Shrinkage and re-expansion of extruded starch acetate foams

Abstract

The effects of concentration of steam, heating time, pretreatment, and cooling conditions on the shrinkage and re-expansion of extruded starch acetate foam were investigated. The re-expansion ratio was determined from the degree by which the foam cells were moistened under the condition tested. The higher the concentration of steam used, the faster and greater the foam re-expanded. Shrinkage was a result of the negative pressure resulting from cooling and condensation of steam or ethanol inside the foam and the unbalanced osmosis of steam transported out of the foam and air transported into the foam and the resistance of foam cell walls. Quenching re-expanded foams led to significant shrinkage, especially for foams re-expanded with highly concentrated steam. Shrinkage cannot be avoided even if the foam is kept at the same temperature it was expanded at the nozzle. Gradually reducing the degree of concentration of steam could reduce the shrinking tendency of the re-expanded foam. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4264–4268, 2006