Flexural Performance and Processing Limitations of Resin-Infused Composites Reinforced with Post-Consumer Jute Coffee Sacks: A Comparative Study with Thermoset and Thermoplastic Matrix Systems

Abstract Post-consumer natural-fibre textiles offer a potential route towards low-impact composite materials. However, their processing-related limitations and mechanical performance variability remain insufficiently understood. This study investigates the flexural performance and processing constraints of polymer composites reinforced with discarded jute coffee sacks and benchmarks them against primary flax fibre laminates and plywood reference materials. Com-posite specimens were manufactured by vacuum-assisted resin infusion using three polymer matrix systems: a petrochemical epoxy, a partially bio-based epoxy, and an infusable thermoplastic (Elium®). Flexural properties were characterised by four-point bending, and apparent fibre volume fractions were estimated to assess laminate consolidation quality. Composites reinforced with recycled jute coffee sacks achieved flexural moduli between 3.2 and 4.9 GPa and flexural strengths between 61 and 90 MPa, depending on the matrix system. In comparison, primary flax laminates exhibited significantly higher moduli (9.9–16.1 GPa) and strengths (153–234 MPa), while plywood reference materials showed comparable stiffness but a distinct failure mode dominated by tensile fracture of the bottom veneer and subsequent interlaminar crack propagation. The reduced performance of recycled jute composites is primarily attributed to processing-induced limitations, including heterogeneous textile architecture, local fibre misalignment, and porosity, which dominate the mechanical response. Differences between matrix systems were found to be secondary relative to the influence of reinforcement quality and impregnation effectiveness. Despite these constraints, the results demonstrate that post-consumer jute coffee sacks can be processed into resin-infused composites with reproducible, moderate flexural performance suitable for semi-structural applications with limited load requirements. The study provides quantitative insight into the processing–structure–property limitations governing the use of reclaimed natural-fibre textiles in engineering composite design.