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Review
Recent Advances in Reactive ExtrusionProcessing of BiodegradablePolymer-Based Compositions
Jean-Marie Raquez, Ramani Narayan, Philippe Dubois*
This review reports on recent advances in the design of biodegradable polymers built frompetroleum and renewable resources using reactive extrusion processing. Reactive extrusionrepresents a unique tool to manufacture biodegradable polymers upon different types ofreactive modification in a cost-effective way. Partially based on our ongoing research, ring-opening polymerization of biodegradable polyesters will be approached as well as thechemical modification of biodegradable polymers, particularly natural polymers. The develop-ment of environmentally friendly poly-mer blends as well as (nano)compositesfrom natural polymers, including naturalfibers and nanoclays, through reactiveextrusion, as an efficient way to im-prove the interfacial adhesion betweenthese components, will be also dis-cussed.
Introduction
Reactive extrusion is an attractive route for cost-effective
polymer processing, which enhances the commercial
viability and cost-competitiveness of these materials, in
J.-M. Raquez, P. DuboisCenter of Innovation and Research in Materials & Polymers(CIRMAP), Laboratory of Polymeric and Composite Materials,Materia Nova & University of Mons-Hainaut, Place du Parc 20,B-7000 Mons, BelgiumE-mail: philippe.dubois@umh.ac.beR. NarayanDepartment of Chemical Engineering & Material Science, 2527Engineering Building, Michigan State University, East Lansing,MI-48824, USAJ.-M. RaquezDepartement Technologie des Polymeres et Composites, Ecoledes Mines de Douai, Rue C. Bourseul 941 B.P. 10838, 59508 DouaiCedex, France
Macromol. Mater. Eng. 2008, 293, 447–470
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order to carry out melt-blending, but also various chemical
reactions including polymerization, grafting, branching
and functionalization as well.[1,2] In addition, reactive
extrusion processes involve introducing reactive agents at
optimum points in the reaction sequence, homogenizing
the ingredients, and allowing sufficient time for the
completion of the reactions. In a typical reactive extrusion
process, the reactants are fed into the extruder, usually
through a feed hopper. However, various liquid or gaseous
reactants can be introduced at specific points in the
reaction sequence by using injection along the extruder
barrel. The reactive mixture is conveyed through the
extruder, and the reaction is driven to the desired degree of
completion. At this point, and after removing any volatile
by-products, the molten polymeric product is pumped out
through a die and subsequently quenched, solidified, and
pelletized. Therefore, production and processing can be
integrated in one-stage processing.[3,4]
The recent environmental regulations, societal concerns
and growing environmental understanding throughout
DOI: 10.1002/mame.200700395 447
J.-M. Raquez, R. Narayan, P. Dubois
Jean-Marie Raquez was born in Paris (France) in 1977. He graduated in Chemistry from the University of Mons-Hainaut (Belgium)in 1999. He received his PhD degree in Polymer Chemistry under the supervision of Professor Philippe Dubois (University ofMons-Hainaut) on the topic dealing with the controlled synthesis of a biodegradable poly(ester-alt-ether), poly(1,4-dioxan-2-one), through ring-opening polymerization. After a postdoctoral stay with Professor Ramani Narayan (MichiganState University, USA), he moved back to University of Mons-Hainaut as assistant research. In 2007 he was appointedAssociated Professor at ‘‘Ecole des Mines de Douai’’ in France. His research work focuses on the chemical modification andsynthesis of polymer-based (nano)composites issued from renewable resource.Ramani Narayan is University Distinguished Professor at Michigan State University in the Department of Chemical Engineering& Materials Science. He has 115 refereed publications in leading journals to his credit, 18 patents, edited three books and oneexpert dossier in the area of bio-based polymeric materials. Under his supervision, 20 students have obtained their Master’sdegree, ten students their Ph.D. degrees and six are working towards their Ph.D. He has major research programs with industryand serves as consultant for several companies.He has won several awards: Awarded the ‘‘University Distinguished Professor’’ title in 2007 highest and very selective honorthat can be bestowed on a faculty member by the university. Those selected for the title have been recognized nationally andinternationally for the importance of their teaching, research and public service achievements; Governors (State of Michigan)University Award for commercialization excellence; University Distinguished Faculty Award, 2006; Withrow DistinguishedScholar award, 2005 awarded to one faculty in the MSU College of Engineering based on exemplary research accomplishments,national & international recognition; Fulbright Distinguished Lectureship Chair in Science & Technology Management &Commercialization (University of Lisbon; Portugal); the William N. Findley Award for ‘‘significant contributions to theapplication of new technologies within the scope of ASTM Committee D20 on Plastic; Award of Excellence from ASTMcommittee D 20 on Plastics for exemplary technical contributions, sustained participation, and valued leadership; 2006. TheJames Hammer Memorial Lifetime Achievement Award, 2006 for outstanding leadership, and research accomplishments in thefield of Degradable Polymers from the BioEnvironmental Polymer Society (BEPS). Research and Commercialization Awardsponsored by ICI Americas, Inc. & the National Corn Growers Association.He is on the Board of Directors of ASTM International-a premier international standards setting organization. He chairs theASTM committee on Environmentally Degradable Plastics and Biobased Products (D20.96) and the Plastics Terminologycommittee D20.92. He is also the technical expert for the USA on ISO TC 61 on Plastics – specifically for Terminology, andBiodegradable plastics. Dr. Narayan also chairs the scientific committee of Biodegradable Products Institute (BPI), NorthAmericaa biodegradable and biobased plastics trade industry organization (www.bpiworld.org).He is a successful entrepreneur having been responsible for commercializing several technologies.Philippe Dubois graduated in Chemical Sciences from the Facultes Universitaires Notre-Dame de la Paix (FUNDP, Namur,Belgium) in 1987. He received his PhD degree in Chemical Sciences from University of Liege (ULg) in 1991. In the same year, heworked as a postdoctoral fellow for Dow Chemical (Terneuzen, Holland) and the Laboratory of Macromolecular Chemistry andOrganic Catalysis directed by Prof. Ph. Teyssie at ULg. Then he joined the National Fund for Scientific Research (FNRS) at ULg till1997. In 1994, he worked as visiting scientist at the Chemical Research Engineering Department of the Michigan State University(MSU). In Oct. 1997, he moved to University of Mons-Hainaut (UMH) where he obtained the chair of macromolecular chemistryand created/directed the Laboratory of Polymeric and Composite Materials (now ca. 35 people). He has co-authored over 300publications in international journals, 180 personal communications at conferences and is co-inventor of 40 patents. Heco-edited 6 books. He is full professor at UMH and invited professor at FUNDP, MSU, and Faculte Polytechnique de Mons (FPMs).He is Scientific Director at the Materia Nova Research Center in Mons and Director of the Center of Innovation and Research inMaterials & Polymers CIRMAP (with ca. 80 members). He is currently Past-President of the Belgian Royal Chemical Society (hewas the President in 2006/7).
448
the world have triggered renewed efforts in plastic
industry to develop new products and processes compa-
tible with our environment.[5–10] The design of biodegrad-
able plastics is an appropriately eco-efficiency approach to
enhance the environmental quality for many products.
Biodegradable plastics can be converted into useful and
friendly environmental products to minimize the waste
disposed in landfills. Different markets are found in the
realm of the biodegradable polymers, including packaging
(trash bags, wrappings, loose-fill foam, food containers and
laminated papers), disposable non-woven (engineered
fabrics), hygiene products (diaper back sheets and cotton
swabs), consumer goods (fast-food tableware, containers,
Macromol. Mater. Eng. 2008, 293, 447–470
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egg cartons, razor handles and toys) and agricultural tools
(mulch films and planters).
Unfortunately, the utilization of biodegradable poly-
mers as bulk commodity materials is still restricted to few
applications because of the strong cost-competition with
cheaper petroleum-based polymers, and their limited
thermo-mechanical properties.[8,11] To permit their com-
mercial scale-up, an appropriate, inexpensive and easy
process to manufacture biodegradable polymers is
highly desirable for their commercial viability and cost-
competitiveness. Besides, while maintaining their overall
biodegradability, melt-blending through a reactive way
these biodegradable polymers with inorganic/organic
DOI: 10.1002/mame.200700395
Recent Advances in Reactive Extrusion Processing . . .
fillers as well as other biodegradable polymeric materials
may represent an additional way of reducing the overall
cost for the resulting materials, but also to modify their
thermo-mechanical properties and (bio)degradation rates
efficiently. Developing such biodegradable polymeric
melt-blends/composites with satisfactory overall thermo-
mechanical behavior however requires the ability to
control interfacial energy, to generate dispersed phases
of limited size and strong interfacial adhesion, and to
improve the stress transfer between the component
phases.[12] This can be effectively completed using proper
interface compatibilization between these different com-
ponents during their reactive processing.[13] Undoubtedly,
reactive extrusion (coined REX) serves on all these issues in
the manufacture of high-performance and inexpensive
biodegradable polymeric materials using a one-stage
continuous reactive processing.
Hence, this review aims at highlighting the recent
developments of novel biodegradable polymeric materials
using REX as an efficient processing technique, partially
based on our ongoing research over the past few years. The
production of biodegradable polymers through REXwill be
described, as well as the reactive modification and melt
blending of biodegradable polymers in the preparation of
useful and environmentally friendly products.
Reactive Extrusion Processing
Figure 1. Schematic representation of a reactive extrusion process including a typical screw forpolymerization of CL under inert atmosphere.
Reactions that previously re-
quired heavy equipments, parti-
cularly with batch operations,
can be completed in a more
efficient continuous way through
REX. Extruders have been used to
resolve heat and mass transfer
problems that arise when dra-
matic viscosity of the reaction
medium (when monomer is con-
verted to polymer) increases
within a magnitude order of 105
in batch polymerization pro-
cesses.[14,15] In a batch reactor,
as the polymerization proceeds,
the viscosity increases and after
a certain point the material
becomes unmanageable in terms
of mixing and heat transfer. In
this respect, REX shows to be a
promising technique for polymer
processing. The ability of these
extruders to create new thin sur-
face layers continuously can
increase the degree of mixing
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and minimize temperature gradients within the polymer
being processed. Another factor that can be controlled via
operating conditions and geometrical specifications of
screw extruders is the residence time in the system.
Generally, the residence time is substantially lower as
compared to that required in a batch reactor for the same
reaction; reducing therefore long exposure to high tem-
peratures that can cause polymer degradation. The ability
of an extruder to handle materials of high viscosities
without any solvents results in a dramatic cost reduction
in raw materials and in solvent recovery equipments, and
polymers produced are in a ready-to-use form. Finally, for
polymer modification reactions, reactive extrusion pro-
cesses offer natural means for polymerization, chemical
cross-linking and grafting via the inherent capability of
stage feeding of reactive agents (Figure 1). This way, one
can tailor-make specialty polymers that are uneconomical
to produce in large scale operations.
The design of extruders for REX applications involves
the manipulation and integration of information and
knowledge from several distinct areas. Extruders for
reactive processing must deal with the continuously
changing nature of the reactive melt. Mixing phenomena
are considerably delayed when the reactive mixture is
highly viscous, leading to important gradient in chemical
composition and temperature that affect the product
quality. Mixing is an important factor when using an
extruder as a reactor. Especially longitudinal mixing must
www.mme-journal.de 449
J.-M. Raquez, R. Narayan, P. Dubois
450
be given significant consideration for radical reactions
since back-mixing can influence the residence time
distribution, that is the course of the reaction, and can
affect the molecular weight distribution (MWD) of the
final product.[20]
Although both single and twin-screw extruder config-
urations are used in REX processes, twin-screw ones are
increasingly being favored over the single-screw ones.
