Electrical Cable And Process

Abstract

An electrical cable which can be made by the tandem extrusion process is provided. The electrical cable comprises (1) a core of an electrical conductor, (2) an aluminum shield surrounding the conductor, at least the outer surface treated with a soluble azo compound cross-linking agent for polyethylene and a complex of fumarato chromic (III) nitrate, (3) a layer of a polyolefin having a density within the range of 0.910 to 0.935 surrounding and adhered to the treated surface of the aluminum shield, and polyolefin being polyethylene or a copolymer of ethylene with at least one .alpha.-olefin having three to eight carbon atoms (4) an outer jacket of polyethylene containing carbon black surrounding and adhered to the polyolefin layer.

Parent Case Text

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 241,240, filed Apr. 5, 1972 now abandoned, and assigned to the assignee of the present application.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electrical cables.

2. Prior Art

The conventional technique for making corrosion resistant electrical cable employs a laminated shield as described in U.S. Pat. No. 3,206,541, issued to Ludwik Jachimowicz on Sept. 14, 1965, and in U.S. Pat. No. 3,233,036, issued to Ludwik Jachimowicz on Feb. 1, 1966. This shield consists of aluminum or copper foil coated on one or both sides with a copolymer of ethylene and acrylic acid or acrylic acid esters of the type described in U.S. Pat. No. 2,987,501 issued to James K. Rieke et al. on June 6, 1961, and U.S. Pat. No. 3,027,346 issued to Frank M. Rugg et al. on Mar. 27, 1962. The coating exhibits moderate to good adhesion and fair durability to moisture. An additional cable structure is described in U.S. Pat. No. 3,586,756 issued to O. G. Garner et al. on June 22, 1971.

Manufacture of electrical cable of the type above described is described in U.S. Pat. No. 3,332,138 issued to O. G. Garner on July 25, 1967. Additional manufacturing methods and cable constructions are described in U.S. Pat. No. 3,272,912, issued Sept. 13, 1966 to Ludwik Jachimowicz and U.S. Pat. No. 3,504,102 issued Mar. 31, 1970 to F. F. Polizzano.

In electrical cable of the type above described, it would be desirable to use polyethylene because of its high electrical resistivity and resistance to chemicals and moisture.

U.S. Pat. No. 3,674,915, issued July 4, 1972 to J. E. Pritchard recommends the use of certain ethylene-1-olefin copolymers as the dielectric in electric cables, with lubricants or slip agents between the various layers of the cable. U.S. Pat. No. 3,681,515 to R. C. Mildner teaches the use of thick or thin adhesive layers of ethylene-acrylic acid copolymer to adhere a polyethylene sheath to an aluminum shield in electrical cable. However, it would be desirable to have a greater bond strength than is available with such copolymers.

Belgian Patent 765,137, granted Oct. 1, 1971 discloses the ability of fumarato chromic nitrates to adhere polyethylene and aluminum, but even greater strengths are desirable in electric cable.

SUMMARY OF THE INVENTION

According to the present invention there is provided an electrical cable comprising, (1) a core of an electrical conductor, (2) an aluminum shield surrounding the conductor, at least the outer surface treated with a soluble azo compound cross-linking agent for polyethylene and a complex of fumarato chromic (III) nitrate, (3) a layer of a polyolefin having a density within the range of 0.910 to 0.935 surrounding and adhered to the treated surface of the aluminum shield, said polyolefin being polyethylene or a copolymer of ethylene with at least one .alpha.-olefin having three to eight carbon atoms and (4) an outer jacket of polyethylene containing carbon black surrounding and adhered to the polyolefin layer.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing is a cross-sectional view of a cable constructed in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the drawing, there is shown an electrical cable comprising a plurality of individually insulated conductors 10 making up a cable core. The core is wrapped longitudinally with an aluminum foil 11 which is treated on one or both surfaces with an aqueous solution containing an azo acid cross-linking agent for polyethylene and a complex of fumarato chromic (III) nitrate. The aluminum foil is folded in such a way that overlap of the edges occurs to form lap seam 12. While the aluminum foil can be of any convenient thickness, it will usually be about 5 to 10 mils thick.

