Corn starch/Carboxymethyl Cellulose Films: Influence of Citric Acid on Water Susceptibility, Morphological and Tensile Properties

The present study aimed to produce and characterize corn starch/carboxymethyl cellulose (TPC/CMC) films cross-linked with different concentrations of citric acid (CA). The films were obtained by solvent evaporation (casting solution) with 5, 10, 15, and 20% citric acid relative to the CMC mass (TPC/CMC-CA-0 to 20). They were qualitatively evaluated for visual aspects, swelling, moisture content, and solubility, and characterized by Fourier transform infrared spectroscopy (FTIR), thickness, and scanning electron microscopy (SEM). The results allowed us to evaluate the cross-linking effect promoted by citric acid. Visually, all films showed excellent optical transparency. FTIR spectroscopy demonstrated the formation of ester groups, indicative of cross-linking of the TPC-CMC films. SEM images revealed a homogeneous surface morphology without porosity in the films with CA. The addition of CA to the TPC-CMC films reduced water swelling, moisture absorption, and permeation. Additionally, the insertion of citric acid promoted an improvement in the tensile strength of TPC-CMC from 6.07 to 8.39 MPa for TPC-CMC/CA-15. This same trend was obtained with an increase of around 115% for the modulus in the TPC-CMC/CA-15 film compared to the film without CA. These results were due to cross-linking promoted by CA which produces a more cohesive material and thus increases the modulus and tensile strength. Thus, our results demonstrate that the cross-linking promoted by CA in TPC-CMC films has a profound influence on their water susceptibility and tensile properties.