COATING COMPOSITION

Abstract: A coating composition which comprises (A) 15 to 65 % by weight (calculated as SiO₂) of colloidal silica, (B) 10 to 80 % by weight (calculated as R¹Si(OH)₃) of a partial hydrolytic condensation product of an alkyl­ trialkoxysilane of the formula: R¹Si(OR²)₃ wherein the group R¹ is methyl or vinyl and the group R² is an alkyl having 1 to 4 carbon atoms, (C) 1 to 50 % by weight (calculated as R³ n Si(OH) 4-n ) of a partial hydrolytic condensation product of an acetoxy­ silane of the formula: R³ n Si(OCOCH₃) 4-n wherein the group R³ is an alkyl having 1 to 3 carbon atoms, vinyl, phenyl, or a mercaptoalkyl having 1 to 3 carbon atoms, and n is an integer of 0 to 2, provided that the total amount of the component (B) wherein the group R¹ is methyl and of the component (C) wherein the group R³ is methyl and n is 1 is in the range of from 10 % by weight to less than 70 % by weight based on the whole amount of the components (B) and (C), and (D) a solvent, which is useful for coating plastic articles and metallic articles in order to improve the surface properties such as surface hardness, chemical resistance, etc.

Description: This invention relates to a coating composition useful for improvement of surface properties such as surface hardness, chemical resistance, etc. of plastic articles or metallic articles. Prior Art There have recently been used various plastic articles in replace of glass articles. For instance, shaped articles of polymers such as polycarbonate, polymethyl methacrylate, etc. have been used in wide range of utilities such as lens for glasses and other optical devices, window glass of buildings, window glass of transports such as trains, automobiles, lump cover, and the like, because of the excellent properties, such as excellent transparency, light weight, easy processability, impact resistance. However, these plastic articles have low surface hardness and less solvent resistance, and hence, the surface is easily injured by rubbing and further easily swollen or dissolved by contacting with solvents. Furthermore, the surface of low-hardness metals such as aluminum, or the surface of plastics and metals coated with paints is easily injured by rubbing with steel wool or sand, and it is also injured during use and lose luster. In order to eliminate these drawbacks, it has been proposed to coat them with various hardening resins, but there has never been found any satisfactory method. The present inventors had previously tried to improve the surface hardness of the plastic articles by applying a paint thereto and had found that a composition comprising co-partial hydrolyzates of tetraalkoxysilane and organic silicon compound and/or a mixture of each hydrolyzate, and acrylic polymer and an etherified methylolmelamine can give excellent surface hardness (cf. Japanese Patent First Publication (Kokai) No. 33128/1976, U.S. Patent 4,028,300 and 4,190,699) and further that a composition comprising the same components as above-mentioned application except the contents thereof are varied can give a coating film having well ballanced surface hardness and physiochemical properties of hot water resistance, heat cycle resistance, weatherability (cf. Japanese Patent Second Publication (Kokoku) No. 33858/1985. U.S. Patent 4,224,211). The coating compositions of these patents are practically usable, but are still inferior to glass in the surface hardness and durability. Japanese Patent Second Publication (Kokoku) No. 39691/1977 (U.S. Patent 3,986,997) and Japanese Patent First Publication (Kokai) No. 94971/1980 (GB Patent 2,036,053) disclose coating compositions comprising as the main components colloidal silica and partial hydrolyzate of methyltrimethoxysilane. The compositions can give a coating film having sufficient hardness, but the coating film has less flexibility and less adhesion to plastic articles, and further, it requires too long time until the coating film shows sufficient hardness. Besides, Japanese Patent First Publication (Kokai) No. 177079/1985 discloses a coating composition comprising colloidal silica, a hydrolytic condensation product of organotrialkoxysilane, a specific curing catalyst which can cure the product at a comparatively low temperature within a short period of time, and a solvent. The composition is, however, hardly produced in short period of time, because aging step for several weeks is required for the preparation of said composition. Summary Description of the Invention The present inventors have intensively studied to find an improved coating composition which can give a coating