biodegradable multiphase systems based on plasticized starch - a review
DESCRIPTION
MULTIPHASE SYSTEMSTRANSCRIPT
Biodegradable Multiphase Systems Based onPlasticized Starch: A Review
Luc Averous*
ECPM (LIPHT), Strasbourg Cedex 2, France
CONTENTS
ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
2. BIODEGRADABLE POLYMERS . . . . . . . . . . . . . . . . . . . 233
2.1. Concepts: Biodegradability and Renewability . . . . . . . . . 233
2.1.1. Biodegradability and Compostability . . . . . . . . . . 233
2.1.2. Renewability and Sustainable Development . . . . . . 234
2.2. Biodegradable Polymers Classifications . . . . . . . . . . . . . 234
2.3. Biodegradable Polyesters . . . . . . . . . . . . . . . . . . . . . 235
2.3.1. Presentation . . . . . . . . . . . . . . . . . . . . . . . . 235
2.3.2. Agro-Resources Based Polyesters . . . . . . . . . . . 235
2.3.2.1. PLA . . . . . . . . . . . . . . . . . . . . . . . 235
2.3.2.2. PHAs . . . . . . . . . . . . . . . . . . . . . . 239
2.3.3. Petroleum-Based Polyesters . . . . . . . . . . . . . . . 240
2.3.3.1. PCL . . . . . . . . . . . . . . . . . . . . . . . 240
2.3.3.2. PEA . . . . . . . . . . . . . . . . . . . . . . . 241
2.3.3.3. Aliphatic Copolyesters . . . . . . . . . . . . 241
2.3.3.4. Aromatic Copolyesters . . . . . . . . . . . . 241
231
DOI: 10.1081/MC-200029326 1532-1797 (Print); 1532-9038 (Online)
Copyright # 2004 by Marcel Dekker, Inc. www.dekker.com
*Correspondence: Luc Averous, ECPM (LIPHT), 25 rue Becquerel, 67087 Strasbourg Cedex 2,
France; Fax: 33-3-90-24-27-16; E-mail: [email protected].
JOURNAL OF MACROMOLECULAR SCIENCEw
Part C—Polymer Reviews
Vol. C44, No. 3, pp. 231–274, 2004
2.4. Agro-Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
2.4.1. Presentation . . . . . . . . . . . . . . . . . . . . . . . . 242
2.4.1.1. Polysaccharides . . . . . . . . . . . . . . . . 242
2.4.1.2. Lignins and Ligno-Celluloses . . . . . . . . 243
2.4.2. Native Starch . . . . . . . . . . . . . . . . . . . . . . . 243
3. PLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
3.2. PLS Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
3.3. Molten State Behavior . . . . . . . . . . . . . . . . . . . . . . 247
3.4. Solid-State Behavior . . . . . . . . . . . . . . . . . . . . . . . . 249
3.4.1. Crystallinity . . . . . . . . . . . . . . . . . . . . . . . . 249
3.4.2. Plasticizer and Water Interactions . . . . . . . . . . . 250
3.4.3. Post Processing Aging . . . . . . . . . . . . . . . . . . 250
3.4.4. PLS Properties . . . . . . . . . . . . . . . . . . . . . . 250
3.4.5. PLS Issues and Strategies . . . . . . . . . . . . . . . . 253
4. PLS-BASED MULTIPHASE SYSTEMS . . . . . . . . . . . . . . . 253
4.1. Structures Classification . . . . . . . . . . . . . . . . . . . . . . 253
4.2. Biodegradable PLS-Based Blends . . . . . . . . . . . . . . . . 254
4.2.1. Blends Properties . . . . . . . . . . . . . . . . . . . . . 256
4.2.2. Blends Compatibilization . . . . . . . . . . . . . . . . 257
4.3. Biodegradable PLS-Based Multilayers . . . . . . . . . . . . . 258
4.3.1. Interfacial Instabilities . . . . . . . . . . . . . . . . . . 258
4.3.2. Interfacial Adhesion . . . . . . . . . . . . . . . . . . . 259
4.4. Biodegradable PLS-Based Composites . . . . . . . . . . . . . 260
4.4.1. Cellulose Fibers Reinforcement . . . . . . . . . . . . . 260
4.4.2. Lignin Fillers . . . . . . . . . . . . . . . . . . . . . . . 261
4.4.3. Nanocomposites and Mineral Microfillers . . . . . . . 261
5. CONCLUSION AND PERSPECTIVES . . . . . . . . . . . . . . . . 262
ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . 263
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
ABSTRACT
The aim of this review is to show the relationships between the structure, the process,
and the properties of biodegradable multiphase systems based on plasticized starch
(PLS), the so-called “thermoplastic starch.” These mutiphase materials are obtained
when associating association between plasticized starches and other biodegradable
materials, such as biodegradable polyesters [polycaprolactone (PCL), polyhydroxyalk-
anoates (PHAs), polylactic acid (PLA), polyesteramide (PEA), aliphatic, and aromatic
copolyesters], or agro-materials (ligno-cellulosic fiber, lignin etc.). Depending on
materials (soft, rigid) and the plastic processing system used, various structures
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(blends, composites, multilayers) can be obtained. The compatibility problematic
between these hetero-materials is analyzed. These starchy products show some interest-
ing properties and have some applications in different fields: packaging, sports,
catering, agriculture and gardening, or hygiene.
Key Words: Plasticized starch; Agro-polymer; Multiphase system; Biodegradable
polyester; Structure; Plastic processing.
1. INTRODUCTION
It is widely accepted that the use of long-lasting polymers for short-lived applications
(packaging, catering, surgery, hygiene), is not entirely adequate. This is not justified when
increased concern exists about the preservation of ecological systems. Most of today’s
synthetic polymers are produced from petrochemicals and are not biodegradable. These
persistent polymers are a significant source of environmental pollution, harming wildlife
when they are dispersed in nature. For example, the effects of plastic bags are well known
to affect sea-life.[1] Besides, plastics play a large part in waste management, and the col-
lectivities (municipalities, regional, or national organizations) are becoming aware of the
significant savings that the collection of compostable wastes would provide. Besides,
valorizing the plastics wastes presents some issues. Energetic valorization yields some
toxic emissions (e.g., dioxin). Material valorization implies some limitations linked to
the difficulties to find accurate and economically viable outlets. In addition, material valor-
ization shows a rather negative eco-balance due to the necessity, in nearly all cases, to
wash the plastic wastes and to the energy consumption during the process phases
(waste grinding and plastic processing). For these different reasons, reaching the con-
ditions of conventional plastic replacements by degradable polymers, particularly for
packaging applications, is of major interest for the different actors of the socio-economical
life (from the plastic industry to the citizen).
The potential of biodegradable polymers and more particularly that of polymers
obtained from agro-resources such as the polysaccharides (e.g., starch) has long been
recognized. However, to this day, these agro-polymers largely used in some applications
(e.g., food industry) have not found extensive applications in the packaging industries to
replace conventional plastic materials, although they could be an interesting way to over-
come the limitation of the petrochemical resources in the future. The fossil fuel and gas
could be partially replaced by greener agricultural sources, which should also participate
in the reduction of CO2 emissions.[1]
2. BIODEGRADABLE POLYMERS
2.1. Concepts: Biodegradability and Renewability
2.1.1. Biodegradability and Compostability
According to ASTM standard D-5488-94d, biodegradable means capable of under-
going decomposition into carbon dioxide, methane water, inorganic compounds, or
Multiphase Systems Based on PLS 233
biomass in which the predominant mechanism is the enzymatic action of micro-organisms
that can be measured by standard tests, over a specific period of time, reflecting available
disposal conditions. There are different media (liquid, inert, or compost medium) to
analyze biodegradability. Compostability is material biodegradability using compost
medium. Biodegradation is the degradation of an organic material caused by biological
activity, mainly micro-organisms’ enzymatic action. This leads to a significant change
in the material chemical structure. The end-products are carbon dioxide, new biomass
and water (in the presence of oxygen: aerobia) or methane (oxygen absent: anaerobia),
as defined in the European Standard EN 13432:2000. Unfortunately, depending on the
standard used (ASTM, EN), different composting conditions (humidity, temperature
cycle) must be realized to determine the compostability level.[2] Then, it is difficult to
compare the results using different standard conditions. We must also take into account
the amount of mineralization as well as the nature of the residue left after biodegrada-
tion.[3] The accumulation of contaminants with toxic residues and chemical reactions of
biodegradation can cause plant growth inhibition in these products, which must serve as
fertilizers. Actually, the key issue is to determine the environmental toxicity level for
these by-products, which is known as eco-toxicity.[4]
Some general rules enable the estimating of the biodegradability evolution. An
increase of parameters such as the hydrophobic character, the macromolecular weight,
the crystallinity or the size of spherulites decreases biodegradability.[5] On the contrary,
the presence of polysaccharides (blends) favors biodegradation.
