GB2360780A

GB2360780A - Silicone Rubber Composition containing Wollastonite

Info

Publication number: GB2360780A
Application number: GB0007535A
Authority: GB
Inventors: Michael Proctor
Original assignee: Dow Corning Corp
Current assignee: Dow Silicones Corp
Priority date: 2000-03-29
Filing date: 2000-03-29
Publication date: 2001-10-03
Also published as: GB0007535D0

Abstract

The invention discloses the use of a curable silicone rubber composition comprising: <SL> <LI>(A) 15 to 90 weight percent of a heat-curable organosiloxane polymer, containing at least 2 alkenyl groups per molecule, <LI>(B) 1 to 65 weight percent of a reinforcing silica filler, based on the total composition, <LI>(C) 5 to 80 weight percent wollastonite having an average particle size of 2 to 30 žm, based on the total composition, and <LI>(D) a curing component sufficient to cure the composition, as a ceramifiable layer in a safety cable composition. </SL> The composition provides flame resistance for insulation and jacketing materials for safety cables.

Description

SILICONE RUBBER COMPOSITION This invention relates to curable silicone rubber compositions with improved flame resistance and to the use of such silicone compositions as insulation and jacketing materials for safety cables.

A safety cable as described in the present application is a cable intended to be used for the wiring and interconnection of electrical systems required to maintain circuit integrity under fire conditions for longer periods than can be achieved with cables of conventional construction. Such safety cables may be used for example to power lift shafts, emergency circuits and emergency lighting systems in the event of a fire by creating a fireproof barrier between a conductor and the fire. Electrical safety cables of this sort have a conducting metallic core which is typically made from copper. A tape usually made from a mica material is wrapped around the metallic core to effect as a ceramifiable layer in the event of a fire. Micas are silicates known as some of the best electrical insulators. The mica tape is typically coated with an extruded layer of silicone rubber or the like, which in turn is surrounded by a thermoplastic filler, the whole wire having an outer or surrounding sheath, typically made from a thermoplastic such as an ethyl vinyl acetate copolymer (EVA) or polyethylene. It has been found that, were a fire to occur, the mica tapes currently used, protect the inner core from the fire, but the mica tapes are expensive and difficult to handle and thereby it is both difficult and expensive to produce mica tape wrapped cores which in turn provide rigid heavy cables which are difficult to utilise.

Recently Sawada., in Japanese patent application 9 55125 described a fireproof electric cable having excellent fire resistance. A fireproof layer of 0.1-1.Omm in thickness is formed by extruding a coating composition that is 200-500 weight parts of four inorganic fillers to 100 weight parts of hot-vulcanized silicone rubber. The four fillers are glass powder, alumina, wollastonite, and mica. Sawada teaches that all four fillers must be present for sufficient fire resistance. The silicone rubber used by Sawada is hot-vulcanized, and may also contain a crosslinking agent consisting of an organic peroxide, but no information on the chemistry of the silicone is specified. The glass powder consists of particles having a diameter of from 50 to 300 gym, present in a range of is from 50 to 125 parts by weight per 100 weight parts of hot- vulcanized silicone rubber. The alumina which is used consists of particles having a diameter of 50-200 #tm present in a range of from 50 to 125 parts by weight per 100 parts by weight of hot-vulcanized silicone rubber. Wollastonite, anhydrous calcium silicate, is used in the form of needle crystals having a particle size of 100-300

gym, present in a range of from 50 to 125 parts by weight per 100 parts by weight of the hot-vulcanized silicone rubber. Mica is preferably in the form of flakes with an aspect ratio of about 30 to 70 and a particle size of from 100 to 300 pm present in a range of from 50-125 parts by weight per 100 parts by weight of the hot-vulcanized silicone rubber. The wire coating of Sawada was tested according to the Fire Services and Fire Resistance Test Act. The test was conducted following a flame curve with a temperature of 840 C during a 30 min burning process.

