Method for producing a decorative surface on a panel, panel, and panel covering

Abstract

The invention relates to a method for producing a decorative surface on a panel or other workpiece. The invention also relates to a panel or other workpiece produced according to the method according to the invention. The invention moreover relates to a panel covering comprising a plurality of intercoupled panels according to the invention.

Description

[0001] Method for producing a decorative surface on a panel, panel, and panel covering

[0002] The invention relates to a method for producing a decorative surface on a panel or other workpiece. The invention also relates to a panel or other workpiece produced according to the method according to the invention. The invention moreover relates to a panel covering comprising a plurality of intercoupled panels according to the invention. The invention moreover relates to a system for carrying out the method according to the invention.

[0003] The objective of decorative surfaces or coated surfaces is to achieve a lifelike appearance. This is typically accomplished by applying a three-dimensional (3D) embossed structure to substrates such as particle boards, MDF boards, HDF boards, polymer based boards that mimic natural materials like wood or stone.

[0004] This embossed structure, also known as haptics, is often synchronized with the underlying decorative image. For instance, in a wood reproduction, a printed knot hole is aligned with a corresponding depression in the embossed structure. This synchronized embossing is referred to as a synchronous pore, which can be produced analogously using an embossing matrix that matches the decorative image.

[0005] It is highly desirable for both the tactile and visual elements of the decorative and printed image to be perceivable. This involves achieving a variation in gloss levels between the recessed areas (pores) and the remaining areas to enhance the 3D effect. Gloss levels are determined in accordance with the method outlined in DIN EN ISO 2813:2015-02. Gloss measurement involves quantifying the amount of light reflected from a surface compared to a polished glass reference standard, measured in Gloss Units (GU). The reflected light quantity depends on the angle of incidence and the surface properties. Gloss measurements can be taken at various angles of incidence (20°, 60°, and 85°), with a preference for the 60° angle. Alternatively, the average value of measurements at these three angles can be utilized. The reflectance measures the percentage of light energy emitted and received by a gloss meter at a specified angle of incidence. In EP 3 928 880 A1 a production method is disclosed to produce a matt board, wherein a liquid curable layer to be structured is firstly semi-cured by a low-energy long-wave ultraviolet (UV) lamp, and then irradiated made by a high-energy UV excimer lamp in an inert gas atmosphere to generate microscopic wrinkles in said layer, and hence to structure said layer leading to a matt impression. However, this method suffers from several drawbacks. One drawback is that this method results in a fully matted board, while in practice position-selective glossy and matt areas are desired. Moreover, the excimer technology used leads to relatively high initial costs, complex maintenance, safety Concerns (the high-energy UV light emitted by excimer lamps (especially at shorter wavelengths like 172 nm) can be hazardous to human skin and eyes), sophisticated, undesired cooling requirements to manage the heat generated during operation, the relatively slow curing process, and, moreover, leads to a limited penetration depth which does not allow curing of thicker layers. Moreover, this technology is not suitable to create a desired haptic effect the consumers are looking for in case of e.g. imitated wood (nerve) patterns.

[0006] It is therefore a first objective of the present invention to develop an improved method and system to provide a panel or other workpiece with a haptic decorative surface.

[0007] It is therefore a second objective of the present invention to develop an improved method and system to provide a panel or other workpiece with a haptic decorative surface and position-selective glossy and matt areas.

[0008] To this end, the invention proposes a method for producing a decorative surface on a panel, comprising the steps of:

[0009] A) applying a curable liquid layer to be structured onto a top surface of said panel (or other workpiece), wherein said liquid layer is preferably substantially immiscible with water,

[0010] B) position-selectively printing, in liquid state, water based structuring ink droplets onto a part of a top surface of said liquid layer to be structured, wherein, preferably, the density of the structuring ink droplets is equal to or greater than the density of the liquid layer to be structured allowing the water based structuring ink droplets to sink, preferably partially sink, into the liquid structure layer, wherein said structuring ink droplets are configured: o to at least partially absorb electromagnetic radiation irradiated during step C), and / or o when in contact with the liquid layer, to produce at least one reaction product which is capable of at least partially absorbing electromagnetic radiation irradiated during step C), and / or o to be less curable compared to the liquid layer during irradiation by electromagnetic radiation during step C), and / or o to be non-curable when irradiated with electromagnetic radiation, in particular in the UV region, during step C);

[0011] C) irradiating the surface of the liquid layer and the applied structuring ink droplets with electromagnetic radiation, in particular in the UV region, wherein the liquid layer will be cured at least partially, and wherein at least a fraction of water present in the water based structuring ink will evaporate resulting in a formation of at least one recessed diffractive microstructure in the at least partially cured structured layer at one or more locations where the structuring ink is printed during step B),

[0012] D) removing, in particular mechanically removing, at least a part of residual fractions of the structuring ink from the at least partially cured structured layer to at least partially expose the at least one recessed diffractive microstructure,

[0013] E) applying a curable liquid top coating onto an unstructured portion of the at least partially cured structured layer, while the structured portion, defined by said at least one recessed diffractive microstructure, of the at least partially cured structured layer is substantially left uncovered by said top coating, and

[0014] F) irradiating the surface of said top coating at least once with electromagnetic radiation, in particular in the UV region, to cure the top coating.

