INSULATION COMPOSITION FOR CABLES

Abstract: Insulation composition for cables comprising a homopolymer of ethylene or a copolymer of ethylene and an alpha-olefin and an ethylene-acrylate copolymer. The compositions have very good thermal and electrical stability properties. They are preferably used for medium and high voltage cable. The insulation composition is preferably cross-linked.

Description: Insulation composition for cables The present invention relates to an insulation com¬ position particularly suitable for medium and h gh vol¬ tage cables. The composition . which is cross-linkable or 5- cross-linked, consists essentially of a com ination of a homopolymer of ethylene, or a copolymer of ethylene and an a Ipha-o Iefi n, and an ethy Iene-a I ky I acrylate or ethylene- alkyl ethacrylate copolymer. The compositions of the in¬ vention have very good thermal and electrical stabi lity ■ 0. properti es . The development in the field of power cables goes towards higher and higher service temperatures and higher demands are thus made on the abi lity of the insulation material to withstand these and to withstand increased, 15. very high, temperatures due to overload and short-circuit currents. It is further extremely important that the mat¬ erial can withstand formation of so-called water trees, which are initiated, among' other things-, from sites with specially high electrical stress, but will grow at a lower 20. stress, "and which gradually can lead to breakdown a.nd con¬ siderably reduce the service life of the wire or cable. Polyethylene is widely used as a cable insulating material. On the other hand, copoLymers of ethylene and polar materials such as vinyl acetate and ethylene acrylate 25. are essentially used only as jacketing material and only to an insignificant extent as insulation material, and then only for low voltage purposes. The thermomechanical stability is generally improved by cross-linking and cross- linked materials are thus predominantly used. CopoLymers 30 • of ethylene and alpha-olefins, linear ethylene polymers, have, as such, better thermomechanical properties than conventional branched homopolymers of correspond ng density. Se eral additives have been suggested in order to solve the problems of water treeing, eg different si lanes. It 35. has also been suggested to add an ethylene-vinyl acetate copolymer as a water treeing inhibitor to polyethylene. According to the present invention it has been found that compositions of ethylene polymers and e hy Iene-a I ky I acrylate or ethylene-al yl methacrylate copoLymers have very good thermal properties and that they are particularly suitable as insulation mater al for high voltage wire and ca.ble, conferring resistance to water tree formation. The compositions of the invention are uti lized cross- 5. linked on the cables. They can be cross-Linked by radiation or by chemical cross-linking without risk of degradation. A particular advantage is that they can stand the very high cross-linking temperatures which are utilized in the more recent cross-lin ing processes such as in dry cross- 10. l nking e.g. in nitrogen atmosphere, without being degraded and giving rise to corrosive degradation products. The present nven ion thus relates to a cross-linkable or cross-l nked cable insulation composition, for medium and high voltage cable, which consists essentially of an 15. ethylene polymer, said ethylene polymer being a homopolymer of ethylene or a copolymer of ethylene and an alpha-olefin, and an ethy lene-a IkyI acrylate or an ethylene-a IkyI meth¬ acrylate copolymer n an amount of 10 to 40 per cent by weight based on the combination of ethylene polymer and 20. acrylate polymer. The invention relates particularly to a composition as stated above which contains a cross-Li nking agent . The base polymer in the present compositions is a homopolymer of ethylene or a linear copolymer of ethylene 25. and one or several alpha-olefins. The latter ethylene poly¬ mers are essentially linear and are produced by copolymer- ization of ethylene and alpha-olefins hav ng 3 to 14 carbon atoms. Generally 1 to 20 per cent by weight of the copolymer is u'sed. The ethylene polymer in the present compositions 30. su tably has a density which does not exceed 940 kg/m , as non-cross-linked material, and the density is suitably within the range of from 915 to 935 kg/πτ3. The melt index for the ethylene base polymer, MI , (measured according to ISO 1133, 2.16 kg weight at 190°C) should preferably 35. be with n the range of 0.1 to 20 g/10 min., and most prefer¬ ably ithin the range of from 0.5 to 10. The base polymer in the composi ons is preferably a homopolymer of ethylene, and preferably high pressure pol ethylene. The acrylate copoLymers n the compos tions are co- polymers of ethylene and alkyl acrylate or alkyl methacryl^ ate wherein the alkyl groups usually have 1 to 8 carbon atoms, eg methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, i-butyl acrylate, methyl methacrylate, 5 _ ethyl methacrylate. CopoLymers of two or more of the men¬ tioned acrylates or methacrylates may of course also be used. The copoLymers can be present in amounts of 10 to 50 per cent by weight and suitably in an amount of 25 to 40 per cent based on the combination with the ethylene 10. base polymer. Ethylen.e-alkyl acrylate copoLymers are prefer¬ red and espec ally copoLymers of ethylene and n-butyl acryl¬ ate which have especially satisfactory