Unpublished PCT patent application, filed under the Internal reference number DC 4729, discloses a silicone rubber composition and its use as a coating for plenum type transmission media cables. The silicone composition comprises: 30 to 90 weight percent of a heat-curable organosiloxane polymer, containing at least 2 alkenyl groups per molecule, 1 to 65 weight percent of a reinforcing silica filler, based on the total composition, 5 to 70 weight percent wollastonite having an average particle size of 2 to 30 gym, based on the total composition, and (D) a curing component sufficient to cure the composition. However, DC 4729 does not indicate or even suggest that such a composition could be used in safety cables.

For the sake of clarification a safety cable needs to achieve the following criteria:- an ability to form a char or ceramified layer in a well defined temperature range a sharp transition to the char in the event of a fire the resulting ceramified layer protecting the core needs to have sufficient mechanical strength to avoid easy fracture iv) in the event of a fire the safety cable needs to have the required electrical properties at desired operating temperatures and

v) the coefficient of thermal expansion of the ceramifiable layer must be appropriate for the conductor being protected. In comparison plenum cables are cables used in hollow spaces of buildings, for example, under floors ,walls, ceilings, air conditioning and lift shafts and the like. Silicone rubber plenum cable materials typically form a relatively fragile char which has a tendency to crack allowing exposure of fresh fuel to the flame thus propagating flame spread, resulting in the conducting core being fractured/broken. Typically safety cables have a much greater overall diameter >25mm compared with approximately 4-5mm for plenum cables.

It has now been found that certain curable silicone

rubber compositions containing 5 to 70 weight percent wollastonite having an average particle size of from 2 to 30Vm have surprisingly good fire resistance and are suitable for use as coatings for safety cables instead of mica tape as they form hard chars on burning.

In accordance with a first aspect of the invention there is provided the use of a curable silicone rubber composition comprising: (A) 15 to 90 weight percent of a heat-curable organosiloxane polymer, containing at least 2 alkenyl groups per molecule, (B) 1 to 65 weight percent of a reinforcing silica filler, based on the total composition, (C) 5 to 70 weight percent wollastonite having an average particle size of 2 to 30 gym, based on the total composition, and (D) curing component sufficient to cure the composition, as a ceramifiable layer in a safety cable composition. Component A, the organosiloxane polymer has the average composition of RaSiO(4-a)/2. In the formula, R is selected from substituted and unsubstituted monovalent hydrocarbon groups and is exemplified by alkyl groups such as methyl, ethyl, and propyl; alkenyl groups such as vinyl, allyl, butenyl, and hexenyl; aryl groups such as phenyl; and aralkyls such as 2-phenylethyl, said substituted groups may be, for example halogen groups preferably fluoro groups and a is a value from 1.95 to 2.05.

The organosiloxane polymer has at least 2 silicon- bonded alkenyl groups in each molecule. The alkenyl groups can be bonded in pendant and/or terminal positions. The molecular structure of the organosiloxane polymer generally has a degree of polymerization (dp) in the range of from 200 to 20,000. This dp range includes polymers which are thick, f lowable liquids as well as those that have a stiff, gum-like consistency. Typically, silicone rubber compositions used in wire and cable applications usually use polymers with a stiff, gum-like consistency to process more readily used in screw-type extruders. Generally, these stiff gum-like polymers have a dp above about 1500 and have a Williams plasticity number (ASTM D926) in the range of from about 3 0 to250, and preferably from 95 to 125. The plasticity number, as used herein, is defined as the thickness in millimeters x 100 of a cylindrical test specimen 2 cubic cm in volume and approximately 10 mm in height after the specimen has been subjected to a compressive load of 49 Newtons for three minutes at 25 C. More recently, silicone rubber made from polymers that are thick flowable liquids have been found to be useful as wire and cable materials. These materials can typically be pumped through a die to coat wire or cable without the use of a screw-type extruder. Because less stress is needed to process these materials, they may be more suitable for coating glass or polymer fiber cables. The polymers that are thick flowable liquids have a dp below about 1500 and have a viscosity of between about 200 to 100,000 mPa-s at 25 C.

