JP2005078835A - Coaxial cable and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coaxial cable with low manufacturing cost and high attenuation characteristics in high frequency transmission, and also to provide the manufacturing method thereof. <P>SOLUTION: The coaxial cable has an inner conductor, an insulating layer made of a fluororesin foam covering the periphery of the inner conductor, an outer conductor installed in the periphery of the insulating layer, and a sheath layer installed in the periphery of the outer conductor, and the fluororesin satisfies following (A) and (B). (A) Melting viscosity at 372°C is 102-107 poise. (B) Fluoride ions extractable in methanol/water mixed solution of volume ratio of 1:1 is 1.5 ppm or less. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a coaxial cable and a method for manufacturing the same.

  Conventionally, as a coaxial cable excellent in heat resistance, flame retardancy, and attenuation characteristics of electromagnetic waves to be transmitted, it has a configuration in which an inner conductor is coated with a fluororesin (this configuration is also referred to as a “fluororesin insulated wire” hereinafter). Coaxial cable is used. Among them, PTFE insulated electric wires obtained by paste extrusion coating of polytetrafluoroethylene (PTFE) and firing, and unfired PTFE tape are stretched, and the fired porous PTFE tape is wound around a conductor. The obtained porous PTFE insulated wire or the like has an advantage of exhibiting excellent attenuation characteristics (that is, attenuation is small) in high-frequency transmission (Patent Document 1). However, since PTFE insulated wires have a firing process that takes a long time, and porous PTFE insulated wires have a low productivity per hour of winding a tape around a conductor, these insulated wires are The manufacturing cost is high.

On the other hand, as fluororesin insulated wires which are excellent in terms of production cost, thermoplastic fluororesin such as tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA) and tetrafluoroethylene / hexafluoropropylene copolymer (FEP) are available. An insulated wire or the like obtained by extrusion-coating a conductor (without firing) is known. However, these conventionally used insulated wires coated with PFA and FEP are inferior to the above-mentioned (porous) PTFE insulated wires in terms of attenuation characteristics, and are dielectric in transmission at high frequencies, particularly at high frequencies of 1 GHz or higher. There are concerns about problems such as high tangent (tan δ), large transmission energy loss, and heat generation of electric wires.
JP 2000-11664 A Japanese Patent Publication No. 4-83

  In view of such a situation, the present invention uses a fluororesin that has thermoplasticity, can be extrusion-coated, and can be used without firing (ie, has a low manufacturing cost), and PTFE insulation. It is an object of the present invention to provide a coaxial cable including a fluororesin insulated electric wire having a damping characteristic comparable to that of an electric wire and a method for manufacturing the same.

  In the past, it was thought that the increase in tan δ when the above-mentioned thermoplastic fluororesin was used was due to the main structure of the fluororesin. However, as a result of detailed investigations by the present inventors, the fluororesin contains fluoride ions that can be extracted into a methanol / water mixture as described later (that is, chemically unstable terminal groups are present). It has been found that the above problem arises due to In addition, conventionally, the foaming of the fluororesin has not been performed in the production of the coaxial cable due to the fact that the chemical foaming agent for the fluororesin has not been known. It has been found that the fluororesin further improves the damping characteristics. Based on these findings, the present invention was completed by using a fluororesin foam having a molecular structure as described later.

