JPH0627151B2 - Crosslinked ethylene copolymer for electrical insulation - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a crosslinked ethylene copolymer for electrical insulation.

The crosslinked product of the present invention has excellent impulse fracture resistance and A
In addition to the C breakdown characteristic, it is particularly excellent in water tree characteristics, and is industrially useful as a material for electric power cables that can prevent electrical deterioration (water tree deterioration) during actual use.

Prior art Crosslinked polyethylene based on low density polyethylene has excellent electrical properties and heat resistance, so CV
It is widely used as a cable insulating material, but dielectric breakdown occurs under ultra-high voltage, and higher performance materials are required.

For this reason, many studies have been conducted to improve the dielectric breakdown characteristics under ultrahigh voltage.

For example, the presence of impurities such as holes, water, and metal causes the concentration of electric charges, which lowers the dielectric breakdown characteristics. Therefore, as a material for ultrahigh-voltage cables, impurity removal technology is being investigated, and 100 μ or more. Clean polyethylene that does not contain contaminants and dry cross-linking technology that does not create voids have been developed. 275 by making full use of these technologies
Even KV cables have come into practical use.

However, insulation deterioration such as water tree deterioration after long-term use under high voltage (for example, Technical Report of the Institute of Electrical Engineers of Japan, Part I,
No. 11 (August 1974)) and the like cannot be completely prevented. Therefore, attempts have been made to use calcium stearate, various aromatic compounds, etc. as voltage stabilizers, but methods using these additives Had a problem in long-term performance retention (for example, Japanese Patent Publication No. 48-24809, West German Patent No. 12).
(See 48773 specification, French patent No. 1464601 specification, etc.).

SUMMARY OF THE INVENTION In view of these circumstances, the present inventors have focused on the development of excellent ethylene copolymer crosslinked products such as dielectric breakdown characteristics, long-term performance retention, flexibility, and no metal catalyst residue. We succeeded in developing a crosslinked ethylene copolymer having excellent properties that satisfy the above requirements.

That is, the invention of the present application is obtained by polymerizing ethylene and N-vinylpyrrolidone at a reaction temperature of 120 ° C. or higher and a reaction pressure of 500 kg / cm ² or higher using a free radical generator to obtain 0.005 N-vinylpyrrolidone units. A crosslinked product obtained by subjecting a random copolymer of ethylene and N-vinylpyrrolidone contained in an amount of -10 mol% to heat crosslinking or electron beam crosslinking in the presence of a chemical crosslinking agent, which is JIS C3005.
The gel fraction of the crosslinked product determined by the measuring method specified in
% Of the ethylene copolymer for electrical insulation is provided.

EFFECTS OF THE INVENTION The crosslinked ethylene copolymer for electrical insulation of the present invention is excellent in withstand voltage characteristics, long-term insulation deterioration prevention characteristics (water tree deterioration prevention characteristics), flexibility, molding processing characteristics, etc. It has excellent performance as an insulating material for power cables.

DETAILED DESCRIPTION OF THE INVENTION The ethylene copolymer used in the present invention is a novel ethylene copolymer not described in the literature, which is a copolymer of ethylene and N-vinylpyrrolidone and is N-vinylpyrrolidone.
The content is 0.005 to 10 mol%, preferably 0.005 to 5 mol%.

The polymer structure of the ethylene copolymer is preferably a random copolymer.

The ethylene copolymer used in the present invention is ethylene and N-
In addition to vinylpyrrolidone, other monomers can be added to modify the resin, and the modifying monomer is 10 mol%.
Can be included. As the modifying comonomer, a monomer known to be copolymerizable with ethylene can be used.

For example, vinyl acetate, vinyl propionate, and other vinyl esters, ethyl acrylate, methyl acrylate, butyl acrylate, and other amolylic acid esters, methyl methacrylate, ethyl methacrylate, butyl methacrylate, and other methacrylic acid esters, acrylic acid, methacrylic acid, and other ethylene esters. α, β unsaturated acids.

