Deformation Resistant Shielding Composition

1. Field of the Invention

This invention relates to ethylene polymer based compositions which are used as shielding materials for electrical conductors which are used in high voltage wire and cable.

2. Description of the Prior Art

Electric wire and cable that is designed to carry electric current under a high voltage, i.e., .gtoreq. 2K volts, usually comprises a core conductor which comprises one or more strands of a conducting metal or alloy such as copper or aluminum, a layer of a semi-conducting conductor shielding, a layer of insulation and a layer of semi-conducting insulation shield. A plurality of neutral wires which are usually made of tinned copped are embedded in or wrapped around the layer of semi-conducting insulation shield. These neutral wires form a concentric ring around the core of the cable.

These semi-conducting shielding compositions have been formed from ethylene polymers and conductive carbon black. However, the present service demands on these high voltage power cables requires the use therein, as the semiconducting insulation shield compositions, of insulation shielding compositions which are adaptable to function at the relatively high temperatures, of the order of 90.degree. to 130.degree.C., to which these cables can heat up to during the transmission of the electrical power through the cable. Unless these insulation shielding materials maintain their physical integrity at such high operating temperatures the concentric neutral wires which are embedded in or wrapped around the insulation shield are subject to penetrating the insulation shielding if the insulation shield experiences any significant amount of deformation as a result of external forces under these elevated use conditions. The ethylene polymer based compositions which have been filled with conductive carbon black and which have been used as the insulation shield for high voltage cables in the past tend to soften and deform precipitously at temperatures of the order of .gtoreq. 70.degree. to 90.degree.C. due to fusion or melting of the ethylene polymer therein. Although a number of solutions to the problem have been proposed, not all of such solutions have provided an insulation shielding composition which will resist deformation at elevated temperatures, and which can be readily fabricated at elevated temperatures, and which will not suffer a concomitant deterioration in embrittlement temperature. These high voltage power cables are used in many instances outdoors, and above ground, in freezing and subfreezing temperatures, and the insulation shielding compositions, therefore, must also resist embrittlement under such external low temperature conditions.

Prior to the present invention, therefore, it has not be readily possible to provide semi-conducting insulation shielding compositions for high voltage power cables which can be readily processed at temperatures of the order of about 125.degree. to 250.degree.C., which will not suffer embrittlement at external temperatures of about 0.degree. to -45.degree.C. and which will resist deformation under internal in-service temperature conditions of about .gtoreq. 100.degree.-120.degree.C.

An object of the present invention is to provide a readily processable, deformation resistant, semi-conducting ethylene polymer based insulation shielding composition for high voltage wire and cable.

Another object of the present invention is to provide high voltage wire and cable which is shielded with a composition which enables the cable to be used under internal elevated temperature conditions while being able to experience low embrittlement temperatures relative to its external environment.

These and other objects of the present invention are achieved with a semi-conducting insulation shielding composition which is formed from a selective combination of thermoplastic olefin polymers, conducting carbon black and elastomers.

The objects of the present invention are achieved by employing as a semi-conducting insulation shield composition for high voltage wire and cable a composition of the following formulation, based on a total weight percent therein of 100:

a. about 25 to 60, and preferably, about 40 to 50, weight percent of thermoplastic low density ethylene copolymer,

b. about 3 to 20, and preferably about 5 to 10, weight percent of thermoplastic crystalline propylene resin or high density ethylene polymer,

c. about 5 to 35, and preferably about 10 to 20, weight percent of a thermoplastic elastomer, and

d. about 30 to 50, and preferably about 32 to 40, weight percent of conducting carbon black,

with the proviso that the weight ratio of such component b) to such component c) in such formulation is 1:1 to 1:3, and is preferably 1:1.5 to 1:2.5.

The insulation shielding compositions of the present invention are used in thermoplastic form, that is, they are not cross-linked. The insulation shielding compositions of the present invention have low embrittlement temperatures of the order of about -45.degree. to -55.degree.C. (as measured by ASTM D-746) and they are substantially resistant to deformation at temperatures of the order of .gtoreq. 120.degree.C. as indicated by their deformation values of about .gtoreq. 1% at .gtoreq. 120.degree.C. to about 40% at .gtoreq. 130.degree.C., (as measured by the IPCEA method).

The insulation shielding compositions of the present invention also have the following good physical properties:

tensile strength values of about 1,000 to 3,000 pounds per square inch (psi) (as measured by ASTM D-412-68),

ultimate elongation values of about 100 to 350 percent (as measured by ASTM D-412-68); and

modulus values of about 20,000 to 50,000 psi (as measured by ASTM D-638-71).

The low density ethylene copolymer which is to be used in the insulation shielding composition of the present invention is a normally solid (i.e. solid at 25.degree.C.) thermoplastic resin having a density of about 0.910 to <0.940, and preferably of about 0.915 to 0.930, grams per cubic centimeter (as measured by ASTM 1505), and a melt index of about 1.5 to 50, and preferably of about 3.5 to 20, decigrams per minute (as measured by ASTM D-1238 at 44 psi test pressure).

