Patent Publication Number: US-10767368-B2

Title: Plastic coated composite building boards and method of making same

Description:
RELATED APPLICATION DATA 
     This application claims priority to and is a continuation of application Ser. No. 15/044,639 filed Feb. 16, 2016 and entitled “Plastic Coated Composite Building Boards and Method of Making Same,” which is a continuation of application Ser. No. 13/941,950 filed Jul. 15, 2013 and entitled “Plastic Coated Composite Building Boards and Method of Making Same,” now U.S. Pat. No. 9,259,888, which is a divisional of application Ser. No. 12/480,159 filed on Jun. 8, 2009 and entitled “Plastic Coated Composite Building Boards and Method of Making Same,” now U.S. Pat. No. 8,486,516, which, in turn, claims priority to provisional application Ser. No. 61/093,167 filed on Aug. 29, 2008 entitled “Thermoplastic Coatings for Glass Reinforced Faced Gypsum Board.” The contents of these applications are fully incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to an improved construction for composite building boards. More particularly, the present invention relates to composite building boards that include an exterior plastic coating and a pre-coated fibrous mat for increasing the durability and surface strength of the resulting board. 
     Description of the Background Art 
     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. 
     One particularly popular form of building board is known as gypsum board. Gypsum board is constructed by depositing a layer of cementitious gypsum slurry between two opposing paper liners. Gypsum slurry is the semi-hydrous form of calcium sulfate and has many physical characteristics that make it suitable for use as a building component. For example, gypsum boards generally have a smooth external surface, a consistent thickness, and allow for the application of finishing enhancements, such as paint. Gypsum board is also desirable because it provides a degree of fire resistance and sound abatement. 
     An example of a paper-covered gypsum board is disclosed in U.S. Pat. No. 2,806,811 to Von Hazmburg. Von Hazmburg discloses a board that primarily consists of a thick gypsum core that is encased in a fibrous envelope consisting of both a manila sheet and a newsprint sheet. These sheet layers can be made from a conventional multi-cylinder paper making process. 
     Although conventional paper faced gypsum board, such as that disclosed by Von Hazmburg, is acceptable for many applications, it also has considerable drawbacks. The biggest drawback is durability. Gypsum board is far more brittle than other building materials, such as wood or masonry based materials. Paper faced gypsum boards, therefore, chip and/or crumble under both compressive and tensile loads. As a result, conventional gypsum board is easily damaged by the normal wear and tear within a dwelling, such as impacts with people and/or furniture. Conventional gypsum board construction often has poor load carrying capacity and inadequate nail pull strength. As a result, traditional gypsum board often cannot support the loads needed to hang pictures or install shelving. 
     As a consequence of these drawbacks, efforts have been made over the years to improve the durability and surface strength of gypsum board. One particularly useful development 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 paper-free gypsum board construction. More specifically, Baehr replaces paper facing sheets with opposing layers formed, in part, from glass fiber mats. This construction provides a stronger and harder external surface and is an improvement over paper faced boards. Although an improvement from the standpoint of durability, the use of exposed fiber mats is problematic. Namely, workers handling such boards are exposed to lose strands of fiber. This poses a health risk and necessitates the use of protective gloves and/or masks. Thus, GRG boards utilizing exposed facing sheets are not ideal. 
     A subsequent improvement is described in commonly owned U.S. Pat. No. 4,378,452 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. However, the glass mat of Pilgrim is slightly 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 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. EGRG boards eliminate, or greatly reduce, the presence of exposed fibers and otherwise provide a smooth working surface. Despite eliminating the safety issues surrounding GRG boards, Pilgrim ultimately failed to provide a board with sufficient strength and durability. 
     A further improved EGRG board is disclosed in commonly owned U.S. Pat. No. 6,524,679 to Hauber, et al. The contents of the Hauber patent are fully incorporated herein by reference. The EGRG board of Hauber adds a polymeric compound to the gypsum slurry. Suitable polymeric compounds may include, for example, polyvinyidene chloride (PVDC), or polyvinylchloride (PVC), or similar polymers. The polymer additive increases durability and board strength and also creates a matrix within the slurry after it sets. Although certainly an improvement over existing EGRG technology, Hauber did not address issues associated with the durability of the exterior face or the complete mechanical and chemical bonding of the exterior face to the underlying gypsum slurry. 
     Thus, there still exists a need in the art for improved building board construction. More specifically, there is a need in the art for a board with a polymer matrix that provides enhanced durability, impact resistance, water repellency, fire resistance, and load carrying capacities. There is also a need in the art for a board that provides these physical properties without unduly increasing the weight or cost of the resulting board. The present invention is aimed at achieving these objectives. 
     SUMMARY OF THE INVENTION 
     It is therefore one of the objects of the present invention to enhance the physical characteristics of conventional building board. 
     It is another object of this invention to increase the durability and load carrying capacity of building board with minimal increases in weight and cost. 
     It is another object of the present invention to coat building boards with a polymer that yields specific physical properties depending upon the intended use of the board. 
     Another object of the present invention is to provide a building board with beneficial physical characteristics via the inclusion of a fibrous mat with a thermoplastic pre-coating. 