2.1.2. Renewability and Sustainable Development
Renewability is linked to the concept of sustainable development. The UN World
Commission on “Environment and Development in our Future” defines sustainability as
the development which meets the needs of the present time without compromising the
ability of future generations to meet their own needs. According to Narayan,[1] the manu-
factured products, e.g., packaging, must be designed and engineered from “conception to
reincarnation,” the so-called “cradle-to-grave” approach. The use of annually renewable
biomass, like wheat, must be understood in a complete carbon cycle. This concept is
based on the development and the manufacture of products based on renewable and bio-
degradable resources: starch, cellulose, etc. By collecting and composting biodegradable
plastic wastes, we can generate much-needed carbon-rich compost: humic materials.
These valuable soil amendments can go back to the farmland and reinitiate the carbon
cycle. Besides, composting is an increasing key point to maintain the sustainability of
the agricultural system by reducing the consumption of chemical fertilizers.
2.2. Biodegradable Polymers Classifications
Biodegradable polymers are a growing field.[6–8] A vast number of biodegradable
polymers have been synthesized or are formed in nature during the growth cycles of all
organisms. Some micro-organisms and enzymes capable of degrading them have been
identified.[6,9,10]
Depending on the evolution of the synthesis process, different classifications of the
different biodegradable polymers have been proposed. Figure 1 shows an attempt at
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classification. We have four different categories. Only, three categories (a–c) are obtained
from renewable resources:
a. polymers from biomass such as the agro-polymers from agro-resources (e.g.,
starch, cellulose),
b. polymers obtained by microbial production, e.g., the polyhydroxyalkanoates
(PHAs),
c. polymers conventionally and chemically synthesized and whose monomers are
obtained from agro-resources, e.g., the polylactic acid (PLA),
d. polymers whose monomers and polymers are obtained conventionally, by
chemical synthesis.
We can also classify these different biodegradable polymers into two main families: the
agro-polymers (category a) and the biodegradable polyesters (categories b–d).
2.3. Biodegradable Polyesters
2.3.1. Presentation
Figure 2 shows the chemical structures of various biodegradable polyesters. Table 1
shows main polyesters, which are commercially available. Besides, physical and mechani-
cal properties of some commercial polyesters are given in Table 2.
2.3.2. Agro-Resources Based Polyesters
2.3.2.1. PLA
Lactic acid can be produced in different ways: chemical or biological, i.e., by fermen-
tation of carbohydrate from lactobacillus.[11] The enantiomeric monomers (D and L) are
polycondensed via its cyclic dimer (lactide) by ring-opening polymerization (ROP) to a
high molecular weight polymer.[10,12,13]
Figure 1. Classification of the biodegradable polymers.
Multiphase Systems Based on PLS 235
Compared to the other biodegradable polyesters, polylactic acid (PLA) is the product
that at the present time has one of the highest potentials due to its availability in the market
and its low price.[14–16] For instance, Cargill–Dow has developed processes that use corn
and other feedstock to produce different PLA grades (NatureWorksw).[18] For this
company, the estimated production in 2002 is 100 ktons. Actually, it is the highest pro-
duction of biodegradable polyester. In 2004, its price is around 3E/kg. Different
Figure 2. Chemical structures of different biodegradable polyesters.
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companies such as Mitsui Chemicals (Japan), Galactic (Belgium), or Dainippon Ink
Chemicals (Japan) produce smaller PLA outputs.
Properties of PLA are highly related to the ratio between the two mesoforms D and L.
Commercially available, we can find 100% L-PLA which present a high crystallinity
(C-PLA) and copolymers of poly(L-lactic acid) and poly(D,L-lactic acid) which are
rather amorphous (A-PLA).[17–19] PLA can show crystalline polymorphism[20] which
can lead to different melting peaks[21] with a main transition at 1528C for the D,L-PLA
(see Table 2). Furthermore, PLA can be plasticized using oligomeric lactic acid (OLA),
citrate ester,[22] or low molecular weight polyethylene glycol (PEG).[21,23] Table 3
shows the evolution of some properties depending on the plasticizer content. We can
notice a Tg shift of 408C by addition of 20wt% of OLA. At room temperature, we are
close to the rubber plateau with high elongations at break (200%). Besides, the effect of
plasticization increases the chain’s mobility and then favors the PLA organization and
crystallinity we obtain after plasticization, a crystallinity ranging between 20% and 30%.
PLA presents a medium water and oxygen permeability level[24] comparable to poly-
styrene.[25] These different properties associated with its tunability and its availability
favor its actual developments in different packaging applications (trays, cups, bottle,
films, etc.).[15,18] McCarthy[26] showed that A-PLA presents a soil degradation rate
Table 1. Classification of main biodegradable polyesters, commercially available.
Trade name Company
Agro-resources based polyesters
PLA Natureworks Cargill-Dow LLC (USA)
Lacty Shimadzu (Japan)
Lacea Mitsui Chemicals (Japan)
Heplon Chronopol (USA)
CPLA Dainippon Ink Chem. (Japan)
PLA Galactic (Belgium)
PHA
PHB, PHBV Biopol Monsanto-Metabolix (USA)a
PHB, PHBV Biocycle Copersucar (Brazil)
PHBHx, PHBO, PHBOd Nodax Procter & Gamble (USA)
Petroleum-based polyesters
PCL CAPA Solvay (Belgium)
Tone Union Carbide (USA)
Celgreen Daicel (Japan)
PEA BAK Bayer (Germany)a
Aliphatic copolyesters (e.g., PBSA) Bionolles Showa Highpolymer (Japan)
EnPol Ire Chemical Ltd (Korea)
Skygreen SK Chemicals (Korea)
Lunare SE Nippon Shokubai (Japan)
Aromatic copolyesters (e.g., PBAT) Eastar Bio Eastman Chemical (USA)
Ecoflex BASF (Germany)
Biomax Dupont (USA)
PHEE Dow Chemicals (USA)a
aThe production of these polyesters has been stopped.
Multiphase Systems Based on PLS 237
Table
2.
Physicalandmechanical
properties
ofsomebiodegradable
polyesters.
PLA
Dow-Cargill
(Natureworks)
PHBV
Monsanto
(BiopolD400G)
HV¼
7mol%
PCLSolway
(CAPA
680)
PEA
Bayer
(BAK1095)
PBSA
Showa
(Bionolle
3000)
PBATEastm
an
(EastarBio
14766)
Density
1.25
1.25
1.11
1.07
1.23
1.21
Meltingpoint(D
SC)(8C)
152
153
65
112
114
110–115
Glass
transition(D
SC)(8C)
58
5261
229
245
230
Crystallinity(%
)0–1
51
67
33
41
20–35
Modulus(M
Pa)
(NFT51-035)
2,050
900
190
262
249
52
Elongationat
break
(%)(N
FT
51-035)
915
.500
420
.500
.500
Tensile
stress
atbreak
ormax
(MPa)
(NFT51-035)
——
14
17
19
9
Biodegradationa
mineralization(%
)
100
100
100
100
90
100
Water
permeabilityWVTRat
258C
(g/m
2/day)
172
21
177
680
330
550
Surfacetensionb(g)(m
N/m
)50
—51
59
56
53
gd(dispersivecomponent)
37
—41
37
43
43
gp(polarcomponent)
13
—11
22
14
11
aAt60daysin
controlled
compostingaccordingto
ASTM
5336.
bDeterminationsfrom
contact
anglesmeasurementsofprobes
liquids.
So
urc
es:Refs.[21,27,33,40,72,132,208] .
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much slower compared with polybutylene succinate/adipate (PBSA). PLA is presumed to
be biodegradable, although the role of hydrolysis versus enzymatic depolymerization in
this process remains open to debate.[27] Composting conditions are found only in industrial
units with a high temperature (above 508C) and a high relative humidity (RH) to promote
chain hydrolysis. But, according to Tuominen et al.,[28] PLA biodegradation does not
exhibit any eco-toxicological effect.
