METHOD OF MANUFACTURING A WEAR RESISTANT SURFACE AND

PRODUCT THEREOF

Technical field

The disclosure relates to the field of digitally created wear resistant surfaces for building panels such as floor and furniture components. The disclosure relates to hard wear resistant particles that are positioned in pre-determined patterns on a surface.

Field of Application

Embodiments of the present invention are particularly suitable for use in floors, which are formed of floor panels comprising a core, a decorative layer and a transparent wear resistant protective layer above the decorative layer. Preferred embodiments are conventional laminate floors, powder based floor, wood floors, plastic based Luxury Vinyl Tile (LVT) floors and ceramic tiles. The following description of techniques, problems of known technology and objects and features of the invention will therefore, as a non-restrictive example, be aimed above all at this field of application and in particular at floorings which are similar to conventional laminated floorings. The embodiments of the invention may also be used to produce wear resistant surfaces on any essentially flat panels preferably furniture components.

Background

The following description is used to describe the background and products, materials and production methods that may comprise specific parts of preferred embodiments in the disclosure of this invention.

The majority of all laminate floors are produced according to a production method generally referred to as Direct Pressed Laminate (DPL). Such laminated floors have a core of 6 -12 mm fibreboard, a 0.2 mm thick upper decorative surface layer of laminate and a 0.1-0.2 mm thick lower balancing layer. The surface layer of a laminate floor is characterized in that the decorative and wear properties are generally obtained with two separate layers of paper, one above the other. The decorative layer is generally a melamine formaldehyde (hereafter shortened to melamine) impregnated printed paper and the wear layer is a melamine impregnated transparent overlay paper, which comprises small wear resistant aluminium oxide particles such as corundum, hereafter shortened to aluminium oxide.

The overlay paper is made of pure cellulose, which is based on delignified pulp. The overlay paper becomes almost completely transparent after lamination and the appearance of the decor paper is visible. Thicker overlay papers with a considerable amount of aluminium oxide particles may give a high wear resistance. The

disadvantage is that they are less transparent and a grey layer that disturbs the printed pattern covers the decorative pattern.

The wear resistant aluminium oxide particles may be included in an overlay paper in several ways during impregnation. They may be mixed into the liquid melamine resin or scattered on the wet overlay paper. Paper based overlay may be replaced with a liquid overlay comprising a mix of aluminium oxide particles and liquid melamine resin that is applied on the impregnated decor paper.

The printed decorative paper and the overlay are laminated to a HDF core in large discontinuous or continuous laminate presses where the resin cures under high heat (about 170 degrees C) and pressure (40 - 60 bars) and the papers are laminated to the core material. An embossed press plate or steal belt forms the surface structure.

Sometimes a structured paper is used as a press matrix. The embossing is in high quality floors made in register with the design.

Laminated floors may also be produced with direct printing technology. Hydro printing inks are used to print the decor by a multicolour printing press. The print is covered with a protective transparent wear layer that may be an overlay, a plastic foil or a lacquer that may comprise wear resistant particles.

Direct printing technology may be replaced with digital printing technology that is much more flexible and small production volumes can be economically manufactured. The difference between these two methods is mainly the printing step where the printing rollers are replaced by a digital non-contact printing process. Recently new "paper free" floor types have been developed with solid surfaces comprising a substantially homogenous powder mix of fibres, binders and wear resistant particles hereafter referred to as WFF (Wood Fibre Floor).

The powder mix may comprise aluminium oxide particles, melamine resins and wood fibres. In most applications colour pigments are included in the mix and all these materials are applied in dry form as a mixed powder on a FIDF core and cured under heat and pressure to a 0.1 - 1.0 mm solid layer. Several advantages over known technology and especially over conventional laminate floorings may be obtained such as increased wear and impact resistance, deep embossing, increased production flexibility and lower costs. Digital powder printing has been developed and it is possible to create very advanced designs by injecting ink into the powder prior to pressing. The powder layer may include one or several powder based base colours and digital ink jet printing may only produce a small part of the total decor. A powder overlay comprising a mix of transparent fibres, wear resistant particles and melamine powder may be used to increase the wear resistance of the digital print. Such protective layer is applied even on the base layer where it is not needed since a base layer comprising wear resistant particles have sufficient wear resistance.

Wood floors are delivered as pre-finished floors with a wood surface that is coated with several transparent layers in the factory. The coating may be made with UV cured polyurethane that comprises wear restante particles.

Ceramic tiles are one of the major materials used for flooring and wall coverings. A tile body comprising clay minerals is covered with one or several layers of glaze that may comprise wear resistant particles.

Luxury Vinyl Tiles, generally referred to as LVT floorings, are constructed as a layered product. The base layer is made primarily of PVC mixed with chalk filler in order to reduce material costs. The base layer has a high quality printed decorative PVC foil on the upper side. A transparent wear layer of vinyl with a thickness of 0.2 - 0.6 mm is generally applied on the decorative foil. The transparent layer may include a coating of polyurethane, which provides additional wear and stain resistance. Such polyurethane layer may comprise wear resistant particles.

As a summary it may be mentioned that wear resistant particles, especially aluminium oxide, are used in many floor types in order to increase the wear resistance of the floor surface.

