The performance of adhesive-hardwood bonds can often be sensitive to humidity and temperature variation. Therefore, it is frequently challenging to achieve standard requirements for structural applications. To gain a better understanding of the wood-adhesive bond, the properties of the individual constituents as well as the local interface of European beech (Fagus sylvatica L.) wood cell walls in contact with structural adhesives were analyzed by means of nanoindentation. These results are compared to classical lap-shear strength. As adhesives two different one-component polyurethane adhesives (1C PUR) and a phenol resorcinol formaldehyde adhesive (PRF) were used. In one case, the beech wood was additionally pre-treated with an adhesion-promoting agent (primer) prior to bonding with 1C PUR. Beech wood joints were analyzed subsequent to several treatments, namely standard climate, after wet storage and in re-dried conditions. In addition, the influence of the primer on the hydroxyl accessibility of beech wood was investigated with dynamic vapor sorption (DVS). The lap-shear strength revealed good performance in dry and re-dried conditions for all adhesives on beech. Both polyurethane adhesives obtained deficits when tested in wet conditions. The use of a primer significantly improved the PUR performance in wet condition. DVS experiment demonstrated a decrease in hydroxyl group accessibility when using a high primer concentration. As novelty, nanoindentation was used for the first time to characterize the local wood–adhesive-interface properties in wet conditions. Nanoindentation showed that all tested 1C PUR perform quite similar in room climate, while PRF achieves considerable higher values for reduced E-modulus and hardness. Wet storage led to a considerable reduction in mechanical properties for all adhesives, while the highest relative change was observed for PRF. After re-drying, the adhesives re-gained a large part of their original mechanical properties in room climate. No distinct effect of the primer on the local micromechanical properties could be detected with nanoindentation in terms of specific work of indentation.

中文翻译：

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通过纳米压痕和拉伸剪切强度表征潮湿条件下的木材胶粘剂

硬木粘合的性能通常对湿度和温度变化敏感。因此，要达到结构应用的标准要求通常是具有挑战性的。为了更好地了解木材与木材之间的粘合力，各个成分的特性以及欧洲山毛榉（Fagus sylvatica）的局部界面，L.）通过纳米压痕分析与结构粘合剂接触的木质细胞壁。将这些结果与经典的搭接剪切强度进行比较。作为粘合剂，使用了两种不同的单组分聚氨酯粘合剂（1C PUR）和苯酚间苯二酚甲醛粘合剂（PRF）。在一种情况下，在与1C PUR粘合之前，还用粘合促进剂（底漆）对榉木进行了预处理。在湿存储后和重新干燥的条件下，对山毛榉木接缝进行了几种处理（即标准气候）后进行了分析。此外，还通过动态蒸气吸附（DVS）研究了底漆对榉木羟基可及性的影响。搭接剪切强度表明山毛榉上的所有粘合剂在干燥和重新干燥条件下均具有良好的性能。在潮湿条件下进行测试时，两种聚氨酯粘合剂均出现缺陷。底漆的使用显着提高了在潮湿条件下的PUR性能。DVS实验表明，使用高引物浓度时羟基可及性降低。作为新颖性，纳米压痕首次用于表征潮湿条件下的局部木材-胶粘剂界面特性。纳米压痕表明，所有测试的1C PUR在室温下的性能都非常相似，而PRF在降低E模量和硬度方面达到了相当高的值。湿存储导致所有粘合剂的机械性能大大降低，而PRF的相对变化最大。重新干燥后，胶粘剂在室温下恢复了很大一部分的原始机械性能。