Fibrous mat for gypsum board

Abstract

The present invention relates to a fibrous mat for a gypsum board comprising: a first layer comprising glass fibers; a second layer placed onto the first layer comprising glass fibers; a coating placed onto the second layer comprising at least one vinyl polymer; wherein the average diameter of the glass fibers in the first layer is larger than the average diameter of the glass fibers in the second layer; and the first layer and the second layer are impregnated with a composition comprising a binder. Further, the present invention relates to the use of said fibrous mat in a gypsum board, a gypsum board containing said fibrous mat, a process for producing a fibrous mat and a process for producing a gypsum board.

Description

[0001] ID 473 HE Ref. 257218 AHLSTROM OYJ FIBROUS MAT FOR GYPSUM BOARD Technical field The present invention relates to a fibrous mat for a gypsum board, a gypsum board comprising said fibrous mat, the use of said fibrous mat in a gypsum board, and a process for producing said fibrous mat and a process for producing a gypsum board. Background art Gypsum boards, drywalls or plasterboards, generally comprise a gypsum core blended to a woven or non-woven material of inorganic and / or organic fibers located on or just embedded below the surfaces. The material may be a mat formed by single or multiple layers and may be reinforced by filaments or webs of fiber strands. Gypsum boards have usually a cellulosic paperboard or a nonwoven material wrapped around a layer of gypsum. The gypsum is typically obtained from a hydratable calcium sulphate such as CaSO4∙ H2O, which is hardened upon wetting and subsequent drying. The gypsum composition may include water resistant additives such as polyvinyl alcohol, wax, polysiloxanes or thermoplastic synthetic resins. Other additives include the fire-resistant glass fibers or mineral fillers, such as clay. Properties of the gypsum board can therefore be tuned according to the intended final use. Water resistant additives, such as fluorocarbon resin or silicone derivatives, are commonly introduced into the mat to bring hydrophobicity to the final gypsum board. The water resistance of a gypsum board is typically characterized by the maximum allowed absorption of liquid water into the board, in accordance with applicable norms in each country. The mats used in gypsum boards generally contain mixtures of cellulosic, glass and polymeric fibers. Polymeric fibers are used for example to increase the strength of the final product. However, polymeric fibers may undergo degradation over time and, moreover, increase the fire load of the material. Mats generally have two sides, each of which serves a purpose. One side is intended to absorb the gypsum slurry, whereas the other side should prevent the slurry from leaking through the mat. For example, in order to prevent the slurry from leaking through the mat, US 10,569,508 B2 describes a facer having two layers, one of which comprises larger fibers than the other one, and in which a coating with a high amount of fillers is added to block the bleed through of the gypsum. Further, microfibers are used to control the slurry leaking. However, using a high amount of fillers in the coating to decrease permeability may block the pores of the mats and further increase the weight of the product. A high amount of fillers, especially on the side of the mat which impregnates with the gypsum slurry, may impair the ability of the mat to interlock with gypsum. Further, when present on the coated side, fillers may interfere with the formation of the coating, which may result more porous. Lastly, a high amount of filler generally leads to a product which is less environmentally friendly. The use of microfibers, that is, fibers having a diameter of 6 µm or less, significantly increases the costs of the final product. Further, microfibers are less optimal than larger fibers in terms of rigidity and resistance to mechanical stress bending of the final product. Moreover, the use of microfibers may be associated with safety concerns not only during production of the mats, but also in the manufacture of the gypsum boards containing them, for example during the cutting of the mats, and in later stages such as during renovation or demolition of walls containing the gypsum boards. Technical problem Accordingly, there is a demand for mats for gypsum boards which have good mechanical properties and allow for good impregnation with gypsum, but at the same time avoid leaking of gypsum through the board, and that are lighter and have a low fire load. Summary of the invention The present invention is aimed at solving the problems of the prior art by providing a fibrous mat for a gypsum board comprising: a) a first layer comprising glass fibers; b) a second layer placed onto the first layer comprising glass fibers; c) a coating placed onto the second layer comprising at least one vinyl polymer; wherein the average diameter of the glass fibers in the first layer is larger than the average diameter of the glass fibers in the second layer; and the first layer and the second layer are impregnated with a composition comprising a binder. Further, the present invention relates to a gypsum board comprising a gypsum core and a fibrous mat according to the present invention, wherein the coating containing a vinyl polymer in the fibrous mat and the gypsum core are at opposite sides of the fibrous mat. Moreover, the present invention relates to the use of the fibrous mat according to the present invention in a gypsum board. The present invention also relates to a process for producing a fibrous mat for a gypsum board, the process comprising the steps of: (a) laying a suspension A comprising glass fibers onto a screen; (b) laying a suspension B comprising glass fibers onto the suspension A after or simultaneously with step (a); wherein the average diameter of the glass fibers comprised in the suspension A is larger than the average diameter of the glass fibers comprised in the suspension B, or wherein the average diameter of the glass fibers comprised in the suspension B is larger than the average diameter of the glass fibers comprised in the suspension A; (c) dewatering the suspensions A and B to form a layer A and a layer B comprising glass fibers; (d) impregnating the layers A and B obtained in steps (a) to (c) with a composition comprising a binder; (e) curing the binder; and (f) applying a composition comprising at least one vinyl polymer onto the layer A or B to form a coating, with the proviso that in step (f), the composition is applied onto layer A if the average diameter of the glass fibers comprised in the suspension B is larger than the average diameter of the glass fibers comprised in the suspension A, and in step (f), the composition is applied onto layer B if the average diameter of the glass fibers comprised in the suspension A is larger than the average diameter of the glass fibers comprised in the suspension B. The present invention also relates to a process