JPH08339720A - Cross-linked polyethylene insulating cable and its manufacture - Google Patents

Abstract

PURPOSE: To enhance maintenance free performance and durability by setting its oxygen concentration not more than a prescribed value, and covering an insulator with an oxygen barrier type synthetic resin film in a cross-linked polyethylene insulator. CONSTITUTION: In a cable, an inside semiconductive layer 2, an insulator 3, an outside semiconductive layer 4 and a barrier film 5 are formed in order on a conductor 1. The insulator 3 is composed of polyethylene cross-linked by a DCP, and the oxygen concentration is set not more than 0.5 capacity %. The barrier film 5 is formed of a vinylidene chloride-vinyl chloride copolymer having a thickness of 0.5mm. Therefore, infiltration of oxygen from outside is checked, and for example, even if it is heated by removal of a winding habit at laying time after it is preserved over a long period of time, water is not generated in the insulator 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a crosslinked polyethylene insulated cable and a method for producing the same, and in particular, the generation of water at the time of heating due to the residue of decomposition of the crosslinking agent in the insulator of the crosslinked polyethylene insulated cable is suppressed, TECHNICAL FIELD The present invention relates to a crosslinked polyethylene insulated cable in which characteristic deterioration of the polyethylene insulated cable is prevented for a long period of time, and a method for producing the same.

[0002]

2. Description of the Related Art Recently, electric wires or cables having an insulating layer of crosslinked polyethylene have been widely used. This cross-linked polyethylene insulated cable (hereinafter simply referred to as "cable") is generally made by cross-linking polyethylene, which is an insulator, with dicumyl peroxide (hereinafter referred to as "DCP") as a cross-linking agent to provide heat resistance. It is said that one of the advantages is that it is excellent in maintenance-free property as compared with the conventional insulating oil-filled cable. However, when this cable is heated to, for example, about 100 ° C. or higher during work prior to installation, water is generated inside the insulator, which causes an inconvenient phenomenon such as inducing a water tree.

For example, when removing the curl of the cable prior to laying, the cable is mechanically linearized as called curl removal and then heated by a heater or the like for annealing.
Water may be generated in the insulator during this operation. Moreover, when connecting a cable, the method called an injection mold joint (EMJ) is performed. This is a method in which a crosslinkable polyethylene compound is extruded around the exposed conductor connection and heated in a mold to form a crosslinked insulation coating.In this case, heat is applied to the insulation near the connection. Water is generated. In these cases, in order to prevent the generation of water, it has conventionally been necessary to take care such as annealing or heating at a temperature as low as possible for a long time, which significantly reduces the work efficiency.

Therefore, the cause of this water generation was sought.
As a result, the water generated in the cross-linked polyethylene is
Was found to be generated by decomposition. That is, upon crosslinking, DCP decomposes according to the following formula (I),
By-produces acetophenone, methane, cumyl alcohol, etc.

[0005]

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When heated, the by-produced cumyl alcohol undergoes a decomposition reaction (hereinafter referred to as "secondary decomposition reaction") as shown in the following formula (II) to produce α-methylstyrene and water. .

[0007]

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In order to remove this water, for example, in Japanese Unexamined Patent Publication (Kokai) No. 4-342731, heating is performed to a temperature of 75 ° C. or lower at which the secondary decomposition reaction (II) of cumyl alcohol is suppressed, and an insulator is previously prepared. A method is proposed in which the water content is reduced and the water is newly heated to 85 to 95 [deg.] C. to cause a secondary decomposition reaction gradually and the generated water is evaporated through an insulator. This method has the effect of suppressing the generation of voids and water trees due to a large amount of water being generated at one time, but it takes a long drying time to gradually diffuse the water, and the production efficiency is poor. In addition, JP-A-4-355013
Proposes a method of volatilizing and removing cumyl alcohol, which is a causative substance of water generation, by heating and drying a crosslinked insulator. However, this method is also inferior in production efficiency because an additional heating and drying step for a long time is required to remove cumyl alcohol and generated water.

In addition to the method of removing cumyl alcohol and generated water by heating, the above secondary decomposition reaction (II) is suppressed by adding, for example, an aliphatic amine or an isocyanic acid ester into an insulator as an antioxidant. A method of doing so has also been proposed (see JP-A-1-243306 and JP-A-63-289715, respectively). However, these anti-aging agents may cause delamination or contamination due to blooming, and may expire relatively quickly due to migration or the like. In any case, it is not preferable to mix a large amount of these due to the insulation characteristics of the cable.