Definition of Some Terms

In the following text, the visible surface of the installed floor panel is called "front side", while the opposite side of the floor panel, facing the sub floor, is called "rear side ". By "up " is meant towards the front side and by "down " towards the rear side. By "vertically" is meant perpendicular to the surface and by "horizontally" parallel to the surface.

By "binder" is meant a substance that connects or contributes to connect two particles or materials. A binder may be liquid, powder based, a thermosetting or thermoplastic resin and similar. A binder may comprise two components that react when in contact with each other.

By "digital printing" is meant a digitally controlled ejection of drops of fluid comprising a colorant from a print head onto a surface. By "paneF is meant a sheet shaped material with a length and width that is larger than the thickness. This rather broad definition covers for example laminate and wood floors, tiles, LVT, sheet shaped wall coverings and furniture components.

Known Technique and Problems thereof

The general technologies, which are used by the flooring industry to provide a wear resistant surface, are mainly based on applying wear resistant particles such as aluminium oxide on an upper part of the floor surface. The particles are applied at random. Due to production tolerances, some parts of the surface may comprise larger amounts than other part and the average amount is generally higher than needed.

Clusters of particles may create grey spots and unwanted shadings. Laminate floors are produced as large sheets that are cut into several panels. Wear resistant particles are applied over the whole sheet and even on areas where the saw blade cuts the sheet into individual panels and where parts of the surface is removed when the locking systems are formed. This creates high wear on the saw blades and on the milling tools. The surface is generally embossed with low and high portions. The wear on the high portions is much higher that on the low portions.

Powder based digitally printed floors may comprise much more wear resistant particles than necessary if they are covered by a powder overlay that covers even the unprinted parts where no protective layer is needed.

It would be a major advantage if the wear resistant particles may be applied in a more precise way and especially if they may be applied in well-defined pre-determined patterns that may cover parts of the floor surface. It is known from pre-published material (IPCOM 000224950D, which is incorporated by reference) and from the web site of Valinge innovation AB that particles may be applied in well-defined patterns with a combination of blank and dry ink. A cost efficient method to apply wear resistant particles on specific surface portions is not described.

WO 201 1/107610 describes a method to create an elevation or an embossing on a floor panel in order to avoid the use of expensive press plates. The method is the same as known methods to create a raised print or 3D structures with powder and a digital print head. It describes a method to produce a floorboard by digitally printing a curable substance for creating an elevation on the panel and not a wear resistant layer. The curable substance may comprise wear resistant particles, for example corundum. The curable substance may be cured by UV radiation or may be a varnish. The wear resistant particles are randomly mixed with the powder and are not applied in a predetermined pattern. The method is not suitable for flooring applications where the surface after printing is cured under heat and pressure.

EP 2 213 476 Al describes that a pre-determined pattern may be digitally printed on a carrier with curable liquid so as to form an embossing decoration pattern, which is pressed on the overlay. The curable liquid may be a plastic, which becomes rather rigid after curing, for example, a plastic containing ink. The method is not used to apply wear resistant particles and is not suitable for floor applications. The digital printing head can only print a very thin layer with a thickness of about 10 - 20 micron.

Thicknesses of at least 100 - 200 micron that are required to form an embossing in laminate and 200- 700 micron to match the requirement of powder based floors are not possible to produce in an economic way. A method to print on a lower side of an overlay is also described. Such print will not increase the wear resistance of the overlay.

Objects and Summary

The main objective of at least certain embodiments of the invention is to provide an improved and cost efficient wear resistant protective layer comprising wear resistant particles.

The invention is based on a main principle where application of the wear resistant particles is divided in two separate steps. The particles are applied on a surface. Some particles are bonded by a preferably digitally formed pattern. Other non-bonded particles are removed and the remaining bonded particles form a pre-determined pattern of wear resistant particles. This two-step process may be repeated and several layers of wear and scratch resistant particles may be applied such that an advanced wear resistant layer with particles spaced from each other with pre-determined distances may be formed.

An advantage compared to conventional random applications is that application of the wear resistant particles may be made in a controlled and very precise way. Contrary to known technology wear resistant particles may be evenly distributed and applied in precise digitally formed raster patterns and only on surface portions where they are needed and in amounts that are adapted to the wear properties of the underlying surface portions and to the wear intensity that surface portions are exposed to, for example, edge portions and upper portions of embossed surfaces where the wear is considerably higher than in other parts of the floor. The invention may provide wear resistant surfaces with surface portions comprising variations in wear properties, scratch resistant properties and gloss levels. Surface portion that are cut and milled may be formed without wear resistant particles in order to reduce tool wear.

A first aspect of the invention is a method of forming a wear resistant surface comprising transparent or semi-transparent wear resistant particles on a surface, wherein the method comprises:

• applying a liquid binder in a pre-determined pattern on the surface,

• applying the wear resistant particles on the surface,

• bonding a part of the wear resistant particles to the surface with the liquid

binder , and · removing non-bonded wear resistant particles from the surface, thereby the bonded wear resistant particles are arranged in the pre-determined pattern and are evenly distributed on at least a first portion of said surface.

The pre-determined pattern may be a raster pattern such that the bonded wear resistant particles are arranged with pre-determined distances between each other. Bonding a part of the wear resistant particles to the surface with the liquid binder may comprise bonding wear resistant particles with a first pre-determined distance between each other at said first portion of the surface, and bonding wear resistant particles with a second pre-determined distance between each other at a second portion of the surface. The second pre-determined distance may exceed the first pre-determined distance.

