Gypsum product and method therefor

Abstract

A method comprising adding a suspension of a hydrophobic substance to a gypsum slurry. The hydrophobic substance may impart water repellent characteristics to the gypsum, may be used as a foaming aid, density reducer, dimension stabilizer, and others, and may increase the mechanical strength and durability of the gypsum product. In another embodiment of the present invention, suspensions of other substances may be used in addition to or in place of the hydrophobic substance to impart these characteristics.

Description

FIELD OF THE INVENTION

[0002] The present invention relates generally to gypsum products, and particularly to a method and system for imparting properties and features to gypsum products, such as with the addition of a suspension into the production line of the gypsum product.

BACKGROUND OF THE INVENTION

[0003] Gypsum (calcium sulfate dihydrate or hemihydrate) is a well-known material used in making industrial and building products, especially gypsum wallboard. It is a plentiful and generally inexpensive raw material, which, through a process of hydration may turn into slurry, which may be cast, molded or otherwise formed to useful shapes, and dried to a final product. Gypsum wallboard generally consists of a rehydrated gypsum core sandwiched between multi-ply paper cover sheets, and is used largely for interior wall and ceiling applications.

[0004] However, gypsum products, e.g., wallboard, tile, block, casts, plaster board, dry wall, sheathing and the like, have relatively little resistance to water. When ordinary gypsum wallboard, for example, is immersed in water, the board may quickly absorb a considerable amount of water, lose a great deal of its strength, and distort or swell in different places. Many attempts have been made in the past to improve the water resistance of gypsum products by adding waterproofing materials within the gypsum slurry. The most common waterproofing material in use today is a hydrophobic emulsion, usually an emulsion of wax, paraffin, asphalt or a silicone compound, e.g., silanes and siloxanes.

[0005] It is important to define clearly the difference between an emulsion and a suspension. An emulsion is defined as a dispersion of one liquid in a second immiscible liquid. One well-known example of an emulsion is milk; another is certain kinds of paint. A suspension, on the other hand, is defined as a dispersion of fine solid or liquid particles in a fluid (liquid or gas), the particles being supported by buoyancy.

[0006] Some emulsified silicone compositions used to make gypsum products water repellent may contain an emulsion of polysiloxane and some parts by weight of a liquid suspension of colloidal silica and other parts by weight of an emulsifier stabilizer. However, the silicone composition that is actually added to the gypsum slurry is an emulsion, not a suspension.

[0007] Other methods have been proposed in the prior art to improve the water resistance of gypsum products by coating the finished gypsum product with water resistant films or coatings. Another method involves spraying molten paraffin, wax, asphalt, and the like, into the aqueous gypsum slurry.

SUMMARY OF THE INVENTION

[0008] The present invention seeks to provide an improved gypsum product and process therefor. In one embodiment of the present invention, contrary to the prior art, the gypsum product is made by adding a suspension of a hydrophobic (that is, water repelling or resisting, the terms being used interchangeably) substance, such as but not limited to, paraffin, wax and the like. Adding a hydrophobic suspension to the gypsum slurry may provide several advantages. Emulsions have to be stored at the site where the gypsum product is produced, and have a limited shelf life. The manufacture of emulsions uses a lot of energy and is relatively costly. In contrast, the raw materials for making the suspension are much cheaper and are much more stable. Production of certain water repellent suspensions requires heat. The suspension may be manufactured at the site where the gypsum product is produced. The excess heat of the gypsum production process may be used as part of the manufacturing process of the suspension. Making the suspension at the site where the gypsum product is produced, and introducing the suspension on-line to the gypsum slurry, may provide significant savings in energy, storage, manpower, etc.

[0009] The hydrophobic substance may not just impart water repellent characteristics to the gypsum. It has surprisingly been found that the hydrophobic substance added as a suspension may be used as a foaming aid, density reducer, dimension stabilizer, and others, and may increase the mechanical strength and durability of the gypsum product. In another embodiment of the present invention, suspensions of other substances may be used in addition to or in place of the hydrophobic substance to impart these characteristics.

