Resin composition, insulated wire using resin composition, communication cable, method for producing insulated wire, and use of resin composition

Abstract

The present invention provides: a resin composition with which it is possible to easily produce an insulated wire that has high communication characteristics; an insulated wire; a communication cable; a method for producing an insulated wire; and use of a resin composition. An insulated wire 2 comprises: a twisted wire conductor 21; and an insulator 22 that covers the outer periphery of the twisted wire conductor 21 so as not to form a gap between itself and the outer periphery. The insulator 22 is formed of a resin composition which contains, as a base polymer, a first resin composition that has a first crystallinity and a second resin composition that has a second crystallinity lower than the first crystallinity at a specific ratio, wherein: the first crystallinity is more than 40% and the second crystallinity is 40% or less; the first resin composition contains at least one resin that is selected from the group consisting of a polyethylene-based resin and a polypropylene-based resin; the second resin composition is a polyolefin-based resin; and the content of the second resin composition is 5 mass% to 50 mass% inclusive when the total of the first resin composition and the second resin composition is taken as 100 mass%. The resin composition may be used as an intermediate sheath of a communication cable.

Description

Resin composition, insulated wire using the resin composition, communication cable, method for producing insulated wire, and use of the resin composition

[0001] The present invention relates to a resin composition, an insulated wire using the resin composition, a communication cable, a method for producing an insulated wire, and use of the resin composition.

[0002] In recent years, small-diameter multi-core cables have been proposed for use as in-vehicle communication cables mounted on automobiles and other vehicles, and as communication cables for use within or between various devices. These cables reduce the gap between the central conductor and the insulator, thereby reducing the skew in signal transmission between pairs of cables (see, for example, Patent Document 1).

[0003] The multi-core cable described in Patent Document 1 is configured by twisting two coaxial wires, each having a center conductor covered with an insulator, together, or by assembling a plurality of pairs of coaxial wires, each having two parallel-arranged coaxial wires, with the center conductor of each coaxial wire being a stranded wire formed by twisting 19 or more strands. With this configuration, by having 19 or more strands in the center conductor, the surface irregularities of the stranded wire are reduced compared to the usual case of seven strands, reducing gaps around the stranded wires and also reducing gaps inside the stranded wires. As a result, the variation in the dielectric constant of the coaxial wires along their longitudinal direction is reduced, making it possible to keep the propagation delay time difference (skew) of transmission signals between the two coaxial wires low.

[0004] JP 2015-72806 A

[0005] In recent years, polypropylene-based resins, which have a lower dielectric constant and a higher melting point than polyethylene-based resins and therefore do not have a cross-linked structure, have come to be used as the insulating material for covering the conductors of insulated wires that make up small-diameter multi-core cables for communication. However, insulated wires that use polypropylene-based resins as the insulating material have experienced high skew and reduced overall communication characteristics, even when the number of strands that make up the conductor is increased to reduce the surface irregularities of the stranded wires.

[0006] An object of the present invention is to provide a resin composition that can easily produce an insulated wire having high communication properties, an insulated wire, a communication cable, and a method for producing an insulated wire using the resin composition. Another object of the present invention is to provide a resin composition that has shape stability and flexibility and is suitable for producing a communication cable having high communication properties, and a method for producing an insulated wire, a communication cable, and an insulated wire using the resin composition.

[0007] [1] A resin composition comprising, as a base polymer, a first resin composition having a first crystallinity and a second resin composition having a second crystallinity lower than the first crystallinity, in predetermined proportions, wherein the first crystallinity is greater than 40% and the second crystallinity is 40% or less, the first resin composition comprises at least one resin selected from the group consisting of polyethylene-based resins and polypropylene-based resins, and the second resin composition is a polyolefin-based resin, and the content of the second resin composition is 5% by mass or more and 50% by mass or less, when the total of the first resin composition and the second resin composition is 100% by mass. [2] The resin composition according to [1], wherein the content of the second resin composition is 5% by mass or more and 25% by mass or less, when the total of the first resin composition and the second resin composition is 100% by mass. [3] The resin composition according to [1], wherein the content of the second resin composition is 25% by mass or more and 50% by mass or less, when the total of the first resin composition and the second resin composition is 100% by mass. [4] An insulated electric wire comprising a stranded conductor formed by twisting together a plurality of strands of wires and an insulator tightly covering the outer periphery of the stranded conductor, the insulator being made of the resin composition according to [1]. [5] The insulated electric wire according to [4], wherein the polypropylene-based resin constituting the first resin composition is a block copolymer polypropylene. [6] The insulated electric wire according to [5], wherein the polyolefin-based resin constituting the second resin composition is a low-crystalline polyolefin resin. [7] The insulated electric wire according to [6], wherein the standard deviation of the capacitance of the insulated electric wire for n = 10 is 10 pF / m or less and the difference between the maximum and minimum for n = 10 is 33 pF / m or less. [8] The insulated electric wire according to [7], wherein the stranded wire is a round wire. [9] A communication cable comprising a pair of insulated wires according to any one of [4] to [8].

