GENERAL FIELD OF THE INVENTION

The invention pertains to a method for producing a multilayer panel provided with a surface coating that is impervious to water, and to a panel produced this way.

BACKGROUND ART In the manufacture of furniture, cabinets, household articles, counter tops, floor and wall decorations and the like, it is commonly known to use multilayer panels. Multilayer panels are dimensionally very stable, even when made from low cost materials such as wood chips and paper. By applying a surface coating, not only a functional and decorative surface can be created but at the same time the impression is raised that the panel is a solid, high cost panel made for example of high quality wood. The surface coating typically consists of a sheet material that is impervious to water and is adhered to one or more of the planar portions of the panel. The surface coating provides for an aesthetic and durable use of the panel. In recent years, a lot of attention has gone to developing sustainable laminates for covering panels which led i.a. to the development of new types of high pressure laminates (HPL, produced by saturating multiple layers of kraft paper with phenolic resin), thermally fused laminates (TFL, wherein a resin- impregnated sheet of decor paper is fused directly to a panel), new types of decorative papers and foils (mostly pre-impregnated with a blend of melamine, acrylic and urea resins) and new types of rigid thermoformable foils (RTF, thermoplastic 2D and 3D coverings). Also, a lot of development has gone into finding alternatives for wood as a resource for making panels. Panels made of the fibrous residue of sugarcane, beets, grain, or panels made of paper waste, stone, recycled and recovered wood materials etc. have been described in the art, aiming at a design that has a lower impact on the availability of natural resources.

The most common type of multilayer panel is a panel made of 7-15 thin layers of wood, i.e. veneer. The manufacture cycle of such a panel starts with the cutting of the log: by rotating it longitudinally against a blade, veneers are obtained. The veneer strips are then accurately dried, sorted out according to quality and then spliced transversally and longitudinally to obtain the desired dimensions. The veneer strips are glued together with different techniques according to their purpose. Water based solvents or resin binders such as urea and melamine binders are typically used since they are able to penetrate into the wood and provide a high strength binding. The panels are then conveyed to the pressing lines, single- or double-compartment, according to their size. Once the panel is formed, it is sanded to make its thickness uniform and squared till obtaining the desired dimensions. Thereafter, the panel is brought over to a coating machine where a surface coating is applied. In most cases, the coating is applied at a remote location in a different factory, providing the additional advantage that residual water, e.g. originating from the glue, can leave the panel before the water impervious coating is applied. Other types of multilayer panels are made correspondingly, having in common that firstly the multilayer panel is made by gluing sub-panels to each other and thereafter a decorative finish is applied. In practice, manufacturing the panel is done by one manufacturing party, while another party applies the surface coating, the type of which depends on the desired use of the panel.

OBJECT OF THE INVENTION

It is an object of the invention to provide an alternative way to manufacture a multilayer panel.

SUMMARY OF THE INVENTION

In order to meet the object of the invention, a method for producing a multilayer panel having a surface coating that is impervious to water is provided, the method comprising providing a first polysaccharide fibre based sub-panel having a bottom side and a top side, providing a second polysaccharide fibre based sub-panel which is congruent to the first sub-panel, the second panel having a bottom side and a top side, applying a layer of a hot melt adhesive on the top side of the first sub-panel, stacking the second subpanel in a spatially aligned relationship on the first sub-panel by lying the bottom side of the second sub-panel on the layer of the hot melt adhesive, applying a coating material on the top side of the second sub-panel, leading to an intermediate product comprising the stacked first and second sub-panel with an intermediate layer of the hot melt adhesive and the coating material on its top side, heating the intermediate product such that the hot melt adhesive reaches a temperature above its melting temperature, while applying pressure on the intermediate product, cooling down the intermediate product, thereby forming the multilayer panel with its water impervious coating.

To applicant’s surprise, against prior art teachings, a multilayer panel can be manufactured in one process step, in which step also the water impervious coating is applied. It was even more surprising that this can be achieved using polysaccharide fibre based sub-panels, since these panels are inherently hydrophilic and thus inherently contain water which was always believed to be detrimental for applying a water impervious surface coating without sufficient drying of the multi-layer panel after its assembly out of multiple sub-panels. Another surprise was that this can be achieved using a hot melt adhesive instead of the usual (hydrophilic) thermoset resins or water based glues. A hot melt adhesive is highly viscous upon cooling and does not penetrate into the panels along the polysaccharide fibres, at least not to the extent for example a water based adhesive does. A good penetration is believed to be a requisite for manufacturing a durable multilayer panel. Another important reason why hot melt adhesives are typically not used for making polysaccharide based panels, is that the polysaccharide used most widely, i.e. cellulose of natural origin (e.g. from wood, plant fibres etc.), cannot be heated above 100-105°C without some level of deterioration such as for example caramelization of residual sugars. Typically, hot melts for use in constructions have a melting point well above 105°C to avoid delamination.

So it came as a great surprise that if in combination, use is made of polysaccharide based sub-panels and hot melt as adhesive, one can apply the surface coating in the same process step as the manufacturing of the multi-layer panel, without needing to firstly remove any residual moist in the (hydrophilic!) sub-panels. The reason for this is not quite clear. Without being bound to theory, it may be that a hot melt layer, given its immediate solid (crystalline) nature right after cooling, provides for a barrier that prevents any residual moist to travel freely through the whole multi-layer panel and interfere with the application of the surface coating. The inherent presence of residual moist may even be advantageous for the use of hot melt adhesive since this moist may prevent that the polysaccharide when contacting the hot molten adhesive (typically above 120°C) reaches a temperature above 100°C.

The invention also leads to a new multi-layer panel, viz. a multilayer polysaccharide fibre based panel provided with a surface coating that is impervious to water, comprising multiple stacked polysaccharide fibre based sub-panels wherein each pair of contiguous sub-panels has an intermediate layer of a hot melt adhesive in between the contiguous sub-panels, the stacked layer forming a panel with a bottom side and a top side, at least one of which sides is provided with a water impervious coating.

DEFINITIONS

A panel is a solid, self-supporting (dimensionally stable) substantially two dimensional object, i.e. a broad and thin, having length and width dimensions that are at least 10 times larger than its height dimension, preferably at least 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 200, 300, 400, 500 up to 1000 times or more longer or wider than its height (i.e. it’s thickness in the direction of its smallest dimension), which object is typically but not necessarily rectangular, typically but not necessarily flat (the panel may be curved, corrugated, etc.), and usually forms or is set into the surface of a larger substrate such as a door, a wall, a ceiling, a piece of furniture, a tray etc. A panel intrinsically has stable dimensions but depending on its thickness a panel may be marginally flexed under stress. Typical examples of types of materials out of which panels are made for use in the construction of buildings, furniture and other household articles are OSB (oriented strand board), MDF (medium density fiberboard), plywood, cardboard etc. Typical weights for panels used in buildings, furniture and household items are between 1.5 and 50 kg/m ² , in particular between 2 and 20 kg/m ² (as opposed to for example veneer or other surface laminates which have weights in the order of 0.4 to 0.8 kg/m ² ).

A multilayer panel is a panel made of at least two stacked congruent sub-panels durably bonded together, typically comprising 3-20 sub-panels. The sub-panel may be any panel such as a compressed panel, a honeycomb panel etc..

A material is water impervious when its intrinsic (virgin) properties do no allow water to pass through. A material that is damaged, for example mechanically drilled, or chemically deteriorated by the influence of UV light, such that it allows water to pass through, is still a water impervious coating in the sense of the invention. The passing of water can be measured using ASTM E96 / E96M - 16 (Standard Test Methods for Water Vapor Transmission of Materials). Using the“water cup” test, when the permeability rating at 20°C is“1 .0 perm” or less, the coating is considered water impervious according to the present invention, although a permeability of less than 1.0 perm, for example 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1 perm or lower is preferred for any water impervious coating to support the stability of the panel in a humid environment.

Congruent means to be similar to or in agreement with, such that two congruent items can be spatially combined.

