Electricity: conductors and insulators – Feedthrough or bushing – Compression
Patent
1989-02-13
1990-09-04
Lee, John D.
Electricity: conductors and insulators
Feedthrough or bushing
Compression
174 23C, G02B 644, H02G 1520
Patent
active
049539469
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to electrical or fibre optic cables of the type comprising one or more filaments, which conduct electricity or light, within a protective sheath. The present invention also relates to the manufacture of such cables.
Electrical or fibre optic cables generally comprise one or more filaments, which conduct electricity or light, within a protective sheath or covering. In such cables, it is preferable to fill the interstices between the sheath and the filaments with a filler comprising a powder such as standard grade hydroxyethyl cellulose, carboxymethyl cellulose, or a cross linked acrylamide polymer. Such powders, which swell when contacted with water, are included so that should water enter the cable e.g. by pores or fractures in the protective sheath, then the powder will swell and block the point of entry.
Each of the powders described above has a disadvantage. Thus, relative to the cellulose ethers, polyacrylamide is expensive. Carboxymethyl cellulose, although cheap and relatively stable, tends to be a less efficient block as it swells more than the others. Finally standard hydroxyethyl cellulose, whilst being cheap and efficient, tends to be unstable and to lose its blocking properties at elevated temperature.
A new grade of hydroxyethyl cellulose has been developed which is not only cheap and efficient but which is stabler than previously used grades.
Accordingly, the present invention provides an electrical or fibre optic cable comprising one or more conducting filaments within a protective sheath characterised in that between the protective sheath and the filaments there is provided a powder consisting essentially of a hydroethylcellulose having an MS value between 0.7 and 1.25 which has not been surface treated with glyoxal.
As regards the hydroxyethyl cellulose powder used, this is one having an MS (molar substitution) value of between 0.7 and 1.25 preferably between 0.8 and 1.1. The MS value of a hydroxyethyl cellulose is defined as the number of ethylene oxide molecules which have reacted with each anhydro-glucose unit in the cellulose.
The hydroxyethyl cellulose is also one which has not been surface treated with glyoxal.
The intrinsic viscosity of the cellulose raw material used to make the cellulose ether should be in the range 6 to 23 dl per gram preferably 15 to 20 dl per gram (as measured by following ASTM D-1795).
Where the cable is to contain substantial amounts of metals such as aluminium it is preferable to treat the hydroxyethyl cellulose after the ethoxylation reaction to remove residual corrosive salts, such as sodium acetate, which are present.
The hydroxyethyl cellulose powder of the present invention is preferably one in which greater the 90% by weight of the powder has a particle size of less than 175 microns.
The cables of the present invention can be prepared by one of a number of standard methods. In a preferred embodiment the cable is prepared by (1) stranding the filaments together to form a bundle (2) coating the bundle successively with a semiconductive layer, an insulator and a channeled semiconductive layer filling the channels of the channeled semiconductive layer with powdered HEC (4) applying an aluminium stripe and (5) finally covering with the protective sheath.
A cable produced by the process described above is shown in cross-section in FIG. 1. The cable comprises a bundle of conductive filaments (1) surrounded by an inner semiconductor (2) which is in turn surrounded by an outer, channeled semiconductor (3). The channels of (3) are filled with the HEC powder. Around the outer channeled semi conductor (3) is a strip of aluminium (4) and finally a protective sheath (5).
To illustrate the improvement which accrues from using the hydroxyethyl cellulose of the present invention reference is now made to the following experiments.
EXPERIMENTS 1-5
The performance of five samples was measured. Experiments 1 and 2 employed respectively a carboxymethyl cellulose and a hydroxyethyl cellulose of the type used previously, whilst Experiments 3 to
REFERENCES:
patent: 4084060 (1978-04-01), Glass, Jr. et al.
patent: 4308416 (1981-12-01), Herman et al.
patent: 4326094 (1982-04-01), Hunn
patent: 4703998 (1987-11-01), Uchioke et al.
Gibson Henry H.
Lee John D.
Union Carbide Chemicals and Plastics Company Inc.
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