CA2381346A1

CA2381346A1 - Surfaces rendered self-cleaning by hydrophobic structures, and process for their production

Info

Publication number: CA2381346A1
Application number: CA002381346A
Authority: CA
Inventors: Edwin Nun; Markus Oles; Bernhard Schleich
Original assignee: Creavis Gesellschaft fuer Technologie und Innovation mbH
Current assignee: Evonik Operations GmbH
Priority date: 2001-04-12
Filing date: 2002-04-10
Publication date: 2002-10-12
Also published as: DE10118351A1; PT1249467E; US20020150725A1; DE50210866D1; DK1249467T3; EP1249467A1; EP1249467B1; JP2002347159A; ATE373039T1; US6852389B2; ES2292653T3

Abstract

The present invention relates to objects having self-cleaning surfaces and processes for their production.
The self-cleaning surfaces produced have structure-forming particles and fixative particles, which together form the surface structure made from elevations and depressions.

Description

Surfaces rendered self-cleaning by hydrophobic structures, and process for their production FIELD OF THE INVENTION
The present invention relates to objects with self-cleaning surfaces and to processes for their production.
BACKGROUND
Objects with surfaces which are extremely difficult to wet have a number of commercially significant features.
The feature of most commercial significance here is the self-cleaning action of low-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 acting between 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 is 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 depasited on surfaces of this type are in turn very easy to remove.

- 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 ~m to nm range. U.S.
Patent 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.

L~
u.s. Patent 3,354,022 and WO 96/04123 describe other processes for reducing the wettability of objects by topological alterations in the surfaces. Here, artificial elevations or depressions with a height of from 'S about 5 to 1000 pm 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.
to 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 15 contact area with water. W0 00158410 describes these structures and claims the formation of the same by spray-application of hydrophobic alcohols, such as 10-nonokosanol, or of alkanediols, such as 5,10-nonokosanediol. A disadvantage here is that the self-cleaning surfaces lack stability, since the structure is removed by detergents.

2 ~~
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.
~-A-11-171592 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.

It is an object of the present invention to provide articles (or objects) having surfaces which are particularly effective in self-cleaning and which have structures in the nanometer range, and also to provide simple processes for producing self-cleaning surfaces of this type.
SUMMARY OF THE INVENTION
The present invention provides an object having a self-cleaning surface having an artificial, at least to some extent hydrophobic, surface structure made from elevations and depressions, wherein the elevations and depressions are formed by structure-forming particles secured to the surface, and also by fixative particles.
The present invention also provides a process for producing a self-cleaning surface on an object, where the surface is an artificial, at least to some extent hydrophobic, surface structure made from elevations and depressions, where the elevations and depressions are formed by structure-forming particles secured to the surface. The process comprises securing the structure-forming particles to the surface by using fixative particles which likewise contribute to forming the elevations and depressions.
"At least to some extent hydrophobic" may refer to the fact that the whole of the surface need not be covered by hydrophobic structure-forming particles or that the whole of the surface be hydrophobicized. 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 ergs/cm2 and preferably less than 25 ergs/cm2.

