Fluororesin fibers and fabrics made from said fluororesin fibers

Abstract

To provide a fluororesin-based fiber suitable for use as an insulation member required in thin, high-functional electrical equipment, electronic equipment and communication equipment, by thinning a fabric with respect to a fiber diameter by focusing on a relationship between a total fineness and a single-fiber fineness and a cross-sectional shape of the fiber, while maintaining properties of the fluororesin-based fiber as a material, such as heat resistance, chemical resistance, or small relative dielectric constant and dielectric tangent.SOLUTION: A fiber contains a fluorine-based resin at least in a portion thereof, has a rectangular cross-sectional shape, and has irregularities continuously formed on a fiber surface along a direction parallel to a longitudinal direction. At least portions of single fibers are adhered to each other to form the fiber having a rectangular cross sectio in the claim 1. The fiber has a single fiber fineness of 2.5 dtex-8.0 dtex and a total fineness of 55 dtex or less in the claim 1 or 2. The fabric is made of the fiber in any of the claims 1-3.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to fluorine-based fibers and fabrics made from these fluorine-based fibers. [Background technology]

[0002] In recent years, electrical equipment, electronic equipment, and communication equipment have developed remarkably. Currently, there is a growing need for miniaturization of these devices. As a result, the various printed circuit boards used in these devices are also becoming lighter, thinner, and shorter, with the thinnest possible designs being required. Furthermore, these devices tend to use higher frequency bands. Therefore, the various printed circuit boards used in these devices are required to have low relative permittivity and dielectric loss tangent. The relative permittivity and dielectric loss tangent required vary depending on the operating environment, but in high-performance electrical equipment, electronic equipment, and communication equipment such as the already operational 5th generation mobile communication system (5G) and its successor, the 6th generation mobile communication system (6G), fluororesin-based resin materials, namely fluororesin-impregnated glass cloth sheets (made by impregnating glass cloth with fluororesin dispersion and sintering it) and fluororesin fiber paper, have been used from the perspective of dielectric properties such as relative permittivity and dielectric loss tangent. However, fluororesin-impregnated glass cloth sheets have drawbacks: their dielectric properties are lower than those of fluororesin alone due to the dielectric properties of the glass cloth, and a planar layer of fluororesin is formed on the surface, resulting in insufficient adhesion to copper foil. On the other hand, fluororesin fiber paper, as disclosed in Patent Document 1, for example, is a porous material with relatively high surface roughness, and therefore exhibits better adhesion to copper foil compared to fluororesin-impregnated glass cloth sheets. However, currently, its thickness is several hundred micrometers, which is insufficient to meet the recent need for miniaturization. Therefore, in order to solve the above problems of fluororesin fiber paper, there is a need for fluororesin fibers with small fiber diameters and sheets using these fibers.

