Development and optimization of polyurethane foam composites reinforced with biosilica nanoparticles from rice husks

The increasing demand for sustainable and eco-friendly materials has driven research on renewable fillers for polymer composites. Conventional fillers are costly, prompting the exploration of alternatives driven by agricultural waste. Rice husks, an abundant agricultural waste, are a low-cost and sustainable source of high-purity biosilica nanoparticles. This study explored the synthesis and characterization of biosilica nanoparticles from rice husks for use as a sustainable and renewable filler in a polyurethane foam matrix. Rice husks characterization involved the analysis of the physical composition. The rice husks exhibited variable moisture content and high ash content of 46–49% before pretreatment with hydrochloric acid and 49–54% after pretreatment. Through controlled combustion at 600°C and acid pretreatment, high-purity amorphous silica nanoparticles (86% purity) with an average crystallite size of 7.33 nm were obtained, as confirmed by X-ray diffraction, Fourier transform infrared spectroscopy, and energy dispersive X-ray analyses. The nanoparticles exhibited a spherical morphology and a stoichiometric Silicon:Oxygen ratio (1:1.7), while surface silanol groups enhanced their compatibility with polyurethane matrices. Response surface methodology was used to optimize the effects of nanoparticle concentration, mold temperature, and curing time on the resultant physical, mechanical, and thermal properties. The developed polyurethane foam nanocomposites were lightweight, with densities in range of 34.6-150.7 kg/m 3 , and their compressive strength peaked at 2.7 MPa. The thermal conductivity ranged between 0.0429 and 0.0507 W/mK. Images obtained using a Zeiss Stemi 508 stereomicroscope showed that the optimized foam had a more uniform cell structure than the control foam. The water absorption of the optimized foam was lower than that of the control foam. The optimized foam was more chemically stable in an oxidizing environment than the control foam. The results demonstrate that rice husk-derived biosilica nanoparticles are a promising eco-friendly alternative to conventional fillers, with potential applications in lightweight and insulating polyurethane foams.