Crosslinking of the polymeric parts of wires and cables is desirable, and sometimes essential, to impart good mechanical toughness and resistance to solvents and thermal deformation. Radiation crosslinking necessitates the installation of plant at very high capital cost, and also gives problems in obtaining a uniform dose (and therefore crosslink density) over the cross-section of the cable. Conventional chemical crosslinking with peroxides and/or sulphur requires pressurised curing chambers at high temperatures. The high temperatures required for cure severely limit line speeds, and have high energy costs plus high scrap rates at start-up due to inability to observe the product until it has emerged from the long curing chamber.

There has therefore been considerable interest in the past decade and a half in a low capital cost chemical crosslinking technique that does not require a pressurised and heated chamber and hence enables the extrusion quality to be observed as easily and quickly as that of a thermoplastic product; it is based on a two-step chemical reaction in which a hydrolysable unsaturated silane is first grafted to a base polymer (usually a homopolymer or copolymer of ethylene) and subsequently crosslinking is brought about by catalysed condensation reaction between pairs of silane groups. This 'Sioplas' process was first introduced by Dow Corning Limited (formerly Midland Silicones Limited) as a 2-stage process using separate machines for the grafting and extrusion steps (U.K. Patent No. 1286-460) but was subsequently improved by the applicants in conjunction with Etabs. Maillefer S A (U.K. Patent No. 1526398) to operate as a single-stage process with

grafting in the extruder. This one-stage process has been used on a significant ^' scale, but its application has been restricted, hitherto, by the inability of experts in the art to provide halogen-free compositions crosslinkable by this process having commercially acceptable flame retardant characteristics.

Experiments with alumina trihydrate, or other lame-retardant fillers that evolve water on heating, in conventional compositions for one-stage Sioplas crosslinking based on ethylene polymers (copolymers with ethyl acetate, ethyl acrylate, propylene etc. and some homopolymers) have failed, partially through unacceptable premature crosslinking (as might have been expected because of the risk of the filler evolving water at grafting temperature) and partially through the production of a porous product that was wholly unacceptable for electrical applications; and experts have drawn the conclusion that water-evolving fillers are incompatible with one-stage Sioplas chemistry (see, for example, Modern Plastics International, December 1984 pp 38-40) .

We have now discovered that this conclusion of the prior art is unjustified and that satisfactory results can be obtained simply by making an unconventional choice of initiator for the crosslinking reaction.

In accordance with the invention, a crosslinkable polymer composition for extrusion comprises: at least one flame-retardant filler of the class that evolve substantial amounts of bound water on heating at temperatures above a value !<_ which value lies in the range 120-250°C; a compatible ethylene polymer base in which the filler is dispersed; a hydrolysable unsaturated silane; and a free-radical grafting initiator having a half-life of less than 10 minutes at a temperature 25°C

below T ₍ - as determined by Differential Thermal Analysis using chlorobenzene as solvent; see for instance "Noury Initiators", Delivery Programme of the Thermoplastics Industry, a current publication of Akzo Chemie U.K. Limited.

A silanol condensation catalyst will also be required to obtain crosslinking in an acceptably short time.

The most readily available fillers of the class defined are gypsum (T^l 28°C) , alumina trihydrate

(minimum T^-l 70°C) , and magnesium hydroxide (minimum T ₍ i=250 ^o C.) Gypsum is not recommended as there are practical difficulties with most polymers if T <_ is below 150°C, and magnesium hydroxide is much preferred as its higher T _jj value not only gives a much wider choice of initiators but also allows a higher extrusion rate because of the higher permissible processing temperature. Fillers coated to enhance compatibility with the polymer are preferred, suitable coating materials including oleates and other fatty acid soaps, silanes, titanates, siloxanes, and carboxylated butadiene polymers (for example those described in British Patent No. 1603300) and other low-molecular weight polymers and oligomers containing carboxyl groups. Particularly suitable magnesium hydroxide fillers are commercially available from Kyowa Chemical Industry Company Limited (Japan) under the trade mark 'Kisuma'. If more than one water-evolving filler is present, the relevant Υ _ will, of course, be the lower or lowest.

The polymer may be any of the well-known types that is capable of accepting the quantity of filler needed to meet flame retardance requirements without unacceptable loss of physical and/or electrical properties; ethylene-vinyl acetate copolymers (EVA) (containing from about 5 to about 70 mole % of vinyl acetate) are most preferred, but ethylene/ethyl acrylate, ethylene/methyl acrylate, ethylene butyl acrylate and rubbery and non-rubbery ethylene/propylene

copolymers (and terpolymers) are also suitable), as are some ethylene homopolymers, such as the one of low molecular weight sold by Union Carbide Group, under the

Trade Mark UCAR as UCAR FLX, and ^' butyl grafted polyethylenes such as that sold by ASEA Kabel AB under the designation ET Polymer. Mixtures of polymers can be used.

