Stock material or miscellaneous articles – Sheet including cover or casing – Encased layer derived from inorganic settable ingredient
Reexamination Certificate
1998-08-21
2002-01-29
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Sheet including cover or casing
Encased layer derived from inorganic settable ingredient
C156S042000, C156S043000, C156S044000, C106S772000, C106S785000, C423S314000
Reexamination Certificate
active
06342284
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and composition for preparing set gypsum-containing products, e.g., gypsum boards, reinforced gypsum composite boards, plasters, machinable materials, joint treatment materials, and acoustical tiles, and methods and compositions for producing them. More particularly, the invention concerns such set gypsum-containing products that have increased resistance to permanent deformation (e.g., sag resistance) by employing one or more enhancing materials. Some preferred embodiments of the invention concern making such products by hydration of calcined gypsum in the presence of an enhancing material that causes the set gypsum produced by such hydration to have increased strength, resistance to permanent deformation (e.g., sag resistance), and dimensional stability (e.g., non-shrinkage during drying of set gypsum). The enhancing material also provides other improved properties and advantages in preparing the set gypsum-containing products. In an alternative embodiment of the invention, set gypsum is treated with one or more enhancing materials to provide similar, if not the same, increased strength, resistance to permanent deformation (e.g., sag resistance), dimensional stability, and other improved properties and advantages in gypsum-containing products. In some embodiments of the invention the set gypsum-containing product of the invention contains relatively high concentrations of chloride salts, yet avoids detrimental effects of such salt concentrations in gypsum-containing products in general.
BACKGROUND
Many well known useful products contain set gypsum (calcium sulfate dihydrate) as a significant, and often as the major, component. For example, set gypsum is the major component of paper-faced gypsum boards employed in typical drywall construction of interior walls and ceilings of buildings (see, e.g., U.S. Pat. Nos. 4,009,062 and 2,985,219). It is also the major component of gypsum/cellulose fiber composite boards and products, as described in U.S. Pat. No. 5,320,677. Products that fill and smooth the joints between edges of gypsum boards often contain major amounts of gypsum (see, e.g., U.S. Pat. No. 3,297,601). Acoustical tiles useful in suspended ceilings can contain significant percentages of set gypsum, as described, for example, in U.S. Pat. Nos. 5,395,438 and 3,246,063. Traditional plasters in general, e.g., for use to create plaster-surfaced internal building walls, usually depend mainly on the formation of set gypsum. Many specialty materials, such as a material useful for modeling and mold-making that can be precisely machined as described in U.S. Pat. No. 5,534,059, contain major amounts of gypsum.
Most such gypsum-containing products are prepared by forming a mixture of calcined gypsum (calcium sulfate hemihydrate and/or calcium sulfate anhydrite) and water (and other components, as appropriate), casting the mixture into a desired shaped mold or onto a surface, and allowing the mixture to harden to form set (i.e., rehydrated) gypsum by reaction of the calcined gypsum with the water to form a matrix of crystalline hydrated gypsum (calcium sulfate dihydrate). This is often followed by mild heating to drive off the remaining free (unreacted) water to yield a dry product. It is the desired hydration of the calcined gypsum that enables the formation of an interlocking matrix of set gypsum crystals, thus imparting strength to the gypsum structure in the gypsum-containing product.
All of the gypsum-containing products described above could benefit if the strength of their component set gypsum crystal structures were increased in order to make them more resistant to the stresses they may encounter during use.
Also there is a continuing effort to make many such gypsum-containing products lighter in weight by substituting lower density materials (e.g., expanded perlite or air voids) for part of their set gypsum matrix. In such cases there is a need to increase the strength of the set gypsum above normal levels just to maintain overall product strength at the levels of the previously higher density product, because there is less set gypsum mass to provide strength in the lower density product.
Further, there is a need for greater resistance to permanent deformation (e.g., sag resistance) in the structure of many of these gypsum-containing products, especially under conditions of high humidity and temperature, or even load. The human eye typically cannot perceive sag of a gypsum-containing board at less than about 0.1 inch of sag per two foot length of board. Thus, there is a need for gypsum-containing products that are resistant to permanent deformation over the useful life of such products. For example, gypsum-containing boards and tiles are often stored or employed in a manner in which they are positioned horizontally. If the set gypsum matrix in these products is not sufficiently resistant to permanent deformation, especially under high humidity and temperature, or even load, the products may start to sag in areas between the points where they are fastened to or supported by an underlying structure. This can be unsightly and can cause difficulties in use of the products. In many applications gypsum-containing products must be able to carry loads, e.g., insulation or condensation loads, without perceptible sag. Thus, there is a continuing need to be able to form set gypsum having increased resistance to permanent deformation (e.g., sag resistance).
There is also a need for greater dimensional stability of set gypsum in gypsum-containing products during their manufacture, processing, and commercial application. Especially under conditions of changing temperature and humidity, set gypsum can shrink or expand. For example, moisture taken up in crystal interstices of a gypsum matrix of a gypsum board or tile exposed to high humidity and temperature can aggravate a sagging problem by causing the humidified board to expand. Also, in the preparation of set gypsum products there is usually a significant amount of free (unreacted) water left in the matrix after the gypsum has set. This free water is usually subsequently driven off by mild heating. As the evaporating water leaves the crystal interstices of the gypsum matrix, the matrix tends to shrink from natural forces of the set gypsum (i.e., the water was holding apart portions of the interlocking set gypsum crystals in the matrix, which then tend to move closer together as the water evaporates).
If such dimensional instability could be avoided or minimized, various benefits would result. For example, existing gypsum board production methods would yield more product if the boards did not shrink during drying, and gypsum-containing products desired to be relied on to hold a precise shape and dimensional proportions (e.g., for use in modeling and mold making) would serve their purposes better. Also, for example, some plasters intended for interior building wall surfaces could benefit from not shrinking during drying, so that the plaster could be applied in thicker layers without danger of cracking, rather than needing to be applied in multiple thinner layers with long pauses to allow adequate drying between layer applications.
Some particular types of gypsum-containing products also exhibit other particular problems. For example, lower density gypsum-containing products are often produced by using foaming agents to create aqueous bubbles in calcined gypsum slurries (flowable aqueous mixtures) that yield corresponding permanent voids in the product when the set gypsum forms. It is often a problem that, because the aqueous foams employed are inherently unstable and therefore many of the bubbles may coalesce and escape the relatively dilute slurry (like bubbles in a bubble bath) before the set gypsum forms, significant concentrations of foaming agents have to be employed to produce the desired concentration of voids in the set gypsum, in order to obtain a product of desired density. This increases costs and risks of adverse effects of chemical foaming agents on other components o
Groza Brent E.
Henkels Paul J.
Jones Frederick T.
Mlinac Raymond J.
Sucech Steven W.
Janci David F.
Jones Deborah
Leydig Voit & Mayer Ltd
Lorenzen John M.
United States Gysum Company
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