Foamed electric wire, communication cable and manufacturing method thereof

Abstract

A foamed electric wire (10) includes: a conductor (12); and a coating layer that covers the conductor and is composed of one or more layers. At least one of the coating layers is a foamed coating layer (14) made of a resin composition containing a polypropylene resin, the foamed coating layer being formed by foam extrusion molding. The average bubble diameter of the foamed coating layer (14) is 30 [mu] m or less in the cross-sectional direction and 60 [mu] m or less in the longitudinal direction. The foaming rate of the foamed coating layer (14) is 25% or more and 55% or less. The arithmetic average height of the surface of the foamed electric wire (10) is 20 [mu] m or less.

Description

Technical Field

[0001] The invention relates to a foamed electric wire, a communication cable and a manufacturing method thereof. Background Art

[0002] In the process of physical foaming extrusion using inert gas for small diameter electric wires used as foamed electric wires for high-speed communication cables in the GHz band, it is considered that foaming is difficult to control, and communication characteristics tend to deteriorate due to changes in foaming. Therefore, a conventional chemical foaming process is generally used, in which the formation of foaming is relatively mild and the bubble diameter is easily stabilized. Patent document 1 discloses that a foamed coating layer is formed by kneading a base resin and a masterbatch containing a thermal decomposition type chemical foaming agent and a polypropylene resin, and the bubble diameter and the foaming rate are controlled, for example, by controlling the melt tension of the resin.

[0003] Reference List

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent No. 5420662 Summary of the invention

[0006] However, in Patent Document 1, a chemical foaming agent is kneaded into a masterbatch and mixed by a foaming extrusion process. Therefore, the chemical foaming agent has poor dispersibility during melting, and it is difficult to achieve a uniform bubble diameter. In addition, since the foaming agent and the masterbatch need to be fully melted in the barrel, there are problems of low productivity and low production efficiency. In addition, since azodicarbonamide (ADCA) is used as a chemical foaming agent, there is a problem of not being able to comply with REACH regulations.

[0007] The present invention aims to provide a foamed electric wire having excellent wear resistance and heat deformation resistance, which is designed for use in an in-vehicle environment while ensuring communication stability and controlling the bubble diameter even in a physical foaming process using an inert gas. In addition, the present invention aims to provide a communication cable using the foamed electric wire and a method for manufacturing the same.

[0008] A foamed electric wire according to an aspect of the present invention includes: a conductor; and a covering layer, which covers the conductor and is composed of one or more layers, wherein at least one layer of the covering layer is a foamed covering layer, the foamed covering layer is made of a resin composition containing a polypropylene resin, the foamed covering layer is formed by foaming extrusion molding, the average cell diameter of the foamed covering 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, and the arithmetic mean height of the surface of the foamed electric wire is 20 μm or less.

[0009] A communication cable according to another aspect of the present invention includes the above-mentioned foamed electric wire.

[0010] A method for manufacturing a foamed electric wire according to another aspect of the present invention comprises: a step of forming a covering layer which covers a conductor and is composed of one or more layers; and a step of forming a foamed covering layer which is made of a resin composition containing a polypropylene resin as at least one layer of the covering layer, the foamed covering layer being formed by foaming extrusion molding, wherein an average cell diameter of the foamed covering layer is 30 μm or less in a cross-sectional direction and 60 μm or less in a longitudinal direction, a foaming rate of the foamed covering layer is 25% or more and 55% or less, and an arithmetic mean height of a surface of the foamed electric wire is 20 μm or less.

[0011] According to the present invention, it is possible to provide a foamed electric wire having excellent wear resistance and heat deformation resistance, which is designed for use in an in-vehicle environment while ensuring communication stability and capable of controlling the bubble diameter even in a physical foaming process using an inert gas. In addition, it is possible to provide a communication cable using the foamed electric wire and a method for manufacturing the same. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] [ Figure 1 ] Figure 1 An example of a case where the covering layer of the foamed electric wire according to the present embodiment is a single layer is shown, and is a cross-sectional view taken perpendicularly to the longitudinal direction of the foamed electric wire.

