Abstract
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.
Highlights
The on-going trend towards mixed forests in Europe and a growing stock of hardwood challenges wood industry and science for an increasing material usage of hardwood
While the phenol resorcinol formaldehyde adhesive (PRF) joints performed significantly higher than the 1C PUR adhesives joints, no significant difference was observed within the 1C PUR variants
All 1C PUR adhesive bonds were characterized by a similar wood failure percentage in standard climate conditions of around 30–40%
Summary
The on-going trend towards mixed forests in Europe and a growing stock of hardwood challenges wood industry and science for an increasing material usage of hardwood. Various studies on wood cell walls, adhesives and their interactions at the interphase followed and contributed to a better understanding of the joint performance (Amman et al 2016; Zhang et al 2015; Jakes et al 2008; Konnerth et al 2006; Rindler et al 2018; Obersriebnig et al 2013). NI is capable of analyzing the properties of the individual components present in the interphase region of wooden bonds (e.g., Zhang et al 2015; Konnerth et al 2006) as well as the performance of the local interface at the micro-scale level (Obersriebnig et al 2013) Studies in this field mainly addressed the interphase region of wood-adhesive-bonds in dry conditions or the influence of moisture on polymer films (Konnerth et al 2010). Performance of the local interface between cell wall and adhesive in dry, wet and re-dried conditions
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