GB2027363A

GB2027363A - Process for the Electrostatic Coating of Workpieces

Info

Publication number: GB2027363A
Application number: GB7919598A
Authority: GB
Original assignee: SUHL ELEKTROGERAETE VEB K
Current assignee: SUHL ELEKTROGERAETE VEB K
Priority date: 1978-06-29
Filing date: 1979-06-05
Publication date: 1980-02-20
Also published as: SE445893B; SE7904609L; BE877331A; DE2914733A1; FR2429620A1; FR2429620B1; HU182784B; NL7903831A; DD137196A1; DD137196B1; CS248254B1; GB2027363B; CH644283A5

Abstract

The invention relates to a process for the electrostatic coating of workpieces which are completely or partly composed of insulating material. In electrostatic coating, the coating material is applied to the workpiece in powder or other freely-flowing form, using the forces of an electrical field, and dried or melted (fused) in subsequent heat treatment. An electrostatically semi- conductively acting surface having a surface resistance of 10<9> to 10<12> ohms is produced in a workpiece to be coated. Such a surface may be provided by coating with quaternary ammonium compounds or by incorporation of quaternary ammonium compounds into the substrate. Thereafter a coating of a powdery or other free-flowing material is applied with the use of coating appliances using the principle of creating an ionized cloud of electrically charged particles of atomized coating material to effect coating of the surface of said workpiece.

Description

SPECIFICATION Process for the Electrostatic Coating of Workpieces The invention relates to a process for the electrostatic coating of workpieces which are completely or partly composed of insulating material.

In electrostatic coating, the coating material is applied to the workpiece in powder or other freely-flowing form, using the forces of an electrical field, and dried or melted (fused) in subsequent heat treatment.

The sphere of application of this process is the coating of parts composed of compressionmoulded materials, plastics or other insulating materials, e.g. for machines and domestic appliances, such as handles, housings and covering hoods. It serves mainly for the production of non-abrasive or chemically resistant surfaces having a decorative effect. Furthermore, the process is used for the coating of sanitary devices composed of ceramics, glass or porcelain.

It also makes possible the electrostatic coating of metallic objects with surface areas of insulating material which are produced when flaws are improved by priming with plastics, and also for the renewed coating of workpieces already provided with a non-conductive coat.

For the electrostatic coating of workpieces composed of insulating materials, it is known for the workpieces to be pre-treated by preliminary application of a conductive base layer and thus make the electrostatic powder and paint layer possible, as in the case of a metailic object (German Offenlegungsschrift 2 450 260; Japanese Patent 50-7635). The pre-requisite for operational capability here is that the surface resistance of the workpiece provided with the conductive base layer be smaller than 108 ohms.

The disadvantage of this process is that in this surface treatment, the applied conductive base layer, after coating, is maintained wholly or partially as a conductive undercoat, and the surface of insulating materials so treated thereby tend to form electrical leakage paths. They are therefore unsuitable for many spheres of application, e.g. when the workpieces are used in conjunction with electrical components or for dirt insulation. Furthermore, the means used to produce the conductive base layer tend to cause pores and bubbles in the coat which forms, as a result of the emergence of solvents, during thermal after-treatment, and thus impair the quality of the coat. Such coat troubles can also be caused by respraying during electrostatic coating which is necessary on account of projecting edges or points or uneven surface conductivity.A further disadvantage of the known method is that it cannot be used in many cases for technical safety reasons. At least, it requires additional earthing control devices. The result of this is that the surface is in fact conductive, but because of the usually puncti-form suspension in screened areas of the workpiece, access to whcih is difficult for the pre-treatment means, no sufficiently low earthing resistance is guaranteed.

For this reason, inflammable spark discharges may be produced at the suspension points during the electrostatic coating.

Furthermore, it is known for an agent for the surface treatment of insulating materials to be used which contains pigments which, before coating, are activated by radiation energy and thereby produce a conductive surface with a surface resistance smaller than 108 ohms (German Offenlegungsschrift 2 042 778). The use of these agents and the radiation generator necessary for this purpose is, however, economically ineffective. In addition, the disadvantaves described above also occur; this also appplies to the danger of spark creation. It arises from the fact that areas around the suspension point, as a result of their generally screened position, are subjected to a smaller dose of radiation and in certain cases, are inadequately activated.

The similarly known processes for (plast) metallization by metal vaporization, electrolytic precipitation or graphitization (rubbing with graphite) require high expenditure on plant technology and have the same disadvantages as the processes already described; this especially applies to workpieces which are intended for electrical devices and appliances.

