Gypsum products

Abstract: A gypsum product is disclosed formed of set gypsum plaster slurry and having an exposed gypsum surface in which the moisture resistance of the gypsum surface is enhanced by the inclusion in the qypsum of a hydrophobic silicone and at least one potassium salt together with fumed silica.

Description: The present invention relates to a set of gypsum products having exposed gypsum surfaces and provides such products with enhanced water repellency or moisture resistance. Prominent among such products are paperless gypsum board, but also included are gypsum boards having liners of paper or other material as one or both faces. In the case of boards having liners in both faces, the gypsum is exposed at the edges. Other gypsum products can also suffer from water absorption. Moisture resistant or water repellent plasterboard in the United Kingdom must meet the requirements of the British Standard for Gypsum Plasterboard [(BS1230:Part 1:1985)]. Moisture resistant wallboard is required to have a water absorption not greater than 5% expressed as the weight percentage of water absorbed by a 300mm X 300mm board sample immersed in water at 20°C for two hours, and can be used in situations where there is a risk of limited exposure of the board to moisture. Moisture repellent wallboard is required to have a surface water absorption not greater than 160g/m 2 measured after two hours exposure of the board surface to water at 20°C held in the ring of a Cobb water absorption apparatus, and can be used in situations where there is a risk of limited exposure to moisture on the surface of the board. To achieve moisture resistance, a hydrophobic additive may be incorporated into the plaster slurry employed in forming the board. The surface water absorption requirement for moisture repellency can be achieved by hard sizing of the lining or facing papers employed. The desired degree of protection against moisture in moisture resistant plasterboard is more difficult to achieve with board of reduced thickness at a given core density, despite the same area of surface and lower area of exposed core edge. The is because the weight of specimens under test decreases as the thickness is reduced. Similarly, the desired degree of protection becomes more difficult to achieve if the weight of the board is reduced at a given thickness. Moisture absorbency is a particular problem with paperless gypsum products. As well as tiles and other unlined or cast articles, such products also include paperless board, for example, that described in our patent GB 2 053 779, which discloses a building board comprising a core of set cementitious material, for example, gypsum, and an inorganic fibrous fabric or web embedded in at least one face thereof, wherein the material of the core penetrates the fibrous fabric or web and extends in a continuous film over the outer surface of the respective fabric or web. If such a product is made without a waterproofing additive it has a highly absorbent surface which poses problems in subsequent plastering or decorating. As has already been indicated, lined boards can also suffer from moisture absorption, whether through the exposed face in board lined only in one face or through the edges of board lined in both faces. In order to reduce the absorbency of the surfaces of board, it is customary to add a hydrophobic agent such as silicone to the plaster slurry employed. When using silicone it has proved impossible to achieve paperless gypsum board of this type which is moisture resistant at all thicknesses. There are several reasons for this, notably that the exposed gypsum surface is inherently much more difficult to make moisture resistant and that it is customary to dry such board at lower temperatures than conventional paper faced gypsum board in order to avoid recalcination of the board surfaces, which results in inadequate curing of the silicone waterproofing agents. Merely increasing the level of silicone addition has not overcome these difficulties. Potassium sulphate is a setting accelerator for plaster slurries and has the effect of increasing the size of gypsum crystals formed from the slurry and thus decreasing the total surface envelope of the set gypsum crystals. It has been found experimentally that, if potassium sulphate is added to a plaster slurry containing silicone, a substantial improvement in water resistance is observed which bears little relation to the change in crystallography of the gypsum. This effect has proved to be due to the potassium ion, although some salts, especially potassium carbonate and potassium sulphate, produce better results than others. Unfortunately, the improvement in water resistance is accompanied by an unacceptable loss of strength of the set gypsum due to the gypsum crystal habit modification. Additionally, various potassium salts have different effects on setting time, ranging from acceleration through neutral to retardation. It has now been found, in accordance with this invention, that if potassium salt solutions are introduced absorbed on fumed silica as a carrier into a gypsum plaster slurry, a significant improvement in moisture resistance can be achieved without unacceptable loss of strength of the set gypsum. A further unexpected discovery is that this result is achieved regardless of any modification of set brought about by the salt addition: where the salts affect the gypsum crystallography and the setting time, one would ordinarily expect a strength loss to occur, Moreover, mixtures of accelerating and retarding potassium salts can be used to achieve a slurry having the same setting time as a control slurry without the potassium additions. The fumed silica is preferably added in an amount of from 0.05 to 10 weight percent of plaster. The potassium salt or salts are preferably added in an amount from 0.05 to 1.0 weight percent plaster. The silicone, which may for example be a methyl hydrogen polysiloxane, may be used in an amount from 0.01 to 2.0 weight percent of plaster and preferably 0.3 to 1.0%. Examples of commercially available silicones are Wacker Chemie's BS 94 fluid and Rhone Poulenc's H68 fluid. Preferably, the potassium salt is potassium carbonate or potassium sulphate or a mixture thereof, preferably in the weight proportion of 1:1 to 3:1 of the sulphate to the carbonate. Further unexpected observations suggest that, although fumed silica is known as a strengthening additive for cement, the presence of fumed silica is not the reason for the maintenance of strength when fumed silica and absorbed salt solutions are added to the plaster slurry used in the production of gypsum boards and other cast gypsum products. Furthermore, it has been observed that, while the presence of fumed silica alone promotes some improvement in the moisture resistance of gypsum board containing silicone, it is the additional presence of potassium salts which is crucial to the development of high levels of water resistance with maintenance of compressive strength of the set gypsum. These observations are illustrated by, and the effects of various additions on setting time, compressive strength and moisture absorption of paperless gypsum boards are shown in, the following Table I. The superior results achieved by this invention are shown in Tests 5, 6 and 7. The application of the invention to a product other than paperless board is illustrated by the following example. Gypsum tiles of 6mm thickness were cast from a slurry of gypsum plaster and dried at 40°C in low humidity. Similar tiles were cast with hydrophobic silicone incorporated in the slurry and others with silicone together with potassium salt solutions absorbed on fumed silica at levels calculated as weight percent of plaster. The density and absorption values of the tiles are shown in Table II. It will be seen that, although silicone alone causes some reduction in moisture absorption, the combination of silicone, potassium salt and fumed silica brings about a much more striking reduction. ADDITIVE ABSORPTION (%) DENSITY (KG/m 3 ) NIL (CONTROL) 40.0 1043 0.5% silicone 26.7 1052 0.5% silicone + 12.5 1035 1.33% fumed silica + 0.3% potassium sulphate + 0.1% potassium carbonate + 1.6% water

