WO2016180610A1

WO2016180610A1 - Process for preparing a multi-layer coated substrate

Info

Publication number: WO2016180610A1
Application number: PCT/EP2016/058690
Authority: WO
Inventors: Matthew Paul Amick; Harmen Jelke Kielstra; Robert Grahn; Peter HÅKANSSON
Original assignee: Akzo Nobel Coatings International BV
Current assignee: Akzo Nobel Coatings International BV
Priority date: 2015-05-08
Filing date: 2016-04-20
Publication date: 2016-11-17
Anticipated expiration: 2017-11-08

Abstract

The invention relates to process of preparing a multi-layer coated substrate comprising the steps, in any suitable order, of a) Providing a substrate of wood or wood-based material, b) Providing a primer sheet comprising a polymeric top layer and an adhesive bottom layer, c) Laminating the adhesive bottom layer of the primer sheet to at least a part of the surface of the substrate of wood or wood-based material, and d) Applying a top coat on the polymeric top layer of the primer sheet.

Description

Process for preparing a multi-layer coated substrate

The invention relates to a process of preparing a multi-layer coated wood or wood-based material substrate. The invention also relates to a multi-layer coated substrate obtainable by the process.

US 8506742 B2 describes a method for laminating a plastic film with a wood- base substrate, in particular for producing a high-gloss surface on a wood-base substrate, in which a plastic film is laminated by means of a laminating agent, preferably an adhesive, with a wood-base substrate and permanently bonded to the wood-base substrate, the plastic film being formed so as to be transparent on its top, and the products obtainable in this manner and their use, in particular in the area of the wood and furniture industry, for example for the production of pieces of furniture and fitments of all kinds.

WO 2009/103847 describes a self-adhesive material for a wood board. The self-adhesive material is formed of polyolefin film containing reactive groups with -OH groups of the wood for forming self-adhesive properties. Further, the invention relates to a wood board in which the self-adhesive material is used as a coating and/or as a glue-line material.

WO 2008/002360 describes the preparation of a laminated composite structure having a base layer comprising a base substrate; at least one polymeric layer laminated onto a surface of the base substrate. The polymeric layer is preferably bonded to the base substrate using a thermosetting adhesive interposed between the substrate surface and the polymeric layer.

A drawback of the methods described in these documents is that the gloss grade, colour and structure of the surface of the substrate is determined by the type of plastic film used, which limits the types of surface appearances which can be realized. Further, there is an interest in the wood industry, in particular in the furniture industry, to use low-density wood-base substrates to reduce the costs of the final products. A problem with low density wood-based substrates is that coatings cannot be directly applied onto such substrates in a satisfactory manner, generally leading to unsatisfactory finishes. In order to achieve better results it is common to use a wood-substrate with an external layer of high or medium density wood and a core layer of low density wood, the high or medium density external layer contributing to a more satisfactory appearance of the final product, and the low density core bringing the costs down. However, when profiling of the wood-substrate is required, the low density core layer is exposed and the problem on achieving unsatisfactory finishes remains.

The application of plastic films on such profiled substrates (e.g. by 3D-pressing) can lead to equally unsatisfactory finishes. The present invention seeks to provide a process which is very flexible in the type of surface to be achieved, and which is in particular suitable for creating a high quality finish on low density wood-based substrates and/or 3D-shaped substrates (e.g. profiled or milled substrates). The invention now provides a process of preparing a multi-layer coated substrate comprising the steps, in any suitable order, of

a) Providing a substrate of wood or wood-based material

b) Providing a primer sheet comprising a polymeric top layer and an adhesive bottom layer,

c) Laminating (e.g. 3D pressing) the adhesive bottom layer of the primer sheet to at least a part of the surface of the substrate of wood or wood-based material,

d) Applying a top coat on the polymeric top layer of the primer sheet In the first step of the process a substrate is provided which is a wooden substrate, or which is a wood-based material. Examples of suitable substrates include fiberboard of low, medium, or high density, particle board, oriented strand board, plywood, or natural woods such as pine or poplar. The process is of particular use for wood based substrates which are difficult to coat with liquid primer compositions, or which have a pronounced tendency to absorb liquid primer compositions. The preferred wood based substrates are fiberboard and particle board. The process according to the invention allows for the easy formation of high quality primers and top coats on inexpensive wood-based substrates, in particular on fiber board or particle board having a low density, such as a density in the range of 520 to 750 kg/m³, in particular 550 to 650 kg/m³.

