EP1448315A2

EP1448315A2 - Method for coating an object with a film and equipment therefor

Info

Publication number: EP1448315A2
Application number: EP02801081A
Authority: EP
Inventors: Jean-Philippe Allen; Maurice University of Western Ontario BERGOUGNOU; Ion University of Western Ontario INCULET; Nicolas Pierru
Original assignee: Atofina SA
Current assignee: Arkema France SA
Priority date: 2001-11-29
Filing date: 2002-11-29
Publication date: 2004-08-25
Also published as: AU2002364800A1; JP2005510351A; CN1617770A; US20050069652A1; KR20040068565A; WO2003045581A3; WO2003045581A2; CA2468758A1; FR2832653A1

Abstract

The invention concerns a method for coating an object with a film derived from melting a thin powder layer which consists in: (a) providing an electrically charged fluidized powder bed in a vessel, said powder being charged with a forced circulation tribocharging device; (b) wetting in the vessel the object connected to a zero potential or sufficient to coat it with powder; (c) placing the powder-coated object in an oven at an adequate temperature until the coating film is obtained by melting the powder. In accordance with an advantageous embodiment of the invention the oven comprises electrodes brought to a high electrical potential so as to produce a corona effect which compensates relaxation of the charge of powder particles when heated. Thus, the charge of the particles is maintained and hence remain on the object and can thus form the film by melting. The invention also concerns the equipment for implementing the method.

Description

METHOD FOR COVERING AN OBJECT WITH A FILM AND APPARATUS FOR CARRYING OUT SAID METHOD

[field of the invention]

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 implementing this method. There are currently on an industrial scale several methods of recovering an object.

The first method is electrostatic powdering, it consists of charging the powder with static electricity and putting it in contact with the object to be covered connected to a zero potential. For example, the powder is injected into an electrostatic gun which will charge the said powder by Corona effect (also called crown effect), by triboelectrification or by a combination of the two. The powder thus charged is sprayed onto the object to be covered, connected to a zero potential. According to another form of electrostatic powdering, the object connected to zero potential is soaked in a fluidized bed of charged powder. Inside the fluidized bed is powder with which 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 state of fluidization inside the bed in the presence of air or any other gas. In the fluidized bed there are electrodes for charging the powder by corona effect or a device for charging it by triboelectric effect. The object covered with powder is then placed in an oven at a temperature sufficient to ensure a coating by fusion of the powder causing the filmification. For example, for a polyamide 11 powder, it suffices to heat to 220 ° C.

The second method consists in preheating the object to be covered to a temperature higher than the melting temperature of the powder. Once hot, the object is immediately immersed in a fluidized bed of the powder, the powder melts on contact with the hot object and films. A solid covering is thus ensured. In this process, a hot object is soaked in a fluidized bed cold and to combat heat loss we need an oven at a temperature higher than that necessary for filming, which leads to increased energy consumption. In addition, if the object consists of parts having very different thermal inertias, these different parts do not have the same temperature and therefore the thickness of the film is not uniform.

The present invention relates to the electrostatic process in which the object is soaked in a fluidized bed of charged powder.

[The prior art and the technical problem]

US Pat. No. 3,248,253 describes electrostatic fluidized beds in which are placed electrodes brought to very high potentials. In such electrostatic fluidized beds, the particles are charged by the corona effect which consists in ionizing the air in the vicinity of a point and therefore in electrically charging the particles in this zone. Objects to be coated, attached to a conveyor system and connected to the earth, pass over the fluidized bed and become covered with powder by electrostatic attraction. According to a variant, the objects to be coated are immersed in the fluidized bed. A similar process is described in US Patent 4,381,728. Good recoveries are obtained in such beds but they present a certain danger due to the presence of electrodes carried at high potential which can give rise to electric arcs with the object to be covered.

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 patent GB 1,487,195.

A disadvantage of these conventional electrostatic fluidized bed systems with crown charge lies in the fact that the powder deposit is not homogeneous. In particular, the concave parts of a room are difficult to access. In US patent 4,689,241 are described limitations such as the lack of thickness in the Faraday cages formed by the object to be covered. Finally, a difference in the thickness of the powder deposit is observed between the most distant parts of the charging electrode. Other descriptions of classic electrostatic fluidized beds with crown effect exist in "Electrostatic fluidized bed, theory, design, application", American Paint Journal 1972, 57 (11) 53-5, 66, 68, 70-2 and in "ANTEC , Lecture Proceedings (Part 2) ", Society of Plastics Engineers, 1994 -

Brookfield, CT, USA - page 2329, 2331.

