**4. Discussion**

Heat treatment made the specimens lose their moisture content completely, resulting in the vessel perforations and pit openings to shrink and be wide open. This shrinkage led to increased specific air permeability, because permeability is influenced by the porous structure of the material, and even slight changes in the porous system may significantly affect permeability [35,36]. The shrinkage is partially permanent due to the irreversible hydrogen bonding that occurs when water is moved in the cell wall [8,21].

NS-impregnation also resulted in significant increases of permeability in all NS-impregnated treatments. This increase was attributed to the high pressure applied in the pressure vessel, resulting in the rupture of vessel perforations, tyloses, and any other physical obstacle in the way of fluid transfer. SEM images showed vessels that were blocked (Figure 4A). The blockage was ruptured and torn open after NS-impregnation (Figure 4B,C). Therefore, fluid could flow more easily, and eventually, permeability increased significantly.

Heat treatment decreased pull-off strength in all three coatings studied here (Figure 3). It is reported that heat treatment results in the occurrence of microcracks in the wood structure and thermal degradation of cell wall polymers [5,37–39]. These microcracks led to unwanted penetration of adhesive film into wood texture, far from being involved in the process of anchoring and sticking dolly to wood substrate. However, in the NS-impregnated specimens, heat was transferred to deeper parts of specimens, and therefore, accumulation of heat did not occur on the surface of specimens, eventually decreasing microcracks in NS-impregnated specimens. This was translated into higher pull-off strength in NS-impregnated specimens of all three types of coatings.

**Figure 4.** SEM images showing cell parts and perforation plates (↓); (**A**) cross-section of a blocked vessel by distorted and ruptured vessel elements; (**B**) longitudinal section of an open vessel; (**C**) longitudinal section of a vessel blocked by broken cell parts and perforation plates.

(**C**)

The fitted-line plot between pull-off adhesion strength versus specific air permeability in all treatments showed no particular trend between these two properties (Figure 5). Regression analysis also showed insignificant *R*<sup>2</sup> between air permeability versus pull-off strength in almost all specimens and treatments (Table 1). The only significant *R*<sup>2</sup> was found in HT-145 polyester specimens. However, this one case cannot be a reliable indicator as to the existence of potentially significant *R*<sup>2</sup> between the two properties of permeability and pull-off strength. Therefore, it can be concluded that air permeability cannot be considered a good criterion to estimate the pull-off strength in beech wood. Both properties (permeability and pull-off strength) are dependent on the porous structure of materials, but permeability is influenced by the continuous pores while pull-off strength is more dependent on the surface pores, whether continuous or isolated. That is, an isolated and blocked vessel can also be active in the penetration and anchoring of adhesive, similar to a continuous vessel (Figure 4A). However, permeability is only dependent on the number of continuous vessels and pores, their attributes, and the way they are connected to one another.

**Figure 5.** Fitted-line plot between pull-off adhesion strength versus specific air permeability (Perm3 = specific air permeability measured at the third vacuum pressure or water column level).



SC = sealer-clear painted; NS-I = nanosilver-impregnated; ns = nonsignificant; HT = heat-treated.
