 June, 2004,
Manchester, UK
x
Table of Contents
Contents Page
Chapter One: A Review of Radiation Curing and Grafting of Charge
Transfer (CT) Monomer Complexes 1
1.1. Introduction 2
1.2. Substrates 4
1.2.1. Cellulose 4
1.2.2. Cellulose grafting with UV and Ionising Radiation 6
1.2.3. Polypropylene 8
1.3. Photografting of Vinyl Monomers 12
1.3.1. Grafting mechanism 12
1.3.2. Factors influencing the grafting reactions 13
1.4. Homopolymerisation 14
1.5. Photoinitiators 16
1.6. Lewis acids 18
1.7. Solvent effects 20
1.8. Curing and Cure Grafting 22
1.8.1. CT Complexes 22
1.8.2. CT complex free radical and cationic polymerisation 26
1.8.3. Role of the acceptor – Maleic anhydride 28
1.8.4. Role of the donor vinyl ethers 29
1.8.5. Analysis of CT Complexes 31
1.8.6. Polyesters as Curing Resins 32
1.9. Thiol-Ene Systems 33
1.10. Spectroscopic Analysis of Polymers 35
xi
1.11. Research Scope of Thesis 36
Chapter Two: Experimental Design and Procedure 37
2.0. UV sources 38
2.1. Ionising source 38
2.1.1. Gamma radiation 38
2.1.2. Electron Beam 39
2.2. Monomer Graft Solution Preparation 39
2.3. Grafting Procedure 40
2.4. Curing Procedure 41
2.4.1. Grafting Yield Calculations 42
2.5. FTIR 42
2.6. Medium Pressure Mercury Lamp Calibrations 42
2.7. Materials 43
2.7.1. Monomers 43
2.7.2 Metal salts 44
2.7.3 Photoinitiators 44
2.7.4 Solvents 44
2.7.5 Substrates 44
Chapter Three: Radiation Induced Graft polymerisation and Cure
Processes Involving Charge Transfer Complexes 45
3.1 Introduction 46
3.2 Grafting Investigations of DA Complexes 46
xii
3.3 DA Complexes for Acceleration of Radiation Grafting of MMA/Styrene
Solutions 49
3.4. Effect of Solvent Structure on UV Grafting of MA/DVE-3
to Cellulose 50
3.5. Comparison of wool with cellulose in photografting
MA/DVE-3 in solvent 54
3.6. Photografting of CT Complexes to PP 55
3.7. Pretreatment of substrates to enhance grafting efficiency 57
3.7.1. Excimer preirradiation of cellulose 57
3.7.2. Corona source pre-treatment of PP 59
3.8. Grafting of CT Complexes using Ionising Radiation 60
3.9. Grafting of Maleate Esters as Acceptors 61
3.10. Effect of double bond molar ratio (DBMR) of DA Monomers
on Grafting 63
3.11. Grafting of Vinyl Acetate (VA) as Donor with MA 64
3.12. Significance of Concurrent Grafting work in Analogous Curing 64
3.13. UV Curing and Cure Grafting with LED Lamps
(Comparison of Con-Trol Cure UV-LED vs Conventional
Mercury UV Lamps) 66
3.14. Thiol-Ene UV Curing Processes 70
3.15. Effect of Thiol Functionality in UV Curing and Cure Grafting 73
3.16. Cure and Cure Grafting on PP 76
3.17. Conclusion 78
xiii
Chapter Four: Additives for Accelerating Photopolymerisation
Processes Involving CT Complexes 79
4.0. Introduction 80
4.1. UV Dose Required to Gel CT Complexes 80
4.2. Solvent Effects in Polymerisation and Grafting 83
4.3. UV Curing and Cure Grafting of CT Complexes with Concurrent
Grafting 85
4.4. Comparison of Effect of Additives in Curing of CT Complexes 88
4.5. Effect of Vinyl Ether Structure on Reactivity of MA Complex 90
4.6. Curing and Cure Grafting with LED Lamp 93
4.7. Mechanism of Polymerisation Process in Curing and Cure Grafting 99
4.8. Conclusion 101
Chapter Five: Electron Beam Curing and Cure Grafting of Charge
Transfer Monomer Complexes to Cellulose 102
5.0. Introduction 103
5.1. EB Curing and Grafting of a Model CT Monomer Complex 104
5.2. Effect of Altering Monomers in CT Complex 105
5.3. Significance of Structure of Monomers on Reactivity of
CT Complexes 107
5.4. EB Curing and Cure Grafting of Thiol-Enes 108
5.5. Effect of Oxygen in EB Curing and Cure Grafting
of Thiol-Enes 111
5.6. Mechanism of EB Curing of Thiol-Enes 112
xiv
xv
5.7. Comparison of EB with UV in Curing and Cure Grafting
of CT Complexes 114
5.8. Conclusion 116
Chapter Six: Spectroscopic Analysis of Polymers After Curing
and Grafting Reactions 117
6.1. Introduction 118
6.2. UV Cured Polymers from Mixtures of MA/DVE-3 118
6.3. Analysis of Cure-Grafted DVE-3/MA on Cellulose 123
6.4. Scanning Electron Micrographs of MA/DVE-3 Polymerised
on to Cellulose 125
6.5. Conclusion 128
Chapter Seven : Conclusions 130
Chapter Eight: Literature references 134
Appendix 143
Chapter One
A Review of Radiation Curing and Grafting of
Charge Transfer (CT) Monomer Complexes
1.1. Introduction
Radiation chemistry is used extensively in polymer industries and in research. The
technology has been in use since the 1950’s and particularly since the 1970’s during an
era where demand for energy conservation became a major concern to the general public
1-3.
Photopolymerisation initiated by ultraviolet (UV) light has received considerable
attention for industrial applications due to the rapid rates of polymerisation as well as
providing solvent-free curing and grafting of polymer films. Many UV polymerisations
proceed rapidly at room temperature with a fraction of the energy required of thermal
processes1-3
. Significant reasons such as environmental concerns associated with volatile
organic emissions and the need for high speed reactions to enhance production rates
encouraged rapid development of these systems. As a result, UV initiated
photopolymerisation processes are finding applications in many industries, including
coatings, inks, adhesives and electronics.
UV curing is a process where a polymeric material is formed after rapid reaction of
usually a monomer/oligomer mixture on a substrate but adhesion of the film is dependent
on intermolecular forces between the polymer and the substrate (Van der Waals or
London dispersion forces). In UV grafting however, the process involves chemical bond
formation between the polymer and the substrate (Figure 1.1)1-4
. Cure grafting involves
formation of chemical bonds to a substrate during the curing of a film. Radiation grafting
is also time dependent whereas radiation curing occurs in a fraction of a second.
2
+ O/Mhv
Curing
+ O/Mhv
Grafting
Figure 1.1: Curing and grafting processes (where O/M represents oligomers/ monomer
mixtures.)
The components of formulations for UV cured coatings usually are comprised of
monomers and oligomers, photoinitiators, additives and pigments. Monomers are
classified by the number of their reactive groups as being either monofunctional,
bifunctional or multifunctional. Monomers such as the acrylate series are highly reactive
only in the presence of a photoinitiator. The use of the acrylates however does have
limitations in that residual monomer in polymerised samples may lead to skin and eye
irritation1-4
.
One possible problem with UV curable coatings is the migration of unreacted material
from the coating. These compounds may be present as reactant residues or degradation
products. Photoinitiators are never fully reacted during polymerisation and may have
adverse occupational health and safety effects5-7
. Multifunctional photoinitiators are
compounds which react with the curing matrix and are incorporated into the finished
film, hence reducing the levels of unreacted photoinitiator that are capable of migrating8,
9.
3
Graft copolymerisation of vinyl monomers to polymer substrates has been intensively
studied. Synthetic polymers for grafting include isotactic polypropylene, poly (ethylene
terephthalate), polycarbonate and nylon-6. Natural polymers such as cellulose have also
been investigated, particularly for industrial applications. Some of the vinyl monomers
for grafting include maleic anhydride and the maleate esters, vinyl ethers, vinyl silanes,
thiolenes, acrylates and methacrylates. Grafting can be used to produce polymeric
materials with improved chemical and physical properties such as adhesion and solvent
resistance. Grafting may also affect compatibility or miscibility with other polymers and
the formation of composites having enhanced mechanical properties and appearance10
.
1.2. Substrates
1.2.1 Cellulose
Two substrates were used as reference materials, cellulose and polypropylene. The
former represents polar materials whilst the latter is non polar. The structures of each of
the substrates will be discussed, and the grafting reactions of each reviewed. This grafting
section will be brief since grafting per se is not the predominant subject of the thesis.
Reference will be made to grafting using both radicals and metal ions since the
intermediates in both systems are similar. Cellulose (α - cellulose) is a long chain
polymer, made up of repeating units of glucose, a simple sugar (Figure 1.2). Cellulose is
the most abundant naturally occurring organic substance, being found as the principal
structural component of cell walls in higher plants. Light scattering methods reveal that
the cellulose chains have 5,000 to 10,000 glucose residues, and there is no branching. In
the cellulose chain, the glucose units are joined by single oxygen atoms (acetal linkages)
between the carbon of one ring and the next ring. Since a molecule of water is lost when
4
an alcohol and a hemiacetal react to form an acetal, the glucose units in the cellulose
polymer are referred to as anhydroglucose units11
.
Figure 1.2: Structure of Cellulose
The beta-glycosidic links play a central role in determining the structural properties of
cellulose, and thus the strength of the cellulose fibres. Because of the beta-links, the chain
assumes an extended rigid conformation (essential for fibres), with each glucose residue
turned 180° from its neighbour. Another consequence of this structure is that OH groups
of adjacent chains allow very extensive H-bonding between chains11-13
. This extensive
inter-chain H-bonding, and rigid beta-conformation makes cellulose fibres very strong
and able to resist reaction in very strong basic solutions11-13
.
The interchain hydrogen bonds in the crystalline regions of cellulose are strong, giving
the resultant fibres good strength and insolubility in most solvents. They also prevent
cellulose from melting (i.e. it is non-thermoplastic). In the less ordered regions, the chains
are further apart and the hydroxyl groups are available for hydrogen bonding to other
molecules, such as water. Most cellulose structures can absorb large quantities of water
(i.e. they are very hydroscopic). Thus, cellulose swells, but does not dissolve, in water
and other polar solvents11-18
.
5
The most chemically reactive groups in the cellulose are the hydroxyls, and reactions
allow cross-linking or grafting to occur of another polymer to the backbone of cellulose14-
18.
1.2.2. Cellulose grafting with UV and Ionising Radiation
Swelling is one of the important parameters influencing grafting reactions to cellulose.
Swelling of cellulosic fibres may increase the surface area of contact with the solution of
the monomer. The pores provide an area for polymer radical diffusion and also hydroxyl
rich surfaces for adsorption and reaction. Cellulose fibres can be swollen with polar
solvents and penetrated with hydrophilic monomer systems. Polar swelling agents like
alcohols can also be effective diffusion promoters of monomers into cellulosic fibres. The
intercrystalline swelling of cellulose by polar swelling agents occurs predominantly in the
amorphous regions of cellulose11
. The polar hydroxyl groups of cellulose can be easily
modified by radiation grafting with monomers19, 20
.
Radicals are formed in cellulose by exposure to radiation, particularly from ionising
sources. Gamma radiation also ruptures carbon-carbon double bonds and produces
radicals, as depicted in Figure 1.3. Chain scission may also generate other radicals. In the
presence of oxygen, the γ-irradiation may produce cellulose diperoxides and
hydroperoxide species by a radical chain reaction process 21-23
.
UV preirradiation particularly with excimer sources can be used to improve the grafting
yield on cellulose in the presence of oxygen. The cellulose material has been irradiated
without monomer to produce surface radicals. However, oxidative reactions that occur at
the surface of the cellulose change other properties of the material24
.
6
H
O
H
OH
OH
CH2OH
H
H
OH
H
O
H
OH
OH
CH2OH
HOH
H
O
OH
OH
CH2OH
H
H
OH
H
O
H
OH
O
CH2OH
H
H
OH
H
O
H
OH
OH
CH2OH
H
H
H
O
H
OH
CH2OH
H
H
OH
hv
. . .
. .
H
O
H
OH
OH
CH2OH
H
H
OHhv
H
O
H
OH
OH
CH2OH
H
H
OH. .
H
O
H
OH
OH
CH2OH
H
H
OH
OH
OHH
CH2OH
H
H
O
H
OH
hv
H
O
H
OH
OH
CH2OH
H
H
OH
OH
OHH
CH2OH
H
H
O
H
OH
.+
Figure 1.3: Effect of UV radiation on cellulose
Gaylord et al15, 16, 24, 25
demonstrated that vinyl grafting to cellulose was enhanced in the
presence of ceric ions without the use of radiation, a result however which is of relevance
to the current radiation project. Grafting of methyl methacrylate (MMA) to cellulose
produced high molecular weight graft copolymers. Gaylord proposed that a complex was
formed between the cellulose, solvent and monomer. Cellulose provides hydroxyl groups,
and can act as a donor or acceptor, depending on the polarity of the monomer. This result
is interesting since it shows that basic complexes in these systems could be formed
without radiation.
7
Liu Yinghai et al26
established that grafting of methyl acrylate to cellulose can be
achieved by using copper salts as an initiator without the use of radiation by forming
metal complex intermediates. Thermogravimetric analysis showed that grafting of methyl
acrylate onto cellulose improved the thermal stability of pure cellulose. Also the presence
of poly (methyl acrylate) enhanced the hydrophobic properties compared to that of pure
cellulose.
Dilli and Garnett19
studied the grafting of vinyl monomers to cellulose using ionising
radiation, however they reported low yields for reactions performed in nonpolar solvents.
Higher yields were reported for systems grafted in polar solvents such as dimethyl
formamide, acetone and a series of alcohols.
Gupta and Sahoo27
developed grafting systems which were initiated by ceric ions using a
mixture of acrylonitrile and methyl acrylate which was again a radiation free system. The
grafting yields were found to be dependent on the monomer concentrations.
1.2.3. Polypropylene
Polypropylene (PP) is an important thermoplastic material with applications as fibres,
films and consumer goods. PP is a hydrocarbon polymer, and it is nonpolar. The
functionalisation of PP by grafting can improve the properties of the polymer. Isotactic
polypropylene is the only stereoisomer of this polymer which has significant industrial
applications (Figure 1.4).
8
CH
2
CH
CH
2
CH
CH
2
CH
C H3
CH
2
CH
C H3
C H3
C H3
CH
2
CH
CH
2
CH
CH
2
CH
CH
2
CH
C H3
C H3
C H3
C H3
CH
2
CH
CH
2
CH
CH
2
CH
CH
2
CH
C H3
C H3
C H3 C H
3
Atactic Polypropylene
Isotactic Polypropylene
n
n
Syndiotactic Polypropylene
n
Figure 1.4: PP derivative structures
PP is usually surface treated before dyeing of fibres, printing of films, painting or gluing
to other components. Chemical modification of PP is required to generate appropriate
functional groups at the surface. When PP is irradiated using γ- or UV radiation in the
presence of air, hydroperoxides (PP-OOH) are preferentially formed from the large
number of labile tertiary hydrogens are available on the polymer. Hydroperoxide
generates a hydroxyl radical upon decomposition, and the polypropyleneoxy radical9, 11,
12, 28, 29 (Figure 1.5).
Polyolefins have a low surface free energy. Corona discharge is used to pretreat the
surface to improve adhesion. In corona discharge treatment, the electrical discharge
contains electrons, ions and radicals. These cause surface reactions which result in polar
groups on the surface12
. Changes in wettability on aging are attributed to migration and
reorientation of functional groups within the modified surface region29, 30
.
9
CH
CH3
CH3
CH3
OO
CH2CH2 C.
O2
CCH2 .
γhv
C
CH3
CH2
OO.
+ C
CH3
CH2
H
C
CH3
CH2
OO
.+ C
CH3
CH2
H
C
CH3
CH2
O
C
CH3
CH2
O M
..
+ M (M)n
OH
(M)n
OH
C
CH3
CH2
OO
C
CH3
CH2
O
C
CH3
CH2
O
H
hv
.+ OH.
.M
M
Figure 1.5: Radical reactions of hydroperoxides after UV irradiation31, 9, 11, 12
.
10
Abstraction of a proton from the tertiary carbon or the methyl pendant group of the PP is
proposed for systems using radical initiators30
(see Figure 1.6).
CCH
2
CH
2CH3
CCH
2
CH
2CH2
.
(I) (II)
.
Figure 1.6: Tertiary PP radicals
Dole32
proposed that other radical species may be produced at high irradiation doses for
PP (Figure 1.7).
CHCH
2
CH
2
CH
CH CH
CH
CH CH
..
alkyl(III)
allyl
(IV)
polyenyl
(V)
.( )
n
Figure 1.7: Tertiary radicals from irradiation of PP
Morra and co-workers33
grafted polar monomers onto polyolefins by radical
intermediates formed using UV radiation. SEM analysis showed that the surface is rough
which may be due to the difference in the reactions of the amorphous and crystalline
domains of the polymer.
Kern et al34
reported that UV induced eximed dimers could give surface modification of
polymers in the presence of reactive gases and modifiers. The depth of the surface
penetration was shown to exceed that of corona and plasma treatments.
11
Borsig et al35
has proposed that PP grafting can be improved by chemical modification of
the PP chain. This can be achieved by chlorination of PP initiated by UV radiation in a
solution of a chlorinated solvent like CCl4. PP grafting may occur by direct irradiation of
PP in the presence of monomers like styrene, vinylpyridine, MMA, methyl acrylate (MA)
and acrylamide. The β-scission of the PP chain occurs with the formation of a terminal PP
radical and a vinylidene end group at which grafting reactions may then occur.
1.3. Photografting of Vinyl Monomers
1.3.1. Grafting mechanism
Grafting involves formation of a chemical bond between the polymer and the reacting
monomers and this may be achieved via free-radical or ionic polymerisation processes.
There are three steps involved in the mechanism for grafting via radical polymerisation.
Firstly, initiation needs to take place for the monomers to become graft reactive through
the formation of monomer radicals. The formation of an active site for graft on the
polymer substrate or backbone must also occur. The source of the initiating radicals is
usually the photochemical decomposition of the photoinitiator. Radicals can not only be
produced by direct irradiation of the photoinitiator (PI) but also via energy transfer from a
sensitiser to the photoinitiator. The radicals on the polymer substrate are formed by
hydrogen abstraction reactions. The surface radicals can undergo additional reactions
including β-scission and addition to a monomer31
.
The substrate, S, is usually irradiated while in direct contact with the monomer, MH. The
levels of grafting are dependent upon the relative concentrations of monomer and
backbone polymer, SH, but also upon the level of photoinitiator, PI (equations 1.1 – 1.4).
12
)4.1(
)3.1(
)2.1(
)1.1(
..
..
..
..
GraftSM
SIHISH
MPHPMH
IPPI
→++→++→+
+→
1.3.2. Factors Influencing the Grafting Reactions
The grafting yield depends on the rate constants for formation of radicals (initiator,
monomer and substrate), propagation rate constants for polymerisation of monomers, and
for termination rate constants of substrate and polymer radicals by combination.
Polymerisation begins by creation of free radical sites by decomposition of the initiator,
and then reaction of these radicals to form monomer radicals. These add more monmers
to produce a growing polymer chain. Reactions of the polymer radicals with substrate
radicals produces covalent bonds between the substrate and the polymer (reactions 1.5-
1.13)36
.
Initiation
I 2R •
(1.5) ⎯→⎯kd
S + R • S
• (1.6) ⎯→⎯ki
S •
+ M SM •
(1.7) ⎯→⎯ki
M + R
• M
• (1.8) ⎯→⎯ki
Propagation
SMn •
+ M SMn+1 •
(1.9) ⎯→⎯kp
Mn •
+ M Mn+1 •
(1.10) ⎯→⎯kp
Termination
SMn •
+ SMn •
graft polymer (1.11) ⎯→⎯kt
SMn •
+ S•
graft polymer (1.12) ⎯→⎯kt
Mn •
+ Mn •
homopolymer (1.13) ⎯→⎯kt
13
The accessible surface area of the substrate may be increased by addition of a swelling
solvent. This is important for increasing the diffusion of reactants into the substrate. The
swelling ability of the solvent is dependent on its dielectric constant which should be
similar to that of the substrate. Solvent structure is also important because solvents such
as alcohols have hydroxyl groups which may form radicals via abstraction reactions.
Termination usually occurs by combination for most vinyl monomers, although
disproportionation to give an alkene and an alkane occurs when the propagating radical is
sterically hindered or possesses easily abstractable β-hydrogens31
.
1.4. Homopolymerisation
A potential competing reaction to grafting is homopolymerisation of the monomer
leading to low grafting yields. The ultimate effect of homopolymerisation is that the
system gels. At this point it is difficult to remove the grafted substrate which is locked
into the gel. The gel point is when the reaction mixture transforms from a liquid to a three
dimensional matrix37
. A polymer gel is a semisolid system consisting of a polymer
network with a significant liquid fraction trapped within the network. The gelation point
occurs when the resin viscosity has increased to a point where it no longer flows. The gel
point of a mixture may be estimated by calculation of the branching coefficient, α. The
branching coefficient represents the probability that a given functional group on a branch
unit is bonded to another branch unit. The functionality of the branch units is represented
as f and the critical value of α for gelation is:
αc = 1/(f-1) (1.14)
14
As the viscosity of the reaction mixture increases, the rate at which polymers diffuse
through the medium should decrease. A considerable amount of the monomer mixture
may be trapped in the polymer network37
.
In model studies by Hstu38
, he proposed that the polymerisation process can divided into
five stages: induction, microgel formation, phase separation, macrogelation and post-
gelation. The induction stage involves the decomposition of initiators and the reaction of
these with monomer to form monomeric radicals. Monomer systems with phase
separation have a co-continuous structure. At the gel point, a crosslinked network was
formed through either the intermolecular reaction among microgels, microgel clusters or
dispersed domains.
Abnormal gelation phenomena were observed during some polymerisation reactions and
these are called popcorn or cauliflower polymers. These structures are hard, white and
opaque in appearance and are readily distinguishable from the rest of the polymer. These
popcorn polymers do not swell in solvent. Baker39
proposed a mechanism for the
formation of popcorn polymers. Initiators nucleate sites that react to the gel point. As the
network is formed, mobility of active initiator sites decreases as the viscosity of the
solution rises. The rate of the monomer diffusion becomes limited, and the chains
surrounding the active site begin to break due to contraction during polymerisation. More
radicals are formed as the result of chain breakage and these radicals initiate new polymer
chains. Growth of the popcorn polymer may proceed as long as unreacted monomer is
available to feed the additional radicals that are formed.
15
1.5. Photoinitiators
Photoinitiators can be used in both curing and cure grafting reactions. The photoinitiators
are common for both systems, and the treatment outlined here will be relevant also to
curing which will be discussed later in this thesis. Two types of photoinitiators (PIs) are
commonly used in photografting, either free radical or cationic. The initiators
predominantly used in this thesis were free radical thus only these PIs will be described.
Initiator free radicals are generated by photochemical homolytic cleavage of covalent
bonds. These primary radicals add to carbon-carbon double bonds of the monomer
resulting in primary propagating radicals. Branching, crosslinking, and graft
copolymerisation may occur as a result of hydrogen abstraction by primary radicals. The
rate of initiation is thus determined by the rate of decomposition of the initiator38-40
.
Free radical photoinitiators (FRPI) are useful especially for polymerisation reactions
using mercury lamps. Photosensitisers may be used as additives with FRPI to improve
rates of polymerisation, however later developments with more efficient FRPI has tended
to eliminate the need for the photosensitisers. The range of FRPI include benzophenone;
Michlers ketone; thioxanthones, benzoin ethers such as α,α-dimethoxy-2-phenyl
acetophenone (DMPA); α,α-diethoxy acetophenone; α-hydroxy-α,α-dialkyl
acetophenones such as α-hydroxy-α,α-dimethyl acetophenone and 1-benzoyl
cyclohexanol; cyclic PIs such as cyclic benzoic methyl esters and benzil ketals; and
finally acylphosphine oxides such as 2,4,6-trimethyl benzylol diphenyl phosphine oxide
and bis (2,6) dimethoxy benzylol ( 2,4,4-trimethyl phenyl) phosphine1.
In early studies, benzoin ethers were used as FRPI and although these materials yield two
radicals when exposed to UV, only the acetophenone radical is very active (Figure 1.6).
The recently developed acylphosphines are much more reactive than most other classes
16
of FRPI (Figure 1.6) because decomposition of one molecule of the acylphosphine leads
to four active radicals 1.
Scherzer41
determined that the absorption spectrum for the photoinitiator must be well
matched to the wavelength of the incident radiation. Most commercial photoinitiators
absorb at wavelengths of the emission spectrum of mercury lamps. The polymerisation
rate is proportional to the square root of the fraction of the absorbed light and to the
square root of the intensity of the incident UV radiation. The polymer conversion
increases with both increasing photoinitiator concentration and the light intensity.
OMe
OMe OMe
OMe
O
P
O
C
O
CH2
CH
CH
2
CMe3
Me
C
C
O
C
OEt
C
O
C
OEt
OMe
C
O
CH2
CH
CH
2
CMe3
Me
OMe
OMe
C
O
OMe
P
O
hv+. (1.6).
hv
+ + (1.7). .. .
Figure 1.6: Radicals formed by the decomposition of benzil ethyl ketal and bis(2,6-
dimethoxybenzoyl)(2,4,4-trimethylpentyl) phosphine oxide.
Deng et al42
, reported that radiation grafting of thick films can be improved by immersion
of the substrate in a solution of acetone containing FRPI before coating with the
monomer formulation and this technique is called the preabsorbing technique. The FRPI
17
was given time to penetrate into the substrate to give a homogeneous distribution. The
grafting yields for the preabsorbing method were higher than for the one step
simultaneous process.
1.6. Lewis acids
The presence of Lewis acids can enhance the reactivity of certain radiation
polymerisation processes. These acids have also been shown to catalyse some non-
radiation induced polymerisation reactions. Simultaneous radiation grafting of styrene to
rayon can be enhanced in the presence of Cu2+
and Fe2+
and it was proposed that the
cations suppressed the homopolymerisation process by scavenging the radicals thus
allowing polymer radical chains to graft43
.
Hirooka and co-workers44
discovered that metal halides form complexes with reacting
monomers because a carbonyl group in the conjugated position reacts spontaneously
with donor monomers. In the presence of alkylaluminium halide, a 1:1 alternating
copolymer was formed with trialkylborons (BR3). It was also suggested that the
trialkylborons may initiate free radical polymerisation.
Gaylord and co-workers45, 46
found that ZnCl2 increased grafting of copolymers of
acrylonitrile-styrene to cellulose, and alternating copolymers were formed in the presence
of the metal halide. Acrylonitrile-styrene may form a coordination complex with ZnCl2
and the cellulose hydroxyl groups. Alternatively the sites for grafting on cellulose may be
the aldehyde groups generated by oxidation or hydrolysis by the ZnCl2.
18
Zhang et al47
, investigated the copolymerisation of MMA and 2-(1-acetoxyethoxy)ethyl
methacrylate (AEEMA). It was found that the solvent, THF, which had been used in the
preparation of the backbone copolymer acted as a weak Lewis base thus stabilising the
propagating carbocation.
Polymerisations involving strong Lewis acids like SnCl4 and SnBr4 spontaneously occur
after the addition of the initiators without exposure to UV radiation. It was reported that a
weak Lewis acid like ZnCl2 gave soluble low molecular weight oligomers and incomplete
monomer conversion. However if HCl was used in combination with ZnCl2, polymers of
relatively high molecular weight were obtained and at high monomer conversion48
.
Dayte49
reported that the mode of catalysis of acrylate and methacrylates by the Lewis
acids appears to be coordination of the carbonyl oxygen atom of the monomer thus
activating the monomer towards nucleophilic attack by the radicals. Acrylates are more
reactive than methacrylates in Lewis acid catalysed reactions.
Lewis acids catalyse carbocationic polymerisation of vinyl ethers due to the resonance
stabilisation of the lone pair of electrons on the oxygen by the complexes. Cationic
palladium complexes with imine ligands increase the rates of polymerisation of vinyl
ethers. Transition metal catalysts for polymerisation of vinyl ethers provide good control
of molecular weights and give high conversions, without using an initiator50
.
Garnett et al51, 52
reported that the presence of mineral acids such as H2SO4 increased the
grafting yield of styrene to polyethylene. Hydrochloric acid was shown to strongly inhibit
grafting of styrene with concurrent significant incorporation of chlorine in the polymer
19
and copolymer. This suggests that the chlorine from HCl can also terminate the
propagating chain.
ZnCl2 has been shown to accelerate the rate of polymerisation of vinyl monomers
containing ester and nitrile groups and this was attributed to complexes formed with the
monomers and ZnCl253
.
Complexes of methyl methacrylate and methacrylonitrile with Lewis acids like SnCl4,
AlCl3 and BF3 have been copolymerised with styrene under irradiation with a high
pressure mercury lamp. The copolymers had equimolar concentrations of methyl
methacrylate (or methacrylonitrile) and styrene, regardless of the molar ratio of the
monomers in the feed. The polymer structures were shown to be alternating copolymers
by NMR54, 55
. Thus these Lewis acid complexes can themselves initiate certain
polymerisation processes after exposure to UV radiation.
1.7. Solvent Effects
Solvents are used to assist in the grafting process. They may be used to control the rate of
grafting to polymers. A good solvent serves to swell the polymer and allows diffusion of
radicals into the substrate. Solvents which had similar polarity to the substrate were
chosen for swelling of polymers. Cellulose is an example of a polar polymer, and
solvents with high dielectric constants were expected to have greater swelling properties
than non polar solvents56, 57
. With cellulose and similar substrates, the polarity of the
solvent was expected to be the determining factor in whether the substrate polymer was
efficiently swelled and diffusion of reactive radicals into the substrate occurred58-60
.
20
However, with polar substrates, the use of non-polar solvents would be expected to
weaken the interaction forces between the substrate and the functional monomers
resulting in low graft yields. The properties of the substrate such as specific surface area
and the pore diameters are dependent on the polarity of the solvent (Table 1.1).
Thus, dimethyl formamide (DMF), a polar solvent, greatly swells polar substrates unlike
hexane. A greater surface area and monomer diffusivity will lead to higher accessibility
to the grafting sites61
. With respect to non polar substrates, typified by PP, swelling of the
substrate again enhances grafting.
Table 1.1: Swelling properties of common organic solvents
Solvent Cellulose PP Dielectric Constant
CCl4 - + 2.24
CHCl3 - + 4.8
CH2Cl2 - + 9.08
Benzene - + 2.28
Toluene - + 2.4
Methanol + - 33.6
Acetonitrile + - 36.6
Ethanol + - 24.3
DMF + - 38.0
DMSO + - 48.0
Ethyl acetate + - 6.0
Acetone + + 20.7
Cyclohexene - + 2.0
THF - + 7.52
Water + - 80.4
(+) – swelling; (-) – no swelling
21
The swelling of PP is more pronounced in chlorinated solvents, such as chloroform and
dichloromethane, than in either acetonitrile or tetrahydrofuran. This swelling behaviour
may lead to changes in the three-dimensional configuration of the functional groups
taking part in the grafting of these the sites resulting in improved grafting 62-64,
.
Halogenated solvents can also act as initiators in some systems. Dass64
reported that
electron-acceptor complexes react with halogenated solvents to produce free radicals,
which initiate vinyl polymerisation. Free radical polymerisation may be inhibited by
transition metal ions, because the primary radicals are consumed by these ions.
1.8. Curing and Cure Grafting
1.8.1. Charge Transfer Complexes
Curing, especially with UV has only been a relatively recent technological development.
The process involved essentially polymerisation of oligomer/monomer mixtures under
the influence of powerful radiation sources such as high pressure, high voltage UV lamps
or electron beam (EB) machines. The oligomers and monomers used are usually
acrylates. The predominant oligomers are epoxy, urethane, polyester and polyester
acrylates whilst the monomers are multifunctional materials like tripropylene glycol
diacrylate. Photoinitiators similar to those already discussed in photografting are used to
accelerate cure.
The technology has remained with this generic acrylate chemistry until recently when
new developments, such as the use of charge transfer (CT) complexes have been
introduced. Much of the work in this thesis has investigated curing of CT complexes.
22
A charge transfer complex (CTC) is formed when a partial electron-transfer takes place
between a pair of alkenes, i.e. a donor and an acceptor, as described by Mulliken’s
theory65
. The electron donor molecule, D, must possess high electron density due to the
presence of nitrogen, oxygen and sulphur. An electron acceptor molecule, A, on the other
hand, is able to accept electron density and stabilize the additional charge through
resonance or induction. A mechanism was proposed where CT complexes are formed
from the single electron transfer from an electron rich to an electron poor alkene. A
proton may also be transferred from a radical cation to a radical anion to give a sp3
carbon radical and a sp2 carbon radical. One or both of these radicals may then initiate
polymerisation66
.
Schmidt et al67
, reported that the reaction mechanism of charge transfer complexes should
be dependent on the strength of the interactions between the monomers which can be
quantified via equilibrium constants. However, steric and charge effects also have an
influence on the complex formation.
CT work which is of value to the present radiation processing has been previously
investigated in non radiation systems68
. Complexes may be formed by the interaction
between donor and acceptor molecules which may be in either ground or excited states.
The excited state complex can be formed alternatively through excitation of either the D
or A molecule, followed by the interaction with the partner, to yield an exciplex in the
presence of UV from the interaction of two DA olefins 69
.
Hall et al70
, investigated the spontaneous reactions of electron rich and electron poor
alkenes to form adducts as shown in Figure 1.7.
23
D
A
+
ionic homopolymerisation
Free radical copolymerisation
[2+2] cycloadduct
[4+2] cycloadduct
other small molecules
Figure 1.7: Reactions of donor and acceptor molecules after UV irradiation.
Alkenes of like polarity usually do not polymerize without initiator via this mechanism
but if there is a moderate difference in electron density of the two olefins this may cause
spontaneous free radical copolymerisation. Tetramethylenes intermediates are formed in
the presence of UV by coupling of the alkenes at their β-positions and can be either
singlet diradical or zwitterionic depending on the terminal α-substituents. The weaker
donor and acceptor form a predominantly diradical tetramethylene intermediate which
can initiate free radical polymerisation. The more nucleophilic and more electrophilic
alkenes form a predominantly zwitterionic intermediate which can initiate ionic
polymerisation. Solvent polarity also influences formation of the tetramethylenes, with
zwitterions being favoured in more polar media, while diradicals are favoured in nonpolar
media. The electrophilic character of acceptor olefins can be increased with the use of
Lewis acids71
.
Weaker donor and acceptor olefins form biradical tetramethylene intermediates that can
initiate free radical copolymerisation, while the more nucleophilic and more electrophilic
olefins form a predominantly zwitterionic intermediate that can initiate ionic
homopolymerisation. If the initiating species is a diradical, then each growing polymer
chain will have two radical ends72-74
.
24
Polymerisation of CT complexes may be dependent on the wavelength of light used to
initiate the process. Hall et al, reported that direct excitation of complexes at red-shifted
UV wavelengths yielded a copolymer, however at lower wavelengths the product was a
cycloadduct. Hydrogen abstraction may take place from the D and A, to give either an
excited state CT complex or an exciplex. Depending on the chemical composition of A
and D, the initiation can start directly by a hydrogen abstraction from the 1,4-biradical
and it will also participate as a chain transfer agent under prolonged irradiation75
.
1.8.2. CT complex free radical and cationic polymerisation
Jönsson and co-workers76
, have expressed serious concerns about the use of initiators in
coatings because of their toxicology especially effects of sensitisation and skin irritation.
The advantages of the CT polymerisation process are that no photoinitiator is
theoretically needed to initiate polymerisation. The direct photolysis of a CT complex
will lead to polymerisation via a free radical process.
Several combinations of acceptors and donors have been proposed for polymerisation of
CT complexes75
(Figure 1.8). Thus if the DA complex is prepared with a 1:1 double bond
molar ratio, alternating copolymerisation should occur. With excess donor, cationic
homopolymerisation would be expected whilst with excess acceptor anionic
homopolymerisation would be predicted.
25
D
A
D
A
D
A
D A
D
A
A D
D
A
)(
Free radical copolymerisation
Cationic polymerisation
Anionic polymerisation
(
(
)
)n
n
n
+-
..
hv
hv
excess
excess
Figure 1.8: Donor and acceptor polymerisation
The rate of polymerisation should be a maximum at a monomer composition ratio of 1:1
where the concentration of DA is greatest. If the DA adds to the chain end in a concerted
manner, then an alternating copolymer should be formed but if the non-complexed
monomers add to the chain end, random composition would be expected. Experimental
parameters such as composition, monomer concentration and donor-acceptor pairs should
affect the composition of the copolymer in DA systems75,76
.
The reactivity of CT complexes in UV systems have been investigated by Nagarajan and
co-workers76
and the results show increased cure speeds for clear coat formulations in the
presence of a maleimide and benzophenone, the latter acting as a sensitiser. Benzoyl
biphenyl (BBP) was a better sensitiser for maleimides than the benzophenone, and 3-
methyl benzophenone has also been shown to give enhancement in UV cure rates.
26
Kuznetsov et al77
reported that complex formation of radicals may be responsible for
production of alternating copolymers. It is now acknowledged that alternating
copolymerisation may take place according to two mechanisms, either as
homopolymerisation of a complex formed by comonomers, or if the reactivity ratios of
both monomers are zero.
According to Neckers and co-workers78
, direct absorption of UV light by the
donor/acceptor complex occurs in the ground state, where the complex absorbs at longer
wavelengths than the individual components. When the irradiation of the mixture occurs
at the specific wavelength of absorption of the complex, instantaneous polymerisation
resulted.
1.8.3. Role of the Acceptor - Maleic anhydride
Although a range of acceptors can be used in radiation processing, maleic anhydride
(MA) was chosen for the bulk of this work as an acceptor. Jönsson et al79
, has performed
extensive analysis via photodifferential scanning calorimetry (DSC) on many acceptor
and donor monomers and the results were used to categorise each of the monomers
according to the rates of initiation and CT complex formation. The results for maleic
anhydride had indicated that it is a strong acceptor.
MA, like most other anhydrides, can be hydrolysed to a corresponding dicarboxylic acid
(Figure 1.9). Simple dissolution of maleic anhydride in water suffices. The process is
exothermic. The maleic acid can be converted to fumaric acid by heat. Maleic anhydride
is the most reactive of the anhydrides, and the relative dissociation constant for
complexes of maleic anhydride was about ten times higher than that of other anhydrides.
27
Maleic anhydride may also give rise to two different ester derivatives upon reaction with
an alcohol.
O
OH
CH
CHOH
O
O
OH
CH
O
OH
CHO
CH
CH
O
O
maleic acid fumaric acidmaleic anhydride
Figure 1.9: Maleic anhydride and its hydrolysed derivatives.
Maleic anhydride - styrene was shown to form an alternating copolymer via thermal
initiation. Halogen derivatives of maleic anhydride were electron acceptor monomers and
although the relative rates of polymerisation varied, polymerisation gave alternating
copolymers80-82
.
Rzaev83
studied the copolymerisation of maleic anhydride with allyl monomers to form
high molecular weight products. The formation of CT complexes in the initial monomer
mixture was shown to inhibit degradative chain transfer. CT complexation of monomers
such as allyl acrylate, allyl methacrylate with maleic anhydride was shown to occur but
with increased cyclisation during formation of the macromolecules.
Polymers that contain anhydride residues are important because of their high reactivity
towards –NH2 and –OH functional groups, while succinic groups are potentially able to
undergo transesterification or alcoholysis reactions with –OH terminated polyesters84-88
.
28
1.8.4. Role of the Donor-Vinyl Ethers
Vinyl ethers such as triethylene glycol divinyl ether (DVE-3) are convenient donors for
the formation of CT complexes because DVE-3 is a liquid which can dissolve solid
monomers such as MA.
Vinyl ethers are light sensitive and readily absorb UV light in the range of 190 – 240 nm.
Vinyl ethers possess electron rich structures and radicals typically form upon irradiation
due to the thermodynamic stability of the radical generated α to the oxygen. Vinyl ethers
are susceptible to attack by radicals. The radical attacks the terminal methylene of the
vinyl ether, and the radical product is stabilised by the oxygen atom of the ether group89-
91 (Figure 1.10).
OO
OO
ground state
hv
OO
OO
.excited state
Figure 1.10: H- radical of a vinyl ether
Vinyl ethers do not homopolymerise readily in the presence of free radicals. Radicals
may be generated through hydrogen abstraction reactions, solvent radicals or the self
29
sensitizing property of the newly formed charge transfer complex formed with an
acceptor monomer. CT complex formation in solution of maleic anhydride and divinyl
ethers have been identified using UV spectroscopy. The formation of a copolymer with a
composition of maleic anhydride/butyl vinyl ether (2:1 molar ratio) has been reported by
Butler and co-workers87, 91
.
Vinyl ethers are known to be very reactive monomers that undergo fast cationic
polymerisation. However, Braun92
reported that vinyl ethers do not undergo free radical
polymerisation because of the highly nucleophilic nature of the double bond which is
caused by induction by the alkoxy group.
Vinyl ethers have been reported to form copolymers with many acceptor monomers. Hao
et al89
had worked with isobutyl vinyl ethers and had produced donor-acceptor complexes
with maleic anhydride in chloroform. Monomers such as maleic anhydride and dimethyl
maleate can be polymerised with alkyl vinyl ethers to form alternating copolymers which
then can be further esterified93, 94, 97
.
Kohli95
had reported that maleimide monomers copolymerise with vinyl ethers to yield
both cis and trans conformers but radical initiation is the rate limiting step. Charge
transfer constants were determined by 1H NMR and the complexes were found to be 1:1.
Vinyl ether complexes have been shown to undergo self initiating reactions by Von
Sonntag96
. The copolymerisation of butyl vinyl ether with MMA charge transfer
complexes occurred only by a radical mechanism.