2.3.2.2. PHAs
PHAs can be produced in different ways, chemically or biologically, by fermentation
from feedstock. This family comprises mainly of a homopolymer, polyhydroxybutyrate
(PHB), and different copolyesters, polyhydroxybutyrate-co-hydroxyalkanoates such as
polyhydroxybutyrate-co-hydroxyvalerates (PHBV) (see Fig. 2), or polyhydroxybutyrate-
co-hydroxyhexanoate (PHBHx), polyhydroxybutyrate-co-hydroxyoctonoate (PHBO),
and polyhydroxybutyrate-co-hydroxyoctadecanoate (PHBOd).
PHB is a natural polymer. It is an intracellular storage product of bacteria and algae.
After fermentation, PHB can be obtained by solvent extraction. Recently, Monsanto has
developed a geneticmodification of plants tomake themproduce small quantities of PHB.[18]
PHB is a highly crystalline polyester (80%) with a high melting point, Tf ¼
173–1808C (Monsanto data), compared with the other biodegradable polyesters. Glass
transition temperature (Tg) is around 58C. The homopolymer shows a narrow window
for the process condition. To ease the transformation, PHB can be plasticized with
citrate ester, but the corresponding copolymer (PHBV) is more adapted for the process.
PHBV can be produced by bacterial fermentation (e.g., with Alcaligens eutrophus)[29]
of bioproducts such as glucose containing propionic or valeric acid, followed by an extrac-
tion step and purification of the polymer. PHBV can be also produced from butyrolactone
and valerolactone with an oligomeric aluminoxane catalyst.[30] According to the synthesis,
we obtain different structures, isotactic with random stereosequences for the bacterial
copolyesters and with partially stereoregular block for the synthetic copolyesters.
A large range of bacterial copolymer grades had been industrially produced by
Monsanto under the Biopolw trade mark, with HV contents reaching 20%. The production
was stopped at the end of 1999. Metabolix bought Biopolw assets in 2001. Currently,
different small companies produce bacterial PHBV, e.g., Copersucar (Brazil) produces
PHBV (HV ¼ 12%) 45% crystalline, from sugarcane molasses.[31] Recently, Procter
Table 3. Plasticization effects on some PLA properties.
Tg (DSC)
(8C)Crystallinity
(%)
Modulus
(MPa)
Elongation
at break (%)
A-PLA 58 1 2,050 (44) 9 (2)
þ10% OLA 37 21 1,256 (38) 32 (4)
þ20% OLA 18 24 744 (22) 200 (4)
þ10% PEG 400 30 26 1,488 (39) 26 (5)
þ20% PEG 400 12 29 976 (31) 160 (12)
Note: Standard deviations are given in brackets.
Source: Ref.[21].
Multiphase Systems Based on PLS 239
and Gamble has begun to develop a large range of polyhydroxybutyrate co-hydroxyalk-
anoates (PHBHx, PHBO, PHBOd). Industrial production is planned in 2005.
Figure 2 and Table 2 give, respectively, the chemical structure and the properties of
some PHBV.Material properties can be tailored by varying the HV content. An increase of
the HV content induces an increase of the impact strength and a decrease of
– melting temperature and glass transition,[32]
– crystallinity,[33]
– water permeability,[33]
– tensile strength.[34]
Besides, PHBV properties can evolve when plasticization occurs, e.g., with citrate ester
(triacetin).[34,35] The PHAs as the PLAs are sensitive to the process conditions. Under
extrusion, we obtain a rapid diminution of the viscosity and the molecular weight due
to macromolecular linkage by increasing the shear level, the temperature and/or the resi-dential time.[36]
The kinetics of enzymatic degradation is variable according to the crystallinity,
the structure,[5,31] and then, to the processing history.[37] Bacterial copolyesters are
more biodegradable than synthetic copolyesters.[38]
2.3.3. Petroleum-Based Polyesters
A large number of biodegradable polyesters are based on petroleum resources
obtained chemically from synthetic monomers.[12–18] We can distinguish (see Table 1),
according to the chemical structures (see Fig. 2), polycaprolactones (PCLs), polyester-
amides (PEAs), aliphatic or aromatic copolyesters. All these polyesters are soft at room
temperature.
2.3.3.1. PCL
Poly(1-caprolactone) is obtained by ROP of 1-caprolactone in the presence of alumi-
num isopropoxide.[12,13,38] PCL is widely used as a PVC solid plasticizer or for poly-
urethane applications. But, it finds also some application based on its biodegradable
character in domains such as controlled release of drugs, soft compostable packaging,
etc. Different commercial grades are produced by Solvay (CAPAw), by Union Carbide
(Tonew), and by Daicel (Celgreenw).
Figure 2 and Table 2 give, respectively, the chemical structure and the properties of
this polyester. PCL shows a very low Tg (2618C) and a low melting point (658C), whichcould be a handicap in some applications. Therefore, PCL is generally blended[27,30,39,40]
or modified (e.g., copolymerization, crosslink[41]).
Tokiwa and Suzuki[42] have discussed the hydrolysis of PCL and biodegradation by
fungi. They have shown that PCL can be enzymatically degraded easily. According to
Bastioli,[27] the biodegradability can be clearly claimed but the homoplymer hydrolysis
rate is very low. The presence of starch can significantly increase the biodegradation
rate of PCL.[39]
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2.3.3.2. PEA
PEA was industrially obtained from the statistical copolycondensation of polyamide
(PA 6 or PA 6-6) monomers and adipic acid.[18,43] Bayer had developed different commer-
cial grades under BAKw trademark but their production stopped in 2001. Figure 2 and
Table 2 show, respectively, the chemical structure and the properties of this polyester.
It is the polyester, which presents the highest polar component, and then it shows good
compatibility with other polar products, e.g., starchy compounds. On the other hand, it
presents the highest water permeability (see Table 2).
Currently, the environmental impact of this copolymer is still open to discussion.
Fritz[44] had shown that this biodegradable polyester presented after composting a nega-
tive eco-toxicological impact but more recently, Bruns et al.[45] have confirmed these
results. These authors discussed Fritz’s experiments and more precisely the composting
methods used.
2.3.3.3. Aliphatic Copolyesters
A large number of aliphatic copolyesters are biodegradable copolymers based on pet-
roleum resources. They are obtained by the combination of diols, such as 1,2-ethanediol,
1,3-propanediol, or 1,4-butadenediol, and dicarboxylic acid: adipic, sebacic, or succinic
acid. Showa Highpolymer (Japan) has developed a large range of polybutylene succinate
(PBS) obtained by polycondensation of 1,4-butanediol and succinic acid. Polybutylene
succinate/adipate (PBSA), presented in Fig. 2, is obtained by addition of adipic acid.
These copolymers are commercialized under the Bionollew trade mark.[18] Table 2
shows the properties of such a terpolymer. Ire chemical (Korea) commercializes exactly
the same kind of copolyesters under EnPolw trade mark. Skygreenw, a product from SK
Chemicals (Korea), is obtained by polycondensation of 1,2-ethanediol, 1,4-butadenediol
with succinic and adipic acids.[46] Nippon Shokubai (Japan) also commercializes an ali-
phatic copolyester with Lunare SEw trademark. These copolyesters’ properties depend
on the structure,[47] i.e., the combination of diols and diacids used.
Biodegradability of these products depends also on the structure. The addition of
adipic acid, which decreases the crystallinity[48] tends to increase the compost biodegra-
dation.[49] According to Ratto et al.,[50] the biodegradation results demonstrate that
although PBSA is inherently biodegradable, the addition of a starch filler significantly
improve the rate of degradation.
2.3.3.4. Aromatic Copolyesters
Compared with totally aliphatic copolyesters, aromatic copolyesters are often based
on terephthalic diacid. Figure 2 and Table 2 show, respectively, the chemical structure
and the properties of such products (e.g., see Eastar Biow from Eastman). Besides,
BASF and DuPont commercialize aromatic copolyesters with Ecoflexw [18] and Biomaxw
trade marks, respectively. Biomaxw shows a high terephthalic acid content which modifies
some properties such as themelting temperature (2008C). But, according toMuller et al.,[47]
an increase of terephthalic acid content tends to decrease the degradation rate, while aro-
matic like aliphatic copolyesters degrade totally in micro-organisms environment
(compost). Ecoflexw biodegradation has been analyzed by Witt et al.,[51] they concluded
that there is no indication for an environmental risk (ecotoxicity) when aliphatic–aromatic
copolyesters of the Ecoflex-type are introduced into composting processes.
