US3451871A - Method of treating metallic surfaces - Google Patents

Description

June 24, 1969' BAUER ET AL 3,451,871

METHOD OF TREATING METALLIC SURFACES Filed May 25, 1965 Sheet of 2 FIG.1

/3 IL IL I l m 20 l I INYENTORS T2, Hanna 13am My. Javier Q TTORNEY June 24, 1969 H. BAUER ET AL 3,451,871

. METHOD OF TREATING METALLIC SURFACES Filed May 25, 1965 Sheet 2 of 2 FIG. 4

- F/ [Cr/6. 6 a

HEXE LZE United States Patent U.S. Cl. 156-244 13 Claims ABSTRACT OF I THE DISCLOSURE A method of treating at least one surface of a metal member, such as for instance an elongated sheet metal member, so as to improve the adherence of a cover layer thereto, in which the surface is subjected to the action of a high voltage, high frequency alternating field Vesta-blished between the member and a rotating electrode spaced therefrom.

The present invention relates to a method of treating metallic surfaces and, more particularly, the present in- ;vention is concerned with an electric pretreatment of metallic surfaces for the purpose of increasing the surface activity thereof with respect to the adherence of cover layers, for instance lacquers, printing inks, adhesives, or synthetic plasticvmaterial which may be applied to such metallic surfaces.

.The metallic surfaces which are to be treated according to the presentinvention may, prior, thereto, be subjected to a pre-treatment by chemical or physical methods for the purpose of cleaning the surfaces ofv impurities adhering thereto. For instance, sheets, bands or foils of aluminum or aluminum alloys maybe subjected to soft annealing in order to remove fat therefrom, or the metallic surface may be cleaned mechanically by brushing, or chemically by applying primers or priming substances which will improve the adherence of subsequently applied cover layers and the like, or by treatment with detergents or lacquer-like bodies. All of these treatments have the purpose of cleaning a metallic surface and/or, at least to a limited extent, activating thesame so that a firmer adherence of the subsequently applied cover layer or the like can be achieved.

Treatment according to the present invention maybe carried out in connection with metallic surfaces which have thus been pretreated but also in connection with metallic surfaces which have thus been' pretreated but also in connection with metallic surfaces which have not been subjected to any pretreatment. I It is an object of the present invention to provide, a

3,451,871 Patented June 24, 1969 With the above and other objects in view, the present invention contemplates a method of treating a metal surface so as to improve the adherence of a cover layer thereto, comprising the step of subjecting hte metal surface to a high voltage, high frequency alternating field.

The present invention also includes a method of treating the surface of a metal body so as to improve the adherence of a cover layer thereto, comprising the steps of subjecting a surface portion consisting essentially of a metal selected from the group consisting of aluminum, copper, lead and tin to a high voltage, high frequency alternating field of between 50,000 and 600,000 volts and between 25 and 400 kHz., and applying a cover layer to the thus treated surface portion.

According to the present invention, the metal surface, which may or may not have been pretreated, for instance as described above, is subjected to a short time exposure to a high voltage, high frequency alternating field, whereby the metal body, the surface of which is to be treated, serves as one of the two electrodes between which the high voltage, high frequency alternating field is formed.

The alternating high frequency field which is to be applied according to the present invention, generally should have a voltage of between 50,000 and 600,000 volts, and preferable between 400,000 and 600,000 volts, and a frequency of between 25 and 40 kHz. and preferably between 25 and 75 kHz. Under these conditions, a corona effect is avoided.

It has been found that in this manner an excellent adherence can be achieved between various covering materials such as printing ink, metalizations, adhesive, etc. and the thus treated metallic surface. For instance, in the case of aluminum and aluminum alloy surfaces, due to the greatly increased ability of thus treated aluminum or aluminum alloy surface to hold or adhere high polymeric synthetic materials, it is possible to form a very firm adherence between aluminum foils and polyolefins which are extruded onto the same (and which generally show poor adherence to aluminum) and, on the other hand, it is possible to form, after the pretreatment of the aluminum foil in accordance with the present invention, cover layers of extruded polyolefins on the aluminum or the like surface at a much greater forward speed than was up to now possible.

By the treatment according to the present invention a continuous electric discharge takes place between the surface of the metal to be treated and the electrodes which are spaced therefrom and this will activate the treated metal surface to such an extent that non-metallic as well as metallic materials can strongly and permanently adhere to the thus treated metal surface.

