Abstract:
The disclosed invention consists of an improved gypsum based, cellulosic containing board and method for applying a resin to an untreated board at a spray station where pMDI resin is sprayed onto the front and back side of the board. A resin distribution system is used to supply the spray nozzles with pMDI. Optionally, a second spray station is included, if desired, to add additional pMDI resin over the surface of the board to achieve complete coverage. The improvement is an increased water resistance and flexural strength.

Description:
This application is a divisional application of application Ser. No. 10/251,461 filed on Sep. 20, 2002 now U.S. Pat. No. 6,951,586, which is a divisional application of application Ser. No. 09/732,608 filed on Dec. 8, 2000, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to the ability to provide a uniform application of polymeric diphenylmethane diisocyanate (pMDI) onto cellulose gypsum panels, boards and other surfaces, to create a substrate with increased strength and water resistance. 
     Exterior wall cladding is used as a barrier to keep exterior air and moisture out of the wall cavity. If water and moisture penetrate the wall cladding surface damage will result to the cladding board itself. Prior art exterior wall cladding was made out of gypsum sheathing or water-resistant gypsum board. It was found that the application of pMDI to a cellulose/gypsum based board greatly increased the board&#39;s strength and water resistance. The disclosed invention applies the pMDI to the cellulose/gypsum based board with an apparatus that provides a uniform coating across the board which results in increased water resistance and flexural strength. 
     SUMMARY OF THE INVENTION 
     The disclosed invention consists of an improved cellulose/gypsum based board, and means for conveying a gypsum and cellulosic board or panel to a spray station where pMDI resin is delivered through a series of spray nozzles to the face and back of the gypsum board or panel. A resin distribution system is used to supply the spray nozzles with pMDI. Optionally, to assist in the spreading of the pMDI resin over the surface of the cellulose/gypsum board to achieve complete coverage of the cellulose/gypsum-based substrate, a second spray system can be included. The nozzles of the second spray system may be adjusted to cover areas of the face and back of the board that are not covered by the first spray system. The resulting panel exhibits dramatically improved water resistance and flexural strength. Atmospheric moisture is sufficient to cure the pMDI matrix. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing illustrating a production line for forming cellulose/gypsum board having a head box, dewatering vacuums, a dewatering primary press, a secondary press, a drying kiln and a resin distribution system all for processing a rehydratable gypsum fiber slurry upon a conveyor, 
         FIG. 2  is a front view of the resin distribution system including a resin drum, a metering pump and the spray system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is directed to an improved cellulose/gypsum based board and to a method for applying polymeric diphenylmethane diisocyanate (pMDI) to a cellulose/gypsum based board, and in particular, the use of one or more spray systems to provide a uniform application of pMDI onto the cellulose/gypsum based board. The forming system, generally designated with the numeral  10  and shown in  FIG. 1 , includes a head box  12 , vacuum boxes  14 , a wet (primary) press  16 , a secondary press  18 , and a drying kiln  20 . The function of the primary press  16  is 1) to nip a gypsum/cellulose fiber filter cake mat to a desired thickness and 2) to remove 80-90% of remaining water. The function of the secondary press  18  is to compress the board during setting to a calibrated final thickness and to aid in achieving flexural strength in the final product. The secondary press  18  has a continuous belt  22  that also aids in achieving smoothness to the board surface as the rehydrating mat expands against the belt  22 . The head box  12  is used to uniformly disperse a calcined slurry having at least about 70% liquid by weight, across the width of the forming table  24 , where vacuum boxes  14  are used to dewater the slurry into a mat of generally 28-41% moisture content (wet basis) (40-70% moisture content on a dry basis). The forming table  24  includes side dams to contain the slurry pond and a conveyor or forming wire  26  to move the slurry away from the head box  12  and towards the primary press  16 . As the slurry moves along the forming table  24 , the vacuum boxes  14  dewater the slurry into a mat, creating a decreasing water content gradient in the slurry going from the head box  12  towards the primary press  16 . At some point along this gradient, there is a zone referred to as the wet line, where it is observable that the slurry is changing into the wet mat. Put another way, one can see that the slurry is no longer fluid as the water is removed. 
     In the preferred embodiment, the slurry pond is further dewatered and formed into a filter cake by the application of additional vacuum boxes  14 . With reference to  FIG. 1 , the conveyor or forming wire  26  carries the filter cake to the primary press  16  which further dewaters the filter cake and nips the material to a desired thickness. During this time, the board begins setting and expands to fill the nip gap. The board exits the primary press  16  and is carried on the conveyor  26  to the secondary press  18 . The secondary press  18  shapes the board to a final calibrated thickness. The board expands against the smooth belt  22  of the secondary press  18  which further aids in rendering a smooth surface and increased flex strength. 