for producing a gypsum board, the process including a step of depositing a gypsum slurry on a fibrous mat according to the present invention. The present invention also concerns a process for producing a gypsum board, the process including a step of producing a fibrous mat according to the process of the present invention. Further, the present invention relates to a fibrous mat and a gypsum board obtainable by the process according to the present invention. It has been found that a combination of glass fibers in two layers and having different average diameters leads to a product in which one side is more porous than the other, resulting in different absorption capabilities on each side of the fibrous mat. The layer with smaller fibers is denser, thus preventing leak through of the gypsum slurry, whereas the other side allows for a good absorption of the gypsum. The coating that is provided onto the side with denser fibers further reduces the leaking through of the gypsum and surprisingly improves the barrier properties of the fibrous mat. Moreover, the binder that impregnates the fibrous layers binds the fibers together and improves the mechanical properties of the final product. The inventors have found that the above combination of features leads to a product that is lightweight, as it can be manufactured with little or no fillers in the composition, and has a low fire load, as it can be produced without adding polymeric fibers. Further, the fibrous mat of the present invention can be prepared with one production line, thus reducing the production costs. Advantageously, the fibrous mat of the present invention may be prepared with a process that uses foam suspensions of glass fibers, which allow for reduced production times and so decreases the environmental impact of the production process. Using foam suspensions also allows for a better formation of the fibrous mat, as the fibers may be distributed more evenly in the web and thus result in more consistent properties. In addition, foam- forming also requires less water than other methods, which allows for additional material and energy savings. Brief description of the drawings Figure 1 is a schematic illustration of a fibrous mat according to the present invention. Detailed description of the invention Definitions Described herein is a fibrous mat for a gypsum board, said fibrous mat comprising a first layer, a second layer and a coating. The fibrous mat is also referred to herein as “mat facer”, “facer” and “mat”. The term “fiber” means a material form characterized by an extremely high ratio of length to diameter (e.g. 50 / 1). In the context of the present invention, suitable fiber length is advantageously from about 0.08 to about 4 cm. The term “fiber” also covers a mixture of fibers of different nature, i.e. organic and inorganic fibers. In the present invention, the term “average” denotes mean average, unless stated otherwise. In the present invention, references to amounts “by weight” are intended to be synonymous with “by mass”. Further, as used herein, the term weight percent (wt%) refers to a percentage amount by weight. In the present invention, the term “polymer” denotes a compound comprising at least ten repeating units such as, for example, a homopolymer, a copolymer, a graft copolymer, a branch copolymer or a block copolymer. Where the present description refers to “preferred” embodiments / features, combinations of these preferred embodiments / features shall also be deemed as disclosed as long as this combination is technically meaningful. Hereinafter, the use of the term “comprising” should be understood as disclosing in a non-limited way, that is to say that additional components or steps can be present or implemented, as long as this is technically meaningful. For a more restricted embodiment, the terms “consisting of” will be used and have to be understood as disclosing in a limited way, that is to say without any additional component or step. Fibrous mat The present invention relates to a fibrous mat for a gypsum board comprising a first layer, a second layer and a coating. With reference to Figure 1, the fibrous mat (1) comprises a first layer (11) which comprises glass fibers (12) (hereinafter also referred to as “first glass fibers”); a second layer (13) placed onto the first layer (11) and comprising glass fibers (14) (hereinafter also referred to as “second glass fibers”); and a coating (15) placed onto the second layer (13) and comprising at least one vinyl polymer. In the fibrous mat (1), the average diameter of the first glass fibers (12) is larger than the average diameter of the second glass fibers (14) and the first layer (11) and the second layer (13) are impregnated with a composition comprising a binder. The fibrous mat may be described as having an “inner side” and an “outer side” which are opposite to each other, and wherein the order of the layers and the coating from the inner side to the outer side is as follows: first layer, second layer and coating. The inner side is the side of the fibrous mat that contacts or is intended to contact the gypsum core of a gypsum board; more particularly this is preferably the side of the first layer which is not in contact with the second layer, that is, which faces away from the second layer. The outer side is the side of the fibrous mat that is or is intended to be farther from the gypsum core, i.e. located in the external side of the gypsum board and thus facing away from the core of the gypsum; more particularly this is preferably the side of the coating which is facing away from the second layer of the fibrous mat. Further, the fibrous mat of the present invention is suitable for being used in a gypsum board. For this purpose, one side of the fibrous mat should be available for impregnation with a gypsum core precursor material, such as a gypsum slurry. Hence, in a preferred embodiment, the second layer is placed onto one side of the first layer, while the opposite side of the first layer is not coated with the coating containing the vinyl polymer and is available for contact and impregnation with gypsum material. When the sides of the fibrous mat are referred to as “inner side” and “outer side” as explained above, the side of the first layer which is facing away from the second layer corresponds to the inner side of the fibrous mat. Preferably, the total basis weight of the fibrous mat is 70 g / m2or more, more preferably 75 g / m2or more, even more preferably 80 g / m2or more, and is 160 g / m2or less, preferably 150 g / m2or less, more preferably 140 g / m2or less. Preferably, the thickness of the fibrous mat is 0.5 mm or more, preferably 0.55 mm or more, more preferably 0.6 mm or more, and 2 mm or less, preferably 1.8 mm or less, more preferably 1.5 mm or less. First and second layers The first and the second layers of the fibrous mat of the present invention each contain glass fibers. However, fibers made of different materials may also be independently present in each of the first and / or the second layer, such as