A further advantage of the activation of the metal surface according to the present invention is found, for inmethod of treating metallic surfaces which will improve the adherence of covering materials such 'as lacquers,

printing inks, adhesives or plastic materials such as synthetic plastic materials, for instance polyethylene films and the'like thereto and which will also improve the adherence of metal applied to the thus treated surface.

:iIt is a further object of the present invention to pro- 7 stance, in applying high pressure polyethylene extrusion layers to the activated metal surface. It is possible, to extrudethe polyethylene at considerably lower temperatures than were required up to now in order to obtain a firm adherence to the metal surface. For instance, it has been found in many cases that the extrusion temperature of the polyethylene may be reduced by between 20 and 30 C. below the extrusion temperature which would be required in order to adhere the extruded polyethylene to a similar metal surface which has not been subjected to a high voltage, high frequency field in accordance with the present invention. Nevertheless, the adherence of the polyethylene which had been extruded at such lower temperature to the metal surface, such as for instance .an

aluminum surface which had been subjected to the high voltage high frequency treatment in accordance with the present invention, has been found to be equally strong as the adherence of extruded polyethylene which had been extruded at the higher temperature to a similar aluminum 3 or the like foil which had not been subjected to the high frequency and high voltage alternating field in accordance with the present invention.

The activation of the metal surface which is achieved by the high frequency high voltage alternating field treatment goes far beyond the activation which, for instance, is accomplished by soft annealing or otherwise removing fat and cleaning the surface of the metal and the activation according to the present invention results in an adherence of the cover layer which is much stronger than could be obtained up to now even on a completely clean metallic surface.

It may be assumed, however, without limiting the present invention to any specific theory, that by subjecting the metal surface to the high frequency, high voltage alternating field, particularly within the above described limits of voltage and frequency, the characteristics of the metal surface will be changed in a manner which has not yet been fully determined.

The present invention is of particular significance with respect to applying polyethylene layers to metal foils, such as aluminum or aluminum alloy foils, since, due to the fact that the polyethylene need not be heated to such high temperature as was previously required, a polyethylene layer will be formed which will not only firmly adhere to the aluminum or the like foil, but which will be, to a much greater extent than was hitherto possible,

free of odors. This is particularly important with respect to the utilization of such polyethylene coated aluminum foils as a packing material in the food industry.

. It is also frequently of great importance that by proceeding in accordance with the present invention, it is frequently unnecessary to take steps in order to improve the ability of the metal foil to have a cover layer firmly adhered thereto. When it is nevertheless desired to carry out the preliminary steps for improving the ability of the metal foil to adhere a cover layer thereto, for instance, when it is desired in the case of aluminum to subject the surface to an anodizing treatment prior to the high frequency, high voltage alternating pretreatment according to the present invention, than, by combining these two treatments, a further improvement in the ability of the metal surface to adhere a cover layer thereto will be achieved so that a coherence between metal surface and cover layer is obtained which is of greater strength than could be achieved up to now.

Generally, it is desirable to utilize for the high frequency, high voltage alternating field treatment a current of between 25 and 75 kHz, or even higher, and of a voltage which-although in some cases might be as low as 50,000 volts, generally will be 300,000 volts or more, preferably between 400,000 and 600,000 volts. Within these limits, the desired surface activation of the metal is obtained with a very high degree of certainty.

According to a preferred embodiment of the present invention, it is a particular advantage of the same that a firm adherence, or an increase in the adherence of the covering material on the metal surface can be accomplished without requiring more or less involved pretreatment steps. Only in rare cases, when an extraordinarily high degree of cohesion between the metal surface and 'the covering material is required, will it be desirable to combine conventional pretreatment steps such as described above, for instance defatting or other purification of a metal surface, or anodizing, with the high frequency, high voltage alternating treatment according to the present" invention. The high frequency, high voltage alternating field treatment according to the present invention can be carried out, for instance, on aluminum foils or other aluminum surfaces, whereby the aluminum may be either in hard condition or in soft annealed condition.

As will be described in more detail in connection with the drawings, it has been found advantageous to form the high frequency, high voltage alternating field between electrodes which preferably are provided with an insulating layer at the portion thereof facing the alternating field. The electrodes may be stationary or, preferably, may be formed as rotating rollers, whereby the metal sheet or foil having a surface which is to be treated is preferably in contact with the opposite relative to the treating electrode, and the metal sheet or foil is moved in tangential direction relative to the treating electrode.

