CA2381134A1 - Surfaces rendered self-cleaning by hydrophobic structures, and process for their production - Google Patents

Abstract

The present invention relates to objects having self-cleaning surfaces, and to processes for their production. The self-cleaning hydrophobic surfaces have particles with a size in the micrometer range to submicrometer range and a fissured structure in the nanometer range. These particles are secured by a carrier, such as a coating, to a surface of the object.

Description

>. , Surfaces rendered self-cleaaiag by hydrophobic structures, and process for their production FIELD OF THE INVENTION
The present invention relates to objects or articles having self-cleaning surfaces, and to processes for their production.
BACKGROUND
Objects with surfaces which are extremely difficult to wet havea number of commercially significant features.
The feature of most commercial significance here is the self-cleaning action of iow-wettability surfaces, since the cleaning of surfaces is time-consuming and expensive. Self-cleaning surfaces are therefore of very great commercial interest. The mechanisms of adhesion are generally the result of surface-energy-related parameters relating to the two surfaces which are in contact. These systems generally attempt to reduce their free surface energy. If the free surface energies between two components are intrinsically very low, it can generally be assumed that there will be weak adhesion between these two components. The important factor here is the relative reduction in free surface energy. In pairings where one surface energy is high and one surface energy is low the crucial factor is very often the opportunity for interactive effects, for example, when water is applied to a hydrophobic surface it is impossible to bring about any noticeable reduction in surface energy. This is evident in that the wetting i poor. The water applied forms droplets with a very high contact angle. Perfluorinated hydrocarbons, e.g. polytetrafluoroethylene, have very low surface energy.
There are hardly any components which adhere to surfaces of this type, and components deposited on surfaces of this type are in turn very easy to remove.

' . CA 02381134 2002-04-10 - la -The use of hydrophobic materials, such as perfluorinated polymers, for producing hydrophobic surfaces is known. A further development of these surfaces consists in structuring the surfaces in the ~Cm to nm range. U.S.
Patent No. 5,599,489 discloses a process in which a surface can be rendered particularly repellent by bombardment with particles of an appropriate size, followed by perfluorination. Another process is described by H. Saito et al. in "Service Coatings International" 4, 1997, pp. 168 et seq. Here, particles made from fluoropolymers are applied to metal surfaces; whereupon a marked reduction was observed in the wettability of the resultant surfaces with respect to water, with a considerable reduction in tendency toward icing.

~ . CA 02381134 2002-04-10 ,Y , ' O.Z. 5749 .2 _ US A 3 354 022 and WO 96104123 describe other processes for reducing the wettabiiity of objects by topological alterations in the surfaces. Here, artificial elevations or depressions with a height of from about 5 to 1000 ~m and with a separation of from about 5 to 500 ~,m are applied to materials which are hydrophobic or are hydrophobicized after the structuring process. Surfaces of this type lead to rapid droplet formation, and as the droplets roll off they absorb dirt particles and thus clean the surface.
This principle is borrowed from the natural world. Small contact surfaces reduce Van der Waals interaction, which is responsible for adhesion to flat surfaces with low surface energy. For example, the leaves of the lotus plant have elevations made from a wax, and these elevations lower the contact area with water. WO 00158410 describes these structures and claims the formation of the same by spray-application of hydrophobic alcohols, such as 10-nonokosanol, or of aikanediols, such as 5,10-nonokosanediol. A disadvantage here is that the self cleaning surfaces lack stability, since the structure is removed by detergents.
Another method of producing easy-clean surfaces has been described in DE 199 17 367 A1. However, coatings based on fluorine-containing condensates are not self-cleaning. Although there is a reduction in the area of contact between water and the surface, this is insufficient.
EP 1 040 874 A2 describes the embossing of microstructures and claims the use of structures of this type in analysis (microfluidics). A disadvantage of these structures is their unsatisfactory mechanical stability.
3 o JP 11171592 describes a water-repellent product and its production, the dirt-repellent surface being produced by applying a film to the surface to be treated, the film having fine particles made from metal oxide and having the hydrolysate of a metal: alkoxide or of a metal chelate. To harden this film the substrate to which the film has been applied has to be sintered at temperatures above 400°C. The process is therefore suitable only for substrates which are stable even at temperatures above 400°C.

