GB2033416A - Conductive Polymer Composition and its Use in Electric Cables - Google Patents

Abstract

Conductive polymer compositions comprise (A) a thermoplastics material which can be crosslinked by the grafting on of one or more silane compounds, and by the subsequent action of moisture, and which comprises a polymer or polymer blend melting at or above 120 DEG C, and (B) an electrically conductive non- hygroscopic highly structured carbon blacks. A composition according to the invention is of particular value for the formation of a conductive layer in an electric medium-voltage or high- voltage cable having a field-limiting internal and external conductive layer underneath and above the insulation.

Description

SPECIFICATION Conductive Polymer Composition and its Use in Electric Cables This invention relates to a conductive polymer composition based on a thermoplastics material, which can be crosslinked by the grafting on of one or more silane compounds, and by the subsequent action of moisture; with or without one or more elastomers as modifiers; the composition incorporating one or more electrically conductive fillers as additives.

As is known, certain ordinary thermoplastics materials, e.g. polyethylene, can be satisfactorily filled with, for example, carbon black or other fillers only to a minor extent. If the degree of filling exceeds this extent, the composition "stiffens", and its mechanical cohesion is adversely affected, so that it is unsuitable for further processing. Copolymers, (e.g. of ethylene and vinyl acetate) which can readily be filled can be used to avoid this difficulty, but their use does entail a substantial increase in cost. Another possibility is to add an elastomer, e.g. polyisobutylene or butyl rubber, to the thermoplastics material as a modifier, so that it becomes substantially easier to fill the composition.

As is known, these polymer compositions which have been rendered conducting, for example, with carbon black, can be used in all cases in which electric fields have to be controlled, or in which high field strengths, which could lead to breakdown discharges, for example in an insulation, have to be dissipated. Thus, for example, a shielding on an electric cable, i.e. a core smoothing applied to the core of a cable, and the radiation protection customary in high-voltage cables, can be made of these conductive polymer compositions. Conductive polymer compositions, however, can also be employed for terminations of electric cables, the compositions, either by themselves or applied to any desired carrier material, being employed to ensure control of the electric field.

The requirements applying to these conductive polymer compositions, e.g. adequate electrical conductivity, adhesion to (or ability to be heat-sealed with) other thermoplastics materials, for example with a cable insulation consisting of polyethylene, low absorption of moisture by the conductive filler and easy processability, coupled with low price, could not, however, always be met by the compositions used hitherto. For example, if the conductivity of the composition left something to be desired, and for this reason the proportion of carbon black was increased, this was liable to entail an increased absorption of moisture, which in turn jeopardised production of a smooth surface on application of the composition, e.g. on to a cable insulation.

Moreover, the problem of absorption of moisture is particularly serious in the case of compositions which are crosslinked by the grafting of one or more silane compounds on to polymer molecules in the presence of moisture (German Laid-out Specification (Auslegesschrift) 1,794,028).

Since grafted polymers prepared in this known procedure have the property of crosslinking in the presence of moisture, their granulation and storage can present serious problems. Thus, a certain degree of preliminary crosslinking can easily occur, and processability is thereby liable to be impaired or even jeopardised. There are particular difficulties, however, if a conductive composition is to be prepared from the grafted polymer, and, for this purpose, as is usual, carbon black is added to the polymer.Since the carbon blacks hitherto used are highly hygroscopic, that is to say they contain a relatively large proportion of moisture which can be removed only with difficulty, and incompletely, even by what are intended to be drying processes, typical conductive compositions prepared by known procedures are unsuitable, or not very suitable, for processing by the moisture-crosslinking process (crosslinking via siloxane bridges) in cable technology.

However, electric cables and lines usually have a core smoothing around the core, to avoid increases in field strength caused by roughness of the core surface, especially when these cables and lines are intended for high or ultra-high voltages. This core smoothing is also particularly advantageous if it is prepared from a cross-linkable material. It is usual to carry out crosslinking of a core smoothing, or of an insulation provided around a core smoothing, by a procedure in which, after extruding, the concentric layers applied to the core are passed through a heat treatment zone, e.g. in what has been called a continuous vulcanisation (C.V.) unit.However, this heat treatment entails increased expenditure, which may be unacceptable particularly if materials are used for the insulation which are not crosslinked like ordinary thermoplastics or elastomers in a C.V. unit, that is to say by the external action of heat.