Themain reasons for this are the extended control of residence
time distribution and mixing, but also their superior heat
andmass transfer capabilities. The single-screw extruder is
currently utilized for simpler jobs likemelting, plasticizing,
and discharging melt for the production of films, pipes,
profiles, etc. The twin-screw extruders, according to their
specific characteristics, can tackle the more complex tasks
such as homogenizing, dispersing pigments and additives,
alloying, reactive compounding, concentrating, devolati-
lizing, polymerizing, etc. The major difference between
single- and twin-screw extruders is the conveying
mechanism. Although in single-screw machines, it
depends on frictional forces in the solids conveying zone
and viscous forces in the melt-pumping zone, in twin-
screw extruders it is largely dependent on the screw
geometrical configuration, and it is of a positive displace-
ment character. The relativemerits and the performance of
twin-screw extruders have been appraised.
Depending on the direction of rotation of the two
screws, twin-screw extruders can be distinguished in
co-rotating and counter rotating machines. In simple
terms, it can be said that co-rotating screws have a radial
and counter-rotating have an axial shearing and plasticiz-
ing effect. Although each type of twin-screw extruder has a
certain uniqueness regarding ingredients, type of reaction,
and polymer produced, and although no machine design
(counter- or co-rotating) provides the complete solution,
co-rotating intermeshing twin-screw extruders have been
found to be suitable for many continuous REX processes.
Compared with the counter-rotating twin-screw extruders
where additional radial forces are present, the two screws
for co-rotating intermeshing twin-screw extruders are set
side by side with minimum clearance between them. The
crest of one screw completely wipes the flights and the
root of the other one. This self-wiping feature eliminates
dead regions where material can stagnate during proces-
sing. Due to the co-rotating design of the screws, high
speeds, thus strong shearing forces, and high outputs can
be obtained, enabling the material to be transferred from
one screw to the other under a constant mixing. In
addition, the intensive and constant surface renewal
creates favorable degassing conditions.
The modular design and assembly arrangement of the
screw and barrel sections of twin-screw machines, along
with the use of special feeding and venting ports provide
adequate flexibility for specific reactive extrusion tasks.
Macromol. Mater. Eng. 2008, 293, 447–470
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The sequence of screw elements is of prime importance for
the control of the filling ratio inside the extruder. Three
types of elements are widely used in co-rotating inter-
meshing twin-screw extruders: kneading blocks, mixing
gears, and conveying screw elements. Kneading blocks are
primarily used for the dispersion of partially melted
polymer particles and solid additives or reactive agents
into themelt. Mixing gears are superior to kneading blocks
for the thorough distribution of finely divided particles, for
achieving isothermal conditions at a given location in the
barrel, and for the homogenization of two or more feed
streams. Conveying elements are the ones that move
the material through the extruder. The arrangement of
the screw elements is what determines the residence time
distribution in twin-screw extruders. The maximum
practical residence time capability of a typical co-rotating
intermeshing 45:1 Length/Diameter twin-screw extruder
is about 7min, at low screw speeds and with short-
pitch elements. Designs with even longer residence times
(10–75min) are also available. Recently, micro-
compounding using reactive extrusion technology has
emerged as a efficient way of handling small amount of
materials, leading to the preparation of expensive
materials such as (nano)composites, biopolymers or
pharmaceuticals at limited cost.[21]
According to its unique characteristic features, the
reactive extrusion processing technology has provided
different types of chemical reactions:[14,20–31]
a. F
ree radical, anionic, cationic, condensation, andcoordination polymerizations of monomers or oligo-
mers to high molecular weight polymers.
b. C
ontrolled degradation and cross-linking of polymersby means of a free radical initiator for preparing a
product with controlled molecular weight distribution.
c. F
unctionalization of commodity polymers for produc-ing materials to be used in grafting applications.
d. P
olymer modification by grafting of monomers ormixture of monomers onto the backbone of existing
polymers for improving various properties of the
starting materials. Free radical initiators and ionizing
radiation can be used to initiate the grafting reactions.
e. I
nterchain copolymer formation: Usually, this type ofreaction involves combination of reactive groups from
several polymers to form a graft copolymer.
f. C
oupling reactions that involve reaction of a homo-polymer with a polyfunctional coupling agent/filler in
the preparation of high-performance products.
Many authors have largely reviewed different synthetic
technology for the preparation of biodegradable polymers,
as well as their types of applications ranging from daily
applications to biomedical ones.[6–8,32] Within the scope of
this review, we will focus on the use of reactive extrusion
DOI: 10.1002/mame.200700395
Recent Advances in Reactive Extrusion Processing . . .
in the design of biodegradable polymeric materials with
useful properties. First, the reactive extrusion synthesis of
biodegradable polymers, particularly aliphatic polyesters
obtained through ring-opening polymerization (ROP) of
cyclic (di)esters will be discussed. These synthetic biode-
gradable polymers have attracted much attention owing
to their physico-chemical properties that can be readily
tailored for more specific applications. Besides, chemical
modification and reactive melt blending of synthetic bio-
degradable polymers, as well as those derived from renew-
able resources will be discussed, again carried out through
REX for the manufacture of useful and fully biodegradable
products. Finally, a special attention will be paid to biode-
gradable polymeric composites, and particularly the
(nano)composites. The reinforcement of biodegradable
polymers using (nano)fillers, particularly layered silicates,
have recently emerged as high-performance materials
having interesting mechanical and barrier properties
achieved at low filler content (less than 5 wt.-%).[33]
Reactive Extrusion (Co)polymerization ofCyclic (Di)esters
Among the synthetic biodegradable polymers, aliphatic
polyesters such as poly(e-caprolactone) (PCL) and polylac-
tides (PLAs) have drawn a lot of interest from both the
academic and industrial media, whose potential applica-
tions cover such widely different fields as packaging for
industrial products, mulching films in agriculture, bio-
resorbable materials for hard tissue replacement and
controlled drug delivery devices.[6,34] PCL has been
thoroughly investigated because of the possibility of
blending this aliphatic polyester with a number of
miscible commercial polymers such as PVC and bisphenol
A polycarbonate. PCL is a highly hydrophobic, biodegrad-
able and semi-crystalline polyester with melting and glass
transition temperatures of ca. 60 8C and �60 8C, respec-tively.[35–37] The homopolymer of L-lactide (or D-lactide),
poly(L-lactide) (or poly(D-lactide)), is semi-crystalline with
melting and glass transition temperatures of ca. 175 8C and
60 8C, respectively. Poly(L-lactide) (PLA) is biocompatible,
and degrades by hydrolytic scission to lactic acid, which is
a natural intermediate in the carbohydrate metabolism.
High tensile strength and low ultimate elongation make
that poly(L-lactide) is rather used for producing porous
scaffolds and load-bearing applications such as in
orthopedic fixations and sutures.[38,39]
Interestingly enough, poly(1,4-dioxan-2-one) (PPDX)
appears to be an attractive candidate as a biodegrad-
able substitute for commodity polymers. This aliphatic
poly(ester-alt-ether) copolymer offers a good compromise
between its processing temperature and the service
Macromol. Mater. Eng. 2008, 293, 447–470
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temperature range with a melting and glass transition
temperatures of 110 8C and –10 8C, respectively.[40–44]
Aliphatic polyesters can be produced by two different syn-
thetic pathways: step polycondensation of a,v-hydroxyacids
and ROP of cyclic (di)esters. However, the traditional
polycondensation usually requires high temperatures,
long reaction time and a continuous removal of water
to recover quite lowmolecular weight polymers with poor
mechanical properties finally. In contrast, ROP provides a
direct and easy access to the corresponding highmolecular
weight polyester within a few minutes.
Both aluminum mono- and trialkoxides have shown to
be very effective initiators[45–51] to promote the ROP of
various (di)lactones such as e-caprolactone (CL) with high
selectivity (restricted occurrence of termination and
transfer reactions). This allows the preparation of high
molecular weight polyesters and a huge range of novel
macromolecular architectures both in solution and in bulk
(i.e., in absence of any solvent). Next to the aforementioned
aluminum derivatives, tin (II, IV) alkoxides and carboxy-
lates (the latter coupled with alcohols as co-initiators), and
more particularly tin(II) bis(2-ethylhexanoate)
(Sn(Oct)2)[52] have been widely used. Such commercial
catalysts can be more readily handled (i.e., do not require
high vacuum equipment), and are relatively easy to purify
(at least down to ca. 2 mol% of proton containing
impurities) by distillation for semi-quantitative synthetic
work,[49] Furthermore, Sn(Oct)2 has been approved as a
food additive by FDA. The most advocated mecha-
nism[50,51] involves a direct catalytic action of Sn(Oct)2.
Actually, Sn(Oct)2 has been first proposed to activate the
monomer forming a donor-acceptor complex, which
further participates directly in the propagation step.
Sn(Oct)2 is liberated in every act of propagation. It follows
from this mechanism that Sn(II) atoms are not covalently
bound to the polymer at any stage of polymerization.
Recently, Penczek et al. have proposed another more likely
scheme proceeding via the ‘‘active chain-end’’ mechanism.
This last one involves the in-situ formation of Sn-alkoxide
bonds at the chain-ends, as observed by MALDI-TOF and
fully confirmed by kinetic studies.[49,52] Thus, through a
rapid exchange equilibrium, Sn(Oct)2, and most probably
any other covalent metal carboxylates, are first converted
by reaction with protic compounds (ROH) into tin (or other
metal) alkoxides as active centers for polymerization
(Figure 2). The polymerization involves a ‘‘coordination-
insertion’’ mechanism similarly to the previously dis-
cussed mechanism for covalent metal alkoxides and
dialkylaluminum alkoxides (Figure 3).
It is worth noting that we have recently and succinctly
reported on ROP of cyclic (di)esters, and to some extent, to
the chemical modification of biodegradable aliphatic
polylactones through reactive extrusion processing.[53]
However, the scope of that mini-review was restricted
www.mme-journal.de 451
J.-M. Raquez, R. Narayan, P. Dubois
Figure 2. Proposed activation mechanism for catalyzed ROP ofe-caprolactone promoted by Sn(Oct)2.
452
only to our own expertise, without referring to the most
relevant results obtained elsewhere. By contrast, the
following section will summarize in a more systematic
way the main advances in REX obtained by the different
research groups that are involved in the field.
Reactive Extrusion Ring-Opening (Co)polymerizationof e-Caprolactone
PCL is an aliphatic polyester currently prepared by ROP of
CL catalyzed with stannous octanoate (Sn(Oct)2) in
the presence of heavy alcohol (initiator) such as
1-dodecanol,[54] using a batch process with a maximum
number-average (Mn) of 80 kg �mol�1, andmarketed under
the trade names ‘‘TONE’’ and ‘‘CAPA’’ by Dow Chemicals
and Solvay, respectively. However, these PCL polymers
suffer from poor processing characteristics like low
melt-strength, and for certain applications, inadequate
thermo-mechanical properties like tear strength.
Figure 3. ‘‘Coordination-insertion’’ mechanism of the ROP of CL.
Macromol. Mater. Eng. 2008, 293, 447–470
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Kim at al. reported bulk ROP of CL carried out both in a
laboratory internal mixer (Brabender Plasticorder) and in a
modular intermeshing co-rotating twin-screw extru-
der.[55–57] Various initiators such as titanium n-butoxide,
aluminum triisopropoxide, and sodium hydride were first
used to polymerize CL in an internal mixer. High
conversion in PCL could be effectively obtained when
aluminum triisopropoxide initiated the polymerization of
CL. This polymerization was then investigated through
reactive extrusion processing for various ratios of
monomer to initiator using aluminum triisopropoxide
(Al(OiPr)3) as initiator as well as under a range of different
processing conditions, including barrel temperature pro-
files, throughput, and screw speed. GPC analyses demon-
strated that high molecular weight PCL, together with
significant quantities of PCL oligomers were produced at
different reaction temperatures. Increasing screw speed
and decreasing the throughput caused a severe reduction
in the PCLmolecular weight once themaximummolecular
weight was obtained after the first mixing segment.
