Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
1997-06-20
2001-08-07
Parker, Frederick (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S475000, C427S485000, C427S486000, C427S375000, C343S872000
Reexamination Certificate
active
06270853
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the powder coating of electrically nonconducting substrates.
Powder coating is a technique used to provide a durable coating on a surface. Powder particles of a curable organic powder-coating compound are electrostatically charged and directed toward the surface of a substrate. When the substrate is a grounded or connected to an oppositely charged metal, the particles are attracted to the surface and adhere to the surface temporarily. The surface is thereafter heated to elevated temperature to cure the curable organic compound to form the final coating.
Powder coating is a preferred alternative to painting or electrophoretic paint coating. In these processes, solvents are used as carriers for the paint pigments and other constituents of the paint coating. The solvents used for high-quality paint coatings include volatile organic compounds (VOCs), which are potentially atmospheric pollutants. Powder coating utilizes no solvents and no VOCs, and is therefore substantially more environmentally friendly.
Powder coating is more difficult when the substrate is an electrically nonconducting material such as a plastic or ceramic. Several techniques have been developed to impart sufficient electrical conductivity to the substrate that it can be electrostatically powder coated. A conductive material such as graphite can be added to the substrate to improve its conductivity, but this technique has the drawback that it requires modification of the character of the substrate. The substrate can be preheated so that the powder particles partially cure and stick when they initially contact the hot surface, but this approach requires that the substrate be heated to temperatures that cannot be tolerated by some types of substrates such as organic-matrix composite materials. In yet another approach, an electrically conductive primer, typically containing metallic or graphite particles, is coated onto the surface of the substrate.
Although this approach is operable, it leaves the finished part with an electrically conductive coating between the substrate and the cured powder coating. This electrically conductive coating can interfere with some uses of the finished part, which otherwise would not exhibit electrical conductivity.
There is a need for an improved approach for electrostatic powder coating of electrically nonconducting objects. Such an approach would find widespread application in the coating of composite materials, ceramics, plastics, and the like. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a method for powder coating of an electrically nonconductive substrate. The method is practiced without heating the substrate during the coating operation. There is no limitation as to the type of powder coating utilized or the apparatus and method for electrostatically charging and depositing the powder onto the substrate. The coated substrate remains electrically nonconducting with a high surface electrical resistance, an important consideration for some applications such as missile parts that must remain transparent to radio frequency signals.
In accordance with the invention, a powder coating method comprises the steps of providing an electrically nonconducting substrate, applying an antistatic material to the surface of the substrate, directing a flow of electrostatically charged powder particles toward the substrate to form a powder coating on the substrate, and curing the powder coating.
The substrate can be any electrically nonconducting material, such as, for example, a plastic, a ceramic, a glass, or a nonmetallic composite material. The antistatic material is preferably a fatty amine salt. A preferred fatty amine salt is ditallow dialkyl ammonium salt, and a most preferred fatty amine salt is ditallow dimethyl ammonium salt. The antistatic material may be applied by any known technique, such as spraying, dipping, and brushing, but spraying is preferred.
To apply the powder particles, a flow of the powder material (also sometimes termed a “powder precursor” material) is formed and electrostatically charged. Application and electrostatic charging can be accomplished by any known technique, such as passing the flow of powder particles through a charged field or inducing a charge on the particles by frictionally contacting the flow of particles with a surface. There is no known limitation on the type of powder particles that can be used. After the powder particles are applied to the substrate surface, the powder is cured by heating the powder coating and the substrate to an elevated temperature according to a curing schedule recommended for the powder coating that is used. This curing step is accompanied by an increase in the resistivity of the underlying antistatic coating, a desirable result inasmuch as the entire coated article becomes once again electrically nonconducting.
A key feature of the present approach is the application of an antistatic material to the substrate prior to powder coating. The antistatic coating, which is typically on the order of a few micrometers thick or less, provides sufficient electrical conductivity to the surface to permit the electrostatic powder coating. The surface conductivity of the antistatic-coated substrate is about 10
12
ohms per square or more, and may be adjusted by heat treatments. This high resistivity does not result in unacceptable electromagnetic wave attenuation for most applications.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.
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“Electrostatic Powder Coating”, J.F. Hughes et al, Research Studies Press Ltd, pp. 1-12, 1984.*
T. L.Ellis, et al, “Selective Electrostatic Coating of Nonconductive Substrates”, vol. 15, No. 9, Feb. 1973; pp. 2726-2727, XP002079692, New York US (See Whole Document).
Brown Larry W.
Leal James A.
McGinnis Arthur
Raghavan Srini
Collins David W.
Lenzen, Jr. Glenn H.
Parker Frederick
Raytheon Company
Rudd Andrew J.
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