Electrical Cable With Protective Coating Or Shielding Tape

Abstract

In order to obtain the tenacious adherence of polyethylene copolymer without the cost of the copolymer, this cable has polyethylene protection on the metal foil of the shielding layer with the polyethylene a homopolymer throughout a substantial part of its thickness and changing to copolymer characteristics adjacent to the face of the foil.

Parent Case Text

BACKGROUND AND SUMMARY OF THE INVENTION

This invention is a continuation of application Ser. No. 627,082, filed Mar. 30, 1967, with parts of the parent application omitted.

Description

This invention is an improved electrical cable construction; the improvement being in the protection of the metallic components from corrosion. The invention will be described as applied to cables having a shielding layer made of aluminum tape, but it will be understood that other metal can be used for the shielding layer such as copper, or even steel.

The metal tape is protected by a plastic coating, such as a polyolefin. It will be described with polyethylene as the polyolefin, but other resins can be used which have equivalent characteristics.

It is one object of the invention to provide improvements on the constructions shown in U.S. Pat. Nos. 3,206,541 and 3,233,036 which show aluminum tapes used for shielding layers of electric cables and with polyolefin layers for protecting the metal. The adhesion must be sufficient to prevent delamination of the polyolefin coating from the metal in the presence of moisture and liquid water containing alkalis, salts or acids which are found in ground water.

In these patents referred to above, the layer of polyolefin for protecting the metal is a copolymer containing carboxyl groups so as to form a permanent chemical bond with the metal, especially aluminum. This type of bonding system has been found superior to other systems on resistance to delamination and resistance to the admission of moisture into the interface between the polyolefin layer and the aluminum tape at the edge of the sandwich formed by the tape and the polyolefin layer.

This prevention of delamination and admission of moisture at the edge of the sandwich is of extreme importance because if the polyolefin layer is allowed to lift slightly at the tape edge, an adjacent area of the interface of the metal becomes exposed to corrosive environment and little by little the corrosion progresses resulting after lapse of time in a greater area of delamination. Once delamination takes place, the film ceases to protect the metal from contact with the corrosive environment and corrosion proceeds on the exposed surface of metal as if there were no protective layer at all.

This invention is based on the discovery that the same results can be obtained at lower cost by having the protecting layer a low cost homopolymer with a concentration of reactive groups in the part of the layer of plastic that is adjacent to the metallic strip or foil or tape and only in that part of the protecting layer. For purposes of this application, the terms "strip," "foil," and "tape" are considered synonymous.

It is sufficient that the protecting layer contains the reactive groups and copolymer characteristics adjacent to the metal surface only. Where the protecting layer is very thin, the use of copolymer protecting layers is not objectionable because there is such a small amount of material involved, but with thicker protecting layers there is a substantial saving in cost by using a lower cost polymer that extends all the way to the surface of the metal but that has different characteristics at the surface of the metal because of the inclusion of localized reactive groups in the polymer adjacent to the metal. The construction has the added advantage of reduced adherence to overlying insulation which must be stripped from the cable where connections are to be made.

It may seem that the theoretical ideal would be to limit the reactive groups to the surface of the protecting layer that confronts the metal. In practice there is some merging of the copolymeric part of the protecting layer that bonds to the metal and the polymer above it, and a decrease in concentration of the reactive groups away from the cable. It may be that this produces a more stable protecting layer.

Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing, forming a part hereof, in which like reference characters indicate corresponding parts in all the views:

FIG. 1 is a fragmentary view of an electric cable with portions broken away to expose the construction of the inner part of the cable;

FIG. 2 is a view similar to FIG. 1 but showing a modified form of cable;

FIG. 3 is a greatly enlarged sectional view on the line 3-3 of FIG. 1;

FIG. 4 is a greatly enlarged sectional view on the line 4-4 of FIG. 2; and

FIG. 5 is a greatly enlarged sectional view illustrating the construction of the protective coating for either of the cables shown in the other views.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a telephone cable 8 comprising a plurality of individually insulated conductors 10 assembled into a cable core about which is applied a core tape 12 of conventional construction such as a GR-S/Mylar core tape, for example, 16 mils thick, applied over the core in a longitudinal wrap. A shield 14 is applied over the core tape. The conductors 10 and the surrounding core tape 12 may, for purposes of this invention, be considered the core of the cable and this core is designated in in FIG. 1 by the reference character 15.

