Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
2002-12-12
2004-10-05
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S485000, C106S272000, C428S484100
Reexamination Certificate
active
06800334
ABSTRACT:
This invention relates to powder coating compositions and to their use in coating substrates, especially substrates of complicated shape, with particular reference to articles having recessed portions.
BACKGROUND OF THE INVENTION
Powder coating compositions generally comprise a solid film-forming resin binder, usually with one or more colouring agents such as pigments, and optionally also contain one or more performance additives. They are usually thermosetting, incorporating, for example, a film-forming polymer and a corresponding curing agent (which may itself be another film-forming polymer), but thermoplastic systems (based, for example, on polyamides) can in principle be used instead. Powder coating compositions are generally prepared by intimately mixing the ingredients (including colouring agents and performance additives) for example in an extruder, at a temperature above the softening point of the film-forming polymer(s) but below a temperature at which significant pre-reaction would occur. The extrudate is usually rolled into a flat sheet and comminuted, for example by grinding, to the desired particle size. Other homogenisation processes also come into consideration, including non-extruder-based processes such as, for example, processes involving mixing using supercritical fluids, especially carbon dioxide.
Powder coating compositions 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.
WO 99/30838 proposes a process which 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.
As compared with processes in which a substantial electric field is generated between charging electrodes and the substrate workpiece, the process of WO 99/30838, which is conducted without ionisation or corona effects in the fluidised bed, offers the possibility of achieving good coating of substrate areas which are rendered inaccessible by the Faraday cage effect.
SUMMARY OF THE INVENTION
The present invention provides a powder coating composition which incorporates a wax in a post-blended form.
The term “post-blended” means that the wax material has been incorporated after the extrusion or other homogenisation process (for convenience referred to hereinafter simply as “extrusion”).
The use of post-blended wax in accordance with the invention offers the possibility of achieving improved Faraday cage penetration in the coating of substrates and, as a result, more uniform coating of substrates having recessed areas or other locations rendered difficultly accessible by the Faraday cage effect, for example, the internal corner regions of microwave ovens. In particular, the invention enables the desired minimum coating thickness to be achieved on such regions without having to apply excess material to other more easily accessible areas of the substrate. Substantial savings of powder coating material are possible.
It will be understood that the use of post-blended wax in accordance with the invention is clearly distinct from prior proposals to incorporate wax for different purposes before or during extrusion. Such proposals can, however, be combined with the practice of the present invention.
The advantages of the invention are best seen in corona application processes, but other application processes may in principle be used instead, although the effect of the invention will generally then be less pronounced.
The invention further provides a process for forming a coating on a substrate, in which a composition according to the invention is applied to the substrate by a powder coating process, preferably a corona application process, resulting in particles of the composition adhering to the substrate, and forming the particles into a continuous coating.
The substrate is advantageously an article having recessed portions subject to the Faraday cage effect, and for an article having multiple faces the ratio of the minimum to maximum coating thickness is advantageously at least 40%, preferably at least 50%.
The invention also provides the use of a powder coating composition of the invention in coating an article having recessed portions which may be, for exa
Care Martyn P.
Park Jae-Kyn
Tinmouth Paul J.
International Coatings Limited
Nixon & Vanderhye
Parker Fred J.
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