Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
2001-06-07
2003-01-14
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S461000, C427S185000, C118S634000, C118SDIG005
Reexamination Certificate
active
06506455
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for covering an object with a film resulting from the melting of a thin layer of powder previously deposited on the object and to the apparatus for implementing this method. The issue is more particularly one of covering all kinds of objects using powder in a fluidized bed. The fluidized bed contains powder with which the object is to be covered. This powder is in the form of small-sized solid particles, for example particles of between 0.01 and 1 mm, of any shape, which are in a state of fluidity within a bed in the presence of air or any other gas.
There are currently a number of coating methods in existence on an industrial scale.
The first is electrostatic powder coating; this consists in charging the powder with static electricity and bringing it into contact with the object that is to be covered, which object is connected to zero potential. For example, the powder kept in the fluidized state is injected into an electrostatic spray gun which will charge the said powder by a corona effect, by triboelectrification or by a combination of the two. The powder thus charged is sprayed onto the object to be covered, which object is connected to zero potential. Coating will be along the electric field lines. Because of this, it will be difficult to achieve coverage in areas exhibiting Faraday cages, such as intersections or hollow parts. Furthermore, a great deal of powder is not deposited on the object and has therefore to be recycled. The object covered with powder is then placed in an oven at a high enough temperature that coating can be achieved by melting the powder, causing it to form a film. For example, for a polyamide 12 powder it is necessary to heat to 200° C.
The second method consists in preheating the object that is to be covered to a temperature above the melting point of the powder. Once hot, the object is immediately immersed in a fluidized bed; the powder melts in contact with the hot object and forms a film. A solid covering is thus ensured. In this method, a hot object is dipped into a cold fluidized bed and in order to combat heat loss it is necessary to have an oven at a temperature which is far higher than the temperature required for film forming, this leading to increased energy consumption. However, all the powder is retained within the bed and coverage is not affected by regions exhibiting a Faraday effect. The thickness depends on the shape of the object and may sometimes not be completely uniform. The present invention relates to electrostatic powder coating.
PRIOR ART
There are in existence conventional electrostatic fluidized beds such as those described, for example, in patent U.S. Pat. No. 4,381,728, in which electrodes raised to very high potentials are arranged. In such electrostatic fluidized beds, the particles are charged by a corona effect which consists in ionizing the air near to a spiked point and therefore in electrically charging the particles in this region. The object that is to be coated is immersed in the fluidized bed. Good coverage is obtained in such beds but there is a certain danger associated with the presence of electrodes at high potential which may cause electrical arcing with the object that is to be covered.
To avoid any electric arcing between the electrode and the object that is to be coated, it is possible to place this electrode under a porous slab as described in patent GB 1,487,195.
One drawback with these conventional corona-charged electrostatic fluidized bed systems lies in the fact that the deposition of powder is not uniform. In particular, the concave parts of an item are difficult to access. U.S. Pat No. 4,689,241 describes limitations such as insufficient thicknesses in the Faraday cages formed by the object that is to be covered. Finally, a difference in thickness of the powder deposit is observed between the parts furthest from the charging electrode. Other descriptions of corona-effect conventional electrostatic fluidized beds are to be found in “Electrostatic fluidized bed, theory, design, application”, American Paint Journal 1972, 57(11) 53-5, 66, 68, 70-2 and in “ANTEC,
15
Conference Proceedings (Part 2)”, Society of Plastics Engineers, 1994—Brookfield, Conn., USA—page 2329, 2331.
Alternative solutions have been proposed in response to these problems. Patent WO 96 11061 describes a charging system which does not use a corona effect but which works by induction. However, this technique is still applicable only to powders of low resistivity.
The publication “Triboelectrification of polymer powders in a fluidized Bed”, Power Engineering; Journal of the Academy of Science of the USSR, Vol. 19, No. 6, page 75-83, describes a triboelectric charging system, but which is nonetheless assisted by electrodes connected to a high voltage.
Finally, the publication “Charge of powdered paint according to a triboelectric mechanism during its fluidisation”, Journal Lakokras, Mater. IKH Primen (1979), (4), 30-2, describes the triboelectric charge in a conventional fluidized bed on the walls of the vat. However, it discloses the limitations of the electric charge over time on account of powder particles covering the walls from as early as the first moments of fluidization.
BRIEF DESCRIPTION OF THE INVENTION
The method of the invention uses a tribocharging device other than the walls of the vat and does not use electrodes connected to a source of electrical energy.
Thus, the present invention is a method for covering an object with a film resulting from the melting of a thin layer of powder, in which method:
(a) a bed of electrostatic fluidized powder is arranged in a vat, this powder being charged essentially by a tribocharging device other than the walls of the vat and located in the vat and/or outside the vat,
(b) the object, connected to zero or sufficient potential, is dipped into the vat in order to cover it with powder,
(c) the object, covered with the powder, is then placed in an oven at a high enough temperature that the coating film can be obtained by melting the powder.
This is an electrostatic fluidized bed tribocharged essentially using a device other than the walls of the vat. The powder is tribocharged; a high volumetric charge density is thus created within the fluidized bed. The powder is charged and fluidized. If an object that is to be covered, connected to zero or sufficient potential, is plunged into the charged bed, there will be an electric field created by the charged volume of powder. This will contribute to good electrodeposition on the earthed object. The dipped object can be at a positive, negative or zero potential. Advantageously the tribocharging device is a honeycomb.
In this invention, the powder is tribocharged, that is to say charged by contact or friction. Friction is provided by the fluidization air or gas which carries the particles of powder and allows these to come into contact with the tribocharging systems which will be described hereinafter. The charging system described in this application is autonomous and requires no supply of energy other than the gas used to fluidize the powder.
The present invention also relates to the apparatus for implementing the method.
DETAILED DESCRIPTION OF THE INVENTION
The objects that can be coated may be of any kind provided that they can be plunged into the fluidization vat and withstand the temperature of the oven. By way of example, mention may be made of metals such as aluminium, aluminium alloys, steel and its alloys. The invention can be used particularly for metal dishwasher baskets.
As far as the powders are concerned, these consist of a substance which, through heating, will form a film to protect the object. By way of example, mention may be made of polyamides, polyolefins, epoxies and polyesters.
Polyamides are to be understood as meaning the products of condensation:
of one or more amino acids, such as aminocaproic acids, 7-aminoheptanoic acid, 11-aminoundecanoic acid and 12-aminododecanoic acid, of one or more lactams, such as caprolactam, oenantholactam and lauryllact
Bru Ariel
Inculet Ion
Tedoldi Arnaud
Atofina
Millen White Zelano & Branigan P.C.
Parker Fred J.
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