Coating compositions, method of forming hydrophilic films, and hydrophilic film-coated articles

Abstract: Coating compositions comprising (a) an organosilicon compound obtained by the hydrolysis in water of a hydrolyzable mixture of specific hydrolyzable silanes or their partial hydrolyzates, (b) water, and (c) a microparticulate photooxidation catalyst, substantially reduce or eliminate the amount of organic solvent required. In addition, these coating compositions provide applied films having good weather resistance and antifouling properties, and are themselves stable even at an alkaline pH.

Description: BACKGROUND The present invention relates to coating compositions suitable for the protection of articles directly exposed to the elements, such as buildings and other structures. This invention relates also to a method of forming hydrophilic films using these compositions, and to coated articles having hydrophilic films obtained with these compositions. Because of their corrosion resistance and weathering resistance, silicone resins and fluorocarbon resins are commonly employed in exterior coatings used on buildings and structures which are directly exposed to the elements. Coats composed of these resins are resistant to degradation by acid rain and long-term exposure to sunlight. These coatings had generally been thought to be endowed also with excellent antifouling properties, but long-term weathering resistance tests showed that, due to the influence of atmospheric grime, dust and grit, iron particles, acid rain and the like, contaminants gradually deposit onto the surface of the coat, causing a deterioration in its appearance that is indicative of modest or inadequate antifouling properties One known method for providing a coat with improved antifouling properties makes use of a self-cleaning effect achieved by adding to the paint or coating an alkyl silicate which is a silanol group precursor, and rendering the surface of the applied coat hydrophilic, so that contaminants are washed from the coat by rainwater. Contaminant adsorption can be prevented for a while in this way, but the self-cleaning effect is short-lived because the hydrophilising substance tends to be carried away on account of poor water resistance, and contaminants eventually become firmly attached to the coat surface. Prior-art techniques for preventing the deposition of contaminants by hydrophilising the coat surface were thus inadequate in the durability and persistence of the desired effect. Another approach for preventing the hydrophilising substance from being carried away involves the introduction of organic functional groups, but the degree of hydrophilisation is unsatisfactory (see JP-A 40907/1997, JP-A 40908/1997, JP-A 40909/1997, JP-A 40911/1997). International Patent Application WO 96/29375 describes a recently discovered method for imparting hydrophilicity to a coat that involves adding a microparticulate photooxidation catalyst, typically titanium oxide, to the coating composition. Irradiation of the photooxidation catalyst-containing coat with UV-containing light renders the coat hydrophilic to an extent where the water-drop contact angle of the surface becomes 10 degrees or less. This is believed to be a hydrophilising system in which electrons and holes form at the surface of the fine photocatalyst particles that have been exposed to light. Oxygen and water in the air are oxidized and reduced at the surface of these particles, generating active radical species which in turn partially decompose the silicone resin serving as the binder in the coat, forming hydrophilic silanol groups at the surface of the coat, where moisture in the air is then adsorbed. Deposited organic contaminants are self-cleaned from the coat by rainwater. The hydrophilic properties of the coat can be maintained because even the contaminating ingredients that adhere to the surface of the coat are completely degraded oxidatively by the radical species which are generated. In addition, the presence of numerous hydroxyl groups at the surface of the fine particles of the photooxidation catalyst, such as titanium oxide, allows the particles to bond securely to the binder resin and not be carried away from the coat, thus making it possible to semi-permanently sustain the antifouling properties. One known example of the generation of radicals by a photooxidation catalyst is the degradative deterioration (or chalking) by sunlight of a paint film in which titanium oxide is used as a white pigment. The surface of pigment-grade titanium oxide is generally coated with a metal oxide such as silica or alumina, which prevents the formation of radicals to a considerable yet incomplete degree. In long-term exposure tests on paint films, the erosion of resin from about the pigment is due to degradation of the resin by the small amount of radicals that are generated. Therefore, the binder resin in a hydrophilisable, photooxidation catalyst-containing coat must not be readily subject to chalking, and only the surface-most layer of the coat must be attacked by radicals to induce a very slight degree of resin degradation for hydrophilisation. Silicone resins and modified silicone resins are suitable as this type of resin. The silicone resins used as binders in paints and coatings are generally diluted with organic solvents. Even the photooxidation catalyst-containing coating compositions in above-mentioned International Patent Application WO 96/29375 invariably contain an organic solvent for dissolving the silicone resin. However, problems associated with the use of organic solvents, such as environmental contamination, toxicity to workers during application, and fire and explosion hazards, have led to a heightened demand for