(1) an inner conductor, an insulating layer made of a fluororesin foam covering the outer periphery of the inner conductor, an outer conductor provided on the outer periphery of the insulating layer, and a sheath layer provided on the outer periphery of the outer conductor; A coaxial cable in which the fluororesin comprises the following (A) and (B).
(A) The melt viscosity at 372 ° C. is 10 ² to 10 ⁷ poise,
(B) The fluoride ion that can be extracted into a methanol / water mixture having a volume ratio of 1: 1 is 1.5 ppm or less on a weight basis.
(2) The coaxial cable according to (1), wherein the foam is obtained by physically foaming a composition containing a fluororesin and a nucleating agent using a physical foaming agent.
(3) The coaxial cable according to (1) or (2), wherein the fluororesin further comprises the following (C).
(C) 80 to 99% by weight of the fluororesin is composed of repeating units represented by —CF ₂ CF ₂ —, and 1 to 20% by weight is —CF (OR _f ) —CF ₂ — (wherein R _f Is a perfluoroalkyl group having 1 to 8 carbon atoms) or a repeating unit represented by —CF (CF ₃ ) —CF ₂ —.
(4) The coaxial cable according to any one of (1) to (3), wherein the outer conductor is a corrugated copper tube.
(5) The coaxial cable according to any one of the above (1) to (4), which is for transmitting an electromagnetic wave having a frequency of 1 GHz or more.
(6) an inner conductor, an insulating layer made of a fluororesin foam covering the outer periphery of the inner conductor, an outer conductor provided on the outer periphery of the insulating layer, and a sheath layer provided on the outer periphery of the outer conductor A method of manufacturing a coaxial cable having
Heating a resin mixture obtained by mixing a fluororesin and a nucleating agent comprising the following (A) and (B) to 340 to 410 ° C. and adding a physical foaming agent to the heated resin mixture to perform physical foaming And a step of coating the outer periphery of the inner conductor with the resin mixture.
(A) The melt viscosity at 372 ° C. is 10 ² to 10 ⁷ poise,
(B) The fluoride ion that can be extracted into a methanol / water mixture having a volume ratio of 1: 1 is 1.5 ppm or less on a weight basis.
(7) The production method according to (6), wherein the content of the nucleating agent in the resin mixture is 0.01 to 20 parts by weight with respect to 100 parts by weight of the fluororesin.
(8) The production method according to (6) or (7), wherein the fluororesin further comprises the following (C).
(C) 80 to 99% by weight of the fluororesin is composed of repeating units represented by —CF ₂ CF ₂ —, and 1 to 20% by weight is —CF (OR _f ) —CF ₂ — (wherein R _f Is a perfluoroalkyl group having 1 to 8 carbon atoms) or a repeating unit represented by —CF (CF ₃ ) —CF ₂ —.

  Thus, using a fluororesin in which the concentration of fluoride ions that can be extracted into a methanol / water mixture is strictly controlled for an insulating layer of a coaxial cable has never been conceived. This is not known that the terminal group of the fluororesin affects the attenuation characteristic in the high frequency region (“GHz” region), but is merely known as a factor causing fouling (patent document). 2). The coaxial cable of the present invention can be manufactured at a low cost and can have both excellent attenuation characteristics (low tan δ) by using a thermoplastic fluororesin having the above requirements.

  The coaxial cable of the present invention uses a fluororesin that has a low transmission loss during high-frequency transmission, does not require firing, has a low manufacturing cost, and has a melting point higher than that of a polyethylene resin generally used for high-frequency coaxial cables. Therefore, it is a coaxial cable with a large power capacity (high heat resistance).

  The coaxial cable of the present invention will be described with reference to the drawings. FIG. 1 is a diagram schematically showing a coaxial cable of the present invention. FIG. 1A is a perspective view, and its II sectional view is FIG. The coaxial cable 1 of the present invention is a long multilayer covered electric wire. The coaxial cable 1 includes an inner conductor 11 that occupies a substantially central portion of a cross section perpendicular to the longitudinal direction, an insulating layer 12, an outer conductor 13, and a sheath layer 14 that cover the outer periphery in order from the center side. Have at least. The coaxial cable 1 of the present invention is characterized by the fluororesin constituting the insulating layer 12 and its molding mode.

  The insulating layer 12 of the coaxial cable 1 of the present invention has a fluororesin foam having the following requirements (A) and (B), preferably the following requirements (A) to (C), on the outer periphery of the internal conductor 11. It is formed by coating.