A polysaccharide fibre based panel is a panel that consists at least for 50% (w/w) of polysaccharide fibres, for example for 51 , 55, 60, 65, 70, 71 , 72, 73, 74, 75, 76, 77, 78, 79, 80, 81 , 82, 83, 84, 85, 86, 87, 88, 89, 90, 91 , 92, 93, 94, 95, 96, 97 ,98, 99 up to 100% w/w of these fibres, i.e. macromolecules composed of long chains of at least 10 monosaccharide units bound together by glycosidic linkages. Polysaccharides fibres belong to the class of so called structural polysaccharides (as opposed to storage polysaccharides such as starch and glycogen) and range in structure from linear to highly branched. Examples include cellulose, chitin, arabinoxylan and pectin. Typical polysaccharide fibre based panel are panes such as plywood, oriented strandboard, particleboard, and fibreboard.

Cellulosic means made from cellulose or a derivative of cellulose (such as for example viscose or rayon).

A layer is a thickness of some material laid on or spread over a surface in a continuous manner, although a layer may have occasional spots or interruptions or may have a regular pattern of spots or interruptions (for example a reticulated layer).

Applying pressure on an object (i.e. the object being pressed) means using force at least at the level of gravitational forces working on the object to push the object in a particular direction (not needing to actually move it, but for example only directed at compacting it). A hot melt adhesive is a thermoplastic adhesive that is designed to be melted, i.e. heated to above a melting temperature to transform from a solid state into a liquid state, (the melting temperature may be a melting range of a few degrees or more) and to adhere materials after solidification. Hot melt adhesives are typically non-reactive, (partly) crystalline and comprise low (less than 5, 4, 3, 2, preferably even less than 1 mass %) or no amount of solvents, so curing and drying are typically not necessary in order to provide adequate adhesion. In the liquid state the adhesive has a suitably low viscosity, is tacky and solidifies rapidly after cooling down to below its melting temperature (typically in a few seconds to one minute), with little or no drying needed. Unlike a pressure sensitive adhesive, a hot melt adhesive is not permanently tacky. Unlike solvent based adhesives, a hot melt adhesive does not shrink substantially or lose thickness as it solidifies.

A binder is a substance used to make other substances or materials stick or mix together, and is mixed with these components (forming one mass), as opposed to an adhesive which is applied in between different objects to connect them. Often a binder is a synthetic resin.

HOT MELT ADHESIVES FOR USE IN THE INVENTION

Useful hot melt adhesives suitable for use in the present invention may comprise a Polymer P present as a main constituent (i.e. in an amount of at least 50% by weight of the adhesive composition). Conveniently the hot melt adhesive comprises at least 60%, more conveniently at least 70%, most conveniently at least 80% of Polymer P by weight of the adhesive composition. Usefully the Polymer P and/or the hot-melt adhesive may be substantially bio-based (i.e. using naturally occurring materials). Polymer P is a thermoplastic polymer that is at least partly crystalline. Conveniently Polymer P is semi crystalline. Polymer P may have a melting point from 40 to 220°C where the polymer is other than polyamide and where Polymer P comprises polyamide a melting point from 40 to 260°C. Usefully Polymer P has a melting point from 40 to 200°C, more usefully from 40 to 150°C and most usefully from 70 to 120°C, for example about 1 10°C.

By“crystalline” is meant herein that polymer has a melting enthalpy (AHm) of at least 5 J/g, preferably at least 8 J/g, more preferably of at least 10 J/g, most preferably at least 15 J/g. A person skilled in the art would appreciate that many crystalline materials are not fully crystalline but have a degree of crystallinity which is less than 100%, preferably from 2 to 98%, more preferably from 5 to 90%, most preferably from 10 to 80%. Such materials comprise a mixture of phases such as domains of amorphous material and domains of crystalline material (e.g. where polymer chains are substantially aligned) and are often referred to by the informal term”semi-crystalline”. Such materials typically crystallize in lamellae. The different domains can be seen for example under a polarised light microscope and/or by transmission electron microscopy (TEM). The degree of crystallinity of a‘semi-crystalline’ material may be measured by any suitable method such as by measuring density, by differential scanning calorimetry (DSC), by X-ray diffraction (XRD), by infrared spectroscopy and/or by nuclear magnetic resonance (NMR).

Polymer P may have a glass transition temperature below 100°C, advantageously below 80°C, more advantageously below 70°C, even more advantageously below 50°C and most advantageously below 40°C.

Polymer P may have a melt viscosity (all measured at 150°C) of less than 500 Pa.s, usefully less than 300 Pa.s, more usefully less than 200 Pa.s, most usefully less than 100 Pa.s. In one embodiment of the invention the Polymer P may have a melting point from 40 to 150°C, a glass transition temperature below 50°C and a melt viscosity at 150°C of less than 500 Pa.s. In another embodiment of the invention the Polymer P may have a melting point from 40 to 150°C, a glass transition temperature below 50°C and a melt viscosity at 150°C of less than 300 Pa.s. In yet another embodiment of the invention the Polymer P may have a melting point from 70 to 120°C, a glass transition temperature below 40°C and a melt viscosity at 150°C of less than 200 Pa.s.

In any of the embodiments the Polymer P is most preferably a (co) polyester. Polymer P may be a polymer obtained and/or obtainable by a polycondensation, a ring opening polymerisation of cyclic monomers and/or a step-growth polymerisation method.

Polymer P may comprise one or more polymers or copolymers selected from the group consisting of: (co)polyurethane(s); (co)polycarbonate(s); (co)polyester(s),

(co)polyamide(s);(co)polyester-amide(s); (co)polyester-carbonate(s); mixtures thereof and/or copolymers thereof. Although Polymer P can also comprise (co)polyurethane and/or a(co)polycarbonate type polymer(s), preferably Polymer P comprises

(co)polyester(s), (co)polyamide(s) and/or poly(ester-amide)(s). More preferred Polymer P is obtained by a polycondensation and/or by a ring opening polymerisation of cyclic monomers (e.g. cyclic ester and/or cyclic amide). Even more preferred Polymer P comprises (co)polyester(s), most preferably polyester(s).

Preferably the weight average molecular weight (Mw) of the Polymer P is < 500000 g / mol, more preferably < 250000 g/mol and most preferably < 100000 g/mol. Preferably the weight average molecular weight (Mw) of the Polymer P is > 1000 g/mol more preferably > 3500 g/mol and most preferably > 5000 g/mol. Preferably the weight average molecular weight (Mw) of the Polymer P is from 100 to 500000 g/mol, more preferably from 3500 to 250000 g/mol and most preferably from 5000 to 100000 g/mol. Preferably the number average molecular weight (Mn) of the Polymer P is < 300000 g / mol, more preferably < 100000 g/mol and most preferably < 50,000 g/mol. Preferably the number average molecular weight (Mn) of the Polymer P is > 500 g /mol more preferably > 1000 g/mol and most preferably > 2000 g/mol. Preferably the number average molecular weight (Mn) of the Polymer P is from 100 to 300000 g/mol, more preferably from 500 to 100000 g/mol and most preferably from 2000 to 50000 g/mol. Usefully the weight average molecular weight (Mw) of the Polymer P (especially where they comprise polyester) is > 3500 g/mol, more usefully > 5000 g/mol, most usefully > 8000 g/mol and especially > 10000 g/mol. Usefully the weight average molecular weight (Mw) of the Polymer P (especially where they comprise polyester) is < 75000 g/mol, more usefully < 60000 g/mol, most usefully < 50000 g/mol and especially < 40000 g/mol. Usefully the weight average molecular weight (Mw) of the Polymer P (especially where they comprise polyester) is from 3500 to 75000 g/mol, more usefully from 5000 to 60000 g/mol, most usefully from 8000 to 50000 g/mol and especially from 10000 to 40000 g/mol, or even from 15000 to 30000 g/mol.

