WO2008061791A1 - Coated board of wood-based material - Google Patents

Abstract

The present application relates to a coated board of wood-based material and a method for coating a board of wood-based material, wherein the board of wood-based material is in particular a wall panel or floor panel or is intended for producing such a panel, comprising a front side and a rear side, wherein at least the surface of the front side is provided with a polymer coating and wherein the polymer coating has a hardness gradient, so that the hardness of the polymer layer decreases with increasing depth from the surface.

Description

 Coated wood-based panel

1. Field of the invention

The present invention relates to a coated wood-based panel, in particular for the production of a floor, ceiling or wall covering, and a method for coating a wood-based panel.

2. Background

From the prior art, a variety of toppings on wood-based material is known. In the simplest case, such a plate is made of solid real wood. However, such solid wood panels are very expensive and can be laid as panels only by well-trained professionals. However, such so-called real wood planks offer a very attractive surface. In order to avoid the high cost of real wood floors and at the same time to enable the attractive surface of such floors, veneer decking panels have been developed. Veneers are thin sheets, usually 0.3 to 0.8 mm, made of high-quality wood, which are glued to a carrier material. The support materials are usually made of cheaper wood materials and are significantly thicker than the veneer layer. A disadvantage of such coverings is the relatively sensitive surface, which can easily be damaged, for example, by moisture or by mechanical action.

Furthermore, laminate panels for floor or ceiling coverings are known from the prior art. In comparison to the initially mentioned lining panels, laminate panels are relatively inexpensive. Laminate panels are usually made of a 4 to 12 mm thick support plate made of MDF or HDF material, so a relatively cheap wood material, on top of a with a decor printed paper is glued on. On the underside of the support plate is usually a so-called Gegenzugpapier which is intended to counteract a delay of the support plate by the applied decorative layer. In order to improve the durability of the decorative layer, it is usually applied to a so-called overlay paper, which is impregnated with a resin such as an amino resin and are applied to the very fine abrasion resistant particles, such as Alumuniumoxidpartikel. By pressing under the action of heat and pressure, the various layers of the laminate panels are joined together and the resins used are cured. It thus creates a durable, abrasion-resistant decorative surface.

In order to improve the durability and thus also the optical properties of wood-based panels, as used for example for wall, ceiling or floor panels, various coating methods and materials have been proposed in the prior art. In principle, such coatings can be applied to any type of wood-based panel to increase the durability of the surfaces, including the real wood and laminate panels listed above.

From WO 2007/042258 Al, for example, a method for coating a wood-based panel is known in which in a single coating step, a relatively thick protective layer of plastic material is applied to the surface of a plate. The plastic material used is a polymerizable acrylate system which can cure via polymerization. The polymerization is initiated by irradiation, so that a complete conversion takes place through the thickness of the applied layer.

Starting from this prior art, the object is a coated wood-based panel, as well as to provide a method for coating a plate, which has particularly advantageous mechanical properties. These and other objects, which will become apparent upon reading the following description or which may be recognized by those skilled in the art, are achieved with a coated wood-based panel according to claim 1 and a method of coating according to claim 9.

With the present invention, with optically good transparency of the coating, and also good brilliance of a printed image applied underneath or in it, abrasion values of the highest abrasion class AC 5 in accordance with prEN 15468 are achieved. The surface is characterized by high micro-scratch resistance (Mar-Resistance) and impact resistance according to Class 33 (prEN 15468). The characteristic values for chemical and water vapor resistance, castor wheel and furniture foot test certainly correspond to prEN 15468. Furthermore, the method allows a surface in which, in addition to the pressure, a deeply embossed decorative structure, for example a brushed wood structure or a stone structure can be introduced. The invention is thus particularly well suited for the provision of floor panels.

3. Detailed description of the invention

The coated wood-based panel is in particular a floor, ceiling or wall panel or a wood-based panel intended for further processing into a floor, ceiling or wall panel, and comprises a front and a back, wherein at least the surface of the front side is provided with a polymer coating , The term wood-based panel is to be understood broadly and includes, for example, both panels of real wood and MDF, HDF, chipboard, composite panels, OSB panels, etc. The panel can continue with additional coatings, papers, veneers or the like on their front and top surfaces / or back side be provided. Thus, if a coating of the surface of the wood-based panel is mentioned, this does not necessarily mean a direct coating of the wood-based panel, but it may for example be provided with a decorative paper, the coating then on the - A -

Dekoφapier is applied. According to the invention, the polymer coating has a hardness gradient after curing, so that the hardness of the polymer layer decreases with increasing depth seen from the surface. That is, the polymer layer preferably has the highest hardness on its outer surface and the lowest hardness near the interface between the coating and the surface of the wood-based panel, with a falling course between the two extremes.

