EP1119422B2 - Method for covering an object with a film - Google Patents

Abstract

The present invention relates to as triboelectric fluidized bed method and apparatus for coating an object with powder prior to heating to form a coating. The methods includes arranging a bed of fluidixed powder in a vat where the powder can be charged by a tribocharging device such as a honeycomb comprising prisms or vertical tubes open at both ends.

Description

The present invention relates to a method of covering an object with a film originating from the melting of a thin layer of powder previously deposited on the object and the apparatus for the implementation of this method. It is more particularly to cover all kinds of objects with the aid of powder in a fluidized bed. Inside the fluidized bed is powder that we want to cover the object. This powder is in the form of small solid particles, for example between 0.01 and 1 mm, of any shape, which are in a fluidized state inside the bed in the presence of air or any other gas.

There are currently several recovery processes on an industrial scale.

The first is the electrostatic powdering, it consists in charging the static electricity powder and putting it in contact with the object to be covered connected to a zero potential. For example, the powder maintained in the fluidized state is injected into an electrostatic gun which will charge said powder by Corona effect, by triboelectrification or by a combination of the two. The powder thus charged is projected onto the object to cover connected to a zero potential. The recovery will be done according to the electric field lines. Because of this, areas with Faraday cages, such as intersections or hollow parts, will be poorly covered. In addition, a lot of powder is not deposited on the object and must be recycled. The powder coated object is then placed in an oven at a temperature sufficient to melt coat the powder causing filming. For example for a polyamide powder 12 it is sufficient to heat to 200 ° C.

The second is to preheat the object to be coated at a temperature above the melting temperature of the powder. Once hot, the object is immediately immersed in a fluidized bed, the powder melts in contact with the hot object and films. A solid recovery is thus ensured. In this process, a hot object is quenched in a cold fluidized bed and to control the heat loss an oven is required at a higher temperature than that required for the filming, resulting in increased energy consumption. However, all the powder is kept in the bed and the coating is not affected by the Faraday effect regions. The thickness depends on the shape of the object and may not be completely uniform. The present invention relates to electrostatic powdering.

[the prior art]

There are conventional electrostatic fluidized beds as described for example in the US Patent 4,381,728 in which are arranged electrodes carried at very high potentials. In such electrostatic fluidized beds, the particles are corona charged which consists of ionizing the air in the vicinity of a tip and thus electrically charging the particles in this zone. The object to be coated is immersed in the fluidized bed. Good recoveries are obtained in such beds but they present a certain danger due to the presence of electrodes raised to high potential which can give rise to arcing with the object to cover.

To avoid any electric arc between the electrode and the object to be coated, it is possible to place this electrode under a porous slab as described in the patent GB 1,487,195 .

A disadvantage of these conventional cored charge electrostatic fluidized bed systems is that the powder deposition is not homogeneous. In particular, the concave parts of a room are difficult to access. In the U.S. Patent 4,689,241 limitations such as lack of thickness in the Faraday cages formed by the object to be covered are described. Finally, a difference in thickness of the powder deposit is observed between the most distant parts of the charging electrode. Other descriptions of conventional electrostatic fluidized beds with corona effect exist in " Electrostatic fluidised bed, theory, design, application ", American Paint Journal 1972, 57 (11) 53-5, 66, 68, 70-2 and in " ANTEC, Conference Proceedings (Part 2) ", Society of Plastic Engineers, 1994 - Brookfield, CT, USA - page 2329, 2331 .

To answer these problems, alternative solutions have been proposed. In the WO 96 11061 a charging system which does not have a corona effect but by induction is described. However, this technique remains applicable only to low resistivity powders.

In the publication " Triboelectrification of polymer powders in a fluidized bed, "Power Engineering, Journal of the USSR Academy of Science Vol 19, No. 6 page 75-83 a triboelectric charging system is described but nevertheless assisted by electrodes connected to a high voltage.

Finally, in the publication "Charge of powdered paint according to a triboelectric mechanism during their fluidization", Lakokras Journal, Mater. IKH Primen (1979), (4), 30-2, the triboelectric charge is described in a conventional fluidized bed on the walls of the vessel. However, it reveals the limitations of the electric charge over time because of the covering of the walls from the first moments of the fluidization by powder particles.

