MXPA06002737A - Thermal protective coating for ceramic surfaces - Google Patents

Abstract

A coating admixture, method of coating and substrates coated thereby, wherein the coating contains colloidal silica, colloidal alumina, or combinations thereof;a filler such as silicon dioxide, aluminum oxide, titanium dioxide, magnesium oxide, calcium oxide and boron oxide;and one or more emissivity agents such as silicon hexaboride, carbon tetraboride, silicon tetraboride, silicon carbide, molybdenum disilicide, tungsten disilicide, zirconium diboride, cupric chromite, or metallic oxides such as iron oxides, magnesium oxides, manganese oxides, chromium oxides, copper chromium oxides, cerium oxides, terbium oxides, and derivatives thereof. In a coating solution, an admixture of the coating contains water. A stabilizer such as bentonite, kaolin, magnesium alumina silicon clay, tabular alumina and stabilized zirconium oxide is also added.

Description

THERMAL COATING PROTECTOR FOR CERAMIC SURFACES RELATED REQUESTS This application claims the benefit of the non-provisional US patent application serial number 10 / 657,850, entitled "PROTECTIVE THERMAL COATING FOR CERAMIC SURFACES", filed on September 9, 2003, the content of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION The present invention relates to a protective coating for use in a ceramic substrate, and more particularly to a thermal protective coating, and to ceramic substrates coated therewith, whose coating provides excellent resistance to high temperatures and repeated thermal shock at elevated temperatures. .

BACKGROUND OF THE INVENTION Protective coatings for use in numerous substrates are known in the prior art. For example, U.S. Patent No. 5,296,288, the content of which is incorporated herein by reference in its entirety, issued March 22, 1994, discloses a protective coating for ceramic materials and a thermal control structure comprising a ceramic material that has the protective coating on it. The protective coating includes, in admixture, powder of silicon dioxide, colloidal silicon dioxide, water and one or more emittance agents selected from the grconsisting of silicon tetraboride, silicon hexaboride, silicon carbide, molybdenum disilicide, Tungsten isilicide, and zirconium diboride. That invention has the disadvantage that the coating has to be used immediately after the composition is prepared. Previous efforts have been made to generate protective coatings having high emissivity characteristics for use on metal surfaces. U.S. Patent Nos. 5,668,072 ('072), and 6,007,873 ('83), issued respectively on September 16, 1997 and December 28, 1999, present a coating composition with high emissivity, and methods of use for coating the interior of ovens, in which the coating composition includes an agent with high emissivity such as a rare earth oxide and a binding agent. The preferred emissivity agent is cerium oxide, or related agents which include mixed oxides of cerium oxide and precursors. Terbium can be replaced by cerium. The binder, which is also used as a suspending agent, includes a solution of aluminum phosphate, peptized aluminum oxide monohydrate, and ethyl alcohol. The inventions of '072 and' 873 make use of organic substances that potentially increase the amount of fumes generated during heating. U.S. Patent No. 4,810, 300 ('300) issued March 7, 1989, discloses a composition for producing an adherent and water-insoluble sediment on the surfaces of the substrate, whose sediment is used for inks, paints and the like. The coating material for the substrate surfaces includes at least water, a pre-reacted lithium silicate and an unreacted lithium hydroxide monohydrate. Preferably, the liquid phase contains a dispersant in the clay form. An appropriate pigment or other refractory material such as graphite, oxides, borides, nitrides, carbides, sulfides, metals and mixtures thereof, may also be incorporated therein. The effective temperature range of the coating material is up to approximately 2000 ° C. The invention '300 adheres to metal surfaces, but does not provide thermal protection to the underlying surface. U.S. Patent No. 5,569,427 ('427) issued on October 29, 1996, discloses a high temperature coating for use in a ceramic substrate and a fireless process for obtaining the high temperature coating. The coating has an operating temperature of up to 1500 ° C. The coating of '427 is used immediately after it is ready, and is formulated for surfaces with ceramic substrate. U.S. Patent No. 6,444, 271 ('271) issued September 3, 2002, discloses a durable refractory ceramic coating having a silicide coating comprising a refractory metal and silicon, which combine to form a silicide. The coating described there is at least partially diffuse in the base structure of at least one surface. The structure of the base is a ceramic material, which preferably is a ceramic oxide material. The invention of '271 uses a polymeric stock solution for a carrier, to apply the coating, thereby potentially increasing the flammable nature of the stock solution. The use of bentonite in heat resistant coatings is also known. U.S. Patent No. 4,072,530 issued February 7, 1978 presents a refractory composition for coating furnace walls containing silicon carbide, a stabilized zirconium oxide or bentonite, a silicon dioxide, a hydrolyzate of poly (ethyl silicate) ), a sodium or aluminum phosphate silicate and water. None of the above inventions and patents, taken either singly or in combination, describe the present invention as claimed.

