JPH0552608B2 - - Google Patents

Description

[Detailed description of the invention] (a) Field of industrial application The present invention provides an insulation made by crosslinking a crosslinked ethylene copolymer having excellent dielectric strength and heat resistance, or an ethylene polymer mainly composed of the copolymer. The present invention relates to power cables using layers. (b) Prior Art Various plastic materials have been used as insulating materials for power cables. In particular, low-density polyethylene produced by high-pressure radical polymerization is inexpensive, has low dielectric loss, has good processability, and can be crosslinked to improve heat resistance. Compared to ionic polymerization, low-density polyethylene is less susceptible to tree phenomenon due to the contamination of foreign substances such as catalyst residues. It is widely used for power cables because it has many advantages such as fewer concerns. One of the problems with such cross-linked polyethylene power cables is the lack of dielectric strength, which results in thickening of the insulation layer of high-voltage power cables, that is, an increase in the outer diameter of the cables, making it difficult to transport and install the cables. This poses a major obstacle. In addition, in crosslinking treatment to improve heat resistance,
Polyethylene has insufficient crosslinking properties. Therefore, there is a desire for highly crosslinked, ie highly heat resistant, materials. (c) Problems to be Solved by the Invention The present invention is directed to crosslinking a novel ethylene copolymer that has extremely high crosslinking properties and can improve dielectric strength, or an ethylene polymer containing the copolymer as a main component. By using it as an insulating layer of a power cable, the above-mentioned drawbacks are improved, and a power cable that maintains high dielectric strength and high heat resistance is provided. (d) Means for solving the problems The present invention is directed to the production of ethylene or a mixture of ethylene and other monomers in the presence of an aromatic compound having at least two carbon-carbon double bonds in one molecule. A copolymer obtained by high-pressure radical polymerization under the conditions of a polymerization pressure of 500 to 4000 Kg/cm ² and a polymerization temperature of 50 to 400°C, or a copolymer containing the copolymer as the main component and 0.005 to 0.005 units derived from the aromatic compound. The present invention provides a power cable characterized by using an insulating layer formed by crosslinking an ethylene polymer having a content of 1 mol %. The aromatic compound in the present invention is a compound having a monocyclic or polycyclic non-fused or fused aromatic ring and at least two carbon-carbon double bonds, and is a hydrocarbon compound or its oxygen, It is a derivative containing sulfur, nitrogen, halogen, etc. Examples of monocyclic compounds include those whose side chain is derived from a cyclic aromatic compound such as benzene or indene, specifically 1-phenyl-2,6
-1,3-butadiene; 9-phenyl-nonadiene; 3-methyl-8-phenyl-1,5-octadiene; allylstyrene; 4-styryl-1-butene; hydrocarbons such as allyl indene, vinyl cinnamate, Vinyl allyl phenyl ether, 1-
Isopropenyl-4-vinylbenzene, (meth)
Representative examples include oxygen-containing compounds such as allylbenzyl acrylate. Examples of the polycyclic compound include diarylalkane derivatives, biphenyl derivatives, naphthalene derivatives, etc., specifically 1-phenyl-1-(4'-vinylphenyl)ethylene;1,1-diphenylbutadiene; 2,4 -diphenyl-1,3-pentadiene; bis(4-isopropenylphenylmethane); 1,2- or 1,1-bis(4-isopropenylphenyl)ethane; 1,1-bis(vinylphenyl)methane; 1,1-bis(vinylphenyl)ethane; 2,2'-divinylbiphenyl;
4,4'-diisopropenylbiphenyl;4,4'-
Diallyl biphenyl; divinylnaphthalene; diallylnaphthalene; diisopropenylnaphthalene and the like. In addition, one or two of these aromatic compounds may be used.
A mixture of more than one species may be used, or a mixture with a by-product derived when producing the above aromatic compound may be used. The ethylene copolymer used in the cable of the present invention is produced by a radical polymerization method under high pressure. That is, the radical polymerization method under high pressure is a polymerization pressure of 500 to 4000 Kg/cm ² , preferably
in a tank or tube reactor in the presence of a free radical catalyst and a chain transfer agent, if necessary auxiliaries, at a reaction temperature of 1000-3500 Kg/cm ² and a reaction temperature of 50-400°C, preferably 100-350°C. A method of contacting and polymerizing ethylene, an aromatic compound, and, if desired, other monomers simultaneously or in stages. When the aromatic compound is solid, it is supplied after being dissolved in an appropriate solvent. The free radical catalysts include customary initiators such as peroxides, hydroperoxides, azo compounds, amine oxide compounds, oxygen, and the like. Chain transfer agents include hydrogen, propylene, butene-1, C ₁ to _{C 20} or higher saturated aliphatic hydrocarbons, and halogen-substituted hydrocarbons, such as
Methane, ethane, propane, butane, isobutane, n-hexane, n-heptane, cycloparaffins, chloroform and carbon tetrachloride, C ₁ ~
Saturated aliphatic alcohols of C ₂₀ or more, such as methanol, ethanol, propanol and isopropanol, saturated aliphatic carbonyl compounds of C ₁ to _{C 20} or more, such as carbon dioxide,
Mention may be made of acetone and methyl ethyl ketone as well as other aromatic compounds such as toluene, diethylbenzene and xylene. Furthermore, the above-mentioned ethylene copolymer may contain other unsaturated monomers in addition to ethylene, and these unsaturated monomers include propylene, butene-1-pentene-1, hexene-1, and hexene-1. 1,4-methylpentene-1, octene-1, decene-1,
