**3. Results**

#### *3.1. Thermally Modified Wood*

Spruce wood samples which were modified with three different temperatures before the plasma treatment—160, 180, and 200 ◦C are hereafter referred to as T160, T180, T200, respectively. Wood that has not been thermally modified is referred to as a reference—Ref. The thermal modification of timber caused a decrease in SFE. From the initial value of 66 mJ/mm2, it decreased to 62, 65, and 58 mJ/mm<sup>2</sup> for T160, T180, and T200, respectively. The reference sample has the highest SFE as shown in Figure 3, i.e., wood thermal modification increases the wood surface hydrophobicity. The reason behind this is the intended disruption of cellulose, hemicellulose, and to a lesser extent of lignin, which is more stable. The hydroxyl groups present in hemicelluloses are strongly involved in wetting the wood by forming hydrogen bonds with water molecules. Their decomposition gives rise to observed hydrophobicity growth [18].

**Figure 3.** Comparison of the surface energy of different TMW without plasma treatment.

The loss of these hydrogen bonds will also affect the ratio of surface energy components. The dispersion component increases from 38 mJ/mm<sup>2</sup> for the reference wood to 44 mJ/mm<sup>2</sup> for the T200. The polar component decreases from 28 to 14 mJ/mm2. There are boundaries between T180 and T200 where the hydrophilic behavior is significantly weakened. This is in agreemen<sup>t</sup> with an increase in the proportion of lignin in the wood. According to the article [10], there is a small difference between woods heated for 2 h at 150 and 180 ◦C—28%, compared to 31% for 200 ◦C.

#### *3.2. Distance from the Electrode*

Plasma treatment at the power of 400 W was done for two different distances between the wood and electrode of 0.15 mm and 1 mm. DCSBD plasma is most intense at the height interval of 0.1–0.3 mm from the electrode. When the sample is exposed to DCSBD plasma at this distance range, the contact angles of the water is expected to drop most significantly, as the SFE grows. In our case, however, when treating the reference spruce samples at 0.15 mm, there was no statistically significant change in SFE (Figure 4). The reason is that untreated spruce wood has already relatively high SFE. The resulting low contact angle (especially for water) interferes with the measurement accuracy. As a matter of fact, a value of 70 mJ/mm<sup>2</sup> could represent an upper limit of reliably measurable SFE, and thus any improvement above this level (e.g., by plasma treatment) is undetectable. For thermally treated samples (Figure 4), where the initial SFE was lower, treatment at 0.15 mm resulted in a statistically significant increase of SFE, chiefly due to the growth of its polar component.

**Figure 4.** Comparison of surface energies of thermally unmodified wood at different distances from the electrode in the air atmosphere.

The picture changed for the gap of 1 mm. At this position, the opposite effect occurred—the SFE dropped from 66 mJ/mm<sup>2</sup> to 56 mJ/mm2, on account of polar from 28 to 11 mJ/mm2, although dispersive component manifested a slight increase from 38 to 45 mJ/mm<sup>2</sup> (Figure 4). In article [7], the authors also reported such anomalous increase in hydrophobicity for the distance of 0.93 mm, where plasma began to quench.

The hydrophobization effect of 1mm distance treatment was observed also for thermally modified samples (Figure 5). The most significant SFE increase occurred for T200 at a distance of 0.15 mm, which is consistent with the results from [19]. This is related to the lower initial SFE of wood T200. After plasma treatment, the total energy was comparable to other samples with different heat treatments. The total values of the SFE of all thermally modified and reference wood for the distance of 0.15 mm from the electrode were in the range of 37–38 mJ/mm<sup>2</sup> for dispersive and 27 to 31 mJ/mm<sup>2</sup> for polar components. For a distance of 1 mm, the dispersion components were in the range of 45–46 mJ/mm<sup>2</sup> and the polar components were in the range of 9–11 mJ/mm2.

**Figure 5.** Comparison of surface free energies for different gases during plasma activation of wood. (**a**) in case of 0.15 mm gap and (**b**) in case of 1 mm gap from the electrode.

