US3835005A

US3835005A - Electrocoating process and apparatus

Info

Publication number: US3835005A
Application number: US00301617A
Authority: US
Inventors: M Dudley; P Claessens
Original assignee: Canada Wire and Cable Co Ltd
Current assignee: 555794 ONTARIO Inc; Noranda Inc
Priority date: 1972-10-27
Filing date: 1972-10-27
Publication date: 1974-09-10
Anticipated expiration: 1991-09-10

Abstract

A CONTINUOUS ELECTROCOATING PROCESS FOR ELECTROCOATING SUBSTRATES SUCH AS WIRE OR STRIP IN WHICH SUBSTRATE IS COATED IN A CELL THROUGH WHICH BATH LIQUID IS CONTINUOUSLY CIRCULATED INCLUDES THE STEPS OF CONTACTING THE SUBSTRATE ENTERING THE CELL WITH FRESHLY INTRODUCED BATH LIQUID AT RELATIVELY HIGH VELOCITY, COATING THE SUBSTRATE IN THE BATH, AND CAUSING FLOW OF COATING AND AT THE BATH SURFACE RADIALLY OUTWARDLY OF THE EMERGING SUBSTRATE TO REMOVE ACCUMULATED GAS BUBBLES FROM THE COATING, THEN WIPING THE WET COATED SUBSTRATE TO REMOVE EXCESS CLINGING BATH MATERIAL, AND THEN CURING AND COATING.

Description

Sept. 10, 1974 M. A. DUDLEY ETAL 3,835,005

ELECTROCOATINGPROCESS AND APPARATUS Filed Oct. 27, 1972 3 Sheets-Sheet 1 FIG. 1

3 18 5 k3?) I i-E E 20a D.C. POWER 2O SUPPLY 14 Sept. 10, 1974 M. A. DUDLEY ET ELECTROCOAI'ING PROCESS AND APPARATUS 3 Shets-Sheet 2 Filed 001:. 27, 1972 Sept. 10, 1974 M. A. DUDLEY. TAL

ELECTROCOATING PROCESS AND APPARATUS 3 Sheets- Sheet 5 Filed Oct. 27, 1972 States Patent 3,835,005 ELECTROCOATING PROCESS AND APPARATUS Michael Alan Dudley, Beaconsfield, Quebec, and Pierre Leon Claessens, Sainte Eustache, Quebec, Canada, as-

signors to Canada Wire and Cable Limited, Toronto, Ontario, Canada Filed Oct. 27, 1972, Ser. No. 301,617 Int. Cl. B01k 5/02; C23b 13/00 US. Cl. 204181 23 Claims ABSTRACT OF THE DISCLOSURE A continuous electrocoating process for electrocoating substrates such as wire or strip in which substrate is coated in a cell through which bath liquid is continuously circulated includes the steps of contacting the substrate entering the cell with freshly introduced bath liquid at relatively high velocity, coating the substrate in the bath, and causing iiow of coating liquid at the bath surface radially outwardly of the emerging substrate to remove accumulated gas bubbles from the coating, then wiping the wet coated substrate to remove excess clinging bath material, and then curing and coating.

This invention relates to electrocoating within a liquid bath and, in particular, to such a process wherein the article to be coated is a continuous length of electrically conductive substrate.

BACKGROUND OF THE INVENTION Electrocoating of articles within a liquid bath is well known and has been practised in one form or another for a number of years. Where the articles to be coated are individual, discrete articles, the practice has been similar to that exemplified by the disclosure of US. Pat. No. 3,362,899 issued to Gilchrist on Jan. 9, 1968. Thus patent discloses an apparatus adapted to carry the articles through the electrocoating bath by means of a conveyor which, one at a time, immerses the articles in the bath, maintains them submerged for a period long enough for the electrocoating to take place and then withdraws them from the bath and carries them to such subsequent drying, curing or other stations as may be appropriate.

In the apparatus illustrated in the Gilchrist patent referred to above, the tank containing the electrocoating bath is a metallic tank, and serves as cathode of the electrocoating apparatus and the article to be coated serves as the anode.

Electrocoating has also been adopted as a means for applying electrically insulating resinous coatings to continuous lengths of metallic conductors such as metal wire and strip. In such cases, the process must provide thorough, continuous coatings of high quality, to obtain the necessary electrical insulation of the conductor.

