CA2192000C - Electrostatic fluidized bath with semiconductor electrode - Google Patents

Abstract

The invention relates to an electrostatic fluidized bath for coating substrates by means of powders which are charged electrostatically when contacting one or a plurality of semiconductor electrodes immersed in the fluidization vat. The semiconductor electrodes have a voluminal resistivity from 10<2> to 10<8> OMEGA .m, and preferably from 10<4> to 10<6> OMEGA .m. The coating of the substrates in such baths is particularly regular and uniform regardless of the geometry of substrate to be coated and is appropriate for large size substrates.

Description

1 ' DESCRIPTION
ELECTROSTATIC FLUIDIZED BATH WITH SEMI-ELECTRODE
CONDUCTIVE
TECHNICAL AREA
The present invention relates to an electrostatic fluidized bath with semiconductor electrode (s) for coating substrates with materials powdery plastics electrostatically charged.
PRIOR ART
There are different techniques for coating metal substrates with using charged thermoplastic or thermosetting powders lo electrostatically:
* by electrostatic projection using an electrostatic spray gun type Corona * or triboelectric in which the particles are charged electrostatically before being directed to the substrate to be coated, maintained at zero potential (mass) * by soaking in a fluidized electrostatic bath consisting of a tank insulating material and a conductive electrode connected to the high voltage, in general in or next to the porous slab - see for example US 4,381,728 in the name of NORTHERN TELECOM LTD.
The main disadvantages of the electrostatic projection technique a limitation of the thickness of the powder coating (thickness in general less than 100 pm) inherent to the applied voltage and to the gun electrostatic 20 due to reverse ionization or repulsion phenomena and the impossibility to put on uniformly parts of complex geometry such as Faraday cages;
so for substrates made of welded wires, zero field lines appear at intersections making it impossible to coat these powder grain intersections.
Soaking in a known fluidized electrostatic bath poses a risk non-negligible electric arc between the substrate to be coated connected to earth and the conductive electrode which can cause ignition and / or explosion of the mass pulverulent from the tank. Because of these risks, it is dangerous to immerse totally the substrates to be coated within the pulverulent mass in fluidization and therefore requires the use of a minimum quantity of powder. Currently, this technique allows the regular and uniform coating only of substrates of small * (trademark) "; i '' -.,: =
(i.; ..

2 21 9 20 00 very simple size and geometry such as wires, cables or profiles.
In DD 242.353, a method of soaking in a bath has been described.
fluidized electrostatic where the charging electrode is a semi-electrode conductive laminated on insulation. This system helps limit risks inflammation and of explosion of the pulverulent mass which is located within the tank;
however, for complex and fairly large parts, the coating is neither regular, nor uniform.
In US 3,248,253, a method of soaking in a bath has been described.
fluidized electrostatic where the charge electrodes are semiconductor, is are in the form of points and are placed at the level of the porous slab and possibly completely immersed in the fluidized powder. The substrates to coat are brought over the fluidized powder creating a attraction of grains of powder electrically charged from the top of the bath fluidized to substrates with zero electrical potential. The quality of the coatings obtained is not very uniform and the risk of electric arc between the electrodes and the substrates to be coated are far from negligible given the pointed shape of electrodes.
STATEMENT OF THE INVENTION

The present invention provides a charging electrode semiconductor with volume resistivity between 102 and 108S2.m, characterized in that it is made up an insulating or semiconductor matrix in which selected particles from the group are incorporated consisting of mineral conductive particles, organic conductive particles, semi-particles mineral conductors and semiconductor particles organic.

The present invention also provides a method for evenly and evenly coat substrates metal of large size and sometimes geometry complex such as welded wire, grids, washing baskets dishes, supermarket carts, using powders ~~ .:

3 electrostatically charged without danger of discharge electric during operation of the fluidized bath according to the invention and achieves thicknesses of deposits powder up to 300 µm without ionization phenomenon reverse or electrostatic repulsion.

