EP0956909B1 - Method and apparatus for continuous electrostatic application of a powder substance to a substrate - Google Patents

Abstract

An electrically conducting oblong substrata (1) in the form of a continuous band moves vertically through a depositing enclosure (2). The powder is introduced into the enclosure continuously and an electric field is created in the enclosure, by means of electrodes (9), enabling the powder (5) to be applied on one of the faces of the substrata. The powder is supplied in the upper part of the enclosure so as to move by gravity along the substrata surface.

Description

The present invention relates to a method for the continuous application of a powdered substance, in particular of powder paint, on an elongated substrate, presenting preferably in the form of a continuous band, running substantially vertically in the direction of its length through an enclosure deposit into which the above powder is introduced in a manner substantially continuous and in which an electric field is created allowing this powder to be applied to at least one of the two faces of the substrate.

It is currently very common to apply paint on metal strips, mainly steel strips and aluminum, running continuously. When this paint is liquid, the application is carried out using machines fitted with rollers.

However, for various ecological reasons, economic or qualitative, we increasingly want to replace liquid paints with 100% dry extract paints, i.e. solvent free. Powder coatings and crosslinked paints under UV radiation or under electron beams are the main representatives of such paintings.

Generally the known line for powder coatings uses electrostatic headlights operating on the principle corona or tribo.

The disadvantages of using such projectors are first of all due to their low flow rate, of the order of 100 to 500 g / min, which requires the simultaneous use of a large number of such projectors to obtain an economically justified return. However, line speeds using this process however do not exceed generally not 30 m / min. In addition, the use of these projectors poses often problems of homogeneity of distribution of the powder on the surface to be painted.

Documents US 4,795,339 and US 5,279,863 describe apparatus for forming a coating on a conductive substrate and electrostatic processes using a powdery substance.

These known methods and devices have the great disadvantage that the supply of the deposit enclosure by powdered substances is very complicated to regulate and develop requiring, among other things, the use of an overpressure at the inlet and a depression at the outlet of the depository, in addition to the expensive pumping. This can lead to the formation of layers of very heterogeneous coating giving poor quality products which are not suitable for most applications.

One of the essential aims of the present invention is to offer an extremely simple and reliable process to very large yield allowing to obtain a highly uniform powder deposit both over the entire width of the substrate to be coated and over its length, and this at very small thicknesses, of the order of 10 microns, up to relatively large thicknesses, of the order of 250 microns, without need to take special precautions.

To this end, according to the invention, there is provided a method as set forth in claim 1. Other embodiments of the method are indicated in claims 2 to 11.

The invention also relates to a device for application of the above process.

According to the invention, a device is provided as indicated to claim 12. Other embodiments of a device according to the invention are indicated in claims 13 to 20.

This device is essentially characterized by the fact that the deposit enclosure has, at its upper part, an opening feed for the powder so that it can fall out freely as a relatively homogeneous and uniform current powdery particles along the substrate surface (s) running in the depository parallel to the downdraft of this powder.

Advantageously, the walls of the enclosure are made made of an electrical insulating material or are covered internally of such material.

According to a preferred embodiment of the invention, the enclosure comprises, along its walls facing the plane according to which the substrate can pass through the enclosure, at least one electrode, but preferably a succession of electrodes made up by a series of conducting wires extending at a certain distance one on the other along the above-mentioned plane, these wires being separated from these walls allowing the passage for powder that can be poured from a continuously and at a controllable flow rate across the top of the enclosure.

La figure 1 est une représentation schématique en élévation et en coupe verticale d'une première forme de réalisation du dispositif suivant l'invention.

La figure 2 est une vue schématique analogue à celle de la figure 1 d'une deuxième forme de réalisation de l'invention.

La figure 3 est, à plus grande échelle, une vue schématique en élévation de l'appareil d'alimentation de poudre utilisé dans les deux formes de réalisation montrées aux figures 1 et 2.

Other details and particularities of the invention will emerge from the description given below, by way of nonlimiting example, of some particular embodiments of the invention with reference to the accompanying drawings.