cellulosic fibers and other polymeric fibers. Glass fibers are useful in that they reinforce the tensile strength, ensure a good mechanical bonding at the interface with a gypsum core thanks to the interlocking of gypsum crystals between the interstices between glass fibers, and stabilise the fibrous mat with regards to variations in humidity. Further, glass fibers have a high chemical resistance, lower fireload and better dimensional stability in different temperatures, without undergoing degradation over time as polymeric fibers generally do. Preferably, the first and the second layers are each a nonwoven material. The term “nonwoven material” indicates a material made from fibers and having a structure of individual fibers which are interlaid, but not in an identifiable manner as in a woven or knitted fabric or material. In a preferred embodiment, the amount of glass fibers in the first layer is 80 wt% or more, preferably 90 wt% or more, even more preferably 100 wt% of the total amount of fibers in the first layer. In a preferred embodiment, the amount of glass fibers in the second layer is 80 wt% or more, preferably 90 wt% or more, even more preferably 100 wt% of the total amount of fibers in the second layer. In a preferred embodiment, the amount of glass fibers in each of the first layer and the second layer is 80 wt% or more, preferably 90 wt% or more, even more preferably 100 wt% of the total amount of fibers in each layer. In the present invention, the diameter relationship between the average diameters of the first glass fibers and the second glass fibers is important to ensure a different porosity through the thickness of the fibrous mat. More specifically, the first layer having larger fibers is more porous than the second layer and can advantageously absorb the gypsum, whereas the second layer having smaller fibers is denser and prevents the bleeding through of the gypsum slurry that is typically used during preparation of the gypsum board described herein. Due to the smaller fiber size, the second layer has a lower air permeability as compared to the first layer. The average diameter of the first and second glass fibers may be measured by any means known in the art, and particularly may be measured by scanning electron microscopy. The method of measurement is however not particularly limited, as long as the same method is used for the average diameter of the first and second glass fibers, such that a comparison can be made. For example, the average diameter of the first and second glass fibers may be measured by scanning electron microscopy and calculated as the arithmetic average of at least 50, preferably at least 100 fibers. Preferably, a ratio between the average diameter of the first glass fibers and the average diameter of the second glass fibers is 1.1 or more, more preferably 1.2 or more, even more preferably 1.3 or more, and is preferably 3 or less, more preferably 2.5 or less, even more preferably 2 or less. In a preferred embodiment, the average diameter of the first glass fibers is 8 µm or more, preferably 9 µm or more, more preferably 10 µm or more, even more preferably 11 µm or more. Preferably, the average diameter of the first glass fibers is 20 µm or less, preferably 18 µm or less, more preferably 17 µm or less, even more preferably 16 µm or less. In a preferred embodiment, the average diameter of the second glass fibers is 7 µm or more, preferably 8 µm or more, more preferably 9 µm or more, even more preferably 10 µm or more. Preferably, the average diameter of the second glass fibers is 16 µm or less, preferably 14 µm or less, more preferably 13 µm or less, even more preferably 12 µm or less. The above ranges for the average diameters of the first and second glass fibers in the fibrous mat are particularly preferable also from a viewpoint of ensuring a good porosity and capability of interlocking with gypsum. Preferably, the amount of glass microfibers in the first and in the second layer is as low as possible. In the context of the present invention, the term “microfibers” denotes fibers having an average diameter of 6 µm or less. In the present invention, preferably the amount of glass fibers having an average diameter of 6 µm or less in each of the first and the second layer is 5 wt% or less, preferably 1 wt% or less, preferably 0 wt% of the total amount of fibers in each of the first and the second layer. That is, preferably the first and the second layers do not contain fibers having an average diameter of 6 µm or less. Preferably, the amount of polymeric fibers in each of the first and the second layer does not exceed 10 wt%, preferably 5 wt%, more preferably 0 wt% of the total amount of fibers in each of the first and the second layer. In this context, examples of polymeric fibers are polyamide, polyaramide, polyethylene and polypropylene fibers. Preferably, the first and the second layer do not contain polymeric fibers. The low amount or absence of polymeric fibers is advantageous in that the amount of burning material is significantly lowered. Preferably, the first layer has a basis weight of 30 g / m2or more, preferably 35 g / m2or more, more preferably 40 g / m2or more, and 120 g / m2or less, preferably 110 g / m2or less, more preferably 100 g / m2or less. Preferably, the second layer has a basis weight of 10 g / m2or more, preferably 15 g / m2or more, more preferably 20 g / m2or more, and 70 g / m2or less, preferably 65 g / m2or less, more preferably 60 g / m2or less. In the present invention, a weight ratio of the first layer to the second layer is preferably 1 or more, more preferably 1.5 or more, even more preferably 2 or more, and 20 or less, more preferably 18 or less, even more preferably 15 or less. The second layer of the fibrous mat of the present invention is placed onto the first layer. The wording “placed onto the first layer” in the context of the present invention denotes that the second layer is laid on at least one side of the first layer and covers at least part of its surface on that side. Preferably, the second layer covers the entire surface of one side of the first layer. Binder The first and the second layer of the fibrous mat of the present invention are impregnated with a composition comprising a binder. The term “impregnated with a composition” as used herein denotes that the composition permeates through at least a portion of the thickness of the first and the second layer. Preferably, the composition permeates through the whole thickness of the first and the second layer. The binder included in the composition that impregnates the first and second layer is preferably an acrylic binder. Examples of the acrylic binder include polymers of acrylic acid, polymers of methacrylic acid, styrene butadiene, styrene acrylic copolymers, urea formaldehyde resins, modified urea formaldehyde resins (for example, modified with one or more of acrylic, styrene butadiene, acrylic copolymer or vinyl acetate resins), melamine formaldehyde resins and mixtures thereof. In a preferred embodiment, the acrylic