TECHNICAL FIELD OF THE INVENTION

The present invention relates to a coating for a construction board, a construction board comprising said coating and a method for producing the construction board. This type of construction boards comprises a gypsum core and is commonly used within the field of construction for panelling walls, ceilings, columns, beams, floors and similar structures. This type of construction boards is also used for room dividing structures and is also used as fireproof and soundproof construction parts.

STATE OF THE ART

There is a plurality of different types of construction boards and coatings for construction boards in the prior art. One conventional construction board, a so called gypsum board, comprises a gypsum core having a first side, a second side opposite the first side and edge sides, wherein at least the first and second sides of the construction board are provided with a coating in the form of paper layers.

The prior art also includes a plurality of construction boards and plaster boards having a plastic layer to provide decorative or water-repellent properties. One such type of construction board is disclosed in CA688763, wherein a paper coated gypsum core is provided with a plastic layer for decorative purposes. Another such type of construction board is disclosed in US2008/0245012, wherein the coating can comprise synthetic fiber and a polymer based binder in the form of latex to provide a hydrophobic construc- tion board.

One drawback with such prior art construction boards is that they are difficult to produce and that the production cost can be relatively high. Within the field of construction boards there is also a need for improved properties for use in wet areas and other environments in which the construction board can be exposed to moisture.

SUMMARY OF THE INVENTION

One object of the invention is to avoid the above mentioned drawback of prior art and provide a coating for a construction board being less sensitive to moisture. The coating according to the invention results in simple and cost efficient production of a construction board having a gypsum core and favourable properties against moisture and microbial growth in the form of mold and similar. Further, the coating according to the invention provides a favourable working environment for construction workers and others who are working with construction boards of this type. The present invention comprises a coating for a construction board having a gypsum core, characterised in that the coating comprises a flexible, water permeable and polymer based first layer for direct contact with the gypsum core to connect the coating thereto, a flexible, water permeable and reinforcing second layer connected to the first layer and being arranged for providing a foldable coating, and a flexible water permeable and polymer based third layer connected to the second layer, wherein the third layer is heat resistant for avoiding melting when passing through a drying kiln for drying the gypsum core. Hence, the coating can be arranged entirely by polymer materials or plastic materials providing improved moisture resistance while the working environment for the handling thereof can be improved.

The first layer of the coating can comprise polypropylene or a polymer or a plastic material having corresponding properties. Polypropylene or a polymer based on polypropylene has been found to give good adhesion to the gypsum core while excellent water permeability can be obtained. Further, the melting point of polypropylene is high enough to allow the first layer to pass through a kiln for drying the gypsum core without any appreciable effect. The first layer can be arranged as a synthetic fiber. The first layer can be arranged with apertures in the form of through apertures or in the form of indentations or recesses for cooperation with the gypsum core. The apertures can be arranged so that gypsum slurry penetrates them for improved adhesion to the first layer. Further, the apertures can further improve the water permeability while the apertures result in material savings. The second layer can comprise a reinforcing and polymer based net structure, such as a net structure based on polypropylene or any other material having corresponding properties. It has been found that a reinforcing layer comprising a reinforcing net structure of polypropylene result in a favourable strength and rigidity of the coating, so that it can be folded to a shape for receiving gypsum slurry and maintain the folded shape during the moulding of the gypsum core. The second layer can be arranged to give the construction board the necessary strength. For example, the coating or the second layer can be arranged with a strength corresponding to a paper coating of a paper coated standard type gypsum board. The net structure can be embedded in a plastic material in the form of a plastic film or a fiber cloth of a synthetic fiber.

The third layer can comprise polyester or any other material having corresponding properties. It has been found that polyester has favourable properties for adhesion to the second layer while it is sufficiently heat- resistant to form an outer surface for passing through a kiln for drying the gypsum core. Further, polyester can be arranged to have favourable properties for further work, such as filling, painting, wallpapering, tiling, etc. Thus, the third layer can be formed in a polyester based polymer. For example, the third layer is formed in a thermoplastic having a melting point above 200 ⁰ C. The third layer can be arranged as a synthetic fiber.

The invention also relates to a construction board comprising the gypsum core as above. The coating on at least one of the sides of the gypsum core can be provided with a waterproof fourth layer, for example, by melting of a surface layer of the third layer. Consequently, a water repellent or wa- terproof construction board can be provided, which is suitable for use in wet areas and similar. For example, the fourth layer can function as a vapour barrier, wherein only the joints between adjacent construction boards re- quires further work, such as by applying tape or a conventional vapour barrier or similar, to provide a vapour barrier for a wet area for example.

