MXPA06009925A - Coating for metal surfaces, method for the production thereof and use thereof as a self-cleaning protective layer, particularly for the rims of automobiles - Google Patents

Abstract

A coating for metal surfaces, consisting of a.) optionally, a scratch-resistant perhydropolysilazane base coat and b.) an upper protective layer containing at least one perhydropolysilazane of formula (1) and photocatalytic titanium dioxide (I), wherein n is a whole number and n is dimensioned in such a way that the perhydropolysilazane has a number average molecular weight of 150 -150.000 g/mol. Preferably, the thickness of the protective layer is at least 1 micrometre, more preferably 2 - 20 micrometres. The invention also relates to a method for the production of the coating, in addition to the use thereof as a self-cleaning protective layer, particularly for the rims of automobiles.

Description

COATING FOR METAL SURFACES, METHODS FOR THE PRODUCTION OF THE SAME AND ITS USE AS A PROTECTIVE LAYER OF SELF-CLEANING PARTICULARLY FOR CAR RIÑES DESCRIPTIVE MEMORY The present invention ST refers to a superhydrophilic, transparent, and photocatalytically active coating for car tires. The coating is based on polysilazanes, which are combined with photocatalytically active metal oxides. The use of aluminum rings in the construction of automobiles has increased enormously in recent years. On the one hand, the lighter aluminum rings offer weight advantages over the steel rings and therefore allow fuel savings, but the essential aspect is that the aluminum rings are used above all aesthetic reasons, since they give to the vehicle a higher value and a refined appearance. A disadvantage of aluminum rings is in particular their susceptibility to corrosion and their propensity to get dirty. In addition, scratches on the shiny surface of an aluminum wheel are more noticeable than in a steel wheel. For this reason the aluminum rings are provided at the end of the manufacturing operation with a coating, which is usually composed of an aluminum pre-treatment (chrome or chrome-free coating), a primer, a coating of pigmented base and, finally, a transparent coating. This complex coating is necessary in order to ensure sufficient protection against corrosion. Despite the coating, corrosion causes problems, through the use, for example, of the abrasive salt in the winter. Finally, the brake dust deposited on the aluminum wheel over time also eats the coating and can no longer be removed. In addition, when snow chains are used, the aluminum rings are easily scraped. Another cause of scrapes is the cleaning of aluminum rings with abrasive tools, such as brushes or sponges. They are also spreading more and more polished or machine-polished aluminum rings, whose surface consists of a shiny, aesthetically attractive surface of pure aluminum, protected only by a thin transparent coating, in order to retain the brightness of the wheel. With this kind of kidney, the protection against corrosion by means of a thin coating film, which must additionally be invisible to the human eye, is very difficult to achieve. Another problem with car wrecks is the ease with which they get dirty and the difficulty involved with rim cleaning, depending on their geometry. Several types of car wrecks are not completely clean even after a visit to a car wash. The sometimes complex geometry of the kidney makes them more difficult to clean by hand. Since, however, the majority of car drivers give a value to having permanently clean scrapes, and want to minimize the effort necessary for permanent cleaning, a problem arises here that still waits for a solution. WO 02/088269 A1 discloses the use of a perhydropolisilazane solution to produce hydrophilic, dirt repellent surfaces. The description includes that the use in the automotive sector (in the body and kidney), and solutions of perhydropolisilazane with a weight fraction of 0.3% to 2% is recommended. Example 1 uses a highly diluted solution with a weight fraction of only 0.5% perhydropolisilazane, with which a very thin steel coating is obtained, with a coating thickness of approximately 0.2 microns. Such a thin coating is first unable to prevent scratches from the paint surface and is also unable to ensure sufficient protection against corrosion or prevent it from being consumed by brake dust. further, the thin coating is not sufficient to level the relatively non-homogeneous transparent coating and produce a crystalline surface, truly smooth, easily treatable for cleaning. The contact angles with the water that could be achieved with a hydrophilic coating described above are about 30 °, which means that when it rains or when the kidneys are cleaned with water, the droplets still form. The relatively hydrophilic crystalline surface makes it easier to clean the kidneys, but the coating does not have the self-cleaning effect.