The process of the invention gives access to self-cleaning surfaces which have Structure-forming particles and fixative particles, which together form the desired surface structure. The use of particles which have a fissured ~~ structure gives access, in a simple manner, to surfaces which have structuring extending into the manometer range.
In order to obtain this structure in the manometer range, it is necessary that there is no substantial wetting of the particles by the fixative particles with which they have been secured to the surface, since otherwise the nanostructure would be lost.
The self-cleaning surfaces of the invention, and also processes for their production, are described by way of example below, but there is no intention to limit the surfaces of the invention or the process of the invention to the embodiments given by way of example.
DESCRIPTION OF PREFERRED EMBODIMENTS
The self-cleaning surface of the invention which has an artificial, at least to some extent hydrophobic, surface structure made from elevations and depressions, is distinguished by the fact that the elevations and depressions are formed by structure-forming particles secured to the surface, and also by the fixative particles used for the securing process. The structure forming particles preferably have a fissured structure with elevations and/or depressions in the manometer range. The elevations preferably have an average height of from 20 to 500 mm, particularly preferably from 50 to 200 mm. The separation of the elevations and, respectively, depressions on the particles is preferably less than 500 mm, very particularly preferably less than 200 mm.
The separations of the structure-forming particles on the self-cleaning surfaces are from 0 to 10 particle diameters, in particular from 0 to 3 particle diameters and very particularly preferably from 1 to 2 particle diameters.
The fissured structures with elevations and/or depressions in the nanometer range may be formed by cavities, pores, grooves, peaks, and/or protrusions. The particles themselves have an average size, generally of less than 50 ~,m, preferably less than 30 Vim, and very particularly preferably less than 20 ~.m, but at least 0.1 ~.m. The dibutyl phthalate adsorption, based on DIN 53 601, gives values, preferably of from 100 to 350 m1/100 g, more preferably from 250 to 350 m1/100 g.
The structure--forming 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 mz/g.
The structure--forming particles used may be of a wide variety of compounds from many branches of chemistry or from the natural world. The particles preferably have at least one material selected from silicates, doped silicates, minerals (e. g. magadiite), metal oxides, silicas, polymers, and coated metal powders. The particles very particularly preferably have fumed silicas or precipitated silicas, in particular Aerosils*, A1z03, Si02, Ti02, Zr02, zinc powder coated with Aerosil* 8974, or pulverulent polymers, e.g.
cryogenically milled or spray-dried polytetrafluoroethylene ( PTFE ) .
The particles secured to the self-cleaning surfaces preferably have not only fissured structures but also hydrophobic properties. The particles here may themselves be hydrophobic, e.g. particles comprising PTFE. However, the *Trade-mark particles secured may also have been hydrophobicized subsequently in a manner known to the skilled worker.
The fixative particles present according to the invention encompass compounds selected from the group consisting of the hot-melt adhesives and/or powder coatings.
these hot-melt adhesives and/or powder coatings are particularly preferably selected from the ethylene-ethyl acrylate copolymers, ethylene-vinyl acetate copolymers, polyamides, epoxy resins, polyether sulfones, polyisobutenes, and polyvinyl butyrals. The hot-melt adhesives used as fixative is particularly preferably a copolymer made from thermoplastic polyamide with caprolactone.
The fixative particles preferably have an average size of less than 50 Vim. The fixative particles preferably 1~~ have an average size which corresponds to the size of the structure-forming particles. However, it can also be advantageous for the average size of the fixative particles to be smaller than that of the structure-forming particles, by from 10 to 70~, preferably from 25 to 50~.
The self-cleaning surfaces of the invention have a roll-off angle, generally of less than 20°, particularly preferably less than 10°, the definition of the roll-off angle being that a water droplet rolls off when applied from a height of 1 cm to a flat surface resting on an inclined plane. The advancing angle and the receding angle are preferably above 140°, particularly preferably above 150°, and have less than 10° of hysteresis.
Depending on the fixative particles used, and on the size and material of the structure-forming particles used, it is possible to obtain semitransparent self-cleaning surfaces. The surfaces of the invention may particularly be 6a -contact-transparent, 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. The finer the particles used, the better the transparency of the self-5. cleaning surfaces.
The self-cleaning surfaces of the invention are preferably produced by i:he process of the invention, intended for producing these surfaces. This process of the invention for producing self-cleaning surfaces which have an artificial, at least to some extent hydrophobic, surface structure made from elevations and depressions, where the elevations and depressions are formed by structure-forming particles secured to the surface, is distinguished by the fact that the structure-forming particles are secured to the 1~~ surface by using fixative particles which likewise contribute to formation of the elevations and depressions.
The structure-forming particles used are preferably those which comprise at least one material selected from silicates, doped silicates, minerals, metal oxides, silicas, and polymers. The particles very particularly preferably comprise fumed silicates or silicas, in particular Aerosils, A1203, Si02, Ti02, Zr02, Zn powder coated with Aerosil 8974, or pulverulent polymers, e.g. cryogenically milled or spray-dried polytetrafluoroethylene (PTFE).
It is preferable to use particles which have fissured structures with elevations and/or depressions in the nanometer range. This method gives access to self-cleaning surfaces which have particularly good self-cleaning performance. It is particularly preferably to use particles with a BET surface area of from 50 to 600 m2/g. It is very - 6b -particularly preferable to use particles which have a BET
surface area of from 50 to 200 m2/g.
It is also preferable to use structure-forming particles having an average size of less than 50 Vim, preferably less than 30 ~,m, and very particularly preferably less than 20 ~,m.
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 fine powders, such as Aerosil VPR411, Aerosil 8974, or Aerosil 88200 (Degussa AG).
The fixatives used as fixative particles are preferably compounds selected from the group consisting of the hot-melt adhesives and/or powder coatings. These hot-melt adhesives and/or powder coatings preferably O.Z. 5750 comprise at least one compound selected from the ethylene-ethyl acrylate copolymers, ethylene-vinyl acetate copolymers, polyamides, polyether sulfones, polyisobutenes, epoxy resins, and polyvinyl butyrals.
The steps encompassed by the process of the invention are preferably a) applying fixative particles and structure-forming particles to a surface, and b) incipient melting of the fixative particles to secure the structure-forming particles and the fixative particles to the surface.
1o The fixative particles and structure-forming particles may be applied one after the other, for example. The fixative particles are usually applied to the surface first, followed by the structure-forming particles. Incipient melting of the fixative particles on the surface prior to application of the structure-forming particles can be advantageous, and incipient melting (or incipient sintering) here is the agglutination of fixative particles at their points of contact.
In one particularly preferred embodiment of the process of the invention, a 2 0 mixture of fixative particles and structure-forming particles is prepared and then applied to the surface. The structure-forming particles used for preparing the mixture made from structure-forming particles and fixative particles are preferably particles whose hydrophobic properties are similar to the properties of the fixative particles (mutually balanced hydrophobic 2 5 properties).
The particles may be applied to the surface in a manner known to the skilled worker, e.g. by spray-application or powder-application. Depending on the use of the object provided with a self-cleaning surface, the surface 3 o may have been given a corrosion-protection coating, or a color coating or a coating for warning purposes.
The incipient melting of the invention is brought about by brief heating, and incipient melting (or incipient sintering) here is softening of the fixative 35 particles in such a way that cooling produces at least some mutual adhesion of the surface of the fixative particles to adjacent surtaces of fixative particles, and/or to structure-forming particles, and also to the surface of the object which is to be provided with a self-cleaning surface.

