JP7835068B2 - Fluororesin fibers and fabrics made from said fluororesin fibers - Google Patents

Description

This invention relates to fluorine-based fibers and fabrics made from these fluorine-based fibers.

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 fifth-generation mobile communication system (5G) and its successor, the sixth-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, thus providing 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, to solve the above problems with fluororesin fiber paper, there is a need for fluororesin fibers with small fiber diameters and sheets using these fibers.

Regarding the spinning technology for 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). In Example 1 of Patent Document 2, viscose is used as the matrix, and 50% by weight of viscose is mixed with 50% aqueous dispersion of PTFE with a concentration of 60%. The molding stock, degassed under reduced pressure of 10 Torr, is extruded into a coagulation bath through a pore size of 0.12 mm, and semi-calcined at a temperature of 280°C while giving a 4% relaxation. Then, calcination is performed using a calcination roller maintained at 350°C, and the material is taken up at a speed of 30 m/min. It is then heat-stretched at a temperature of 350°C, crimped, and cut to obtain a PTFE staple with a round cross-section. It is also described that a web is obtained by carding the obtained PTFE staple. Another known technique involves kneading PTFE powder with a wax-like lubricant, forming it into a rod or film, then stretching and firing it, followed by splitting the film (see Patent Document 3). Example 1 of Patent Document 3 describes a process where fine PTFE resin powder is mixed with the lubricant Isopar® K at a ratio of 0.184 g/g, packed into a tubular body to produce pellets, then extruded and stretched, and the resulting film is split to obtain rectangular cross-section PTFE fibers. A woven fabric is then obtained by twill weaving these PTFE fibers.

Japanese Patent Publication No. 2003-49387 Japanese Patent Publication No. 2006-207097 Special table 2016-531218 publication

However, Patent Document 2 only describes a web with a basis weight of 750 g/ ^m² using PTFE staples with a fineness of 2.2 dtex at the example level, and its thickness is too thick to meet the miniaturization needs of high-performance electrical equipment, electronic equipment, and communication equipment, leaving room for improvement. Similarly, Patent Document 3 only describes a fabric with a basis weight of 135 g/ ^m² and a thickness of 200 μm using PTFE filaments with a fineness of 172 dtex at the example level, and its thickness is too thick to meet the miniaturization needs of high-performance electrical equipment, electronic equipment, and communication equipment, leaving room for improvement. Therefore, the object of the present invention is to provide a fluororesin fiber and a fabric made of 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.

To solve the above problems, the present invention is configured as follows.
[1] A fluororesin fiber having a rectangular cross-sectional shape in a cross-section perpendicular to the longitudinal direction, and having continuous irregularities formed on the fiber surface along a direction parallel to the longitudinal direction.
[2] A method for producing fluororesin fibers according to claim 1, wherein at least a portion of the single fibers of fluororesin fibers are bonded together to form fluororesin fibers having a rectangular cross-section.
[3] The method for producing fluororesin fibers according to claim 2 , wherein the fluororesin fibers have 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 the fibers described in claim 1 .

According to the present invention, while maintaining the material properties of fluorine-based fibers, such as heat resistance, chemical resistance, and low relative permittivity and dielectric loss tangent, it is possible to make the fabric thinner relative to the fiber diameter, thereby providing a fabric made of fluororesin-based fibers that can meet the needs for miniaturization of high-performance electrical equipment, electronic equipment, and communication equipment.

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. This is an example showing the surface state of a fabric (woven material) made of fluororesin-based fibers according to the present invention.

<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 as the fluororesin fiber.

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

In the present invention, the rectangular cross-section of the fluororesin fiber is preferably formed by bonding (or fusing) at least a portion of the individual fibers together, creating a cross-section (Figure 1) in which the individual fiber cross-sections are continuously joined in a straight line. It is preferable to limit the number of these joints to 10; exceeding 10 joints disrupts the straight-line continuous jointing during fabric production, making it easier for the individual fibers to overlap, thus making the fabric unsuitable for thinning. By creating a cross-sectional shape in which the individual fiber cross-sections are continuously joined in a straight line, overlapping of individual fibers is suppressed during fabric production, allowing the fabric to be thinned relative to its fiber diameter. Furthermore, this also improves processing stability in weaving, knitting, and other processing steps described later. Moreover, this shape, in which fine fiber cross-sections are continuously joined in a straight line, can form irregularities along a direction parallel to the fiber's longitudinal direction. When single fibers (referring to a single fluororesin fiber of the present invention) are not adhered and are scattered, stress concentration on the single fibers during processing steps such as weaving and knitting can cause them to break (hereinafter referred to as single-fiber breakage), and the broken fibers can become entangled, leading to significant defects. Because fluororesin fibers have lower strength than general-purpose fibers such as polyester and nylon, the above phenomenon is more likely to occur. The thinner the single fiber diameter, the more pronounced single-fiber breakage becomes, significantly reducing processability. In contrast, since at least a portion of the single fibers of the present invention are adhered to each other, processing tension during weaving and knitting is prevented from concentrating on specific single fibers, suppressing single-fiber breakage and thus achieving extremely excellent processing stability. The cross-section of the joined fine fibers is not particularly limited; the cross-section may be round, β-shaped, C-shaped, triangular, or flattened, but a round cross-section is preferred.