The object on which the present invention is based was to develop a coating that exhibits a self-cleaning effect, is hard and scratch resistant, and protects the aluminum wheel against corrosion and against burns by brake dust. . The self-cleaning surfaces can be obtained through a coating with a photocatalytically active metal oxide, especially titanium dioxide in the anatase modification. The self-cleaning effect is essentially based on the following mechanism: in photocatalysis, the valence band electrons of titanium dioxide are excited through the light and cross through the conduction band. The lifespan of these excited species is long enough to allow some of the electron and electron holes to also diffuse to the surface. There the holes of the electrons abstract the electrons from the water molecules that adhere to the surface, and the free electrons in the conduction band are transferred to the oxygen molecules. This results in -OH radicals, which have a very large oxidation potential (close to that of elemental fluorine), and superoxide anions (O2-), which also have a strongly oxidative effect. The highly reactive • OH and - O2- species react with the organic compounds (in the form of dirt particles, for example) to form water and carbon dioxide, so that the organic dirt is completely separated. As well as the photocatalytic effect, which causes the oxidation of organic impurities on the surface of the Ti02 particles, there is an additional effect that is important for the production of self-cleaning surfaces: the superhydrophilic effect, through which the surface spontaneously moistens with water This effect can be quantified through the measurement of the contact angle, with the superhydrophilic effect reaching an angle of < 5. As a result of the combination of the two phenomena described, the photocatalytically active surfaces exhibit a self-cleaning effect: on the one hand, the dirt particles on the surface are destroyed (where they comprise organic material), and additionally, the humidification with effective water means that both dirt particles and oxidation products are washed more easily from the surface. The superhydrophilic surface additionally provides an anti-deposit effect. Photocatalytically active titanium dioxide has made inroads in Japan in particular as a coating material for self-cleaning surfaces. There are numerous patent applications and patents in this field. However, due to the strongly oxidative effect of the radicals formed, the application is often restricted to coatings of inorganic substrates such as glass, ceramic, stone, etc. For use as a coating on organic substrates such as plastics, varnishes, and paints, etc., a protective coating between the substrate and the titanium dioxide coating is necessary, and must meet the following criteria: it must be inorganic in nature, for that does not destroy itself through the photocatalytic effect of titanium dioxide under exposure to sunlight; it must have adequate adhesion to both the substrate and the titanium dioxide coating; it must be transparent, so as not to devalue the appearance of the substrate. In the case of aluminum rings, it must also provide sufficient protection against scrapes and corrosion protection, and must be capable of inexpensive application using conventional coating techniques. To meet these conditions this inorganic protective coating must have a very high degree of entanglement, in order to constitute a barrier for ions and gases. Protective coatings that possess the aforementioned properties can be produced, for example, of perhydropolisilazane (PHPS). In a variety of substrates PHPS forms very thin SiOx coatings that depend on the choice of reaction parameters that can have very high level within interlacing. The use of a SiOx protective coating obtained from PHPS between the substrate and the photocatalytic titanium dioxide coating has been described in a number of patents. JP 2000 025 156 thus describes a self-cleaning protective coating consisting of a silica layer, produced from a polysilazane, and an additional layer comprising titanium dioxide as photocatalyst in a sol-gel matrix. JP 2000 017 620 claims the same system for use as an anti-fog coating on traffic mirrors. As described above, superhydrophilic surfaces exhibit anti-fog properties since a film of water is formed instead of droplets. JP 2000 017 619 discloses a system comprising a protective coating PHPS and photocatalytic titanium dioxide deposited with steam, or titanium dioxide in a siloxane matrix, for self-cleaning noise-proof walls, of polycarbonate and polymethyl methacrylate. The systems described in the aforementioned patent are used exclusively in polycarbonate, polymethyl methacrylate and glass. The pencil hardness of these coatings at 2-3 H, moreover, is unsuitable for use in aluminum rings. In all cases titanium dioxide is applied either through vapor deposition or as a sol-gel matrix. No details are given of the effectiveness of the photocatalytic effect or of self-cleaning. In JP 11 035 887 a mixture of PHPS and photocatalytic titanium dioxide or a glass substrate is applied. This system is unsuitable for organic substrates, since in this case an inorganic protective coating is not used. In JP 11 227 091 no PHPS inorganic protective coating was also used, and organic substrates can not be used without being destroyed after a certain time.