More particularly, the invention provides a method coating in a fluidized bath of substrates using powdery electrostatically charged compositions, characterized in that the load of the compositions powdery is provided by one or more electrodes semiconductors as defined above, connected to a stabilized high voltage generator and in that the powder compositions have a volume resistivity between 1012 and 1017S2.m.

In addition, the process of the applicant allows, for substrates having spikes - such as dishwasher baskets - to have, at level of these tips, a powder coating of greater thickness than the others parts of the substrate, thus providing increased protection against corrosion, the corrosion phenomena generally appearing at these points.
The electrostatic fluidized bath according to the invention consists of a tank in insulating material, for example polyvinyl chloride (PVC), polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF) most often rectangular, or cylindrical in shape. We prefer adapt the shape of the tank according to the geometry of the substrate to be coated :
when the substrate is a wire or a tube, it is advantageous to coat it in circular section tank (cylindrical tank), when the substrate is a plate one tank of square or rectangular section is more suitable.
The tank has a double bottom: a supply of compressed air is provided in the lower part of the tank, called the stilling chamber (2), which is surmounted by a porous element (3) which can be a slab porous, a woven or sintered material (e.g. PE fabric, sintered PE), substantially horizontal also in insulating material, through which is blown with the air - ,;

3a pressurized intended to put the pulverulent particles in suspension. The air necessary for the fluidization of the powder particles is introduced into the bedroom intake / stillment located at the bottom of the tank below the element porous by an air supply pipe (1) in which can be integrated temperature and humidity control systems as well as air dust filtration systems. In order to avoid pollution of air due to untimely passage of powders from the upper part of the tank to the bedroom air intake, it is necessary to provide a good seal between the air intake chamber on the one hand, and the part of the tank where are confined them powdery particles on the other hand. This seal can for example @ be assured by a rubber seal allowing the porous element to be firmly anchored to the walls of the tank maintained by permanent or non-permanent fixing means, such as than screw nut. After passing through the holes of the porous element (slab, fabric or sintered), the air arrives in the tank (4) proper in which the materials pulverulent have been introduced and ensures the suspension of said particles. The air pressure necessary to properly fluidize the bath powder is approximately proportional to the mass of powder introduced into your tank.
The volume expansion of the powders during fluidization is generally understood between 15 and 60% (expressed relative to the volume at rest) and depends on the nature coating powders used. During the implementation of the process according to the invention, the substrate is completely submerged below the level superior of the fluidized mass (9).
One or more semiconductor electrodes (8) and the one or more substrates to be coated (5) are arranged inside the tank, in the area same as fluidization of particles. The suspended powder particles charge then electrostatically by contact with the electrode (s) and are then attracted by the grounded substrate (5) (6) (zero potential) that they are coming cover.
The semiconductor electrodes (8) according to the invention have a resistivity volume between 102 and 108 S2.m, and preferably between 104 and 106S2.m measured according to standard ASTM D257, and are presented preferably in the form of a parallelepiped, possibly truncated.

3b 21 9 20 00 They can for example be made up of an insulating matrix (ie electrically neutral) in which particles are incorporated conductive or mineral and / or organic semiconductors. We adjust the concentration of conductive or semi-conductive particles within the matrix so as to achieve the desired volume resistivity.
As an example of materials constituting the insulating matrices, one can cite all materials based on thermoplastic polymers, thermosetting

