NO141871B - VULCANIZABLE SEMI-CONDUCTING INSULATION SHELTER FOR ELECTRIC conductors - Google Patents

Description

The present invention relates to vulcanizable semi-conductive insulating sheathing compounds containing ethylene vinyl acetate copolymer and carbon black.

Construction of insulated electrical conductors such as wires

and cables intended for medium to high voltage are known in the field and usually comprise a core conductor consisting of one or more strands of conductive metal or alloy such as copper or aluminium, a layer of semi-conductive insulating material, an insulating layer of e.g. cross-linked polyethylene and a layer of semi-conductive insulating mass on top of the insulation. Several neutral wires which are usually made of copper or aluminum can be embedded in or wound around the layer of semi-conductive insulating mass, if desired, in the form of a concentric ring around the insulated cable.

The insulating layer and its overlying semi-conductive insulating sheath mass are usually produced by a method called tandem extrusion, whereby these layers are formed in sequence from tandem extruders and cured simultaneously in a single operation to reduce the number of manufacturing steps. However, it leads to simultaneous hardening of the two layers

by heat and pressure so that the layers apparently mix in the contact surface and to the formation of cross-links in this contact surface. This seems to be the case even with the so-called two-stage method where the insulation is cured in a first step and the semi-conductive insulating mass is then extruded and cured on top of the insulation.

Formation of these cross-links between insulation and shielding makes subsequent separation of the two layers (insulation and semiconductor shielding), which occurs when splicing or end connections of wires and cables, very difficult and time-consuming. Such a strong bond between the layers also tends to make the semi-conductive layer leave a carbon residue on the insulation even when the layer is pulled off. A semi-conductive sleeve that can be easily and cleanly pulled off from the insulation on an insulated conductor is therefore highly desired in the area.

Previously known methods for the production of removable casings and insulations are described in the American patents no. 3,424,631 and 3,719,769, in the British patents no. 1,103,098 and 1,103,099 and in the German Offentlegungsschrift no. 2,117,247.

U.S. patent no. 3,424,631 describes an insulation containing ethylene and a maximum of 45% copolymerized vinyl acetate. The patent, however, mentions nothing about a content of 10 - 45% soot and also does not mention a content of "Vulcup" or "Varox", i.e. of a, a'bis(t-butylperoxy)diisopropylbenzene or 2,5-dimethyl-2 <1>,5<1->di(t-butylperoxy)hexane. This U.S. the patent describes the stripping of insulation from a metal sheath, while the desired goal according to the present invention is a semi-conductive sheath that can be stripped from a polymer insulation even if the two are cured at the same time.

U.S. patent no. 3,719,769 is directed to insulated electrical conductors where the semi-conductive insulation sheaths are derived from vulcanizable preparations containing (A) an ethylene vinyl acetate copolymer (with 25 - 55% vinyl acetate), (B) carbon black and (C) 2,5-dimethyl-2 <1>,5<1->di(t-butylperoxy)hexyne-3.

The use of only one cross-linking agent is described here

which is different from that used according to the present application. However, the only cross-linking agent described in this section is not always available.

Nor do the two British patents shed any light

the goal that is desired to be achieved with the present application, in that they do not use the intended cross-linking agents.

Regarding the latter document, German Offentlegungsschrift No. 2,117,247 applies substantially the same as stated for the U.S. patent No. 3,719,769.

The aim of the present invention is an easily peelable semi-conductive insulating sheath mass as a layer where the adhesion between the insulation and the sheath mass in an insulated electrical conductor is not over 5.7 kg/cm. It has now been discovered that such types of easily removable semi-conductive insulation sheathing compounds for combination with cross-linked polyethylene insulation can be made from vulcanizable semi-conductive insulation sheathing compounds, and the invention thus relates to such extensive, intended for

the total weight of the composition,

(a) 55-90% by weight, preferably 60-75% by weight ethylene vinyl acetate copolymer containing from 27-45% by weight and preferably 29-35% by weight vinyl acetate based on the weight of the copolymer, and

(b) 10 - 45% by weight and preferably 30 - 40% by weight

conductive soot, and the invention is characterized by the pulp as the only cross-linking agent containing from 0.2-5% by weight

of an agent selected from α,α'-bis-(tert-butylperoxy)diisopropylbenzene and 2,5-dimethyl-2',5'-di(tert-butylperoxy)

witch hazel and mixtures thereof.

