transmission line

Abstract

[Problem] To provide a transmission line having excellent transmission characteristics. [Solution] Provided is a transmission line comprising a conductor and a dielectric layer, wherein the transmission line is characterized in that the dielectric layer has a plurality of air bubbles and a plurality of needle-shaped nucleating agents and, when nucleating agents longer than the median length are classified as a long nucleating agent group and nucleating agents shorter than the median length are classified as a short nucleating agent group based on the observed length of the plurality of needle-shaped nucleating agents observed in a cross-section of the dielectric layer, the average length of nucleating agents of the long nucleating agent group is 3.5 times or less the average length of nucleating agents of the short nucleating agent group.

Description

[Technical Field]

[0001] The present disclosure relates to a transmission line having a dielectric layer, and more particularly to a transmission line including a dielectric layer having bubbles and a conductor, the conductor being a transmission path for an electrical signal. [Background technology]

[0002] Foamed dielectrics, which contain air bubbles in a resin, can achieve a dielectric constant lower than that of the resin itself, making them useful as dielectrics for transmission paths of high-frequency electrical signals. For example, by using such foamed dielectrics as an insulating layer formed between the center conductor and outer conductor of a coaxial cable, a cable with low electrical signal attenuation and excellent transmission characteristics can be obtained. In these applications, attempts are being made to improve the transmission characteristics of the transmission line by controlling the foaming state of the foamed dielectric. For example, the bubbles in the foam must be fine and uniformly distributed. Additives called nucleating agents can be used to form such foams.

[0003] JP 2008-174752 A discloses a resin composition containing a fluororesin and boron nitride, in which the boron nitride has a d99 of 15 μm or less. This composition is said to provide a foam with fine bubbles uniformly distributed. In this disclosure, the foam nucleating agent functions as a starting point for generating bubbles during foam molding.

[0004] Japanese Patent Laid-Open No. 2005-206745 discloses a foamable composition containing a fluororesin and electrically insulating whiskers, a foam obtained by foaming the foam, and a coaxial insulated cable using a foam layer containing a fluororesin as an insulator. It has been found that the use of whisker-like insulating whiskers allows for a high expansion rate and the formation of fine bubbles. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-174752 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-206745 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the foam compositions described in these patent documents and the transmission lines using them as dielectric layers still have room for improvement in terms of cell shape and transmission characteristics.

[0007] The present invention provides a transmission line having a foamed dielectric layer with excellent cell shape and excellent transmission characteristics. [Means for solving the problem]

[0008] In order to solve the above problems, the configurations described in the claims can be adopted. A transmission line comprising a conductor and a dielectric layer, wherein the dielectric layer has a plurality of bubbles and a plurality of needle-shaped nucleating agents, and when, based on the observed lengths of the plurality of needle-shaped nucleating agents observed in the cross section of the dielectric layer, needle-shaped nucleating agents longer than the median length are classified as a long nucleating agent group, and needle-shaped nucleating agents shorter than the median length are classified as a short nucleating agent group, the average length of the needle-shaped nucleating agents in the long nucleating agent group is 3.5 times or less than the average length of the needle-shaped nucleating agents in the short nucleating agent group. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is an example of a cross-sectional view illustrating a transmission line 10 according to a first embodiment of the present disclosure. [Figure 2] FIG. 2 is a conceptual diagram for explaining details of a dielectric layer 30 of the transmission line 10. [Figure 3] FIG. 1 is a diagram conceptually illustrating the action of an acicular nucleating agent during bubble growth. [Figure 4] Electron microscope image of needle-shaped nucleating agents intersecting the bubble interface. [Figure 5]FIG. 1 is a diagram illustrating the formation process of needle-shaped nucleating agents that intersect with the bubble interface. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments (or examples) of the present disclosure will be described in detail with reference to the drawings. Note that the embodiments described below do not limit the scope of the invention as claimed, and not all combinations of features described in the embodiments are necessarily essential to the solution of the invention. Furthermore, the respective embodiments in each example may be freely combined within the scope that does not lose the technical significance of the present invention.

[0011] (First embodiment) In the first embodiment of the present disclosure, the dielectric layer of the transmission line has a plurality of acicular nuclei, and among these acicular nuclei, a group of acicular nuclei with a large length has a suppressed average length compared to a group of acicular nuclei with a small length. That is, a transmission line comprising a conductor and a dielectric layer, the dielectric layer having a plurality of bubbles and a plurality of needle-shaped nucleating agents, and based on the observed lengths of the plurality of needle-shaped nucleating agents observed in a cross section of the dielectric layer, when a group consisting of a plurality of needle-shaped nucleating agents longer than the median length is classified as a long nucleating agent group and a group consisting of a plurality of needle-shaped nucleating agents shorter than the median length is classified as a short nucleating agent group, the average length of the needle-shaped nucleating agents in the long nucleating agent group is 3.5 times or less the average length of the needle-shaped nucleating agents in the short nucleating agent group. This makes it possible to obtain a transmission line with excellent transmission characteristics.

