3.7.2. Wood Etching

The structural changes of the sample surface after the plasma treatment were monitored using SEM. No significant changes in the structures were observed after the thermal nor plasma modification (Figures 10 and 11).

**Figure 10.** SEM images of plasma-treated samples at a distance of 0.15 mm from the electrode (**a**) 1000× magnification, (**b**) 50,000× magnification.

**Figure 11.** SEM images of plasma treated samples at a distance of 1 mm from the electrode (**a**) 1000× magnification, (**b**) 50,000× magnification.

Previously it has been reported [25], that DCSBD treatment can etch a wood surface. Though the time needed for such a modification is in order of minutes. In [3], researchers laid wood directly on the DCSBD and did not observe the formation of nanostructures even after 60 s.

#### 3.7.3. Influence of UV Radiation

To evaluate the sole effect of UV radiation emitted by plasma discharge a quartz glass was placed between the plasma and the wood. This allowed transmission of UV radiation, but at the same time preventing the passage of ions, electrons, radicals, and other molecules from the plasma, such as ozone or nitrogen oxides. The sample was positioned 1 mm above the electrode, and the air was used as the working gas.

Figure 12a,b show the resulting total SFEs and their corresponding disperse (dis.) and polar (pol.) components. Both plasma and UV treatment resulted in a statistically significant reduction of SFE. In both cases, the reduction could be attributed to the lowering of SFE's polar component, which was more pronounced for plasma treatment. The sensitivity of thermally modified spruce to UV exposure contradicts our former observation in [26], where thermally modified European beech (Fagus sylvatica) did not exhibit any statistically significant response. The reason for this remains unclear yet. One explanation can be higher initial SFE of spruce (58 vs. 49 mJ/mm<sup>2</sup> for T200), which may allow the effect to manifest itself. Another explanation may lay in the different chemical composition of coniferous spruce and deciduous beech.

**Figure 12.** Comparison of surface energies of reference samples, plasma treated at 1 mm gap and treated by plasma generated UV. (**a**) Total SFE, (**b**) division into SFE components.

To better understand this discrepancy, further thorough testing with different wood species and UV intensity would be needed. This is beyond the scope of this article and can be considered for further studies. UV radiation has been reported to damage lignin and cellulose crystallinity upon prolonged exposure [27]. However, during the presented experiments, the UV expose of samples was for a short time. The effect of UV radiation on color was studied in articles [28,29] and FTIR tests were performed. They observed darkening of the wood in the reference samples and lightening of the heat modified samples.
