DE10233829A1 - Process for preparation of surfaces with selfcleaning properties by application of nano particles with production of a lotus effect useful for coating objects subjected to high soiling and water loads, e.g. in outdoor sports - Google Patents

Abstract

A process for preparation of surfaces with selfcleaning properties by application of nano particles to the surface, where hills separated by 20 nm to 100 micron of height 20 nm to 100 micron are produced, and the nano particles are applied dry by electrostatic spraying of powder particles with a spray gun is new. Independent claims are included for: (1) a selfcleaning surface obtained as above; (2) an object with a surface as described above and below, including films, practical use objects, sports articles, textiles, clothing items, and advertising material.

Description

The present invention relates to a process for the production of surfaces with self-cleaning Properties by means of powder coating.

The production of self-cleaning surfaces that can be cleaned of impurities by moving water has been described many times. Rolling off the water droplets on hydrophobic surfaces, especially if they are structured, but without recognizing self-cleaning, was already described in 1982 by AA Abramson in Chimia i Shisn russ. 11, 38. In addition to a suitable structure, special surface chemistry is also required for self-cleaning surfaces. A suitable combination of structure and hydrophobicity makes it possible for even small amounts of moving water to take dirt particles adhering to the surface with them and to clean the surface (WO 96/04123; US 3,354,022 , C. Neinhuis, W. Barthlott, Annals of Botany 79, (1997), 667). This combination of structure and chemistry can be achieved, for example, using an embossing process in a hydrophobic lacquer. Injection molding and hot stamping processes are also possible.

State of the art with regard to self-cleaning surfaces is according to EP 0 933 388 that an aspect ratio of> 1 and a surface energy of less than 20 mN / m is required for such self-cleaning surfaces. The aspect ratio is defined here as the quotient of the medium height to the medium width of the structure. The aforementioned criteria are realized in nature, for example in the lotus leaf. The surface of a plant formed from a hydrophobic, wax-like material has elevations that are up to a few μm apart. Water drops essentially only come into contact with the tips of the elevations. Such water-repellent surfaces are widely described in the literature. An example of this is an article in Langmuir 2000, 16, 5754, by Masashi Miwa et al, which describes that the contact angle and roll angle increase with increasing structuring of artificial surfaces, formed from boehmite, applied to a spin-coated lacquer layer and then calcined.

• a) durch eine Trägerschicht oder
• b) durch eine direkte Einlagerung der Partikel ins Polymer/Substrat.

In addition to this molding of structures using suitable tools, particulate systems have also been developed. The Swiss patent CH-PS 268 258 describes a process in which structured surfaces are produced by applying powders such as kaolin, talc, clay or silica gel. The powders are fixed on the surface by oils and resins based on organosilicon compounds. More recently, particulate systems have been developed based on nanoparticles with a very hydrophobic surface, such as in DE 101 29 116 . DE 101 38 036 and DE 101 34 477 described. The nanoparticles are either bound to the substrate

• a) through a carrier layer or
• b) by direct storage of the particles in the polymer / substrate.

Appropriate procedures are described for case a). For case b), a process could be developed that uses a solvent or alcohol. When the solvent is used, the plastic is dissolved and the nanoparticle is embedded in the polymer matrix. With the evaporation of the solvent, the plastic solidifies again and the nanoparticle is firmly integrated in the polymer matrix. This method is also described above. When using a suspension of alcohol that does not dissolve the substrate and nanoparticles, the suspension is sprayed onto the polymer. There is a temporary connection of the nanoparticles to the substrate. The exact mechanisms behind this technology are not yet known. However, the alcohol probably acts as an antistatic and reduces the local charge gradients. This method has also already been described, for example in DE 102 05 007 ,

In the mentioned procedures electrostatic powder coating processes are also used. In particular such processes have been used in the production of self-cleaning Surfaces under Use of a carrier layer used, the powder particles by means of electrostatic coating was applied to the wet glue. Alternatively, this became However, the method is also used to apply to a moistened (usually with alcohol) surface dusting the nanoparticles. All of these methods have in common that the workpiece is damp becomes. This requires a very expensive drying must be connected downstream. Especially with textile webs this is a problem. In addition, the evaporating solvents (Alcohols) is an environmental problem.