[0012] 1 is an example of a cross-sectional view illustrating a transmission line 10 according to a first embodiment of the present disclosure. The signal transmission direction of the transmission line shown in this figure is perpendicular to the plane of the paper, that is, this figure shows the cross section of the transmission line in a plane perpendicular to the longitudinal direction of the cable-like transmission line. The transmission line 10 has a core 40 including a central conductor 20 and a dielectric layer 30 formed around the central conductor 20. The central conductor 20 can be a thin metal wire containing a low-resistance metal such as silver or copper. The dielectric layer 30 is made of an insulating material such as resin, and contains many fine bubbles (not shown) inside, thereby forming a layer with a low dielectric constant. The transmission line 10 may further include a coating layer formed around the core 40. In this figure, the transmission line 10 has a coaxial cable structure including an outer conductor 50 formed around the core 40 and an outer jacket 60 formed around the outer conductor 50.

[0013] 2 is a conceptual diagram for explaining the details of the dielectric layer 30 of the transmission line 10. For example, it shows the region 310 indicated by the dashed line in FIG. The dielectric layer 30 includes multiple bubbles 320 formed in the resin layer 340. For simplicity, the bubbles are shown as independent spheres and ellipsoids with circular and elliptical cross sections, respectively. However, they may be partially connected to adjacent bubbles or may have irregularly deformed shapes. From the perspective of mechanical properties, the more circular the cross-sectional shape of the bubbles, the better. Meanwhile, in the case of a transmission line dielectric, increasing the proportion of bubbles in the dielectric layer has the advantage of reducing the dielectric constant. Therefore, it is preferable to increase the proportion of bubbles until the resin layer between adjacent bubbles is stretched into a film-like shape, creating multiple regions with a planar extension of approximately uniform thickness.

[0014] The dielectric layer 30 also includes a plurality of needle-shaped nucleating agents 330. Here, the term "needle-shaped nucleating agents" refers to fine needle-shaped objects made of an insulating material with an aspect ratio of 3 or more, preferably 6 or more. The aspect ratio is the value obtained by dividing the length of the needle-shaped nucleating agent in the extension direction, i.e., the length in the major axis direction, by the diameter (or width). For example, when the needle-shaped nucleating agent is placed on a sample stage, the size of the needle-shaped nucleating agent in the extension direction is the length, and the size in the direction perpendicular to the extension direction is the diameter. The needle-shaped nucleating agents used in the dielectric layer of the transmission line must be fine to suppress an increase in the dielectric constant of the dielectric layer. Specifically, the diameter is preferably 0.1 μm (micrometer) or more and 10 μm or less, more preferably 0.1 μm or more and 3 μm or less, and particularly preferably 0.1 μm or more and 1 μm or less. For example, electrically insulating whiskers as disclosed in Patent Document 2 can be used as the material for such needle-shaped nucleating agents.

[0015] The needle-shaped nucleating agents 330 are preferably uniformly dispersed in the resin layer 340. The direction and distribution position of the needle-shaped nucleating agents do not need to have a clear regularity. However, the length distribution of the individual needle-shaped nucleating agents in the transmission line 10 in this embodiment has the characteristics described below. In other words, when the length of the needle-shaped nucleating agent observed in the cross section of the dielectric layer of the transmission line 10 is taken as the observed length, and needle-shaped nucleating agents longer than the median are classified as the long nucleating agent group, and needle-shaped nucleating agents shorter than the median are classified as the short nucleating agent group, the average length of the needle-shaped nucleating agents in the long nucleating agent group is 3.5 times or less than the average length of the needle-shaped nucleating agents in the short nucleating agent group. The transmission line 10 having such characteristics makes it possible to obtain excellent transmission characteristics. Furthermore, from the viewpoint of obtaining even more stable and excellent transmission characteristics, it is more preferable that this ratio, i.e., the value obtained by dividing the average length of the acicular nucleating agents in the long nucleating agent group by the average length of the acicular nucleating agents in the short nucleating agent group, be 3 or less, and it is particularly preferable that it be 2.7 or less.

