JP7707742B2 - Power transmission cable and method for manufacturing the same - Google Patents

Description

The present invention relates to a power transmission cable and a method for manufacturing the power transmission cable, in particular, a power transmission cable using a halogen-free flame-retardant resin composition and a method for manufacturing the same.

Cables used in railway vehicles and other vehicles are required to have properties such as flame retardancy and low smoke generation to minimize damage in the event of a fire. To achieve high flame retardancy, materials are used in which halogen-based flame retardants such as chlorine-based or bromine-based ones are added to polyolefin. However, substances that contain large amounts of these halogen-based flame retardants generate large amounts of toxic and harmful gases when burned, and depending on the incineration conditions, can generate highly toxic dioxins. For this reason, cables that use non-halogen materials (halogen-free materials) as coating materials that do not contain halogen substances are becoming more common, from the perspective of fire safety and reducing environmental impact.

For example, Patent Document 1 discloses a power cable that uses a base polymer containing an ethylene-vinyl acetate copolymer with a vinyl acetate content of 50% by weight or more as a sheath layer, and a halogen-free flame-retardant resin composition that contains a metal hydrate and silica in a total amount of 100 to 180 parts by weight per 100 parts by weight of the base polymer, in order to achieve high flame retardancy and low smoke generation.

Patent Document 2 discloses a power transmission cable that includes an inner semiconductive layer formed on the outer periphery of a conductor, an insulating layer formed on the outer periphery of the inner semiconductive layer, an outer semiconductive layer formed on the outer periphery of the insulating layer, a semiconductive tape layer formed by wrapping a semiconductive tape around the outer periphery of the outer semiconductive layer, a shielding layer formed by wrapping a wire around the outer periphery of the semiconductive tape layer, and a sheath layer formed on the outer periphery of the shielding layer. In this power transmission cable, the wire that constitutes the shielding layer suppresses dents in the outer semiconductive layer and the insulating layer.

JP 2016-100140 A JP 2016-100148 A

The power transmission cable is formed by coating a core portion having a conductor and an insulating layer formed around the conductor with a non-halogen flame-retardant resin composition that forms a sheath layer. After coating with the non-halogen flame-retardant resin composition, the cable is formed by cross-linking the resin material by heating.

Here, when the non-halogen flame-retardant resin composition that forms the sheath layer is heated, the internal insulating layer and sheath layer each thermally expand, and contract during the cooling process. At this time, an excessive gap may occur between the insulating layer and the sheath layer due to the difference in the shrinkage rate between the insulating layer and the sheath layer. This excessive gap is a factor that impairs the connection reliability at the connection point between the power transmission cable and other components. For example, the occurrence of an excessive gap between the insulating layer and the sheath layer causes one of the members between the insulating layer and the sheath layer to move in the longitudinal direction of the power transmission cable, causing sheath misalignment. When such sheath misalignment occurs, the connection reliability between the power transmission cable and other components may be impaired.

In addition, if the sheath layer has high physical strength, such as tensile strength, the risk of damage to the power transmission cable is reduced, resulting in a long and good product life.

The present invention aims to provide a power transmission cable and a method for manufacturing a power transmission cable that suppresses the gap between the sheath layer and the insulating layer, improves the pull-out strength inside the sheath layer, and has a high tensile strength of the sheath layer.

The power transmission cable of the present invention has (a) a core portion having a conductor and an insulating layer formed on the outer periphery of the conductor, and (b) a sheath layer formed on the outer periphery of the core portion, the insulating layer being thicker than the sheath layer, the insulating layer having a linear expansion coefficient greater than the linear expansion coefficient of the sheath layer, the sheath layer having a tensile strength greater than 12.0 MPa, and the pull-out strength inside the sheath layer being 10 kgf or greater.

The method for producing a power transmission cable of the present invention includes the steps of (a) coating a core portion having a conductor and an insulating layer formed on the outer periphery of the conductor with a halogen-free flame-retardant resin composition that will become a sheath layer, and (b) cross-linking the sheath layer by heating, wherein the insulating layer is thicker than the sheath layer, the linear expansion coefficient of the insulating layer is greater than the linear expansion coefficient of the sheath layer, the tensile strength of the sheath layer is greater than 12.0 MPa, and the pull-out strength inside the sheath layer is 10 kgf or greater.

According to one aspect of the power transmission cable and the method for manufacturing the power transmission cable of the present invention, it is possible to suppress the gap between the sheath layer and the insulating layer, and improve the pull-out strength inside the sheath layer. Furthermore, it is possible to improve the tensile strength of the sheath layer, and it is possible to make both the pull-out strength and tensile strength characteristics good.

FIG. 2 is a cross-sectional view showing a configuration of a power transmission cable. 1 is a schematic diagram showing a manufacturing process of a power transmission cable. 1 is a schematic diagram showing a manufacturing process of a power transmission cable. FIG. 11 is a cross-sectional view showing a state of a pull-out test. 1 is a graph showing the relationship between crosslinking temperature and tensile strength and pull-out strength in the examples.

(Embodiment)
(Configuration of power transmission cable)
The power transmission cable according to the present embodiment will be described below. Fig. 1 is a cross-sectional view showing the configuration of the power transmission cable.