So there was a task Develop processes with which nanoparticles are applied dry to the workpieces can be.

Completely surprisingly it was found that general powder coating processes are suitable. So surprisingly by spraying of electrostatic powder on one surface this surface. sufficient be durable equipped with self-cleaning properties, without the powder using a carrier, glue, solvent or otherwise on the surface had to be attached.

The subject of the present invention is therefore a process for the production of surfaces with self-cleaning properties by applying particles to the surface, whereby elevations which are at a distance of 20 nm to 100 μm and a height of 20 nm to 100 μm are formed, which is characterized in that the The particles are applied dry by electrostatically spraying on the powder.

Also the subject of the present Invention are self-cleaning surfaces, produced according to the method according to the invention as well as objects, those surfaces exhibit.

The method according to the invention has the advantage that no solvents are used, which is complex and often the environment must be removed. In contrast to conventional processes is the inventive method very low in emissions and waste water free. In addition, the method according to the invention very easy to do. It is especially very important in large-scale processes suitable because the coating can be very homogeneous and on the high web speeds in production are adjusted can. Electrostatic coatings are also in the textile industry widespread and therefore it is often not even necessary to purchase new ones Equipment.

The manufacturing method according to the invention of surfaces with self-cleaning properties and the surfaces according to the invention described below by way of example, without the invention being based on this limited should be. The process is based on the use of electrostatic spraying for creating self-cleaning surfaces.

The process of making surfaces with self-cleaning properties by applying particles to the surface whereby elevations that are a distance of 20 nm to 100 microns and a Height of 20 nm to 100 μm have, are formed, is characterized in that the application the particles dry by electrostatic spraying the powdery Particles using a powder spray gun he follows. The method is applicable to all surface materials. In particular, the method is applicable to surfaces that a material selected from plastics, lacquers, wood, aluminum, glass or metals. The surface can do the surface a film, a three-dimensional object, a membrane, a textile / nonwoven web, a lamp, an aluminum rim, a piece of furniture or a plastic part.

Spraying powders with electrostatic spraying is on and for known. Usually becomes a powder spray gun with outside High voltage electrodes used. By external high-voltage electrodes that So-called corona electrodes are used with the electrostatic powder spray gun the powder charge. Due to the contact with the corona electrode High voltage of e.g. B. approx. 100 kV ions are generated in the air, which electrostatically charge the powder particles. The powder particles are negatively charged here. Usually become electrostatic Charging high voltage values in the range of 30 to 110 kV, preferred from 40 to 90 kV and very particularly preferably from 50 to 80 kV. The current is preferably from 20 to 75 mA, preferably from 25 to 60 mA. The amount of powder transported in the air stream can range from 0.1 to 100 g / min. The for the promotion of the conveying air used preferably has a pressure of above 0 to 4 bar, preferably from 0.5 to 1.5 bar.

The powder particles are separated by two different force effects to the surface of the with a self-cleaning surface workpiece to be equipped. On the one hand, the transport takes place due to aerodynamic forces the flow velocity from the gun muzzle escaping spray. On the other hand, it is transported by electrical forces. Between the spray gun and the workpieces there is a potential difference (model of a capacitor), in which accelerates the electrically charged particles. The on the workpiece impinging powder particles give because of their very low conductivity their load only slowly, usually within hours. Influence causes opposing parts of the same size to appear on the workpiece surface Charges generated, which cause the hold due to coloumb attraction. The powder particles remain even after the particles have been discharged due to van der Waals forces on workpiece be liable. The one from the spray gun to the workpiece flowing Air ion flow causes such a strong charge on the separated Powder layer that the breakthrough field strength for the air was exceeded even with thin layers becomes. The unloading operations ionize the air between the powder particles and lead to a positive internal current, the so-called counter corona, to the negative Corona electrode. Newly arriving, negatively charged powder particles are unloaded and no longer adhere to the workpiece, causing another Increase in layer thickness is prevented. With corona discharge two forces act. The electrostatic repulsion among the negatively charged particles and the attractions of the positively charged workpiece. As long as the positive attractions predominate, the layer builds up on. But if the repulsive Forces under dominate the particles, no further particles are stored.

