Properties of injection moulded blends of starch and modified biodegradable polyesters

Abstract

The properties of blends of starch and aliphatic biodegradable polyesters are presented in this paper. The aliphatic polyesters used include: polyε-caprolactone, polybutylene succinate and a butanediol–adipate–terephthalate copolymer. To improve the compatibility between the starch and the synthetic polyester, a compatibilizer containing an anhydride functional group incorporated on to the polyester backbone was used. The blends were melt compounded using a twin-screw extruder. The concentration of the starch in the blend was varied from 10% to 70% by weight. The amylopectin content of the starches varied from 30% to 99%. The addition of a small amount of compatibilizer increased the strength significantly over the uncompatibilized blend. For the compatibilized blend, the tensile strength was invariant with starch content when compared to the original polyester, while it decreased with increase in starch content for the uncompatibilized blend. Blends displayed a sharp intake of water and those containing butanediol–adipate–terephthalate copolymer had a higher water absorption than those containing the other polyester. Each blend displayed two glass transition; one corresponding to the polyester and the other corresponding to starch. For compatibilized blends, the glass transition temperature of starch in the blend is lower than that observed for the uncompatibilized blend. For all polyester blends, those containing 99% amylopectin starch at 70% level had the lowest crystallinity which otherwise decreased with decreased amylopectin level in the starch. Blend morphology indicates that the starch phase become finer as the amylopectin in the blend increased. Also, a higher starch content led to greater melting of the starch granules and at 70% starch by weight, a cocontinuous phase between the starch and the synthetic polymer exists.