30
1.8.5. Analysis of Polymers Prepared from CT Complexes
Many instrumental methods have been employed for analysis of these polymers. Of
particular significance has been the results obtained using real time infrared (RTIR)
spectroscopy. Decker et al98
found that vinyl ethers in the presence of cationic
photoinitiators would rapidly polymerise upon UV exposure and this continued even after
reaction had ceased. Unlike free radical polymerisation, cations do not terminate by
combination. Increasing the formulation viscosity by the introduction of oligomers was
found to have a retarding effect on the UV curing rates, because of the reduced mobility
of the reactive species. RTIR may give the polymerisation profile of each one of the two
monomers during the whole polymerisation process. The chemical structure of vinyl
ethers was found to play a key role on both the cure speeds and the final conversions.
EPR studies by Knolle99
had shown that radical cations of vinyl ethers led to the
formation of a dimer like cation structures. Their EPR spectra and thermal changes were
similar for all of the vinyl ethers despite the different monomer structures for the vinyl
ethers studied.
Radical trapping was used by Jones and co-workers100
to determine whether alkyl
radicals undergo concerted additions and the extent to which complex addition competes
with simple addition of uncomplexed alkenes. The following results had shown that the
consumption of N-phenylmaleimide by the 1-butyl radical generated in N-
phenylmaleimide/2-chloroethyl vinyl ether solutions occurs essentially only by simple
addition of the olefin. No evidence for concerted addition of a comonomer EDA complex
was found.
31
The complexes between vinyl ethers and other monomers usually have a 1:1
stoichiometry, and if the double bond concentration of each monomer are equal, then the
concentration of the complex is proportional to the square of the monomer concentration
and the equilibrium constant. The rate of copolymerisation also depends on the square of
the total monomer concentration and equilibrium constant.
1.8.6. Polyesters as Curing Resins
A convenient method for utilising the UV curing of DA complexes particularly those
involving vinyl ethers is to combine them with reactive unsaturated polymers. For this
purpose, unsaturated polyester resins are valuable. Unsaturated polyester resins are
important for production of composites, textiles and clothing. The polyester may be
dissolved in a reactive monomer such as styrene or vinyl ethers to give lower viscosity
liquids. When these resins are cured, the copolymer is a solid thermoset101
.
The mechanism for radiation curing is a reaction between the polyester and the biradical
from the CT complex or even the vinyl monomer itself. The reaction results in the
polyester chains being crosslinked or grafted by vinyl monomers. The curing behaviour
of polyester resins is very different to epoxies, urethanes or phenolics. Most epoxies and
urethanes begin to increase in viscosity as soon as they are catalyzed and continue to
increase until they are fully cured. Polyesters provide a specific working time (gel time)
during the polymerisation reaction with very little viscosity increase or temperature
change until close to gelation102
.
The polyesters used in radiation curing are essentially linear unsaturated materials. Linear
unsaturated polyesters are prepared commercially by the reaction of a saturated diol with
32
a mixture of unsaturated dibasic acids. For example, a polyester may be obtained from
esterification of propylene glycol and maleic anhydride. Appreciable cis-trans
isomerisation generally occurs during the esterification reaction. If hyperbranched
polyesters are used, the crosslinking density also considerably increases, while at the
same time the glass transition temperature decreases103-105
.
1.9. Thiol-Ene Systems
Thiols such as trimethylolpropane tris 3-mercaptopropionate (TTP) have become
increasingly important in radiation curing. They have predominantly been polymerised
with alkenes. The thiol-ene polymerisation involves a two step propagation sequence
(reactions 1.6 -1.12). The first step involves the addition of a thiyl radical onto the alkene
to generate a carbon-carbon centred radical, the second step is a subsequent hydrogen
abstraction by the carbon centred radical produced in the first step. The first step involves
a free radical addition but the second involves a chain transfer reaction and effectively
produces an addition of a thiol across a carbon double bond. The thiol acts like a chain
transfer agent and limits chain propagation to a single step. The use of higher functional
monomers would result in higher molecular weight crosslinked polymers. Since the
process is free radical, the conversion rate is high106
.
33
)23.1('
22'
2'
2
)22.1('
2'
2''
)21.1('''
2
)20.1('
22''
2'
)19.1(22
'
)18.1(''
)17.1(
.
..
.
..
..
..
..
SRCCHRRHCCHSHRCCHRSHR
CCHRSRSHRCCHRSHRSR
RSSRSR
SRRCCHSHRSHRCCHRSHR
CCHRRSHCHRCHSR
SRIHSHRI
PIhv
PI
→→+
→+→+
→=++→+
+⎯→⎯
Cramer et al108
, found that thiol-ene polymerisation processes are not inhibited by oxygen
and are self polymerisable systems. Photocurable mixtures comprising a trithiol and a
triallyl ether readily polymerise without the requirement of a photoinitiator. Hoyle107, 109
reports that the self-initiated process may occur via direct excitation of the thiol to cleave
the sulfur hydrogen bond and give hydrogen and thiyl radicals capable of initiating the
free radical chain process. Another explanation is that a charge transfer complex forms
between the thiol and the alkene and this is responsible for the ease of polymerisation of
these mixtures.
Bowman and co-workers110
studied the chain transfer of dodecanethiol to dimethacrylates
and diacrylates to determine whether the chain length affects the termination kinetics
when multifunctional methacrylates or acrylates are cured. Mercaptans are often used as
chain transfer agents in free radical polymerisation and are used to control the molecular
weight of linear polymers. The reactivity of thiol-ene systems is greater for vinyl ethers
than for the acrylates and simple alkenes. The reactivity is attributed to the electrophilic
nature of the thiyl radical. Thiol-ene systems can photopolymerise in the absence of a
34
traditional photoinitiator if a strong acceptor such as N-substituted maleimide is included
in the formulation111
.
1.10. Spectroscopic Analysis of Polymers
Charge transfer (CT) complexes are novel self sensitising monomer systems which can
be cured and grafted onto substrates. The amount of concurrent grafting during cure can
be evaluated using appropriate spectroscopic analytical techniques. Mechanistic studies
of the polymerisation process and grafting reactions may be confirmed using these
spectroscopic results. Fourier Transform Infrared (FTIR) and NMR techniques are often
used for monitoring reactions of monomers and may also provide evidence of the
polymer structure.
Solution and solid state NMR which is also an important tool for polymer analysis
provides compositional information on the bulk and the graft materials. NMR can
provide information about polymer stereochemistry and degree of crystallinity.
Additional information may also be obtained to demonstrate changes in the mobility of
monomers or polymers in reaction media. These results provide useful information on the
kinetics of the process, the polymer structure and stereoregularity112, 113
.
Scanning Electron Microscopy (SEM) is an important technique for examining
composites prepared by radiation curing techniques. Characteristic properties like
grafting depth, arrangement of fibres, surface texture, relative structure of the material,
and surface coverage are all important qualities of these materials.
35
1.11. Research Scope of Thesis
The objectives of the project were to study:
(i) Bulk grafting reactions in solution
(ii) UV and EB curing of specific monomer systems
(iii) UV & EB cure grafting of the same monomer systems
The effectiveness of donor and acceptor monomers in forming UV and EB curable CT
monomer complexes was evaluated and compared to results obtained from the curing and
grafting experiments.
The curing of CT monomer complexes are discussed, and the factors which influence the
physical and chemical aspects of these coatings were examined. Determining the relative
rates of polymerisation is important for optimisation of the cure and grafting processes.
Complementary studies such as the effect of additives on cure rates and film properties
have also been studied and the results will aid the understanding of the polymerisation
processes of these CT complexes. Additives which were studied include commercial
initiators, and Lewis mineral acids.
Investigation of the parameters affecting grafting included the use of the preirradiation
and simultaneous methods. Pre-irradiation involved the use of a corona discharge or an
excimer laser as the treatment steps. Grafting yields for all CT complexes and the solvent
effects on the graft yields were determined for several substrates.
36
Chapter Two
Experimental Design and Procedure
37
2.0: UV Sources
For bulk curing, the UV units used comprised four components: a high pressure
200W/inch mercury lamp, reflector units and shielding, a control unit and a cooling
system. For grafting a Phillips 90W medium pressure mercury lamp model 93110E2 was
mounted in a vertical configuration at the centre of an enclosed drum which was sealed to
reduce loss of UV light during sample exposure. A circular slotted rack which could hold
up to 36 sample tubes rotated the samples at a distance of 30 cm from the lamp.
For curing onto substrates, two UV curing facilities were used. These were Primarc 200
Watt/inch unit and a Fusion 300 Watt/inch with H-bulb. The latter delivered
approximately 1.02×10-2
J/cm2 depending on the line speed. All data for UV doses are
calculated from the number of passes required to cure the coating.
For the curing and grafting preirradiation experiments, an excimer Fusion source
(600watts/inch) was utilised. UV light intensity measurements were made with an Int
Light IL – 390 radiometer and oxalate actinometry.
2.1: Ionising Radiation Sources
2.1.1: Gamma radiation
The experiments were performed using the gamma pond facility at the Australian Nuclear
Science and Technology Organisation (ANSTO). The water in the pond provides
shielding from the radioactive rods. The depth of the pond is 5 meters and most of the
radiation is absorbed within a meter of the source.
38
The rods are positioned on the bed of the holding tank and aligned in a spherical manner.
Sample test tubes were placed in cardboard containers which were encased in a stainless
steel canister. The canister was then lowered into the gamma field for a required time.
Calibration of the gamma source was carried out by the technical staff at the facility using
the Fricke dosimetry and the doses were calculated in kG/hr. Total exposure doses were
calculated from the exposure time.
2.1.2: Electron Beam
Two EB sources were used for the curing experiments, namely a Nissan 500 kV unit and
an ESI 175 kV facility. Using these facilities, mixtures of low molecular weight
unsaturated monomers and oligomers were converted in a fraction of a second to fully
cured coatings. The depth of penetration of the radiation in the polymer coating is
inversely proportional to the intensity of the radiation. The optimal conditions were 2.8
Mrad under nitrogen unless otherwise specified. The monomer mixtures were drawn
down as thin coatings onto the cellulose substrates. The coated samples were attached to
a moving line through the beam source.
2.2: Monomer graft solution preparation
The grafting solutions usually consisted of two monomers, solvents and various additives
used to either enhance or inhibit grafting. The temperature during preparation of mixtures
was monitored throughout the process. In the case of monomers that were solids at room
temperature, mild heating was required to prepare the solutions for coating the substrates.
The samples during the preparation were shielded away from the UV light ensuring no
polymerisation had occurred prior to irradiation.
39
2.3: Grafting Procedure
For grafting studies using cellulose, samples were strips of Whatman number 41 acid
washed chromatography filter paper which were cut to the size of 3 cm x 4 cm. They had
a nominal thickness of 200 microns. The polypropylene samples were cut from a sheet of
isotactic polypropylene thickness of 200 micron thickness. The dimensions of the
samples were also 3 cm x 4 cm.
All samples were equilibrated to 65% RH before and after irradiation treatment. All
substrates were kept in the desiccators for at least 24 hours to stabilize to constant weight.
The samples were weighed and used immediately after removal from the desiccator. No
chemical pre-treatment was used in grafting experiments.
Immediately after the substrates were marked and weighed they were fully immersed in
monomer solutions which included solvents and other specific additives determined by
experimental design. Order of addition was important to ensure no sample gelling prior to
irradiation. The grafting solution was then stirred until homogeneity was established. The
strips are allowed to swell in the monomer/solvent mixtures for known times, then
subjected to UV irradiation.
The samples were irradiated with UV until the grafted solution was close to the gel point.
This observation was important because autoacceleration after gelation causes a large
increase in the viscosity of the reacting monomeric solution thus making removal of
grafted substrate from the test-tube difficult. Accumulated crosslinked gel/polymer onto
the surface of the substrate is a common problem when irradiation periods have exceeded
the induction period of gel formation. Gel on the surface of the substrate would give
40
inconsistent values after solvent extraction for the grafting yields. After irradiation, the
grafted samples were extracted with suitable organic solvents (usually chloroform) to
remove any homopolymer from the substrate. The grafted samples were extracted for 72
hours to ensure complete removal of the homopolymer from the grafted substrate.
For gamma radiation experiments, samples were prepared one night before irradiation
and were refrigerated at –2oC. Prior to exposure, the samples were equilibrated to room
temperature.
2.4: Curing Procedure
Mixtures containing monomers, oligomers and additives were applied to substrates using
a drawdown bar to produce uniform thin coatings. After curing, the samples were
weighed and the Cure(%) was calculated from:
Cure(%) = (Wcs – Wi) / Wi x 100 (2.1)
where Wcs is the weight of cured substrate before extraction, and Wi is the initial weight
of substrate.
2.4.1: Grafting yield calculations
After calculation of the percent cure, the solvent extracted samples were reweighed to
determine the remaining polymer. The percentage graft was calculated from 2.2.
Graft(%) = (Wf – Wi)/ Wi x 100 (2.2)
where Wf is the final weight of cured substrate after extraction, and Wi is the initial
weight of substrate.
41
Experiments for cure(%) and graft(%) were done in duplicate and data reported in Tables
are the averages of these.
2.5: FTIR
FTIR analyses of the polymers and grafted samples were performed on a FTS 3000MX
BIORAD Excalibur series spectrophotometer using the Merlin Software. The
experimental parameters consisted of a wavelength range of 450 to 4000 cm-1
, and 16
sampling scans with a resolution of 4 cm-1
.
Sample preparation for liquids consisted of squeezing a drop of the sample between two
NaCl plates. Solid samples were ground thoroughly with KBr at approximately 1 to 3%
by weight and pressed into a pellet with a thickness of about 1 mm. The pellet was then
placed in a sample holder and analysed.
Cellulose grafted samples were analysed by preparing KBr pellets. Layers of fibres were
removed using a scalpel under a microscope and these were ground with KBr and the
pressed into pellets. This process was repeated a number times to ensure consistency in
sampling. Peak subtraction was used to determine peaks attributed to grafting. Polymers
were prepared from the monomers used in the grafting experiments and specta of these
were used as standards. These were subtracted from the spectra of the grafted samples to
identify peaks which could be attributed to grafting.
2.6: Medium Pressure Mercury Lamp Calibrations
Lamp calibrations were achieved by using the uranyl oxalate actinometry method. The
method determines the total number of photons absorbed by the sample. The uranyl ion
42
(UO22+
) readily forms salts with many organic acids (oxalic acid was chosen). The
excited UO22+
ions decompose oxalic acid as follows (2.3-2.4).
UO22+
+ HO2CCO2H → UO22+
+ CO2 + CO + H2O (2.3)
UO22+
+ HO2CCO2H → UO2H+ + H
+ + 2CO2 (2.4)
The amount of oxalate remaining after exposure was determined by titration against
standardised KMnO4 under acidic conditions. In acidic solution, KMnO4 will convert
oxalic acid to carbon dioxide and water:
2MnO4- + 5H2C2O4 + 6H
+ → 2Mn
2+ + 10CO2 + 8H2O (2.5)
2.7: Materials
2.7.1: Monomers
Monomers (AR grade) were obtained from Sigma Aldrich Co and used without further
purification included maleic anhydride, dimethyl maleate , diethyl maleate, diallyl
maleate, dibutyl maleate, bis (2-ethylhexyl) maleate, mono-2-ethylhexyl maleate, mono-
butyl maleate, styrene, methyl methacrylate, butyl methacrylate, methyl acrylate, ethyl
acrylate, butyl acrylate, hexanediol diacrylate, thiophene, furan, epoxy acrylate, urethane
acrylate, unsaturated polyester, p-methoxy styrene, α-methyl styrene, 4-chlorostyrene, 4-
bromostyrene, acrylic acid , acrylonitrile, trimethylolpropane tris (3-mercaptoprionate),
maleimide, methyl maleimide, ethyl maleimide, phenyl maleimide, hydroxy ethyl
maleimide, hydroxy propyl maleimide, vinyl acetate, N-vinyl pyrrolidone, hydroxy ethyl
methacrylate. Triethylene glycol divinyl ether, ethyl vinyl ether, n-propyl vinyl ether, n-
butyl vinyl ether, iso-butyl vinyl ether, tert-butyl vinyl ether, ethylene glycol divinyl
ether, hexanediol divinyl ether, tetraethylene glycol divinyl ether, trimethylolpropane
trivinyl ether, diethylamino ethyl vinyl ether, 1,4-cyclohexane dimethanol divinyl ether,
43
2-ethylhexyl vinyl ether, t-butyl vinyl ether, isobutyl vinyl ether, hydroxy butyl vinyl
ether were donated by BASF.
2.7.2: Metal salts
Antimony (III) chloride, antimony (V) chloride, copper chloride, cobalt chloride, zinc
chloride, ferric chloride, ferrous chloride, lead chloride, nickel chloride, magnesium
chloride, and lead acetate were obtained from Sigma-Aldrich (AR grade).
2.7.3: Photoinitiators
Irgacure 184 (1-hydroxycyclohexyl phenylketone) and Irgacure 819 (bis-(2,4,6-
trimethylbenzoyl)-phenylphosphineoxide) and Irgacure 2020 were supplied by Ciba-
Geigy. The cationic PIs (CPIs), Degacure KI85 (Degussa) and Cyracure UVI-6974
(Union Carbide), were sulfonium salts, the former containing PF6- with the latter SbF6
-.
2.7.4: Solvents
Solvents (AR grade) were purchased from Aldrich: methanol, ethanol, propanol, acetone,
N,N-dimethylformamide (DMF), chloroform, dichloromethane, carbon tetrachloride,
tetrahydrofuran (THF), ethyl acetate, acetonitrile, dimethyl sulfoxide, toluene, benzene,
ethyl acetate, 1,2-dichloroethane, tetrachloroethylene, dibromobutane, trichloroethylene,
bromotrichloromethane, 2-chloroethanol, and 2-chloro-2-methyl propane.
2.7.5: Substrates
The cellulose was Whatman 41 grade filter paper, the wool was Belmerino quality
supplied by Geelong Labs Australia) and polypropylene films (isotactic 5.0 x 4.0 cm, 200
micron thickness) from Sicpa Industries.
44
Chapter Three
Radiation Induced Graft Polymerisation
and Cure Processes Involving Charge
Transfer Complexes
45
3.1. Introduction
For this section of the thesis the literature survey originally discussed needs to be
expanded to cover aspects of the chemistry used for these specific monomers. Recently
the use of CT complexes in photoinitiator free curing with ultraviolet light and excimer
sources has been reported. These CT complexes are compounds of donor (D) and
acceptor (A) molecules which are monomers capable of polymerising in the presence of
the above sources as DA species70-76
. In earlier preliminary studies, it was also shown that
in the presence of UV, grafting to a variety of substrates can be accomplished using DA
complexes. This observation which has been confirmed by other workers, is important
since it demonstrates that concurrent grafting may occur during the UV curing of these
DA complexes leading to improvements in properties such as increased adhesion in the
finished film.
3.2. Grafting Investigation of DA Complexes
The study of the polymerisation of the DA complexes can be separated into four
categories: (i) their use, either the complex itself or components of the complex, as
additives in accelerating the grafting of monomers (ii) grafting of the DA complex (iii)
homopolymerisation of the DA complex and (iv) curing and cure grafting of the DA
complex. Each of these were examined in detail.
In this chapter, experiments using different DA systems to those previously published
have been used. In addition, the UV conditions needed to polymerise these DA
complexes in bulk will be examined, demonstrating the value of this technique in
46
preparing novel DA polymers in a very simple convenient manner at room temperature.
Such unique polymers can be used in other applications besides surface coatings.
Finally the value of ionising radiation from gamma sources like cobalt-60 in developing
analogous polymerisation processes to those reported with UV will be discussed. These
include grafting of the DA complexes to typical substrates like cellulose and wool and
polypropylene (PP).
In the UV curing of CT complexes, radicals are formed by the mechanism shown in
reactions (3.1- 3.4 )114
D + A [ D – A]* R• (3.1) ⎯⎯→⎯ γ,hv ⎯⎯→⎯ γ,hv
R• + [DA] R-DA • (3.2) ⎯⎯→⎯ γ,hv
R-DA• + [DA] R-DA-DA • R-(DA)x • (3.3) ⎯⎯→⎯ γ,hv ⎯⎯→⎯ γ,hv
R-(DA)x • P-P or 2P (3.4) ⎯⎯→⎯ γ,hv
This process has been investigated in great detail by Jönsson, Hoyle and Decker69
.
Homopolymerisation then results from reactions involving these primary radical
monomer species as shown in equations 3.5-3.9:
SH S • + H • (3.5) ⎯⎯→⎯ γ,hv
S • + R • GRAFT (3.6) ⎯⎯→⎯ γ,hv
CCl4 CCl3 • + Cl • (3.7) ⎯⎯→⎯ γ,hv
CHCl3 CHCl2 • + Cl • (3.8) ⎯⎯→⎯ γ,hv
Cl • + SH S • + HCl (3.9) ⎯⎯→⎯ γ,hv
47
Substrate radicals, S●, are directly formed either with UV or gamma rays (3.5) which can
subsequently lead to grafting (3.6). When solvent is involved in the process, for example
with the simple halogenated hydrocarbons, additional radicals can be formed (3.7)-(3.9).
With PI in the system, radicals can be formed via pathways (3.10) and (3.11).
PI P • + I • (3.10) ⎯⎯→⎯ γ,hv
SH + P • S
• + PH (3.11) ⎯⎯→⎯ γ,hv
MMA grafting was accelerated by the presence of a vinyl ether additive like DVE-31,2
,
because MMA acts as an acceptor and forms a CT complex with the DVE-3 monomer. In
such systems, DVE-3 has a dual role. Firstly it can swell the substrate to permit ease of
access of grafting monomer and, secondly, grafting is enhanced via CT complex
formation and subsequent relevant radical reactions similar to those described above.
The mechanistic role of CT complexes in grafting is complicated by the following
possibilities: (i) the CT complex (DA) grafts as one entity; (ii) one component, D, grafts
followed by attack on the immobilised D by A; (iii) the roles of D and A in process (ii)
are reversed; (iv) same as process (ii) except attack by DA; (v) same as process (iii)
except attack by DA and (vi) DA homopolymerises and crosslinks within the pores of the
substrate to yield predominantly an interpenetrating polymer network with some grafting.
In the above processes, the possible reactions (i)-(v) will essentially determine the
sequencing in the graft copolymer. Process (vi) tends to occur as the grafting solution
becomes viscous and approaches gel point. The presence of an appropriate solvent not
48
only reduces this problem but also assists with the grafting process as described
previously where the importance of partitioning phenomena in these reactions was
emphasised. If the crosslinking density is high, there are problems with solubility of the
homopolymer and its subsequent extraction from the polymer mixture.
Because these DA complexes cure very quickly without PI, the cured film of polymer
may simply be physically bonded to the substrate and thus potentially capable of facile
delamination in subsequent applications. Grafting may be limited because in the absence
of PI, energetically it is difficult for the CT complex radical to abstract hydrogen atoms
from the surface of the substrate to create grafting sites. Thus the optimisation of
concurrent grafting during cure, even if low, could minimise delamination in the final
product which would be beneficial in many applications of the technology.
3.3. DA Complexes for Acceleration of Radiation Grafting of MMA/Styrene Solutions
The data in Table 3.1 show the effect of the vinyl ethers: triethylene glycol divinyl ether
(DVE-3), 1,4-cyclohexane dimethanol divinyl ether (CHVE) and hydroxy butyl vinyl
ether (HBVE) in additive amounts on the radiation grafting of mixtures of MMA and
styrene in methanol to cellulose. Styrene was included in the MMA mixtures since earlier
work demonstrated that its presence could restrict MMA homopolymer formation. The
use of styrene slows the reaction as it is an extremely weak donor monomer compared
with DVE-3, preferentially forming a homopolymer.
The three ethers are important donor monomers used with both UV and gamma radiation
systems. Inclusion of the vinyl ethers with the exception of several 20% MMA samples,
49
leads to a large enhancement in grafting of the monomers to cellulose. In this respect,
DVE-3 and CHVE are particularly effective in enhancing the grafting process.
Table 3.1: Radiation grafting of MMA/Styrene mixtures in methanol to cellulose using
styrene comonomer with vinyl ether additives (1% w/w).
Graft(%) at concentration (%v/v)
(ST:MMA:MeOH)
Technique Dose and
dose rate
System
16:64:20 12:48:40 8:32:60 4:16:80
CONTROL 125 77 9.1 4.7
DVE-3 250 294 339 1.0
CHVE 244 308 35 63
UV 8.82 x
102J at 3.7
x 10 J/hr
HBVE 107 95 59 1.5
CONTROL 67 81 58 33
DVE-3 218 252 222 105
CHVE 221 244 221 101
Gamma 8.0 x 103
Gy at 3.4
x
103Gy/hr HBVE 138 159 128 49
3.4. Effect of Solvent Structure on UV Grafting of MA/DVE-3 to Cellulose
UV cure grafting of MA:DVE-3 to cellulose may be dependent on the structure of the
solvent. Results have shown that there is a dependence of efficiency on the polarity of the
solvent (Figure 3.1 and Table 3.2). At 80% monomer in THF the yield was 75% at 55 J.
Acetone gave the highest yield of 87% in 80% MA:DVE-3 solution.
50
THF
Acetone
Methanol
0
20
40
60
80
100
20 40 60 80 100
[DA]/Solvent (%v/v)
% G
raft
Figure 3.1. UV Grafting of MA:DVE-3 to cellulose dose of 162 J at 1.02×10-2
J s-1
.
Table 3.2: UV Grafting of MA:DVE-3 to Cellulose in Various Solvents with PIa.
Graft as a function of Complex Concentration (%) Solvent
80% Dose (J) 60% Dose (J) 40% Dose (J) 20% Dose (J)
THF 624 2 188 7 376 11 283 11
Toluene 148 6 54 9 103 6 6 6
CCl4 248 17 145 9 178 17 97 18
CHCl3 202 17 173 6 107 17 28 17
CH2Cl2 272 4 201 9 76 13 62 18
ACN 180 2 85 15 106 17 41 17
Acetone 377 4 270 5 217 10 140 15
aUV dose rate 1.02×10
-2 Js
-1.Irgacure 1800 (0.05%w/w).
Polar solvents such as acetone and THF wet the cellulose much more effectively than
non-polar solvents such as toluene. This is reflected in the much higher grafting yields
for reactions in polar solvents. The grafting yields using chlorinated solvents are less per
J, and this may be due to chain transfer reactions with these halogenated compounds.
When PI is included in the monomer solution, reactivity is significantly increased. Thus
in the presence of PI, the radical concentration is increased further leading to significant
51
competing homopolymerisation and early termination of the grafting process as shown by
the shorter irradiation times needed to achieve high grafting (Table 3.2).
Previous reports have demonstrated the importance of solvents in grafting reactions
involving conventional monomers initiated by UV and ionising radiation. As expected,
solvents which wet and swell the substrates can influence the grafting reactivity. In
addition, solvents can interact with the donor or acceptor to yield intermediates
possessing unique charge transfer properties.
As indicated by Table 3.2, higher yields are observed in the presence of the chlorinated
solvents. In the absence of PI, grafting can actually be enhanced at certain concentrations
in particular solvents, especially dichloromethane (CH2Cl2). With the inclusion of PI
grafting yields are strongly enhanced as demonstrated not only by the amount of grafting
but also by the much lower UV doses required to achieve graft.
A plausible mechanism whereby the CT complex DA (represented by DVE-3 and MA
respectively) is grafted to substrate SH is shown in reactions 3.12, 3.13 and 3.14.
D + A [D-A] [D-A]*
R • (3.12) ⎯⎯→⎯ γ,hv ⎯⎯→⎯ γ,hv ⎯⎯→⎯ γ,hv
SH S • + H • (3.13) ⎯⎯→⎯ γ,hv
S • + R • GRAFT (3.14) ⎯⎯→⎯ γ,hv
52
A process for the formation of R• has been discussed by Jönsson
69, 72, 73. A path for
radical generation from DA is shown in equation 3.12 whereby photopolymerisation of
MA:DVE-3 leads to the formation of an exciplex containing a cyclobutane ring which
decomposes to the 1,4-biradical species which can be active in the initiation of the
grafting and curing process as shown in Figure 3.2.
OO
OO
O OO
Maleic anhydride DVE-3
+
hv
O OO
OO
OO+
*
Exciplex
OO
OO
O OO
1,4-biradical formation
.
*
OO
OO
OO O
.
Figure 3.2. Formation of 1,4-biradical species69
53
Precipitation of polymer in these photografting reactions can also, in certain solvents,
particularly halogenated solvents lead to turbidity in the grafting solution restricting
transmission of UV through the supernatant and terminating the polymerisation process.
Higher grafting yields were observed for cellulose than for wool (Figure 3.3) as a large
proportion of labile acetal hydrogens of cellulose are easily abstractable, therefore
leading to an increase in grafting sites. The lower grafting results with wool may be due
to the extensive interlinkages across the structure which restricted formation of surface
radicals. The radicals formed from CT complexes may give extensive
homopolymerisation instead of grafting with the substrate.
3.5. Comparison of wool with cellulose in photografting MA/DVE-3 in chlorinated
solvent
The photografting efficiency of solutions of MA/DVE-3 with cellulose, cellulose with
0.05% PI and wool with 0.05% PI are shown in Figure 3.3. PI must be added to
chlorinated solvents to give good grafting yields.
54
CELLULOSE
0 (147)
12 (147)10 (147)
02468
101214
DCM Chloroform Carbon
Tetrachloride
% G
raft
CELLULOSE + 0.05% PI 282 (3)
46 (3)
135 (3)
050
100150
200250300
DCM Chloroform Carbon
Tetrachloride
% G
raft
WOOL + 0.05% PI
148 (3)97 (3)
65 (3)
-50
50
150
250
350
DCM Chloroform CarbonTetrachloride
% G
raft
Figure 3.3. Photografting of MA:DVE-3 (1:1) at (60% w/w) to cellulose and wool in
halogenated solvents with and without PI (Irgacure 1800, 1% w/w); data in brackets dose
of UV in joules at a dose rate of 1.02 x 10-2
J s-1
.
3.6. Photografting of CT Complexes to PP
The problem of photografting CT complexes to PP is clearly demonstrated by the data
shown in Figures 3.4 and 3.5.
Even with halogenated solvents, photografting of MA:DVE-3 to PP is poor. If PI is
included in the grafting solution (Figure 3.5), the dose to achieve 4% graft was reduced
from 147 J to 3 J. CHVE was chosen as a donor monomer for these experiments because
preliminary work indicated that this donor monomer was the most reactive of the series.
55
CELLULOSE
1114
00 0*
18*
0
5
10
15
20
DMF Acetonitrile DMSO THF Ethyl Acetate Cyclohexane
% G
raft
PPE
0 0
22*
0 00
0.5
1
1.5
2
2.5
DMF Acetonitrile DMSO THF Ethyl Acetate Cyclohexane
% G
raft
*
Figure 3.4: Comparison of cellulose with PP for photografting of MA:CHVE CT
complex in a range of solvents.
PPE
4 (147)
0 (147) 0 (147)0
1
2
3
4
5
DCM Chloroform Carbon Tetrachloride
% G
raft
P P E + 0 .05% P I
4 (3)
0 (3) 0 (3)0
1
2
3
4
5
D C M C hlo ro fo rm C arbo n
T etrachlo ride
% G
raft
Figure 3.5: Photografting of MA:DVE-3 (60%w/w) to PP with and without Irgacure 1800
(0.05% w/w) in various halogenated solvents; dose in Joules (J) in brackets at a dose rate
of 1.02 x 10-2
J s-1
.
56
3.7. Pretreatment of substrates to enhance grafting efficiency
Pretreatment of the substrate prior to grafting may be useful for enhancing subsequent
photografting processes. Pretreatment techniques usually lead to an increase in grafting
sites in the substrate. In the present UV grafting work two analogous procedures have
been used to improve yields: i.) preirradiation of cellulose with a Fusion excimer source
and ii.) pretreatment with a corona discharge (only PP).
3.7.1. Excimer preirradiation of cellulose
When the excimer source (222 nm, 80 mW/cm2) is used to preirradiate cellulose prior to
grafting, solvents such as methanol, ethyl acetate and acetonitrile are effective in
increasing photografting yields with the MA:DVE-3 complex particularly at high
monomer concentrations (Table 3.3).
Table 3.3: UV Grafting of MA:DVE-3 Complex at 80 and 60% w/w Concentrations in
Various Solvents to Cellulose Preirradiated with Excimer Sourcea.
Graft (%) at Complex Concentration
80% 60%
Solvent
Graft(%) Dose (J) Graft(%) Dose (J)
MeOH 26(10) 3(162) 16(11) 132(162)
Toluene 21(29) 41(46) 17(11) 132(73)
EtAc 38 (78) 73(37) 30(75) 132(69)
CH2Cl2 150(187) 41(30) 86(39) 132(52)
CHCl3 94(36) 73(30) 21(32) 132(52)
CCl4 20(110) 73(30) 15(10) 132(52)
Acetonitrile 47(35) 6(30) 12(9) 132(52)
Acetone 103(86) 89(162) 26(20) 110(162)
aUV dose rate 1.02×10
-2 J s
-1. Data in brackets for runs without preirradiation; EtAc=
Ethyl Acetate. Samples were irradiated by passing under the lamp on a conveyor.
57
With methanol, the dose used with 80% solution was the lowest of the series, since
methanol was so reactive. Fusion excimer preirradiation in air leads to bond rupture to
cellulose, and with oxidation to an increase in carboxyl and carbonyl groups. The initially
peroxidised substrate is decomposed by UV irradiation in the presence of CT complex
leading to the formation of the graft copolymer (reactions 3.15 and 3.16, Ce = cellulose).
Ce
excimer
excimer
O 2
O 2
Ce
Ce
Ce
O O
Ce
Ce
2
Ce
Ce
Ohv nC
2
Ce
O C (3.15)
O
Ce
hvCe
O
2nC
Ce
(3.16)O
Ce
Ce
2
Ce
In addition to diperoxides, hydroperoxides can also be formed on the irradiated polymer,
leading to graft after subsequent UV treatment of the CT monomer solution. UV
dissociation of the hydroperoxide gives rise to an equivalent number of graft copolymer
and homopolymer molecules (reactions 3.17 and 3.18). With cellulose, competing
homopolymer yields in this system are relatively small which indicates that diperoxide
formation initially predominates in grafting with this substrate.
OCCe
Ce
excimer
excimer
O2
O2
Ce
Ce
Ce
OOHhv
hvCe
O
2.
nC
O
Ce
Ce
Ce
O OH+nC
Ce
+ Cm
O (3.17)
OOH
+ 2OH 2 + 2 C m OH (3.18)
..
.
2
Ce
Ce
58
When PI is included in the photografting solution, all solvents studied show an increase
in yields of most DA complexes studied, presumably due to abstraction reactions between
radicals from the PI and substrate molecules leading to an increase in grafting sites. Even
with solvents like toluene and chloroform, grafting enhancement is significant when PI is
used.
3.7.2. Corona source pre-treatment of PP
The contrast between cellulose and PP in these preirradiation processes is quite marked.
The previous data show that photografting to PP using the current CT complexes is low
even with the inclusion of a PI in the monomer solution. However, the situation is
drastically altered if PP is pretreated with a corona discharge prior to grafting. Corona
discharge pretreatment in air was expected to lead to diperoxide and hydroperoxide
formation. In this instance hydroperoxides are formed during corona discharge treatment.
(192)
(77)
(86)
(115)
(192)(192) (191)
(20)0
50
100
150
200
250
2030405060708090100
Com plex Concentration (% )
% G
raft
Figure 3.6. Effect of complex concentration (MA:DVE-3) in DCM on photografting yield
with corona discharge treated PP. Numbers in brackets are dose of irradiation (J).
59
The data in Figure 3.6 show that high grafting yields of MA:DVE-3 can be achieved by
reaction with corona discharge pretreated PP. Even in neat solution the yields are very
high. Inclusion of a halogenated solvent such as dichloromethane gave good yields.
3.8. Grafting of CT Complexes using Ionising Radiation
The preceding data have shown how difficult it can be to photograft CT monomers to
untreated PP. With ionising radiation initiation, such limitations do not exist since the
more energetic gamma rays from a Cobalt-60 source can readily break bonds in the PP,
thus directly creating grafting sites in the substrates.
MA:DVE-3 and CHMI:DVE-3 in acetonitrile and DMF (60%v/v) respectively have been
radiation grafted to cellulose. No previous information involving the relative reactivities
of cellulose and wool with PP in the grafting of CT complexes in the presence of ionising
radiation have been reported, however this situation is corrected in Table 3.4.
The data in Table 3.4 for the grafting of MA:CHVE in acetonitrile initiated by Co-60
gamma rays show that grafting is high for both cellulose and wool and marginally
effective for PP. The components of the complex either MA or CHVE show much lower
grafting yields. Mechanistically the grafting process is similar to that proposed for the
UV system except that grafting sites can be directly formed in the substrate in high yield
with the more energetic gamma rays. With gamma radiation, ions may also participate as
intermediates in the grafting process, however current evidence indicates that the
participation of ionic intermediates is small.
60
In the presence of ionising radiation, grafting of these CT complexes occurs as efficiently
as with UV. With ionising radiation care with the level of radiation used is important
since degradation of the backbone polymer, particularly with naturally occurring
macromolecules like cellulose and wool, may occur. The results in Table 3.4 for the
ionising radiation grafting of MA/CHVE in acetonitrile to cellulose and PP show clearly
that the grafting of the complex itself is very much more efficient than the grafting of
each of the component monomers individually.
Table 3.4: Grafting of MA/CHVE CT Complex (60%) in Acetonitrile using Ionising
Radiation
Monomer (%w/w) Substrate Irradiation Dose
(kGy)
Graft (%)
Cellulose 0.92 4.4
Wool 0.92 5.8
30% CHVE
PP 0.92 0.0
Cellulose 0.92 3.2
Wool 0.92 4.5
30% MA
PP 0.92 0.0
Cellulose 0.41 211
Wool 0.92 160
60% MA/CHVE
PP 0.41 7.3
Irradiation at dose rate of 8.1 x 10-1
kGy/hr.
3.9. Grafting with Maleate Esters as Acceptors
The use of MA as a component in the DA charge transfer complex in these studies
possesses a number of disadvantages especially when used without solvent. There can be
solubility problems and also difficulty with the speed of the reaction. In some
experiments, e.g. neat monomers in bulk, reaction speed can be extremely fast whereas
under other conditions such as coating applications the reverse may be observed. In
61
addition grafted MA may hydrolyse in the copolymer to alter the properties of the
finished product.
Table 3.5: Maleate/DVE-3 grafting to cellulose in CHCl3 in the presence of 0.05%
Irgacure 819
Additive
NA PI SbCl3
Acceptor
D G D G D G MBMA 154
b 47 3 44 83 30
BEHMA 220b 71 9 137 135 103
MEHMA 220b 13 6 62 62 33
DAMA 220b 5 12 199 154 94
DBMA 220b 16 17 49 154 15
aComplex conc (90% w/w); D = dose; G = Graft(%); bno gel
The use of maleate esters may overcome some of these problems (Table 3.5). Consistent
with the MA/DVE-3 results, the corresponding maleate ester/DVE-3 systems show a
maximum in graft at the highest monomer concentrations studied, namely 90% v/v.
Inclusion of PI in these ester systems again enhances graft considerably as expected.
When compared with the analogous MA/DVE-3 chloroform data previously reported, the
reactivity of the ester systems is lower than that of the corresponding MA complex,
suggesting that both steric and electronic effects of the maleates influence subsequent
reactivity of these compounds.
The present results with the esters and cellulose, especially with PI present, show clearly
that at relatively low UV doses, very efficient grafting of the DA complex to cellulose.
This result suggests that DVE-3 is the active component which is swelling the cellulose
and leading to grafting of the CT complex. In addition, the esters themselves may be
62
contributing to this reaction. Once the cellulose has swollen, radicals from the PI can
then accentuate the grafting process.
3.10. Effect of Double Bond Molar Ratio (DBMR) of DA Monomers on Grafting
The double bond molar ratio, DBMR, for the donor and acceptor in the CT complex
should be 1.0/1.0 for efficient reactivity in Table 3.6. In grafting work however, there
may be a significant deviation from this statistical ratio since absorption of polar DVE-3
into the substrate could be expected to alter the above ratio. In the relevant DBMR results
for the MA/DVE-3 system, for the neat monomers in the absence of PI, significantly
higher doses of UV are needed for the 1.0/1.5 (MA/DVE-3) system compared with the
statistical ratio to achieve significant grafting.
Table 3.6: Significance of double bond molar ratio (DBMR) of DA components on
photografting CT complexes
No PI PI CT Complex
DBMR
Substrate
Solvent Dose (J) Graft
(%) Dose (J) Graft
(%) PPE 0 3 6 2 7 MA/DVE-3 1.0/1.0
Cellulose 0 13 90 2 180
PPE 0 123 19 7 14 DMMA/DVE-3 1.0/1.0
Cellulose 0 160 30 7 80
PPE 0 113 39 4 7 MA/DVE-3 1.0/1.5
Cellulose 0 113 30 4 30
PPE 0 114 14 10 69 DMMA/DVE-3 1.0/1.5
Cellulose 0 128 24 10 58
Monomer concentration (90% w/w); PI (if present), Irgacure 1800 (0.05% w/w)
When additives are included in the system, either PI or solvents, the situation becomes
more complicated since solvents may themselves complex with the donor or acceptor and
the PI itself contributes additional radicals to the process. When dimethyl maleate
(DMMA) replaces MA in the DVE-3 complex as neat monomers, the DBMR again does
not appear to be as important. This result reflects the fact that the DMMA system is
63
much slower to react than the MA complex as evidenced by the higher UV doses needed
when compared with MA/DVE-3 to achieve reaction especially in the neat 1.0/1.0
complexes. These conclusions are consistent for both PP and cellulose.