[brief description of the invention]

The method of the invention uses a tribocharging device other than the walls of the tank and does not use electrodes connected to a source of electrical energy.

Thus the present invention is a method of covering an object with a film resulting from the fusion a thin layer of powder as defined in claim 1.

It is an electrostatic fluidized bed tribocharged essentially using a device other than the walls of the tank. The powder is tribocharged, thus creating a high density of charge within the fluidized bed. The powder is charged and fluidized. If one plunges into the loaded bed an object to cover connected to a potential zero or enough, we will be in the presence of an electric field created by the volume of powder loaded. This will contribute to good electrodeposition on the object to the ground. The object can be at a positive or negative polarization or zero. Advantageously the tribochargeur device is a honeycomb.

In this invention, the powder is tribocharged, that is, loaded by contact or friction. Friction is ensured by the air or fluidization gas which entrains the powder particles and allows them to come into contact with the tribocharging systems which will be described later. The charging system described in the present application is autonomous and does not require energy input other than the gas ensuring the fluidization of the powder.

The present invention also relates to apparatus for carrying out the method.

[detailed description of the invention]

The objects that can be coated can be of any kind provided they can be immersed in the fluidization tank and withstand the temperature of the furnace, for example, mention may be made of metals such as aluminum, alloys aluminum, steel and its alloys. The invention is particularly useful for dishwashing metal baskets.

As for the powders they consist of a substance which by heating will form a protective film of the object. By way of example, mention may be made of polyamides, polyolefins, epoxies and polyesters.

• d'un ou plusieurs aminoacides, tels les acides aminocaproïques, amino-7-heptanoïque, amino-11-undécanoïque et amino-12-dodécanoïque d'un ou plusieurs lactames tels que caprolactame, oenantholactame et lauryllactame ;
• d'un ou plusieurs sels ou mélanges de diamines telles l'hexaméthylène-diamine, la dodécaméthylènediamine, la métaxylyènediamine, le bis-p aminocyclohexylméthane et la triméthylhexaméthylène diamine avec des diacides tels que les acides isophtalique, téréphtalique, adipique, azélaïque, subérique, sébacique et dodécanedicarboxylique ;

ou des mélanges de tous ces monomères ce qui conduit à des copolyamides.Polyamide is understood to mean the condensation products:

• one or more amino acids, such as aminocaproic, amino-7-heptanoic, amino-11-undecanoic and amino-12-dodecanoic acids of one or more lactams such as caprolactam, oenantholactam and lauryllactam;
• one or more diamine salts or mixtures such as hexamethylenediamine, dodecamethylenediamine, metaxylyenediamine, bis-p-aminocyclohexylmethane and trimethylhexamethylenediamine with diacids such as isophthalic, terephthalic, adipic, azelaic, suberic, sebacic acids and dodecanedicarboxylic;

or mixtures of all these monomers which leads to copolyamides.

Polyolefins are understood to mean polymers comprising olefin units such as, for example, ethylene, propylene, butene-1 units, etc.

• le polyéthylène, le polypropylène, les copolymères de l'éthylène avec des alphaoléfines. Ces produits pouvant être greffés par des anhydrides d'acides carboxyliques insaturés tels que l'anhydride maléique ou des époxydes insaturés tels que le méthacrylate de glycidyle.
• les copolymères de l'éthylène avec au moins un produit choisi parmi (i) les acides carboxyliques insaturés, leurs sels, leurs esters, (ii) les esters vinyliques d'acides carboxyliques saturés, (iii) les acides dicarboxyliques insaturés, leurs sels, leurs esters, leurs hemiesters, leurs anhydrides (iv) les époxydes insaturés. Ces copolymères de l'éthylène pouvant être greffés par des anhydrides d'acides dicarboxyliques insaturés ou des époxydes insaturés.

By way of example, mention may be made of:

• polyethylene, polypropylene, copolymers of ethylene with alphaolefins. These products may be grafted with unsaturated carboxylic acid anhydrides such as maleic anhydride or unsaturated epoxides such as glycidyl methacrylate.
• copolymers of ethylene with at least one product chosen from (i) the unsaturated carboxylic acids, their salts, their esters, (ii) the vinyl esters of saturated carboxylic acids, (iii) the unsaturated dicarboxylic acids, their salts, their esters, their hemiesters, their anhydrides (iv) the unsaturated epoxides. These copolymers of ethylene may be grafted with unsaturated dicarboxylic acid anhydrides or unsaturated epoxides.

