GB2131045A - Compositions for use in electric cables - Google Patents

Abstract

A filling medium for fully-filled telecommunication cables with cellular insulation comprises (by weight):- (a) 70-98% a liquid polybutene with a number average molecular weight between 200 and 4000; (b) 1-15% of a block copolymer which comprises at least one block tending to confer solubility in (a) and at least one block tending to inhibit such solubility, the proportions of the blocks being such that the copolymer tends to form a gel with (a); and (c) 1-15% of a further hydrocarbon material that is miscible with the medium above 135 DEG C but forms solid dispersed particles on cooling at a temperature not lower than 65 DEG C. m

Description

SPECIFICATION Electric cables and compositions for use in them This invention relates to fully-filled telecommunication cables and compositions for use as a filling medium in them. These cables comprise a multiplicity of conductors each with individual insulation, grouped in any appropriate way and enclosed in a sheath, the interstices between the insulated conductors, and between them and the sheath, being filled with a waterproof filling medium.

The function of the filling medium is to provide a dielectric medium, the properties of which remain at the required level even when water gains access to the interstices. Furthermore the filling medium must inhibit the passage of water along the interstices in the event that the sheath or a joint enclosure is damaged when the cable is exposed to wet conditions, for instance while buried in the ground.

The insulation on the individual conductors of such cables is almost always of polyethylene or polypropyiene (including certain of their semicrystalline thermoplastic copolymers of low permittivity), and in many cases the material is used in cellular form. To be effective the filling medium must be hydrophobic and highly deformable throughout the temperature range experienced by telecommunication cables in service (which can extend from -400C to +800C). It must also contact the insulation in such a way as to prevent passage of water between itself and the insulation.There are very few non-volatile materials that satisfy these requirements as well as the other important requirements of low permittivity and low dielectric loss, but for cables operating at temperatures up to about 500C hydrocarbon oils gelled with waxes (e.g. petroleum jellies) or hydrocarbon oils gelled with other suitable gelling agents have been found to be suitable.

It is however desirable to use fully-filled cables in some circumstances at temperatures up to about 80"C--for example when they are associated with and run alongside large power cables-and in this case two difficulties arise. First, petroleum jelly shows a tendency to fill cells in the insulation to an extent that may be appreciable in a few months. This can result in partial loss of the medium from interstitial spaces in the cable leading to the developement of longitudinal channels and the consequent impairment of the waterproofness of the cable; in addition, the displacement of air (permittivity approximately 1.0) in the cellular insulation by the filling medium (permittivity typically 2.3) will lead to changes in cable capacitance and hence transmission characteristics of the cable.

Second, the viscosity of the medium decreases to the point at which it may flow along the interstices under the hydrostatic pressures that may occur in an installed cable, allowing the water to flow after it.

In our European Patent Application 78300524.2, Publication Number 0001713, we have described a filling medium, and cables incorporating it, in which both of these problems have been overcome. The filling media there described, however, in common with conventional petroleum jelly based filling compounds, adhere tenaciously to insulation and require repeated mechanical wiping away from the insulation to prepare individual insulated wires for jointing and terminating purposes.

The present invention defines a filling medium which is less tacky and more easily removed from the insulation and is generally more convenient to handle.

Other filling media that satisfy this desire are known, but these are particularly unsatisfactory in their cell filling characteristics, even at relatively low temperatures and it is the object of te present invention to provide a filling medium (and cables incorporating it) that satisfies the requirement of effectively inhibiting cell filling in conjunction with that of the improved handling characteristics and which can be formulated for use at temperatures up to about 800C if required.

The filling medium in accordance with the invention comprises (by weight): (a) 7098% a liquid polybutene with a number average molecular weight between 200 and 4000; (b) 1-1 5% of a block copolymer which comprises at least one block tending to confer solubility in the base and at least one block tending to inhibit such solubility, the proportions of the blocks being such that the polymer tends to form a gel with the base; and (c) 1-1 5% of a further hydrocarbon material that is miscible with the medium above 1 350C but forms solid dispersed particles on cooling at a temperature not lower than 650C.

