METHOD AND APPARATUS FOR FORMING A SURFACE STRUCTURE

[001] The present invention relates to the technical field of methods aimed at generating three- dimensionality and embossing on surfaces of pieces provided with supports of different kinds of materials, like e.g. mainly flat panels for producing floors, coverings, furniture, and more generally for architectural and design surfaces.

[002] The present invention relates to a method and an apparatus for applying such method for embossing panels using an inkjet printer, improving the embossing process and the quality of the final product. In its preferred embodiment, such method is applied to panels previously provided with a decorative pattern, e.g. a pattern mimicking a wood grain, and realizing an embossing in register. Nonetheless, the method according to the present invention can be applied to obtain an embossing not in register with the already provided pattern, or even to obtain a generic embossing on a panel which is not provided with a decorative pattern.

[003] As known, panels are coated to provide them with particular features of resistance and an aesthetical effect. In particular, panels coated according to the present invention are typically made of medium/low-cost materials, like wooden materials (MDF, HDF, particleboard, plywood), or plastic materials (PVC, polyolefins) that are ennobled giving them the look of high-end wood essences or also other kinds of decoration (e.g. stone). Among the materials that can be treated according to the present method, there are also the so-called Stone Plastic Composite (SPC) floorings.

[004] Mainly flat panel means a panel wherein two out of three dimensions are much wider than the third dimension. Typically, the measures of such panels range 100x500x3 mm to 1220x3050x30 mm.

[005] If the look of the panel mimicking a wood essence has to be credible and convincing, in the art it is well known that, in addition to its purely visual aspect, the panel must be provided with a tactile (haptic) surface with pores corresponding to the eye perception. The working allowing to provide panels with a surface with pores is called embossing. In particular, it is desirable to have an embossing in register with the printed image (Embossing In Register, EIR). In other words, there must be a correspondence between the concavities/convexities perceived by touch and the printed image perceived by eyes. Before the advent of digital (inkjet) printing, embossing was provided to panels through moulds, rollers or belts. Since the advent of digital printing, embossing can be provided through this technology, so obtaining excellent aesthetic results.

[006] In the art, there are provided methods and apparatuses for creating a haptic surface structure on a surface already provided with a drawing which mimics e.g. the grain of a wood essence.

[007] EP2943346B1 owned by Ceraloc Innovation AB describes a method and a device for creating a three- dimensional digital print on a panel. The said method for forming a digital print (P) on a panel (1) comprising a surface (2) and which surface being part of a panel for building (1) such as a panel for floors (1) comprises the following steps: feeding the panel below a digital head (30') for applying droplets on it; applying on the surface (2) a liquid binder (11) by means of the digital application head (30'); applying dyes (7) on the liquid binder (11) and on the surface (2) and in which the dyes (7) are macro particles (64) of dye being larger than 20 micron; binding a part of the dues (7) to the surface (2) by means of the liquid binder (11); removing the dyes which are not bound to the surface (2) in such a way to form a digital print (P) with the said dyes; and applying heat and pressure on the panel (1), on the surface (2) and on the bound dyes (7) in such a way that the dyes (7) are permanently bound to the surface (2).

[008] EP3109056A1 of Hymmen, published in 2016, describes a method and an apparatus for applying said method, capable of obtaining a surface embossed in register. Such method comprises the following steps:

(A) Application of a liquid base layer 2 on the surface of the workpiece 1;

(B) Spraying of a plurality of droplets 3 on the still liquid base layer in a manner, that the thickness of the base layer 2 changes on the positions, where the droplets are sprayed on, wherein indentations are formed in the liquid foundational layer 2 by the spraying of the droplets 3; (C) Fixation of the liquid base layer (2).

[009] In the art, it is known that the base layer (resin) applied to the surface of the panel to be treated and the droplets are provided with a different chemical nature:

Generally speaking, all combinations are possible, as shown in the above table. Nonetheless, experimentally it was shown that the best results are obtained using a hydrophobic base layer and hydrophilic and/or hydrophobic droplets. [010] In particular, the preferably photopolymerizable resin used as fluid base layer can be acrylate (acrylate or methacrylate), or vinyl monomers, or cationic resins such as epoxy resin, polyurethane, hotmelt, polyols and monomers such as oxetanes and vinyl ethers. The hydrophilic droplets can be water or water supplemented with surface tension modifiers, like e.g. silicones. The hydrophobic droplets can be e.g. silicones and/or fluorinated compounds. [0011] W02020039361A1 of Giorgio Macor, published in 2020, describes a method wherein to the steps A, B, C a step of removal of droplets and at least part of the base resin layer in the areas in correspondence to the deposition spots of droplets is added: (A) Applying a resin R on the surface of a material;

(B) Applying a liquid L on at least one portion of the resin R, when the resin R is liquid or partially solidified;

(C) Polymerizing, at least partially and also separately, the resin R and the liquid L;

(D) Mechanically removing the polymerized liquid L and at least part of the resin R in correspondence to the deposition areas of the liquid L.

