Patent Description:
More particularly the invention is related to a method for manufacturing panels, especially made of thermoplastic material, wherein said panels at least comprise a substrate material and a provided thereon top layer with a printed motif. The method of the invention is used for manufacturing panel the top layer of which is formed from PVC, including at least one thermoplastic decor foil having a printed motif. The panels of the invention may relate to furniture panels, ceiling panels, flooring panels or similar, wherein these panels comprise a substrate. According to the preferred embodiment, the panels comprise a filled synthetic composite material substrate or a mineral based substrate or cement based. These latter panels are also referred to as LVT panels (Luxury Vinyl Tiles), SPC panels (Solid Polymer Composite), WPC (Wood Polymer Composite).

Traditionally, the decor or pattern of such panels is printed on thermoplastic foil by means of offset or rotogravure printing. The obtained foil is taken up as a decor foil in a so-called LVT panel. For manufacturing the panels, preferably at least a lamination of the decor foil and a transparent thermoplastic wear layer is carried out in order to form the top layer of the panel. The mutual connection or adherence of the decor foil and the transparent wear layer is preferably obtained through a thermal lamination process, e.g. by using one or more heated press rollers. The obtained top layer may then be glued or thermally laminated to the substrate. In order to possibly form a relief in the top layer a press treatment or pressing operation may be used. Namely by bringing the thermoplastic top layer in contact with a structured press element, for example a structured press roller. The press element is preferably cooled, while the thermoplastic top layer is presented to the roller in a heated condition, such that the thermoplastic top layer may be cooled down and frozen while in contact with the press element, thereby taking over the negative of the structure of the press element.

The printing of thermoplastic foil by means of an analog printing process, such as by rotogravure or offset printing, at affordable prices inevitably leads to large minimal order quantities of a particular decorative foil and restricts the attainable flexibility. A change of decor or pattern necessitates a standstill of the printing equipment of about <NUM> hours. This standstill time is needed for exchange of the printing rollers, the cleaning of the printing equipment and for adjusting the colors of the new decor or pattern to be printed.

Instead of analog printing techniques, digital printing techniques, especially inkjet printing techniques, are becoming increasingly popular for the creation of decors or patterns, be it on paper, on foil or directly on a plate-shaped substrate possibly with the intermediary of preparatory layers. Such digital techniques can enhance the flexibility in the printing of decors significantly. Reference is amongst others made to the <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and the <CIT>, where such techniques are disclosed.

<CIT>, <CIT> and <CIT> disclose the use of an inkjet receiver coating to enhance the printing quality on a raw decor paper. Such inkjet receiver coating comprises (water) absorbing particle, especially silica, and a binder, usually a crosslinked polymer such as polyvinyl alcohol (PVA). <CIT> discloses a primer composition comprising a metallic salt and a wetting agent for printing on substrates to be laminated with thermoplastic wear layers.

If on one hand the absorbing particles can improve the quality of the print, they can stiffen the foil thereby interfering with the embossing of a structure in the top layer of the panel. Moreover, since the water absorbing particles have bad affinity with thermoplastic material used for the foil and for the wear layer, in particular PVC, and are inert during lamination, they can cause adhesion issues during the lamination process between the foil and the wear layer. Furthermore, the presence of water absorbing particles in the coating might cause delamination when the panel, in use, is cleaned with water since water can be attracted by the particles.

For the thermal lamination process to be effective it is important to guarantee good adhesion between the different thermoplastic layers. Such adhesion can be affected by the kind of inks, for example UV ink can have bad adhesion to PVC, and by the presence of liquid, for example water between the layers. <CIT> proposes to improve the adhesion between the layer by adding an adhesion promoting layer between the thermoplastic layers.

The present invention aims in the first place at an alternative method for manufacturing panels having a decorative surface and seeks, in accordance with several of its preferred embodiments, to solve one or more of the problems arising in the state of the art.

Therefore the present invention relates to a method for manufacturing a panel in accordance with claim <NUM>.

