Water repellent surface treatment

A method and article are disclosed wherein a metal, polymeric or inorganic coated substrate is provided with a durable non-wetting surface by treatment with a perfluoroalkyl alkyl silane and a fluorinated olefin telomer.

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. 
THE PRIOR ART 
U.S. Pat. No. 4,263,371 to Franz teaches a method for reducing the surface 
energy of a glass article by chemisorption of an organotin compound at a 
temperature insufficient to thermally decompose the compound. Such 
compounds as methyltin chloride are shown to increase the contact angle of 
a drop of water on the surface from 10.degree. for untreated glass to 
45.degree. to 90.degree. for various compounds. 
U.S. Pat. No. 4,276,350 to Franz discloses a method for reducing the 
reactivity of a glass surface by blocking reactive sites such as silanol 
groups at the glass surface with a molecular layer of fluorocarbon. The 
method involves absorbing multivalent cations at the glass surface to 
provide anchoring sites for the chemisorption of otherwise nonreactive 
fluorocarbons. 
U.S. Pat. No. 4,301,197 to Franz et. al. discloses the formation of highly 
efficient release surfaces on glass substrates by treatment of the glass 
with poly alkyl hydrogen siloxane. The treated glass surface effectively 
releases such materials as polycarbonates, acrylics, and polyurethanes 
contacted in press polishing or laminating processes. 
U.S. Pat. No. 4,529,657 to Franz discloses reducing the surface energy of 
glass by absorbing multivalent cations such as chromium onto a glass 
surface, then treating the surface with an aqueous solution of an alkali 
metal alkylate such as sodium stearate or oleate to bond the long chain 
paraffinic or olefinic acid anion to the glass surface by ionic bonding of 
the alkylate anion to the absorbed multivalent cation, thereby rendering 
the glass surface non-wettable and lubricated. 
SUMMARY OF THE INVENTION 
The present invention provides a substrate surface with high water 
repellancy and high lubricity. Durable water and dirt repellency of a 
substrate surface are provided by applying to the substrate surface a 
select combination of fluorinated compounds. High water repellency is 
provided by perfluoroalkylsilanes which bond to the substrate surface, 
while high lubricity is provided by addition of fluorinated olefin 
telomers. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
A perfluoroalkyl alkyl silane is combined with a fluorinated olefin telomer 
to produce the compositions of the present invention. The silane/olefin 
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 perfluoroalkyl alkyl silanes have the general formula R.sub.n 
R'SiX.sub.4-n, wherein R is a perfluoroalkyl radical, n is preferably 1, 
R' is an alkyl, preferably ethyl, vinyl or propyl, and X is preferably a 
radical such as alkyl, halogen, and/or alkoxy. Preferred perfluoroalkyl 
radicals range from CF.sub.3 to C.sub.30 F.sub.61, preferably C.sub.6 to 
C.sub.16, and most preferably C.sub.8 to C.sub.10. R' is preferably ethyl. 
Preferred radicals for X include chloro, iodo, methyl, methoxy, ethoxy and 
acetoxy radicals. Preferred perfluoroalkyl ethyl silanes in accordance 
with the present invention include perfluoroalkyl ethyl-trichlorosilane, 
perfluoroalkyl ethyl-trimethoxysilane, perfluoroalkyl 
ethyl-dichloro(methyl)silane and perfluoroalkyl 
ethyl-diethoxy(methyl)silane. These perfluoroalkyl ethyl silanes appear to 
react with bonding sites at the substrate surface on a molecular basis. 
There does not appear to be polymerization or crosslinking. Strong surface 
bonding of the perfluoroalkyl ethyl silanes produces a substrate surface 
which exhibits a high contact angle with a drop of water, indicating high 
water repellency. The fluorinated olefin telomer, which does not on its 
own bond to the substrate surface, but which is bonded in combination with 
the perfluoroalkyl silane, provides lubricity to promote dirt repellency. 
The olefin structure provides for crosslinking to produce a durable 
surface. Preferred olefin telomers have the general formula C.sub.m 
F.sub.2m+1 CH.dbd.CH.sub.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 4 to 10. 
Suitable solvents include isopropanol, ethanol, hexane, heptane, acetone, 
toluene and naphtha. Preferred solvents are fluorinated hydrocarbon 
solvents such as trichlorotrifluoroethane, and methylene chloride, and 
perfluorinated organic compounds such as perfluorocarbons. Concentrations 
of about 0.005 to 5, preferably about 0.05 to 2.5, percent of each 
component are preferred. 
The solvent is preferably evaporated simply by drying in air at ambient 
temperature. The composition may be cured by heating the treated surface. 
A cure cycle of about 200.degree. F. (about 93.degree. 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.degree. to 
500.degree. F. (about 204.degree. to 260.degree. C.) may be preferred, 
particularly about 3 minutes at about 470.degree. F. (about 243.degree. 
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 Gartner Scientific Goneometer optics. 
The surface to be measured is placed in a horizontal position, facing 
upward, in front of a point source light such as a slide projector lamp. A 
drop of water is placed on top of the surface in front of the light source 
so that the contours of the sessile drop can be viewed and the contact 
angle measured through a goneometer telescope equipped with circular 
protractor graduation.