In addition to the use of powder spray guns with external high-voltage electrodes, powder spray guns can also be used, which provide the necessary electrostatic charging of the powder can only be achieved through triboelectric processes. In this process, the powder particles are positively charged by friction. In triboelectric spray guns, the layer thickness is only limited due to the electrostatic repulsion from the already deposited powder layer.

Powder spray guns can be used, how to buy them are to be acquired. In larger industrial plants become common equipment used up to twelve Have automatic or manual pistols. Manufacturer of such systems or suitable powder spray guns is for example the company Nordson Deutschland GmbH in Erkrath.

Such particles can be used as which is at least a non-conductive Material selected from silicates, minerals, metal oxides, silicas, pigments or polymers exhibit. Can be particularly preferred the particles silicates, doped silicates, minerals, metal oxides, Alumina, silicas or fumed silicates, aerosils or powdered polymers, e.g. spray-dried and be agglomerated emulsions or cryomilled PTFE. Preferably particles are used which have hydrophobic properties. Especially silicas are preferably used as hydrophobic particles.

Particles are preferably used which an average particle diameter of 0.02 to 100 μm, particularly preferably from 0.01 to 50 μm and very particularly preferably have from 0.1 to 30 μm. But are suitable also particles that are made up of primary particles Store together to form agglomerates or aggregates with a size of 0.2 to 100 μm.

It can be advantageous if the particles used have a structured surface. Particles which have an irregular fine structure in the nanometer range, that is to say in the range from 1 to 1000 nm, preferably from 2 to 750 nm and very particularly preferably from 10 to 100 nm, are preferably used on the surface. Fine structure is understood to mean structures which have heights, widths and distances in the areas mentioned. Such particles preferably have at least one compound selected from pyrogenic silica, precipitated silica, aluminum oxide, silicon dioxide, pyrogenic and / or doped silicates or powdered polymers. The particles with the irregular fine structure preferably have elevations with an aspect ratio of greater than 1, particularly preferably greater than 1.5. The aspect ratio is defined as the quotient from the maximum height to the maximum width of the survey. In 1 is shown schematically what should be understood by surveys, which are formed by the fine structure of the particles. The figure shows a particle P which has elevations. A selected elevation E, which is present on the particles due to the fine structure of the particles, has an aspect ratio of 2.5, calculated as a quotient from the maximum height of the elevation mH ', which is 2.5 and the maximum width mB', which is 1 in proportion.

The hydrophobic properties of the Particles can be inherently present by the material of the particles, such as for polytetrafluoroethylene (PTFE). But it can also be hydrophobic particles be used after a suitable treatment hydrophobic Have properties such as with at least one connection from the group of alkylsilanes, fluoroalkylsilanes or disilazanes treated particles. Particularly suitable particles are hydrophobicized pyrogens silicas, so-called aerosils. example for hydrophobic particles are e.g. the Aerosil VPR 411 or Aerosil R 8200. Examples of by treatment with perfluoroalkylsilane and subsequent annealing Hydrophobizable particles are e.g. Aeroperl 90/30, Sipernat silica 350, Aluminum oxide C, zirconium silicate, vanadium-doped or Aeroperl VP 25/20.

As particles, especially as particles, which is an irregular fine structure in the nanometer range on the surface are preferred such particles are used which have at least one compound selected from pyrogenic or precipitated silica, aluminum oxide, Silicon oxide or powdery Have polymers. It can be advantageous if the used Particles have hydrophobic properties. Particularly suitable as particles, among other things, hydrophobicized fumed silicas, so called aerosils.

It can be beneficial if particles are used that have hydrophobic properties. The hydrophobic Properties of the particles can inherent to the particle material used. But it can hydrophobic particles are also used, e.g. by a Treatment with at least one compound from the group of the alkylsilanes, Perfluoroalkylsilanes, paraffins, waxes, fatty acid esters, functionalized long chain alkane derivatives or alkyl disilazanes, hydrophobic properties exhibit.

It may be advantageous to use the surfaces that are the surface structure have been equipped, subsequently to hydrophobize again. This can be done by treating the surfaces with those for the hydrophobization of the particles specified compounds take place.

