See also, biodegradable polymers or bioplastics

AGRO-POLYMERS

Agro-polymers are mainly extracted from plants. They are compostable and renewable polymers. We find different families such as the polysaccharides or the proteins. 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.

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. According to the botanical specie, we can measure different cellulose contents. It is a cheap semi-crystalline material, which is widely used in paper production but also as reinforcing elements in polymer matrixes. To obtain a thermoplastic material, cellulose is modified by acetylation (cellulose acetate), which production actually is low. After acidic treatment, and elimination of the amorphous parts of cellulose microfibrils, we obtain the whiskers (mono-crystals), which are used to develop nanocomposites materials [52-53].

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.
Starch granules can be isolated from plants. Main sources come from wheat, potato, maize, rice, cassava, pea, waxy maizes, amylomaizes … According to the resource, native starches have dimensions ranging from 0.5 to 175 microns and appear in a variety of shape [55]. For instance, Figure 4 shows micrographs of the shape of different starch granules (wheat and pea).

Figure 4: Micrographs of native starch (SEM observations): wheat starch (left) and pea starch (left). White scale = 10 microns.

Starch is a polysaccharide consisting in D-glucose units, referred to as homoglucan or glucopyranose. Starch is composed of two different macromolecules, amylose and amylopectin. Amylose (Figure 5a) 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 (Figure 5b) is a highly multiple-branched polymer with a high molecular 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].

(a)

(b)

Figure 5: Amylose structure (a), amylopectin structure and cluster model (b).

Starch composition is variable according to the botanical origin. Some mutant plant species present some special composition. We can find rich-amylose starch with amylomaize (till 80 %) and some rich-amylopectine starch with waxy maize (>99%).
After the different industrial stages of isolation and refining, starch shows usually 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 hydroxyls 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 cristallinity are observed in the granule denoted as A, B, C or V type. Figure 6 shows that the crystalline regions (20 to 45 %) are arranged as thin lamellar domains, perpendicular to the radial axis.

Figure 6: Radial structure of a starch granule (amorphous and crystalline region). Source: [210]

PLASTICIZED STARCH

Except for 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 gelatinisation 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 gelatinisation. By decreasing the moisture content (less than 20 wt%), the melting temperature tends to be close to the degradation temperature. For instance, for pure dry starch Tm=220-240°C [60] compared to 220°C, which is the temperature of 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 others polyols (sorbitol, polyethylene glycol  ...) [62-63]. A mixture of different polyols can also be held [64]. Other compounds such as those containing nitrogen (urea, ammonium derived, amines …) can be used [68-70]. These plasticized starches (PLS) are also commonly called “thermoplastic starches” or TPS [66]. The first patents and articles on these processable materials were published at the end of the eighties [71-73]. PLS combines starch, a non-votalile and high-boiling [71] plasticizer and often water. Figure 7 shows that plasticized starches 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).

Figure 7: Presentation of different starchy products, depending on water content and destructuring level.

Plasticized Starch Process

The disruption of granular starch is the transformation of the semicristalline 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 combination of thermal and mechanical inputs can be obtained by extrusion, a common plastic processing technique. 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 give the different stages of the extrusion. The starch granules are fragmented. Under temperature and shearing, starch is destructured, plasticized, melted but also partially depolymerised. After the processing, we obtain a homogeneous molten phase.

Figure 8: Schematic of starch process by extrusion.

 Molten State Behaviour

The extrusion and viscous behaviour of molten plasticized starch is known to depend on temperature, moisture content and thermomechanical treatment [73-76]. Viscosity data and rheological models that take into account these variables have been reported in literature. Martin et al. (2003) [77] have summarized the main rheological studies performed on starch, including the measurement technique and models used. They all report a thermoplastic-like behaviour 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 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 dependence of starch melt viscosity upon structural factors or variables directly involved in its transformation. Some authors have introduced terms relating the modification of starch, such as conversion, degree of transformation, or extent of degradation [78-82]. Lai and Kokini (1990) [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 viscosity measurements. Zheng and Wang (1994) [82] identified the contribution of shear and thermal energies in the conversion of waxy cornstarch. Della Valle et al. (1995 and 1996) [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 Equations 1, which present a pseudo-plastic model with the pseudo-plastic index (m) and the consistency (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].

