Foamed electric wire, communication cable and its manufacturing method

Abstract

To provide a foamed wire, a communication cable and a manufacturing method thereof that enable foam diameter control even by physical foaming method using inert gas, and that have excellent abrasion resistance and heat deformation resistance assuming an automotive environment while ensuring communication stability.SOLUTION: A foamed electric wire 10 comprises a conductor 12, and a cover layer that covers the conductor, the cover layer comprising one or more layers. At least one layer of the cover layer is a foamed cover layer 14 comprising a resin composition, which is formed by foam extrusion molding and includes a polypropylene resin. The average cell diameter of the foamed cover layer 14 is 30 μm or less in the cross-sectional direction and 60 μm or less in the longitudinal direction, and the foaming rate of the foamed cover layer 14 is from 25% to 55% inclusive. The arithmetic mean height in the surface of the foamed electric wire 10 is 20 μm or less.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a foamed electric wire, a communication cable, and a method for producing the same. [Background technology]

[0002] In the case of foamed electric wires used in high-speed communication cables for the GHz band, it has been thought that foaming control is difficult in the physical foaming extrusion method using an inert gas for small-diameter electric wires, and that communication characteristics are likely to deteriorate due to foaming variations. For this reason, conventional chemical foaming methods, which generate foam relatively gently and tend to stabilize the foam diameter, are widely used. For example, Patent Document 1 discloses that a foamed coating layer is formed after kneading a base resin with a master batch containing a thermal decomposition type chemical foaming agent and a polypropylene-based resin, and the foam diameter and foaming rate are controlled by controlling the melt tension of the resin. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5420662 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in Patent Document 1, the chemical foaming agent is kneaded into the master batch and mixed in the foam extrusion process, so the dispersion of the chemical foaming agent during melting is poor, making it difficult to achieve a uniform foam diameter. In addition, since it is necessary to melt the agent sufficiently in the cylinder, there is a problem that the production speed is slow and the production efficiency is poor. Furthermore, since azodicarbonamide (ADCA) is used as the chemical foaming agent, there is a problem that it cannot comply with the REACH regulation.

[0005] The present invention has been made in consideration of the problems inherent in the conventional techniques. An object of the present invention is to provide a foamed electric wire that allows the foam diameter to be controlled even in a physical foaming method using an inert gas, and that has excellent abrasion resistance and heat deformation resistance in consideration of the vehicle-mounted environment while ensuring communication stability. Another object of the present invention is to provide a communication cable using the foamed electric wire and a manufacturing method thereof. [Means for solving the problem]

[0006] A foamed electric wire according to an embodiment of the present invention comprises a conductor and a covering layer which covers the conductor and consists of one or more layers, at least one of the covering layers being a foamed covering layer made of a resin composition containing a polypropylene resin formed by foam extrusion molding, the foamed covering layer having an average foam diameter of 30 μm or less in the cross-sectional direction and 60 μm or less in the longitudinal direction, a foaming ratio of the foamed covering layer of 25% or more and 55% or less, and an arithmetic mean height on the surface of the foamed electric wire of 20 μm or less.

[0007] A communication cable according to another aspect of the present invention includes the foamed electric wire described above.

[0008] A manufacturing method for a foamed electric wire according to another embodiment of the present invention includes the steps of covering a conductor to form a covering layer consisting of one or more layers, and forming a foamed covering layer consisting of a resin composition containing a polypropylene resin by foam extrusion molding for at least one of the covering layers, wherein the foamed covering layer has an average foam diameter of 30 μm or less in the cross-sectional direction and 60 μm or less in the longitudinal direction, an expansion ratio of the foamed covering layer is 25% or more and 55% or less, and an arithmetic mean height on the surface of the foamed electric wire is 20 μm or less. Effect of the Invention

[0009] According to the present invention, it is possible to control the foam diameter even with a physical foaming method using an inert gas, and it is possible to provide a foamed electric wire and communication cable that have excellent abrasion resistance and heat deformation resistance taking into account the vehicle-mounted environment while ensuring communication stability, and a manufacturing method thereof. [Brief description of the drawings]