[0013] [ Figure 2 ] Figure 2 An example of a case where the covering layer of the foamed electric wire according to the present embodiment has two layers is shown, and is a cross-sectional view taken perpendicularly to the longitudinal direction of the foamed electric wire.

[0014] [ Figure 3 ] Figure 3 An example of a case where the covering layer of the foamed electric wire according to the present embodiment has three layers is shown, and is a cross-sectional view taken perpendicularly to the longitudinal direction of the foamed electric wire.

[0015] Reference Mark List

[0016] 10 Foam wire

[0017] 12 Conductor

[0018] 14 Foam coating

[0019] 16 Surface

[0020] 18 Inner layer DETAILED DESCRIPTION

[0021] The foamed electric wire according to the present embodiment will be described in detail below with reference to the accompanying drawings. For convenience of explanation, the dimensional ratios of the drawings are exaggerated and may be different from the actual ratios.

[0022] [Foam wire]

[0023] The foamed electric wire of the present embodiment includes a conductor and a coating layer covering the conductor. The coating layer is composed of one or more layers. At least one layer of the coating layer is a foamed coating layer made of a resin composition containing a polypropylene resin, and the foamed coating layer is formed by foaming extrusion molding. Preferably, the foamed coating layer is formed by foaming extrusion molding (physical foaming process) using an inert gas.

[0024] like Figure 1 As shown, the foamed wire 10 includes a conductor 12 and a foamed coating layer 14 covering the outer periphery of the conductor 12. The conductor 12 and the foamed coating layer 14 form an insulated wire. Figure 1 As shown, when the covering layer of the foamed wire 10 is a single layer, the conductor 12 in the foamed wire 10 is covered only by the foamed covering layer 14. Such a foamed wire 10 having a single covering layer can be manufactured by covering the conductor 12 with a resin composition containing a polypropylene resin by foam extrusion molding.

[0025] like Figure 2 As shown, when the covering layer of the foamed electric wire 10 has two layers, the foamed electric wire 10 includes a conductor 12, a foamed covering layer 14 covering the outer periphery of the conductor 12, and a surface layer 16 covering the outer periphery of the foamed covering layer 14. Similar to the foamed electric wire having a single covering layer, the foamed covering layer 14 of the foamed electric wire 10 having two covering layers is formed by covering the conductor 12 with a resin composition containing a polypropylene resin by foam extrusion molding. Meanwhile, the surface 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 covering layer 14 and the surface layer 16 by co-extrusion.

[0026] like Figure 3 As shown, when the covering layer of the foamed electric wire 10 has three layers, the foamed electric wire 10 includes a conductor 12, an inner layer 18 covering the outer periphery of the conductor 12, a foamed covering layer 14 covering the outer periphery of the inner layer 18, and a surface layer 16 covering the outer periphery of the foamed covering layer 14. In order to manufacture the foamed electric wire 10 having three covering layers, the conductor 12 is first covered with the inner layer 18 by extrusion molding. Then, as described above, in the same manner as when the foamed electric wire 10 has two covering layers, the foamed electric wire can be manufactured by covering the inner layer 18 with the foamed covering layer 14 and the surface layer 16.

[0027] The coating layer of the foamed wire 10 may have four or more layers. In addition, there is no particular limitation on the number of coating layers or the configuration of the foamed wire 10. The outer layer or inner layer of the foamed coating layer 14 does not necessarily include a non-foamed layer, and the coating layer of the foamed wire 10 may be formed of only the foamed coating layer 14.

[0028] The arithmetic mean height of 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 characteristics (surface roughness). Since the arithmetic mean height is 20 μm or less, it is possible to ensure wear resistance when used as a communication cable. According to the international standard ISO 25178 that defines the evaluation method of surface roughness, the arithmetic mean height of 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 captured using a 3D optical profiler (manufactured by KEYENCE CORPORATION) or the like.

[0029] [Foamed coating layer]

[0030] 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 (random PP), block polypropylene (block PP) and copolymers with components such as other olefins copolymerizable with propylene. Examples of other olefins 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.