The shield is a laminate consisting of a metallic strip with coating on both its inner and outer surface as will be described more fully in connection with FIG. 3. An outer extruded jacket 18, preferably of polyethylene, surrounds the shield 14 and provides the outer layer of the cable.

FIG. 2 shows a modified cable construction in which individually insulated conductors 10' are surrounded by a shield 20, the construction of which will be described more fully in connection with FIG. 4. This cable shown in FIG. 2 can also have an outer jacket if desired.

FIGS. 3 and 5 show the shield 14 made of an aluminum strip 24 which is of greater width than the circumference of the core 15 so that the strip, when extending longitudinally of the cable, has edge portions forming a seam 26.

Along this seam 26, the edge portions of the tape overlap one another so that the shield is of double thickness at the seam 26. On the inner surface of the aluminum tape 24, there is a plastic coating 32, and on the outer surface of the aluminum tape 24, there is a plastic coating 34. The inner coating 32 may be formed from an inner part 36 and an outer part 38. The inner part 36 is a copolymer with reactive groups which cause the copolymer to bond chemically to the surface of the aluminum tape 24. This inner part 36 is preferably a copolymer of polyethylene with reactive carboxyl groups for obtaining the chemical bonding. The outer part 38 may be homopolymer polyethylene, preferably a high molecular weight polyethylene which is preferably merged with the inner part 36 by fusion to produce a homogeneous system containing no internal physical boundaries, as illustrated in FIG. 5. The outer coating 34 is preferably made in the same way as the inner coating 32.

A practical technological process for obtaining the desired coatings on the tape 24 is to apply the copolymer 36 and the homopolymer 38 in two stages. In the first stage, a thin application of the copolymer, containing the reactive groups, is deposited over the metal tape 24. This deposition can be done by any of the known processes. For example, it can be by means of a hot melt extrusion process, in which case the thickness of the application will be greater than by some other processes; the thickness by hot melt extrusion being of the order of 1.5 to 2 mils. An application of copolymer 36 can also be deposited by heat sealing to the metal a thin film of the copolymer having a thickness between one and two mils. On the other hand, a much thinner application of the order of 0.1 mil can be obtained by depositing the copolymer by a known electrophoretic process in which particles of copolymer suspended in fluid are driven to the metal by an applied electrical field.

The method by which the coating is applied to the metal tape 24 is not a part of this invention. It is sufficient that the inner part 36 be applied in any known manner and to the desired thickness and that the outer part 38 be then deposited over the inner part in such a way as to unite the parts, preferably by fusion. Thus the outer part 38 may be applied by a hot melt extrusion process, or by fusion of a film of homopolymer of polyethylene to the underlying inner part 36 with the necessary heat and pressure.

It will be understood that there are other means for accomplishing the desired differential dispersion of copolymer in the protective coating of polyolefin film. For example, a polyethylene film can be chemically treated on one surface to modify that surface into copolymer containing reactive groups. After this treatment the film or coating can be heat fused to the metal strip 24 with the copolymerized side towards the metal.

The figures on thickness of the layers, as set forth above, are given merely by way of illustration. With a shielding strip having a thickness of 8 mils, electrophoretic deposition of copolymer can be made on both sides of the strip to a thickness of 0.1 mil. A film of high molecular weight polyethylene having a thickness of 2 mils can then be heat fused to the electrophoretically deposited layer of copolymer on the metal. It is advantageous, from a cost standpoint, to have the inner part 36 as thin as practical and to use the polyethylene homopolymer for the necessary remaining thickness of the protecting coating on the metal tape.