[0010] [Figure 1] FIG. 2 is a cross-sectional view of the foam-in-place electric wire according to the embodiment, taken perpendicularly to the longitudinal direction of the foam-in-place electric wire, showing an example in which the covering layer of the foam-in-place electric wire according to the embodiment is one layer. [Diagram 2] FIG. 2 is a cross-sectional view taken perpendicular to the longitudinal direction of the foam-foamed electric wire according to the embodiment, showing an example in which the covering layer of the foam-foamed electric wire according to the embodiment has two layers. [Diagram 3] FIG. 2 is a cross-sectional view taken perpendicular to the longitudinal direction of the foam-foamed electric wire according to the embodiment of the present invention, showing an example in which the covering layer of the foam-foamed electric wire has three layers. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The foamed electric wire according to the present embodiment will be described in detail below with reference to the drawings. Note that the dimensional ratios in the drawings are exaggerated for the convenience of explanation and may differ from the actual ratios.

[0012] [Foam wire] The foamed electric wire of this embodiment includes a conductor and a covering layer covering the conductor. The covering layer is made of one layer or multiple layers. At least one layer of the covering layer is a foamed covering layer made of a resin composition containing a polypropylene resin, formed by foam extrusion molding. The foamed covering layer is preferably formed by foam extrusion molding using an inert gas (physical foaming method).

[0013] As shown in Fig. 1, the foamed electric wire 10 includes a conductor 12 and a foamed covering layer 14 covering the outer circumference of the conductor 12. The conductor 12 and the foamed covering layer 14 form an insulated electric wire. When the foamed electric wire 10 has a single covering layer as shown in Fig. 1, the conductor 12 of the foamed electric wire 10 is covered only with the foamed covering layer 14. Such a foamed electric wire 10 having a single covering layer can be produced by covering the conductor 12 with a resin composition containing polypropylene resin by foam extrusion molding.

[0014] As shown in Fig. 2, when the foamed electric wire 10 has a two-layer coating layer, the foamed electric wire 10 includes a conductor 12, a foamed coating layer 14 that coats the outer periphery of the conductor 12, and a skin layer 16 that coats the outer periphery of the foamed coating layer 14. In the foamed electric wire 10 having such a two-layer coating layer, the foamed coating layer 14 is formed by coating the conductor 12 with a resin composition containing a polypropylene resin by foam extrusion molding, as in the case of a single-layer coating layer. On the other hand, the skin layer 16 can be formed by a general extrusion molding method, but from the viewpoint of improving productivity, it is preferable to form the foamed coating layer 14 and the skin layer 16 by simultaneous extrusion.

[0015] As shown in Fig. 3, when the foamed electric wire 10 has three coating layers, the foamed electric wire 10 includes a conductor 12, an inner layer 18 that coats the outer periphery of the conductor 12, a foamed coating layer 14 that coats the outer periphery of the inner layer 18, and a skin layer 16 that coats the outer periphery of the foamed coating layer 14. To manufacture such a foamed electric wire 10 having three coating layers, the conductor 12 is first coated with the inner layer 18 by extrusion molding. Then, as described above, the inner layer 18 is coated with the foamed coating layer 14 and the skin layer 16 in the same manner as in the case of the foamed electric wire 10 having two coating layers.

[0016] The covering layer of the foamed electric wire 10 may be four or more layers. The number and configuration of the covering layer of the foamed electric wire 10 are not particularly limited. The outer layer or inner layer of the foamed covering layer 14 does not necessarily need to include a non-foamed layer, and the covering layer of the foamed electric wire 10 may be formed only from the foamed covering layer 14.

[0017] The arithmetic mean height on the surface of the foamed electric wire 10 is 20 μm or less. The arithmetic mean height is a parameter for evaluating three-dimensional surface properties (surface roughness). By setting the arithmetic mean height to 20 μm or less, it is possible to ensure abrasion resistance when used as a communication cable. The arithmetic mean height on the surface of the foamed electric wire 10 is measured as the arithmetic mean height Sa by analyzing an image of the surface of the foamed electric wire 10 taken with a one-shot 3D shape measuring instrument (manufactured by Keyence Corporation) or the like, in accordance with the international standard ISO25178, which defines a method for evaluating surface roughness.