BRIEF DESCRIPTION OF THE INVENTION The present invention relates to a protective thermal coating containing colloidal silica, a filler such as a refractory material with fine particle size, one or more emissivity agents, and a stabilizer. The filler is taken from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide, magnesium oxide, calcium oxide, and boron oxide. The emissivity agents may be silicon hexaboride, carbon tetraboride, silicon tetraboride, silicon carbide, molybdenum disilicide, tungsten disilicide, zirconium diboride, copper chromite, or metal oxides such as iron oxides, magnesium oxides, manganese oxides, chromic copper oxides and other chromic oxides, and their derivatives. A clay stabilizer with large molecules, such as bentonite or kaolin, is added. In a coating solution according to the present invention, a mixture of the protective thermal coating contains colloidal silica, a filler, one or more emissivity agents, a stabilizer, and water. The stabilizer is added to extend the shelf life of the coating solution. A dye can be added to form a paint with thermal protection. Colloidal alumina can be added to the colloidal silica, or replaced by it. The present invention can be used to coat a variety of refractory materials that are comprised of ceramic composites. The coating provides a means to re-irradiate heat during exposure to temperatures ranging from 648¡.8 ° C (1200 ° F) to 1926.6 ° C (3500 ° F). Some main products are refractory bricks composed of primary ceramic fibers and various flexible compounds that contain cloths made of ceramic fiber. The coating is capable of withstanding repeated thermal shock without degradation, and has proven to prolong the working life of crucibles, incinerators, insulators, metallurgical ovens and reusable ceramic furniture that is subjected to high temperature conditions. The coating can be applied to substrates ceramic cloths, which are commonly used for welding and flame protection covers, and can also be used in sealing and insulation applications. The coating contains one or more emissivity agents, crystalline compounds and an amorphous matrix that constitutes a thermal protective system that has many potential uses. The coating has reduced surface temperatures from 148.8 ° C (300 ° F) to as high as 537.7 ° C (1000 ° F). Subsequently, the temperature of the back face versus the front face of a coated substrate has been reduced by as much as 371.1 ° C (700 ° F). One aspect of the present invention is to provide a protective thermal coating that has an extended shelf life. The addition of a stabilizer makes it possible to prepare a solution for coating according to the present invention, and to use it at a later date. Another aspect of the present invention is to provide a thermal protective coating that improves the optical properties, that is, by increasing the emissivity of the coated substrate, which improves the radiative heat transfer and reduces the catalytic efficiency of the. materials with ceramic substrate at temperatures substantially higher than the melting point or the dissociation point of the substrate material. This protection allows the exposure of the substrate materials to higher thermal conditions than would normally be allowed by the uncoated substrate, thereby extending the useful range of thermal conditions of the substrate material. A related aspect of the present invention is that the protective thermal coating extends the useful life of ceramic materials. The present invention is more cost efficient than uncoated ceramics. For example, the ceramic bricks that are used in metallurgical furnaces, undergo constant degradation and require replacement. The replacement of these ceramic materials is expensive. Ceramic bricks coated with the present invention require less frequent replacement, resulting in a significant reduction in the operating cost of a given furnace. The protective thermal coating of the present invention decreases the catalytic efficiency of the surface, the low catalytic efficiency is due to the low thermal conductivity and the high emissivity characteristics of the coating. This provides a means to re-irradiate more thermal energy to the surrounding area instead of transferring it to the underlying substrate. This aspect of the present invention provides thermal protection for coated ceramic substrates. Additionally, another aspect of the present invention is that it is capable of increasing performance temperatures for all types of non-metallic substrates, including, but not limited to, woven ceramics, ceramic fibers, bulk ceramic fibrous materials, ceramics alone or pressed, and combinations of them. Additionally, the present invention provides increased resistance to abrasion and corrosion when applied to structures such as motors, turbines, raceways, refractory liners, and other ceramic applications. Another aspect of the present invention is to provide a protective thermal coating that does not produce toxic fumes when heated. The protective thermal coating can be applied to furnace refractory surfaces in the field, and is 100% inorganic. The protective thermal coating of the present invention, therefore, does not produce toxic fumes when heated, and can be safely applied to ceramic surfaces in the field as desired. Still another aspect of the present invention is to provide a protective thermal coating that does not significantly increase the weight of the coated substrate. These and other aspects of the present invention will become apparent upon further review of the following specification.