-4-and / or elastomers and / or thermoplastic elastomers (TPE) electrically neutral, such as polyolefins, natural and / or synthetic rubbers, TPE to polyamide base. The conductive particles are introduced into the matrix insulating according to any type of process adapted to the nature of the polymer (s) and or elastomers of the insulating matrix, such as by extrusion, injection, calendering, by dissolved or precipitated, during any vulcanization of the insulating matrix.
The conductive particles or charge centers can be based on carbon black, metal oxides, powders of metal compounds (Fe, HAVE, io Zn, Ni, Cu, bronze, brass, ...), graphite, and generally any additive with a volume resistivity much lower than that of the insulating matrix.
In the context of the present invention, the semiconductor electrode can also be made of semiconductor material, ie of volume resistivity understood between 102 and 108 S2.m, and preferably between 104 and 106 S2.m. A
advantage not negligible of such electrodes lies in the fact that a contact between the operator and the electrode are no longer dangerous.
The electrodes according to the invention are generally in the form of parallelepipeds, possibly truncated in their upper part (the section horizontal is higher at the bottom of the tank), the truncated face being oriented towards the center of the tank (where the substrate to be coated is generally placed) and willing substantially perpendicular to the porous bottom.
As indicated above, the horizontal section of the electrodes according to the invention is greater than or equal at the bottom of the tank to the section in the top from (to tank; this variation can be continuous or not, for example linear (the cross section of the electrode is a trapezoid), hyperbolic, parabolic or in form of stair treads.
They are connected to a stabilized high voltage generator (7) delivering an adjustable voltage between 0 and 60 kV, positive or negative of low amperage (generally less than 200 pA) via a contact junction (10) placed at the rear of the electrode, i.e. on the side facing the wall of the tank. The junction contact can be punctual; in this case, we prefer the to place in the upper rear part of the electrode.
It is preferred that the electrical contact at the electrode is not provided by a conductive element dangerous to operators during a unexpected disconnection but be ensured by means of a semiconductor element who extends beyond the tank, eliminating any risk of electric shock to the vicinity of the tank.

-5-Another preferred embodiment of the electrode according to the invention consists to apply to the rear face of the electrode described above an element of lower resistivity (11), therefore more conductive, for example by bonding, co-extrusion, etc. This more conductive element can completely cover or partially the rear face of the electrode. This composite electrode allows to improve the distribution of charge carriers on the surface of the electrode of load ie on the face in contact with the powders.
The electrodes according to the invention can be independent or integral with the fluidization tank. In the latter case, they can be attached to the walls io vertical of the tank and / or at the porous bottom.
In order to avoid clogging of the electrode surface due to agglomeration powders, if the electrodes are independent of the fluidized bath, we can make them undergo vibratory movements intended to take off the powder clusters, when the electrodes are integral with the tank, the tank + electrode (s) assembly can to be subjected to a mechanical vibration system.
Substrates to be coated according to the process developed by the applicant can be semiconductor or conductive. When coating with powders electrostatically charged, their temperature must not exceed C. It is therefore possible to coat the substrate at room temperature, this 2o which is economically interesting.
As an example of substrates, mention may be made of metallic substrates or comprising a metallic part, for example iron, ordinary steel or galvanized, aluminum or aluminum alloy, wooden substrates, materials plastics, glass, cement, terracotta, and generally materials composites of which at least one of the elements can be chosen from the list previous.
Prior to coating powders, the substrate may undergo one or several surface treatments such as alkaline degreasing, pickling, brushing, shot blasting, phosphating, hot rinsing, etc. Unlike the process of soaking in conventional fluidized baths (non-electrostatic) which requires a heating the substrate to temperatures of the order of 300 C, some treatments which degrade at high temperature can be used, i.e.
phosphating Fe, Zn, galvanization, as well as prior coating using of primary.
In order to improve the adhesion of the coating and / or its resistance to corrosion, an adhesion primer can be applied either in liquid form under powdery form.
The coating powders usable within the framework of the present invention can constitute the adhesion primer and / or the coating superficial;