The inkjet receiving coating comprises a content of water absorbing substance below <NUM>%wt. and is free from water absorbing mineral particles, like silicate or aluminum silicates, for example talc, clays, calcined clays, kaolin, silica. By reducing the quantity of water absorbing substance the foil becomes more flexible so that it can be rolled again after printing without deteriorating the print and it is also possible to improve embossing of the structure on the top surface panel. On the other hand the inkjet receiving layer allow to improve the print quality on the foil. Within the context of the present application with water absorbing particle or substance is meant a substance, in particular in form of particle that is able to absorb liquid in particular the vehicle of the ink. Thus, the invention is not limited to water absorbing particle but is meant to encompass liquid absorbing substances.

Preferably, the inkjet receiver layer comprises polymeric substance, also called binder, preferably a thermosetting substance.

In the preferred embodiment, the binder can comprise water-based dispersion of terpolymer vinyl chloride, vinyl acetate ethylene; water-based dispersion or copolymer vinyl chloride, vinyl acetate; water-based aliphatic PU dispersion; other non-blocking water-based acrylics dispersions or a water-based styrene acrylic dispersion or combinations thereof.

In an embodiment, as polymeric substance in said inkjet receiver coating at least or mainly polyvinyl alcohols are used.

According to variants, the inkjet receiver coating includes, a polymer selected from the group consisting of hydroxyethyl cellulose; hydroxypropyl cellulose; hydroxyethylmethyl cellulose; hydroxypropyl methyl cellulose; hydroxybutylmethyl cellulose; methyl cellulose; sodium carboxymethyl cellulose; sodium carboxymethylhydroxethyl cellulose; water soluble ethylhydroxyethyl cellulose; cellulose sulfate; vinylalcohol copolymers; polyvinyl acetate; polyvinyl acetal; polyvinyl pyrrolidone; polyacrylamide; acrylamide/acrylic acid copolymer; polystyrene, styrene copolymers; acrylic or methacrylic polymers; styrene/acrylic copolymers; ethylene-vinylacetate copolymer; vinyl-methyl ether/maleic acid copolymer; poly(<NUM>-acrylamido-<NUM>-methyl propane sulfonic acid); poly(diethylene triamine-co-adipic acid); polyvinyl pyridine; polyvinyl imidazole; polyethylene imine epichlorohydrin modified; polyethylene imine ethoxylated; ether bond-containing polymers such as polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene glycol (PEG) and polyvinyl ether (PVE); polyurethane; melamine resins; gelatin; carrageenan; dextran; gum arabic; casein; pectin; albumin; chitins; chitosans; starch; collagen derivatives; collodion and agar-agar. The most preferred variants for the binder are polyvinyl acetates, ethylvinylacetates, block copolymers based on polyvinylacetate, block copolymers based on polyvinylalcohol, acrylates, latexes, polyvinyl derivaties, VCVAC derivatives, polyurethanes based on polyols and isocyanates, polyurethanes based on polycarbamates and polyaldehydes, e.g. both as a waterbaseddispersion/emulsion or a waterbased or solvent solution.

It is to be noted that, the ink receiver coating can comprise also a crosslinking agent for the crosslinking reaction of the polymeric substance itself. In case of presence of a crosslinking agent this is preferably selected from the group comprising: aldehydes, polyaldehydes, dialdehydes, alcohols, boronic acid, borax, polyalcohols, carbamates, polycarbamates, carbonic acids, glyoxal based agent, zirconium-based agents, titanates and polycarbonic acids.

The inkjet receiver coating can further comprise a dispersant. A dispersant is an oligomer or polymer which stabilize the liquid dispersions of pigment against flocculation. The dispersant can comprise polycarboxylates, polyphosphates, a polyionic polymer, preferably polyDADMAC (Polydiallyldimethylammonium chloride) polyamine or alumina salts.

Preferably, the inkjet receiver coating is provided with less than <NUM> %, more preferably less than <NUM>% based on dry coating weight of dispersant, for example between <NUM> and <NUM>%.