The novel ethylene copolymer used in the present invention has a number average molecular weight of 1,000 or more. Molecular weight is 1000
If it is smaller, long-term performance is degraded. The molecular weight is preferably 3000 or more.

Since the ethylene copolymer used in the present invention falls into the category of thermoplastic resin, other thermoplastic resins such as polyethylene, polypropylene and ethylene-vinyl acetate copolymer, which are commonly used for resin materials of this type, are used. It can also be used by blending with polymers, petroleum resin,
A wax, a stabilizer, an antistatic agent, an antiaging agent, a voltage stabilizer, a carbon black, an ultraviolet absorber, a synthetic rubber or a natural rubber, a lubricant, an inorganic filler and the like can also be blended and used.

The ethylene copolymer used in the present invention is produced by subjecting a predetermined monomer to copolymerization conditions, and can be produced by a known high-pressure polyethylene production apparatus by radical polymerization.

This polymerization is preferably carried out continuously. As the polymerization apparatus, a continuous stirring tank reactor or a continuous tube reactor generally used in the high pressure radical polymerization method of ethylene can be used.

The ethylene copolymer used in the present invention is obtained by supplying ethylene and N-vinylpyrrolidone to the above-mentioned polymerization apparatus and radically polymerizing in the presence of the above-mentioned catalyst. In this case, the ratio of ethylene and N-vinylpyrrolidone is appropriately selected so as to obtain an ethylene copolymer having a desired composition, but since N-vinylpyrrolidone has a higher polymerization ability than ethylene, it is usually N-vinylpyrrolidone. Is based on the total amount in the polymerization system
Polymerization is carried out in the state of ethylene containing 0.0015 to 5 mol%, preferably 0.0015 to 3.0 mol%.

The polymerization pressure employed is a pressure exceeding 500 kg / cm ² , and preferably in the range of 1000 to 4000 kg / cm ² . The polymerization temperature is at least 120 ° C, but preferably in the range of 150 to 300 ° C.

The polymer formed in one or more reactors can be separated from the unreacted monomer and processed as in conventional high pressure polyethylene manufacture. The unreacted monomer mixture is mixed with an additional amount of the same monomer,
Repressurize and circulate in reactor. The additional amount of monomer added as described above is of a composition that restores the composition of the mixture to the original composition of the polymerization system, and generally this additional amount of monomer was separated from the polymerization vessel. It has a composition almost corresponding to that of the polymer.

In the above method, a tank reactor is preferable in order to obtain an ethylene copolymer having a uniform composition.

The catalyst is usually dissolved in a solvent having a small chain transfer effect and directly injected into the reactor by a high pressure pump. Concentration is 0.5-30
About wt% is desirable.

Suitable solvents include, for example, hexane, heptane, white spirit, hydrocarbon oils, cyclohexane, toluene, higher branched saturated fatty acid hydrocarbons, and mixtures of these liquids.

In addition, in the injection of N-vinylpyrrolidone, it is dissolved alone or in a solvent having a small chain transfer effect and directly injected into the reactor by a high pressure pump. Examples of the solvent include aromatic compounds such as ethyl benzoate, toluene and methyl benzoate, and fatty acid esters such as ethyl acetate.

In the high-pressure radical polymerization, a chain transfer agent is generally used for adjusting the molecular weight except for a special case.

In the above method, as the chain transfer agent, all chain transfer agents used in ordinary high pressure radical polymerization can be used.

These gaseous substances are injected into the suction side of the compressor, and liquid substances are injected into the reaction system by a pump.

The ethylene copolymer produced in the reactor and used in the present invention may be separated from the monomer in a separator and used as it is according to a conventional method of high-pressure radical polymerization method. Various post-treatment steps, such as those used in the product obtained according to, may be carried out.