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

In the publication "Triboelectrification of polymer powders in a fluidized Bed", Power Engineering; Journal of the Academy of Science of the USSR Vol 19, n ° 6 page 75-83, a triboelectric charging system is described but nevertheless assisted by electrodes connected to a high voltage. In the publication "Charge of powdered paint according to a triboelectric mechanism during its fluidization", Journal Lakokras, Mater. IKH Primen (1979), (4), 30-2, it describes the triboelectric charge in a conventional fluidized bed on the walls of the tank. 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. Patent WO 0076677 describes a process for covering an object with a film originating from the fusion of a thin layer of powder in which:

(a) there is a bed of electrified fluidized powder in a tank, this powder being charged essentially by a tribocharging device, other than the walls of the tank, located in the tank and / or outside the tank,

(b) the object connected to zero potential or sufficient to cover it with powder is soaked in the tank,

(c) the object covered with the powder is then placed in an oven at a sufficient temperature until the coating film is obtained by melting the powder. 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 volume density of charge inside the fluidized bed. The powder is loaded and fluidized. If we immerse in the charged bed an object to be covered connected to a zero or sufficient potential, we will be in the presence of an electric field created by the volume of charged powder. This will contribute to good electrodeposition on the grounded object. The object can be at a positive or negative polarization or zero.

In this patent the powder is tribocharged, that is to say charged by contact or friction. The friction is provided by the air or the fluidizing 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 any energy input other than the gas ensuring the fluidization of the powder.

Advantageously, a "honeycomb" is used as the tribocharging device. It is a structure made up of geometric elements whose section can go from any type of polygon (the elements are then prisms) up to the circle (the elements are then tubes). These elements are hollow, arranged vertically and preferably have a thickness of between 1 and 10 mm; their length is for example between 15 and 25 cm. These tubes are joined to each other so as to constitute a solid and homogeneous whole. The interstices between tubes are plugged by any means such as aluminum foil. Although any type of polygonal section can be envisaged, the cylindrical structure is preferred. A cylindrical geometry is preferred so as to allow homogeneous fluidization. Edge effects will be limited by an adapted length of the tubes constituting the honeycomb, that is to say that these tubes are advantageously greater than 15 cm in length. This "honeycomb" is placed at the bottom of the bed. Sufficient space must be left at the top of the bed to immerse the object and there must be around the said object a volume density of charge sufficient to ensure electrodeposition. The "honeycomb" is placed as low as possible in the bed, so as to optimize the contact in the tubes without however disturbing the fluidization.

The diameter of the tubes is chosen as small as possible in order to increase the contact surface, but it is nevertheless necessary to ensure that the tubes will not get blocked and are therefore large enough to ensure correct fluidization. The longer these tubes, the better the electrical charge generated on the powder particles, however, there is a limit on the space to be left for soaking the article. By way of example, tubes 25 mm in diameter and 150 mm in length can be used. They are advantageously made of PVC or PTFE.

The process based on the honeycomb in the fluidized bed described in this patent WO 0076677 has the advantage of being very simple and very energy efficient since there are no electrodes connected to a potential however the particle charge is not always perfect. It has now been found that using a forced circulation tribocharger tube the loading of the powder particles is done very well. The term "forced circulation" is used in opposition to the circulation in the tubes of the honeycomb which is done only by the flow of the fluidization gas of the bed. "Forced circulation" is understood to mean that part of the content of the fluidized bed, that is to say a mixture of fluidizing gas and powder, is withdrawn and using a pump or an equivalent device such as a gas ejector (preferably operating with the same gas as the fluidized bed) and passes through one or more tribocharging tubes. Preferably the pump and the tribocharging tube are outside the bed so as not to disturb its operation, the gas and the tribocharged powder after passing through the tribocharging tube are returned to the fluidized bed. The principle of tribocharging powders using a gun in which the powder to be charged is circulated under the effect of a gas flow is known from US Pat. No. 4,399,945. The particles are charged by friction on a surface , a grounding system allows the charges to be evacuated. However, in this prior art, it is neither described nor suggested that the powder comes from a fluidized bed and returns there after being loaded. US Pat. No. 5,622,313 describes an improvement on the preceding one, that is to say that in order to remove the charges from the tribocharged surface it is treated by effect corona. The charges provided by the corona effect not only neutralize the charges accumulated on the tribocharged surface but also pass into the powder and therefore add to the charges created by triboelectric effect. As in US patent 4,399,945, it is neither described nor suggested that the powder comes from a fluidized bed and returns there after being loaded.