     Yet another object of this invention is to provide a building board with an exterior coating that is engineered to enhance one or more physical properties, such as UV resistance, electrical conductivity, EMF resistance, sound attenuation, and water and fire resistance. 
     It is also an object of the present invention to cross-link an external polymer coating to polymer additives present within the core of a gypsum board. 
     It is still yet another object of the present invention to provide a building board construction that includes a thermoplastic coating that is applied to an underlying polymer modified dense slurry layer to thereby create a low cost, lightweight, durable board. 
     Yet another object of this invention is to offer process improvements that significantly reduce manufacturing costs and which allow for the economical production of building board. 
     It is also an object of the present invention to utilize a pre-coated fibrous mat in conjunction with an external thermoplastic coating. 
     Still yet another object of the invention is utilize a thermoplastic pre-coating on a glass mat that is embedded within a dense slurry layer to thereby produce a truly composite building board. 
     Another object of the present invention is to bind together a mat of organic and/or inorganic fibers with a holt melt thermoplastic polymer, as opposed to traditional thermal setting binders. 
     Yet another object of the present invention is to bind together a mat of continuous and/or non-continuous fibers with a hot melt thermoplastic polymer, as opposed to traditional thermal setting binders. 
     It is a further object of the present invention to provide a building board with significant cost advantages and performance characteristics. 
     Another object of the present invention is to apply a continuous and uniform layer of thermoplastic upon the fibers of a mat, whereby the thermoplastic cools instantly to thereby eliminate the energy costs associated with drying steps needed in traditional board manufacturing processes. 
     Still yet another object of the present invention is to eliminate fiber loss and/or disengagement via the application of a thermoplastic layer to there by eliminate costs associated with the dust and/or debris removal steps necessary in traditional board manufacture. 
     Finally, it is an object of the present invention to use thermoplastics to create instantly strong and bound fibrous mats, whereby manufacturing speeds can be greatly increased. 
     These and other objects are carried out by providing a composite building board that includes a porous mat with a thermoplastic pre-coating. The board further includes a cementitious slurry layer that penetrates the porous mat to thereby form a boundary layer that substantially covers the mat&#39;s exterior surface. The board also includes an external polymer coating that is mechanically and chemically adhered to the boundary layer and which forms forming a polymer matrix within the composite building board. 
     The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood so that the present contribution to the art can be more fully appreciated. Additional features of the invention will be described hereinafter which form the subject of the claims of the invention. 
     It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a perspective view of a building board constructed in accordance with the present invention. 
         FIG. 2 a    is a cross sectional view of the building board taken along line  2 - 2  of  FIG. 1 . 
         FIG. 2 b    is a cross sectional view of an alternative building board construction taken along line  2 - 2  of  FIG. 1 . 
         FIG. 3  is a view of a manufacturing process that can be employed in constructing building boards of the present invention. 
         FIGS. 4 a -4 c    are detailed views taken from  FIG. 3 . 
         FIG. 5  is a view of an alternative manufacturing process that can be employed in constructing the building boards of the present invention. 
         FIGS. 6 a -6 c    are detailed view taken from  FIG. 5 . 
         FIG. 7  is a detailed view of a roller coater used to apply the plastic coating of the present invention. 
         FIG. 8  is a detailed view of a curtain coater used to apply the plastic coating of the present invention. 
         FIG. 9  is a detailed view of a knife coater used to apply the plastic coating of the present invention. 
         FIG. 10  is a detailed view of a spray coater used to apply the plastic coating of the present invention. 
     
    
    
     Similar reference characters refer to similar parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention relates to composite building boards and associated manufacturing methods. In particular, the invention relates to composite boards that may include, for example, one or more slurry layers with embedded fibrous mats. In accordance with the invention, an exterior plastic coating is mechanically adhered to an underlying slurry layer. The plastic coating chemically bonds and cross-links with polymer additives within the slurry layer. Additional benefits are realized by precoating one or more of the fibrous mats prior to embedment within the slurry layer. The result is a fully integrated polymer matrix with greatly improved durability and surface strength with only minimal increases in cost or weight. 
     Composite Board Construction 
     With reference now to  FIG. 1 , the composite building board  20  of the present invention is depicted. Building board is typically formed in long sheets in a continuous production line process. The sheets are thereafter cut to a desired length. Boards are produced in standardized widths of either 4 or 9 feet, depending upon the intended use. However, the present invention is by no means limited to any specific board dimensions or geometry. As noted in more detail hereinafter, board  20  includes an upper surface  22  that includes a plastic coating. Plastic coatings can likewise be applied to the bottom  24  and side edges  26  of board  20 . 
