Starch blends with other plastics

The biodegradability of starch in the plastic matrix mainly depends on the accessibility of starch to microbes and on the coimectivity of starch particles each other. Wool and Cole (8) described a simulation model based on percolation theory for predicting accessibility of starch in LDPE to microbial attack and add hydrolysis. This model predicted a percolation threshold at 30% (v/v) starch irrespective of component geometry and other influential factors. Two critical aspects, the bioavailability and the kinetics of the starch hydrolysis in the plastic matrix must be examined before such blends could be applied as controlled release formulation of pestiddes. The goal of this work was to develop a kinetic model describing the degradation and release of starch blended with hydrophobic plastics. 

Starch is made thermoplastic at elevated temperatures ia the presence of water as a plasticizer, aHowiag melt processiag alone or ia blends with other thermoplastics (192—194). Good solvents such as water lower the melt-transition temperature of amylose, the crystalline component of starch, so that processiag can be done well below the decomposition—degradation temperature. 

Blending is one of the most promising alternatives to make starch useful as a polymer in replacement of otha-plastics, and the fast progress occurring in this field is attested by several reviews published recently [82, 110, 111, 115]. Indeed, the commercial plastics based on starch presently available are in the form of blends with other... 

A lowering in PLA modulus and Tg can be avoided if PLA is blended with other polymers. However, it is not miscible with many plastics, and the use of block copolymers or the use of reactive blending is generally necessary. Candidate polymers may be biodegradable or nonbiodegradable. In the former category are starch. 

The most common strategy to decrease the price or improve the properties of polylactide to fulfill the requirements of different applications is blending. Polylactide has been blended with degradable and inert polymers, natural and synthetic polymers, plasticizers, natural fibers and inorganic fillers. The most common blends include blends with other polyesters such as polycaprolactone or PLA/starch blends. Usually the compatibility between the two components has to be improved by addition of compatibilizers such as polylactide grafted with starch or acrylic acid (114,115). Recently a lot of focus was concentrated on the development of polylactide biocomposites, nanocomposites and stereocomplex materials. In addition various approaches have been evaluated for toughening of polylactide. 

Although starch is itself a thermoplastic polymer (thermoplastic starch, TPS) (Li et al, 2008) and can be processed via extrusion or molding with the aid of a plasticizer (Chuayuljit et al, 2009), this natural polymer is a rapidly degrading material with minimal moisture resistance. Application possibilities of TPS are as fillers or blends with other thermoplastics (Arvanitoyannis et al, 1997, 1998). Chemical modification of starch, on the other hand (Gandini, 2008 Lee et al, 2007), can be used to adjust degradation properties as well as the water stability of the resulting materials. 

Starch is one of the most widely used biopolymer in biocomposites because of its low cost and versatility. A plasticizer like glycol is sometimes used to make it suitable for processing. It is also blended with other polymers like aliphatic polyesters to improve its physical and mechanical properties. Biocomposites based on starch matrices show improved properties, which are comparable to E-glass/epoxy composites. Tensile, flexural, impact, and creep properties of these biocomposites are significantly better than those of neat starch. Various biofiber surface treatments have been shown to improve the properties of starch-based biocomposites. 

Other effective plasticizers for starch for imparting melt processibility include a variety of low molecular weight compounds, such as glycerol and diethylene glycol, and also polymers such as poly(ethylene-co-vinyl alcohol) [55]. Furthermore, starch plasticized in that manner can be melt blended with minor amounts of hydrophobic thermoplastics, such as polyethylene and poly( methyl methacrylate), to obtain biodisintegratable molded articles with good mechanical properties. 

Several companies have developed starch-based plastics. By using carefully selected starch feedstocks, and water as a plasticizer, they produce thermoplastics from nearly 100% starch, or from blends of starch with other biodegradable components. Many of these materials are water-soluble in addition to being biodegradable. The major target application has been as a replacement for polystyrene foam, including both molded cushions and loosefill. 

Plasticized starch has widely been studied in a mixture with other polymers [AVE 04a, AVE 00a, AVE 00b, AVE 01a, MAR 01a, MAR 01b, SCH 04]. A large nrrmber of patents have been pubUshed on this subject [AVE 04a]. These research efforts have led to the commercialization of different biodegradable blends - some of them based on plasticized starch (Table 9.4). 

However, plasticizers can make brittle films less strong and, as a result, blending [101] or laminating [102] with other materials has been used to overcome this disadvantage. Aqueous blends of soluble starch and cellulose acetate have been studied intensively [103-107] as they have properties that make them suitable... 

PLA PLA-based block copolymers include diblock, triblock, and multiblock copolymers Jeon et al. (20(B), Chen et al. (2003), Pospiech et al. (2005) Blend with starch, other polyesters, and low molecular weight plasticizers such as glyc ol, sorbitol, and triethyl citrate Cargill (2007)... 

Blends of starch with polar polymers containing hydroxyl groups, such as poly(vinyl alcohol), copolymers of ethylene and partially hydrolyzed vinyl acetate have been prepared since the 1970s, as described by Otey et al. [61, 68-72]. Since starch and other natural polymers are hydrophilic, water has been commonly used as a plasticizer for these materials. The possibility of using water as plasticizer makes it possible to add the polymer to be blended as an aqueous emulsion, as for example, in the case of natural rubber latex [112], poly(vinyl acetate) and other synthetic polymer lateces [71,113,114]. Blends of starch and biodegradable polymers and polymers from renewable resources have been reviewed recently due to their growing importance [82, 110, 111, 115, 116]. Table 15.3 gives some polymers commonly used in blends with starch. 

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