Method And Arrangement For Continuous Etching And Anodizing Of Aluminum

Abstract

Elongated aluminum bodies such as aluminum sheets, foils and the like, the term aluminum denoting not only pure aluminum but also aluminum alloys which lend themselves to electrolytic etching and anodizing, are continuously passed through two electrolytic cells to be etched in the first and anodized in the second cell, at least one of the two cells comprises two adjacent compartments each equipped with a stationary electrode and containing electrolyte. The other cell may be similarly constructed or may be a single compartment electrolytic cell including electrolyte and a stationary electrode wherein the elongated aluminum body passing therethrough forms the electrode of opposite polarity. Alternating current is applied to the double-compartment cell and, if one of the electrolytic cells is of the single-compartment type, directed current is applied thereto. The elongated aluminum body forms during its passage through the double-compartment cell a bipolar conductor having portions within the respective compartments of this cell. It is possible in this manner, i.e. by utilizing two cells at least one of which being a two-compartment cell, to carry out etching and anodizing in a single continuous pass of the elongated aluminum body.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a method and arrangement for the electrolytic etching (roughening) and electrolytic oxidation of elongated aluminum bodies such as aluminum foils, bands or sheets. The term "aluminum" is meant to denote not only pure or technically pure aluminum, but also aluminum alloys which lend themselves to electrolytic etching and anodization.

Etched or roughened foils or strips of aluminum are well known and used for many industrial purposes. The etching of the surface of the foils or strips may serve for technical or decorative purposes. Such decorative purposes included the roughening of the surface of aluminum wall coverings and the like. Industrial and technical purposes are served by the roughening of aluminum foils or strips which are to be used as printing foils or which are to be coated or covered with pigments.

The roughening of aluminum surfaces by mechanical treatment, chemical etching or electrochemical methods also is well known. According to the conventional electrochemical methods for the purpose of roughening an aluminum surface, primarily electrolytic processes are utilized in which the aluminum body which is to be treated serves as electrode in an acid bath which is in contact with a further electrode. Depending on the other process conditions, generally the electrolyic roughening requires a period of the magnitude of several minutes.

The electrolytic oxidation of etched or roughened aluminum surfaces, particularly in order to harden such surfaces and to make them chemically resistant, also has been proposed. In accordance with these methods, the roughened aluminum body which is to be electrolytically oxidized is introduced as one of the electrodes into an acid bath including an electrode of opposite polarity, i.e., an electrolytic cell is formed in which the aluminum body represents the anode.

Since according to the above-mentioned processes for the electrolytic etching or roughening of aluminum bodies, as well as for the electrolytic oxidation, the aluminum body serves as one of the electrodes in an acid electrolyte, it was not possible up to now to carry out electrolytic etching and electrolytic oxidation in a continuous manner, i.e. so that the aluminum foil, strip, sheet or the like would pass as a continuous strip sequentially through the electrolytic etching and the electrolytic oxidizing device. By proceeding in this manner, it would have been unavoidable that short circuits from one cell to the other were caused by the connecting aluminum strip. Consequently, it was necessary to carry out the two electrolytic processes separated from each other, thereby requiring relatively large expenditures for equipment, operational personnel and also long periods of time for completing the two processes.

Alternating current operated electrolytic cells are known which comprise two chambers or compartments adjacent each other, each equipped with a stationary electrode and containing electrolyte. Alternating current is applied to the electrodes respectively located in the two chambers, and the aluminum strip or the like passing from one of the chambers to the other forms a bipolar conductor therebetween.

It is also known to clean electrochemically etched aluminum surfaces by brushing and rinsing, or chemically by immersion in a bath of dilute aqueous sodium hydroxide solution.

It is an object of the present invention to provide a method and arrangement for the etching and anodizing of aluminum which will permit to carry out the etching and anodizing of strips and the like in a single continuous process, whereby a continuous strip passes sequentially through an electrolytic etching cell and therefrom through an electrolytic oxidizing cell. By proceeding in this manner, i.e. in accordance with the present invention, substantial savings on equipment, space, operating personnel and process time are achieved and a product of highly uniform and desired quality is obtained.