organic solvent-free water-based coatings. In the design of a water-based silicone resin coating, methods which use a water-insoluble silicone resin as an emulsion obtained by dispersion of the resin in water and methods which use a water-soluble silicone resin may be contemplated. Silicone resin emulsion coatings do not require hydrophilic groups within the resin, in addition to which they have excellent water resistance, making them suitable as water-based coatings. On the other hand, depending on the type of emulsifying agent used, they may be subject to a strong pH influence, resulting in poor stability. Moreover, the processes involved in the preparation of silicone resin emulsion coatings tend to be complex and to require the use of large-scale emulsifying equipment, which can lead to considerable expense. As for the commonly used water-soluble silicone resins, condensation reactions readily occur to induce gelation in pH regions other than the weakly acidic pH levels at which the silanol groups are stable, and especially at an alkalinity of pH 9 or more. In addition, microparticulate photooxidation catalyst-containing suspensions generally remain stably dispersed at a strong acidity of pH 4 or less and at a strong alkalinity of pH 9 or more. However, on account of problems associated with rusting and corrosion in actual use, it is advantageous to make appropriate use of microparticulate photooxidation catalyst-containing suspensions having an alkaline pH. JP-A-9/227829 describes further photocatalytic coating compositions carried in organic solvent. Water for hydrolysis of the silane may be added to the solvent-borne preparation. Suitable hydrolysable silanes include alkyl, alkenyl, aryl , epoxy alkyl and aminoalkyl silanes, with hydrolysable alkoxy groups. A general aim herein to be provide new and useful water-based silicone resin coating compositions able to form hydrophilic coatings. Other aspects are methods of preparation and use of such compositions, and articles coated with them. Preferred aims include the provision of a novel coating composition comprising a silicone resin and a microparticulate photooxidation catalyst, which forms a hydrophilic coating having substantial antifouling properties and light resistance and which is stable even at alkaline pH levels. In our work, we have found that the above aims are achievable with coating compositions comprising a water-based silicone resin compound composed primarily of certain silicone resins specified below and a particulate photooxidation catalyst for rendering hydrophilic the surface of an applied film of the coating composition. Specifically, in a first aspect the present invention provides a coating composition comprising: (a) 100 parts by weight of an organosilicon compound obtainable by hydrolysing a silane mixture containing (i) 100 parts by weight of a nitrogenous organic group-containing silane of the following general formula (1): YR 1 m SiR 2 3-m wherein R 1 is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 8 carbons, R 2 is an alkoxy or acyloxy group having 1 to 4 carbons, Y is a nitrogenous organic group, and m is 0 or 1, or a partial hydrolysate thereof, and (ii) from 5 to 200 parts by weight of a silane of the following general formula (2): R 3 n SiR 4 4-n wherein R 3 is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 8 carbons, R 4 is an alkoxy or acyloxy group having 1 to 4 carbons, and n is 0, 1 or 2, or a partial hydrolysate thereof; (b) from 10 to 2,000 parts by weight of water; and (c) fine particles of a photooxidation catalyst, the composition being substantially free of organic solvent. The invention also provides a method of forming a hydrophilic film by applying the above composition onto a substrate to form a film and irradiating UV-containing light to a surface of the film, e.g. so that the film surface has a contact angle with water of up to 60 degrees. The invention provides as well a hydrophilic film-coated article comprising a substrate having on a surface thereof a film of the above composition, which preferably has a contact angle with water of up to 60 degrees. Preparations of the composition are also covered. We find that a water-based silicone resin composition which is stable in an aqueous solution and, in particular, is stable even in an alkaline aqueous solution is obtainable simply by hydrolysing a mixture of (i) a nitrogenous organic group-containing silane of formula (1) or a partial hydrolysate thereof, and (ii) a silane of formula (2) or a partial hydrolysate thereof, in water or in at least an amount of water necessary for hydrolysis. The composition comprising components (a) to (c) is applied to a substrate to form a film. Upon exposure to UV-containing light, this film becomes hydrophilic. More particularly, at room temperature or under heating, the reactive groups present in the resin at the interior of the film formed by the coating composition of the invention undergo crosslinking reactions that induce curing, thereby giving a firm coat. Fine particles of the photooxidation catalyst are dispersed within the film. When the particles are composed of a metal oxide, chemical bonds form between hydroxyl groups at the surface of the particles and silanol groups within the silicone resin, thereby immobilising the photooxidation catalyst particles on the film and in the film interior. Irradiating this coat with UV-containing light, typically sunlight, triggers the oxidising action of the photooxidation catalyst, causing the contact angle of the surface with water to become 60 degrees or less. Continued exposure to sunlight, and the attendant rise in the dose of light incurred by the coat, leads to a further decline in the contact angle, with a