Water repellent surface treatment with integrated primer
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. application Ser. No. 08/220,353 filed Mar. 30, 1994, now abandoned, which is a continuation-in-part of U.S. application Ser. No. 07/589,235 filed Sep. 28, 1990, now U. S. Pat. No. 5,308,705, which is a continuation-in-part of application Ser. No. 07/503,587 filed Apr. 3, 1990, which is now U.S. Pat. No. 4,983,459. 
    
    
     BACKGROUND 
     The present invention relates generally to the art of surface treatment and, more particularly, to the art of producing a water repellent surface on various substrates, and most particularly to improving the durability of such water repellent surfaces. 
     THE PRIOR ART 
     European Patent Application No. 92107814.3 (Publication Number 0 513 690 A2) of Yoneda et al. describes a surface-treated substrate having at least two treated surface layers wherein the first outermost layer is obtained by treatment with a compound forming a surface having a contact angle of at least 70° against water and the second underlayer is obtained by treatment with at least one reactive silane compound selected from isocyanate silane compounds and hydrolyzable silane compounds. 
     U.S. Pat. Nos. 4,983,459 and 4,997,684 to Franz et al. disclose an article and method respectively for providing a durable nonwetting surface on glass by treatment with a perfluoroalkylalkyl silane and a fluorinated olefin telomer. 
     In U.S. Pat. No. 5,308,705 Franz et al. describe providing nonwetting surface properties to substrates other than glass by treatment with a perfluoroalkylalkyl silane and a fluorinated olefin telomar. 
     In U.S. Pat. No. 5,328,768 Goodwin discloses a glass substrate the surface of which is treated with first a silica primer layer and second a perfluoroalkylalkyl silane. 
     SUMMARY OF THE INVENTION 
     The present invention provides a substrate surface with high water repellency and high lubricity. The durability of the water and dirt repellency of a substrate surface is improved by applying to the substrate surface a perfluoro-alkylalkylsilane compound and a hydrolyzable silane compound. The hydrolyzable silane compound is a compound capable of hydrolyric condensation to form silica gel and functions as an integral primer compound. The surface treatment of the present invention provides enhanced durability to the water and dirt repellent surface without requiring a separate primer layer. High water repellency and lubricity are provided by perfluoroalkylalkylsilane. The hydrolyzable silane also provides for reactive drying of the solvent. The perfluoroalkylalkylsilane and hydrolyzable silane surface treatment of the present invention also provides enhanced abrasion resistance to the substrate surface. Increased resistance to humidity, ultraviolet radiation and mechanical abrasion are provided by the silane compound capable of hydrolytic condensation to silica gel. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Durability of rain and soil repellency provided by application of a perfluoroalkylalkylsilane to a substrate surface is enhanced by mixing a silane or mixture of silanes capable of hydrolysis to silica gel with a perfluoroalkyl-alkylsilane prior to application. In accordance with the present invention, a mixture of a perfluoroalkylalkylsilane and silane or mixture of silanes capable of hydrolysis to silica gel is applied to the surface of glass to form a coating which is more durable than would have been formed without the silane or mixture of silanes capable of hydrolysis to silica gel. 
     Perfluoroalkylalkylsilane and hydrolyzable silane are applied to the surface of a substrate to produce the article of the present invention preferably as a colloidal suspension or solution, preferably in an aprotic solvent, preferably an alkane or mixture of alkanes, or a fluorinated solvent. The preferred solution of the present invention is applied to a substrate surface by any conventional technique such as dipping, flowing, wiping or spraying. The solvent is evaporated and the composition forms a durable, non-wetting, lubricating surface with improved abrasion resistance. The present invention provides the durability benefits of a primer without the additional step of applying a separate primer layer. The use of the completely hydrolyzable silane improves the humidity, ultraviolet light, and abrasion resistance of the silane surface treatment as measured by the Cleveland Condensing Cabinet, QUV (with FS40 or B313 lamps), and wet sled abrasion tests, indicating a longer useful product lifetime. 