Further, the coatings should be substantially uniform, in nature and thickness, over the entire length of the metal substrate. Holes in the coatings cannot be tolerated if the coating is to provide insulation. It is difiicult to provide an electrocoating process for making resin-insulated metal wire and strip which can be run continuously for long periods of time, at an economically fast rate, and still produce coatings of the necessary high quality uniformly over long lengths of substrate. The process involves the steps of electrodeposition of resin from solution, drying and curing of the deposited resin, with handling of the continuously moving substrate before, during and after the coating and curing steps. When initially applied, the coating is extremely vulnerable, and care has to be taken not to damage it.

Previously proposed processes have not proven wholly satisfactory in providing electrically insulated conductors 3,835,005 Patented Sept. 10, 1974 of the nature of metallic wire or strip on a continuous basis, at an economically satisfactory rate, and with high quality of applied coatings.

BRIEF SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved process for continuously electrocoating electrically conducting substrate such as wire or strip, with electrically insulating resinous coating.

It is a further object to provide such an electrocoating process in which the resin is deposited in such a manner that the final cured coating is of high quality, continuous and substantially uniform over the length of the substrate.

The present invention provides a continuous electrocoating process as aforesaid in which the coating is applied in a highly compact condition to the substrate. It has been found that among the important features affecting the quality of the finally applied, cured coating are the quality and speed of deposition of the initial layer of resin deposited on the bare metal, and the avoidance of having gas bubbles entrapped in or adhered to the coating, when it passes into the curing oven.

In the process of the present invention, for obtaining improved results in these respects, there is provided a system whereby bare metal substrate entering the cell is initially contacted with freshly recirculated and hence replenished liquid coating composition under conditions of relatively high velocity, and whereby as the coated substrate emerges from the bath composition, flow of liquid composition is arranged to be radially away from the strip.

The manner in which the substrate is first contacted with electrocoating composition is important in obtaining resin coatings of high compaction and thus good overall quality. This initial contact takes place preferably where the concentration of coating resin in the coating bath is highest. Since, as electrocoating proceeds, resin is removed from the liquid composition and deposited on the substrate, this location is near to the place of introduction of fresh electrocoating composition. As the concentration of resin in the composition decreases, the conductivity of the composition increases. As the conductivity increases there is an increasing likelihood of electrolysis of water occurring, generating bubbles of gas at the electrodes. If this occurs, there is a risk that gas generated at either electrode will become entrapped in the coating being deposited. It is particularly important, in obtaining electrically insulated coatings, that gas entrapment in the first applied layer of coating be avoided. Subsequently, the coating is subjected to high tempera tures to cure it, whereupon trapped gas will expand and perhaps escape, leaving a hole in the coating. In addition, at constant applied voltage and because of increased conductivity due to resin depletion from the electrocoating composition, there is a increase in the current flowing and hence increased heat generation. Since the coating process is temperature sensitive, this should be avoided as far as possible, in order to obtain uniform coating over extended continuous runs.

Further in order to obtain good compaction of the resin, the bare substrate entering the bath initially contacts the coating composition under conditions of relatively high velocity. This assists in ensuring highest concentration of resin at point of initial contact, as discussed above, and also helps to prevent any gas bubbles from clinging to the substrate, either from electroylsis or any other source, by mechanical action.

By arranging that incoming substrate initially contacts freshly entering composition, it is ensured that the coating composition initially contacting the substrate is of constant composition. Since the major proportion of the coating is deposited on the substrate in the initial stage of the electrodeposition process, this helps to improve uniformity of the final coatings obtained.

On emergence from the bath composition, flow of composition at the bath surface away from the substrate helps to convey gas bubbles away from the emerging substrate. Some generation of gas at the electrodes by electrolysis of water at various locations in the cell is not entirely avoidable. Also, foam tends to be generated in the process, after it has been running for a time, due to the mechanical action of continuously moving substrate contacting continuously circulating liquid composition. This is also led away from the substrate by the flow .of liquid at the bath surface, so that it does not pass into the curing oven with the coated substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferably the substrate moves vertically upwardly in the process of the invention, through an electrocoating bath and a curing oven arranged vertically above the bath. This vertical path of travel allows the substrate to follow a closely predetermined path, without sag: so that its distance from an electrode is constant and controlled. Further it allows the substrate to pass through the electrocoating bath and the oven without having to make direction-changnig contact with structures, so that the coating is not physically damaged prior to curing.

