EP0349749A1 - Use of a plasma pretreatment in order to enhance the adhesion of a subsequent second lacquer layer to be applied - Google Patents

Abstract

The use of a plasma pretreatment for painted components to increase the adhesiveness of a second coat of paint to be applied subsequently is proposed.

Description

The invention relates to the use of a plasma pretreatment to increase the adhesiveness of a second lacquer layer to be applied subsequently.

Plasma treatments for producing an adhesive surface have been known for some time. They are used to paint, print, or create a primer for subsequent bonding, especially those made from polyolefin. Such a method is described for example in DE-OS 36 38 719.

The plasma treatment can be carried out, for example, in so-called low-pressure plasma. According to this method, the parts to be treated are placed in a vacuum chamber and exposed to a process gas there. This process gas can be, for example, nitrogen, oxygen or an inert gas. The process gas is exposed to a high-frequency voltage field, which causes the process gas to ionize. In the plasma created by the ionization, the ions and radicals as well as ultraviolet radiation react with the surface of the part to be treated, whereby - depending on the type of process gas used - thin layers are removed or the molecules on the surface of the workpiece are crosslinked. This conversion improves the wettability of the surface, so that the adhesiveness of a layer applied thereon is improved.

The so-called corona pretreatment of plastics, in particular films and molded parts made of plastics, is also known. In the corona treatment, a high-tension field is generated and a corona spark discharge is generated towards the workpiece by means of an electrode. The resulting plasma is transferred to the surface of the workpiece to be treated and leads to an increase in wettability.

Compared to the first-mentioned plasma treatment, the corona treatment has the advantage that no vacuum container is necessary. Corona treatment can therefore be used to treat large workpieces or only areas of them without complex equipment. In the known corona pretreatment systems, the treatment is carried out by manually or automatically moving the electrode along the workpiece. The electrode radiates freely, so that a ground-leading counter electrode is not necessary.

The invention has for its object to show a new use for such plasma pretreatments.

The new use is that the plasma pretreatment is carried out on components which have already been painted and for which the adhesiveness of the paint surface is to be improved for a second coat of paint to be applied subsequently.

The known plasma pretreatments are used almost exclusively for the treatment of plastics in order to ensure good adhesion of paints, varnishes, adhesives or foams. In special cases, metal treatment has already been carried out.

In order to make layers of paint more adhesive for subsequent further painting, grinding or the application of an adhesion promoter is usually carried out. In most cases, grinding is preferred as a proven and economical working method. An adhesion promoter is therefore often only used if the areas to be treated are difficult to access for grinding or if the component to be treated is already installed and would have to be dismantled for grinding. Finally, profiled or grained surfaces cannot be treated by grinding, but only by an adhesion promoter if the surface structure is to be retained.

The grinding of workpieces made of soft polyurethane foam, for example, is complex since the workpiece can be easily pressed in during grinding. To make matters worse, highly elastic paints must be used for such workpieces, which on the one hand are difficult to sand, but on the other hand, without the sanding of the first coat, the required liability for the second coat is insufficient. This is especially true if a metallic paint is to be used for the second coat.

Some paint systems with excellent weathering stability, which are particularly scratch- and car wash-proof, could not be used so far because the difference in surface tension between the cross-linked and the wet paint layer applied to the second coat is so great that there is no wetting. This disadvantage can only be almost eliminated by very intensive, so-called "matt grinding". For reasons of economy, however, grinding can usually not be carried out, so that these coating systems are rarely used.

The use of a plasma pretreatment for painted components to increase the adhesiveness of a second coat of paint to be applied subsequently avoids the above-mentioned disadvantages of sanding and those of the conventional adhesion promoters, and the use of any paint systems while ensuring the desired adhesion is possible.

The plasma pretreatment should advantageously be used where grinding is not possible, where the application of the adhesion promoter in the production process is difficult (spraying systems, covering the parts not to be treated), where new coating systems could not previously be used for reasons of economy and where, because of grinding of the workpiece made of elastic material is only possible under difficult conditions.

The proposed plasma pretreatment of the painted surface creates oxygen-containing, possibly also nitro-containing, functional groups which make the greatest contribution to the adhesion of a further layer of paint to be applied thereon. A suitable measure for the adhesion is the surface tension. For example, this can be up to 20 mN / m before the plasma treatment. A value of at least 50 mN / m should be achieved for the desired adhesion, which is easily possible with the proposed plasma pretreatment.

