METHOD TO PRODUCE A BUILDING PANEL AND A SEMI-FINISHED PRODUCT

Abstract

A method to produce a building panel, including: providing a substrate, applying a thermosetting binder in dry form on the substrate for forming a sub-layer, applying a sheet on the sub-layer, and pressing the substrate, the sub-layer and the sheet together to form a building panel, thereby the thermosetting binder of the sub-layer impregnates the sheet from below. Also, a semi-finished product.

Claims

1. A method to produce a building panel, comprising providing a substrate, applying a thermosetting binder in dry form on the substrate for forming a sub-layer, controlling the moisture content of the sub-layer when arranged on the substrate, applying a sheet on the sub-layer, wherein the sheet comprises one or more of a paper sheet, a non-woven, a woven fibre sheet, or a fibre sheet, and wherein the sheet further comprises pigments and/or fillers, and pressing the substrate, the sub-layer and the sheet together to form a building panel, thereby the thermosetting binder of the sub-layer impregnates the sheet.

2. The method according to claim 1, wherein controlling the moisture content of the sub-layer comprises one or more of the following: applying steam on the substrate prior to applying the sub-layer, applying steam on the sub-layer when arranged on the substrate, applying steam on the sheet, applying liquid on the substrate prior to applying the sub-layer, applying liquid on the sub-layer when arranged on the substrate, applying liquid on the sheet, and/or drying the sub-layer and/or the sheet.

3. The method according to claim 1, wherein the moisture content of the sub-layer when arranged on the substrate is 5-35 wt %.

4. The method according to claim 1, wherein the sheet being applied on the sub-layer is unimpregnated or impregnated by a fluid comprising less than 20 wt % of a thermosetting binder.

5. The method according to claim 1, wherein the sub-layer consists essentially of the thermosetting binder.

6. The method according to claim 1, further comprising printing a décor on the sheet when arranged on the sub-layer.

7. The method according to claim 6, wherein the sheet is adhered to the sub-layer prior to printing.

8. A method to produce a building panel, comprising providing a substrate, controlling relative humidity (RH) of ambient air enclosing an application device adapted to apply a thermosetting binder in dry form, applying a thermosetting binder in dry form on the substrate by said application device for forming a sub-layer, applying a sheet on the sub-layer, wherein the sheet comprises pigments and/or fillers, and pressing the substrate, the sub-layer and the sheet together to form a building panel, thereby the thermosetting binder of the sub-layer impregnates the sheet.

9. The method according to claim 8, wherein the relative humidity (RH) is 35-60% RH.

10. The method according to claim 8, wherein the sheet being applied on the sub-layer is unimpregnated or impregnated by a fluid comprising less than 20 wt % of a thermosetting binder.

11. The method according to claim 8, wherein the sheet comprises one or more of a paper sheet, a non-woven, a woven fibre sheet, or a fibre sheet.

12. The method according to claim 8, wherein the sub-layer consists essentially of the thermosetting binder.

13. The method according to claim 8, further comprising printing a décor on the sheet when arranged on the sub-layer.

14. The method according to claim 13, wherein the sheet is adhered to the sub-layer prior to printing.

15. The method according to claim 8, further comprising applying moisture to the thermosetting binder, the sub-layer, the sheet, and/or the substrate prior to pressing.

16. The method according to claim 8, wherein a moisture content of the sub-layer when arranged on the substrate is 5-35 wt %.

17. A method to produce a building panel, comprising providing a substrate, applying at least one thermosetting binder on the substrate, controlling reactivity of said at least one thermosetting binder by applying different types and/or forms of thermosetting binders on the substrate, applying a sheet on said at least one thermosetting binder, wherein the sheet comprises one or more of a paper sheet, a non-woven, a woven fibre sheet, or a fibre sheet, and wherein the sheet further comprises pigments and/or fillers, and pressing the substrate, said at least one thermosetting binder and the sheet together to form a building panel, thereby said at least one thermosetting binder impregnates the sheet.

18. The method according to claim 17, wherein controlling reactivity of said at least one thermosetting binder comprises applying a first thermosetting binder in liquid form and applying a second thermosetting binder in powder form on the first thermosetting binder.

19. The method according to claim 17, wherein the sheet being applied on said at least one thermosetting binder is unimpregnated or impregnated by a fluid comprising less than 20 wt % of a thermosetting binder.