Preferably also, after emerging from the bath, the substrate passes close to wiping means, arranged to remove clinging, drag-out coating composition from the substrate before it enters the curing oven. It is preferred to use physical structures as wiping means, finely adjusted as to their position relative to the coating substrate, rather than air wipes, since the latter tend to cause roughening of the surface of the electrodeposited coatings. When the substrate is fiat metal strip, the wiping means constitutes a structure adjacent each fiat surface of the strip. Smooth surfaced structures are preferred, such as glass rods, plastic rods, bars, strips, sheets and the like, mounted transversely to the direction of travel of the strip. The Wiping means are arranged to contact the drag-out material clinging to the substrate, but not to damage the electrodeposited coating, which is uncured at this stage and hence vulnerable to physical damage. This positioning is a straight-forward matter, and adjustments can be made from visual observations, since the drag-out material is different in appearance from the electrodeposited material, so that the wiping means can be readily adjusted to remove only the drag-out material and not to damage the electrodeposited material. Another form of wiping means which has been successfully employed is a pair of thin plastic sheets or films, one on each side of the strip, which are secured at one of their edges, but loose at their opposite edges near the passage of the strip. These opposite edges overlap, and the strip passes between them so that they are dragged upwardly by the upwardly moving strip to trail on the coated surfaces thereof. Such sheets remove drag-out material, but their contact with the electrodeposited coating is so light that it is not damaged thereby. Thin films of poiyethylene are suitable for this purpose.

Preferably after initial coating and whilst still within the coating bath, the moving substrate encounters a flow of coating composition under conditions of high velocity again at at least one location. This also assists in removing any deposited or forming gas bubbles from the coating surface. This may be arranged by a suitable arrangement of baffies and apertures therein at any location adjacent the moving substrate in the bath, causing high velocity flow of the composition continually circulating through the bath.

As noted, at the emergence of the coated substrate from the bath, flow of liquid composition is arranged to be radially away from the substrate. This is conveniently arranged by means of an overflow or weir at at least two diametrically opposed locations on either side of the substrate. In such arrangement, new bath composition enters at the bottom of the bath, is moved upwardly through the bath and overflows at the top, from where it is recirculated. Preferably the weir surrounds the emerging strip. In addition to causing flow of composition away from the emerging substrate, this also provides another advantage. As noted, electrolysis of Water is likely to occur during the process with generation of hydrogen gas at the cathode. Some generated hydrogen gas tends to dissolve in the composition, so that it gradually accumulates as the composition is recirculated, until it is released as gas bubbles on a subsequent pass through the electrocoating bath. However, the provision of a weir, over which the composition flows in a very thin layer, facilitates the escape of dissolved hydrogen and hence reduces the problem of hydrogen accumulation in the composi tion and the risk of hydrogen bubble deposition on the substrate.

In order to facilitate this escape of occluded gases from the bath composition still further, it is desirable to have a relatively long residence time of the composition in the reservoir and associated parts. In a continuously circulating system as in the present invention, this is best expressed in terms of the relationship of volume of composition in the cell to volume of composition in the total system. It is preferred that the volume of composition in the total system should be at least 4 times that in the cell alone; most preferably the volume of composition in the total system should be at least 7 times the volume in the cell. Normally it is not practicable for the total volume to be more than about 12 times that in the cell, because of the large solution volumes which such would entail.

The process of the invention is designed to run continuously over long periods of time on substrate such as metal strip, foil and wire. Electrocoated insulated, metal strip is used in transformer windings. Electrocoated metal foil is used for capacitor fabrication. Special care has to be taken in coating metal foil because of its lack of tensile strength. Means has to be provided to avoid subjecting it to excessive tension in the process whilst maintaining substantially constant speed of passage of foil through the process. A suitable such means is described in detail below.

Preferred processes and apparatus of the invention will now be described with reference to the accompanying drawings, for illustrative but not limitation purposes.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 is a diagrammatic view for carrying out the process of the present invention;

FIG. 2 is a view in section on the line 22 of FIG. 1;

FIG. 3 is a detail, in section, of the top part of the electrocoating bath of FIG. 1;

FIG. 4 is a diagrammatic vertical cross-section of the wiping arrangement of the apparatus of FIG. 1, located above the bath;

FIG. 5 is a diagrammatic view in vertical cross-section of the lower part of an alternative bath arrangement for carrying out the process of the invention; and

FIG. 6 is a diagrammatic view in vertical cross-section of an alternative apparatus for carrying out the process of the invention, particularly adapted for use in coating metal foil.