[0016] Here, the observation length is the length obtained by observing the cross section of the dielectric using, for example, a scanning electron microscope (SEM). Preferably, an electron microscope image of the cross section is obtained, and the observation length is the extension direction length of each of the multiple needle-shaped nucleating agents measured in that image. The observation direction and the actual extension direction of the needle-shaped nucleating agents are not necessarily perpendicular to each other, and part of the entire length of the needle-shaped nucleating agents may be hidden by the shadow of the resin or other nucleating agents, so the observation length is in most cases shorter than the actual length of the needle-shaped nucleating agents. The median is the middle value when the observation length values of each nucleating agent are arranged in order of size. If the number of data points is even, the average of the two middle values can be used.

[0017] The mechanism by which the transmission line 10 having the above characteristics achieves excellent transmission characteristics is believed to be, but not limited to, the contribution of the mechanism described below. The inventors of the present application compared prototype levels using various materials such as nucleating agents and resins, as well as varying the manufacturing conditions of the transmission line. Through these evaluations, they confirmed that acicular nucleating agents can provide transmission lines with excellent transmission characteristics, and also found that there were differences in the smallness of the dielectric constant and its stability when comparing transmission lines using acicular nucleating agents.

[0018] To address this issue, we first considered the role of needle-shaped nucleating agents in bubble formation in dielectric layers. It is generally believed that the surface of a nucleating agent in a resin layer can serve as a starting point for bubble generation. Dispersing nucleating agents in a resin layer simultaneously generates multiple starting points for bubble generation throughout the resin layer, resulting in a foam containing widely and uniformly dispersed bubbles with minimal variation in diameter. Considering the shape of the nucleating agent from this perspective, needle-shaped nucleating agents with a large aspect ratio are considered to be more effective than granular nucleating agents with an aspect ratio close to 1. That is, even if the weight of each individual nucleating agent is the same, the increased surface area of the nucleating agent can expand the area that serves as a starting point for bubble generation. Furthermore, for example, when the same mass and number of nucleating agents are distributed in a given volume of resin, nucleating agents with a higher aspect ratio can increase the area adjacent to the nucleating agent in the resin layer. In transmission lines, where the dielectric constant deteriorates simply by increasing the amount of additive, the use of acicular nucleating agents with a specified aspect ratio, which can increase the area and range of existence of the additive per unit weight, is thought to be a great advantage.

[0019] The inventors of the present application have considered that the needle-shaped nucleating agent has a favorable effect not only in the bubble generation stage described above but also in the bubble growth stage. Figure 3 is a diagram conceptually explaining the effect of the needle-shaped nucleating agent during bubble growth. Figure 3(a) shows the state immediately after bubble generation, and Figure 3(b) shows the state after the bubble has grown over a short period of time. The resin layer (340) is a resin layer in a molten state during bubble generation and bubble growth in the manufacturing process of the transmission cable 10 (for example, the foamed resin layer formation process described below). When this layer cools and solidifies, it becomes the resin layer 340 of the transmission cable 10. The molten resin layer (340) contains bubbles 320 and needle-shaped nucleating agents 330. In addition to the illustrated needle-shaped nucleating agents 330, there may be other needle-shaped nucleating agents that are the origin of the bubbles 320, but these are not shown here. At the stage shown in FIG. 3(a), the bubbles 320 and the illustrated needle-shaped nucleating agents 330 are spaced apart to a certain extent, and the needle-shaped nucleating agents 330 do not significantly affect the growth of the bubbles 320. As the bubbles 320 grow over time, they eventually come into contact with the needle-shaped nucleating agents 330, either directly or through a thin resin film. As shown in FIG. 3(b), even after contact, the bubbles 320 continue to grow while the needle-shaped nucleating agents 330 rotate and move. However, because the needle-shaped nucleating agents must move, growth is suppressed compared to when the needle-shaped nucleating agents 330 are not present. The more a bubble grows and enlarges compared to other bubbles, the greater the probability of contact with the needle-shaped nucleating agent, and therefore the growth of such bubbles that have grown first is more easily suppressed, thereby reducing the size difference between the bubbles. In this case, the needle-shaped nucleating agent exists over a longer distance than the granular nucleating agent, so it is more likely to increase the probability of contact with bubbles, and furthermore, the rotational and movement resistance after contact is also greater, so it is thought that the effect of suppressing the growth of bubbles that grow first is also significantly greater. In particular, when needle-shaped nucleating agents are arranged along the resin film between the bubbles, the mechanical strength of the resin film between the bubbles increases, which is expected to have the effect of suppressing the coalescence of bubbles due to film rupture.

[0020] Through observation of evaluation samples, the inventors of the present invention discovered the presence of needle-shaped nucleating agents fixed in the resin in a unique state. In electron microscope observations, most needle-shaped nucleating agents are observed at an angle that follows the film of the resin layer or the inner surface of the bubbles. However, it was discovered that, although rare, there are needle-shaped nucleating agents in which a portion of their entire length is embedded in the resin layer and the remainder protrudes into the inner surface of the bubbles, i.e., they intersect with the bubble interface. Figure 4 is an electron microscope image of a needle-shaped nucleating agent intersecting with the bubble interface. In this image, the rod-shaped object observed near the center of the image, extending slightly inclined in the vertical direction of the page, is the needle-shaped nucleating agent. It can be seen that a portion of the needle-shaped nucleating agent located in the upper left corner of the page is embedded in the resin layer, and the remainder protrudes into the space inside the bubbles.

