1. Field of the Invention
The present invention relates to a gypsum board used in building construction and to a process for its manufacture; and more particularly, to a non-woven glass fiber mat comprising a blend of glass fibers having different diameters and lengths bonded together with a resinous latex binder, a gypsum board or similar product in panel form faced on at least one side with such a mat, and processes for the manufacture thereof.
2. Description of the Prior Art
Wallboard formed of a gypsum core sandwiched between facing layers is used in the construction of virtually every modern building. In its various forms, the material is employed as a surface for walls and ceilings and the like, both interior and exterior. It is relatively easy and inexpensive to install, finish, and maintain, and in suitable forms, is relatively fire resistant.
Although paper-faced wallboard is most commonly used for finishing interior walls and ceilings, other forms with different kinds of facings have superior properties that are essential for other uses. One known facing material is non-woven fiberglass mat.
Gypsum wallboard and gypsum panels are traditionally manufactured by a continuous process. In this process, a gypsum slurry is first generated in a mechanical mixer by mixing at least one of anhydrous calcium sulfate (CaSO4) and calcium sulfate hemihydrate (CaSO4.½H2O, also known as calcined gypsum), water, and other substances, which may include set accelerants, waterproofing agents, reinforcing mineral, glass fibers, and the like. The gypsum slurry is normally deposited on a continuously advancing, lower facing sheet, such as kraft paper. Various additives, e.g. cellulose and glass fibers, are often added to the slurry to strengthen the gypsum core once it is dry or set. Starch is frequently added to the slurry in order to improve the adhesion between the gypsum core and the facing. A continuously advancing upper facing sheet is laid over the gypsum and the edges of the upper and lower facing sheets are pasted to each other with a suitable adhesive. The facing 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 sheets. Such a flat strip of unset gypsum is known as a facing or liner. The strip is conveyed over a series of continuous moving belts and rollers for a period of several minutes, during which time the core begins to hydrate back to gypsum (CaSO4.2H2O). The process is conventionally termed “setting,” since the rehydrated gypsum is relatively hard. During each transfer between belts and/or rolls, the strip is stressed in a way that can cause the facing to delaminate from the gypsum core if its adhesion 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.
After the cutting step, the gypsum boards are fed into drying ovens or kilns so as to evaporate excess water. Inside the drying ovens, the boards are blown with hot drying air. 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. The boards are commonly sold to the building industry in the form of sheets nominally 4 feet wide and 8 to 12 feet or more long and in thicknesses from nominally about ¼ to 1 inches, the width and length dimensions defining the two faces of the board.
While paper is widely used as a facing material for gypsum board products because of its low cost, many applications demand water resistance that paper facing cannot provide. Upon exposure to water either directly in liquid form or indirectly through exposure to high humidity, paper is highly prone to degradation, such as by delamination, that substantially compromises its mechanical strength. Gypsum products typically rely on the integrity of the facing as a major contributor to their structural strength. Consequently, paper-faced products are generally not suited for exterior or other building uses in which exposure to moisture conditions is presumed.
In addition, there is growing attention being given to the issue of mold and mildew growth in building interiors and the potential adverse health impact such activity might have on building occupants. The paper facing of conventional gypsum board contains wood pulp and other organic materials that may act in the presence of moisture or high humidity as nutrients for such microbial growth. A satisfactory alternative facing material less susceptible to growth is highly sought.
A further drawback of paper-faced gypsum board is flame resistance. In a building fire, the exposed paper facing quickly burns away. Although the gypsum itself is not flammable, once the facing is gone the board's mechanical strength is greatly impaired. At some stage thereafter the board is highly likely to collapse, permitting fire to spread to the underlying framing members and adjacent areas of a building, with obvious and serious consequences. A board having a facing less susceptible to burning would at least survive longer in a fire and thus be highly desirable in protecting both people and property.
To overcome these and other problems, a number of alternatives to paper facing have been proposed. U.S. Pat. No. 4,647,496 discloses an exterior insulation system including a fibrous mat-faced gypsum board having a set gypsum core that is water-resistant. The fibrous mat is preferably sufficiently porous for the water in the gypsum slurry to evaporate during the production drying operation as the gypsum sets. The mat comprises fibrous material that can be either mineral-type or a synthetic resin. One preferred mat comprises non-woven glass fibers, randomly oriented and secured together with a modified or plasticized urea formaldehyde resin binder, and sold as DURA-GLASS® 7502 by the Manville Building Materials Corporation.