Usefully the number average molecular weight (Mn) of the Polymer P (especially where they comprise polyester) is > 1500 g/mol, more usefully > 2000 g/mol, most usefully > 3000 g/mol and especially > 5000 g/mol. Usefully the number average molecular weight (Mn) of the Polymer P (especially where they comprise polyester) is < 60000 g/mol, more usefully < 50000 g/mol, most usefully < 40000 g/mol and especially < 30000 g/mol. Usefully the number average molecular weight (Mn) of the Polymer P (especially where they comprise polyester) is from 1500 to 60000 g/mol, more usefully from 2000 to 50000 g/mol, most usefully from 3000 to 40000 g/mol and especially from 5000 to 30000 g/mol.

The molecular weight distribution (MWD) of the polymer may influence properties such as the equilibrium viscosity of the compositions comprising them. MWD is conventionally described by the polydispersity index (PDI). PDI is defined as the weight average molecular weight divided by the number average molecular weight (Mw/Mn) where lower values are equivalent to lower PDI’s. Preferably the value of PDI is < 30, more preferably < 15, most preferably < 10 and especially < 5.

Mw may be measured by any suitable conventional method for example by Size Exclusion Chromatography (SEC), sometimes also referred to as Gel Permeation Chromatography (GPC). The number average molecular weight (also denoted as Mn) may also be determined by a similar method to that for Mw and/or may be calculated theoretically.

Suitably, the SEC analyses are performed on an Alliance Separation Module (Waters e2695), including a pump, auto injector, degasser, and column oven. The eluent is Hexafluoroisopropanol (HFIP) with the addition of PotassiumTriFluoroAcetate (PTFA) 0.1 wt%. The injection volume is 100mI. The flow is established at 0.8 ml/min. The sample to be tested is applied to three silica modified 7pm PFG columns at a temperature of 40°C. The detection is performed with a differential refractive index detector (DRI, Waters 2414) and a photo diode array detector (Waters 2996 PDA). The sample solutions are prepared with a concentration of 5 mg solids per ml HFIP (+ 0.1 % PTFA wt%). The solubility is judged with a laser pen after 24 hours stabilization at room temperature; if any scattering is visible the samples are filtered first. Calibration is performed with eleven narrow poly methylmethacrylate standards ranging from 645 to 1 ,677,000 Da. The calculation is performed with Empower 3 software (Waters) with a third order calibration curve. The obtained molar masses are poly methylmethacrylate equivalent molar masses (Da or g/mol).

Although polyesters can be produced without the formation of a condensation, e.g. by polymerising epoxides with anhydrides, generic (co)polyester-amide may be formed by the condensation reaction of for example molecules having acid or anhydride functionalities or acid chlorides or acid halides with molecules having alcohol and/or amine functionalities. Thus for example polycondensation of suitable polyfunctional acids (preferably diacids) with suitable polyols (preferably diols or (mixtures with) tri- or tetrafunctional alcohols) or polycondensation of hydroxy acids can produce polyesters. Also ring opening polymerization of cyclic esters, such as caprolactone,

pentadecalactone, ambrettolide and similar materials can produce polyesters. Similarly, polycondensation of suitable poly functional acids (preferably diacids) with suitable polyamines (preferably diamines or mixtures with trifunctional amines) or

polycondensation of amino acids can produce polyamides. Ring opening polymerisation of cyclic amides, such as caprolactam, laurolactam and similar materials can produce polyamides. Analogously polycondensation of suitable poly functional acids (preferably diacids) with suitable polyamino alcohols (preferably dialkanol amine), polyols (preferably diols) and/or polyamines (preferably diamines) can produce poly(ester amides). Polyester amides can also be produced by (co)polymerization of lactones and/or lactams (as described herein). By having more than one of such functional groups on one molecule, polymers may be formed. If an amine such as dialkanol amine is used the resulting polyester resin is generally named as“polyester amide”. By having even more functional groups on one molecule it is possible to form hyperbranched polyesters as are well known in the art. By including polyisocyanate components urethanised polyesters (also known as polyester urethanes) or urea containing polyamides may be formed.

Preferred amines and derivatives thereof that may be used to obtain a Polymer P comprise any alkyl-, alkanol-, alkoxyalkyl-, di- and polyamines, as well as amino acids, lactams and similar materials; ethylene diamine, butylene diamine, hexamethylene diamine, isophorone diamine, 2-Methylpentamethylenediamine, 1 ,3-pentanediamine, dimer fatty diamine (e.g. available from Croda under the trade mark Priamine®), ethanolamine, diethanol amine, isopropanol amine, diisopropanol amine, caprolactam, laurolactam, lysine, glycine and/or glutamine. Thus, it is well known that polyesters, which contain carbonyloxy (i.e. -C(=0)-0-) linking groups may be prepared by a condensation polymerisation process in which monomers providing an“acid

component” (including ester-forming derivatives thereof) are reacted with monomers providing a“hydroxyl component”.

The monomers providing an acid component may be selected from one or more polybasic carboxylic acids such as di- or tri-carboxylic acids or ester-forming derivatives thereof such as acid halides, anhydrides or esters. The monomers providing a hydroxyl component may be one or more polyhydric alcohols or phenols (polyols) such as diols, triols, etc. It is to be understood, that the polyester resins described herein may optionally comprise autoxidisable units in the main chain or in side chains’ and such polyesters are known as autoxidisable polyesters. If desired the polyesters may also comprises a proportion of carbonylamino linking groups -C(=0)-NH- (i.e. amide linking group) or -C(=0)-N-R ² - (tertiary amide linking group) by including an appropriate amino functional reactant as part of the hydroxyl component or alternatively all of the hydroxyl component may comprise amino functional reactants, thus resulting in a polyester amide resin. Such amide linkages are in fact useful in that they are more hydrolysis resistant.

There are many examples of carboxylic acids (or their ester forming derivatives such as anhydrides, acid chlorides, or lower (i.e. Ci- ₆ )alkyl esters) which can be used in polyester synthesis for the provision of the monomers providing an acid component. Examples include, but are not limited to monofunctional acids such as (alkylated) benzoic acid and hexanoic acid; and C4-20 aliphatic, alicyclic and aromatic dicarboxylic acids (or higher functionality acids) or their ester-forming derivatives. Preferred examples of suitable acids and derivatives thereof that may be used to obtain a polyester comprise any of the following: adipic acid, fumaric acid, maleic acid, citric acid, succinic acid, itaconic acid, azelaic acid, sebacic acid, suberic acid, pimelic acid nonanedioic acid, decanedioic acid, 1 ,4-cyclohexanedicarboxylic acid, 1 ,3- cyclohexanedicarboxylic acid, 1 ,2-cyclohexanedicarboxylic acid, terephthalic acid, isophthalic acid, sulfoisophthallic acids and/or metal salts thereof (e.g. 5-sodiosulpho isophthalic acid), phthalic acid , tetrahydrophthalic acid, 2,5-furanedicarboxylic acid (FDCA), any suitable mixtures thereof, combinations thereof and/or any suitable derivatives thereof (such as esters, e.g. di(Ci- ₄ alkyl) esters, metal salts and/or anhydrides). Suitable anhydrides include succinic, maleic, phthalic, trimellitic, tetrahydrophthalic and hexahydrophthalic anhydrides. More preferred (co)polyesters may be obtained from the following acids: terephthalic acid, isophthalic acid, succinic acid, suberic acid, pimelic acid, adipic acid, fumaric acid, maleic acid, itaconic acid, dimer fatty acid, sebacic acid, azelaic acid, sulfoisophthallic acid (and/or its metal salt),

1 ,3-cyclohexanedicarboxylic acid, 1 ,4-cyclohexane dicarboxylic acid, 2,5-furane dicarboxylic acid, trimellitic anhydride, esters thereof (e.g. dialkyl esters thereof), combinations thereof and/or mixtures thereof.

