JP2003022716A - Insulating power cable made of crosslinked polyethylene - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an insulating power cable made of crosslinked polyethylene, which can be manufactured at a low price, and in which tan δ in a high temperature/high electric field is reduced. SOLUTION: This is the insulating power cable made of crosslinked polyethylene wherein the insulator layer is composed of a crosslinked material of a resin composition in which 0.2 to 0.5 parts by weight of antioxidant prepared by mixing bis[2-methyl-4-(3-n-alkylthiopropionyloxy)-5-t-butylphenyll sulfide 95 to 80 wt.% and 4,4'-thiobis (3-methyl-6-t-butylphenol) 5 to 20 wt.% are compounded against 100 parts by weight of polyethylene, and wherein grafting rate of bis[2-methyl-4-(3-n-alkylthiopropionyloxy)-5-t-butylphenyll sulfide component to the polyethylene is 75% or more in its crosslinked material.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a crosslinked polyethylene insulated power cable. 2. Description of the Related Art Insulation of electric power cables has electrical characteristics and characteristics.
Crosslinked polyethylene is widely used because of its excellent heat resistance and easy maintenance. Cross-linked polyethylene insulated power cables are generally formed by extruding a resin composition comprising a low-density polyethylene and a predetermined amount of a cross-linking agent and antioxidant on a conductor, and then heating under pressure to decompose the cross-linking agent and cross-link the polyethylene Molded in. In recent years, the voltage of cross-linked polyethylene insulated cables has been increased, and accordingly, the insulator is exposed to severe conditions. Therefore, it is necessary to minimize foreign matter, voids, etc., which significantly lower the electrical characteristics. Therefore, in order to prevent foreign substances from being mixed into the insulator, a method of filtering foreign substances in the base resin and the additives such as a crosslinking agent and an antioxidant through a screen mesh having small openings has been adopted. Dicumyl peroxide (DCP), which is commonly used as a cross-linking agent, has a melting point of about 40 ° C., and can be liquefied by heating. However, 4,4′-thiobis (3-methyl-6-t-butylphenol) used as an antioxidant (antiaging agent) has a melting point of about 155 ° C., so that liquefaction is difficult and liquefaction is difficult. Since the solubility in the liquefied DCP is low, the required amount cannot be dissolved in the liquefied DCP. Therefore, 4,4'-thiobis (3-methyl-6-t-butylphenol) cannot be used after being previously filtered through a mesh having a small opening. Further, there is a method in which a required amount of DCP and 4,4′-thiobis (3-methyl-6-t-butylphenol) are sprinkled on polyethylene using a blender or the like, and the resulting mixture is introduced into an extruder and filtered. Even in this case, the extrusion temperature is 130 ° C
4,4'-thiobis (3-methyl-6-t
-Butylphenol) is a solid and has a problem that the screen mesh is clogged due to the presence of particles larger than the screen openings. On the other hand, bis [2-methyl-4- (3-n-alkylthiopropionyloxy) -5-t-butylphenyl] is used as an antioxidant.
Application of sulfide (trade name: ADK STAB AO23: manufactured by Asahi Denka Kogyo Co., Ltd.) has also been studied (Japanese Patent Application Laid-Open No. Hei 9-18).
No. 8785). Since this antioxidant is liquid at room temperature, it is possible to effectively remove foreign matter with a fine mesh. However, this AO23 becomes a charge trap site in the crosslinked polyethylene. In particular, there is a problem that the amount of charges trapped in a high electric field increases, and that mobility increases at a high temperature and tan δ at a high temperature and a high electric field increases. The rise of tan δ under high temperature and high electric field is caused by deterioration of insulation characteristics due to heat generation of the insulator layer,
This causes a decrease in the transmission capacity of the power cable due to an increase in the dielectric loss. As a method for solving this, a compound represented by the following formula (1) is added to a compound similar to AO23 by 90 /
A method of mixing at a ratio of 10 to 50/50 (weight% ratio) is known (JP-A-7-118463). Formula (1) (Wherein R is -C ₁₂ H ₂₅ or -C ₁₄ H
Represents ₂₉ . However, since the compound represented by the formula (1) has a special chemical structure, it is not a commercially available product that can be easily obtained, and the cost is high. In addition, the addition of the compound represented by the formula (1) increases the viscosity of the solution, which inevitably makes filtering with a fine mesh difficult. SUMMARY OF THE INVENTION An object of the present invention is to provide a crosslinked polyethylene insulated power cable which can be manufactured at a low cost and has a reduced tan δ at a high temperature and a high electric field. Means for Solving the Problems The present inventors have made intensive studies on means for solving these problems, and as a result,
AO23 and 4,4'-thiobis (3-methyl-6-
(t-butylphenol) and 95/5 to 80 by weight%.