The power transmission cable 1 shown in FIG. 1 has a conductor 2, an internal semiconductive layer 3 formed on the outer periphery of the conductor 2, an insulating layer 4 formed on the outer periphery of the internal semiconductive layer 3, an external semiconductive layer 5 formed on the outer periphery of the insulating layer 4, a semiconductive tape layer 6 formed on the outer periphery of the external semiconductive layer 5, a shielding layer 7 formed on the outer periphery of the semiconductive tape layer 6, a pressure tape layer 8 formed on the outer periphery of the shielding layer 7, and a sheath layer 9 formed on the outer periphery of the pressure tape layer 8.

The power transmission cable of this embodiment is suitable for use as a special high voltage power transmission cable for transmitting high voltages of, for example, 7000 V or more. The outer diameter (diameter) of the power transmission cable is, for example, 30 mm or more and 60 mm or less. Such a power transmission cable is arranged along the roof or wall so as to connect, for example, a pantograph arranged on the roof of a railway vehicle to a multi-voltage transformer arranged under the floor.

The conductor 2 is formed by twisting together multiple strands. Examples of strands that can be used include conductor wires and copper alloy wires. The strands may also be plated with a metal such as tin. The conductor 2 transmits a high voltage of, for example, 7000 V or more as mentioned above. A separator tape may also be wrapped around the conductor 2.

The internal semiconductive layer 3 and the external semiconductive layer 5 are provided to alleviate electric field concentration, and are made of a material made conductive by dispersing conductive powder such as carbon in rubber such as ethylene propylene rubber or butyl rubber. If there is a minute gap between the conductor 2 and the insulating layer 4, or between the insulating layer 4 and the shielding layer 7, electric field concentration is likely to occur, so it is preferable that each of the internal semiconductive layer 3 and the external semiconductive layer 5 is formed so as to adhere closely to the insulating layer 4. By sandwiching the insulating layer 4 between the internal semiconductive layer 3 and the external semiconductive layer 5, it is possible to alleviate electric field concentration between the conductor 2 and the insulating layer 4, or between the insulating layer 4 and the shielding layer 7.

The insulating layer 4 is made of a material such as ethylene propylene rubber, vinyl chloride, cross-linked polyethylene, silicone rubber, or a fluorine-based material. The insulating layer 4 may also contain clay.

Since the insulating layer 4 is required to have high insulating properties, the thickness of the insulating layer 4 is greater than the thicknesses of the inner semiconductive layer 3, the outer semiconductive layer 5, the shielding layer 7, and the sheath layer 9. The thickness of the insulating layer 4 is, for example, about 8 mm or more and 16 mm or less.

Here, the laminate consisting of the conductor 2, inner semiconductive layer 3, insulating layer 4, and outer semiconductive layer 5 from the inside may be referred to as the resin core part C.

A semiconductive tape 6, a shielding layer 7, and a holding tape layer 8 are provided on the outer periphery of this resin core portion C.

The semiconductive tape layer 6 on the outer periphery of the external semiconductive layer 5 (resin core part C) is, for example, a semiconductive tape wound in a spiral shape along the cable axial direction. The semiconductive tape may be, for example, a base fabric or nonwoven fabric woven with warp and weft threads made of nylon, rayon, PET, or the like, impregnated with a conductive powder such as carbon dispersed in rubber such as ethylene propylene rubber or butyl rubber. The thickness of the semiconductive tape is, for example, 0.1 mm to 0.4 mm, and the width of the semiconductive tape is, for example, 30 mm to 70 mm. The semiconductive tape may be wound in an overlapping manner, for example, such that 1/4 to 1/2 of the tape width overlaps.

The shielding layer 7 is formed by winding a wire around the outer circumference of the semiconductive tape layer 6 in a spiral shape, for example along the cable axis. The wire is made of a conductive material, for example tin-plated soft copper, and may be a wire material with a diameter of 0.4 mm to 0.6 mm. This shielding layer 7 is connected to ground when in use.

The pressure tape layer 8 is, for example, a pressure tape wound in a spiral shape around the outer circumference of the shielding layer 7 in the cable axial direction. The pressure tape may be a tape made of plastic or rayon. A polyester nonwoven fabric may also be used. The thickness of the pressure tape is, for example, 0.03 mm or more and 0.2 mm or less, and the width of the pressure tape is, for example, 50 mm or more and 90 mm or less.

The laminate from the conductor 2 to the pressure tape layer 8 is sometimes called the core part with a shielding layer.

A sheath layer 9 is provided on the outer periphery of the pressure tape layer 8 (core portion with shielding layer). The sheath layer 9 is formed by, for example, extrusion molding a halogen-free flame-retardant resin composition on the outer periphery of the pressure tape layer 8. This sheath layer 9 is cross-linked. The sheath layer 9 is a protective layer that protects the core portion with shielding layer (laminate from the conductor 2 to the pressure tape layer 8). The thickness of the sheath layer 9 is, for example, 2.5 mm or more and 3.0 mm or less.

The non-halogen flame-retardant resin composition that constitutes the sheath layer 9 contains a base polymer (resin component), a flame retardant, a cross-linking agent (silane coupling agent and peroxide), and other additives.

As the base polymer (resin component), for example, ethylene vinyl acetate copolymer (EVA) can be used. In particular, it is preferable to use EVA with a vinyl acetate content (VA amount) of 40% by mass or more. If the vinyl acetate content is 40% by mass or more, the resulting cinders will be strong, resulting in good flame retardancy and low smoke generation.