[0003] As a technique for spinning fluorine-based fibers, a method is known in which an aqueous dispersion of polytetrafluoroethylene (PTFE) is dispersed in a matrix component, spun from a spindle, and then calcined (see Patent Document 2). Example 1 of Patent Document 2 describes using viscose as the matrix, mixing 50% by weight of viscose with 50% of a 60% concentration PTFE aqueous dispersion, then degassing the molding stock under reduced pressure of 10 Torr, and extruding it into a coagulation bath through a pore size of 0.12 mm. Partial calcination is performed at a temperature of 280°C while giving it 4% relaxation, and then calcination is performed using a calcination roller maintained at 350°C, taken up at a speed of 30 m / min, heat-stretched at a temperature of 350°C, and then crimped and cut to obtain a round cross-section PTFE staple. It also describes obtaining a web by carding the obtained PTFE staple. Other known techniques include kneading PTFE powder with a wax-like lubricant, forming it into a rod or film, then stretching and firing it, and subsequently splitting the film (see Patent Document 3). Example 1 of Patent Document 3 describes mixing fine PTFE resin powder with Isopar(registered trademark) K, a lubricant, at a ratio of 0.184 g / g, filling it into a cylindrical body to produce pellets, then extruding and stretching them, and splitting the resulting film to obtain PTFE fibers with a rectangular cross-section. A woven fabric is then obtained by twill weaving the resulting PTFE fibers. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2003-49387 [Patent Document 2] Japanese Patent Publication No. 2006-207097 [Patent Document 3] Special Publication No. 2016-531218 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, Patent Document 2 describes an example using PTFE staples with a fineness of 2.2 dtex and a basis weight of 750 g / m². 2 The website only contains a description of the technology, and there is still room for improvement in addressing the need for miniaturization of high-performance electrical equipment, electronic devices, and communication equipment. Furthermore, Patent Document 3 describes an example using a PTFE filament with a fineness of 172 dtex and a basis weight of 135 g / m². 2 The existing description only mentions a fabric with a thickness of 200 μm, which is too thick to meet the miniaturization needs of high-performance electrical equipment, electronic equipment, and communication equipment, leaving room for improvement. Therefore, the objective of the present invention is to provide a fluororesin fiber and a fabric made from said fluororesin fiber that can meet the miniaturization needs of electrical equipment, electronic equipment, and communication equipment by thinning the fabric relative to the fiber diameter, while maintaining the properties of fluororesin fibers such as heat resistance, chemical resistance, and low relative permittivity and dielectric loss tangent, by focusing on the relationship between total fineness and single fiber fineness and the cross-sectional shape of the fiber. [Means for solving the problem]

[0006] To solve the above problems, the present invention is configured as follows. [1] A fluororesin fiber having a rectangular cross-sectional shape in the cross-section perpendicular to the longitudinal direction, and having continuous irregularities formed on the fiber surface in a direction parallel to the longitudinal direction. [2] A method for producing fluororesin fibers according to claim 1, comprising bonding at least a portion of the single fibers of fluororesin fibers together to form fluororesin fibers having a rectangular cross-section. [3] Fluororesin-based fibers Single fiber fineness: 2.5 dtex to 8.0 dtex, total fineness: 55 dtex or less Claim 2 As described Manufacturing method of fluororesin-based fibers . [4] Claim 1 A fabric made from the fibers described above. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a fabric made of fluororesin-based fibers that can meet the needs for miniaturization of high-performance electrical equipment, electronic equipment, and communication equipment by making the fabric thinner relative to the fiber diameter, while maintaining the material properties of fluororesin-based fibers such as heat resistance, chemical resistance, and low relative permittivity and dielectric loss tangent. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic cross-sectional diagram illustrating the cross-section of the fluororesin fiber of the present invention, showing an example with eight joined single fibers. [Figure 2] This is an example showing the surface state of a fabric (woven material) made of fluororesin-based fibers according to the present invention. [Modes for carrying out the invention]

[0009] <Fluororesin-based fibers> In the present invention, any fluororesin fiber can be used as long as 90% or more of the repeating structural units of the polymer are composed of monomers containing one or more fluorine atoms in the main chain or side chain. However, resins composed of monomers with a higher number of fluorine atoms are preferred. Examples include polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), polyvinylidene fluoride (PVDF), ethylene-tetrafluoroethylene copolymer (ETFE), and ethylene / chlorotrifluoroethylene copolymer (ECTFE). Among these, PTFE is preferred for the fluororesin fiber.

[0010] In the present invention, the cross-sectional shape of the fluororesin fiber is rectangular, and a rectangular cross-section with irregularities on its outer circumference is preferred. Here, "rectangular" does not mean that the four corners are right angles; for example, if the surface has irregularities, it is sufficient if the outer edge is roughly rectangular when bordered. Furthermore, it is preferable that these irregularities are formed continuously on the fiber surface in a direction parallel to the longitudinal direction of the fiber (the recesses and protrusions are formed continuously along the longitudinal direction). The continuity of the irregularities in the direction parallel to the longitudinal direction of the rectangular fiber improves adhesion to dissimilar materials. As a result, good adhesion can be obtained when resin is impregnated into a fabric using the fluororesin fiber of the present invention, or when copper or the like is pressed onto it.