The organosiloxane polymer can be a homopolymer or a copolymer or a mixture of such polymers. The siloxy units in the organosiloxane polymer are exemplified by dimethylsiloxy, viny7_methylsiloxy, and methylphenylsiloxy. The molecular terminal groups in the organosiloxane polymer

are exemplified by trimethylsiloxy, and vinyldimethylsiloxy groups. The organosiloxane polymer is exemplified by vinyldimethylsiloxy-endblocked dimethylsiloxane vinylmethylsiloxane copolymer, vinyldimethylsiloxy endblocked polydimethylsiloxane, vinylmethylhydroxysiloxy endblocked dimethylsiloxane-vinylmethylsiloxane copolymer, and vinyldimethylsiloxy-endblocked dimethylsiloxane- methylphenylsiloxane-vinylmethylsiloxane copolymer Preferably the component A is present in an amount of from 15 to 29 weight percent. Most preferably component A is present in an amount of from 18 to 27 weight percent.

Component B is a reinforcing silica filler, to provide increased mechanical properties in the present heat cured silicone rubber composition. The filler can be any appropriate silica filler, which may be treated or untreated, provided it is of a type which is known to reinforce polydiorganosiloxane. Preferred silica fillers may be selected from finely divided, fumed and precipitated forms of silica and silica aerogels having a specific surface area of at least about 50 M2/g, and preferably 150 to 400 M2/g. The filler is typically added at a level of about 1 to 65 weight percent of the weight of the total composition, and preferably in a range of 5 to 25 weight percent of the total composition.

It is preferred to treat the reinforcing silica filler to render its surface hydrophobic, as typically practiced in the silicone rubber art. This can be accomplished by reacting the reinforcing silica filler with a liquid organosilicon compound which contains silanol groups or hydrolyzable precursors of silanol groups. Compounds that can be used as filler treating agents, also referred to as anti-creping agents or plasticizers in the silicone rubber art, include such ingredients as low molecular weight liquid hydroxy- or alkoxy-terminated polydiorganosiloxanes,

including a,o-silanediols, hexaorganodisiloxanes, cyclodimethylsiloxanes and hexaorganodisilazanes. Component (C) is 5 to 70 weight percent of wollastonite having an average particle size of 2 to 30 pim. Preferably component (C) is present in an amount of from 40 to 60 weight percent. Wollastonite, also known as calcium metasilicate, is a naturally occurring mineral. The wollastonite used in,. this invention is generally a mined form, which may have an acicular morphology, that is a needle-like shape. Typically, the ratio of the particle length to its diameter or aspect ratio is at least 2:1 or greater. It is preferred for the wollastonite to have an average particle size of from about 2 to 15 #tm and an aspect ratio of about 2:1 or greater, most preferably a particle size of about 2 #tm and an aspect ratio of about 2:1. The wollastonite used in this invention has a low BET surface area, typically less than 25 M2/g. A preferred form of wollastonite is supplied by NYCO Minerals, Inc., Willsboro NY. Compositions wit=h less than about 5 weight percent wollastonite do not exhibit the char formation and low heat release rate of the present invention. The upper limit of wollastonite that is useful will depend on the properties desired in the uncured and cured composition. Generally, wollastonite present at greater than about 80 percent by weight results in uncured compositions that are too stiff and therefore difficult to process, and results in cured compositions that have reduced tensile strength and elongation. The curing component (D) can be any of the well-known curing components known in the silicone elastomer art. For example, the curable silicone elastomer compositions of this invention may be cured to the elastomeric state by exposure to electron beams, ultraviolet rays, electromagnetic waves, or heat. Where heat is used as the curing mechanism, an organic peroxide curing catalyst may be used as component D. Examples of suitable organic peroxide curing catalysts include 2,5-dimethyl-2,5-di(tert- butylperoxy)hexane, 2,2-bis(t-butylperoxy)-p- diisopropylbenzene, l,l,bis(t-butylperoxy)-3,3,5- trimethylcyclohexane, 2,5-dimethyl-2,5-di(tert- butylperoxy)hexyne-3, di-t-butylperoxide, benzoyl peroxide, p-chlorobenzoyl peroxide, dicumyl peroxide, tertiary butyl peracetate, tertiary butyl perbenzoate, monochlorobenzoyl peroxide, 2,4-dichlorobenzoyl peroxide, and tertiary butyl cumyl peroxide. The amount of catalyst used will depend on the type of catalyst and can be determined by experimentation. Generally, peroxide catalysts are useful in amount from about 0.05 to 10 parts, and more preferably 0.1 to 5 parts by weight catalyst per 100 parts by weight ingredient (A).