Similarly there are many examples of polyols which may be used in (optionally autoxidisable) polyester resin synthesis for the provision of the monomers providing a hydroxyl component. The polyols preferably have from 1 to 6 (more preferably 2 to 4) hydroxyl groups per molecule. Suitable monofunctional alcohols include for example eicosanol and lauryl alcohol. Suitable polyols with two hydroxy groups per molecule include diols such as 1 ,2-ethanediol, 1 ,3-propanediol, 1 ,4-butanediol, 2,3-butanediol,

1 ,6-hexanediol, 2,2-dimethyl-1 ,3- propanediol (neopentyl glycol), the 1 ,2-, 1 ,3- and 1 ,4- cyclohexanediols and the corresponding cyclohexane dimethanols, diethylene glycol, dipropylene glycol, and diols such as alkoxylated bisphenol A products, e.g. ethoxylated or propoxylated bisphenol A. Suitable polyols with three hydroxy groups per molecule include triols such as trimethylol propane (TMP) and 1 ,1 ,1 -tris (hydroxymethyl)ethane (TME). Suitable polyols with four or more hydroxy groups per molecule include bis-TMP, pentaerythritol (2, 2-bis(hydroxymethyl)-1 , 3-propanediol), bis-pentaerythritol and sorbitol (1 ,2,3,4,5,6-hexahydroxyhexane). Examples of hydroxyl functional amines with both hydroxyl functionality and amine functionality are described in, for example, WO

00/32708, use of diisopropanol or ethanol amine is preferred. These can be used to prepare polyester amide resins.

Elastomeric polyols may also be used as building blocks to prepare the Polymer P (e.g. a polyester) and suitable polyols may comprise dihydroxy-terminated

polytetrahydrofuran (polyTHF), dihydroxy-terminated polypropylene glycol, dihydroxy- terminated polybutylene succinate, dihydroxy-terminated polybutylene adipate; other aliphatic polyesters with Tg below zero and two OH end groups; and/or any mixtures thereof and/or any combinations thereof. Examples of suitable copolyester elastomers that may be obtainable and/or obtained from such polyols are those available from DSM under the trade mark Arnitel®.

Preferred examples of suitable alcohols that may be used to obtain a polyester Polymer P comprise any of the following; isosorbide, ethylene glycol, 1 ,2-propanediol, 1 ,3- propanediol, 1 ,5-pentanediol, neopentyl glycol, diethylene glycol, triethylene glycol, 1 ,8- octanediol, 2,2,4-trimethyl-1 ,3-pentanediol, polyethylene glycol, polypropylene glycol, 2,2,4,4-tetramethyM ,3-cyclobutanediol, 2,4-dimethyl-2-ethylhexane-1 ,3-diol, 2,2- dimethyl-1 ,3-propanediol, 2-ethyl-2-butyl-1 ,3-propanediol, 2-ethyl-2-isobutyl-1 ,3- propanediol, 1 ,3-butanediol, 2,3-butanediol (e.g. from a renewable source) 1 ,5- pentanediol, 1 ,6-hexanediol, 1 ,4-butanediol, 1 ,10-decanediol, 1 ,12-dodecanediol, dimer fatty acid diol, glycerol, pentaerythrithol, di-pentaerythritol, any suitable combinations and/or mixtures thereof.

In yet another embodiment the (co) polyester may be built up from an acid selected from terephthalic acid, isophthalic acid, succinic acid, suberic acid, pimelic acid, adipic acid, fumaric acid, maleic acid, itaconic acid, dimer fatty acid, sebacic acid, azelaic acid, sulfoisophthallic acid or its metal salt, 1 ,3-cyclohexanedicarboxylic acid, 1 ,4- cyclohexane dicarboxylic acid, furane dicarboxylic acid, trimellitic anhydride and/or dialkyl esters thereof, mixtures thereof together with an alcohol selected from: ethylene glycol, 1 ,2-propanediol, 1 ,3-propanediol, 1 ,5-pentanediol, neopentyl glycol, diethylene glycol, triethylene glycol, 1 ,8-octanediol, 2,2,4-trimethyl-1 ,3-pentanediol, polyethylene glycol, polypropylene glycol, 2,2,4,4-tetramethyM ,3-cyclobutanediol, 2,4-dimethyl-2- ethylhexane-1 ,3-diol, 2, 2-dimethyl-1 , 3-propanediol, 2-ethyl-2-butyl-1 , 3-propanediol, 2- ethyl-2-isobutyl-1 , 3-propanediol, 1 ,3-butanediol, 2,3-butanediol, 1 ,5-pentanediol, 1 ,6- hexanediol, 1 ,4-butanediol, dimer fatty acid diol, glycerol, pentaerythrithol, di- pentaerythritol and/or mixtures thereof. Dimer fatty acids, dimer fatty diols and/or dimer fatty diamines (e.g. available from Croda) may also be used as potential building blocks to obtain Polymer P. The esterification polymerisation processes for making the polyester for use in the invention composition are well known in the art and need not be described here in detail. Suffice to say that they are normally carried out in the melt optionally using catalysts such as titanium- or tin-based catalysts and with the provision for removing any water (or alcohol) formed from the condensation reaction. Preferably if the polyester resin comprises carboxylic acid functionalities, they are derived from a polyacid and or anhydride.

The (co)polyester and other resins described herein as suitable for Polymer P may also comprise acidic moiet(ies) other than carboxylic acid moieties for example where the resin is prepared from a strong acid such as sulfonated acids, phosphonated acids, derivatives thereof (e.g. esters) and/or salts thereof (e.g. alkali metal salts). Preferred non-carboxylic acid moiet(ies) comprise neutralized or partially neutralized strong acid group selected from sulfonated moieties, phosphonated moieties and/or derivatives thereof, more preferably is an aromatic sulfonated acid or salt thereof, most preferably is an alkali metal sulfo salt of a benzene dicarboxylic acid, for example is represented by formula:

Sodium salt of 5-(sulfo)isophthalic acid (SSIPA) Preferably the weight average molecular weight (Mw) of the polyester amide resin or urethanised polyester(-amide) resin is < 40,000 g / mol, more preferably < 24,000 g/mol and most preferably < 18,000 g/mol. Preferably the polyester amide resin or autoxidisable urethanised polyester(-amide) resin has a PDI less than 8, more preferably a PDI less than 5.5, most preferably a PDI less than 4.0. Preferably the polyester amide resin or urethanised polyester(-amide) resin has a carbonyl amine content (defined as the presence of NH-C=0 or N-C=0 in mmoles / 100 g solid resin) of at least 10 mmoles/100 g solid resin, more preferably at least 20 mmoles/100 g, most preferably at least 50 mmoles/100 g solid resin and especially at least 65 mmoles/100 g solid resin.

In addition the polyester amide resin or urethanised polyester(-amide) resin preferably has a carbonyl amine content (defined as the presence of NH-C=0 or N-C=0 in mmoles/100 g solid resin) of less than 500 mmoles/100 g solid resin, more preferably less than 400 mmoles/100 g solid resin, most preferably less than 300 mmoles/ 100 g solid resin and especially less than 225 mmoles/100 g solid resin. In one embodiment Polymer P comprises a (co) polyester, characterised in that the

(co)polyester is obtained and/or obtainable from reacting at least one acid selected from terephthalic acid, 2,5-furanedicarboxylic acid, adipic acid, fumaric acid, dimer fatty acid, sebacic acid, azelaic acid, succinic acid, and/or combinations thereof with at least one alcohol selected from ethylene glycol, 1 ,6-hexanediol, 1 ,4-butanediol, dimer fatty acid diol and/or combinations thereof. Usefully the hot melt adhesive used in the present invention comprises (in addition to the Polymer P) up to maximally 50 % by weight of optional ingredients selected from, tackifiers, waxes, plasticizers, nucleating agents, anti-static agents, neutralising agents, adhesion promoters, pigments, dyes, emulsifiers, surfactants, thickeners, heat stabilisers, levelling agents, anti-cratering agents, fillers, sedimentation inhibitors, UV absorbers, antioxidants, dispersants, defoamers, co solvents, wetting agents, reactive diluents and the like and/or combinations thereof introduced at any stage of the production process or subsequently. If present any reactive diluents have an Mn > 200 g/mol, more preferably > 500 g/mol and most preferably > 1000 g/mol and preferably an Mn < 7000 g/mol, more preferably < 5000 g/mol and especially < 3500 g/mol. It is also possible to include fire retardants like antimony oxide in the adhesive to enhance the fire retardant properties of the adhesive. Some non-limiting examples of tackifiers include tall oil, gum or wood rosin either unmodified, partially hydrogenated, fully hydrogenated or disproportionated, polymerized rosins, rosin derivatives such as rosin esters, phenolic modified rosin esters, acid modified rosin esters, distilled rosin, dimerised rosin, maleated rosin, and polymerized rosin; hydrocarbon resins including aliphatic and aromatic resins, coumarone-indene resins, polyterpenes, terpene-phenolic resins, maleic resins, ketone resins, reactive resins, hybrid resins and polyester resins.