The invention also relates to a method for producing such a construction board, comprising the steps of a) advancing a first coating sheet comprising a first polymer based layer, a reinforcing and polymer based second layer and a heat resistant and polymer based third layer, b) folding the first coating sheet along each long side and forming a mould having a bottom and sides and by means of the reinforcing second layer maintain the shape of the first coating sheet, c) applying liquid gypsum slurry on the first coating sheet, so that the first layer of the first coating sheet is brought into direct contact with a first side of the gypsum slurry, d) applying a second coating sheet in direct contact with the opposite side of the gypsum slurry, e) cutting the gypsum slurry and the coating sheets arranged on the gypsum slurry to desired length and thereby forming boards, f) feeding the boards into a kiln for drying, wherein water in the gypsum slurry leave through the coatings and wherein the heat resistant third layer prevents the coating from melting when passing through the kiln.

The method can also comprise the step of melting a surface layer of the third layer forming a waterproof fourth layer of the coating.

Further characteristics and advantages of the present invention will become apparent from the description of the embodiments below, the ap- pended drawings and the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to the accompanying drawings, in which Fig. 1 is a schematic section view of a construction board having a gypsum core and a coating according to one embodiment of the invention,

Fig. 2 is a schematic section view of the coating according to a general embodiment of the invention, wherein the coating comprises first, second and third layers.

Fig. 3 is a schematic section view of the coating according to one embodiment of the invention, wherein the second layer comprises a reinforcing structure arranged inside a plastic based film,

Fig. 4 is a schematic section view of the coating according to one embodi- ment of the invention, wherein the first layer is formed with through apertures.

Fig. 5 is a schematic section view of the coating according to one embodiment of the invention, wherein the first layer is formed with apertures in the form of recesses,

Fig. 6 is a schematic section view of the coating according to one embodiment of the invention, wherein the first layer is formed with a surface structure for favourable adhesion to the gypsum core,

Fig. 7 is a schematic section view of the coating according to one embodiment of the invention, wherein the third layer is provided with openings, and

Fig. 8 is a schematic section view of the coating according to one embodiment of the invention, wherein the third layer is provided with a waterproof fourth layer.

DESCRIPTION

The drawings have been designed to illustrate the invention. Dimensions and proportions do not necessarily correspond to a real embodiment.

With reference to Fig. 1 a construction board 10 is illustrated comprising a gypsum core 11 having a first side 12 and a second side 13 opposite the first side 12. The gypsum core 11 also comprises a plurality of edge sides, which extend between the first side 12 and the second side 13 and are not illustrated in the drawings. The gypsum core 11 is for example a conventional gypsum core for construction boards. For example, the gypsum core 11 is arranged in a corresponding manner as the gypsum core of a con- ventional gypsum board for construction.

The gypsum core 11 is provided with a coating 14. The coating 14 is for example arranged on the first side 12 and the second side 13 of the gypsum core 11. For example, the coating 14 also extends over one or more of the edge sides of the gypsum core 11. According to one embodiment the first side 12 of the gypsum core 11 is provided with a first coating 14a, wherein the second side 13 of the gypsum core 11 is provided with a second coating 14b. The first coating 14a is for example of a different type than the second coating 14b. Alternatively, the first coating 14a corresponds to the second coating 14b. With reference to Fig. 2 the coating 14 according to one embodiment of the present invention is illustrated. The coating 14 comprises a first layer 15, a second layer 16 and a third layer 17. Hence, the coating is arranged as a laminate or a foil having at least three layers. The coating 14 is polymer based and comprises synthetic or semi-synthetic polymers or plastic materi- als. For example, the coating 14 is arranged entirely of plastic materials and has no cellulose based materials, such as paper. According to one embodiment of the invention the coating 14 is formed also without glass fiber and corresponding materials. The coating 14 is flexible, wherein the coating 14 is foldable. Simultaneously, the coating 14 is arranged with a rigidity to be able to maintain a folding, such as a folding by means of a notch along the coating 14, so that a mould shape having a bottom and upwards folded sides of a sheet of the coating can be maintained. According to one embodiment of the invention the coating 14 is water permeable, at least until the gypsum core 11 of the construction board 10 has dried. Further, the coating 14 is arranged with a heat-resistant surface to prevent melting thereof when coming into contact with transport rolls and similar inside a kiln for drying the construction boards 10. The coating 14 is for example produced by conventional lamina- tion techniques, such as by gluing or that a sheet of the first layer 15 is melted together with sheets of the second and third layers 16, 17.