The use of a treating electrode of roller-like configuration will have the advantage that the discharge will take place along a line across the entire width of the material, i.e. the metal sheet, foil or the like, whereby the line of discharge on the surface of the roller electrode will continuously change its position due to rotation of the roller electrode. This will prevent undue heating of the roller treating electrode or localized over-heating of the same. Furthermore, an electrode in the shape of a roller afiords an easy opportunity for cooling of the electrode. Preferably, the roller is supported by bearings of highly insulating material in order to avoid voltage losses. 7 I

The width of the gap which is to be maintained between the metal surface which is to be treated and the treating electrode will depend only to a slight degree on the electric conductivity of the dielectric medium within the gap between treating electrode and metal surface, which dielectric medium generally will be air enriched with ozone formed by the discharge, and will primarily depend on the voltage which is applied. Generally, the width of the gap should be between 0.3 and 10 mm., and preferably between 0.5 and 3 mm.

Since the extent to which the surface characteristics of the metal with respect to the firmness of adherence of cover layers thereto are improved according to the present invention will depend on the type of metal or alloy, it is sometimes advantageous to repeat the high frequency, high voltage alternating field treatment several times, for instance by arranging several treating electrodes in. sequence so that the metal surface which is to be treated passes successively through the alternating fields created between the metal body the surface of which is to be treated and the respective treating electrodes. On the other hand, it may be advantageous to arrange several treating electrodes in succession because thereby the speed of passage of the metal foil or the like through'the treating device can be increased.

Furthermore, the metal sheet or foil which is to be treated may be moved along such a path, or several treating electrodes may be arranged in such a position, that not only one but both surfaces of the metal sheet, foil or the like will be exposed to the high frequency, high voltage alternating field.

Preferably, the metal sheets or foils or semifinished products, for instance metal tubes, which are to be subjected to a high frequency, high voltage alternating field in accordance with the present invention, are treated in conventional production lengths which, for instance in the case of metal sheets or foils, may be from several hundred to several thousand meters, because in this manner it is possible to operate a substantially continuous process. In the case of shorter metal bodies, for instance cut sheets, the alternating field treatment can be either carried out intermittently, separately for each cut sheet, or the individual cut sheets are arranged in sequence with very little distance between adjacent individual cut sheets so that the voltage of the treating electrode can be maintained permanently, i.e. for the entire length of time during which successive cut sheets are passed through the treating device.

It is advisable to form the guide rollers for the sheets, foils, tubes or the like which are located above or below these metal bodies in the vicinity of the treating electrode, of non-conductive materials such as rubber or the like in order to avoid flash-over of electric current in the vicinity of the treating electrode. Provided that the voltage is sufiiciently high, the width of the sheet or the like which is to be treated can be varied as desired without marked 15 influence on the improved surface characteristics which are achieved by the treatment. It is of course desirable that the treating electrode has at least the same width as the metal sheet or the like which is to be treated.

The method of the present invention is suitable not only for the treating of the surfaces of metal sheets and foils, but also for treating the metal surfaces of differently shaped bodies, for instance pipes, whereby the shape of the treating electrode must be adjusted to that of the surface which is to be treated so that a gap of substantially even width throughout is formed there'between.

The high frequency, high voltage alternating field treatment of the metal surface in accordance with the present invention may be carried out immediately prior to applying a cover layer to the thus treated metal surface, for instance by extrusion, printing, lining or coating in different manners. However, it is also possible to store the metal bodies, for instance sheets or foils which have been exposed to the alternating field in accordance with the present invention, and to carry out the subsequent application of a cover layer or the like by lacquering, printing, extrusion application of a polyolefin or the like, etc., at some later date.

As described above, the counter electrode to the treating electrode is formed by the metal body the surface of which is to be subjected to the high voltage, high frequency alternating field.

There is a certain. dependency between the voltage and the length of time for which the alternating field is to be applied. In certain cases, for instance, when operating with an alternating field having a voltage of 600,000 volts, 1

the improvement in the surface characteristics of an aluminum foil for the purpose of adhering high pressure polyethylene thereto, will be achieved by passing the aluminum foil or the like through the high frequency, high voltage alternating field at speeds of up to 200 meters per minute. When the voltage is reduced, the speed of passage ofthe metal surface through the high frequency field must be correspondingly reduced. In accordance with the foregoing, for instance, optimum surface characteristics will be achieved with the same aluminum surface and under otherwise similar conditions but at a voltage of only 500,- 000 volts if the speed of passage of the foil through the alternating field is reduced to 160 meters per minute.

