Processing and properties of extruded starch/polymer foams

Blends of starch and various thermoplastic resins were extruded into foams using a twin-screw extruder. Resins included poly(vinyl alcohol), cellulose acetate (CA), and several biodegradable polyesters. Foams of corn starch with poly(lactic acid) (PLA), poly(hydroxyester ether) (PHEE), or poly(hydroxybutyrate-co-valerate) (PHBV) had significantly lower densities and greater radial expansion ratios than the control starch. Blends with other polyesters and CA had densities and expansion ratios between those of the control starch and the other polyesters. Most of the polymer occupied spherical to elongated domains 1 - 10 mum long although PLA domains were much smaller. Surface polymer concentrations were larger than the bulk and correlated with foam expansion and resistance to fragmentation. Foams were also extruded using blends of PLA or PHEE with high amylose starch (70% amylose), wheat starch, and potato starch. Addition of either resin significantly reduced the foam density and increased expansion. At constant relative humidity, compressive strength was a function of foam density only and not the type of resin or starch in the blend. Addition of the resins reduced the water sensitivity of the foams and increased the time needed for complete dissolution. Blends with PLA, PHEE, or PHBV produced foams with densities comparable to commercial starch-based loose-fill foams. (C) 2002 Published by Elsevier Science Ltd.