Electrical conductors with strippable polymeric materials

Abstract

A composite of polymeric materials which are adheringly joined to each other and which can be easily and cleanly separated by stripping apart with a low pulling force whereupon the contacting surfaces of their interface each separate cleanly without retention of any residue from the other, and which comprises the combination of a body of peroxide curable ethylene-containing polymer adjoined to a body of a sulfur curable elastomer comprising chlorosulfonated polyethylene. The combination of materials is especially advantageous when used in wire and cable constructions comprising a composite of an electrical insulation and an overlying strippable semiconductive layer.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application is related to U.S. application Ser. No. 336,146, filed Feb. 26, 1973, for Insulated Conductor With Strippable Layer, by Thaddeus D. Misiura and Joseph E. Vostovich, now U.S. Pat. No. 3,793,476, and assigned to the same assignee as this application.

BACKGROUND OF THE INVENTION

A common type of construction for electrical wires or cables designed for medium to high voltage applications, for example about 15 to 35 KV, as well as other classes of electrical service, comprises combinations of one or more insulating layers and semiconductive layers. In a typical cable structure, for instance, the metallic conductor may be provided with an organic polymeric insulation such as cross-linked polyethylene, and an overlying body of semiconducting material comprising an organic polymeric composition which has been rendered electroconductive by the inclusion therein of electrical conductivity imparting agents or fillers such as carbon black. Although these cable constructions may vary in certain elements, and often include an intermediate component disposed between the metallic conductor and the primary body of dielectric insulation such as a layer of separating tape or inner layer of semiconductive material, or are enclosed within protective covering sheaths, all such cable constructions conventionally include therein at least a body of primary insulation surrounding the conductor with an overlying body of semiconducting material in physical contact with the insulation. However, this arrangement of a layer of insulation with a superimposed layer of semiconductive material thereover incurs certain handicaps.

For example, to prevent the occurrence of ionization or corona formation resulting from internal voids or pockets within the cable construction and consequent ultimate breakdown of the insulation, it is necessary to eliminate the presence or possible occurrence of any free space or voids within or resulting from the interface between the adjoining surfaces of the body of the insulation and the body of semiconducting material. U.S. Pat. No. 3,677,849 deals with this problem of intermediate void spaces at the interface of the insulation and semiconductive material by applying a heat treatment to the assembled product to induce a shrinkage of the semiconductive material tightly about the insulation. U.S. Pat. No. 3,259,688 proposes a different solution to this problem comprising a distinctive construction and an irradiation treatment. Further, U.S. Pat. No. 3,646,248 discloses that this problem is resolved by the use of an olefinic terpolymer insulation with a first curing system and an olefinic terpolymer semiconductive material with a second curing system comprising a different curing agent.

Further, the insulation layer and overlying semiconductive layer for electrical cable can be formed concurrently about the wire or metal conductor by means of a continuous simultaneous extrusion process with one extruder apparatus, or these layers are formed in sequence employing tandem extruders, and both layers are thereafter cured at the same time in a single operation and unit to minimize manufacturing steps and apparatus. However, the simultaneous curing of both layers together, or even the curing of only one layer alone while it is in a contiguous arrangement with the other, can result in the apparent formation of cross-linking bonds bridging the interface between the adjoining surfaces of each phase. The occurrence of such cross-linking bonds bridging the interface between the surfaces of said phases renders very difficult their subsequent separation, such as in the removal of a portion of the body of semiconductive material from about the insulation by stripping for the purpose of making cable splices or terminal connections. Such separation requires the application of great force, and, upon being peeled off, the semiconductive material is prone to leaving a substantial residue of its mass firmly adhering to the other surface or insulation. As is known in the art, it is necessary when splicing and treating cable ends that the semiconductive material be cleanly stripped or completely removed from the terminal section of the cable end without any damage or material loss to the underlying surfaces of the insulation, and thus the separation can require an appreciable amount of added labor time and costs when the semiconductive material is difficult to remove by stripping and/or a residue thereof is retained tanaciously adhering to the surface of the insulation. The difficulties of this aspect of such cable constructions are the subject of U.S. Pat. Nos. 3,684,821 and 3,643,004.

SUMMARY OF THE INVENTION

This invention comprises a combination of specific organic polymeric materials, which are cured with dissimilar curing systems, and a composite construction formed therewith wherein the two phases or bodies of the curable materials are adheringly united with each other at their abutting surfaces to provide a substantially continuous and secure union of their contacting surfaces extending over their common interface. The combination and cure systems of the invention effectively obviate the occurrence of intermediate void spaces between the interface of the two phases or bodies, while at the same time providing an interface union between the phases which is easily separated with a relatively small pulling force whereupon the components part with clean surfaces each free of any residue from the other. The invention also comprises a method of producing electrically insulated and shielded cables.