[0011] The rectangular cross-section of the fluororesin-based fiber in the present invention is preferably formed as a cross-section (FIG. 1) in which at least a part of the single fibers are adhered (or fused) to continuously connect the single-fiber cross-sections linearly. It is preferable that the number of such connections is up to 10. If it exceeds 10, the continuous connection on a straight line will collapse during fabric production, and overlapping of single fibers is likely to occur, which is not suitable for making a thin fabric. By adopting a cross-sectional shape in which the single-fiber cross-sections are continuously connected linearly, when obtaining a fabric, overlapping of single fibers can be suppressed, and the fabric can be made thinner with respect to the fiber diameter. Furthermore, it also has the effect of improving processing stability in processing steps such as weaving and knitting described later. And such a shape in which the thin-fiber cross-sections are continuously connected linearly can form irregularities along the direction parallel to the fiber longitudinal direction. When single filament fibers (referring to one fluororesin-based fiber of the present invention) are not adhered and are scattered, when stress concentrates on a single filament during processing steps such as weaving and knitting, the single filament fiber breaks (hereinafter referred to as single-filament breakage), and the broken fibers become entangled and develop into a major defect. Since the fluororesin-based fiber has lower strength compared to general-purpose fibers such as polyester and nylon, the above phenomenon is likely to occur, and the single-filament breakage becomes more prominent as the single-fiber diameter becomes thinner, and the processability significantly deteriorates. In contrast, since at least a part of the single fibers of the fluororesin-based fiber of the present invention are adhered, it is possible to prevent the processing tension during weaving or knitting from concentrating on a specific single filament fiber, suppress single-filament breakage, and thus obtain extremely excellent processing stability. The cross-section of the thin fibers to be connected is not particularly limited, and the cross-section may be any shape such as round, β-shaped, C-shaped, triangular, flat, etc., but a round cross-section is preferred.

[0012] The single-fiber fineness of the fluororesin-based fiber in the present invention (referring to the fineness of one single fiber constituting the fluororesin-based fiber) is preferably such that the single-fiber fineness is 2.5 dtex or more and 8.0 dtex or less. If the single-fiber fineness is less than 2.5 dtex, the spinnability deteriorates significantly, making industrial production difficult. Also, if it exceeds 8.0 dtex, since the single-fiber diameter becomes large, the irregularities with respect to the thickness become small, and the adhesion to different materials decreases.

[0013] As the total fineness of the fluororesin-based fiber in the present invention (referring to the fineness of one fluororesin-based fiber), it is preferable that the total fineness is 55 dtex or less. If it exceeds 55 dtex, the effect of a thin fabric becomes low, and it is not suitable for the insulating member applications required in high-functional electrical equipment, electronic equipment, and communication equipment.

[0014] <Manufacturing method of fluororesin-based fiber> Known manufacturing methods of fluororesin-based fibers include the split peeling method, paste extrusion method, melt spinning method, matrix spinning method (also called emulsion method), etc.

[0015] The split peeling method is a manufacturing method in which a powder of a fluororesin is compressed in a cylinder, then sintered, split and peeled, and then drawn.

[0016] The paste extrusion method is a manufacturing method in which a powder of a fluororesin is kneaded with a wax-like lubricant, formed into a rod shape or a film shape, then the lubricant is removed, and stretched and fired (it may not be fired). However, in these two manufacturing methods, the cross-section of the final fibrous material obtained by cutting thinly inevitably has a flat shape, and moreover, it is random and inferior in uniformity, and there is a drawback that fabric processing is difficult.

[0017] Also, the melt spinning method is a manufacturing method in which a powder of a fluororesin is heated at a temperature above the melting point, the melted resin is spun from a die, and fiberized. Although this manufacturing method can obtain fluororesin-based fibers with high uniformity by spinning from a die, it cannot be applied to PTFE etc. which have a high melting point and show almost no fluidity even when exceeding the melting point.