SUMMARY OF THE INVENTION

The present invention thus proposes a continuous method for sequentially etching and anodizing aluminum sheets, foils and the like, according to which such sheets, foils or strips of aluminum or aluminum alloy which may be electrolytically etched and anodized are passed through two sequentially arranged electrolytic devices each connected to a source of electric current, so as to subject the strip in the first of the electrolytic devices to etching and in the second of the electrolytic devices to anodic oxidation. At least one of the two electrolytic devices comprises two adjacent compartments, each equipped with a stationary electrode and containing electrolyte. Alternating current is applied to the stationary electrodes in the two compartments and the strip forms during its passage through the compartments a bipolar conductor having portions within the respective compartments of the device. The other of the two electrolytic devices may be of similar structure, or may be an electrolytic single chamber cell including electrolyte and a stationary electrode, wherein then the strip passing therethrough will form the electrode of a polarity opposite to that of the stationary electrode.

The first of the two devices will serve for etching and the second for anodizing the strip. The electrolyte of the etching cell preferably will consist of dilute hydrochloric or hydrofluoric acid, whereas the electrolyte of the anodizing cell preferably will consist of dilute sulfuric acid.

The external ohmic resistance between the sources of alternating current (if two of the two-compartment cells are used) should be such that no significant amount of current will flow in an external conduit between the two sources of current, and no significant amount of current should flow within the portion of the aluminum or the like strip located between the two electrolytic devices.

The same conditions with respect to ohmic resistance will also apply in cases where only one of the two electrolytic devices is of the two-compartment type described above and the other is of the above-described single chamber structure.

Preferably, the sources of electric current for the two electrolytic devices will be independent from each other. Cleaning baths, rinsing and drying devices may be interposed along the path of the aluminum strip between the first and second electrolytic devices, i.e. between the etching and anodizing of the strip, and the anodized strip may be rinsed and sealed after leaving the anodizing electrolytic device.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic illustration of an arrangement according to the present invention in which both electrolytic devices are of the two-chamber alternating current type;

FIG. 2 illustrates schematically an arrangement according to the present invention according to which the etching is carried out in a two-chamber alternating current electrolytic device and the anodizing is carried out in a direct current-operated single-cell electrolytic device; and

FIG. 3 is a schematic illustration of an arrangement according to which the etching is carried out in a direct current, single-cell electrolytic device and the oxidizing is carried out in an alternating current two-chamber electrolytic device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, the continuous, sequential etching and anodizing of the aluminum strip or the like is carried out by first electrolytically etching and thereafter electrolytically oxidizing the continuous moving strip whereby at least one of these two electrolytic processes is carried out in an electrolytic device which comprises at least two chambers each equipped with one electrode, whereby the electrodes are connected to a source of alternating current. The strip passing sequentially through the two chambers of this electrolytic device forms during passage from one to the other of the two chambers a bipolar conductor.

Thus, it is essential according to the present invention to utilize for the etching and/or oxidation of the aluminum strip or the like a two-chamber, alternating current electrolytic cell in which the foil or strip which is to be treated, i.e. which is to be etched or oxidized does not form one of the electrodes but only a bipolar conductor whereby it is achieved that electric shunts between the above-described electrolytic device and the other of the two electrolytic devices will be avoided.

Thus, at least one of the two electrolytic devices must be of the above-described two chamber type, whereas the other of the two electrolytic devices may be of the same type or may be a direct current electrolytic cell in which the aluminum strip passing therethrough forms one of the two electrodes.

It is thus possible, in accordance with the present invention, to carry out electrolytic etching as well as electrolytic oxidation successively on one and the same continuous strip and the like which passes in the indicated sequence through the two electrolytic devices. It has been found that proceeding in accordance with the present invention does not only substantially reduce the total time requirement for etching and anodizing due to the fact that the etching and anodizing may now be carried out in a continuous manner, but furthermore, that the entire surface treatment can be carried out in a period of less than 1 minute and will result in aluminum or the like surfaces of very high quality. The total surface treatment, i.e. the etching as well as the anodizing, is carried out, in accordance with the present invention, in a substantially automatic manner which also simplifies the control of the process and particularly of the treating baths.

