The effects of blend ratio and storage time on thermoplastic starch/poly(butylene adipate--terephthalate) films.

The objective of this work was to investigate blend ratio and storage time effects on the morphological, mechanical, and thermal properties of thermoplastic starch/poly(butylene adipate--terephthalate) (TPS/PBAT) films. TPS was prepared from plasticized cassava starch using a twin-screw extruder. TPS was subsequently melt-blended with PBAT with varied weight ratios (i.e., 20/80, 40/60 and 60/40) and blown to form TPS/PBAT films. It was found that increasing the TPS/PBAT ratio to 40/60 led to improved distributions of polymeric components and increased PBAT crystallization temperatures while reducing TPS melting transitions and tensile properties of TPS/PBAT films. After three months of storage at 30 °C, the tensile strength and secant modulus at 2% strain of TPS/PBAT films increased due to recrystallization of both TPS and PBAT. Blend ratios were the primary determinant for changes in TPS/PBAT film elongation at break with this storage time. Elongation at break decreased at low TPS:PBAT ratios (i.e., 20/80) and increased at high blend ratios (i.e., 60/40). The recrystallization of both TPS and PBAT components were observed from XRD and DSC analyses. Results obtained from both techniques confirmed the formation of additional crystalline structures of individual components during storage. The recrystallization phenomena also affected thermal transition temperatures of blend components. The crystallization temperature of PBAT-rich phase increased as starch could act as nucleating sites for PBAT. Using DMA, the curve of TPS/PBAT film exhibited two sharp individual peaks corresponding to the glass transitions of PBAT-rich and starch-rich phases. The of TPS-rich phase shifted to higher temperature due to recrystallization of TPS-rich phase.