COMPOSITION FOR ELECTRIC CABLES

Abstract: A peroxide-cross-linkable ethylene polymer composition for an insulating layer of an electric cable is described. The composition is characterised in that the additives of the composition comprise an N-substituted 2,2,6,6-tetramethylpiperidine compound as an antioxidant and light stabilising agent; and that the composition after 21 days at 135 DEG C has a retained ultimate tensile strength of at least 75 % and a retained ultimate elongation of at least 75 % when tested in accordance with IEC 811. This additive acts as a combined light and thermo-oxidative stabilising agent and inhibits the generation of moisture thereby reducing the risk of water tree formation. Preferably, the composition contains no conventional antioxidants, such as phenolic antioxidants, organic phosphite oxidants and sulphur containing anti-oxidants.

Description: COMPOSITION FOR ELECTRIC CABLES Field of the Invention The present invention relates to a composition for electric cables, more particularly an ethylene polymer composition for the insulating layer of an electric cable, preferably a medium, or high or very high voltage electric power cable. The composition comprises an ethylene polymer and additives, including a peroxide cross-linking agent and stabilising agents. Background of the Invention Electric cables and particularly electric power cables for medium voltage (MV; 1-35 kV) , high voltage (HV; 35-500 kV) and extra high voltage (EHV; >500 kV) may be composed of a plurality of polymer layers extruded around the electric conductor. In power cables the elect- ric conductor is usually coated first with an inner semiconductor layer followed by an insulating layer, then an outer semiconductor layer followed by water barrier layers, if any, and on the outside a sheath layer. In addition, some HV and EHV cables are enclosed in a tube, usually of aluminium. The layers of the cable are based on different types of ethylene polymers, which usually are crosslinked. Crosslinked ethylene polymers are used for the insulating layer of electric cables. By the expression "ethylene polymer" is meant, generally and in connection with the present invention, a polymer based on polyethylene or a copolymer of ethylene, wherein the ethylene monomer constitutes the major part of the mass. Thus, ethylene polymers may be composed of homopolymers or copolymers of ethylene, wherein the copolymers may be copolymers of ethylene and one or more monomers which are copolymerisable with ethylene or graft copolymers. LDPE (low-density polyethylene, i.e. polyethylene prepared by radical polymerisation at a high pressure) is today the predominant cable insulating material. As mentioned above the ethylene polymer may be an ethylene copolymer, and in that case it includes from 0 to about 25% by weight, pre- erably about 1-20% by weight of one or more comonomers which are copolymerisable with ethylene. Such monomers are well known to those skilled in the art and no extensive enumeration will be required, but as examples, mention can be made of vinylically unsaturated monomers, such as C 3 -Cs alpha olefins, for instance propene, butene; dienes, for instance 1, 7-octadiene, 1, 9-decadiene; vinylically unsaturated monomers containing functional groups, such as hydroxyl groups, alkoxy groups, carbonyl groups, carboxyl groups and ester groups. Such monomers may consist of, for instance, (meth) acrylic acid and alkyl esters thereof, such as methyl-, ethyl- and butyl- (meth) acrylate; vinylically unsaturated, hydrolysable silane compounds, such as vinyl trimethoxysilane; vinyl acetate etc. However, if the ethylene polymer is an ethylene copolymer the amount of polar comonomer should be kept low, such that the polar comonomer comprises at most 10% by weight of the ethylene polymer in order not to increase the dissipation factor too much. Besides the additives described in more detail below, the remainder of the composition according to the present invention is made up of the ethylene polymer specified above. This means that the amount of ethylene polymer in the composition should lie in the range from about 95% by weight to about 99.7%, preferably about 96-99% by weight of the composition. In order to improve the physical properties of the insulating layer of the electric cable and to increase its resistance to the influence of different conditions, the ethylene polymer contains additives the total amount of which usually is about 0.3-5% by weight, preferably about 1-4% by weight. These additives include stabilising additives such as antioxidants to counteract degradation due to oxidation, radiation, etc.; lubricating additives, such as stearic acid; additives for water-tree resistance, such as polyethylene glycol, silicones etc.; and crosslinking additives such as peroxides which decompose upon heating and initiate crosslinking of the ethylene plastic of the insulating composition, optional-ly used in combination with unsaturated compounds having the ability to form crosslinks when initiated by radical forming agents. In electric cables of the type described above the presence of water or moisture should be avoided, particularly