Curable Polyurethane-Acrylate Copolymers
Se-Chan Jang
A dissertation submitted to the Department of Chemical Engineering,
College of Engineering
Hanyang University
in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY
Curable Polyurethane-Acrylate Copolymers
2006 2
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Curable Polyurethane-Acrylate Copolymers
Prof. Sung-Chul Yi, Ph.D., P.E.
UV curing has now become a well-accepted technology that has found
its
main applications in various industrial sectors in which ultrafast
cure and
high quality product are required. UV curable adhesives containing
dual-curable
acrylic oligomer (AO) with alkoxy silyl group, fluorine atoms, and
vinyl group as
pendent groups were synthesized by two-stage reactions. The dual
curing behaviors,
e.g. thermal and UV cure, were studied by using photo-DSC and
real-time FT-IR.
An optimum adhesive formulation, based on AO 15 wt(g), epoxy
acrylate 80 wt(g),
isobonyl methacrylate 17 wt(g) and photoinitiator 3 wt(g), was
obtained. As the
content of AO increased in optical adhesive formulation, refractive
index decreased
II
but transmittance increased with the increasing the fluorine
content. The optical
transmittance at the range of 1.3 to 1.55 was over 90%. The
addition of
colloidal silica to the above formulation played role in reducing
of cross-linking
volume shrinkage and the increasing of glass fiber adhesion. The
required properties
for the optical adhesive, including chemical resistance and thermal
resistance,
dimension stability, and etc., were also investigated.
We investigated whether the weathering performance of UV curable
coatings
could be significantly improved by using both a reactive light
stabilizer and an
optimized photoinitiator combination. A reactive light stabilizer
(UCHA) containing
a UV absorbance group, a radical scavenger, and acryl groups in a
molecule was
synthesized, and its performance was investigated. Experimental
studies of UV
curing kinetics indicated that a combination of Irgacure 819 and
Darocure 1173 in
the ratio 1:3 provided the optimal performance and production cost.
The outdoor
resistance of the coated films was tested in an accelerated QUV
weatherometer. The
exposure results obtained with UCHA showed that the UV curable
coatings are
useful in outdoor applications
UV curable urethane-acrylate dispersions were prepared for use as
anodic
electrodeposition coating. Two acrylate groups were introduced to
the ends of the
polyurethane main chains by a sequential condensation reaction. The
neutralized
urethane-acrylate polymer containing photoinitiator with/without a
multifunctional
acrylate oligomer (EB-9970) could be dispersed into a stable
dispersion without any
phase separation. From methylethylketone (MEK) resistance
measurements, it was
confirmed that the three-functional acrylate, pentaerythritol
triacrylate (PETA), was
III
a more efficient coupling agent than 2-hydroxyethyl acrylate in the
design of UV
curable urethane-acrylate polymer. Kinetic studies using real-time
FT-IR and
photo-DSC revealed that the cross-linked films containing EB-9970
gave a higher
conversion factor than those without EB-9970, resulting in a better
MEK resistance.
IV
TABLE OF CONTENTS
Abstract Table of Contents List of Tables List of Figures List of
Schemes
Chapter 1
Background 1
Chapter 2
Adhesives 15
PreparedPoly(methacrylate) Copolymer 18
V
Physical Properties 20
2.3.1 Preparation of Dual Curable Poly(methacrylate) Copolymer
23
2.3.2 Curing Behaviors of Optical Adhesives 28
2.3.3 Optical Characteristics and Physical Properties of the
Optical
Adhesives 36
UV Curable Coatings 41
3.2.3 Photodifferential Scanning Calorimetry (Photo-DSC) 44
3.2.4 UV-visible Spectroscopy 44
3.3 Results and Discussion 49
3.3.1 Synthesis of the Reactive Light Stabilizer (UCHA) 49
VI
3.3.2 UV Absorption Spectra of Light Stabilizers and
Photoinitiators 50
3.3.3 Effects of Photoinitiators on Curing Behavior 52
3.3.4 Outdoor Performance of the Reactive Light Stabilizer 57
3.4 Conclusion 65
4.1 Introduction 66
4.2 Experimental 69
4.2.1 Materials 69
4.2.3 Electrodeposition of UV Curable Dispersion 72
4.2.4 Real-Time FT-IR 73
4.3.1 Synthesis of Urethane-acrylate Polymer 74
4.3.2 Particle Size Control 75
4.3.3 Electrodeposition Behaviors of UV Curable Urethane-acrylates
75
4.3.4 Curing Behaviors of UV Cured Electrodeposition Films 77
4.4 Conclusion 88
poly(methacrylate) copolymers 19
Table 2.2 Radical polymerization of poly(methacrylates) copolymer
27
Table 2.3 The properties of the films with silica filler
concentration 39
Table 3.1 The UV curable coating formulations including various
photoinitiators
and light stabilizers. Data values are weight percentages. 51
IX
Figure 1.2 UV radiation curing process 3
Figure 1.3 Application fields of UV curable materials 4
Figure 1.4 UV curable adhesives for optical devices 6
Figure 1.5 Comparison of additive stabilizer with reaction
stabilizer 8
Figure 1.6 Electrodeposion application field 10
Figure 1.7 Schematic diagram of the anodic electrodeposition
process and the
anodic resin micell and counter ions 14 Figure 2.1 1H-NMR spectrum
of AO monomer 25
Figure 2.2 19F-NMR spectrum of AO monomer 26
Figure 2.3 IR curves of siloxane group cross-linking in the AO-15
adhesive
formulation 29
Figure 2.4 Real-time FTIR/ATR spectra of the UV exposed AO-15
formulation 31
Figure 2.5 The conversion profile from real-time FTIR/ATR
conversions according
to AO content 32
Figure 2.6 Photo-DSC curves of UV adhesive formulations according
to the AO
contents 35
Figure 2.7 Plots of refractive index and transmittance versus AO
contents 37
Figure 3.1 1H NMR spectrum of the reactive light stabilizer (UCHA)
46
Figure 3.2 Absorption spectra of photoinitiators and UV absorbers
(A: Darocur
1173, B: Tinubin 400, C: UV curable light stabilizer, D:
BAPO(Irgacure
X
A-D 55
formulations A-D 56
E-H 58
formulations E-H 59
Figure 3.7 IR spectra of stabilized UCHA(formulation G) before and
after QUV
aging (2000 h) 61
Figure 3.8 QUV weathering of an unstabilized coating and stabilized
coating with
2wt.% UCHA 62
Figure 3.9 Yellow index (ΔYI) of formulations D-I versus the QUV-A
exposure
time 64
Figure 4.2 The particle size distribution of UV curable
urethane-acrylate
dispersion 76
the migration of anionic urethane-acrylate dispersion 78
Figure 4.4 Electrodeposition yields of urethane-acrylate
dispersions according to
the EB-9970 contents 79
Figure 4.5 The relationship between MEK resistance and EB-9970
content 81
XI
Figure 4.6 Real-Time FT-IR spectra of UV exposed UA1 83
Figure 4.7 The conversion curves of photopolymerization for UA1,
UA1-50 and
UA1-80 84
Figure 4.8 Photo-DSC exotherms for the photopolymerization of UA1,
UA1-50 and
UA1-80 86
Scheme 2.1 Synthesis of poly(methacrylate) copolymers 24
Scheme 3.1 Synthesis scheme of the reactive light stabilizer UCHA,
PETA,
pentaerythritol triacrylate 45
Scheme 4.1 The synthetic route of UV curable urethane-acrylate
polymer (U;
urethane group) 70
The popularity of environmentally safe coating (ESC) systems
is
increasing due to global regulations to reduce the volatile organic
components
in the coating formulations. Among ESC systems such as radiation
cured,
water based, high solid, powder, and supercritical carrier
coatings, UV
radiation cured coating systems continue to have the brightest
future [1-4].
The present popularity of UV radiation curing is due to a number of
reasons
[5]. First, UV curables are comprised of 100% reactive components,
which
provide an environmentally acceptable coating. Second, UV curing
systems
are energy efficient process; they require a small fraction of the
power
normally consumed by thermally cured coatings. Finally UV curables
can be
easily formulated to meet a variety of applications since
functionalized
monomers and oligomers are available covering a wide range of
properties.
Due to their advantages in important industrial applications,
including
adhesives, coatings, inks, varnishes, and electronics [6-9].
However, UV curable adhesives still has a problem in adhesion
of
complex devises having recessed area that can be reached by the UV
beam
[10], and poor outdoor weatherability [11, 12].
Therefore we studied "Dual Curable Fluorinated
poly(methacrylate)
2
Additives (0.01~2%) Surfactants, Pigment, Stabilizer, etc.
UV
4
Electronics/PC (Encapsulation)
Adhesives/Products Assembly
Electronics/PC (Encapsulation) Electronics/PC (Encapsulation)
5
Stabilizers for Weather-Resistant UV Curable Coatings".
