JP2005248377A - Polytetrafluoroethylene fiber, method for producing the same, and cloth given by using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polytetrafluoroethylene (PTFE) fiber having a fiber section with a plurality of convexities or a flat fiber section of which the each has not been obtained yet, to provide a method for producing the same, and to provide a cloth given by using the same. <P>SOLUTION: This PTFE fiber having the fiber section with the plurality of the convexities or the flat fiber section is produced by matrix spinning processes. The method for producing the PTFE fiber comprises using viscose as a matrix and spinning the fiber by discharging a liquid mixture of the viscose and an aqueous PTFE dispersion into a coagulation bath which is regulated to contain specified components and have a specified concentration through a nozzle having a plurality of the convexities, wherein the spun fiber is characteristically baked so that the fiber is passed through a semi-baking process at a specified temperature, while being relaxed at a specified relaxation ratio, and then baked at a specified temperature. The cloth having collecting properties of fine dust more excellent than ever is obtained by using the PTFE fiber having the profiled section. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a polytetrafluoroethylene fiber having an irregular cross section with a uniform fineness obtained by a matrix spinning method, which has not been heretofore, and a method for producing the same. In addition, a fabric using the fiber and a fabric obtained by fibrillating the fabric have high dust collection efficiency, and can be suitably used as a filter material or a fine particle sealing material.

  Fluororesin fibers represented by polytetrafluoroethylene (hereinafter also abbreviated as PTFE) fibers are used for industrial materials because of their excellent heat resistance, chemical resistance, electrical properties, and low friction coefficient. Widely used.

  Since PTFE is insoluble and has a very high melt viscosity when heated and melted, its production method is known as a matrix method known as Patent Document 1, Patent Document 2, etc. (also referred to as an emulsion method), and Patent Document 3 For example, a split exfoliation method known from the above, or a paste extrusion method known from Patent Literature 4, Patent Literature 5, Patent Literature 6, and the like. On the other hand, a copolymer-based fluororesin copolymerized with PTFE (tetrafluoroethylene-6-fluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkoxy group copolymer (PFA) or 4-fluoroethylene). An ethylene-olefin copolymer (ETFE) or the like) can be produced by melt spinning.

  A modified cross-section PTFE fiber by a split peeling method and a felt-like filter material using the modified cross-section fiber are known (for example, Patent Document 7). However, the irregular cross-section fiber obtained by the split peeling method has a flat cross-sectional shape due to its manufacturing method, and its shape and fineness are random and non-uniform, so that it is difficult to produce nep during felt processing and difficult to produce was there.

  Further, a modified cross-section fiber by melt spinning using a copolymer type fluororesin and a bag filter using the same are known in Patent Document 8, Patent Document 9, and the like. However, since the copolymer-type fluororesin that performs melt spinning is copolymerized for the purpose of providing fluidity at the time of melting, the chemical resistance and heat resistance are inevitably inferior to those of PTFE. The method for producing PTFE fibers is known from Patent Document 10 and Patent Document 11. However, with respect to Patent Document 10, the purpose is to facilitate the mixing with other fibers by reducing the fineness of PTFE fiber, and the coagulation bath concentration and alkali bath concentration relating to the production method are wide, and the present invention. This is different from the range of manufacturing the irregular cross-section PTFE fiber. The purpose of Patent Document 11 is also related to increasing the strength of PTFE fibers, and the manufacturing method is also different from the method for manufacturing modified cross-section PTFE fibers of the present invention.

  That is, PTFE fibers having an irregular cross-sectional shape with a uniform fineness have not been obtained so far.

  Among the applications, PTFE fibers are particularly widely used for bag filters of garbage incinerators, and composite products of fluorine fibers and glass fibers are widely used. Alternatively, bag filters using heat-resistant fibers other than glass fibers, such as aramid, polyphenylene sulfide, polyimide or polyparaphenylene benzoxazole, are also widely used.

  In contrast to the bag filter using the above heat-resistant fiber, a bag filter having higher dust collection efficiency is currently required. This is a bag filter used in, for example, a gasification melting furnace and the like, and is a high collection efficiency filter capable of collecting dust having a small particle diameter.

  Alternatively, a fine particle sealing material using a fluororesin fiber is also widely used. For example, it is a toner sealing material for a printer and seals a minute amount of toner at a temperature of 150 ° C. or higher. As the colorization of toner progresses, a material capable of encapsulating a finer amount of toner is required.

  In view of this, a method has been proposed in which a fluororesin microporous film is bonded to the felt surface of the heat-resistant fiber, and dust is collected with high efficiency by the microporous film (Patent Document 12, etc.). This method certainly has a high dust collection efficiency of 0.5 μm or less, but has a problem of peeling because the adhesion between the fluororesin microporous film and other materials is poor. Further, when used for a bag filter, there is a problem that separation occurs due to this frictional force because friction occurs with the retainer when a backwash pulse is applied.