An alternative curing component (D) which is applicable is an organohydrogensiloxane crosslinker used in the presence of a platinum group metal -containing catalyst. The organohydrogensiloxane crosslinker can contain an average of at least two silicon-bonded hydrogen atoms per molecule, and no more than one silicon-bonded hydrogen atom per silicon atom, the remaining valences of the silicon atoms being satisfied by divalent oxygen atoms or by monovalent hydrocarbon radicals comprising one to seven carbon atoms. The monovalent hydrocarbon radicals can be, for example, alkyl groups such as methyl, ethyl, propyl, tertiary butyl, and hexyl; cylcoalkyl groups such as cyclohexyl; and aryl groups such as phenyl and tolyl. Such materials are well known in the art. The molecular structure of the organohydrogensiloxane may be linear, linear with a degree of branching, cyclic, or of a network- form. There are no particular restrictions on the molecular weight of the organohydrogensiloxane, however it is preferable that the viscosity at 25 C be 3 to 10,000 mPa-s. Furthermore, the amount of component (C) that is added to the composition is in an amount such that the ratio of the number of moles of hydrogen atoms bonded to silicon atoms to the number of moles of alkenyl groups bonded to silicon atoms is in the range of 0.5:1 to 20:1, and preferably in the range of 1:1 to =):1. If this molar ratio is less than 0.5:1, curing of the present composition becomes insufficient, while if this molar ratio exceeds 20:1, hydrogen gas is evolved resulting in foaming during the curing process.

The platinum group metal -containing catalyst can be any such catalyst which is known to catalyze the reaction of silicon-bonded hydrogen atoms with silicon-bonded vinyl groups. By platinum group metal, it is meant ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum based catalysts are particularly preferred. Examples of such platinum based catalysts include chloroplatinic acid, alcohol solutions of chloroplatinic acid, complexes of chloroplatinic acid with olefins, complexes of chloroplatinic acid with divinylsiloxane, platinum black, metallic platinum, and catalysts in which metallic platinum is supported on a support. The amount of component (D) that is added varies according to the type of catalyst that is used, and is not especially restricted; ordinarily, however, the amount added is 1 to 1,000 parts by weight, preferably 5 to 100 parts by weight platinum group metal, per 1,000,000 parts by weight of component (A).

When the organosiloxane polymer is a vinyldimethylsiloxane-containing gum, it is preferred that the curing component be selected from the organic peroxide curing agents. When the organosiloxane polymer is a vinyldimethylsiloxane-containingliquid with viscosity of less than 150,000 mPa-s, it is preferred that the curing component comprise an organohydrogensiloxane crosslinker and a platinum group metal -containing catalyst.

optional smoke reducing agents may be added to the ingredients in the composition of the present invention. These smoke reducing agents may be selected from materials frequently used in the silicone rubber industry to reduce smoke, including platinum, aluminum trihydrate and magnesium oxide. These materials may help in allowing the silicone to pass smoke generation criteria. As shown below in the examples, aluminum trihydrate can have the effect of

increasing heat release of the polymer on burning. Therefore, aluminum trihydrate should be used only after experimentation to confirm that the increased heat release is acceptable for the amount of smoke reduction achieved. On the other hand, use of platinum in the present composition does not have an adverse impact on heat release, and can be used at levels normally used for smoke reduction.

Additional ingredients may optionally include colorants, pigments, tensile modifiers, heat stabilizers, lubricants and/ or ot=her fillers known in the art, including but not limited to ground quartz, diatomaceous earth, calcium carbonate titanium dioxide and mica.