[0021] When needle-shaped nucleating agents intersect with bubble interfaces in this manner, the contribution of the regions of the needle-shaped nucleating agents protruding into the bubbles to the aforementioned bubble growth suppression effect is significantly reduced. As a result, if the variation in bubble diameter increases, the difference in dielectric constant between regions of the transmission line may increase and the transmission characteristics may deteriorate. Furthermore, at least the needle-shaped nucleating agents in the regions protruding into the bubbles do not particularly contribute to improving the transmission characteristics or mechanical properties, but their presence increases the dielectric constant. Therefore, it is preferable to suppress the occurrence of such needle-shaped nucleating agents intersecting with bubble interfaces. In considering this suppression method, we considered how the intersection of needle-shaped nucleating agents and bubble interfaces occurs.

[0022] FIG. 5 illustrates the formation process of a needle-shaped nucleating agent intersecting with a bubble interface. Similar to FIG. 3, a bubble 320 and a needle-shaped nucleating agent 330 are shown growing in a molten resin layer (340). FIG. 5(a) shows the bubble 320 and the needle-shaped nucleating agent 330 separated from each other, while FIG. 5(b) shows the bubble growing and then contacting the needle-shaped nucleating agent, resulting in further growth. The difference from FIG. 3 is that the existing needle-shaped nucleating agent 330 is longer, and as a result of the growth of the bubble 320, the needle-shaped nucleating agent intersects with the bubble interface. Similar to FIG. 3, the growing bubble attempts to rotate and move the needle-shaped nucleating agent after its interface comes into contact with the needle-shaped nucleating agent. However, even in a molten resin with a relatively high viscosity, the longer the needle-shaped nucleating agent, the stronger its fixation in the resin. As a result, it was thought that with long needle-shaped nucleating agents, the pressure from the bubbles is likely to exceed the strength of the resin film at the contact point between the bubbles and the needle-shaped nucleating agent, making it more likely that the bubbles will break through the resin film.

[0023] In other words, the inherent effect of needle-shaped nucleating agents, namely, inhibiting the growth of pre-growing bubbles, is obtained by the characteristic shape of needle-shaped nucleating agents, which have a large longitudinal length (or aspect ratio). However, if the longitudinal length becomes excessively large, the needle-shaped nucleating agents are more likely to break through the bubble interface, thereby reducing the inherent effect. In contrast, in the transmission line of this embodiment, the lengths of the acicular nucleating agents are controlled to have a predetermined distribution. More specifically, the ratio of the average length of the short acicular nucleating agents to the average length of the long acicular nucleating agents is suppressed to a certain level or less. This makes it possible to achieve a high level of bubble control effect inherent to the acicular nucleating agents while relatively minimizing the occurrence of a decrease in control effect due to the acicular nucleating agents crossing bubble interfaces. It is not necessary to completely eliminate the occurrence of needle-shaped nucleating agents that intersect with the bubble interface; they may coexist with needle-shaped nucleating agents that extend along the interface. It is particularly difficult to completely eliminate their occurrence in resins such as fluororesins, which are difficult to ensure wettability with the nucleating agent material. Rather, such materials that are difficult to ensure wettability can be considered to be inherently in a state where interface crossings are likely to occur, and it is understood that this configuration, which controls the length distribution of the needle-shaped nucleating agents, can achieve significant improvements by suppressing the number of needle-shaped nucleating agents that intersect the interface.

[0024] (Method of manufacturing the transmission line 10) Next, a method for manufacturing the transmission line of this embodiment will be described. First, an acicular nucleating agent is prepared. The acicular nucleating agent is preferably composed of an insulating material that has little effect on the electrical properties of the transmission line. Examples of such materials include metal compounds and ceramic materials. While it may be possible to form acicular nucleating agents from granular materials to obtain acicular nucleating agents with a predetermined length distribution, it is preferable to prepare fibrous materials with a high aspect ratio and adjust them to the desired length distribution by appropriate processing. For example, potassium titanate and aluminum borate are commercially available as fibrous or acicular materials for applications such as reinforcing plastics and adjusting brake wear, and the use of such commercially available materials is being considered. However, these fibrous materials used to enhance mechanical properties often contain many with a large aspect ratio to maximize their effectiveness. Therefore, to obtain a length distribution of acicular nucleating agents that can achieve excellent properties in the dielectric layer of a transmission line, it is necessary to adjust the length distribution by, for example, applying appropriate mechanical stress to such fibrous materials.

