GB2134534A

GB2134534A - High voltage electrical apparatus

Info

Publication number: GB2134534A
Application number: GB08400454A
Authority: GB
Inventors: Albert Arthur Pinching
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1983-01-10
Filing date: 1984-01-09
Publication date: 1984-08-15
Also published as: CA1208000A; EP0115910A1; GB2134534B; EP0115910B1; GB8300535D0; GB8400454D0; DE3463095D1; ATE26501T1; ZA839660B

Abstract

Electrical apparatus in which a body of insulating material is subjected to an electrical stress along its surface of at least 10<4> V/m is distinguished in that the body comprises particulate calcium carbonate dispersed in a compatible thermoplastic or elastomeric matrix based on a carbon-chain polymer. Surprisingly this inexpensive white and apparently inert filler is capable of providing compositions that have a failure voltage (according to the tracking resistance test set out in ASTM specification D-2303) of 3.0 kV and above.

Description

1 GB 2 134 534 A 1

SPECIFICATION

High voltage electrical apparatus 1 Th is i nvention rel ates to electrica 1 a ppa ratus in which a body of i nsu 1 ati ng materia 1 is su bjected to a n electrical stress along its surface of at least 104Wrn.

Theuse of plastics materiaisfor such bodies, especiallyout of doors, is severely inhibited by the liability of manysuch materials to failure by "tracking" 75 (theformation of conductive paths by carbonisation of the organic constituent(s) of the plastics material), rapid erosianorflame.

A numberof rigid materials that avoid premature failure of this kind have been developed, but availabil- 80 ity of flexible materials isvery limited; the most popularflexible materials currently commercially available are expensive materials based on the use of alumina trihydrate filler either in very large amounts or in combination with iron (111) oxide (which presum- 85 ably catalyses some reaction that competes with carbonisation). Large amounts of filler limit physical properties that can be achieved, whereas iron (111) oxide imparts an undesirable intrinsic colour.

In accordancd'with the present invention, electrical 90 apparatus in which a body of insulating material is subjectedto an electrical stress along its surface of at least 104WM is distinguished bythe factthatthe said body comprises particulate calcium carbonate dis persed in a compatible thermoplastic or elastomeric 95 carbon-chain polymer matrix in an amount sufficient to raisethefailure voltage of the body (as determined bythe tracking resistancetest method setout in ASTM specification D-2303) to a value not less than 3.0 W.

Bythe use of the present invention, ultimate 100 breakdown voltage in the range 4-6.2 W have been attained; it is entirely surprising that such excellent tracking resistance can be achieved bythe use of an inexpensive, white and apparently inertfiller.

Commercially avaiiablefiller grades of calcium 105 carbonate, which generally have a mean particle size in the range 0.01 - 50 micrometre (but preferably below 10 micrometrey can be used, and comparable results are obtained with natural and synthetic (pre cipitated) types. Relatively coarse crushed limestone 110 is also effective in. raising failure voltage, but may have a detrimental effecton physical properties.

The polymer matrix may be of any carbon-chain polymer (by which is meant a polymer not having any significant number of hetero-atoms in its mai n chain) 115 or carbon- chaikt polymer mixtu re that is sufficiently inert and capable of accepting the loading of calcium carbonate required to achieve the specified tracking resistancei Preferred are hydrocarbon-chain polymers with side-chain hetero-atoms to enhance compatibility and loading capacity, and the most preferred side-chainsare esters and other carboxylic groups. Typical preferred polymers are the semi-crystalline copolymers of ethylene with vinyl acetate, ethyl 6Q acrylate, methyl acrylate, butyl acrylate and acrylic acid; amorphous copolymers of this class as well as ethylene-propylene copolymer rubbers and ethylene propylene - diene terpolymer rubbers can also be used.

The minimum loading required and the maximum acceptable will varyto some extentwith thetype of polymer matrix and with the size, shape and surface condition of the calcium carbonate particles. Usually however a loading of at least40 parts by weightfor 70 each hundred parts byweight of polymer matrix (hereinafter abbreviated to phr) will be needed to achieve the defined tracking resistance and a loading over 120 phr is likelyto result in poor mechanical properties. 50-90 phr is usually recommended and 80 phr may be considered a starting pointfor optimisation.

The calcium carbonate particles may be coated with a dispersing agent, such as calcium stearate orwith a coupling agent, such as a silane ortitanate, or an organic material polymerised in situ on the particles.

The polymeric matrix may have been crosslinked, preferably by irradiation, with orwithoutthe use of promoters (such astriallyl cyanurate, trimethylolpropane trimethacrylate, ethylene glycol dimethyacrylate, ortrially] trimellitate); conventional crosslinking reagents, e.g. peroxides, can usually be used alternatively, with orwithoutthe sametype of promoter.

Other inorganic fillers can be used if needed to achieve special requirements for flame retardance or other properties, butthe content is preferably keptto a minimum; activefillers such as alumina trihydrate, magnesium carbonate or dolomite are likely to be acceptable up to about half the calcium carbonate loading (provided the total loading is notthereby made excessive) and inertfillers such as silica flour or talc up to about 20 phr (subjectto the same proviso).

Minor amounts of other conventional additives may be present, important examples being antioxidants, U.V. stabilisers, flame retardants and processing aids. Pigments can also be used but need to be chosen with care as some (notably titanium dioxide) have a deleterious effect on tracking properties.

Formulations in accordancewith the invention may be usedforthe whole, ora superficial layerof, extruded and injection moulded componentsfor insulators, cablejoints and terminations (including heat-shrinkable components for these applications), switchgear and overload cut-outs.

