Macro- and micromechanical characterization of wood-adhesive bonds exposed to alternating climate conditions

Three-part specimens were produced from Norway spruce wood (Picea abies Karst.) and bonded with the following adhesives: melamine-urea-formaldehyde (MUF), phenol-resorcinol-formaldehyde (PRF), and a two-component emulsion polymer isocyanate (EPI). The effect of alternating climate conditions on bond strength was studied by tensile tests. The specimens were exposed to a three-step ageing cycle lasting for 7 days [50 degrees C/95% relative humidity (RH), -20 degrees C/65-70% RH and 75 degrees C/15% RH] which was repeated 24 times. In general, a decrease in internal bond strength of all exposed specimens was observed but it was highest in the case of MUF-bonded joints. Furthermore, a significant decrease of the tensile strength of the wood adherend perpendicular to the grain in the tangential direction was determined after the cyclic climatic changes. The mechanical performance of the different adhesives in the bond line was tested by means of nanoindentation. Reduced values of elastic modulus, hardness, and total indentation were observed after climatic treatment, particularly for the rigid MUF adhesive, whereas the flexible adhesive EPI did not show such changes.