According to one embodiment of the present invention, electrolytic etching as well as electrolytic oxidation is carried out in the double or multichamber electrolytic devices described above each of which comprises at least two chambers, each equipped with a single stationary electrode and work, such as an aluminum strip or the like extending through both chambers forming a bipolar conductor but not one of the electrodes thereof.

According to another embodiment of the present invention, the electrolytic etching is carried out in a direct current electrolytic device in which the aluminum foil, strip or the like forms the cathode, and the electrolytic oxidation is carried out in a multichamber, alternating current electrolytic device as described above.

However, this sequence may also be reversed, and thus, according to still another embodiment of the present invention, the electrolytic etching is carried out in the multichamber, alternating current electrolytic device whereas the electrolytic oxidation is carried out in a direct current cell in which the foil or strip acts as the anode.

It is also within the scope of the present invention to cleanse the foil or strip after the electrolytic etching and prior to the electrolytic oxidation in a, per se, known manner, for instance by passage through a bath of dilute aqueous sodium hydroxide.

It is also possible and within the scope of the present invention to subject the foil or strip during the continuous etching and anodizing process to coloration. The sealing of the roughened and electrolytically oxidized aluminum surfaces in boiling, de-ionized water may also be combined with the above-described steps of the process of the present invention. All of these additional treating steps such as cleansing, coloring and sealing may be incorporated into the single continuous pass of the aluminum strip, foil or the like and thus are carried out in a particularly simple, economical and quick manner.

The roughened or etched surface obtained by the method of the present invention appears to be of better quality than that obtained by conventional methods. For certain technical purposes, particularly the utilization of the aluminum or aluminum foil or strip as printing foil or plate, it has been found advantageous to carry out the etching so as to achieve a surface roughness of between 2 and 3 microns, which may be accomplished by subjecting the strip, foil or the like to electrolytic etching at a current density of 20 amperes per 100 square centimeters and for a period of between 0.5 and 0.75 minutes.

If a surface roughness of only between 1 and 2 microns is desired, preferably a current density of about 16 amperes per 100 square centimeters will be applied for a period of between 0.5 and 0.75 minutes.

According to one embodiment of the present invention, the arrangement for carrying out the above-described process will comprise an electrolytic etching device represented by a two-chamber, alternating current electrolytic cell in which the aluminum strip or the like forms a bipolar conductor but is not used as one of the electrodes, and which contains for instance a dilute aqueous hydrochloric acid as electrolyte in both of the chambers. The device will include suitable rollers or the like for guiding the foil or band sequentially through the two chambers. The foil or strip leaving the second or last of the chambers of the etching device passes through a more or less conventional rinsing bath or the like and then into a second two-chamber, alternating current electrolytic device of the type described above in which, however, the electrolyte is dilute aqueous sulfuric acid and which also is equipped with suitable guide means for passing the foil or strip sequentially through the two chambers of this second or anodizing cell.

According to another embodiment of the present invention, the etching is carried out in a direct current electrolytic cell in which the minus terminal of the source of direct current is in contact with a guide roller for the foil or strip so that the foil or strip while passing through this cell will become the negative electrode. The cell contains dilute aqueous hydrofluoric acid as electrolyte and is equipped with a stationary electrode connected to the positive terminal of the source of direct current. Again, suitable devices for the guiding of the strip through the direct current etching cell are provided and the etched strip passes through a rinsing bath or the like into an anodizing device which, in this case, must be a two- or multichamber alternating current electrolytic cell of the type described further above and holding in both chambers dilute aqueous sulfuric acid as the electrolyte. Again, suitable arrangements are made for guiding the foil or strip sequentially through the two chambers of the anodizing device.

According to yet another embodiment of the present invention, the etching device is a two-chamber alternating current electrolytic cell of the type described above, holding dilute aqueous hydrochloric acid as the electrolyte and provided with suitable arrangements for guiding the foil or strip sequentially through the two chambers of the electrolytic device and again the strip does not form one of the electrodes but is in the nature of a bipolar conductor passing through the two chambers of the cell. The thus-etched strip is then passed through a rinsing device and from there into a direct current, single-cell electrolytic device holding dilute sulfuric acid as the electrolyte and provided with guide means for passing the strip through the cell. In this case, the guide means preferably include a conductive roller connected to the positive terminal of a source of direct current so that the strip upon contact with this roller will become the positive electrode of the direct current electrolytic cell which is also provided with a negative electrode connected to the negative terminal of the source of direct current.