  Alternatively, as disclosed in Patent Document 13, the ultrafine fiber layer and the felt base material layer are integrated by needle punching, and the distribution of the ultrafine fiber is gradually reduced from the front surface to the back surface, and then by a high-pressure water punch. A filter with high collection efficiency is known in which a fiber that can be made ultrafine is divided into ultrafine fibers. However, in this method, it is necessary to laminate two or more kinds of different fibers, and there is a problem that there are many processing steps. Further, the ultrathinnable fiber of Patent Document 13 is only exemplified as a polyamide / polyester split fiber.

  Patent Document 14 describes a filter medium using a fluororesin fiber having branches and / or loops. The fiber is carded to form a web, and then branched by needle punching. And / or fibers that produce loops. The fabric having branches and / or loops obtained by this method has a problem that the strength of the web is lowered because the fluororesin fibers are divided on the entire surface and inside.

Patent Document 15 discloses a method of using PTFE fiber to generate a fibril using physical impact such as high-pressure jet water flow treatment to obtain a fabric excellent in collecting dust and excellent in wear resistance. It is disclosed in Reference 16. This patent document describes a technique using a round cross-section PTFE fiber produced by a matrix spinning method that has been commercially available. The present invention relates to a PTFE fiber having a convex portion for greatly surpassing such a conventional technique, a manufacturing method thereof, and a fabric using the PTFE fiber.
Japanese Patent Publication No.52-25453 JP-A-1-139840 Japanese Patent Publication No.51-88727 Japanese Patent Publication No. 51-18991 Japanese Patent Publication No.58-30406 JP-A-2-286220 JP 2001-276528 A Japanese Patent Publication No. 3-10723 JP 2002-282627 A Japanese Patent No. 2571379 Japanese Patent No. 3327027 JP 2000-140588 A Japanese Patent Laid-Open No. 4-032649 JP 2000-61224 A JP 2001-146633 A JP 2002-348765 A

  The subject of this invention is providing the PTFE fiber which has the convex part which was not obtained until now, its manufacturing method, and a fabric using the same. When a PTFE fiber having a fiber cross section including a plurality of convex portions or a flat fiber cross section obtained in the present invention is used, a fabric excellent in the ability to collect finer dust can be obtained.

  In order to solve the above-described problems, the polytetrafluoroethylene fiber according to the present invention has a fiber cross section including a plurality of convex portions or a flat fiber cross section.

  Further, in the method for producing polytetrafluoroethylene fiber according to the present invention, a mixture liquid with an aqueous dispersion of polytetrafluoroethylene is used as a matrix in a coagulation bath adjusted to a specific component and concentration, that is, using viscose as a matrix. Then, a sulfuric acid concentration of 7 to 13% and a sodium sulfate concentration of 7 to 15% are discharged from a die having a plurality of convex portions into a coagulation bath, and spinning is performed. In particular, when firing is performed, a method of performing firing at a specific temperature after passing through a semi-firing step at a specific temperature while giving relaxation at a specific relaxation rate. That is, after spinning and scouring, a baking roller is used, and a baking is performed at a temperature of 320 to 380 ° C. after a half baking step of 250 to 320 ° C. while giving relaxation of 1 to 5%.

  In addition, the fabric according to the present invention is characterized in that a fabric is obtained using the above-mentioned modified cross-section PTFE fiber, or a fabric in which fibrils are generated by applying a physical impact to the surface of the fabric, for example. .

  Furthermore, the present invention also provides a filter material and a fine particle sealing material using such a fabric.

  ADVANTAGE OF THE INVENTION According to this invention, the polytetrafluoroethylene fiber which has the fiber cross section containing the several convex part which was not before, or the flat fiber cross section can be provided, and when this polytetrafluoroethylene fiber is used, it will be more than before. A fabric excellent in the collection property of minute dust can be obtained.

Hereinafter, the present invention will be described in detail together with preferred embodiments.
The present invention has been intensively studied on the above-described problem, i.e., a fabric excellent in the ability to collect fine dust without impairing the properties of the PTFE fiber, and a fiber cross section including a plurality of convex portions on the surface of the fabric, or It has been clarified that, when a PTFE fiber having a flat fiber cross section or a fibrillated PTFE fiber is arranged, this problem can be solved all at once.

