CN1114928C - Composition for electric cable - Google Patents

Abstract

A peroxide-crosslinkable ethylene polymer composition for cable insulation is described. The composition is characterized in that its additives include N-substituted 2,2,6,6-tetramethylpiperidine compounds as antioxidants and light stabilizers alone; and when tested according to IEC811, the combination The article has a residual ultimate tensile strength of at least 75% and a residual ultimate elongation of at least 75%. The additive acts as a combined light stabilizer and thermo-oxidative stabilizer and inhibits the generation of moisture, thereby reducing the risk of water tree formation. Preferably the composition is free of conventional antioxidants such as phenolic antioxidants, organophosphite antioxidants and sulfur-containing antioxidants.

Description

Composition for cables

technical field

The present invention relates to a composition for electrical cables, more particularly, the present invention relates to an ethylene polymer composition for the insulating layer of electrical cables, preferably medium, or high or very high voltage power cables. The composition comprises an ethylene polymer and additives including peroxide crosslinkers and stabilizers.

Background technique

Cables, especially medium voltage (MV, 1-35kV), high voltage (HV, 35-500kV) and extremely high voltage (EHV, >500kV) power cables, may consist of a number of polymer layers extruded around an electrical conductor. In power cables, the conductor is usually first covered with a semiconducting inner layer, then covered with an insulating layer, and then wrapped with a semiconducting outer layer. sets of layers. Furthermore, certain HV and EHV cables are enclosed in tubes, usually made of aluminum. The layers of the cable are based on different types of ethylene polymers, which are usually cross-linked.

Cross-linked ethylene polymers are used for cable insulation. The term "ethylene polymer" generally and in the present invention refers to polymers based on polyethylene or ethylene copolymers in which ethylene monomers make up the major part. Thus, the ethylene polymer may consist of a homopolymer or a copolymer of ethylene, where the copolymer may be a copolymer or a graft copolymer of ethylene and one or more monomers copolymerizable with ethylene. LDPE (Low Density Polyethylene, ie polyethylene prepared by free-radical polymerization under high pressure) is currently the main cable insulation material. As noted above, the ethylene polymer may be an ethylene copolymer, and when it contains 0 to about 25 wt%, preferably about 1 to 20 wt%, of one or more comonomers copolymerizable with ethylene. Such monomers are well known to the person skilled in the art and need not be enumerated in detail, but as examples mention may be made of ethylenically unsaturated monomers such as C ₃ -C ₈ alpha olefins, for example propylene, butene; diolefins , such as 1,7-octadiene, 1,9-decadiene; ethylenically unsaturated monomers containing functional groups such as hydroxyl, alkoxy, carbonyl, carboxyl and ester groups. Such monomers may include, for example, (meth)acrylic acid and its alkyl esters, such as methyl, ethyl and butyl (meth)acrylic acid; ethylenically unsaturated hydrolyzable 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 so that the polar comonomer constitutes at most 10% by weight of the ethylene polymer, in order not to increase the loss factor too much . Apart from the additives which will be described in more detail below, the remainder of the composition of the invention consists of the ethylene polymers described above. This means that the amount of ethylene polymer in the composition should be about 95-99.7%, preferably about 96-99%, by weight of the composition.

In order to improve the physical properties of the cable insulation and increase its resistance to the influence of different conditions, the total amount of additives contained in the ethylene polymer is usually about 0.3-5 wt%, preferably about 1-4 wt%. These additives include stabilizing additives such as antioxidants against degradation caused by oxidation, radiation, etc.; lubricating additives such as stearic acid; additives for water-tree resistance such as polyethylene glycol, polysilicon oxane, etc.; and crosslinking additives such as peroxides that decompose when heated and initiate crosslinking of the ethylene plastic of the insulating composition, optionally combined with non-polymers having the ability to form crosslinks when initiated by free radical forming agents Saturated compounds are used in combination.

In cables of the type mentioned above, the presence of water or moisture should be avoided, especially in the insulation, since they have a detrimental effect on the performance of the cable. Moisture leads to the formation of dendritic branching defects, so-called water trees, which in turn can lead to breakage and possible circuit failure. The higher the voltage on the cable, the greater the risk of water tree formation. Therefore, there is a strong desire to minimize and if possible eliminate moisture in cables, especially power cables (MV, HV and EHV cables).