[0010] There is thus provided in accordance with an embodiment of the present invention a method comprising adding a suspension of a hydrophobic substance to a gypsum slurry. The suspension may be added to the gypsum slurry on-line at a production line where a gypsum product is produced. The suspension may be manufactured at a site where the gypsum product is produced. The suspension may be added during the preparation of the gypsum slurry, or to the already prepared gypsum slurry.

[0011] In accordance with an embodiment of the present invention excess heat of producing the gypsum product is used as part of manufacturing the suspension.

[0012] Further in accordance with an embodiment of the present invention the method may comprise storing the hydrophobic substance in a first storage container, storing a liquid in a second storage container, and forming the suspension by dispersing the hydrophobic substance in the liquid in a mixing device. The suspension may be added directly from the mixing device to the gypsum slurry. The suspension may be fed from the mixing device to an auxiliary mixing device, and the suspension may be added from the auxiliary mixing device to the gypsum slurry. The flow and/or production of the suspension to the gypsum slurry may be controlled with a control feeder valve.

[0013] Producing and adding the suspension to the gypsum slurry may be done in a production line, wherein the production line and the control feeder valve operate in a closed control loop.

[0014] Still further in accordance with an embodiment of the present invention the hydrophobic suspension may be prepared at the gypsum production line.

[0015] In accordance with an embodiment of the present invention adding the suspension of the hydrophobic substance to the gypsum slurry imparts at least one of the following characteristics to the product: water resistance, foaming, density reduction, dimension stabilization, and increase of mechanical strength and durability.

[0016] Further in accordance with an embodiment of the present invention the method may include adding an additional substance to the suspension of the hydrophobic substance, wherein the additional substance imparts at least one of the following characteristics to the product: water resistance, foaming, density reduction, dimension stabilization, and increase of mechanical strength and durability.

[0017] In accordance with an embodiment of the present invention the suspension may comprise a host particle, comprising a macroscopic particle of a substance other than gypsum.

[0018] Further in accordance with an embodiment of the present invention the suspension may comprise a surface active material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawing in which:

[0020] FIG. 1 is a simplified block diagram of a method and system for making a gypsum product, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0021] Reference is now made to FIG. 1, which illustrates a method and system for making a gypsum product, in accordance with an embodiment of the present invention.

[0022] A hydrophobic substance 10, such as but not limited to paraffin, wax, (e.g., paraffin wax, montan wax, candelilla wax, carnauba wax, beeswax, polyethylene wax, maleated hydrocarbons and others), asphalt and the like or any combination thereof, may be stored in a storage container 12. Hydrophobic substance 10 may be in the form of liquid, particles, flakes, pellets and other shapes, of any size. Storage container 12 may comprise a mixer 14. Dispersing aids (not shown) may be added to hydrophobic substance 10 in storage container 12. Mixer 14 may be used to reduce hydrophobic substance 10 to particles of any desired size. Provision may be made for controlling the temperature of the contents of storage container 12, such as, but not limited to, to a temperature range of 60-98° C.

[0023] Another storage container 16 may be provided for storing therein a liquid 18, such as but not limited to, water, which will be used to disperse therein hydrophobic substance 10 to form a suspension. Storage container 16 may also comprise a mixer 20. Dispersing aids (not shown) may be added to liquid 18 in container 16. Provision may be made for controlling the temperature of the contents of storage container 16, such as, but not limited to, to a temperature range of 60-98° C.

[0024] The contents, or any portion thereof, of storage containers 12 and 16 may be fed via tubing 22 and 24, respectively, to a batch mixing device 26, where hydrophobic substance 10 is dispersed in liquid 18 to form a suspension 27. Control valves 28 and 30, respectively, may control the flow of material from storage containers 12 and 16 to batch mixing device 26. Batch mixing device 26 may comprise a mixer 32, which may be any suitable dispersing mixer, such as but not limited to, a propeller, a stirrer, or dissolver moving at any suitable rate to form or maintain suspension 27. A control unit 34 may be provided that controls operation of batch mixing device 26, such as but not limited to, the weight or volume of the substances being mixed, particle size, the pressure or temperature of batch mixing device 26, and the time duration of mixing. Dispersing aids (not shown) may be added to the suspension in storage container 26. Provision may be made for controlling the temperature of the contents of storage container 16, such as but not limited to, to a temperature range of 60-98° C.

