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The performance of adhesive-hardwood bonds can often be sensitive tohumidity and temperature variation. Therefore, it is frequently challenging to achievestandard requirements for structural applications. To gain a better understanding of thewood-adhesive bond, the properties of the individual constituents as well as the localinterface of European beech ( Fagus sylvatica  L.) woodcell walls in contact with structural adhesives were analyzed by means ofnanoindentation. These results are compared to classical lap-shear strength. Asadhesives two different one-component polyurethane adhesives (1C PUR) and a phenolresorcinol formaldehyde adhesive (PRF) were used. In one case, the beech wood wasadditionally pre-treated with an adhesion-promoting agent (primer) prior to bonding with1C PUR. Beech wood joints were analyzed subsequent to several treatments, namelystandard climate, after wet storage and in re-dried conditions. In addition, theinfluence of the primer on the hydroxyl accessibility of beech wood was investigatedwith dynamic vapor sorption (DVS). The lap-shear strength revealed good performance indry and re-dried conditions for all adhesives on beech. Both polyurethane adhesivesobtained deficits when tested in wet conditions. The use of a primer significantlyimproved the PUR performance in wet condition. DVS experiment demonstrated a decrease inhydroxyl group accessibility when using a high primer concentration. As novelty,nanoindentation was used for the first time to characterize the localwood–adhesive-interface properties in wet conditions. Nanoindentation showed that alltested 1C PUR perform quite similar in room climate, while PRF achieves considerablehigher values for reduced E-modulus and hardness. Wet storage led to a considerablereduction in mechanical properties for all adhesives, while the highest relative changewas observed for PRF. After re-drying, the adhesives re-gained a large part of theiroriginal mechanical properties in room climate. No distinct effect of the primer on thelocal micromechanical properties could be detected with nanoindentation in terms ofspecific work of indentation.

Characterization of wood-adhesive bonds in wet conditions by means of nanoindentation and tensile shear strength