Structure and properties of urea-plasticized starch films with different urea contents

•The paper provided some structure-performance data for urea-plasticized thermoplastic starch.•The existence state of urea in the starch film was characterized by FTIR–ATR spectra as well as by analysis of properties.•It was revealed that the antiplasticization or plasticization of the starch film was dependent on the urea content used.•Urea could restrain the crystallization of starch film. Films of thermoplastic starch (TPS) plasticized with different contents of urea were prepared by using a solution casting method. Scanning electron microscopy, X-ray diffraction and IR spectroscopy were used to characterize structures of the TPS films, respectively. Water vapor sorption isotherms and tensile properties of the films were determined. TPS films showed more smooth and transparent in appearance and less B-type crystallinity than the starch film without urea. The effect of urea content on the structure and behavior of the TPS film could be divided in three stages: (1) below urea 10% where urea interacted with starch via H-bonding and the films showed an antiplasticization effect, (2) from urea 10% to 30% where an apparent plasticization effect appeared on the starch because of free urea molecules as the effective plasticizer, and (3) a macroscopic phase separation occurred due to supersaturation of urea when urea content was more than 30%.