The second pre-determined distance may essentially correspond to surface portions where the surface is to be cut and/or where locking systems will be formed. The second pre-determined distance may essentially correspond to the width of the cut or of the locking system.

The surface may be formed with embossed surface portions comprising upper and lower surface portions. The first portion of the surface may correspond to upper surface portions and the second portion of the surface may correspond to lower surface portions.

The surface may comprise a printed decor and the wear resistant particles may be spaced from each other and coordinated in register with the printed decor.

The surface may be formed with embossed surface portions comprising upper and lower surface portions and wherein the content of the wear resistant particles may be higher in the upper portions than in the lower portions.

The wear resistant particles may comprise aluminium oxide such as corundum. The wear resistant particles may be coated or mixed with a thermosetting resin. The surface may be a powder layer, a paper layer or a foil. The surface may comprise a melamine formaldehyde resin. The surface may be a part of a floor panel. The wear resistant particles may be removed by an airstream.

The liquid binder may be a blank ink comprising a liquid substance that is applied by a digital drop application head. The liquid substance may be water based and comprises glycol or glycerine.

The liquid substance may be exposed to IR light or hot air.

The liquid substance may be applied with a Piezo ink head.

The liquid substance may be applied with a thermo ink head.

The surface with the wear resistant particles may be is heated and pressed. The wear resistant particles may be applied by scattering.

The wear resistant particles may be arranged in a wood grain or a stone pattern.

A second aspect of the invention is a panel with a decorative surface comprising a pattern of wear resistant particles, wherein the wear resistant particles are arranged in a raster pattern with pre-determined distances between the wear resistant particles.

The wear resistant particles may be evenly arranged in at least a first portion of the surface.

At a first portion of the surface, the wear resistant particles may be arranged with a first pre-determined distance between each other, and at a second portion of the surface, the wear resistant particles may be arranged with a second pre-determined distance between each other. The second pre-determined distance may exceed said first predetermined distance.

The surface may be an embossed surface comprising upper and lower surface portions. The first portion of the surface may correspond to upper surface portions and the second portion of the surface may correspond to lower surface portions.

The surface may comprise a printed decor and the wear resistant particles may be spaced from each other and coordinated with the printed decor.

The surface may comprise a printed and embossed decor with an upper and a lower surface portion and wherein the content of wear resistant particles may be higher in the upper surface portion than in the lower surface portion.

An edge portion may comprise a higher content of wear resistant particles than an inner surface portion spaced from the edge portion.

The surface may be a paper layer or a foil.

The surface may comprise a powder layer. The surface may be a part of a building panel.

The surface may be a part of a floor panel.

The wear resistant particles may comprise aluminium oxide such as corundum. The wear resistant particles may be arranged in a wood grain or a stone pattern. The surface may be a part of a panel that is a laminate or wood floor, a powder based floor, a tile or a LVT floor.

A third aspect of the invention is a sheet with a decorative surface comprising a pattern of wear resistant particles, wherein the wear resistant particles are arranged in a raster pattern with pre-determined distances between the wear resistant particles.

The wear resistant particles may be evenly arranged in at least a first portion of the surface.

At a first portion of the surface, the wear resistant particles may be arranged with a first pre-determined distance between each other, and at a second portion of the surface, the wear resistant particles may be arranged with a second pre-determined distance between each other. The second pre-determined distance may exceed said first predetermined distance.

The surface may be an embossed surface comprising upper and lower surface portions. The first portion of the surface may correspond to upper surface portions and the second portion of the surface may correspond to lower surface portions.

The second pre-determined distance may essentially correspond to surface portions where the sheet is to be cut into several panels and/or where locking systems will be formed.

The surface may comprise a printed decor and the wear resistant particles may be spaced from each other and coordinated in register with the printed decor.

The surface may comprise a printed and embossed decor with upper and lower surface portions and wherein the content of wear resistant particles may be higher in the upper portions than in the lower portions.

The wear resistant particles may comprise aluminium oxide such as corundum. A fourth aspect of the invention is a paper comprising a pattern of wear resistant particles wherein the wear resistant particles are arranged in a raster pattern with predetermined distances between the wear resistant particles.

The paper may comprise a melamine formaldehyde resin.

The paper may be a transparent overlay with wear resistant particles arranged in patterns. The paper may be a decorative paper with wear resistant particles arranged in patterns.

A fifth aspect of the invention is a method of forming a wear resistant surface comprising transparent or semi-transparent wear resistant particles on a surface, wherein the method comprises: · applying a liquid binder in a pre-determined pattern on a surface,

• applying a wear resistant particles on the surface,

• bonding a part of the wear resistant particles to the surface with the liquid

binder , and

• removing non-bonded wear resistant particles from the surface, thereby the bonded wear resistant particles are arranged in the pre-determined pattern.

Brief Description of the Drawings

The invention will in the following be described in connection to preferred

embodiments and in greater detail with reference to the appended exemplary drawings, wherein,

Figs la-e Illustrate surfaces comprising wear resistant particles;

Figs 2a-b Illustrate a sheet and a floor panel having a surface with wear

resistant particles;

Figs 2c-e Illustrate bonding of wear resistant particles; Figs 2f Illustrate a method and equipment to apply wear resistant particles in pre-determined patterns.