Poly(methacrylate) materials are increasingly finding applications
in optical
and electro-optical devices due to their high transparency, light
weight, easy
of processability, and possible molecular tailoring through
controlled synthesis
and/or blending techniques [13,14]. The primary applications of
these
materials include optical wave guides, optical fibers, and
compact/optical disks
[15-17]. These devices generally consist of many optical and
electro-optical
parts, and it is often necessary to bond these components by simply
adjusting
refractive index to match that of the substrates while sustaining
their good
properties such as glass-like transparency and low birefringence to
prevent
optical adhesives much remain transparent and miscible even after
curing.
In order to circumvent the insufficient UV curing of the recessed
area,
we sought to introduce not only vinyl groups but also siloxane
groups in the
fluorinated PMMA by either grafting or copolymerization so that the
prepared
PMMA can be dual-cured with UV irradiation or heat adjusting.
In this thesis, dual-curable acrylic oligomers (AO) having alkoxy
silane
group, fluorine atoms and vinyl group as a pendent group were
synthesized
by two-stage reactions. The dual curing behaviors, e.g., thermal
and UV cure,
were studied by using photo-DSC and real-time FT-IR. The
required
properties for the optical adhesive, including chemical resistance
and thermal
resistance, dimension stability and etc., were also
investigated.
6
7
And most UV curable films contain residues of oligomers or
monomer
with unreacted photoinitiators having a photochromophore. These
residues
accelerate the polymer degradation due to harsh weathering
conditions
associated with solar radiation, atmospheric oxygen, humidity,
pollutants, and
heat. The polymer degradations occurring in UV exposed coating
films are
responsible for the observed changes in their performance [18]. To
overcome
this problem, the stabilization of UV cured films has been
attempted by
adding light stabilizers and a combination of UV absorbers and
hindered
amine light stabilizer (HALS) radical scavengers [19-21].
The light-stabilization performance of UV curable coatings has been
well
documented in several reports, but there are several reasons why no
major
industrial applications have yet emerged [22-27]. First, the
inner-filter effect
of the added UV absorber both slows down the cure speed and reduces
the
cure depth. The incomplete polymerization at the coating/substrate
interface
usually leads to poor adhesion. Second, the performance of light
stabilizers
deteriorates rapidly, although it is adequate at the initial stage
of formulation.
The light stabilizers are low-molecular-weight analogs that often
migrate to
the surface of the film to be finally wiped off or lost in some
other way.
To reduce this detrimental effect, we have attempted to chemically
graft light
stabilizer groups to UV curable films.
8
Additive Stabilizer
Low molecular weight Migration to the surface Finally wiped off or
lost
Good compatibility
Semi-permanent weatherability
Coating layer
9
The objective of this study was to produce UV curable coatings
with
weathering performance comparable to that of conventional
thermosetting
coatings. A reactive light stabilizer containing a UV absorbance
group, a
radical scavenger, and acryl groups in the same molecule was
synthesized for
this study. The curing behaviors and outdoor resistance of a
coating
formulation including the synthesized light stabilizers were also
investigated.
And we also studied "UV Curable Urethane-acrylate Dispersions for
the
Anodic Electrodeposition" Since Reuss of Moscow University
discovered
electrophoresis in 1809, many processes based on elctrophoretic
deposition
have been developed, including deposition of thick films,
laminates, and body
shaping [28, 29]. Some of these processes are in commercial uses.
For
example, electrodeposition process were introduced to the
automotive coating
industry in the early 1960s, which offers important advantages such
as
excellent corrosion protection, higher paint utilization and low
environmental
contamination [30]. Electrodeposition offers precise control of
film thickness,
uniformity, and deposition rate, and is especially attractive owing
to its low
equipment cost and inexpensive starting materials [31, 32]. Due to
the use of
the electric field, electrodeposition is particularly suited to the
formation of
uniform films on the substrates of complicated shape, impregnation
of porous
substrates, and deposition on the selected areas of the substrates.
Two
electrodeposition processes for coating applications have been
developed,
based on the electrode polarity of the coated metal : in
anodic
electrodeposition
10
11
Advantages Disadvantages
Easy control of film thickness Good uniformity Rapid deposition
rate Low cost of equipment and materials Safe process
Limited by thermosensitive substrates Poor weatherability
Difficulty in selecting variety color.
12
(AED), the coated metal acts as an anode, and in cathodic
electrodeposition
(CED), it acts as a cathode.
Electrodeposition research and development over the past 40 years
has
shown many significant achievements, including highly corrosion
protection
CED based on thermosetting epoxy-urethane, and nearly 100%
paint
utilization by the employment of ultra-filtration membrane [33].
However,
further development of the electrodeposition coating is required to
overcome
some limitations that are inherent in electrodeposition process.
One of these is
the need for high temperature thermocuring in the range of
160-180.
Therefore, a low-temperature curing system in electrodeposition
coating has
been developed to allow decrease in the power consumption and to
coat
thermosensitive substrates [34]. In 1994, Furakawa et al.
reported
low-temperature curable electrodeposition of 110-120, based
on
N-methacrlyoylcarbamate (N-MAC) [35]. But the low dissociation
temperature
of N-MAC can have an effect on bath stability during the
electrodeposition
procedure. Recently we reported the combination of UV curing and
CED,
which make it possible to cure at room temperature in a few seconds
after a
short flash-off stage [36, 37]. In addition to low-temperature
curing, such
UV curable electrodeposition has many advantages described above,
such as
high paint utilization, control of coating thickness, and
coatability of recessed
area. More recently, Fieberg and Reis reviewed the development of a
UV
curable electrodeposition coating system [38].
Although AED tends to dissolve the metal, leading to corrosion, its
use
13
for coating acid resistant metals such as Al, Ni, and Al alloy
continues,
because AED involves cheapter equipment and easier resin design
than does
CED [39]. Thus, the objective of the present work was to
demonstrate that
UV curable urethane-acrylate dispersions stabilized by carboxylate
groups can
be applicable to AED, thereby broadening the application range of
UV
curable electrodeposition. In this study, a urethane-acrylate
polymer was
synthesized and neutralized with an amine to give anionic
dispersion for the
AED process. We investigated the electrodeposition behaviors of UV
curable
urethane-acrylate dispersions with and without the multifunctional
acrylate
oligomer. We also studied the kinetics of the high-speed UV curing
reaction,
and investigated the performance of UV cured films.
14
Anode +
Cathod -
++
++ ++
-- --
--
Figure 1.7 Schematic diagram of the anodic electrodeposition
process and the
anodic resin micell and counter ions
15
for Optical Adhesives
Poly(methacrylate) materials are increasingly finding applications
in optical
and electro-optical devices due to their transparency, light
weight, easy of
processability, and possible molecular tailoring through controlled
synthesis
and/or blending techniques [13, 14]. The primary applications of
these
materials include optical wave guides, optical fibers, and
compact/optical disks
[15-17]. These devices generally consist of many optical and
electro-optical
parts, and it is often necessary to bond these components by simply
adjusting
refractive index to match that of the substrates while sustaining
their good
properties such as glass-like transparency and low birefringence to
prevent
optical adhesives much remain transparent and miscible even after
curing.
In pursuit of the above goals, dimethacylates based on
fluorinated
bisphenol-A or diols have been selected because of undergoing
rapid
photopolymerization to generate cross-linked polymers with good
optical and
16
mechanical properties [40]. UV curable adhesives have many
advantages in
production time and arranging of the phases of jointing, which can
be
deformed by heating in thermally curing adhesives. However, UV
curable
adhesives still have a problem in adhesion of complex devices
having
recessed area not to reach the UV beam [10]. In order to circumvent
the
insufficient UV curing of the recessed area, we sought to introduce
not only
vinyl groups but also siloxane groups in the fluorinated PMMA by
either
grafting or copolymerization so that the prepared PMMA can be dual
cured
with UV irradiation or heat adjusting.
In this thesis, dual curable acrylic oligomers (AO) having alkoxy
silane
group, fluorine atoms and vinyl group as a pendent group were
synthesized
by two-stage reactions. The dual curing behaviors, e.g., thermal
and UV cure,
were studied by using photo-DSC and real-time FT-IR. The
required
properties for the optical adhesive, including chemical resistance
and thermal
resistance, dimension stability and etc., were also
investigated.
17
Corporation. 3-(trimethoxysilyl)propyl methacrylate,
2-hydroxylethyl
methacrylate, 2,2’-azobis(2-methylpropinitrile)(AIBN) and
dibutyltin dilaurate
were purchased from Aldrich, and were used without further
purification. The
fluorinated methacrylate monomer, Zonyl TM fluoromonomer was
obtained
from Du Pont. 1-Hydroxycyclohexylphenyl ketone (Igacure 184,
Ciba-Geigy)
was used as a photoinitiator. Methylisobutyl ketone was dried over
molecular
sieves.
