Employing a mixture of fine-particle PKS, glycerol, and citric acid as an eco-friendly binder for plywood production from rubberwood (Hevea brasiliensis) veneer

Biomass-based adhesives, which are environmentally friendly and sustainable materials enabling low formaldehyde wood composites, have garnered interest. Therefore, palm kernel shells (PKS), available as industrial agricultural residue and rich in lignin, are mixed in form of fine particles with glycerol and citric acid, and tested as a candidate for binder in plywood production. The study focused on examining the effects of two factors: the quantity of adhesive used and the pressing temperature. Glycerol and citric acid are low-cost non-toxic chemicals that activate the functional groups and induce changes in the PKS component during hot pressing. Consequently, the mixtures with PKS as fine particles could cross-link with rubberwood veneer, forming a plywood panel with shear strength and bending strength that meet the requirements outlined in ISO 12466-2: part 2, and in Thai industrial standard (TIS 178-2549) for indoor use. The properties of plywood were primarily influenced by the pressing temperature rather than by the quantity of adhesive. Specifically, the temperatures 180 °C and 200 °C enhanced the extent to which the molten binder penetrated the rubberwood surface, consequently improving the mechanical properties and water resistance of the bonding.