Abstract:
Disclosed are building board manufacturing techniques that minimize the build-up of slurry on associated forming equipment and also produce panels with enhanced physical properties. The methods involve applying a dissolvable film laminate to one or more fiber mats at the outset of the forming process. In the un-dissolved state, the film acts as a containment envelope for the gypsum slurry and any free floating glass fibers. During subsequent curing, the film is dissolved by vaporized water. In its dissolved state, the film is liquefied and coats the fibers of the underlying mat. This results in a building board with improved physical properties.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and is a continuation-in-part of U.S. patent application Ser. No. 12/794,959 filed on Jun. 7, 2010 and entitled “Method for Constructing Composite Building Boards using Dissolvable Films.” The contents of this co-pending application are fully incorporated herein for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to a system and method for constructing composite building boards. More particularly, the present invention relates to the use of thermoplastic coatings in the construction of composite gypsum building boards. 
         [0004]    2. Description of the Background Art 
         [0005]    Building board, also known as wallboard, plasterboard, or drywall, is one of the most commonly used building components in the world today. Building board is frequently used within the interior of a dwelling, where it functions both as a finished wall covering and as a structural room partition. Building board can also be used on the exterior of a dwelling, where it serves as a sheathing to provide weather protection and insulation. Building board can also be used as an interior facing for other structures as well, such as stairwells, elevator shafts, and interior ducting. 
         [0006]    One particularly popular form of building board is known as glass reinforced gypsum (GRG) board. An example of one such board is disclosed in U.S. Pat. No. 4,265,979 to Baehr et. al. Baehr discloses a building board constructed from opposing glass fiber mats with an intermediate gypsum core. This construction provides a hardened external surface and is an improvement over earlier paper faced boards. 
         [0007]    Current GRG manufacturing techniques have some significant drawbacks. Namely, during construction, some of the individual mat fibers are not covered by the gypsum slurry core and are therefore exposed. These fibers have a tendency to dry out and disengage from the board. As a result, free floating glass fibers tend to accumulate on and damage associated forming equipment, such as forming tables, forming plates, motor drives, bearings, and the like. The presence of disengaged fibers also presents a significant hazard to workers who must wear appropriate safety masks so as not to ingest the fibers. The most common way to combat this problem is through the use of expensive dust collection equipment and/or the periodic and repeated cleaning of the forming equipment. 
         [0008]    A subsequent board manufacturing technique is described in commonly owned U.S. Pat. No. 4,378,405 to Pilgrim. The contents of the Pilgrim patent are fully incorporated herein by reference. Pilgrim discloses a GRG board that is faced on one or both sides with a porous, nonwoven glass mat. The glass mat of Pilgrim is slightly but fully embedded into the slurry core. This is accomplished by vibrating the gypsum slurry to cause it to pass through the porous openings in the mat. Embedding the mat within the core as taught in Pilgrim results in a thin film of slurry being formed on the outer surface of the board. Building boards with this construction are referred to as embedded glass reinforced gypsum (EGRG) boards. 
         [0009]    EGRG boards eliminate, or greatly reduce, the presence of exposed fibers and greatly reduce the presence of free floating fibers. However, the construction of EGRG boards also has its drawbacks. Namely, EGRG boards require the application of low viscosity gypsum slurry. This slurry leaks from the boards during manufacture and accumulates on associated forming equipment. Thus, during manufacture, the forming tables, forming belts, and associated rollers and motors are exposed to substantial build-ups of gypsum slurry. Over time, if not regularly cleaned, the manufacturing process comes to a complete stop. Thus, in traditional GRG and EGRG building board manufacturing techniques there is a substantial capital investment in equipment designed to clean the forming areas. 
         [0010]    Additionally, even in the construction of EGRG boards, there is a continuing problem with some fibers becoming exposed, dried and detached. This, in turn, results in the accumulation of free fibers on the forming tables, forming belts and associated rollers and motors. As with the excess gypsum slurry, these fibers must be removed in order to prevent equipment failure resulting in downtime. 