O.Z. 5750 _ g _ The adhesion may have been produced by chemical bonding, or else by physical forces.
The selection of the temperature at which the incipient melting is carried out, and also of the duration of the incipient melting, should be such that there is only partial melting of the fixative particles, and that the structure of the structure-forming particles, in particular the structure in the nanometer range, is retained.
1o The heating may take place in a manner known to the skilled worker, e.g.
by means of an oven or of some other heat source. The heating preferably takes place by means of infrared radiation. However, it can also be advantageous for a mixture made from at least fixative particles and structure-forming particles, or for the fixative particles alone, to be applied to a heated surface, which is cooled after the application process. This may be particularly advantageous when the surface of the object itself is such that the mixture applied does not remain on the surface in a stable manner. Examples of reasons for this include the geometry of the object and insufficient adhesion of the pulverulent coating agent (fixative particles 2 0 or structure-forming particles) to the substrate.
The mixture used in the preferred embodiment and encompassing at least fixative particles and structure- forming particles, preferably has from 10 to 90% by weight of structure--forming particles and from 90 to 10% by weight 2 5 of fixative particles. The mixture used particularly preferably comprises from 25 to 75% by weight of structure-forming particles and from 25 to 75% by weight of fixative particles. The mixtures may be prepared by simple mixing of the solids. However, the mixing process may also use mixing assemblies familiar to the skilled worker. It can be advantageous 3 o for the mixing to take place with heating, the current consumed by the mixer being monitored. When agglomeration begins, this being easy to detect from a rise in the current consumed, the mixture is recooled. The slight heating has by this time firmly bonded the structure-forming particles at least to some extent to the fixative particles, but without surrounding the 35 structure-forming particles by the molten fixative, since this would result in loss of the structure of the structure-forming particles in the nanometer range.