As the base polymer (resin component), the above EVA may be used in combination with maleic acid modified polyolefin, styrene butadiene rubber, etc.

Metal hydroxides can be used as flame retardants. Examples of metal hydroxides that can be used include magnesium hydroxide and aluminum hydroxide. The amount (content) of the metal hydroxide added is preferably 100 parts by mass or more and 150 parts by mass or less per 100 parts by mass of the base polymer. If the amount is 100 parts by mass or more and 150 parts by mass or less, good heat aging properties and low smoke generation can be obtained. From the viewpoint of achieving both high flame retardancy and low smoke generation, the amount (content) of the metal hydroxide added is more preferably 100 parts by mass or more and 125 parts by mass or less per 100 parts by mass of the base polymer.

As the metal hydroxide, the above-mentioned aluminum hydroxide, magnesium hydroxide, etc. can be used. By using either magnesium hydroxide or aluminum hydroxide, high flame retardancy can be achieved. In addition, it is preferable to use aluminum hydroxide or magnesium hydroxide whose surface is coupled with a fatty acid or a silane compound. By using such coupled compounds, good tensile strength and breaking elongation can be obtained in a tensile test.

The metal hydroxide may be a combination of the aluminum hydroxide and magnesium hydroxide described above. In this case, it is preferable to adjust the mass ratio of magnesium hydroxide:aluminum hydroxide to within the range of 40:60 to 60:40. This is because the stepwise dehydration method is more effective in suppressing the temperature rise of the cable and solidifying the scum after the start of combustion in a halogen-free flame-retardant resin composition. The dehydration start temperatures of aluminum hydroxide and magnesium hydroxide are around 210°C and 280°C, respectively, and by using the above mass ratio, stepwise dehydration proceeds effectively, suppressing the temperature rise of the cable after the start of combustion and promoting the solidification of the scum.

As the crosslinking agent, a peroxide and a silane coupling agent can be used. Here, the silane coupling agent is grafted to the base polymer by the peroxide, and is then used to crosslink the base polymer by silane crosslinking. As the peroxide (a compound having the -O-O- structure), tert-butylperoxy(2-ethylhexyl)carbonate, 1,1-bis(tert-butylperoxy)cyclohexane, tert-butylperoxyisopropylcarbonate, tert-amylperoxyisopropylcarbonate, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, di-tert-butylperoxide, di-tert-amylperoxide, 1,1-di(tert-amylperoxy)cyclohexene, etc. can be used.

As the silane coupling agent (R-Si-X ₃ , R: organic group, X: functional group, where X may be a different functional group containing H), those having a functional group (X) of vinyl, epoxy, styryl, methacryl, amino, isocyanurate, mercapto, or acid anhydride can be used. Wherein X may be a different functional group containing H. Specifically, examples of the silane coupling agent include vinyl silane compounds such as vinyl trimethoxy silane, vinyl triethoxy silane, and vinyl tris (β-methoxyethoxy) silane; epoxy silane compounds such as β-(3,4-epoxycyclohexyl) ethyl trimethoxy silane, γ-glycidoxypropyl trimethoxy silane, and γ-glycidoxypropyl methyl diethoxy silane; styryl silane compounds such as p-styryl trimethoxy silane; and methacryloxypropyl triethoxy silane, 3-methacryloxypropyl trimethoxy silane, 3-methacryloxypropyl methyl dimethoxy silane, and 3-methacryloxypropyl methyl diethoxy silane. Examples of the silane compounds that can be used include aminosilane compounds such as tacrylsilane compounds, γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane, N-β-(aminoethyl)γ-aminopropyltrimethoxysilane, β-(aminoethyl)γ-aminopropylmethyldimethoxysilane, and N-phenyl-γ-aminopropyltrimethoxysilane, isocyanurate silane compounds such as tris-(trimethoxysilylpropyl)isocyanurate, mercaptosilane compounds such as 3-mercaptopropyltrimethoxysilane and 3-mercaptopropyltriethoxysilane, and acid anhydride silane compounds such as 3-trimethoxysilylpropylsuccinic anhydride. Two or more of these silane compounds can also be used in combination.

The amount (content) of the peroxide added is preferably 4 parts by mass or more per 100 parts by mass of the base polymer, and the amount (content) of the silane coupling agent added is preferably 2 parts by mass or more per 100 parts by mass of the base polymer. By setting the amount of the peroxide and the silane coupling agent added as crosslinking agents in the above range, the mechanical strength of the sheath layer can be maintained even when low-temperature crosslinking is performed. The upper limit of the amount (content) of the peroxide added is 10 parts by mass. By setting the amount of the peroxide added to 10 parts by mass or less, crosslinking does not proceed unnecessarily, and processing (especially extrusion processing) can be performed in a state with good handleability. In addition, by adding 2 parts by mass or more of the silane coupling agent per 100 parts by mass of the base polymer, the decrease in tensile strength can be suppressed. In addition, the upper limit of the amount (content) of the silane coupling agent added is 6 parts by mass. By setting the amount of the silane coupling agent added to 6 parts by mass or less, the decrease in the breaking elongation can be suppressed. The amount (content) of the silane coupling agent added refers to the amount added when the silane coupling agent itself is added, and does not include the amount of silane coupling agent used for surface treatment on the surface of the metal hydroxide.

Other additives that can be used include crosslinking agents, stabilizers, antioxidants, colorants, lubricants, etc.