         [0011]    Thus, there exists a need in the art for improved building board manufacturing techniques. More specifically, there is a need in the art for manufacturing techniques that minimize the accumulation of gypsum slurry and/or free floating fibers on associated forming equipment. There also exists a need to minimize capital investment needed to construct GRG and EGRG building boards. There is yet another need to economically produce GRG and EGRG building boards with improved physical characteristics. The present invention is aimed at achieving these objectives. 
       SUMMARY OF THE INVENTION 
       [0012]    One advantage of the present method is realized by applying a thermoplastic film to the surface of a composite building board. 
         [0013]    Another advantage is achieved by applying a thermoplastic film to the surface of a building board in an in-line manufacturing process. 
         [0014]    Another advantage is accomplished by limiting the build-up of slurry on forming equipment associated with board production. 
         [0015]    Yet another advantage is achieved by constructing fiber reinforced building boards that minimize and/or eliminate the presence of exposed and/or free floating fibers. 
         [0016]    Still yet another advantage is accomplished by forming a thin slurry layer between a mat and an adjacent thermoplastic layer. 
         [0017]    Another advantage is attained by using a thermoplastic layer in the construction of building boards, wherein the layer acts as a containment envelope for slurry and glass fibers. 
         [0018]    Still yet another advantage is realized by forming a building board using an outermost thermoplastic layer, wherein the thermoplastic layer minimizes slurry leaking from the face of the board. 
         [0019]    Still yet another advantage is that, once produced, the boards obtain a thermoplastic layer that may be utilized as a base layer for additional coatings or surface treatments. The use of an existing plastic layer in this manner can be in reduction of additional coating weights, modification of texture and or compounding with secondary coating layers. All of these advantages lead to more economical panel production and increased panel functionality and customization. 
         [0020]    Various embodiments of the invention may have none, some, or all of these advantages. Other technical advantages of the present invention will be readily apparent to one skilled in the art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which: 
           [0022]      FIG. 1  is an elevational view of the first part of the manufacturing process wherein the slurry is supplied between facing mats; 
           [0023]      FIG. 2  is an elevational view of the second part of the manufacturing process wherein the building panels are dried. 
           [0024]      FIGS. 3   a - 3   e  are successive cross sectional views of the building panels taken from  FIG. 1 . 
           [0025]      FIG. 3   f  is a bottom plan view of the board showing the slurry bleeding through the board surface. 
           [0026]      FIG. 4  is a detailed view of the curtain coater and/or slot die coater used in applying a thermoplastic layer. 
           [0027]      FIG. 5  is a detailed view of a spray coater used in applying a thermoplastic layer. 
           [0028]      FIG. 6  is a detailed view of a knife coater used in applying a thermoplastic layer. 
       
    
    
       [0029]    Similar reference characters refer to similar parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0030]    The present invention relates to a system and method for manufacturing thermoplastic coated building boards. The system and method can be used to produce both glass reinforced gypsum board (GRG) and embedded glass reinforced gypsum board (EGRG). The thermoplastic coating is applied to a mat prior to board formation. The coating can be applied in-line or off-line with respect to the remaining production line. The coating acts as a containment envelope between an exterior, or contacting, surface of the mat and the underlying forming belt. The coating also retains a thin layer of gypsum slurry on the exterior surface of the mat. This reduces contamination of the production line and produces boards with increased physical properties. 
         [0031]      FIG. 1  illustrates a gypsum board production line that has been modified in accordance with the present disclosure. Line  20  includes a series of forming tables  22  for supporting the building panel  24  during its formation. As is known in the art, the mats that form panel  24  are under tension by way of a series of downstream belts. Once panel  24  has been formed, it is passed to a series of board dryers  26 . Dryers  26  function in driving out excess moisture and causing the gypsum slurry to set. This results in the formation of a dried composite panel  24 . 
         [0032]    As further noted in  FIG. 1 , gypsum board  24  is formed from first and second mats ( 28  and  32 ) between which a volume of gypsum slurry is deposited. These mats are initially stored in large rolls ( 28   a  and  32   a ) that are unwound to provide a continuous length of mat. First roll  28   a  is unwound onto forming table  22 . Slurry  34  is thereafter deposited upon the mat from an overhead mixer  36  at various downstream locations. Second roll  32   a  is ideally positioned downstream of first  28   a  roll and is unwound over top of the deposited gypsum core to create sandwich or panel. 
