JP5500788B2 - cable - Google Patents

Description

  The present invention relates to a cable.

  A coaxial cable, which is a type of communication cable, includes an inner conductor, an insulating layer that covers the inner conductor, an outer conductor that surrounds the insulating layer, and a sheath that covers the outer conductor and protects the outer conductor. In a communication cable having low loss and high heat resistance, a fluororesin having excellent heat resistance and dielectric properties is generally used as an insulating layer covering an inner conductor.

  Although such a fluororesin is excellent in heat resistance, it is brought to a high temperature state when being processed when forming the insulating layer. At this time, since the processing temperature and decomposition temperature of the fluororesin are generally close, it is known that a part of the fluororesin is decomposed to generate corrosive gas or that fluoride ions are eluted. For this reason, when a bare conductor, that is, a conductor that is not plated, is used as the inner conductor covered with the fluororesin, the surface of the inner conductor is corroded by corrosive components such as the above corrosive gas and eluted fluoride ions. The resistance value of the inner conductor surface may increase. In particular, in a high-frequency communication cable, current flows through the surface portion of the inner conductor in contact with the fluororesin, so that the increase in resistance due to corrosion becomes even more remarkable. Therefore, in a cable using a fluororesin as an insulating layer, a plated wire, particularly a silver plated wire is generally used as an internal conductor for corrosion resistance.

  On the other hand, the thinner a communication cable such as a coaxial cable, the greater the contribution of the conductor to cost and attenuation. In other words, when the plated wire is used, the proportion of the plated portion in the conductor is increased as compared with the case where a bare conductor having the same diameter is used. For this reason, although the electrical characteristic as a communication cable becomes favorable, since silver is generally used as a plating part, there existed a problem that cost became high.

In addition, although the conductor contributes greatly, it is also important to reduce the loss of the insulating layer. For this reason, a cable using a fluororesin as a foam as well as polyethylene has been proposed (Patent Document 1 below).
JP 2005-78835 A

  However, in the cable of Patent Document 1, since the fluororesin is a foam, there are minute recesses due to voids in the fluororesin (hereinafter referred to as “foam cells”) on the conductor surface side of the foam. It is thought that it is formed. For this reason, the conductor surface will be in the state exposed to the foam cell. At this time, as described above, corrosive components such as corrosive gas and eluted impurities are generated in the fluororesin at a high temperature, but these corrosive components tend to collect in the foamed cells present in the foam. For this reason, when the inner conductor is made a bare conductor in order to reduce the cost of the cable, the corrosive gas filled in the gap contacts the inner conductor surface and corrodes the surface portion of the inner conductor. As a result, the surface resistance of the inner conductor is greatly increased, and the signal attenuation characteristics of the cable are greatly deteriorated accordingly. Further, if the surface of the inner conductor is corroded discontinuously in a patchy manner, the VSWR (Voltage Standing Wave Ratio) becomes small, which also causes deterioration of signal attenuation characteristics in the cable. Therefore, there has been a demand for a cable that can sufficiently suppress deterioration of signal attenuation characteristics even when a bare conductor is used as a conductor.

  Therefore, an object of the present invention is to provide a cable that can sufficiently suppress deterioration of signal attenuation characteristics even when a bare conductor is used as a conductor.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have determined that a non-foamed layer, which was thought to deteriorate the attenuation characteristics of the cable when coated on the conductor, is replaced with a foamed layer containing a fluororesin and the conductor. It has been found that the above-mentioned problems can be solved by providing them in between, and the present invention has been completed.

That is, the present invention includes a conductor and an insulating layer that covers the conductor, and the insulating layer includes a foam layer that includes a foam containing a fluororesin, and a non-foam that includes the fluororesin. The non-foamed layer is disposed between the foamed layer and the conductor, and the fluororesin contained in the foam and the fluororesin contained in the non-foam are both The cable has an elution fluoride ion concentration of 1.5 ppm or less and a heating weight change rate of 0.02 to 0.04 mass% .

  According to this cable, since the non-foamed layer exists between the foamed layer and the conductor, the surface of the conductor is not exposed to the foamed cells in the foam constituting the foamed layer. For this reason, even if corrosive components such as gas and elution impurities corrosive to the conductor are present in the fluororesin constituting the insulating layer, the surface portion of the conductor is sufficiently corroded by the corrosive component. To be suppressed. For this reason, it is possible to suppress an increase in the resistance of the surface portion of the conductor, and to sufficiently suppress the deterioration of the signal attenuation characteristics in the cable. In particular, when a high-frequency current is passed through a conductor covered with an insulating layer, this high-frequency current flows in a concentrated manner on the surface of the conductor due to the skin effect. It is extremely effective for suppression.