[0015] The method makes use of a water based structure ink which does not properly mix with the surrounding liquid layer (during step B)) due to the preferred immiscibility of both liquids, which leads to a relatively clear interface between the liquid layer and the ink droplets sank, preferably partially sank, into the liquid layer, which is preferred to generate one or more relatively well-defined recessed microstructures into the liquid layer. To this end, the liquid layer may comprise e.g. a polymerizable acrylate mixture and / or may comprise or be formed by a non-aqueous and / or nonpolar solvent-based lacquer system.

[0016] Secondly, by using a water based ink, the water fraction of the ink will at least partially, and typically instantaneously, evaporate during step C), which leads to a sudden significant volume increase and even a steam explosion, which typically deform the shape of the interface of the liquid layer and the structuring ink. For example, the initial shape of said interface may be substantially hemispherical in case of a single ink droplet, and may become wider and / or deeper and / or bagshaped and / or crater-shaped during the irradiation step according to step C). The resulting interfacial surface will be coarse and microstructure, and optionally even nanostructured. This micro / nano-structured surface will eventually scatter light reflection in all directions and will therefore result in an optically matte impression, while also resulting in a desired haptic effect which can be felt by an end-user. This matt effect is secured as the top coating is preferably not applied within the microstructure(s). Unprinted portions of the liquid layer, wherein the structuring ink is not printed, will be covered by the top coating, which is preferably a relatively glossy top coating. The optical contrast between the glossy top coating and the matt microstructure(s) will be seen, and appreciated, by end-users of the panel according to the invention. The depth of the microstructure(s) may vary and is dependent on the volume of the ink droplets used and the amount of ink droplets used at the same location, but is generally situated in between 0.03 and 1 .5 mm.

[0017] The density of the liquid structuring ink droplets and the density of the liquid layer to be structured may be situated between 1 and 1.2. This secures that the droplets will penetrate fully, and / or partially, into the liquid layer. It is preferred that the droplets will remain positioned in a top section, more preferably at a top surface, of the liquid layer, to allow proper formation of the one or more recessed microstructures.

[0018] Preferably, the surface tension of the liquid structuring ink droplets is substantially equal to the surface tension of the liquid layer to be structured. Typically, this surface tension is situated between 65 and 80 mN / m, such as 72-73 mN / m. During step B) at least a fraction of the ink droplets applied is partially situated within (penetrated into) the liquid layer, such that between 40% and 60% of an outer surface of each of these droplets is covered by the liquid layer. The remaining surface area is typically left uncovered during step B). It is imaginable that one or more ink droplets fully penetrate into the liquid layer and will be entirely surrounded by the liquid layer during step B). Additionally or alternatively, it is imaginable that a fraction of the ink droplets fully penetrates into the liquid layer and will be entirely surrounded by the liquid layer during step B), while another fraction of the ink droplets partially penetrates into the liquid layer and will be partially surrounded by the liquid layer during step B).

[0019] The water based ink droplets applied during step B) may be fine droplets and / or may larger droplets. Preferably, the fine droplets have a volume of 0.1 pl to 1 pl, more preferably 0.3 pl to 0.8 pl, in particular 0.5 to 0.6 pl. The larger droplets may have a volume from 1 pl to 80 pl, more preferably from 3 pl to 12 pl, in particular preferably from 5 pl to 10 pl. Preferably, the average diameter of the ink droplets is situated between 50 and 500 pm.

[0020] The speed of the droplets and / or the fine droplets is especially between 0.5 m / s and 12 m / s, preferably between 3 m / s and 7 m / s, especially preferably between 5 m / s and 6 m / s. This printing speed typically also determines the level of penetration of the droplets into the liquid layer.

[0021] To secure an accurate, and flexible, position-selective application of the water based ink droplets, acting as mask (or mask ink), is preferably realizing by means of digital printing during step B).