[0031] The melt tension of the resin composition measured at 200 ° C using a capillary rheometer is 15mN or more and 45mN or less, and preferably 18mN or more and 35mN or less. Since the melt tension of the resin composition is more than 15mN, the bubble on the surface of the foaming coating layer is prevented from bursting, and the surface of the foaming coating layer is unlikely to be rough. In addition, since the melt tension of the resin composition is less than 45mN, the bubble is prevented from over-extending in the length direction of the foaming wire, and the bubble is easy to continue to have a spherical shape, and the communication stability when used as a communication cable can be ensured. The melt tension can be measured using a capillary rheometer such as CAPILOGRAPH (registered trademark) (manufactured by Toyo Seiki Seisaku-sho, Ltd.).

[0032] The melt viscosity of the resin composition measured at 200 ° C using a capillary rheometer is 120Pa s or more and 200Pa s or less, and preferably 130Pa s or more and 170Pa s or less. Since the melt viscosity of the resin composition is more than 120Pa s, the formation of bubbles becomes favorable. In addition, since the melt viscosity of the resin composition is less than 200Pa s, it is possible to prevent bubble aggregation, and the bubble diameter can be easily controlled to the desired bubble diameter. The melt viscosity can be measured according to JIS K 7199: 1999 using a capillary rheometer such as CAPILOGRAPH (registered trademark) (manufactured by Toyo Seiki Seisaku-sho, Ltd.).

[0033] The average bubble diameter of the foamed coating layer 14 is 30 μm or less in the cross-sectional direction of the foamed wire 10 and 60 μm or less in the length direction of the foamed wire. In addition, the average bubble diameter of the foamed coating layer 14 is preferably 20 μm or less in the cross-sectional direction of the foamed wire 10 and 55 μm or less in the length direction of the foamed wire 10. Since the average bubble diameter of the foamed coating layer 14 is 30 μm or less in the cross-sectional direction and 60 μm or less in the length direction, it is easy to form nearly spherical and fine bubbles with uniform bubble diameters. Therefore, the communication stability when used as a communication cable can be ensured. The average bubble diameter can be confirmed by observing the cut surface obtained by cutting the foamed wire 10 in the cross-sectional direction or the length direction using an electron microscope, measuring the bubble diameter, and calculating its average value. Alternatively, the average bubble diameter can be confirmed by nondestructively observing the cut surface obtained by cutting the foamed wire 10 in the cross-sectional direction or the length direction using an X-ray microscope CT, measuring the diameter of the bubble, and calculating its average value.

[0034] The foaming rate of the foamed coating layer 14 is 25% or more and 55% or less. The characteristic impedance of the vehicle-mounted communication cable is preferably in the range of 95Ω to 105Ω. Since the foaming rate of the foamed coating layer 14 is 25% or more and 55% or less, the characteristic impedance of the foamed electric wire 10 can be controlled within the above range. The foaming rate is taken as the area occupied by the bubbles in the cross-sectional area of ​​the foamed coating layer 14. By observing the cut surface of the foamed electric wire 10 and calculating the average value of the foaming rate in the cross-sectional direction and the longitudinal direction, the foaming rate can be confirmed in the same manner as the average bubble diameter.

[0035] The foamed cover layer 14 is preferably formed by foaming extrusion molding using an inert gas. Specifically, during molding, an inert gas such as a foaming agent is injected into the heated molten resin composition to foam. The above-mentioned foaming rate can be controlled by adjusting the injection amount, temperature and inert gas pressure.

[0036] Examples of inert gases for foaming extrusion molding include nitrogen, carbon dioxide, argon, water vapor, helium and isobutane. From the perspective of solubility and environmental considerations for polypropylene resin, inert gas is preferably selected from at least one of the group consisting of nitrogen, carbon dioxide and argon, and more preferably nitrogen or carbon dioxide. Nitrogen or carbon dioxide are used for general foaming extrusion molding of polypropylene resin. One inert gas can be used alone, or two or more can be used in combination.