The method according to the invention can be used to produce self-cleaning surfaces which preferably have elevations formed from particles, the elevations being at a distance of 20 nm to 100 μm and a height of 20 nm to 100 μm. The coating formed by the particles preferably has a layer thickness of 20 nm to 120 μm, particularly preferably from 0.1 to 50 μm and very particularly preferably from 1 to 20 μm. It should be noted that the coating does not have a continuous coating but must be understood as coating in the sense of the present invention, particles applied to the surface, wherein the individual particles can be present on the surface at intervals of 0 to 10 particle diameter, in particular at intervals of 0 to 3 particle diameter.

The surfaces according to the invention preferably have at least a covering with elevations with an average height of 20 nm to 25 μm and one average distance from 20 nm to 25 μm, preferably with an average Height of 50 nm to 10 μm and / or an average distance of 50 nm to 10 μm and entirely particularly preferably with an average height of 50 nm to 4 μm and / or an average distance of 50 nm to 4 μm. Very particularly preferred have elevations on the surfaces according to the invention with an average height of 0.25 to 1 μm and an average distance of 0.25 to 1 μm. Under the middle distance For the purposes of the present invention, the elevations become the distance the highest One survey to the next highest Survey understood. Has an elevation in the shape of a cone so represents the top of the cone the highest Collection of the survey. If the survey is one Cuboid, the top surface of the cuboid was the highest elevation of the survey.

The wetting of bodies and the self-cleaning property can be determined by the contact angle, that a drop of water forms with the surface. On Contact angle of 0 degrees means complete wetting of the surface. The The static contact angle is usually measured using Devices, where the contact angle is determined optically. On smooth hydrophobic surfaces are common static contact angle of less than 125 ° measured. The present self-cleaning surfaces have static contact angles of preferably greater than 130 °, preferably greater than 140 ° and entirely particularly preferably greater than 145 °. It was also found that a surface only has good self-cleaning properties if these has a difference between the advancing and retreating angle of at most 10 °, which is why surfaces according to the invention are preferred a difference between the advance and retreat angles of less than 10 °, preferably less than 5 ° and very particularly preferably have less than 4 °. For the determination of the At a progressive angle, a drop of water is placed on the needle surface set and by adding water through the cannula of the drops on the surface increased. During the Magnification slides the edge of the drop over the surface and the contact angle is determined to advance angle. The retreat angle is measured on the same drop, only through the cannula the drop Deprived of water and during of reducing the drop of the contact angle measured. The difference between both angles is called hysteresis. The smaller the difference is, the less the interaction of the water drop with the surface the pad and the better the lotus effect.

The formed by the particles Surveys are only about electrostatic interactions or van der Waals forces on the Fixed surface. Despite these relatively weak forces, the particles adhere preferentially at least as hard on the surface, that a pulse of more than 12 mNs is required to remove the particles from the surface replace. It is obvious that such a self-cleaning surface Textiles do not survive a normal wash cycle.

The surface according to the invention can be a surface of a Textile, a film, a three-dimensional object, one Membrane, a textile / fleece web, a truck tarpaulin, a hygiene fleece, a diaper or other household appliance.

The method according to the invention can e.g. to Coating objects, exposed to high levels of dirt and water, especially for the outdoor area, skiing, alpine sports, motor sports, motorcycle sports, Motorcross sports, sailing, textiles for the leisure sector as well for coating technical textiles, selected from tents, awnings, umbrellas, tablecloths, Convertible tops, technical textiles or work clothes are used become.

Objects with a surface according to the invention can e.g. Foils, everyday objects, Sporting goods, textiles, clothing and advertising media, awnings include.

Surfaces produced according to the invention are shown in the figures 1 to 3 explained in more detail without the invention being restricted to these special embodiments.

The figure 1 schematically shows a particle P which has elevations. A selected elevation E, which is present on the particle due to the fine structure of the particles, has an aspect ratio of 2.5, calculated as a quotient from the maximum height of the elevation mH ', which is 2.5 and the maximum width mB', which is 1 in proportion.

2 shows a scanning electron microscope (SEM) image of a polypropylene fleece coated with Aerosil R 8200.

3 shows a SEM image of the polypropylene fleece coated with Aerosil R 8200 2 in a tenfold magnification. The particles applied to the fiber are very easy to see.

The process according to the invention is described by way of example using the following examples ben, without the invention being limited thereto.

