GB2295562A

GB2295562A - Electrostatic spray painting of plastic articles using an electrically-conductive coating pre-treatment

Info

Publication number: GB2295562A
Application number: GB9500414A
Authority: GB
Inventors: Peter Mamo
Original assignee: BRITISH CONSULTING GROUP Ltd
Current assignee: BRITISH CONSULTING GROUP Ltd
Priority date: 1994-12-01
Filing date: 1995-01-10
Publication date: 1996-06-05
Also published as: IT1271296B; ITMI942563A1; ITMI942563A0; GB9500414D0

Abstract

Plastic articles are electrostatically spray painted by pre-treating the articles with an electrically-conductive coating after ionisation of the article and prior to the application of a base coat of pigmented and/or metallic paint and the subsequent application of a coating of transparent paint followed by baking to polymerise the paints. The electrically-conductive coating is a monocomponent polyester resin made conductive by adding carbon black, the coating having a specific conductivity of 50,000-200,000 Siemens/cm<2>.

Description

NETHOD OF PAINTING PLASTIC ARTICLES This invention relates to a method of painting plastic articles.

As experts in the field know, the traditional method of painting plastic objects is essentially made up of the following phases: 1) the ionisation of the article to be painted, usually by means of an ionised current of air; 2) the application to the same article, by a basic coating, of pigmented and/or metallic paint; 3) a successive application of a coating of transparent paint or lacquer; and finally 4) baking in an oven for the necessary time to polymerise the paints.

In this known method the application of the above mentioned paints is effected by means of known compressed air pistols or spray guns.

Such a traditional method presents various disadvantages.

In particular there is the fact that the plastic article to be painted is charged with static electricity after its ionisation, making it attract any airborne dust particles, which detracts from the aesthetic result of the painting.

Besides, as is known to the experts in the art, in the above mentioned procedure the deposition efficiency on the painted products is only 50t; that means the amount of paint that is supplied by the spray gun utilised at present is double that which is effectively applied to the item.

Finally, the actual efficiency limits productivity due to the fact the painting spray guns cannot supply more than 350-400 cc of paint per minute, which constitutes an obvious technological limitation that controls the duration of the painting process.

This invention proposes to overcome the disadvantages and limitations of the above mentioned known procedure.

This object is attained by the present invention which provides a method of painting plastic articles comprising the steps of:- ionising the article to be painted, applying to said article a basic coating of pigmented and/or metallic paint; subsequently applying a coating of transparent paint; and baking in an oven for the necessary time to polymerise the paints; wherein after said ionising step and before the application of the pigmented and/or metallic paint an electrically conductive further coating is applied; and wherein the application of the electrically conductive further coating and of the transparent paint is effected by means of a conventional electrostatic spray painting technique.

The application of all of the pigmented and/or metallic paint is preferably effected by means of conventional electrostatic spray guns.

The procedure described above permits an increase in productivity of 40-50%, when compared to the traditional procedures of painting plastic objects. This is due to the utilisation of the known method of electrostatic painting, until now utilised only for objects of conductive material (in particular metallic objects), but now rendered possible by the presence of the conductive coating on the surface of the plastic. Therefore a significant advantage follows, namely the avoidance of having to spray directly all the areas of the object which are intended to be painted. This is due to the so-called "wrapping" effect typical of the electrostatic painting method.Besides in comparison to the known procedures of painting plastic objects, one has a significant saving (30-40k reduction) of paint since the electrostatic painting process reduces the rebounding of the particles of paint from the surface of the article being sprayed. Besides, the presence of the conductive coating on the plastic article avoids the manifestation of a static charge, and thus dust is no longer attracted by the object. Furthermore, one has a significant reduction of polluting emissions due to the fact that for an equal surface area to be painted, the emissions of solvents due to the reduced consumption of the paint medium diminish by 30-40%.Finally the presence of an additional coating (the conductive coating) makes it possible according to this invention to obtain a total paint cover that has a thickness greater than that obtained by the traditional procedure. The techniques of electrostatic painting permit therefore paint layers of greater thickness. Due to this technique the small defects caused during the moulding of plastic objects can be better hidden, obtaining a better aesthetic quality of the finished painted article.

The procedure according to the invention involves slight changes to the system of traditional painting mainly in providing an added station or cabin for the application of the conductive paint coating as well as substitution of normal air compressed painting spray guns by electrostatic ones for at least the application of the metallic and/or pigmented paint and the covering transparent paint or lacquer, and optionally for the electrically conductive base coating paint.

Obviously one has to use paints having a resistivity that is suitable for electrostatic paints. Painting can be done equally well by using manual or automatic equipment, or both.

The conductive coating can be conveniently obtained at a low cost by using a monocomponent paint, for example a polyester resin base which is rendered electrically conductive by adding a post-treated carbon black pigment, with a specific conductivity of 50,000 to 200,000 Siemens/cm2. This paint can easily be prepared by any producer of paints conventional for such purposes, once he is given the desired characteristics.

At first sight the fact of adding a method step (that of the application of the conductive coating) as compared with the traditional procedure, can be seen as a lengthening of the overall process, with a consequent increase of costs.

However, due to the advantages obtained and illustrated above, the balance becomes very positive, even keeping in mind that, due to greater thickness of the various layers of paint, the duration of the painted objects in the oven needs to be increased by approximately 10%.

Claims

1. A method of painting plastic articles comprising the steps of:- ionising the article to be painted, applying to said article a basic coating of pigmented and/or metallic paint; subsequently applying a coating of transparent paint; and baking in an oven for the necessary time to polymerise the paints; wherein after said ionising step and before the application of the pigmented and/or metallic paint an electrically conductive further coating is applied; and wherein the application of the electrically conductive further coating and of the transparent paint is effected by means of a conventional electrostatic spray painting technique.

2. A method according to claim 1, in which the ionisation of the article to be painted is performed by means of an ionised current of air.

3. A method according to claim 1 or 2, in which electrostatic spray guns are used for the application of said pigmented and/or metallic paint.

4. A method according to claim 1, 2 or 3, wherein the paint used to form the electrically conductive coating has a specific conductivity of 50,000 to 200,000 Siemens/cm2.

5. A method according to claim 4, wherein the paint used is the monocomponent type of polyester resin made conductive by adding pigment of treated carbon black.

6. A method of painting plastic articles, according to claim 1 and substantially as hereinbefore described.