A high curing efficiency sucrose-based adhesive via two-step modification by toluene-4-sulfonic acid and methylenediphenyl diisocyanate

Sucrose-based adhesives utilize renewable biomass material sucrose as the main ingredient, which are eco-friendly and non-toxic, making them attractive potential substitutes for formaldehyde-based wood adhesives. However, wood-based panel processing using sucrose-based adhesives requires extraordinarily harsh conditions with high hot-pressing temperatures (≥ 160 °C), limiting practical applications. In this study, the sucrose-based adhesive (SD) was optimized by toluene-4-sulfonic acid (TsOH) and methylenediphenyl diisocyanate (MDI). The optimum preparation conditions were as follows: sucrose-based solution (SS) was synthesized at 100 °C for 3 h with 5%TsOH, then adding 20%MDI after SS was mixed with dephenolic cottonseed protein (DCP) at room temperature. After 63 ± 2 °C hot water soaking for 3 h, the wet shear strength (0.80 MPa) of the obtained optimal adhesive (SDTM20) meets the GB/T 9846-2015 China National Standard (≥ 0.7 MPa) for class II plywood, and is 135.3% higher than the SD adhesive. Regarding the curing behavior, when heating at 150 °C for 7 min, the insoluble mass proportion of SDTM20 adhesive (61.2%) increased by 141.9% compared with the SD adhesive. In TG-DTG and DSC curves, the rapid weight loss temperature (145.1 °C) corresponded to the endothermic reaction temperature (148.4 °C), implying that the thermal decomposition and polymerization temperature decreased about 20 °C. In terms of the curing mechanism of SDTM20 adhesive, on the one hand, TsOH promoted the conversion of 5-hydroxymethylfurfural (increased by 158.8%) and other furan compounds during SS synthesis, then the active groups of these furan compounds (such as aldehyde group and hydroxymethyl group) could react with DCP by Maillard reaction. On the other hand, during the curing process, the MDI curing agent enhanced the reactivity of SD adhesive system and formed an insoluble polymer with dimethyl ether bridge as the main bonding bond. The surface morphology of the cured adhesive was changed by TsOH/MDI optimization, with a mass of irregular holes, which might be due to the reaction between the compounds containing hydroxyl in SDTM20 adhesive and the formation of carbon dioxide.