[0018] For these reasons, it is preferable to manufacture the fluororesin-based fibers of the present invention by employing a matrix spinning method. In the matrix spinning method, a mixture of a viscose matrix and an aqueous dispersion of a fluororesin is extruded from a die into a solidification bath to form fibers, which are then refined and fired. By firing at a temperature above the melting point of the fluororesin, the majority of the matrix polymer is fired and scattered, the fluororesin is melted, and the particles are fused together, thereby imparting subsequent stretchability. In this firing process, by controlling the arrangement of the single fibers of the running yarn, at least some of the single fibers constituting the fluororesin-based fibers can be bonded (or fused) together. Also, at this stage, some of the matrix polymer remains in the fluororesin-based fibers as carbides, which can be removed by high-temperature furnace treatment. After firing, the undrawn yarn can be directly drawn in 1 or 2 steps to obtain fluororesin-based fiber drawn yarn. Strength is developed during this drawing process, which allows for the production of the fluororesin-based fibers used in the present invention. However, even without high-temperature furnace treatment, the fluororesin-based fibers used in the present invention can be obtained. This manufacturing method yields highly uniform fluororesin-based fibers by spinning them from a spinneret. Furthermore, by changing the spinneret design and spinning / drawing conditions, the cross-sectional shape of the single fibers, fineness, and total fineness can be easily controlled, making it a suitable method for thinning fabrics relative to fiber diameter.

[0019] In the spinning solution, the proportion of fluororesin is preferably 75 to 93% by mass, and in this case, the proportion of matrix components is preferably 7 to 25% by mass. When the proportion of fluororesin in the spinning solution is 75% by mass or more, fusion between fluororesin particles progresses during the firing process, making yarn breakage less likely and resulting in fluororesin fibers with high process stability. On the other hand, when the proportion of fluororesin is 93% by mass or less, yarn breakage is less likely to occur during the spinning process. Viscose, polyvinyl alcohol, sodium alginate, and hydroxypropyl cellulose can be used as matrix components, but in the present invention, viscose is preferred. An aqueous solution of inorganic mineral acid and / or inorganic salt can be used as the coagulation bath, but in the present invention, a mixed aqueous solution of sulfuric acid and sodium sulfate is preferred.

[0020] <Fabric made from fluororesin fibers> The fabric made from the fluororesin fiber of the present invention is a fabric that uses the fluororesin fiber of the present invention in at least part. When forming a fabric using the fluororesin fiber of the present invention, twisting may be performed to improve the processing stability. The twisting process referred to herein includes not only the twisting of a single fluororesin fiber of the present invention, but also the combined twisting process in which multiple fluororesin fibers of the present invention are twisted together, and the combined twisting process in which fluororesin fibers of the present invention are twisted together with fibers other than the fluororesin fiber of the present invention. The number of twists can be appropriately set according to the desired characteristics, but it is preferable that the twist coefficient k is 100 or more and 25000 or less. Here, the twist coefficient k is calculated by the following formula, where T [t / m] is the number of twists per meter and D [dtex] is the fineness of the composite yarn.

[0021] k = T × D 0.5 More preferably, the value is between 200 and 10000, and particularly preferably between 1000 and 3000. If the value is 3000 or higher, the adhesion between the single fibers may be peeled off due to stress in the circumferential direction of the fibers, and the effect of suppressing overlap between the single fibers constituting the fluororesin-based fibers may not be fully exhibited. If the value is 200 or lower, the required processing stability may not be sufficiently obtained.

[0022] The fabric using at least a part of the fluororesin-based fiber of the present invention preferably has through-holes of 100 μm 2 / piece or more and 25,000 μm 2 / piece or less. By having through-holes, an anchor effect can be obtained when impregnated with a resin or the like, and the adhesiveness is improved. When the area is 100 μm 2 / piece or more, the resin or the like can sufficiently penetrate, so that the resin can continuously exist on the front and back of the fabric, and high adhesiveness between the resin or the like and the fabric can be obtained. When it is 25,000 μm 2 / piece or more, there is a possibility that sufficient morphological stability of the fabric cannot be obtained, and when impregnated with a resin or the like, a difference in physical properties is likely to occur between the part where the fabric exists and the through-holes. When it is less than 100 μm 2 / piece, there is a possibility that sufficient impregnation property of the resin cannot be obtained. From the viewpoint of sufficient impregnation of the resin and obtaining the adhesion between the fabric and the resin, the area of the through-holes is 500 μm 2 / piece or more and 15,000 μm 2 / piece or less is more preferable, and particularly preferably 1000 μm 2 / piece or more and 4000 μm 2 / piece or less. The area ratio of the through-holes in the fabric is preferably 1% or more and less than 40%. If it is less than 1%, there is a possibility that the effect of improving the adhesiveness by the through-holes cannot be sufficiently obtained. If it is 40% or more, the fabric is likely to be misaligned, the fabric strength is reduced, and fibers with a large fineness are required for shape retention, resulting in a concern that the thickness of the fabric increases. From the balance between preventing misalignment and the resin impregnation property, it is more preferably 2% or more and 20% or less, and particularly preferably 3% or more and 10% or less.

