Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
2001-12-11
2004-11-09
Dixon, Merrick (Department: 1774)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C428S304400, C428S070000, C428S307300, C428S312200, C428S312400, C428S316600
Reexamination Certificate
active
06815049
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to gypsum compositions. More particularly, the invention relates to set gypsum-containing compositions that exhibit enhanced strength, dimensional stability, and resistance to permanent deformation, and to methods, gypsum-containing 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 (e.g., plaster-surfaced internal building walls) and also in paper-faced 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 mold-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 (calcium sulfate hemihydrate and/or calcium sulfate anhydrite) and water (and other components, as appropriate). The mixture is cast into a desired shape or onto a surface, and then allowed 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). 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. Mild heating is utilized to drive off the remaining free (i.e., unreacted) water to yield a dry product.
One problem with such gypsum-containing products is that they often are subject to permanent deformation (e.g., sag), especially under conditions of high humidity, temperature, or load. For example, the possibility of sag is particularly problematic where gypsum-containing boards and tiles are stored or employed in a manner in which they are positioned horizontally. In this respect, if the set gypsum matrix in these products is not sufficiently resistant to permanent deformation, the products may start to sag in areas between the points to which they are fastened, or supported by, an underlying structure. This can be unsightly and can cause difficulties during use of the products. Furthermore, in many applications, gypsum-containing products must be able to carry loads, e.g., insulation or condensation loads, without perceptible sag.
Another problem with such set gypsum-containing products is that dimensional stability can be compromised during their manufacture, processing, and commercial application. For example, in the preparation of set gypsum products, there is usually a significant amount of free (i.e., unreacted) water left in the matrix after the gypsum has set. Upon drying of the set gypsum in order to drive off the excess water, the interlocking set gypsum crystals in the matrix tend to move closer together as the water evaporates. In this respect, as the water leaves the crystal interstices of the gypsum matrix, the matrix tends to shrink from natural forces of the set gypsum that were resisting capillary pressure applied by the water on the gypsum crystals. As the amount of water in the aqueous calcined gypsum mixture increases, lack of dimensional stability becomes more of a problem.
Dimensional stability is also of concern even after the final dried product is realized, especially under conditions of changing temperature and humidity where set gypsum is susceptible to, for example, expansion and shrinkage. 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.
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 upon to hold a precise shape and dimensional proportions (e.g., for use in modeling and mold making) would serve their purposes better.
International Publication No. WO 99/08979 describes the use of polyphosphates as an enhancing material to achieve the goals of resistance to permanent deformation (e.g., sag) and enhanced dimensional stability. According to WO 99/08979, polyphosphates in general produce the benefits of resistance to permanent deformation and enhanced dimensional stability whether the polyphosphates are used to treat a calcium sulfate material before it rehydrates to form set gypsum (i.e., a “pre-set treatment”) or to treat an already-set gypsum (i.e., a “post-set treatment”). In many situations, carrying out a post-set treatment is more complicated and difficult to achieve than a pre-set treatment because, for example, once the set gypsum is in place, it can be difficult to absorb the enhancing material into the set gypsum or to contact the set gypsum with the enhancing material (e.g., where the set gypsum is covered with paper). Thus, it generally is more desirable to impart resistance to permanent deformation and enhanced dimensional stability by way of a pre-set treatment of a calcium sulfate material. However, almost all of the polyphosphates described in WO 99/08979 have a negative effect on the strength of the resulting set gypsum-containing product when used in a pre-set treatment.
Thus, there is a continuing need to impart resistance to permanent deformation and enhanced dimensional stability in a pre-set treatment while maintaining or, preferably, even increasing the strength (e.g., compressive strength) of the resulting set gypsum-containing product. Of the polyphosphate enhancing materials disclosed in WO 99/08979, only the trimetaphosphate compounds were found to actually increase strength upon addition in a pre-set treatment. It would be desirable to identify alternative enhancing materials for achieving resistance to permanent deformation and enhanced dimensional stability in a pre-set treatment of a calcium sulfate material while maintaining or, preferably, even increasing the strength of the resultant set gypsum product so as to afford flexibility in manufacture and in choosing raw materials.
Accordingly, it will be appreciated from the foregoing that there is a need in the art for an alternative pre-set treatment of a calcium sulfate material for increasing resistance to permanent deformation (e.g., sag) and dimensional stability while maintaining or, preferably, even increasing the strength of the resultant set gypsum product. The invention satisfies this need. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a composition comprising a calcium sulfate material, a set gypsum-containing product, and a method for producing a set gypsum-containing product having increased resistance to permanent deformation, while maintaining or increasing strength.
In accordance with the present invention, it has been found that the inclusion of a tetrametaphosphate compound (e.g., salt or ion) in a pre-set treatment of a calcium sulfate material enhances the resistance to permanent deformation (e.g., sag) while maintaining or, preferably, increasing compressive strength of the composition. Furthermore, it is expected that the addition of the tetrametaphosphate in a pre-set treatment will enhance the dimensional stability of the resulting set gypsum-containing product. It is to be noted that, although the singular is utilized for convenience to describe certain terms such as the tetrametaphosphate compo
Frank William A.
Veeramasuneni Srinivas
Yu Qiang
Dixon Merrick
Janci David F.
Leydig , Voit & Mayer, Ltd.
Lorenzen John M.
United States Gypsum Company
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