Detailed Description of Embodiments

Figure la shows a conventional application of wear resistant particles such as aluminium oxide particles on a paper based overlay surface 2 used in laminate floorings. The particles, which have a size of about 0.1 mm, are applied at random and the whole surface is covered. Some surface portions comprise larger amounts and some smaller amounts of the wear resistant particles. Two to five particles and even more may be connected to each other in clusters and some particles may be spaced from each other with a distance of up to about 1 mm. Figure lb shows schematically an embodiment of the invention, which is based on a preferred principle where a binder pattern BP is formed digitally by an ink head, hereafter referred to as digital drop application head, that preferably only applies a binder 1 1, hereafter referred to as blank ink, on a surface 2. Wear resistant particles hereafter referred to as dry overlay 15 that comprises for example small aluminium oxide particles, are applied such that they are in contact with the binder pattern BP. The wear resistant particles may be transparent or semi-transparent. By the wear resistant particles being transparent or semi-transparent is meant that the visual impression of the underlying layer is not affected in a significant way. The blank ink 1 1 connects some particles that form the same pattern as the binder 1 1 and a pattern BP of wear resistant particles is formed on the surface 2 when other non-bonded particles 15 are removed from the surface 2 by for example vacuum. This method allows that the surface 2 may be covered with wear resistant dry overlay comprising particles that are evenly distributed on the surface with pre-defined distances Dl, D2 between the major parts of the particles. An ideal distance between the particles is about 0.2 -0.6 mm and no clusters of connected particles should occur. Such evenly distributed particles of dry overlay provide a high quality surface with high wear resistance and transparency combined with low material costs.

The blank ink 1 1 and the dry overlay 15 may be applied in many alternative ways. The blank ink may be applied on the dry overlay or the dry overlay may be applied on the blank ink. The surface may point upwards or downwards and the blank and/or the dry overlay particles may be applied from above or from below. A surface with blank ink may for example point downwards and may be brought into contact with a dry overlay layer. Non-bonded dry overlay particles may be removed by gravity when the surface is separated from the dry overlay layer. In order to simplify the description, the majority of the preferred embodiments show a surface pointing upwards.

Figure lc shows a powder-based surface 2 comprising a base colour 2a and a digital print P applied on the base colour. The base colour may comprise wear resistant particles and a second layer of dry overlay particles 15 is only applied on the printed parts P. The application is made in two steps as described above with blank ink and dry overlay where the dry overlay applied on the non printed portions is removed.

Figure Id shows a surface 2 with embossed upper 17a and lower 17b surface portions. The wear resistant particles are preferably only applied on the upper portions 17a, which are exposed to high wear. Surface portions may also be formed with different amounts of particles per cm2. The amount of particles may for example be larger in the lower portions than in the upper portions.

Figure le shows a sheet 1 which is after pressing divided into two floor panels la, lb. The wear resistant particles are applied with a distance Dl that corresponds to the part of the surface that is removed when the sheet 1 is cut and the locking systems are formed on the individual panels la, lb. The distance Dl is preferably larger than a few mm, which corresponds to the width of a saw blade SB. The distance may also be about 5 - 10 mm, which corresponds to the surface portion that is needed to form the major part of the mechanical locking system.

A protective layer of for example bleached fibres and melamine resin or only melamine or only fibres may be applied on the wear resistant particles in order to for example protect press plates during pressing or to create different gloss levels.

Figure 2a shows a panel la with a core 3, a balancing layer 4 and an embossed surface 2 comprising upper 17a and lower parts 17b. The panel edges are formed with a mechanical locking system comprising a strip 6, with a locking element 8 in one edge that cooperates with a locking groove 14 in an adjacent edge of another panel for horizontal locking of the adjacent edges and a tongue 10 in one edge that cooperates with a tongue groove 9 in another edge for vertical locking of the panels. The panel comprises bevels 5 at the upper edges. The panel may comprise different amounts of dry overlay on the upper and lower surface portions

Figure 2b shows a sheet 1 which is cut into two individual panels la and lb. The wear resistant particles are applied in patterns with a distance D2 between the particles such that the wear properties of surface portions, which are removed in connection with cutting, and milling of the locking systems and the bevels are lower in such portions than in other parts of the sheet. Preferably such portions should be produced such that the content of wear resistant particles is as small as possible, preferably less than 10% of the average content of the panel surface.

Floor panels may warp in different humidity and the wear on the edges that generally warp upwards in dry conditions is much higher than on the inner part of the panel.

Increase amounts of wear resistant particles may be applied at surface portions 2a adjacent to the panel edge. Figures 2c - 2e shows how wear resistant particles, preferably aluminium oxide 63, may be bonded and position in well-defined patterns. A binder of blank ink 1 1 is applied on a surface with preferably a conventional digital ink head. The binder may also be applied with rollers and other similar methods. Water may be sufficient to bond the particles until they are pressed.

Figure 2c shows that a binder is preferably included in the surface 2 and may react with the liquid pattern of blank ink 1 1 applied by the digital drop application head.

