Water repellent surface treatment for plastic and coated plastic substrates
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. application Ser. No. 07/220,353 filed Mar. 30, 1994, 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 durable water repellent surface on various plastic and coated plastic substrates. 
     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 telomer. 
     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 plastic or coated plastic substrate surface with high water repellency and high lubricity. Durable water and dirt repellency of a substrate surface are provided by applying to the substrate surface a silica primer layer followed by a perfluoroalkylalkylsilane compound. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates the improvement in durability of a nonwetting perfluoroalkylalkylsilane surface treatment employing a primer on plastic in accordance with the present invention. The figure shows the contact angle as a function of exposure in a Cleveland Condensing Cabinet for untreated, treated but unprimed, and primed and treated acrylic surfaces. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Improved durability of rain and soil repellency provided by a perfluoroalkylalkylsilane surface treatment of plastic and coated plastic substrate surfaces is obtained by the use of a silica primer layer. In accordance with the present invention, the silica primer layer is preferably deposited by magnetron sputtering, or applied by a sol-gel condensation reaction, i.e. from alkyl silicates or hydrolyzable silicon-containing compounds such as tetra-acetoxysilane or chlorosilanes, preferably tetrachlorosilane, hexachlorodisiloxane, or partially hydrolyzed and condensed mixtures thereof. 
     A perfluoroalkylalkylsilane is applied to the silica-primed surface of a plastic or coated plastic substrate to produce the article of the present invention. The perfluoroalkylalkylsilane composition is preferably employed as a solution, preferably in a fluorinated solvent. The 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. 
     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, acyl, 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. Preferred radicals for X include chloro, bromo, iodo, methoxy, ethoxy and acetoxy radicals. Preferred perfluoroalkylethylsilanes in accordance with the present invention include perfluoroalkylethyltrichlorosilane, perfluoroalkylethyltrimethoxysilane, perfluoroalkylethyltriacetoxysilane, perfluoroalkylethyldichloro(methyl)silane and perfluoroalkylethyldiethoxy(methyl)silane. 
     These perfluoroalkylethylsilanes appear to react with bonding sites at the silica primed plastic 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. The perfluoroalkylalkylsilane may be combined with a fluorinated olefin. The fluorinated olefin telomer, which does not on its own bond to the substrate surface, provides for delayed hydrolysis of the Si-X species to produce a durable surface via enhanced reactivity with the glass surface. Preferred olefin telomers have the general formula C m  F 2m+1  CH═CH 2  wherein m may range from 1 to 30. The more preferred olefin telomers are a mixture of compounds of the above formula wherein m ranges from 1 to 16, preferably 8 to 12. 
     The perfluoroalkylalkylsilane and optional fluorinated olefins are preferably applied in solution. Suitable solvents include isopropanol, ethanol, hexane, heptane, mineral spirits, acetone, toluene and naphtha. Preferred solvents are 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. 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, limited to temperatures which do not degrade the substrate. A preferred maximum temperature for a polycarbonate substrate may be about 160° C. A preferred maximum temperature for acrylic may be about 100° C. Stretched acrylic substrates are more preferably heated only to about 80° C. 
     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. 
     The use of a relatively thick (about 1000 Angstroms) silica primer layer, intermediate thickness (about 200 Angstroms) magnetron sputtered silica primer layer, or a thinner (about 100 Angstroms) sol-gel applied silica primer layer on the surface of plastic substrates such as acrylic and polyurethane and coated plastics such as acrylate or urethane coated polycarbonate increases the resistance of the silane surface treatment in accelerated weathering tests. 
     Various other materials suitable for the preparation of silica films include silicon tetrahalides (or partially hydrolyzed/condensed silicon halides, preferably chlorides), silicon tetracarboxylates (preferably acetate), and other silanes or polysiloxanes which will hydrolyze with atmospheric and physisorbed water to produce a silica coating on a plastic or coated plastic surface. Sources of silica which do not readily hydrolyze are also suitable if a thin film of silica (or other inorganic oxide) can be prepared. Examples include tetraalkylammonium silicates, sodium silicates, and colloidal silicates. Variations which include other metal oxide salts or colloids are also useful. 
     The use of a mixed oxide primer will be understood to be a variation of the primer composition. Materials suitable for mixture include halides, alkoxides, and carboxylates of alumina, titanium, zirconium, and sodium. The material is chosen so as to hydrolyze with moisture and condense to produce an oxide coating. 
     The tetrachlorosilane or chlorosiloxane primers are also preferred since the application method requires simple methods and this primer can be applied to any plastic or coated plastic. Suitable solvents for the primer include anhydrous solvents such as perfluorocarbons, 1,1,2-trichlorotrifluoroethane, trichloroethane, methylene chloride, hydrocarbons, and other solvents without an active hydrogen. Most preferable are perfluorocarbons and hydrocarbons due to their inherent dryness and environmental considerations. Concentrations can range from about 0.01 to 100 percent by weight depending upon material and application method with a most preferable concentration range of 0.1 to 3 percent by weight. 
