Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
2000-11-22
2002-06-25
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C106S772000
Reexamination Certificate
active
06409825
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to gypsum compositions. More particularly, the invention relates to wet gypsum accelerators for accelerating the hydration of calcined gypsum to calcium sulfate dihydrate, as well as to methods, compositions, and products related thereto.
BACKGROUND OF THE INVENTION
Set gypsum (calcium sulfate dihydrate) is a well-known material that is included commonly in many types of products. By way of example, set gypsum is a major component of end products created by the use of traditional plasters, for example, plaster-surfaced internal building walls, and also of gypsum boards employed in typical drywall construction of interior walls and ceilings of buildings. In addition, set gypsum is the major component of gypsum/cellulose fiber composite boards and products, and also is included in products that fill and smooth the joints between edges of gypsum boards. Also, many specialty materials, such as materials useful for modeling and making that are precisely machined, produce products that contain major amounts of set gypsum.
Typically, such gypsum-containing products are prepared by forming a mixture of calcined gypsum, that is, calcium sulfate hemihydrate and/or calcium sulfate anhydrite, and water, as well as other components, as desired. The mixture typically is cast into a predetermined shape or onto the surface of a substrate. The calcined gypsum reacts with water to form a matrix of crystalline hydrated gypsum or calcium sulfate dihydrate. It is the desired hydration of the calcined gypsum that enables the formation of an interlocking matrix of set gypsum crystals, thereby imparting strength to the gypsum structure in the gypsum-containing product. Mild heating can be used to drive off unreacted water to yield a dry product.
Regardless of the type of gypsum-containing product being made, accelerator materials commonly are included in the mixture comprising calcined gypsum and water in order to enhance the efficiency of hydration and to control set time. Typically, the accelerator material includes finely ground dry calcium sulfate dihydrate, commonly referred to as “gypsum seeds,” which generally have a median particle size smaller than about 25 &mgr;m. Even smaller particles are preferred, for example, having a median particle size below about 10 &mgr;m, because the efficiency of the gypsum seeds in accelerating the rate of hydration increases as the particles become finer. The gypsum seeds enhance nucleation of the set gypsum crystals, thereby increasing the crystallization rate thereof. As is known in the art, gypsum seed accelerator materials progressively lose their effectiveness upon aging, even under normal conditions. In this respect, some efficiency of the accelerator is lost even as it is ground, and the gypsum seeds dramatically lose potency over time during handling or storage. The loss of acceleration efficiency of conventional accelerator materials is exacerbated when the accelerator is exposed to heat and/or moisture.
To combat the loss of efficiency of the gypsum seeds over time, particularly under conditions of heat, it is customary to coat the calcium sulfate dihydrate accelerator material with any of a number of known coating agents, such as, for example, sugars, including sucrose, dextrose and the like, starch, boric acid, or long chained fatty carboxylic acids including salts thereof. Conventional heat resistant accelerator materials are both ground and provided in dry form inasmuch as accelerator loses efficiency upon contact with moisture, for example, because the accelerator particles undesirably agglomerate and/or because the coating agents often are soluble in water.
Existing approaches for extending the longevity of gypsum seed accelerator materials have not been fully satisfactory. In particular, despite the availability of the foregoing approaches, conventional accelerator materials still can be stored only for a few days such that the accelerator material must be prepared freshly a short time prior to use. Accordingly, conventional accelerator materials cannot be stored for more than a few hours or transported over long distances and then used. Furthermore, conventional accelerator materials are relatively expensive to make because commonly used additives, such as sugar, typically are included in relatively large amounts, such as, for example, about 5% by weight of the accelerator material. The expense in manufacturing set gypsum-containing products often is increased further in conventional manufacturing processes because a second accelerator material, such as potash or aluminum sulfate, typically is included to enhance crystal growth after nucleation in order to compensate for the loss of efficiency of the gypsum seeds.
In addition, conventional accelerator materials lose their effectiveness upon exposure to moisture and, thus, must remain in a dry environment prior to use. As such, conventional heat-resistant accelerator materials cannot be used at all, or, if used, require special precautions, in the manufacture of certain set gypsum-containing products that are made using a wet mixing process, as opposed to a dry feed system. By way of example, a wet mixing process is utilized in the preparation of gypsum-cellulosic fiber composite panels. Particularly, water and calcined gypsum are wet mixed and allowed to penetrate a porous cellulosic fiber structure such that, after hydration, set gypsum forms in situ within the pores of the cellulosic fibers to enhance the strength of the composite. Conventional heat resistant accelerators generally are not utilized in such a wet mixed process because the efficiency of the wet gypsum accelerator is compromised due to extended exposure to water.
Accordingly, it will be appreciated from the foregoing that there is a need in the art for an accelerator material that does not lose its effectiveness over long periods of time, even upon exposure to heat and/or moisture, and which uses relatively small amounts of additives such that the manufacture of the accelerator material is relatively inexpensive.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a wet gypsum accelerator, a method of preparing a wet gypsum accelerator, a method of hydrating calcined gypsum to form an interlocking matrix of set gypsum, a set gypsum-containing composition, and a set gypsum-containing product.
The wet gypsum accelerator of the invention comprises particles of calcium sulfate dihydrate, water, and at least one additive elected from (i) an organic phosphonic compound; (ii) a phosphate-containing compound; or (iii) a mixture of (i) and (ii). The wet gypsum accelerator preferably is prepared via wet grinding. Water, the additive, and gypsum are combined (in any order) to form a mixture, with other optional components added, as desired. When combined with the water, the gypsum can be in the form of calcium sulfate dihydrate, or alternatively, at least some of the gypsum can be in the form of calcined gypsum, that is, calcium sulfate hemihydrate and/or calcium sulfate anhydrite, such that the calcined gypsum is converted to calcium sulfate dihydrate in the presence of the water, with excess water being required in the mixture to accommodate the wet grinding. Preferably, the gypsum is in the form of calcium sulfate dihydrate when grinding is initiated, but grinding can begin before all of the calcined gypsum is converted to calcium sulfate dihydrate. The calcium sulfate dihydrate is wet ground in the presence of the additive to form the wet gypsum accelerator.
The wet gypsum accelerator according to the invention is used to facilitate the preparation of a set gypsum-containing composition, as well as a product comprising the set gypsum-containing composition. In particular, the wet gypsum accelerator of the invention can be combined with water and calcined gypsum (in any order) to form an aqueous mixture in which the calcined gypsum is hydrated to form an interlocking matrix of set gypsum. In accordance with the present invention, the wet gypsum acceler
Veeramasuneni Srinivas
Yu Qiang
Janci David F.
Leydig , Voit & Mayer, Ltd.
Lorenzen John M.
Marcantoni Paul
United States Gypsum Company
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