Abstract:
The invention provides a transparent and tenacious coating film using an inorganic paint composition prepared by adding a borate salt into an alkali metal silicate, and further mixing thereto lepidoblastic transparent silica with a thickness of 0.01 to 0.5 μm and a surface diameter of 2 to 5 μm. The alkali metal silicate is solidified with metal ions issued from the borate salt to form a coating film, which contains a glass formed by solidifying dissolved boric acid in the coating film. The coating film can be made tenacious without impairing transparency by dispersing many flakes of transparent silica as laminated layers.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to an article that exhibits its characteristics by receiving or reflecting a light, particularly an inorganic paint composition useful for protecting the surface of a solar panel of a solar generator system and a mirror capable of obtaining a transparent and tenacious coating film.  
           [0003]    2. Description of the Related Art  
           [0004]    Various inorganic paint compositions using an alkali metal silicate as a binder have been proposed.  
           [0005]    However, most of the coating films obtained are fragile, and fine cracks are generated by distortion when the coated subjects are expanded and contracted due to temperature changes to impair weather resistance and pollution resistance of the coating film.  
           [0006]    Accordingly, the inventors of the invention have invented an inorganic paint composition prepared by adding a polyvalent metal-containing hardener such as calcium silicate and zinc phosphate to an alkali metal silicate, and by further adding, as an inorganic filler, a fine powder of a natural glass having a borate component-dissolving action mainly comprising colemanite and ulexite (see Japanese Patent Application Publication Nos. 3,140,611 and 3,140,612).  
           [0007]    A tenacious coating film could be obtained in these inventions since boric acid is dissolved by mixing with water, and dissolved boric acid and the binder are simultaneously solidified when the mixed solution is dried after coating. These inorganic paint compositions have been widely recognized as coating film-forming agents for protecting the surfaces of commonly used exterior and interior construction materials and interior materials of underground passages.  
           [0008]    However, the inorganic paint compositions described above are naturally almost impermeable to light since they contain a natural glass fine powder. Accordingly, they are not suitable as materials exhibiting their characteristics by receiving a light, for example as a solar panel of a solar generator system, and as materials that exhibits their characteristics by reflecting a light, for example as a surface protecting film of a mirror.  
           [0009]    On the other hand, since natural minerals such as kaolin, talc and bentonite were added as fillers for improving tenacity of the coating film in inorganic paint compositions proposed by other inventors, light permeability thereof has remained insufficient.  
         SUMMARY OF THE INVENTION  
         [0010]    Accordingly, it is an object of the invention for solving the problems above to provide a tenacious coating film without impairing light permeability by forming a coating film by solidifying an alkali metal silicate with metal ions originating from a borate salt. Such inorganic paint composition is prepared by adding the borate salt to the alkali metal silicate, and by further adding thereto lepidoblastic transparent silica with a thickness of 0.01 to 0.5 μm and a surface diameter of 2 to 5 μm as an inorganic filler. Transparency of the coating film is assured by allowing the coating film formed to contain a glass formed by solidification of dissolved boric acid while many thin flakes of transparent silica are dispersed and laminated. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0011]    The embodiments of the invention will be described hereinafter.  
         [0012]    The inorganic paint composition of the invention is prepared by adding a borate salt into an alkali metal silicate, and further mixing thereto lepidoblastic transparent silica with a thickness of 0.01 to 0.5 μm and a surface diameter of 2 to 5 μm.  
         [0013]    A titanium oxide fine particle may be blended with the composition above.  
         [0014]    The alkali metal silicate is a silicate salt of an alkali metal such as lithium silicate, sodium silicate and potassium silicate, and is solidified by dehydrating contraction reaction and gelatinization by adding a borate salt as described below.  
         [0015]    Examples of the borate salt include magnesium borate, calcium borate, barium borate, strontium borate, zinc borate and aluminum borate. These borate salts may be used alone or as a mixture.  
         [0016]    An example of silica having the properties as described above is commercially available Sun Lovely LFS (trade name, manufactured by Asahi Glass Co., Ltd.), which is commercialized as a slurry prepared by dispersing silica as described above in water.  