[10] A coaxial communication cable comprising a coaxial wire whose central conductor is covered with an insulator made of the resin composition according to [2].

[11] A double-core insulated cable having two conductors covered with an insulator made of the resin composition according to [3].

[12] A communication cable comprising an intermediate sheath made of the resin composition according to [3].

[13] A method for producing an insulated wire according to [4], comprising: preheating the stranded conductors at 130°C or less; and coating the outer periphery of the preheated stranded conductors with the resin composition by extrusion molding to form the insulator having a thickness of 0.5 mm or less.

[14] A method for producing a coaxial communication cable according to

[10] , comprising: preheating the central conductor at 150°C or less; and coating the outer periphery of the preheated central conductor with the resin composition by extrusion molding to form the insulator having a thickness of 0.6 mm or less.

[0008] According to the present invention, an insulated wire having high communication characteristics can be easily manufactured, and a communication cable capable of communicating differential signals with little skew can be provided.

[0009] FIG. 1 is a cross-sectional view showing an example of a resin composition according to a first embodiment of the present invention and an insulated wire using the same. FIG. 2 is a flowchart showing a method for manufacturing an insulated wire using the resin composition according to the first embodiment of the present invention. FIG. 3 is a cross-sectional view showing an example of a communication cable according to a second embodiment of the present invention. FIG. 4 is a cross-sectional view showing an example of a coaxial communication cable according to a third embodiment of the present invention. FIG. 5 is a flowchart showing a method for manufacturing a coaxial wire used in the coaxial communication cable according to the third embodiment of the present invention. FIG. 6 is a cross-sectional view of a dual-core insulated cable according to a fourth embodiment of the present invention. FIG. 7 is a cross-sectional view of a communication cable for differential signal transmission according to a fifth embodiment of the present invention. FIG. 8A is a cross-sectional photograph of the insulated wire of Comparative Example 7, in which the insulation was not sufficiently filled and solidified due to a low conductor preheating temperature. FIG. 8B is a cross-sectional photograph of the insulated wire of Comparative Example 8, in which sink marks occurred in the insulation due to a high conductor preheating temperature. FIG. 9 is a cross-sectional photograph of the insulated wire of Example 2, in which no sink marks occurred.

[0010] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, components having substantially the same functions are designated by the same reference numerals, and redundant description thereof will be omitted.

[0011] 1 is a cross-sectional view showing an example of a resin composition and an insulated wire using the resin composition according to a first embodiment of the present invention. The insulated wire 2 includes a stranded conductor 21 formed by twisting together a plurality of strands 21 a, and an insulator 22 covering the outer periphery of the stranded conductor 21.

[0012] As described above, insulated wires used in communication cables for vehicles and those used for intra-device and inter-device communication are typically made of polypropylene-based resins, which have a lower dielectric constant and a higher melting point than polyethylene-based resins and therefore do not have a cross-linked structure. However, in actual multi-core cable products, even when the number of strands constituting the stranded conductor is increased (e.g., by increasing the number to 19 or more without changing the conductor diameter) to reduce the surface irregularities of the stranded conductor, high skew and reduced overall communication characteristics have occurred. After extensive research, the inventors discovered that when polypropylene-based resins are used as the insulator material for the insulated wire, multiple voids (cavities) are likely to form between the stranded conductor and the insulator, even when the surface irregularities of the stranded wire are reduced.

[0013] Furthermore, the present inventors have noted that, depending on the type of polypropylene-based resin, the volume change of the resin composition when it melts and solidifies can be significant, leading to the formation of voids (cavities) on the conductor surface or in the insulating layer, known as "sink marks." In particular, homopolymer polypropylene and block copolymer polypropylene have high melting points and high crystallinity (high thermal shrinkage). Therefore, even if the temperature of the conductor is raised to fill the conductor with an insulating material made of a resin composition, sink marks are likely to occur around the conductor due to the temperature difference between the outside and inside when the resin composition cools and solidifies. If a portion of the insulator does not adhere to the conductor's outer periphery, multiple cavities are formed around the conductor, and the distribution of these cavities varies, resulting in changes in the capacitance of the insulated wire along its length, which in turn causes skew. However, highly precise control of manufacturing conditions is required to prevent sink marks. One aspect of the present invention is to provide a resin composition suitable for producing insulated wires and communication cables with excellent communication properties, without requiring the need for sophisticated control of manufacturing conditions for insulated wires.

[0014] The present inventors also noted that when a conventional resin composition is used for the insulator of a double-core insulated electric wire, the area between the two conductors constituting the two cores (i.e., the center of the double-core insulated electric wire) is the least likely to cool and is therefore prone to shrinkage and the formation of voids (cavities). Here, when the resin composition of the present invention is used for the insulator of a double-core insulated electric wire, by increasing the proportion of the second resin composition compared to the resin composition used for the insulator of a coaxial cable, the flexibility of the insulator is increased, and the formation of voids (cavities) can be suppressed and the shape of the insulator can be maintained even when the conductor preheating is low.