         [0033]    Mats ( 28  and  32 ) are preferably constructed from a series of nonwoven, randomly aligned glass fibers. Mats ( 28  and  32 ) may also comprise continuous or non-continuous fibers, organic or inorganic fibers, woven or nonwoven fibers, or blends thereof. The fibers may also be continuous in length, chopped non-continuous in length, identical or random in length, of blends thereof. The fibers within mats ( 28  and  32 ) ideally have lengths of between ½″ to 2.″ Mats ( 28  and  32 ) preferably have a thickness of between about 0.0625″ to 0.5″ 
         [0034]    Mats ( 28  and  32 ) are also preferably pre-coated with an organic or inorganic resin binder to hold the individual fibers together. Additionally, mats ( 28  and  32 ) can be supplied uncoated, with the resin binder being applied at a point along production line  20 . However, the disclosed method can be carried out with a variety of other mat constructions. 
         [0035]    In accordance with the present disclosure, a volume of hot, molten thermoplastic  38  is applied to an external surface  40  of first mat  28  prior to forming table  22 . More specifically, as noted in  FIG. 1 , first mat  28  is routed by way of guide rollers  42  beneath a coating device  44 , which applies a uniformly thin layer  46  of liquefied plastic  38  over external surface  40  first mat  28  in a continuous in-line process. Any of a variety of hot melt thermoplastics can be utilized. In one non-limiting example, molten acrylonitrile butadiene styrene (ABS) plastic is used. In accordance with this disclosure, hot melt thermoplastic refers to a thermoplastic that is applied in a liquid state and that forms an adhesive bond upon cooling to a solid state. 
         [0036]    Although ABS plastic is one example, any of the following plastics can also be used, alone or in combination with one another: Celluloid, Cellulose Acetate, Ethylene-Butyl Acrylate, Ethylene-Methyl Acrylate, Ethylene Vinyl Acetate (EVA), Ethylene Vinyl Alcohol (EVAL), Fluoroplastics (PTFEs, including FEP, PFA, CTFE, ECTFE, ETFE), Ionomers, Liquid Crystal Polymer (LCP), Metallocene, Polyacetal (POM or Acetal), Polyacrylates (Melt and Cure Acrylics), Polyacrylonitrile (PAN or Acrylonitrile), Polyamide (PA or Nylon), Polyamide-imide (PAI), Polyaryletherketone (PAEK or Ketone), Polybutyadiene (PBD), Polybutylene (PB), Polybutylene Terephthalate (PBT), Polybutylene Terephthalate (PET), Polycyclohexylene Dimethylene Terephthalate (PCT), Polycarbonate (PC), Polyketone (PK), Polyester, Polyethylene/Polythene/Polyethane, Polyether Block Amide (PEBA), Polyetheretherketone (PEEK), Polyetherimide (PEI), Polyethersulfone (PES), Polyethylenechlorinates (PEC), Polyimide (PI), Polylactic Acid (PLA), Polymethylpentene (PMP), Polyphenylene Oxide (PPO), Polyphenylene Sulfide (PPS), Polyphthalamide (PPA), Polypropylene (PP), Polystyrene (PS), Polysulfone (PSU), Polyvinyl Chloride (PVC), Spectralon, thermoplastic Olefinic Elastomer (TPO). 
         [0037]    The thermoplastic pre-coating can be any of the foregoing hot melt thermoplastics or various blends thereof. The holt melt thermoplastic may also incorporate secondary additives blended into said hot melt thermoplastic to impart specific enhancements to the precoating, the precoated fibrous mat, or the resulting building panel. These secondary additives may provide improved strength, improved flexibility, improved hardness, improved impact resistance, improved abrasion resistance, UV resistance, mold and mildew resistance, bacterial resistance, viral resistance, formaldehyde scavenging, carbon dioxide scavenging, structural characteristics, improved fire resistance, EMF resistance (as a shielding sheathing, interior wall board, roof deck board, or underlayment), frequency specific resistance (as a shielding sheathing, interior wall board, roof deck board, or underlayment), solar collecting (as a roofing panel), piezoelectric energy generation (as an underlayment), water drainage, or improved sound resistance. 