JP 2000 053 920 and JP 2002 301 429 describe formulations comprising PHPS and photocatalytic titanium dioxide, the PHPS solid content of the formulation being between 0.1% and 5%. The use of these formulations is restricted to exterior façade coatings. JP 2003 170 060 describes a system composed of a PHPS main coating and a coating of photocatalytic titanium dioxide, the thickness of the total coating of this system being between 0.01 and 0.5 μm. The thickness of the coating of this kind is too low for scratch-resistant aluminum scratches. JP 2000 189 795 and JP 2000 191 960 likewise discloses a system wherein PHPS is used as the main coating. Applied to this main coating is a coating of titanium dioxide that is integrated into a sol-gel matrix. None of the photocatalytic systems indicated above is suitable for use as a self-cleaning coating for aluminum rings, since all of them lack at least some of the requirements for this application. Any of the coatings are too thin and therefore not resistant to scratching, nor do they contain corrosion inhibition, or no main coating is used at all, so after prolonged exposure to the sun the varnish of the wheel it could be destroyed through the photocatalytic action of titanium dioxide or the activity of titanium dioxide is too low, since there is insufficient titanium dioxide on the surface and is therefore unable to develop its photocatalytic action. The object on which the present invention is based was to develop a coating exhibiting a hard and scratch-resistant self-cleaning effect, and which protects the aluminum wheel against corrosion and against brake dust burns. Surprisingly it has now been found that with the perhydropolysilazane solution it is possible first of all to produce a sufficiently thick protective coating and a barrier coating which is scratch resistant and which prevents corrosion of the aluminum wheel and the chemical separation of the transparent coating through of the photocatalytic action of titanium dioxide and also prevents the burning of brake dust. The subsequently applied formulation comprising titanium dioxide (anatase) and perhydropolisilazane provides a self-cleaning effect and adheres exceptionally to the PHPS coating, due to chemical similarity. The invention therefore provides a coating for metal surfaces which is composed of: a) optionally a scratch-resistant perhydropolisilazane-based coating, comprising a perhydropolisilazane of the formula (1) and b) a protective topcoat comprising at least less a perhydropolisilazane of the formula (1) and photocatalytic titano dioxide.

