JPH0673248B2 - Electrical insulation material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to an electric insulating material having excellent breakdown strength against impact voltage.

(B) Conventional technology Various plastic materials have been used as electrical insulating materials for power cables and the like. In particular, olefin polymers are excellent in various properties such as electrical properties, mechanical properties and chemical stability. Among them, low-density polyethylene produced by high-pressure radical polymerization is inexpensive, has low dielectric loss, has good processability, and can be crosslinked to greatly improve its heat resistance. It is widely used for electric wires and power cables because it has many advantages such as little concern about tree phenomenon due to mixing.

The current problem with such insulating materials for power cables is that when the transmission voltage rises in line with the expected increase in transmission capacity in the future, the thickness of the insulator must be increased by an amount commensurate with the rising voltage. Is. That is, with current materials such as polyethylene, dielectric breakdown occurs unless the insulating layer is made very thick in response to high voltage.

Various improvement methods have been proposed for this problem.
For example, several methods have been proposed in which styrene is graft-polymerized with polyethylene in order to improve the puncture resistance against impact voltage, especially in a high temperature range. Japanese Patent Publication No. 54-18760 discloses one of them, but this method has a problem that the cross-linking of polyethylene is unavoidable and the impulse strength in the low temperature region is lowered.

Also, a method of blending an aromatic polymer such as polystyrene with a polyethylene or olefin polymer (Japanese Patent Publication No. 38-20717).
Japanese Patent Laid-Open No. 50-142651 and Japanese Patent Laid-Open No. 52-54187), but they have the drawback of poor compatibility between the polyethylene or olefin polymer and the styrene polymer.

A method of blending polyethylene with a block copolymer of styrene and conjugated dienes (Japanese Patent Laid-Open No. 52-41884) has also been proposed, but this method deteriorates heat resistance and extrusion processability.

In addition, a method of impregnating polyethylene with electric insulating oil has been proposed (Japanese Patent Laid-Open No. 49-33938). However, this method causes bleeding of the electric insulating oil after long-term use or due to environmental changes. Therefore, there is a drawback that the effect is impaired.

(C) Problems to be Solved by the Invention As a result of extensive studies to solve the above problems, the present invention does not have the drawbacks of the prior art as described above, and has an electric resistance with increased breakdown strength against impact voltage. It provides an insulating material.

(D) Means for Solving the Problem The present invention includes a polymer chain containing 0.005-1 mol of a unit having an aromatic ring in an amount of 0.005 to 1 mol with respect to 100 mol of an ethylene unit, and crystallinity by X-ray diffraction. The present invention provides an electrically insulating material composed of an ethylene-based polymer obtained by a high-pressure radical polymerization method having a content of 30% or more.

The unit having an aromatic ring used in the present invention is derived from an aromatic compound having a saturated group or an unsaturated group outside the aromatic ring having a monocyclic or polycyclic (condensed or non-condensed) aromatic ring. . It is preferably an aromatic compound having 1 to 3 rings. Specifically, benzene, indane, indene, naphthalene, biphenyl, diarylalkane, acenaphthylene, anthracene, phenanthrene, terphenyl, triaryldialkane, triarylalkane, arylnaphthalene, aralkylnaphthalene, etc. or their partial hydride or dehydration. It is a compound having an aromatic ring skeleton having a saturated group or an unsaturated group outside the aromatic ring, represented by elementary products. Furthermore, as a derivative of a compound having these aromatic ring skeletons, an alkyl group, a cycloalkyl group, a hydrocarbon group having a double bond or a triple bond; a group containing a halogen atom such as chlorine or bromine; a methoxy group, Oxygen-containing group having carboxyl group, carbonyl group, ether bond, ester bond, phenolic hydroxyl group, alcoholic hydroxyl group, etc .; unsaturated carboxylic acid such as acrylic acid, methacrylic acid, maleic acid, fumaric acid or its derivative; vinyl ester group ;
One or more substituents such as a group having a sulfur atom; a group having a nitrogen atom; and the like are bonded to the hydrocarbon compound having the aromatic ring skeleton.

As will be described later, in the present invention, it is an essential requirement that a unit having an aromatic ring is contained in the ethylene polymer chain in a predetermined amount, and if it can be introduced into the polymer chain, its compound In principle, it does not matter what the specific chemical structure of is.

