US3354022A

US3354022A - Water-repellant surface

Info

Publication number: US3354022A
Application number: US356155A
Authority: US
Inventors: Dettre Robert Harold; Jackson Harold Leonard; Jr Rulon Edward Johnson
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1964-03-31
Filing date: 1964-03-31
Publication date: 1967-11-21
Anticipated expiration: 1984-11-21

Description

Nov. 21, 1967 R. H. DETTREI ET AL 3,354,022

'y wATER-REPELLANT SURFACE Filed March 31, 1964 v 5 Sheetsfsheet s MEMS/NG ROLLER COL@ /7//P United States Patent 3,354,022 WATER-REPELLANT SURFACE Robert Harold Dettre, Wilmington, Harold Leonard Jackson, Hociressin, and Rulon Edward Johnson, Jr., Wilmington, Del., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Mar. 31, 1964, Ser. No. 356,155 8 Claims. (Cl. 161--123) ABSTRACT OF THE DISCLOSURE This invention relates to water repellent, solid surfaces and to a process for their preparation. More particularly, this invention relates to solid surfaces having configurations which endow the surfaces with improved water repellent properties and a method for imparting these configurations to solid surfaces.

Prior to this invention, major efforts in combatting the problem of producing water repellent materials centered around the application of coatings to substrates. A variety of water impervious coatings have been used, but they have the inherent disadvantages characteristic 4of coatings in general, namely, gradual absorption of water by the coating and contact angle hysteresis both of which reduce repellency. These and other problems have prevented the art from producing a durable, highly water repellent surface.

Literature concerning the theoretical requirements for good water repellency has created certain misleading postulates. For example, it is generally believed that water repellency is greatest when the contact angle 0 is as large as possible. Contact angle 6 is defined as the angle (measured through the liquid) which a liquid makes with a solid'. While it is true that increases in the advancing contact angle (the largest experimentally measured contact angle, 0a) and receding contact angle (the smallest experimentally measured contact angle, 0r) can improve repellency, in certain instances increases in both these angles result in decreased depellency. Water repellency, which is defined in terms of the angle a (relative to the horizontal) to which a surface must be tilted in order for a water drop to roll off, depends on the differences in the cosines of the advancing and receding contact angles. This relationship is given by:

` (1) Sin a=KW (cos r-cos 0,) where K'=a constant for a given volume of water W=width of drop Therefore, as mentioned above, repellency can be reduced if the increases in 0r and 0a are such that the difference in their cosines increases.

. The existing literature also gives the impression that formation of a composite surface, i.e., a surface composed of high portions and low portions which when contacted with water will create air-liquid and solid-liquid interfaces between the water drop and the repellent surface, will give improved repellency regardless of the air content of the composite surface. The air content of a composite surface is determined by taking an imaginary plane parallel to the surface passing through the tops of the high portions of the surface and measuring at this plane the percentage of the total surface area which is air.

This hypothesis found in the prior art is partially based on the premise that an increase in contact angle alone is sufficient to improve repellency. However, composite surfaces can be obtained which are less repellent than their smooth counterparts. FIGURE l illustrates graphically the effect of the air content of a surface on the advancing contact angle 9a and the receding contact angle 0!- for a representative solid surface of this invention having a constant chemical composition and an intrinsic advancing water contact angle (0a) of 110 and an intrinsic receding water contact angle (0r) of 92. The intrinsic contact angle (for 0 and 0,) is defined as the contact angle exhibited by a given solid surface when smooth. By a smooth surface is meant a surface having high portions not greater thanabout 0.5 micron in height. Whether a composite surface will be more or less repellent than the corresponding smooth surface depends on the air content of the composite surface. Actually, there is a reduction in repellency for composite surfaces having low percentages of air in the surface when compared to their smooth solid counterparts. This is due to the fact that the advancing contact angle 0 increases more rapidly than does the receding angle 0r when the air content of the surface is increased within certain limits, i.e., from about 0 to about 60% as shown in FIGURE 1. This increase in air content results in an increase in the difference between the contact angles. Referring to Equation 1, this increase in difference causes a concomitant increase in the tilt angle u.

FIGURE 2 further illustrates graphically the relationship between tilt angle a and air content of the fsurface. As shown in the graph, initial increases in air content decrease water repellency until at a certain air content the l tilt angle equals the tilt angle for the smooth surface.

Beyond this critical air content tilt angles decrease drastically. The critical air content is about 60% or morerfor most solid surfaces. Surfaces having low tilt angles when contacted with water droplets exhibit characteristics similar to mercury droplets on glass.

The manufactures, process and theory of this invention will be better appreciated by .reference to the drawing wherein several embodiments of the invention are illusi trated.

FIGURE 1, discussed above, is a graph summarizing the effect of increasing aircontent on 0a and 0r for a given solid surface. Y y

FIGURE 2, discussed above, is a graph summarizing the variations in tilt angle a with increasing air content for a given solid surface.

FIGURE 3 is a plan view in partial section of one embodiment of the present invention wherein b is the distance between the centers of adjacent high portions,

' c is the distance between the outer extremities of adjacent high portions and d is the diameter of a high portion.

FIGURE 4 is a cross-sectional view of the embodiment represented in FIGURE 3 taken along line 2 2 wherein h is the height of a given high portion.