So far, it has always been desirable to achieve the highest possible hardness over the entire layer thickness. The coating according to the invention deviates from this teaching and nevertheless surprisingly leads to very good mechanical durability values. An explanation for this could be that by a uniform as possible drop in hardness no large jumps in the properties of the coating occur and this is therefore particularly durable.

The present invention also relates to a method for coating a wood-based panel, in particular a floor, ceiling or wall panel, or a wood-based panel is further processed to floor panels, wherein a first liquid coating agent is applied to a wood-based panel in a first step and on the still a first liquid coating agent is applied to wet first coating agents, the liquid layers penetrating each other according to the physics of liquids. This results in a gradient of the concentration of both liquids to each other. While in the outer regions of the total layer (upper side or lower side of the total layer), the respective liquid of the original single layers is dominant, there is a concentration gradient of the first liquid or the second liquid to the middle and further to the other side of the layer. Ideally, the respective concentration curve corresponds to a straight line. Since with higher viscosity liquids with short mixing times disturbances can occur ideally, one must assume that the actual concentration curves correspond only approximately straight lines and deviations are possible. Are the fluids For example, polymerizable acrylate systems that differ in the double bond fraction, it follows from the above, that analogous to the concentration gradient of the two liquids to each other a gap in the number of double bonds from one side to the other side of the layer is formed. If polymerization is initiated in such a layer, for example by UV excitation, and it is assumed that almost complete conversion of the double bonds takes place under inert conditions, a polymer layer with a gradient of the crosslinking sites is formed. While the side with high double bond concentration is correspondingly strongly crosslinked, the other side with the lower double bond fraction has a correspondingly lower crosslinking. According to polymer physics, the hardness in such a system gives information on the crosslink density. If, for example, the microhardness (Martens hardness DIN EN ISO 14577) is measured within a layer produced correspondingly from two polymerizable liquids, a hardness gradient results analogously to the crosslinking density. For example, the layer can be removed stepwise using a Taber Abraser test according to EN 13329. The curve of the hardness gradient corresponds in completely the same way to the concentration gradient of both liquids described above. Ideally, the mixing of the liquids occur straights. In practice, however, deviations from the straight line will occur. Mathematically, it can therefore be expected that the function y = f (x) has a course deviating from a straight line (where y is the Marten hardness and x the removal depth in the layer).

The person skilled in the illustrated relationship is illustrated by the following example:

About a Walzenauftragswerk a first layer of 45 g / m ^{2 is} rolled on an HDF support plate, wherein the coating composition of the first layer, for example, to 35% of a 1, 6-hexanediol diacrylate and 65% of a polyester acrylate. Immediately thereafter, a second layer having a mass of 40 g / m ^{2 is} applied to this layer, wherein the coating agent the second layer consists for example of a mixture of 70% polyurethane acrylic acid ester and 30% dipropylene glycol diacrylate. Both layers in the present case contain a photoinitiator. The total liquid layer thus produced is exposed to UV radiation under a nitrogen atmosphere and the entire layer is polymerized. The double bond conversion is about 98%.

To investigate the resulting coating, the coating was subsequently removed stepwise by the Taber Abraser test, in each case by 200 revolutions (described in EN 13329). Each time an abrasion step was reached, Marten hardness was measured. If the measured Martens hardness in N / mm ^{2 is} plotted on a y-axis in a rectangular coordinate system and the corresponding abrasion depth in μm on the x-axis, the result is approximately a straight line with the function y = 134.8 - 1 , 03 x. The coefficient of determination was determined to be 87.8%, which shows a very high accuracy of this mathematical relationship for wood-based materials.

In addition, when the coatings of the invention are used for a durable floor covering, for example, the layers may additionally be provided with abrasion resistant particles, e.g. with fine corundum particles. These may be present in a dispersion, for example, before the coating process in one of the two or even in both coating compositions or they may be sprinkled in a separate process step on the still moist, but already applied coating compositions.