The inkjet receiver coating can also comprise a flocculant, preferably a metal salt, preferably a cationic metal salt. Preferably said metal salt is chosen from the list consisting of CaCl<NUM>, MgCl<NUM>, CaBr<NUM>, MgBr<NUM>, CMA (Calcium Magnesium Acetate), NH<NUM>Cl, Calcium Acetate, ZrCl<NUM>, calcium nitrate and Magnesium Acetate. The positive ion of the dissolved metal salt will tend to neutralize the electrosteric stabilization function of the pigment. The most preferred cationic metal salts are CaCl<NUM>, MgCl<NUM>, CMA, Calcium Acetate, calcium nitrate and Magnesium Acetate, as the inventors have obtained the best results with these ink reactive compounds. Said flocculant can also be chosen from the list consisting of sodiumaluminate, a double sulphate salt such as alum, polyaluminumchloride, polyacrylate, dicyandiamide (e.g. Floquat DI5 from SNF) and polyacrylamide. The flocculating agent pulls the ink pigments out of the ink dispersion. Thereby the pigments are prevented from penetration to far down into the ink receiver coating. Mainly the vehicle of the ink, e.g. the water in the case of waterbased inks, is absorbed deeper down into the ink receiver coating.

Preferably, inkjet receiver coating is provided with <NUM> to <NUM> %, based on dry coating weight of flocculating agent, in particular of metal salt.

In a particular embodiment the inkjet receiving coating can be acidic. In particular can comprise one or more acid component. Said acid component can be either organic or inorganic. Preferred examples of acid component are citric acid, formic acid, lactic acid, propionic acid or a combination thereof. Preferably said acid component can show a pH <NUM>, more preferably below <NUM>,<NUM>. Said acid component has the function of destabilizing the ink dispersion and is generally used as an alternative to the above mentioned metal salts.

The inkjet receiver coating may also comprise one or more of the following agents:.

Preferably, said foil is provided with <NUM> to <NUM>/m<NUM>, and preferably between <NUM> and <NUM>/m<NUM>, dry weight of said inkjet receiver coating.

Preferably the thermoplastic foil onto which the inkjet receiver coating is applied has a base weight of <NUM> to <NUM> grams per square meter, e.g. between <NUM> and <NUM> grams per square meter.

The thermoplastic foil can be of any material, such as polyvinylchloride (PVC) foil, polypropylene (PP) foil, polyethylene (PE) foil, polyethylene-terephthalate (PET) foil or thermoplastic polyurethane (TPU). The preferred thermoplastic material is PVC, and the preferred binder for use on such thermoplastic foils is polyurethane based, acrylate based, or polyvinyl acetate based.

In the method of the invention, the foil is printed using waterbased inks in particular comprising pigmented inks. Water-based ink are chosen because thanks to their flexibility they allow to improve embossment on the panel, moreover they represent a sustainable and ecological choice for the ink. It is also to be noted that by using water-based inks it is possible to reduce the risk of damaging the print heads thanks to the absence, or low quantity, of polymeric substances that can dry and clog the printhead.

The ink comprises a binding agent. Preferred examples of binding agent are VC-VAC copolymer or polymer latexes, either acrylates or urethane dispersion or combinations thereof components. Preferably, the inks show an amount of binding agent which is as high as possible, such that an optimum subsequent lamination with the wear layer can be obtained with an acceptable jet behavior. An amount of binder or binding agent of at least <NUM>, at least <NUM> or even at least <NUM> percent by weight of the liquid ink seems viable. Due to the jet behavior, the amount of binding agent preferably should be kept lower than <NUM> percent by weight. The binder has a MFFT (Minimum Film Forming Temperature) below <NUM>, more preferably below <NUM>, for example <NUM> or less. Such a low value of MFFT provides for an easily dryable ink so that the ink itself can be effectively fixed onto the ink receiving layer, even in absence of absorbing substances, and without excessively heating the foil.

According to the most preferred embodiment said inkjet receiver coating is applied in one step in order to form a unique layer having the inkjet receiving coating composition. Anyway, it is not excluded that said inkjet receiver coating is applied in at least two partial steps, wherein respectively a first layer with a first composition and, subsequently, a second layer is applied with a second composition wherein said first and second composition may be either the same or different compositions.