[0037] In addition to the polypropylene resin, various additives can be added to the resin composition in appropriate amounts to the extent that the effects of the present embodiment are not inhibited. Examples of the additives include flame retardants, inorganic fillers, flame retardant aids, antioxidants, processing aids, crosslinking agents, metal passivators, copper inhibitors, anti-aging agents, fillers, reinforcing agents, ultraviolet absorbers, stabilizers, plasticizers, pigments, dyes, colorants, and antistatic agents.

[0038] (Flame Retardant)

[0039] The flame retardant enhances the flame retardancy of the resin composition. The flame retardant may be, for example, at least one of an organic flame retardant or an inorganic flame retardant. As an organic flame retardant, for example, halogen flame retardants, such as bromine flame retardants and chlorine flame retardants, and phosphorus flame retardants, such as phosphate esters, condensed phosphate esters, cyclic phosphorus compounds, and red phosphorus can be used. As an inorganic flame retardant, at least one metal hydroxide selected from the group consisting of aluminum hydroxide, magnesium hydroxide, and calcium hydroxide can be used. One of the flame retardants may be used alone, or a mixture of multiple flame retardants may be used. 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 effect on mechanical properties.

[0040] (Antioxidant)

[0041] The antioxidant inhibits oxidation of the resin composition. As the antioxidant, known antioxidants for thermoplastic resins and the like can be used, including: free radical chain inhibitors such as phenolic antioxidants, hindered phenolic antioxidants, and amine antioxidants; peroxide decomposers such as phosphorus antioxidants and sulfur antioxidants; and metal deactivators such as hydrazine antioxidants and amine antioxidants. One of the antioxidants may be used alone, or a mixture of multiple antioxidants may be used. The amount of the antioxidant added may be appropriately adjusted in consideration of the antioxidant effect and any defects caused by bleed-out.

[0042] (Copper inhibitor)

[0043] When copper or a copper alloy is used for the conductor 12, copper may cause degradation of the coating layer of the foamed electric wire 10, which is called copper damage. Therefore, a copper inhibitor can be added to the resin constituting the coating layer of the foamed electric wire 10. As the copper inhibitor, for example, a salicylic acid-based copper inhibitor or a hydrazine-based copper inhibitor is used. The amount of the copper inhibitor added can be appropriately adjusted in consideration of the copper damage prevention effect and any defects caused by oozing out.

[0044] As a method for adding the above-mentioned additives to the polypropylene resin to obtain a resin composition, known means can be used. The resin composition can be obtained by, for example, mixing the resin and the additive using a known mixing device such as a Banbury mixer, a mixing mill, a roll mill, a twin-screw extruder or a single-screw extruder.

[0045] [conductor]

[0046] As the conductor 12, a single wire composed of one element wire can be used, or a stranded wire conductor formed by twisting a plurality of element wires can be used. For the stranded wire conductor, it is possible to use all concentric stranded wires in which element wires surrounding one or more element wires are twisted concentrically; aggregated stranded wires in which a plurality of element wires are twisted together in the same direction; and composite stranded wires in which a plurality of aggregated stranded wires are twisted concentrically. There are no particular restrictions on the diameter of the conductor and the diameters of the individual element wires constituting the stranded wire conductor. The conductor 12 may be a compressed conductor or an uncompressed conductor. In addition, there are no particular restrictions on the materials of the conductor and the stranded wire conductor, and it is possible to use, for example, known conductive metal materials such as copper, copper alloys, aluminum, and aluminum alloys. In addition, the surfaces of the conductor and the stranded wire conductor may be plated, and tin plating, silver plating, or nickel plating may be applied thereto.

[0047] The outer diameter of the conductor 12 is not particularly limited, but is preferably 0.45 mm or more. Since the outer diameter of the conductor 12 is 0.45 mm or more, the resistance of the conductor 12 can be reduced. In addition, the outer diameter of the conductor 12 is preferably 0.80 mm or less. Since the outer diameter of the conductor 12 is 0.80 mm or less, the foam wire 10 can be easily arranged in the path even if the path is narrow and short.