Filler to polymer ratios will depend upon the balance of flame retardance to physical and electrical properties required, usually in the range from 3:1 to

1:2 by weight (subject to limits of compatibility).

The most usual silane is vinyl trimethoxy silane

(VTMOS), but any of the silanes used in conventional

Sioplas processes can be used. A list is included in the specification of U.K. Patent No. 1286460. Amounts required are broadly as for the conventional process, based upon the active polymer content of the mixture. The grafting initiator will normally be peroxide, examples of suitable commercially available peroxides being as follows:

(a) for gypsum, alumina trihydrate and magnesium hydroxide

Tertiary butyl per pivalate

Tertiary amyl per pivalate Di(3,5,5 trimethyl hexanoyl) peroxide

Bis (2 methyl benzoyl) peroxide

Di normal octanoyl peroxide

Di - decanoyl peroxide

Dilauroyl peroxide (b) for both alumina trihydrate and magnesium hydroxide

(but not for gypsum):

Tertiary butyl per 2-ethyl hexanoate (TBPEH) .

Tertiary butyl per isobutyrate

Tertiary butyl peroxy diethyl acetate Dibenzoyl peroxide

1,1 Di-tertiary butyl peroxy - 3,3, 5-trimethyl cyclohexane

Tertiary butyl peroxy 3,5,5, trimethyl hexanoate

Tertiary butyl peroxy isopropyl carbonate Tertiary butyl peracetate Tertiary butyl perbenzoate

1,1 Bis (tertiary butyl peroxy) cyclohexane 2,2 Bis (tertiary butyl, peroxy) butane.

Normal butyl 4,4 bis (tertiary butyl peroxy) valerate and (c) for magnesium hydroxide (but not for alumina trihydrate or gypsum): Dicumyl peroxide 2,5-Dimethyl 2,5-di (tertiary butylperoxy) hexane 1,4-Di (tertiary butylperoxy iso propyl) benzene Tertiary butyl cumyl peroxide Di-tertiary butyl peroxide

Quantities of grafting initiator can be determined by trial in the usual way.

Dibutyl tin dilaurate (DBTDL) (at a level of the order of 0.01?) is the only silanol condensation catalyst in regular use for Sioplas crosslinking and is preferred; many alternatives are listed in the specification of U.K. Patent No. 1286460.

The compositions may also include conventional Sioplas-compatible antioxidants and other stabilisers, processing aids, lubricants, pigments and usually minor amounts of other fillers such as magnesium carbonate, calcium carbonate, double carbonates of magnesium and calcium (such as dolomite), antimony trioxide, antimony pentoxide and zinc borate.

Preferably the polymer and filler are pre-blended and dried prior to the addition of the other ingredients.

In the following examples, all parts are by weight unless otherwise indicated. Example 1 ^■

49.7 Parts of the ethylene-vinyl acetate. copolymer (28 weight % vinyl acetate) sold under the trademark Evatane and the grade designation 28-05 was melted and homogenised on a two roll mill at 80-90°C and blended on the mill with 49.8 parts of the magnesium

hydroxide filler sold under the trademark Kisuma and the grade designation 5B (believed to be oleate coated and of the kind described and claimed in U.K. Patent No.1514081) and 0.5 parts of the antioxidant sold under the trademark Flectol as Flectol pastilles.

The blend was dried for at least 16 hours at 55°C using a dehumidified hot air dryer (sold under the designation Drymaster). To this dried base mix were added 0.9 parts VTMOS, 0.45 parts TBPEH and 0.01 parts DBTDL and the mixture was extruded in a 38mm 20D laboratory extruder with a screw of nominal compression ratio 2:1 operating at a uniform temperature of 130°C to give an insulating covering free of porosity with a radial thickness of 0.75mm on a round copper wire 1.2mm in diameter at a line speed of 1.8m/min. Curing was in water at 70°C for 24 hours.

The insulation had a tensile strength of 12.7 MPa, elongation at break of 172? and in a hot set test at 2 bar and 200°C showed an elongation of 15? and a set of -5?. Its volume resistivity (at 75°C and 600 V d.c.) was higher than 10 ¹ ° Ωm after 30 weeks immersion in water at that temperature. (Two samples were tested; one had unchanged volume resistivity after 62 weeks; the other had fallen to 8x10 ⁸ Ωm). Example 2

Substantially the same procedure was followed as in Example 1 but using:

(a) as the base mix 37 parts of Evatane 28-05, 5 parts of another EVA sold by Allied Chemical Company under the designation AC400A (to improve extrusion characteristics) and 57.5 parts of Kisuma 5A (similar to Kisuma 5B but believed to be stearate coated) and 0.5 parts of Flectol pastilles; and

(b) as curative system 0.65 parts VTMOS, 0.25 parts TBPEH, and 0.01 parts DBTDL.

The line speed in this case was 2.6 mm/min.