Example 1:

A PP fleece (VS2050, Freudenberg) with 50 g / m ² is placed in an electrostatic coating chamber (type Surecoat, Nordson). The following parameters were selected for the electrostatic coating system:
Atomizing air 0.5 bar
Gun supply 1 bar
Vortex air 1 bar
Current 25 mA at 40 kV
Particles used: Aerosil R 8200

The Aerosil was directly on the Lying fleece applied and the pistol was fired at a speed of approx. 6 m / min led the surface. The supernatant Aerosil was made using an electrically uncharged metal roller, the above the treated fleece led was picked up.

The behavior of the treated fleece was then characterized. The treated fleece showed a very good lotus effect. Water droplets dripped off very well. The roll angle, i.e. the angle to the horizontal at which a drop rolls off independently, was 2.4 ° for a 60 μl drop of water. The water penetration through the fleece thus finished only took place when the water column built up exceeded 30 cm (measured according to DIN EN13562). 2 shows a SEM image of the nonwoven coated in this way.

Example 2:

The parameters of the coating system were as set in Example 1. However, the was changed The speed at which the pistol was moved over the fleece. The speed here was only 2 m / min. As in Example 1 here too the Aerosil is applied directly to the surface. The subsequent one Characterization of the surface resulted in a very good roll angle of 1.8 °, which is 0.6 ° smaller was as in example 1. A water column of 60 cm.

By slowing down the application the particle was able to significantly improve the self-cleaning effect, characterized by the roll angle. This is probably because the fact that with a slower application speed a more even one and denser particle layer can be generated.

Claims (16)

Process for the production of surfaces with self-cleaning properties by applying particles to the surface, whereby elevations which are at a distance of 20 nm to 100 μm and a height of 20 nm to 100 μm are formed, characterized in that the application of the particles is dry by electrostatic spraying of the powdery particles using a powder spray gun. Method according to claim 1, characterized in that the surfaces are a material selected from Has plastics, paints, wood, glass or metals. Method according to claim 1 or 2, characterized in that the surface of the Surface of a Foil, a three-dimensional object, a membrane or is a textile / nonwoven web. Procedure according to at least one of the claims 1 to 3, characterized in that a powder spray gun with outside High voltage electrodes is used. Procedure according to at least one of the claims 1 to 4, characterized in that the electrostatic powder spray gun a high voltage of 30 to 90 kV for charging the powder particles starts. Procedure according to at least one of the claims 1 to 5, characterized in that particles that have an irregular fine structure have in the nanometer range on their surface. Procedure according to at least one of the claims 1 to 6, characterized in that particles that have a medium Have particle diameters from 0.02 to 100 microns can be used. Method according to claim 7, characterized in that particles having an average particle diameter from 0.1 to 30 μm have, are used. Procedure according to at least one of the claims 1 to 8, characterized in that particles selected from Silicates, minerals, metal oxides, silicas, pigments or polymers, be used. Procedure according to at least one of the claims 1 to 8, characterized in that particles selected from fumed silicas, Precipitated silica, aluminum oxide, Silicon dioxide, doped silicates, pyrogenic silicates or powdered polymers, be used. A method according to at least one of claims 1 to 10, characterized in that the particles by treatment with at least one Compound from the group of alkylsilanes, fluoroalkylsilanes and / or disilazanes with hydrophobic properties. Self-cleaning surface, produced by means of a method according to at least one of claims 1 to 11th surface according to claim 12, characterized in that they have elevations formed from particles have, the surveys a distance of 20 nm to 100 microns and a Height of 20 nm to 100 μm exhibit. surface according to claim 12 or 13, characterized in that the surface is a surface of a Textile, a film, a three-dimensional object, one Membrane or a textile / nonwoven web. Use of the method according to one of claims 1 to 11 for coating objects, exposed to high levels of dirt and water, especially for the outdoor area, skiing, alpine sports, motor sports, motorcycle sports, Motorcross sports, sailing, textiles for the leisure sector as well for coating technical textiles, selected from tents, awnings, umbrellas, Tablecloths, convertible tops, technical textiles or work clothes. objects with a surface according to one of claims 12 to 14, characterized in that the objects are foils, Utensils, Sporting goods, textiles, clothing and advertising media.