Generally, it is noted that, although the foil obtained with the method of the invention is printable with an inkjet printer, it is a non-claimed possibility that the foil eventually is printed using other techniques, such as rotogravure or offset printing. Also, in such case, the diminished dust release and the potentially better printing quality is of interest. This is especially the case when aqueous inks are being used.

Preferably, said inkjet receiving coating is a liquid substance which is deposited on said foil. Preferably the liquid substance is a water-based suspension of at least said binder.

The deposition of said liquid substance of the inkjet receiver coating can be obtained in any way, possibly by means of printing, e.g. inkjet printing, but preferably by means of coating techniques, such as roller coating, e.g. by means of one or more gravure rollers, spraying, metering rollers, bead coating, scattering, slot die coating. With the latter techniques, preferably a coating is obtained that covers at least <NUM>% of the surface of the foil. Inline measurement systems may be desirable to steer and control the weight of the inkjet receiver coating. Such technique brings down the risk of obtaining uncoated areas of the foil , which could lead to local flaws in the printed motif. A preferred equipment for application of the liquid substance is a rotogravure or anilox coating device.

The deposition of the liquid substance for the ink receiving coating may be performed in a rotogravure coating line or, alternatively, on the printing equipment, immediately before the printing operation. This last case solves any possible issues with limited shelf life of the inkjet receiver coating. Preferably the deposition of the liquid substance is performed while the foil is still in an "endless" shape, namely taken from the roll without cutting. Such techniques allow for a more uniform application of the inkjet receiver coating. In the case the coating is wholly or partially done on the printing equipment, the printing equipment is preferably a roll-to-roll or a roll-to-sheet printer, comprising a coating device upstream of the print heads, for example a gravure coater and/or additional printing heads suitable for printing the liquid substance for the respective sublayer of the inkjet receiver coating. Such additional printing heads, for example an additional row of printing heads, may have nozzles with a larger diameter than those used for the actual printing of the pattern. A resolution of <NUM> to <NUM>, or even <NUM> to <NUM> dots per inch may suffice for these nozzles.

After printing, the ink is dried to remove water or vehicle residues thereby improving the thermal lamination on the panel. According to the invention, multiple drying steps are performed, such that each drying step is performed after printing a corresponding color ink. More in detail it is possible that after each printing head or printing unit a drying unit is provided, for example a drying NIR (Near Infra-Red) pin, to perform one drying step between the printing units. It is also possible that, during printing, the foil is placed on a heated support, for example a heated printing drum. According to another possibility it is possible that the printing operation is performed at a temperature different, preferably higher, to environment temperature, for example said printing operation is performed at a temperature below <NUM>, for example below <NUM>.

In order to improve the lamination between the printed foil and the upper wear layer, it is possible to provide a first adhesion layer on top of the printed foil. Preferably, the adhesion layer comprises, or is formed by, the same substance forming the binder of the ink receiving layer. Preferably the adhesion layer can comprise water-based dispersion of terpolymer vinyl chloride, vinyl acetate ethylene; water-based dispersion or copolymer vinyl chloride, vinyl acetate; water-based aliphatic PU dispersion; other non-blocking water-based acrylics dispersions or a water-based styrene acrylic dispersion or combinations thereof. Component of the adhesion layer can be self-cross linkable or the adhesion layer can comprise a crosslinker. The adhesion layer can comprise additives like wetting agent, biocide, leveling agent, antifoam, solvent, coloring agent like pigment.

According to an embodiment of the invention, a second adhesion layer can be provided below the printed foil, i. e between the printed foil and the substrate. This is particularly the case when the substrate is not made of a thermoplastic material or when the substrate is made of a thermoplastic material that is different than that forming the foil. The second adhesion layer can be formed by the same substances forming the first adhesion layer although, according to the specification of the materials, can be formed by different substances.

The wear layer comprises a transparent or translucent sheet made of PVC. The wear layer can comprise hard particles to improve abrasion resistance, for example aluminum oxide particles.

