Sustainable Thermoset Foams From Epoxidized Soybean Oil and Agricultural Waste as Substitutes for Their Petroleum‐Based Plastic Counterparts

ABSTRACT Foamed materials are essential for applications requiring mechanical or thermal protection, such as packaging and construction. Conventional foams like polystyrene and polyurethane perform well but pose major environmental challenges due to difficult disposal and fossil‐derived feedstocks. To address these issues, we develop fully biobased, biodegradable soybean‐oil‐derived composites in both compact and foamed form, with compact materials serving as essential benchmarks for comparison and characterization. All materials are synthesized from epoxidized soybean oil cross‐linked with liquid Pripol 1040 and cured at 80°C, enabling incorporation of agricultural residues such as carrot skin, tomato peel, and spinach stems without thermal degradation. Among the compact composites, those containing 50 wt.% carrot powder shows the best performance, increasing tensile strength from 0.15 ± 0.06 MPa for the neat thermoset to 1.16 ± 0.13 MPa. Controlled humidity conditioning introduces moisture that acts as an intrinsic foaming agent, enabling the transition from compact to naturally foamed composites. The foams containing 40 wt.% carrot exhibit a density reduction from 0.980 ± 0.010 g cm ^{− 3} to 0.660 ± 0.012 g cm ⁻³ , competitive thermal insulation (0.12 ± 0.004 W m ⁻¹ K ⁻¹ ), and marine biodegradability, with biochemical oxygen demand increasing from 1.88 to 33.05 mg O ₂ /100 mg. Overall, this work provides a sustainable alternative to fossil‐based foams, coupling high performance with substantial waste valorization.