Particularly preferred substances are polyamide 11 and polyamide 12. The size of the powder is advantageously between 0.01 mm and 1 mm.

The term "thin layer of powder" means a thickness up to 2 mm and advantageously between 0.1 and 0.6 mm.

The fluidized bed is sized so as to totally immerse the part to be covered. Its shape does not matter as long as it contains the necessary volume of powder, the part to be covered can be completely immersed and the fluidization is correct.

In order to choose a material that will correctly tribocharge the powder, a first choice can be made by comparing the working functions of the powder and the material envisaged. This can be done by looking at the values of the electronvolts working functions of the two species concerned and their respective positions in a triboelectric series. Plus the difference: | Ft powder - Ft material | is larger the powder will load easily. It is recommended that this value be greater than 0.5 eV in absolute value. "Ft" refers to the work function, these values are read in triboelectric series tables such as for example ELECTROSTATICS by JA CROSS, IOP Publishing, 1987. Lower values can be considered, while knowing that the tribocharge will therefore be less good and thus the recovery less effective.

However, these values are only theoretical and the fact that a good tribocharge is obtained between the material and the powder can be verified by the experiment described by II Inculet et al. US Patent 5,289,922 and which consists in tribocharging the powder in a cylinder constituted by the material tribochargeur in rotation and then in measuring the charge obtained. With this type of test, if the ratio Q / m (mass load) obtained for the powder is greater in absolute value than 0.5 10 ** (- 6) C / kg, then the volume load that will be obtained in a more great that the object to be coated will be sufficient. We can always try materials giving lower values while knowing that the recovery will be affected. As examples of tribocharging materials, mention may be made of PVC, PTFE and stainless steel.

The powder is loaded by triboelectrification, that is to say by friction or contact with a good tribocharging material. The tribocharging material is chosen according to the previously defined criteria.

The tribocharge device is a "honeycomb" device located in the tank, this device is described in detail later in this text.

• On peut augmenter la surface de contact entre la poudre et le matériau tribochargeur. Par exemple on peut ainsi modifier en jouant sur la rugosité de la surface ou en collant des tubes ou des demi tubes sur les parois. On peut aussi ajouter un système de chicanes vibrantes au fond du lit ou un système composé de tout autre objet présent dans le lit, ne perturbant pas la fluidisation et assurant une bonne tribocharge.

It would not be outside the scope of the invention also having a tribocharging material on the walls of the tank, it is only a complement to the main tribochargeur device.

• The contact area between the powder and the tribocharging material can be increased. For example, it is possible to modify by playing on the roughness of the surface or by sticking tubes or half-tubes on the walls. It is also possible to add a system of vibrating baffles at the bottom of the bed or a system composed of any other object present in the bed, not disturbing the fluidization and ensuring a good tribocharge.

It is important to mention that we can combine many of the techniques described previously. We can also combine several materials.

We use a "honeycomb" (see figures 1 and 2 ). It is a structure composed of geometric elements whose section can go from any type of polygon (the elements are then prisms) to the circle (the elements are then tubes). These elements are hollow, have a thickness preferably between 1 and 10 mm; their length is for example between 15 and 25 cm. These tubes are contiguous to each other so as to form a solid and homogeneous assembly. The interstices between tubes are plugged by any means such as aluminum foils. Although any type of polygonal section can be envisaged, the cylindrical structure is preferential. A cylindrical geometry is preferred so as to allow homogeneous fluidization. Edge effects will be limited by a suitable length of the constituent tubes of the honeycomb that is to say that these tubes are advantageously greater than 15 cm in length.

The outside of the tubes is advantageously covered with a metallic paint or any other conductive material and connected to a zero potential or sufficient to eliminate the charges. The advantage of this solution is that it will allow a tribocharge of the continuous powder over time. Indeed, by friction on the material, the powder acquires a given load, the material is loaded with the opposite polarity. However, to have a continuous charge phenomenon, it is necessary to evacuate the charges of polarity opposite to that of the powder and which accumulate on the inner walls of the tubes. These charges will actually be discharged to the outside of the tube and advantageously to the earth. This allows permanent availability of the tribocharging surface.