FIGURE 5 is a plan view in partial section of another specific embodiment of the present invention wherein b, c and d have the same meaning as in FIGURE 3.

FIGURE 6 is a cross-sectional view of the embodiment represented in FIGURE taken along line 4--4 wherein h has the same meaning as in FIGURE 4.

FIGURE 7 is a photomicrographic plan view of a representative non-uniform composite surface of this invention wherein the dark areas indicate low portions and the light areas indicate high portions. The angle of illumination of the original photograph was to the plane of the' surface.

FIGURE 8 is a diagrammatic side elevation view of apparatus for preparing surfaces having the configurations dis-closed in this application.

It is an object of this invention to provide a permanent,

solid, water repellent surface. Another object is to provide a method for the production of solid water repellent surfaces. A further object is to provide intermediate means utilizable in processes for the production of these surfaces. These and other objects will become readily apparent from the following description and claims.

These objects are accomplished in accordance with lthe present invention whereby a surface is produced having configurations which impart a greater degree of water repellency to the surface than the corresponding unmodified solid surface.

The tendency for a solid surface to repel water is dependent upon contact angle hysteresis, Le., the difference between advancing (0a) and receding (0,.) contact angles of a water droplet on the solid surface. Where the difference in the above angles is great, there is a tendency for the water droplet to remain on the solid surface, i.e., the surface does not repel water. However, when hysteresis is reduced by lessening the difference between 0a and c1I water droplets are repelled from the solid surface and roll off the surface. When solid surfaces consisting of compositionsv having a high intrinsic contact angle are suiciently modified to produce a composite surface having an air content greater than the critical air content, low contact angle hysteresis is observed and high repellency is obtained.

This can be theoretically explained by considering the thermodynamic properties of a water drop on a composite surface. Qualitatively, these considerations show that when a drop moves over a composite surface the advancing portion of its periphery rnoves rapidly over the solid areas and comes to rest at solid-air boundaries or edges. At these points the contact angle 0 increases to very high values, approaching 180 This behavior reinforces the tendency of the drop, due to its surface tension, to minimize its surface area. The receding portion of the drop periphery tends to remain on solid regions and the contact angle 0r at thesey points approaches the receding contact angle for water on the smooth solid. The extent to which this takes place depends on how much high solid area is present in the composite surface. Opposing the tendency to remain on solid regions is the previously mentioned tendency to minimize surface area. Therefore, as the air content of the composite surface increases and less solid is present to restrain the periphery, the surface tension of the water predominates and the receding contact angle' increases to high values, approach ing those of the advancing portions of the drop.

Specifically, the present invention is directed to a solid surface having an intrinsic advancing water contact angle 0a of greater than 90 and a receding water contact angle 0r of at least 75, Said SOlid surface comprising high portions and low portions, said low portions being at least 60% of the total surface area of the solid surface (i.e., an air content of at least 60% air).and said high portions having an average distance between adjacent high portions of not greater than 1000 microns, the average height of the high portions above the low portions. being at least .5 times the averagedistance between adjacent high portions, said high portions having con gurations which produce a lower tilt angle on said solid surface than exhibited on the corresponding smooth surface.

One embodiment of the present invention involves a solid surface comprising an @ordered arrangement of projections (high portions) consisting of non-contiguous vertically oriented columns of solid rising from the low surface, the maximum distance between the columns being given by where Cm is distance in centimeters, 0a is the intrinsic advancing water contact angle and f is air content expressed as a fraction (i.e., the fraction of the total surface area that consists of low portions), the height of the columns being at least .5 times the distance between adjacent columns, said surface having an air content of at least air (low portions) and the advancing intrinsic water contact angle being greater than 90 and the receding intrinsic water contact angle being at least Another embodiment of the present invention involves a solid surface comprising high portions having therein an ordered arrangement of depressions (low portions), said high portions consisting of interconnected vertical walls of solid which enclose chambers of air, the maximum distance between the walls being given by Cm=0.587 cos 0a where C,m is the distance in centimeters, Qa is the intrinsic advancing water contact angle and the depth of the depressions being at least .5 times the distance between the walls, said surface having an air content of at least 60% air (depressions), the solid surface having an intrinsic advancing water contact angle of greater than and an intrinsic receding water contact angle of at least 75.

A third embodiment involves solids having rough surfaces comprising non-uniform arrangements of high and low portions which on an average fall within the necessary height, distance and air content requirements generic to this invention.

Preferred embodiments of the present invention involve solid surfaces having intrinsic advancing water contact angles of greater than and intrinsic receding Water Contact angles of greater than 85 with air contents of 75% air or above and heights of high portions above low portions about equal to the average distances between adjacent high portions.

In systems comprising ordered arrangements of high portions (projections) the preferred distance between adjacent high portions is about 250 microns or below. On surfaces having an ordered arrangement of depressions surrounded by interconnected vertical walls the preferred distance between adjacent walls is 600 microns or below.

The water repellent surfaces of this invention may be made from any solid surface having an intrinsic advancing water contact angle 0a of greater than 90 and `a receding Water contact angle (if of at least 75. It is necessary that the surfaces have high intrinsic advancing contact angles to prevent liquid penetration in the composite surface due to the hydrostatic pressure of the liquid drops. Receding contact angles of at least 75 are necessary to avoid having to increase the air content of the surface to impractically high values to overcome the attractive forces between the drops and the high solid portions.

