Starch-aliphatic polyester blends

Other blends such as polyhydroxyalkanoates (PHA) with cellulose acetate (208), PHA with polycaprolactone (209), poly(lactic acid) with poly(ethylene glycol) (210), chitosan and cellulose (211), poly(lactic acid) with inorganic fillers (212), and PHA and aliphatic polyesters with inoiganics (213) are receiving attention. The different blending compositions seem to be limited only by the number of polymers available and the compatibility of the components. The latter blends, with all natural or biodegradable components, appear to afford the best approach for future research as property balance and bio degradability is attempted. Starch and additives have been evaluated in detail from the perspective of structure and compatibility with starch (214). 

Starch can be destructured in the presence of more hydrophobic polymers such as aliphatic polyesters. Aliphatic polyesters with low melting points are difficult to process by conventional techniques such as film blowing and blow moulding. Films such as polycaprolactones (PCL) are tacky as extruded and have a low melt strength (over 130 °C). Also, the slow crystallisation of the polymer causes the properties to change with time. Blending starch with aliphatic polyesters improves processability and biodegradability. 

Polycaprolactone aliphatic polyesters have long been available from companies such as Solvay and Union Carbide (now Dow Performance Chemicals) for use in adhesives, compatibilisers, modifiers and films as well as medical applications. These materials have low melting points and high prices ( 4-7 per kg in 2005). PCL is predominantly used as a component in polyester/starch blends such as... 

In the context of this chapter, the use of thermoplastic starch in blends with thermoplastic resins is of the main interest. As shown in Table 16.11, several blends have been developed, e.g., with vinyl alcohol copolymers (EVAl), polyolefins, aliphatic polyesters such as poly-e-caprolactone (PCL) and its copolymers, or polymers of glycols (e.g., 1,4-butanediol) with succinic, sebacic, adipic, azelaic, decanoic or brassihc acids, PCL + PVC. Compatibilization is possible by amylose/EVAl V-type complexes, starch grafted polyesters, chain extenders like diisocyanates, epoxies, etc. [Bastioli et al., 1992, 1993]. 

Mater-Bi (Novamont, Environmental Packaging, L. P.) Thermoplastic, compatibiUzed blends of starch with aliphatic polyesters, EVAl, PVAl,... 

Some synthetic polymers like, polyurethanes, specifically polyether-polyurethanes, are likely to be degraded by microbes but not completely. However, several polymers such as, polyamides, polyfluorocarbons, polyethylene, polypropylene, and polycarbonate are highly resistant to microbial degradation. Natural polymers are generally more biodegradable than synthetic polymers specifically, polymers with ester groups like aliphatic polyesters [1]. Therefore, several natural polymers such as cellulose, starch, blends of those with synthetic polymers, polylactate, polyester-amide, and polyhydroxyalkanoates (PHAs) have been the focus of attention in the recent years [3]. 

It is difficult to process aliphatic polyesters having low melting points by conventional techniques for thermoplastic materials, such as film blowing and blow moulding. It has been found that the blending of starch with aliphatic polyesters allows their processability and biodegradability to be improved. 

Poly(lactic acid) (PLA) (Fig. 1.16) is an aliphatic polyester polymerised by lactic acid which is made by fermentation of natural raw materials, for example, com starch and sugarcanes. Due to the chiral nature of lactic acid and its effects on the polymer s characteristics, the biodegradability and mechanical properties of PLA can be tailored by varying the proportion of different forms. Meanwhile, PLA can also copolymerise with other monomers or blend with other polymers to improve some properties of the material, eg, flexibility. PLA and PLA-based copolymers are the most popular biodegradable materials for the production of absorbable sutures (Li, 1999) (Fig. 1.16). 

Blending of starch with aliphatic polyesters improves their processability and biodegradability Particularly suitable polyesters are poly(s caprolactone) and its copolymers, or polymers of higher melting point formed by the reaction of 1,4-butandiol with succinic acid or with sebacic acid, azelaic acid, or polydactic acid), poly(hydroxyalkanoates), and aliphatic-aromatic polyesters. 

The first synthetic biodegradable aliphatic polyester commercially available was poly(e-caprolactone) (PCL), produced under the trade name Tone by the Union Carbide Corporation in the USA. The product was the subject of a number of biodegradability studies [7] and was originally employed for medical sutures, and then as a component in biodegradable polyester/starch blends (e.g., Mater-Bi Z-grade, Novamont, Novara, Italy) [13]. 

Researchers at Michigan State University have patented the use of aliphatic polyester-grafted starch as a compatibiliser between starch and aliphatic polyesters such as PCL [86]. These grafted compatibilisers provide enhanced interfacial adhesion between the starch and polyester phases. Compositions with two phases can be generated with a variety of morphologies that affect the properties of the blend. Interfacial adhesion is one factor... 

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