A further serious disadvantage of the known conductive compositions lies in the temperaturedependent resistance characteristics of the conductive material. In fact, in the case of a composition of a copolymer, for example, the conductivity decreases to a greater or lesser extent, that is to say the electrical resistance increases, as the temperature increases. This increase in resistance can be two to three powers of ten even for a change in temperature from e.g. 20 to 900C, so that, when such a conductive composition is used to build up layers of an electric cable, and when the increases in temperature occur which are unavoidable during use, completely undefinable electric field factors may come into play. Damage to the insulation and discharges in the cable may then result.

It is an object of this invention to provide a conductive polymer composition which, when it is to serve for example as an internal or external conductive layer in a high-voltage cable, can be crosslinked by the action of moisture, without the quality of an extruded layer of the composition being adversely influenced, mechanically or electrically, and which moreover will retain satisfactory mechanical and electrical properties even when the temperature changes.

According to the present invention, we provide a composition as first mentioned herein, wherein a polymer or polymer blend melting at or above 1 200C, to which one or more non-hygroscopic highly structured carbon blacks are added as conductive fillers, is used as the base material on to which the one or more silane compounds are grafted. The highly structured carbon blacks referred to in the present description and claims are carbon blacks of a type which has a large surface area but also has a high tendency of the carbon black particles to aggregate. These blacks include blacks produced by cracking, and gas blacks, as they are called, which have, it being understood that they are to be highly structured, a low tendency to take up water from the atmosphere by adsorption.A compositon containing one or more of these carbon blacks, however, can still be crosslinked by the action of moisture. Nevertheless the ease of extrusion is not affected by any substantial risk of premature crosslinking being made possible by the presence of absorbed moisture.

A composition according to the invention, after extrusion of the grafted material, has in typical cases a perfect smooth surface; the quality of the surface is largely independent of the concentration of the grafting agent(s); the loss of electrical conductivity at increased operating temperatures, mentioned earlier, which is evidently to be attributed to the carbon black particles pulling apart when the crystalline portion of the base polymer melts, and to the sudden increase in volume associated therewith, is no longer apparent; and the operative reliability of electric cables incorporating the conductive polymer composition is thus considerably increased.

A further substantial advantage of a composition according to the invention is that the conductive composition can be grafted and shaped, e.g. by being extruded on to the core of an electric cable, in one operation. A strand of the composition issuing at a temperature below 2200C normally has neither bubbles nor pores; on the other hand, the presence of bubbles and pores in the case of prior-art compositions which were used in the cable industry meant that the conductive composition had to be prepared in a two-step process comprising grafting (first step), intermediate storage in sealed metal drums, and processing with insulating ingredient(s) (second step). In this procedure, it was exceptionally troublesome that storability was only limited, even when the drums were sealed carefully.After even 12-1 6 days, such an increase in viscosity was liable to occur, as a result of partial premature crosslinking, that the processability of the composition was jeopardised. This affected both the surface and the electrical properties of the composition.

A composition according to the invention has particular advantages if a high-density polyethylene ( > 0.94/g/cm3) is used as the base material. These advantages comprise, inter alia, a high grafting yield, good processability, and a degree of crosslinking which is adequate for an internal conductive layer of a cable.

It can sometimes be appropriate, however, to employ a blend, e.g. of polypropylene and one or more elastomers, as the base material.

We prefer to employ non-hygroscopic carbon blacks which have an average particle size of 180-350 , more preferably 200-300 , and a specific surface area of 50-120 m2/g, more preferably 70-90 m2/g. Examples of these carbon blacks are blacks produced by cracking or gas blacks which are available under the names Shawinigan black, Akzo black, Ketjen black EC and acetylene black.