A typical poly(methacrylate) copolymer was prepared according to
the
following procedure. 337.5 g of methyl isobutyl ketone was placed
into a 1
four-necked flask equipped with a reflux condenser, stirrer,
thermometer, and
nitrogen inlet, and heated to 110oC. The monomer mixture of
2-methacryloyloxyethyl isocyanate (116.25 g),
3-(trimethoxysilyl)propyl
metharylate (15.50 g) and Zonyl TM fluoromonomer (66.75 g),
including 15
g of AIBN was slowly added for 1 hour, and maintained at 110oC for
5
hours. After NCO equivalent of the synthesized polymer was measured
by
titration method and cooled into the room temperature, 0.075 g of
dibutyl tin
dilaurylate and 0.5 g of 4-methoxy phenol were added. 97.5 g (0.75
mol, the
18
corresponding equivalents of NCO) of 2-hydroxyethyl methacrylate
was slowly
added at room temperature and stirred for 6 hours. The resulting
polymer
was obtained in brownish liquid after solvent evaporation with
vacuum, and
characterized by 1H NMR(CDCl3), 6.02 and 5.65 ppm
(CH2=C(CH3)CO2-),
1.846 ppm (CH2=C(CH3)CO2-) and 3.36 ppm (-Si(CH3)3), and 19F
NMR,
-81.14 ppm (-CF3), -113.91 ppm (-CF2CF2CH2-) (122~123) ppm (-CF2-)n
and
-126 ppm (-CF2CF3).
Poly(methacrylate) Copolymer
Table 2.1 shows the typical UV curable adhesive formulations
including
the poly(methacrylate) copolymer (Mn=3,500) as prepared above. A
series of
UV curable adhesive formulations with varying contents of the
poly(methacrylate) copolymer was prepared without changing of
other
ingredients content.
The real-time FT-IR spectra were obtained using a ReactIR
1000
spectrometer (ASI Applied System) equipped with a
diamond-composite
insertion probe. The samples were exposed for 30 s to the UV
radiation
from a metal halide lamp (EFOS UV System) though a fiber-optic
light
guide. The UV light intensity at the sample was 465 mW/cm2 over
a
wavelength range of 320-420 nm.
19
Table 2. 1 UV curable adhesive formulations including the
prepared
poly(methacrylate) copolymer
a Diacrylate of bisphenol A diglycidyl ether (SK UCB, Korea)
b 1-Hydroxycyclohexyl phenyl-ketone (Ciba-Geigy )
scanning calorimeter equipped with a photocalorimetric accessory
(TA
5000/DSC 2920). The initiation light source was a 200 W
high-pressure
mercury lamp, the UV light intensity at the sample was 50 mW/cm2
over a
wavelength range of 285-440 nm, and the sample was placed in
an
uncovered aluminum pan. The conversion was measured after 1 min
of
exposure. TA Instruments software was employed to obtain the
results from
the photo-DSC experiments.
Properties
To measure the refractive index, the optical adhesive specimens
were
prepared cast films on the test slides. The sample thickness ranged
from 8 to
10 . The optical adhesives were cured for 60 sec. at UV
illumination
devices with 35 W/cm metal halide lamp (MUV-35U, Moritex), and
then
heated at 80 for 10 min. The refractive indices nD of the cured
optical
adhesives at 1.55 wavelength were determined ATAGO Abbe’s
Refractometer at 20 .
The transmittance of the cured optical adhesive was measured by
a
UV-Vis spectrometer (Varian-3-Bio) and near IR as prepared. The
optical
adhesive with 1 mm of film thickness was applied to the quartz and
cured
according to the above procedure. In the case of optical adhesives,
the
transmittance at range of near IR than UV-visible is more
important. Thus
21
Shear strength was determined by conducting shear tests in
accordance
with ASTM D3163-92. The 3 mm-thick quartz were cut in to 50×10
mm
strips and bonded using a single-lab joint on a 100 mm2 section. A
bond
line thickness of 1 mm was maintained by using for 1 mm diameter
glass as
spacers and was removed after curing. The loading was applied using
a
Honsfield tensometer. The tensometer cross head speed was at 10
mm/min.
Total cure shrinkage of the optical adhesives was calculated from
the
densities of the resin before and after curing. The densities of
the optical
adhesives before cure were measured with a pycnometer. The
densities of the
optical adhesives after cure were measured in a 50 burette with
distilled
water as the liquid medium indicating the volume of the cured
adhesives.
Shrinkage of the adhesives was calculated by means of the
following
equation :
V L ×100
where : S : Shrinkage
VS : Volume of UV cured adhesive.
The coefficient of thermal expansion of films of rigid adhesives
was
22
carried out with the use of thermal mechanical analyzer (TMA 2940,
TA
Instrument) in the expansion mode. The TMA used for this work
was
equipped with a fused silica expansion sample holder. Pushrod
contact forces
of 10cN were employed. Calibration of these instruments (correction
of the
sample holder influence) was done by using a fused silica standard.
The
calibration run was, of course, conducted under the same conditions
as used
for the samples. The measurements were conducted in a dynamic
helium
atmosphere (gas flow rate : 50 ml/min) to avoid the condensation of
water
on the sample surface during cooling. The samples were heated from
-20°C
to +230°C at a heating rate of 5 K/min. The sample was about 15
mm×10
mm×0.1 mm.
A dual curable poly(methacrylate) copolymer having alkoxy silane
group,
fluorine atoms and vinyl group as a pendent group was synthesized
by
two-stage reactions as shown in scheme 2.1. The isocyanate group
containing
oligomers was firstly synthesized by radical polymerization of
acrylic
monomers, and followed by urethane reaction with 2-hydroxy
ethyl
methacrylate. The optical and physical properties of the prepared
copolymer
can be varied with the weight ratio of the methacryl monomers,
e.g., n/m/l in
scheme 2.1. For example, optical transmittance of AO depends on the
content
of fluorinated alkyl methacrylate (Zonyl TM fluoromonomer), and UV
and
thermal curing behaviors can be strongly influenced by the content
of
3-(trimethoxysilyl)propyl methacrylate and post-introduced
2-hydroxyethyl
methacrylate, respectively.
When we make UV curable adhesives by using the above prepared
poly(methacrylate) copolymer, the adhesive should be composed of
the
poly(methacrylate) copolymer and the diacrylate of bisphenol-A
diglycidyl
ether (Ebecryl 600, SK UCB Korea), which is a widely accepted
fixing
adhesives. There is a incompatibility of these two different types
of resins
[10]. We attempted to solve this problem by controlling the
molecular weight
of the poly(methacrylate) copolymer. As shown in table 2.2, the
number
average molecular weight was decreased with the amount of
radical
24
CH3
25
Figure 2.1 1H-NMR spectrum of AO monomer (l:m:n = 0.083 : 1 :
0.0125)
26
Figure 2.2 19F-NMR spectrum of AO monomer (l:m:n = 0.083 : 1 :
0.0125)
27
No. AIBN wt(%)
Mn Compatibility with
epoxy resina Viscosity(cps)
1 2 9,800 Bad 6,000
2 4 5,500 Bad 2,000
3 7 4,000 Fair 1,500
4 10 3,500 Good 1,100
a Compatibility was evaluated by the dielectric loss tangent for
blends of AO, 50 wt(%) and
Ebecryl 600, 50 wt(%).
initiator, AIBN, and the synthesized copolymer with low molecular
weight
was more compatible with epoxy resin. In the polymerization of
methacryl
monomers, the increased amount of radical initiator resulted in
lowering the
molecular weight of AO. Miscibility of AO to the epoxy resin can
be
characterized by transparency observation with naked eye,
measurement of
single Tg or dielectric relaxation study. In our experiment,
compatibilities of
AO with Ebecryl 600 were evaluated by the dielectric relaxation
study, e.g.,
the measurement of dielectric loss tangent [10]. AO with the
molecular
weight of 3500 for 50/50 copolymer/epoxy blend showed a single
loss
tangent peak, indicating that the blend is miscible. However, AO
with the
higher molecular weight than 5500 showed two corresponding loss
peaks to
give the immiscible blend.
AO is a dual curable poly(methacrylate) copolymers having alkoxy
silane
group, fluorine atoms and vinyl group as a pendent group. The
alkoxy silane
groups of the above synthesized AO absorb react with moisture
in
atmosphere to form Si-O-Si bonds leading to a network structure and
curing.
In order to identify the curing of the alkoxy silane group, The
curing
behaviors of the adhesive formulation (AO-15, 15 wt.(g)) in table
2.1
observed at the relative humidity of 60% and 85 by FT-IR/ATR
(figure
2.3). The absorbance at 1080 cm-1of Si-O-Si stretching band after
curing
shows that the alkoxy silane groups were cross-linked to form a
network.
29
Before crosslinking
After crosslinking
1080 cm-1
Figure 2.3 IR curves of siloxane group cross-linking in the AO-15
adhesive
formulation
30
The UV curable adhesives were also cross-linked by exposure to
UV
radiation. The effect of incorporating the prepared
poly(methacrylate)
copolymer into the adhesive formulations was examined by real-time
FT-IR
and photo-DSC exotherm curves. The typical real-time FT-IR spectra
of UV
exposed AO-15 are shown in Figure 2.4. The spectra are highlighted
by the
disappearance of 812 cm-1 band characteristic of the C-H
deformation modes
of =CH2 group. Since the decrease in intensity of this peak is
attributable to
the gradual disappearance of the (meth)acrylic double bond as the
cure
proceeds, the extent of photopolymerization can be
spectroscopically recorded
in real-time.
