CA1144324A

CA1144324A - Jacketing, coating, or enveloping elongated stock, preferably electrical cable

Info

Publication number: CA1144324A
Application number: CA000346904A
Authority: CA
Inventors: Hermann-Uwe Voigt; Hans-Peter Stehmann; Franz Danekas
Original assignee: KM Kabelmetal AG
Current assignee: KM Kabelmetal AG
Priority date: 1979-03-05
Filing date: 1980-03-04
Publication date: 1983-04-12
Also published as: JPS55121215A; BE882046A; AU5606780A; IN153905B; FR2450684B1; GB2044165B; AU531328B2; GB2044165A; AT368317B; ATA29780A; FR2450684A1; NL8001294A; DE2908457A1; DE2908457C2

Abstract

ABSTRACT OF THE DISCLOSURE A polyethylene blend is extruded through a fine mesh sieve to remove impurities; and the extrudate is fed to a second extruder whose head applies a coating to a conductor, or the like. A cross-linking agent is added to the material in the second extruder well upstream from the head.

Description

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D-6815 (Schnei) l BA Y~GROUND_O~ TI~I_IN_r.~TIOM

2~

3~ The present invention relates to continuously

4 covering ~nd jacketincJ elongated stock (cable, tubes, wires, etc.) with cross-linkable, mixed olefin polymerizate; cross-6 linking to be provided for by means of peroxides. More 7 particularly, the invention relates to making such linings, 8 jackets, or envelops for purposes of electrically insulating 9 medium- or high-voltage cables.

ll Existing cable networks are used to an increasing 12 extent, and new networks or extensions are operated more 13 and more frequently,at higher voltages. Accordingly, 14 the quality requirements for such cable are becoming l5¦ increasingly stringen-t, and specifications are raised for 16 I permitting operation of the cable at such higher voltages.
17 Aside from improving the quality of the insulation (electric 18 strength) generally, one has to consider specific problems l9 that may arise during operation at higher voltages. For 20~ example, glow discharges in the insulation have to be avoided 211 as they result, ultimately in voltage breakthrough and local 22~¦ destruction of the cable insulation. These glow discharges 23 I can be prevented by providing and using an insulation jacket 2a I which is free from impurities.
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D-6815 (Schnei) 1l In the case of thermoplastics, such as polyethylene, 21~ one has to remove metal and other dirt particles, e.g., 3ll by means of sieving. Such sievincJ, however, is not always 4I desirable because shear forces are set up in the material

5 1I duriny the process, particularly when subjected to pressure,

6 ¦ causing the temperature to rise to 130C and above. Aside

7 from possible softening, the sieved blend may contain

8 additives for cross-linking, and undue heating may result

9 in premature cross-linking. This so-called scorching or scorch formation reduces the qua]ity of the material, 11 particularly as far as its insulative properties are con-12 cerned. Thus, one has avoided sieving of blends which con-13 ¦ tain already cross-linking agents; or wide mesh sieves have 1~ I been used through which small dirt and metal particles may 15 ~ readily pass. This then leaves the problem of purity of 16 I the material unsolved, which problem becomes particularly 18 ~ pressing for high-voltage cable.
19 l 21 ~ - DESCRIPTION OF THE INVENTION
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231 It is an object of the present invention to provide 2~11 a new and improved method and equipmen-t for applying an 25~l impurity-free coating, jacket, or envelope to elongated stock.

2~ - -2-Il D-6~15 (Schnei) l¦l It is another object of the present invention to 2~1 provide a jacket, coating, or envelope to elongated stock 3l under conditions avoidillc3 premature cross-linking.
4l 5¦ It is a further object of -the present invention to 61 improve the quality of cable, having a cross-llnked polymer 71 jacket, particularly as far as insulative stren~gth is con-~ cerned.