     After exiting the secondary press  18 , the board is dried in a kiln  20 . A non-aqueous pMDI resin is spray-applied to the face and backside of the cellulose/gypsum board by using a spray system  28  that sprays at a preferable rate from about 9 to about 25 pounds per 1000 square feet of cellulose/gypsum board. The pMDI penetrates efficiently into the board. As the pMDI migrates through the board, a reaction takes place between water that is in the ambient air, plus any remaining/evaporating water in the board, and the pMDI that permeates into the board. The interaction between the pMDI and the water transforms the pMDI into polyurethane, which forms urethane linkages with the cellulosic fibers at and slightly below the surface of the board to seal the faces of the board. The polyurethane does not increase the overall thickness of the board but rather seeps into the board. The resin applied to the board by the spray system  28  thus does not remain suspended to cure as a mere coating on the surface due to the polymeric resin, like pMDI, interacting with the cellulosic fibers. Water from the ambient surroundings is sufficient to start the curing of the pMDI, and, thus the resin is applied to a dried board, which may have a small percentage of evaporating remaining free water that has not yet evaporated. The non-aqueous resin soaks into the board and reacts with the cellulosic fibers in the board. A polyurethane/cellulose matrix is formed. By treating the entire cellulose/gypsum board with pMDI, a polyurethane/cellulose matrix is formed that completely seals the board. 
     The resultant cellulose/gypsum board treated with the pMDI has an increase in flexural strength of 20-35% over the non-treated board. The typical curing time to allow for complete transportation of the pMDI into the polyurethane cellulose matrix within the board is approximately three days, but may vary depending upon ambient conditions. 
     The polyurethane/cellulose matrix formed does not increase the overall thickness of the board. The matrix becomes a water resistant layer of the board that is approximately ⅛ inch thick. A cellulose/gypsum board treated with pMDI on one side, allowed to cure, and completely submersed in water resulted in the deterioration of the untreated portion of the board. The treated portion of the board remained intact and was about ⅛ inch thick. 
     A water absorption test was performed on the surface of both an untreated cellulose/gypsum board and a board treated with pMDI to determine the quantity of water absorbed by the board. During the test, 100 square centimeters of the surface of the board was subjected to 100 milliliters of 70° F. water for two hours. The untreated board absorbed 92-100 grams of water during the two hour test period. The board treated with pMDI absorbed 0.5 grams of water for the 2 hour test period which is well below the acceptable limit for exterior cladding. Boards treated with pMDI were more scuff resistant than untreated boards and were less dusty when handled. These desirable qualities are beneficial because they enhance the marketability of the resultant product. 
     The spray system  28 , as shown in  FIG. 2  includes a horizontal spray bar  30  equipped with equally spaced spray nozzles  32 , a manifold  34 , feed tubes  36 , a filtering system  40 , a positive displacement pump  38  and a storage container  42 . The spray bar  30  is an elongated tube that spans the width of the board. Typical sheets of cellulose/gypsum board are 48 inches in width. In the preferred arrangement, the spray nozzles  32  are attached to the spray bar  30  in three inch intervals. It has been found that placing the spray nozzles  32  three inches apart provides for enough spray overlap to adequately wet the board with pMDI. The spray nozzles  32  spray in a fan pattern and are positioned 8-10 inches above the board. Placing the nozzles  32  close to the board reduces the amount of overspray that is typically associated with spray systems. The spray nozzles  32  are not air assisted since it is desirable to reduce atomization of the pMDI so overspray can be kept to a minimum. Overspray decreases the pMDI transfer rate onto the board, which increases the amount of pMDI required to coat the cellulose/gypsum board and the amount of overall product required. The spray bar  30  is connected to the manifold  34  that delivers pMDI to different locations on the spray bar  30  by use of feed tubes  36 . The feed tubes  36  are vertically oriented and connect the spray bar  30  to the manifold  34 . The manifold  36  is supplied with pMDI under pressure from a positive displacement pump  38 . The pump  38  is connected to the storage container  42  by use of a supply line  44 . The supply line  44  also connects the pump  38  to the filter system  40  and the filter system  40  to the manifold  34 . The storage container  42  is typically a storage drum that is positioned upon a drum cart  46 . The storage container  42  also includes a valve  48  and a breather  50  to allow for the removal of pMDI. The breather  50  is utilized to allow air to displace the pMDI removed from the storage container  42 . The pump  38  is adjusted to the desired flowrate and pumps the pMDI through the filter system  40  and to the manifold  34 . The filter system  40  includes two filters  41  connected in parallel to filter out any particles that may clog the nozzles  32 . The filter system  40  is equipped with valves  52  to allow the supply line  44  to be closed off to prevent the leakage during the replacement of the filters  41 . By utilizing two filters  41  that are large enough handle the flowrate of the pMDI from the pump  38 , one filter  41  can be taken off-line for a filter replacement while the other filter  41  remains in service. 
     The invention is also useful for paper coated gypsum board wherein the paper provides the cellulosic fibers for forming the urethane linkages with the curing pMDI. 
     Various features of the invention have been particularly shown and described in connection with the illustrated embodiments of the invention. However, it must be understood that these particular arrangements, and their method of manufacture, do not limit but merely illustrate, and that the invention is to be given its fullest interpretation within the terms of the appended claims.