Higher molecular weight PCL produced by increasing
ratios of monomer to initiator resulted in a more severe
reduction of the molecular weight during reactive extru-
sion. This was ascribed to both specific mechanical energy
(i.e. the energy consumed per unit of mass of the material
extruded) and the molecular weight.[57,58] A more detailed
study about rheological developments for reactive proces-
sing of PCL upon both experimental and modeling aspects
has been reported elsewhere,[16–19] and has recently been
reviewed in International Polymer Processing.[60–63]
Recently, we have reported that alumi-
num sec-butoxide (Al(OsecBu)3) was a more
suitable initiator in the reactive extrusion
ROP of CL. Although Al(OiPr)3 has shown to
efficiently initiate the ROP of CL, it requires
being sublimated first, and then dissolved
in an organic solvent like toluene in order
to control the ROP of CL. In contrast,
Al(OsecBu)3 is commercially available as a
pure liquid, and therefore does not require
any previous purification step before use.
In addition, Al(OsecBu)3 has shown to be an
efficient initiator in the bulk ROP of CL
carried out in small reactors.[64,65] Inter-
estingly, when Al(OsecBu)3 was used as
initiator in reactive extrusion ROP of CL, a
well-controlled synthesis of PCL in terms of
molecular weight and polydispersity
(Mw=Mn � 1.7) was successfully obtained
at a temperature ranging from 130 to
180 8C in an intermeshing twin-extruder
with the screw configuration made up
with only conveying elements. The con-
veying elements were selected to avoid as
DOI: 10.1002/mame.200700395
Recent Advances in Reactive Extrusion Processing . . .
Figure 4. Three-arm star-shaped PCL as prepared by ROP of CL initiated with Al(OsecBu)3in reactive extrusion.
much as possible the undesirable thermal degradation
reaction that occurs when kneading elements are used.
Like Al(OiPr)3, the polymerization of CL promoted by
Al(OsecBu)3 proceeds through the so-called ‘‘coordination-
insertion’’ mechanism, yielding polyester chains end-
capped by a growing aluminum alkoxide bond (see
Figure 3).[64,65] Due to the trifunctionality of Al(OsecBu)3,
three-arm star shaped PCL with Mn of each arm as high as
200 kg �mol�1 could be prepared with monomer conver-
sions larger than 95% and within residence times of less
than five minutes in the extruder (Figure 4). In blown film
applications, the resulting PCL displayed significant better
dart and tear properties than commercially available
linear PCL.
As potential compatibilizing agents in polyamides/PCL
blends, lactam-CL block copolymers were prepared
through reactive extrusion as well.[66,67] Continuous
copolymerization of CL with e-caprolactam (CLa) and
v-lauryl lactam (LLa) were carried out in a modular
intermeshing co-rotating twin-screw extruder. Sodium
hydride (NaH) and N-acetyl caprolactam were employed,
respectively, as co-initiators in the synthesis of lactam-
lactone copolymers. In the presence of N-acetyl caprolac-
tam, it is considered that CLa is unable to act as a faster
activator for polymerization of CL than N-acetyl capro-
lactam as described by Goodman et al.[68] In this respect,
NaH was added as a co-initiator together with N-acetyl
caprolactam for polymerization of CLa before the reaction
of sodiocaprolactam with CL. It has been demonstrated
that the order of addition is of prime importance for the
formation of blocky copolymer. Simultaneous feeding of
bothmonomers with NaH and N-acetyl caprolactam in the
first hopper of the twin-screw extruder produces amixture
of homopolymers. In contrast, both high molecular weight
P(CLa-b-CL) and P(LLa-b-CL) block copolymers have been
successfully achieved by adding the lactam (LLa and CLa)
into the first hopper and the CL sequentially into
the second hopper. The respective block lengths of the
copolymer could be adjusted by controlling the feed rate of
each monomer during reactive extrusion. To modulate the
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stiffness of lactam part, lactone-lactam
terpolymers having block and random
caprolactam structure, i.e. P(LLa-b-
CLa-b-CL) and P(LLa/CLa-b-CL) have
also been prepared through the same
procedure.
For seat belt applications, new fibers
derived frompoly(ethylene terephthalate)-
block-PCL block copolymers have been
developed through REX.[69] Such fibers
provide the desired load-limiting perfor-
mance for the design of safety seat belt.
The synthesis of poly(ethylene ter-
ephthalate)-block-PCL block copolymers
was carried out by ROP of CL initiated by hydroxyl-
terminated poly(ethylene terephthalate) (PET) together
with Sn(Oct)2. A block copolymer with minimal transes-
terification reactions could be obtained in reactive
extrusion at 290 8C because of the fast distributive mixing
of CL into the high melt-viscosity PET and the short
reaction time. After fiber spinning, these PET-b-PCL block
copolymers exhibited high degree crystalline orientation.
Reactive Extrusion Ring-Opening (Co)polymerizationof L,L-Lactide
A tremendous demand is raising on the synthesis of PLA as
obtained by ring-opening polymerization of L,L-lactide (LA)
because LA derives from renewable resources, avoiding the
depletion of petrochemical resource, and the emission of
green gas (CO2). The cyclic dimer of lactic acid is recovered
after fermentation of corn or sugar beets followed by a
free-solvent polymerization/depolymerization process
(Figure 5).
Interestingly, a continuous one-stage reactive extru-
sion[70–74] has been reported by some of us on the
manufacture of economically viable PLA via ROP of LA
promoted by Sn(Oct)2. This technique requires that the
bulk polymerization be close to completeness within a
very short time (5–7 min), which is predetermined by the
residence time distribution of the extrusion system, and
that the PLA stability is high enough at the high processing
temperature. Although Sn(Oct)2 can promote quite fast
polymerization of LA, it is well-known to provide adverse
effects on both the molecular weight and properties of PLA
as a result of back-biting and intermolecular transester-
ification reactions, not only during the polymerization of
LA, but also during any further melt-processing.[75,76] In
this respect, an equimolar amount of triphenylphosphine
(P(C6H5)3) has been added to Sn(Oct)2 in order to enhance
the rate of LA polymerization significantly, but also to
suppress (or at least delay) any degradation reactions such
as transesterification reactions.[70,71] This kinetic effect has
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J.-M. Raquez, R. Narayan, P. Dubois
Figure 5. PLA production via prepolymer and lactide.
454
been accounted for the coordination of the Lewis base onto
the tin atom, making easier the insertion of the monomer
into the metal alkoxide bond of the initiator/propagation
active species. This tin alkoxide bond is formed in situ by
reaction of alcohol and the tin(II) dicarboxylate, and
proceeds through the aforementioned ‘‘coordination-
insertion’’ mechanism.[50,51] The addition of one equiva-
lent of P(C6H5)3 onto Sn(Oct)2 allows reaching an
acceptable balance between propagation and depolymer-
ization rates, so that the polymerization is fast enough to
be performed through a continuous one-stage process in
an extruder.
Using a closely intermeshing co-rotating twin-screw
extruder (with a suitable processing and screw concept),
the equimolar Sn(Oct)2/P(C6H5)3 complex was used as a
catalyst system in ROP of LA yielding high molecular
weight PLA within a residence time of ca. 7 min at high
monomer conversions (ca. 98%) and at a temperature of
about 180–185 8C. Adding alcohol as (co)initiating system
could easily adjust molecular weights of as-recovered PLA.
In addition, by reducing the amount of the catalytic
complex ([LA]0/[Sn]¼ 5 000), the resulting PLA exhibited
good melt-stability during further melt processing such as
melt-spinning. For more specific applications, the
melt-stability for the resulting PLA[77,78] could be further
enhanced by adding stabilizers (Ultranox 626) during
reactive extrusion, without influencing the course of the
polymerization reaction. Interestingly enough, some of us
demonstrated that reactive extrusion processing could
enhance much more the kinetics of ROP of LA in bulk than
the conventional batch polymerization technology such as
glass reactors. Indeed, although the conversions in LA
reached the same equilibrium values (ca. 98%) whichever
the polymerization system, the time required for reaching
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this monomer conversion was
approximately 40min using glass
ampoules as reactor, compared to ca.
7min in the reactive extrusion pro-
cess. At lowmonomer conversion, the
relative enhancement on polymeriza-
tion kinetics can be ascribed to the
molten temperature of reaction me-
dium inside the extruder, being a
key-parameter in reactive proces-
sing.[59] In batch polymerization,
depending on the extent of polymer-
ization, there is a temperature gradi-
ent with a bad thermal transfer for
the reaction medium, leading to
longer reaction times. By contrast,
the heat transfer is better due to high
mixing phenomena in extruders.[60,61]
However, at high monomer conver-
sion, the rate of the reaction gets
limited not only by the molten temperature and the
reactivity of the chemicals, but also by the diffusion of the
monomers (and other low molecular weight compounds)
inside the high viscous melt to find a reactive partner,
when high molecular weight polymer is reached. This
physical movement is limited to the Brownian movement
in the glass ampoule, but is well supported in the
twin-screw extruder by the mixing elements and by the
shearing of the polymer inside the intermeshing zone.
The preparation of block copolymers based on PLA has
also been carried out in a co-rotating twin-screw
extruder.[73,74] Different lengths of v-hydroxylated pre-
polymers such as PCL or poly(ethylene oxide) (PEO) as
macroinitiators enable to prepare a multitude of possible
block polymers with the same processing concept and
equipment. Other authors have attempted to prepare
multi-block copolymers based on PLA through a controlled
number of transesterifications.[79] In the first attempt,
various catalysts (nBu3SnOMe, Sn(Oct)2, Ti(OBu)4, Y(Oct)3,
and para-toluene sulfonic acid) were added to promote
these transesterification reactions of PCL against PLA and
PEO prepolymers. In blends of PLA and PCL (50:50 by
weight), the use of nBu3SnOMe was reported to catalyze
the transesterification reactions between PLA and PCL. If
2wt.-% in nBu3SnOMe was added to the blend, some
transesterification reactions occurred during reactive
extrusion, but substantial PLA degradation took place also
as evidenced by the formation of large amounts of LA
monomer (�12wt.-%). Smaller amounts in nBu3SnOMe
were not effective in promoting transesterification under
these reaction conditions. The use of the other studied
catalysts, i.e. Sn(Oct)2, Ti(OBu)4, Y(Oct)3, and para-toluene
sulfonic acid, revealed that no sufficient transesterification
reactions occurred, and in all cases the PCL was recovered
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unaffected, while PLA degradation was observed as
indicated by the formation of LA and fast decrease in
molecular weight. In this respect, Stevels et al. have
preferred to effectively prepare multi-blocky PLA-b-
PCL-b-PLA and PLA-b-PEO-b-PLA terpolymers with moder-
ate molecular weights at 180 8C by reactive extrusion ROP
of LA initiated with a,v-hydroxyl PCL and PEO in presence
of Sn(Oct)2. High yield (more than 90wt.-% in LA converted
into polymer) could be achieved within a few minutes.
Reactive Extrusion Ring-Opening (Co)polymerizationof 1,4-Dioxan-2-one
Poly(1,4-dioxan-2-one) (PPDX) obtained by ROP of
1,4-dioxan-2-one (PDX) appears to be another attractive
candidate as a biodegradable substitute for commodity
polymers. This aliphatic poly(ester-alt-ether) copolymer
offers a good compromise between its processing tem-
perature and its service temperature range because of a
melting and glass transition temperatures of 110 8C and
�10 8C, respectively.[40,41] Furthermore, PPDX has proven
to be tougher than polylactides and even HDPE with a
tensile strength close to 50 MPa for an ultimate elongation
ranging from 500% to 600%. The use of PPDX materials as
bulk materials is however restricted as a result of its low
ceiling temperature (265 8C). This favors the unzipping
depolymerization reactions from the hydroxyl end-group
in the molten state (Figure 6).