The advantage of heating the particles during mixing is that agglomeration produces larger particles in the mixture mentioned, and these are easier to process, since when the mixture is applied to the surface by spraying-application or powder-application it is possible very substantially to eliminate dusting, and there is no longer any opportunity for separation resulting from mechanical effects, e.g. density differences between fixative particles and structure-forming particles.
The fixative particles used according to the invention preferably have an average size of less than 50 ~,m. The fixative particles preferably have an average size which corresponds to the size of the structure forming particles. However, it can also be advantageous for the average size of the fixative particles to be smaller than that of the structure-forming particles, by from 10 to 70°~, preferably from 25 to 50%.
If hydrophilic structure-forming particles are used with hydrophilic fixative particles for producing the self-cleaning surface structure, this is treated with at least one compound from the group consisting of the alkylsilanes, alkyldisilazanes, waxes, paraffins, fatty esters, fluorinated andlor 2o functionalized alkanes, and perfluoroalkylsilanes, in order to give the self-cleaning surface hydrophobic properties. The manner of the treatment is preferably that the surface comprising the particles and intended to be hydrophobicized is dipped into a solution which comprises a hydrophobicizing reagent, e.g. alkylsilanes, excess hydrophobicizing 2 5 reagent is allowed to drip off, and the surface is annealed at the highest possible temperature. However, the treatment may also be carried out by spraying the surface with a medium comprising a hydrophobicizing reagent, followed by annealing. A treatment of this type is preferred for treating steel beams or other heavy or bulky objects, for example. The 30 limitation on the temperature arises from the softening points of the fixatives, of the structure-forming particles, and of the substrate to which the self-cleaning surface has been applied.
The process of the invention 35 gives excellent results in the production of self cleaning surfaces on planar or non-planar objects, in particular on non-planar objects. This is possible only to a limited extent with the conventional process. In particular, processes in which prefabricated films are applied to a surface and processes in which the intention is to produce a structure by embossing are not capable, or have only very limited capability, for use on non-planar objects, e.g. sculptures. However, the process of the invention may, of 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 for producing self-cleaning surfaces on greenhouses has particular advantages, since the process can also produce self-cleaning surfaces on transparent materials, for example, such as glass or Plexiglas, and the self-cleaning surface can be made 1 o transparent at least to the extent that the amount of sunlight which can penetrate the transparent surtace equipped with a self-cleaning surface is sufficient for the growth of the plants in the greenhouse. Greenhouses which have a surface of the invention can be operated with intervals between cleaning which are longer than for 1 ~~ conventional greenhouses, which have to be cleaned regularly to remove leaves, dust, lime, and biological material, e.g. algae.
The process of the invention may also be used advantageously for providing load-bearing or non-load-bearing elements of buildings, above 2 o ground level, with self-cleaning surfaces, very particularly if the surface has corrosion protection, a signal marking, as is the case on warning panels with yellow and black stripes, or has a color coating. This can prevent long lasting soiling of these elements, and thus increase the intervals between cleaning and make colored signage lastingly visible 25 without impairment by soiling.
In addition, the process of the invention can be used for producing self-cleaning surfaces on non-rigid surfaces of objects, e.g. umbrellas or other surfaces required to be flexible. The process of the invention 3o ~ - may very particularly preferably be used for producing self-cleaning surfaces on flexible or inflexible 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).
35 , The examples below are intended to provide further description of the surtaces of the invention and the process for producing the surfaces, without limiting the invention to these embodiments.

Example 1:
A mixture composed of 50% by weight of Aeroperl 90130 from Degussa AG, a spray-dried fumed silica with a BET surface area of 90 mZ/g, and 50% by weight of polyamide hot-melt adhesive powder (Vestamelt~P06, Degussa AG) with an average particle size below 50 ~m is spray-applied electrostatically to a polymethyl methacrylate (PMMA) sheet of thickness 2 mm. To secure the particles to the sheet and produce a fissured structure, the sheet is annealed for 5 min at 108°C. The sheet is then treated with it) Antispread~ (Dr. Tillwich GmbH), a surface-hydrophobicizing agent, making the particles and, respectively, the surface hydrophobic. The surface was first characterized visually and recorded as +++, meaning that there is virtually complete formation of water droplets. The advance angle and receding angle were each measured as greater than 150°. The 1 ~~ associated hysteresis is below 10°.
Example 2:
A powder caoating (FREOPOX~EKP-7, Emil Frei GmbH & Co.) was doctor applied cold to give a layer of 200 ~.m thickness on a nickel plate, and 2ci sprinkled with a hydrophobic Aerosil (R 8200, Degussa AG). This mixture and the nickel plate were exposed for 3 minutes to a temperature of 180°C. After cooling, there was only a slight improvement in the run-off behavior of water.
25 Example 3:
The experiment of Example 2 was repeated, but a metal roller was used to press the Aerosil R 8200 into the molten surface coating. The material was post-annealed, again for 3 minutes. The run-off behavior of the cooled sheet was only slightly improved over that of the pure powder coating.

