JPS63170476A - Coating composition - Google Patents

Abstract

PURPOSE:To obtain the titled composition curable at a low temperature and giving a coating film having excellent heat-resistance, etc., by compounding a specific organopolysiloxane, colloidal alumina, water, alcohols and an amine- based silane coupling agent at specific ratios. CONSTITUTION:The objective composition contains (A) 100pts.wt. of an organopolysiloxane (in terms of organoalkoxysilane) consisting of a condensate of an organoalkoxysilane of formula RSi(OR')3 (R is 1-8C organic group; R' is 1-5C alkyl or 1-4C acyl) (preferably methyltrimethoxysilane), (B) 5-50pts. wt. of colloidal alumina (in terms of solid), (C) 10-80pts.wt. of water (including water in the component B if any), (D) >=100pts.wt. of an alcohol (preferably methanol, etc.) and (E) 0.1-15pts.wt. of an amine-based silane coupling agent.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a coating composition, and more particularly, the present invention relates to a coating composition that coats the surface of stainless steel, aluminum, ceramics, cement, fiber, paper, glass, plastic, etc. with heat-resistant coating composition. We manufacture coatings with excellent properties such as water resistance, seawater resistance, organic chemical resistance, acid resistance, alkali resistance, corrosion resistance, heat and light absorption radiation, abrasion resistance, weather resistance, moisture resistance, and adhesion. The present invention relates to a coating composition that cures at a low temperature of 100° C. or lower, which is suitable for the purpose of the present invention.

[Conventional technology]

In recent years, heat resistance, water resistance, seawater resistance, organic chemical resistance, acid resistance, alkali resistance, corrosion resistance, heat and light absorption radiation,
There is a need for a coating composition that has excellent abrasion resistance, weather resistance, moisture resistance, adhesion, etc., and is capable of forming a highly hard coating film.

As a coating composition that partially satisfies these requirements, an acidic aqueous composition containing a partial condensate of silanol and colloidal silica has been proposed (Japanese Patent Publication No.
2-39691).

[Problem that the invention seeks to solve]

However, in the coating composition described in Japanese Patent Publication No. 52-39691, the condensation of the trisilanol contained in the composition does not proceed sufficiently because the resulting composition is aged at room temperature. When dried at room temperature, curing is insufficient, resulting in poor adhesion and decreased hardness. Furthermore, even if a coating film is formed by heat curing using the composition, the adhesion of the coating film to the substrate is still insufficient.

The present invention was made against the background of the above-mentioned conventional technical problems, and has properties such as heat resistance, water resistance, seawater resistance, organic chemical resistance, acid resistance, alkali resistance, heat and light absorption/radiation, abrasion resistance,
An object of the present invention is to provide a coating composition that cures at a low temperature of 100° C. or lower, which has excellent weather resistance, moisture resistance, etc., and can form a coating film with excellent adhesion and high hardness.

[Means for solving problems]

That is, the present invention provides (a) general formula R51(OR')3
(In the formula, R is an organic group having 1 to 8 carbon atoms, and R is an alkyl group having 1 to 5 carbon atoms or an acyl group having 1 to 4 carbon atoms.) (b) 5 to 50 parts by weight of colloidal alumina in terms of solid content; (c) 10 to 80 parts by weight of water (however, present in component (b)) water).

The present invention provides a coating composition characterized by containing (d) 100 parts by weight or more of an alcohol, and (e) 0.1 to 15 parts by weight of an amine-based silane coupling agent.

Next, the composition of the present invention will be explained in detail in terms of constituent elements.

(a) Organopolysiloxane The organopolysiloxane has the general formula RSi
It is obtained by hydrolyzing and polycondensing an organoalkoxysilane represented by (OR")3, and functions as a binder in the composition obtained by the present invention.

R in this organoalkoxysilane has 1 to 1 carbon atoms.
8 organic groups, such as alkyl groups such as methyl group, ethyl group, n-propyl group, i-propyl group, and other γ
-chloropropyl group, vinyl group, 3,3.3-trifluoropropyl group, γ-glycidoxypropyl group, γ-methacryloxypropyl group, γ-mercaptopropyl group,
Examples include phenyl group and 3,4-epoxycyclohexylethyl group.