Solids found to have the necessary intrinsic contact angles are for example: alkyl polysiloxanes; polyfluoroalkyl polysiloxanes; waxes having a softening temperature greater than 40 C. which are essentially halogen free and water insoluble; and polymers comprising polymerizable or copolymerizable ethylenic compounds which will form polymers having the following surface constitutions:

as described by Shafrin and Zisman, l. Phys. Chem. 64, 519, 521 (1960).

Speciiic examples of operable ethylenic compounds which fall within the definition of the invention are as follows:

where n is from 3 to 13.

Examples of polymers which arev not satisfactory are the homo-polymers of: CF2=CC12, CHZZCHOI,

CHFCClz, CHFCHCN The waxes which are operable in this invention are:v Natural waxes such as beeswax, candelilla wax, carnauba Wax, esparto wax; petroleum waxes such as paratiin waxes, micro-crystalline waxes; fossil or earth waxes such as montan wax, ozocerite, peat wax and synthetic waxes such as Fischer-Tropsch waxes and hydrogenated waxes.

While most solid surfaces exhibiting the required intrinsic contact angle show improved repellency over thesmooth unmodified surface when the air content is above about 60% and the other conditions mentioned above are met certain surfaces (e.g., polyethylene) do not show improved repellency until the air content of the surface is about 75% to 80% air.

The expressions listed above for the distance between adjacent high portionsare derived from a consideration of the water penetration pressure on the surfaces as follows: When a cylindrical capillary is made of a material which when smooth has an advancing water contact angle, 0a, greater than 90, a pressure, P, is required in order 6. to force the water into the capillary. The magnitude of the pressure is given by 2 P=47 cos 6a where Fy is the surface tension of water and c is the diameter of the capillary. Using the value for the surface tension of water at 25 C. and expressing P in centimeters of water, Equation 2 becomes cos 6a cos 0, c

where 0r is the receding contact angle. if Hr is less than the water will show no tendency to come out of the capillary. For most solids with non-zero Contact angles, @a and 0, are not equal (0r being less than 0a). In this situation the angles 0a and 0r are not to be confused with the angles actually observed forl water drops on the embossed surface. The present angles 9a and 0r refer to the water angles on the solid regions of the embossed surface. In some instances surfaces can be prepared where 6:0r but this requires special procedures to obtain smooth surfaces and elaborate purication methods to obtain homogeneous surfaces; It is probably impossible to obtain homogeneous polymer surfaces; even adsorbed monolayers are, to some extent, heterogeneous. Many materials possess surfaces when smooth which have 0g greater than 90 and 0r less than 90. It is therefore necessary to emboss the surfaces of these materials in order to have high repellency. For a given surface (with a 9a 90) the penetration pressure is determined by the distance, c,- in Equation 3. The maximum distance that could be tolerated would be one which prevents water drops from penetrating under their own hydrostatic pressure. The maximum 'height that a water drop can attain in the presence of gravity is about 0.5 cm. The maximum distance, Cm (in cm.), is then given by i (5) Cm=0.587 cos 0,L

edv cos 0a where d is the diameter of the cylinders and b is the center-to-center distance between cylinders. Expressed in terms of f (the fraction 'of the total surface area that is air) and the distance, c, between the outer walls of the cylinders (c=b-d) and using the value for the surface tension of water at 25 C., Equation 6 becomes P= [\/2.721 1-f) -rraau-fo'g) @OS 0 Where P is expressed in centimeters of water. ImposingV the condition that a water drop should not penetrate into the surface under its own hydrostatic pressure gives for the maximum distance between cylinder walls,

In a system of random dispersed high portions (rough surfaces) it is difficult to give exact mathematical definitions of distance ranges. However, prepared surfaces having average distances between adjacent high portions falling within the generic requirements of this invention provide improved water repellency. Preferred non-uniform surfaces having distances of from about 20 microns to about 400 microns have been found to impart superior water repellency to surfaces provided the largest average spacing between adjacent high portions is below about 300 microns.

The high portions of the composite surfaces of this invention must have certain configurations in order to provide the proper air content while at the same time avoiding penetration of the liquid into the low portions of the surface. Optimum results are achieved when the sides of the walls or projections are perpendicular to the plane of the surface, however, it is not necessary that the sides of the projections or walls be absolutely vertical or perpendicular to the surface. It is sufficient if somewhere on all sides of a given high portion there is a point at which a line drawn tangent to the side will make an angle between the tangent and the horizontal, measured through the high portion, of about 80 or more.

Methods of preparing water repellent surfaces The present invention is also directed to a process for the preparation of water repellent solid surfaces which comprises contacting a solid surface having an intrinsic advancing water conta-ct angle of greater than 90 and an instrinsic receding contact angle of at least 75 with means for softening said solid surface, contacting the solid surface with a die capable of producing a positive configuration comprising high portions and low portions on the solid surface wherein the height of the high portions and the distance between adjacent high portions will conform to the requirements previously stated and removing said die from contact with the solid surface.

Alternatively, the softening step may be dispensed with provided the die form or mold is of sufficient mechanical strength to withstand pressures necessary to impart the desired configurations to the surface materials of this invention without damage to the die or loss of detail in the resulting surface configuration.