20. A method to produce a building panel, comprising providing a substrate, applying a thermosetting binder in dry form on the substrate for forming a sub-layer, applying a sheet on the sub-layer, wherein the sheet comprises one or more of a paper sheet, a non-woven, a woven fibre sheet, or a fibre sheet, and wherein the sheet further comprises pigments and/or fillers, and pressing the substrate, the sub-layer and the sheet together to form a building panel, thereby the thermosetting binder of the sub-layer impregnates the sheet.

21. The method according to claim 20, wherein the sheet being applied on the sub-layer is unimpregnated or impregnated by a fluid comprising less than 20 wt % of a thermosetting binder.

22. The method according to claim 20, wherein the sub-layer consists essentially of the thermosetting binder.

23. The method according to claim 20, wherein the sub-layer comprises the thermosetting binder and non-absorbing fillers.

24. The method according to claim 20, further comprising applying moisture to the thermosetting binder, the sub-layer, the sheet, and/or the substrate prior to pressing.

25. The method according to claim 20, wherein a moisture content of the sub-layer when arranged on the substrate is 5-35 wt %.

26. The method according to claim 20, further comprising printing a décor on the sheet when arranged on the sub-layer.

27. The method according to claim 26, wherein the sheet is adhered to the sub-layer prior to printing.

28. A semi-finished product adapted to form, or form part of, a building panel, comprising a substrate, a sub-layer arranged on the substrate, a sheet arranged on the sub-layer, wherein the sheet comprises one or more of a paper sheet, a non-woven, a woven fibre sheet, or a fibre sheet, and wherein the sheet further comprises pigments and/or fillers, and wherein the moisture content of the sub-layer is 5-35 wt % such that the sheet is adhered to the sub-layer.

29. The method according to claim 28, wherein the sub-layer consists essentially of the thermosetting binder.

30. The method according to claim 28, wherein the sub-layer comprises the thermosetting binder and non-absorbing fillers.

31. The semi-finished product according to claim 28, wherein the thermosetting binder is a B-stage thermosetting binder.

32. The semi-finished product according to claim 28, wherein the thermosetting binder is in A-stage and B-stage.

33. The semi-finished product according to claim 28, wherein the sub-layer has impregnated less than half the thickness of the sheet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0160] The present invention will by way of example be described in more detail with reference to the appended schematic drawings, which show embodiments of the present invention.

[0161] FIG. 1 shows an embodiment of a method to produce a building panel.

[0162] FIG. 2 shows an embodiment of a semi-finished product.

[0163] FIG. 3 shows an embodiment of a method to produce a building panel.

[0164] FIG. 4 shows an embodiment of a semi-finished product.

DETAILED DESCRIPTION

[0165] An embodiment of a method to produce a building panel 10 will now be described with reference to FIG. 1. In FIG. 1, a substrate 1 is provided. The substrate 1 may be a wood-based board. The substrate 1 may be a wood-fibre based board such as MDF, HDF, particleboard, OSB, etc. The substrate 1 may also be a WPC (Wood Plastic Composite) or other type of substrate comprising a thermoplastic material, optionally with fillers. The substrate may be a mineral board or a plasterboard. The substrate may be a phenolic paper or HPL.

[0166] The substrate 1 is conveyed, for example be a conveyor belt 2 or similar, under an application device 3. The application device 3 applies, preferably scatters, a dry powder material on an upper surface of the substrate 1 such that a sub-layer 4 is formed on the substrate 1. The dry powder material may substantially consist of a thermosetting binder.

[0167] The sub-layer 4 thereby formed may substantially consist of the thermosetting binder. The thermosetting binder may be in dry form, preferably dry powder form, when applied on the substrate 1. By substantially consisting of a thermosetting binder is meant that the sub-layer 4 comprises at least 90 wt % of the thermosetting binder, such as at least 95 wt % of the thermosetting binder. Preferably, the sub-layer 4 is filler-free, or preferably at least free from fillers absorbing moisture and thereby not absorbs the thermosetting binder, such as free from wood fibres. Preferably, the fillers may absorb at least less moisture, and thereby thermosetting binder, than wood fibres. The sub-layer 4 may comprise additives such as wetting agents, anti-stat agents, release agents, catalysts, etc. The sub-layer 4 may also comprise pigment. Further, the sub-layer 4 may comprise inert fillers, such as fillers not absorbing moisture, and thereby not absorbs the thermosetting binder. In one embodiment, the sub-layer 4 comprises the thermosetting binder and non-absorbing filler such as glass fibres, barium sulphate, calcium carbonate, talc, etc. The non-absorbing filler may be a non-porous filler. The filler may be an inorganic filler. Such non-absorbing filers and/or pigments may cover the colour of the substrate and provide a desired colour of the sub-layer.