Again, like other cyclic esters such as CL, aluminum
alkoxides can efficiently promote ROP of PDX, which offers
the possibility for the synthesis of high molecular weight
PPDX through a fast and continuous process in a twin-
screw extruder.[41] Besides these kinetic considerations,
developing PPDX as biodegradable thermoplastics requires
reducing its thermal degradation essentially due to
unzipping reactions, while preserving as much as possible
the semi-crystalline properties of PPDX (high melting
temperature slightly above 100 8C). Therefore, simulta-
Figure 6. Unzipping depolymerization mechanism of v-hydroxyl PPD
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neous bulk (co)polymerization of PDX promoted by
Al(OsecBu)3 (([PDX]0þ [CL]0)/[Al]¼ 2 500) was carried out
with lowmolar fractions in CL ranging from 0 to 16 mol-%
in a co-rotating twin-screw extruder at 130 8C. Indeed,random distribution of a few units of CL all along the PPDX
backbone[43] represents the best way to prevent or at least
limit undesirable unzipping reactions of PPDX chains. As
far as the homopolymerization of PDX is concerned, the
polymerization conversion did not exceed more than 65%,
corresponding to the equilibrium value obtained at 130 8C,in a good agreement with previously published data
performed in bulk and in sealed glass ampoules.[41]
Interestingly, with a limited amount in CL, the PDX
(co)polymerization yield reached the completion, attesting
for the formation of a blocky structure for the resulting
P(PDX-co-CL) copolymers. Both 13C and 1H NMR spectros-
copy indicated that multiple short PPDX homosequences
separated from each other by CL unit(s) composed the
resulting copolyester chains. As shown by TGA and DSC
experiments, this allowed enhancing significantly the
thermal stability of PPDX chains, without preventing the
crystallization of the resulting copolymers with a melting
temperature as high as 95 8C at a CL molar fraction close to
11mol-%. These relevant features open the door to the
manufacture of low cost PPDX-based materials at large
scale using a continuous one-step process.
Reactive Extrusion Modification ofBiodegradable Polymers
Chemical modification is usually employed to enlarge the
potential applications of biodegradable polymers. For
instance, to better suit their properties to specific
applications, chemical modification of starch is often
required because of the dominant hydrophilic character,
and unsatisfactory mechanical properties, particularly in
wet environment of starch. Typical examples of reactive
extrusion modification of biodegradable polymers will be
X chains.
outlined in the following sections.[18]
‘‘Grafting onto’’ Reactions ofVarious Monomersonto FunctionalBiodegradable Polymers
Over the past decades, starch, an
anhydroglucose polymer, has attracted
considerable attention as an interest-
ing structural platform for the
manufacture of sustainable and bio-
degradable plastic packaging due to
its natural abundance and low cost.[7]
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J.-M. Raquez, R. Narayan, P. Dubois
456
Starch is a polymer made from anhydroglucose (C6H10O5)
units attached either by a-1,4 linkages or by a-1,6
linkages.[80] The a-1,4 linkages yield a linear homopolymer
called amylose with molecular weights ranging from
100000 to 500 000 g �mol�1, while the a-1,6 linkages are at
the origin of the branching points in the polysaccharide
structure called amylopectine with molecular weights
higher than millions (Figure 7). It is worth noting that the
proportion of both amylose and amylopectine is geneti-
cally established and is relatively constant for each species
of plant.
Due to its high functionality in hydroxyl functions,
various types of monomers have been grafted onto
unmodified starch through reactive extrusion. In paper-
making and textile applications, cationic starches have
been successfully prepared through reactive extrusion
in the presence of 3-chloro-2-hydroxypropyltrimethyl-
ammonium chloride (CHPTMA) as monomer, and of an
alkaline catalyst, yielding a quaternary ammonium cationic
starch ether [starch–O–CH2–CHOH–CH2Nþ(CH3)3].
[81] Using
a Clextral BC 45 twin-screw extruder as a reactor, a
reaction efficiency of up to 82% was obtained in extrusion
processing with wheat starch within only a few minutes.
Carr’s et al. proposed a similar study[82] with high reaction
efficiency of up to 90% or more for the system of
unmodified starch reacting with CHPTMA (molar ratios
monomer/starch of 1:2) using NaOH as catalyst in a ZSK 30
twin-screw extruder. Reaction temperature was kept at
70 8C for all experiments. The combined effect of the high
temperature (90 8C), intensive mixing, high-starch solids
(65%), and appropriate level of catalyst contributed to the
unusually high reaction efficiency values (90%), exceeding
maximum values previously reported using laboratory-
batch reaction procedures.
Carr et al. reported grafting of acrylic acid (AA) and
acrylamide (AC) onto starch free-radically promoted
by aqueous ceric ammonium nitrate (CAN), carried out
using a twin-screw extrusion.[83] Presumably, free-radicals
are formed at carbon atoms 2 or 3 in starch that are able to
initiate the polymerization with acrylic compounds as
Figure 7. Structure of amylose (a) and amylopectine (b).
Macromol. Mater. Eng. 2008, 293, 447–470
� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
evidenced by electron spin resonance. In the presence of
35 wt.-% in water, and at 80 8C, levels of add-on (wt.-% of
total synthetic polymer) of the starch grafted with
polyacrylic compounds (St-g-PAC) were of 27–44% for
the extrusion process. Interestingly, the average residence
times in the extruder were about 2–3 min, largely below
two hours, which are currently required in a batch process.
When AA was used as monomer, the saponification of
resulting St-g-PAC led to highly water-absorbent materials
for hygienic and cosmetic applications.
Cellulose, a polysaccharide constituted by anhydroglu-
cose (C6H10O5) units attached via b-1,4 linkages, is themost
abundant biomass on the surface of the earth. Cellulose
can be converted into a wide range of derivatives with
desired properties and applications. The most important
derivative from cellulose is cellulose acetate used in many
common applications including toothbrush handles and
adhesive tape backing. The common synthesis of cellulose
acetate is the processing of high-grade cellulose in the
presence of a mixture of methylene chloride and acetic
anhydride.[84] The resulting cellulose acetate is processed
into strands, sheets and films by addition of liquid
additives using the extrusion technique. Cellulose acetate
corresponds to a thermoplastic with good barrier proper-
ties to grease and oil, and is used for the packaging coating
of food. Until the mid-1990s, plastic-grade cellulose
acetates were believed to be non-biodegradable due to
their high substitution degree (SD).[84–87] Between two and
three hydroxyl groups of the glucose repeat unit are most
often acetylated. However, it has been shown that
cellulose acetates with SD up to 2.5 are biodegradable in
stimulated composting.[87] A decrease in SD from 2.5 to 1.7
results in a large increase in the rate of their biodegrada-
tion. In this respect, thermoplastic cellulose acetates are
regaining interest as potentially biodegradable plastics for
composting of plastic waste without encountering the
water-solubility problems typical for starch-based materi-
als. Low molecular weight plasticizers like phthalates,
glycerol, triacetine, or cyclic lactones are preferred for easy
processability.[88] However, their main disadvantage is
related to plasticizer migration
that can account for loss ofmecha-
nical properties. Novel families of
thermoplastic cellulose acetate
starting from cellulose-2,5-acetate
were produced through reactive
extrusion technology that grafted
cyclic lactones, simultaneously
onto polysaccharide, hydroxy-
functional plasticizer, optionally
also hydroxyfunctional fillers.[89]
Organosolv ligin, cellulose, starch,
and chitin were added to rein-
force the polymer matrix. It was
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Recent Advances in Reactive Extrusion Processing . . .
demonstrated that cyclic lactones such as CL, glycolide,
and lactide could be successfully grafted onto the rigid
polysaccharide backbone, and hydroxyfunctional plastici-
zers in a twin-screw extruder system (length/diameter
ratio was of 48) at 190 8C. Low molecular weight
hydroxyfunctional plasticizers such as glycerol were
in situ converted into non-migrating higher molecular
weight oligopolyester plasticizer. The simultaneous graft-
ing of cellulose-2,5-acetate and hydroxyfunctional plasti-
cizer substantially improved compatibility between these
two components. These resulting oligolactone-modified
cellulose-2,5-acetate exhibited high compatibility with
fillers such as organosolv ligin, cellulose, starch, and chitin
when these latter were added in a subsequent down-
stream reactive processing.
Acid/acetyl Derivatization Carried out ontoFunctional Biodegradable Polymers
Acid derivatization of starch is a well-known technique to
obtain lower viscosity products, which are dispersible at
higher solids than one made from the native starch and
one whose dispersions are still able to be pumped and
handled.[90–93] Miladinov et al. reported the reactive
extrusion preparation of starch-fatty acid esters contain-
ing 0.01–0.03 mol levels in co-organic acid anhydrides
(acetic, propionic, heptanoic, and palmitic anhydrides)
with regard to the degree of substitution of starch, in the
presence of sodium hydroxide as catalyst.[94] Some
molecular weight reduction of the amylopectin fraction
could be detected that lowered the specific mechanical
energies for REX, particularlywhen heptanoic and palmitic
anhydrides were used as co-organic acid anhydrides.
Reactive extrusion preparation of starch esters has been
carried out using maleic anhydride (MA) as cyclic dibasic
acid anhydrides, yielding a free carboxylic group. Such a
free carboxylic group has shown to be valuable to promote
acid-catalyzed esterification reactions with biodegradable
Figure 8. Hydrolysis (a) and glucosydation (b: for sake of clarity, only thglycerol is represented) reactions present during the in situ maleatio
Macromol. Mater. Eng. 2008, 293, 447–470
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poly[(butylene adipate)-co-terephthalate], leading to the
formation of a graft copolymer in subsequent downstream
blending operations.[53,95] In situ reactive modification of
starch by 0–8wt.-% inMAas esterification agent and in the
presence of 20wt.-% glycerol as plasticizer has been carried
out through reactive extrusion at 150 8C. When 2.5 wt.-%
MAwas used, the recovery yield for the resultingmaleated
thermoplastic starch (MTPS), as determined by Soxhlet
extraction, was almost complete. This attests for the
reaction of some glycerol moieties to starch backbone
during the extrusion process. Increased MA content
decreased the recovery yield of resulting MTPS due to its
partial solubilization in the solvent used for Soxhlet
extraction (acetone). Besides the in situ esterification of
TPS, intrinsic viscosity, FT-IR and 2D liquid-phase NMR
spectroscopy measurements proved the occurrence of
some hydrolysis and glucosidation reactions as promoted
by MA moieties grafted onto the starch backbone. Such
reactions reduced the relative molecular weight of MTPS
(Figure 8). Therefore, the resulting MTPS had improved
processability due to its reducedmelt viscosity. Indeed, TPS
has found to display a gel-like viscoelastic behavior. This is
related to the formation of a crystalline elastic network
produced by the complexation of amylose molecules with
lipids/plasticizers, and the physical entanglement of
starch chains caused by its high molecular weight. Such
a physical entanglement is responsible for the incomplete
homogenization in the melt blend of TPS with other
polymers such as PCL, affecting the final properties for the
resulting melt-blends.[96] WAXS diffraction analyses con-
firmed the complete disruption of the granular structure of
native starch in MTPS during the reactive extrusion
processing. Tomasik et al.[90] reported a similar chemical
modification of cornstarch using MA and the like, and by
varying amounts of water (18, 20 and 30%) as plasticizer,
the whole process was carried out by extrusion. Carbonate
buffer, either at pH 8 or pH 9, was added during extrusion.
Extrusion of starch with cyclic anhydrides in alkaline
medium represented a facilemethod for the preparation of
e reaction between starch and the hydroxymethylene function fromn of TPS.
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J.-M. Raquez, R. Narayan, P. Dubois
458
anionic thermoplastic starches. However, such modified
hydrophilic thermoplastic contained large amounts in
water, e.g. rendering difficult their subsequent melt
blending with hydrophobic biodegradable polymers.
Soy proteins are the first biopolymers derived from
agriculture, which have been used for the manufacture of
molded materials. Among plant protein sources, soy protein
is of relatively low cost with vast available supplies. Soy
proteins are available in three different forms as soy flour,
soy isolate, and soy protein concentrate. Soy proteins are
complex macromolecules containing 20 amino acids that
supply enough available sites to react with coupling
agents. As biomaterials, soy proteins can be masterfully
converted into soy protein plastics through extrusion with
a plasticizers.[97,98] Common plasticizers used in the
manufacture of soy protein plastics include glycerol,
ethylene glycerol, propylene glycerol, 1,4-butanediol, 1,3-
butanediol, poly(ethylene glycol) sorghum wax, and
sorbitol.[99] Thermoplastic processing of proteins with
plasticizers induces marked changes in the connectivity of
the protein network, which limits the processing window.