[0023] The fabric using at least a part of the fluororesin-based fiber of the present invention preferably has 5 or less layers of single fibers in the thickness direction. More preferably, it is 3 or less, and particularly preferably 2 or less. When it exceeds 5, the fabric becomes thick with respect to the fiber diameter, and there is a concern that it cannot sufficiently meet the needs for miniaturization of electrical equipment, electronic equipment, and communication equipment. Since two or more fibers are required to adhere or intertwine to form the fabric, the substantial lower limit of the number of layers of single fibers in the thickness direction is 2.

[0024] The form of the fabric using at least a portion of the fluororesin-based fibers of the present invention is not particularly limited, and can be woven or knitted, wet-laid nonwoven fabric, dry-laid nonwoven fabric, etc., but a woven fabric is preferred from the viewpoint of thickness and dimensional stability.

[0025] When forming a fabric using the fluororesin-based fibers of the present invention, a fabric in which the fluororesin-based fibers of the present invention are used in at least one of the warp and weft threads is preferred, a fabric in which the fluororesin-based fibers of the present invention are used in at least the weft threads is more preferred, and a fabric in which the fluororesin-based fibers of the present invention are used in both the warp and weft threads is particularly preferred. By using the fluororesin-based fibers of the present invention in both the warp and weft threads, dielectric properties and thinning effects can be maximized. When the fluororesin-based fibers of the present invention are used in the weft threads, it is more preferable that the weft threads are untwisted. This configuration suppresses the overlapping of single fibers constituting the fluororesin-based fibers of the present invention caused by twisting, and makes it easier to control the number of layers of single fibers in the thickness direction to a preferred range. When the fluororesin-based fibers of the present invention are used in the warp threads, the warp threads can be selected as either twisted or untwisted depending on the situation. Selecting twisted yarn improves productivity. Selecting untwisted yarn suppresses the overlapping of single fibers constituting the fluororesin-based fibers, making it possible to obtain a thinner fabric. The weave structure is not particularly limited; twill, satin, plain, modified, and multi-layered structures using these in each layer can be employed. However, a single plain weave is preferred because it allows for relatively easy production of thin fabrics.

[0026] A fabric using at least a portion of the fluororesin-based fibers of the present invention can be combined with known fibers such as cotton, polyester fibers, polyamide fibers, poly(p-phenylene terephthalamide) fibers, poly(metaphenylene terephthalamide) fibers, polyphenylene sulfide fibers, poly(p-phenylene benzobisoxazole) fibers, polyethylene fibers, ultra-high molecular weight polyethylene fibers, liquid crystal polyester fibers, glass fibers, carbon fibers, and silicon carbide fibers, depending on the properties required for the fabric. When applied to applications requiring dielectric properties, such as printed circuit boards, glass fibers and liquid crystal polyester fibers are preferred from the viewpoint of dielectric properties, and liquid crystal polyester fibers are particularly preferred.

[0027] The fabric using at least a portion of the fluororesin-based fibers of the present invention may be subjected to additional pressure processing to further reduce its thickness, depending on the desired properties. Here, any known method may be selected for the pressure processing, such as calendering or hot pressing.

