Olivier Martin,
Emmanuelle Schwach,
Luc Averous,
Yves Couturier
UMRINRA/URCA(FARE), CERME: Materials and Packaging Research Centre, ESIEC, BP1029,
51686 Reims Cedex 2, France
Properties of Biodegradable Multilayer Films Based on Plasticized Wheat Starch
Multilayer films based on plasticized wheat starch (PWS) and various biodegradable aliphatic
polyesters have been prepared through flat film coextrusion and compression molding. Poly(lactic
acid) (PLA), polyesteramide (PEA), poly(e-caprolactone) (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. The main goal of the polyester layers was to
improve significantly the properties of PWS in terms of mechanical performance and moisture
resistance. Since no specific compatibilizer or tie layer were added, the properties of subsequent
films rely on the compatibility between the respective materials only. The effects of glycerol content in PWS, polyester type, and film composition on the mechanical properties and adhesion
strength of multilayers were investigated. The conditions for optimal product performance were
examined. The multilayer films may be suitable for applications in food packaging or disposable
articles.
Keywords: Plasticized wheat starch; Polyesters; Multilayer structures; Adhesion strength
1 Introduction
Over the last decades, there has been growing needs to find alternatives to petroleum-based
products because of environmental concerns. Starch is an inherently biodegradable, renewable and
low-cost material, and has high potential in food or non-food applications [1]. In that respect, the
applications of starch have given rise to a great number of studies [2-5]. Native starch, when plasticized with glycerol and water, can be processed by conventional extrusion techniques, and will be
referred herein as plasticized wheat starch (PWS).
Unfortunately, there are some strong limitations to the development of starch-based products, due to
its poor mechanical properties and high sensitivity to moisture. One strategy to overcome these
weaknesses is to associate starch with a moisture resistant polymer with good mechanical
properties, while maintaining the overall biodegradability of the product. We have melt-blended
PWS together with various biodegradable polyesters, such as poly(e-caprolactone) (PCL) [6],
poly(lactic acid) (PLA) [7], poly(butylene succinate adipate) (PBSA) [8], and polyesteramide
(PEA) [9]. Blending PWS with these polyesters resulted in a significant improvement of the
properties of plasticized starch. However, although a "protective" polyester skin layer was formed
at the surface of
Correspondence: Luc Averous, ESIEC, BP1029, 51686 Reims Cedex 2, France. Phone: (+)33326913399, Fax: (+)326913803, e-mail: luc.averous@univ-reims.fr.
most blends during injection molding [6-9], the moisture sensitivity of plasticized starch was not yet
fully addressed. In an effort to develop the applications of starch, coating PWS with hydrophobic
biodegradable polyester layers should be preferred. Multilayer coextrusion has been widely used in
the past decades to combine the properties of two or more polymers into one single multi-layered
structure [10-12]. To date, there has been only few published studies reporting the use of plasticized
starch and polyesters in coextrusion [13, 14] and some examples have been described in the patent
literature [15-17]. The preparation of starch-based multilayers may also be carried out through
compression molding of plasticized starch and polyesters. This technique has been used by
Hulleman et al. [18-20] to shape plasticized starch into low thickness disks or films, and they
studied the properties of subsequent compression molded materials. Although coating waterresistant polyesters on molded starch substrates was disclaimed [21], no published studies
investigated the use of multi-step compression molding to prepare stratified products with plasticized starch as the central layer.
In this work, the properties of "plasticized starch-polyester" coextruded and compression molded
films are reported. Besides the PWS and polyester components, no specific compatibilizer or tie
layer were used. The effects of the film composition, polyester type and plasticizer content in PWS
on the adhesion strength between respective layers were examined. Finally, the conditions to
improve the adhesion between layers are critically discussed.
2 Materials and Methods
2.1 Materials
Native wheat starch was purchased from Chamtor (France). The starch contains 74% amylopectin
and 26% amylose, with residual protein and lipid contents less than 0.2% and 0.7%, respectively.