The PHPS perhydropolisilazane, both in the base coat and in the protective coat, has the following formula (1) wherein n is an integer and is measured in such a way that the polysilazane has a number average molecular weight of 150 to 150,000 g / mol. The protective coating (b) has a thickness of at least 1 micrometer, preferably 2 to 20 micrometers, more preferably 3 to 10 micrometers, and ensures sufficient protection against corrosion and scratching. The coating of the invention is especially suitable as a protective coating for car tires, where it prevents burns by brake dust in the wheel and at the same time prevents the destruction of the organic transparent coating by virtue of the second coating, which is applied additionally and comprises photocatalytic titanium dioxide. This second coating comprises a mixture of PHPS of the formula (1) and nano-scale photocatalytic titanium dioxide, The nanoscale titanium dioxide is preferably of the anatase type and has a particle size of 0.001 - 0.5 μm. The ratio of perhydropolisilazane (based on the solids content of PHPS) with titanium dioxide in the photocatalytic coating is from 1: 0.01 to 1: 100, preferably 1: 0.1-1: 50, more preferably 1: 1-1: 5. Perhydropolisilazane exhibits very good adhesion to a wide variety of substrates, including metal and ceramic surfaces, and also to polymeric materials such as, for example, plastics and varnishes. The invention further provides a method for producing a self-cleaning coating for metal surfaces, wherein first of all in an optional first step: a) a perhydropolisilazane solution comprising a catalyst and if desired one or more co-binders in a solvent they are applied to the metal surface as a base coat and subsequently; b) an additional protective coating is applied to this base coating or on the metal surface directly, said protective coating comprising at least one perhydropolisilazane of the formula (1) and photocatalytic titanium dioxide. wherein n is an integer and is such that the perhydropolisilazane has a number average molecular weight of 150 to 150,000 g / moles. The perhydropolysilazane solution can therefore be applied, for example, to a coated metal surface, for example, to a coated aluminum rim, that is, in the transparent coating directly, in order to protect the rim further from scratching. , corrosion, or burns from brake dust. There is also an increase in gloss after the coating has been applied, as compared to the clear coat. Alternatively, it is possible to do so without the transparent coating, and apply the perhydropolisilazane solution directly on the pigmented base coat, which allows a saving of one coating step. In the case of polished or machine-polished aluminum rings, it is also possible to use the perhydropolisilazane solution as the sole protective coating, replacing the transparent coating normally used. It is therefore possible to produce a protective coating that is much thicker than conventional coatings, in tandem with the reduced consumption of reduced solvent emission material, said coating additionally having superior properties than conventional coatings. Both, the first and second protective coatings are applied in solution. For this purpose the perhydropolisilazane is dissolved or dispersed in a solvent, with the addition of a catalyst if desired. Solvents particularly suitable for the formulation of perhydropolisilazane are organic solvents that do not contain water and do not contain protic substances (such as alcohols or amines, for example). Said solvents are, for example, aliphatic or aromatic hydrocarbons, halogenated hydrocarbons, esters such as ethyl acetate or butyl acetate, ketones such as acetone, or methyl ethyl ketone, ethers such as tetrahydrofuran, or dibutyl ether, and also dialkyl ethers of mono and polyalkylene glycol (glymes) or mixtures of these solvents. The concentration of perhydropolisilazane in the solvent for the basecoat and the protective coating is in the range of 0.01 to 40% by weight, preferably in the range of 1% to 25% by weight. As an additional constituent, the perhydropolisilazane formulation may comprise catalysts, such as organic amines, fine metal particles or metal salts, or organic acids, which accelerate the formation of the silica film, or adhesives having influence, for example, on the viscosity of the formulation, the wetting of the substrate, the formation of the film or the behavior of the evaporation, or the organic and inorganic UV absorbers or photoinitiators. Suitable catalysts are N-heterocyclic compounds, such as 1-methylpiperazine, 1-methylpiperidine, 4,4'-trimethylenediperipine, 4,4'-trimethylene (1-methylpiperidine), diazobicylic (2.2.2) octane and cis- 2,6-dimethylpiperazine. Additional suitable catalysts are mono-, di- and trialkylamines, such as methylamine, dimethylamine, trimethylamine, phenylamine, diphenylamine and triphenylamine, DBU (1,8-diazabicyclo (5.4.0) -7-undecene), DBN (1, 5-diazabicyclo (4.5.0) -5-nonene), 1, 5,9-triazacyclododecane and 1, 4,7-triazacyclononane. Additional suitable catalysts are organic and inorganic acids such as acetic acid, propionic acid, butyric acid, valeric acid, maleic acid, stearic acid, hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, doric acid, and hypochlorous acid. Additional suitable catalysts are metal carboxylates of the formula (RCOO) nM of saturated or unsaturated aliphatic or alicyclic CrC22 carboxylic acids and metal ions such as Ni, Ti, Pt, Rh, Co, Fe, Ru, Os, Pd, Go, and Al; n is the charge of the metal ion. Suitable additional catalysts are acetylacetonate complexes of metal ions such as Ni, Pt, Pd, Al and Rh. Additional suitable catalysts are metal powders such as Au, Ag, Pd or Ni with a particle size of 20 to 500 nm. Additional suitable catalysts are peroxides such as acid peroxide, metal chlorides and organometallic compounds such as ferrocenes and zirconocenes. The coating can take