       FIG. 2 a    illustrates the composite cross section of board  20 . Namely, board  20  includes upper and lower fibrous mats ( 28  and  32 ) that are embedded within a multi-layered gypsum slurry. The depicted embodiment illustrates two fiber mats ( 28 , 32 ), although those skilled in the art will appreciate that any number of mats can be utilized. In the preferred embodiment, these mats ( 28 ,  32 ) are formed from a grouping of nonwoven and randomly aligned fibers that are held together in a binder. Suitable binders include resins, such as urea-formaldehyde. The fibers of the mat are preferably long inorganic fibers, such as glass fibers. The fibers can also be continuous or non-continuous or blends of both. The fibers can be formed from organic or inorganic filaments. In one embodiment, mineral fibers are used. Small diameter fibers are preferred; namely, fibers with an average diameter of between approximately 13-16 μm. The resulting mat is sufficiently porous to allow for the passage of gypsum slurry between the individual fibers, whereby the interior and exterior faces of the mat can be coated, or substantially coated, with gypsum slurry. Although porous, mats ( 28 ,  32 ) nonetheless serve to strengthen the face of the resulting board  20 . Suitable fiber mats are more fully described in commonly owned U.S. Pat. No. 6,524,679 to Hauber, the contents of which are fully incorporated herein. Furthermore, as described below, mats ( 28 ,  32 ) can be pre-coated in order to increase composite bonding and board strength. 
     Lower mat  32  optionally includes a pair of folds ( 34 ,  36 ) within each side. First folds  34  create the upstanding side edges  38  of lower mat  32 . Side edges  38  are generally formed at a 90° angle to the remainder of mat  32  and, thereby, serve to reinforce the side edges  26  of board  20 . Second folds  36  form the inwardly directed upper edges  42  of lower mat  32 . Upper edges  42  are sometimes referred to as machine edges. Upper edges  42  are preferably spaced from one another to form a gap  44 . The result is a partially closed channel. Upper edges  42  provide a surface for supporting upper mat  28 . As described in greater detail hereinafter, folds ( 34 ,  36 ) within lower mat  32  are formed by creaser wheels during the manufacturing process. 
     In addition to strengthening board  20 , side edges  38  of lower mat  32  also form a channel for receiving a volume of slurry. Once a sufficient amount of slurry has been deposited into the channel, the upper mat  28  is secured over top of the inwardly directed upper edges  42 . The result is a closed fiber mat that reinforces all four sides ( 22 ,  24 ,  26 ) of building board  20 . 
     Composite board  20 , in the preferred embodiment, is formed from three discrete cementitious gypsum slurry layers. More specifically, board  20  includes upper and lower slurry layers ( 46  and  48 ) as well as an intermediate core layer  54 . As described in U.S. Pat. No. 6,524,679 to Hauber, the upper and lower layers ( 46  and  48 ) are formed from a denser slurry than the intermediate core layer  54 . This construction has the added benefit of strengthening the exterior faces of board  20  without unnecessarily increasing the overall weight. 
     During the manufacturing process, the upper and lower dense slurry layers ( 46 ,  48 ) are coated and adhere to upper and lower mats ( 28 ,  32 ), respectively. Rollers are then used to push the dense slurry ( 46 ,  48 ) through the pores within mats ( 28 ,  32 ). Because mats ( 28 ,  32 ) are porous, the dense gypsum slurry is allowed to fully penetrate the mats ( 28 ,  32 ). As a result, and as depicted in  FIG. 2 a   , a layer of dense gypsum slurry ( 46 ,  48 ) substantially covers both the internal and external faces of upper and lower mats  28 , 32 ). Dense gypsum slurry ( 46 ,  48 ) generally forms a thicker layer on the interior surfaces. As noted in  FIGS. 2 a - b   , the internal dense slurry layer ( 46 ,  48 ) interfaces with slurry core  54 . The dense slurry layer ( 46 ,  48 ) also forms a thin boundary layer  52  of between 0.01 to 2.0 millimeters (mm) as measured from the exterior surface of fiber mat ( 28 ,  32 ). 
     Boundary layer  52  thereby forms a topographically that undulates into and out of the underlying glass mats ( 28 ,  32 ) but which nonetheless covers the individuals fibers of mats ( 28 ,  32 ). Alternatively, boundary layer  52  can form a uniform and smooth covering surface over mats ( 28 ,  32 ). In either event, complete embedment of mats ( 28 ,  32 ) is achieved as a result of the slurry penetration. In the preferred embodiment, the slurry penetrates between 95% to 100% of mats ( 28 ,  32 ) and forms a smooth and relatively level and uniform polymer modified composite dense gypsum outer surface layer. 
     Boundary layer  52  also interfaces with the external plastic coating as illustrated in  FIG. 2 a   . Alternatively, in the event plastic coating  56  is not utilized on all surfaces, as noted in  FIG. 2 a   , boundary layer  52  cures to form the exterior surface of board  20 . In this case, boundary layer  52  prevents fibers from mats ( 28 , 32 ) from being exposed. 
     The outer dense slurry layers ( 46 ,  48 , and  52 ) all preferably include a polymer additive to increase the overall durability and surface strength of the board. The polymer additive also preferably facilitates a strong chemical bond between itself and the exterior plastic coating  56 . Suitable polymer additives will provide a root structure to which coating  56  can attach. Suitable polymeric compounds may include, for example, polyvinyidene chloride (PVDC), or polyvinylchloride (PVC), or similar polymers. Another suitable polymer additive is a functionalized styrene butadiene (SBD) latex that is available from Omnova Solutions of Fairlawn, Ohio. Yet another suitable additive is silane or a functionalized silane (SiH4) Silane compounds are ideally used in conjunction with other polymers to facilitate coupling between the polymer to glass fibers. Silane is also known as a stabilizing agent. Suitable silane compounds are sold by Down Corning. Still yet other polymer additives are described in U.S. Pat. No. 6,524,679 to Hauber. Whatever additive is utilized, it should be capable of providing covalent, allyl, Vanderwal, single and double bonding to the exterior plastic coating  56 . 
