Electrically conductive materials

Details Electrically conductive materials Electrically conductive materials

1980-06-19

2001-04-03

Kopec, Mark (Department: 1751)

Compositions

Electrically conductive or emissive compositions

Elemental carbon containing

C174S268000, C174S07400A, C428S035700

Reexamination Certificate

active

06210607

ABSTRACT:

This invention relates to electrically conductive materials and more particularly to conductive materials comprising a polymeric matrix and a conductive filler.
It is known, in order to suppress the occurrence of electrical discharges in areas of high electrical stress, for example at the shield end of high voltage cable termination, to provide a stress relieving material which extends around the insulation for some distance from the end of the shield. A variety of stress relieving materials have been proposed for this purpose, for example, polymeric materials containing carbon black as the conductive filler. Exemplary of these prior art materials is that described in British Patent No. 1,394,272 in which there is disclosed an elastomeric dielectric composition having a permittivity of at least 20 and comprising an elastomeric component, from 15 to 130 parts by weight, per 100 parts of the elastomeric component, of carbon black, and from 0.4 to 25 parts by weight, per 100 parts by weight of the elastomeric component, of one or more plasticising and/or peptizing agents.
However, it has been found in practice that the resistivity and impedance of such materials is very sensitive to small variations in the loading of the conductive filler, thus making it very difficult to obtain consistent electrical properties in manufacture. This problem, which arises for conductive materials generally, is especially acute with those whose electrical properties make them of interest as stress relieving materials in high voltage applications. An increase in the loading of the conductive filler produces larger values of both the permittivity and the loss tangent.
Whilst larger values of the former are desirable. When the impedance opposite is true for the loss tangent. When the impedance is sensitive to changes in loading it then becomes difficult to obtain the desired values of permittivity and loss tangent simultaneously, and consistently. An increase in the resistive nature of the material (associated with the loss tangent) due to the above may result in an unusable product.
Suprisingly, it has now been found that the susceptibility of a conductive material to small variations in loading of the conductive filler may be markedly reduced by the use of more than one such conductive filler in the material.
According to the present invention there is provided a conductive material which comprises a polymeric matrix having dispersed therein a conductive filler system comprising a minor proportion of a relatively more conductive filler as hereinafter defined and a major proportion of a relatively less conductive filler as hereinafter defined.
The invention also provides a composition suitable for processing, for example by moulding or extrusion, into a conductive material according to the invention, and an electrical component comprising such a conductive material.
The invention will now be more particularly described with reference to conductive materials having stress relieving properties, but it is to be understood that the invention is not limited thereto and may have application to conductive materials generally.
In one aspect, therefore, the present invention provides a material having stress relieving properties, which comprises a polymeric matrix having dispersed therein a filler system comprising a minor proportion of a relatively more conductive filler and a major proportion of a relatively less conductive filler as hereinafter defined.
For use as a stress relieving material in high voltage application, typically from 5 kV to 69 kV or even higher, for example about 20 kV, the composition is required to exhibit a high permittivity, usually in excess of 20. This corresponds to a specific impedance close to 10
9
ohm cm. In the materials to be described the specific impedance is desirably less than 10
9
ohm cm, and most preferably between 10
7
and 10
9
ohm cm, all values at a frequency of 50 Hz. Preferably the polymeric matrix and the filler system are so chosen that, if a curve is constructed of filler loading, in parts by weight per 100 parts by weight of polymeric matrix, against impedance of the conductive material, the rate of change of log
10
(impedance) with loading at an impedance value of 10
8
ohm cm lies in the range of from 0.1 to 0.017, preferably from 0.07 to 0.025.
Polymeric materials suitable for use as the polymeric matrix may include resins comprising, for example, polyolefins and olefin copolymers such as polyethylene, polypropylene, ethylene/propylene copolymers, and polybutenes; substituted polyolefins, particularly halogen-substituted polyolefins such as polyvinyl chloride, polyvinylidene chloride, polyvinylidene fluoride, Teflon 100 (a polytetrafluoroethylene manufactured by Du Pont), Teflon FEP (a copolymer of tetrafluoroethylene and hexafluoro-propylene manufactured by Du Pont) Teflon PFA (a copolymer of tetrafluoroethylene and perfluoroalkoxy moieties manufactured by Du Pont), Tefzel (a terpolymer of ethylene, tetrafluoroethylene and a fluorinated monomer manufactured by Du Pont), and Halar (a copolymer of ethylene and chlorotrifluoroethylene manufactured by Allied Chemicals); polyesters, particularly segmented copolyester polymers such as Bytrel (a segmented polyether ester copolymer derived from terephthalic acid, polytetramethylene ether glycol and 1,4-butanediol manufactured by Du Pont); and polyurethane.
Other suitable polymeric materials for use as the polymeric matrix include elastomers comprising, for example, copolymers of dienes with olefinically unsaturated monomers such as ethylene/propylene
on-conjugated diene terpolymers, styrene/butadiene polymers, butyl rubbers and copolymers of dienes with unsaturated polar monomers such as acrylonitrile, methyl methacrylate, ethyl acrylate, vinyl pyridine and methyl vinyl ketone; halogen-containing elastomers such as chloroprene polymers and copolymers, for example neoprene, chlorinated polyethylene, chlorosulphonated polyethylene, and Viton (a copolymer of vinylidene fluoride and hexafluoropropylene manufactured by Du Pont); copolymers of olefins with olefinically unsaturated esters such as elastomeric ethylene/vinyl acetate polymers, ethylene/acrylic acid ester copolymers such as ethylene/ethyl acrylate and methacrylate copolymers, particularly ethylene/acrylic rubbers such as Vamac (a terpolymer of ethylene, methyl acrylate and a cure-site monomer manufactured by Du Pont), and acrylic rubbers such as polyethyl acrylate, polybutyl acrylate, butyl acrylate/ethyl acrylate copolymers, and butyl acrylate/glycidyl methacrylate copolymers; silicone elastomers such as polydiorganosiloxanes, copolymers, block copolymers and terpolymers of monomethylsiloxanes, dimethylsiloxane, methylvinylsiloxane and methylphenylsiloxane, fluorosilicones for examples these derived from 3,3,3-trifluoropropyl siloxane and carborane siloxanes, elastomeric polyurethanes; and polyethers such as epichlorohydrin rubbers.
Blends of the above mentioned elastomers and thermoplastic resins may also advantageously be used. Particularly good results have been obtained using polyolefins, olefin copolymers and halogen-substituted olefin copolymers and these are the preferred polymeric materials for use in the present invention.
The relatively more conductive fillers may be chosen from among those particulate fillers currently used in the production of conductive materials. In
FIG. 1
, curve A, there is shown a plot of impedance against filler loading, in parts by weight per 100 parts by weight of base material for a typical conductive filler of this type.
It will be observed that as the loading is increased the curve is first shallow and then falls steeply in the range of specific impedance values 10
9
to 10
6
. Within a range of impedance values 10
9
to 10
7
which is the region of interest in the production of stress relieving materials, the impedance falls by a factor of two orders of magnitude with a change in filler loading of 10% by weight. It can thus be seen that a relatively small change in filler loading, such as may be occasioned by n

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