Figure 1 is a schematic representation in elevation and in vertical section of a first embodiment of the device according to the invention.

Figure 2 is a schematic view similar to that of Figure 1 of a second embodiment of the invention.

FIG. 3 is, on a larger scale, a schematic elevation view of the powder supply apparatus used in the two embodiments shown in FIGS. 1 and 2.

In the different figures, the same figures of reference relate to the same items.

In general, the method according to the invention is on the continuous electrostatic application of a substance in powder on an elongated substrate, consisting for example of a continuous strip, such as a steel sheet in the form of a continuous rollable tape.

The powdered substance can be very varied in nature, allowing for example to form on a substrate a layer of protection or an electrically insulating layer on a substrate conductive or a decorative layer in case the powder contains pigments or dyes.

According to this process, this substrate is scrolled vertically following the direction of its length through a deposit enclosure in which the above-mentioned powder is introduced continuously, according to a uniform current, and at an adjustable rate while creating a field electric allowing to apply this powder on one or on two sides of the substrate.

This process is characterized in relation to the processes known by the fact that the powder is fed to the upper part of the depository by means of a device or installation suitable, in such a way as to allow this powder to move essentially by simple gravity along the surface (s) of the substrate to which it is to be fixed.

Figure 1 relates to a first embodiment of a particular device for implementing this method.

This is in particular a device for the application electrostatic continuous powder paint 5 on one side of a continuous strip of a substrate 1 which runs vertically, from bottom to top or top to bottom, in a storage enclosure 2.

It is an enclosure 2 whose walls 3 are made of an electrical insulating material, preferably a plastic material transparent, like Plexiglas or polycarbonate. In some case, the interior faces of these walls can simply be covered by electrical insulating material.

This enclosure is formed by a standing box having at its upper end an opening 4 for feeding the powder 5 in the deposition chamber 2 and for the passage of the substrate 1 moving vertically over the entire height of the latter. So, a slot or opening 6 for the passage of the substrate 1 is provided in the bottom 7 of enclosure 2.

Furthermore, this bottom 7 has a hopper 8 making it possible to harvest the excess powder which may not have been deposited on the substrate, which can then be evacuated through an orifice 8 '.

This 8 'orifice can be connected to an installation of recycling, not shown, in which the collected powder is sucked so that it can be reused.

The electric field is created in the deposit enclosure 2 at means of electrodes 9 which can be brought to a high DC voltage, preferably negative, with respect to substrate 1, the latter being preferably grounded.

In the particular embodiment shown in Figure 1, these electrodes 9 consist of a series of conductive wires extending a certain distance above each other opposite the face of the substrate 1, on which the powder 5 must be fixed, transversely to the direction of travel of the latter, parallel to each other and fixed by their ends to the side walls extending in planes perpendicular to the plane in which the substrate 1 moves.

In addition, these conductive wires are located at a certain distance from the wall 3 of the box 2 opposite to the face of the substrate 1 to cover with powder 5. They can, for example be made of copper, in tungsten or steel.

As already mentioned above, this powder 5 is moves in the deposit enclosure 2 up and down only under the effect of gravity. The conductive wires, forming the electrodes, are arranged in such a way that, when the powder 5 falls, it basically goes in the area of enclosure 2 between these wires and the substrate 1 and is negatively electrostatically charged by corona effect to then be attracted to substrate 1 thanks to the field intense electric prevailing in the powder passage zone between the substrate and the wires. Working parameters and conditions can be set in such a way that the powder 5 arises with a very high application yield on substrate 1, generally greater than 95%.

The fact that in a preferred embodiment of the invention, the conducting wires are brought to a high voltage continuous negative and that the substrate is grounded reduces maximum risk of breakdown and powder deposition on the wires.

Ideally, the value of the high voltage should be the most high possible so as to form a crown discharge around the wires, but in practice this value is limited by the breakdown limit which could ignite the powder. Furthermore, too low a voltage does not not achieve the desired crown discharge around the wires and negatively influences the efficiency of the deposit on the substrate. next the invention, an ideal and safe value of the electric field is located around 3500 volts per cm.