The object having been covered with powder, it is taken out of the fluidized bed and it is heated so that the powder melts and forms a film on the object. As soon as we start to heat the object covered with powder, there is a relaxation of the charges and depending on the nature of the powder, the nature of the object to be covered and its geometry, part of the powder can come off before having melted and formed the film. We have now found that it suffices to have the electrodes in the oven, these electrodes are brought to a high electrical potential in order to cause a corona effect which compensates for the relaxation of the charge of the powder particles during their heating. This maintains the charge of the particles and therefore they remain on the object and can thus form the film by fusion. This has not been described in the prior art. In the prior art US 3248253 the corona effect is used to electrically charge the powder in the bed while in the present invention the powder has been tribocharged, has deposited on the object and the corona effect is not used only to hold the powder on the object while it melts to form the film.

[Brief description of the invention]

The present invention is a method of covering an object with a film originating from the fusion of a thin layer of powder in which:

(a) there is a bed of electrified fluidized powder in a tank, this powder being charged by a tribocharger device with forced circulation,

(b) the object connected to zero potential or sufficient to cover it with powder is soaked in the tank, (c) the object covered with the powder is then placed in an oven at a sufficient temperature until the coating film is obtained by melting the powder.

According to an advantageous form of the invention, the oven comprises electrodes brought to a high electrical potential in order to cause a corona effect which compensates for the relaxation of the powder particles during their heating. This maintains the charge of the particles and therefore they remain on the object and can thus form the film by fusion. The present invention also relates to the apparatus for implementing the method.

[Detailed description of the invention]

Regarding the objects to be coated they can be of any kind provided that they can be immersed in the fluidization tank and withstand the temperature of the oven. By way of example, mention may be made of metals such as aluminum, aluminum alloys, steel and its alloys.

Regarding powders, they consist of a substance which, when heated, will form a protective film for the object. By way of example, mention may be made of polyamides, polyolefins, epoxies and polyesters. Polyamide means 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 hexamethylene diamine, dodecamethylenediamine, metaxylyenediamine, bis-p aminocyclohexylmethane and trimethylhexamethylene diamine with diacids such as isophthalic, terephthalic, adipic, azelaic, azelaic, dodecanedicarboxylic; or mixtures of several of these monomers which leads to copolyamides.

The term “polyolefins” means polymers comprising olefin units such as, for example, ethylene, propylene, butene-1 units, etc. By way of example, mention may be made of:

- polyethylene, polypropylene, copolymers of ethylene with alphaolefins. These products can be grafted with anhydrides of unsaturated carboxylic acids such as maleic anhydride or unsaturated epoxides such as glycidyl methacrylate. - copolymers of ethylene with at least one product chosen from

(i) unsaturated carboxylic acids, their salts, their esters, (ii) vinyl esters of saturated carboxylic acids, (iii) unsaturated dicarboxylic acids, their salts, their esters, their hemiesters, their anhydrides (iv) epoxides unsaturated. These ethylene copolymers can be grafted with anhydrides of unsaturated dicarboxylic acids 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 of up to 2 mm and advantageously between 0.1 and 0.6 mm.

As regards the fluidized bed, it is dimensioned so as to completely immerse the part to be covered. Its shape does not matter as long as it contains the necessary volume of powder, that the part to be covered can be completely submerged and that the fluidization is correct. As regards the forced circulation tribocharger device and first of all the material which will correctly tribocharge the powder, a first choice can be made by comparing the working functions of the powder and of the material envisaged. This can be done by looking at the values of the work functions in electronvolts of the two species concerned and their respective positions in a triboelectric series. The more the difference: | Ft powder - Ft material I is larger the more the powder will load easily. It is recommended that this value be greater than 0.5 eV in absolute value. "Ft" denotes the function of work, these values are read in tables of triboelectric series such as for example ELECTROSTATICS of JA CROSS, IOP Publishing, 1987. Lower values can be considered, while knowing well that consequently the tribocharged will be less good and therefore the recovery less efficient.

However, these values are only theoretical and the fact that a good tribocharged is obtained between the material and the powder can be verified by the experiment described by ll Inculet et al in US patent 5,289,922 and which consists in tribocharging the powder in a cylinder made of tribocharging material in rotation and then 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 large that the object to be coated will be sufficient. We can always try materials giving lower values while knowing that the overlap will be affected. Examples of tribochargers include PVC, PTFE, PVDF and stainless steel.

