JPH09298012A - Composition for semi-conducting layer of power cable - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a composition which has excellent molding workability at the time of extrusion molding, a small change with temperature in volume resistivity of an acquired semi-conducting layer, and also an excellent heat deformation characteristic. SOLUTION: Carbon black, which has DBP oil absorption not less than 300[ml/100g] and a specific surface area not smaller than 1000[m<2> /g], is added to an ethylen copolymer wherein content of a monomer copolymerizing with ethylene is not less than 30wt.% and MFR is not more than 25[g/10min] by not less than 4 PHR but less than 10 PHR. Further, modulus of heat deformation at the time when cross-linking is conducted by adding a cross-linking agent is caused to be not greater than 30% at 120 deg.C when a 34N load is applied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductive composition for forming a semiconductive layer of a power cable, and more particularly to a composition for a semiconductive layer of a power cable whose volume resistivity changes little with temperature.

[0002]

2. Description of the Related Art A cross-linked polyethylene insulation cable, which has been the mainstream of electric power cables in recent years, is shown in FIG. 1 for the purpose of relaxing electric field concentration at the insulator interface and preventing partial discharge. The inner semiconductive layer 2 and the outer semiconductive layer 4 are provided on the boundary surface between the cable conductor 1 and the insulator 3 and the outer surface of the insulator 3, and the metal shielding layer 5 is formed on the outer surface of the outer semiconductive layer 4. In addition, the press-wrap tape 6 and the sheath layer 7 are generally provided. The inner semi-conductive layer 2 and the outer semi-conductive layer 4 are formed from a semi-conductive composition which is made semi-conductive by using polyethylene or an ethylene copolymer as a base resin and adding conductive carbon black in an appropriate amount. And is extruded at the same time as the insulator 3 when the power cable is manufactured, and is formed on the cable conductor 1 and the insulator 3.

By the way, the semiconductive layers 2 and 4 are required to have a conductivity of about 10 ⁵ Ω-cm or less in practical use. Further, the volume resistivity shows temperature dependency, tends to increase as the temperature rises, and tends to decrease at a certain temperature. However, such temperature dependence of the volume resistivity is not preferable for practical use, and in the case of a power cable of crosslinked polyethylene insulation, the volume resistance is in the range of room temperature to about 130 ° C (considering use under a short-time permissible current). It is said that it is preferable that the displacement of the rate (difference between the maximum value and the minimum value) is small.

In order to meet the requirement regarding the volume resistivity, increasing the blending amount of carbon black can be an effective solution, but on the other hand, the mechanical properties such as tensile elongation value and brittleness property are There are problems such as a decrease, an increase in viscosity during melting and deterioration of extrusion processability, and an increase in water absorption, which promotes generation and growth of a water tree that causes dielectric breakdown of the cable. In addition, in order to meet the same requirements regarding volume resistivity, Japanese Patent Laid-Open No. 25573/1992.
Japanese Patent Publication No. 6 proposes to add an alkyl phosphite as a substance that suppresses an increase in volume resistivity at high temperature. However, Japanese Patent Application Laid-Open No. 4-255736 also describes that when the amount of the alkyl phosphite added is large, the workability during extrusion molding decreases.

The present invention has been made in view of the above problems, and is an electric power cable which is excellent in workability at the time of extrusion molding, has less volume change of the volume resistivity of the obtained semiconductive layer, and is excellent in heat deformation characteristics. It is an object of the present invention to provide a semiconductive layer composition.

[0006]

Means for Solving the Problems The above object is to achieve a content of the ethylene copolymerizable monomer of the present invention of 30% by weight.
Above, the ethylene-based copolymer having an MFR of 25 [g / 10 min] or less has a DBP oil absorption of 300 [ml / 10].
0 g] or more and a specific surface area of 1000 [m ² / g] or more is added in an amount of 4 PHR or more and less than 10 PHR and a cross-linking agent is added to obtain a thermal deformation rate of 1
It is achieved by a composition for a semiconductive layer of a power cable, which is 30% or less when a load of 20 ° C. and a load of 34 N is applied.

According to the semiconductive composition of the present invention, by defining the oil absorption amount and the specific surface area of carbon black, the conductivity required for practical use can be obtained with a smaller amount than the conventional amount. Moreover, since the compounding amount is small, the processability during extrusion molding of the semiconductive composition can be improved. On the other hand, regarding the decrease in the stability of the volume resistivity with respect to temperature changes (increased displacement) due to the small amount of carbon black compounded, the monomer that copolymerizes with ethylene in the ethylene-based copolymer (hereinafter, To improve the heat resistance by setting the content of the (comonomer) to be 30% or more, and by setting the MFR (melt flow rate) of the ethylene-based copolymer to 25 [g / 10 min] or less. You can

