JPH03127412A - Manufacture of bridging polyolefine insulated cable - Google Patents

Abstract

PURPOSE:To prevent generation of a water tree of a bridging polyolefine insulating coat by, heat-drying the insulated coat while evacuating after bringing. CONSTITUTION:A polyolefin composition in which an organic peroxide is included as a bridging gear, is excursively spread and bridged on a conductor, so as to form a bridging polyolefin insulating coat. The bridging polyolefine insulating coating heat-dried, while the conductor is being evacuated, then it is returned under a normal pressure in an inert gas atmosphere. Due to the drying process, not only moisture but also a resolved residue of the bridging agent are reduced. When the resolved residue left undergoes the subsequent process of returning the normal pressure in the inert gas atmosphere, secondary resolution is suppressed, and abrupt generation of water is suppressed thereby. The generation of a water tree of the insulating coat is thus prevented, and a bridging polyolefin insulated cable having good and stable characteristic can thus be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a method for manufacturing a crosslinked polyolefin insulated cable having a crosslinked polyolefin insulation coating formed by crosslinking with an organic peroxide.

(Prior technology) In manufacturing cross-linked polyolefin insulated cables,
A polyolefin composition containing an organic peroxide such as DCP (dicumyl peroxide) as a crosslinking agent is extrusion coated onto a continuously fed conductor, and then heat treated to form a crosslinking structure. A commonly used method is to provide a cross-linked polyolefin rim cover.

However, in the second method, the crosslinking reaction of the polyolefin composition proceeds as shown in the following formula, and the resulting moisture remains in the insulation coating along with the decomposition residue of the crosslinking agent.

That is, the following formulas [11-[III] are shown as an example of the crosslinking reaction of polyethylene by DCP, and are produced as a decomposition product of water or cumyl alcohol.

e-C-0-0-Cbe) - groan 2 第>C-0・[1
1Ct13CII3C
lhC113 The amount of water generated in such a crosslinking process reaches about 200 ppH, even in the case of so-called dry crosslinking, in which the heat treatment is performed using an inert gas instead of high-temperature, high-pressure steam, and the amount of water generated in the crosslinking process reaches approximately 200 ppH, which exceeds the permissible saturated moisture content of polyethylene (m501) I )III
It is considerably oversaturated.

In addition, the cumyl alcohol produced in formula [■] remains in the insulation coating as a residue, and due to the heat generated after using the cable, the secondary decomposition reaction shown in formula [III] progresses and produces water, so the cumyl alcohol is left in the insulation coating. The water contained in is temporarily several hundred pp.
Concentrations as high as ra can be reached.

Such high concentrations of water cause water trees to form in the insulation coating, degrading the electrical properties of the cable.

(Problems to be Solved by the Invention) As described above, in the conventional method, there was a problem in that a high concentration of water remained in the polyolefin insulation coating of the cable frame (16), causing water trees and deteriorating the cable properties.

The present invention was made in order to solve such conventional problems, and reduces the water content in the cross-linked polyolefin insulation coating to at least the allowable saturated water content not only immediately after cross-linking but also after the cross-linking. It is an object of the present invention to provide a method for manufacturing a crosslinked polyolefin insulated cable that prevents the occurrence of water tree etc. in the insulation coating.

[Structure of the Invention] (Means for Solving the Problems) The method of the present invention involves extruding a polyolefin composition blended with an organic peroxide as a crosslinking agent onto a conductor and crosslinking it to form a crosslinked polyolefin insulation coating. After forming, the crosslinked polyolefin insulation coating is heated and dried under vacuum, and then returned to normal pressure in an inert gas atmosphere.

In the present invention, the polyolefin composition containing an organic peroxide as a crosslinking agent includes polyethylene, polypropylene, ethylene/propylene copolymer, ethylene/propylene copolymer,
Based on polyolefins such as ethylene copolymers such as vinyl acetate copolymer (EVA), DCPSl and 3-bis(t-butyl peroxy isobrohivil) benzene are generally known as crosslinking agents for this type of polymer. Select and blend those suitable for the base polymer to be used from among those that are used, and add inorganic chargers, if necessary.
Examples include those mixed with anti-aging agents and other additives.

In the present invention, after such a polyolefin composition is extruded onto a conductor and crosslinked, the crosslinked polyolefin insulation coating is dried.
Vacuum to below 5 Torr.