place by means of processes such as those conventionally used for surface coatings. The process in question can be, for example, spraying, dipping, or flow coating. Then there may be a subsequent thermal treatment, in order to accelerate the healing of the coating. Depending on the formulation of perhydropolisilazane used and the catalyst, the cure takes place even at room temperature, but can be accelerated by heating. Due to the high reactivity of perhydropolisilazane the coating is cured in principle even at room temperature or below, but its cure can be accelerated through an increase in temperature. The maximum possible cure temperature depends essentially on the substrate to which the coating is applied. In the case of bright aluminum, relatively high temperatures are possible, 180-200 ° C for example. If the coating is applied to a coating that is already present (either base coat or clear coat) it is advisable to work at a lower temperature, so that the bottom coat does not soften, preferably from 25 to 160 ° C, more preferably from 80 to 150 ° C. Before the second protective coating is applied it is preferred to cure the base coat initially from room temperature to temperatures of 200 ° C, depending on the coating material. The healing of the coating is also affected by atmospheric humidity. At a relatively high humidity the healing takes place more quickly, which can be an advantage; conversely, curing in an atmosphere with only low humidity, such as a drying cabinet, involves a slow and uniform curing process. The curing of the coating of the invention can therefore take place at a relative atmospheric humidity of 0 to 100%. The base coating produced by means of the perhydropolisilazane formulation described above alone already forms a surface that is easy to clean, due to its hydrophilic nature. The contact angles for water are around 30 °, so drops that are already very flat are formed. This surface, however, does not have self-cleaning properties. It is resistant to scratching, protects against corrosion, adheres exceptionally to the transparent base, transparent coating, base coat or polished aluminum, and presents an excellent barrier to the photocatalyst comprising the coating to be applied subsequently. It also increases the brightness of the metal surface. A second coating, comprising a photocatalyst, is applied to the above-described silica base coating. Typical photocatalysts are titanium dioxide (T02), iron oxide (Fe2O3), tungsten oxide (WO2), zinc oxide (ZnO), zinc sulphide (ZnS), cadmium sulfide (CdS), titanate strontium (SrT¡O2) and molybdenum sulfide (MoS2), and neutralized species of the aforementioned photocatalysts. It is preferred to use titanium dioxide in the anatase modification. Then this second coating is also transparent and does not adversely affect the original brightness of the substrate, the size of the titanium dioxide particles should be placed on a scale of 0.001 - 0.5 μm. Particles of this class are commercially available, either in the form of powder in the form of dispersions. There are a variety of methods for applying a photocatalytic coating of this kind to the silica coating. One method involves the deposition of chemical vapor (CVD). In this case, a vapor of titanium dioxide particles is produced which is then deposited on the respective surface. The coatings are generally very thin (20-30 nm) and the process is technically demanding and expensive. Titanium dioxide can also be produced in situ from a sol-gel system and applied to the surface along with the sol-gel matrix. For sol-gel systems, a chemical step is needed in each case and a thermal after-treatment is required in order to allow these systems to cure. Another version, which is less expensive and easier to use compared to the methods referred to above, involves the mixing of dispersed titanium dioxide with a solution of perhydropolisilazane. There are a number of advantages in this case: there is no need for expensive and demanding vapor deposition technology, there is no need to carry out the additional synthesis step, and the compatibility of this formulation with the silica coating already present It is excellent, since perhydropolisilazane is present in both cases. In this case, then the perhydropolisilazane acts on the one hand as a binder for the titanium dioxide particles and on the other hand as a promoter for the adhesion of the silica film. It is advisable to disperse the titanium dioxide particles in the same solvent where the perhydropolisilazane was also dissolved. The titanium dioxide dispersion and the perhydropolysilazane formulation are subsequently mixed in a defined ratio and the resulting dispersion is applied to the silica coating by dipping, flow coating or spraying. This second coating can be cured at room temperature, although the curing operation can also be accelerated through heat. The concentration of perhydropolisilazane in a solvent is between 0.01% and 40%, preferably between 1% and 25%. The concentration of the titanium dioxide dispersion is between 0.01% and 70%, preferably between 0.5% and 30%. The ratio of the solids between perhydropolisilazane and titanium dioxide is from 1: 0.01 to 1: 100, preferably from 1: 0.1 to 1: 50. The concentration of the combined solutions of perhydropolisilazane and titanium dioxide is from 0.01% to 50%. In order to achieve an excellent photocatalytic or self-cleaning effect in the part of the coating, it is necessary that the content of titanium dioxide has an amount with particular preference of 1-5 parts by PHPS. This ensures the presence in the higher coating of a sufficient amount of reactive titanium dioxide particles that provide the photocatalysis and superhydrophilic effect. The present invention further provides in particular the use of the coating described above as a self-cleaning protective coating for automotive buffers, especially aluminum buffers.