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES The present invention relates to a protective thermal coating, containing from about 55 to about 355 colloidal silica, from about 23% to about 79% of a filler, from about 2% to about 20% of one or more emissivity agents , and from about 1.5% to about 5.0% of a stabilizer in a dry mix. In a coating solution according to the present invention, a wet mixture of the protective thermal coating contains from about 15% to about 455 of colloidal silica, from about 23% to about 55% of a filler, from about 1% to about 10. % of one or more emissivity agents, from about 0.5% to about 2.5% of a stabilizer and from about 18% to about 40% of water. The wet mix coating solution contains between about 40% and about 70% total solids. As used in the present application, all percentages (%) are weight-to-weight percentages, also expressed as% weight / weight,% (w / w), w / w,% w / w, or simply%, unless that is indicated otherwise. Also, as used herein, the term "wet mix" refers to relative percentages of the protective thermal coating composition in solution and "dry mix" refers to the relative percentages of the composition of the dry protective thermal coating composition. , before the addition of water. In other words, the percentages of dry mix are those that are present without taking water into account. The wet mixture refers to the mixture in solution (with water). "Wet weight percentage" is the weight in a wet mix, and "dry weight percentage" is the weight in a dry mix without considering the wet weight percentages. The term "total solids", as used herein, refers to the total sum of silica / alumina and alkali or ammonia (NH3), plus the fraction of all solids, including impurities. The weight of the solid component divided by the total mass of the complete solution, one hundred times, produces the percentage of "total solids". The colloidal silica preferably is a monodispersed distribution of colloidal silica, and therefore, has a very narrow range of particle sizes. The filler is preferably a refractory material with fine particle size, taken from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide, magnesium oxide, calcium oxide and boron oxide, the emissivity agent or agents, preferably they are taken from the group consisting of silicon hexaboride, boron carbide (also known as carbon tetraboride), silicon tetraboride, silicon carbide, molybdenum disilicide, zirconium diboride, copper chromite and metal oxides such as iron oxides , oxides of magnesium, oxides of manganese, chromic copper oxides, chromium oxides, cerium oxides, and oxides of terbium, and derivatives thereof. Chromic copper oxide, as used in the present invention, is a mixture of copper chromium and cupric oxide. The stabilizer can be taken from the group consisting of bentonite, kaolin, silicon-magnesium clay and alumina, alumo-tabular and stabilized zirconium oxide. the stabilizer is preferably bentonite. Other clay stabilizers in spheres can be substituted here as a stabilizer. Colloidal alumina, in addition to or instead of colloidal silica, may also be included in the mixture of the present invention. When the colloidal alumina and the colloidal silica are mixed together, one or the other requires surface modification to facilitate mixing, as is known in the art. Dye can be added to the protective coating of the present invention to form a heat resistant paint. Inorganic pigments may be added to the protective coating to form a heat resistant paint without generating toxic fumes. In general, the inorganic pigments are divided into the subclasses: colored (salts and oxides), black, white and metallic. Suitable inorganic pigments include, but are not limited to, orange cadmium, red cadmium, deep orange cadmium, cadmium orange lithopon, and red lithium cadmium. A preferred embodiment of the present invention contains a dry blend from about 0.01% to about 30.0% colloidal silica, from about 50% to about 79% silicon dioxide powder, and from about 2% to about 154% of one or more emulsifying agents taken from the group consisting of cerium oxide, boron silicide, boron carbide, silicon tetraboride, molybdenum silicon carbide d isilicide, tungsten disilicide, zirconium diboride, and from about 1 .5% to about 5.0% bentonite powder. The corresponding solution coating (wet mix) for this embodiment contains from about 20.0% to about 35.0% colloidal silica, from about 25.0% to about 55.0% silicon dioxide, from about 18.0% to about 35.0% water, and from about 2.0% to about 7.5% of one or more emittance agents, and from about 0.50% to about 2.50% bentonite powder. Deionized water is preferably used. Preferred embodiments of the wet mix have a total solids content ranging from about 50% to about 65%. A more preferred thermal protective coating of the present invention contains a dry blend from about 15.0% to about 25.0% colloidal silica, from about 68.0% to about 78.0% silicon dioxide powder, about 2.00% to about 4.00% bentonite powder, and from about 4.00% to about 6.00% of an emittance agent. The emitting agent is taken from one or more of the following: zirconium boride, boron silicide, and boron carbide. A more preferred wet mixture contains about 27.0% colloidal silica based on a colloidal silica solids content of about 40%, from about 255 to about 50% silicon dioxide powder, about 1.50% bentonite powder, and from approximately 2.50% up to approximately 5.50% of an emittance agent, with the rest of water. The emitting agent is more preferably taken from the group consisting of zirconium boride, boron silicide, and boron carbide. Preferred embodiments include those in which the emitting agent contains about 2.50% zirconium diboride, about 2.50% boron silicide, or from about 2.50% to about 7.50% boron carbide. The pH of a more preferred wet mixture according to the present invention is about 9.0 ± 1.0, the specific gravity is about 1.40 to 1.50, and the total solids content is about 50% up to 60%.