  In addition to PTFE, fluoropolymers include tetrafluoroethylene-6fluoropropylene copolymer (FEP) copolymerized with PTFE, tetrafluoroethylene-perfluoroalkoxy group copolymer (PFA), or tetrafluoride. There are ethylene-olefin copolymers (ETFE), and these are produced by melt spinning. However, PTFE is most excellent from the viewpoint of heat resistance, and the present invention is a fiber having a uniform shape and fineness having a fiber cross section including a plurality of convex portions made of PTFE, or a flat fiber cross section, which has not been disclosed so far. It is about.

  Conventionally, a matrix spinning method (also called an emulsion method), a split peeling method, a paste extrusion method, and the like are known as methods for producing PTFE fibers. The split peeling method is a manufacturing method in which PTFE powder is compressed by cylinder, sintered, split peeled and then stretched. The paste extrusion method is a method in which PTFE powder is kneaded with a wax-like lubricant without using a matrix polymer, molded into a rod-like or film-like form, then removed, stretched and fired (may not be fired). It is a manufacturing method to do. However, in these two production methods, the final fibrous material obtained by slicing the production method has a flat cross section, and it is random and inferior in uniformity. There was a drawback that it was easily generated.

  In order to obtain a modified cross-section PTFE fiber according to the present invention, it is necessary to perform a matrix spinning method. In the matrix spinning method, viscose or the like is used as a matrix and a mixed solution of PTFE with an aqueous dispersion of PTFE is discharged into a coagulation bath to be fiberized, and then refined and then fired. By firing at a temperature equal to or higher than the melting point of the polymer, PTFE is melted and the particles are fused while the majority of the matrix polymer is fired and scattered. After firing, the undrawn yarn is drawn directly in one step or two steps to develop strength.

  The PTFE fiber having a fiber cross section including a plurality of convex portions or a flat fiber cross section as referred to in the present invention can be obtained for the first time by using the matrix spinning method and performing yarn production under specific conditions. And cannot be obtained by paste extrusion.

  The fiber according to the present invention needs to be a PTFE fiber having a fiber cross section including a plurality of convex portions or a flat fiber cross section. Compared with a fiber having a round cross-sectional shape, a fiber including a plurality of convex portions in the fiber cross-sectional shape has a significantly large surface area, so that high dust collection efficiency is obtained.

  The cross-sectional shape of the fiber cross-section including the plurality of convex portions in the present invention is not particularly limited, and any of multi-leaf cross-sections such as triangles, squares and three to eight leaves, irregular cross-sections such as Y and H shapes, and other irregular cross-sections These cross-sectional shapes can also be used and are not particularly limited. Further, the convex portion may have a flat cross-sectional shape. The number of convex portions of the fiber is preferably 3 to 8 leaves from the viewpoint of increasing production stability from the surface of the yarn breakage during spinning, increasing the surface area of the fiber, and ease of fibril formation after the fabric. More preferably, it is 3-5 leaves.

  Moreover, it is preferable that the modified cross-section PTFE fiber of the present invention has a fineness of 1.5 dtex or more and 18.0 dtex or less. In general, in order to improve dust collection efficiency, a tendency to finer is required to increase the surface area, but on the other hand, increasing the fineness is also required in order to improve air permeability. When the irregular cross-section fiber of the present invention is used, even if the fineness is increased to about 18.0 dtex, the surface area is increased, so that a felt having a high dust collection performance can be obtained while maintaining a high level of air permeability. More preferably, it is 2.0 dtex or more and 15.0 dtex or less from the viewpoint of felt workability, and within this range, a felt that far surpasses the conventional dust collection performance is obtained.

  Next, the variation in fineness of the modified cross-section PTFE fiber of the present invention is preferably 10% or less of the fineness of the fiber. As described above, the cross section of the fiber obtained by the split peeling method or paste extrusion method is random and the fineness is not uniform. Therefore, the fineness variation is very large. For this reason, the dust collecting performance is good, but on the other hand, there is a drawback that a nep or the like is easily generated during the felt processing, and the processing is difficult. Inventing a modified cross-section PTFE fiber that has been deformed and suppressed variation in fineness according to the present invention makes it possible to achieve both of these. When the fineness variation exceeds 10% of the fineness of the fiber, it means that the cross-sectional shape and the fineness are not uniform, and it is difficult to perform stable processing, which is not preferable.

  On the other hand, irregular cross-section fibers with different fineness variations obtained by the present invention during felt processing are suppressed to 10% or less, or irregular cross-section fibers with different fineness variations suppressed to 10% or less and round cross-section fibers or split peeling. Even if the fiber obtained by the method or paste extrusion method is used at an appropriate mixing ratio, the process can be carried out without any problem.

  Furthermore, when the PTFE fiber of the present invention is cut and used as a short fiber, the fiber length may be about 30 to 100 mm, but is not particularly limited.

