GB1580006A - Cross-linking of cable insulation - Google Patents

Description

(54) CROSS-LINKING OF CABLE INSULATION (71) We, INDUSTRIE PIRELLI SOCIETA PER AZIONI, an Italian Company, of Centro Pirelli, Piazza Duca d'Aosta 3, Milan, Italy, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention is concerned with electric cables and, more particularly, with a method of forming a cross-linked polyolefin insulation thereon, particularly on medium and high tension cables (e.g. 6 to 350 kV).

In the conventional method of forming a cross-linked polyolefin insulation on a single or stranded conductor, a coating of the polyolefin is extruded continuously onto a moving conductor, and the conductor then passes through a long curing tube (which is hermetically sealed to the extruder head), in which tube the coated cable is subjected to heat and pressure to cure the coating. The space between the conductor and the inside wall of the tube is conventionally filled with steam at a pressure of up to about 250 psi, the steam serving as a heat-transfer medium. For convenience, the curing tube may be suspended so as to take up a catenary form. After leaving the curing tube, the coated conductor may be cooled by passage through a cooling tube.

One of the more serious problems associated with this conventional method is that the cured polyolefin coating tends to contain voids which are usually irregularly spaced throughout the coating. The presence of voids can significantly reduce or impair the effectiveness of the coating as an electrical insulation, and the problem tends to become more acute with increasing thickness of the insulation.

There have been numerous proposals and suggestions in the past for overcoming this problem, including effecting the cross-linking in the absence of a vapour or in an anhydrous medium, and among the media which have been proposed are inert gases, mercury, diathermic liquids such as high molecular weight glycols, and low melting alloys such as Wood's alloy. For a variety of reasons, none of these proposals has been entirely satisfactory.

In U.K. patent specification no. 1,479,027, there is described and claimed a new apparatus for manufacturing electric cables covered with a cured polyolefin as the insulating material.

In the apparatus there described, which operates essentially according to the conventional process described above (except that the particular apparatus is different from conventional arrangements), there is used as a combined heat transfer medium and sealing medium a silicone oil. The oil desirably has a high viscosity, preferably of from 5000 to 100,000 centistokes, in order inter alia to act effectively as a sealant in that particular apparatus.

Reference should be made to specification no. 1,479,027 for further details of the apparatus there described.

In practice, there are certain problems generally in using high viscosity silicone oils as fluid heat transfer media. For example, the higher the viscosity, the more difficult it is to pump the oil around the circuit and the more energy is required for degassing and heating. As a result of extensive tests, we have found, however, that silicone oils of low viscosity are very effective heat transfer media in curing polyolefin insulations in conventional processes. In particular, we have found that such oils should not have a viscosity of above 500 centistokes (measured at 25"C). We have further found that, surprisingly, there is a minimum viscosity for the silicone oil, below which voids are formed in the polyolefin insulation. This minimum is between 100 and 200 centistokes, i.e. about 150 centistokes. Thus, by operating the conventional curing method using a silicone oil of viscosity from 150 to 500 centistokes (measured at 25"C), we have found that excellent results can be obtained.

According to the invention, therefore, there is provided a method of cross-linking a cross-linkable polyolefin insulation on an electrical conductor, which comprises contacting the insulation with a heated silicone oil in a long tube under pressure for a time and at a temperature so as to cause cross-linking of the polyolefin, wherein the silicone oil is one which is inert towards the polyolefin and which has a viscosity of from 150 to 500 centistokes measured at 25"C.

It is to be understood that the invention provides an improvement in the conventional process outlined hereinbefore. It is not applicable, for example, to recent rather different techniques such as that known as the "mini-pipe" process. The latter process uses a much shorter (e.g. 50 to 100 feet as opposed to 250 feet) curing tube which is electrically heated, and the internal diameter of the curing tube is very close to the diameter of the coated cable passing therethrough (whereas in the conventional process the internal diameter of the curing tube is usually rather larger than the coated cable diameter). The internal pressure in the tube in the "mini-pipe" process is much higher (e.g. of the order of 1000 psi) than in the longer curing tube of the conventional process.

In the method of the invention, the silicone oil preferably has a viscosity, measured at 25"C, of from 200 to 300 centistokes. Among the preferred silicone oils are the alkyl polysiloxanes, such as methyl polysiloxane, and the alkyl-aryl polysiloxanes such as methylphenyl polysiloxane.

After the insulation layer has been heated to effect cross-linking, it may then be cooled by, for example, using a silicone oil of viscosity (measured at 25"C) of from 150 to 500 centistokes, or by using an inert gas atmosphere such as nitrogen.