To increase the effectiveness of the honeycomb, it is strongly recommended to drill a large number of small holes perpendicular to the tube in order to multiply the discharge paths of the charges from the inside to the conductive outer surface. These small holes may be between 0.05 and 2 mm in diameter.

Another solution consists in including in the thickness of the constituent material of the tribocharging tube conductive elements electrically connected to the metallic paint or to the conductive material itself electrically connected to a ground.

This "honeycomb" is placed at the bottom of the bed (see figure 3 ). Sufficient space should be left at the top of the bed to immerse the object and dispose around the said object of a volume density sufficient charge to ensure electrodeposition.

The "honeycomb" is placed as low as possible in the bed, so as to optimize the contact in the tubes without disturbing the fluidization. The diameter of the tubes is chosen as small as possible in order to increase the contact surface, but it must be ensured that the tubes will not clog and are therefore wide enough to ensure fluidization. correct. The longer these tubes are, the better the electrical charge generated on the powder particles, however it is limited by the space to be left for soaking the article. By way of example, tubes with a diameter of 25 mm and a length of 150 mm can be used. They are advantageously PVC.

As one can see figure 3 the selected air or fluidization gas is injected into a wind box placed under the bed. The air then passes through a porous, or perforated metal grid or plate, the pressure drop of which is chosen so as to correctly fluidize the powder. The air velocity used is between Umf, minimum fluidization speed, and Umb, minimum bubbling rate. It is inadvisable to place well above Umb as this causes bubbling and projection of charged fine particles on the outside of the bed. We must stand above Umf to be able to easily introduce the object to be covered in the powder.

By way of example, the Applicant has made a honeycomb by juxtaposing PVC tubes 2.5 cm in diameter, of standardized thickness and 15 cm in length. Each tube is covered on the outside with a layer of conductive paint. This honeycomb section equivalent to that of the fluidized bed used for the covering is placed. This bed is of size equal to 40 by 40 cm and 60 cm high. The "honeycomb" is positioned at a distance of 5 cm above the fluidizing air distributor.

A calculation makes it possible to ensure that one will dispose and bring to the bed, thanks to the envisaged system, enough electrical charges to allow a recovery of objects at an industrial rate.

Example: production of dishwasher baskets covered with polyamide 11 of particle size 200 microns sold under the trademark RILSAN® by the applicant. An ammeter is placed between the "honeycomb" and the earth, the measurement of the current makes it possible to know the quantity of charge generated in the bed; the tribocharge on the walls of the bed or on any surface other than the "honeycomb" is ignored here. The mass of powder deposited on a conventional dishwasher basket is: 130 g. The charge acquired by triboelectrification in this bed is 0.5 10 ^-6 C / kg. Each coated basket therefore requires a load of 0.065 10 ^-6 C. An industrial dish rack production line manufactures 1 basket or multiple of 1 basket every 10 seconds. This multiple depends on the configuration of the line and the size of the fluidization vessel. At the rate of 1 basket every 10 seconds, take 0.065 10 ^-6 C, ie a continuous current of 6.5 10 ^-9 Ampere. It is therefore necessary that the supplied current is identical or higher, preferably. In our example, a maximum of 10 ^-9 amperes was measured.
According to a particular form of the invention, the operation is carried out at a low temperature. Since the kinetics of discharge are minimized at low temperature, the bed previously described is surrounded by an envelope containing a cold fluid or any means of cooling the bed. For the purposes of the present invention "low temperature" means less than 20 ° C.

It is also possible to use air or cold fluidization gas, that is to say less than 20 ° C. According to another form of the invention, it is possible to pulsate the air or the fluidization gas. Indeed, if the air velocity is high, the friction material powder is increased, which increases the amount of load supplied to the bed. On the other hand, when an object is immersed, it is necessary to ensure a maximum electrodeposition of the highest density of volume charge, which implies a low fluidization speed, however, maintaining the fluidized state. A bed can be fluidized at a speed lower than Umf by adding a vibration. We can create a restless state then calm during the immersion and so on.

According to another form of the invention, a vibrating mechanism is used to release the powder particles which remain fixed on the tribocharging surfaces.