     With the consistency:           (1)
MC=moisture content,
GC=glycerol content,
a, a’ 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 measurement. This material shows a behaviour not totally thermoplastic but thermo-mecano-plastic [77]. Both, mechanical 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 behaviour of low-moisture starches. In that respect, extruder-fed slit rheometry is well adapted [89-93]. Martin et al. (2000 et 2003) [77, 88] have also shown the great interest combining capillary and slit dies on an in-line viscometer attached to the head of an extruder (A large shear rate range can be covered: 4 decades) with thermomechanical conditions (shear rate, SME, temperature) comparable to those used during the processing. Also of interest is the RheoplastÒ, 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 behaviour. The measurement of elastic component of plasticized starch 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, plasticized starch was shown to behave mainly as a solid-like material, because subjected to insufficient mechanical treatment [96]. As an alternative, Senouci and Smith (1988) [90] related the entrance and exit pressure losses in a slit viscometer die 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.

Solid State Behaviour

• Cristallinity

Compared to native starch, plasticized starch shows a lower cristallinity. According to Van Soest et al. (1996) [97], two kinds of cristallinity are obtained after PLS processing: residual cristallinity from native starch (A, B, C types) and processing induced cristallinity (V and E types). Critallinity induced by processing is influenced by parameters such as the extrusion residence time, the screw speed or the temperature. It is mainly caused by the fast recristallysation of amylose into single-helical structure. After post processing ageing, Van Soest (1996) [98] proposed a complex model for plasticized starch with amorphous and crystalline amylose and amylopectin and probable co-crystallization between amylose and amylopectin.

• Plasticizer and water interaction

The evolution of the moisture content are of consequence on the evolution of properties or transitions such as the glass transition, because water acts also 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. (1997) [100], Mathew and Dufresne (2002) [63] have determined according to the relative humidity, the nature and content of the plasticizer and, the moisture content after equilibrium on starch. They have shown that for low relative humidity, water content decreases when plasticizer increased. In this case, plasticizer molecules then occupy some sites initially occupied by water. For higher activities (> 43 %HR) water content tend to increase with the plasticizer content due to plasticizer-water interactions. According to Lourdin et al. (1997) [100], this evolution is not linear but very well marked for high plasticizer content (e.g., more than 18%wt, for glycerol) i.e., above the limit where phase separation between polysaccharide and plasticizer can occur, with constitution of plasticizer-rich phases [101]. This concentration threshold corresponds to one glycerol molecule per three-anhydroglucose unit.
Lourdin et al. (1997) [102] have shown that at low plasticizer content an antiplastification effect occur. Due to strong interaction between plasticizer and starch, a hydrogen links network appears and we obtain a material reinforcement. Then, when the plasticizer contents increase (e.g., more than12%wt for glycerol into potato starch), interactions plasticizer-plasticizer occur with a material swelling and a plastification effect.

• Post processing ageing.

Different authors [40, 103-104] have shown that after processing, plasticized starch shows an ageing with a strong evolution of mechanical properties such as the tensile modulus, which increase during several weeks. PLS present 2 kinds of ageing behaviour depending on glass temperature value. In the sub-Tg domain, PLS shows a physical ageing versus time, with a material densification [104-105]. At a temperature above Tg, PLS shows retrogradation phenomena with the evolution of the cristallinity and rearrangements of plasticizer molecules into the material [103] versus storage time. The retrogradation kinetic depends on the macromolecules mobility, on the plasticizer type and content [104].

PLS properties

According to the plasticizer/starch ratio, PLS present a large range of attributes. Figure 9 shows the thermograms evolution for different formulations (S=Starch, G=Glycerol and W=water in %wt).

Figure 9: DSC evolutions with different glycerol/starch ratios. Source: [72]

Figure 9 illustrates that the Cp variations at glass transitions are very low although DMTA determinations are generally more desirable to obtain PLS glass transitions. DMTA evolutions are drawn on Figure 10 for the same formulations. The evolutions of tang delta versus temperature show 2 transitions. Main and broad relaxation (a transition) can be linked to the PLS glass transition [40]. Secondary relaxation (between –50 and –60°C) could be connected to glycerol glass transition [40]. According to Lourdin et al. (1997) [101], this latter relaxation could be an indicator of the level of interactions between the plasticizer and the polysaccharides. In the explored domains, Figure 10 shows that this relaxation temperature decreases when glycerol content increases and then, the phase segregation.
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 compared to most polyesters [24]. Permeability increases drastically at the glass transition and continues to rise on the rubber plateau, with the plasticizer content.