Tan δ at high temperature and high electric field is reduced by using a crosslinked product of a composition obtained by mixing 0.2 to 0.5 part by weight of the antioxidant mixed at / 20 with 100 parts by weight of polyethylene as an insulator. It has been found that a crosslinked polyethylene insulated power cable can be provided, and based on this finding, the present invention has been accomplished. That is, according to the present invention, the insulator layer is composed of bis [2-methyl-4- (3-n-alkylthiopropionyloxy) -5-t-butylphenyl] sulfide 95 to 80 parts by weight based on 100 parts by weight of polyethylene.
% By weight and 5 to 20% by weight of 4,4'-thiobis (3-methyl-6-t-butylphenol) and 0.2 to 0.5 part by weight of an antioxidant, crosslinked resin Bis [2-methyl-4- (3-n-alkylthiopropionyloxy)] in the crosslinked resin composition
[-5-t-butylphenyl] sulfide component having a graft ratio of 75% or more to polyethylene is provided. The polyethylene used in the present invention includes, for example, high- and medium-pressure polyethylene, low-pressure polyethylene, ultra-low-density polyethylene, linear low-density polyethylene, ethylene-propylene copolymer, Examples include an ethylene-α-olefin copolymer, an ethylene-vinyl acetate copolymer, an ethylene-ethyl acrylate copolymer, an ethylene-styrene copolymer, and other copolymers. The crosslinked product of the resin composition of the present invention can be obtained by blending an organic peroxide conventionally used as a crosslinking agent with the above resin composition, and extruding the mixture to carry out heat crosslinking. The organic peroxide is not particularly limited as long as it is usually used for crosslinking. For example, benzoyl peroxide, dicumyl peroxide (DC
P), t-butylcumyl peroxide, 2,5-dimethyl-2,5-di-tert-butylperoxyhexene, di-tert-butyl peroxide, 2,4-dichloro-benzoyl peroxide and the like. Can be. The crosslinking agent used in the present invention is preferably dicumyl peroxide. The compounding amount of the crosslinking agent is polyolefin 10
1.0 to 2.5 parts by weight per 0 parts by weight is preferred. If the amount is too small, crosslinking will not be performed sufficiently, and the mechanical properties and heat resistance of the insulating layer will decrease.If the amount is too large, burning will occur when the resin composition is extruded, and the electrical characteristics will decrease. . Next, the crosslinking reaction of the resin composition according to the present invention will be described. Antioxidant (hereinafter represented by AH)
Is crosslinked with an organic peroxide (hereinafter, represented by R'OOR '), the crosslinking reaction is generally considered to proceed as follows. ing. First, radicals are generated based on R'OOR '→ 2R'O. By thermal decomposition of R'OOR'. Then, this radical abstracts hydrogen from AH and PH, and the following formula: 3R′O · + AH + PH
→ 2R'OH + ・ A ・ R'O ・ + PH → 3R'O
Based on H + P. +. A., An antioxidant and a radical of polyethylene (.A. And P. in the formula) are generated. Then, at the same time as the crosslinking between the radicals of the polyethylene proceeds, the following formula: A · + 2P · → PA
By -P, the antioxidant is graft-polymerized to the polyethylene. Eventually, in the overall crosslinking reaction, the antioxidant is graft-polymerized to the ethylene main chain and fixed there. In the present invention, a mixed antioxidant is used as described above. For the sake of explanation, 4,4'-thiobis (3-methyl-6-t-butylphenol) is referred to as "antioxidant A", Bis [2-methyl-4- (3-n-alkylthiopropionyloxy) -5-t-butylphenyl]
The sulfide is also called “antioxidant B”. Since 4,4'-thiobis (3-methyl-6-t-butylphenol) (antioxidant A) has a phenolic hydroxyl group in the molecule, hydrogen is more easily extracted by the above radicals than polyethylene. Therefore, at the time of crosslinking, the antioxidant A
Since hydrogen abstraction occurs preferentially, the polymer is completely grafted to the polymer after crosslinking. When the antioxidant A is grafted onto the polymer, an increase in mobility at high temperature can be suppressed, and thus an increase in tan δ at high temperature and high electric field can be suppressed. On the other hand, bis [2-methyl-4- (3-n-
Alkylthiopropionyloxy) -5-t-butylphenyl] sulfide (antioxidant B) does not have a phenolic hydroxyl group, so hydrogen abstraction is carried out by n-
Will occur in the alkyl group. Therefore, the ease of hydrogen extraction is almost the same as that of polyethylene, so that a part of the antioxidant B remains ungrafted,
This causes tan δ to rise in a high electric field. The n-alkyl group is not particularly limited,
An n-alkyl group having 3 to 18 carbon atoms is preferable, for example,
n-butyl, n-propyl, n-hexyl, n-dodecyl, n-tetradecyl and the like. For example, bis [2-methyl-4- {3-n-alkyl (C ₁₂
Or Adecastab AO23 (trade name, manufactured by Asahi Denka Kogyo KK) containing C ₁₄ ) thiopropionyloxy {-5-t-butylphenyl] sulfide as a main component.
