JP2008226772A - Foam-insulated wire - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a foam-insulated wire manufacturable with high productivity by suppressing generation of a blowhole inside while keeping the extent of foaming of the whole of a foaming insulation layer. <P>SOLUTION: This foam-insulated wire is formed by adding a forming nucleating agent in a base resin arranged on the circumference of a center conductor 11, and foaming the base resin by injecting gas therein. In the foam-insulated wire, the base resin is a polyethylene or polyolefin-based resin of which the melting tensile force before forming is 10 mN at 160°C; and a part or the whole of the base resin is foamed to concentrically form, on the circumference of the center conductor 11, a plurality of foaming insulation layers 13 and 14 formed of the polyethylene or polyolefin-based resin after foaming. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a foamed electric wire that can reduce loss during high-frequency transmission.

  In a high-frequency transmission cable that transmits power at a communication frequency in the GHz band, it is required that the dielectric constant ε and the dielectric loss tangent tanδ are small as the electrical characteristics of the insulator covering the central conductor, and in particular, the dielectric loss tangent tanδ is small. Desired.

  The transmission loss is expressed by the following equation. The transmission loss is proportional to the frequency fw and tan δ, and is proportional to the square root of ε. Therefore, tanδ is relatively important.

(Transmission loss) ∝ √ε × fw × tanδ
To reduce ε and tan δ, it is effective to use polyethylene (PE) as an insulator and foam the PE to form a foamed electric wire. By foaming PE and forming bubbles inside the insulator, ε and tanδ of the insulator can be made smaller than when using PE alone (solid PE with no bubbles), thus increasing the transmission signal frequency. Along with this, the importance of foamed electric wires is increasing.

  For example, Patent Document 1 discloses a small-diameter foamed electric wire obtained by extruding a polyolefin resin to high foam.

  Patent Document 2 discloses that an insulating layer is formed by using a filler having a low affinity with a fluororesin as a foam nucleating agent and uniformly dispersing fine bubbles.

  Patent Document 3 discloses that a bubble distribution is uniform by adding a foam nucleating agent having a particle size of 1 to 2 μm to a resin.

  Patent Document 4 discloses a high-frequency coaxial cable in which a foamed insulating layer is formed on the outer periphery of a conductor. It is characterized in that the density of the resin used for the foamed insulating layer, the ratio of the number of long chain branches, the relationship between melt tension and MFR (Melt Flow Rate), and the endothermic peak in DSC measurement are one.

  Patent Document 5 discloses a high foam insulated electric wire having an internal foamed polyethylene layer on a core wire and an external high foamed polyethylene layer outside the internal foamed polyethylene layer. This highly foamed insulated electric wire is obtained by adding a chemical foaming agent to the polyethylene layer and chemically foaming it.

JP-A-11-213759 International Publication No. 2003/000792 Pamphlet JP 2004-171942 A JP 2006-100160 A Japanese Patent Laid-Open No. 7-326241

  However, a highly foamed foamed wire with low transmission loss has a problem that it is difficult to manufacture and productivity is low. Specifically, a so-called “nest” in which a plurality of bubbles break and coalesce inside the foam layer, or giant bubbles are generated due to abnormal growth of the bubbles, resulting in deterioration of electrical characteristics such as transmission loss, outer diameter fluctuations, machine It causes a decrease in the strength of the target.

  This problem is caused when coating the PE with the foaming gas injected during the manufacture of the electric wire (at the time of extrusion of the molten PE) on the central conductor, and cooling and solidifying while growing the bubbles. Also occurs because it takes time until the cooling effect appears. That is, the cooling is slow inside and PE maintains a molten state for a long time, so that bubbles grow excessively and a nest is easily generated.

  For this reason, a normal foamed electric wire has a large bubble near the center conductor and a high foaming degree, and the bubble diameter in the outer layer portion tends to be small and the foaming degree tends to be low. Therefore, in order to produce a high-performance foamed wire with low transmission loss in the high-frequency region, it is necessary to achieve both a high degree of foaming and no generation of nests inside the foamed layer, but the production conditions are stably maintained. Since it is difficult to do so, productivity is lowered.

  Accordingly, an object of the present invention is to provide a foamed electric wire that solves the above-mentioned problems, suppresses the formation of nests inside, and can be manufactured with high productivity while maintaining the foaming degree of the entire foamed insulating layer. There is to do.

  In order to achieve the above object, the foamed electric wire of the present invention is a foamed electric wire formed by adding a foam nucleating agent to a base resin provided on the outer periphery of a central conductor and injecting a gas to foam the base resin. The base resin is a polyethylene or polyolefin resin having a melt tension of 10 mN at 160 ° C. before foaming. A part or all of the base resin is foamed, and a foam insulating layer made of polyethylene or polyolefin resin after foaming is formed. A plurality of concentric layers are formed on the outer periphery of the central conductor.