In order to avoid early drying of the ink at the printheads of the printer, it is preferable that the printer is equipped with an ink recirculating system. Preferably the ink recirculating system comprises a recirculating capacity above <NUM>/min, more preferably above <NUM>/min, for example of <NUM>/min or more. In this way it is possible to reduce risk of damaging the print heads even in case of use of ink with low MFFT, and in case of printing at high temperature.

According to the invention, the thermoplastic foil is printed by means of an inkjet printer and is attached to said substrate material preferably by means of a hot-pressing treatment. Said inkjet printer operates on the basis of water-based inks, wherein, more particularly, an inkjet printer of the single-pass type and/or an inkjet printer operated in single-pass mode is preferred.

The method can also comprise a step of embossing the upper surface of the panel, for example by embossing the wear layer and/or the printed foil. The embossing step can be before, after or during thermal lamination of the thermoplastic foils on the substrate. It is also possible that the wear layer is provided with an embossed structure.

The foil of the invention may be a colored, pigmented and/or dyed base foil. The use of a colored and/or dyed base layer enables further limiting the dry weight of deposited ink for attaining a particular pattern or color, so that that flexibility of printed foil is improved. According to an alternative embodiment the ink receiving layer on said foil to be printed is colored or pigmented with colored pigments.

With the intention of better showing the characteristics according to the invention, in the following, as an example without limitative character, an embodiment is described, with reference to the accompanying drawings, wherein:.

<FIG> illustrates a decorative panel <NUM> comprising a substrate <NUM> made of a thermoplastic material, preferably rigid PVC in the form of the so called SPC and a top layer <NUM> provided with a printed motif <NUM>, in this case imitating a wood décor. The panel <NUM> can have the shape of a rectangular and oblong floor panel, with a pair of long sides <NUM> and a pair of short sides <NUM>. In this case the panel <NUM> is provided at least at the long sides <NUM> with coupling means <NUM> allowing to lock the respective sides <NUM> together with the sides of a similar panel both in a direction R1 perpendicular to the plane of the coupled panels, as in a direction R2 perpendicular to the coupled sides and in the plane of the coupled panels. As illustrated in <FIG> such coupling means or coupling parts can basically have the shape of a tongue <NUM> and a groove <NUM>, provided with additional cooperating locking means <NUM> allowing for said locking in the direction R2.

<FIG> show that the top layer <NUM> comprises a decorative foil <NUM> and a wear layer <NUM> provided on top of the decorative foil <NUM>. The decorative foil <NUM> comprises a thermoplastic foil <NUM> for example made of PVC, and the digitally printed décor <NUM>. The decorative foil <NUM> further comprises an inkjet receiving layer <NUM>. In the example the inkjet receiving layer comprises a binder and is free from any water absorbing substance. The binder preferably comprises water-based dispersion of terpolymer vinyl chloride, vinyl acetate ethylene; water-based dispersion or copolymer vinyl chloride, vinyl acetate; water-based aliphatic PU dispersion; other non-blocking water-based acrylics dispersions. The inkjet receiving layer further comprise a cross linker, a metal salt and other additives like dispersant, coupling agent, wetting agent or PH modifiers.

In the example, the wear layer <NUM> is a transparent thermoplastic foil made of PVC. Between the decorative foil <NUM> and the wear layer <NUM> is provided an adhesion layer <NUM> comprising water-based dispersion of terpolymer vinyl chloride, vinyl acetate ethylene; water-based dispersion or copolymer vinyl chloride, vinyl acetate; water-based aliphatic PU dispersion; other non-blocking water-based acrylics dispersions, thereby strongly improving lamination between the decorative foil and the wear layer.

<FIG> shows some step of a method for manufacturing the panel <NUM>. In the method is illustrated a step S1 of providing the thermoplastic foil <NUM> by uncoiling it from a first roll <NUM>. The foil <NUM> is then provided, on at least one surface, with the inkjet receiving layer <NUM>, for example with a doctor blade or a sprayer, in a step S2.

The coated foil <NUM> is then printed (step S3) in a single pass printer <NUM> using water-based ink. The ink comprises a binding agent having preferably a MFFT below <NUM>. for example the binding agent are VC-VAC copolymer or polymer latexes, either acrylates or urethane components, dispersion or combinations thereof.