The ethylene copolymer thus obtained is used alone or with a thermoplastic resin such as polyethylene and ethylene-vinyl acetate copolymer, particularly a composition with an ethylene-based thermoplastic resin is crosslinked to form a crosslinked product. This cross-linking can be carried out by compounding an ordinary chemical cross-linking agent with 0.5 to 4 parts by weight, preferably 1 to 3 parts by weight, based on 100 parts by weight of the ethylene copolymer, or cobalt 60 or a linear accelerator. May be irradiated with about 5 to 20 megarads for electron beam crosslinking, or an ethylene copolymer may be crosslinked with a vinyl monomer having an alkoxysilane such as vinyltrimethoxysilane beforehand or afterwards. You can also When used for an electric cable, it is preferable to use the chemical crosslinking method among the above.

As the chemical cross-linking agent, for example, the following free radical generators can be used. Specifically, dicumyl peroxide, t-butyl cumyl peroxide, 2,5-
Dicumyl-2,5-di (t-butylperoxy) hexane, 2,5-dimethyl-2,5-di (t-butylperoxy) hexene-3, benzoyl peroxide, etc. which are usually used as chemical crosslinking agents Is.

The crosslinked product of the present invention obtained as described above contains a copolymer of ethylene and N-vinylpyrrolidone, and the crosslinked product contains 0.005 to 10 mol% of N-vinylpyrrolidone units, preferably 0.005 to 5 mol%. It will be included by mol%. The crosslinked product has a gel fraction of 40% or more determined by the measuring method specified in JIS C3005. If the gel fraction is smaller than this value, the heat deformation rate obtained by the above-mentioned measurement method becomes large, which is a practical problem.

Experimental Example Reference Example 1 Using an agitated autoclave-type continuous reactor with an internal volume of 1.5, ethylene was dissolved at a rate of 40 kg / hour and N-vinylpyrrolidone (abbreviated as NVP) at a rate of 200 g / in ethyl acetate. 600m / h, 3 propylene
20 / hour, 500 g of a solution of n-hexane dissolved in n-hexane at a rate of 3 g of t-butylperoxypivalate as a catalyst.
Supply continuously at a rate of m / h, polymerization pressure 2400 Kg
Polymerization was carried out at a polymerization temperature of 200 ° C./cm ² to produce an ethylene copolymer.

The obtained ethylene copolymer has MFR = 3 g / 10 min,
Molecular weight Mn = 20,000, Mw = 130,000, N in polymer
The VP content was 0.25 mol%.

Reference Example 2 Using the same reactor as in Reference Example 1, without using propylene, N
VP dissolved in ethyl acetate at a rate of 200 g / 6.0 was 6.0 / hour, and the same catalyst used in Reference Example 1 was 320
Polymerization was carried out in the same manner as in Reference Example 1 except that the polymerization temperature was set to m / h and the polymerization temperature was set to 201 ° C. to produce an ethylene copolymer.

The obtained ethylene copolymer has MFR = 2 g / 10 minutes,
The content of NVP in the polymer was 2.7 mol%.

Examples 1 and 2, Comparative Example 1 Samples of ethylene copolymers produced in Reference Examples 1 and 2 and commercially available high-pressure polyethylene "Yukaron ZF30R" [Mitsubishi Yuka Co., Ltd., MFR = 1.0 g / 10 min] The polymer was used as a polymer to evaluate the characteristics of these polymer cross-linked products.

The production of the crosslinked polymer and the preparation of the sample for property evaluation were carried out by setting the temperature of the Brabender mixer at 110 ° C., 100 parts by weight of each sample polymer, 2 parts by weight of dicumyl peroxide as a crosslinking agent, and “ Santonox R ”0.
After adding 3 parts by weight and kneading for 5 minutes each, the kneaded sample was preformed to a thickness of 5 mm by a hot plate press kept at 130 ° C.
It was heated and pressed at Kg / cm ² gauge pressure for 20 minutes to obtain a crosslinked body of a sheet having a thickness of 5 mm.

The measurement method used for the property evaluation performed using this crosslinked product is
It is as follows.

