Effects of extrusion temperature and moisture content on mechanical properties of starch-based foams

Blends composed of wheat and corn starch, poly(ethylene-ca-vinyl alcohol), polystyrene, plasticizers, and nucleating and blowing agents were equilibra ted at several relative humidities and extruded at several temperatures and 100 rpm screw speed using a single-screw C. W. Brabender laboratory-scale extruder Bulk compressibility, bulk resiliency, and relaxation time of the starch-based foams were studied. A mathematical model was developed to pred ict bulk compressibility of the starch-based foams using a dimensional anal ysis method. Bulk compressibility decreased as extrusion temperature and mo isture content increased; whereas, the rate of relaxation in stress relaxat ion tests increased. Bulk resiliency increased as extrusion temperature inc reased and moisture content decreased. The mechanical properties of starch- based foams were compared to those of commercial polystyrene foams. The res ults indicated the optimum extrusion temperatures of the blends for making foamed cushioning materials (bulk compressibility: 120 to 300 N, bulk resil iency: 65 to 100%): 150 degrees C for a blend using normal wheat starch, an d 160 degrees C for a blend using hydroxypropylated wheat starch.