contact angle as low as 10 degrees or less being attainable. Hence, the coating compositions of this invention are effective for protecting the surfaces of articles directly exposed to the elements, such as buildings and other structures. Moreover, because of the presence of particles having a photooxidation catalytic activity, the surface of the applied coat is rendered hydrophilic when exposed to UV-containing light, thus exhibiting excellent antifouling properties. The microparticulate photooxidation catalyst also has light resistance (i.e., it is UV light-absorbing), and is thus able to protect the substrate from deterioration, discoloration, fading and other undesirable changes due to sunlight. In particular, the coating composition of the present invention uses a water-soluble silicone resin as the binder resin and may be substantially free of organic solvents. Hence, environmental contamination is minimal and safety during coating operations is excellent. Morever, we find that the compositions can be stable even in the alkaline region of the pH scale. In addition, because the microparticulate photooxidation catalyst absorbs ultraviolet light and assists the silicone resins in manifesting antifouling effects, the coating compositions may also serve to protect the underlying substrate from harmful UV radiation, thereby making it possible to prevent the fading and discoloration of signs and billboards, printed articles, and the like. DETAILED DESCRIPTION OF THE INVENTION As described above, component (a) of the coating composition according to the present invention is an organosilicon compound obtained by hydrolyzing a silane mixture in water or an organic solvent containing at least an amount of water necessary for hydrolysis. The hydrolyzable silane mixture includes (i) 100 parts by weight of a nitrogenous organic group-containing silane of formula (1) or a partial hydrolysate thereof and (ii) from 5 to 200 parts by weight of a silane of formula (2) or a partial hydrolyzate thereof. YR 1 m SiR 2 3-m R 1 is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 8 carbons, R 2 is an alkoxy or acyloxy group having 1 to 4 carbons, Y is a nitrogenous organic group, and m is 0 or 1. R 3 n SiR 4 4-n R 3 is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 8 carbons, R 4 is an alkoxy or acyloxy group having 1 to 4 carbons, and n is 0, 1 or 2. The nitrogenous organic group-containing silane as component (i) functions to make the system water-soluble. In order to confer water-solubility to the organosilicon compound serving as a primary component of the desired water-based silicone resin coating composition, use is made here of a compound having general formula (1): YR 1 m SiR 2 3-m wherein R 1 is a nitrogen-free, substituted or unsubstituted, monovalent hydrocarbon group having 1 to 8 carbons, R 2 is an alkoxy or acyloxy group having 1 to 4 carbons, Y is a nitrogenous organic group, and m is 0 or 1. The nitrogenous organic group-containing compounds of formula (1) may be used alone or as a mixture of two or more. Partial hydrolysates of these may also be used. In formula (1), R 1 represents a substituted or unsubstituted monovalent hydrocarbon group having 1 to 8 carbons, such as alkyl, cycloalkyl, alkenyl, aryl, and aralkyl groups, as well as groups in which some or all of the hydrogen atoms in the foregoing groups are substituted with halogen atoms such as fluorine. Illustrative examples include -CH 3 , -CH 2 CH 3 , -CH 2 CH 2 CH 3 , -CH(CH 3 ) 2 , -CH 2 CH 2 CH 2 CH 3 , -CH(CH 3 )CH 2 CH 3 , -CH 2 CH(CH 3 )CH 3 , -C(CH 3 ) 3 , -C 6 H 5 , and -C 6 H 13 . R 2 in formula (1) represents an alkoxy or acyloxy group having 1 to 4 carbons. Illustrative examples include -OCH 3 , -OCH 2 CH 3 , -OCH 2 CH 2 CH 3 , -OCH(CH 3 ) 2 , -OCH 2 CH 2 CH 2 CH 3 , -OCH(CH 3 )CH 2 CH 3 , -OCH 2 CH(CH 3 )CH 3 , -OC(CH 3 ) 3 , -OCOCH 3 , and -OCOCH 2 CH 3 . Of these, -OCH 3 and -OC 2 H 5 are preferred. Y is a nitrogenous organic group. Preferably it contains N in one or more aliphatic amino or amido moieties. Examples include Herein, R 5 , R 6 , and R 9 to R 13 are hydrogen atoms or monovalent hydrocarbon groups having 1 to 8 carbons, with R 5 and R 6 , R 9 and R 10 and R 11 , and R 12 and R 13 , respectively, being mutually the same or different; R is a halogen atom; R 7 and R 8 are divalent hydrocarbon groups having 1 to 8 carbons atoms, with R 7 and R 8 being mutually the same or different; and p is 0 or an integer from 1 to 3. The aforementioned monovalent hydrocarbon groups having 1 to 8 carbons are as defined above for R 1 . Illustrative examples of the divalent hydrocarbon groups having 1 to 8 carbons include alkylene groups. Nitrogenous group examples follow. H 2 NCH 2 -, H(CH 3 )NCH 2 -, H 2 NCH 2 CH 2 -, H(CH 3 )NCH 2 CH 2 -, H 2 NCH 2 CH 2 CH 2 -, H(CH 3 )NCH 2 CH 2 CH 2 -, (CH 3 ) 2 NCH 2 CH 2 CH 2 -, H 2 NCH 2 CH 2 HNCH 2 CH 2 CH 2 -, H(CH 3 )NCH 2 CH 2 HNCH 2 CH 2 CH 2 -, (CH 3 ) 2 NCH 2 CH 2 HNCH 2 CH 2 CH 2 -, H 2 NCH 2 CH 2 HNCH 2 CH 2 HNCH 2 CH 2 CH 2 -, H (CH 3 )NCH 2 CH 2 HNCH 2 CH 2 HNCH 2 CH 2 CH 2 -, Cl - (CH 3 ) 3 N + CH 2 CH 2 CH 2 -, Cl - (CH 3 ) 2 (C 6 H 5 -CH 2 -)N + CH 2 CH 2 CH 2 -, Illustrative examples of the nitrogenous organic group-containing silanes of formula (1) are given below. H 2 NCH 2 Si(OCH 3 ) 3 , H 2 NCH 2 Si(OCH 2 CH 3 ) 3 , H 2 NCH 2 SiCH 3 (OCH 3 ) 2 , H 2 NCH 2 SiCH 3 (OCH 2 CH 3 ) 2 , H 2 NCH 2 CH 2 Si(OCH 3 ) 3 , H 2 NCH 2 CH 2 Si(OCH 2 CH 3 ) 3 , H 2 NCH 2 CH 2 SiCH 3 (OCH 3 ) 2 , H 2 NCH 2 CH 2 SiCH 3 (OCH 2 CH 3 ) 2 , H 2 NCH 2 CH 2 CH 2 Si(OCH 3 ) 3 , H 2 NCH 2 CH 2 CH 2 Si(OCH 2 CH 3 ) 3 , H 2 NCH 2 CH 2 CH 2 SiCH 3 (OCH 3 ) 2 , H 2 NCH 2 CH 2 CH 2 SiCH 3 (OCH 2 CH 3 ) 2 , H(CH 3 )NCH 2 CH 2 CH 2 Si(OCH 3 ) 3 , H(CH 3 )NCH 2 CH 2 CH 2 