binder is an acrylic acid resin. The amount of binder in the fibrous mat may be 10 wt% or more, preferably 15 wt% or more, more preferably 20 wt% or more, even more preferably 22 wt% or more, and preferably 40 wt% or less, more preferably 30 wt% or less, even more preferably 26 wt% or less of the total weight of the fibrous mat. The total weight of binder and coating in the fibrous mat may be 1 wt% or more, preferably 5 wt% or more, more preferably 10 wt% or more, even more preferably 12 wt% or more, and 50 wt% or less, preferably 40 wt% or less, more preferably 30 wt% or less, even more preferably 25 wt% or less of the total weight of the fibrous mat. Coating The fibrous mat of the present invention includes a coating placed on the second layer and which contains at least one vinyl polymer. The coating is placed onto the second layer, and specifically onto the surface of the second layer which faces away from the first layer. That is, the coating and the first layer are on the opposite sides of the second layer of the fibrous mat according to the invention. Preferably, the coating is present on the surface of the second layer, and not on the surface of the first layer which faces away from the second layer. Preferably, the surface of the first layer which faces away from the second layer is uncoated, so as to enable exposure of the fiber surface of the glass fibers that are present in the inner side area and ensure sufficient penetration with a gypsum-containing material and interlocking with the gypsum crystals. The coating that is placed onto the second layer may comprise more than one vinyl polymer, and preferably comprises two vinyl polymers which are different from each other. The total amount of vinyl polymer(s) in the coating may be 5 wt% or more, preferably 7 wt% or more, more preferably 10 wt% or more of the total weight of coating and binder. Further, the total amount of vinyl polymer(s) in the coating may be 50 wt% or less, preferably 45 wt% or less, more preferably 40 wt% or less of the total weight of coating and binder. The wording “total weight of the coating and binder” refers to the sum of the weight of the coating and the weight of the binder that is present in the composition impregnating the first and the second layers. In a preferred embodiment, the coating comprises polyvinyl alcohol. Polyvinyl alcohol may be present in the coating in an amount of 5 wt% or more, preferably 7 wt% or more, more preferably 10 wt% or more of the total weight of coating and binder. Further, the amount of polyvinyl alcohol in the coating may be 50 wt% or less, preferably 45 wt% or less, more preferably 40 wt% or less of the total weight of coating and binder. In a preferred embodiment, the coating comprises a polymer comprising a monomer unit of a vinyl ester of an alpha branched aliphatic monocarboxylic acid having a chain length from 5 to 20 carbon atoms. Preferred examples of such polymer are vinyl esters of versatic acid, also referred herein as vinyl versatate polymers. In a preferred embodiment, the coating comprises a vinyl versatate polymer. Preferably, the vinyl versatate polymer is selected from a vinyl ester of versatic acid 9, a vinyl ester of versatic acid 10, a vinyl ester of versatic acid 11 and mixtures thereof. The vinyl versatate polymer may be present in the coating in an amount of 1 wt% or more, preferably 3 wt% or more, more preferably 5 wt% or more of the total weight of the coating and binder. Further, the vinyl versatate polymer may be present in the coating in an amount of 18 wt% or less, preferably 17.5 wt% or less, more preferably 17 wt% or less, even more preferably 15 wt% or less or 13 wt% or less of the total weight of the coating and binder. In a more preferred embodiment, the coating comprises polyvinyl alcohol and a vinyl versatate polymer. Preferably, a weight ratio between polyvinyl alcohol and vinyl versatate polymer may be 0.5 or more, preferably 0.8 or more, more preferably 1 or more, even more preferably 1.1 or more, and 5 or less, preferably 4 or less, more preferably 3 or less. It has been found that the use of at least one vinyl polymer in the coating and preferably the combination of polyvinyl alcohol and a vinyl versatate polymer is particularly advantageous in the context of the present invention from the viewpoint of achieving optimal barrier properties even with a low amount of coating, thus resulting in a fibrous mat with reduced bleed through of the gypsum and good water barrier properties, which is at the same time light weight. The barrier properties of the fibrous mat of the present invention may be determined in terms of Cobb value as determined according to a modified ISO 535 standard as described below in the examples. Preferably, the Cobb value as determined on the side of the fibrous mat which is coated with the coating is 0.5 g / m2or more, preferably 0.7 g / m2or more, more preferably 1 g / m2or more, and 10 g / m2or less, preferably 8 g / m2or less, more preferably 6 g / m2or less. The Cobb value as measured with the method above on the side of the first layer which faces away from the side coated with the coating is higher than the Cobb value as measured on the side of the fibrous mat which is coated with the coating. The use of at least one vinyl polymer in the coating allows to achieve a Cobb value within the above ranges on the side which is coated with the coating, which preferably does not exceed 10 g / m2. In this case, optimal barrier properties can be ensured. Due to the advantageous combination of different fiber sizes in the first and second layers, impregnation with the binder and coating with a composition comprising at least one vinyl polymer, such barrier properties can be achieved even in the absence of polymeric fibers and fillers, with low amounts of coating and by using materials having a low basis weight. The coating may comprise other components in addition to the at least one vinyl polymer. For example, the coating may include fillers such as calcium carbonate, calcium sulphate, clay, kaolin, sand, talc, mica, glass powder, titanium dioxide, magnesium oxide, alumina, alumina trihydrate, aluminium hydroxide, antimony oxide, silica and silicate. Preferably, the amount of fillers in the coating and in the binder composition is as low as possible and is 10 wt% or less, preferably 5 wt% or less, more preferably 0 wt% of the total weight of the coating and binder. Preferably, the coating and binder composition of the fibrous mat of the present invention do not contain fillers. The total amount of coating in the fibrous mat is preferably 1 wt% or more, preferably 2 wt% or more, more preferably 3 wt% or more, and preferably 10 wt% or less, preferably 8 wt% or less, more preferably 7 wt% or less of the total weight of the fibrous mat. The coating may have a basis weight of 1 g / m2or more, preferably 1.5 g / m2or more, more preferably 2 g / m2or more, and preferably 10 g / m2or less, more preferably 8 g / m2or less, further preferably 6 g / m2or less, and even more preferably 5 g / m2or