[0048] [surface layer]

[0049] Composition Figure 2 and Figure 3 The material and thickness of the surface layer 16 of the outermost portion of the foamed electric wire 10 shown are not particularly limited as long as electrical insulation from the conductor can be ensured. For the surface layer 16, electrical insulating resins can be optionally used, including: olefin resins such as cross-linked polyethylene and polypropylene; and vinyl chloride resins. Specifically, as the resin material constituting the surface layer 16, 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, perfluoroalkoxy resin, natural rubber, chloroprene rubber, butyl rubber, ethylene-propylene rubber, chlorosulfonated polyethylene rubber or silicone rubber can be used. One of the materials can be used alone, or two or more materials can be used in combination.

[0050] like Figure 2 and Figure 3 As shown, the foaming coating 14 can be covered by a skin 16 using known means. Skin 16 can be formed by, for example, general extrusion molding. As the extruder for extrusion molding, for example, a single screw extruder or a twin screw extruder with a screw, a crushing plate, a crosshead, a distributor, a joint and a die can be used. When extrusion molding is carried out, the resin material is put into the extruder set at a temperature where the resin is fully melted. At this point, in addition to the resin material, if desired, the various additives added to the above-mentioned resin combination, such as an antioxidant, can be put into the extruder.

[0051] [Inner layer]

[0052] right Figure 3 The material and thickness of the inner layer 18 covering the conductor 12 shown are not particularly limited as long as electrical insulation with the conductor 12 can be ensured, and a resin material similar to the above-mentioned surface layer 16 can be used. In addition, the conductor 12 can be covered with the inner layer 18 using known means, and the same general extrusion molding method as in the case of forming the above-mentioned surface layer 16 can be used.

[0053] As described above, the foamed wire 10 includes a conductor 12 and a coating layer covering the conductor 12, the coating layer being composed of one or more layers. At least one layer of the coating layer is a foamed coating layer 14 made of a resin composition containing a polypropylene resin, the foamed coating layer being formed by foaming extrusion molding. The average bubble diameter of the foamed coating layer 14 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 coating layer 14 is 25% or more and 55% or less. The arithmetic mean height of the surface of the foamed wire 10 is 20 μm or less. Therefore, even by a physical foaming process using an inert gas, the bubble diameter can be controlled, and a foamed wire having excellent wear resistance and heat deformation resistance can be provided, which is designed for a vehicle-mounted environment while ensuring communication stability.

[0054] Furthermore, as described above, the method for manufacturing the foamed electric wire 10 includes: a step of forming a coating layer that covers the conductor 12 and is composed of one or more layers; and a step of forming a foamed coating layer 14 that is made of a resin composition containing a polypropylene resin as at least one layer of the coating layer, the foamed coating layer 14 being formed by foaming extrusion molding. The average bubble diameter of the foamed coating layer 14 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 coating layer 14 is 25% or more and 55% or less. The arithmetic mean height of the surface of the foamed electric wire 10 is 20 μm or less.

[0055] [Communication Cable]

[0056] The communication cable according to the present embodiment includes the above-mentioned foamed wire 10. The communication cable can be manufactured by a known method, and can be manufactured by, for example, a general extrusion molding method. Specifically, the sheath can be formed by bundling more than one foamed wire 10, and then extruding a sheath material onto the outer surface of the foamed wire 10 and covering the outer surface. The communication cable with the foamed wire 10 can be used as a coaxial cable or a twisted pair cable. In addition, as a resin for the sheath of the communication cable, a known insulating resin can be optionally used, and the known insulating resin includes olefin resins such as cross-linked polyethylene and polypropylene and vinyl chloride, and may include a plasticizer. As a plasticizer, a known plasticizer added to polyvinyl chloride can be used.

[0057] The communication cable according to the present embodiment includes the foamed wire 10. Due to the foamed wire 10, the bubble diameter can be controlled even by a physical foaming process using an inert gas. The foamed wire 10 has excellent wear resistance and heat deformation resistance, and is designed for a vehicle-mounted environment while ensuring communication stability. Therefore, for example, a communication cable including such a foamed wire 10 can be preferably used as a vehicle-mounted transmission cable.

[0058] [Example]

[0059] The present embodiment will be described in more detail below through examples and comparative examples, but the present embodiment is not limited to these examples.