Die forms or molds utilizable in the practice of this invention include commercially available screens or mesh plates, photosen-sitive polymers which after exposure to actinic light are treated to produce the negative of the desired surface configuration, metallic dies produced by contacting the metal with photosensitive polymers having the positive configuration desired to be reproduced on the solid surface and masses of hollow fibers rigidly held together by suitable means. These holiow fiber die forms or molds are aligned together in parallel manner in bundles and cut or sliced approximately perpendicular to their length to create a die surface of their cross-section. The outer diameter of these hollow fibers must be on an average in the range of from about l to about 450 microns and the inner diameter must be on an average less than about .6 times the outer diameter. The length of hollow fibers used in the die must be at least .5 times the difference between the inner and outer diameters of the hollow fibers.

I. Surfaces consisting of projections A novel and convenient method for preparing a surface consisting of vertical projections has now been provided by utilizing a mass of hollow fibers as a die form or mold. The hollow fibers may be obtained as described in U.S. Patent 2,999,296 and French Patent 990,726. The outer diameter of the fibers used measure within the range of 10 to 450 microns and are 1.2 to 3.0 times the inner diameter. The fibers are compacted together and cut or sliced approximately perpendicular to their length exposing the cross-section. The truncated ends are then used as a mold or die. The surface to be treated is softened slightly, by heating or other suitable means (e.g., solvents) to facilitate impression of the surface by the mold. The surface hardens as it cools, whereby high portions (projections) resembling vertical columns corresponding to the hollow cores of the fibers are permanently developed on the surface. The hollow fibers may be assembled into bundles of various shapes and sizes or may be attached to a roller having the truncated ends of the fibers as the roller surface. A large quantity of the object having the surface to be treated may be conveyed in such a manner under the roller so that the pattern is stamped on the surface.

II. Surfaces consisting of depressions One method of producing a pattern of depressions on a surface is now provided by first preparing a -grid conforming to the required surface dimensions by drawing the pattern on a sheet of paper which transmits actinic light. The pattern so drawn is a model of the pattern desired on a large scale. By reducing photographically, the pattern is obtained in the small scale desired. A halftone screen, such as is used in photography, may also be used in place of the paper pattern. A relief image of the pattern is made on photosensitive plastic by placing the paper pattern on halftone screen over the plastic and exposing to actinic light, as described in U.S. Patent 2,760,- 863. There is formed on the plastic a relief image of the pattern. This plastic grid is now used as a die and impressed onto another surface capable in itself of being used as an embossing die, eig., Woods metal. This die, bearing a reverse pattern of the grid, is impressed on the surface to be made water repellent in the same manner as described above for the hollow fiber surface. The lines of the original paper pattern thus become the walls of the depressions. The embossing dies may also be made of any metal or non-metal which can be chemically or mechanically modified to give the desired surface pattern with the desired spacings and dimensions, providing the material is of sufficient strength to permit its use as a die. Alternatively, a photosensitive glass available commercially, such as described in Glass Engineering Handbook, E. B. Shund, McGraw-Hill, New York, 2nd ed., 1958, p. 363, may be used in the preparation of such a surface. By following the lmethod described in the above reference for preparing patterned glass, a surface having an array of depressions may be produced directly.

However, glass in itself does not possess the requisite advancing and receding water contact angles. It must therefore be treated with a material having the necessary intrinsic advancing and receding water contact angles to achieve the desired water repellency by a physical coating process. Or, for example, monoor multilayers of a substance having the required Contact angles may be chemisorbed on the glass to form a strongly adherent coating. The coating acquires the configuration of the glass substrate thereby forming a highly water repellent surface.

The following examples are given to illustrate various specific embodiments of this invention and do not limit the invention in any way.

Contact angles were measured directly on profiles of sessile drops using the general method described in Surface Chemistry by I. J. Bikerman, Academic Press, Inc., New York (1958), 2nd ed., p. 343. In these examples contact angles were measured on profiles of sessile drops using a microscope fitted with a goniometer eyepiece; magnification was about 20X. Error in angle measurements by this method is estimated to be about if. Drop addition and size changes were made using a hypodermic syringe. Advancing angles were measured after the drop size was increased and the periphery advanced over the surface; receding angles Were measured after the drop size was reduced and the periphery receded. Unless otherwise stated, readings were taken with 30 seconds of drop formation and/or size change, and drop volumes were between 0.03 and 0.10 ml. Receding angles were measured on drops that had been in contact with the surface for some time (several minutes) before being receded and on fresh drops which were placed on the surface and immediately (several seconds) reduced in size. There was little difference between these two methods as long as the drop size was kept below 0.1 ml. Each contact angle value is the average of at least 8 measurements. All measurements were made at 25 il C. and 50% relative humidity.

In these examples the air content of the surface was determined by photomicrographs (plan view) of the surface. The photomicrographs are taken with the plane formed by the tops of the high portions of the surface in focus. The fraction of the total area of the photomicrographs that is air is taken as the air content of the surface. The magnification is chosen such that each photomicrograph includes enough of the area of the surface to give a reliable and reproducible value for surface porosity (air content).