3.11. Grafting of Vinyl Acetate (VA) as Donor with MA
DVE-3 is a relatively expensive monomer and, if CT work is to be commercialised, lower
cost donors would be required. VA is a much cheaper alternative. However, the data in
Table 3.7 for the photografting of MA/VA on both PP and cellulose show that VA is
much less reactive than DVE-3 in this work even with the incorporation of PI in the
system.
Table 3.7: MA/VA Photografting to PP and cellulose
80% 60% 40% 20%
Substrate
PIa
(%)
Dose
(J)
Graft
(%)
Dose
(J)
Graft
(%)
Dose
(J)
Graft
(%)
Dose
(J)
Graft
(%)
PP 1 37 8 110 14 220 5 220 9
PP 0 220 0 220 0.1 220 1 220 0.4
Cellulose 1 37 26 110 5 110 1 110 3
Cellulose 0 220 6 220 6 220 8 220 6
Irgacure 1800 (1.0 % w/w); DBMR, MA/VA (1.0/1.0); other conditions as in Table 1.
3.12. UV Cure and Cure Grafting with Cellulose as Substrate
This section deals with curing and cure grafting of CT complexes. This contrasts with
previous section where grafting only was investigated. In contrast, curing and cure
grafting are rapid processes and solvent free. Grafting per se can occur over any time
64
permitted. In Table 3.8 are shown typical curing and cure grafting data obtained with
representative CT complexes using cellulose as model substrate chosen because UV
curing on cellulose is widely used industrially. MA/DVE-3 is utilised as typical CT
complex in this work, the grafting yield with this complex being almost quantitative
under the curing conditions used.
When DVE-3 is replaced with other difunctional vinyl ethers, the situation is similar,
however with the monofunctional vinyl ethers (EGBVE) curing is not as efficient and the
corresponding graft is low. Even with EGBVE, where only partial curing occurs,
significant grafting is found, an observation which indicates that grafting and curing can
be concurrent processes. When MI replaces MA as acceptor with DVE-3, grafting yields
are less than half the curing yields, the results with the MMI complex being even poorer.
Table 3.8: Effect of CT complexes on UV curing and cure graft onto cellulosea
Complex Passes to Cure Cure (%) Graft (%) Cure Degree
MA/DVE-3 3 150 145 complete
MA/BDDVE 3 56 52 complete
MA/HDDVE 3 51 47 complete
MA/EGBVE 3 10 4 partial
MI/DVE-3 3 270 110 complete
MMI/DVE-3 6 175 26 complete
Fusion F300 lamp, D bulb, 16 m min-1
at peak UV intensity of 1.4 watts cm-2
and dose
0.20 J cm-2
per pass. Monomers (1:1) molar double bond ratio; other details, Table 3.1.
65
3.13. UV Curing and Cure Grafting with LED Lamps
Most curing lines in industry use lamps which have wavelengths <385 nm. These are
very hazardous and require shielding. Recently, light emitting diode, LED lamps, have
been developed to emit safer UV frequencies. An example of a LED is the Con-Trol-Cure
UV-LED line 100. The lamp has a cut off for all wavelengths < 385 nm. Body exposure
to these lamps is therefore safer than with conventional mercury arc facilities. Curing and
cure grafting processes with and without photoinitiator using these lamps were studied
for conventional oligomer/monomer acrylates and vinyl ethers, charge transfer monomer
complexes and thiol-enes. Cellulose and polypropylene were used as substrates. The
performance of the lamp is compared with Fusion equipment and EB sources.
The data in Table 3.9 refer to curing and cure grafting of conventional oligomer/
monomer mixtures which have been used for many years for formulation of UV coatings.
The formulations consist predominantly of acrylates with some experiments performed
with vinyl ether capped oligomers. The substrate was Whatman 41 filter paper which is a
relatively pure cellulose and very porous. In these experiments, time required to cure was
reported since these times are much longer than in conventional UV with mercury arc
lamps where a much higher UV dose is used. Time of exposure is a more meaningful
concept for the types of applications which may be used with these new lamps. Both
acrylate oligomers, namely the aromatic urethane and epoxy, when included with a
multifunctional acrylate did not cure within reasonable times without the inclusion of PI
(the curing time was effectively 60 s in these examples). The two PIs used absorb in the
region corresponding to the output of the lamp, IR 369 being expected to be more
efficient than the ITX material. Of the multifunctional acrylates used as diluents, both di-
66
and trifunctional monomers were equally reactive. Cure grafting yields in the presence of
IR 369 were also relatively high.
When the acrylate system was replaced with vinyl ether capped oligomer (VE 1312)
dissolved in a difunctional vinyl ether monomer, the results were similar to the acrylates
except the cure grafting yields in the vinyl ether systems were much lower.
Table 3.9: Curing and Cure Grafting of Monomer Oligomer Mixtures to Cellulose with
LED UV Lamp
System
Oligomer Monomer
PI Cure(%
)*
Cure(%)*
Graft
Cure time
UA TPGDA - 160 109 Post Cure 24hr
UA TPGDA IRG 369 187 180 Cure 60 sec
UA TPGDA ITX 209 165 Residual tack
UA TMPTA - 176 108 Just cured 24hrs
UA TMPTA IRG 369 179 168 Almost cured 5min
UA TMPTA ITX 222 180 Almost cured 5min
EPA TPGDA - 175 108 Cured 24hrs
EPA TPGDA IRG 369 255 238 Cured 5min
EPA TPGDA ITX 196 153 Just cured 5min
EPA TMPTA - 184 108 Cured 24hrs
EPA TMPTA IRG 369 182 174 Cured 2min
EPA TMPTA ITX 220 177 Just cured 5min
VE 1312 DVE-3 - 178 109 Just cured 5min
VE 1312 DVE-3 IRG 369 180 107 Just cured 5min
VE 1312 DVE-3 ITX 171 110 Just cured 5min
*Note that the cure is the increase in weight of sample after reaction thus % cure and cure
graft can be in excess of 100%.
67
When, for comparison, the new LED lamp was replaced with the conventional UV
source, the monomer/oligomer systems and the thiol acrylate hybrid monomer complexes
are cured on cellulose in much shorter times as expected (Table 3.9). Concurrent cure
grafting was also observed, for the LED but was lower with the vinyl ether system,
consistent with the data in the table for the other UV source.
When the acrylates are replaced with CT complexes, firstly the simple MA/DVE-3
mixture, curing is not efficient without PI and, even then, cure times of up to 5 min are
needed (Table 3.10). In contrast the thiol acrylate (TTP/HDDA) system, especially at 1:5
molar ratios, cures faster even without PI, the 2.5 min needed being marginally
satisfactory for many of the new potential commercial applications. Inclusion of PI
accelerates the cure and also improves cure grafting yields.
Table 3.10: Curing and Cure Grafting of CT Complexes onto Whatman 41 Cellulose with
LED UV Lamp
CT Complex
Donor Acceptor
PI
Cure
Cure
Graft
Observation
DVE-3 MA - 160 116 Post Cure 24 hrs
DVE-3 MA IRG 369 144 106 Cured 5 min
DVE-3 MA ITX 212 181 Cured 5 min
HDDA (1) TTP (1) - 211 135 Cured 3.5 min
HDDA (1) TTP (1) IRG 369 224 211 Cured 60 sec
HDDA (1) TTP (1) ITX 216 186 Cured 2.5 min
HDDA (5) TTP (1) - 174 103 Cured 2.5 min
HDDA (5) TTP (1) IRG 369 230 225 Cured 1.0 min
HDDA (5) TTP (1) ITX 218 212 Cured 1.0 min
68
The data in Table 3.11 indicates that the UA/TPGDA mixture forms a weaker CT
complex when initiated. However when stronger complexes such as the HDDA/TTP are
initiated under the same conditions, the higher reactivity leads to grafting with and
without PI’s. VE 1312 in the presence of DVE-3 is less efficient as an acceptor, thus
leading to homopolymerisation.
Table 3.11: Curing and Cure Grafting of Monomer/Oligomer Mixtures and CT
Complexes with Irgacure 369 and ITX using Fusion F300 Lamp on Cellulose
SYSTEM PI CURE
(%)
CURE
GRAFTING (%)
OBSERVATION
(Passes to cure)
- 11 10 3 passes
IRG 369 60 25 2 passes
UA/TPGDA
ITX 18 10 2 passes
- 92 10 3 passes
IRG 369 69 13 1 pass
VE1312/DVE-3
(6:4) ITX 72 14 1 pass
- 120 100 3 passes
IRG 369 180 160 1 pass
HDDA/TTP
(1:1) ITX 130 41 1 pass
Replacement of Irgacure 369 and ITX with the more recently developed Irgacure 819 and
184, using the LED lamp leads generally to faster polymerisation rates for the
monomer/oligomer systems shown in Table 3.12 with the exception of the vinyl ethers
where rates are comparable with both PI groups. Concurrent cure grafting is also very
efficient for most systems particularly those using Irgacure 819.
69
Table 3.12: Curing and Cure Grafting of Monomer/Oligomer Mixtures and CT
Complexes with Irgacure 819 and Irgacure 184 using LED UV Lamp on Cellulose
SYSTEM PI CURE CURE
GRAFT
OBSERVATION
IRG 819 175 170 CURE 90min EPA/TMPTA
IRG 184 200 150 2min
IRG 819 165 105 CURE 5min VE1312/DVE-3
IRG 184 160 100 p.cure 24 hr
MA/DVE-3 IRG 184 185 125 60secs
IRG 819 230 220 60secs HDDA/TTP (1:1)
IRG 184 240 220 60secs
IRG 819 225 225 60secs HDDA/TTP (5:1)
IRG 184 200 180 60secs
3.14. Thiol-Ene UV Curing Processes
The possible mechanistic role of thiol-ene CT complexes in the photopolymerisation
process has been investigated. Thiol-enes are known to form CT complexes which have
been characterised by spectroscopic techniques. These complexes may participate in the
initiation process during radiation polymerisation. Earlier studies have shown there are
differences in reactivity between thiol-ene systems and the corresponding DVE-3 CT
complexes. In addition, recent work using UV initiation has shown the difficulty of
interpreting the curing of thiol-enes in terms of the participation of the CT complex, in
particular the degree to which the complex may be involved in the initiation step.
Specifically the problem is that thiyl radicals (reactions 3.19-3.21) are only produced
slowly from the complex whereas in UV processes, a large flux of additional radicals are
produced from photolysis.
70
The UV curing of thiol-enes occurs by a step growth addition mechanism that is
propagated by a free radical chain process involved in the addition of a thiol group across
an alkene double bond as shown in reactions 3.19 – 3.21.
)21.3('
22
'
2
)20.3('
2
'
2
)19.3(
..
...)(
RSRCHCRSHRSHHRCCRSH
HRCCRSHCHRCHRS
PRODUCTSRSrelevantifPIRSH
+−→+−−→=+
+→+
The trithiol, trimethylolpropane tris (3-mercaptopropionate) (TTP) , in the presence of the
alkene, triallyl-1,3,5-triazine-2,4,6-(1H, 3H, 5H)-trione (TAT) , cures in two passes using
the Fusion lamp (Table 3.13). Cure grafting yields are relatively poor for this system.
The presence of PI also accelerates cure but does not affect the cure grafting yield
significantly. High coating weights and hence high cure yields (>100%, based on an
increase in weight of substrate) were deliberately used in these experiments in order to
accurately examine the cure grafting yields which were expected to be low. The cure
yields, i.e. extremely high or low, can be easily controlled by the amount of coating
applied to the substrate by the laboratory metering rod. When a multifunctional acrylate
like HDDA replaces TAT, the UV curing reactivity with TTP is similar to the analogous
TAT system, curing occurring in two passes and is accelerated by the inclusion of PI.
71
Table 3.13: UV Curing and Cure Graft of Thiol-Enes on Cellulose using Fusion Lampa
Thiol-Ene System P.C.b Cure (%) Cure Graft (%)
TTP/TAT 2 340 67
TTP/TAT + 6% H2O 1 58 0
TTP/TAT + PI 1 470 77
TTP/DVE-3 3 10 9
TTP/HDDA (1:5) 2 250 9
TTP/HDDA (1:5)+ PI 1 330 70
TTP/DVE-3 + 5% H2O 1 11 10
TTP/DVE-3 + PI 1 13 5
TTP/DVE-3/ 20 % EPA 1 6 2
TTP/DVE-3 + 5% H2O 1 14 8
TTP/CHVE 3 7 6
TTP/CHVE/ 20 % EPA 1 13 9
TTP/UA 1 84 34
TTP/EPA 1 43 24
MA/DVE-3 3 150 145
MA/DVE-3 + PIc 1 100 90
MA/DVE-3/UA 1 56 32
MA/DVE-3/EPA 1 37 27
MI/DVE-3 1 270 110
aRatios of reactants (1:1 by weight). ; PI = 2% Irgacure 2020.
bP.C. = passes to cure under UV lamp, Fusion F300, D bulb, at 16 m min
-1.
Extremely poor cure grafting was obtained with the HDDA system, however this is
significantly improved with the inclusion of PI as expected. The combination of a strong
donor, like DVE-3, with TTP leads to curing in three passes and almost quantitative cure
grafting. Addition of PI to this system improves cure speed, as does the presence of an
acrylate oligomer like an epoxy acrylate (EPA) however cure grafting yields are lowered
72
under these conditions. When the difunctional vinyl ether, DVE-3, is replaced by a
monofunctional ether such as CHVE, similar results to those with TTP are observed.
TTP itself can also be used in direct combination with acrylate oligomers like the
aromatic urethane (UA) and EPA. These results are consistent with the data obtained for
HDDA, especially with UA and EPA oligomers which in commercial practice usually
contain up to 20% of a difunctional acrylate such as HDDA to permit easy processing in
the plant.
3.15. Effect of Thiol Functionality in UV Curing and Cure Grafting
Preliminary studies of thiol-ene systems in radiation curing have shown that trifunctional
thiols are required to achieve efficient crosslinking. Further it would be expected that
crosslinking efficiency in the thiol-ene system would be improved with increasing
functionality of the alkene. Thus the effect of increasing the thiol functionality from the
tri- to the tetra- compound should improve the reactivity of the resulting thiol-ene system.
In Table 3.14, the results of comparing TTP, with a tetrathiol, pentaerythritol tetrakis-3-
mercatopropionate (PTP), in these curing and cure grafting reactions is reported. The
cellulose in this work is a chrome coated label stock since this type of paper is known to
possess difficult adhesion problems. The paper is coated with an acrylic finish on one
side only and the current UV experiments have been performed separately on each side of
the paper for comparison.
The data in Table 3.14 for coatings on the chrome coated paper side are consistent with
the previous UV curing and cure grafting results obtained with TTP and a range of enes
73
in Table 3.13 on pure cellulose. All combinations cured in one pass except for the TAT
and HDDA (1:5) systems incorporating PTP and TTP where two passes were required.
Table 3.14: UV Curing and Cure Grafting of Thiol-Enes on Label Stock Cellulosea
Thiol
PTP TTP
Ene System
CURE C. GRAFT CURE C. GRAFT
TAT 7b
3 6b
0
TAT + PI 12
9 9 3
TAT + 6% H2O 12 5 6 2
TAT + 6% H2O + PI 10 3 10 3
DVE-3 14 5 7 3
DVE-3 + PI 14 4 9 2
TMPTVE 8 0 11 6
TMPTVE + PI 32 20 9 2
HDDA 15 2 7 2
HDDA + PI 13 0 13 0
HDDA (1:5) 15c
8 6 0
HDDA (1:5) + PI 8 0 8 0
UA (5:1) 47 6 - -
UA (5:1) + PI 74 35 - -
EPA 43 39 - -
EPA + PI 46 38 - -
EPA (5:1) 65 37 - -
EPA (5:1) + PI 57 50 - -
aIrradiations with 80 W cm
-1mercury lamp. All samples cured in one pass unless
specified. As expected MA and DMMA, strong acceptors do not cure with PPT after six
passes in reference experiments. bTwo passes to cure.
cSix passes to cure.
74
Inclusion of PI accelerates the curing to one pass in these slower systems. The significant
feature of the data in Table 3.15 where the uncoated label stock is used is that the cure
grafting is very poor in all PI free systems especially with the trifunctional thiol.
Inclusion of PI with certain tetrafunctional systems improves these yields. PI was also
included in separate experiments in order to determine whether the presence of PI
improved cure graft yields. Exceptions to the above general observations are the runs
containing oligomer acrylates (UA, EPA), where cure grafting results are significantly
improved compared to the other formulations.
Table 3.15: UV Curing and Cure Grafting of Thiol-Enes on Chrome Uncoated label
Stock Cellulosea
Thiol
PTP TTP
Ene System
CURE C. GRAFT CURE C. GRAFT
TAT 23 6 -
-
TAT + PI 17 2 - -
DVE-3 32 2 15 5
DVE-3 + PI 25 5 12 0
TMPTVE - - 19 2
TMPTVE + PI - - 30 6
HDDA 19 0 10 0
HDDA + PI 23 3 31 0
HDDA (1:5) 30b
2 11 5
HDDA (1:5) + PI 24 2 22 5
UA (5:1) 85 81 - -
UA (5:1) + PI 45 39 - -
EPA 39 6 - -
EPA + PI 38 3 - -
EPA (5:1) 31 3 - -
EPA (5:1) + PI 91 67 - -
aIrradiation conditions as in Table 3.12.
bSix passes to cure.
75
Similar results to those in Table 3.15 are obtained when the coatings are cured on the
uncoated side of the stock cellulose. Again cure grafting is very poor with both tri- and
tetra- systems, the oligomer acrylates especially UA being an improvement in this
respect. The HDDA system at high dilution (1:5) with PTP also cures very poorly again
consistent with the data in Table 3.15.
3.16. Cure and Cure Grafting on PP
All runs with PP required inclusion of the wetting agent, DC 31, which is a commercial
silicone. As expected, curing of both oligomer/monomer acrylate and CT complexes by
the LED lamp was readily achieved on this PP, but cure grafted samples delaminated
after curing, a property that is well known in UV curing on PP. However cure grafting
was achieved with two samples (Table 3.16), the EPA/TMPTA acrylate system and the
MA/DVE-3 CT complex.
When the more powerful Fusion lamp replaced the Con-Trol-Cure source in these current
experiments, much faster rates of polymerisation were again found (Table 3.17).
Concurrent cure grafting yields were still low and this was consistent with the data in
Table 3.16. The results indicate that radical sites can be more readily obtained in the PP
with the high energy Fusion lamps.
If PI is added to the formulation, then reasonable yields can be achieved with the LED
lamp (Figure 3.18).
76
Table 3.16: Curing and Cure Grafting of CT Complexes on Polypropylene Film with the
LED UV Lamp.
System PI Cure Cure
Graft
Observation
EPA/TMPTA - - - No cure
EPA/TMPTA IRG 369 144 112 Cure 90 s
EPA/TMPTA ITX 128 - Cure 60 s
MA/DVE-3 - - - No cure
MA/DVE-3 IRG 369 - - Very tacky 5 min – no cure
MA/DVE-3 ITX 136 111 Cure 5 min
DC 31 (0.05 g); Other systems examined and cured but demonstrated very poor cure
grafting on PP film were as follows:- aromatic UA/TMPTA; TTP/HDDA (1:1),
TTP/HDDA (1:5) and VE 1312/ VE 5015.
Table 3.17: Curing and Cure grafting of CT Complexes with Irgacure 369 and ITX on
Polypropylene using Fusion Lampa
SYSTEM PI CURE
(%)
CURE
GRAFTING (%)
OBSERVATION
- 13 0 4 passes, no cure
IRG 369 29 3 3 passes
AUA/TPGDA
ITX 14 3 3 passes
- 34 19 3 passes
IRG 369 12 8 2 passes
HDDA/TTP (1:1)
ITX 15 10 2 passes
- 22 7 5 passes, no cure
IRG 369 26 11 3 passes
HDDA/TTP (5:1)
ITX 15 10 3 passes
aAromatic urethane acrylate (UA) with TPGDA, also VE1312/DVE-3 cured poorly and
yielded very low cure grafting with these PIs.
77
Table 3.18: Curing and Cure Grafting of CT Complexes with Irgacure 819 and Irgacure
184 on Polypropylene using LED UV Lampa
SYSTEM PI CURE (%) CURE GRAFT
(%)
OBSERVATION
EPA/TMPTA IRG 819 140 110 cure 5 min
HDDA/TTP(1:1) IRG 819 130 105 cure 5 min
aDC31 (0.5%) added. The other systems examined and cured but showed
very poor grafting was HDDA/TTP (5:1). 184 with EPA/TMPTA, MA/DVE-3,
HDDA/TTP (1:1) and (1:5), VE1312/DVE-3 did not cure.
3.17. Conclusion
CT complexes can be directly grafted to substrates in the presence of UV or ionising
radiation. It appears that CT complexes can act as intermediates in a wide range of
polymerisation processes. The effect that such CT complex formation will exert on the
final polymerisation process will depend upon the other parameters of the system. Thus,
in grafting, swelling of the substrate can be very important and may well compete with
CT complex formation in determining the extent of grafting.
78
Chapter Four
Additives for Accelerating
Photopolymerisation Processes
Involving CT Complexes
79
4.0. Introduction
The significance of concurrent grafting during radiation curing has previously been
discussed in terms of improvement in properties of the finished product and also possible
limitations to recycling such materials. The possibility of accelerating the polymerisation
process by the inclusion of additives could be of value both theoretically and practically.
The use of additives to control speed of polymerisation is reported in this chapter. Lewis
acids have been shown to be effective additives hence antimony salts were chosen for this
study as a representative system from the considerable number salts that are potentially
available. The first part of the chapter has been devoted to photografting of the CT
complexes and related work with polymerisation of complexes in the bulk. The latter part
of the chapter concerns UV curing and cure grafting of the CT complexes.
4.1. UV Dose Required to Gel CT Complexes
For photografting studies where the substrate is immersed in the CT complex or its
solution, the UV dose to gel is important since this is the limit to which reactions should
be taken. If the solution gels then it is difficult to remove the substrate from the gel. The
data in Table 4.1 indicate the UV doses required to gel the various CT complexes of
differing structures, thus reflecting the various rates of polymerisation of the different
complexes.
80
Table 4.1. UV Dose for CT complexes to gel a.
System Dose
(J)
Appearance System Dose
(J)
Appearance
MA/PMS 10 Clear gel MA/DVE-3 1 Clear gel
DMMA/DVE-3
(1:1)b
52 Clear gel MA/EGBVE 1 Clear gel
DMMA/DVE-3
(2:1)b
104 Clear gel MA/TGVE 2 Yellow clear gel
ACN/S 368 Clear viscous polymer MA/BDDVE 1 Yellow clear gel
MMA/NVP 384 Clear viscous polymer MA/HDDE 1 Clear gel
DEMA/NVP 210 Clear viscous polymer MA/EVE 12 Yellow clear gel
DMMA/NVP 457 Clear viscous polymer MA/TBVE 31 Red viscous liquid
aTemp 20
oC;
bDouble bond molar ratio; dose (joules), with standard 15g sample used in
calibration; dose rate 1.02 x 10-2
J/sec. Vinyl ether and maleate structures in
experimental.
The vinyl ether complexes, especially with the maleates, generally polymerise much
faster than the other complexes in the absence of PI. Inclusion of PI reduces the dose to
gel using the MMA/DVE-3 complex as a representative system (Table 4.2). Likewise the
presence of SbCl3 accelerates the photopolymerisation of the DMMA/DVE-3 and HEMA
complexes. The data in these Tables thus establishes the UV limits of dose needed for the
subsequent grafting experiments discussed in Tables 4.3 and 4.4 where substrates must be
removed from the grafting solution before gelling has occurred.
81
Table 4.2. Effect of SbCl3 and PI additives on UV dose to gel of DMMA, MMA and
HEMA Polymerizations.
System Additive Concentration Dose to gel (J) Gel
appearance
MMA/DVE-3 NA NA 184 No gel
MMA/DVE-3 SbCl3 1% 1M (in acetone) 129 Clear
MMA/DVE-3 SbCl3 1% 5M (in acetone) 55 Clear
MMA/DVE-3 PI 1% (w/w) 73 Yellow
MMA/DVE-3 PI + SbCl3 1% PI + 1% 5M SbCl3 37 Yellow
DMMA/DVE-3 NA NA 106 Clear
DMMA/DVE-3 SbCl3 1% 1M (in acetone) 4 Clear
HEMA/NVP NA NA 808 No gel
HEMA/NVP SbCl3 1% (w/w) 282 Yellow-
Orange
HEMA/MA/DVE-3 NA NA 57 Yellow
HEMA/MA/DVE-3 SbCl3 1% (w/w) 46 Yellow
aPI, Irgacure 819, bis-(2,4,6-trimethylbenzoyl)-phenyl phosphine oxide; 20
oC; NA- non
applicable; other conditions as in Table 4.1.
In Table 4.3, the photografting efficiency of maleates and imides in CT complexes was
evaluated and compared with maleic anhydride and DVE-3 was the only donor used with
these acceptors. Of the series, BEHMA and MEHMA are particularly effective in
photografting especially in the presence of PI and SbCl3 (1% w/w) as additives whilst the
two imides studied are less reactive. Overall maleic anhydride is the most efficient of the
acceptors studied even in the presence of PI and SbCl3. This observation is reflected in the
fact that lower UV doses are required to achieve reasonable graft levels and as PI and
SbCl3 are included as additives, the UV dose to obtain reasonable graft is reduced even
further.
82
Table 4.3. Additive effect in photografting to cellulose of DVE-3 CT complexes a.
Additive
NA PI SbCl3
Acceptor monomer
D G D G D G
MA 117 159 1.8 432 0.2 425
MMA 220b 5 94 15 106 34
DMMA 220b 5 9 270 82 15
EMI 220b 4 37 133 73 35
PMI 220b 18 50 98 21 29
MBMA 154b 171 3 144 83 30
BEHMA 220b 13 9 337 135 103
MEHMA 220b 67 6 112 62 233
DAMA 220b 5 12 199 154 94
DBMA 220b 16 17 49 154 215
aComplex conc (90% in acetone w/w); conditions as in Table 6.2; D = dose; G =
Graft(%); bno gel; NA- non applicable.
4.2. Solvent Effects in Polymerisation and Grafting
In general previous photografting work using simple monomer systems, the presence of
solvent was important especially if the solvent swells the substrate and permits access of
the monomer to the substrate. In Table 4.4 the first report of the effect of substrates on
photografting CT complexes to cellulose is shown. Without solvent, the grafting
monomer solution rapidly becomes viscous due to homopolymerisation, grafting is
correspondingly low and difficulty is experienced in removing the substrate from the
monomer. Inclusion of polar solvents like THF and acetone overcome this problem, the
former solvent being particularly effective in producing relatively high grafting yields. If
a PI is included in the monomer solution, the dose required to gel was dramatically
reduced and the corresponding grafting yield was enhanced. A similar result was
83
observed for the SbCl3 additive, however, the effect whilst appreciable is not as dramatic
as with the PI.
Table 4.4. Additive photografting acceleration of DVE-3 CT Complex grafting to
cellulosea.
Additive Acceptor monomerb Solvent Dose (J) Graft (%)
EA THF 330 135
NA EA Acetone 330 6
MAc THF 330 175
EA THF 3 180
1800 EA Acetone 3 78
( 0.05% w/w) MAc THF 3 260
EA THF 18 65
SbCl3 EA Acetone 67 77
(1% w/w) MAc THF 18 200
a Complex 60% (w/w); PI, Irgacure 1800, 25% bis-(2,4,6-trimethyl benzoyl)-2,4,4-
trimethyl pentyl phosphine oxide + 75% 1-hydroxy cyclohexyl phenyl ketone;b
Supernatant mobile liquid without solvent viscous; other conditions as in Table 4.3.
Previous results in Tables 4.3 and 4.4 were obtained with CT solutions of fixed CT
concentrations. Photografting of the DMMA/DVE-3 complex dissolved in chloroform
was studied at concentrations of complex varying from zero to 100% with and without
SbCl3 additive. Although the grafting was highest with neat CT complex, the uniformity
of graft was lower than when some solvent was present. It was thus better to graft with
percentages of solvent (at least 10-20%) to overcome this problem.
84
4.3. UV Curing and Cure Grafting of CT Complexes with Concurrent Grafting
The preceding part of this chapter covers work related to photografting of CT complexes
predominantly to cellulose. In these studies there were no time restraints on the
photografting reactions i.e. photografting could proceed for any time scale, even over
days. The results of this work with CT complexes were consistent mechanistically with
previous photografting studies using simple monomers. Thus grafting proceeded by
reaction between radical sites formed in the substrate and monomer adsorbed in the
surface of the polymer. Inclusion of additives like PIs enhanced the photografting process
by creating a higher concentration of initiator radicals. The important feature of the
grafting studies is that the resulting copolymer is chemically bonded to the substrate,
hence this work demonstrates the experimental conditions under which you would expect
to optimize the level of grafting for a system.
The principles of this work are relevant to industrial UV curing processes where
monomers and oligomers are exposed to UV sources for a fraction of a second to cure. In
these reactions, it is important to know whether concurrent grafting occurs during the
fraction of a second that curing takes place. As mentioned in the literature survey, the
occurrence of concurrent grafting with curing is relevant to the properties of the resulting
film, as it is increases the strength of the adhesion between film and substrate. The ability
to observe concurrent grafting in curing with CT complexes is important since these
monomers do not readily abstract hydrogen atoms from the surface of the appropriate
substrate and create grafting sites. The remainder of this chapter is devoted to UV curing
and cure grafting of various CT complexes on cellulose, with some comparisons with
polypropylene. The results in Table 4.5 show the degree to which the CT compositions
85
cure on cellulose under the UV conditions used, and also give the cure grafting
percentage.
Table 4.5.Curing and Concurrent Photografting of Neat CT Complexes with Lewis
Acidsa.
Additive
System
Process NA PI Me PI + Me
Cure(%) 152 (3) 155 (1) 115 (1) 165 (1) MA/DVE-3
(1% CuCl2) Graft(%) 146 154 46 153
Cure(%) 152 (3) 155 (1) 123 (6) 150 (1) MA/DVE-3
(1% MnCl2) Graft(%) 146 154 89 146
Cure(%) 152 (3) 155 (1) 132 (6) - MA/DVE-3
(1% FeSO4) Graft(%) 146 154 65 -
Cure(%) 122 (3) 113 (1) 73 (1) -
MA/PMS Graft(%) 97 20 13 -
Cure(%) 57 46 77 49
MMA/DVE-3 Graft(%) 1 2 3 4
Cure(%) 6 12 6 6
ACN/S Graft(%) 2 3 2 3
Cure(%) 4 12 6 8
AA/S Graft(%) 1 2 3 2
Cure(%) 46 97 56 110
MAc/DVE-3 Graft(%) 4 50 14 60
a PI, Irgacure 184 (1% w/w), Me = salt, 1% w/w; % cure is yield % by weight after
curing before extraction to give Graft(%); figure in brackets = number of passes to cure
(samples exposed under an F300 lamp having a D bulb with a line speed of 16 m/min at a
peak UV intensity of 1.4 W/cm2 and a dose of 0.20 J/cm
2 per pass); some salts were only
marginally soluble and an appropriate solvent to solubilise the salt was used in small
amounts.
86
A variety of CT complexes and conditions for curing are shown in the Table 4.5.
MA/DVE-3 being the simplest of the complexes in structure has been the predominant
complex used. The efficiency of cure with and without the additives used in MA/DVE-3
system is high. Additives like PI and Lewis acids (metal salts) are most effective not only
in curing efficiency but generally in concurrent grafting with the same system.
The other monomers whether as donor or acceptor materials, were significantly less
reactive, particularly in concurrent grafting. The monomers used were methyl
methacrylate (MMA), acrylonitrile (ACN), acrylic acid (AA), methyl acrylate (MAc),
styrene (S) and para-methoxy styrene (PMS). These results showed that with this group
of monomers, it was difficult to achieve radical formation from the initial CT complex.
When the synthetic polymer PP replaces cellulose as substrate in these curing
experiments the results are significantly different (Table 4.6). Thus all acceptors even
MA with DVE-3 (except BEHMA) show virtually no concurrent grafting. Inclusion of PI
with MA alters this situation. The presence of antimony chloride is beneficial with
BEHMA, however the best concurrent grafting yields are obtained when both PI and
antimony chloride are included in the same monomer mixture. The MA/DVE-3 result
using the PI as initiator is important because it demonstrates that radicals from PI lead to
concurrent grafting presumable by abstraction reactions with the substrate. Inclusion of
the Lewis acid, SbCl3, nullifies the effect again due to complexation of the monomer with
the additive. These reactions will be discussed later, in more detail in this thesis.
87
Table 4.6. Curing and concurrent photografting of neat CT complexes with salts and PI
on PP filma.
System Process Additive System Process Additive
NA PI Sb Sb + PI NA PI Sb Sb + PI
Cure(%) 42 93 26 8 Cure(%) 4 1 12 30
MA Graft(%) 0 73 0 0
MMA Graft(%) 1 0 0 9
Cure(%) 54 147 6 20 Cure(%) 28 42 83 98
DMMA Graft(%) 0 1 0 11
BEHMA Graft(%) 13 8 18 22
aConditions as in Table 4.5.
4.4. Comparison of Effect of Additives in Curing of CT Complexes
In Table 4.7, the inclusion of HCl was shown to be useful as an additive to increase the
reactivity of the two MA vinyl ether complexes. The grafting yields are relative to those
of the commercial PIs. This supports the theory that the Cl radicals have excellent
hydrogen abstraction reactivity. When combined with PIs the grafting is increased by the
higher concentration of radicals produced.
88
Table 4.7. Effect of additives on curing and cure grafting of MA complexes with BDDVE
and EGDVE on cellulosea
MA/BDDVE MA/EGDVE Additive Passes
to cure Cure(%) Graft(%
)
Passes
to cure Cure(%) Graft(%
)
N.A. 3 56 52 3 94 76
IRG 184 1 100 91 1 79 68
IRG 819 1 60 58 1 50 41
SbCl3 1 110 110 1 120 120
HCl 1 170 160 1 130 115
IRG 184 + HCl 1 130 125 1 115 115
UVI 6974 2 34 6 2 98 76
UVI 6974 + HCl 1 125 100 1 150 150
KI85 2 150 76 2 120 69
aN.A. = no additives; all PIs and SbCl3 (1% w/w); HCl (1% 0.1M)
The effect of additives including PIs (both free radical and cationic), Lewis acids and
combinations of these components have been utilised in curing and cure grafting with two
representative vinyl ether donors and MA as the acceptor. The two vinyl ethers were the
difunctional monomers, BDDVE and EGDVE, and the results show that both free radical
and Lewis acid initiators can be used to increase curing with these monomers. The
cationic photoinitiators (UVI components KI85) demonstrates poor reactivity in cure
grafting where abstraction processes would be difficult.
89
4.5. Effect of Vinyl Ether Structure on Reactivity of MA Complex
Earlier data in this thesis, especially the results in Table 4.3, indicated that MA is a strong
acceptor especially when used with DVE-3 in these UV curing reactions. It is thus of
interest to examine the reactivity of other vinyl ethers. In Table 4.8 the data for
photocuring of a representative number of MA/VE complexes on to cellulose is reported.
It was observed that the higher functionality vinyl ethers cure more efficiently than the
lower ones (e.g. EGBVE). Concurrent grafting also occurs irrespective of the degree of
cure, however if curing is not complete, the level of concurrent grafting is reduced.
Table 4.8. Photocuring and Grafting of MA/VE Complexes to Cellulose Without
Additivesa
System
Oligomer Monomer
PI Cure Cure
Graft
Cure time
UA TPGDA - 160 109 Post cure 24 hr
UA TPGDA IRG 369 187 180 Cure 60 s
UA TPGDA ITX 209 165 Tacky 5 min, post cure 24 hr
UA TMPTA - 176 108 Just cured 24 hrs
UA TMPTA IRG 369 179 168 Almost cured 5 min
UA TMPTA ITX 222 180 Almost cured 5 min
EPA TPGDA - 175 108 Cured 24 hrs
EPA TPGDA IRG 369 255 238 Cured 5 min
EPA TPGDA ITX 196 153 Just cured 5 min
EPA TMPTA - 184 108 Cured 24 hrs
EPA TMPTA IRG 369 182 174 Cured 2 min
EPA TMPTA ITX 220 177 Just cured 5 min
VE 1312 DVE-3 - 178 109 Just cured 5 min
VE 1312 DVE-3 IRG 369 180 107 Just cured 5 min
VE 1312 DVE-3 ITX 171 110 Just cured 5 min
90
The results using a traditional free radical PI, Irgacure 184, as an additive are shown in
Table 4.9. It was observed that inclusion of the PI lead to enhanced rates of
polymerisation when compared to the reference results in Table 4.8, and concurrent
grafting also occurs in these systems. Potential problems with the use of PIs are their
expense and the fact that unreacted PI in the cured films may migrate through the films
and lead to adverse toxicological problems.
Table 4.9. Photocuring and Grafting of MA/VE Complexes to Cellulose with Irgacure
184a
CT Complex
Donor Acceptor
PI Cure Cure
Graft
Observation
DVE-3 MA - 160 116 Post Cure 24 hrs
DVE-3 MA IRG 369 144 106 Cured 5 min
DVE-3 MA ITX 212 181 Cured 5 min
HDDA (1) TTP (1) - 211 135 Cured 3.5 min
HDDA (1) TTP (1) IRG 369 224 211 Cured 60 s
HDDA (1) TTP (1) ITX 216 186 Cured 2.5 min
HDDA (5) TTP (1) - 174 103 Cured 2.5 min
HDDA (5) TTP (1) IRG 369 230 225 Cured 1.0 min
HDDA (5) TTP (1) ITX 218 212 Cured 1.0 min
aMonomer abbreviations, legends and conditions as in Table 4.8.
In Table 4.10, the data shows that the use of HCl with a variety of MA/vinyl ether
complexes can lead to enhanced curing with all vinyl ether complexes when compared to
the data without this additive in Table 4.8.
91
Table 4.10. Photocuring and Grafting of MA/VE Complexes to Cellulose with HCla
SYSTEM PI CURE
(%)
CURE
GRAFTING (%)
OBSERVATION
- 11 10 3 passes
IRG 369 60 25 2 passes
UA/TPGDA
ITX 18 10 2 passes
- 92 10 3 passes
IRG 369 69 13 1 pass
VE1312/DVE-3
(6:4) ITX 72 14 1 pass
- 120 100 3 passes
IRG 369 180 160 1 pass
HDDA/TTP
(1:1) ITX 130 41 1 pass
aMonomer abbreviations, legends and conditions as in Table 4.8.
Mechanistically HCl may be considered to act as a mild Lewis acid in these reactions and
its reactivity can be interpreted in a manner similar to that for the Lewis acid metal salts
discussed later in this chapter. With HCl there is an additional process which can
contribute to the acceleration of the UV polymerisation. In the presence of UV, HCl can
initiate free radical polymerisation under certain experimental conditions where ethylene
is used as typical olefin.
In the curing of VE complexes, it has been found that, especially under mercury arc lamp
conditions, unreacted VE can remain in the cured films. A method for overcoming this
problem is to include a cationic photoinitiator as an additive. This PI may then remove
the excess VE by cationic polymerisation. The results in Table 4.11 show that the
cationic PI, UVI 6974, will enhance polymerisation rates, however certain products may
be discoloured with this initiator so that the ultimate fate of the cured films is important
in deciding whether to use this additive.
92
Table 4.11. Photocuring and Grafting of MA/VE Complexes to Cellulose with UVI 6974a
SYSTEM PI CURE CURE
GRAFT
CURE TIME
IRG 819 175 170 90 min EPA/TMPTA
184 200 150 2 min
IRG 819 165 105 5 min VE1312/DVE-3
184 160 100 Post cure 24 hr
MA/DVE-3 184 185 125 60 s
819 230 220 60 s HDDA/TTP (1:1)
184 240 220 60 s
819 225 225 60 s HDDA/TTP (5:1)
184 200 180 60 s
aMonomer abbreviations and legends as in Table 4.8.
4.6. Curing and cure grafting with LED lamp
A recent outstanding discovery in UV curing is the development of the light emitting
diode (LED) lamps. These facilities are less powerful that the corresponding mercury arc
equivalents however because their wavelength of operation is in the 385 – 395 nm region
compared with 365 nm for mercury, the former lamps are potentially much safer to use
especially on a commercial scale.
This is the first study reported on curing and cure grafting systems using the Con-trol-
cure LED (385 nm). For potential applications of these lamps, curing times of 1-5
minutes are satisfactory. The problem with their application is whether they are energetic
enough to initiate cure grafting during curing. The data in Table 4.12 is a preliminary
screening test of the LED lamp with traditional oligomers such as aromatic urethane
acrylates and epoxy acrylate dissolved in a reactive monomer like TPGDA. Such
93
formulations are traditional and known to cure efficiently under a mercury arc lamp. The
system thus acts as an appropriate reference for further LED work.
Table 4.12. Curing and Cure Grafting of Monomer Oligomer Mixtures onto Whatman 41
Cellulose with LED UV Lamp
System
Oligomer Monomer
PI Cure Cure
Graft
Cure time
UA TPGDA - 160 109 Post Cure 24 hr
UA TPGDA IRG 369 187 180 Cure 60 s
UA TPGDA ITX 209 165 Tacky 5 min, post
cure 24 hrs
UA TMPTA - 176 108 Just cured 24 hrs
UA TMPTA IRG 369 179 168 Almost cured 5 min
UA TMPTA ITX 222 180 Almost cured 5 min
EPA TPGDA - 175 108 Cured 24 hrs
EPA TPGDA IRG 369 255 238 Cured 5 min
EPA TPGDA ITX 196 153 Just cured 5 min
EPA TMPTA - 184 108 Cured 24 hrs
EPA TMPTA IRG 369 182 174 Cured 2 min
EPA TMPTA ITX 220 177 Just cured 5 min
VE 1312 DVE-3 - 178 109 Just cured 5 min
VE 1312 DVE-3 IRG 369 180 107 Just cured 5 min
VE 1312 DVE-3 ITX 171 110 Just cured 5 min
The formulations reported in Table 4.12 consist predominantly of acrylates with some
experiments performed with vinyl ether capped oligomers using LED lamps. The
substrate used was Whatman 41 filter paper which is a relatively pure cellulose and very
porous. In these experiments time required to cure is reported since these times are much
longer than in conventional UV using mercury arc lamps. Both acrylate oligomers,
94
namely the aromatic urethane and epoxy, when included with a multifunctional acrylate
did not cure within reasonable times without the inclusion of PI. The curing time was
effectively 60 s in these examples. The two PIs used absorb in the region corresponding
to the output of the lamp, IRG 369 generally being more efficient than the ITX material.
