The Effect of Processing Conditions and Composition on Starch Microcellular Foam Properties.

Abstract

Starch microcellular foams (SMCF) are prepared by pore preserving drying or formation processes and contain pores in the micron size range. SMCF have high specific surface area and are useful for applications such as opacifying pigments or as adsorbent materials. The objective of this research was to determine how the processing conditions and use of a sizing and crosslinking agent would affect the foam structure and properties.

To produce SMCF materials in this research, cooked starch solutions were subjected to different solvent exchange processes involving the exchange of water for ethanol. Drying from the low surface tension ethanol allows pore formation on drying to produce low density and high opacity microcellular foams; if the starches are dried from water, large capillary forces destroy the pore structure, producing dense translucent solids. It was found that the microcellular foam density passed through a minimum with respect to the solvent exchange times.

SMCF (crosslinked and uncrosslinked) were prepared from molded aquagels and carbon dioxide extruded samples separately and then solvent exchanged. Extruded samples showed macroscopic pores whereas samples from aquagels showed a much finer micro pore structure. Aquagel based SMCF samples had lower density and higher brightness than extruded samples. The starch foams with micro pore structure had low density and high brightness. The solvent exchange process was the most important variable in generating a microcellular structure. Micro pores and not macro pores contributed to increased brightness of these materials. The brightness and density of the foams were found to be linearly related. Crosslinking with epichlorohydrin imparted significant water resistance to the extruded samples as evidenced in lower water swelling and higher contact angles. Equilibrium moisture content was correlated with the micro-porous structure.

In order to investigate the effect of adding a hydrophobicizing agent on the water sensitivity several samples of SMCF-AKD were produced and the results showed that the samples had similar brightness, higher density, and higher water contact angle than starch foams alone. SMCF materials blended with Kymene, a common wet strength agent for paper and a crosslinking component for carbohydrates, did not show a significant increase in water resistance.