WO1994008068A1 - Pretreatment of plastic components for electrostatic enameling - Google Patents

Abstract

The invention pertains to a method of pretreating plastic components for electrostatic enamelling by applying a thin film of an electrically conductive material in a plasma atmosphere.

Description

Pretreatment of plastic parts for electrostatic painting

description

The present invention relates to a method for pretreating plastic parts for electrostatic painting and a method for electrostatic painting.

The principle of electrostatic painting is based on charging the paint particles to be applied to a component in an electrical field. For this purpose, the electrostatic lacquer application device is placed at a generally negative high-voltage potential, while the component to be lacquered represents the (grounded) counter electrode. With a suitable spray device, e.g. sharp-edged bell-shaped rotating bodies, sharp-edged rotating disks or pressure guns, the paint particles to be applied are atomized into the electric field and charged. Because of their charge, the component to be coated causes an electrostatic attraction. Since the movement path of the particles essentially corresponds to the electrical field lines opening onto the component, the paint particles hit the component to be painted with a high probability. In comparison to conventional painting processes, the overspray in electrostatic painting is therefore greatly reduced. In this way, significant advantages can be achieved in painting systems with regard to an increase in paint utilization, a reduction in painting time, a reduction in paint sludge, a reduction in the exhaust air and a reduction in the return rate. Electrostatic painting processes have therefore already been widely used in painting metallic components.

In contrast, the use of electrostatic painting processes on plastic components still poses great problems. The reason for this lies in the very low electrical conductivity of the plastic surface, which prevents the formation of a field line density that is as high and uniform as possible and the derivation of the electrical charge impinging on the component. As a result, the plastic component is charged, which causes subsequent paint particles to be electrostatically repelled and thus in turn drastically increases the overspray.

Various methods for improving the electrostatic paintability of plastics are already known. For example, the electrical conductivity of plastics can be increased by suitable additives such as graphite, carbon black or stainless steel fibers. As a rule, however, this equipment leads to a significant increase in the cost of the plastic or to an intolerable impairment of its mechanical properties or its visual appearance. This type of conductivity equipment for plastics can therefore not be used in the majority of painting applications.

It is also known that the electrostatic paintability can be improved by depositing the plastic parts with an electrically conductive material. The deposit leads to the desired formation of a high density of electrical field lines which penetrate the plastic part and ensure the impact of the paint particles. In the case of painting step by step, the electrical charge can be discharged through the freshly applied paint, although good electrical contact between the paint and the conductive mask must also be ensured. Since the conductive mask covers part of the component, the process is only suitable for the partial painting of plastic parts. In addition, complicated geometries, e.g. in the case of plastic parts with ribs, problems with regard to the formation of the desired field line density were observed.

The electrostatic paintability of plastics can also be improved by applying a conductive solution. The lead solution usually contains quaternary ammonium salts or amines. It is usually applied inline with the paint using the usual spraying methods. The guiding solution must have good compatibility with both the plastic and the paint system, with a slight dissolving of the plastic being desirable in order to achieve good paint adhesion. However, these compatibility requirements restrict the use of control solutions to selected plastics and coating systems. In addition, the application of the master solution is associated with an undesirable increase in the solvent emission in the paint shop.

A frequently used method for improving the electrostatic paintability is the application of primers which are made electrically conductive by additives such as graphite, metal salts or metal particles. With this method too, the conductivity or antistatic equipment can impair the optical and sometimes also the mechanical properties of the plastic. The production of thin layers by plasma-assisted chemical vapor deposition is known (see Kirk-Othmer, Encyclopedia of Chemical Technology, Third Ed., P. 47f.-, Wiley Intererscience, John Wiley and Sons, New York, 1982) .

It is also known to produce thin layers by sputtering in an argon plasma (cf. Kirle-Othmer, Third Ed., P. 692f.). This technology is mainly used for the production of magnetic hard disks.

It was an object of the present invention to find a process which can be used economically for a broad class of plastic substrates and coating systems, without influencing the optical and mechanical properties, and. ensures a high adhesive strength of the paint layer.