The fumarato chromic (III) nitrate complex used to treat the aluminum surface is described in U.S. application Ser. No. 210,833, filed Dec. 22, 1971, in the name of Alden J. Deyrup and assigned to the assignee of the present application, the disclosure of which is hereby incorporated by reference. A preferred complex is the Type II complex described therein. This complex has the approximate formula:

[Cr(H.sub.2 O).sub.4 (C.sub.4 H.sub.2 O.sub.4)].sup.+ NO.sub.3 .sup.-

Cross-linking agents for polyolefins are generally either azo compounds or peroxide compounds. For commercial applications it is desirable to have stable treating solutions with long shelf life at a wide range of temperatures from near freezing to over 40.degree.C. such as might be encountered in transporting the materials. Available azo compounds are more likely than peroxide compounds to be stable in the highly acidic (pH about 1.5 to 2.0) solution of chromium complex.

It is also desirable to have a minimum of organic solvents in the treating solutions, both to minimize the ecological and economic need for solvent recovery and also to avoid flammability in the materials being worked with. Treating solutions may desirably contain from about 10 to about 30 percent, preferably less than 20 percent, isopropanol and still not present flammability hazards. Parts and percentages herein are by weight except where indicated otherwise. Thus, the azo compounds must be soluble and stable in acidic water containing 10 to 30 percent isopropanol and the chromium complexes. This potentially includes the so-called water soluble azo compounds and some that are not soluble in pure water.

A preferred water soluble compound for use in the invention is 4-t-butylazo-4-cyanovaleric acid which has the following structure: ##SPC1##

This material is sold by the Lucidol Company under the designation R-A73.

A less effective but still useful water soluble azo compound is 4,4'azobis[4-cyanovaleric acid], having the structure: ##SPC2##

Another effective water soluble azo compound is the monoamide derivative of azo bisisobutyronitrile, known as 2-cyano-2'-carbamoylazobisisopropane and having the structure: ##SPC3##

Other suitable azo compounds will be readily apparent to those skilled in the art. Although various levels of azo compounds are suitable in treating solutions of the invention, 0.1 to 0.4 percent concentrations are desirable.

Sodium salts and chloride salts are undesirable as the azo compounds because of reaction with the acid chromium complex solutions and deleterious effects on adhesion.

Application of an aqueous solution of the azo compound and the complex to the aluminum surface can be by any convenient technique such as dipping, spraying, brushing or rolling. After contacting the aluminum surface with the solution, the surface may be dried by heating. The effect of the treatment is to cause an invisible, nonetching surface alteration, probably the adsorption of a monomolecular layer of the complex which improves the speed and durability of adhesion of polymers to the substrate.

The treated aluminum foil which is wrapped around the cable core to form the shield is then surrounded with an adhesive layer 13 of a polyolefin which is usually 1 to 10 mils in thickness but can be 50 mils or more in thickness. The polyolefin used has a density between 0.910 to 0.935. Polyolefins useful in the present invention and which have the specified density are polyethylene and copolymers of ethylene with at least one .alpha.-olefin having three to eight carbon atoms. The upper limit of the .alpha.-olefin in the copolymer is only determined by the specific .alpha.-olefin used which will give a copolymer having a density within the specified density range.

In order to protect the coated aluminum shield, a protective outer jacket 14 of polyethylene filled with carbon black surrounds and adheres to the polyolefin adhesive layer. The outer jacket is usually 40 to 80 mils in thickness and is polyethylene having a density between 0.910 to 0.935. Carbon black is dispersed throughout the outer jacket at a concentration lower than the saturation level of carbon black in the polyethylene, but it will usually be at a concentration between 2 to 3 percent.

The electrical cable of the present invention can be manufactured by techniques known to those skilled in the art. For instance, the method of British Patent 1,080,778 can be employed by extruding the two polyethylene layers by the well-known tandem extrusion process.

Advantages of the present invention include the ability to use polyethylene as the adhesive layer and obtain good adhesion to the aluminum foil. By having the aluminum foil treated with an azo compound and the complex of fumarato chromic (III) nitrate, the use of a prelaminated aluminum composite can be avoided. Thus, manufacture of the electrical cable is simplified because of the in-line coating of the aluminum shield with polyethylene by tandem extrusion process.

The invention can be further understood by the following example:

Wettable, dead soft aluminum foil (3 .times. 0.008 in.) is primed on both sides with a dilute solution of Type II chromium (III) fumarate coordination complex as described and prepared in Example 6 of U.S. Ser. No. 210,833 of Deyrup.

In this example, a solution of Type II is prepared. The preparation is at a total volume of 1 liter, maintained by addition of water to make up for evaporating losses during boiling.

