JP2021006609A - Coating composition, heat-resistant coating, and method of forming the same - Google Patents

Abstract

To provide: a storage-stable coating composition capable of forming a heat-resistant coating which exhibits excellent non-wettability even for simulated ash, such as molten phosphate glass, and has high strength and adhesion to a substrate; a heat-resistant coating that can be formed using such a coating composition; and a method of forming the same.SOLUTION: A coating composition comprises (a) an alkali metal silicate, (b) an activator reactive to the alkali metal silicate, which comprises a layered silicate and at least one selected from the group consisting of hydroxides, oxides and silicates of alkaline earth metal or group 13 elements, and (c) boron nitride.SELECTED DRAWING: None

Description

The present disclosure relates to coating compositions, heat resistant coatings and methods thereof.

Boron nitride is chemically inert and exhibits low wettability to many metal and glass melts. Boron nitride is stable in a high temperature environment of up to about 900 ° C., and when it exceeds 900 ° C., it oxidizes to form boron oxide (B ₂ O ₃ ). As described above, boron nitride is an excellent anti-adhesion agent at 900 ° C. or lower, and a ceramic coating in which boron nitride particles are mixed with an inorganic binder is used in applications where heat resistance or durability is required, for example, a boiler of a coal power plant. It is used for the purpose of preventing molten ash such as slag generated by combustion of coal from adhering to the surface of structures such as the inner wall and steam pipe of the coal.

Patent Document 1 (Japanese Unexamined Patent Publication No. 2012-251228) states that "a film formed on the surface of a base material, in which pores of a porous film formed containing hexagonal boron nitride particles and silica particles are hexagonal. A film formed by sealing with a resin material having permeability and curability mixed with boron nitride particles ”is described.

Patent Document 2 (Japanese Patent Laid-Open No. 2005-534478) describes "fine boron nitride in a method for producing a ceramic coating on a metal and / or ceramic surface and a ceramic coating on a product in a reactor, a process plant, and a combustion plant. A mixture of at least one medium particle size inorganic binder in the nanometer range and at least one solvent and / or water is applied onto the metal and / or ceramic surface or product and the coating is applied. A method characterized by firing and coating the mixture by heating "is described.

Patent Document 3 (Japanese Patent Laid-Open No. 2006-527090) is a sizing agent for producing a release layer having long-term stability, and A) silicon oxide, zirconium oxide, aluminum oxide, boehmite or a mixture thereof. A group containing an inorganic molding agent containing colloidal inorganic particles based on the above, and a group containing SiO ₂ , TiO ₂ , ZrO ₂ , Al ₂ O ₃ , AlOOH, Y ₂ O ₃ , CeO ₂ , SnO ₂ , iron oxide, and carbon. When the inorganic filler selected from the above, the additive, and B) the molding agent (i) containing silicon oxide are used, a suspension of boron nitride particles in an organic solvent, the molding containing no silicon oxide. When the agent (ii) is used, a suspension of boron nitride particles in water, C) an organic solvent when the molding agent (i) is used, and water when the molding agent (ii) is used. In the molding agent (i), the molding agent contains a specific one or more silanes, a semi-chemical amount of water based on the hydrolyzing group of the silane component, and any organic. The molding agent (ii) further optionally contains water as a solvent, comprising a solvent, to form a nanocomposite sol under the conditions of the sol-gel process, if hydrolysis and condensation are required. Describes a sizing agent for producing a release layer having long-term stability, including.

Japanese Unexamined Patent Publication No. 2012-251228 Special Table 2005-534478 Special Table 2006-527090

The ash adhesion prevention property of the ceramic coating can be evaluated by measuring the contact angle of the molten simulated ash on the coating surface using soda glass or phosphoric acid glass as the simulated ash. Phosphate glass has higher wettability than soda glass and therefore requires a larger amount of boron nitride. Increasing the content of boron nitride in the coating can increase the non-wetting property of the coating, while reducing the strength of the coating or its adhesion to the substrate. That is, the coating is likely to be damaged or lost, or the coating is likely to peel off from the surface of the substrate. Further, if the coefficient of thermal expansion between the ceramic coating and the base material is different, the adhesion to the base material tends to be further lowered. Since slag collides with the inner wall of a boiler of a coal power plant and the surface of a structure such as a steam pipe, it is desired to improve the strength of the coating and the adhesion to the base material.

