The effect of moisture on the mechanical response of wood, adhesive and their interphase by means of nanoindentation

The single sections of a wood/adhesive bond, namely the adherend wood, the pure adhesive and the interphase are expected to contribute differently to the mechanical behaviour of a joint. Moisture-related changes of the stress/strain states between the adherends tend to result in deformation or even material failure. Therefore, beside the elastic, the moisture-related mechanical properties of the involved components are of significant interest. Adhesively bonded samples of spruce (Picea abies) using adhesives that are common in furniture and flooring production (emulsion polymer isocyanate (EPI), polyvinyl acetate (PVAc), urea formaldehyde (UF) and ultra-low formaldehyde-emitting amino resin (ULEF)) were manufactured under laboratory conditions. Quasi-static nanoindentation was performed on the pure adhesive polymer, the S2 layer of pure wood cells and interphase wood cells, to obtain the reduced elastic modulus, hardness and the indentation creep. To investigate the moisture-related mechanical properties, the testing environment was held under constant temperature and different controlled humidity levels. The pure wood cell walls generally tend to show a more stable behaviour to humidity changes than the investigated adhesives. It can be assumed that the ULEF adhesive penetrated the cell walls, as differences between pure wood cells and those located in the interphase could be identified. EPI, PVAc and UF showed no evidence of modifying the interphase cell walls but revealed different behaviours in relation to pure wood cells.