Degradation of starch-plastic blends

Kinetic Model for Degradation of Starch—Plastic Blends mth ControDed-Release Potential... 

ZHANG ETAL. Degradation of Starch-Plastic Blends... 

The second section deals with the degradability of commodity plastics and specialty potymers. Emphasis is on the biodegradation of polyethylene, its blends with starch, and constraints in the decay of such composites. Additionally, the biodegradability of different functional groups (polyethers, carbotylic adds, esters, and dioxanones) is mcamined with respect to composition and miaostructure. 

Since incorporating starch into a hydrophobic matrix retards the starch degradation rate (2), it should be possible to use starch-plastic blends in lieu of the starch alone as controlled release formulation of pesticides. The release of pesticides which are either adsorbed to (Figure la) or covalently bonded to (Figure lb) the starch could be controlled primarily by the rate of starch degradation. 

Biodegradability of the Starch in the Blends. The availability of starch to contact with amylase in starch-plastic blends is a prerequisite for starch digestion. The release rate of non-bioavailable starch granules which were completely occluded by plastic is simply zero, unless the plastic matrix is broken by other means. Hence, the discussion about the kinetics of starch degradation in the following sections refers to that portion of starch which was biodegradable. 

This chapter reviews the general context of starch as a material. After a survey of the major sources of starch and their characteristic compositions in terms of amylase and amylopectin, the morphology of the granules and the techniques applied to disrupt them are critically examined. The use of starch for the production of polymeric materials covers the bulk of the chapter, including the major aspect of starch plasticization, the preparation and assessment of blends, the processing of thermoplastic starch (TPS), the problems associated with its degradation and the preparation of TPS composites and nanocomposites. The present and perspective applications of these biodegradable materials and the problems associated with their moisture sensitivity conclude this manuscript. 

Reis RL, Mendes SC, Cunha AM et al (1997) Processing and in vitro degradation of starch/ EVOH thermo plastic blends. Polym Int 43 347-352... 

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. 

Oxygen Availability in Degrading Films. A major difference between natural materials and starch-plastic or cellulose-plastic blends is that the hydrophilic and relatively permeable matrix of materials like wood and hydrated polysaccharide films allows diffusion of O2 and release of nutrients from sites at a distance from the invasion site. As colonization proceeds, pore enlargement occurs when the pore walls are degraded (8) or as the polymer matrix of amylose or PVA films is hydrolyzed (10.12). In contrast, the LDPE matrix supplies no nutrients, hinders diffusion of water and O2, and the pore diameter cannot be increased. The consequence of impermeability is that the sole means of obtaining O2 and nutrients is by diffusion through water-filled pores. 

In its application in biodegradable plastics, starch is either physically mixed, with the native granules kept intact, or melted and blended on a molecular level with the appropriate polymer as described in Sects. 5.7 and 5.8. In either form, the fraction of starch in the mixture which is accessible to the enzymes can be degraded by either, or both, amylases and glucosidases. 

To coimter the loss of water dining the process and a significant degradation of the material, to prevent, e.g., the formation of cellular stractures output from an extruder, and to obtain a material with controlled properties, we use a non-volatile plasticizer such as glycerol or other polyols (sorbitol, xylitol, fructose, polyethylene glycol, etc.) [AVE 04a, XIE 12]. Blends of these different polyols are also used [CHI 10b]. Other, notably nitrogenous plasticizers (urea, ammonium derivatives, amines, etc.) can also be used. Plasticized starches are thus created. They are... 

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. 

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