Starch-based microcellular foams

The present study was initiated to develop alternative methods of preparing microcellular starch-based foams from semirigid aqueous gels (aquagels) and to characterize the impact of the preparative method on the physical and mechanical properties of the foams. Semirigid aquagels were made from 8% solutions of wheat starch, corn starch, and high-amylose corn starch. The aquagels were freeze-dried or dehydrated in ethanol (alcogels), and either dried in air, extracted with liquid CO2 and dried in CO2 vapor, or critical point-dried (CPD). The wheat and corn starch foams prepared by air-drying alcogels had densities and mechanical properties similar to those that were extracted by liquid CO2 or the CPD samples. Foams of high-amylose corn starch could only be made from alcogels by liquid CO2 extraction and CPD. The mean densities of CPD wheat, corn, and high-amylose corn starch foams were 0.23, 0.24, and 0.10 g/cm3, respectively. The compressive strength and modulus of elasticity of the foams were positively correlated with density. The wheat and corn starch foams were weaker under tension when compared to compression, due probably to the abundant voids and imperfections in the foam matrix that provided sites for cracks to propagate. Wheat and corn starch foams deformed under compressive stress also had a high range in elastic modulus (21-35 MPa) and low elastic recovery (13%) compared to freeze-dried and high-amylose corn starch samples (3-8 MPa and 27-36%, respectively). The range in thermal conductivity of the starch foams (0.024-0.043 W/m.K) was comparable to that of commercial insulation materials. The foam matrix was composed of pores (<2 micrometer) defined by a network of strands in which were embedded remnants of starch granules. The remnants were most abundant in wheat and corn starch samples. Freeze-dried foams had large, nonuniform pores with a continuous cell-wall structure that conferred relatively high tensile strength.