Development and evaluation of a novel fructose-based bio-adhesive for plywood applications

Formaldehyde-based adhesives raise serious environmental and health concerns, prompting the wood-based panel industry to seek sustainable alternatives. In this study, a formaldehyde-free plywood adhesive (PP/FAT) was developed using fructose as the main raw material, ammonium dihydrogen phosphate (ADP) and toluene-4-sulfonic acid (TsOH) as catalysts, peanut protein (PP) as a filler, and methylenediphenyl diisocyanate (MDI) as a curing agent. The formulation was optimized under the following conditions: fructose/ADP mass ratio of 80/20, 10 % TsOH, 80 wt% solid content, synthesized at 90 °C for 3 h, followed by mixing with PP at 4/1 ratio and adding 5 % MDI. Three-ply plywood bonded with this adhesive and hot-pressed at 150 °C for 10 min achieved dry and wet shear strengths of 1.02 MPa and 0.97 MPa, exceeding the GB/T 9846–2015 standard (≥0.7 MPa). Curing analysis confirmed 150 °C as the optimal temperature, producing an insoluble mass proportion of 68.2 %. Chemical analyses revealed that, during the synthesis process, ADP and TsOH catalyzed the conversion of fructose into furan compounds. During the curing process, PP reacted with saccharide and its derivatives via the Maillard reaction, MDI underwent addition reactions with the hydroxyl groups in saccharide and its derivatives or with the hydroxyl and amino groups in PP, forming carbamate and urea linkages. Ultimately, the adhesive system formed insoluble polymers with benzene rings and dimethyl ether bridges as the main bonding linkages. With eco-friendly components, this study develops a novel formaldehyde-free plywood adhesive, offering a sustainable pathway for plywood industry.