Evaluating fundamental position-dependent differences in wood cell wall adhesion using nanoindentation

Abstract

Spruce wood specimens were bonded with one-component polyurethane (PUR) and urea-formaldehyde (UF) adhesive, respectively. The adhesion of the adhesives to the wood cell wall was evaluated at two different locations by means of a new micromechanical assay based on nanoindentation. One location tested corresponded to the interface between the adhesive and the natural inner cell wall surface of the secondary cell wall layer 3 (S3), whereas the second location corresponded to the interface between the adhesive and the freshly cut secondary cell wall layer 2 (S2). Overall, a trend towards reduced cell wall adhesion was found for PUR compared to UF. Position-resolved examination revealed excellent adhesion of UF to freshly cut cell walls (S2) but significantly diminished adhesion to the inner cell wall surface (S3). In contrast, PUR showed better adhesion to the inner cell wall surface and less adhesion to freshly cut cell walls. Atomic force microscopy revealed a less polar character for the inner cell wall surface (S3) compared to freshly cut cell walls (S2). It is proposed that differences in the polarity of the used adhesives and the surface chemistry of the two cell wall surfaces examined account for the observed trends.

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

“…As a consequence, stiffening of the wood cell walls is frequently observed (e.g., Gindl et al 2004;Konnerth et al 2006). However, this effect was not visible for the hardness of wood cells near PRF measured in other studies (Obersriebnig et al 2013). Between the 1C PUR variants, wood cells of 1C PUR A and 1C PUR C did not show any significant difference, while 1C PUR B was significantly lower.…”

“…1) of the indents. As recommended by Obersriebnig et al (2013), indents were performed in a displacement-controlled mode with a maximum indentation depth of 850 nm. Load was applied in a three-segment load ramp with a load increase for 3 s, peak load holding for 20 and 3 s of unloading.…”

“…The usage of nanoindentation to determine the properties of wood cell walls was introduced by Wimmer et al (1997). 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 has also been used in high humidity environment as shown by Jakes et al (2015), but not yet applied to water-stored glue lines of hardwood in combination with a primer.…”

“…However, indentation values reported for studies on wood cell walls have to be interpreted carefully due to the threedimensional stress state in combination with the anisotropic nature of wood, as well as the importance of proper sample preparation (Konnerth et al 2009). 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).…”

“…Little information is available on micromechanical properties including the influence of moisture and the performance of the interface at the local level, possibly due to a lack in available methodology. One possible approach to test interface performance was proposed by Obersriebnig et al (2013). Knowledge of moisture-dependent mechanical properties of single constituents present in hardwood bonds could therefore help to better understand the behavior of the joint and possible influence of a primer.…”