JPH0548564B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a polyethylene insulator having excellent electrochemical resistance. (Background) Insulators made by extruding polyethylene at a temperature above its melting point are widely used as insulation coatings for various electric wires and cables. However, such insulators using polyethylene have electrochemical problems. In other words, when moisture exists at the interface between polyethylene and the conductor and inside the polyethylene, the interaction between this moisture and electrical stress creates a dendritic microscopic water droplet array (water tree) that moves from the interface toward the inside of the insulator. Furthermore, when foreign matter or voids are present inside the polyethylene, a fine dendritic micro water droplet array (bow tree) is generated around these foreign matter or voids. Therefore, when these electrochemical trees grow, there is a problem in that the insulation properties of polyethylene are significantly deteriorated. (Objective) Accordingly, an object of the present invention is to provide an insulator made of polyethylene having excellent electrochemical resistance and insulation properties. (Structure) The insulator of the present invention is made by extruding a precursor made of polyethylene in a solid state below the melting point of polyethylene, and has an extrusion ratio [R=(with respect to the extrusion direction of the precursor). cross-sectional area in the right angle direction)/
(Cross-sectional area of the molded product (polyethylene insulator) in the same direction)] is 2 or more, or the extrusion ratio R and the expansion ratio [E = (peripheral length of the cross-section of the molded product in the same direction)/
(peripheral length of the cross section of the precursor in the same direction)] and [R×E] is set to be 2 or more, so that it has electrochemical resistance. FIG. 1 shows an example of the electrical insulator of the present invention, and reference numeral 1 in the figure indicates a polyethylene insulator. The insulator 1 is usually a cylindrical body, and is used, for example, as an insulator that directly covers an electric conductor or as a cable sheath. This insulator 1 is made by extruding a precursor made of polyethylene in a solid state at a temperature below the melting point of polyethylene, and has an extrusion ratio [R = cross-sectional area of the precursor in a direction perpendicular to the extrusion direction. )/(cross-sectional area of the molded product in the same direction)] is 2 or more, or extrusion ratio R and expansion ratio [E = (peripheral length of the cross-section of the molded product in the same direction)/(precursor in the same direction) The product [R×E] of the peripheral length of the cross section) is 2 or more. Here, the extrusion ratio means the ratio of the cross-sectional area of the molded article (polyethylene insulator) in the same direction to the cross-sectional area of the polyethylene precursor in the direction perpendicular to the extrusion molding direction. Further, the expansion ratio means the ratio of the length of the periphery of the cross section of the precursor in the same direction to the length of the periphery of the cross section of the molded article in the direction perpendicular to the extrusion molding direction. For example, if a circular hole is formed in a cross section perpendicular to the extrusion direction of the molded product, the length of the outer periphery of the cross section plus the circumference of the circular hole is This is the perimeter of the cross section of the molded product. The above-mentioned polyethylene includes all polyethylenes used in electrical insulation materials, copolymers thereof, and blends thereof. The melting point of polyethylene that satisfies the conditions of the present invention varies depending on the type, but is usually in the range of 100 to 140°C. In this way, if the extrusion ratio R is 2 or more, or if the product R×E of the extrusion ratio R and the expansion ratio E is 2 or more, the obtained molded product will have excellent electrochemical resistance. . and again,
In order to significantly improve the dielectric strength of this molded article, it is desirable that the extrusion ratio R be 4 or more. In the present invention, the above provisions are provided when the extrusion ratio R is less than 2, or when the product [R x E] of the extrusion ratio R and the expansion ratio E is less than 2, the obtained This is because electrochemical tree occurs in the molded product, and its dielectric strength is not much different from the dielectric strength of a normal melt extrusion molded product, so the expected effect cannot be obtained. Next, when the polyethylene insulator of the present invention is solid phase extruded, the relationship between the precursor and the molded product is expressed by the extrusion ratio R and the expansion ratio E: (i) R>1, E<1.