[0018] [Foam covering layer] The foamed covering layer 14 is made of a resin composition containing a polypropylene resin. Examples of the polypropylene resin used in the resin composition include homopolypropylene (homoPP), random polypropylene (randomPP), block polypropylene (blockPP), and copolymers with other olefins and other components that are copolymerizable with propylene. Examples of other olefins that are copolymerizable with propylene include α-olefins such as ethylene, 1-butene, isobutylene, 1-pentene, 3-methyl-1-butene, 1-hexene, 3,4-dimethyl-1-butene, 1-heptene, and 3-methyl-1-hexene.

[0019] The resin composition has a melt tension of 15 mN or more and 45 mN or less, preferably 18 mN or more and 35 mN or less, measured by a capillary rheometer at 200°C. By setting the melt tension of the resin composition to 15 mN or more, bubbles on the surface of the foamed coating layer are prevented from breaking, and the surface of the foamed coating layer is less likely to become rough. In addition, by setting the melt tension of the resin composition to 45 mN or less, bubbles are prevented from stretching too much in the longitudinal direction of the foamed electric wire, and the bubbles are more likely to maintain their spherical shape, ensuring communication stability when used as a communication cable. The melt tension can be measured using a capillary rheometer such as Capillograph (registered trademark) (manufactured by Toyo Seiki Seisakusho Co., Ltd.).

[0020] The resin composition has a melt viscosity of 120 Pa·s or more and 200 Pa·s or less, preferably 130 Pa·s or more and 170 Pa·s or less, measured by a capillary rheometer at 200°C. By making the melt viscosity of the resin composition 120 Pa·s or more, bubble generation becomes good. In addition, by making the melt viscosity of the resin composition 200 Pa·s or less, bubbles are prevented from coalescing, and it becomes easy to control the bubble diameter to a desired size. The melt viscosity can be measured using a capillary rheometer such as Capillograph (registered trademark) (manufactured by Toyo Seiki Seisakusho Co., Ltd.) in accordance with JIS K 7199:1999.

[0021] The foamed covering layer 14 has an average foam diameter of 30 μm or less in the cross-sectional direction of the foamed electric wire 10 and 60 μm or less in the longitudinal direction of the foamed electric wire 10. The average foam diameter of the foamed covering layer 14 is preferably 20 μm or less in the cross-sectional direction of the foamed electric wire 10 and 55 μm or less in the longitudinal direction of the foamed electric wire 10. By having the average foam diameter of the foamed covering layer 14 of 30 μm or less in the cross-sectional direction and 60 μm or less in the longitudinal direction, bubbles that are close to spherical and have a fine and uniform foam diameter are easily formed, and communication stability can be ensured when used as a communication cable. The average foam diameter can be confirmed by a method in which the cross-sectional or longitudinal direction of the foamed electric wire 10 is cut and observed with an electron microscope to measure the diameter of the bubbles and calculate the average value. Alternatively, it can also be confirmed by a method of observing a cross-sectional or longitudinal cut surface of the foamed electric wire 10 non-destructively using an X-ray microscope CT, measuring the diameters of the bubbles, and calculating the average value.

[0022] The foamed covering layer 14 has a foaming rate of 25% or more and 55% or less. The characteristic impedance of an in-vehicle communication cable is preferably 95 to 105 Ω, and by setting the foaming rate of the foamed covering layer 14 to 25% or more and 55% or less, the characteristic impedance of the foamed electric wire 10 can be controlled within that range. Note that, like the average foam diameter described above, the foaming rate can be confirmed by observing the cut surface of the foamed electric wire 10, regarding the ratio of the area occupied by bubbles in the cross-sectional area of ​​the foamed covering layer 14 as the foaming rate, and calculating the average foaming rate in each of the cross-sectional direction and the longitudinal direction.

[0023] The foamed coating layer 14 is preferably formed by foam extrusion molding using an inert gas. Specifically, during molding, the resin composition is foamed by injecting an inert gas as a foaming agent into the resin composition in a heated and molten state. The foaming rate can be controlled by adjusting the injection amount, temperature, and pressure of the inert gas.