WO 95/3357

6,219 2 0 0 0 PCT / FR95 / 00751 they have a volume resistivity of between 1012 and 1017 S2.m and of preferably between 1013 and 1016 S2.m.
The volume resistivity of the powders is measured using a resistivity whose electrometer has a resistance greater than 1011 S2. We place in the cell a powder height of 1 cm after tamping, apply to the cell a voltage V of 1 kV and measures the intensity I of the current flowing through the cell.
The resistivity p is given by the formula p = (V * A) / (I * d) where A is the section of the cell, d the height of the powder, here 1 cm, V the tension and I
the intensity of current.
The powders according to the invention are based on plastics or resins thermoplastics and / or thermosets.
- As an example of thermoplastic resins, there may be mentioned * polyolefins such as PE, PP, their copolymers or alloys * PVC
* aliphatic, cycloaliphatic and / or aromatic polyamides, such as PA-11, PA-12, PA-12.12, PA-6, PA-6.6, PA-6.12, elastomers polyamide-based thermoplastics, alone, in admixture and / or copolymerized.
- As an example of thermosetting resins, there may be mentioned * epoxy, epoxy / phenolic resins, * polyester resins, * epoxylpolyester hybrids.
- Acrylic and polyurethane resins which are also suitable can be either thermoplastic or thermosetting, as some polyester resins.
Suitable acrylic resins, polyesters and polyurethanes also can be either thermoplastic or thermosetting The coating powders according to the invention are compositions with several constituents with one or more major polymers such as PA, PE and may contain various additives and / or fillers.
The volume resistivity to be taken into account is that of the composition final and not that of the majority constituent.
The powders which can be used in the context of the invention have an average diameter generally between 80 and 150 pm and preferably close to 100 pm. Their maximum diameter is generally less than 500 µm, and preferably less at 300 pm. These are the grain sizes commonly encountered for soaking in non-electrostatic fluidized baths It is also possible to use granulometry powders commonly encountered for application using an electrostatic gun _._ _ i 2,192,000

-7-average diameter between 20 and 40 pm. In this case, we prefer to add a fluidizing agent, such as AI203, in an amount generally between 0.5 and 2 % by weight of the composition of the coating powder.
You can choose to apply a layer on the substrate to be coated depending on the process described above and applying the other layer (s) according to a technical different application (liquid coating, electrostatic gun, bath fluidized, ...).
If the primer is applied in powder form, for example in a bath electrostatic fluidized according to the invention, the coating can be applied superficial in another fluidized bath according to the invention without first carrying out its filming by intermediate cooking.
If a liquid primer is used, we prefer to let it dry a few minutes, for example at room temperature, before proceeding with the soaking in the fluidized bath according to the invention.
To ensure the most regular and uniform powder coating possible and this, over the entire surface of the substrate to be coated, it is preferable to ensure a relative movement between the substrate to be coated and the fluidized bath of particles powdery.
We can keep the tank fixed and immerse the substrate to be coated in the mass pulverulent which is kept under rotating, ascending stirring, descending, 2o etc. The substrate can also be kept immobile and by a system of the sink appropriate from the fluidization tank lift said tank until the substrate is immersed in the pulverulent material and ensure stirring of the bath powder by means of a tank vibration system.
WAYS TO IMPLEMENT THE INVENTION
The following examples illustrate the invention without, however, limiting it.
The fluidized bath and the electrodes are as shown in the Figure.
Two semiconductor electrodes with volume resistivity between 104 and 106 S1m were carried out by the applicant by incorporating 7% in weight from carbon black to natural rubber. Their dimensions are 500 * 300 * 6 mm;
they were placed in a PVC tank with a porous bottom (PE sintered) of dimension 530 * 330 * 300 mm (Examples 1 to 10).
In all the examples, the viscosity in solution is measured in the metacresol at 20 C in solution with 0.5 g of polymer per 100 ml of solvent and East expressed in dl / g.

The substrate is constituted by a mesh of dimension 100 * 150mm obtained by right angle welding of degreased and shot blasted ordinary steel wire of diameter 3.25 mm, the mesh size of the mesh being 50 * 50 mm.