less. The amount of coating with respect to the sum of the weight of the coating and the weight of the binder may be 5 wt% or more, preferably 7 wt% or more, more preferably 10 wt% or more, even more preferably 12 wt% or more, and may be 50 wt% or less, preferably 40 wt% or less, more preferably 35 wt% or less, even more preferably 25 wt% or less. Gypsum board The present invention also relates to a gypsum board comprising a gypsum core and a fibrous mat according to the present invention, wherein the coating containing a vinyl polymer in the fibrous mat and the gypsum core are at opposite sides of the fibrous mat. That is, when the sides of the fibrous mat are referred to as the “inner side” and the “outer side” as above, and the side of the first layer which faces away from the second layer corresponds to the inner side, the gypsum core is in contact with the inner side of the fibrous mat. Thanks to the combination of two layers comprising glass fibers having different sizes, the gypsum board of the present invention is characterized by an improved bonding strength between the gypsum core and the facer, and the leaking through of the gypsum is prevented thanks to the smaller fibers and to the coating on the outer side of the fibrous mat. The gypsum board according to the present invention includes a gypsum core. Besides gypsum, the gypsum core may further comprise water resistant and / or fire resistant agents. Water resistant additives, such as or silicone derivatives or wax, are typically introduced into the gypsum core to increase the hydrophobicity of the gypsum board. The water resistance of a gypsum board is typically characterized by the maximum allowed absorption of liquid water into the board, in accordance with either of norm ASTM C-473 or EN-520 or EN15283-1. Examples of water-resistant additives include poly (vinyl alcohol), molten wax, emulsified wax / asphalt, emulsified wax, asphalt, metallic soaps, resins, polysiloxanes and synthetic thermoplastic synthetic materials such as polyvinylchloride or polyvinylacetate. The amount of water-resistant additives may vary between 0.05 wt% to about 5 wt% in reference of the total weight of the gypsum core. Examples of fire-resistant additives include mineral fibres (glass fibres, basalt fibres), and mineral fillers (clay, vermiculite, silica, alumina). The amount of fire-resistant additives can be from about 0.03 wt% to about 10 wt%. Further additives can be used, such as biocides. Foaming agents may be also used in the gypsum core slurry in order to decrease the total core weight. Typical gypsum core weight density ranges from 700 to 1000 kg / m3. Thickening or fluidizing additives are also typically used to control the rheology of the gypsum slurry. These could be respectively carboxymethylcellulose (CMC), hydroxymethylcellulose (HMC), hydroxyethylcellulose (HEC), starch, guar gums as thickening agents, and polycarboxylate ether as fluidizing agent. Starch, CMC, HMC or HEC are also used to impart better bonding between the gypsum core and the mat facer. Process for producing a fibrous mat The present invention also relates to a process for producing a fibrous mat for a gypsum board, the process comprising the steps of: a) laying a suspension A comprising glass fibers onto a screen; (b) laying a suspension B comprising glass fibers onto the suspension A after or simultaneously with step (a); wherein the average diameter of the glass fibers comprised in the suspension A is larger than the average diameter of the glass fibers comprised in the suspension B, or wherein the average diameter of the glass fibers comprised in the suspension B is larger than the average diameter of the glass fibers comprised in the suspension A; (c) dewatering the suspensions A and B to form a layer A and a layer B comprising glass fibers; (d) impregnating the layers A and B obtained in steps (a) to (c) with a composition comprising a binder; (e) curing the binder; and (f) applying a composition comprising at least one vinyl polymer onto the layer A or B to form a coating, with the proviso that in step (f), the composition is applied onto layer A if the average diameter of the glass fibers comprised in the suspension B is larger than the average diameter of the glass fibers comprised in the suspension A, and in step (f), the composition is applied onto layer B if the average diameter of the glass fibers comprised in the suspension A is larger than the average diameter of the glass fibers comprised in the suspension B. In step (a), a suspension A comprising glass fibers is laid on a screen. The composition and type of glass fibers may be such that the layer resulting from such suspension is as described above for the first or for the second layer of the fibrous mat of the present invention. The screen is not particularly limited and may be any screen used in the art for the deposition of glass fiber-containing nonwovens, including a forming fabric or a wire. In step (b), a suspension B comprising glass fibers is laid onto the suspension A after or simultaneously with step (a), preferably simultaneously with step (a). The composition and type of glass fibers in the suspension B may be such that the layer resulting from such suspension is as described above for the first or for the second layer of the fibrous mat of the present invention. In the process of the present invention, the average diameter of the glass fibers comprised in the suspension A is larger than the average diameter of the glass fibers comprised in the suspension B, or the average diameter of the glass fibers comprised in the suspension B is larger than the average diameter of the glass fibers comprised in the suspension A. The average diameters of the glass fibers comprised in the suspensions A and B may be as described above for the first and the second glass fibers described above. Preferably, the average diameter of the glass fibers comprised in the suspension A is larger than the average diameter of the glass fibers comprised in the suspension B. In this case, the average diameter of the glass fibers comprised in the suspension A and the average diameter of the glass fibers comprised in the suspension B may be as described above for the first and the second glass fibers, respectively. Preferably, the suspension A and / or the suspension B is a foam suspension. The foam suspensions may be prepared by mixing the fibers with foam in a pulper. The foam suspension may be formed by mechanical agitation and may be obtained in the presence of a foaming aid such as a surfactant, which can be useful to stabilize the suspension by lowering the surface tension between the gas bubbles and the water in the foam. For example, a foam suspension of glass fibers in the context of the present invention may comprise glass fibers, water and air, and optionally a foaming aid as described above. The use of foam suspensions allows for an advantage when compared with methods using wet-laid techniques, because the foam process ensures a good dispersion of the glass fibers, and