JPH03280309A - Manufacture of cross-linked polyolefin insulated cable - Google Patents

Abstract

PURPOSE:To improve the heating efficiency of a cable sheathed with a cross- linked polyolefin insulator forming its cross-link using an organic peroxide so as to dry the cable in a short time by heating and drying the cable while vacuum-drawing it. CONSTITUTION:A cable sheathed with a cross-linked polyolefin insulator forming its cross-link using an organic peroxide is heated and dried while being vacuum- drawn. This suppresses the generation of water as the result that the resolved residue of the organic peroxide causes the secondary resolution thereof, and also removes the generated water so that a stable characteristic cable prevented from the generation of a water tree can be manufactured. Moreover, since the core of the cable is inductively heated by an inductive heating coil installed in a drying kiln at the time of heating and drying the cable while vacuum- drawing it, the heating efficiency of the cable can be substantially increased to quicken the heating speed thereof while the whole of the cross-linked polyolefin insulator can be heated and dried uniformly. Because the cable can be dried in a short time, the stable characteristic cross-linked polyolefin insulated cable not generating water tree can be manufactured with excellent manufacturing efficiency.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to an improvement in the M-threading method for a crosslinked polyolefin insulated cable coated with a crosslinked polyolefin formed by crosslinking with an organic peroxide.

(Prior art) Conventionally, as a method for crosslinking the insulator of a crosslinked polyolefin insulated cable, a crosslinking agent made of an organic peroxide such as dicumyl peroxide (DCP) is added to an insulator composition and crosslinking is carried out by heating. is widely practiced.

However, in crosslinking using such an organic peroxide, the heat applied during the crosslinking process causes further secondary decomposition of the decomposition residue of the organic peroxide (cumyl alcohol when the crosslinking agent is DCP) to produce water. As a result, moisture remains in the insulation. This residual moisture increases the crosslinking process! ! ! Even in the case of so-called dry crosslinking, which is carried out using an inert gas without using high-pressure steam, the amount reaches about 200 ppm, which is a supersaturation state with respect to the permissible saturated water content of polyethylene of 50 ppm. Furthermore, the decomposition residue of organic peroxides undergoes secondary decomposition due to the thermal history that occurs during subsequent use of the cable, so the residual moisture in the insulator can temporarily reach several hundred ppm. Such a high concentration of residual moisture inside the cable promotes the occurrence of water trees and causes deterioration of the electrical properties of the cable.

To solve this problem, the secondary decomposition of organic peroxide decomposition residue is further accelerated in the presence of air (oxygen), so after crosslinking the polyolefin insulation on the conductor, it is necessary to attach it to the cable drum. A cable manufacturing method has already been proposed in which the cable is wound, stored in a drying pot, and then heated and dried under vacuum to reduce residual moisture. however,
Gable dry soot ovens usually use a heating method in which water vapor is filled between the double walls to raise the wall temperature, and the cable is heated by heat radiation from the walls and convection of air inside the drying oven. For this reason, if heating is performed while vacuuming, the presence of air, which is a heat medium, is extremely thin, so you will rely only on heat radiation from the wall surface.
A new problem has arisen in that the heating efficiency of the cable is very poor. Therefore, the heating and drying time is prolonged, resulting in a decrease in productivity and an increase in fuel consumption, as well as uneven heating of the cable.

(Problems to be Solved by the Invention) In view of the above points, the present invention aims to reduce the residual moisture content of a cross-linked polyolefin insulated cable by organic peroxide cross-linking and to prevent the occurrence of water trees by heating while vacuuming. In the drying process, cross-linked polyolefin P increases the heating efficiency of the couple, significantly shortens the required heating time, and evenly heats and dries the entire cable. It is an object of the present invention to provide a method for manufacturing an edge couple.

(Means for Solving the Problems) The present invention consists of coating a conductor with a polyolefin composition containing an organic peroxide as a crosslinking agent and crosslinking it to form a crosslinked polyolefin insulator, and then installing an induction heating coil. The present invention relates to a method for manufacturing a crosslinked polyolefin insulated cable, which comprises storing the cable in a dried drying pot and heating and drying it by induction heating while drawing a vacuum.

The polyolefin composition in the present invention includes polyethylene, polypropylene, ethylene-propylene copolymer (EP), and ethylene-vinyl acetate copolymer (EVA).
Based on polyolefins such as ethylene-based copolymers such as It is possible to use a composition in which the composition is blended with an inorganic filler, an anti-aging agent, and other additives as necessary.

In the present invention, such a polyolefin composition is extruded and coated onto a conductor, crosslinked by a conventional method such as steam crosslinking or dry crosslinking, and then placed in a drying pot equipped with an induction heating coil and evacuated. The film is then dried by induction heating at a temperature of 80 to 90°C for a predetermined period of time. The heat drying is carried out to the extent that no deterioration of the electrical properties of the insulator appears, that is, the residual moisture content is the permissible saturated moisture content of the polyolefin.
It is preferable to continue the process at the elbow where the temperature is below Elpm.

Next, a cable induction heating method will be explained with reference to the drawings. FIGS. 1 and 2 are cross-sectional views showing an embodiment of the manufacturing method of the present invention.