[0010]

As a result of research on the above problems, the present inventors have previously found that the oxygen concentration in the insulator is involved in the thermal decomposition of cumyl alcohol. We proposed a cross-linked polyethylene insulated cable whose oxygen concentration was less than 0.5% by volume (Japanese Patent Application No. 5-29).
1048). Although this cable has good control of water generation due to heating, since polyethylene originally has high oxygen permeability, it can be used as a part of cable storage or processing work immediately after crosslinking until installation. It was necessary to keep the whole in a poor oxygen atmosphere with an oxygen partial pressure of 80 mmHg or less. In practice, this is complicated because it is necessary to regularly inspect the atmospheric gas components and replace the gas, and the maintenance-free property of the crosslinked polyethylene insulated cable is impaired from this viewpoint. Therefore, the present invention is a cross-linking method that keeps the oxygen concentration in the insulator at 0.5% by volume or less at all times without the need to store it in an oxygen-deficient atmosphere after production, thereby suppressing the generation of water during heating for a long period of time. The purpose is to provide a polyethylene insulated cable and its manufacturing method.

[0011]

The above object is to provide a cable in which the oxygen concentration in the insulator is 0.5% by volume or less, and the insulator is covered with an oxygen barrier synthetic resin film. Can be solved by The oxygen-barrier synthetic resin film (hereinafter referred to as “barrier film”) basically has a lower oxygen permeability than that of cross-linked polyethylene, but particularly contains vinylidene chloride or (meth) acrylonitrile as a main component. A homopolymer or copolymer,
It is preferably composed of a composition of a synthetic resin selected from the group consisting of ethylene-vinyl alcohol copolymer, polyethylene terephthalate, and nylon-6. Also,
The thickness of this barrier film is preferably 0.03 mm or more and 1 mm or less.

According to the present invention, when the above cable is manufactured, the insulator after cross-linking is kept in a poor oxygen atmosphere having an oxygen partial pressure of 80 mmHg or less so that the oxygen concentration is 0.5% by volume or less. A method for coating an insulator with an oxygen barrier film is provided. In particular, in the process of manufacturing a cable, a methane drying step of drying and removing methane generated by decomposition of DCP according to the formula (I) by heating is introduced into a poor oxygen atmosphere having an oxygen partial pressure of 80 mmHg or less to remove and deoxidize methane. It is preferable to carry out at the same time.

[0013]

Since the oxygen concentration in the insulator is suppressed to 0.5% by volume or less and the barrier film prevents the invasion of oxygen from the outside, this crosslinked polyethylene insulated cable is heated even after being manufactured. No water is generated in the insulator.

Hereinafter, the present invention will be described in more detail by way of examples. FIG. 1 shows a cable which is an embodiment of the present invention. This cable includes an inner semiconductive layer 2, an insulator 3, an outer semiconductive layer 4, and a barrier film 5 on a conductor 1 in this order.
Are formed. This insulator 3 is made of polyethylene crosslinked with DCP and has an oxygen concentration of 0.5% by volume.
It is as follows. The barrier film 5 has a thickness of 0.5 mm
Of vinylidene chloride / vinyl chloride copolymer.

In the cable of this embodiment, the insulator 3 has an oxygen concentration of 0.5% by volume or less and is covered with the barrier film 5 to prevent oxygen from entering from the outside. After being stored for a certain period of time, water may not be generated in the insulator 3 even if it is heated by, for example, removing curl at the time of installation.

This cable can be manufactured by the following method. An inner semiconductive layer 2, an insulator 3, and an outer semiconductive layer 4 are formed on the conductor 1 by three-layer coextrusion, and then heated in a methane drying furnace in a poor oxygen atmosphere with an oxygen partial pressure of 80 mmHg or less, The generated methane is diffused and oxygen in the insulator is removed until the volume becomes 0.5% by volume or less, and then a vinylidene chloride / vinyl chloride copolymer is formed as a barrier film 5 to a thickness of 0.5 mm. It was extruded so that the cable of the example was obtained. This cable can be further formed with a shielding layer, a rubberized tape layer, a sheath, etc. on the barrier film 5 by a conventional method.

