Process for making a chemically-resistant porcelain enamel

Abstract

A process for preparing a chemically resistant coating in which a substrate is sequentially contacted with a ground coat, fired, contacted with an intermediate coat, fired, and contacted with a cover coat and fired. The intermediate coat is comprised of a minor amount of inorganic fiber which has a softening point in excess of 950 degrees Centigrade, an average length of from about 100 to about 750 microns, an average diameter of from about . .515.!. .Iadd.5-15 .Iaddend.microns, and an average aspect ratio of from about 10:1 to about 75:1.

I claim: 
     
       1. A process for preparing a chemically-resistant coating upon a substrate, comprising the steps of sequentially: (a) contacting said substrate with a first slurry to produce a first coated substrate with a wet film thickness of from about 0.3 to about 0.75 millimeters, wherein: 1. said first slurry contains from about 60 to about 70 weight percent of solid material . .and has.!. .Iadd.comprising glass frit having .Iaddend.a particle size distribution such that at least about 5 weight percent of the particles in said first slurry are smaller than 44 microns and at least about 20 weight percent of the particles in said first slurry are larger than 150 microns,   2. said solid material in said .Iadd.glass frit in said .Iaddend.first slurry is comprised of, by weight of dry solid material, from about 48 to about 58 weight percent of silica, from about 12 to about 22 weight percent of boric oxide, from about 1 to about 9 weight percent of potassium oxide, and from about 1 to about 9 weight percent of alumina,     (b) drying said once coated substrate to a moisture content of less than about 10 weight percent, thereby producing a once dried substrate,   (c) subjecting said once dried substrate to a temperature of from about 810 to about 910 degrees Centigrade for at least about 20 minutes, thereby producing a once fired substrate,   (d) contacting said once fired substrate with a second slurry to produce a second coated substrate with a wet film thickness of from about 0.75 to about 1.5 millimeters, wherein:   
     
     
       1. said second slurry is comprised of from about 60 to about 75 weight percent solid material and has a particle size distribution such that at least 10 weight percent of the particles in such second slurry are larger than 150 microns and less than about 10 percent of the particles in such second slurry are smaller than 44 microns, 2. said solid material in said second slurry is comprised of from about 2 to about 10 weight percent of inorganic fiber by total weight of solid material and at least about 90 weight percent of frit, wherein: (a) said frit is comprised, by dry weight of frit, of from about 68 to about 74 weight percent of silica, from about 0.5 to about 2.5 weight percent of alumina, from about 7 to about 15 weight percent of sodium oxide, from about 1 to about 5 weight percent of lithium oxide, and from about 2 to about 9 weight percent of zirconium oxide,   (b) said inorganic fiber has a softening point in excess of 950 degrees Centigrade, an average length of from about 100 to about 750 microns, an average diameter of from about 5 to about 15 microns, and an average aspect ratio of from about 10:1 to about 75:1,     (e) drying said twice coated substrate to a moisture content of less than about 10 weight percent, thereby producing a twice dried substrate,   (f) subjecting said twice dried substrate to a temperature of from about 750 to about 850 degrees Centigrade for at least about 20 minutes, thereby producing a twice fired substrate,   (g) contacting said twice fired substrate with a thrice slurry which is substantially identical to said twice slurry with the exception that it does not contain any of said inorganic fiber, wherein to produce a thrice coated substrate with a wet film thickness of from about 0.75 to about 1.5 millimeters, wherein   
     
     
       1. said third slurry is comprised of from about 60 to about 75 weight percent solid material and has a particle size distribution such that at least 10 weight percent of the particles in such third slurry are larger than 150 microns and less than about 10 percent of the particles in such third slurry are smaller than 44 microns, (h) drying said thrice coated substrate to a moisture content of less than about 10 weight percent, thereby producing a thrice dried substrate, and   (i) subjecting said thrice dried substrate to a temperature of from about 740 to about 840 degrees Centigrade for at least about 20 minutes.   
     
     
       2. The process as recited in claim 1, wherein each of said first slurry, said twice slurry, and said thrice slurry is comprised of water. 
     
     
       3. The process as recited in claim 1, wherein said solid material in said first slurry is comprised of from about 1 to about 6 weight percent of cobalt oxide. 
     
     
       4. The process as recited in claim 1, wherein said solid material in said first slurry is comprised of from about 0.2 to about 4 weight percent of nickel oxide. 
     
     
       5. The process as recited in claim 1, wherein each of said first slurry, said twice slurry, and said thrice slurry is contacted with said substrate by spraying it onto said substrate. 
     
     
       6. The process as recited in claim 1, wherein said inorganic fiber consists essentially of amorphous silica. 
     
     
       7. The process as recited in claim 6, wherein said inorganic fiber has a coefficient of thermal expansion of from about 0 to about 20×0.0000001 centimeters per centimeter per degree Centigrade. 
     
     
       8. The process as recited in claim 6, wherein said inorganic fiber has a coefficient of thermal expansion of from about 4 to about 10×0.0000001 centimeters per centimeter per degree Centigrade. 
     
     
       9. The process as recited in claim 6, wherein said inorganic fiber has an aspect ratio of from about 15:1 to about 25:1. 
     
     
       10. The process as recited in claim 1, wherein said substrate is comprised of a substantially planar surface. 
     
     
       11. The process as recited in claim 1, wherein said substrate is comprised of a substantially concave surface. 
     
     
       12. The process as recited in claim 1, wherein said substrate is comprised of a substantially convex surface. 
     
     
       13. The process as recited in claim 1, wherein said twice slurry is comprised of an alcohol of the formula ROH wherein R is lower alkyl containing from about 1 to about 3 carbon atoms. 
     
     
       14. The process as recited in claim 13, wherein said thrice slurry is comprised of an alcohol of the formula ROH wherein R is lower alkyl containing from about 1 to about 3 carbon atoms. 
     
     
       15. The process as recited in claim 14, wherein said alcohol is ethanol.