  The single yarn strength of the PTFE fiber of the present invention is preferably 0.7 cN / dtex or more, and the single yarn elongation is preferably 50% or less. If the single yarn strength is less than 0.7 cN / dtex and the single yarn elongation exceeds 50%, when the fiber is post-processed, the single fiber is stretched, resulting in poor processability.

  Moreover, it is preferable that the dry heat shrinkage rate in 300 degreeC * 30 minutes of the short fiber of this invention is 30% or less. In fact, when making and using felt etc., because of the heat resistance of the material, there are many applications that are used under high temperature, and if the dry heat shrinkage rate is too high, the felt will shrink and clogging will easily occur, It is not preferable. The dry heat shrinkage is more preferably 20% or less.

  The modified cross-section PTFE fiber of the present invention uses viscose as a matrix, and a plurality of protrusions are formed in a coagulation bath in which a mixed solution of PTFE with an aqueous dispersion is controlled to a sulfuric acid concentration of 7 to 13% and a sodium sulfate concentration of 7 to 15%. After discharging, spinning, and scouring from the die having a part, a baking roller is used to perform a baking at a temperature of 320 to 380 ° C. after passing through a half baking step of 250 to 320 ° C. while giving relaxation of 1 to 5%. Can be manufactured.

  The viscose used in the above method is preferably one usually used for rayon production, that is, a cellulose concentration of 5 to 10% by weight, an alkali concentration of 4 to 10% by weight, and carbon disulfide 27 to 32% by weight (based on cellulose). Further, as a matrix, a hydroxypropyl cellulose solution or a hydroxyethyl cellulose solution can be used instead of viscose as disclosed in JP-T-2001-511222.

  The aqueous dispersion of PTFE used in the above method preferably has a concentration of 50 to 70% by weight and contains a nonionic or anionic surfactant as a stabilizer in an amount of 3 to 10% by weight based on the PTFE polymer. The size of the dispersed particles of the PTFE aqueous dispersion is 0.5 μm or less, preferably 0.3 μm or less.

  These viscose and PTFE aqueous dispersions are mixed to prepare a mixed solution. At this time, the PTFE concentration in the mixed solution is 20 to 40%, preferably 25 to 35%, while the cellulose concentration is about 2 to 6%, preferably about 3 to 5%. At this time, if the PTFE concentration exceeds 40% and is too high, the yarn is difficult to coagulate in the coagulation bath. In addition, PTFE particles fall off from the yarn in a scouring bath / alkaline bath, and stable spinning cannot be performed. Further, it is not preferable because the fusion between the PTFE particles becomes strong at the time of firing, the single yarn is strongly fused, and the fibrillation of the single yarn itself hardly occurs. If the PTFE concentration is less than 20%, it is easy to coagulate in the coagulation bath, but it becomes difficult to maintain the irregular cross-sectional shape during firing, and a lot of carbonized components remain in the fired fiber. The strength decreases, which is not preferable.

  The mixed liquid thus mixed is defoamed, but if the temperature is high at this time, there is a concern that the viscose will solidify, and there is a concern that the moisture evaporates and PTFE aggregates. Therefore, it is necessary to control to a low temperature of 15 ° C. or less during defoaming. The degree of vacuum is preferably about 10 Torr. Regarding the timing of mixing the viscose and PTFE, a method of mixing the viscose and the PTFE aqueous dispersion before defoaming or mixing them immediately before defoaming and introducing them to a die using a static mixer or the like can be employed.

  Next, this spinning mixture is discharged from a molding die composed of discharge holes having a plurality of convex portions immersed in a coagulation bath and solidified. As the coagulation bath, an aqueous solution of an inorganic mineral acid and / or an inorganic salt is used. In the present invention, a mixed aqueous solution of sulfuric acid and sodium sulfate is used. At this time, the sulfuric acid concentration is preferably 7 to 13%. If the sulfuric acid concentration is less than 7%, the rate at which the yarn solidifies in the coagulation bath becomes very slow, which makes it difficult to obtain the desired irregular cross-sectional shape. On the other hand, if the sulfuric acid concentration exceeds 13%, the sulfuric acid attached to the fiber surface is not easily deoxidized, and yarn breakage occurs frequently in the firing process, and the rate at which the yarn solidifies in the coagulation bath becomes very fast. This is not preferable because it is difficult to control the irregular cross-sectional shape.

