Material Development and Foam Extrusion Processing of Thermoplastic Starch-Based Compounds

An investigation into the properties of starch-based compounds is conducted to determine their foamability in the foam sheet extrusion process. Three compounds containing different starch types (wheat, corn, and acetylated waxy corn starch with high amylopectin content) as well as poly(butylene adipate-co-terephthalate) (PBAT) and further additives are compounded, and their thermal, morphological, and rheological properties are analyzed. Based on these results, the most promising compound is chosen and processed by foam sheet extrusion in a second process step to evaluate its foamability using chemical and physical blowing agents (PBA). The type of the starch has an influence on the blend morphology. A change in the rheological properties (loss factor tan δ, melt drawability) is also observed between the compounds. The foamability analysis indicates that a minimum foam density of 0.46 g/cm3 with a degree of foaming of 0.65, which equals a density reduction of 65%, is achievable for the thermoplastic starch (TPS) compound with wheat starch using carbon dioxide (CO2) as the PBA. For maximization of the degree of foaming and achieving lower foam densities, the melt extensibility must increase to ensure that thin cell walls can be realized without the occurrence of cell coalescence. Also, further process optimization with different physical blowing agent compositions is needed.