Vulcanizable copolymers of ethylene vinyl acetate as well as methods for producing such mixtures which can be used according to the invention are known in the field. However, it has been discovered that for the production of a semi-conductive casing compound which can be easily pulled off from the cross-linked polyethylene layer, it is important to use an ethylene vinyl acetate copolymer which cannot be combined with the cross-linked polyethylene insulation. This incompatibility means in this connection that there is no physicochemical bond or affinity between the polyethylene and the ethylene vinyl acetate resin during extrusion and curing. The incompatibility gives a mutual repulsion between the resins which in turn prevents mixing and bond formation between the layers.

If a special vulcanizable semiconducting mass will

have such an incompatibility and will provide an electrical conductor insulated with cross-linked polyethylene having an easily peelable semi-conductive sheath can be found by measuring the adhesion between a laminate of cross-linked polyethylene and the cross-linked product of the vulcanizable mass, according to ASTM-D903. In order to be accepted as an easily removable casing, the binding force between the mass and the cross-linked polyethylene must not exceed

5.7 kg/cm when the force is measured according to the stated test method.

Thus, ethylene vinyl acetate copolymers in this connection should have from 27 - 45 and preferably 29 - 35 weight-% vinyl acetate based on the total weight of the copolymer, assuming that copolymers with less than 27 weight-% vinyl acetate will give casings that

has too strong a bond to the insulation of cross-linked polyethylene, while copolymers with more than 45% by weight of vinyl acetate can give too weak adhesion to cross-linked polyethylene. The amount of ethylene vinyl acetate copolymer contained in the vulcanizable casing according to the invention can be from 55 - 90% by weight,

and preferably 60-75% by weight, based on the total weight of vulcanizable mixture. Such ethylene vinyl acetate copolymers and/

or methods for producing such are well known in the field.

The use of conductive carbon black in semi-conductive casing compound

is well known in the area and all types of conductive soot in a suitable form can be used including flue soot, oil furnace soot or acetylene soot, provided that the soot is conductive. The amount of conductive soot in the vulcanizable mass according to the present invention can be between approx. 10-45% by weight, preferably 30-40% by weight, based on the total weight of the vulcanizable composition.

The only cross-linking agents used for semiconductor mixtures according to the invention consist of ot,a'-bis-(t-butylperoxy)-diisopropylbenzene ("Vulcup"), 2,5-dimethyl-2',5'-di (t-butylperoxy)hexane ("Varox") or mix-

ings of these. Although the preferred amount of cross-linking agent will vary depending on the particular ethylene vinyl acetate copolymer used and other such obvious factors, an

it is generally assumed that the amount of crosslinking agent will normally be 0.2 - 5 and preferably 0.6 - 2% by weight, based on the total weight of the vulcanizable semiconducting mixture.

It will of course be understood that these border areas possibly

will not be suitable for all possible masses according to the present invention, and that for a particular vulcanizable mass the use of cross-linking agents which give a

cross-linked product with an adhesion force of more than 5.7 kg/cm measured as defined above, to cross-linked polyethylene, will not be suitable, and that the relevant quantities must then be determined by routine experiments.

It will of course also be understood that vulcanizable semi-conductive insulation sheathing compound according to the invention can, if desired, contain other common additives in the quantities that are usually used in such compounds.

Examples of such additives are anti-aging auxiliaries, stabilizers, antioxidants, cross-linking accelerators and retarders, pigments, fillers, lubricants, ultraviolet stabilizers, anti-blocking agents and the like. The total amount of such additives that are usually used is at most approx. 0.05 - 3% by weight based on the total weight of the casing mass.