     Core slurry layer  54  generally comprises the majority of board  20  and extends fully between, and bonds with, the upper and lower dense slurry layers ( 46 ,  48 ). In one possible manufacturing method, core slurry layer  54  is deposited over top of lower slurry layer  48  (along with embedded lower fiber mat  32 ). Thereafter, upper dense slurry layer  46  (along with embedded upper fiber mat  28 ) is applied over core slurry  54 . As with the upper and lower slurry layers ( 46 ,  48 ), core slurry  54  can likewise include a polymer additive for the purpose of adding durability and surface strength. The polymer additive within core slurry  54  preferably chemically bonds with, and cross-links to, the polymer additives within the other slurry layers ( 46 ,  48 , and  52 ). 
     External Plastic Coating 
     After the various slurry layers have been assembled an outer plastic coating  56  is applied to the dense slurry boundary layer  52 . Coating  56  can be optionally applied to any or all board surfaces. In the embodiment depicted in  FIG. 2 a   , plastic coating  56  is applied to the top and bottom surfaces ( 22 ,  24 ) as well as the peripheral edges  26 . In the embodiment depicted in  FIG. 2 b   , plastic coating  56  is only applied to the top most surface  22 . The number of coated surfaces will depend upon the intended use of the board. The intended use will also dictate the composition of coating  56 . Plastic coating  56  is ideally chosen to give the exterior surfaces ( 22 ,  24 , and  26 ) enhanced surface strength and load carrying capacity. Other desirable characteristics provided by coating  56  include flexibility, sound attenuation, water, mold and mildew resistance, as well as a variety of architectural effects. 
     To achieve this, coating  56  can be any of a variety of synthetic, semi-synthetic, or organic polymers. Both reactive and nonreactive polymers can be used. Isotactic and atactic polymers can likewise be used. 
     Additionally, multilayer laminated polymer coatings can be used to provide even greater strength and durability. When a polymer coating according to the present invention has been applied to the board, the top-most coating then can serve as an additional foundation on which other coatings and/or laminates can be applied. The properties of the different layers may be made compatible so as to form a strong chemical bond between the successively applied layers. This results in laminates with strong bonding capabilities. The laminated layers can also be incorporated into, or made to form, complete composite structures. It is further noted that the external thermoplastic layer  56  can be used on a board that does not include fibrous mats ( 28 ,  32 ). For example, a paper facing layer can be used in lieu of mats ( 28  and  32 ). 
     The following is a list of various polymers that can be used, either individually or in combination with one another, for polymer coating  56 : Acrylonitrile butadiene styrene (ABS), Celluloid, Cellulose Acetate, Ethylene-Butyl, Acrylate, Ethylene-Methyl Acrylate, Ethylene Vinyl Acetate (EVA), Ethylene-Acrylic-Acid-copolymer (EAA); Ethylene Vinyl Alcohol (EVAL), Fluoroplastics (PTFEs, including FEP, PFA, CTFE, ECTFE, ETFE), ionomers, Liquid Crystal Polymer (LCP), 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), Polyactic Acid (PLA), Polymethylpentene (PMP), Polyphenylene Oxide (PPO), Polyphenylene Sulfide (PPS), Polyphthalamide (PPA), Polypropylene (PP), Polystyrene (PS), Polysulfone (PSU), Polyvinyl Chloride (PVC), Spectralon, and thermoplastic Olefinic Elastomers (TPO). 
     Of these, it is preferred to use a hot melt thermoplastic with a melting point of between 100° F. to 500° F. Either natural or synthetic hot melt thermoplastics can be used. Additionally, the present inventors have determined that hot melt thermoplastics having a melting point within the specified range allows a plastic coating to be applied in a liquefied form without calcining the underlying gypsum. Preferred hot melt thermoplastics include both EVA and EAA polymers, as both have suitable melting points and otherwise provide sufficient bonding points for the polymer additives in the dense gypsum layers of the board. 
     In addition to the foregoing, polyolefin polymers can also be used, including polar or nonpolar polyolefenic compounds, crystalline or amorphous polyolefenic compounds, natural or synthetic tacifying resins as part of polyolefenic compounds, and low viscosity polyolefenic compounds. The selected polymer can also be used to generate a variety of films, including microscopically continuous and/or non-continuous films engineered for molecular water permeability, non-oriented polymer films, planar oriented polymer films, randomly oriented polymer films, and films with low thermal conductivity. 