[0168] The thermosetting binder may be an amino resin. The thermosetting binder may be melamine formaldehyde resin, urea formaldehyde resin, phenol formaldehyde resin, or a combination thereof. The thermosetting binder may be spray-dried.

[0169] The thermosetting binder is applied in an amount of 50-200 g/m2.

[0170] The relative humidity (RH) of the ambient air enclosing the application device 3 may be controlled. The relative humidity of the ambient air enclosing the application device 3 may be 35-60% RH, preferably 40-55% RH. The application device 3 may be enclosed by a housing 34 such that the relative humidity (RH) of the ambient air can be controlled.

[0171] By controlling the relative humidity (RH) of the ambient air, it is possible to apply the thermosetting binder in dry form without the thermosetting binder getting sticky and adheres to the application device 3, or at least reducing the difficulties involved with applying, such as scattering, a powder material having a high concentration of thermosetting binder without any fillers absorbing the moisture. When applying a mix comprising both thermosetting binder and absorbing fillers, the fillers may absorb the moisture without the thermosetting binder getting too sticky. When applying a thermosetting binder without any absorbing fillers, problems associated therewith can be at least reduced by controlling the relative humidity (RH) of the ambient air enclosing the applying device 3.

[0172] In order to get the thermosetting binder of the sub-layer 4 sticky enough to obtain adherence to a sheet 6, moisture 5 may be applied. The moisture content may be controlled such that desired properties, such as reactivity, adherence, etc., are obtained.

[0173] It is desired that the moisture content of the sub-layer 4 when arranged on the substrate 1 is in the range of 5-35 wt %, preferably 15-25 wt %. The desired moisture content may be controlled by applying moisture 5 on the substrate prior to applying the sub-layer 4, on the sub-layer 4, and/or on the sheet 6. For example, moisture 5 may be applied and/or may be controlled by one or more of the following:

[0174] applying steam on the substrate 1 prior to applying the sub-layer 4,

[0175] applying steam on the sub-layer 4 when arranged on the substrate 1,

[0176] applying steam on the sheet 6 prior to being applied on the substrate 1,

[0177] applying steam on the sheet 6 when being arranged on the substrate 1,

[0178] applying liquid on the substrate 1 prior to applying the sub-layer 4,

[0179] applying liquid on the sub-layer 4 when arranged on the substrate 1,

[0180] applying liquid on the sheet 6 prior to being applied on the substrate 1,

[0181] applying liquid on the sheet 6 when being arranged on the substrate 1,

[0182] drying the sheet 4, and/or

[0183] drying the sub-layer 4 when arranged on the substrate 1.

[0184] In the embodiment shown in FIG. 1, moisture 5 is applied on the sub-layer 4 when arranged on the substrate 1.

[0185] In the embodiment wherein moisture is applied to the sub-layer 4 by means of the sheet 6, the moisture content of the sub-layer 4 is to be measured when the sheet 6 is arranged on the sub-layer 4.

[0186] In one embodiment, moisture is applied by applying moist fillers on the sub-layer 4 and/or on the substrate 1.

[0187] When the sub-layer 4 is arranged on the substrate, the thermosetting binder of the sub-layer is substantially in B-stage.

[0188] The sub-layer 4 adheres to an upper surface of the substrate. A sheet 6 is applied on the sub-layer 4. The sheet may be a paper sheet, a non-woven, a woven fibre sheet such as textile, or a fibre sheet. The sheet may be fibre reinforced. The fibres may be cellulose fibres and/or cotton fibres. The sheet may include pigments and fillers.

[0189] The sheet 6 may be a conventional paper intended for laminate flooring, such as a paper intended to be used for rotogravure. The sheet may be an overlay paper. The sheet may be a paper adapted for ink jet printing.

