[Technical field of the invention]

The present invention concerns a method for manufacturing embossed decorative substrates, notably panels and boards, in particular based on wood fibers such as MDF (Medium Density Fiberboard) or HDF (High Density Fiberboard) substrates on which a decor is formed by digital printing.

The invention further relates to substrates such as panels and boards, notably useful as flooring, that are obtainable by such methods.

[Background of the invention]

Digital printing refers to methods of printing from a digital based image directly to the substrate. It usually refers to professional printing where small run jobs from desktop publishing and other digital sources are printed using large format and/or high volume laser or inkjet printers. Digital printing has a higher cost per printed object than traditional offset printing methods but this price is usually offset by the cost saving achieved by avoiding all the technical steps in between needed to make printing plates. It also allows for on demand printing, short turn around, and even a modification of the image with each printed copy. The savings in labour and the increased capability of digital presses means digital printing is reaching a point where it could match or supersede offset printing technology's ability to produce larger print runs of several thousand copies at a low price.

However, when using digital printing on substrates such as high density fiberboard, the decor is generally still printed on a separate support of paper or fleece which is applied onto the substrate. Indeed, while a basic layer may be applied in order to ensure sufficient adherence of the applied inks, such basic layers may affect the printing quality. Indeed, a porous layer may absorb the ink leading to insufficient covering while a non-porous basic layer may lead to their coalescence and affect color quality. The use of a separate support for printing however makes the process more costly and complex.

Typically, the manufacture of floors or similar products comprises an embossing step carried out after the printing (for example digital printing) of a decor on wood-based panels and boards such as wood particles boards, MDF or HDF panels. Therefore, embossed reliefs are directly transferred on top coat (lacquer, liquid...) when engraved rollers are used, or on top layer (melamine, paper, foil...) when short cycle press is used. Thus, the embossed reliefs are not transferred directly on the substrate, but rather on the top coat/top layer. Consequently, known floors, such as standard laminates, exhibit embossed reliefs with a maximum depth limited to the thickness of said top coat/top layer, which is about 30-300 μηι.

There is thus a need for alternative processes allowing reaching floors exhibiting deeper embossed surfaces.

The present invention aims to provide a method for the manufacture of substrates comprising a decor formed by digital printing, exhibiting deep embossed reliefs/embossed impressions.

The present invention also aims to provide a method for the manufacture of such substrates which allows for a good image quality.

The present invention also aims to provide a method for the manufacture of such substrates which exhibit an improved natural appearance.

[Detailed description of the invention]

The present invention relates to a method for the manufacture of wood fiber based decorative substrates, comprising the steps of:

a) mechanical embossing a substrate so as to form an embossed surface ;

b) applying a primer on the embossed surface of the substrate ;

c) drying or curing the primer coated on the embossed surface so as to form a base layer;

d) digitally printing a decor onto the base layer using a liquid ink; and

e) drying or curing the liquid ink forming the digitally printed decor.

The substrates may be in particular chosen from the group consisting of panels and boards.

Wood-based boards, in particular wood-fiber boards are preferably used.

In one embodiment, the substrates are chosen from the group consisting of HDF, MDF, cardboards and WPC (Wood Plastic Composite). In particular, the substrate is HDF.

In case of floor panels, the printed fiberboards obtained in the process are cut to form a final floor panel that preferably has a surface of less than 0.5 m ² , or of even less than 0.3 m ² . Prior to applying the method according to the invention, the surface of the substrates may be filled with a material layer comprising a filler in order to obtain an even surface. In the method of the invention, step a) may be carried out with engraved embossing cylinders or press plates.

In one embodiment, the embossing step (a) is carried out with hot or cold embossing cylinders. In particular, the hot embossing cylinders are cylinders heated to a temperature comprised from 120 C° to 170°C, preferably up to 150°C. In particular, the cold embossing cylinders are cylinders used from 20°C to 35°C.

Advantageously, both hot and cold embossing cylinders transfer relief on substrate with no reverse deformation on substrate.

Typically, embossing cylinders may be engraved embossing cylinders which allow transferring the relief of said cylinders onto the substrate. The embossing cylinders may have a surface structure relief up to 12 mm, preferably up to 1 .5 mm.

Preferably, when embossing cylinders are used, the mechanical embossing step (a) is carried out with a transportation speed of the substrate comprised between 5 and 20 m/min, preferably at 8 m/min. In one embodiment, step (a) is carried out with press plates, for example with short cycle press.