In a further aspect of the present invention there is provided a silicone rubber composition comprising (A) 15 to 29 weight percent of a heat-curable organosiloxane polymer, containing at least 2 alkenyl groups per molecule, (B) 1 to 65 weight percent of a reinforcing silica filler, based on the total composition, (C) 5 to 70 weight percent wollastonite having an average particle size of 2 to 30 #tm, based on the total composition, and (A) a curing component sufficient to cure the composition. Components (A) , (B) , (C) and (D) are preferably as defined above. In a still further aspect of the present invention there is provided a safety cable comprising i) an inner conducting core ii) a silicone rubber ceramifiable layer surrounding the metallic core comprising A) 15 to 90 weight percent of a heat-curable organosiloxane polymer, containing at least 2 alkenyl groups per molecule, B) 1 to 65 weight percent of a reinforcing silica filler, based on the total composition, C) 5 to 80 weight percent wollastonite having an average particle size of 2 to 30 gym, based on the total composition, and D) a curing component sufficient to cure the composition, and at least one of iii) an additional protective layer of silicone rubber or the like iv) a thermoplastic filler; or v) a thermoplastic outer surrounding sheath.

Preferably the inner conducting core (i) is a metallic conductor but it may be any type of conductor which might be used in a safety cable such as a fibre optic light conductor. An example of a possible metallic conductor suitable for use as a wire core, is copper. Components (A) , (B) , (C) and (D) are preferably as defined above. Protective layer (iii) is preferably made of a suitable silicone rubber composition, which is preferably highly temperature resistant and will undergo endothermic decomposition to silica upon burning. The thermoplastic filler (iv) can be any appropriate filler. The thermoplastic outer surrounding sheath (v) is preferably made from a thermoplastic such as ethyl vinyl acetate (EVA) or polyethylene, but any appropriate thermoplastic material may be utilised. Such a safety cable would not require a mica tape and would therefore be significantly easier to prepare (extrusion), would be lighter and more flexible prior to a fire.

In UK safety cables have to meet British Standard BS 6387 and in particular C, W and Z ratings of BS 6387.

C Rating A piece of cable is heated with a gas flame at 950 C for 3 hours and no failure of the electrical integrity of the cable is permitted.

W Rating A piece of cable is burnt for 15 minutes with a gas flame at 650 C, the resulting ceramified cable is then sprayed with water and the burning phase repeated. Again no failure of the electrical integrity of the cable is permitted during the duration of the test.

Z Rating A piece of cable is burnt for 15 minutes with a gas flame at 950 C for 15 minutes and the burnt cable is subjected to a defined mechanical shock test every 30 seconds and no failure of the electrical integrity of the cable is permitted during the duration of the test. Safety cables in accordance with the present invention are considered suitable to pass the BS 6387 (c,w,z).

In the following examples test samples of the silicone rubber compositions for use in safety cables were prepared by mixing the following compositions in a Baker Perkins mixer and cured in a hot press at 170 C for 10 minutes. Two comparative samples were also analysed. The samples were post cured in an air circulating oven for 4 hours at 200 C. The catalyst was milled into the rubber.

Compound 1, a comparative example is a commercially available product for use in safety cables known as WX 2988 from Wacker GmbH.

Compound 2 is a further comparative example which comprises10.8% by weight of base 1,17.85% by weight of base 2 and 35 5% by weight of base 3, 0.6o by weight of a heat stabilizer,0.6% by weight of a tensile strength modifier, 6.4% by weight of additional fire retardants, 3.2% by weight of a pigment and 25.5% by weight of ground quartz. A varox peroxide catalyst was utilised in an amount of 1 part by weight per hundred parts by weight of (A). Base 1 contains 32% by weight of dimethyl vinylsiloxy- terminated dimethyl methyl vinyl siloxane, 27% by weight of a dimethylvinylsiloxy-terminated dimethyl siloxane, 45 by weight of a hydroxy terminated dimethyl siloxane and 34% by weight of amorphous silica.

