GB2044165A

GB2044165A - Extruding cross-linked sheaths around elongate material, in particular electric cables

Info

Publication number: GB2044165A
Application number: GB8005407A
Authority: GB
Original assignee: KM Kabelmetal AG
Current assignee: KM Kabelmetal AG
Priority date: 1979-03-05
Filing date: 1980-02-18
Publication date: 1980-10-15
Also published as: JPS55121215A; BE882046A; AU5606780A; IN153905B; CA1144324A; FR2450684B1; GB2044165B; AU531328B2; AT368317B; ATA29780A; FR2450684A1; NL8001294A; DE2908457A1; DE2908457C2

Abstract

The sheath comprises one or more olefin polymers or copolymers crosslinked by means of one or more peroxides, the initially peroxide-free olefin (co)polymer being melted and homogenised in an extruder (2), from which it is delivered to the extrusion die (6) by a further extruder (5), one or more peroxide crosslinking agents being metered into the molten and homogenised material between the extruders (6,5), or in the further extruder (5). The process enables the melted polymer to be filtered, and degassed, prior to the addition of the cross-linking agent, thereby avoiding incipient cross-linking which would otherwise arise with shear heating of the agent(s) during filtering, and thereby also avoiding the distribution of the agent(s) being affected by the degassing. <IMAGE>

Description

SPECIFICATION Process for the continuous production of sheaths for elongate material, in particular electric cables The present invention relates to a process for the continuous production of a sheath for an elongate material from one or more olefin polymers or copolymers which can be crosslinked by means of one or more peroxides, in particular an insulation for an electric medium-voltage or high-voltage cable.

The increasing loading of existing cable networks and the transition to higher voltages, and indeed the demand for a higher operating voltage, have also entailed increasingly more strigentdemands on the quality of the cables. Thus, to prevent glow discharges in the cable insulation, which eventually can lead to breakthrough and finally to the destruction of the cable, particular value is attached to a cable insulation which is free from impurities. In the case of polyethylene, it is known to remove what has been called polyethylene tobacco, as well as metal particles and other dirt particles, by passing the polymeric material through a screen or filter.As temperatures of 130 C and higher are liable to occur in this screening or filtering step, as a result of, in particular, the shear to which the material is necessarily subjected, this measure intended to provide insulations which are free from impurities is not applicable if thermoplastic or elastomeric materials compounded with crosslinking agents are to be employed. The reason for this is that the increased pressure occurring when the material is screened, and the shear forces arising concurrently, cause an at least incipient crosslinking even in this state; this may for example occur in a melt ofcrosslinkable polyethylene if one or more crosslinking agents are present.This "scorch" effect entails a considerable loss in quality of the finished extruded material, so that hitherto the filtering-off of impurities present in the material has been entirely omitted in the case of materials containing crosslinking agents, or has been carried out merely with screens with very wide meshes. Particularly in the case of crosslinked materials intended for high-voltage applications, however, it is most important to apply an insulating material of high purity to the conductors. This purity can in the long rung only be effectively ensured by a fine-meshed screening or filtering step, since this enables both impurities pre-existing in polymer granules and impurities coming in during processing to be removed.

It is an object of the invention to make possible the use of a purification step, such as is known for the processing of thermoplastic materials, even in the processing of materials which contain crosslinking agents and with which there is a risk of premature crosslinking at initiation temperature determined by the choice of cross-linking agents.

According to the invention, we provide a process for the continuous production of a sheath for an elongate material, from one or more olefin polymers or olefin copolymers which can be crosslinked by means of one or more peroxides, characterised in that an initially peroxide-free olefin (co)polymer material is melted and homogenised, and, if the same operation but in a station which is spatially separate from the melting and homogenising station, is subsequently shaped, one or more peroxide crosslinking agents being metered into the molten and homogenised material before the shaping step.

This procedure has the advantage that the one or more crosslinking agents, which may for example be introduced in liquid form, meet a homogeneous melt in which they can be rapidly and thoroughly distributed, so that the intake and melting of the polymeric material are unaffected by their introduction and can be adjusted to give the optimum throughput without making allowance for the presence of any peroxide entailing a scorch risk.

Our starting point in evolving the invention was a type of equipment which is in itself known for certain other purposes (German Published Specification (Offenlegungsschrift) No. 2,458,776). The lattercomprises two extruders, usually disposed in an Lshaped arrangement, the first extruder being used for melting and homogenising and the second being used for discharging or final shaping of the polymeric material. In the case of the invention, one or more inlets for introducing one or more crosslinking agents are provided in a connection joining the two extruders and or in the second extruder. In preferred equipment, downstream of the first extruder, a screening or filtering device is provided, whereby (in use of the equipment) the material is transferred to the second extruder with a reduced impurity content.The production of insulations which are as free as possible from impurities can thus be ensured.

Moreover, in the case of the invention, extruder screw deposits due to antioxidants being washed off the surface of polymeric material granules can be avoided. Use can be made of an extruder screw comprising mixer components which promote rapid and homogeneous distribution of the one or more crosslinking agents.

In a preferred process according to the invention, impurities are filtered out of the molten and homogenised material before it undergoes shaping.

This has the particular advantages that a closemeshed screen or filter can be used, and then it is not necessary to make allowance for scorch risks due to increased shearing forces.