Typically more liquid primer coating is needed in conventional coating processes to effectively cover the surface of lower density board in order to get a good appearance. Low density board is typically used for low end finishes and cheaper products. With the process of the invention it is possible to eliminate primer painting steps and also reduce the quantity of topcoat used and still achieve an excellent appearance. The primer film serves as a leveling layer which covers surface imperfections in lower quality board and allows high quality finishes to be achieved on inexpensive wood-based substrates.

Hence, the process of the invention allows a furniture manufacturer to use lower density and inexpensive wood-based substrates for furniture making which gives them a cheaper and lower weight product that is better for transportation costs and easier to handle by consumers.

The process of the invention also comprises the step of providing a primer sheet comprising at least two layers, i.e. polymeric top layer and an adhesive bottom layer.

Examples of suitable polymers for the polymeric top layer of the primer sheet include polyester, such as polyethylene terephthalate, polyolefin, such as polypropylene (e.g. cavitated biaxially oriented polypropylene (BOPP)), polyurethane, polyvinylchloride, fluoropolymers (e.g. polyvinylidene fluoride), acrylics or other suitable resins. Also mixtures or hybrids of the afore-mentioned polymers can be used. Polyurethanes, fluoropolymers, polypropylenes, or polyesters, may be particularly preferred. It is preferred that the polymeric top layer has good moisture barrier properties and the ability to cover or level any surface irregularities of the wood-based substrate.

Particularly preferred are polymers which allow for satisfactory lamination even on 3-D shaped substrates (e.g. profiled or milled substrates). For instance, monolayer or multilayer films comprising polypropylene, modified polyethylene terephthalate (e.g. with isophthalic acid (PEI) or modified with cyclohexanedimethanol, CHDM, (PET-G)), thermoplastic polyurethanes (e.g. based on aromatic polyether or aromatic polyester urethanes), acrylates (such as polymethyl methacrylate, acrylonitrile butadiene styrene), fluoropolymers (such as polyvinyl fluoride, polyvinylidene fluoride, fluorinated ethylene propylene, perfluoroalkoxy alkane, ethylene tetrafluoroethylene, or ethylene chlorotrifluoroethylene), or combination thereof. Particularly useful examples of polymeric primer top layer films include cavitated biaxially oriented polypropylene (BOPP), such as those commercialized under the designation Torayfan®), thermoplastic polyurethane such as those commercialized under the designation Dureflex®, polymethylmethacrylate such as those commercialized under the designation Acrylite®, and polyvinylidene fluoride / acrylic alloys such as those commercialized under the designation Fluorex®.

An advantage of using a cavitated film (such as cavitated BOPP) is its good results for 3D-shaped substrates and in particular its conformance to small radius curves upon 3D pressing (e.g. for profiled or milled substrates such as a profiled kitchen front).

The polymeric top layer may be a clear layer or an opaque layer containing pigments. A white or gray pigmented polymeric top layer has been found very suitable for most top coats to be applied on the polymeric top layer of the primer sheet. If so desired, the pigmentation of the polymeric top layer of the primer sheet can be adapted to match the color of the top coat to be applied. In this case irregularities in the hiding power of the top coat will be less visible. Also minor damages or scratches occurring during use of a piece of furniture will be less visible if the color of the polymeric top layer matches the color of the top coat.

For a high quality multi-layer coated substrate it is important to have sufficient inter-layer adhesion between the various layers. In order achieve good inter- layer adhesion, it may be suitable to use a polymeric top layer which has undergone an adhesion improving surface treatment. Examples of adhesion improving surface treatments include corona treatment, plasma treatment, flame treatment, or chemical treatments. Depending on the needs, one or both surface of the polymeric top layer may be subjected to the afore-mentioned adhesion improving surface treatment. Such adhesion improving surface treatment is of particular importance in cases wherein the polymer used for the polymeric top layer has a low surface energy or a low surface polarity, or otherwise a surface with insufficient adhesion properties. Polymeric top layers based on polypropylene, polyethylene terephthalate or fluoropolymers have preferably undergone an adhesion improving surface treatment.