#### *3.3. Influence of Gas Composition on Processing*

As a working gas, gases commonly used in industry, i.e., N2, O2, Ar, and air were studied. The wood treatment took place in each atmosphere at two different distances of the material from the electrode (0.15 and 1 mm). At the distance of 0.15 mm from the electrode, the SFE increases slightly in all atmospheres. In particular, the dispersion component slightly decreases, and the polar component increases. This can be explained by oxidation on the surface either directly during plasma activation or after extraction into atmospheric air, where the interaction of activated wood with O2 can take place. So, it could be concluded that while keeping the gap between the sample and the plasma in the range of 0.1–0.3 mm the working gas does not influence significantly the plasma treatment effect. In practice, however, it is hard to keep such a high precision during wood processing.

On the contrary, in the case of a 1 mm distance, the working gas played an important role. Air and N2 treatment displayed a more hydrophobic wood surface. With this respect, air gas showed a more pronounced effect. For both gases, the dispersion component increased, and the polar component decreased. Interestingly enough, 1 mm distance treatment in pure O2 had no hydrophobization effect. This points out the important role of NOx chemistry in the effect. An ample amount of nitric oxide compounds (NO, NO2, N2O5) and nitric acid in the presence of H2O is formed within the DCSBD and can strongly interact with the surface. These compounds oxidize the surface, thus rather causing an increase in SFE [20,21]. At the moment, it is an unresolved question to what extent one can manipulate the resulting SFE by altering the N2 to O2 ratio. This question was partially addressed in [22], where authors tested different atmospheres for DBD ATMOS plasma activation and monitored the change in wetting. They go<sup>t</sup> an increase in the contact angle for 1:2 and 3:1 ratios N2:O2 and a decrease for the case 1:1. However, they did not explain this behavior but only stated that "the combination of structural change (induced by UV radiation, the impact of metastable particles, or both) and chemical change due to surface oxidation are responsible for the observed surface modification of wood samples [22].

In the case of O2 and Ar, the total SFE increased, comparable to the case where the wood was treated at a distance of 0.15 mm from the electrode. The increase of the polar component was slightly larger (max. 4 mJ/mm2, but mostly up to 1 mJ/mm2) and in most cases, the decrease of the dispersion component was slightly smaller (max. 5 mJ/mm<sup>2</sup> for T200 Ar, but most up to 1 mJ/mm2). The behavior of T200 wood was most different from other wood samples.

The polar component in the N2 atmosphere after treatment at 1 mm distance increased with a temperature of thermal modification, but for T200 fell again close to the reference wood (Ref 1 mm) value.

#### *3.4. Comparison of Wood and Polymer*

To verify that the increase in water contact angle at 1 mm is specific to wood chemical composition, the contact angle of water for the polymer treated was measured. Specifically, polymethyl methacrylate (PMMA) commonly known as plexiglass was used. After plasma treatment, the hydrophilicity of the plexiglass increased, see Figure 6. For PMMA, the water contact angle was reduced for both cases. Keeping a 0.15 mm gap, the water contact angle decreased more markedly than for a 1 mm gap. Wood, on the other hand, acquired more hydrophilic properties only for 0.15 mm, and, at a distance of 1 mm, it was hydrophobized. Therefore, it is a matter of wood material and its specific morphological and chemical composition. For some polymers, plasma treatment can also increase the hydrophobicity of the surface due to etching, the formation of nanostructures and can achieve even superhydrophobic behavior [23]. However, in Section 3.7.2 Wood etching it will be shown that the increase in hydrophobicity in the wood was not caused by emerging nanostructures, as in these cases.

**Figure 6.** Comparison of water contact angles between polymer and wood.

#### *3.5. Fourier Transform Infrared Spectroscopy*

FTIR spectra for T200 and both distances treatment are shown in Figure 7. There was no observable change in the absorbance band of the bonds of water and OH groups (3550–3150 cm<sup>−</sup>1). The absorbance in the CHx band (2947 cm<sup>−</sup>1) decreased after plasma treatment. This decrease is due to chemical reactions on the sample surface [2]. In the area of C=O conjugate bonds (1655 cm<sup>−</sup>1) there was a significant increase in absorbance for samples treated at 0.15 mm compared to the reference sample. These bonds are affected by plasma oxidation [24]. In [24] for spruce, both regions corresponding to C=O increased, of which the unconjugated band was more significant. In the case of a 1 mm gap, this peak decreased. Unconjugated bonds (1727 cm<sup>−</sup>1) increased after plasma treatment in both cases.