In general, a cross-linked polyethylene insulation cable is extruded from DCP-containing polyethylene and then cross-linked in an atmosphere of steam, nitrogen gas, silicone oil or the like, so that oxygen is hardly present in the insulator in this state. However, this insulator absorbs oxygen in the air after crosslinking and the oxygen concentration gradually increases. For example, in the above-mentioned methane removal step, the cable after crosslinking is heated to 40 to 90 ° C for about 1 day to 1 month. If this is performed in the atmosphere, the oxygen concentration in the insulator exceeds 0.5% by volume during this period. This is also clear from the fact that the equilibrium value of the oxygen concentration under the atmospheric pressure of 25 ° C. is about 1% by volume based on the volume of the insulator. Therefore, this methane removal process is carried out at an oxygen partial pressure of 80 m.
It is performed in a poor oxygen atmosphere of mHg or less.

To reduce the partial pressure of oxygen to 80 mmHg or less, there are a method of reducing the pressure of the atmosphere and a method of diluting with an inert gas. Any of these can be adopted, or these methods can be combined. In any case, since the oxygen partial pressure under atmospheric pressure is about 160 mmHg, the object can be achieved by reducing the oxygen partial pressure to about half or less. In particular, in the methane drying step, if the inert gas is circulated while the atmosphere is kept at a reduced pressure, the removal of methane and oxygen is promoted together, so that the step is greatly improved in efficiency.

The inert gas is not particularly limited as long as it is a non-corrosive gas other than oxygen, for example, N ₂ ,
SF ₆ , He, CO ₂ or the like can be used. Of these, if SF ₆ is used as the inert gas, not only oxygen is removed but also the pressure resistance of the insulator is improved. Generally, it is preferable to use N ₂ gas because it is easily available.

When the oxygen concentration becomes 0.5% by volume or less as described above, the barrier film 5 is coated on the outer semiconductive layer 4. The barrier film 5 of the embodiment is applied directly on the outer semiconductive layer 4, but the present invention is not limited to this, and another layer is interposed between the outer semiconductive layer 4 and the barrier film 5. May be. The barrier film 5 may be made of any synthetic resin as long as it has a lower oxygen permeability than the outer semiconductive layer 4. However, if an attempt is made to enhance the oxygen barrier effect without making the film thickness excessively large, the oxygen transmission rate (30 ° C.) will be 0.1 × 10 ⁻¹⁰ cm ³ (STP) · cm ·
It is preferable to use a synthetic resin composition having a cm ⁻² · s ⁻¹ · kPa ⁻¹ or less.

Examples of the synthetic resin composition include homopolymers or copolymers containing vinylidene chloride or (meth) acrylonitrile as a main component, ethylene-vinyl alcohol copolymer, polyethylene terephthalate, and nylon-6. Can be mentioned.
This synthetic resin composition may contain a synthetic resin, a plasticizer, a filler, and the like other than the above, as long as the oxygen barrier property and other properties are not significantly impaired.

The thickness of the barrier film is, as long as the oxygen concentration in the insulator is maintained at 0.5% by volume or less during the storage period of the cable, its material (oxygen permeability), operating environment conditions, production Although not particularly limited, it is generally preferably 0.03 mm or more and 1 mm or less in relation to the difficulty of the film and the like. If the thickness is less than 0.03 mm, uniform film formation by extrusion is difficult and the film strength is insufficient. If it exceeds 1 mm, the cable diameter increases and the quality becomes excessive.

[0023]

【Example】

(Example 1) 2 PH of DCP was formed on a conductor having a diameter of 60 mm.
R-containing polyethylene is extruded and dry-crosslinked to a thickness of 2
A 5 mm cross-linked polyethylene insulator was formed into a cable body. Next, this was subjected to methane drying in a methane drying furnace maintained in a nitrogen atmosphere of 1 atm (oxygen partial pressure of 80 mmHg or less). Methane was dried at 80 ° C. for 10 days. After drying with methane, a vinylidene chloride / vinyl chloride copolymer resin was extruded on the cable body to a thickness of 0.5 mm to form a barrier film, and the cable of Example 1 was obtained. The vinylidene chloride-vinyl chloride copolymer resin used had a vinylidene chloride: vinyl chloride molar ratio of 70:30 and had an oxygen permeability (3
0 ° C) is 0.008 × 10 ^-10 cm ³ (STP) · cm ·
It was cm ^-2 · s ⁻¹ · kPa ⁻¹ .

Comparative Example A cable body was prepared in the same manner as in Example 1, except that a barrier film was not applied after drying methane under a nitrogen atmosphere to obtain a cable of a comparative example.