Both IRG 369 and ITX are commercial free radical photoinitiators however the former
absorbs more strongly in the region of the spectrum from the LED lamp and thus is more
reactive in these processes.
Of the multifunctional acrylates used as diluents, both di (TPGDA) and trifunctional
(TMPTA) monomers were equally reactive. Cure grafting yields in the presence of IRG
369 were also relatively high reflecting the effect of porosity of the cellulose on the
absorption properties of the monomers in the formulation. When the acrylate system was
replaced with a vinyl ether equivalent, using a mixture of a vinyl ether capped oligomer
dissolved in a difunctional vinyl ether monomer, the results were similar to the acrylates
except for cure grafting where yields in the vinyl ether systems were much lower. These
results especially with cure grafting are most significant because they indicate that
energetically when used with appropriate PIs that the LED lamps can initiate reactions,
such as abstraction processes, which lead to radical formation in the substrate and
subsequent cure graft. This is the first report of such processes occurring.
When the acrylates are replaced with CT complexes, firstly the simple MA/DVE-3
mixture, curing is not efficient without PI and, even then, cure times of up to 5 min are
needed (Table 4.13). Cure grafting yields with this complex are also not high relative to
the results with the more energetic mercury lamp. In contrast the thiol acrylate
(TTP/HDDA) system, especially at 1:5 molar ratios, cures faster even without PI.
95
Inclusion of PI accelerates the cure and also improves the cure grafting yields. The
binding species shown in reaction 4.1 involves charge transfer intermediates which have
been proposed in metal catalysis.
S HR
Su
S
R
Su
Su
H
.. +
(4.1)
Replacement of the two PIs already used, namely Irgacure 369 and ITX, with the recently
developed more efficient Irgacure 819 and 184 and using the LED lamp leads generally
to faster polymerisation rates in the monomer/oligomer systems studied in Table 4.11
with the exception of the vinyl ethers where rates are comparable with both PI groups.
96
Table 4.13 Curing and Cure Grafting of CT Complexes onto Whatman 41 Cellulose with
LED UV Lamp
CT Complex
Donor Acceptor
PI Cure Cure
Graft
Cure time
DVE-3 MA - 160 116 Post Cure 24 hrs
DVE-3 MA IRG 369 144 106 Cured 5 min
DVE-3 MA ITX 212 181 Cured 5 min
HDDA (1) TTP (1) - 211 135 Cured 3.5 min
HDDA (1) TTP (1) IRG 369 224 211 Cured 60 s
HDDA (1) TTP (1) ITX 216 186 Cured 2.5 min
HDDA (5) TTP (1) - 174 103 Cured 2.5 min
HDDA (5) TTP (1) IRG 369 230 225 Cured 1.0 min
HDDA (5) TTP (1) ITX 218 212 Cured 1.0 min
These results are consistent with the observation that Irgacure 819 is more efficient in
free radical formation leading to enhanced cure and also abstraction reactions with the
substrate resulting in high concurrent grafting. In the presence of UV, Irgacure 819 yields
four radical species each of which is capable of initiating polymerisation.
Even though curing is much faster with the PIs used in Table 4.14, concurrent cure
grafting is also very efficient for most systems particularly those using Irgacure 819.
97
Table 4.14. Curing and Cure Grafting of CT Complexes with Irgacure 819 and Irgacure
184 using the LED UV Lamp
SYSTEM PI CURE CURE
GRAFT
CURE TIME
IRG 819 175 170 cure 90 min EPA/TMPTA
184 200 150 2 min
IRG 819 165 105 cure 5 min VE1312/DVE-3
184 160 100 Post .cure 24 hr
MA/DVE-3 184 185 125 60 s
819 230 220 60 s HDDA/TTP (1:1)
184 240 220 60 s
819 225 225 60 s HDDA/TTP (5:1)
184 200 180 60 s
When, for comparison, the new LED lamp is replaced with the conventional Fusion UV
source, the monomer/oligomer systems and the thiol acrylate hybrid monomer complexes
are cured on cellulose in much shorter times as expected (Table 4.15).
Table 4.15. Curing and Cure Grafting of CT Complexes with Irgacure 369 and ITX using
Fusion F300 Lamp
System PI Cure (%) Cure Grafting (%) Observation
- 11 10 5 minutes
IRG 369 60 25 3 minutes
UA/TPGDA
ITX 18 10 3 minutes
- 92 10 5 minutes
IRG 369 69 13 2 minutes
VE1312/DVE-3 (6:4)
ITX 72 14 2 minutes
- 120 100 5 minutes
IRG 369 180 160 2 minutes
HDDA/TTP (1:1)
ITX 130 41 2 minutes
98
Overall from the above work, it is significant that even with a low powered LED lamp,
surface processes are energetic enough to be involved in bond rupture and subsequent
surface grafting.
4.7. Mechanism of Polymerisation Process in curing and cure grafting
Cure and cure grafting involves the photopolymerisation of a CT complex whether it be
as a film with a fast curing step or in mixtures of monomers for bulk polymerization. A
relevant important mechanistic problem, particularly in bulk polymerisation, is the
function of the CT complex in both initiation and propagation.
Hall and Padias74-75
found that using UV irradiation that the initiating species is a
zwitterion biradical which may initiate either ionic homopolymerisation or free radical
copolymerisation. For the current work with PIs as additives, radicals from the initiation
obviously contribute to polymerisation and enhance the radical concentrations for
grafting sites.
The inclusion of certain Lewis acids in appropriate CT monomer solutions enhances the
rate which is an advantage in both curing and bulk processes like grafting. At this time,
based on the current data (typically with antimony chloride which was the most reactive
salt), Lewis acids which tend to be the most active accelerants broadly follow the Pearson
Hard and Soft (HSAB) principle with the present active range being classified as
borderline between H and S. The role of a Lewis acid in these reactions would thus be
consistent with other alternating copolymerisation studies where acceptor monomer
properties are enhanced by complexation with a Lewis acid. This leads to further
99
reduction in electron density of the vinyl group thus increasing the polarity between D
and A monomers as shown in reactions (4.2) and (4.3) where L is the Lewis acid and R1.,
indicates radical formation via the modified pathway.
(4.2)
) RL)](A[DhvL)(AD
LALA
→∗−−⎯⎯ →⎯−+−→+
(4.3
In the present work, the interpretation of these effects is complicated by two factors (i) in
certain of the reactions studied, the presence of solvent which may also form complexes
with D and A monomers and (ii) certain donors possess polar functional groups which
will also complex with the Lewis acids, e.g. the vinyl ethers which may undergo cationic
polymerisation. Mechanistically such processes need to be separated from the DA
reaction, however, in practical terms, if such processes assist fast polymerisation
concurrently, they will be of value commercially. Finally in grafting, salts can complex
with OH groups in the cellulose creating more sites for grafting, whereas analogous
reactions with less reactive non polar PP do not occur unless the substrate is pretreated.
Inclusion of certain Lewis acids in the CT monomer systems studied increases the
photopolymerisation rates of these complexes as does the addition of PIs. A synergistic
effect in certain systems is observed when the two types of additives are included in the
same solution. A similar accelerating effect is observed with the addition of these
additives in certain curing processes on cellulose and PP film, concurrent grafting being
found under certain experimental conditions. Mechanistically the current results are
consistent with previous literature theories in the photopolymerisation of CT complexes
100
where the participation of zwitterion biradical intermediates have been proposed leading
to ionic or free radical polymerisation, the latter being favoured in the present PI system.
4.8. Conclusion
For the first time CT complexes are shown to be capable of participation in curing and
cure grafting processes. Inclusion of certain Lewis acids in the CT monomer systems
studied increases the photopolymerisation rates of these complexes as does the addition
of PIs. A synergistic effect in certain systems is observed when the two type additives are
included in the same solution. A similar accelerating effect is observed with the addition
of these additives in certain curing processes on cellulose and PP film, concurrent
grafting being found under certain experimental conditions.
Mechanistically the current results are consistent with previous literature theories in the
photopolymerisation of CT complexes where the participation of zwitterion biradical
intermediates have been proposed leading to ionic or free radical polymerisation, the
latter being favoured in the present PI system115. The principles of radiation curing and
cure grafting developed with mercury arc lamps can also be applied to the lower energy,
safer, LED equivalents.
101
Chapter Five
Electron Beam Curing and Cure
Grafting of Charge Transfer Monomer
Complexes to Cellulose
102
5.0: Introduction
The use of electron beam (EB) is potentially valuable for curing of CT complexes, and its
implementation would overcome the previous problems experienced with UV because it
requires no photoinitiator and it can be used to cure thick or pigmented films. CT
complexes are attractive candidates for EB systems because they have low viscosities
which make these systems amenable to application at the higher line speeds at which the
EB operates. In addition with the EB technique, there is the advantage of direct electron
irradiation of the substrate which then creates sites where cure grafting can occur
simultaneously with cure thus improving adhesion between the cured film and the
substrate. One major drawback of using EB is that it normally requires a nitrogen purge
to reduce oxygen inhibition.
Preliminary studies of EB curing involving CT complexes have already been reported to
demonstrate the feasibility of the cure grafting technique. This chapter will outline the
value of EB as a tool in the curing of CT complexes for the film formation. The work will
also include novel thiol-ene systems which have become important110. The EB results
will be compared with the corresponding UV systems.
A range of charge transfer donor/acceptor monomer complexes were selected for EB
curing onto cellulose. The effect of a number of parameters on the EB curing efficiency
was investigated including structure of donor and acceptor monomer, radiation dose and
presence of additives. The reactivity of conventional monomer CT complexes have been
compared with thiol-ene systems. The magnitude of concurrent grafting during cure, i.e.
cure grafting, has been simultaneously determined. Analogous curing processes initiated
by UV have also been studied and the results compared with data from the corresponding
103
EB systems. Common mechanistic relationships between EB and UV systems in curing
and cure grafting have been examined.
5.1. EB Curing and Cure Grafting of a model CT Monomer Complex
The results in Table 5.1 show that the MA/DVE-3 complexes cure readily under EB at
radiation doses commercially used for coating paper. Normally in these CT complexes
1:1 double bond molar ratios are used, however in the current system at these ratios, MA
solubility in DVE-3 can be a problem, so reactions have been performed with a slight
excess of DVE-3.
The results in Table 5.1 show that MA/DVE-3 can also be EB cured in the presence of
other monomers like N-vinyl pyrrolidone (NVP), also commercially available oligomers
such as epoxy acrylate (EPA), polyester acrylate (PEA) and unsaturated polyesters to
give polymers with widely varying properties.
Table 5.1. EB Curing and Cure Grafting of MA/DVE-3 Complexes with and without
Additives on Cellulose.
Additive Reactant Ratio Cure (%) C.Ge (%) Nil
1:1 27 9
PE a 1:2:2
d 24 7
PEb
1:2:2 22 8
PE a 3:4:4 13 8
PEc
3:4:4 22 9
PE, NVP a 3:4:4 26 7
EPA a 1:1:2 22 9
EPA a 2:2:1 49 47
PEA a 1:1:2 22 6
a Dose 2.8 x 10
4 Gy;
b 1.5 x 10
4 Gy;
c1.0 x 10
4 Gy;
d Typical ratio MA:DVE-3:PE : 1:2:2
(by weight); eC.G. = cure graft, NVP was added at 10% w/w.
104
Certain formulations can be EB cured at relatively low radiation doses such as with the
fifth sample in Table 5.1 where only 10 kGy are needed. Cure grafting yields are also
reasonable in all of these runs, being particularly high in the presence of the unsaturated
polyester and also one of the EPA runs.
5.2. Effect of Altering Monomers in CT Complex
When the monomer components of the CT complexes are altered, EB curing and cure
grafting can be achieved with a wide range of appropriate donors and acceptors (Table
5.2). The acceptors include maleates, maleimides and acrylates with vinyl ethers as
donors. These complexes can also be used with oligomers like epoxy acrylates and
aromatic urethane acrylates (AUA).
The other important feature of the data in Table 5.2 are the results of changing the molar
ratio of the reactants in terms of double bonds. The molar ratio of double bonds in a range
of CT complexes was varied and the mixture cured using EB. The results demonstrate,
that EB curing of representative CT monomer complexes occurred, optimum results were
obtained for mixtures with double bond molar ratios of 1:1 for both the donor and
acceptor monomers in the complex. With MA/DVE-3, the complex cures readily at 2.8
x104 Gy under nitrogen. However, in this instance, the cure grafting yield was
significantly lower than the percentage cure thus indicating that some of the cured
polymer was only physically bonded to the substrate rather than chemically bonded i.e. it
was not grafted.
105
Table 5.2. EB curing and cure grafting of CT monomer complexes on cellulosea.
Complex Reactant
ratio
Dose x104 Gy N2/Air Cure
(%)
C.G.
(%)
C.G.E.
(%)
MA/DVE-3 1:1 2.8 N2 27 9 33
MA/DVE-3/PE 1:1:2 1.5 N2 23 22 96
MA/DVE-3/PE 1:1:2 1.0 N2 - - -
MA/DVE-3/PE 3:4:4 1.0 N2 22 9 41
MA/DVE-3/PE 3:4:4 2.8 Air 34 33 97
MA/DVE-3/EPA 2:2:1 2.8 N2 46 44 96
DMMA/DVE-3 1:1 2.8 N2 14 14 100
MI/DVE-3 1:1 2.8 N2 26 18 69
MMI/DVE-3 1:1 2.8 N2 27 23 85
EA/DVE-3 1:1 1.5 N2 100 95 95
TMPTEA
/TMPTVE
1:1 2.8 N2 32 25 78
aDose 2.8 x 10
4 Gy at 40 m min
-1 under N2 unless otherwise specified; cure is weight
increase of substrate after EB exposure; C.G. = cure graft is weight increase of substrate
after solvent extraction of cure; C.G.E. = cure graft efficiency = C.G. ÷ cure.
Inclusion of unsaturated polyester (PE) in the MA/DVE-3 leads to greater efficiency of
cure and the cure grafting yields, the dose to cure being reduced to almost one third at
reactant ratios of 3:4:4 (MA/DVE-3/ PE). PE was used in polymerisations of MA/DVE-3
to stabilise the complex over long periods of time and provide an increase in viscosity so
that the coating weight is effectively increased and improved flexibility in coating
applications achieved. Inclusion of PE also permitted curing of the complex in air, a
property which is significant since this is normally not possible due to the formation of
peroxides upon EB irradiation which inhibits polymerisation. Mechanistically the fact
that the peroxides scavenge oxygen and stabilise the CT complex means that the
106
complexes can be used in a wide variety of different unique reactions where lifetimes of
the complex are important.
In the presence of polyester (PE), a ratio of 1:1:2 for MA/DVE-3/PE indicates 50% of
formulae by weight was PE. With MA/DVE-3, the complex cures readily at 2.8 x104 Gy
under nitrogen and the yield is quantitative i.e. complete cure of complex on the paper
occurs. However, in this instance, the cure grafting yield is significantly lower thus
indicating that some of the cured polymer is only physically bonded to the substrate.
However significant cure grafting does occur indicating that chemical bonding between
polymer and substrate is present and adhesion of the entities are strong.
5.3. Significance of Structure of Monomers on Reactivity of CT Complexes
Inclusion of appropriate donor/acceptor pairs in EB reactions accelerate both the cure and
cure grafting processes such that curing is achieved within minimal passes due to the
increase in radical concentration in the system. Acceptor monomers like the maleates,
maleimides and acrylates were shown to be less efficient than MA in curing with vinyl
ethers however yields were acceptable.
MA/DVE-3 is shown to polymerise with other vinyl monomers, forming terpolymers as
well as with other oligomers such as EPA, PEA and unsaturated polyesters to give
polymers with widely varying properties. Certain formulations can be EB cured at
relatively low radiation doses with the inclusion of reactive oligomeric diluents, where
only 10 kGy are needed.
107
108
When the monomer components of the CT complexes are altered, EB curing and cure
grafting can be achieved at relatively low doses of 5 kGy. The other interesting feature of
EB is that no external initiator is needed since the radiation itself generates free radicals
with the result that no unreacted initiator will be left in the coating unlike in UV cured
processes. However, prolonged irradiation will allow radical scavengers like oxygen to
eliminate initiating alkyl radicals like the alkyl hydroperoxide radicals.
5.4. EB Curing and Cure Grafting of Thiol-Enes
Radiation curing of thiol-enes has recently become important in UV work. There is now a
need to evaluate the value of EB systems in this field. Thiol-enes are specific types of CT
complexes, however the mechanism of cure which will be discussed in detail later in this
chapter remains controversial.
In order to demonstrate the EB curing of these thiol-enes, a typical CT complex has been
chosen involving strongly interacting donor and acceptor molecules whose structures are
illustrated in Figure 5.1.
CH2 C
CH2
CH2 O
O
O
CH2
CH2
CH2 O CH2
O
CH2 SH
O
CH2 SH
CH2 SH
CH3
N N
N OO
O
CH2
CH
CH2CH2
CH
CH2
CH2
CH
CH2
(I) (II) (II) Figure 5.1. Structures of the trithiol, trimethylol propane tris(3-mercaptopropionate)
(TTP, I) and an olefin containing electron rich double bonds, triallyl-1,3,5-triazine-2,4,6-
(1H, 3H, 5H)-trione (TAT, II).
In Table 5.3, the EB curing and cure grafting data is shown for the TTP/TAT complex,
and other TTP systems including the last two entries for MA and MI CT complexes with
DVE-3. All data were obtained under nitrogen irradiation which is standard practice for
EB polymerization.
The data in Table 5.3 show that a variety of alkenes can be used with the trithiol. TAT is
especially useful as the alkene since concurrent grafting with this compound is almost
quantitative. Inclusion of water in the system is not detrimental to curing and cure
grafting and this may be important since the presence of water in a cured film can
improve the swelling of certain substrates and thus improve adhesion.
109
Table 5.3. EB Curing of Thiol-Enes In Nitrogen on Cellulosea
Thiol-Ene System Cure (%) Cure Graft
(%)
Comment
TTP/TAT 34 33 gloss
TTP/TAT 35 32 corona
TTP/TAT + 6% H2O 29 28 gloss
TTP/HDDA (1:1) 38 20 gloss
TTP/HDDA (1:5) 41 20 gloss
TTP/HDDA (1:1) + 20% PST 17 15 gloss
TTP/DVE-3 16 10 gloss
TTP/TMPTVE 23 21 gloss
TTP/DMMA 25 13 matt
TTP/HBVE 21 10 matt
TTP/UA 31 29 gloss
TTP/EPA 26 22 gloss
MA/DVE-3 27 9 gloss
MI/DVE-3 26 18 gloss
aDose 2.8 x 10
4 Gy at 40 – 60 m min
-1. Reactant ratios (1:1 by weight) unless
specified.
Multifunctional acrylate monomers like HDDA are effective with TTP even at higher
dilution (1:5). Concurrent grafting in this system is relatively poor, however this situation
is significantly improved by the inclusion of a binder resin like polystyrene (PST). Strong
donor monomers like DVE-3 and trimethylopropane trivinyl ether (TMPTVE), especially
the latter, also constitute effective EB curing systems with TTP. Polar monomers such as
DMMA and HBVE cure readily with TTP however the final film is matt whereas all
others are gloss. This observation would suggest incompatibility in the cured film of the
two components and indicates that homopolymerisation of each of the monomers occurs
independently rather than copolymerisation, and probably not even uniform
interpenetrating IPN’s are formed. The result is mechanistically interesting since DMMA
110
is a strong acceptor. Acrylate oligomers, typically aromatic urethanes and epoxies, can
also be cured with TTP which is a useful result since the unique properties of these
oligomers can be incorporated into the finished films. The last two entries in Table 5.3
are used for mechanistic comparison purposes and demonstrate the reactivity of typical
CT complexes in this work. MA and MI are traditionally strong acceptors which have
been coupled with a strong donor like DVE-3. Although these latter CT systems are very
efficient with EB, concurrent cure grafting yields are not exceptional and contrast
markedly with the TTP results. Even pretreatment of the cellulose with a corona
discharge which is usually used to create more sites in a substrate and improve adhesion
through grafting, does not improve either the curing or cure grafting yields.
5.5. Effect of Oxygen in EB Curing and Cure Grafting of Thiol-Enes
All the data reported in Table 5.3 have been obtained using a inert nitrogen blanket over
the film during the EB processing. If EB curing could be accomplished in air, this would
dramatically reduce the costs for commercial processing. The data in Table 5.4 show that
certain TTP systems can be EB cured in air at doses similar to those used with nitrogen.
TTP/TAT, EB cures well in air, although the concurrent grafting yields are not
exceptional even if the substrate was corona treated. These cure grafting results are
surprising since corona pretreatment is an energetic process which breaks C-H bonds in
the substrate to create additional grafting sites. The presence of oxygen in the surface
may well inhibit reactions at these sites. The results for the polymerization of TTP/TAT
complex without corona pretreatment are in marked contrast to the HDDA system
especially at higher dilution (1:5) and particularly in the presence of binder polymer PST.
The PST result is consistent with what is found with a traditional CT complex like
111
MA/DVE-3 where inclusion of the binder polymer enables not only curing to be achieved
in air but also improves the yield of the concurrent cure graft.
5.6. Mechanism of EB Curing of Thiol-Enes
Irradiation of a substrate by EB leads to the limited formation of free radicals and ions.
The EB initiated polymerization is essentially a free radical process. It is thus similar to
UV mechanistically except that higher doses of more energetic radiation can be utilized
in a very short period of time, thus there is no necessity to accelerate rates of
Polymerisation in EB with additives such as PIs which are used by UV. The EB curing
and cure grafting processes discussed here are thus similar mechanistically to analogous
UV systems previously treated in earlier chapters.
In this respect the oxygen effect is important since oxygen inhibition is observed with
both UV and EB systems, being as serious in the latter case a nitrogen blanket is the only
method for keeping doses absorbed to a level where radiation degradation of the substrate
doesn’t occur during curing. A significant observation in earlier UV curing of acrylates
was that PIs were needed to overcome the oxygen inhibition problem. However when
thiol-enes were photopolymerised there was no oxygen inhibition. The EB results
followed a similar pattern in that no oxygen inhibition was observed during curing.
Mechanistically EB and UV thiol-ene curing systems are thus similar. The lack of oxygen
inhibition in curing with both EB and UV systems can be attributed to the reaction
between the β-thioether carbon radical species reaction 5.1) and molecular oxygen to
yield a β-thioether peroxy radical in a co-oxidation process (reaction 5.2).
112
RSH2C CHR' O
2 RSH2C CHR'
O2
RSH2C CHR'
O2
RSH2C CHR'
O2H
+
+ RS
. (5.1)
(5.2)
. .
.
This peroxy radical may then undergo a chain transfer reaction with the thiol to preserve
the propagation radical. The insensitivity of the thiol-ene process to oxygen can thus be
attributed to the fact that the thiol can undergo a rapid chain transfer reaction with the
peroxy radical. By contrast, in acrylate and methacrylate systems, the analogous peroxy
intermediates are chain stoppers, resulting in the formation of nonpropagating species111
.
The interesting aspect of this acrylate/thiol-ene curing situation where oxygen inhibition
impedes the former but does not affect the latter system is shown in the hybrid runs in
Table 5.3 i.e. the acrylate thiol complexes where both technologies are present. In such
examples, typified by using dilute thiols in acrylates such as TTP/HDDA (1:5) in Table
5.3, the process is essentially an acrylate homopolymerisation which is initiated by the
radical from the CT complex of the thiol and acrylate carried out in the presence of a
strong chain transfer agent such as the thiol which acts to attenuate the effects of oxygen.
The MA/DVE-3 CT complex with unsaturated polyester will also cure in air under these
EB conditions, whereas the CT complex itself cures only with difficulty.
In this latter system, the unsaturated polyester scavenges the oxygen, permitting the
polymerisation of the CT complex to occur readily. With respect to cure grafting, it is
significant that this reaction also occurs efficiently in air with EB.
113
5.7. Comparison of EB with UV in Curing and Cure Grafting of CT Complexes The fact that EB can initiate both curing and cure grafting of CT complexes is important
since the results indicate that the scope of polymerising systems available for high speed
coating and related processes can be widened considerably. This could replace
dependence on acrylate technology with its technical limitations. The CT complexes can
also copolymerise with other oligomers yielding hybrid products with versatile properties
such as improved flexibility, abrasion resistance and the like. The technique can also be
used with specific complexes such as MA/DVE-3 to prepare graft copolymers with
almost quantitative cure graft yields. The MA/DVE-3 and MI/DVE-3 complexes are
particularly effective in both EB and UV curing and cure grafting work. These results are
consistent with the previously reported data on these systems in commercial UV
photoinitiator free curing. Finally, the thiol-ene chemistry results with EB cure, offers
outstanding potential for providing new products especially in combination with acrylate
monomers and oligomers.
When compared with EB processes, the corresponding UV systems are less reactive
especially in cure grafting. In Table 5.4 a summary of the relevant UV data for this
discussion is presented after direct comparison purposes with the EB.
114
Table 5.4. Summary of UV Curing of Thiol-Enes on Cellulosea
Thiol-Ene System P.C.b Cure (%) Cure Graft (%)
TTP/TAT 2 340 67
TTP/TAT + 6% H2O 1 58 0
TTP/TAT + PI 1 470 77
TTP/DVE-3 3 10 9
TTP/HDDA (1:5) 2 250 9
TTP/HDDA (1:5)+ PI 1 330 70
TTP/DVE-3 + 5% H2O 1 11 10
TTP/DVE-3 + PI 1 13 5
TTP/DVE-3/ 20 % EPA 1 6 2
TTP/DVE-3 + 5% H2O 1 14 8
TTP/CHVE 3 7 6
TTP/CHVE/ 20 % EPA 1 13 9
TTP/UA 1 84 34
TTP/EPA 1 43 24
MA/DVE-3 3 150 145
MA/DVE-3 + PIc 1 100 90
MA/DVE-3/UA 1 56 32
MA/DVE-3/EPA 1 37 27
MI/DVE-3 1 270 110
aRatios of reactants (1:1 by weight). ; PI = 2% Irgacure 2020;
bP.C. = passes to cure under
UV lamp, Fusion F300, D bulb, at 16 m min-1
; c0.5% Irgacure 184;
d1.0% Irgacure 184.
It is observed that certain of the CT complexes require the use of PIs, although in low
amounts, to achieve reasonable UV cure speeds, this observation being particularly
relevant to the bulk photopolymerisation processes under the UV processing conditions
used. The fact that a cationic PI accelerates the bulk photopolymerisation of
115
DMMA/DVE-3, but not as efficiently as the free radical initiators, indicates that ionic
processes may be involved in polymerisation in this system and therefore may also
contribute to the mechanism of the EB process. Current evidence would indicate that this
ionic contribution would be low.
The fact that in this UV work, CT monomer complexes may be polymerised under certain
specific conditions need to use PIs suggest that, with modification, polymerisation of
similar CT monomer complexes may be initiated by EB. This conclusion is supported by
the evidence for EB curing discussed in this thesis.
5.8. Conclusion
Curing and cure grafting of a range of CT monomer complexes have been demonstrated
using EB and the results compared with equivalent UV systems. The complexes studied
include the thiol-ene monomers which have been shown to copolymerise with the vinyl
ethers as donors. The UV/EB technique has been extended to include curing and cure
grafting of thiol-ene systems. EB curing at relatively low radiation doses with specific
complexes is observed as is curing in air.
116
CHAPTER 6
Spectroscopic Analysis of Polymers After
Curing and Grafting Reactions
117
6.1. Introduction
An important aim of this project was developing reactive monomer systems where the
degree of grafting could be controlled. The chemical analysis of the polymers formed
from UV curing and grafting of CT complex represents several challenges. The cured
materials are usually highly crosslinked, and are insoluble in all solvents. This eliminates
chemical characterization techniques such as solution phase NMR, gel permeation
chromatography, and mass spectroscopy experiments to examine the products of these
systems. Coatings were grafted onto substrates such as cellulose and analysis of the
polymer and polymer-substrate bonding could provide useful information about the
properties of these materials. However, characterization of these composite materials is
even more difficult than the crosslinked polymers115
.
6.2. UV Cured Polymers from Mixtures of MA/DVE-3
The polymerisation of MA/DVE-3 gave highly crosslinked materials and this was
verified in many extraction experiments on the cured samples grafted onto cellulose (see
Chapter 3). FTIR analysis was expected to be a very convenient method for examining
the chemistry occurring in these polymerisation reactions, both in the cured coatings
which contained no cellulose and for the cure grafted composites which were expected to
have polymers grafted onto cellulose. The sample preparation procedure was simple. The
polymer coating could easily be removed using a scalpel when the surface was viewed
using a dissecting microscope, and KBr pellets were prepared from these samples.
Samples of the cellulose-polymer material could also be prepared by this technique.
118
Figure 6.1 shows reference spectra obtained for the monomers, MA and DVE-3, and a
sample of the dried gel (90oC, 30 min) produced from bulk UV polymerisation of
MA/DVE-3 with Irgacure 819 as the photoinitiator. The carbonyl/carboxyl region of the
infrared spectrum was expected to be very informative about the reactions during
polymerisation. MA and DVE-3 are expected to form charge-transfer complexes and it
was expected that the copolymerisation reaction would produce an alternating copolymer.
The spectrum of the copolymer does not have the peaks expected for the reaction of
DVE-3 and MA. MA has a broad peak at 1783 cm-1
(C=O stretching frequency), DVE-3
has a sharp peak at 1637 cm-1
(C-O stretching frequency) but the gel has peaks at 1854,
1780, 1729 and 1639 cm-1
. The last peak could be attributed to either partially
polymerised or residual DVE-3, and the peak at 1780 cm-1
to an acid anhydride. The
peak at 1729 cm-1
is typical of a carbonyl functional group. The presence of this peak was
not expected from this polymerisation reaction.
40
60
80
100
120
Tra
nsm
ittan
ce
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
Maleic anhydride
DVE-3
Gel
Figure 6.1: FTIR spectra of the MA, DVE-3 and the gel produced by bulk curing a
mixture of DVE-3-and MA
119
Succinic anhydride and succinic acid are some of the possible products which may be
formed from MA during polymerisation. Figure 6.2 shows the FTIR spectra of succinic
anhydride, succinic acid, and the DVE-3 and MA gel.
Succinic acid has a very broad C=O stretch at 1695 cm-1
and succinic anhydride has C=O
stretches at 1863 and 1784 cm-1
. The 1784 cm-1
peak is much sharper than the
corresponding peak for maleic anhydride. The wavelengths and relative intensities for
the both C=O stretching frequencies of succinic anhydride are similar to those observed
for the polymer (1854 and 1780cm -1
) but have different relative intensities. Since MA
doesn’t homopolymerise, this is direct evidence for the formation of a MA/DVE-3
copolymer by this UV curing reaction.
0
20
40
60
80
100
120
140
Tra
nsm
ittan
ce
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
DVE-3 + Maleic anhydride
Succinic anhydride
Succinic acid
Figure 6.2: FTIR spectra of the succinic acid, succinic anhydride, and the gel produced
by bulk curing a mixture of DVE-3-and MA.
120
However, the peak at 1730 cm-1
has not been assigned by these models. This peak is due
to a carbonyl group, and would be usually assigned to an ester. Figure 6.3 shows spectra
obtained for mixtures of MA/DVE-3 (1:1 double bond molar ratio) which were acquired
immediately after preparation and no exposure to UV light. The bands in the carboxyl
region are certainly not those expected for a simple mixture of DVE-3 and MA. Two
possible reasons for the appearance of these bands are:
• Formation of a charge transfer complex by the two monomers
• Hydrolysis of a proportion of the vinyl groups of the DVE-3 and subsequent
reaction of the alcohol with MA to give an ester
0
20
40
60
80
100
120
05001000150020002500300035004000
Wavenumber cm-1
% T
ran
smit
tan
ce
MA/DVE-3 NO UV EXPOSURE
MA/DVE-3 NEAR GEL POINT
Figure 6.3: Infrared spectra of solutions of MA and DVE-3
Triethylene glycol (TEG) was used instead of DVE-3 to examine these two alternate
possibilities. TEG would not be expected to form a charge transfer complex with MA, but
it may react with DVE-3 to give an ester. Figure 6.4 shows the spectra obtained for a
mixture of TEG and MA. It is interesting that the carbonyl region of these spectra is very
similar to those observed for the MA/DVE-3 mixtures. It can be concluded that the peak
at 1730 cm-1
has not arisen from the formation of a charge complex between DVE-3 and
121
O
O
O
OH
O
O
O
OHC CHO OH+
heat
6.1
OHO
OH
O
O
Oheat 6.2
MA but must be due to an esterification reaction between these two monomers. The
reaction between an alcohol and an anhydride can produce two products by either
monoesterification or diesterification.
0
20
40
60
80
100
600110016002100260031003600
Wavenumber (cm-1)
% T
ran
smit
tan
ce
MAc
MAc/TEG mixture
Figure 6.4: FTIR spectrum of MAc and TEG mixture (1:1 double bond molar ratio).
Maleic anhydride usually has traces of maleic acid present. The maleic acid may react
with the DVE-3 which could hydrolyse the vinyl ether to produce acetaldehyde and an
alcohol. This alcohol may then react with the MA to give an ester which was observed for
mixtures of TEG and MA. Mixtures of DVE-3 and maleic acid (MAc) were prepared and
122
the infrared spectra are shown in Figure 6.5. The peak at 1730 cm-1
confirmed that DVE-
3 can react with maleic acid to form an ester.
0
20
40
60
80
100
600110016002100260031003600
Wavenumber (cm-1)
% T
ran
smit
tan
ce
MAc
MAc/DVE-3
Figure 6.5: Infrared spectra of DVE-3 and maleic acid mixture (1:1 double bond molar
ratio)
6.3: Analysis of Cure-Grafted DVE-3/MA on Cellulose
The curing and grafting reactions of the MA/DVE-3 copolymer with cellulose are very
important. Figure 6.6 shows the spectra of the cellulose used in these curing/grafting
experiments, a cured sample of MA/DVE-3/celluose and a grafted sample where the
cured sample had been extracted with chloroform to remove unreacted monomers and
polymeric materials which had not been crosslinked. The cured MA/DVE-3/cellulose
samples were prepared by irradiating an equimolar mixture of DVE-3 and MA with 1%
Irgacure 184.
123
Cellulose has a peak at 1638 cm-1
(C-O stretching frequency) which unfortunately
overlaps with the C-O stretching frequency for DVE-3 at 1637 cm-1
. However, both the
cured and the grafted samples have a more intense stretch at 1638 cm-1
than the cellulose
alone, and this is evidence for the presence of DVE-3 in the cured and grafted samples. A
strong stretch at 1730 cm-1
is also observed for these samples.
0
10
20
30
40
50
60
Tra
nsm
ittan
ce
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
Cellulose
Grafted
Cured
Figure 6.6: FTIR spectra of cellulose, grafted and cured samples of MA/DVE-3/cellulose
using Irgacure 184 as an additive.
This wavelength (1730 cm-1
) is characteristic of the formation of an ester which was
initially attributed to be from chemical grafting of the MA/DVE-3 copolymer to the
cellulose with the formation of ester bonds between the succinic anhydride repeat units in
the copolymer and hydroxyl groups on the cellulose. However, the previous section has
shown that mixtures of DVE-3 and MA undergo esterification reactions and this could
not be distinguished from esterification reactions with cellulose. The results thus do not
demonstrate unequivocally evidence of MA/DVE-3 grafting.
124
6.4: Scanning Electron Micrographs of MA/DVE-3 Polymerised onto Cellulose
Scanning Electron Microscopy (SEM) was used to record images of fractured surfaces of
cellulose samples before and after grafting with MA/DVE-3 to examine whether the
morphology of the polymer films on the cellulose surface could be used to determine
whether the copolymer was physically bonded on the surface of cellulose.
Figure 6.7: SEM of cellulose
The SEM micrograph of cellulose showed the paper is a mat of independent fibres with
numerous voids between the strands. It was expected that micrographs of grafted
cellulose would show changes to the fibres especially to the crystalline regions of the
cellulose. The contours of the fibres may also change upon grafting due to variations due
to intense swelling by the more polar monomers/solvents as well as polymer deposition
within the fibre networks which causes disturbances in their arrangement.
Figures 6.8 and 6.9 are micrographs of polymerised MA/DVE-3 and MMA/DVE-3 on
cellulose (no solvent extraction). The MA/DVE-3 polymer coated the surface of the
cellulose fibres but did not fill the void spaces but the MMA/DVE-3 polymer gave a
continuous film. This is important because choice of MA/DVE-3 as the coating material
125
would give a material where the cellulose fibres are selectively coated but the composite
would be porous, but the MMA/DVE-3 product would act as a barrier film.
Figure 6.8: SEM of MA/DVE-3 grafted onto cellulose
Figure 6.9: SEM of MMA/DVE-3 grafted onto cellulose
Figure 6.10 and 6.11 are micrographs of DMMA/DVE-3 and MMA/ST polymerised onto
cellulose. Both of these polymers coat the cellulose fibres with negligible polymer in the
void spaces. The images particularly for MMA/ST show excellent interfacial adhesion
between the polymer chains and the cellulose fibres.
126
Figure 6.10: SEM of DMMA/DVE-3 on cellulose
Figure 6.11: SEM of MMA/ST on cellulose
Figure 6.12 is a micrograph of the cured MMA/DVE-3 sample shown in Figure 6.9 after
extraction with chloroform for 24 hours. Although these are of different regions on the
surface of this material, it is apparent that some of the coating has been removed in this
region. However, the bulk of the coating is still an intact film.
127
Figure 6.12: SEM of MMA/DVE-3 on cellulose
6.5: Conclusion
The aim of this section was to gain information about the composition of the copolymer
formed during UV polymerization of DVE-3 and MA on the surface of cellulose. A
second aim was to examine whether the polymer was grafted to the cellulose or whether
the film adhered to the paper because of the penetration of the monomer into the void
spaces between the fibres.
The analysis of crosslinked polymers is difficult. However, the analysis of the products
of photopolymerization of DVE-3 and MA were also complicated by an esterification
reaction which occurred between the two monomers. The formation of this ester also
made analysis of the grafting by this polymer on cellulose using FTIR not feasible.
The analysis of the polymer film on paper using SEM showed some interesting properties
for these materials. Although the films appear to be similar when viewed by eye, some of
128
the systems only coated the cellulose fibres to give porous materials, while other polymer
systems gave continuous films which would have greatly increased barrier properties.
129
Chapter Seven
Conclusions
130
UV curing and grafting has been shown to be a valuable new technique when
polymerising acrylic and charge transfer (CT) complex hybrid systems. The thesis has
demonstrated the possibility of utilising a wide range of resins in both UV and EB
processing using the donor/acceptor complexes to complement traditional acrylate
technology.
CT complexes were used in a number of radiation polymerisation experiments which
investigated both curing and grafting. The studies emphasised the importance of
donor/acceptor reactivity of monomer pairs. Vinyl ethers were shown to be the most
reactive of the donor monomers whilst maleic anhydride was the most reactive acceptor
monomer. The most significant observation was that low UV doses were required to
achieve complete cure of CT complexes compared to conventional acrylate systems.
Most importantly, the complexes were shown to graft directly to using either UV or EB
radiation in the absence of PI’s.
The yields of concurrent grafting for the CT complexes were relatively high compared to
PI assisted reactions, with rapid gelling observed when exposed to UV or EB. MA/DVE-
3 complex was used as the control throughout the studies, as it was the most reactive of
the series producing high grafting yields at low UV doses.
Grafting to cellulose under appropriate radiation conditions, in most instances resulted in
excellent adhesion to cellulose. However grafting to low surface energy films like PP was
poor, because the CT complexes were ineffective in diffusing into the substrate. The PP
unlike cellulose is less porous, thus did not allow any physically bonding or anchoring of
131
crosslinked polymer chains. The grafting was only enhanced by using the pre-irradiation
technique with the excimer laser source.
A range of novel additives were also used to enhance the rates of CT complex
polymerisation in these UV processes. Inclusion of certain Lewis acids in the CT
monomer systems increased the photopolymerisation rates as observed for the PIs. A
synergistic effect in some systems was observed when PI were combined with CT
complexes.
The UV/EB studies were also extended to thiol-ene curing and cure grafting to cellulose.
UV and EB were shown to be efficient for the curing and cure grafting of thiol-enes.
Changing the structure and the functionality of the olefin in the thiol-ene system
enhanced the rate of polymerisation. When thiols were combined with acrylate
monomers/oligomers, this enhanced the speed of cure. EB curing and cure grafting of
these thiol-enes in air was also successfully shown to occur. A plausible mechanism for
the process was proposed that reaction occurs by a step growth addition mechanism
where free radical propagation reaction involved the addition of a thiol group across a
double bond.
A new UV lamp, the Con-Trol-Cure UV LED line 100, was also tested and shown to be
useful for a range of curing and cure grafting processes involving oligomer/monomer
acrylates and vinyl ethers combinations. Since the lamp has a cut off wavelength of 385
nm, cure times were longer than conventional UV Fusion and EB systems, and as
expected, oxygen inhibition was more prevalent with the new lamp. The performance of
the lamp when compared to the Fusion lamp and EB sources, was acceptable as reactive
132
CT complexes particularly the thiol-enes were shown to be quite efficient in grafting to
cellulose with relatively short doses.