In addition, R'' in organoalkoxysilane has 1 carbon number.
~5 alkyl group or an acyl group having 1 to 4 carbon atoms, such as methyl group, ethyl group, n-propyl group, i-propyl group, n-butyl group, 5ec-butyl group, tert
-butyl group, acetyl group, etc.

Specific examples of these organoalkoxysilanes include:
Methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, i-propyltrimethoxysilane,
i-propyltriethoxysilane, T-chloropropyltrimethoxysilane, γ-chloropropyltriethoxysilane, vinyltrimethoxysilane, vinyl I-ethoxysilane, 3,3.3-1-lifluoropropyltrimethoxysilane, 3.3.3-1-Lifluoropropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, T-glycidoxypropyltriethoxysilane, γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropyl triethoxysilane,
γ-mercaptopropyltrimethoxysilane, γ-mercaptopropyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, 3.4-epoxycyclohexylethyltrimethoxysilane, 3.
Examples include 4-epoxycyclohexylethyltriethoxysilane, but methyltrimethoxysilane and methyltriethoxysilane are preferred.

These organoalkoxysilanes can be used alone or in combination of two or more.

In addition, (a) 80 of organoalkoxysilane
It is preferable that mol % or more is CH, S i (OR').

The polystyrene equivalent weight average molecular weight of such organopolysiloxane is preferably 3,000 to 50,000,
Particularly preferably 4,000 to 20°000, 3,
If it is less than 5,000, the adhesion of the resulting coating film will decrease;
If it exceeds 0.000, the storage stability of the composition will be poor and gelation will occur, and when the resulting composition is coated, the adhesion of the coating may decrease.

(b) Colloidal Alumina Colloidal alumina prevents gelation of the compositions of the present invention;
Increased viscosity and addition of other fillers used as necessary, and improvement of the heat resistance, hardness and adhesion of the coating film formed by coating the resulting composition, as well as antistatic properties, Others are used for the purpose of providing water necessary for hydrolysis of component (a).

Such colloidal alumina is usually an alumina sol with a pH in the range of 5 to 6 using water as a dispersion medium, and contains 5 to 25% of alumina, pseudoboehmite, boehmite, etc.
The average particle size is usually 5 to 200 mμ,
Preferably it is 10 to 100 mμ.

Examples of this alumina sol include Alumina Sol-100, Alumina Sol-200, and Alumina Sol-520 manufactured by Nissan Chemical Industries.

In addition, colloidal alumina may be one produced by high-temperature hydrolysis of anhydrous aluminum chloride, or one with the general formula Al (OR") l, (where RI + is an alkyl group having 1 to 4 carbon atoms, such as methyl group, ethyl group, n-butyl group, etc., where n is an integer of 1 to 4), and has an average particle size of 5 to 200 mμ, preferably 10 ~100 mμ fine particle alumina 5-25% by weight,
75-95% by weight of water and 0.0% of an acid such as acetic acid or hydrochloric acid.
It is also possible to use colloidal alumina consisting of 5 to 5% by weight.

Such colloidal alumina is strongly positively charged in contrast to the other fixed components in the compositions of the invention.

This colloidal alumina thus forms stable aggregates with other fillers optionally used in the solutions of the compositions of the invention.

Further, as an example of colloidal alumina, an acidic aqueous dispersion of finely particulate alumina having an average particle size of 5 to 200 mμ, preferably 10 to 100 mμ and a pH of 2 to 6 can also be used.

Examples of such fine particulate alumina include aluminum oxide C manufactured by Degussa, Germany.

The proportion of the above colloidal alumina in the composition is
(a) Based on 100 parts by weight of component, 5 to 5 in terms of solid content
The amount is 0 parts by weight, preferably 8 to 40 parts by weight, and if it is less than 5 parts by weight, the gelation prevention and thickening of the composition of the present invention, as well as the dispersibility of other fillers used as necessary, are insufficient. Moreover, when the resulting composition is subjected to coating, the coating film will not have sufficient hardness and will be easily cracked.On the other hand, if it exceeds 50 parts by weight, the amount of other components will be relatively small and the viscosity will be too thick. or the adhesion of the paint film decreases.

(c) Water Water is an essential component for the hydrolysis of the organoalkoxysilane used in component (a), and also
) serves as a dispersion medium for the components.