The blank ink may preferably mainly comprise distilled or deionized water. The adhesion of water may be sufficient to connect particles to the surface with a force that is sufficient to allow removal of the non-bonded particles. The production costs for such a liquid substance are extremely low and clogging of the nozzles when a binder dries may be avoided. Some chemicals may be added, for example glycol or glycerine, that are needed to reach the viscosity and surface tension of the liquid substance that may be needed for a proper function of a print head. Water-soluble polyethylene glycol (PEG), that is available in many different molecular weights, is especially suitable to modify water such that a blank ink with an appropriate viscosity that works, for example, with Piezo heads may be obtained. Low monocular weight formulations such as, for example, PEG 400 are especially suitable to use in blank ink and preferably together with dry ink or a surface that comprises thermosetting resins such as melamine. Water and PEG are compatible with melamine resins and allows easy and fast curing when heat and preferably also pressure is applied. A preferred non-drying solvent that is compatible with thermosetting resins should be miscible with water, have a boiling point above 100°C and a melting point lower than the application temperature. Examples of such, but not restricted to, are ethylene glycol, propylene glycol, polyethylene glycol, diethylene glycol, butane diol and glycerine. Combinations can also be used. In some applications some other minor amounts of chemicals may be included in the blank ink, for example, wetting agents release agents.

Most Piezo print heads are designed to work with a viscosity in the range of 2 to 12 centipoise (cps). The water based blank ink may easily be adapted to meet all possible viscosity requirements.

A suitable blank ink that preferably may be used in a low viscosity print head designed to operate with a viscosity of about 5 cps such as a Kyocera print head may be a water based glycol solution comprising, for example, about 75 wt% (weight) ethylene glycol or 55 wt% diethylene glycol or 50 wt% propylene glycol or 38 wt% polyethylene glycol PEG 400. A water based glycerine solution comprising about 40% glycerine may also be used. De-ionized water may also be mixed with glycerine and glycol. A suitable blank ink for a low viscosity print head may, for example, comprise about 40 wt% water, 50 wt% glycerine and 10 wt% diethylene glycol.

A suitable blank ink that preferably may be used in a high viscosity print head designed to operate with a viscosity of about 10 - 12 cps such as a Fuji print head may be a water based glycol solution comprising, for example, about 95 wt% ethylene glycol or 75 wt% diethylene glycol or 70 wt% propylene glycol or 50 wt% polyethylene glycol PEG 400. A water based glycerine solution comprising about 65 wt% glycerine may also be used. De-ionized water may also be mixed with glycerine and Glycol. A suitable blank ink for a high viscosity print head may, for example, comprise about 30 wt% water, 60 wt% glycerine and 10 wt% diethylene glycol. The water content for blank ink adapted for low and high viscosity Piezo print heads may be increased further if high viscosity glycols are used; for example, polyethylene glycol with a higher molecular weight than PEG 400. A preferred blank ink that preferably is suitable for Piezo print heads may comprise 0 - 70 wt% water and 30 - 100 wt% glycol and/or glycerine. Even more preferred is a formulation comprising 10 - 70 wt% water and 30 - 90 wt% glycol and/or glycerine. Blank ink that is suitable for thermal bubble jet print heads that are designed for very low viscosities; for example, 2 - 4 cps may comprise more than 70 wt% water.

All blank ink formulations may comprise small amounts, about 1 %, of wetting agents such as BYK or Surfinol and chemicals such as Actidice intended for control of bacteria and fungi.

The blank ink is preferably essentially a non-curable liquid substance that is used to obtain the application bonding and to bond the particles until the final permanent bonding takes place preferably with heat and pressure and with resins that are a part of the substrate material and/or the dry ink particles. Such blank ink will not bond particles when it dries or when heat is applied. The blank ink may comprise special curable binders; preferably a binder comprising a thermally cross linkable acrylate copolymer dispersion preferably water based acrylic emulsion, which preferably is compatible with water, glycol or glycerine. Preferable binder content is 5-20%. Acrylic emulsions will bond particles when the water content evaporates and they will create a strong bond under high heat and pressure.

A high water content of at least 50% gives the advantages that the material cost may be low. The "decap time", which is the amount of time a print head can be left uncapped and idle and still fire ink drops properly, will be rather short, less than one hour, since water evaporates. A low water content combined with a high glycol or glycerine content will increase the decap time considerably. Blank ink with a water content below 40 wt% may have a decap time of several hours. Water content below 20 wt% will give a very long decap time that may exceed 6 hours. It is possible to use blank ink that comprises more than 90 wt% glycol and this may increase the decap time to several days. Blank ink may be made without water and high viscosity print heads may handle blank ink that comprises, for example, 100% ethylene glycol.

An ink circulation system may be avoided in industrial printers when blank ink is used that does not comprise any pigment dispersions or binders and that is mainly a water based solution as described above. This will decrease the cost for the printing equipment considerably. Ink circulations systems may be used even when binder free blank ink is used that comprises a high water content of for example more than 50 wt%. Circulation will increase the decap time and problems related to evaporation of water when the print heads are not in operation will be avoided.

The binder that bonds the particles to the surface may comprise two components. The first binder component may be included in the blank ink. The second binder component may be included in the dry ink or the surface and activated by the blank ink. This makes it possible to use, for example mainly water or water mixed with a binder free glycol or glycerine mix in order to obtain the bonding. Water and/or may react with a binder that may be included in the particles or in the surface.

