JPH0667985B2 - Ethylene copolymer for power cables - Google Patents

Description

TECHNICAL FIELD The present invention relates to a novel ethylene copolymer for power cables.

INDUSTRIAL APPLICABILITY The novel ethylene copolymer of the present invention is a material for electric power cables, which is excellent in withstand voltage characteristics, particularly in electric breakdown resistance such as electric tree characteristics and water tree characteristics.

Prior art Cross-linked polyethylene based on low density polyethylene is widely used as a CV cable insulation material because it has excellent electrical properties and heat resistance, but dielectric breakdown occurs under ultra-high voltage and High performance materials are required.

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

For example, if impurities such as holes, water, and metals are present, the concentration of electric charges will occur, and the dielectric breakdown characteristics will be degraded.Therefore, as a material for ultra-high 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. Making full use of these technologies, 275KV cables have been put to practical use.

On the other hand, however, such a high-voltage cross-linked polyethylene insulated power cable has a drawback in that its insulation thickness is significantly increased, and a material having more excellent withstand voltage characteristics is desired.

If the material itself has a function of avoiding the concentration of electric charges, it can be manufactured with a thin material.Therefore, attempts have been made to use calcium stearate, various aromatic compounds, etc. as voltage stabilizers. However, bleeding out of the additive occurs and there is a problem in long-term performance retention (see, for example, Japanese Patent Publication No. 48-24809, West German Patent No. 1248773, French Patent No. 1464601, etc.).

SUMMARY OF THE INVENTION In view of these circumstances, the present inventors have focused on the development of an ethylene copolymer excellent in dielectric breakdown properties, long-term performance retention, flexibility, and no catalyst residue of metal, resulting in excellent ethylene. Succeeded in developing a copolymer.

That is, the present invention has the general formula (I): (Wherein R ¹ represents H or a methyl group, R ² represents a C _1-8 alkyl group, m represents an integer of 0 to 2, n represents an integer of 1 to 3 and m + n = 3, respectively). And ethylene obtained by polymerizing ethylene containing 0.003 to 10 mol% (based on the total amount) of a vinyl silane compound at a reaction temperature of 120 ° C. or more and a reaction pressure of 500 kg / cm ² or more using a free radical generating initiator. A random copolymer with a compound, a vinylsilane compound unit, To provide an ethylene copolymer for a power cable, which contains 0.005 to 10 mol% of ethylene glycol and has a number average molecular weight of 3000 or more.

Advantageous Effects of Invention The novel ethylene copolymer of the present invention satisfies all the required performances as an insulating material for power cables, such as withstand voltage characteristics, long-term performance retention, flexibility, no metal residue, and cable coating processing characteristics. In particular, it shows extremely excellent performance as an insulating material for high-voltage power cables.

DETAILED DESCRIPTION OF THE INVENTION The ethylene copolymer for electric power cables of the present invention is a novel copolymer which has not been described in the literature. (Wherein R ¹ represents H or a methyl group, R ² represents a C _1-8 alkyl group, m represents an integer of 0 to 2, n represents an integer of 1 to 3 and m + n = 3, respectively). The vinyl silane compound is a random copolymer containing a vinyl silane compound unit in an amount of about 0.005 to 10 mol%, preferably about 0.005 to 5 mol%.

Specific examples of R ² in the vinylsilane compound represented by the general formula (I) include groups such as methyl, ethyl, propyl, butyl, pentyl, hexyl and octyl.

Further specific examples of the compound represented by the general formula (I) include diphenylmethylvinylsilane, triphenylvinylsilane, diphenylethylvinylsilane, diphenylbutylvinylsilane, diphenylpentylvinylsilane, phenyldimethylvinylsilane, phenyldiethylvinylsilane, Phenylmethylethylvinylsilane, 2
Examples thereof include-(diphenylmethylsilano) propylene, 2- (triphenylsilano) propylene, and 2- (diphenylethylsilano) propylene.

The ethylene copolymer of the present invention comprises ethylene and general formula (I)
In addition to the monomer represented by, other monomers can be added to modify the resin.
Can be included. 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. They are α, β unsaturated acids.

The novel ethylene copolymer of the present invention has a number average molecular weight of 3000.
That is all.

The ethylene copolymer of the present invention can be applied to various uses other than the above-mentioned use as an insulating material for power cables. For example, it can be used as an electric wire and cable insulating material by extrusion molding, or can be used as an insulating film or a general-purpose film formed into a film by a T-die method or an inflation method.