When water is present in the component (b), in addition to this water, -C tap water, distilled water, ion-exchanged water, etc. can be used separately.

Distilled water or ion-exchanged water is particularly preferred when the composition of the present invention is to be highly purified. The proportion of water in the composition is 10 to 80 parts by weight, preferably 15 to 60 parts by weight, based on 100 parts by weight of component (a), and if it is less than 10 parts by weight, the resulting composition may be used for coating. If the amount exceeds 80 parts by weight, the storage stability of the composition will decrease, which is not preferable.

(d) Alcohols Alcohols are used as a dispersion medium for other fillers used as necessary, and are mainly used to uniformly mix components (a) and (b), and to This is to prevent gelation when hydrolyzed by water.

Such alcohols include monohydric alcohols or dihydric alcohols.
Examples of monohydric alcohols include saturated aliphatic alcohols having 1 to 8 carbon atoms, specifically methanol, ethanol, n-propyl alcohol, i-propyl alcohol, 5ec-7
Examples of the dihydric alcohol include ethyl alcohol, tert-butyl alcohol, and ethylene glycol.

These alcohols are preferably i-propyl alcohol and 5ec-butyl alcohol. These alcohols can be used alone or in combination of two or more.

Moreover, ethylene glycol derivatives such as ethylene glycol monobutyl ether and acetic acid ethylene glycol monoethyl ether can also be added to the alcohols.

The proportion of alcohol in the composition is 100 parts by weight or more, preferably ioo to 4 parts by weight, per 100 parts by weight of component (a).
If the amount is less than 100 parts by weight, the dispersibility of (a) organopolysiloxane and other fillers used as necessary will be poor, and the viscosity of the resulting composition will be too high, making it difficult to apply a homogeneous coating. On the other hand, if the amount exceeds 400 parts by weight, the amount of other components will be relatively small, and the adhesion of the coating film formed by applying the obtained composition to coating may decrease, or the film may be too thin to be used for the purpose. In some cases, it may become impossible to obtain the desired coating film.

(e) Amine-based silane coupling agent The amine-based silane coupling agent is a curing accelerator for the composition of the present invention, and has the effect of curing the coating film at a low temperature of 100° C. or lower. Examples of such amine-based silane coupling agents include γ-aminopropyltriethoxysilane, T-(2-aminoethyl)-aminopropyltrimethoxysilane, T-(2-aminoethyl)-aminopropylmethyldimethoxysilane, and γ-aminopropyltriethoxysilane. -Anilinopropyltrimethoxysilane and the like. These (e
) Amine-based silane coupling agents can be used alone or in combination of two or more. The proportion of the amine-based silane coupling agent in the composition is (a)
The amount is 0.1 to 15 parts by weight, preferably 0.3 to IO parts by weight, per 100 parts by weight of the component. If it exceeds 15 parts by weight, storage stability will deteriorate and pot life will be shortened.

As described above, the coating composition of the present invention can be used as described above (
It contains components a) to (e), and its total solid concentration is preferably 10 to 50% by weight, particularly preferably 15% by weight.
~35% by weight, and if it is less than 10% by weight, the solid content concentration is too low and the properties such as heat resistance, water resistance, chemical resistance, weather resistance, etc. of the coating film formed by coating the resulting composition are impaired. The characteristics may not be expressed, and pinholes may occur in the coating film that is formed.
If it exceeds % by weight, the solid content concentration may be too high, resulting in adverse effects such as deterioration of the storage stability of the composition or difficulty in forming a uniform coating film even when the composition is subjected to coating. .

In addition, it is also possible to add other fillers to the composition of the present invention as needed.

Such fillers are water-insoluble and may include, for example, organic or inorganic pigments.

In addition, particulate or fibrous metals and alloys other than pigments, and their oxides, hydroxides, carbides, nitrides, sulfides, etc. Specific examples include particulate or fibrous iron, copper, aluminum, nickel, Silver, zinc, ferrite, carbon black, stainless steel, silicon dioxide, titanium oxide, aluminum oxide, chromium oxide, manganese oxide, iron oxide, zirconium oxide, cobalt oxide, synthetic mullite, zircon (zirconia silicate), aluminum hydroxide, water Examples include, but are not limited to, iron oxide, silicon carbide, silicon nitride, boron nitride, and molybdenum disulfide. The average particle size or average length of these fillers is usually 0.05 to 50 μm, preferably 0.05 to 50 μm.
If it is less than 0.05 μm, the viscosity of the composition will increase, while if it exceeds 50 μm, the dispersibility of the resulting composition may deteriorate.