[0025] To adjust such a length distribution, mechanical processing that applies shear stress to the acicular nucleating agent can be applied, such as processing using a stirrer, a Henschel mixer, a tumbler, or a mill. These methods are sometimes used to break down material agglomerates or improve dispersibility. If they were used to break down fibrous or needle-like nucleating agents, some of the nucleating agents, especially those with long lengths, might be destroyed, potentially changing the length distribution. However, achieving a desired length distribution requires the application of a greater external stress than simply breaking down the agglomerates. For example, in Comparative Example 1 described below, electron microscopy revealed that the agglomerated nucleating agents were broken down to the point where they were barely visible, resulting in a uniformly dispersed state. However, the average length of the long nucleating agents was significantly greater than that of the short nucleating agents. This is thought to be due to the fact that the above materials are fine and highly hard, making them difficult to break unless mechanical processing energy appropriate for the material is applied. Furthermore, the nucleating agents are often surface-modified to suppress agglomeration, which may also affect the materials.

[0026] The method for controlling the length distribution of needle-shaped nucleating agents is explained in more detail below. First, a fibrous material with a certain aspect ratio is prepared. For example, commercially available materials containing a large amount of fibrous material with a diameter of less than 3 μm and a length of 10 μm or more can be used. Many of these materials contain a certain number of pieces that are too long to be used as needle-shaped nucleating agents in the dielectric layer of a transmission cable. However, the length can be adjusted by applying external mechanical stress and breaking them. However, in particular, when needle-shaped nucleating agents longer than the median length are classified as a long nucleating agent group and needle-shaped nucleating agents shorter than the median length are classified as a short nucleating agent group based on the observed lengths of the multiple needle-shaped nucleating agents observed in the cross section of the dielectric layer, in order to control the value obtained by dividing the average length of the needle-shaped nucleating agents in the long nucleating agent group by the average length of the needle-shaped nucleating agents in the short nucleating agent group (hereinafter simply referred to as the average length ratio) within a specific range, the following must be considered.

[0027] The presence of very long solids in commercially available fiber materials before processing increases the average length of the long nucleating agent group. Applying mechanical stress to such fiber materials often initially acts to decrease the average length ratio. This is because, although it depends on the type of mechanical stress, in many methods, the destruction of very long solids often occurs preferentially over the destruction of short solids. However, as the destruction progresses and the destruction of very long solids approaches saturation, the destruction of medium-length solids becomes relatively more prevalent. As a result, the number of short solids increases significantly, leading to a decrease in the median length. If the decrease in the median becomes more pronounced than the decrease in the average length of the long nucleating agent group, the average length ratio may increase again.

[0028] The average length ratio is also affected by the relationship between the strength of the fiber material and the magnitude of the mechanical stress required for fracture, as well as the distribution of the mechanical stress. An example of a disruption method using a Henschel mixer and a tumbler will be described below. The Henschel mixer is a device that mixes and stirs materials to be processed, either alone or together with other materials, in a container equipped with a metal blade. The rotating metal blade directly impacts the fibrous material, effectively destroying it. This is particularly suitable for reliably destroying high-strength fibrous materials. On the other hand, since large stress is applied only to the localized area of the workpiece where the blade strikes, short-time processing may result in uneven processing when processing large batch sizes, etc. In contrast, simply extending the processing time may also promote the destruction of medium-length fibrous materials, resulting in a larger average length ratio. A tumbler is a device that mixes and stirs materials by storing them in a container, either alone or with other materials, and changing the orientation of the container (i.e., the direction of gravity). When used to break down fibrous materials, the breaking energy comes from collisions with the container or other contents within the container, making it less effective than a Henschel mixer at breaking down fibrous materials with extremely high mechanical strength. However, it is expected to have the advantage of suppressing the breaking of fibrous materials of medium length. It is suitable for fibrous materials with relatively low mechanical strength, and because it is easy to apply breaking stress uniformly to the entire object in the container, it is also advantageous for increasing the batch size of processing. In this way, by setting appropriate processing conditions depending on the strength of the applied fibrous material, the processing batch size, etc., it is possible to obtain a dielectric in which needle-shaped nucleating agents having a desired average length ratio are distributed.

[0029] For example, a process for preparing a foam nucleating agent with a desirable average length ratio includes preparing a fibrous material with an average diameter of 1 μm or less and breaking the fibrous material using a tumbler, with the processing (breaking) conditions set to predetermined conditions. It is also effective to combine treatment using a Henschel mixer as a pretreatment for the tumbler. Here, the Henschel mixer not only breaks the fibrous material but also deflocculates the fibrous material, thereby reducing the variability in the subsequent breakage of the fibrous material using the tumbler. Therefore, a stable average length ratio can be easily obtained even when the fibrous material is significantly agglomerated.

