CA2393226A1 - Surfaces rendered self-cleaning by hydrophobic structures and a process for their production - Google Patents

Abstract

The present invention relates to an object having a self-cleaning surface which has a self-regenerating self-cleaning effect, and to a process for its production. The object has a self-cleaning hydrophobic surface on which there are first particles secured by means of a carrier, where the carrier itself is a mixture made from second particles and a binder. Although the particles on the surface may be ablated by erosion, new particles are released from the carrier and the self-cleaning effect is thus self-regenerating. The object may be used with advantage outdoors, e.g. on vehicles, since that is where the self-cleaning materials are particularly exposed to the effects of the environment.

Description

Z ~ CA 02393226 2002-07-12 3urfacss rendered self-clsa~ b h ro hcrbic structures and a process for their production FIELD OF THE INVENTION
The present invention relates to objects having a self-cleaning surface and to a process for their production.
In particular, the present invention relates to an object having a self-cleaning surface which maintains self-cleaning for a prolonged period despite natural erosion.
BACKGROUND OF THE INVENTION
Articles 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 the surface-energy-related parameters relating to interaction of the two surfaces which are in contact. The 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 - la -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.
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 um 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 "Surface 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 x K CA 02393226 2002-07-12 - 2 - 0.2. 5792 with respect to water, with a considerable reduction in tendency toward icing.
US Patent 3 354 022 and WO 96104123 describe other processes for reducing the wettability of articles 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 1 o formation, and as the droplets roll off they absorb dirt particles and thus clean the surface.
This principle has been borrowed from the natural v~norld. Small contact areas reduce Van der Waal's interaction, which is responsible for adhesion to flat surtaces with low surtace energy. For example, the leaves of the lotus plant have elevations made from a wax, and these elevations tower the contact area with water. WO 00158410 describes these structures and claims the formation of the same by spray-application of hydrophobic alcohots, such as 10-nonacosanol, or of alkanediots, such as 2 0 5,10-nonacosanediot. The separations of the elevations in the structures are in the range from 0.1 to 200 ~m and the heights of the elevations are from 0.1 to 100 ~,m. However, no information is given concerning the shape of the elevations. A disadvantage here is that the self-cleaning surtaces lack stability, since the structure is removed by detergents.
Another method of generating easy-clean surtaces has been described in DE 19917 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.
Processes- for producing the structured surfaces are likewise known.
Besides the precision-casting reproduction of these structures by way of a master structure, by injection molding, or by embossing processes, there are other known processes which utilize the application of particles to a surtace, e.g. in US 5 599 489. Common features of all casting processes - 3 - O.Z. 5792 are that the self-cleaning behavior of the surfaces can be described by way of a very high aspect ratio, and that the structures have three-dimensional periodicity.
High aspect ratios in three-dimensional space, i.~. objects which are tall ' and narrow and stand in isolation, are difficult to produce industrially and have low mechanical stability.
There has been much relatively n3cent work concerned with the three-1 o dimensional structuring of surtaces, an example being US 6 093 754, where a three-dimensional structure is achieved by way of multiple printing of the surtace, some of the printing inks repelling the next layer so that a structure is formed.
C. Bernard and D. Lebellac describe in FR 2792003 A1 a process for producing structured surtaces which are both water-repellent and oil-repellent, by way of vacuum deposition, using a CVD technique. These layers, too, have insufficient mechanical stability.
2o Reducing the aspect ratio generally also increases the stability of the layers. For example, in "Physikaiische Blotter" 56 (2000), No. 4, 49 et seq.
Frank Burmeister describes a process for obtaining nanostructures by means of capillary forces. It is emphasized that structures from a few atomic layers up to the particle radius can be varied in such a way that it is also possible to generate structures with an aspect ratio > 1. However, it is also emphasized that processes of this type are only useable for relatively small areas, since otherwise stress cracks can form in the drying process.
In "Advanced Materials°, 2001, 13, No. 1, pp. 51 et seq., Hideshi Hattore 3o describes a process for electrostatic coating, emphasizing that self organization of the particles occurs if the surtace to be coated and the particles themselves carry opposite electrical charges. However this process does not generate aspect ratios > 1, and these layers are therefore merely antireflection layers.
An interesting process for generating set-cleaning surfaces is described by Akira Nakaiima, Langmuir 2000, 16, 5754-5760, where the structure is generated by subliming aluminum acetylacetonate. 2°~ of titanium dioxide also has to be added to the hydrophobic material in order to achieve self cleaning properties. The cause of the effect here is certain to ~ be. the catalytic decomposition properties of the tit~ium dioxide in combination with light, rather than the structure and the hydrophobic properties.
None of these negative properties is found in the case of the present invention. DE 101 18 351 and DE 101 18 352 say that stable self-cleaning surfaces can be obtained by securing structure focmers which have to have a fissured structure. If use is made here of hydrophobic stnicture-formers and hydrophobic carrier materials, these surfaces are to some 1 o degree mechanically stable and to some degree resistant to erosion by wind, weather and light. However, it is impossible to avoid ablation of the active layers, in particular when damage is caused by UV light. As is generally the case, attack by wind and weather leads to gradual smoothing of the surface and thus to fall-off in the s~If-cleaning effect. The self cleaning action falls away asymptotically. The limiting value reached is dependent on the amount of residual stn,~ure remaini~ on the carrier, and the extent of hydrophobic properties and smootimess possessed by the resultant surface.
DE.199 44 169 A1 achieves self-cleaning surfaces by' way of incipient erosion of an "outer layer". In this process, the effect does not appear until erosion has occurred. Further erosion causes the self-cleaning effect to reduce or disappear entirely. There is no regeneration of the self-cleaning surface.
It was therefore an object of the present invention to provide surtace structures which have high mechanical stability and which self-clean by way of the movement of water, even after natural erosion has occurred, and also to provide a simple process for producing these self-cleaning 3 0 surfaces.
Surprisingly, it has been found that surface structures which also have the structure-former in the carrier have a regenerative effect, since although erosion ablates some of the structure~orming particles it also releases, 3 5 from the carrier, new particles whidi have activity for the self-leaning effect, thus giving self-regeneration of the self-cleaning effect.