The thickness of the polymeric top layer is suitably in the range of 20 to 200 μιτι, preferably in the range of 25 to 75 μιτι. The primer sheet further comprises an adhesive bottom layer. The adhesive bottom layer suitably comprises a hot melt or pressure sensitive adhesive composition. Non-reactive adhesives may be preferred. For the purposes of the instant description a non-reactive adhesive means and adhesive which does not contain groups that chemically react with -OH groups of the wood substrate. Advantages of non-reactive adhesives include low to no toxicity, excellent gap filling, high initial tack or green strength, and reprocess-ability. In particular, it is advantageous in a production environment to be able to re-process wood substrates, e.g., when they show certain coating defects. With reactive adhesives it is difficult to remove the laminated primer sheet without significant destruction of the surface of the wood substrate making reprocessing impossible. The adhesive composition may be a reactive, or a thermoplastic hot melt adhesive composition, a non-reactive hot melt adhesive composition may be preferred.

The adhesive composition is suitably selected to allow sufficient melt-flow of the adhesive at the lamination temperature to allow the adhesive to flow into any surface irregularities of the substrate surface.

Non-reactive hot melt adhesive materials that are useful may contain as a major constituent an adhesive polymer such as an acrylic-type polymer; block copolymer; natural rubber, reclaimed rubber, or synthetic rubber (e.g. styrene- butadiene rubber); tackified natural or synthetic rubber; a copolymer of ethylene and vinyl acetate; an ethylene-vinyl-acrylic terpolymer; polyisobutylene; poly (vinyl ether); aromatic polyether urethane; polyolefin (e.g. poly-alpha-olefins); and crystallizable polyesters (e.g. crystallizable thermoplastic copolyesters). Other materials may be included in the adhesive such as tackifying resins, plasticizers, antioxidants, fillers, waxes, etc.

Pressure sensitive adhesive materials that are useful may be an acrylate or methacrylate based composition typified by low glass transition temperature acrylic esters. Examples of useful acrylic esters include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutylacrylate, tert-butyl acrylate, amyl acrylate, hexyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, undecyl acrylate, or lauryl acrylate. Glass transition temperatures of such acrylic esters may generally be in the range of 0 to about -80 °C.

Particularly preferred adhesive materials for flat line or 3D press lamination include acrylic pressure sensitive adhesives such as those commercialized under the designation Aroset®, aromatic polyether urethanes such as those commercialized under the designation Macroplast®, crystallizable thermoplastic copolyesters such as those commercialized under the designation Dynacoll® S, and poly-alpha-olefins (APAO) such as those commercialized under the designation Vestoplast®.

The adhesive composition can be applied to the polymeric top layer as an organic solvent-based composition, a water-based composition, or as a composition without volatile diluents.

In one embodiment, the adhesive layer is a non-pigmented clear layer. If so desired, the adhesive comprises pigments or filler particles to adapt color and opacity properties.

A suitable manufacturing process for the primer sheet is to solvent cast the adhesive onto one surface of a pre-formed polymeric sheet in a web process or roll to roll process and force dry the applied adhesive layer in a multistage continuous oven. Suitable application methods include slot die, roll coating, reverse roll coating, gravure coating, or other roll casting methods well known to the art. A preferred application method is slot die or roll coating. The dry thickness of the adhesive layer is suitably in the range of 5 to 50 μιτι, preferably in the range of 12 to 40 μιτι.

