Improvement of the bonding performance of sucrose and ammonium dihydrogen phosphate adhesive by addition of dephenolized cottonseed protein

ABSTRACT Research on adhesives made from recycled and renewable biomass materials is an important direction in wood material industry. The eco-friendly plywood adhesive named as SADP adhesive, according to its composition (Sucrose and Ammonium Dihydrogen Phosphate), developed in our research group previously has good bonding performance but bleed-through veneer. In previous studies, we found that plant proteins, such as defatted soybean flour (DSF) and dephenolized cottonseed protein (DCP) material could improve this phenomenon. However, although DCP considerably promoted the bonding performance of the SADP adhesive, DSF did not. Therefore, this study will determine the optimal preparation and hot pressing conditions of the developed adhesive, and investigate the curing mechanism. First, the evaluation of the effects of the plant protein type, mass ratio, hot pressing temperature and time on the bonding performance of these adhesives revealed that plywood prepared by hot pressing at 170°C for 7 min using DCP/SADP-1/3 ratio as adhesive had the highest wet shear strength (1.17 MPa). Additionally, the comparative analysis of the two plant proteins showed that the DCP has a higher crude protein content, more tryptophan and arginine, which might be the reason for its better bonding performance with SADP solution. Second, in the analysis of curing behavior, the result of TG-DSC analysis corresponded to the insoluble mass proportion measurement, indicating that the optimal curing temperature of DCP/SADP-1/3 adhesive was about 170°C. In addition, ATR FT-IR analysis indicated that the curing mechanism was complex, involving caramelization and Maillard reaction to form a dense crosslinking structure, with dimethylene ether bridge as the main linkage. Finally, a comparison of various scanning electron microscopy (SEM) chromatograms revealed that, with the increase of the amount of DCP added, the cured adhesive became less porous and had smoother surface, which further confirmed that DCP and SADP solution formed a novel network crosslinked structure after curing process. Furthermore, compared with the wet shear strength of SADP adhesive (0.88 MPa), the plywood prepared using the novel DCP/SADP-1/3 adhesive (1.17MPa) was increased by 33%.