SUMMARY OF THE INVENTION
The present invention therefore provides an object having a self-cleaning surface with a self-regenerating self-cleaning effect, and having an artificial, at least to some extent (namely at least partially) hydrophobic, surface structure made from elevations and depressions, where the elevations and depressions are formed by first particles secured to the surface by means of a carrier, wherein the carrier is a mixture made from second particles and a binder.
The present invention also provides a process for producing an object having a self-cleaning surface, by achieving a suitable, at least to some extent hydrophobic, surface structure on a surface by securing first particles by means of a carrier, wherein the carrier used comprises a mixture of second particles and a binder.
For the purposes of the present invention, self-cleaning effect is an effect which renders it more difficult to wet surfaces. The poor wettability of the surfaces, in particular by water, removes contamination, e.g. dust, from the surface by way of roll-off of liquid droplets. The liquid may be rain, for example.
For the purposes of the present invention, the first and second particles may be composed of like materials.
The process of the invention has the advantage that the surface coating systems (carriers) produced by intimate incorporation of structure-forming materials (particles) into the binder system have a) excellent adhesion to the surface and b) excellent adhesion to the particles also applied. This method gives self-cleaning surfaces in which new structure-forming particles are released by erosion which occurs naturally by way of UV light and the effects of wind and + CA 02393226 2002-07-12 weather, the result being self-regeneration of what is called the lotus effect. With the process of the invention it is unnecessary for the structure-formers to be dispersed without any agglomerates in the carrier, as demanded by U.S. Patent No. 6,020,419. On the contrary, agglomerates in the carrier are desirable in the present invention, since agglomerates have appropriately advantageously structured surfaces, the agglomerates being particles in the size range less than 50 ~m and complying with the descriptions in DIN 53 206. Since the use of agglomerates is also possible, and there is no need for the complicated process of breaking down the agglomerates, the cost for producing self-cleaning surfaces by the process of the invention is substantially lower.
The self-regenerating self-cleaning effect gives objects having the self-cleaning surfaces of the invention particularly high suitability for applications in an aggressive environment, in particular for open-air application. In the case of prior art self-cleaning surfaces, the effects of weathering and the environment cause relatively rapid impairment of the self-cleaning properties, e.g. by way of erosion, since erosion removes the surface structure responsible for the self-cleaning properties. In the case of objects having the surfaces of the invention, the self-cleaning effect self-regenerates, since although particles are ablated by erosion, new particles come to prominence from the carrier, likewise ablated. Depending on the thickness of the carrier layer and on the number of particles present therein, the self-cleaning effect is retained substantially longer on the surfaces of the invention than is the case with conventional self-cleaning surfaces.
Substances used for securing particles on a surface are hereinafter termed "carriers".

