Effect of tartaric acid and castor oil on the characteristics of low-density polyethylene/thermoplastic starch composite films

Low-density polyethylene (LDPE) and thermoplastic starch (TPS) films containing varying concentrations of tartaric acid (0, 1.5, and 3 %) and castor oil (0, 1, and 2 %) were produced using an extrusion process. Field-emission scanning electron microscopy images confirmed a smooth and homogeneous surface with the combination of tartaric acid (TA) and castor oil (CO), indicating good compatibility between the polymer components. The formation of starch ester bonds with TA and CO was observed in the Fourier transform infrared spectroscopy spectrum. Dynamic mechanical thermal analysis results demonstrated that the addition of TA and CO to the LDPE/TPS films resulted in a decrease in the glass transition temperature and a positive change in the storage modulus. The sample with the highest biodegradability percentage was LDPE/TPS/TA3 %/CO1 %, which achieved a biodegradability rate of 34.46 %. This was attributed to acid hydrolysis and a reduction in the polymer's molecular weight over a six-month period of storage in soil. The water solubility, contact angle, and water vapor permeability of the sample LDPE/TPS/TA3 %/CO1 % were measured at 9 %, 88.35°, and 3.51 × 10^–6 gs^-1m^-1Pa^-1 respectively. The mechanical properties of the composite films improved with the addition of TA and CO due to the cross-linking effect; specifically, the tensile strength, elongation at break, and heat seal strength of LDPE/TPS/TA3 %/CO1 % were 9.77 MPa, 8.48 %, and 8.45 MPa, respectively. The highest opacity value was recorded for the sample LDPE/TPS/TA3 %/CO2 %, which measured 12.55, surpassing the other samples. The melt flow index of the sample LDPE/TPS/TA3 %/CO1 % was determined to be 6.27 g/10 min, while the sample LDPE/TPS/TA3 % had a melt flow index of 9.70 g/10 min. The addition of TA and CO also enhanced the oxygen barrier properties of the films.