[0024] Examples of the inert gas used in the foam extrusion include nitrogen gas, carbon dioxide gas, argon gas, water vapor, helium gas, and isobutane gas. From the viewpoint of solubility in polypropylene resin and environmental consideration, the inert gas is preferably at least one selected from the group consisting of nitrogen gas, carbon dioxide gas, and argon gas, and more preferably nitrogen gas or carbon dioxide gas. In addition, nitrogen gas or carbon dioxide gas is used in general foam extrusion of polypropylene resin. Such inert gases may be used alone or in combination of two or more.

[0025] In addition to the polypropylene resin, the resin composition may contain various additives in appropriate amounts within a range that does not impair the effects of the present embodiment. Examples of the additives include flame retardants, inorganic fillers, flame retardant assistants, antioxidants, processing assistants, crosslinking agents, metal deactivators, copper inhibitors, antiaging agents, fillers, reinforcing agents, UV absorbers, stabilizers, plasticizers, pigments, dyes, colorants, and antistatic agents.

[0026] (Flame retardant) The flame retardant improves the flame retardancy of the resin composition. The flame retardant may be, for example, at least one of an organic flame retardant and an inorganic flame retardant. As the organic flame retardant, for example, a halogen-based flame retardant such as a bromine-based flame retardant and a chlorine-based flame retardant, and a phosphorus-based flame retardant such as a phosphate ester, a condensed phosphate ester, a cyclic phosphorus compound, and red phosphorus can be used. As the inorganic flame retardant, at least one metal hydroxide selected from the group consisting of aluminum hydroxide, magnesium hydroxide, and calcium hydroxide can be used. These flame retardants may be used alone or in combination. The flame retardant may include, for example, an organic flame retardant and an inorganic flame retardant. The amount of the flame retardant added may be appropriately adjusted in consideration of the flame retardant effect and the influence on the mechanical properties.

[0027] (Antioxidants) The antioxidant suppresses the oxidation of the resin composition. As the antioxidant, known antioxidants used in thermoplastic resins, such as radical chain inhibitors such as phenol-based antioxidants, hindered phenol-based antioxidants, and amine-based antioxidants, peroxide decomposers such as phosphorus-based antioxidants and sulfur-based antioxidants, and metal deactivators such as hydrazine-based antioxidants and amine-based antioxidants, can be used. The antioxidant may be used alone or in combination. The antioxidant may be appropriately adjusted in the amount added, taking into account the antioxidant effect and problems caused by bleed-out.

[0028] (Copper damage inhibitor) When copper or a copper alloy is used for the conductor 12, the copper may cause deterioration of the coating layer of the foamed electric wire 10, that is, so-called copper damage. For this reason, a copper damage inhibitor may be added to the resin constituting the coating layer of the foamed electric wire 10. As the copper damage inhibitor, for example, a salicylic copper damage inhibitor or a hydrazine copper damage inhibitor is used. The amount of the copper damage inhibitor added may be appropriately adjusted taking into consideration the copper damage prevention effect and problems due to bleed-out.

[0029] The resin composition can be prepared by mixing the additives with the polypropylene resin using known means, such as a Banbury mixer, a kneader, a roll mill, a twin-screw extruder, or a single-screw extruder.

[0030] [conductor] The conductor 12 may be a single wire made of one strand, or a stranded conductor made of multiple strands twisted together. The stranded conductor may be a concentric stranded wire made of one or more strands twisted together around the central strand; a bunched stranded wire made of multiple strands twisted together in the same direction; or a composite stranded wire made of multiple bunched strands twisted together in a concentric manner. The diameter of the conductor and the diameter of each strand constituting the stranded conductor are not particularly limited. The conductor 12 may be a compressed conductor or a non-compressed conductor. Furthermore, the materials of the conductor and the stranded conductor are not particularly limited, and may be known conductive metal materials such as copper, copper alloy, aluminum, and aluminum alloy. The surfaces of the conductor and the stranded conductor may be plated, for example, tin-plated, silver-plated, or nickel-plated.

[0031] The outer diameter of the conductor 12 is not particularly limited, but is preferably 0.45 mm or more. By making the outer diameter of the conductor 12 0.45 mm or more, the resistance of the conductor 12 can be reduced. Moreover, the outer diameter of the conductor 12 is preferably 0.80 mm or less. By making the outer diameter of the conductor 12 0.80 mm or less, the foamed electric wire 10 can be easily routed even in a narrow and short path.