WO 95/33576, 2`1 9 * 2 OO 0 PCT / FR95l00751

-8-The coating powders consist of * 92 parts by weight of PA-1 1 viscosity in solution of approximately 1 dl / g of average diameter 100 pm, maximum diameter less than 300 pm and of which 1% in particle weight has a diameter less than 40 µm * 8 parts by weight of submicron Ti02 (average diameter 0.05 pm) * 0.05 part by weight of fluidizing agent.
Their volume resistivity is 2.1013 S2.m The substrate to be coated is plunged vertically (the long side (3 stitches) of the mesh is dipped perpendicular to the bottom of the tank) in a bath fluidized io in which there are also 2 semiconductor electrodes (see Fig) and of volume resistivity 104-106 S2.m; the back side of this electrode is incorporated of a material of volume resistivity 103-105 S2.m laminated.
By way of comparison, the substrate to be coated is immersed in a fluidized bath electrostatic in which there are 2 semiconductor electrodes of resistivity volume 104-106 S2.m whose rear face is partially laminated sure which was laminated insulating material according to DD 242.353.
The voltage applied to the electrodes is +8 kV. We measure the time of soaking of the substrate which is necessary so that the thickness of the coating at center of the mesh is equal to 130 μm and the thicknesses of coating 2o at the periphery of the grid (at the 4 peaks and in the middle of the 4 sides).
The results are collated in Tables 1 (electrodes according to the invention) and 2 (electrodes according to DD 242.353).

e (Nm) Right Middle Left Top 230 140 240 Middle 220 130 230 Low 240 130 240 Soaking time: 5 s e (Nm) Right Middle Left Top 330 200 320 Middle 200 130 200 Low 230 150 250 Soaking time: 8 s -~~ `2000 WO 95/33576 2`1 PCT / FR95 / 00751

-9-It can be seen that the variation in thickness between the center of the mesh and the edges is 77% with a semiconductor electrode according to the invention then that it reaches 146% with a semiconductor electrode according to DD 242.353.
There is also a greater number of pitting of the coating at the intersections of the wire mesh (no coating) The substrate consists of a steel plate with a side of 100 mm 3 mm thick. It is coated under the same operating conditions as that of substrate of Example 1 (same coating powders; coating thickness io at the center of the room: 130 pm) and the thickness of the coating to the 4 peaks of the room.
The same measurements are carried out in a fluidized bath provided with 2 electrodes according to DD 242.353.
The results are collated in Tables 3 and 4.

e (Nm) Right edge Left edge Top edge 250 240 Bottom edge 230 230 e (Nm) Right edge Left edge Upper edge 330 340 Bottom edge 300 290 It can be seen that the variation in thickness between the center of the plate and the peaks is only 77% with 2 semiconductor electrodes according to the invention so that it reaches 146% with 2 semiconductor electrodes according to DD 242.353.

After coating with a wire mesh as defined in Example 1 using a liquid primer of epoxy / phenolic type with thickness 10-15 pm, it is left to dry 15 min at room temperature before proceeding with the surface coating of the roasting with 6.1013 S2.m volume resistivity powders, consisting of * 93 parts by weight of PA-12 viscosity in solution of approximately 1 dl / g of average diameter 100 pm, maximum diameter less than 300 pm * 7 parts by weight of submicron Ti02 (average diameter 0.05 pm) * a fluidizing agent

-10-under similar operating conditions (soaking time: 8 sec voltage applied to the semiconductor electrode: 10 kV).
Next, the wire mesh thus coated is cooked by passing through an oven at 220 C for 5 minutes.
Uniform coating is obtained as well as excellent overlapping of the wires, especially at the intersections of the mesh with a very Well appearance of the coating.