The ceramic coating can be formed using a coating composition containing a reactive binder component that forms a coating or matrix that retains the boron nitride particles by heating. If the reaction of the reactive binder component proceeds over time during storage of the coating composition, the coating composition may gel or thicken, making work impossible or difficult during coating.

The present disclosure is storage stability capable of forming a heat resistant coating that exhibits excellent non-wetting properties even with simulated ash such as molten phosphoric acid glass and has high strength and adhesion to a substrate. Provided are an excellent coating composition, and a heat-resistant coating and a method for forming the same, which can be formed by using such a coating composition.

According to one embodiment of the present disclosure, from (a) alkali metal silicates, (b) layered silicates, and hydroxides, oxides and silicates of alkaline earth metals or Group 13 elements. Provided is a coating composition comprising (c) boron nitride and an activator that is reactive with the alkali metal silicate, including at least one selected from the group.

According to another embodiment of the present disclosure, there is provided a method of forming a heat resistant coating, which comprises spraying the coating composition onto the surface of a metal or ceramic.

According to yet another embodiment of the present disclosure, a group consisting of a matrix of alkali metal silicates, layered silicates, and hydroxides, oxides and silicates of alkaline earth metals or Group 13 elements. Provided is a heat resistant coating comprising boron nitride and an activator that is reactive with the alkali metal silicate, including at least one selected from the above.

The coating composition of the present disclosure is excellent in storage stability, exhibits excellent non-wetting property against simulated ash such as molten phosphate glass, and has high strength and heat resistance having adhesion to a substrate. A coating can be formed.

It should be noted that the above description shall not be deemed to disclose all embodiments of the present invention and all advantages relating to the present invention.

Hereinafter, the present invention will be described in more detail for the purpose of exemplifying typical embodiments of the present invention, but the present invention is not limited to these embodiments.

The coating composition of one embodiment comprises (a) an alkali metal silicate, (b) a layered silicate, and an alkali earth metal or Group 13 element hydroxide, oxide and silicate. It contains (c) boron nitride and an activator that is reactive with alkali metal silicates, including at least one selected from the group.

The alkali metal silicate functions as a binder that adheres boron nitride to the surface of the substrate. The alkali metal silicate has a siloxane bond (Si—O—Si) when forming a coating, a bond via a metal ion existing in the system (Si—O—MO—Si, M is a metal), etc. Form a matrix containing. The matrix can function as a binder that retains boron nitride in the heat resistant coating. The matrix is generally amorphous.

Examples of the alkali metal silicate include lithium silicate, sodium silicate, potassium silicate and the like. Since a coating that is stable in the coating composition during preparation and storage of the coating composition, has appropriate strength and cohesive force, and has high adhesion to the substrate can be obtained, it can be used as an alkali metal silicate. It is advantageous to use potassium silicate.

The alkali metal silicate may be a mixture containing at least two selected from the group consisting of lithium silicate, sodium silicate, and potassium silicate. Among these, a mixture of sodium silicate and potassium silicate is advantageously used. The mass ratio of sodium silicate to potassium silicate is from about 50:50 to about 0: 100. By using potassium silicate as the main component of the alkali metal silicate, the storage stability of the coating composition can be enhanced.

In some embodiments, the content of the alkali metal silicate in the coating composition is about 20% by weight or more, about 25% by weight or more, or about 30% by mass or more based on the solid content of the coating composition. , About 55% by mass or less, about 50% by mass or less, or about 45% by mass or less. By setting the content of the alkali metal silicate to about 20% by mass or more based on the solid content of the coating composition, a coating having high strength can be provided. By setting the content of the alkali metal silicate to about 55% by mass or less based on the solid content of the coating composition, shrinkage due to curing can be suppressed, and a coating having high adhesion can be provided.

The alkali metal silicate may be provided, for example, in the state of an aqueous solution (water glass), and the concentration of the alkali metal silicate is about 10% by mass or more, about 20% by mass or more, or about 30% by mass or more. It can be about 90% by mass or less, about 80% by mass or less, or about 70% by mass or less. In this embodiment, the water in the aqueous solution also functions as a solvent for the coating composition.