The person skilled in the art recognizes from the present description of the invention that, depending on the application, it is possible to use coating compositions having preferred concentrations other than those indicated in the example. Preferably, the concentration of 1,6-hexanediol diacrylate may be between 10 and 60%, more preferably between 20 and 40%; the concentration of polyester acrylate between 40 and 90%, more preferably between 50 and 80% lie; the concentration of polyurethane acrylic ester is between 45 and 95%, more preferably between 55 and 75%, and the concentration of dipropylene glycol diacrylate is between 5 and 55%, more preferably between 15 and 35%. The substances mentioned are intended to illustrate the principle of a layer with hardness gradients on the basis of a preferred embodiment. Of course, a variety of other or other polymerizable substances can be used instead. Polymerizable acrylates are particularly preferred substances for the coatings described herein.

The coating agent of the first, as well as both the second and possibly further layers may consist of a single polymerisable substance or of mixtures of substances. Particularly suitable substances are polymerizable acrylates in general and here in particular the substances: 1,6-hexanediol diacrylate, polyester acrylate, polyurethane acrylic acid ester and dipropylene glycol diacrylate. Particularly suitable for the first layer is a mixture of 1,6-hexanediol diacrylate and polyester acrylate. For the second layer, a mixture of polyurethane acrylic acid ester and dipropylene glycol diacrylate is particularly suitable.

The coating compositions may contain further additives, such as, for example, flow aids, wetting aids, dyes, abrasion-resistant particles, etc. It is essential that these further constituents permit the above-described crosslinking or penetration and that polymerization is still possible.

When selecting the coating compositions for the individual layers, the specified substances are preferred, but the person skilled in the art recognizes that it is not explicitly the use of the specified substances, but essentially the provision of polymerizable coating compositions. 4. Detailed description of exemplary embodiments

A detailed description of exemplary embodiments will now be given below with reference to the accompanying diagrams and drawings.

Fig. 1 is a schematic representation of a

Coating process;

Figs. 2A to 2C are schematic diagrams showing the flow of mixing second liquid layers;

FIG. 3 is a graph showing the course of hardness in FIG

Dependency on the depth of the coating;

Fig. 4 is a diagram illustrating the upper and lower limits of the hardness gradient according to a preferred embodiment of the invention;

Fig. 5 is a diagram illustrating the upper and lower limits of a more preferred embodiment of the invention; and

Fig. 6 is a diagram illustrating the upper and lower limits of the hardness gradient of a still further preferred embodiment.

In Fig. 1, a coating system for coating wood-based panels 10 is shown schematically. The wood-based panels 10, such as solid wood panels, HDF, MDF or chipboard, are passed through a roller conveyor 12 through the various stations of the coating plant. In a first coating station 14, a first liquid coating agent is produced by means of a rotating application roller 15 20 applied in a continuous coating on the wood-based panels 10. The application roller 15 is supplied via a supply device 16 with coating agent. In the second coating station 17, a second liquid coating agent 21 is applied to the still moist first coating agent 20 via a further rotating application roller 18. The applicator roll 18 is supplied by means of a feed device 19 with the second liquid coating agent. The job can of course be done with any other suitable application method, such as a sprayer or a doctor blade or the like. It is only important that the application of the second layer takes place, as long as the first layer is still wet enough, so that there is a partial mixing of the layers. In addition, after the second coating station 17, it is of course also possible to provide further coating stations in order, for example, to apply a third liquid coating agent to the still moist second coating agent 21 or also additional stations in order to introduce abrasion-resistant particles onto or into the moist layers.

After leaving the coating station 17, the coated plates 10 are transported to a curing station 30, where the layers are cured by means of UV lamps 31. On their way from the coating station 17 to the curing station 30, there is a partial mixing of the liquid coating agent 20 and 21, which occurs in particular at the interfaces of the two coating agents. The mixing is naturally greater, the closer you are to this interface of the two layers. The hardening of the layers in the curing station 30 stops the mixing process and sets the once set mixing ratio and thus the mechanical properties of the coating produced. The extent of mixing at the interfaces - which takes place by itself and preferably without external mechanical action - depends on the time that elapses between the application of the second coating agent 21 to the still moist first coating agent 20 and the curing in the curing station 30 , In addition, the mixing the two coating agents are also influenced by the respective viscosity of the coating agent, the rule of thumb being that the higher the viscosity, the lower the mixing per unit time.

The principle of thorough mixing of the two coating agents applied can best be seen from the schematic representation of FIGS. 2A to 2C. FIG. 2A shows the condition of the two coating agents 20 and 21 applied to a wood-based panel 10 immediately after application of the second coating agent 21. At this point in time, no mixing has taken place. The coating agents 20 and 21 are polymers in the present case, each having different numbers of C-C carbon double bonds. As indicated schematically in FIG. 2A, the first coating agent 20 has a smaller number of CC double bonds than the second coating agent 21. Because of the greater number of CC double bonds in the coating agent 21, this will have a greater hardness after curing than the coating agent 20, which is provided with fewer CC double bonds.