The antioxidant B used in the present invention may be a mixture having different n-alkyl groups. In the present invention, by mixing the antioxidant A with the antioxidant B as a mixed antioxidant, the grafting efficiency of the antioxidant B to polyethylene can be increased, thereby increasing the temperature at high temperatures. Tan at high electric field
δ rise can be suppressed. In order to suppress the rise of tan δ at high temperature and high electric field, the graft ratio of antioxidant B to polyethylene needs to be 75% or more. This is because if the graft ratio is lower than this, a sufficient effect cannot be obtained in suppressing the increase in tan δ. The graft ratio is preferably from 80 to 100%. The graft ratio of antioxidant B to polyethylene can be determined by slicing the obtained crosslinked polyethylene insulator and measuring IR before and after extracting the antioxidant B with a good solvent according to the following formula. Graft ratio (%) = absorbance at 1725 cm ⁻¹ after solvent extraction / absorbance at 1725 cm ⁻¹ before solvent extraction (1725 cm ⁻¹ is a characteristic peak of antioxidant B in IR.) In order to keep the graft ratio of the antioxidant B to polyethylene within the above range, it is preferable to increase the temperature at the time of crosslinking the insulator. If the crosslinking temperature is too low, the grafting reaction does not proceed sufficiently, so that the crosslinking temperature is generally preferably 200 ° C. or higher. In the mixed antioxidant of the present invention, the amount of the antioxidant A added to the antioxidant B is
/ Antioxidant A is expressed as a ratio of 95% to 80% by weight.
/ 20. If the amount of the antioxidant A is less than this, the graft ratio of the antioxidant B component to polyethylene cannot be increased to 75% or more, so that tan δ at a high temperature and a high electric field increases. When the amount of the antioxidant A is larger than this, the viscosity of the mixed antioxidant increases, so that not only filtering with a fine mesh becomes difficult, but also the separated antioxidant separates. As a result, stable characteristics (for example, antioxidant performance) cannot be obtained. The ratio of the amount of each antioxidant added is preferably 90/10 to 72/18. The added amount of the mixed antioxidant must be 0.2 to 0.5 parts by weight based on 100 parts by weight of polyethylene. If the addition amount is less than this, sufficient antioxidant properties cannot be obtained, and if it is more than this, tan δ at high temperature and high electric field will increase due to the effect of ungrafted components present in the insulator in trace amounts. It is. The added amount of the mixed antioxidant is based on 100 parts by weight of polyethylene.
0.3 to 0.4 parts by weight is preferred. Further, since the antioxidant A has an effect of preventing burning during extrusion, the addition of the antioxidant A to the antioxidant B also has the effect of being advantageous for long extrusion. The antioxidants A and B contained in the above-mentioned mixed antioxidants are both easily available or easily synthesized, and are advantageous from the viewpoint of reducing production costs. In the production of the insulated power cable of the present invention, the step of extrusion-coating the conductor with an insulator layer comprising a crosslinked body of the resin composition and providing the conductor can be carried out in a conventional manner. In addition, the conductor used for the insulated power cable of the present invention can be appropriately selected depending on the application and is not particularly limited, and a conductor generally used for a power cable can be used. The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the invention is limited thereto. In the following tables, numerical values indicate parts by weight unless otherwise specified. Examples 1 to 3 and Comparative Examples 1 to 2 The resin composition shown in Table 1 was extrusion-coated on the outer periphery of a 150 mm ² conductor together with the resin for the inner semiconductive layer.