The activator is at least one selected from the group consisting of layered silicates that are reactive with alkali metal silicates and hydroxides, oxides and silicates of alkaline earth metals or Group 13 elements. Includes seeds. However, the activator does not contain aluminum oxide (alumina), which has low reactivity with alkali metal silicate, as an active ingredient. The activator can be a source of metal ions that can be used to form the alkali metal silicate matrix, or can react with the alkali metal silicate to form part of the matrix.

The layered silicate has a plurality of layered structures laminated with or without interlayer ions, and some or all of the layered structures are dispersed in the coating composition. It is considered that the layered structure thus dispersed is incorporated into the matrix of alkali metal silicates at the time of forming the coating, and the strength of the coating or the adhesion to the substrate can be enhanced.

In particular, the aluminosilicate contained in the layered silicate can form part of the matrix by a geopolymer reaction with the alkali metal silicate. Specifically, when the aluminosilicate reacts with the alkali metal silicate, the aluminosilicate having a geopolymer structure represented by (-Si-O-Al-O-) _n (n is the degree of polymerization) of the aluminosilicate. A three-dimensional network is formed. In this reaction, (1) the Al or Si cationic species of the aluminosilicate is dissolved by the hydroxy group, (2) the silicate monomer is condensed by the metal cation supplied from the aluminosilicate, and (3) silicon. It involves the formation of a three-dimensional network by condensation of silicates. By forming this three-dimensional network, it is possible to effectively suppress the shedding of silicon nitride, further increase the strength of the coating, and enhance the adhesion to the substrate. As a result, a coating having particularly excellent blast resistance can be formed.

The layered silicate is selected from the group consisting of, for example, mica such as muscobite, frogopite, smectite such as hectorite, montmorillonite (including those provided in the form of bentonite), talc, pyrophyllite and kaolin. At least one is mentioned. Mica such as phlogopite and phlogopite can form a high-density coating because the layered structure is easily dispersed. Smectites such as hectorite and montmorillonite contain alkali metal ions (interlayer ions) between the layered structures and can hold a large amount of water, so that they promote the geopolymer reaction and form a high-density matrix. Can be done. It is advantageous that the layered silicate contains a large amount of aluminosilicate as a component, and it is particularly advantageous that the layered silicate is mica or smectite.

The crystal structure of layered silicate is a laminated composite layer in which both sheets of a tetrahedral sheet having a two-dimensionally continuous net-like connection of Si—O tetrahedra and an octahedral sheet such as Al—O are combined. It is a three-dimensional structure.

Hydroxides of alkaline earth metals or Group 13 elements include magnesium hydroxide, calcium hydroxide, boric acid, and aluminum hydroxide. Examples of oxides of alkaline earth metals or Group 13 elements include magnesium oxide, calcium oxide, and boron oxide. Examples of silicates of alkaline earth metals or Group 13 elements include calcium silicate and aluminum silicate. Among these, at least one selected from the group consisting of aluminum hydroxide, magnesium hydroxide and magnesium oxide can be advantageously used.

In some embodiments, the average particle size of the activator is about 0.1 μm or greater, about 0.2 μm or greater, or about 0.5 μm or greater, about 10 μm or less, about 5 μm or less, or about 3 μm or less. When the average particle size is in the above range, the fluidity of the coating composition can be increased, and excellent spraying characteristics can be obtained, for example, when a spraying composition is obtained. Factors of spray properties include surface tension and thixotropy. The average particle size of the activator is defined as the effective diameter, which is measured by the laser diffraction / scattering method and assumes that the particles are spherical.

In some embodiments, the content of the activator in the coating composition is about 3% by weight or more, about 10% by weight or more, or about 15% by weight or more, about 50, based on the solid content of the coating composition. It is mass% or less, about 45 mass% or less, or about 40 mass% or less. By setting the content of the activator to about 3% by mass or more based on the solid content of the coating composition, the formation of a three-dimensional network can be sufficiently promoted. By setting the content of the activator to about 50% by mass or less based on the solid content of the coating composition, shrinkage due to curing can be suppressed, and a coating having high adhesion can be provided.