thermo-oxi dative stabi lity in contact with copper. The acrylate copolymer suitably has a melt index of 2 to όO g/10 min. (MI 2 measured 15. as stated earlier) . The content of acrylate in the copolymer should suitably be within the range of from 9 to 50 per cent by weight. Higher acrylate contents in the copoLymers seem to lead to shorter trees and lower tree dens ty, and thus a copolymer with an ac.rylate content of 15 to 35 per 20. cent by weight is preferably used, and most preferably a copolymer with an acrylate content within the range of from 18 to 30 per cent by weight. In the present composi¬ tions the acrylate copolymer is present in an amount of 10 to 40 per cent by weight and preferably in an amount 25. of 10 to 35 per cent by weight. The mentioned values are per cent of the acrylate copolymer based on the combination of this and the ethylene polymer. The insulation composition is particularly suitable for medium and high voltage cable and hereby is meant wire 30. and cable for an electrical field strength above 1 kV/mm and particularly above 1.8 kV/mm. The compositions according to the invention are used in a cross-linked stage on the cables and they can be cross- - 1 inked in a known manner by radiation cross-linking or 35. using che ical cross-lin ing agents. Chemical cross-linking agents are conventional such agents such as organic per¬ oxides and azo compounds and also si lanes. Che ical cross- linking agents can be present in the compositions of the . invention or be added afterwards, at the cable production. Preferred compositions contain an effective amount of a cross-linking agent selected from organic peroxides, azo compounds and s lanes. Organic peroxides are preferred and as examples of such can be mentioned dicumyl peroxide 5. and di-tert. butyl peroxide. The required amount of cross- link ng agent depends among other th ngs on the desired degree of cross-lin ing, the type of cross-linking agent and the type of ethylene base polymer. When the base mater¬ ial is polyethylene the amount of cross-linking agent s 10. suitably within the range of from 0.5 to 6 per cent by weight, based on the combination of ethylene base polymer and acrylate copolymer. For linear ethylene polymers smaller amounts of cross-linking agent can be used to obtain the same degree of cross-lin ng and the amount is here suitably 15. within the range of from 0.2 to 5 per cent by weight. Particularly preferred compositions according to the present invention comprise polyethylene as base poly¬ mer, an ethy lene-a Iky I acrylate copolymer and a cross-link¬ ing agent, preferably an organic peroxide. 20. For high voltage cables an outer bonded semicon¬ ducting layer of an ethy lene-a Ikylac ry late copolymer is often used. The present compositions give the highest com¬ patibility with such layers and thus the highest adhesion. Medium and high voltage cables with a bonded outer semicon- 25. duct ng layer of an ethylene-acrylate copolymer and an insulation of the present composition will thus have the best possible total electrical properties. If an eth lene-vi ny I acetate copolymer, or a non- -acrylate copolymer, is used in the outer semiconducting 30. layer for strippable such layers, which are sometimes de¬ sired, the strippability ill presumably be improved in combination with the present insulation composition. The present insulation compositions consist essential¬ ly of the ethylene polymer and the ethylene-acrylate co- 35. polymer as previously defined. Even if other polymeric materials may be included the compositions do preferably not contain any other polymeric materials, and at least not more than 10 per cent by weight of such materials. The compositions can of course contain add tives w ich are conventional for this type of materials such as anti- oxidants, cross-linking accelerators and cross-linking reta rdant s , • other additives to inhibit water treeing, pro- rads, field strength regulators etc, which are used in 5. minor amounts w th regard to the base composition. Inorganic materials, for example fi llers such as aluminium hydroxides, kaolin and antimony o ides, should preferably not be present in the compositions, and at least not in higher amounts than 2 per cent by weight as most such materials have a 10. negative effect on dielectric losses in higher amounts. The compositions can be prepared in a conventional manner by ixing in conventional, batch or continuous, mixing equipment. The added ethylene-acrylate copolymer can suitably be used as a carrier for other additives such 15. as cross-linking agents, antioxidants etc. The composit ons are applied to a metal conductor, over the on this usually present semi-conducting layer, as insulation. One embodiment of the present invention also relates to an insulated cable comprising a metal con- 20. ductor, a semi-conducting layer and an insulation of the present composition and an outer semiconducting layer ex¬ truded over this and then cross-linked. The compositions can be cross-linked and at this the recently developed and increasingly uti lized high temperature cross-linking 25. processes, dry cur ng processes where the temperatures are at about 250°C or higher, can be used. In contrast to other types of copolymers, such as ethy Iene-vi ny I acetate copolymers, the present compositions with acrylate copoly¬ mers can be treated at these high temperatures without 30. any substantial degradat on and without