Because of the low absorption of moisture by the present carbon blacks, we can usually employ 20-90 parts, preferably over 25 but not over 50 parts, per 100 parts of polymer in a composition according to the invention. The grafting and crosslinking by the action of moisture are unaffected, and high conductivity values can be achieved for the crosslinkable mixture, as a result of the high proportion of fine-particled carbon black.

The invention is illustrated by the following specific examples of compositions which may be used.

Composition I Hard polyethylene (density: 0.95 g/cm3) 70 parts Plasticizing or elastifying additive 30 parts Non-hygroscopic carbon black 70 parts Antioxidant (2,2,4-trimethyl-dihydroquinoline) 0.7 part Silane 2.0 parts Peroxide 0.2 part This composition may, for example, use a carbon black with the following properties: specific weight, 1.6-2.0 g/cm3; average particle size, 260-290 ; specific surface area, 70-90 m2/g; carbon content, 99.5%. An investigation of the conductivity at room temperature and at an elevated operating temperature (for example 900C) established that in this case the electrical conductivity of the composition according to the invention was virtually constant, i.e. approximately 3.5--5.0 1 02Q.

cm.

Another composition was prepared which also proved to be a high-grade electrically conductive composition with no increase in the tgA value at 900C.

Composition II Polypropylene 65 parts EPR (ethylene/propylene rubber) 35 parts Non-hygroscopic carbon black 60 parts Silane 2.2 parts Peroxide 0.2 part Antioxidant (dimethyldihydroquinoline) 0.5 part Electrical stabiliser (e.g. of alkylated aromatic type) 2.0 parts In both cases, as also in certain other compositions similarly formulated, it is essential that the base material (i.e. the polymer or polymer blend) should melt at or above 1 200C, and that use should be made of one or more carbon blacks which are not hygroscopic, and thus do not have an adverse effect on the stages wherein crosslinking occurs by the action of moisture.

In addition to a conductive polymer composition as already specified, the invention includes an electric medium-voltage or high-voltage cable having a field-limiting internal and external conductive layer underneath and above the insulation, wherein the internal and/or the external conductive layer comprises a composition according to the invention.

The invention further includes a process for the production of a cable as just mentioned, wherein in a single operation the grafting on of the silane compound(s) is effected, and the conductive polymer composition is then applied to the core and/or the insulation of the cable forthwith.

Claims (10)

Claims

1. Conductive polymer composition based on a thermoplastics material which can be crosslinked by the grafting on of one or more silane compounds, and by the subsequent action of moisture; with or without one or more elastomers as modifiers; the composition incorporating one or more electrically conductive fillers as additives; wherein a polymer or polymer blend melting at or above 1 200 C, to which one or more non-hygroscopic highly structured carbon blacks are added as conductive fillers, is used as the base material on to which the one or more silane compounds are grafted.

2. Composition according to claim 1, wherein a high-density polyethylene, having a density of at least 0.94 g/cm3, is used as the base material.

3. Composition according to claim 1 or 2, wherein one or more non-hygroscopic carbon blacks with an average particle size of 180-350 A and a specific surface area of 50--1 20 m2/g are used.

4. Composition according to claim 3, wherein one or more non-hygroscopic carbon blacks with an average particle size of 200-300 and a specific surface area of 70-90 m2/g are used.

5. Composition according to any preceding claim, wherein the proportion of carbon black added is 20 to 90 parts per 100 parts of polymer.

6. Composition according to claim 5, wherein the proportion of carbon black added is over 25 but not over 50 parts per 100 parts of polymer.

7. Composition according to any preceding claim, wherein a blend based on polypropylene and one or more elastifying additives is used as the base material.

8. Composition according to claim 1, obtained from starting materials substantially as specified in either of the foregoing examples.

9. Electric medium-voltage or high-voltage cable having a field-limiting internal and external conductive layer underneath and above the insulation, wherein the internal and/or the external conductive layer comprises a composition according to any preceding claim.

10. Process for the production of a cable according to claim 9, wherein in a single operation the grafting on of the silane compound(s) is effected, and the conductive polymer composition is then applied to the core and/or the insulation of the cable forthwith.