BUILDING BOARD AND ITS PRODUCTION

Abstract: A building board comprising cementitious fibre board has first and second surface layers (20, 23) of fibres admixed with cementitious materials; and a core layer (21) of fibres admixed with cementitious material but of different composition from either of the surface layers and disposed between the surface layers and bonded thereto by interpenetrating crystal growth; and at least one layer of fibrous mesh (22) interposed between the core layer and at least one of the surface layers and bonded to each of the adjacent layers by the interpenetrating crystal growth. The board may be produced by successively spreading layers of surface and core mixes to form a fibrous cementitious assembly, with interposition of at least one layer of fibrous mesh between at least one pair of adjacent layers of the mixes; compressing the assembly to a predetermined thickness; and maintaining the pressure until hydration of the cementitious material in the assembly is substantially complete.

Description: BUILDING BOARD AND ITS PRODUCTION The present invention relates to the production of building board of superior strength, Modulus of Rupture (MOR) , Limit of Proportionality (LOP) and stiffness, which make it suitable for use in load-bearing situations, for example in suspended flooring. In its preferred aspects the invention relates to an improvement in layered gypsum fibre board having surface layers formed from compositions having lower water : plaster ratios than used in the production of traditional plasterboard from aqueous slurries of gypsum plaster. Gypsum fibre board of this kind and a semi-dry method of manufacturing it have been described in International Patent Publication WO93/01932 of Carl Schenck AG. Over the past thirty years a variety of composite materials have been developed for use as building materials. For example, it is .known to introduce a relatively strong and stiff glass fibre mesh or tissue to reinforce a weaker core material. There have been problems in achieving a good bond between the glass and the core material, either because of chemical incompatibility or because a large difference in strength means that the core cannot hold the glass in place when force is applied. Two approaches to overcoming this problem are described in GB 2 053 779A and GB 2 316 693 A. In the case of cement boards it has been necessary to use a slurry of cement and water to hold the mesh on the mortar core. Because the slurry layer exhibits greater shrinkage than the core on drying, it is common to find microcracks in the slurry layer, which reduce the benefits obtained from the glass mesh. In another line of development, improvements in the properties of gypsum have been obtained by incorporating cellulose fibres or wood chippings . Such gypsum fibre board composites have been used as impact resistant boards. Nevertheless, the flexural strength of these composites (commonly 8-10MPa) is less than half that of chipboard (about 22MPa) , so that they cannot at present be used in place of the latter in, for example, suspended flooring. Moreover, given that floors are usually specified to have a very small deflection (less than 1/300) , it would also be necessary to increase the stiffness. It is also known to increase the impact resistance of gypsum fibre board by bonding glass fibre mesh to one external face of the board. The mesh is applied in a secondary process but gives the face of the board a rough appearance. It is an object of this invention to provide a layered cementitious fibre board having at least one smooth face and very efficient bonding of a fibrous structure to the core. In accordance with this invention a building board comprises cementitious fibre board comprising first and second surface layers of fibres admixed with cementitious material; a core layer of a structural solid additive, such as fibres or lightweight aggregate, admixed with cementitious material but of different composition from either of the surface layers and disposed between the surface layers and bonded thereto by interpenetrating crystal growth; at least one layer of fibrous mesh being interposed between the core layer and at least one of the surface layers and bonded to each of the adjacent layers by the interpenetrating crystal growth. The board of this invention may be produced by preparing a mix or mixes containing fibrous or structural solid material, as the case may be, hydratable cementitious material and water to constitute the surface layers and core of the product; spreading a layer of a surface mix on a supporting surface; and thereafter successively spreading layers of the core mix and a surface mix to form a fibrous cementitious assembly; compressing the assembly to a predetermined thickness; and maintaining the pressure until hydration of the cementitious material in the assembly is substantially complete; in which at least one layer of fibrous mesh is interposed between at least one pair of adjacent layers of the mixes during formation of the assembly. The cementitious material is preferably gypsum, and the surface layers are preferably produced from semi-dry compositions having lower water: plaster ratios than those used in the manufacture of traditional plasterboard, which are slurries and require to be handled differently. For example, such slurry compositions have water: plaster ratios in excess of 60% whereas, in the production of gypsum fibre board according to this invention, the compositions used to form the surface layers preferably have water: plaster ratios below 58%, and especially in the range 50-55% by weight. The core layer may contain up to 20% fibre, calculated on the dry weight of the core layer in the finished board. Whether or not it includes fibre, the core should always contain sufficient structural solid additive to provide a semi-dry mix when admixed with the cementitious material and hydration water. The additive referred to here is a solid material capable of conferring a physical structure on the layer when bonded by the cementitious material, and is typified by fibrous materials and aggregates, notably lightweight aggregates. The fibres in the layers may be cellulose fibres, such as paper or wood fibres. The fibre contents of the layers may differ from one another, the core layer preferably having a lower fibre content than the surface layers. For example, while the core may contain from 0 to 20% paper fibre and typically about 8%, the surface layers may contain from 5 to 20% paper fibre on the same basis, values about 14% being typical. The core layer preferably has a lower density than the surface layers. In particular, if an overall less dense product is required, the layers, but more