allows for a shorter drying time to form the layers, thus resulting in a quicker and environmentally friendlier process. The step (c) of dewatering the suspensions A and B to form a layer A and a layer B comprising glass fibers may be carried out by application of vacuum. The step (d) of impregnating the layers A and B obtained in steps (a) to (c) with a composition comprising a binder may be preferably carried out with a curtain coater. Other methods of application of the binder composition are also possible, such as with a dip and squeeze applicator or spraying. The binder in the composition used in step (d) may be as described above for the fibrous mat of the present invention. Excess binder may be removed by application of vacuum. In step (e), the binder is cured. The curing step may be carried out by heating the impregnated first and second layer obtained in step (d) at a temperature of 100 to 200°C, preferably 120 to 180°C, for 0.5 to 5 minutes. In step (f), a composition is applied onto the layer A or B, the composition comprising at least one vinyl polymer to form a coating. The vinyl polymer(s) included in the composition used in step (f) may be as described above for the vinyl polymer(s) of the coating of the fibrous mat. The composition may be applied onto the layer A or B preferably by size press. Other methods of application of the composition are also possible, such as spray coating. In the process according to the invention, the composition comprising a vinyl polymer is applied onto the layer which comprises the glass fibers having the smallest average diameter. That is, the composition comprising a vinyl polymer is applied onto layer A if the average diameter of the glass fibers comprised in the suspension B is larger than the average diameter of the glass fibers comprised in the suspension A, or it is applied onto layer B if the average diameter of the glass fibers comprised in the suspension A is larger than the average diameter of the glass fibers comprised in the suspension B. In a preferred embodiment, the average diameter of the glass fibers comprised in the suspension A is larger than the average diameter of the glass fibers comprised in the suspension B, and in step (f), the composition is applied onto layer B. In a preferred embodiment, in step (f), the total amount of vinyl polymer(s) in the composition applied onto the layer A or B is 1 to 10 wt%, preferably 3 to 7 wt% of the total weight of the obtained fibrous mat. Advantageously, the steps (a) to (f) of the process according to the present invention may be performed on a papermaking machine comprising a head box. Further, it is possible to perform the steps (a) to (f) by using only a single head box. The present invention also relates to a fibrous mat obtainable by the above process. The fibrous mat obtainable by the above process may be the fibrous mat of the present invention described above. That is, the process above may be a process for producing a fibrous mat for a gypsum board according to the present invention. Process for producing a gypsum board The present invention also concerns a process for producing a gypsum board, including a step of depositing a gypsum slurry on a fibrous mat according to the present invention. Further, the present invention also relates to a process for producing a gypsum board including a step of producing a fibrous mat according to the present invention. Preferably, the gypsum slurry is deposited on the first layer of the fibrous mat which faces away from the second layer of the fibrous mat. Preferably, the gypsum slurry is deposited on the inner side of the fibrous mat as described above, which corresponds to the side of the first layer which faces away from the second layer. Upon deposition of the gypsum slurry on the first layer of the fibrous mat, a gypsum core which is covered by the fibrous mat of the present invention may be formed. The present invention also relates to a gypsum board obtainable by the processes described above. The gypsum board obtainable by the processes described above may be the gypsum board according to the present invention. Examples Example 1 to 3 In Example 1, fibrous mat was prepared by depositing a foam suspension A of glass fibers having an average diameter of 10 µm on a wire, wherein the amount of glass fibers was 100 wt% of the total amount of fibers in the foam suspension A, and a foam suspension B of glass fibers having an average diameter of 6 µm, wherein the amount of glass fibers was 100 wt% of the total amount of fibers in the foam suspension B. Upon dewatering, the suspensions A and B formed a first and a second layer, respectively. In Examples 2 and 3, a fibrous mat was prepared by depositing a foam suspension A of glass fibers having an average diameter of 13 µm (K13 fibers, 13 mm length) on a wire, wherein the amount of glass fibers was 100 wt% of the total amount of fibers in the foam suspension A, and a foam suspension B of glass fibers having an average diameter of 10 µm (G10 fibers, 9-10 mm length), wherein the amount of glass fibers was 100 wt% of the total amount of fibers in the foam suspension B. Upon dewatering, the suspensions A and B formed a first and a second layer, respectively. In all examples, the average fiber diameter of the fibers of each layer was calculated as the arithmetic average of the fiber diameter of 100 fibers determined by SEM imaging. The respective average fiber diameters of the fibers in the first and in the second layers were as reported in Table 1. The weight ratio of the first to the second layer was 70:30, that is, the second layer represented 30% of the total fiber mass and the first layer represented 70% of the total fiber mass. The mat of each example was impregnated with a binder composition comprising 16 wt% of an acrylic binder (Examples 1 and 2) or 26 wt% of an acrylic binder (Example 3). For these examples, an acrylic acid resin (Acrodur from BASF) was used as the acrylic binder. The basis weight of the mat before coating was 80 g / m2in Example 1. In Example 2, the basis weight of the mat before coating was 97.9 g / m2, and in Example 3 the basis weight of the mat before coating was 55.4 g / m2. The second layer was coated with a mixture of 50:50 polyvinyl alcohol and vinyl versatate with a coating amount as indicated in Table 1. The basis weights of the coated products are as reported in Table 1. Comparative Examples 1 to 4 Fibrous mats were prepared in a similar way as in Examples 1 to 3, except that the average fiber diameters of the first and second layer were as reported in Table 1, and the coating was either absent (Comparative Example 2) or containing styrene acrylate (Comparative Examples 1, 3 and 4). The basis weight of the mat before coating was 80 g / m2in Comparative Examples 1 and 2. The basis weight of the coated products are reported in Table 1. In Comparative Examples 3 and 4, the coating mixture was also applied to the opposite side of the fibrous mat, that is, onto the side of the first layer which faced away from the second layer.

