Field of the Invention

The invention relates to the field of wood coating, in particular to a multilayer wood coating system for wood surface treatment in the exterior and a method of its application. The invention is specifically applicable for surface treatment of larch wood.

Background Art of the Invention

There are many ways of modifying the surface of wood, in particular by plasma, thermally, chemically, enzymatically or by individual nanoparticles, which are used to modify the coating system. The original appearance of wood exposed to exterior conditions can only be preserved by transparent, partly lightly pigmented, and semi-transparent surface treatment.

However, transparent, semi-transparent, and lightly pigmented coatings on wood in the exterior lack the required durability with faster degradation and flaking compared to pigmented ones. This is due to the enhanced penetration of solar radiation into the layer of transparent or partially transparent coating itself and into a wooden substrate where the radiation induces the reactions of photodegradation. Such reactions accelerate the degradation of the coating system and induce chemical changes in the wooden substrate with a negative impact on the coating adhesion to the wood. These chemical changes are more significant in the kinds of wooden substrates that contain an increased portion of extractive substances. From among wooden substrates widely used in Europe, in particular oak and larch are concerned. Due to the swelling and shrinking of wood caused by humidity changes in the exterior conditions, the flaking of coatings is easier.

To extend the service life of transparent and semi-transparent coatings are nowadays used UV stabilizers such as HALS (hindered amine light stabilizers), and nanoparticles, in particular based on metal oxides, or their combinations. CZ 33183 U discloses wood-based material, equipped with a layer of penetrating primer containing HALS and ZnO nanoparticles, and a glazing layer containing UV stabilizers. The aforementioned components are admixed into the solutions of individual coatings and the coatings are applied onto a wooden substrate in the following order: the primer layer, the glazing layer, and a top hydrophobic layer, containing water-borne synthetic resins with nanoparticles of multivalent metallic AsS chelate complex. The disadvantage is that the UV stabilizers only work in a certain wavelength range of UV light and do not react in the visible light region, thus providing only partial protection of the wooden substrate against photodegradation.

Some sources mention polymeric materials to stabilize wood-based materials. For example, polyethylene glycol (PEG) in water solution is widely used as a “green wood stabilizer” to prevent cracking and shrinkage of wood. In other studies, in particular the additions of individual distributed nanofibers with various organic and inorganic compositions were used in the wood coating system to enhance its the resistance to mechanical stress. An example is the use of cellulose nanofibers to improve wood coating system described in article Kluge M., et. al., Nanocellulosic fillers for water-borne wood coatings: reinforcement effect on freestanding coating films, Wood Science and Technology, 2017, 51 (3), pp. 601 -613, describes cellulose nanofibers in concentration 2.0% to modify a water-borne coating system to improve the mechanical resistance of the treated wooded substrate. Even though this solution provides an increase in mechanical resistance, the disadvantage is that it does not provide the stabilization of the polymer and the durability of the wood substrate in case of solar radiation in the UV-Vis wavelength region.

The term “UV-Vis region” is defined as the region of radiation absorbing in the UV wavelength mainly in the range of 100 to 400 nm and in the visible Vis region in the range of 400 to 800 nm.

Another article (Veigel S., et. aL, Improving the mechanical resistance of water-borne wood coatings by adding cellulose nanofibre, Reactive and Functional Polymers, 2014, 85. pp. 214- 220) describes the use of cellulose microparticles to modify a water-borne acrylic or polyurethane coating to improve the mechanical resistance of the treated wooden substrate. Another possibility is to use polyvinyl alcohol called PVA or SiOs nanofibers, according to Kumar A., Coating of wood by means of electrospun nanofibers based on PVA/SiO2 and its hydrophobization with octadecyltrichlorosilane (OTS), Holzforschung, 2016, 71 (3), while nanoparticles are admixed to the coating in the form of freely suspended nanofibers. The disadvantages are low stability of the polymer and the relatively low durability of the wooden substrate in case of solar radiation in the UV-Vis wavelength region. EP 1585703 A1 discloses the method of porous inorganic materials production or matrix material containing nanoparticles with highly homogeneous widths. The porous material is produced by vapour deposition of a separating agent onto a carrier to form a layer of the separating agent; simultaneously the material and the separating agent are applied by the vapour deposition onto the separation agent. The material is modified by SiOs particles. However, this procedure is suitable for metal coating of metallic surfaces, not natural wood.

