Comparative Evaluation of Natural and Acid-Based Plasticisers for Biodegradability, Compatibility, and Functional Enhancement of PLA/Wheat Gluten Bioplastic for Packaging Applications

Polylactic acid (PLA) and wheat gluten (WG) are biodegradable polymers with promising potential for sustainable packaging; however, their limited flexibility and barrier properties hinder broader application. This study investigates the modification of PLA/WG composite films using acetic acid and rosemary essential oil (REO) to improve their structural and functional properties. Microscopic analysis showed that rosemary essential oil (REO) made the surface rough and created small holes, while acetic acid helped make the structure denser and more even. Differential Scanning Calorimetry (DSC) showed a decrease in the cold crystallisation temperature from 148.9 °C (PGW) to 118.1 °C (PGW-A) and a reduction in melting enthalpy to 65.9 J/g with acetic acid, indicating a more amorphous structure. In contrast, REO increased the melting enthalpy to 100.9 J/g, suggesting partial recrystallisation and enhanced flexibility. Mechanical testing revealed that a mixture of 5% REO and 3% acetic acid yielded the optimal balance of strength and stretch before breaking. Barrier analysis showed a reduced water vapour transmission rate (WVTR) of 91.65 g/m2 per day. Biodegradation studies indicated 8.8% weight loss over eight weeks, while antimicrobial and antifungal assays confirmed REO's strong bioactivity. Overall, the synergistic effect of acetic acid and REO produced flexible, semi-crystalline, and bioactive PLA/WG films suitable for environmentally friendly packaging applications.