[0025] Suspension 27 may be added directly, if desired, from batch mixing device 26 to a gypsum slurry mixer 41. Gypsum slurry 40 may flow along a production line 42 for making a gypsum product 44, such as but not limited to, pre-cast gypsum products, e.g., wallboard, tile, block, casts and the like. The amount of suspension 27 added to gypsum slurry 40 and the rate of its addition thereto may be functions of various factors, such as but not limited to, the speed of the production line 42, the type of the gypsum product, the desired quality or thickness or other property of the product, the ratio of hydrophobic substance 10 to liquid 18 in suspension 27, etc. A control process feeder valve 50 may control the flow of suspension 27 to the gypsum slurry mixer 41 in production line 42. The production line 42 and control process feeder valve 50 may operate in a closed control loop, wherein the rate of production line 42 automatically controls the feeding of suspension 27 to gypsum slurry mixer 41.

[0026] Instead of adding suspension 27 directly from batch mixing device 26 to gypsum slurry 40, suspension 27 may first flow from batch mixing device 26 to an auxiliary mixing device 46 via tubing 48. A control batch feeder valve 50 may control the flow of material from batch mixing device 26 to auxiliary mixing device 46. Auxiliary mixing device 46 may also comprise a mixer 52, which may be any suitable dispersing mixer, such as but not limited to, a propeller, stirrer, or dissolver moving at any suitable rate to form or maintain suspension 27. Provision may be made for controlling the temperature of the contents of storage container 16, such as but not limited to, to a temperature range of 30-98° C. A control unit 54 may be provided that controls operation of auxiliary mixing device 46, such as but not limited to, the weight or volume of the substances being mixed, the pressure or temperature of auxiliary mixing device 46, and the time duration of mixing. A control process feeder valve 56 may control the flow of suspension 27, via tubing 58, to the gypsum slurry mixer 41 in production line 42. The production line 42 and control process feeder valves 50 and 56 may operate in a closed control loop, wherein the rate of the production line 42 automatically controls the production and feeding of suspension 27 to gypsum slurry mixer 41.

[0027] It is noted that any of the mixing devices (e.g., “batch” or “auxiliary”) may comprise provision for mixing discrete batches or continuous mixing of substances. It is further noted that the devices and equipment used in the above-described process are exemplary only, and the present invention is not limited to these devices or equipment.

[0028] The addition of suspension 27 may impart water resistance to the final gypsum product 44. However, suspension 27 may not just impart water repellent characteristics to the gypsum. It has surprisingly been found that hydrophobic substance 10 added as suspension 27 may be used as a foaming aid, density reducer, dimension stabilizer, and others, and may increase the mechanical strength and durability of the final gypsum product 44. In general, suspension 27 may be added by itself to the gypsum slurry 40, or other substances may be added to the suspension 27 before its addition into the gypsum slurry 40, or suspension 27 may be added to other components before adding to the gypsum slurry 40.

[0029] For example, in another embodiment of the present invention, suspensions of other substances may be used in addition to or in place of the hydrophobic substance 10 to impart these characteristics to the gypsum product 44. For example, PCT published patent application WO 99/35103 describes a wax emulsion comprising an emulsifying system containing a sulfated anionic surfactant and a non ionic surfactant having high water solubility (cloud point) and high foaming ability is added to a gypsum slurry to improve the moisture resistance of gypsum board. The wax is a mixture of a petroleum derived hydrocarbon wax and a montan wax. In the present invention, instead of a wax emulsion, suspension 27 may be employed, wherein suspension 27 further comprises a surface active material as a foaming agent, such as but not limited to, a nonionic surfactant (e.g., possessing a foaming ability of at least 300 and a cloud point (in saline per DIN 53917) of at least 50) and an anionic dispersing agent which is a sulfated compound. These additional substances in suspension 27 may not only serve as foaming agents, but also may reduce the density and increase the dimensional stability of the finished gypsum product.

[0030] As another example, U.S. Pat. No. 6,010,596 to Song (assigned to US Gypsum Co.), describes a gypsum product with uniformly good water resistance, resistance to nail and screw pull-out, dimensional stability, and fire resistance. The product may be formed by adding a wax emulsion to a heated slurry of calcium sulfate hemihydrate and a host particle of a stronger material, passing the heated slurry onto a porous flat forming surface to form a filter cake which is dewatered and pressed to form a board before the hemihydrate is completely rehydrated to gypsum.