Accordingly, a process for the pretreatment of plastic parts for electrostatic painting has been found, which is characterized in that the plastic parts are processed in a plasma-assisted process with a thin, electrically conductive

Layer provides. Furthermore, a process for the electrostatic painting of plastic parts has been found, which is characterized in that the plastic parts are first provided with a thin electrically conductive intermediate layer in a plasma-assisted process and then a layer of paint is applied by electrostatic painting.

The electrically conductive, thin intermediate layer can be a metallic layer which contains at least one of the metals mentioned below: aluminum, copper, silver, gold, titanium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, Iron, cobalt, nickel, palladium or platinum. Furthermore, the electrically conductive intermediate layer can consist of carbon (graphite). Layers of copper or stainless steel made of, for example, VA 4 steel are preferred.

Furthermore, the electrically conductive thin layer can consist of conductive oxides which contain at least one of the following elements from the series indium, tin, cadmium or zinc or alloys of such elements. Mixed oxides of indium and tin are preferred, the tin content being 5 to 20 atom% of the indium content and oxygen not being stoichiometrically bound, for example In ₉ oSnιoO _x . The person skilled in the art knows how to adjust the oxygen content to achieve the desired conductivity via the process oxygen pressure.

The electrically conductive intermediate layer according to the invention should be as neutral as possible in color and have very good electrical conductivity.

The thickness of the intermediate layer should be in the range from 0.5 to 5000 nm, preferably 1 to 100 nm and particularly preferably between 5 and 20 nm.

The setting of the desired layer thickness and its chemical composition can be achieved by setting the process parameters such as process gas pressure, process gas composition, atomization performance, coating time. The person skilled in the art knows how to adjust the individual parameters.

In order to improve the electrostatic paintability of plastics, the intermediate layer does not necessarily have to form a closed film. For example, the coating can also be in the form of islands which do not cover the entire area but are interconnected or form a rather irregular network.

Furthermore, the electrically conductive layer can be applied as a so-called gradient layer, i.e. by varying the process parameters accordingly, the chemical composition within the layer can be changed or a different morphology, for example different crystallites, can be achieved.

The intermediate layer can be applied by various methods known per se. Both physical vapor deposition (PVD = Physical Vapor Deposition) and chemical vapor deposition (CVD = Chemical Vap. Dep.) Can be considered.

One method is the sputtering of a metallic or oxidic target in a low-pressure plasma. Various method variants of cathode sputtering, such as magnetron sputtering, direct current sputtering (DC sputtering), radio frequency sputtering (RF sputtering), bias sputtering or reactive sputtering, and combinations thereof, are suitable for producing the layers. In magnetron sputtering, the target to be atomized is located in an external magnetic field, which causes a high plasma density in the region of the target and thus an increase in the atomization rate. With DC or RF sputtering, the excitation of the Dust plasma in a manner known per se by DC or RF generators. In the case of bias sputtering, the molded body to be coated is usually subjected to a negative bias (bias), as a result of which the component to be coated is exposed to an intensive interaction with the plasma.

The targets to be atomized can either be in the form of flat plates, cylindrical hollow bodies or in the form of flat or three-dimensionally shaped sheets or nets. The component to be coated is arranged so that the most uniform possible coating is achieved. Several targets can be used to improve the homogeneity of the coating. It is also possible to move the object to be coated during the treatment. .

Cathode sputtering is carried out in a plasma-assisted manner in a noble gas plasma, usually made of argon, but also from other noble gases such as He, Ne, Kr or Xe.

The application of the electrically conductive layer is usually carried out at pressures in the range from 10 " ⁴ to 10 ^-1 , preferably 10 ^-3 to 5 x 10" ² mbar. The temperatures during the treatment are in the range from 20 to 150 ° C.

Oxidic layers are usually produced by sputtering appropriate oxide targets, or metal targets in a reactive plasma which contains oxygen or gases containing oxygen.

Alternatively, the intermediate layer according to the invention can be applied by sputtering a metallic or oxide target using an arc plasma. It is also conceivable to atomize the metallic or oxidic target in a laser-induced plasma.

Finally, plasma-assisted vapor deposition processes can also be used to produce the intermediate layers, with hot metal vapors being condensed on the substrate.