Si(OCH 2 CH 3 ) 3 , H (CH 3 )NCH 2 CH 2 CH 2 SiCH 3 (OCH 3 ) 2 , H (CH 3 )NCH 2 CH 2 CH 2 SiCH 3 (OCH 2 CH 3 ) 2 , (CH 3 ) 2 NCH 2 CH 2 CH 2 Si(OCH 3 ) 3 , (CH 3 ) 2 NCH 2 CH 2 CH 2 Si(OCH 2 CH 3 ) 3 , Cl - (CH 3 ) 3 N + CH 2 CH 2 CH 2 Si(OCH 3 ) 3 , Cl - (CH 3 ) 3 N + CH 2 CH 2 CH 2 Si(OCH 2 CH 3 ) 3 , Cl - (CH 3 ) 2 (C 6 H 5 -CH 2 -)N + CH 2 CH 2 CH 2 Si(OCH 3 ) 3 , Cl - (CH 3 ) 2 (C 6 H 5 -CH 2 -)N + CH 2 CH 2 CH 2 Si(OCH 2 CH 3 ) 3 , H 2 NCH 2 CH 2 HNCH 2 CH 2 CH 2 Si(OCH 3 ) 3 , H 2 NCH 2 CH 2 HNCH 2 CH 2 CH 2 Si(OCH 2 CH 3 ) 3 , H 2 NCH 2 CH 2 HNCH 2 CH 2 CH 2 SiCH 3 (OCH 3 ) 2 , H 2 NCH 2 CH 2 HNCH 2 CH 2 CH 2 SiCH 3 (OCH 2 CH 3 ) 2 , H 2 NCH 2 CH 2 HNCH 2 CH 2 HNCH 2 CH 2 CH 2 Si(OCH 3 ) 3 , H 2 NCH 2 CH 2 HNCH 2 CH 2 HNCH 2 CH 2 CH 2 Si(OCH 2 CH 3 ) 3 , H 2 NCH 2 CH 2 HNCH 2 CH 2 HNCH 2 CH 2 CH 2 SiCH 3 (OCH 3 ) 2 , H 2 NCH 2 CH 2 HNCH 2 CH 2 HNCH 2 CH 2 CH 2 SiCH 3 (OCH 2 CH 3 ) 2 , Of these, H 2 NCH 2 CH 2 HNCH 2 CH 2 CH 2 Si(OCH 3 ) 3 and H 2 NCH 2 CH 2 HNCH 2 CH 2 CH 2 Si(OCH 2 CH 3 ) 3 are especially preferred. Hydrolyzates of the above may also be used. The silane serving as component (ii) has the general formula (2): R 3 n SiR 4 4-n wherein R 3 is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 8 carbons, R 4 is an alkoxy or acyloxy group having 1 to 4 carbons, and n is 0, 1 or 2. The silanes may be used alone or as a mixture of two or more thereof. Partial hydrolysates of these may also be used. In formula (2), R 3 represents a nitrogen-free, substituted or unsubstituted, monovalent hydrocarbon group having 1 to 8 carbons, such as alkyl, cycloalkyl, alkenyl, aryl, and aralkyl groups, as well as these groups in which some or all of the hydrogen atoms are substituted with halogen atoms such as fluorine. Illustrative examples include -CH 3 , -CH 2 CH 3 , -CH 2 CH 2 CH 3 , -CH(CH 3 ) 2 , -CH 2 CH 2 CH 2 CH 3 , -CH(CH 3 )CH 2 CH 3 , -CH 2 CH(CH 3 )CH 3 , -C(CH 3 ) 3 , -C 6 H 5 , and -C 6 H 13 . R 4 in formula (2) represents an alkoxy or acyloxy group having 1 to 4 carbons. Illustrative examples include -OCH 3 , -OCH 2 CH 3 , -OCH 2 CH 2 CH 3 , -OCH(CH 3 ) 2 , -OCH 2 CH 2 CH 2 CH 3 , -OCH(CH 3 )CH 2 CH 3 , -OCH 2 CH(CH 3 )CH 3 , -OC(CH 3 ) 3 , -OCOCH 3 , and -OCOCH 2 CH 3 . Of these, -OCH 3 and -OC 2 H 5 are preferred. Examples of the silanes of formula (2) are given below. Si(OCH 3 ) 4 , Si(OCH 2 CH 3 ) 4 , Si (OCH 2 CH 2 CH 3 ) 4 , Si (OCH 2 CH 2 CH 2 CH 3 ) 4 , CH 3 Si(OCH 3 ) 3 , CH 3 Si(OCH 2 CH 3 ) 3 , CH 3 Si(OCH 2 CH 2 CH 3 ) 3 , CH 3 Si(OCH 2 CH 2 CH 2 CH 3 ) 3 , (CH 3 ) 2 Si(OCH 3 ) 2 , (CH 3 ) 2 Si(OCH 2 CH 3 ) 2 , (CH 3 ) 2 Si(OCH 2 CH 2 CH 3 ) 2 , (CH 3 ) 2 Si(OCH 2 CH 2 CH 2 CH 3 ) 2 , Of these, Si(OCH 3 ) 4 , Si(OCH 2 CH 3 ) 4 , CH 3 Si(OCH 3 ) 3 , CH 3 Si(OCH 2 CH 3 ) 3 , and partial hydrolyzates of these are especially preferred. When the nitrogenous organic group-containing silane having formula (1) or a partial hydrolysate thereof (i) and the silane having formula (2) or a partial hydrolysate thereof (ii) are mixed and used together, the mixing proportions are from 5 to 200 parts by weight, and preferably from 10 to 150 parts by weight, of the silane of formula (2) or a partial hydrolysate thereof (ii) per 100 parts by weight of the nitrogenous organic group-containing silane of formula (1) or a partial hydrolysate thereof (i). More than 200 parts by weight of component (ii) is undesirable because the stability within the alkaline region of the pH scale is unacceptably low. The method for preparing component (a) of the coating composition preferably uses primarily water as the solvent, although a solvent obtained by adding to water a water-miscible organic solvent (e.g., an alcohol, ester, ketone, or glycol) may be used if necessary. Examples of suitable organic solvents include alcohols such as methanol, ethanol, 1-propanol, and 2-propanol; esters such as methyl acetate, ethyl acetate, and ethyl acetoacetate; ketones such as acetone and methyl ethyl ketone; and glycols such as glycerol and diethylene glycol. The amount of solvent is preferably from about 400 to about 5,000 parts by weight, and more preferably from about 1,000 to about 3,000 parts by weight, per 100 parts by weight of the starting silanes. At less than 400 parts by weight of solvent, the reaction may proceed so far that a homogeneous system is not achieved, in addition to which the liquid may become less stable during storage. On the other hand, an amount of solvent in excess of 5,000 parts by weight would be economically disadvantageous. The amount of water in the solvent is preferably such that the molar ratio of water to the starting silanes is from 5/1 to 50/1. At a molar ratio of less than 5, hydrolysis does not readily proceed to completion, which adversely effects the stability of the liquid, whereas a molar ratio of more than 50 tends to be economically undesirable. Examples of suitable reaction methods include those given below. (1) Dropwise addition of the mixed silanes to water or an organic solvent containing at least an amount of water necessary for hydrolysis. (2) Dropwise addition of water to the mixed silanes which either do or do not contain an organic solvent. (3) Dropwise addition of the silane or a partial hydrolysate thereof serving as component (ii) to water or an organic solvent containing at least an amount of water necessary for hydrolysis, followed by dropwise addition of the nitrogenous organic group-containing silane or a partial hydrolysate thereof serving as component (i). (4) Dropwise addition of the nitrogenous organic group-containing silane or a partial hydrolysate thereof serving as component (i) to water or an organic solvent containing at least an amount of water necessary for hydrolysis, followed by dropwise addition of the