**Figure 7.** FTIR spectra of wood T200 to compare the change in chemical composition at different distances from the electrode. The treatment was done in the air.

The 1596 cm<sup>−</sup><sup>1</sup> peak decreased for 0.15 mm that supports the theory that in the active plasma region CHx components undergo degradation. At the same time after plasma treatment at a 1 mm distance, this peak decreased less. For plasma-treated samples, absorbance increases in the syringyl and guaiacyl regions (aromatic groups from lignin) and OH groups from cellulose (1313–1336 cm<sup>−</sup>1).

Figure 8 compares measurements in N2 and air. The samples behave very similarly. For T180, the band of unconjugated C=O after the plasma treatment in the air hardly changes. At a distance of 1 mm they are almost identical, except for a large increase in C=O (1727, 1655 cm<sup>−</sup>1) and an increase in vibrations of 1264, 1219 cm<sup>−</sup><sup>1</sup> also bound to C=O bonds. These differences are also evident in the treatment at a distance of 0.15 mm from the electrode, especially in the vicinity of the areas 1727 and 1655 cm<sup>−</sup>1. Greater oxidation occurred in the N2 atmosphere.

**Figure 8.** FTIR spectra comparison of the T180 wood after plasma treatment under N2 and air atmosphere.

#### *3.6. X-ray Photoelectron Spectroscopy*

Due to the complex irregular surface topography, the random distribution of the wood components, and their random size, the errors of some measurements are significant.

Some samples contained a small percentage of N2 (up to 1.5%) in addition to O2 and carbon. After plasma treatment in a N2 atmosphere, they reached a maximum of 2% at 1 mm distance from the electrode and 9% at 0.15 mm distance. This increase did not occur in the air. The oxygen and carbon atomic percentages and O/C ratio obtained from XPS spectra for various gases and distances from the electrode are presented in Table 1.

**Table 1.** Oxygen and carbon atomic percentage and O/C ratio obtained from XPS spectra for various gases and distances from the electrode. Data are in [%], except for O/C, which is dimensionless.


Component C1 corresponds to bonds C–C and C–H, component C2 is attributed to bonds C–O, component C3 can correspond to either the group O–C–O or the bond C=O and component C4 corresponds to the groups O=C–O. The main contribution to peak C1 comes from lignin and extracts, in peaks C2 and C3 it comes from functional groups in lignin and polysaccharide. The C4 peak is attributed to hemicellulose [19].

It is not surprising that after plasma treatment of a sample, the oxidation on its surface increased when it was directly in contact with the plasma. There was also a decrease in the Cl peak due to the decomposition of the extracts. This decrease was more pronounced in the air than in the N2 atmosphere. There was no measurable change in Ar. An increase in C3 and C4 peaks was also observed. A graphical representation of component changes can be seen in Figure 9a,b, where the drop of the C1 component and the increase of the C3, C4 components are clearly shown. It is known that an increase in the polar part of SFE occurs due to oxidation [19].

**Figure 9.** The XPS spectrum deconvolution of Cls band: (**a**) untreated Ref sample, (**b**) Ref sample plasma treated in the air at 0.15 mm gap (**c**) Ref sample plasma treated in the air at 1 mm gap, (**d**) T200 plasma treated at 1 mm gap.

In the case of a 1 mm gap under a N2 and air atmosphere, the obtained O/C and heights of Cl peaks values were similar to the reference sample. A decrease in the O/C ratio on TMW was recorded. This corresponds to the increase in SFE measured by the SeeSystem. Figure 9c,d show examples of XPS spectra of reference and T200 samples with the corresponding individual components. It is seen that even after plasma treatment, a similar ratio of peaks between the reference and heat-treated wood was obtained.

For a 1 mm gap, the increase in O/C was greater for O2, thanks to the easiest oxidation. There was also an obvious decrease in the Cl component, from 48% to 17% for the reference wood and from 46% to 29% for T200.

At a 0.15 mm gap from the electrode for Ar, N2, and air atmospheres a significant increase in O2 values was recorded. At a 1 mm gap, lower O/C values for TMW after plasma treatment were obtained compared to the reference ones, though they were within the error bars.
