Blends of hydroxypropylated high amylose starch and various functional aids listed below were extruded into foams using a twin-screw extruder ZSK - 30. In this study, the hydrophobic character and mechanical properties of starch foams were improved by using other biodegradable polymers, such as poly-caprolactone (PCL), poly (butylene adipate-co-terephthalate) (PBAT), cellulose acetate (CA), methylated pectin (MP), and polyvinyl alcohol (PVA), and crosslinkers like glyoxal. The hydrophobic character was improved in terms of a reduction in steady state weight gain, and an increase in dimensional stability (reduction in loss of radial and longitudinal dimensions) on moisture sorption. At the same time, efforts were made to maximize the expansion ratios by reduction of unit densities. Formulations of these foams (in terms of additive content and other processing parameters) were optimized. Particular formulations with PVA, polyesters like PCL and PBAT, and glyoxal with PVA gave foams with unit densities lower than 25 kg/m(3). The dimensional stability increased with an increase in the polyester content, but the density increased beyond an optimum polyester content, too. The loss in radial and longitudinal dimensions under steady state conditions was 12-20% with polyesters as compared to about 50-55% for control starch. Addition of these processing aids did decrease the water sensitivity of the starch foams. Foams with CA and methylated pectin, in the presence/absence of glyoxal, had marginally lower unit densities and slightly higher expansion ratios, as compared to those of control starch. (c) 2006 Wiley Periodicals, Inc.