[0190] The weight of the sheet 6 may for instance be 20-120 g/m2, preferably 45-80 g/m2, such as about 80 g/m2. The sheet 6 may be provided as separate sheets or as a continuous web. The weight of the sheet may depend on the amount of ink printed. If less ink is applied, the weight of the sheet can be reduced.

[0191] In one embodiment, the sheet 6 may be unimpregnated, or at least substantially unimpregnated. By unimpregnated is meant that the sheet 6 is substantially resin-free. By substantially unimpregnated is meant that the sheet may have been impregnated by a fluid comprising less than 20 wt % of a thermosetting binder, preferably comprising less than 10 wt % of a thermosetting binder. By substantially unimpregnated is meant that the surface of the sheet is still permeable. By the sheet still being permeable, the sheet may still absorb ink during printing, such that bleeding of the print is reduced. The sheet may have passed an impregnation bath comprising an impregnating fluid comprising less than 20 wt % of a thermosetting binder, preferably comprising less than 10 wt % of a thermosetting binder. The side of the sheet adapted to face the substrate may have passed the impregnation bath. The sheet may thereafter be dried, preferably from the side of the sheet adapted to be printed, i.e. the side of the sheet adapted to be facing away from the substrate. In this embodiment, the sheet 6 may contain a small amount of thermosetting binder.

[0192] The sheet 6 may be steamed, or liquid may be applied, prior to being applied on the sub-layer 4, in order to swell the sheet and in order to avoid that wrinkles are formed in the sheet 6 when arranged on the sub-layer 4. In this manner, moisture 5 may be applied also on the sub-layer 4. The sheet 6 may also be stretched by a roller (not shown) prior to being applied to the sub-layer 4. In this embodiment, wherein moisture is applied to the sheet 6, the sub-layer may be applied in dry form, such as having a moisture content of less than 10% when being applied.

[0193] The sheet 6 is applied on the sub-layer 4 by a roller 15. The roller 15 may be a flexible roller. If the sub-layer and/or the substrate has an uneven surface, a flexible roller follows any variation in thickness such that variations in pressure are reduced. Variations in pressure may otherwise result in poor printing quality in portions being subjected to high pressure.

[0194] It is desired that the moisture content of the sub-layer 4 when the sheet is applied is in the range of 5-35 wt %, preferably 15-25 wt %.

[0195] By the moisture 5 applied, the thermosetting binder of the sub-layer 4 gets sticky such that the sheet 6 is adhered to the sub-layer 4. Thereby, the sheet 6 is adhered to the substrate 2 by means of the sub-layer 4. However, the upper side of the sheet 6 facing away from the sub-layer 4 remains unimpregnated by the thermosetting binder of the sub-layer 4 and substantially from the thermosetting binder of the sub-layer 4.

[0196] Thereby, a semi-finished product 11 is obtained, which is shown in more detail in FIG. 2. The semi-finished product 11 is a pre-preg for a building panel suitable for printing on the upper surface of the sheet 6. The thermosetting binder, forms the sub-layer 4 arranged on the substrate 1. In the semi-finished product 11, the thermosetting binder of the sub-layer 4 is substantially in B-stage. The moisture content is preferably in the range of 5-35 wt %, preferably 15-25 wt %. Due to the moisture, the thermosetting binder of the sub-layer 4 gets sticky such that the sub-layer 4 adheres to the substrate 1 and to the sheet 6. The semi-finished product 11 may therefore be handled and stored, for example, for a printing step performed later on.

[0197] When the sheet 6 is adhered to the sub-layer 4, the upper surface of the sheet 6, facing away from the sub-layer 4 may be printed. A décor may be printed on the upper surface of the sheet 6 by digital printing. The printer 7 may be an inkjet printer. The digital printing technique may be any non-contact printing method, such as using a Drop-on-Demand (DOD) technique. Examples of DOD techniques are piezoelectric DOD inkjet and thermal DOD inkjet. The ink used may be curable inks such as UV curable inks, solvent based inks, and aqueous inks (also called waterborne or water based inks).

[0198] If the sheet 6 is provided in form of a continuous web, the web may be cut into separate sheets prior to printing.

[0199] Prior to pressing, the printed décor may be dried by means of IR, NIR, hot air, etc. (not shown).

[0200] Prior to pressing, the printed décor may be dried by means of IR, NIR, hot air, etc. (not shown).