Boron nitride has releasability and imparts ash adhesion prevention performance to the heat resistant coating. The ash adhesion prevention performance can be evaluated by the contact angle between the molten ash and the heat-resistant coating surface. The ash adhesion prevention performance of the heat-resistant coating is exhibited even at a high temperature of, for example, 500 ° C to 900 ° C. Hexagonal (hcp) boron nitride has a layered structure similar to graphite, and exhibits high releasability due to its layered structure and chemically inert properties, and is therefore preferably used. be able to. Further, since boron nitride has high thermal conductivity, it is possible to provide a coating that suppresses the influence on heat exchange efficiency when applied to the inner wall of a coal power plant boiler or the surface of a steam pipe, for example.

The particle shape of boron nitride may be flat, scaly, agglutinating, or the like.

In some embodiments, the particle size of boron nitride is about 1 μm or more, about 2 μm or more, or about 3 μm or more, about 300 μm or less, about 290 μm or less, or about 280 μm or less. By setting the particle size of boron nitride within the above range, the fluidity of the coating composition at the time of spray coating can be increased and a more uniform film can be formed. The particle size of boron nitride is defined as the median diameter determined by laser diffraction / scattering particle size distribution measurement.

In some embodiments, the specific surface area of the boron nitride, as measured by the BET method is specific surface area measured about ^3m 2 / g or more, about 3.2 m ² / g or more, or about 3.5 m ² / G or more, about 25 m ² / g or less, about 23 m ² / g or less, or about 20 m ² / g or less. By setting the specific surface area of boron nitride within the above range, it is possible to provide a coating composition and coating in which boron nitride is densely dispersed.

The boron nitride used in the preparation of the coating composition may contain impurities or additives in an amount of, for example, about 10% by mass or less. Impurities or additives include borate, boron oxide, carbon, alkali metals and alkaline earth metal borates. These impurities or additives may act as activators or form part of the activators. The purity of boron nitride used in the preparation of the coating composition is preferably about 90% by mass or more, or about 95% by mass or more.

In some embodiments, the content of boron nitride in the coating composition is about 15% by weight or more, about 20% by weight or more, or about 25% by weight or more, about 80, based on the solid content of the coating composition. It is mass% or less, about 75 mass% or less, or about 70 mass% or less. By setting the content of boron nitride within the above range, a heat-resistant coating having excellent ash adhesion prevention performance can be formed.

In some embodiments, the mass ratio of activator to alkali metal silicate (mass of activator / mass of alkali metal silicate) is about 5:95 or greater, about 10:90 or greater, or about 15 :. 85 or more, about 50:50 or less, about 45:55 or less, or about 40:60 or less. By setting the mass ratio of the activator to the alkali metal silicate to about 50:50 or less, it is possible to effectively suppress an excessive increase in the viscosity of the coating composition and secure a desired fluidity. By setting the mass ratio of the activator to the alkali metal silicate to about 5:95 or more, the thermal stability of the heat-resistant coating can be improved.

The coating composition may contain a surfactant as an optional component. By using a surfactant, even when a large amount of boron nitride is mixed, separation (for example, precipitation) of boron nitride particles during the formation of the coating can be suppressed, and a more uniform coating can be formed. As the surfactant, for example, a long-chain alkyl sulfate such as ammonium lauryl sulfate which is an anionic surfactant, a polyoxyethylene alkyl ether which is a nonionic surfactant, and the like can be used.

In some embodiments, the content of the surfactant in the coating composition is about 0.01% by weight or more, about 0.02% by weight or more, or about 0.%, based on the solid content of the coating composition. It is 05% by mass or more, about 2% by mass or less, about 1.5% by mass or less, or about 1% by mass or less.

The coating composition may include aggregate as an optional component. By using an aggregate, a coating having high wear resistance can be provided. As the aggregate, for example, at least one selected from the group consisting of alumina, silica, and zirconia can be used.

In some embodiments, the average particle size of the aggregate is about 10 nm or more, about 20 nm or more, or about 50 nm or more, about 10 μm or less, about 5 μm or less, or about 3 μm or less. The average particle size of the aggregate is defined as the effective diameter measured by the laser diffraction / scattering method, assuming that the particles are spherical.

