Enhancing Corn Starch-Poly(Vinyl Alcohol) and Glycerol Composite Films with Citric Acid Cross-Linking Mechanism: A Green Approach to High-Performance Packaging Materials

Corn starch (CS) is a renewable, biodegradable polysaccharide valued for its film-forming ability, yet native CS films exhibit low mechanical strength, high water sensitivity, and limited thermal stability. This study improves CS-based films by blending with poly(vinyl alcohol) (PVA) or glycerol (GLY) and using citric acid (CA) as a green, non-toxic cross-linker. Composite films were prepared by casting CS–PVA or CS–GLY with CA at 0%–0.20% (w/w of starch). The influence of CA on physicochemical, mechanical, optical, thermal, and water barrier properties was evaluated. CA crosslinking markedly enhanced the tensile strength, water resistance, and thermal stability of CS–PVA films while increasing transparency in CS–GLY films. At 0.20% CA, the composite achieved 34.99 MPa tensile strength, reduced water vapor permeability, and minimized water uptake. FTIR confirmed ester bond formation between CA and hydroxyl groups of CS, PVA, and GLY, whereas thermal analysis showed higher decomposition temperatures and lower weight loss in crosslinked films. Increasing CA levels also decreased opacity and improved light transmittance, indicating greater homogeneity and reduced crystallinity. This dual-polymer matrix combined with a natural crosslinking strategy provides a sustainable route to high-performance, biodegradable CS-based packaging materials.