The ethylene copolymer of the present invention can also be manufactured into various molded products by blow molding, injection molding, extrusion molding and the like. As a specific example of extrusion molding, a film made of other plastic or a material other than plastic (metal foil, paper, cloth, etc.) may be extrusion-coated to obtain a laminated phase film.

Since the ethylene copolymer of the present invention falls within the category of thermoplastic resin, other thermoplastic resins such as polyethylene and polypropylene ethylene-vinyl acetate copolymer, which are commonly used for resin materials of this type, are used. It can be blended with other materials such as petroleum resin, wax, stabilizer, antistatic agent, antiaging agent, voltage stabilizer, carbon black, UV absorber, synthetic rubber or natural rubber, lubricant, inorganic filler. It is also possible to mix and use the above.

Also, it can be used as a cross-linked product by chemical crosslinking agent, electron beam, or graft copolymerization of vinyltrimethoxysilane or the like. In that case, as the chemical cross-linking agent, generally used ones such as dicumyl peroxide and percadox can be used, and the use of anti-aging agent and other additives in combination is not hindered.

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

The catalyst used in this radical polymerization is a compound that generates a free radical, for example, oxygen, ditersyl butyl peroxide, tersyl butyl cumyl peroxide,
Dialkyl peroxides such as dicumyl peroxide, acetyl peroxides, isobutyl peroxide, diacyl peroxides such as octanoyl peroxide, diisopropyl peroxydicarbonate, peroxydicarbonates such as di2-ethylhexyl peroxydicarbonate, and tertiary. Peroxyesters such as ributyl peroxyisobutyrate, tert-butyl peroxypivalate, and tert-butyl peroxylaurate, ketone peroxides such as methyl ethyl ketone peroxide, cyclohexanone peroxide, 1,1 bister sialy butyl peroxide Peroxyketals such as oxycyclohexane, 2,2 bister butyl peroxyoctane, tert butyl hydroperoxide, cumene hydro Hydroperoxide such as chromatography oxide, 2,2-azobisisobutyronitrile azo compounds such as nitriles.

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 polymerization can be carried out as a single-zone process using these single reactors, but many reactors can be used in series, optionally with a cooling device, or internally in a multi-zone process. It is also possible to use a single reactor in which the is effectively divided into several zones. In the multi-zone method, the reaction conditions in each zone are made different, and the characteristics of the polymer obtained in each of the reactors or each zone are controlled so that the monomer in each reactor or zone is controlled. It is usual to adjust the composition, catalyst concentration, molecular weight modifier concentration, etc. When using a plurality of reactors connected in series, in addition to a combination of two or more tank type reactors or two or more tube type reactors, one or more tank type reactors and one or more tubes are used. Combinations with type reactors can also be 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 catalyst. In this case, the proportion of ethylene and the monomer represented by the general formula (I) is appropriately selected so as to obtain the ethylene copolymer of the present invention having a desired composition. Since the polymerization ability of the body is slightly larger than that of ethylene, the amount of the ethylene represented by the general formula (I) is usually 0.003 to 10 mol%, preferably 0.003 to 5 mol% based on the total amount in the polymerization system. Polymerize in the state.

It employed the polymerization pressure is a pressure in excess of 500 Kg / cm ^2, preferably in the range of 1000~4000Kg / cm ^2. The polymerization temperature is at least 120 ° C, preferably 150-
It is in the range of 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 present invention, the reactor is preferably a tank reactor in order to obtain a 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. The concentration is preferably about 0.5 to 30% by weight.

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), the monomer 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 high-pressure radical polymerization, a chain transfer agent is generally used to control the molecular weight, except in special cases.

In the present invention, as the chain transfer agent, all those used in ordinary high pressure radical polymerization can be used.

For example, alkanes such as ethane, propane, butane, hexane, heptane, alkenes such as propylene, butene and hexene, alcohols such as ethanol, methanol and propanol, ketones such as acetone and methyl ethyl ketone, acetaldehyde, propionaldehyde, etc. Aldehydes and many other compounds used in the high pressure method 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 of the present invention produced in the reactor is separated from the monomer in the separator according to a conventional high-pressure radical polymerization method to be a product as it is. 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.