In a liter of total aqueous solution are placed together 20.0 g. chromium (III) nitrate, Cr(NO.sub.3).sub.3.sup.. 9H.sub.2 O, and 40.0 g. of fumaric acid. The solution is boiled for 1 hour, then cooled to 24.degree.C., resulting in a clear blue solution with pH 1.18. To this are added with stirring 34.0 ml. of a 1.000 normal sodium bicarbonate solution, resulting in a pH of 1.43. The solution is boiled again 1 hour, then cooled to 24.degree.C., resulting in a pH of 1.33. To this are added 33.0 ml. of the 1.000 normal sodium bicarbonate solution, resulting in a pH of 1.63. The solution is boiled again for 1 hour then cooled to 24.degree.C., resulting in a pH of 1.44. To this are added 33.0 ml. of the 1.000 normal sodium bicarbonate solution, resulting in a pH of 1.84. The solution is boiled 3 hours longer, resulting in a pH of 1.67. It is then cooled to 4.degree.-5.degree.C. and filtered to remove excess fumaric acid. The resulting solution had a clear medium green color with an absorption spectrum characteristic of Type II. This example illustrates the preparation of a fairly completely converted solution of II which is essentially pure except for the presence of by-product sodium nitrate.

The Type II complex solution is diluted with water containing 10-30 percent of isopropanol, such as from 3 to 12 parts per part of solution, preferably about 7:1. The treating solution of the invention is made by adding azo compound to this complex solution, such as about 0.1 to 0.4 percent azo compound, preferably about 0.4 percent based on the weight of the treating solution or 1 to 3 percent based on the chromium complex. Preferably about 0.4 percent of 4-t-butylazo-4-cyanovaleric acid is dissolved completely in the solution. This then forms the preferred treating solution which is ready to use and which has a substantial shelf life stability. It is applied to aluminum foil in any of a number of manners, and air-dried in place. A preferred method is the use of gravure roll application techniques as known in the art.

The treated aluminum foil is used for making cable by a double extrusion technique. The foil is fed into a cable wrapping guide where it is folded with the lap seam extending lengthwise of the cable. No oil is used during the folding process. The resulting aluminum shield is then pulled into a 2-inch extruder and coated at 230.degree.C. at a speed of 15 feet per minute with approximately 0.05 in. of a commercial grade of unstabilized low density polyethylene having a density of 0.9201 and a melt index of 1.40. The polyethylene is made by coordination catalysis at low pressure with butene-1 comonomer. The coated cable is preheated (ca. 50.degree.C.), fed into an extruder with a different tubing die, and a jacket of approximately 0.05 inch of black polyethylene having a density of about 0.93 and containing 2.5 percent carbon black is coated over it. Excellent adhesion is found between the treated aluminum and polyethylene.

Comparative tests were made with samples of treated aluminum 0.7 mil thick adhered to 2.5 mil low density polyethylene film having no antioxidant, "Alathon" 20 film sold by E. I. du Pont de Nemours and Company. The tests showed much stronger bonds for samples made with a treating solution of the invention containing azo compound and Type II chromium complex than for samples made with a treating solution which contained the Type II chromium complex but not the azo compound.

The tests were made with a 7:1 dilution in water containing 30 percent isopropanol of the above-described Type II chromium complex concentrate. The azo compound was 4-t-butylazo-4-cyanovaleric acid at four concentrations. The treating solution was applied to the aluminum foil by means of a gravure roll having 180 quadrangular cells per square inch, and it was then dried at 65.degree.C. The polyethylene was laminated onto the treated aluminum by passing rapidly over a roll at 204.degree.C., with less than one second contact time, and then promptly cooled. Then another sample of each type was produced by passing over the roll at double the initial speed. Samples were cut from the coated strip thus produced. The samples had a width of 1 inch. The aluminum and polyethylene were separated if possible and then pulled apart for a 180.degree. separation in an "Instron" tester at a jaw separation rate of 2 inches per minute. Certain tests were also made after soaking in water and in a 1:1 by volume water-ethanol solution for three weeks. The force required to separate the aluminum and polyethylene is reported in Table I below in grams for the one inch specimen width. --------------------------------------------------------------------------- TABLE

I 180.degree. Peel Strength (g/in) Water- Treating Solution Line Speed Dry Water Alcohol __________________________________________________________________________ Complex 1x 24 5 5 do. 2x 22 5 5 Complex+0.15% 1x (insepar- 220 380 azo acid able do. 2x (do.) 50 90 Complex+0.23% 1x (do.) 180 250 azo acid do. 2x 1230 30 40 Complex+0.32% 1x (insepar- 170 220 azo acid able do. 2x 307 Complex+0.41% 1x (insepar- 170 290 azo acid able do. 2x 449 0.15% azo acid 1x 40 __________________________________________________________________________

These data show the superiority of the invention under dry and wet conditions and after exposure to wateralcohol solutions.