Glycerol (99.5% purity) was used as a plasticizer. Various types of PWS (PWS1, PWS2 and PWS3)
were prepared, differing in the glycerol to starch ratio. The procedure to prepare PWS from native
starch was detailed in previous studies [6]. Tab. 1 shows the different PWS formulations prepared
(with the starch, water and glycerol contents before and after extrusion). Both powdery and
extruded products were used in the study.
The aliphatic polyesteramide (PEA: LP BAK 404) was kindly supplied by BayerAG (Germany).
Poly(£-caprolac-tone) (PCL: CAPA 680) was purchased from Solvay (Belgium). Poly(lactic acid)
(PLA: 92% L-lactide and 8% meso-lactide) was obtained from Cargill-Dow Polymers LLC (USA).
Poly(butylene succinate adipate) (PBSA: Bionolle #3000) was obtained from Showa Highpolymer
Co. (Japan). Finally, poly(hydroxybutyrate-co-valerate) (PHBV: Biopol D400G, 8% HV and
Biopol D600G, 12% HV) was kindly supplied by Monsanto (USA). The choice of these polyesters
was based on requisites such as mechanical properties, moisture barrier properties and
biodegradability. Their properties and characteristics are listed in Tab. 2.
2.2 Processing and procedures
Multilayer coextrusion was performed with a lab-scale co-extrusion line consisting of two singlescrew extruders, a feedblock and a 350mm wide flat film forming die, followed by a three-roll
calendering system. A specially designed torpedo-like screw was used for the extrusion of
plasticized starch, and a 30mm diameter, 26:1 L/D single screw extruder was used for the outer
layers. Fig. 1 shows a schematic view of the three-layer coextrusion set-up. The feedblock was
designed to receive two feed streams, and the melt stream forming the outer layers is divided by
half (in white) and then merges with the main polymer stream (inner layer, in gray) right before the
flat die entry. As may be seen in Fig. 2, the combined melt streams originating from the feedblock
may spread uniformly across the entire width of die, thanks to the coathanger geometry, consisting
of a restriction in the flow channel section along the flow direction. The 350 mm wide flat die lips
are adjustable in height from 0.5 to 2 mm. As the multilayer film exits the die, it may be taken up by
a three-roll calendering system, allowing to obtain a smooth film with uniform thickness across the
entire width.
In typical experiments, the coextrusion line was run until steady-state operation was reached. The
flow rates of the polymers used were measured independently for each extruder, at the
corresponding screw speeds, so that the A:B flow ratio could be controlled. In all experiments, the
outer layers constituted the minor component, for cost efficiency reasons. The PWS were processed
in a ternTab. 1. Plasticized starch formulations used and densities of solid products.
* compositions are given in % (w/w) (total wet basis), and values between brackets were
determined after extrusion at 105°C and equilibration at 23 °C, 50% RH.
Tab. 2. Physical and molecular characteristics of the polyesters used in the study.
n.d. means the characteristic was not disclaimed by the supplier.
Fig. 1. Schematic of the lab-scale three-layer coextrusion line (the gray stream is the inner layer,
and the white polymer stream constitute the outer layers).
Fig. 2. Upper schematic view of the feedblock and flat coathanger die used in coextrusion experiments.