The layer thickness of the polymeric top layer and the adhesive bottom layer may be adapted so as to achieve the required quality of lamination and adhesive flow during the lamination process. It is preferred to select a hot-melt adhesive which is sufficiently non-tacky at ambient temperature to allow winding up the primer sheet in a roll without blocking for storage and transportation of the primer sheet. Adhesives which are sufficiently non-tacky can be selected by testing. Generally, suitable adhesives are those having a softening point or the beginning of the melt range above a temperature of 60°C, preferably above 80°C. When adhesives are used which can crystallize or partly crystallize, it is generally beneficial to allow the adhesive to crystallize or partly crystallize after application in order to minimize tackiness.

The process further comprises the step of laminating the adhesive bottom layer of the primer sheet to at least a part of the surface of the substrate of wood or wood-based material. A suitable lamination process of the primer sheet to wood substrate involves heat laminating the sheet to the substrate. The substrate may be an essentially flat or board like substrate. Alternatively, the substrate may be a 3D-shaped substrate such as a substrate having three dimensional surface contours (e.g. a profiled or milled substrate) to which the primer sheet is laminated such that the primer sheet adopts the surface contour of the substrate. If so desired, the primer sheet may be wrapped around the edges of the substrate where needed. Lamination temperature and pressure required depend on line speed and wood substrate type. The lamination can be done on a flat surface profile or a 3D surface profile such as a milled or profiled surface profile of the wood substrate, e.g., by 3D-pressing.

In a further embodiment, the primer sheet is laminated to a flat and formable wood based substrate, and subsequently the wood based substrates and the layers already applied to it are together subjected to a shaping process so as to create a three-dimensional shape. The shaping process can be carried out before or after application of the top coat on the polymeric top layer of the primer sheet. The process further comprises the step of applying a top coat on the polymeric top layer of the primer sheet.

In one embodiment, the top coat is applied as a liquid coating composition to the polymeric top layer of the primer sheet. Suitable application methods for the liquid top coat are well known to the skilled person and include roller coating and spray coating. Spray coating may be preferred, e.g., for 3D-shaped substrates after lamination of the primer sheet. The liquid coating composition may be a water-based coating composition, and organic solvent-based coating composition, or a solvent-free liquid coating composition. After application, the liquid coating composition is solidified by evaporation of volatile diluent or chemical curing, or a combination thereof. The coating composition may be thermally curable or curable by actinic radiation, such as UV light or electron beam radiation. Particular examples of suitable coating compositions include polyesters (e.g. thermosetting polyesters, free radical curing unsaturated polyesters, and acid curing saturated polyesters), fluoropolymers (e.g. polyvinylidene fluoride), acrylates (e.g. UV-curing acrylates), polyurethanes (including crosslinkable aliphatic polyurethane dispersions, in particular acid or hydroxyl functional polyurethane dispersions) or mixtures thereof (e.g. polyvinylidene fluoride / acrylic alloys). Particularly useful coating compositions include free radical curing unsaturated polyester compositions such as AkzoNobel Wood Finishes IC301 and crosslinkable aliphatic polyurethane dispersions such as those commercialized under the designations Sancure®, Bayhydrol®, Daotan®, or Hauthane.

Generally, the liquid top coat is applied to the polymeric top layer after the primer sheet has been laminated to the wood-based substrate. However, if so desired, the liquid top coat may also be applied to the polymeric top layer before the primer sheet is laminated to the wood-based substrate. In this case, the top coat should have sufficient flexibility to withstand the lamination process, such as coatings based on aliphatic polyurethane dispersions (crosslinkable aliphatic polyurethane dispersions in particular), acrylic latexes, high molecular weight (>150,000 MW) thermoplastic acrylics, or coatings based on polyvinylidene fluoride / acrylic alloys. Such coatings are generally applied to the polymeric top layer and cured or dried before laminating (e.g. 3D pressing) the wood based substrate.