[0028] The fabric obtained with the above configuration may be subjected to a surface treatment to improve its wettability. Generally, plasma processing is used, and in addition to atmospheric pressure plasma processing, plasma processing using reactive gases such as argon, nitrogen, helium, carbon dioxide, and ammonia gas, either alone or in mixtures, may be selected.

[0029] The fabric obtained with the above configuration can also be used after impregnating it with resin. Here, the resin used for impregnation can be a thermosetting resin or a thermoplastic resin. Although not particularly limited, examples of thermosetting resins include phenolic resin, melamine resin, urea resin, unsaturated polyester resin, epoxy resin, polyurethane resin, diallyl phthalate resin, silicon resin, polyimide resin, vinyl ester resin, and modified resins thereof. For thermoplastic resins, examples include vinyl chloride resin, polystyrene resin, ABS resin, polyethylene resin, polypropylene resin, fluororesin, polyamide resin, polyacetal resin, polycarbonate resin, polyester resin, and more preferably, thermoplastic polyurethane, butadiene rubber, nitrile rubber, neoprene, polyester elastomer, and other synthetic rubbers or elastomers. Among these, resins mainly composed of phenolic resin and polyvinyl butyral resin, unsaturated polyester resin, vinyl ester resin, polyethylene, polyolefin resins such as polypropylene, and polyester resins are preferably used due to their impact resistance, dimensional stability, strength, and cost. Such thermosetting resins and thermoplastic resins may contain various additives that are commonly used industrially for their purpose, application, productivity in manufacturing and processing processes, or to improve their properties. For example, they may contain modifiers, plasticizers, fillers, mold release agents, colorants, diluents, etc. The term "main component" here refers to the component with the largest mass ratio among the components excluding the solvent. In the case of a resin whose main components are phenolic resin and polyvinyl butyral resin, this means that the mass ratios of these two types of resins are the first and second largest (in any order).

[0030] As for the method of impregnating the fabric with resin, when using a thermosetting resin, a common method is to dissolve the thermosetting resin in a solvent to prepare a varnish, and then impregnate the fabric with it using knife coating, roll coating, comma coating, or gravure coating. When using a thermoplastic resin, melt extrusion lamination is commonly used. [Examples]

[0031] Next, the present invention will be specifically described based on the examples. However, the present invention is not limited to these examples. Various modifications and alterations are possible without departing from the technical scope of the present invention. The measurement methods for the various characteristics used in these examples are as follows.

[0032] [Measurement and Evaluation Methods] (1) Cross-sectional shape (confirmation of adhesion between single fibers) Using a KEYENCE VE-9800 scanning electron microscope, 20cm x 20cm test pieces were taken from fabric samples. Cross-sectional observations were performed at 1000x magnification at three different locations on the sample to check for adhesion and confirm whether the cross-section was rectangular.

[0033] (2) Single fiber fineness An initial load (6% of the fineness load (g)) is applied to accurately collect a 1m length sample of fluororesin fiber, and the mass of the sample is measured to 1 / 10,000 of a g, and the following formula is used: The single fiber fineness (dtex) was calculated using the formula: Single fiber fineness (dtex) = Mass of sample / (Length of sample × Number of holes in spinning nozzle) × 10,000, and the average of three measurements was taken as the single fiber fineness (dtex).

[0034] (3) Total fineness An initial load (6% of the fineness load (g)) is applied to accurately collect a 1m long multifilament sample, and the mass of the sample is measured to 1 / 10,000 of a g, and the following formula is used. The total fineness (dtex) was calculated using the formula: Total fineness (dtex) = Mass of sample / Length of sample × 10,000, and the average of three measurements was used as the total fineness (dtex).

[0035] (4) Arithmetic mean roughness Using a KEYENCE VHX-7000 microscope, the arithmetic mean roughness was measured in a direction perpendicular to the longitudinal direction of the fiber, and the average value at 10 locations was defined as the arithmetic mean roughness (μm).