In the drawings, like reference numerals indicate like parts.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1 of the drawings, the continuous, elongated substrate to be coated is shown at 10, being withdrawn from a supply which is illustrated as reel 11 carried upon a stand 12. In this specific embodiment, the substrate 10 is an aluminum metal strip, of width about 1 inch and thickness about 0.01 inch, to be provided with an electrically insulating coating of resin. The strip is passed over a first guide roller 13 which changes the direction of travel of the strip from substantially horizontal to vertical and enters the electrocoating cell or bath container 14 through a seal structure comprising opposed teflon-tipped

strips

15, 16 with which the strip 10 makes sliding contact, in the bottom wall 26 of the bath container 14. The container 14 for the electrocoating bath is rectangular in horizontal cross-section and contains an auxiliary electrode 17. The electrode 17 is of fork-like configuration and is arranged within the bath relative to the strip 10 with

arms

17a, 17b thereof one on either side of the strip 10 and adjacent the side walls of the cell 14, so as to ensure total coverage of the substrate by the coating that is electro-deposited within the bath. In the specific embodiment illustrated a DC. power source 18 is provided with its negative terminal electrically connected to the auxiliary electrode 17, so that it constitutes the cathode of the cell. The auxiliary electrode 17 is mounted in the cell 14 in a manner so that it does not make electrical contact with any electrically conductive parts of the bath structure. If the cell 14 is of electrically conductive metal the cathode is insulated therefrom by suitable means, whilst remaining adjacent to the side walls thereof. Preferably, however, the cell 14 is a dielectric material such as polyvinyl chloride or other plastic. The positive terminal of the DC. power source 18 is electrically connected to the strip 10 by means of a contact roller 19. The strip 10 and all exposed conductive structures are grounded.

The cell 14, which as noted is generaly rectangular in cross-section, holds the liquid electrocoating bath composition 20. This composition is continuously circulated through the cell 14 from a reservoir (not shown) where it is continuously monitored and replenished with fresh amounts of ingredients to compensate for material deposited on the strip 10 moving continuously through the cell 14, so as to keep the composition 20 substantially constant. The composition 20 enters the cell 14 through

inlet ports

21, 22 located near the bottom of the tank and in close proximity to the point of entry of the strip 10 into the cell 14 through

seal

15, 16.

The lower portion 23 of the cell 14 has

lateral extensions

24, 25 on two sides thereof so that its lower portion is generally rectangular in horizontal cross-section (FIG. 2). The

inlet portions

21, 22 through which liquid coating composition is continuously introduced into the cell connect with the

lateral extensions

24 and 25 respectively. The

electrode arms

17a and 17b, extend downwardly to positions close to the bottom wall 26 of the cell 14. As shown in FIG. 2, the

electrode arms

17a, 17b have substantially the same width as the cell 14. Thus,

lateral extensions

24, 25 communicate with main body of the cell 14 only by way of the small gaps 27 and 28 left between the bottoms of the

electrode arms

17a, 17b and the bottom wall 26 of the cell. Thus fresh coating composition introduced into the cell 14 continuously contacts the newly entering strip 10 adjacent the point of entry of the strip into the cell 14, and after passing through the narrow gaps 27 and 28. By this arrangement, fresh coating composition contacts the strip 10 under relatively high velocity, the

electrode arms

17a, 17b having acted as baffles for the solution. Thus a compact initial layer of resin is coated on the substrate.

In the main body of the cell 14, a short distance above the

lateral extensions

24, 25, is provided a baffle plate 2 9. The baflie plate 29 is seen in plan in FIG. 2, and comprises a generally rectangular plate of dielectric material, e.g. PVC, centrally apertured at 30 to allow passage of the strip 10 therethrough, without contacting the sides of the aperture 30. The baflie plate 29 is secured to the sides of the cell 14 and provision is made for passage of the

electrode arms

17a, 17b through it. The baffle plate pro- 6 vides a location at which the strip encounters solution under conditions of high shear, the solution being pumped continuously through cell 14 in an upward direction and hence through aperture 30. This high shear location provides a means of removing bubbles which may be form ing on or adhering to the coating.