Claims

What is claimed is:

1. An electrical cable comprising (1) a core of an electrical conductor, (2) an aluminum shield surrounding the conductor, at least the outer surface treated with an aqueous solution containing from about 10 to about 30 percent by weight isopropanol, and a dissolved azo compound cross-linking agent for polyethylene, and a complex of fumarato chromic (III) nitrate, (3) a layer of a polyolefin having a density within the range of 0.910 to 0.935 surrounding and adhered to the treated surface of the aluminum foil, said polyolefin being selected from the group consisting of polyethylene and a copolymer of ethylene with at least one .alpha.-olefin having three to eight carbon atoms and (4) an outer jacket of polyethylene containing carbon black surrounding and adhered to the polyolefin layer.

2. The electrical cable of claim 1 wherein the azo compound is selected from the group consisting of 4-t-butylazo-4-cyanovaleric acid, 2-cyano-2'-carbamoylazobisisopropane, and 4,4'-azobis[4-cyanovaleric acid].

3. The electrical cable of claim 2 wherein the azo compound is 4-t-butylazo-4-cyanovaleric acid.

4. The electrical cable of claim 1 wherein the aluminum strip is about 5 to 10 mils in thickness.

5. The electrical cable of claim 1 wherein the polyolefin layer is polyethylene about 1 to 10 mils in thickness.

6. The electrical cable of claim 1 wherein the outer jacket is 40 to 80 mils in thickness and is polyethylene having a density within the range of 0.910 to 0.935 containing about 2 to 3 percent by weight of carbon black.

7. An electrical cable comprising (1) a core of an electrical conductor, (2) an aluminum shield 5 to 10 mils in thickness surrounding the conductor, at least the outer surface treated with 4-t-butylazo-4-cyanovaleric acid and a complex of fumarato chromic (III) nitrate, (3) a layer of polyethylene about 1 to 10 mils in thickness, having a density within the range of 0.910 to 0.935 surrounding and adhered to the treated surface of the aluminum shield and (4) an outer jacket 40 to 80 mils in thickness of polyethylene having a density within the range of 0.910 to 0.935 which contains about 2 to 3 percent by weight of carbon black.

8. The electrical cable of claim 1 wherein the treated aluminum shield has a lap seam extending lengthwise of the cable.

9. The electrical cable of claim 7 wherein the treated aluminum shield has a lap seam extending lengthwise of the cable.

10. A process for making an electrical cable of claim 1 in which the azo compound and complex are applied to the aluminum shield by contacting said shield with an aqueous solution of said complex and said azo compound.

11. The process of claim 10 in which said azo compound is selected from the group consisting of 4-t-butylazo-4-cyanovaleric acid, 2-cyano-2'-carbamoylazobisisopropane, and 4,4'-azobis[4-cyanovaleric acid].

12. The process of claim 11 in which said azo compound is 4-t-butylazo-4-cyanovaleric acid.

13. A double extrusion process for making electrical cable in which a treated aluminum shield is wrapped around a conductor core with a lap seam, a layer of polyolefin having a density within the range of 0.910 to 0.935 is extruded onto said treated aluminum foil, said polyolefin being selected from the group consisting of polyethylene and a copolymer of ethylene with at least one .alpha.-olefin having three to eight carbon atoms, and then an outer jacket of polyethylene containing carbon black is extruded onto said polyolefin layer, at least the outer surface of said aluminum shield having been treated by contacting it with a solution in water and about 10 to 30 percent by weight isopropanol of an azo compound cross-linking agent for polyethylene and a complex of fumarato chromic (III) nitrate to provide adhesion between said aluminum shield and said polyolefin layer.

14. A process of claim 13 in which said azo acid is selected from the group consisting of 4-t-butylazo-4-cyanovaleric acid, 2-cyano-2'-carbamoylazobisisopropane, and 4,4'-azobis[4-cyanovaleric acid].

15. A process of claim 14 in which said azo acid is 4-t-butylazo-4-cyanovaleric acid.