The powder is charged by triboelectrification, that is to say by friction or contact with a good tribocharger material. The tribocharger material is chosen according to the criteria previously defined. If the powder to be loaded is made of polyamide, the tribocharging material is advantageously made of PTFE (polytetrafluoroethylene). This principle of tribocharged is known in itself, this is what is used in the tribocharging pistols of the prior art.

Advantageously, part of the powder and of the gas from the fluidized bed is removed and it is forced to pass through tubes of the tribocharging material and returns to the fluidized bed or it remixes with the powder present in the bed. It is not necessary to pass all of the gas and powder from the fluidized bed through the tubes of the tribocharger material. The gas flow containing the powder which is forced to pass through the tubes of the tribocharging material can be from 0.5 to 20% of the fluidization gas flow and advantageously from 1 to 10%. We can also define this flow of gas containing the powder which is passed through the tubes of the tribocharging material by its hourly flow which is a fraction of the volume of the fluidized bed. This fraction can be any, but advantageously it is from 0.5 to 30% of the volume of the fluidized bed. In order to have a maximum of powder-tribocharger material contacts, the tubes of the tribocharger material can be arranged in a spiral around a cylindrical part. Inside the tubes, charges will be created due to triboelectrification between the powder and the tribocharging material. A person skilled in the art can adjust the flow rate of gas charged with powder in the tribocharger device by observing the quantity of powder which is deposited on the object to be coated and which is soaked in the bed. The tubes made of tribocharging material can for example be between 1 and 6 m in length and between 4 and 15 mm in diameter, advantageously between 4 and 8 mm. The tubes having a relatively small diameter, the powder-tribocharger material contacts will be numerous and the triboelectrification will be all the better. Similarly, triboelectrification will be all the better as the tubes of tribocharging material are longer. It is recommended to remove the charges generated by the triboelectrification which are inside the teflon tubes. For this, we are based on the same principle as the tribogun (or tribocharger gun). There are several solutions for removing these charges. Advantageously, the outside of the tubes made of tribocharging material is covered with a conductive paint. After three or four hours of operation the charges accumulate. This accumulation creates a discharge which causes the appearance of a conductive charred part, which allows the evacuation of the charges. Charges develop inside the tubes of tribocharging material. After three to four hours of operation, sparks appear and a "breakdown" occurs. This breakdown shows that a conductive part between the interior (non-conductive) and the exterior (conductive) of the tubes appears. The charges can then flow through this conductive part.

Advantageously, the conductive parts are "artificially" created by drilling a hole with a needle in the tribocharger tube. This hole is covered with a layer of conductive paint based on nickel. Holes can be made every 10 or 30cm. The tube is then covered on the outside of Aquadag®, a conductive paint. It is also recommended to connect the cylindrical part around which the tribocharger tube is wound.

As for the means of causing this forced circulation, it is for example a pump or an equivalent device such as a gas ejector (preferably operating with the same gas as the fluidized bed). At the outlet of the tribocharger device, the gas and the charged powder are remixed with the fluidized bed. This tribocharging device with forced circulation can be placed inside the fluidized bed but it risks disturbing the operation of the bed. In addition, the tribocharging tubes must be electrically isolated from the fluidized bed by covering them with rubber or with an insulator while removing the charges from the tribocharging tube towards the outside of the bed. It is much simpler for this device to be outside the bed. The pump (or the forced circulation means) sucks powder and gas through tubes which pass through the wall of the fluidized bed and sends them into the tribocharging tube (s) then through another orifice drilled in the wall of the fluidized bed returns the powder and gas to the bed. The pump can be replaced by a gas ejector preferably operating with the same gas as that which is in the fluidized bed. The sampling in the fluidized bed of the gas loaded with powder and its return to the bed after having passed through the tribocharging tubes must be done as much as possible without disturbing the fluidization and therefore the deposition of powder on the object to be covered. One can for example use low flows in the tribocharger tube, have one or more compartments in the fluidized bed or even carry out the return in the fluidized bed by a system of cyclones and / or compartments.

The electric charge of the powder in the fluidized bed increases with the flow of powder in the tribocharger. The thickness of the powder layer on the object which is soaked in the fluidized bed increases with the electric charge of the powder.