A cross-linking treatment was performed at 0 ° C. to produce a power cable having an insulator layer thickness of 6 mm. Note that DCP and the mixed antioxidant were filtered through a # 400 mesh having a mesh size of 33 μm in advance to remove foreign substances, and injected into an extruder using a pump. The following measurements and tests were performed on each of the obtained power cables. Measurement of Graft Ratio A 0.3 mm sliced sample was taken from the insulator layer of each power cable, and the sample was subjected to FTIR measurement. Thereafter, this sample was subjected to Soxhlet extraction with methylene chloride for 8 hours, dried, and subjected to FTIR measurement again to determine the graft ratio of the AO23 component to polyethylene. Measurement of tan δ For each power cable, use an automatic sharing bridge (manufactured by Soken Electric Co., Ltd., model: DAU-PSC-UA), frequency 50 Hz, measurement temperature 90 ° C., average electric field 25 k
The tan δ (%) was measured under the condition of V / mm. It should be noted that the tan δ value under this condition is 0.05% or less as a standard for acceptance. Measurement of residual elongation (aging test) A sample was taken from the insulation layer of each power cable and the thickness was 1 mm.
After processing into the No. 4 dumbbell shape specified in JIS K7113-1981,
In a gear oven specified in 1977, at a temperature of 150
An aging test was carried out at ℃ for 168 hours, and when the residual elongation rate (%) at that time was 80% or more, it was evaluated as ○, and when it was less than 80%, as X. Table 1 shows the above results. [Table 1] As is clear from Table 1, Comparative Example 1 shows that 4,4'-thiobis (3-
(Methyl-6-t-butylphenol) content is too low and the grafting ratio is low, so that the tan δ value becomes large and not at a practical level. In Comparative Example 2, the viscosity of the mixed antioxidant was too high because the content of 4,4′-thiobis (3-methyl-6-t-butylphenol) in the mixed antioxidant was too high compared to the range specified in the present invention. Is high, # 400
Due to the inability to filter on the mesh, it was impossible to manufacture the cable. On the other hand, the power cables (Examples 1 to 3) of the present invention have better tan δ values and residual elongation ratios than those of Comparative Examples 1 and 2, and particularly, the tan δ values are significantly improved. I understand. Examples 4 to 5 and Comparative Examples 3 to 4 The mixing ratio of the antioxidant was determined based on the weight% of AO23 / 4,4'-thiobis (3-methyl-6-t-butylphenol).
And a cable was manufactured in the same manner as in Example 1 except that a resin composition in which the antioxidant was added as shown in Table 2 was used, and the above measurement and test were performed. . The results are shown in Table 2. [Table 2] As is clear from Table 2, Comparative Examples 3 and 4
In any case, the graft ratio of AO23 is within the range specified in the present invention. However, in Comparative Example 3, since the amount of the mixed antioxidant was too small, the residual elongation was small and not practical. In Comparative Example 4, since the content of the mixed antioxidant was too high, the tan δ value was large and not at a practical level. On the other hand, the power cables (Examples 4 and 5) of the present invention have better tan δ values and residual elongation rates than Comparative Examples 3 and 4. The crosslinked polyethylene insulated power cable of the present invention has an excellent effect that it can be manufactured at a low cost, and tan δ at a high temperature and a high electric field is extremely reduced while maintaining the anti-aging performance. Play. Therefore, the crosslinked polyethylene insulated power cable of the present invention does not cause deterioration of the insulation characteristics or a decrease in the transmission capacity of the power cable due to an increase in the dielectric loss, and is suitable as an insulated power cable particularly at a high temperature and a high electric field.

Claims (1)

Claims: 1. An insulator layer comprising bis [2-methyl-4- (3-n-alkylthiopropionyloxy) -5-t-butylphenyl] based on 100 parts by weight of polyethylene.
95-80% by weight of sulfide and 4,4′-thiobis (3
-Methyl-6-t-butylphenol) 5 to 20% by weight
And a crosslinked resin composition containing 0.2 to 0.5 parts by weight of an antioxidant containing the following: bis [2-methyl-4- (3-n- A crosslinked polyethylene insulated power cable having a graft ratio of alkylthiopropionyloxy) -5-t-butylphenyl] sulfide component to polyethylene of 75% or more.