(Manufacturing methods for power transmission cables)
A method for producing the power transmission cable according to the present embodiment will be described below. Figures 2 and 3 are schematic diagrams showing the steps for producing the power transmission cable.

The resin core C of the power transmission cable is formed (prepared). First, the conductor 2 is prepared, and the internal semiconductive material, which is the raw material of the internal semiconductive layer 3, the insulating material, which is the raw material of the insulating layer 4, and the external semiconductive material, which is the raw material of the external semiconductive layer 5, are extruded around the periphery of the conductor 2. For example, the internal semiconductive material (3M) is extruded around the periphery of the conductor 2 from an extruder (200a), and the insulating material and the external semiconductive material are extruded from other extruders (not shown). In this way, the internal semiconductive layer 3, the insulating layer 4, and the external semiconductive layer 5 can be extruded together, for example, so as to surround the periphery of the conductor in sequence.

As a modified example, the inner semiconductive layer 3, the insulating layer 4, and the outer semiconductive layer 5 may be extruded in sequence, thereby forming a resin core C made of the conductor 2, the inner semiconductive layer 3, the insulating layer 4, and the outer semiconductive layer 5.
Next, the rubber contained in the layer constituting the resin core portion C is crosslinked (first crosslink).

Such a resin core part C can be formed, for example, by using the device shown in FIG. 2. The single-screw extruder 200a shown in FIG. 2 has a screw 220 arranged in a cylinder and a material inlet 221. For example, the material 3M of the internal semiconductive layer 3 is introduced from the material inlet (hopper) 221. In addition, another single-screw extruder (not shown) also has a screw arranged in a cylinder and a material inlet, and the material of the insulating layer 4 is introduced from the material inlet (hopper). In addition, another single-screw extruder (not shown) also has a screw arranged in a cylinder and a material inlet, and the material of the external semiconductive layer 5 is introduced from the material inlet (hopper). In this way, the conductor 2 passes through the extrusion head 230, and the materials of the internal semiconductive layer 3, the insulating layer 4, and the external semiconductive layer 5 are extruded from the inside to the outside of the conductor 2, and crosslinked while passing through the crosslinking tube (steam tube) 240 (crosshead extrusion). Using such a continuous crosslinking device, the internal semiconductive layer 3, insulating layer 4, and external semiconductive layer 5 of the resin core part C are heated and crosslinked (crosslinked by passing through pressurized steam). For example, crosslinking is performed for 30 to 60 minutes in a steam atmosphere at 150°C to 180°C. In this way, the resin core part C can be formed.

In the above, the materials of the internal semiconductive layer 3, the insulating layer 4, and the external semiconductive layer 5 are extruded together on the outer periphery of the conductor 2, and the three layers of the internal semiconductive layer 3, the insulating layer 4, and the external semiconductive layer 5 extruded in order on the outer periphery of the conductor 2 are crosslinked together. However, it is also possible to extrude each layer on the outer periphery of the conductor 2 and crosslink the three layers together, or it is also possible to extrude and crosslink each layer on the outer periphery of the conductor 2.

Next, the cross-linked resin core C is cooled. For example, the resin core C (conductor 2, inner semiconductive layer 3, insulating layer 4, and outer semiconductive layer 5) unrolled in FIG. 2 is continuously immersed in water in a cooling tank (not shown) and cooled (water cooling method).

Next, a semiconductive tape is spirally wound around the outer circumference of the outer semiconductive layer 5 along the cable axial direction to form the semiconductive tape layer 6. The semiconductive tape may be wound in an overlapping manner, for example, so that 1/4 to 1/2 of the tape width overlaps.

Then, a wire is wound spirally around the outer circumference of the semiconductive tape layer 6 along the cable axial direction to form the shielding layer 7. The diameter of the wire used here is, for example, about 0.6 to 0.8 mm. Next, a pressure tape is wound spirally around the outer circumference of the shielding layer 7 along the cable axial direction to form the pressure tape layer 8. In this way, a core part with a shielding layer (a laminate from the conductor 2 to the pressure tape layer 8) can be formed.

Next, the above-mentioned halogen-free flame-retardant resin composition is extruded around the outer periphery of the holding tape layer 8 (core portion with shielding layer) to form the sheath layer 9. After that, the sheath layer 9 is crosslinked (second crosslinking).

For example, material 51, which is pellets of the above-mentioned halogen-free flame-retardant resin composition, is supplied from hopper 101 of extruder 100 shown in FIG. 3(a). Note that some of the components of the above-mentioned halogen-free flame-retardant resin composition (e.g., crosslinking agent) may be added from an inlet (not shown) in the middle of extruder 100. Then, the above-mentioned halogen-free flame-retardant resin composition is coated on the outer periphery of the core part with the shielding layer (laminate from conductor 2 to pressing tape layer 8, see FIG. 3(b)) supplied from the upstream side to form sheath layer 9.

In this embodiment, the silane coupling agent can be added by, for example, the integral blending method. The integral blending method is a method in which the silane coupling agent is added without being used for pretreatment of other materials. In this case, the silane coupling agent and other materials may be added simultaneously, or the timing of adding the silane coupling agent and other materials may be staggered. For example, it is preferable to add the silane coupling agent after adding the resin component that will become the base polymer, and it is more preferable to add the silane coupling agent immediately after adding the base polymer. In this case, it is not to be added immediately after adding an inorganic filler such as a flame retardant.