perature range between 100 °C and 130 °C, and the outer layer polyesters were processed at 110 °C,
120°C, 140 °C, 160 °C and 180 °C for PCL, PBSA, PEA, PLA and PHBV, respectively. Because
melt temperature differences between respective layers should not be too high, [23], the PLA had to
be plasticized to lower its processing temperature. The effect of various plasticizers on the thermal
properties of PLA has been reported previously [7]. In addition, a lower melting temperature PHBV
grade could be used (Biopol D600G). Once the steady-state conditions were reached, the multilayer
films were collected and set apart for further analyses. The compression molded multilayered
products were prepared through a two-step process. The first step consisted of preparing the
plasticized starch and polyester molded disks separately. The powdery plasticized starch and dried
polyester pellets (2 h at 50 °C in a vented oven) were compression molded with a Carver automatic
press (model 3891, Carver Inc., Indiana, USA). In a second step, the PWS and
polyester disks were stacked together and compression molded on the same press, without initial
pressure. The conditions for the compression molding of starch and polyesters and for that of the
stratified starch-polyester structures are shown in Tab. 3. The temperature settings of the hot press
were above the melting point of the respective materials for the molding of individual polymers,
and slightly above the lower melting polymer for the molding of multilayers. PET films were used
to facilitate de-molding, and the compression molded disks were cooled by air jet. After cooling, the
compression molded disks were stored under controlled temperature and humidity conditions prior
to further tests.
2.3 Sample analysis
Tensile testing was performed with a DY.25 MTS-Adamel Lhomargy tensile testing machine
(France), with a crosshead speed of 50mm/min. Rectangular strips
Tab. 3. Compression molding conditions of the PWS, the polyesters, and the subsequent multilayer
products.
(150x25 mm2) were cut from the defects-free area of the coextruded films (thickness: 0.8-1.5 mm)
using a sharp blade. Ten samples were tested per film, after a 1 week conditioning period at 50%
and 23 °C. Young's modulus, maximum tensile strength and elongation at break were determined.
Peel tests were carried out on a Thwing-Albert peel tester (model 225-100), at a rate of 50mm/min.
The test specimens were conditioned at 22 °C and 50% RH, and cut from the multilayer films into
100 x 20mm2 strips before testing. The outer polyester layer was delaminated manually and gripped
onto the load cell, while the film was secured on a sliding plate, so that a constant 90° angle between the polyester layer and PWS was maintained during the test. Load data collection started after
3s of pre-peel. A mean value was determined from each test, and each sample was tested five times.
A Kruss G23 goniometer (Germany) was used to measure the water contact angle and to determine
the kinetics of water sorption (slope of the curve contact angle = f(time)), related to the hydrophobic
character of the material. For more details, the reader may refer to published studies [6, 8].
Fig. 3. Chemical structures of the polyesters.
3 Results and Discussion
The properties of the products used in the coextrusion and compression molding experiments are
presented in Tabs. 1 and 2, and the chemical structure of the polyesters is shown in Fig. 3. As may
be seen from Tab. 1, for a given formulation, the glycerol and moisture compositions of plasticized
wheat starch (PWS) vary between the powdery form and the pellet (i.e., extruded) form. All kinds
of powdery PWS and extruded PWS pellets were used in coextrusion experiments, whereas the hot
pressed multilayers were prepared with PWS3 only. Most starch-polyester film combinations were
processed through coextrusion, because, contrary to compression molding, coextrusion is a
continuous and one-step process. Whatever the process used to prepare multilayers, the peel
strength, i.e. the force that has to be applied to delaminate one layer from another, was measured for
all PWS/polyester laminates.
3.1 Adhesion properties of PWS/polyesters systems
Fig. 4 shows a comparison between the peel strength results of hot pressed and coextruded films,
based on the PWS3 substrate. Hot pressed laminates were prepared under the conditions reported in
Tab. 3, and coextruded structures processing is described in a yet unpublished paper. The most
striking observation is that coextruded films have a higher adhesion (about twice as much) than
compression molded films. In effect, smaller peel strength
Fig. 4. Effect of the polyester type and preparation technique on the peel strength of PWS3polyester films (the white bars represent the hot pressed films and the dark bars represent
coextruded films).
values may be interpreted as lower adhesion between layers, related to the compatibility between
the respective materials. Moreover, the same trend of peel strength increase can be seen, from the
left to the right (Fig. 4) between both kinds of laminates, which is rather satisfactory. The
polyesteramide has the highest peel strength, PBSA and PCL show medium values, and the values
of PLA and PHBV are the lowest. Although peel strength does not constitute an absolute measure
of the adhesion, it reflects the compatibility or affinity between PWS and the respective polyesters.
