KR20010052091A - Composition for electric cables - Google Patents

Abstract

The present invention relates to peroxide crosslinkable ethylene polymer compositions for insulating layers of electrical cables. The compositions of the present invention comprise N-substituted 2,2,6,6-tetramethylpiperidine compounds as antioxidants and light stabilizers, and when tested according to IEC 811, the composition is 75 after 21 days at 135 ° C. Have a final retention tensile strength of at least% and a final retention elongation of at least 75%. The additive acts as a combined light and thermal-oxidative stabilizer, reducing the risk of water tree formation by inhibiting moisture production. Preferably, the composition of the present invention contains no conventional antioxidants such as phenolic antioxidants, organic phosphite antioxidants and sulfur containing antioxidants.

Description

Composition for electric cable {COMPOSITION FOR ELECTRIC CABLES}

Electrical cables and especially power cables for medium voltage (MV; 1-35 kV), high voltage (HV; 35-500 kV) and ultra high voltage (EHV;> 500 kV) may consist of a plurality of polymer layers extruded around the electrical conductor. In power cables, electrical conductors are usually first covered with an inner semiconductor layer and then with an insulating layer, followed by an outer semiconductor and then a water barrier layer, optionally with an outer layer. In addition, some HV and EHV cables are surrounded by tubes, usually tubes of aluminum. The layers of the cable are usually based on different types of ethylene polymers that are crosslinked.

The crosslinked ethylene polymer is used as the insulating layer of the electrical cable. The term "ethylene polymer" generally means in the context of the present invention an ethylene copolymer or a polymer based on polyethylene in which the ethylene monomer makes up most of its mass. Thus, the ethylene polymer may consist of a homopolymer or copolymer of ethylene, and the copolymer may be a copolymer of ethylene or one or more monomers copolymerizable with ethylene or graft copolymers. Low-density polyethylene (LDPE), ie polyethylene produced by radical polymerization at high pressures, is currently the main cable insulation material. As mentioned above, the ethylene polymer may be an ethylene copolymer, in which case the ethylene polymer comprises 0 to about 25%, preferably about 1 to 20%, by weight of one or more comonomers copolymerizable with ethylene . Such monomers are well known to those skilled in the art and need not be enumerated individually, but include, for example, vinyl unsaturated monomers such as C ₃ -C ₈ alpha olefins (eg, propene, butene); Dienes (eg, 1,7-octadiene, 1,9-decadiene); Mention may be made of vinylically unsaturated monomers containing hydroxyl groups, alkoxy groups, carbonyl groups, carboxyl and ester groups. Such monomers include, for example, (meth) acrylic acid and alkyl esters thereof (eg, methyl-, ethyl-, and butyl (meth) acrylates; vinylic unsaturated hydrolyzable silane compounds (eg, vinyl trimes). Oxysilane), however, if the ethylene polymer is an ethylene copolymer, the amount of polar comonomer should be kept at a low level, so that the polar comonomer does not increase excessively by the dissipation factor. Up to 10% by weight In addition to the additives to be described in more detail below, the remaining components of the composition according to the present invention consist of the ethylene polymers specified above, wherein the amount of ethylene polymer in the composition is from about 95 to about 99.7% by weight of the composition. Preferably in the range of about 96 to 99% by weight.

In order to improve the physical properties of the insulating layer of the electric cable and to increase the resistance to the influence of different conditions, the ethylene polymer usually contains about 0.3 to 5% by weight, preferably about 1 to 4% by weight of the total amount. Such additives include additives such as antioxidants that resist degradation by oxidation, radiation, and the like; Lubricating additives such as stearic acid; Additives for water-tree resistance, such as polyethylene glycol, silicone, and the like, and crosslinking additives that decompose upon heating to initiate crosslinking of ethylene plastics of the insulating composition, such as peroxides, optionally initiated by radical forming agents. If used, unsaturated compounds capable of forming crosslinks are used together.

In electrical cables of the type described above, the presence of water or moisture should be avoided due to the detrimental effect on the cable properties, especially in the insulation layer. Moisture can lead to the formation of dendritic branched defects, the so-called water trees, and then to failure and electrical breakdown. The risk of water tree formation is greater with higher cable voltages. Therefore, there is a strong demand for minimizing and possibly removing water from electrical cables, in particular power cables (MV, HV and EHV cables).