Example 4:
The experiment of Example 2 was repeated, but a hot-melt adhesive (Vestamelt P 06, Degussa AG) was used instead of the powder coating.
After cooling there was only a slight improvement in the run-off behavior of 35 water.
*Trade-mark ,, O.Z. 5750 Example 5:
The experiment of Example 2 was repeated. A hydrophilic silica (Sipernat 350, Degussa AG) was used instead of the hydrophobic Aerosil R 8200.
Subsequent hydrophobicization by means of a hydrophobicizing reagent (Antispread~, Dr. Tillwich GmbH) was carried out after the sheet had cooled. This example showed full development of the lotus effect.
It can be seen from Examples 2 to 5 that bonding of the structure-former to the matrix is impeded if the fixative particles (powder coating or hot-melt 1o adhesive) and the structure-forming particles (Aerosil 88200) differ in their hydrophobic properties. The bonding is not impeded when a more hydrophilic precipitation silica (Sipernat 350) is used, and the structure-former sinters firmly to the powder coating particles or adhesive particles.
Comparative 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, 14% by weight of Plex 4092 F, an acrylic copolymer from Rohm GmbH and 2% by weight of Darokur 1173 UV curing 2 o agent were added, and the mixture stirred for at least 60 min. This mixture was applied as carrier at a thickness of 200 ~,m to a PMMA sheet of thickness 2 mm. The layer was dried to some extent, for 5 min. A
suspension of 10% by weight of spray-dried fumed silica, Aeroperl 90/30 Degussa AG, a silica with a BET surface area of 90 m2/g, in ethanol, was doctor-applied to this layer, which had been dried to some extent. The assessment of the surface after curing in UV light and treatment with the hydrophobicizing agent Dynasilan 8262 is only +, i.e. there is poor droplet formation and the droplet adheres to the surface until high angles of inclination have been reached.
The poor Lobes effect in the comparative example is attributable to filling-in of the fissured structures. This takes place by way of solution of monomers in ethanol. Prior to curing, the ethanol evaporates and the curing agent preferentially remains behind in the fissured structures.

Claims

1. An object having a self-cleaning surface having an artificial surface structure which is at least to some extent hydrophobic and comprises elevations and depressions, wherein the elevations and depressions are formed by structure-forming particles secured to the surface and fixative particles.

2. The object having the self-cleaning surface as claimed in claim 1, wherein the structure-forming particles have a fissured structure with elevations or depressions in the nanometer range.

3. The object having the self-cleaning surface as claimed in claim 1 or 2, wherein the elevations have an average height from about 20 to about 500 nm.

4. The object having the self-cleaning surface as claimed in any one of claims 1 to 3, wherein the elevations are separated from each other by less than 500 nm.

5. The object having the self-cleaning surface as claimed in any one of claims 1 to 4, wherein the structure-forming particles on the surface are separated by from 0 to particle diameters.

6. The object having the self-cleaning surface as claimed in any one of claims 1 to 5, wherein the fixative particles are made of a material selected from the group consisting of hot-melt adhesives and powder coatings.

7. The object having the self-cleaning surface as claimed in claim 6, wherein the hot-melt adhesives and powder coatings are selected from the group consisting of ethylene-ethyl acrylate copolymers, epoxy resins, ethylene-vinyl acetate copolymers, polyamides, polyether sulfones, polyisobutenes and polyvinyl butyrals.

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

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

10. The object having the self-cleaning surface as claimed in any one of claims 1 to 9, wherein the structure forming particles have an average size of less than 30 µm.

11. The object having the self-cleaning surface as claimed in any one of claims 1 to 10, wherein the fixative particles have an average size that is 10 to 70% smaller than that of the structure-forming particles.

12. The object having the self-cleaning surface as claimed in at least one of claims 1 to 10, wherein the structure-forming particles axe composed of at least one material selected from the group consisting of silicates, doped silicates, minerals, metal oxides, silicas, polymers, and metal powders.

13. The object having the self-cleaning surface as claimed in claim 11, wherein the structure-forming particles are hydrophobic.

14. The object having the self-cleaning surface as claimed in any one of claims 1 to 12, wherein the structure-forming particles have a BET surface area from about 50 to about 600 m2/g.

15. The object having the self-cleaning surface as claimed in any one of claims 1 to 14, wherein the self-cleaning surface has a roll-off angle of less than 20°.

16. The object having the self-cleaning surface as claimed in any one of claims 1 to 15, wherein the self-cleaning surface has an advancing and receding angle of a water droplet of above 140°.

17. The object having the self-cleaning surface as claimed in any one of claims 1 to 16, wherein the self-cleaning surface has a contact angle hysteresis of a water droplet of less than 10°.

18. A process for producing an object having a self-cleaning surface having an artificial, at least to some extent hydrophobic, surface structure made from elevations and depressions, where the elevations and depressions are formed by structure-forming particles secured to the surface and fixative particles, which process comprises:

securing the structure-forming particles to a surface of the object by using fixative particles.