silane or a partial hydrolysate thereof serving as component (ii). Reaction method (1) is especially preferable because of the stability of the resulting water-based silicone resin composition. With these methods, the silicone resin is obtained in the form of an aqueous solution. Water may be added or removed in order to adjust the amount of water to 10 to 2,000 parts by weight, and preferably 100 to 1,000 parts by weight, per 100 parts by weight of the silicone resin, and thereby form a water-based binder composition. Less than 10 parts by weight of water gives the organosilicon compound a poor storage stability, whereas more than 2,000 parts by weight of water requires to apply a too much amount of the water-based silicone resin composition, which is not cost-effective. The substantial absence of organic solvent within this water-based binder composition is a feature herein; up to 50 parts by weight, and especially up to 30 parts by weight, of organic solvent may be included per 100 parts by weight of the silicone resin. Now the particulate photooxidation catalyst used as component (c) is described. The photooxidation catalyst. when present at the surface of the applied coat, should have a high enough photooxidation catalytic activity to degrade the organic groups that make the surface hydrophobic. Examples of suitable photooxidation catalysts include titanium oxide, cerium oxide and zinc oxide. Anatase-type titanium oxide is especially preferable on account of its chemical stability and low cost. Because the photooxidation catalytic activity generally becomes higher the smaller the mean particle size, it is advantageous to use particles no larger than about 0.1 micron in size. The phrase "mean particle size," as used herein, refers to the value obtained by the Scherrer formula from the integrated width of the highest peak in the x-ray diffraction pattern for crystals in a powder sample of the photooxidation catalyst particles. For reasons having to do in part with the transparency of the applied film, the photooxidation catalyst in the coating compositions of the invention is most advantageously used in the form of a sol of the photooxidation catalyst particles colloidally dispersed within a dispersing medium. However, the photooxidation catalyst particles may also be used as a water- or solvent-containing paste, or as a powder. Examples of dispersants which may be used in the colloidal sol include water, alcohols such as methanol, ethanol, isopropanol, n-butanol and isobutanol, and Cellosolves. Of these, water is the most preferable in terms of the dispersion stability of the colloid and its mixing stability with the aqueous silicone resin composition. If an organic solvent-dispersed sol is employed, this should preferably be used such that the content of organic solvent in the overall coating composition is up to 5% by weight. At an organic solvent content of more than 5% by weight, the above-described concerns over environmental pollution, safety during application, and storage stability of the liquid coating composition may not be adequately resolved. Moreover, it is advantageous that an alkali be included in the sol of photooxidation catalyst particles colloidally dispersed in this dispersant so that the sol has a pH of at least 9, and preferably 10 or more. If, on the other hand, the photooxidation catalyst is used in the form of a powder or a paste, it is advantageous to mix the catalyst with the water-based silicone resin composition, then thoroughly disperse the catalyst using an apparatus such as a sand mill or a paint shaker. The amount of microparticulate photooxidation catalyst added to the water-based silicone resin composition is not critical, although an amount of about 10 to 400 parts by weight per 100 parts by weight of the resin solids is preferable. Increasing the photooxidation catalyst content is effective for speeding up the rate at which the surface of the applied coat becomes hydrophilic and enhancing antifouling properties, but at the sacrifice of crack resistance. Hence, use of the photooxidation catalyst within a range of 50 to 200 parts by weight per 100 parts by weight of the resin solids is especially preferable. In addition to the three above-described essential components (a) to (c), the composition of the invention may also contain additives, diluents and curing catalysts as described below. The microparticulate photooxidation catalyst is inherently deodorizing and antimicrobial on account of its powerful oxidising effects when irradiated with light. In order to further enhance these capabilities, substances containing one or more metallic elements selected from among silver, copper, platinum, gold, palladium, iron, nickel, and zinc may also be included. Commonly known surfactants, titanate coupling agents and silane coupling agents may be added to improve the stability of the inventive compositions in coating liquid form. The compositions of the invention can be applied onto various substrates, and film-forming ingredients, leveling agents and organic solvents may be added to enhance the film formability. Film-forming ingredients that may be added include water-soluble polymers for increasing the viscosity of the liquid coating for enhancing film formability. Examples include polyvinyl alcohol, polyacrylic acid, carboxymethyl cellulose, polyvinyl acetamide and polyvinyl pyrrolidone. Any commonly known leveling agent may be used, although polyether-modified silicones are preferable. Further, the addition of a small amount of organic solvent is acceptable if it is effective for enhancing the film formability. Preferable solvents for this purpose include alcohols having up to 5 carbon atoms, ethyl cellosolve, butyl cellosolve, ethylene