GB2190092A - Improvements in electric cables - Google Patents

Abstract

A method for sealing joints and terminations in electric cables comprises applying to the joint or termination a composition comprising (A) a resinous copolymer of R3SiO0.5 units and SiO2 units in which R represents a monovalent hydrocarbon group, (B) a polydiorganosiloxane having a viscosity of from 0.02 Pa.s to 100 Pa.s at 25 DEG C and (C) a ceramifiable filler. The method is particularly suitable for mineral-insulated and other cables which may be subject to fire conditions.

Description

SPECIFICATION Improvements in electric cables This invention relates to an improved meansfor sealing ajoint,termination orotherdiscontinuityin the insulating sheath of an electric cable.

Electric cables comprising one or more conductors separated from one another and from a surrounding metallic sheath by a compacted, refractory mineral are now widely employed. They are generally referred to as mineral insulated cables and their nature and construction are intended to enable continued operation underfire conditions. Such cables thus find employment in hazardous environments where they form part of the circuits of, for example, alarm systems and emergency lighting.

It is the practice to seal the terminal points and joints of such cables to prevent the ingress of moisture and other contaminants and to restore any loss of the insulating barrier resulting from stripping back the sheath and/or removal ofthe mineral insulation. The said sealing procedure normally involves the application to the termination orjoint of a mechanicalterminatororconnectorwhich may comprise for example a pot, ferrule or gland, usually metallic and which may be threaded internally or externally and through which the conductors may or may not extend. The remaining voids between the conductors and sheath or between the conductors and the mechanical terminator/connectorarefilled with an insulating sealing compound.It is desirable that the sealing compound besufficientlyfluidtofill all cavities and for any surplus two exude during filling or internal pressure arising from, for example, expansion during use. Another desirable characteristic of the sealing compound is that it retains, asfaras possible, its integrity and insulating properties when exposed to high temperatures, e.g.

500 to 1 000 C, as underfire conditions. Typical sealing compounds are those of putty-like consistency which are mixtures of organic oils and inorganic fillers. Although such compounds have found general application the advent of new, more stringent standards for mineral insulated cables has created a need for products having improved performance under emergency conditions, in particular exposure to fire, water and mechanical shock. We have now discovered that such an improvement can be obtained if there is employed a sealing compound comprising certain organosiloxane polymers in admixture with a ceramifiable filler.

British Patent 688 185 discloses a cable capable of functioning at high temperatures the cable being characterised in that its conductor or conductors are provided with insulation comprising a plasticised compound consisting of talc, mica or like silicious substance and an organosilicon polymer. Said patent does not, however, disclose the use of the organosiloxane polymers employed according to the present invention.

G.B. 2046 771A discloses compositions comprising certain organosiloxane polymers, a ceramic-forming filler and an organic peroxide. Said compositions are stated to be useful as electrical insulating materials.

According to the present invention there is provided a method for sealing a joint, termination or other discontinuity in an electric cable which comprises applying to the said joint, termination or other discontinuity a composition comprising (A) a resinous copolymer of R3SiO0.5 and SiO2 units, wherein each R represents a monovalent hydrocarbon group having from 1 to 6 inclusive carbon atoms and the ratio of R3SiO0.5to SiO2 units is in the range from 0.5:1 to 1.2::1, (B) a polydiorganosiloxane having a viscosity of from 0.02 Pa.sto 100 Pa.s at 25"C, wherein at least 60 percent of the total silicon-bonded organic substituents are methyl groups and any remaining substituents are selected from phenyl, vinyl, alkoxy, alkoxyalkoxy and hydroxy groups, and (C) a ceramifiable filler, the said composition being substantially free of a curing agentforone or both of (A) and (B).

Also included within the scope ofthis invention is an electric cable obtained by the hereinabove defined method.

The method and cables ofthis invention are characterised by the use of a sealing composition comprising (A), (B) and (C). Component (A) is a resinous copolymer of R3SiOo 5 units and SiO2 units in a ratio of 0.5 to 1.2, preferably 0.6 to 0.9. The R groups may be for example methyl, ethyl, butyl, phenyl or vinyl. Preferred in view of their commercial availability are the copolymers wherein the R groups are methyl or both methyl and vinyl.

Resinous copolymers (A) and their methods of preparation are well known in the art. They are solid, solvent-soluble products and may be prepared as described in, for example, British Patent 706719.

According to the said preparative process a silica hydrosol is treated at low pH with a source of R3Si00.5 siloxane units, such as a hexaorganodisiloxane e.g.