Figure 2.5 shows the conversion curves of photopolymerization
for
adhesive without AO, AO-15, AO-20 and AO-35 containing 15 wt(g),
20
wt(g), and 30 wt(g) of AO, respectively. The conversion (α) was
calculated
at various intervals using the following equation:
α(%) =
[I812]0
×100 ---(1)
where [I812]o and [I812]t are the sample absorbance at 812 cm-1,
before and
after UV exposure, respectively. Usually, relative intensity is
required
because the absolute intensity varies from sample to sample. Since
it is
difficult to find the reference peak from the infrared spectra of
cross-linking
31
=CH2 def., 812cm-1
Figure 2. 4 Real-time FTIR/ATR spectra of the UV exposed AO-15
formulation
32
Time (min)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
% C
according to AO content
system, conversion of photopolymerization was deduced based on the
absolute
intensities. The kinetics of photopolymerization were also analyzed
using the
method described by Decker [41] and the steady state equation given
below
Rp(%/sec)= -------(2) α t2 - α t2 t2 – t1
where Rp is the rate of polymerization in %/sec, αt2 and αt1 are
the
conversions in the straight line region of the sigmoid curve, and
t1 and t2 are
the times in seconds that correspond to the conversions αt2 and
αt1. Figure
2.5 shows the overall rate of polymerization (Rp) and final
conversion (α) as
a function of AO content in adhesive formulation. The final
conversion (α)
increased as the content of AO increased. It is presumably due to
the
increase of methacrylate groups of AO. However, the overall rate
of
polymerization (Rp) was decreased as the content of AO increased.
At the
initial stage, the increased siloxane groups of AO reacted with
water in air to
give a cured interface, and the cured interface subsequently
inhibited the
mobility of the acrylate or methacrylate groups to retard the
overall rate of
polymerization (Rp).
The curing behavior was also studied by photo-DSC method and
results
supporting the conclusion derived from real-time FT-IR were
obtained. The
34
heat flow obtained from the photo-DSC experiment can be also
converted
directly to the ultimate percentage conversion and rate of
polymerization for a
given sample. Figure 2.6 shows the photo-DSC exotherm curves
for
photopolymerization as a function of AO concentration in
adhesive
formulation. The values of exotherm and conversion increased until
the
content of AO increased up to 15 wt(g), and then decreased when
the
content exceeds. The photo-DSC results show the optimum content of
AO,
15 wt(g), in curing behavior contrast to those of the above
real-time FT-IR
studies. These results indicate that the excess content of AO in
adhesive
formulation inhibited the diffusion mobility of propagating chains
due to the
increase of the fluorinated long chain alky group and siloxane
groups. From
those data it could be concluded that the content of the dual
curable
poly(methacrylate) copolymer in adhesive formulations, AO, was
fixed in 15
wt(g) in further studies of optical characteristics and physical
properties.
35
H ea
80 AO AO-10 AO-15 AO-20 AO-35
Figure 2.6 Photo-DSC curves of UV adhesive formulations according
to the
AO contents
2.3.3 Optical Characteristics and Physical Properties of the
Optical Adhesives
Optical adhesives for light signal pathway play an important role
in fixing
the components and minimizing any interference to traverse of light
signal.
Thus, it is required to good transmittance of light and
controllability in
refractive index. For example, meth(acrylate) or epoxy polymer
including
fluorine atoms and vinyl polymers having sulfur atoms were used for
low
and high refractive indices, respectively. Various optical
adhesives from 1.45
to 1.70 of refractive indices have been developed with ±0.005 of
extent. To
evaluate the controllability of refractive index of the above
poly(methacrylate)
copolymer in adhesive formulation (table 2.1), the refractive
indices are
plotted in relation to wt(%) of poly(methacrylate) copolymer in
Figure 2.7
The refractive indices were found to decrease with increasing AO
content due
to the presence of fluorine atoms.
Transmittance of optical adhesives is an importance factors engaged
with
refractive index, which is effective in optical loss during
transmittance of
light signal. Especially, Transmittances and absorbencies at 1.3
and 1.55
are directly related to optical loss because most of optical
communication
conducted at these two wavelengths. Figure 2.7 shows transmittances
of
optical adhesives range of 1.1 to 1.7 with content of AO. It was
found
that transmittances of optical adhesives increased with increasing
AO content.
Higher transmittance at the increased content of AO can be
explained from
37
R ef
ra ct
iv e
in de
x (n
Transmittance at 1.3µm Transmittance at 1.55µm
Figure 2.7 Plots of refractive index and transmittance versus AO
contents
38
that fluorine atoms substitute hydrogen atoms to minimize the
C-H
absorbance at near IR range (1.1~1.7 ) [42]. In our study, the
optimized
optical adhesive including 15 wt(g) of AO showed more than 90
%
transmittance, which is comparable with the commercialized optical
adhesives.
Most of optical adhesives were presumably included inorganic filler
such
as silica in order to decrease the thermal expansion or cure
shrinkage, which
are can be resulted in distortion of the ingredients array of
optical devices
for curing process. In our experiment, colloidal silica with nano
size (Hanse
Chem. Germany, XP/0768) was formulated into the optimized
formulation
(AO-15). Table 2.3 summarized physical properties of the AO-15
formulation
as a function of colloidal silica content. Little change in
refractive index was
shown even in high content of colloidal silica up to 30 wt%.
However, both
chemical resistance and shear adhesion strength were improved as
the
content of colloidal silica. The optical adhesive containing 20 wt%
colloidal
silica showed 1.8 % of moisture absorption and 9.6 x 10-5 of
thermal
expansion coefficient, which are improved physical properties in
comparison
with commercialized product [43]. These improved chemical and
physical
properties from by adding colloidal silica presumably resulted from
the
process that strong Si-O-Si chemical bond was formed among the
colloidal
silica, siloxane group of glass fiber and alkoxy silane group of
AO.
39
Table 2.3 The properties of the films with silica filler
concentration
Physical properties Silica 0 wt% Silica 10 wt% Silica 20 wt% Silica
30wt%
Pendulum hardness (s) 228 230 236 232
MEK double rubs 62 >200 >200 196
Refractive index(nD 25) 1.4982 1.4982 1.4987 1.4992
Transmittance (%) 91 91 90 90
Tensile shear adhesive strength (Kgf/cm2)
Dry 208 216 220 211
Wet 165 187 193 191
Coefficient of thermal expansion (10-5 )
Before Tg
40
We have newly synthesized dual cross-linkable
poly(methacrylate)
copolymer having alkoxy silane group, fluorine atoms and vinyl
group as
pendent groups for UV adhesives by radical addition polymerization.
The dual
curing behaviors, e.g. thermal and photo-cure, were studied by
using
photo-DSC and real-time FT-IR, and an optimum adhesive formulation
was
obtained from the studies of curing behaviors. As the content of AO
was
increased in optical adhesive formulation, refractive index was
decreased but
transmittance was increased due to the increasing of fluorine
content. The
addition of colloidal silica to the above formulation was helpful
to the
decreasing of cross-linking volume shrinkage and the increasing of
glass fiber
adhesion.
41
for Weather-Resistant UV Curable Coatings
3.1 Introduction
UV curable coatings have become a well-accepted technology that
has
found a variety of industrial applications because of its unique
advantages,
i.e., a solvent-free resin that is transformed almost instantly
into a highly
cross-linked polymer at ambient temperature [44-46]. Although there
has been
a steady research effort in both the reactivity (aimed at faster
and more
complete cure) and the final properties of UV cured coatings,
outdoor
applications of UV curable coatings have been limited due to their
weak
weathering resistance [11, 12]. Most UV curable films contain
residues of
oligomers or monomer with unreacted photoinitiators having a
photochromophore. These residues accelerate the polymer degradation
due to
harsh weathering conditions associated with solar radiation,
atmospheric
oxygen, humidity, pollutants, and heat. The polymer degradations
occurring in
UV exposed coating films are responsible for the observed changes
in their
performance [18]. To overcome this problem, the stabilization of UV
cured
42
films has been attempted by adding light stabilizers and a
combination of UV
absorbers and hindered amine light stabilizer (HALS) radical
scavengers
[19-21].
The light-stabilization performance of UV curable coatings has been
well
documented in several reports, but there are several reasons why no
major
industrial applications have yet emerged [22-27]. First, the
inner-filter effect
of the added UV absorber both slows down the cure speed and reduces
the
cure depth. The incomplete polymerization at the coating/substrate
interface
usually leads to poor adhesion. Second, the performance of light
stabilizers
deteriorates rapidly, although it is adequate initially. The light
stabilizers are
low-molecular-weight analogs that often migrate to the surface of
the film to
be finally wiped off or lost in some other way. To reduce this
detrimental
effect, we have attempted to chemically graft light stabilizer
groups to UV
curable films.