Examples include vinyl acetate, ethyl acrylate, methacrylic acid or its ester, and mixtures thereof. The content of unsaturated monomers in the ethylene copolymer is preferably 0 to 3 mol%, particularly 1 mol% or less. The density of the above ethylene polymer is 0.890 to 0.950
A range of g/cm ³ is preferred. In addition, the melt index (hereinafter abbreviated as MI) is preferably 0.05 to 50.
g/10 minutes, more preferably in the range of 0.1 to 20 g/10 minutes. The content of units derived from the aromatic compound to be included as a polymer component in the ethylene copolymer is 0.005 to 1.0 mol%, preferably 0.01 to 0.7 mol%. If it is less than 0.005 mol%, almost no improvement effect is observed, and if it exceeds 1.0 mol%, the impulse rupture strength is not only lower than that when no unit derived from the aromatic compound is contained, but also high-pressure radicals It is uneconomical due to the large consumption of polymerization initiators and the use of large amounts of expensive aromatic compounds, and the chain transfer reaction becomes intense, resulting in a significant decrease in the molecular weight of the ethylene copolymer and This makes the polymer unsuitable as an insulating material. In the present invention, an ethylene polymer containing no other aromatic units may be blended with the ethylene copolymer. Compositions containing other ethylene-based polymers are also preferred embodiments of the present invention, and when the aromatic unit content in the composition is within the above range, the impulse rupture strength of the composition is improved. Other ethylene polymers to be blended with the ethylene copolymer used in the cable of the present invention include ethylene homopolymer, ethylene and propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1 , copolymers with α-olefins having 3 to 12 carbon atoms such as octene-1 and decene-1, ethylene and vinyl acetate, acrylic acid, ethyl acrylate, methacrylic acid, ethyl methacrylate, maleic acid, maleic anhydride Copolymers with polar group-containing monomers such as ethylene homopolymers or ethylene and α-olefin copolymers modified with unsaturated carboxylic acids such as acrylic acid and maleic acid or derivatives thereof, and the like. A mixture of the following may be mentioned. The ethylene copolymer used in the cable of the present invention or the ethylene polymer containing the ethylene copolymer as its main component is characterized by the use of an aromatic compound having a small number of multiple carbon-carbon double bonds. , double bonds that are not involved in copolymerization remain in the polymer chain, and have excellent crosslinking properties. Therefore, when this material is used for the insulating layer of a power cable, the crosslinking reaction (crosslinking efficiency) is fast, and the insulating layer with a high crosslinking rate (gel fraction) is formed, and high heat resistance is maintained. . Furthermore, high dielectric strength is maintained for the following reasons. In other words, polyethylene, which has traditionally been widely used as an insulating material for power cables, has a lower breakdown strength against impact voltage (impulse breakdown strength) when the degree of crystallinity is lowered, and a decrease in workability when the degree of crystallinity is increased. It has the disadvantage that it deteriorates. It is generally known that when other components are introduced into the polyethylene chain, the degree of crystallinity decreases due to steric hindrance. However, the present inventors have discovered that when units derived from a specific aromatic compound are contained in the polymer chain at a specific ratio as described above, the range in which the impulse rupture strength increases even if the degree of crystallinity decreases is I discovered that it exists. The present invention is based on the unexpected finding that an improvement in impulse rupture strength can be achieved by introducing a very small amount of aromatic units into an ethylene polymer. Therefore, as mentioned above, the crystallinity of the ethylene copolymer or the ethylene polymer containing the ethylene copolymer as a main component is also an important factor for improving the dielectric strength. It is preferable that the crystallinity by diffraction is within 30% of the crystallinity of an ethylene homopolymer containing no aromatic rings obtained under substantially the same conditions. The upper limit of the crystallinity is naturally determined to be equal to or lower than the crystallinity of an ethylene homopolymer obtained under substantially the same conditions as the ethylene copolymer. In the present invention, olefin polymers (including copolymers) other than the other ethylene polymers, polyacrylonitrile, polyamide, etc. , polycarbonate,
Thermoplastic resins such as ABS resin, polystyrene, polyphenylene oxide, polyvinyl alcohol resin, vinyl chloride resin, vinylidene chloride resin, polyester resin, petroleum resin, coumaron-
Synthetic or natural rubber such as indene resin, ethylene-propylene copolymer rubber (EPR, EPDM, etc.), SBR, NBR, butadiene rubber, IIR, chloroprene rubber, isoprene rubber, styrene-butadiene-styrene block copolymer, etc. It can be used in combination with at least one of the following. On the other hand, in the present invention, antioxidants, lubricants, ultraviolet inhibitors, dispersants, copper damage inhibitors, neutralizers, plasticizers,