[0022] During step C) the liquid layer may be partially cured. During step C) the structuring ink droplets may (also) be cured partially, although it is also imaginable that the structuring ink droplets do not cure at all during step C). This facilitates removal of the residual fraction of the ink droplets during step D). Final curing of both liquid layer, and - if still present - traces of the structuring ink droplets may occur during step F). In a preferred embodiment, the electromagnetic radiation used during step C) has a wavelength below 300 nm, wherein preferably use is made of at least one LED UV radiation source. It is imaginable that the electromagnetic radiation used during step C) has a wavelength which is adjusted during the execution of step C). In order to improve the curing process, electromagnetic radiation of different wavelengths in different time intervals may be used during step C) (and / or optionally during step F)). For example, a wavelength of less than 200 nm may be used first, then a wavelength of less than 250 nm and finally a wavelength of less than 300 nm.

[0023] During step C), it is imaginable that each portion of the liquid layer and ink droplets is irradiated at least to times by the electromagnetic radiation. For successive irradiation steps, the same and / or purposively mutually other wavelengths may be used.

[0024] During step D), preferably at least a part of residual fractions of the structuring ink is removed from the at least partially cured structured layer by means of at least one (rotating) brush roller, in particular at least one textile brush roller (having textile brushing fibers) and / or at least one nylon brush roller (having nylon (polyamide) brushing fibers) and / or at least one polypropylene based brush roller (having propylene brushing fibers) and / or at least one metal, in particular steel, based roller (having metal fibers, in particular steel fibers). The fibers of the brush roller(s) should be sufficiently hard and / or stiff to remove the residual (uncured or semi-cured) fraction of the structuring ink, preferably substantially entirely. To this end, it may be advantageous in case the brush roller has an axial rotation speed between 1 ,000 and 2,000 rpm. It may be advantageous in case the brush roller is (also) alternately laterally displaced in an axial direction of the brush roller, wherein the lateral movement speed is preferably situated between 2 and 3 meter per minute. This provides the fibers of the brush roller(s) a movement both in length direction and in width direction, and hence a movement in the XY plane. In this manner the recessed microstructure(s) can be cleared of residues of the structuring ink in a relatively accurate and efficient manner.

[0025] During step E) the top coating is applied by means of digital printing and / or by means of rolling. The top coating may comprise a single layer or a plurality of layers applied on top of each other. Conventionally, the, preferably transparent, top coating is applied by means of one or more rollers. However, this traditional technique may be less suitable in case the panel is provided with one or more bevels or grout line. In this latter case, digitally printing could be more suitable to apply the top coating. Moreover, in case digital printing would be used to apply the top coating, the printing pattern could be, preferably completely, complementary to the printing pattern of the structuring ink

[0026] The top coating, and optionally underlying semi-cured layers, may be cured during step F) by using electromagnetic radiation having a wavelength greater than 300 nm, wherein preferably use is made of at least one Hg (mercury) UV radiation source although other suitable UV radiation sources are not excluded.

[0027] The structured layer formed during step C) and the top coating are preferably transparent and / or translucent (at least after step F), which allows an optional decorative image borne by panel to remain visible.

[0028] As indicated above, the structuring ink droplets preferably do (substantially) not mix with the liquid layer to secure and maintain a substantially clear interface between both liquids. The water based structuring ink comprises water. Additionally or alternatively, the water based structuring ink may comprise another polar solvent, such as ethanol. Typically the presence is water is desired due to the beneficial volatility of water allowing workable time for application and processing, due to the steam formation during curing, as well as for safety and cost reasons.

[0029] It is imaginable that the water based structuring ink only consists of water. It is also imaginable that this structuring ink comprises at least one of the following ingredients in the indicated concentration (vol %) in addition to water having a total content of 10-99%: (i) a substance from the group of hindered amines in a concentration of 0-20%, and / or (ii) a substance from the group of N,N'- diphenyleoxamides in a concentration of 0-20%.

[0030] It is imaginable that the structuring ink comprises as solvent: alcohol, and / or glycol, and / or water, and / or a mixture of two or more of these liquids. The solvent content in the ink is preferably 10-99% by volume. In addition to this solvent, the structuring ink preferably comprises at least one of the following ingredients in the indicated concentration (vol %): (i) a substance from the group of hindered amines in a concentration of 0-20%, and / or (ii) a substance from the group of N,N'- diphenyleoxamides in a concentration of 0-20%.

[0031] The curable liquid layer initially applied during step A) preferably comprises a (curable) polymerizable acrylate mixture. Additionally or alternatively, the liquid layer may comprise a solvent, preferably a non-polar solvent, such as hexane, cyclohexane, toluene, xylene, heptane, and / or mineral spirits. Mineral spirits are a mixture of aliphatic and alicyclic C7 to C12 hydrocarbons.

