Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
2000-07-24
2001-08-28
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S461000, C118S634000, C118SDIG005
Reexamination Certificate
active
06280798
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for the application of powder coating compositions to substrates.
2. Background Information
Powder coatings form a rapidly growing sector of the coatings market. Powder coatings are solid compositions which are generally applied by an electrostatic spray process in which the powder coating particles are electrostatically charged by the spray gun and the substrate (normally metallic) is earthed. The charge on the powder coating particles is normally applied by interaction of the particles with ionised air (corona charging) or by friction (tribostatic or “tribo” charging). The charged particles are transported in air towards the substrate and their final deposition is influenced inter alia by the electric field lines that are generated between the spray gun and the workpiece. A disadvantage of this process is that there are difficulties in coating articles having complicated shapes, and especially articles having recessed portions, as a result of restricted access of the electric field lines into recessed locations (the Faraday cage effect), especially in the case of the relatively strong electric fields generated in the corona-charging process. The Faraday cage effect is much less evident in the case of tribostatic charging processes, but those processes have other drawbacks.
As an alternative to electrostatic spray processes, powder coating compositions may be applied by fluidised-bed processes, in which the substrate workpiece is preheated (typically to 200° C.-400° C. ) and dipped into a fluidised bed of the powder coating composition. The powder particles that come into contact with the preheated surface melt and adhere to the workpiece. In the case of thermosetting powder coating compositions, the initially-coated workpiece may be subjected to further heating to complete the curing of the applied coating. Such post-heating may not be necessary in the case of thermoplastic powder coating compositions.
Fluidised-bed processes eliminate the Faraday cage effect, thereby enabling recessed portions in the substrate workpiece to be coated, and are attractive in other respects, but have the well-known disadvantage that the applied coatings are substantially thicker than those obtainable by electrostatic coating processes.
Another alternative application technique for powder coating compositions is the so-called electrostatic fluidised-bed process, in which the fluidising air is ionised by means of charging electrodes arranged in the fluidising chamber or, more usually, in the plenum chamber below the porous air-distribution membrane. The ionised air charges the powder particles, which acquire an overall upwards motion as a result of electrostatic repulsion of identically charged particles. The effect is that a cloud of charged powder particles is formed above the surface of the fluidised bed. The substrate workpiece (earthed) is introduced into the cloud and powder particles are deposited on the substrate surface by electrostatic attraction. No preheating of the substrate workpiece is required.
The electrostatic fluidised-bed process is especially suitable for coating small articles, because the rate of deposition of the powder particles becomes less as the article is moved away from the surface of the charged bed. Also, as in the case of the traditional fluidised-bed process, the powder is confined to an enclosure and there is no need to provide equipment for recycling and reblending the overspray that is not deposited on the substrate. As in the case of the corona-charging electrostatic process, however, there is a strong electric field between the charging electrodes and the substrate workpiece and, as a result, the Faraday cage effect operates to a certain extent and leads to poor deposition of powder particles into recessed locations on the substrate.
DD-A-126 791 discloses an electrostatic fluidised-bed process employing an apparatus including a fluidised bed of powdered material in a fluidised layer of which are located charging electrodes. In the discussion of the prior art, suitable charging electrodes are indicated as being in the form of needles, wires or plates maintained at a high voltage for the purpose of generating ions which attach themselves to powder particles and cause them to be deposited on a workpiece in the fluidised bed. DD-A-126 791 is directed to arrangements including porous charging electrodes.
GB-A-1 059 166 discloses an apparatus, which does not include a fluidised bed, in which an article to be coated is connected to a source of high voltage while suspended in a tank. Finely powdered plastics material contained in the tank is made to form a mist and to coat the article by an electromotive force exerted on the powdered plastics material by the high voltage of the article to be coated combined with a degree of agitation of the powdered plastics material.
SUMMARY OF THE INVENTION
The present invention provides a process for forming a coating on a conductive substrate, which comprises establishing a fluidised bed of a powder coating composition, thereby effecting tribostatic charging of the powder coating composition, immersing the substrate wholly or partly within the said fluidised bed, applying a voltage to the substrate for at least part of the period of immersion, whereby charged particles of the powder coating composition adhere to the substrate, withdrawing the substrate from the fluidised bed and forming the adherent particles into a continuous coating over at least part of the substrate.
In general, the process comprises the steps of establishing a fluidised bed of a powder coating composition, immersing the substrate wholly or partially within the said fluidised bed, applying a voltage to the substrate for at least part of the period of immersion, whereby particles of the powder coating composition are charged substantially by friction alone and adhere to the substrate, withdrawing the substrate from the fluidised bed and forming the adherent particles into a continuous coating over at least part of the substrate.
Conversion of the adherent particles into a continuous coating (including, where appropriate, curing of the applied composition) may be effected by heat treatment and/or by radiant energy, notably infra-red, ultra-violet or electron beam radiation.
In the process of the present invention, particles of the powder coating composition adhere to the substrate as a result of the frictional charging (tribostatic or “tribo” charging) of the particles as they rub against one another in circulating in the fluidised bed. As compared with processes in which a substantial electric field is generated between charging electrodes and the substrate workpiece, the process of the present invention offers the possibility of achieving good coating of substrate areas which are rendered inaccessible by the Faraday cage effect.
The process of the present invention is conducted without ionisation or corona effects in the fluidised bed.
The voltage applied to the substrate is sufficient to attract the frictionally charged powder coating particles to the substrate while resulting in a maximum potential gradient that is insufficient to produce either ionisation or corona effects in the fluidised bed of powder coating composition. Air at atmospheric pressure usually serves as the gas in the fluidised bed but other gases may be used, for example, nitrogen or helium.
Since the voltage applied to the substrate is insufficient to produce either ionisation or corona effects in the fluidised bed of powder coating composition, the substrate is, in effect, electrically isolated and there is effectively no current flow in the substrate. If there is any current flow, it is anticipated that it is unlikely to be more than 10 mA, probably unlikely to be more than 5 mA and expected to be less than 1 mA and more likely to be of the order of a few microamps; that is, the current is, in practice, expected to be too small to be measured by conventional current-measuring instruments.
As com
Arpe Gianfranco
Kittle Kevin J.
Ring John
Burns Doane Swecker & Mathis L.L.P.
International Coatings Limited
Parker Fred J.
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