After printing, the obtained decorative foil <NUM> is coiled again in a second roll <NUM> and then stocked, as indicated in step S4.

For the production of the panel <NUM>, the decorative foil <NUM> can be uncoiled from second roll <NUM> in a step S5 and then coated with the adhesion layer <NUM> in a step S6. The decorative foil <NUM> can then be cut into sheets <NUM>, in a step S7. The sheets <NUM> are then sandwiched between a support <NUM> and a wear layer <NUM> and then thermally laminated to form the panel <NUM>.

In <FIG> it is shown an alternative method that differs from that of <FIG> in that after printing step S3 the decorative foil <NUM> is immediately coated with the adhesion layer <NUM> and then laminated onto the substrate <NUM>. In this case, the printing line can be in line with a support manufacturing equipment, not show, for example an extruder.

<FIG> illustrates that the foil <NUM> having the inkjet receiver coating <NUM> may be printed by means of an inkjet printer <NUM>, which, in this example comprises a central cylinder <NUM> upon which the foil <NUM> is partially wound and several printing units <NUM>, each comprising one or more print heads, disposed radially around the central cylinder <NUM> and over the area of the foil <NUM> to be printed. The printer <NUM>, in this example, relates to a printer of the single pass type, wherein the provision of the printed motif involves a relative motion of said inkjet printer <NUM>, more particularly the printing unit <NUM>, and said foil <NUM> during printing in a printing direction D. In this case, the printing unit <NUM> and the print heads are at standstill, while the foil <NUM> moves during ejection of inks onto the foil <NUM>, more precisely onto the inkjet receiver coating <NUM> applied to the foil <NUM>. The foil <NUM> gets printed during a single continuous movement of the foil <NUM> itself relative the printer <NUM>.

Preferably each printing unit <NUM> is configured for printing only one color. Preferably each printing unit <NUM> extends above the entire width of the foil <NUM> to be printed. For example, each printing unit is composed by a so called colorbar.

In the embodiment illustrated in the example the printer <NUM> is provided with a plurality of NIR pins <NUM> each disposed after a respective printing unit <NUM> so that each pin <NUM> immediately dries the ink just printed by the respective printing unit <NUM>. In the example the central cylinder <NUM> comprises a heating device <NUM> to heat the foil <NUM> during printing to further help drying of the ink. Preferably the printing operation is performed at a temperature above external environment temperature but, in any case, below <NUM> to prevent deformation of the foil.

In the example each printing unit comprises an ink recirculating circuit <NUM> configured to recirculate a flow of ink <NUM>/min or more.

It is generally noted that the dimensions of the represented top layer <NUM> and its components is, in the figures, drawn out of scale in order to better illustrate the invention.

Claim 1:
A method for manufacturing a panel (<NUM>) of the type comprising a support (<NUM>) and a top layer (<NUM>), wherein the top layer (<NUM>) comprises a printed motif (<NUM>) and a wear layer (<NUM>) disposed above said motif (<NUM>), wherein said wear layer (<NUM>) comprises PVC, the top layer (<NUM>) comprising an inkjet receiving layer (<NUM>) and said inkjet receiving layer (<NUM>) comprising a content of water absorbing substance below <NUM>%wt and being free from water absorbing mineral particles, wherein the method comprises the step of providing a printing substrate in the form of a thermoplastic foil, the step of coating said printing substrate with said inkjet receiving layer (<NUM>), the step of digitally inkjet printing said motif (<NUM>) with a set of water based inks of multiple color on said coated thermoplastic foil, wherein said inks comprise a binding agent and wherein said step of digital inkjet printing comprises one or more printing operations, each of said printing operations being dedicated to print one color, wherein the coated and printed thermoplastic foil (<NUM>) is subsequently attached to a substrate, characterized in that the method comprises multiple ink drying steps, wherein each of said drying steps is performed after performing one or more respective printing operations, and wherein each ink drying step is performed after printing a correspondent ink, and in that said binding agent of the ink shows a minimum film forming temperature below <NUM>, preferably below <NUM>.