Preparation and Characterization of a Robust, High Strength, and Mildew Resistant Fully Biobased Adhesive from Agro-Industrial Wastes

Recent decades have witnessed an increasing trend of replacing conventional formaldehyde-based adhesives with sustainable and ecofriendly biobased adhesives in the commercial wood industry. The present study developed a full biomass utilization design for adhesives consisting of a cheap soybean meal and a lignin-based crosslinking agent (EXL). The primary component of the latter, i.e., epoxidized lignin, was synthesized using alkaline lignin and epichlorohydrin under simple reaction conditions. The Fourier transform infrared, proton nuclear magnetic resonance, and carbon-13 nuclear magnetic resonance analyses revealed the epoxy characteristic peaks in EXL, further validating the efficacy of the design developed. This simple design successfully synthesized the EXL with more epoxy groups. Moreover, a fully biobased adhesive with a wet bonding strength of up to 0.96 MPa was produced by incorporating the EXL as a sole crosslinking agent, which satisfies the requirements for interior plywood use. Furthermore, the effect of the lignin-based crosslinking agent with more epoxy groups on the structural changes, thermal stability, and micromorphology of the resulting adhesive was investigated. The incorporation of EXL exhibited a beneficial impact on the soy adhesive with improved wet bonding strength and thermal stability. Additionally, the antimold observation experiment under a high humidity environment demonstrated that the EXL contributed to the excellent antimold performance of the adhesive.