in the insulating layer, because of its detrimental effect on the properties of the cable. Moisture leads to the formation of dendritically branched defects, so-called water trees, which in turn can lead to breakdown and possible electric failure. The risk of formation of water trees is higher the higher the voltage of the cable. It is therefore a strong desire to minimise and if possible eliminate moisture from electric cables, espe- cially electric power cables (MV, HV and EHV cables) . Moisture in electric cables may either be derived from moisture in the ambient atmosphere that migrates into the cable or moisture that is generated in situ in the cable due to chemical reactions. In electric cables with peroxide-crosslinked polymers, such as peroxide-crosslinked ethylene polymer insulating layers, moisture is generated due to decomposition of the peroxide and interaction with additives in the polymer. The prevailing peroxide-crosslinking agent is dicumyl peroxide, which during crosslinking inter alia gives rise to cumyl alcohol, which in turn is prone to decompose to α-methylstyrene and water. This reaction is strongly catalysed by acids, i.e. the decomposition and formation of water is strongly increased if the polymer composition of the insulating layer contains acidic substances. Antioxidant additives in polymer compositions of electric cables are usually sulphur containing com- pounds that due to oxidation and decomposition form acids, such as sulphenic acids, and these acidic substances strongly influence the decomposition of peroxide to formation of water and decomposition products such as α-methylstyrene . In order to minimise or inhibit moisture in peroxide-crosslinked polymers of electric cables, such as an peroxide-crosslinked ethylene polymer of the insulating layer of an electric cable, it is therefore essential that the generation of moisture due to peroxide decomposition should be decreased as much as possible. Summary of the invention It has now been found that generation of moisture due to peroxide decomposition can be substantially reduced with retention of excellent ageing resistance by using certain hindered amine light stabilising (HALS) agents as a combined antioxidant and light stabilising agent while excluding any conventional antioxidants, such as phenolic antioxidants, sulphur containing antioxidants and organic phosphite antioxidants. Surprisingly, the HALS compound acts not only as an effective light stabilising agent, but also as an effective antioxidant making it possible for the composition to pass stringent requirements for thermo-oxidative stability in spite of the fact that the composition contains little or no conventional antioxidants. More particularly, the present invention provides a peroxide-crosslinkable ethylene polymer composition for an insulating layer of an electric cable, which composi- tion contains up to about 5% by weight of additives including a peroxide crosslinking agent and stabilising agents, characterised in that the stabilising agents comprise an N-substitued 2,2,6,6-tetramethylpiperidine compound as an antioxidant and light stabilising agent; and that the composition after 21 days at 135°C has a retained ultimate tensile strength of at least 75% and a retained ultimate elongation of at least 75% when tested in accordance with IEC 811. Other distinguishing features and advantages of the invention will appear from the following specification and the appended claims . Detailed description of the invention While as indicated above sulphur containing antioxidants are prone to form acidic substances on oxidation and decomposition which accelerate moisture formation by peroxide decomposition, it has been found that certain N-substitued hindered amine stabilisers comprised of 2,2,6,6-tetramethylpiperidine compounds can be used as antioxidants that do not form acidic substances and thus do not contribute to moisture generation but at the same time give excellent ageing resistance. The 2,2,6,6-tetramethylpiperidine compounds are preferably used alone as antioxidants. Different 2, 2, 6, 6-tetrametylpiperidine compounds may be used singly or in combination with each other as stabilising agents in the composition according to the present invention. Preferably, the composition includes little or no conventional antioxidants. This means that the combined amounts of conventional antioxidants, such as phenolic antioxidants, organic phosphite antioxidants and sulphur containing antioxi- dants are at most 0.15% by weight of the composition, preferably at most 0.10% by weight of the composition. Most preferably the composition does not contain any such conventional antioxidant at all. The 2,2,6,6-tetramethylpiperidine compounds can be incorporated in the ethylene polymer composition by compounding together with other additives, such as peroxide crosslinking agent, lubricating additives, additives for water tree resistance, etc. Generally, the total amount of antioxidant (s) should lie in a range of about 0.1-1.0% by weight, preferably about 0.1-0.5% by weight . As indicated above, the 2,2,6,6-tetramethylpiperidine compounds of the present invention not only act as