The invention includes the combination of a first body of peroxide curable ethylene-containing polymer with a second body composed of a sulfur curable chlorosulfonated polyethylene rubber (Hypalon). The sulfur curable chlorosulfonated polyethylene can contain minor amounts of an ethylene-propylene rubber dispersed therethrough to enhance the plastic properties of the chlorosulfonated polyethylene rubber.

The compositions and the attributes of this combination are uniquely suitable and advantageous for use in the construction of insulated electrical wires and cables in the formation of a composite insulation of an ethylene-containing polymer with an easily and cleanly strippable semiconductive material superimposed over the insulation. The polymeric material comprising the said chlorosulfonated polyethylene elastomer can be rendered suitably electroconductive so as to perform as a semiconductive material by appropriately filling it with a typical electrical conductivity imparting agent or filler such as carbon black, dispersed therethrough, or some other electrically conductive particulate material such as silicon carbide, iron, aluminum, etc., in such amounts so as to impart the desired degree of conductivity.

OBJECTS OF THE INVENTION

It is a primary object of this invention to provide curable polymeric materials that can be joined in a contiguous relationship with their interfacial surfaces adheringly united together so as to eliminate the presence or any occurrence of intermediate void spaces therebetween, and which thereafter can be separated by the application of a very low pulling force with the interfacial surfaces of the bodies cleaving cleanly and free of any adhering residual material.

It is also a primary object of this invention to provide insulative electrical conductors or wire with coverings including a combination of bodies of cured organic polymeric materials comprising a first layer of insulation with a surface thereof adheringly joined to a surface of a second layer which may be of any suitable thickness down to less than about 1 millimeter, and wherein the second layer of the polymeric material is easily and cleanly strippable from the first layer of insulation with a very low peeling effort of only a few pounds pulling force per 1/2 inch wide strip of material, leaving the separated surface of each layer intact, and clean and free of any residue, and a method of making the same.

It is an additional and specific object of this invention to provide an insulated electrical wire or cable having a multilayered covering about a metallic conductor comprising a combination of cured polymeric materials consisting of an insulation and an overlying semiconductive shield which is free of intermediate voids or spaces at the interface of said materials, and wherein the material consisting of the semiconductive shield comprising a polymeric carrier or matrix for particulate conductive filler material dispersed therethrough can be peeled or stripped off the underlying insulation with little effort or pull and it separates or parts cleanly from the surface of the insulation leaving it intact and without adhering material, and a method of making the same.

It is a further object of this invention to provide a curable composite polymer system of two layers or bodies which can be substantially simultaneously formed and joined together such as in a dual-head extruder and simultaneously cured joined in intimate physical contact with each other, and the thus formed layers or bodies can thereafter be cleanly separated or stripped apart with a very small effort or force.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 comprises a perspective view of a portion of an insulated conductor having a semiconductive shield thereon; and

FIG. 2 comprises a cross-sectional view of the insulation and overlying semiconductive layer about a portion of metallic conductor.

DESCRIPTION OF A PREFERRED EMBODIMENT

This invention is hereinafter described in relation to its principal field of application and utility, namely, the construction and method of producing electrical wire and cable, although other areas of application are contemplated.

The invention specifically comprises a novel combination of given curable polymeric materials, or combined bodies composed thereof, which provide unique interfacial characteristics when their contiguous surfaces are adheringly joined together by curing the polymeric material of at least one of the combined bodies. Curable polymeric materials of the invention comprise for the one phase, a body or unit of peroxide curable ethylene-containing polymer, and for the other phase of the composite, a body or unit of a sulfur curable chlorosulfonated polyethylene elastomer.

The peroxide curable ethylene-containing polymer of one phase of the combined polymeric bodies includes polyethylene, a common and extensively used electrical insulation material for wire and cable, which is cross-link curable with peroxide curing agents to a thermoset state in keeping with the requirements of the invention. Also included are similar copolymers of ethylene and other polymerizable materials, and blends of such polymers and copolymers which are at least predominantly composed of ethylene and are known in the art to provide effective cross-link curable electrical insulations. For example, copolymers of ethylene and vinyl acetate and similar copolymers wherein the ethylene content is a majority of more than 50% by weight, and preferably at least about 75% by weight of ethylene content. The latter class of copolymers of ethylene and blends for electrical insulating materials for wire and cable are disclosed in the above mentioned U.S. Pat. Nos. 3,259,688 and 3,684,821, and other prior art publications.

