JPH0269543A - Insulating composition and power cable - Google Patents

Abstract

PURPOSE:To obtain an insulating composition, consisting of ultralow-density polyethylene and ethylene-vinyl acetate copolymer with a vinyl acetate content limited to a specific range, improved in water tree resistance and useful as insulators for power cables. CONSTITUTION:An insulating composition which is a mixture of (A) ultralow- density polyethylene having 0.91-0.88g/cm<3> density with (B) ethylene-vinyl acetate copolymer and having 0.5-10wt.% vinyl acetate content and obtained by, as necessary, blending 1-3wt.% crosslinking agent (e.g., dicumyl peroxide) and 0.1-0.3wt.% 4,4'-thiobis-(6-tert-butyl-3-methylphenol), pentaerythrityl-tetrakis[3-(3,5- di-tert-butyl-4-hydroxyphenyl)propionato]methane, etc., as an antioxidant and power cable using the above-mentioned composition as an insulator.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an insulating composition and a polyethylene electrical cable, and particularly to an insulator with improved water resistance.

<Conventional technology> Cross-linked polyethylene cable (X
LPE cable) is widely known.

The weak point of this XLPE cable is that water treeing occurs due to moisture in the insulator and the presence of local abnormal electric fields, which is a phenomenon unique to the cable, which deteriorates the insulation performance of the cable. This water tree in the XLPE insulating layer is thought to occur when water is concentrated in polyethylene, which is a hydrophobic polymer, when there is an abnormal electric field locally.

Therefore, it is considered that by blending a polymer that has a polar group and is hydrophilic to some extent, it is possible to prevent water from concentrating in the local abnormal electric field and to improve the water resistance.

In fact, there have already been some proposals to improve water resistance by blending ethylene-vinyl acetate copolymer (EVA) or ethylene-ethyl acrylate copolymer (EEA).

<Problems to be Solved by the Invention> However, even with the above-mentioned conventional blended insulating compositions, the water tree prevention effect is still insufficient, especially when the insulation composition is immersed in water, such as in distribution class electric cables. Under certain severe conditions, further improvement in water resistance has been desired.

The present invention was made based on such a request.

Means for Solving the Problems and Their Effects> The feature of the present invention is that the polyethylene is ultra-low density polyethylene (density 0.91 to 0°88 g/cm', hereinafter referred to as
EVA using ULDPE) and blending
in a mixture with vinyl acetate in a specific amount (0,
5 to 10% by weight) to improve water resistance.

More specifically, the present invention provides a mixture of ULDPE and EVA, wherein the vinyl acetate content in the mixture is from 0.5 to
The present invention relates to an insulating composition having a weight of 10% by weight and an electric cable using this insulating composition as an insulator.

In the present invention, EVA is blended to improve water resistance, and this EVA and ULDPE
The mixing ratio with tJLDPE is not particularly limited, but in the mixture with tJLDPE, the vinyl acetate content is 0.5 to
The reason why the amount of M is set at % is that if it is less than 0.5% by weight, the water tree inhibiting effect will be small, and if it exceeds 10% by weight, the electrical properties of the cable, especially the dielectric loss tangent (tan δ) will deteriorate.

The following are reasons why the use of U L D P , H improves water resistance. First, water trees are thought to develop as water condensing in abnormal electric field areas in a polymer is accompanied by the formation of micropaths, cracks, etc. in the polymer due to Maxwell stress and the like. However, ULD
In the case of PE, since it has a rubber-like structure with low crystallinity, coagulated water is difficult to concentrate and tends to be uniformly diffused in ULDPE, making it difficult for microbaths and cracks to occur. It is thought that the development will be prevented.

In addition, as a means to prevent flow deformation of the insulator, which is a concern when this electric cable is used at high temperatures, it is also possible to perform appropriate crosslinking treatment. Methods for this crosslinking treatment include chemical crosslinking using an organic peroxide-based crosslinking agent, irradiation crosslinking using electron beam irradiation, and silane crosslinking using a silane coupling agent. Here, as the organic peroxide-based crosslinking agent, dicumyl peroxide (DCP),
13-bis-(L-butylperoxy-isoprobyl)
Benzene and the like are preferably used, and the amount of these mixed is preferably 1 to 3% by weight.

Furthermore, if necessary, 4,4'-thiobis-(6-L
-butyl-3-methylphenol), pentaerythyl-tetrakis (3-(3,5-di-1-butyl-4-hydroxyphenyl)propionate comethane, etc.) alone or in combination; It may be blended in an amount of about .3% by weight.

<Example I> Insulating compositions according to the present invention (Examples ■ to ■) and insulating compositions that do not meet the conditions of the present invention (Comparative Examples ■ to ■) were created using the formulations shown in Table 1. .

In addition, the EVA used has M, F, and R of 3 to 5.
DCP is used as a crosslinking agent, and 4,4'-thiobis-(6-L-butyl-3) is used as an anti-aging agent.
-methylphenol) and pentaerythyl-tetrakis (3-(3,5-di-t-phthyl-4-hydroxyphenyl)propionate comethane) in a ratio of 4:1.