The method used to determine tilt angles (a) in these examples is essentially that described in I. Colloid. Sci., vol. 17, p. 309 (1962), by C. G. L. Furmidge. The surface to be studied was placed on a hinged brass plate. A drop of water of the required volume was placed on the experimental surface using a hypodermic syringe. The hinged plate was immediately `rotated until' the 'drop started to roll off the surface. The angle to which the plate was rotated was then measured .to the nearest degree lwith a protractor. With only slight differences the tilt anglennecessary to cause the drop to commence rolling is equal to the angle necessary to keep the drop rollmg.

EXAMPLE 1 An embossing die is made in the following manner: A bundle of hollow fibers of a copolymer of tetraiiuoroethylene and hexauoropropylene is compressed, using a hose clamp, to give a close-packed arrangement of the fibers. The hollow fibers have an inside diameter of 250 microns and an outside diameter of 430 microns. The clamped Ibundle is then cut with a microtome knife so that the surface thereby formed is iiush with one end of the hose clamp. This surface (1.5 cm. in diameter), which consists of the truncated ends of the hollow fibers, is used as an embossing die.

The die described above is pressed (by hand) against the warm surface of a piece (2 cm. x 2 cm. x 1 cm.) of paraffin wax. The surface of the Wax is previously heated to 35 to 40 C. (to a depth of about 1 mm.) in order to soften it and thereby facilitate fiow into the holes of the die. There is negligible adhesion of the paralin wax to the hollow fiber die. The embossed surface consists of an array of cylindrical projections oriented at right angles to the original wax surface. There is negligible penetration of wax into the spaces between the fibers of the die because these spaces are sufficiently small, the applied pressure during embossing is low enough and the flow properties of the wax at 35 to 40 C. are poor enough to prevent penetration.

The cylindrical projections are about 250 microns in diameter and 150 to 200 microns high. The distance between centers of adjacent projections is about 430 microns and the area of the surface, measured at the plane formed by the tops of the projections, is approximately 70% air. The advancing contact angle for a water drop has increased from 113 on the original surface to 144 on the embossed surface and the receding angle has increased from 100 to 129. The tilt angle necessary for a 0.05 ml. water drop to roll olf the surface has decreased-from to 9.

A lower tilt angle for one surface than for another means that if both surfaces are vertical the surface with the lower tilt angle will permit roll-off of smaller water drops. This is equivalent to saying that liquid 'retention will be lower on the surface with the lower tilt angle.

In actual practice the hollow fiber dies can be many times larger in area then the one described above and the embossing process can be carried out using conventional presses. For example, the fibers can be mounted in closepacked array on any rigid backing such as a metal or hard plastic plate, the dimensions of the plate being determined by the size of the surface to be embossed or by the size of the embossing press. A plate of several square feet in area is possible. The close packing can be achieved and maintained by exerting lateral pressure at the periphery of the plate or by sealing the fiber ends to the plate using a suitable adhesive. l

Alternatively, for applications where continuous embossing of large areas is desirable, the fibers can be mounted on a roller of several feet in length and of sufficient diameter so that close packing of the fibers is possible at the outer surface of the die.

The parafiin wax in this example need not be a solid piece of the wax alone. It can just as well be a thin coating of wax (as thin as 0.1 mm.) on another surface, such as a sheet of glass or metal. I'

EXAMPLE 2 An embossing die is made in the manner described in Example 1 using hollow fibers of polyethylene. These fibers have an inside diameter of 80 to 95 microns and an outside diameter of 220 microns. This die (1.5 cm. in diameter) is used to emboss the parafiin wax surface of Example 1 using the method described in Example 1.

The resulting -cylindrical projections of the embossed surface are 80 to 95 microns in diameter and 200 to 300V microns high. The distance between centers of adjacent projections is 240 to 300 microns and the area of the surface, measured at the plane formed by the tops of the projections, is approximately 87% air. The advancing contact angle for a drop of Water has increased from 113 to 156 and the receding angle has increased from 100 to 148. The tilt angle necessary for a 0.05 ml. water drop to roll off the surface has decreased from 15 to 4.

As with the hollow fiber dies of Example 1, it is also possible to fabricate dies which are much larger than the one used in this example.

EXAMPLE 3 The hollow ber die of Example 1 is pressed (by hand) against the surface of a piece of polypropylene film (2 cm. x 2 cm. x 0.05 cm.) having a melting point of about 165 C. and a melt index of about 0.5. The film is previously heated to 150 to 155 C. to soften it and thereby facilitate flow into the holes of the die. The die is also heated (to to 120 C.). The embos-sed surface consists of an array of cylindrical projections 250 microns in diameter and 500 to 6.00 microns high. There is negligible penetration of polypropylene between the fibers of the die. The average distance between centers of adjacent projections is about 450 microns and the area of the surface, measured at the plane formed by the tops of the projections, is approximately 75% air. The ad` vancing contact angle for a water drop has increased from on the original fiat surface to 139 on the embossed surface and the receding Contact angle has increased from 85 to 115. The tilt angle necessary for a 0.10 ml. water drop to roll off the surface has decreased from 21 to 16.

The larger dies described in Example 1, particularly the roller type, can be used to emboss large sheets of polypropylene to produce the surface described in this ex. ample.