Me3SiOSiMe3,ViMe2SiOSiMe2Vi or PhMe2SiOSiMe2Ph, or a triorganochlorosilane e.g.

Me3SiCI, Me, Vi and Ph representing methyl,vinyl and phenyl respectively. An alternative procedure involves the cohydrolysis of the triorgano-substituted silanes and silanes free of organic substitutents. Depending on the method of preparation and the degree of molecular condensation the copolymer may have up to about 3 or 4 percent by weight of silicon-bonded hydroxyl groups. If desired the hydroxyl content may be reduced byfurther condensation or by reaction with a silylating agent, for example a silazane such as hexamethyldisilazane or 1 ,3-divinyltetramethyldisilazane.

The polydiorganosiloxanes (B) are well-known, commercially available materials. They may be described as linearorsubstantially linear organosiloxane polymers, that is they are composed predominantly of diorganosiloxane units. Such polydiorganosiloxanes therefore include those which can be represented by the average unit formula R'nSiO4 n, wherein at least 60 percent ofthe R' groups are methyl, any remaining R' groups being selected from phenyl and vinyl, and n has a value of about 2, preferably from about 1.9 to about 2.1.The polydiorganosiloxane (B) may be end-stopped, for example with -OX groups, wherein X represents H or an alkyl or alkoxyalkyl group, orwith triorganosiloxy groups, for example, trimethylsiloxy, vinyldimethylsiloxy or phenylmethylvinylsiloxy groups. The existence or nature ofthe end-blocking groups is, however, not critical.

In general the preferred polydiorganosiloxanes are those wherein all of the silicon-bonded substituents (R') are methyl or are methyl with upto about 5 percent of the total being vinyl and/or phenyl. Examples of the preferred polydiorganosiloxanes (B) therefore are polydimethylsiloxanes, polydimethylsiloxanes end-stopped with trimethylsiloxy, vinyldimethylsiloxy or phenylmethylvinylsiloxy units, and copolymers of dimethylsiloxy and methylvinylsiloxy units end-stopped with trimethylsiloxy units. The viscosity ofthe polydiorganosiloxane (B) mayvaryfrom 0.02 Pa.sto 100 Pa.s at25 C. Generally the viscosity will be chosen having regard to the handling properties, particularlythe plasticity, desired in the sealing compound,the range from 0.5 Pa.sto 50 Pa.s being normally preferred.The polydiorganosiloxane (B) may comprise a single species, for example with regard to its molecular size or the nature ofthe silicon-bonded substituents. It may, however, comprise several species and may thus consist of blends of polydiorganosiloxanes of different viscosities and/or different organic substituents.

Components (A) and (B) are best employed in relative proportions in the range from about 35 to about 65 parts by weight per 100 parts total weight of (A) and (B). Less than about 35 parts of (A) is believed to lead to a seal of reduced strength underfire conditions. More than about 65 parts of (A) can give rise to difficult mixing conditions, particularly when iarge amounts ofthefillter(C) are to be incorporated.

Component (C) comprises at least one ceramifiablefiller. Examples of such fillers are aluminium silicate, magnesium silicate, zirconium silicate, zinc silicate, mica, calcium aluminium silicate, magnesium aluminium silicate,talc, kaolinite, montmorillonite, silica, magnesium oxide, aluminium oxide, zirconia, beryllia, titania, potassium titanate, silicon carbide and silicon nitride. Preferred from an economic standpoint are the naturally occurring silicates and oxides e.g. mica and silica. A particularly preferred form of mica is mica which has been ground to a fine particle size buy which still retains its plate-like structure. The proportion offiller employed will depend to some extent on the properties desired in the sealing material both during normal use and under emergency conditions.In general we preferto employfrom about 50 to about 250 parts byweight offiller per 100 parts total weight of (A) and (B).

Preparation ofthe sealing compound can be achieved by simple manual or machine mixing ofthe components. Most conveniently the resin copolymer (A) and polydiorganosiloxane (B) are mixed together priortothe incorporation of the filler (C).

The sealing compound should be substantially free of organic peroxide orothersiloxane curing agentwhich would cause hardening of the sealing compound during storage or normal use. Preferably the compound should also not contain organic components, in particular volatile solvents such as toluene and xylene which can lead to the formation of voids in the sealing compounds,the emission of inflammable vapours during use and difficulty in handling the sealing compound during manufacture and use.