[0201] Prior to pressing, a protective layer (not shown) may be applied on the sheet 6 provided with the décor. The protective layer may be any kind of protective layer, such as a resin impregnated overlay paper with wear resistant particles, a powder overlay comprising a thermosetting binder applied in dry form with wear resistant particles, and/or a protective layer comprising a thermoplastic material.

[0202] The substrate 1, the sub-layer 4, the sheet 6, and any optional protective layer (not shown) are thereafter pressed together such that a building panel 10 is formed. The press 8 may be a static or continuous press. Preferably, both heat and pressure is applied. Pressing may take place during 8-35 seconds at 140-210° C. at about 40 bar. During pressing, the thermosetting binder of the sub-layer 4 impregnates the sheet 6 such that an impregnated décor sheet of the building panel 10 is formed. After pressing, the thermosetting binder/binders is substantially in its C-stage. During pressing, the sheet 6 provided with the décor may be embossed, preferably in register with the printed décor.

[0203] As an alternative or complement to applying a protective layer prior to pressing, a protective coating (not shown), such as a radiation curing coating, may be applied to the impregnated décor sheet after pressing.

[0204] The building panel 10 may be provided with mechanical locking system for joining with an adjacent building panel. The mechanical locking system may be of the type described in WO 2007/015669, WO 2008/004960, WO 2009/116926, or WO 2010/087752, the entire contents of each is expressly incorporated by reference herein.

[0205] The building panel 10 may be a floor panel, a ceiling panel, a wall panel, a door panel, a worktop, a furniture component, etc. The printed sheet 6 forms a décor layer of the building panel 10.

[0206] An embodiment of a method to produce a building panel 10 will now be described with reference to FIG. 3. In FIG. 3, a substrate 1 is provided. The substrate 1 may be a wood-based board. The substrate 1 may be a wood-fibre based board such as MDF, HDF, particleboard, OSB, etc. The substrate 1 may also be a WPC (Wood Plastic Composite) or other type of substrate comprising a thermoplastic material, optionally with fillers. The substrate may be a mineral board or a plasterboard.

[0207] The substrate 1 is conveyed, for example be a conveyor belt 2 or similar, under an application device. The application device 3 applies thermosetting binder/binders on an upper surface of the substrate 1. In the embodiment shown in FIG. 3, the application device 3 comprises a first application device 31 and a second application device 32.

[0208] The first application device 31 applies a first thermosetting binder in a first step such that a first layer 12 comprising the first thermosetting binder is formed. In a second step, the second application device 32 applies a second thermosetting binder on the first layer 12 such that a second layer is formed 13. The first layer and the second layer together form a sub-layer. Preferably, the first thermosetting binder has a higher reactivity compared to the second thermosetting binder. In one embodiment, the first thermosetting binder may be urea formaldehyde resin and the second thermosetting binder may be melamine formaldehyde resin. In one embodiment, the first and second thermosetting binders may be of the same type but may be applied in different forms, for example, a first thermosetting binder applied in liquid form and a second thermosetting binder applied in dry form. As a further example, the first thermosetting binder may be urea formaldehyde resin applied in liquid form, and the second thermosetting binder may be melamine formaldehyde resin applied in dry form. As an example, the first thermosetting binder may have moisture content exceeding 55 wt % and the second thermosetting binder may have moisture content of less than 30 wt %.

[0209] The first and/or the second thermosetting binder may be an amino resin. The first and/or the second thermosetting binder may be melamine formaldehyde resin, urea formaldehyde resin, phenol formaldehyde resin, or a combination thereof. Additives such as wetting agents, anti-stat agents, release agents, catalysts, etc., may be added the first and/or the second thermosetting binder.

[0210] Thereby, by combining different thermosetting binders and/or different forms of thermosetting binders, such as liquid and dry, reactivity may be controlled. By applying the first layer 12 of a thermosetting binder having high reactivity on the substrate 1, during pressing, the thermosetting binder with high reactivity prevents flowing of any other thermosetting binder with lower reactivity into the substrate since the thermosetting binder with high reactivity cures faster than the other thermosetting binder with lower reactivity applied thereon. Thereby, the other thermosetting binder with lower reactivity will instead flow into the sheet and impregnate the sheet.