~ CA 02393226 2002-07-12 DETAILED DESCRIPTION
In the invention an object having a self-cleaning surface with a self-regenerating self-cleaning effect, and having an artificial, at least to some extent hydrophobic, surface structure made from elevations and depressions, where the elevations and depressions are formed by first particles secured to the surface by means of a carrier, the carrier is a mixture made from second particles and a binder.
The particles may be particles in the sense of DIN 53 206. According to that standard, particles may be separated particles or aggregates or agglomerates, where, according to DIN 53 206, aggregates have primary particles in edge- or surface-contact, while agglomerates have primary particles in point-contact. The particles used may also be those formed by combining primary particles to give agglomerates or aggregates. The structure of these particles may be spherical, strictly spherical, moderately aggregated, almost spherical, extremely highly agglomerated, or porous-agglomerated. The preferred size of the agglomerates or aggregates is from 20 nm to 100 Vim, particularly preferably from 0.2 to 30 Vim.
The particles which form the structure preferably have a fissured structure with elevations and/or depressions in the nanometer range. The average height of the elevations is preferably from 20 to 500 nm, particularly preferably from 50 to 200 nm. The separation of the elevations and, respectively, depressions on the particles. is preferably below 500 nm, very particularly preferably below 200 nm.
The fissured structures with elevations and/or depressions in the nanometer range may be formed by cavities, pores, grooves, peaks and/or protrusions, for example. The ' CA 02393226 2002-07-12 - 7a -particles themselves have an average size of less than 50 ~Cm, preferably less than 30 ~.m, and very particularly preferably less than 20 Vim. The separations of the particles on the surface are preferably from 0 to 10 particle diameters in particular from 0 to 3 particle diameters.
The particles preferably have a BET surface area of from 50 to 600 square meters per gram, and very particularly preferably from 50 to 200 m2/g.
The structure-forming particles used may be a very wide variety of compounds from a large number of fields of chemistry. The particles preferably comprise at least one material selected from the group consisting of silicates, doped silicates, minerals, metal oxides, silicas, polymers, and"silica-coated metal powders. The particles very particularly preferably comprise fumed silicas or precipitated silicas, in particular Aerosils*, A1z03, Si02, Ti02, Zr02, zinc powder coated with Aerosil* R 974, and preferably having a particle size of from 1 Vim, or pulverulent polymers, e.g. cryogenically milled or spray-dried polytetrafluoroethylene (PTFE), or perfluorinated copolymers, or copolymers with tetrafluoroethylene.
The particles for generating the self-cleaning surfaces preferably have hydrophobic properties, besides the fissured structures. 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 very fine powders, such as Aerosil* R 8200 (Degussa AG), these materials being commercially available.
*Trade-mark ' CA 02393226 2002-07-12 - 7b -The silicas whose use is preferred preferably have a dibutyl phthalate adsorption, based on DIN 53 601, of from 100 to 350 m1/100 g, preferably from 250 to 350 m1/100 g.
The particles are secured to the surface by means of a carrier. Applying the particles to the surface in a tightly packed layer permits the self-cleaning surface to be generated. According to the invention, the carrier comprises a mixture made of a binder and second particles, and these second particles may be the same as the above-mentioned first particles. The mixture of the binder and the second particles preferably comprises from 1 to 50% by weight, particularly preferably from 5 to 25~ by weight, and very particularly preferably from 7.5~ to 15~ by weight, of the second particles, based on the mixture.

