The effects of plasticizers containing amide groups on the properties of thermoplastic starch

Several amide groups-containing plasticizers for thermoplastic starch (TPS), such as formamide, acetamide, and urea, were studied in this paper with glycerol as reference. The hydrogen bond interaction between starch and plasticizers in TPS was tested by Fourier transform infrared (FT-IR) spectroscopy. Both the oxygen of the C-O-H group and the oxygen of the C-O-C group in starch could form hydrogen bonds with these plasticizers. The order of the hydrogen bond-forming abilities is as follows: urea > formamide > acetamide > polyols. The retrogradation of formamide-plasticized TPS (FPTPS), acetamide-plasticized TPS (APTPS) and urea-plasticized TPS (UPTPS) was investigated at three levels of relative humidity (RH=0, 50 and 100%) using X-ray diffractometry. Urea and formamide could effectively improve the resistance of TPS towards retrogradation. The studied mechanical properties demonstrated that FPTPS had a good breaking strain but poor breaking stress, while UPTPS had opposite characteristics. The properties of TPS mainly relied on the hydrogen bond-forming abilities between plasticizers and the starch matrix. On the other hand, the water resistance of TPS mainly depended on the plasticizer. The higher the water absorption of the plasticizer was, the better was the water resistance of the TPS.