[0211] The first and the second layers 12, 13 formed may substantially consist of the first thermosetting binder and the second thermosetting binder, respectively. By substantially consisting is meant that the first and/or second layers comprises at least 90 wt % of the thermosetting binder, such as at least 95 wt % of the thermosetting binder. Preferably, the first and the second layers 12, 13 are filler-free, or preferably at least free from fillers absorbing moisture and thereby not absorbs the thermosetting binder, such as free from wood fibres. Preferably, the fillers may absorb at least less moisture, and thereby thermosetting binder, than wood fibres. The first and the second layers 12, 13 may comprise additives such as wetting agents, anti-stat agents, release agents, catalysts, etc. The first and the second layers 12, 13 may also comprise pigment. Further, the first and the second layers 12, 13 may comprise inert fillers, such as fillers not absorbing moisture and thereby not absorbs the thermosetting binder. In one embodiment, the first and second layers 12, 13 comprise the thermosetting binder and non-absorbing filler such as glass fibres, barium sulphate, calcium carbonate, talc, etc. The non-absorbing filler may be a non-porous filler. The filler may be an inorganic filler. Such non-absorbing filers and/or pigments may cover the colour of the substrate and provide a desired colour of the first and second layers 12, 13.

[0212] As described above, the relative humidity (RH) of the ambient air enclosing the application device 3 may be controlled. Especially, the ambient air enclosing the second application device 32 applying the second thermosetting binder may be controlled, especially if the second thermosetting binder is applied in powder form.

[0213] The relative humidity of the ambient air enclosing the application device, or at least the second application device 32 applying a thermosetting binder in powder form, may be 35-60% RH, preferably 40-55% RH. The second application device 32 may be enclosed by a housing 33 such that the relative humidity (RH) of the ambient air can be controlled.

[0214] By controlling the relative humidity (RH) of the ambient air, it is possible to apply the thermosetting binder in powder form without the thermosetting binder getting sticky and adhere to the second application device 32, or at least reducing the difficulties involved with applying, such as scattering, a powder material having a high concentration of thermosetting binder without any fillers absorbing the moisture.

[0215] In order to get the thermosetting binder of the second layer 13 sticky enough to obtain adherence to a sheet 6, moisture 5 may be applied. The moisture content may be controlled such that desired properties, such as reactivity, adherence, etc., are obtained.

[0216] It is desired that the moisture content of the second layer 13 is in the range of 5-35 wt %, preferably 15-25 wt %. The desired moisture content may be controlled by applying moisture 5 on the substrate 1, on the first layer 12 of the first thermosetting binder, on the second layer 13 of the second thermosetting binder, and/or on the sheet 6. Sufficient amount of moisture 5 may also be achieved by applying the first thermosetting binder in liquid form. For example, moisture 5 may be applied and/or may be controlled by one or more of the following:

[0217] applying steam on the substrate 1 prior to applying the first layer 12 of the first thermosetting material,

[0218] applying steam on the second layer 13 of the second thermosetting binder,

[0219] applying steam on the sheet 6 prior to being applied on the substrate 1,

[0220] applying steam on the sheet 6 when being arranged on the substrate 1,

[0221] applying liquid on the substrate 1 prior to applying the first layer 12 of the first thermosetting material,

[0222] applying liquid on the second layer 13 of the second thermosetting binder

[0223] applying liquid on the sheet 6 prior to being applied on the substrate 1,

[0224] applying liquid on the sheet 6 when being arranged on the substrate 1, and/or

[0225] drying the any of the first and/or second layers 12, 13 when arranged on the substrate.

[0226] In the embodiment shown in FIG. 3, moisture 5 is applied on the second layer 13.

[0227] When arranged on the substrate 1, the first thermosetting binder may be substantially in A-stage and the second thermosetting binder may be substantially in B-stage.

[0228] The sub-layer 14 formed by the first and second layers 12, 13 adheres to the substrate 1.

[0229] A sheet 6 is applied on the second layer 13. The sheet 6 may be a paper sheet, a non-woven, woven fibre sheet such as textile, or a fibre sheet. The sheet may be fibre reinforced. The fibres may be cellulose fibres and/or cotton fibres. The sheet may include pigments and fillers.

[0230] The sheet 6 may be a conventional paper intended for laminate flooring, such as a paper intended for rotogravure. The sheet may be an overlay paper. The sheet may be a paper adapted for ink jet printing.

