JPH0627149B2 - 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 present invention, ethylene and the general formula (I), (However, R represents H or —CH ₃ ) and a monomer having a phenyl ester group, and a random copolymer containing 0.005 to 10 mol% of the phenyl ester group unit is chemically crosslinked. Which is a crosslinked product obtained by heat crosslinking or electron beam crosslinking in the presence of
The present invention provides a crosslinked ethylene copolymer for electrical insulation, wherein the gel fraction of the crosslinked product is 40% or more as determined by the measuring method specified by C3005.

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 exhibits extremely 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, and ethylene and the general formula (I), (Wherein R represents H or —CH ₃ ), which is a random copolymer with a phenyl ester group-containing monomer represented by 0.005 to 10 mol% of phenyl ester group units, and preferably 0.005 to 5 mol%. It contains about mol%.

The monomer represented by the general formula (I) is specifically phenyl acrylate or phenyl methacrylate.

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

In the ethylene copolymer used in the present invention, in addition to ethylene and the monomer represented by the general formula (I), other monomer can be added for modifying the resin, and the modifying monomer is 1
It can be contained up to 0 mol%. As the modifying comonomer, a monomer known to be copolymerizable with ethylene can be used.

For example, vinyl esters such as vinyl acetate and vinyl propionate, acrylic esters such as ethyl acrylate methyl acrylate and butyl acrylate, methacrylic acid esters such as methyl methacrylate, ethyl methacrylate and butyl methacrylate, ethylene such as acrylic acid and methacrylic acid. 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 100
When it is less than 0, long-term performance is deteriorated. The molecular weight is preferably 3000 or more.

Since the ethylene copolymer used in the present invention falls within the category of thermoplastic resins, other thermoplastic resins such as polyethylene, polypropylene, ethylene-vinyl acetate are commonly used for resin materials of this type. It can be used by blending with copolymers, 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.

In the ethylene copolymer used in the present invention, ethylene and the monomer represented by the general formula (I) are supplied to the above-mentioned polymerization apparatus and radical-polymerized in the presence of the above-mentioned catalyst. In this case, the proportion of ethylene and the monomer represented by the general formula (I) is appropriately selected so as to obtain an ethylene copolymer having a desired composition, but the polymerization of the monomer represented by the general formula (I) Since the ability is higher than that of ethylene, the amount of the monomer represented by the general formula (I) is usually 0.01 to 2 mol% based on the total amount in the polymerization system,
Polymerization is preferably carried out in the state of ethylene containing 0.01 to 1 mol%.

It employed the polymerization pressure is a pressure exceeding 500 kg / cm ^2, preferably in the range of 1000~4000kg / 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.

Further, in injecting the monomer represented by the general formula (I), it is dissolved alone or in a solvent having a small chain transfer effect,
Inject directly into the reactor with 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 used in the present invention, which is produced in the reactor, is separated from the monomer in the separator according to a conventional high pressure radical polymerization method, and is directly used as a product. This product may be used as it is, but may be subjected to various post-treatment steps such as those already used for products obtained by the high pressure radical polymerization method.

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 crosslinking can be carried out by adding 0.5 to 4 parts by weight, preferably 1 to 3 parts by weight of a normal chemical crosslinking agent to 100 parts by weight of the ethylene copolymer, or cobalt 60 or a linear accelerator. Electron beam cross-linking may be performed by irradiating with about 5 to 20 megarads, or the ethylene copolymer may be cross-linked with a vinyl monomer having an alkoxysilane such as vinyltrimethoxysilane in advance or afterwards. You can also let it. 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 a monomer represented by the general formula (I), wherein the crosslinked product has a phenyl ester group unit of 0.005 to 10 mol. %, Preferably 0.005 to 5 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 a stirring autoclave type continuous reactor having an internal volume of 1.5, ethylene was 32 kg / hour, benzyl methacrylate (abbreviated as BMA) was 64 g / hour, and propylene was 58.
At 0 / hr, a solution of tert-butyl peroxyisobutyrate as a catalyst dissolved in n-hexane at a rate of 2 g / hour was continuously supplied at a rate of 600 m / hour to obtain a polymerization pressure of 2
It was polymerized at 400 kg / cm ² and a polymerization temperature of 220 ° C. to produce an ethylene copolymer.

The obtained ethylene copolymer has MFR = 3.0 g / 10
Min, number average molecular weight 20,000, BMA content in polymer
It was 0.16 mol%.

Reference Example 2 Using the same reactor as in Reference Example 1, BMA was 320 g / hr,
An ethylene copolymer was produced by the same procedure as in Reference Example 1 except that propylene was 500 / hour, the same catalyst as used in Reference Example 1 was 320 m / hour, and the polymerization temperature was 218 ° C.

The obtained ethylene copolymer has MFR = 3 g / 10 min,
Number average molecular weight 19,900, BMA content in the polymer is 0.83
It was mol%.

Reference Example 3 Instead of BMA in Reference Example 1, phenyl methacrylate (P
MA) (abbreviated as MA) at 67 g / hour and a polymerization temperature of 22
An ethylene copolymer was produced by polymerizing in the same manner as in Reference Example 1 except that the temperature was 3 ° C.

The obtained ethylene copolymer has MFR = 4 g / 10 min,
Number average molecular weight 18,900, PMA content in the polymer is 0.18
It was mol%.

Example 1, cross-linked products 1-2 Samples of ethylene copolymers produced in Reference Examples 1-3 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 a pressure of kg / cm ² gauge 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.

Example 2, crosslinked products 3 to 5 Ethylene copolymers obtained in Reference Examples 1 to 3 and "Yukaron Z"
"F30R" was used as each sample polymer, and a 6 KV grade, 1 x 250 mm ² crosslinked polyethylene insulated cable (insulation thickness 3.5 mm) was prepared at 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 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 code Internal reference number FI technical display location C08F 220: 18) (72) Inventor Shinichi Irie 6 Hachiman Kaigan Dori, Ichihara City, Chiba Furukawa Electric Co., Ltd. Chiba Cable In-house (72) Inventor Hitoshi Kimura, 6 Yawata Kaigan Dori, Ichihara-shi, Chiba Furukawa Electric Co., Ltd. In Chiba Electric Wire Works (56) Reference JP-A-61-110907 (JP, A) JP-A-61-206108 (JP, A)

Claims (1)

[Claims] 1. Ethylene and general formula (I), (Wherein R represents H or —CH ₃ ) and a monomer having a phenyl ester group represented by the formula, a random copolymer containing 0.005 to 10 mol% of the phenyl ester group unit is chemically crosslinked. Which is a cross-linked product obtained by heat-crosslinking or electron beam cross-linking in the presence of
A crosslinked ethylene copolymer for electrical insulation, wherein the gel fraction of the crosslinked product is 40% or more as determined by the measurement method specified in 3005.