[0002] Table 1 Average Amount of Amount of Amount of Basi rage fiber fiber binder and s Ave Coating Amount of Coating coating coa coating versatate Weight of diameter of first diameter of Amount ting (wt% (wt layer (µm) second layer Chemistry (wt% of % of (wt% of Coated final (g / m2) of coating + bi final coating + Product (µm) nder)2product) product) binder) (g / m) Example 1 10 6 PVA + versatate 19.0 3.0 23.4 3.6 11.7 83.0 Example 2 13 10 PVA + versatate 18.4 2.9 13.0 2.4 6.6 120.0 Example 3 13 10 PVA + versatate 30.8 3.8 15.0 4.8 7.7 80.0 Comparative Example 1 10 6 styrene acrylate 17.7 10.0 80.5 7.7 0.0 130.3 Comparative Example 2 10 6 None 0.0 0.0 0.0 0.0 0.0 80.0 Comparative Example 3 13 10 styrene acrylate 32.0 20.0 62.5 20.0 0.0 100.0 Comparative Example 4 13 10 styrene acrylate 32.0 20.0 62.5 20.0 0.0 100.0

[0003] Characterization of the fibrous mats The properties of the fibrous mats prepared in the examples and comparative examples are reported in Tables 2 and 3. The air permeability was determined according to ISO 9237 and measured at 200 Pa. The tensile strength in machine direction and in cross direction was determined according to ISO 3341. The Cobb value was determined at 120 seconds on the coated side (for all examples and comparative examples, indicated as “top side” in Table 3), and on the side of the first layer that was not in contact with the second layer, which was either uncoated or coated (for Examples 2 and 3 and Comparative Examples 3 and 4, indicated as “bottom side” in Table 3). More specifically, the Cobb value was measured according to a IS0 535 method. The method consists of measuring the cobb values as in IS0535, but after 120 seconds contact time with water. For this measurement, the water is in contact with the outer surface, i.e. the side coated with the coating, of the fibrous mat. All testing were conducted under laboratory conditions at a temperature of 23.0 ± 1.0°C and 50.0 ± 2.0% relative humidity (RH). After that samples were stabilised in weight under these conditions for at least 24 hrs. Table 2 Tensil Sum of Thickness Air e Tensile CD+MD Cobb value P Strength Strength (mm) ermeability MD Tensile coated side (l / m2s) CD Strengt2N / 50mm N / 50mm h (g / m) N / 50mmExample 10.655 2220 248 81 329 4.2Comparative Example 10.71 2080 228 182 410 0.70.63 2500 400 147 547fiails as thereComparative Example 2s no coatingTable 3 Cobb value Cobb value top side bottom side (g / m2) (g / m2) Example 2 0.7 1.9 Example 3 3.3 15.8 Comparative Example 3 400 + 400 + Comparative Example 4 10.3 10.8 In Example 1, which is made using polyvinyl alcohol and vinyl versatate in the coating, an optimal water resistance can be achieved with only 3.6 wt% of coating. As shown in Example 1, it is possible to achieve optimal low Cobb value (less than 5 g / m2) with low basis weight facer starting materials (80 g / m2) and a small coating amount. When working with the same basis weight materials, the conventional styrene acrylic coating is not sufficiently water repellent and a higher amount of coating is necessary to achieve low Cobb values. Such large amounts are detrimental to the air permeability of the material. The fiber size may also influence the final properties of the product. Examples 2 and 3, which contained slightly larger fibers, achieved particularly good Cobb values with a relatively low amount of coating and binder.