Multiphase Systems Based on PLS 241
Another kind of polyester can be classified into the group of the aromatic polyesters,
the polyhydroxy ester ethers (PHEE). They are obtained from adipic acid and the
diglycidil ether of bisphenol A. A commercial PHEE had been developed by Dow chemi-
cals (USA), but the production has stopped.
2.4. Agro-Polymers
2.4.1. Presentation
Agro-polymers are mainly extracted from plants. They are compostable and renew-
able polymers. We find different families such as the polysaccharides or the lignins.
They show some common characteristics such as a hydrophilic character. Most of them
can be processed directly, either plasticized as fillers or modified by chemical reactions.
2.4.1.1. Polysaccharides
This family is represented by different products such as starch or cellulose based on
glucose units linked in macromolecular chains.
Cellulose is the first agro-polymer in the biosphere. It is a linear polymer consisting of
b(1–4) linked D-glucose synthesized by plants and bacteria. Table 4 shows that according
to the botanical species, we can measure different cellulose contents. It is a cheap semi-
crystalline material, which is widely used in paper production but also as a reinforcing
element in polymer matrixes. To obtain a thermoplastic material, cellulose is modified
by acetylation (cellulose acetate), whose production actually is low. After acidic treat-
ment, and elimination of the amorphous parts of cellulose microfibrils, we obtain the whis-
kers (mono-crystals), which are used to develop nanocomposites materials.[52,53]
Table 4. Composition of ligno-cellulose fibers, from various botanical origins.
Fibers
Cellulose
content (%)
Lignins
content (%)
Hemicellulose
content (%)
Ash
(silice, . . .) (%)
Straw fibers
Wheat 29–35 16–21 27 5–9
Rice 28–36 12–16 23–28 15–20
Rye 33–35 16–19 27–30 2–5
Wood fibers
Conifer 40–45 26–34 7–14 ,1
Leafwood 38–49 23–30 19–26 ,1
Others
Flax 43–47 21–23 16 5
Jute 45–53 21–26 15 0.5–2
Cotton linters 80–85 — 1–3 0.8–2
Source: Ref.[192].
Averous242
2.4.1.2. Lignins and Ligno-Celluloses
Lignins are second in natural earth abundance. They are aromatic amorphous poly-
mers. Figure 3 shows that the lignin structure is built with a lot of different chemicals
functions. Depending on botanical resources and fractionation, we obtain a diversity of struc-
tures. Lignins are obtained from almost all types of natural plants, in association with cellu-
lose: ligno-cellulose. The architecture of the vegetal tissue is based on a complex composite
structure in which the matrix is based on lignins and the reinforcing elements are cellulose
fibers. Table 4 gives the lignins contents for different botanical species.
Lignins are by-products of pulp and paper mills and are conventionally treated as a
waste material having low economical usage. They are generally used for their calorific
energy. Lignins are separated from the cellulose either by strong alkaline or acidic sol-
utions, or by a high-pressure steam treatment followed by solvent extraction. They can
be used after fractionation as fillers or as antioxidants.[54] Lignins can be modified, e.g.,
by esterification, to decrease the polar character for inclusion in apolar matrixes.
2.4.2. Native Starch
Starch is the main storage supply in botanical resources (cereals, legumes, and tubers).
It is a widely available raw material on earth. It presents different industrial applications in
fields such as food, paper, textile, and adhesive.
Figure 3. Structural and chemical presentation of wood lignin. Source: Ref.[54].
Multiphase Systems Based on PLS 243
Starch granules can be isolated from plants. Main sources come from wheat, potato,
maize, rice, cassava, pea, waxy maizes, amylomaizes, etc. According to the resource,
native starches have dimensions ranging from 0.5 to 175mm and appear in a variety of
shapes.[55] For instance, Fig. 4 shows micrographs of the shape of different starch granules
(wheat and pea).
Starch is a polysaccharide consisting in D-glucose units, referred to as homoglucan or
glucopyranose. Starch is composed of two different macromolecules, amylose and amylo-
pectin. Amylose [Fig. 5(a)] is a linear or sparsely branched carbohydrate based on a(1–4)
bonds with a molecular weight of 105–106. The chains show spiral shaped single or double
helixes. Amylopectin [Fig. 5(b)] is a highly multiple-branched polymer with a high mole-
cular weight: 107–109. It is based on a(1–4) bonds but also on a(1–6) links constituting
branching points occurring every 22–70 glucose units.[56]
Table 5 shows that according to the botanical origin, starch composition is variable.
Some mutant plant species present some special composition. We can find rich-amylose
starch with amylomaize (till 80%) and some rich-amylopectin starch with waxy maize
(.99%).
After the different industrial stages of isolation and refining, starch usually shows
some traces of lipids, gluten, or phosphate, which can interfere with the starch properties,
e.g., by the formation of lipid complexes or with the gluten, by Maillard reactions.
Starch shows a special granular organization,[57] a high degree of radial organization
from hilum. Macromolecules are mainly oriented according to the radial axis. The
ultra-structure is obtained by inter-macromolecules hydrogen links, between hydroxyl
groups, with the participation of water molecules. Amylose and the branching regions
of amylopectin form the amorphous zone in the granule. Amylopectin is the dominating
crystalline component in native starch with double helix organizations; we can also find
co-crystallization with amylose and single-helical crystallization between amylose and
free fatty acids or lipids. Several types of crystallinity are observed in the granule
denoted as A, B, C, or V-types. Figure 6 shows that the crystalline regions (20–45%)
are arranged as thin lamellar domains, perpendicular to the radial axis.
Figure 4. Micrographs of native starch (SEM observations): wheat starch (left) and pea starch
(right). White scale ¼ 10mm.
Averous244
3. PLS
3.1. Definitions
Except for the purpose as filler to produce reinforced plastics,[58] native starch must be
modified to find applications such as destructured starch. The destructuring agent is
usually water. We obtain starch gelatinization with the combination of water (high
content) and heat. Gelatinization is the disruption of the granule organization. The
starch swells forming a viscous paste with destruction of most of inter-macromolecule
hydrogen links. We obtain a reduction of both, the melting temperature (Tm or Tf) and
the glass transition (Tg). Figure 7 shows that according to the level of destructuration
and the water content, we obtain different products and applications; e.g., we can obtain
expanded structures with a rather high water content. Such closed cells structures
(foams) have been developed to obtain shock absorbable and isothermal packaging.[59]
Most starch applications require water dispersion and partial or complete gelatinization.
By decreasing the moisture content (,20wt%), the melting temperature tends to be close to
the degradation temperature. For instance, for pure dry starch Tm ¼ 220–2408C[60] com-
pared to 2208C, which is the temperature at the beginning of starch decomposition.[61] To
overcome this last issue, we add a non-volatile (at the process temperature) plasticizer to
decrease Tm, such as glycerol or other polyols (sorbitol, PEG. . .).[62,63] Amixture of different
polyols can also be held.[64] Other compounds such as those containing nitrogen (urea,
ammonium derived, amines . . .) can be used.[65–67] These plasticized starches (PLS) are
also commonly called “thermoplastic starches” or TPS.[66] The first patents and articles on
Figure 5. Amylose structure (a), amylopectin structure and cluster model (b).
Multiphase Systems Based on PLS 245
Table
5.
Compositionandcharacteristicsfordifferentkindsofstarch.
Starch
Amylose
contenta(%
)
Amylopectin
contenta(%
)
Lipid
contenta(%
)
Protein
contenta(%
)
Phosphorus
contenta(%
)
Moisture
contentb(%
)
Granule
diameter
(mm)
Crystallinity
(%)
Wheat
26–27
72–73
0.63
0.30
0.06
13
25
36
Maize
26–28
71–73
0.63
0.30
0.02
12–13
15
39
Waxystarch
,1
99
0.23
0.10
0.01
N.d.
15
39
Amylomaize
50–80
20–50
1.11
0.50
0.03
N.d.
10
19
Potato
20–25
79–74
0.03
0.05
0.08
18–19
40–100
25
No
te:N.d.,notdetermined.
aDetermined
onadry
basis.
bDetermined
afterequilibrium
at65%
RH,208C
.
So
urc
e:Refs.[66,209] .