The object of the invention is to make the electrostatic coating of workpieces which are wholly or partiallly composed of insulating material possible without their insulating properties being impaired, such as for example, by preliminary application of an electrically conductive layer.

Furthermore, it is intended to prevent the occurrence of inflammable spark discharges on the surfaces of the workpiece.

According to the invention we provide a process for the electrostatic coating of workpieces which are wholly or partly composed of insulating material or provided with an insulating coating including the step of providing an electrostatically semi-conductively acting surface, and thereafter creating an ionised cloud of electrically charged particles of atomized coating material to effect coating of the surface of said workpiece.

The state in which the surface can actually no longer be regarded as electrically conductive, but still does not have the high surface resistance which is characteristic of electrical insulating materials is denoted as an electrostatically semiconductive surface. Its surface resistance preferably lies within the range of 109 to 1012 ohms, measured between 10 cm long cutting electrodes located at a spacing of 1 cm. Thus it has been found that such hitherto non-conducting surfaces can be electrostatically coated, if coating devices which create a weakly ionized cloud of electrically charged particles of the atomized coating material are used for the application of the powdered or other free-flowing coating material. As a result, conveyance of a charge from the coating appliance to the workpiece occurs only through the charged particles.

On the other hand, a charge is not additionally conveyed to the workpiece by a current or airions, so that an electrically conductive coat can be relinquished.

Amongst coating appliances with low air ions are included spraying appliances with an inner charge and therefore without an outwardly acting corona electrode, e.g. with an electrokinetic powder charge (East German Patent 106 308), or turbulence (fluidization) plants with electrokinetic charging devices (East German Patent 1 13 289) or with charging electrodes which are switched off during coating (East German Patent 123 644).

The electrostatically semi-conductingly acting surface of the insulating material can be produced, according to a development of the invention, by anti-statically acting additives being mixed with the insulating material before the shaping treatment and the workpieces produced being subjected to a heating process after or during the shaping, in which the anti-static additives diffuse onto the surface.

This diffusing-out process can also be produced by storing the workpiece for a period of one day or longer.

Another possibility of producing electrostatically semi-conductively acting surfaces of insulating materials consists of applying to the workpiece an aqueous solution containing 1 to 10% by weight of a quaternary ammonium compound and 0 to 5% of a defoaming agent, or a mixture on a base of organic solvents containing 5 to 40% by weight of a quaternary ammonium compound and 95 to 60% by weight of an ester of low aliphatic monocarbonic acid-preferably butyl acetate.

Here, application can occur by known methods, e.g. spraying, immersing, painting or vaporizing.

The effect can be increased by thermal drying of the workpiece and application of the powdery materials to the workpieces which still retain some residual heat.

One measure essential to the process consists of fastening the workpieces composed of insulating material, before coating, by means of a suspension device which is similarly composed of insulating material, at least in the area which comes in contact with the material.

Here, the surface of this part is provided with an electrostatically semi-conductively acting surface according to one of the aforedescribed processes, before suspension of the workpieces or together with them.

The advantage of the solution as per the invention is that it permits the electrostatic coating of insulating materials without their surfaces having to be provided-with a conductive lining beforehand. The use of an electrostatic semi-conductively acting surface, on the contrary, does not impair the subsequent use of the workpieces in conjunction with electrical appliances and apparatus, as the materials so treated can be included amongst electrical insulating materials. An increase in technological safety is also linked with the use of the solution, because by the electrostatic coating of insulating materials having a surface resistance larger than 109 ohms, no inflammable spark discharges can be created by the latter.Moreover, the quality of the coat is improved, because as a result of the merely electrostatically semi-conductive surface, projecting edges and points produce field non uniformities only to a very limited extent, the signs of re-spraying no longer occur. The occurrence of pores or craters through re-spraying is, moreover, eliminated. The latter is also brought about by the fact that solvents do not occur in a quantity which leads to the formation of pores or craters, as the thickness of the electrostatically semi-conducting layer liies within molecular dimensions.

Various exemplified embodiments are explained hereafter in order to illustrate different variations of the solution according to the invention.

The invention is firstly explained with an example of electrostatic coating of a polyester housing.

Manufacture of the coated part takes place in successive process stages.

To begin with, an anti-statically acting agent, e.g. an aliphatic amino compound or an alkyl sulphonate in the range of 0.02 to 2% by weight is admixed with the initial material before processing in an extruder. Thereafter, pressing in a mould under the influence of temperature takes place, so that the anti-static agent diffuses in an acceierated manner onto the surface. Then there is the electrostatic plast coating, in which the parts are fixed to an earthed metal hanger and immersed in a turbulence bed from where powder is continually sucked up for the purpose of electrical charging, and after electrical charging has occurred, is fed to the turbulence tank again.