Since the two coating agents 20 and 21 are applied wet-on-wet, starting from the interface 22 between the two layers, they are mixed, as indicated in FIG. 2B. This means that more double bonds are present in the region close to the border to the boundary layer 22 as a result of the mixing process in the underlying layer and, accordingly, in the area close to the border of the overlying layer, somewhat fewer double bonds than before mixing. Fig. 2C shows the two

Layers after the mixing has progressed a little further and has reached a suitable degree of mixing. If at this time the curing of the coating agent, for example by means of UV radiation, takes place, this degree of mixing is determined, since naturally no mixing can take place in the hardened layers. In the diagram of FIG. 3, the hardness profile of a coating according to the invention (example with hardness gradient) and a coating according to the prior art is plotted. The inventive example consisted of a ground and provided with a primer wood-based panel, were applied to the two different coating wet-on-wet. The first applied coating agent consisted of about 35% 1, 6-hexanediol diacrylate and about 65% polyester acrylate and was applied at 45 g / m. The second coating agent, which was applied to the still moist first layer, consisted of about 70% polyurethane acrylic acid ester and about 30% dipropylene glycol diacrylate and was applied at 40 g / m ² . After the second layer was applied, it was waited 10 seconds to allow the viscous liquid materials to mix. Subsequently, the two layers were completely cured together.

The prior art example consisted of a conventional coating wherein several thin layers of material were applied one at a time, and between the respective application processes, the previously applied layer was cured. The lower 3 layers consisted of a mixture of 70% polyester acrylate and 30% 1, 6-hexanediol diacrylate with an application thickness of 12 g / m ² . The two upper layers consisted of 70% polyurethane glycol diacrylate and 30% dipropylene acrylic acid ester and the two upper layers contained 15% corundum with a mean particle size of D 50 of 25 microns.

The test was carried out according to the European standard for laminate flooring DIN EN 13329 with a Taber Abraser tester 5151 from Taber Industries. Each time after 200 revolutions with S-41 abrasive paper, the hardness and trace depth of the samples were determined. The determination of Martens hardness (registering hardness test under test force effect) was carried out according to DIN EN ISO 14577. The test instrument used was a "Fischerscope H100" made by Helmut Fischer GmbH The following test parameters were used: maximum force: 50/30 mN and measurement duration: 20 seconds Spurt depth was performed with a mechanical palette measuring device. As a tester, a Perthometer S3P Perthen was used.

When measuring the samples, it has been found that, due to the relatively soft materials used, it is probable that there will be more or less large variations in the hardness of a given layer depth. It is therefore necessary to measure at several points in order to obtain meaningful, representative data by averaging. In the measurements made, the hardness and the trace depth were measured at four points after 200 revolutions of the abrasive paper. It has been shown that four measurement points provide sufficient accuracy in most cases. Of course you get even more accurate results when you use more than four measurement points, such as eight.

The table reproduced below shows the individual measured values for the sample according to the invention of the example. The measurement was carried out on the finished cured coating, ie the state in which corresponding products would also be used in real terms as floor panels.

Table 1: Example with hardness gradient

In the table above, the column "Revolve" indicates the number of revolutions performed with the Taber Abraser tester The column "Depth trace" indicates how many micrometers of material of the coating from the original surface at the four measuring points. 4 were removed. The "depth hardness measurement" column indicates by how many micrometers the test mandrel penetrated the coating at each of the four measuring points 1-4 In the "Marten hardness" column, the hardness in Newton per mm ² for the four measuring points 1 is then determined -4 indicated. Below the individual values, the respective mean value for the four measuring points is indicated. From the table presented above, it can be clearly seen that the hardness of martensite decreases as one goes deeper into the hardened, finished layer. It can also be seen that at 800 and 1000 (total) revolutions there is a slight increase in Martens hardness. This is due to an irregular mixing of the two coating compositions used, which is difficult to completely avoid in practice. Nonetheless, it can be clearly seen from the diagram of FIG. 3 that in the example with a hardness gradient overall there is an almost continuous decrease in hardness without large jumps. The comparative example according to the prior art, however, shows no such continuous course of hardness, but rather has at a depth of 60 to 80 microns on a pronounced discontinuity to the original initial hardness.

The mean values of the test specimen according to the invention are reproduced in Table 2 below.

