Coffee Silverskin as a Sustainable Alternative Filler for Plywood: Characterization and Performance Analysis

Coffee silverskin, a by-product of coffee processing, was studied using microscopic (SEM), spectroscopic (FTIR), and thermogravimetric (TGA, DSC) methods to assess its use as a substitute filler in the manufacturing of plywood. TGA showed that the material was compatible with plywood hot pressing temperatures (140 °C) and that it was thermally stable up to 50 °C, with a notable decomposition event at 335 °C. Functional groups like hydroxyl and carbonyl were detected by FTIR analysis, indicating possible hydrogen bonds and chemical adaptability. DSC analysis confirmed structural alterations by highlighting endothermic processes associated with dehydration and an exothermic transition over 150 °C. Coffee silverskin substituted rye flour in plywood adhesive compositions at different concentrations (0%, 1%, 5%, 10%, and 20%). Due to the structural and chemical constraints of the filler, larger concentrations (10% and 20%) dramatically lowered bonding strength, whereas low silverskin amounts (1% and 5%) attained strengths equivalent to rye flour, reaching up to 5 N mm^-2, according to internal bond strength tests. SEM images revealed smaller, more fragmented, and porous silverskin particles than larger, compact rye flour particles, which affected mechanical interlocking and adhesion. The findings point to coffee silverskin as an environmentally friendly and performance-balancing substitute for conventional fillers, especially at medium levels.