[0060] [Resin composition]

[0061] (resin)

[0062] PP1: Polypropylene: manufactured by Japan Polypropylene Co., Ltd., product name: WAYMAX (registered trademark) EX4000

[0063] PP2: Polypropylene: manufactured by Prime Polymer Co., Ltd., product name: Prime Polypro (registered trademark) E150GK

[0064] PP3: Polypropylene: manufactured by Prime Polymer Co., Ltd., product name: Prime Polypro (registered trademark) J715M

[0065] PP4: Polypropylene: manufactured by Prime Polymer Co., Ltd., product name: Prime Polypro (registered trademark) J-452HP

[0066] PP5: Polypropylene: manufactured by SunAllomer Ltd., product name: Qualear (registered trademark) CM688A

[0067] EP1: Soft polypropylene (propylene / ethylene copolymer resin): manufactured by SunAllomer Ltd., product name: Adflex (registered trademark) Q200F

[0068] PE1: Low-density polyethylene (LDPE): manufactured by DOW-MITSUIPOLYCHEMICALSCO.,LTD., product name: MIRASON (registered trademark) 3530

[0069] (Antioxidant)

[0070] Phenol antioxidant: manufactured by ADEKA CORPORATION, product name: ADK-STAB (registered trademark) AO-60

[0071] (Copper inhibitor)

[0072] Hydrazine copper inhibitor: manufactured by BASF SE, product name: IRGANOX (registered trademark) MD1024

[0073] [Evaluation of resin composition]

[0074] The resin compositions of Examples 1 to 10 and Comparative Examples 1 to 7 were prepared by melt-kneading the above resins, antioxidants, and copper inhibitors in the blending amounts shown in Tables 1 and 2. Then, as material properties of each resin composition, melt tension and melt viscosity were measured by the following method. Tables 1 and 2 show the evaluation results.

[0075] (Melt tension)

[0076] Melt tension was measured using CAPILOGRAPH (registered trademark) (manufactured by Toyo Seiki Seisaku-sho, Ltd.). Specifically, the capillary L / D was set to 10, and the furnace temperature was set to 200°C. After the material was added, the material was kept for 3 minutes and dissolved. The resin was extruded at a piston speed of 20 mm / min, and the resin was extracted by an extraction roller (manufactured by Toyo Seiki Seisaku-sho, Ltd.) at a speed increment of 10 mm / min, while the extraction speed increased in the range of 10 mm / min to 200 mm / min. The resin was kept at each extraction speed for 30 seconds, and if the resin was not cut off, the extraction speed was changed to the next extraction speed. The tension when the resin was cut off was set as the melt tension.

[0077] (Melt viscosity)

[0078] The melt viscosity was measured using CAPILOGRAPH (registered trademark) (manufactured by Toyo Seiki Seisaku-sho, Ltd.). As measurement conditions, the capillary L / D was set to 10, and the furnace temperature was set to 200° C. The value measured at a piston speed of 100 mm / min when the piston speed was changed to values ​​of 300 mm / min, 200 mm / min, 100 mm / min, 50 mm / min, and 10 mm / min was set as the melt viscosity.

[0079] [Table 1]

[0080]

[0081] [Table 2]

[0082]

[0083] [Evaluation of Foamed Wire]

[0084] Using the resin compositions of Examples 1 to 10 and Comparative Examples 1 to 9, the following method was used to prepare Figure 3 The foam wire (triple-layer insulated wire) shown.