The objective of this study was to produce UV curable coatings
with
weathering performance comparable to that of conventional
thermosetting
coatings. A reactive light stabilizer containing a UV absorbance
group, a
radical scavenger, and acryl groups in the same molecule was
synthesized for
this study. The curing behaviors and outdoor resistance of a
coating
formulation including the synthesized light stabilizers were also
investigated.
43
from Rhodia. Dibutyltin dilaurate (DBTDL), 4-methoxy phenol,
4-hydroxy-2,2,6,6-tetramethyl piperidin, and pentaerythritol
triacrylate were
purchased from Aldrich, and were used without further purification.
The
photoinitiators Darocure 1173 and Irgacure 819 were obtained from
Ciba
Specialty Chemicals. The UV absorber hydroxyphenyl-s-triazine
(Tinubin 400)
was also obtained from Ciba Geigy. Aliphatic urethane acrylate,
trimethylol
propane triacrylate, and 1,6-hexanediol diacrylate were purchased
from SK
UCB (Korea). Methylethylketone was dried over molecular
sieves.
3.2.2 Synthesis of Reactive Light Stabilizer
A reactive light stabilizer was prepared according to the
following
procedure. 30 g of methyl ethyl ketone, 5.85 g (0.01 mol) of
hexamethylene
isocyanurate (Tolunate HDT), 0.03 g of 4-methoxy phenol, and 0.03 g
of
DBTDL were placed in a 0.5 four-necked flask that was equipped with
a
reflux condenser, stirrer, thermometer, and inlet of nitrogen gas,
and stirred
continuously at 60oC. 4-Hydroxy-2,2,6,6-tetramethyl piperidin (1.56
g, 0.01
mol) and 6.23 g (0.01 mol) of hydroxyphenyl-s-triazine-1 (Tinubin
400) were
slowly added, and the extent of reaction was confirmed by
measurement of
NCO values after 2 h thereafter 2.98 g (0.01 mol) of
pentaerythritol
44
triacrylate and 0.3 g of 4-methoxyphenol were added. Measuring an
additional
2 h, the progress of the reaction was monitored by NCO values of
zero. The
resulting reactive light stabilizer was characterized by 1H
NMR(CDCl3) as
follows (values in ppm; see Figure 3.1): 8.11 (HO-Ar) 1.12~1.15
(-(CH2)4-,
(CH3)2C<) 1.91 (>NH) 5.86, 6.09, and 6.4 (CH2=CH-) 4.27
(-C(CH2O-)3) 3.78
(-CH2OCONH-) 3.15 (-CH2OCONHCH2-) 4.05 (-CH2OCONH-) and 2.2
(CH3-Ar) and by FT-IR (cm-1): 1722 (C=O, acrylate stretching) 1684
(-C=C-,
acrylate stretching) 1583 (-C=C-, aromatic stretching) and 811
(=C-H,
deformation).
The photo-DSC experiments were conducted by using a
differential
scanning calorimeter equipped with a photocalorimetric accessory
(TA
5000/DPC System). The initiation light source was a 200-W
high-pressure
mercury lamp, which gave a UV light intensity at the sample of 35
mW/cm2
over a wavelength range of 200-400 nm. Samples weighing 4.0 ± 0.1
(mean
±SD) mg were placed in uncovered aluminum pans (thickness ~500
μm).
Isothermal UV curing experiments were performed at 25 . TA
Instruments
software was employed to acquire the data and analyze the results
from the
photo-DSC experiments.
The absorption spectra of the photoinitiator and UV absorbance
were
45
1.0 eq of 4-hydroxy-2,2,6,6 -tetramethyl piperidine 1.0 eq of
PETA
1.0 eq of Tinubin 400
Scheme 3.1 Synthesis scheme of the reactive light stabilizer UCHA,
PETA,
pentaerythritol triacrylate
46
Figure 3.1 1H NMR spectrum of the reactive light stabilizer
(UCHA)
47
measured using a Cary 3 Bio UV-Visible spectrophotometer. Diluents
of 0.02
g/ in methylene chloride were prepared, and quart cells with a path
length
of 1.0 cm were used in the analysis.
3.2.5 Evaluation of Weather Resistance
The weathering resistances of various coating formulations were
evaluated
by exposing the samples in the QUV-accelerated weatherometer
equipped with
either UVA-340 or UVB-313 fluorescent lamps and operated under wet
cycle
conditions: 8 h UV irradiation at 70, followed by 4 h of dark
condensation
at 50. The thickness of the coating films was typically 15 μm. The
extent
of the coating-film degradation was evaluated by yellow index (ΔYI)
values
which were calculated using
ΔYI = {(1.28×X-1.06×Z)/Y}×100 ---------(1)
where X, Y, and Z are the red, green, and blue values,
respectively, as
measured with a chromameter (CR-300, Minolta) under the CIE
(commission
international del’Eclairage, 1970) color regulation.
Infrared spectroscopy was also used to monitor the curing reaction
and
the chemical changes occurring in each QUV-exposed sample. The
extent of
the photodegradation of QUV-exposed samples was evaluated from
the
decrease in the IR band centered at 2930 cm-1 (C-H stretching). The
relative
amounts of remaining functional groups were determined by
calculating the
48
ratio of the IR absorbance at the corresponding wavenumber after a
given
QUV exposure to the IR absorbance in the unexposed sample. The
formation
of oxidation products (C=O and OH) was observed by the increase in
their
related IR absorption values (1780 and 3500 cm-1,
respectively).
49
3.3.1 Synthesis of the Reactive Light Stabilizer (UCHA)
As described above, utilizing a combination of UV absorbers and
radical
scavenger is preferable when designing the light stabilizer. The
general
product profile for suitable UV absorbers has already been
disclosed in earlier
publications [47-49]. The four most important UV absorber classes
are the
hydroxyphenyl-benzotriazoles, hydroxyphenyl-s-triazines,
hydroxyl-
benzophenones, and oxalic anilides. From a technical point of
view,
hydroxyphenyl-s-triazine is the most interesting class of UV
absorbers. In our
study, the reactive light stabilizer was incorporated with a
hydroxyphenyl-s-triazine using a urethane reaction.
It is well known that HALSs trap radicals during polymer
degradation.
Earlier experiments in polyolefins have shown the important role of
the
nitroxyl radicals formed during the stabilization cycle [50, 51].
The
tetramethyl piperidin group was also introduced as a radical
scavenger into
the reactive light stabilizer in addition to a
hydroxyphenyl-s-triazine group.
As shown in Scheme 3.1, a reactive light stabilizer containing a
UV
absorbance group, a radical scavenger, and acryl groups in the same
molecule
was synthesized in order to realize UV curable coatings. The
urethane
reaction of a hydroxyphenyl-s-triazine,
4-hydroxy-2,2,6,6-tetramethyl piperidin,
and hexamethylene isocyanurate with DBTDL was carried out in methyl
ethyl
ketone at 60. Three acryl groups were introduced to the reactive
light
50
stabilizer by a subsequent reaction of pentaerythritol triacrylate
with the
remained isocyanate group. The theoretical acryl-group content of
the resulting
reactive light stabilizer (UCHA) was calculated as 1.80 mEq/g,
which was
considered sufficient for grafting into the UV curable film matrix
during the
UV curing process. The prepared light stabilizer was not purified
further
when making the UV curable coating formulations, and was studied in
terms
of UV curing behaviors and outdoor resistance.
3.3.2 UV Absorption Spectra of Light Stabilizers and
Photoinitiators
We knew that the addition of a HALS radical scavenger (1 wt.%)
would
not inhibit the UV curing process, since the polymerization of
acrylates
proceeded in an O2-depleted medium where nitroxyl radicals are not
formed
and therefore do not interfere with the polymerization. However, it
was found
that the addition of a UV absorber slow the polymerization, because
this
stabilizer competes with the photoinitator in the absorption of
incident
photons (radiation inner-filter effect). Therefore, it is important
to select active
photoinitiators whose absorption band does not overlap with that of
the UV
absorbers. Figure 3.2 shows the spectra of Darocure 1173, Irgacure
819,
Tinubin 400,and the reactive light stabilizer (UCHA). Because the
UCHA has
a hydroxyphenyl-s-triazine group, UCHA and Tinubin 400 have
similar
absorption bands. Igacure 819, a long-wavelength photoinitiator,
showed
stronger absorption from 360 to 430 nm than did
hydroxyphenyl-s-triazine.
Darocure 1173, a short-wavelength photoinitiator, showed a strong
absorption
51
Table 3.1 The UV curable coating formulations including various
photoinitiators
and light stabilizers. Data values are weight percentages
Ingredient A B C D E F G H I
EB284a 60 60 60 60 60 60 60 60 60
HDDAb 20 20 20 20 20 20 20 20 20
TMPTAc 20 20 20 20 20 20 20 20 20
Darocure 1173 4 - 3 1 4 - 3 1 1
Irgacure 819 - 4 1 3 - 4 1 3 3
UCHA - - - - 2 2 2 2 -
Tinubin 400 - - - - - - - - 1
b HDDA is 1,6-hexanediol diacrylate.
c TMPTA is trimethylol propane triacrylate.
d HALS-1 is a hindered amine light stabilizer (Tinubin 292) of Ciba
Specialty Chemical.