- 8 - O.Z. 5792 For the purposes of the present invention, binders are surface coatings or surface coating systems, or adhesives or adhesive systems. In principle, any surtace coating system or adhesive system may be used as binder.
In one preferred embodiment of the self-cleaning surface of the invention, the binder is a surtace 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 pref~rably taking place by polymerization or crosslinking. The cured surface coating particularly 1 o preferably comprises polymers andJor copolymers made from mono-and/or polyunsaturated acrylates and/or methacrylates. The mixing ratios may be varied within wide limits. It is also possible for the cured surface coating to comprise compounds having functional groups, e.g. hydroxyl groups, epoxy groups, or amine groups, or fluorine-containing compounds, e.g. perfluorinated acrylic esters. This is advantageous in particular when the compatibilities of surface coating and hydrophobic particles are balanced with respect to one another, as is the case, for example, using N-[2-(acryloyloxy)ethyl]-N-ethylperfluorooctane-1-sulfonamide with Aerosil R 8200. The surface coatings which may be used are not only surface 2 o coatings based on acrylic resin but also surface coatings based on polyurethane, and also surface coatings which comprise polyurethane acrylates or silicone acrylates.
The self-cleaning surfaces of the invention have a roll-off angle of teas 2 5 than 20°, particularly preferably less than 10°, the definition of the roll-off angle being such 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 above 140°, preferably above 150°, and have less than 15° hysteresis, preferably less than 10°. Particularly good self 3 o cleaning surfaces are obtainable when the surfaces of the invention have an advancing and receding angle above at least 140°, preferably above 150°.
Depending on the binder used and on the size and material of the particles 35 used, it is possible to achieve semitransparent self-cleaning surfaces. In particular, the surfaces of the invention may be contact-transparent, i.e.
when a surface of the invention is produced on an article on which there is writing, this writing remains legible if its size is adequate.