Preferably, short cycle press is heated to temperature in the range of 30°C to 300°C, preferably in the range of 120°C to 250°C.

Preferably, a heating for a period comprised between 2 seconds to 10 minutes, preferably 2 to 30 seconds is sufficient.

In the method of the invention, the embossing step (a) is advantageously carried out directly on the substrate, and not on any top coat/top layer present on the substrate. Thus, the embossed surface is advantageously present directly on the substrate.

In the method of the invention, the embossing step (a) is thus carried prior to the digital printing step.

In one embodiment, the step (a) is carried out at least on one side of the substrate. In particular, the embossed surface is present on at least one side of the substrate.

In one embodiment, the primer is selected from the group consisting of transparent or tinted UV lacquer, transparent or tinted UV oil, water-based stains, and mixtures thereof.

A UV-priming material consists preferably essentially of UV curable lacquer, pigments, hardener, reactive diluents or radical formers as chain starter. The water-based pigmented primer contains preferable at least one pigment of a bright color, in particular preferably at least one white pigment. Preferably, the primer is a white tinted UV lacquer.

In particular, the primer is a transparent UV lacquer known under the trade name UV BASE COAT commercialized by Akzo Nobel, or the primer is a tinted UV lacquer known under the trade name UV BASE COAT WHITE commercialized by Akzo Nobel.

In particular, the primer is a water-based stain known under the trade name AQUA PIGMENTS STAIN NORDIC WHITE commercialized by Akzo Nobel, or a transparent UV oil known as OIL BASE COAT commercialized by Osmo Polyx.

The applied quantity of primer used in the method of the invention may be comprised between 5 and 50 g/m ² , preferably between 25 and 30 g/m ² .

Advantageously, the primer allows a good transfer of the decor inside the embossing impressions, such as holes, of the embossed surface. Besides, the primer also advantageously achieves adhesion between the embossed surface and the digitally printed decor.

In one embodiment, in step (c), the coated substrate with UV-priming material is pre- gelled by UV radiation.

In one embodiment, the coated substrate with water-based pigmented primer is dried in convection dryer by heating the coated substrate to a temperature in the range of 50°C to 90°C, preferably to a temperature in the range of 60°C to 80°C.

In one embodiment, the coated substrate with water-based pigmented primer is dried by infrared dryer.

In step (d), the substrate coated with a base layer is submitted to digital printing to form the decor.

Generally, the printed decor represents a wood pattern. Other patterns such as stone patterns, parquet imitations or fantasy patterns are however also possible. As digital printing is very versatile, any pattern may be printed, even for small quantities and within short time and without requiring an exterior provider such as a printing company.

In step (d), the liquid ink may be a water-based ink or an UV ink. Water-based inks are less expensive than solvent-based inks. Thus, water-based inks are preferred for said digital printing. Preferably, said digital printing techniques are of the type which can deposit ink droplets in a point-wise manner, such as this is the case, for example, with Drop-on-Demand (DOD) inkjet systems.

In a preferred embodiment, the substrate has during printing a speed of more than 10 m/min and better of more than 80 m/min, or in particular up to 150 m/min.

In another preferred embodiment, the substrate is standing still and the printhead of the inkjet system are passing over the substrate.

In step (e), the ink forming the printed decor is dried or cured. Drying of the water- based ink may be effected under ambient conditions or optionally be accelerated using conventional means such as a heater, a fan or an infrared dryer. The time required for the ink to dry depends on a number of factors such as the composition of the ink, its water content, the drop size, and of course, temperature of the surface, temperature and humidity of the surrounding air, and the amount of air circulation in the immediate area of the printed decor.

In particular, UV inks are cured by UV radiation.

The quantity of digital printing ink may be between 5 and 15 g/m ² , preferably between 6 and 8 g/m ² .

Preferably, the digital printing is a direct digital printing, and do not require the use of a separate support of paper or fleece which is applied onto the substrate.

According to the invention, the decor is digitally printed onto the base layer, said base layer lying onto the embossed surface of the substrate. In particular, the resulting decorative substrates comprises an embossed surface formed directly on the substrate, said embossed surface being below the base layer and the decorative layer (digitally printed decor).

In one embodiment, the method of the invention further comprises a step consisting in: f) coating the digitally printed decor with a top coat or finishing layer.

Preferably, the top coat is selected from the group consisting of lacquers, oils and resins. Among the oils, mention made be for example of UV or oxidative oils.