effective light stabilising agents, but surprisingly also as very effective antioxidants providing thermo-oxidative stability to the composition. The thermo-oxidative stability provided by the N-substituted 2,2,6,6-tetramethylpiperidine compound is usually sufficient for the requirement of an electric cable insulating layer composition, so that no other antioxidants are required for thermo-oxidative stability. That the 2,2,6,6- tetramethylpiperidine compound alone is able to provide sufficient thermo-oxidative stability is particularly surprising in view of the fact that the requirement for thermo-oxidative stability is very rigourus for electric cables which have a service life of about 30-40 years. The thermo-oxidative stability is determined according to the International Standard IEC 811. According to IEC 811 dumbbell test pieces are made of the composition to be evaluated and are tested for thermo-oxidative ageing. Normal test temperature is 135°C but the testing has been performed also at 150°C. The ultimate tensile strength at break and the ultimate elongation at break of the composition are determined before the testing is started and thereafter at predetermined time intervals. The results are expressed as percent retained ultimate tensile strength at break (RUTS) and percent retained ultimate elongation at break (RUE) , the initial values (ageing time 0 days) being given as 100%. The requirement according to IEC 811 is that after 21 days at 135°C the retained ultimate tensile strength at break (RUTS) should be least 75% and that the retained ultimate elongation at break (RUE) should be at least 75%. An increasingly common request i the cable industry is, however, that 75% RUTS and RUE should be kept also after 10 days at 150°C. It is a requisite that the 2,2,6,6-tetramethylpiperidine compound is N-substituted. The substituent is preferably a Cι-C 8 alkyl, C 6 -Cι 2 cycloalkyl, Cχ-Cιo acyl or acyloxy group or a C -C 3 alkoxy group. Among these substituents are preferably Cι-C 8 alkyl or Cι-C 8 alkoxy groups are preferred. Particularly preferred are Cι~C 4 alkyl groups, such as methyl, ethyl, propyl or butyl, or Cι-C 4 alkoxy groups, such as methoxy, ethoxy, propoxy or butoxy. By way of example 2,2,6,6-tetramethylpiperidine compounds for use as antioxidants in accordance v/ith the present invention may be selected from the following: Structure Trade name R R R— NH(CHj),— N(CH,) 2 — N(CH 2 ),NH— R CHIMASSORB 119 R R Major component TINUVIN 765 For comparison purposes also this compound has been evaluated: 4 Among the above mentioned compounds Chimassorb 119 is particularly preferred at present as an antioxidant according to the present invention. Preferably the N-substituted 2, 2, 6, 6-tetrametyl- piperidine compound should be compatible with the ethylene polymer resin of the composition. By "compatible" in this connection is meant that it should be possible to homogeneously blend the 2,2,6,6-tetramethylpiperidine compound v/ith the ethylene polymer resin without migration or exudation of the 2,2,6,6-tetramethylpiperidine compound. The N-substituted 2,2,6,6- -tetramethylpiperidine compound is preferably incorporated in the ethylene polymer composition by compounding together with the other additives of the composition. To further facilitate the understanding of the invention, some illustrative, non-restrictive examples v/ill be given below. All parts and percentages refer to weight, unless otherwise stated. Example 1 Compositions for insulating layers of electric cables v/ere made by compounding an ethylene polymer resin consisting of low density polyethylene (LDPE) (density 922 kg/m 3 , MFR 2 0.9 g/10 min) v/ith various additives listed in Table 1. Three compositions according to the present invention (A, B and C) and tv/o comparative compositions (D and E) v/ere made. The additives v/ere compounded v/ith the ethylene polymer resin at a temperature of 220°C. The contents of the polymer compositions A-E are shown in Table 1. Table 1 Composition in % by weight Component A B_ C D E LDPE 97.9 97.7 97.7 97.7 97.7 Chimassorb 119 0.2 0.4 CGL-116 0.4 Chimassorb 944 0.4 Irganox 1035 0.2 Irganox ® PS 802 0.2 Methylstyrene dimer 0.4 0.4 0.4 0.4 0.4 Dicumylperoxide 1.5 1.5 1.5 1.5 1.5 The following properties of the compositions B-E were evaluated: the peroxide response determined as the change in Gδttfert elastograph-value in Nm after 10 min at 180°C; and the α-methylstyrene content after 40 min at 220°C and 250°C, respectively (which is a measure of moisture generation originating from the decomposition of the peroxide), determined by HPLC analysis. The results are shown in Table 2. Table 2 Properties of compositions B-E Test B C D E Elastograph, 180°C, 0.81 0.81 0.81 0.66 10 min α-Methyl- 100 130 90 3500 styrene 220°C, 40 min, (ppm) -methyl- 200 320 190 4100 styrene 250°C, 40 min, (ppm) It is evident from Table 2 that the peroxide res- ponse of the compositions B-C according to the invention, and also of composition D was clearly better than that of the comparative composition E, both in terms of peroxide response and low water formation. With regard to the level of α-methylstyrene it is noted from Table 2 