Sustainable Chitosan-Citric Acid Films via Homogeneous Cross-Linking: Comparative Structure–Property Analysis for Green Packaging Applications

Abstract Sustainable chitosan-citric acid films were developed through homogeneous cross-linking, for potential application as biodegradable food packaging. Chitosan (CS) from food waste offers antimicrobial properties, but suffers from poor mechanical strength and high water affinity compared to conventional plastics. This study presents an efficient cross-linking strategy, using citric acid (CA, 2%w/v) and glycerol (Gly, 2%w/v), and comparing commercial CS with agricultural-fertilizer-grade one (biological CS). Films were characterized before/after thermal curing (150 °C, 10 min) using ATR-FTIR, WAXS, TGA, mechanical analysis, and water uptake analysis. Curing converted ionic CS-CA interactions to covalent amide bonds, reducing water uptake >90% (1027% → 94% for commercial CS; 926% → 87% for biological CS) and enhancing tensile strength while maintaining flexibility via glycerol plasticization. WAXS revealed comparable crystallinity indices (69–75%) between CS sources, confirming agricultural-grade CS viability. Cu particles incorporation (1 g/L) could impart enhanced antimicrobial functionality, without compromising mechanical performance. This homogeneous cross-linking eliminates multistep drying of heterogeneous methods, offering an environmentally friendly, scalable route to high-performance bioplastic films as petroleum plastic alternatives.