In electrical cables, moisture may be derived from moisture being transferred to the cable in the ambient atmosphere or may be moisture generated in the cable itself due to chemical reactions.

In electrical cables having peroxide crosslinked polymers, such as peroxide crosslinked ethylene polymer insulation layers, moisture is produced by decomposition of the peroxide and interaction with the additives of the polymer. The predominant peroxide crosslinker is dicumyl peroxide, which tends to generate cumyl alcohol, especially during crosslinking, and then break it down into α-methylstyrene and water. This reaction is strongly catalyzed by acid. That is, the decomposition and formation of water greatly increases when the polymer composition of the insulating layer contains an acidic substance. Antioxidant additives in the polymer composition of electrical cables are usually sulfur-containing compounds, which, due to oxidation and decomposition, form acids such as sulfenic acid, and these acidic substances decompose peroxides to form decomposition products such as α-methylenestyrene and water. Contribute greatly to making

In order to minimize or inhibit the moisture in the peroxide crosslinked polymer of the electrical cable, such as the peroxide crosslinked ethylene polymer of the insulation of the electrical cable, it is necessary to reduce the production of moisture due to peroxide decomposition as much as possible.

Summary of the Invention

Moisture production due to peroxide decomposition has been particularly impaired as antioxidants and light stabilizers mixed without the use of phenolic antioxidants and conventional antioxidants such as sulfur containing antioxidants and organic phosphide antioxidants. It has been found that by using amine light stabilizers (HALS) can be reduced while retaining good aging resistance. Surprisingly, the HALS compound acts not only as an effective light stabilizer but also as an effective antioxidant, despite the fact that the composition contains little or no conventional antioxidants, the composition meets the stringent requirements for thermal-oxidative stability. To match.

More specifically, the present invention relates to a peroxide crosslinkable ethylene polymer composition for an insulating layer of an electrical cable comprising up to about 5% by weight of an additive comprising a peroxide crosslinker and a stabilizer, wherein the stabilizer is an antioxidant and a light stabilizer. As an N-substituted 2,2,6,6-tetramethylpiperidine compound, and after 21 days at 135 ° C. the composition had a final retained tensile strength of at least 75% and was tested according to IEC 811 A composition is characterized by having a final retention elongation of at least%.

Other salient features and advantages of the present invention will become apparent from the following specification and the appended claims.

The present invention relates to compositions for electric cables, in particular ethylene polymer compositions for insulating layers of electric cables, preferably medium or high or ultra high voltage power cables. Compositions of the present invention comprise ethylene polymers and additives, including peroxide crosslinkers and stabilizers.

As described above, sulfur-containing antioxidants tend to form acidic substances upon oxidation and degradation, promoting the formation of water by peroxide decomposition, but consisting of 2,2,6,6-tetramethylpiperidine compounds It has been found that N-substituted hindered amine stabilizers can be used as antioxidants to form no acidic material, thereby preventing moisture from being produced and at the same time providing good aging resistance. The 2,2,6,6-tetramethylpiperidine compound is preferably used alone as an antioxidant. Different 2,2,6,6-tetramethylpyriridine compounds can be used alone or in admixture with one another as stabilizers of the compositions according to the invention. Preferably, the compositions of the present invention contain little or no conventional antioxidants. This means that the combined amount of conventional antioxidants such as phenolic antioxidants, organic phosphite antioxidants and sulfur containing antioxidants is 0.15% or less by weight of the composition, preferably 0.10% or less of the composition. Very preferably, the composition contains no such conventional antioxidants.

The 2,2,6,6-tetramethylpiperidine compound can be mixed into the ethylene polymer composition by mixing with other additives such as peroxide crosslinkers, lubricating additives, additives for water tree resistance, and the like. Generally, the total amount of antioxidant should be in the range of about 0.1 to 1.0% by weight, preferably about 0.1 to 0.5% by weight.