[0032] [Skin layer] As shown in Figs. 2 and 3, the skin layer 16 constituting the outermost layer of the foamed electric wire 10 is not particularly limited in material and thickness as long as it can secure electrical insulation against the conductor. The skin layer 16 can be made of any electrically insulating resin, such as an olefin resin, such as cross-linked polyethylene or polypropylene, or a vinyl chloride resin. Specifically, the resin material constituting the skin layer 16 can be, for example, polyvinyl chloride, heat-resistant polyvinyl chloride, cross-linked polyvinyl chloride, polyethylene, cross-linked polyethylene, foamed polyethylene, cross-linked foamed polyethylene, chlorinated polyethylene, polypropylene, polyamide (nylon), polyvinylidene fluoride, ethylene-tetrafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene, perfluoroalkoxyalkane, natural rubber, chloroprene rubber, butyl rubber, ethylene propylene rubber, chlorosulfonated polyethylene rubber, or silicone rubber. These materials may be used alone or in combination of two or more.

[0033] As shown in Figs. 2 and 3, the foamed covering layer 14 can be covered with the skin layer 16 by a known method. For example, the skin layer 16 can be formed by a general extrusion molding method. As the extruder used in the extrusion molding method, for example, a single screw extruder or a twin screw extruder having a screw, a breaker plate, a crosshead, a distributor, a nipple and a die can be used. When performing extrusion molding, the resin material is charged into the extruder set at a temperature at which the resin is sufficiently melted. At this time, in addition to the resin material, various additives to be added to the above-mentioned resin composition, such as an antioxidant, can be charged into the extruder as necessary.

[0034] [Inner layer] 3, the material and thickness of the inner layer 18 covering the conductor 12 are not particularly limited as long as it can ensure electrical insulation against the conductor 12, and the same resin material as the above-mentioned skin layer 16 can be used. Furthermore, the method of covering the conductor 12 with the inner layer 18 can be a known method, and like the above-mentioned skin layer 16, a general extrusion molding method can be used.

[0035] As described above, the foamed electric wire 10 includes the conductor 12 and the covering layer, which covers the conductor 12 and is composed of one or more layers. At least one layer of the covering layer is a foamed covering layer 14 made of a resin composition containing a polypropylene resin, formed by foam extrusion molding. The average foam diameter of the foamed covering layer 14 is 30 μm or less in the cross-sectional direction and 60 μm or less in the longitudinal direction, and the foaming ratio of the foamed covering layer 14 is 25% or more and 55% or less. The arithmetic mean height on the surface of the foamed electric wire 10 is 20 μm or less. Therefore, it is possible to control the foam diameter even with a physical foaming method using an inert gas, and it is possible to provide a foamed electric wire that is excellent in abrasion resistance and heat deformation resistance in consideration of the vehicle environment while ensuring communication stability.

[0036] As described above, the manufacturing method of the foamed electric wire 10 includes a step of covering the conductor 12 to form a covering layer consisting of one or more layers, and a step of forming the foamed covering layer 14 consisting of a resin composition containing polypropylene resin by foam extrusion molding for at least one layer of the covering layer. The foamed covering layer 14 has an average foam diameter of 30 μm or less in the cross-sectional direction and 60 μm or less in the longitudinal direction, and an expansion ratio of the foamed covering layer 14 of 25% or more and 55% or less. The arithmetic mean height on the surface of the foamed electric wire 10 is 20 μm or less.

[0037] [Communication cable] The communication cable according to the present embodiment includes the above-mentioned foamed electric wire 10. The communication cable can be manufactured by a known method, for example, a general extrusion molding method. Specifically, after bundling one or more foamed electric wires 10, the sheath material can be extruded and covered on the outer surface of the foamed electric wire 10 to form a sheath. The communication cable including the foamed electric wire 10 may be used as a coaxial cable or a twisted pair cable. In addition, the type of resin used for the sheath of the communication cable may be any known insulating resin such as olefin resin such as cross-linked polyethylene or polypropylene, or vinyl chloride, and may contain a plasticizer. The plasticizer may be a known plasticizer added to polyvinyl chloride.