The method for introducing the aromatic ring into the ethylene polymer chain is not particularly limited, but there are those by copolymerization, those utilizing chain transfer, those by graft modification, and the like. Both polymerizations are possible. In high-pressure radical polymerization, there is no catalyst residue that adversely affects the electrical properties, and there is no restriction due to the chemical structure of the aromatic compound because the aromatic ring can be reliably introduced into the polymer chain by both copolymerization and chain transfer. In particular, it is particularly preferable in that it does not generate a gel which is often observed in graft modification.

A typical polymerization method is shown below.

First, in the ionic polymerization using a Ziegler type catalyst, a solid catalyst component containing at least magnesium and titanium, such as metal magnesium, magnesium hydroxide, magnesium carbonate, magnesium oxide, magnesium chloride, etc., and a metal and magnesium selected from silicon, aluminum and calcium. Atoms-containing double salts, double oxides, carbonates, chlorides or hydroxides, and further these inorganic solid compounds are treated with oxygen-containing compounds, sulfur-containing compounds, aromatic hydrocarbons, halogen-containing substances or Polymerization similar to the polymerization reaction of olefins by a conventional Ziegler type catalyst in the presence of a catalyst in which an inorganic solid compound containing magnesium such as a reacted product is supported by a titanium compound by a known method and an organoaluminum compound is combined. By doing It obtained Te.

That is, the reaction is carried out in a gas phase or in the presence of an inert solvent, or using the monomer itself as a solvent, in a state where oxygen, water and the like are substantially cut off. Polymerization conditions are temperature 20-300
℃, preferably 40 ~ 200 ℃, the pressure is normal pressure to 70k
g / cm ² · g, preferably ^{2 to} 60 kg / cm ² · g. The molecular weight can be adjusted to some extent by changing the polymerization conditions such as the polymerization temperature and the molar ratio of the catalyst, but it is effectively performed by adding hydrogen to the polymerization system. Further, it is possible to carry out a two-step or more multi-step polymerization reaction with different polymerization conditions such as hydrogen concentration and polymerization temperature without any trouble. In particular, the ethylene polymer of the present invention is preferably produced by a radical polymerization method under high pressure. That is, the radical polymerization method under high pressure means a polymerization pressure of 500
~ 4000 kg / cm ² , preferably 1000-3500 kg / cm ² , reaction temperature
Under conditions of 50-400 ° C., preferably 100-350 ° C., ethylene and aromatic compounds, optionally further in a tank or tube reactor in the presence of a free radical catalyst and a chain transfer agent, if necessary auxiliary agents. It is a method of contacting or polymerizing other monomers simultaneously or stepwise. When the aromatic compound is a solid, it is dissolved in a solvent and supplied.

Examples of the above-mentioned free radical catalyst include conventional initiators such as peroxides, hydroperoxides, azo compounds, amine oxide compounds and oxygen.

Further, as the chain transfer agent, hydrogen, propylene, butene-
1, C ₁ -C ₂₀ or more saturated aliphatic hydrocarbons and halogen-substituted hydrocarbons such as methane, ethane, propane, butane, isobutane, n-hexane, n-heptane, cycloparaffins, chloroform and tetrachloride. Carbon, C _{1 to} C ₂₀ or higher saturated aliphatic alcohols such as methanol, ethanol, propanol and isopropanol, C _{1 to} C ₂₀ or higher saturated aliphatic carbonyl compounds such as compounds such as acetone and methyl ethyl ketone, etc. Can be mentioned.

Polyethylene, which has been widely used as an insulating material for power cables, lowers its crush strength against impulse voltage (impulse puncture strength) when its crystallinity is lowered, and its workability deteriorates when its crystallinity is raised. It has drawbacks. It is generally known that the introduction of other components into the polyethylene chain lowers the crystallinity due to steric hindrance. However, the present inventors have found that when an aromatic ring is contained in a polymer chain in an extremely small amount in a specific ratio, there is a range in which the impulse fracture strength increases even if the crystallinity decreases. That is, it was found that there is a range in which the impulse fracture strength is improved in the ratio of aromatic rings contained in the ethylene polymer chain and the crystallinity of the ethylene polymer. This improving effect is recognized in a wide temperature range from low temperature to high temperature, and is particularly remarkable in the high temperature range.

In the present invention, the unit having an aromatic ring in the ethylene polymer chain is from 0.005 to 100 mol of ethylene unit.
It is applied in the range of 1 mol, preferably 0.01-0.7 mol.