Cross-linking of proteins occurs in situ as a temperature-
controlled phenomenon, but high shear conditions can
significantly decrease the activation energy of this
reaction.
Interestingly, Vaz et al. used the high functionality of
soy proteins in the reactive extrusion for the design of
biodegradable soy matrix drug delivery systems.[100,101]
Glyoxal was used as a cross-linker, which has dialdehyde
functionality able to react with the free e-amine groups of
the lysine (hydroxylysine) residues of soy protein
(Figure 9). In situ cross-linking reactions of soy protein
were carried out at ca. 130 8C under rotation speed at 100
RPM, in the presence of glyoxal and of an encapsulated
drug (theophylline) through one-stage reactive extrusion
process at pH 4 and 7. The resulting drug delivery systems
offer several advantages as follows: (i) ease of production,
(ii) suitability for a large variety of polymeric matrices,
(iii) applicability to different types of drugs, and
(iv) biodegradability. The drug release patterns could be
adjusted during processing by cross-linking, changing the
net charge (effect of pH), and by a filler reinforcement such
as hydroxyapatite.
Corn gluten meal chemically modified with citric
derivatives has also been prepared in a continuous reactive
extrusion process.[102] Corn gluten meal is a mixture of
corn starch, fiber and corn protein obtained, as a
by-product, from the wet-milling of corn in the ethanol
Figure 9. Acetyl functionalization of soy proteins from e-amine gro(hydroxylysine) residues.
Macromol. Mater. Eng. 2008, 293, 447–470
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industry. In this study, citric anhydride was reacted with
this corn product in reactive extrusion to generate value-
added, acid-insoluble reaction products with enhanced
metal-binding properties for the treatment of industrial
wastewaters. Pendant carboxylic groups were so obtained
from citric anhydride that reacted with the different
nucleophilic groups of corn gluten meal from the part of
starch/fibers (hydroxyl functions), and corn protein
(hydroxyl, sulfhydryl, and amino groups). Interestingly,
these derivatives exhibited high metal binding ability for,
at least nine metals (Cd2þ, Co2þ, Cu2þ, Fe2þ, Pb2þ, Mn2þ,
Ni2þ, Agþ, and Zn2þ). Respirometry testing revealed that
biodegradability of corn gluten meal was unaffected after
treatment with citric anhydride.
Free-Radical Grafting of Functional Groups ontoBiodegradable Polymers through Reactive Extrusion
In order to incorporate functional pendant groups all along
the polymer backbone, free-radical grafting of unsaturated
monomers such as maleic anhydride, and acrylic acid and
its derivatives has received much attention in the reactive
extrusion technology.[24–26,30,31] The most preferred unsa-
turatedmonomer is actuallymaleic anhydride (MA) that is
useful for further compatibilization reactions with, e.g.,
hydrophilic fillers like starch granules.[103] The reason
of this interest is that MA does not readily polymerize
under the conditions employed in grafting reactions, and is
therefore grafted at high efficiency without the accom-
panying formation of homopolymer. A number of
methods are available in the literature to produce such
a graft polymer that includes melt-grafting, solid state
grafting, solution grafting, suspension grafting in aqueous
or organic solvents, and redox-induced grafting.[104] The
most widespread method is the melt state process
performed by REX, which can alleviate the difficulties
owing to diffusion-controlled grafting reactions of high
molten viscosities polymeric matrix in bulk.
Tang et al. investigated thoroughly the grafting of MA
onto biodegradable aliphatic/aromatic (co)polyesters
through reactive extrusion using dicumyl peroxide,
benzoyl peroxide, and di-tert-butyl peroxide as free-radical
initiators.[1] The effect of various factors such as free-
radical initiator concentration, MA concentration, and
reaction temperature on the percent grafting of MA onto
the copolyesters was investigated. The structures of the
grafted (co)polyesters were characterized using FT-IR and
ups of the lysine
NMR spectroscopy. It has been demon-
strated that MA can be grafted onto any
copolyesters. The grafting reaction takes
place selectively at aliphatic dicarboxylic
acid units of the copolyesters (Figure 10).
Minimal degradation of the polyester
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Figure 10. Free-radical grafting of PLA backbone.
chains was also observed from intrinsic measurements.
The desired graft content can be controlled by the
aforementioned factors, but there is, however, an opti-
mum radical concentration, which depends on the [free-
radical initiator]/[MA] ratio to promote grafting efficiency,
beyond which the chain scission and termination reac-
tions become predominant. In PLA, low grafting efficiency
was observed due to its limited availability of free-radical
sites on the polymer backbone for grafting.
Some of us have carried out a more consistent study on
the free-radical grafting reaction of MA onto the PLA
backbone using a co-rotating intermeshing twin-screw
extruder (L/d ratio of 14) at two reaction temperatures (180
and 200 8C).[104] For all experiments, 2 wt.-% MA was used,
while varying the free-radical initiator concentration,
i.e. 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane (Luperox
L101), between 0 and 5 wt.-% by PLA. Triple-detector size
exclusion chromatography (TriGPC), melt-flow index, and
thermal gravimetric analysis were used as main char-
acterization tools. Whichever the processing temperature,
increasing free-radical initiator content increases the
content of MA moieties grafted onto PLA chains, as
determined by back titration of an excess of morpholine
with HCl. However, the molecular weight for the resulting
MA-g-PLA decreased with further addition of Luperox 101
during the reactive extrusion processing. Such a behavior
is more likely due to a competition between the molecular
weight increase through chain branching, and the
molecular weight decrease by b-chain scission triggered
by grafted MA (Figure 11). In contrast, the simple addition
of Luperox 101 to the extruded PLA allows increasing its
molecular weight to some extent through a free-radical
self-branching reaction. However, highly branched PLA
chains until the formation of microgel could occur at
higher temperature as shown by TriGPC. This was more
Figure 11. Free-radical b-scission of PLA backbone.
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� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
likely due to the hydrogen radical
abstraction in a-position of the car-
bonyl groups followed by radical
addition onto the carbon–carbon
double bond of the enolate forms in
equilibrium within the polyester
chains. Additional free-radical chain
scissions might occur and participate
in the chain branching process as
well.
Interestingly, when these low-
maleated PLA (0.7 wt.-% MA) were melt-blended with
granular cornstarch (30 and 40 wt.-%) again by reactive
extrusion, improvement in the interfacial adhesion of
PLA-based composites could be successfully achieved.
These grafted reactive functions can therefore react with
the hydroxyl groups of starch macromolecules to form
covalent bonds, and thus, they provide better control of the
size of phase and strong interfacial adhesion.
Reactive Extrusion Oxidation Reactions ofBiodegradable Polymers
Commercial oxidized starches are batch-prepared at room
temperature conditions and low (3%) concentrations of
oxidant (usually hypochlorite) for adhesive applica-
tions.[105] During product isolation (filtration and aqueous
washing), some of the product is dissolved (due to
molecular breakdown) and gets lost. Hypochlorite has
been the oldest and most frequently used oxidant. Other
oxidants such as permanganate, hydrogen peroxide,
persulfate, periodate and dichromate have also been used.
The different oxidation procedures result in variations in
molecular structure and properties. Wing and Willett[106]
used the reactive extrusion to prepare oxidized starches.
Three types of cornstarches (waxy cornstarch, pearl
cornstarch, and amylomaize) were oxidized by a reactive
extrusion-drum drying process with hydrogen peroxide
and a ferrous-cupric sulfate catalyst. A ZSK 30 corotating
twin-screw extruder, which had a temperature profile of
88/115/88/77/71/65/54/48 8C (feed to die) and a screw
speed of 110 rpm, with a starch feed of 180 g �min�1 (10%
moisture) was used. Thermo-chemical oxidation of starch
by reactive peroxide extrusion-drum drying represents a
rapid, continuous procedure for making water-soluble
products containing high carboxyl
and carbonyl contents. Increasing
the peroxide level increased the
carboxyl and the carbonyl content,
whereas increasing the amylose con-
tent decreased the solubility. The
residual granule structure was still
present in high amylose starch
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J.-M. Raquez, R. Narayan, P. Dubois
460
extrudates with or without peroxide. Solution viscosities
indicate a significant molecular degradation by reactive
extrusion.
Reactive Extrusion Melt Blending ofBiodegradable Polymers
Polymer melt-blending is a well-used technique whenever
modification of properties is required, because it uses
conventional technology at low cost.[107] The objective for
preparation of novel melt-blends between two or more
polymers is not to change the properties of the entire
components drastically, but to capitalize on themaximum
possible performance of the blend. Substantial efforts have
been realized in the preparation of biodegradable starch-
basedmelt-blends using the reactive extrusion technology,
due to the abundance, renewability and low cost produc-
tion of starch.[108–122] Interestingly, hydroxyl groups cause
starch to behave as an alcohol during chemical reactions
generally. This property of starch is important when
considering reactive melt blending of starch with syn-
thetic polymers. The presence of such large numbers of
hydroxyl groups affords starch hydrophilic properties, and
therefore adds affinity for moisture and dispersability in
water. However, hydrophilicity is undesirable in many
plastic packaging applications, and hence it is a major
limitation in using starch as a homopolymer. Melt-
blending starchwithmoisture resistant and biodegradable
polymer having goodmechanical properties represents the
best method to prepare useful products, while maintain-
ing the biodegradability of overall products.
Besides, fillers such as talc and kaolin are frequently
incorporated in thermoplastics to reduce the costs of
molded products.[123] These fillers improve the properties
of the polymers such as strength, rigidity, durability, and
hardness. Particularly, worldwide for at least twenty years,
there has been a new and intense desire to tailor the
structure and composition of materials applying sizes
about the nanometer. Nanofillers having at least one of
their dimension in the nanometer scale, exhibit high
specific surface areas able to significantly improve the
properties of polymeric (nano)composites at low content
in contrast to microfillers.[33]
Developing such melt-blends/(nano)composites with
satisfactory overall physico-mechanical behavior however
requires the ability to control interfacial tension, to
generate a dispersed phase of limited size and strong
interfacial adhesion, and to improve the stress transfer
between the component phases.[12] Compatibilization is
therefore called upon. Usually, this is achieved by adding
or creating in situ during the blending process, a third
component, often called an interfacial agent, emulsifier, or
compatibilizer. The latter can be a graft or a block
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� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
copolymer. Effective compatibilizers must be located at
the interface between the phase domains of the immis-
cible blend. Most importantly, the degree of compatibiliza-
tion in a particular system depends on the reactivity of the
compatibilizer used. It has been found that a compati-
bilizer is most effective when its sections are of higher
molecular weight than the corresponding blend compo-
nents. Several theories have attempted to explain the role
of compatibilizers, of which two mechanisms are con-
sidered plausible. The first mechanism is thermodynamic
in nature in that the compatibilizer reduces the interfacial
tension between the phases. The second mechanism is
kinetic in nature in that the presence of the compatibilizer
at the interface reduces the agglomeration of domains by
steric stabilization.[1,124] Recently, Macosko et al. have
reported an excellent review about reactions at the polymer-
polymer interface for blend compatibilization.[125] It has
been shown that the major factors influencing the
interfacial reaction, and therefore blend compatibilization
are: inherent reactivity of functional polymers used as
compatibilizer, thermodynamic interactions between dif-
ferent polymeric partners, functional group location along
the compatibilizer chain, and the effect of processing
flows. Concerning the last factor, flow in melt mixers is
well-known to accelerate the coupling rate tremendously,
by changing the concentration profile of functional groups
from compatibilizer at the interface and/or increase the
collision probability. However, in this case, it is often
unclear which of the mechanisms is predominately
involved under simple model flows.