Moisture in the cable may be derived from moisture in the surrounding atmosphere migrating into the cable or from moisture formed in situ in the cable due to chemical reactions.

In cables with insulation of peroxide crosslinked polymers, such as peroxide crosslinked ethylene polymers, moisture is generated due to decomposition of the peroxide and interaction with additives in the polymer. A commonly used peroxide cross-linking agent is dicumyl peroxide, which gives especially cumyl alcohol during the cross-linking process, which in turn decomposes easily into α-methylstyrene and water. The reaction is strongly catalyzed by acids, ie the decomposition and the formation of water are strongly increased when the polymer composition of the insulating layer contains acidic substances. Antioxidant additives in cable polymer compositions are usually sulfur-containing compounds, which form acids such as sulfenic acid due to oxidation and decomposition, and these acidic substances strongly affect the decomposition of peroxides to form water and decomposition products such as α-methylbenzene vinyl.

Therefore, in order to minimize or suppress moisture in peroxide crosslinked polymers of cables, such as peroxide crosslinked ethylene polymers of cable insulation, it is necessary to reduce as much as possible Moisture produced by decomposition.

Contents of the invention

It has been found that by using certain hindered amine light stabilizers (HALS) as composite antioxidants and light stabilizers while excluding any conventional antioxidants such as phenolic antioxidants, organophosphite antioxidants, and sulfur-containing antioxidants, it can be significantly reduced due to Moisture generation caused by peroxide decomposition while retaining excellent aging resistance. Surprisingly, the HALS compound acts not only as an effective light stabilizer, but also as an effective antioxidant, allowing the composition to pass the stringent requirements of thermo-oxidative stability despite the fact that the composition contains little or no Free from conventional antioxidants.

More specifically, the present invention provides a peroxide-crosslinkable ethylene polymer composition for use in cable insulation, which composition contains up to about 5% by weight of an additive comprising a peroxide crosslinker and a stabilizer, It is characterized in that the stabilizer includes an N-substituted 2,2,6,6-tetramethylpiperidine compound as an antioxidant and a light stabilizer; and when tested according to IEC811, the residual limit of the composition after 21 days at 135°C is pulled The tensile strength is at least 75% and the residual ultimate elongation is at least 75%.

Other distinctive features and advantages of the present invention will become apparent after reading the following specification and appended claims.

As mentioned above, sulfur-containing antioxidants tend to form acidic substances during oxidation and decomposition to accelerate the decomposition of peroxides to form moisture. It has been found that certain N - Substituted hindered amine stabilizers as antioxidants, they do not form acidic species and thus do not lead to moisture generation, but at the same time impart excellent aging resistance. The 2,2,6,6-tetramethylpiperidine compound is preferably used alone as an antioxidant. Different 2,2,6,6-tetramethylpiperidine compounds can be used alone or in combination with each other as stabilizers in the compositions according to the invention. Preferably the composition contains little or no conventional antioxidants. This means that conventional antioxidants such as phenolic antioxidants, organophosphite antioxidants and sulfur-containing antioxidants are combined in an amount of at most 0.15 wt%, preferably at most 0.10 wt% of the composition. Most preferably the composition does not contain any such conventional antioxidants at all.

2,2,6,6-Tetramethylpiperidine compounds can be incorporated into ethylene polymer compositions by compounding with other additives such as peroxide crosslinkers, lubricity additives, additives for water treeing resistance, etc. middle. Generally speaking, the total amount of antioxidants should be about 0.1-1.0 wt%, preferably about 0.1-0.5 wt%.

As mentioned above, the 2,2,6,6-tetramethylpiperidine compounds of the present invention not only act as effective light stabilizers, but surprisingly also act as very effective antioxidants, providing heat to the composition. oxidation stability. The thermo-oxidative stability provided by the N-substituted 2,2,6,6-tetramethylpiperidine compounds is usually sufficient for the requirements of the cable insulation composition, so that there is no need to use other antioxidants to provide thermal stability. oxidation stability. In view of the fact that the requirements for thermo-oxidative stability are very stringent for cables with a service life of about 30-40 years, the use of this 2,2,6,6-tetramethylpiperidine compound alone provides sufficient Thermo-oxidative stability is particularly surprising.