The present invention will be further understood from the descriptions of 
specific examples which follow. 
EXAMPLES I TO III 
A solution is prepared comprising 2.5 grams of perfluoroalkyl ethyl 
trichlorosilane and 2.5 grams of fluorinated olefin telomer in 95 grams of 
Freon TF solvent, available from DuPont. The perfluoroalkyl ethyl 
trichlorosilane composition comprises a range of perfluoroalkyl radicals 
CF.sub.3 (CF.sub.2).sub.n wherein the average n=9.0 and the approximate 
distribution is C.sub.6 =6 percent, C.sub.8 =50 percent, C.sub.10 =29 
percent, C.sub.12 =11 percent and C.sub.14+ =4 percent. The fluorinated 
olefin telomer has a similar distribution with C.sub.4 =4 percent, C.sub.6 
=35 percent, C.sub.8 =30 percent, C.sub.10 =17 percent, C.sub.12 =8 
percent and C.sub.14+ =6 percent. This solution is Treatment A. A solution 
of the same perfluoroalkyl ethyl trichlorosilane and fluorinated olefin 
telomer (2.5 grams of each) is prepared in a perfluorinated solvent, 
Fluorinert FC-77 from 3M. This solution is Treatment B. Metal plates 
measuring 6.times.6 inches (about 15.times.15 centimeters) are cleaned in 
dilute hydrochloric acid, washed with isopropanol and water, rinsed with 
deionized water and dried in air. The above solutions are applied with a 
cotton pad, and the excess removed with isopropanol. The contact angles of 
the treated surfaces are compared with the contact angles of untreated 
surfaces in the following table. 
TABLE I 
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Contact Angle (.degree.) 
Treatment 
Treatment 
Example 
Metal Untreated A B 
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I galvanized steel 
71 90 84 
II stainless steel 
77 110 112 
III aluminum 78 107 102 
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Chrome plate, copper and brass do not show similar increases in contact 
angle. This is believed to be a result of the absence of a surface 
oxidation layer with which the silane can bond. Steel and aluminum both 
comprise a durable, coherent metal oxide skin which is believed to react 
with the silane to yield a water-repellent surface. 
EXAMPLES IV TO VI 
The above Treatment A solution is applied to the surfaces of steel panels 
coated with three polymer topcoats used as automotive parts finishes as in 
the previous examples. Another set of polymer coated steel panels is 
treated with a solution similar to Treatment A except that half of the 
olefin is replaced with perfluoroiodide CF.sub.3 (CF.sub.2).sub.n I, 
wherein n is from about 4 to 14, mostly 6 to 10 in a similar distribution 
as the silane and olefin. This is Treatment C. The contact angles are 
compared in the following table. 
TABLE II 
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Contact Angle (.degree.) 
Treatment 
Treatment 
Example 
Polymer Untreated A C 
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IV epoxy 75 92 112 
V melamine 73 96 105 
crosslinked 
polyurethane 
VI acrylic 74 82 110 
melamine 
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EXAMPLES VII TO X 
Various inorganic coatings are deposited on glass substrates. The inorganic 
coated surfaces are then wiped with a solution of Treatment A as in 
previous examples. The contact angles are compared in the following table. 
TABLE III 
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Contact Angle (.degree.) 
Example Coating Untreated Treatment A 
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VI sputtered tin oxide 
54 106 
VII sputtered indium/tin 
55 104 
oxide 
VIII sputtered antimony/tin 
44 109 
oxide 
IX pyrolytic chrome/ 
64 110 
iron/cobalt oxide 
X pyrolytic tin oxide 
30 108 
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A sputtered low emissivity coating was damaged by Treatment A. It is 
believed that the silver layer in such a coating is not stable in the 
presence of chloride. 
The above examples are offered to illustrate the present invention. Various 
perfluoroalkyl silanes, fluorinated olefin telomers, solvents and 
concentrations may be applied by any conventional technique, and cured at 
suitable temperatures for adequate times to provide durable non-wetting 
surfaces to any of a variety of 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 is especially 
suitable in automobile and aircraft parts as well as in building 
components.