Figure 10: DSC evolutions with different glycerol/starch ratios.Source: [72]

PLS issues and strategies

As a material, PLS shows strong attributes: a total compostability without toxic residues, the renewability of the resource. Besides, compared to synthetic thermoplastic, it is a rather cheap material. Compared to fossil resources, the price of starch resources remains stable and even tends to decrease due to cereals overproductions in the world. In addition, PLS can be easily processed with plastic processing machines. It shows according to the plasticizer level and the starch botanical source, a wide properties range. But unfortunately, PLS shows various issues for some applications (e.g., packaging) such as a great moisture sensitivity, rather weak mechanical properties compared to 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 XXth century, in the continuity of researches on modified cellulose. Yet in 1942, Mullen and Pacsu [108] published a critical analysis of the different method of starch (tri)ester preparation. In 1943, the same authors [109] presented an industrial usage of such a compound. Since then, a large 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 e.g., 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 purification (to eliminate the by-products). Besides, the chemical reactions lead to some incidences 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 developed the association of PLS with others 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.

PLASTICIZED STARCH-BASED MULTIPHASE SYSTEMS

Structures Classification

Two different materials can be associated to PLS to obtain compostable materials: biodegradable polyesters or agro-materials (lignins, cellulose …). Figure 11 shows the different sorts of structures, which can be obtained by associations, and the related process.

Figure 11: Schematic of multiphase systems based on plasticized starch - Process and structures

Biodegradable PLS-Based Blends

Blend shows different attributes [112-113]. Blending is the easier process to associate together different polymers. Blending provides a powerful route to obtain materials with improved property/cost performances. This approach is cheaper than the development 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 e.g., to multilayer structures.
PLS has been widely used in blends with other polymers [114-115]. A lot of patents were published on this topic. These considerable research efforts have led to starch-based blends being commercialised, Mater-Bi [30, 71] from Novamont (Italy) or Bioplast 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., polycaprolactone). Applications concern packaging, disposable cutlery, gardening, leisure, hygiene …
In the past, blends with synthetic polymers, such as PE [116], or EVOH [117], were developed leading to non-fully biodegradable materials [118-119]. These controversial materials, because presented as biodegradable, 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. We find some associations of PLS with agro-polymers such as proteins [118-121] or pectins [120, 122]. But, most of the researches are focused on the blending of PLS with biodegradable polyesters: polycaprolactone (PCL) [32, 39-40, 123-129], polyesteramide (PEA) [39], polyhydroxybutyrate-co-hydroxyvalerate (PHBV) [126-127, 130-131] (PHBV), polybutylene succinate-adipate (PBSA) [50, 132], poly(butylene adipate-co-terephtalate) (PBAT) [132], polylactic acid (PLA) [21] or poly(hydroxy ester ether (PHEE) [27]. These polyesters commercially available show some interesting and reproducible properties such as a more hydrophobic character, a lower water permeability and some improved mechanical properties, compared to PLS.

• 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. equations 2) and the parallel model from Voight (Cf. equations 2) for respectively, the lower and the higher boundary [21, 40, 72]. In the case of a co-continuous structure, such as shown on some PLS/PLA blends [21, 133], modulus can be evaluated with a rather good agreement by Davies model (Cf. equations 2) [134].
 

Ei and fi are, respectively, the modulus and the volume fraction of the ith phase.             (2)

Starch and more hydrophobic compounds such as biodegradable polyester are rather immiscible and mixing produces blends with separated phases with rather poor interfacial 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 compared to 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, PEA presents the highest surface tension, a high polar component, and PLA the lowest. These different determinations allow the establishing of a classification from the least compatible (PLA) to the most compatible polyesters (PEA).
However, this low compatibility induces special behaviours 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 to 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 deposits decreases drastically for contents lower than 10 wt% of polyesters in the blend [132]. Biodegradability of such blends is modified, degradation occurs from the starchy core toward the skin [141].

• Blends compatibilisation

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 a coupling agent such as peroxides [41, 151-152] or polyisocianates [153-157],
- The development of copolymers: starch-graft polyester. To compatibilise starch and PCL, different authors have developed polycaprolactone-grafted starch or dextran [158-163]. The grafting is obtained by ROP of e-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.

Biodegradable PLS-Based Multilayers

Compared to blends, multilayer structures present some attributes. Moisture sensitivity 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. For instance, Bastioli et al. [164] (1996) proposed different processes (coextrusion, casting and hot melt techniques) to protect starch-based materials with wax layers. Stratified materials can be obtained by a multi-step 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 liquids 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 phenomenon corresponds to the surrounding of the more viscous polymer by the less viscous one, as shown by Lee and White (1974) [169]. Figure 12 illustrates interfacial instabilities with wavy interfaces. Yih (1967) [170] and Hickox (1971) [171] pioneered studies on interfacial instabilities, suggesting that viscosity differences may cause instabilities of stratified flow. Schrenk and Alfrey (1978) [172] investigated the factors responsible for the onset of instabilities, and suggested the existence of a critical shear stress value beyond which interfacial instabilities are likely to occur. Han and Shetty (1976 and 1978) [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 (1990) [175-176], Su and Khomani (1992) [177-178] examined 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 mechanism of instabilities. Wilson and Khomani (1992 and 1993) [179-181] studied experimentally and numerically the propagation of periodic flow disturbances, 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 plasticized starch and polyester in coextrusion 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].

