Valorization of Spent Engine Oil as a Sustainable Polyol Substituent in Flexible Polyurethane Foam Synthesis: A Comparative Study of Starch and Natural Zeolite Filler Systems

Abstract

The accumulation of used engine oil poses severe ecological threats to soil and groundwater systems, particularly in developing industrial regions where proper waste disposal systems are lacking. Simultaneously, the manufacturing of flexible polyurethane foams remains heavily dependent on volatile, petroleum-derived polyols, contributing to a high carbon footprint. This study presents a circular economy approach by chemically converting hazardous used engine oil into a polyhydroxylated organic extender (oil polyol) to partially replace commercial polyether polyols in flexible polyurethane foam formulations. The non-polar hydrocarbon chains of used engine oil were functionalized via an acid-catalyzed hydration reaction, converting thermally cracked alkenes into reactive secondary hydroxyl groups capable of forming urethane linkages. To investigate structural preservation and prevent cellular collapse, two composite foam series were synthesized using a one-shot free-rise foaming method: one containing  natural starch as an organic biodegradable filler, and another containing  natural zeolite as an inorganic nucleating agent and chemical scavenger. The substitution level of polyhydroxylated oil was varied systematically from . Physical and mechanical characterization showed that the starch-filled foams suffered a severe bulk density drop from and catastrophic mechanical failure at  oil substitution due to thermodynamic phase separation and cell wall thinning. Conversely, the natural zeolite-filled foams maintained highly stable bulk densities (ranging between ) across all substitution levels, with compressibility rising to  at  oil. Optical microscopy confirmed that natural zeolite acts as an exceptional cell stabilizer, facilitating uniform heterogeneous bubble nucleation and adsorbing residual heavy metals and acidic impurities from the waste oil. These results demonstrate that natural zeolite provides a robust physical and chemical framework that permits high-ratio waste-oil valorization without compromising the mechanical performance of flexible polyurethane foams.