Plasticisers can be used to reduce the glass transition temperature (Tg) of the polymer. Some non-limiting examples of a plasticizer include benzoate esters, phthalate esters, citrate esters, phosphate esters, terephthalate esters, isophthalate esters, isosorbide esters or combinations thereof. As is well known to a skilled person other suitable commercially available plasticisers can also be used to prepare hot melt adhesives for use in the present invention.

The adhesive also can comprise one or more compatible waxes to improve the bond strength, prevent or reduce cold flow, and to decrease set time. Some non-limiting examples are 12-hydroxystearamide, N-(2-hydroxy ethyl)-12-hydroxystearamide, stearamide, glycerine monostearate, sorbitan monostearate, 12-hydroxy stearic acid, N,N’-ethylene-bis-stearamide, hydrogenated castor oil, oxidized synthetic waxes, and functionalized synthetic waxes such as oxidized polyethylene waxes.

Nucleating agents may be used with the adhesive composition to modify and control crystal formation. The terms“nucleating agent” and“nucleator” are synonymous and refer to a chemical substance which when incorporated into polymers form nuclei for the growth of crystals in the polymer melt. Any incompatible material can serve as a nucleator provided that it rapidly separates into particles as the molten adhesive cools. There are a wide variety of organic and inorganic materials known as nucleating agents that skilled person would be able to select as suitable for use in the present invention. Low molecular weight polyolefins and/or olefinic ionomers with a melt temperature from 70°C to 130°C or talcum or sodium benzoate are non-limiting examples of suitable nucleating agents that could be used in the present invention. Also chemical nucleating agents can be applied, such as for example sodium benzene sulfonate end groups.

Preferred hot melt adhesives that are suitable for use in the present invention exhibit one or more (more preferably all) of the following properties: They can be applied (in a molten state) at a temperature from 100 to 250 °C, preferably from 1 10 to 160°C; they have a melt viscosity of less than 500 Pa.s, preferably < 250 Pa.s at 150 °C. In particular preferred are such hot melt adhesives when exhibiting a melt viscosity typically even below 150 Pa.s. Hot melt adhesives that are particularly suitable for use in the present invention are polyester adhesives having a crystallinity between 1 % and 50% and a melt viscosity of 5-55 Pa.s at 150°C.

Regarding the crystallinity, this preferably can have any value between 5 and 40% such as 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, 38 and 39%. Typical ranges are 2-40 and 4-20%. The determination of the crystallinity is based on ASTM standard D3418 ("Standard Test Method for Transition Temperatures and Enthalpies of Fusion and Crystallization of Polymers by Differential Scanning Calorimetry") using a Mettler STARe differential scanning calorimeter. For the actual measurement an adhesive sample of 10 mg is placed in a sample cup. This sample is kept in an oven for 15 minutes at 150°C. After this, the sample is cooled to 50°C and then heated to 250°C at a speed of 5°C/min. The sample is kept at 250°C for 1 minute and thereafter directly cooled to 25°C at a speed of 5°C/min. From the obtained DSC data the percentage of crystallinity in the sample polymer is calculated using the Mettler STARe SW 9.2 software.

Regarding the melt viscosity, this may have any value between 5 and 55 Pa.s such as 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29,

30, 31 , 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 , 42, 43, 44, 45, 46, 47, 48, 49, 50, 51 , 52,

53 and 54 Pa.s. T _m and T _c may be obtained by any suitable method such as Differential Scanning Calorimetry (DSC). If melting and/or crystallisation of a sample is observed over a temperature range, the T _m and/or T _c values are recorded as the peak (maximum) temperature observed in this range. The viscosity of a polymer adhesive can be measured by using a cone and plate viscometer (Brookfield CAP 2000+, available from Brookfield Ametek, Middleboro, MA, USA) with a 24 mm diameter spindle and a cone angle of 1.8 degrees (Brookfield Cap 2000+ spindle #4). Samples are heated to 150°C. At 150°C the spindle is lowered on the sample. The sample is measured at 21 rpm for 30 seconds. The viscosity is determined automatically by the viscometer’s default algorithm.

SURFACE COATINGS FOR USE IN THE INVENTION As a coating material for the multi-layer panels, all known types of coating can be applied, such as wood based veneers, metal veneers, ceramics, fibreglass reinforced plastics, high pressure laminates, glass finishes, paper based phenolics such as Formica, fabrics etc. It was found that heat curable one component powders as known from WO 2010/136315 are particularly useful for providing a water impervious coating on panels according to the present invention. Such powders typically comprise a thermal curing initiation system and a resin system, wherein the resin system comprises a unsaturated resin and a co-crosslinker wherein the co-crosslinker is chosen from the group of acrylates, methacrylates, vinylesters, vinylethers, vinyl amides, alkyn ethers, alkyn esters, alkyn amides, alkyn amines, propargyl ethers, propargyl esters, itaconates, enamines and mixtures thereof.

With heat curable is meant that curing of the powder coating composition can be effected by using heat. A thermal initiation system is present in the resin to make this heat curing possible. Heat curing has the advantage that in the one step process of the present invention, the heating the powder coating composition can be done at the same time without the use of additional equipment, for instance equipment that generates UV light or accelerated electrons. Preferably, the powder coating composition of the present invention can be cured at a temperature of from 60 to 150°C, for example between 70 to 140°C, or between 100 to 140°C, such as for example between 120 to 130°C.

With powder is meant a composition that can be applied to a substrate as a dry (without solvent or other carrier) finely divided solid, which when melted and fused, forms a continuous film that adheres to the substrate.

With a one component powder, also called a 1 K system, is meant that all (reactive) components of the powder coating composition form part of one powder. This as opposed to a two component system, also called 2K system, wherein the powder coating composition is composed of at least two different powders with different chemical compositions, which keeps the reactive components physically separated.

With the term thermal initiation system is meant the system that triggers the radical polymerization of the reactive unsaturations in the resin and the co-crosslinker. The thermal initiation system comprises a radical initiator. Initiation systems suitable for use in the present invention are initiation systems that in the so called BDDMA test (see WO 2010/136315) have a geltime of between 0.5 and 1000 minutes. Preferably a thermal initiation system having a geltime of at least 2 minutes, more preferably having a geltime of at least 4 minutes and/or at most 800 minutes, for example at most 600, for example at most 400, for example at most 200 minutes, is used.

The curing of the powder coating composition according to the invention takes place by means of heat; that is the powder coating composition is heat curable. The thermal initiator in the thermal initiation system upon heating generates (free) radicals able to initiate polymerization of the reactive unsaturations in the resin in combination with the unsaturated groups in the co-crosslinker or the polymerization of the reactive unsaturations in the resins. Solid initiators are preferred over liquid ones.

Flow characteristics (flow) of the powder coating compositions on the substrate can be determined by comparing the flow of the coating with PCI Powder Coating Flow panels (ACT Test Panels Inc., APR22163 (A) Batch: 50708816) at a coating thickness of approximately 75 pm. The rating of flow is from 1 to 10, with 1 representing the roughest coating and 10 representing the coating with the best flow.