As shown in more detail in FIG. 3, at its upper end, the cell 14 is provided with a weir 31 and a peripheral, overflow-receiving channel 32, both of which extend around all four sides of the cell 14. An outlet port 33 is connected to the channel 32, through which overflow composition 20a is removed from the cell and returned to the reservoir previously referred to, for replenishment and recirculation. The top edges of the

electrode arms

17a and 17b are disposed just below the top edges of the walls of the cell 14. Thus it will be appreciated that, in the system illustrated, bath composition 20' is being continuously circulated through the cell 14, generally upwardly, from

inlet ports

21 and 22, until it overflows over the weir 31 which surrounds the strip 10, into the channel 32; the overflow composition 20a is removed from channel 32 through outlet port 33. By means of this arrangement, at the upper surface of the liquid bath, the liquid composition flows radially outwardly of the strip emerging from the bath, as the liquid flows over the surrounding weir 31.

Such an arrangement tends to draw away from the coated surfaces of the strip 10 foam which tends to form at the surface, due to the agitation effect of the continuously moving strip and solution, and bubbles of gas generated in the cell 14 and clinging to the coating. Removal of such bubbles from the coating before it is cured helps to ensure a final cured coating free from holes, cracks, blisters or similar discontinuities, and surface defects.

The bath composition is an aqueous system, suitably comprising a dilute aqueous solution and/or dispersion of a heat curable water soluble resin together with other components as is well known in the art. Other bath compositions, of course, may be employed depending upon the nature of the coating that is to be applied and the purpose that it is to serve.

It will be noted that from the roller 13, the substrate 10 passes vertically into the electrocoating cell 14 through the

seal structure

15, 16 in the bottom wall 26 thereof and that it continues its vertical travel, emerging from the cell at 34. As it emerges from the cell at 34, the sub strate is wet and carries with it, in addition to the deposited coating, clinging liquid material which is referred to as drag-out. The drag-out material should be removed before curing, to give a satisfactorily uniform final dielectric coating.

For this purpose, there are provided

smooth glass rods

35 and 36, located on each side of the emerging, coated, wet strip 10. This arrangement is illustrated in more detail in FIG. 4. The

rods

35 and 36 are finely adjusted in relation to the moving strip 10, so that they remove the drag-out material 37 on the surface of the strip and destroy any clinging gas bubbles, but they do not harm the electro-deposited coating 38 on the strip. In the case illustrated, where as previously mentioned a metal strip of 1 inch width and 0.01 inch thickness is being coated, the

rods

35 and 36 are typically 1 cm. in diameter. For convenience, they may be mounted on the top of the cell structure but in any case, they should be close to the liquid surface of the bath.

After passing the smooth surfaced

rods

35 and 36, the strip 10 continues its vertical travel upwardly into the drying and curing oven 39 of a suitable design wherein the coating is heated and cured in a manner that is well known in the art. The vertical length of the curing oven will depend upon the speed of travel of the substrate and the length of time that the curing operation requires for a particular coating material at the temperatures that exist within the oven. These are parameters which are well understood and which require no detailed disclosure.

As the substrate 10 emerges from the curing oven 39, it is preferably cooled, for example, by means of air blowers or air coolers 40. As illustrated in FIG. 1, these are preferably arranged to blow generally downwardly onto the strip 10 so as generally to assist in reducing the amount of fumes issuing from the top of the oven 39, and depositing on roll 41. It will be appreciated that, throughout the electrocoating and curing processes, since leaving roll 13, the strip has not made physical, direction changing contact with any structure. After emerging from the oven 39 and being cooled, however, the coating is now in its final durable state, and the risk of damage to it is very much decreased. Thus, after subjection to the air coolers 40, the strip passes over an upper roller 41 which constitutes the first physical contact the substrate has encountered since entering the electrocoating cell 14 through the

seal structure

15, 16. Thereafter, the substrate may pass over a further direction changing roller 42, if one is necessary, and is then lead to a point of accumulation constituted by a rewind stand 43.

The substrate 10 is moved through the apparatus solely by the application of a tension applied to the substrate at the point of accumulation by the rewind stand 43. Thus, the greatest physical force is applied to the substrate at the point where the coating is best able to Withstand it, namely, at a point considerably downstream from the coating and curing stations.

Further, it will be appreciated that the preferred vertical travel of the substrate through the electrocoating cell, past the smooth surface rollers and through the drying and curing oven, enables an arrangement whereby the coated substrate makes contact with no physical structure from the time it is coated until that coating has been cured. Travel other than in a vertical direction, over the lengths that are involved, introduces a tendency for the substrate to sage and depart from a closely predictable path thereby rendering accidental contact and resulting damage more likely.