Finally, the last section of the thesis, involved chemical characterization of the polymers
formed as a result of the UV curing and grafting reactions. The analysis did present
several challenges as the cured materials were highly crosslinked, and were insoluble in
solvents. Coatings which were grafted onto cellulose were shown by SEM micrographs to
form composites with the paper by physical diffusion into the cellulose. The
polymer/cellulose matrix did not show any morphological differences from ungrafted
cellulose, with only polymer deposited in between the spaces of the cellulose fiber
network.
133
Chapter Eight
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Appendix
143
The information presented in the appendix is a collection of all literature journal articles
published in support of the thesis. The articles were published over the course of the PhD
work and subsequently supplements the thesis discussion. The publications will be listed
in a chronological order.
Ref: Garnett, J.L., Ng, L.-T., Viengkhou, Hennessy, I., V., Zilic, E.F., 2000. In Proc.
RadTech North America 2000, Baltimore, USA. p. 804
Photoinitiator Free UV Grafting and Curing using CT Complexes on
Polypropylene. Comparison with Cellulose and Wool as Substrates.
John L Garnett1, Loo-Teck Ng2, Visay Viengkhou2, Iain Hennessy2 and Elvis Zilic2
1Radcure Australia Inc, University of Western Sydney Nepean, PO Box 10, Kingswood,
NSW 2747, Australia
2 School of Civic Engineering and Environment, University of Western Sydney Nepean,
PO Box 10, Kingswood, NSW 2747, Australia
Abstract
The experimental conditions have been determined for the photografting to cellulose and
wool of charge transfer (CT) complexes containing an acceptor monomer, maleic
anhydride (MA) with donor ethers, DVE-3 and CHVE which are used in UV curing. The
effect of solvent and photoinitiator (PI) on these reactions has been examined. These
grafting data with naturally occurring macromolecules have been compared to analogous
reactions performed on synthetic polypropylene (PP) which previously has proved to be
relatively inert in such grafting processes. The effect of pretreatment of substrate on the
photografting process has been studied. For cellulose the pretreatment consisted of
exposure to a 600watt/inch excimer source whereas for PP a corona discharge facility
144
was utilised. The significance of the grafting work, particularly the corona pretreatment
of PP, in commercial curing is discussed. The relevance of these studies to the UV
processing of banknotes produced from PP is evaluated.
Introduction
Photoinitiator (PI) free curing using charge transfer (CT) complexes is now an
established industrial process (1). There are a number of advantages in using this
technique for UV curing including improved economics, better physical properties of the
finished coating including odour and the possibility of bypassing acrylate materials which
currently constitute the conventional chemistry predominantly used in UV curing.
Recently to complement this new UV curing process preliminary studies of the analogous
PI free UV grafting involving CT complexes predominantly to cellulose and wool were
reported (2-4). Such a process leads to the opportunity of synthesising new products not
readily capable of being prepared by other means.
In the present paper the PI free UV grafting process is extended to synthetic substrates
typified by PP. Previously, because of the lack of polar groups in PP, PI free UV grafting
to this substrate was extremely difficult because of the problem energetically of creating
grafting sites in relatively inert PP either by abstraction reactions or direct photolytic
cleavage CH bonds in the substrate. In the current work, this problem has been overcome
and high grafting yields to PP can now be readily achieved in the absence of PI.With
respect to the curing process, this current work is important since the results are relevant
to the possibility of concurrent grafting occurring during PI free curing on PP. The
occurrence of such grafting has two important ramifications, i.) if present it can lead to
improved adhesion between cured coating and substrate, a property particularly relevent
145
to curing on PP and ii.) with substrates like cellulose recycling of the finished product can
become difficult (4).
The significant feature of the present work is that the inherent difficulties in photografting
to PP have now been overcome and UV conditions for obtaining high grafting yields
have been obtained. Such conditions can be related to analogous processing conditions
used in PI free UV curing. These PP results are compared to similar studies with cellulose
and wool previously reported (5). The effect of swelling agents on the grafting process is
discussed as well as the possible role of PIs in activating the backbone polymer and
enhancing the grafting yields further.
Experimental
The following procedures used for grafting and curing were modifications of those
previously published (6). Irgacure 1800 ( bis(2,4,6-trimethylbenzoyl phenyl phosphine
oxide )) was used as PI and was obtained from Ciba-Geigy. The monomers studied
included maleic anhydride (MA) purchased from Aldrich with vinyl ethers namely
triethylene glycol divinyl ether (DVE-3) and 1,4-cyclohexane dimethanol divinyl ether
(CHVE) obtained from ISP. For the irradiation grafting procedure, PP films (isotactic 5.0
x 4.0 cm, 200 micron thickness) or strips of cellulose (Whatman No.41 acid washed
chromatography filter paper) or wool (Belmerino quality supplied by Geelong Labs
Australia) of comparable size were fully immersed in the solution and irradiated with a
90 watt medium pressure mercury lamp at room temperature. For the UV grafting,
samples were exposed at 30cm from the lamp at a dose of 1.02 x 10-2 Js
-1 for the times
146
needed to give the total doses recorded in the figures and tables. At the completion of the
irradiation samples were exhaustively extracted to constant weight as previously
described (6). For the radiation curing experiments which were used to confirm the
reactivity of the monomers studied in grafting, appropriate mixtures of monomers and,
where applicable, PIs were applied to the substrate by drawdown bar technique as thin
coatings and the materials exposed on a conveyor belt to the appropriate radiation source.
For the curing and grafting preirradiation experiments, Primarc (200watt/inch) and
Fusion (300watt/inch) facilities with H and D bulbs were utilised as well as an excimer
Fusion source (600watts/inch). UV light intensity measurements were made with an Int
Light IL – 390 radiometer and oxalate actinometry. The corona PP was treated by Sicpa
Australia and was given one pass on one side on a commercial corona line.
Results and Discussion
Grafting of neat MA:DVE-3 complex to cellulose and wool
In previous preliminary studies CT complexes have been grafted to substrates other than
PP, predominantly cellulose. However no systematic investigation of the process has
been performed, especially the effect of solvent. In this earlier work, the MA:DVE-3
complex has been used as an additive for enhancing the grafting of a methacrylate
monomer, MMA, with cellulose, wool and PP as substrates (4,5,7). Both UV and ionising
radiation were utilised as sources in this work with a range of solvents including PI.
Additional previous reports include photografting of MA:DVE-3 in DMF to cellulose
with a range of monomer concentrations (7), photografting with the same substrate in
acetonitrile, DMSO, THF and ethyl acetate at only one monomer concentration (60%v/v)
(5,8) and grafting to cellulose initiated by gamma rays in DMF at one concentration (4).
147
The present work constitutes the first detailed investigation of solvent effects in these CT
photografting processes.
As reference for these studies, data for the direct photografting of a typical CT complex,
MA:DVE-3 without solvent is useful. The results of such work using cellulose and wool
as substrates are shown in Table 1. Wool appears to be less reactive than cellulose in
these data, however the lower grafting result with wool may reflect the fact that
competing homopolymerisation is extensive with this substrate and limits the grafting
yield. As expected, inclusion of a PI like Irgacure 1800 enhances the grafting yield
significantly with both substrates. The results are consistent with the following
mechanism where D, A represent DVE-3 and MA respectively and SH the substrate
(reactions 1-3).
Table 1
UV Grafting of neat MA:DVE-3 Complex to
Cellulose and Wool with and without PIa.
Substrate PIb
Dose (J) Graft(%)
Cellulose 0 34 585
Cellulose 0.1 4 34
Wool 0 49 87
Wool 0.1 4 18.5 a
Dose rate 1.02×10-2
Js-1
. Dose determined by gel formation.
bPI is Irgacure 1800(1%w/w)
D + A [D A] [D A]*hv
R.
(1)
SHhv
S + H.
(2)
hvS + R
. .Graft (3)
The mechanism whereby species R. is formed from DA has been discussed by Jönsson,
Hoyle and Decker (3). This mechanism is also consistent with the data recently obtained
by the present authors for the direct photografting of MA:DVE-3 to cellulose and wool
148
using the styrene comonomer technique to overcome the homopolymerisation (5). Prior
to the present work, direct photografting of MA:DVE-3 to the two substrates has been
difficult because of the presence of extensive homopolymerisation restricting the ease of
recovery of the grafted materials. In this respect inclusion of solvent in the system is an
advantage.
Effect of solvent structure on UV grafting of MA:DVE-3 to cellulose
Photografting MA:DVE-3 to cellulose is dependent on the structure of the solvent. As the
data in Figure 1 show halogenated solvents like CH2Cl2 and CCl4 (footnote) are the most
efficient at the highest monomer concentrations studied. The shape of the curves for
acetone and THF systems are typical of most solvents, reaching a grafting maximum at
80% complex concentration. Presumably the high reactivity of the halogenated solvents
in these reactions is due to facile formation of additional radicals in the presence of UV
as shown by the following reactions with chloroform representing the effect of inclusion
of hydrogen in the molecule. Fundamental studies of the process(9) suggest that reaction
(5) is the initiating step for subsequent processing after exposure of chloroform to UV.
CCl4
hvCCl
3 + Cl
hv CHCl
2 + ClCHCl
3 (5)
(4).
.
.
.
In the presence of PI radical concentration is increased further leading to significant
competing homopolymerisation and early termination of the grafting process as shown by
the shorter irradiation times to achieve high grafting (Table 2).
149
THF
Acetone
Methanol
0
20
40
60
80
100
20 40 60 80 100
Monomer Complex (1:1) concentration
% G
raft
Figure 1. UV Grafting of MA:DVE-3 to cellulose dose of 162J at 1.02×10-2
Js-1
. At 80%
monomer THF yield was 75% at 55J. Chlorinated solvents CH2Cl2 and CCl4 gave the
highest yield of 187% and 110% graft respectively at 30J in 80% monomer solution.
Table 2
Grafting of MA:DVE-3 to Cellulose in Various Solvents with PIa.
Graft (%) of Complex Concentration
Solvent 80% Dose (J)
60% Dose (J)
40% Dose (J)
20% Dose (J)
THF 624 2 188 7 376 11 283 11
MeOH 11 6 7 9 18 162 8 162 Toluene 148 6 54 9 103 6 6 6
CCl4 248 17 145 9 178 17 97 18 CHCl3 202 17 173 6 107 17 28 17 CH Cl2 2 272 4 201 9 76 13 62 18
Acetonitrile
180 2 85 15 106 17 41 17
Acetone 377 4 270 5 217 10 140 15
aUV dose rate 1.02×10
-2 Js
-1.Irgacure 1800 (0.05%w/w).
Comparison of wool with cellulose in photografting MA:DVE-3 in solvent
In the preceeding work halogenated solvents have been reported as being amongst the
most efficient for photografting CT complexes to cellulose. When wool replaces cellulose
in these reactions the photografting efficiency is maintained in these same solvents
150
especially when PI is incorporated into the monomer solution, dichloromethane (DCM)
being the best solvent with PI at the monomer concentration studied, 60%w/w
(Figure 2).
CELLULOSE
0 (147)
12 (147)10 (147)
02468
101214
DCM Chloroform Carbon
Tetrachloride
% G
raft
CELLULOSE + 0.05% PI 282 (3)
46 (3)
135 (3)
050
100150
200
250300
DCM Chloroform Carbon
Tetrachloride
% G
raft
WOOL + 0.05% PI
148 (3)97 (3)
65 (3)
-50
50
150
250
350
DCM Chloroform CarbonTetrachloride
% G
raft
Figure 2. Photografting of MA complex with DVE-3 (1:1) at (60%w/w) to cellulose and
wool in halogenated solvents with and without PI (Irgacure 1800, 1% w/w); data in
brackets dose of UV in joules at a dose rate of 1.02 x 10-2
Js-1
. Adjacent figure actual
grafting yield %.DCM= dichloromethane.
When DVE-3 is replaced as donor compound by CHVE in the MA complex, a similar
trend in reactivity in the halogenated solvents is observed, however wool now appears to
be more reactive than cellulose especially in the absence of PI (Figure3).
151
WOOL 347 (147)
10 (147)0 (147)
050
100150200250300350400
DCM Chloroform Carbon Tetrachloride
% G
raft
CE LLU LO SE
0 (147)
69 (147)
0 (147)0
10
20
30
40
50
60
70
80
D C M C hloroform C arbon
Tetrachloride
% G
raft
CELLULOSE + 0.05% PI
108 (3)
41 (3)
10 (3)
0
50
100
150
DCM Chloroform Carbon Tetrachloride
% G
raft
WOOL + 0.05% PI 286 (3)
14 (3) 28 (3)
050
100150200250300350400
DCM Chloroform Carbon Tetrachloride
% G
raft
Figure 3. Photografting of MA complex with CHVE (1:1) (60%w/w) to cellulose
and wool in various halogenated solvents with and without PI (Irgacure 1800, 1% w/w);
data in brackets dose of irradiation in joules at a dose rate of 1.02×10-2
Js-1
. Adjacent
figure actual grafting yield %.
152
This CHVE result is consistent with the previous DVE-3 data where the high reactivity of
wool with DVE-3 leads to excessive homopolymerisation and early termination of the
grafting reaction. With CHVE competing homopolymerisation is not so severe allowing
the grafting process to proceed further before termination. The relatively high reactivity
of wool in these reactions reflects the facile rupture of S-S bonds in this substrate,
creating sites where grafting may occur. Overall the results show that MA vinyl ether CT
complexes can photograft very efficiently to naturally occurring polar macromolecules
like cellulose and wool.
Photografting of CT complexes to PP
The problem of photografting CT complexes to PP is clearly demonstrated by the data in
Figure 4 where a comparison is made of the photografting efficiencies of wool and
cellulose versus PP in the presence of MA:CHVE complex using a variety of non
halogenated solvents.
WOOL
9 9
2013*
53*
18*
0102030405060
DMF Acetonitrile DMSO THF Ethyl Acetate Cyclohexane
% G
raft
CELLULOSE
1114
00 0*
18*
0
5
10
15
20
DMF Acetonitrile DMSO THF Ethyl Acetate Cyclohexane
% G
raft
PPE
0 0
22*
0 00
0.5
1
1.5
2
2.5
DMF Acetonitrile DMSO THF Ethyl Acetate Cyclohexane
% G
raft
*
153
Figure 4. Comparison of wool and cellulose and PP for photografting of MA:CHVE CT
complex in various solvents ; 108J except for asterisk runs (dose of 84J).
Even with halogenated solvents photografting of MA:DVE-3 to PP is poor unless PI is
included in the grafting solution (Figure 5, where the dose to achieve 4% graft drops from
147J to 3J when PI is included in the monomer solution). Thickness of PP also influences
the efficiency of photografting to this substrate particularly as shown by the data in Table
3 where photografting to banknote PP is much higher than to the analogous packaging
material even without solvent. Banknote PP consists of two sheets of PP laminated
together and is much thicker than the packaging PP used in current experiments.
The presence of appropriate solvent will swell the PP, especially the thick banknote
substrate, trapping the complex within the sheets and facilitating subsequent grafting. In
photografting processes involving neat monomers the vinyl ether component is the liquid
and semi-polar and can act in a similar swelling capacity to assist grafting.
PPE
4 (147)
0 (147) 0 (147)0
1
2
3
4
5
DCM Chloroform Carbon Tetrachloride
% G
raft
PPE + 0.05% PI
4 (3)
0 (3)
0 (3)
0
5
10
15
20
DCM Chloroform Carbon Tetrachloride
% G
raft
154
Figure 5. Photografting of MA:DVE-3 (60%w/w) to PP with and without Irgacure 1800
(1%w/w) in various halogenated solvents; dose in Joules (J) in brackets at a dose rate of
1.02 x 10-2
Js-1
. Adjacent figure actual grafting yield %.
Pretreatment of subtrates to enhance grafting efficiency
Methods for pre-excitation of the substrate prior to grafting are potentially useful for
enhancing subsequent photografting processes. In ionising radiation systems,
preirradiation is an accepted technique for achieving efficient grafting. In the present UV
grafting work two analogous procedures have been used to improve yields, i.)
preirradiation with a Fusion excimer source ( cellulose) and ii.) pretreatment with a
corona disharge (PP).
Excimer source preirradiation of cellulose
When the excimer source is used to preirradiate cellulose prior to grafting, methanol,
ethyl acetate and acetonitrile are effective solvents in increasing photografting yields with
the MA:DVE-3 complex particularly at the highest monomer concentrations studied
(Table 4 and 5).
This higher reactivity of certain solvents is reflected in the lower UV doses required to
achieve a particular percentage graft. It is significant that these active solvents are oxygen
containing polar materials which would be compatible with the oxygen containing polar
substrate and thus favour grafting processes.
Table 3 Photografting of CT Complex MA:DVE-3 to PP Film used in Label manufacturing and
Banknote Production a
Graft(%) at CT concentration PP Source
100 90 80 70
2.6 (110) 9.6 (129) 1.9 (156) 2.7 (188) Packaging
Banknote 56 (74) 68 (112) 14 (158) 14 (194)
a Solvent used DCM; data in brackets, dose of irradiation (J); dose rate 1.02×10
-2 Js
-1.
155
Table 4
UV Grafting of MA:DVE-3 Complex at 80 and 60% w/w concentrations in
Various Solvents to Cellulose Preirradiated with Excimer Sourcea.
Graft (%) at Complex Concentration Solvent 80% 60%
Graft(%) Dose (J) Graft(%) Dose (J) MeOH 26(10) 3(162) 16(11) 132(162) Toluene 21(29) 41(46) 17(11) 132(73)
EtAc 309(78) 73(37) 38(75) 132(69) CH2Cl2 150(187) 41(30) 86(39) 132(52) CHCl3 94(36) 73(30) 21(32) 132(52) CCl4 20(110) 73(30) 15(10) 132(52)
Acetonitrile
469(35) 6(30) 12(29) 132(52)
Acetone 103(86) 89(162) 26(20) 110(162)
aUV dose rate 1.02×10-2 Js-1. Data in brackets for runs without preirradiation;
EtAc= Ethyl Acetate.
Table 5
UV Grafting of MA:DVE-3 Complex at 40 and 20% w/w concentrations in Various
Solvents
to Cellulose Preirradiated with Excimer Sourcea.
Graft (%) at Complex Concentration Solvent 40% 20%
Graft (%) Dose (J) Graft(%) Dose (J) MeOH 7(13) 162(162) 4(21) 162(162) Toluene 10(9) 162(130) 11(1) 162(162)
EtAc 12(4) 162(120) 4(5) 162(162) CH2Cl2 43(7) 162(114) 8(8) 162(162) CHCl3 16(19) 162(96) 8(8) 162(162) CCl4 6(9) 162(130) 4(74) 162(162)
Acetonitrile 9(13) 162(162) 5(8) 162(162) Acetone 15(18) 162(162) 14(17) 162(162)
aConditions as in Table 4
156
The mechanism of the Fusion excimer preirradiation process may be related to the
analogous ionising radiation system where very extensive work has been published
(9,10). Because of energetics, UV is less efficient in rupturing CH bonds than ionising
radiation however, in both cases, irradiation in air leads to cellulose oxidation with an
increase in carboxyl and carbonyl groups. With respect to grafting, initially peroxidised
substrate is formed which is decomposed subsequently by a UV treatment in the presence
of complex (C) leading to the formation of the graft copolymer ( CE, cellulose) as shown
in reactions 6 and 7.
Ce
excimer
excimer
O2
O2
Ce
Ce
Ce
O O
Ce
Ce
2
Ce
Ce
Ohv nC
2
Ce
OC (6)
O
Ce
hv
Ce
O
2
.
nC
O
Ce
C
(7)
.
O
Ce
Ce
2
In addition to diperoxides, hydroperoxides can also be formed in the irradiated polymer,
leading to graft after subsequent UV treatment of the CT monomer solution. As reactions
8 and 9 show UV dissociation of the hydroperoxide gives rise to an equivalent number of
graft copolymer and homopolymer molecules. With cellulose, competing homopolymer
yields in this system are relatively small which indicates that diperoxide formation
initially predominates in grafting enhancement with this substrate.
Ce
Ce
excimer
excimer
O2
O2
Ce
Ce
Ce
OOH hv
hv
Ce
O
2
.
nC
O
Ce
C
Ce
Ce
O OH+nC
Ce
Ce
+ Cm OH (8)
OOH
+ 2OH 2 + 2 Cm
OH (9)
..
.2
OC
157
When PI is included in the photografting solution after excimer irradiation, all solvent
studied show an increase in yield (Tables 6 and 7), presumably due to abstraction
reactions between radicals from the PI and substrate molecules leading to an increase in
grafting sites (reactions 10 and 11). Even with solvents like toluene and chloroform
grafting enhancement is significant when PI is used.
Table 6
UV Grafting of MA:DVE-3 Complex at 80 and 60% w/w concentration in various
solvents including PI to Cellulose Preirradiated with Excimer Source.
Graft (%) at Complex Concentration Solvent 80% 60%
Graft(%)
Dose (J) Graft(%) Dose (J)
MeOH 24(11) 1(6) 15(7) 10(9) Toluene 60(148) 1(6) 41(54) 133(9)
EtAc 123(148)
1(40) 94(38) 4(9)
CH2Cl2 118(272)
4(4) 80(201) 4(9)
CHCl3 83(202) 1(17) 49(173) 6(6) CCl4 198(248
) 4(17) 28(145) 4(9)
Acetonitrile 80(180) 1(2) 36(85) 12(15) Acetone 20(377) 1(4) 21(270) 11(5)
aUV dose rate 1.02×10-2 Js-1. Data in brackets for runs without preirradiation.
Table 7
UV Grafting of MA:DVE-3 Complex at 40 and 20% w/w concentration in various
solvents including PI to Cellulose Preirradiated with Excimer Source.
Graft (%) at Complex Concentration Solvent 40% 20%
Graft(%) Dose (J) Graft(%) Dose (J) MeOH 6(18) 162(162) 6(8) 162(162) Toluene 23(103) 9(6) 18(6) 18(6)
EtAc 58(114) 6(6) 34(6) 9(6) CH2Cl2 67(76) 6(13) 9(18) 10(18) CHCl3 21(107) 6(17) 12(17) 12(17) CCl4 15(178) 4(17) 15(17) 14(17)
Acetonitrile
30(106) 12(17) 8(17) 24(17)
Acetone 14(217) 14(10) 9(15) 162(15) a
UV dose rate 1.02×10-2
Js-1
. Data in brackets for runs without
preirradiation.
158
Corona source pretreatment of PP.
The contrast between cellulose and PP in these preirradiation processes is quite marked
the previous data show that photografting to PP using the current CT complexes can be
very difficult even with the inclusion of a PI in the monomer solution. However, the
situation is drastically altered if PP is pretreated with a corona discharge prior to grafting.
The data in Figure 6 show that high grafting yields of MA:DVE-3 can be achieved to
corona discharge pretreated PP.
PI hv
P.
+ I.
(10)
S (11).
SH + P.
+ PH
(192)
(77)
(86)
(115)
(192)(192) (191)
(20)0
50
100
150
200
250
2030405060708090100
Com plex Concentration (% )
% G
raft
Figure 6. Effect of complex concentration (MA:DVE-3) in DCM on
photografting yield to corona disharge treated PP. Numbers on graph, dose of irradiation
(joules).
Even in neat solution the yields are very high. Inclusion of a halogenated solvent such as
dichloromethane especially at high monomer concentrations as previously found for
grafting of these complexes leads to good yields.
159
PPE 195195
195184
165
880
2
4
6
8
10
020406080100Monomer Concentration (%w/w)
% G
raft
BANKNOTE PPE
0
187
194
186159
0510152025
020406080100
Monomer Concentration (%w/w)
% G
raft
CDT PPE
194187195
186
159
051015202530354045
020406080100Monomer Concentration (%w/w)
% G
raft
Figure 7. Comparison of various PPEs in photografting CT complex
MA:DVE-3 in CCl4; CDT PP =corona discharge treated PP; details of other PPEs in
experimental section; numbers on graph, dose of irradiation (joules); dose rate 1.02×10-2
Js-1
.
The accelerating effect of the corona treatment of PP is confirmed by the data in Figure 7
where yields from such pretreatment lead to much higher grafting yields than the non
pretreated material.
160
Mechanistically, the corona pretreatment may be related to the analogous ionising
radiation pretreatment with PP especially its application in grafting (6,11) in a similar
manner to that previously discussed above for cellulose. Thus corona discharge
pretreatment in air can lead to diperoxide and hydroperoxide formation. With ionising
radiation systems in air, it is known (6) that PP suffers very marked oxidative degradation
even at low doses. This type of process with PP is attributed to the large number of
tertiary hydrogens present in the molecule. In this instance hydroperoxides are formed
facily in the reaction by a short chain process (reactions 12 and 13).
C
CH3
H
dischargeC
CH3
C
CH3
.O
2
O2
(12)
C C (13)
CH3
O2
+
CH3
H
C
CH3
O2
H
.
+ C
CH3
.
..
..
Similar types of processes would be expected for the corona discharge treatment however
in this instance compared to ionising radiation the former process possesses the
advantages of simply being a surface pretreatment whereas ionising radiation has an
effect throughout the bulk of the whole molecule.
Grafting of MA:DVE-3 complex using ionising radiation
The preceeding data have shown how difficult it can be to photograft CT monomers to
untreated PP due to the energetics of the UV process. With ionising radiation initiation,
such limitations do not exist since the more energetic gamma rays from a Cobalt-60
source can readily break bonds in the PP, thus directly creating grafting sites in the
substrates. The present authors previously reported isolated studies of the use of ionising
radiation in this work. Thus MA:DVE-3 and CHMI:DVE-3 in acetonitrile and DMF
(60%v/v) respectively have been radiation grafted to cellulose (4). However no previous
comparative information involving the relative reactivities of cellulose and wool with PP
in the grafting of CT complexes in the presence of ionising radiation have been reported.
161
The data in Table 8 for the grafting of MA:CHVE in acetonitrile initiated by Cobalt-60
gamma rays show that grafting is facile for both cellulose and wool and marginally
effective for PP. The components of the complex namely MA and CHVE independently
graft to varying degrees ( but an order of magnitude lower than the complex itself) to
both cellulose and wool, however PP remains inert to each of these monomers under
these conditions. The results show that the complex does radiation graft efficiently to
substrates, doses as low as only 0.4kGy being required to achieve high yields of over to
200% in some systems. Mechanistically the ionising radiation process is similar to that
proposed for the UV system except that grafting sites can be directly formed in the
substrate in high yield with the more energetic gamma rays.
Table 8
Grafting of MA/CHVE CT Complex (60%) in Acetonitrile using Ionising Radiation
Irradiation at dose rate of 8.1 x 10-1 kGy/hr.
Monomer
(%w/w)
Substrate Irradiation Dose
(kGy)
Graft (%)
30% CHVE Cellulose 0.92 4.4
Wool 0.92 58
PP 0.92 0.0
30% MA Cellulose 0.92 3.2
Wool 0.92 4.5
PP 0.92 0.0
60% MA/CHVE Cellulose 0.41 211
Wool 0.92 160
PP 0.41 7.3
Irradiation at dose rate of 8.1 x 10-1 kGy/hr.
Significance of current grafting work in analogous curing
The present photografting studies are important in the related UV curing systems,
particularly the recently reported PI free process (1). The fact that CT complexes of the
type used in UV curing can be photografted to cellulose indicates that during analogous
162
curing, concurrent grafting may occur under certain processing conditions. The
ramifications of this situation have previously been discussed at earlier meetings (4)
particularly the problems that may be encountered in recycling inks and coatings made
with this technology i.e. ideally processing conditions for avoiding concurrent grafting
with the substrate would be preferable for cellulose type materials. By contrast, recycling
of synthetics like PP do not present problems. The fact that corona pretreated PP can lead
to high photografting yields with CT complexes indicates that if such corona processing
conditions are used on commercial curing lines to improve adhesion, these adhesion
advantages may be due to concurrent grafting. In this respect the results are very relevant
to the well known banknote process using PP where printing and coating can be
improved by pretreatment.
References
1. Jönsson, S., Hasselgren, C., Ericsson, J. S., Johanson, M., Clark, S., Miller, C., Hoyle,
C.E., Proc RadTech, North America ’98, Chicago, 1998, p.189.
2. Garnett, J.L., Viengkhou, V. and Ng, L-T., proc. RadTech Asia ’95, Bangkok,
Thailand, 1995, p.381.
3. Decker, C., Morel, F., Jönsson, S., Clark, S., ., Hoyle, C.E., Proc RadTech,Europe
’97, Academic Days, Lyon, 1997, p.169.
4. Garnett, J.L., Ng, L-T and Viengkhou, V., Proc RadTech ’98, North America,
Chicago, USA, 1998,. p.627.
5. Garnett, J.L., Ng, L-T and Viengkhou, V., Hennessy, I.W., Shah, N. and Zilic, E.,
Proc RadTech Europe ‘99, Berlin, 1999,. p.677.
6. Dworjanyn, P.A. and Garnett, J.L.. In Radiation Curing in Polymer Science and
Technology-Vol 1. Fouassier, J.P. and Radek, F.J., Elsevier, London, 1983, p.263.
7. Garnett, J.L., Ng, L-T and Viengkhou, V., Hennessy, I.W., and Zilic, E., Proc Int
Meeting Rad Proc 11, Melbourne, Australia, 1999, Rad. Phys. Chem. In press.
8. Garnett, J.L., Ng, L-T and Viengkhou, V. and Zilic, E., Polymer Int., 48, 1-11 (1999).
9. Ottolinghi, M. and Stein, G., Rad. Res., 14, 281 (1961).
10. Dilli, S. and Garnett, J.L., J. Polymer. Sci., A-1, 4, 2323 (1966).
11. Chapiro, A., Radiation Chemistry of Polymeric Systems, Wiley, New York (1962).
163
164
ELECTRON BEAM CURING AND CURE GRAFTING OF
CHARGE TRANSFER MONOMER COMPLEXES ON
CELLULOSE- COMPARISON WITH UV SYSTEMS.
Gary R. Dennis, John L. Garnett and Elvis F. Zilic.
School of Science, Food and Horticulture, University of Western
Sydney, Locked Bag 1797, Penrith South DC 1797 AUSTRALIA.
Fax: +61 2 9685 9915
Email: [email protected]
ABSTRACT
A range of charge transfer donor/acceptor monomer complexes have been
EB cured onto cellulose. The effect of a number of parameters on the EB
curing efficiency has been investigated including structure of donor and
acceptor monomer, radiation dose and presence of additive. The reactivity
of conventional monomer CT complexes have been compared with thiol-ene
systems in preliminary work. The magnitude of the occurrence of
concurrent grafting during cure, i.e. cure grafting, has been simultaneously
determined. Analogous curing processes initiated by UV have also been
studied and the results compared with data from the corresponding EB
systems. Photopolymerisation of the CT complexes in bulk has also been
investigated and related to the efficiency of curing. Common mechanistic
relationships between EB and UV systems in curing and cure grafting have
been examined. The potential significance of the current work in industrial
applications has been evaluated.
1. INTRODUCTION
The recent application of charge transfer (CT) monomer complexes has
been a significant development in UV curing since, in some systems the
presence of photoinitiator (PI) is not necessary to achieve fast reaction1. In
these CT curing processes, the occurrence of concurrent grafting has been
shown to be important2. The term cure grafting has been proposed for this
concurrent reaction to distinguish this type of graft from conventional pre-
irradiation and simultaneous methods3. The advantages of adopting cure
grafting technology have recently been discussed3.
The fact that in this UV work, CT monomer complexes may be polymerised
without the need to use PIs suggests that, with modification, polymerisation
of similar CT monomer complexes may be initiated by EB. In the present
paper, results are reported of preliminary experiments which show that this
EB proposal is valid. A range of CT monomer complexes has been EB
cured onto cellulose, in some examples, under relatively low radiation
doses. The cure grafting yields from these runs have been simultaneously
determined. These EB data have been compared with results from analogous
CT monomer complexes cured by UV. This UV curing work has been
extended to studies of the photopolymerisation of the same CT monomer
complexes in bulk. The effect of including appropriate additives to improve
the reactivity of the CT formulations has also been investigated. Preliminary
studies of analogous thiol-ene systems have been performed and the results
compared with those of conventional monomer CT complexes.
2. EXPERIMENTAL
165
UV curing and cure grafting procedures including lamp calibrations were
similar to those previously described2. Curing was performed under a F300
Fusion lamp, with a line speed of 16 m/min at peak UV intensity of 1.4
W/cm2 and dose of 0.20 J/cm per pass.
For UV irradiations of monomer samples in bulk, a medium pressure 90W
mercury arc lamp with dose rate 1.02 x 10-2
J/s was used. The monomers
utilised were donated by ISP, BASF or purchased from Aldrich and were
maleic anhydride (MA), triethylene glycol divinyl ether (DVE-3), N-vinyl
pyrrolidone (NVP), maleimide (MI), methyl maleimide (MMI), dimethyl
maleate (DMMA), ethyl acrylate (EA), trimethylolpropane ethoxylate
triacrylate (TMPTEA), trimethylolpropane trivinyl ether (TMPTVE),
trimethylolpropane tris (3-mercaptopropionate) (TTP), triallyl-1,3,5-
triazine-2,4,6-(1H, 3H, 5H)-trione (TAT) and 1,6-hexanediol diacrylate
(HDDA). The oligomers used were all supplied by Ballina and were epoxy
acrylate (EPA), polyester acrylate (PEA), aromatic urethane acrylate (AUA)
whilst the unsaturated (PE) was supplied by Orica. PIs were donated by
Ciba Geigy (Irgacure series) or GE (UVI 6974, cationic).
3. RESULTS
In the EB and UV work, concurrent grafting has been determined and
expressed as cure grafting. This term was proposed at the recent Tihany
Radiation Chemistry Conference3 in order to distinguish grafting obtained in
166
curing work from the conventional pre-irradiation and simultaneous
radiation methods3. The predominant paper used in the experiments was
Whatman 41 filter paper.
The results in Table 1 show that the MA/DVE-3 complexes listed cure
readily under EB at radiation doses commercially used for coating paper.
Normally in these CT complexes 1:1 double bond molar ratios are used,
however in the current system at those ratios MA solubility in DVE-3 can
be a problem, so reactions have been performed with a slight excess of
DVE-3. Previous studies have shown that this change in ratio of reactants
only marginally effects the reactivity.
Table 1
EB Curing and Cure Grafting of MA/DVE-3 Complexes
with and without Additives on Cellulose.
Additive Reactant
Ratio
Cure
(%)
C.Ge
(%)
-a
1:1 27 9
PE a 1:2:2
d 24 7
PEb
1:2:2 22 8
PE a 3:4:4 13 8
PEc
3:4:4 22 9
PE, NVP a 3:4:4 26 7
EPA a 1:1:2 22 9
EPA a 2:2:1 49 47
PEA a 1:1:2 22 6
a Dose 2.8 x 10
4 Gy;
b 1.5 x 10
4 Gy;
c1.0 x 10
4 Gy
d Typical ratio
MA:DVE-3:PE ::
1:2:2 (by weight); eC.G. = cure graft.
167
The results in Table 1 show that MA/DVE-3 can also be EB cured in the
presence of other monomers like NVP, also oligomers such as EPA, PEA
and unsaturated polyesters to give polymers with widely varying properties.
Certain formulations can be EB cured at relatively low radiation doses such
as with the fifth sample in Table 1 where only 10 kGy are needed. Cure
grafting yields are also reasonable in all runs of the table, being particularly
high in the presence of the unsaturated polyester and also one of the EPA
runs.
When the monomer components of the CT complexes are altered, EB curing
and cure grafting can be achieved with a wide range of appropriate donors
and acceptors (Table 2). The acceptors in this table include maleates,
maleimides and acrylates with vinyl ethers as donors. These complexes can
also be used with oligomers like epoxy acrylates and aromatic urethane
acrylates (AUA). Curing can also be achieved at the relatively low dose of 5
kGy (TMPTEA/TMPTVE, first of the three runs with this complex in Table
2). The other interesting feature of these data is the EB reactivity of the
thiol-ene type systems. TTP reacts with a range of monomers and
oligomers, the HDDA result being outstanding.
The thiol-ene systems are also important because there has been a
resurgence of interest in the UV curing of these materials4. The results of
UV curing of such systems when compared with EB (Table 3) are thus
significant. The TTP mixtures cure well with UV however the cure grafting
yields are poor even in the presence of PI which improves cure. These
168
conclusions are also valid for the other complexes in Table 3, with the
exception of the MA and MI complexes where cure grafting is particularly
high.
Table 2
EB Curing and Cure Grafting of Miscellaneous CT Monomer Complexes
with and without Additives on Cellulose. Comparison with Thiol-Ene
Systems.
Complex Reactant
Ratioe Cure
(%)
C.G. (%)f
DMMA/DVE-3 c 1:1 14 14
DMMA/DVE-3a,c
1:1 36 19
MI/DVE-3 c 1:1 26 18
MMI/DVE-3 c 1:1 27 23
EA/DVE-3 c 1:1 23 9
TMPTEA/TMPTVEd 1:1 17 1
TMPTEA/TMPTVEa,c
1:1 31 2
TMPTEA/TMPTVEb,c
1:1 48 15
TTP/TAT c 1:1 34 33
TTP/HDDA c 1:1 38 20
169
TTP/HDDA c 1:5 41 20
TTP/AVA c 1:1 40 36
TTP/EPA c 1:1 34 17
aAdditive EPA(40% by weight);
bAdditive AUA (50% by weight);
cDose =
2.8 x 104 Gy;
dDose = 0.5 x 10
4 Gy;
eMonomer ratio by weight;
f C.G. = cure
graft.
Table 3
UV Curing and Cure Grafting of CT Monomer Complexes with and
without Additives on Cellulose. Comparison with Thiol-Ene Systems.
Complexd P.C
e. Cure (%) C.G. (%)
f
MA/DVE-3 3 150 145
MA/DVE-3a
1 100 90
MA/DVE-3c
3 32 5
DMMA/DVE-3 6 37 5
MI/DVE-3
3 270 110
MI/DVE-3 a 1 250 230
MMI/DVE-3
6 175 26
MMI/DVE-3 a 2 255 90
TTP/TAT 2 340 67
TTP/TATb
1 470 77
TTP/HDDA 2 250 9
TTP/HDDAb
1 330 70 aAdditive, Irgacure 184 (1% w/w);
bAdditive, Irgacure 2020 (1% w/w);
cAdditive, PE;
dMonomer ratio by weight 1:1 except PE run where ratio
MA:DVE-3:PE::1:2:1; ePasses to cure;
fC.G. = cure graft.
With UV polymerisation in bulk, a similar trend to the curing of these CT
complexes is experienced (Table 4). Inclusion of small amounts of PI,
even cationic PI, leads to gel at relatively low UV doses. In this respect the
170
TTP results are particularly interesting especially with HDDA where
gelling readily occurs in a large excess of HDDA at very low UV doses.
Table 4
Bulk Photopolymerisation of CT Complexes with and without Additives.
Comparison with Thiol-Ene Systems.
Complexa Additive Gel dose(J)
MA/DVE-3 - 55
MA/DVE-3 2020 (1.0%)c
1
DMMA/DVE-3 - 110
DMMA/DVE-3 819 (1.0%)c
2
DMMA/DVE-3 UVI 6974
(1.0%)c
9
EA/DVE-3 - 220
EA/DVE-3 1800 (0.05%)c
3
TTP/HDDA
- 61
TTP/HDDA
2020 (1.0%)c
1
TTP/HDDAb - 11
TTP/HDDAb 2020 (1.0%)
c 1
TTP/EPA - 17
TTP/EPA 2020 (1.0%)c
6
TTP/TAT - 117
TTP/TAT 2020 (1.0%)c
2 a
Monomer ratios by weight 1:1; bMonomer ratio 1:5;
cBy
weight.
4. DISCUSSION
4.1.Comparison of EB with UV in Curing and Cure Grafting
The fact that EB can initiate both curing and cure grafting of CT complexes
is important since the results indicate that the scope of polymerising systems
available for high speed coating and related processes can be widened
considerably. There is no longer a dependence on acrylate technology with
its technical limitations for this purpose. The CT complexes can also
171
copolymerise with other oligomers yielding hybrid products with versatile
properties. The technique can also be used with specific complexes such as
MA/DVE-3 to prepare graft copolymers with almost quantitative cure graft
yields. The MA/DVE-3 and MI/DVE-3 complexes are particularly effective
in both EB and UV curing and cure grafting work. These results are
consistent with the previously reported data on these systems in commercial
UV photoinitiator free curing. Finally, the thiol-ene chemistry results with
EB cure, offer outstanding potential for providing new products especially
in combination with acrylate monomers and oligomers.
When compared with EB processes, the corresponding UV systems are less
reactive especially in cure grafting. Certain of the CT complexes require the
use of PIs, although in low amounts, to achieve reasonable UV cure speeds,
this observation being particularly relevant to the bulk photopolymerisation
processes under the UV processing conditions used. The fact that a cationic
PI accelerates the bulk photopolymerisation of DMMA/DVE-3, but not as
efficiently as the free radical initiators, indicates that ionic processes may be
involved in polymerisation in this system and therefore may also contribute
to the mechanism of the EB process.
4.2. Mechanism of Process
From studies in a number of groups5-12
, the following mechanism which is
applicable to the present work has been proposed to explain UV curing of
CT complexes (reaction 1).
172
)9(
)8(
)7(
)6(
)5()(
)4(
)3(
)2(
*
••••
••••
••
+•••
+→++⎯⎯⎯ →⎯
→++⎯⎯⎯ →⎯
→→→⎯⎯⎯ →⎯+→+
+⎯⎯⎯ →⎯+⎯⎯⎯ →⎯
−
SPHSHP
IPPI
graftSR
HSSH
polymerRADAD
MHeMH
eMHMH
HMMH
radiation
radiation
radiation
radiation
radiation
D
A*
D
A
D
A
A
D
DA D A
D
A
D
A
radiation
+-
n)(
Free radical copolymerisation
( )n
Cationic homopolymerisation
( )n
Anionic homopolymerisation
..
(1)
+
The scheme shows the formation of a zwitterion biradical which leads to (i)
free radical alternating copolymerisation or (ii) with excess donor cationic
homopolymerisation and (iii) with excess acceptor anionic
homopolymerisation.