[0015] Furthermore, according to the resin composition of the present invention, since the first resin composition is a polypropylene-based resin, crosslinking is not required, and flexibility can be achieved by increasing the proportion of the second resin composition. Therefore, focusing on shape stability and flexibility, the resin composition can be used not only as an insulator for insulated wires and communication cables, but also for applications other than as an insulator for communication cables, such as a material for intermediate sheaths. Another object of the present invention is to provide a resin composition that has shape stability and flexibility and is suitable for producing insulated wires and communication cables with excellent communication properties.

[0016] Here, the propagation delay time Td of the insulated wire, which is a factor in generating the skew, can be calculated from the following equations (1) and (2): Td=ε ０．５ × 0.33 (1) ε = C × log (D / d) / 24.1 (2) (ε: relative permittivity, C: capacitance, D: coating diameter, d: conductor diameter)

[0017] That is, the propagation delay time Td is determined by the relative dielectric constant ε of the insulated wire, and the relative dielectric constant ε can be calculated from the conductor diameter d of the insulated wire, the coating diameter (insulator diameter) D of the insulated wire, and the capacitance C of the insulated wire. Therefore, since changes in capacitance have an adverse effect on communication characteristics, in this embodiment, the material and manufacturing conditions of the insulator 22 are selected so as to prevent the occurrence of a gap between the outer periphery of the stranded conductor 21 and the insulator 22. Details of the material and manufacturing conditions of the insulator 22 will be described later.

[0018] (Stranded Conductor Configuration) The stranded conductor 21 has an outer diameter equivalent to, for example, 20 to 30 AWG (American Wire Gauge). This results in a small-diameter communication cable suitable for high-speed digital signal transmission. One example of the stranded conductor 21 is one in which seven 0.16 mm diameter round wires 21a are twisted together, resulting in an outer diameter equivalent to 26 AWG (approximately 0.48 mm). The number of wires 21a constituting the stranded conductor 21 is not limited to seven, and may be six or less, or eight or more. Furthermore, a solid wire may be used instead of the stranded conductor 21.

[0019] The wire 21 a may be, for example, a round wire having a circular cross section. Alternatively, the wire 21 a may be a rectangular wire having a square cross section or a rectangular wire having a rectangular cross section. The wire 21 a may be made of, for example, a tin-plated mild steel wire or a tin-plated copper alloy wire.

[0020] (Configuration of the Insulator) The insulator 22 is made of a resin composition containing a first resin composition having a first crystallinity and a second resin composition having a second crystallinity lower than the first crystallinity, in a predetermined ratio, as a base polymer. That is, by mixing the first resin composition having a high crystallinity with the second resin composition having a low crystallinity to form a base polymer for the insulator 22, the crystallinity of the entire resin composition is reduced. By extruding this resin composition to coat the stranded conductor 21, the occurrence of sink marks due to cooling shrinkage of the insulator 22 can be reduced. This stabilizes the shape of the insulator 22 after cooling, ensures that the insulator 22 is tightly attached to the outer periphery of the stranded conductor 21 throughout its entire length, and minimizes voids between the stranded conductor 21 and the insulator 22. Even if voids exist, the distribution of the voids is minimal. As a result, an insulated electric wire 2 is obtained with minimal change in capacitance along the length. The content of the base polymer in the resin composition does not include fillers or flame retardants that change the dielectric constant of the insulator 22, and the content of antioxidants and heavy metal deactivators is kept to the minimum necessary to obtain heat resistance, preferably 95% by mass or more, and even more preferably 98% by mass or more. In this specification, "close contact" and "almost no voids" refer to a state in which the void ratio is less than about 5%.

[0021] The first crystallinity of the first resin composition is preferably greater than 40%, and the second crystallinity of the second resin composition is preferably 40% or less. Furthermore, the content of the second resin composition is preferably 5% by mass or more and 50% by mass or less, more preferably greater than 10% by mass and 40% by mass or less, when the total of the first resin composition and the second resin composition is 100% by mass. This allows the crystallinity of the entire resin composition to be reduced by mixing the first resin composition with a second resin composition having a lower crystallinity to form a base polymer. The first and second crystallinity degrees in the present invention are evaluated by differential scanning calorimetry (DSC) (for example) in accordance with ASTM D3418, with a heating and cooling rate of 10°C / min or less and in a nitrogen atmosphere.

[0022] The first resin composition includes at least one resin selected from the group consisting of polyethylene-based resins and polypropylene-based resins. Examples of polypropylene-based resins that can be used include homopolymer polypropylene, random copolymer polypropylene, and block copolymer polypropylene (e.g., ethylene-propylene copolymer). Among these, block copolymer polypropylene, which has a high melting point and good impact properties, may be used.

[0023] The second resin composition may be, for example, a polyolefin-based resin. Examples of polyolefin-based resins that can be used include homopolymer polypropylene, propylene-α-olefin copolymer elastomer, ethylene / octene copolymer, and but-1-ene-ethene polymer. Note that a propylene-based resin may also be used in consideration of compatibility with the polypropylene-based resin of the first resin composition.

