DE10064520A1 - Method for producing structured surface with self-cleaning effect on vehicles, comprises producing micro- or nano-structure on carrier by anodic oxidation and transferring this structure to the surface - Google Patents

Abstract

Method for producing a structured surface with a self-cleaning effect comprises producing a micro- or nano-structure on a carrier by anodic oxidation and transferring this structure to the surface.

Description

The present invention relates to a method for producing micro or nanostructured surface structures, the so-called self-cleaning show effect. It can be used in particular for the surface coating of components Find modes of transport.

If this application speaks of nano- or microstructure, it is included always understand that dimensions in at least one structural dimension Nano or micro range are available.

Surfaces or coatings of parts or attachments of modes of transport (Motor vehicles, airplanes, rail-bound vehicles, etc.) must be the highest Requirements with regard to corrosion protection, scratch resistance, appearance, color exercise, chemical resistance and resistance to other environmental influences such as B. Bird droppings are sufficient. On the one hand, this places the highest demands on the materials, on the other hand, due to the high competitive pressure in the Automotive manufacturing processes of parts, the coating material lien and its application under great cost pressure.

The use of e.g. B. moldings and surface produced by chemical etching Chen in the production of decorative, grained surfaces of plastic parts len is well known. The structure sizes that can be achieved are in the upper range µm to mm range.

Structured bodies of steel structured in this way for producing self-cleaning Surfaces restrict the surface materials that can be used for self-cleaning surfaces  in the sense of the occurrence of the self-cleaning effect on for Non-usable coating materials or plastic masses on.

There are self-cleaning surfaces based on the so-called lotus effect made of structured materials with a low surface energy.

The surface energy of the materials used must be so low that a Drops of water on a smooth surface of the material have a contact angle of has more than 90 °.

The lower the surface energy, the worse the surfaces become also due to lower-occurring substances in technology and in nature Surface energy, such as B. waxes, oils, tree resins.

The structure of surface materials with low surface energies for self-cleaning surfaces must be such that

• 1. The contact area for drops with the surface material is minimized (solid-water interface) and the amount of air enclosed between the micro- or nanostructures is maximized (water-air interface). This leads to a quantitatively calculable increase in the contact angle.
  These conditions do not correspond to the increase in the roughness of surfaces. On the contrary, an increase in roughness can U. lead to a drastic deterioration or loss of self-cleaning. Conversely, water drops on surfaces with structures that by definition have a low roughness can have large contact angles. (J. Bico et al. Europhysics Letters; 47 (1999), 220).
• 2. The interaction of dirt particles with the surface if possible is minimized.

A characteristic of self-cleaning surfaces recognized by experts is a what drop, the one that is on this surface when rolling off the surface Dirt and dust particles are carried away. Self-cleaning only occurs after contact angle for water of about 130 °.

The condition of low surface energy is best of all Materials met, whose surface energy is less than 30 mN / m. As for traffic suitable materials for coatings or the production of attachments thus come z. B. polypropylene, polyethylene, fluorinated polyethylene copolymers, Polyvinyl fluoride or other fluorinated plastic materials.

There are a large number of methods for making them extremely hydrophobic Known surfaces. All publications have in common that self-cleaning is not described per se.

In CH 268 258 A a method is described in which fine particles such. B. Clay particles are made hydrophobic and by a curable organosilicone resin fixed on a surface.

US 3,354,022 relates to structured water and oil repellent surfaces chen and their manufacture. The structures are created by embossing processes. As Embossing tools are hollow fibers fixed on one side or with holes Nickel plates (mesh plate) are used.

EP 0825 901 B1 describes a thin pigment layer of the organic pigment C. I. Pigment Red 149 in vacuum on a metallized polyimide film and recrystallized by heating to 220 ° C. Discrete whiskers form which is vaporized with a thin metal film (Pd, Pt, Ag or Au) and then be made hydrophobic by a monomolecular fluoroalkylthiol film.

Furthermore, a large number of writings are known, in which a plasma is used Parts of surfaces are etched away or exposed. (EP 476 510 A1 or US 5,693,236).  

The self-cleaning effect itself is described in various publications ben.

