Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Noninterengaged fiber-containing paper-free web or sheet...
Reexamination Certificate
2001-12-21
2004-05-25
Jones, Deborah (Department: 1771)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Noninterengaged fiber-containing paper-free web or sheet...
C428S292100, C428S304400, C428S307300, C428S703000, C106S772000, C106S773000, C106S774000, C106S785000
Reexamination Certificate
active
06740395
ABSTRACT:
BACKGROUND OF THE INVENTION
Certain properties of gypsum make it very popular for use in making industrial and building plasters and other building products, especially gypsum wallboard. It is a plentiful and generally inexpensive raw material which, through a process of dehydration and rehydration, can be cast, molded or otherwise formed to useful shapes. It is also noncombustible and relatively dimensionally stable when exposed to moisture. However, because it is a brittle, crystalline material, which has relatively low tensile and flexural strength, its uses are typically limited to non-structural, non-load-bearing and non-impact-absorbing applications.
Gypsum wallboard, also known as drywall, consists of a rehydrated gypsum core sandwiched between multi-ply paper cover sheets and is used largely for interior wall and ceiling applications, as well as other specialty uses. The paper cover sheets contribute significantly to the strength of wallboard; without the paper up to 80% of the flexural strength is lost. Furthermore, because of the brittleness and low nail and screw holding properties of its gypsum core, conventional wallboard by itself cannot support heavy appended loads or absorb significant impact.
A material which may be used to make a wallboard having numerous properties superior to those of paper-covered gypsum wallboard is taught in a patent assigned to United States Gypsum Company, U.S. Pat. No. 5,320,677 (the '677 patent). The '677 patent teaches a gypsum composite material which is strong enough so as not to require a paper covering. The gypsum composite material is made from a combination of gypsum and a “host particle” such as a lignocellulosic material. This combination produces a composite material useful for making strong cast products, plasters, building products and for other applications. The composite material may be used to make a paperless gypsum wallboard also known as “fiberboard”. This fiberboard is fire resistant, has non-directional strength, including resistance to nail and screw pull-out throughout its expanse, and a harder finish than paper-covered wallboard. It also has better dimensional stability than paper-covered wallboard and has the ability to maintain its strength even in a humid environment. In addition, fiberboard has fewer defects related to the use of paper, e.g., problems such as the tearing of the paper covering or bubble formation under the paper are eliminated. Finally, fiberboard can be produced at a competitive cost.
A disadvantage of the fiberboard of the '677 patent is that the surface is not as smooth as would be desirable, because of the inclusion of the fibrous lignocellulosic material. A smooth, flat and level surface is preferred for applications such as ceilings and wall assemblies. Current methods used to smooth the rough surface of fiberboard and to minimize the surface imperfections may involve many laborious steps and add significant expense to fiberboard production. Rough-surfaced materials may be covered with conventional paints or coatings to help hide the imperfections, but because these coatings tend to shrink as they dry, the finished surface follows the contours of the rough surface and a flat, level, and smooth surface does not result. Rough-surfaced materials may also be sanded or polished after coating to yield a smooth surface and multiple coating/sanding operations may be needed to produce a smooth surface. Multi-step coating and sanding (or polishing) operations have been used to prepare the surface of fiberboard substrates such as medium density fiberboard. At each coating step, the wet coating must be dried—a process that involves energy and requires physical space on a production line. Multi-step surface preparation procedures require much longer production lines and increase energy costs, as well as adding expense for sanding or polishing materials, and for disposal of waste materials.
If the fiberboard could be coated with a material that does not shrink as it dries, then the rough surface could be covered, leaving a flat, level and smooth surface. It would be possible to make a smooth, flat-surfaced fiberboard in fewer surface preparation steps, e.g., in only one or two steps without sanding or polishing.
In principle, the fiberboard could be coated with calcium sulfate hemihydrate (plaster of Paris), which will convert to calcium sulfate dihydrate (gypsum) when mixed with water, resulting in hardening or setting of the plaster. The reaction inexorably proceeds to completion in a period of time as adjusted by various accelerators and retarders, usually 5-300 minutes after mixing with water. A typical calcium sulfate hemihydrate coating would thus need to be kept in a dry form, then mixed with water just before being used to coat the fiberboard. But, the machinery used to coat the fiberboard would have to be frequently cleaned to remove accumulation of calcium sulfate dihydrate. Thus, using a coating based on calcium sulfate hemihydrate using conventional set control additives is not amenable to a continuous in-line production process. But, if the gypsum composition did not set when mixed with water, constant cleaning of the machine would not be needed. Then, if the gypsum composition only set after being coated on the fiberboard panel, then the desired smooth finish on the rough panel could be achieved.
The present inventors have discovered how to produce a smooth, flat gypsum-coated fiberboard. Their invention includes a paperless fiberboard with a surface acceptable for wall and ceiling applications and a process for making such fiberboard.
SUMMARY OF THE INVENTION
The present invention is a process for preparing a smooth-surfaced gypsum-coated substrate from a rough-surfaced substrate, especially from fiberboard that previously could not be smoothed in an economical manner and the resulting smooth-surfaced substrate.
In one embodiment of the present invention, a rough-surfaced fiberboard substrate is made smooth by the application of a calcium sulfate hemihydrate-containing coating composition with a reverse roll coater. As the fiberboard is fed through a reverse roll coater, a layer of the coating composition is deposited onto the surface of the board where hydration occurs to set the coating. The surface of the finished substrate is smooth, flat and level at the time the fiberboard emerges from the reverse roll coater. Since the coating composition contains a set preventer, it does not set until after deposited on the surface of the fiberboard where an activator initiates the setting reaction only on the panel and not in the coating contained within the reverse roll coater, thus permitting the use of a reverse roll coater for continuous operation. A continuous operation would not be possible using conventional set controlled calcium sulfate hemihydrate coatings, as the setting reaction would proceed inexorably to completion once mixed with water, necessitating shut down of the equipment to clean out the set gypsum in and on the roll coater.
In a preferred embodiment, the set preventers are the non-calcium-bearing phosphate compounds of U.S. Pat. No. 5,746,822. After coating the fiberboard substrate with the coating composition, and the coating composition sets, i.e., the calcium sulfate hemihydrate in the coating composition hydrates to form calcium sulfate dihydrate (gypsum). In order for this setting process to occur, a set initiator must be present, either in or on the surface of the fiberboard, or alternatively introduced into the coating as it is applied. Set initiators include the accelerators of the '822 patent. While it was suggested in the '822 patent that aluminum sulfate and ferrous ion-containing compounds could be used, they were not recommended. However, aluminum sulfate is a preferred set initiator in the present invention. The amount of the set initiator will depend on the amount required to overcome the set preventing effect of the non-calcium-bearing phosphates. Generally, about 0.01 to 0.03 wt % of the set initiator, based on the weight of the
Attard Peter M.
Carbo Aida L.
Fults Therese A.
Halm John D.
Lynn Michael R.
Bahta Abraham
Janci David F.
Jenkens & Gilchrist
Jones Deborah
Lorenzen John M.
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