In this invention, the single fiber fineness (referring to the fineness of a single fiber constituting the fluororesin fiber) of the fluororesin-based fiber is preferably 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. Furthermore, if it exceeds 8.0 dtex, the single fiber diameter increases, resulting in less surface irregularity relative to the thickness, thus reducing adhesion to dissimilar materials.

In this invention, the total fineness of the fluororesin fibers (referring to the fineness of a single fluororesin fiber) is preferably 55 dtex or less. If the total fineness exceeds 55 dtex, the thinness effect is reduced, making it unsuitable for insulating material applications required in high-performance electrical equipment, electronic equipment, and communication equipment.

<Method for manufacturing fluororesin fibers>
Known methods for manufacturing fluororesin fibers include the split extrusion method, paste extrusion method, melt spinning method, and matrix spinning method (also known as emulsion method).

The split-peel method is a manufacturing process in which fluororesin powder is compressed in a cylinder, sintered, split-peeled, and then stretched.

The paste extrusion method involves mixing a fluororesin powder wax-like lubricant with the paste, forming it into a rod or film, removing the lubricant, stretching, and firing (sometimes without firing). However, both of these methods have the drawback that the resulting fibrous material, obtained by thinly cutting the material, has a flattened cross-section, is random and lacks uniformity, and is difficult to process into fabric.

Furthermore, melt spinning is a manufacturing method in which fluoropolymer powder is heated to a temperature above its melting point, and the molten resin is spun out through a die to form fibers. While this method yields highly uniform fluoropolymer fibers through spinning, it cannot be applied to materials like PTFE, which have high melting points and exhibit almost no fluidity even above their melting point.

For these reasons, it is preferable to manufacture the fluororesin-based fibers of the present invention by employing a matrix spinning method. The matrix spinning method involves extruding a mixture of a viscose matrix and an aqueous dispersion of a fluororesin into a coagulation bath via a die, forming fibers, followed by refining and then firing. By firing at a temperature above the melting point of the fluororesin, the majority of the matrix polymer is fired and scattered, while 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 in the running yarn, at least some of the single fibers constituting the fluororesin-based fibers can be bonded (or fused) together. Furthermore, 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 one or two steps to obtain fluororesin-based fiber drawn yarn. Strength is developed during this drawing process, allowing 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.

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, reducing the likelihood of yarn breakage 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. The matrix component can be viscose, polyvinyl alcohol, sodium alginate, or hydroxypropyl cellulose, but in this invention, viscose is preferred. The coagulation bath can be an aqueous solution of inorganic mineral acid and/or inorganic salt, but in this invention, a mixed aqueous solution of sulfuric acid and sodium sulfate is preferred.

<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 here includes not only twisting a single fluororesin fiber of the present invention, but also twisting a plurality of fluororesin fibers of the present invention together, and twisting a fluororesin fiber of the present invention together with fibers other than the fluororesin fiber of the present invention. The number of twists can be set appropriately 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.

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.

The fabric using the fluororesin fibers of the present invention in at least a portion of it preferably has through-holes with an area of 100 ^μm² /hole to 25,000 ^μm² /hole. Having through-holes provides an anchoring effect when impregnated with resin, etc., improving adhesion. An area of 100 ^μm² /hole or more allows sufficient penetration of the resin, etc., so that the resin can be continuously present on both the front and back surfaces of the fabric, resulting in high adhesion between the resin, etc. and the fabric. If the area is ^{25,000 μm²} /hole or more, the morphological stability of the fabric may not be sufficient, and when impregnated with resin, etc., differences in physical properties are likely to occur between the areas where the fabric is present and the through-holes. If the area is less than ^{100 μm²} /hole, the resin may not be able to penetrate sufficiently. From the viewpoint of ensuring sufficient resin impregnation and achieving good adhesion between the fabric and the resin, the area of the through-holes is more preferably 500 ^μm² /hole or more and less than 15,000 ^μm² /hole, and particularly preferably 1,000 ^μm² /hole or more and less than 4,000 ^μm² /hole. The area ratio of the through-holes to the fabric is preferably 1% or more and less than 40%. If it is less than 1%, the effect of improving adhesion by the through-holes may not be sufficiently obtained. If it is 40% or more, the fabric is more prone to weave slippage, the strength of the fabric decreases, and thicker fibers are required to maintain the shape, which may result in an increase in the thickness of the fabric. From the balance between preventing weave slippage and resin impregnation, it is more preferably 2% or more and 20% or less, and particularly preferably 3% or more and 10% or less.