[0012] In such method, three-dimensionality is generated by partially/integrally removing the mixture of liquid L and resin R. The function of said liquid L is obtaining, after polymerization, a mixture of liquid L and resin R having a glass transition temperature (Tg) lower than that of the polymerized resin R per se. In case the resin R is a photopolymerizable resin, said liquid L inhibits the photopolymerization , and consequently a mixture of resin R and liquid L partially solidified is generated. The partially solidified/polymerized mixture can appear as a sticky high viscosity liquid/solid.

[0013] It was experimentally found that the step D of removal of liquid L and at least part of the resin R in correspondence of the deposition areas of liquid L, performed mainly through brushing, allows to obtain a finished panel provided with haptic features that are even better than those obtained with the process according to EP3109056.

[0014] US2020346484A1 again of Hymmen, published in 2020, discloses that the brushes used for the removal of masking and/or of resin layer can be a disk brush and/or a roller brush and/or a brush band and/or, as stationary brush, a beam having a brush trimming, wherein the contact element, particularly the brush, comprises textile and/or elastic fibres, in particular, nylon fibres, Anderton, and/or metal, particularly steel, brass or copper as bristles.

[0015] W02022064072A1 of Hymmen, too, published in

2022, describes a method for producing a three- dimensional surface similar to that described in US2020346484A1 . Paragraphs 0062, 0063 state: The removal of the material of the base layer results in depressions and/or through-holes being formed in the base layer at these locations, the removal of the masking or the removal of the masking and the base layer or the removal of material of the base layer only being preferably physical and/or done chemically. In particular, brushing and/or sandblasting can be mentioned as physical removal, which can be supported by suction. If a brush or, alternatively or additionally, a sanding or planing element is used to remove the material of the base layer, the base layer and/or the masking is treated by means of electromagnetic radiation in such a way, or its hardness or viscosity is adjusted in such a way that the brush, the sanding element and/or the planing element does not clog the base coat and/or the masking with dissolved material. It is thus avoided that the brush, the sanding element and/or the planing element has to be cleaned during the production of the workpieces . The irradiation preferably takes place immediately before and/or during the removal.

[0016] The methods according to W02020039361A1, US2020346484A1 and W02022064072A1 are based on the principle of creating areas on the surface of the panel wherein the protective coating (or resin) is provided with a lower tenacity with respect to the adjacent areas thanks to a partial polymerization of said areas. Such methods provide a removal from said areas of the partially polymerized fluids through brushing with suitable brushes (generally steel brushes or made of other materials provided with suitable hardness).

[0017] The stickiness of the partially polymerized fluids leads to the soiling of the brushes and of the panels on which inevitably part of said partially polymerized fluids is smeared during the removal step. Due to said stickiness, such fluids adhere to the surface of panels, soiling them.

[0018] The soiling of brushes can be remedied at least partially by drying through irradiation the fluids that adhered to the bristles of said brushes, and successive removal through impact/vibration of said bristles.

[0019] A drawback of said solutions lies in the difficulty of removing the partially polymerized liquids through said brushing. This is due to the high soiling that said fluids cause on the apparatus applying the method, and especially on the panels themselves. This hinders the continuous production of the panels, while the frequent cleaning of the apparatus is expensive. The irradiation treatment of the base layer sticking to the brushes allows the removal through impact/vibration of the bristles of brushes, but does not allow to prevent/remedy the soiling of panels due to the smearing of fluids on the surface of panels, which leads to a high production waste.

[0020] The present invention aims to solve the above-described drawback.

[0021] Aim of the present invention is providing a method and an apparatus for applying such method for forming three-dimensional (haptic) surfaces, allowing to overcome the problem of the soiling of panels during their production and of the soiling of the means used to remove the non-polymerized fluids. A further aim is obtaining better results in terms of quality of coated panel.

[0022] This object is achieved by a method and an apparatus having the features of the independent claims. Advantageous embodiments and refinements are specified in claims dependent thereon.