[0038] The communication cable according to the present embodiment includes a foamed electric wire 10. The foamed electric wire 10 allows the foam diameter to be controlled even by a physical foaming method using an inert gas, and has excellent abrasion resistance and heat deformation resistance in consideration of the vehicle-mounted environment while ensuring communication stability. Therefore, a communication cable including such a foamed electric wire 10 can be preferably used as, for example, an in-vehicle transmission cable. EXAMPLES

[0039] The present embodiment will be described in more detail below with reference to examples and comparative examples, but the present embodiment is not limited to these examples.

[0040] [Resin composition] (resin) PP1: Polypropylene: Made by Japan Polypropylene Co., Ltd. Product name: Waymax (registered trademark) EX4000 PP2: Polypropylene: Made by Prime Polymer Co., Ltd. Product name: Prime Polypro (registered trademark) E150GK PP3: Polypropylene: Made by Prime Polymer Co., Ltd. Product name: Prime Polypro (registered trademark) J715M PP4: Polypropylene: Made by Prime Polymer Co., Ltd. Product name: Prime Polypro (registered trademark) J-452HP PP5: Polypropylene: Sanallomer Co., Ltd. Product name: Qualia (registered trademark) CM688A EP1: Soft polypropylene (propylene-ethylene copolymer resin): manufactured by SunAllomer Co., Ltd. Product name: Adflex (registered trademark) Q200F PE1: Low-density polyethylene (LDPE): manufactured by Dow Mitsui Polychemicals Co., Ltd. Product name: Mirason (registered trademark) 3530

[0041] (Antioxidants) Phenolic antioxidant: ADEKA Corporation, product name: Adeka STAB (registered trademark) AO-60

[0042] (Copper damage inhibitor) Hydrazine-based copper inhibitor: BASF Ltd. Product name: Irganox (registered trademark) MD1024

[0043] [Evaluation of resin composition] The resin compositions of Examples 1 to 10 and Comparative Examples 1 to 7 were prepared by melt-kneading the above-mentioned resins, antioxidants, and copper inhibitors in the amounts shown in Tables 1 and 2. The melt tension and melt viscosity of the resin compositions were measured as material properties by the following methods. The evaluation results are shown in Tables 1 and 2.

[0044] (Melt tension) The melt tension was measured using Capillograph (registered trademark) (manufactured by Toyo Seiki Seisakusho Co., Ltd.). Specifically, the capillary L / D was set to 10, the furnace temperature was set to 200°C, and the material was filled and then held for 3 minutes to melt. The resin was extruded at a piston speed of 20 mm / min, and taken up at a take-up roller (manufactured by Toyo Seiki Seisakusho Co., Ltd.) at a take-up speed in increments of 10 mm / min in the range of 10 to 200 mm / min. Each take-up speed was held for 30 seconds, and if the material was not cut, the take-up speed was changed to the next take-up speed. The tension at the time of cut was taken as the melt tension.

[0045] (Melt Viscosity) The melt viscosity was measured using Capillograph (registered trademark) (manufactured by Toyo Seiki Seisakusho, Ltd.) The measurement conditions were capillary L / D = 10, furnace temperature set at 200°C, and the piston speed was changed to 300, 200, 100, 50, and 10 mm / min, and the value at 100 mm / min was recorded as the melt viscosity.

[0046] [Table 1]

[0047] [Table 2]

[0048] [Evaluation of foamed electric wire] Using the resin compositions of Examples 1 to 10 and Comparative Examples 1 to 9, foamed electric wires (triple-insulated electric wires) as shown in FIG. 3 were produced by the following method.

[0049] The foamed electric wire was produced by extruding a polypropylene resin (extruder temperature 170-220°C) onto a compressed or uncompressed twisted conductor (outer diameter 0.45mm-0.80mm) and covering it with an inner layer thickness in the range of 0.03-0.1mm. This electric wire was covered with the resin compositions of Examples 1-10 and Comparative Examples 1-9 by foam extrusion molding to an outer diameter of 1.00-2.20mm to form a foamed covering layer. The extruder temperature for the foam extrusion molding was set to 170-220°C, and high-pressure nitrogen gas pressurized by a gas booster was injected midway through the extruder and mixed. At that time, the foaming ratio of the foamed covering layer was measured under conditions of a low foaming ratio (25% or 20%) and a high foaming ratio (55% or 60%) as shown in Tables 3 and 4. Furthermore, a polypropylene-based resin was extruded simultaneously with the foamed covering layer using a sub-extruder (set temperature 210-240°C) to cover the skin layer to a thickness of 0.03-0.1 mm. The electric wire properties of the foamed electric wire thus obtained were evaluated by the following methods: foaming ratio, average foaming diameter, arithmetic mean height, abrasion resistance, heat deformation resistance, LCTL, and characteristic impedance. The evaluation results are shown in Tables 3 and 4.