The term "cross-linked polyethylene" naturally includes insulation preparations which are derived from a cross-linked polyethylene homopolymer or a cross-linked polyethylene copolymer which has a comonomer content which does not adversely affect the desired result according to the invention. Normally, the cross-linked polyethylene insulation will be pure cross-linked polyethylene homopolymer. The use of polyethylene insulating compounds and semi-conductive casing compounds, the method of manufacture and the actual manufacture of insulated conductors are so well known that no further description is required here to enable those skilled in the art to understand how the polymer components are manufactured and used for the manufacture of insulated conductors. E.g. LD polyethylene compositions can, if desired, contain common additives such as fillers, aging inhibitors, talc,

clay, calcium carbonate and other processing aids as well as common cross-linking agents which are well known in the field, as well as common semi-conductive casing compounds. Insulated electrical conductors can also be produced in the usual way by e.g. tandem extrusion whereby the insulation layer is extruded over the conductor which is previously coated with a conventional extruded semi-conductive sheath followed by the vulcanizable layer on which the insulation and sheath layers are cured (cross-

are tied) simultaneously under pressure. Another common method consists in hardening the insulation layer before contact with the vulcanizable mass which is then itself hardened while the layer is in contact under pressure with said hardened insulation layer. It is believed to be beneficial to prevent any pre-mixing of the insulating mass and the vulcanizable mass before the mixtures harden since any such mixing could cause the cross-linking agent to affect the adhesion between the two layers by mutual cross-linking between the surfaces of the two layers. Other special aspects of insulated electrical conductors that use the present invention can also be the same as for ordinary insulated electrical conductors and are not decisive in that they depend for the most part on the desired area of use for the product.

The insulated electrical conductors using the present invention are unique in that the cross-linked insulating sheathing compound can be easily and cleanly pulled off, usually in one piece, from cross-linked polyethylene insulation.

The following examples illustrate the invention. All quantities, percentages and relative ratios stated are on a weight basis where nothing else is stated.

EVA - ethylene vinyl acetate copolymer

VA - % vinyl acetate by weight in the copolymer

MI - melting index

Dicup - di-a-cumyl peroxide

"Lupersol-130" - 2,5-dimethyl-2',5<1->di(t-butylperoxy)-hexyne-3 "Vulcup" - a,a 1-bis-(t-butylperoxy)-diisopropylbenzene

"Varox" - 2,5-dimethyl-2',5'-di(t-butylperoxy)-hexane Example_l_2_16

A series of vulcanizable semi-conductive masses were produced in which the weight % of vinyl acetate in the copolymer of ethylene vinyl acetate was varied in the same way as the cross-linking agents. The ingredients in each mixture are listed in Table I and each composition contained, in addition to the listed ingredients, 40% by weight of conductive carbon black and 0.4% by weight of polymerized 1,2-dihydro-2,2,4-trimethyl-quinoline , an antioxidant, and so that the quantities of all components in each preparation are based

on the total weight of the composition.

The compositions are produced by evenly mixing the component parts in a laboratory mixer of the "Banbury" type, and approx. 1300 g of each mass.

To assess the peelability of the compositions as semiconducting sheaths, each composition was used to prepare a laminate of polyethylene/ethylene vinyl acetate. The laminates were made from laboratory test plates where the polyethylene plate was in all cases produced by a. polyethylene homopolymer mass consisting of polyethylene homopolymer (98%), di-α-cumyl peroxide (2%) and bis(2-methyl,5-t-butyl-4-hydroxyphenyl)-sulphide (0.2%) as antioxidant.

In examples 1-9, the laminates were made of polyethylene/ethylene vinyl acetate by first casting polyethylene sheets (with dimensions 20 x 20 cm x 6.3 mm in thickness) at 175°C for 15 minutes and cross-linking the sheet, then vulcanizable sheets of ethylene vinyl acetate (20 x 20 cm x 3.2 mm thickness) cast separately, but not cross-linked, and laminates prepared by pressing each vulcanizable ethylene vinyl acetate sheet together with one of the cross-linked polyethylene sheets at 200°C and 14 kg/cm^

for 20 minutes during which the vulcanizable ethylene vinyl acetate mass was cross-linked.

In Examples 10-16, the polyethylene/ethylene vinyl acetate laminates were made in the same manner as above except that the polyethylene sheets were cast at 110°C so that they were not cross-linked. Cross-bonding of both boards took place at the same time as the laminate was formed.