In some embodiments, the content of aggregate in the coating composition is about 3% by weight or more, about 5% by weight or more, or about 10% by weight or more, about 20 based on the solid content of the coating composition. It is mass% or less, about 18 mass% or less, or about 15 mass% or less.

The coating composition may contain an inorganic or organic viscosity modifier as an optional component. By using the viscosity modifier, the surface tension of the coating composition can be reduced, and fluidity suitable for the intended use, for example, spraying properties can be imparted to the coating composition. The viscosity modifier can also impart thixotropy suitable for the coating method to the coating composition. As the viscosity modifier, a polymer-based viscosity modifier such as polyurethane and its modified product (for example, urea-modified polyurethane), acrylic resin, silicone resin, acrylic-modified silicone resin, polyethylene glycol, polypropylene glycol and the like can be used. It is preferable to use an inorganic thixotropy agent as the viscosity modifier because it does not cause carbonization at high temperatures. For example, fine silica powder, calcium carbonate and the like can be used as the inorganic thixotropy agent.

In some embodiments, the content of the viscosity modifier in the coating composition is about 0.1% by mass or more and about 5% by mass or less based on the solid content of the coating composition.

The coating composition generally contains a solvent. As a solvent, water, methanol, ethanol, n-propanol, isopropanol (2-propanol), butanol, alcohol such as ethylene glycol, diethyl ether, diethylene glycol monomethyl ether, ether such as diethylene glycol monobutyl ether, ketone such as acetone and methyl ethyl ketone, acetic acid. Ethers such as ethyl and butyl acetate, mixed solvents thereof and the like can be used. The water used to prepare the coating composition is preferably deionized water.

In one embodiment, the coating composition is an aqueous composition containing water, alcohol, or a mixed solvent of water and alcohol as a solvent. The water-based coating composition is easy to handle, has excellent spraying characteristics, and can form a heat-resistant coating having excellent adhesion to the surface of the base material.

The coating composition may also contain, as other optional components, a curing accelerator such as metal alkoxide, a molding agent such as polyvinyl butyral, polyethylene glycol, polyethyleneimine, polyvinyl alcohol, polyvinylpyrrolidone, a pigment, a pH adjuster, a dispersant and the like. Good.

The coating composition can be prepared by mixing alkali metal silicates, activators, boron nitride, and other optional components such as surfactants, aggregates and the like. For example, the coating composition can be prepared by the following procedure. The activator and boron nitride are mixed. Alkali metal silicate is added to the resulting mixture. The alkali metal silicate is added, for example, in the form of a 50% by weight aqueous solution. Surfactants, aggregates and the like are added to the mixture as needed, and a solvent is added as needed to obtain a desired solid content, and the mixture is mixed for about 30 minutes to about 10 hours. In order to prevent an unnecessary reaction from occurring during mixing, components other than the alkali metal silicate may be mixed, and finally the alkali metal silicate may be added and mixed. Aggregates may be mixed immediately prior to use of the coating composition. For mixing, a paint shaker and a mixing device such as beads such as zirconia beads, alumina beads, silica beads, or a homomixer can be used.

In some embodiments, the solid content of the coating composition is about 10% by weight or more, about 20% by weight or more, or about 25% by weight or more, about 70% by weight or less, about 65% by weight or less, or about 60% by mass. It can be less than or equal to%. By setting the solid content of the coating composition in the above range, the composition can be obtained in which the components are uniformly dispersed, and the spraying characteristics of the coating composition can be enhanced.

In some embodiments, the viscosity of the coating composition is optimized by shear rate. That is, at a low shear rate, the high viscosity does not cause liquid dripping even on the vertically provided coated surface, and at a high shear rate, the low viscosity results in good sprayability. Specifically, as a low shear rate region, the viscosity at a shear rate of 5 sec ^-1 is about 500 mPa · s or more, about 700 mPa · s or more, or about 1000 mPa · s or more. As a high shear rate region, the viscosity at a shear rate of 200 sec ^-1 is about 100 mPa · s or less, about 80 mPa · s or less, or about 50 mPa · s or less. The viscosity of the coating composition is measured by a rheometer (Thermo Scientific ^TM HAAKE RheoStress 1: manufactured by Thermo Fisher, Mass., USA).