Further , since the corrosive component in the fluororesin is sufficiently reduced, the amount of the corrosive component capable of corroding the conductor surface is greatly reduced. For this reason, deterioration of the attenuation characteristic of a cable can be suppressed more fully.

  The cable may further include an outer conductor that surrounds the insulating layer, and a sheath that covers the outer conductor. In this case, the cable can be used as a coaxial cable.

  According to the present invention, even if a bare conductor is used as a conductor, a cable that can sufficiently suppress the deterioration of the signal attenuation characteristics is provided.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to FIG.

(cable)
FIG. 1 is a partial side view showing an embodiment of a cable according to the present invention, and FIG. 2 is a sectional view taken along line II-II in FIG. As shown in FIG. 1, the cable 10 of the present embodiment is a coaxial cable and includes an internal conductor 1. The inner conductor 1 is covered with a tubular insulating layer 2, and the insulating layer 2 is surrounded by a tubular outer conductor 3. The outer conductor 3 is covered with a tubular sheath 4.

  As shown in FIG. 2, the insulating layer 2 includes a tube-shaped foam layer 2a made of a foam containing a fluororesin and a tube-like non-foamed layer 2b made of a non-foamed material containing a fluororesin. It is composed of a laminate. The foam constituting the foam layer 2a has foam cells. Here, the non-foamed layer 2b is coaxially disposed inside the foamed layer 2a. That is, the non-foamed layer 2b is disposed between the inner conductor 1 and the foamed layer 2a.

  According to this cable 10, since the non-foamed layer 2b is disposed between the foam layer 2a and the inner conductor 1, the surface of the inner conductor 1 is exposed to the foam cells in the foam constituting the foam layer 2a. It will not be done. For this reason, even if corrosive components such as gas and elution impurities that are corrosive to the inner conductor 1 are present in the fluororesin constituting the insulating layer 2, the surface of the inner conductor 1 due to the corrosive component. Corrosion of the part is sufficiently suppressed. For this reason, it is possible to suppress an increase in the resistance of the surface portion of the inner conductor 1, and thus sufficiently suppress the deterioration of the attenuation characteristics of the cable 10. In particular, when a high-frequency current flows through the inner conductor 1 covered with the insulating layer 2, the high-frequency current flows concentrated on the surface portion of the inner conductor 1 due to the skin effect. The suppression is extremely effective for suppressing the deterioration of the attenuation characteristics.

  Next, each of the inner conductor 1, the insulating layer 2, the outer conductor 3, and the sheath 4 will be described in detail.

  The inner conductor 1 is a bare conductor. Here, the bare conductor is made of a single metal material and is not covered with a plated conductor made of a metal material different from this material. This is for greatly contributing to the cost reduction of the cable 10 and facilitating reuse by melting the inner conductor 1. Examples of the bare conductive wire include a copper wire, a copper alloy wire, and an aluminum wire, and the internal conductor 1 can be a single bare wire or a stranded wire. The bare conductor may be hollow or solid.

  The foam layer 2a constituting the insulating layer 2 is composed of a foam containing a fluororesin. The reason why it is made of a foam containing a fluororesin is that the fluororesin is excellent in heat resistance and dielectric properties.

  Examples of the fluororesin include tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA) and tetrafluoroethylene / hexafluoropropylene copolymer (FEP). In addition, as said perfluoroalkyl vinyl ether, perfluoropropyl vinyl ether, perfluoromethyl vinyl ether, etc. can be used conveniently.

The foaming degree of the foam is expressed by the following formula:
Foaming degree (%) = 100-100 × (specific gravity after foaming / specific gravity before foaming) (1)
Can be obtained. The foaming degree in the foam layer 2a is preferably 40% to 60%, more preferably 45% to 55%. When the foaming degree is within the above range, the loss reduction of the insulating layer 2 and the amount of the corrosive component can be well balanced, and the deterioration of the attenuation characteristic can be more sufficiently suppressed.

  In addition to the fluororesin, the foam may contain, for example, a resin generally used for coating the inner conductor 1, such as polypropylene and polyethylene.

  However, the content of the fluororesin in the foam is preferably 90% by mass to 100% by mass from the viewpoint of low loss and heat resistance of the insulating layer 2.