         [0017]    Solidification of the coating film is insufficient when the content of the borate salt in 100 parts by weight of the alkali metal silicate is less than 0.5 parts by weight, while solidification is so excessively rapid that fine cracks are formed in the coating film when the content exceeds 35 parts by weight. Accordingly, the proportion of the borate salt is preferably in the range of 0.5 to 35 parts by weight.  
         [0018]    Coating power of the coating film is insufficient when the content of silica is less than 0.5 parts by weight, while transparency decreases and coating with a uniform thickness becomes impossible due to high and unstable viscosity of the paint when the content exceeds 50 parts by weight. Accordingly, the proportion of silica is preferably in the range of 0.5 to 50 parts by weight.  
         [0019]    An appropriate proportion of the titanium fine powder is 10 to 35 parts by weight.  
         [0020]    The function of the coating film formed by coating the inorganic paint composition of the invention will be described below.  
         [0021]    The coating film obtained in the invention has many silanol groups (—SiOH) on the surface, and the film exhibits super-hydrophilicity by the silanol groups.  
         [0022]    Accordingly, tough contamination such as oily contaminants adhered on the surface can be readily washed away merely by rinsing with water.  
         [0023]    This is because strongly adsorbed water on the super-hydrophilic coating film invades the space between the coating film and contaminants, which is peeled off by being floated up from the costing film.  
         [0024]    Super-hydrophilicity is improved in the coating film blended with the titanium oxide fine powder.  
         [0025]    Titanium oxide contains chemically adsorbed water, and physically adsorbed water binds to the chemically adsorbed water. Physically adsorbed water is stabilized by being incorporated into gelled silica originating from the alkali metal silicate by surface diffusion. Hydrophobic organic impurities and microorganisms floating in air are decomposed by a photocatalytic reaction of titanium oxide even when they are adhered on the surface of the coating film. Accordingly, chemically adsorbed water always remains exposed.  
         [0026]    The invention will be described in more detail with reference to examples.  
         [0027]    Mixed in a ball mill for 10 minutes by adding water were 100 parts by weight of sodium silicate, 11 parts by weight of calcium borate and 48 parts by weight of Sun Lovely LFS (7.2 parts by weight as converted into dry silica). The mixture obtained was coated on a SUS 304 stainless steel test plate and glass test plate by spray coating, and coating films with a thickness of about 10 μm were obtained by drying for about 50 minutes in a hot air stream heated at 220 to 250° C.  
         [0028]    The results of physical property tests of the coating film formed on the SUS 304 stainless steel plate are shown in Table 1, the results of dewetting tests are shown in Table 2, the results of chemical resistance tests are shown in Table 3, and the results of pollution resistance test are shown in Table 4.  
                       TABLE 1                       Test Items   Test Conditions   Results                   Checkerboard   adhesive checkerboard tape   100/100       Square   method according to JIS       Test   K5400(1990) 8.5 (with an           interval of 1 mm)       Specular   surface glossiness   113%       Surface       Glossiness   according to JIS K5400       Test   (1990) 7.6       Pencil   test method according to   no less than 9H       Scratch   JIS K5400 (1990) 8.4.1       Test       Impact   ball drop method according   no problem by dropping       Resistance   to JIS K5400 (1990) 8.1.3   the ball from a height       Test       of 100 cm       Heat   heat stability test of   slight color change       Resistance   coating film according to   (color change by       Test   JIS K5400 (1990) 8.1.3   oxidation of SUS 304)       Surface   surface resistivity   1 × 10 8  (Ω)       Resistivity   according to JIS K8911       Test       Antibacterial   according to JIS Z2801     Staphylococcus aureus  &lt; 10       Test   (established in 2000)     Escherichia coli  &lt; 10       Weather   Test Machine: weather   Initial gloss: 124.3%       Resistance   meter manufactured by   Gloss after 250 hours:       Test   Dai-Nippon Plastic Co.   104.8%           Test time: 500 hours   Gloss after 500 hours:           Operation Condition in One   98.1%           Cycle: metal halide lamp   ΔE 2.58           (60 mW/cm 2  at 360 nm)           L: irradiation at 63° C.,           50%           RH, 6 hours           R: no irradiation at 70° C.,           90% RH, 2 hours           D: no irradiation at 30° C.,           98% RH, 4 hours           water shower for 5 seconds           before and after D       Brine   500 hours according to JIS   No rust and swelling       Spray   K5400   were observed at both       Test       cross-cut and flat               portions.       Moisture   500 hours according to JIS   No damage and rust were       Resistance   K5400   observed at both cross-       Test       cut and flat portions.                  