Averous246
these processable materials were published at the end of the eighties.[68–70] PLS combines
starch, a non-votalile and high-boiling[71] plasticizer and often water. Figure 7 shows
that PLS are obtained both with low water content and high level of destructuration.
PLS is usually transformed under thermomechanical treatment as a thermoplastic, using
conventional machines for plastic processing (e.g., by extrusion).
3.2. PLS Process
The disruption of granular starch is the transformation of the semi-crystalline granule
into a homogeneous, rather amorphous material with the destruction of hydrogen bonds
between the macromolecules. Disruption can be accomplished by casting (e.g., with dry
drums) or by applying thermomechanical energy in a continuous process. The combi-
nation of thermal and mechanical inputs can be obtained by extrusion, a common
plastic processing technique. This process can be in one or two stages. In a one-stage
process, the extruder, usually a twin-screw extruder, is fed with native starch. Along the
barrel, water and liquid plasticizer are successively introduced. In a two-stage process,
the first stage is a dry blend preparation.[72] Into a turbo-mixer, under high speed, the
plasticizer is added slowly into the native starch until a homogeneous dispersion is obtained.
Then, the mixture is placed in a vented oven allowing the diffusion of the plasticizer into the
granule. The plasticizer swells the starch. After cooling, the right amount of water is added to
the mixture using a turbo-mixer. This dry blend is then introduced into an extruder. Figure 8
gives the different stages of the extrusion. The starch granules are fragmented. Under temp-
erature and shearing, starch is destructured, plasticized, melted but also partially depolymer-
ized. After the processing, we obtain a homogeneous molten phase.
3.3. Molten State Behavior
The extrusion and viscous behavior of molten PLS is known to depend on tem-
perature, moisture content, and thermomechanical treatment.[73–76] Viscosity data and
rheological models that take into account these variables have been reported in literature.
Figure 6. Radial structure of a starch granule (amorphous and crystalline region). Source: Ref.[210].
Multiphase Systems Based on PLS 247
Martin et al.[77] have summarized the main rheological studies performed on starch,
including the measurement technique and models used. They all report a thermoplastic-
like behavior of low hydrated starch, with an Arrhenius dependence on temperature and
similar for moisture content. Conversely, structural modifications of starch, which affect
the viscosity of the product, are reflected differently by specific mechanical energy
(SME), by the screw speed, or even by the extruder barrel pressure, which depend on
the machine characteristics. This discrepancy underlines the need to ascertain the depen-
dence of starch melt viscosity upon structural factors or variables directly involved in its
transformation. Some authors have introduced terms relating to the modification of starch,
such as conversion, degree of transformation, or extent of degradation.[78–82] Lai and
Kokini[83] studied the rheological properties of high amylose (70%) and high amylopectin
(98%) cornstarches, and showed the strong influence of the processing history undergone
by products prior to viscosity measurements. Zheng and Wang[82] identified the contri-
bution of shear and thermal energies in the conversion of waxy cornstarch. Della Valle
et al.[84,85] took into account the starch transformation and the resulting macromolecular
degradation, evaluated by chromatography (SEC profiles) and intrinsic viscosity,[86,87]
by confirming the importance of the term noted b. This latter is reported in Eq. (1),
Figure 8. Schematic of starch process by extrusion.
Figure 7. Presentation of different starchy products, depending on water content and destructuring
level.
Averous248
which presents a pseudo-plastic model with the pseudo-plastic index (m) and the consist-
ency (K). The influence of SME on the degradation of starch products was evaluated by
intrinsic viscosity [h] measurements. The gradual decrease of [h] with increasing SME
confirmed that macromolecular degradation occurred.[77,88]
h ¼ Kj _gg jm�1 with the consistency:K ¼ K0 � expE
R
1
T� aMC� a0GC� b � SME
� �
ð1Þ
where MC, moisture content; GC, glycerol content; a, a0, and b, dimensionless
coefficients.
A common difficulty of rheological studies is that a thermomechanical treatment is
needed to obtain a homogeneous molten starch phase prior to measurement. This material
shows a behavior not totally thermoplastic but thermo-mechano-plastic.[77] Both, mechan-
ical energy and temperature are required to obtain a molten material to cause it to flow.
Consequently, traditional rheometry (rotational, capillary) is not appropriate for the
study of the rheological behavior of low-moisture starches. In that respect, extruder-fed
slit rheometry is well adapted.[89–93] Martin et al.[77,88] have also shown great interest
in combining capillary and slit dyes on an in-line viscometer attached to the head of an
extruder (a large shear rate range can be covered: 4 decades) with thermomechanical con-
ditions (shear rate, SME, temperature) comparable to those used during the processing.
Also of interest is the Rheoplastw, known as a reliable commercial tool,[94,95] to
perform capillary viscometry measurements or to simulate extrusion of starchy products.
Finally, starch melts are commonly considered to exhibit viscoelastic behavior. The
measurement of elastic components of PLS molten phases, associated with the first
normal stress difference (N1), is not trivial because conventional rheometers do not
allow to perform mechanical treatment and cannot prevent volatilization of plasticizers.
In a recent study using plane-plate geometry, PLS was shown to behave mainly as a
solid-like material, because subjected to insufficient mechanical treatment.[96] As an
alternative, Senouci and Smith [90] related the entrance and exit pressure losses in a slit
viscometer dye to the elastic properties of potato starch-based materials. Entrance and
exit pressure effects have been also used to evaluate the elasticity of melt of plasticized
wheat starch. Some authors[77] have concluded that significant elastic properties may be
expected.
3.4. Solid-State Behavior
3.4.1. Crystallinity
Compared with native starch, PLS shows a lower crystallinity. According to Van
Soest et al.,[97] two kinds of crystallinity are obtained after PLS processing: residual crys-
tallinity from native starch (A, B, C types) and processing induced crystallinity (V and E
types). Crystallinity induced by processing is influenced by parameters, such as the extru-
sion residence time, the screw speed, or the temperature. It is mainly caused by the fast
recrystallization of amylose into single-helical structure. After post processing aging,
Multiphase Systems Based on PLS 249
Van Soest[98] proposed a complex model for PLS with amorphous and crystalline amylose
and amylopectin and probable co-crystallization between amylose and amylopectin.
3.4.2. Plasticizer and Water Interactions
The evolution of the moisture content are of consequence on the evolution of proper-
ties or transitions such as the glass transition, because water also acts as a plasticizer[99] but
it is a volatile plasticizer which is equilibrated in mechanisms of sorption–desorption with
the environment.[66] Lourdin et al.,[100] and Mathew and Dufresne[63] have determined
according to the RH, the nature and content of the plasticizer and the moisture content
after equilibrium on starch. They have shown that for low RH, water content decreases
when plasticizer increased. In this case, plasticizer molecules then occupy some sites
initially occupied by water. For higher activities (.43% RH) water content tends to
increase with the plasticizer content due to plasticizer–water interactions (e.g., see
Table 6). According to Lourdin et al.,[100] this evolution is not linear but very well
marked for high plasticizer content (e.g., more than 18wt%, for glycerol), i.e., above
the limit where phase separation between the polysaccharide and the plasticizer can
occur, with constitution of plasticizer-rich phases.[101] This concentration threshold corre-
sponds to one glycerol molecule per three-anhydroglucose unit.
Lourdin et al.[102] have shown that at low plasticizer content an antiplastification
effect occurs. Due to strong interaction between the plasticizer and the starch, a hydrogen
links network appears and we obtain a material reinforcement. Then, when the plasticizer
contents increase (e.g., .12wt% for glycerol into potato starch), interactions between
plasticizer–plasticizer occur with a material swelling and a plastification effect.
3.4.3. Post Processing Aging
Different authors[40,103,104] have shown that after processing, PLS shows an aging
with a strong evolution of mechanical properties such as the tensile modulus, which
increase during several weeks. PLS presents two kinds of aging behavior depending on
glass temperature value. In the sub-Tg domain, PLS shows a physical aging versus time,
with a material densification.[104,105] At a temperature above Tg, PLS shows retrogradation
phenomena with the evolution of the crystallinity and rearrangements of plasticizer mol-
ecules into the material[103] versus storage time. The retrogradation kinetic depends on the
macromolecules mobility, on the plasticizer type and content.[104]
3.4.4. PLS Properties
According to the plasticizer/starch ratio, PLS presents a large range of attributes.