Aluminum tapes as thin as from 0.003 to 0.001 inch can be used and effectively protected by the coating of this invention and polyethylene sheets of from 0.005 to 0.010 inch in thickness can be used as protecting films for the coating.

FIG. 4 shows a modified construction in which the confronting faces of the edge portions of the shield are parallel to each other and upstanding in a substantially radial direction at the time that they are fused together. This type of seam is a conventional construction as illustrated in U.S. Pat. No. 3,206,541, previously referred to. After the upstanding edge portions are bonded together, they are bent over to form the seam 46 shown in FIG. 5. The tape used for the construction shown in FIG. 4 is the same as that shown in FIG. 5.

The preferred embodiment of this invention has been illustrated and described, and the invention is defined in the appended claims.

Claims

We claim:

1. In an electrical cable of the type having a core including a conductor and polyolefin electrical insulation around the conductor, a shield around the insulation, and an outer jacket surrounding the shield, the improvement which comprises the shield including a metal strip with corrosion protective polyolefin coating on the outside of the strip, and which is in addition to the jacket surrounding the shielding, said jacket being a polyolefin extrusion in contact with the coating on the shield, the coating being a mass of the same polyolefin for the full thickness thereof with no internal physical boundaries and with resulting mechanical and chemical radial continuity of the coating from the metal outwardly, the inner part of the coating in contact with the metal being a polymer having carboxyl groups dispersed therein for obtaining strong adherence to the metal and the part of the coating outward from the metal strip being of the same material as the inner part but without the carboxyl groups whereby it will not bond to the metal of the strip or to the jacket as effectively as the inner part of the coating, the reactive groups being concentrated in the part of the coating that contacts with the strip and decreasing as the coating extends radially away from the strip.

2. The combination described in claim 1 characterized by the polyolefin being polyethylene and being polar because of the reactive carboxyl groups, and the polyolefin of the outer part of the coating also being polyethylene containing substantially no carboxyl groups and being nonpolar, the outer part of the coating completely covering the inner part whereby the polar polyethylene is protected from reaction with metal salts in ground water which contacts with the outside surface of the coating on the cable shield, and the outer part and inner part of the coating merging into one another the carboxyl groups distributed through the polyethylene of the inner part of the coating giving said inner part polar characteristics for obtaining tenacious adherence to the metal.

3. The combination described in claim 2 characterized by the metal strip being an aluminum tape having corrosion protective coating on both sides, the coating on at least the outside of the tape being the coating described in claim 1.

4. The combination described in claim 1 characterized by the outer part of the coating being a high molecular weight polyethylene homopolymer, and the inner part of the coating being polar polyethylene with reactive carboxyl groups.

5. The combination described in claim 1 characterized by the strip extending around the circumference of the core and having a longitudinal seam with opposite edge portions of the strip in contact with one another at the seam, and corrosion protecting coating on the contacting opposite edge portions of the strip bonded to one another to close the seam.

6. The combination described in claim 1 characterized by the inner and outer parts of the coating being fused together where they merge into one another to form the mass of the same polyolefin for the full thickness thereof with no internal physical boundaries and with resulting mechanical and chemical continuity of the coating from the metal outward.

7. The combination described in claim 1 characterized by the reactive groups being supplied to from 5 to 50 percent of the radial extent of the coating depending upon the total thickness of the coating.

8. The combination described in claim 7 characterized by the metal strip having a thickness of 1 to 3 mils and the coating on one side of the strip having a radial extent of approximately 2 to 4 mils.

9. The combination described in claim 7 characterized by the inner part of the coating having the carboxyl groups being of a radial thickness of from 0.1 to 2.0 mils.

10. The combination described in claim 2 characterized by the part of the coating that has the carboxyl groups being an electrophoretically deposited copolymer of polyethylene.