IE44382B1 - Electrolytic production of perforated metal foil - Google Patents

Abstract

1524985 Electrolytic perforation of metal foil INCO EUROPE Ltd 24 Aug 1976 [26 Aug 1975] 35178/75 Heading C7B An anodically polarized continuous metal foil is perforated in an electrolytic bath, one face contracting an inert endless surface e.g. a Ti or rubber surfaced roll or band, and the other contacting a perforated Ti band mask which may be pressed against the foil by an adjustable or sprung roller. A curved hollow and perforated cathode may be supplied internally with electrolyte which may be filtered or ion-exchanged in a recycle line. Ni, Cu or Fe may be so formed into battery electrodes.

Description

This invention relates to a process for the electrolytic production of continuous lengths of perforated metal foil and apparatus therefor.

There is a commercial requirement for continuous 5 lengths of thin metal foil, having a repeating pattern of perforations therethrough, for many applications including components for the optical, electronics and battery making industries.

This need has hitherto been supplied by producing . Ιθ perforated foil continuously in an electroforming process in which the foil is electrodeposited on a pitted mandrel. The process has been described in a paper entitled Electoforming of Nickel Screens delivered by J Van der Waals at the Symposium on Nickel Deposition in the Engineering Industries held in October 1963 and abstracted.in the Nickel Bulletin of October 1963 page 235-236.

Whilst the process works well, it proves to be uneconomic due to the comparatively short life of the mandrels, their initial cost and that of their reclamation after they have been mechanically and/or chemically degraded during use in the process.

Anodic etching techniques have been used for shaping metal bodies for example UK Patent 561 788, and in UK Patent 1 009 518 metal foil has been perforated batch-wise by sandwiching the foil between two confirming masks of the desired configuration and subjecting the foil to an anodic etching treatment. This process however does not lend itself to the continuous production of perforated foil because of the problems of aligning the masks and of ensuring that they are conforming.

The present invention is based on the discovery that considerable economic savings may be achieved by an anodic dissolution process by which a regular pattern of perforations of controlled size may be readily achieved.

According to the present invention a process for the production of a continuous length of perforated metal foil comprises passing a continuous length of foil through an electrolytic etching bath with one face of the foil maintained in contact with an endless Burface which is inert with respect to the electrolyte and the other face of the foil maintained in contact with an endless perforated titanium mask, and applying a potential difference of less than 10V across the foil and a cathode immersed in the bath whereby the foil exposed to the bath through the perforated mask is anodically etched away. Although other materials are available which could be used for masking, it has been found that only titanium is both corrosion - 3 44383 resistant under anodic conditions and has the dimensional stability required. Moreover the mask may readily be produced, perforated with any desired hole pattern by conventional techniques . The endless surface may be that of an endless belt or roll.

The process is most suitably applied to metal foils of nickel, copper, iron and alloys based thereon and up to about 125 Jim thick.

Xn accordance with a further feature of the invention apparatus for perforating metal foil comprises a tank suitable for holding an electrolytic etching solution, an endless belt or roll having a surface which is inert to the electrolyte to be used, and an endless perforated titanium mask both supported so that in use a foil to be perforated may be passed through the tank held with one face in contact with the surface of the'endless belt Or roll and the other face in contact with the titanium mask, means for supplying, in use, anodic current to a foil to be perforated, a I cathode located adjacent but spaced apart from the titanium mask and means for supplying current to the cathode in use.

The endless belt or roll is advantageously made of,or covered with, a non-conducting flexible material such as rubber.

In a preferred process foil is passed across a large diameter roll so that the foil is in contact 44383 with the roll over at least about 50% of the circumference of the roll. For 4. jim thick foil a typical roll would be 15 cm in diameter.

Preferably the cathode substantially conforms in shape to the belt or roll so that an approximately constant distance is maintained between the foil and the cathode during the major proportion of the passage of the foil through the electrolytic bath. The space between the foil and cathode is preferable less than 20 mm, and is normally about mm.

It has been found to be advantageous for the cathode to be provided with regularly located holes along its length, and to be connected to a manifold through which electrolyte is supplied so that in use electrolyte may be pumped through the holes and jetted on to the surface of the foil exposed through the mask.