The first layer 15 is arranged for direct abutment against the gypsum core 11 to connect the coating 14 therewith. Hence, the first layer 15 is ar- ranged with a first side 18 and a second side 19 opposite the first side 18, wherein the first side 18 forms an inner side of the coating 14 and is formed for direct contact with the gypsum core 11. The second side 19 of the first layer 15 is directly connected to the second layer 16. For example, the first layer 15 is directly connected to the second layer 16. The first layer 15 is di- rectly connected to the gypsum core 11 of a construction board 10 according to the invention.

The first layer 15 is flexible, water permeable and polymer based. For example, the first layer 15 is a plastic film or a synthetic fiber of a suitable plastic material arranged for water permeability, favourable adhesion to the gypsum core 11 and favourable adhesion or connection to the second layer 16. It has been found that a particularly suitable material for the first layer 15 is based on polypropylene or consists of polypropylene plastic. It is possible that the first layer 15, possibly with suitable additives, can be based on other types of plastic materials having properties corresponding to polypropylene. For example, it is possible that other thermoplastics comprising polystyrene, polyvinylchloride, ABS-plastic or polyolefins, such as polyethylene, or mixtures thereof, are useful as plastic materials in the first layer 15. It is also possible that other olefin fibers than polypropylene fibers are useful for the first layer 15. The second layer 16 is arranged between the first layer 15 and the third layer 17. For example, the second layer 16 is arranged in direct contact with the first layer 15 and the third layer 17. The second layer 16 is flexible, water permeable and reinforcing to provide a foldable coating 14. The second layer 16 is arranged to provide the coating 14 with strength and rigidity to make it possible to fold the edge sides of the coating 14 upward and thus forming a mould for liquid gypsum slurry. Hence, the second layer 16 is formed with a rigidity so that the coating 14 can be folded to a shape having a bottom and sides and maintain the mould shape obtained by the folding. For example, the second layer 16 is formed as a plastic film or a synthetic fiber of a suitable plastic material to provide reinforcing properties and a rigidity making the folding possible. Alternatively, the second layer 16 is formed as a reinforcing net structure of suitable plastic material. It has been found that a particularly suitable material for the second layer 17 is based on polypropylene or consists of polypropylene plastic. It is possible that the second layer 16, possibly with suitable additives, can comprise other types of thermoplastics, such as polyethylene or other polyolefins, polystyrene, poly- vinylchloride, ABS-plastic or similar, or mixtures thereof.

The third layer 17 is connected to the second layer 16. For example, the third layer 17 is directly connected to the second layer 16 in a first side 20. According to one embodiment of the invention a second side 21 opposite the first side 20 of the third layer 17 forms an outer surface of the coating 14. The third layer 17 is flexible, heat resistant, water permeable and polymer based. The third layer 17 is arranged to provide a heat resistant outer surface of the coating 14, so that the second side 21 of the third layer 17 can bear against transport devices arranged in a kiln for drying construction boards 10 having a gypsum core 11. For example, the third layer 17 is formed in a plastic material having a melting point above 15O ⁰ C, or above 200 ⁰ C and for example above 25O ⁰ C, so that the construction board 10 can pass through a kiln for drying the construction board 10 without melting of, or any substantial effect on, the outer surface of the second side 21 of the third layer 17. According to one embodiment of the invention the third layer 17 is further arranged to provide a favourable surface for further working, such as filling, painting, tiling, wallpapering, etc. Hence, the third layer 17, according to one embodiment of the invention, is formed with a structure of the second side 21 of the third layer 17 and the outer surface of the coating 14 making such further working possible for finishing, decoratively or functionally, a room dividing structure, such as a wall surface or ceiling surface, for example. The third layer 17 is for example formed as a plastic film or synthetic polymer fiber of suitable plastic material to provide a heat resistant outer surface for passing through a kiln and an outer surface that is suitable for further processing. For example, the third layer 17 is formed with a fluffy outer surface. For example, the third layer 17 is based on a plastic material different from the second layer 16. It has been found that a particularly suitable material for use in the third layer 17 comprises polyester. For example, the third layer 17 is formed in polyester fiber, such as PET-fiber or any other polyester fiber. For example, the third layer 17 is formed in one or more thermoplastic materials, wherein the third layer 17 or a part thereof can be melted. It is possible that the third layer 17, possibly with suitable additives, can comprise olefin fiber, acrylic fiber or polyamide fiber, such as aliphatic polyamides and aramides.