By adding the silane coupling agent in this manner, it is well dispersed and mixed into the base polymer. As a result, the sheath layer 9 formed from the non-halogen flame-retardant resin composition mixed in this manner can have improved tensile strength.

Next, the power transmission cable (laminate from the conductor 2 to the sheath layer 9) sent out from the extruder 100 is supplied to a lead coating layer forming device 110 arranged downstream of the extruder 100. While moving the power transmission cable (laminate from the conductor 2 to the sheath layer 9, see FIG. 3(c)), a lead coating layer 10 is continuously formed on the outer circumference of the power transmission cable (sheath layer 9) (see FIG. 3(d)), and the power transmission cable is wound up on a winding drum 120. By providing the lead coating layer 10, steam does not come into contact with the sheath layer 9 in the crosslinking process described below, so that deformation of the surface of the sheath layer 9 due to steam pressure can be suppressed. Note that the material of the coating layer (coating material) is not limited to lead.

Next, the power transmission cable (laminate from the conductor 2 to the lead coating layer 10, see FIG. 3(d)) wound on the winding drum 120 is subjected to a crosslinking treatment. Specifically, the winding drum 120 on which the power transmission cable (laminate from the conductor 2 to the lead coating layer 10) is wound is placed in a crosslinking facility (kettle crosslinking facility) 130, and the sheath layer 9 is crosslinked (heated). For example, it is left to stand in a steam atmosphere at 90°C for 72 hours (h).

Of the heating conditions (crosslinking temperature, crosslinking time), the crosslinking temperature is preferably 90°C or higher and lower than 105°C. If it is lower than 105°C, the formation of gaps between the sheath and the core portion with the shielding layer can be suppressed, and if it is 90°C or higher, the crosslinking speed does not become extremely slow. Furthermore, it is more preferable that the crosslinking temperature is 90°C or higher and 100°C or lower. Furthermore, it is preferable that the crosslinking time is 5 hours or higher and 270 hours or lower, and more preferably 24 hours or higher and 72 hours or lower.

Then, the sheath layer 9 after crosslinking is cooled. For example, the power transmission cable (the laminate from the conductor 2 to the lead coating layer 10) wound on the winding drum 120 is removed from the crosslinking equipment (boiler crosslinking equipment) 130, and cooled by being left to stand at room temperature (e.g., 25°C), and the lead coating layer 10 is peeled off, thereby producing the power transmission cable.

In this embodiment, a silane coupling agent and a peroxide are added to the sheath layer as crosslinking agents in the above ranges, so that crosslinking can be allowed at relatively low temperatures while maintaining mechanical strength, and gaps between the insulating layer and the sheath layer that may occur due to differences in the shrinkage rates of the insulating layer and the sheath layer can be suppressed. This makes it possible to suppress a decrease in connection reliability due to misalignment of the core portion (insulating layer).

The effect of this embodiment will be described in detail below. When the conventionally commonly used kettle cross-linking method is applied as a method for bridging the sheath layer 9, a gap may occur between the insulating layer 4 and the sheath layer 9, more specifically, between the outer semiconductive layer 5 and the shielding layer 7. This gap is considered to occur due to the difference in the shrinkage rate of each layer (particularly the insulating layer 4 and the sheath layer 9) formed in sequence to cover the periphery of the conductor 2. In the case of kettle cross-linking, the already cross-linked inner semiconductive layer 3, insulating layer 4, and outer semiconductive layer 5 other than the sheath layer 9 to be cross-linked are also exposed to high temperatures for a long time (for example, exposed to a high temperature of 145°C for 2 hours). As a result, each of the inner semiconductive layer 3, insulating layer 4, and outer semiconductive layer 5 expands due to heat and shrinks in the subsequent cooling process. At this time, the shrinkage rate of each layer is different, so a gap occurs according to the difference in the shrinkage rate.

For example, if ethylene propylene rubber is used for the insulating layer 4 and EVA is used for the sheath layer 9, the ratio of the linear expansion coefficients, "linear expansion coefficient of insulating layer/linear expansion coefficient of sheath layer," is 1.3 or more.

Even if the linear expansion coefficient of the insulating layer 4 is greater than that of the sheath layer 9, if the insulating layer 4 is thin, excessive gaps will not occur. However, in the case of extra-high voltage power transmission cables, there is a tendency to make the insulating layer 4 relatively thick in order to improve the insulating properties, and the insulating layer 4 is thicker than the sheath layer 9, and it is preferable that the insulating layer 4 is three or more times thicker than the sheath layer 9. Thus, if the insulating layer 4 is thick, the amount of deformation of the insulating layer 4 caused by the difference in linear expansion coefficient becomes large, making it easier for gaps to occur.

When gaps occur between the layers that make up the power transmission cable in this way, the characteristics of the power transmission cable are reduced due to the gaps. In particular, when the conductor 2 is disposed at the center of the sheath layer 9, and the inner semiconductive layer 3, insulating layer 4, outer semiconductive layer 5, and sheath layer 9 are disposed around the conductor 2, sheath misalignment is likely to occur between the layers with gaps between them. When this sheath misalignment occurs, it may impair connection reliability at the connection points between the power transmission cable and other components. Here, "sheath slippage" refers to the phenomenon in which the sheath moves between layers with gaps when the conductor 2 is placed at the center of the sheath layer 9 and the inner semiconductive layer 3, insulating layer 4, outer semiconductive layer 5, and sheath layer 9 are placed around the conductor 2. For example, it refers to the phenomenon in which the sheath moves in the longitudinal direction of the power transmission cable due to the occurrence of excessive gaps between the holding tape layer 8 and the shielding layer 7, between the shielding layer 7 and the semiconductive tape layer 6, or between the core with the shielding layer (the laminate from the conductor 2 to the holding tape layer 8, see Figure 3(b)) and the sheath layer 9.