(A) The melt viscosity at 372 ° C. is 10 ² to 10 ⁷ poise.
The melt viscosity of the resin is an apparent viscosity at a piston speed of 10 mm / min and a temperature of 372 ° C. measured using a corrosion resistant barrel with an inner diameter of 9.6 mm and a die with a hole diameter of 1.0 mm and a length of 10 mm on the Toyo Seiki Capillograph 1B. is there.

The melt viscosity at 372 ° C. of the fluororesin used in the present invention is 10 ² to 10 ⁷ poise. If the viscosity is less than 10 ² poise, extrusion coating cannot be performed well, and if it exceeds 10 ⁷ poise, the plasticity is too low and extrusion coating becomes difficult. The viscosity is preferably 10 ⁴ poise or more from the viewpoint of good extrusion coating, and preferably 10 ⁵ poise or less from the viewpoint of reducing the processing speed and the load on the motor of the extrusion molding machine.

(B) The fluoride ion that can be extracted into a methanol / water mixture having a volume ratio of 1: 1 is 1.5 ppm or less on a weight basis.
The concentration of fluoride ions that can be extracted into a methanol / water mixture having a volume ratio of 1: 1 is a value obtained by the following measurement. A 10 g sample is placed in a polyethylene bottle, 10 ml of a methanol / water mixture (1: 1 volume ratio) is added, and an additional 10 ml of Orion 94-09-09 total ionic strength adjusting buffer is added. These mixtures are stirred and allowed to stand for 24 hours, and then measured using a fluoride ion electrode (Orion 96-90-00).

Thus, the fluoride ion measured needs to be 1.5 ppm or less on a weight basis with respect to the fluororesin whole quantity. When the fluoride ion exceeds 1.5 ppm, there is a problem that tan δ in the case of an insulated wire is increased. The fluoride ion is preferably 1.0 ppm or less, and more preferably 0.5 ppm or less. From the above point, the smaller the fluoride ion, the better. According to the process for converting the terminal groups of which will be described later fluororesin -CF ₃ end, it is known that fluorine resins satisfying this requirement is obtained (Patent Document 2).

The “fluororesin” in the present invention refers to a resin comprising 80% by weight or more of repeating units represented by —CF ₂ CF ₂ —. In addition to the above requirements (A) and (B), the fluororesin of the present invention preferably further includes the following requirement (C).

(C) 80 to 99% by weight of the fluororesin is composed of repeating units represented by —CF ₂ CF ₂ —, and 1 to 20% by weight is —CF (OR _f ) —CF ₂ — (wherein R _f is A perfluoroalkyl group having 1 to 8 carbon atoms) or a repeating unit represented by —CF (CF ₃ ) —CF ₂ —.
The weight ratio is determined by infrared spectroscopy. Based on a correction curve made with a reference film of a known weight ratio using an absorption at 10.07 μm and an absorption at 4.25 μm in a nitrogen atmosphere of a film sample having a thickness of about 0.05 mm. Then, the weight ratio is calculated.

The reason why 80 to 99% by weight of the fluororesin used in the present invention is preferably composed of a repeating unit represented by —CF ₂ CF ₂ — is that the production can be facilitated. The fluororesin used in the present invention is a repeating unit represented by —CF (OR _f ) —CF ₂ — except for the repeating unit represented by —CF ₂ CF ₂ — (wherein R _f is as described above. It is more preferable that

In the above formulas, perfluoroalkyl group having 1 to 8 carbon atoms represented by R _f is the formula -C _n F _{2n + 1 (n} is an integer from 1 to 8.) A group represented by any That's fine.

The method for obtaining such a fluororesin is not particularly limited, and may be produced by a known method, or may be a commercially available product (for example, NEW PFA Teflon (registered trademark) HP series manufactured by Mitsui DuPont Fluorochemical Co., Ltd.) ) May be used. In particular, among the above requirements (A) to (C), a fluororesin satisfying (A) and (C) can be easily synthesized by a conventional method. Hereinafter, as an example of a method for obtaining the fluororesin of the present invention, a method of converting an unstable terminal group of the fluororesin satisfying (A) and (C) to a —CF ₃ terminal (Patent Document 2) will be described.

