**1. Introduction**

Wood is a well-known industrial material, and its surface properties have a huge impact on durability, quality, and product aesthetics. If the wood has been properly processed and treated, it will last for a very long time, which can be demonstrated on historic buildings, utility and artistic objects, musical instruments, and other wood products. By modifying wood surfaces, it is possible to achieve even better properties, streamline its use, or find new possibilities for its application in various areas of human activity [1].

Plasma treatment can change the surface properties of the wood and have a positive effect e.g., when applying varnishes and adhesives. This could not only affect the amount of the used paint or adhesives but also obtain a higher strength of the glued joints in stressed places.

Diffuse coplanar surface barrier discharge (DCSBD) enables homogeneous surface treatment of various flat surface materials such as wood [2,3], polymers [4], or glass [5]. Concerning the treatment of lignocellulose materials, it was observed that the effect of DCSBD treatment depends on the distance between the treated wood surface and the DCSBD electrode [6,7].

Understanding the reasons behind this effect could help us determine appropriate conditions for plasma treatment and maximize the desired effect, which may find its use in the construction industry, the production of furniture, musical instruments, decorative objects, etc.

**Citation:** Košelová, Z.; Ráhel', J.; Galmiz, O. Plasma Treatment of Thermally Modified and Unmodified Norway Spruce Wood by Diffuse Coplanar Surface Barrier Discharge. *Coatings* **2021**, *11*, 40. https:// doi.org/10.3390/coatings11010040

Received: 3 December 2020 Accepted: 27 December 2020 Published: 1 January 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In this research a spruce wood was chosen for treatment, owing to its large abundance in Central Europe, mainly due to the recent bark beetle calamity. Nowadays, the spruce wood is frequently thermally modified, usually in the range of 160 ◦C to 220 ◦C at low oxygen environment, to improve its dimensional stability against the moisture content and biological resistance against decay [8]. The heat treatment results in the loss of hemicelluloses mainly, which causes the growth in the relative abundance of hydrophobic lignin. This is considered to be the main reason behind the obtained reduced hygroscopicity (ability to absorb and retain water moisture) of wood. At the same time, however, the low surface wettability of thermally modified wood (TMW) complicates its further processing when applying water-based varnishes and adhesives or wood preservatives. For instance, in [9] authors observed a considerable, almost 1/3 reduction in adhesion strength of alkyd-reinforced acrylate paint with TMW spruce, already after a mild treatment (<200 ◦C).

TMW exhibits higher content of hydrophobic lignin, which is known to be quite sensitive to plasma treatment [10]. Thus, properly applied plasma treatment can be used to revert the obtained hydrophobic surface characteristics of heat-modified spruce as well as other wood species, such as beech, pine core, and other woods with higher lignin content.

In this research, spruce with different heat pre-treatment (160, 180, and 200 ◦C) was studied. The effect of DCSBD plasma treatment was evaluated by measuring the changes in wood surface free energy (SFE), chemical composition, and micro-morphology. An important parameter that affects the plasma treatment of wood is the composition of plasma treatment working gas. For this sake, the most common industrial gases such as air, nitrogen (N2), oxygen (O2), and argon (Ar) were investigated. Finally, factors such as substrate heating and ultra-violet (UV) radiation on the resulting hydrophilicity of the material were studied.

#### **2. Materials and Methods**

#### *2.1. Samples and Wood Thermal Modification*

The spruce samples were cut into regular cuboids. Because wood is a biological material that is exposed to varying different conditions during its growth, the resulting material is inhomogeneous. It has different surface and internal properties in different places. Only smooth parts of the samples without visible defects were used for the measurement. The samples were stored at room temperature in a closed container that maintained a constant 35–37% humidity without direct sun contact. The moisture content of the samples was measured to be less than 3%. The density of spruce and spruce heat-treated at 160, 180 and 200 ◦C was measured to be 480, 440, 410 and 400 kg/m3, respectively. The size of samples varied depending on the analytical technique that was used. Usually, at least 3 samples for each parameter were measured. The Katres s.r.o pilot plant laboratory chamber was used to perform thermal modification of the wood, which thermally treats the wood in the water vapor environment. The process has three basic phases: high-temperature drying, thermal modification, and final cooling. First, the temperature in the chamber, due to hot steam, rises rapidly to 100 ◦C. It then increases further, but more slowly, to 130 ◦C. During this, the humidity in the wood drops to a close to zero value. After drying, thermal modification follows. In our case, it took place for three hours at temperatures of 160, 180 or 200 ◦C. Steam is used during drying and heat treatment as protection against ignition. Finally, the chamber is cooled to a temperature of about 90 ◦C. At this stage, wetting is important for the final properties of the wood to be usable, it should have a moisture content above 4% [11].