It has been observed that the breakdown limit changes within air when it contains powder. So the powder, at the concentration used, halves the breakdown voltage, which is around 10 kV per cm in dry air. However, none notable influence of the humidity level of the air in the enclosure on the effectiveness of the deposit has not been established. According to the invention, it is however recommended to work in a controlled atmosphere of dry air and fresh at a temperature of at most 20 ° C, in order to avoid, on the one hand, agglomeration of the powder and, on the other hand, any chemical preaction anticipated.

Lead wires should be located at a certain distance from the substrate which must at least be such as to avoid the physical contact between the wires and the substrate. This distance cannot be too large for economic and technical reasons, in particular to avoid having to use too high voltages. A possible value of wire-substrate distance is approximately 5 to 12 cm. A preference is given for a distance of the order of 7 cm with a voltage of the order of 25 kV.

The number of conducting wires depends on the flow of the powder 5 which enters the deposit vessel 2 and which must be deposited on the substrate 1. An ideal value for obtaining a deposit yield greater than 95%, the other conditions being optimal, is of the order of 700 to 900 g of powder per minute, per meter of width of the substrate and by common thread.

The distance between the wires can also have an effect on the yield of the deposit formed by the powder 5 on the substrate 1. In fact, the closer the wires are to each other, the less they behave independently and minus the total passing electrical current between the wires and the substrate 1, formed for example by a strip continues, is high. Ideally, this distance will be as large as possible. According to the invention, it has been found that a distance giving excellent results is around 20 cm between two consecutive wires.

As already mentioned above, it is preferable that the conductive wires are not applied against the wall 3 of the enclosure 2 opposite and parallel to the substrate 1. It has indeed been found that, in this case, the deposition efficiency is relatively reduced. Ideally, the wires 9 must be at least 5 to 10 cm away from this wall 3.

The diameter of the wires is also a parameter important, since the crown effect is all the more intense as the wire diameter is small due to the tip effect. It was found that excellent results have been obtained with wires of a diameter about 100 to 250 microns.

Figure 2 relates to a second embodiment of a device according to the invention.

This device is distinguished from that shown in the Figure 1 by the fact that it is provided for the simultaneous fixing of powder 5 on both sides of a substrate 1.

Thus, in this second embodiment, the enclosure deposit 2 extends symmetrically on either side of the plane in which must pass the substrate 1 on which the powder 5 must be fixed. The powder is fed on either side of this plane and the wires conductors are also provided on each side of this plane.

However, in certain cases where the deposit to be formed on a of the sides must be different from that of the other side, it is possible to adapt the parameters and working conditions on one side of this plan entirely independent of the parameters and conditions of work on the other side of this plan. This can, for example, be the case for location and number of wires and voltage applied.

The apparatus 10 used to introduce the powder by gravity in the upper part of the enclosure can be very varied in nature. What matters above all is that it makes it possible to obtain a distribution uniform and controllable flow rate of the powder 5 introduced by gravity by the feed opening 4 in the deposit enclosure 2. In addition, this device 10 should preferably make it possible to adapt the width of feeding the powder as a function of the width of the substrate to be coated. In addition, it must make it possible to deliver a relatively large quantity powder in a continuous, regular and homogeneous way across the width desired. The powder flow can vary from 0 g to a value of the order 10,000 g per minute and per meter of width of the substrate.

It may also be important that this device 10 allows to regulate the powder flow linearly.

A supply apparatus 10 meeting these requirements has was shown schematically in Figure 3. It is a kind of sprinkler that is mounted directly above the opening supply 4 of the storage enclosure 2.

This device includes a mounted feed hopper 11 above a cylinder 12 which can rotate, at an adjustable speed, around a horizontal axis 13 in the direction of arrow 14. The surface of the cylinder 12 is coated with metal studs 15, so as to allow, during the rotation of the cylinder 12 about its axis 13, to cause, by the lower opening 16 of the hopper 11, a metered quantity of powder, contained in the hopper, on the cylindrical surface of cylinder 12 in the direction of the arrow 14.