In the fabric using at least a portion of the fluororesin-based fibers of the present invention, it is preferable that the number of single fiber layers in the thickness direction is 5 or less. More preferably, it is 3 or less, and particularly preferably 2 or less. If it exceeds 5, the fabric becomes thick relative to the fiber diameter, raising concerns that it may not adequately meet the miniaturization needs of electrical equipment, electronic equipment, and communication equipment. Since two or more fibers need to be bonded or intertwined to form a fabric, the practical lower limit for the number of single fiber layers in the thickness direction is 2.

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 include woven fabrics, wet-laid nonwoven fabrics, dry-laid nonwoven fabrics, etc. However, from the viewpoint of thickness and dimensional stability, a woven fabric is preferred.

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 weaves incorporating these in each layer can be used. However, a single-layer plain weave is preferred because it allows for relatively easy production of thin fabrics.

The fabric using at least a portion of the fluororesin-based fibers of the present invention can be compounded 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.

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 of pressure processing may be selected; calendering and hot pressing are common examples.

The fabric obtained using the above configuration may be subjected to 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, either individually or in mixtures, may be selected.

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, polypropylene and other polyolefin resins, and polyester resins are preferably used due to their impact resistance, dimensional stability, strength, and cost. Such thermosetting and thermoplastic resins may contain various additives commonly used industrially for their intended purpose, application, and to improve productivity or properties in manufacturing and processing processes. For example, they may contain modifiers, plasticizers, fillers, release agents, colorants, diluents, etc. Here, "main component" refers to the component with the largest mass ratio among the components excluding the solvent. In the case of a resin primarily composed of phenolic resin and polyvinyl butyral resin, this means that the mass ratios of these two resins are the first and second largest (in any order).

Regarding the method of impregnating the fabric with resin, when using thermosetting resins, 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 thermoplastic resins, methods such as melt extrusion lamination are commonly used.

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.

[Measurement and Evaluation Methods]
(1) Cross-sectional shape (confirmation of adhesion between individual fibers)
Using a KEYENCE VE-9800 scanning electron microscope, 20 cm x 20 cm 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.

(2) Single fiber fineness A sample of fluororesin fiber with a length of 1 m was taken by applying an initial load (6% load of fineness (g)), and the mass of the sample was measured to 1/10,000 g. The single fiber fineness (dtex) was calculated using the following 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).

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

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

(5) Area of through-holes and area ratio of through-holes to fabric Using a KEYENCE VHX-7000 microscope, the surface of the fabric was photographed at 500x magnification, and the photographed area was denoted as S _tot , the area occupied by through-holes as S _A , and the number of through-holes as N. The area of through-holes and the area ratio of through-holes to fabric were calculated using the following formulas.
Area of through-hole = S _A / N [ ^μm² / hole]
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, 20 cm x 20 cm test pieces were taken from the fabric sample, and cross-sectional observations were performed at 500x magnification at 10 different locations on the sample to measure the number of single fiber layers in the thickness direction. 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.

(7) Thickness of the fabric The thickness of the fabric was measured under 23.5 kPa in accordance with JIS L1096:2010 "Testing methods for woven and knitted fabrics" 8.4. a).

[Example 1]
46% by mass of viscose (cellulose concentration 9.0 wt%, alkali concentration 6.0 wt%) and 54% of a PTFE aqueous dispersion with a concentration of 60% were mixed, and then 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 a temperature of 70°C, and then refined in an alkaline bath containing a 0.12% aqueous solution of caustic soda to remove acidic components. Subsequently, 8H of unfired yarn, drawn from the alkaline bath, was concentrated into a single strand and squeezed with a nip roller. Then, while providing 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 together. 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/2.54 cm and a weft density of 132 threads/2.54 cm was produced on a loom.

[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.

[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.

[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.

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

[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.

[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 6.4 cc/min/spindle to 0.8 cc/min/spindle, 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 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.

[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.

[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.

For the fluororesin-based fibers and fabrics made from these fluororesin-based fibers prepared in Examples 1-6 and Comparative Examples 1-3, the following evaluations were performed: (1) cross-sectional shape (confirmation of adhesion between individual fibers), (2) individual fiber fineness, (3) total fineness, (4) arithmetic mean roughness, (5) area of through-holes and the ratio of the through-hole area to the fabric, (6) number of single fiber layers relative to 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-based fibers and fabrics made from these fluororesin-based fibers in the present invention can maintain the material properties of fluororesin-based fibers, such as heat resistance, chemical resistance, and low relative permittivity and dielectric loss tangent, while thinning the fabric relative to the fiber diameter, thus providing a fabric made from fluororesin-based fibers suitable for insulating material applications required in high-performance electrical equipment, electronic equipment, and communication equipment.

1: Fluororesin fiber 2: Recess 3: Protrusion

Claims (4)

A fluororesin-based fiber having a rectangular cross-sectional shape perpendicular to its longitudinal direction, and possessing continuous irregularities formed on its surface along a direction parallel to its longitudinal direction. 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. A method for producing fluororesin fibers according to claim 2 , wherein the fluororesin fibers have 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. A fabric made of fluororesin-based fibers as described in claim 1 .