The terms‘thermal initiator’,‘radical initiator’ and‘initiator’ are used interchangeably herein. The radical initiator may be any radical initiator known to the person skilled in the art. Examples of radical initiators include, but are not limited to azo compounds such as for example azo isobutyronitrile (AIBN), 1 ,1’- azobis(cyclohexanenitrile), 1 ,1’- azobis(2,4,4-trimethylpentane), C-C labile compounds such as for example

benzpinacole, peroxides and mixtures thereof. Preferably, the initiator in the initiating system is a peroxide. The peroxide may for example be a percarbonate, a perester or a peranhydride. Suitable peranhydrides are for example benzoylperoxide (BPO) and lauroyl peroxide (commercially available as Laurox™). Suitable peresters are for instance t-butyl perbenzoate and 2-ethylhexyl perlaurate. Suitable percarbonates are for example di-t-butylpercarbonate and di-2-ethylhexylpercarbonate or monopercarbonates. The choice of peroxide is in principle not critical and can be any peroxide known to the person skilled in the art for being suitable for use in radical curing of unsaturated resins. Such peroxides include organic and inorganic peroxides, whether solid or liquid

(including peroxides on a carrier); also hydrogen peroxide may be applied. Examples of suitable peroxides include for example, peroxy carbonates (of the formula -OC(O)O-), peroxyesters (of the formula -C(O)OO-), diacylperoxides, also known as peranhydride (of the formula -C(0)00C(0)-), dialkylperoxides or perethers (of the formula -00-), hydroperoxides (of the formula -OOH), etc. The peroxides may also be oligomeric or polymeric in nature. An extensive series of examples of suitable peroxides can be found, for instance in US 2002/0091214-A1 , paragraph [0018], hereby incorporated by reference. Preferably, the peroxide is chosen from the group of organic peroxides. Examples of suitable organic peroxides are: tertiary alkyl hydroperoxides (such as, for instance, t-butyl hydroperoxide), other hydroperoxides (such as, for instance, cumene hydroperoxide), special class of hydroperoxides formed by the group of ketone peroxides (perketones, being an addition product of hydrogen peroxide and a ketone, such as, for instance, methyl ethyl ketone peroxide, methyl isobutylketone peroxide and acetylacetone peroxide), peroxyesters or peracids (such as, for instance, t-butyl peresters, benzoyl peroxide, peracetates and perbenzoates, lauroyl peroxide, including (di)peroxyesters, -perethers (such as, for instance, peroxy diethyl ether). It is of course also possible to use mixtures of peroxides in the powder coating composition of the present invention. Also, the peroxides may be mixed peroxides, i.e. peroxides containing any two different peroxy-bearing moieties in one molecule. Especially suitable for use in the present invention are any of the following initiators:

peranhydrides, for example benzoyl peroxide or lauroyl peroxide; peroxydicarbonates, for example di(4-t-butylcyclohexyl)-peroxydicarbonate, dicetyl peroxydicarbonate, dimyristylperoxydicarbonate.

Examples of suitable resins include polyesters, polyacrylates (= acrylic resins), polyurethanes, epoxy resins, polyamides, polyesteramides, polycarbonates, polyureas etc., as well as mixtures thereof. Preferably the resin is a polyester. The reactive unsaturations (the carbon carbon double bonds connected directly to an electron withdrawing group) may be present in the backbone of the resin, pendant on the (backbone of the) resin, at the terminus of the resin or at a combination of these locations. Preferably resins having reactive unsaturations based on fumaric acid, maleic acid and/or itaconic acid, more preferably resins having reactive unsaturations based on fumaric acid and/or maleic acid are used in the powder coating composition of the present invention.

The reactive unsaturation may be built into the resin backbone, for instance by reacting a hydroxyl functional monomer (such as the polyalcohols mentioned before) with an unsaturated carboxylic acid or anhydride, such as for example fumaric acid, maleic acid, citraconic acid, itaconic acid or mesaconic acid. Resins where it is possible to build reactive unsaturation into the backbone by reacting a hydroxylfunctional monomer with an unsaturated carboxylic acid are for example polyesters.

Also, the reactive unsaturation may be connected to a side-group on the resin, by reacting an epoxide functional pendant group in the resin, for example a glycidyl functional acrylate, with an unsaturated carboxylic acid, such as for example methacrylic acid or acrylic acid or the monoesters of fumaric acid, maleic acid, citraconic acid, itaconic acid or mesaconic acid.

Also, the reactive unsaturation may be connected to a side-group on the resin, by reacting a hydroxyl functional pendant group in the resin, for example a hydroxyl functional acrylate, with an unsaturated carboxylic acid, such as for example methacrylic acid or acrylic acid or an unsaturated carboxylic anhydride, such as for example the anhydrides of itaconic acid, maleic acid or citraconic acid.

It is also possible to connect the reactive unsaturation to the terminus (or termini) of the resin, for example by reacting a hydroxyl functional, epoxide functional or amine functional terminal group with an unsaturated carboxylic acid, such as for example fumaric acid, maleic acid, citraconic acid, itaconic acid, mesaconic acid or the monoesters thereof, methacrylic acid or acrylic acid. So, a resin having a hydroxyl, amine or glycidyl terminal group may be reacted with such carboxylic acids.

Also, or alternatively, hydroxyl or amine functional resins may be modified with hydroxyl functional compounds containing a reactive unsaturation via reaction with a

diisocyanate forming urethane and/or urea bonds. This modification may be done both on pendant hydroxyl groups and on terminal hydroxyl groups.

A resin is classified as acid functional in case the hydroxyl value is lower than the acid value. In case a carboxylic functional resin is desired, the hydroxyl value of the resin is usually below 10 mg KOH per g resin. A resin is classified as hydroxyfunctional in case the acid value is lower than the hydroxyl value. In case a hydroxyl functional resin is desired, the acid value of the resin is usually below 10mg KOH per g resin. The hydroxyl value of the resin in the powder coating composition of the present invention is usually in the range of from 0 to 70 mg KOH per g resin. It is desired to have a resin, preferably a polyester, with an acid value of less than 5 mg KOH per g resin in case a vinylether or vinylester co-crosslinker is used in the powder coating composition of the present invention. In case a co-crosslinker other than a vinylether or a vinylester is used, the acid value of the resin, preferably a polyester may range from 0 to 250, for example from 0 to 60 mg KOH per g of the resin.

The number average molecular weight (Mn) of the surface coating resin is in principle not critical and can for example be from 1 ,000 to 20,000 Da. Preferably, the Mn of the resin is at least 1 ,500 Da, for example at least 2,000 Da and/or preferably at most 8,000, for example at most 4,000 Da in case of an amorphous resin and/or preferably at most 15,000 Da in case of a crystalline resin. Preferably, the resin is a polyester having a number average molecular weight (Mn) in the range of from 1 ,500 to 8,000, for example in the range of from 2,100 and 4,000Da.

In the powder coating composition also a co-crosslinker is present. With co-crosslinker is meant a compound having carbon carbon double bonds which are reactable with the reactive unsaturations (the carbon carbon double bonds directly connected to an electron withdrawing group) in the resin. The co-crosslinker for use in the composition of the present invention, is chosen from the group of acrylates, methacrylates, vinylesters, vinylethers, vinyl amides, alkyn ethers, alkyn esters, alkyn amides, alkyn amines, propargyl ethers, propargyl esters, itaconates, enamines and mixtures thereof, preferably from the group of vinylethers, vinylesters, (meth)acrylates and mixtures thereof.

In an embodiment the powder coating composition of the present invention comprises a resin, preferably a polyester, for example a polyester based on fumaric acid, comprises a co-crosslinker, for example a vinylether, for example the vinylether as commercially available such as Uracross™ P3307 from DSM Resins and a thermal initiator, for example a perdicarbonate, for example di(4-t-butylcyclohexyl)peroxydicarbonate or dimyristyl peroxydicarbonate which are commercially available from AkzoNobel under the names Perkadox™ 16 and Perkadox™ 26, respectively and an inhibitor, for example a hydroquinone, for example tert-butylhydroquinone or 2,3,5-trimethyl hydroquinone.The invention therefore especially relates to a powder coating composition according to the present invention, wherein the resin is a polyester based on fumaric acid, wherein the co-crosslinker is a vinylether and wherein the thermal initiation system comprises a perdicarbonate, preferably di(4-t-butylcyclohexyl)peroxydicarbonate or dimyristyl peroxydicarbonate and a hydroquinone, preferably tert-butylhydroquinone or 2,3,5- trimethylhydroquinone. In another embodiment of the invention, the powder coating composition of the present invention comprises a resin, preferably a polyester, for example a polyester based on fumaric acid, a co-crosslinker, for example a vinylether, for example the vinylether as commercially available such as Uracross™ P3307 from DSM Resins and a thermal initiator, for example benzoyl peroxide (BPO). The invention therefore especially relates to a powder coating composition according to the present invention, wherein the resin is a polyester based on fumaric acid, wherein the co-crosslinker is a vinylether and wherein the thermal initiator is benzoyl peroxide.