According to this method, the end group is changed by bringing the fluororesin granules, flakes, pellets and the like satisfying the above (A) and (C) into contact with a compound (preferably fluorine gas) that generates a fluorine radical. Can be converted to CF ₃ . When fluorine gas is used, the fluorine gas is preferably diluted to 10 to 25% by weight with an inert gas (such as nitrogen), and is 150 to 250 ° C. (preferably 200 to 250 ° C.), 1 to 10 atm (preferably (At atmospheric pressure), the fluororesin is allowed to act for 4 to 16 hours (preferably 8 to 12 hours). At this time, it is preferable to stir in order to continuously expose the new surface of the fluororesin. By performing such a reaction until the fluororesin exhibits the above-described property (B), a fluororesin satisfying the above (A) to (C) can be obtained. As described above, the requirement (C) is not necessarily required in the present invention.

  In the present invention, the above-mentioned fluororesin foam is used as the insulating layer 12. The resin foam means a porous resin molded body containing continuous or independent bubbles by some method. Conventionally, loss of loss in high-frequency transmission has not been achieved by foaming a fluororesin in an insulating layer of a coaxial cable. Upon completion of the present invention, it has been found that the loss can be further reduced by using a foamed fluororesin for the insulating layer.

  The degree of foaming in the foam can be expressed by a parameter that is the degree of foaming. The degree of foaming means the rate of change in the specific gravity of the foam before and after foaming, and the rate of change between the specific gravity of the material constituting the foam (hereinafter also referred to as true specific gravity) and the apparent specific gravity of the foam. be equivalent to. The foaming degree of the fluororesin foam constituting the insulating layer 12 of the coaxial cable 1 is calculated through the apparent specific gravity measurement and the true specific gravity measurement of the foam as follows.

The apparent specific gravity of the foam constituting the insulating layer 12 is determined by subjecting the insulating layer 12 taken out by removing the inner and outer conductors of the coaxial cable to an underwater replacement method in accordance with JIS K7112. More specifically, the weight in air of a sample of the foam is M _0, specific gravity substitution fluid known ([rho); the weight M ₁ when immersed in (eg water), the sample Is obtained by V = (M ₀ −M ₁ ) / ρ, and using this volume V, the apparent specific gravity is obtained as M ₀ / V.

  The procedure for measuring the true specific gravity of the foam material constituting the insulating layer 12 is as follows. First, the foam is subjected to hot pressing at a temperature equal to or higher than the melting point of the material to crush the bubbles, thereby creating a sheet sample without bubbles. Next, the specific gravity of the sheet sample is measured by an underwater substitution method according to JIS K 7112 described above. The specific gravity thus obtained is the true specific gravity. Examples of the temperature at the time of hot pressing are 330 ° C. if the foam material is PFA and 300 ° C. if it is FEP.

From the apparent specific gravity of the foam obtained as described above and the true specific gravity of the material, the foaming degree of the foam is calculated by the following formula.
Foaming degree (%) = (true specific gravity−apparent specific gravity) × 100 (%) / true specific gravity

  The degree of foaming of the fluororesin foam constituting the insulating layer 12 of the coaxial cable 1 of the present invention is not particularly limited, but is preferably 35% or more from the viewpoint of reducing the design constraints and transmission loss. Preferably it is 40% or more. The restriction on the design is that if the foaming degree is too small, the outer conductor 13 must be thickened or the inner conductor 11 must be thinned when the characteristic impedance of the coaxial cable is 50Ω. The higher the degree of foaming, the better from the viewpoint of reducing transmission loss, but the degree of foaming is preferably 65% or less from the viewpoint of ease of production, and more preferably from the viewpoint of obtaining a stable and uniform foaming degree. Is 55% or less.