  It is preferable to adjust the sodium sulfate concentration to 7 to 15%. Sodium sulfate suppresses rapid coagulation of cellulose. When the sodium sulfate concentration is less than 7%, the rate at which the yarn solidifies in the coagulation bath becomes very fast, and it becomes difficult to control the irregular cross-sectional shape, which is not preferable. On the other hand, when the concentration of sodium sulfate exceeds 15%, the rate at which the yarn solidifies in the coagulation bath becomes very slow, which makes it difficult to obtain the desired irregular cross-sectional shape. That is, in the present invention, the PTFE modified cross-section fiber could be produced by adjusting both the sulfuric acid concentration and the sodium sulfate concentration within a specific range using the matrix method.

  For firing, a contact-type firing roller or a non-contact-type firing heater can be used, but a contact-type firing roller is preferably used. After squeezing the unfired yarn derived from the scouring bath or the alkaline bath as it is or with a nip roller, it is necessary to perform a half-baking step at 250 to 320 ° C. while giving a relaxation of 1 to 5%. The unfired yarn guided to the roller in the contact-type semi-baking process maintained at 250 to 320 ° C. rapidly shrinks on the roller to increase the tension. If the relaxation rate is less than 1%, the tension becomes too high and it becomes difficult to maintain an irregular cross-sectional shape, and yarn breakage due to shrinkage tends to occur. If it exceeds 5%, the relaxation rate is too high, and the yarn loosens, causing a problem in the processability. However, the relaxation of 1 to 5% can be performed not only once before entering the semi-baking but also between the rollers in the semi-baking process or between the rollers in the baking process. The semi-baking process is a process that must be performed before entering the subsequent baking process. When the roller temperature in the semi-baking step is lower than 250 ° C., the fiber is heated at a stretch in the subsequent baking step, so that the deformed cross section is easily deformed or fusion between single yarns is not preferable. On the other hand, when the temperature is higher than 320 ° C., the fibers are heated at a stretch in the semi-firing stage, so that the deformed cross section is easily deformed or fusion between single yarns is not preferable. Therefore, it is necessary to keep the roller in the semi-baking step in the range of 250 to 320 ° C.

  The semi-fired yarn is then fired at a temperature of 320-380 ° C. At this stage, most of the cellulose is burned and scattered, and the PTFE particles in the cellulose are thermally fused in a fibrous form to obtain an unstretched PTFE yarn. When the firing temperature is lower than 320 ° C., the PTFE particles in the fiber are not sufficiently fused, and yarn breakage frequently occurs during stretching after firing, and the fiber strength is also lowered. On the other hand, if the firing temperature is higher than 380 ° C., the deformed cross-sectional shape is deformed by heat and a desired sharp convex shape cannot be obtained, the fusion of PTFE particles in the fiber becomes strong, and the carbonization component remains. Since the amount is reduced, it becomes an irregular cross-section fiber that is difficult to fibrillate. Further, it is not preferable because fusion between single yarns also occurs, resulting in an adverse effect on the openability of the product.

  Next, the PTFE undrawn yarn is heat-drawn at a temperature of 300 to 400 ° C. by a commonly used drawing method to obtain a PTFE drawn yarn. Further, it can be produced in the same manner as described above using a non-contact type firing heater during firing.

  After the refining, it is preferable to perform a washing step with an alkali at an alkali concentration of 0.08 to 0.16% before performing the semi-baking and baking steps. In such an alkali cleaning bath, an aqueous solution of a compound selected from alkali metal or alkaline earth metal hydroxides, carbonates and bicarbonates is used, but generally an aqueous alkali metal solution, particularly an aqueous caustic soda solution is preferably used. It is done. The concentration of the compound is preferably 0.08 to 0.16 wt%. Generally, although depending on the firing temperature range of the next step, when PTFE fiber enters the firing step, yarn breakage frequently occurs in the firing step if an acid component remains on the fiber surface. In the case of a conventional round cross-sectional shape, cleaning is sufficient even if only the refining process takes into account the refining time. However, in the case of producing a fiber having a plurality of convex portions or a flat fiber cross-section as referred to in the present invention, the acid component inside the irregularities is alkalinized because of the large surface area derived from the irregularities on the fiber surface. Neutralization and washing are preferred. Washing with alkali affects not only the yarn breakage due to deoxidation, but also the degree of firing, that is, the color and fibrillation. If it is in the range of the semi-baking and baking temperature of this invention, the density | concentration of an alkali bath has the preferable range of 0.08-0.16 wt%. When the concentration of the alkaline bath is less than 0.08 wt%, the cellulose content is difficult to decompose during firing, and as a result, a large amount of cellulose remains that cannot be decomposed in the fiber after firing, making subsequent stretching difficult and stretching. Yarn breakage tends to occur frequently in the process. On the other hand, when the concentration of the alkaline bath exceeds 0.16 wt%, the cellulose starts to dissolve during alkali cleaning, and the residue tends to accumulate in the alkaline bath or in the guide. Further, the strength of the unfired yarn at the time of entering the semi-firing / firing process becomes weak, and troubles in passing through the process are likely to occur, which is not preferable. Further, the fusion of the PTFE particles inside the fibers during firing becomes strong and is difficult to fibrillate. A more preferable alkali concentration is in the range of 0.10 to 0.14 wt%.

