A High Curing Efficiency Sucrose-based Adhesive via two-step modification by Toluene-4-sulfonic Acid and Methylenediphenyl Diisocyanate

Abstract

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 extraordinary harsh conditions with high hot-pressing temperatures (≥170 ℃), limiting practical applications. In this study, the sucrose-based adhesive (SD adhesive) was modified by toluene-4-sulfonic acid (TsOH) and methylenediphenyl diisocyanate (MDI) in two steps. The sucrose-based solution (SS) of the optimal adhesive in the study (SDTM20 adhesive) was synthesized with 5%TsOH catalyst at 100 ℃ for 3h, and then 20%MDI curing agent was added, followed by mixing. After that, the curing behavior, curing mechanism and microscopic characterization was used to analyze the optimal prepared materials. The wet shear strength (0.80 MPa) of the prepared adhesive meets the GB/T 9846-2015 China National Standard (≥0.7 MPa). 13C NMR showed that the TsOH catalyst promoted the formation of aromatic compounds, oligosaccharides and carbonyls, and the 5-HMF content of the SS in SDTM20 adhesive (44.0 g/L) was 158.8% higher than that of SD adhesive. In addition, the thermal curing behavior of SDTM20 adhesive was explained by changes in insoluble mass proportion (IMP) and TG-DSC thermal analysis. From the IMP measurement, SDTM20 adhesive (61.2%) increased by 141.9% compared with the SD adhesive. Additionally, in TG-DTG and DSC curves, the rapid weight loss temperature (145.1 ℃) corresponded to the endothermic reaction temperature (135.3 ℃), implying that the main curing reaction occurred at approximately 140 ℃, indicating that it is possible to improve curing efficiency of SD adhesive. ATR-FTIR was used to evaluate the curing mechanism of the SDTM20 adhesive. After adding TsOH/MDI, the dehydration reaction of sucrose, caramelization reaction, and Maillard reaction were accelerated to improve the curing efficiency. A complex polymeric cross-linking network structure with isocyanurate rings was generated, mainly connected by dimethyl ether bridges. SEM analysis of the microstructure of the adhesive revealed unknown crystal substances and small bubble pores on the fracture surface of the modified SDTM adhesive, showing that a novel cross-linking reaction had occurred.