[0024] The thickness of the insulator 22 may be determined by the content of the second resin composition. When the insulator 22 is thin, the temperature difference between the outside and inside when solidifying by cooling is small, so the content of the second resin composition can be reduced. Conversely, when the insulator 22 is thick, the content of the second resin composition is increased. When the total of the first resin composition and the second resin composition is 100% by mass, and the content of the second resin composition is in the range of 5% by mass or more and 50% by mass or less, or more than 10% by mass and 40% by mass or less, the thickness of the insulator 22 is preferably 0.2 mm or more and 0.6 mm or less, and more preferably 0.3 mm or more and 0.5 mm or less, when the characteristic impedance is 100±10 Ω.

[0025] The polyolefin resin composition used as the material for the insulator 22 may contain additives such as stabilizers (e.g., antioxidants, heavy metal deactivators, etc.) to improve heat resistance. Examples of antioxidants include 2,2'-dimethyl-2,2'-(2,4,8,10-tetraoxaspiro[5.5]undecane-3,9-diyl)dipropane-1,1'-diyl bis[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propanoate] (ADEKA: Adekastab AO-80). Examples of heavy metal deactivators include N'1,N'12-bis(2-hydroxybenzoyl)dodecane dihydrazide (ADEKA: Adekastab CDA-6S).

[0026] (Method for Manufacturing Insulated Wire) Next, an example of a method for manufacturing an insulated wire 2 will be described. FIG. 2 is a flowchart showing a method for manufacturing an insulated wire using the resin composition according to the first embodiment of the present invention. Specifically, first, a plurality of wires 21a (e.g., seven in this embodiment) are twisted together to form a stranded conductor 21. Next, the stranded conductor 21 is preheated to a predetermined temperature (e.g., 100°C or higher and 130°C or lower), and a polyolefin resin composition constituting the insulator 22 is extruded at a predetermined linear speed (e.g., 200 m / min) to coat the outer periphery of the stranded conductor 21 with the insulator 22. Generally, when using a polypropylene resin with a melting point of 160°C, the conductor preheating temperature must be set high, around 140°C, or higher, to ensure that the insulator 22 does not fill around the stranded conductor 21. On the other hand, a high conductor preheating temperature of 140°C can cause sink marks. Even if it were possible to address this issue by adjusting the extrusion temperature and cooling conditions, other problems would arise, such as a decrease in tensile elongation due to distortion during cooling and a flattened (oval) shape, and advanced control has been required to obtain a stable insulated wire 2. However, by using a polyolefin-based resin as the second resin composition, it is possible to provide flexibility in the conditions for conductor preheating, extrusion temperature, and cooling, so the advanced control required in the past is not necessary.

[0027] (Advantages of the Present Embodiment) The resin composition according to the present embodiment provides the following advantages. (a) By using a resin composition containing a base polymer obtained by mixing a polypropylene-based resin (e.g., block copolymer polypropylene) as an example of a first resin composition having a high degree of crystallinity with a polyolefin-based resin (e.g., a low-crystalline polyolefin resin or a low-crystalline olefin elastomer) as an example of a second resin composition having a low degree of crystallinity as the material for the insulator 22 covering the stranded conductor 21, the crystallinity of the entire resin composition can be reduced. As a result, cooling shrinkage of the insulator 22 when the resin composition is extruded to cover the stranded conductor 21 can be suppressed. This stabilizes the shape of the insulator 22, ensures that the insulator 22 adheres closely to the outer periphery of the stranded conductor 21 throughout its entire length, and virtually eliminates voids between the stranded conductor 21 and the insulator 22. As a result, an insulated electric wire 2 can be obtained that exhibits low longitudinal capacitance change and excellent communication characteristics. (b) Not only when the number of strands constituting the stranded conductor is large (e.g., 19 or more) and the surface of the stranded conductor is small in unevenness, but also when the number of strands is small (e.g., 7 or less) and the surface of the stranded conductor is large in unevenness, it is possible to prevent the formation of voids between the stranded conductor 21 and the insulator 22. (c) The resin composition according to this embodiment makes it extremely easy to control the manufacturing conditions for manufacturing the insulated wire 2.

[0028] Second Embodiment Fig. 3 is a cross-sectional view showing an example of a communication cable according to a second embodiment of the present invention. This communication cable 1 uses the insulated wire 2 shown in Fig. 1 as signal wires 2a to 2d. The following describes this embodiment, focusing on differences from the first embodiment. Here, the signal wires 2a to 2d are an example of insulated wires.

[0029] The communication cable 1 includes a pair of signal wires 2a, 2b for transmitting differential signals, a pair of signal wires 2c, 2d for receiving differential signals, a shielding layer 3 that twists these signal wires 2a to 2d together to cover the entire cable, and a sheath 4 that covers the outer periphery of the shielding layer 3. Note that the pair of signal wires 2a, 2b for transmitting differential signals may be twisted together, and the pair of signal wires 2c, 2d for receiving differential signals may be twisted together. Alternatively, the signal wires 2a to 2d may be arranged in parallel without being twisted together.