that all HALS-based compositions B-D, which included the 2,2,6,6-tetramethylpiperidine compound Chimassorb 119, CGL-116 and Chimassorb 944, respectively, instead of conventional sulphur-containing antioxidant additives, gave a substantially reduced level of α- methylstyrene and thus a substantially reduced moisture generation. Example 2 Thermo-oxidative ageing properties The compositions A-D in Example 1 were also tested in a thermo-oxidative ageing test. In this example the heat ageing properties were determined. Dumbbell test pieces were punched out from crosslinked, compression moulded plaques made of the compositions and tested for thermo-oxidative ageing at 135°C (Compositions C and D) and at 150°C (Compositions A-D) for various periods of time. The ultimate tensile strength and the ultimate elongation at break of the compositions were determined before the testing started and subsequently at predetermined time intervals. In Table 2 the values are expressed as percent retained ultimate tensile strength at break (RUTS) and percent retained ultimate elongation at break (RUE) The initial values, ageing time 0 days, being given as 100%. The requirement is that RUTS and RUE after 21 days at 135°C should not be lower than 75%. As stated before, new, coming requirements may prescribe that RUTS and RUE not decrease below 75% after 10 days at 150°C. The testing was carried out in accordance with the International Standard IEC 811. The results are shown in Table 3. Table 3 Composition Ageing time at RUTS (%) RUE (%) 135°C (days) C 0 100 100 14 97 91 21 92 84 0 100 100 14 96 82 21 85 72 Composition Ageing time at RUTS (%) RUE (%) 150°C (days) A 0 100 100 6 85 86 14 75 78 B 0 100 100 10 89 93 C 0 100 100 5 86 78 15 84 76 0 100 100 6 86 69 10 79 62 From the results it can be seen that the innovative compositions A-C all pass both requirements while the non-N-substituted Chimassorb 944 does not confer sufficient RUE to compound D. Example 3 Scorch properties The scorch properties were evaluated at 135°C in a Brabender Plasticorder PL 2000-6. The oil-heated kneader 350, 287 cm 3 with walzenkneaders W 7646 was used. The torque was measured as a function of time and the reported value, T10, is the time when an 10 Nm increase in torque, using the minimum value as a reference point, was observed. Composition B was tested with and without the methylstyrene dimer present in a scorch test. The scorch retardant effect of the methylstyrene dimer is easily seen from the tests since a TIO value of 33 min was measured in the composition without the methylstyrene dimer compared to a TIO value of 55 min for the composi- tion containing the methylstyrene dimer. Another potential scorch additive, Irganox HP-136, was also tested replacing the methylstyrene dimer in composition A, with everything else in composition A remaining unchanged. It was found to lead to somewhat inferior crosslinking and a shorter TlO-value but still offers an alternative to methylstyrene dimer.

Claims: CLAIMS 1. A peroxide-crosslinkable ethylene polymer compo- sition for an insulating layer of an electric cable, which composition contains up to about 5% by weight of additives including a peroxide crosslinking agent and stabilising agents, c h a r a c t e r i s e d in that the stabilising agents comprise an N-substituted 2,2,6,6- -tetramethylpiperidine compound as an antioxidant and light stabilising agent; and that the composition after 21 days at 135°C has a retained ultimate tensile strength of at least 75% and a retained ultimate elongation of at least 75% when tested in accordance with IEC 811. 2. A composition as claimed in claim 1, wherein it contains at most 0.15% by weight in total of phenolic, organic phosphite and sulphur containing stabilisers. 3. A composition as claimed in claim 1 or 2, wherein the 2,2,6,6-tetramethylpiperidine compound is N-substi- tuted with a C x -C 8 alkyl, C 6 -Cι 2 cycloalkyl, Cι-Cι 0 acyl or acyloxy group or an Cι-C 8 alkoxy group. 4. A composition as claimed in claim 3, wherein the 2,2,6,6-tetramethylpiperidine compound is N-substituted with an Cι-C alkyl group. 5. A composition as claimed in claim 1, wherein the additives comprise a 2,2,6,6-tetramethylpiperidine compound selected from the group consisting of R R Major component 6. A composition as claimed in claim 5, wherein the ,2,6,6-tetramethylpiperidine compound is 7. A composition as claimed in any one of claims 1-6, wherein the composition includes an N-substituted 2,2,6,6-tetramethylpiperidine compound in an amount of 0.1-0.5% by weight. 8. A composition as claimed in any one of claims 1-7, wherein the composition after 10 days at 150°C has a retained ultimate tensile strength of at least 75% and a retained ultimate elongation of at least 75% when tested in accordance with IEC 811.

Inventors: DAMMERT RUTH, GUSTAFSSON BILL, LIEBIG KARIN, SMEDBERG ANNIKA, DAMMERT, RUTH, GUSTAFSSON, BILL, LIEBIG, KARIN, SMEDBERG, ANNIKA
Applicants: BOREALIS AS, DAMMERT RUTH, GUSTAFSSON BILL, LIEBIG KARIN, SMEDBERG ANNIKA, BOREALIS A/S, DAMMERT, RUTH, GUSTAFSSON, BILL, LIEBIG, KARIN, SMEDBERG, ANNIKA
Application year: 1998
Main classification: H01B3
IPC codes: C08K5, C08L23, H01B3