     Preferred silanes capable of hydrolysis to silica gel have the general formula SiX 4 , wherein X is a radical such as halogen, alkoxy, or acyl, preferably chloro, bromo, iodo, methoxy, ethoxy, and acetoxy. Preferred hydrolyzable silanes. include tetrachlorosilane and tetraacetoxysilane. 
     The silanes, SIX 4 , serve two functions. One is to become part of the coating and impart resistance to weathering and abrasion. Another function is to dry the solvent. Typical hydrocarbon solvents can contain 50 to 200 ppm of water. Other solvents can be much higher in water content. For example, a solvent containing 200 ppm water would have enough water present to deactivate a perfluoroalkylalkylsilane at 0.5 weight percent concentration. The completely hydrolyzable silane is capable of eliminating or reducing the water content of the solvent prior to perfluoroalkylalkylsilane addition. Otherwise, water deactivation of the perfluoroalkylalkylsilane would lead to insufficient coating deposition or very poor durability. 
     Preferred perfluoroalkylalkylsilanes have the general formula R m (R&#39; n )SiX 4-m-n , wherein R is a perfluoroalkylalkyl radical; m is typically one, n is typically zero or one, and m+n is less than 4; R&#39; is a vinyl or an alkyl radical, preferably methyl, ethyl, vinyl or propyl; and X is preferably a radical such as halogen, acyloxy, and/or alkoxy. Preferred perfluoroalkyl moieties in the perfluoroalkylalkyl radicals range from CF 3  to C 30  F 61 , preferably C 6  F 13  to C 18  F 37 , and most preferably C 8  F 17  to C 12  F 25  ; the alkyl moiety is preferably ethyl. R&#39; is preferably methyl or ethyl. Preferred radicals for X include hydrolyzable chloro, bromo, iodo, methoxy, ethoxy and acetoxy radicals. Preferred perfluoroalkylalkylsilanes in accordance with the present invention include perfluoroalkylethyltrichlorosilane, perfluoroalkylethyltrimethoxysilane, perfluoroalkylethyltriacetoxysilane, perfluoroalkylethyldichloro(methyl)silane and perfluoroalkylethyldiethoxy(methyl)silane. 
     These preferred perfluoroalkylethylsilanes appear to react with bonding sites at the substrate surface on a molecular basis. Strong surface bonding of the perfluoroalkylethylsilanes produces a durable substrate surface which exhibits a high contact angle with a drop of water, indicating high water repellency. 
     Suitable solvents include isopropanol, ethanol, hexane, heptane, mineral spirits, acetone, toluene and naphtha. Preferred solvents are alkanes or halogenated hydrocarbon solvents such as trichlorotrifluoroethane, and methylene chloride, and perfluorinated organic compounds such as perfluorocarbons. Concentrations of about 0.005 to 50, preferably about 0.05 to 5, percent by weight of silane are preferred. The solvent is preferably evaporated simply by drying in air at ambient temperature, or may be removed by wiping. The silanes may also be crosslinked to form a more durable coating. Preferably, curing is accomplished by heating the silane treated surface. Typically, curing temperatures of at least 150° F. (about 66° C.) are preferred, particularly above 200° F. (about 93° C.). A cure cycle of about 200° F. (about 93° C.) for about 30 minutes is suitable. Higher temperatures and shorter heating times may be more efficient. A cure cycle of 2 to 5 minutes at 400° to 500° F. (about 204° to 260°  C.) may be preferred, particularly about 3 minutes at about 470° F. (about 243° C.). The substrate surfaces may, in the alternative, be contacted with perfluoroalkylalkylsilane in vapor form. 
     The contact angles recited herein are measured by the sessile drop method using a modified captive bubble indicator manufactured by Lord Manufacturing, Inc., equipped with Gaertner Scientific goniometer optics. The surface to be measured is placed in a horizontal position, facing upward, in front of a light source. A drop of water is placed on top of the surface in front of the light source so that the profile of the sessile drop can be viewed and the contact angle measured through the goniometer telescope equipped with circular protractor graduation. 
     Weathering chambers include the Cleveland Condensing Cabinet (CCC) and QUV Tester (products of The Q-Panel Company, Cleveland, Ohio). The CCC chamber was operated at a vapor temperature of 140° F. (60° C.) in an indoor ambient environment which resulted in constant water condensation on the test surface. The QUV Tester is operaned with cycles of 8 hours UV (either B313 or FS40 lamps) at black panel temperature of 65°-70° C. and 4 hours condensing humidity at 50° C. 
     The present invention will be further understood from the descriptions of specific examples which follow. 
    