Thermo-oxidative stability was determined according to the international standard IEC 811. According to IEC 811, dumbbell test pieces were made from the composition to be evaluated and tested for thermo-oxidative aging. The usual test temperature is 135°C, but the test has also been performed at 150°C. The ultimate tensile strength at break and the ultimate elongation at break of the composition were determined before and at predetermined time intervals after the start of the test. Results are expressed as % Residual Ultimate Tensile Strength at Break (RUTS) and % Residual Ultimate Elongation at Break (RUE), starting values (ageing time of 0 days) taken as 100%. According to the requirements of IEC 811, the residual ultimate tensile strength at break (RUTS) should be at least 75% and the residual ultimate elongation at break (RUE) should be at least 75% after 21 days at 135°C. However, it is an increasingly common requirement in the cable industry that RUTS and RUE should also remain at least 75% after 10 days at 150°C.

It is essential that the 2,2,6,6-tetramethylpiperidine compound is N-substituted. The substituent is preferably C ₁ -C ₈ alkyl, C ₆ -C ₁₂ cycloalkyl, C ₁ -C ₁₀ acyl or acyloxy or C ₁ -C ₈ alkoxy. Among these substituents, C ₁ -C ₈ alkyl or C ₁ -C ₈ alkoxy is preferred. Particularly preferred are C ₁ -C ₄ alkyl, such as methyl, ethyl, propyl or butyl, C ₁ -C ₄ alkoxy, such as methoxy, ethoxy, propoxy or butoxy .

As an example, the 2,2,6,6-tetramethylpiperidine compound used as an antioxidant in the present invention may be selected from the following:

CGL-116R＝

TINUVIN 622(MW为3100-4000)

TINUVIN 765为了对比，也评价下一化合物：

CHIMASSORB 944(MW 2500-4000) Structural brand name CHIMASSORB 119

CGL-116R＝

TINUVIN 622 (MW 3100-4000)

TINUVIN 765 For comparison, the next compound was also evaluated:

CHIMASSORB 944 (MW 2500-4000)

Among the above-mentioned compounds, Chimassorb 119 is currently particularly preferred as the antioxidant of the present invention.

Preferably the N-substituted 2,2,6,6-tetramethylpiperidine compound should be compatible with the ethylene polymer resin of the composition. "Compatible" as used herein means that it should be possible to homogeneously blend the 2,2,6,6-tetramethylpiperidine compound with the ethylene polymer resin without the 2,2,6,6-tetramethylpiperidine compound migration or exudation. The N-substituted 2,2,6,6-tetramethylpiperidine compounds are preferably incorporated into the ethylene polymer composition by compounding with other additives of the composition.

In order to further facilitate the understanding of the present invention, some illustrative but non-limiting examples are given below. All parts and percentages are by weight unless otherwise indicated.

Detailed ways Example 1

Compositions for cable insulation layers were prepared by compounding an ethylene polymer resin consisting of low-density polyethylene (LDPE) (density: 922 kg/m ³ , MFR ₂ : 0.9 g/10 min) with various additives shown in Table 1 .

Three compositions of the invention (A, B and C) and two comparative compositions (D and E) were prepared. The additives were compounded with the ethylene polymer resin at 220°C. The contents of polymer components A-E are shown in Table 1.

Table 1

Composition, wt% 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 Over 0.4 Dicumyl oxide 1.5 1.5 1.5 1.5 1.5

Compositions B-E were evaluated for the following properties: peroxide response measured as a change in Gottfert Elastograph-value (Nm) after 10 minutes at 180°C; and α-methylbenzene after 40 minutes at 220°C and 250°C, respectively Ethylene content, which is a measure of moisture production from the decomposition of the peroxide, was determined by HPLC analysis. These values are listed in Table 2.

Table 2

Performance test of composition BE B C D E Elastograph, 180 ° C, 10 min 0.81 0.81 0.81 0.66 α-methylstyrene, 220 ° C, 40 minutes, 100 130 90 3500 (ppm) α-methyl styrene, 250 ° C, 40 minutes, 200 320 190 4100(ppm)

From Table 2 it can be seen that the peroxide response of inventive compositions B-C and also composition D is significantly better than comparative composition E, both in terms of peroxide response and low water formation.