Accordingly there is a need for surfaces which are particularly effective in self-cleaning and which have structures in the manometer range, and for simple processes for producing self-cleaning surfaces of this type.
SUMMARY OF THE INVENTION
The present invention provides an object or article with a self-cleaning surface which has an artificial, at least to some extent hydrophobic, surface structure of elevations and depressions, where the elevations and depressions are formed by particles secured by means of a carrier on the surface, wherein the particles have a fissured structure with elevations and/or depressions in the manometer range.
The present invention also provides a process for producing a self-cleaning surface of an object by producing a suitable, at least to some extent hydrophobic, surface structure. The process comprises securing particles by means of a carrier on a surface, wherein the particles have a fissured structure with elevations and/or depressions in the manometer range.
The process of the invention gives access to self-cleaning surfaces which have particles with a fissured structure. The use of particles which have a fissured structure gives simple access to surfaces with structuring extending into the manometer range. For this structure in the manometer range to be retained, it is necessary for the particles not to have been wetted by the carrier by which they have been secured to the surface, since otherwise the structure in the manometer range would be lost.
An advantage of the process of the invention is that surfaces sensitive to scratching are not damaged by particles present in the carrier when the particles are applied, since when surface coatings are used with subsequent application of the particles to the carrier, the surface sensitive to scratching has prior protection by the carrier.
Substances used for securing particles to a surface are hereinafter termed carriers.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows a scanning electron micrograph (SEM) of particles used to form structures of aluminum oxide C
(Degussa AG) .
Fig. 2 shows an SEM of a surface of particles of Sipernat* FK 350 silica (Degussa AG) on a carrier.
DESCRIPTION OF PREFERRED EMBODIMENTS
The self-cleaning surface of the invention, which has an artificial, and at least to some extent hydrophobic, surface structure is made from elevations and depressions, the elevations and depressions being formed by particles secured to the surface by means of a carrier. These particles have a fissured structure with elevations and/or depressions in the nanometer range. The elevations preferably have an average height of from 20 to 500 nm, particularly preferably from 50 to 200 nrn. The separation of the elevations and, respectively, depressions on the particles is=preferably less than 50O nm, very particularly preferably less than 200 nm.
"At least to some extent hydrophobic" may refer to the fact that the whole of the surface need not be covered *Trade-mark - 4a -by hydrophobic structure-forming particles or that the whole of the surface be hydrophobici.zed. Preferably, greater than 50~ of the surface area has hydrophobic properties.
"At least to some extent hydrophobic" also refers to a surface having an average free surface energy of less than 30 erge/cmz and preferably less than 25 ergs/cm2.
The fissured structures with elevations and/or depressions in the nanometer range may be formed for example by cavities, pores, grooves, peaks, and/or protrusions. The particles themselves have an average size of less than 50 ~Cm, preferably less than 30 Vim, and very particularly preferably less than 20 Vim, and preferably at least 0.2 ~,m, more-preferably at least 0.5 ~,m.
The particles preferably have a BET surface area of from 50 to 600 square meters per gram. The particles very particularly preferably have a BET surface area of from 50 to 200 m2/g.
The particles used and forming the structure may be of a wide variety of compounds from many branches of chemistry. The particles preferably have at least one material selected from silicates, doped silicates, minerals, metal oxides, silicas, polymers, and silica-coated metal powders. The particles very particularly preferably have fumed silicas or precipitated silicas, in particular Aerosils*, A1203, Si02, Ti02, ZrOz, zinc powder coated with Aerosil* 8974, preferably with a particle size of 1 ~,m, or pulverulent polymers, e.g. cryogenically milled or spray-dried polytetrafluoroethylene (PTFE) or perfluorinated copolymers or copolymers with tetrafluoroethylene.
*Trade-mark - 4b -Besides the fissured structures, the particles also preferably have hydrophobic properties in order to generate the self-cleaning surfaces. The particles themselves maybe hydrophobic; e.g. particles comprising PTFE, or the particles used may have been hydrophobicized. The hydrophobicization of the particles may take place in a manner known to the skilled worker.. Examples of typical hydrophobicized particles are commercially available particles, for example fine powders, such as Aerosii 88200 (Degussa AG).
The siiicas whose use is preferred preferably have a dibutyl phthalate adsorption to DIN 53 601 of from 100 to 350 mIl10Q g, the values preferably being from 250 to 350 m1/100 g.
A carrier is used to secure the particles to the surface. The self-cleaning 1 o surface can be generated by applying the particles in a densely packed layer to the surtace.
in one :preferred embodiment of the self-cleaning surtace of the invention, the carrier is a surface coating cured by means of thermal energy and/or the energy in light, or a two-component surface coating system, or some other reactive surface coating system, the curing preferably taking place by polymerization or crossfinkag. The cured surface coating particularly preferably comprises polymers and/or copolymers made from singly and/or multiply unsaturated acryiates and/or methacrytates. The mixing ratios may be varied within wide,boundaries. ltis also possible for the cured surtace coating: to comprise compounds . having :functional groups, e:g. hydroxyl groups, epoxy groups, amine groups, or fluorine-containing compounds, , e.g. pertluorina~ed acrylic esters. This is advantageous particularly if the compatibilities of 5urtace coating and hydrophobic particles are balanced with respect to one another; as is the case; for example, using N-(2 (aaryloyloxyjethylj-N-ethylperfluorooctane-1-sulfonamide with Aerosil 88200. The surface coatings which may be used are not only surtace coafings baseu on acrylic resin but also surface coatings based on polyurethane, and also surface coatings which comprise polyurethane 3 o acrylates or silicone acrylates.