The adhesion between the test plates (cut into strips

20 x 2.5 cm) was measured according to ASTM method D9 03 which measures the adhesion or pull-off strength between two plates in the laminate expressed as kg per cm and which is used as a measure of the degree of peelability of semi-conductive insulation sleeves made of ethylene vinyl acetate from cross-linked polyethylene insulation. The test results for each polyethylene/ethylene vinyl acetate sheath laminate prepared as discussed above are also listed in Table I.

Examples 5, 7, 9 and 12 - 15, which reproduce results for products according to the present invention, show the excellent pull-off capability of the present casing masses, and the casing layer in each example is pulled off cleanly and in one piece from the insulation layer.

Example_el_17

The example illustrates the manufacture of an insulated electric cable.

A common aluminum conductor was sequentially coated with a common semiconducting sheath (8.63 mm), a polyethylene insulating layer (6.78 mm) and a semiconducting insulating sheath (1.40 mm) consisting of polyethylene/vinyl acetate (29% vinyl acetate, MI - 20 ),

and 0.6% "Vulcup".

When manufacturing the cable, the extruded sheath and the insulation layer were cured in a steam vulcanization tube (17.5 kg/cm 2 steam pressure) before extruding the insulation sheath mass over the insulation, the sheath mass then being hardened by renewed passage through the steam vulcanization tube. This method is known in the art as two-stage extrusion.

Two parallel incisions in the sheath of the insulated cable were made 13 mm apart in the axial direction of the cable and the insulating layer underwent a tensile pull-off test to determine the adhesion or tack force of the insulation. The insulation sleeve was pulled off cleanly and in one piece from the insulation and exhibited a binding force of 34 - 46 kg/cm, which indicates excellent pull-off capability for the sleeve according to the invention.

An electrically insulated cable was made for comparison purposes, which was tested in the same way, using a mixture of vinyl acetate and 2% "Di-

cup" which produced a cable with a sheath that showed a binding force of 114 - 142 kg/cm and which did not pull off cleanly,

but in pieces from the insulation.

Example_18

The example illustrates the manufacture of an insulated electric cable.

A plain aluminum conductor was sequentially coated with plain semi-conductive sheath (0.63 mm), polyethylene insulating layer (6.78 mm) and a semi-conductive insulating sheath (1.40 mm) consisting of polyethylene/vinyl acetate (29% vinyl acetate, Mi - 20) and 0, 6% "Vulcup".

When manufacturing the cable, all three layers consisting of conductor sleeve, insulation and insulation sleeve were extruded one after the other and cured simultaneously in steam vulcanization tubes (17.5 kg/cm 2 ). This method can be described as one-stage triple extrusion.

The binding force for the insulation sleeve on the insulation above the cable was measured as described in example 17. The insulation sleeve was pulled off cleanly and in one piece from the insulation and had a binding force of 80 - 91 kg/cm which shows the outstanding pulling ability of the insulation sleeve according to the invention.

For comparison, an insulated electric cable was made which was tested in the same way, with the insulation sheath mass consisting of vinyl acetate and 2% "Dicup", which gave a cable with such a strong binding force between the insulation sheath and the insulation that the pull-off capacity could not be measured in the specified test since the two the layers were strongly fused at the contact surface.

Claims (2)

1. Vulcanizable semi-conductive insulating sheath composition comprising, based on the total weight of the composition, (a) 55-90% by weight, preferably 60-75% by weight ethylene vinyl acetate copolymer containing from 27-45% by weight and preferably 29-35% by weight vinyl acetate calculated on the weight of the copolymer, and (b) 10 - 45% by weight and preferably 30 - 40% by weight conductive carbon black, characterized in that the mass as the only cross-linking agent contains from 0.2-5% by weight of an agent selected from a , α'-bis(tert-butylperoxy)diisopropylbenzene and 2,5-dimethyl-2',5'-di(tert-butylperoxy)-hexene and mixtures thereof. 2. Mass according to claim 1, characterized in that it contains 0.6-2% by weight cross-linking agent.