US3397132A - Treatment of metal surfaces - Google Patents

Description

Aug. 13, 1968 L. E. WOLINSKI 3,397,132

TREATMENT OF METAL SURFACES Filed Oct. 16, 1964 METAL SURFACED FILM STRUCTURE.

oncmuc VAPOR +0ARRIER GAS INVENTOR LEON EDWARD WOLIN SKI BY fizz/@444.

ATTORNEY United States Patent 3,397,132 TREATMENT OF METAL SURFAGES Leon Edward Wolinski, Butfalo, N.Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del, a corporation of Delaware Filed Get. 16, 1964, Ser. No. 404,234 10 Claims. (Cl. 204-165) ABSTRACT OF THE DISCLOSURE Metal surfaces are rendered more adherent to polymer coatings by subjecting the metal surface to the action of a pulsating electrical discharge at voltages of from about 1000 to about 100,000 and at frequencies of from about 350 to 500,000 cycles per second, at atmospheric pressure in an atmosphere consisting of an inert gaseous carrier medium which will sustain said electrical discharge and up to about 5% by volume of an organic agent having a vapor pressure of at least 0.25 mm. of mercury at 60 C., said agent being selected from the group consisting of polymerizable organic compounds, nonpolymerizable organic compounds having replaceable hydrogen and perhalohydrocarbons.

This invention relates to the treatment of metal surfaces and more particularly to the treatment of metallic surfaces of flexible shaped structures having at least one surface of metal, e.g., metal foils, metalliz/ed polymeric film structures, and metal foil-organic polymeric film laminates, to produce improved structures.

A number of problems arise in the utilization of shaped structures having a metallic surface such as metal foils, rigid metal structures, as well as various plastic sheets or films bearing a metallic surface. In the fabrication and utilization of various metal coated plastic films in condensers, for example, difficulties arise through premature breakdown of the metal coated film by the action of corona or there results a corona breakdown at a lower voltage than is desired. Frequently it is desirable to adhere either a metal coated plastic film or a metal foil to other surfaces or structures with very strong bonds which will withstand exceedingly heavy usage, and available technology does not provide the extremely firm bonding that is required for such applications. For example, organic films deposited on conductive substrates by glow discharge polymerization show a tendency to peel or separate from the substrate (Electrical Properties of Thin Organic Films, Bradley and Hammes, Radiation Research Corporation, Journal of the Electrochemical Society, volume 110, No. 1, January 1963). Further, such processes are not well adapted to large scale commercial operation. In still other situations, the bright metal surface on various metal structures is subject to corrosion by various chemical actions or through the effects of weathering under adverse atmospheres. Still other metal structures such as flat silverplate show the effect of tarnishing from the adverse atmospheres in which such articles may be stored.

It is therefore an object of this invention to provide a simple and economical process for rendering various shaped structures having metallic surfaces resistant to such adverse effects as noted above. A further object of this invention is to provide a simple and economical process for treating the metal surface of flexible metal-surfaced structures such as metal foils, metallized film, metal foilto-film laminates and the like to produce improved structures. It is a still further object to provide various shaped structures having metallic surfaces with greatly improved properties. The foregoing and related objects will more clearly appear from the detailed description which follows.

These objects are realized by the present invention which, in essence, comprises subjecting the metal surface of a metal-surfaced shaped structure to the action of an electrical discharge at substantially atmospheric pressure in a gaseous atmosphere containing up to about 5% by volume of the vapor of at least one organic agent having a vapor pressure of at least 0.25 mm. of mercury at 60 C. in an inert gaseous carrier medium which will sustain said electrical discharge.