    
     EXAMPLE I 
     A solution was prepared by mixing one gram of tetrachlorosilane and one gram of perfluoroalkylalkylsilane in 40 grams of trichlorotrifluoroethane (Freon® TF solvent, a product of DuPont). The perfluoroalkylalkylsilane comprised perfluoroalkylethyltrichlorosilanes, wherein the perfluoroalkyl moieties comprised primarily C 6  F 13  to C 18  F 37 . For comparison, a control solution was mixed without the tetrachlorosilane. The solutions were applied to the atmosphere surface of 3.9 millimeter thick Solex® float glass (product of PPG Industries, Inc.) with a cotton pad. Coupons were cured at 200° F. (93° C.) for one hour. The excess silane was removed from the glass surfaces by solvent washing. Coupons were weathered in the CCC and QUV-FS40 weathering cabinets. Coating efficiency was measured by the contact angle of a sessile drop of water. The results are shown in the following table. 
     
                       TABLE I                                                     
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CCC               OUV-FS40                                                
Hours  Primer  No Primer  Hours Primer                                    
                                      No Primer                           
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0      105°                                                        
               105°                                                
                          0     107°                               
                                      106°                         
496    102°                                                        
               87° 319   106°                               
                                      102°                         
927    67°                                                         
               60° 1332  91°                                
                                      89°                          
1669   49°                                                         
               40° 2115  83°                                
                                      70°                          
                          2498  78°                                
                                      70°                          
                          2943  72°                                
                                      57°                          
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     EXAMPLE II 
     Four solutions were prepared, each of which was 0.5 percent by weight perfluorohexylethyltrichlorosilane in Isopar L solvent (product of Exxon), a mixture of alkanes. The solutions had tetrachlorosilane concentrations of 0.0, 0.2, 0.45, and 0.79 percent by weight. The order of addition was Isopar L, tetrachlorosilane and perfluoroalkylethyltrichlorosilane to utilize the reactive drying of the solvent by the hydrolyzable tetrachlorosilane. These four solutions were coated on the tin surface of coupons of 0.182 inch (4.6 millimeter) thick clear float glass. Samples were tested in the CCC chamber. Coating efficiency was measured by the contact angle of a sessile drop of water. It can be seen in the following table that increasing the concentration of hydrolyzable silane within this range improves the durability of the perfluoroalkylalkylsilane surface treatment. 
     
                       TABLE II                                                    
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         CCC Contact Angle (°)                                     
         Primer Concentration                                             
         (Percent by Weight)                                              
Hours      0      0.2         0.45 0.79                                   
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0          115    115         114  114                                    
122        81     89          105  105                                    
284        54     65          77   81                                     
475        36     44          58   69                                     
642        --     --          --   47                                     
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     EXAMPLE III 
     Four solutions were prepared, each of which was 2.5 percent by weight of the perfluoroalkylethyltrichlorosilane described in Example I and 2.5 percent by weight perfluoroalkylethylene in Fluorinert® FC-77 fluorocarbon solvent (a product of 3M). The solutions had tetrachlorosilane concentrations of 0.0, 1.0, 2.0 and 5.0 percent by weight. These four solutions were coated on the tin surface of coupons of 0.187 inch (4.7 millimeter) thick clear float glass. Coupons were cured at 300° F. (149° C.) for 15 minutes. Samples were tested in the CCC and QUVB-313 chambers. Coating efficiency was measured by contact angle of a sessile drop of water. Results are given in the following tables. 
     