(ii) Two possibilities are possible: R>1 and E>1. In other words, in case (i), it means that the peripheral length of the cross section of the molded product is shorter than the peripheral length of its precursor. For example, if the precursor is a cylinder and the molded product is a cylinder, In some cases, the molded article will be smaller in diameter and longer than the precursor. In the case of (ii), the cross-sectional area of the molded product is smaller than the cross-sectional area of its precursor compared to the case of (i), but the peripheral length of the cross-section of the molded product is smaller than that of the precursor. For example, if the precursor is a cylinder and the molded product is a cylinder, the wall thickness of the molded product is the same as the wall thickness of the precursor. It is thinner and longer than the precursor, and at least its inner diameter is larger than that of the precursor. Next, the method for manufacturing the polyethylene insulator of the present invention will be explained in the two cases described above with reference to FIGS. 2 and 3. First, FIG. 2 shows an example of a manufacturing apparatus suitable for manufacturing the insulator of the present invention under the conditions of R>1 and E<1. This manufacturing apparatus is generally composed of a mold 2, a die 3, a plunger 4, and a mandrel 5. The mold 2 has a cylindrical cylinder 6 at its center, and a cylindrical plunger 4 is inserted into the cylinder 6 so as to be movable in the axial direction of the cylinder 6. At the bottom of this plunger 4,
One end of the rod-shaped mandrel 5, which is longer than the stroke of the cylinder 6, is fixed. A heater (not shown) is provided on the outside of the mold 2, and a die 3 is connected to the lower part. This die 3 has
A through hole 7 whose diameter decreases from the top to the bottom is formed in the center. The upper part of the through hole 7 communicates with the cylinder 6. A cylindrical precursor 8 made of polyethylene is charged into the cylinder 6, and the mandrel 5 is passed through the axial center of the precursor 8. A molded product (polyethylene insulator) 1 is extruded from the lower end of the mandrel 5 by pushing down the plunger 4. Further, inside the cylinder, a pressure medium 9 is filled above and outside the precursor 8. This pressure medium may be any pressure medium as long as it has a lubricating effect during extrusion molding and is not absorbed by polyethylene, and glycerin or the like is usually used. To manufacture an insulator made of polyethylene using such an apparatus, first, a precursor 8 is charged into the cylinder 6 of the mold 2, and the mandrel 5 is passed through the axial center of the precursor 8. Next, such a precursor 8 is heated with a heater provided outside the mold 2. The temperature of the precursor 8 at this time is at most below its melting point, and is determined in the range from above the glass transition point to below the melting point, taking into consideration workability and the like. Next, the heated precursor 8 is pushed out into the lower through hole 7 by the plunger 4 via the pressure medium 9. The pressure applied to the precursor 8 during extrusion depends on the temperature of the precursor; low pressure is applied at high temperatures, and high pressure is applied at low temperatures, but it is usually in the range of 50 to 5000 kg/ ^cm2 . Ru. The thus extruded precursor 8 is cooled to obtain a polyethylene electrical insulator 1. In this manufacturing method, hydraulic pressure from the plunger 4 and the pressure medium 9 is used to pressurize the precursor 8, but the pressure can be applied using only the plunger. Further, a through hole for the mandrel 5 to pass through is provided in the center of the precursor 8, and the peripheral length of the precursor 8 in this case is the sum of the outer peripheral length and the peripheral length of the through hole. Next, FIG. 3 shows an example of a manufacturing apparatus suitable for manufacturing the polyethylene insulator of the present invention under the conditions of R>1 and E>1. This manufacturing equipment is
Generally, it is composed of a mold 10, a die 11, a plunger 12, and a core 13. The mold 10 has a cylindrical cylinder 14 in the center, and a cylindrical plunger 1 is disposed within the cylinder 14 so as to be movable in the axial direction of the cylinder 6.
2 is inserted. At the bottom of this mold 10,
A die 11 is connected, and a through hole 15 is formed in the center of the die 11, the diameter of which increases from the top to the bottom. The through hole 15 communicates with the cylinder 14 at the upper part, and has the same diameter and is elongated at the lower part. Furthermore, this through hole 1
A core 13 is provided within 5. This core 1
3 has an enlarged diameter portion 1 at the upper end in the longitudinal direction of the cylindrical portion 16.
7, and a tapered portion 18 is formed on the outside of this enlarged diameter portion 17. The through hole 15 and the core 13 are coaxially arranged with a predetermined clearance. A precursor 19 made of polyethylene is charged into the cylinder 14 . Further, a heater (not shown) is provided on the outside of the mold 10. In order to manufacture an insulator made of polyethylene using such an apparatus, first, a precursor 19 is charged into the cylinder 14 of the mold 10, and the precursor 19 is
is heated by a heater provided outside the mold 10. The temperature of the precursor 19 at this time is high but below its melting point, and is determined in the range from above the glass transition point to below the melting point, taking into account workability and the like. Next, this heated precursor 19 is pushed out into the lower through hole 14 by the plunger 12. The pressure applied to the precursor 19 during extrusion also depends on the temperature of the precursor; low pressure is applied at high temperatures and high pressure is applied at low temperatures, but it is usually in the range of 80 to 8000 kg/ ^cm2 . Ru. Next, the extruded precursor 19 is formed into a cylindrical shape between the through hole 15 and the tapered surface 18 and between the through hole 6 and the enlarged diameter part 17, the inner diameter is determined, and the precursor is extruded outside and cooled. A molded product (polyethylene insulator) 1 is obtained. In this manufacturing method, the plunger 12 is used to pressurize the precursor 19, but hydraulic pressure can also be used. Further, although a through hole is not provided in the center of the precursor 19, a through hole may be provided in the center of the precursor in order to facilitate extrusion molding. In the above example, the cross-sectional shape in the direction perpendicular to the molding direction is annular, but any shape may be used. For example, the inner opening may be oval and the outer opening may be circular. It may be hot. Example 1 Polyethylene with a density of 0.95 g/cm ³ (melting point 138°C)
Among the insulators of the present invention, the expansion ratio E is 1.