Figure 2d shows that aluminium oxide particles 63, may be coated with a thermoplastic or thermosetting resin, for example melamine 13. Fluidized bed equipment comprising a cylinder may be used to apply a resin coating on the particles, for example a melamine formaldehyde coating on aluminium particles. Pressurized hot air is used to bring the particles to free-float under gravity and to behave as a fluid in the cylinder. Liquid melamine resin is sprayed into the cylinder with the particles and a resin coating may be made under temperatures of 50 - 120 C°. The coated particles are after coating and drying preferably brought into a separation mixer and connected particles are separated and sieved.

Figure 2e shows that dry overlay particles 15 may also be mixed with a spray dried binder in powder form such as melamine particles 13 that melt when they are in contact with the blank ink 1 1. Figure 2f shows schematically a digital particle application equipment 40 that may be used to create a digital pattern BP of wear resistant particles on a panel la comprising a surface 2, a core 3 and a backing layer 4. A blank ink application station 36 comprising a digital drop application head 30', that preferably is a Piezo head or a thermal print head, applies a binder pattern (BP) with blank ink 1 1. Several heads 30' may be positioned side by side in order to cover the width of the surface that is printed. The binder pattern is created digitally in the same way as in conventional digital printing. The digital drop application head is connected with a feeding pipe 32 to a container 31 with blank ink. The digital drop application heads 30' are digitally connected with preferably data cables 33 or wireless to a digital control unit 34 that controls the application of the drops, the speed of the conveyor 21, the function of a dry ink application unit 27 and all other equipment that is used to bond and remove particles.

The water based drops of the blank ink 1 1, which in this embodiment serve as an application binder, should be wet until they pass the dry ink application unit 27 that in this preferred embodiment is a scattering station. Dry overlay 15, that in this preferred embodiment comprises aluminium oxide particles mixed with a resin of spray dried melamine powder, is scattered on the liquid blank ink 1 1.

The scattering equipment comprises a hopper 45 that contains dry overlay 15, a doctor blade 47 that together with a roller 46, preferably comprising an engraved, embossed, etched or sand blasted roller surface 44, acts as a dispensing device that moves a pre- determined amount of dry overlay 15 from the hopper 45 and to the surface 2. The roller 46 may oscillate and may also have a roller surface 44 that comprise small needles. A material-removing device that may be an oscillating or rotating brush 48 may also be used in some applications together with one or several rotating or oscillating meshes 49 that may oscillate or rotate in different directions.

The doctor blade 47 may be rigid or flexible and may have an edge that is adapted to the structure of the roller surface. The oscillating or rotating meshes 49 may also be formed such that they spread the dry overlay 15 in a pre-defined way and they may be combined with one of several nets that may be used to sieve the particles before they are applied as a layer. The rotation of the roller, the position of the doctor blade and the speed of the surface that is intended to be covered with the dry overlay may be used to control the layer thickness. The liquid blank ink 1 1 and the dry overlay is in this embodiment heated and stabilized when it is displaced under preferably a hot IR lamp 23, which is located preferably after the digital drop application head 30' in the feeding direction.

A dry overlay removal station 28, that in this embodiment is based on air streams and vacuum, removes dry overlay particles that are not wet and not bonded by the binder pattern and a perfect dry overlay pattern P is provided. Preferably a combination of air pressure created by an air knife and vacuum is used to remove non-bonded particles. The dry overlay removal station may be located after the IR lights 23 or between the IR lights and the scattering unit 27. This production step may be repeated and several types of wear resistant particles may be applied at different portions of the surface. The removed particles may pass through a sieve or a filter and they may be recycled and reused again several times.

The dry overlay may in addition to wear resistant particles also comprise melamine particles and/or pigments and/or fibres, preferably bleached transparent or semi- transparent wood fibres. Heat and pressure is applied to the surface 2 such that a wear resistant surface is formed.

The method to apply wear resistant particles in patterns in order to reach cost saving and increased transparency may also be used together with a conventional overlay paper or decorative paper. Wear resistant particles may be applied in patterns on the overlay prior or after impregnation and the overlay paper with the wear resistant particles applied in patterns may be applied on a decorative paper. Wear resistant particles may be applied in patterns on the decorative paper preferably after impregnation when the decorative paper is positioned on a carrier, preferably a sheet material such as HDF. Impregnation of the decorative paper may be avoided if the paper is applied on a layer comprising thermosetting resin, for example a powder layer.

A transparent overlay paper without any wear resistant particles or with only small scratch resistant particles may be applied on the wear resistant pattern in order to provide additional properties such as different gloss levels or to provide a layer that protects the press plate against wear during pressing.

Powder based surfaces may be applied with a basic mix that does not include any aluminium oxide particles. Such wear resistant particles may be applied in patterns in a second step and surface portions without any wear resistant particles may be formed even in powder based surfaces in order to reduce tool wear.

All described embodiments may be partly or completely combined.

