Modification of novel bio-based adhesive made from citric acid and sucrose by ZnCl2

Abstract

Abstract Due to environmental and health concerns, a substitute for conventional wood adhesives has been sought, with bio-based adhesives being one of the most-researched alternatives. Recently, a bio-based adhesive made from citric acid and sucrose was used to manufacture sweet sorghum bagasse particleboard. However, the pressing conditions (200 °C, 10 min) consume a high amount of energy, and so optimization of the pressing condition is required. Therefore, the effects of ZnCl2 as a catalyst for the curing reaction of citric acid-sucrose adhesive were investigated. Thermal analyses such as differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) as well as Fourier transform-infrared (FT-IR) spectroscopy and insoluble matter analysis were used to investigate the effect of the catalyst on the curing behaviour of a citric acid-sucrose adhesive. DSC and TGA showed that ZnCl2 decreased the curing temperature, enthalpy reaction, and apparent activation energy. The apparent activation energy obtained using the Kissinger method decreased from 123.7 kJ/mol to 108.7 kJ/mol after 1% ZnCl2 catalyst was added. The level of insoluble matter in the cured adhesive after 4 h of boiling treatment at 150 °C, 180 °C, and 200 °C with various heating times (6 and 10 min) was higher after the addition of 1% ZnCl2. FT-IR analysis of the cured adhesive showed that ZnCl2 could accelerate the thermal degradation of sucrose, the formation of 5-hydroxymethylfurfural, and the formation of polymeric humins.