EXAMPLES The perhydropolisilazanes used are products of Clariant Japan K. K. The solvents used are mixtures of xylene and pegasol (designation NP) or di-n-butyl ether (designation NL). The solutions contain either amines, metals, or metal salts as catalysts. The titanium dioxide used comprises nanoscale anatase dispersions in xylene. In the following examples, the parts and percentages are by weight. Aluminum rings are standard commercial aluminum rings such as those obtainable through the automobile accessory trade or parts of these rings can be obtained by looking for complete rings, or metal test panels consisting of the appropriate material. The coating is carried out either by spraying with a standard commercial spray gun or by immersion in a standard commercial immersion apparatus.

Scratch resistance is determined through repeated loading (5 forward or backward movements) with a grade 00 steel wool with a force of N. Scratches were visually evaluated according to the following scale: very good (no scrapes), good (few scratches), satisfactory (distinctive scratches), adequate (severely scratched) and deficient (very severely scratched). The adhesion of the coating was determined through the test in cross section according to DIN. EN ISO 2409, adhesion being on the scale of 0 (best score) to 4 (worst score). The model substance used to determine the photocatalytic activity or the self-cleaning effect was methylene blue, and its separation was visually monitored (disappearance of the coloration).

EXAMPLE 1 (Coating of an aluminum foil coated with base coat and transparent coating through immersion) A coated aluminum foil that had been provided with a standard commercial pigmented basecoat and a clearcoat was immersed in an immersion apparatus that was filled with 20% perhydropolisilazane solution at 20% strength in n-dibutyl ether (NL 120A -20, containing palladium propionate as a catalyst), and removed from the apparatus at a rate of 120 cm / min. Subsequently, it was left in the air for about 10 minutes, for evaporation, and dried at 80 ° C for 60 minutes. The result is a clear, transparent, and crack-free coating on the surface. The gloss of the sheet was increased by 5 gloss units as compared to the uncoated sheet. This coating is at least 2 μm thick. A mixture of 3.5 parts by weight of photocatalytic titanium dioxide in xylene and 1 part by weight of perhydropolisilazane in xylene (NL 110-20, containing 4,4'-trismethylene (1-methylpiperidine)) was subsequently applied to this barrier coating. , which was applied equally through immersion. The sheet was extracted from the immersion bath at a speed of 120 cm / min. It was left in the air for 10 minutes to evaporate. This gave a clear, transparent, and crack-free coating which in the service test is much easier to clean than an uncoated aluminum sheet and, in addition, it is also much less prone to pick up dirt. After a number of days of exposure to sunlight, a film of thin water was formed, instead of droplets, on the surface. When a methylene solution was applied to the sheet, and the sheet was allowed to stand in the sunlight, the blue color disappeared after a short period.

EXAMPLE 2 (Coating of a polished aluminum sheet without immersion coating) In the case of a polished aluminum sheet without a clear coating, no barrier coating of perhydropolisilazane was applied, since the substrate is not composed of an organic coating material but rather of a polished aluminum, which is not attacked by the photocatalytic action of titanium dioxide. This sheet was immersed in an immersion apparatus that was filled with a mixture of 3.5 parts by weight of photocatalytic titanium dioxide in xylene and 1 part by weight of perhydropolisilazane in xylene (NL110-20, containing 4,4'-trismethylene- (1-methylpiperidine)) and extracted at a rate of 120 cm / min. This sheet was subsequently left in the air for about 10 minutes for evaporation and then dried at 80 ° C for 60 minutes. This gave a clear, transparent, and crack-free coating. This coating is scratch resistant, protects against corrosion, prevents burn-in brake dust and has self-cleaning. In the test the coated, polished aluminum sheet is easier to clean than an aluminum sheet coated with a clear coating, and it is also much less prone to pick up dirt. After the number of days of exposure to sunlight a thin film of water was formed, instead of droplets, on the surface. When the methylene blue solution was applied to the sheet and the sheet was allowed to stand in the sunlight, the blue color disappeared after a short period of time.

