Sustainable Strong Adhesive from Fully Biobased Itaconate Polyester and Brewer’s Spent Grain via Molar Ratio Optimization

Itaconic acid (IA), being biomass-derivable, has garnered significant attention for its pendant C═C, enabling free-radical polymerization for sustainable adhesive development. In this work, we developed a sustainable adhesive using PPDDI-BSG matrices synthesized through catalyst-free, solventless polyesterification with varying IA and brewer’s spent grain (BSG) contents. Tailoring IA and BSG content resulted in P1-SGx with a semicrystalline nature and P2-SGx with an amorphous nature, enabling exploration of tailoring’s impact on the matrix cohesive and interfacial adhesive, advancing high-performance and eco-friendly adhesive solutions. Increased BSG content reduced cross-link density and enhanced crystallinity in the polymer matrix, with BSG inducing transcrystallinity in the semicrystalline P1-SGx network, contributing to improved local reinforcement. Mechanical testing demonstrated progressive improvements in Young’s modulus, increasing from 12.0 to 24.4 MPa for P1-SGx and from 167.9 to 270.1 MPa for P2-SGx. IA tuning also resulted in modulus enhancements of up to 2000%. Lap shear tests highlighted superior adhesive strength (∼7.3 MPa) in highly cross-linked P2-SGx matrices, which exhibited strong matrix cohesion but were prone to adhesive failure. Conversely, P1-SGx matrices with lower cross-link density experienced cohesive failure due to weaker matrix integrity, despite localized reinforcement from transcrystalline regions. All samples exhibit hydrolytic degradability to different extents, ensuring a sustainable end-of-life solution. This work highlights the critical role of IA and BSG contents in modulating matrix cohesion and interfacial adhesion, advancing the design of strong and sustainable adhesives and contributing to adhesive science.