The insulation had a tensile strength of 11.9 MPa, elongation at break of 145? and in the hot set test

showed an elongation of about 40? and sets of -2.5 and 0 on two samples. Its volume resistivity after 37 weeks under the same conditions as in Example 1 was better than 10 ¹⁰ Ωm. Example 3

Substantially the same procedure was followed as in Example 1 but using as the base mix 40.5 parts Evatane 28-05, 5 parts of AC400A and 54 parts of the hydrated alumina sold under the trademark Hydral and the grade number 710. The line speed was 2m/min.

The insulation had a tensile strength of 14.2 MPa, elongation at break of 199? and in the hot set test showed an elongation of about 20? and a set of -3?. Its volume resistivity (at 75 °C and 600 V d.c.) was higher than 5x10* Ωm after 45 weeks immersion in water at that temperature. Examples 4-37

The procedure of example 3 was followed with a variety of base mixes (all including approximately 0.5 parts Flectol pastilles, not listed in the tables) with the results indicated below. (Line speed and radial thickness varied somewhat).. Ingredients used in these examples and not previously referred to are as follows: Fillers: Marinco H: Uncoated magnesium hydroxide from Merck & Co. Inc. USA.

Kisuma 5E: Anther coated magnesium hydroxides from Kyowa Chemical Industry Co. Ltd. Japan. DC: Magnesium hydroxide, received uncoated from Morton Thiokol

Inc. U.S.A. and coated with 1.6? Y9774. Lycal 96HSE and Uncoated magnesium hydroxides PDM/PH10/200- respectively from Steetley Magnesia Division U.K. and

Pennine Darlington Magnesia Limited, U.K. Ultracarb U5: A mixed filler of magnesium

hydroxy carbonate and calcium magnesium carbonate from Microfine Minerals Limited, U.K. Ultracarb M5: a version of Ultracarb U5 with a polymeric coating (The T _ζ -'s of Ultracarb fillers have not yet been accuraturely measured) Ethylene polymers: DPDM 6182 Ethylene/ethyl acrylate copolymer from BP Chemicals Limited, U.K. DEFD1225 A polyethylene of very low density from Union Carbide

Corp . , U.S.A. Other AC6A: Polyethylene, from Allied ingredients: Chemicals, U.S.A.

Omya EXH1 : Calcium carbonate from Croxton and Garry Limited, U.K.

A174: Gamma methacryloxypropyl trimethoxy silane from Union Carbide Corporation, U.S.A. DC1107: Methyl hydrogen polysiloxane from Dow Corning Limited, U.K.

Y9774: A proprietary silane from Union Carbide Group, U.S.A. Surlyn 9450: An ionomeric polymer from Du Pont, U.S.A. In the case of precoated fillers, the quantity shown in the tables includes the weight of the coating, the percentage of which, relative to the total weight of that coated filler, is given immediately below.

All compositions of the Examples showed low values in the hot set test, indicative of a sufficient degree of crosslinking for typical wire and cable applications and all, except only Example 20, passed the Horizontal Specimen Flame Test defined in paragraph 102

of UL 62 Standard for Flexible Cord and Fixture Wire. The compositions of Examples 2 and 25 also passed the IEC 332 Part 1 vertical burning test, which is a very stringent test for the small wire size of these samples.

All the Examples except Examples 7-12 and Example 37 met the electrical property requirements required of conventional PVC-based materials by the applicable British Standard (BS 6746:1984).

TABLE 1

- 11 -

TABLE .2

- 12 -

TABLE 3

- 13 -

TABLE' 4

- 14. -

TABLE 5

- 15 -

TABLE 6

- 16 -

TABLE 7

TABLE 8

TABLE 9

REMARKS TO TABLES B VTMOS and TOPEH reduced to 0.7 parts and 0.35 parts respectively. C VTMOS and TOPEH reduced to 0.8 parts and 0.4 parts respectively.

D Peroxide content increased to 0.75 parts.

E Failed horizontal burning test (because of low filler content) . F Coating is silane Ϊ9774. G Passed vertical burning test. H Coating is DC 1107 J Coating is magnesium stearate.

K The curative systems used in these Examples were as follows (in weight ?) Example VTMOS TBPEH DBTDL

31 0.65 0.20 0.01

32 0.72 0.33 0.01

33 0.75 0.35 0.01

3h 0.75 0.35 0.01 35 0.65 0.175 0.01

36 0.80 0.40 0.01

37 0.77 0.36 0.01 Example 38

In a trial on a commercial extruder, a composition comprising 37 parts Evatane 28-05, 57.5 parts Kisuma 5B, 5 parts AC400A, 0.05 parts Flectol pastilles, 0.65 parts VTMOS, 0.30 parts TBPEH and O.OIparts DBTDL was successfully extruded at line speeds up to 550 m/min and at temperatures up to 190°C. Samples extruded at 130°C had a tensile strength of 10.7 MPa, Elongation at break of 182? and in the Hot Set test recorded Extensions of 25 to 30? and Sets of -2.5? to 0. These samples also passed the IEC 332 vertical burning test.