[0231] The weight of the sheet 6 may for instance be 20-120 g/m2, preferably 45-80 g/m2, such as about 80 g/m2. The sheet 6 may be provided as separate sheets or as a continuous web. The weight of the sheet may depend on the amount of ink printed. If less ink is applied, the weight of the sheet can be reduced.

[0232] In one embodiment, the sheet 6 may be unimpregnated, or at least substantially unimpregnated. By unimpregnated is meant that the sheet 6 is substantially resin-free. By substantially unimpregnated is meant that the sheet may have been impregnated by a fluid comprising less than 20 wt % of a thermosetting binder, preferably comprising less than 10 wt % of a thermosetting binder. By substantially unimpregnated is meant that the surface of the sheet is still permeable. By the sheet still being permeable, the sheet may still absorb ink during printing, such that bleeding of the print is reduced. The sheet may have passed an impregnation bath comprising an impregnating fluid comprising less than 20 wt % of a thermosetting binder, preferably comprising less than 10 wt % of a thermosetting binder. The side of the sheet adapted to face the substrate may have passed the impregnation bath. The sheet may thereafter be dried, preferably from the side of the sheet adapted to be printed, i.e. the side of the sheet adapted to be facing away from the substrate. In this embodiment, the sheet 6 may contain a small amount of thermosetting binder.

[0233] The sheet 6 may be steamed, or liquid may be applied, prior to being applied on the sub-layer 14, in order to swell the sheet and in order to avoid that wrinkles are formed in the sheet 6 when arranged on the sub-layer 14. In this manner, moisture 5 may be applied also on the sub-layer 14. The sheet 6 may also be stretched by a roller (not shown) prior to being applied to the sub-layer 14.

[0234] The sheet 6 is applied on the sub-layer 14 by a roller 15. The roller 15 may be a flexible roller. If the sub-layer 14 and/or the substrate 1 have an uneven surface, a flexible roller follows any variation in thickness such that variations in pressure are reduced. Variations in pressure may otherwise result in poor printing quality in portions being subjected to high pressure.

[0235] When applying the sheet 6, it is desired that the second layer 13 has a moisture content the range of 5-35 wt %, preferably 15-25 wt %.

[0236] By the moisture 5 applied, the thermosetting binder of the second layer 13 gets sticky such that the sheet 6 is adhered to the second layer 13. Thereby, the sheet 6 adheres to the substrate 1 by means of the first and the second layers 12, 13. However, the upper side of the sheet 13 facing away from the sub-layer remains unimpregnated by the thermosetting binder of the second layer 13 and substantially free from the thermosetting binders of the second layer 13.

[0237] Thereby, a semi-finished 21 product is obtained, which is shown in more detail in FIG. 4. The semi-finished product 21 is a pre-preg for a building panel suitable for printing on the upper surface of the sheet 6. The first and the second layers 12, 13 form the sub-layer 14, which may substantially consist of thermosetting binders. By substantially consisting of a thermosetting binder is meant that the sub-layer comprises at least 90 wt % of the thermosetting binder, such as at least 95 wt % of the thermosetting binder. Preferably, the sub-layer is filler-free, or preferably at least free from fillers absorbing moisture, and thereby not absorbing the thermosetting binder. Preferably, the fillers may absorb at least less moisture, and thereby thermosetting binder, than wood fibres. The sub-layer may comprise inert fillers such as non-absorbing fillers. In one embodiment, the sub-layer 4 comprises the thermosetting binder and non-absorbing filler such as glass fibres, barium sulphate, calcium carbonate, talc, etc. The non-absorbing filler may be meant a non-porous filler. The filler may be an inorganic filler. Such non-absorbing filers and/or pigments may cover the colour of the substrate and provide a desired colour of the sub-layer.

[0238] In the semi-finished product 21, the thermosetting binder/binders is substantially in A-stage and in B-stage. The first thermosetting binder may be in A-stage. The second thermosetting binder may be in B-stage. The moisture content of the second layer 13 is preferably in the range of 5-35 wt %, preferably 15-25 wt %. Due to the moisture, the thermosetting binder of the second layer 13 gets sticky such that the first and second layers 12, 13 adheres to the substrate 1 and to the sheet 6. The semi-finished product may therefore be handled and stored, for example, for a printing step performed later on.