Claims: A gypsum product formed of set gypsum plaster slurry and having an exposed gypsum surface in which the moisture resistance of the gypsum surface is enhanced by the inclusion in the gypsum of a hydrophobic silicone and at least one potassium salt together with fumed silica. A product according to claim 1 in which the potassium salt or salts are present in an amount from 0.05 to 1.0 weight percent of the plaster. A product according to claim 1 or 2 in which the fumed silica is present in an amount of from 0.05 to 10 weight percent of the plaster. A product according to claim 1, 2 or 3 in which the silicone is present in an amount from 0.01 to 2.0 weight percent of the plaster. A product according to any preceding claim in which the product is in the form of paperless gypsum board. A product according to any of claims 1 to 4 in which the product is a gypsum board lined in at least one face, for example with a paper liner. A product according to claim 6 lined in both faces with for example a paper liner. A product according to any of claims 1 to 7 which is a gypsum tile. A product according to claim 5, 6, 7 or 8 in which the potassium salt is potassium carbonate or potassium sulphate or a mixture thereof, preferably in the weight proportion of 1:1 to 3:1 of the sulphate to the carbonate. A product according to any preceding claim in which the said at least one potassium salt is absorbed onto the said fumed silica. A process for making a set gypsum product from a plaster slurry in which a hydrophobic silicone is added to the slurry as well as a potassium salt solution absorbed into fumed silica. A process according to claim 11 in which silicone, potassium salt or salts and fumed silica are employed in any of the percentage amounts defined in any of claims 2 to 4. A process according to claim 11 or 12 in which the said at least one potassium salt is absorbed onto the said fumed silica.

Inventors: BOOT FRANK LYN, BOOT, FRANK LYN
Applicants: BPB PLC
Application year: 1999
Main classification: C04B28
IPC codes: C04B20, C04B28