  In general, means for obtaining a foamed resin molded article includes chemical foaming using a chemical foaming agent that generates a gas by chemical change, and physical foaming using a physical foaming agent that can be gasified by the gas itself or phase change. The foam constituting the insulating layer 12 of the coaxial cable 1 of the present invention can be made by either foaming means because there is no difference in terms of the effects and effects of the invention by chemical foaming or physical foaming. Good. Since chemical foaming agents for fluororesins are not known, physical foaming will be described below.

  Physical foaming is performed by mixing a physical foaming agent into a resin composition for forming a foam. A physical foaming agent is a gas or a substance that can change into a gas upon phase change. As a specific example, a fluororesin composition to be coated on the inner conductor 11 is heated in an extruder to be softened or melted, and a physical foaming agent is added to the heated fluororesin composition on the outer periphery of the inner conductor 11. A method for producing a foam by covering and forming the insulating layer 12 may be mentioned.

  The temperature at the time of heating a fluororesin in an extruder is 340-410 degreeC, Preferably it is 360-400 degreeC. The physical foaming agent used in the present invention may be any substance that can become a gas at this temperature. Specifically, it can be a flon gas, a rare gas (helium, neon, argon, krypton, xenon, radon), hydrocarbon (propane, butane). , Pentane, hexane, etc.), carbon dioxide gas, nitrogen gas and the like, and preferably carbon dioxide gas, nitrogen gas and the like. The method of adding a physical foaming agent in such an extrusion process is preferable in that it can simultaneously cover the inner conductor 11 and foam the resin. When foaming the fluororesin by such a method, in order to increase the foaming degree, the injection pressure of the gas should be increased or the flow rate of the gas should be increased. To lower the foaming degree, the reverse operation should be performed. Good. Specific conditions for obtaining a fluororesin foam exhibiting a preferable foaming degree will be described in detail in Examples described later.

  In physical foaming, only a physical foaming agent may be added, but preferably a nucleating agent is also added. The nucleating agent is specifically boron nitride (BN), silicon dioxide, titanium dioxide, alumina, magnesia or a mixture of two or more thereof. Since such a nucleating agent has poor wettability with a fluororesin (it is difficult to fit in), it becomes a core when foaming. More specifically, since the fluororesin and the nucleating agent are hardly compatible, the physical foaming agent (gasified by heating) tends to accumulate at the interface between the two, and the pressure is released when foaming is applied. Is promoted. When adding a nucleating agent when foaming a fluororesin in the above-described extrusion molding, a fluororesin and a nucleating agent are added to an extruder to obtain a resin mixture, and then foaming and extrusion molding are performed as described above. It is good to do. At this time, the content of the nucleating agent in the resin mixture is preferably 0.01 to 20 parts by weight with respect to 100 parts by weight of the fluororesin from the viewpoint of obtaining sufficient foaming and preventing transmission loss due to the nucleating agent. More preferably, it is 0.1 to 5 parts by weight.

  What is necessary is just to determine arbitrarily the shape and dimension of the internal conductor 11 used for the coaxial cable 1 of this invention according to the objective. The material of the internal conductor 11 is not limited as long as it is a metal, and a known material may be applied. For example, a single wire such as a copper wire, a copper alloy wire, a silver-plated copper wire, a tin-plated copper wire, an aluminum wire, or a twisted wire. Lines and the like are exemplified. The inner conductor 11 may be hollow, and may be hollow and the thickness may not be constant like a corrugated tube.