EXAMPLE 3 (Coating of an aluminum wheel through spraying) A standard commercial aluminum wheel as obtainable through an automobile accessory trade was sprayed with a solution of perhydropolisilazane with 20% strength in n-dibutyl ether (NL 120A-20, containing palladium propionate as a catalyst). The rin was then left in the air for 10 minutes, for evaporation, and subsequently dried at 80 ° C for 60 minutes. This resulted in a clear, transparent and crack-free coating on the surface. The gloss of the coated wheel was increased by 5 units of brightness compared to the uncoated wheel. This coating is at least 2 μg thick. Subsequently a mixture of 3.5 parts by weight of photocatalytic titanium dioxide in xylene and 1 part by weight of perhydropolisilazane in xylene (NL 110-20, containing 4,4'-trismethylene (1-methylpiperidine)) was applied to this barrier coating through aspersion. The evaporation was allowed to take place for 10 minutes. This gave a clear, transparent and crack-free coating which in the service test compared to an uncoated aluminum wheel made in the same vehicle was much easier to clean and also much less prone to pick up dirt. After a number of exposure days in sunlight a thin film of water was formed, instead of droplets, on the surface. When the methylene blue solution was applied to the wheel and the wheel was left to rest in the sunlight, the blue color disappeared after a short period of time.

EXAMPLE 4 (Coating of a polished aluminum wheel through spray) An aluminum rim polished with machine or polished without a clear coating was purchased from a kidney manufacturer. In the case of this aluminum rim, a barrier coating of perhydropolisilazane was applied since the substrate is not composed of an organic coating but rather of polished aluminum, which is not attacked by the photocatalytic action of titanium dioxide. This rin was coated by spraying with a mixture of 3.5 parts by weight of photocatalytic titanium dioxide in xylene and 1 part by weight of perhydropolisilazane in xylene (NL 110-20, containing 4,4'-trismethylene (1-methylpiperidine). ). The rin was subsequently left in the air for 10 minutes, for evaporation, and then dried at 80 ° C for 60 minutes. This gave a clear, transparent and crack-free coating. This coating is resistant to scratching, protects against corrosion, and prevents burn by brake dust, and is self-cleaning. In the service test, the polished, coated aluminum rim is much easier to clean compared to the uncoated aluminum rim in the same vehicle, and it is also much less prone to pick up dirt. After a number of days of exposure to sunlight a thin film of water was formed, instead of droplets, on the surface. When a solution of methylene blue was applied to the wheel and the wheel was left to rest in the sunlight, the blue color disappears after a short period of time.

Claims (11)

NOVELTY OF THE INVENTION CLAIMS

1. A coating for metal surfaces that is composed of a) a scratch-resistant perhydropolisilazane-based coating and b) a top protective coating comprising at least one perhydropolisilazane of the formula (1) and photocatalytic titanium dioxide wherein n is an integer and is such that the perhydropolisilazane has a number average molecular weight of 150 to 150,000 g / ml.

2. The coating according to claim 1, further characterized in that the protective coating has a thickness of at least 1 micrometer, preferably 2 to 20 micrometers, more preferably 3 to 10 micrometers.

3. The coating according to claims 1 and / or 2, further characterized in that the ratio of perhydropolisilazane to titanium dioxide in the photocatalytic coating is from 1: 0.01 to 1: 100, preferably from 1: 0.1-1: 50, and more preferably from 1: 1 - 1: 5.

4. - The coating according to at least one of the preceding claims, further characterized in that the titanium dioxide used is the anatase modification.

5. The coating according to at least one of the preceding claims, further characterized in that the particle size of the titanium dioxide particles is in the range of 0.001-0.5 μm.

6. A process for producing a self-cleaning coating for metal surfaces, wherein first of all in a first step a) a solution of perhydropolisilazane comprising a catalyst and if desired one or more co-binders in a solvent is apply to the metal surface as a base coat and subsequently b) an additional protective coating is applied to this base coat or to the metal surface directly, said protective coat comprises at least one perhydropolisilazane of the formula (1) and photocatalytic titanium dioxide wherein n is an integer and is such that the perhydropolisilazane has a number average molecular weight of 150 to 150,000 g / ml.

7. The process according to claim 6, further characterized in that the concentration of perhydropolisilazane in the solvent for the basecoat and the protective coating is in the range of 0.01% to 40% by weight.

8. The process according to claim 6 and / or 7, further characterized in that the curing of the coatings takes place at room temperature at 200 ° C.

9. The method according to at least one of the preceding claims 6 to 8, further characterized in that when a base coat is applied in step a) it is first cured before the protective coating is applied.

10. The use of a coating according to at least one of claims 1 to 5 as a self-cleaning protective coating for metal surfaces.

11. The use as claimed in claim 10, wherein the protective coating is on the kidneys, especially aluminum bands.