Hybrid Organic–Mineral Fillers in Polyurethane Foam Composites for Sustainable Nursery-Farming Materials: A Review

Biodegradable polyurethane (PU) foams reinforced with hybrid organic–mineral fillers are increasingly explored as sustainable alternatives to conventional nursery-farming materials, combining functional performance with controlled end-of-life degradation. This review synthesizes current literature on the incorporation of lignocellulosic agro-wastes—corn husk, guinea corn husk, millet bran, and wheat husk—alongside calcium carbonate-rich snail-shell powder into PU foam matrices. The molecular chemistry of PU formation, including toluene diisocyanate synthesis and polyol–isocyanate reactions, is examined alongside the roles of catalysts, blowing agents, and surfactants in governing foam morphology. Emphasis is placed on how hybrid organic–mineral fillers influence rise time, cure time, density, tensile and compressive strength, flexural performance, hardness, and thermal stability, with the mineral fraction providing rigidity and nucleation sites while the lignocellulosic fraction promotes biodegradability. Environmental and green-chemistry perspectives, including life-cycle considerations and soil-burial degradation mechanisms, are discussed to highlight the ecological benefits of agro-waste valorization. Despite extensive research on single-filler PU systems, this review identifies a significant gap in hybrid filler systems specifically designed for nursery applications requiring seedling support, water retention, and controlled biodegradability. The findings underscore the potential of hybrid agro-waste/snail-shell PU foams as innovative, low-cost, and environmentally sustainable materials for agricultural applications.