Generally, aluminum foils having a purity of at least 98.5% and including the conventional impurities such as iron, silicon, copper, etc. in accordance with the ASTM norms will give good results, as well as aluminum alloys of the type AlMn (3003), AlMgMn (3004), AlMg 3 (5052), AlMg 5 (5056), AlMnCuMg (2024), AlMgSi (6066), A1990 (1100), A1 99.5 (1050), A1 99.7 (1070), A1 99.9 (1090), Al 99.99 (1099 Similar results are achieved by treating metal surfaces other than aluminum or aluminum alloys, particularly metal surfaces which consist essentially of copper, lead and tin, including tinned lead and tinned strip steel.

The greater the length of the treating electrode, or the greater the width of the metal sheet or fo'il which is to be treated, the higher should be the voltage, thus, in the case of treating electrodes having a length of 3 meters, it is desirable to apply 600,000 volts. Theoretically, it is possible to subject sheets of foil of any desired width to the alternating field treatment accord-ing to the present invention by arranging a plurality of electrodes, each having a length of for instance about 3 meters, next to each other with slightly overlapping end portions, however, for all practical purposes it is hardly necessary to treat sheets or foils having a width of more than 3 meters and thus, it is generally possible to produce the required alternating field with a single treating electrode.

The novel features which are considered as characteristic for 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 drawings, in which:

FIG. 1 is a diagrammatic view of an extrusion arrangement according to the present invention for applying polyolefins, for instance high pressure polyethylene onto carrier webs such as metal foils, particularly aluminum foils and compound foils 'such as paper-aluminum foils and paper-aluminum-synthetic material foils;

FIG. 2 is a diagrammatic view of an arrangement according to the presentinvention for applying lacquer or baking enamel to a metal sheet;

FIG. 3 is a diagrammatic view station for subjecting a metal and high voltage alternating air cooling device;

FIGS. 4 and 5 are diagrammatic views of arrangeof an electric treating surface to a high frequency field, including a compressed ments for subjecting a metal foil or the like to two suc- Although it is possible to store metal foilsand the like, the surface of which has been treated according to the present invention, and to apply a cover layer thereto after such storage, and still achieve superior adherence,

generally, it is desirable to carry out the adherence of the tained metal surface of improved surface characteristics and guide rollers of the conveying equipment for the metal sheet, foil or the like. 7

When it is desired to pretreat the aluminum foil by soft annealing, prior to subject-ing the same to the alternating field treatment of the present invention, such soft annealing preferably is carried out at temperatures between about 450 and 550 C., depending on the specific composition of the aluminum or aluminum alloy.

, been pretreated andactivated cessively arranged high frequency, ing fields; and

FIGS. 6 and 6a are respectively schematic front and side views of a treating device for applying a high frequency, high voltage alternating field to a metal sheet, including a stationary treating electrode.

Referring now to the drawing and particularly to FIG. 1, reference numeral 4 denotes the arrangement for successively applying two high voltage, high frequency alternating fields to a metal foil such as an aluminum or aluminum alloy foil. The two rollers 4 beneath continuous metal sheet or foil30 are guide rollers and consist of an ozone-resistantinsulating material, for instance silicone rubber. Rollers 4 serve for adjusting the exact width of the gap between the upper surface of aluminum sheet 30 and treating electrodes 4". Treating electrodes 4" are roller electrodes of the typeillustrated in more detail in FIGS. 4, '5 and 6. v

Starting now with the left hand portion of FIG. 1, it will be seen that the aluminum foil or sheet 30 passes from an unwinding station 1 through. alacquer applying station 2 and a drying channel 3. to the alternating field treating station 4. The aluminum sheet whichthus has passes then between pressure roller 5 andcooling roller 8, and during the passage between these two rollers, molten high pressure polyethylene which has been molten in extrusionscrew conveyor 6 is applied to the activated surface'of aluminum high voltage alternat sheet 30 through slot-shaped nozzle 7. The thus formed composite sheet with the polyethylene firmly adhering to thealuminumiswthen wound at coil winder orw-inding station -9. I

According tothe present inventionit is also possible, as will be described in some. of. the examples herein, to eliminate the priming stations 2 and .3 and to pass the aluminum sheet or the like directly from unwinding device 1 to the alternating field treating arrangement 4.

However, by using the entire device as illustrating in FIG. 1, including the priming arrangement, the improvement in adherence between the aluminum surface and the extruded polyethylene which is achieved by application of the chemical primer at the lacquering arrangement 2 and evaporation of the solvent for the lacquer in drying channel 3, is further increased by the application of the alternating field in treating device -4 so that the adherence of the polyethylene to the aluminum sheet will be much stronger than could be achieved by only priming the aluminum sheet without application of the alternating field.

