An Investigation of the Physical Properties of Extruded Glycerol- and Formamide-Plasticized Cornstarch

Glycerol and formamide are used as plasticizers to prepare dry thermoplastic starch (DTPS) in a single-screw extruder. Glycerol/formamide—water plasticized starch (TPS) is also studied as a standard. Scanning electron microscopy reveals that more residual granules exist in glycerol-plasticized dry starch than in other blends. After the removal of water, DTPS becomes rigid and fragile. Increasing the formamide content results in a dramatic increase in the elongation of DTPS, and it maintains a high tensile strength. Dynamic mechanical thermal analysis demonstrates that formamide can decrease the glass transition temperature (Tg) of TPS and replace water to form formamide—starch mixtures in DTPS. Thermogravimetric analysis shows that destructurization of the polysaccharide chain is accelerated after removal of water, but it is alleviated by increasing the formamide content. It is also shown using Fourier transform infrared spectroscopy that formamide can form stronger hydrogen-bond interactions with starch than glycerol. At the same time, X-ray diffractometry shows that starch is plasticized more completely both in TPS and DTPS with a high formamide content. Finally, rheological study shows that formamide can decrease the shear viscosity and improve the fluidity of TPS and DTPS.