For the content of α-methylstyrene, it was found from Table 2 that all the compounds containing 2,2,6,6-tetramethylpiperidine compounds Chimassorb 119, CGL-116 and Chimassorb 944 instead of conventional sulfur-containing antioxidant additives were found The HALS-based composition BD gives a significantly lower alpha-methylstyrene content and thus also a significantly lower moisture production. Embodiment 2 thermo-oxidative aging performance

Compositions A-D in Example 1 were also tested in a thermo-oxidative aging test.

In this example, heat aging properties were determined. Dumbbell test pieces were punched from crosslinked compression molded sheets prepared from the compositions and tested for thermo-oxidative aging at 135°C (Compositions C and D) and 150°C (Compositions A-D) for various times. The ultimate tensile strength at break and the ultimate elongation at break of the compositions were determined before starting the test and at predetermined time intervals thereafter. Values are expressed in Table 2 as % Residual Ultimate Tensile Strength at Break (RUTS) and % Residual Ultimate Elongation at Break (RUE). The starting value at an aging time of 0 days was taken as 100%. The requirement is that RUTS and RUE are not less than 75% after 21 days at 135°C. As mentioned earlier, emerging requirements may stipulate that RUTS and RUE do not drop below 75% after 10 days at 150°C. Test according to the international standard IEC811. The results are shown in Table 3.

Table 3 Aging time (days) of the composition at 135° C. RUTS (%) RUE (%) C 0 100 100

14 97 91

21 92 84D 0 100 100

14 96 82

Aging time of 21 85 72 composition at 150°C (days) RUTS(%) RUE(%)

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

D 0 100 100

6 86 69

10 79 62

From these results it can be seen that compositions AC of the present invention meet both requirements, while the non-N-substituted Chimassorb 944 does not impart sufficient RUE to composition D. Embodiment 3 scorch performance

Scorch performance was evaluated in Brabender Plasticorder PL 2000-6 at 135°C. An oil heated kneader 350, 287 cm ³ with Walzenkneaders W7646 was used. Torque is measured over time and the reported value T10 is the time at which an increase in torque of 10 Nm is observed, using a minimum value of 10 Nm as a reference point. Composition B was tested in the scorch test with and without methylstyrene dimer. The scorch retarding effect of methylstyrene dimer is easily seen from these tests, since the T10 value measured in the composition without the methylstyrene dimer was 33 minutes, while in the composition containing the methylstyrene dimer A T10 value of 55 minutes was measured for the composition of styrene dimer.

Another potential scorch additive, Irganox HP-136, was also tested in place of methylstyrene dimer in Composition A, where other conditions were kept constant. It was found to be slightly less crosslinked and have a shorter T10 value, but still capable of displacing methylstyrene dimer.

Claims (8)

1. N-substituted 2,2,6,6-methylpiperidine compound as the only antioxidant and light stabilizer in the peroxide-crosslinkable ethylene polymer composition for cable insulation The use of the composition comprising up to 5% by weight of additives including peroxide crosslinkers and stabilizers, the composition has a residual ultimate tensile strength of at least 75 after 21 days at 135°C when tested according to IEC811 % and a residual ultimate elongation of at least 75%. 2. purposes according to claim 1, wherein 2,2,6,6-tetramethylpiperidine compounds use C ₁ -C ₈ alkyl, C ₆ -C ₁₂ cycloalkyl, C ₁ -C ₁₀ acyl or acyl Oxygen or C ₁ -C ₈ alkoxy N-substituted. 3. Use according to claim 2, wherein the 2,2,6,6-tetramethylpiperidine compound is N-substituted with _C1 - _C4alkyl . 4. The use according to claim 1, wherein the addition comprises 2,2,6,6-tetramethylpiperidine compounds selected from the group consisting of:

R=

MW is 3100-4000; 5. The use according to claim 4, wherein the 2,2,6,6-tetramethylpiperidine compound is

6. The use according to any one of claims 1-5, wherein the composition comprises an N-substituted 2,2,6,6-tetramethylpiperidine compound in an amount of 0.1-0.5 wt%. 7. Use according to any one of claims 1-5, wherein the composition has a residual ultimate tensile strength of at least 75% and a residual ultimate elongation of at least 75% after 10 days at 150°C when tested according to IEC811 . 8. Use according to claim 6, wherein the composition has a residual ultimate tensile strength of at least 75% and a residual ultimate elongation of at least 75% after 10 days at 150°C when tested according to IEC811.