Adhesive strength and micromechanics of wood bonded at low temperature

Abstract

Load-bearing structural timber requires durable and reliable adhesive bonding. Hence manufacture is determined by strict regulations for instance regarding wood temperature specifications. This research investigates bonding at significantly lower substrate temperatures providing sufficient time for cure to ascertain whether appropriate bond strength can be achieved. Scarf joints were produced at different wood temperatures with melamine-urea-formaldehyde (MUF). Resin and hardener were applied on separate sides. Adhesive strength was verified by macroscopic testing of the bond strength and microscopic examination of the properties of the adhesive. Resulting tensile shear strength varied marginally for different temperatures even after water immersion. Penetration of the adhesive into the wood in fiber direction was deeper on hardener than on resin application side due to differences in viscosity with no dependence on temperature. Nanoindentation visualized adhesive penetration into the cell wall with increased hardness and reduced modulus of elasticity from unfilled cells to adhesive filled cells independent of temperature. The results indicate least influence of the substrate temperature at the time of adhesive application on the macro- and microscopic level permitting bonding at low temperatures, given that enough time for cure at sufficient temperature is allowed.