The low density ethylene copolymer contains, based on the total weight of the copolymer, about 65 to 98, and preferably about 75 to 95, weight percent of interpolymerized ethylene and about 2 to 35, and preferably about 5 to 25, weight percent of one or more of the monomers vinyl acetate, vinyl propionate, vinyl butyrate, acrylic acid, methacrylic acid, and the C.sub.1 to C.sub.8 alkyl esters of such acids such as methyl acrylate, ethyl acrylate, methyl methacrylate, butyl methyl acrylate and 2-ethyl hexyl acrylate.

Up to about 5, and preferably about 1 to 3, weight percent of the interpolymerized ethylene in the low density ethylene copolymer may be replaced by one or more C.sub.3 to C.sub.6 monoolefins such as propylene, butene-1, pentene-1 and hexene-1.

The ethylene copolymers may be used individually or in combination with each other.

The second thermoplastic resin component of the composition of the present invention may comprise one or more normally solid crystalline polymers having a melting point of .gtoreq. 115.degree.C. and being one or more of propylene resins and high density polymers of ethylene.

The propylene resin may be an injection molding or extrusion grade isotactic homopolymer, or a copolymer containing .gtoreq. 94 weight percent of interpolymerized propylene and > 0 to 6 weight percent of one or more interpolymerized monomers consisting of ethylene and the C.sub.4 to C.sub.6 monoolefins such as butene-1, isobutene-1, and hexene-1. Such polymers of propylene usually have a melt index of about 2 to 40 decigrams per minute.

The high density polymer of ethylene has a density of .gtoreq. 0.940 and a melt index of about 2 to 30, and preferably of about 4 to 15, decigrams per minute. The high density polymer of ethylene may be a homopolymer or a copolymer of at least 94 weight percent of interpolymerized ethylene and up to 6 weight percent of one or more interpolymerized C.sub.3 to C.sub.6 monoolefins such as propylene, butene-1, pentene-1, and hexene-1.

These second thermoplastic resin components may be used individually or in combination with each other.

The third resin component of the compositions of the present invention is a normally solid elastomer. These thermoplastic elastomers are non-crystalline materials which, in an unvulcanized state, at room temperature, can be stretched repeatedly to at least twice their original length and, upon release of the stress, will return immediately with force to their approximate original length.

These elastomer resins include ethylene-propylene copolymers which contain, based on the total weight of such resins, about 10 to 90 weight percent of interpolymerized ethylene and about 10 to 90 weight percent of interpolymerized propylene.

The elastomer may also be an ethylene-propylenetermonomer terpolymer which contains, based on the total weight of the terpolymer, about 10 to <90 weight percent of interpolymerized ethylene, about 10 to <90 weight percent of interpolymerized propylene and about > 0 to 5 weight percent of interpolymerized termonomer. The ethylene and propylene are preferably used in the terpolymers in a weight ratio, to each other, of about 4:6 to 6:4, and most preferably of about 1:1. The termonomer is usually one or more conjugated or non-conjugated C.sub.4 to C.sub.10 polyenes such as 1,4- and 1,5-hexadiene, dicyclopentadiene, methyl cyclopentadiene and 5-ethylidene bicyclo[2.2.1]hept-2-ene.

Other elastomer resins which may be used are

butyl rubber,

polybutadiene,

polychlorobutadiene,

styrene-butadiene copolymers (block and random),

butadiene-acrylonitrile copolymers,

polyisoprene,

polyacrylates such as copolymers of methyl or ethyl-acrylate or methyl-acrylate with chloroethyl vinyl ether or acrylonitrile,

chlorosulfonated polyethylene, and

chlorinated polyethylene.

Carbon Black

Any conducting carbon black may be used in the compositions of the present invention, including channel blacks, oil furnace blacks and acetylene blacks. The carbon blacks should have a particle size of the order of about 10 to 60 millimicrons.

In addition to the low density ethylene copolymer, the propylene resin or high density ethylene polymer, the elastomer, and the carbon black, the insulation compositions of the present invention may also contain one or more adjuvant materials of the types normally employed in ethylene polymer based insulation compositions.

The total amount of such adjuvants which are usually used amounts to no more than about 0.05 to 3 percent by weight based on the total weight of the insulation composition.

These adjuvants would include about 0.05 to 1.0 weight percent of one or more of the high temperature anti-oxidants which are commonly employed in ethylene polymer based insulation compositions. Such compounds would include

1,3,5-trimethyl-2,4,6-tris(3,5-ditertiary butyl-4-hydroxy benzyl)benzene,

1,3,5-tris(3,5-ditertiary butyl-4-hydroxy benzyl)-5-triazine-2,4,6-(1H,3H,5H)trione,

tetrakis [methylene-3-(3'.5'-di-t-butyl-4'-hydroxy phenyl)-propionate]methane, and

polymerized 1,2-dihydro-2,2,4-trimethyl quinoline.