[0239] When the sheet 6 is adhered to the second layer 13, the upper surface of the sheet 6, facing away from the second layer 13, may be printed. A décor may be printed on the upper surface of the sheet 6 by digital printing. The printer 7 may be an inkjet printer. The digital printing technique may be any non-contact printing method, such as using a Drop-on-Demand (DOD) technique. Examples of DOD techniques are piezoelectric DOD inkjet and thermal DOD inkjet. The ink used may be curable inks such as UV curable inks, solvent based inks, and aqueous inks (also called waterborne or water based inks).

[0240] If the sheet 6 is provided in form of a continuous web, the web may be cut into separate sheets prior to printing.

[0241] Prior to printing, the printed décor may be dried by means of IR, NIR, hot air, etc. (not shown).

[0242] Prior to pressing, the printed décor may be dried by means of IR, NIR, hot air, etc. (not shown).

[0243] Prior to pressing, a protective layer (not shown) may be applied on the sheet 6 provided with the décor. The protective layer may be any kind of protective layer, such as a resin impregnated overlay paper with wear resistant particles, a powder overlay comprising a thermosetting binder applied in dry form with wear resistant particles, and/or a protective layer comprising a thermoplastic material.

[0244] The substrate 1, the sub-layer 14 formed by the first and the second layers 12, 13, the sheet 6, and any optional protective layer (not shown) are thereafter pressed together such that a building panel 10 is formed. The press 8 may be a static or continuous press. Preferably, both heat and pressure is applied. Pressing may take place during 8-35 seconds at 140-210° C. at about 40 bar. During pressing, the thermosetting binder of at least the second layer 13 impregnates the sheet 6 such that an impregnated décor sheet of the building panel 10 is formed. The first layer 12 of the first thermosetting binder having higher reactivity compared the second thermosetting binder prevents, or at least reduce, flowing of the second thermosetting binder into the substrate 1, since the first thermosetting binder cures faster than the second thermosetting binder. Thereby, the flowing of the second thermosetting binder can be controlled to flow into the sheet to impregnate the sheet. During pressing, the sheet 6 provided with the décor may be embossed, preferably in register with the printed décor. After pressing, the thermosetting binder/binders is substantially in its C-stage.

[0245] As an alternative or complement to applying a protective layer prior to pressing, a protective coating (not shown), such as a radiation curing coating, may be applied to the impregnated décor sheet after pressing.

[0246] The building panel 10 may be provided with mechanical locking system for joining with an adjacent building panel. The mechanical locking system may be of the type described in WO 2007/015669, WO 2008/004960, WO 2009/116926, or WO 2010/087752, the entire contents of each is expressly incorporated by reference herein.

[0247] The building panel 10 may be a floor panel, a ceiling panel, a wall panel, a door panel, a worktop, a furniture component, etc. The printed sheet forms a décor layer of the building panel 10.

[0248] It is also contemplated the décor may already be printed on the sheet 6 when arranged on the sub-layer 4 or on the second layer 13 of the above described type. The décor may be printed on the sheet 6 by conventional printing techniques, such as rotogravure. In this embodiment, the sub-layer 4 or the second layer 13 adheres the sheet 6 to the substrate 1. During pressing, the printed sheet 6 is impregnated by means of the thermosetting binder of the underlying sub-layer 4 or of the underlying second layer 13.

[0249] It is contemplated that there are numerous modifications of the embodiments described herein, which are still within the scope of the invention as defined by the appended claims. For example, it is contemplated that more than one wear resistant foil may be arranged on a core for forming a building panel.

[0250] It is also contemplated that in embodiments, for example when the substrate is a phenolic paper or HPL, the substrate, the sub-layer and the sheet may be attached to a board, for example, by an adhesive, in a separate process after pressing.

EXAMPLE

[0251] 150 g/m2 of a spray dried melamine formaldehyde resin powder was applied on a HDF board. 20 g/m2 of water was sprayed on the melamine formaldehyde resin layer. Steam is applied on an unimpregnated paper adapted for inkjet printer. The unimpregnated paper is applied on the melamine formaldehyde resin layer by a roller. A décor is printed on the unimpregnated paper by an inkjet printer with aqueous inks when the paper is arranged on the melamine formaldehyde resin layer. An overlay paper is arranged on the unimpregnated paper after printing. The assembly is pressed during 15 seconds at 175° C. and 40 bar.