     The present invention will be further understood from the descriptions of specific examples which follow. 
     EXAMPLE I 
     A 3 by 4 inch (7.6 by 10.2 centimeter) sheet of cast acrylic was cleaned with hexane then methanol. A primer solution comprising 0.8 percent by weight tetrachlorosilane in Fluorinert® FC-77 (product of 3M) fluorocarbon solvent was applied. The solution was allowed to dry at ambient temperature. The primed acrylic surface was then contacted with a solution of 2.5 weight percent each of perfluoroalkylalkyltrichlorosilane and perfluoroalkylethylene in FC-77 fluorocarbon solvent. The perfluoroalkyl moiety comprised primarily C 6  to C 18  perfluoroalkyl groups, and the alkyl moiety was ethyl. After 2 minutes at ambient temperatures, the excess solution was washed off the surface using FC-77 solvent. The contact angle was 113° to 115°. 
     EXAMPLE II 
     Three different acrylic substrates from different sources, Lucite L (DuPont), Plex MC (Rohm and Haas) and Acrylite FF (Cyro Industries), were treated as follows. One set of each was treated respectively with the perfluoroalkylalkylsilane of the previous example (unprimed), and with the primer and perfluoroalkylalkylsilane of the previous example. The contact angles for all samples were in the range of 117° to 123° initially. 
     Some of these treated acrylic samples were exposed in a Cleveland Condensing Cabinet (CCC), constantly condensing water vapor at 140° F. (60° C.). Periodically, the coupons were checked for wettability as measured by contact angle of a sessile drop of water. These results are reported in the following table. 
     
                       TABLE                                                       
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Contact Angle (°)                                                  
Lucite L       Plex MC      Acrylite FF                                   
CCC    Un-             Un-          Un-                                   
(hours)                                                                   
       primed  Primed  primed Primed                                      
                                    primed                                
                                          Primed                          
______________________________________                                    
1      117     123     119    121   119   123                             
115    85      100     80     99    84    113                             
328    73      98      74     96    81    108                             
609    83      96      75     91    79    108                             
1015   72      94      72     87    75    101                             
1299   73      94      74     87    75    100                             
1676   69      89      69     84    72    93                              
2252   70      90      69     85    70    91                              
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     EXAMPLE III 
     Primer solution and perfluoroalkylalkylsilane solution were prepared as in Example I. Lucite L acrylic substrates were treated with perfluoroalkylalkylsilane solution with and without primer solution. 
     These treated acrylic samples were exposed in a Cleveland Condensing Cabinet and QUVB-313 apparatus. The results are reported in the following table. 
     Some of these treated acrylic samples were exposed in a QUV-Test, QUVB-313, which is a cyclic light and humidity test using UVB-313 lamps, cycled 8 hours UV light at 65° C. black panel temperature followed by 4 hours dark humidity cycle at 50° C. Periodically, the coupons were checked for wettability as measured by contact angle of a sessile drop of water. These results are reported in the following table. 
     
                       TABLE                                                       
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CCC    Contact Angle (°)                                           
                     QUVB     Contact Angle (°)                    
(hours)                                                                   
       Unprimed  Primed  (hours)                                          
                                Unprimed                                  
                                        Primed                            
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0      117       121     0      113     118                               
122    95        116     144    65      118                               
219    94        107     310            113                               
454    81        98      566            114                               
712    68        93      1255           85                                
930    71        91                                                       
1219   70        87                                                       
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     EXAMPLE IV 
     Samples of urethane-ester polymer prepared from the reaction of diisocyanate and a polyol reaction product of trimethylolpropane and caprolactone were treated with the perfluoroalkylalkyl solution of Example I, with and without the primer solution of Example I. The surfaces of untreated, perfluoroalkylalkylsilane treated, and silane primer and perfluoroalkylalkylsilane treated polyurethane were evaluated by measuring the contact angle of a sessile drop of water. The untreated polyurethane surface had a contact angle of 84°, the perfluoroalkylalkylsilane treated surface had an initial contact angle of 114°, and the polyurethane surface treated with both primer and perfluoroalkylalkylsilane had a contact angle of 125°. 
     The above examples are offered to illustrate the present invention. Various hydrolyzable silanes, perfluoroalkylalkylsilanes, solvents and concentrations may be applied by any conventional technique, and optionally cured at suitable temperatures for adequate times to provide durable non-wetting surfaces to any of a variety of plastic and coated plastic substrates such as acrylic, urethane, polycarbonate and other polymers. The treated substrates of the present invention are especially suitable in automobile and aircraft parts, as well as in building components.