[0030] The shield layer 3 includes an inner shield layer 3a provided on the inside and formed by winding a resin tape (e.g., polyester tape), an outer shield layer 3b provided on the outside of the inner shield layer 3a and formed by winding a conductive tape (e.g., a tape laminated with aluminum and polyester), and a metal shield layer 3c provided on the outside of the outer shield layer 3b and formed from a metal braid (e.g., a tin-plated annealed copper wire braid). Note that the configuration of the shield layer 3 is not limited to the above.

[0031] The sheath 4 is made of, for example, a polyolefin such as polyethylene or polypropylene. However, the resin constituting the sheath 4 is not limited to polyolefin.

[0032] (Effects of this embodiment) According to the communication cable 1 of this embodiment, there is almost no gap between the stranded conductor 21 and the insulator 22, and by using the signal lines 2a to 2d with little change in capacitance in the longitudinal direction, differential signals with little skew can be communicated.

[0033] Although the first and second embodiments of the present invention have been described above, the embodiments of the present invention are not limited to the first and second embodiments and various modifications and implementations are possible. For example, a twin-ax cable may be formed by a pair of insulated electric wires 2 that transmit differential signals, or a coaxial cable may be formed by a pair of coaxial wires that transmit differential signals. [Third Embodiment]

[0034] 4 is a cross-sectional view showing an example of a coaxial communication cable 30 according to a third embodiment of the present invention. The coaxial communication cable 30 includes a center conductor 31, an insulator 32 covering the outer periphery of the center conductor 31, a shield layer 33 covering the outer periphery of the insulator 32, and a sheath 34 provided on the outside of the shield layer 33. Here, a coaxial line consisting of the center conductor 31, the insulator 32 covering the outer periphery of the center conductor 31, and the shield layer 33 (i.e., the outer conductor) is referred to as a coaxial line 300.

[0035] The central conductor 31 is a stranded wire formed by twisting together a plurality of (e.g., seven) strands 31a made of copper or a copper alloy. The strands 31a may be, for example, annealed copper wires or tin-plated annealed copper wires. Alternatively, a solid wire may be used instead of the central conductor 31.

[0036] The insulator 32 is composed of a first resin composition having a first crystallinity and a second resin composition having a second crystallinity lower than the first crystallinity, and the content of the second resin composition is preferably 5% by mass or more and 50% by mass or less, and more preferably 5% by mass or more and 25% by mass or less, when the total of the first resin composition and the second resin composition is 100% by mass.

[0037] As an example, the thickness t of the insulator 32 is 0.6 mm or less, preferably 0.52 to 0.55 mm, and the outer diameter of the insulator is 1.52 to 1.65 mm.

[0038] The insulator 32 is formed in close contact with the outer periphery of the central conductor 31 with almost no voids remaining, and the void ratio is less than 5% in terms of cross-sectional area ratio.

[0039] 4, the shield layer 33 may have a two-layer structure including an inner shield layer 33a formed by winding a conductive tape (e.g., a tape laminated with aluminum and polyester) and an outer shield layer 33b formed from a metal braid (e.g., a tin-plated annealed copper wire braid) around the outer periphery of the inner shield layer 33a. Note that the configuration of the shield layer 33 is not limited to the above.

[0040] The sheath 34 is made of, for example, a polyolefin such as polyethylene or polypropylene. However, the resin that forms the sheath 34 is not limited to polyolefin.

[0041] (Method of Manufacturing Coaxial Cable) FIG. 5 is a flowchart showing a method of manufacturing a coaxial cable 300 used in a coaxial communication cable 30 according to a third embodiment of the present invention. Specifically, first, a plurality of wires 31a (e.g., seven in this embodiment) are twisted together to form a central conductor 31. Next, the central conductor 31 is preheated to 150°C or less, preferably 135 to 145°C. The resin composition is extruded around the preheated central conductor 31 to form an insulator 32 having a thickness of 0.6 mm or less. The extrusion temperature is preferably 210 to 230°C. The extruded insulator 32 is cooled in a cooling water tank to solidify the insulator 32, and a shielding layer 33 is then formed around the outer periphery thereof, completing the coaxial cable 300.

[0042] (Effects of this embodiment) According to this embodiment, it is possible to suppress the generation of gaps between the central conductor 31 and the insulator 32, reduce the variation in capacitance in the longitudinal direction, and obtain a coaxial communication cable 30 suitable for high-frequency communication, such as for use in an in-vehicle camera.

[0043] [Fourth embodiment] The resin compositions of the insulator 22 of the insulated wire 2 according to the first embodiment of the present invention, the insulator 22 of the communication cable 1 according to the second embodiment, and the insulator 32 of the coaxial communication cable 30 according to the third embodiment can also be used as insulators for double-core insulated wires. Figure 6 is a cross-sectional view of a double-core insulated cable 40 according to the fourth embodiment of the present invention. The double-core insulated cable 40 includes a pair of conductors 41, 41 made of twisted or solid wires, an insulator 42 that covers the outer peripheries of the pair of conductors 41 and fills the gap between the conductors 41, 41, a shielding layer 43 that covers the outer periphery of the insulator 42, and a sheath 44 provided on the outside of the shielding layer 43. Here, a specially shaped insulated wire having an oval cross section and including the pair of conductors 41, 41 and the insulator 42 that covers the outer peripheries of the pair of conductors 41 and fills the gap between the conductors 41, 41 is referred to as an insulated wire 400.