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The composition for a semiconductive layer of a power cable according to the present invention will be described below. The base resin of the semiconductive composition of the present invention is an ethylene-based copolymer,
The comonomer that is copolymerized with ethylene is not particularly limited, and examples thereof include ethylene-acrylic acid ester copolymers (for example, ethylene-methyl acrylate copolymer and ethylene-ethyl acrylate copolymer), ethylene-acetic acid. Vinyl copolymers and the like can be preferably used. The content of this comonomer is preferably 30% by weight or more of the ethylene copolymer, and if it is less than this, the improvement of stability due to temperature change of volume resistivity becomes insufficient. FIG. 2 shows that the content of vinyl acetate as a comonomer is 10, 2
Carbon black having a DBP oil absorption of 495 [ml / 100 g] and a specific surface area of 1270 [m ² / g] was uniformly added to an ethylene-vinyl acetate copolymer of 0,33% by weight.
Graph showing the temperature dependence of the volume resistivity of the outer semiconductive layer of the cable (6 kV class, conductor cross-sectional area 60 mm ² ) produced by three-layer coextrusion crosslinking using the semiconductive composition produced by adding PHR. Is. As is clear from the figure,
The displacement of the volume resistivity tends to decrease as the content of vinyl acetate increases, and in particular, the content of vinyl acetate is 33%.
In% by weight, the displacement is within one digit.

The upper limit of the comonomer content is not particularly limited. On the other hand, on the other hand, the ethylene-based copolymer tends to deteriorate in heat distortion characteristics when it is formed into a semiconductive layer as the comonomer content increases. Regarding the heat deformation characteristics, there is no specification for the semiconductive layer, but for insulators, the test method is specified in JIS C 3005. Generally, the heat deformation rate at a temperature of 120 ° C when a 34N load is applied is 40% or less. Is practically preferred. Therefore, it is considered that it is desirable to maintain the same or higher characteristics for the semi-conductive layer, and in the present invention, the thermal deformation rate at a load of 34 N at 120 ° C. is 30.
% Or less. Therefore, the upper limit of the comonomer blending amount should be determined so as to satisfy the above-mentioned heat deformation rate. In addition, in the present invention, the crosslinking degree is increased and the MFR is increased.
Is specified to be 25 [g / 10 min] or less, heat resistance is improved.

The carbon black according to the present invention has D
It is preferable that the BP oil absorption is 300 [ml / 100 g] or more and the specific surface area is 1000 [m ² / g] or more, and it is more preferable that both values are large. Where D
The BP oil absorption is defined as the amount of dibutyl phthalate (unit: ml) that can be contained per 100 g of carbon black. Examples of the carbon black having the above DBP oil absorption and specific surface area include “Ketjen Black EC DJ-600” manufactured by Lion Corporation and “# 3950” manufactured by Mitsubishi Chemical Corporation. Also,
This carbon black was added to the ethylene-based copolymer.
More than PHR and less than 10 PHR are mixed. When the blending amount of carbon black is less than 4 PHR, 10 ⁵ Ω-c practically required as the volume resistivity of the obtained semiconductive layer.
A conductivity of m or less cannot be obtained. Further, when the amount of carbon black compounded is 10 PHR or more, the viscosity of the semiconductive composition becomes too high, the fluidity at the time of molding is lowered, and the mechanical properties of the obtained semiconductive layer are poor. Become.

In the present invention, a crosslinking agent is added to improve the heat resistance and mechanical properties of the semiconductive layer. As the cross-linking agent according to the present invention, as long as it is usually used for the cross-linking reaction of the ethylene-based copolymer, it can be used without particular limitation, as a cross-linking agent satisfying such a demand, for example, Dicumyl peroxide (DCP), 2,
Examples thereof include 5-dimethyl-2,5-di (t-butylperoxy) hexyne-3. Further, the amount of the crosslinking agent added may be the amount added to the crosslinking reaction of the ordinary ethylene-based copolymer, but if a large amount increases the scorch anxiety, heating at the time of forming the semiconductive layer It is desirable to experimentally find an amount that allows the deformation rate to satisfy the above-mentioned numerical value, and to set the range.

The semiconductive composition according to the present invention contains the above-mentioned ethylene copolymer, carbon black and a crosslinking agent as essential components, but other additives may be added, if necessary, within a range that does not impair the effects of the present invention. , Eg antioxidants (eg,
An appropriate amount of 4,4′-thiobis- (6-tert-butyl-m-cresol), a polymer of 2,2,4-trimethyl-1,2-dihydroquinoline) or the like can be added.

The composition for the semiconductive layer of the power cable according to the present invention will be clarified based on Examples and Comparative Examples. Examples 1 to 3 and Comparative Examples 1 to 4 As base resins, two kinds of ethylene-vinyl acetate copolymers EVA1 (vinyl acetate content: 33% by weight) and EVA2 (vinyl acetate content) having different vinyl acetate contents are used. Amount: 20% by weight)
Is used, and three types of carbon blacks (1) to (3) having different specific surface areas and DBP oil absorptions, a cross-linking agent (dicumyl peroxide) and an antioxidant (4,4′-thiobis- (6- 3-butyl-m-cresol)) was added in the compounding amount (PHR) shown in Table 1 to prepare a semiconductive composition.