Preferably, in all of these embodiments of the present invention, a bath holding dilute aqueous sodium hydroxide is interposed between the etching and the anodizing device, again equipped with suitable guide means for passing the etched strip therethrough, and this bath will serve for cleansing of the etched foil prior to anodizing of the same.

It is also within the scope of the present invention to include in the continuous process of etching and anodizing and in the arrangement for carrying out this process, a rinsing and sealing device through which the anodized strip is passed and whereby the sealing device may be a bath of boiling di-ionized water.

The following examples are given as illustrative only without, however, limiting the invention to the specific details of the examples.

In the examples, reference will be made to the Figures of the drawing in which identical elements are identified by identical reference numerals.

EXAMPLE 1

The present invention is described in the present example with reference to FIG. 1 of the drawing as utilizing two double chamber alternating current electrolytic devices. Aluminum strip 12 is fed from coil 10 into the alternating current, two-chamber electrolytic device 14.

Electrolytic device or cell 14 comprises two chambers 16 and 18 separated from each other by wall 17. Each of the chambers contains dilute aqueous hydrochloric acid. Each of the chambers 16 and 17 is equipped with one electrode, 32 and 34 respectively, which electrodes are connected to a source of alternating current identified by reference numeral 36. Coil or strip 12 passes over guide roller 24 into first chamber 16 in the bottom portion of which a further guide roller 26 is arranged. Another guide roller 20 is arranged substantially above separating wall 17 so that the strip will pass through the electrolyte of chamber 16, then out of the same over guide roller 20 into the electrolyte of chamber 18. In second chamber 18 a further guide roller 28 is arranged near the bottom thereof and the aluminum strip is guided thereby through chamber 18 and from there by guide roller 30 out of the electrolytic, alternating current etching device. It will be noted that strip 12 does not form one of the electrodes and thus may be called, particularly in the vicinity of guide roller 20, i.e. between chambers 16 and 18, a "bipolar conductor."

The thus-etched strip is passed from guide roller 30 through a rinsing device 38 in which electrolyte adhering to the strip is removed. From there, strip 12 passes, suitably guided by rollers 42, through a cleansing device 40 which is represented by a container holding dilute aqueous sodium hydroxide.

Electrolytic two-chamber cell 14, as well as cleansing device 40, are provided with suitable, per se conventional, heating arrangements which, for sake of clarity, have been omitted in the drawing.

From cleansing container 40, the strip 12 is passed through a second rinsing device 38 and from there through a conventional heating device 46 for the purpose of being dried.

The thus-etched, cleansed and dried strip is then passed through an electrolytic oxidation device 48, represented by a second two-chamber, alternating current electrolytic cell which again comprises two chambers 50 and 52 separated from each other by wall 51. The electrolyte in chambers 50 and 52 consists of dilute aqueous sulfuric acid. Each of chambers 50 and 52 is equipped with one electrode, 54 and 56 respectively, and electrodes 54 and 56 are connected to a source of alternating current identified by reference numeral 58. Somewhat different from the etching cell 14, the strip is passed from chamber 50 to chamber 52 through a suitable opening in wall 51, for instance through a rubber gasket arranged in wall 51 so as to permit passage of the strip therethrough without causing intermingling of the electrolyte of the two chambers.

The arrangement used according to electrolytic cell 14 for the passage of the strip from the first to the second chamber is practical only if the strip is of sufficient flexibility, whereas the arrangement utilized in electrolytic cell 48 will permit the treatment of a strip of greater stiffness since the strip need not be bent while passing from chamber 51 to chamber 52.

Thus, in any specific given case, it will have to be decided, depending on the mechanical characteristics of the strip whether the arrangement of cell 14 or the arrangement of cell 48 is best suitable for the passage of the strip from the first to the second chamber of the respective electrolytic device.

After leaving electrolytic device 48, the thus-anodized strip is passed through another rinsing device 38 to a conventional drying device 46 and the thus-dried, etched and anodized strip 12 may then be wound to form a coil 68.