Base 2 contains 59% by weight of dimethylvinylsiloxy terminated dimethyl siloxane, 31.9% by weight of amorphous

silica and 1.7o by weight of dimethylvinylsiloxy terminated dimethyl methylvinyl siloxane Base 3 contains 67% by weight of dimethylvinylsiloxy terminated dimethyl methylvinyl siloxane 24o by weight of amorphous silica and 5% by weight of a hydroxy terminated phenylmethylsiloxane. Compound 3 contained 39.1o by weight of base 4, 1% by weight of a tensile strength modifier, 59.8% by weight of wollastonite having a density of 2.9g/cm' and an average particle size of 123m and 0.1% by weight of a platinum based smoke reducing agent. A varox peroxide catalyst was utilised in an amount of 1 part by weight per hundred parts by weight of (A).

Base 4 contained 51 % by weight dime thylvinylsiloxy- terminated dimethyl, methylvinyl siloxane; 19.o by weight amorphous silica with a surface area of 250 M2/g; 22 % by weight hydroxy-terminated dimethyl siloxane; and 3% by weight hydroxy-terminated phenylmethyl siloxane. Base 4 had a plasticity of 1.8 to 1.9 mm.

Compound 4 contained 49.5% by weight of base 4, to by weight of a tensile strength modifier, and 49.5o by weight wollastonite having a density of 2.9g/cm' and an average particle size of 2@Lm and 0.1% by weight of a platinum based smoke reducing agent. A varox peroxide catalyst was utilised in an amount of 1 part by weight per hundred parts by weight of (A).

The performance of the two compounds in accordance with this invention and the two comparative compounds on the basis of a) physical properties, b) processability (as assessed by capillary rheometry) and c) weight loss by thermo-gravimetric analysis.

Example 1 Physical properties of samples of each compound were analysed using a selection of standard tests and the results are provided in Table 1 Plasticity was measured using a Wallace Plastometer. The Mooney viscosity test was carried out at 25 C on uncured composition with a view to analysing the processability of the uncured composition using a Monsanto Mooney MV 2000. Shore A hardness was measured using a Zwick 144 Materials testing durometer. Tensile strength measurements were taken using a Zwick tensile test Bank. Table 1. Physical Property data

Compound Compound Compound Compound 1 2 3 4 Specific 1.3 1.4 1.8 1.752 Gravity Plasticity (mm) 550 280 295 320 Mooney 85.6 43.4 46.5 57.9 (initial) Mooney (final) 60.6 40.9 32.4 48.7 Delta Mooney 25 2.5 14.1 9.2 Hardness (NPC) 68 60 69 58 Hardness (PC) 69 63 70 59 Modulus 100 % 2.53 2.34 2.67 2.44 (NPC) Modulus 100 0 2.72 2.58 3.46 2.24 (PC) Tensile (NPC) 9.1 7.6 3.0 5.4 Tensile (PC) 8.6 7.4 3.7 5.8 Elongation 458 398 159 338 (NPC) Elongation (PC) 443 365 141 342 Tear (NPC) 27.1 27.5 20.9 19.0 Tear (PC) 25.7 26.1 21.8 19.8 Example 2. Processability The shear viscosity vs. shear rate curves are provided below as Fig.l Samples were analysed using a Ro sand Capillary Rheometer with a 16/1 die with a lmm diameter. for compounds compound 3 had the lowest viscosity as a function of shear rate of all of the compounds tested. It will be noted that Compound 4 in accordance with the invention had the best processability according to the capillary rheometry tests carried out.

Examples 3. Thermo-crravimetric analysis The compounds were heated in air from ambient to 950 C at 20 C min-'. Onset decomposition temperatures and total weight loss were recorded (Table 3, Figures 2 - 6). Table 2. Decomposition parameters

Compound Onset ( C) finish weight Theoretical ( C) loss (o) loss (owt) Compound 1 300 630 43 Compound 2 430 830 28 43 compound 3 420 610 15 29 compound 4 300 435 23 36 The theoretical weight losses were calculated by assuming all the filler material in the compound would not decompose and that the non-filler components, could be considered to be polydimethylsiloxane which would oxidise to silica.