In the preferred embodiment of this invention, illustrated diagrammatically in the accompanying drawing, a continuous web of flexible pellicular structure having a metal surface such as a plastic film bearing a thin metal coating, a laminate of a plastic film such as polyester film to a thin metal foil such as aluminum, zinc, silver, chromium, copper, etc., or metal foil per se, is continuously passed between a set of spaced electrodes consisting of a rotating metal roll 1 which is connected electrically to ground, and one or more stationary hollow metal tubes 2 disposed parallel to the longitudinal axis of the roll and closely and uniformly spaced from the surface thereof. The tubes are each connected electrically to a suitable power source which supplies to each tube electrode an alternating (or pulsating direct) current of from 0.3 to 5.5 r.m.s. (root mean square) amperes at a voltage in excess of about 2000 volts, and at a frequency within the range of from about 200,000 to about 500,000 cycles per second. A mixture of an inert carrier gas which will sustain an electrical discharge, e.g., nitrogen, and the vapor of a polymerizable organic monomer having a vapor pressure of at least 0.25 mm. of mercury at 60 C., e.g., acrylonitrile or glycidyl methacrylate, said vapor constituting not over 5% by volume of the mixture, is continuously fed-to the hollow interior of the tube electrodes through distributor ducts 3 and issues from the tubes at the gap between each tube and the grounded roll through suitable openings distributed along the length of the tubes whereby the electrical discharge across said gap and onto the exposed metal surface takes place in an atmosphere containing said vapor. The mixture of nitrogen and vapor may, of course, be introduced into the reaction zone through one or more tubes separate from the electrode assembly and the stationary electrodes may be of solid construction. The assembly just described is suitably enclosed in a chamber 4 maintained at atmospheric pressure and provided with the necessary openings to facilitate maintenance of the vapor-containing atmosphere in the treating zone and controlled exhaust therefrom, and to minimize operational hazards.

In carrying out the surface treatment of this invention the potential difference between the electrodes providing the electrical discharge may vary from very low voltages in the order of 1000 volts up to pulsating voltages of 100,000 and above. In general, however, it is preferred to maintain the voltage in excess of about 2000 volts. Frequencies from 350 cycles per second up to 500,- 000 cycles per second and above can be used. Frequencies in the range of 200,000 to 500,000 cycles are preferred for many applications in order to obtain effective treatment at commercially acceptable exposure times. For other applications, frequencies in the range of about 1000 cycles to 50,000 cycles provide very satisfactory results.

While the current to the electrodes may range up to 5.5 RF amperes or more, for optimum results a range of about 0.3 RF amperes to 3.5 RF amperes is preferred. Power to the electrodes may range from about 1 watt per lineal inch to the electrode length to 400 watts or more per lineal inch of the electrode length.

Any suitable means known to the art for providing an electrical stress field (i.e., an electrostatic field) of alternating or pulsating character between spaced electrodes (e.g. a high frequency spark generator of the type hereinafter identified or a motor generator setup) may be employed as the power source in the electrical discharge treatment of this invention.

The electrodes are preferably spaced from about .01 inch' to about 0.125 inch; Useful results can be obtained when the electrode gap is as low 'as 0.005 inch to as much as 0.25 inch provided suitable adjustments in such features as amount of current. electrode dimension and exposure time are made. Time of exposure to the electrical discharge treatment is not especially critical and effective treatments are realized at exposure times as short as 1 10- second and no adverse effects are noted at times as long as 60 seconds. Preferably, the exposure time should be not less than 4 l0 second. For economic reasons, exposure times as short as possible consistent with effective treatment would normally be employed.

Any organic agent selected from the group consisting of polymerizable organic compounds, nonpolymerizable organic compounds having replaceable hydrogen and perhalohydrocarbons having a vapor pressure of at least 0.25 millimeter of mercury at 60 C. may be employed for purposes of this invention. Typical examples of suitable polymerizable compounds include acrylonitrile, glycidyl methacrylate, methyl methacrylate, cyclopentadiene, styrene, p-chlorostyrene, vinyl butyl ether, methyl vinyl acetate, l-hexene, n-vinyl-Z-pyrrolidone, ethylene imine, tetrafluoroethylene, hexafluoropropene, dichlorodifluoroethylene and acrylic acid. Typical nonpolymerizable compounds having a replaceable hydrogen include xylene, hexane, cyclohexane, chloroform, tetrahydrofurane, diethylsulfone, tetramethylxylene, tetraisopropyltitanate, methylamine, ethylamine, dimethylamine, diethylene triamine and butylamine. Typical perhalohydrocarbons include carbon tetrachloride, trichlorofluoromethane, dichlorodifluoromethane, monochlorotrifiuoromethane, tetrachlorodifluoroethane, trichlorotrifiuoroethane, dichlorotetrafluoroethane, bromotrichloromethane, dibromodifluoromethane, dibromomonochlorotrifiuoroethane, dibromotetrafluoroethane and monochloroheptafluoropropane.

Carrier gases which are inert, that is, do not interfere with the action of the organic vapor such as nitrogen or carbon dioxide, are preferably employed to facilitate transport of the organic vapor to the electrode gap or to facilitate discharge when gases having high dielectric strength are used as the modifying agent. In some instances ammonia and various volatile amines may be employed as a carrier either along or with nitrogen or carbon dioxide.