especially the core layer, may additionally contain a lightweight aggregate, such as conventionally expanded perlite vermiculite or the like. Suitable levels of such aggregate are from 10% to 60%, for example about 30%, perlite by weight of the layer in the finished board. The water necessary for the hydration and setting of the cementitious binder material can be introduced to the mix wholly or in part as water absorbed in the fibrous constituent of the mix. Water can also be introduced, more especially into the core mix, by the use of wet or damp lightweight aggregate material. Any shortfall in the water necessary for hydration can be made up by a separate addition or by spraying the layers or assembly with water. The fibrous mesh may be of inorganic fibre such as glass or carbon fibre, or of organic fibres, such as polypropylene or high density oriented polyethylene. Not only may one or more layers of, for example, inorganic fibrous mesh be interposed between one or both surface layers and the core layer, but one or more layers of organic fibrous mesh may additionally be interposed in either or both of these locations. For example, a glass fibre mesh may be included to increase stiffness and an organic mesh to improve impact properties. As indicated, the fibrous mesh may be of glass fibres and preferably takes the form of an open mesh glass fibre fabric or crenette. The open texture of the fabric helps the compositions of the two adjacent layers to combine during the pressing operation. The size of opening of the mesh is preferably from 1.0 to 20mm, a typical crenette having about 8 tows per inch (about 3 per cm) . In producing the board of this invention, the support surface may be a moving belt or table in a continuous production line, and the board, after release from pressure, may be cut into lengths before being dried. Pressure may be varied at different times during the process, or at different positions along the line, to provide a pressure profile that encourages combination of the layers and fibrous mesh. The thickness of the various layers in the board can be adjusted by varying the amount and thus the thickness of the respective compositions spread on the support surface. By this means the relative thickness of each of the surface and core layers in relation to the others can be adjusted. This in turn enables the distance of the one or more layers of fibrous mesh from the central plane of the board to be controlled. The compositions may contain other substances, including additives well known in the production of building board. For example, the compositions may contain setting accelerators to enable the setting and hydration times of the compositions to be adjusted to suit the particular product or circumstances of production. This is in addition to the presence in one or more of the layers, and especially in the core layer, of an aggregate, for example a lightweight aggregate such as expanded perlite. The compositions may also include a viscosity modifier. In the production of this board it is preferred that the proportion of water in the mixes should be kept low, provided the total quantity of water applied during manufacture is at least stoichiometrically sufficient. The mixes will therefore have a semi-dry rather than a flowable consistency, and the pressure regime must be adjusted to ensure that the fibrous mesh is firmly embedded between the compressed layers of the board. In a preferred embodiment of the invention, now to be described, a fibrous mesh is introduced into an assembly of gypsum fibre layers for the manufacture of layered gypsum fibre board by this kind of semi-dry process. In the accompanying drawings : Fig. 1 is a schematic view of one example of a line for producing board in accordance with this invention; and Fig.2 is a cross-section of one example of board embodying the invention. In this example, wet recycled paper fibres for a bottom surface layer are spread on a first preforming belt 10, followed by a layer of hemihydrate plaster together with additives. The belt 10 carries the materials to a mixing head 11, which deposits the resulting mix on a main forming belt 12. Then, in accordance with the invention, glass fibre crenette is drawn from a roll 13 and laid over the bottom surface layer on the main belt 12. Wetted aggregate and fibres for the core mix are supplied to a blender 14, from where it is spread on a second pre-forming belt 15, passing through a mixing head 16 to be spread on top of the bottom surface layer and crenette on the main forming belt 12. At the same time fibres and plaster for a top surface layer are supplied to a third pre-forming belt 17 are mixed in a mixing head 18 and spread on top of the core layer on the main forming belt. Carried by the belt 12, and with any necessary additional water sprayed on to the surface, the assembled layers pass into the press 19 where pressures are applied according to a profile which ensures that the mixes set together and incorporate the crenette into the crystal structure of the product. The pressures applied to mixes are controlled such that the dry finished board has a density in the range 350kg/m 3 to 1350kg/m 3 When the setting and hydration of the gypsum are substantially complete, the assembly leaves the press region and is trimmed and cut into lengths for drying. The product exhibits the structure shown in Fig. 2, and comprises a bottom surface layer 20 of gypsum with a relatively higher fibre content, for example of paper fibres, a core layer 21 of gypsum with a lower fibre content, and preferably containing a lightweight aggregate such as perlite, a web of inorganic fibrous mesh 22, such as the glass fibre crenette already discussed, and an upper surface layer 23 of gypsum with a higher fibre content, which may have the same composition as the bottom surface layer, or different, depending on the properties required of the board. Board in accordance with the invention may be made with two smooth surfaces. It can, depending on its composition and density, exhibit a flexural strength up to 15 MPa or more. If it is required to manufacture board with more than one layer of fibrous mesh between the core layer and one or both surface layers, or with a layer or layers of organic fibrous mesh in addition to an inorganic fibrous mesh in either or both of these positions, the apparatus shown in Fig. 1 can be modified to provide the necessary additional supply rolls 13 for other fibrous meshes, which are led between the appropriate mixing heads 11, 16 or 18. Fibrous mesh can also be applied externally during formation of the assembly, either before the mixing head 11 or after the mixing head 18, but the resulting product may not have smooth surfaces.