Antifoaming agents, flame retardants, crosslinking aids, flow improvers,
Additives such as weld strength improvers and nucleating agents may be added to the extent that they do not significantly impair the effects of the present invention. The crosslinking method used in producing a crosslinked power cable using the ethylene copolymer may be chemical crosslinking, which is generally widely used, or radiation crosslinking. (E) Functions and Effects of the Invention The power cable of the present invention uses a novel ethylene copolymer in the polymer chain in which an aromatic compound capable of improving crosslinking properties and dielectric strength as described above is present in the insulating layer. The ultra-high voltage power cable has a high crosslinking rate and crosslinking efficiency, and has high heat resistance and dielectric strength, that is, excellent impulse rupture strength. (f) Examples 1 to 3 Next, examples will be described in comparison with comparative examples. Examples 1-3 Internal volume fully substituted with nitrogen and ethylene
Approximately 1700 g of ethylene, the aromatic compounds shown in Table 1, and a predetermined amount of n-hexane were charged into a metal autoclave type reactor equipped with a stirrer (3.8), and di-tert-butyl peroxide as a polymerization initiator was further injected. , polymerization temperature 170℃, polymerization pressure 1600
Kg/cm ² and polymerization time of 60 minutes to prepare various ethylene polymers containing units derived from aromatic compounds as shown in Table 1. A part of the produced polymer was dissolved in heated carbon tetrachloride, and reprecipitated by pouring it into a large amount of acetone. This operation was repeated several times for purification, and then vacuum-dried. Table 1 shows the properties of the obtained copolymer. A predetermined amount of an organic peroxide crosslinking agent and an antiaging agent were blended into the copolymer to prepare an insulator compound. Apply the above compound on a conductor with a conductor area of 325 mm ² .
Extrusion coating was performed at a temperature of 120°C to an insulation thickness of 11 mm. An extruded semiconducting layer was applied directly above the isobody and on the surface of the insulator. After extrusion coating, the cable was introduced into high-temperature heated oil and crosslinked to obtain a power cable having a cross-sectional structure as shown in FIG. The impulse breaking strength (measured by a conventional method), gel fraction (crosslinking rate), and thermal deformation rate (heat resistance) of the obtained cable were measured and are shown in Table 1. Example 4 1 instead of the aromatic compound used in Example 1
- Aromatic compound consisting of a mixture of 4-ethyl-1-isopropenylbenzene and 1-isopropyl-4-vinylbenzene (4/1/2 molar ratio) produced as a by-product during the production of isopropenyl-4-vinylbenzene. Polymerization was carried out in the same manner as in Example 1 using the compound. A power cable was produced using the above copolymer in the same manner as in Example 1, and the results of evaluation are shown in Table 1. Comparative Examples 1 to 3 Ethylene homopolymers shown in Table 1 and copolymers with comonomer contents outside the range of the present invention were prepared under the same polymerization conditions as in Example 1, and cables were manufactured in the same manner as in Example 1. The results of physical property evaluation are shown in Table 1. (g) Evaluation results As is clear from Table 1, Examples 1 to 4 show that the dielectric strength and heat resistance of the power cables using the ethylene copolymer of the present invention in the insulation layer are higher than that of the conventional crosslinked polyethylene power cable ( This shows that it is superior to that of Comparative Example 1). On the other hand, when the aromatic content was set outside the range of the present invention in Comparative Examples 2 and 3, no improvement effect was observed. The test method is as follows. 1) Gel fraction The molded product was crushed into 20 mesh passes, extracted with xylene at 120°C for 10 hours, and the residual ratio was determined. 2) Heat denaturation rate The sample was pressurized with a load of 2.64 kg in an oil bath at 120°C, and the deformation rate after 30 minutes was determined. 3) Crystallinity Based on X-ray diffraction method. 【table】

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an example of the cable of the present invention. DESCRIPTION OF SYMBOLS 1... Conductor, 2... Internal semiconducting layer, 3... Insulating layer, 4
...external semiconducting layer.

Claims (1)

[Claims] 1. Ethylene or a mixture of ethylene and other monomers is polymerized at a pressure of 500 to 4000 kg/cm in the presence of an aromatic compound having at least two carbon-carbon double bonds in one molecule. ^2. The main component is a copolymer obtained by high-pressure radical polymerization at a polymerization temperature of 50 to 400°C, and contains 0.005 units derived from the aromatic compound.
A power cable characterized by using an insulating layer formed by crosslinking an ethylene polymer having a content of 1 mol %. 2. The power cable according to claim 1, wherein the aromatic compound is at least one selected from the group consisting of allylstyrene, 4-styryl-1-butene, and 1-phenyl-1-styrylethylene. 3. The crystallinity of the copolymer measured by X-ray diffraction shows a decrease in crystallinity compared to that of an ethylene homopolymer containing no aromatic rings obtained under substantially the same conditions.
The power cable according to claim 1 or 2, characterized in that the range is within 30%.