Table 2: Mean values of the example with hardness gradient

The values of the comparative test piece according to the prior art are shown in Tables 3 and 4 below.

Table 3: Sample according to the prior art

Table 4: Mean values of the sample according to the prior art It has been found experimentally that particularly good mechanical properties of the finished overall layer can be achieved if the hardness gradient of the finished overall layer-as illustrated by way of example in FIG. 3-substantially corresponds to the following relationship:

(-3.0 x) + C ≤ Y (x) ≤ (-0.2 x) + C where: each absolute value of the depth in μm of the coating of the

Surface of the coating is seen ago; Y (x) is the absolute value of hardness in N / mm ² at a given

Depth x is; and

C is the absolute value of the initial hardness in N / mm ^{2 of} the coating at about x ~ 0-5 microns depth.

The "absolute" values mean that only the pure numerical values are entered into the above formula, ie without the corresponding unit of measure "μm" or "N / mm ² " If, for example, the initial value of the above example with hardness gradient 140 , 8 N / mm ² (see Table 2), only the absolute values, ie C = 140.8, are used in the above table Likewise, only the absolute values, ie, for example, x = 3.5, are used for x For example, upper and lower bounds for Y (x = 3.5) are 140.1 and 130.3, respectively. At a depth of x = 40 μm, 132.8 for the upper limit and 20.8 for, for example, are obtained These upper and lower limits for Y (x) have the unit of measure N / mm ^2. It is important that the absolute values are used in the formula starting from the designated units of measurement "μm" or "N / mm ² ", respectively not for example from "mm" or "N / m ^2. " It should be clear to the person skilled in the art that the above formula k is a mathematical formula for describing the hardness gradient itself, but rather defines an area in which it should run.

The initial hardness value of the coating is the value in the first few microns of the coating. Due to the commonly used measuring method it is difficult to determine the hardness for the penetration depth "0 μm" by means of a test mandrel which penetrates a few μm into the coating The term "substantially" is therefore chosen because it is difficult to achieve a perfectly uniform mixing of the materials. so that it can come in reality again and again to individual small outliers, such as the hardness value of 104.2 Newton / mm ² at a depth of 42.1 microns (see Table 2) of the example discussed above with hardness gradient. In addition, the values extremely close to the surface of the wood-based panel are generally inaccurate, since the residual layer thickness to be measured must have a certain minimum thickness in order to allow meaningful measurements. The residual layer thickness should therefore be at least 5 μm, preferably 10 μm and even more preferably at least 20 μm, for meaningful measurements. In other words, the last 20 μm of the layer, close to the wood-based panel, does not necessarily have to follow the preferred hardness gradient described above, although this is of course preferred.

In a further preferred embodiment, the hardness gradient essentially follows the following relationship:

(-2.5 -x) + C ≤ Y (x) ≤ (-0.4-x) + C

And in a still further preferred embodiment, essentially:

(-2.0 x) + C ≤ Y (x) ≤ (-0.6 x) + C

FIGS. 4 to 6 illustrate the meaning of the above-described relationships of the hardness gradients on the basis of the example with hardness gradient. It should be understood that the given absolute values for hardness and depth are merely exemplary. Of course, it is also possible to apply overall layers with significantly greater thicknesses or lower thicknesses. In addition, the absolute value of the hardness of course depends on the materials used and may also be greater or less than the values of the example with Hardness gradient. However, the magnitude of the given values for the hardness gradient example is particularly preferred and suitable for use in a floor panel.

The skilled artisan recognizes by the detailed description of the method according to the invention how he can achieve a coating according to the invention of a wood-based panel. This means, of course, that all materials mentioned or mentioned in connection with the description of the method, such as, for example, the substances for the coating compositions, can also be used in the coating of the wood-based panel according to the invention.

The proposed method is particularly suitable for the coating of floor panels, or for the coating of wood-based panels which are further processed to floor panels, since here the beneficial mechanical properties of the hardness gradient most affect. Likewise, the presented coated wood-based panel is for the same reason preferably a floor panel, or a coated wood-based panel, which is intended for further processing to a floor panel.