This result confirms well our previous findings on the compatibility of plasticized starch with
polyesters [6-9] through the study of blended systems. Moreover, these results are in good
agreement with those of Biresaw and Carriere [22], who showed that the compatibility between
starch and PLA or PCL, based on surface energy measurements, is in the same order of magnitude
as in our results. The process parameters of the hot pressed laminates may also influence the final
adhesion between layers as well. However, the effect of varying molding time or pressure were not
investigated in this work since the resulting changes would lie within the range of peel strength
experimental error (5 to 15%).
It is well know that plasticizers migrating toward the surface of polymers can significantly affect
the adhesion. The amount of plasticizer (water, glycerol) added to starch for processing purposes
probably has an adverse effect on the adhesion strength between the layers of plasticized starchpolyester structures. We have previously reported the tendency of highly plasticized starch to exude
toward the surface with time [6]. Tab. 4 presents the peel strength data of PWS/polyester
coextruded structures, with the three types of PWS as the central layer, and with PEA or PLA as the
outer layers, respectively. From Tab. 4, it is obvious that the peel strength of PEA/PWS/PEA and
PLA/PWS/PLA films decreases as the glycerol/starch (G/S) ratio increases. For instance, the peel
strength of PWS/PLA laminates ranges from 0.12 to 0.05 N/mm for G/S ratios between 0.14 and
0.54. These results seem to confirm that migration of plasticizers to the interface readily occurs,
whatever the polyester skin layer, which is in agreement with the results of Wang et al. [13].
However, the multilayers prepared with PWS pellets (lines 7 to 10, Tab. 4) did not show the same
trend, since the peel strength data lie around 0.2 N/mm while the G/S ratio varied from 0.14 to 0.50.
This result may be interpreted by a diminution of the glycerol and water contents in extruded PWS
(see Tab. 1). Another explanation takes into account the rough surface texture of the PWS extruded
from pellets as opposed to the generally smooth surfaces obtained by extrusion of powdery PWS,
since rough surfaces may favor stronger ties with the adjacent layer. At last, the aging of the films is
known to influence the peel strength data [13]. All our tests were performed on one week aged and
equilibrated multilayers (at 23 °C, 50% R.H.), unless specified otherwise. It was noticed that the
Tab. 4. Effect of PWS composition on the peel strength of starch-polyester multilayers.
* The multilayer film contained some defects (i.e.. interfacial instabilities) at the interface.
1
The thickness was measured at the center of the coextruded films with a precision thickness tester.
2
The PWS used to prepare laminates bad been previously extruded and pelletized.
adhesion strength varied with time. For instance, the peel strength of PWS/PHBV ranged from
slightly adhesive (0.02 N/mm, Fig. 4) to virtually non-adhesive, since the PHBV layers
spontaneously peeled off the starch layer after a 50 weeks period of time. In that case, it was not
clearly identified whether the aging, the glycerol migration or the low compatibility was responsible
for the loss of adhesion.