EXAMPLE 1 - DIGITALLY FORMED POWDER OVERLAY

A HDF sheet with a thickens of 8 mm was sprayed with deionized water and a powder mix of about 200 g/m2 of powder comprising wood fibres, melamine particles, brown colour pigments and aluminium particles was applied by scattering equipment on the HDF sheet. The water penetrated into the lower parts of the mix. The upper dry part of the mix was removed by vacuum and a very even powder mix of 150 gr/m2 was obtained. The mix was cold pressed with a metal roller and a hard stabilized powder based surface with a brown basic colour was obtained. The panel with the stabilized powder surface was put on a conveyer and displaced under a digital Piezo print head that provided a conventional ink jet print on the brown base coloured surface. The digital print covered about 20% of the surface and the basic colour. A digital print head was thereafter used to applied drops of blank ink comprising mainly water on the digitally printed surface portions. A higher intensity of drops was applied on the printed portions that were intended to form upper parts of the surface than on the printed portions that were intended to form lower parts of the surface after the final pressing operation. A dry mix of aluminium particles (85 wt%) with an average size of 100 microns and spray dried melamine formaldehyde particles (15 wt%) with a similar size was scattered on the whole surface. The sheet was thereafter displaced under a vacuum- sucking pipe where essentially all non-bonded aluminium oxide particles and melamine formaldehyde particles were removed. A protective transparent wear resistant layer or a so-called overlay was formed with aluminium oxide particles applied on essentially only the digitally printed surface portions. The panel was thereafter pressed against an embossed press plate during 15 seconds under a temperature of 170 degrees C in a 40 bars press. The surface with the protective layer was cured to a hard wear resistant surface with a high quality wear resistant digital print comprising a higher amount of aluminium oxide particles in the upper parts of the printed and embossed surface portions than in the lower surface portions.

EXAMPLE 2 - DIGITALLY FORMED PAPER OVERLAY

A digital Piezo print head was used to applied drops of blank ink comprising mainly water on a melamine impregnated overlay paper sheet. The drops were applied in a raster pattern with a drop distance of about 1 mm. A higher intensity of drops with a drop distance of 0.5 mm was applied on the surface portions that were intended to form upper parts of the surface. No drops were applied on a 12 mm wide surface portion that extended over the whole length of the overlay paper and that corresponds to the surface portion where a saw blade cuts the pressed sheet and where the locking system is formed. A dry mix of aluminium particles with an average size of 100 microns was scattered on the whole overlay paper surface. The overlay paper was thereafter displaced under a vacuum- sucking pipe where essentially all non-bonded aluminium oxide particles were removed. The bonded aluminium oxide particles formed a pattern, which was essentially identical to the applied drops. The overlay with the aluminium oxide particles was displaced under an IR lamp and applied on a HDF sheet with a decorative melamine impregnated paper. The sheet was thereafter pressed against an embossed press plate during 15 seconds under a temperature of 170 degrees C in a 40 bars press. The surface with the decorative and overlay papers was cured to a hard wear resistant surface with a high quality wear resistant overlay comprising a base structure with accurately positioned aluminium oxide particles with a pre-defined distances between the particles and with a higher amount of aluminium oxide particles in the upper parts of the printed and embossed surface portions than in the lower surface portions. The sheet was thereafter cut along the surface area without any aluminium oxide particles and the locking system was formed in edge portions, which were almost completely free from aluminium oxide particles. The wear on the saw blade and the milling tool was considerably lower. EXAMPLE 3 - DIGITAL WEAR LAYER ON DECORATIVE PAPER

A digital Piezo print head was used to applied drops of blank ink comprising mainly water on a melamine impregnated decorative paper applied on a HDF core. The drops were applied in a raster pattern with a drop distance of about 0.6 mm on surface portions that were intended to form lower parts of the pressed surface. A higher intensity of drops with a drop distance of about 0.3 mm was applied on surface portions that were intended to form upper parts of the pressed surface. No drops were applied on a 12 mm wide surface portion that extended over the whole length of the decorative paper and that corresponded to the surface portion where a saw blade cuts the pressed sheet and where the locking system is formed. A dry mix of aluminium particles with an average size of 100 microns was scattered on the whole surface of the decorative paper. The sheet was thereafter displaced under a vacuum-sucking pipe where essentially all non-bonded aluminium oxide particles were removed. The bonded aluminium oxide particles formed a pattern, which was essentially identical to the applied drops. The sheet with the decorative paper and with the aluminium oxide particles was displaced under an IR lamp. A conventional melamine impregnated overlay without any aluminium oxide particles was applied over the decorative paper and the sheet with the two papers was thereafter pressed against an embossed press plate during 15 seconds under a temperature of 170 degrees C in a 40 bars press. The surface with the decorative and overlay papers was cured to a hard wear resistant surface with a high quality wear resistant surface comprising a base structure with accurately positioned aluminium oxide particles with a pre-defined distances between the particles and with a higher amount of aluminium oxide particles in the upper parts of the embossed surface portions than in the lower surface portions. The sheet was thereafter cut along the surface area without any aluminium oxide particles and a locking system was formed in edge portions, which were almost completely free from aluminium oxide particles. The wear on the saw blade and the milling tool was considerably lower.

Embodiments 1. A method of forming a wear resistant layer comprising transparent or semi transparent particles (15) on a surface (2) wherein the method comprises the steps of:

• applying a liquid binder (1 1) in a pattern (BP) on the surface (2), • applying the wear resistant particles (15) on the surface (2),

• bonding a part of the wear resistant particles (15) to the surface (2) with the liquid binder (1 1), and

• removing non-bonded wear resistant particles (15) from the surface (2) such that the wear resistant layer is formed with evenly distributed wear resistant particles (15).