[0044] Here, the shield layer 43 may have a two-layer structure including an inner shield layer 43a formed by winding a conductive tape (e.g., a tape laminated with aluminum and polyester) and an outer shield layer 43b formed from a metal braid (e.g., a tin-plated annealed copper wire braid) around the outer periphery of the inner shield layer 43a. Note that the configuration of the shield layer 43 is not limited to the above.

[0045] The sheath 44 is made of, for example, a polyolefin such as polyethylene or polypropylene. However, the resin that makes up the sheath 44 is not limited to polyolefin. The configurations of the other members are the same as those in the third embodiment, so detailed description thereof will be omitted.

[0046] The insulator 42 is composed of a first resin composition having a first crystallinity and a second resin composition having a second crystallinity lower than the first crystallinity. The content of the second resin composition can be 25% by mass or more and 50% by mass or less, where the total of the first resin composition and the second resin composition is 100% by mass. In this case, the resin composition that penetrates between the two conductors 41, 41 constituting the two cores (i.e., the center of the insulated electric wire 400) is least likely to cool and therefore is more likely to shrink, forming a void (cavity). To prevent the formation of a void (cavity) between the two conductors 41, 41, the resin composition used for the insulator 42 of the insulated electric wire 400 contains a higher proportion of the second resin composition than the resin composition used for the insulator 32 of the coaxial cable 300, thereby increasing the flexibility of the insulator 42.

[0047] (Effects of this embodiment) According to this embodiment, even if the conductor preheating during manufacturing is low, it is possible to obtain a two-core insulated cable 40 that suppresses the formation of voids (cavities) between the two conductors 41, 41 that make up the two cores and maintains the shape stability of the insulator 42.

[0048] Fifth Embodiment The resin composition of the present invention can be applied to components other than cable insulators. FIG. 7 is a cross-sectional view showing a communication cable 50 for differential signal transmission according to a fifth embodiment of the present invention. The communication cable 50 for differential signal transmission includes a pair of insulated wires 51A and 51B, an intermediate sheath 52 covering the outer periphery of the pair of insulated wires 51A and 51B, a shield layer 53 covering the outer periphery of the intermediate sheath 52 and including an inner shield layer 53a and an outer shield layer 53b, and an outer sheath 54 provided on the outside of the shield layer 53. The insulated wire 51A includes a conductor 51A1 and an insulating layer 51A2 covering the outer periphery of the conductor 51A1. The insulated wire 51B includes a conductor 51B1 and an insulating layer 51B2 covering the outer periphery of the conductor 51B1. The configurations of the shield layer 53 and the outer sheath 54 are similar to those of the shield layer 43 and sheath 44 of the fourth embodiment, and therefore detailed description thereof will be omitted.

[0049] The intermediate sheath 52 is made of a first resin composition having a first crystallinity and a second resin composition having a second crystallinity lower than the first crystallinity, and the content of the second resin composition can be 25% by mass or more and 50% by mass or less when the total of the first resin composition and the second resin composition is 100% by mass. As in the fourth embodiment, flexibility can be obtained by increasing the proportion of the second resin composition compared to the resin compositions used in the first to third embodiments, and crosslinking is not required because the first resin composition is a polypropylene-based resin.

[0050] (Effects of the Present Embodiment) According to the present embodiment, a communication cable 50 for differential signal transmission that is highly flexible and suitable for in-vehicle high-speed communication can be obtained. Furthermore, since crosslinking is not required in the manufacturing process, the production process is simplified, contributing to reduced manufacturing costs and improved productivity. Furthermore, the present embodiment has a primary objective of ensuring shape stability of the cable outer diameter rather than suppressing sink marks. Disturbances in the cable outer diameter cause fluctuations in characteristic impedance, resulting in signal attenuation and reflection, which can easily degrade communication characteristics. However, by using the present resin composition, communication characteristics can be maintained and propagation delay time can be stabilized. This is particularly effective in evaluating longitudinal conversion loss (LCL) and longitudinal conversion transmission loss (LCTL), which indicate the amount of conversion of a differential signal to an in-phase signal.

[0051] Table 1 shows the materials, configurations, manufacturing conditions, and characteristics of Examples 1 to 14. Table 2 shows the materials, configurations, manufacturing conditions, and characteristics of Examples 15 to 17 and Comparative Examples 1 to 6. Table 3 shows the capacitance variations of Examples 1 to 5 and Comparative Examples 2, 6 to 9. Examples 1 to 17 correspond to the first embodiment.

[0052] A 50 mm single-screw extruder was used for extrusion molding of the insulator 22. Round wires made of tin-plated mild steel wires were used as the stranded conductor 21. The extrusion temperature was a maximum of 210 to 230°C, the conductor preheating was in the range of 100 to 130°C, and the screw rotation speed was in the range of 30 to 50 rpm.