When the amount of the unit having the aromatic ring is less than 0.005 mol per 100 mol of the ethylene unit, almost no improvement effect is observed, and when it exceeds 1 mol, the impulse breaking strength is higher than that when the unit does not contain the aromatic ring. On the contrary, it decreases, and in the case of high pressure radical polymerization, the consumption of the initiator is large, and a large amount of expensive aromatic compounds are used, which is uneconomical. Further, in the present invention, the crystallinity of the ethylene polymer by X-ray diffraction is 30% or more.
Preferably, the reduction rate of the crystallinity of the ethylene homopolymer containing no aromatic ring obtained under substantially the same conditions as the ethylene polymer is suppressed to 25%.
The upper limit of the crystallinity is naturally determined to be equal to or lower than the crystallinity of the ethylene homopolymer obtained under substantially the same conditions as the ethylene polymer. When the crystallinity is 30% or less, the impulse fracture strength is rather reduced and the volume resistivity is also reduced as compared with the ethylene homopolymer. In the present invention, it is necessary to satisfy both conditions of the content of units having an aromatic ring and the crystallinity at the same time.

The density of the ethylene polymer of the present invention is 0.890 to 0.950 g / cm.
A range of ³ is preferred. In addition, the melt index (hereinafter M
(Abbreviated as I) is preferably in the range of 0.05 to 50 g / 10 minutes, more preferably 0.1 to 20 g / 10 minutes.

The ethylene polymer of the present invention may contain other unsaturated monomer in addition to ethylene, and as the unsaturated monomer, propylene, butene-1, pentene-1, and sekihen-1 can be used. , 4-methylpentene-1, octene-1,
Examples thereof include decene-1, vinyl acetate, ethyl acrylate, methacrylic acid or its ester, and mixtures thereof.

The content of unsaturated monomer in the ethylene polymer is 0 to
It is preferably 3 mol%, particularly preferably 1 mol% or less.

The ethylene polymer of the present invention can be used by blending with another ethylene polymer containing no aromatic unit. A composition containing another ethylene polymer is also a preferred embodiment of the present invention, and the impulse breakdown strength of the composition is improved when the content of the aromatic unit in the composition is within the above range.

Other ethylene-based polymers to be added to the ethylene-based polymer of the present invention include ethylene homopolymer, ethylene and propylene, butene-1, pentene-1, hexene-1, 4-.
Copolymers of α-olefins having 3 to 12 carbon atoms such as methylpentene-1, octene-1, and decene-1, ethylene and vinyl acetate, acrylic acid, ethyl acrylate, methacrylic acid, ethyl methacrylate, malein Acid, a copolymer of a polar group-containing monomer such as maleic anhydride, or the ethylene homopolymer or ethylene-α-olefin copolymer modified with an unsaturated carboxylic acid such as acrylic acid or maleic acid or a derivative thereof. Examples include coalescing and the like and mixtures thereof.

(E) Action and Effect of the Invention The ethylene-based polymer of the present invention has excellent dielectric strength as an electric insulating material, and in particular, it has excellent puncture resistance against impact voltage in a high temperature region, and therefore, as an insulating material for an ultra-high voltage power cable. Very useful.

In the present invention, an olefin polymer (including a copolymer), polyacrylonitrile, polyamide, polycarbonate, ABS resin, polystyrene, polyphenylene oxide, polyvinyl alcohol resin, as long as the properties of the ethylene polymer are not significantly impaired. , Vinyl chloride resin, vinylidene chloride resin, thermoplastic resin such as polyester resin, petroleum resin, chroman-indene resin, ethylene-propylene copolymer rubber (EPR, EPDM, etc.), SB
It can be used by mixing with at least one kind of synthetic rubber such as R, NBR, butadiene rubber, IIR, chloroprene rubber, isoprene rubber, styrene-butadiene-styrene block copolymer or natural rubber.

On the other hand, in the present invention, organic / inorganic fillers, antioxidants, lubricants, various organic / inorganic pigments, UV inhibitors, dispersants, copper damage inhibitors, neutralizing agents, foaming agents, plasticizers, bubbles Additives such as an inhibitor, a flame retardant, a cross-linking agent, a flowability improver, a weld strength improver and a nucleating agent may be added within a range that does not significantly impair the effects of the present invention.