In FIG. 1, an iron core 2 is installed vertically at the center of a cable drum 6 around which a manufactured cable core 5 is wound, and an induction heating coil 4 is wound around a portion below the floor surface 3 of the drying oven 1. In this case, both ends of the cable core 5 are electrically connected in a conventional manner, and if a current is caused to flow through the induction heating coil 4 in this way to cause a change in the magnetic field,
A circuit current flows through the cable conductors, heating the cable core.

In addition, as shown in Fig. 2, it is installed on the lid of the drying pot 1.
', - When a current is passed through the induction heating coil 4, an eddy current flows through the cable conductor due to S induction, heat is generated due to the conductor resistance, and the cable core 5 is heated.

If this is not the case and an electric current is used, the induction heating coil may be installed at the bottom of the drying oven, or may be used at both the lid and the bottom. Furthermore, even if two magnetic poles are placed facing each other along the inner wall surface of the drying pot 1, and two induction heating coils are placed outside the drying pot 1 on the m-polarity side, eddy currents will not be generated in the cable conductor. can be generated and heated by induction.

By heating the cable core from the inside by induction heating in this manner, the heating efficiency of the crosslinked polyolefin insulator is greatly improved, and the drying time for residual moisture can be shortened.

Note that such a drying process reduces not only moisture but also decomposition residues of the crosslinking agent. For example, in the case of DCP, mill alcohol will later undergo secondary decomposition and generate water.
The amount is reduced to 0.1 to 0.5% by weight, which is about one-half or less compared to the case where such drying treatment is not performed.

In the present invention, the cable core is heated by induction while the inside of the drying pot is evacuated, and after heating and drying, the inside of the drying pot is returned to normal pressure. is more desirable.

This is because by returning the pressure to normal pressure using an inert gas, the subsequent intrusion and diffusion of oxygen and secondary decomposition of the crosslinking agent decomposition residue are suppressed, and the amount of increase in residual moisture becomes extremely small.

(Function) In the method of the present invention, a cable having a cross-linked polyolefin insulator made of organic peroxide cross-links is heated and dried while being vacuumed, so that the decomposition residue of the organic peroxide secondary decomposes and generates moisture. Since it suppresses the occurrence of water tree and removes the generated moisture, it is possible to manufacture a cable with stable characteristics that prevents the occurrence of water trees. Furthermore, when heating and drying while vacuuming, the cable core is heated by induction using an induction heating coil installed in the drying oven, which greatly increases heating efficiency and speeds up the heating rate. The entire surface can be uniformly heated and dried.

(Example) Examples of the present invention will be described.

Implementation PA] A crosslinkable semiconducting composition (A) based on EVA for forming a semiconducting layer and adding Perhexine (trade name) as a crosslinking agent, and a polyethylene based composition (A) for forming an insulator with DCP added. A crosslinkable insulating composition (B) was prepared. Place this on a conductor with a diameter of 45 mm (A), (B)
, (A) were coated in three layers by co-extrusion so that the thicknesses were 1.3 m, 27 m, and 1.0 mm in this order, and dry heat crosslinking was performed to form an inner semiconductive layer, an insulating layer, and an outer semiconductive layer. After that, it was wound up on a cable drum and then placed in a drying pot with an induction heating coil attached to the lid as shown in Fig. 2, and heated to below 0.5 TOrr using a vacuum pump (not shown). While the inside was evacuated, a current was passed through the induction heating coil to heat the cable core from inside. It took 24 hours for the insulator surface temperature to reach 90°C. When heating was continued for another 20 to 25 days to maintain this temperature, the moisture content in the insulator was 151) p
It became ll. After heating, the pressure was returned to normal pressure in a nitrogen gas atmosphere.

Comparative Example] A cable core similar to that in Example 1 was wound onto a cable tram, and then placed in a conventional drying oven in which the gap between the inner and outer walls was filled with water vapor, and heated to 0.5 Tor with a vacuum pump.
The cable core was heated while the inside of the drying pot was evacuated to below r. The time required for the surface temperature of the insulator to reach 90"C was 261 hours. When heating was continued to maintain this temperature, the moisture content in the insulator increased to 151".
It took an additional 25-30 days of heating to reach I)it.

Comparative Example 2 The cable core was heated from the inside by induction heating in the same manner as in Example 1 except that the inside of the drying pot was not evacuated. The time required for the insulator surface temperature to reach 90°C was 5 to 10 hours.

Continue heating to maintain this temperature for an additional 30 days C-
), the moisture content in the insulator did not fall below 50 pI).

(Effects of the Invention) As is clear from the above examples, according to the method of the present invention, when removing residual moisture in an insulator by heating and drying while vacuuming, heating efficiency is greatly improved and a short time is achieved. Since drying can be carried out during the drying process, a crosslinked polyolefin insulation couple with stable characteristics without water tree generation can be produced with high production efficiency.

[Brief explanation of drawings]

1 and 2 are cross-sectional views showing an embodiment of the manufacturing method of the present invention. 1...Drying pot 4...Induction heating coil 5...Cable core Fig.

Claims (1)

[Claims] (1) After coating a conductor with a polyolefin composition containing an organic peroxide as a crosslinking agent and crosslinking it to form a crosslinked polyolefin insulator, it is placed in a drying pot equipped with an induction heating coil and evacuated. A method for producing a crosslinked polyolefin insulated cable, characterized by heating and drying by induction heating.