In EB initiated processes, radicals, cations and anions are capable of
participating (reactions 2-4), however in practice free radical reactions
173
predominate. Cationic intermediates are formed by secondary electron
capture processes (reaction 4). There are thus common mechanistic
pathways to explain both UV and EB curing and cure grafting processes.
These are depicted in reactions 5-9, the first three referring to curing and
cure grafting whilst the last two demonstrate the role of PIs in UV systems.
These mechanistic pathways have been discussed in more detail
elsewhere3. With the thiol-ene system, CT complexes in the Mulliken
concept appear to be formed, however the role of the complexes in the
actual curing and cure grafting processes remain controversial. In the thiol-
ene systems, polymerisation occurs by a step growth process that proceeds
by a free radical mechanism involving addition of a monofunctional thiol
to a monofunctional ene depicted in reactions 10-12.
)12('
22
'
2
)11('
2
'
2
)10()(
....
.Pr
RSRCHCRSHRSHCHRCRSH
CHRCRSHCHRCHRS
productsRSrelevantifPIRSH
opagation
Initiation
+−→+−
−→=++→+
4.3 Significance of Current Work in Industrial Processing
The present EB and UV curing and cure grafting results are of potential
importance in industrial applications. The fact that monomers of different
generic chemistry to the commonly used acrylates are available for use is
valuable particularly in formulation for coatings and inks, especially with
EB initiation. The polar nature of certain monomers for the CT complexes
174
such as the acceptors MA and MI leads to improvement in physical
properties of the coated products particularly adhesion to the substrate.
Under certain processing conditions, the occurrence of cure grafting can be
important since it results in improved interfacial bonding between cured
film and substrate. In other applications cure grafting, if excessive, can lead
to problems in the recycling of the finished product. There are thus
conditions where curing can be achieved with and without concurrent cure
grafting. Overall the ability to cure these CT monomer complexes especially
with EB and particularly involving the thiol-enes improves the versatility of
products capable of being obtained with the technology.
5. CONCLUSIONS
EB is shown to cure a wide range of CT monomer complexes at radiation
doses, at least comparable to, and in some examples significantly lower than
used in conventional industrial processing. EB is more efficient than UV in
both curing and cure grafting processes, certain CT complexes requiring the
use of small amounts of PIs to achieve photopolymerisation in reasonable
times. This latter observation is also relevant to photopolymerisation of CT
complexes in bulk systems. The results indicate that ionic intermediates in
addition to free radical may contribute to the EB process. The EB CT
monomer system can also be used with acrylates in hybrid formulations. EB
also initiates curing of thiol-ene systems, coatings being obtained similar to
those produced by UV. The present work provides a new basis for coatings
and related processes in industry.
175
6. ACKNOWLEDGEMENTS
The authors thank AINSE, Ballina Pty Ltd, Fusion UV systems and the
Paint Research Association (Catherine Haworth) for support.
7. REFERENCES
1. S. Jönsson, J. Hulgren, P.E. Sundel, M. Shimose, J. Owens, K. Vaughn,
C.E. Hoyle, Proc. RadTech Asia ’95, Bangkok, Thailand, 1995, p.283.
2. J.L. Garnett and E.F. Zilic, Proc. RadTech Europe 2001, Basle,
Switzerland, 2001, p.233.
3. G.R. Dennis, J.L. Garnett, E.F. Zilic, Proc. Tihany Rad. Chem. Conf.,
Sopron, Hungary, 2002; published in Rad. Phys. Chem., 67, 391 (2003).
4. C.E. Hoyle, M. Cole, M. Bachemin, W. Kuang, S. Jönsson, Proc.
RadTech 2002, North America, Indianapolis, USA, 2002, p.674.
5. J.L. Garnett, G.R.Dennis, E.F. Zilic, Proc. RadTech 2002, North
America, Indianapolis, USA, 2002, p.1002.
6. H.K. Hall, Jr., A.B. Padius, J. Polym. Sci., Part A, Polym. Chem., 39,
2069 (2001).
7. M.L. Coote, T.P. Davis, Prog. Polym. Sci., 24 (9), 1217 (2000).
8. C. Decker, Polym. International, 45, 133 (1998).
9. P.A. Dworjanyn and J.L. Garnett., In Radiation Curing in Polymer
Scienece and Tachnology, Vol 1. Fouassier, J.P. and Rabek, J.F.Eds,
Elsevier, London, 1993, p263.
176
177
10. J. von Sonntag, D. Beckert, W. Knolle, R. Mehnert, Rad. Phys. Chem.,
55, 609 (1999).
11. J.M.G. Cowie, In Comprehensive Polymer Science, Vol 4, Part II, G.C.
Eastmond, A. Ledwith , S. Russo, and P. Sigwalt, Eds, Pergamon, England,
1989, p377.
12. S. Dilli and J.L. Garnett, J. Polym. Sci., A-1. 4. 2323 (1966).
EB Curing and Cure Grafting of Thiol-Enes. Comparison with UV Systems and
Potential Industrial Applications of the Products.
G. R. Dennis, J. L. Garnett and E. F. Zilic.
University of Western Sydney, School of Food, Science and Horticulture, Parramatta
Campus, NSW, Australia.
Abstract
Variables affecting the UV/EB curing and cure grafting of thiol-enes on cellulose have
been studied. These include effect of varying the type of olefin, increasing the
functionality of the thiol, use of acrylate monomers and oligomers in hybrid systems,
altering the surface structure of the cellulose and finally the role of air in these processes
particularly with EB. Photopolymerisation of the thiol-enes in bulk has also been
investigated. The potential of these thiol-ene systems in industrial applications is briefly
discussed.
Introduction
Thiol-ene systems are becoming increasingly important in UV curing (1-4). Originally
developed in the eighties (5) the thiol-enes ultimately proved to be unsuitable for general
commercial applications at the time. Recent work, especially with new thiol-enes, has
shown that this system can be attractive in UV cure. Preliminary studies have also been
reported for analogous EB curing of thiol-enes (2).
178
A significant feature of radiation curing of thiol-enes are the unique properties that these
materials can impart to the product film. An important aspect of these properties is the
adhesion of the coating to the substrate after radiation curing. Adhesion of cured coating
can be dramatically improved if concurrent grafting occurs (6, 7). Curing and grafting are
depicted in reactions 1 and 2 where O/M denotes the oligomer/monomer resin system.
+ O/M
+ O/M
grafting
curing
(1)
(2)UV
UV
An important difference between grafting and curing is the nature of the bonding
occurring in each process. In grafting, covalent carbon – carbon bonds are formed
whereas in curing bonding usually involves weaker Van der Waals or London dispersion
forces. The process depicted in reaction 2 has been termed cure grafting to differentiate it
from conventional methods such as preirradiation and simultaneous techniques (7). Cure
grafting, per se, has been shown to be a useful technique for synthesising novel
copolymers possessing unique properties not capable of being achieved by standard
preirradiation and simultaneous methods. The scope of the process has been outlined in
detail elsewhere (7).
In terms of properties of cured coatings the occurrence of concurrent cure grafting is
significant since, if present, cure grafting would not only minimise delamination of the
179
coated film but could also render difficult recycling of the finished product especially if it
was cellulose. Hence the conditions for the control and optimisation of cure grafting
during radiation curing is important. In this respect there are potential differences in the
proportion of cure grafting if UV or EB sources are used. With more energetic ionising
radiation such as EB, radicals at sites for grafting are formed more readily than with UV.
Thus concurrent cure grafting would be expected to occur more efficiently in EB
systems. With UV, in the presence of photoinitiators (PIs) abstraction reactions with the
backbone polymer can occur leading to a graft copolymer. Even with PI free systems in
uring, the occurrence of cure grafting has been observed and discussed (2).
discussed. The potential of this work in industrial
application will be briefly mentioned.
c
Thiol-ene systems may be very useful for controlling cure grafting because preliminary
studies indicate that concurrent cure grafting in such systems will generally be low even
with EB (2). These studies were performed with the trifunctional thiol (PPT, I) and thus
the use of other thiol-enes particularly higher functionality materials with other
components such as oligomers is relevant. The purpose of this paper is to describe
radiation curing of such thiol-enes on cellulose and to compare the trifunctional and
tetrafunctional thiols in the process. The use of thiol-enes combined with other oligomer
systems to improve surface properties will also be investigated since the thiol-enes may
be used as components to induce specific properties into films. The effect of this process
in both curing and cure grafting will be
180
CH2
CH2 C
CH2
CH2 O
O
O
CH2CH
CH2
CH2 O CH2
O
CH2 SH
CH2 SH
CH3
CH2
O
CH2 SHN N
N OO
O
CH2
CH
CH
CH2CH2
CH2
(I) (II) (II)
Figure 1. Structures of typical trithiol, trimethylol propane tris(3-mercaptopropionate)
(TTP, I) and olefin containing electron rich double bonds triallyl-1,3,5-triazine-2,4,6-(1H,
3H, 5H)-trione (TAT, II).
Experimental
Monomers and oligomers were supplied by Aldrich, BASF, Monocure Pty Ltd and
Ballina Pty Ltd and include pentaerythritol tetrakis-3-mercatopropionate (PTP),
trimethylolpropane tris (3- mercaptopropionate (TTP, I), triallyl-1,3,5-triazine-2,4,6-(1H,
3H, 5H)-trione (TAT, II), triethyleneglycol divinyl ether (DVE-3), trimethylol propane
trivinyl ether (TMPTVE), hydroxybutyl vinyl ether (HBVE), cyclohexyl vinyl ether
(CHVE), maleic anhydride (MA), maleimide (MI), dimethyl maleate (DMMA),
hexanediol diacrylate (HDDA), aromatic urethane acrylate (UA), epoxy acrylate (EPA)
and polystyrene (PST). Photoinitiators (PIs) were donated by Ciba Geigy (Irgacure
series). The procedures used in EB and UV curing and cure grafting including lamp
calibrations were modifications of those previously described (2, 6). For the EB work two
machines were used, namely Nissan 500 kV and ESI 175 kV facilities. For UV, two
lamps were used, a Fusion F300 with H and D bulbs (line speed of 16 mm-1 at peak UV
intensity of 1.4 Wcm-2
and dose of 0.20 Jcm-1
per pass together with a mercury lamp
facility of 80 Wcm-1
with a dose of 0.17 Jcm-1
per pass. The UV lines were calibrated
with an INT Light IL’ 390 radiometer. For the bulk UV irradiations a Philips 90 W
181
medium pressure mercury lamp model 93110ET fitted with a quartz envelope and
mounted in a vertical configuration was used.
Results
In both UV and EB work, the data have been expressed as percent curing and percent
cure grafting (7). These terms have previously been defined as follows. Curing is the
crease in weight of the substrate after being coated, exposed to the radiation source and
the coating tested for cure by the mination procedures.
Curing ( C ) is defined as follows where WO
substrate after the cured sample has been
lvent extracted to constant weight to remove residuals in the traditional radiation
r the conventional preirradiation and
multaneous methods (7). Cure grafting (C.G.) is defined as follows where We is weight
s has indicated the occurrence
in
conventional solvent rub and dela
is the weight of substance before coating and
WC is the weight of the substrate after coating has been cured.
C = (WC - WO)/ WO
Cure grafting is the weight increase of the
so
grafting procedures originally developed fo
si
of coated substrate after solvent extraction.
C.G. = (We - WO)/ WO
The cure and cure grafting terms are important since the presence of cure grafting in a
cured film can significantly influence the properties of the end product in its final use
especially in recycling. These concepts are particularly relevant to the current thiol-ene
work since preliminary exploratory work with these system
182
of poor cure grafting in certain of the cured films (2). The essential difference between
iable influencing coating weight
elivered by the rod was the viscosity of the medium, higher viscosities, such as when
higher coating weights. In some examples in
Tables 1 and 2 the EB curing and cure grafting of thiol-ene systems is reported, the
sults in Table 1 being obtained for curing under nitrogen whilst those in Table 2 were
radiation curing and cure grafting as defined above is that to obtain the cure grafted
material, residuals in the cured film such as unreacted monomers, low molecular weight
oligomers and PIs or their fragments need to be removed.
The types of cellulose substrates used in this work have also been specifically chosen
since the results will be of particular value to certain industries, especially the graphic arts
field including printing, overproof varnishing and label applications. For the data reported
in Tables 1-3, three types of papers were examined including Niklakett medium 70 gsm,
Algrofiness 80 gsm and Whatman 41. As expected similar degrees of reactivity in curing
and cure grafting were observed with all papers thus Whatman 41 was adopted as model
paper for most of the EB runs. The results in Tables 4 and 5, especially the former table
are particularly relevant to the label market thus the substrate used was a chrome coated
label stock paper, coated on one side which is the side used for label printing and coating.
The reverse side was uncoated cellulose. If cure grafting was efficient i.e. quantitative,
the yield could be predetermined before cure by the weight of coating applied to the
cellulose by the metering rod (7). The essential var
d
oligomers were present, generally leading to
the tables, high coating weights were deliberately applied in runs to examine specific film
properties which are discussed in the relevant sections.
EB Curing and Cure Grafting of Thiol-Enes
In
re
183
cured in air. The last two entries in Table 1 are data obtained for the MA and MI charge
ansfer (CT) complexes with DVE-3 as donor.
ble 1
g of Thiol-Enes In Nitroge Cellulosea
Cure (%) Cure Graft (%) C t
tr
Ta
EB Curin n on
Thiol-Ene System ommen
TTP/TAT 34 33 gloss
TTP/TAT 35 32 corona
TTP/TAT + 6% H2O 29 28 gloss
TTP/HDDA (1:1) 38 20 gloss
TTP/HDDA (1:5) 41 20 gloss
TTP/HDDA (1:1) + 20% PST 17 15 gloss
T TP/DVE-3 16 10 gloss
TT E P/TMPTV 23 21 gloss
184
TTP/DMMA 25 13 matt
TTP/HBVE 21 10 matt
TTP/UA 31 29 gloss
TTP/EPA 26 22 gloss
MA/DVE-3 27 9 gloss
MI/DVE-3 26 18 gloss
aDose 2.8 x 10
4 Gy at 40 – 60 m min
-1. Reactant ratios (1:1 by weight) unless specified.
Extensive information on curing has been previously been reported for these systems.
The current results are included for comparative purposes and are relevant to the
mechanism of the thiol process discussed in the following sections. The data in Table 1
show that a variety of enes can be used with the TTP trithiol. TAT is especially useful as
e olefin since concurrent grafting with this compound is almost quantitative. Inclusion th
of water in the system is not detrimental to curing and cure grafting and this may be
important since the presence of water in a cured coating can improve the swelling of
certain substrates and thus improve adhesion.
Multifunctional acrylate monomers like HDDA are effective with TTP even at higher
dilution (1:5). Concurrent grafting in this system is relatively poor, however this situation
is significantly improved by the inclusion of a binder resin like PST. Strong donor
monomers like DVE-3 and TMPTVE, especially the latter, also constitute effective EB
curing systems with TTP. Polar monomers such as DMMA and HBVE cure readily with
TTP however the finished coating is matt whereas all others are gloss. This observation
would suggest incompatibility in the cured coating of the two components. The result is
185
mechanistically interesting since DMMA is a strong acceptor. Acrylate oligomers,
typically aromatic urethanes and epoxies, can also be cured with TTP which is a
practically useful result since the unique properties of these oligomers can be
incorporated into the finished coatings. The last two entries in Table 1 are used for
mechanistic comparison purposes and demonstrate the reactivity of typical CT complexes
this work. MA and MI are traditionally strong acceptors which have been coupled with
t to the HDDA system especially at higher dilution (1:5) and particularly in the
resence of binder polymer PST. The PST result is consistent with what is found with a
traditional CT complex like MA/DVE-3 where inclusion of the binder polymer enables
not only curing to be achieved in air but also improves the yield of the concurrent cure
graft.
ble 2
in
a strong donor like DVE-3. Although these latter CT systems are very efficient with EB,
concurrent cure grafting yields are not exceptional and contrast markedly with the TTP
results.
All data reported in Table 1 have been observed using an inert nitrogen blanket over the
film in the EB processing. The possibility that EB curing could be accomplished in air
would dramatically reduce the costs for commercial processing. The data in Table 2
show that certain TTP systems can be EB cured in air at doses similar to those used with
nitrogen (Table 1). TTP/TAT EB cures well in air, although the concurrent grafting yields
are not exceptional even if the substrate is corona treated. These results are in marked
contras
p
Ta
186
EB Curing of Thiol-Enes in Air on Cellulosea
Cure (%) Cure Graft (%) Com ent Thiol-Ene System m
TTP/TAT 19 14
TTP/TAT 25 14 Corona
TTP/HDDA 18 15
TTP/HDDA (1:5) 24 23
TTP/HDDA (1:1) + 20% PST 23 22
MA/DVE-/PE (3:4:4) 34 33
aConditions as in Table 1.
UV Curing and Cure Grafting of Thiol-Enes
Essentially the same types of thiol-ene systems as reported in Tables 1 and 2 for EB
processing have been studied using UV curing on a similar type of cellulose to that in
Table 1. The trithiol, TTP (I), in the presence of the ene, TAT (II), cures in two passes
using the Fusion lamp. Cure grafting yields are relatively poor in this system and are even
lower with the inclusion of water although this additive speeds up cure. The presence of
PI also accelerates cure but does not affect the cure grafting yield significantly. High
coating weights and hence high cure yields (>100%, based on an increase in weight of
substrate) were deliberately used in these experiments in order to accurately examine the
cure grafting yields which were expected to be low. The cure yields i.e. extremely high or
low can be easily controlled by the amount of coating applied to the substrate by the
laboratory metering rod. When a multifunctional acrylate (MFA) like HDDA replaces
TAT, the UV curing reactivity with TTP is similar to the analogous TAT system, curing
occurring in two passes and is accelerated by the inclusion of PI. Extremely poor cure
187
grafting is obtained with the HDDA system, however this is significantly improved with
the inclusion of PI as expected. The combination of a strong donor, like DVE-3, with
TTP leads to curing in three passes and almost quantitative cure grafting. Addition of
water or PI to this system improves cure speed, as does the presence of an acrylate
oligomer like an epoxy acrylate (EPA) however cure grafting yields are lowered under
these conditions. When the difunctional vinyl ether, DVE-3, is replaced by a
monofunctional ether such as CHVE similar results with TTP are observed. TTP itself
an also be used in direct combination with acrylate oligomers like the aromatic urethane
A) and EPA. These results are consistent with the data obtained above for HDDA,
specially with UA and EPA oligomers which in commercial practice usually contain up
20% of an MFA like HDDA to permit easy processing in the plant.
c
(U
e
to
188
Table 3
UV Curing of Thiol-Enes on Cellulosea
Th P.C.b Cure (%) Cure Graft (%) iol-Ene System
TTP/TAT 2 340 67
TTP/TAT + 6% H2O 1 58 0
TTP/TAT + PI 1 470 77
TTP/DVE-3 3 10 9
TTP/HDDA (1:5) 2 250 9
TTP/HDDA (1:5)+ PI 1 330 70
TTP/DVE-3 + 5% H2O 1 11 10
TTP/DVE-3 + PI 1 13 5
TTP/DV EPA E-3/ 20 % 1 6 2
TTP/DVE-3 + 5% H2O 1 14 8
TTP/CHVE 3 7 6
TTP/CHVE/ 20 % EPA 1 13 9
TTP/UA 1 84 34
TTP/EPA 1 43 24
MA/DVE-3 3 150 145
MA/DVE-3 + PIc 1 100 90
MA/DVE-3/UA 1 56 32
MA/DVE-3/EPA 1 37 27
MI/DVE-3 1 270 110
aRatios of reactants (1:1 by weight). ; PI = 2% Irgacure 2020.
bP.C. = passes to cure under UV lamp, Fusion F300, D bulb, at 16 m min
-1.
189
c0.5% Irgacure 184.
d1.0% Irgacure 184
The last five entries in Table 3 list the results of UV curing and cure grafting of two
aditional CT monomer complexes on cellulose, namely MA and MI complexes of DVE-
. These results are shown for comparison purposes with the TTP data and are relevant to
compound should improve the reactivity of the resulting thiol-ene system. In
Table 4, the results of comparing a trithiol, TTP, with a tetrathiol, PTP, in these curing
and cure grafting reactions is reported. The c llulose in this work is a chrome coated label
stock.
tr
3
the following discussion section.
Effect of Thiol Functionality in UV Curing and Cure Grafting
Preliminary studies of thiol-ene systems in radiation curing has shown that trifunctional
thiols are required to achieve efficient crosslinking. Further it would be expected that
crosslinking efficiency in the thiol-ene system would be improved with increasing
functionality of the ene. Thus the effect of increasing the thiol functionality from the tri to
the tetra
e
190
Table 4
UV Curing and Cure Grafting of Thiol-Enes on Chrome Coated lab k Cellulosea
Thiol
el Stoc
PTP TTP
Ene System
CURE C. G FT CU E C. GRAFT RA R
TAT 7b
3 6b
0
12
9 9 3 TAT + PI
T AT + 6% H2O 12 5 6 2
TAT + PI + 6% H2O 10 3 10 3
DVE-3 14 5 7 3
DVE-3 + PI 14 4 9 2
TMPTVE 8 0 11 6
TMPTVE + PI 32 20 9 2
HDDA 15 2 7 2
HDDA + PI 13 13 0 0
HDDA (1:5) 15c
8 6 0
HDDA (1:5) + PI 8 0 8 0
UA (5:1) 47 6 - -
UA (5:1) + PI 74 35 - -
EPA 43 39 - -
EPA + PI 46 38 - -
EPA (5:1) 65 37 - -
EPA (5:1) + PI 57 50 - -
191
aIrradiations with 80 W cm
-1mercury lamp. All samples cured in one pass unless
ecified. As expected MA and DMMA, strong acceptors do not cure with PPT after six
was specifically chosen as a model so that the work could be optimised for
e label industry since much UV curing and printing is performed in that field with this
coatings on the chrome coated paper side are consistent with the
revious UV curing and cure grafting results obtained with TTP and a range of enes in
able 3 on relatively pure cellulose. All combinations cured in one pass with the
very poor in all PI free systems
especially with the trifunctional thiol. In f PI with certain tetrafunctional systems
PI, PI was also included in separate experiments with th tems in order to determine
whether the presence of PI improved cure graft yields.
sp
passes in reference experiments.
bTwo passes to cure.
cSix passes to cure.
This substrate
th
type of cellulose. The paper is coated with an acrylic finish on one side only and the
current UV experiments have been performed separately on each side of the paper for
comparison.
The data in Table 4 for
p
T
exception of the TAT and HDDA (1:5) systems incorporating PTP and TTP where two
passes were required.
Inclusion of PI accelerates the curing to one pass in these slower systems. The significant
feature of the data in the table is that cure grafting is
clusion o
improves these yields. Even though most systems in the table cured in one pass without
ese sys
192
Tabl
UV Curing and Cure Grafting of Thiol-Enes on Chrome Uncoated label Stock Cellulosea
Thiol
e 5
PTP TTP
Ene System
CURE C. G FT C C. GRAFT RA URE
TAT 23 6 - -
TAT + PI 17 2 - -
DVE-3 32 2 15 5
DVE-3 + PI 25 5 12 0
TMPTVE - - 19 2
TMPTVE + PI - - 30 6
HDDA 19 0 10 0
HDDA + PI 23 3 31 0
HDDA (1:5) 130b 2 1 5
HDDA (1:5) + PI 22 24 2 5
UA (5:1) 85 81 - -
UA (5:1) + PI 45 39 - -
EPA 39 6 - -
EPA + PI 38 3 - -
EPA (5:1) 31 3 - -
EPA (5:1) + PI 91 67 - -
aIrradiation conditions as in Table 4.;
bSix passes to cure.
193
Exceptions to the above general observations are the runs containing oligomer acrylates
(UA, EPA), where cure grafting results are significantly improved compared to the other
formulations.
Similar results to those in Table 4 are obtained when the coatings are cured on the
ince the process can yield unique products containing thick
ctions. The data in Table 6 demonstrate the effect of functionality of thiol on the speed
f bulk photopolymerisation of various thiol-ene combinations. As expected, the
trathiol is significantly more reactive than the trifunctional compound except with the
ifunctional vinyl ether when both thiols are extremely reactive. This result confirms
nequivocally that the higher the functionality of the thiol and ene the faster the rate of
photopolymerisation.
uncoated side of the stock cellulose (Table 5). Again cure grafting is very poor with both
tri and tetra systems, the oligomer acrylates especially UA being an improvement in this
respect. The HDDA system at high dilution (1:5) with PTP also cures very poorly again
consistent with the data in Table 4.
Effect of Thiol Functionality in Bulk Photopolymerisation of Thiol-Enes
Photopolymerisation of thiol-enes in bulk is becoming increasingly attractive to
specialised markets, s
se
o
te
tr
u
194
Table 6
Co of TTP with P in Bulk P topolymerisation of Thiol-enes with and
without Photoinitiatora.
Gel dose (J)
mparison PT ho
PTP TTP
Th
PI
iol-ene system
NPI PI NPI
TAT 238 1 184 1
HDDA 19 3 42 4
DVE-3 17 1 35 1
TMPTVE 4 0.5 4 0.5
MA/DVE-3 55 1 - -
aDose rate 1.02 x 10
-2 J/s; NPI=no PI; PI=1% Irgacure 2010.
Inclusion of PI dramatically accelerates rates of polymerisation to the degree where it is
istinguish the reactivity of the two thiols. Again the thiol-enes with TAT are
ata where these thiol-
difficult to d
the slowest to polymerise consistent with the analogous curing d
enes required more passes to cure than the other systems studied. For mechanistic
discussion purposes the results for the MA/DVE-3 CT complex are used for comparison
and indicate this complex slower in UV bulk polymerisation than any of the thiol-enes
studied in the table, however inclusion of PI compensates for this difference.
Discussion
Mechanism of Radiation Curing and Cure Grafting of Thiol-Enes
195
In the earlier thiol-ene UV curing work the possible mechanistic role of thiol-ene CT
complexes in the photopolymerisation process was considered (5). Thiol-enes are known
to form such CT complexes which have been characterised by spectroscopic techniques.
With respect to the current radiation curing and cure grafting work, the problem to be
resolved is the degree, if any, these complexes participate in the initiation process of the
radiation polymerisation. Other types of CT complexes have already been used
successfully in this work (2, 8). Typical donor/acceptor (D/A) complexes donors like
DVE-3 with the acceptors MA and MI. Data from EB and UV curing and cure grafting
with these DA complexes are reported for comparison purposes in Tables 1-3 and in
Table 6 for photopolymerisation in bulk. The data show that the thiol-enes and the DVE-
3 CT complexes exhibit comparable reactivities in radiation curing, cure grafting and
exhibiting slower speeds of polymerisation. These results would suggest that there may
be common mechanistic pathways between the two systems. With respect to the DVE-3
type CT monomer systems used in this work as reference, extensive studies by a number
of authors on the UV initiated process have led to the following mechanism being
proposed to explain the curing process. With donors such as DVE-3 and acceptors like
MA and MI, reactions 3 and 4 were proposed to explain UV curing with these complexes.
bulk polymerisation processes with the latter CT complexes in some specific runs
196
Reaction 3 shows the initial effect of radiation on the DA complex whilst reaction 4
DD
outlines subsequent possibilities depending on the reaction path.
Reaction 4 shows the formation of a zwitterion biradical which leads to (i) free radical
alternating copolymerisation or (ii) with excess donor, cationic homopolymerisation or
ii) with excess acceptor, anionic homopolymerisation.
extended to explain UV concurrent cure grafting (7) in reactions 5-9. Reaction 5
summarises the overall process. Reaction 6 shows site formation in the substrate whilst
reaction 7 demonstrates the specific cure grafting process using the radicals formed in
reaction 5. Inclusion of PI leads to higher radical concentrations (reactions 8 and 9).
(i
The formation of the intermediate in reaction 3 involves interaction between a ground
state donor with an acceptor in the excited state. The above mechanism has also been
A
radiation
A*
D
A*
D
A
.
D
A
+
-
A
D
D
A D A
n
Free radical copolymerisation
)(
D
( )n
Cationic homopolymerisation
A
( )
Anionic homopolymerisation
(3)
n
(4)
.
197
)9(SPHSHP +→+
The above mechanism developed for UV processes can also be used to explain radiation
curing and cure grafting of these complexes in the presence of EB sources. Because EB
sources are more energetic than UV then additional reactions to those found for UV are
expected for EB. In EB, radicals, cations and anions are form
)8(
)7(
..
..
..
IPPI
graftSRradiation
radiation
+⎯⎯⎯ →⎯→+
)6(),(
)5()( .
**
*
substrateSHHSSH
polymerRADAD radiation
+⎯⎯⎯ →⎯→→→⎯⎯⎯ →⎯+
MH M + H
MH MH + e
ed during irradiation with
inating (reactions 10-12 where MH is monomer).
the
relevant tables show, there are differences in reactivity in specific runs between the thiol-
ene and the DVE-3 complexes. Recent studies by other authors using UV initiation have
indicated the difficulty of interpreting the curing of the thiol-enes in terms of the
participation of the analogous CT complex, in particular the degree to which the thiol-ene
CT complex may be involved in the initiation step. Specifically the problem is that thiyl
dicals (see below) are only produced slowly from the complex whereas in radiation
rocesses, a large flux of additional radicals are produced by radiolysis (or photolysis as
MH + e MH
+
-
.(10)
(11)
(12)
the first process predom
Ionic reactions can thus contribute to the overall EB process, however, in practice free
radical reactions tend to predominate under most processing conditions. As the data in
ra
p
.
. .
198
relevant). Detailed kinetic studies thus need to be performed with the thiol-ene system
before this aspect of the mechanism can be resolved.
with
e lower energetic UV system, even with PI present, are relatively poor, this result
lectrons from EB to generate sites on
ccurs late in the free
radical chain process and the medium does not become viscous until the later stages of
the polymerisation. This property has the effect of effectively increasing the residence
time of the coating, whilst still low in viscosity, on the substrate before cure, thus
RSH + PI (if relevant)
With respect to the thiol-ene system it is generally accepted that the basic reaction occurs
by a addition mechanism that is propagated by a free radical chain process involving the
addition of a thiol group across an ene double bond as shown in reactions 13-15 (1).
In the current work, when comparing EB and UV as initiating systems, differences
between the two thiol-ene systems are observed.
Thus while cure grafting of thiol-enes with
RS + Products
2RS + CH CHR' RSH C CHR'2
RSH2C CHR' + RSH RSH2C CH2R + RS
. .
'
(13)
(15)
.
..(14)
EB is efficient, the corresponding yields
th
presumably reflecting the ability of the energetic e
the substrate where grafting may occur. The relatively fast line speeds of EB (500 m/min)
can lead to problems in adhesion i.e. cure grafting can be difficult even with polar
substrates like cellulose. The fact that with the thiol-ene system, being essentially a step
growth process involving a free radical chain mechanism, gelation o
199
enhancing the penetration of coating into the substrate leading to graftin
the post curing contribution to grafting would also be in
since ultimate gelation is delayed and complete curing would not be
time after removal from the source.
g. In this respect
creased under these conditions
achieved until some
ffect of Oxygen in EB Curing and Cure Grafting
be attributed to the fact that the thiol can undergo a rapid chain transfer reaction
ith the peroxy radical. By contrast, in acrylate and methacrylate systems, the analogous
RSH2C CHR'
E
An important observation from earlier UV thiol-ene photopolymerisation was the lack of
oxygen inhibition in curing (5). This was attributed to the reaction between the
betathioether carbon radical species (reaction 14) and molecular oxygen to yield a
betathioether peroxy radical in a co-oxidation process (reaction 16).
O2
This peroxy radical may then undergo a chain transfer reaction with the thiol to preserve
the propagation radical (reaction 17). The insensitivity of the thiol-ene process to oxygen
can thus
w
peroxy intermediates are chain stoppers, resulting in the formation of nonpropagating
species.
This rationale developed for UV thiol-ene systems can theoretically be extended to EB
curing of the same systems. With EB, efficient curing of acrylates can only be achieved
+ O2 RSH2C CHR'
RS 2H C CHR'
O2
+ RSH RSH C CHR'
O H
2
2
. + RS (17)
.
(16).
.
200
in an inert nitrogen atmosphere, an observation consistent with the above discussion.
Acrylates generally have been the predominant technology used commercially in EB
curing of coatings to the present time, hence the requirement for inert conditions during
curing.
The data in Table 2 demonstrate the conditions where curing can be observed on
cellulose using a range of thiol-ene systems in the presence of air. Even if acrylates are
one of the thiol-ene components, EB curing in air is still possible with this hybrid system.
This result is consistent with earlier UV studies of hybrid thiol acrylate systems where
rapid photopolymerisation uninhibited by oxygen was observed (5). The result of EB
curing in air is particularly significant for systems involving dilute thiols in acrylates
(TTP/HDDA-1:5, in Table 2) where oxygen inhibition is minimised. In this system, the
process is essentially an acrylate homopolymerisation carried out in the presence of a
strong chain transfer agent such as the thiol which acts to attenuate the effects of oxygen.
he MA/DVE-3 CT complex with unsaturated polyester will also cure in air under these
EB conditions, whereas the CT complex itself cures only with diff ulty. In this latter
stem, the unsaturated polyester scavenges the oxygen, permitting the polymerisation of
With respect to cure grafting, it is significant that this
T
ic
sy
the CT complex to occur readily.
reaction also occurs efficiently in air with EB, This result suggests that ionic processes
may also be involved in the mechanism of this reaction to complement the free radical
process. Oxygen, especially at the surface of cellulose, is known to react readily with
impinging electrons according to reaction 18, the resulting O2- species, being desorbed
from the surface, may well influence the resulting cure grafting process via an ionic
process.
201
)18(22
−− →+ OeO
UV Curing of Thiol-Enes in Bulk
Thiol-enes are potentially useful in a range of areas where bulk photopolymerisation
properties are important, such as in composite formation. A number of the thiols
particularly the tri- and tetra- derivatives with a trifunctional ether can gel at reasonably
low UV doses without the necessity of using a PI as the data in Table 6 show. The ability
of the thiol-ene to crosslink efficiently is thus important. In this respect the higher
functional tetrathiol is more efficient than the trithiol with the divalent ene system in the
bulk photopolymerisation process. However in these UV systems care still needs to be
exercised to obtain uniform polymerisation in the bulk due to the relatively poor
transmission coefficient of UV through such systems. The photopolymerisation in bulk
systems is further complicated if performed in the presence of air. Generally, in air, the
reaction between the thiol and olefin essentially involves the addition co-oxidation
process whilst in low air concentrations the addition reaction predominates (5). In bulk
photopolymerisation two situations arise, (i) at the surface (the air interface) a high
concentration of air is expected whilst (ii) in the bulk further removed from the interface,
lower concentration of oxygen is present. When the concentration of oxygen in the
process
a
thiol-ene mixture is low, the expected addition chain transfer-addition
predominates, whilst in the presence of excess oxygen the co-oxidation process is
predominant. These two processes thus yield different intermediates during the
polymerisation process depending upon the concentration of oxygen present at a
particular position in the bulk at the time of the irradiation. Thus in such bulk
photopolymerisation, the nature of the properties and polymer formed at the interface and
in the bulk may vary significantly. Uniformity of product formation in bulk runs may thus
202
be detrimentally affected and may significantly influence the physical properties of the
final product especially the strength of the product.
Commercial Potential of Thiol-Ene EB/UV Curing and Cure Grafting Processes
The thiol-ene system has recovered its potential as an excellent monomer in UV curing
after an unsatisfactory start in the eighties with the W.R. Grace process in floor tile work.
ven since that time thiol-enes have continued to be used in small scale niche markets.
been renewed interest in the thiol-enes with the advent of new technology.
he results of the present studies show that the process, using either EB or UV, provides
n to be efficient initiators for the curing and cure grafting of thiol-
useful hybrid systems for these polymerisation processes. Increasing the
of the thiol enhances the speed of polymerisation in the system. EB curing
fting of these thiol-enes in air can be accomplished. A plausible mechanism
E
There has now
T
an efficient coating process which can be used, either alone, or with a variety of other
systems as hybrids, including acryate monomers and olgimers and the CT complexes
typified by the DVE-3/MA, MI systems. With EB, the thiol-enes are particularly
attractive since they can be cured in air which is a significant economic advantage. With
both EB and UV, the thiol-enes can be used to modify substrates by cure grafting (as
distinct from curing) technologies. This system complements the conventional radiation
grafting processes using the simultaneous and preirradiation techniques.
Conclusions
UV and EB are show
enes on cellulose. Changing the structure of the olefin in the thiol-ene affects the
reactivity of the system. Acrylate monomers and oligomers when used with the thiols
constitute
functionality
and cure gra
203
for the processes has been proposed. The UV/EB curing and cure grafting of the thiol-
t Research Association (Catherine Haworth) for
002,
, Proc. RadTech Asia ’03, Yokahama,
a,
4. N. B. Cramer, J. P. Scott, C. N. Bowman, Macromolecules, 2002, 35, 361.
5. Jacobine, A. T. in Radiation Curing in Polymer Science and Technology:
Photopolymerisation Mechanisms, Eds. J. P. Fouasier and J. F. Rabek, Elsevier
Applied Science: London, pp. 219-268, 1993.
6. Dworjanyn, P.A. Garnett, J.L., in: Radiation Curing in Polymer Science and
Technology Volume 1 Fundamenta abek,
(Eds). Elsevier. London, p. 63, 1993.
7. G. R. Dennis, J. L. Garnett, E. F. Zilic., 2003, Rad. Phys. Chem. 67, 391-395.
enes is shown to be of value in a number of industries, such as the graphic arts, especially
for printing and coating on paper.
Acknowledgments
The authors thank Ballina Pty Ltd, Concept Paints Pty Ltd, Monocure Pty Ltd, Fusion
UV systems and the library of the Pain
support.
References
1. C. E. Hoyle, M. Cole, M. Bachemin, W. Kuang, S. Jönsson, Proc. RadTech 2
North America, Indianapolis, USA, 2002, p.674.
2. G. R. Dennis, J. L. Garnett, E. F. Zilic
Japan, 2003, p. 254.
3. B. K. Christmas, H. Ho, T. Ngo, D. Brenes, Proc. RadTech 2002, North Americ
Indianapolis, USA, 2002, p713.
ls and Methods. J. P. Fouassier, F. J. R
204
8. S. Jönsson, J. Hulgren, P. E. Sundel, M. Shimose, J. Owens, K. Vaughn, C. E.
Hoyle, Proc. RadTech Asia ’95, Bangkok, Thailand, 1995, p. 283.
205
206
Abstract
Radiation Curing of Composites
Gary R. Dennis, John L. Garnett and Elvis F. Zilic.
School of Science, Food and Horticulture, University of Western Sydney, NSW,
Australia.
The use of radiation curing in the synthesis of composites is discussed including the
value of ultraviolet light (UV) versus electron beam (EB) as sources. The
advantages and limitations of the UV process are examined. The technique is
briefly described in terms of equipment, types of resins used including
donor/acceptor monomers and specific fibres and fillers appropriate for the process.
The significance of concurrent grafting on the fibres during curing of composites is
discussed. Additives for enhancing rates of curing and grafting of the matrix resins
are reported. Mechanisms to explain the observed polymerisation processes are
described including spectroscopic studies. Emerging technologies including the
future of nanocomposites are outlined. A unique Australian application of UV
composites is discussed – the banknote.
Introduction
Radiation curing using electron beam (EB) and ultraviolet light (UV) as sources for
initiation of polymerisation is now an established technology in the ink and surface
coatings industries. Current developments in these fields are continuously reported
in RadTech conferences in Europe, Asia and North America. With respect to the
synthesis of composites including fibres, EB curing has been used commercially for
a number of years (1). The basis of the technique is that energetic electrons can
penetrate relatively thick sections of material leading to both surface and through
cure, an essential property for composite formation. However EB sources are
relatively expensive and this has limited their use in the synthesis of composites.
Recent developments with cheaper analogous UV curing has enabled this lower
energy source of radiation to now be considered for use in composite processing
with wide potential.
The purpose of this present article is to discuss the latest developments in UV
curing and their relevance in composite processing. In this respect, in addition to
cost, UV has a number of advantages which will be compared to EB systems. The
types of composites treated will be both fibre reinforced materials that exhibit
structural properties and also essentially inorganic filler dispersions. This article
will include the recently reported extension of the field to the UV curing of
nanocomposites.
207
The UV Technique
In its simplest form, the system involves the irradiation of a resin containing fibres
or filler material preferably at room temperature and at relatively fast line speeds.
The resin- fibre mixture can be premixed or the two components progressively
built-up in successive layers as in boat manufacture. Until recently, in order to
achieve cure in a reasonably short period of time, the presence of photoinitiator (PI)
and / or photosensitizer (PS) were needed in the mixture, especially to achieve
through cure of thick samples.
For the irradiation, a range of high performance UV lamps including microwave
and mercury arc lamps are available for curing composite formulations. Ideal
systems for on-line running are lamps supplying 400 W/inch which fit simply
across a conveyor belt designed to run at convenient curing speeds. At the recent
RadTech 2002 conference in Indianapolis, lamps of 800 W/inch were on exhibition.
The heat output from such facilities is high and, if used, the relevant lines may need
cooling. However, for the curing of some products, the ability to use both UV and
heat concurrently to speed up rates of polymerisation can be an advantage and such
high wattage lamps could be valuable for these applications. For the curing of
composites in moulds, the above line technique can be readily modified.
Resins
A wide range of polymers have been used in traditional composite processes cured
by catalytic or thermal means. Typically the polymers used in these conventional
composites are polyolefins, polyamides, polyesters, polycarbonates, epoxies,
208
acrylics and isocyanate oligomers. Theoretically these polymers can also be used in
UV cure systems however their relatively high molecular weights severely restrict
their application in these processes. For UV cure, until recently, the most frequently
utilised resin types were acrylates. These systems were developed for UV curing of
inks and coatings, however experience from these fields can be readily extrapolated
to fibre composite work. In addition, in many classifications, inks and filled
coatings are considered to be composites (1). Normally, a UV cured system
contains a resin which is an oligomer, usually highly viscous, and a lower viscosity
multifunctional monomer to speed up cure (2). Generally, the higher the proportion
of the oligomer present in the mixture, the more favourable the properties of the
resulting UV cured film. Typical acrylate oligomers and monomers used in current
UV curing are those shown in Table 1.