  The polyethylene may be a single polyethylene or a blend of two or more types of polyethylene.

  It is preferable that the minimum thickness of each foam insulating layer is 10% or more of the total thickness of the foam insulating layer.

  Also, an inner skin layer made of a solid polyethylene layer formed immediately above the central conductor, an outer skin layer made of a solid polyethylene layer formed on the outer periphery of the outermost foam insulation layer, and a plurality of foam insulation layers You may have at least one of the intermediate | middle skin layers which consist of a solid polyethylene layer formed.

  According to the present invention, an excellent effect that a foamed electric wire having a high foaming degree and a small transmission loss can be produced with high productivity is exhibited.

  Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a sectional view showing a preferred embodiment of a foamed electric wire according to the present invention.

  A foamed electric wire 10 according to the present embodiment has a foamed insulating layer 12 having a large number of bubbles on a central conductor 11. The foam insulating layer 12 is formed concentrically on the outer periphery of the center conductor 11, and includes an inner foam layer 13 formed on the outer periphery of the center conductor 11 and an outer foam layer 14 formed on the outer periphery of the inner foam layer 13. Become.

  In the present embodiment, an inner skin layer 15 made of a solid polyethylene layer is formed immediately above the center conductor 11 (between the center conductor 11 and the inner foam layer 13), and the inner foam layer 13 and the outer foam layer 14 are formed. In between, an intermediate skin layer 16 made of a solid polyethylene layer is formed. Further, an outer conductor 17 is formed on the foamed insulating layer 12, and an outermost coating layer (sheath layer) 18 is coated on the outer conductor 17 to constitute the foamed electric wire 10 (coaxial cable).

  Here, the foamed insulating layer 12 will be described.

  The foamed insulating layer 12 is formed by adding a foaming nucleating agent to the base resin and injecting a foaming gas when the base resin is extrusion coated on the central conductor 11.

  The ratio of the thickness of each layer (for example, the internal foam layer 13 and the external foam layer 14) of the foam insulating layer 12 is not particularly limited. However, if one of the layers is extremely thin, the effect of forming the foamed insulating layer 12 with a plurality of layers is reduced. Therefore, the minimum thickness of each layer is 10% or more of the thickness of the foamed layer insulating 12. Is desirable.

  As the base resin constituting the foamed insulating layer 12, polyethylene or polyolefin resin is used. These polyethylene or polyolefin resins are selected from materials having a melt tension before foaming of 10 mN or more at 160 ° C.

  This material is not particularly defined as long as it has polyethylene having a melt tension. LDPE (low density polyethylene), HDPE (high density polyethylene), LLDPE (linear low density polyethylene), MDPD (medium density polyethylene) , UHMWPE (ultra high molecular weight polyethylene) and the like, and those obtained by individually or blending a plurality of types of polyethylene. Examples of the polyolefin resin include polypropylene, polyester, polymethylpentene-1, and the like. These materials can each be adjusted to a desired melt tension when forming the insulating layer (foamed insulating layer 12).

  The material of the foamed insulating layer 12 is considered to have a standard of 10 mN (@ 160 ° C.) or more because it is easier to control the generation and growth of bubbles when it has a certain melt tension as a practical characteristic. . Even if the melt tension is less than this, the foamed insulating layer 12 can be molded by changing the conditions such as lowering the pressure of the injected gas. However, since the manufacturing conditions are not sufficient, it is not practical. In order to obtain a melt tension of 10 mN or more as a guide, it is possible to appropriately blend (mix) the above-mentioned various PEs in addition to selecting PE satisfying a condition with a single grade.

  The melt tension of the material forming the foamed insulating layer 12 is measured under the conditions shown in Table 1 below using a capillary rheometer.

Next, the manufacturing method of the foamed electric wire of this Embodiment is demonstrated.
(1) The inner skin layer 15 and the inner foam layer 13 are coated on the center conductor 11. The inner skin layer 15 may be formed simultaneously with the inner foam layer 13 on the center conductor 11, or may be coated on the center conductor 11 in advance before the inner foam layer 13 is formed.
(2) The intermediate skin layer 16 is coated on the internal foam layer 13.
(3) The outer foam layer 14 is coated on the intermediate skin layer 16.
The steps (1) to (3) may be sequentially performed from the steps (1) to (3), or the steps (1) to (3) may be performed simultaneously.
(4) The outer conductor 17 and the outer covering layer (sheath layer) 18 are formed on the outer periphery of the outer foam layer 14 to obtain the foamed electric wire 10 of the present embodiment.