The method ofthis invention may be employed to seal joints, terminations and other discontinuities in any type of cable construction which lends itself to sealing with a putty-like material. It is, however, particularly useful for use with mineral-insulated cables. Application of the sealing compound to a void in a cable according to the method ofthis invention may be carried out manually or mechanically or by a combination of both. If desired, the adhesion between the sealing compound and the cable surface may be increased by the application of a suitable primer, e.g. a silane.

In common with practices known in the art the use of the sealing compound may be carried out in connection with sleeves, end caps, glands, connectors or other mechanical fittings normally associated with the formation ofterminations and joints in electric cables. As an illustration of such use the accompanying drawing depicts in longitudinal section a simple form of sealed end termination of a mineral-insulated ca bie. In the drawing a cable sheath (1 ) surrounds two conductors (2) which are insulated from each other and the sheath by a pulverulent mineral (3), which may be for example magnesium oxide.Fastened to the end of the sheath (1) is a cylinder (4) which contains a sealing compound (5) surrounding the conductors (2); the sealing compound being held in position byan insulating disc (6) through which the conductors extend.

The cylinder (4) is usually fabricated from a metal e.g. brass or copper and may be fastened to the sheath by any suitable means e.g. by crimping, brazing or by means of an internal screw thread.

Sealing compounds which may be employed according to the method of this invention are described in the following examples which are for illustration only. Unless otherwise indicated the parts are expressed by weight and in the formulae Me represents the methyl group and Vi the vinyl group.

Example 1 The siloxane component employed to prepare the sealingcompoundofthisexamplewasa blendof(1) 46 parts of a resinous copolymer of Me3SiO05, Me2ViSiOO 5 and SiO2 units in which the ratio of triorganosiloxy units to SiO2 units was about 0.8 to 1.0, and (2) 54 parts of a dimethylvinylsiloxy terminated copolymer of dimethylsiloxy units and methylvinylsiloxy units having a viscosity of about 0.4 Pa.s at 25"C.

To 100 parts of this siloxane componentwas added, with thorough mixing, 90 parts of finely-ground (micronised) mica. The resulting product was a readily deformable putty. When the putty was subjected to heating in a furnace at 950 C for 3 hours it was converted to a dense, hard solid. A sampleofsilicone rubberwhich was similarly treated formed a weak friable solid having an open structure.

Example 2 To 100 parts ofthesiloxanecomponentdescribed in Example 1 was added, with thorough mixing, 75 parts of a mixture of 56 parts fine silica, 22 parts aluminium oxide and 22 parts of magnesium oxide.

The resulting product was a putty which could readily be kneaded in the hand. When subjected to 750 Cfor3 hours samples of the putty were converted to a hard, dense solid.

Example 3 A sealing compound was prepared as described in Example 1 except that the micronised mica was replaced by the same quantity of a fine particle size mica in which the particles retained their plate-like configuration.

When the sealing compound was used to seal the termination of a mineral-insulated cable a satisfactory seal was obtained which retained its integrity when exposed to temperatures up to 950"C.

Claims (7)

1. A method for sealing a joint, termination or other discontinuity in an electric cablewhich comprises applying to the said joint, termination or other discontinuity a composition comprising (A) a resinouscopolymerof R3SiOo.5 and SiO2 units, wherein each R represents a monovalent hydrocarbon group having from 1 to 6 inclusive carbon atoms and the ratio of R3SiOo.5to SiO2 units is in the range from 0.5:1 to 1.2::1, (B) a polydiorganosiloxane having a viscosity of from 0.02 Pa.s to 100 Pa.s at 250C, wherein at least 60 percent of the total silicon-bonded organic substituents are methyl groups and any remaining substituents are selected from phenyl, vinyl, alkoxy, alkoxyalkoxy and hydroxygroups, and (C) a ceramifiable filler, the said composition being substantiallyfree of a curing agent for one or both of (A) and (B).

2. A method as claimed in Claim 1 wherein the R groups in (A) comprise both methyl groups and vinyl groups.

3. A method as claimed in Claim 1 or Claim 2 wherein the R' groups in (B) comprise methyl groups and vinyl groups.

4. A method as claimed in any one ofthe preceding claimswhereinthefiller(C)isselected from silica and mica.

5. A method as claimed in any one of the preceding claims wherein the filler (C) is present in a proportion of from 50 to 250 parts by weight per 100 parts of the weight of (A) and (B).

6. A method as claimed in any one of the preceding claims wherein the electric cable is a mineral insulated cable.

7. A method as claimed in Claim 1 wherein the applied composition is substantially as described in any one ofthe Examples herein.