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 we re melt blended in a corotating twin-screw extruder with the synthetic polymers, ethylene-vinyl acetate, low density polyethylene and high d ensity polyethylene having anhydride functionality. Four different sta rches were used: unmodified waxy maize, containing 100% amylopectin; i ndustrial corn,containing approximately 70% amylopectin and 30% amylos e; common corn, possessing approximately 50% amylopectin and 50% amylo se and an unmodified high amylose starch, consisting of approximately 30% amylopectin and 70% amylose. The concentration of starch in the bl ends was kept at 70% by weight. The influence of starch type on proces sing parameters (pressure, torque and energy), tensile strength, perce nt 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 contai ning the highest amylose content required the lowest torque. Degradati on or debranching of the branched macromolecular components of the sta rch 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 p roperties and storage time are discussed. The tensile and flexural str ength increased at the amylose content of 70% and it is higher in waxy maize starch (highly branched) blends, presumably due to more crossli nking. Samples with weldline had a lower tensile strength, flexural mo dulus and percent elongation. Influence of starch type on water absorp tion is also discussed. Blends containing waxy maize starch absorb mor e water than those containing high amylose content of 70%. In most of the samples, the equilibrium water uptake was between 20 to 24% over t he period of 80 days. (C) 1998 Elsevier Science Ltd. All rights reserv ed.