Ludox (trademark) TM 50 colloidal silica, and Ludox (trademark) AS 40 colloidal silica, are available from Grace Davidson (of Columbia, MD). The particles in the Ludox colloidal silica (trademark) are uniform discrete spheres of silica that have no detectable internal surface area or crystallinity. Most are dispersed in an alkaline medium that reacts with the surface of the silica to produce a negative charge. Due to the negative charge, the particles repel one another, resulting in stable products. While most classifications are stable between a pH of 8.5 and a pH of 11.0, some classifications are stable in the neutral pH range. The colloidal silicas Ludox (trade mark), are colloidal aqueous dispersions of very small silica particles. They are opalescent with milky white liquids. Due to its colloidal nature, the Ludox colloidal silica particles (trademark) are available in two primary families: mono-dispersed, very narrow particle size distribution of the Ludox colloidal silica (trademark), and poly-dispersed, distribution of Large particle size of the colloidal silica Ludox (trademark). The colloidal silica Ludox (trademark) is converted to a dry solid, usually by gelation. Colloidal silica can be gelled: (1) by removing water, (2) changing the pH, or (3) by adding a salt or water miscible organic solvent. During drying, the hydroxyl groups on the surface of the particles condense by cleaving the water to form siloxane bonds (Si-O-Si), resulting in coalescence and inter-bonding. The dry particles of colloidal silica Ludox (trademark) are chemically inert and resistant to heat. The particles develop strongly adhesive and cohesive bonds, and are effective binders for all types of granular and fibrous materials, especially when their use at high temperature is required. The filler may be a silicon dioxide powder such as Min-U-Sil (trademark) 5, silicon dioxide available in U.S. Silica (from Berkeley Springs, WV). This silicon dioxide is finely ground silica. The chemical analysis of silicon dioxide Min-U-Sil (trademark) indicates contents of 98.5% of silicon dioxide, 0.060% of iron oxide, 1.1% of aluminum oxide, 0.02% of titanium dioxide, 0.04% of calcium oxide, 0.03% magnesium oxide, 0.03% sodium dioxide, 0.03% potassium oxide and 0.4% is lost in ignition. Typical physical properties are a compacted bulk density of 656.7 kg / m3 (41 pounds / ft3), a non-compacted bulk density of 576.6 kg / m3 (36 pounds / ft3), a Mohs hardness of 7, a Hegman index of 7.5, average diameter of 1.7 microns, an oil absorption (D-1483) of 44, a pH of 6.2, 97% - 5 microns, 0.005% + sieving of 325, a reflectance of 92%, a yellowness index of 4.2 and a specific gravity of 2.65. The emitting agents are available from several sources. Emissivity is the power of a surface to emit heat by radiation, and the proportion of radiant energy emitted by a surface with respect to the radiant energy emitted by a black body at the same temperature. The emittance is the energy radiated by the surface of a body per unit area. Boron carbide, also known as carbon tetraboride, which can be used as an emissivity agent in the present invention, is sold as 1000 V boron carbide, and is available from Electro Abrasives (of Buffalo, NY). Boron carbide is one of the hardest materials available made by man. Above 1300 ° C, it is even harder than diamond and cubic boron nitride. It has a four-point flexural strength of 3515.3 kg / cm2 at 4921.4 kg / cm2 (50,000 to 70,000 psi) and a compressive strength of 29,107 kg / cm2 (414,000 psi) depending on density. Boron carbide also has a low thermal conductivity (29-67 V / mK) and has an electrical resistivity ranging from 0.1 to 10 ohm-cm. typical chemical analysis indicates 77.5% boron, 21.5% carbon, 0.2% iron and the total boron plus carbon is 98%. The hardness is 2800 Knoop and 9.6 Mohs, the melting point is 2350 ° C (4262 ° F), the oxidation temperature is 932 ° F (500 ° C) and the specific gravity is 2.52 g / cc. Green silicon carbide 1000 V, an optional emissivity agent, is also available from Electro Abrasives. The green silicon carbide is a man-made, extremely hard mineral (2600 Knoop or 9.4 Mohs) that possesses high thermal conductivity (100 V / m-K). It also has high resistance to high temperatures (at 1100 ° C, green SiC is 7.5 times stronger than AI2O3). Green SiC has a modulus of elasticity of 410 GPa, with no decrease in strength up to 1800 ° C, and does not melt at normal pressures, but instead dissociates at 2815.5 ° C. Green Silicon Carbide is a batch composition made from silica sand and coke, and is extremely pure. The physical properties are as follows for silicon carbide: the hardness is 2600 Knoop and 9.4 Mohs, the melting point is 2600 ° C (4712 ° F), and the specific gravity is 3.2 g / cc. The typical chemical analysis is 99.5% SiC, 0.2% SiO2, 0.03% Si in total, 0.04% Fe in total, and 0.1% of3 C in total. The silicon carbide and commercial molybdenum disilicide may need cleaning, as is well known in the art, to remove the flammable gas that is generated during production. Boron silicide (B6Si) (Item number B-1089), is available from Cerac (of Milwaukee, Wisconsin). Boron silicide, also known as silicon hexaboride, available from Cerac, has a sieve of -200 to -325 (about 2 to 6 microns on average) and a typical purity of about 98%. Zirconium boride (ZrB2) (Article number Z-1031) is also available in Cerac with a typical average of 10 microns or less (sieving of -325), and a typical purity of approximately 99.5%. The mixture of the present invention preferably includes bentonite powder, tabular alumina, or other magnesium and alumina silica clay as a stabilizer. The bentonite powder allows the present invention to be prepared and used at a later time. Examples provided for the present invention include PolarGel bentonite powder (Article number 354) available from Whittaker Clark &; Daniels (from South Plainfield, NJ). Bentonite technical classification is generally used for the purpose of suspending agents, emulsifiers and binders, as well as rheological modifiers. The typical chemical analysis is 59.00% to 61.00% of silicon dioxide (SiO2), 20.00% to 22.00% of aluminum oxide (AI2O3), 2.00% to 3.00% of calcium oxide (CaO), 3.50% to 4.30% of magnesium oxide (MgO), 0.60 to 0.70% ferric oxide (Fe2O3), 3.50% to 4.00% of sodium oxide (Na2O), 0.02% to 0.03% of potassium oxide (K2O), and 0.10% to 0.20 % titanium dioxide and a maximum of 8.0% humidity. The pH value ranges from 9.5 to 10.5. Typical physical properties are from 83.0 to 8.0 dry gloss, 2.50 to 2.60 specific gravity, 2.14 kg / liter (20.82 pounds / solid gallon), 0.46 L per one kilo (0.0480 gallons per one pound of mass), 24 mL of minimum foaming power, maximum gel formation of 2 mL, and 100.00% through a 200 sieve. Tabular alumina (Alumina Tab T64, Article number 635) and silica clay of magnesium and alumina (Mag Alum Sil Technical, item number 105), are also available at Wittaker Clark & Daniels The dyes, which may be added to the present invention, include, but are not limited to, inorganic pigments. Suitable inorganic pigments, such as yellow iron oxide, green chromium oxide, roo iron oxide, black iron oxide, titanium dioxide, are available from Hoover Color Corporation. Additional suitable pigments, such as copper chromium black spinel, chromium green-black hematite, yellow titanium rutile, antimony and nickel, polished titanium rutile, antimony and manganese, and blue-green spinel of chromite and cobalt, are available from The Shepherd Color Company (Cincinnati, Ohio). The protective thermal coating of the present invention is prepared by placing the liquid ingredients in a clean, relatively dry container for mixing. While mixing, the remaining ingredients are slowly added to the mixture to prevent the powders from clumping together and adhering to the side of the mixing vessel. The mixture is then mixed at high speed, such as 5000 rpm, for at least 20 minutes, depending on the configuration of the mixer. Preferably, a high shear driving blade is used, such as a blade are high shear saw teeth, where the mixing is performed at revolutions ranging from about 3000 rpm to about 5000 rpm. A ball grinder or glass grinder may be used in place of a conventional mixer having a blade. The solids in the protective thermal coating mixture can settle during transport or storage.

Before use, the mixture can be completely mixed again, to ensure that settled solids and lumps are redispersed completely. For containers of 500 mL and above, a flat blade with high cutting force and high speed is used, the blade is connected to a hand drill, drill press or mandrel mixer motor, and mixed at high power (3000 rpm or more) ), while moving the blade up and down inside the container to impact and break the settled lumps. To ensure complete dispersion, the mixture has to be mixed again for 10 to 15 minutes. You can use a spatula to paint manually, to remove the wet mixture in the container, and verify that the mixture and dispersion are complete. You can shake small containers by hand for 5 minutes, and stir with a stick to paint to ensure that the settled clumps are dispersed and homogeneous. The present invention is frequently applied to a surface as a substrate in the field. The substrate surface can be a ceramic brick, a ceramic slab, ceramic fiber, ceramic cloth, or the like. The coating can be applied wet and left to dry in the air or dried with heat. Ceramic substrates can be the internal ceramic surfaces of refractory kilns, automotive, marine or aerospace ceramic parts, and any other ceramic surface that may be subject to high temperatures. Initially, a ceramic surface can be cleaned of all dirt, loose material, surfactants, oils and the like. When possible, the ceramic surfaces to be coated must be scraped with sandblasting (SiO2) / gravel with a coarse medium, followed by removal of loose particles with a brush or air gun. The surface of some ceramic substrates can be cleaned with a gravel jet with a coarse medium of silica sand or with an amorphous bleed medium, such as Black Beauty shot (trademark), as needed. Black Beauty is a trademark of H arsco Corporation. The shot blasting method of the ceramic substrate is not limited to coarse medium of silica sand and the like, but comprises any equivalent alternative method. The preferred blasting medium will vary with the type of refractory. All substrates have to be baked or cured by the substrate manufacturer's recommendations. If available, heating to a minimum of 498.8 ° C (930 ° F) for one hour to remove moisture, chemical additives and oil sludge. The coating has to be applied to the substrate as soon as possible