  Furthermore, the temperature of the alkaline bath is preferably 20 ° C. or less. When the temperature of the alkaline bath exceeds 20 ° C., the cellulose begins to dissolve during alkali washing, as in the case where the alkali concentration is too high, and the residue tends to accumulate in the alkaline bath and the guide. This is not preferable because the unfired yarn strength becomes weak and troubles in passing through the process easily occur. The temperature of the alkaline bath is preferably 15 ° C. or lower.

  In the present invention, at least a part of the fabric using the PTFE fiber of the present invention can be prepared by mixing with the PTFE fiber of the present invention together with glass fiber, aramid, polyphenylene sulfide, polyimide, polyparaphenylenebenzoxazole or the like. . However, although aramid has an excellent decomposition temperature of 500 ° C. or higher, it has a weak point of low acid resistance. Polyphenylene sulfide has excellent chemical resistance, but its melting point is 285 ° C., which is slightly low in heat resistance. In the case of polyimide, there is a slight problem in alkali resistance, and polyphenylenebenzoxazole has high strength, but the market price is very expensive. Glass fiber has a decomposition point of 700 ° C. or higher and no heat resistance, but has a slight problem with alkali resistance. In contrast, fluororesin fibers, especially PTFE, exhibit excellent chemical resistance except that they melt into a specific perfluorinated organic liquid at 299 ° C or higher and are slightly attacked by molten alkali metal. As for heat resistance, since the melting point is as high as 327 ° C., the fluororesin fiber exhibits the most balanced and excellent performance as a whole. Therefore, it is most suitable for filter applications. As the mixing ratio, it is preferable to mix the PTFE fiber of the present invention at a ratio of 20 to 100%, preferably 40 to 100%. Although the PTFE fiber of the present invention can be used as a fabric as it is, it is more effective when used as a fibrillated fabric as described below.

  As the fabric having fibrillation in the present invention, it is preferable that the fibrillated PTFE fiber occupies 50% or more of the total area of the fabric surface. This is because when less than 50% of the total area is occupied, only a fabric having a low dust collection efficiency of 0.5 μm or less can be obtained.

  Next, the manufacturing method of this fabric is demonstrated. That is, the fabric made of such fibrillated PTFE fibers uses a means for generating fibrillated PTFE fibers on the surface of the fabric made of non-fibrillated PTFE fibers by applying a physical impact. Are manufactured.

  In the fabric according to the present invention, there are fibrillated PTFE fibers having a minimum fineness of 1.1 dtex or less and further 0.1 dtex or less on the surface, and the proportion of PTFE fibers having a high fineness gradually increases from the surface toward the inside. A fabric is preferred. This is because a cloth made of PTFE fibers fibrillated in the thickness direction of the cloth is not preferable because all the constituent fibers are finely divided, and the strength of the cloth is lowered. Moreover, the PTFE fiber which maintained the irregular cross section which exists in the inside of a fabric without fibrillation with the PTFE fiber fibrillated on the fabric surface improves dust collection efficiency. The fabric made of non-fibrillated PTFE fibers can be used without any problem as long as it is a felt in which a web made of PTFE short fibers is integrated with a needle punch. Further, the felt here may be a laminate of a web made of short PTFE fibers and a base fabric made of a woven fabric made of heat-resistant multifilaments or monofilaments, or a fluororesin fiber (4 fibers). Woven fabrics and knitted fabrics made of fluorinated ethylene-6-propylene propylene copolymer (FEP), tetrafluoroethylene-perfluoroalkoxy group copolymer (PFA), tetrafluoroethylene-olefin copolymer (ETFE), etc.) But it can be used without problems. Further, the fabric made of non-fibrillated PTFE fiber may be a single web made of PTFE fiber, or may be used without any problem even with a fabric calendered with this web or a resin bonded nonwoven fabric cured by attaching a resin. it can.

  The physical impact in the present invention is preferably high-pressure jet water flow treatment. This is because high-pressure jet water flow treatment can provide a physical impact while minimizing the occurrence of needle holes or fiber breaks caused by, for example, needle punching.