[0032] In a specific embodiment, the liquid layer comprises or consists of an acrylic lacquer containing 25-35% by weight of a HDDA bi-acrylate, and / or 35-45% by weight of a DPGDA bi-acrylate, and / or 5-15% by weight of a TM PTA crosslinker, and / or 1-5% by weight of an industrial photoinitiator and / or 15-19% by weight of other components. If applied, the acrylic lacquer preferably has a viscosity of 80- 500 mPa s, preferably 150-400 mPa s, measured at 25° C and normal pressure with a rheometer.

[0033] As indicated above, the gloss level is preferably determined in accordance with the method according to DIN EN ISO 2813:2015-02. For gloss measurement, a quantity of light is measured that is reflected from a surface in relation to a reference standard of polished glass. The unit of measurement used is GU (Gloss Units). The amount of light reflected by the surface depends on the angle of incidence and the properties of the surface. For gloss measurement, different angles of incidence (20°, 60° and 85°) can be used to measure the reflectance, preferably with an angle of incidence of 60°. Alternatively, the mean value of measurements for the three angles of incidence can also be used. The reflectance compares the light energy emitted and received by a gloss meter in percent at a certain angle of incidence.

[0034] All surface portions of the panel which, according to the standard, achieve less than 20 gloss units when measured with a gloss meter are defined as “matt” and all surface portions of the panel which achieve more than 60 gloss units are defined as “glossy”. The at least recess microstructure preferably has a gloss level of 0 to 20 Gloss Units (GU) as determined in accordance with the method according to DIN EN ISO 2813:2015-02. Preferably, during step C) a plurality of microstructures is formed. Typically, the collective of microstructures preferably visualizes and / or represents a wood nerve pattern, which is preferably aligned and / or synchronized at least partially with a printed wood image (including printed wood nerves) of a decorative layer of the panel positioned underneath the structured layer.

[0035] As said, the top coating preferably has a higher gloss level than the at least one recessed microstructure. Preferably, the top coating has a gloss level of at least 60 Gloss Units (GU) as determined in accordance with the method according to DIN EN ISO 2813:2015-02.

[0036] The top coating preferably has a smooth upper surface, although it is not excluded that the top coating is provided, for example during step E) and / or step F), with a structured top surface.

[0037] The thickness of the structured layer may vary, but is preferably thicker than half of the thickness of the largest ink droplets. An excessive thickness of the structured layer is undesired from an economic point of view as well as for curing reasons. In practice, the thickness of the structured layer is preferably situated between 1 and 2 mm.

[0038] The invention also relates to a panel, in particular decorative panel, produced by the method according to the invention.

[0039] The panel preferably comprises:

[0040] - a core, which may be a single core layer and / or a plurality of core layers,

[0041] - a decorative print layer applied, directly or indirectly, on top of said core,

[0042] - the structured layer applied, directly or indirectly, on top of said decorative print layer, and

[0043] - the top coating applied on top of said structured layer.

[0044] Preferably, at least one transparent and / or translucent wear layer situated in between the decorative layer and the structured layer. This will protect decorative print layer during normal use. The thickness of the wear layer will preferably be smaller than the thickness of the structured layer. The thickness of each wear layer is preferably situated in between 0.2 and 0.3 for residential use, and between 0.4 to 0.7 mm for commercial use, of the panel according to the invention.

[0045] The decorative print layer comprises and / or consists of a printed decorative image, which is preferably realized by means of digital printing. This latter also facilitates to realize that the at least one recessed microstructure of the structured layer is at least partially synchronized with a image represented by the decorative print layer. This synchronization effect is also known as embossing-in-register (EIR). The decorative print layer may comprise a carrier layer, such as a, preferably white, polymer film (e.g. a PVC film or PU film) and / or a, preferably white, paper film, onto which the decorative image is printed. The carrier layer may be fused and / or glued onto the core. The carrier layer may also be a, preferably white, primer layer directly applied onto the core onto which the decorative image is directly printed.

[0046] The core may comprise MDF or HDF. However, additionally or alternatively, the substrate may comprises one or more other materials. The core may, for example, comprise magnesium oxide (MgO) or other mineral-based materials. Examples of other mineral-based cores are for example gypsum-based substrates, cementbased substrates, etc. Also one or more edges of the core at least partially composed of one or more of these materials may be treated by one or more water barrier agents. Additionally or alternatively it is imaginable that the core comprises at least one thermoplastic material, such as polypropylene (PP), polyurethane (PU), thermoplastic polyurethane (TPU), polystyrene (PS), polyethylene (PE), polyethylene terephthalate (PET), and / or polyvinyl chloride (PVC), polylactic 5 acid (PLA), and / or polyvinyl butyral (PVB). In case one or more of these thermoplastic materials is / are used for manufacturing the substrate, this thermoplastic material may be virgin, recycled, or a mixture thereof.