Further, the ethylene polymer of the present invention may be used without being cross-linked, or may be subjected to a cross-linking treatment if necessary. As the crosslinking method, a general crosslinking method is applied.

(F) Examples Examples will be shown below.

Examples 1 to 11 A metal autoclave equipped with a stirrer, which was sufficiently replaced with nitrogen and ethylene, was charged with a predetermined amount of ethylene, an aromatic compound and n-heptane which was a chain transfer agent, and di-tert-butyl which was a polymerization initiator. Polymerization was carried out under the conditions of injection of peroxide, pressure of 1900 kg / cm ² , polymerization temperature of 180 ° C. and polymerization time of 40 minutes (Production Method 1).

A part of the produced polymer was dissolved in heated carbon tetrachloride, poured into a large amount of acetone and reprecipitated, and this operation was repeated several times for purification and then vacuum drying.

The purified polymer was molded into a sheet with a thickness of 500 μm by heat compression, and by infrared spectroscopy by compensation method using a sheet of ethylene polymer containing no aromatic compound of the same thickness as a control, the Quantitation of units derived from aromatic compounds was performed.

The amount of the unit derived from the aromatic compound contained in each produced polymer was determined mainly from the absorbance of absorption attributed to the aromatic ring near 1600 cm ^-1 . Further, the melt index of each produced polymer was measured according to JIS K-6760, and the crystallinity was measured by an X-ray diffraction method. The impulse breakdown test was performed as follows, and the results are shown in Table 1.

The sample was formed into a sheet having a thickness of 50 μm by heat compression molding.

The electrode system includes a fixed electrode, a so-called McKeon electrode (first
Figure) was used. The electrode system substrate 4 is made of polymethylmethacrylate and has a hole having a diameter of 1/2 inch in the center thereof. A 1/2 inch stainless steel ball 1 was used as the electrode. Sample 2 was cut into 8 to 10 mm square pieces and sandwiched between the electrodes. A degassed epoxy resin 3 was filled between the sample 2 and the electrode and cured. Immerse such a McKeon electrode in a container filled with silicone oil and apply it at 20 ° C and 80 ° C.
The measurement was carried out by placing in a constant temperature bath at ℃. The voltage waveform used for destruction was a negative polarity, 1 × 40 μs impulse waveform, the waveform was observed with an oscilloscope, and the one that was destroyed at the wave front was used as data, and the average value of 20 points or more was taken.

Comparative Example 1 Under the same polymerization conditions as in the above Production Method 1, an ethylene homopolymer was prepared without injecting an aromatic compound, and the same measurement was carried out. The results are shown in Table 1.

Comparative Example 2 and Comparative Example 3 Under the same polymerization conditions as in the above Production Method 1, an ethylene-based polymer was prepared in which the content of the unit having an aromatic ring was outside the range of the present invention, and the same measurement was carried out. The results are shown in Table 1.

Comparative Example 4 Polymerization was carried out under the same conditions except that propylene was used instead of n-heptane as the chain transfer agent in the above Production Method 1 (Production Method 2) to prepare an ethylene-based polymer, and the same measurement was performed. The results are shown in Table 1.

Evaluation results As is clear from Table 1, in Examples 1 to 11, the polymers of the present invention have superior fracture resistance to ethylene homopolymer (Comparative Example 1) at low temperature and high temperature, and are particularly excellent in high temperature region. It means that.

On the other hand, in Comparative Examples 2 and 3, when the aromatic compound content was outside the range of the present invention, no improvement effect was observed.

Further, in Comparative Example 4, when the crystallinity was out of the range of the present invention, the similar improving effect was not observed.

[Brief description of drawings]

FIG. 1 is a schematic side view showing a McKeon electrode for impulse breakdown test in the present invention. 1 ... Stainless steel ball 2 ... Sample 3 ... Epoxy resin 4 ... Polymethyl methacrylate substrate

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shigenobu Kawakami 1-15-10 Hachiman, Ichikawa City, Chiba (56) References JP-A-58-11511 (JP, A)

Claims (2)

[Claims] 1. A high-pressure radical polymer containing 0.005 to 1 mol of a unit having an aromatic ring in 100 mol of ethylene units in a polymer chain and having a crystallinity of 30% or more by X-ray diffraction. An electrically insulating material made of an ethylene polymer obtained by a legal method. 2. The electrical insulating material according to claim 1, wherein the ethylene polymer contains another monomer.