Cross-Linked Biocomposites with Both Matrix and Fillers Made from Soy-Derived Ingredients

Natural resources, such as wood components (cellulose, hemicellulose, and lignin) and plant oils, have drawn significant interest for the development of polymeric biocomposites. Despite some advantages of soybean hull (SH) and soybean meal (SM), such as high abundance, low cost, and high functionality, both materials lack film-forming properties and mechanical performance and are highly hydrophilic, which makes them incompatible with most polymer matrices. This study demonstrates the suitability of using various ratios of SH and SM in combination with other soy-based derivatives—soy oil-derived polymers—simultaneously in the development of cross-linked biocomposites. For this purpose, we reacted SH or SM with maleic anhydride (via hydroxyl groups) to introduce reactive sites for free-radical polymerization, followed by the bulk polymerization of the maleinized SH and SM in the presence of high-oleic soybean oil-based acrylic monomer (HOSBM). As a result, simultaneous “grafting from” polymerization on the filler surface and formation of the HOSBM homopolymer occur. The synthetic procedure results in a homogeneous distribution of fillers, both modified with soy-derived polymeric chains in the biocomposite matrix (polyHOSBM). In the study, up to 35 wt.% of total SH and SM was incorporated into the biocomposites, further cross-linked via post-polymerization autoxidation of polyHOSBM unsaturated functionalities. The mechanical characterization shows that incorporating 25 wt.% soybean hull leads to an enhanced Young’s modulus and tensile strength in comparison to other investigated biocomposites. Overall, the resulting cross-linked biocomposite films exhibit Young’s modulus in a range of 50–140 MPa, tensile strength of 1–2.9 MPa, and elongation at break of 18–55%. This work demonstrates the potential of the developed synthetic procedure to homogeneously distribute two abundant natural fillers simultaneously in cross-linked biocomposites.