When it is desired to subject the anodized strip to sealing, it is proposed to interpose a container 62 between last rinsing device 38 and last heating and drying device 46, through which the strip is passed by suitable guide rollers 64, in which sealing of the oxide layer is accomplished by contact with boiling deionized water.

The method of the present invention may be carried out in the above-described device, for instance by utilizing as electrolyte in the chambers 16 and 18 of etching cell 14 dilute aqueous hydrochloric acid having a concentration of between 1.5 percent by volume. The temperature of the dilute hydrochloric acid preferably will be maintained between 20.degree. and 80.degree. C. and most preferably at about 50.degree. .+-.10.degree. C.

The source of alternating current has a voltage of between 30 and 60 volts and a current density of between 5 and 25 amperes per 100 square centimeters is maintained. The strip is passed through the etching cell at such speed as to obtain a treating time of between 30 and 45 seconds.

The second electrolytic two-chamber device 48 contains as electrolyte dilute aqueous sulfuric acid having a concentration of 20 percent by volume and being maintained at a temperature of between about 30.degree. and 50.degree. C. Anodizing is carried out at a current density of between 10 and 30 amperes per 100 square centimeters.

It would be possible, of course, to carry out in the context of the present invention the cleansing of the etched strip by conventional brushing of the same, but it is preferred to carry out chemical cleansing in container 40 by passing the strip therein through dilute aqueous sodium hydroxide having a concentration of about 0.5 percent by weight and being maintained at a temperature of about 40.degree. C.

Sealing of the etched and anodized strip is carried out in container 62 by passing the strip therein through boiling deionized or distilled water having a pH of between 5.6 and 6.0.

A surface roughness of between 2 and 3 microns was obtained by a current density of 20 amperes per 100 square centimeters and a treating time of between 30 and 45 seconds. Foils or plates of a surface roughness within the above range are excellently suitable for producing printing plates thereof.

A lesser degree of surface roughness, such as between about 1 and 2 microns, which generally suffices if the strip or foil is to be used for decorative purposes, is obtained by reducing--under otherwise equal conditions--the current density to about 16 amperes per 100 square centimeters.

EXAMPLE 2

Another embodiment of the present invention will be described in the present example with reference to FIG. 2 of the drawing in which for sake of simplicity the arrangements for cleansing and sealing have been omitted, although cleansing and sealing arrangements may be included in the overall arrangement schematically illustrated in FIG. 2 at the points thereof corresponding to the points at which these arrangements appear in FIG. 1.

The process and arrangement according to FIG. 2 differs from that described in connection with FIG. 1 particularly by utilizing as oxidizing electrolytic cell 70 a direct current electrolytic device. The strip is passed through an etching device and ancillary devices corresponding to those of FIG. 1 until it reaches guide roller 74 which is connected to the positive terminal of a source of direct current. From guide roller 74, the strip passes through the electrolyte of electrolytic cell 70 which consists of dilute sulfuric acid. For this purpose, three further guide rollers 80 are indicated. A negative electrode 78 connected to the source of direct current is immersed in the electrolyte.

The direct current anodizing device 70 may contain as electrolyte dilute sulfuric acid having a concentration of about 20 percent by volume and being maintained at a temperature of between about 35.degree. and 50.degree. C., and anodizing may be carried out at a current density of between 3 and 8 amperes per 100 square centimeters.

EXAMPLE 3

The embodiment of the present example will be described with reference to FIG. 3 in which again certain ancillary devices corresponding to those of FIG. 1 have been omitted.

The essential difference between the embodiment of FIG. 3 as compared with that of FIG. 1 is that the etching is carried out in a direct current electrolytic cell 82, whereas anodizing is carried out in a two-chamber, alternating current electrolytic cell 48 which is substantially identical with cell 48 of FIG. 1.

According to the presently described example, strip 12 is fed from coil 10 over guide roller 86 which is connected to the negative terminal of the source of direct current and from there guided by guide rollers 92 through the electrolyte of the cell which consists of dilute hydrofluoric acid. The positive electrode 90, connected to the positive terminal of the source of direct current is immersed in the electrolyte.

The dilute hydrofluoric acid may have a concentration of 2 percent by volume and preferably will be maintained at about 45.degree. C. Etching is carried out at a current density of about 10 amperes per 100 square centimeters.