A chemical primer which may be advantageously used by proceeding in accordance with FIG. 1, is shellac which must be free of wax and which is preferably applied in an amount of 0.01 gram of shellac per square meter of aluminum surface.

FIG. 2 shows unwinding device 10 from which the aluminum band or the like passes through an alternating field pretrea'ting station 11 including treating electrode rollers 11' located beneath aluminum sheet 30 and guide rollers 11" formed of insulating material and serving for maintaining the desired gap between the metal sheet 30 and treating rollers 11'. After being exposed to the alternating field in treating station 11, aluminum sheet 30 passes through a conventional lacquer application device 12 and a drying or baking channel 13, and from there to coil forming device 14.

According to FIG. 3, the metal foil 21 is connected to earth by means of metallic guide rollers 17. The treating station for applying the alternating field comprises guide roller 16 formed of ozone resistant insulating material such as silicone rubber or the like, and treating electrode 15, the details of which are described in FIGS. 4, and 6. The distance between treating electrode 15 and the surface of foil 21 is shown in an exaggerated manner, in fact, and as described further above, the gap between the free surface of foil 21 and treating roller 15 should not exceed mm., and preferably will be between 0.5 and 3 mm.

Guide roller 16 is supported by sliding bearings so as to permit adjustment of the gap between guide roller 16 and electrode roller 15. At low forward movement of metal sheet or foil 21, such as 5 meters per minute and less and high intensity of the alternating leld, it may happen that the surface of treating roller or electrode will be unduly heated. Excessive working temperatures at the surface of electrode 15 would cause accelerated wear and tear of the same and could lead to short circuits. Generally, such low forward speed of the aluminum foil or the like will not be encountered with the exception of a combination of the alternating field treating device with a relatively old fashioned arrangement for applying a baking enamel. Whenever the-operating temperature of the roller electrode 15 rises above a desired upper limit, it is possible to cool roller electrode 15 by applying compressed air through nozzles 18. However, if compressed air emanates from nozzles 18 at a very high speed, then an undesirable fluttering of a relatively thin metal foil could occur between rollers 16 and 17. Thus there are certain practical limitations with respect to the speed at which compressed air may be released through nozzles 18. However, by cooling the compressed air, it is possible to achieve the desired cooling of roller 15 even without excessive speed of the compressed air. H

Treating electrode 15 is connected to high frequency transformer 19, the other terminal of which is grounded. The transformer receives the primary voltage from high frequency generator 20. I Suitable apparatus of this type, is available for instance under the trade name Vetaphone Treater T300/ 3. FIGS. 4 and 5 are diagrammatic illustrations of arrangements for exposing the-metal sheet, such as an aluminum foil 21 to two successively arranged high frequency, high voltage alternating fields.

Guide rollers 22 are formed of insulating material and are turnably arranged in sliding bearings so that the gap between foil 21 and the treating rollers can be adjusted. The treating electrodes according to FIGS. 4 and 5 are turnably supported by bearings made of insulating material. The treating electrodes or rollers are rotated by suitable drive means, for instance V-belts and it is not necessary that the surface speed of the electrode rollers is equal to the forward speed of metal foil 21. When operating with relatively small forward speeds of metal foil 21, it has been found advantageous to permit the electrode rollers to rotate at somewhat greater surface speed than would correspond to the forward speed of foil 21 since thereby a cooling effect is achieved on the surface of the electrode rollers.

In order to achieve an even distribution of the voltage applied to the treating electrode, the same must consist of a body of high electric resistance and simultaneously must be arranged as condenser in an oscillatory circuit, whereby the metal foil 21 forms the zero potential.

The treating electrodes or rollers shown diagrammatically in FIGS. 4 and 5 consist in their innermost portion of a steel core 26 to which the voltage is applied. Metal foil 25, preferably consisting of aluminum, serves for even distribution of the charge. The entire outer surface of metal foil 25 is covered with a layer 24 consisting of metal powder or of a pulverulent semi-conductor, preferably graphite. By changing the thickness of layer 24 or by choosing as the material thereof more or less highly conductive pulverulent material, it is possible to change and control the resistance values and thus the condenser effect of the entire treating electrode. The outermost layer 23 of the roller or treating electrode consists of a heatresistant insulating material, for instance of the material known under the trade name Pertinax (Phenolic Resin).