Table 1
Representative Modified Acrylate Oligomers and Monomers used in
UV Curea
Oligomers Monomers
Epoxies TMPTA and
derivatives
Aromatic
urethanes
TPGDA
Aliphatic
urethanes
HDDA
209
Polyesters PETA
Polyethers IBA
a TMPTA = trimethylol propane triacrylate; TPGDA = tri(propylene glycol)
diacrylate; HDDA =1,6-hexanediol diacrylate; PETA = pentaerythritol triacrylate;
IBA = isobornyl acrylate.
Oligomers are essentially low molecular weight unsaturated resins (m.w. ≈ 2,500 g
mol-1
) capable of rapidly crosslinking with a multifunctional monomer on exposure
to UV (usually 365 nm). Acrylates have been used in this work because of the rapid
response of that functional group to UV.
Modified acrylates are used to obtain specific physical properties in the cured
product. Thus epoxy acrylates are particularly fast curing, give tough films when
cured but tend to be brittle. Urethanes yield films that are slower to cure but are
generally flexible. Aromatic urethanes are faster to cure than aliphatics but suffer
from yellowing. Polyesters are generally slowest to cure of the series and form
properties between urethanes and epoxies.
Monomers used with these oligomers are also predominantly multifunctional
acrylates as shown in Table 1.
All of the resin systems discussed here cure essentially by a free radical process.
There are also systems particularly those using epoxy resins that can cure by
cationic processes. These latter systems are much more expensive than the free
radical analogues and therefore possess limited potential.
210
In addition to providing the desired physical properties, these monomers and
oligomers must possess acceptable environmental and toxicological parameters e.g.
draize values which reflect the degree of skin irritation produced by exposure to
monomers must be low. N-vinyl pyrrolidone (NVP) has been used as a comonomer
in many formulations but its odour does limit the degree to which it can be utilised.
In addition to acrylates, polyester resins dissolved in styrene have been used for
many years in UV cured composites, usually as fibreglass systems. However these
systems cure relatively slowly even in the presence of appropriate PIs. Since
styrene is carcinogenic, elimination of styrene fumes is important during
manufacture and there are questions about the effects of traces of residual styrene
uncured in the finished product. Their current use is limited.
The range of resins capable of being used in UV composites has been expanded by
the recent discoveries in curing of charge transfer (CT) complexes (3).
Table 2
Typical Donor/Acceptor Monomers with
Potential in UV Cure
Donor Acceptor
Alkyl vinyl ethers Acrylonitrile
Furan Methyl acrylate
N-vinyl pyrrolidone Maleic anhydride
211
Indole Maleimide
α-methyl styrene Dialkyl maleates
p-methoxy styrene Alkyl maleates
Stilbene Dialkyl fumarates
Thiophene Divinyl sulfone
Styrene Fumaronitrile
Vinyl acetate Methyl
methacrylate
Vinyl chloride Ethyl acrylate
In these systems donor (D) and acceptor (A) monomers can form CT complexes
capable ofbeing rapidly cured by UV of appropriate wavelength. Typical D and A
monomers are listed in Table 2, this list being representative but by no means
complete.
The speed of polymerisation of the monomers listed in Table 2 may vary
considerably depending on which complex is formed, however this problem can be
overcome by the use of appropriate additives (4). When exposed to relevant sources
of UV radiation especially from excimer sources of wavelength 308 nm, the DA
complexes polymerise readily and may also crosslink. One advantage of this
process is that for some systems, no PI is required to initiate polymerisation.
However, when used on lines fitted with mercury arc lamps, the rate of
polymerisation may be too slow for many industrial applications and under these
conditions addition of PIs may be needed to attain adequate rates of polymerisation.
212
The process of photopolymerisation involving CT complexes formed from the DA
monomers is shown in reaction 1.
D
A
D
A*
D
A
D
A
A
D
DA D A
D
A
hv
+-
n)(
Free radical copolymerisation
( )n
Cationic homopolymerisation
( )n
Anionic homopolymerisation
(1)
..
Extensive polymerisation studies have previously been performed with DA
complexes, using both catalytic and thermal initiation (5). UV studies have also
been performed with these monomers however the conditions for fast on-line
polymerisation as required in curing have only recently been determined (3).
A further advantage of the use of these CT complexes in UV curing of composites
is that they can be mixed with acrylates and similar resins to form copolymers
which possess unique properties for specific applications. Problems to be overcome
in formulating these mixtures are the degree of mutual solubility and compatibility
of different generic chemistry resin types.
Donor monomers such as the ether monomers (especially the polyfunctional vinyl
ethers) are particularly useful in UV curing. These include the divinyl ethers of
polyols having a number of hydroxyl groups including ethylene glycol, propylene
glycol, butylene glycol and similar materials.
213
A unique group of compounds that can be used as acceptors are polyfunctional
materials. These include polyunsaturated compounds containing a number of
unsaturated groups and involve particularly polyethylenically unsaturated
polyesters such as polyesters synthesised from fumaric or maleic acids and
anhydrides. The advantage of these materials is that they will readily copolymerise
with other donor monomer systems.
Bifunctional compounds containing both donor and acceptor groups can also be
used to produce CT complexes. Suitable bifunctional compounds could include
those made by condensing maleic anhydride with a vinyl ether derivative.
Photoinitiators
Photoinitiators (PI) and photosensitisers (PS) are useful additives in many curing
formulations especially if processing of the material on a mercury arc line is
intended. Initially in UV curing, both PS and PI were used, however later
developments with efficient PIs has tended to eliminate the need for the PS which
can still be used in cheaper formulations. The range of PIs include intermolecular
hydrogen abstraction PIs like benzophenone, Michlers ketone, thioxanthones;
benzoin ethers such as α,α-dimethoxy-2-phenyl acetophenone (DMPA); α,α-
diethoxy acetophenone; α-hydroxy-α,α-dialkyl acetophenones such as α-hydroxy-
α,α-dimethyl acetophenone and 1-benzoyl cyclohexanol; cyclic PIs such as cyclic
benzoic methyl esters and benzil ketals; and finally acylphosphine oxides such as
214
2,4,6-trimethyl benzylol diphenyl phosphine oxide and bis (2,6) dimethoxy
benzylol ( 2,4,4-trimethyl phenyl) phosphine.
In early studies benzoin ethers were used as PI but these materials yield two
radicals when exposed to UV, only one of which is very active (reaction 2).
The recently developed acylphosphines are much more reactive than most other
classes of PI (reaction 3) because one molecule of the acylphosphine shown leads
to four active radicals and is thus more efficient in initiating polymerisation than
the other classes.
C
O
C
OEthv
C
O
C
OEt
+. . (2)
OCH
C
O
OCH 3
3
O
C
OCH O
3
CH O3
P
(DMBAPO)
hvO
C
OCH O
3
CH O3
P
OCH
C
O
OCH 3
3
.
+
+
C
OCH O
3
CH O3
+
O
P R
(3)
. .
.
Fibres and Fillers
The presence of components like fibres and fillers in the matrix resin may effect
the UV processability of the composite. These additives may preferentially absorb
UV, reducing the rate of polymerisation. Thus inclusion of pigment dispersions
may be extremely detrimental to the rate of UV curing especially black pigmented
formulations which may require excessively large amounts of PI to achieve cure
in a reasonable period of time. Table 3 shows the variation in level of PI needed to
cure a film containing different pigments at a typical UV dose and dose rate used
with curing of inks as a model system.
215
Table 3
Typical Levels of PI required to Cure UV Inksa
Colour Conventional
Acrylate Inks
CT Resin
Inks
Black 10 3.0
Blue 10 3.0
Red 10 1.5
Yellow 10 6.0
White 4 0.3
aPI (% w/w) of ink, 200 W/inch mercury arc lamp, line running at 20 m min
-1;
levels of CT resin system upperlimit; some systems run at 0.1% PI such as white
on an excimer 400W/inch line at 20 metres min-1
; inclusion of matting agent e.g.
OK412 silica, does not markedly affect cure with concurrent graft; CT resins used
are maleate/VE derivatives.
Under particular circumstances, some pigments and fillers can enhance the
polymerisation rate of the matrix resins.A wide variety of fibres both organic and
inorganic can be used in these composites. These include cellulosics like jute,
aramids such as kevlar, carbon and fibre glass. Fillers include low priced
inorganics such as calcium carbonate, quartz and various silicates which are in the
formulation to reduce material costs of monomers and polymers as well to
improve physical and chemical properties like tensile strength, abrasion
resistance, chemical and thermal resistance, or optical, magnetic or conductive
216
properties. Because of potential problems during processing i.e, increase in
viscosity or thixotropy, the filler load is usually limited below the level desirable
for performance maximisation. A solution to this problem may be the use of
monodispersed nanoscale particles and this technique will be discussed later in
this paper.
Advantages and Disadvantages of using UV Curing
In previous sections of this paper, the methodology associated with UV to initiate
curing of composites has been discussed. It is now appropriate to outline the
overall advantages and potential limitations of using UV technology for
composite formation shown in Table 4. Several years ago a summary of the status
of UV was outlined based on industrial experience with inks over the preceeding
twenty five years. Since this evaluation, the technology has developed strongly
especially with the onset of new chemistry. A number of the limitations, have
now been surmounted. These include cost of equipment and materials, difficulty
with filled products, problems with the application of 100% solids, toxicity and
the limitation of earlier chemistries.
Table 4
Advantages and Limitations of UV
Processing in Composite Formation
Advantages Limitations
Faster production Material – relatively
217
rates costly
V.O.C. reduction Equipment – relatively
costly
Reduced process
costs
Difficulty with filled
products
Unique product
properties
Line- of- sight cure
Room temperature
cure
Difficult to apply 100%
solids
Energy reduction Toxicity – skin irritation
Space savings Adhesion problems
Lower waste and
rejects
Limited chemistry
available
Ease of use
Grafting During UV Curing of Composites
When the resin matrix is UV cured in the presence of an additive such as a fibre,
the strength and other physical properties of the finished product may be affected
by the proportion of matrix/fibre adhesion or the interfacial bonding between resin
and fibre. With organic fibres there is the possibility that surface grafting can
occur concurrently with curing leading to an enhancement in interfacial
properties. By definition grafting involves chemical bonding between resin and
fibre. Concurrent grafting is already an advantage in EB curing of composites and
218
a similar situation is now possible with UV systems, even though the latter are of
lower energy. Even with inorganic fibres like glass, UV is currently used
commercially to coat glass fibre optic strands to endow unique properties to the
finished product. Such coatings strongly adhere even though only physical forces
are involved between the glass and the polymer.
As a model to simulate the composite formed from cellulose fibres and resin,
experiments have been performed in the current work on the resin coating of
cellulose paper.The extent of grafting during UV curing of a resin matrix onto a
sheet of fibre, e.g. cellulose, may be followed by spectroscopic techniques such as
Fourier Transform Infrared Spectroscopy. The qualitative and quantitative
changes in chemical bonding can give information about both the polymer
composition and the bonds formed between the polymer and the fibre, and these
techniques will be discussed later in this paper.
Typical data for UV curing of a range of CT complexes onto cellulose are shown
in Table 5. When the structure of the vinyl ether is varied using MA as the
acceptor, it was observed that the monofunctional vinyl ether NBVE complex
does not cure as efficiently as the two other more highly functionalised vinyl
ethers. Replacement of MA by maleate analogues again leads to lower reactivity
with DEMA and BEHMA. When PMS is used as donor in place of the vinyl
ether, the reactivity of the MA complex is similar to that of the DVE-3 analogue.
219
Table 5
UV Curing and Grafting of CT
Complexes on Cellulosea
Complex PC CY GY
MA/DVE-3 3 150 145
DEMA/DVE-3 5 71 5
BEHMA/DVE-
3
4 54 22
MA/TMPTVE 3 190 140
MA/NBVE*
3 70 6
MA/PMS 3 122 97
aPC = passes to cure under Fusion F300 lamp, D bulb, line speed 16 m min
-1 at
peak UV intensity of 1.4 W/cm-2
and dose of 2.0 x 10-1
J cm-2
per/pass;
*Partially cured; CY = cure yield which is yield % by weight after curing but
before extraction to give Graft(%) (GY); MA = maleic anhydride; DVE-3 =
triethylene glycol divinyl ether; DEMA = diethyl maleate; TMPTVE =
trimethylol propane trivinyl ether; NBVE = n-butyl vinyl ether; PMS= p-
methoxy styrene; BEHMA= bis-ethyl hexyl maleate.
Importantly, in all samples reported in Table 5, concurrent grafting occurred
with cure. This implies that strong adhesion between the resin matrix and fibre
occurs during cure in these samples. Where the reactivity in cure is relatively
low, concurrent grafting is also low, particularly with the NBVE complex where
curing is incomplete.
220
When cellulose is replaced by the organic fibre, polypropylene (PP), the results
indicate that there may be a problem with creating sites on this fibre where
grafting may occur (Table 6). Under the experimental conditions used, only the
BEHMA complex demonstrates significant grafting yields. Further work (6) has
shown that by pretreatment of PP, grafting can be achieved in these reactions.
Table 6
UV Curing and Grafting of CT Complexes
onto PPEa
Complex PC CY GY
MA/DVE-3 3 42 0
DMMA/DVE-
3
4 54 0
BEHMA/DVE-
3
4 28 13
aConditions
and monomer abbreviations as in Table 5; DMMA = dimethyl
maleate.
Additives to Accelerate UV Curing and Grafting Processes
Recently a range of novel additives have been found to increase the
photopolymerisation rate in curing and grafting processes. These developments
are important since inclusion of appropriate additives can lead to lower UV doses
221
to achieve cure and industrial lines can then be run at higher speeds. Also CT
complexes which only cure at line speeds that are marginally satisfactory can, in
the presence of suitable additives, become economically viable.
When additives are used for thicker composite systems, the speed of cure is not
necessarily as critical as in the previous process. However, if lower UV doses can
be used for polymerisation, this can be valuable for composite work since, under
these conditions the process is more economical and radiation damage to the
reagents is minimised. This latter comment is particularly relevant to cellulose
curing systems, where degradation of the substrate in the presence of excess
radiation has been observed under certain specific processing conditions,
especially with EB curing.
In Table 7 the data for photocuring a representative number of MA/VE complexes
on to cellulose is reported. This data should be used as a reference for the data in
Tables 8-11 which give the results when additives are used in these reactions. It
was observed that the higher functionality vinyl ethers cure more efficiently than
the lower ones (e.g. EGBVE). Concurrent grafting also occurs irrespective of the
degree of cure, however if curing is not complete, the level of concurrent grafting
is reduced. Mechanistically these processes are depicted in reactions 4-6.
polymerRADAD hv →→→⎯→⎯+ .)( * (4)
.. HShv
SH +⎯→⎯ (5)
graftSR →+ ..
222
It is to be noted in reaction 4 that the formation of the excited state of the complex
occurs via electron donation from the donor to the excited state acceptor (7).
Table 7
Photocuring and Grafting of MA/VE
Complexes to Cellulose Without Additivesa
VE P CY GY COMMEN
T
TMPTVE 2 190 140 CC
EGBVE 3 10 4 PC
HDDVE 3 51 47 CC
DVE-3 3 89 65 CC
aEGBVE= ethylene glycol butyl vinyl ether; HDDVE = hexanediol divinyl
ether; remaining monomers, legends and conditions as in Table 5.
The results using a traditional PI, Irgacure 184 ( 1-hydroxycyclohexyl phenyl
ketone), as an additive are shown in Table 8. It was observed that inclusion of
the PI leads to enhanced rates of polymerisation when compared to the results in
Table 7, and concurrent grafting also occurs in these systems. Potential problems
with the use of PIs is their expense and the fact that unreacted PI in the cured
films may migrate through the films and lead to adverse toxicological problems.
Table 8
Photocuring and Grafting of MA/VE
Complexes to Cellulose with Irgacure 184a
223
VE P
CY GY COMMEN
T
TMPTVE 1 160 115 CC
EGBVE 2 57 10 PC
NBVE 2 47 6 PC
HDDVE 2 145 145 CC
DVE-3 1 100 90 CC
aMonomer abbreviations, legends and conditions as in Table 7.
In addition to PIs, a range of Lewis acids have recently been observed to
accelerate these curing processes as shown in Table 9 (8). The inclusion of a
typical Lewis acid (SbCl3) in the curing system leads to enhanced reactivity
similar to that shown with the PIs, however mechanistically the additives appear
to react via different pathways (reactions 7-10, where SH is the substrate and L
the Lewis acid).
.. IPhv
PI +⎯→⎯ (7)
.. SPHSHP +→+ (8)
LALA −→+ (9)
.1
])([)( * RLADhvLAD →−→⎯→⎯−+ (10)
The PI process is a simple free radical reaction whereas the role of Lewis acid is
consistent with factors that effect conventional alternating copolymerisation
systems, especially the principle that acceptor monomer properties are enhanced
224
by complexation with a Lewis acid (9). This complexation process decreases the
electron density at the vinyl group of the acceptor monomer thus increasing the
polarity difference between donor and acceptor monomers (reactions 9 and 10)
which should increase the concentration of the DA complex. Based on current
data the Lewis acids which tend to be the most active accelerants in this work
appear to follow the Pearson Hard and Soft (HSAB) principle with the present
active range being classified as borderline H and S.
Table 9
Photocuring and Grafting of MA/VE
Complexes to Cellulose with SbCl3a
VE P CY GY COMMEN
T
TMPTVE 1 220 120 CC
EGBVE 2 42 12 PC
NBVE 2 72 9 PC
HDDVE 1 71 68 CC
DVE-3 1 75 52 CC
aMonomer abbreviations and legends as in Table 8.
In earlier thermal polymerisation studies of analogous systems, Davis and co-
workers (10) proposed that the CT complex was not involved in the propagation
reactions. For UV work, Hall and Padius (11) concurred for both initiation and
propagation in the polymerisation of DA monomer complexes. In these systems,
Hall proposed that the initiating species is a zwitterion biradical which may
225
initiate ionic homopolymerisation or free radical polymerisation (reaction 1).
Changes in the magnitude of the propagation rate constant was ascribed to polar
effects on the zwitterion species in free radical reactions. In the present studies it
was observed that concurrent grafting yields may be reduced in the Lewis acid
systems when compared with the free radical process. This suggests that ionic
processes may contribute significantly to polymerisation in the presence of Lewis
acid metal salt additives.
A potential side reaction, particularly if halide salts are used, is the generation of
HCl by in situ reduction between salt and the organic compound in a process
called self activation. This process is used in metal catalysis to activate catalysts
like reduction of Pt(IV) to Pt (II) and Pt (II) to Pt (0) (8). Reactions 11 and 12
using CuCl2 as typical Lewis acid show the processes involved.
)12(222322
)11(222223222
ClRCHHClClCuRCHCuCl
RCHRCHHClClCuRCHCuCl
++→+++→+
Studies of the use of HCl as an additive (Table 10) show that this compound can
enhance rates of polymerisation.
Table 10
Photocuring and Grafting of MA/VE
Complexes to Cellulose with HCla
VE P CY GY COMMEN
T
TMPTVE 1 72 33 CC
226
EGBVE 2 29 7 PC
NBVE 2 26 8 PC
HDDVE 1 180 110 CC
DVE-3 1 185 95 CC
aMonomer abbreviations, legends and conditions as in Table 9.
Mechanistically the enhanced rates of curing and grafting may be interpreted in a
manner similar to that proposed for the Lewis acid metal salts (4). With HCl there
is an additional process which can contribute to the acceleration of the UV
polymerisation. In the presence of UV, HCl can initiate free radical
polymerisation (12) under certain experimental conditions depicted in reactions 13
and 14 where ethylene is used as typical olefin. Thermodynamically reaction (14)
is not favoured over reaction (13) which promotes the polymerisation.
)14(92.20
)13(784.108
1
322
1
222
2
2−
−
+=Δ+→+−=Δ↔=+
••••
molekJHClCHClCHHClCHClCH
molekJHCHClCHCHCHCl
In the curing of VE complexes, it has been found that, especially under mercury
arc lamp conditions, unreacted VE can remain in the cured films. A method for
overcoming this problem is to include a cationic photointiator as additive. This PI
will then remove the excess VE by cationic polymerisation, a process consistent
with the previous zwitterion intermediate discussion (reaction 1). The results in
Table 11 show that cationic PI UVI 6974 will enhance polymerisation rates,
however certain products may be discoloured with this initiator.
227
Table 11
Photocuring and Grafting of MA/VE
Complexes to Cellulose with UVI 6974a
VE P
CY GY COMMEN
T
TMPTVE 2 120 94 CCb
EGBVE 4 29 5 PC
NBVE 4 250 98 CCb
HDDVE 2 155 120 CCb
DVE-3 2 160 120 CCb
aMonomer abbreviations, legends and conditions as in Table 10;
bDiscolouration
UV Curing in Bulk Systems
Composite production may involve the curing of thick sections of monomer
polymer mixtures. When observations obtained from studies of photocuring of
films are extrapolated to bulk systems, several considerations are important. If the
curing is done on line, then speeds for achieving reaction in the bulk will usually
be slower than for the analogous film application. In addition the transmission
coefficient of light through the resin matrix becomes important and can influence
the degree to which through cure in the bulk, as distinct from surface cure, can be
achieved.
228
The use of additives typically those previously described to assist this process can
thus be beneficial. The fact that from the previous work discussed, grafting will
occur concurrently with polymerisation in the bulk is important. This grafting
process will enhance the properties of the composite by improving the interfacial
bonding between the fibre and resin matrix. In this respect the problem with
comparing the reactivities of different fibre composites in bulk UV processing is
that the fibre or filler itself may or may not be transparent to UV. The relative
reactivities of several CT resins after bulk polymerisation are shown in Table 12.
The results show the effect of vinyl ether structure on the dose to gel for each of
the complexes. The actual dose listed is approximate because of problems with
penetration of UV through a bulk resin system. Inclusion of appropriate Lewis
acid and PI additives reduced the dose to gel, consistent with their accelerating
effect in UV curing already reported in previous Tables.
Table 12
Bulk Photopolymerisation of MA/DVE-3 Complexes with Additivesa
Dose (J) to gel with additive
Control 1% SbCl3
(0.1M)
% Irgacure
819
Vinyl
ether
Dose G/NG Dose G/NG Dose G/NG
EGDV
E
21 G 16 G 2 G
DVE-3 55 G 8 G 4 G
TEGD 185 NG 170 G 22 G
229
VE
aG=gelled; NG=not gelled; lamp used was 90W medium pressure mercury with
dose rate of 1.02 x 10-2
J sec-1
; TEGDVE = tetraethylene glycol divinyl ether;
remaining monomer abbreviations as in Table 12.
Infrared Analysis of Polymer Composites
Infrared spectroscopy has many applications for qualitative and quantitative
analysis of polymers, and many comprehensive treatises exist on these techniques
(13,14). Some of these applications include identification of polymers,
copolymers and additives, measurement of kinetic parameters, and confirmation
of grafting of polymers to surfaces.
The kinetics of polymerisation reactions can be followed in real time using
sampling techniques such as Attenuated Total Reflection (ATR). One of the
advantages of ATR is the small depth of penetration into the sample (0.5 – 15 um
for ZnSe depending on the wavelength of the IR radiation, angle of incidence, and
refractive index of the sample). ATR is particularly useful for quantitative
analysis of soft samples which give good surface contact with the ATR crystals
(typically ZnSe). Decker (15) has used this technique to monitor UV curing of
vinyl ethers, and blends of vinyl ethers with epoxides, maleates and
maleimides.The disappearance of the C=C peak at 1623 cm-1 corresponded to the
rate of polymerisation of the VE double bond while polymerisation of the
comonomer was followed at 790 cm-1
for epoxides, 1415 cm-1
for acrylates or
230
maleates, and 697 cm-1
for maleimides. For the maleate-VE system, the rate of
disappearance of both monomers was independent of the initial composition, and
this was attributed to homopolymerisation of the CT complex formed by these
two monomers.
Transmission experiments for grafting of polymers onto thin films have also been
extensively investigated. The films have to be about 50 um thick or less for
sufficient IR to reach the detector. Deng (16) used transmission FTIR to monitor
UV photografting of maleic anhydride onto low density polyethylene films. The
spectra gave conclusive evidence that the anhydride was grafted onto the PE.
Preparation of potassium bromide (KBr) pellets is another very convenient
method of sample preparation especially for hard crosslinked samples which have
poor surface contact with ATR crystals. The curing and grafting reactions of the
MA/DVE-3 copolymer with cellulose are very important (20). Figure 1 shows the
spectra of KBr pellets of the cellulose used in these curing/grafting experiments, a
cured sample of MA/DVE-3/cellulose and a grafted sample where the cured
sample had been extracted with chloroform to remove unreacted monomers and
polymeric materials which had not been crosslinked. The cured MA/DVE-
3/cellulose samples were prepared by irradiating an equimolar mixture of DVE-3
and MA with 1% Irgacure 184.
231
0
10
20
30
40
50
60
Tra
nsm
ittan
ce 4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
Cellulose
Grafted
Cured
Figure 1: FTIR spectra of cellulose, grafted and cured samples of MA/DVE-
3/cellulose using Irgacure 184 as an additive.
The carbonyl/carboxyl region of the infrared spectrum was expected to be very
informative about the reactions occurring during polymerisation, especially
whether copolymerisation of MA occurred with DVE-3. It was also expected to
give information about grafting reactions of the MA to the cellulose substrate
through formation of esters with the cellulose. MA has a broad peak at 1783 cm-1
(C=O stretching frequency), and DVE-3 has a sharp peak at 1637 cm-1
(C-O
stretching frequency).
Cellulose has a peak at 1638 cm-1
(C-O stretching frequency) which unfortunately
overlaps with the C-O stretching frequency for DVE-3 at 1637 cm-1
. However,
both the cured and the grafted samples have a more intense stretch at 1638 cm-1
than the cellulose alone, and this is evidence for the presence of DVE-3 in the
cured and grafted samples. A strong stretch at 1730 cm-1
is also observed for these
samples but there are no peaks at 1863 or 1784 cm-1
which would have been
attributed to succinic anhydride repeat units of the copolymer. This wavelength
(1730 cm-1
) is characteristic of the formation of an ester which may be because
the MA/DVE-3 copolymer is chemically grafting to the cellulose with the
232
formation of ester bonds between the succinic anhydride repeat units in the
copolymer and hydroxyl groups on the cellulose. This is a very important result
because it means that the polymer is not simply a gel coat on the cellulose but is
chemically bonded to the surface.
Relevant to composite formation, these infrared results are significant since they
demonstrate that grafting can occur during composite synthesis leading to
improved properties of the product.
Emerging Applications of UV - Nanocomposites
The use of UV curing to synthesise nanocomposites with unique properties is
becoming increasingly important. At the recent RadTech USA 2002 meeting,
three papers were presented on this topic. Inorganic fillers can be used to
advantage in polymer industries and material science. The application of these
fillers in UV cured formulations maybe restricted particularly in coatings due to
problems like loss of transparency, high viscosity build-up or thixotropy and
sedimentation. Some losses in physical properties of the composites may arise
because of poor interfacial bonding between the filler and the matrix resin.
Recently developed reinforced composites which consisted of monodispersed
nanometre sized silica particles with radiation cured resins (unsaturated acrylates
and cationic curing epoxies), gave a great enhancement in scratch and abrasion
resistance, toughness and hardness. In these studies even formulations with loads
233
of silica up to 50%, water clear products with low viscosity and no sediment
formation were realised (17).
These developments make nanocomposites a very useful and versatile raw
material for many applications. Zahouily and collegues (18) reported the
fabrication of organic clay-UV curable nanocomposite materials. These
composites were formed within seconds at room temperature by UV irradiation of
chemically modified clay powder impregnated with an acrylate. These products
gave enhanced physical properties such as increased heat resistance, strength,
stiffness and barrier properties. Because of the very small particle size of the filler,
the optical properties such as transparency of these materials do not change
substantially in contrast to the optical properties of micro composites.
Nanosize silica particles were also used as fillers for acrylates (19). In order to
improve interaction between silica particles and the acrylate matrix, the surface of
the fillers was chemically modified by reaction with a polysiloxane. The acrylate
nanocomposite formulations contained up to 35% by weight of polysiloxane
covered nanosized silica. The polysiloxane shell carries methacrylate functional
groups which could be copolymerised with acrylates. After UV curing,
polyacrylate nanocomposites were obtained. These composites exhibited
markedly improved properties as compared to pure acrylate polymers e.g.
increased modulus and heat resistance, improved scratch and abrasion resistance
as well as reduced gas permeabilities. These nanocomposites prepared by UV
curing offer excellent possibilities for development of future products.
234
Industrial Significance of Current UV Curing of Composites – Banknote
Processing
The ability to cure rapidly, at room temperature, dispersions of inorganic fillers
and fibres in matrix resins with UV to give products with unique properties has
wide potential for a large range of industries. Novel resin systems recently
developed for UV inks and coatings can be used, not only to replace conventional
materials like styrene/polyester but may also provide the basis for the
development of completely new products. As previously mentioned, inks and
paints are considered to be forms of composites, particularly a clear coating which
contains silica fillers to give a matt finish (1).
A novel application of this technology, which is important in Australia, is in the
production of banknotes. In this respect Australia has a unique position in the
world since it has recently developed the only UV cured banknotes in circulation
anywhere in the world (6). These banknotes which are based on polypropylene
films utilise a final UV cured clear matt coating to achieve specific properties.
This banknote has proved to be very successful in combating forgeries and the
technology has been licensed to a number of overseas countries. The final steps in
the production of the note consist of UV curing a clear gloss coating directly over
the Intaglio printed polypropylene sheet to improve the adhesion of ink to
polypropylene. This process is followed by a UV cured matt coating to impart to
the note a paper-like feel.
235
Adhesion of UV coatings to polypropylene is normally extremely difficult, so a
prime coat of clear gloss resin, where concurrent grafting, and therefore adhesion,
is high, is first applied to the printed polypropylene, followed by a matt coating
i.e. a composite based on the same resin as in the full gloss. Interfacial bonding
between the two like resins is strong and delamination of coating from the note is
minimised. This is thus a unique application of a UV cured composite which is
essentially a lamination of a PP film containing an inorganic filler with
appreciable loading.
Conclusions
UV is shown to be a valuable new technique for curing composites containing
typical fibres or inorganic fillers. UV possesses advantages when compared with
EB, predominantly economic and simplicity. Much of the earlier UV work
developed for inks and coatings can be directly applied to the curing of
composites containing thicker resin sections. A wide range of resins can now be
used in this UV work including the recently developed donor/acceptor monomers
to complement the traditional acrylate technology. In the presence of fibre under
appropriate radiation conditions, concurrent grafting during cure can occur
leading to better interfacial bonding between resin and fibre with improved
product properties. A range of novel additives has been discovered to enhance the
rates of polymerisation in these UV processes thus lowering the radiation doses
required to achieve cure and graft. A mechanism involving the role of CT
complexes observed to explain the reactivity in curing and grafting reactions is
proposed. Detailed spectroscopic studies of these systems have also been reported.
236
Three representative examples of UV cured nanocomposites are described to
demonstrate the potential of this new emerging technology. A unique Australian
discovery in UV cured composites is discussed involving the development of the
banknote which is a first on world currency markets.
Acknowledgements
The authors thank AINSE, Ballina Pty Ltd and Fusion UV systems for support.
References
1. A .J. Berejka and C. Eberle, Rad. Phys. Chem., 63, 551 (2002).
2. G.Webster, Chemistry and Technology of UV and EB Formulation for
Coatings, Inks and Paints, Vol II, Prepolymers and Reactive Diluents,
John Wiley, Chichester, 1997.
3. S. Jönsson., J. Hulgren., P.E. Sundell, M. Shimose, J. Owens, K. Vaughn,
and C.E. Hoyle, Proc,. RadTech Asia 95, Bangkok, Thailand 1995 p283.
4. J.L. Garnett and E.F. Zilic, Proc. RadTech Europe 2001, Basle, 2001,
p233.
5. D.J.T. Hill, J.H. O’Donnell and P.W. O’Sullivan, Prog. Polym. Sci., 8, 215
(1982).
6. G.R. Dennis, J.L. Garnett and E.F. Zilic, unpublished work.
237
7. J. von Sonntag, D. Beckert, W. Knolle and R. Mehnert, Rad. Phys. Chem.,
55, 609 (1999).
8. J.L.Garnett, G.R. Dennis and E.F. Zilic, Proc. RadTech 2002, North
America, Indianapolis, USA, 1002 (2002).
9. J.M.G. Cowie, in Comprehensive Polymer Science, Vol 4, Part II, G.C.
Eastmond, A. Ledwith, S. Russo and P. Sigwalt, Eds Pergamon, England,
1989, p377.
10. M.L. Coote and T.P. Davis, Prog. Polym. Sci., 24, (9), 1217 (2000).
11. H.K. Hall. Jr and A.B. Padius, J. Polym. Sci Part A. Polym. Chem., 39,
2069 (2001).
12. C. Walling, M.S. Kharasch and F.R. Mayo, J. Am. Chem. Soc., 61, 2693
(1939).
13. D. Hummel, F. Scholl, Atlas of Polymer and Plastics Analysis, Carl
Hanser Verlag (1988)
14. M. Urban, Attenuated Total Reflectance
Spectroscopy of Polymers, Polymer Surfaces and Interfaces Series, ACS
(1996).
15. C. Decker, C. Bianchi, D. Decker, F. Morel, Progress in Organic
Coatings, 42, 253-266 (2001).
16. J.P. Deng, W.T Yang, B. Ranby, European Polymer Journal, 38, 1449-
1455 (2002).
17. C. Roscher, J. Adam, C. Eger and M. Pyrlik, Proc. RadTech 2002, North
America, Indianapolis, USA, 321 (2002).
18. K. Zahouily, C.Decker, S. Benfarki and J. Baron, Proc. RadTech 2002, North
America, Indianapolis, USA, 309 (2002).
238
19. A. Tauber, E. Hartmann, H-J Gläsel, F.Bauer, R. Mehnert, J. Monkiewicz and
R. Edelmann, Proc. RadTech 2002, North America, Indianapolis, USA, 300
(2002).
20. G.R. Dennis, J.L. Garnett and E.F. Zilic, Proc. Radcure Coatings and Inks,
Bredbury, June 2002 in press.
239
Additives for accelerating photopolymerisation processes involving
CT complexes. Applications in grafting, curing, composite and IPN
formation, also controlled release.
John L. Garnett and Elvis Zilic.
School of Science, Food and Horticulture, University of Western
Sydney, PO Box 10, Kingswood, NSW, 2747, Australia.
Abstract
The accelerating effect of Lewis acids in photoinitiator free UV grafting
and curing involving charge transfer (CT) complexes with cellulose and
polypropylene (PP) as representative substrates has been investigated. The
data have been compared with the use of photoinitiators (PIs) in these
systems, also the combined effects of PI and Lewis acid. The results are
shown to be of value in the related fields of composite and interpenetrating
polymer network (IPN) formation, also controlled release of reagents. A
mechanism for the modus operandi of the additives is proposed.
Introduction
The significance of the occurrence of concurrent grafting during radiation
curing has previously been discussed in terms of improvement in
properties of the finished product and also possible limitations to recycling
240
such materials (1,2). The concept of simultaneous grafting occurring with
cure is particularly relevant to UV initiated processes, especially the
recently developed PI free system involving CT complexes (3) where the
absence of PI can effect the ability to create grafting sites in the substrate.
In this earlier work effect of double bond molar ratio, role of solvent and
structural significance of donors and acceptors were discussed. In addition
to the importance of concurrent grafting in these curing systems, grafting
per se, as a process independent of curing and involving CT monomers,
with or without solvent, can be used to modify substrates or synthesise
new materials (4). These techniques of grafting and curing with CT
monomer complexes have been extended to the related processes of
composite and IPN formation (5) also in the synthesis of novel controlled
release processes involving work which was presented at the recent
Avignon, 12th International Meeting on Radiation processing (6).
In the above systems, the possibility of accelerating the polymerisation
process by the inclusion of additives could be of value both theoretically
and practically. In particular, with curing involving CT monomer
complexes, it is found in practice that, for many systems, rates of
polymerisation can be too slow for industrial lines especially those
equipped with the conventional mercury lamps. For grafting, composite
and IPN formation also controlled release work, there is generally no
restriction on reaction time. However if the time of irradiation can be
lowered, the process can become industrially more viable especially for
curing also minimising radiation damage to the reagents in the other
241
related processes.The use of additives to overcome the above difficulties
involving speed of polymerisation is reported in this paper. The additives
used are Lewis acids. In particular inorganic salts of antimony, iron,
copper and manganese have been chosen as representative systems from
the considerable number salts which are potentially available and have
been studied in the current project. Zinc chloride at relatively high
concentrations has previously been used to enhance CT complex formation
in styrene/acrylonitrile for photo-DSC work (7), however the
concentration of salt required for the current studies is generally
significantly lower than that used by these latter authors. The inclusion of
salt has been compared with the effect of PIs in accelerating these
polymeric reactions.
Experimental
Grafting and curing procedures were similar to those previously described
(1,4). Actinometry was performed with the uranyl nitrate oxalic acid
system and the INT Light IL’390 radiometer. Monomers, donated by ISP
and BASF, were triethylene glycol divinyl ether (DVE-3), N-
vinylpyrrolidone (NVP), ethylene glycol butyl vinyl ether (EGBVE), t-
butyl vinyl ether (TBVE), 1,4-butandiol divinyl ether (BDDVE),
hexandiol divinyl ether (HDDE), ethyl vinyl ether (EVE), tetraethylene
glycol divinyl ether (TGVE), ethyl maleimide (EMI), phenyl maleimide
(PMI), monobutyl maleate (MBMA), mono-2-ethyl hexyl maleate
(MEHMA), diallyl maleate (DAMA), dibutyl maleate (DBMA), bis-ethyl
242
hexyl maleate (BEHMA), hydroxy ethyl methacrylate (HEMA), maleic
anhydride (MA), methyl methacrylate (MMA), dimethyl maleate
(DMMA), diethyl maleate (DEMA), methyl acrylate (MAc), ethyl acrylate
(EA), acrylonitrile (ACN), acrylic acid (AA), styrene (S) and p-methoxy
styrene (PMS). In addition to whatman 41 filter paper, light weight
printing bond paper was used in curing.
Results And Discussion
UV Dose to gel CT complexes-significance in composite, IPN and
controlled release.The data in Table 1 indicate the wide spread of UV
doses required to gel the CT complexes of differing structures. The vinyl
ether complexes, especially with the maleates, generally polymerise much
faster than the other complexes in the absence of PI. Inclusion of PI
reduces the dose to gel using the MMA/DVE-3 complex as representative
system (Table 2). Likewise presence of SbCl3 accelerates the
photopolymerisation of the DMMA/DVE-3 and HEMA complexes, the
data for the last system being valuable in hydrogel synthesis for the
controlled release of drugs where lower radiation doses minimise
photodegradation of the drugs. A similar principle applies to the use of
imides in this work (6).The presence of a Lewis acid like SbCl3 can be
even more efficient than the PI, a synergistic effect of the two additives
occurring when included together. Inclusion of a substrate like cellulose
does not markedly effect this trend (Table 3), such results being of value in
243
composite, semi IPN and IPN formation using CT complexes without PI
(6). Of the systems studied in Table 3 the reactivity of MA is outstanding,
reflecting its very efficient interaction with DVE-3 leading to strong
crosslinking near the end of the reaction. This conclusion is consistent with
the data in Figure 1 where the presence of SbCl3, particularly at the higher
concentrations of monomer, accelerates the reaction. BEHMA and
MEHMA are also effective in grafting, especially with antimony whilst the
two imides studied are marginally less reactive.
Table 1 .UV Dose for CT complexes to gel – Application in IPN,
composite formation and controlled releasea.
System Dose Appearance System Dose Appearance
MA/PMS 10 Clear gel MA/DVE-3 1 Clear gel
DMMA/DVE-3
(1:1)b
52 Clear gel MA/EGBVE 1 Clear gel
DMMA/DVE-3
(2:1)b
104 Clear gel MA/TGVE 2 Yellow clear gel
ACN/S 368 Clear viscous polymer MA/BDDVE 1 Yellow clear gel
MMA/NVP 384 Clear viscous polymer MA/HDDE 1 Clear gel
DEMA/NVP 210 Clear viscous polymer MA/EVE 12 Yellow clear gel
DMMA/NVP 457 Clear viscous polymer MA/TBVE 31 Red viscous liquid
aTemp 20
oC;
bDouble bond molar ratio; dose (joules), with standard 15g
sample used in calibration; dose rate 1.02 x 10-2
J/sec.
244
Solvent effects in polymerisation and grafting.
Previous initial studies in this work have shown that dilution of the CT
complex with solvent reduces the polymerisation rate but can enhance
grafting especially within the bulk of the substrate if the solvent swells this
material (1,4). In these appropriate solvent systems, the grafting yield
virtually increases dramatically above 60-80% monomer concentration.
The data in Table 4 demonstrate the UV dose required in efficient solvents
like THF to graft acrylate/DVE-3 complexes to cellulose close to gel point
of the supernatant. If a PI is included in the monomer solution, the dose
required to gel is dramatically reduced and the corresponding grafting
yield is enhanced.
A similar result is observed for the SbCl3 additive however the effect
whilst appreciable is not as dramatic as with the PI.
Table 2. Effect of SbCl3 and PI additives on UV dose to gel of DMMA,
MMA and HEMA complexes.