 
         [0029]    [0029]                             TABLE 2                           Measurements of Contact Angle θ by Dewetting Test according to       JIS K6894 (1996) 8.4                Contact Angle θ               after 4 Hours&#39;   Contact Angle θ of           Boiling Water Test   Blank Sample               SUS 304 Plate   9   14       Plate Glass   8   18       Comparative Product   74        (Commercially Available       Product Coated with       Fluorinated Paint)                    
         [0030]    [0030]                                   TABLE 3                       Chemical   Judge   Chemical   Judge   Chemical   Judge                   hydrochloric acid   A   hydrofluoric   D   sat. iron (II)   A       (10%)       acid (46%)       chloride       hydrochloric acid   A   sulfuric acid   A   acetone   A       (36%)       (10%)       aqua regia   A   sulfuric acid   A   methyl   A               (98%)       acetate       perchloric acid   A   phosphoric acid   A   carbon   A       (60%)       (20%)       tetrachloride       saturated chromic   A   phosphoric acid   A   methanol   A       acid mixture       (85%)       conc. sulfuric   A   acetic acid   A   chloroform   A       acid + conc.       (20%)       hydrochloric acid       (1:1)       conc. sulfuric   A   aq. ammonia   A   xylene   A       acid + conc.       (28%)       nitric acid (1:1)       20% nitric acid   A   sodium hydroxide   A   petroleum   A               (20%)       benzin       60% nitric acid   A   sat. aq. sodium   A   ethanol   A               hydroxide       20% hydrofluoric   D   sat. potassium   C       acid       permanganate                                                                    
         [0031]    [0031]                                     TABLE 4                           Pollution Substance   Judge                                1   Sauce   A       2   Soy Sauce   A       3   Soybean Paste   A       4   Roast Meat Sauce   A       5   Mayonnaise   A       6   Fry Oil   A       7   Salad Oil   A       8   Table Salt   A       9   Nicotine   A       10   Smoke   A       11   Woolong Tea   A       12   Vinegar   A       13   Detergent “Smile”   A       14   Soap   A       15   Coffee   A       16   Japanese Tea   A       17   Blue Ink   A       18   Red Ink   A       19   Oily Marker Ink   A       20   Domestic Sewage   A       21   Iron Rust   A       22   Oily Marker Ink (Red)   A                                                                            
         [0032]    Transparency of the coating film was evaluated by measuring the changes of spectral transmittance before and after forming the coating film according to the method prescribed in JIS Z8722 (2000: color measuring method—reflection color and transmission color).  
         [0033]    The results of measurements of spectral transmittance of the articles before and after coating are shown in Tables 5 and 6, respectively.  
