Patent Publication Number: US-6217946-B1

Title: Method for applying polymeric diphenylmethane diisocyanate to cellulose/gypsum based substrate

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to the ability to provide a uniform application of polymeric diphenylmethane diisocyanate (pMDI) onto gypsum boards, cellulose gypsum panels and other surfaces. More particularly, the present invention relates to the use of rotary cylinder brush technology to provide a uniform application of pMDI onto cellulose/gypsum based substrates. 
     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 gypsum board greatly increased the board&#39;s strength and water resistance; however, early attempts at applying pMDI to cellulose/gypsum based substrate membranes have met with little success. The prior art method of applying the pMDI was to use a spray apparatus which atomized the pMDI so it could be applied to gypsum board. The spray technique has several problems. First, the spraying of the pMDI results in a non-uniform application of the coating which prevents the achieving uniform water resistance across the gypsum board. Second, the atomization of pMDI creates a health concern by introducing small particles of pMDI into the air that can be inhaled by persons in the vicinity. The disclosed invention applies the pMDI with an apparatus that provides a uniform coating across the gypsum board which results in an increase in water resistance. 
     SUMMARY OF THE INVENTION 
     The disclosed invention consists of a means for conveying a gypsum board or panel to a rotary cylinder brush station where pMDI resin is delivered onto the rotary cylinder brush just as the panel passes under the brush. A resin distribution system is used to coat the rotary cylinder brush applicator with pMDI. Optionally, to assist in the spreading of the pMDI resin over the surface of the gypsum board to achieve complete coverage of the cellulose/gypsum based substrate, a second rotary cylinder brush can be included. The bristles of the second rotary cylinder brush may be finer than the bristles of the first rotary brush. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing illustrating a production line for forming gypsum fiberboard having a head box, dewatering vacuums, a dewatering primary press, a secondary press, and a drying kiln all for processing a rehydratable gypsum fiber slurry upon a conveyor; 
     FIG. 2 is a perspective view of the first brush station of the present invention having a gearmotor drive and a rotary applicator brush; 
     FIG. 3 is a perspective view of the an optional second embodiment of the invention including a first application brush station and a second smoothing brush station, the first station including a gear motor drive, a resin distribution system and a first application brush, the second smoothing station including a gear motor drive and a second smoothing brush; and 
     FIG. 4 is a front view of the resin distribution system of FIG. 3, including a resin drum, an application manifold and a metering pump. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is directed to a method for applying a polymeric solution for achieving water resistance, and preferably applying a resin such as polymeric diphenylmethane diisocyanate (pMDI) to a cellulose/gypsum based substrate, and in particular, the use of one or more rotary cylinder brushes to provide a uniform application of pMDI onto the cellulose/gypsum based substrate. 
     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 . After the board is completely dried, the conveyer  26  carries the board to the primary rotary brush station  28 , as best seen in FIGS. 2 and 3. Preferably, the primary rotary brush station  28  is comprised of an rotary cylinder brush  30  having bristles  32  and a resin distribution system  34 . One suitable brush for use as the brush  30  is made of nylon and manufactured by INDUSCO (Fairfield, N.J.). The function of the resin distribution system  34  is to continuously supply pMDI resin to the rotary cylinder brush  30 , as will be explained more fully below. The brush  30  is rotatably driven by a gearmotor  36 , such that as the board passes under the brush  30 , the bristles  32  of the brush  30  repeatedly come into contact with the board. The resin distribution system  34  continuously coats the bristles  32  of the rotary brush  30  with pMDI resin as the brush  30  rotates. As the bristles  32  coated with pMDI resin come into contact with the board, the pMDI resin is uniformly applied to the surface of the board. The amount of pMDI resin applied can vary, although preferably a range of 9-20 lbs. of pMDI resin is brush-coated per every 1,000 square feet of board. 
     In the exemplary embodiment, the rotary cylinder brush  30  includes a core  38 , an applicator sleeve  40  and bristles  32 . The core  38  is approximately fifty-one inches long, has an inner diameter of approximately two inches and an outer diameter of approximately three inches. The applicator sleeve  40  in the disclosed embodiment is approximately fifty inches long with an inner diameter that corresponds to the outer diameter of the core  38 , and an outer diameter of approximately six inches. The diameter, however, may vary depending upon the treated board texture, its width, and the application rate. All along the outer circumference of the applicator sleeve  40  are bristles  32 , preferably made of nylon. It is preferred that the bristles  32  be somewhere in the range of 0.011 inches to about 0.016 inches in diameter, although it is known that the diameter of the bristles  32  can range anywhere from 0.010 inches to 0.018 inches. 
     Although core  38  and applicator sleeve  40  are described as being fifty-one inches and fifty inches in length, respectively, it is known that this length can vary depending upon the width of the gypsum panels to be treated. 
     The resin distribution system  34 , as best seen in FIG. 4, includes a resin drum  42  for storage of the pMDI resin, and an application manifold  44  for spraying the resin onto the bristles  32  of the application brush  30 . The pMDI resin is conveyed from the drum  42  to the manifold  44  via a metering pump  46  through series of conduits. The pump  46  controls the amount of resin that is applied to the substrate. The application manifold  44  includes a series of sprayers  45  extending along the length of the application brush  30  to ensure that the bristles  32  of the brush are coated with the resin throughout the length of the brush  32 . The sprayers  45  of the exemplary embodiment are about 3 inches on center, but may vary depending upon the application rate. The sprayers  45  comprise nozzles, as shown in the figures, but other spray or non-spray fluid application means may be used to coat the brush  32 . For example, a perforated pipe is one alternative. 
     Optionally, a second (smoothing) rotary cylinder brush station  48  can be included adjacent to the first brush station  28  and subsequent to the first brush station  28  in the coating process, as illustrated in FIG.  3 . The structure of the second brush station  48  is similar to the first brush station  28 . However, preferably the bristles  50  of the smoothing brush  52  are finer than the bristles  32  of the application brush  30 . The smoothing brush  52  is used, if desired, to assist in the spreading and smoothing out of the pMDI resin over the surface of the gypsum and cellulose fiber board to achieve complete coverage of the gypsum board panel. 
     In a second application (not shown), the disclosed apparatus and method for applying pMDI resin is used to apply pMDI onto a gypsum board with paper, allowing the pMDI to absorb into the panel paper, thereby adding considerable strength to the wallboard when the pMDI is allowed to dry and set within the paper. The resultant gypsum boards with pMDI resin exhibit superior properties when compared to regular gypsum sheathing or water-resistant gypsum boards. 
     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.