glycol ethers, propylene glycol ethers and lactic acid esters. To maintain the stability of the coating composition, the amount of solvent added is set at 5% by weight or less, based on the overall composition. Any curing catalyst known to the prior art may be used in the compositions of the invention. Exemplary curing catalysts include basic compounds such as lithium hydroxide, sodium hydroxide, potassium hydroxide, sodium methylate, sodium acetate, sodium formate, n-hexylamine, tributylamine, and diazabicycloundecene; metal-containing compounds such as tetraisopropyl titanate, tetrabutyl titanate, aluminum triisobutoxide, aluminum triisopropoxide, aluminum acetylacetonate, aluminum perchlorate, cobalt octylate, cobalt acetylacetonate, iron acetylacetonate, tin acetylacetonate, and dibutoxytin octylate; and acidic compounds such as p-toluenesulfonic acid, trichloroacetic acid, hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid. These are used in a catalytic amount. Combinations of acidic and basic compounds that serve as buffers for regulating the pH may be added. Examples include acetic acid with sodium acetate, and disodium hydrogen phosphate with citric acid. Use can also be made of other ingredients, such as pigments, dyes, and storage stabilizers, so as to provide an excellent coat performance. A hydrophilic film can be formed as follows. First, the diluent, various additives, and the curing catalyst are added to and mixed with the above composition to prepare a coating liquid. The coating liquid is then applied to the substrate, and the applied film is air-dried or heat-cured to form a cured film. This cured film is subsequently irradiated with UV-containing light, preferably making the water-drop contact angle of the film surface 60° or less. Examples of the substrate material used here include, without particular limitation, plastic mouldings, ceramics, glass, metals, wood, cellulose, paper, and composites thereof. The method used to apply the coating composition of the invention onto the substrate is not critical. Some examples of suitable methods of application are spray coating, roller coating, dip coating, spin coating, ink jet coating, and brush coating, although use may be made of other methods as well. Curing of the applied film may be carried out in the same manner as in commonly employed methods for curing applied paint or coating films. That is, the applied film may be air-dried by letting it stand in air, or sufficient curing may be achieved at from room temperature to 250°C for a period of about 2 minutes to about 6 hours, although there are no strict limits on the heating temperature, time and other conditions so long as the temperature does not exceed the heat resistance temperature of the substrate. Sunlight will do as the UV-containing light needed to render the film surface hydrophilic, but any light source at any intensity may be used, provided it includes light having a wavelength of 300 to 400 nm. Examples of suitable light sources run the gamut from high ultraviolet intensity sources, such as commercial black light lamps, mercury-vapor lamps and metal halide lamps, to indoor lighting such as fluorescent lamps and incandescent lamps. Although the dose of light required to hydrophilise the film surface varies depending on the content of photooxidation catalyst particles within the applied film and the wavelength distribution of the light source, the film surface can generally be rendered hydrophilic to a contact angle with water of up to 30° in several days time with excitation at an ultraviolet light dose of 0.001 mW/cm 2 . Because the dose of ultraviolet light present in the sunlight that falls on the Earth's surface is about 0.1 to 1 mW/cm 2 , the film surface can be hydrophilised in even less time by exposing it to sunlight. Once the microparticulate photooxidation catalyst is added to and dispersed in the above-described aqueous silicone resin composition which is stable within the alkaline region of the pH scale, there is obtained the coating composition embodying the invention which is ready for use. By applying and curing this composition, a coat is formed. When exposed to UV-containing light, the coat is hydrophilized to a water-drop contact angle of 60 degrees or less and exhibits outstanding antifouling properties. Because the coating compositions can be resistant to light, coating the composition onto signs and billboards or topcoating it onto printed articles is effective for preventing the fading and discoloration of colored images. EXAMPLES Examples are given below to illustrate the invention, and are not intended to limit the scope thereof. All parts and percents are by weight. Synthesis Examples 1 to 5 describe the synthesis of water-based silicone resin compositions, and Examples 1 to 4 and Comparative Examples 1 and 2 describe the preparation of coating compositions. Synthesis Example 1 A 200-ml reactor equipped with a stirrer, a thermometer, and a condenser was charged with 120 g (6.67 mol) of water. While stirring the water in the reactor, a mixture of 44.4 g (0.2 mol) of H 2 NCH 2 CH 2 NHCH 2 CH 2 CH 2 Si(OCH 3 ) 3 and 13.6 g (0.1 mol) of CH 3 Si(OCH 3 ) 3 was added dropwise at room temperature over a period of 10 minutes, whereupon the temperature within the reactor rose from 27°C to 50°C. The reaction mixture was then heated to a temperature of 60 to 70°C in an oil bath, and stirred in this state for one hour. Next, an ester adapter was attached, the temperature within the reactor was raised to 98°C, and the methanol generated as a by-product was removed, giving 149 g of Water-Based Silicone Resin Composition 1. The nonvolatiles content of this composition was 25.1% as determined by heating at 105°C for 3 