film having excellent surface hardness and adhesion and also excellent durability in hot water resistance, weatherability, heat cycle resistance, etc. by heating in a short-period of time with high productivity and have found that the desired coating composition can be obtained by using a specific combination of colloidal silica, a partial hydrolytic condensation product of alkyltrialkoxysilane, and a partial hydrolytic condensation product of acetoxysilane. An object of the invention is to provide an improved coating composition which is useful for improving the surface conditions such as surface hardness, chemical resistance, of plastic articles or metallic articles. Another object of the invention is to provide a coating composition which can form a coating film having well ballanced properties within a short period of time. These and other objects and advantages of the invention will be apparent to those skilled in the art from the following description. The coating composition of the invention comprises (A) 15 to 65% by weight calculated as SiO₂ of colloidal silica; a partially hydrolyzed and partially co-condensed product of the following component (B) and component (C): (B) 10 to 80% by weight calculated as R¹Si(OH)₃ of a trialkoxysilane of the formula: R¹Si(OR²)₃ wherein R¹ is methyl or vinyl and R² is an alkyl having 1 to 4 carbon atoms, (C) 3 to 35% by weight calculated as R³ Si(OH) 4-n of an acetoxysilane of the formula: R³ Si(OCOCH₃) 4-n wherein R³ is vinyl and n is 1, provided that the amount of component (B) wherein R¹ is methyl is in the range of from 10% by weight to less than 70% by weight based on the whole amount of components (B) and (C); and (D) a solvent. The colloidal silica (A component) is commercially available in the form of an aqueous or alcoholic dispersion containing silica particles having a particle size of 5 to 15 mµ. Such products can be used as they stand. Examples of these commercially available products are "Adelite®" (manufactured by Asahi Denka Kogyo K.K., Japan), "Snowtex®" (manufactured by Nissan Chemical Industries, Ltd., Japan), "Cataloid®" (manufactured by Catalysts & Chemicals Industries Co., Ltd., Japan), and "Rudox®" (manufactured by E.I. DuPont, U.S.A.). The colloidal silica includes both of acidic and basic products, and is incorporated in an amount of 15 to 65 % by weight, preferably 20 to 55 % by weight, (calculated as the solid component SiO₂). The incorporation of the colloidal silica is essential in order to give the composition the desired surface hardness. When it is used in an amount of less than the above lower limit, the coating film formed from the coating composition has insufficient surface hardness, and further, the coating composition has undesirably shorter pot life. On the other hand, when the amount is over the above upper limit, the coating film obtained from the composition showes lower flexibility and less hot water resistance and is liable to be easily cracked or blushed. In the partial hydrolytic condensation product of an alkyltrialkoxysilane of the formula: R¹Si(OR²)₃ (B component), the group R¹ means methyl or vinynl, which is essential in order to obtain a coating film having excellent surface hardness with excellent flexibility. The group R² means an alkyl having 1 to 4 carbon atoms, such as methyl, ethyl, isopropyl, n-butyl, and t-butyl. The group R² releases the corresponding alcohol when hydrolyzed, and the produced alcohol participates at least a part of the solvent (alcohol component) contained in the coating composition. The partial hydrolytic condensation product (B) is incorporated in an amount of 10 to 80 % by weight, preferably 20 to 70 % by weight. This component is used in order to give the coating film the desired ballance between the surface hardness and the durability, and when the amount is less than the above lower limit, the coating film does not show the desired hardness and is liable to show lower hot water resistance and weatherability and hence to be easily cracked, and on the other hand, when the amount is over the above upper limit, the coating film is diadvantageously inferior both in surface hardness and in adhesion. In the partial hydrolytic condensation product of an acetoxysilane of the formula: R³ n Si(OCOCH₃) 4-n (C component), the group R³ means vinyl, and n is 1, because when n is 1, the properties of the coating film are most improved without deterioration of surface hardness. When R³ is this group, the coating film obtained showes excellent flexibility with retaining the excellent surface hardness. The partial hydrolytic