EMBODIMENTS

In an embodiment of the method according to the invention, the bottom side of the first sub-panel is additionally provided with the coating material while providing the intermediate product. This way, a panel can be created having a water impervious coating on both of its sides. The potential problem of residual moist does not prevent that a two-sided finished multi-layer can be made in line with the present invention.

In another embodiment, the intermediate product is heated and pressed in a mould.

This method has found to be particularly useful for producing a panel according to the invention. In a mould, simultaneous heating and pressing can be accomplished according to art known methods. The closed“nature” of a mould has not been found to lead to any principal problems with residual moist.

In again another embodiment the intermediate product is at least partly cooled down while being pressed in a cold mould (cold meaning having a temperature below the crystallisation temperature of the hot melt adhesive, for example between 10 and 50°C, typically around 20-30°C). Although to arrive at adequate mechanical stability of the panel there is no need to take particular measures for cooling down the intermediate product, it has been found that when the cooling down process at least partly takes place in a cold mould while the product is being pressed, any warping that arises during the cooling down process (for example since different materials have different volume changes when cooling down) can be minimised or even prevented. Such cooling is done for 0.5 to 30 minutes, such as 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28 or 29 minutes. The pressure applied during cooling will best be adjusted to the size of the multilayer panel, for example a pressure between 20 and 300 bar can be applied, such as 21 , 22, 23, 24, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180, 200, 220, 240, 260, 280 or 290 bar.

In yet another embodiment the hot melt adhesive has a melt temperature between 100°C and 250°C, for example between 1 10°C and 160°C, such as 1 1 1 , 1 12, 1 13, 1 14,

1 15, 1 16, 1 17, 1 18, 1 19, 120, 121 , 122, 123, 124, 125, 126, 127, 128, 129, 130, 131 ,

132, 133, 134, 135, 136, 137, 138, 139, 140, 141 , 142, 143, 144, 145, 146, 147, 148,

149, 150, 151 , 152, 153, 154, 155, 156, 157, 158 or 159°C. Although it is commonly known that in particular cellulose of natural origin might deteriorate when heated even only at 100°C (see for example Iowa State University thesis Oxidative degradation of cellulose-actetate rayon, II Thermal degradation of some cellulosic textile by steam", by Ester, Virginia Charlotte, 1943; source: Retrospective Theses and Dissertations, 13518; https://lib.dr.iastate.edu/rtd/13518), when using polysaccharide based sub-panels of this or corresponding fibres and adhering the sub-panels with a hot melt adhesive no hindering deterioration may need to take place. As explained here above, the inherent residual moist in such panels may even be the reason for this surprising effect, making sure the temperature of the sub-panel at the interface with the layer of hot melt adhesive remains below the boiling temperature of the water. Regarding a glass transition temperature of a polymer or polymer mixture that constitutes the thermoplastic adhesive, it is preferred that this temperature is below 20°C, even below 0°C or even below -10°C.

Still, in another embodiment the hot melt adhesive comprises a polyester polymer. A polyester polymer has found to be useful for application in the present invention. In particular useful is a condensation polymer. The polymer may have a weight averaged molecular weight (Mw) between 15,000 and 30,000 g/mol. In particular, the weight averaged molecular weight advantageously has a value of 15001 , 15500, 16000,

16500, 17000, 17500, 18000, 18500, 19000, 19500, 20000, 20500, 21000, 21500, 22000, 22500, 23000, 23500, 24000, 24500, 25000, 2550, 26000, 26500, 27000,

27500, 28000, 28500, 29000, 29500 up to 29999 g/mol or any other value in between two consecutive values of these. In particular, the polymer may have a crystallinity of between 1 and 50%. Regarding the crystallinity, this can preferably have any value between 5 and 40% such as 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, 38 and 39%. Typical ranges are 2-40 and 4-20%. In again another embodiment the polysaccharide fibres are cellulosic fibres. Cellulosic fibres were found to be particularly useful in the present invention. Sub-panels made form cellulosic fibres can be any panels made form for example wood, wood chips, wood pulp, wood dust (e.g. saw dust), plant fibres (such as hemp, ramie, cotton, flax, linen), paper, recycled paper, etc. Preferably, the polysaccharide fibres are of plant origin.

In yet another embodiment the first and second sub-panels contain (throughout their mass) less than 10% binder, preferably less than 4, 3, 2 or 1 % binder up to even no binder. It was found that high quality panels can be provided even when hardly any or no binder is present in the sub-panels. Although typically a binder is used when making sub-panels made from pulped material in order to increase the mechanical strength of the sub-panels and hence of the multi-layer panel, it was found that in the present invention, even with low amounts of binder or no binder at all, good multi-layer panels can be made. Again, without being bound to theory, it is believed that a hot melt adhesive is better able to create a strong bond with a panel if a low amount or no binder is present in the material from which the sub-panel is made, since a binder may slow down the penetration of a molten hot melt adhesive into the sub-panel. An example of a sub-panel with no binder that can be used in the present invention is the ECOR panel available from Ecor, San Diego, USA.

In an embodiment wherein the coating material is a self-supporting layer of a water impervious material (such as for example a wood based veneer, a metal veneer, a layer of ceramics, a fibreglass reinforced plastic, a high pressure laminates, Formica etc.) the top side of the second sub-panel is provided with a layer of the hot melt adhesive before the coating material is applied on the second sub-panel. In this embodiment the coating material is applied as a self-supporting layer that can be adhered using the hot melt adhesive.

In alternative embodiment, the coating material is heat curable powder coating composition (which may be a one component or multicomponent system), wherein the intermediate product is heated to a temperature which is equal to or higher than a temperature at which the heat curable powder coating composition cures to form the water impervious coating. In this embodiment, the coating material is applied as a powder on the stack of sub-panels to form the intermediate product, whereafter this product is heat and pressure treated to form the ultimate multi-layer panel. In this embodiment, the intermediate product is heated to a temperature which is equal to or higher than a temperature at which the heat curable component powder cures to form the water impervious coating. Useful heat curable (one component) powder systems are described here above. Preferably, the powder comprises a thermal initiation system comprising a peroxide, preferably an organic peroxide. The powder may comprise a polyester resin and a co-crosslinker chosen from the group of vinylethers, vinylesters, methacrylates, acrylates, itaconates and mixtures thereof.

Further embodiments of a multi-layer panel according to the invention correspond to the latter two methods of using different types of coating material. In a first embodiment of the multilayer polysaccharide fibre based panel according to the invention, the water impervious coating is a self-supporting layer of a water impervious material adhered to the panel using the said hot melt adhesive. In a second embodiment of the multilayer polysaccharide fibre based panel according to the invention, the water impervious coating is a layer that is in situ formed on the panel by heat curing a heat curable one component powder applied to the panel without using an additional layer of adhesive.

The invention will now be further explained using the following examples.

EXAMPLES

Example 1 heat curable one component powder coating on panel

In a first experiment an attempt was made to apply a heat curable one component powder on a polysaccharide sub-panel to establish whether such a powder can be used to make a water impervious coating on such a panel. The panel used was an ECOR FlatCOR - ECOR White panel (available from Ecor, San Diego, USA via Ecor R&D, Venlo) having dimension of roughly A5 format. The coating material was a heat curable one component powder based on Uralac P 1021 R and Uralac P 1910C obtained from DSM Resins (Zwolle, The Netherlands). The coating powder was evenly distributed over the surface of the cold (room temperature) ECOR panel at a density of about 240 g/m ² to form an intermediate product. The intermediate product was placed under a hot (150°C) textile press (Adkins ASMC 28 Heat Press), heated on the top side only, and pressed for 25 seconds using hand pressure. The press was lifted to remove the heated plate from the coated panel. The coating material was cured and formed a smooth white water impervious layer on top of the ECOR panel.