The process of this invention is particularly adapted to the treatment of metal foils such as those based on aluminum, steel, copper, tin, etc., as well as various metallized plastic films such as films of polyethylene terephthalate bearing a coating of aluminum or zinc or laminates of a plastic film to the metal foils. However, it is also within the scope of the process of this invention to treat rigid structures having a metal surface, using a suitable arrangement of electrodes as will be evident to persons skilled in the art.

The following specific examples will serve to further and more fully illustrate the principles and practice of this invention.

Example 1 A one-mil thick film of biaxially oriented polyethylene terephthalate, heat set at 200 C., and which bore a 0.0004 mil coating of aluminum on one surface was drawn through an apparatus as illustrated in the accompanying drawing at a speed of feet per minute. The electrodes of the apparatus were connected to a high frequency spark generator (Model HFSG-Lepel High Frequency Laboratories, Inc.). The stationary electrodes were spaced 0.04 inch from the surface of the roll and the power setting of the generator was set at 70 corresponding to the current of approximately 1.3 RF ainperes to the electrodes. An atmosphere of about 3% by volume of trichlorofiuoromethane in nitrogen was maintained between the electrodes.

The treated metallized film was made up into a capacitor and tested for resistance to corona breakdown. It was observed that the treated film showed no evidence of corona breakdown until a voltage of 1100 was reached. The same metallized film without the electrical discharge treatment showed a breakdown at 450 volts.

The capacitor for the evaluation of the treated film is constructed as follows: Each capacitor is constructed of two strips of Z-inch wide polyethylene terephthalate film with the coating of the metal approximately 0.004 mil thick on one side. The metal coating is 1% inches in width on each surface. The two strips of metallized film are interwound with two strips of /4 mil aluminum foil 1% inches in width. The capacitor is then subjected to the corona starting voltage which is indicated by a cathode ray oscilloscope. The increase in starting voltage permitted in the tests with a capacitor made from the test film and a corresponding control made without electrical discharge treating are compared.

Examples 2-5 Corona Discharge Metal Gaseous Example Surface Atmos- Treated Non-Treated phere Film Control (V olts) (Volts) 2 Zinc CF3G1 1,100 450 3 Copper C2FB 1,000 425 4- Silver. (3213501.... 1,000 475 Chr0m1nm CCIQFL... 1,050 470 Example 6 A strip of aluminum foil three mils thick was drawn through the apparatus as in Example 1 at a rate of 10 feet per minute. An atmosphere of nitrogen and acrylonitrile was provided in the space between the electrodes by bubbling a stream of nitrogen through acrylonitrile (approximately 1% of acrylonitrile by volume) maintained at room temperature at a rate of about four cubic feet per minute. There was then applied to the treated. foil a thin layer of epoxy resin (Epon 1004 with Epon U Curing Agent) and the resin bearing surfaces were pressed together to form a laminate. The laminate was pulled apart on a Suter Tester and showed a bond strength of over 3600 grams per inch. A control aluminum foil which had not been given the treatment in the electrical discharge showed a bond strength of 480 grams per inch; a second control aluminum foil which had been drawn through the electrical discharge but in the absence of an atmosphere of acrylonitrile showed a bond strength of about 600 grams per inch.

Example 7 Following the procedure of the preceding example, a strip of two mil thick cold rolled steel was drawn between the electrodes ofthe apparatus described in Example 1 in an atmosphere of nitrogen and glycidyl meth acrylate; The resulting treated foil was made into a laminate through the use of Swift X-7071 epoxy resin adhesive and the laminate was then pulled apart on a Suter Tester. The laminate showed a bond strength of over 4000 grams per inch; a control foil which had not been subjected 'to the electrical discharge treatment showed a bond strength of 500 grams per inch; another control foil which had been subjected to an electrical discharge treatment but in the absence of the gaseous atmosphere of glycidyl methacrylate showed a bond strength of about 1500 grams per inch. Still another foil exposed for a comparable time to an atmosphere of glycidyl methacrylate under reduced pressure in a glow discharge and made into a laminate as described above showed a bond strength of 800 grams per inch.

Example 8 A two-mil strip of cold rolled steel bearing a chrome coating of about 0.001 mil was drawn between the electrodes of the apparatus as in Example 1 at a rate of feet per minute in an atmosphere of nitrogen and diethylenetriamine. Power was supplied by a motor generator delivering 2500' volts to the electrodes at a frequency of 3000 cycles. A corrosion test wherein a strip was dipped into a brine solution when alternately permited to dry in air with the alterations taking place over a 10-hour period. At the end of this time there was no indication of any formation of rust spots indicating corrosion on the treated strip. In contrast, a control strip of chromium coated steel foil which had not been subjected to the electrical discharge treatment in the atmosphere of diethylene triamine, showed various spots wherein corrosion had started in this period of time.