[0036] (5) Area of ​​the through-hole and the ratio of the area of ​​the through-hole to the fabric Using the KEYENCE VHX-7000 microscope, the surface of the fabric was photographed at 500x magnification, and the photographed area was S tot Of that area, the area occupied by the through-hole is S A The number of through-holes was denoted as N. The area of ​​the through-holes and the ratio of the area of ​​the through-holes to the fabric were calculated using the following formula. Area of ​​the through hole = S A / N [μm 2 / piece] Area ratio of through holes in fabric = S A / S tot ×100[%] (6) Number of single fiber layers in the thickness direction of the fabric Using a KEYENCE VHX-7000 microscope, 20cm x 20cm test pieces were taken from fabric samples. Cross-sectional observations were performed at 500x magnification at 10 different locations on the sample, and the number of single fiber layers in the thickness direction was measured. The average value of the obtained results was calculated and rounded to the first decimal place to determine the number of single fiber layers in the thickness direction of the fabric.

[0037] (7) Thickness of the fabric The thickness of the fabric was measured under a pressure of 23.5 kPa in accordance with JIS L1096:2010 "Testing Methods for Woven and Knitted Fabrics," section 8.4.a).

[0038] [Example 1] 46% by mass of viscose (cellulose concentration 9.0 wt%, alkali concentration 6.0 wt%) and 54% of a 60% aqueous PTFE dispersion were mixed, and the mixture was degassed under reduced pressure of 10 Torr to obtain a spinning mixture with a cellulose / PTFE ratio of 12.8%. The above spinning mixture was wet-spun using a round-hole die with a pore size of 130 μm, a pore length of 130 μm, and 16H holes in a spinning bath with a sulfuric acid concentration of 9.0%, a sodium sulfate concentration of 10.0%, and a temperature of 15°C, at a spindle discharge rate of 6.4 cc / min. The solidified unfired yarn was then divided into two parts, washed with warm water at 70°C, and then refined in an alkaline bath containing a 0.12% aqueous caustic soda solution to remove acidic components. Subsequently, 8H of unfired yarn, drawn from the alkaline bath, was gathered into a single strand and squeezed with a nip roller. Then, while allowing 3% relaxation, it was fired using a firing roller that was gradually heated to a temperature of 250°C to 360°C, and drawn at a speed of 32 m / min to obtain undrawn fluororesin fiber. Next, the undrawn yarn was heat-treated in a high-temperature furnace at 310°C, and then heat-stretched at a draw ratio of 10 times at a temperature of 365°C. The cross-sectional shape of the single fibers before joining was circular, and two spindles of PTFE drawn yarn with a rectangular fiber cross-section (Figure 1) were obtained by continuously joining eight single fibers in a straight line. The PTFE fiber obtained from one spindle had a single fiber fineness of 5.3 dtex and a total fineness of 42 dtex. Furthermore, observation of the cross-section revealed that some of the single fibers constituting the fluororesin fiber were bonded to each other. The obtained PTFE fiber untwisted yarn was used as both the warp and weft threads, and a plain weave fabric with a warp density of 142 threads / 2.54cm and a weft density of 132 threads / 2.54cm was produced on a loom.

[0039] [Example 2] A plain weave fabric was produced using the same method as in Example 1, except that the weft density was changed to 142 threads / 2.54 cm. Compared to Example 1, the increased weft density caused some of the adjacent weft threads to overlap, resulting in a slight increase in thickness.

[0040] [Example 3] A plain weave fabric was produced using the same method as in Example 1, except that the warp threads were twisted to 300 T / m, the warp density was changed to 90 threads / 2.54 cm, and the weft density was changed to 137 threads / 2.54 cm. Compared to Example 1, the addition of twist to the warp threads resulted in two warp single threads overlapping in the thickness direction, slightly increasing the thickness.

[0041] [Example 4] A plain weave fabric was produced using the same method as in Example 3, except that the weft density was changed to 175 threads / 2.54 cm.

[0042] [Example 5] A plain weave fabric was produced using the same method as in Example 3, except that the weft density was changed to 216 threads / 2.54 cm.

[0043] [Example 6] A plain weave fabric was prepared using the same method as in Example 3, except that the weft density was changed to 94 threads / 2.54 cm. The resulting fabric was more prone to thread slippage compared to Examples 1-5.