52
band at 325 nm and became transparent around 360-430 nm where the
UV
absorber shows absorption bands. These UV absorption spectra
indicate that
the UV curing behavior of coating formulations is strongly
influenced by the
selected photoinitiators and UV absorbers.
3.3.3 Effects of Photoinitiators on Curing Behavior
As indicated in Table 3.1, two series of UV coating formulations
were
prepared with/without the reactive light stabilizer (UCHA) at 1
wt.%, and
their UV curing behaviors were compared. Photoinitiators were fixed
at 4
wt.% based on the total formulation, which is generally known to be
the
optimal amount in UV curable coating formulations. Figure 3.3 shows
the
photo-DSC exotherms for the UV curing kinetics of formulations A-D
without
the reactive light stabilizer. The photo-DSC method assumes that
for a cure
process the measured heat flow is proportional to the conversion
rate. This
assumption is valid for materials with a single reaction and no
other
enthalpic events, such as the evaporation of solvent or volatile
components,
enthalpy relaxation, or significant changes in heat capacity with
conversion.
Therefore the rate of change in the conversion can be defined as
follows:
totalH dt
is the conversion rate or the polymerization rate, dt dα
is the
measured heat flow, and totalH is the total exothermic heat of
reaction. The
degree of conversion is calculated from the integrated form of
Equation.(2):
total t H
Figure 3.4 plots the percentage conversion versus irradiation time
derived
from Figure 3.3 for the photopolymerization of the above systems.
From the
data of Figures 3.3 and 3.4, formulation A (containing the
long-wavelength
photoinitiator, Irgacure 819) shows higher exotherms and conversion
than does
formulation B (containing the short-wavelength photoinitiator,
Darocure 1173).
This indicates that formulation A leads to a denser UV curing
network than
does formulation B. Formulations C and D (containing both Irgacure
819 and
Darocure 1173) show nearly the same (or slightly lower) exotherms
and
conversion compared with formulation A.
54
Figure 3. 2 Absorption spectra of photoinitiators and UV
absorbers
(A: Darocur 1173, B: Tinubin 400, C: UV-curable light stabilizer,
D:
BAPO(Irgacure 819)
H ea
Figure 3.3 Photo-DSC exotherms for the photopolymerization of
formulations A-D
Pe rc
en ta
ge c
on ve
rs io
formulations A-D
57
Figure 3.5 shows the photo-DSC exotherms for the UV curing kinetics
of
formulations E-H containing the reactive light stabilizer (UCHA).
Figure 3.6
plots the percentage conversion versus irradiation time derived
from Figure
3.5 for the photopolymerization of the above systems. Figures 3.5
and 3.6
reveal that formulation E (containing UCHA and Darocure 1173)
showed the
lowest exotherm and conversion compared with formulations F H.
As
indicated above, these results might be caused by the reactive
light stabilizer,
UCHA, absorbing UV light so as to inhibit the initiation of
short-wavelength
photoinitiator (Darocure 1173). However, formulations G and H
(containing a
mixture of Darocure 1173 and Irgacure 819) showed only slightly
lower
exotherms and conversions than did formulation F (containing
Irgacure 819
only). The long-wavelength photoinitiator had a greater effect on
the curing
behavior when the light stabilizer was present. Experimental
studies of UV
curing kinetics revealed that a mixture of Irgacure 819 and
Darocure 1173 in
the ratio 1:3 was preferable when considering both the cost and
performance
of photoinitiators.
3.3.4 Outdoor Performance of the Reactive Light Stabilizer
It is well known that the weather resistance of the coated films
can be
evaluated by measuring the yellow index or the gloss reduction.
Recently
Schwalm et al. reported the results of a quantitative evaluation of
the weather
58
H ea
Figure 3.5 Photo-DSC exthertherms for the photopolymerization
of
formulations E-H
Pe rc
en ta
ge c
on ve
rs io
formulations E-H
60
resistance based on the decrease of absorption peak at 2993
cm-1(C-H
stretching) using IR spectroscopy [52]. In the present study, the
weather
resistance of polycarbonate sheet coated with UV coating
formulations was
evaluated by measuring the yellow index and using IR spectroscopy.
Figure
3.7 shows IR spectra of stabilized UCHA (formulation G) before and
after
QUV aging (2000 h). The main change occurred in the region
associated
with absorption by CH groups (2930 cm-1), which decreased upon
QUV
aging. A small increase in the carbonyl absorbance at 1780 cm-1 and
in the
hydroxyl absorbance around 3500 cm-1 was also observed. Figure 3.8
shows
QUV weathering of an unstabilized coating and a coating stabilized
with 2
wt.% UCHA. The stabilizing effect of the reactive light stabilizer
(UCHA) on
the photodegradation of the UV cured films was clearly apparent,
with a
13% reduction in the C-H content requiring an exposure of 2000
h,
compared to only 400 h for the unstabilized sample.
Figure 3.9 shows the changes in the yellow index (ΔYI) upon
QUV
weathering for coating formulations D-I applied over polycarbonate
sheet. The
ΔYI values increased with the exposure time in the UV cured film
without
light stabilizer (sample D), which may be attributable to polymer
degradation
caused by UV irradiation. Formulation I, containing the
additive-type light
stabilizer, showed a high weather resistance at the initial stage
and increased
ΔYI values after 500 h of exposure time. In contrast, formulations
E-H
(containing the reactive light stabilizer UCHA) did not show large
increases
in the yellow index and still had high gloss after weathering. The
superior
61
3 Before QUV ageing After QUV 2000 h
Figure 3.7 IR spectra of stabilized UCHA(formulation G) before and
after
QUV aging (2000 h)
R em
ai ni
ng C
H g
ro up
D (UCHA X) H (UCHA 1wt%)
Figure 3.8 QUV weathering of an unstabilized coating and stabilized
coating
with 2 wt% UCHA
63
weather resistance of formulations E-H is attributed to the
reactive light
stabilizer (UCHA) in this study containing three acryl groups that
are
chemically incorporated in the UV cured film matrix. The above
experiments
clearly show that UV cured films containing the reactive light
stabilizer
(UCHA) have excellent weather resistance and hence would be useful
in
outdoor applications.
Y I
D E F G H I
Figure 3.9 Yellow index (ΔYI) of formulations D-I in relation to
the QUV-A
exposure time
3.4. Conclusion
With the aim of producing UV curable coatings for use in
outdoor
applications, a specialized reactive light stabilizer (containing a
UV
absorbance group, a radical scavenger, and acryl groups in the
same
molecule)was synthesized and its performance was investigated.
The
weathering performance of UV curable coatings significantly
improved by
using both a reactive light stabilizer and an optimized
photoinitiator
combination. The UV curing kinetics experiments revealed that a
combination
of Irgacure 819 and Darocure 1173 at the ratio 1:3 was to be
optimal
considering both cost and performance of photoinitiators. The
weather
resistance of the coated films was quantitatively evaluated by
measuring the
effect of QUV irradiation on the IR spectra and the yellow index.
The
exposure data indicates that UV curable coatings containing the
reactive light
stabilizer would be useful in outdoor applications.
66
the Anodic Electrodeposition
Since Reuss of Moscow University discovered electrophoresis in
1809,
many processes based on elctrophoretic deposition have been
developed,
including deposition of thick films, laminates, and body shaping
[28, 29].
Some of these processes are in commercial uses. For example,
electrodeposition process were introduced to automotive coating
industry in
the early 1960s, and offered important advantages such as excellent
corrosion
protection, higher paint utilization, and low environmental
contamination [30].
Electrodeposition offers precise control of film thickness,
uniformity, and
deposition rate, and is especially attractive owing to its low
equipment cost
and inexpensive starting materials [31, 32]. Due to the use of the
electric
field, electrodeposition is particularly suited to the formation of
uniform films
on the substrates of complicated shape, impregnation of porous
substrates, and
deposition on the selected areas of the substrates. Two
electrodeposition
processes for coating applications have been developed, based on
the
67
electrode polarity of the coated metal: in anodic
electrodeposition(AED), the
coated metal act as an anode, and in cathodic
electrodeposition(CED), it acts
as a cathode.
Electrodeposition research and development over the past 40 years
has
shown many significant achievements, including highly corrosion
protection
CED based on thermosetting epoxy-urethane, and nearly 100%
paint
utilization by the employment of ultra-filtration membrane [33].
However,
further development of the electrodeposition coating is required to
overcome
some limitations that are inherent in electrodeposition process.
One of these is
the need for high temperature thermocuring in the range of
160-180.
Therefore, a low-temperature curing system in electrodeposition
coating has
been developed to allow drecrease in the power consumption and to
coat
thermosensitive substrates [34]. In 1994, Furakawa et al.
reported
low-temperature curable electrodeposition of 110-120, based
on
N-methacrlyoylcarbamate (N-MAC) [35]. But the low dissociation
temperature
of N-MAC can have an effect on bath stability during the
electrodeposition
procedure. Recently we reported the combination of UV curing and
CED,
which make it possible to cure at room temperature in a few seconds
after a
short flash-off stage [36, 37]. In addition to low-temperature
curing, such
UV curable electrodeposition has many advantages described above,
such as
high paint utilization, control of coating thickness, and
coatability of recessed
area. More recently, Fieberg and Reis reviewed the development of a
UV
curable electrodeposition coating system [38].