  The non-foamed layer 2b constituting the insulating layer 2 is made of a non-foamed material containing a fluororesin. The non-foamed material refers to one having a foaming degree of 0% obtained by the above formula (1). As the fluororesin constituting the non-foamed body, the same fluororesin as the foamed layer 2a can be used. The fluororesin contained in the non-foamed layer 2b may be the same as or different from the fluororesin contained in the foamed layer 2a, but the same is preferable because the adhesion between the foamed layer 2a and the non-foamed layer 2b is increased. .

  As a method for forming the insulating layer 2, the following method can be exemplified. One method includes a method in which the inner conductor 1 is coated with the non-foamed layer 2b and then the non-foamed layer 2b is coated with the foamed layer 2a.

  Here, in order to coat the inner conductor 1 with the non-foamed layer 2b, the fluororesin is heated with an extruder or the like to be in a molten state, and a tubular shape is formed without injecting a foaming agent into the molten fluororesin. What is necessary is just to carry out extrusion coating. At this time, it is preferable to evacuate the space between the tube-shaped fluororesin and the inner conductor 1. In this case, the coating time of the fluororesin on the inner conductor 1 can be shortened, and the degree of adhesion of the fluororesin can be increased.

  Further, in order to coat the non-foamed layer 2b with the foamed layer 2a, the fluororesin is heated with an extruder or the like to be in a molten state, a foaming agent is injected into the molten fluororesin, and the tube is extruded and coated. Good. Here, as the blowing agent, in addition to hydrocarbons such as propane, inert gases such as nitrogen gas and carbon dioxide can be used, but in view of cost and the like, it is preferable to use an inert gas. In addition, before inject | pouring a foaming agent into a fluororesin, it is preferable to mix a foaming nucleus agent in a fluororesin beforehand. In this case, foaming can be promoted starting from the foam nucleating agent. As the foam nucleating agent, inorganic substances such as boron nitride, silicon dioxide, titanium dioxide, and alumina can be used. For these, if a fluorinated fatty acid metal salt or the like is added as a dispersing agent that also functions as a foam nucleating agent, the dispersibility of the inorganic substance can be increased, and the entire fluororesin can be uniformly distributed. Foaming can occur. At this time, it is preferable that the space between the tube-shaped fluororesin and the non-foamed layer 2b is evacuated as described above.

  As another method, a common crosshead is connected to the first extruder into which the foaming agent is not injected and the second extruder into which the foaming agent is injected. This is a method in which the non-foamed layer 2b is extrusion coated from the first extruder onto the inner conductor 1 and then the foamed layer 2a composed of a foam is extrusion coated from the second extruder to the non-foamed layer 2b.

  As yet another method, the fluororesin is melted with an extruder, a foaming agent is injected into the fluororesin, and then the fluororesin into which the foaming agent has been injected is extrusion coated onto the internal conductor 1. In this method, when the fluororesin is extrusion coated on the inner conductor 1, the portion of the fluororesin close to the inner conductor 1 is cooled and foaming is suppressed, and foaming is likely to occur in a portion far from the inner conductor 1. become. As a result, the inner conductor 1 is covered with the non-foamed layer 2b, and the non-foamed layer 2b is covered with the foamed layer 2a.

  The fluororesin contained in the foam layer 2a and the non-foam layer 2b preferably has an elution fluoride ion concentration of 1.5 ppm or less and a heating weight change rate of less than 0.5% by mass. In this case, since the corrosive component in the fluororesin is sufficiently reduced, the amount of the corrosive component that can corrode the surface of the inner conductor 1 is greatly reduced. For this reason, deterioration of the attenuation characteristic of the cable 10 can be more sufficiently suppressed.

  Here, the “heating weight change rate” refers to the weight change rate before and after heating the fluororesin at 360 ° C. for 1 hour.

  “Eluted fluoride ion concentration” means that a fluororesin pellet is melted by heating at 380 ° C. for 5 minutes, and then the melt is cut to the size of the original pellet. After adding 10 ml of ORION's TISAB (II) to 10 ml of a mixed solution of 1: 1 by volume ratio, the mixture was stirred at room temperature (23 ° C. ± 2 ° C.) and allowed to stand for 24 hours. The value measured with a fluorine ion electrode manufactured by ORION.

  In addition, as described above, a fluororesin having an elution fluoride ion concentration of 1.5 ppm or less and a heating weight change rate of less than 0.5% by mass is commercially available and can be easily obtained. .

