Walnut Meal as Formaldehyde-free Adhesive for Plywood Panels

Abstract

Walnut meal and polyethyleneimine (PEI) were used as raw materials for producing an environmentally friendly adhesive to bond plywood, and the shear strength was investigated. The results showed that the walnut meal-based mixture adhesive can successfully be used for the production of plywood. However, the shear strength of plywood clearly changed with different raw materials ratios. Both the wet and hot water strength were improved with increasing polyethyleneimine (PEI) content. This indicated that PEI had an important contribution to the water-resistance of the mixture system, and considering the cost of PEI, a walnut meal powder and PEI weight ratio of 70:30 was recommended. The effects of walnut protein solution thermal processing time and temperature were also studied. It was beneficial to enhance the wet and hot water shear strength of plywood to increase the thermal processing time and temperature. The investigation of curing characteristic of mixture system and monomer ingredient via differential scanning calorimetry (DSC) revealed that the curing peak temperature of the mixture system was similar to PEI, and that a chemical reaction should be made between them during the heating process. Lastly, the thermal characteristic of the mixture system was investigated via thermogravimetric (TG) analysis, where good thermal resistance was displayed.