Reactive Extrusion Starch-Based Melt-Blending
Starch is largely used as filler for environmentally friendly
plastics since about two decades. Although starch may be
added as filler, its more interesting and technologically
challenging uses have been in the area of using starch as a
binder, as a thermoplastically processable constituent
within thermoplastic polymer blends, and as a thermo-
plastic material by itself.[126] While native starch does not
typically behave as a thermoplastic material by itself, it is
a thermoplastic in the presence of a plasticizer when
heated and sheared.[127,128] Glycerol and water are the
most widely used plasticizers. The role of plasticizers is to
destructurize by cleaving hydrogen bonds between the
starch macromolecules, and by inducing partial depoly-
merization of starch polymers. It contributes to lower the
melting and the glass transition temperatures below its
decomposition temperature (230 8C).[13,129,130] However,
the physical properties of polyol-plasticized starch tend to
be modified after being stored for a long period because of
its re-crystallization (retrogradation), with migration of
plasticizers. To limit the retrogradation phenomenon,
amide compounds as plasticizers such as urea and
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formamide were added in the plasticized starch prepara-
tion, but the resulting materials were very rigid and
brittle.[131,132] The starch structure may be modified by,
e.g., acetylation to reduce the hydrophilic character of the
macromolecules. This hydrophobic starch acetate was
shown to be useful in starch-based extruded foams.[133–135]
However, this chemical process results again in inferior
mechanical properties and greater product cost for these
starchy materials.[136]
Therefore, some authors have preferred to melt-blend
TPS with biodegradable hydrophobic polymers such as PCL
and cellulose acetate in order to manufacture environ-
mentally friendly products.[108–119,137,138] As excepted,
melt-blending TPS with these polyesters resulted in a
significant improvement in the properties of plasticized
starch. However, although a ‘‘protective’’ polyester skin
layer is formed at the surface of most blends during
injection molding, the moisture sensitivity of TPS can be
not fully addressed.
Averous et al. prepared different compositions of wheat
thermoplastic starch and PCL through a two-stage extru-
sion in order to determine the thermo-mechanical proper-
ties of resulting melt-blends.[139] A large range of blends
was analyzed with different glycerol (plasticizer):starch
content ratios (0.14:0.54) and various PCL concentrations
(up to 40 wt.-%). Whatever the composition, it resulted a
phase-separation occurring in the melt-blend as observed
by DSC and DTMA. Depending on the content in glycerol,
two distinct behaviors could be obtained. When the starch
matrix has a glassy behavior (low content in glycerol),
extrusion blending with PCL resulted in a decrease of
the material modulus, but the impact resistance was
improved. On the other hand, when the starch has a
rubbery behavior, PCL increased the modulus of the mate-
rials. However, ageing studies showed a structural evolu-
tion for the resulting melt-blends after processing during
several weeks. A significant increase of Young’s modulus
and of the maximum strength was observed for all
melt-blends. This is due to post-crystallization and water
evolution inside melt-blends.
Other efforts have been employed to develop a coating
of TPS with hydrophobic biodegradable polyester through
reactive extrusion. Multilayer coextrusion has beenwidely
used in the past decades to combine the properties of two
or more polymers into one single multi-layered structure.
Martin et al. preparedmultilayer films based on plasticized
wheat starch (PWS) and various biodegradable aliphatic
polyesters through flat film coextrusion.[140] PLA, poly-
esteramide (PEA), PCL, poly(butylene succinate adipate)
(PBSA), and poly(hydroxybutyrate-co-valerate) (PHBV)
were chosen as the outer layers of the stratified ‘‘polyester/
PWS/polyester’’ film structure. Different levels of peel
strength were found, depending on the compatibility of
plasticized starch with the respective polyesters. In
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� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
particular, PEA presented the best adhesion to the PWS
layer, probably due to its polar amide groups. PCL and PBSA
showed medium adhesion values, and both PLA and PHBV
were the least compatible polyesters. The same trend in
the magnitude of adhesion strength was observed which-
ever the multilayer techniques. However, some inherent
problems rise due to the multilayer flow conditions
encountered in co-extrusion such as an encapsulation
and interfacial instabilities phenomena because of the
difference of hydrophilic balance between TPS and the
polyesters. For formation of in situ bonding, electron
irradiation carried out TPS layers has also been attempted,
but the results were quite unsatisfactory.
Anhydride functionalization of the biodegradable
hydrophobic polymers represents another method of
compatibilization largely utilized in the reactive extrusion
preparation of starch-based melt-blends with useful
end-properties.[1,90–93,138,141–144] For instance, MA and
dicumyl peroxide (DCP) were used as cross-linking agent
and initiator respectively for blending plasticized starch
with a biodegradable aliphatic/aromatic (co)polyesters,
called EnPol1, using a two-stage reactive extrusion process
(L/D¼ 40:1).[145] The first step was the preparation of
maleated polyester, and the resulting polyester was then
melt-blended with 40 wt.-% in starch plasticized with
glycerin as a plasticizer. It has been demonstrated that at a
peroxide initiator constant varying MA content higher
than 2wt.-%were found to be unsuitable for compounding
with starch as observed on the tensile properties for the
melt-blends. In contrast, improvement in Young’smodulus
and stress at break were achieved in all blends containing
less than 1.5 wt.-% of MA. Melt-blends with 1.0 wt.-% in
MA showed larger improvement in break elongation. This
could be explained by the improvement in interfacial
adhesion between plasticized starch and maleated poly-
esters as shown by SEM analyses.
In melt-blends between TPS and polyesters, some
authors[146] have tried to develop chemically modified
TPS as a valuable coupling agent with polyesters through a
reactive extrusion free-radical grafting process using
ricinoleic oxazoline maleate derivatives (Figure 12). Direct
insertion of the fatty acid oxazoline moiety into the starch
backbone represents a suitable way of hydrophobic
modification for the polysaccharide, but also the bifunc-
tionality of the grafting agent provides the opportunity for
reactive coupling of the modified starch with hydrophobic
polymers such as polycondensates with small amounts of
reactive groups, e.g. free carboxylic acid functions. In a
mini-scale extruder, the grafting reaction was promoted
by bis(a,a-dimethylbenzyl) peroxide as free-radical initia-
tor in high grafting yields (between 1 and 30 wt.-% to
starch) for potato starch plasticized with 20 wt.-% in
glycerin. From torque measurement, the grafting was
completed within 15 min. The presence of oxazoline
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J.-M. Raquez, R. Narayan, P. Dubois
Figure 12. Reaction scheme for the radical grafting of starch with ricinoleic oxazoline maleate.
Figure 13. Proposed coupling reaction between oxazoline groups and free acid groups inbiodegradable PBAT.
462
moieties onto starch backbone was proved by 1H NMR and
FT-IR spectroscopy.
Subsequently, coupling reactions were carried out from
these TPS derivatives with poly[(butylene adipadate)-
co-terephthalate] (PBAT), biodegradable a,v-hydroxy-
carboxy-polycondensate, again in a laboratory extruder
(Figure 13). PBAT is a biodegradable aromatic/aliphatic
copolyester obtained by polycondensation from tere-
phthalate acid, adipic acid, and 1,4-butanediol, wherein
themaximum amount of terephthalate acid in copolyester
is close to 40 wt.-%, enhancing its mechanical strength,
while retaining the biodegradability of the resulting
copolyester (Figure 14).
Interestingly, a uniform blend could be obtained with
TPS chemically modified by 10% ricinoleic oxazoline
maleate derivatives as shown by SEM. In contrast, the
polymer obtained from untreated TPS exhibits a well-
separated phase. Interestingly, the tensile properties of
films obtained from the PBAT/TPS grafted with 10%
ricinoleic oxazoline maleate derivatives melt-blends
(50:50) were enhanced. Biodegradation studies did not
show an impairment of the degradation values compared
to the unmodified TPS/PBAT melt-blend.
However, although many have attempted for years to
discover the ‘‘perfect’’ starch/polymer blend that would
Figure 14. Synthesis of PBAT.
Macromol. Mater. Eng. 2008, 293, 447–470
� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
yield an environmentally sound
polymer while, at the same time,
fulfilling desired mechanical and
cost criteria, such a combination
has been difficult to achieve. The
reason for this is that the empha-
sis has been on finding the opti-
mal polymer or mixture of poly-
mers and other admixtures in
order to ‘‘optimize’’ the properties of the starch/polymer
blend thereby.
Furthermore, a major issue that most authors have not
addressed yet, is that the morphology, particularly the size
of dispersed domain, and thus the mechanical properties
for the resulting melt-blend of both immiscible partners, is
strongly affected by their difference in melt-viscosity. It
is very critical since starch,[141] and also TPS, exhibits high
melt-viscosity related to its high molecular weight
(ranging from 100 000 to 500 000 g �mol�1 and more than
millions, respectively, for the amylose and amylopectin).
As a consequence, at least 20% plasticizer is required in the
extrusion of TPS-based melt-blends at temperatures of ca.
130 8C.[127]
In contrast to this approach, we have originally prepared
through reactive extrusion a novel in situ chemically
modified TPS, so called maleated TPS (MTPS) with
improved processing and high reactivity in blown film
applications.[142] As aforementioned, MTPS was prepared,
through reactive extrusion processing of starch in the
presence of glycerol as plasticizer, and of MA as
esterification agent. In addition to derivatization of starch
backbone with MA, the MA moieties grafted onto the
starch backbone could promote some hydrolysis and
glucosidation reactions that reduced the relative molecu-
lar weight of MTPS. This reduction
in molecular weight is a major
issue in the melt blending of TPS
with its polymeric partner that
most of the authors have not
addressed. In addition to reduced
melt-viscosity, the interest in
using MTPS is the presence of free
carboxylic acid groups, i.e. MA
moieties grafted onto the starch
backbone, which could promote
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Recent Advances in Reactive Extrusion Processing . . .
acid transesterification reactions with polyester chains
such as PBAT, leading to graft copolymers[147–149]
(Figure 15). We selected PBAT as a good candidate for
melt-blending with starch-based products due to its
interesting thermal and mechanical properties.[138]
As far as MTPS/PBAT melt-blends are concerned, effect
of polyester and MA contents was studied on the
physico-chemical parameters for the resulting melt-
blends, again prepared through a downstream extrusion
operation. For high polyester fractions, i.e., 60 and 70wt.-%,
PBAT-g-MTPS graft copolymers were obtained through
transesterification reactions promoted by the MA-derived
acidic moieties grafted onto the starch backbone. This was
determined by selective Soxhlet extraction experiments
and FT-IR spectroscopy analyses. At lower polyester
content, no significant reaction occurred more likely due
to an inversion in phase morphology between PBAT and
MTPS. Interestingly, tensile properties of blown films
derived from the PBAT-g-MTPS graft copolymer containing
70 wt.-% polyester, were much higher than those of the
melt-blend of TPS and PBAT performed in the presence of
MA. This difference in mechanical performances resulted
from the low melt-viscosity of MTPS, yielding a finer
morphology of the dispersed phase in the continuous PBAT
matrix, together with an increased interfacial area for the
grafting reaction. Thiswas attested by ESEM. IncreasedMA
content in the preparation of MTPS did not affect the
tensile values, suggesting that the entanglement of PBAT
and MTPS chains, responsible for these values, did not
change after reactive extrusion melt blending. Moreover,
WAXS diffraction analyses evidenced that the MTPS
crystalline structure was completely disrupted in the
‘‘PBAT-g-MTPS’’ reactive blends suggesting the grafting
Figure 15. Proposed mechanism of transesterification reactions betweby the MA-derived acidic moieties grafted onto the starch backbone
Macromol. Mater. Eng. 2008, 293, 447–470
� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
reaction/homogenization of the MTPS in the polyester
continuous phase.