After coating with a liquid primer according to Example 3, we proceed to io surface coating of the mesh with the same powders features that those of Example 1, under similar operating conditions ( soaking: 3 sec; voltage applied to semiconductor electrodes: +10 kV).
Next, the wire mesh thus coated is cooked by passing through an oven at 220 C for 5 minutes.
Uniform coating is obtained as well as excellent overlapping of the wires, especially at the intersections of the mesh with a very Well appearance of the coating.
EXAMPLE 5 (comparative) We proceed to the surface coating of a mesh as defined in the example.
1 with powders of volume resistivity 3.1010 S2.m and consisting of PA-6 in the form of microporous beads with an average diameter of 40 μm and a diameter maximum less than 80 µm, viscosity in solution of about 0.83 dl / g in a bath fluidized as described in Example 1 followed by baking in an oven.
It can be seen that whatever the potential to which the semi-electrode is worn conductive, the deposit of powder on the mesh is not very homogeneous with including too much overlap at the ends of the mesh.
EXAMPLE 6 (comparative) We proceed to the coating of a mesh as defined in Example 1 with powders with a volume resistivity of less than 1010 S2.m and made of PVC
from K
WERT 57, plasticized with dioctylphthalate and stabilized with tin, diameter way 150 µm and maximum diameter less than 400 µm in a fluidized bath such as described in Example 1.
No powder deposit forms on the screen, whatever the voltage and polarity of the electrodes.

We proceed to the coating of a mesh as defined in Example 1 with powders of volume resistivity 8.1014 S2.m and made of copolymer ethylene / acrylic acid containing 7% by weight of acrylic acid, diameter ~. _ _

-11-medium 100 µm and maximum diameter less than 300 µm in a fluidized bath Phone as described in Example 1 and under similar operating conditions (time to soaking: 6 s; voltage applied to the semiconductor electrode: +10 kV).
Next, the wire mesh thus coated is cooked by passing through an oven at 210 C for 10 minutes.
Uniform coating is obtained as well as excellent overlapping of the wires, especially at the intersections of the mesh with a very good coating filming.

We proceed to the coating of a mesh as defined in Example 1 with powders with a volume resistivity greater than 1016 S2.m and made up of 100 parts by weight of PA-11 viscosity in solution of approximately 1.15 dl / g, diameter medium 100 pm, maximum diameter less than 300 pm and of which 1% by weight of particles less than 40 µm in diameter and 0.05 parts by weight of agent of fluidization in a fluidized bath as described in Example 1 and in similar operating conditions. The PA-1 1 used was polymerized into presence of H3PO4 whereas the PA-1 1s of Examples 1, 2, 4 were polymerized in presence of H3P02.
The deposition of the powder on the substrate is difficult.

After coating with a wire mesh as defined in Example 1 using a liquid primer of epoxy / phenolic type with thickness 10-15 pm, it is left to dry 15 min at room temperature before proceeding with the surface coating of the grill with powders consisting of * 100 parts by weight of PA-1 1 from Example 8, * 5 parts by weight of calcium carbonate * 0.5 part by weight of submicron carbon black * 0.5 part by weight of antioxidant, * 0.4 part by weight of anti-crater * 0.05 part by weight of fluidizing agent obtained by extrusion of the constituents in a fluidized bath as described in Example 1 and under conditions similar procedures (soaking time: 3 s; voltage applied to the electrode semiconductor: +10 kV).
Next, the wire mesh thus coated is cooked by passing through an oven at 220 C for 5 minutes.

WO 95/33576 `, 't 9` v OO PCTIFR95 / 00751

-12-Uniform coating is obtained as well as excellent overlapping of the wires, especially at the intersections of the mesh with a very Well appearance of the coating.

After coating with a wire mesh as defined in Example 1 using a primary powder epoxy / dicyandiamide in proportions stoichiometric at using an electrostatic gun (voltage +30 kV), we proceed to coating surface of the mesh with powders with the same characteristics as those of Example 9 under similar operating conditions (soaking time: 10 s io voltage applied to the semiconductor electrode: +15 kV).
Next, the wire mesh thus coated is cooked by passing through an oven at 200 C for 5 minutes.
A uniform deposit of the coating with a thickness between 160 is obtained and 300 μm as well as an excellent covering of the wires, in particular at intersections wire mesh with very good filming of the coating.
EXAMPLE 11 (comparative) The substrate consists of a mesh of dimension 100 "150mm obtained by right angle welding of degreased and shot blasted ordinary steel wire of diameter 3.25 mm, the mesh size of the mesh being 50 * 50 mm. he East 2o coated under the same operating conditions as that of the substrate of example 1 (same coating powders; coating thickness at the center of the part: 130 pm) except for the 2 semiconductor electrodes which have a volume resistivity less than 104 S2.m and were carried out by the plaintiff by incorporating carbon black into natural rubber.
An electric arc occurs when the distance between the tips of the electrodes and the substrate is less than 5 cm, the voltage applied to electrodes being +8 kV.