after any cleaning, subsequent drying, or once the substrate has reached room temperature in the case of any previous heat treatment process. The coating mixture must be applied using a spray gun or a brush, depending on the desired application. The spray method is a preferred method of applying the coating to a substrate. It is desirable to use oil-clearing air pressure from 0.35 kg / cm2 to 10 kg / cm2 (5 to 10 psi) and 1 mm nozzle. The flow is established in the spray gun, and the air configurations, to achieve uniform coverage with the coating, at the desired densities according to the application to which it is directed. The coating can be applied using a brush. The brush method is generally used for permeable and non-permeable ceramics / refractories. You can use a brush with thin nylon fences. The coating is applied with firm, equal strokes, while trying to place the coating in a single layer to avoid passing the brush through the same area more than once. It is advisable to experiment on a test sample to achieve uniform coverage with the coating. While you can use a spray gun or a brush, the ceramic substrate has to be at room temperature and the splashes and the equipment should be cleaned immediately with water. The control of the coverage density of the product is desirable for a uniform coating that adheres to the ceramic substrate and protects it. The desired coverage density depends on the porosity, the solids content of the coating and other factors. Optimum cover densities range from 150 to 300 grams of dry coating weight per square meter of substrate surface area. Most substrates require an approximate coverage of 3.68 m2 / liter up to 4.91 m2 / liter (150 ft2 / gallon up to 200 ft2 / gallon). The dry coating should not be thicker than about 1 to 10 thousand for ceramic refractories. The thickness has to be adjusted according to the use of the ceramic surface to be coated. Thicker coatings than what is desirable can be peeled, and do not perform as well. After the coating on the substrate has been air-dried for a minimum of two hours, the following curing process is recommended for optimum adhesion when coating a refractory surface. The temperature is increased to approximately 93.3 ° C (200 ° F) per hour until a peak temperature of approximately 815.5 ° C (1500 ° F) is reached. The peak temperature is maintained for approximately two hours. After two hours, the temperature is reduced to room temperature at a rate of up to two 93.3 ° C (200 ° F) per hour. The coating is then inspected to verify its uniformity, and once the inspection is completed, the refractory substrate can be brought to the operating temperature by the manufacturer's refractory specifications. Example 1 contains 23% by dry weight of colloidal silica TM50 Ludox (trademark) and 28.6% by wet weight based on Ludox (trademark) solids content of 50.0%, 63.7% by dry weight of min-U-Sil (trademark) 5 SiO2 powder and 31.9% wet weight, 4.03% dry weight of an emissivity agent and 2.00% wet weight, where the emissivity agent is taken from the group consisting of boron carbide powder, green silicon carbide, zirconium boride, and boron silicide, and 2.91% by dry weight of PolarGel bentonite powder (article number 354) and 1.40% by wet weight, and 36.1% of water, based on the content of solids of Ludox (commercial brand) of 50.0%. The pH of Example 1 is 8.5 ± 1.0, and the total solids content is 50 + 0.3%. Example 1 is prepared by placing the liquid ingredients in a clean, relatively dry mixing vessel. While mixing, the remaining ingredients are added slowly to the mixture, to prevent the dusts from forming lumps and adhering to the sides of the mixing vessel. The mixture is then mixed at high power for at least 20 minutes, depending on the configuration of the mixer. The mixing is carried out in a high shear mixer, with a Cowles Hi-Shear Impeller blade with a 0.37 kilowatt (0.5 horsepower) motor that generates 7500 rpm without load. Example 2 contains 29.3% by dry weight of colloidal silica TM50 Ludox (trademark) and 28.6% by wet weight based on the solids content of Ludox (trademark) of 50.0% or, 52.8% or by dry weight of 5 SiO2 in min-U-Sil powder (trademark) and 26.4% by wet weight, 15.0% by dry weight of boron carbide powder or silicon carbide powder and 7.5% by wet weight, 2.91% by dry weight of PolarGei bentonite powder (article number 354) and 1.45% by wet weight, and 36.1% water, based on Ludox (trademark) solids content of 50.0%). The pH of Example 2 is 8.5 ± 1.0, and the total solids content is 50 ± 0.3%. Example 2 is prepared in the same way as example 1 |. Example 3 contains 16.5% by dry weight of colloidal silica AS-40 Ludox (trademark), and 26.8% by wet weight based on the Ludox (trademark) solids content of 40%, 76.3% by dry weight of 5 SiO2 min-U-Sil powder (trademark) and 49.6% by wet weight, 4.30% by dry weight of powdered boron carbide or boron silicide powder, and 2.80 by wet weight, 2.90% by dry weight of PolarGel bentonite powder (article number 354), and 1.88% by wet weight, and 18.9% by weight. water, based on the solids content Ludox (trademark) of 40%. The pH of Example 3 is 8.5 ± 1.0, the specific gravity is 1.64 ± 0.05, and the total solids content is 65 ± 0.9%. example 3 is prepared in the same way as example 1. Example 4 contains 21.84% by dry weight of colloidal