         [0038]    The present disclosure also contemplates using any of a variety of coating devices  44 . The preferred coating apparatus is a curtain coater  48  and/or slot die coater and is illustrated in  FIG. 4 . However, a spray coater  52  ( FIG. 5 ) or a knife coater  54  ( FIG. 6 ) can alternatively be used. The hot melt thermoplastic can alternatively be applied to the fibrous mat via a hot melt roll coater, forward, reverse, or multi-stage forward and reverse application methods. Still yet other alternatives include a hot melt slot coater, or a hot melt flow coater. An emersion bath can also be employed. Whatever coating device is employed, it is preferred to position the coating device  44  in-line with the remaining components of production line  20 . In still yet other embodiments, off-line coating processes can be employed. 
         [0039]    Whatever coating device  44  is used, a uniform layer  46  of the hot thermoplastic  38  should be applied in a continuous process. However, it is within the scope of the present invention to apply layer  46  non-uniformly or to control the porosity layer  46 . It is also preferred that layer  46  be thin with a thickness of approximately 0-50% of the thickness of underlying mat  28 . However, in the preferred embodiment, layer  46  has a thickness that is between approximately 1% to 10% of the thickness of mat  28 . It is also preferred that thermoplastic layer  46  have a minimum thickness weight of between approximately 0.01 g/sqft to 45 g/sqft.  FIG. 3   a  illustrates the thermoplastic layer  46  applied to external surface  40  of mat  28 . Once applied, layer  46  will generally cover between approximately 90% to 99.999% of the entire surface of mat  28 . Thus, even after layer  46  is deposited, approximately less than 1% and up to 90% of the underlying mat  28  will be exposed. This exposed region will permit a limited degree of bleed through by the deposited slurry.  FIG. 3   f  is a bottom plan view showing the slurry bleeding through the surface of the board. 
         [0040]    Coating device  44  may also include internal channels  56  within which a heating fluid, such as a hot oil from reservoir  57 , may be circulated (note  FIG. 4 ). These channels  56 , thereby, act as a heat exchanger to raise or maintain the temperature of the plastic  38  prior to its application. This ensures that thermoplastic  38  remains in a molten or liquefied state prior to its delivery upon mat  28 . The coating head may also have electrically heated elements contained integrally or any other means of providing stable elevated temperature of said thermoplastic delivery apparatus  44 . 
         [0041]    After thermoplastic layer  46  has been applied, mat  28  is routed over additional guide rollers  42  prior to arriving at forming table  22 . This gives the molten thermoplastic layer  46  sufficient time to come into contact with the ambient air and cool. Layer  46 , however, is still warm as it travels over forming table  22 . Additionally, thermoplastic layer  46  is permitted to adhere to the external surface  40  underlying mat  28 , as well as to the individual fibers comprising mat  28 , prior to forming tables  22 . 
         [0042]    Mat  28  with the applied thermoplastic layer  46  is also inverted prior to arriving at forming table  22 . This inversion is achieved via one or more guide rollers  42 . More specifically, after thermoplastic layer  46  has been applied, mat  28  is turned upside down to expose the internal uncoated surface  58  of mat  28 . This results in thermoplastic layer  46  contacting and facing underlying forming table  22 . It also results in interior surface  58  of first mat  28  being exposed. This is noted in the cross section of  FIG. 3B . 
         [0043]    Subsequent downstream processing may include the application of a first gypsum slurry layer  62 , and the passage of the slurry layer and mat through a pair of roller coaters  64 . This results in the creating of a first dense slurry layer adjacent the exposed internal surface  58  of first mat  28 . Layer  46  will still be warm as first gypsum slurry layer  62  is applied. Slurry  62 , as well as the additional slurry that is deposited downstream, will assist in cooling thermoplastic layer  46 . As slurry  62  is deposited, thermoplastic layer  46  is expanded and slightly displaced. 
