Green design of biodegradable packaging films via renewable coffee waste and PVA matrix tuning

Abstract

Biodegradable packaging materials derived from renewable resources have attracted increasing attention as sustainable alternatives to conventional petroleum-based plastics. In this study, biodegradable composite films based on poly(vinyl alcohol) (PVA) reinforced with spent coffee grounds (SCG), an abundant agro-industrial waste, were successfully fabricated via a simple and environmentally benign water-based solution casting method. The effect of PVA matrix concentration (6–14% w/v) was systematically investigated to elucidate its role as a key structural parameter governing film formation, intermolecular interactions, thermal behaviour, and mechanical performance. Fourier transform infrared (FTIR) analysis provided direct evidence of hydroxyl-driven intermolecular interactions between PVA chains and lignocellulosic components of SCG, confirming the formation of hydrogen-bonded networks within the composite films. Thermogravimetric analysis (TGA) revealed a predictable multi-step degradation mechanism, with the main decomposition occurring between 250 and 500 °C and the formation of residual carbonaceous char at higher temperatures due to lignin-rich SCG fractions. Differential scanning calorimetry (DSC) showed a distinct melting transition around 170 °C and a degradation-related thermal event above 300 °C, indicating that crystalline domains and melting stability of PVA were preserved after SCG incorporation. Mechanical testing demonstrated that PVA concentration strongly influenced tensile behaviour. The composite films exhibited ductile deformation with high elongation at break (300–890%). An optimum formulation was identified at 12% PVA, which achieved the best balance between strength and flexibility, reaching a maximum tensile strength of 1.70 MPa and elongation at break of 889%. At higher PVA concentration (14%), excessive solution viscosity reduced filler dispersion homogeneity, leading to a slight decrease in tensile strength while maintaining high flexibility. Overall, this work highlights the importance of polymer matrix concentration as a critical design parameter beyond filler loading alone and demonstrates the potential of spent coffee grounds as a renewable, circular-economy reinforcement for sustainable PVA-based biodegradable packaging films.