Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Noninterengaged fiber-containing paper-free web or sheet...
Reexamination Certificate
2002-06-14
2004-10-05
Dixon, Merrick (Department: 1774)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Noninterengaged fiber-containing paper-free web or sheet...
C428S703000, C428S319100, C428S182000, C428S913000, C428S309900
Reexamination Certificate
active
06800361
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved gypsum-based substrate faced with a polymeric nonwoven sheet material, the gypsum-based substrate suited for use in construction materials such as wall panels, ceiling panels, floor underlayment and interior and exterior sheathing.
BACKGROUND OF THE INVENTION
Gypsum board is traditionally manufactured by a continuous process. In the process, a gypsum slurry is first generated in a mechanical mixer by mixing calcium sulfate hemihydrate (also known as calcined gypsum), water, and other agents. These various additives are used in the gypsum slurry as set accelerators (such as ground gypsum, potassium sulphate), set retarders (such as diethylene triamine tetra acetic acid), water reducing agents (such as condensed naphthalene sulphonates), foaming agents (such as lauryl alcohol ether sulphates), liner bonding agents (such as starch), anti-burning agents (such as boric acid), glass fibers for improved physical properties and fire resistance, other agents to improve reaction to fire properties (such as clay), water proofing agents (such as wax or silicones), or other agents. The gypsum slurry is deposited on a paper sheet which has had each edge scored or creased to facilitate the folding of the edges to make a sidewall of height equal to board thickness and a further flap of width about 1 inch wide folded back over the board. An upper continuously advancing paper sheet is then laid over the gypsum slurry and the edges of the upper and lower sheets are pasted to each other using glue at the edges of the top and/or bottom sheet. The paper sheets and gypsum slurry are passed between parallel upper and lower forming plates or rolls in order to generate an integrated and continuous flat strip of unset gypsum sandwiched between the paper sheets that are known as facing or liners. This strip is conveyed over a series of continuous moving belts and rollers for a period of 2 to 5 minutes during which time the core begins to hydrate back to gypsum and hardens. During each transfer between belts and/or rolls, the strip is stressed in a way that can cause the paper facing to delaminate from the gypsum core if the adhesion between the gypsum core and the facing is not sufficient. Once the gypsum core has set sufficiently, the continuous strip is cut into shorter lengths or even individual boards or panels of prescribed length. Once again, it is important for there to be good adhesion between the paper sheets and the set, but still wet, gypsum core or the cutting action will pull the edges of the paper facing sheet away from the gypsum core. Good adhesion between the top and bottom paper sheet at the edges which are pasted with glue is also important here.
After the cutting step, the gypsum boards are separated and grouped through a series of belts and rollers and then flipped over before being fed into drying ovens or kilns where the boards are dried so as to evaporate excess water. The hydration from hemihydrate to gypsum must be essentially complete by this point, normally between 7 and 15 minutes after mixing. When the gypsum boards are accelerated, flipped and fed into the drying ovens, the boards are subjected to a variety of stresses that can cause the facing to peel away from the gypsum core of the boards unless there is good adhesion between the set (but still wet) gypsum core and the facing material. Inside the drying ovens, the boards are blown with hot drying air at speeds up to 4000 feet/minute which can cause further delamination of the paper facing if there is not good wet adhesion between the gypsum and the paper liners. If portions of the facing sheets delaminate from the gypsum core during drying in the oven, the liner can become entangled in the rollers and the gypsum crumbles as it dries, jamming the oven, which then requires the line to be shut down while the loose gypsum and liner is cleaned out of the ovens. Poor wet bond between liner and the gypsum core can also result in blisters due to delamination during the drying process. The gypsum boards are dried in the ovens for anywhere from 30 to 75 minutes. After the dried gypsum boards are removed from the ovens, the ends of the boards are trimmed off and the boards are cut to desired sizes. Good adhesion between the top and bottom paper sheet at the edges which are pasted with glue is also important throughout the board forming process as well as during use of the board.
Gypsum board has been the subject of numerous patents, such as U.S. Pat. No. 4,057,443, Canadian Patent No. 1,189,434, as well as co-pending U.S. patent application Ser. Nos. 09/512,921 and 09/513,097, all of which are incorporated herein by reference.
For years it has been recognized that high toughness and abuse resistance are desirable properties in gypsum-based board for use in buildings. High toughness and abuse resistance are here defined in terms of high initial modulus, high flexural strength corresponding to high-to-moderate initial modulus, high maximum flexural strength and high work-to-break. In addition to high toughness, it is desirable for gypsum board to have an abrasion resistant property in order to resist abuse. Further, it is desired to have gypsum board with some flexibility under load.
Standard gypsum boards are produced with a cellulosic paper liner. Paper has good wet adhesion with the gypsum slurry during board formation. It is believed that cellulose draws moisture from the slurry and pulls the slurry into close contact with the paper fibers. As the gypsum sets, there is some interlocking of the gypsum crystals with the paper fibers at the surface of the liner, as well as some chemical bonding between the wet gypsum matrix and the hydrophilic paper fibers. Paper does not allow the gypsum slurry to seep through during board forming, provides reasonable strength and a paintable surface to the finished gypsum board.
However, there are several disadvantages to the use of paper as a liner for gypsum board. Paper acts as a food source for mold and mildew, and it becomes especially weak and subject to delamination either directly from the gypsum core or between the layers of the multi-layer sheets when the paper becomes damp due to water leaks or high humidity.
In addition, standard paper-lined gypsum board has lower work-to-break and abrasion resistance than is needed for certain applications. Work-to-break (WTB) is defined as the force (or stress) required to break the sample times the distance (or strain) that the sample is deformed before failure. On a stress- strain curve, WTB is represented by the area under this stress-strain or breaking curve.
In use, paper-faced gypsum boards are generally coated with another material, such as specialty paint or wall coverings, in order to achieve high abrasion resistance. To overcome these durability problems, paper-faced board is frequently covered with a wall paper of hard sheet or plastic film when used in high traffic areas.
There are international and foreign building materials standards that also classify conventional gypsum boards in the combustible category. There have been efforts to make panel products from gypsum that can achieve noncombustible status. The weight fraction of 5-6% paper in standard paper-lined gypsum would most likely cause this building material to fail the test for combustibility as described in ASTM E136, were it not for the fact that some building materials, such as gypsum board, have been defined in section X1.2.3b of the standard as noncombustible, based on their composition and flame spread properties alone. U.S. Pat. No. 6,221,521 concerning gypsum boards made without liners describes how even these gypsum/fiber boards that are reinforced with internal cellulosic fibers instead of external paper liners are deemed combustible as tested by ASTM E136 because of the presence of more than 3-4% of organic fibers in the core of the board.
A normal paper liner contains about 170-220 g/m
2
of cellulosic content. Similar standards exist in other countries and new European standards being implemented do
Bruce Robert B.
Shah Ashok Harakhlal
Dixon Merrick
E. I. du Pont de Nemours and Company
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