_ g _ To allow self-regeneration of the self-cleaning effect to be achieved, it is necessary for there to be differences in the properties of the material used for the particles and for the binder. The differences may be mechanical, physical, or else chemical in nature. To achieve self-regeneration it is ' important that the binder is ablated (whether chemically, mechanically, or physically) more rapidly than the particles present therein. In relation to mechanical stability, therefore, it is preferable that the hardness of . the particles is greater than the hardness of the binder used, by ~ 0°~;
preferably 20%, and very particularly preferably 50°~: In this way the 1 o ablation achieved by abrasion of the binder lying on the surface is more rapid than that of the particles, and when a particle is lost new particles come to prominence from the binder and replace those lost. Depending on the environmental factors to which the surf~e of the invention is exposed, the properties of the materials may be optimized by selection and combination of the binder used and the particles used.
Different UV resistance of particle and binder can bring about an effect of just this type. Particles such as Aerosils have unlimited UV resistance.
However, the UV light can be transmitted through the particles to the 2o binder layer, and may cause damage to the binder, which frequently compris~s a polymer matrix. The result is that over time the ac~esion of the structure forming particles is weakened, and possibly that the particle is released from the carrier. The surface is temporarily exposed to the W
light at this location. This then attacks the organic compounds of the binder in the usual way. However, degradation of the corresponding polymer chains releases new particles at the surtace, and these ~ are structure-fonners which again ensure that the surface has self-leaning properties.
3 o Although yes made from quartz (Aerosils) have high UV
transmittance, only very little UV radiation reaches the polymeric carrier matrix through the particles, since the particles have numerous angled surfaces and the associated light scattering means that only a small part of the UV radiation penetrates the particles.
Objects having the self-cleaning surface of the invention are preferably produced by the process of the invention for producing these surfaces. In this invention process for ' CA 02393226 2002-07-12 producing self-cleaning surfaces by achieving a suitable, at least to some extent hydrophobic, surface structure on a surface by securing first particles by means of a carrier, the carrier used comprises a mixture of second particles and a binder.
The first particles used preferably comprise at least one material selected from the group consisting of silicates, doped silicates, minerals, metal oxides, silicas, and polymers. The particles very particularly preferably comprise fumed silicates or silicas, in particular Aerosils, minerals such as magadiite, A1203, Si02, Ti02, Zr02, zinc powder coated with Aerosil R 974, 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 of particles which have a BET surface area of from 50 to 200 m2/g.
The first particles for generating the self-cleaning surfaces preferably have hydrophobic properties, besides the fissured structures. 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 very fine powders, such as Aerosil R 974 or Aerosil R 8200 (Degussa AG), these materials being commercially available.
The process of the invention preferably comprises the steps of a) applying a mixture of the binder and the second particles as a carrier to a surface, where the binder is in an uncured form, b) applying the first particles which comprise fissured structures to the carrier, and c) securing the first particles by curing the carrier (i.e., the binder).
Examples of methods of applying the mixture are the use of a spray, a doctor blade, a brush, or a jet. The mixture is preferably applied at a thickness of from 1 to 200 hem, with preference at a thickness of from 5 to 100 ~Cm, and very particularly preferably at a thickness of from 25 to 50 ~.m. Depending on the viscosity of the mixture, it may be advantageous to allow the mixture to undergo to some extent (i.e., partially) curing or drying prior to application of the first particles. The viscosity of the mixture is ideally selected in such a way that the first particles applied can sink into the mixture at least to some extent, but in such a way as to prevent flow of the mixture and of the first particles applied thereto when the surface is placed vertically.
The first particles may be applied by commonly used processes, such as spray application or powder application. In particular, the first particles may be applied by spray application using an electrostatic spray gun. Once the first particles have been applied, excess particles, i.e. particles not adhering to the mixture, may be removed from the surface by shaking, or by being brushed off or blown off. These unattached particles may be collected and reused.

' CA 02393226 2002-07-12 - lla -The binder used in the carrier may be generally an organic substance and may be a surface coating or a surface coating system, or an adhesive or an adhesive system. It is preferable for the binder used to comprise a surface coating system or surface coating which at least comprises a mixture made from mono- and/or polyunsaturated acrylates and/or methacrylates. The mixing ratios may be varied within wide limits. The binder used particularly preferably comprises a surface coating which can be cured by means of thermal or chemical energy, and/or the energy in light.
If the particles used have hydrophobic properties, the~binder selected preferably comprises a surface coating or a surface coating system which has hydrophobic properties.
It can be advantageous for the mixtures used as surface coating for the binder to comprise compounds having functional groups, e.g. hydroxyl groups, epoxy groups, or amine groups, or fluorine-containing compounds, e.g.
perfluorinated acrylic esters. This is advantageous in particular when the compatibilities (relating to the hydrophobic properties) of surface coating and hydrophobic particles are balanced with respect to one another, as is the case, for example, using N-[2-(acryloyloxy)ethyl]-N-ethylperfluorooctane-1-sulfonamide with Aerosil VPR 411.
The surface coatings which may be used as binder are not only surface coatings based on acrylic resin but also surface coatings based on polyurethane, and also surface coatings which comprise polyurethane acrylates. Two-component surface coating systems or other reactive surface coating systems may also be used as binder.