condensation product (C) is incorporated in an amount of 3 to 35 % by weight. The (C) component is essential for giving excellent adhesion to the coating film so that the coating film showes well ballanced properties between the surface hardness and the durability, and further, it is effective for shortening the curing time of the coating film, which is also advantageous from the viewpoint of productivity. When the (C) component is used in an amount less than the above lower limit, the desired adhesion is not exhibited and hence the coating film is inferior both in the surface hardness and in the hot water resistance, and on the other hand, when the amount is over the above upper limit, the coating film is liable to be easily blushed, and the coating composition has disadvantageously shorter pot life. The total amount of the component (B) wherein R¹ is methyl group is in the range of from 10 % by weight to less than 70 % by weight, preferably 30 to 50 % by weight, based on the whole amount of the whole components (B) and (C). When the above total amount of the specific components (B) and (C) is less than the above lower limit, the coating film shows insufficient surface hardness and is liable to show lower weatherability (to be cracked), and on the other hand, when the amount is over the above upper limit, the coating film is liable to show lower weatherability, which results in undesirable cracking, low adhesion and low hardness of the coating film. The coating composition of this invention can be prepared in the following procedure. Firstly, an alkyltrialkoxysilane and acetoxysilane are added to a prescribed amount of colloidal silica dispersion, and the mixture is stirred with heating, by which the former two compounds are partially hydrolyzed and simultaneously partially condensed. The hydrolysis can proceed very easily and mildly without necessity of using any catalyst and further without gelation or any other problem. The addition of the alkyltrialkoxysilane and acetoxysilane to the colloidal silica dispersion is preferably done dropwise in view of more improvement of the properties of the coating film. The commercially available colloidal silica dispersions have usually a solid component of 20 to 40 % by weight, and the commercially available products can be used as they stand. When the colloidal silica dispersion has higher concentration of the solid component, the final coating composition has less water content, which is advantageous because there can be omitted a step of distillation of water which is required for regulating the water content in the final product. The above partial hydrolytic condensation reaction is carried out at a temperature of 20 to 100°C, preferably 50 to 90°C, for 0.5 to 5 hours, preferably 1 to 3 hours. The partial hydrolytic condensation products thus obtained are directly diluted with a solvent including a lower fatty alcohol without any intermediate step such as distillation of alcohol and water produced. The mixture is optionally incorporated with other additives such as curing promoter catalysts, surfactants and the like, and the mixture is filtered to give the desired coating composition. The solvent used for preparing the coating composition includes alcohols (e.g. methanol, ethanol, propanol, butanol, etc.), ketone (e.g. acetone, methyl ethyl ketone, etc.), esters (e.g. ethyl acetate, butyl acetate, etc.), ethers (e.g. diethyl ether, dipropyl ether, dioxane, etc.), cellosolves (e.g. methyl cellosolve, butyl cellosolve, cellosolve acetate, etc.), halogenated products (e.g. halogenated hydrocarbons such as trichlorethane, etc.), carboxylic acids (e.g. lower alkylcarboxylic acids having 1 to 4 carbon atoms, such as formic acid, acetic acid, propionic acid, etc.), aromatic compounds (e.g. benzene, toluene, xylene, etc.), and the like, which may be used alone or in combination of two or more thereof. Most suitable solvent is selected depending on the used polymer materials, evaporation speed, and the like. The lower alkylcarboxylic acids (e.g. formic acid, acetic acid, etc.) are particularly preferable because they are effective for giving the coating film excellent adhesion to the substrate. Other preferred solvents are combinations of lower alcohols (e.g. methanol, ethanol, propanol, or butanol), lower alkylcarboxylic acids (e.g. formic acid, acetic acid, or propionic acid), aromatic compounds (e.g. benzene, toluene, or xylene), and cellosolves (e.g. methyl cellosolve, butyl cellosolve, or cellosolve acetate). The amount of the solvent may vary depending on the desired thickness of the coating film