2. The method as in embodiment 1, wherein the wear resistant particles (15) are applied in a raster pattern with pre-determined distance (Dl, D2) between the wear resistant particles (15). 3. The method as claimed in embodiment 2, wherein the pre-determined distance (Dl, D2) essentially corresponds to surface portions where the surface (2) is to be cut and/or where locking systems will be formed.

4. The method as in any one of the embodiments 1 - 3, wherein the surface (2) comprises a printed decor (P) and the wear resistant particles (15) are spaced from each other and coordinated in register with the printed decor (P).

5. The method as in any one of the embodiments 1 - 4, wherein the surface (2) is formed with embossed surface portions comprising upper (17a) and lower (17b) surface portions and wherein the content of wear resistant particles are higher in the upper portions (17a) than in the lower portions (17b). 6. The method as in in any one of the preceding embodiments, wherein the wear resistant particles (15) comprise aluminium oxide (63) such as corundum.

7. The method as in in any one of the preceding embodiments, wherein the wear resistant particles (15) are coated or mixed with a thermosetting resin.

8. The method as in any one of the preceding embodiments, wherein the surface (2) is a powder layer, a paper layer or a foil.

9. The method as in any one of the preceding embodiments, wherein the surface (2) is a part of a floor panel (1).

10. The method as in any one of the preceding embodiments, wherein the wear resistant particles (15) are removed by an airstream. 1 1. The method as in any one of the preceding embodiments, wherein the binder is a blank ink (1 1) comprising a liquid substance that is applied by a digital drop

application head (30').

12. The method as in embodiment 1 1, wherein the liquid substance is water based. 13. The method as in embodiment 1 1 or 12, wherein the liquid substance is exposed to IR light (23) or hot air.

14. The method as in embodiment 1 1, wherein the liquid substance is applied with a Piezo ink head.

15. The method as in embodiment 1 1, wherein the liquid substance is applied with a thermo ink head.

16. The method as in any one of the preceding embodiments, wherein the surface (2) with the wear resistant particles (15) is heated and pressed.

17. The method as in any one of the preceding embodiments, wherein the wear resistant particles (15) are applied by scattering. 18. The method as in any one of the preceding embodiments, wherein the wear resistant particles (15) are arranged in a wood grain or a stone pattern.

19. A panel (1) with a decorative surface (2) comprising a pattern (P) of wear resistant particles (15) wherein the wear resistant particles (15) are applied in a raster pattern with pre-determined distance between the wear resistant particles (15). 20. The panel as in embodiments 19, wherein the surface (2) comprises a printed decor (P) and the wear resistant particles (15) are spaced from each other and coordinated with the printed decor (P).

21. The panel as in embodiment 19 or 20, wherein the surface (2) comprises a printed and embossed decor with an upper (17a) and a lower (17b) surface portion and wherein the content of wear resistant particles (15) are higher in the upper surface portion (17a) than in the lower surface portion (17b).

22. The panel as in any one of the embodiments 19 - 21, wherein an edge portion comprises a higher content of wear resistant particles than an inner surface portion spaced from the edge portion 23. The panel as in any one of the preceding embodiments 19 - 22, wherein the surface (2) is a paper layer or a foil.

24. The panel as in any one of the preceding embodiments 19 - 22, wherein the surface (2) comprises a powder layer. 25. The panel as in any one of the preceding embodiments 19 - 24, wherein the surface (2) is a part of a building panel (1).

26. The panel as in any one of the preceding embodiments 19 - 25, wherein the surface (2) is a part of a floor panel (1).

27. The panel as in any one of the preceding embodiments 19 - 26, wherein the wear resistant particles (15) comprise aluminium oxide such as corundum.

28. The panel as in any one of the preceding embodiments 19 - 27, wherein the wear resistant particles (15) are arranged in a wood grain or a stone pattern.

29. The panel as in any one of the preceding embodiments 19 - 28, wherein the surface (2) is a part of a panel (1) that is a laminate or wood floor, a powder based floor, a tile or a LVT floor.

30. A sheet (1) with a decorative surface (2) comprising a pattern (P) of wear resistant particles (15) wherein the wear resistant particles (15) are applied in a raster pattern with pre-determined distance (Dl, D2) between the wear resistant particles.

31. The sheet as in embodiment 30, wherein the pre determined distance (D l, D2) essentially corresponds to surface portions where the sheet (1) is to be cut into several panels (la, lb) and/or where locking systems will be formed.

32. The sheet as in any one of the embodiments 30 or 31, wherein the surface (2) comprises a printed decor (P) and the wear resistant particles (15) are spaced from each other and coordinated in register with the printed decor (P). 33. The sheet as in any one of the embodiments 30 - 32, wherein the surface (2) comprises a printed and embossed decor with upper (17a) and lower (17b) surface portions and wherein the content of wear resistant particles (15) are higher in the upper portions (17a) than in the lower portions (17b).

34. The sheet as in in any one of the preceding embodiments 30 - 33, wherein the wear resistant particles (15) comprise aluminium oxide (63) such as corundum. 35. A paper comprising a pattern (P) of wear resistant particles (15) wherein the wear resistant particles (15) are applied in a raster pattern with pre-determined distance between the wear resistant particles (15).