Chapter 7 - Mechanical Properties of Starch-Based Plastics

Abstract

Mechanical properties of starch depend on similar factors to those of synthetic polymers but with added considerations of water sensitivity and the influences of starch source, formulation, processing, and aging/retrogradation. Amylose/amylopectin ratios vary with starch source with high-amylose starches generally giving materials of higher strength and ductility. The amylose/amylopectin ratio also affects gelatinization processes, which generate the uniform amorphous thermoplastic by the action of heat and mechanical work in the extruder. Gelatinization is aided by suitable plasticizers, usually low-molecular-weight polyols and water. These also lower melt viscosity and Tg. Correct selection of plasticizers increases the toughness of the starch but decreases strength and modulus. Water content can vary with ambient conditions and directly affects mechanical properties. Increased water content has a similar effect to decreased strain rate or increased temperature. Quantifying these variables is important for use of starch materials in different applications.