The object of the invention is to prepare a multilayer wood coating system that would increase its resistance to exterior conditions with extended overall service life, while preserving the original appearance of the wooden substrate. Another object of the invention to prepare such a multilayer coating system which can be applied to all types of wooden substrates, in particular on larch and oak wooden substrate.

Disclosure of the Invention

This object is achieved by development of a multilayer wood coating system comprising at least one primer layer and at least one glazing layer. The primer layer and the glazing layer are formed by an acrylate water-borne coating based on polymers of acrylic and/or methacrylic acid esters. It is the subject matter of the invention that the primer layer comprises dispersed ZnO nanoparticles in range from 1 .0 to 5.0% (w/w), preferably the dispersed ZnO nanoparticles are in range from 1.8 to 2.5% (w/w). Further the object of the invention is that the multilayer coating system further comprises a porous middle layer formed by a polyamide nanofibrous nonwoven fabric arranged between the primer layer and the glazing layer. The surface and/or subsurface regions of the primer layer and the middle layer are interpenetrated in at least part of their volume, wherein in this interpenetrated region the porous middle layer is saturated by the primer layer and the total thickness of the multilayer coating system is in the range of 80 to 140 pm. This arrangement of the multilayer coating system provides increased resistance to the exterior conditions due to the protective effect of the porous middle layer, thereby extending the service life and preserving the original appearance of the wooden substrate. At the same time, the dispersed ZnO nanoparticles ensure the photostability of the wooden substrate.

In a preferred embodiment, the primer layer and the glazing layer comprise 30 to 95% (w/w) of acrylic and/or methacrylic acid ester polymers. This concentration ensures easy application of the primer layer to the wooden substrate and the glazing layer to the middle layer, while being optimal for maintaining the original appearance of the wooden substrate.

In another preferred embodiment, the middle layer is made from material selected form the group of: polyamide 6, polyamide 610, polyamide 8, polyamide 12. Polyamide is a material known for its high abrasion resistance, high elasticity and strength in dry and wet conditions, and high biological resistance. The numerical designation of polyamides directly characterizes the shape of the chemical formula, where the type of chemical reaction results in the production of a given type of polyamide with specific properties such as brittleness, acid solubility, flexibility, etc.

In another preferred embodiment, the middle layer has grammage in range of 100 to 500 mg/m ² and thickness in the range of 1 to 5 nm. The diameter of the nanofibers of the middle layer is in range of 1 to 4 nm and the length of the nanofibers of the middle layer is from 5 to 500 mm. Polyamide nanofibers of such length and diameter are used for reinforcement in composite materials which are intended to be lightweight and at the same time mechanically resistant. In the multilayer coating system according to the invention, this ratio of fiber weight and size has been experimentally verified as optimal.

In a preferred embodiment, the size of the ZnO nanoparticles dispersed in the primer layer is in range of 20 to 40 nm. Such a nanoparticle size provides a sufficiently large surface area in such a thin layer to still ensure photostability of the wooden substrate.

In another preferred embodiment, the multilayer coating system further comprises fungicidal agents and/or UV stabilising agents. The fungicidal agent is iodo-2-propynyl-N- butylcarbamate, and the UV stabilising agents are based on branched and linear C7-C9 alkyl 3-[3-(2H-benzotriazole-2-yl)-5-(1 ,1 -dimethyl-ethyl)-4-hydroxyphenyl] propionates, wherein the selected UV stabilizing agents and fungicidal agent react favourably with the selected polymer.