[0047] The overall thickness of the core may vary, wherein the thickness is preferably between 5 and 12 millimetre, and even better between 6.5 and 9.5 millimetre.

[0048] The panel according to the invention may be used and / or may be configured as floor panel, wall panel, ceiling panel, and / or furniture panel. It is imaginable that the panel comprises at least one grout line or bevel at at least one panel edge, wherein at least one recessed microstructure of the structured layer is located on said grout line or on said bevel, such that at least a part of an upper surface of said grout line or of said bevel is defined by said at least one recessed microstructure. This leads to a textured upper surface of at least one bevel(s) and / or at least one grout line(s). This could further improve the look and feel of the panel according to the invention.

[0049] The panel may comprise coupling profiles, preferably complementary coupling profiles, at at least one pair of opposing panel edges to allow mechanical intercoupling of adjacent panels. The complementary coupling profiles may e.g. comprise a tongue and / or a groove. Preferably, the complementary coupling profiles are configured to mutual lock intercoupled panels both in horizontal as well as in vertical direction. It is typically preferred at each panel edge comprises at least one coupling profile.

[0050] The invention also relates to a panel covering comprising a plurality of mechanically intercoupled panels according to the invention.

[0051] The invention moreover relates to a system for carrying out the method according to the invention, comprising:

[0052] - at least one coating station for coating a panel with a liquid layer to be structured,

[0053] - at least one structuring ink printing station to position-selectively print, preferably digitally print, ink droplets onto the liquid layer to be structured,

[0054] - at least one first curing station configured to irradiate the liquid layer to be structured and applied onto the panel with electromagnetic radiation, preferably within the UV region, to at least partially cure and to microstructure said layer,

[0055] - at least one brushing station for removal of at least a part of a residual fraction of the structuring ink from the structured layer,

[0056] - at least one top coating station for applying a top coating onto of the brushed structured layer,

[0057] - at least one second curing station configured to irradiate at least the top coating to cure said top coating, and at least one conveyor to successively displace the panel along each station.

[0058] The invention will be further elucidated by several illustrative examples and with reference to the appended non-limitative figures, wherein:

[0059] - figures 1-7 show schematically successive process steps of a method for producing a decorative surface on a panel according to the invention;

[0060] - figure 8 shows an alternative edge finishing of the panel according to the invention as compared to the panel shown in figure 1-7, and

[0061] - figure 9 shows an exploded perspective view of a panel according to the invention, which may be the panel as shown in figure 7.

[0062] Figure 1 schematically shows a panel 1 according to the invention. The panel 1 is shown in more detail in figure 7. On top of this panel 1 a decorative 3D structure will be applied, which becomes integral part of the panel 1 as such. A first step is shown in figure 1 , wherein a curable liquid layer 2, which is preferably acrylate based, is applied onto a top surface of the panel 1. The application of the liquid layer 2 can be realized, for example, by means of rolling and / or spraying, e.g. in a coating station, and / or printing, in particular digital printing, in a printing station. Typically though not necessarily the entire top surface of the panel 1 is covered by the liquid layer 2. The thickness of the layer 2 is smaller than the thickness of the panel 1 situated underneath said layer 2 and it typically situated in the range of 1-3 mm. The panel 1 provided with the liquid layer 2 is transported by means of at least one conveyor (not shown) in a direction indicated by the arrow (A), along successive stations, firstly to a printing station 20, preferably a digital printing station, for printing preferably digital printing, water based structuring ink droplets 3 onto the still liquid layer 2. As shown in figure 3, the water based structuring ink droplets 3 partially penetrate into the liquid layer 2 and partially float onto the liquid layer 2, which is also shown in figure 3a. This can be realize by aligning the physical (and chemical) properties of the liquid layer 2 and the structuring ink droplets 3. Preferably the density of the ink droplets 3 are equal to or slightly (up to 20%) higher than the density of the liquid layer 2 to facilitate partial penetration of the droplets 3. Preferably, the surface tensions of both liquids 2, 3 is more or less the same (+ / - 20%). The ink droplets 3 are preferably partially or entirely immiscible with the liquid layer 2 which impedes penetration and which secures and temporarily maintains a substantially clear interface between both liquids 2, 3. The droplets 3 are printed position-selectively at / on the locations wherein one or more recesses in the form of microstructures are to be created.