The method involves heating at a temperature of 80 to 90°C for a predetermined period of time until the water in the insulation coating reaches an allowable saturated moisture content of polyolefin, that is, a moisture content of about 50 pp11 that does not affect the properties of the insulation coating, followed by drying. is suitable. The heating time varies depending on the insulation wave of the cable and the thickness of the sheathing; for example, in the case of an insulating sheath with a thickness of 27 m11, it takes about 25 to 30 days under the above heating conditions.

Note that such drying treatment reduces not only moisture but also decomposition residues of the crosslinking agent. For example, taking the aforementioned crosslinking by CDP as an example, since it later undergoes secondary decomposition and generates water, the amount of mill alcohol is approximately 1/2 or less compared to the case where such drying treatment is not performed. It decreases to about 0.5wt%.

However, the remaining decomposition residue is subjected to the next step of returning to normal pressure in an inert gas atmosphere, thereby suppressing secondary decomposition and suppressing rapid generation of water.

In other words, this type of secondary decomposition reaction proceeds with oxygen in the atmosphere as a catalyst, so by returning the pressure to normal pressure in an inert gas atmosphere, such as nitrogen gas, the subsequent invasion and diffusion of oxygen is suppressed, resulting in decomposition. The speed slows down and water production becomes very slow, and during this time the moisture naturally evaporates, so the moisture in the insulation coating remains almost the same as it was immediately after the drying process.

In the method of the present invention, as described above, by heating and drying while drawing a vacuum after cross-linking the gun edge wave crest, the water generated by cross-linking is almost completely removed, and the cross-linking decomposition residue is also removed to some extent. By returning the pressure to normal pressure in an inert gas atmosphere after heating and drying, the secondary decomposition reaction of the slight remaining crosslinking residue is suppressed, and the generation and dissipation of water are balanced, not only immediately after crosslinking but also afterwards. This also prevents water from rapidly increasing in the insulation coating. Therefore, the occurrence of water trees in the insulation coating is prevented, and a crosslinked polyolefin insulated cable with stable characteristics can be obtained.

(Example) Next, examples of the present invention will be described.

A crosslinkable semiconductive composition (A) based on EVA for forming a semiconducting layer and containing Verhexin (trade name) as a crosslinking agent; a semiconductive composition (A) based on low density polyethylene for forming an insulator and containing Verhexin (trade name) as a crosslinking agent; The crosslinkable insulating composition (B) containing DCP was applied onto a conductor having an outer diameter of 45 IDI11 in the order of (A), (B), and (A), each having a jl of 1.
．． Three layers were simultaneously extruded and corrugated to give a thickness of 3 mm, 27n+Ia, 1,011110, and then subjected to IQ in a conventional manner to form an inner semiconductive layer, an insulator, and an outer semiconductive layer, and then placed in a drum. Next, this was placed in a heating vacuum chamber, and the temperature was reduced to 0.5
It was kept at a temperature of 80 to 90°C for 25 to 30 days without evacuation to Torr or less. Thereafter, the pressure was returned to normal using nitrogen gas, and the chamber was taken out from the heating vacuum chamber.

The moisture content in the insulator of the cable thus obtained was 15 ppm immediately after being taken out from the heating vacuum chamber. Moreover, this cable was left as it was, and the moisture content in the insulator was measured in the same manner after tol], and the moisture content hardly increased.

For comparison, we measured the moisture content in the insulator of a cable that had been heat-dried under vacuum as described above, taken out of the drying chamber, and returned to normal pressure, and found to be 60 ppm.

[Effects of the Invention] As is clear from the above examples, according to the method of the present invention, a polyolefin composition containing an organic peroxide as a crosslinking agent is extrusion coated, crosslinked, and then heated under vacuum. Since the cable is dried and then returned to normal pressure in an inert gas atmosphere, the moisture content in the insulation coating of the cable decreases and does not increase rapidly. Therefore, the occurrence of water trees in the insulation coating is prevented, and a crosslinked polyolefin insulated cable having good and stable properties can be obtained.

Claims (1)

[Claims] (1) A polyolefin composition containing an organic peroxide as a crosslinking agent is extrusion coated onto a conductor, crosslinked to form a crosslinked polyolefin insulation coating, and then the crosslinked polyolefin insulation coating is heated and dried while vacuuming. A method for producing a cross-linked polyolefin insulated cable, which is then returned to normal pressure in an inert gas atmosphere.