The intermediate layer can also be applied by plasma-assisted chemical vapor deposition of an organometallic compound such as metal carbonyls, for example nickel tetracarbonyl, iron pentacarbonyl or metal alkyls, metal alkenyls, metal alkynyls, aromatic metal complexes or substituted metal complexes. A gas mixture which contains at least one corresponding organometallic compound is let into the plasma treatment chamber. The plasma conditions are set so that in the plasma area there is a decomposition of the organometallic compound with simultaneous formation of a metallic or oxidic coating on the plastic component. In addition to the organometallic compound, the gas mixture can contain oxygen or nitrogen and noble gases such as helium, neon, krypton, xenon or, preferably, argon. Due to the excellent splitting ability of the plasma, the plasma-assisted CVD deposition is particularly suitable for the uniform coating of components with complicated geometries. The deposition of the metal-organic starting compound can take place both in vacuum and at atmospheric pressure. A vacuum deposition at a pressure of 10 ^-3 to 10 mbar is preferred.

Either direct current (DC), high frequency (RF, e.g. 13.56 MHz) or microwave generators (frequency e.g. 2.45 GHz) can be used for the electrical supply of the plasma. The DC voltage is fed into the plasma chamber in a known manner via a round, rod-shaped, cylindrical, box-shaped or insulated electrode provided with another suitable geometry. The RF voltage is supplied in a comparable manner, but an electrical tuning unit between the generator and the electrode is used to maximize the coupled-in and to minimize the reflected electrical power. The microwave excitation which can be used alternatively is carried out in a known manner without electrodes, with hollow or coaxial conductors being used outside the coating space to supply the electrical power. Horn antennas or rod antennas, for example, can be used to feed the microwave into the plasma area.

The treatment time for applying the thin electrically conductive intermediate layer is in the range from 0.1 to 1000 s, preferably 1 to 20 s.

Preferred methods for applying the intermediate layer are plasma-assisted cathode sputtering and plasma-assisted chemical vapor deposition, with RF bias cathode sputtering being preferred.

Thermoplastic, thermosetting and thermosetting plastic parts are suitable for the pretreatment process according to the invention, which can contain additives for mechanical reinforcement, coloring, heat stabilization, UV stabilization, fire protection equipment or other additives. The method is preferably used for the improvement of the electrostatic paintability of homopolymers, copolymers or blends of polymers from the series polyethylene, polypropylene, polyvinyl chloride, polystyrene, polymethyl methacrylate, polycarbonate, diethylene glycol dialyl carbonate (CR 39), styrene / butadiene Copolymer, acrylonitrile / butadiene / styrene polymer, polyethylene terephthalate, polybutylene terephthalate, polyoxymethylene, polyamide, polysulfone, polyether sulfone, poly (aryl) ether ketone, polyimide or polyurethane. The method is particularly preferably used for blends of polypropylene and an ethylene-propylene diene copolymer. The electrical surface conductivity of the plastics mentioned is significantly increased by the method according to the invention, which ensures the formation of a high field line density and the discharge of the electrical charge in the electrostatic coating. The intermediate layers used show very good adhesive strength on the plastic substrates mentioned. If necessary, the adhesive strength of the intermediate layer can be additionally improved by a previous plasma treatment of the carrier, for example by a low-pressure plasma treatment in an oxygen-containing gas mixture.

The electrostatic painting of the parts equipped according to the invention can be carried out in a known manner

- Rotation bell systems

Disc systems

air-free or compressed air operated gun systems

respectively.

The method is suitable for

- Lacquers based on chemically crosslinking binder systems such as Polyurethanes, polyacrylates

Paints based on a physically drying binder system such as polyurethane dispersions or polyacrylate dispersions. Both paint systems based on organic solvents (eg esters, ketones, glycol ethers, aromatics) and water-based paints are suitable. Lacquers with a high solids content ("high solids") and powder lacquers such as, for example, epoxy polyester, epoxy polyester hybrid, polyurethane, acrylate, polyester triglycidyl isocyanate lacquers can also be used.

In this case, multi-layer structures consisting of

- primer / filler / top coat - filler / top coat

getting produced.