  The high-pressure jet water flow treatment here is preferably a treatment pressure of 3 MPa or more. Compared with a fluorine fiber having a round cross section, by using the deformed fiber obtained in the present invention, fibrils can be expressed even if the treatment pressure is set to be as low as 3 MPa, and damage to the fiber can be suppressed as much as possible. On the other hand, if the treatment pressure is less than 3 MPa, the fibers are not fibrillated, and only a fabric having a low dust collection efficiency such as 0.5 μm or less can be obtained.

  The fabric obtained by the present invention can be suitably used for filter materials or fine particle sealing materials. According to the fabric of the present invention, not only a filter that collects fly ash and dust, but also a filtration filter for liquid can be used without any problem. Further, as a fine particle sealing material, for example, a toner seal of a printer. Although it can be used as a stopping material, it is not limited to these uses.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated concretely, this invention is not limited to these. In addition, the measuring method of each physical property of a fabric is as follows.

(1) Fineness variation A sample is taken at random from the PTFE drawn yarn before fibrillation, and a cross-sectional photograph is taken by the embedding method as described below. Then, each cross-sectional photograph was cut out and the cross-sectional area was obtained by measuring the weight. The fineness of the PTFE fiber of the present invention was calculated using a specific gravity of 2.30 g / cm ³ . Thirty samples are randomly measured and the average value is calculated. The average value, the minimum fineness, and the degree of variation of the larger maximum fineness were measured.

<Embedding method>
Apply a little tension to the forming frame and fix it with cello tape. Heat at 200 ° C. to melt the mixture of paraffin and stearic acid. When the temperature reaches 130 ° C., ethyl cellulose is added little by little, and the mixture is kept warm for 1 hour with stirring to remove bubbles. After dropping to 100 ° C., it is poured into a forming frame. After cooling and solidifying, cut into blocks of appropriate size. Using a microtome, cut a section (thickness of about 7 μm) from the block and place it on a slide glass. At this time, thinly spread arbumen on a slide glass (albumen is an egg white and glycerin equivalent, 1% by weight of sodium salicylate added as a preservative). It is left to stand in a dryer maintained at 70 ° C. for 20 minutes, heat-treated and dried, then immersed in an isoamyl acetate bath for about 1 hour, decapsulated, and then air-dried. Put a drop of liquid paraffin on the slide glass, place the cover glass gently so that air does not enter, and take a picture using a microscope.

(2) Card passing property The room temperature is 30 ° C., the relative humidity is 60%, and while the raw cotton of 2 g / m to 10 g / m is put into the card machine, the winding of the cylinder roller when passing through the roller, the occurrence of the nep, Evaluation was performed as follows.
○: Good, △: Somewhat bad, ×: Very bad

(3) The proportion of fibril yarns in the total area of the fabric surface (hereinafter referred to as the proportion of fibrils)
A photograph of the surface of the fabric was taken to determine the area occupied by the fibril yarn. The photographing magnification was 50 times, and a video high scope was used.

(4) Minimum fiber diameter A cross-sectional photograph of the fabric is taken with an electron microscope, the fiber with the smallest fiber diameter is selected, and the fiber diameter of the fiber is read. The shooting magnification is 1000 times.

(5) Collection efficiency The collection efficiency was measured by the atmospheric dust counting method. The dust particle size is 0.5 μm or less, the filtration wind speed is 1.0 m / min, and the data is obtained by measuring the dust collection efficiency in the atmosphere using a particle counter.

Examples 1-5
Viscose maturity (salt point) 8.0, cellulose concentration 9.0%, alkali concentration 6.2% viscose 50% by weight and 60% concentration PTFE aqueous dispersion 50% mixed, then reduced pressure of 10 Torr Defoaming was performed below to obtain a forming stock solution having a polymer concentration of 30%. The PTFE resin content relative to the polymer in the stock solution was 87.0%, and the stock solution viscosity at 30 ° C. was 132 poise. This stock solution was introduced into a molding die having a plurality of convex portions, and the spinning die was changed so as to have the cross-sectional shape and fineness shown in Table 1, and discharged into a coagulation bath. The coagulation bath was a mixed aqueous solution having a sulfuric acid concentration of 10.0% and a sodium sulfate concentration of 11.0%, and the temperature was 10 ° C. Next, the solidified unfired yarn was washed with warm water having a temperature of 80 ° C., and then introduced into an alkaline bath containing an aqueous caustic soda solution having a concentration of 0.12%, followed by scouring to completely remove the acid component. Thereafter, the unfired yarn led from the alkaline bath is squeezed with a nip roller, then subjected to half firing at a temperature of 280 ° C. while giving 4% relaxation, and then fired using a firing roller maintained at 350 ° C. for 30 m. An undrawn yarn was obtained by taking up at a speed of / min. Next, the undrawn yarn was heat-drawn at a temperature of 350 ° C. to obtain a modified cross-section PTFE drawn yarn shown in Table 1.

