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 
     1. Field of the Invention 
     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. 
     2. Description of the Related Art 
     Various inorganic paint compositions using an alkali metal silicate as a binder have been proposed. 
     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. 
     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). 
     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. 
     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. 
     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 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph showing spectral transmittance versus wavelength before coating; and 
         FIG. 2  is a graph illustrating spectral transmittance versus wavelength after coating. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of the invention will be described hereinafter. 
     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. 
     A titanium oxide fine particle may be blended with the composition above. 
     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. 
     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. 
     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. 
     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. 
     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. 
     An appropriate proportion of the titanium fine powder is 10 to 35 parts by weight. 
     The function of the coating film formed by coating the inorganic paint composition of the invention will be described below. 
     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. 
     Accordingly, tough contamination such as oily contaminants adhered on the surface can be readily washed away merely by rinsing with water. 
     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. 
     Super-hydrophilicity is improved in the coating film blended with the titanium oxide fine powder. 
     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. 
     The invention will be described in more detail with reference to examples. 
     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. 
     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 
                 according to JIS K5400 
               
               
                 Glossiness 
                 (1990) 7.6 
               
               
                 Test 
               
               
                 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. 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 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) 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
             
           
               
                 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 
               
               
                   
               
               
                 Judgment of results:  
               
               
                 A - no change,  
               
               
                 B - slightly changed,  
               
               
                 C - evidently changed,  
               
               
                 D - completely corroded  
               
               
                 Test Method: Each chemical (0.2 ml) is dripped on the sample, the surface of the sample is covered with a petri dish, the sample is washed with water after allowing to stand for 24 hours at room temperature, and the surface is observed after cleaning it by wiping.  
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
             
           
               
                 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 
               
               
                   
               
               
                 Judgment of results:  
               
               
                 A - no change,  
               
               
                 B - slightly changed,  
               
               
                 C - evidently changed,  
               
               
                 D - completely corroded  
               
               
                 Test Method: Each pollution substance (0.2 ml) is dripped on the sample, the surface of the sample is covered with a petri dish, the sample is washed with water after allowing to stand for 24 hours at room temperature, and the surface is observed after cleaning it by wiping.  
               
               
                 Tested at Aichi Industrial Technology Center  
               
             
          
         
       
     
     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). 
     The results of measurements of spectral transmittance of the articles before and after coating are shown in Tables 5 and 6, respectively. The graph of  FIG. 1  illustrates the spectral transmittance at various wavelengths before coating as set forth in Table 5. The graph of  FIG. 2  illustrates the spectral transmittance at various wavelengths after coating as set forth in Table 6. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 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 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 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 
               
               
                   
                   
               
             
          
         
       
     
     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. 
     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. 
     Since the coating film is super-hydrophilic, the panel can be automatically cleaned with even when rain water coating film is polluted with contaminants. 
     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. 
     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. 
     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.