Further the object of the invention is a specific method of applying the multilayer coating system on the wooden substrate. In this process, first the primer layer is applied on the surface of a wooden substrate with humidity in range of 8 to 22% (w/w). The middle layer is then applied on top of the primer layer which is not dried yet. Finally, at least one glazing layer is applied on top of the middle layer. In a preferred embodiment of the method of application, the middle layer is applied to a auxiliary carrier layer before application to the primer layer, which is removed after application of the middle layer to the primer layer.

In another preferred embodiment of the method of application, the auxiliary carrier layer is a textile made of bonded under-jet carded yarn, for which the term “spunbond” is commonly used also in Czech technical terminology. The middle layer is formed on this spunbonded yarn by spinning the polyamide solution in an electrostatic field. In this way, for example, a nanonetwork can be created which can be used as a carrier for a fungicide and to increase the bioresistance of wood.

In another preferred embodiment of the method of application, the polyamide solution used contains 16% (w/w) polyamide, 28% (w/w) formic acid at concentration of 99% (w/w) and 56% (w/w) acetic acid with concentration of 99% (w/w).

Further the object of the invention is that it includes also a product, i.e., a wooden substrate treated with the multilayer coating system according to the present invention. The wooden substrate is preferably made of larch or oak wood thick 8 mm at least. In a preferred embodiment the wooden substrate is in form selected from group consisting of: plank, board, beam, prism, glued laminated timber, bio-board or window frame.

The advantage of the multilayer coating system for wood according to the present invention consist in particular in its increased resistance to exterior conditions with extended overall service life, while it preserves the original appearance of the wooden substrate. Another advantage of the multilayer coating system for wood according to the present invention further consist of applicability to all types of wood, in particular on larch and oak wood. Another advantage is that no synthetic resin-based hydrophobic top layer is needed, thus the cost of the production and application is simplified and reduced. Brief Description of Drawings

The present invention will be explained in detail by means of the following figures where:

Fig. 1 shows image of the nanosheet forming the middle layer taken from scanning electron microscope called SEM, described in Example 1 at magnification of 500x (A) and 10,000x (B),

Fig. 2 shows axonometric drawing of a wood-based material for the manufacture of exterior wood products and structures, comprising the wooden substrate, the primer layer comprising an acrylic water-borne coating with dispersed ZnO nanoparticles, the porous middle layer comprising a polyamide nanofibrous nonwoven fabric, and the glazing layer comprising an acrylic water-borne coating,

Fig. 3 shows scan of each type of wooden substrate samples coated with a multilayer coating system according to the invention, tested according to Example 5 after 9 weeks of aging,

Fig. 4 shows a detailed arrangement of the layers forming the multilayer coating system according to Fig. 2,

Fig. 5 shows the separation of the auxiliary carrier layer from the middle layer applied on the primer layer.

Preferred embodiments of the Invention

Example 1 : Multilayer coating system exterior application of an extended overall service life on a larch heartwood plank

The composition of the material was optimized based on the results of experiments with accelerated artificial weathering in the UV-chamber combined with frost cycles in the cycle chamber for the evaluation of change in colour shade, gloss, contact wetting angle, analysis performed by the confocal laser microscope and visual assessment of the quality of the coating.

A board from larch solid heartwood with the thickness of 8 mm and the humidity content of 12% (w/w) was treated by the primer layer 3 of the modified commercially available transparent exterior coating Impranal, manufactured by Stachema CZ s.r.o. Czech Republic. This commercially available coating was modified by the admixture of ZnO nanoparticles manufactured by ROTI®nanoMETIC with the size ~ 25 nm. The content of ZnO nanoparticles was tested for 1 .0% (w/w), 2.0% (w/w), and 3.0% (w/w) before the primer layer 3 was dried. The ZnO nanoparticles were admixed into the acrylic coating by the ultrasonic mixing equipment for a period of 10 minutes and then the coating was applied onto the wooden substrate 2. The quantity of the applied modified acrylic coating containing ZnO nanoparticles in the primer layer 3 was 120 g/m ² . This primer layer 3 of the multilayer coating system 1 was applied by brush.