In an alternative embodiment, the top coat is applied to the polymeric top layer of the primer sheet as a polymer based top coat sheet. The polymer based top coat sheet may be laminated to the polymeric top layer of the primer sheet after the primer sheet has been laminated to the substrate. If so desired, the polymer based top coat sheet may be laminated to the polymeric top layer of the primer sheet before the primer sheet is laminated to the substrate. In that case, the top coat sheet and the primer sheet are laminated to the wood or wood-based substrate as a multilayer coating sheet. Examples of suitable polymer based top coat sheet materials include polyurethanes, acrylics, polyesters, fluoropolymers, and their combinations (e.g. polyvinylidene fluoride / acrylic alloys) in single layer or multilayer solution cast or extruded films. The polymer based top coat sheets may be clear or pigmented or printed with high quality graphics by a suitable print method such as gravure or digital print. The polymer based top coat sheets may also be embossed with a texture such as for the tactile sensation of wood grain. Suitable polymer based top coat sheet films are available from AkzoNobel under the designation Soliant®.

The invention also relates to a multi-layer coated substrate obtainable by a process as described above. The multi-layer coated substrate comprises

a) A wood or wood-based substrate

b) A primer sheet comprising a polymeric top layer and an adhesive bottom layer, laminated to at least a part of the surface of the wood- based substrate via the adhesive bottom-layer, and

c) A top coat facing the polymeric top layer of the primer sheet.

Particularly preferred examples of such multi-layer coated substrates are, e.g., 3D-shaped substrates, in particular 3D-shaped substrates with low density such as medium density fiberboard (MDF), comprising an acrylic sheet or a cavitated BOPP as polymeric top layer of the primer sheet, a hotmelt polyurethane as adhesive bottom-layer of the primer sheet and a thermosetting polyester-based top coat or a polyvinylidene fluoride (PVDF)/acrylic alloy top coat sheet.

Example 1

A white opaque polyethylene terephthalate sheet with chemical adhesion promoting surface coating treatment on both surfaces was used. The layer thickness was 25 or 50 μιτι. Suitable sheets are available from Mitsubishi Polyester under the trade designation Hostaphan® W54B or from SKC Films under the trade designation Skyrol® SW84G. To one surface a polyurethane hot melt adhesive from Chase Corporation, available under the trade designation Macroplast® QA4849, was applied in a dry film thickness of 13 or 25 μιτι. These sheets served as primer sheets and were heat laminated to a flat, low density wood-based substrate. Subsequently a liquid top coat was applied to the top layer of the primer sheet. The top coat was a thermosetting polyester- based top coat composition available from Akzo Nobel Wood Finishes under the trade designation IC301 . The topcoated primer sheet showed exemplary appearance and moisture resistance when compared to low density wood- based substrate coated with liquid primer and liquid topcoat.

Example 2

A 175 μιτι clear acrylic primer sheet available from Evonik under the trade designation Acrylite® was coated with 50 μιτι of the polyurethane hot melt adhesive Macroplast® QA4849 that had been pigmented white. The hot melt adhesive was first coated and dried on a 50 μιτι BOPP release carrier and then thermally transfer laminated to the clear acrylic sheet. The primer sheet with hot melt adhesive was thermoformed (3D-pressed) over a MDF substrate that had been 3D profiled as in a kitchen front. The pressing temperature was 160 °C and the pressure was less than 5 Bar. The 3D wrapped MDF substrate was subsequently coated with the thermosetting polyester-based top coat composition IC301 . The resulting coated profiled board show exemplary appearance in the profiled areas and the A side radius of the board.

Example 3

A laminated wood sample was prepared according to the process set forth in Example 2 except using a 75 μιτι white acrylic primer sheet and a clear hot melt adhesive with similar results. Example 4

A laminated wood sample was prepared according to the process set forth in Example 2 except using a 50 μιτι clear, surface treated cavitated BOPP primer sheet available from Toray Plastics under the trade designation Torayfan®. The conformance of the primer sheet in the areas of small profile radius was excellent.

Example 5

A clear 25 μιτι polyvinylidene fluoride (PVDF)/acrylic top coat sheet with a wood grain pattern gravure print available from AkzoNobel under the trade designation Fluorex® was thermally laminated to a clear 50 μιτι corona treated polyester based primer sheet available from Mitsubishi Polyester under the trade designation Hostaphan® MG47. The polyurethane hot melt adhesive Macroplast® QA4849 was coated and dried on a 50 μιτι BOPP release liner and then thermally transfer laminated to the polyester primer sheet on the side opposite the PVDF/acrylic top coat sheet. The multilayer coating sheet was then thermally laminated to a flat MDF substrate to transfer the wood grain pattern appearance to the wood-based substrate with good results.