The theoretical weight loss of each compound is much greater than the actual weight loss suggesting that not all of the polydimethylsiloxane present was oxidised. One explanation for this may be that the outside of the sample oxidised and densified, through ceramification, with the result that the interior was protected from further attack by oxygen.

It was found that compound 4 had the narrowest decomposition range of 135 C and the lowest onset temperature (300 C) which may off=er an advantage in the fire safety performance of cables coated with this material.

Hence it can be seen that compound 4 was the easiest to process by capillary rheometry and the lowest Thermogravimetric analysis weight losses were seen for compounds 3 and compound 4, i.e. the two examples which are in accordance with the present invention.

Claims

CLAIMS 1. Use of a curable silicone rubber composition comprising: (A) 15 to 90 weight percent of a heat-curable organosiloxane polymer, containing at least
2 alkenyl groups per molecule, (B) 1 to 65 weight percent of a reinforcing silica filler, based on the total composition, (C) 5 to 80 weight percent wollastonite having an average particle size of 2 to 30 gm, based on the total composition, and (D) curing component sufficient to cure the composition, as a ceramifiable layer in a safety cable composition. 2. The use in accordance with claim 1 wherein component (A) is a vinyl -containing polydimethylsiloxane.
3. The use in accordance with any preceding claim wherein component (C) comprises 40 to 60 weight percent wollastonite.
4. The use in accordance with any preceding claim wherein component (C) has an aspect ratio of about 2 to 1.
5. The use in accordance with any preceding claim wherein component (C) has an average particle size of from about 2 to 15 @tm.
6. The use in accordance with any preceding claim wherein component (D) is a peroxide catalyst, selected from the group consisting of 2,5-dimethyl-2,5-di(tert butylperoxy)hexane,2,2-bis(t-butylperoxy)-p diisopropylbenzene,l,1,bis(t-butylperoxy)-3,3,5- trimethylcyclohexane, 2,5-dimethyl-2,5-di(tert- butylperoxy)hexyne-3, di-t-butylperoxide, benzoyl peroxide, p-chlorobenzoyl peroxide, dicumyl peroxide, tertiary butyl peracetate, tertiary butyl perbenzoate, monochlorobenzoyl peroxide, 2,4-dichlorobenzoyl peroxide, and tertiary butyl cumyl peroxide.
7. The use in accordance with any preceding claim wherein component (D) consists of an organohydrogensiloxane crosslinker, and a platinum group metal catalyst.
8. The use in accordance with any preceding claim wherein the ingredients further comprise an effective amount of a smoke reduction agent.
9. The use of claim 8 wherein the smoke reduction agent is a platinum compound.
10. The use in accordance with any preceding claim wherein the ingredients further comprise an additional filler selected from the group consisting of ground quartz, diatomaceous earth, calcium carbonate titanium dioxide and mica.
11. A silicone rubber composition comprising (A) 15 to 29 weight percent of a heat-curable organosiloxane polymer, containing at least 2 alkenyl groups per molecule, (B) 1 to 65 weight percent of a reinforcing silica filler, based on the total composition, (C) 5 to 80 weight percent wollastonite having an average particle size of 2 to 30 gym, based on the total composition, and (D) a curing component sufficient to cure the composition,
12. A safety cable comprising i. an inner conducting core ii. a silicone rubber ceramifiable layer surrounding the metallic core comprising A) 15 to 90 weight percent of a heat-curable organosiloxane polymer, containing at least 2 alkenyl groups per molecule, B) 1 to 65 weight percent of a reinforcing silica filler, based on the total composition, C) 5 to 80 weight percent wollastonite having an average particle size of 2 to 30 VLm, based on the total composition, and D) a curing component sufficient to cure the composition,

and at least one of iii. an additional protective layer of silicone rubber or the like iv. a thermoplastic filler; or v. a thermoplastic outer surrounding sheath.