Fungal-mediated modification of spent coffee grounds for enhanced interfacial compatibility and biodegradability of green composites

This study prepared sustainable polylactic acid (PLA) biocomposites reinforced with 20 wt% untreated spent coffee grounds (SCG_NT) and fungal biomass (FB) produced from the fermentation of SCG through solid-state fermentation (FB-SCG/SSF) and submerged liquid fermentation (FB-SCG/SmL), upcycling coffee waste into high-performance biofillers for packaging. Phytochemical, morphological (SEM), and chemical (FTIR) analyses were performed on raw SCG_NT and FB, whereas thermal (TGA/DSC) and mechanical analyses were performed on the biocomposites. DSC analysis revealed substantially enhanced crystallinity (Xc: 26–27%, vs. 9% in neat PLA; FB-SCG/SmL max 27%). TGA showed SSF-mediated thermal stabilization (higher Tmax than SCG_NT) and increased char residues (12–18% at 600 °C vs. <1% neat PLA). The thermal enhancements were attributed to the insulating properties of the mycelium and the improved fiber-matrix interfacial adhesion. These findings identify FB-SCG/SSF as an ideal filler, which includes a nucleating agent, a thermal-resistance booster, and a compatibilizer of the matrix and filler. Biodegradation studies under simulated aerobic composting (ASTM D5338) revealed that biological pre-treatment is a critical determinant of mineralization efficiency in composting with the PLA20%FB-SCG/SmL variant showing the best performance and reaching 96±3.2% mineralization after 90 days.