To improve a surface of plastics lenses that have been used increasingly frequently in recent years, metal oxide sols having a high refractive index are used as a component of a hard coat agent applied to the surface thereof.
For example, Japanese Patent Publication No. Sho 63-37142 describes a molded part having a transparent cover layer containing fine inorganic substance particles comprised by at least one metal oxide selected from Al, Ti, Zr, Sn and Sb having a particle size of 1 to 300 nm.
Japanese Patent Publication No. Hei 4-27168 discloses crystalline titanium oxide-tin oxide sol having particles with a particle size of 50 nm or less. This is obtained by a method in which a water-soluble titanium compound and a water-soluble tin compound are reacted with an alkali metal hydroxide or its carbonate and/or an ammonium compound and hydrothermally treated.
Japanese Patent Publication No. Hei 5-29363 discloses a cosmetic having incorporated therein a titanium oxide-cerium oxide composite sol obtained by adding hydrogen peroxide to a dispersion of titanium oxide hydrate and cerium oxide hydrate to dissolve the titanium oxide hydrate and cerium oxide hydrate and heating the solution.
Further, Japanese Patent Application Laid-open No. Hei 2-178219 and Japanese Patent Publication No. Hei 4-45453 disclose a method for producing a titanium oxide-iron oxide composite sol.
In the above-mentioned prior art, the metal oxide particles comprised by Al, Ti, Zr, Sn, Sb, etc. having a particle size of 1 to 300 nm described in Japanese Patent Publication No. Sho 63-37142 are undesirable since where they are coated on a plastics substrate and cured, the resulting coating film has insufficient water resistance even when they are used each alone as a component of a hard coat agent. Also, when high refractive titanium oxide is used in this application, its primary particle size must be 20 nm or less, preferably 15 nm or less in relation to transparency. However, a problem arises that it is colored to blue color when irradiated with ultraviolet rays.
In the case where the sols described in Japanese Patent Publication No. Hei 4-45453, Japanese Patent Application Laid-open No. Hei 2-178219 and Japanese Patent Publication No. Hei 5-29363 are used, addition of a small amount of iron oxide or cerium oxide to the titanium oxide can inhibit the discoloration attributable to the irradiation of ultraviolet rays. However, because of the iron oxide or cerium oxide added in such an amount as can inhibit the coloration, the sols themselves are colored to brown or yellow and coating films prepared from them are also colored to and are undesirable.
Also, the crystalline titanium oxide-tin oxide sol described in Japanese Patent Publication No. Hei 4-27168 is undesirable in that hydrothermal treatment at 100.degree. C. or higher is essential to convert it into a composite form, with the result that firm and stable secondary flocculates are generated to considerably decrease the transparency of the resulting sol.
Whereas plastic molded products are used in large amounts making the best of their advantages such as lightness in weight, ready processing, and impact resistance, on the contrary, they have defects that their hardness is insufficient and they tend to be marred, corroded by solvents, electrically charged to adsorb dusts, have insufficient heat resistance and so on, so that they are less sufficient in practical use as lenses for glasses windowpanes, etc. than inorganic glass molded parts.
Accordingly, it has been proposed to provide a protective coat on plastic molded parts. So many kinds of coating compositions have been proposed for use as a coat and for example, a coating composition containing an organosilicon compound or its hydrolysates as a main component (resin component or coating film-forming component) has been used as a hard coat for lenses for glasses. However, this coating agent has insufficient marring resistance. Further, Japanese Patent Application Laid-open No. Sho 53-111336 discloses use of a coating agent containing colloidal silica particles obtained by addition of silica sol as a hard coat for lenses for glasses.
Conventionally, plastic lenses for glasses have mostly been produced by cast polymerization of diethylene glycol bisallyl carbonate in the state of monomer. The lens thus produced has a refractive index of about 1.50, which is lower than the refractive index of glass lens being 1.52, so that there arises the problem that in the case of lenses for glasses for short sight the thickness of frame increases. For this reason, recently, the development of monomers having a higher refractive index than that of diethylene glycol bisallyl carbonate have been promoted. Such high refractive resin materials are disclosed in, for example, Japanese Patent Application Laid-opens No. Sho 55-13747, No. Sho 56-166214, No. Sho 57-23611, No. Sho 57-54901, No. Sho 59-133211, No. Sho 60-199016 and No. Sho 64-54021.
In contrast to high refractive lenses using the above-described high refractive resin materials, Japanese Patent Application Laid-opens No. Sho 62-151801 and No. Sho 63-275682 disclose coating agents containing a colloidal dispersion of fine particles of an oxide of a metal such as Sb or Ti.
The coating agent containing colloidal silica by addition of a silica sol has such a problem that a film obtained by coating and curing the coating agent generates interference fringes, making the appearance of the lens unacceptable. Also, in the case of lenses, it is often the case that an antireflection film (a multilayer structure film comprising inorganic oxide thin films based on the optical interference theory) is provided on the coating film. In this case, there has been such a problem that the antireflection film has a reflection color, for example, very pale green, and the reflection color varies and is uneven depending on the position on the surface of the lens.
In a coating agent containing colloidal tin oxide by addition of a tin oxide sol, the tin oxide sol has low compatibility with organosilicon compounds such as silane coupling agent or silicon-containing substances such as the hydrolysates of organosilicon compounds and therefore, it has decreased stability, with the result that films obtained by coating and curing the coating agent have insufficient water resistance.
In the coating agent containing colloidal titanium oxide by addition of a titanium oxide sol, the titanium oxide sol has also low compatibility with organosilicon compounds such as silane coupling agent or silicon-containing substances such as the hydrolysates of organosilicon compounds and therefore, it has decreased stability, with the result that films obtained by coating and curing the coating agent have insufficient water resistance. Also, there has been the problem that it is discolored to blue by irradiation with ultraviolet rays.
The coating agent containing colloidal antimony oxide by addition of an antimony oxide sol has improved stability because compatibility between the antimony oxide sol and organosilicon compounds such as silane coupling agent or silicon-containing substances such as the hydrolysates of organosilicon compounds is high. However, there is such a problem that films obtained by coating and curing the coating agent have insufficient refractive index.
The present invention is to provide a coating composition that gives rise to a film showing no interference fringes and having no irregularity in reflection color, the film being obtained by coating and curing the coating agent on a high refractive optical element that uses a high refractive resin material having a high refractive index of n.sub.D =1.54 to 1.70 and an optical element utilizing the coating composition.
Also, the present invention is to provide a coating composition for plastics that is excellent in marring resistance, surface hardness, wear resistance, flexibility, transparency, antistatic properties, dyeability, heat resistance, water resistance, chemical resistance, etc. and an optical element utilizing the coating composition. In particular, the present invention is to provide a coating agent that gives rise to a film that is excellent in marring resistance, adhesion, water resistance, transparency, light resistance and generates no interference fringe.
The present invention provides a method for producing composite sols, coating compositions utilizing such composite sols, and optical elements that can obviate the above-mentioned problems.