  Foaming nucleating agents used for foaming the base resin include BN (boron nitride), talc, inorganic fine particles typified by silica fine particles, fluororesin fine particles, ADCA (azodicarbonamide), OBSH (p, Organic fine particles such as p′-oxybisbenzenesulfonylhydrazine) may be used and are not particularly limited.

As the gas used as the foaming agent (foaming gas), a so-called inert gas such as N ₂ or Ar is suitable, but a desired gas such as CO ₂ , isobutane gas, or compressed air may be used.

  The injection pressure of the foaming gas can be set according to the desired degree of foaming, the melt tension of the material used, and the type and amount of the foam nucleating agent.

  The specific range of the optimal foaming gas injection pressure varies greatly depending on factors such as the size of the extruder (cable diameter), the type and grade of resin, the target degree of foaming, and the production speed. The “optimal gas pressure range for the structure” is 3 MPa to 150 MPa. If the gas injection pressure is less than 3 MPa, sufficient bubbles for obtaining the effect of the foamed electric wire are not formed. On the other hand, if the gas injection pressure exceeds 150 MPa, the bubbles are excessively large and cannot be maintained. is there.

  Further, the gas injection pressure at the time of forming the inner foam layer 13 may be set to a low gas pressure in anticipation of the cooling delay with respect to the outer foam layer 14. Further, the gas injection pressure of the external foam layer 14 may be set lower than the gas injection pressure of the internal foam layer 13 in order to prevent gas escape when forming the external foam layer 14.

  The operation of the present embodiment will be described.

  In the present embodiment, the foam insulating layer 12 is formed using a “physical foaming method” in which a foam is obtained by injecting a foaming gas and adding a foam nucleating agent. For example, in the chemical foaming method, a chemical foaming agent is used (foaming is carried out by chemical reaction in the base resin), resulting in foaming residues, which lower the electrical characteristics (dielectric constant ε, dielectric loss tangent tanδ) of the foamed wire. Sometimes. In contrast, in the physical foaming method, the amount of the foam nucleating agent used is as small as about 1/10 to 1/100 compared with the case of using the chemical foaming agent, so that the deterioration of electrical characteristics is minimized. Can be suppressed.

  The temperature at which the foamed insulating layer 12 is formed is an item (parameter) only for suppressing the viscosity of the base resin. That is, the foamed electric wire 10 according to the present embodiment has a secondary effect that can ease restrictions on temperature control during extrusion of the base resin.

Conventional foamed electric wires are generally effective in preventing abnormal growth of bubbles because the material with increased melt tension has a large viscous resistance when the resin is stretched against the growth of bubbles. The goal was to control bubble growth only at high melt tension. However, in a thick cable (when the foam insulation layer is thick), (1) foaming starts immediately after extrusion (2) because it enters the cooling water immediately, the outer layer portion of the foam layer hardens while the internal temperature does not drop easily (foaming) The layer will be a good insulation)
(3) In the inner periphery of the foam layer, the resin will remain in a molten state for a long time, and the growth of bubbles is likely to continue. (4) Gas that has nowhere to go because the outer layer is cured is a cause of “nest” such as abnormal growth of bubbles. There was a possibility.

  On the other hand, in the foamed electric wire 10 of the present embodiment, even if the cable (particularly the foam insulation layer) is thickened, the foam insulation layer 12 is formed in a plurality of layers concentrically with the central conductor. Therefore, in particular, the internal foamed layer can be set to a low gas pressure in anticipation of the cooling delay (in conventional foamed electric wires, when the foamed insulating layer is formed by setting the gas pressure low, the foamed insulating layer as a whole Shortage).

  Therefore, a sufficient degree of foaming can be obtained even if the gas pressure is lowered, so that it is not necessary to increase the viscous resistance of the resin against the foaming gas, and the range of material selection is expanded. Therefore, if an appropriate gas pressure (and other production conditions) can be selected, materials can be selected in a wide range of a melt tension of 10 mN (160 ° C.) or more.

  As described above, according to the foamed electric wire 10 of the present embodiment, a polyethylene or polyolefin resin having a melt tension of 10 mN at 160 ° C. before foaming is used, and a plurality of foamed insulating layers are concentrically formed on the outer periphery of the center conductor. Thus, while maintaining the foaming degree of the entire foamed insulating layer, it is possible to suppress the generation of nests inside and to manufacture with high productivity.