FIG. illustrates diagrammatically an alternative arrangement for the bottom part of the cell 14. In this arrangement the

auxiliary electrode arms

17a and 17b extend down to near the bottom Wall 26 of the cell as before, but do not act as baflies. Inlet ports for the incoming electrocoating bath composition comprise

pipes

44 and 45 which enter through the cell walls and through the

auxiliary electrode arms

17a and 17b, to points on either side of, but in close proximity to, the strip at its point of entrance into the cell. The

pipes

44 and 45, which are insulated so that they are not part of the auxiliary electrode, ensure high velocity of the composition initially contacting the strip 10. In a similar manner to the previous embodiment, a baflie plate 29 is provided, through a gap in which the strip passes, providing a high shear location.

FIG. 6 illustrates diagrammatically an alternative embodiment of the process of the invention, which is particularly suitable for coating very thin metal foils, e.g., aluminum. Such foils when provided with insulated resin coatings are used as capacitor foils. As will be appreciated, however, continuous electrocoating of very thin foils provides a particular problem because of the lack of physical strength of the foils, particularly at the temperatures likely to be encountered in the process. Any application of tension to the foil risks breaking the foil, necessitating shut-down of the process for repairs.

The process illustrated in FIG. 6 provides a constant tension arrangement for raising the foil through the cell and the curing oven, so that the tensile strength of the foil is not over-reached. This arrangement includes dancer roll 46 which is set to apply a predetermined tension to the strip. When the tension tends to increase, the dancer roll 46 responds by rising with the foil. Sensing means 47 are associated with the dancer roll 46, which sensor means are coupled to the driven feed roll 48 to vary the speed of drive to correct the tension variation determined by the dancer roll. The feed roll 48 feeds the foil strip by frictional engagement therewith, assisted by an idle weight roll 49 located above the strip 50 and feed roll 48 and bearing on the strip 50. The arrangement also includes similar means provided downstream of the oven, for feeding the foil strip through the apparatus and to the windup roll 43. There is provided a drive roll 51 and associated weight roll 52 frictionally driving the strip 50. The drive roll 51 is driven by a motor at a carefully controlled, constant speed, by means of an SCR speed control circuit. Normally the speed of the feed roll 43 is the same as that of the drive roll 51, but the precise speed of feed roll 48 is controlled by dancer roll 46 so that the tension in the foil strip 59 is kept constant at a predetermined value.

A similar dancer roll 53 and associated sensing means 54 is provided downstream of the drive roll 51. By this means, the speed of windup reel 43 is automatically ad'- justed to maintain constant tension in the foil 50 during windup.

The tension followed by dancer rolls 53 and 46 is preset so that it will not exceed the strength of the foil strip 50 under the conditions of operation, but also so that it will keep the foil strip 50 taut and following a predetermined path through the apparatus. Foil is so thin and flexible that unless kept taut it will flap and thereby vary its position in the cell 14 relative to the

electrode arms

17a, 17b, which would lead to non-uniform electrocoating. This is avoided by proper tensioning using the dancer rolls.

The apparatus illustrate diagrammatically in FIG. 6 also varies from that in FIG. 1 in respect of the means for introducing the foil strip 50 into the cell 14. In the embodiment of FIG. 1, seal was effected at the point of entry of the strip 10 by frictional engagement with Teflon tipped

rods

15, 16 defining the entrance to the cell. Such a frictional arrangement is not satisfactory for foil however, because of the risk of breakage. Consequently in the arrangement in FIG. 6, the foil 50 first enters a lower vessel 55 through a gap in the bottom wall 56 thereof. On moving vertically upwards through vessel 55, the strip 50

encounters dragging foils

57, 58 of sheet material e.g. metal foil. Conveniently, these can be made of molybdenum. These foils 57, 58 are anchored to the bottom wall 56 of the vessel 55. As the foil strip 50 moves upwardly it tends to drag

foils

57 and 58 upwardly With it, until they are in substantially sealing, frictional engagement with the moving strip 50. Such frictional engagement of foil with foil does not risk breaking strip 50. Then the strip enters cell 14 through an aperture 59 in the bottom wall 26 thereof, without making contact with the sides of the aperture. There is consequently a leakage of bath composition 20 through this aperture 59. The leaked composition 60

encounters dragging foils

57 and 58, accumulates in the bottom of vessel 55 from where it is led via conduit 61 back to the composition gaservoir (not shown) for replenishment and recircula- The remainder of the apparatus of FIG. 6 is substantially as described with reference to FIG. 1. The cell is of the same general configuration, having an electrode 17 with

arms

17a and 17b disposed as described, an apertured bafiie plate 30, a weir 31, overflow channel 32, oven 39. air coolers 40 and guide roll 41.