                       TABLE IIIA                                                  
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         CCC Contact Angle (°)                                     
         Primer Concentration                                             
         (Percent by Weight)                                              
Hours      0      1.0         2.0  5.0                                    
______________________________________                                    
0          113    113         115  114                                    
162        95     109         114  112                                    
348        73     81          98   93                                     
684        31     41          43   34                                     
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                       TABLE IIIB                                                  
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         QUVB Contact Angle (°)                                    
         Primer Concentration                                             
         (Percent by Weight)                                              
Hours      0      1.0         2.0  5.0                                    
______________________________________                                    
0          113    114         117  116                                    
566        107    111         111  109                                    
1375       82     95          95   87                                     
2095       72     80          84   71                                     
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     EXAMPLE IV 
     Solutions were prepared comprising 0.5 percent by weight perfluoroalkylethyltrichlorosilane with and without 0.5 percent by weight tetrachlorosilane in Isopar L solvent. Three perfluoroalkylethyltrichlorosilanes were employed: 1H,1H,2H,2H-tridecafluorooctyltrichlorosilane (&#34;octyl&#34;), 1H,1H,2H,2H-heptadecafluorodecyltrichlorosilane (&#34;decyl&#34;), or a mixture of perfluoroalkylethyltrichlorosilanes described in Example I. Tempered Solex® glass coupons and clear float glass coupons that had undergone a heat treatment which simulates a bending cycle (without bending significantly) were used in this study. The Solex glass was 0.157 inch (4 millimeters) thick, the clear float was 0.090 inch (2.3 millimeters) thick, and the tin surfaces were treated. Samples were tested in the QUVB-313 chamber and on a Wet Sled Abrader (Sheen Instruments LTD, Model 903). The Wet Sled Abrader was custom modified with an aluminum block which held two automotive windshield wiper blades. The wet sled abrasion test thus configured has an unusually high pressure wiper arm loading and is done partially wet and partially dry. These wiper strokes are much more severe than normally used in vehicles. Coating efficiency was measured by the contact angle of a sessile drop of water. The plus sign &#34;+&#34; refers to the presence of tetrachlorosilane in the coating formulations in the following tables. 
     
                       TABLE IVA                                                   
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OUVB-313 Contact Angle (°)                                         
Hours  octyl  octyl+   decyl decyl+  mix  mix+                            
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0      116    112      111   115     111  118                             
163    102    105      87    112     102  116                             
352    95     95       84    107     100  111                             
496    82     88       74    102     89   106                             
659    79     80       66    93      82   99                              
827    70     85       60    89      82   103                             
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                       TABLE IVB                                                   
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Wet Sled Abrasion Contact Angle (°)                                
Cycles octyl  octyl+   decyl decyl+  mix  mix+                            
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0      113    117      111   116     115  117                             
200*   86     104      79    108     86   108                             
600*   52     99       78    106     79   105                             
5000   35     84       47    91      82   92                              
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 *These data (at 200 and 600 cycles) were obtained with a slurry of 0.5   
 percent by weight HiSil ® 233 synthetic precipitated silica in water.
 The data for 5000 cycles were obtained using deionized water only.       
 
    
     EXAMPLE V 
     A control solution was prepared by mixing 95 grams of FC-77 solvent, 2.5 grams of perfluoroalkylethyltrichlorosilanes (perfluoroalkyl=C 6  F 13  to C 18  F 37 ), and 2.5 grams of perfluoroalkylethyleneo A primer-containing solution was prepared by mixing 188 grams of FC-77 solvent, 5 grams of perfluoroalkylethyltrichlorosilanes, 5.0 grams of perfluoroalkylethylene (perfluoroalkyl=C 6  F 13  to C 18  F 37 ), and 2 grams of tetrachlorosilane. A primer only solution was prepared from 198.4 grams of FC-77 solvent and 1.6 grams of tetrachlorosilane. These solutions were applied to the tin surface of 4.9 millimeter thick clear float glass with a cotton pad. Selected coupons were coated with primer solution prior to coating with either control solution or the solution containing perfluoroalkylalkylsilane and tetrachlorosilane. Coupons were cured at 300° F. (149° C.) for 15 minutes. The excess silane was removed from the glass surfaces by solvent washing. Coupons were weathered in the CCC. Coating efficiency was measured by the contact angle of a sessile drop of water. 
     
                       TABLE V                                                     
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CCC Contact Angle (°)                                              
No primer layer     Primer layer                                          
       No integral                                                        
                  Integral  No integral                                   
                                     Integral                             
Hours  primer     primer    primer   primer                               
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0      114        114       113      114                                  
232    116        116       117      115                                  
398    100        110       109      110                                  
590    49         78        75       86                                   
918    29         39        31       41                                   
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     The solution containing a silane hydrolyzable to silica led to more durable coatings whether or not the glass was preprimed with a silica layer separately with a solution of a hydrolyzable silane. 
     The above examples are offered to illustrate the present invention. Various perfluoroalkylalkylsilanes, hydrolyzable silanes, solvents and concentrations may be applied by any conventional technique, and optimally cured at suitable temperatures for adequate times to provide durable non-wetting surfaces to any of a variety of glass and plastic substrates, as well as other inorganic surfaces such as metals, ceramics, enamels, and metal or metal oxide films. The treated substrates of the present invention are especially suitable in automobile and aircraft parts, as well as in building components.