As described above, the 2,2,6,6-tetramethylpiperidine compounds of the present invention not only act as effective light stabilizers, but also surprisingly as very effective antioxidants that provide thermal-oxidative stability to the composition. The thermal-oxidative stability provided by the N-substituted 2,2,6,6-tetramethylpiperidine compound generally meets the requirements of the electrical cable insulation layer composition, thus providing other properties for thermal-oxidative stability. Does not require antioxidants. The fact that the 2,2,6,6-tetramethylpiperidine compound alone can provide sufficient thermal-oxidative stability for electrical cables having a lifetime of about 30 to 40 years is a requirement for thermal-oxidative stability. It is particularly surprising in light of the fact that it is very strict.

Thermal-oxidative stability is measured according to the international standard IEC 811. According to IEC 811, dumbbell test pieces consist of the composition to be evaluated and tested for thermal-oxidative aging. The standard test temperature is 135 ° C., but the test was also performed at 150 ° C. The final tensile strength at break and the final elongation at break of the composition are measured before the test begins, and then at predetermined time intervals. This result is expressed as a percentage of the final retained tensile strength at break (RUTS) and the final retained elongation at break (RUE), with an initial value (zero aging time) given as 100%. The requirement according to IEC 811 is that the final retained tensile strength (RUTS) at break after 21 days at 135 ° C must be at least 75% and the final retained elongation (RUE) at break must be at least 75%. However, an increasing demand in the cable industry is that 75% of RUTS and RUE should still be maintained after 10 days at 150 ° C.

It is required that the 2,2,6,6-tetramethylpiperidine compound be N-substituted. Such substituents are preferably C ₁ -C ₈ alkyl, C ₆ -C ₁₂ cycloalkyl, C ₁ -C ₁₀ acyl or acyloxy groups or C ₁ -C ₈ alkoxy groups. Among these substituents, preferably a C ₁ -C ₈ alkyl or C ₁ -C ₈ alkoxy group is preferable. Particularly preferred are C ₁ -C ₄ alkyl, for example methyl, ethyl, propyl or butyl, C ₁ -C ₄ alkoxy, for example methoxy, ethoxy, propoxy or butoxy.

For example, the 2,2,6,6-tetramethylpiperidine compound for use as an antioxidant according to the present invention may be selected from:

Structure

CHIMASORB 119

TINUVIN 622 (MW 3100-4000)

Tinuvin 765

For comparison, the compounds were evaluated:

Kimassorb 944 (MW 2500-4000)

Among the above-mentioned compounds, Chimassorb 119 is particularly preferred as an antioxidant according to the present invention.

Preferably, the N-substituted 2,2,6,6-tetramethylpiperidine compound should be compatible with the ethylene polymer resin of the composition. In this context, "commercially available" means that the 2,2,6,6-tetramethylpiperidine compound is homogeneous with the ethylene polymer resin without transfer or exudation of the 2,2,6,6-tetramethylpiperidine compound. It must be able to be mixed. The N-substituted 2,2,6,6-tetramethylpiperidine compound is preferably incorporated into the ethylene polymer composition by mixing with the remaining additives of the present composition.

Furthermore, to help understand the present invention, some illustrative and non-limiting examples will be provided below. All parts and percentages are by weight unless otherwise indicated.

Example 1

The composition for the insulation layer of the electric cable was prepared by mixing an ethylene polymer resin consisting of low density polyethylene (LDPE) (density 922 kg / m ³ , MFR ₂ 0.9 g / 10 min) with the various additives listed in Table 1.

Three compositions (A, B and C) and two comparative compositions (D and F) according to the invention were provided. The additive was mixed with the ethylene polymer at a temperature of 220 ° C. The content of polymer compositions A-E is listed in Table 1.

Table 1

The following properties of the composition BE were evaluated: peroxide reaction measured as the Goettfert elastograph-value changed at 180 ° C. after 10 minutes and 220 ° C. and 250 measured by HPLC analysis respectively. Α-methylstyrene content after 40 minutes at < RTI ID = 0.0 > C, < / RTI > this is a measure of moisture production derived from peroxide decomposition. The results are shown in Table 2 below:

TABLE 2

From Table 2 it was shown that the peroxide reactions of Compositions B-C and D according to the invention are clearly superior in peroxide reaction and less water formation than Comparative Composition E.