The cables of Example 1 and Comparative Example above were subjected to a water generation test by secondary decomposition of the crosslinking agent.
The cable of Example 1 was kept in air at 70 ° C. for 3 days, then wrapped with aluminum tape to prevent evaporation of water, and then kept in an air oven at 120 ° C. for 10 days.
Heated for hours. The amount of water generated in this insulator was measured by the Karl Fischer method. The comparative example halves the sample, one (Comparative sample 1) kept in air at 70 ° C. for 3 days, the other (Comparative sample 2) kept in nitrogen at 70 ° C. for 3 days, and then each of the examples and Similarly, an aluminum tape was wound and heated, and the amount of water generated in this insulator was measured. The above results are shown in Table 1.

[0026]

[Table 1]

In Table 1, in Comparative Sample 1, although the initial oxygen concentration in the insulator was low, since there was no barrier film, oxygen invaded while being kept at 70 ° C. in the atmosphere,
Cumyl alcohol was decomposed by heating to generate water. Since the comparative sample 2 was held in nitrogen and oxygen did not penetrate, the amount of water generated was relatively small even when heated.
On the other hand, the cable of Example 1 had a low initial oxygen concentration in the insulator, even though it was kept at 70 ° C. for 3 days in the atmosphere, as in Comparative Sample 1, and the barrier film prevented the external oxygen from entering from the outside. The generation of water was extremely small even when heated, because the invasion of oxygen was blocked.

(Examples 2 to 5) A cable body was prepared in the same manner as in Example 1 and dried with methane in a nitrogen atmosphere at 1 atm (oxygen partial pressure of 80 mmHg or less), and then the following synthetic resin compositions were used. Was extruded to form a barrier film, which was used as each example. Oxygen permeability of each synthetic resin composition (3
0 ° C) (cm ³ (STP) · cm · cm ^-2 · s ^-1 · kP
a ^-1 ) and thickness (mm) are shown below.

Synthetic resin composition Oxygen permeability × 10 ⁻¹⁰ Thickness mm Example 2 Polyethylene terephthalate 0.045 0.5 Example 3 Ethylene 44 / vinyl alcohol 56 copolymer 0.0003 0.1 Example 4 Nylon-6 0.051 0.5 Example 5 Ropaque ^{( a)} 0.0047 0.2 ^(a) Acrylonitrile copolymer manufactured by Monsanto USA

The cables of the respective examples were kept at 70 ° C. in the atmosphere for 3 days in the same manner as in the case of Example 1, and then aluminum tape was wrapped around the cables to prevent evaporation of water, and then in an air oven. Heated to 120 ° C. for 10 hours. The amount of water generated in this insulator was measured by the Karl Fischer method. Table 2 shows the results.

[0031]

[Table 2]

From the results in Table 2, it is clear that the barrier films of Examples 2 to 5 prevented the invasion of oxygen and prevented the generation of water during heating.

[0033]

INDUSTRIAL APPLICABILITY The crosslinked polyethylene insulation cable of the present invention has an oxygen concentration of 0.5% by volume or less in the insulator, and since this insulator is covered with a barrier film, it is heated in the atmosphere. Even if this happens, water is not generated in the insulator, and a cable having excellent maintenance-free property and durability can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of the present invention.

[Explanation of symbols]

3 ... Cross-linked polyethylene insulator, 5 ... Oxygen barrier synthetic resin film.

Claims (3)

[Claims] 1. A crosslinked polyethylene insulated cable in which the oxygen concentration in the crosslinked polyethylene insulator is 0.5% by volume or less, and the crosslinked polyethylene insulator is covered with an oxygen barrier synthetic resin film. 2. A synthetic resin film having an oxygen barrier property comprises a homopolymer or copolymer containing vinylidene chloride or (meth) acrylonitrile as a main component, an ethylene-vinyl alcohol copolymer, polyethylene terephthalate, and nylon-6. The crosslinked polyethylene insulated cable according to claim 1, comprising a composition of a synthetic resin selected from the group. 3. When producing a cross-linked polyethylene insulation cable, the cross-linked polyethylene insulation is kept in a poor oxygen atmosphere having an oxygen partial pressure of 80 mmHg or less so that the oxygen concentration is 0.5% by volume or less. A method for producing a crosslinked polyethylene insulated cable in which a crosslinked polyethylene insulator is covered with an oxygen barrier synthetic resin film.