[0050] (Expansion rate, average expansion diameter) An X-ray CT microscope (nano3DX) was used to take CT images (cross-sectional and longitudinal directions) of the foamed electric wire. The photographing conditions were: spatial resolution 4.31 μm / voxel, X-ray camera lens L1080, binning 3, exposure time 4 seconds, X-ray source Cu (40 kV, 30 mA), field of view 3.626 × 2.719 mm, and number of photographs 400. The photographed images were analyzed using the ImageJ analysis software to calculate the foaming ratio and average foaming diameter as average values.

[0051] (arithmetic mean height) The surface of the foamed electric wire was photographed using a one-shot 3D shape measuring instrument (manufactured by Keyence Corporation), and the arithmetic mean height Sa of the foamed electric wire surface was calculated using analysis software. The photographing conditions were a magnification of 25 times, a measurement range of 10 × 10 mm, and five photographs.

[0052] (Wear resistance) Abrasion resistance tests were conducted in accordance with the scrape abrasion test stipulated in ISO19642-12. Specifically, a needle was placed perpendicularly against the cut-out foamed electric wire test sample, and the needle was moved back and forth with a constant load (4N) to abrade the coating layer, and the number of times it was moved back and forth before it came into contact with the conductor was measured. Tests that could be moved back and forth 300 times or more before touching the conductor were rated as passing (◎), tests that could be moved back and forth 100 times or more but less than 300 times were rated as passing (○), and tests that could be moved back and forth less than 100 times were rated as failing (×).

[0053] (Heat deformation) The heat deformation resistance test was conducted in accordance with the high temperature pressure test specified in ISO19642-12. Specifically, a constant load was applied from above to the cut-out foamed electric wire test sample for 4 hours at a set temperature of 100°C (Class B), and then a voltage of 1 kV was applied to the conductor of the test sample using a voltage resistance device to conduct a voltage resistance test. The applied load differs depending on the thickness of the coating layer and the outer diameter of the electric wire, and can be calculated using the following formula (1). Tests that maintained insulation for 1 minute were evaluated as passing (○), and tests that maintained insulation for less than 1 minute were evaluated as failing (×). Load (N) = 0.8 x (thickness of coating layer x (2 x outer diameter of electric wire - thickness of coating layer)) 0.5 (1)

[0054] (LCTL) The communication stability when used as a communication cable was evaluated by LCTL (longitudinal transfer conversion loss). Specifically, a test sample was prepared by twisting two foamed electric wires at a pitch of 30 mm, wrapping a metal film around them vertically or horizontally, covering them with a tin-plated soft copper wire braid, and covering the braid with a 0.5 mm thick PVC (polyvinyl chloride resin) sheath. The same voltage was applied to the two foamed electric wires of the obtained test sample using a network analyzer, and the LCTL was calculated by converting the ratio of the potential difference caused by the imbalance of the foamed electric wires into decibels. LCTL of 18 dB / m or less was passed (◎), 20 dB / m or less was passed (○), and over 20 dB / m was failed (×).

[0055] (characteristic impedance) The communication stability when used as a communication cable was evaluated by its characteristic impedance. Specifically, the characteristic impedance of the cut-out foamed electric wire test samples was measured using a vector network analyzer (VNA) (E5071C manufactured by Keysight Technologies). Those with a characteristic impedance of 95 to 105 Ω were rated as pass (○), and those with a characteristic impedance of less than 95 Ω or more than 105 Ω were rated as fail (×).