Claims

CLAIMS 1. A fibrous mat (1) for a gypsum board comprising: a) a first layer (11) comprising glass fibers (12); b) a second layer (13) placed onto the first layer (11) comprising glass fibers (14); c) a coating (15) placed onto the second layer (13) comprising at least one vinyl polymer; wherein the average diameter of the glass fibers (12) in the first layer (11) is larger than the average diameter of the glass fibers (14) in the second layer (13); and the first layer (11) and the second layer (13) are impregnated with a composition comprising a binder.

2. The fibrous mat according to claim 1, wherein the coating comprises polyvinyl alcohol.

3. The fibrous mat according to claim 1 or 2, wherein the coating comprises a polymer comprising a monomer unit of a vinyl ester of an alpha branched aliphatic monocarboxylic acid having a chain length from 5 to 20 carbon atoms, preferably wherein the polymer is a vinyl versatate polymer.

4. The fibrous mat according to any one of claims 1 to 3, wherein the coating comprises polyvinyl alcohol and a vinyl versatate polymer.

5. The fibrous mat according to claim 3 or 4, wherein the vinyl versatate polymer is selected from a vinyl ester of versatic acid 9, a vinyl ester of versatic acid 10, a vinyl ester of versatic acid 11 and mixtures thereof.

6. The fibrous mat according to any one of claims 3 to 5, wherein the amount of vinyl versatate polymer in the coating is 18 wt% or less of the total weight of the coating and binder.

7. The fibrous mat according to any one of claims 1 to 6, wherein a Cobb value as measured on the side of the fibrous mat coated with the coating (15) is 10 g / m2or less.

8. The fibrous mat according to any one of claims 1 to 7, wherein the amount of fillers in the coating is 10 wt% or less, preferably 5 wt% or less, and more preferably 0 wt% of the total weight of the coating and binder.

9. The fibrous mat according to any one of claims 1 to 8, wherein the total weight of the coating and binder represents 1 wt% or more and 30 wt% or less of the total weight of the fibrous mat.

10. The fibrous mat according to any one of claims 1 to 9, wherein the amount of glass fibers in each of the first layer and the second layer is 80 wt% or more, preferably 90 wt% or more, even more preferably 100 wt% of the total amount of fibers in each layer.

11. The fibrous mat according to any one of claims 1 to 10, wherein the average diameter of the glass fibers in the first layer is 8 µm or more and 20 µm or less and the average diameter of the glass fibers in the second layer is 7 µm or more and 16 µm or less, preferably 8 µm or more and 16 µm or less.

12. The fibrous mat according to any one of claims 1 to 11, wherein the amount of glass fibers having an average diameter of 6 µm or less in each of the first layer and the second layer is 5 wt% or less, preferably 1 wt% or less of the total amount of fibers in each layer.

13. The fibrous mat according to any one of claims 1 to 12, wherein the binder in the composition impregnating the first layer and the second layer is an acrylic binder.

14. The fibrous mat according to any one of claims 1 to 13, wherein the amount of binder in the composition impregnating the first layer and the second layer is 10 wt% or more and 30 wt% or less of the total weight of the fibrous mat.

15. The fibrous mat according to any one of claims 1 to 14, wherein the amount of polymeric fibers in each of the first layer and the second layer does not exceed 10 wt% of the total amount of fibers in each layer.

16. The fibrous mat according to any one of claims 1 to 15, wherein the total basis weight of the fibrous mat is from 70 to 160 g / m2.

17. The fibrous mat according to any one of claims 1 to 16, wherein a weight ratio of the first layer to the second layer is 1 or more and 20 or less.

18. A gypsum board comprising a gypsum core and a fibrous mat according to any one of claims 1 to 17, wherein the coating containing a vinyl polymer in the fibrous mat and the gypsum core are at opposite sides of the fibrous mat.

19. Use of the fibrous mat according to any one of claims 1 to 17 in a gypsum board.

20. A process for producing a fibrous mat for a gypsum board, the process comprising the steps of: (a) laying a suspension A comprising glass fibers onto a screen; (b) laying a suspension B comprising glass fibers onto suspension (A) after or simultaneously with step (a); wherein the average diameter of the glass fibers comprised in the suspension A is larger than the average diameter of the glass fibers comprised in the suspension B, orwherein the average diameter of the glass fibers comprised in the suspension B is larger than the average diameter of the glass fibers comprised in the suspension A; (c) dewatering the suspensions A and B to form a layer A and a layer B comprising glass fibers; (d) impregnating the layers A and B obtained in steps (a) to (c) with a composition comprising a binder; (e) curing the binder; and (f) applying a composition comprising at least one vinyl polymer onto the layer A or B to form a coating, with the proviso that in step (f), the composition is applied onto layer A if the average diameter of the glass fibers comprised in the suspension B is larger than the average diameter of the glass fibers comprised in the suspension A, and in step (f), the composition is applied onto layer B if the average diameter of the glass fibers comprised in the suspension A is larger than the average diameter of the glass fibers comprised in the suspension B.

21. The process according to claim 20, wherein the average diameter of the glass fibers comprised in the suspension A is larger than the average diameter of the glass fibers comprised in the suspension B, and in step (f), the composition is applied onto layer B.

22. The process according to claim 20 or 21, wherein the suspension A and / or the suspension B is a foam suspension.

23. The process according to any one of claims 20 to 22, wherein in step (f), the amount of composition comprising at least one vinyl polymer applied onto the layer A or B is 1 to 10 wt%, preferably 3 to 7 wt% of the total weight of the obtained fibrous mat.

24. The process according to any one of claims 20 to 23, wherein the steps (a) to (f) are performed on a papermaking line comprising a head box.

25. A fibrous mat for a gypsum board obtainable by the process according to any one of claims 20 to 24.

26. A process for producing a gypsum board, including a step of depositing a gypsum slurry on a fibrous mat according to any one of claims 1 to 17.

27. A process for producing a gypsum board, including a step of producing a fibrous mat according to any one of claims 20 to 24.

28. A gypsum board obtainable by the process according to claim 26 or 27.

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