Then, each of the above-mentioned compounds was kneaded under heat in a roll mill according to the respective compounding amounts, and then a sheet-like article having a size of locmXlocm and a thickness of 1 mm or 3 mm was obtained from each resin mixture by press molding. The pressing conditions at this time were a temperature of 180° C. and 1 hour and 30 minutes.

Note that the gel fraction of the insulating sheets made of each resin mixture thus obtained was 85% or more. This iL
t is based on a method in which the sample is immersed in warm xylene at 110'C for 24 hours to melt only the uncrosslinked portion, and then dried and the gel fraction is measured.

Next, the number of water tree occurrences and the dielectric loss tangent (tan δ) of these insulating sheets were measured, and the results are also listed in Table 1.

A water tree test method and a dielectric loss tangent (tan δ) measurement method for measuring the number of water trees generated in this case are shown below. The water tree test method will be explained based on FIG. 1.

Water tree test method In FIG. 1, 1 is an insulating sheet used as a test sample. This insulating sheet 1 is a sheet formed by press-molding the above-mentioned resin mixture, and in this water tree test, a sheet having a thickness of 3 mm is used. A coating layer 2 of conductive paint is provided on the bottom surface of the sheet 1 to serve as a ground electrode, and a water tank 4 is provided on the top surface of the sheet 1 to form a water electrode, to which a high voltage of 10 kV, 1 kHz is applied. The configuration is such that the voltage can be applied from the electrode 3. After applying the voltage between the electrodes for 30 days, the sheet 1 was boiled and water trees generated on the sheet 1 were observed. At this time, we focused only on water trees with a diameter of 50 μm or more, and measured the number of these.

Note that here, the number of occurrences is based on conventional cross-linked polyethylene (X
The number is expressed as a relative number when the number of water trees generated on a comparison sheet prepared with the intention of 100 was 100.

Dielectric Loss Tangent (tan δ) Measuring Method For measuring the dielectric loss tangent (tan δ), a 1 mm thick sheet made of each of the above formulations is used as a sample. To this, 1
A voltage of kV and 50 kHz was applied, and the dielectric loss tangent (tan δ) was measured using a Schering bridge.

From Table 1 above, in the case of Examples ① to ② of the present invention, compared to Comparative Example ② (XLPH insulator), there is an effect of suppressing water tree generation, and in particular, the vinyl acetate content is reduced from 0.5 to It can be seen that the effect is remarkable when the content is in the range of 10% by weight, and as is clear from the value of the dielectric loss tangent, the insulation property is also comparable to that of ordinary XLPE.

On the other hand, in the case of Comparative Examples ① to ② which do not satisfy the limiting conditions of the acrylate content of the present invention, it can be seen that the effect of suppressing water tree generation is small and the insulation properties are poor.

Next, electric cables were prepared using the insulating compositions of Example (2) and Comparative Example (2) as insulators. The structure of this cable is
It has a 3N structure in which an insulating layer with a thickness of 3 mm is provided on the conductor, and an inner semiconducting layer and an outer semiconducting layer are formed, and shielding or sheathing that is normally applied to the outside is omitted. On this occasion,
Copper was used as the conductor, and the cross-sectional area of the conductor was 100 mm.

A semiconductive mixture of ethylene-vinyl acetate copolymer and conductive carbon black was used for the internal and external semiconductive layers, and the coating layers were formed by extrusion coating.

For each of the above-mentioned power cables thus prepared, the following submergence electrification test was conducted to determine the dielectric breakdown voltage, and the results are shown in Table 2.

Water immersion charging test Each of the above cables was immersed in 70°C warm water, and 1
After applying a voltage of kV, 1 kHz for 90 days, further AC (50)! The voltage of z) was increased under step-up conditions of 5 kV/30 minutes, and the dielectric breakdown voltage was measured.

Table 2 From this Table 2, it can be seen that the dielectric breakdown voltage after water immersion electrification is higher in the case of Example 2 of the present invention than in Comparative Example 2.

<Effects of the Invention> As is clear from the above explanation, according to the present invention, ULD
PE (density 0.91-0.88g/cm3) and EVA
, the vinyl acetate content in the mixture is 0.
．． Since it is 5 to 10% by weight, insulation performance (jan δ etc.)
It is possible to obtain an insulating composition that has the same level of properties as conventional XLPE and has a large water tree suppression effect, and if this insulating composition is used as an insulator, it has good insulation performance (tan δ, etc.) and water tree resistance. In addition, there is no decrease in dielectric breakdown voltage after submerged electrification, and furthermore, due to the low crystallinity of ULDPE,
A cable with excellent flexibility can also be obtained.

The insulating composition of the present invention can be used not only as an insulator for cables but also for connection parts of cables, etc., if necessary.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram for explaining the water tree test method in the present invention.

Claims (2)

[Claims] (1) Ultra-low density polyethylene (density 0.91-0.88
g/cm^3) and an ethylene-vinyl acetate copolymer, the vinyl acetate content in the mixture is from 0.5 to
10% by weight of an insulating composition. (2) A power cable using the insulating composition according to claim 1 as an insulator.