  In the present embodiment, the foamed insulating layer 12 has a two-layer structure of an inner foamed layer 13 and an outer foamed layer 14, but when the foamed insulating layer 12 is thick, the foamed insulating layer 12 may be divided into three or more layers. Is possible. As long as the configuration of the extruder (resin injection pressure, gas injection pressure, foam nucleating agent, foaming agent (type of foaming gas), etc.) can be adjusted, the foaming degree of multiple layers of foam insulation layers is finely adjusted for each foam insulation layer. it can.

  Further, at least one skin layer made of solid polyethylene may be formed on the inner circumference or the outer circumference of each foam insulation layer (inner foam layer 13, outer foam layer 14) of the foam insulation layer 12.

  For example, an internal skin layer 15 may be provided between the central conductor 11 and the foamed insulating layer 12 in order to improve the adhesion between the central conductor 11 and the foamed insulating layer 12. An outer skin may be provided on the outer periphery of the foamed insulating layer 12 to prevent fluctuations in the outer diameter and to maintain foaming gas and improve foaming efficiency. A skin layer 16 may be provided between the inner foam layer 13 and the outer foam layer 14. Providing the intermediate skin layer 16 can effectively prevent the internal foamed layer 13 from degassing when forming a foam layer having an ultra-high foaming degree.

  Each foamed insulating layer may be formed of the same material or different materials.

  In Examples 1 to 9 and Comparative Examples 1 to 7, a high-frequency coaxial cable of 20D (center conductor diameter 9.0 mm, insulating layer thickness 7.0 mm, outer conductor diameter 25.0 mm, sheath diameter 28.0 mm) was prototyped. Evaluation was performed. Table 2 shows the parameters of the prototype high-frequency coaxial cable.

  The nucleating agent and foaming agent used when forming the foamed insulating layer, the inner skin layer and the outer skin layer are shown.

  Examples and comparative examples were prototyped by varying the presence or absence of the internal skin layer and the external skin layer and the nitrogen gas injection pressure as parameters.

  As the material of the foam insulating layer, the materials shown in Table 4 were blended to obtain a material having a desired melt tension.

  In Examples 1 to 8, in order to clearly show the effects of the invention, the inner foamed insulating layer and the outer foamed insulating layer were made of the same material. In Example 9, the inner, intermediate and outer foamed insulating layers were made of the same material.

  FIG. 2 shows an outline of the prototype device and the trial procedure of the example.

  As shown in FIG. 2, first, the central conductor is supplied from the feeder 21, and if necessary (Examples 3 to 9 and Comparative Examples 2 and 4 to 7), an internal skin layer is formed in the internal skin extruder 22. Coating. At that time, the coating thickness is about 50 μm.

  Subsequently, the foaming insulating layer is covered by the main extruder 23. The main extruder 23 can simultaneously extrude three layers, and simultaneously coats an internal foam layer, an external foam layer, and, if necessary, an external skin layer.

  After the coating of each layer is completed, it is cooled in the cooling bath 24 and wound up by the wire winder 25. Thereafter, an outer conductor and a sheath layer are provided, and various characteristics of the obtained foamed electric wire (coaxial cable) are evaluated.

The evaluation items are as follows.
(1) Foaming degree (%)
Ten thousand meters are manufactured, and the foaming degree is measured at a total of 10 places (by alcohol specific gravity method) every 1000 m, and the difference between the average value and the maximum value and the minimum value is shown as the fluctuation amount.

  When it has a skin layer, the degree of foaming including that is shown.

  Evaluation was made by an alcohol specific gravity method (a method for obtaining a solid specific gravity from a difference in weight between air and liquid. A specific gravity meter uses a DH type manufactured by Toyo Seiki).

A foam having a foaming degree of at least 65% is accepted.
(2) Presence / absence of giant bubbles In the foamed insulating layer (particularly in the vicinity of the central conductor), the presence or absence of abnormal bubble growth or bubble breakage is confirmed using a microscope or an electron microscope.

Giant bubbles can actually be classified into the following two types according to the background.
1. Overgrowth type: One bubble grows large and is several times larger than other bubbles. Because it is a single bubble, the outer circumference draws a relatively beautiful ellipse.
2. Combined type (“nest”): A combination of adjacent bubbles that grow. The outer periphery tends to be uneven, leaving the shape of each bubble.

  In the present embodiment, bubbles having a diameter exceeding approximately 1.5 mm are collectively treated as giant bubbles.

  The size of a giant bubble is a factor of both the number and size of the bubble. Therefore, the determination of the presence or absence of bubbles is expressed by the area occupied by the giant bubbles in the cross-sectional area of the foamed insulator.