1 1 EXAMPLE 4 The hollow fiber die of Example 1 is pressed (by hand) against the surface of a piece of polyethylene film (2 cm. x 2 cm. x 0.1 cm.) having a melting point of about 110 C. and a melt index of 1.8. The film is previously heated to 102 to 105 C. to soften it and thereby facilitate ow into the holes of the die. The die is also heated (to 95 to 100 C.). The embossed surface consists of an array of cylindrical projections 250 microns in diameter and 450 to 500 microns high. There is negligible penetration of polyethylene between the fibers of the die. The average distance between centers of adjacent projections is about 450 microns and the area of the surface, measured at the plane formed by the tops of the projections, is about 75% air. The advancing contact angle for a water drop has increased from 98 on the original at surface to 140 on the embossed surface and the receding contact angle has increased from 76 to 105 The tilt angle necessary for an 0.05 ml. water drop to roll off the surface has decreased from 30 to 22.

EXAMPLE 5 A relief image of the configuration desired is prepared according to the method described in U.S. Patent 2,760,863, whereby the surface of a photosensitive polymer (1 mm. thick coating of the polymer on a 2.5 mm. thick sheet of aluminum) is irradiated with actinic light which has first passed through a photographic transparency on which there is an ordered array of opaque squares separated by ne spacings that transmit the actinic light. The exposed regions of the photosensitive polymer undergo a chemical reaction which makes them more resistant than the unexposed regions to attack by caustic solution. After a thorough washing with caustic solution the resulting polymer surface consists of a network of interconnecting walls (intersecting at right angles) enclosing square-shaped depressions. The walls are 27 microns thick and 130 microns high. The distance between walls (centerto-center) is 195 microns. The network is approximately 1 cm. square in area.

An impression of this network is obtained by covering the surface with molten (80 to 100 C.) Woods metal (a 0.5 cm. thick layer of the liquid metal covers the surface), applying a vacuum (0.1 mm. of mercury) to remove air from between the Woods metal and the polymer surface, and then restoring atmospheric pressure in order to force the liquid metal into the depressions of the polymer surface. rIhe solidified metal impression is used to emboss the paraiin wax surface of Example 1. The same embossing procedure is used as in Example 1.

Alternatively the negative form of the desired surface configuration can be produced on the photosensitive polymer. However, a caustic Washing procedure is necessary to -completely flush out the narrow (27 microns) channels which result if the negative form is made. Therefore, the procedure utilizing the Woods metal impression is preferred in order to obtain the negative form for use in embossing other surfaces.

The embossed wax surface is a replica of the original photopolymer surface. However, some loss in surface detail has resulted from the above procedure of making an impression of an impression. The center-to-center dis tance between walls is 190` microns; the walls are 37 microns thick and 130 microns high. The area of the surface, measured at the plane formed by the tops of the walls, is 65% air. The advancing contact angle for a water drop has increased from 113 on the original flat surface to 137 on the embossed surface and the receding angle has increased from 100 to 116. No decrease in tilt angle was obtained here since this example represents the lower limit to the air content of a parafin wax surface.

EXAMPLE 6 An embossing die is produced in the manner described in Example 5 and a at parafn wax surface is embossed in the manner described in Example 1. The embossed wax surface consists of interconnecting walls 15 microns thick and 130 microns high. The distance between walls (centerto-center) is 195 microns and the area of the surface, measured at the plane formed by the tops of the walls, is about air. The advancing contact angle for water has increased from 113 on the original flat surface to 155 on the embossed surface and the receding contact angle has increased from 100 to 145. rl`he tilt angle necessary for a 0.015 ml. water drop to roll off the surface has decreased from 15 to 4.

In actual practice the embossing dies described in this example and in Example 5 can be many times larger and can be used in plate or roller form. The Woods metal die described here would be applicable only to surfaces of materials which have the desirable flow characteristics at temperatures below 65 C. For other materials, dies of higher melting metals or alloys must be used.

If suficient care is taken in washing technique, the negative photopolymer surfaces can be fabricated and used directly as embossing dies.

EXAMPLE 7 A fluorocarbon wax dispersion in 1,1,2-trichloro-1,2,2 triiiuoroethane, obtained by reacting methanol and tetraiiuoroethylene in the manner described in U.S. Patent 3,067,262 (the Wax having a crystalline melting point of 278 C., an approximate molecular weight of 2000) and containing about 20% solids, is diluted with a solution of 50 parts of trichloroiiuoromethane and 50 parts of dichlorodifluoromethane to give a solids concentration of about 1%. This mixture is sprayed onto a 1" x 3 glass microscope slide to give a coating which is 0.1 to 0.2 mm. thick. All components of the above mixture except the wax are volatile. The wax particles at the top of the coating are 5 to 80 microns in diameter, the spacing between them is 20 to 160 microns and the area of the surface in the plane formed by the top of the coating, is approximately 88 to 90% air. The advancing contact angle for a water drop on this surface is 159 and the receding angle is 156; the corresponding angles for a smooth, flat surface of the wax are 111 and 95 re spectively. The tilt angle necessary for a 0.05 ml. water `drop to roll off the surface is 18 for the smooth wax and about 1 for the sprayed wax.