The proportion of other fillers used as necessary in the composition is usually 50 to 100 parts by weight of component (a).
500 parts by weight, preferably 80 to 300 parts by weight,
If the amount is less than 50 parts by weight, the performance imparted by the filler to the composition cannot be fully exhibited, while if it exceeds 500 parts by weight, the resulting composition may gel, and when coated, The hardness of the resulting coating film may deteriorate, as well as its adhesion to the substrate, and workability may also deteriorate.

Note that the selection of other fillers to be used as necessary is made based on the purpose of the coating film to be obtained, for example, based on the following selections.

■ Silicon dioxide, silicon dioxide,
Use materials with excellent corrosion resistance such as titanium oxide, aluminum oxide, chromium oxide, zirconium oxide, synthetic mullite, zircon, silicon carbide, and silicon nitride.

■ As a filler for making an electrically insulating film, use an electrically insulating metal oxide, carbide, or nitride that does not contain alkali metals.

■ Fillers for making decorative films include iron oxide, titanium dioxide, cobalt oxide, zinc oxide, tin oxide, lead oxide,
Use oxides such as aluminum oxide.

■ Oxides such as iron oxide, copper oxide, cobalt oxide, manganese oxide, chromium oxide, silicon dioxide, titanium oxide, aluminum oxide, and zircon are used as fillers for making the heat radiation film.

(2) Copper, aluminum, nickel, aluminum, carbon blank, tin oxide, etc. are used as fillers for making conductive or semiconductive films.

■ Metal oxides, hydroxides, nitrides, etc. with low thermal conductivity are used as fillers to create the heat insulating film.

■ Zinc, lead, chromium, etc. are used as fillers to create anti-rust films.

(2) In addition, in order to create a coating film that takes advantage of the characteristics of various fillers, two or more of the above-mentioned fillers can be used in combination.

Furthermore, the composition of the present invention contains various surfactants, silane coupling agents other than amine-based coupling agents, titanium coupling agents, and alkali metal salts such as naphthenic acid, octylic acid, nitrous acid, sulfite, aluminic acid, and carbonic acid. Other additives such as , dyes, etc. may also be added.

The composition of the present invention containing the above-mentioned components (a) to (d) can be prepared, for example, by first adding (a) 100 parts by weight of "Organoalfuki 229" as a starting material for the component (a). On the other hand, (b) colloidal alumina has a solid content of 5~
50 parts by weight, (c) 10 to 80 parts by weight of water (however, (b)
) Contains water that may be present in the ingredients. ),and(
d) Mix 11 to 40 parts by weight of alcohol and perform hydrolysis and polycondensation to form (a) organoalkoxysilane with a weight average molecular weight of 3,000 to 50 in terms of polystyrene.
,000 (a) organopolysiloxane and (b) then add the alcohol (d) to make the total amount of component (d) 100 parts by weight or more.
It can be manufactured more easily.

Here, the amount of component (d) added in step (a) is (
a) 1 to 40 parts by weight, preferably 2 to 25 parts by weight, based on 100 parts by weight of the component, and less than 1 part by weight of (a)
゛Hydrolysis and polycondensation of organoalkoxysilane become too rapid, making it difficult to adjust the weight average molecular weight to a specified polystyrene equivalent, and a gelled product may precipitate.On the other hand, if the amount exceeds 40 parts by weight, organoalkoxysilane Hydrolysis and polycondensation are slowed down and the reaction takes a long time, which is not preferable.

In addition, the molar ratio of component (c)/component (a) in step (a) is preferably 1.5 to 6. If it is less than 1.5, hydrolysis and polycondensation will be difficult to proceed, while if it exceeds 6. and the storage stability of the composition may decrease.

In addition, the amount of component (d) added in this step (a) is:
It is preferable to adjust the amount to 10 to 40% by weight of component (c); if it is less than 10% by weight, the amount of water, which is component (c), will be relatively too large, resulting in rapid hydrolysis and polycondensation. If it is too much, the storage stability of the composition will deteriorate, while if it exceeds 40% by weight, the amount of water will be relatively too small, resulting in slow hydrolysis and polycondensation, and the reaction may take a long time.