According to FIG. 41, both treating rollers will affect the same face of metal sheet or foil 21, while FIG. 5

' shows an arrangement whereby the successively arranged treating rollers will affect opposite faces of foil 21, respectively, so that both sides of foil 21 will be treated and improved with respect to the surface characteristics in connection with the subsequent adherence of a cover layer thereto.

FIG. 6 is a schematic illustration of a stationary treating electrode. It is characteristic for the use of stationary treating electrodes that a very highly heat resistant insulating material must be employed.

Thus, for treating electrodes of this type, Pyrex glass layer 28 has been found to form a suitable insulating layer, since Pyrex glass can withstand temperatures up to 200. The inner surface of insulating layer 28 is preferably completely covered with an aluminum foil 29, in order to assure even voltage distribution over the entire electrode. The outer electrode surface is formed by open loops 27 of electric resistance wire, for instance of a manganese or Konstantan (alloy: 54% Cu; 45% Ni; 5%

- Mn) wire of 0.1 mm. diameter. The individual wire loops 27 which are interrupted as illustrated, serve primarily for achieving an even distribution of the electric discharge. The metal surface which is to be treated, as well as the turnably supported electrodes are past at the desired distance tangentially to the surface of the stationary electrode, whereby the gap between the stationary treating electrode and the metal sheet or foil 21 can be adjusted by means of an insulated guide roller.

In accordance with the method of the present invention A and with the devices diagrammatically illustrated in the drawing and described hereinabove, it is always possible to achieve an improvement of the surface characteristics of a metal sheet or other metal body which is subjected to the high frequency, high voltage alternating field, provided that the body which is to be thus treated is either a metallic conductor, such as a metal sheet or foil, or a composite foil which includes at least one conductive metal layer. 7

It is known to improve the adherence characteristics of polyolefin foils, cellophane and various paper and cardboard types by application of a high frequency alternating field which will result in corona discharges. In this manner, it is possible, for instance, to change the surface of a polyolefin layer so as to make it possible to imprint the same. However, this effect cannot be achieved by means of the conventional electric pretreatment of these carrier materials if the same are already connected to a metal web, for instance an aluminum foil. Similarly, the conventional method is inoperative in the case of a non conductive carrier sheet or web, for instance of paper which is coated with a synthetic material in which a metal powder, for instance aluminum bronze is incorporated. Due to the applied high voltages, these metal powders will cause the entire layer to become conductive and this makes it impossible to utilize the conventional method which is connected with corona discharges.

g The high frequency, high voltage treatment in accordance with the present invention overcomes the above discussed difliculties and permits not only the treatment of exposed metallic surfaces but also of non-metallic surfaces provided that the same are included in a composite structure which includes a metallic layer. The effect achieved thereby gives results which somewhat correspond to those which are obtained by means of the corona elfect in the case of non-conductive materials or composite layers which are free of conductive components.

While the present invention is described herein primarily with respect to the treatment of aluminum and aluminum alloy foils, it is emphasized that the present method is also suitable for improving the surface characteristics with respect to adherence of cover layers to other metal bodies such as sheets or foils of steel, tinned steel, zinc-coated steel, copper, lead, tinned lead and tin.

Foils of the above-mentioned metals and metal combinations were subjected to the high frequency, high voltage alternating field as described above, and then in an extrusion device coated with about 50 grams per square meter of high pressure polyethylene. It was found in all cases that the adherence of the polyethylene layer to the metal surface was much stronger when the metal surface had been subjected, prior to application of the polyethylene, to the alternating field treatment according to the present invention. In these cases, the alternating field pretreatment was generally carried out at a distance of about 1 meter from the extrusion nozzle for the polyethylene. The temperature of the polyethylene at the extrusion nozzle was about 280 C., and an alternating field of. 600,000 volts and -50 kHz. was applied. The gap between the metal surface which was to be treated and the treating electrode was maintained at 2 mm.

Lead and tin sheets were passed through the alternating field treating device at a speed of meters per minute and the other metals mentioned above at a speed of 60 meters per minute.

Metal sheets of commercial quality were used and it was found that the purity or the analysis of the material was nearly without any influence on the effect of the high voltage, high frequency alternating field treatment. Generally, the width of. the thus treated metal sheets ,was between 30 and 80 cm.

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

EXAMPLE 1 A soft annealed aluminum foil of 0.015 mm. thickness and a width of 1,000 mm. is coated in a conventional extrusion device with a layer of high pressure polyethylene. If the aluminum foil has not been subjected to any kind of" surface pretreatment, it will be found that at a forward speed of 30 meters per minute or more the adherence between aluminum surface and polyethylene, by application of the polyethylene at a nozzle temperature of about 280 C., will be unsatisfactory.