It will be understood from the description hereinabove that it is essential according to the present invention to carry out at least one of the two electrolytic processes, i.e., either the etching or the anodizing or both, in a cell of the type identified by reference numerals 14 and 48, respectively. It becomes possible thereby to carry out the electrolytic etching and the electrolyte oxidation of aluminum or aluminum alloy foil, strip 12 or the like in a sequential, continuous manner and thereby to realize substantial savings with respect to equipment, time and operating personnel. Furthermore, the actual treating periods according to the present invention are shorter than conventional treating periods and the quality of the foils or strips etched and anodized in accordance with the present invention compares favorably with the results obtained by conventional methods particularly with respect to the uniformity and evenness of the roughening or etching of the strip. The control of the process is also greatly simplified.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of etching and anodizing devices differing from the types described above.

While the invention has been illustrated and described as embodied in a continuous electrolytic etching and anodizing arrangement for the treatment of aluminum and aluminum alloy foils, strips and the like, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.

Claims

What is claimed as new and desired to be protected by Letters Patent is:

1. A continuous method for sequentially etching and anodizing aluminum sheets, foil and the like comprising the steps of continuously passing an elongated body, consisting essentially of aluminum or aluminum base alloy through two sequentially arranged electrolytic means each connected to a separate source of electric current so as to subject said elongated body in the first of said electrolytic means to etching and in the second of said electrolytic means to anodic oxidation, at least one said electrolytic means comprising at least two adjacent compartments each equipped with a stationary electrode and containing electrolyte; and applying alternating current by one of said sources of current to the electrodes of said at least one electrolytic means, said elongated body forming during its passage through the compartments of said at least one electrolytic means a bipolar conductor having portions within the respective compartments of said at least one electrolytic means so that the current from said one source will flow from one of said electrodes to the portion of said elongated body in one of said compartments and to the portion in the other compartment and back to the electrode in the other compartment without flowing along said elongated body beyond said one

2. A method as defined in claim 1, wherein each of the two sequentially arranged electrolytic means is connected to a separate source of alternating current and comprises at least two adjacent compartments each equipped with a stationary electrode and containing electrolyte, and wherein the electrolyte of the first of said two electrolytic means causes etching of aluminum and the electrolyte of the second of said two

3. A method as defined in claim 2, wherein the electrolyte said first electrolytic means is aqueous hydrochloric acid and the electrolyte of

4. A method as defined in claim 1, wherein the external ohmic resistance between said sources of alternating current is such that no significant amount of current will flow between the two sources of current within the portion of said elongated body located between said first and second

5. A method as defined in claim 1, wherein the other of said electrolytic means consists essentially of an electrolytic cell including electrolyte and a stationary electrode contacting said electrolyte, and wherein the other source of current is a source of direct current connected with one pole thereof to said stationary electrode and with the other pole thereof

6. A method as defined in claim 5, wherein said elongated body is subjected to etching in said other of said electrolytic means and the stationary

7. A method as defined in claim 5, wherein said elongated body is subjected to anodic oxidation in said other of said electrolytic means and the

8. A method as defined in claim 1, and including the step of passing said elongated body leaving said electrolytic etching means through a cleansing bath prior to introduction of said elongated body into said electrolytic

9. A method as defined in claim 8, wherein said cleansing bath consists

10. A method as defined in claim 1 and including the steps of rinsing and drying said elongated body after passage of the same through said first

11. A method as defined in claim 1, and including the step of sealing the anodized elongated body leaving said second electrolytic means by passing

12. A method as defined in claim 1, wherein said elongated body is subjected to etching at a current density and for a period of time such as to obtain by said etching a surface roughness of between about 2 and 3

13. A method as defined in claim 12, wherein said etching is carried out at a current density of about 20 amperes per 100 square centimeters and for a

14. A method as defined in claim 1, wherein said elongated body is subjected to etching at a current density and for a period of time such as to obtain by said etching a surface roughness of between about 1 and 2

15. A method as defined in claim 14, wherein said etching is carried out at a current density of about 15 amperes per 100 square centimeters and for a

16. A method as defined in claim 1, and including the step of continuously imparting color to said elongated body.