Claims

claims 1. Coated Holzwerkstofφlatte, in particular a wall, ceiling or floor panel, comprising a front and a back, wherein at least the surface of the front side is provided with a polymer coating, characterized in that the polymer coating has a hardness gradient, so that the hardness of the polymer layer decreases substantially continuously as the depth increases from the surface of the coating. 2. Coated wood-based panel according to claim 1, characterized in that the hardness gradient substantially corresponds to the following relationship: (-3.0 - JC) + C <7 (JC) <(-0.2 - JC) + C where: each Absolute value of the depth in μm of the coating of the Surface of the coating is seen ago; Y (x) is the absolute value of the hardness in N / mm ² at a certain depth x; and C is the absolute value of the initial hardness in N / mm ^{2 of} the coating at about x ~ 0-5 microns depth. 3. Coated Holzwerkstofφlatte according to claim 1, characterized in that the hardness gradient substantially following Relationship corresponds to: (-2.5 x) + C <F (Jt) <(- ^0.4 x) + C where: the absolute value of the depth in μm of the coating is seen from the surface of the coating; Y (x) is the absolute value of the hardness in N / mm ² at a certain depth depending; and C is the absolute value of the initial hardness in N / mm ^{2 of} the coating at approx. JC ~ 0-5 μm depth. 4. Coated wood-based panel according to claim 1, characterized in that the hardness gradient substantially following Relationship corresponds to: (-2.0 • x) + C ≤ Y (x) ≤ (-0.6 ^■ x) + C where: JC is the absolute value of the depth in μm of the coating seen from the surface of the coating; Y (x) is the absolute value of hardness in N / mm ² at a given Depth x is; and C is the absolute value of the initial hardness in N / mm ^{2 of} the coating at about x ~ 0-5 microns depth. 5. Coated wood-based panel according to one of the preceding claims, characterized in that the flap is a chipboard, MDF board, HDF board, OSB board or real wood board. 6. Coated wood-based panel according to one of the preceding claims, characterized in that the polymer coating consists of polymers which are curable by means of radiation. 7. Coated wood-based panel according to one of the preceding claims, characterized in that the polymer coating has an initial Martens hardness at a depth of about 0-5 microns from 120 N / mm ² to 250 N / mm ² measured according to DIN ISO 14577. 8. Coated wood-based panel according to one of the preceding claims, characterized in that the polymer coating has an initial Martens hardness at a depth of about 0-5 microns from 130 N / mm ² to 200 N / mm ² measured according to DIN ISO 14577. 9. A method for coating a wood-based panel, comprising the following steps: a) providing a wood-based panel; b) applying a first liquid coating agent; c) applying at least one second liquid coating agent to the still moist first coating agent, so that a partial mixing of the coating agent takes place; d) curing the applied coating agent by means of radiation, so that the hardened resulting coating has a hardness gradient, wherein the hardness of the coating decreases with increasing depth from the surface of the resulting coating. 10. A method for coating a wood-based panel according to claim 9, characterized in that prior to step d) further coating agents are applied to the still moist previously applied coating agent. 11. A method for coating a wood-based panel according to claim 9 or 10, characterized in that the hardness gradient substantially corresponds to the following relationship: (-3.0 x) + C ≤ Y (x) ≤ (-0.2 x) + C where: each absolute value of the depth in μm of the coating of the Surface of the coating is seen ago; Y (x) is the absolute value of hardness in N / mm ² at a given Depth x is; and C is the absolute value of the initial hardness in N / mm ^{2 of} the coating at about x ~ 0-5 microns depth. 12. A method for coating a wood-based panel according to claim 9 or 10, characterized in that the hardness gradient substantially corresponds to the following relationship: (-2.5 -x) + C ≤ Y (x) ≤ (-0.4-x) + C where: Λ: the absolute value of the depth in μm of the coating of the Surface of the coating is seen ago; Y (x) is the absolute value of the hardness in N / mm ² at a certain depth x; and C is the absolute value of the initial hardness in N / mm ^{2 of} the coating at about x ~ 0-5 microns depth. 13. A method for coating a wood-based panel according to claim 9 or 10, characterized in that the hardness gradient substantially corresponds to the following relationship: (-2.0 x) + C ≦ Y (x) ≦ (-0.6 -x) + C wherein: each of the absolute values of the depth in μm of the coating is seen from the surface of the coating; Y (x) is the absolute value of hardness in N / mm ² at a given Depth x is; and C is the absolute value of the initial hardness in N / mm ^{2 of} the coating at about x ~ 0-5 microns depth. 14. A method for coating a wood-based panel according to any one of claims 9 to 13, characterized in that the first and second layers are polymer layers, wherein the second polymer layer contains more C-C double bonds than the first polymer layer. 15. wood-based panel coated by a method according to any one of claims 9 to 14. 16. Use of a coated wood-based panel according to claim 1 or 15 as a floor, ceiling or wall panel.