  The outer conductor 13 and the sheath layer 14 of the coaxial cable 1 of the present invention can be formed of a conventionally used material for a coaxial cable by a conventionally known method. The outer conductor 13 is not limited as long as it is a metal, and the formation method thereof may be tubular conductor sinking, formation of a tubular conductor by tape welding, horizontal winding or braiding of a conducting wire, vertical attachment of tape, or the like. From the viewpoint of facilitating bending of the coaxial cable 1, the outer conductor 13 is preferably a corrugated copper tube. As a further advantage of using a corrugated copper tube as the outer conductor 13, the gap formed between the outer peripheral surface of the insulating layer 12 and the inner peripheral surface of the outer conductor 13 acts as if it were a bubble in a foam. To contribute to the reduction of transmission loss. The sheath layer 14 may be any material that can protect the outer conductor 13 from physical and chemical damage. Specific examples include fluororesin, PVC (polyvinyl chloride), PE (polyethylene), and general rubber. Is done. From the viewpoint of low smoke generation, low toxicity, low corrosion, heat resistance, chemical resistance, and cold resistance, it is preferable to form the sheath layer 14 by extruding a fluororesin.

In the coaxial cable 1 of the present invention, the thickness (diameter) of the inner conductor 11 and the coating thicknesses of the insulating layer 12, the outer conductor 13, and the sheath layer 14 are not particularly limited, but are preferably as follows. . Here, the thickness (diameter) of the inner conductor 11 means the diameter of a circle having the same area as the cut area of the inner conductor 11 in a cross section perpendicular to the longitudinal direction of the coaxial cable. When the inner conductor 11 is hollow, the cut area means the area of the combined area of the cut surface and the hollow portion surrounded by the cut surface. When the cut area of the internal conductor 11 varies depending on the cut location (when the thickness is not constant), the maximum thickness of the internal conductor 11 is preferably in the following range.
Inner conductor 11 0.1-10 mm,
Insulating layer 12 0.5-10 mm,
Outer conductor 13 0.1-1.0 mm,
Sheath layer 14 0.1-2.0 mm.

  The coaxial cable 1 of the present invention may further include a support wire or the like in addition to the above-described inner conductor 11, insulating layer 12, outer conductor 13, and sheath layer 14.

  The coaxial cable of the present invention can be adapted for use in transmitting various types of electromagnetic waves, but can be suitably used particularly for transmitting high frequencies because of its excellent attenuation characteristics. Here, the high frequency means an electromagnetic wave having a frequency of 1 GHz or more, preferably 2 to 20 GHz.

  Hereinafter, the present invention will be described in more detail based on each example, but the present invention is not limited to only the example.

[Example 1-8, Comparative Example 1-10]
The above-mentioned end group conversion was performed on a commercially available fluororesin (PFA). Melt viscosity at 372 ° C. of the obtained fluororesin, extractable fluoride ion, —CF (OR _f ) —CF ₂ — (wherein R _f represents a C 1-8 perfluoroalkyl group) or The amount of the repeating unit represented by —CF (CF ₃ ) —CF ₂ — is as shown in Tables 1 and 2. 100 parts by weight of this fluororesin and the nucleating agent of the kind and amount shown in Table 1 were put into a biaxial kneader, kneaded and extruded at 340 ° C., and stirred to obtain a resin mixture. The resin mixture was put into an extruder, the foaming agents shown in Tables 1 and 2 were injected, and the outer periphery of the inner conductor made of silver-plated annealed copper wire was coated while stirring to obtain a fluororesin insulated wire. In order to control the foaming degree of the insulating layer, the injection pressure of the foaming agent was adjusted. For example, the injection pressure was 2 MPa in order to achieve a foaming degree of 30%, and the injection pressure was 6 MPa in order to achieve a foaming degree of 50%. The outer periphery of this fluororesin insulated wire is covered with a 0.5 mm thick copper tube (see Tables 1 and 2 for the presence or absence of corrugating), and then covered with a 0.5 mm thick sheath layer made of FEP. Got. FIG. 2 is a schematic view of the coaxial cable thus obtained.

[Evaluation]
Each parameter of the fluororesin and its foam was measured as described above. Attenuation characteristics (dB / m) with respect to radio waves with frequencies of 2 GHz and 20 GHz were measured for the insulated wires of each Example and Comparative Example using a network analyzer. These measurement results are summarized in Tables 1 and 2.