     Whatever polymer coating  56  is utilized, it should provide excellent mechanical adhesion to the underlying dense slurry layers and also chemically bond to polymer additives included therein. Mechanical adhesion can be achieved via topographic mirroring of the polymeric coating  56  to the underlying dense slurry boundary layer  52 . This topographic mirroring can be accomplished by controlling the hardness and pressure of the application rollers. Topographic mirroring can also be accomplished by varying application heat, thereby employing vacuum like forces upon cooling which draws the coating into the topography, which is an unexpected result discovered by the inventors during development. 
     Chemical bonding between coating  56  and the underlying slurry layers  46 ,  48 ,  52 , and  54  is achieved by cross-linking polymer coating  56  with the polymer additives contained within the various slurry layers: namely, upper slurry layer  46 , lower slurry layer  48 , slurry boundary layer  52 , and core slurry layer  54 . The respective polymers are chosen to ensure adequate cross-linking and the creation of long, high molecular weight polymer chains that extend throughout composite board  20 . For instance, correct selection of the polymeric coating  56  allows for the creation of ionic, valent and covalent bonding, as well as bonding via van der Waals forces. Moreover, if a nonreactive polymeric coating is selected, coating  56  will initiate with the underlying polymer additives after the coating  56  is taken through a phase change. Alternatively, if a reactive polymeric coating is selected, coating  56  will polymerize upon application to the underlying boundary layer  52 . 
     Performance enhancing fillers and/or modifiers can also be added to polymer coating  56 . These fillers and modifiers can provide any of the following physical enhancements: UV resistance, electrical conductivity, electromagnetic force (EMF) resistance, lower polymeric densities, sound attenuation, water resistance, and flame retarding, heat transfer resistance, elastomeric performance enhancers, strength modification, weather stabilization, improved esthetics, and phosphorescence, photochromatic or polychromatic enhancements. 
     The flame retardant properties may be chemical, intumescent, expanding, natural or synthetic. Density modifiers and sound attenuators may include gasses, for example nitrogen, solids, liquids or nano-particles, and micro-fine particulate rubbers. Strength modification may be provided by fillers or modifiers that are metals, organic or inorganic compounds, including fibrous or synthetic fiber compounds, flakes or nano materials. Weather stabilization may include synthetic and natural light stabilizers for providing protection against infrared, visible and ultraviolet light. Other stabilizers may also be added for improved performance, such as free radical scavengers, oxygen scavengers, and the like. 
     As illustrated in the cross-sectional views of  FIG. 2 , plastic coating  56  is directly applied to the slurry boundary layer  52  that is formed on the external surface of mats  28  and  32 . Polymer coating  56  can be laid down by any of a wide variety of hot melt coating applications. Suitable coating applications are more fully described hereinafter in conjunction with  FIGS. 7-10 . In the preferred embodiment, polymer coating  56  is applied in a thickness of between 0.01 to 500 mils. 
     Although 0.01 to 500 mils is disclosed as preferred, different applications may require different thicknesses. For example, for glass reinforced gypsum boards used for sheathing, a coating of between 0.25 to 3 mil is preferred. Similarly, for more robust applications, for example, underlayment or shaft lining, the coating is increased to a range of from 1.8 g to 3.5 g by weight. For tile backing applications, which require more integrity and water resistance, a thicknesses of about 1-10 mils is required. For varying the level of smoothness in interior gypsum boards, a level 2 finish board may require a 0.25-0.9 mil coating; a level 3 finish may require a 1.0-1.9 mil coating; a level 4 finish may require a 2.0-2.9 mil coating; and a level 5 finish may require a 2.5-5.0 mil coating. Another very rugged, structural, or highly thermally insulating, product may have thicker coatings. The weight and the resulting thickness of the coating may be varied at will based upon the performance criteria required in the finished product and the inherent chemical and physical properties of the particular coating applied to a particular product. 
     A coating of between 0.01 to 500 mils will result in a building board with an exterior surface hardness of between 50 to 150 on the Rockwell hardness scale, or a minimum of about 15 to a maximum of about 70 on a Shore A and D hardness scale. The preferred water vapor permeability of the final coating  56  is anywhere between a minimum of about 0.01 (nearly water impervious) to a maximum of about 98 (nearly complete water transmission). Also, depending upon the intended use of the board, coating  56  may be applied with a topography that ranges from smooth to coarse, or even fluted to chamfered. Additives can also be included within polymer coating  56  to provide for a film translucence ranging from 0.001% to 100%. 
     Another possible improvement is providing sound attenuation by having offset patterns and or shapes or lines in the face and or back surfaces of boards  20 . Thus, when two or more boards  20  are placed in facing relation, the offset portions come into contact with each other, thereby creating air pockets between the two boards  20 . Air pockets provide excellent insulation and sound dampening as well as heat insulation for fire retardation purposes. 
     Additionally, injection of micro air bubbles or heat activated expanded polymeric cells or expanded polymeric cells into the molten thermoplastic prior to the thermoplastic coating  56  being applied to the polymer modified gypsum surface has resulted in another unexpected discovery of the invention herein disclosed. That is the entrained air bubbles or shallow cells can provide built in sound attenuation at the application surface of the composite board  20 . Introduction of any filler material into the molten thermoplastic results in the filler becoming permanently suspended within the thermoplastic, in either a molten or cooled state. Injecting micro air bubbles into the thermoplastic during the coating process to reduce density also results in significant sound attenuation of the exposed surface on which the coating  56  has been applied. Air as used herein should be understood to include any type of gaseous materials, for example, nitrogen or an inert gas. 