~ CA 02393226 2002-07-12 - 12 - O.Z. 5792 To prepare the mixture made from binder and particles and used as can-ier, the binder is intimately and thoroughly mixed with the particles.
The mixing may take puce in a manner known to the skilled worker.
The particles are secured to the carrier by curing the carrier, preferably by way of thermal and/or chemical energy, and/or the energy in light, depending on the surface coating system used. The curing of the carrier, initiated by chemical or thermal energy, andlor the energy in light, may take place by polymerization or crosslinking of the constituents of the 1 o surface coatings or, respectively, surface coating systems, for example.
It is particularly preferable for the curing of the carrier to take place by way of the energy in light, and it is very particularly preferable for the polymerization of the carrier to take place by way of the fight in the UV
region from a medium-pressure mercury lamp. It is preferable for the curing of the carrier to take 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 can be necessary to restrict the time which expires between application of the particles and curing of the carrier, in order to avoid complete immersion of the particles in the carrier. It is preferable for the carrier to be cured within from 0.1 to 10 min, preferably within from 1 to 5 min, after application of the particles.
In carrying out the process of the invention it can be advantageous to use particles which have hydrophobic properties, andlor particles which have hydrophobic properties as a result of treatment with at least one compound selected from the group consisting of the alkylsilanes, alkyldisilazanes, and perfluoroalkylsilanes. The hydrophobicization of particles is known, 3 o and an example of a description is found in the Degussa AG series of publications Pigments [Pigments], Number ~18.
It can also be advantageous for the particles to be given hydrophobic properties after the process of securing to the carrier. One way in 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 selected from the group consisting of the alkylsilanes and the perfluoroalkylsilanes, e.g. those which can be purchased from Degussa AG. The preferred method of treatment is 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 self-cleaning surfaces with a self-regenerating self-cleaning effect on planar or non-planar articles, in particular on non-planar articles. This is possible to only a limited extent with the conventional processes. In particular, non-planar articles, 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 may, of course, also be used to produce self-cleaning surfaces with a self-regenerating self-cleaning effect on articles 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 transparent at least to the extent that the amount of sunlight which can penetrate the transparent surface 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 conventional greenhouses, which have to be cleaned regularly to remove leaves, dust, lime, and biological material, e.g.
algae.
The process of the invention can also be used for producing self-cleaning surfaces with a self-regenerating self-cleaning effect on non-rigid surfaces of articles, e.g.
umbrellas or on other surfaces required to be 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 being partitions dividing public toilets, partitions of shower cubicles, of swimming pools, or of saunas, and also shower curtains (flexible partition).
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 gives a scanning electron micrograph (SEM) of a carrier system with a self-regenerating self-cleaning effect. It is clear that, when the superficial particles are lost, particles lying underneath these take over their function, and the self-cleaning property is therefore retained.
Figure 2 is a diagram of the mode of operation of the surface of the invention. Particles have been secured to the surface O using a carrier T which comprises binder and particles P (a). If this surface is exposed to erosion for a period, the result is a surface O as in b, with a markedly thinner layer T'. It is clearly seen that the uppermost layer of particles which were secured by way of the carrier (a) have been ablated by erosion (b), and the structure of the self-cleaning surface is now formed by particles which were previously present within the carrier.