Before the primer layer 3 of the acrylic transparent exterior coating dried, the middle layer 4 consisting of a polyamide nanofibrous nonwoven fabric was applied. The nanofibrous nonwoven fabric used in this example was formed on a spinning device by a method of spinning a polymer solution in an electrostatic field. The polymer solution for spinning in an electrostatic field was prepared from synthetic polyamide (PA6) components of small white granules, labelled as Ultramid B24 by BASF France. The polymer for forming the matrix was used in an amount of 16 % (w/w). The other components of the solution were solvents, namely 28% (w/w) of a solution containing formic acid CH2O2 at a concentration of 99% (w/w) and 56% (w/w) of a solution containing acetic acid CH3COOH at concentration of 99% (w/w). The three components of the solution were mixed together in exact proportions in a glass beaker and then stirred for 8 hours at +80 °C on a magnetic stirrer.

Subsequently, the polymer solution was applied onto the spinning string of the spinning device Nanospider NS1WS5000 type. In a preferred embodiment, the spinning process takes place in a spinning chamber at the boundary conditions +22 °C and 65% relative humidity of air in the electrostatic field at the voltage of 6,000 V, where the polymer solution was drifted by an electron beam of the electrostatic field, thus pulling out fibres from the string towards the anode at a distance of 45 cm. During the procedure, components of formic and acetic acids solvents, were evaporated from the polymer solution, in a manner allowing the produced nanofibers to be entrapped on an auxiliary carrier layer 6 made of bonded under-jet carded yarn or “spunbond” which was placed in front of the cathode. Thus, in the electrostatic field region, nanofibers consisting only of PA6 Ultramid B24 polymer in length of 5 to 500 mm and diameters of 1 to 4 nm were produces and entrapped on the spunbond auxiliary carrier layer 6. The deposition of individual nanofibers on the auxiliary carrier layer 6 formed a planar compact nanolayer with a thickness ranging from 1 to 5 nm and a mass ranging from 100 to 500 mg/m ² .

The middle layer 4 in the form of a nanofibrous nonwoven fabric on the auxiliary carrier layer 6 was created from electrostatically conductive spunbond fabric in the spinning device. The nanofibrous nonwoven fabric was applied on the primer layer 3 of the modified acrylic coating so that the wooden substrate 2 with the first layer of modified acrylic coating was placed inverted, i.e., with the treated side of the primer layer 3 directly on the nanofibrous nonwoven fabric. The nanofibrous nonwoven fabric was pressed into the primer layer 3 which was not dried yet only by the own weight of the wooden substrate 2. After the primer layer 3 had dried, the wooden substrate 2 was separated from the auxiliary carrier layer 6 of the spunbond after the coating had dried, thereby the middle layer 4 with the nanofibrous nonwoven fabric was separated from the auxiliary carrier layer 6 so the middle layer 4 was incorporated into the primer layer 3 of the water-borne exterior acrylic transparent wood coating system, which was modified and contained ZnO nanoparticles.

Two additional layers of commercial acrylic transparent coating were then applied over the already dried primer layer 3 coated with ZnO nanoparticles and the middle layer 4, forming the glazing layer 5. Each of these layers was applied at mass 120 g/m ² . The coating used was the same commercial coating that was used for the primer layer 3, but was not modified by the addition of ZnO nanoparticles. The next glazing layer 5 was applied after drying of the previously applied primer layer 3 and middle layer 4, while fine grinding with sanding paper grits greater than 180 was provided for better adhesion of the coating system.

Overall, the surface coating of the larch wooden substrate 2 was 120 pm thick after all layers had dried.