The titanium mask endless belt, is conveniently perforated in the desired configuration by photomechanical etching techniques. In a preferred technique titanium strip is thoroughly cleaned and dip-coated with photo-resist prior to air drying and baking. The coated titanium is then inserted between and in contact with two identical photomasks connected in register and both sides exposed. The exposed mask is then developed to remove unexposed reset, baked and etched on 44382 both sides until complete perforation ie achieved.

To compensate for under-cutting during the etching stage, each dot on the photomask is smaller in size by an amount equal to the undercut. Subsequently photo-resist is removed with solvent, the surface of the mask cleaned ad the mask trimmed to length and spot welded to form an endless belt.

Xn preferred apparatus for carrying out the present process the titanium mask is preferably mounted on three or more rotatable rolls, at least one of which is adjustable, or sprung, so that in use the mask is held tightly against the foil during its passage the electrolytic bath. Conveniently drive is applied to one of the rolls on which the mask is mounted and the foil is transported by friction between itself and the mask. The bearing friction reinforces the clamping pressure between the mask and foil held against the inert belt or roll. Current is preferably fed to the foil by passage over a current-feeding roll positioned before the inert belt or roll, or via the belt or roll itself if made of a conducting material such as titanium.

Any suitable etching bath may be used in 25 processes of the present invention and typical electro-chemical machining electrolytes are particularly useful. For the perforation·of nickel foil a high chloride electrolyte is necessary to obtain good perforations without passivation occuring. Where the electrolyte bath is not agitated, very low pHs, i.e. about 1 may also be necessary to prevent passivation. It has been found generally that forced circulation of the electrolyte allows the perforation rate to be increased. Ά preferred electrolyte for the production of perforated nickel foil comprises approximately 20% sodium chloride solution at a pH of 1 to 7, and preferably •of 4 to 6. At higher pH values the nickel precipitates as the hydroxide and if allowed to build up in the sodium chloride electrolyte it is deposited on the titanium mask. This is undesirable for long term operation and at these pHs it may be desirable to continuously separate the precipitated nickel hydroxide from the sodium chloride electrolyte. At lower pHs the nickel is retained in solution and if the concentration is limited, for example by ion exchange techniques, there will be no deposition of nickel at the cathode.

It is important that the potential on the titanium mask surface should not be allowed to exceed 10 volts anodic which would allow immediate corrosion to occur. Such a circumstance 44383 would only be likely to occur when complete passivation Of the foil occurred due, for example, to failure of the pump circulating the electrolyte or to failure of the foil-driving means causing all exposed foil to be dissolved. Preferably when operating the process of the present invention means for automatically cutting off power supply to the foil in the event of a pump of drive failure should be incorporated into the apparatus.

It has been found, surprisingly, that in processes of tha present invention, the current required for perforation may be less than theoretical. The etching process tends to occur from the outside of the holes inwards and normally small discs of the metal foil drop out as the hole is trepanned in the foil. Thus the current theoretically required to dissolve the central portion of the hole is not utilised in practice. Although the process may be operated at current densities of about lOOA/dm i20 it is advantageous to operate at as high a current density as possible, conveniently about 600A/dm for the perforation of 4 jum foil· Higher current densities are possible, the perforation rate being governed by the current carrying capacity of the foil.

The perforated foil leaving the electrolyte bath may conveniently be passed through a suitable - 8 44382 washing bath, and then dried by passage through an oven. The fine metal foil can be caused to crease by normal air convection in the environment of the apparatus. This is overcome in the preferred process by cooling the perforated foil leaving the oven by jets of air, and ths cooled foil ig interleaved with tissue as it passes onto the take-up reel.

The process of the present invention may be used to produce perforated foil having holes up to greater than 6mm in diameter and in which up to 50% of the foil is perforated, and is particularly useful for the production of perforated foil for use in tha production of battery electrodes, for example as disclosed and claimed in UK patent specification no. 1 246 048.

A process in accordance with the invention will now be described by way of example only, with reference to the accompanying drawing which shows schematically apparatus for carrying out the process.