[0063] Subsequently, as shown in figure 4, the panels 1 provided with the liquid layer 2 and the partially penetrated water based structuring ink droplets 3 are moved along a first curing station 21 with at least one first irradiation source 22 for emitting electromagnetic radiation in the UV region. This first irradiation source is preferably configured to emit electromagnetic radiation in the Middle Ultraviolet (MUV) region (200-300 nm). Preferably, the first irradiation source is formed by a LED UV lamp, preferably one that emits a wavelength of 254 nm, and / or 265 nm, and / or 280 nm. It is imaginable that the wavelength is adjusted during use of the lamp, and in particular during (partial) curing of the liquid layer 2 and the ink droplets 3. During this (first) curing step, the liquid layer 2 will typically be more cured (to a higher curing level) than the droplets 3. The heat generated by the first irradiation source will force evaporation of at least a part of the water (H2O) fraction of the aqueous ink droplets 3. This will evaporation may be accompanied by a micro-explosion of water molecules within the droplets 3 which may deformed the interface between the droplets 3 and the surrounding layer 2. The evaporation of water from the droplets 3 will dry the droplets and - dependent on the exact composition of the droplets 3 - may make the residual dry fraction of the droplets 3 relatively brittle. This curing step as shown in figure 4 can be realized at relatively high speed, such as 30 meter per minute, which is also referred to as rapid curing.

[0064] In a next step, as shown in figure 5 the top surface of the panel 1 (including the applied decorative top structure 2, 3) will be subjected to a brushing action in a brushing station 22 by using one or more axially rotating (or rotatable) brushes 23. Preferably, the brush(es) is / are rotated in a direction opposite to the movement direction of the panel 1 to intensify the brushing action. The brush roller(s) may comprise fibers at least partially made of metal, such as steel, and / or polymer, such as nylon, and / or textile, and / or animal hair, and / or natural fibers. In this embodiment, the brush(es) rotate(s) with a rotation speed of 1 ,460 rpm. Preferably, the brush(es) is / are also alternately moved in lateral direction (sideward direction), more preferably with a speed of approximately 2,5 meter per minute. This brushing action removes at least a residual fraction, preferably the entire fraction, of the ink droplets 3, thereby creating partially or entirely empty recessed microstructures 4 (micro pockets) and / or nanostructures 5 (nano pockets), as in shown in more detail in figure 5a. Due to the nanostructured or microstructured (textured) surfaces of said recessed microstructures (and / or said nanostructures) a diffuse reflection will occur which will be result optically in a matt expression. This is, for example, beneficial in case the structured layer 2 is used to imitate a 3D wood nerve pattern. As indicated with arrow B, dust may be discharge by using a vacuum discharge in the brushing station 22.

[0065] In a further step, as shown in figure 6, the panel 1 is guided along a second coating station 24 to apply a top coating 6 (in liquid) on top of the non-recessed parts (unstructured parts) of the structured layer 2, and preferably only on these parts. The top coating 6 may consist of a single layer or of multiple layers. The top coating 6 may be applied, e.g. by means of spraying and / or roller, and / or by means of printing, preferably digital printing.

[0066] In a subsequent step (figure 7), the top coating 6 is cured by subjecting the top coating 6 to one or more irradiating sources, such as mercury lamps (Hg UV lamps) of a second curing station 25, which preferably to expose the top coating 6 (and layers underneath said coating 6) to wavelengths between 300 and 400 nm. During this curing step, the top coating is preferably fully cured. During this curing step, the semi-cured structured layer 2 may also become more cured, and even fully cured. The top coating, at least once cured, is transparent. The same applies to the structured layer 2.

[0067] As shown in figure 8, at least one panel edge can be provided with a bevel 7 and / or a grout line, which may be caused by a recessed edge portion of the panel, which can e.g. be a compressed edge portion and / or a portion where material has been removed. The bevel and / or grout line may have an inclined planar and / or inclined curved and / or inclined angular surface. The structured layer 2 will follow the shape of the (part of the) panel 1 positioned underneath. The bevel 6 and / or grout line may be provided with one or more microstructures to provide the bevel 6 and / or grout line with a matt appearance.

