Abstract:
A method of producing a ceramic device having thin walls of microporous ceramic material, channels open at both ends, and a large exposed internal surface area. The method comprises preparation of a unique bilayer film and assembly of this film into a ceramic device by rolling it upon itself into a spiral cylinder or by arranging it in stacks of sheets. One layer of the film is composed of ceramic powder in an organic binder; the other of a material which may be removed by physical or chemical means.

Description:
SUMMARY DESCRIPTION OF THE DEVICE PRODUCED BY THIS INVENTION 
     The ceramic device of this invention has a plurality of parallel walls connected to each other by spacers and arranged in a regular pattern which may be in the form of a spiral or be in the form of flat sheets stacked upon each other as shown in FIGS. 1-2 respectively. The arrangement of the walls forms a multiplicity of channels running through the device which are interrupted only by the spacers joining adjacent walls, and are open at both ends. Spacers connect adjacent walls at regular intervals through the structure to give the device integrity and strength. The spacers may have any cross section but circular or rectangular cross sections are preferred. They may be arranged regularly (e.g., in parallel rows) or irregularly. The composition of the walls and spacers may be of any desired ceramics: examples include petalite, cordierite, alumina silicates, alumina-silica-magnesia, zircon, mullite or alpha alumina. If desired the walls may have one or more layers of differing ceramic materials. 
     In the preferred embodiment of the invention the walls range between 0.005 inch to 0.012 inch in thickness and the distance separating walls is in the range 0.005 inch to 0.030 inch. The width of the spacers is in the range 0.010 inch to 0.300 inch: they may be of any desired length. This distance between spacers is in the range 0.010 inch to 0.300 inch. This invention is not however limited to producing a device with the dimensions here indicated. 
     The object of the invention is to provide a method of constructing a ceramic device which may serve as a catalyst support, whereby the device is formed in a substantially continuous process on conventional machinery with or without minor modifications obvious to those skilled in the art. It should be further understood that this ceramic catalyst support, on coating with an appropriate catalyst, for instance a metal or metals from Group IIA, IB, VB, VIB, VIIB, and VIII, could be suitable for use as a catalytic converter for the conversion of harmful automobile pollutants into innocuous materials. 
     Several patents describe methods of constructing a ceramic device having thin microporous walls and channels open at both ends. U.S. Pat. Nos. 3,824,196 and 3,983,283 describe the extrusion of structures containing many channels from a plastic composition of the desired ceramic. Other patents describe methods of producing sheets of ceramic. These sheets are then stacked together in a manner that provides for channels between adjacent sheets. In most cases the sheets must be wavy or corrugated. 
     U.S. Pat. No. 3,926,702 describes the preparation of sheets by preparing a fluid suspension of ceramic powder in a phenolic resin that has been dissolved in alchohol and flowing this into a wavy mold that is subsequently heated to drive off the alchohol, then further heated to fire the ceramic. The single sheet produced from the mold is combined with sheets produced similarly to produce the device. 
     U.S. Pat. No. 3,982,981 describes a method whereby sheets of ceramic are prepared in a continuous process on a fourdrinier paper machine from a suspension of ceramic powder and natural or synthetic fibers. After sheets so produced have been corrugated they are assembled with each other into a device. 
     U.S. Pat. No. 3,948,810 describes a device whereby sheets of ceramic (formed in a process not described in the patent) are assembled in layers alternating with layers of ceramic balls or sheres that have been formed in a separate process not specified in the patent. 
     U.S. Pat. No. 3,963,504 describes a process by which a thermoplastic composition of ceramic powder, high molecular weight polyolefin, and plasticizer are formed into a sheet by pressing between two platens in an hydraulic press. After the sheets are assembled together the plasticizer is removed by solvent causing the sheets to become inflexible. 
     The instant invention described herein differs from those mentioned in that it describes formation of a ceramic sheet by coating onto a substrate sheet. None of the patents cited make use of a coating process. Further the substrate sheets performs several unique functions: 
     1. It supports the ceramic film 
     2. In the second embodiment of the invention, it serves to define the form of the ceramic sheet (the production of extensions in in the film that become spacers). 
     3. When the sheets are assembled into a device, it serves to space the sheets of ceramic equidistant and prevent their premature collapse by physically occupying the space that will remain open in the finished device. 
     4. In the first embodiment, it physically occupies the space that later will become open; thus allowing spacers to be produced by the technique to subsequently be described herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view of a ceramic device made by rolling a bicomponent sheet upon itself. 
     FIG. 2 is a view of a ceramic device made by stacking bicomponent sheets one upon the other. 
    
    
     DESCRIPTION 
     The essence of the present invention lies in the construction of a unique bicomponent film. The film is formed by coating a suspension of ceramic powder in an organic binder over a film composed of wax, plastic, or paper. In the simplest embodiment of this invention, a fluid mixture of solvent, organic binder, and ceramic powder is coated onto a flexible sheet by any of the many methods common to the art. This flexible sheet may be self-supporting or be supported by a paper web. The flexible sheet may also be formed by coating the film-forming material directly onto a properly prepared paper web. 
