Polyolefins thermoplastic starch

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]. 

Polyolefins, PO Thermoplastic starch. Compatibihzed blends. Bastioli et a/., 1993, 1997... 

Thermoplastic starch is also blended with other polymers to improve its properties for particular applications. For example, a bag for collection of household food waste for composting that readily dissolved when it got wet would not function very well In applications such as this, the resins used for blending are also biodegradable, so that they do not interfere with the composting operation. In other cases, starch is blended with nonbiodegradable resins such as polyolefins. 

Thermoplastic starch can also be blended with other polymers such as polyolefins [115]. In this sort of blend, a compatibilizer such as ethylene-maleic anhydride copolymer can be used in order to make hydroxyl starch groups and anhydride copolymer groups to react and obtain ester bonds. This sort of esterification helps to compatibilize the starch (hydrophilic) with polyolefins (lipophilic). 

Thermoplastic starch can also be blended with polyolefins, possibly in the presence of a compatibilizer. Starch/cellulose derivative systems are also reported in the literature [12]. 

Thermoplastic starch can also be blended with polyolefins [131 ]. In this case about 50% of thermoplastically processable starch is mixed with 40% of polyethylene and 10% of ethyl acrylate-maleic anhydride copolymer. During this mixing process an esterification reaction takes place between the maleic anydride groups in the copolymer and the free hydroxyl groups in starch. 

Thermoplastic starch alone can be moulded as traditional thermoplastic materials its sensitivity to humidity, however, makes it unsuitable for most applications. Starch-filled degradable polyolefins show good disintegration properties. There is no evidence, however, of a significant biodegradation of the fragments produced by disintegration. 

In the last five years Asian countries, and specifically China and Korea, have performed impressively in the sector of blends of thermoplastic starch with polyolefins, in terms of intellectual property and products range offered to the market. The non compliance of these products with the international norms of biodegradability and compostability, however, did not permit a significant market growth in western countries where low environmental impact products have more market potential. 

Thermoplastic properties can be varied by mixing low- or high-density polyolefines, com starch and plasticizers. They all can be processed by typical plastics technologies, such as molding or film-blowing. Such resins contain up to 60-75% natural materials and up to 40% synthetic polymers. 

Starch can be nsed as a natural filler in traditional plastics (11,23-33) and par-ticnlarly in polyolefins. When blended with starch beads, polyethylene films (34) biodeteriorate on exposure to a soil environment. The microbial consumption of the starch component, in fact, leads to increased porosity, void formation, and the loss of integrity of the plastic matrix. Generally (32,35-38), starch is added at fairly low concentrations (6-15%) the overall disintegration of these materials is achieved by the use of transition-metal compounds, soluble in the thermoplastic matrix, as pro-oxidant additives which catalyze the photo- and thermooxidative process (39-44). 

Polylactic acid or polylactide (PLA) is a thermoplastic aliphatic polyester that can be derived from renewable resources, such as corn starch or sugarcanes. Although PLA has been known for more than a century, it has become of great commercial interest in recent years because of its renewability and degradability to natural metabolites. In addition, the properties of PLA can be varied over a wide range which makes it suitable to be used as a substitute to many petroleum based commodity plastics, such as polyolefins,... 

The thermoplasticity of material proteins has been utilised to produce materials using thermal or thermomechanical processes under low hydration conditions, as already employed for starch- or polyolefin-based materials [136, 152]. According to the thermoplastic behaviour of synthetic polymers, the Tg of the proteins involves sudden variations in their physical properties (thermal, mechanical, dielectric properties and so on). The molecular response associated with the transition from the glassy to the rubbery state involves an overall increase in the free volume and macromolecule mobility [153,154]. As for synthetic polymers, the Tg of the proteins is affected by the MW, chain rigidity, size and polarity of the lateral groups, presence of intermolecular bonds or crystalline zones, and also by the plasticiser type and concentration [155,156]. 

A family of natural polymers which are thermoplastic and water stable, but nevertheless biodegrade, are industrial biotechnology mimics of polyolefins. These molecules, functionally akin to starch are the food reserve of the bacterial world. They are simple polyesters with properties like polypropylene, but environmentally benign because in soil or water they are substrates for microorganisms. 

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