Example 2: Facade board made of larch heartwood

Onto a side of a facade board from larch heartwood with the dimensions of 20 x 80 x 3,500 mm was applied a multilayer coating system 1 according to Example 1 and the facade board was exposed to exterior conditions. The facade board can be used for external wooden facades of buildings exposed to exterior conditions, however, without a direct contact with the ground. Example 3: Parts for the manufacture of non-jointed parts of exterior wooden structures

The larch wood-based material, produced in the manner described in Example 1 , was subsequently used to produce the components for the manufacture of a beam bridge exposed to exterior conditions. The bridge's supporting beams were 100 x 100 mm wide and 9,000 mm long. The railing of the bridge was made of 50 x 50 mm prisms with a length of 9,000 mm and 20 x 100 mm boards with a length of 1 ,000 mm. The bridge pieces were structurally connected by a spigot and a mortise.

Example 4: Window frames

For the production of the glued window were used lamellas made of three segments from larch heartwood with dimensions of 24 x 80 x 1 ,500 mm, which were connected to a greater required length using the finger joint or shortened. The segments were then glued together and a multilayer coating system 1 described in Example 1 was applied to the top segment, which was exposed to the weather in the wooden window.

Example 5: Artificial accelerated weathering tests

Samples of wooden substrate 2 made of larch heartwood with dimensions 20 x 40 x 160 mm were ground by sandpaper with the grit size 120 and then conditioned to acquire an equilibrium humidity of 10 ± 2% (w/w). Subsequently, the wood samples were coated on all sides with a multilayer coating system 1 according to Example 1 .

Testing of the samples by artificial accelerated weathering was performed in a UV chamber QUV Weathering Tester (Q-Lab, USA) according to CSN EN 927-6 standard and in a cycle temperature chamber at the temperature cycles of 80 °C and -25 °C. The test was performed in the following cycles: 24 h with the temperature of 45 °C without light. Then, for the period of 144 h, UV radiation phases with an intensity of 1.10 W/m ² at a wavelength of 340 nm and temperature on a black panel 65 °C (2.5 h) were rotated with 0.5 h of spraying distilled water without light. During each 144-h cycle of the radiation and spraying combination, the samples were removed from the UV chamber and exposed to three cycles of changing temperatures of 80 °C and -25 °C in the climate chamber (a total of 6 one-hour cycles with a total duration of 6 h). The total test time in the UV chamber was 1 ,500 h. On the tested samples, colour was measured using a spectrophotometer, gloss using a glossmeter, contact wetting angle using a goniometer and a laser scanning microscope was used for visual evaluation in addition to observation with a magnifying glass. The hardness of the multilayer coating system 1. was evaluated using the pencil hardness test and the Brinell hardness test.

For comparison, the same tests were carried out on larch wood samples that were also ground by sandpaper with the grit size 120 and then conditioned to an equilibrium humidity content of 10 ± 2% (w/w) and to which the following surface treatments were applied: a) the primer layer 3 in the form of a commercial water-borne acrylic coating applied in 3 layers; b) ZnO or ZrOs nanoparticles were mixed into the primer layer 3 at a concentration of 3.0% (w/w) and the coating was applied in three layers and thus the nanoparticles were contained in all three layers of the coating; c) different concentrations (1.5% (w/w), 2.0% (w/w) and 3.0% (w/w)) of ZnO and ZrOs nanoparticles were admixed into the first layer of the primer layer 3 of the coating, the other two layers of the coating were free of nanoparticles; d) the primer layer 3 was applied in one layer, into which the middle layer 4, made according to Example 1 , was placed, and after drying two more layers of the glazing layer 5 were applied; e) the primer layer 3 was applied in a single layer, in which ZnO or ZrOs nanoparticles were admixed at different concentrations (1.5% (w/w), 2.0% (w/w) and 3.0% (w/w)), and in which the middle layer 4, made according to Example 1 , was further placed, and after drying two more layers of the glazing layer 5 were applied.