In addition to the ingredients specified, the medium may include effective amounts of antioxidants and other stabilisers, including gel stabilisers, and also low molecular weight hydrocarbon resins, particularly those, said to be derived largely from alpha-methylstyrene, sold by Hercules Powder Company under the trade name Kristalex and having ring-and-ball softening points (ASTM E28) between 200 and 1 200C and more particularly between 600 and 100 C.

The block copolymer (b) preferably forms 28% of the medium, and may be of various types, such as those designated "ABA", "ABAB", "ABCBA", "AnB" and (AB)n.

At least when the oil has an aromatic content less than 1 5%, a preferred material for use as ingredient (b) is a block copolymer of the ABA type in which the centre block (B) is a random copolymer of ethylene and butylene with end blocks (A) formed by graft polymerisation of styrene on to the centre block, for example a commercial material with a styrene to ethylene-butylene ratio of approximately 0.4, sold by Shell Chemicals Limited udner the trade mark Kraton as Kraton G1 650 or Go 652.

The temperatures at which ingredient (c) must become miscible with the medium is determined by the temperature at which the medium can be introduced into the cable. The maximum level of this temperature will amost always be determined by the softening characteristics of the insulation of the cable in which the medium is to be used and will be lower than the limit of 1 350C for insulation materials with a lower softening point than polypropylene.For example, for the following commonlyused cellular insulating materials, the ingredient (c) must be miscible with the medium down to the temperature indicated (at least): medium density polyethylene-95 (preferably 90)OC high density polyethylene105 (preferably 100)OC polypropylene135 (preferably 130)OC For maximum versatility of use, it is preferable that this ingredient does not form solid dispersed particles until the temperature falls to at least 900 C, and preferably to 850C.

Preferably there is at least 1.5%, and more especially 1.55%, of ingredient (c). This ingredient is preferably a low molecular-weight polyethylene with a ring-and-ball softening point (ASTM-E28) in the range 110-1 300 C. Suitable ethylene homopolymers are sold by Allied Chemical, for example grade AC 8 which has a ring-and-ball softening point (ASTM-E28) of 11 60C and a viscosity (Brookfield-LVT at 1 400C) of 350 cP.

A preferred gel stabiliser is the "synthetic iso-propylated phenyl orthophosphate" sold by Ciba Geigy Limited under the trade mark "Reofos" as Reofos 95. Alternatives can be selected from among other phosphate esters, alkyl ether phosphates, di(alkyl ether) esters, di-and poly(alkylene oxy) glycol diesters, alkyl-alkyl ether diesters, aikyl ether monoesters and aromatic diesters.

Conventional antioxidants can be used in appropriate amounts, a suitable example being 0.5 1% of the material sold by Vulnax international Limited under the trade mark Permanax as Permanax WSP.

Other stabiliser additives, such as metal deactivators, flame retardants, can be added as appropriate.

The following table shows the composition of an example of a filling medium in accordance with the invention (Example 1) together with some other media (Examples A-D) for comparison purposes.

Selected properties are reported.

The exact compositions of Examples B, C and D are not known; they are commercially available materials as follows:- Example B: Penreco FW-Non-petrolatum based cable filling compound sold by Penreco Inc., of Butler, Pennsylvania, U.S.A.

Example C: Silkqlene 947-Petroleum jelly compound sold by Dalton 8 Company Limited, of Belper, Derbyshire, U.K.

Example D: Insojell 3332-High temperature petroleum jelly compound sold by Dussek Cambell Limited, of Crayford, Kent, U.K.