The extrusion ratio R was varied within the range of R>1 to produce molded products that were smaller than 1 (the molded products were thinner than the precursor). The molding temperature during production was 90 to 100°C, and the molded product was a pipe with an inner diameter of 6 mm and a wall thickness of about 1 mm. In addition, a polyethylene insulator was manufactured by ordinary melt extrusion molding using the same polyethylene as above at a molding temperature of 120°C. This molded product was a pipe with approximately the same dimensions as the above-mentioned pipe. Next, electrochemical tests were conducted on these pipes. The test conditions were (1) using water electrodes inside and outside the polyethylene pipe, (2) at room temperature, and (3) applying a voltage of 3 kV at a high frequency of 1 KHz. After each period of time, the sample was stained with methylene blue using a conventional method, cut into thin sections, and observed under a microscope to investigate the occurrence of electrochemical toxicity. The results are shown in Table 1.

[Table] Number of trees generated: pieces/mm ³
WT: water tree BT: boat tree As is clear from Table 1, the conditions of the present invention (R
It can be seen that those satisfying ≧2) do not generate either water tree or bow tree tree, and have excellent electrochemical tree resistance compared to the molded products of comparative examples. Example 2 Polyethylene with a density of 0.92 g/cm ³ (melting point 108°C)
An insulator of the present invention (E=1.5, R=1.4) was produced at a molding temperature of 60°C. This molded product has an inner diameter of 20
mm, and the wall thickness was approximately 1 mm. In addition, a polyethylene insulator was manufactured by ordinary melt extrusion molding using the same polyethylene as above at a molding temperature of 145°C. This molded product was a pipe with approximately the same dimensions as the above-mentioned pipe. Next, an electrochemical tree test was conducted on these two pipes under the same conditions as in Example 1, and the state of tree generation after 40 days of energization was investigated. In the solid-phase extruded pipe, there was no occurrence of water trees or bow tree trees, but in the pipe melt-extruded at 145°C, which is above the melting point of polyethylene, the occurrence of 0.35 water trees/ ^mm3 was observed. Ta. As is clear from this result, products that satisfy the conditions of the present invention (R×E≧2) do not generate water tree or bow tree tree, and have better electrochemical resistance than the molded products made by melt extrusion molding of comparative examples. It can be seen that it has excellent tree performance. (Effects of the Invention) The polyethylene insulator of the present invention is obtained by extruding a polyethylene precursor in a solid state below the melting point of polyethylene, and has an extrusion ratio [R
= (Cross-sectional area of the precursor in the direction perpendicular to the extrusion molding direction) / (Cross-sectional area of the molded product in the same direction)] is 2
or above, or the product [R×E] of the extrusion ratio R and the expansion ratio [E=(peripheral length of the cross section in the same direction of the molded product)/(peripheral length of the cross section of the precursor in the same direction)] is 2
With the above configuration, a polyethylene insulator having excellent electrochemical resistance can be obtained. Furthermore, when the extrusion ratio R is 4 or more, a polyethylene insulator having excellent insulation properties can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing an example of the polyethylene insulator of the present invention, and FIGS. 2 and 3 each show an example of a manufacturing apparatus for manufacturing the polyethylene insulator of the present invention by solid-phase extrusion molding. FIG. 2, which is a schematic configuration diagram, is used when the extrusion ratio R>1 and the expansion ratio E<1. FIG. 3 is a diagram showing a manufacturing apparatus used when the extrusion ratio R>1 and the expansion ratio E>1. 1...Polyethylene insulator.

Claims (1)

[Claims] 1 A polyethylene insulator formed by extruding a polyethylene precursor in a solid state below the melting point of polyethylene, where extrusion ratio R = (cross-sectional area of the polyethylene precursor in a direction perpendicular to the extrusion molding direction) )/(cross-sectional area of the molded product in the same direction)
is 2 or more, or the product of extrusion ratio R and expansion ratio E = (peripheral length of the cross section in the same direction of the molded product) / (peripheral length of the cross section of the precursor in the same direction) [R × E]
A polyethylene insulator having electrochemical resistance of 2 or more.