With reference to Fig. 3 the coating 14 according to one embodiment of the invention is illustrated, wherein the second layer 16 comprises a reinforcing structure 22 inside a plastic based film or fiber cloth, wherein the reinforcing structure 22 is enclosed by a reinforcing enclosure layer 23. For example, the reinforcing structure 22 is arranged as a net structure in a suitable plastic material, which plastic material for example is polypropylene or possi- bly other plastic materials as mentioned above. The reinforcing enclosure layer 23 is formed in a suitable plastic material. For example, the second layer 16 is produced by melting together or joining a first reinforcing enclosure layer 23a and a second reinforcing enclosure layer 23b having the reinforcing structure 22 arranged between them. With reference to Fig. 4 the coating 14 according to one embodiment of the invention is illustrated, wherein the first layer 15 is arranged with through apertures 24. The through apertures 24 are arranged to further improve the adhesion of the coating 14 to the gypsum core 11 and to further improve the water permeability of the first layer 15 while material savings can be obtained simultaneously. For example, the through apertures 24 are arranged so that liquid gypsum slurry can penetrate into them, wherein the through apertures 24 receive portions of the gypsum core 11 for improved attachment to it. According to one embodiment of the invention the first side 18 of the first layer 15 is provided with apertures in the form of recesses 25, as illustrated in Fig. 5, to improve the adhesion of the coating 14 to the gypsum core 11. Alternatively, the first side 18 of the first layer 15 is provided with another structure to increase the surface for adhesion to the gypsum core 11. Examples of such structures are rough structures, fiber structures, fluffy structures, recesses, grooves and similar, which is illustrated by means of Fig. 6. Such an increased surface against the gypsum core 11 , as the apertures 24, recesses 25 or rough structure of the first layer 15 result in, can possibly also result in that the water evaporation surface of the gypsum core is increased, which can result in more efficient drying thereof.

With reference to Fig. 7 the coating 14 according to one embodiment of the invention is illustrated, wherein the third layer 17 is arranged with openings 26. The openings 26 are formed to further improve the possibilities of further processing of the second side 21 of the third layer 17 and to further improve the water permeability of the third layer 17 while material savings are obtained simultaneously. According to the illustrated embodiment the openings 26 are through openings. Alternatively, the openings 26 are formed as recesses or indentations in the second side 21 of the third layer 17. With reference to Fig. 8 the coating 14 according to one embodiment of the invention is illustrated, wherein the third layer 17 is provided with a waterproof fourth layer 27. For example, the fourth layer 27 is formed by melting the outer surface or second side 21 of the third layer 17. For example, the outer surface of the third layer 17 is heated to form the waterproof fourth layer 27 after drying of the gypsum core 11. For example, the fourth layer 27 is formed after the construction board 10 has passed through a kiln for drying the gypsum core 11.

The construction board 10 is for example produced by that a first coating sheet formed by the coating 14 is supplied to a side folding device for upward folding of the long sides of the coating sheet. For example, the first coating sheet is supplied in a conventional manner. After that, the first coating sheet is folded along each long side wherein a mould having a bottom and sides is formed for receiving liquid gypsum slurry. According to one embodiment of the invention the first coating sheet is folded in a conventional manner. Alternatively, a folding notch is applied on the first coating sheet by means of a special device to avoid melting of the coating 14. The mould formed by the first coating sheet is filled with liquid gypsum slurry, so that the gypsum slurry is brought into direct contact with the first layer 15 of the coating 14. After that, a second coating sheet is applied directly into contact with an opposite side of the gypsum slurry. For example, also the second coating sheet is formed by the coating 14 described above. Hence, the gypsum slurry is moulded between two coating sheets, wherein one of the coating sheets is folded along its long sides and is for example glued or melted together with the second coating sheet. After that, the gypsum slurry and the coating sheets arranged on the gypsum slurry is cut into desired length forming boards, after which the boards are brought into a kiln for drying the gyp- sum slurry to form the construction board 10. The temperature in the kiln is for example at least 15O ⁰ C, or above 200 ⁰ C, for example above 25O ⁰ C or for example about 26O ⁰ C. As described above a surface layer of the third layer 17 can be melted after drying the gypsum slurry to form a construction board having a waterproof surface layer. Alternatively, a waterproof fourth layer 27 can be applied on the third layer 17 by lamination.