The substrate is constituted by a mesh of dimension 100 * 150mm obtained by right angle welding of degreased and shot blasted ordinary steel wire of diameter 3.25 mm, the mesh size of the mesh being 50 * 50 mm. he East coated under the same operating conditions as that of the substrate example 1 (same coating powders; coating thickness at the center of the part: 130 pm) except for the 2 semiconductor electrodes which have a volume resistivity equal to 108 S2.m and were produced by the plaintiff in incorporating carbon black into natural rubber.
An immersion time of 25 s is necessary while only 5 s are sufficient by using the electrodes of Example 1.

WO 95/33576 2 1 - / 20 O'O PCT / FR95 / 00751

-13-EXAMPLE 13 (comparative) From the electrodes of Example 1, 5 electrodes are cut in a triangle dimensions 20 * 200 * 3 mm (base / height / thickness) and fixes them on the slab porous of the fluidization tank described above and proceeds to the coating of a steel substrate using the operating conditions of Example 1 (even substrate, same coating powders; same voltage applied to the electrodes ;
coating thickness at the center of the part: 130 μm).
The results are collated in Table 5.

e (pm) Right Middle Left High 110 100 120 Middle 280 130 270 Bottom 310 190 290 Soaking time: 16 s

Claims (14)

1. Semiconductor charging electrode of volume resistivity between 10 2 and 10 8.OMEGA..m, characterized in that it consists of a matrix insulating or semiconductive in which are incorporated particles selected from the group consisting of particles conductive minerals, conductive particles organic, inorganic semiconductor particles and organic semiconductor particles. 2. Electrode according to claim 1, in which the volume resistivity is between 10 4 and 8 .OMEGA..m. 3. Electrode according to any one of claims dications 1 and 2, characterized in that the matrix insulation is made from natural rubber and the particles incorporated conductors are carbon black. 4. Electrode according to any one of claims dications 1 to 3, characterized in that it is presented in the form of a parallelepiped, truncated or not. 5. Composite electrode part of which is consisting of an electrode as defined by one any one of claims 1 to 4 and its rear face is constituted by an element of lower resistivity. 6. Coating process in a fluidized bed of substrates using powder compositions electrostatically charged, characterized in that the loading of the pulverulent compositions is ensured by a or more semiconductor electrodes such as defined by any one of claims 1 to 5, connected to a stabilized high voltage generator and what powder compositions have a resistivity volume between 10 12 and 10 17.OMEGA..m. 7. Coating method according to claim 6, in which the powder compositions have a volume resistivity between 10 13 and 10 16.OMEGA..m. 8. Coating method according to any of claims 6 and 7, characterized in that the powder compositions are based on a selected compound from the group consisting of polyamide, polyolefins, resins epoxies and polyester resins. 9. Coating method according to any of claims 6 to 8, characterized in that the one or more electrodes have a junction contact to the generators of punctual voltage or not, placed at the back of the electrodes. 10. Coating method according to claim 9, in which the junction contact consists of a semiconductor element which extends beyond the bath fluidized. 11. Coating method according to any of claims 6 to 10, characterized in that the one or more electrodes are subjected to vibratory movement. 12. Coating method according to claim 11, in which the bath is subject to movements vibratory. 13. Electrically non-insulating substrates, coated according to the process as defined by one any of claims 6 to 12, and selected from the group consisting of dishwasher baskets and trolleys supermarket. 14. Non-insulating substrates according to claim tion 13, in the form of wires or gratings.