System Additive Concentration Dose to gel (J) Gel
245
appearance
MMA/DVE-3 NA NA 184 No gel
MMA/DVE-3 SbCl3 1% 1M (in acetone) 129 Clear
MMA/DVE-3 SbCl3 1% 5M (in acetone) 55 Clear
MMA/DVE-3 PI 1% (w/w) 73 Yellow
MMA/DVE-3 PI + SbCl3 1% PI + 1% 5M SbCl3 37 Yellow
DMMA/DVE-3 NA NA 106 Clear
DMMA/DVE-3 SbCl3 1% 1M (in acetone) 4 Clear
HEMA/NVP NA NA 808 No gel
HEMA/NVP SbCl3 1% (w/w) 282 Yellow-
Orange
HEMA/MA/DV
E-3
NA NA 57 Yellow
HEMA/MA/DV
E-3
SbCl3 1% (w/w) 46 Yellow
aPI, Irgacure 819, bis-(2,4,6-trimethylbenzoyl)-phenyl phosphine oxide;
Temp 20oC; other conditions as in Table 1.
Table 3. Salt and PI additive effect in photografting to cellulose and
photopolymerisation of DVE-3 CT complexes involving maleate and
imide monomers, also MMA in acetone solutiona.
Additive
Acceptor monomer NA PI SbCl3
246
D G D G D G
MA 117 159 1.8 432 0.2 425
MMA 220b 5 94 15 106 34
DMMA 220b 5 9 270 82 15
EMI 220b 4 37 133 73 35
PMI 220b 18 50 98 21 29
MBMA 154b 171 3 144 83 30
BEHMA 220b 13 9 337 135 103
MEHMA 220b 67 6 112 62 233
DAMA 220b 5 12 199 154 94
DBMA 220b 16 17 49 154 215
aComplex conc (90% w/w); conditions as in Table 2; D = dose; G =
Graft(%); bno gel.
Curing of CT complexes with concurrent grafting. The ability to
observe concurrent grafting in curing with CT complexes is important
since these monomers do not readily abstract hydrogen atoms from the
surface of the appropriate substrate and create grafting sites. The results in
Table 5 show the degree to which the CT compositions listed cure on
cellulose under the UV conditions used, also the extent to which the
percentage of the original cured coating remains (i.e. grafted).
MA/DVE-3 also retains a high percentage (greater than 90%) of the
original coating as graft after solvent extraction. With respect to other
acrylates and maleates studied, the BEHMA complex with DVE-3 is the
247
most effective in curing and grafting. In this respect the results are
consistent with earlier data (Tables 1 and 3) on the reactivity of MA/DVE-
3, this CT complex being extremely effective in curing in one pass on
cellulose under optimum additive conditions, on either whatman
paper or printers light weight bond material.
Table 4. Effect of salts and PI in accelerating photografting to cellulose
and photopolymerisation of DVE-3 CT complexes involving acrylate
acceptor monomers in solventa.
Additive Acceptor
monomerb
Solven
t
Dose (J) Graft
(%)
EA THF 330 135
NA EA Aceton
e
330 6
MAc THF 330 175
EA THF 3 180
1800 EA Aceton
e
3 78
( 0.05% w/w) MAc THF 3 260
EA THF 18 65
SbCl3 EA Aceton
e
67 77
(1% w/w) MAc THF 18 200
248
a Complex 60% (w/w); PI, Irgacure 1800, 25% bis-(2,4,6-trimethyl
benzoyl)-2,4,4-trimethyl pentyl phosphine oxide + 75% 1-hydroxy
cyclohexyl phenyl ketone;
0102030405060708090
0 20 40 60 80 100 120
Monomer conc (% w/w)
% G
raft
ing
yie
ld
b Supernatant mobile liquid, all others highly viscous; other conditions as
in Table 3.
Figure 1: Photografting of DMMA/DVE-3 in chloroform to cellulose and
PP without PI; (♦) cellulose, no additives; (σ) cellulose,1% w/w SbCl3;
PP, no graft with or without additive; other conditions as in Table 3.
The two styrene complexes with ACN and AA cure slowly and graft
poorly. Inclusion of the PI and salt additives leads to dramatic changes to
some of the grafting patterns. Thus with the DVE-3 complexes of MMA,
DEMA, MAc and particularly BEHMA inclusion of PI leads to improved
concurrent grafting yields. A similar result is observed with the inclusion
of salt, but the absolute yields are lower than with the PI, demonstrating
the ability of radicals from the PI creating grafting sites in the substrate. In
the other examples the dose to cure is lower with PI than with salt however
the yields can be higher with certain salts, this result reflecting the
importance on grafting of the higher dwell time of the complex on the
substrate prior to complete cure. The best results are obtained when both
249
PI and salt are included at concentrations comparable to the PI alone in the
same monomer solution.
When PP replaces cellulose as substrate in these curing experiments the
results are significantly different (Table 6). Thus all acceptors with DVE-3
except BEHMA show virtually no concurrent grafting. Inclusion of PI
with MA alters this situation. The presence of antimony is beneficial with
BEHMA, however the best concurrent grafting yields are obtained when
both PI and antimony are included in the same monomer mixture.
Mechanism of polymerisation process. In all studies reported in this
paper, the basic concept involves the photopolymerisation of a CT
complex whether it be in film form from a fast curing step or in bulk for
the other applications. A relevant important mechanistic problem,
particularly in bulk polymerisation, is the function of the CT complex in
both initiation and propagation.
Table 5. Curing and concurrent photografting of neat CT complexes with
salts and PI on cellulose in sheet forma.
System Process Additiv
e
System Process Additive
NA PI Me PI +
Me
NA PI Me PI +
Me
MA/DVE-
3
Cure(%
)
152 (3) 155 (1) 115
(1)
165 (1) MMA Cure(%
)
57 46 77 49
(1% Graft(% 146 154 46 153 /DVE- Graft( 1 2 3 4
250
CuCl2) ) 3 %)
MA/DVE-
3
Cure(%
)
152 (3) 155 (1) 123
(6)
150 (1) ACN/S Cure(%
)
6 12 6 6
(1%
MnCl2)
Graft(%
)
146 154 89 146 Graft(
%)
2 3 2 3
MA/DVE-
3
Cure(%
)
152 (3) 155 (1) 132
(6)
- AA/S Cure(%
)
4 12 6 8
(1%
FeSO4)
Graft(%
)
146 154 65 - Graft(
%)
1 2 3 2
MA/PMS Cure(%
)
122 (3) 113 (1) 73 (1) - MAc Cure(%
)
46 97 56 110
Graft(%
)
97 20 13 - /DVE-
3
Graft(
%)
4 50 14 60
DEMA Cure(%
)
71 188 86 117 BEHM
A
Cure(%
)
54 130 112 140
/DVE-3 Graft(%
)
5 100 31 59 /DVE-
3
Graft(
%)
22 81 27 61
a PI, Irgacure 184 (1% w/w), 1-hydroxycyclohexyl phenyl ketone; Me =
salt, 1% w/w SbCl3 unless otherwise specified; % cure is yield % by
weight after curing before extraction to give Graft(%); figure in brackets,
no of passes to cure (samples exposed under an F300 lamp having a D
bulb with a line speed of 16 m/min at a peak UV intensity of 1.4 watt/cm2
and a dose of 0.20 J/cm2 per pass); some salts were only marginally
251
soluble and appropriate solvent to solubilise the salt was used in small
amounts. Other conditions as in Table 4.
Table 6. Curing and concurrent photografting of neat CT complexes with
salts and PI on PP filma.
aConditions as in Table 5.
System Process Additive System Process Additive
NA PI Sb Sb + PI NA PI Sb Sb + PI
MA Cure(%) 42 93 26 8 MMA Cure(%
)
4 1 12 30
Graft(%
)
0 73 0 0 Graft(
%)
1 0 0 9
DMM
A
Cure(%) 54 147 6 20 BEHM
A
Cure(%
)
28 42 83 98
Graft(%
)
0 1 0 11 Graft(
%)
13 8 18 22
252
In thermal work Davis and coworkers concluded that the CT complex was
not involved in propagation (8). Hall and Padias concur for both initiation
and propagation (9). In these systems, Hall proposes that the initiating
species is a zwitterion biradical which may initiate either ionic
homopolymerisation or free radical copolymerisation. Propagation is
ascribed to polar effects in free radical reactions. For the current work with
PIs as additives, radicals from the initiation obviously contribute to
polymerisation and enhance the original radical concentrations (R.),
reactions (1) – (5), for curing and grafting (4) where SH is the substrate
and S. the grafting site.
(5)SPHSHP
(4)IPhv
PI
(3)graftSR
(2)HShvSH
(1)polymerR*
A)(Dhv
AD
••
•+
•
••
••
•
+→+
→++
→→−+
⎯→⎯
⎯→⎯⎯→⎯
The inclusion of certain Lewis acids in appropriate CT monomer solutions
enhances the rate which is an advantage in both curing and bulk processes
like grafting, composite and IPN formation also the synthesis of controlled
release polymer systems (4-6). At this time, based on the current data
(typically Table 5 with antimony the most reactive of metals listed), Lewis
acids which tend to be the most active accelerants broadly follow the
Pearson Hard and Soft (HSAB) principle with the present active range
being classified as borderline between H and S. The role of Lewis acid in
these reactions would thus be consistent with other alternating
253
copolymerisation studies where acceptor monomer properties are
enhanced by complexation with a Lewis acid. This leads to further
reduction in electron density of the vinyl group thus increasing the polarity
between D and A monomers as shown in reactions (6) and (7) where L is
the Lewis acid and R1., indicates radical formation via the modified
pathway.
ss
active non polar PP do not occur unless the substrate is pretreated (4).
onclusion
(7)1RL)](A[DhvL)(AD →−−−+ ⎯→⎯In the present work, the interpretation of these effects is complicated by
two factors (i) in certain of the reactions studied, the presence of solvent
which may also form complexes with D and A monomers and (ii) certain
donors possess polar functional groups which will also complex with the
Lewis acids, ethers, themselves, being vulnerable to cationic
polymerisation. Mechanistically such processes need to be separated from
the DA reaction, however, in practical terms, if such processes assist the
fast polymerisation concurrently, they will be of value commercially.
Finally in grafting, salts can complex with OH groups in the cellulose
creating more sites for grafting, whereas analogous reactions with le
(6)LALA
•∗−→+
re
C
Inclusion of certain Lewis acids in the CT monomer systems studied
increases the photopolymerisation rates of these complexes as does the
addition of PIs. A synergistic effect in certain systems is observed when
254
the two type additives are included in the same solution. These results are
of value in bulk polymerisation of CT complexes such as in composite,
IPN and semi IPN formation also controlled release of reagents. A similar
accelerating effect is observed with the addition of these additives in
certain curing processes on cellulose and PP film, concurrent grafting
being found under certain experimental conditions. Mechanistically the
current results are consistent with previous literature theories in the
photopolymerisation of CT complexes where the participation of
zwitterion biradical intermediates have been proposed leading to ionic or
free radical polymerisation, the latter being favoured in the present PI
system.
rs thank AINSE, Ballina Pty Ltd and Fusion UV systems for
g, Lewis acids, CT complexes, Composites, IPNs,
Controlled release.
. Fouassier, J.P. and Rabek, J. F.Eds,
C.E., Proc.
RadTech Europe ’97, Academic Days, Lyon, 1997, p.169.
Acknowledgements
The autho
support.
Keywords
Curing, Photograftin
References
1. Dworjanyn, P.A. and Garnett, J.L., In Radiation Curing In Polymer
Science and Technology- Vol 1
Elsevier, London, 1993, p.263.
2. Decker, C., Morel, F., Jönsson, S., Clark, S.C. and Hoyle,
255
3. Jönsson, S., Hasselgren, C., Ericsson, J.S., Johanson, M., Clark, S.,
Miller, C. and Hoyle, C.E., Proc. RadTech, North America ’98,
Chicago, 1998, p.189.
4. Garnett, J.L., Ng, L-T., Viengkhou, V., Hennessey, I. And Zilic, E.,
Proc. RadTech 2000, North America, Baltimore, USA, 2000, p804.
5. Ng, L-T., Garnett, J.L., Zilic, E. and Nguyen, D., Rad. Phys. Chem., in
press.
6. Garnett, J.L., Ng, L-T., Nguyen, D., Swami, S. and Zilic, E.F., Proc
12th
International Meeting Radiation Proceedings, Avignon, France,
March 2001, Rad. Phys. Chem., in press.
7. Cole, M.C., Bachemin, M., Nguyen, C.K., Viswanathan, K., Hoyle, C.,
Jönsson, S. and Hall, H.K.Jr., Proc. RadTech 2000, North America,
Baltimore, USA, 2000, p211.
8. Coote, M,L. and Davis, T.P., Prog. Polym. Sci., 24 (9), 1217 (2000).
9. Hall, H.K.Jr and Padias, A.B., J.Polym.Sci Part A, Polym. Chem., 39,
2069 (2001).
256
Curing and Cure Grafting Using Novel UV Lamps - Comparison with EB
systems
G. R. Dennis, J. L. Garnett, G. McKean and E. F. Zilic.
University of Western Sydney, Parramatta, School of Food, Science and
Horticulture.
ABSTRACT
The use of a novel UV lamp, the Con-Trol-Cure UV-LED line 100 for
curing and cure grafting work is reported. The lamp has a cut off for all
wavelengths < 385nm. Body exposure to these lamps is therefore safer
than with conventional mercury arc facilities. Curing and cure grafting
processes with and without photoinitiator using the lamps are discussed
using polymer systems conventional oligomer/monomer acrylates and
vinyl ethers, charge transfer monomer complexes and thiol-enes. Cellulose
and polypropylene are used as substrates. The performance of the lamp is
compared with Fusion equipment and EB sources. The possible potential
of the lamp in new commercial applications is discussed.
257
1.0. INTRODUCTION
Since the commercialisation of UV curing processes over thirty years ago,
the predominant emphasis of the field has been on systems which use
lamps that are shielded down to the line. The reason for this is that these
UV systems utilise a wide variety of peaks in the UV spectrum to initiate
polymerisation with emphasis on the 365nm region. This area is dangerous
to body exposure, particularly the eyes, hence the necessity to screen the
sources down to the line.
Because of this restriction, the largest area of application until recently of
these UV systems has been predominantly in inks, clear coatings and
adhesives for paper and plastics applications in the graphic arts field. In
addition this limitation in application of the technology has been further
exacerbated by the fact that pigmented systems, especially heavily
pigmented formulations such as are found in paints, could not be readily
cured on these lines because of poor UV penetration of the films. The
recent development of new photoinitiators (PIs) has now essentially
overcome these problems and large volume use of UV curing in the paint
industry is being predicted.
The other problem with commercially curing paints, when compared with
inks and clear coatings on these UV lines using conveyor belt systems with
shielded lamps, is the geometric shape of many objects. Recently lamps
have been developed for these applications, especially compact sources
with filtered UV light containing no peaks in the spectrum below 385nm
258
which is the region of most concern for body exposure. These lamps do
not cure as efficiently as their equivalents at 365nm, however they are
designed for systems where cure times for up to several minutes are
satisfactory for the purpose. These modified lamps are thus available for
new areas where there is large potential for application of this UV
technology.
In the present manuscript, details of curing and cure grafting experiments
performed with one of these lamps will be described. The results will be
compared with the UV curing of the same monomer/oligomer
formulations performed with conventional 300W/inch Fusion lamps. For
comparison purposes some experiments will also be performed using a low
energy EB source. The model monomer/oligomer systems used will be
conventional acrylates as well as charge transfer (CT) monomer
complexes including the thiol-enes. The thiol-enes are of particular
importance since they have recently been rediscovered for UV curing
purposes (Charles Hoyle and ourselves, ref). They have also been shown
to be valuable in EB curing systems (our work, ref). Typical substrates
used in these radiation curing and cure grafting experiments are Whatman
41 filter paper representing pure cellulose, acrylic coated paper used as
label stock in printing applications for the graphic arts field and
polypropylene (PP) film typical of the plastic field. Both curing and cure
grafting experiments will be performed, the latter results being particularly
significant since very little previous radiation grafting work has been
reported with the above CT monomer systems (our reference). This is also
259
the first report of UV cure grafting of any of the monomer/oligomer
systems using this new type of UV lamp termed “The Con-Trol-Cure UV-
LED linear 100” which filters all UV below 385nm and is supplied by UV
Process Supply Chicago.
2.0. EXPERIMENTAL
Because a novel UV lamp with unique physical properties has been used in
these studies, certain experimental procedures previously reported (our
reference) required modification. Thus calibration of the lamp was
performed using chemical actinometry with uranyl nitrate. Several basic
assumptions were made during this calibration, firstly the change in
wavelength of UV from 365 to 385nm did not markedly effect the
chemical reaction of the uranyl ion and secondly the Ø value of the
process was similarly wavelength independent over the range of UV used.
The results from this new lamp were compared with curing of the same
monomer/oligomers on a conventional UV line namely Fusion F300 with
H and D bulbs operating at a line speed 16 m min-1 with a peak UV
intensity of 1.4 W/cm2 and dose 0.20 J/cm. This latter lamp are calibrated
with an INT light IL’390 radiometer. For the EB work two machines were
used, namely Nissan 500kv and ESI 175kv facilities. Monomers and
oligomers were supplied by, or purchased from, Aldrich, BASF,
Monocure, Morflex and Ballina and included maleic anhydride (MA),
triethylene glycol divinyl ether (DVE-3), trimethylol propane triacrylate
(TMPTA), hexanediol diacrylate (HDDA), tripropylene glycol diacrylate
(TPGDA), epoxy acrylate (EPA), aromatic urethane acrylate (UA),
260
aliphatic urethane acrylate (AUA), maleimide (MI), unsaturated polyester
(PE), trimethylol propane tris(3-mercaptopropionate) (TTP), vinyl ether
terminated aromatic ester oligomer (VE 1312) and tris(4-vinyloxybutyl)
trimellitate (VE 5015). Photoinitiators (PIs), donated by Ciba-Geigy were
2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 (Irgacure
369) and 2-isopropylthioxanthone (ITX), 1-hydroxy-cyclohexyl-phenyl-
ketone (Irgacure 184) and bis 92,4,6-trimethylbenzoyl)-
phenylphosphineoxide (Irgacure 819). Wetting agent DC31 was supplied
by Dow.
3.0. RESULTS
3.1. Curing and Cure Grafting on Pure Cellulose
The data in Table 1 refer to curing and cure grafting of conventional
oligomer/monomer mixtures which have been used for many years in UV
work. The formulations consist predominantly of acrylates with some
experiments performed with vinyl ether capped oligomer. The substrate
used was Whatman 41 filter paper which is a relatively pure cellulose and
very porous. In these experiments time required to cure is reported since
these times are much longer than in conventional UV where the UV dose
is used. Time of exposure is a more meaningful concept for the types of
applications used with these new lamps. Both acrylate oligomers, namely
the aromatic urethane and epoxy, when included with a multifunctional
261
acrylate did not cure within reasonable times without the inclusion of PI.
(The curing time was effectively 60sec in these examples). The two PIs
used absorb in the region corresponding to the output of the lamp, IR 369
generally being more efficient than the ITX material. Of the
multifunctional acrylates used as diluents, both di and trifunctional
monomers were equally reactive. Cure grafting yields in the presence of IR
369 were also relatively high reflecting the effect of porosity of the
cellulose on the absorption properties of the monomers in the formulation.
When the acrylate system was replaced with a vinyl ether equivalent, using
a mixture of a vinyl ether capped oligomer dissolved in a difunctional
vinyl ether monomer, the results were similar to the acrylates except for
cure grafting where yields in the vinyl ether systems were much lower.
Table 1
Curing and Cure Grafting of Monomer Oligomer Mixtures onto Whatman
41 Cellulose with Con-Trol-Cure UV Lamp
System
Oligomer Monomer
PI Cure Cure
Graft
Cure time
UA TPGDA - 160 109 Post Cure 24hr
UA TPGDA IRG 369 187 180 Cure 60 sec
UA TPGDA ITX 209 165 Tacky 5min, post cure
24hrs
UA TMPTA - 176 108 Just cured 24hrs
UA TMPTA IRG 369 179 168 Almost cured 5min
262
UA TMPTA ITX 222 180 Almost cured 5min
EPA TPGDA - 175 108 Cured 24hrs
EPA TPGDA IRG 369 255 238 Cured 5min
EPA TPGDA ITX 196 153 Just cured 5min
EPA TMPTA - 184 108 Cured 24hrs
EPA TMPTA IRG 369 182 174 Cured 2min
EPA TMPTA ITX 220 177 Just cured 5min
VE 1312 DVE-3 - 178 109 Just cured 5min
VE 1312 DVE-3 IRG 369 180 107 Just cured 5min
VE 1312 DVE-3 ITX 171 110 Just cured 5min
When the acrylates are replaced with CT complexes, firstly the simple
MA/DVE-3 mixture, curing is not efficient without PI and, even then, cure
times of up to 5min are needed (Table 2). Cure grafting yields with this
complex are also not high. In contrast the thiol acrylate (TTP/HDDA)
system, especially at 1:5 molar ratios, cures faster even without PI, the
2.5min needed being marginally satisfactory for many of the new potential
commercial applications being considered. Inclusion of PI accelerates the
cure and also improves cure grafting yields.
Table 2
Curing and Cure Grafting of CT Complexes onto Whatman 41 Cellulose
with Con-Trol-Cure UV Lamp
CT Complex PI Cure Cure Graft Observation
263
Donor Acceptor
DVE-3 MA - 160 116 Post Cure 24hrs
DVE-3 MA IRG 369 144 106 Cured 5min
DVE-3 MA ITX 212 181 Cured 5min
HDDA (1) TTP (1) - 211 135 Cured 3.5min
HDDA (1) TTP (1) IRG 369 224 211 Cured 60 sec
HDDA (1) TTP (1) ITX 216 186 Cured 2.5min
HDDA (5) TTP (1) - 174 103 Cured 2.5min
HDDA (5) TTP (1) IRG 369 230 225 Cured 1.0min
HDDA (5) TTP (1) ITX 218 212 Cured 1.0min
When, for comparison, the new Con-Trol-Cure lamp is replaced with the
conventional UV source, the monomer/oligomer systems and the thiol
acrylate hybrid monomer complexes are cured on cellulose in much
shorter times as expected (Table 3). Concurrent cure grafting is also
observed, being particularly lowwith the vinyl ether system, consistent
with the data in the table for the other UV source.
Table 3
Curing and Cure Grafting of Monomer/Oligomer Mixtures and CT
Complexes with Irgacure 369 and ITX using Fusion F300 Lamp (on
Cellulose)
SYSTEM PI CURE CURE OBSERVATION
264
(%) GRAFTING (%)
- 11 10 3 passes
IRG 369 60 25 2 passes
UA/TPGDA
ITX 18 10 2 passes
- 92 10 3 passes
IRG 369 69 13 1 pass
VE1312/DVE-3
(6:4) ITX 72 14 1 pass
- 120 100 3 passes
IRG 369 180 160 1 pass
HDDA/TTP
(1:1) ITX 130 41 1 pass
Replacement of the two PIs already used, namely Irgacure 369 and ITX,
with the more recently developed Irgacure 819 and 184 using the Con-
Trol-Cure lamp leads generally to faster polymerisation rates in
monomer/oligomer systems studied in Table 4 with the exception of the
vinyl ethers where rates are comparable with both PI groups. Even though
curing is much faster with the PIs used in Table 4, concurrent cure grafting
is also very efficient for most systems particularly those using Irgacure
819.
Table 4
Curing and Cure Grafting of Monomer/Oligomer Mixtures and CT
Complexes with Irgacure 819 and Irgacure 184 using Con-Trol-Cure UV
Lamp on Cellulose (W41)
SYSTEM PI CURE CURE OBSERVATION
265
GRAFT
IRG 819 175 170 CURE 90min EPA/TMPTA
184 200 150 2min
IRG 819 165 105 CURE 5min VE1312/DVE-3
184 160 100 p.cure 24hr
MA/DVE-3 184 185 125 60secs
819 230 220 60secs HDDA/TTP (1:1)
184 240 220 60secs
819 225 225 60secs HDDA/TTP (5:1)
184 200 180 60secs
3.2. Curing and Cure Grafting on Label Stock Cellulose
The data in the previous tables utilise a relatively pure cellulose as
substrate. This Whatman 41 paper is porous and, whilst suitable for some
applications, is unsatisfactory for others particularly in the graphic arts
area such as printing and coating on labels and the like.
For this latter purpose, acrylic precoated label stock is the substrate used,
this curing and cure grafting with this surface coated cellulose is important
commercially. The surface tension properties of this stock were also
different and necessitated the use of a wetting agent (DC 31) to achieve
uniform surface coating.
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The data in Table 5 demonstrate the effect of curing and cure grafting with
the Con-Trol-Cure UV lamp using the label stock cellulose with
conventional aromatic urethane and epoxy acrylate oligomers when
dissolved in TMPTA. The reference stock paper had previously been pre-
extracted in solvent to confirm that the preformed coating was chemically
bonded to the cellulose.
Curing of the acrylate mixtures is efficient on the substrate only if PI is
present, consistent with the data in Tables 1 and 2 where Whatman 41
paper was used. Cure grafting data in Table 5 however are significantly
lower than observed in Tables 1 and 2, this result reflecting the effect of
the difficulty experienced in chemical bonding of the acrylate to the
preformed coating on the paper. In contrast, the data for the vinyl ether
capped oligomer/monomer mixture in cure grafting is higher than for the
acrylates indicating the advantageous effect of the presence of vinyl ether
on the reactivity with the performed coating on the label stock.
Table 5
Curing and Cure Grafting of Monomer/Oligomer Mixtures onto Label
Stock Cellulose with Con-Trol-Cure UV Lamp
267
System PI Cure Cure
Graft
Observation RUN
Oligomer Monomer
1 UA TMPTA - - - No cure 5min
2 UA TMPTA IRG 369 181 113 Cure 60sec
3 UA TMPTA ITX 160 104 Cure 60sec
4 EPA TMPTA - - - No cure 5min
5 EPA TMPTA IRG 369 160 103 Cure 60 sec
6 EPA TMPTA ITX 170 106
7 VE 1312 VE 5015 - - - No cure
8 VE 1312 VE 5015 IRG 369 115 106
9 VE 1312 VE 5015 ITX 114 110
DC 31 (0.05g in all)
When the conventional oligomer/monomer mixtures are replaced by CT
monomer complexes (Table 6), curing with the Con-Trol-Cure lamp again
occurs efficiently only if PI is present. All CT complexes studied post cure
within 24hrs of removal from the lamp. Of the two PIs used, ITX is
marginally better than IR 369 for the CT complexes.
Table 6
Curing and Cure Grafting of CT Complexes on Label Stock Cellulose
Con-Trol-Cure UV Lamp
CT Complex PI Cure Cure Observation
268
Donor Acceptor Graft
DVE-3 MA - 117 105 Post Cure 24hrs
DVE-3 MA IRG 369 116 109 Tacky 5min
DVE-3 MA ITX 129 129 Cured 5min
HDDA (1) TTP (1) - 107 101 Post cure 24hrs
HDDA TTP IRG 369 158 106 Cure 90sec
HDDA TTP ITX 144 106 Cure 60sec
HDDA (5) TTP (1) - 109 104 Post Cure 24hrs
HDDA TTP IRG 369 122 102 Slight tack 5min
HDDA TTP ITX 156 101 Cured 2min
DC 31 (0.05g in all)
With respect to cure grafting of these systems, the MA/DVE-3 complex is
efficient in the presence of PI, consistent with the results in Table 5, (runs
8 and 9), where inclusion of vinyl ethers led to significant cure grafting
yields. Cure grafting of the hybrid acrylate CT complex (TTP/HDDA) was
not as efficient as with the DVE-3 complexes, especially at 1:1 monomer
ratios.
3.3. Cure and Cure Grafting on PP
When a synthetic substrate like PP replaced cellulose, surface tension
problems were experienced in application, even more difficult than those
enountered with the label coated stock. All runs with PP required inclusion
269
of wetting agent DC 31. As expected, curing of both oligomer/monomer
acrylate and CT complexes which the Con-Trol-Cure lamp was readily
achieved on this PP, however extreme difficulty was encountered with the
cure grafting process, most samples tending to delaminate after curing, a
property that is well known in UV curing on PP. However cure grafting
was achieved with two samples (Table 7), the EPA/TMPTA acrylate
system and the MA/DVE-3 CT complex, however the former coating
tended to be brittle and hard whereas the latter was more flexible.
Table 7
Curing and Cure Grafting of Monomer/Oligomer Mixtures and CT
Complexes on Polypropylene Film with the Con-Trol-Cure Lamp.
System PI Cure Cure
Graft
Observation
EPA/TMPTA - - - No cure
EPA/TMPTA IRG 369 144 112 Cure 90sec
EPA/TMPTA ITX 128 - Cure 60sec
MA/DVE-3 - - - No cure
MA/DVE-3 IRG 369 - - Very tacky 5min – no cure
MA/DVE-3 ITX 136 111 Cure 5min
DC 31 (0.05g in all); Other systems examined and cured but demonstrated
very poor cure grafting on PP film were as follows:- aromatic
UA/TMPTA; TTP/HDDA (1:1), TTP/HDDA (1:5) and VE 1312/ VE
5015.
270
When the more powerful Fusion lamp replaces the Con-Trol-Cure source
in these current experiments, much faster rates of polymerisation are again
experienced as expected (Table 8). Concurrent cure grafting yields are also
not high especially with the aliphatic urethane acrylate mixture which is
used as model for all other acrylate oligomers, this result being consistent
with the data in Table 7. Similar observations can be made when Irgacure
819 replaces Irgacure 369 and ITX in these reactions using the Con-Trol-
Cure source.
Table 8
Curing and Cure grafting of Monomer/Oligomer Derivatives and CT
Complexes with Irgacure 369 and ITX on Polypropylene using Fusion
Lampa
SYSTEM PI CURE
(%)
CURE GRAFTING
(%)
OBSERVATION
- 13 0 4 passes, no cure
271
IRG 369 29 3 3 passes AUA/TPGDA
ITX 14 3 3 passes
- 34 19 3 passes
IRG 369 12 8 2 passes
HDDA/TTP (1:1)
ITX 15 10 2 passes
- 22 7 5 passes, no cure
IRG 369 26 11 3 passes
HDDA/TTP (5:1)
ITX 15 10 3 passes
aaromatic urethane acrylate (UA) with TPGDA, also VE1312/DVE-3 cured
poorly and yielded very low cure grafting with these PIs.
Table 9
Curing and Cure Grafting of Monomer/Oligomer Mixtures and CT
Complexes with Irgacure 819 and Irgacure 184 on Polypropylene using
Con-Trol-Cure UV Lampa
SYSTEM PI CURE (%) CURE GRAFT
(%)
OBSERVATION
EPA/TMPTA IRG 819 140 110 CURE 5min
HDDA/TTP(1:1) IRG 819 130 105 CURE 5min
aDC31 (0.5%) added. The other systems examined and cured but showed
very poor grafting was HDDA/TTP (5:1). 184 with EPA/TMPTA,
MA/DVE-3,
HDDA/TTP (1:1) and (1:5), VE1312/DVE-3-3 did not cure.
272
3.4. Cure and Cure Grafting on Cellulose using EB
The results in Table 10 show that the type of CT complexes reported in the
earlier tables can be very efficiently cured with EB without the need to
incorporate catalysts such as PIs consistent with earlier preliminary reports
with other CT monomer complexes. Cure grafting data from these runs is
also relatively high, especially for the MA/DVE-3/PE system. PE was used
in this run to (i) increase viscosity of the MA/DVE-3 resin which is
normally low and (ii) to reduce penetration into the cellulose which is a
strong medium paper used in the graphic arts area. EB is essentially a
surface curing technique and the cellulose chosen was ideal for the
concurrent curing and cure grafting experiments. The data also show that
other acceptors besides MA, like the maleimide MI, are also efficient in
these curing and cure grafting procedures.
Table 10
EB Curing of CT Complexes on Cellulosea
System Cure Cure Graft Observation
MI/DVE-3 (1:1) 26 18 Complete cure
MA/DVE-3/PE (3:4:4) 37 36 Partial cure
TTI/HDDA (1:1) 36 27 Complete cure
273
TTI/HDDA (1:5) 32 24 Partial cure
aAll runs performed at 2.8 Mrad N2. Cellulose used was media paper.
4.0. DISCUSSION
4.1. Curing with con lamp
The data in the tables show that the Con-Trol-Cure lamp can cure a wide
range of monomers and oligomers typically those in Tables 1 and 2 after
exposure for 60 seconds. Within this time frame there are now a number of
potential commercial applications which may be exploited by taking
advantage of the safety of the lamp with its cut-off at 385nm (check).
Certain combinations such as the thiol acrylate hybrid (TTP/HDDA) can
cure within several minutes without the need to use PI which is
advantageous from economic and safety aspects. Inclusion of PIs
accelerates the curing process due to the more efficient absorption of the
incident UV and the corresponding increase in the number of initiating
radicals. Irgacure 819 and 184 are the most efficient PIs used.
Certain of the monomers studied, when complexed, are capable of self-
initiating the polymerisation process themselves. Thus CT monomer
complexes like MI/DVE-3 and MA/DVE-3 can cure rapidly upon
exposure to UV of appropriate wavelength. The MA/DVE-3 system is
typical of the donor/acceptor (DA) complexes which have previously been
274
used in UV curing work over the past nine years (Hall and others) with
lamps similar to the high intensity Fusion used in these current studies for
comparison purposes. These DA complex have not, to our knowledge
previously been used to cure with the Con-Trol-Cure lamp. From the
previous extreme work by a number of authors the following mechanism
has been proposed to explain the UV curing process developed with the
high intensity Fusion type lamp. This mechanism outlines the manner by
which radicals are initially formed and subsequently lead to
polymerisation. It is logical to assume that this mechanism developed for
curing with the higher intensity lamps would also be applicable to the
current Con-Trol-Cure system.
The initial effect of UV on the DA complex is depicted in reaction 4,
which involves the interaction between a ground state donor with an
acceptor in the excited state.
D
A
D
A*
radiation (4)
Reaction 5 shows the subsequent formation of a zwitterion biradical which
leads to free radical polymerisation to form an alternating copolymer
(reaction Y will be top part of reaction four in the e/5). Inclusion of PI
leads to high radical concentrations.
275
D
A*
D
A
.
D
A
+
-
A
D
D
A D A
n)(
Free radical copolymerisation
D
( )n
Cationic homopolymerisation
A
( )n
Anionic homopolymerisation
(5)
.
4.2. Thiol-Ene Processes
The above CT mechanism is theoretically applicable to the radiation
curing of thiol-enes which have recently been successively reinvestigated
after a lapse of twenty years (Hoyle). In the earlier thiol-ene UV curing
work the possible mechanistic role of thiol-ene CT complexes in the
photopolymerisation process was considered (ref 5 e/5). Thiol-enes are
known to form CT complexes which have been characterised by
spectroscopic techniques. With respect to the current radiation curing work
the problem to be resolved is the degree to which, if any, these complexes
participate in the initiation process of the radiation polymerisation. Earlier
studies (e/5 paper) have shown there are differences in reactivity between
276
thiol-ene systems and the corresponding DVE-3 CT complexes. In
addition, recent detailed work by other authors using UV initiation have
confirmed the difficulty of interpreting the curing of thiol-enes in terms of
the participation of the CT complex, in particular the degree to which the
complex may be involved in the initiation step. Specifically the problem is
that thiyl radicals (reaction 7, see below) are only produced slowly from
the complex whereas in UV processes, a large flux of additional radicals
are produced from photolysis. Detail kinetic studies thus need to formed
with the thiol-ene system before this aspect of the mechanism can be
resolved.
With respect to the UV curing of thiol-enes it is generally accepted that the
basic reaction occurs by a step growth addition mechanism that is
propagated by a free radical chain process involved in the addition of a
thiol group across an ene double bond as shown in reactions 7 to 9.
)9('22
'2
)8('2
'2
)7()(
....
.
RSRCHCRSHRSHHRCCRSH
HRCCRSHCHRCHRS
PRODUCTSRSrelevantifPIRSH
+−→+−−→=++→+
4.3. Effect of type of Cellulose
277
The structure of the cellulose used is particularly important in these
experiments because porosity can effect the uniformity of the coatings
applied. This is due in part to the viscosity of the coatings and can vary
dramatically. Thus the viscosity of monomer/oligomer mixtures typically
EPA/TMPTA, in Table 4 is usually high enough to enable the coating of
porous papers like Whatman 41 filter paper without being strongly
absorbed into the substrate. In contrast the viscosity of the CT complexes
is quite low and can be strongly absorbed during the coating application.
For commercial purposes, such as in the graphic arts field, acrylic coated
paper stock is used to overcome these problems and can be coated by any
combination of monomer and/or oligomer to give a uniform surface film.
The surface of these papers thus tends to resemble the structure of plastics
like the polyolefins which have surface tension problems during UV
curing. In order to coat the label stock efficiently a wetting agent was
needed. This enabled uniform coating with efficient curing to occur on the
acrylic coating of the paper by mechanisms analogous to those discussed
above (Tables 5 and 6).
4.4. UV Curing on PP
Surface tension problems have always been experienced in UV curing of
coatings to PP. As with the coated label stock previously discussed it was
necessary to incorporate a wetting agent to enable a uniform finish to be
applied to the PP studied. The inclusion of this wetting agent tended to
retard the curing of the coating more so on the PP then with the previous
278
precoated stock, where the acrylic polymer coating was more polar than
PP. As a consequence, it was difficult to cure a number of coatings within
5 minutes using the Con-Trol-Cure lamp even in the presence of PI
(Tables 7 and 9). The performance of some of the coatings was better than
the larger fusion lamp, this result emphasising the value of the intensity of
the radiation when curing films containing retarders.
4.5. EB Curing
The data in Table 10 show that the thiol acrylate hybrids and DVE-3
complexes exhibit comparable reactivity in EB curing. These results would
suggest that the mechanism of EB curing of the two systems is similar to
that already proposed for UV. However because EB sources use ionising
radiation they are more energetic than UV thus additional processes to
those described in UV can be applied to EB. In EB curing, radicals, cations
and anions are formed with the first process predominating (reactions 10 –
12) ionic reactions can thus contribute to overall EB process, however, in
practice free radical reactions tend to predominate under these processing
conditions.
)12(
)11(
)10(
..
..
−+
⎯→⎯++⎯→⎯
+⎯→⎯
MHeMH
eMHMH
HMMH
hv
hv
hv
4.6. Cure Grafting
Curing and cure grafting occur concurrently during the irradiation of
typical oligomer/monomer systems (Table 1) and CT monomer complexes
279
(Table 2) used in the current experiments. The two processes are depicted
in reactions 1 and 2 where O/M denotes the oligomer/monomer system.
+ O/M
+ O/M
grafting
curing
(1)
(2)UV
UV
The difference between grafting and curing is the nature of the bonding
occurring in each process. In grafting, covalent carbon-carbon bonds are
formed whereas in curing bonding usually involves weaker Van der Waals
or London dispersion forces. The process depicted in reaction 2 has been
termed cure grafting to differentiate it from previous conventional methods
used for grafting synthesis namely the preirradiation and simultaneous
techniques (ref our work , ref 7 e/5). Cure grafting has been shown to be a
useful technique for synthesising novel copolymers possessing unique
properties not capable of being achieved by either of the standard
preirradiation and simultaneous methods. The scope of the process has
been outlined in detail elsewhere (ref 7 e/5). Overall the occurrence of
concurrent cure grafting can be an advantage in curing in that it improves
the bonding between the coating and substrate, thus minimising
delamination. Cure grafting can also lead to problems with recycling of
papers coated by the radiation curing process. Before processing, it is thus
280
convenient to analyse the end use of the product being cured and to
determine whether ultimate recycling is required. Thus decision may then
determine the chemistry of the radiation curing mixture to be used in the
coating process.
The concept of cure grafting is particularly relevant to the present work
since the irradiation exposure times are longer with the current Con-Trol-
Cure lamp than with the Fusion system. The residual time of the coating
on the substrate is thus longer with the former lamp and leads to the
opportunity of improved concurrent grafting during cure because of
appropriate low molecular weight components in the film formulation may
wet the substrate more efficiently and thus be absorbed more readily into
the substrate where grafting may occur. Thus the curing properties of the
Con-Trol-Cure lamp are most important in discussing the current
experiments, particularly the lamp exposure time required to achieve
complete polymerisation of the monomer/oligomer mixture.
For grafting to occur sites need to be formed in the substrate by carbon-
hydrogen bond rupture (reaction x). Once these sites are formed grafting
may occur via reactions (5-9, e/5).
281
)9(
)8(
)7(
)6(),(
)5()(
....
..**
*
SPHSHP
IPPI
graftSR
substrateSHHSSH
polymerRADAD
radiation
radiation
radiation
+→++⎯⎯⎯ →⎯
→++⎯⎯⎯ →⎯
→→→⎯⎯⎯ →⎯+ .
Kinetically if surface polymer formation is more efficient than bond
rupture in the substrate followed by the grafting reaction then surface
homopolymer with physical bonds to the substrate may result. Such a
situation is relevant in the present work with certain systems particularly
with thiol-ene complexes where very poor grafting yields have been
obtained in some formulations. The property reflects poor chemical
adhesion to the substrate by the film and may significantly limit the
ultimate applications of the coated product.
Again the mechanisms for cure grafting via UV and EB processes are
similar except, as discussed for curing, with EB, additional processes from
the ionising radiation are available to enhance the cure grafting yields.
When cure grafting, particularly with the porous Whatman 41 paper occurs
with the CT type complexes, the low viscosity of the reactants leads to
appreciable monomer incorporation into the substrate. When cured, the
resulting product is swollen indicating that this type of process is
applicable to one stop synthesis of composites with potentially unique
properties.
282
CONCLUSIONS
A new UV lamp, the Con-Trol-Cure UV LED line 100, is shown to be
useful for a wider range of curing and cure grafting processes involving
oligomer/monomer acrylates and vinyl ethers. Since the lamp has a cut off
at 385nm, cure times are longer than with conventional UV Fusion and
low energy EB systems. Mechanism for curing and cure grafting are
similar to these proposed for conventional UV facilities however, as
expected, oxygen inhibition is more significant with the new lamp. The
possible potential of the new lamp in novel applications is discussed.
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