The cro:;s-linkable polyolefin insulation may be any cross-linkable polyolefin composition suitable as a cable dielectric and may be, for example, a cross-linkable elastomeric polyolefin copolymer.

The following Tests illustrate the use of various silicone oils for curing polyolefins.

TESTS The following tests were carried out in a static manner, on lengths of cables insulated with cross-linkable polyethylene, making use of the commercial product 'HFDB 4201' (Union Carbide) and using, as heat-transporter fluids, silicone oils of various viscosities at 25"C. The silcone oils used were Rhodorsil oils of the P47 series from Rhone Poulenc. These are pure methyl-polysiloxanes, and pure methyl-phenyl polysiloxanes (641V200). The cable lengths were kept stationary, in a bath of the silicone oil, for a period of time corresponding to the average time a moving cable would be kept in the cross-linking chamber in conventional practice, and also (for comparison) for much longer periods, at a temperature which would be usual in practice when cross-linking polyethylene, under a pressure as indicated using an inert gas (nitrogen). At the end of the tests, each cable was examined so as to ascertain whether or not voids were present.

The results are shown in the Table. These results show that the formation or non-formation of voids and discontinuities in the dielectric, is not due directly to the temperature, or to the cross-linking time, or to the pressure of the inert gas. It has been thought hitherto that these were important factors (in connection with void formation in rubber insulated cables) and, indeed, if it were not for this erroneous thinking, the problem of voids in polyolefin dielectrics might well have been resolved at an earlier date. The test results show that the dominant variable factor is actually the viscosity of the silicone oils. From the Table, it is evident that, not only is there a lower limit of viscosity, but also that it is not necessary at all, to have recourse to silicone oils of a high viscosity.

The advantages of avoiding high viscosities are significant, especially in energy saving for pumping, degassing and heating the oil. Furthermore, with low viscosity oils, a turbulent motion can be set up in the oil in the curing tube and inside any cooling chamber, so improving the heat transfer.

TABLE

EXAMPEL SILICONE VISCOSITY CENTISTOKES TIME AND TEMPERATURE PRESSURE VOLDS-FORMATION NO. OIL TYPE -at 25 C - OF CROSS-LINKING -N2- INDIELECTRIC Hours ... C Kg/cm2 1. 47 V 20 20 1 200 5 YES 2. 47 V 20 20 4 200 14 YES 3. 47 V 100 100 4 200 5 YES (rate) 4. 47 V 100 100 4 200 14 YES (rate) 5. 47 V 200 200 4 200 14 NO 6. 47 V 300 300 4 200 14 NO 7. 47 V 300 300 4 200 14 NO 8. 47 V 1000 1000 4 200 5 NO 9. 47 V 1000 1000 4 200 14 NO 10. 47 V 12500 12500 4 200 14 NO 11. 641 V 200 200 4 200 14 NO

Claims (11)

WHAT WE CLAIM IS:

1. A method of cross-linking a cross-linkable polyolefin insulation on an electrical conductor, which comprises contacting the insulation with a heated silicone oil in a long tube under pressure for a time and at a temperature so as to cause cross-linking of the polyolefin, wherein the silicone oil is one which is inert towards the polyolefin and which has a viscosity of from 150 to 500 centistokes measured at 250C.

2. A method according to claim 1 wherein the viscosity of the silicone oil is from 200 to 300 centistokes measured at 250C.

3. A method according to claim 1 or 2 wherein the silicone oil is an alkyl polysiloxane.

4. A method according to claim 3 wherein the silicone oil is a methyl polysiloxane.

5. A method according to claim 1 or 2 wherein the silicone oil is an alkyl-aryl polysiloxane.

6. A method according to claim 5 wherein the silicone oil is a methyl-phenyl polysiloxane.

7. A method according to any of claims 1 to 6 wherein after contacting the insulation with the heated silicone oil to cause cross-linking, the insulation is then cooled in a silicone oil having a viscosity of from 150 centistokes to 500 centistokes, measured at 250C.

8. A method according to any of claims 1 to 6 wherein after contacting the insulation with the heated silicone oil to cause cross-linking, the insulation is then cooled in an inert gas atmosphere.

9. A method according to claim 8 wherein the inert gas is nitrogen.

10. A method according to any preceding claim wherein the cross-linkable polyolefin insulation is a cross-linkable elastomeric polyolefin copolymer.

11. An electric cable having a cross-linked polyolefin insulation which has been crosslinked by the method of any preceding claim.