However, gypsum board products incorporating such conventional fibrous mats have proven to have certain drawbacks. While fibrous mats are undesirably more costly than the traditionally used kraft paper, there are other, more troublesome issues as well. Some persons are found to be quite sensitive to the fiberglass mat, and develop skin irritations and abrasions when exposed to the mat at various stages, including the initial production of the mat, the manufacture of composite gypsum board with the mat facing, and during the cutting, handling, and fastening operations (e.g., with nails or screws) that attend installation of the end product during building construction. Handling of the mat, and especially cutting, is believed to release glass fibers responsible for the irritation. The fibers may either become airborne or be transferred by direct contact. As a result, workers are generally forced to wear long-sleeved shirts and long pants and to use protective equipment such as dust masks. Such measures are especially unpleasant in the sweaty, hot and humid conditions often encountered either in manufacturing facilities or on a construction jobsite.
There have been suggestions that a small portion of the glass fiber in such mats be replaced by polymer fiber materials and that an acrylic binder be used instead of urea formaldehyde resin. While gypsum boards incorporating such mats have somewhat improved strength and handling characteristics, they are undesirably more expensive to make and stiffer and less fire resistant. Moreover, the problems of irritation from dust released, e.g. during cutting, remain.
In addition, many of the available non-paper faced gypsum boards have further features that make them undesirable for many of the wall facing applications for which they are intended. For example, the surface roughness of current fiber-faced boards makes them difficult to finish satisfactorily by normal painting, because the texture of the mat remains perceptible through the paint. The fibers in the mat themselves give rise to various asperities, and to additional, larger sized irregularities often termed in the industry with descriptives such as “orange peel”, “cockle”, or similarly evocative terms describing surface non-planarity. The perceived smoothness of a board surface is the result of a complex interplay between various topographic features of the board, including the size, depth, spacing, and regularity of the features. In most instances, the smoothness of different board surfaces may readily be compared and ranked by visual inspection, especially under illumination by obliquely incident light. In addition, image analysis techniques are useful in quantifying certain of the topological features seen on various gypsum board surfaces.
Many of the aforementioned surface defects arise during the drying or curing of the mat or gypsum board. Even after painting, these defects and the underlying fibrous texture remain perceptible and aesthetically unappealing. As a result, a uniform, smooth finish can be achieved only in conjunction with a prior ameliorative treatment. Typically it is necessary to skim coat the surface with drywall joint compound or the like and then sand to a requisite smoothness to achieve a surface that will accept paint satisfactorily. This treatment must be accomplished at the construction site, resulting in added labor and materials cost. The additional steps entail inconvenience and delay, the consequences of the time needed for applying and drying the coating, and the generation of further nuisance dust. These difficulties are not encountered with paper-faced gypsum board, whose as-produced surface is smooth enough to accept paint readily with a minimum of surface preparation. Accordingly, current fiber-faced gypsum board is seldom if ever used for interior finished walls.
Another form of mat-faced gypsum board is known from U.S. Pat. No. 4,879,173, which discloses a mat of non-woven fibers having a reinforcing resinous binder that can comprise a single resin or a mixture of resins, either thermoplastic or thermosetting. Exemplary resins disclosed include a styrene-acrylic copolymer and a self-crosslinking vinyl acetate-acrylic copolymer. A small amount of the binder is applied to the surface of the mat and penetrates but part of the way therethrough. The board is said to be useful as a support member in a built-up roof. The highly textured surface of the mat binder provides many interstices into which can flow an adhesive used to adhere an overlying component. However, considerable care is required in using a mat containing substantial numbers of voids as a facer for gypsum board. Conventional processing that incorporates deposition of a relatively wet slurry is generally found to result in considerable intrusion of the slurry through the mat and onto the faced surface, which is frequently undesirable. Prevention of this excess intrusion typically requires very careful control of the slurry viscosity, which, in turn, frequently leads to other production problems. Alternative mats, which inherently limit intrusion, yet still have sufficient permeability to permit water to escape during the formation and heat drying of the gypsum board are thus eagerly sought as a simpler alternative.
A fibrous mat facer with improved strike-through resistance and useful as a facer substrate or carrier for receiving a curable substance in a fluid state is disclosed by U.S. Pat. No. 4,637,951. The porous, non-woven mat comprises a blend of microfibers intermixed and dispersed with base fibers and bound with a binder comprising a water miscible combination of a heat settable polymer. The mat is said to be useful in forming composite materials employing a curable thermoset, preferably foamable material such as a polyurethane or polyisocyanurate rigid foam board and as a carrier web in the vinyl flooring industry where the settable polymer comprises a vinyl plastisol.
Notwithstanding the advances in the field of gypsum boards and related articles, there remains a need for a readily and inexpensively produced mat-faced gypsum board having one or more of a smoother surface, a stronger internal bond to prevent delamination of the facer when subjected to prolonged wetness after installation, a surface requiring less paint to produce an aesthetically acceptable finished wall, ceiling, or the like, and better flame and mold resistance.