The plastic parts treated according to the invention can be used as

Components in the automotive exterior such as bumpers, flaps, doors, spoilers, side panels, ram protection strips, wheel covers, spreading discs,

Components in the automotive interior such as steering column covers or phono housings

Household items

Housing in consumer electronics.

The substrates used in the following examples are injection-molded sheets (15 × 15 cm ² ) made from a blend of polypropylene and ethylene-propylene-diene copolymers, with an ethylene content of 15 to 20% by weight. , a rubber content of approx. 30% by weight and a melt flow index according to DIN 53735 of 2.5 g / 10 min at 230 ° C / 2.16 kg.

example 1

A plate-shaped copper target (0 200 mm) was attached to a magnetron cathode in a cathode sputtering system. In parallel to the target, an injection-molded plastic plate (150x150 mm ² ) would be attached to a metallic holder (0 700 mm) at a distance of 60 mm. The system was evacuated to a pressure of 10 ^"7 mbar. Thereafter, argon was introduced into the system and a pressure of 8xl0 ^{~ 3} mbar was set. An AC voltage with a frequency was applied to the Cu target with the aid of a radio frequency generator of 13.56 MHz, with a power output of the generator of 200 W. An alternating voltage was also applied to the metallic holder of the plastic plate the frequency applied 13.56 MHz, with a generator power of 500 W. The treatment time of the plastic plate was 10 s.

The treated plastic plate was coated with an aqueous primer based on epoxy resin and polyamine components by means of electrostatic high-speed rotation (layer thicknesses approx. 20 μ).

For comparison, untreated plastic plates were coated analogously.

After the paint had dried, the application efficiency, i.e. the ratio of the amount of paint applied to the atomized amount of paint is determined by measuring the weight of the two plates.

The application efficiency of the treated plate was 2.5 times greater than that of the untreated plate.

Analogously, a treated or an untreated plastic plate was coated with a solvent-based metallic basecoat and the application efficiency was determined. The treated plates had a higher application efficiency by a factor of 2.6.

Example 2

A plate-shaped target (0 150 mm) made of I goSnio was attached to a magnetron cathode in a cathode sputtering system. In parallel to the target, an injection-molded plastic plate (150x150 mm) was attached to a metallic holder (0 700 mm) at a distance of 90 mm. The facility was opened

10 " ⁷ mbar. Thereafter, argon and oxygen were at a ratio of 74.1% by volume Ar to 25.9% by volume 0 ₂ (at an argon flow of 40 nm [standard eubie cm / min], and a 0 ₂ -Flow of 14 seem) and a pressure of 5xl0 ^-3 mbar set.

Then a DC voltage (power of 500 W) was applied and the IngoSnio target was atomized. The treatment time of the plastic plate was 2 minutes.

Boards treated in this way were each coated with an aqueous primer or a metallic basecoat, as in Example 1. After drying, the application efficiency was determined. Compared to untreated plates coated in a similar manner, the treated plates had a coating efficiency which was higher by a factor of 2.7 (coating with aqueous primer) or 3.7 (metallic base lacquer).

Claims

Claims 1. A process for pretreating plastic parts for electrostatic painting, characterized in that the plastic parts are provided with a thin electrically conductive layer in a plasma-assisted process. 2. The method according to claim 1, characterized in that the thin electrically conductive layer consists of a metal. 3. The method according to claim 1, characterized in that the thin electrically conductive layer consists of an oxide which contains at least one element selected from the group consisting of In, Sn, Cd and Zn. 4. The method according to any one of claims 1 to 3, characterized gekenn¬ characterized in that the application of the thin layer is carried out by plasma-assisted sputtering. 5. The method according to any one of claims 1 to 3, characterized gekenn¬ characterized in that the application of the thin layer is carried out by plasma-assisted chemical vapor deposition. 6. Use of plastic parts treated according to the method according to one of claims 1 to 5 as substrates for electrostatic painting. 7. A process for the electrostatic painting of plastic parts, characterized in that the plastic parts are first provided with a thin electrically conductive intermediate layer in a plasma-assisted process before the electrostatic painting. 8. Object coated by electrostatic painting, obtainable according to the method of claim 7.