68
Although AED tends to dissolve the metal, leading to corrosion, its
use
for coating acid resistant metals such as Al, Ni, and Al alloy
continues,
because AED involves cheapter equipment and easier resin design
than does
CED [39]. Thus, the objective of the present work was to
demonstrate that
UV curable urethane-acrylate dispersions stabilized by carboxylate
groups can
be applicable to AED, thereby broadening the application range of
UV
curable electrodeposition. In this study, a urethane-acrylate
polymer was
synthesized and neutralized with an amine to give anionic
dispersion for the
AED process. We investigated the electrodeposition behaviors of UV
curable
urethane-acrylate dispersions with and without the multifunctional
acrylate
oligomer. We also studied the kinetics of the high-speed UV curing
reaction,
and investigated the performance of UV cured films.
69
dilaurate (DBTDL), dimethylolpropionic acid, 2-hydroxyethyl
acrylate and
pentaerythritol triacrylate (PETA) were purchased from Aldrich, and
were
used without further purification. The photoinitiator, Darocure
1173 was
obtained from Ciba-Geigy, and multifunctional acrylate oligomer, EB
9970
(Mn=600, five acrylate functionalities), was obtained from SK UCB,
Korea.
The acetone and isopropyl alcohol used in the process were dried
over
molecular sieves.
The urethane-acrylate polymer was prepared according to the
following
procedure. A 1 four-necked flask was equipped with a reflux
condenser,
stirrer, thermometer, and inlet for nitrogen gas 250 g of
poly(tetramethyleneglycol) (Mn = 1000), 111 g of IPDI, 16.7 g
of
dimethylolpropionic acid, 0.5 g of DBTDL and 200 g of acetone were
placed
in the flask and stirred continuously for 6 h at 60oC. The reaction
mixture
was cooled to 40oC, and 60.6 g of pentaerythritol triacrylate and
0.3 g of
4-methoxyphenol were added. The polycondensation reaction was
continued
for 5 h at 40 oC. The resulting polymer, UA1, was characterized by
1H
NMR(CDCl3) as follow : δ = 5.80 and 6.34 ppm (CH2=CHCO2-), 6.05
ppm
70
CO2H
CH3
CO2H
CH3
NCO
CO2H
CH3
3 UA1 :
CO2 -
CH3
Scheme 4.1 The synthetic route of UV curable urethane-acrylate
polymer
(U; urethane group)
72
(-OCH2CH2CH2CH2O-)n and 1.55 ppm (-OCH2CH2CH2CH2O-)n as shown
in
Figure 4.1.
330 g of the UA1 (68.1 wt% in acetone), and 10.12 g of Darocure
1173
were charged to a reactor equipped with an electric stirrer, and
neutralized
with 5.57 g of dimethyl ethanolamine. Deionized water (1500 g) was
added
slowly into the mixture, which was agitated violently to disperse
the polymer
into the water phase to form a stable dispersion having a resin
concentration
of 12.5 wt%. The particle size of the polyurethane dispersion was
57 nm, as
measured by a Coulter N4 Plus submicron particle sizer (Beckman
Coulter).
Three additional dispersions containing EB-9970 at weight
percentages of
25%, 50%, and 80% were prepared without changing the resin
concentration
or the degree of neutralization. UA2 dispersions terminated
with
2-hydroxyethyl acrylate instead of pentaerythritol triacrylate were
also
synthesized by the same procedure.
4.2.3 Electrodeposition of Photo Curable Dispersion
Anode panels were cut from nickel-plated ASB
(acrylonitrile-butadiene-styrene). They had an area of 130 cm2,
while the area
of the stainless steel counter electrode (cathode) was 50 cm2.
The
anode-to-cathode distance was 10 cm. The bath was stirred at a
moderate
rate, by mechanical means. After electrodeposition with 40 voltage
for 2 min,
the coating was rinsed with distilled water and exposed to the
radiation of a
73
200 W/inch2 medium-pressure mercury lamp, in the presence of air,
at a
scanning speed of 6 m/min (light intensity: 970 mJ cm-2).
4.2.4 Real-Time FT-IR
Real-time FT-IR spectra were obtained using a React IR 1000
spectrometer (ASI Applied System) equipped with a diamond
composite
insertion probe. The samples were exposed for 30 sec to the UV
irradiation
from a metal halide lamp (EFOS UV System), via a fiber optic light
guide.
The UV light intensity was 4650 mW/cm2 and the wavelength range was
320
to 410 nm.
The photo-DSC experiments were conducted using a differential
scanning
calorimeter equipped with a photocalorimetric accessory (TA
5000/DSC 2920).
The initiation light source was a 200-W high-pressure mercury lamp,
and the
UV light intensity was 80 mW/cm2. The wavelength range was over 285
to
440 nm and the samples were placed in uncovered aluminum pans.
TA
instruments software was employed to process the data from the
photo-DSC
experiments.
74
UV curable urethane-acrylate polymers were prepared by
modifying
conventional procedures as indicated in Scheme 4.1. The
polycondensation
reaction of poly(tetramethyleneglycol), dimethylolpropionic acid
and IPDI with
DBTDL was carried out in acetone at 60oC. In order to investigate
the
curing efficiencies, two different acrylate groups were introduced
to the
terminals of the poly(urethane) main chains by subsequent reaction
with
pentaerythritol triacrylate and 2-hydroxyethyl acrylate.
Uretane-acrylate
introduced with pentaerythritol triacrylate (UA1) has six acrylate
groups at the
poly(urethane) chain ends. UA1 exhibited molecular weights of Mn =
4,500
and Mw = 8,000 and a vinyl group content of 1.36 mEq/g. On the
other
hand, urethane-acrylate polymer (UA2) terminated with
2-hydroxyethyl acrylate
has two acrylate groups at of poly(urethane) chain ends. UA2
exhibited
molecular weights of Mn = 4,100 and Mw = 7,300 and a vinyl
group
content of 0.45 mEq/g. Hirose et al. reported that the content of
vinyl bonds
must be higher than a threshold value (~2.0 mEq/g) in order to
attain a
satisfactory methylethylketone (MEK) resistance [53]. In our
experiment, the
vinyl group contents of both UA1 and UA2 were too low to provide
MEK
resistance. Therefore, three additional dispersions containing 25
wt%, 50, wt%
and 80 wt% of EB-9970, as a multifunctional acrylate oligomer,
were
prepared in UA1 and UA2 respectively, and compared with those
without
75
4.3.1 Particle Size Control
Stable dispersions were obtained by the 100 % neutralization of
the
dimethylol propionic acid groups of UA1 and UA2 with dimethyl
aminoethanol. Darocure 1173 and EB-9970 were used as a liquid
type
photoinitiator and as a multifunctional acrylate, respectively. The
content of
the photoinitiator was fixed at 4.4 wt% of the total solid weight
of the
dispersion. The neutralized uretane-acrylates containing Darocure
1173 and
EB-9970 could be used to produce stable dispersions without any
phase
separation, until the content of EB-9970 reached 80 wt%. The
particle sizes
of these polyurethane dispersions based on UA1 were around 57 nm,
and
grew with the incorporation of EB-9970 as shown in Figure
4.2.
4.3.3 Electrodeposition Behaviors of Photo Curable
Urethane-acrylates
Figure 4.3 shows a schematic representation of the AED process,
displaying
the migration of the anionic polymer dispersion. During the
process, the
ionized polymer that is dispersed in deionized water is deposited
on the
metal surface and most of water is expelled from the coagulated
coating film
through electro-osmosis. In our previous works, it was observed
that the
residual water content of the deposited films was in range of
20-30%
(wt./wt.) [37]. Such relatively low water contents in the
electrodeposition
films is an advantage of the short flash-off zone procedure
over
76
UA1
Figure 4.2 The particle size distribution of UV curable
urethane-acrylate
dispersion
77
The electrodeposition behaviors of the anodic urethane-acrylate
dispersions
were directly influenced by the polymer concentration of
dispersion, the
applied voltage, and the hydrophilicity of the polymer. Figure 4.4
shows the
electrodeposition yields of anionic urethane-acrylate dispersions
at 40 direct
current voltages as a function of the content of EB-9970.
Increased
concentrations of EB-9970 in the dispersion produced higher
deposited yields
under the same applied voltage, as shown in Figure 4.4. This can
be
explained by that EB-9970, which is a low-molecular-weight
oligomer, acts as
a coalescing agent to increase the conductivity of the deposited
film. Figure
4.4 also shows that the deposited yield of UA1 is higher than those
of UA2
with or without EB-9970. This may be because UA1 - in terms of
the
relatively low concentration of the poly(tetramethyleneglycol)
segment - is
more hydrophobic than UA2, thereby inhibiting redissolution of the
coagulated
polymer [29].