Table 6 shows for increasing glycerol/starch ratio, the evolutions of different proper-
ties, including physical and mechanical behaviours. Figure 9 shows the thermograms’
evolution for the formulations given in Table 6. Figure 9 illustrates that the Cp variations
at glass transitions are very low, although DMTA determinations are generally more desir-
able to obtain PLS glass transitions. DMTA evolutions are drawn in Fig. 10 for the same
formulations. The evolutions of tang delta versus temperature show two transitions.
Main and broad relaxation (a-transition) can be linked to the PLS glass transition.[40]
Secondary relaxation (between 2508C and 2608C) could be connected to glycerol
Averous250
Table
6.
Plasticized
starch—
form
ulationsandproperties.
Glycerol/
dry
starch
ratioa(w
/w)
Water
a(w
t%)
Density
aTg(D
SC)(8C)
a-Transition
(DMTA)(8C)
Modulusa
(MPa)
Max
tensilea
strength
(Mpa)
Elongationat
break
a(%
)
S74G10W10b
0.14
91.39
43
63
1144(42)
21.4
(5.2)
3(0)
S74G18W12b
0.25
91.37
831
116(11)
4.0
(1.7)
104(5)
S67G24W9b
0.35
12
1.35
27
17
45(5)
3.3
(0.1)
98(5)
S65G35W0b
0.50
13
1.34
220
111(1)
1.4
(0.1)
60(5)
No
te:Standarddeviationsarein
brackets.
aContentsandproperties
afterequilibrium
at238C
and50%
RH,6weeks.
bInitialform
ulation(S
xG
yW
z):S,starch
(xwt%
);G,glycerol(y
wt%
);W,water
(zwt%
).
So
urc
e:Refs.[40,132] .
Multiphase Systems Based on PLS 251
glass transition.[40] According to Lourdin et al.,[101] this latter relaxation could be an indi-
cator of the level of interactions between the plasticizer and the polysaccharides. In the
explored domains, Figure 10 shows that this relaxation temperature decreases when gly-
cerol content increases and then, the phase segregation.
Figure 9. DSC evolutions with different glycerol/starch ratios. Source: Ref.[72].
Figure 10. DSC evolutions with different glycerol/starch ratios. Source: Ref.[72].
Averous252
Table 6 exemplifies that we can achieve a large range of mechanical properties by
variation of the plasticizer content. Further, PLS shows different levels of permeability
(moisture and oxygen).[100,106,107] Although PLS water permeability is high due to its
polar character, oxygen permeability is found low comparedwithmost polyesters.[24] Perme-
ability increases drastically at the glass transition, and continues to rise on the rubber
plateau, with the plasticizer content.
3.4.5. PLS Issues and Strategies
As amaterial, PLS shows strong attributes: a total compostability without toxic residues,
the renewability of the resource. Besides, compared with synthetic thermoplastic, it is a
rather cheap material. Compared with fossil resources, the price of starch resources
remains stable and even tends to decrease due to overproduction of cereals in the world. In
addition, PLS can be easily processedwith plastic processingmachines. It shows awide prop-
erties range according to the plasticizer level and the starch botanical source. But unfortu-
nately, PLS shows various issues for some applications (e.g., packaging) such as a great
moisture sensitivity, rather weak mechanical properties compared with synthetic polymers.
To overcome these weaknesses, during the last decades different strategies were elaborated.
Chemical starch modification has been carried out, since the first half of the twentieth
century, in the continuity of research on modified cellulose. Yet in 1942, Mullen and
Pacsu[108] published a critical analysis of the different methods of starch (tri)ester
preparation. In 1943, the same authors[109] presented an industrial usage of such a com-
pound. Since then, a large volume of literature has been published on this subject.
Starch esterification (e.g., by acetylation) improves its water resistance.[110] In addition,
we can control the degree of substitution (DS, between 0 and 3) to obtain the accurate
hydrophobic character. Besides, these compounds can be plasticized with, for instance,
ester citrate.
But, the strategy of chemical modification is strongly limited as far as toxicity and
diversity of by-products obtained during the chemical reactions are concerned. Another
limitation lies in the cost of both process stages, that of the modification and product puri-
fication (to eliminate the by-products). Besides, the chemical reactions lead to some inci-
dences on the polysaccharide molecular weight, with a decrease due to chain linkages.
Consequently, the mechanical properties are altered.[111] Such products do not fulfil the
requirements for the substitution of PLS for material applications. Yet for one or two
decades, another more promising strategy has been developed for the association of
PLS with other biodegradable compounds to obtain compostable multiphase materials.
We can obtain different structures with corresponding properties. This approach induces
some problematics linked with the quality of the interfaces, i.e., concerning the continuity
between the phases and the compatibility between the different materials.
4. PLS-BASED MULTIPHASE SYSTEMS
4.1. Structures Classification
Two different materials can be associated to PLS to obtain compostable materials:
biodegradable polyesters or agro-materials (lignins, cellulose, etc.). Figure 11 shows the
different sorts of structures, which can be obtained by associations, and the related process.
Multiphase Systems Based on PLS 253
4.2. Biodegradable PLS-Based Blends
Blend shows different attributes.[112,113] Blending is the easier process to associate
different polymers together. Blending provides a powerful route to obtain materials
with improved property/cost performances. This approach is cheaper than the develop-
ment of new polymers. Besides, blends are also commonly used as models to test the
compatibility between different polymeric phases because a blend presents a great
surface of interphase compared with, e.g., multilayer structures.
PLS has been widely used in blends with other polymers.[114,115] A lot of patents were
published on this topic (see Table 7). These considerable research efforts have led to
starch-based blends being commercialized, Mater-Bi#[30,71] from Novamont (Italy) or
Bioplastw from Biotec (Germany).[115] Starch blends production, for this latter, is now
managed by Novamont. To produce these commercial blends, starch is blended with
non-biodegradable polymers (polyolefins) or with biodegradable polyesters (e.g., PCL).
Applications concern packaging, disposable cutlery, gardening, leisure, hygiene and
the like.
In the past, blends with synthetic polymers, such as PE[116] or EVOH,[117] were devel-
oped leading to non-fully biodegradable materials.[118,119] Because these controversial
materials were presented as biodegradable, they have been named bio-fragmentable. To
maintain the compostability feature, different biodegradable blends were developed. A
great number of patents have been published on this topic (see Table 7). We find some
associations of PLS with agro-polymers such as proteins[118–121] or pectins.[120,122] But,
most of the research is focused on the blending of PLS with biodegradable polyesters:
PCL,[32,39,40,123–129] PEA,[39] PHBV,[126,127,130,131] PBSA,[50,132] poly(butylene adipate-
co-terephtalate) (PBAT),[132] PLA[21] or PHEE.[27] These commercially available poly-
esters show some interesting and reproducible properties such as a more hydrophobic
Figure 11. Schematic of multiphase systems based on plasticized starch—process and structures.
Averous254
Table 7. Main patents disclosing biodegradable PLS-based materials.
Patents no. Inventors (assignee) Comments
Plasticized starch (PLS)
EP 282 451 Dobler et al. (1988) Destructured starch and PLS
EP 304 401 Tomka et al. (1989) Destructured starch and PLS
WO 90/05161 Tomka (1990) PLS
EP 400 531 Bastioli et al. (1990) PLS
WO 92/04408 Cole and Egli (1992) PLS and process
US 5,275,774 Bahr et al. (1994) Extrusion with water removing
during the process
EP 609 983 De bock and Bahr (1994) Plasticizers: mixture of sugar
alcohols
WO 94/28029 Lorcks et al. (1994) PLS and process
WO 96/35748 Haschke and Tomka (1996) Plasticizers: graft copolymers
EP 758 669 Bastioli et al. (1997) PLS
WO 97/48764 Loerck et al. (1997) Plasticizers: polymers system
WO 98/51466 Feil et al. (1998) PLS foam moulding
Biodegradable PLS-based blends
EP 327 505 Rehm et al. (1989) PLSþ PHB
EP 535 994 Tokiwa et al. PLSþ PCL
FR 2 691 467 Van Hoegaerden (1993) PLSþ proteinaceous material
US 5,280,055 Tomka (1994) PLSþ cellulose ester
EP 580 032 Dehennau and Depireux
(1994)
PLSþ PCL
EP 596 437 Tomka (1994) PLSþ biodegradable polymers
WO 94/25493 Fishman and Coffin (1994) PLSþ pectines
WO 95/24447 Narayan and Krishnan (1995) PLSþ PCL, in absence of water
EP 704 495 Muramatsu and Hino (1996) PLSþ PBSA or PHBV, or PCL
WO 97/03120 Seppala et al. (1997) PLS–polyester compatibilization
with poly-isocyanate
JP 9 137 069 Shimoozono and Hino (1997) PLSþ aliphatic bio. polyester
JP 9 294 482 Shimoozono and Hino (1997) PLSþ aliphatic bio. polyester
WO 98/20073 Bastioli et al. (1998) PLSþ bio. polyesterþ non-ionic
surfactant
WO 99/02595 Bengs et al. (1999) PLSþ biodegrable
polymerþ phosphate
EP 947 559 Bastioli et al. (1999) PLSþ bio. polyestersþ non-ionic
surfactant
EP 965 615 Bellotti et al. (1999) PLSþ bio. polyesters (heterophase)
US 6,025,417 Willett et al. (2000) PLSþ PHEE
US 6,218,321 Lorcks et al. (2001) Production of fibers: PLSþ bio.