DE 198 60 137 A1 discloses a method with which ultraphobic surface be made on the basis of structured aluminum. For this, the Surface of aluminum with a periodic microstructure with a roughness depth from 1 to 1000 µm, after anodizing and calcining with an adhesive middle layer and then with a hydrophobic or oleophobic Provide coating. The disclosure is only on aluminum surfaces limits. The specialist in the field of automotive painting or coating knows, however, that surfaces manufactured in this way meet the requirements with regard to Corrosion protection, long-term resistance (UV or Florida test, salt spray tests etc. pp.) are not sufficient.

WO 96/04123 describes processes for the production of microstructured described self-cleaning surfaces on objects. These surfaces have an artificial surface structure of elevations and depressions. The distance between the elevations and depressions should be in the range of 5 up to 200 µm and the height of the elevations are in the range from 5 to 100 µm and the elevations are made of hydrophobic or durable hydrophobic materials hen.

In DE 198 03 787 A1 u. a. for shaping or structuring surfaces pouring, injection molding and other processes with hydrophobic properties give. Corresponding negative forms of the desired structure are required for this Lich. To produce these negative forms, for. B. specified the league technique. According to this method, several masks must first be passed through electron beam lithography graphie are produced, which then for exposure of a photore serve by deep x-ray lithography, creating a positive shape arises. Then the gaps in the photoresist are replaced by galvanic  Deposition of a metal filled, so that a metal structure as a negative form of desired structure arises.

The process described here is complicated and costly. This process is especially for the production of large negative molds with nanostructured ones Surfaces, especially for three-dimensional bodies, very complex and therefore to generate modes of transport components with structured, easy to cleaning surfaces not usable.

All of the methods described so far have in common that they are suitable for Requirements for coatings in the area of modes of transport not industrial are usable.

The anodic oxidation of aluminum has been a process that has been described for a long time. For example, a large number of references to the production of anodic Oxidized sheet materials and their use in the production of Offset printing plates are known, e.g. B. EP 0 139 111 B1. For the printing plates they are Wettability with the printing inks and the resistance to the developer decisive. An average surface roughness Rz of 1-15 µm is applied roughened surface specified. The non-directional roughening of the surface serves the increase in wettability and does not have the function of a targeted one Structuring in the nanometer range.

The object of the invention is an industrially usable, inexpensive method for the production of micro- and especially nanostructures on surfaces to be specified which have self-cleaning properties.

This object is achieved with the subject matter of patent claim 1. advantageous Comments are the subject of subclaims.

According to the present invention are used to manufacture self-cleaning Surfaces on the surface of an auxiliary carrier (e.g. a molding tool) Negative structures in the micro or nanometer range in one or two process steps  generated. The micro or Na is then transferred structure on the substrate to be structured.

To create the finest regular structures on large surfaces primarily suitable methods based on self-organizing mechanisms based. According to the known anodic oxidation of Aluminum and other metals used. It can be used for structures with very regular cylindrical pores.

The achievable structural dimensions, i.e. distance and diameter of the pores, are between about 10 nanometers and 1 micrometer, i.e. in the range in which the effects of self-cleaning occur.

The substrate whose surface is to be structured can be foil or act like layered bodies. After surface structuring can so that the desired components or attachments are coated or coated. It is also possible to structure the film-like or layer-like substrate on the one hand and the coating of the component with this substrate in one process step (for smooth surfaces see e.g. EP 0 123 374 B1, EP 0320 925 B1).

On the other hand, it is also possible that with the method according to the invention the surface of a three-dimensional component is structured directly.

The condition of low surface energy is best of all Materials whose surface energy is less than 30 mN / m are met. As for traffic particularly suitable materials for coatings or components thus z. B. polypropylene, polyethylene, fluorinated polyethylene copolymers, polyvinylfluor oride or other fluorinated plastic materials.

As the examples below show, by choosing the electrolyte and other anodizing parameters (e.g. anodizing voltage, bath temperature and anodizing time)  the surface structure to be generated with regard to structure diameter, the Structure depth and the number of structural elements per unit area can be set.