This step (a) is usually carried out by mixing the components (a) to (d) and stirring at a temperature of 40 to 90 degrees Celsius, preferably 50 to 80 degrees Celsius, and a reaction time of 1 to 10 hours, preferably 2 hours.
The process is carried out for about 8 hours, and as described above, a composition containing organopolysiloxane (a) having a polystyrene equivalent weight average molecular weight of 3,000 to 50,000 is obtained.

According to the method for producing a composition of the present invention, component (d) is then further added in step (b).

That is, step (b) suppresses the organopolysiloxane (a) produced after the hydrolysis and polycondensation in step (a) from further hydrolyzing and polycondensing during storage and increasing its molecular weight. It also acts as a diluent to adjust the solid content concentration of the entire composition.

The amount of component (d) added in this step (b) is (a)
'The total amount of component (d) is 100 parts by weight of component lOO.
It is added in an amount of at least part by weight.

Furthermore, in the present invention, in the composition containing components (a) to (d) prepared in this way, (
c) Component (e) is additionally added.

In addition, in the manufacturing method of the composition of the present invention, when the other fillers used as necessary are further blended, the following manufacturing method may be adopted in accordance with the manufacturing method described above. .

That is, the components (a) to (d) and the filler are mixed, and the step (a) is performed, followed by the step (b), and then the step (c).

Alternatively, the filler may be added separately in step (b) after mixing the components (a) to (d) (a).

The coating composition of the present invention can form a coating film with a dry film thickness of about 5 to 50 μm in one coat on the surface of the target substrate using a coating method such as brushing, spraying, or dipping. , dry film thickness 10-1 after 2-3 coatings
Film thickness can be made to about 50μm, and temperature range from room temperature to 100℃
If an insulating material is used as a filler by drying and curing at a temperature of 10 minutes to 12 hours, it will pass a discharge test of 1,000 volts or more with a commercially available discharge pinhole tester without pinholes. It is possible to form a coating film.

Examples of substrates to be coated with the composition of the present invention include stainless steel, aluminum, ceramics, cement, fibers, paper, glass, and plastics.

〔Example〕

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples.

In the examples, parts and percentages are based on weight unless otherwise specified.

In addition, gel permeation chromatography (G
The measurement of the weight average molecular weight in terms of polystyrene by the PC method and various measurements in the examples are as follows.

For measurement by the GPC method, a sample was prepared by dissolving 1 g of organopolysiloxane in 100 cc of tetrahydrofuran using tetrahydrofuran as a solvent under the following conditions. Further, as the standard polystyrene, standard polystyrene manufactured by US Pressure Chemical Company was used.

Equipment: Made by Waters, USA, high-temperature high-speed gel permeation chromatogram (Model 150-CALC/GPC) Column: Made by Showa Denko 1, 5HODEX A-8M,
Length: 50 cr++ Measurement temperature: 40° C. Flow rate: lcc/min Adhesion was determined by performing a tape peel test three times after a substrate test according to JIS K5400, and giving an average of 1.

The hardness was based on the pencil hardness according to JIS K5400.

Heat resistance was determined by holding the sample at 400°C for 240 hours in an electric furnace, allowing it to cool naturally, and observing the state of the coating film.

Thermal cycle properties are as follows: heating at 200°C for 30 minutes and heating at -20°C.
Cooling was repeated 100 times for 30 minutes at °C, and the state of the coating film was observed.

Storage stability was determined by storing the product in a polyethylene bottle and visually determining the presence or absence of gelation.

In the pinhole test, a discharge type pinhole tester (manufactured by Sanko Denshi Kenkyusho ■, PRD) was used to discharge at 2,000 volts, and the presence or absence of pinholes was examined.

Boiling water resistance was determined by boiling in tap water for 8 hours and observing the state of the coating film.

Water resistance was determined by immersing the film in tap water for 60 days and observing the state of the coating film.

For acid resistance (1), one drop of 20% hydrochloric acid was added onto the coating film, and after standing for 10 minutes in a petri dish with a lid, the coating was washed with water and the condition of the coating film was observed.

The condition of the coating film was then observed.