  The outer conductor 3 may be a known one that has been conventionally used. For example, the external conductor 3 can be configured by winding a conductive wire or a tape configured by sandwiching a conductive sheet between resin sheets along the outer periphery of the insulating layer 2. Further, the outer conductor 3 can be constituted by a corrugated metal tube, that is, a corrugated metal tube. In this case, the flexibility of the cable 10 can be improved.

  The sheath 4 protects the outer conductor 3 from physical or chemical damage, and examples of the material constituting the sheath 4 include fluororesin and polyethylene. From the viewpoint of heat resistance and dielectric characteristics, Is preferably a fluororesin.

  The present invention is not limited to the above embodiment. For example, in the above embodiment, the case where the cable 10 includes the inner conductor 1, the insulating layer 2, the outer conductor 3, and the sheath 4 is described. However, in the cable of the present invention, the outer conductor and the sheath are omitted. May be.

  Hereinafter, the content of the present invention will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

(Examples 1-4)
Two types of tetrafluoroethylene / perfluoroalkyl vinyl ether copolymers (PFAs) shown in Table 1 below were prepared as fluororesins. In Table 1, 440HP-J and 940HP-plus are processed to suppress elution of fluoride ions and generation of decomposition gas, and 440HP-J and 940HP-plus are as shown in Table 1. Are different from each other in terms of the elution fluoride ion concentration and the heating weight change rate.

  On the other hand, bare copper wires shown in Table 1 were prepared as the types of conductors. The fluororesin was put into an extruder and melted and kneaded at 360 ° C., and then coated on the conductor to form a non-foamed layer having a thickness of 70 μm on the conductor. Note that the blowing agent was not injected in the extruder.

  Next, a small amount of boron nitride as a foam nucleating agent was added to the fluororesin to prepare a foam compound.

  The foamed compound was charged into the extruder and melt-kneaded at 360 ° C., and then nitrogen gas was injected into the foamed compound at a pressure of 20 MPa to dissolve it. Subsequently, foamed cells were formed in the foamed compound and then coated on the non-foamed layer. Thus, a foamed layer having the thickness shown in Table 1 was formed on the non-foamed layer. A cable was thus obtained.

(Comparative Examples 1-4)
The non-foamed layer was not formed on the conductor, the foamed layer was formed directly on the conductor, and the type of conductor, conductor diameter, fluororesin constituting the foamed layer, and the thickness of the foamed layer were as shown in Table 1. Except for the above, a cable was obtained in the same manner as in Example 1.

  The following evaluation was performed about the cable obtained in Examples 1-4 and Comparative Examples 1-4.

(Discoloration of conductor)
About the cable obtained in Examples 1-4 and Comparative Examples 1-4, the heating test was done. At this time, the heating test was performed by leaving the cable in air at 105 ° C. for 1000 hours. And the discoloration of the conductor was evaluated by visually observing the surface of the conductor of the cable after the test. The results are shown in Table 1.

(Conductor resistance increase rate)
For the cable after the heating test, the resistance value R1 of the conductor was measured, and using this R1 and the initial resistance value R0 measured before the heating test, the rate of increase in the conductor resistance was measured according to the following formula. The results are shown in Table 1.
Rate of increase in conductor resistance = 100 × [(R1 / R0) −1]

  From the results shown in Table 1, in the cables of Examples 1 to 4, the resistance increase rate of the conductor is suppressed to be considerably small even when a bare conductor is used as the conductor as compared with the cables of Comparative Examples 1 to 4. It was found that the deterioration of the signal attenuation characteristics can be sufficiently suppressed.

  From this, it was confirmed that according to the cable of the present invention, it is possible to sufficiently suppress the deterioration of the signal attenuation characteristics even when a bare conductor is used as the conductor.

It is a partial side view which shows one Embodiment of the cable of this invention. It is sectional drawing along the II-II line of FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Internal conductor (conductor), 2 ... Insulating layer, 2a ... Foamed layer, 2b ... Non-foamed layer, 3 ... External conductor, 4 ... Sheath, 10 ... Cable.

Claims (2)

Conductors,
An insulating layer covering the conductor,
The insulating layer is
A foam layer composed of a foam containing a fluororesin;
Having a non-foamed layer composed of a non-foamed material containing a fluororesin,
The non-foamed layer is disposed between the foamed layer and the conductor ;
Both the fluororesin contained in the foam and the fluororesin contained in the non-foam have an elution fluoride ion concentration of 1.5 ppm or less and a change in heating weight of 0.02 to 0.04% by mass. Having a rate,
Cable characterized by. The conductor is an inner conductor;
The cable according to claim 1, further comprising an outer conductor surrounding the insulating layer and a sheath covering the outer conductor.