However, by pretreating the aluminum foil surface by subjecting the same to an alternating electric field of 600,000 volts and kHz. at a distance of about 1 meter before the extrusion nozzle, the adherence between the high pressure polyethylene and the aluminum surface was found to be so strong that a high pressure polyethylene layer of 0.050 mm. thickness could be applied even at a forward speed of the aluminum foil of 60 meters per minute and more. The thus formed composite aluminumpolyethyelne foil could not be separated into its components without tearing of the polyethylene film. The distance between the treating electrode roller and the aluminum surface, or the gap therebetween was maintained at 1.2 mm. 1

For the purpose ofthis example, a roller electrode arrangement produced by the Deutsche Vetaphone G.m.b.H. Hamburg, Model No. T300/3 was used.

EXAMPLE 2 Substantially in the manner illustrated in FIG. 1, however, without any priming steps, an aluminum band having a thickness of 0.050 mm. and a width of 800 mm. was subjected to the alternating field treatment described in Example 1 at a forward speed of 25 meters per minute, and without being prior thereto subjected to any chemical or mechanical or other pretreatment. The thus surface activated sheet was then formed into a coil and stored for 15 days. Thereafter, the sheet was uncoiled and to the activated surface thereof high pressure polyethylene was applied in a conventional extrusion arrangement at a temperature of the polyethylene of 280 C. at the nozzle orifice and in a thickness of 70 grams of polyethylene per square meter. Very strong adherence between the polyethylene and aluminum layers was observed which far surpassed that which could be achieved by conventional methods.

of intermediate hardness, having a thickness of 0.2 mm. and a width of 600 mm. was combined with a layer of baking enamel without any pretreatment of the aluminum band. The forward speed of the aluminum band was 5 meters per minute, the baking temperature 230 C. and the thickness of the lacquer film 5 grams per square meter.

By proceeding in this manner, only insufficient adherence of the lacquer to the aluminum band could be achieved. However, upon first subjecting the surface of the aluminum band to the alternating field treatment described in Example 1, with a distance between the treating electrode and the lacquer applying and baking device of 1.5 meter, it was found that thebaked enamel layer adhered so strongly to the aluminum surface that even difficult deformation operations such as deep drawing of the lacquered aluminum sheet could be carried out without weakening the bond between the baked enamel and the aluminum surface.

EXAMPLE 4 meter from the nozzle of the polyethylene extrusion de- EXAMPLE By adhering to an aluminum foil as received from the rolling mill and having a thickness of 0.015 mm. and a width of 750 mm. a polypropylene foil of 0.050 mm. under interposition of a 2-component adhesive on the basis Desmophen-Desmodur, it will be found that the adherence of the adhesive to the aluminum surface, and thus the adherence of the polypropylene foil is unsatisfactory.

If, however, the surface of the aluminum foil is pretreated by being exposed to an alternating field as described in Example 4, with a distance of 2 meters between the treating electrode and the device for applying the adhesive, and subsequent adherence of the polypropylene foil to the adhesive covered aluminum surface, the desired high strength of adherence between the aluminum foil and the polypropylene foil was achieved.

EXAMPLE 6 An aluminum foil as received from the rolling mill (i.e. without any pretreatment) having a thickness of 0.009 mm. and a width of 1,400 mm., was rewound at a speed of 200 meters per minute and simultaneously subjected to the alternating field treatment according to the present invention substantially as described in Examples 4 and 5, however, so that both sides of the aluminum foil were exposed to the high frequency, high voltage alternating electric field, substantially as illustrated in FIG. 5.

The improvement in the surface characteristics of both surfaces of the foil which was achieved in this manner made it possible, without further pretreating, to imprint the aluminum foil, to apply baking enamel thereto and to apply non-metallic foils thereto, for instance a polypropylene foil in the manner described in Example 5.

EXAMPLE 7 A laminated foil consisting of a soft annealed aluminum foil having a purity of 99.5, a thickness of 0.012 and a width of 1,000 mm., and glued thereto a kraft paper having the weight of 60 grams per square meter, was extrusion coated at the free face of the paper layer with high pressure polyethylene in a thickness of 50 grams per square meter.

At a forward speed of the composite foil of 200 meters per minute and a temperature of the polyethylene of about 280 C. at the nozzle orifice, the adherence between the paper surface and the extruded polyethylene was unsatisfactory.

