Expansion, Morphological, and Mechanical Properties of Starch−Polystyrene Foams Containing Various Additives

Starch and polystyrene (PS) were mixed at 70:30, 80:20, and 85:15 ratios with 0.5 and 1% talc and extruded into loose-fill packaging foams. Azodicarbonamide (ADC), at 0, 0.2, and 0.4 wt %, and citric acid, at 0, 0.25, and 0.5%, were added as blowing agents to enhance the radial expansion and functional properties of foams. As the concentration of ADC in the starch−PS mixtures was increased from 0 to 0.2%, the expansion ratios increased, and consequently the bulk densities decreased. However, with a further increase in the concentration of ADC to 0.4%, the expansion ratios dropped considerably. The effects of citric acid on radial expansion and other properties were similar to those of ADC. Compared to ADC, citric acid produced foams with higher radial expansion and correspondingly lower densities. Extrudates containing citric acid had large-sized cells, but the cell walls were structurally damaged due to degradation of starch. The spring indices of foams treated with various additives were not significantly different, indicating that spring index may not be a reliable measure of the elasticity. On the other hand, compressibility and modulus of elasticity varied depending on the starch, talc, and blowing agents. Foams extruded with ADC were superior in terms of their cushioning ability and other functional properties.