[0085] In the method for manufacturing a foamed electric wire, a polypropylene resin is first extruded (at an extruder temperature of 170°C to 220°C) onto a compressed or uncompressed stranded wire conductor (outer diameter of 0.45mm to 0.80mm), and the conductor is covered with an inner layer having a thickness of 0.03mm to 0.1mm. Using an electric wire, while the outer diameter of the electric wire is in the range of 1.00mm to 2.20mm, the resin composition of Examples 1 to 10 and Comparative Examples 1 to 9 is covered by foaming extrusion molding to form a foamed coating layer. The temperature of the extruder used for foaming extrusion molding is set in the range of 170°C to 220°C, and high-pressure nitrogen obtained by pressurization of a gas supercharger is injected from the middle part of the extruder and mixed. At this time, as shown in Tables 3 and 4, the manufacture of the electric wire is carried out under the conditions that the foaming rate of the foamed coating layer is a low foaming rate (25% or 20%) and a high foaming rate (55% or 60%). In addition, while the covering layer is foamed, a polypropylene resin is extruded by a sub-extruder (set temperature is 210° C. to 240° C.), and the foamed covering layer is covered with a surface layer having a thickness of 0.03 mm to 0.1 mm. The foaming rate, average cell diameter, arithmetic mean height, wear resistance, heat deformation, LCTL and characteristic impedance are evaluated by the following methods as the characteristics of the foamed electric wire obtained in this way. Tables 3 and 4 show the evaluation results.

[0086] (Foaming rate and average bubble diameter)

[0087] X-ray microscope CT (nano3DX) was used to capture CT images (in cross section and length direction) of the foamed wire. The image capture conditions were as follows: spatial resolution 4.31 μm / voxel, X-ray camera lens L1080, pixel combination (binning) 3, exposure time 4 seconds, X-ray source Cu (40 kV, 30 mA), field of view 3.626 mm*2.719 mm, and the number of captured images 400. The foaming rate and average bubble diameter were calculated as the average value of the captured images using the analysis software ImageJ.

[0088] (Arithmetic mean height)

[0089] The image of the surface of the foamed wire was captured using a 3D optical profiler (manufactured by KEYENCE CORPORATION), and the arithmetic mean height Sa of the surface of the foamed wire was calculated using analysis software. The image capturing conditions were as follows: magnification 25, measurement range 10*10 mm, and number of captured images 5.

[0090] (Abrasion resistance)

[0091] The wear resistance test was conducted according to the scratch wear test specified in ISO 19642-12. Specifically, a needle was placed perpendicular to the test sample of the cut foamed electric wire, the coating layer was worn by reciprocating the needle with a fixed load (4N), and the number of reciprocations of the needle until it came into contact with the conductor was measured. When the number of reciprocations of the needle until it came into contact with the conductor was 300 times or more, it was evaluated as excellent (symbol ◎), when the number of reciprocations was 100 times or more and less than 300 times, it was evaluated as qualified (symbol ○), and when the number of reciprocations was less than 100 times, it was evaluated as unqualified (symbol x).

[0092] (Thermal deformation)

[0093] The heat deformation test is carried out according to the high temperature pressure test specified in ISO 19642-12. Specifically, at a set temperature of 100°C (Class B), a fixed load is applied from above to the test sample of the cut foamed wire for four hours, and then a voltage of 1 kV is applied to the conductor of the test sample using a voltage withstand device for a voltage withstand test. The applied load varies according to the thickness of the coating and the outer diameter of the wire, and can be determined based on the following formula (1). When the insulation is maintained for one minute, it is evaluated as qualified (symbol ○), and when the insulation is maintained for less than one minute, it is evaluated as unqualified (symbol x).

[0094] Load (N) = 0.8*(coating thickness*(2*wire outer diameter-coating thickness)) 0.5 (1)

[0095] (LCTL)

[0096] The communication stability when used as a communication cable is evaluated by the longitudinal conversion transmission loss (LCTL). Specifically, two foamed wires are twisted at a pitch of 30 mm, and a metal film is vertically arranged or horizontally wrapped around them. Then a tinned soft copper wire braid is covered, and a vinyl chloride resin (PVC) having a thickness of 0.5 mm is covered thereon as a sheath to manufacture a test sample. For the obtained test sample, the same voltage is applied to the two foamed wires using a network analyzer, and the LCTL is calculated by converting the potential difference ratio caused by the imbalance of the foamed wires into decibels. When the LCTL is below 18 dB / m, it is evaluated as excellent (symbol ◎), when the LCTL is below 20 dB / m, it is evaluated as qualified (symbol ○), and when the LCTL is greater than 20 dB / m, it is evaluated as unqualified (symbol x).

