Formulation of tamarind polysaccharide-lignin-PVA based biodegradable films using response surface methodology (RSM)

The current study focuses on optimizing tamarind polysaccharide (TP)/lignin/PVA films incorporated with citric acid. TP (5 g) and PVA (0.875 g) were blended together in a weight ratio of 85% and 15%, respectively. The effects of different concentrations of lignin (0-2 g i.e. 0-40% w/w, calculated per mass of TP), citric acid (CA) (0-2 g i.e. 0-40% w/w, calculated per mass of TP) and glycerol (1-5 ml i.e. 20-100% w/w, calculated per mass of TP) on the water vapor transmission rate (WVTR), tensile strength (TS), and elongation-at-break (EB) were studied by employing a second-order Box-Behnken design. The obtained models for all the responses were found to be significant (p < 0.05) and fitted well with the experimental data (0.97 < R² < 0.88). Statistical analysis revealed that the effects of glycerol on WVTR, glycerol and citric acid on TS, and citric acid on EB were significant (p < 0.05). Based on the maximum desirability function, the optimum conditions were found to be 2 g of lignin and 1 ml glycerol per 100 ml of film-forming solution. The films developed under the optimized conditions showed a WVTR of 36.09 g/m²/h, TS of 16.29 MPa, and EB of 47.936%. The TP/lignin/PVA composite films were evenly distributed, flexible, and brown. The optimized films did not initially contain citric acid, so they were subsequently cross-linked with citric acid at lower concentrations (0.25-0.75 g). The effects of these low citric acid levels on WVTR, TS, and EB were investigated. The results indicated that the addition of 0.25 g citric acid per 100 ml of blend resulted in an overall improvement of film properties. The chemical structures of the optimized films (with and without citric acid) were characterized using Fourier-transform infrared spectroscopy and X-ray diffraction. The thermal stability was determined using differential scanning calorimetry, and structural analysis was performed using scanning electron microscopy.