[0031] In the present invention, instead of a wax emulsion, suspension 27 may be employed, wherein suspension 27 further comprises a “host particle”. The term “host particle” is meant to cover any macroscopic particle, such as a fiber, a chip or a flake, of a substance other than gypsum. The particle, which is generally insoluble in the slurry liquid, may also have accessible voids therein; whether pits, cracks, fissures, hollow cores, or other surface imperfections, which are penetrable by the slurry menstruum and within which calcium sulfate crystals can form. The substance of the host particle may have desirable properties lacking in the gypsum, and, preferably, at least higher tensile and flexural strength. A ligno-cellulosic fiber, particularly a wood fiber, is an example of a host particle.

[0032] It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of the features described hereinabove as well as modifications and variations thereof which would occur to a person of skill in the art upon reading the foregoing description and which are not in the prior art.

Claims

1. A method comprising: adding a suspension of a hydrophobic substance to a gypsum slurry.

2. The method according to claim 1, wherein said suspension is added to said gypsum slurry on-line during the preparation of said slurry.

3. The method according to claim 1, wherein said suspension is added to said gypsum slurry on-line at a production line where a gypsum product is produced.

4. The method according to claim 1, wherein said suspension is manufactured at a site where the gypsum product is produced.

5. The method according to claim 4, wherein excess heat of producing the gypsum product is used as part of manufacturing said suspension.

6. The method according to claim 1, further comprising storing said hydrophobic substance in a first storage container, storing a liquid in a second storage container, and forming said suspension by dispersing said hydrophobic substance in said liquid in a mixing device.

7. The method according to claim 1, wherein said suspension is added directly from said mixing device to said gypsum slurry.

8. The method according to claim 1, wherein said suspension is fed from said mixing device to an auxiliary mixing device, and said suspension is added from said auxiliary mixing device to said gypsum slurry.

9. The method according to claim 7, further comprising controlling flow of said suspension to said gypsum slurry with a control feeder valve.

10. The method according to claim 8, further comprising controlling flow of said suspension to said gypsum slurry with a control feeder valve.

11. The method according to claim 9, comprising producing and adding said suspension to said gypsum slurry in a production line, wherein said production line and said control feeder valve operate in a closed control loop.

12. The method according to claim 10, comprising producing and adding said suspension to said gypsum slurry in a production line, wherein said production line and said control feeder valve operate in a closed control loop.

13. The method according to claim 7, further comprising controlling production of said suspension to said gypsum slurry with a control feeder valve.

14. The method according to claim 8, further comprising controlling production of said suspension to said gypsum slurry with a control feeder valve.

15. The method according to claim 1, further comprising forming a product from said gypsum slurry.

16. The method according to claim 15, wherein adding said suspension of the hydrophobic substance to the gypsum slurry imparts at least one of the following characteristics to said product: water resistance, foaming, density reduction, dimension stabilization, and increase of mechanical strength and durability.

17. The method according to claim 15, further comprising adding an additional substance to said suspension of the hydrophobic substance, wherein said additional substance imparts at least one of the following characteristics to said product: water resistance, foaming, density reduction, dimension stabilization, and increase of mechanical strength and durability.

18. The method according to claim 1, wherein said suspension comprises a host particle, comprising a macroscopic particle of a substance other than gypsum.

19. The method according to claim 18, wherein said host particle has at least higher tensile and flexural strength than gypsum.

20. The method according to claim 18, wherein said host particle is generally insoluble in said gypsum slurry.

21. The method according to claim 18, wherein said host particle has accessible voids therein, which are penetrable by said gypsum slurry and within which calcium sulfate crystals are formable.

22. The method according to claim 18, wherein said host particle comprises a lignocellulosic fiber.

23. The method according to claim 1, wherein said suspension comprises a surface active material.

24. The method according to claim 23, wherein said surface active material comprises at least one of a foaming aid, a density reducer, and a dimension stabilizer.

25. The method according to claim 23, wherein said surface active material comprises a nonionic surfactant and an anionic dispersing agent which is a sulfated compound.