A rotary brush 17 which can rotate at a speed adjustable around a horizontal axis 18 in the direction of arrow 19 is applied against this cylindrical surface, on the side of the hopper 11 where the powder 5 is removed from the latter by the pins 15 on this cylindrical surface and is applied against said surface to allow detach the powder, break up any agglomerates and spray the powder down.

The axis of rotation 18 of this brush 17 is preferably located substantially at the same level as the axis 13 of the cylinder 12 and its rotation speed is generally continuously adjustable between 0 and 4500 rpm. In addition, this brush 17 includes pins relatively rigid metals, for example steel, of a length between 1 and 2 cm, and the distance from the axis 13 of cylinder 12 is such that the end of the pins 20 is just in contact with the surface of it, as these pass into the plane defined by the axes of rotation 13 and 18.

The width of the powder feed on the surface of the cylinder 12 can be adjusted by positioning two cheeks, not shown, located in the hopper 11 and parallel to the walls transverse of the latter. Thus, a width of more than 2 m is easily conceivable.

The powder flow is linearly adjustable by playing only on the speed of rotation of the cylinder 12 around its axis 13.

1) il permet de délivrer une quantité importante de poudre de manière continue, régulière et homogène à travers l'ouverture d'alimentation 4 et ceci sur toute la largeur du substrat 1 défilant dans l'enceinte de dépôt 2;

2) la quantité de poudre délivrée est proportionnelle à la vitesse de rotation du cylindre 12 qui peut être asservie à la vitesse de ligne de l'installation entière ;

3) le principe de fonctionnement de cet appareil 10 est extrêmement simple et ne nécessite qu'un paramètre de réglage, à savoir la vitesse de rotation du cylindre 13.

The advantage of this supply device 10 is threefold:

1) it makes it possible to deliver a large quantity of powder in a continuous, regular and homogeneous manner through the feed opening 4 and this over the entire width of the substrate 1 passing through the deposition enclosure 2;

2) the quantity of powder delivered is proportional to the speed of rotation of the cylinder 12 which can be controlled by the line speed of the entire installation;

3) the operating principle of this device 10 is extremely simple and requires only one adjustment parameter, namely the speed of rotation of the cylinder 13.

After fixing the powder 5 on the substrate 1, the latter can optionally be heated so as to subject this powder to at least partial melting to form a coating substantially uniform and durable on this substrate. This heating can be provided directly at the place where the substrate leaves the deposit enclosure.

In the case where the substrate has a face to be coated made of an electrically conductive material, such as a sheet metal steel, it can advantageously be subjected to firing by induction after fixing the powder on the substrate.

This subsequent heating is advantageously carried out as the substrate moves vertically, in the same way so that when applying the powder and this in order to reduce the surface on the ground, to precisely position the substrate 1 between the turns of the inductor, not shown in the figures, and avoid the need use intermediate support cylinders.

Such heating means have not been shown in the figures, since it is a technique known per se.

The object of the invention is further illustrated by a few concrete examples of applying a powder paint on a substrate formed by a steel strip in a device of the type such as shown in the accompanying figures.

Examples Example 1

• vertical application on one side; the strip scrolls from bottom to top.
• powder: polyester-TGIC with particle size distribution included between 1 and 150 µ with a maximum of the curve at 50 µ.
• line speed: 30 m / min.
• powder flow: 5280 g / min * m wide
• 6 copper wires of 100 µm in diameter spaced 20 cm apart and placed at 7 cm from the sheet
• applied voltage = 25 kV
• average thickness measured after baking: 80 µ (with an Δ of 5% maximum across the width)

Example 2

• vertical application on one side; the strip scrolls from bottom to top
• powder: polyester-TGIC with particle size distribution included between 1 and 150 µ with a maximum of the curve at 50 µ
• line speed: 50 m / min
• powder flow: 8800 g / min * m wide
• 12 copper wires of 100 µm diameter spaced 20 cm apart and placed at 7 cm from the sheet
• applied voltage = 25 kV
• average thickness measured after baking: 78 µ (with an Δ of 5% maximum across the width)