Example 6

A laminated wood sample was prepared according to the process set forth in Example 5 except an additional transparent, low gloss 38 μιτι top coat sheet containing a soft touch tactile sensation was thermally laminated on top of the clear PVDF/acrylic top coat sheet with the wood grain pattern. This multilayer top coat sheet was then thermally laminated to the polyester primer sheet. The resulting laminated MDF substrate has the appearance and tactile sensation of soft, stained real wood. Example 7

A laminated wood sample was prepared according to the process set forth in Example 3. After application of the topcoat, the laminated wood sample was reprocessed by heating the sample with IR heat to 120 °C in order to sufficiently soften the hot melt adhesive allowing the primer sheet with topcoat to be removed from the wood substrate. The wood substrate was lightly sanded and brushed and then prepared again with primer sheet and topcoat as above. It is advantageous in a production environment if wood substrates can be reprocessed when they show certain coating defects. With heat reactive hot melt adhesives it is difficult to remove the laminated primer sheet without significant destruction of the surface of the wood substrate making reprocessing impossible.

Claims

A process of preparing a multi-layer coated substrate comprising the steps, in any suitable order, of

a) Providing a substrate of wood or wood-based material,

c) Laminating the adhesive bottom layer of the primer sheet to at least a part of the surface of the substrate of wood or wood-based material, and

d) Applying a top coat on the polymeric top layer of the primer sheet.

The process according to claim 1 , wherein the top coat is applied as a liquid coating composition.

The process according to claim 1 , wherein the top coat is applied as a polymer based top coat sheet.

The process according to claim 3, wherein the polymer based top coat sheet is laminated to the polymeric top layer of the primer sheet after the primer sheet has been laminated to the substrate.

The process according to claim 3, wherein the polymer based top coat sheet is laminated to the polymeric top layer of the primer sheet before the primer sheet is laminated to the substrate.

The process according to any one of the preceding claim, wherein surface of the polymeric top layer of the primer sheet has undergone an adhesion improving treatment.

The process according to any one of the preceding claims, wherein the polymeric top layer of the primer sheet comprises a polymer selected from a polyurethane, a fluoropolymer, an acrylate, a polypropylene, or a polyester, in particular comprising cavitated biaxially oriented polypropylene (BOPP) or polyvinylidene fluoride/acrylic alloy.

8. The process according to any one of the preceding claims, wherein the adhesive bottom layer of the primer sheet comprises a hotmelt adhesive or a pressure sensitive adhesive, in particular a non-reactive adhesive, and more in particular a hotmelt polyurethane, an acrylic pressure sensitive adhesive, a crystallizable thermoplastic copolyester, or a poly- alpha-olefin.

9. The process according to any one of the preceding claims, wherein the top coat on the polymeric top layer of the primer sheet comprises a polymer selected from a polyester, an acrylate, a fluoropolymer and a polyurethane.

10. The process according to any one of the preceding claims, wherein step c) is a heat laminating process and/or a 3D-pressing laminating process.

1 1 . The process according to any one of the preceding claims, wherein the polymeric top layer of the primer sheet is a pigmented layer.

12. The process according to any one of the preceding claims, wherein the wood-based substrate is a low density wood-based substrate and/or a 3D-shaped substrate.

13. The process according to any one of the preceding claims, wherein the wood-based substrate is selected from fiber board and particle board and in particular, and more in particular is fiber board or particle board with a density in the range of 520 to 680 kg/m³.

14. The process according to any one of the preceding claims, wherein after step c) or after step d) the substrate and the layers already applied to it are subjected to a shaping process so as to create a three-dimensional shape.

15. A multi-layer coated substrate obtainable by the process according to any one of the preceding claims comprising

a) A wood or wood-based substrate

b) A primer sheet comprising a polymeric top layer and an adhesive bottom layer, laminated to at least a part of the surface of the wood- based substrate via the adhesive bottom-layer,

c) A top coat facing the polymeric top layer of the primer sheet.