In another preferred process according to the invention, metering-in of the one or more crosslinking agents is effected intermediately, with regard to time and space, between the homogenisation and the beginning of shaping. In addition to the one or more crosslinking agents, one or more other additives, e.g. one or more stabilisers, or one or more antioxidants, can be added at the point(s) concerned.

Especially in a case in which, for example, degassing of the polymeric material is to be effected in a transition zone from a first to a second extruder, it has proved advantageous, in a further preferred process according to the invention, to meter in the one or more crosslinking agents during a pressure build-up in the shaping step. The degassing step then has no adverse effect on the distribution of the one or more cross-linking agents in the sheath produced.

The crosslinking agents which can be used include agents of the organic peroxide type. Thus use may be made of the agent known under the trade name Trigonox D 140; one may also use di-tert.-butyl peroxide, or the agent known under the trade name Perkadox 14 or dicumyl peroxide. Insofar as these additives, with the exception of di-tert.-butyl peroxide, are solid at room temperature, it has proved advantageous in a process according to the invention to introduce the one or more crosslinking agents in dissolved form. In this connection, it is particularly advantageous for the solution containing the one or more crosslinking agents to be a solution which also has a voltage-stabilising action or contains one or more voltage stabilisers. The latter are usually in dissolved form.Moreover, solvents for peroxidic crosslinking agents, which in numerous cases comprise aromatic hydrocarbons, may advantageously contain unsaturated groups, e.g. vinyl or propenyl, so that they can be grafted wholly or partially onto the polyethylene or other olefin (co)polymer macromolecules in the course of peroxidic crosslinking.

The peroxides or peroxide mixtures which can be used in the production of crosslinked polymeric materials in a process according to the invention include, as well as co-agents (e.g. dicumyl peroxide), peroxides having higher decomposition temperatures, e.g. di-isopropylbenzene monohydroperoxide or tert.-butyl hydroperoxide.

The invention will be explained in more detail with reference to the accompanying drawing, in which the single Figure is a diagrammatic representation of certain equipment used in carrying out a process according to the invention.

In the use of the equipment shown in the Figure, a polymer composition, for example a polyethylene composition, is introduced into a hopper 1 of a first extruder 2. The latter has a revolving screw 3 which initially effects mixing and subsequently effects melting and homogenising ofthe polymeric material. At an appropriate rate, the homogenised material is fed via a connection 4 to a second extruder 5, which acts as what may be called a discharge extruder. With the aid of the latter, the material delivered is applied to an electric conductor 7, for example a conductor for a high-voltage cable, in an extrusion head 6.

To ensure that the material applied is sufficiently free from impurities to satisfy the relevant requirements, a screening device indicated diagrammatically at 8 is provided, the homogenised material being passed through this screening device. Certain elevated temperatures are attained in the material, due to shearing forces, but these are immaterial inasmuch as the material is still, so far, free from crosslinking agents.

Instead of introducing one or more crosslinking agents together with the polymeric material, as was hitherto customary, an inlet is provided as indicated at 9 in the second or discharge extruder 5, through which inlet the crosslinking agent(s) can be directly introduced into the polymeric material which has been freed from impurities and already homogenised, in a phase of rising pressure. The one or more crosslinking agents are readily absorbed, distributed homogeneously through the polymeric material, and carried outwards through the extrusion head 6 together with the polymeric material, without a risk of premature crosslinking.

Claims

1. Process for the continuous production of a sheath for an elongate material, from one or more olefin polymers or olefin copolymers which can be crosslinked by means of one or more peroxides, characterised in that an initially peroxide-free olefin (co)polymer material is melted and homogenised, and, in the same operation but in a station which is spatially separate from the melting and homogenising station, is subsequently shaped, one or more peroxide crosslinking agents being metered into the molten and homogenised material before the shaping step.

2. Process according to claim 1, characterised in that impurities are filtered out of the molten and homogenised material before it undergoes shaping.

3. Process according to claim 1 or 2, characterised in that metering-in of the one or more crosslinking agents is effected intermediately, with regard to time and space, between the homogenisation and the beginning of shaping.

4. Process according to claim 1 or 2, characterised in that metering-in of the one or more crosslinking agents is effected during a pressure build-up in the shaping step.

5. Process according to claim 1,2,3 or4, characterised in that the one or more crosslinking agents are added in dissolved form.

6. Process according to claim 5, characterised in that the solution containing the one or more crosslinking agents is a solution which also has a voltage-stabilising action or contains one or more voltage stabilisers.

7. Process according to claim 1, substantially as described with reference to the accompanying drawing.

8. Equipment for carrying out a process according to claim 1, comprising two extruders and a connection whereby the extruders are joined to one another, characterised in that, in the connection and/or in the second extruder, one or more inlets are provided for introducing one or more crosslinking agents.

9. Equipment according to claim 8, characterised in that, downstream of the first extruder, a screening or filtering device is provided, whereby (in the use of the equipment) the material is transferred to the second extruderwith a reduced impurity content.

10. Equipment according to claim 8, substantially as described with reference to the accompanying drawing.

11. Process for the continuous production of a medium-voltage or high-voltage electric cable comprising a core enclosed in an insulating sheath, wherein the core is provided with its sheath by a process according to any of claims 1 to 7 or in equipment according to claim 8, 9 or 10.

12. An electric cable produced by a process according to claim 11.