Preferably, the top coat is based on an amino resin, such as a melamine provided in the form of a resin or sheet. As an alternative, translucent or transparent lacquers may also be used. Preferably, the top coat is coated onto the board in liquid form, for example, by means of one or more rollers. Alternatively, the top coat is coated onto the board by using melamine sheets.

Said top coat further may also comprise hard particles, such as aluminum oxide, by which an improved wear resistance is obtained at the surface of the final substrate. This top coat shows an improved abrasion resistance than a lacquer. Preferably, such particles are provided on the still wet top coat.

The final thickness of the top coat is generally comprised between 30 and 300, preferably between 50 and 220 μηι.

According to one embodiment, in order to improve abrasion resistance on top smooth surface (walking area), a polyurethane top coat is applied according to the Kleiberit HotCoating® technology, and UV lacquer is applied, in particular by sponge roller, in deep embossed area. According to this embodiment, no hot coat is applied in embossed areas in order not to lose the embossed structure.

The Kleiberit HotCoating® technology consists in the application of KLEIBERIT PUR

HC 717 to the surface. KLEIBERIT PUR HC 717 is solid at room temperature.

In one embodiment, the method of the invention further comprises a step consisting in: g) drying or curing the top coat. In particular, the top coat with UV material is cured by UV radiation.

The conditions required to ensure curing of the top coat notably based on melamine sheets, include in particular the temperature range, duration and pressure. Suitable temperature range, duration and pressure for curing the top coat may vary as they are function notably of the specific polymer and hardener used. However, generally, a temperature in the range of 100 to 300°C, preferably 120°C to 250°C is appropriate. A heating for a period comprised between 2 seconds to 10 minutes, preferably 2 to 30 seconds is generally sufficient. Further, a pressure of 20 to 60 kg/cm ² is preferred.

In one embodiment, the method of the invention, further comprises the step consisting of:

h) providing the back side of the substrate with a balance paper or a layer of lacquer. In particular, the layer of lacquer is selected from the group consisting of: tinted UV lacquer, transparent UV lacquer, tinted UV oil, transparent UV oil, water-based stains, and mixtures thereof. According to an embodiment, in the method of the invention, step h) consists in providing the back side of the substrate with a PES needle felt with woven backing (PP). Such step is carried out in order to improve surface soundness and water resistance properties.

Currently, during the installation of a standard laminate and/or parquet, a foil is laid between "ground" and flooring. According to this embodiment, the PES needle felt with woven backing replaces the foil.

In one embodiment, the embossed surface comprises embossed impressions extending to a maximum depth of up to the thickness of the substrate, for example of up to 12 mm in particular when the substrate is HDF. Preferably, the embossed impressions are selected from the group consisting of holes, cracks, pores, and mixture thereof.

According to the invention, the decor is digitally printed on the embossed surface covered with the basic layer.

In one embodiment, the digitally printed decor is in register with the embossed impressions. In particular, during the digital printing step, the embossed surface is positioned under the printing heads in order to apply the digitally printed decor in exact positions on embossed impressions.

The inventors found that the method according to the invention advantageously allows the preparation of floors with deep embossed surfaces, and more particularly with deeper embossed surfaces than in existing known floors.

More particularly, the method according to the invention advantageously allows the formation of embossed surface directly on the substrate, and not on top coat or top layer such as in typical known processes. Thus, the embossed impressions of the embossed surface of the substrate have a maximum depth limited to the thickness of the substrate and not limited to the thickness of the top coat. In particular, the method of the invention allows the obtaining of substrate with embossed impressions having a maximum depth of up to 12 mm (thickness of HDF), preferably between 0.8 mm and 12 mm, more preferably between 1 mm and 12 mm. The method also advantageously allows the obtaining of floors with good image quality, and which look more natural than existing products. In particular, the method allows a good transfer of the decor on/inside the embossing impressions of the embossed surface. Advantageously a highly realistic imitation of wood pattern is attained.

The present invention also concerns a floor panel or board obtained according to the method as defined above.

The present invention concerns a floor panel or board comprising:

i) an embossed wood fiber based substrate;

ii) a base layer coated onto the embossed substrate;

iii) a digitally printed decor coated onto the base layer; and

iv) a top coat layer coated onto the digitally printed decor.

As used herein, the "embossed wood fiber based substrate" means a wood fiber based substrate comprising an embossed surface, said embossed surface comprising at least one embossing impression.