[0097] (Characteristic impedance)

[0098] The communication stability when used as a communication cable was evaluated by the characteristic impedance. Specifically, the characteristic impedance of the test sample of the cut foamed electric wire was measured using a vector network analyzer (VNA) (E5071C, manufactured by Keysight Technologies). When the characteristic impedance was within the range of 95 to 105Ω, it was evaluated as acceptable (symbol ○), and when the characteristic impedance was less than 95Ω or greater than 105Ω, it was evaluated as unacceptable (symbol x).

[0099] [Table 3]

[0100]

[0101] [Table 4]

[0102]

[0103] As shown in Table 1, the resin compositions of Examples 1 to 10 had a melt tension at 200° C. 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, measured using a capillary rheometer.

[0104] As shown in Table 3, in both cases where the foaming rate was 25% and the foaming rate was 55%, the average cell diameter of the foamed wires produced using the resin compositions of Examples 1 to 10 was 30 μm or less in the cross-sectional direction of the foamed wire and 60 μm or less in the longitudinal direction of the foamed wire. In addition, the arithmetic mean height of the foamed wire was 20 μm or less, and the results of wear resistance, heat deformation, LCTL, and characteristic impedance were good.

[0105] Meanwhile, as shown in Table 2, the melt tension of the resin compositions of Comparative Examples 1, 2, 4, 8 and 9 measured using a capillary rheometer at 200°C is less than 15 mN or greater than 45 mN. In addition, the melt viscosity of the resin compositions of Comparative Examples 1, 3, 7 and 9 measured using a capillary rheometer at 200°C is less than 120 Pa·s or greater than 200 Pa·s.

[0106] As shown in Table 4, in both cases where the foaming rate is 25% and 55%, the average bubble diameter of the foamed wires manufactured using the resin compositions of Comparative Examples 1, 2 and 4 is greater than 30 μm in the cross-sectional direction. At the same time, in both cases where the foaming rate is 25% and 55%, the average bubble diameter of the foamed wires manufactured using the resin compositions of Comparative Examples 8 and 9 is greater than 60 μm in the length direction. In addition, in both cases where the foaming rate is 25% and 55%, the arithmetic mean height of the foamed wires manufactured using the resin compositions of Comparative Examples 1, 2 and 4 is greater than 20 μm. The resin compositions of Comparative Examples 1, 2, 4 and 7 to 9 are unqualified in any of the wear resistance, thermal deformation and LCTL results. In addition, Table 2 reveals that the melt tension of the resin compositions of Comparative Examples 5 and 6 is 15 mN or more and 45 mN or less, and the melt viscosity of the composition is 120 Pa·s or more and 200 Pa·s or less. Table 4 reveals that the foaming rates of the resin compositions of Comparative Examples 5 and 6 were 20% or 60%, and thus the characteristic impedance of the compositions was unqualified. The resin composition of Comparative Example 3 could not be foamed, and thus a foamed electric wire could not be produced.

[0107] Although the present embodiment has been described above, the present embodiment is not limited to the above description, and various modifications can be made within the scope of the gist of the present embodiment.

[0108] The entire contents of Japanese patent application No. 2022-161080 (filed on October 5, 2022) are incorporated herein.

Claims

1. A foamed wire, comprising: conductor; as well as a coating layer covering the conductor and consisting of one or more layers, wherein: At least one of the covering layers is a foamed covering layer, the foamed covering layer is made of a resin composition containing a polypropylene resin, and the foamed covering layer is formed by foaming extrusion molding. The average cell diameter of the foamed covering 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 coating layer is 25% or more and 55% or less, and The arithmetic mean height of the surface is 20 μm or less.

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

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

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

5. A method for manufacturing a foamed electric wire, comprising: a step of forming a coating layer covering the conductor and consisting of one or more layers; as well as A step of forming a foamed covering layer, the foamed covering layer being made of a resin composition containing a polypropylene resin as at least one layer of the covering layer, the foamed covering layer being formed by foaming extrusion molding, wherein: The average cell diameter of the foamed covering 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 coating layer is 25% or more and 55% or less, and The arithmetic mean height of the surface of the foamed electric wire is 20 μm or less.

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