[0053]

[0054]

[0055]

[0056] In Tables 1 and 2, material A is manufactured by LCY Chemical under the trade name "Globalene 7633," material B is manufactured by Mitsui Chemicals under the trade name "Tafmer PN2060N," material C is manufactured by Idemitsu Kosan under the trade name "L-Modu S901," and material D is manufactured by Mitsui Chemicals under the trade name "Tafmer XM7090."

[0057] In Tables 1 and 2, the characteristic "Contraction around the conductor / stable cross-sectional shape" means that the propagation delay time difference (skew) of the transmission signal between two pairs of insulated wires in a multi-core cable can be kept low. "Characteristics 2" and "Characteristics 3" in Table 3 show the standard deviation and range (maximum and minimum values) of the measurement results measured at 10 points within a 1m length of wire.

[0058] (Results of investigation into materials and conductor preheating temperatures) As shown in "Characteristic 1" in Tables 1 and 2, stable (◯) results were obtained in Examples 1 to 17, and unstable (×) results were obtained in Comparative Examples 1 to 6. From this, it can be said that the ratio of material A to other materials (materials B, C, or D) is preferably 50% by mass or more and 90% by mass or less for material A, and 10% by mass or more and 50% by mass or less for other materials.

[0059] When the thickness of the insulator 22 is 0.3 mm, it was found that the amount of other materials could be reduced to 10% by mass if the conductor preheating temperature was set to 130°C, as shown in Example 15. When the thickness of the insulator 22 is 0.5 mm, it was found that the amount of other materials could be reduced to 20% by mass if the conductor preheating temperature was set to 130°C, as shown in Example 1 and Comparative Example 3. When the thickness of the insulator 22 is 0.5 mm, it was found that the conductor preheating temperature could be reduced to 100°C if the amount of other materials was set to 30% by mass or more, as shown in Examples 2, 4, 8-10, 12-14, and Comparative Example 6.

[0060] (Study Results of Capacitance Variation) As a result of measuring capacitance variation with n = 10, as shown in Table 3, in Examples 1 to 5, the standard deviation was 0.06 to 0.10 pF / m, and the difference (range) between the maximum and minimum values ​​was 0.14 to 0.33 pF / m. On the other hand, in Comparative Examples 2 and 6-9, the standard deviation was 0.15 to 0.34 pF / m, and the range was 0.36 to 0.99 pF / m. Since the relative dielectric constant ε of air is approximately 1, it is believed that the capacitance variation reflects the voids inside the stranded conductor 21 and the multiple voids formed between the stranded conductor 21 and the insulator 22. Therefore, compared to Comparative Examples 2 and 6-9, it is believed that in Examples 1 to 5, the insulator 22 is in close contact with the outer periphery of the stranded conductor 21 throughout its entire length, resulting in smaller void distribution variation (or almost no voids at all) between the stranded conductor 21 and the insulator 22.

[0061] Fig. 8A is a cross-sectional photograph of the insulated wire of Comparative Example 7, in which the insulation was not sufficiently filled and solidified due to a low conductor preheating temperature. Fig. 8B is a cross-sectional photograph of the insulated wire of Comparative Example 8, in which sink marks occurred in the insulation due to a high conductor preheating temperature. Fig. 9 is a cross-sectional photograph of the insulated wire of Example 2 of the present invention, in which no sink marks occurred. Note that Figs. 8A, 8B, and 9 show cross-sectional photographs after the stranded conductor has been removed. The stranded conductor of Comparative Example 7, Comparative Example 8, and Example 2 was configured by twisting together seven round wires with a diameter of 0.18 mm, and the outer diameter of the insulated wire was 1.54 mm. As shown in Table 3, the insulators of Comparative Examples 7 and 8 contained 100% by mass of material A (ethylene-propylene copolymer) and did not contain the other materials B, C, or D. The conductor preheating temperature of Comparative Example 7 was 100°C, and that of Comparative Example 8 was 130°C. As shown in Table 3, the insulator of Example 2 contains 70% by mass of material A (ethylene-propylene copolymer) and 30% by mass of material B (propylene-1-butene-ethylene copolymer), and the conductor preheating temperature is 100°C.

[0062] 8A and 8B , the insulated wire of Comparative Example 7 solidified without being sufficiently filled with insulation due to the low conductor preheating temperature, while the insulated wire of Comparative Example 8 had multiple voids between the stranded conductor and the insulation even when the conductor preheating temperature was increased. The inventors confirmed that the distribution of voids varied throughout the entire length of the insulated wire. When the conductor preheating temperature was low, there was little sink marking, but the insulation was not filled around the stranded conductor. When the conductor preheating temperature was high, the insulation was filled around the stranded conductor, but there were many sink marks. This is thought to be due to the difficulty in controlling the manufacturing conditions, particularly the conductor preheating and cooling conditions.

[0063] 9, almost no voids were formed between the stranded conductor and the insulator in the insulated wire of Example 2. The inventors confirmed that there were no areas where voids were concentrated, and that there was no variation in the distribution of voids over the entire length of the insulated wire.