3.2 Strategies to improve adhesion
As stated in a previous paper [7], the combination of plas-ticized starch with PLA is of great
advantage, for unquestionable reasons of cost, renewability and biodegradabili-ty. However, the
apparent lack of affinity between both polymers seems difficult to overcome. Two different
strategies were attempted to increase the adhesion strength between starch and PLA. The first
approach consisted of lowering the melting temperature of PLA down to 130°C by a plasticizer
(polyethylene glycol. PEG [7]). so to avoid melt temperature differences between the respective
layers during coextrusion. In effect, processing temperatures above 160°C can be critical for starch
in that substantial changes may arise in the PWS layer, for example vapour bubbles growth and
starch degradation. The other strategy consisted of melt blending PLA with other biodegradable
polyesters in a first step, and then to coextrude the blended materials (PLA + other polyester)
together with PWS, in order to increase the ties between layers. Blend compositions with
respectively 10% and 20% (w/w) of PEA, PCL and PBSA were prepared and tested. These attempts
were carried out through coextrusion only. The former strategy relies on obtaining better surface
characteristics, whereas the latter relies on a stronger bonding network between layers. The results
of the different strategies on the peel strength data are compiled in Tab. 5. Plasticizing the PLA with
polyethylene
glycol (lines 1 to 3) allowed to decrease the melting temperature, but had a detrimental effect on the
total peel strength. The peel strength decreased from 0.05 to 0.02 N/mm for PEG contents ranging
from 0 to 20%. Excess plasticizer is believed to have a lubricating effect on the PLA layer. The
melting temperature of PLA/polyester blends (lines 4 to 12) decreased by 10 to 15 °C, whatever the
blend system. But only moderate changes could be seen on the peel strength data of the "plasticized
starch-blended polyester" films, except in the case of PEA and PBSA (lines 8, 9 and 12), the peel
strength increased by about 40%. A third strategy was tested to improve the adhesion strength
between plasticized starch and some polyesters. Contrary to the above described trials, the blended
composition was used in the inner starch layer, still to enhance the ties between starch and polyester
layers. The polyesters constituted the minor phase of blend (i.e., 5% and 10%, w/w) and were
blended with PWS as described in previous papers [6-9]. For instance, when PLA was used as cap
layers, PWS/PLA blended compositions were prepared and coextruded together with pure PLA.
The influence of a blended PWS inner layer was tested with a selection of the three most interesting
polyesters, i.e., PEA, PLA and PCL. The effects on the peel strength data are presented in Fig. 5.
The dark bars correspond to the peel strength results of coextruded starch-polyester films from Fig.
4. The gray and white bars represent the peel strength values of blended starch-polyester
multilayers, with the starch layer comprising 5% and 10% of polyester, respectively. An increase in
peel strength of 19 to 31% was observed for PEA-based films, 37 to 50% for PCL-based films, and
a 40% increase was observed for PLA based films.
Modifying PLA with additives such as plasticizers and other polyesters led to mixed results. When
plasticizers were used, the final peel strength results were even lower than
Tab. 5. Effect of various additives (plasticizer and polyesters) on the thermal properties of PLA and
on the resulting peel strength data of PWS/PLA coextruded structures.
* Standard deviations are given between brackets.
Fig. 5. Effect of the added polyester (0%, 5% and 10%, w/w) in the PWS phase on the total peel
strength of subsequent coextruded films.
with pure PLA, due to excess of plasticizers (lubricating effect). When other polyesters such as
PEA, PCL and PB-SA were blended together with PLA, the melting temperature of the blends
decreased, even at low polyester composition, but the consequences on the adhesion strength were
moderate. However, the accuracy of the measurement is not high enough to monitor small changes.
Furthermore, the presence of some defects on the films, referred to as interracial instabilities [13],
may alter the results. However, much care was taken in the sampling of multilayer films for testing.
The last strategy (blending polyesters with PWS) yielded satisfactory results. From 20% to 50%
increase in peel strength was obtained when the blended phase was the inner layer. But these improvements were obtained at the cost of a supplementary operation and utilization of more
polyester, which is not desirable from an economical point of view. There was no significant effect
of the film composition on the peel strength, since films with cap layers ranging from 50 to 200 μm
roughly led to the same data.
3.3 Mechanical and moisture resistance properties
The mechanical properties of the starch polyester multilayers obtained by coextrusion are presented
in Tab. 6. The properties of the individual polyesters are indicated in Tab. 2. During each
coextrusion run, the flow ratio was set to obtain similar film compositions, with polyesters as the
minor component (20-25% of the total film weight). The cap layers were delaminated manually and
weighed to determine the exact composition of films. Tab. 6 shows that properties of the
multilayered products clearly lie between those of individual constituents, but are closer to the
characteristics of extruded starch. The deformation properties of brittle PSWI did not vary much,
even when combined with ductile polyesters such as PCL and PBSA. The value of low modulus
PWS3 significantly raised thanks to the higher modulus polyesters. The effect of the film
composition (i.e., cap layers thickness) was not investigated. These results somehow differ from
those of Wang et al. [13] who found that the properties of the laminates were close to those of
extruded starch sheets, because their cap layers constituted as low as 2% of the mass of products.