hours. Synthesis Example 2 A 200-ml reactor equipped with a stirrer, a thermometer, and a condenser was charged with 120 g (6.67 mol) of water. While stirring the water in the reactor, a mixture of 44.4 g (0.2 mol) of H 2 NCH 2 CH 2 NHCH 2 CH 2 CH 2 Si(OCH 3 ) 3 and 15.2 g (0.1 mol) of Si(OCH 3 ) 4 was added dropwise at room temperature over a period of 10 minutes, whereupon the temperature within the reactor rose from 25°C to 56°C. The reaction mixture was then heated to a temperature of 60 to 70°C in an oil bath, and stirred in this state for one hour. Next, an ester adapter was attached, the temperature within the reactor was raised to 98°C, and the methanol generated as a by-product was removed, giving 137 g of Water-Based Silicone Resin Composition 2. The nonvolatiles content of this composition was 27.3% as determined by heating at 105°C for 3 hours. Synthesis Example 3 A 200-ml reactor equipped with a stirrer, a thermometer, and a condenser was charged with 120 g (6.67 mol) of water. While stirring the water in the reactor, a mixture of 33.3 g (0.15 mol) of H 2 NCH 2 CH 2 NHCH 2 CH 2 CH 2 Si(OCH 3 ) 3 and 22.8 g (0.15 mol) of Si(OCH 3 ) 4 was added dropwise at room temperature over a period of 10 minutes, whereupon the temperature within the reactor rose from 25°C to 53°C. The reaction mixture was then heated to a temperature of 60 to 70°C in an oil bath, and stirred in this state for one hour. Next, an ester adapter was attached, the temperature within the reactor was raised to 98°C, and the methanol generated as a by-product was removed, giving 117 g of Water-Based Silicone Resin Composition 3. The nonvolatiles content of this composition was 27.5% as determined by heating at 105°C for 3 hours. Synthesis Example 4 A 200-ml reactor equipped with a stirrer, a thermometer, and a condenser was charged with 120 g (6.67 mol) of water. While stirring the water in the reactor, a mixture of 55.6 g (0.25 mol) of H 2 NCH 2 CH 2 NHCH 2 CH 2 CH 2 Si(OCH 3 ) 3 and 10.4 g (0.05 mol) of Si(OCH 2 CH 3 ) 4 was added dropwise at room temperature over a period of 10 minutes, whereupon the temperature within the reactor rose from 27°C to 49°C. The reaction mixture was then heated to a temperature of 60 to 70°C in an oil bath, and stirred in this state for one hour. Next, an ester adapter was attached, the temperature within the reactor was raised to 98°C, and the methanol and ethanol generated as a by-product were removed, giving 137 g of Water-Based Silicone Resin Composition 4. The nonvolatiles content of this composition was 31.1% as determined by heating at 105°C for 3 hours. Synthesis Example 5 A 1.0-liter reactor equipped with a stirrer, a thermometer, and a condenser was charged with 200 g of ion-exchanged water and 30.0 g of a 15% solution of anionic emulsifying agent Hitenol No. 8 (from Dai-Ichi Kogyo Seiyaku Co., Ltd.), then heated to 50°C under stirring. Next, 72.0 g (0.47 mol) of Si(OCH 3 ) 4 and 84.5 g (0.62 mol) of CH 3 Si(OCH 3 ) 3 were added dropwise over a period of one hour, and stirring was continued for one hour at 50°C. This was followed by cooling, thereby giving 383 g of Water-Based Silicone Resin Composition 5. The nonvolatiles content of this composition was 18.0% as determined by heating at 105°C for 3 hours. Examples 1 to 4 The aqueous silicone resin compositions prepared in the synthesis examples, photocatalytic anatase-type titanium oxide sol (as an aqueous dispersion having a solids content of 15%) with a pH of 9 to 10, colloidal silica (as a methanol dispersion having a solids content of 15%), and ion-exchanged water were formulated in the proportions shown in Table 1 to give coating liquids. These liquids were applied and cured under the conditions indicated below, and the resulting coats were subjected to the tests described below. Comparative Examples 1 and 2 Coating liquids were prepared by formulating the aqueous silicone resin composition obtained in Synthesis Example 5 in the proportions indicated in Table 1. The coats obtained from these coating liquids were subjected to the same tests as in Examples 1 to 4. (1) Application of Coat Substrate: aluminum sheet Method of Application: The coating liquid was applied with a No. 14 bar coater, then air-dried for 30 minutes. Thereafter, it was cured at 150°C for 20 minutes, thereby forming a coat having a thickness of about 0.5 to 5 µm. (2) Outdoor Exposure Samples of the applied film were mounted on a weathering panel (tilted at 45°) set up on the grounds of Silicone-Electronics Materials Research Center of Shin-Etsu Chemical Industry Co., Ltd. (located in Matsuida-cho, Usui-gun, Gunma Prefecture, Japan). They were exposed to sunlight for an outdoor exposure period of two months. (3) Evaluation of Applied Film Surface Contact Angle: With 5 pl of pure water dropped onto the sample surface, the contact angle was measured 30 seconds later using a contact angle measuring device (CA-X150, manufactured by Kyowa Kaimen Kagaku K.K.). Contaminant Adhesion, Cracking Resistance: Contaminant adhesion and cracking resistance of the applied film before and after outdoor exposure were visually evaluated. (4) Storage Stability of Coating Composition The coating composition was stored at 30°C for 1 month. The appearance of the aged liquid (presence or absence of precipitate, change in viscosity) and the condition of the applied film (appearance, ability to be hydrophilized) were examined and compared with the results of the fresh liquid. (5) Evaluation of Light Resistance Plain paper (PPC paper) was single-colour printed over its full surface by means of an ink jet printer (BJC-600S, manufactured by Canon Inc.) using magenta ink supplied with the printer. Thereafter, the coating composition was full-surface printed