The gaps between the layers constituting the power transmission cable as described above are caused by thermal expansion and contraction when the sheath layer 9 is heated (cross-linked), so the occurrence of gaps can be suppressed by lowering the heating temperature. However, cross-linking at low temperatures may reduce the degree of cross-linking of the sheath layer, resulting in a decrease in mechanical strength.

In this embodiment, therefore, in order to increase the mechanical strength even when crosslinking at low temperatures, the type and amount of crosslinking agent in the halogen-free flame-retardant resin composition that constitutes the sheath layer are adjusted, and the order in which each component is added is specified, thereby successfully suppressing gaps that occur in the power transmission cable and improving the mechanical strength. This is explained in more detail below based on examples.

[Example]
The halogen-free flame-retardant resin composition used in the power transmission cable of the present embodiment will be described more specifically below with reference to examples.

(Material name)
<Base polymer>
EVA1: "Evaflex EV45LX" manufactured by Mitsui DuPont Polychemicals (VA content: 46% by mass)
EVA2: "Evaflex V9000" manufactured by Mitsui DuPont Polychemicals (VA content: 41% by mass)
<Crosslinking agent>
Silane coupling agent: Shin-Etsu Chemical Co., Ltd. "KBM-503" (3-methacryloxypropyltrimethoxysilane)
Peroxide: "Trigonox 22-70E" (1,1-bis(tert-butylperoxy)cyclohexane) manufactured by Kayaku Akzo Co., Ltd.

<Crosslinking assistant>
Triallyl isocyanate: "TAIC" manufactured by Nippon Kasei Co., Ltd.
<Stabilizer>
Zinc oxide: "Zinc Oxide No. 3" manufactured by Sakai Chemical Industry Co., Ltd.
<Antioxidants>
2,2,4-trimethyl-1,2-dihydroquinoline polymer: "Nocrac 224" manufactured by Ouchi Shinko Chemical Co., Ltd.
<Flame retardants>
Magnesium hydroxide: "Kisuma 5L" manufactured by Kyowa Chemical Industry Co., Ltd.
Aluminum hydroxide: "BF-013STV" (silane 1.0 μm) manufactured by Nippon Light Metal Co., Ltd.

<Coloring agent>
Carbon: "FT Carbon" manufactured by Asahi Carbon Co., Ltd.
<Lubricant>
Lithium hydroxstearate: "LS-6" manufactured by Nitto Kasei Kogyo Co., Ltd.
Zinc stearate: "EZ-101" manufactured by Nitto Kasei Kogyo Co., Ltd.

(Examples 1 to 3)
A halogen-free flame-retardant resin composition was prepared with the ingredients shown in Table 1, and kneaded. The laminate (see FIG. 3(b)) from the conductor 2 to the pressing tape layer 8 was then coated (solid extrusion) with the halogen-free flame-retardant resin composition to form a sheath layer 9. The laminate was then covered with a lead coating layer 10 (see FIG. 3(d)), wound on a winding drum 120, and the winding drum 120 was placed in a crosslinking facility (kettle crosslinking facility) 130 to crosslink (heat) the sheath layer 9. The halogen-free flame-retardant resin composition and treatment conditions (crosslinking temperature, crosslinking time) used for the sheath layer 9 are as shown in Table 1. Finally, the sheath layer 9 was cooled to obtain a power transmission cable.

When extruding the halogen-free flame-retardant resin composition, the flame retardant (metal hydroxide) was first added to the kneader, followed by the additives other than the silane coupling agent, the base polymer, and the silane coupling agent, in that order. After all materials were added, the mixture was thoroughly kneaded and extruded.

The conductor 2 was a stranded wire (outer diameter 12.53 mm) made of 19 composite strands of 27 tin-plated soft copper wires. A separator tape was placed between the conductor 2 and the internal semiconductive layer 3, and the nylon separator tape was wrapped around the outer circumference of the conductor 2 with 1/2 wrap. The internal semiconductive layer 3 was 1.000 mm thick and was made by solid extrusion of conductive ethylene propylene rubber containing carbon. The outer diameter after the internal semiconductive layer 3 was formed was 14.97 mm. The insulating layer 4 was 15.165 mm thick and was made by solid extrusion of ethylene propylene rubber containing clay. The outer diameter after the insulating layer 4 was formed was 45.30 mm. The semiconductive tape layer 6 was 0.500 mm thick and 40 mm wide, and was made by solid extrusion of nylon tape containing carbon with 1/2 wrap. The outer diameter after the semiconductive tape layer 6 was formed was 46.30 mm. The shielding layer 7 was a horizontally wound shield in which 30 tin-plated soft copper wires were wound horizontally at a pitch of 136 mm. The thickness of the shielding layer 7 was 0.800 mm, and the outer diameter after the shielding layer 7 was formed was 47.90 mm. The pressure tape layer 8 was 0.220 mm thick and 90 mm wide, and was wound with a pressure tape made of nylon in a 1/2 wrap pattern. The outer diameter after the insulating layer 4 was formed was 48.34 mm. The thickness of the sheath layer 9 was 2.5 mm, and the outer diameter after the sheath layer 9 was formed was 53.34 mm.