19. A process for producing an object having a self-cleaning surface having an artificial, at least to some extent hydrophobic, surface structure made from elevations and depressions, where the elevations and depressions are formed by structure-forming particles secured to the surface and fixative particles used for the securing process and where the structure-forming particles have a fissured structure with elevations and depressions in the nanometer range which comprises:

securing the structure-forming particles to the object by using the fixative particles.

20. The process as claimed in claim 18 or 19, comprising the steps of a) applying fixative particles and structure-forming particles to an object, and b) incipient melting of the fixative particles to secure the structure-forming particles and the fixative particles to the object.

21. The process as claimed in any one of claims 18 to 20, wherein the application takes place by spray-application or powder-application.

22. The process as claimed in any one of claims 19 to 21, wherein the incipient melting is brought about by brief heating whereby the structure in the nanometer range is retained.

23. The process as claimed in claim 22, wherein the heating takes place by means of infrared radiation.

24. The process as claimed in any one of claims 18 to 23, wherein the structure-forming particles used comprise at least one material selected from the group consisting of silicates, doped silicates, minerals, metal oxides, silicas, metals, and polymers.

25. The process as claimed in claim 24, wherein the structure-forming particles used have an average size of less than 50 µm.

26. The process as claimed in any one of claims 18 to 24, wherein the fixative particles used comprise compounds selected from the group consisting of hot-melt adhesives and powder coatings.

27. The process as claimed in claim 26, wherein the hot-melt adhesives and powder coatings are selected from ethylene-ethyl acrylate copolymers, ethylene-vinyl acetate copolymers, epoxy resins, polyamides, polyether sulfones, polyisobutenes, and polyvinyl butyrals.

28. The process as claimed in any one of claims 18 to 27, wherein use is made of fixative particles having an average size of less than 50 µm.

29. The process as claimed in any one of claims 20 to 28, wherein in step a) a mixture comprising the structure-forming particles and the fixative particles is applied to the surface.

30. The process as claimed in any one of claims 20 to 29, wherein the use is made of a mixture of structure-forming particles and fixative particles, having from 25 to 75% by weight of structure-forming particles and from 25 to 75% by weight of fixative particles.

31. The process as claimed in any one of claims 18 to 30, wherein the structure-forming particles and fixative particles used have mutually balanced hydrophobic properties.

32. The process as claimed in any one of claims 20 to 31, wherein the surface structure is given hydrophobic properties after the structure-forming particles have been secured.

33. The process as claimed in claim 32, wherein the surface structure is given hydrophobic properties by treatment with at least one compound selected from the group consisting of alkylsilanes, perfluoroalkylsilanes, alkyldisilazanes, waxes, paraffins, fatty esters, fluorinated and functionalized alkanes.

34. A process for producing an object having a self-cleaning surface which has an artificial surface structure that is at least to some extent hydrophobic and comprises elevations and depressions formed by structure-forming particles and fixative particles both secured to the surface, which process comprises:

(A) applying, to a non-self-cleaning surface of the object, the fixative particles, wherein the fixative particles are made of a hot melt adhesive or a powder coating and have an average size of less than 50 µm;

(B) applying, to the non-self-cleaning surface of the object, the structure-forming particles, wherein the structure-forming particles have an average size of less than 50 µm and are inherently hydrophobic or have been made hydrophobic;

(C) heating the object, thereby incipient melting only the fixative particles to such an extent (i) that the fixative particles are softened and agglutination of the fixative particles is brought about and (ii) that a cooling of the fixative particles produces adhesion of the fixative particles to themselves, to the structure-forming particles and to the non-self-cleaning surface of the object; and (D) finally cooling the object, to secure the structure-forming particles as well as the fixative particles to the non-self-cleaning surface, wherein steps (A) and (B) are conducted simultaneously or one after the other in any order or by applying a mixture of the fixative particles and the structure-forming particles prior to step (C); or step (B) is conducted after steps (A) and (C).

35. Use of the process as claimed in any one of claims 18 to 34, for producing self-cleaning surfaces on planar or non-planar objects.

36. Use of the process as claimed in any one of claims 18 to 34, for producing self-cleaning surfaces on non-rigid surfaces of objects.

37. Use of the process as claimed in any one of claims 18 to 34, for producing self-cleaning surfaces on flexible or inflexible partitions in the sanitary sector.

38. Use of the process as claimed in any one of claims 18 to 34, for producing self-cleaning surfaces on corrosion-protected elements in buildings above ground level.