Biodegradable Polymeric (Nano)composites Preparedthrough Reactive Extrusion
By definition, a composite material is formed by the
combination of different phases, which have distinct
structural and chemical compositions, leading to a synergy
of physical, chemical and/or mechanical properties
compared to each component taken separately. As
reinforcing elements, one can distinguish fibers, particles,
clays, minerals, and so on. In addition, the matrix can
include various types of materials: organic, mineral, and
metallic. The classification of composites is based on
either the shape of the filler (fibers or particles) or on the
matrix.
In recent years, the use of natural/bio-fiber reinforced
composites has rapidly expanded due to the availability of
natural/bio-fiber derived from annually renewable
resources, as reinforcing fibers in both thermoplastic
and thermosetting matrix composites as well as for the
positive environmental benefits gained by such materi-
als.[150,151] Most of research is also being conducted on the
potential of natural fibers as reinforcement for polymers,
because natural fibers have low density, acceptable
specific strength properties, easy preparation, and biode-
gradability.
Fully green biocomposites were prepared from PLA and
recycled cellulose fibers (from newsprint) by extrusion
followed by injection molding processing.[17] The physico-
mechanical and morphological properties of the result-
en MTPS and PBAT promoted.
ing composites were investi-
gated by varying the amount
of cellulose fibers. Compared
to the neat resin, the tensile
and flexural moduli of the
composites were significantly
higher because of the high
modulus provided by cellu-
lose. Increase in stiffness of
resulting composites was also
confirmed by DMA analyses.
The authors claimed that PLA/
natural fiber composites have
mechanical properties of suf-
ficient magnitude to compete
with conventional thermo-
plastic composites. However,
a significant decrease in ten-
sile strength at high amount
of fibers could be observed,
because of the lack of interac-
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J.-M. Raquez, R. Narayan, P. Dubois
464
tion between cellulose and PLA. Cellulose has a strong
hydrophilic character due to three hydroxyl groups per
monomeric unit in contrast to the rather hydrophobic PLA.
Wood fiber was used as a reinforcing fiber in the reactive
extrusion preparation of corn gluten meal-based compo-
sites.[152] Corn gluten meal mainly contains oil (4%), starch
(20%), zein protein (60–70%), which is an alcohol-soluble
protein extracted from corn that has low water resistance.
Homogeneous dispersion ofwood fiber in corn glutenmeal
could be obtained through reactive extrusion in the
presence of glycerol, ethanol, and water as plasticizers.
The mixture of ethanol/water was an excellent plasticizer
to disrupt the intermolecular interaction of zein, leading to
the improvement of melt-mobility. Corn gluten meal-
based composites were successfully prepared with 10–
50wt.-% wood fiber. It is worth pointing out that the
melt-viscosity of the medium increased with increasing
wood fiber content, and with a decreasing water content,
which led to a decrease of melt-mobility. From the flexural
testing, it has been demonstrated that the flexural
strengths of these biocomposites increased after the
addition of 10–30 wt.-% wood fiber, but decreased by
the addition of 40–50 wt.-% wood fiber. These results are
ascribed to the difference in interfacial tension between
wood fiber and corn gluten meal. Morphological studies
revealed that breaking occurred in the matrix for these
bio-composites at high content of wood fiber (30 wt.-%),
breaking occurred at the interface of the fiber and the
matrix.
Chemically pre-treated natural fibers were also
employed in reactive extrusion in order to enhance the
interfacial adhesion between natural fibers and polymers,
and hence the properties of resulting composites. Indian
grass fiber reinforced soy based biocomposites were
accordingly fabricated using twin-screw extrusion and
injection molding technology.[153] Using extrusion tech-
nology, soy protein can be masterfully converted to soy
protein plastics.[154] However, soy protein plastic products
tend to have lower strength and higher moisture
absorption. Currently, biodegradable polymers in the
melt-blend of soy protein plastic are used to overcome
these drawbacks, including polyester amide and PCL,
whose processing windows match that of soy protein
plastic. Liu et al. incorporated PBAT with the soy protein
polymer to form a soy-based bioplastics.[153] To get higher
strength and modulus materials from soy based bioplas-
tics, raw Indian grass and alkaline-treated Indian grass
were added as natural fibers. Alkaline treatment
removes the fraction of lignin contained in this fiber,
and reduces its size. Tensile and flexural properties as well
as the heat deflection temperature of soy-based bioplastics
were improved, but the impact strength of the biocompo-
sites did not improve after reinforcement with raw Indian
grass fiber. The impact fracture of raw Indian grass fiber
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� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
reinforced biocomposites was found to occur on the outer
surface of the fiber, due to intrinsic differences in the
morphological structure between the outer and inner
surfaces of the grass fiber as shown by ESEM. Interestingly,
the alkali solution treated Indian grass fiber significantly
increased the tensile strength and impact strength as well
as the flexural strength due to the improved dispersion of
the fiber in thematrix, and the enhanced aspect ratio of the
fiber.
Other authors have preferred to melt-blend biodegrad-
able polymers and natural fibers by adding maleic
anhydride-modified polymers as a compatibilizer. Nitz
et al. reported the melt-compounding of PCL with wood
flour and lignin in a Werner & Pfeiderer twin-screw
extruder.[155] Wood flour contains about 25 wt.-% lignin,
which together with cellulose forms the structural
component of trees and various plants. As a cheap
phenolic biopolymer, lignin offers attractive potential as
filler and additive, especially with respect to the modifica-
tion of biodegradable polymers. Because of the presence of
phenolic groups in lignin, it is expected that lignin fillers
can influence both oxidative stability and biostability of
such compounds. Reactive extrusion technology was
carried out to prepare several families of thermoplastic
PCL compounds containing wood flour and lignin in the
presence of MA-grafted PCL as a compatibilizer. In the first
step, grafting of MA onto PCL was performed with a
reactive extrusion process in the presence of Luperox 101
as free-radical initiator. Appropriate MA/peroxide ratios
led toMA-grafted PCLwith aMA content close to 1.44wt.-%
as determined by titration. When a low amount of this
MA-grafted PCL (less than 2.5 wt.-% of overall product) was
added, attractive properties could be obtained in these
PCL-based composites reinforced with wood flour and
lignin. For a PCL composites containing 2.5 wt.-% in
MA-g-PCL and 40 wt.-% wood flour, Young’s modulus
increased by 450%, and the tensile strength increased by
115% in comparison with the properties of neat PCL. The
mechanical properties of the wood flour composites were
much better as those of the lignin-based compounds.
More than 70 wt.-% lignin was added without mechanical
properties being impaired, while compositions containing
40 wt.-% lignin showed break elongation exceeding 500%.
According to morphological studies, very effective lignin
dispersion was achieved within the PCL matrix due to
better compatibilization provided by MA-g-PCL. Interest-
ingly, biodegradation studies revealed that the addition of
lignin enhanced the biostability of PCL compounds,
affording to enhance the lifetime of PCL-based compounds
in outdoor applications.
Inorganic fillers have also been utilized in the prepara-
tion of biodegradable polymeric composites. Talc is a
common filler used for the improvement of properties of
the polymers such as strength, rigidity, durability, and
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Recent Advances in Reactive Extrusion Processing . . .
hardness. Talc has a plate-like geometry in which an
edge-shared octahedral sheet of Mg(OH)2 is sandwiched
between tetrahedral sheets of silica (SiO2). A bacterial
polyester, i.e. PHBV has been melt-compounded with
different talc weight contents (15–50 wt.-%) through
extrusion combinedwith an injectionmolding process.[156]
PHBV belongs to the family of polyhydroxyalkanoates,
which are biocompatible and biodegradable thermoplas-
tics with potential applications in different fields like
agriculture, marine, and medicine. The synthesis of
polyhydroxyalkanoates occurs normally, when there is
an excess of carbon and energy and limitation of at least
one nutrient (N, P, Mg, Fe, and so on) needed for
microorganism growth. When PHBV was melt-blended
with talc, moderate to significant improvements in the
tensile, flexural, and storage moduli of talc-filled PHBV
were obtained compared to those of neat PHBV. This can be
explained by the poor filler dispersion and filler-matrix
adhesion as revealed by scanning electron microscopy in
the talc-filled PHBV composites.
We reported the preparation of new biodegradable and
high-performance talc/PBAT hybrid materials through
reactive extrusion in blown films applications.[157] In the
first step, the polyester backbone was reactively modified
through free-radical grafting of MA in order to improve
the interfacial adhesion between PBAT and talc. The
resulting MA-g-PBAT was then reactively melt-blended
with talc through esterification reaction of MA moie-
ties grafted onto the polyester chains with the silanol
present at the edge surface of talc.[158,159] Sn(Oct)2 and
4-dimethylaminopyridine (DMAP) were studied as ester-
ification catalysts (Figure 16). The interfacial adhesion
between both partners was substantially enhanced as
evidenced by SEM and selective extraction of the polyester
part. As a result, the biaxial tensile properties measured on
blown films prepared from these compatibilized compo-
sites were considerably improved, as compared to those of
the conventional PBAT-talc melt-blends. Extrapolation to a
one-step reactive extrusion process was successfully
achieved by preparation of in situ chemically modified
PBAT-talc compositions containing up to 60 wt.-% talc.
Interestingly, the highest tensile properties were obtained
Figure 16. Surface grafting mechanism of MA moieties onto the silanolof talc.
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� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
by melt-blending 50 wt.-% of native PBAT and 50 wt.-% of
such a chemically modified PBAT/talc hybrid filled with
60wt.-% of talc, therefore used as a masterbatch. Such an
approach allowed reducing the degradation of native
polyester chains through undesirable reactions such as
b-scission and transesterification reactions promoted,
respectively, by the MA free-radical treatment and
esterification catalysts used along the reactive extrusion
process. Finally, X-ray photoelectron spectroscopy mea-
surements carried out on the reactivelymodified PBAT-talc
compositions, e.g. containing 60wt.-% talc, attested for the
formation of covalent ester bonds between the silanol
functions available at the edge surface of talc particles, and
the maleic anhydride moieties grafted onto the polyester
backbones.
Recently, polymer-layered silicate nanocomposites have
emerged as a new class of organic-inorganic materials that
have shown unexpected properties such as large increase
in the thermal stability, mechanical strength, and imper-
meability to gases such as water and oxygen. Such
improvements in their properties achieved at low content
in layered silicate (<5 wt.-%) are relied on the interactions
between the clays and polymers, which can yield inter-
calation and/or exfoliation structures. In the intercalated
‘‘hybrid’’ structure, a monolayer of extended polymer
chains is sandwiched between the silicate sheets, resulting
in a well-ordered multilayer of alternating polymer and
inorganic sheets. In the exfoliated (or delaminated)
nanostructure, the silicate nanolayers are individually
dispersed in the polymer matrix. Exfoliation of the silicate
layers usually provides nanocomposite materials with
the highest improvement in properties aforemen-
tioned.[160–167]
Most of works have focused on the development of
(nano)composites based on aliphatic polyesters. Aliphatic
polyesters are among the promising materials for the
production of high-performance, and environment-
friendly biodegradable plastics. However, gas-barrier
properties, melt-viscosity for further processing, etc. are
not often sufficient for various end-use applications.
Okamoto et al. reported detailed studies about the
structure-property relationship in designing desired prop-
functions at the edge
erties for layered silicate nanocom-
posites PLA.[168–170] These PLA/
layered silicate nanocomposites were
prepared by melt-extrusion from
montmorillonite organicallymodified
with octadecylammonium (MMT-C18),
wherein silicate layers of the clay
were intercalated and randomly dis-
tributed in the matrix. MMT-C18 was
prepared by cation-exchange be-
tween octadecylammonium and the
naturally occurring Cloisite Naþ.
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J.-M. Raquez, R. Narayan, P. Dubois
466
Incorporation of very small amounts of oligoPCL as
compatibilizer led to better parallel stacking of the silicate
layers, and also much stronger flocculation due to the
hydroxylated edge-edge interaction of the silicate
layers.[168] The PLA/layered silicate nanocomposites exhib-
ited a remarkable improvement of materials properties in
both solid (loss and storage moduli) and melt-states
(melt-viscosity) compared to the matrix without clay.