The sulfur curable chlorosulfonated polyethylene (Hypalon) elastomer phase, which when joined with the peroxide curable ethylene-containing polymer phase together produce the distinctive interfacial characteristics and functions of this invention, may include minor amounts of an ehtylene-propylene copolymer or terpolymer rubber as a softening agent or plasticizer to render the chlorosulfonated polyethylene more tractable in compounding with the electroconductive filler and other ingredients and in processing and forming. Being curable with the same sulfur curing systems as the chlorosulfonated polyethylene, the terpolymers of ethylene, propylene and a diene can be included in amounts of up to about 30 % by weight of the blend with the chlorodulfonated polyethylene. The copolymers of ethylene and propylene which are not curable with sulfur systems, cannot be tolerated in proportions of more than about 20% by weight of the blend with the chlorosulfonated polyethylene elastomer. Terpolymers of ethylene-propylene include commercially available rubbers produced by the copolymerization of ethylene and propylene together with minor proportions of dienes such as ethylidiene norbornene, or dicyclopentadiene or 1,4-hexadiene. The terpolymers of ethylene-propylene with dienes, as is well known in the art, give greater latitude in the available curing systems in relation to the copolymers of only ethylene and propylene. Specifically, the copolymers require a free radical curing mechanism as provided by a peroxide compound, whereas the terpolymers with this additional unsaturated radicals can also be cured with a conventional sulfur-accelerator curing system, as well as with a peroxide free radical system.

Sulfur-bearing curing systems for the chlorosulfonated polyethylene (Hypalon), and ethylene-propylene terpolymers, are conventional and well known in the art, for example, tetramethylthiuram disulfide (METHYL TUADS), tetraethylthiuram disulfide (ETHYL TUADS), dipentamethylene thiuram tetrasulfide (SULFADS), and the like commercial sulfur bearing curing agents.

For service in electrical applications such as a semiconductive component in cable for medium to high voltage service, the sulfur curable chlorosulfonated polyethylene elastomer can be easily rendered electroconductive to any appropriate degree desired by the filling or inclusion therethroughout of a suitable amount of an electrical conductivity imparting agent, such as about 15 to 75 parts of carbon black or metal particles by weight of the polymeric ingredients according to conventional practices. When aptly rendered electroconductive with a suitable amount of a conductive material, dispersed therethroughout, the chlorosulfonated polyethylene can fulfill the required electrical functions of a semiconducting material in electrical cable, and when combined with an ethylene-containing polymer insulation and cured in accordance with requirements of this invention, it provides the unique interfacial properties which effectively eliminate the occurrence of intermediate void spaces between the interfaces of insulation and semiconductive materials and also enables an easy and clean separation of the semiconductive material from the insulation.

The ethylene-containing polymeric material or phase of the combination of this invention is curable to a substantially thermoset condition by cross-linking with a free radical forming peroxide according to conventional practices such as described in U.S. Pat. Nos. 2,888,424 and 3,079,370, and subsequent relevant prior art. A preferred cross-linking curing system for the ethylene-containing polymeric material comprises the use of a tertiary peroxide curing agent such as a dicumyl peroxide.

In the practice of this invention it is required that at least one of the polymeric bodies or phases, either the peroxide curable ethylene-containing polymer or the sulfur curable chlorosulfonated polyethylene, undergoes curing while the surface thereof is in intimate physical contact with the surface of the other polymeric body or phase whereby the curing mechanism of one phase can effect the apparent bonds bridging the surfaces to adheringly unite the contacting surfaces of the interface. However, as a practical matter the most expedient manufacturing systems such as the sequential or tandem extrusion of the dual layers of ethylene-containing polymer and overlying chlorosulfonated polyethylene upon the wire core followed by simultaneous curing of both phase together, would incur the preferred curing of each polymeric phase or material of the combination at the same time to achieve the optimum effects thereof.