(1) Molecular weight: By gel permeation chromatography.

(2) Comonomer content: By infrared spectroscopy.

(3) MFR: JIS K6760 (4) Density: JIS K6760 (5) Gel fraction: JIS C3005 (6) Electrical tree characteristics: 20 mm x 20 each formed into a 5 mm thick sheet
Cut out to mm. The cut piece has a diameter of 1 mm and a tip radius of curvature of 5
A 15 μm needle was inserted. On the other hand, a silver paste was applied to the surface opposite to the side where the needle was inserted to prepare a test piece (see FIG. 1).

An alternating voltage was applied to this test piece at a boosting rate of 500 V / sec, and the generation start voltage of the electrical tree was measured.

(7) Water tree characteristics: 20 mm x 20 each molded into a 5 mm thick sheet
Cut out to mm. An injection needle with a diameter of 1 mm is 20 mm on this piece.
After the insertion, distilled water was poured in to pull out 15 mm, and a 10 mm wide aluminum foil was attached to the side surface opposite to the side where the injection needle was inserted to obtain a test piece (see FIG. 2).

After applying an alternating voltage of 60 Hz and 10 KV to this test piece for 50 hours, the growth average length of the water tree was observed with an optical microscope.

Table 1 shows the evaluation results of the properties of the crosslinked product.

Examples 3-5, Comparative Examples 2-3 Ethylene copolymers obtained in Reference Examples 1-2 and "Yukaron"
ZF30R "was used as each sample polymer, and a 6 KV grade, 1 × 250 mm ² crosslinked polyethylene insulated cable (insulation thickness 3.5 mm) was prepared with the compounding ratio shown in Table 2. An extrusion type semiconductive layer compound was used for the inner and outer semiconductive layers.

Immediately after manufacturing, an impulse breakdown test was performed on the obtained cable. Breakdown voltage is 200KV / 3 times, then 10
KV / 3 step-ups required. Further, the obtained cable was flooded under the conditions of 6 KV and 1 KHz. After the flooding, the insulator was sliced to a thickness of 0.5 mm, boiled, and then the number of votitley in the insulator was measured with an optical microscope at 400 times. In addition, for samples after flooding and charging, 40
After applying KV / 30 minutes, the AC breakdown value (AC breakdown value) was determined under the condition of 5 KV / 30 minutes step-up.

From the results shown in Tables 1 and 2, the cross-linked product of the present invention has excellent impulse breakdown resistance and AC breakdown resistance, as well as water tree characteristics and bow tie tree characteristics that are important as insulating materials for high voltage power cables. Is particularly excellent, and it is clear that it is useful for electrical insulation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a second test piece used for measuring electrical tree characteristics.
The figure is a schematic view of a test piece used for measuring water tree characteristics.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Internal reference number FI Technical indication C08F 226: 10) (72) Inventor Shinichi Irie 6th Hachiman Kaigan Dori, Ichihara-shi, Chiba Furukawa Electric Co., Ltd. Company Chiba Electric Wire Works (72) Inventor Hitoshi Kimura 6th Hachiman Kaigan Dori, Ichihara City, Chiba Furukawa Electric Co., Ltd. Chiba Electric Wire Works (56) Reference JP-A-50-46788 (JP, A)

Claims (1)

[Claims] 1. N-vinylpyrrolidone units obtained by polymerizing ethylene and N-vinylpyrrolidone at a reaction temperature of 120 ° C. or higher and a reaction pressure of 500 kg / cm ² or higher using a free radical generator have an N-vinylpyrrolidone unit content of 0.005 to. 10 mol% ethylene and N-
A cross-linked product obtained by subjecting a random copolymer with vinylpyrrolidone to heat-crosslinking in the presence of a chemical cross-linking agent or electron beam cross-linking, wherein the cross-linked product determined by the measurement method specified in JIS C3005 A crosslinked ethylene copolymer for electrical insulation having a gel fraction of 40% or more.