EXAMPLE 8 A mixture containing 90 parts of a 12.5% solution of paraiiin wax in n-hexane and 10 parts of glass beads of a diameter in the range 3 to 12 microns, is heated to 40 to 50 C. and sprayed onto a 1" x 3 glass microscope slide to give a coating which is 0.1 to 0.2 mm. thick. The n-hexane rapidly evaporates from the surface leaving agglomerates of wax-coated glass beads. The agglomerates at the top of the coating are 15 to 250 microns in diameter, the spacing between them is 50 to 350 microns and the area of the surface in the plane formed by the top of the coating, is approximately 88% air. The advancing contact angle for a water drop on this surface is 158 and the receding angle is 156; the corresponding angles on a smooth, flat paran wax surface are 113 and 101 respectively. The tilt angle necessary for a 0.05 ml. water drop to roll off the surface is 15 for the smooth wax and about 1 for the sprayed wax-glass bead mixture.

In Examples 7 and 8 above, all parts are by weight.

EXAMPLE 9 A 400 line per inch nickel mesh plate, commercially available from the Buckbee Mears Co., St. Paul, Minn. (6" x 6) with a square array of holes (28 to 29 microns square with a center-to-center distance of 60 microns and a depth of 25 microns) is used as a die to emboss a lm (3 cm. x 6 om. x 15 cm.) of a copolymer of tetrafluoroethylene and hexaiiuoropropylene. The plate iS 13 pressed against the film in a Carver laboratory press at 12,000 to 14,000 p.s.i. and 100 C. for 15 minutes. The plate` is then peeled from the film; there is negligible adhesion between plate and film. The embossed surface consists. of a regular 'l arrayof square-shaped projections oriented at right angles to the original film surface..'l`hese projections are 28 to 29 microns on a side and 22 to 25 microns high. The centertocenter distance of adjacent projections is 60 microns and the area of the surface, measured at the plane formed by thetops of the' projections is 75 to 78% air. The advancing vangle for a water drop has increased from 114V on the original surface to 157H on the embossed surface and the receding angle has 4increased from 97 to 134. Thetilt angle necessary for a 0.05 ml. drop to roll off the surface has decreased from'23 on the smooth surface to 12 on the embossed surface. v EXAMPLE 10 The nickel mesh plate of Example 9 above is pressed against the surface of -a film of polypropylene (3 cm. x 6 cm. x 0.05 cm.) at 16,000 p.s.i.,and 140 C. for 20 minutes. The plate is peeled from"the film; there is negligible adhesion between plate and film. The embossed surface is asdescribed in Example 9 above. The advancing angle for a-water drop has increased from 107 on the original surface to 158 on the embossed surface and the receding angle has increased from, 83 to 110. The tilt angle necessary-forv a 0.05 mlfdrop to roll off the surface has decreased from 34 to 22.

EXAMPLE 1l A fiuorocarbon wax having a melting range of 95 to 135 C. and the general formula where n varies from 3 to 9 but is predominately 7 and 8, is dissolved in 1,1,2-trichloro-1,2,2-triuoroethane to the extent of about 5%. About 100 parts of the above solution and parts of glass beads having diameters in the range of 3 to 12 microns are mixed together, heated to about 40 C. and sprayed onto a 1 by 3 glass microscope slide to give a coating which is 0.1 to 0.2 mm. thick. The 1,1,2-trichloro-1,2,2-trifluoroethane rapidly evaporates from the surface leaving agglomerates of waxcoated glass beads. The agglomerates at the top of the coating are to 300 microns in diameter, the spacing between them is 80 to 300 microns and the area of the surface in the plane formed by the top of the coating, is approximately 82% air. The advancing contact angle for a water drop on this surface is 157 and the receding angle is 154; the corresponding angles on a smooth, flat surface of the wax are 128 and 114 respectively. The tilt angle necessary for a 0.05 ml. water drop to roll off the surface is 13 for the smooth wax and about 1 for the sprayed wax-glass `bead mixture. In the above example, all parts are by weight.

EXAMPLE 12 A continuous film of polyethylene (3 ft. wide, 150 microns thick) having a melting point of 110 C. and a melt index of 1.8 is passed beneath a suitable heater which heats the top half of the film to 102 to 104 C. The lm is then passed between two rollers. The surface of one roller is covered with the close-packed ends of hollow fibers of a copolymer of tetrauoroethylene and hexafluoropropylene. The hollow fibers have an inside diameter of 50 microns and an outside diameter of 123 microns. The surface of the embossing roller is heated to 95 to 100 C. just before it comes into Contact with the film as shown in FIGURE 8. This facilitates 4fiow of the polyethylene into the holes of the die. Before the film is separated from the embossing roller, a stream of cold air is directed -at the film in order to cool it to about C. Adhesion of the polyethylene to the copolymer is negligible and the film readily separates from the surface of the embossing roller.

The embossed surface consists of an array of cylindrical projections 50 microns in diameter and 200 microns high. The average distance between centers of adjacent projections is 123 microns and the area of the surface, measured at the plane-formed by the tops of the projections, is air. The film thickness, excluding the projections,v isnow microns. The advancing Contact angle for a water drop has increased from 98 on the original fiat surface to on the embossed surface and the receding -anglehas increased from 76 to 120. The tilt angle necessary for a 0.05 ml. water drop to roll olf the surface has decreased from 30 to 18".