An apparatus similar in principle to that illustrated in the accompanying drawings is used for electrocoating wire of circular or rectangular cross-section, diifering mainly in the shape of the auxiliary electrode and cell bottom mechanism. Preferably, the auxiliary electrode is arranged to surround the wire.

In other embodiments of suitable apparatus, the auxiliary electrode can be the bath structure itself, so'that the upper wall of the auxiliary electrode constitutes the weir. It is, however, preferred to have an electrically insulating bath structure, for reasons of safety.

The weir arrangement illustrated in the accompanying drawings arranges radial flow of composition away from the emerging strip. It is not essential that the weir extend all the way around the emerging strip to achieve this. A weir disposed at substantially diametrically opposed locations on either side of the strip would provide a similar effect. This flow of liquid away from the emerging strip removes the gas bubbles deposited thereon, to a substantial degree. A surrounding weir is, however, preferred.

It will of course be appreciated that the smooth surfaces provided for removal of drag-out material can be of a wide variety of materials and design. The material and design selected is a routine matter, to fit most conveniently in any specific apparatus chosen. Glass rods are merely examples of a suitable structure.

In the preferred process of the invention, liquid bath composition is moved continuously through the electrocoating bath, as described. In such processes, temperature control of the cell is preferably provided, since the thickness of the coating deposited by electro-deposition is dependent upon the cell temperature, and this temperature has a tendency to rise in a continuous process as the process proceeds. This is conveniently done by provision of a heat exchanger means in the recycle system, remote from the cell itself. Preferred temperature ranges in the bath depend upon specific ingredients of the cell composition, but are normally from 10 to 50 C.

For best electrocoating at economically satisfactory rates, it is desirable to have a relatively fast flow of liquid upwardly through the cell, i.e. a relatively short residence time. Satisfactory flow rates with given apparatus can readily be determined by a simple trial experimentation. In a typical example, a cell of capacity litres, of rectangular cross-sectional area about 100 sq. centimeters and arranged generally as illustrated in FIG. 1, was satisfactorily operated at a liquid flow rate of about litres per minute, with a speed of travel of the strip through the cell of from about 2 to 7 meters per minute. In this same cell, each limb of the cathode was spaced about 5 cms. from the strip.

Having described the preferred method of carrying out the process of the invention, it is pointed out that this description is intended to be illustrative only and that the scope of the invention is defined in the appended claims.

What we claim as our invention is:

1. A continuous process for coating an electrically conductive substrate by electrocoating within a liquid bath, comprising the steps of:

(a) withdrawing the substrate from a source of supply and continuously passing the substrate into an electrocoating cell;

(b) continuously introducing into the cell from a reservoir an electrocoating bath liquid containing a freshly replenished coating material and causing initial contact of the substrate entering the cell with bath liquid being introduced into the cell under conditions of relatively high velocity;

(0) continuously performing an electrocoating step on that portion of the moving substrate that is within the cell to apply a coating to the substrate;

(d) passing the substrate through a high shear zone at a location dawnstream from the location of said initial contact thereby subjecting the substrate for a second time to electrocoating 'bath. liquid flowing at a relatively high velocity;

(e) causing a flow of electrocoating bath liquid on the surface of the liquid bath radially outwardly of the substrate emerging from the bath to remove gas bubbles from the coating surface;

(f) continuously removing the outwardly flowing bath liquid from the cell and returning it to said reservoir;

(g) replenishing the coating material in the bath liquid at said reservoir;

(h) leading the substrate out of the electrocoating cell,

and then curing the applied coating;

(i) and subsequently leading the coated substrate to a point of accumulation.

2. The process of claim 1 wherein the coating material in the electrocoating bath liquid is a coating resin.

3. The process of claim 1 wherein the electrocoating bath liquid is continuously monitored and replenished with fresh amounts of ingredients to compensate for material deposited as a coating on said substrate in said cell.

4. The process of claim 1 wherein radially outward flow of bath liquid from the emerging substrate is arranged by provision of a weir over which bath liquid leaving the bath flows continuously, positioned at least two diametrically opposed locations on either side of the substrate.

5. The process of claim 4 wherein the liquid coating composition is continuously cycled through the cell in a generally upward direction, recovered from the weir overflow, and returned to the reservoir where it is replenished with coating resin and recycled to the bottom part of the cell.