Experimental Example Example 1 Using a stirred autoclave type continuous reactor having an internal volume of 1.5 l, ethylene was 32 kg / hour, diphenylmethylvinylsilane (abbreviated as DPMVSi) was 1.0 l / hour, and propylene was 20 l.
/ H, 400m of a solution of tert-butyl peroxyisobutyrate as a catalyst dissolved in n-hexane at a rate of 5g / l
Polymerization pressure 2600Kg / c
An ethylene copolymer was produced by polymerization at m ² and a polymerization temperature of 220 ° C.

The ethylene copolymer obtained had an MFR of 50 g / 10 minutes, a number average molecular weight of 18,500 and a DPMVSi content of 0.52 mol% in the polymer.
It was.

Example 2 Using the same reactor as in Example 1, 2.5 liter / hour of DPMVSi, 12 liter / hour of propylene, 320 ml of the same catalyst used in Example 1
/ H, polymerization was carried out in the same manner as in Example 1 except that the polymerization temperature was 218 ° C. to produce an ethylene copolymer.

The obtained ethylene copolymer had MFR≡3.0 g / 10 minutes, a number average molecular weight of 19,900, and a DPNVSi content in the polymer of 1.37 mol%.

Reference Examples 1 to 3 (Characteristic evaluation of uncrosslinked product) The ethylene copolymer of the present invention produced in Examples 1 and 2 and a commercially available high-pressure polyethylene "Yukaron ZF30R"
The properties of these polymers were evaluated by using [MFR ≡1.0 g / 10 min, manufactured by Mitsubishi Petrochemical Co., Ltd.] as a sample polymer.

The measuring method used for this characteristic evaluation 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: Sample polymer is put into a 5 mm thick sheet under the conditions of 160 ° C and 150 Kg / cm ² respectively. The molded product was cut into 20 mm x 20 mm. A needle having a diameter of 1 mm and a tip curvature radius of 5 μ was inserted into the cut piece by 15 mm. 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).

AC voltage was applied to this test piece at a boost rate of 500 V / sec,
The generation start voltage of the electric tree was measured.

(7) Water tree characteristics: A sample polymer was molded into a 5 mm thick sheet under the conditions of 160 ° C. and 150 kg / cm ² and cut into 25 mm × 25 mm. After inserting a 1 mm diameter injection needle 20 mm into this cutout piece, withdrawing 15 mm while injecting distilled water, and sticking an aluminum foil in 10 mm on the side opposite to the side where the injection needle was inserted to give a test piece. (See Figure 2).

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

Table 1 shows the characteristic evaluation results of the ethylene copolymers of the present invention produced in Examples 1 and 2.

Reference Examples 4 to 6 (Evaluation of properties of crosslinked product) To prepare a sample for evaluation of properties of a crosslinked product, the temperature of a Brabender mixer was set to 110 ° C, 100 parts by weight of each sample polymer, and dicumyl peroxide 2 as a crosslinking agent. Parts by weight and 0.3 parts by weight of "SANTONOX R" as an anti-aging agent, kneading for 5 minutes each, and pre-molding the kneaded sample to a thickness of 5 mm with a hot plate press kept at 130 ° C, and then applying these to the same hot plate. A press was heated and pressed at 180 ° C. and 100 kg / cm ² gauge pressure for 20 minutes to obtain a crosslinked body.

Test pieces similar to those of Reference Examples 1 to 3 were produced using this crosslinked body.

The results of the characteristic evaluation are shown in Table 2.

From the results shown in the above-mentioned reference examples, the novel ethylene copolymer of the present invention has extremely excellent electrical tree properties and water tree properties, which are particularly important as materials for high-voltage power cables, and usually has a crosslinked body as a power cable. Although it is used, it is clear that this crosslinked product is particularly excellent in water tree characteristics as compared with the non-crosslinked product, and this excellent effect is obtained for the first time by the ethylene copolymer of the present invention.

[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.

Claims (1)

[Claims] 1. A general formula (I) (Wherein R ¹ represents H or a methyl group, R ² represents a C _1-8 alkyl group, m represents an integer of 0 to 2, n represents an integer of 1 to 3 and m + n = 3, respectively). Ethylene containing 0.003 to 10 mol% (based on the total amount) of a vinyl silane compound obtained by polymerization using a free radical generating initiator at a reaction temperature of 120 ° C. or more and a reaction pressure of 500 kg / cm ² or more, and the vinyl silane. A random copolymer with a compound, a vinylsilane compound unit, Is an ethylene copolymer for power cables, containing 0.005 to 10 mol% of ethylene glycol and having a number average molecular weight of 3000 or more.