Properties of injection mouldet starch synthetic polymer blends - III. Effect of amylopectin to amylose ratio in starch

Corn starch with different contents of amylopectin to amylose ratio were melt blended in a corotating twin-screw extruder with the synthetic polymers, ethylene-vinyl acetate, low density polyethylene and high density polyethylene having anhydride functionality. Four different starches were used: unmodified waxy maize, containing 100% amylopectin; industrial corn,containing approximately 70% amylopectin and 30% amylose; common corn, possessing approximately 50% amylopectin and 50% amylose and an unmodified high amylose starch, consisting of approximately 30% amylopectin and 70% amylose. The concentration of starch in the blends was kept at 70% by weight. The influence of starch type on processing parameters (pressure, torque and energy), tensile strength, percent elongation, flexural strength and water absorption was investigated. Torque and energy data showed that the blends with starch containing high amylopectin content had the maximum value while the blend containing the highest amylose content required the lowest torque. Degradation or debranching of the branched macromolecular components of the starch during the thermomechanical processing treatment was observed from a gel-permeation chromatograph. Tensile strength and water absorption were correlated with the torque generated during blending. The effect of amylose to amylopectin in starch on tensile properties, flexural properties and storage time are discussed. The tensile and flexural strength increased at the amylose content of 70% and it is higher in waxy maize starch (highly branched) blends, presumably due to more crosslinking. Samples with weldline had a lower tensile strength, flexural modulus and percent elongation. Influence of starch type on water absorption is also discussed. Blends containing waxy maize starch absorb more water than those containing high amylose content of 70%. In most of the samples, the equilibrium water uptake was between 20 to 24% over the period of 80 days. (C) 1998 Elsevier Science Ltd. All rights reserved.