Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
2000-05-11
2002-09-24
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S485000, C428S474400
Reexamination Certificate
active
06455110
ABSTRACT:
TECHNICAL FIELD
This application pertains to the electrostatic painting, and in particular to the electrostatic painting of non-conductive plastics.
BACKGROUND OF THE INVENTION
In recent years, the automotive industry has increased its use of plastic materials for exterior car body panels and trim parts. The predominant reasons are weight-reduction and the availability of more sophisticated high impact strength plastics, such as Noryl GTX® resin, a poly(arylene ether)/polyamide composition. To a large extent, the future success of plastics for car body panels will depend on their ability to be painted “on-line” in the assembly plant with a high quality appearance similar to painted metal car body panels.
When a metal body panel is painted, it is relatively simple to maintain the metal at ground potential. In the electrostatic painting process, particles (paint droplets) are charged by an electrode in the spray head, and a charged spray cloud from the spray head is attracted to the metal surface by the high voltage difference. This process greatly reduces over-spray and produces a high quality surface on the painted metal parts. For these reasons, and others, electrostatic spray painting techniques are the methods of choice in the automotive industry for spray painting exterior body panels made of sheet metal.
Difficulties arise when using electrostatic spray techniques for painting plastic substrates. The problem is particularly difficult when the objective is to paint plastics with the same high quality and appearance as metal parts using electrostatic paint spray equipment. In order to electrostatically spray paint plastic substrates, a number of technical problems must be overcome. For instance, electrostatic charges accumulate on the surface of a plastic substrate during the electrostatic spray painting process. The charges that accumulate do not dissipate as readily as with metals. This accumulation of charges reduces the potential between the spray head and the substrate, leading to weaker electrical forces on the charged paint droplets. The accumulated charges on the substrate surface also cause an opposing electrical field that repels air-borne paint particles, and tend to produce a non-uniform field across the surface. These phenomena produce a self-limiting effect of yielding less paint deposition and producing less uniformity in the build-up of the paint film when compared with painting metal substrates. In addition, some plastics have retained charges that may continue to exist for long time periods after the paint has been sprayed, making the painted surface more vulnerable to dust attraction. As a result of these problems, it has been difficult to achieve a high quality paint coat by electrostatic painting of plastics. The problem is particularly difficult when the objective is to apply uniform paint coats to plastic panels having complex three-dimensional shapes.
Conductive plastic substrate materials comprised of plastic and conductive filler, can be painted by electrostatic spray techniques. The conductive filler alleviates the build up of surface charges and the resulting low deposition and non-uniform build up of paint films on plastics. Conductive plastic substrates do have drawbacks however. The base color is typically black and hence it is difficult to achieve good paint coverage, especially with light colors. More significant, however, is that many conductive fillers diminish the desirable physical properties, such as ductility, of the finished part.
Another approach has been to develop electrically conductive primers which are air-sprayed onto the plastic substrate prior to electrostatically spraying on the finished paint film. Conductive primers can reduce the problems of accumulated electrostatic charges, low paint film build-up and non-uniform conductivity and film thickness, but in order to effectively use such conductive primers, certain technical problems first must be overcome. Such problems have been controlling the surface smoothness of the primer and achieving good adhesion to the polymer. The conductive primer needs a good level of surface conductivity along with humidity insensitivity, uniformity of conductivity across the primer surface, and durability. If surface conductivity is too low, non-uniform build-up of the paint film can result. Surface conductivity, as measured in terms of “resistivity” (ohms), should be reasonably insensitive to humidity; otherwise non-uniformities in conductivity and in the paint film build-up are produced. Coating thickness can alter the uniformity of surface conductivity. When coating thickness varies as the primer is applied it is difficult to achieve uniform surface conductivity.
Generally speaking, the use of conductive primers for plastic substrate panels in the automotive industry has not been successful in economically producing a high quality finish. Because of non-uniform conductivity and primer film thickness, this priming technique has resulted in a generally poor appearance of the finished paint film. A non-uniform primer, even though an undercoat in the process, can cause a poor appearance of the finished exterior paint coat. It is difficult to produce a uniform paint film thickness with a primer applied by non-electrostatic air spray techniques, followed by air spraying a charged-particle paint film.
As a result of the above mentioned problems, the technique of using conductive primers has resulted in a high scrap rate and increased production time. The current method of priming plastic parts for electrostatic paint spraying involves the additional step of either shipping to a separate location for priming, or priming on the paint line at the assembly plant. This amounts to high transportation and handling costs and a higher than normal scrap return rate.
U.S. Pat. No. 5,490,893, to Enlow et al., herein included by reference, discloses the use of a thermoformable conductive laminate to provide a conductive, paintable surface to the non-conductive surface and thereby overcome the problems of non-uniform conductivity and primer thickness. Enlow et al. form a matte release-coated casting sheet, then cast a thin film of an electrically conductive polymer so as to form a conductive primer coat. The conductive primer coat is transfer-laminated to a thermoformable plastic sheet to form a conductive face sheet which is thermoformed and bonded to the plastic substrate article. The article is then painted by applying the paint to the electrostatically paintable conductive face sheet. Paint is not applied to the non-conductive surface but to the conductive face sheet covering the non-conductive surface. Although Enlow et al. solve some problems, the dark base color of the thermoformable conductive laminate as well as the large number of steps in the process make this process uneconomical. Dark base colors generally make it difficult to obtain good paint coverage, especially with light colors. These disadvantages are compounded by the fact that a dark base color will easily show through when the paint layer is damaged such as when the article is scratched.
Accordingly, there remains a need in the art for a means to economically provide conductivity to non-conductive polymer surfaces with uniform conductivity and non-dark base color for electrostatic painting.
BRIEF SUMMARY OF THE INVENTION
The present invention relates to a method for painting a non-conductive polymer article. The method comprises applying a conductive layer, such as a metal foil, adjacent to at least a portion of a second surface of the article so as to provide sufficient conductivity to enable electrostatic painting of a first surface of the article, and electrostatically painting the first surface.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description.
DETAILED DESCRIPTION OF THE INVENTION
The above described drawbacks and deficiencies in the prior art are overcome by the invention of a method for electrostatic
Fortuyn Johannes E.
Hochberg Gerd Claus
Hoorn Ron Van
Koevoets Christiaan Henricus
Wagenaar Jan
General Electric Company
Parker Fred J.
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