- 5a -The self-cleaning surfaces of the invention preferably have a roll-off angle of less than 20°, particularly preferably less than 10°. The definition of the roll-off angle is that a water droplet rolls off when applied from a height of l cm to a flat surface resting on an inclined plane. The advancing angle and the receding angle are preferably above 140°, more preferably above 150°, and preferably have less than 1f° of hysteresis, more preferably less than 10°: The fact that the surfaces of the invention have an advance angle and receding angle of at least ~ CA 02381134 2002-04-10 140°; preferably more than 150°; means that it is possible to obtain particularly good self-cleaning surfaces.
Depending on the surface coating sysfem used, and on the size and material of the particles used, it is possible to obtain semitransparent self cleaning surfaces. The surfaces of the invention may particularly be contacttransparent, i:e. when a surface of the invention is produced on an object on which there is writing, this writing remains legible if its size is adequate.
to The self-cleaning surfaces of he invention are preferably produced by the process of the invention intended for producing these surfaces. The process of the invention for producing self-cleaning surtaces by producing a suitable, at 'least to some extent hydrophobic, surface structure by securing particles by means of a carrier on a surface; uses particles which have fissured structures with elevations andlor depressions in 'the nanometer range. .
Use is preferably made of particles which comprise at least one material selected from silicates, doped silicatesd minerals, metal oxides, silicas, metal powders and polymers. The particles particularly preferably comprise fumed silicates or silicas, in particular Aerosils, minerals, such as magadiite, A1203, SiOa, Ti02, ZrQ2; zinc powder coated with Aerosil 8974, or pulverulent polymers, e:g. cryogenically milled or spray-dried polytetrafluoroethylene (PTFE).
Particular preference is given to the use of particles with a BET surface area of from 50 to 600 m2/g. Very particular preference is given to the use 3 0 of particles which have a BET surface :area of from 50 to 200 m2lg.
The particles for generating the self-cleaning surfaces preferably have not only the fissured structures but also hydrophobic properties. The particles may themselves be hydrophobic, e.g: particles comprising PTFE, or the particles used may have been hydrophobicized. The hydrophobicization of the particles may take place in a manner known to the skilled worker.
Examples of typical hydrophobicized particles are commercially available particles, for example fiine powders, such as Aerosil 8974, or Aerosil 88200 (Degussa AG).