However, by pretreating the composite paper-aluminum foil in a device corresponding to that schematically illustrated in FIG. 1 with an alternating field of 500,000 volts and 50 kHz., and with a distance of about 1 meter between the treating electrode and the extrusion nozzle for the polyethylene, and while maintaining a gap of about 1.5 mm. between the free surface of the paper and the turnably supported treating electrode roller, it was found that the high pressure polyethylene adhered very strongly to the paper surface. It was not possible to tear off the polyethylene film without at the same time also tearing the entire composite or laminated foil structure.

EXAMPLE 8 A soft annealed aluminum foil having a purity of 99.5 a thickness of 0.009 mm. and a width of 600 mm. is combined by means of a lacquer adhesive with a cellophane foil weighing 35 grams per square meter. The thus formed composite foil is coated at the free aluminum face thereof in an extrusion device with 30 grams per square meter of high pressure polyethylene.

At a forward speed of the aluminum-cellophane foil the polyethylene and aluminum surface is entirely insufficient. Application of a chemical primer, namely waxfree shellac in a quantity of 0.05 grams per square meter, in a device such as illustrated in FIG. 1 and identified therein with reference numerals 2 and 3, did not suffice for sufficiently improving adherence betweenthe aluminum and polyethylene surfaces at forward speeds of the aluminum-cellophane foil of 60 meters per minute and more.

Upon applying in addition to the shellac primer also the alternating field pretreatment of the aluminum surface in accordance with the present invention, utilizing two electrodes arranged as illustrated in FIG. 4 and applying 600,000 volts and 60 kHz., it was found that the adherence between the high pressure polyethylene and the aluminum surface became so strong that the polyethylene could no longer be removed from the aluminum surface without destruction. Even by increasing the forward speed of the foil through the alternating field and through the polyethylene extrusion device to 150 meters per minute, the same high degree of adherence between polyethylene and aluminum surface was obtained.

The two treating electrodes were located 70 and cm. distant, respectively, from the extrusion nozzle and the gap between the aluminum surface and the turnably supported treating roller electrodes amounted to 1.2 mm.

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.

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

1. A method of treating at least one surface of a metal member so as to improve the adherence of a cover layer thereto, comprising the steps of subjecting said surface to the action of a high voltage, high frequency alternating field by passing said member in one direction closely adjacent but spaced from an electrode and by connecting said member as counter pole to said electrode; and rotating said electrode about an axis extending transverse to said one direction.

2. A method as defined in claim 1, wherein said metal member is an elongated sheet metal member moved in longitudinal direction past said electrode.

3. A method as defined in claim 1, wherein said alternating field has a voltage of between 50,000 and 600,000 volts and a frequency between 25 and 400 kHz.

4. A method as defined in claim 1 and including the step of cooling said electrode.

5. A method as defined in claim 4, wherein said metal member is subjected to the action of at least two high voltage, high frequency alternating fields spaced in said one direction from each other.

6. A method as defined in claim 4, wherein said metal member is subjected at opposite surfaces to the action of high voltage, high frequency alternating fields.

7. A method as defined in claim 1, and extruding onto the thus treated surface a layer of polyethylene.

8. A method as defined in claim 1 and adhering to the thus treated surface a sheet of polypropylene.

9. A method as defined in claim 1 and applying printing ink to the thus treated surface.

10. A method as defined in claim 1 and applying a lacquer to the thus treated surface.

-11. A method as defined in claim 1 and applying enamel to the thus treated surface.

12. A method as defined in claim 1 and extruding a layer of an extrudable synthetic plastic material to the thus treated surface.

References Cited UNITED STATES PATENTS 3,205,094 9/1965 Erlandson 118-620 X 3,281,347 10/1966 Winder 25049.5 X 2,522,082 9/ 1950 Arnold 11793.1 X

14 2,867,912 1/1959 Kritchever 156272 X 3,037,886 5/1962 Ryznar 1117-49 X 3,323,965 5/1967 Hanle et a] l56--244 EARL M. BERGERT, Primary Examiner. T. R. SAVOIE, Assistant Examiner.

US. Cl. X.R. 117-49, 93.1; 156--272; 204-440; 25049.5

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,451,871 June 24, 1969 Heinrich Bauer et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading to the printed specification, line 7, "Dessau Vetaphone Gesellschaft" should read Deutsche Vetaphone Gesellschaft Signed and sealed this 21st day of April 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer WILLIAM E. SCHUYLER, JR.

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