The adjuvants may also include effective amounts, for the intended purposes of the adjuvant, of metal deactivators, lubricants, antiblock agents, waterproofing fillers, inorganic fillers and voltage stabilizers.

All of the components of the insulation compositions of the present invention are usually blended or compounded together prior to their introduction into the extrusion device from which they are to be extruded onto the core electrical conductor or onto the insulation. The various resins and the other desired constituents may be blended together by any of the techniques used in the art to blend and compound thermoplastics to homogeneous masses. For instance, the components may be fluxed on a variety of apparatus including multi-roll mills, screw mills, continuous mixers, compounding extruders and Banbury mixers, or dissolved in mutual or compatible solvents.

When all the solid components of the composition are available in the form of a powder, or as small particles, the compositions are most conveniently prepared by first making a blend of the components, in a Banbury mixer or a continuous extruder, and then masticating this blend on a heated mill, for instance a two-roll mill, and the milling continued until an intimate mixture of the components is obtained. Alternatively, a master batch containing one or more of the resins and the carbon black and, if desired, some or all of the other components, may be added to the remaining mass of polymer. Where one or more of the resins are not available in powder form, the compositions may be made by introducing such resin(s) to the mill, masticating them until they form a band around one roll, after which a blend of the remaining components is added and the milling continued until an intimate mixture is obtained. The rolls are preferably maintained at a temperature which is within the range of about 50.degree.C. to 150.degree.C. The composition, in the form of a sheet, is removed from the mill and then brought into a form, typically dice-like pieces, suitable for subsequent processing.

After the various components of the insulation shielding compositions of the present invention are uniformly admixed and blended together, they may be readily processed in conventional extrusion apparatus temperatures of at about 125.degree. to 250.degree.C.

The following examples are merely illustrative of the present invention and are not intended as a limitation upon the scope thereof.

A series of four compositions were prepared to illustrate the novel and unexpected properties of the insulation shielding compositions of the present invention. Compositions 1-3, as described below, were comparative compositions. Composition 4, as described below, represents the novel insulation shielding compositions of the present invention.

The components of each of these four compositions are listed below in Table I. The compositions were formed by uniformly admixing the components thereof in a laboratory sized Banbury mixer. About 2,000 grams of each of such compositions was thus prepared.

To evaluate the properties of these compositions, 100 gram samples of each of the compositions were compression molded at 190.degree.C. under a pressure of 10,000 pounds per square inch to form test plaques measuring 8 inches .times. 8 inches, which are also 50, 75 or 125 mils thick.

The test plaques were then physically tested in a variety of test procedures to demonstrate the substantially superior properties of the compositions of the present invention. These test results are listed below in Table II.

These test results indicate that the compositions of the present invention provide good ease of processing, low embrittlement temperatures and high resistance to deformation under load at elevated temperatures, without suffering any substantial loss in other physical properties which are essential in wire and cable insulation shielding compositions such as tensile strength, elongation and modulus.

These test results demonstrate that only composition 4 provides the desired combination of good processability, low embrittlement temperature and resistance to deformation under elevated temperatures.

Although compositions 1 and 2 provide good processability and a low embrittlement temperature they show precipitous deformation at temperatures as low as 75.degree.-90.degree.C.

Composition 3 shows some improved resistance to deformation at elevated temperatures but it has poor processability and has a relatively high embrittlement temperature.

A series of two additional compositions were prepared to illustrate the novel and unexpected properties of the insulation shielding compositions of the present invention. Compositions 1-2, as described above, and new composition 5 as described below, were comparative compositions. Composition 6, as described below, represents the novel insulation shielding compositions of the present invention.

The components of each of these four compositions are listed below in Table III. The compositions were formed and tested as disclosed above in Examples 1-4. The test results are reported in Table IV.

These test results indicate that the compositions of the present invention provide good ease of processing, low embrittlement temperatures and high resistance to deformation under load at elevated temperatures, without suffering any substantial loss in other physical properties which are essential in wire and cable insulation shielding compositions such as tensile strength, elongation and modulus.

These test results demonstrate that only composition 6 provides the desired combination of good processability, low embrittlement temperature and resistance to deformation under elevated temperatures.

Although compositions 1 and 2 provide good processability and a low embrittlement temperature they show precipitous deformation at temperatures as low as 75.degree.-90.degree.C.

Composition 5 shows improved resistance to deformation at elevated temperatures but it has poor processability and has a relatively high embrittlement temperature.

The ethylene copolymer and the propylene homopolymer used in the examples each had a melting point of .gtoreq. 115.degree.C.