The resulting scans of the larch wood surfaces of the 2 substrates are shown in Fig. 3 and their characteristics are summarized in Table 1 . Table 1 : Quantification of defects in surface treatment found in the compared tested variants after 9- weeks exposure to artificial accelerated weathering. Based on the results shown in Fig. 3 and Table 1 , it is evident that the surface treatment according to the present invention improves the overall durability of the samples of larch heartwood compared to the samples treated with only by a commercially available coating system or by other types of tested treatments. The use of the primer layer 3 containing ZnO nanoparticles and the middle layer 4 containing a polyamide nanofibrous nonwoven according to the present invention significantly improved the overall durability of the part after a 9-week artificial weathering test.

By the application of the most preferable ZnO modification no reduction in colour fastness, hardness, and hydrophobicity in the form of contact wetting angle of the treated larch heartwood occurred compared to the reference commercially available coating systems, prior to or after the accelerated artificial weathering. The surface gloss was slightly decreased immediately after the application of the surface treatment applied according to Example 1 , however, such decreasing was less significant than in the commercially available surface treatment during the accelerated artificial weathering.

Example 6: Testing different surface treatments of larch heartwood and their comparison

1) Commercially available coating systems

Different variations were investigated on a wooden substrate 2 following the same artificial accelerated weathering methodology described in Example 5. The results of the solutions were not fully satisfactory. Commercial coating systems on larch wood are subject to faster degradation, in particular due to the high content of arabinogalactans. Furthermore, they are also susceptible, at the elevated temperatures, which are common in exposed areas during the summer months, to be damaged by resin leakage through the coating system. Resin leakage forms droplets on the surface of the coating system in which impurities get deposited to an increasing extent, leading to aesthetic impairment of the wooden substrate. In all tested samples of transparent coating systems, the results of commercial coating systems were inferior compared to the present invention. This concerned in particular the defoliation of the coatings from the wooden substrate 2 after the test, and also the leakage of the resin, to which the multilayer coating system 1 according to the present invention was not prone, mainly due to the synergistic effect of the applied middle layer 4 and the ZnO nanoparticles in the primer layer 3.

For direct comparison for the artificial accelerated weathering test in Example 5, a commercially available exterior acrylic transparent coating system was applied without any treatment or modification in 3 layers onto larch heartwood.

2) Acrylic coating versus other nanofiber nonwoven coating bases (defoliation)

In the experiments, other types of exterior coatings of a different polymer base were also tested, in which the middle layer 4 consisting of a nanofibrous nonwoven fabric made of polyamide according to the present invention was inserted during the process of forming the coating system. Specially for exterior use on wooden substrate, transparent coatings on the basis of a) acrylates; b) polyurethanes; c) natural oils mixed with naphtha; d) alkyds were used. After the application of the middle layer 4 consisting of a nanofibrous nonwoven polyamide fabric to the primer layer 3 and the application of a glazing layer 5 of the tested commercial coating, the tested samples were subjected to 6 weeks of artificial accelerated weathering according to Example 5. In this case, ZnO nanoparticles were not used in the first layer of the coating. The experiment only compared the effectiveness and ability to bond the polyamide nanofibrous nonwoven tightly to the various polymeric coating substances. After the test, the defoliation of the samples and also the general change in the colour shade of the substrate was evaluated. The results are shown in Table 2.

Table 2: Comparison of the results of the durability of coating systems on different polymer bases with a nanotextile applied on larch wood. Based on these preliminary tests and their evaluation, an acrylate-based coating system was used for further detailed proposal of the solution, including the application of ZnO nanoparticles to improve the resistance to UV-radiation and VIS spectra.