Alkali resistance was determined by dropping 1 ml of a 1% sodium hydroxide aqueous solution onto the paint film, leaving it in a petri dish with a lid for 1 day,
After washing with water, the condition of the coating film was observed.

Weather resistance was determined by conducting a 2,000 hour irradiation test using a weather meter according to JIS K5400, and observing the state of the coating film.

Example 1 and Comparative Example 1 (a) in a reactor equipped with a reflux condenser and a stirrer.

100 parts of methyltrimethoxysilane, (b) alumina sol (aqueous dispersion manufactured by B-San Kagaku Kogyo ■, solid content concentration 20%)
), 15 parts of i-propyl alcohol (d) were added, and the mixture was heated to 60°C and reacted for 4 hours to obtain a composition. Alumina was removed from the resulting composition using a centrifuge, and the supernatant was filtered through a filter with a pore size of 0.45 μm.
When this filtrate was measured by GPC method, the weight average molecular weight in terms of polystyrene was 7,300.

To the composition obtained in the same manner as above, (d) an additional 100 parts of i-propyl alcohol (hereinafter simply referred to as "additional i-propyl alcohol") and (e) 3 parts of γ-aminopropyltriethoxysilane were added. A coating composition A was obtained.

In the same manner as above, the types, amounts, reaction temperatures, and reaction times of (a)'organoalkoxysilane, (b) colloidal alumina, (d) alcohols, and (e) amine-based silane coupling agent were determined in Table 1. Coating compositions B-F and GL were obtained by changing the coating compositions as shown in .

The compositions A-F and 11-K shown in Table 1 were spray-coated onto the following substrates (a) and (b), respectively, and cured at a low temperature of 100°C or less to form a cured coating film. .

Substrate (a) Nislate plate (JIS A 540) Substrate (b) Nislate plate (JIS l-14000,
A1050P) These cured coating films were tested for adhesion, hardness, heat resistance, and cold/hot cycle properties. The results are shown in Table 2.

Example 2 and Comparative Example 2 From the compositions AF and HK shown in Table 1 (e
Each of the compositions excluding component () was used as a binder, and various fillers shown in Table 3 and component (d) were added thereto and mixed in a ball mill for 5 hours.
)' components are added as shown in Table 3. Coating compositions MR and SV were obtained. The compositions M-R and S-V shown in Table 3 were spray-coated onto the substrates of (a) and (b), respectively, and cured at a low temperature of 100°C or less to form a cured coating film. .

These cured coating films were tested for adhesion, hardness, heat resistance, cold/hot cycle resistance, boiling water resistance, water resistance, acid resistance, alkali resistance, and weather resistance. The results are shown in Table 4.

〔Effect of the invention〕

The coating film obtained by coating the composition of the present invention has excellent heat resistance and adhesion, is free from pinholes, and is made of stainless steel, aluminum, ceramics, cement, fibers, paper, glass, plastics (excluding Teflon, (Excluding those without functional groups such as polyethylene and polypropylene) can be used for almost all materials, and is characterized by curing at a low temperature of 100°C or less.

Furthermore, by changing the filler added as necessary, the composition of the present invention can be improved in heat resistance, water resistance, seawater resistance, organic chemical resistance, acid resistance, alkali resistance, heat and light absorption and radiation properties. , corrosion-proofing films, electrical insulation films, decorative films, radiation-absorbing films, rust-proofing films, conductive films, semiconductor films, flame-retardant films, with excellent abrasion resistance, weather resistance, moisture resistance, adhesion, storage stability, etc. It can be used to create heat insulating films, hardened films, protective films, etc.

Claims (1)

[Claims] (1) (a) General formula RSi(OR')_3 (wherein, R is an organic group having 1 to 8 carbon atoms, and R' is an alkyl group having 1 to 5 carbon atoms or an acyl group having 1 to 4 carbon atoms. 100 parts by weight of an organopolysiloxane consisting of a condensate of organoalkoxysilane represented by (b) 5 to 50 parts by weight of colloidal alumina in terms of solid content, (c) 10 parts by weight of water. ~80 parts by weight (including water that may be present in component (b)) (d) 100 parts by weight or more of alcohol, and (e) 0.1 to 15 parts by weight of an amine-based silane coupling agent A coating composition characterized by containing