The semiconducting composition prepared as described above was tested for conductivity, volume resistivity stability, heat resistance, water absorption and processability. The test methods and criteria are as follows. [Conductivity] The semiconductive composition was co-extruded and crosslinked in three layers to be processed into an outer semiconductive layer of a cable (6 kV class, conductor cross-sectional area 60 mm ² ), and the volume resistivity was measured at room temperature. A value of 10 ⁵ Ω-cm or less was evaluated as “◯”, and a value above 10 ⁵ Ω-cm was evaluated as “x”. [Volume resistivity stability] A cable (6 kV class, conductor cross-sectional area 60 m) obtained by coextruding and cross-linking three layers of a semiconductive composition.
m ² ) was processed into an outer semiconductive layer, and the volume resistivity was measured over a temperature range from room temperature to 130 ° C. The displacement of the volume resistivity in the above temperature range is less than one digit is “◯”, and one digit or more is “x”. [Heat resistance] Based on JIS C 3005, the heat deformation rate at a load of 34 N was measured at 120 ° C., and the heat deformation rate was less than 30% as “◯”, and 30% or more as “x”. [Water absorption] A water content measured by the Karl Fischer method is "○" when less than 200 ppm, and "X" when it is 200 ppm or more.
And [Workability] When the outer semiconductive layer is extrusion-molded to produce the above cable, the case where the motor of the molding machine is overloaded or the occurrence of melt fracture is seen as "x", When there was not, it was marked as "○". The above test results are shown in Table 1.

[0015]

[Table 1]

As is clear from Table 1, each of the semiconductive compositions of Examples 1 to 3 of the present invention is excellent in conductivity, volume resistivity stability, heat resistance, water absorption and processability. It was confirmed. On the other hand, since the semiconductive composition of the comparative example uses carbon black (3) whose DBP oil absorption and specific surface area are outside the scope of the present invention, there is a problem particularly in the stability of conductivity and volume resistivity. is there. Further, the semiconductive composition of the comparative example is a composition in which carbon black (1) having both the DBP oil absorption amount and the specific surface area is within the range of the present invention, but the upper limit value (10P
(Less than HR), there is a problem in water absorption and processability. On the other hand, the semiconductive composition of the comparative example is a composition in which carbon black (1) having a DBP oil absorption amount and a specific surface area within the scope of the present invention is blended.
Since the content is less than the lower limit (4 PHR) specified in the present invention, there is a problem in heat resistance in addition to conductivity and volume resistivity stability. Further, the semiconductive composition of Comparative Example has a problem in stability of volume resistivity because the comonomer content is less than the lower limit value (30% by weight) specified in the present invention.

[0017]

As described above, according to the present invention,
By defining the oil absorption amount and specific surface area of carbon black, the conductivity required for practical use can be obtained with a smaller compounding amount than before, and the processability of the semi-conductive composition can be improved due to the small compounding amount. Can be improved. On the other hand, the problem that the range of change in volume resistivity becomes large due to the small amount of carbon black compounded is improved by setting the comonomer content in the ethylene copolymer to 30% or more. By setting the MFR of the polymer to 25 [g / 10 min] or less, heat resistance can be ensured. Thus, the semiconductive composition according to the present invention is a composition suitable for forming a semiconductive layer of a power cable.

[Brief description of drawings]

FIG. 1 is a sectional view of essential parts showing a basic configuration example of a semiconductive layer of a crosslinked polyethylene insulated power cable.

FIG. 2 is a cable (6 kV) produced by three-layer coextrusion crosslinking using a semiconductive composition produced by uniformly adding 4 PHR of carbon black to an ethylene-vinyl acetate copolymer having a different vinyl acetate content. Class, conductor cross section 60m
3 is a graph showing the temperature dependence of the volume resistivity of the outer semiconductive layer of m ² ).

[Explanation of symbols]

 1 conductor 2 inner semi-conductive layer 3 insulator 4 outer semi-conductive layer 5 metal shielding layer 6 hold-down tape 7 sheath

Claims (1)

[Claims] 1. An ethylene-based copolymer having a content of a monomer copolymerizable with ethylene of 30% by weight or more and an MFR of 25 [g / 10 min] or less has a DBP oil absorption of 300.
[Ml / 100g] or more and specific surface area of 1000 [m ² /
g] or more of carbon black of 4 PHR or more and 10 P or more
A composition for a semiconductive layer of a power cable, characterized in that the heat deformation rate when added below HR and when a crosslinking agent is added to effect crosslinking is 120% or less and 30% or less when a 34N load is applied.