[0030] The method for adjusting the length distribution of the nucleating agent is not limited to the above. For example, it is also possible to prepare a group of nucleating agents having a wide distribution, and then extract a group of nucleating agents having a desired length distribution by a known method such as classification.

[0031] Regardless of the adjustment method, when the needle-shaped nucleating agents longer than the median length are classified as a long nucleating agent group and the needle-shaped nucleating agents shorter than the median length are classified as a short nucleating agent group based on the observed lengths of the multiple needle-shaped nucleating agents observed in the cross section of the foamed dielectric layer of the final transmission line, a transmission line with excellent transmission characteristics can be obtained by setting adjustment conditions such that the ratio of the average length of the needle-shaped nucleating agents in the long nucleating agent group to the average length of the needle-shaped nucleating agents in the short nucleating agent group becomes the desired value. The average length of the acicular nucleating agents in the long nucleating agent group is preferably 1.5 times or more the average length of the acicular nucleating agents in the short nucleating agent group, because if the fibrous material is broken down to a ratio of 1.5 or less, the function of the nucleating agents will become similar to that of particulate nucleating agents, potentially reducing the inherent effect of the acicular nucleating agents.

[0032] Next, the nucleating agent with the adjusted length distribution is dispersed in a resin. The resin serving as the dispersion medium for the acicular nucleating agent may be the same resin that will form the dielectric resin layer. In particular, fluororesins, which themselves have low dielectric constants, can form a dielectric layer for a transmission line with excellent transmission characteristics. Suitable fluororesins include thermoplastic fluororesins such as tetrafluoroethylene-hexafluoropropylene copolymers, tetrafluoroethylene-alkyl vinyl ether copolymers such as tetrafluoroethylene-perfluoropropyl vinyl ether copolymers, ethylene-tetrafluoroethylene-perfluorobutylethylene copolymers, ethylene-chlorotrifluoroethylene copolymers, and polyvinylidene fluoride. The melt flow rate (MFR) of these resins is preferably 10 g / 10 min to 40 g / 10 min, and particularly preferably 20 g / 10 min to 30 g / 10 min. The melt flow rate can be adjusted by selecting the grade of the material resin or by mixing resins with appropriate grades.

[0033] The dispersion of the needle-shaped nucleating agent in the resin can be carried out by kneading a mixture of the needle-shaped nucleating agent and the resin using a mixer, etc. The kneaded mixture of the needle-shaped nucleating agent and the resin can be formed into resin pieces called pellets, which can improve storage properties and ease of use in downstream processes. It is conceivable that the length distribution of the needle-shaped nucleating agent may change during this dispersion process, pelletization process, or foamed resin layer formation process described below. Even in such cases, the stress applied to the nucleating agent in each process may be adjusted so that the observed length in the transmission line has a predetermined length distribution. However, since it becomes more difficult to directly control the length distribution in later processes, it is preferable to adjust the needle-shaped nucleating agent in a process as upstream as possible so that it has a length distribution close to the required one.

[0034] Next, a foamed resin layer is formed. When the transmission line includes a central conductor and a foamed resin layer disposed around the central conductor, the resin layer is preferably formed by extrusion. The formed resin layer can be made into a foamed resin layer by injecting high-pressure gas into the resin obtained by melting the pellets in a mold and then reducing the pressure applied to the molten resin layer. Alternatively, a foamable substance that generates gas by chemical reaction or thermal decomposition may be mixed into the pellets or the resin obtained by melting the pellets, and foaming may be performed using the generated gas.

[0035] Although not limited to this, the transmission line 10 of this embodiment can be obtained by the above process. If the transmission line 10 is a coaxial cable, the above process can be followed by forming an outer conductor and an outer jacket. The outer conductor can be formed by winding metal foil around the core or by forming a braid of thin metal wires. Either the metal foil wound layer or the thin metal wire braided layer may be formed, or both may be formed.