The smaller the number of giant bubbles, the better. The minimum (small) or less is acceptable.
(3) Bubble diameter (μm)
The respective bubble diameters in the vicinity of the center conductor of the inner foam layer and the outer periphery of the outer foam layer are measured. The bubble diameter should be as small as possible, and it is desirable not to exceed 800-1000 μm.
(4) Transmission loss (dB/100m@2.2μm)
This is a loss when a 2.2 GHz signal wave is transmitted through a coaxial cable. The transmission loss is desirably 6.5 (dB / 100 m@2.2 GHz) or less.
(5) Comprehensive judgment Production stability (yield) and characteristics were comprehensively evaluated.

  Even if the manufactured foamed electric wire had high performance, the evaluation was low for those with low manufacturing stability.

(Four-grade evaluation from x to ◎. Applicable to ○ or more)
Manufacturing stability Manufacturing stability is evaluated based on the amount of change in foaming, outer diameter, etc. during manufacturing. In this embodiment, since the degree of foaming is particularly focused, the emphasis is placed on the degree of foaming and the amount of variation with respect to the degree of foaming.

  Table 6 shows the materials, conditions, and evaluation results of Examples 1 to 9 and Comparative Examples 1 to 7.

Comparison between Examples and Comparative Examples In Examples 1 to 4, in order to mold the internal foamed layer in which bubbles tend to grow at a low gas pressure and the external foamed layer at a high gas pressure, variations in the bubble diameters at the inner and outer peripheral parts are performed. And abnormal growth can be suppressed. For this reason, it was confirmed that the characteristics of the product were stabilized and the productivity was greatly improved.

  On the other hand, in Comparative Examples 1 to 7 in which the foamed insulating layer is one layer, as pointed out in the problems of the prior art, if the foaming degree is increased in order to reduce the transmission loss, the growth in the inner layer proceeds and the huge It tends to become bubbles.

  Examples 5-8 and Comparative Examples 5-7 compare the foamed states by changing the melt tension of polyethylene used in the foamed layer.

  Examples 5 and 6 and Comparative Examples 5 and 6 are examples using a low melt tension material, and since the growth of bubbles proceeds rapidly, the occurrence of giant bubbles in the inner layer is unavoidable. In Examples 5 and 6, the degree is small and acceptable. Further, by reducing the gas pressure of the outer foam layer, the degree of foaming can be prevented from degassing, and the degree of foaming that can withstand practical use can be maintained.

  In Example 7 and Comparative Example 7, the same high melt tension material was used for comparison. In Example 7, it was confirmed that coalescence of the internal foamed layer could be prevented by maintaining a lower gas pressure. In contrast, in Comparative Example 7, the melt tension is high, and bubbles hardly grow on the outer peripheral portion.

  Furthermore, in Example 7, since the degree of foaming was slightly reduced, it was confirmed that the gas pressure in the outer peripheral portion was increased in Example 8 to maintain a balance between characteristics and productivity. In Example 9, three foamed insulating layers were formed, and finer gas pressure adjustment was performed, thereby succeeding in forming more uniform bubbles.

  As described above, according to the present invention, the conventional technology with only one foam layer solves the problem that it was difficult to achieve both productivity and transmission performance, and stabilizes a foamed wire with low transmission loss. And has the effect of enabling production.

It is sectional drawing which shows the foamed electric wire which concerns on suitable one Embodiment of this invention. It is a block diagram which shows the manufacturing method of a foamed electric wire.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Foam electric wire 11 Center conductor 12 Foam insulation layer 13 Internal foam layer 14 External foam layer 15 Internal skin layer 16 Intermediate skin layer 17 External conductor 18 Sheath layer (outer covering layer)

Claims (4)

In the foamed electric wire formed by adding a foam nucleating agent to the base resin provided on the outer periphery of the center conductor, injecting a foaming gas and foaming the base resin,
The base resin is a polyethylene or polyolefin resin having a melt tension of 10 mN at 160 ° C. before foaming, foaming a part or all of the base resin, and forming a foam insulating layer made of polyethylene or polyolefin resin after foaming, A foamed electric wire comprising a plurality of concentric layers formed on the outer periphery of the central conductor.   The foamed electric wire according to claim 1, wherein the polyethylene is a single polyethylene or a blend of two or more types of polyethylene.   The foamed electric wire according to claim 1 or 2, wherein the minimum thickness of each foamed insulating layer is 10% or more of the total thickness of the foamed insulating layer.   An inner skin layer made of a solid polyethylene layer formed immediately above the central conductor, an outer skin layer made of a solid polyethylene layer formed on the outer periphery of the outermost foamed insulating layer, and formed between a plurality of foamed insulating layers. The foamed electric wire according to claim 1, which has at least one of an intermediate skin layer made of a solid polyethylene layer.

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