Low temperature and moisture dependent curing behavior of selected wood adhesives

Curing kinetics of adhesives are a highly influencing factor in the manufacture of engineered wood products. Various parameters such as temperature and humidity can affect the reactivity of the adhesive and thus the curing speed and the resulting adhesive strength. This research investigates the influence of very high and low wood moisture content (MC) and low temperature on the curing kinetics of adhesives. The examined melamine-urea-formaldehyde (MUF) and one-component polyurethane (1C PUR) are used for structural timber products where mechanically stable and durable adhesive bonds are crucial. A rheometer set-up with conditioned poplar plates allowed the interaction of adhesive and substrate to simulate realistic curing conditions. Both adhesives, MUF and 1C PUR, had delayed gel times at lower temperatures and curing slowed down. For 1C PUR water absorption from the wood to the adhesive had a further important role on the curing reaction. For MUF, substrate temperature was the main influencing factor with much longer gel times at low temperatures compared to 1C PUR. Gel times increased to such an extent for a low MC of 5%, as the moisture required was not sufficiently available, that temperature influence became negligible. Resulting tensile shear strength supported the rheological results. Lower substrate temperature affected tensile shear strength of MUF negatively, while 1C PUR kept its tensile strength. Both adhesives retained their tensile properties after water immersion.