Examples of electrolytes are sulfuric acid, phosphoric acid and oxalic acid or other known acids in question. The type of electrolyte to use also depends on the material composition of the to be structured Tool.

The in the electrolyte bath between the tool and a suitable cathode anodizing voltage can be in the range of 10 to 200 V. The temperature in the electrolyte bath is between 0 and 30 ° C. The anodizing time can be of a few Minutes to several hours.

The tool surfaces can be smoothed before the actual anodizing step. This can be done either by mechanical polishing of the surfaces or chemical electropolishing. As an electropolishing bath comes e.g. B. a mixture of 25: 75 parts by volume of chloric acid and ethanol for use. The average roughness of the polished surface should be about 3 nm over an area of 3 µm ² .

Methods have proven to be particularly advantageous in which the tool in a two-stage process is anodized, by varying the Para meters of electrolyte composition, anodizing voltage, bath temperature and anodi the cylindrical pore structure to be created on the tool can be influenced.

Before anodizing, it is advisable to use suitable tool surfaces Process to clean and degrease. After the first anodizing step, the generated porous oxide layer removed by a selective etching process. This can e.g. B. with a mordant consisting of phosphoric acid and chromic acid elevated temperature (e.g. 50-70 ° C). Despite the application of the etching process, residues of the oxide layer are retained, which are then during the the subsequent second anodizing step act as germs.  

The second anodizing step can be carried out with the same parameters that were already used in the first anodizing step, but are also other parameters possible.

One possibility is the subsequent widening of the pores knife in the upper area. For example, due to the short-term action a selective pickling agent (phosphoric acid and chromic acid) can be achieved that a pyramidal structure forms towards the base of the pores. This targeted pores expansion, d. H. Broadening of the pore diameter and tapering to the pores basically leads to the later transfer of the structure to a suitable substrate to a pyramidal whisker structure and thus to an increased mechanical Stability.

According to the specified 2-step process for anodic oxidation, the Use of 0.3 M oxalic acid as electrolyte at a bath temperature of 5 ° C applying a DC voltage of 40 V to an aluminum substrate into one Pore diameter of approx. 100 nm.

If larger pore diameters are required, z. B. through use of about 10% phosphoric acid as electrolyte at a bath temperature of about 3 ° C and an applied DC voltage of approx. 160 V pore diameter of up to 300 nm are generated. With an anodizing time of 10 minutes, the pore depth results of approx. 300 nm.

For the production of regular porous nano and micro structures on tools surfaces that are not metallic or consist of a metal that is not can be anodically structured, can be used as an auxiliary layer on the tool surface thin aluminum layer can be applied. This can e.g. B. by galvanic Deposition processes or can also be achieved by vapor deposition processes. The fat this auxiliary layer can be between approximately 100 nm and several μm.  

There are several options for transferring the micro and nanostructure to suitable substrates in the sense of a replication process:

• - hot stamping of a film in a continuous process;
• - Injection molding into a mold that is nano or micro structured Surface wearing, with a thermoplastic;
• - Filling a micro-structured tool shape or a calender with one Monomer and then polymerizing using chemi , thermal or UV starters and combinations;
• - Filling a micro-structured tool shape or a calender with one not or not fully cured or physically drying Be Layering compound (so-called in-mold coating compound, in-mold-coating com position), as described in EP 0 123 374 B1 for smooth surfaces ben and subsequent polymerization using chemical, thermal or UV starters and combinations;
• - In principle, a method according to EP 0320 925 B1, in which a Manufacturing process for molded plastic parts with a decorative embossed surface stratification is described. For this purpose, an embossed, thermoformable film is placed in inserted a tool that at least on parts of its inner surface che has a structure, and then injection molded. The one for Components for transport modes preferred micro and especially nanostructures tion (small wavelength visible light) by the in the present invention However, the tools described can only be replaced by a completely different one than in the Process design described in EP 0320 925 B1 and completely different materials properties of the thermoformed film can be achieved. Nano injection molding requires own developments regarding tool temperatures, process control and aspect ratio of the nanostructures to be molded. The to pre The surface of the film can be made of a thermoplastic polymer or from a single or multi-layer coating structure based on z. For example: polyesters, polyurethanes, acrylates and their derivatives, epoxy resins, Nitrocellulose, phenolic resins, melamine resins or naturally renewable Resins exist, which are usually used for the representation of organic be layers are used. The individual coating materials can  be filled with artificial or naturally occurring raw materials. The Additive composition can consist of co-crosslinking and non-reactive additives best impact materials based on halogen-containing or silicon-containing products hen. The curing can take place by means of radiation or thermally after or during of the embossing process and the molded nanostructure fixie Ren. The curing of the coating after removal from the Tool or within the dwell time of the film in the tool. As Another method is to cure the coating materials Start the above methods and the nanostructure during the curing process molding.