onto the single-color printed area by means of the BJC-600S ink jet printer. This coated paper was subjected to a light resistance test in a desktop accelerated weathering unit (Suntest CPS+, manufactured by Atlas). The results are shown in Table 2. The results obtained when uncoated samples were similarly tested are also shown in Table 2. The light resistance test was carried out for 100 hours in the Suntest CPS+ desktop accelerated weathering unit, and the color difference ΔE* (color difference of the L*a*b* color space) before and after irradiation was computed. ΔE* values of 0 to less than 5 are rated as A, values of 5 to 10 are rated as B, and values greater than 10 are rated as C. Components (parts by weight) Examples Comparative Example 1 2 3 4 1 2 Water-Based Silicone Resin Composition 1 100 Water-Based Silicone Resin Composition 2 100 Water-Based Silicone Resin Composition 3 100 Water-Based Silicone Resin Composition 4 100 Water-Based Silicone Resin Composition 5 100 100 Titanium oxide sol 175 185 185 210 175 175 Silica sol 8 9 9 12 8 8 Aluminium perchlorate 1 Ion-exchanged water 817 803 800 790 850 850 Contact angle (before irradiation) (before irradiation) 77 80 78 83 98 88 Contact angle (after 2 weeks) 7 6 6 8 19 24 Contact angle (after 2 months) 2 3 5 3 16 19 Contaminant adhesion excellent excellent excellent excellent fair fair Cracking resistance excellent excellent excellent excellent fair poor Storage stability excellent excellent excellent excellent poor poor Coating composition Light resistance Example 1 A Example 2 A Example 3 A Example 4 A Uncoated C In addition to substantially reducing or eliminating the amount of organic solvents contained, which is a very high priority in the field of paints and coatings, the coating compositions of the present invention provide applied films having both weather resistance and antifouling properties, and are themselves stable even in the alkaline region of the pH scale. That is, the use as the binder of a water-soluble silicone resin which is stable even under alkaline conditions prevents the agglomeration of photocatalyst particles in the alkaline region of the pH scale and confers excellent storage stability. Moreover, the system, which is alkaline, does not cause undesirable effects such as corrosion when applied to metal substrates.

Claims: A coating composition comprising: (a) 100 parts by weight of organosilicon compound obtainable by hydrolysing a silane mixture containing (i) 100 silane mixture parts by weight of nitrogenous organic group-containing silane of the following general formula (1): YR 1 m SiR 2 3-m wherein R 1 represents substituted or unsubstituted monovalent hydrocarbon group having 1 to 8 carbons, R 2 represents alkoxy or acyloxy group having 1 to 4 carbons, Y is a nitrogenous organic group and m is 0 or 1, or partial hydrolysate thereof, and (ii) 5 to 200 silane mixture parts by weight of silane of the following general formula (2): R 3 n SiR 4 4-n wherein R 3 represents substituted or unsubstituted monovalent hydrocarbon group having 1 to 8 carbons, R 4 represents alkoxy or acyloxy group having 1 to 4 carbons and n is 0, 1 or 2, or partial hydrolysate thereof; (b) 10 to 2,000 parts by weight of water; and (c) microparticulate photooxidation catalyst; the composition being substantially free of organic solvent. A coating composition of claim 1, wherein at least one said nitrogenous group is selected from those represented by formulae (3) to (6) as follows: wherein R 5 , R 6 , and R 9 to R 13 are hydrogen atoms or monovalent hydrocarbon groups having 1 to 8 carbons, with R 5 and R 6 , R 9 and R 10 and R 11 , and R 12 and R 13 , respectively, being mutually the same or different; R is a halogen atom; R 7 and R 8 are divalent hydrocarbon groups having 1 to 8 carbons atoms, with R 7 and R 8 being mutually the same or different; and p is 0 or an integer from 1 to 3. A coating composition of claim 1, wherein the silane of formula (1) is H 2 NCH 2 CH 2 HNCH 2 CH 2 CH 2 Si(OCH 3 ) 3 or H 2 NCH 2 CH 2 HNCH 2 CH 2 CH 2 Si(OCH 2 CH 3 ) 3 . A composition of claim 1, 2 or 3 wherein the silane of formula (2) is Si(OCH 3 ) 4 , Si(OCH 2 CH 3 ) 4 , CH 3 Si(OCH 3 ) 3 , or CH 3 Si(OCH 2 CH 3 ) 3 . A composition according to any one of the preceding claims wherein the microparticulate photooxidation catalyst comprises particles of at least one metal oxide selected from titanium oxide, cerium oxide, and zinc oxide. A coating composition according to any one of the preceding claims having a pH of at least 9. A method comprising the preparation of a coating composition according to any one of the preceding claims by blending the ingredients (a),(b) and (c) thereof, optionally with prior preparation of ingredient (a) by hydrolysis of the silane mixture containing silane components (i) and (ii). A method of forming a hydrophilic film, comprising the steps of applying a coating composition according to any one of claims 1 to 6 onto a substrate to form a film, and irradiating a surface of the film with UV-containing light. A hydrophilic film-coated article comprising a substrate having on a surface thereof a film of a coating composition according to any one of claims 1 to 6. An article according to claim 9 in which the film has a contact angle with water of not more than 60 degrees.

Inventors: FURUYA MASAHIRO, MATSUMURA KAZUYUKI, YAMAMOTO AKIRA, YAMAYA MASAAKI, FURUYA, MASAHIRO, MATSUMURA, KAZUYUKI, YAMAMOTO, AKIRA, YAMAYA, MASAAKI, Furuya, Masahiro, PVC Tech.Res.Center, Matsumura, Kazuyuki, PVC Tech. Res. Center, Yamamoto, Akira, PVC Tech.Res.Center, Yamaya, Masaaki, PVC Tech.Res.Center
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1998
Main classification: C09D183
IPC codes: C09D183, B05D5, B05D7