or the coating member of the composition, but is usally in such an amount that the concentration of the silicon compound in the final coating composition becomes in the range of 10 to 50 % by weight, preferably 15 to 25 % by weight. The coating composition of this invention can give a coating film by coating on a substrate and baking at a temperature of 70°C or higher. In order to lower the curing temperature, the coating composition may be incorporated by a curing promoter catalyst, such as acids (e.g. hydrochloric acid), toluenesulfonic acid, etc.), organic amines (e.g. triethylamin, etc.), metal salts of organic carboxylic acids (e.g. sodium acetate, etc.), metal thiocyanates (e.g. sodium thiocyanate, etc.), metal nitrites (e.g. sodium nitrite, potassium nitrite, calcium nitrite, etc.), organic tin compounds (e.g. dibutyl tin dilaurate, etc.), and the like. Moreover, for the purpose of obtaining a coating film having a particularly thick thickness, the coating composition may also be incorporated by surfactants (particularly, a small amount of a block copolymer of an alkyleneoxide and dimethylsiloxane) in order to prevent undesirable surface state such as orange peel or shrinkage, and also to prevent cissing of the coating film. These curing promoter catalysts and sufactants can be used only in a small amount, for example, in an amount of 5 % by weight or less, by which the desired effects can be exhibited. The coating composition of this invention can be applied to the substrate by any conventional method, such as spraying, dipping, brushing, and the like. After coating, the coated composition is cured by baking at a temperature of 70°C or higher, and thereby, there is formed the desired coating film having excellent properties, such as transparency, hardness, water resistance, hot water resistance, chemical resistance, heat cycle resistance, and weatherability. The coating composition of this invention is particularly useful for applying to acrylic substrates, but is also useful for applying to other plastic substrates, such as polycarbonate, polystyrene, polyvinyl chloride, ABS resins, and the like, and also to paper, wooden, metallic, and ceramic substrates. As to the substrates having less adhesion, the substrates may previously be under-coated with an acrylic primer such as a solution of polymethyl methacrylate in a solvent, a conventional acrylic lacquer, a thermosetting acrylic paint, or the like, by which the desired adhesion is well exhibited. The coating composition of this invention can give the desired coating film having excellent surface hardness and adhesion and further excellent durability in hot water resistance, weatherability, heat cycle resistance, and the like, within a short period of time, and the coating composition can be prepared in high productivity. This invention is illustrated by the following Examples and Reference Examples, but should not be construed to be limited thereto. In Examples and Reference Examples, "%" means % by weight, unless specified otherwise. Examples 1 to 10 and Reference Examples 1 to 10 (1) Preparation of coating composition: A vessel for hydrolysis provided with a reflux condenser was charged with the (A) component, a: Adelite® AT-30S (manufactured by Asahi Denka Kogyo K.K., Japan) or b: Adelite® AT-30 (manufactured by the same company as above) in the kinds and amounts as shown in Table 1, and while stirring the mixture at a bath temperature of 60°C, at 200 r.p.m., there was added dropwise a mixture of the (B-1) component: CH₃Si(OC₂H₅)₃, the (B-2) component: CH₂=CHSi(OC₂H₅)₃, and the (C) component: CH₂=CHSi(OCOCH₃)₃ in the amounts as shown in Table 1 over a period of time of one hour, and the mixture was continuously stirred with heating for one hour to give co-partial hydrolytic condensation products. The compositions of the co-hydrolytic condensation products (silicon compound) are shown in Table 1. To the co-hydrolytic condensation product (100 parts by weight) was added a solvent mixture consisting of n-butanol (50 %), isopropanol (40 %) and cellosolve acetate (10 %) in such an amount that the concentration of the co-hydrolytic condensation product became 20 %, and to the mixture were further added sodium acetate (0.6 part by weight) and a surfactant SH-28PA (manufactured by Toray Silicone K.K., Japan) (0.6 part by weight), and the resulting mixture was filtered with a filter made of 1 µ glass fiber to give the desired coating compositions. All products has a solid component of 20 %. (2) Coating with the compositions and test of the properties of the coated products: A