Adhesive strength and micromechanics of wood bonded at different moisture contents

The adhesive bonding of wood is preferably carried out at levels of moisture content close to those expected during service. In this study, the influence of non-ideal wood moisture content (MC) during adhesive bonding is investigated, using lap shear testing, nanoindentation and microscopy. Spruce lamellas were bonded at 5, 12, 25% MC and largely above fiber saturation point (∼55% MC) with melamine-urea-formaldehyde (MUF) and one-component polyurethane adhesive (1C PUR). Tensile shear strength decreased with increasing MC for both adhesive systems, even though the specimens were given ample time to post-cure, as testing was at standard MC 12%. Particularly above FSP, with the presence of free water, tensile shear strength was significantly reduced. For MUF, this is consistent with reduced mechanical properties measured with nanoindentation presuming that excessive moisture hindered adhesive cure. For 1C PUR, micromechanical properties of the bulk adhesive were not significantly affected by MC, the influencing factor was rather insufficient adhesion to the wood surface due to free water. The results propose the applicability of the adhesive beyond the recommended moisture ranges, but advise caution when bonding wood above FSP without process adjustments.