polyester
Biodegradable PLS-based multilayers
US 5,391,423 Wnuk et al. (1995) Multilayers with biodegradable
polymers
US 5,512,378 Bastioli et al. (1996) Multilayers PLS/wax fabrication
WO 97/27047 Shogren and Lawton (1997) Coating by spraying solutions
(continued)
Multiphase Systems Based on PLS 255
character, a lower water permeability and some improved mechanical properties, com-
pared with PLS.
4.2.1. Blends Properties
Solid-state properties of the blends depend on the nature of the polyester phase. At
ambient temperature, polyesters can be rigid (e.g., PLA) or soft (e.g., PCL, PBSA,
PBAT). Then, corresponding mechanical properties are tuneable.
Young’s Modulus can be evaluated between two boundaries determined by the serial
model from Reuss [cf. Eq. (2)] and the parallel model from Voight [cf. Eq. (2)] for respect-
ively, the lower and the higher boundary.[21,40,72] In the case of a co-continuous structure,
as shown on some PLS/PLA blends,[21,133] modulus can be evaluated with a rather good
agreement by Davies model [cf. Eq. (2)].[134]
EReuses ¼ f1E1 þ f2E2
1
EVoight
¼f1
E1
þf2
E2
E1=5Davies ¼ f1E
1=51 þ f2E
1=52
ð2Þ
where Ei and fi are, respectively, the modulus and the volume fraction of the ith phase.
Starch and more hydrophobic compounds such as biodegradable polyester are rather
immiscible and mixing produces blends with separated phases with rather poor interfacial
Table 7. Continued.
Patents no. Inventors (assignee) Comments
FR 2 791 603 Averous et al. (2000) Coextrusion: bio. polyester/PLS/bio. polyester
Biodegradable PLS based composites
WO 95/04111 Rettenbacher and Mundigler
(1995)
PLSþwood cellulose
WO 97/03121 Happonen et al. (1997) PLSþ cellulose fibers
US 5,666,216 Tomka (1997) PLSþ natural cellulose fibers
US 5,705,536 Tomka (1998) Foam: PLSþ cellulose fibers
EP 842 977 Rothe (1998) PLSþ natural fibers (milling
industry)
WO 99/56556 Wang (1999) Foam: PLSþ cellulose
fibersþ proteins
US 6,136,097 Heuer et al. (2000) PLSþ ligno-cellulose
fibersþ proteins
US 6,231,970 Andersen et al. (2001) PLSþ natural fibersþmineral
fillers
WO 01/68762 Fischer and Fischer (2001) Nanocomposites:
PLSþ organoclays
US 6,406,530 Boehm and Bengs (2002) PLSþ lignins
Note: Bio. polyester ¼ biodegradable polyester.
Averous256
properties. This is illustrated by the results of different micro- or macro-structural
approaches:
– The shifts of both glass transitions before and after blending which can be com-
pared with the evaluations determined with the Couchmann-Karasz’s model.[135]
– MEB observations of the structure and phase dispersion.
– Tensile test values at break,[21] elongations and strengths.
– Peel test measurements.[21]
– Calculation of the theoretical work of adhesion from contact angles of probes
liquids.[136–138]
For instance, Table 2 shows that PEA presents the highest surface tension, a high polar com-
ponent, and PLA the lowest. These different determinations allow the establishment of a
classification from the least compatible (PLA) to the most compatible polyesters (PEA).
However, this low compatibility induces special behaviors and properties. During the
injection moulding process, we have a preferential migration of the polyester, the low-
viscosity polymer,[139] towards the mould surface. After cooling, we obtain a polyester-
rich skin and a starchy core.[40,140] Imaging NMR has highlighted this pseudo-multilayer
structure. Compared with PLS, this stratified structure gives to the blend rather good water
resistance[40,72] due to the polyester surface protection. Water sensitivity, determined by
contact angle measurements with water drops deposit decreases drastically for contents
lower than 10wt% of polyesters in the blend.[132] Biodegradability of such blends is
modified, degradation occurs from the starchy core toward the skin.[141]
4.2.2. Blends Compatibilization
To improve the compatibility between two phases, compatibilization strategies are
generally developed. This strategy implies the addition of a compound: the compatibilizer,
which can be obtained by modification of at least one polymer initially present in the
blend.[142] For compatibilization, authors have followed different ways:
– The functionalization of the polyester, with maleic anhydride,[143–145] with
pyromellitic anhydride[146] with polyacylic acid[147] or by producing telechelic
polyester phosphate.[148]
– The functionalization of starch with polyglycidyl methacrylate[149] or, with
urethane functions by reaction of n-butylisocyanate.[150]
– The starch–polyester reticulation with coupling agents such as peroxides,[41,151,152]
or polyisocianates.[153–157]
– The development of copolymers: starch-graft polyester. To compatibilize starch
and PCL, different authors have developed PCL-grafted starch or dextran.[158–163]
The grafting is obtained by ROP of 1-caprolactone on the polysaccharide. Reaction
can be catalyzed with stannous octoate and initiated with aluminum alkoxydes.
The length of the grafts can be controlled to obtain a comb structure.[162,163] The
same approaches can be used with PLA-grafted polysaccharides (unpublished work).
Results are improving for the interphase continuity, observed by MEB and by DSC, and
for the mechanical properties.
Multiphase Systems Based on PLS 257
4.3. Biodegradable PLS-Based Multilayers
Compared to blends, multilayer structures present some attributes. Moisture sensi-
tivity is not fully addressed in a blend because of starch phase distribution close to the
surface. Development of moisture-resistant starch-based products shall be undertaken
through multilayers, allowing for the preparation of sandwich-type structures with PLS
as the central layer and the hydrophobic biodegradable component as the surface outer
layers. On this topic, some patents were published (see Table 7). For instance, Bastioli
et al.[164] proposed different processes (coextrusion, casting and hot melt techniques)
to protect starch-based materials with wax layers. Stratified materials can be obtained
by a multistep process based on compression moulding.[165] The case of coating is
also mentioned in literature. Coating has been achieved by spraying[166] or painting[167]
different dilutes liquid such as biodegradable polyester solutions onto the starch-based
material.
Coextrusion seems the best option, since it offers the advantages of being a one-step,
continuous and versatile process. Multilayer coextrusion has been widely used in the past
decades to combine the properties of two or more polymers into one single multilayered
structure.[168] However, some problems inherent to the multiphasic nature of the flow are
likely to occur during coextrusion operations, such as non-uniform layer distribution,
encapsulation, and interfacial instabilities, which are critical since they directly affect
the quality and functionality of the multilayer products. The layer encapsulation phenom-
enon corresponds to the surrounding of the more viscous polymer by the less viscous one,
as shown by Lee and White.[169] Figure 12 illustrates interfacial instabilities with wavy
interfaces. Yih[170] and Hickox[171] pioneered studies on interfacial instabilities,
suggesting that viscosity differences may cause instabilities of stratified flow. Schrenk
and Alfrey[172] investigated the factors responsible for the onset of instabilities, and
suggested the existence of a critical shear stress value beyond which interfacial instabil-
ities are likely to occur. Han and Shetty[173,174] described in detail the factors responsible
for the occurrence of instabilities, such as critical shear stress at the interface, viscosity and
elasticity ratio, and layer thickness ratio. Khomani,[175,176] Su and Khomani[177,178] exam-
ined theoretically the elastic and viscosity effects on the interfacial stability, according to
the die geometry and layer depth ratio. They determined the role of elasticity in the mech-
anism of instabilities. Wilson and Khomani[179–181] studied the propagation of periodic
flow disturbances experimentally and numerically and determined the stable and unstable
flow conditions, in a good agreement with models.