- 14a -EXAMPLES
The examples below are intended to give further illustration of the inventive surfaces and, respectively, the process for producing the surfaces, but there is no intention that the invention be restricted 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 with one another.
Based on this mixture, 14% by weight of Plex* 4092 F, an acrylic copolymer from Rohm GmbH, and 2% by weight of W
curing agent Darokur* 1173 were added, and the mixture was stirred for at least 60 min. 8.45% by weight of hydrophobicized fumed silica particles, Aerosil* VPR 411 (Degussa AG) were added to this mixture made from binder, with vigorous stirring, and stirring was continued until the particles had been completely and thoroughly mixed with the binder and had been completely wetted by the binder.
This mixture made from the binder and the particles was applied at a thickness of 50 ~m as carrier to a polymethyl methacrylate (PMMA) sheet of thickness 2 mm.
Initial drying of the layer was carried out for 5 min. The particles then sprayed on by means of an electrostatic spray gun were hydrophobicized fumed silica, Aerosil* VPR 411 (Degussa AG). After 3 min, the carrier was cured under nitrogen at a wavelength of 308 nm. Once the carrier had been cured, excess Aerosil* VPR 411 was removed by brushing.
Initial characterization of the surface took place visually and was recorded as +++, meaning that *Trade-mark 23443-7$3 there is alrinost complete formation of water droplets. The rolhff angle was 2.6°.
Example 2:
The experiment of Ex~ple 1 is repeated, but Aerosit R 8200 (Degussa AG), which has a BET surface area of 200 t 25 m2lg, is used instead of Aerosit~VPR 411. The assessment of the surface is +++.
Example 3:
10% by weight (based on the total wei~t of the surface coating mixture) of 2-(N-ethyiperfl~orooctanesulft~amido)ethyl acrylate was also added to the surface coating of Example 1, which had previously been mixed with the UV curing agent. This mixture, too; was again stirred for at Ieast.60 min.
This mixture was applied at a thickness of 50 ~cm as carrier to a PIVIMA
sheet of thickness 2 mm. Initial drying of the layer was carried out for 5 min. The particles then applied by means of an electrostatic spray ' gun were hyd~rophobicized fumed silica, Aerosil VPR 491 (Degussa AG). After 3 min, the carrier was cured under nitrogen at a wavelength. of 308 nm.
Once the carrier had been cured, excess Aerosii~VPR 411 was removed by 2 o brushing. Initial characterization of the surfaces was carried out visually and recorded as +++, meaning that there is almost complete formation of water droplets. The roll-off angle was 0.5°.
*Trade-mark

Claims (39)

1. An object having a self-cleaning surface with a self-regenerating self-cleaning effect, and having an artificial, at least to some extent hydrophobic, surface structure made from elevations and depressions, where the elevations and depressions are formed by first particles secured to the surface by means of a carrier, wherein the carrier is a mixture comprising second particles and a binder.

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

3. The object having the self-cleaning surface as claimed in claim 2, wherein the cured surface coating comprises a mixture of mono- or polyunsaturated acrylates, methacrylates or polyurethane.

4. The object having the self-cleaning surface as claimed in any one of claims 1 to 3, wherein the first and second particles each have an average size of less than 50 µm.

5. The object having the self-cleaning surface as claimed in any one of claims 1 to 4, wherein the first and second particles each have an average size of less than 30 µm.

6. The object having the self-cleaning surface as claimed in any one of claims 1 to 5, wherein the first and second particles comprise at least one material selected from the group consisting of silicate, doped silicate, mineral, metal oxide, silica, polymer, and metal powder.

7. The object having the self-cleaning surface as claimed in any one of claims 1 to 6, wherein the first and second particles have hydrophobic properties.

8. The object having the self-cleaning surface as claimed in any one of claims 1 to 7, wherein the first and second particles are separated particles, aggregates or agglomerates.

9. The object having the self-cleaning surface as claimed in any one of claims 1 to 8, wherein the first and second particles have a fissured structure with elevations and depressions in the nanometer range.

10. The object having the self-cleaning surface as claimed in any one of claims 1 to 9, wherein the self-cleaning surface has a roll off angle of less than 20°.

11. The object having the self-cleaning surface as claimed in any one of claims 1 to 10, wherein the binder is capable of being ablated more rapidly than the second particles.

12. The object having the self-cleaning surface as claimed in any one of claims 1 to 11, wherein the second particles have a hardness of at least 10% greater than that of the binder.

13. The object having the self-cleaning surface as claimed in any one of claims 1 to 12, wherein the second particles have a greater UV resistance than the binder.