• Interfacial instabilities

Martin and Averous (2002) [183] have shown on a PLS/PEA/PLS systems, that the key parameters are the skin-layer viscosity and thickness, the global extrusion rate and the die 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. Viscosity differences, high extrusion rates and low cap layer thickness have been shown to promote the occurrence of instabilities. Conversely, moderate extrusion rate, appropriate die geometry and lesser viscous component at the die wall have favoured the stability of the combined flow.

(a)

(b)

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: [77]

• 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 by Figure 13. Elsewhere, Wang et al. (2000) [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 (2002) [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 13: Peel test results at 90°, 50 mm/mn, 23°C and 50%RH. Effect of the polyester type and process on the peel strength of PLS-polyester films.Source: [165]
(White bars represent the hot pressed films by compression moulding. Dark bars represent coextruded films).

Biodegradable PLS-Based Composites

• Cellulose fibres reinforcement

Different types of fibres or microfibrils were tested in association with plasticized starch: microfibrils from potato pulp [52-53], bleached leafwood fibres [186-189], fibres from bleached eucalyptus pulp [190], flax and jute fibres [191]. The different authors have shown high compatibility between both polysaccharides. For instance, Averous et al. (2001) [187] have found a Tg increase of 28°C by addition of 10 wt% of cellulose fibres into a PLS matrix. This evolution is linked to the fibres-matrix interactions, which decrease starch chains mobility. Other example, Figure 14 shows a MEB image of a cryogenic fracture; the cellulose fibres are imbedded in the starchy matrix. Similar results have been found by Curvelo et al. (2001) [190].

Figure 14: MEB observation. Cryogenic fracture of composites: PLS-leafwood cellulose fibres. (white scale = 100 microns). Source: [187]

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 fibre-matrix:

• higher modulus [186-189],
• reduced water sensitivity due to fibre-matrix interactions and to the higher hydrophobic character of the cellulose, linked to its high cristallinity [52-53, 186-187, 190],
• higher thermal resistance [52], due to the transition shift and an increase of the rubber plateau,
• reduced post processing ageing, due to the formation of a 3D network between the different carbohydrates, through hydrogen bonds [189].

On this topic, a large number of patents were published during the last decade with some complex formulations, also based on proteins and even on biodegradable polyesters.

• Lignin fillers

Different kinds of fractioned or modified lignins (Kraft lignins, ) have been investigated in association with PLS obtained by film casting preparation or by extrusion [193-195]. According to Braumberger (2002) [194], both systems are quite compatible with lignins acting as fillers or as extender of the PLS matrix, with the soluble lignin fractions.

• Nano-biocomposites 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 1-2 mm 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 interfacial phenomena then properties are improved, compared to PLS-cellulose composites. Tunicin whiskers favour the starch cristallization due to a nucleating effect of the filler [198].
Different authors [199-201] developed layered silicate nanocomposites to be compatibilized with the biodegradable polyester phase such as e.g., 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 alkylammonium [200-202]. From this latter, Kubiers et al. (2002) [203] have in-situ polymerised polyesters by ring-opening polymerisation (ROP) of e-caprolactone using tin alkoxide catalysis, 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. (2001) [201], thermal stability determined by TGA (thermogravimetric analysis) is improved. Krook et al. (2002) [204] have shown that oxygen and water transmission rates are rather decreased. According to Lee et al. (2002) [46], the kinetic of biodegradation is decreased.
On a recent patent based on PLS-montmorillonites [204], Fischer and Fischer (2001) [205] present clay-based compounds as processing aids, which decrease the viscosity during extrusion. Besides, the mineral compounds act to prevent plasticizer lost during ageing and then, improves the PLS stability. On PLS-biodegradable polyester blends, McGlashan and Halley (2003) [206] have shown that nanocomposites tend to improve the mechanical properties (tensile and impact tests), the barrier properties and the transparency by cristallinity modification.
Mineral micro-fillers were tested into a PLS matrix [207]. Kaolin particles, with an average size of 0.5 mm, were incorporated by extrusion. Due to significant inter-compatibility, subsequent behaviours such as a glass transition decrease, a reduction of water uptake and an increase of the stiffness can be observed.

REFERENCES

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