                                           TABLE 5                           Spectral transmittance (%)       026048 plate glass before coating                    Spectral           Wavelength λ   transmittance (%)                            380   85.83           385   86.70           390   87.68           395   88.44           400   88.30           405   88.32           410   88.51           415   88.93           420   88.36           425   88.97           430   88.27           435   88.56           440   88.69           445   88.66           450   88.58           455   89.15           460   89.04           465   89.48           470   88.84           475   89.79           480   89.16           485   89.61           490   89.20           495   89.77           500   89.38           505   89.79           510   89.58           515   89.84           520   89.75           525   90.13           530   89.80           535   89.69           540   89.43           545   89.82           550   89.62           555   89.87           560   89.59           565   89.58           570   89.37           575   89.49           580   89.15           585   89.43           590   89.37           595   89.59           600   88.96           605   89.22           610   89.35           615   88.57           620   88.97           625   88.93           630   88.61           635   88.74           640   88.57           645   88.19           650   88.41           655   87.82           660   88.15           665   87.91           670   88.04           675   87.81           680   87.49           685   87.91           690   87.43           695   87.11           700   87.26           705   87.14           710   87.03           715   86.79           720   86.72           725   86.82           730   86.81           735   86.47           740   86.45           745   86.33           750   86.29           755   86.11           760   85.97           765   86.47           770   85.60           775   86.54           780   85.50           Total   71.5561           Average   88.34                      
 
         [0034]    [0034]                                                     TABLE 6                           Spectral transmittance (%)       026048 plate glass after coating                    Spectral           Wavelength λ   transmittance (%)                            380   86.60           385   87.52           390   88.55           395   89.32           400   89.36           405   89.37           410   89.46           415   89.69           420   89.53           425   89.74           430   89.28           435   89.63           440   89.71           445   89.52           450   89.82           455   89.97           460   90.29           465   90.39           470   89.97           475   91.07           480   89.96           485   90.60           490   90.48           495   90.93           500   90.78           505   90.86           510   90.70           515   90.90           520   90.84           525   90.92           530   90.95           535   90.62           540   90.59           545   91.06           550   90.56           555   90.91           560   90.57           565   90.79           570   90.76           575   90.61           580   90.40           585   90.12           590   90.46           595   90.53           600   90.03           605   90.09           610   90.18           615   90.08           620   89.82           625   89.86           630   89.59           635   89.70           640   89.67           645   88.95           650   89.19           655   88.99           660   89.04           665   89.04           670   88.55           675   88.96           680   88.85           685   88.75           690   88.56           695   88.35           700   88.50           705   88.39           710   87.97           715   87.92           720   87.79           725   88.01           730   87.89           735   87.52           740   87.37           745   87.57           750   87.36           755   86.92           760   86.85           765   87.23           770   86.57           775   87.46           780   86.53           Total   72.3876           Average   89.37                        
         [0035]    [0035]           
         [0036]    As shown in the results above, the coating film formed by the inorganic paint composition of the invention exhibits excellent durability, weather resistance and pollution resistance, and is quite excellent in transmission of light without exhibiting any changes of spectral transmittance before and after coating the paint on the glass surface.  
         [0037]    The inorganic paint composition of the invention is formed by adding a borate salt in an alkali metal silicate, and lepidoblastic transparent silica with a thickness of 0.01 to 0.5 μm and a surface diameter of 2 to 5 μm is further mixed as an inorganic filler. Accordingly, a solar panel of the solar generator system, which is always exposed to the sunlight, wind and rain, can be protected without impairing power generation efficiency by coating the paint on an external panel of the solar generator system, since the coating film obtained is highly transparent while being excellent in durability and pollution resistance.  
         [0038]    Since the coating film is super-hydrophilic, the panel can be automatically cleaned with even when rain water coating film is polluted with contaminants.  
         [0039]    Other application examples include coating of a bath room mirror, whereby the mirror does not become cloudy since adhered water is diffused due to the super-hydrophilic nature of the coating film even when vapor is condensed on the surface of the coating film.  
         [0040]    Since the inorganic paint composition of the invention comprises 0.5 to 35 parts by weight of the borate salt, 0.5 to 50 parts by weight of silica relative to 100 parts by weight of the alkali metal icate, the coating film is able to be endowed with a good balance among the characteristics such as durability, weather resistance and pollution resistance.  
         [0041]    Since blending titanium oxide fine powder as a photocatalyst permits super-hydrophilicity and self-cleaning ability of the coating film to be improved in addition to antimicrobial property, the inorganic paint composition of the invention is most suitable for painting interior walls of an old-age home and hospital. The inorganic paint composition of the invention has a quite large practical effect that enables the paint to be widely employed.