Referring to the drawing, a typical cable of medium-to-high voltage capacity of the type to which this invention is especially applicable and advantageous, is shown in perspective in FIG. 1, and a short portion of such a cable is also shown with the insulation and semiconductive layer in longitudinal cross section about the conductor in FIG. 2. The overall cable product 10, primarily comprises a metallic conductor 12, a relatively thick first body of a peroxide cured ethylene-containing polymeric insulation 14 surrounding the conductor; and overlying the insulation is a second body or layer of a sulfur cured chlorosulfonated polyethylene semiconductive material 16. Other components can be included in the cable structure following known designs, for example separating paper or tape, or a semiconductive layer located between the metallic conductor 12 and the primary insulation 14, such as are shown in the aforementioned U.S. Pat. Nos. 3,259,688 and 3,684,821, and the means of this invention apply thereto with its attendant advantages whenever the insulation abuts the semiconductive component, as is conventional in medium-to-high voltage capacity cables. Upon curing at least one component of the superimposed combination, either the body of the peroxide curable ethylene-containing polymer insulation 14 or the body of the sulfur curable chlorosulfonated polyethylene, filled semiconductive material 16, and preferably both together, the insulation and semiconductive material covering the insulation become adheringly joined to each other producing a united interface 18 of unique attributes which eliminate intermediate voids. Moreover, the thus formed adhering bond between these components can be separated upon the application of a small pulling force of only a few pounds and the surfaces at the interface separate cleanly leaving each surface free of adherents from the other.

The following comprise specific examples of suitable and preferred polymeric materials for the application of this invention in the construction of high-voltage cable comprising a body of peroxide curable polyethylene insulation combined with an overlying body of semiconductive material of a polymeric carrier or matrix comprising sulfur curable chlorosulfonated polyethylene elastomer filled with particulate conductive material.

The ethylene-containing polymer composition comprising the insulation, or one phase or polymeric body of the combination of this invention, consisted of the following typical commercial insulating formula:

EXAMPLE A Percent Parts by By Weight Weight ______________________________________ Polyethylene, low density - R-4 62.70 100.00 Sinclair Koppers Company Calcined Clay - Whitetex Clay 31.04 50.00 Titanium Dioxide Pigment 3.10 5.00 Titanox RA-NC Antioxidant - Monsanto Flectol-H, 1.09 1.75 polytrimethyldihydroquinoline Vinyl Silane 0.93 1.50 Curing Agent - Hercules Di-Cup T, 1.77 2.85 di .alpha.- cumyl peroxide ______________________________________

These ingredients were compounded in a suitable mixer comprising a roll mill, until substantially homogeneously dispersed. However, pursuant to conventional practices, all ingredients, except for the peroxide, were first admixed at elevated temperatures of about 250.degree.F, or within a range of about 200.degree.F to 300.degree.F, to flux the polymer and expedite the mixing. Thereafter the mix was cooled to below the decomposition temperature of the particular peroxide curing agent, in this case down to below about 220.degree.F, whereupon the peroxide curing agent was added and dispersed through the mix. The compound was then ready for forming to a given shape and curing by the application of heat.

The following comprises examples of the sulfur-cured chlorosulfonated polyethylene elastomer, which as a body or layer in combination with a body or layer of peroxide-cured ethylene-containing polymer, produces the unique interface characteristics of this invention. In these examples the sulfur curable chlorosulfonated polyethylene component is filled with an electrically conductive carbon black for the purpose of performing as a semiconductive material in an electrical cable in combination with a polyethylene insulation of the above formulation.

EXAMPLES I - IV

In the following examples, samples composed of the peroxide curable polyethylene composition given in Example A, and a sample of sulfur curable chlorosulfonated polyethylene elastomer and a blend thereof with an ethylenepropylene terpolymer formulation given in Examples I and II were individually sheeted on a hot mill, and a warm strip, measuring about 0.060 to 0.075 inch thickness, of the polyethylene composition was combined with a similar warm strip of each one of the formulations of Examples I and II of about the same thickness. The thus formed combined strip specimens comprising composite Examples A-I and A-II were each individually molded as composite slabs in a press and cured at 310.degree.F for about 45 minutes to simulate a sequential extrusion molding of one warm layer upon the other followed by a simultaneous curing.

Upon cooling each specimen to room temperature and conditioning each at ambient conditions for approximately 16 hours, a 4 inch long and 1/2 inch wide section of each composite cured specimen was tested in a Scott tester for strippability, and the pulling force in pounds required to separate the adhering layers of each specimen is given in the following table for Examples I and II.

The formulations given in Examples III and IV, were respectively extruded in a thickness of about 0.045 inch over an uncured polyethylene insulation of the composition of Example A which had been formed with an extruder around a core of a number 20 AWG wire conductor in a thickness of about 0.175 inch. Each of said wire specimens of the composite of peroxide curable polyethylene and sulfur curable chlorosulfonated polyethylene blends were then cured with steam at a temperature of about 210 psig (392.degree.F) for a dwell period of about 3 minutes. After cooling and conditioning at room temperature the pull required for stripping or separating the layer of each sample of polymer composite and its parting characteristics were determined. The pulling force to strip a one half inch wide section of the chlorosulfonated polyethylene formulation given in Examples III and IV from the adhesively joined polyethylene composition of Example A is also given in the following Table for Examples I- IV. Also each of the specimens were found to separate clean and free of any residue.