(Comparative Examples 1 to 7)
The component composition and processing conditions (crosslinking temperature, crosslinking time) were as shown in Table 1, and a power transmission cable was obtained in the same manner as in the example.

In the comparative example, when extruding the halogen-free flame-retardant resin composition, the flame retardant (metal hydroxide) was first added to the kneader, followed by the silane coupling agent, additives other than the silane coupling agent, and the base polymer, in that order. After all materials were added, the mixture was thoroughly kneaded and extruded.

The amounts of each component shown in Table 1 are expressed in parts by weight per 100 parts by weight of the base polymer.

(evaluation)
(Tensile test)
The shielded core portion (a laminate from the conductor 2 to the pressure tape layer 8, see FIG. 3(b)) of the obtained power transmission cable was pulled out, and the sheath layer 9 was punched out with a dumbbell to obtain a sample (test piece). The test piece was a dumbbell-shaped No. 6 with a gauge length of 20 mm.

A tensile test was conducted on this sample. The tensile test was conducted based on the IEC 60811-1-1 standard. Specifically, the sample was pulled at a speed of 200 mm/min using a tensile tester, and the tensile strength, breaking elongation, and 100% modulus were measured. The tensile strength is the stress corresponding to the maximum force applied during the test. The breaking elongation is the permanent elongation after breaking expressed as a percentage of the original length. The 100% modulus is the stress at the point when the test piece is 100% elongated.

(Pull-out test)
The obtained power transmission cable was cut into a length of 20 cm to prepare a sample (test piece). In the pull-out test, the core part with the shielding layer (a laminate from the conductor 2 to the pressing tape layer 8, see FIG. 3(b)) was pressed into the cut surface of the power transmission cable, and the force (kgf) until the core part with the shielding layer moved (shifted or peeled off) from the sheath layer 9 was measured. FIG. 4 is a cross-sectional view showing the state of the pull-out test. Specifically, as shown in FIG. 4, the test is performed using a scale 300 and a convex jig 310. The convex jig 310 has a convex part 310a that contacts the core part with the shielding layer of the power transmission cable. The diameter Rc of the convex part 310a is equal to or larger than the diameter Ra of the resin core part (a laminate from the conductor 2 to the outer semiconductive layer 5) and equal to or smaller than the diameter Rb of the core part with the shielding layer (a laminate from the conductor 2 to the pressing tape layer 8). Here, a cylindrical member with a diameter of 3 cm was used. A convex jig 310 was placed on a scale 300, the shielded core of the power transmission cable was aligned with the convex portion 310a, the power transmission cable was pressed into the convex portion 310a, and the maximum value (kgf) of the scale 300 was measured until the shielded core moved relative to the sheath layer 9. If the measured value (pull-out strength) was greater than 10 kgf, it was determined that sheath slippage would not occur and was marked as pass (◯), and if it was less than 10 kgf, it was determined that sheath slippage would occur and was marked as fail (×).

In this way, when a force (load) is applied in an opposite direction between the sheath layer and the part inside the sheath layer in a power transmission cable, the sheath layer and the part inside the sheath layer will shift relative to each other. This force (load) is defined as the "pull-out strength" and is measured by a test (pull-out test) using the above-mentioned scale and convex jig.

(result)
As shown in Table 1, the sheath layers of Examples 1 to 3 obtained good values in both the tensile test and the 100% modulus test, and the measured values in the pull-out test were 10 kgf or more, which suggests that gaps between the core portion with the shielding layer and the sheath layer are suppressed. Note that, when observing the comparative example, the location where gaps occur is not limited to between the sheath layer 9 and the holding tape layer 8, and gaps may occur between the holding tape layer 8 and the shielding layer 7, or between the shielding layer 7 and the semiconductive tape layer 6.

In contrast, in the sheath layers of Comparative Examples 1 to 4, the silane coupling agent was added immediately after the flame retardant was added, and the pull-out test measured values were 10 kgf or more, suppressing sheath slippage, but the tensile strength was 12.0 MPa or less, which did not achieve the desired characteristics.

In addition, the sheath layers of Comparative Examples 5 and 6 had pull-out test values of 10 kgf or more, which prevented sheath slippage, but the crosslinking temperature was low and the tensile strength was insufficient.

In Comparative Example 7, the tensile strength was sufficient, but the crosslinking temperature was high, so the measured value in the pull-out test was lowered to less than 10 kgf.

In contrast, according to the present embodiment, by adjusting the amount and timing of addition of the cross-linking agent as described above, it is possible to suppress the gap between the shielded core and the sheath layer while maintaining the mechanical properties of the power transmission cable, even with low-temperature cross-linking.

Figure 5 shows a graph plotting the relationship between the tensile strength and pull-out strength obtained in the examples and comparative examples, with the crosslinking temperature on the horizontal axis. From Figure 5, it was found that in the low-temperature crosslinking region, the tensile strength of the sheath layer can be improved and the pull-out strength can also be made good by adding the silane coupling agent after the base polymer is added. Note that regardless of the timing of addition of the silane coupling agent, when the crosslinking temperature is the same, the pull-out strength was the same value.

