**1. Introduction**

In outdoor expositions, unprotected wood surfaces are easily attacked by sunlight. The penetration depth of UV-radiation into wood is approximately 75 micrometers and of the visible light 125–500 micrometers [1]. However, free radicals created from the initially photodegraded lignin and hemicellulose macromolecules can further damage wood components up to a depth of approximately 2000 micrometers. Surfaces of wet wood are synchronously susceptible to deterioration processes by bacteria and molds causing color defects and also health problems for people [2].

Coatings recommended for wooden products that are exposed exteriorly, e.g., windows, pergolas, façades, or terrace boards, must protect them against sunlight, water, and microorganisms. This means that the commercial acrylic, alkyd, polyurethane, epoxy, or other coating types should contain suitable additives—pigments, UV-additives, hydrophobic substances, and biocides.

The pigments and UV-additives absorb sunlight, decrease destructive effects of sunlight, or inhibit its transfer through coatings into wood [3–6]. The hydrophobic substances, e.g., natural and synthetic waxes, increase a contact angle between the coated wood and the water drops and decrease the penetration of water through the coating into wood [7]. The environmentally acceptable bactericides and fungicides protect coatings and wood against bio-deteriorations [8].

The transparent, unpigmented coatings used in exteriors should contain effective UV-additives that preserve the macromolecules of coatings from radical depolymerization reactions leading to their destruction and protect the painted wood from color changes and other aesthetical defects [9]. Today, the following substances are applied as UV-additives in coatings: (a) UV absorbers such as 2-hydroxyphenyl-s-triazines [5,10], 2-(2-hydroxyphenyl)-benzotriazoles (BTZ) [3,5], or 2-hydroxyphenyl-benzophenones [11], (b) UV blockers-screeners such as zinc oxide (ZnO), titanium dioxide (TiO2) or cesium dioxide (CsO2), usually used in the form of nanoparticles [3,5,6,12–14], (c) hindered amine light stabilizers (HALS) [15,16], (d) sometimes also with other photo-stabilization mechanisms, for example, imidized nanoparticles which also have a hydrophobic e ffect [17], or lignin stabilizers such as succinic anhydride in combination with epoxidized soybean oil [11]. Unfortunately, the e fficiency of UV-additives in coatings does not always last long-term [18,19].

Coatings for wood products exposed outdoors are increasingly modified with environmentally acceptable organic bactericides and fungicides, for example 3-iodo-2-propinyl-N-butyl-carbamate (IPBC), propiconazole, tebuconazole, or quaternary ammonium compounds (QACs), or less frequently with inorganic biocides, for example nano ZnO, or nano silver [7,8,20,21]. These biocides, as well as other ones, can also be used for wood pre-treatment before the application of coatings [22].

The functional, protective, and decorative properties of coatings used for the treatment of wood surfaces exposed exteriorly should be su fficient and long-term [23]. However, the adhesion strength between coatings and wood surfaces usually worsens over time in relation to: (1) the type and intensity of environmental factors, i.e., the sun-irradiation, water precipitation, wind, emission of carbon blacks and aggressive chemicals, (2) the presence of bacteria, molds or other pests, (3) the used wood species with specific surface characteristics and its physical, chemical or biological pre-treatments before painting with coating, and (4) the used coating type, and the technology of its application.

Individual wood species have specific surface characteristics—wettability, free surface energy, porosity, roughness, pH-value, resistance to bacteria, molds, decaying fungi, etc. [24–29]. All surface characteristics of wood are directly connected with its geometrical, morphological, anatomical, and molecular structure [30]. Di fferences in the wood surface texture of various wood species or of specimens of the same wood species are given by (a) the geometrical level, such as the radial or tangential surface, knots, roughness influenced by machining, etc., (b) the morphological and anatomical levels, such as the diameter of cell elements "fibers, vessels, rays, ... " and their lumens, etc., and (c) the molecular level, such as the type and amount of hydroxyl and other polar functional groups, crystallinity of cellulose, polarity of lignin–polysaccharide components and various extractives, migration of extractives, etc. The surface texture of wood significantly influences the wettability and penetration processes of liquid coatings into wood as well as the adhesion of created coating films to wood.

