Effects of LDPE and glycerol contents and compounding on the microstructure and properties of starch composite films

Corn starch–low density polyethylene (LDPE) blends, at ratios of 95:5, 90:10 and 85:15, were processed into thin films by either single-step twin-screw extrusion or by a two-step process involving compounding (pelleting) of the ingredients before film formation. The microstructure, X-ray patterns, and the tensile, thermal and water vapor permeability (WVP) properties of the films, prepared by both methods, were evaluated and compared. SEM micrographs of the single-step processed films were characterized by the presence of cracks and a discontinuous interface between starch and LDPE. Consequently, their tensile and WVP properties were impaired. Compounding, on the other hand, improved the dispersion of LDPE on to the starch matrix and the interface between the starch and LDPE phases by lowering the viscosity of the thermoplastic starch (TPS) melt. As a result, the stress transfer between the starch and LDPE phases was improved, enhancing the tensile and water vapor barrier properties of the films considerably. In general, the tensile strengths and moduli increased as the LDPE content increased from 5 to 10%, but then decreased with further increase in LDPE content to 15%. Addition of glycerol decreased the tensile strengths and moduli but did not necessarily improve the tensile strains. Differential scanning calorimetry scans and Fourier-transform infrared spectra did not indicate any significant interactions between the two immiscible polymers even after compounding. Phase separation was observed in the TPS. The water vapor barrier properties of the composites were improved by 7.3–25.4% after compounding, but were adversely affected by glycerol content.