Despite the number and diversity of studies on multilayer flows and stability, only
few articles[165,182,183] and patents[164,184] reported the use of PLS and polyester in coex-
trusion processes. Different stratified structures were processed by coextrusion and
studied: with PCL,[165,182] PBSA,[165,182] PEA,[165,182] PLA,[165,182,183] PBAT,[182] or
with PHBV.[183,185]
4.3.1. Interfacial Instabilities
Martin and Averous[183] have shown on a PLS/PEA/PLS systems, that the key para-
meters are the skin-layer viscosity and thickness, the global extrusion rate and the dye
geometry after determining the stable and unstable flow conditions. They have found
that the occurrence of instabilities is strongly related to the shear stress at the interface.
Averous258
Viscosity differences, high extrusion rates, and low cap layer thickness have been shown
to promote the occurrence of instabilities. Conversely, moderate extrusion rate, appropri-
ate dye geometry and lesser viscous component at the dye wall have favored the stability
of the combined flow.
4.3.2. Interfacial Adhesion
Different authors[165,182] have shown that in the absence of compatibilizer according
to the level of PLS–polyester compatibility and to the process, we obtain different levels
of interfacial strength, as illustrated in Fig. 13. Elsewhere, Wang et al.[182] have shown that
the addition of plasticizer to the starch, or a decrease of the polyester molecular weight
(PCL), tends to decrease the peel strength. Martin and Averous[183] have found that the
interfacial strength of biodegradable multilayer films can be improved by a controlled
extent of instabilities at the interface, via mechanical interlocking between respective
layers.
Figure 12. (a) Schematic of a multiplayer film exhibiting wavy instabilities at the interface. (b)
Photographs of a delaminated film (PLS/PLA/PLS) exhibiting wave-like instabilities. Source:
Ref.[77].
Multiphase Systems Based on PLS 259
4.4. Biodegradable PLS-Based Composites
4.4.1. Cellulose Fibers Reinforcement
Different types of fibers or microfibrils were tested in association with PLS: micro-
fibrils from potato pulp,[52,53] bleached leafwood fibers,[186–189] fibers from bleached euca-
lyptus pulp,[190] flax and jute fibers.[191] The different authors have shown high
compatibility between both polysaccharides. For instance, Averous et al.[187] have
found a Tg increase of 288C by addition of 10wt% of cellulose fibers into a PLS
matrix. This evolution is linked to the fiber–matrix interactions, which decrease starch
chains mobility. In another example, Fig. 14 shows a MEB image of a cryogenic fracture;
the cellulose fibers are imbedded in the starchy matrix. Similar results have been found by
Curvelo et al.[190] After mixing, authors have found high improvements of the material
performance; some of them are linked to a usual matrix reinforcement,[192] some others
are brought by the inter-relations fiber-matrix:
– higher modulus,[186–189]
– reduced water sensitivity due to fiber–matrix interactions and to the higher hydro-
phobic character of the cellulose, linked to its high crystallinity,[52,53,186,187,190]
– higher thermal resistance,[52] due to the transition shift and an increase of the rubber
plateau,
– reduced post processing aging, due to the formation of a 3D network between the
different carbohydrates, through hydrogen bonds.[189]
Figure 13. Peel test results at 908, 50mm/mm, 238C and 50%RH. Effect of the polyester type and
process on the peel strength of PLS–polyester films. (White bars represent the hot pressed films by
compression moulding. Dark bars represent coextruded films). Source: Ref.[165].
Averous260
On this topic, a large number of patents were published (see Table 7) during the last
decade with some complex formulations, also based on proteins and even on biodegrad-
able polyesters.
4.4.2. Lignin Fillers
Different kinds of fractioned or modified lignins (Kraft lignins, Acellw lignins) have
been investigated in association with PLS obtained by film casting preparation or by extru-
sion.[193–195] According to Baumberger,[194] both systems are quite compatible with
lignins acting as fillers or as extender of the PLS matrix, with the soluble lignin fractions.
4.4.3. Nanocomposites and Mineral Microfillers
Two kinds of nanoparticules have been tested into PLS composites: whiskers obtained
from cellulose and organoclays.
By acid hydrolysis of cellulosic materials, we obtain whiskers, which are mono-crystals
from cellulose. Some authors[196–198] used tunicin (an animal cellulose) whiskers, slender
parallelepiped rods of 500 nm to 1–2mm length and 10-nm-width,[196] into PLS matrixes.
As for cellulose fibers, whiskers–matrix interactions are important. The high shape ratio
of the nanoparticles (50–200), the high specific area (�170m2/g) increases the interfacialphenomena then properties are improved, compared with PLS–cellulose composites.
Tunicin whiskers favor the starch crystallization due to a nucleating effect of the filler.[198]
Figure 14. MEB observation. Cryogenic fracture of composites: PLS-leafwood cellulose fibers
(white scale ¼ 100mm). Source: Ref.[187].
Multiphase Systems Based on PLS 261
Different authors[199–201] developed layered silicate nanocomposites to be compatibi-
lized with the biodegradable polyester phase, such as PLA,[202] PCL,[201] or PBSA.[46] The
organoclay incorporation is generally between 1 and 10 wt%. These nanocomposites are
based on montmorillonites, which are generally modified by cations exchange with alkyl-
ammonium.[200–202] From this latter, Kubier et al.[203] have in situ polymerized polyesters
by ROP of 1-caprolactone using tin alkoxide catalyst, to obtain intercalated or exfoliated
nanocomposites.
The addition of modified organoclay into biodegradable polyester phase tends to
modify the material properties such as the mechanical properties. The tensile modulus
and strength are increased; on the other hand, the elongation at break is slightly
decreased.[46,201,204] According to Lepoittevin et al.,[201] thermal stability determined by
thermogravimetric analysis (TGA) is improved. Krook et al.[204] have shown that
oxygen and water transmission rates are rather decreased. According to Lee et al.,[46]
the kinetic of biodegradation is decreased.
On a recent patent based on PLS–montmorillonites,[204] Fischer and Fischer[205]
present clay-based compounds as processing aids, which decrease the viscosity during
extrusion. Besides, the mineral compounds act to prevent plasticizer loss during aging
and then, improves the PLS stability. On PLS-biodegradable polyester blends, McGlashan
and Halley[206] have shown that nanocomposites tend to improve the mechanical proper-
ties (tensile and impact tests), the barrier properties and the transparency by crystallinity
modification.
Mineral micro-fillers were tested into a PLS matrix.[207] Kaolin particles, with an
average size of 0.5mm, were incorporated by extrusion. Due to significant inter-compa-
tibility, subsequent behaviors, such as a glass transition decrease, a reduction of water
uptake, and an increase of the stiffness can be observed.
5. CONCLUSION AND PERSPECTIVES
In this review, the relationships between the structure, the process, and the properties
of biodegradable multiphase systems based on PLS, the so-called “thermoplastic starch”
was analyzed. Besides, this review tried to show the high potential presented by plasticized
starch-based materials. Because of its low cost, its renewable and biodegradable character,
starch seems to be nowadays an important challenger for the development of future agro-
based products. Compared to petrochemical polymers, plasticized starch should be a
future good competitor, when petroleum resources will drastically decrease. Besides,
these agro-materials are totally in agreement with the emergent concept of sustainable
development.
The association of plasticized starch with other biodegradable polymers is a good sol-
ution to overcome main starch issues, e.g., compared to the limitations linked to starch
chemical modifications (process cost, toxicity of the by-products etc). Besides, with the
different structures, (blend, composite, multilayer) we can fulfil the requirements of differ-
ent applications fields, such as packaging or other short-lived applications (catering,
agriculture, sport, hygiene) where long-lasting polymers are not entirely adequate.
Averous262
ACKNOWLEDGMENTS
The author wishes to thank Europol’Agro (Reims) for its financial support and also
Patrice Dole, Catherine Joly (UMR INRA-FARE, Reims) for our helpful discussions.
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Received April 8, 2003
Accepted February 12, 2004
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