Table Example: 1 A B C D Composition White technical oil 90 (Puremore WOT 45) SEE Polybutene TEXT Hyvis 03 (number average 90 molecular weight 250) Block copolymer: Kraton G1652) 6 6 Polyethylene (AC8) 4 4

Properties Cell filling (at 700C):: % mass increase 5 weeks 24.5 28.4 31.4 28.5 24.1 10 weeks 32.2 32.5 36.3 48.6 29.11z1 % increase in density # 5 weeks 9.3 16.3 21.5 6.5 2.9 10 weeks 14.4 19.8 22.2 14.7 7.121 % air replacement # 5 weeks 20.4 37.4 47.7 14.0 5.3 10 weeks 42.2 48.4 55.8 47.5 15.2123 Drop point (OC) (lP31/66) 36 71 60 75 85 Drainage test at 700C (passes in 3) 3 3 0 3 Viscosity (P) at 900C 1 .8si 6.4 1.3 < 1 2.3 1125.4s- 5.2 0.9 < 1 1.0 Finger test A B C E E * but rates 3 at 650C # 20 weeks Properties reported in the table were tested as follows: 1. Cell filling Based on weighing in air and water samples of insulated conductor, as described by S. M. Beach, K. R. Bullock and D. F.Cretney, 'Cellular Polyethylene Insulated Filled Communication Cable and its Increasing Use', 24th Wire 8 Cable Symposium, 1 975. The samples used in these tests were of copper wire, 0.5 mm in diameter, covered with 0.2 mm radial thickness of an expanded medium-density polyethylene; the polyethylene was supplied by BXL Plastics Limited under the reference BXL PN 225/6 and the degree of expansion was 30%. Note that, because of thermal annealing effects, density increases below 1% do not imply any cell filling.

2. Drop point By Institute of Petroleum test IP31/66.

3. Drainage Three samples of polyethylene tube, each 1 50 mm long, 3 mm bore and 1.5 mm wall thickness, were filled with the medium to be tested, using a syringe and if necessary softening the medium by heating. The tube ends were both open throughout the test. Filled tubes were suspended vertically in an air oven at 700C for 24 hours. No visible drainage is allowable.

4. Viscosity Measured with the Ferranti Portable Viscometer, Model VM, at the shear rates of 11.8 and 125.4s-' as indicated.

5. Finger test A subjective test of handling properties based on squeezing a ball of the medium between thumb and finger and rating on the scale: A-cohesive enough to stick to only one of thumb and finger and leaving no appreciable oil residue on the other.

B-cohesive but very slight oil residue left.

C-cohesive but slight oil residue left.

D-leaves substantial oil residue and/or is not cohesive enough to avoid risk of leaving some residue of medium.

E-cohesion so low that medium sticks to both thumb and finger.

Claims (6)

Claims

1. A filling medium for fully-filled telecommunication cables with cellular insulation comprising (by weight): (a) 7098% a liquid polybutene with a number average molecular weight between 200 and 4000; (b) 1-1 5% of a block copolymer which comprises at least one block tending to confer solubility in the base and at least one block tending to inhibit such solubility, the proportions of the blocks being such that the polymer tends to form a gel with the base; and (c) 1-1 5% of a further hydrocarbon material that is miscible with the medium above 1 350C but forms solid dispersed particles on cooling at a temperature not lower than 650 C.

2. A medium as claimed in Claim 1 comprising in addition a low molecular weight hydrocarbon resin.

3. A medium as claimed in Claim 1 or Claim 2 in which ingredient (b) is a block copolymer of the ABA type in which the centre block (B) is a random copolymer of ethylene and butylene with end blocks (A) formed by graft polymerisation of styrene on to the centre block.

4. A medium as claimed in any one of the preceding claims in which there is 1.5 to 5% of ingredient (c).

5. A filling medium for fully-filled telecommunication cables with cellular insulation having substantially the composition specified for any one of Examples 1-26 herein.

6. A fully-filled telecommunication cable with cellular insulation distinguished by a filling medium comprising (by weight):- (a) 7098% a liquid polybutene with a number average molecular weight between 200 and 4000; (b) 1-1 5% of a block copolymer which comprises at least one block tending to confer solubility in the base and at least one block tending to inhibit such solubility, the proportions of the blocks being such that the polymer tends to form a gel with the base; and