14. A process for producing an object having a self-cleaning surface with a self-regenerating self-cleaning effect, and having an at least to some extent hydrophobic surface structure, comprising: securing first particles by means of a carrier to the surface, wherein the carrier comprises a mixture of second particles and a binder.

15. The process as claimed in claim 14, wherein the first and second particles comprise at least one material selected from the group consisting of silicate, doped silicate, mineral, metal oxide, silica, metal powder, and polymer.

16. The process as claimed in claim 14 or 15, wherein the first and second particles each have an average size of less than 50 µm.

17. The process as claimed in any one of claims 14 to 16, wherein the first and second particles each have an average size of less than 30 µm.

18. The process as claimed in any one of claims 14 to 17, wherein the first and second particles are separated particles, aggregates or agglomerates.

19. The process as claimed in any one of claims 14 to 18, wherein the first and second particles have a fissured structure with elevations and depressions in the nanometer range.

20. The process as claimed in any one of claims 14 to 19, wherein the second particles have a hardness of at least 10% greater than that of the binder.

21. The process as claimed in any one of claims 14 to 20, wherein the second particles have a greater UV
resistance than the binder.

22. The process as claimed in any one of claims 14 to 21, wherein the amount of second particles in the carrier is from about 1% to about 50% by weight of the carrier.

23. The process as claimed in any one of claims 14 to 22, which process comprises the steps of:
a) applying the carrier to the surface, wherein the carrier contains a curable substance as the binder, b) applying the first particles to the carrier, and c) securing the particles by curing the curable substance in the carrier.

24. The process as claimed in claim 23, wherein the curable substance in the carrier is cured by thermal, chemical or light energy.

25. The process as claimed in claim 23 or 24, wherein the curable substance is a compound functionalized with a functional group selected from the group consisting of hydroxyl, epoxy, amine or fluorine containing compound.

26. The process as claimed in any one of claims 23 to 25, wherein the curable substance when cured is a surface coating which comprises a mixture of mono- or polyunsaturated acrylates, methacrylates, polyurethanes, silicone acrylates or urethane acrylates.

27. The process as claimed in claim 26, wherein the first and second particles and the surface coating have hydrophobic properties.

28. The process as claimed in any one of claims 14 to 27, wherein the first and second particles have hydrophobic properties.

29. The process as claimed in any one of claims 14 to 28, wherein the first and second particles are equipped with hydrophobic properties after the process of securing to the carrier.

30. The process as claimed in any one of claims 14 to 29, wherein the first and second particles have hydrophobic properties as a result of treatment with at least one compound selected from the group consisting of alkylsilane, perfluoroalkylsilane and alkyldisilazane.

31. The process as claimed in any one of claims 23 to 30, wherein the curing step (c) is conducted in an inert gas atmosphere.

32. The process as claimed in any one of claims 23 to 31, wherein the time between the application of the first particles in step (b) and the curing in step (c) is restricted to avoid complete immersion of the first particles in the carrier.

33. The process as claimed in any one of claims 23 to 32, wherein the time between the application of the first particles in step (b) and the curing in step (c) is between about 0.1 minutes to about 10 minutes.

34. The process as claimed in any one of claims 14 to 33, wherein the carrier is applied using a spray, doctor blade, brush or jet.

35. The process as claimed in any one of claims 14 to 34, wherein the carrier is applied in a thickness of from about 1 µm to about 200 µm.

36. The process as claimed in any one of claims 14 to 35, wherein the first particles are applied by spray or powder application.

37. The use of the process as claimed in any one of claims 14 to 37, for producing a self-cleaning surface on a planar or non-planar surface of an object.

38. The use of the process as claimed in any one of claims 14 to 37, for producing a self-cleaning surface on a non-rigid surface of an object.

39. The use of the process as claimed in any one of claims 14 to 37, for producing a self-cleaning surface on a flexible or non-flexible object used in the sanitary sector.