[0023] The method according to the present invention comprises the following steps:

A. Applying a resin R on the surface of a panel to be treated;

B. Applying a liquid L on at least a portion of the resin R, when said resin R is still liquid or partially polymerized or solidified;

C. Polymerizing/solidifying, at least partially and also separately, the resin R and the liquid L;

D. Mechanically removing the polymerized liquid L and at least part of the resin R in correspondence to the deposition areas of liquid L, wherein in the step of mechanical removal of the partially polymerized liquid L and resin R an absorbent power is applied.

[0024] The application of an absorbent powder has the aim of providing a substantially inert material on the surface to be treated, capable of physically adhering to the sticky areas/parts and allowing their more efficient removal, preventing the adhesion effect of the sticky fluids (only partially polymerized) to the bristles of brushes and the smearing of the residues of sticky fluids on panels. [0025] Absorbent powders are provided with a high surface area and are used in order to absorb liquids. Examples of known absorbent powders are: sepiolite (hydroxyl hydrous magnesium silicate), kieselguhr (diatomaceous earth), talc (hydrated magnesium silicate), calcium carbonate, cellulose, and lignin. [0026] In a first embodiment, the absorbent powder is delivered on the surface of panels before the panel reaches the brushing station, while the absorbent powder is suitably levelled out/pressed on the surface of the panel before the removal of the partially polymerized fluids. [0027] In a second embodiment, the absorbent powder is delivered on the brush/es and in part by fall, in part through conveying by the brittles of brushes, is transferred on the panels during the brushing, and therefore at the same time of the step of partial polymerized fluids removal. [0028] The advantage of the first embodiment is the possibility to distribute the absorbent powder in a very controlled manner on the entire surface of the workpieces to be treated, and to facilitate its adhesion to the partially polymerized fluids thanks to suitable devices, like pressing rollers or rotating brushes having a vertical axis or a horizontal axis and made of soft materials. [0029] The advantage of the second embodiment is the limited footprint of the brushing workstation and a better efficacy of the absorbent powder in terms of brush cleaning and workpieces cleaning.

[0030] The particle size (granulometry) of the absorbent powder is in the range 1 to 50 micrometres, preferably 5 to 25 micrometres.

[0031] The needed quantity of absorbent power is 2 to 20 g/m, preferably 5 to 15 g/m with reference to the panels' surface to be treated, obviously depending on the kind of absorbent powder and the kind of fluids to be removed.

[0032] Experimentally, good results were obtainedwith industrial talcum powder or calcium carbonate (CaCO ). An alternative is using other kinds of powders, in particular 3 even resides of wood working, as long as they are provided with the above-described granulometry.

[0033] The preferential choice with the two-above powders (talcum, calcium carbonate) derives from considerations linked to their industrial availability, cost, and functionality.

[0034] Industrial talc is widely used in industry, both as a product component and as dry lubricant. The naturally low granulometry of this powder is ideal for the above-described requirement.

[0035] On the other hand, calcium carbonate is of the same chemical nature of the main component of some pieces to be treated in the sector of floorings with polymeric bases (Stone Plastic Composite, SPC), and advantageously could be recycled for the production of said SPC.

[0036] The consumption of these absorbent powders is considered to be in the range of 5 to 40 tons/year per plant. [0037] The increase in cost of panels due to the use of such powders appears to be limited to few cents of euro/m2 of panel, therefore decidedly modest with respect to the overall process cost, which is about 1.50 to 3.00 euro/m2 of panel, i.e. 2-4% of the overall cost of the process.

[0038] As only a fraction of the absorbent powder delivered on panels is really involved in the adhesion process to the sticky fluids to be removed, in an embodiment of the method there is provided a cyclone separator in order to recover the non-adhered absorbent powder for its successive re-use, so reducing the cost of this step of the process.

[0039] The advantages of the present invention are manifold.

[0040] The first advantage is solving the problem of soiling of brushes and panels surface.

[0041] The second advantage lies in the use of absorbent powders (talcum, calcium carbonate, wood sawdust) which are cheap.

[0042] The third advantage is in preventing the downtime needed for cleaning/maintaining apparatuses and brushes.

[0043] The fourth advantage is preventing the waste of finished panels due to the imperfections generated by the fall of partially polymerized material on the surface of said panels.

[0044] Further advantages and properties of the present invention are disclosed in the following description, in which exemplary embodiments of the present invention are explained in detail based on the drawings:

Figure 2 Schematic representation of the second embodiment according to the present invention

[0045] Figure 1 shows the first embodiment of the method according to the present invention. The dotted portion shows the new and inventive part of the method.