Fig. 6 shows the results obtained from the water contact angle measurements performed on starch
polyester blends, as well as on pure polyesters and plasticized starch. PWS3-blends based on PEA,
PLA and PCL were chosen. Low values of the slope (in absolute value) indicate stronger
hydrophobic character of the products. For all blends, it was observed that the hydrophilic character
decreased rapidly from 0 to 10% polyester content and kept decreasing until the value of pure
polyester was reached. PWS/PLA blends, whatever the composition, showed the best
characteristics, i.e. the highest hy-drophobicity. As expected, better barrier properties can
Tab. 6. Mechanical properties of "polyester/starch/polyester" films.
* Compositions are expressed in weight percentage of the total film, for each three layers.
Fig. 6. Effect of the polyester content (PEA. PLA and PCL, % w/w) on the hydrophilic character of
the product.
be obtained with starch products laminated with moisture resistant polyesters than with blends.
Furthermore, immersion tests of starch-polyester multilayers in water were also performed to check
the ability of the multilayers to resist water penetration. The penetration of water has detrimental
effects on the multilayers such as the swelling of PWS layer, resulting in a decrease of elastic
modulus, and spontaneous delamination of the cap layers. In the immersion tests, in a few days
time, no significant product swell or delamination was observed. That result is satisfactory since it
indicates that the multilayers have a good moisture resistance. This confirms results of Shogren
[24] who indicated that, although polyesters have lower water barrier properties than polyethylene,
they can provide sufficient protection to starch products for short-terms applications. Finally, it is
interesting to note that the moisture resistance of polyesters (PLA > PCL > PEA) is in an inverse
order compared to the ranking of adhesion properties. The polarity of polyesters (Fig. 3), with PEA
polar amide groups for instance, support that observation.
4 Conclusion
Developing compostable and low-cost multilayer materials based on plasticized starch and
biodegradable polyesters is interesting in more than one sense. One prerequisite in the preparation
of multilayered products based on starch is to obtain sufficient adhesion between layers, moisture
barrier properties and uniform layer thickness distribution. Two different techniques were used to
prepared multilayers, i.e., coextrusion and compression molding. Different levels of peel strength
were found,
depending on the compatibility of plasticized starch with the respective polyesters. In particular,
polyesteramide presented the best adhesion to the PWS layer, probably thanks to its polar amide
groups (see Fig. 3). The PCL and PBSA showed medium adhesion values, and PLA or PHBV were
the least compatible polyesters. The same trend in the magnitude of adhesion strength was observed
between coextruded and hot pressed multilayers. These results are in good agreement with previous
findings on starch-polyester blends [6-9] or laminates [13], and on surface energy properties [22].
We also observed the gradual decrease of peel strength values as the glyc-erol content in PWS was
increased. It was possible to increase the adhesion properties of the film by up to 50%, by
introducing polyesters blends in the cap layer or PWS/polyester blends in the central layer. Finally,
the polyester cap layer slightly improved the mechanical properties of plasticized starch, especially
for the low-modulus PWS type. It was also checked that the multilayers have satisfactory water
resistance properties.
This work allowed to validate the use of the coextrusion technique to prepare compostable
multilayer films based on plasticized starch. However, some inherent problems exist due to the
multilayer flow conditions encountered in coextrusion, such as encapsulation and interfacial instabilities phenomena. Addressing these problems is a crucial issue because they can be detrimental to
the product, affecting the quality and functionality. Such investigations are currently underway in
our laboratory.
Acknowledgements
This work was funded by Europol'Agro through a research program devoted to the development of
packaging materials based on agricultural resources.
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(Received: June 7, 2001)