The obtained deformed cross-section PTFE drawn yarn was combined, crimped and cut to obtain a 7.7 dtex × 70 mm PTFE deformed cross-section staple. The staple was carded to obtain a web. Thereafter, the above webs were laminated on both the front and back sides of a woven fabric made of PTFE multifilament (TOYOFLON # 4300 manufactured by Toray Fine Chemical Co., Ltd.) and integrated by needle punching at 350 pieces / cm ² to obtain a felt. Water jet punching was performed three times from the front and back surfaces of the felt at a treatment water pressure of 15 MPa and a feed rate of 5 m / min to obtain a fibrillated fabric. Table 1 shows the results of measurement of the fineness, fineness variation, card passage property, fibril ratio, minimum fiber diameter, and collection efficiency of the fiber.

Comparative Example 1
A fabric was obtained in the same manner as in Example 1 using PTFE staple fiber (TOYOFLON 7.4 dtex × 70 mm; round cross section manufactured by Toray Fine Chemical Co., Ltd.). The measurement results are shown in Table 1.

  Compared to Comparative Example 1, it can be seen that the fabric with fibrillation produced using the modified cross-section PTFE fiber of the present invention has a large proportion of fibrils, a small minimum fiber diameter, and consequently high dust collection efficiency. However, in the 10 leaves, the convex portion was small and approached a round cross section, or the ratio of fibrils decreased and the minimum fiber diameter tended to increase. However, the collection efficiency exceeded that of Comparative Example 1.

Example 6 and Comparative Example 2
For the non-fibrillated felt fabric obtained in Example 1 and Comparative Example 1, the fibril ratio, the minimum fiber diameter, and the collection efficiency were measured without performing fibrillation. The results are shown in Table 1.

  Comparing Example 6 which is a non-fibrillated fabric and Comparative Example 2, the dust collection efficiency of Example 6 using the modified cross-section PTFE fiber of the present invention was far superior. Further, the collection efficiency was almost the same as that of a fabric obtained by fibrillation of a round cross-section fiber (Comparative Example 1).

Examples 7-11
In Examples 7 to 11, yarn production was performed in the same manner as in Example 1 except that the fineness was changed. The results are shown in Table 2. In Examples 7 to 10, the fineness of Example 7 was 1.2 dtex which was less than 1.5 dtex, the fineness variation was about 10%, and the card passability was slightly poor. Further, in Example 11, when the fineness was 20.0 dtex exceeding 18.0 dtex, the minimum fiber diameter was slightly increased, and the dust collection efficiency tended to be slightly low. In addition, due to the large fineness, there was a tendency for yarn breakage during drawing to be seen due to insufficient firing.

  As is clear from these results, a fabric produced using PTFE fibers having a fiber cross section including a plurality of convex portions or a flat fiber cross section obtained by the present invention or a fibrillated fabric captures fine dust. It can be seen that the collection is excellent.

Claims (12)

  A polytetrafluoroethylene fiber having a fiber cross section including a plurality of convex portions or a flat fiber cross section.   The polytetrafluoroethylene fiber according to claim 1, wherein the number of convex portions is 3 to 8 leaves.   The polytetrafluoroethylene fiber according to claim 1 or 2, wherein the fineness is 1.5 dtex or more and 18.0 dtex or less, and the fineness variation is 10% or less of the fineness.   Viscose is used as a matrix, and a mixed liquid with an aqueous dispersion of polytetrafluoroethylene is discharged from a die having a plurality of convex portions in a coagulation bath controlled to have a sulfuric acid concentration of 7 to 13% and a sodium sulfate concentration of 7 to 15%. Then, after spinning and scouring, a baking roller is used, and after a half baking step of 250 to 320 ° C. while giving relaxation of 1 to 5%, baking is performed at a temperature of 320 to 380 ° C. A method for producing polytetrafluoroethylene fiber.   The method for producing polytetrafluoroethylene fibers according to claim 4, wherein a contact-type firing roller is used.   6. The polytetrafluoroethylene fiber according to claim 4, wherein the polytetrafluoroethylene fiber has a washing step of washing with an alkali at an alkali concentration of 0.08 to 0.16% before performing the semi-baking and baking steps. Production method.   A fabric using the polytetrafluoroethylene fiber according to any one of claims 1 to 3 at least in part.   The fabric according to claim 7, comprising fibrils.   The fabric according to claim 8, wherein the fibril is formed by physical impact.   The fabric according to claim 9, wherein the physical impact is a high-pressure jet water flow treatment.   A filter material using the fabric according to claim 7.   The material for fine particle sealing formed using the fabric in any one of Claims 7-10.