Preparation and Characterization of Starch-Based Bioplastic Films Modified by Citric Acid-Epoxidized Soybean Oil Oligomers

In this work, citric acid (CA) was successfully used to improve the features of starch-based bioplastic films containing epoxidized soybean oils (ESO). Citric acid-epoxidized soybean oil oligomers (CESO) with different degrees of polymerization were synthesized by controlling the molar ratios of CA to ESO (nCOOH/epoxy = 1 ~ 4). The molecular weight and degree of polymerization of CESO decreased as more CA was incorporated. Starch-based bioplastic films with CESO were produced via solution casting and characterized by structural, thermal, tensile, water resistance, and optical properties. The hydrogen bonding interactions between starch and CESO were confirmed by Fourier transform infrared spectroscopy (FTIR). Field emission scanning electron microscopy (FESEM) micrographs suggested that CESO exhibited better compatibility with starch than ESO. Bioplastic films with CESO presented lower swelling degree and opacity as well as higher thermal property and tensile strength than the film with ESO. The composites with CESO2 (nCOOH/epoxy = 2) showed the most comprehensive properties including thermal stability, tensile strength, opacity, and water resistance than those with other CESO. Higher contents of CA in CESO (nCOOH/epoxy > 2) might cause the degradation of starch and deteriorate the properties of the films.