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

Abstract

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 regained 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.

Monitoring of Beech Glued Laminated Timber and Delamination Resistance of Beech Finger-Joints in Varying Ambient Climates

“…This can be associated with negative consequences for the long-term durability and load-bearing capacity of bonded beech building products, since MC changes can occur primarily in the outer layers and the delamination can thus occur with the greatest possible influence, especially in the compression and tension zones of the glulam. Consequently, further research is needed with respect to the influence of short-and long-term alternating climatic conditions on the strength of finger-joint bonding, since a climate-induced loss of strength was observed, as previously described [22,23]. Only the durability of the PUR bonding was increased by a higher MC bond , which was also found for the strength of the PUR bonding in [24].…”

Selected previous findings on the factors influencing the gluing quality of solid wood products in timber construction and possible developments: A review

“…For example, digital image correlation was used to measure crack initiation and crack propagation in adhesive joints under load (Knorz et al 2016). Atomic force microscopy (AFM), scanning electron microscopy, and related nanoindentation (NI) techniques enable the determination of the mechanical properties of the wood cell wall at the nanometer scale (Sonderegger et al 2015, Casdorff et al 2018, Bockel et al 2020. A similar level of observation can be achieved by analyzing the materials in situ when performing tests in a synchrotron.…”

“…The detection limits of instruments used for glue bond analysis are key in determining their suitability for delivering valuable reference data and for their successful integration with industrial monitoring systems. The presence of a primer on the wood surface before gluing can be detected by analytical instruments only when applied in sufficiently high concentrations (Casdorff et al 2018, Bockel et al 2020, Figure 7). Nevertheless, it was identified that a 10% water solution of the primer applied at a rate of no more than 20 g m −2 was optimal when gluing ash wood using 1C-PUR.…”

“…), and ash (Fraxinus sp. ), for gluing is known to vary due to their diverse extractive contents (Lüdke et al 2015, Konnerth et al 2016, Bockel et al 2019, Bockel et al 2020. In general, hardwoods have a higher density than softwoods and, consequently, higher mechanical strength and swelling/shrinkage.…”

The potential of improving wood preservative penetration into glued wood products bonded with one-component polyurethane by discontinuous adhesive bondline design

“…Similar trend is observed for wood composites bonded with 1C-PUR adhesive. Bockel et al (2020) determined that the shear strength of bonded beech wood specimens decreased by more than a half when testing them in wet conditions. However, after re-drying the shear strength regained its original value.…”