         [0044]    Vibrators  65  are preferably spaced along the length of forming tables  22  to ensure the uniform distribution of slurry and the elimination of voids. The vibrators also act in embedding mat  28  within the deposited gypsum. Thereafter, additional gypsum slurry  66  is applied over the interior surface  58  to form the core of building board  24  (note  FIG. 3   c ). The deposited gypsum slurry  66  is preferably delivered from a overhead mixer  36 . Slurry  66  will act in further cooling and displacing thermoplastic layer  46 . This, in turn, permits a thin layer of slurry  68  to be formed between the external surface  40  of first mat  28  and thermoplastic layer or barrier  46  ( FIG. 3   d ). 
         [0045]    As is known in the art, additives can be included in the gypsum slurry to achieve desired performance characteristics, such as polymers to provide increased strength and reduced weight. One suitable polymer additive is a styrene butadiene latex that is substantially stable against divalent ions. 
         [0046]    The fibers of the first mat are sufficiently spaced to permit core slurry  66  to fully penetrate the individual glass fibers. This ensures that individual fibers are coated and that mat  28  is completely penetrated. This, in turn, results in the applied gypsum ( 62  or  66 ) coating the exterior surface  40  of mat  28 . Thermoplastic coating  46 , however, limits the amount of deposited slurry ( 62  or  66 ) that contacts the forming belts  22 . In this manner, thermoplastic coating  46  acts as a barrier preventing the discharge of slurry from the exterior surface  40  of mat  28 . This prevents forming table  22 , as well as associated belts, pulleys, and motors, from getting contaminated by gypsum or gypsum particles. 
         [0047]    Thermoplastic barrier  46  and thin gypsum layer  68  together prevent fiber disengagement from mat  28 . Barrier  46  and layer  68  also impart desired physical properties to the resulting building board  24 .  FIG. 3C  is a cross section of board  24  immediately after the gypsum slurry  66  has been applied to interior surface  58 . As noted, slurry does not immediately penetrate mat  28 .  FIG. 3D  is a depiction of the subsequent cross section after gypsum slurry  66  has had time to fully penetrate the thickness of mat  28  and encounter thermoplastic barrier  46 . Because layer  46  is only applied to between 90% to 99.999% of mat  28 , a limited amount of slurry  66  will bleed through to the external surface of the building board. 
         [0048]    Thereafter, a second length of mat  32  is deposited over top of gypsum slurry core  66 . This second mat  32  can likewise comprise a plurality of non-woven randomly aligned glass fibers. Second mat  32  may have a small volume of gypsum  72  applied to its surface before it is applied to gypsum core  66 . Thereafter the resulting panel  24  is formed into a desired thickness by way of a forming plate  74  and pinch point  76 . 
         [0049]    In accordance with conventional gypsum board manufacturing techniques, the resulting panel is then delivered to a series of board dryers  26  ( FIG. 2 ). Dryers  26  are utilized heating the gypsum slurry within the panels and vaporizing any non-crystalline water. Four dryer zones are preferably included. However, the number of dryer zones employed is not critical to the present invention. Dryers are designed to heat the building boards  24  to a degree sufficient to cure the gypsum. This is typically achieved at a temperature of approximately 212° F. The presence of entrained water within the gypsum core will delay the temperature of the gypsum core from raising above 212° F. Dryers of the depicted embodiment utilize a conventional construction and run at temperature levels that range anywhere between approximately 180° F. to 650° F., which is typical for gypsum drying operations. As a result of this heating process, water is vaporized at the surface and delivered upwardly through second mat  28 . Panel  24  can then be cut to desired lengths depending upon the intended use. Notably, thermoplastic layer  46  is not dissolved in dryers  26  and remains intact upon the final cut panels  24 . 
         [0050]    The present disclosure includes that contained in the appended claims, as well as that of the foregoing description. Although this invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention. 
         [0000]    Now that the invention has been described,