In some embodiments, the surface tension of the coating composition is about 10 mN / m or more, about 12 mN / m or more, or about 15 mN / m or more, about 40 mN / m or less, about 36 mN / m or less, or about 34 mN /. It is less than m. A coating composition having a surface tension in the above range is particularly suitable for spray coating on the surface of a metal or ceramic.

When the coating composition is an aqueous composition, the pH of the coating composition can be, for example, about 7 or more and about 14 or less. By setting the pH of the aqueous coating composition in the above range, the stability of the components in the composition can be enhanced and the storage stability of the coating composition can be enhanced.

In some embodiments, the content of the alkaline earth metal in the coating composition is about 30% by weight or less, about 25% by weight or less, or about 20% by weight or less based on the solid content of the coating composition. .. By setting the content of the alkaline earth metal to about 30% by mass or less, gelation or thickening of the coating composition over time can be prevented or suppressed, and storage stability can be enhanced.

In some embodiments, the silicon / alkali metal molar ratio of the coating composition is about 2.0 or greater. By setting the silicon / alkali metal molar ratio of the coating composition to 2.0 or more, the degree of polymerization or the crosslink density of the alkali metal silicate matrix can be increased, and a high-strength coating can be obtained.

The coating composition can be applied to the substrate surface by spraying, dipping, casting, spin coating, knife coating, brush coating and the like. In one embodiment, the coating composition is sprayed onto the surface of the substrate.

In one embodiment, the substrate is a metal such as iron, chromium, copper, nickel, aluminum, titanium, tin, zinc, and alloys thereof, or ceramics such as alumina, silica. The surface of the base material may have a primer treatment.

The thickness (wet thickness) at the time of application of the coating composition can be about 30 μm or more, about 50 μm or more, or about 80 μm or more, about 1000 μm or less, about 900 μm or less, or about 800 μm or less. By setting the thickness of the coating composition at the time of application within the above range, peeling from the surface of the base material can be more effectively prevented or suppressed.

A heat resistant coating is formed by curing the applied coating composition at room temperature or by heating. The curing temperature can be about 50 ° C. or higher, about 100 ° C. or higher, or about 200 ° C. or higher. Curing of the coating composition may be carried out in the environment in which the substrate is actually used. For example, the coating composition may be applied to the inner wall of a coal power plant boiler or the surface of a steam pipe, and the coating composition may be cured on the spot by operating the boiler.

In one embodiment, the heat resistant coating is selected from the group consisting of a matrix of alkali metal silicates, layered silicates, and hydroxides, oxides and silicates of alkaline earth metals or Group 13 elements. It contains an activator having a reactivity with an alkali metal silicate containing at least one of the above, and boron nitride.

In some embodiments, the thickness of the heat resistant coating (dry thickness) is about 25 μm or more, about 35 μm or more, or about 50 μm or more, about 300 μm or less, about 280 μm or less, or about 250 μm or less. By setting the thickness of the heat-resistant coating within the above range, strength and adhesion to the base material can be ensured.

The coating composition of the present disclosure can be used in various applications in which heat resistance and ash adhesion prevention performance are required. For example, the coating composition can be used to form a heat resistant coating on a ceramic inner wall or the surface of a steel steam pipe that reaches a temperature of 500-600 ° C. inside a coal power plant boiler.

The following examples exemplify specific embodiments of the present disclosure, but the present invention is not limited thereto. All parts and percentages are by mass unless otherwise stated.

Table 1 shows the raw materials used for preparing the coating composition of this example.

Example 1 to Example 16
Preparation of coating composition The activator and boron nitride were mixed, and an aqueous solution of alkali metal silicate, a solvent and other components were added to the obtained mixture. Then, the mixture was well stirred with a homomixer to obtain a coating composition. Table 2 shows the composition of the coating composition.

The coating composition was evaluated for the following items.

Storage stability Rheometer (Thermo Scientific ^TM HAAKE RheoStress 1: Thermo Fisher, USA, USA) measures the viscosity (mPa · s) immediately after preparation of the coating composition and the viscosity (mPa · s) after storage at room temperature for January. Measured using (Massachusetts). When the rate of increase in viscosity after storage in January (= (viscosity after storage in January-initial viscosity) / initial viscosity x 100) with respect to the initial viscosity is 20% or less, it is described as "good" in Table 2 below. ..