Example 3

• vertical application on one side; the strip scrolls from bottom to top
• powder: polyester-TGIC with particle size distribution included between 1 and 150 µ with a maximum of the curve at 50 µ
• line speed: 70 m / min
• powder flow: 8800 g / min * m wide
• 12 copper wires of 100 µm diameter spaced 20 cm apart and placed at 7 cm from the sheet
• applied voltage = 25 kV
• thickness measured after firing: 58 µ (with a Δ of 5% maximum across the width)

Example 4

• vertical application on one side; the strip scrolls from bottom to top
• powder: polyester-TGIC with particle size distribution included between 1 and 150 µ with a maximum of the curve at 50 µ
• line speed: 30 m / min
• powder flow: 5280 g / min * m wide
• 6 copper wires of 1 mm in diameter spaced 5 cm apart and placed at 7 cm from the sheet
• applied voltage = 22 kV
• thickness measured after baking: 45 µ (with a Δ of 5% maximum across the width).

It is understood that the invention is not limited to embodiments described above, but many variations can be envisaged without departing from the scope of the invention.

Thus, the invention is not limited to the use of a specific powder supply system. The only requirement is that this powder can move substantially by gravity through the depository, and this in a continuous and homogeneous manner at a relatively high throughput.

It is however not excluded that a slight aspiration could be carried out by the bottom of the enclosure to somewhat control the downward movement of this powder. This could for example be the case for very light powders, but it should however be noted that gravity will always be the essential parameter for the displacement of the powder.

Furthermore, the method according to the invention is not only applicable on a metallic substrate, such as steel, coated steel or aluminum for example, but also on a substrate made electrically conductive, such as paper, cardboard, plastic, textiles, etc., for example covered with a film driver.

Claims (20)

