L.Yu et al Prog.Polym Sel.31 (2006)576-602 579 List of natural polymers arch.cellulose.pectin.konjac.alginate, serum,albumin.collagen/gelatine,silks,resilin.polylysine.polyamino acids.poly(y-glutamic acid),elastin. s,surfactants,emulsan overwhelming abundance and its a ewa [131.Ho itself propciple.the erties of natural p rs can be eplac significantly improved by blending with synthetic mostwater soluble.difficult to process and brittle polymers.Polymer blending is a well-used technique when used without the addition of a plasticizer.In whenever modification of properties is required addition,its mechanical properties are very sensitive because it uses conventional technology at low cos to moisture content.Blending two or more chemi The usual objective for preparing a novel blend of not to change the pr ercom components dras ally,but er has blended h starch for on the e 1970s and 1980s.n app with various polyolefins were developed.However. on gelatinized starch nds ann and 1 4trans these blends were not biodegradable and thus the (gutta percha)for food packaging or biomedical advantage of using a biodegradable polysaccharide applications.Components are mixed to an adequate was lost.In this section,polymer blends only from degree of dispersion by thermal pressing.A series o natural raw materials are discussed blends of gutta percha with gelatinized tarch.with are water and without plasticizers or compatibilizers. wa water has s a solvent,di t to pres plas s121 processing ma ty f gutta peren polysaccharides are the main tue The and wate polymers.their interaction with water and with each other in a water medium give the structure-property intermediate values between the two components relationships in these materials.An analysis of the Carvalho et al.I151 studied the blending of starch glass transition temperature and thermal profile with natural rubber.Thermoplastic starch/natura gives one of the best illustrations of the role of water nds were prepared using natural er-reinforced e of ere prepared in intens tch mixer detail in a later section dispersion of the starc matrix was homoge neous because of the presence of 2.1.Melt processed blends the aqueous medium,with rubber particles ranging in size from 2 to &um.The results revealed a Starch is one of the most promising natural reduction in modulus and tensile strength,making polymers because of its inherent biodegradability. the blends less brittle than thermoplastic starch rate and, in some cases, unsatisfactory mechanical properties, particularly under wet environments. In principle, the properties of natural polymers can be significantly improved by blending with synthetic polymers. Polymer blending is a well-used technique whenever modification of properties is required, because it uses conventional technology at low cost. The usual objective for preparing a novel blend of two or more polymers is not to change the proper￾ties of the components drastically, but to capitalize on the maximum possible performance of the blend. In the 1970s and 1980s, numerous blends of starch with various polyolefins were developed. However, these blends were not biodegradable, and thus the advantage of using a biodegradable polysaccharide was lost. In this section, polymer blends only from natural raw materials are discussed. Since the majority of natural polymers are water soluble, water has been used as a solvent, dispersion medium and plasticizer in the processing of many natural polymer blends [12]. Since proteins and polysaccharides are the main constituents of natural polymers, their interaction with water and with each other in a water medium give the structure–property relationships in these materials. An analysis of the glass transition temperature and thermal profile gives one of the best illustrations of the role of water in natural polymers. Natural fiber-reinforced composites are one of the successful examples and will be discussed in detail in a later section. 2.1. Melt processed blends Starch is one of the most promising natural polymers because of its inherent biodegradability, overwhelming abundance and its annual renewal [13]. However, by itself, pure starch is not a good choice to replace petrochemical-based plastics. It is mostly water soluble, difficult to process and brittle when used without the addition of a plasticizer. In addition, its mechanical properties are very sensitive to moisture content. Blending two or more chemi￾cally and physically dissimilar natural polymers has shown potential to overcome these difficulties. Natural rubber has been blended with starch for a number of different applications. Arvanitoyannis et al. [14] reported on biodegradable blends based on gelatinized starch and 1,4-transpolyisoprene (gutta percha) for food packaging or biomedical applications. Components are mixed to an adequate degree of dispersion by thermal pressing. A series of blends of gutta percha with gelatinized starch, with and without plasticizers or compatibilizers, was prepared in an attempt to preserve the excellent biocompatibility of gutta percha. A low amount of plasticizer was incorporated into the blends to improve mechanical properties. The gas and water permeability values of the blends were found to be intermediate values between the two components. Carvalho et al. [15] studied the blending of starch with natural rubber. Thermoplastic starch/natural rubber polymer blends were prepared using natural latex and cornstarch. The blends were prepared in an intensive batch mixer at 150 1C, with the natural rubber content varying from 2.5% to 20%. The dispersion of rubber in the thermoplastic starch matrix was homogeneous because of the presence of the aqueous medium, with rubber particles ranging in size from 2 to 8 mm. The results revealed a reduction in modulus and tensile strength, making the blends less brittle than thermoplastic starch ARTICLE IN PRESS Table 1 List of natural polymers Polysaccharides
Plant/algal: starch, cellulose, pectin, konjac, alginate, caragreenan, gums
Animal: hyluronic acid
Fungal: pulluan, elsinan, scleroglucan
Bacterial: chitin, chitosan, levan, xanthan, polygalactosamine, curdlan, gellan, dextran Proteins Soy, zein, wheat gluten, casein, serum, albumin, collagen/gelatine, silks, resilin, polylysine, polyamino acids, poly(g-glutamic acid), elastin, polyarginyl–polyaspartic acid Lipids/surfactants Acetoglycerides, waxes, surfactants, emulsan Speciality polymers Lignin, shellac, natural rubber L. Yu et al. / Prog. Polym. Sci. 31 (2006) 576–602 579