Several of these 3-feet sheets, sealed together, can be used in shower curtains to make them more resistant to accumulation of undesirable film since much of this accumulation on ordinary curtains results from wetting of the surface and subsequent evaporation of the water thereon. l y

The embossed surfaces prepared as described in the preceding examples exhibit a lower tilt angle and thereby a vastly improved water repellency. The configuration of projections or depressions causes the advancing and receding contact angles to increase so that the drop width a'nd the difference in the cosines of the receding and advancing contact angles are decreased. The high water repellency is thus due to the intrinsic structure of the surface.

The novel water repellent surfaces of the invention are useful in the production of numerous articles which shed water easily, such as ranwear, shower curtains, tenting material and plastic and ceramic tiles. These surfaces may also constitute condenser surfaces of solar stills land serve as surfaces where ice formation is to be minimized.

While the composite surfaces of this invention have ben shown to have utility in repelling water, they also sh'ow improved repcllency to other liquids both water and oil based which exhibit intrinsic advancing contact angles of greater than 90 and intrinsic receding contact angles of at least 75. For example, liquids with surface tensions considerably lower than that lof water ('yf=72.8 dynes/cm. at 20 C.) will show increased repellency on the spray-coated surface of Example 11. That composite surface has shown improved repellency over the corre sponfding smooth, flat surface to motor oils with surface tensions las low as 32 dynes/cm. Examples of other liquids to which the surface of Example 11 will show improved repellency include:

'y (dynes/cm. at 20 C.)

Composite surfaces of this invention have shown improved repellency to liquids with viscosities in the range of from 5,000 to 10,000 centipoises and surface tensions greater than 40 Idynes/cm. and viscous, aqueous sugar solutions with viscosities of 3000 centipoises and surface tensions close to that of water. Aqueous solutions of inorganic salts will also be repelled more readily since their surface tensions range from 72 to 85 dynes/cm.

As many apparently wi'dely different embodiments of this invention may be made Without departing from the spirit and scope thereof, yit is to be understood that this invention is not limited to the specific embodiments thereof.

What is claimed is:

1. A solid composition having projections extending from the surface thereof, said projections having an average rdistance of not greater than 1000 microns between adjacent projections and having an average height of -at least .5 times the avenage distance, said projections spaced such that the air content of the surface of the composition is at least 60% air, said projections shaped such thiat a line drawn tangent to the sides thereof makes an angle between the tangent land the plane of said surface of =at least 80 when measured through the projection, said surface 'of said projection having an intrinsic advancing Water contact angle of greater than 90 and an intrinsic receding water contact :angle of aft least 75, said surface having a lower tilt angle for water than the corresponding smooth surface.

2. The composition vof claim 1 wherein the average distance between adjacent projections is not greater than 600 microns and the avenage height of the projections is equal to the average distance between adjacent projections, wherein said |air content is at least 75% air, and wherein the intrinsic advancing Water Contact angle is greater than 100 and the intrinsic receding water Contact angle is at least 85.

3. The composition of claim 1 wherein said composition is polyethylene @and wherein the ail content is at least 75 air.

4. The composition Iof claim 1 wherein said projection-s are 11o-contiguous, vertically oriented celums, and wherein the miaximum distance between columns is given by the equation {1339} COS wherein Cm is the distance in centimeters, 6EL is the intrinsic advancing water contact angle and A is the ai-r content expressed las 'a fraction.

5. The composition of claim l-wh'ere'in said projections are ordered interconnected vertical walls and wherein the maximum distance between |adjacent Walls is given by the equation 'Cm=0.587 cos 6a where Cm is the distance in centimeters and 0a is `the intrinsic advancing Water content langle.

6. The composition of claim 1 wherein `said projections are nonuniformlly arranged, and wherein the average distance between adjacent projections is not greater than 300 microns. .f

7. The composition of claim 1 in the form of a film having said projections Ion one side thereof. v

8. The composition of claim 1 in the form of a sheet having said projections on one `side therof.

References Cited Bird Y 161- 116 MORRIS sUssMAN, Primary Examiner.

Claims

1. A SOLID COMPOSITION HAVING PROJECTIONS EXTENDING FROM THE SURFACE THEREOF, SAID PROJECTIONS HAVING AN AVERAGE DISTANCE OF NOT GREATER THAN 100 MICRONS BETWEEN ADJACENT PROJECTIONS AND HAVING AN AVERAGE HEIGHT OF AT LEAST .5 TIMES THE AVERAGE DISTANCE, SAID PROJECTIONS SPACED SUCH THAT THE AIR CONTENT OF THE SURFACE OF THE COMPOSITION IS AT LEAST 60% AIR, SAID PROJECTIONS SHAPED SUCH THAT A LINE DRAWN TANGENT TO THE SIDES THEREOF MAKES AN ANGLE BETWEEN THE TANGENT AND THE PLANE OF SAID SURFACE OF AT LEAST 80* WHEN MEASURED THROUGH THE PROJECTION, SAID SURFACE OF SAID PROJECTION HAVING AN INTRINSIC ADVANCING WATER CONTACT ANGLE OF GREATER THAN 90* AND AN INTRINSIC RECEDING WATER CONTACT ANGLE OF AT LEAST 75*, SAID SURFACE HAVING A LOWER TILT ANGLE FOR WATER THAN THE CORRESPONDING SMOOTH SURFACE.