According to another embodiment of the invention, the electric charge created within the bed by the tribocharging material is reduced by reducing the humidity of the fluidization air. This is a simple and effective way to improve electrodeposition. This moisture reduction is achieved by air drying or compression.

The figure 4 represents an industrial plant according to the present invention.

According to another form of the invention a pre-surface treatment is performed on the object before it is brought into the bed. These are conventional pre-treatments used in the plastic coating industry: phosphating, degreasing, blasting, application of liquid primer or powder, etc. This list is not exhaustive. The objects to be covered are brought by a conveyor grounded. The powder is then loaded into the tribocharged bed described above. During soaking, the electroplating is done. Depending on the load level of the bed, it is important to shake the room more or less steadily. This agitation can be created by small hammers present on the conveyor or any other system. A tapping system makes it possible to eliminate the excess powder at the outlet of the object of the fluidized bed.

With this system and method, non-metallic objects such as wood or plastic can also be coated with powder.

For coating powders that require a primer it can be applied beforehand on the object before soaking in the fluidized powder tank, it can be a liquid or solid primer.

In the case of a solid primer, it can be applied by electrostatic powdering, Corona gun, tribo or both. The primary can also be applied with a tribocharged bed. The primary particles are very small, the primary can not be fluidized alone. But if we mix in a first the primer with the powder to be coated is used, a primary content of at least 1% by weight (based on the weight of the powder) is used, and preferably 5 to 10% by weight, while the fluidization of the Primary particles are ensured by large particles of fluidization powder. This first tribocharged bed is of the same type as those described above. The charge acquired by a particle is more or less inversely proportional to its radius. The smaller, more charged primary particles will provide most of the electrodeposition. It has thus been coated with a solid primary. The second layer is then coated in a tribocharged bed containing coating powder alone. During operations with the primary, it is possible, if desired, to carry out a first firing of this primary, it is also possible to avoid this intermediate firing and to carry out the second firing and to carry out a global firing.

Once the object is covered in the bed, it is brought into an oven (see figure 4 where a cooking is ensured. Depending on the geometry of the object, the properties of the powder and the desired production rate, a convection, infrared or induction furnace can be used.

The process of the present invention is particularly useful for polyamide powders, furthermore it has excellent safety. Explosive tests were carried out with this tribocharged bed. For a tribocharged polyamide bed, high potentials (30 kV) were applied as well as high energies (1 Joule) were discharged into the bed while the ignition energy of the powder is only a few millijoules . The breakdown of air was observed in the bed, with the appearance of sparks. No explosion could be caused.

1. 1 Revêtement métallique (peut être éventuellement mélangé à de la colle).
2. 2 Tubes, constitués d'un matériau tribochargeur.
3. 3 Papier aluminium.
4. 4 Structure en nid d'abeille.
5. 5 Particules de poudre.
6. 6 Lit fluidisé, constitué d'un matériau adéquat.
7. 7 Boîte à vent, le matériau est peu important.
8. 8 Dalle poreuse.
9. 9 Objet à recouvrir.
10. 10 Convoyeur.
11. 11 Pré-traitement des objets à recouvrir (à définir pour optimiser la qualité du recouvrement).
12. 12 Four pour traitement thermique afin de filmifier le recouvrement.
13. 13 Arrivée d'air.
14. 14 Pied isolant, plaçant le lit au-dessus du sol.
15. 15 Ampèremètre relié au nid d'abeille.

La figure 1 montre une vue en perspective de 4, la structure en « nid d'abeille ».
La figure 2 montre une vue de dessus de cette structure en « nid d'abeille ».
La figure 3 détaille le lit fluidisé dans lequel la poudre est fluidisée et tribochargée.
La figure 4 est une vue générale du système de recouvrement qui réalise un revêtement selon la présente invention.The Figures 1 to 4 represent the collection system in which the key elements are numbered from 1 to 15.
The legend for these numbers is given below:

1. 1 Metal coating (may be mixed with glue).
2. 2 Tubes, made of a tribocharging material.
3. 3 Aluminum foil.
4. 4 Honeycomb structure.
5. 5 Particles of powder.
6. 6 Fluidized bed made of a suitable material.
7. 7 Wind box, the material is not important.
8. Porous slab.
9. 9 Object to be covered.
10. Conveyor.
11. 11 Pre-treatment of the objects to be covered (to be defined to optimize the quality of the recovery).
12. 12 Furnace for heat treatment to film the coating.
13. 13 Air supply.
14. 14 Insulating foot, placing the bed above the ground.
15. 15 ammeter connected to the honeycomb.