Ash adhesion prevention (contact angle)
The coating composition was applied to an alumina plate, dried at 90 ° C. for 30 minutes, and then calcined at 600 ° C. for 5 hours. Phosphate glass frit irregularly shaped (VY0144, deformation point 357 _℃, P 2 _{O 5} content of 35 to 40 wt%, Nippon frit Ltd. (Aichi Handa, Japan)) as the simulated ash, on the baked coating I put it in. The sample was heated at 600 ° C. for 1 hour, naturally cooled, and then the appearance of the phosphoric acid glass once melted and solidified on the coating was observed to determine the contact angle between the phosphoric acid glass and the coated surface from the photographic image. When the glass frit is melted at a high temperature, it is repelled by a coating containing boron nitride, deformed into a spherical shape by its surface tension, and naturally cooled to maintain its shape in a spherical shape. Those having a contact angle of 100 degrees or more are particularly excellent in ash adhesion prevention property.

The blast-resistant coating composition was applied to a SUS304 plate, dried at 90 ° C. for 30 minutes, and then fired at 600 ° C. for 5 hours. The coating was blasted under the following blasting conditions. The blast resistance was evaluated as a percentage of the residual coating amount when the mass of the initial coating was 100%.
-Blast medium: glass beads (FGB-100, soda lime glass beads, average particle size 150-180 μm, Fuji Seisakusho Co., Ltd. (Edogawa-ku, Tokyo, Japan))
・ Air pressure: 0.2MPa
・ Blast gun nozzle diameter: 2.7 mm
・ Distance between nozzle and sample: 200 mm
・ Blasting time: 1 minute ・ Sample coating area: 20 mm x 20 mm

Table 2 shows the evaluation results of the coating composition.

It will be apparent to those skilled in the art that the above embodiments and examples can be modified in various ways without departing from the basic principles of the present invention. It is also clear to those skilled in the art that various improvements and modifications of the present invention can be carried out without departing from the gist and scope of the present invention.

Claims (15)

(A) Alkali metal silicate and
(B) For the alkali metal silicate containing at least one selected from the group consisting of layered silicates and hydroxides, oxides and silicates of alkaline earth metals or Group 13 elements. Reactive activator and
(C) A coating composition containing boron nitride. The coating composition according to claim 1, which is an aqueous composition. The coating composition according to claim 1 or 2, further comprising an aggregate selected from the group consisting of alumina, silica, and zirconia. The coating composition according to any one of claims 1 to 3, wherein the content of the alkaline earth metal is 30% by mass or less based on the solid content of the coating composition. The coating composition according to any one of claims 1 to 4, wherein the average particle size of the activator is 0.1 μm to 10 μm. The coating composition according to any one of claims 1 to 5, wherein the layered silicate comprises at least one selected from the group consisting of mica, smectite, talc, pyrophyllite and kaolin. The coating composition according to any one of claims 1 to 6, wherein the molar ratio of silicon / alkali metal is 2.0 or more. The coating composition according to any one of claims 1 to 7, further comprising a surfactant. The coating composition according to any one of claims 1 to 8, wherein the viscosity at a shear rate of 5 sec ^-1 is 500 mPa · s or more, and the viscosity at a shear rate of 200 sec ^-1 is 100 mPa · s or less. The coating composition according to any one of claims 1 to 9, which contains 15% by mass to 80% by mass of the boron nitride based on the solid content of the coating composition. The coating composition according to any one of claims 1 to 10, wherein the alkali metal silicate is potassium silicate. The coating composition according to any one of claims 1 to 11, which is applied to the surface of metal or ceramics. A method for forming a heat-resistant coating, which comprises spraying the coating composition according to any one of claims 1 to 12 onto the surface of a metal or ceramic. The alkali metal comprising a matrix of alkali metal silicates and at least one selected from the group consisting of layered silicates and hydroxides, oxides and silicates of alkaline earth metals or Group 13 elements. A heat resistant coating containing an activator reactive with silicates and boron nitride. The heat resistant coating according to claim 14, which has a thickness of 25 to 300 μm.