A 4 jum thick nickel foil 1 is fed from a supply reel 2 over a current input roll 3 and over guide rolls 4, 5 and a large diameter inert rubber25 covered roll 6 positioned so that the foil contacts the roll 6 over about 50% or more of the circumference of the roll 6. A titanium mask 7 in the form of an endless belt was prepared from 100 pm thick fully-annealed titanium strip perforated in a desired hole pattern by the photo-mechanical etching technique described above. The titanium mask 7 is mounted on three rolls 8, 9, and 10, one of which 10 is adjustable and is set so that the titanium mask is held tightly against the nickel foil 1 passing between the mask .7 and the rubber covered roll 6. The rolls 6, 8, 9 and 10 are encased within a polymethyl methacrylate, tank, not shown The foil 1 is driven by friction between itself and the titanium mask 7 which in turn is' driven by a motor controlling the roll 9. A nickel cathode 11 conforming in shape to t he curvature of the rubber covered roll 6 is positioned facing the titanium mask 7 and the portions of nickel foil 1 exposed through the perforations of the mask.

The cathode 11 is provided with a series of holes along its axis of symmetry, and is linked to an electrolyte manifold 12 through which a suitable electrolyte, such as 20% chloride solution at pH 1 to 5 is jetted. The gap between the cathode and the foil is of the order of 2mm. Electrolyte overflowing from the edges of the cathode are recovered in the tank and recirculated io44383 by a suitable pumping device, after passage over a suitable cation exchange resin, if desired, to remove nickel ions. The current and rate of throughput of the foil are adjusted so that suitable perforations are produced. Typically a potential difference of about 5V is passed to give a current 2 density of about 6OOA/dm at a foil throughput rate of about lOOfV'hour.

The perforated nickel foil is pulled through 10 the remaining stages by the take-up reel 13 driven via a slipping clutch, not shown. The perforated foil is passed through tanks 14 containing a suitable wash solution, such as 10% HCl, then through a water rinse 15 and a drying oven 16 heated by eight 250 watt silica infra red heaters. As the foil emerges from the oven it is cooled to room temperature by jets of compressed air 17 and passes onto the takeup reel 13 interleaved with tissue supplied from a roll 18. Guide rolls on the apparatus are provided with conventional systems for maintaining the alignment of the foil.

Claims (13)

1. A process for lhe production of a continuous length of perforated metal foil which comprises passing a continuous length of foil through an electrolytic etching bath with one face of the foil being maintained in contact with an endless surface which is inert with respect to the electrolyte and the other face of the foil maintained in contact with an endless perforated titanium mask, and applying a potential difference of less than 1OV across the foil and a cathode immersed in the bath whereby the foil exposed to the bath through· the perforated . mask i 8 anodically etched away.

2. A process as claimed in claim 1 wherein the metal foil is nickel, copper, iron or alloys based thereon to - ‘thickness of 125 pm.

3. A process as claimed in claim 1 or claim 2 wherein the endless surface is a roll and the foil is' maintained in contact with the roll over at least 50% of the circumference of the roll.

4. A process as claimed in any preceding claims wherein nickel foil is perforated in a chloride bath at a pH of 1 to 7, the solution being maintained in agitation.

5. A process as claimed in claim 4 wherein the electrolyte is agitated by forced circulation and the pH of the electrolyte is ' ^ roni 4 t° 6; - 12 44382

6. A process as claimed in any preceding claim wherein the current density is maintained at 2 about 6OOA/dm .

7. A process as claimed in any preceding claim wherein the perforated foil is subsequently washed and dried and then cooled by air jets to prevent creasing.

8. Apparatus suitable for perforating metal foil by a process as claimed in claim 1 comprising a tank suitable for holding an electrolytic etching solution, an endless belt or roll having a surface which is inert to the electrolyte to be used, and an endless perforated titanium mask both supported so that in use a foil to be perforated may be passed through the tank held with one face in contact with the surface of the endless belt or roll and the other face in contact with the titanium mask, means for supplying anodic current, in use, to a foil to be perforated, a cathode located adjacent but spaced apart from the titanium mask and means for supplying current to the cathode in use.

9. Apparatus as claimed in claim 8 wherein the endless belt or roll is made of or covered with a non-conducting flexible material.

10. Apparatus as claimed in claim 8 or claim 9 wherein the cathode substantially conforms in - 13 44382 shape to the belt or roll, and.is spaced apart therefrom by less than 20mm.

11. Apparatus as claimed in claim 10 wherein the cathode has holes along its length and is 5 connected to a manifold through which electrolyte may be supplied so that in use electrolyte is pumped through the holes and is jetted on to the surface of the foil exposed through the mask.

12. Apparatus substantially as hereinbefore 10 described with reference to the accompanying drawing.

13. A method of perforating metal foil substantially as hereinbefore described with reference to the example.