[0036] (Example) Each transmission line with the configuration shown in Table 1 was fabricated, and the observed length and dielectric constant of the needle-shaped nucleating agent were measured. A copper solid wire was used as the central conductor. In Comparative Example 1, commercially available whiskers (Arborex Y, manufactured by Shikoku Kasei) made of aluminum borate, which tends to leave long nucleating agents even after various processes, were used as the raw nucleating agent. Pellets were prepared from a mixture of the fluororesin and the whiskers under conditions that sufficiently deflocculated the whiskers and ensured dispersibility, but without any particular control of the length distribution, such as suppressing the average length of the long nucleating agent group. In contrast, in Examples 1, 2, and 3, commercially available whiskers (Tismo D, manufactured by Otsuka Chemical) were used, which are thought to have similar mechanical properties to the whiskers described above, but which have a slightly smaller diameter and therefore are relatively easy to control the length distribution. Nucleating agents with an average diameter of less than 1 μm are particularly suitable for this embodiment because their length distribution is easily controlled. In Examples 1, 2, and 3, conditions were applied that not only deflocculated the whiskers and improved dispersibility, but also clearly changed the length distribution. Although there was no significant difference in the average length of the nucleating agents as a whole, needle-shaped nucleating agents with a suppressed average length of the long nucleating agents compared to the comparative example were prepared, and pellets were formed from a mixture of these nucleating agents and a fluororesin, which was a tetrafluoroethylene-hexafluoropropylene copolymer.

[0037] [Table 1]

[0038] A dielectric layer was formed on the central conductor using each of the pellets. The dielectric layer was formed by extruding the molten pellet resin through the discharge holes of an extrusion die and cooling it. A cable-shaped core was continuously formed by running a thin metal wire that would become the central conductor in the same direction as the resin was discharged from the same discharge hole. Nitrogen gas was injected into the molten resin in the die, and the reduction in pressure around the resin immediately after it exited the discharge hole was used to simultaneously form the dielectric layer on the conductor and create bubbles inside the dielectric layer. The capacitance of the cable-shaped core thus obtained was measured using a capacitance monitor, and the dielectric constant ε was calculated based on the following formula 1, from which the relative dielectric constant was determined. Equation 1 ε=(C×log(D / d)) / 24.16 Here, C is the capacitance per meter (pF), D is the outer diameter of the core (mm), and d is the diameter of the central conductor (mm).

[0039] (Measurement of observation length) After cutting out the dielectric layer of the core, a dielectric layer including a cross section to be used as an observation sample was prepared. The cross section of the observation sample may be formed using common sample preparation means such as cutting and polishing or a microtome, but a preferred method is to mechanically break the cut-out dielectric layer while cooling it with liquid nitrogen or the like, as this has the advantage of causing little change in the foaming state of the resin layer. The cross section was observed using a tabletop microscope (TM4000PLUS / Hitachi High-Technologies Corporation). The magnification can be selected appropriately from 500x to 2500x depending on the condition of the sample to be observed.

[0040] Rectangular regions were randomly set within the acquired electron microscope image, and all acicular nucleating agents entirely within the rectangular region were measured. However, nucleating agents observed as extremely small here may be granular nucleating agents generated during the length distribution adjustment stage, and such granular nucleating agents cannot be expected to have the bubble control effect inherent to acicular nucleating agents. Furthermore, depending on the observation magnification and image clarity, measurement errors in the observed length may increase, potentially reducing reproducibility. Therefore, a lower limit was set and these nucleating agents were excluded from measurement or calculation. For example, within the above observation magnification range, nucleating agents with an observed longitudinal length of less than 1 μm on the image were excluded from calculation, allowing for an accurate understanding of the distribution of acicular nucleating agents. In this example, nucleating agents with an observed major axis size clearly smaller than 1 μm were excluded from measurement. Nucleating agents with a major axis size close to 1 μm were measured, and those with observed lengths less than 1 μm were excluded before calculating the median and average lengths.

[0041] To understand the distribution, it is preferable to secure the number of data points for each level of observation length. In this example, the number of data points used to calculate the median and average length was 28 to 80 or more, depending on the level. If the variation in observation length is large, it is preferable to further increase the number of data points. By securing at least 28, preferably 80 or more, the distribution can be accurately understood. If the distribution density of the nucleating agent is small and it is difficult to secure the required number of data points, the observation area can be expanded or added.

[0042] Using the observation length data obtained as described above, the median, the average observation length of the long nucleating agent group, and the average observation length of the short nucleating agent group at each level were calculated, and the average observation length of the long nucleating agent group was divided by the average observation length of the short nucleating agent group to obtain the average length ratio of the nucleating agent group at each level. This length ratio indicates the distribution state of long needle-shaped nucleating agents among multiple needle-shaped nucleating agents, and it can be determined that in needle-shaped nucleating agents with a large length ratio, there are relatively more long needle-shaped nucleating agents in the population.

[0043] The calculated length ratios and dielectric constants are shown in Table 1. In Comparative Example 1, in which the length distribution was not controlled, the average length ratio of the nucleating agent group was as large as 4.7, and the dielectric constant of the obtained core was also relatively large at 1.44. In contrast, in each Example in which the average length ratio of the nucleating agent group was suppressed to 3.5 times or less, it was confirmed that a core with a low dielectric constant could be stably formed.