[0068] Figure 9 schematically shows a perspective view of a panel according to the invention, which may be the panel 1 as shown in figure 7. The panel 1 comprises a core 8, a backing layer 9 attached, e.g. glued or fused, to a lower side of the core, and a decorative print layer (2D) 10 attached, directly or indirectly, to an upper side of the core. Preferably, the decorative print layer contains a digitally printed image. Preferably, the decorative print layer is covered by one or more transparent wear layers (not shown), on top of which the structured layer 2 and the (discontinuous or interrupted) top coating 6 as shown in figure 7 are applied. The core 7 further comprises complementary coupling profiles 11 at at least one pair, and preferably each pair, of opposite panel edges. The coupling profiles 11 enable the mutual locking of decorative panels 1 for forming a floor covering, wall covering, ceiling covering or furniture covering. Preferably, the coupling 11 profiles are configured to interlock adjacent decorative panels both in a direction parallel to a plane defined by the panels 1 and / or in a direction perpendicular to said plane defined by the panels 1 . The backing layer may e.g. be a polymer based layer and / or a cork layer. The one or more wear layers are preferably UV cured coating layers. Preferably, the decorative image of the decorative layer 10 represents a wood pattern, a tile pattern, a marble pattern, a natural stone pattern, or a concrete pattern. Preferably, the embossing of the structured layer 2, as defined by the recessed structures (microstructures and / or nanostructures) is at least partially, and possibly entirely, aligned (synchronized) with said decorative image to create a realistic texture that closely matches the visual pattern of the panel 1. The panel 1 may for example be used as floor panel or wall panel. The panel may be provided with at least one bevel and / or grout line at at least one panel edge, as e.g. shown in figure 8.

[0069] The verb “comprise” and conjugations thereof used in this patent publication are understood to mean not only “comprise”, but are also understood to mean the phrases “contain”, “substantially consist of”, “formed by” and conjugations thereof.

[0070] The ordinal numbers used in this document, like “first”, “second” are used only for identification purposes. By "horizontal", it is meant as a direction which extends parallel to a plane defined by the panel, and which may intersect a core of the panel. By “vertical”, it is meant as a direction which is perpendicular to said plane defined by the panel. By "complementary" coupling parts, it is meant that these coupling parts of adjacent panels can cooperate with each other. However, to this end, the complementary coupling parts do not necessarily have to have fully complementary forms (inverted designs). The expression “panel” may be replaced by the expression “workpiece” in this disclosure, as also other workpieces than panels may be subjected to the method according to the invention. The “water based structuring ink” comprises a water fraction, although it is imaginable that the water fraction is replaced by another polar solvent, such as ethanol, isopropanol, glycol ether, glycol, acetone, and dimethyl sulfoxide, as a result of which the expression “water based structuring ink” could be replaced by the more generic expression “polar solvent based structuring ink”.

Claims

Claims1. A method for producing a decorative surface on a panel, comprising the steps of:A) applying a curable liquid layer to be structured onto a top surface of said panel, wherein said liquid layer is substantially immiscible with water,B) position-selectively printing, in liquid state, water based structuring ink droplets onto a part of a top surface of said liquid layer to be structured, wherein the density of the structuring ink droplets is equal to or greater than the density of the liquid layer to be structured allowing the water based structuring ink droplets to partially sink into the liquid structure layer, wherein said structuring ink droplets are configured to at least partially absorb electromagnetic radiation, or which, in contact with the liquid layer, produce at least one reaction product which is capable of at least partially absorbing electromagnetic radiation irradiated during step C), and / or are less curable compared to the liquid layer and / or are non-curable when irradiated with electromagnetic radiation, in particular in the UV region, during stepC);C) irradiating the surface of the liquid layer and the applied structuring ink droplets with electromagnetic radiation, in particular in the UV region, wherein the liquid layer will be cured at least partially, and wherein at least a fraction of water present in the water based structuring ink will evaporate resulting in a formation of at least one recessed diffractive microstructure in the at least partially cured structured layer at one or more locations where the structuring ink is printed during step B),D) removing, in particular mechanically removing, at least a part of residual fractions of the structuring ink from the at least partially cured structured layer to at least partially expose the at least one recessed diffractive microstructure,E) applying a curable liquid top coating onto an unstructured portion of the at least partially cured structured layer, while the structured portion, defined by said at least one recessed diffractive microstructure, of the at least partially cured structured layer is substantially left uncovered by said top coating, andF) irradiating the surface of said top coating at least once with electromagnetic radiation, in particular in the UV region, to cure the top coating.

2. Method according to claim 1 , wherein the ratio of the density of the liquid structuring ink droplets and the density of the liquid layer to be structured is situated between 1 and 1.2.

3. Method according to claim 1 or 2, wherein the surface tension of the liquid structuring ink droplets is substantially equal to the surface tension of the liquid layer to be structured.

4. Method according to any of the previous claims, wherein during step B) at least a fraction of the ink droplets applied is partially situated within the liquid layer, such that between 40% and 60% of an outer surface of each of these droplets is covered by the liquid layer.

5. Method according to any of the previous claims, wherein during step B) the ink droplets have a volume 0.1 pl to 80 pl and / or wherein the ink droplets have an average diameter of 50 to 500 pm.

