**4. Discussion**

Thermal modification of wood should raise the relative content of hydrophobic lignin. The results of SFE measurements and the O/C ratio of TMW (Figure 3, Table 1) confirmed this expectation. A large jump of 4.6 mJ/mm<sup>2</sup> in SFE occurred between T180 and T200. The O/C ratio for TMW gave significantly higher values for T160, while there was only a slight decrease between T180 and T200. However, the measurement on sample T160 is suspicious due to the lower proportion of the Cl component compared to other plasma untreated samples. Deviation from other measurements may be due to sample inhomogeneity. According to [19,30], one would expect thermal modification to reduce the O/C ratio. From

the XPS measurements, it was observed that the components C2, C3, C4 did not change significantly (except for T160 wood). The Cl component slightly decreased compared to the reference wood, although an increase due to its association with lignin was expected. However, within the measurement accuracy, values remain the same as do the O/C ratios of Ref, T180, T200.

Only for the T200 sample the FTIR measurements (Figure 7) of showed an unambiguous increase in the absorbance regions of the aromatic skeletal bonds in lignin and the C=O bonds (1596, 1655, 1727 cm<sup>−</sup>1).

The SFE of the samples treated at a 0.15 mm gap from the electrode showed some increase for all types of the atmosphere (Figure 5a). In general, the starting SFE difference of TMW made at different temperatures was lost after the 0.15mm plasma treatment. XPS data showed a large increase in the O/C ratio and an increase in components C2, C3, and C4 at the expense of C1. This suggests a higher presence of polar functional groups on the surface. These results, together with the increase in C=O bonds from FTIR measurements (Figures 7 and 8) signify surface oxidation. The XPS data indicate greater oxidation of pristine spruce for air and N2 and lower for Ar. Plasma treatment of TMW in Ar and N2 showed similar O/C ratio, as well as values of SFE. This may be due to a different process of increasing SFE. FTIR shows a larger increase in C=O under N2 compared to air (Figure 8). The surface energies obtained from laid liquid droplets are in accordance with these FTIR measurements, i.e., higher in a N2 atmosphere than in air.

Zanini et al. studied chemical changes in wood after treatment with Ar RF plasma [31]. An increase in the concentration of phenoxy radicals was observed. The formation of radicals occurs first on lignin, which becomes significantly modified. The resulting radicals can then react with other monomers to form C–C or C–O bonds. However, the RF plasma used in [31] has a distinct set of discharge operation conditions to DCSBD, thus one should expect that both types of plasma would act differently on wood chemical bonds.

For a 1 mm gap, the surface energy strongly depended on the used working gas (Figure 5b). DCSBD plasma in air forms reactive intermediates such as O2\*, O2, O3, O, O+, O3 +, <sup>e</sup><sup>−</sup>, OH, N, CO2, N2\* [6], but only long-lived neutrals and UV is capable to reach the sample surface positioned at 1mm distance. For air, this would be ozone or nitrogen oxides. Considering the frequency range 15–50 kHz AC, 15 kHz is the most suitable for generating O3 and NO [32].

Samples treated in the air at a 1 mm gap showed a reduction in the polar component of the SFE. Changes in values obtained from XPS showed no statistical difference. FTIR measurements showed a small increase in the proportion of unconjugated C=O bonds and a decrease in aromatic skeletal bonds in lignin.

It has been shown that treatment at the 1 mm gap increases the wettability of the cellulose slightly and therefore cannot cause an increase in the hydrophobicity of the wood surface [26]. Not only chemical interaction and UV radiation could cause the change of morphology but the closing of micropores could also be a reason [33]. Partial closure of surface pores has been observed during ionic irradiation of wood [34]. In our conditions, at a 1 mm gap and a treatment time of 10 s, ion radiation should not be significant [22]. The morphology of the wood surface using SEM was mapped. It was found that 10 s is too short a time to etch or otherwise affect the wood morphology. Some morphological changes occur only after a longer treatment period.

The effect of hydrophobization after plasma treatment was not observed on PMMA material after treatment under the same conditions. The effect is therefore associated with the wood material itself, and its chemical changes.

Isolation of the temperature effect has shown that it had no influence on SFE or morphology. Again, the treatment time and temperature were too short to cause any changes.

By isolating the effect of UV radiation, it was found out that it contributed to an increase in the dispersion and a decrease in the polar part with an overall decrease in SFE (Figure 12). By comparing spruce with beech wood, it can be stated that the effect of UV irradiation differs depending on wood type.

For a 1 mm distance treatment, the results in N2 and air are very similar. Lack of O2 during treatment may contribute to a slightly lower increase in hydrophobicity. But part of the water vapor desorbed from the sample could provide O2 molecules needed for the formation of nitrogen oxides and nitric acid. It should be stated, however, that the effects of secondary nitrogen oxides reactions that may take place after samples removed from the chamber could not be eliminated.

Article by Bihani et al. states that wood-meal (in the presence of O2) binds gaseous molecules NO and NO2 relatively quickly [35]. According to the research, reactions with NO and NO2 did not cause a decrease in SFE [35,36]. Wood-meal was modified with nitrogen oxides for de-lignification. In wool or delignified materials, nitrogen dioxide is converted to nitric acid in the presence of O2. These reactions should not be significantly affected by lignin, nitric acid can then strongly oxidize wood [36]. This should increase the polar component of SFE.

When treating the T200 sample at 1 mm in air, the decrease of the polar part was negligible. After plasma treatment in N2, O2, and Ar, the polar part raised (Figure 5b). A possible explanation is that thermal modification at 200 ◦C had actually achieved a hydrophobicity threshold/maximum for spruce wood. The source for further chemical reactions resulting in SFE decline is depleted for the T200 sample. Taking into account, that thermal treatment results in the decomposition of hemicelluloses mainly, the following hypothesis may be drawn: the hydrophobization effects it due to chemical reactions of long-lived plasma generated particles and hemicellulose on the wood surface. The same conclusion was derived for thermally modified European beech (*Fagus sylvatica*) in [26].

From the previous experiments, it is known that the plasma treatment of the cellulose increases its hydrophilicity. In a paper by Talviste et al. [26], it was reported that plasma treatment by DCSBD at 1 mm gap has the same tendency as the reference sample on the cellulose paper water uptake. It was concluded that the cellulose itself contributes to the increased hydrophilicity.

O2 treatment at 1mm caused SFE to increase by increasing its polar component. This is due to the strong oxidizing effects of ozone [21,37]. The selectivity of ozone in reactions with lignin and carbohydrates is strongly dependent on the pH of the environment. When activated by plasma, acidic components such as formic acid (HCOOH) and acetic acid (CH3COOH) are formed, which select the reactions of ozone with lignin [38]. Ozone reacts mainly with unbound bonds, carbonyl, ether, and hydroxyl groups. DCSBD plasma under an O2 atmosphere forms up to 2000 ppm O3 [39]. The O/C ratio also indicates strong oxidation. With respect to the hydrophobization effect, the ozone would definitely contribute to hemicellulose degradation. However, this effect could be completely overshadowed by the formation of novel polar groups on the surface.