     Pre-Coating of Fibrous Mats 
     In a further embodiment of the present invention, fibrous mats ( 28  and  32 ) are pre-coated with a hot melt thermoplastic prior to embedment within dense slurry layers  46  and  48 . The thermoplastic coating  56  is thereafter applied over the outer surface of board  20  as described above. Pre-coating mats  28  and  32  yields a strong, light weight composite board with a chemical matrix that interconnects fibrous mats ( 28  and  32 ), dense slurry layers ( 46 ,  48 ), exterior gypsum coating  56 , and gypsum core  54 . 
     As noted above, a wide range of materials can be used for the filaments (both organic and inorganic) comprising fibrous mats  28  and  32 ; however, randomly aligned glass fibers are preferred. Similarly, mats  28  and  32  can be formed from continuous and/or non-continuous filaments. The individual fibers are held together with a binder, such as urea-formaldehyde. 
     Although both the upper and lower fibrous mats ( 28  and  32 ) can be pre-coated, the pre-coating is preferably limited to lower mat  32 . This is because lower mat  32  underlies the facing side board  20  and therefore has greater strength requirements. Any of a variety of plastics can be used to pre-coat the fibers of mat  32 . Suitable plastics include any of the polymers described above in connection with external plastic coating  56 . For sake of brevity, this list has not been duplicated, but is instead incorporated by reference. In the preferred embodiment, the plastic pre-coating is a hot melt thermoplastic with a melting point of between 100° F. and 500° F.; and more specifically, either an ethylene vinyl acetate (EVA) or ethylene methyl acrylonitrile (EAA). 
     The thermoplastic pre-coating can be applied in a smooth layer or in a layer the follows the topography of the underlying mat. In the preferred embodiment, the thermoplastic is applied in a coating that is between 0.1 to 10 mils thick. The thermoplastic pre-coating can also be filled, or not filled, colored or translucent. 
     Prior to the pre-coating being applied, mat  32  is treated with a combination of acid, isopropyl alcohol, and a silane coupling agent. This pre-treatment step effectively prepares the individual fibers of mat  32  for the subsequent thermoplastic coating. The acid of the pre-treatment step activates binders present within the mat  32  to thereby facilitate bonding. The acid also permits the fibers of mat  32  to bind with the silane coupling agent. The silane coupling agent, in turn, ensures a tight bond between the individual fibers of mat  32  and the surrounding thermoplastic. Silane is a known coupling agent that facilitates bonding between polymers and glass fibers. Silane is also known as a stabilizing agent. Suitable silane compounds are sold by Dow Corning. 
     Following the pre-treatment step, the thermoplastic pre-coating is applied in a liquefied state. Any of the hot melt coating techniques depicted in  FIGS. 7-10  can be utilized in this step. As noted in  FIG. 6 a   , the pre-coating adheres to the individual fibers of mat  32  so as to maintain the porosity of mat  32 . Thus, even with precoating applied, mat  32  can be embedded within dense slurry layer  48 . Once applied, the silane promotes bonding between the individual glass fibers and the surrounding thermoplastic. In this manner, the hot melt thermoplastic pre-coating forms a strong chemical and mechanical bond with the individual fibers of mat  32 . 
     The silane facilitates other bonding as well. Namely, the silane promotes bonding between the thermoplastic pre-coating and binders present within mat  32 . Bonding is also promoted between the thermoplastic pre-coating and the polymers present within dense slurry layers ( 46 ,  48 ), gypsum core  54  and the external thermoplastic coating  56 . Still yet further bonding is promoted between the thermoplastic pre-coating and the calcium and sulfur within the surrounding gypsum. The result is a truly composite building panel, with all components of board  20  being chemically and mechanically bound together. Moreover, the composite panel can be achieved with or without the thermoplastic surface coating described above. 
     The present invention contemplates the complete replacement of traditional thermal setting binders (used to bind an organic and/or inorganic fibrous mat) with a hot melt thermoplastic polymer. An entirely thermoplastic bound fiber mat, while expected to improve manufacturing costs, has unexpectedly offered further advantages. These newly discovered advantages range from significantly improved “X”, “Y”, and “Z” axis strengths of the mat, as well as substantial ductility and flexibility improvements above those seen in mats incorporating traditional thermal setting binders. 
     The present inventors have further discovered that organic and/or inorganic fibers that are preconditioned with silane prior to the application of the hot melt thermoplastic binder, result in increased mat strength. The silane preconditioning also increases the bond at the interface between the hot melt thermoplastic and the organic and/or inorganic fibers. This, in turn, provides a “stretch” like characteristic to the mat comparably similar to that of organic muscle fibers and the known molecular memory pattern propensity for elastomeric stretch and precise rebound. 