__________________________________________________________________________ INGREDIENTS EXAMPLES I II III IV __________________________________________________________________________ Chlorosulfonated polyethylene-duPont, Hypalon 40S 100.0 70.0 80.0 90.0 Ethylene-propylene terpolymer-duPont, Nordel 1320 -- 30.0 20.0 10.0 Conducting carbon black-Cabot, Vulcan XC-72 45.0 45.0 45.0 45.0 Hydrocarbon processing oil-Sun Oil, Circosol 4240 17.0 17.0 17.0 17.0 Litharge (90%) in ethylene-propylene binder-TLD-90 20.0 20.0 20.0 20.0 Microcrystalline wax - Sunoco Anti-Chek 2.0 2.0 2.0 2.0 Antioxidant-dihydro-trimethylquinoline-Vanderbilt, 0.5 0.5 1.0 1.0 Agerite Resin D N,N'-m-phenylenedimaleimide - duPont HVA-2 0.5 0.6 0.5 0.5 Curing agent-dipentamethylene thiuram hexasulfide- 0.6 0.6 0.6 0.6 Vanderbilt, Sulfads Approximate force in pounds needed to strip cured Hypalon composition from cured polyethylene com- position of Example A. 0.78 1.58 2.4 2.1 __________________________________________________________________________

Claims

We claim:

1. An insulated metallic electrical conductor having a covering thereon comprising, a combination of polymeric compositions including a composite of an electrically-insulating body of a peroxide-cured product of an ethylene-containing polymer with a surface adheringly joined to a contacting surface of an easily and cleanly strippable overlying semi-conductive body of a sulfur-cured product of an elastomer of chlorosulfonated polyethylene, with said contacting surfaces of the insulating body and overlying semiconductive body being adheringly joined to each other as a result of at least one of said polymeric materials having been cured while the said surfaces of each of the bodies are in adjoining physical contact with each other.

2. The insulated metallic electrical conductor of claim 1, wherein the said contacting surfaces of the insulating body and overlying semiconductive body are adheringly joined to each other as a result of both the ethylene-containing polymer and the elastomer of chlorosulfonated polyethylene having been cured simultaneously while the surfaces of their bodies are in adjoining physical contact with each other.

3. The insulated metallic electrical conductor of claim 1, wherein the body of the sulfur-cured product of an elastomer of chlorosulfonated polyethylene contains an electrically conductive filler dispersed therethroughout.

4. The insulated metallic electrical conductor of claim 3, wherein the electrically conductive filler is present in an amount of about 15 to about 75% by weight of the chlorosulfonated polyethylene elastomer.

5. The insulated metallic electrical conductor of claim 3, wherein the electrical conductive filler comprises carbon black.

6. The insulated metallic electrical conductor of claim 1, wherein the semiconductive body of a sulfur-cured product of an elastomer of chlorosulfonated polyethylene includes up to about 30% by weight of an ethylenepropylene terpolymer.

7. The insulated metallic electrical conductor of claim 1, wherein the semiconductive body of a sulfur-cured product of an elastomer of chlorosulfonated polyethylene includes up to about 20% by weight of an ethylene-propylene copolymer.

8. The insulated metallic electrical conductor of claim 1, wherein the electrically insulating body of a peroxide-cured product of an ethylene-containing polymer is polyethylene cured with an organic tertiary peroxide curing agent.

9. An insulated metallic electrical conductor having a covering thereon comprising, a combination of polymeric compositions including a composite of an electrically insulating body of a peroxide-cured product of polyethylene and an organic tertiary peroxide curing agent, with a surface thereof adheringly joined to a contacting surface of an easily and cleanly strippable overlying semiconductive body of a sulfur-cured product of an elastomer of chlorosulfonated polyethylene and a sulfur-containing curing agent having an electrically conductive filler dispersed therethroughout in an amount of about 15 to about 75% by weight of the elastomer, said contacting surfaces of the insulating body and overlying semiconductive body being adheringly joined to each other as a result of both the polyethylene and the elastomer of chlorosulfonated polyethylene having been cured simultaneously while the surfaces of their bodies are in adjoining physical contact with each other.

10. The insulated metallic electrical conductor of claim 9, wherein the semiconductive body of a sulfur-cured product of an elastomer of chlorosulfonated polyethylene includes up to about 20% by weight of at least one ethylene-propylene rubber selected from the group consisting of copolymers and terpolymers of ethylene and propylene.