6. The process of claim 5 wherein the volume of coating composition in the total system is from 4 times to 12 times the volume of coating composition in the cell.

7. The process of claim 1 wherein the electrocoating bath liquid is introduced into the cell at a location in close proximity to the entry point of the substrate.

8. The process of claim 7 wherein the substrate comprises thin electrically conductive foil and is fed under predetermined controllable tension from the source of supply to the point of accumulation.

9. The process of claim 7 which includes the additional step of wiping drag-out coating composition from the surface of the coated substrate, after its emergence from the bath and before curing thereof.

10. The process of claim 9 wherein said wiping is accomplished by passing the substrate between smooth surfaced structures positioned to contact the drag-out ma terial clinging to the substrate but not to damage the electrocoated deposited thereon.

11. The process of claim 10 wherein said smooth surfaced structures comprise glass rods and the substrate has flat surfaces.

12. The process of claim 9 wherein said wiping is accomplished by passing the substrate between thin flexible sheets or films arranged to trail on the surface of the continuously moving substrate.

13. The process of claim 9 wherein the substrate moves vertically upwardly continuously through the electrocoating cell and through the curing stage.

14.. The process of claim 13 wherein the substrate constitutes one electrode of the electrocoating cell and the auxiliary electrode of the cell comprises at least one arm on each side of the moving substrate.

15. The process of claim 14 wherein the arms of the auxiliary electrodes act as baffles between the entering bath liquid and the entering substrate and are arranged for providing the high velocity flow of entering bath liquid where it initially contacts the substrate.

16. In apparatus for continuously electrocoating elongated electrically conductive substrate; and electrocoating cell, comprising: a container for electrocoating bath liquid having an aperture in its bottom wall for the passage of said substrate; a baflle extending transversely across said cell separating the interior thereof into upper and lower portions, said bafile having an aperture therein in alignment with the opening in the bottom wall for the passage therethrough of said substrate, said aperture being of a size to permit the substrate to pass therethrough without contacting said baflie and to form a restricted flow passage and high shear area for the electrocoating bath liquid flowing from the lower to the upper portion of the cell; electrode means disposed within the upper and lower portions of the cell; said cell having inlet means in the lower portion thereof for introducing electrocoating bath liquid into the cell, and means connected with the upper end of the cell for withdrawing electrocoating bath liquid and causing surface flow thereof radially outwardly 'of the substrate, said inlet means being arranged to direct electrocoating bath liquid under relatively high velocity against the substrate immediately after it enters the cell.

17. Apparatus as claimed in claim 16 wherein the electrode means comprises auxiliary electrode arms located on each side of the moving substrate and extending downwardly in the cell to positions close to the bottom wall of said cell, arms separating the location of introducing the bath liquid from the location of entry of the substrate, and acting as bafiies to cause relatively high velocity flow of the introduced bath liquid under said arms to contact the substrate.

18. Apparatus as claimed in claim 16 wherein said means for causing radially outward surface flow comprises a weir and associated overflow receiving channel, said Weir being positioned at at least two diametrically opposed locations on either side of the substrate, and said overflow channel being adapted to receive liquid bath composition overflowing the weir and return it for replenishment and recirculation to the cell.

19. Apparatus as claimed in claim 16 additionally including an oven and Wiping means located between the cell and the oven, said wiping means comprising smooth surfaced physical structures between which the substrate passes, in a position to contact dragout material on the surface oi": the electrocoated substrate but not to damage electrodeposited coating.

20. Apparatus as claimed in claim 16 wherein a vessel is disposed immediately below the bottom wall of the cell, said vessel having a gap in its bottom wall for the passage therethrough of substrate to be coated a dragging foil of sheet material fixed to the bottom Wall of the vessel on either side of said gap for sealing, frictional engagement with said substrate.

21. Apparatus as claimed in claim 20 in which said dragging foils are formed of metal.

22. Apparatus as claimed in claim 21 in which said metal is molybdenum.

23. Apparatus as claimed in claim 16 wherein said inlet means comprises a pipe disposed on either side of said substrate and closely adjacent said bottom wall, the outlet end of said pipes being directed at said substrate and located closely adjacent thereto.

References Cited UNITED STATES PATENTS 2,127,413 8/1938 Leguillon 204300 2,956,937 10/1960 Thomson 204-181 3,496,082 2/1970 Orem et a1 204-181 HOWARD S. WILLIAMS, Primary Examiner US. Cl. X.R.