O.Z. 5749 _. ~ _ The process of the invention preferably has the following steps a) applying a curable substance as carrier to a surface, b) applying, to the carrier, particles which have fissured structures, and c) curing the carrier to secure the particles.
The curable substance may be applied for example using a spray, a doctor, a brush or a jet. The curable substance is preferably applied at a thickness of from 1 to 100 ~,m, preferably at a thickness of from 5 to 50 ~,m.
Depending on the viscosity of the curable substance, it may be z o advantageous to allow the substance to undergo some extent of curing or of drying prior to applying the particles: The viscosity of the curable substance 'is preferably selected so that the particles applied can sink into the curable substance at least to some extent, but so as to prevent flow of the curable substance and, respectively, of the particles applied thereto when the surface is placed vertically.
The particles may be applied by commonly used processes, such, as spray application or powder application. In particular, the particles may be applied by pray application using an electrostatic spray gun. Once the 2 o particles have been applied, excess particles, i.e. particles not adhering to the curable substance; may be removed from the surtace by shaking, or by being brushed oft or blown off. These particles may be collected and reused.
The curable substance used as carrier may be a surface coating which at least comprises mixtures made from singly and/or multiply unsaturated acrylates andlor methacrylates. The mixing ratios may be varied within wide limits. It is particularly preferable to use a surface coating curable by means of thermal or chemical energy, and/or the energy present in light.
If the particles used have hydrophobic properties, the curable substance selected is a surface coating, or a surface coating system, which has hydrophobic properties. On the other hand, if the particles used have hydrophilic properties, the curable substance selected will be a surface coating having hydrophilic properties.
It can be advantageous for the mixtures used as surface coating to comprise compounds having functional groups, e.g. hydroxyl groups, ' O.Z. 5749 epoxy groups, amine groups, or fluorine-containing compounds, e.g.
perfiiuorinated acrylic esters. This is advantageous particularly if the compatibilities of surface coatirvg and hydrophobic particles (in relation to hydrophobic properties) are balanced with respect to one another, as is the case, for example, using N-[2-(acryloyloxy)ethyl]-N-ethyl-perfluorooctane-1-sulfonamide with Aerosil VPR411. The curable substances which may be used are not only surface coatings based on acrylic resin but also surtace coatings based on polyurethane, and surface coatings which comprise polyurethane acrylates or silicone acrylates. The 1 o curable substances used may also be two-component surface-coating systems or other reactive surface coating systems.
The particles are secured to the carrier by: curing of the carrier, preferably, depending on the surface coating system used, by thermal andlor chemical energy, andlor the energy present in light. The curing of the carrier, brought about by chemical or thermal energy, and/or the energy present in, light, may take place for example by polymerization, or crosslinking of the constituents of the surface coatings or surface coating systems. The curing of the carrier, particularly preferably takes place by 2 o way of the energy present in light, and the polymerization of the carrier very particularly preferably takes place by way of the light from a medium-pressure Hg lamp; in the UV region. The curing of the carrier preferably takes place in an inert gas atmosphere, very particularly preferably in a nitrogen atmosphere.
Depending on the thickness of the curable substance applied and the diameter of the particles used, it may be necessary to limit the time which expires between applying the particles and curing the curable substance, in order to avoid complete immersion of the particles in the curable 3 0 substance. The curable substance is preferably cured within a period of from 0.1 to 10 min, preferably within a period of from 1 fo 5 min, after application of the particles.
In carrying out the process of he invention it can be advantageous to use particles which have hydrophobic properties andlor which have hydrophobic properties by way of treatment with at least one compound from the group consisting of the alkylsiianes, alkyldisilazanes, or perfluoroalkylsilanes. The hydrophobicization of particles is known, and ' " CA 02381134 2002-04-10 the Degussa AG series of publications Pigrnente; . number 18, may be consulted in this connection, for example.
It can also be advantageous for the particles to be given hydrophobic s properties after securing to the carrier. One way ire which this may take place is that the particles of the treated surface are given hydrophobic properties by way of treatment with at least one compound from the group consisting of the alkylsilanes, he perftuoroalkylsilanes, e:g. those which can be purchased from Sivento GmbH; and alkyldisilazanes. The 20 method of treatment is preferably that the surface which comprises particles and which is to be hydrophobicized is dipped: into a solution which comprises a hydrophobicizing reagent, e.g. alkylsilanes, excess hydrophobicizing reagent is allowed to drip off, and the surface is annealed at the highest possible temperature. The maximum temperature which may be used is limited by the softening point of carrier or substrate.
The process of the invention gives excellent results when used for producing elf-cleaning surfaces on planar or nonplanar objects, in particular on nonplanar objects. This is 2o possible to only a limited extent with the conventional processes. In particular, nonplanar objects, e:g. sculptures, are inaccessible or only accessible to a limited extent when using processes which apply prefabricated films to a surface or processes intended to produce a structure by embossing. However; the process of the invention rnayof 25 course, also be. used to produce self-cleaning: surfaces on objects with planar surfaces, e.g: greenhouses or public conveyances. The use of the process of the invention #or producing self-cleaning surfaces on greenhouses has ,particular advantages, since the process can also produce self-cleaning surfaces on transparent materials; for example, such 3 0 as glass or Plexiglas~; and the self-cleaning surface can be made transparent at least to the extent that the amount of sunlight which can penetrate the transparent surtace i;quipped with a self-cleaning surface is sufficient for the growth of the plants in the greenhouse. Greenhouses which have a surface of the invention 3 5 can be operated with intervals betwreen cleaning which are longer than for conventional greenhouses; which have to be cleaned regularly to remove leaves, dust, lime; and biological material, e.g. algae:

' ~ CA 02381134 2002-04-10 In addition, the process of the invention can be used for producing self-cleaning surfaces on non-rigid surfaces of objects, e.g. um'brellas or other surfaces required tobe flexible. The process of the invention may very particularly preferably be used for producing self-cleaning surfaces on flexible or non-flexible partitions in the sanitary sector, examples of partitions of this type are partitions dividing public toilets, partitions of shower cubicles, of swimming pools, or of saunas, and also shower curtains (flexible partition) .
The examples below are intended to provide further description of the surfaces of the invention and the process for producing the surfaces, without limiting the invention to these embodiments.
Example 1:
20~ by weight of methyl methacrylate, 20% by weight of pentaerythritol tetraacrylate, and 60~ by weight of hexanediol dimethacrylate were mixed together. Based on this mixture; i4% by weight of Plex* 4092 F, an acrylic copolymer frbm Rohm GmbH and 2% by rweight of Darokur* 1173 W curing agent were added, and the mixture stirred for at least 60 min: This mixture was applied as a carrier, at a thickness of 50 aria to a PMMA sheet of thickness 2 mm. The layer was dried to some extent, for 5 min. The particles of hydrophobicized Aerosil VPR 411 fumed silica (Degussa AG) were then applied by means of an electrostatic spray gun.
After 3 min, the carrier was cured at a wavelength of 308 nm under nitrogen. Once the carrier had been cured, excess Aerosil VPR 411 was brushed off. The surface was first characterized'visually and recorded as +++, meaning that there is almost completeformation of water droplets. The *Trade-mark ' ' CA 02381134 2002-04-10 - 10a -roll-off angle was 2.4°. The advance angle and receding angle were each measured and found to be above 150°. The associated hysteresis is below 10°.