In Tables 1 and 2, “repeating unit” means —CF (OR _f ) —CF ₂ — (wherein R _f represents a perfluoroalkyl group having 1 to 8 carbon atoms) in the fluororesin used or The ratio of the repeating unit represented by —CF (CF ₃ ) —CF ₂ — is expressed in weight%. Moreover, the addition amount of the nucleating agent being “2 phr” indicates that 2 parts by weight of the nucleating agent was added with respect to 100 parts by weight of the resin.

  In addition to the measurements shown in Tables 1 and 2, the winding ability of the obtained coaxial cable around a mandrel having a diameter of 20 mm was evaluated. As a result, when the corrugated copper tube was used as the outer conductor, the winding property was good, and no buckling occurred even when it was repeatedly wound 10 times or more. On the other hand, the thing using the copper pipe which does not corrugate as an external conductor has bends when winding is repeated 3 to 10 times.

The coaxial cable of this invention is represented typically. FIG. 1A is a perspective view, and its II sectional view is FIG. The coaxial cable manufactured in the Example and the comparative example is represented typically.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Coaxial cable 11 Inner conductor 12 Insulation layer 13 Outer conductor 14 Sheath layer

Claims (8)

An inner conductor, an insulating layer made of a fluororesin foam covering the outer periphery of the inner conductor, an outer conductor provided on the outer periphery of the insulating layer, and a sheath layer provided on the outer periphery of the outer conductor A coaxial cable in which the fluororesin comprises the following (A) and (B).
(A) The melt viscosity at 372 ° C. is 10 ² to 10 ⁷ poise,
(B) The fluoride ion that can be extracted into a methanol / water mixture having a volume ratio of 1: 1 is 1.5 ppm or less on a weight basis.   The coaxial cable according to claim 1, wherein the foam is obtained by physically foaming a composition containing a fluororesin and a nucleating agent using a physical foaming agent. The coaxial cable according to claim 1 or 2, wherein the fluororesin further comprises the following (C).
(C) 80 to 99% by weight of the fluororesin is composed of repeating units represented by —CF ₂ CF ₂ —, and 1 to 20% by weight is —CF (OR _f ) —CF ₂ — (wherein R _f Is a perfluoroalkyl group having 1 to 8 carbon atoms) or a repeating unit represented by —CF (CF ₃ ) —CF ₂ —.   The coaxial cable according to claim 1, wherein the outer conductor is a corrugated copper tube.   The coaxial cable according to any one of claims 1 to 4, which is for transmitting electromagnetic waves having a frequency of 1 GHz or more. A coaxial having an inner conductor, an insulating layer made of a foam of fluororesin covering the outer periphery of the inner conductor, an outer conductor provided on the outer periphery of the insulating layer, and a sheath layer provided on the outer periphery of the outer conductor A cable manufacturing method comprising:
Heating a resin mixture obtained by mixing a fluororesin and a nucleating agent comprising the following (A) and (B) to 340 to 410 ° C. and adding a physical foaming agent to the heated resin mixture to perform physical foaming And a step of coating the outer periphery of the inner conductor with the resin mixture.
(A) The melt viscosity at 372 ° C. is 10 ² to 10 ⁷ poise,
(B) The fluoride ion that can be extracted into a methanol / water mixture having a volume ratio of 1: 1 is 1.5 ppm or less on a weight basis.   The manufacturing method of Claim 6 whose content of the nucleating agent in a resin mixture is 0.01-20 weight part with respect to 100 weight part of fluororesins. The method according to claim 6 or 7, wherein the fluororesin further comprises the following (C).
(C) 80 to 99% by weight of the fluororesin is composed of repeating units represented by —CF ₂ CF ₂ —, and 1 to 20% by weight is —CF (OR _f ) —CF ₂ — (wherein R _f Is a perfluoroalkyl group having 1 to 8 carbon atoms) or a repeating unit represented by —CF (CF ₃ ) —CF ₂ —.

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