The figure 1 shows a perspective view of 4, the structure in "honeycomb".
The figure 2 shows a top view of this structure in "honeycomb".
The figure 3 details the fluidized bed in which the powder is fluidized and tribocharged.
The figure 4 is a general view of the covering system which provides a coating according to the present invention.

The figures 1 and 2 detail the structure in "honeycomb". This structure 4 consists of tubes of adequate tribocharging material. The outer surface and the ends of the tubes 2 are metallized or covered with a conducting layer 1. 1 is grounded as can be seen figures 3 and 4 . The tubes 2 are glued to each other thanks to the metallic paint 1 or by a little glue. The interstices between the tubes 2 are plugged with aluminum foil 3.

The figure 3 represents a fluidized bed 6, constructed of suitable material, supported and isolated from the ground by the foot 14. compressed air or not cooled and / or dried or not or any other fluidization gas is introduced into the wind box 7 by 13. The air then passes through the porous slab 8 which is mounted horizontally on the bed and placed between the bed 6 and the wind box 7 which are screwed to them. At a certain distance above the porous slab 8, the honeycomb structure 4 is disposed horizontally. This honeycomb structure is the one which will mainly ensure the tribocharge of the powder 5 in the fluidized bed 6. The honeycomb structure 4 is grounded. The ammeter 15 controls the charge level.

On the figure 4 it can be seen that the objects to be covered 9, grounded via the conveyor 10, leave the pre-treatment zone 11, where a suitable pretreatment is performed, before being conveyed to the fluidized bed 6 by the Conveyor 10. The conveyor 10 brings the objects 9 into the tribocharged fluidized bed 6, the bed 6 can also be brought to the objects 9. The objects 9 therefore enter entirely into the fluidized and tribocharged bed, a powder electrodeposition 5 then occurs with enough to ensure a good recovery. The conveyor 10 continues its movement and the objects 9 are taken out of the bed 6 and brought into the oven 12 in which the powder is filmed and forms the desired coating.

The terms and expressions that are used here are purely descriptive and do not constitute limitations. There is no intention in the use of these terms to exclude any equivalent of the described material and it is therefore recognized that modifications are possible within the scope of the invention.

Claims (9)

1. Method of covering an object with a film formed by the melting of a thin layer of powder, in which:

   (a) an electrostatic fluidised powder bed is placed in a vessel, this powder being essentially charged by a tribocharger device, other than the walls of the vessel, said device being located in the vessel, and this tribocharger device is a honeycomb and does not use electrodes connected to a source of electrical energy;

   (b) an object, connected to a zero or sufficient potential, is dipped into the vessel in order to cover it with powder; and

   (c) the object covered with the powder is then placed in an oven at a sufficient temperature until the coating film is obtained by the powder melting.
2. Method according to Claim 1, in which the powder is a nylon-11 or nylon-12 polyamide powder.
3. Method according to Claim 1 or 2, in which the tribocharger device is a honeycomb, which is located in the bottom part of the vessel and consists of vertical tubes or prisms open at both ends.
4. Method according to Claim 3, in which the honeycomb tubes are covered on the outside with a metallic paint or with a conductive material.
5. Method according to Claim 4, in which the wall of the honeycomb tubes is pierced with small holes.
6. Method according to Claim 4, in which the wall of the tubes contains conducting elements.
7. Method according to any one of the preceding claims, in which the object to be coated is covered beforehand with a powder primer, is then optionally put in an oven, to cure the primer, and is then dipped into the vessel in order to be covered with the powder.
8. Method according to Claim 7, in which the coating with the powder primer is carried out in an electrostatic fluidised powder bed consisting of the powder of the method of Claim 1 and containing at least 1% and preferably 5 to 10% by weight of primer.
9. Method according to Claim 7 or 8, in which the prior coating with primer is carried out in an electrostatic fluidised powder bed, this powder being charged by a tribocharger device.

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