The operating conditions of the fluidized bed are known in themselves, these are the usual conditions, they are described in the prior art.

Fig 1 shows an embodiment of the invention. As seen in Figure 1, the air or the chosen fluidizing gas is injected in 3 in a box under the bed. The air then passes through a porous, or a grid or a perforated metal plate, the pressure drop of which is chosen so as to correctly fluidize the powder. In 1 the pump is shown, in 2 the tribocharger tube (we used PTFE sold under the brand Teflon® by Dupont) and in 4 the fluidized bed.

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

For covering powders which require a primer it can be applied beforehand on the object before dipping it 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. You can also apply the primer with a tribocharged bed. The primer particles are very small, so the primer cannot be fluidized alone. But if the primer is mixed in a first bed with the powder which one wants to cover, a primer content of at least 1% by weight (relative to the weight of powder) is used, and preferably 5 to 10% by weight, then the fluidization of the small primary particles is ensured by the 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 electroplating. We thus coated the object a solid primer. The object is then coated with a second layer in a tribocharged bed containing coating powder alone. During operations with primer, it is possible, if desired, to carry out a first firing of this primer, it is also possible to avoid this intermediate firing and to carry out the second covering then to carry out an overall firing.

Once the object is covered in the bed, it is brought into an oven where 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 oven can be used. This step of the method is known in itself and has already been described in the prior art.

According to an advantageous form of the invention, the oven comprises electrodes brought to a high electrical potential in order to cause a corona effect which compensates for the relaxation of the charge of the powder particles during their heating. This maintains the charge of the particles and therefore they remain on the object and can thus form the film by fusion. This process is particularly useful when there is no primer on the object to be coated.

A space charge generation system is placed inside the baking oven to prevent the powder from falling off the metal parts. Indeed, with certain powders and with the increase in temperature, the relaxation time of the powder decreases, thus the powder is detached from the metal part. A system generating a charge of the same sign as that of the powder is placed. To generate a space charge, an electrical potential is applied to a metal part isolated from ground. If this metal part contains pointed parts, a Corona effect appears. For example, for an oven with dimensions of 0.4x0.3x0.5m, four vertical copper tubes of 1.2 cm in diameter are enough, placed on a metal plate inside in the lower part of the oven and in each corner. Each tube is blocked at its upper end by a convex convex bottom in order to avoid the unnecessary corona effect. The corona effect must be concentrated with the needles mounted on the copper tubes. These needles are the cause of the Corona effect. A potential electric is applied to the metal part, insulated by high temperature resistant insulators (ceramic insulators for example).

The temperature is an important factor in the baking of a coating of a part. Polyamide 11, for example, melts at 186 ⁹ C, the oven temperature is adjusted to 220 ^g C. The cooking time varies according to the oven temperature. In fact, the higher the oven temperature, the shorter the cooking time of the part. For an oven temperature of 220 -C and a powder of PA 11 the cooking time is generally between .6 and 10 minutes. In fact, the higher the temperature of the oven and the shorter the relaxation time of the powder charge, that is to say that the powder discharges faster when the temperature increases.

Fig 2 shows an embodiment of the electrodes in the oven. 1 is the power supply, 2 the ceramic insulation, 3 a copper needle, 4 the electrical connection by a copper cable, 5 a copper tube and 6 the oven.

Claims

1 Method of covering an object with a film originating from the fusion of a thin layer of powder in which: (a) there is a bed of electrified fluidized powder in a tank, this powder being charged by a tribocharging device with forced circulation,

(b) the object connected to zero or sufficient potential to cover it with powder is soaked in the tank, (c) the object covered with powder is then placed in an oven at a sufficient temperature until the film is obtained coating by melting the powder.

2 Method according to claim 1 wherein the tribocharging device consists of tubes of tribocharging material supplied by a pump or a gas ejector.

3 The method of claim 1 or 2 wherein the tribocharger device with forced circulation is outside the tank of the fluidized bed.

4 Method according to any one of the preceding claims wherein the length of the tubes of the tribocharging device is between 1 and 6m.

5 Method according to any one of the preceding claims wherein the diameter of the tubes of the tribocharging device is between 4 and 15mm.

6 The method of claim 5 wherein the diameter of the tubes of the tribocharging device is between 4 and 8mm. 7 Method according to any one of the preceding claims wherein the oven comprises electrodes brought to a high electric potential in order to cause a corona effect which compensates for the relaxation of the charge of the powder particles during their heating.