The pretreatment is preferably carried out by a corona spark discharge in a high-voltage field. This requires voltages of several kilovolts. The treatment distance of the electrode can be up to 20 mm, but shorter distances are preferred.

Both movable corona electrodes, which are manually guided over the paint surfaces to be treated, and stationary corona electrodes, which act on the painted surface at a precisely defined or programmed distance, can be used. Depending on the intensity of the desired treatment, treatment times of just a few seconds are necessary per unit area. The shortest possible distance between the electrode and the workpiece is advantageous. When using an insulated electrode, a short electrode spacing can be achieved by guiding the electrode along the workpiece surface without complex control.

Experiments have shown that the corona pretreatment does not cause any visible change in the painted workpiece surface. This enables the corona pretreatment to be used particularly advantageously in practice. It is therefore irrelevant if the pretreated area turns out to be larger than the area that is subsequently covered with another layer of lacquer. This procedure would not be justifiable when using an adhesion promoter or even when sanding.

The proposed pretreatment can be used with great advantage if only a part of a completely pre-assembled unit has to be repainted. This can be the case, for example, with vehicle bodies. With such parts, it is also possible for the large areas to be treated by grinding or with an adhesion promoter and for the edge areas, that is to say those areas to which other components which are not to be coated with another layer of paint to be connected, are processed by means of corona pretreatment. Both the corona and the other plasma pretreatments enable an adherent substrate for each subsequently applied coating system in a simple manner. The surface tensions of the cross-linked and wet lacquer layers can be very different.

In practice, it can be observed that irregularities in the form of craters sometimes already appear during the initial painting. It has been found that these craters are caused by work errors or air pollution. The craters occur more or only if the bodies have stood for a long time before the first painting - for example over a weekend. Due to the longer service life, environmental influences act on the surface, which creates various surface tensions that cause the craters. In some cases, pretreatment errors (fat residues) can also trigger crater formation.

In order to rule out that the craters can also be seen in the second painting, the surface of the first painting could be treated with a suitable agent. Apart from the fact that this procedure is very complex, success can only be achieved if the entire body is then washed and dried again, which increases the effort. A partial treatment of these surfaces without subsequent washing and drying of the entire body is excluded because the edge areas cannot be cleaned without residues.

The craters occur almost exclusively in the somewhat horizontal surfaces of the vehicle body, because the contaminants settle there. In order to eliminate the wetting disturbances caused by the impurities on the paint surface, it is proposed according to patent claim 5 that the components which were first painted a relatively long time ago are subjected to a plasma pretreatment. This easily compensates for the different surface tensions and creates a good basis for the subsequent secondary painting. In most cases it is sufficient if the plasma pretreatment is only carried out on the approximately horizontal surfaces of the components. In the production process, it is possible to move the horizontal surfaces - in the case of a vehicle body, the bonnet, the roof and the trunk lid - fully automatically by means of a device which has the electrodes for the plasma treatment.

As stated above, plasma pretreatment for components that have already been painted and to which a further layer of paint is subsequently to be applied offers considerable advantages. The proposed use opens up completely new areas of application. Although plasma pretreatments have been carried out on plastics for some time, they have never been used to treat paint layers in order to increase the adhesiveness of another paint layer.

In the drawing, the front right section of a passenger car is shown in perspective, the fender of which is to be repainted due to a work error. For this purpose, the surface area marked with 1 is ground and then the area designated with 2 and adjoining the headlight unit 3 is subjected to a brief manual corona treatment. The desired paint layer can then be applied to the fenders prepared in this way without problems. The great advantage of the procedure described is that components which have already been assembled can be pretreated for painting without removing and without risk of damage to adjacent components.

Claims (6)

1. Use of a plasma pretreatment for painted components to increase the adhesion of a second coat of paint to be applied subsequently. 2. Plasma pretreatment according to claim 1, characterized in that it is carried out in a manner known per se by a corona spark discharge in a high-voltage field. 3. Plasma pretreatment according to claim 1, characterized in that the pretreatment is carried out in a manner known per se in a vacuum chamber with the supply of a suitable process gas and under the influence of a high-frequency voltage field which ionizes the process gas. 4. Use according to one of claims 1 to 3, characterized in that the plasma pretreatment on painted, consisting of soft elastic material, for. B. foamed components is carried out. 5. Use according to one of claims 1 to 4, characterized in that the plasma pretreatment is carried out on components which were first painted a relatively long time ago. 6. Use according to claim 5, characterized in that the plasma pretreatment is carried out only on the approximately horizontal surfaces of the components.

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