The electrodeposited films were cross-linked by optical radiation,
after the
flash-off process. The effect of incorporating EB-9970 into the
emulsion
particles was examined in terms of the MEK resistance of UV
cured
electrodeposited films with and without EB-9970. MEK double rubs
were
measured according to standard methods (ASTM D 4752).
78
Anode +
Cathod -
displaying the migration of anionic urethane-acrylate
dispersion
79
0 25 50 75
according to the EB-9970 contents
80
Figure 4.5 shows the correlation between the MEK resistance of UV
cured
films and the EB-9970 content in the dispersion. The MEK resistance
is
raised significantly when the content of EB-9970 is higher than 50
wt.%, and
UA1 gave higher MEK resistance than did UA2 with the same content
of
EB-9970. The higher acrylate functionality of UA1 at the terminals
of the
urethane-acrylate is effective in raising cross-linking density by
copolymerizing
with the pendent acrylate groups in the EB-9970. The MEK resistance
data
clearly show that three-functional acrylate, PETA, is a more
efficient coupling
agent than other mono-functional acrylates such as 2-hydroxyethyl
acrylate
and 2-hydroxyethyl methacrylate in the design of a UV curable
urethane-acrylate polymer. Therefore, the subsequent UV curing
kinetic studies
of the electrodeposited films focused on UA1 rather than UA2.
Typical real-time FT-IR spectra of UV exposed UA1 are shown in
Figure
4.6. The spectra are highlighted by the disappearance of the
811
cm-1characteristic bands of the C-H deformation modes of the acryl
group.
Since the decrease in intensity of this peak is attributable to the
gradual
disappearance of the acrylic double bond as the cure progresses,
the extent of
photopolymerization can be recorded spectroscopically in real time.
Figure
4.7 shows the conversion curves of photopolymerization for UA1
without
EB-9970, UA1-50, and UA1-80 containing 50 wt% and 80 wt%
EB-9970.
The conversion factor (α) was calculated at various intervals,
using the
following equation:
M E
K re
si st
an ce
(t im
Figure 4.5 The relationship between MEK resistance and EB-9970
content
82
( ) [ ] [ ] [ ] 100%
811
811811 × −
= o
t
I II οα --------- (1)
where [I811]o and [I811]t are the sample absorbances at 811 cm-1
before and
after UV exposure, respectively. Usually, relative intensity is
required
because the absolute intensity varies from sample to sample. Since
it is
difficult to find the reference peak from the infrared spectra of
the
cross-linking system, the progress of the photopolymerization was
deduced
based on the absolute intensities. The kinetics of
photopolymerization were
also analyzed using the method described by Decker and Moussa, and
the
following steady-state equation [41] :
αα (2)
where Rp is the rate of polymerization expressed as a percentage
per second,
, αt1 and αt2 are the conversion factor in the linear region of the
sigmoid
curve, and t1 and t2 are the times (in seconds) that correspond to
αt1 and αt2.
As shown in Figure 4.7, the overall rate of polymerization (Rp) as
well as
83
84
Rp = 14.5 %/sec
Figure 4.7 The conversion curves of photopolymerization for UA1,
UA1-50
and UA1-80
85
the final conversion factor (α) of UA1 is lower than those of
UA1-50 and
UA1-80. It is well known that the conversion factors of UV curable
systems
strongly depend on the mobility of the polymer [55]. In our system,
the high
viscosity of UA1 (due to its high molecular weight) presumably
influence the
mobility of the pendent acrylate groups, and consequently result
in
insufficient curing of the materials. On the other hand, the lower
molecular
weight of EB-9970 (Mn=600) can give a higher mobility than those
of
terminal acrylate groups of UA1, and possibly leads to ready
copolymerization with the terminal acrylate groups. Therefore,
the
incorporating of EB-9970 into UA1 raises the initial rate of
polymerization
and the conversion factors of the UV cured electrodeposited films.
As
mentioned above, it can be understood that incorporating a
multifunctional
acrylate oligomers increase the cross-linking density in the UV
cured
electrodeposited films, and thus improves the MEK resistance.
The curing behavior was also studied by a photo-DSC method, and
results
supporting the conclusions derived from real-time FT-IR were
obtained. The
heat flow obtained from the photo-DSC experiment can be also
converted
directly to the ultimate percentage conversion and rate of
polymerization for a
given sample. Figure 4.8 shows the photo-DSC exotherm curves for
the
photopolymerization of UA1, UA1-50 and UA1-80. The values of
exotherm
peak and conversion factor increase as more EB-9970 is added to the
coating
formulations. These results indicate that the inclusion of EB-9970
in the
dispersion decreased the viscosity of total formulation and
increased the
86
H ea
Figure 4.8 Photo-DSC exotherm curves for the photopolymerization of
UA1,
UA1-50 and UA1-80
diffusion mobility of propagating chains. Therefore, the initial
cure rate
became more rapid, and a high exotherm peak and conversion factor
were
obtained.
From these data, it is also apparent that EB-9970 acts as a bridge
during
cross-linking, leading to harder films.
88
In pursuit of broadening the application range of UV curable
electrodeposition, urethane-acrylate anionic dispersions were
studied for AED.
UV curable urethane-acrylate dispersions containing acrylate group
at the
terminals and anionic salts groups were prepared by a
sequential
polycondensation reaction of polyols and neutralization with an
amine. The
urethane-acrylate dispersions containing EB-9970, a multifunctional
acrylate
oligomer, had larger particle sizes and showed higher deposition
yield than
those of the dispersion without EB-9970. From MEK resistance
studies, it
was clearly that the three-functional acrylate, PETA, was a more
efficient
coupling agent than other monofunctional acrylates such as
2-hydroxyethyl
acrylate and 2-hydroxyethyl methacrylate in the design of a UV
curable
urethane-acrylate polymer. Real-time FT-IR and photo-DSC studies
showed
that the cross-linked films containing EB-9970 gave higher
conversion rate
than those without EB-9970, resulting in the better MEK
resistance.
89
Summary and Conclusions
The present popularity of UV radiation curing is due to a number
of
reasons. First, UV curables are comprised of 100% reactive
components,
which provide an environmentally acceptable coating. Second, UV
curing
systems are energetically efficient; they require a small fraction
of the power
normally consumed by thermally cured coatings. Finally UV curables
can be
easily formulated to meet a variety of applications since
functionalized
monomers and oligomers are available enable to adjust
properties.
However, UV curable adhesives still has a problem in adhesion of
complex
devices having recessed area not to reach the UV beam, and poor
outdoor
weatherability.
experiment were performed. And subsequent results were obtained. We
have
newly synthesized dual crosslinkable poly(methacrylate) copolymer
having
alkoxy silane group, fluorine atoms and vinyl group as a pendent
group for
optical adhesives by radical addition polymerization. The dual
curing
behaviors, e.g. thermal and UV cure, were studied by using
photo-DSC and
real time IR and an optimum adhesive formulation was obtained from
the
90
studies of curing behaviors. As the content of AO was increased in
optical
adhesive formulation, refractive index was decreased, but
transmittance was
increased due to the increasing of fluorine content. The addition
of colloidal
silica to the above formulation was helpful to the decreasing of
crosslinking
volume shrinkage and the increasing of glass fiber adhesion. With
the aim of
producing UV curable coatings for use in outdoor applications, a
specialized
reactive light stabilizer (containing a UV absorbance group, a
radical
scavenger, and acryl groups in the same molecule)was synthesized
and its
performance was investigated. The weathering performance of UV
curable
coatings significantly improved by using both a reactive light
stabilizer and
an optimized photoinitiator combination. The UV curing kinetics
experiments
revealed that a combination of Irgacure 819 and Darocure 1173 at
the ratio
1:3 was to be optimal when considering both cost and performance
of
photoinitiators. The weather resistance of the coated films was
quantitatively
evaluated by measuring the effect of QUV irradiation on the IR
spectra and
the yellow index. The exposure data results indicate that UV
curable coatings
containing the reactive light stabilizer would be useful in outdoor
applications.
In pursuit of broadening the application range of UV curable
electrodeposition, urethane-acrylate anionic dispersions studied
for AED. UV
curable urethane-acrylate dispersions containing acrylate group at
the terminals
and anionic salts groups were prepared by a sequential
polycondensation
reaction of polyols and neutralization with amine. The
urethane-acrylate
dispersions containing EB-9970, a multifunctional acrylate
oligomer, have
91
lagerer particle size and showed the higher deposition yield than
those of the
dispersion without EB-9970. From the results of MEK resistance, it
was
clearly seen that three-functional acrylate, PETA, was more
efficient coupling
agent than other mono-functional acrylates such as 2-hydroxyethyl
acrylate
and 2-hydroxyethyl methacrylate in the design of UV curable
urethane-acrylate polymer. The real-time FT-IR and photo-DSC
studies
showed that the crosslinked films containing EB-9970 gave higher
conversion
rate than those without EB-9970 resulting in the better MEK
resistance.
92
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96
UV
.
,
(AO) 2
.
photo-DSC real time IR . AO
15wt(g), 80 wt(g), 17
wt(g) 3wt(g). AO
