Comparative adhesion analysis at glue joints in European beech and Norway spruce wood by means of nanoindentation

To gain a better insight into the delamination behaviour of glue joints in hardwood, the adhesion at the cell wall level was investigated. By the use of nanoindentation techniques, the adhesion, hardness and Young's modulus of adhesively bonded European beech wood (Fagus silvatica L.) was analysed. To highlight differences between soft- and hardwood adherends, Norway spruce wood (Picea abies KARST.) was also investigated. One-component polyurethane (PUR) and phenol resorcinol formaldehyde resin (PRF) were used for bonding. Untreated and aged samples (artificially and naturally weathered) were analysed and compared to silylated samples as reference, assuming the silylation reduces the adhesion to a minimum. From the gathered results it can be concluded that artificial ageing has the same effect as natural weathering on, both, adhesion and the properties of the single components of a bond. In beech wood, weathering increases the adhesion of PUR significantly. For PUR in spruce wood, the adhesion is not affected by any treatment. Tensile shear tests signified a reduced adhesion in all silylated samples. Additionally, the stiffness and hardness of both adhesives were found to be reduced by approximately 10%. As a consequence, the applied silylation is not considered as adequate treatment for reference samples.