## *2.2. Plasma Treatment*

DCSBD was used for plasma treatment (Figure 1). The electrode system consisted of 32 parallel silver electrodes, which were 1.5 mm wide, 220 mm long, and had gaps of 1 mm between them As the dielectric, 96% alumina ceramics were used (Figure 2). For the plasma activation in a certain gaseous atmosphere, a DCSBD in a closed vessel with holes for the gas outlet, and an inlet with a controlled current flow was used. Then the specific gas for 5 min at a flow rate of 2 L/min was filled, so that the chamber was flushed many times with the given gas and the presence of atmospheric gases was minimal. The sinusoidal voltage was in the amplitude of 10 kV at a frequency of 15 kHz. The generator output power was 400 W for all gases except Ar, where it was 250 W. The efficiency of the whole system is approximately 90%. The distance from the sample to the electrode was controlled with the glass plates (0.15 and 1 mm) positioned on the electrode.

**Figure 1.** Photo of the DCSBD plasma reactor.

**Figure 2.** Schematic setup of the DCSBD electrode.

## *2.3. Surface Characterization*

The indirect determination of SFE and its polar and dispersive components was done using the Owens-Wendt regression method described previously in more detail [12]. Four liquids were used in this study: distilled water (γ<sup>D</sup> = 21.9 mJ/mm2, γP = 51.0 mJ/mm2), ethylene glycol (γ<sup>D</sup> = 29.0 mJ/mm2, γP = 19.0 mJ/mm2), diiodomethane (γ<sup>D</sup> = 50.8 mJ/mm2, γP = 0 mJ/mm2) and glycerol (γ<sup>D</sup> = 28.3 mJ/mm2, γP = 36.9 mJ/mm2) [13]. Surface Energy Evaluation System (Advex Instruments, Brno-Komín, Czech Republic) was used to measure contact angles (CA) directly from the camera images. For each testing liquid, the contact angle of 15 droplets (1 μL) was measured and the average values were used for the Owens-Wendt regression. The contact angles were determined at the time when the wetting rate becomes constant (dθ/dt = const) [14]. The obtained data were analyzed by Analysis ToolPak of MS Excel 2016 software (Microsoft Corp., Redmond, WA, USA). The normality of the data distribution was verified by its descriptive statistics tool. The significance of differences among the results was tested using the Student's *t*-test, with the significance level of rejecting the null hypothesis being equal to 0.05. The surface morphology of the examined spruce was studied with a scanning electron microscope (SEM) MIRA3 from TESCAN (Brno, Czech Republic). Samples were cut into small pieces of about 15 × 10 × 5 mm3. Before SEM imaging, the samples were coated with a 10-nm Au–Pd composite layer. The sample surface was electrically connected with the sample holder to reduce surface charge accumulation. All measurements were taken using a secondary electron detector with an accelerating voltage of 7 kV to ensure minimal damage to the surface. The focus was mainly on the internal structures of wood, its tissues, vascular bundles, and especially on places where the effect of plasma etching was evident.

X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ES-CALAB 250Xi (Thermo Fisher Scientific, East Grinstead, UK). An X-ray beam with a power of 200 W (650 μm<sup>2</sup> spot size) was used. The survey spectra were acquired with a pass energy of 50 eV and a resolution of 1 eV. High-resolution scans were acquired with a pass energy of 20 eV and a resolution of 0.1 eV. To compensate for the charges on the surface, an electron flood gun was used. Spectra were referenced to the hydrocarbon type C1s component set at a binding energy of 284.8 eV. Spectra calibration, processing, and fitting routines were done using Avantage software.

Attenuated total reflectance (ATR) infrared spectra were measured with Bruker Vertex 80 V spectrometer (Optik Instruments s.r.o., Brno, Czech Republic) utilizing a diamond crystal for ATR. All measurements were taken in an evacuated regime at a maximum pressure of 5 hPa. The spectra were acquired in a range of 4000–800 cm<sup>−</sup><sup>1</sup> with a resolution of 4 cm<sup>−</sup>1. It was confirmed that the absorption of the band in the region 1031–1053 cm<sup>−</sup><sup>1</sup> did not change as a result of plasma treatment, and each spectrum was normalized to the intensity at 1024 cm<sup>−</sup>1. At least, four samples of each parameter with a minimum of 3 points on each sample were measured. The results presented are the average of the obtained data.

## *2.4. Thermal Camera*

For temperature measurement, a non-contact method using a thermal camera was taken. This method relies on electromagnetic radiation from objects. For the accuracy of measurement, it is essential to choose the value of emissivity correctly. During the recording we used e = 0.86 (mentioned e.g., in [15]). Emissivity in the range e = 0.82–0.89 is reported in the sources (e.g., [16,17]).