Examples 1-13

The following table of examples illustratesthe invention; in all cases the composition was moulded into electrically insulating bodies in theform of sheets, crosslinked by irradiation at 200 kGy (20 Mrad) and tested fortracking resistance according toASTM D-2303:

2 GB 2 134 534 A 2 Example No

TABLE

Composition: (parts by weight) Ethylene/vinyl acetate copolymer/(28% vinyl acetate) Evatane 28-05 1 2 3 4 5 6 7.a 100 100 Ethylene/ethyl acrylate copolymer (15% ethyl acrylate), DPDM 6182 Calcium Carbonate: Snowcal 7ML Winnofil S Crushed Limestone (200 mesh) Alumina Trihydrate (Hydral 710) Dolomite China Clay Antioxidant Flectol Pastilles Irqanox 565 Lead Phthalate Stearic Acid 100 100 100 100 100 60 - 80 - 100 - - - 60 - 80 0.2 0.2 0.2 0.5 0.5 0.5 0.5 0.5 Properties:

Erosion inception voltage (kv) (mean of 2 or 3 measurements) Ultimate breakdown voltage (kV) 3.5 - 3.25 3.25 3.0 >3.5 3.25 3.5 4.0(f) 4.0(t)>4.25 >6.25(f) 6.25 >5.5 6.0(f)4.0(f) TABLE - Continued Example No

9 10 11 12 13' Composition: (parts by weight) Ethylene/vinyl acetate copolymer/(28% vinyl acetate) Evatane 28-05 Ethylenelethyl acrylate copolymer (15% ethyl acrylate), D 6182 Calcium Carbonate: Snowcal 7ML Winnofil S Crushed Limestone (200 mesh) Alumina Trihydrate (Hydral 710) Dolomite China Clay Antioxidant Plectol Pastilles Irganox 565 Lead Phthalate Stearic Acid 100 100 100 100 35 80 150 so - - - 0.5 0.5 2.0 1.5 3.25 >3.5 3.0 3.253.25 0.5 0.5 0.5 2 1.5 2.0 1.5 Properties:

Erosion inception voltage (kV) (mean of 2 or 3 measurements) Ultimate breakdown voltage (M) 4.0M 3.25M The calcium carbonates used in the examples are asfollows:

Snowcal 7MI- is a superfine whiting grade of natural calcium carbonate with a mean particle size of 2.9 micrometre and a particle distribution (by sieving) asfollows:

finerthan 25 micrometres- practically 100% 25 finerthan20micrometres-99% finerthanlOmicrometres-95% finerthan 5micrometres-75% finerthan 3micrometres-53% Winnofil S is a precipitated calcium carbonate with a nominal particle size of 75 nm, surface coated with calcium stearateto inhibit agglomeration and prom- otedispersion.

The crushed limestone was specified only as passing 100% through No. 200 British Standard mesh.

In the Table, (t) denotes ultimate breakdown by tracking and (f) ultimate breakdown byflaming; in other cases the mode of ultimate breakdown has not yet been determined. Example 14 In a further example, a commercially available material (sold in the UK by BXL Plastics Limited under the designation BP D 2979 FR) analysing as 100 parts ethylene-ethyl acrylate polymer, 80 parts of a natural calcium carbonate (estimated range of particle sizes very approximately 0.5-2 micrometre), small amount of Flectol H, was moulded into similar sheet insula tors. Comparable test results were as follows..

erosion inception volage 3.5 W ultimate breakdown voltage (byflaming) 6.25 W The compositions of Examples 1 -11 and 14 are extrudable priorto crossi inking; some are useful for heat-shrink applications when crosslinked.

1 1.

3 GB 2 134 534 A 3

Claims

1. Electrical apparatus in which a body of insulating material is subjected to an electrical stress along its surface of at least 104 Wm distinguished bythe fact thatthe said body comprises particulate calcium carbonate dispersed in a compatible thermoplastic or elastomeric carbon-chain polymer matrix in an amount suff icientto raise the failure voltage of the bodyas determined by the tracking resistance test method set out in ASTM specification D- 2303to a valbanot less than 3.OkV.

2.5- 51"ectrical apparatus as claimed in Claim 1 in whfchWe matrix is of a ftydrocarbon chain polymer with gide-chain hetero-atoms.

3. 51leatrical apparatus as claimed in Claim I in which the polymer matrix isselected from the sern i-crysta [line copolymerg!of ethylene with vinyl acetate, ethyl acrylate, methyfacrylate, butyl acrylate and acrylibacid.

4. Electrical apparatus in which a body of insulating material is subjected to an electrical stress along its su rface of at least 104 Wrn distinguished bythe fact that th e said body is a crossl inked composition su bsta ntial ly confo rming to the form u lation set out in anyone of Examples 1 to 8.

5. Electrical apparatus in which a body of insulating material is subjected to an electrical stress along its surface of at least 104 Wrn distinguished bythe fact thatthe said body is a crosslinked composition substantially conforming tothe formulation set out in any one of Examples 9to 13.

6. Electrical apparatus in which a body of insulating material is subjected to an electrical stress along its surface of at I east 104 Wrn distinguished bythe fact that the said body is a crosslinked Gom position substantially conforming totheformufation set outin Example 14.

Printed for Her Majesty's StationeryOffice byTherweeddal'e Press Ltd., Berwick-upon-Tweed, 1984. Published atthe PatentOffice, 25Southampton BuildinW3,London WC2A lAY, from which copies may beebtained.