Adhesive bonding and miscellaneous chemical manufacture – Methods – Plaster board making
Reexamination Certificate
2001-06-22
2004-03-16
Aftergut, Jeff H. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Plaster board making
C156S039000, C106S680000, C106S772000, C264S042000
Reexamination Certificate
active
06706128
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the production of low weight, high strength gypsum wallboard and gypsum core compositions. More specifically, the invention is directed to a method of providing void spaces in gypsum wallboard and in a gypsum core composition.
2. Description of Related Technology
A common method of constructing walls and ceilings includes the use of inorganic wallboard panels or sheets, such as gypsum wallboard, often referred to simply as “wallboard” or “drywall.” Wallboard can be formulated for interior, exterior, and wet applications. The use of wallboard, as opposed to conventional boards made from wet plaster methods, is desirable because the installation of wallboard is ordinarily less costly and less cumbersome when compared to the installation of conventional plaster walls.
A major ingredient of the gypsum wallboard core (hereinafter “wallboard core” or “core”) is calcium sulfate hemihydrate, commonly referred to as “calcined gypsum,” “stucco,” or “plaster of Paris.” Stucco has a number of desirable physical properties including, but not limited to, its fire resistance, thermal and hydrometric dimensional stability, compressive strength, and neutral pH. Typically, stucco is prepared by drying, grinding, and calcining natural gypsum rock (i.e., calcium sulfate dihydrate). The drying step in the manufacture of stucco includes passing crude gypsum rock through a rotary kiln to remove any free moisture present in the rock. The dried rock is then passed through a roller mill (a type of pulverizer), wherein the rock is ground or comminuted to a desired fineness. The degree of comminution is determined by the ultimate use. The dried, fine-ground gypsum can be referred to as “land plaster,” regardless of its intended use.
The land plaster is used as feed in calcination processes for conversion to stucco. The calcination step in the manufacture of stucco is performed by heating the land plaster to liberate a portion of the chemically bound water molecules. The calcination of stucco can generally be described by the following chemical equation which shows that heating calcium sulfate dihydrate yields calcium sulfate hemihydrate (stucco) and water vapor:
CaSO
4
.2H
2
O+heat→CaSO
4
.½H
2
O+1½H
2
O.
This calcination process step is performed in a “calciner,” of which there are several types known by those of skill in the art.
Uncalcined calcium sulfate (i.e., land plaster) is the “stable” form of gypsum. However, calcined gypsum, or stucco, has the desirable property of being chemically reactive with water, and will “set” rather quickly when the two are mixed together. The setting reaction is a reversal of the above-described chemical reaction that occurs during the calcination step, and is generally described by the following chemical equation showing that calcium sulfate hemihydrate is rehydrated to its dihydrate state:
CaSO
4
.½H
2
O+1½H
2
O→CaSO
4
.2H
2
O+heat.
The actual time required to complete the setting reaction generally depends upon the type of calciner and the type of gypsum rock that is used to produce the gypsum, and can be controlled within certain limits by the use of additives such as, for example, retarders, set accelerators, and/or stabilizers. The time required for rehydration can be as little as about two minutes to as long as about eight hours depending on the quantity of retarders, set accelerators, and/or stabilizers present.
Generally, wallboard is produced by enclosing a core of an aqueous slurry of calcined gypsum and other materials between two large sheets of board cover paper. Methods for the production of gypsum wallboard generally are described, for example, by Michelsen, T. “Building Materials (Survey),”
Kirk
-
Othmer Encyclopedia of Chemical Technology,
(1992 4th ed.), vol. 4, pp. 618-619, the disclosure of which is hereby incorporated herein by reference.
A conventional process for manufacturing the core composition of gypsum wallboard initially includes the premixing of dry ingredients in a high-speed mixing apparatus. The dry ingredients can include calcium sulfate hemihydrate (stucco), an accelerator, and an antidesiccant (e.g., starch). The dry ingredients are mixed together with a “wet” (aqueous) portion of the core composition in a pin mixer apparatus. The wet portion can include a first component, commonly referred to as a “paper pulp solution,” that includes a mixture of water, paper pulp, and, optionally, one or more fluidity-increasing agents, and set retarders. The paper pulp solution provides a major portion of the water that forms the gypsum slurry of the core composition. A second wet component can include a mixture of strengthening agents, foaming agents, and other conventional additives, if desired. Together, the aforementioned dry and wet portions comprise an aqueous gypsum slurry that eventually forms the gypsum wallboard core.
In the production of exterior sheathing and moisture-resistant wallboard cores, various materials, such as silicone water repellents, waxes, and asphalt emulsions, can be incorporated into the gypsum wallboard core to impart increased water absorption resistance to the board. These materials are typically supplied as water emulsions to facilitate ease of incorporation into the board core, and can be added directly into the mixing apparatus or incorporated into the pulp solution prior to addition to the mixing apparatus.
After the aqueous gypsum slurry is prepared, the slurry and other desired ingredients are continuously deposited to form a gypsum wallboard core slurry between two continuously-supplied moving sheets of cover paper. Various types of cover paper (or “cover sheets”) are known in the art. The two cover sheets typically comprise a pre-folded face paper and a backing paper. As the slurry is deposited onto the face paper, the backing paper is brought down atop the deposited core slurry and bonded to the prefolded edges of the face paper. The core slurry is then allowed to cure or set (i.e., react with the water present in the aqueous slurry), whereby calcium sulfate hemihydrate is converted to calcium sulfate dihydrate.
The setting reaction produces gypsum crystals which are interwoven. The resulting crystal-to-crystal interactions contribute to the strength of the wallboard core. The gypsum crystals also preferably interlock with paper fibers protruding from the surface of the cover sheets, thereby bonding the cover sheets to the wallboard core. This bonding-type interaction also increases the strength of the wallboard product.
After the core has set, the formed sheet is dried to remove any excess water, and the board is cut into standard sizes. Standardized sheets (or panels) of wallboard typically are about four feet (about 1.22 meters) wide and about 8 feet to about 16 feet (about 2.4 meters to about 4.9 meters) in length. Sheets typically are available in thicknesses varying in a range of about ¼ inch to about one inch (about 0.6 centimeters to about 2.6 centimeters).
In order to provide satisfactory strength, commercially-available gypsum wallboard generally requires a density of about 1675 to 1700 pounds per thousand square feet (lbs/MSF) of one-half inch board. Because heavy or high-density gypsum wallboards are more costly and difficult to manufacture, transport, store, and manually install at job sites when compared with lighter or low-density boards, various attempts have been made to reduce board weight and density without sacrificing board strength. Often, however, where wallboard is formulated to have a density less than about 1675 to 1700 lbs/MSF of one-half inch board, the resulting strength is unacceptable for commercial sale.
It has been previously disclosed that reduced density wallboard can be obtained by mixing an aqueous foam into the gypsum slurry. The density of the wallboard is reduced because the foam introduces air voids into the gypsum wallboard core composition. However, if the foam substantially degrades during mixing
Aftergut Jeff H.
Howrey Simon Arnold & White , LLP
Kilkenny Todd J.
National Gypsum Properties LLC
Nimmo Anthony
LandOfFree
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