In particular, the embossed surface is directly present on the surface of the substrate, and not on top coat/top layer.

In one embodiment, the substrate is embossed to a maximum depth of up to the thickness of the substrate. In particular, the substrate is embossed to a maximum depth of 12 mm.

Preferably, the embossed surface of the substrate comprises at least one embossed impression having a depth comprises from 1 mm to 12 mm, preferably of 1 mm or 1 .5 mm.

Preferably, in the present invention, the substrate, at the location of the embossed impressions, is at least embossed over 0.5 mm, preferably over 0.8 mm. In particular, the substrate, at the location of the embossed impressions, is embossed to a depth comprised from 1 mm to 12 mm, preferably to a depth of 1 mm or 1 .5 mm.

[Brief description of the drawings]

Figure 1 schematically represent a substrate manufactured using a preferred embodiment of the method according to the invention.

Primer can be applied as a filler, both to cover and to fill in the embossed impressions of the embossed surface, as in particular shown in Figure 1 . Indeed, in Figure 1 , the basic layer 3 covers entirely the embossed surface S, and in particular fill the embossing impressions E such as the wholes, cracks, pores, and mixture thereof.

Figure 2 schematically represent a substrate manufactured using another preferred embodiment of the method according to the invention. In this figure 2, the basic layer 3 preferably covers but not fills in the embossed impressions E of the embossed surface S. Preferably, the primer is applied by soft rubber roller (25 ShA).

According to a preferred embodiment of the method of the invention shown in Fig. 1 , a panel 1 is manufactured as follows.

A substrate 2 made of a high-density fiberboard (HDF) is embossed in particular with engraved cylinders or short cycle press, to form an embossed surface on the substrate. The embossed surface is coated with a reactive composition such as transparent or tinted UV lacquer, transparent or tinted UV oil, water-based stains, and mixtures thereof, which forms a basic layer 3.

More specifically, the reactive composition is then applied in one application step onto the substrate 2 by a suitable means, for instance by way of a roller.

The applied quantity of primer used in the method of the invention may be comprised between 5 and 50 g/m ² , preferably between 25 and 30 g/m ² . In one embodiment the coated substrate with UV-priming material is pre-gelled by UV radiation. In one embodiment, the coated substrate with water-based pigmented primer is dried in convection dryer by heating the coated substrate to a temperature in the range of 50°C to 90°C, preferably to a temperature in the range of 60°C to 80°C. In one embodiment, the coated substrate with water-based pigmented primer is dried by infrared dryer.

The embossed surface of the substrate 2 thus coated with a basic layer 3 is then provided with a decor 4 using digital printing and water-based ink or UV ink, using for instance a Drop-on-Demand (DOD) inkjet system. The specific properties of the basic layer 3 improve adherence of the ink on the substrate 2 while preventing excessive absorbing or on the contrary coalescence of the ink drops forming the decor 4.

In one embodiment after printing the decor 4 with water-based ink, the panel 1 is introduced into an oven and heated to a temperature of 70 to 250°C, for a period of 2 to 30 seconds in view of drying and/or curing the ink drops forming the decor 4. In one embodiment UV inks are cured by UV radiation.

Subsequently, the decor applied onto the panel 1 is protected by application of a top coat 5, notably based on an amino resin, in particular a melamine resin. Such resin may be coated onto the panel 1 in liquid form, notably by means of one or more rollers. Alternatively the overlay is provided onto the panel 1 in solid form, notably by means of one or more sheets. The dimensions of the overlay sheet are chosen according to the dimensions of the panel. As an example, the overlay sheet has a length of 2620 mm, a width of 2070 mm and a grammage of 150 gr/m ² .

After drying, if a resin is used, the top coat is submitted to a press operation under a pressure of about 60 kg/cm ² at a temperature of between 100 and 300 °C for a period of 10 seconds. If required, the water based ink may also be cured during this step. After curing, the basic layer 3 has notable adhesive properties while however maintaining its elasticity and flexibility. Thereby, cohesion between the different layers of the panel 1 is reinforced and the risk of delamination, notably under stress, is reduced.

In particular, the top coat with UV material is cured by UV radiation.

The final thickness of the top coat 5 is generally comprised between 2 and 100, preferably between 5 and 50 μηι. In the case of manufacturing floor panels, profiled edge regions may be formed at the panels, either before or after applying the process of the invention.

The back of the panel may further be protected by application of a suitable protection layer 6 such as a balance paper or of a suitable lacquer.