polymethyl methacrylate sheet plate (thickness 3 mm, Sumipex®000, manufactured by Sumitomo Chemical Company, Limited, Japan) was washed with water and ethanol, and thereto the above coating compositions were applied by dipping method at 23°C, 55 % relative humidity. The coated product was cured by drying with a hot air dryer of 100°C for 2 hours. The thus coated polymethyl methacrylate plate was subjected to observation of appearance and also to the following tests. (i) Adesion test by a cross-hatch test: A number of nicks which reach to the substrate were made in the coating film by a reticulate knife so that a hundred cells having an area of 1 mm² were formed, and thereon was bonded a cellophane tape (manufactured by Sekisui Chemical Co., Ltd., Japan). Thereafter, the cellophane tape was strongly torn up at an angle of 90° against the coating film. The adhesion was evaluated by the number of cells remaining untorn/100 cells. (ii) Surface hardness: it was tested by pencil hardness, steel wool hardness, and Taber abrasion test. (a) Pencil hardness: The surface was scratched with a pencil hardness tester at a load of 1 kg, and it was shown by the maximum pencil hardness giving no scratch. The uncoated product (control) showed the pencil hardness of 5H. (b) Steel wool hardness: The surface of the coated product was rubbed with a commercially available #000 steel wool, and the resulting surface was observed by naked eyes and was evaluated by the degree of scratchability which was graded as follows: A: No scratch B: A few scratches C: Plenty of scratches The uncoated product (control) showed the grade of C. (c) Taber abrasion test: The surface of the coated product was rubbed with a Taber abrasion tester (truck wheel CS-10F, load: 500 g, abrasion cycle: 500 cycle), and the haze was measured by a haze meter. The result was evaluated by the increasing degree of haze: ΔH (%). The uncoated product (control) showed ΔH of 38 %. (iii) Hot water resistance: The coated product was dipped in a boiling water for one hour, and the appearance and adhesion property of the coating film were observed. (iv) Weatherability: The sample was irradiated with a sunshine weather meter (WE-SUN-HCA-1 type, manufactured by Suga Shikenki K.K., Japan) for 2,000 hours, and thereafter, the state of the coating film was evaluated by the appearance, adhesion and steel wool hardness. The above results are shown in Table 2. The compositions of Reference Example 2, 3, 6, 9 and 10 was turbid and could not be used for coating, and hence, the products were not tested. It is to be understood that the invention includes methods of making the coating compositions of this invention.

Claims: A coating composition which comprises (A) 15 to 65% by weight calculated as SiO₂ of colloidal silica; a partially hydrolyzed and partially co-condensed product of the following component (B) and component (C): (B) 10 to 80% by weight calculated as R¹Si(OH)₃ of a trialkoxysilane of the formula: R¹Si(OR²)₃ wherein R¹ is methyl or vinyl and R² is an alkyl having 1 to 4 carbon atoms, (C) 3 to 35% by weight calculated as R³ Si(OH) 4-n of an acetoxysilane of the formula: R³ Si(OCOCH3) 4-n wherein R³ is vinyl and n is 1, provided that the amount of component (B) wherein R¹ is methyl is in the range of from 10% by weight to less than 70% by weight based on the whole amount of components (B) and (C); and (D) a solvent. The composition according to claim 1, wherein the component (A) and the component (B) are incorporated in an amount of 20 to 55 % by weight and 20 to 70 % by weight, respectively. A method of making a coating composition as claimed in any either of claims 1 or 2, wherein the alkyltrialkoxysilane and the acetoxysilane are added to the colloidal silica, and the mixture is stirred with heating whereby the two silane compounds are partially hydrolysed and simultaneously partially condensed. A method as claimed in claim 3 wherein the alklytrialkoxysilane and the acetoxysilane are added separately to the colloidal silica, the addition being dropwise. A method as claimed in claim 3 or claim 4 wherein the partial hydrolytic condensation reaction is carried out at a temperature of 50 to 90°C for 1 to 3 hours. A plastic article or a metallic article which is provided with a coating derived from a coating composition as claimed in any one of claim 1 or 2 or which is produced by the method claimed in any one of claims 3 to 5.

Inventors: FUTAGAMI, MIKIO, MORITANI, MASAHIKO, YUYAMA, MASAHIRO
Applicants: SUMITOMO CHEMICAL COMPANY, LIMITED
Application year: 1987
Main classification: C09D183
IPC codes: C09D183