' ' CA 02381134 2002-04-10 tO. Z . 5749 _ .11 Example 2:
The experiment bf Example 1- was repeated, particles made from aluminum oxide C (Degussa AG), an aluminum oxide with a BET surface area of 100 m2lg, being applied by electrostatic spraying. Once the carrier had been cured, as in Example 1, and excess particles had been brushed off, the cured, brushed-off _ sheet was dipped into a , formulation of tridecafluorooctyitriethoxysilane in ethanol (Dynasilan 8262; Sivento GmbH), for hydrophobicization. Once excess Dynasilan 8262 had dripped off, the sheet was annealed at a temperature of 80°C. The surface was 1 o classified as ++, i.e. water droplet development is nofi ideal, and the roll-off angle is below 20°.
Example 3::
Sipernat 350 silica from Degussa AG is sprinkled onto the sheet from Example 1; treated with the carrier. After 5 min of penetration time, the treated sheet is cured under nitrogen in UV light at 308 nrn. Once again, excess particles are brushed off, and the sheet is then in turn dipped in Dynasilan 8262, and then annealed at 80°C. The surface is classified as +++:
Example 4:
The experiment of Example 1 is repeated, but Aerosil R 8200 (Degussa AG), which has a BET surtace area of 200 ~ 25 m2lg, is used instead of Aerosil VPR 411. The assessment of the surface is +++, The roll-off angle is determined as 1.3°. Advance angle and reGed'tng angle were also measured and each was greater than 150°.: The associated hysteresis is below 10°.
Example 5:
3 o The surface coating from Example 1, after mixing with the UV curing agent;
was additionally provided with 10% by weight (based on the total weight of the surface coating mixture) of 2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate. Thi mixture, too, was again stirred for at least 60 min, and applied as carrier at a thickness of 50 ~,m to a PMMA sheet of thickness 2 mm. The layer was dried to some extent, for 5 min. The particles then applied by means of an electrostatic spray gun were hydrophobicized Aerosil VPR 411 fumed silica (Degussa AG): After 3 min; the carrier was cured at a wavelength of 308 mm under nitrogen. Once the carrier had r O.Z. 5749 cured, excess Aerosil VPR 411 was brushed off. The surface was first characterized visually and recorded as +++, meaning that there is almost complete formation of water droplets. The roll-off angle was 0.5°.
Advance angle and receding angle were each measured and were greater than 150°. The associated hysteresis is below 10°.
Comparative Example 1:
A suspension of 10% by weight of spray-dried fumed silica, Aeroperl 90130, Degussa AG, a silica with a BET surface area of 90 m2/g, in ethanol, was doctor-applied to the carrier of:Example 1, the carrier having been applied at a thickness of 200 ~.m and dried to some: extent. After curing in UV light and treatment with Dynasilan 8262 hydrophobicizing agent, the surface is assessed as only +, i.e. droplet formation is poor and the droplet adheres to the surface until the angle of inclination is high.
The poor cleaning efifect is attributable to filling-in of the fissured structures. This probably takes place by way of solution of monomers of the as yet uncured lacquer system in ethanol. Prior to curing, the ethanol evaporates and the monomers remain behind in the fissured structures, 2 o where they likewise cure during the curing procedure, the result being filling-in of the fissured structures. This markedly impairs the self-cleaning effect.

Claims (39)

1. An object having a self-cleaning surface which has an artificial, at least to some extent hydrophobic,surface structure of elevations and depressions, where the elevations and depressions are formed by particles secured by means of a carrier on the surface, wherein the particles have a fissured structure with elevations or depressions in the nanometer range.

2. The object having the self-cleaning surface as claimed in claim 1, wherein the carrier is a surface coating cured by thermal or chemical energy or by light.

3. The object having the self-cleaning surface as claimed in claim 2, wherein the cured surface coating comprises a member of the group consisting of (a)-polymers or copolymers of singly or multiply unsaturated acrylates or methacrylates, and (b) polyurethane.

4. The object having the self-cleaning surface as claimed in claim 3, wherein the polymers or copolymers of singly or multiply unsaturated acrylates or methacrylates have one or more functional group selected from hydroxyl, epoxy, amine or perfluorinated acrylic esters.

5. The object having the self-cleaning surface as claimed in claim 2, wherein the surface coating is of N-[2-(acryloyloxy)ethyl]-N-ethylperfluorooctane-1-sulfonamide.

6. The object having the self-cleaning surface as claimed in claim 1 or 2, wherein the carrier comprises a mixture of methyl methacrylate, pentaerythritol tetraacrylate, hexanediol dimethacrylate, and a curing agent.

7. The object having the self-cleaning surface as claimed in claim 6, wherein the mixture further comprises 2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate.

8. The object having the self-cleaning surface as claimed in any one of claims 1 to 7, wherein the particles have an average size of less than 50 µm.

9. The object having the self-cleaning surface as claimed in claim 8, wherein the particles have an average size of less than 30 µm.

10. The object having the self-cleaning surface as claimed in any one of claims 1 to 9; wherein the particles are of at least one material selected from silicates; doped silicates, minerals, metal oxides, silicas, polymers and metal powders.

11. The object having the self-cleaning surface as claimed in any one of claims 1 to 10, wherein the particles are hydrophobic.

12. The object having the self-cleaning surface as claimed in any one of claims 1 to 11, wherein the elevations have an average height of from 20 to 500 nm.

13. The object having the self-cleaning surface as claimed in any one of claims 1 to 12, wherein the elevations and depressions on the particles are separated less than 500 nm.