[0044] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. [Explanation of symbols]

[0045] 10 transmission line, 20 center conductor, 30 dielectric layer, 40 core, 320 air bubble, 330 needle-shaped nucleating agent, 340 resin layer.

Claims

1. A transmission line comprising a conductor and a dielectric layer, the dielectric layer has a plurality of bubbles and a plurality of needle-shaped nucleating agents; A transmission line characterized in that, based on the observed lengths of the multiple needle-shaped nucleating agents observed in the cross section of the dielectric layer, needle-shaped nucleating agents longer than the median length are classified as a long nucleating agent group, and needle-shaped nucleating agents shorter than the median length are classified as a short nucleating agent group, and the average length of the needle-shaped nucleating agents in the long nucleating agent group is 3.5 times or less the average length of the needle-shaped nucleating agents in the short nucleating agent group.

2. 2. The transmission line according to claim 1, wherein the average length of the acicular nucleating particles in the long nucleating particles is three times or less the average length of the acicular nucleating particles in the short nucleating particles.

3. 3. The transmission line according to claim 1, wherein the average length of the acicular nucleating agents in the long nucleating agent group is 1.5 times or more the average length of the acicular nucleating agents in the short nucleating agent group.

4. 4. The transmission line according to claim 1, further comprising an outer conductor formed around the dielectric layer, and an outer jacket formed around the outer conductor.

5. 5. The transmission line according to claim 1, wherein the dielectric layer contains a fluororesin.

6. 6. The transmission line according to claim 1, wherein the acicular nucleating agent is an insulator made of a metal compound or a ceramic material.

7. 7. The transmission line according to claim 1, wherein the average diameter of the acicular nuclei is less than 1 μm.

8. 8. The transmission line according to claim 1, wherein the acicular nucleating agent includes an acicular nucleating agent having an aspect ratio of 3 or more.

9. 8. The transmission line according to claim 1, wherein the acicular nucleating agent includes an acicular nucleating agent having an aspect ratio of 6 or more.

10. A method for manufacturing a transmission line having a conductor and a foamed dielectric layer, comprising: Providing a fibrous material; breaking the fibrous material by mechanical processing to form needle-shaped nuclei; Dispersing a plurality of the needle-shaped nucleating agents in a resin; and extruding the resin in which the plurality of needle-shaped nucleating agents are dispersed to form the foamed dielectric layer around the conductor, When the needle-shaped nucleating agents having a length longer than the median length are classified into a long nucleating agent group and the needle-shaped nucleating agents having a length shorter than the median length are classified into a short nucleating agent group based on the observed lengths of the plurality of needle-shaped nucleating agents observed in the cross section of the foamed dielectric layer, A method for manufacturing a transmission line, wherein the average length of the needle-shaped nucleating agents in the group of long nucleating agents is 3.5 times or less than the average length of the needle-shaped nucleating agents in the group of short nucleating agents.

11. Forming an outer conductor around the foamed dielectric layer; The method of claim 10 further comprising forming a jacket around the outer conductor.

12. A method for manufacturing a transmission line as described in claim 10 or claim 11, wherein the average length of the needle-shaped nucleating agents in the long nucleating agent group is 1.5 times or more and 3 times or less than the average length of the needle-shaped nucleating agents in the short nucleating agent group.

13. The foamed dielectric layer comprises a fluororesin, The method for manufacturing a transmission line according to any one of claims 10 to 12, wherein the acicular nucleating agent is an insulator made of a metal compound or a ceramic material.

14. The average diameter of the needle-shaped nucleating agent is less than 1 μm, The method for manufacturing a transmission line according to any one of claims 10 to 13, wherein the acicular nucleating agents include those having an aspect ratio of 6 or more.

15. A method for manufacturing a transmission line described in any one of claims 10 to 14, wherein the mechanical processing includes processing using a Henschel mixer.

16. A method for manufacturing a transmission line described in any one of claims 10 to 15, wherein the machining includes processing using a tumbler.

17. A method for manufacturing a transmission line described in any one of claims 10 to 16, wherein the resin in which the plurality of needle-shaped nucleating agents are dispersed is a pelletized mixture of the plurality of needle-shaped nucleating agents and the resin.

18. Melting the resin in which the plurality of needle-shaped nucleating agents are dispersed; injecting high-pressure gas into the molten resin; and reducing the pressure applied to the resin into which the high-pressure gas has been injected to form the foamed dielectric layer.

19. A method for manufacturing a transmission line described in any one of claims 10 to 17, wherein the resin in which the multiple needle-shaped nucleating agents are dispersed contains a foaming substance that generates gas by chemical reaction or thermal decomposition.

Images