[0064] (Performance Evaluation of Intermediate Sheath) Based on the differential signal transmission communication cable according to the fifth embodiment of the present invention, Example 18 and Comparative Example 10 were prepared with the configuration and materials shown in Table 4, and their characteristics were evaluated. In Example 18, an intermediate sheath was provided between the insulated wire and the shielding layer. The resin composition constituting the intermediate sheath was the resin composition of Example 4 in Table 1. That is, when the total of the first resin composition (material A) and the second resin composition (material B) was taken as 100% by mass, the first resin composition (material A) accounted for 60% by mass, and the second resin composition (material B) accounted for 40% by mass. In Comparative Example 10, a polyester tape was used instead of the intermediate sheath. The characteristics were evaluated based on the 1000BASE-T1 class 1 standard evaluation, with longitudinal conversion loss (LCL) and longitudinal conversion transmission loss (LCTL). Those that did not comply with the above standards were rated "fail," those that complied were rated "pass," and those that complied with the standards and had excellent characteristics were rated "good."

[0065] The resin composition of the present invention can be used to produce insulated wires that combine excellent dimensional stability and communication properties, and is therefore suitable for use in various cables for automobiles, industrial applications, and information and communication equipment. It is particularly suitable for communication cables for differential signals with reduced skew. Furthermore, as an insulator or intermediate sheath, it can also achieve shape stability and flexibility in communication cables, making it suitable for a wide range of industrial applications, including in-vehicle networks, data center communications, and consumer high-speed digital interfaces, all of which require high-speed transmission.

[0066] 1: Communication cable 2: Insulated wire 2a-2d: Signal wire 3: Shield layer 3a: Inner shield layer 3b: Outer shield layer 3c: Metal shield layer 4: Sheath 21: Stranded conductor 21a: Wire 22: Insulator 30: Coaxial communication cable 31: Center conductor 31a: Wire 32: Insulator 33: Shield layer 33a: Inner shield layer 33b: Outer shield layer 34: Sheath 40: Two-core insulated cable 41: Conductor 42: Insulator 43: Shield layer 43a: Inner shield layer 43b: Outer shield layer 44: Sheath 50: Differential signal transmission communication cable 51A: Insulated wire 51B: Insulated wire 52: Intermediate sheath 53: Shield layer 54: Outer sheath 300: Coaxial wire 400: Insulated wire

Claims

1. A resin composition comprising, as base polymers, a first resin composition having a first crystallinity and a second resin composition having a second crystallinity lower than the first crystallinity in predetermined proportions, wherein the first crystallinity is greater than 40% and the second crystallinity is 40% or less, the first resin composition comprises at least one resin selected from the group consisting of polyethylene-based resins and polypropylene-based resins, the second resin composition is a polyolefin-based resin, and the content of the second resin composition is 5% by mass or more and 50% by mass or less when the total of the first resin composition and the second resin composition is 100% by mass.

2. The resin composition according to claim 1, wherein the content of the second resin composition is 5% by mass or more and 25% by mass or less when the total of the first resin composition and the second resin composition is 100% by mass.

3. The resin composition according to claim 1, wherein the content of the second resin composition is 25% by mass or more and 50% by mass or less when the total of the first resin composition and the second resin composition is 100% by mass.

4. An insulated wire comprising a stranded conductor formed by twisting together a plurality of wires, and an insulator tightly covering the outer periphery of the stranded conductor, the insulator being made of the resin composition according to claim 1.

5. The insulated wire according to claim 4, wherein the polypropylene resin constituting the first resin composition is a block copolymer polypropylene.

6. The insulated wire according to claim 5, wherein the polyolefin resin constituting the second resin composition is a low-crystalline polyolefin resin.

7. The insulated wire according to claim 6, wherein the standard deviation of the capacitance of said insulated wire for n=10 is 10 pF / m or less, and the difference between the maximum and minimum for n=10 is 33 pF / m or less.

8. The insulated wire according to claim 7, wherein the wire is a round wire.

9. A communication cable comprising a pair of insulated wires according to any one of claims 4 to 8.

10. A coaxial communication cable comprising a coaxial wire whose central conductor is covered with an insulator made of the resin composition of claim 2.

11. A two-core insulated cable in which two conductors are covered with an insulator made of the resin composition according to claim 3.

12. A communication cable having an intermediate sheath made of the resin composition according to claim 3.

13. A method for producing an insulated wire according to claim 4, comprising: preheating the stranded conductor at 130°C or less; and covering the outer periphery of the preheated stranded conductor with the resin composition by extrusion molding to form the insulator having a thickness of 0.5 mm or less.

14. A method for manufacturing a coaxial communication cable according to claim 10, comprising: preheating the central conductor at 150°C or less; and covering the outer periphery of the preheated central conductor with the resin composition by extrusion molding to form the insulator having a thickness of 0.6 mm or less.

15. Use of the resin composition according to claim 1 for forming the insulation of an insulated electric wire, the insulation of a communication cable, or the intermediate sheath of a communication cable.

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