3) Other types of polymers for nano-network production using polyamide compared to other polymers

For the production of the nanofibrous nonwoven textile forming the middle layer 4, a synthetic polymer of polyamide PA6, known as Ultramid B24, was used. This polymer was chosen because it is inherently conductive in the electrostatic field, and therefore its spinning is easy. The acrylate-based coating system, which is modified with an admixture of ZnO nanoparticles and modified with a nanofibrous nonwoven fabric of polyamide PA6, had the highest resistance to UV radiation, temperature changes and direct contact with water of all the combinations tested. This is due to the fact that the PA6 polyamide polymer is a thermoplastic material and its glass transition temperature (Tg) is in the temperature range from +50 to +55 °C. In case of higher temperature induced by UV radiation on the surface of the treated wooden substrate, the temperature on the surface may increase, which may approach the glass transition point. Other polymers which have similar properties and can be used instead of polyamide PA6 are polyamide PA610, PA8, PA11 , PA12. The other tested types of polymers that can be subjected to the spinning process in an electrostatic field to the level of nanofiber and nanolayer dimensions while chemically interacting with acrylic coating systems lack the aforementioned properties. Thanks to its content of polar oxygen and nitrogen atoms, polyamide PA6 is a strong polar thermoplastic material, which means that is it chemically well bound to the molecules of water present in the acrylic primer layer 3 of multilayer coating system 1 as solvent immediately upon application and also to the polar groups of the adhesion upon its curing and water evaporation. On the other hand, it does not react with non-polar solvents and can only be dissolved in highly concentrated acids, making it very resistant to biotic and abiotic agents.

4) Admixture of nanoparticles (ZnO, ZrO2) only in all layers of the coating system

For comparison, ZnO and ZrOs nanoparticles only were admixed into the acrylic primer layer 3, which have a protective function against degradation caused by UV radiation. Both types of nanoparticles were added to the commercial transparent exterior acrylic primer layer 3 at 3.0% (w/w) concentration, present in all three layers and were applied to larch heartwood. The tested samples were then exposed to the artificial accelerated aging test according to Example 5. The results of the testing are shown in Table 1 , as solution b).

5) Admixtures of only nanoparticles in various concentrations present only in the primer layer of a multilayer coating system

For comparison, only ZnO or ZrOs nanoparticles were admixed into the primer layer 3 of the multilayer coating system 1., which have a protective function against degradation caused by UV radiation. Both types of nanoparticles were added to the commercial transparent exterior primer layer 3 at 1 .5% (w/w), 2.0% (w/w) and 3.0%(w/w) concentration present only in the first layer applied to the larch heartwood. The tested samples were then exposed to the artificial accelerated aging test according to Example 5. The results of the testing are shown in Table 1 , as solution c).

6) Use of only the nano-network in the primer layer of a multilayer coating system

To compare the efficiency of the treatment, only the middle layer 4 consisting of the polyamide nanofibrous nonwoven fabric according to Example 1 with no admixture of nanoparticles was inserted into the primer layer 3 of the multilayer coating system 1_. Then other two glazing layers 5 of a commercial transparent exterior acrylic coating were applied. The tested samples were then exposed to the artificial accelerated aging test according to Example 5. The results of the testing are shown in Table 1 , as solution d).

7) Utilization of different concentrations of ZnO and ZrO2 nanoparticles in the primer layer of a multilayer coating system including a middle layer

The ZnO or ZrOs nanoparticles in different concentrations, namely 1 .5% (w/w), 2.0% (w/w) and 3.0% (w/w), were admixed into the primer layer 3 of the multilayer coating system 1.. The middle layer 4 consisting of a polyamide nanofibrous nonwoven textile was then added to the coating and further two glazing layers 5 without the addition of nanoparticles were applied, as described in Example 1 . The tested samples were then exposed to the artificial accelerated weathering test according to Example 5. The results of the testing are shown in Table 1 , as solution (e). Industrial Applicability

The multilayer wood coating system and the method of its application according to the present invention can be used mainly in the wood industry, construction and architecture, in particular for the surface treatment of larch wooden substrates exposed in exterior conditions.