[0044] [Comparative Example 1] In Example 1, two spindles of fluororesin-based fibers were obtained using the same method, except that the number of holes was changed from 16H to 2H, the discharge conditions from 6.4cc / min / spindle to 0.8cc / min / spindle, and the draw ratio from 10 times to 7.2 times. The PTFE fibers obtained from one spindle had a single fiber fineness of 6.9 dtex and a total fineness of 6.9 dtex. Subsequently, by plying eight strands together, PTFE fibers with a single fiber fineness of 6.9 dtex and a total fineness of 55 dtex were obtained. Furthermore, when the cross-section of the PTFE fibers with a single fiber fineness of 6.9 dtex and a total fineness of 55 dtex was observed, no adhesion between the single fibers constituting the fluororesin-based fibers was confirmed. Using the obtained PTFE fibers, a plain weave fabric was produced in the same method as in Example 6, except that the number of twists in the warp threads was changed to 190 T / m.

[0045] [Comparative Example 2] In Example 1, fluororesin fibers were obtained in two spindles using the same method, except that the number of holes was changed from 16H to 2H, the discharge conditions from 6.4 cc / min / spindle to 0.8 cc / min / spindle, and the draw ratio from 10 times to 7.2 times. The PTFE fibers obtained in one spindle had a single fiber fineness of 6.9 dtex and a total fineness of 6.9 dtex. Subsequently, by plying 16 strands together, PTFE fibers with a single fiber fineness of 6.9 dtex and a total fineness of 110 dtex were obtained. Furthermore, when the cross-section of the PTFE fibers with a single fiber fineness of 6.9 dtex and a total fineness of 110 dtex was observed, no adhesion between the single fibers constituting the fluororesin fibers was confirmed. A plain weave fabric was prepared in the same method as in Example 1, except that the obtained PTFE fibers were used.

[0046] [Comparative Example 3] In Example 1, two spindles of fluororesin-based fibers were obtained using the same method, except that the number of holes was changed from 16H to 2H and the draw ratio was changed from 10 times to 7.2 times. The PTFE fibers obtained from one spindle had a single fiber fineness of 55 dtex and a total fineness of 55 dtex. Furthermore, observation of the cross-section revealed no adhesion between the single fibers constituting the fluororesin-based fibers. A plain weave fabric was prepared using the same method as in Example 1, except that the obtained PTFE fibers were used.

[0047] [Table 1]

[0048] For the fluororesin fibers and fabrics made from the fluororesin fibers prepared in Examples 1-6 and Comparative Examples 1-3, the following evaluations were performed: (1) cross-sectional shape (confirmation of adhesion between single fibers), (2) single fiber fineness, (3) total fineness, (4) arithmetic mean roughness, (5) area of ​​through holes and ratio of through hole area to fabric, (6) number of single fiber layers in the thickness direction of the fabric, and (7) thickness of the fabric. The results are shown in Table 1. As a result, it was clear that the fluororesin fibers and fabrics made from the fluororesin fibers of the present invention can provide fabrics made from fluororesin fibers that are suitable for insulating material applications required in high-performance electrical equipment, electronic equipment, and communication equipment, while maintaining the material properties of fluororesin fibers such as heat resistance, chemical resistance, and low relative permittivity and dielectric loss tangent, and thinning of the fabric relative to the fiber diameter. [Explanation of Symbols]

[0049] 1: Fluororesin fiber 2: Recess 3: Convex part

Claims

1. A fluororesin fiber having a rectangular cross-sectional shape perpendicular to its longitudinal direction, and having continuous irregularities formed on its surface along a direction parallel to its longitudinal direction.

2. A method for producing fluororesin fibers according to claim 1, comprising bonding at least a portion of the individual fibers of fluororesin fibers together to form fluororesin fibers having a rectangular cross-section.

3. The method for producing a fluororesin fiber according to Claim 2, wherein the fluororesin fiber has a single fiber fineness of 2.5 dtex or more and 8.0 dtex or less, and a total fineness of 55 dtex or less.

4. A fabric made of fluororesin-based fibers as described in Claim 1.

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