6. Method according to any of the previous claims, wherein during step B) the ink droplets are applied with a speed of between 0.5 m / s and 12 m / s, preferably between 3 m / s and 7 m / s, more preferably between 5 m / s and 6 m / s.

7. Method according to any of the previous claims, wherein during step B) the structuring ink droplets are applied by digital printing.

8. Method according to any of the previous claims, wherein the at least one recessed diffractive microstructure formed in the structured layer during step C) has at least partially a nanostructured surface.

9. Method according to any of the previous claims, wherein during step C) the liquid layer is partially cured, and wherein the structuring ink droplets are preferably also partially cured.

10. Method according to any of the previous claims, wherein the electromagnetic radiation used during step C) has a wavelength below 300 nm, wherein preferably use is made of at least one LED UV radiation source.11 . Method according to any of the previous claims, wherein the electromagnetic radiation used during step C) has a wavelength which is adjusted during the execution of step C) .

12. Method according to any of the previous claims, wherein during step C) each portion of the liquid layer and ink droplets is irradiated at least to times by the electromagnetic radiation.

13. Method according to any of the previous claims, wherein during step D) at least a part of residual fractions of the structuring ink is removed from the at least partially cured structured layer by means of at least one brush roller, in particular at least one textile brush roller and / or at least one nylon brush roller.

14. Method according to claim 13, wherein during step D) the brush roller has a rotation speed between 1 ,000 and 2,000 rpm.

15. Method according to claim 13 or 14, wherein during step D) the brush roller is alternately laterally displaced in an axial direction of the brush roller, wherein the lateral movement speed is preferably situated between 2 and 3 meter per minute.

16. Method according to any of the previous claims, wherein during step E) the top coating is applied by means of digital printing and / or by means of rolling.

17. Method according to any of the previous claims, wherein the electromagnetic radiation used during step F) has a wavelength greater than 300 nm, wherein preferably use is made of at least one Hg UV radiation source.

18. Method according to any of the previous claims, wherein the structured layer formed during step C) is transparent and / or translucent.

19. Method according to any of the previous claims, wherein the at least recess microstructure scatters light reflection resulting in an optically matte impression.

20. Method according to any of the previous claims, wherein the at least recess microstructure has a gloss level of 0 to 20 Gloss Units (GU) as determined in accordance with the method according to DIN EN ISO 2813:2015-02.21 . Method according to any of the previous claims, wherein during step C) a plurality of microstructures is formed, which preferably together visualize a wood nerve pattern.

22. Method according to any of the previous claims, wherein the top coating has a higher gloss level than the at least one recessed microstructure.

23. Method according to any of the previous claims, wherein the top coating has a gloss level of at least 60 Gloss Units (GU) as determined in accordance with the method according to DIN EN ISO 2813:2015-02.

24. Method according to any of the previous claims, wherein the top coating has a smooth upper surface.

25. Panel, in particular decorative panel, produced by the method according to any of the previous claims, wherein the panel comprises: a core a decorative print layer applied, directly or indirectly, on top of said core, the structured layer applied, directly or indirectly, on top of said decorative print layer, and the top coating applied on top of said structured layer.

26. Panel according to claim 25, wherein the panel comprises at least one transparent and / or translucent wear layer situated in between the decorative layer and the structured layer.

27. Panel according to claim 25 or 26, wherein the at least one recessed microstructure of the structured layer is at least partially synchronized with a image represented by the decorative print layer.

28. Panel according to any of claims 25-27, wherein the panel comprises at least one grout line or bevel at at least one edge, wherein at least one recessed microstructure of the structured layer is located on said grout line or on said bevel, such that at least a part of an upper surface of said grout line or of said bevel is defined by said at least one recessed microstructure.

29. Panel according to any of claims 25-28, wherein the panel comprises coupling profiles at at least one pair of opposing panel edges to allow mechanical intercoupling of adjacent panels.

30. Panel covering comprising a plurality of mechanically intercoupled panels according to claim 29.31 . System for carrying out the method according to any of claims 1 -24, comprising: at least one coating station for coating a panel with a liquid layer to be structured, at least one structuring ink printing station to position-selectively print, preferably digitally print, ink droplets onto the liquid layer to be structured, at least one first curing station configured to irradiate the liquid layer to be structured and applied onto the panel with electromagnetic radiation, preferably within the UV region, to at least partially cure and to microstructure said layer, at least one brushing station for removal of at least a part of a residual fraction of the structuring ink from the structured layer, at least one top coating station for applying a top coating onto of the brushed structured layer, at least one second curing station configured to irradiate at least the top coating to cure said top coating, and at least one conveyor to successively displace the panel along each station.

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