[0056] [Table 3]

[0057] [Table 4]

[0058] As can be seen from Table 1, the resin compositions of Examples 1 to 10 had melt tensions of 15 mN or more and 45 mN or less, and melt viscosities of 120 Pa·s or more and 200 Pa·s or less, measured at 200° C. with a capillary rheometer.

[0059] As shown in Table 3, the foamed electric wires produced using the resin compositions of Examples 1 to 10 had an average foam diameter of 30 μm or less in the cross-sectional direction of the foamed electric wire and 60 μm or less in the longitudinal direction of the foamed electric wire, both when the foaming ratio was 25% and when it was 55%. Furthermore, the arithmetic mean height of the foamed electric wire was 20 μm or less, and the results of abrasion resistance, heat deformation resistance, LCTL, and characteristic impedance were good.

[0060] On the other hand, as seen from Table 2, the resin compositions of Comparative Examples 1, 2, 4, 8, and 9 had melt tensions measured by a capillary rheometer at 200°C of less than 15 mN or exceeded 45 mN. Also, the resin compositions of Comparative Examples 1, 3, 7, and 9 had melt viscosities measured by a capillary rheometer at 200°C of less than 120 Pa s or exceeded 200 Pa s.

[0061] From Table 4, the foamed electric wires produced using the resin compositions of Comparative Examples 1, 2, and 4 had an average foam diameter exceeding 30 μm in the cross-sectional direction in both cases where the foaming ratio was 25% and 55%. On the other hand, the foamed electric wires produced using the resin compositions of Comparative Examples 8 and 9 had an average foam diameter exceeding 60 μm in the longitudinal direction in both cases where the foaming ratio was 25% and 55%. In addition, the foamed electric wires produced using the resin compositions of Comparative Examples 1, 2, and 4 had an arithmetic average height exceeding 20 μm in both cases where the foaming ratio was 25% and 55%. The resin compositions of Comparative Examples 1, 2, 4, and 7 to 9 failed in any of the results of abrasion resistance, heat deformation resistance, and LCTL. Furthermore, from Table 2, the resin compositions of Comparative Examples 5 and 6 had a melt tension of 15 mN or more and 45 mN or less, and a melt viscosity of 120 Pa·s or more and 200 Pa·s or less, but because the foaming ratio was 20% or 60% in Table 4, the characteristic impedance failed. Incidentally, the resin composition of Comparative Example 3 could not be foamed, and a foamed electric wire could not be produced.

[0062] Although the present embodiment has been described above, the present embodiment is not limited to this, and various modifications are possible within the scope of the gist of the present embodiment. [Explanation of symbols]

[0063] 10. Foamed wire 12 Conductors 14 Foam covering layer 16 Skin Layer 18 Inner layer

Claims

1. A conductor; a coating layer that coats the conductor and is made up of one or more layers; Equipped with At least one of the coating layers is a foamed coating layer formed by foam extrusion molding and made of a resin composition containing a polypropylene resin, The average foam diameter of the foamed coating layer is 30 μm or less in the cross-sectional direction and 60 μm or less in the longitudinal direction, The foaming rate of the foamed covering layer is 25% or more and 55% or less, A foamed electric wire having an arithmetic mean height on a surface of 20 μm or less.

2. The foam coating layer is formed by foam extrusion molding using an inert gas, 2. The foamed electric wire according to claim 1, wherein the resin composition has a melt tension of 15 mN or more and 45 mN or less, and a melt viscosity of 120 Pa·s or more and 200 Pa·s or less, as measured by a capillary rheometer at 200°C.

3. 3. The foamed electric wire according to claim 2, wherein the inert gas is at least one selected from the group consisting of nitrogen gas, carbon dioxide gas, and argon gas.

4. A communication cable comprising the foamed electric wire according to claim 1 or 2.

5. A step of coating the conductor to form a coating layer consisting of one or more layers; and forming a foamed coating layer made of a resin composition containing a polypropylene resin by foam extrusion molding for at least one of the coating layers, The average foam diameter of the foamed coating layer is 30 μm or less in the cross-sectional direction and 60 μm or less in the longitudinal direction, The foaming rate of the foamed covering layer is 25% or more and 55% or less, A method for producing a foamed electric wire, in which the arithmetic mean height on the surface of the foamed electric wire is 20 μm or less.

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