1. Method of continuously applying a powdered substance (5), in particular a powdered paint, to an oblong substrate (1) which is electrically conductive or has been rendered electrically conductive, having two faces, preferably in the form of a c ontinuous strip, which consists in
      feeding the substrate (1) substantially vertically in its lengthwise direction through a deposition enclosure (2),
      introducing the powder into the enclosure in a substantially continuous manner, and
      creating an electric field in the enclosure by means of electrodes (9), enabling this powder to be applied to the substrate,
   characterised in that it further consists in
      creating at least one electric field between at least one electrode (9) and coating the face or faces of the substrate to be coated,
      feeding in the powder (5) from a top part of the deposition enclosure,
      allowing a uniform stream of powder particles to flow by gravity along the face or faces of the substrate to be coated, between the latter and said at least one electrode (9), and
      electrostatically charging the powder particles during said flow, attracting them onto the corresponding face of the substrate (1) to be coated due to said at least one electric field created.
2. Method as claimed in claim 1, character ised in that an enclosure (2) is used which has walls (3) made from or internally lined with an electrical insulating material, preferably a transparent plastics material.
3. Method as claimed in either of claims 1 and 2, characterised in that the electric field inside the deposition enclosure (2) is created by means of electrodes (9) to which a continuous high voltage is applied, preferably negative relative to the substrate (1), the latter preferably being connected to ground.
4. Method as claimed in any one of claims 1 to 3, characterised in that the electric field is created in the deposition enclosure (2) by means of a series of conductor wires extending at a certain distance apart, one above the other, facing at least one of the faces of the substrate, spaced apart from the walls (3) of the enclosure opposite these faces, and transversely relative to the feed direction of the substrate.
5. Method as claimed in any one of claims 1 to 4, characterised in that the powder in the deposition enclosure (2) is electrostatically charged in a negative manner by a corona effect so that it is subsequently attracted to the substrate due to the presence of the electric field.
6. Method as claimed in any one of claims 1 to 5, characterised in that an electric field in the order of 3,500 Volts per cm is created in the deposition enclosure (2).
7. Method as claimed in any one of claims 1 to 6, characterised in that the powder is introduced into the deposition enclosure (2) at a rate of up to 10,000 g per minute and p er metre of width of the substrate to be treated.
8. Method as claimed in claim 7, characterised in that the flow rate of the powder in the deposition enclosure (2) is regulated between 700 and 900 grams per minute, per metre of width of the substrate and per conductor wire.
9. Method as claimed in any one of claims 1 to 8, characterised in that, after fixing the powder (5) on the substrate (1), the latter is heated, causing this powder to fuse, at least partially, enabling a substantially uniform coating to form on the substrate.
10. Method as claimed in claim 9, characterised in that, if the substrate (1) has a face to be coated that is made from an electrically conductive material, it is heated by induction after fixing the powder on this face.
11. Method as claimed in either of claims 9 and 10, characterised in that said heating is effected immediately at the outlet from the deposition enclosure (2) as the substrate is vertically displaced.
12. Device for continuously applying a powdered substance (5), in particular a powdered paint, to an oblong substrate (1) that is electrically conductive or has been rendered electrically conductive, having two faces, preferably in the form of a continuous strip, comprising
       a deposition enclosure (2) with an openi ng at its top part (4) and at its bottom part (6) through which said substrate (1) is fed in this enclosure (2),
       a supply apparatus (10) enabling said powder (5) to be introduced into this deposition enclosure (2) in a substantially continuous manner as the substrate (1) is fed through and
       means for creating at least one electric field in this enclosure (2), enabling this powder (5) to be applied to at least one of the faces of the substrate (1),
    characterised in that the enclosure has a feed inlet opening (4) for the powder (5) at its top part, enabling the latter to be introduced into this enclosure,
    and in that said means for creating at least one electric field comprise at least one electrode and the face or faces of the substrate to be coated, between which a stream of particles of said introduced powder flows by gravity, along said at least one face of the substrate to be coated, the powder particles thus being electrostatically charged due to said at least one created electric field and attracted onto the corresponding face of the substrate to be coated.
13. Device as claimed in claim 12, characterised in that the walls of the enclosure (2) are made from an electrical insulating material, preferably a transparent plastics material.
14. Device as claimed in either of claims 12 and 13, characterised in that the enclosure (2) has, along its walls facing the plane along which the substrate (1) can be fed through the enclosure (2), at least one electrode (9), to which a continuous high voltage can be appli ed relative to this substrate (1).
15. Device as claimed in claim 14, characterised in that the enclosure (2) contains a series of successive electrodes (9) distributed across the height of the enclosure (2) facing the plane in which the substrate (1) can be fed through the latter.
16. Device as claimed in claim 15, characterised in that said electrodes (9) consist of a series of conductor wires extending at a certain distance apart from one another in the plane along which the substrate can be fed through the enclosure, these wires being spaced apart from the walls of the enclosure (2) opposite this plane, leaving a passage for the powder so that it can be displaced by gravity through this enclosure.
17. Device as claimed in any one of claims 12 to 16, characterised in that means (8') are provided in order to recycle any powder which has not been applied to the substrate (1) and collects at the bottom of the enclosure.
18. Device as claimed in claim 17, characterised in that the conductor wires have a di ameter of 100 to 250 microns.
19. Device as claimed in any one of claims 12 to 18, characterised in that the supply apparatus (10) comprises a hopper (11) mounted above a cylinder (12) capable of rotating about a substantially horizontal axis (13) and having an opening (16) at its base, against which the surface of the cylinder (12) is disposed, this surface being provided with driving members (15), such as metal pins, so that a metered quantity of powder (5) can be drawn out of the hopper (11) when the cylinder (12) is rotated about its axis (13), through said opening (16) and onto the cylindrical surface of the cylinder (12), a rotary brush (17) being provided in order to lift the powder off said cylindrical surface in a continuous manner.
20. Device as claimed in claim 19, characterised in that the axis of rotation (18) of the brush (17) is located substantially on a level with the axis of rotation (13) of the cylinder (12), this brush (17) having relatively rigid pins (20) extending as far as the surface of the cylinder (12) at the instant at which they are disposed substantially in the plane coinciding with the axes of rotation (13) and (18).

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