Starch-Based Microcellular Foams

Cereal Chem. 72(2): 155-161 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 highamylose corn starch. The aquagels were freeze-dried or dehydrated in ethanol (alcogels), and either dried in air, extracted with liquid C02 and dried in C02 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 C02 or the CPD samples. Foams of high-amylose corn starch could only be made from alcogels by liquid C02 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 pm) 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. Starch is the principal constituent of wheat endosperm (64-74%) and of other cereal grains (MacMasters et a1 1971), and it is a valuable source of food. The United States has depended heavily on exports for many years to market the wheat produced each year. However, some countries that were once largely dependent on U.S. exported wheat have become nearly self-reliant or have become exporters themselves. Efforts have been initiated t o generate new markets for domestic wheat by developing wheat-based, nonfood products. Starch is an important resource for nonfood products because of its low cost and abundant supply. Approximately 4.5 billion pounds of starch, primarily corn starch, are already used annually in the United States for industrial applications (U.S. Congress 1991). Some current nonfood uses for starch include chemicals derived from starch fermentation, starch-containing plastics, starch adhesives, starch-based sizing products, soil conditioners, and extruded-starch packing materials (Kennedy and Fischer 1984, Mentzer 1984, Otey and Doane 1984, Mohamed 1990, U.S. Congress 1991). To further expand nonfood markets for cereal grains, new starch-based products must be developed. One potential industrial application for starch is in the fabrication of microcellular foams. Microcellular foams are low density, solid foams composed of a solid matrix with air-filled pores a few micrometers or smaller in diameter. Microcellular foams are of commercial interest because of their unique physical and mechanical properties (Miller 1994). Polyurethane foams are commercially important microcellular foams with low thermal conductivity that can contain starch as a filler (Cunningham and Carr 1990, Cunningham et a1 1991). Cunningham et a1 (1991) found that 10% starch, or less, in polyurethane formulations did not adversely affect the thermal and mechanical properties of the foams. Another class of microcellular foams that has gained a renewed interest is the aerogels (Fricke 1985). Aerogels were first described in 1931 (Kistler 1931). They are produced from firm aqueous gels (aquagels) by displacing water within the gel matrix with air. Aerogels cannot be prepared by simply air-drying aquagels, because surface tension compresses the gel into a dense material 'Western Regional Research Center, USDA-ARS, Albany, CA