silica TM 50 Ludox (trademark) and 27.09% by wet weight based on the solids content Ludox (trademark) 50.0%, 70.87% or by dry weight of 5 SiO2 min-U-Sil powder (trademark), and 43.94% by wet weight, 4.369% by dry weight of boron silicide powder and 2.709% by wet weight, 2,913% by dry weight of PolarGel bentonite powder (article number 354) and 1,806% wet weight, and 24.46% water, based on Ludox (trademark) solids content of 50.0%. The pH of Example 4 is 8.5 ± 1.0, the specific gravity is 1.59 ± 0.05 and the total solids content is 62.0 ± 0.3%. Example 4 is prepared in the same way as Example 1. An example of a refractory application of the present invention involves coating a ceramics refractory brick such as a Harbison-Walker Clipper DP Super Duty refractory brick. Harbison-Walker is a subsidiary of AN H Refractories, located in Moon Township, PA. The coating has been applied to the refractory brick walls of an oven that has been in operation for a few months. Due to the previous operation, the refractory is free of any volatile substance, both on the surface and within the body of the material. The surface was swept with a nylon brush to remove loose material attached to the surface, and the dust was removed into small particles from the surface by a compressed air blow gun at 4.92 kg / cm2. The wet mixture described in example 4 was applied by means of a spray gun supplied with high pressure suction, with a nozzle of 1 .27 cm (1/2 inch) at 3.51 kg / cm.sup.2 to 4.92 kg / cm.sup.2. An example of a ceramic cloth application involves coating Nexel 440 cloth, available from TMO Industries, Inc. of Huntington Park CA, with the mixture of Example 3. In this application, the substrate is mounted on a wooden frame in order to Secure the substrate during the coating process. The coating of Example 3 was applied by the Paasche airbrush model "H" in a single action, with a nozzle for fluid number "5" and an air cap "5". The target coverage density is 200 to 400 grams of dry coating weight per square meter of substrate area. It should be understood that the present invention is not limited to the embodiments described above, but comprises any of the embodiments within the scope of the following claims.

Claims (5)

1. A protective thermal coating, comprising: in dry mix, a. from about 5% to about 35%) of colloidal silica, colloidal alumina, or combinations thereof; b. from about 23% to about 79% of a filler taken from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide, magnesium oxide, calcium oxide, and boron oxide; c. from about 2% to about 20% of one or more emissivity agents taken from the group consisting of silicon hexaboride, silicon carbide, silicon tetraboride, silicon carbide, molybdenum disilicide, tungsten disilicide, zirconium dibouror, chromite copper and metal oxides; and d. from about 1.55 to about 5.0% of a stabilizer taken from the group consisting of bentonite, kaolin, silicon clay, alumina and magnesium, tabular alumina, and stabilized zirconium oxide.

2. The content of claim 1, further characterized in that: a. the emissivity agents are one or more metal oxides taken from the group consisting of iron oxide, magnesium oxide, manganese oxide, chromic copper oxide, chromium oxide, cerium oxide, terbium oxide, and derivatives thereof.

3. The coating of claim 1, further comprising: a. water, forming a wet mixture having a total solids content ranging from about 40% to about 70%.

4. The coating of claim 1, further characterized in that: the dry mixture contains a. from about 10% to about 30% colloidal silica, b. from about 50% to about 79% of silicon dioxide powder, and c. from about 2% to about 15% of one or more emissivity agents taken from the group consisting of iron oxide, boron silicide, boron carbide, silicon tetraboride, silicon carbide, molybdenum disilicide, tungsten disilicide and diboride of zirconium; and d. from about 1.5% to about 5.0% of a stabilizer taken from the group consisting of bentonite, kaolin, aluminum-magnesium silica clay, tabular alumina, and stabilized zirconium oxide. 5. The coating of claim 4, further characterized in that: a. The stabilizer is bentonite powder. 6. The coating of claim 5, further comprising: water, forming a wet mixture having a total solids content ranging from about 40% to about 70%. 7. The coating of claim 1, further comprising: a. a colorant 8. The coating of claim 1, which further contains: a. a dye 9. A protective thermal coating, containing: in dry mix, a. from about 5% to about 35% colloidal silica, b. from approximately 50% to approximately 79% or from a filler, and c. from about 2% to about 20% of one or more emissivity agents taken from the group consisting of silicon hexaboride, boron carbide, silicon tetraboride, silicon carbide, molybdenum disilicide, tungsten disilicide, zirconium diboride, chromite copper, and metal oxides; and d. from about 1.5% to about 5% of a stabilizer taken from the group consisting of bentonite, kaolin, silica clay, aluminum and magnesium, tabular alumina, and stabilized zirconium oxide. 10. The coating of claim 9, further characterized in that: a. The emissivity agent is a metal oxide taken from the group consisting of iron oxide, magnesium oxide, manganese oxide, chromium oxide, chromic copper oxide, cerium oxide, terbium oxide, and derivatives thereof. 