Thermal and mechanical properties of sustainable rigid polyurethane foam composites reinforced with coffee husk fillers

Coffee husk waste fibers (CHWFs) offer a sustainable solution for reinforcing bio-based polyurethane foams. This study investigates the use of CHWFs in rigid polyurethane foams (RPUFs) synthesized from cashew nutshell-based polyol and evaluates mechanical, thermal, and combustion properties. Uniquely, three CHWF treatments based on untreated, mercerized, and grafted processes were comparatively analyzed to assess overall impact on foam performance. Mercerization increased cellulose content and crystallinity, whereas grafting enhanced moisture resistance and ash content. The CHWF addition reduced pore size and improved thermal stability, with onset temperature increasing from 279.9 °C to 334.4°C for neat foam, whereas the peak decomposition temperatures from 318.7 °C, 403.9 °C, 449.2°C to 321.5°C, 408.1°C, 459.9°C respectively for reinforced foams. Cone calorimeter tests at 30 kW/m 2 showed significant reduction in release rate of heat, CO/CO 2 emissions, and smoke with increasing fiber content. This work presents a novel dual-waste composite approach with enhanced fire safety and environmental value by combining two underutilized agro-wastes (coffee husks and cashew nutshells). Thus, these results highlight the potential of CHWFs as green reinforcements in high-performance, fire-safe RPUF composites. • Sustainable materials reinforced PU foams with CHWFs and cashew nutshell-based polyol. • Enhanced thermal stability raised decomposition temperatures to 350.1°C. • Improved fire performance reduced heat release, toxic gases, and smoke. • Mechanical boost increased compressive strength to 70.1 kPa with grafted CHWFs. • Refined microstructure reduced pore size and improved foam uniformity.