[0045] In its first embodiment, the method according to the present invention comprises the following steps:

A. Applying a layer 3 of a fluid resin R to a panel 1 suitably prepared with a drawing visually reproducing e.g. the vein of a wood essence; the application of said resin R is performed through an apparatus 2 employing any coating method (inkjet, roller, spraying, curtain-coating, etc.);

B. Jetting a plurality of droplets 5 of a liquid L through an inkjet printer on said fluid resin R layer 3;

C. The sinking of said droplets 5 in said base resin layer leads to the formation of areas 7 which are a mixture of said liquid L and resin R; said areas 7 are stabilized through the partial polymerization of said fluid base resin R layer and of said droplets 5 of liquid L, performed through a known photopolymerizing device 6, capable of irradiating a suitable quantity of a light of a suitable wavelength;

DI. Applying an absorbent powder 9 through a device 8 for delivering said powder; the powder 10 adheres to the partially polymerized base layer 3 of resin R and droplets 5 of liquid L; D2. Brushing through a brush device 11, preferably provided with rotating steel brushes, removing said powder 10, the liquid L of droplets 5, and at least partially the base layer 3 of said resin R in correspondence of the areas of droplets 5 deposition, which leads to the formation of cavities 12;

E. Said panel 1 is now provided with a base layer 3 of resin R with a plurality of cavities 12.

The following steps are optional.

F. Applying a photopolymerizable top coat layer 14 to said panel 1 provided with a three-dimensional embossing through a coating device 13 using any kind of technology (roller, inkjet, etc.);

G. Polymerizing said top coat layer 14 through a photopolymerizing device 15.

H. The finished panel is provided with, from bottom to top :

Base panel 1;

Fully polymerized base layer 3;

Cavities 12 in register with the base image;

Fully polymerized top coat layer 14.

[0047] In the first embodiment, said absorbent powder 9 is provided on the surface of panels before the panel reaches the brushing station, while the absorbent powder is suitably levelled out/pressed on the surface of the panel (not shown) before the removal of the partially polymerized fluids.

[0048] Figure 2 shows the second embodiment of the method according to the present invention. The dotted portion shows the new and inventive part of the method.

[0049] In its second embodiment, the method according to the present invention comprises the following steps:

A. Applying a layer 3 of a fluid resin R to a panel 1 suitably prepared with a drawing visually reproducing e.g. the vein of a wood essence; the application of said resin R is performed through an apparatus 2 employing any coating method (inkjet, roller, spraying, curtain-coating, etc.);

B. Jetting a plurality of droplets 5 of a liquid L through an inkjet printer on said fluid resin R layer 3;

C. The sinking of said droplets 5 in said base resin layer leads to the formation of areas 7 which are a mixture of said liquid L-resin R; said areas 7 are stabilized through the partial polymerization of said fluid base resin R layer 3 and of said droplets 5 of liquid L, performed through a known photopolymerizing device 6, capable of irradiating a suitable quantity of a light of a suitable wavelength ;

D. Applying an absorbent powder 9 through a device 8' capable of delivering said powder on the brush/es, and at the same time also on the surface of the panel; preferably rotating steel brushes are used; said device 8' is capable of delivering the powder 9 and brushing at the same time; the powder 10 adheres to the partially polymerized base layer 3 of resin R and droplets 5 of liquid L; the removal of said powder 10, of the base resin R layer 3 in correspondence to the areas of deposition of liquid L, and of the liquid L of droplets 5 leads to the formation of cavities 12;

E. Said panel 1 is provided with a base layer 3 of resin R with a plurality of cavities 12. The following steps are optional.

F. Applying a photopolymerizable top coat layer 14 to said panel 1 provided with a three-dimensional embossing through a coating device 13 using any kind of technology (roller, inkjet, etc.);

G. Polymerizing said top coat layer 14 through a photopolymerizing device 15.

H. The finished panel is provided with, from bottom to top :

Base panel 1;

Fully polymerized base layer 3;

Cavities 12 in register with the base image; Fully polymerized top coat layer 14.

[0050] Obviously and in a known way, in order to obtain an embossing in register, the droplets 5 forming said cavities 12 must be jetted by said inkjet printer 4 according to a pre-defined scheme in concordance with the decorative pattern previously applied on the panel 1 previously provided with the desired decorative pattern.

[0051] The material of the base panel 1 can be a medium/low-cost material like wooden materials (MDF, HDF, particleboard, plywood) , or plastic material (PVC, polyolefins).