Example 9 A three-mil thick strip of copper foil was drawn through the apparatus as in Example 1 wherein the atmosphere was a mixture provided by bubbling nitrogen through glycidyl methacrylate at approximately 4 cubic feet per minute and a second gas produced by passing nitrogen through ethyleneimine at the same rate. The treated film was laminated to a S-mil thick film of tetrafluoroethylene/hexafiuoropropene copolymer (weight ratio 85/15) of the type described by Bro and Sandt U.S.P. 2,946,763. The lamination was carried out by pressing the films together with a slight pressure (less than 1 pound per square inch at 275 C.). A very strong bond was obtained; the copper foil was torn in attempting to separate the copper foil from the plastic film. A laminate made from a control copper foil which had been subjected to the electrical discharge but in the absence of the gaseous atmosphere showed a bond strength of about 1500 grams per inch.

Example 10 A strip of fiat silverplate was drawn through an electric discharge in a -mil gap between spaced electrodes, in an atmosphere of nitrogen and tetrafluoroethylene, the electric discharge being produced by a motor generator operating at 20,000 cycles frequency and a voltage of 3000 across the electrodes and with the silverplate strip being in continuous contact with the ground electrode. The treated silverplate showed no evidence of tarnishing when exposed for one week to an atmosphere containing sulfide vapors, whereas a control sample of the same silverplate which had not been treated in the electrical discharge became blackened when exposed to the same atmosphere for the same period of time.

It will be evident from the foregoing description and examples that the process of this invention results in a wide variety of beneficial effects on shaped structures having metal surfaces including improved corona resistance, resistance to corrosion and chemical attack as well as improved adherability of the metallic surface to other structures. Moreover, the process is well adapted to large scale industrial use in that cumbersome and expensive equipment for providing such special conditions as reduced pressure are not required.

What is claimed is:

1. A process for modifying and improving the characteristics of a metal surface which comprises subjecting the metal surface of a metal-surfaced shaped structure to the action of an electrical discharge at substantially atmospheric pressure in a gaseous atmosphere consisting of an inert carrier gas which will sustain an electrical discharge and of the vapor of at least one organic agent having a vapor pressure of at least 0.25 mm. of mercury at 60 C. and selected from the group consisting of polymerizable organic compounds, nonpolymerizable organic compounds having replaceable hydrogen and perhalohydrocarbons said vapor being present in an amount up to 5% by volume said electrical discharge being formed between spaced electrodes to which are applied an alternating current at a voltage within the range of from about 1000 to 100,000 volts and at a frequency of from about 350 to about 500,000 cycles per second effective to create an electrical discharge between said electrodes.

2. A process for modifying and improving the characteristics of a metal surface which comprises continuously passing a continuous web of a pellicular structure having a metal surface between spaced electrodes spaced a distance of from 0.005 inch to 0.25 inch, continuously applying to said electrodes an alternating current at a voltage within the range of from about 0 to 100,000 volts and at a frequency in the range of from about 350 to about 500,000 cycles per second effective to create an electrical discharge between said electrodes and onto said metal surface, and maintaining between said spaced electrodes at atmospheric pressure an atmosphere consisting of an inert carrier gas which will sustain said electrical discharge and the vapor of at least one organic agent having a vapor pressure of at least 0.25 mm. of mercury at 60 C., and selected from the group consisting of polymerizable organic compounds, nonpolymerizable organic compounds having replaceable hydrogen, and perhalohydrocarbons, said vapor being present in an amount up to 5% by volume of said atmosphere.

3. The process of claim 2 wherein said carrier gas is nitrogen.

4. The process of claim 2 wherein said organic agent is a polymerizable organic monomer.

5. The process of claim 2 wherein said organic agent is acrylonitrile.

6. The process of claim 2 wherein said organic agent is glycidyl methacrylate.

7. The process of claim 2 wherein said pellicular structure is a plastic film having on at least one surface thereof a continuous coating of metal.

8. The process of claim 7 wherein said plastic film is polyethylene terephthalate film.

9. The process of claim 8 wherein said metal coating in aluminum.

10. The process of claim 2 wherein said pellicular structure is metal foil.

References Cited UNITED STATES PATENTS l/ 1959 Kritchever 156-272 X 4/1960 Goodman 117-201

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