Regarding the levels of α-methylstyrene, from Table 2, instead of the conventional sulfur containing antioxidants, 2,2,6,6-tetramethylpiperidine compounds, chimassorb 119, CGL-116 and chimassorb It can be seen that BD, all HALS based compositions comprising 944, significantly reduces the level of α-methylstyrene and thus significantly reduces moisture.

Example 2

Thermal-oxidative aging characteristics

Compositions A-D of Example 1 were used for the thermal-oxidative aging test.

In this example, thermal aging characteristics were measured. Dumbbell test pieces were obtained by drilling from crosslinked compression molded plaques of the composition and tested for thermal-oxidative aging at various time intervals at 135 ° C. (composition C and D) and 150 ° C. (composition AD). . Final tensile strength and final elongation at break of the composition were measured before the test began, and then at predetermined time intervals. In Table 2, the values are expressed as percentage of final retained tensile strength (RUTS) at break and final retained elongation (RUE) at break. An initial value of zero days of aging time is given as 100%. The requirement is that RUTS and RUE should not be lower than 75% at 135 ° C. after 21 days. As already mentioned, the new requirement may be described as that the RUTS and RUE should not decrease below 75% at 150 ° C. after 10 days. This test was performed according to the international standard IEC 811. The results are shown in Table 3.

TABLE 3

From these results it can be seen that the innovative compositions A-C both meet both requirements, but the non-N-substituted chimassorb 944 does not provide sufficient RUE for compound D.

Example 3

Scorch Characteristics

Scotch properties were evaluated at 135 ° C. in Brabender Plasticorder PL 2000-6. Oil heated kneader 350, 287 cm ³ with walzenkneaders W 7646 was used. Torque was measured as a function of time, and the reported value, T10, is the time when the torque was observed to increase by 10 Nm using the minimum value as the reference point. Composition B was tested in the presence and absence of the methylstyrene dimer present in the scotch test. Since the T10 value of 33 minutes was measured in the composition without the methylstyrene dimer, compared to the T10 value of 55 minutes for the composition containing the methylstyrene dimer, the scorch delay effect of the methyl styrene dimer was easily obtained from the above test. Able to know.

As another effective scorch additive, Irganox HP-136 was tested without replacing the methylstyrene dimer in Composition A and all the remaining components of Composition A. It has been shown to induce somewhat crosslinking, but has been shown to be able to be used as a substitute for methylstyrene dimers.

Claims (8)

In a peroxide crosslinkable ethylene polymer composition for an insulating layer of an electrical cable containing up to about 5% by weight of an additive comprising a peroxide crosslinker and a stabilizer, the stabilizer is N-substituted as an antioxidant and a light stabilizer; 2,6,6-tetramethylpiperidine compound, tested according to IEC 811, after 21 days at 135 ° C. the composition has a final retention tensile strength of at least 75% and a final retention elongation of at least 75% Characterized in that the composition. The composition according to claim 1, which contains not more than 5% by weight of phenolic stabilizer, organic phosphite stabilizer, and sulfur-containing stabilizer. The 2,2,6,6-tetramethylpiperidine compound is a C ₁ -C ₈ alkyl, C ₆ -C ₁₂ cycloalkyl, C ₁ -C ₁₀ acyl or acyloxy group. Or N-substituted with a C ₁ -C ₈ alkoxy group. 4. The composition of claim 3, wherein the 2,2,6,6-tetramethylpiperidine compound is N-substituted with a C ₁ -C ₄ alkyl group. The method of claim 3 wherein the additive (MW 3100-4000) A composition comprising 2,2,6,6-tetramethylpiperidine compound selected from the group consisting of: 6. The compound of claim 5, wherein the 2,2,6,6-tetramethylpiperidine compound is A composition characterized in that. 7. A composition according to any one of claims 1 to 6, comprising N-substituted 2,2,6,6-tetramethylpiperidine compounds in an amount of 0.1 to 0.5% by weight. The composition according to claim 1, wherein after 10 days at 150 ° C., the composition has a final retention tensile strength of at least 75% and a final retention elongation of at least 75% when tested according to IEC 811. .