I-t is a particular feature of the present invention ll to improve a coating method and equipment in which a first 12~ extruder melts and homogenizes a polymer, and feeds it to l3ll a second extruder which applies it to elongated stock.
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In accordance with the preferred embodiment of the 16 present invention, it is suggested to melt and homogenize 17 a polymerisate, preferably in an extruder but without inclu-l8 I sion of a cross-linking agent such as a peroxide. The melted 19¦ and homogenized material is fed to an applicator such as 20l a second extruder to coat, jacket, or envelop elongated 21¦ s-tock such as a cable. In order to avoid premature cross-22¦ linking, the requisite agent or additive, or at least one 23 1¦ Of these - such as peroxide --, is added to melted material 2~ upstream from the application of that material; preferably, 25 1l this acent is fed to an inlet of -the second extruder do~nstream 26~l from its reaular charge inlet. In order to remove, i.e., filter, 27 impurities (metal particles, etc.), a sieve is provided in 2~ -3-4~Z~
I D-6815 (Schnei) 1~ the first extruder and melted and homogenized polymerizate 21 (still free of peroxide) is forced throucJh that sieve.
3~
4I The cross-linking agent is, thus, added to a host 5I material which is molten and ihomogellized, and will uniformly 6 ~ blend into the melt quite rapidly. i~lelting and~homogeniza-7 ¦ tion is not interfered with by premature cross~-linking 8~ so that scorch formation is avoided. The sieve may be of a 9 very narrow mesh variety. Any shear forces can be tolerated;
they raise the temperature of the melt, but that enhances 11 homogeneity, still without scorching. The sieve will prevent 12 metal and dirt particles from being included in the final coatiny 1~l Two of us, and another inventor, have suggested a two-e~truder arrangement (see U.S. Patent 4,117,063) to be 16 used for silane-grafting, e.g., polyethylene, and applying 17 the material to elongated stock for subsequent in situ cross-18 linking in the presence of moisture. A similar kind of a 19 dual extruder arrangement can be used for practicing the present invention which, however, practices a different method 21 and is modified as outlined above.
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23~ The preferred embodiment of the invention, the objects 2a ¦ and features of the invention, and further objects, features 25 ~ and advantages thereof, will be better understood from the 26 I following description taken in connection with the accompany-27~ in~ drawings.
281 ~4~

3~4 Il D-6815 (Schnei) 1 D~SCRIPTION O~ T~[E D'~AI~ING

3 '['le fic,ure illustrates an e.;am~le for practicing the 4 best mode of the invention in accordance with the ~referred embodiment tnereof.

7 A polymer blend, e.g., a blend of differ~ent density 8l polyethylenes, or just regular polyethylene, is charged into 9 a funnel or hopper 1 of an extruder 2. The charge is pre-10l ferably granulated and may already contain some additives, 111 but positively not a cross-linking agent such as a peroxide.
12~
13¦ The extruder worm 3 enhances blend homogeneity and 1~¦ increases its temperature so that downstream from the funnel, 15¦ the blend will melt, and is continued to be homogenized.

17 The outlet and discharge nozzle 4 of this extruder 2 18 is connected to a second extruder 5, having a head 6 con-19 structed with a ring nozzle for applying a continuous coating onto a wire lead, cable 7, etc., passing through. Generally 21 speaking~ the dual extruder arrangement may be as shown in 22 Patent 4,117,063 mentioned earlier.

24 ~s a specific feature of this arrangement, a fine mesh sieve 8 is interposed between the chamber of extruder 2 26 and the connecting nozzle 4. Extruder 2, there~ore, forces ! .

~ 3~4 I¦ D-6~15 (Schnei) 1 molten and hi~hly homocJeniz.ed material through the sieve.
2 Basically, the sieve can be placed anywhere, but it should 31 be placed to be passed throuqh by completel~ liquified, 4~ flowing material; and the best place here is at the end of 5~ the barrel of extruder 2. The sieve Eurther introduces 6 ~ shear forces into the material, raisin~ its temperature 7 further to some extent; but that is not detrimental at all 81 as the blend is still free from at least one agent for 9¦ the cross-linking.

11 The cross-linking agent is added to the blend in 12 extruder 5. An inlet nipple 9 is provided for feeding the 13 cross-linking agent to the blend, to be blended into the 1~1 material as advanced by the extruder worm toward head 6.

16¦ The material is, therefore, very clean and free from 17¦ impurities when the cross-linking agent is added. This 18 additive is readily blended into the liquidous material 19 ¦ which is homogenized as to this additive before reaching 20 ¦ head 6. Premature cross-linking is avoided.

22 The cross-linking agent, such as a peroxide, is added 23 to the blend or molten polymer at a location in which pres-24 sure is build up in the barrel of the second extruder, 5.
Eroadly speaking, the cross-linking agent is added at a point 26¦l in which the material is molten and well homogenized, but well 228~ -6--~ 4 3 D6815 (Schnei) 1 upstream fro~ the applicator (head 6) since the cross-linking 2 agen~ must be intimately blended with the material before 3l beiny ap?lied.