14. The object having the self-cleaning surface as claimed in anyone of claims 1 to 13, wherein the particles have a BET surface area of from 50 to 600 m2/g.

15. The object having the self-cleaning surface of any one of claims 1 to 14, wherein the particles and the carrier have mutually balanced hydrophobic properties.

16. The object having the self-cleaning surface of any one of claims 1 to 15, wherein the self-cleaning surface has a roll-off angle of less than 20°.

17. The object having the self-cleaning surface of any one of claims 1 to 16, wherein the self-cleaning surface has an advancing angle and receding angle of a droplet of water applied to the surface of greater than 140°.

18. The object having the self-cleaning surface of any one of claims 1 to 17, wherein the self-cleaning surface has a hysteresis of a droplet of water applied to the surface of less than 15°

19. A process for producing an object having a self-cleaning surface having an artificial, at least to some extent hydrophobic, surface structure of elevations and depressions, which comprises:
securing particles having a fissured structure with elevations or depressions in the nanometer range to the object by means of a carrier.

20. The process as claimed in claim 19, wherein the particles are of at least one material selected from silicates, doped silicates, minerals, metal oxides, silicas, metal powders, and polymers.

21. The process as claimed in claim 19 or 20, which comprises:

a) applying a curable substance to a surface of the object;
b) applying, to the curable substance, the particles which have a fissured structure with elevations or depressions in the nanometer range; and c) curing the curable substance to secure the particles to the surface.

22. The process as claimed in claim 21, wherein the curing of the curable substance takes place by thermal or chemical energy or light.

23. The process as claimed in claim 22, wherein the light is in the UV region.

24. The process as claimed in any one of claims 21 to 23, wherein the curing takes place in an inert gas atmosphere.

25. The process as claimed in any one of claims 21 to 24, wherein the curable substance used is a surface coating comprising a selection from the group consisting of:
(a) mixtures of members of the group consisting of (i) singly unsaturated acrylates, (ii) multiply unsaturated acrylates, and (iii) methacrylates; (b) polyurethanes;
(c) silicone acrylates; and (d) urethane acrylates.

26. The process as claimed in claim 25, wherein the singly unsaturated acrylates, multiply unsaturated acrylates or methacrylates are functionalized by at least one member selected from the group consisting of hydroxyl, epoxy, amine and perfluorinated acrylic esters.

27. The process as claimed in any one of claims 19 to 26, wherein the carrier is N-[2-(aryloyloxy)ethyl]-N-ethyl-perfluorooctane-1-sulfonamide.

28. The process as claimed in any one of claims 19 to 26, wherein the carrier comprises a mixture of methyl methacrylate, pentaerythritol tetraacrylate, hexanediol dimethacrylate, and a curing agent.

29. The process as claimed in claim 28, wherein the mixture further comprises 2-(N-ethylperfluorooctane-sulfonamido)ethyl acrylate.

30. The process as claimed in any one of claims 19 to 29, wherein the coating and the particles have mutually balanced hydrophobic properties.

31. The process as claimed in any one of claims 19 to 30, wherein the particles used are hydrophobic.

32. The process as claimed in any one of claims 19 to 31, wherein the particles have been made hydrophobic as a result of treatment with at least one compound selected from the group consisting of the alkylsilanes, perfluoroalkylsilanes, and alkyldisilazanes.

33. The process as claimed in any one of claims 19 to 31, wherein the particles are made hydrophobic after being secured to the carrier.

34. The process as claimed in claim 33, wherein the particles are made hydrophobic by treatment with at least one compound selected from the group consisting of the alkylsilanes, perfluoroalkylsilanes, and alkyldisilazanes.

35. The process as claimed in anyone of claims 21 to 24, wherein the curable substance is applied at a thickness of from 1 to 100 µm.

36. The process as claimed in any one of claims 21 to 24, wherein the curing step is performed within 0.1 to 10 m of applying the particles to the carrier.

37. The process as claimed in any one of claims 19 to 36, wherein the object is nonplanar.

38. The process as claimed in any one of claims l9 to 36, wherein the self-cleaning surface is formed on a nonrigid surface of the object.

39. The process as claimed in any one of claims 19 to 36, wherein the self-cleaning surface is on a flexible or inflexible partition in a sanitary product.