Thermoformability of Biopolymer Composites with Coffee Silverskin

The valorisation of agro-industrial residues in polymer composites represents a promising strategy for waste valorisation and the development of sustainable packaging materials. In this study, coffee silverskin (CSS), a lignocellulosic by-product, was added at concentrations up to 15 wt.% and processed into sheets via extrusion, followed by thermoforming using moulds with different draw ratios. Processability (MFI) and structural (FTIR), morphological (SEM, optical microscopy), thermal (TGA, DSC), and mechanical characterizations (tensile tests) were performed. Although the SEM images showed that CSS particles were well dispersed in the polymer matrix, and the mechanical behaviour was negatively affected when compared to the neat biopolymer. On the other hand, the addition of CSS increased the melt flow index, suggesting a lubricating/plasticizing effect. DSC results showed a reduction in cold crystallization temperature with CSS addition, confirming a nucleating effect, while glass transition and melting temperatures remained unchanged. Despite a narrower thermoforming temperature window with increasing CSS content, defect-free parts with adequate mould replication were successfully obtained for all formulations. Overall, the incorporation of CSS into PLA matrix provides a viable pathway for producing thermoformable as potential compostable composites, enabling waste valorisation within a circular bioeconomy framework.