This supplement:al feeding of a cross-linking agent 6 (pero~ide) may be accompanied by adding other substances to the blend at the same location, such as stabilizers, anti-81 oxidants, etc. An antioxidan-t when blended with the polymer 9j granules is often washed off in the first extruder and forms

10~ a coating. ~owever, if the antioxidant is fed to the fully

11 melted plastic, this problem is readily avoided. Extruder 5

12 should be constructed -to include blender parts to ensure

13 that the additives fed to the inlet are, indeed, rapidly 1~ mixed with the polymer.

16 The connecting part, 4, between the two extruders may 17 be provided with a degassing opening, or the like, at a point 18 upstream from the adding of the cross-linking agent. As 19 stated, the latter is preferably added to the extruder in a 20 ¦ pressure build-up zone so that degassing of the polymer is 21 I not interfered with, and the latter does not interfere with 22¦1 the adding and distribution of the cross-linking agent.
23 , 2~ ~ The most commonly used polyolefin is polyethylene and, 25 , as stated, one may use a blend of high-, and/or medium-, 26¦ and/or low-density polyethylene. The cross-linking agent 2'~1~
281'- _7_ Il ~44324 D-6815 (Schnei) 1 is preferably an organic perox:ide of, for example, the type ~1~ traded under the designation TRIGONOX D 140 or a peroxide of the Di-tert-butyl-peroxide variety. Still another suitable 411 peroxicie is ,~nown as dicumyl peroxide or PERY~DOX 1~. ~11 of 5 i! these peroxides can be used sinqly or in suitable blends; the 6j invention is directed toward applying these peroxides effec-7~ tively without the side effects of premature cross-linkillg, 8j independently from the specific peroxide used.

The peroxides mentioned above (except Di-tert-bulyl 11 peroxide) are all solid at room temperature. It is thus 12 advisable and appropriate to solve the additive prior to 131 its application to the melted PE. This, in turn, lends 1~ itself to the use of a solvent t~hich does also act as a voltage stabilizer in the material after having been applied there-16 with to the conductor, 7. Alternatively, a voltage stabi-17 lizing additive may also be solved in the same solvent, 18 containing ~he peroxide.

The solvent for the peroxide is preferably a hydro-21 carbon with aromatic rings. Such a solvent may further 22 contain unsaturated groups of vinyl, propenyl, or the like.
23 I They will be grafted upon the polyethylene macromolecules 24 I pursuant to the peroxide cross-linking.

~71 ~ 24 D-6815 (Schnei) l In lieu of the peroxides or peroxide blends mentioned 2 above, one may use peroxides havi.ng a higher .temperature of decompositioning; for example, di-isopropyl-benzelle-monohydro-4¦ peroxide or tert-butyl-hyclro-peroxide.

6 ¦ , The invention is not limited to the embodiments 7 ¦ described above; but all changes and modifications thereof, 81 not constituting departures from the spirit and scope of 9 the inven-tion, are intended to be included.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Method of coating, jacketing, or enveloping elongated stock, comprising the steps of feeding an olefin polymerizate to an extruder, without the addition of a cross-linking agent, for melting and homogenization therein; filtering and extruding the polymerizate by means of said extruder to obtain a homogenized liquidous blend; feeding the resulting extrudate into a second extruder having a head for applying a jacket, coating, or envelope to elongated stock as fed through the head; and adding peroxide to the extrudate in the second extruder, upstream from the head so that the extruder homogeneously blends the peroxide in the extrudate

2. Method according to claim 1, wherein the adding step includes concurrently adding a voltage-stabilizing component.

3. Method according to claim 1 or 2, wherein the peroxide is added in a liquidous solution.

4. In an apparatus for applying a jacket, coating, or envelope to elongated stock, the apparatus including a first extruder for melting and homogenizing a polymerizate, and having a discharge outlet, further including a second extruder connected to the discharge outlet and having a nozzle head through which the stock is fed, the head provided for applying the material as extruded by the second extruder to the stock, the improvement comprising:
the first extruder including a sieve through which melted and homogenized polymerizate is forced for removing impurities prior to feeding the polymeri-zate into the second extruder; and means for feeding peroxide to the polymer-izate when in the second extruder, the means being disposed downstream from the sieve and from an inlet of the discharge outlet means, but well upstream from said head.