Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
1999-12-30
2003-03-11
Weisberger, Richard (Department: 2164)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C428S307300
Reexamination Certificate
active
06531210
ABSTRACT:
The present invention relates to an improved composite material; more particularly to a composite gypsum/cellulose fiber material having improved physical properties which are especially useful for making building products. Specifically, the present invention relates to a methylenediphenyldiisocyanate (MDI) impregnated gypsum/wood fiber building board having enhanced physical properties through the addition of an MDI emulsion to the gypsum and wood fiber during the board manufacturing process.
BACKGROUND OF THE INVENTION
Certain properties of gypsum (calcium sulfate dihydrate) make it very popular for use in making industrial and 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 into 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; i.e., also known as plasterboard or drywall, consists of a rehydrated gypsum core sandwiched between multi-ply paper cover sheets, and is largely for interior wall and ceiling applications. Because of the brittleness and low nail and screw holding properties of its gypsum core, conventional drywall by itself cannot support heavy appended loads or absorb significant impact. Accordingly, means to improve the tensile, flexural, nail and screw holding strength and impact resistance of gypsum wallboard and building products have long been, and still are, earnestly sought.
Another readily available and affordable material, which is also widely used in building products, is lignocellulosic material particularly in the form of wood and paper fibers. For example, in addition to lumber, particleboard, fiberboard, waferboard, plywood and “hard” board (high density fiberboard) are some of the forms of processed lignocellulosic material products used in the building industry. Such materials have better tensile and flexural strength than gypsum wallboard. However, they are also generally higher in cost, have poor fire resistance and are not able to provide adequate strength in lower density products. Therefore, affordable means to remove these use-limiting properties of building products made from cellulosic material are also desired.
Previous attempts to combine the favorable properties of gypsum and cellulosic fibers, particularly wood fibers, are described in detail in U.S. Pat. Nos. 5,817,262 and 5,320,677, both herein incorporated by reference, and assigned to the United States Gypsum Company. It is an object of the present invention to improve upon the teachings of the '262 and '677
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patents, and provide a gypsum/wood fiber board (GWF) product having improved strength, impact resistance, resistance to screw and nail pullout, and dimensional stability.
In general and as taught by the '262 and '677 patents, the process for making a composite GWF material begins with mixing between about 0.5% to about 30%, and preferably between 3% to 20%, by weight, wood fibers with the respective complement of ground, uncalcined gypsum. The dry mix is combined with enough liquid, preferably water, to form a dilute slurry having about 70%-95% by weight water. The slurry is processed in a pressure vessel, such as an autoclave, at a temperature of approximately 285 to 305 degrees F., which is sufficient to convert the gypsum to acicular calcium sulfate hemihydrate crystals. It is desirable to continuously agitate the slurry with gentle stirring or mixing to break up any fiber clumps and keep all the particles in suspension. After the hemihydrate has formed and has precipitated out of solution as hemihydrate crystals, the pressure on the product slurry is relieved when the slurry is discharged from the autoclave. It is at this point that any other desired additives are added to the slurry. While still hot, the slurry is added to a head box which distributes the slurry onto a porous felting conveyor. While on the conveyor, the slurry is dewatered by the action of vacuum pumps which draw the water through the felting conveyor, causing a filter cake to form on the conveyors surface. As much as 90% of the uncombined water may be removed from the filter cake by vacuum pumps. The temperature of the heated slurry is maintained at a temperature above about 160° F. until it has been substantially dewatered and wet pressed into a board. As a consequence of the water removal, the filter cake is cooled to a temperature at which point rehydration may begin. However, it may still be necessary to provide external cooling to bring the temperature low enough to accomplish the rehydration within an acceptable time.
Before extensive rehydration takes place, the filter cake is preferably wet-pressed into a board of desired thickness and/or density. If the board is to be given a special surface texture or a laminated surface finish, it would preferably occur during or following this step of the process. During the wet pressing, which preferably takes place with gradually increasing pressure to preserve the products integrity, two things happen: (1) additional water, for example about 50%-60% of the remaining water, is removed; and (2) as a consequence of the additional water removal, the filter cake is further cooled to a temperature at which rapid rehydration occurs. The calcium sulfate hemihydrate hydrates to gypsum, so that the acicular calcium hemihydrate crystals are converted to gypsum crystals in-situ in and around the wood fibers. After rehydration is complete, the boards can be cut and trimmed, if desired, and then sent through a kiln for drying. Preferably, the drying temperature should be kept low enough to avoid recalcining any gypsum on the surface.
The GWF product has historically relied on gypsum as the sole internal binder. While the use of gypsum as the sole core binder has worked well for the higher density GWF products such as the 55-pcf (pounds per cubic foot) exterior sheathing and 65-pcf underlayment products, it has not been able to provide adequate strength for lower density products. In particular, the gypsum-only GWF product has proved to possess inadequate flexural strength and stiffness for key low-density products such as furniture components and other wood-based applications. Thus, ways to improve the physical properties of the GWF product have been sought.
Beginning in the early 1980's, aqueous dispersions of isocyanate became commercially available. One of these dispersions, based on the use of a particular diisocyanate, MDI, has gained acceptance as a binder for particleboard and oriented strand board (OSB) in place of conventional phenol-formaldehyde resins. In considering the application of MDI to the GWF product, the MDI emulsion system described herein has a number of advantages over gypsum as a binder. MDI is a thermoset resin capable of forming chemical bonds with both hydroxyl groups on the cellulosic fibers as well as cross-linking with itself by reaction of the isocyanate group with water. Through judicious use of catalysts, one can preferentially catalyze either of those reactions if desired. MDI is also resistant to moisture and humidity and, if properly employed in the GWF panel, provides improved physical properties, such as flexural and tensile strength, resistance to nail and screw pull out and impact resistance.
Any binder, such as MDI, added to the GWF slurry must satisfy two conditions: (1) it must be stable in the temperature and chemical conditions present in the headbox, and (2), the binder must be retained in the basemat. These requirements have thwarted previous attempts to incorporate MDI into the GWF product.
Previous unsuccessful attempts have been made to utilize neat MDI as a core additive in a GWF product at a low level, approximately 2% by weight based on the total solids in t
Janci David F.
Lee Mann Smith McWilliams Sweeney & Ohlson
Lorenzen John M.
United States Gypsum Company
Weisberger Richard
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