Of course, there are also the most varied ones known from plastics technology Variants of these basic procedures can be used. For example, the embossing process or the back injection is carried out with a partially crosslinked polymer, which is only after the detachment from the mold or the pressure matrix by irradiation polymerized out.

Tools made of aluminum alloys such as AlZnMgCu 1.5, steel standard number: 3.4565 tensile strength: 530 N / mm ² , or AlZnMgCu 0.5 steel standard number: 3.4345 tensile strength: 450 N / mm ² are particularly suitable for direct anodic oxidation. Likewise the wrought aluminum alloy WL3.1354 used in the aerospace industry, tensile strength 440 N / mm ² . All three materials meet the requirements for a molding insert in plastic processing, especially with regard to tensile strength, but also other parameters such as. B. Wear resistance and service life.

The samples for the production of plastic components with self-cleaning Surfaces were made on an injection molding machine from Krauss-Maffei, type KM 150-460 B2, with a max. Closing force of 1500 kN and a screw through knife of 45 mm.  

A first proof of the functionality of the nanostructured injection mold was made using polypropylene (PP), Novolen 1100 UC, from BASF carried out.

In the second spray test, an ASA / PC carrier film made from Luran S KR 2861/1 C with a UV-curing and a thermally cross-linking clear coat system coated and then the carrier film on the injection molding machine with Luran S KR 2861/1 C, manufacturer BASF, injection molded. The one brought into the injection mold The nano / micro structure should be applied to the surface of the un- / partially cross-linked clear coat are embossed.

For all samples (with PP, ASA / PC carrier film with varnish) were ge below named parameters varied in the specified ranges.

The contact angle between surface and water as an indication of the presence a self-cleaning surface was above. Embodiments in one Range from 130 ° -140 ° C.

Claims (11)

1. Process for the production of surface structures which show the self-cleaning effect, with the following process steps:

• - Generation of a micro or nanostructure on the surface of an auxiliary carrier by anodic oxidation;
• - Transfer of the surface structure of the subcarrier to the substrate to be structured.

2. The method according to claim 1, characterized in that the anodic Oxidation is carried out in two process stages, with the first th and second anodizing stage that at the first anodizing stage witnessed oxide layer is removed by selective etching. 3. The method according to claim 2, characterized in that in the second Anodization level compared to the first anodization level parameters can be changed. 4. The method according to any one of the preceding claims, characterized net that the auxiliary carrier consists of a metal or plastic. 5. The method according to any one of the preceding claims, characterized net that the auxiliary carrier has an additional Al auxiliary layer. 6. The method according to any one of the preceding claims, characterized net that the transfer of the structure of the subcarrier to the structure rende substrate by hot stamping, injection molding or deep drawing. 7. The method according to claim 6, characterized in that the hot stamping or injection molding with a monomer or partially cross-linked polymer  which polymerizes only after detachment from the molding tool be settled. 8. The method according to any one of the preceding claims, characterized is characterized by the fact that the substrate to be structured is film-like or layer-like forms is. 9. The method according to any one of the preceding claims, characterized net that the substrate to be structured is a three-dimensional component forms is. 10. The method according to any one of the preceding claims, characterized in net that the substrate to be structured from a material with a surface energy less than 30 mN / m, in particular polypropylene, polyethylene, fluorinated polyethylene copolymers, polyvinyl fluoride or other fluorinated Plastic materials. 11. Use of a method according to one of the preceding claims Surface coating of components for modes of transport.