     The solvent is evaporated off to provide a bicomponent film composed of a continuous layer of ceramic in binder and a continuous layer of the flexible sheet. The organic binder may be any one of a number of substances capable of being cured to a rigid insoluable form. Preferred are thermosetting resins such as phenolics, urea-formaldehyde, melamine-formaldehyde, and epoxy resins. The flexible sheet is composed of any one of many materials that either form films or are available commercially as films and can be removed by heat, solvents or chemical attack. Suitable materials include waxes and modified celluloses. The bicomponent film so produced may be rolled up into a spiral cylinder or cut into sheets and the sheets stacked together. 
     If a paper web has been used for support, this is separated and discarded. Paper coated with release agent is preferable since it facilitates this separation and may be re-used. Gaps are formed perpendicular to the plane of the sheets by drilling, punching, or sawing. These gaps are filled with a suspension of ceramic in binder preferably but not necessarily similar to that used in the coating process. 
     The ceramic in the gaps ultimately forms the supporting spacers. When these operations have been completed, the organic binder is cured to a rigid insoluable form. Following this, the removable layer is extracted typically, though not necessarily, by dissolution in a suitable solvent, or by melting. The structure so formed is heated to carbonize the binder and then heated further to fire the ceramic. Similar carbonization and firing sequences are described in U.S. Pat. No. 3,926,702 and other patents. 
     In a second embodiment of this invention, a unique bicomponent film is prepared having spacers as an integral part of the film rather than requiring their formation at a later stage, as described in the first embodiment of the invention. This film is produced by the following steps: 
     1. A sheet of suitable material is perforated. The composition of this sheet would correspond to that of the removable layer previously described. The perforations, whose shape determines the cross section of the spacers, may be of any design, but may conveniently be made circular or rectangular. 
     2. The sheet so prepared is laminated to a support sheet (for instance a paper web) in such a way that the two sheets may be separated by peeling them apart. This second sheet will be-called the release sheet. 
     3. A coating material is prepared from a suitable binder and selected ceramic powder and may include a solvent for the binder. This suspension is coated by conventional methods well known in the art on to the composite sheet prepared according to step two. The coating must remain fluid long enough for it to fill the perforations. The solvent is then removed by evaporation. 
     4. Prior to assembling the film into the structure of the support, the release sheet is removed as indicated in the description of the first embodiment. 
     5. The finished support is heated to carbonize the binder and then heated further to fire the ceramic. During the firing process the carbon burns off. 
     Bicomponent films of this type differ from these discussed in the first embodiment of the invention in having a multiplicity of ridges or pillars that penetrate the second layer and are flush with the surface of this layer. When this bicomponent film is rolled up or assembled into a stack of sheets these pillars or ridges make contact with the adjacent ceramic layer. During the curing process these points of contact band together, thus forming the spacer supports. In either embodiment, application of the ceramic binder layer is not limited to one coating pass. In some cases it may be desirable to have two ceramics in the finished support or may be desirable to use two different binders to facilitate the shaping of the support. Both of these objectives can be accomplished by applying a second coating over the first after the first layer is hardened. In another variation of this invention, wax is coated directly onto release paper. This wax may be coated using a special roll that leaves portions of the release sheet without a coating of may be coated solid and later perforated without perforating the release sheet. 
     EXPERIMENTAL 
     100 Parts of wt. of resin PMH707 were added to 45 parts of wt. of PMH707 hardener (Palmer Products, Inc.) and the resulting mixture dissolved in tolulene. To one part of the resin and hardener 4 parts by wt. of #613 calcined alumina A-10 (Whittaker, Clark, and Daniels, Inc.) were added. The alumina, resin and solvent were thoroughly mixed to produce a coating slurry. An 0.018 inch thick beeswax sheet from A.I. Root Co. was perforated with holes about 0.035 inch in diameter and spaced about 0.250 inch apart in a staggered pattern using a hollow punch. 
     Layers of tape were placed 0.500 inch apart on the punched out section of the beeswax sheet and the sheet was placed on release paper. The coating was spread between the pieces of tape by use of a stiff blade. The coating was about 0.009 inch thick. After the solvent was evaporated, the coated area was cut from the beeswax sheet and wrapped around a 0.500 inch spindle. The sample had seven layers. The epoxy was allowed to harden at room temperature for 10 days. The sample was put in an oven at 200° F. and the wax melted out. Subsequently the binder was carbonized at 500° F. 
     The resulting sample was circular and measured 13/16 inch in diameter, had a 1/2 inch open area in the center, and was 3/8 inch high. The sample had six channels which were open along their entire length except for their spacers.