Example 2 - two-layer panel on basis of pre-coated panel

In a second experiment an ECOR FlatCOR - ECOR Brown panel of 10*17cm was firstly coated with the same coating material as described in Example 1 . The intermediate product was placed under the hot (150°C) textile press, heated on the top side only, and pressed for 25 seconds using hand pressure. The press was lifted to remove the heated plate from the coated panel. The coating material was cured and formed a smooth white water impervious layer on top of the ECOR panel.

This coated panel was used in a next process step to arrive at a multilayer panel made of two ECOR panels adhered together using a hot melt adhesive, while at the same time providing the coating layer at the other side of the multi-layer panel. For this, the above mentioned pre-coated panel was placed up-side-down, and another ECOR FlatCOR - ECOR Brown panel of 10*17cm, coated with a polyester hot melt adhesive (available from DSM Resins Zwolle, The Netherlands) was placed with the adhesive side on top of the above mentioned panel. The upper ECOR FlatCOR - ECOR Brown panel was provide with a layer of powder of the heat curable powder coating as described in accordance with Example 1 . This way, a multi-layer intermediate product was provided, consisting of a stack of two polysaccharide sub-panels and an intermediate hot melt adhesive, one panel of which is provided with a cured water impervious surface coating, the other with a non-cured powder coating material. This intermediate product was placed under a hot (150°C) textile press, heated on the top side only, and pressed for 3 minutes using hand pressure. The press was lifted to remove the heated plate from the coated panel. The coating material was cured and formed a smooth white water impervious layer on top of a two-layer ECOR panel, adhered durably together with the hot melt adhesive.

Example 3 - two-layer panel with coating pressed in one step

In a third example two ECOR FlatCOR panels size A4 were stacked with an intermediate layer of the same polyester hot melt adhesive and at the same amount as described here above, and the same heat curable one component powder, at the same amount as described here above was sprayed on top of the top panel. The intermediate product so obtained was pressed for 4 minutes at 150°C as described here above. Thereafter the product was pressed in a cold press (room temperature) for 4 minutes. The finished multi-layer panel was mechanically stable, had a smooth water impervious coating and was warp free.

Example 4 - clear coating instead of white coating

A fourth experiment was carried out to confirm the results of example 3, however using a heat curable powder that leads to a clear coating instead of a white coating (the curable components as such being equal). The amount of hot melt adhesive in between each pair of sub-panels was the same as in example 3, while the amount of heat curable powder sprayed on the top panel was lowered to 195 g/m2. Hot and cold pressing took place as described in example 3. The finished multi-layer panel was mechanically stable, had a clear water impervious coating and was warp free.

Example 5 - three-layer panel

A fifth experiment was carried out to confirm the results of example 4, however using three ECOR FlatCOR panels as sub-panel. The amount of hot melt adhesive in between each pair of sub-panels was the same as in example 4, while the amount of heat curable powder sprayed on the top panel was slightly raised to about 210 g/m2. Hot pressing took place for 20 minutes, and cold pressing for 15 minutes. The finished multi-layer panel was mechanically stable, had a Clear water impervious coating and was warp free.

Example 6 - multi-layer panel with wood veneer

A sixth experiment was carried out in line with example 4, albeit that a layer of thin wood veneer (about 0.6 mm) was placed (as yet another sub-panel) on top of the upper ECOR panel before the heat curable powder was sprayed. All other variables were the same, except for the pressing steps which in each case took 5 instead of 4 minutes. The finished multi-layer panel was mechanically stable and was warp free. However, the topcoating appeared to be a bit too tick for the wood veneer, resulting in a somewhat less nice appearance due to turbidity of the coating layer (resulting in a partly white appearance). Example 7 - multi-layer panel with wood veneer and less coating

A seventh experiment was carried out in line with example 6, albeit that the amount of heat curable powder use was lowered to 1 12 g/m2. The pressing times for the hot and cold press were both 12 minutes. The resulting coating was clear with no visible turbidity.

Example 8 - MDF panels instead of ECOR panels

In a further experiment, 2 layers of 4 mm Medite Premier MDF sized approximately 30*20 cm were used instead of ECOR panels. 150 g/m2 of a polyester hot melt adhesive (available from DSM Resins Zwolle, The Netherlands) was applied between the two MDF layers. The heat curable one component powder of experiment 1 was sprayed onto the top surface of the upper MDF layer and the intermediate product so obtained was pressed for 10 minutes at 150°C using hand pressure and on the same press as used in experiment 1.

This procedure resulted in a multilayer panel of acceptable quality, although the time required for the heat to go through the MDF layer to fully melt the adhesive was considerably longer.

Example 9 - alternative heat curable one component powder

The experiment of example 8 has been performed using ECOR FlatCOR - ECOR brown panels sized approximately 15*22 cm. As heat curable one component powder Uralac P3050 (obtained from DSM Resins, Zwolle, The Netherlands) was used and pressing was done for 20 minutes at 150°C.

The resulting multilayer panel has a coating which is still somewhat soft. Probably this is due to the type of heat curable one component powder selected, as this Uralac P3050 seems to be less reactive as compared to the one component powder of experiment 1.

Example 10 - alternative polyester hot melt adhesive

The experiment of example 8 has been performed using ECOR FlatCOR - ECOR brown panels sized approximately 15*22 cm. An alternative polyester hot melt adhesive, based on a different polyol and also obtained from DSM Resins (Zwolle, The Netherlands), has been applied. All other variables were kept the same as used in experiment 8. The resulting multilayer panel was of good quality, i.e. the panel was mechanically stable, had a smooth water impervious coating and was warp free.

Example 11 - HPL coated multi-layer panel

In experiment 1 1 a five layer panel of 244x122 cm was created, which is covered on the top as well as bottom side with a high pressure laminate (HPL). The HPL used was Duropal 0.8 mm matt white (producer Pfleiderer). The layer panels used were ECOR FlatCOR - ECOR brown (FlatCORs). Four panels were provided with the polyester hotmelt adhesive of experiment 1 (obtainable from DSM Resins, Zwolle, The

Netherlands) on one side. The adhesive was applied in an amount of approximately 170 g/m2 with a roller coater. One FlatCOR of the multilayer panel was provided with the polyester hotmelt adhesive on both sides (top and bottom of the panel).

An aluminum anodized carrying plate was placed on a loading table. HPL was placed on this carrying plate, top down. The FlatCOR with adhesive on both sides was placed on the HPL. The four other FlatCORs were placed on top, with the adhesive side facing up. Then HPL was placed on the last FlatCOR, top up. An aluminum anodized top plate was placed on top of the HPL.

The intermediate product so obtained was placed as a complete stack in a hot press Joos HP90, operated at 50 bar, 170°C top and bottom, for 10 minutes.

Subsequently the hot stack was transferred for cooling in a cooled press Joos HP70, operated at 200 bar, 20°C top and bottom, for 7 minutes.

The finished multilayer panel was ready for sawing to straighten the edges of the panel.

Comparative example - PVA glue instead of hot melt adhesive

In this example a multilayer panel was made using one ECOR panel and one layer of the wood veneer as used in the above described examples 6 and 7. An intermediate layer of a regular water based PVA glue (Bison Houtlijm, Goes, Netherlands) was applied as an intermediate adhesive layer between the ECOR panel and the wood veneer. 192 grams/m2 of the heat curable coating was applied on the wood veneer. After hot and cold pressing (each only 2 minutes due to the total panel being markedly thinner), the panel showed considerable warping. Probably this is the result of the water present in the PVA glue that cannot easily escape the multi-layer panel due to the simultaneous formation of the water impervious coating, even though this was only applied one-sided.