release of corrosi e degradation products. This means that insulation according to the invention has an extremely good resistance to thermal degradation and thus an increased service life. Insulation according to the invention also has a very good electrical 35. stabi lity and reduced or inhibited water tree formation. The invention is further i llustrated in the following examples, which, however, are not intended to limit the same. Parts and per cent relate to parts by weight and per cent by weight, respect ely, unless otherwise stated. Example __ The ability of compositions according to the invention to withstand formation of water trees was examined by test¬ ing on Rogowsky objects. The presence of so-called bow 5. tie trees and so-called vented trees were examined and also the average size of the longest trees. Bow tie trees grow from irregularities inside the nsulation wh le vented trees grow from the interface between insulation and semi¬ conductor. Rogowsky objects give a homogeneous electrical 10. field as in a cable in ser ice. The objects look Like small containers and from the earth side there is an aluminium sheet serving as a water diffusion barrier and earth elec¬ trode. Next to this sheet there is a semiconductor and the two sheets are moulded together. Next to the semicon- 5 _ ductor is the insulation, prepressed from extruded tape. Upon the nsulation a water container s placed and the water is connected with a " high voltage generator. The con¬ tainer, insulation and semiconductor with the aluminium sheet are assembled in a mould and pressed and the insula- 0. tion is cross-lin ed. The samples are conditioned three days at 90°C in an oven with air circulation. The objects are then subjected to an electrical stress of 6kV/mm at 90°C for 3 weeks. Slices are then microto ed from the ob¬ ects, dyed and examined in a microscope. 25. The following compositions were exam ned as described above (In all compositions the base polymer was a poly¬ ethylene with a density of 922 kg/ and a I of 2): a) 70 % polyethylene and 30 % of an ethylene-n-butylacrylate copolymer (MI2 = 4) with an acrylate content of 27 %. 30. b) 81.5 % of polyethylene and 18.5 % of ethy Lene-n-butyl- acrylate copolymer according to a) c) 70 % of polyethylene and 30 % of an et ylene-n-butyL- acrylate copolymer (M1 = 4) ith " an acrylate content of 17 %. 35. d) 75 % of polyethylene and 25 % of ethyIene-n-but LacryLate copolymer according to a) Comparison was made with the polyethylene base polymer ( e) ref.) and also with an ethylene-ethyLacryLate copolymer containing 1.8 % of ethylacrylate (f) ref.). AIL samples were cross-linked with peroxide and tested in the cross? Linked stage . The results are shown in the following table. The tree density has been classified according to a scale where 5. 0 = 0 trees per cm 3 , 1 = up to 300 trees, 2 = 301 to 800 trees, 3 =801 to 1500 trees, 4 = 1501 to 2000 trees and 5 = more than 2000 trees. Compos i t i on Bow tie trees Vented trees d e n s i t y s i : z e ^m ddensity size jo 10 aa)) 0 0--11 ≤ ≤1100 0 0 0 b ) 0- 1 35 0 0 c ) 1 25 0 0 d ) 0- 1 1 5 0 0 e ) r e f . 3 90 2 180 15. ff))rreeff.. 1 1--22 2 277 1 1--22 35 It should be noted that all compositions according to the invention were completely free from vented trees while both the reference compositions showed high amounts of such trees and also higher amounts of bow tie trees. 0 It should also be mentioned that most commercial cross-link¬ ed polyethylene cable insulations at these test show a bow tie tree density according to the above scale of 3 to 5.

Claims: C l a i s 1. Crdss-liπkable or cross-linked cable insulation composition for medium and high voltage cables characterized in that it consists essentially of an ethylene polymer, 5. which is a homopolymer of ethylene or a copolymer of ethyl¬ ene and an a Ipha-o lefi n, and an ethylene-alkyI acrylate or ethy lene-a I kyI methacrylate copolymer in an amount of 10 to 40 per cent by weight, based on the combination of ethylene polymer and acrylate copolymer. 10. 2. A composition according to claim 1, cha acterized in that it contains a cross-l nking agent. 3. A composition according to claim 2, characterized in that the cross-linking agent is an organic peroxide. 4. A composition according to claim 1, characterized 15. n that it contains 10 to 35 per cent by weight of acrylate copolymer, based on the combination of ethylene polymer and acrylate copolymer. 5. A composition according to any of the preceding claims, characterized in that the acrylate copolymer is 20. ethylene-n-butyI acrylate copolymer. 6. A composition according to any of the preceding claims, characterized in that the acrylate content in the acrylate copolymer is within the range of from 9 to 50 per cent by weight. 25. 7. An insulated cable comprising a metal conductor, a sem conducting layer and an insulation of a composit on consisting essent ally of an ethylene polymer, which is a homopolymer of ethylene or a copolymer of ethylene and an a Ipha-olefin, and an ethylene-a IkyI acrylate or ethyl- 30. ene-alkyl methacrylate copolymer in an amount of 10 to 40 per cent by weight, based on the combination of ethylene polymer and acrylate copolymer, and an outer semiconducting layer extruded over this and then cross-li nked .

Inventors: FARKAS ANDRAS ANTAL, FARKAS, ANDRAS, ANTAL
Applicants: UNIFOS KEMI AB
Application year: 1985
Main classification: H01B3
IPC codes: C08K5, C08L23, H01B3