However, the wood species, its roughness, and other surface characteristics are not always the most important factors that a ffect the adhesion strength, as the role of the coating, and similarly of the glue, is usually more important [31,32]. As already mentioned in the previous paragraphs, pigmented coatings are preferentially recommended for exterior usage, while the transparent coatings can only be used after adjusting them with e ffective and stable UV-additives, hydrophobic and antimicrobial substances. K údela and Liptáková [33] and several other researchers identified the "coating—wood" interface as a very important parameter for overall stability and the lifetime of coatings. The adhesion strength between coatings and wood surfaces can be improved by wood pre-treatment with chemical penetrating systems, as well as by wood modification with specific physical methods.

Modification of wood surfaces with plasma is a well-known physical method for their activation before the application of coatings or glues [34–37]. The plasma barrier discharge serves to activate wood surfaces prior to the application of coatings to ensure a higher adhesion strength and increase the service lifetime of coated or glued woods. Plasma can improve the surface properties of wood, wooden composites, and some other materials [37,38]. Plasma forms a thin protection layer on a wooden surface, which is e ffective against the sun, water, and biological influences. Several experiments have shown that plasma can change the chemical composition of wood surfaces, thereby also changing their wettability together with improving the adhesion strength of coatings and glues to wood [35,37–44]. The plasma discharge in wood surfaces activates polysaccharides, lignin and extractives and in the presence of air generates new hydroxyl, carbonyl, carboxyl, peroxide or ether functional groups and radicals, and simultaneously liberates reactive intermediates, such as O2 +, 1O2, O3, O<sup>+</sup>, ionized ozone, free electrons, N, CO2, excited states of N2, etc. [37,45]. These substances react with the wood surface and provide a convenient resource for its activation and purposeful alteration of its wettability [46].

In presence of ozone, wood components are transformed into smaller and also more polar molecules. Thus, newly formed compounds in wood surfaces can form new chemical bonds with the surrounding lignin–polysaccharide components of wood and also with molecules of the used coating system. Following this, an assumption can be pronounced—newly bonded joints in the plasma modified wood will ensure better adhesion of the coating to its surface with a positive e ffect on the final quality and lifetime of the wooden product. In addition, the surface modification of wood by plasma carried out in the air, with the production of ozone, is an e ffective sterilizing method for the killing of bacteria or molds [34,42].

In the last few years, the cold low-temperature plasma (LTP) is increasingly used for the surface modification of wood, wood composites, wood–plastic composites, and other technical materials, because the degrading thermal e ffect of the cold plasma on these materials is neglected [38,47]. LTP does not require high temperatures that are needed in the preparation of ThermoWoods (thermally modified wood at 180–220 ◦C), and it is e ffective without the presence of activating chemical agents that otherwise maximize the benefits of subsequently applied natural or synthetic coatings [48]. In addition, the reaction time of LTP with wood components is relatively short, usually only from a few seconds to several minutes, additionally, the process is performed in a dry environment, and no by-products are produced. The surface of the plasma modified wood becomes more polar, more hydrophilic, and with better wettability with water-based coating systems, for example with acrylic water dispersions. However, gradually, over a period of time, usually after 7 to 30 days of exposure to plasma, the wood surfaces become chemically inert and even more hydrophobic [45]. The hydrophobic character of pinewood surface was also achieved by Moghaddam et al. [49], who created a thin transparent superhydrophobic layer, consisting of nanoparticulate TiO2 and the following deposition of plasma polymerized organic substances, e ffective for controlling its wettability and water repellency.

Generally, the permanent quality of painted wood is connected with its color stability, resistance to the creation of cracks, to thickness reduction and to biological attacks, as well as the good and long-term adhesion strength of the coating system with the wood surface. In exterior exposures, the durability of wood surfaces painted with transparent coatings are often limited to just their adhesion strength to wood substrate. Adhesion of coatings to wood is commonly susceptible to weathering conditions, but less information exists about the e ffect of wood pre-treatment with fungicides and plasma and also about the presence of UV-additives in coatings. The Norway spruce is a common and very important wood species, mainly in Central Europe, used for industrial and building structures, bridges, furniture, musical instruments, sport equipment and other uses. From this point of view, research aimed at improving its surface properties was performed in this work.

The basic aim of this work was to examine changes in the adhesion strength between transparent acrylic and alkyd coatings and the Norway spruce wood due to weathering, in parallel evaluating some other impacts, including the presence of HALS or BTZ UV-additives in coatings, the fungicidal pre-treatment of wood with boric acid or benzalkonium chloride and the plasma modification of wood surfaces.

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