A further exemplified embodiment is the electrostatic powder coating of one part of the housing composed of compression-moulded material. This is firstly fastened to a suspension device with plastic hooks. By immersing the hanger in a solution which is composed of a mixture of 5% by weight dimethyl amino acetic acid dodecyl (amide) carbamoyl chloride and 95% by weight water, and an electrostatically semiconductive surface is produced on the housing parts and the hooks. After a certain dripping-off and vaporizing period, there occurs by means of spraying appliances with electrostatic powder charging, the electrostatic application of an epoxide resin powder coat, which is melted down and hardened by thermal after-treatment in a baking oven. In the aforementioned exemplified embodiment, larger coat thicknesses or coats having a greater uniformity in coat thickness can be produced by the workpiece being dried at temperatures of about or slightly above 1 000C before coating, and the powder coat being applied to the workpiece, which still retains some residual heat.

Another exemplified embodiment consists in carrying out a repeated electrostatic coating of workpieces composed of metal or insulating material, which are already provided with a coat of varnish or plastics in order to improve surface faults or to produce larger coat thicknesses. An electrostatically semi-conductively acting surface is produced on these workpieces before the second coating, by spraying on an organic solution containing 85% by weight of butyl acetate and 15% by weight dimethyl amino acetic acid dodecyl (amide) carbamoyl chloride, and thereafter the powdery coating material is applied by means of spraying pistols which are low in air ions and have no outwardly acting corona electrode, but electrically charge the powder by means of a charging and counter-electrode.

Fusing and hardening-out then takes place in known manner.

A further exemplified embodiment of the process according to the invention is the electrostatic (plast) coating of castings, in which flaws and other surface irregularites have been compensated with electrically insulating priming masses. By smearing the primed surface areas with a solution of the aforedescribed type, an electrostatically semi-conductively acting surface is produced at these points. After drying-off, which is assisted, if necessary, by heating, coating of the workpiece by immersing in a turbulence bed with electrostatic powder charging or by means of another coating appliance which operates according to the principle of creating an ionised cloud of electrically charged particles of atomized coating material to effect coating of the surface of said workpiece.

Claims

1. Process for the electrostatic coating of workpieces which are wholly or partly composed of insulating material or provided with an insulating coating including the step of providing an electrostatically semi-conductively acting surface, and thereafter creating an ionised cloud of electrically charged particles of atomized coating material to effect coating of the surface of said workpiece.

2. A process as claimed in claim 1, wherein the electrostatically semi-conductively acting surface has a surface resistance of 109 to 1012 ohms measured between two 10 cm. long cutting electrodes located at a spacing of 1 cm.

3. A process as claimed in claim 1 or 2, wherein said ionized cloud is created by coating appliances which operate according to the electrokinetic charging principle or by other means in which an electrically charged particle cloud of the atomized coating material is created.

4. A process as claimed in claim 2 or 3, wherein anti-statically acting additives are admixed with the insulating material before the shaping treatment in order to produce the electrostatically semi-conductively acting surface, and the workpieces produced are subjected to a heating process after or during shaping, in which the antistatic additives diffuse onto the surface.

5. A process as claimed in claim 2 or 3, wherein antistatically acting additives are admixed with the insulating material before the shaping treatment, in order to produce the electrostatically semi-conductively acting surface, and after shaping, the workpieces produced are stored for a period of a day or longer, during which the antistatic additives diffuse onto the surface.

6. A process as claimed in claim 2 or 3, wherein an aqueous solution is applied to the insulating material in order to produce the electrostatically semi-conductively acting surface, which solution contains 1 to 10% by weight of a quaternary ammonium compound and 0 to 5% by weight of a de-foaming agent.

7. A process as claimed in claim 2 or 3, wherein in order to produce the electrostatically semi-conductively acting surface there is applied to the insulating material a mixture on a base of organic solvents which contains 5 to 40% by weight of a quaternary ammonium compound and 95 to 60% of an ester of low aliphatic monocarbonic acid, preferably butyl acetate.

8. A process as claimed in any one of claims 1 to 7, wherein the workpiece is subjected to thermal treatment before the electrostatic coating, and the application of the coating material is effected on the workpiece, which has residual heat.

9. A process as claimed in claim 1, wherein the workpieces, before coating, are fastened to a suspension device which is wholly or partially composed of insulating material and on whose insuiating material parts an electrostatically semiconductively acting surface is produced before suspension of the workpieces or together with them.

10. A process as claimed in claim 1, substantially as described herein.