Other types of organically modified layered silicates
(synthetic fluorine mica, synthetic fluorine mica modi-
fied with N-alkyl-N,N- [bis(2-hydroxyethyl)-N-methyl-
ammonium, and smectite) have also been developed by
Okamoto et al. in reactive extrusion.[169,170] It has been
demonstrated that intercalated, exfoliated, and a mixture
of both structures were achieved in the PLA/layered
silicate nanocomposites. Interestingly, better dispersion
and thus better barrier properties for PLA/layered silicate
nanocomposites could be achieved when smectite was
used as nanoclays. In addition, all nanocomposites
exhibited a remarkable improvement of various materials
properties with simultaneous improvement in biodegrad-
ability compared to neat PLA. Same efforts were provided
by the authors in the preparation of polybutylene
succinate (PBS)/layered silicate nanocomposites through
an extrusion process.[171–173] PBS is chemically synthesized
by polycondensation of 1,4-butanediol with succinic acid.
In all (nano)composites, intercalated structures were
obtained, with a remarkable improvement in tensile
properties, thermal stability, and biodegradation of
resulting PBS-based (nano)composites.
New (nano)hydrids were developed from PCL through
reactive extrusion. Organically layered double hydroxide
was used as nanoclay.[174] Although a very low content of
filler was added into the PCL matrix, no good dispersion,
particularly exfoliation was obtained. However, some
mechanical and physical properties were improved with
respect to neat PCL.
Nanocomposites were made from natural biodegrad-
able polymers chemically modified or not. For instance, to
limit the retrogradation phenomenon of plasticized starch,
green (nano)composites were successfully prepared
through reactive extrusion from activated montmorillo-
nite and thermoplastic cornstarch.[175] The thermoplastic
cornstarch was plasticized with urea, ethanolamine as
plasticizers (one equivalent with respect to starch), but
also with natural montmorillonite activated by ethanol-
amine. Exfoliated structures could be so obtained as
attested by WAXS analyses. TEM and SEM images showed
that the resulting nanocomposites presented reticulating
fiber structure after rapid cooling in liquid nitrogen.
The mechanical properties of (nano)composites evi-
dently improved such as tensile stress, and Young
modulus, but also their thermal stability and water
resistance.
Macromol. Mater. Eng. 2008, 293, 447–470
� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
‘‘Green’’ nanocomposites from cellulose acetate were
also prepared through reactive extrusion. Eco-friendly
triethyl citrate was used as a plasticizer, while Cloisite 30B
as organically modified montmorillonite with a methyl
tallow bis(2-hydroxyethyl) quaternary ammonium and
maleic anhydride grafted cellulose acetate butyrate
(CAB-g-MA) were used as organoclay and as compatibili-
zer, respectively.[176,177] CAB-g-MA was previously pre-
pared through reactive extrusion by grafting MA onto
cellulose acetate butyrate as promoted by Luperox 101. The
objective of adding CAB-g-MA was that the MA moieties
react with the hydroxyl functions from Cloisite 30B.
Nanocomposites containing from 0 to 7.5 wt.-% in Cloisite
30B prepared with 5 wt.-% of CAB-g-MA (MA content was
of 0.86 wt.-%) exhibited the best morphology, i.e. the
complete exfoliation of nanoclay within the matrix, but
also the best mechanical properties in terms of tensile and
flexural properties.
In melt-blends derived from hydrophilic plasticized
starch and hydrophobic biodegradable polyester, layered
silicates were also added to improve the compatibility
between both partners. Ikeo at al.[178] developed nanoclay
reinforced biodegradable plastics of PCL/plasticized starch
blends through reactive extrusion. Starch was first
plasticized with glycerin and water, following by its melt
blending with PCL. Maleic anhydride grafted PCL as
recovered through reactive extrusion was added as a
compatibilizer, enabling to improve the compatibility
between thermoplastic starch and PCL. Significant
improvement could be however obtained when natural
montmorillonite (Cloisite Naþ) was added as nanoclay.
Interestingly, the nanoclay acted positively in these
melt-blends after their electron irradiation as attested
by the increase in their modulus and yield stress by
more than 50 wt.-% without any reduction in their
elongation.
In situ REX processing was performed in the preparation
of plasticized starch/PCL nanocomposites.[179,180] Native
wheat starch (ca. 60 wt.-%), PCL (ca. 40 wt.-%), glycerin as
plasticizer, organo-modifiedmontmorillonite, and Fentons
reagent (H2O2 and Fe2þ from ferrous sulfate) were
extruded in a conical twin-screw micro-extruder at
120 8C, and injection-molded at 150 8C. Native starch
was partially oxidized by the peroxide, enabling ester
groups from PCL to cross-link with carbonyl and/or
carboxyl groups as generated from oxidized starch through
a peroxide-initiated free process. Ferrous ion catalyzes the
decomposition of H2O2 into highly reactive hydroxyl
radical that initiates this free-radical chain reaction
process. Addition of 3wt.-% organo-modified clay (MMT-C18)
in this chemically modified starch/PCL blends increased
elongation almost fourfold over that of unmodified starch/
PCL blends. Better solvent-resistance properties were also
achieved.
DOI: 10.1002/mame.200700395
Recent Advances in Reactive Extrusion Processing . . .
Biodegradable nanoscale-reinforced starch-based pro-
ducts were also prepared from an in situ chemically
modified thermoplastic starch and PBAT through reactive
extrusion. Four nanoclays were employed in this study:
hydrophilic Cloisite Naþ, organophilic Cloisite 30B, Ben-
tone 111, and Bentone 166.[181,182] First, thermoplastic
starch was in situ chemically modified in the presence of
nanoclay previously swollen in glycerol as plasticizer
together with MA. Melt blending of these nanoscale-
reinforced MTPS with PBAT was carried out in the
subsequent downstream blending operation. It is worth
noting that the swelling treatment was beneficial in order
to get a better dispersion of nanoclays within the
starch-based melt-blends. As shown previously, the MA
moieties thus grafted onto the starch backbone were able
to promote some glucosidation and hydrolysis reactions
during the preparation of MTPS, reducing the molecular
weight of native starch. This allows a better interpenetra-
tion of the resulting starch ester into the nanoclay
galleries, togetherwith a beneficial swelling pre-treatment
of nanoclay in glycerol. Interestingly enough, the resulting
formulations exhibited superior tensile strength and high
elongation at break, particularly with Cloisite 30B in
blown film applications. In this case, the presence within
the clay galleries of a quaternary ammonium ion bearing
two primary hydroxyl groups could form strong hydrogen
bond interactions with the MA-derived acidic moieties
grafted onto the starch backbone in the MTPS and to some
extent, to react with MTPS and PBAT through transester-
ification reactions promoted by MA. Within the
PBAT-g-MTPS graft copolymer-cloisite 30B nanocomposite,
WAXS and TEM analyses attested for the partial exfolia-
tion of some clay platelets. As a result, water vapor and
oxygen barrier properties of nanoscale-reinforced MTPS-g-
PBAT nanocompositions were enhanced as compared to
the precursors.
Abbreviations
AA acrylic acid;
Concluding Remarks and Outlook
Reactive extrusion is a versatile tool for cost-effective
polymer processing, which enhances the commercial
viability and cost-competitiveness of these materials, in
order to carry out melt-blending, and various chemical
reactions including polymerization, grafting, branching
and functionalization as well. For instance, the obvious
advantages of reactive extrusion polymerization process
are as follows:
AC acrylamide;
Al(OiPr)3 aluminum isopropoxide;
– SMa
�
olvent free-melt process
Al(OsecBu)3 aluminum sec-butoxide;
– CCAN ceric ammonium nitrate;
ontinuous processing, starting from monomer, and
resulting in polymer or finished product
CHPTMA 3-chloro-2-hydroxypropyltrimethylammo-
– Cnium chloride;
ontrol over residence time and residence time dis-
tribution
cromol. Mater. Eng. 2008, 293, 447–470
2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
– In
tegration of other extrusion streams along with thepolymerization process
Environmental concerns have triggered many efforts in
the development of environmentally friendly plastics, i.e.
biodegradable polymers tominimize thewaste disposed in
landfills. However, the use of these biodegradable poly-
mers as bulk materials is still restricted by their relatively
high production cost and poor mechanical properties com-
pared with commodity plastics such as polyethylene.
This review has hence outlined the substantial research
and the development that have been undertaken in the
realm of biodegradable polymers using high-performance
continuous reactive extrusion. When combined to both an
adapted chemistry such as the right selection of the
catalytic system, and a fine selection of extrusion para-
meters, reactive extrusion has demonstrated a remarkable
ability in the synthesis of biodegradable polymers through
ring-opening polymerization using effective initiating
systems, chemical modification of biodegradable polymers
and reactive melt-blending of natural polymers with
aliphatic (co)polyesters. Biodegradable polymers-based
(nano)composites have effectively been prepared from
common fillers such as wood fiber, but also using layered
silicates as nanoclays through reactive extrusion. What-
ever reactive melt-blending, biodegradable polymeric
melt-blends/composites with satisfactory overall physico-
mechanical behavior however require the ability to control
interfacial tension, to generate a dispersed phase of limited
size and strong interfacial adhesion, and to improve the
stress transfer between the component phases, i.e. the
reinforcement of their interface through the formation of
strong covalent bonds. This can be effectively completed
using proper interface compatibilization between differ-
ent components, as reactive extrusion is able to provide
during the reactive processing of these biodegradable
polymeric melt-blends/composites. The reactive extrusion
technology serves on the sustainability and future growth
of biodegradable polymers, particularly in the realm of
compatibilizing mechanisms, surface modifications, and
advanced processing techniques, and it is through an
understanding of these points that they are expected to
replace more and more commodity plastics.
www.mme-journal.de 467
J.-M. Raquez, R. Narayan, P. Dubois
CL e-caprolactone;CLa e-caprolactam;
DCP dicumylperoxyde;
DMA dynamic mechanical analyzer;
DMAP 4-dimethylaminopyridine;
DSC differential scanning calorimetry;
DTMA dynamic thermo-mechanical analysis;
ESEM environmental scanning electronic micro-
scopy;
FT-IR Fourier Transform Infrared;
LA L,L-lactide;
LLa v-lauryl lactam;
Luperox 101 2,5-dimethyl-2,5-di-(tert-butylperoxy)hex-
ane;
MA maleic anhydride;
MMT-C18 montmorillonite organicallymodifiedwith
octadecylammonium;
NaH sodium hydride;
NMR nuclear magnetic resonance;
REX reactive extrusion;
ROP ring-opening polymerization;
PBAT poly[(butylene adipate)-co-terephathalate];
PBS poly(butylene succinate);
PBSA poly(butylene succinate adipate);
P(C6H5)3 triphenyl phosphine;
PCL poly(e-caprolactone);PEA polyesteramide;
PEO poly(ethylene oxide);
PHBV poly(hydroxybutyrate-co-valerate);
PLA polylactide;
PDX 1,4-dioxan-2-one;
PPDX poly(1,4-dioxan-2-one);
SD substitution degree;
SEM scanning electronic microscopy;
Sn(Oct)2 tin (II) bis(2-ethylhexanoate);
TriGPC triple-detection gel-permeation chromato-
graphy;
WAXS wide angle X-ray scattering.
468
Acknowledgements: This research was partly funded by CornProducts International. The authors are very grateful to ‘‘RegionWallonne’’ and the European Community (FEDER, FSE) for generalsupport in the frame of ‘‘Objectif 1-Hainaut: Materia Nova’’. Thiswork was partly supported by the Belgian Federal GovernmentOffice of Science Policy (SSTC-PAI 6/27).
Received: December 6, 2007; Revised: January 30, 2008; Accepted:January 30, 2008; DOI: 10.1002/mame.200700395
Keywords: biodegradable polymer; chemical modification; melt-blending; nanocomposites; reactive extrusion; ring-openingpolymerization
Macromol. Mater. Eng. 2008, 293, 447–470
� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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