(Application example)
In the above embodiment, the power transmission cable is formed of a plurality of laminates as shown in Fig. 1, but the power transmission cable may have a core portion made of an insulated electric wire having a conductor 2 and an insulating layer 4 provided therearound, and a sheath layer 9 provided around the core portion. Also, a plurality of insulated electric wires may be used as the core portion. As the sheath layer of the power transmission cable having such a configuration, the above-mentioned halogen-free flame-retardant resin composition may be used, and a sheath layer may be formed around the core portion in the same manner as in the above embodiment.

In the above embodiment, the power transmission cable is constructed from a laminate of multiple layers as shown in FIG. 1, but for example, the conductor 2 and insulating layer 4 may be essential components of the resin core C, and the inner semiconductive layer 3 or the outer semiconductive layer 5 may be omitted. Furthermore, the semiconductive tape 6, shielding layer 7, or holding tape layer 8 on the outer periphery of the resin core C may be omitted. In such a case, the part inside the sheath layer may be used as the core, and tensile tests, 100% modulus, and pull-out tests may be performed.

The present invention is not limited to the above-described embodiments and examples, and various modifications are possible without departing from the spirit of the invention.

Reference Signs List 1 Power transmission cable 2 Conductor 3 Inner semiconductive layer 3M Material 4 Insulation layer 5 Outer semiconductive layer 6 Semiconductive tape layer 7 Shielding layer 8 Pressing tape layer 9 Sheath layer 10 Lead coating layer 51 Material 100 Extruder 101 Hopper 110 Lead coating layer forming device 120 Winding drum 130 Crosslinking equipment (kettle crosslinking equipment)
200a: Extruder 220: Screw 221: Material inlet (hopper)
230 Extrusion head 240 Steam pipe (crosslinking pipe)
300: Scale 310: Convex jig 310a: Convex portion C: Resin core portion

Claims (12)

(a) a core portion having a conductor and an insulating layer formed on the outer periphery of the conductor;
(b) a sheath layer formed on the outer periphery of the core portion,
the insulating layer is thicker than the sheath layer;
the insulating layer has a linear expansion coefficient greater than that of the sheath layer;
The tensile strength of the sheath layer is greater than 12.0 MPa;
A power transmission cable having a pull-out strength of 10 kgf or more inside the sheath layer. The power transmission cable according to claim 1,
the sheath layer is made of a halogen-free flame-retardant resin composition containing a base polymer, a silane coupling agent, a peroxide, and a flame retardant containing a metal hydroxide;
The content of the silane coupling agent is 2 parts by mass or more relative to 100 parts by mass of the base polymer,
A power transmission cable, wherein the content of the peroxide is 4 parts by mass or more per 100 parts by mass of the base polymer. The power transmission cable according to claim 1,
The thickness of the insulating layer is three times or more the thickness of the sheath layer,
A power transmission cable, wherein the ratio of the linear expansion coefficients of the sheath layer and the insulating layer, i.e., insulating layer linear expansion coefficient/sheath layer linear expansion coefficient, is 1.3 or more. The power transmission cable according to claim 1,
A power transmission cable having a shielding layer between the sheath layer and the insulating layer. The power transmission cable according to claim 4,
A power transmission cable having a pressure tape layer between the sheath layer and the shielding layer. The power transmission cable according to claim 2 ,
the base polymer comprises an ethylene vinyl acetate copolymer;
A power transmission cable comprising 100 to 150 parts by mass of a metal hydroxide per 100 parts by mass of a base polymer. The power transmission cable according to claim 1,
The power transmission cable, wherein the core portion is a resin core portion including a conductor, an inner semiconductive layer, an insulating layer, and an outer semiconductive layer. (a) a step of coating a core portion having a conductor and an insulating layer formed on the outer periphery of the conductor with a halogen-free flame-retardant resin composition to form a sheath layer;
(b) crosslinking the sheath layer by heating;
having
the insulating layer is thicker than the sheath layer;
the insulating layer has a linear expansion coefficient greater than that of the sheath layer;
The tensile strength of the sheath layer is greater than 12.0 MPa;
The method for manufacturing a power transmission cable, wherein the pull-out strength of the portion inside the sheath layer is 10 kgf or more. 9. The method for producing a power transmission cable according to claim 8,
the sheath layer is made of a halogen-free flame-retardant resin composition containing a base polymer, a silane coupling agent, a peroxide, and a flame retardant containing a metal hydroxide;
The content of the silane coupling agent is 2 parts by mass or more relative to 100 parts by mass of the base polymer,
The content of the peroxide is 4 parts by mass or more relative to 100 parts by mass of the base polymer,
The content of the metal hydroxide is 100 parts by mass or more and 150 parts by mass or less with respect to 100 parts by mass of the base polymer.
A method for manufacturing power cables. 10. The method of claim 9 , further comprising the steps of:
The non-halogenated resin composition is obtained by adding the silane coupling agent by an integral blending method and kneading the composition. 9. The method for producing a power transmission cable according to claim 8,
The method for producing a power transmission cable, wherein the step (b) comprises heating at a temperature of 90° C. or higher and lower than 105° C. 9. The method for producing a power transmission cable according to claim 8,
The step (b) comprises heating the sheath layer in a state where the sheath layer is covered with a coating material.