THERMAL AND MECHANICAL PROPERTIES OF POLY(LACTIC ACID) AND STARCH BLENDS WITH VARIOUS LASTICIZERS

Abstract

Poly(lactic acid) (PLA) has potential for use in disposable and biodegradable plastics. Blending starch with PLAwould increase biodegradability and reduce costs. However, the brittleness of the starch/PLA blend is a major drawback formany applications. The objective of this study was to characterize thermal and mechanical properties of PLA/starch blendswith addition of six plasticizers at various levels. Three groups of plasticizers were used: (1) acetyl triethyl citrate (AC) andtriethyl citrate (TC); (2) poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) of low molecular weight andcontaining an ether group that might interact with PLA; and (3) glycerol and sorbitol, which are miscible with starch. ThePLA and cornstarch were blended at a 60:40 weight ratio by using a labscale twinscrew extruder with addition of theseplasticizers ranging from 5% to 25%. The thermal transition temperatures (glass transition, crystallization, and melting) ofthe extruded PLA/starch blends decreased as AC, TC, and PEG contents increased. The PPG almost had no effect on thetransition temperatures of the blends. Cold crystallization and melting temperatures of PLA increased slightly with increasingsorbitol content. Glycerol reduced the cold crystallization and melting temperatures but had no effect on the glass transitiontemperature. Starch formed a continuous phase in the presence of 25% sorbitol or 15%, 20%, or 25% glycerol. The TC andPEG enhanced the crystallization of PLA. The annealed samples containing glycerol had the highest crystallinity. Asconcentration of AC, TC, PEG, PPG, and glycerol increased, tensile strength and Youngs Modulus of the extruded andmolded blends decreased, and elongation increased in comparison to the blend without plasticizers.