Claims: CLAIMS 1. Building board comprising cementitious fibre board comprising first and second surface layers (20, 23) of fibres admixed with cementitious material; a core layer (21) of a structural solid additive admixed with cementitious material but of different composition from either of the surface layers and disposed between the surface layers and bonded thereto by interpenetrating crystal growth; at least one layer of fibrous mesh (22) being interposed between the core layer and at least one of the surface layers and bonded to each of the adjacent layers by the interpenetrating crystal growth. 2. Building board according to claim 1, in which the core layer has a lower fibre content than the surface layers. 3. Building board according to claim 1 or 2, in which the admixed fibres in the surface layers, and in the core layer when present, are cellulose fibres. 4. Building board according to claim 2 or 3 in which the core contains from 0 to 20%, preferably about 8%, paper fibre calculated by dry weight of the finished board, and the surface layers contain from 5 to 20%, preferably about 14%, fibre on the same basis. 5. Building board according to any of claims 1 to 4, in which the core layer has a lower density than the surface layers. 6. Building board according to any of claims 1 to 5, in which the core layer contains lightweight aggregate, for example from 10 to 60%, preferably about 30%, expanded perlite, vermiculite or the like calculated on the dry weight of the core layer. 7. Building board according to any of claims 1 to 6, in which the cementitious material is gypsum and the surface layers are the product of compositions having a water :plaster ratio below 58%, preferably 50-55% by weight. 8. Building board according to any of claims 1 to 7, in which the fibrous mesh is an open mesh glass fibre fabric or crenette. 9. Building board according to claim 8, in which the size of opening of the mesh is from 1.0 to 20mm, and preferably has about 8 tows per inch (about 3 per cm) . 10. Building board according to claim 1, in which at least one layer of inorganic fibrous mesh is interposed between one of the surface layers and the core layer. 11. Building board according to claim 10, in which at least one organic fibrous mesh is additionally interposed between the core layer and at least one of the surface layers. 12. A process for making building board according to claim 1, which comprises: preparing a mix or mixes containing fibrous and/or structural solid additive material, hydratable cementitious material and water to constitute the surface layers and core layer of the product; spreading a layer of a surface mix on a supporting surface; and thereafter successively spreading layers of the core mix and a surface mix to form a fibrous cementitious assembly; compressing the assembly to a predetermined thickness; and maintaining the pressure until hydration of the cementitious material in the assembly is substantially complete; in which at least one layer of a fibrous mesh is interposed between at least one pair of adjacent layers of the mixes during formation of the assembly. 13. A process according to claim 12, in which the mix for the core layer has a lower fibre content than the mix or mixes for the surface layers. 14. A process according to claim 12 or 13, in which the proportion of water in the mixes is sufficiently low to ensure that they have a semi-dry rather than a flowable consistency, provided that they contain sufficient water for hydration or that any necessary additional water is supplied before setting. 15. A process according to any of claims 12 to 14, in which the cementitious material is gypsum plaster and the surface layers are formed from compositions having a water : plaster ratio below 58%, preferably 50-55% by weight. 16. A process according to any of claims 12 to 15, in which the core mix contains a setting accelerator or lightweight aggregate. 17. A process according to any of claims 12 to 16, in which at least one layer of inorganic fibrous mesh is laid on the first layer of surface mix spread on the supporting surface. 18. A process according to claim 17 in which at least one layer of an organic fibrous mesh is additionally interposed between at least one pair of adjacent layers of the mixes during formation of the assembly. 19. A process according to any of claims 12 to 18, in which the fibres are cellulose fibres.

Inventors: HORSLEY STUART EDWARD, HORSLEY, STUART, EDWARD
Applicants: BPB PLC
Application year: 1999
Main classification: E04C2
IPC codes: B28B1, C04B28, E04C2