     In addition to the foregoing, various fillers and additives can be included with the thermoplastic pre-coating in order to impart desired physical characteristics. For example, the pre-coating can be compounded with a gas to volumize the plastic and thereby reduce the amount of plastic needed to completely coat mat  32 . Still yet other fillers can be applied to create cost savings. As those skilled in the art will realize, other additives can be included within the pre-coating to yield other properties and for instant product variation. The fillers can also be pretreated with silane upon incorporation into, or compounding with, the hot melt thermoplastic. Such pretreated fillers will be mechanically bound by the surrounding thermoplastic and will also be subsequently chemically bonded and locked into place. 
     Finally, it should be noted that the pre-coating of mats ( 28 ,  32 ) can be carried out with or without application of external plastic coating  56 . For instance, the pre-coated mats described herein (i.e. mats singularly coated with thermoplastic, or coated with a thermoplastic that includes silane and/or fillers) can be used in connection with conventional gypsum faced boards. Alternatively, the pre-coated mats can be used in conjunction with cellulose or paper faced boards. In short, the pre-coating can be beneficially incorporated into any conventional building board construction. 
     Manufacturing Methods 
     Suitable manufacturing methods for the building board of the present invention are depicted in  FIGS. 3-6 .  FIG. 3  discloses a plant with a particular process  58  for compiling composite board. This process is more fully described in commonly owned U.S. Pat. No. 6,524,679 to Hauber, the contents of which are fully incorporated herein. A brief review of this preferred manufacturing process is provided herein for sake of clarity. Those of ordinary skill in the art will recognize other manufacturing methods that are suitable for producing plastic coated boards in accordance with the present invention. 
     Process  58  includes a first supply roll  60  of fiber mat that forms lower fibrous mat  32 . As noted in the detailed view of  FIG. 4 a   , mat  32  is a porous structure formed from a grouping of individual fibers. Although fibers are preferred, longer fiber rovings can also be employed. After being rolled out, upper and lower creaser wheels  62  are used to form the first and second folds ( 34 ,  36 ). These folds ( 34 ,  36 ), in turn, create both upturned sides  38  and the inward edges  42  of lower mat  32 . The upturned sides  38  serve to create a channel for receiving gypsum slurry core  54 . 
     The first gypsum layer  48  is thereafter deposited unto mat  32 . The gypsum slurry is supplied from mixer  68  and slurry outlet  64 . The slurry dispensed from this outlet  64  is preferably a denser gypsum slurry. The slurry is deposited directly upon the lower mat  32  over a forming table  66 . Thereafter, roll coaters  72  are used to press the deposited slurry  48  through lower mat  32 . The porosity of mat  32  allows the dense slurry layer  48  to completely penetrate and coat both the interior and exterior surfaces of mat  32 .  FIG. 4 b    depicts mat  32  embedded within the slurry  48  after passage through the roll coaters  72 . Ideally, a thin layer of dense gypsum  48  adheres to both the interior and exterior surfaces of mat  32 . 
     Core slurry layer  54  is thereafter deposited at a downstream location via a second slurry outlet  74 . Core slurry layer  54  comes from the same mixer  68  as dense slurry layer  48 . However, as noted above, core slurry layer  54  is preferably less dense than the upper and lower slurry layers ( 46 ,  48 ) and forms the majority of the resulting panel  20 . One or more vibrators  76 , which are positioned between adjacent forming tables  66 , are used to force air voids from the deposited gypsum slurry. 
     An additional supply of fiber mat  28  is then supplied from a second supply roll  78 . Fiber mat  28  forms the upper fibrous mat of composite board  20 . A third slurry outlet  82  is included for delivering a dense upper slurry layer  46  to fiber mat  28 . This is done upon a separate forming table  84 . Mat  28  is guided to forming table  84  via a series of guide rollers  88 . A roller coater  86  is used to press dense slurry layer  46  through the porous fiber mat  28  so as to coat the internal and external surfaces. The coated fiber mat  28  is thereafter routed to the upper surface of core gypsum  54  via guide rollers  88 . Upper mat  28  is then secured to the core slurry layer  54  and lower slurry layer  48  at a forming station. The forming station includes a forming plate  92  with an associated hinge  94 . Forming station ensures that the various layers are pressed together to create a smooth composite gypsum board  20  with a uniform thickness.  FIG. 4 c    is a detailed view showing the assembled components of board  20 . As more fully described in the Hauber patent, a downstream edging assembly  96  can be used to smooth the edges of board  20 . 
       FIG. 5  illustrates a modified process wherein mat  32  is first pre-coated with a thermoplastic layer. This pre-coating process is described in greater detail hereinabove. Other than the pre-coating step, the process depicted in  FIG. 5  is the same as that depicted in  FIG. 3 . Pre-coating station  61  is located immediately downstream from supply roll  60 . Thus, as mat  32  is unwound from supply roll  60 , it is routed to station  61  via a series of guide rollers. At station  61 , a series of roller coaters are used to apply the liquefied hot melt thermoplastic pre-coating. This pre-coating is applied in a continuously flowing and uniform layer to the individual fibers of mat  32 . The thermoplastic then immediately cools, thereby eliminating any drying steps.  FIG. 6 a    shows the individual fibers of mat  32  coated with the pre-coating.  FIG. 6 b    is a detailed view showing the coated fibers after the slurry  48  has been applied. As noted, the pre-coating ideally leaves gaps between the individual fibers to allow for the passage of slurry  48 . Finally,  FIG. 6 c    is a cross sectional view showing the mats  28 ,  32 ), along with core  54  and upper and lower dense slurry layers ( 46 ,  48 ). 