11. The coating of claim 9, further comprising: a. water, forming a wet mixture having a total solids content ranging from about 50% to about 65%. 12. The coating of claim 9, further characterized in that it contains: from about 2% to about 15% of an emissivity agent. The coating of claim 12, further characterized in that: the emittance agent is taken from the group consisting of boron silicide, boron carbide, silicon carbide, and zirconium boride. 14. The coating of claim 9, further characterized in that: a. The filler is a metal oxide taken from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide, magnesium oxide, calcium oxide, and boron oxide. 15. The coating of claim 9, further comprising: a. a dye 16. The coating of claim 9, further characterized in that: a. The stabilizer is bentonite powder. 17. A method for preparing a protective thermal coating for a substrate, comprising the steps of: a. add a dry mix to a mixing container, characterized in that the dry mix contains i. from about 5% to about 35% or of colloidal silica, colloidal alumina, or combinations thereof, ii. from approximately 23% to approximately 79% > of a filler, and iii. from about 2% to about 20% of one or more emissivity agents taken from the group consisting of silicon hexaboride, boron carbide, silicon tetraboride, silicon carbide, molybdenum disilicide, tungsten disilicide, zirconium diboride, copper chromite, and metal oxides; and iv. from about 1% to about 5% of a stabilizer taken from the group consisting of bentonite, kaolin, aluminum and magnesium silica clay, tabular alumina, and stabilized zirconium oxide; b. adding water to the mixing vessel such that the total solids content of the wet mixture ranges from about 40% to about 60%; and c. Mix the contents of the mixing vessel forming a protective thermal solution for coating. 18. The method of claim 17, further characterized in that: a. The filler is taken from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide, magnesium oxide, calcium oxide, and boron oxide. 19. The method of claim 17, further characterized in that: a. The emissivity agent is a metal oxide taken from the group consisting of iron oxide, magnesium oxide, manganese oxide, chromium oxide, chromic copper oxide, cerium oxide, terbium oxide and derivatives thereof. The method of claim 17, further comprising the steps of: a. Place the liquid ingredients in the mixer container before mixing them in dry ingredients. The method of claim 17, further comprising the steps of: a. Mix with high shear until the content is well dispersed. 22. The method of claim 17, further comprising the steps of: a. provide a storage container, and b. Place the solution for protective thermal coating in the storage container, for future use. 23. The method of claim 22, further comprising the steps of: a. Mix again the solution for protective thermal coating in the container for storage after storage for current use. The method of claim 17, further comprising the steps of: a. Cover a substrate with the protective thermal coating solution. 25. The method of claim 23, further comprising the steps of: a. Cover a substrate with the protective thermal coating solution. 26. The method of claim 17, further comprising the steps of: a. cure the coating on the substrate. 27. The method of claim 17, further characterized in that: a. the dry mixture also contains a colorant. 28. The method of claim 26, further characterized in that: a. the curing of the coating on the substrate comprises the steps of: i. air-drying the coating on the substrate for about two hours; ii. increasing the temperature of the coated substrate at a rate of approximately 93.3 ° C (200 ° F) per hour until reaching a peak temperature of approximately 815.5 ° C (1500 ° F); iii. maintain the peak temperature for approximately 2 hours; and iv. Reduce the temperature of the coated substrate to room temperature at a rate up to approximately 93.3 ° C (200 ° F) per hour. 29. A substrate coated with a thermal protective coating, comprising: a. a substrate having a thermal protective coating on it; and b. the protective thermal coating that contains i. from about 5% to about 35% of a colloidal silica, alumina or colloidal, or combinations thereof, ii. from approximately 23% to approximately 79% of a filler, and c. from about 2% to about 20%) of one or more emissivity agents taken from the group consisting of silicon hexaboride, boron carbide, silicon tetraboride, silicon carbide, molybdenum disilicide, tungsten disilicide, zirconium diboride, copper chromite, and metal oxides; and d. from approximately 1.5% to approximately

5. 0% of a stabilizer taken from the group consisting of bentonite, kaolin, magnesium aluminum and magnesium clay, tabular alumina, and stabilized zirconium oxide. 30. The coated substrate of claim 29, further characterized in that: a. The filler is taken from the group consisting of silicon dioxide, aluminum oxide, titanium dioxide, magnesium oxide, calcium oxide, and boron oxide. 31. The coated substrate of claim 29, further characterized in that: a. The emissivity agent is a metal oxide taken from the group consisting of iron oxide, magnesium oxide, manganese oxide, chromium oxide, chromic copper oxide, cerium oxide, terbium oxide, and derivatives thereof. 32. The coated substrate of claim 29, further comprising: a. a dye