[0052] The photopolymerizable resin R layer 3 can be acrylate (acrylate or methacrylate), or vinyl monomers, or cationic resins such as epoxy resin, polyurethane, hotmelt, polyols and monomers such as oxetanes and vinyl ethers.

[0052] The droplets 5 can be water-based or can be photopolymerizable monomers comprising products capable of inhibiting the polymerization/solidification of said resin R. [0054] According to the invention, the particle size (granulometry) of the absorbent powder is in the range 1 to 50 micrometres, preferably 5 to 25 micrometres .

[0055] Said absorbent powder is preferably industrial talc, calcium carbonate (CaCO) or 3 wooden sawdust, although even other agents could be employed. Said absorbent powder is preferably employed in a quantity ranging 2 to 20 g/m, preferably 5 to 15 g/m of panel 1 to be treated.

[0056] The brushes provided for the removal of the partially polymerized fluids must be provided with bristles of suitable hardness preferably made of steel, preferably with a Brinnell hardness in the range 150-250 kg/mm. The diameter of the brushes preferably is in the range 200-400 mm, preferably 250-350 mm. Said bristles have a diameter ranging 100-600 micrometers, preferably 150-300 micrometers, and a length ranging 20- 60 mm, preferably 25-50 mm.

[0057] The industrial production line allowing the application of the method according to the present invention comprises:

A first apparatus 2 for applying the base resin R layer 3;

An inkjet printer 4;

A first photopolymerizing device 6;

A device 8 for delivering said absorbent powder 9 and a device 11 for brushing, or alternatively, A device 8' capable of delivering said powder 9 at the same time on the brush and on the passing panel 1 and of removing said powder through one or more brushes;

Optionally a second apparatus 13 for applying a top coat layer 14;

Optionally a second photopolymerizing device 15. [0058] According to the present invention, with respect to the known art, in the industrial production line there is provided a brushing workstation, provided with at least a brush, to which brush a dispenser is coupled. Such dispenser dispenses an absorbent anti- adhesion compatible powdery material, i.e. chemically/physically intended to bond or agglomerate to polymeric materials or resins that are non-polymerized and/or non- hardened or partially polymerized or hardened. Said dispenser is provided with at least a dispensing nozzle or with a plurality of nozzles distributed along the area provided with brushing bristles, and such dispenser is oriented with its dispensing cone toward said bristles, or at least a portion of said bristles.

[0059] In an embodiment, in the industrial production line, in correspondence of said devices 8 for delivering said absorbent powder and 11 for brushing, or alternatively, in correspondence of said device 8' capable of delivering said powder on said brush/es and to remove said partially polymerized fluids, there is provided a (not shown) cyclone separator.

[0060] Cyclone separators are known in the powder coating industry as devices for the recovery of powders suspended in the airflow used for the continuous washing of spraying booths. Their normal use is separating the solid particulate, having a granulometry over a given minimum, from the air of a certain gas flow and allowing the recovery of powder through its fall in a special container, allowing the successive conveying of powder to the recovery and recycling area. In the case of the present invention, the cyclone separator has two aims: (1) separating the delivered absorbent powder 9 from the powder 10 entered into contact (adhered) to the partially polymerized fluids and therefore having a granulometry higher than that of the non-adhered powder 9; (2) separating the non-adhered absorbent powder 9 from the air of the main gaseous flow outputting from the brushing device. The cyclone separator is a two-stage device, suitably dimensioned in order to work on two minimum different granulometries of the particulate to be treated.

[0061] In an embodiment, the cyclone separation step leads not only to the recovery of the anti- adhesion absorbent powdery material, but also to the recovery of the powdery material that adhered to the removed resin or polymeric materials. Said material is subsequently used as natural charge of vegetal or mineral fibres already at least partially bonded to the specific polymeric material or to the specific resin in the preparation of synthetic materials provided with vegetal and/or powdery charges. This allows to recover a bigger quantity of powdery material, with an overall cheaper production process.

[0062] Although not shown in Figures, said industrial production line comprises known means for conveying said panel 1 to the different working stations, e.g. band conveyors. 1 base panel

2 apparatus for applying a base layer

3 base layer

4 inkjet printers 5 droplets

6 photopolymerizing device

7 droplets sunk in the base layer

8 device for powder administration

8' device for powder administration and brushing 9 absorbent powder

10 delivered and partially adhered powder

11 brushing device

12 cavities

13 apparatus for applying a top coat 14 top coat

15 photopolymerizing device