     An additional station (not shown) can also be included for the pre-treatment step. Namely, a coater can be positioned upstream of station  61  to apply the acid, isopropyl alcohol and silane coupling agent as detailed above. Alternatively, the pre-treatment step can be carried out prior to mat  32  being wound onto supply roll  60 . 
     Hot Melt Applicator 
     After the forming steps are completed board  20  is then routed to a downstream hot melt applicator for application of polymer coating  56 . Polymer coating  56  can applied to board  20  prior to the curing of slurry layers ( 46 ,  48 ,  54 ) and prior to board  20  passing through associated dryers. Application of polymer coating  56  prior to curing allows for increased mechanical and chemical bonding as coating  56  dries together with underlying slurry layers ( 46 ,  48 ,  52 ,  54 ), thereby resulting in a stronger and more durable surface coating. 
     Alternatively, polymer coating  56  can be applied to board  20  after the slurry layers ( 46 ,  48 ,  52 ,  54 ) have been cured and dried. This can also provide a strong bond between coating  56  and the underlying boundary layer  52 . The bond is strong enough such that coating  56  is considered to be integral with board  20 . 
     In either event, prior to application of polymer coating  56 , board  20  passes through a cleaning station. The cleaning station ensures that dust and debris are removed prior to application of polymer coating  56 . Polymer coating  56  can thereafter be applied by any of a wide variety of applicator methods. Ideally, the applicator heats the polymer coating  56  to a temperature of between 100 to 500° F. to thereby keep the polymer in a liquid state during application. Various applicators are disclosed in  FIGS. 5-7 . Although  FIGS. 5-7  disclose single applicators, it is understood that a series of coating applicators, either in series or parallel, or a combination of serials and parallel, can likewise be used. 
     One suitable applicator is the roller coater depicted in  FIG. 5 . Roller coater  98  includes both a metering roller  102  and an adjacent application roller  104 . Roller  104  is spaced on top of the board assembly line, preferably at a height that matches the thickness of board  20 . A supply of heated, liquefied polymer is delivered to the gap between the metering and application rollers ( 102 ,  104 ). Metering roller  102  ensures that a uniform layer of polymer is adhered to application roller  104 . Application roller  104  is heated via internal heating elements (not shown). For instance, a heated oil may be passed through piping internal to the roller. This ensures that the temperature of the polymer is maintained and prevents premature curing or solidification. Application roller  104  then applies an even layer of liquid polymer to the dense slurry boundary layer  52  of the underlying board  20 . A support roller  106  is utilized beneath application roller  104 . The thickness of the polymer layer is determined by the gap between the metering roller  102  and the application roller  104 . In the preferred embodiment, upon curing, polymer layer  56  is between 0.01 to 500 mils. The advantage of the roller coater  98  is that application roller  104  is in contact with board  20  for only a short period of time. Thus, the underlying slurry is not exposed to high temperatures for prolonged periods of time. This helps avoid unwanted calcination of the underlying gypsum slurry. 
       FIG. 6  discloses a curtain type coater  108 . This coater includes a heated applicator  112  with an elongated slot or opening  114 . The width of slot  114  is made to match the width of the board being produced. However, slightly increasing or decreasing the width of the slot can impart beneficial effects, such as smoothing, texturing, or building up architectural features. A supply of liquid polymer is delivered under the force of gravity through the slot  114  and is directly deposited upon the dense slurry boundary layer  52 . In this embodiment, the thickness of the polymer coating  56  is determined, in part, by the size of the applicator opening  114  and the speed of the underlying conveyor. 
     In  FIG. 7  a knife or edger coater  116  is depicted. Knife coater  116  includes an outlet supply  118  for the liquefied polymer. This outlet supply  118  delivers a volume of liquid polymer directly onto the dense slurry boundary layer  52 . A dam is thereafter created as the liquid polymer encounters scraper  122 . Scraper  122 , which may be heated, is then used as an applicator to apply a uniformly thin layer of polymer to board  20 . In this case, the thickness of the polymer coating  56  is determined by the spacing between scraper  122  and the underlying board. Finally,  FIG. 10  illustrates the application of coating  56  via a conventional spray coater  124 . 
     Other applicator types, beyond those described above, can also be utilized. For example, polymer coating  56  can be applied by a hot melt flood coater, a hot melt submersion coater, a hot melt co-extrusion coater, or any combination of the foregoing. 
     Regardless of the application method, the result is a continuous process for applying a polymer coating  56  to the surface of board  20 . Moreover, coating  56  is applied in a continuously flowing and uniform layer to the entire surface of board  20 . The thermoplastic then immediately cools, thereby eliminating any drying steps. The physical characteristics of the boards being produced can be easily modified during production by changing the polymer or by the inclusion of polymer additives. This enables a variety of different boards, each having unique physical characteristics for differing applications, to be advantageously produced on a single production line. 
     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.