Porous aramid fibers and their fiber products

Abstract

To provide aramid fibers with high strength and a porous structure that is easy to handle. [Solution] An aramid fiber having a porous structure in which a non-porous layer is formed on the surface of the fiber, wherein the non-porous layer occupies at least 90% of the total surface area of ​​the surface layer, and the diameter of the fiber is 500 μm or less. Also, a textile product containing an aramid fiber having a porous structure in which a non-porous layer is formed on the surface of the fiber, wherein the non-porous layer occupies at least 90% of the total surface area of ​​the surface layer, and the diameter of the fiber is 500 μm or less.

Description

Technical Field

[0001] The present invention relates to aramid fibers and fiber products having a porous structure.

Background Art

[0002] In fiber products such as clothing and bedding, natural feathers are often used in many products that appeal for heat retention and heat insulation properties. The excellent fluffiness due to fine barbules and the excellent heat retention properties due to the complex void structure formed inside are exhibited.

[0003] However, products using natural feathers have drawbacks such as being unable to be washed at home in principle, and it may be difficult to additionally impart desired functions because natural feathers are composed of proteins. Also, from the perspective of animal protection in recent years, the demand for synthetic fiber batting made of synthetic fibers capable of imparting functionality has been increasing, and among them, the pursuit of synthetic fiber materials that exceed the excellent heat retention and heat insulation properties of natural feathers has been continuing.

[0004] In order to enhance heat retention and heat insulation properties, based on the heat transfer principle, it is important to lower the thermal conductivity of the material, suppress heat transfer by convection of heat inside the material, and suppress the release of radiant heat to the outside. One of the materials attracting attention as a material capable of achieving these is an aerogel material.

[0005] Aerogel materials are porous materials in which colloidal particles and polymers accumulate with each other to form a three-dimensional network structure. Also, the dispersion medium is a gas, and it is generally a material with a low apparent density and a high porosity, resulting in a low thermal conductivity. In particular, the pore diameter of the aerogel is several tens of nm, which is smaller than about 70 nm, which is the average free path of air under atmospheric pressure, and forms dead air by inhibiting heat transfer by convection, exhibiting excellent heat retention and heat insulation properties.

[0006] While possessing such excellent functionality, aerogels generally have poor mechanical properties and are brittle; therefore, aerogel materials are usually used in a block form. Many aerogel materials utilize silica particles, and aerogel materials incorporating cellulose nanofibers, carbon nanofibers, etc., have also been reported. However, in all cases, obtaining continuous fibrous aerogels has been difficult.

[0007] Among these, aerogels using poly(p-phenylene terephthalamide) (PPTA) possess excellent mechanical properties derived from the PPTA molecular chain, and continuous fiber formation is being considered as the most practical material.

[0008] Patent Document 1 discusses gels and nanocomposite materials containing branched aramid nanofibers, and discloses that aramid aerogel fibers having a three-dimensional network structure can be obtained by solution spinning a dispersion of aramid nanofibers, with the aim of improving the mechanical properties of the composite material.

[0009] Furthermore, Patent Document 2 discloses an aramid aerogel fiber in which the aramid is formed from hydrophobized aramid to suppress collapse caused by the shrinkage of the skeleton due to water absorption by the amide bonds that are abundant in the aramid structure, thereby improving the mechanical properties of the aramid aerogel fiber. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Special Publication No. 2019-511387 [Patent Document 2] Special Publication No. 2023-536381 [Overview of the Initiative] [Problems that the invention aims to solve]

[0011] The aramid aerogel fibers obtained by the technology described in Patent Document 1 are characterized by having a branched structure of aramid nanofibers in order to enhance the mechanical properties of the composite material. However, while the branched structure is suitable for dispersion in a matrix, its mechanical properties are insufficient for use as a continuous fiber.

[0012] Patent Document 2 describes how hydrophobicizing aramid can suppress the breakdown of the aerogel structure due to water absorption, but this also tends to weaken the intermolecular interactions between aramid molecules, making it difficult for the inherent mechanical properties of aramid to be exhibited.

[0013] Both Patent Documents 1 and 2 describe aramid fibers having a porous structure and possessing excellent heat insulation properties. However, due to their low strength, they are prone to breakage during the process of forming fabrics such as woven or knitted materials due to friction from process tension and thread guides, making it difficult to form them into fabrics. Therefore, there has been a need for aramid fibers with high strength and a porous structure that are easy to handle. [Means for solving the problem]

[0014] To solve the above problems, the aramid fiber of the present invention has the following configurations (1) to (3). (1) An aramid fiber having a porous structure in which a non-porous layer is formed on the surface of the fiber, wherein the non-porous layer occupies at least 90% of the total surface area of ​​the surface layer, and the diameter of the fiber is 500 μm or less. (2) The tensile strength of the fiber measured in accordance with JIS L1013:2021 is 35 MPa or more, the elongation of the fiber is 10% or more, and the density of the fiber is 0.2 g / cm³. 3 The aramid fiber described in (1) above is as follows: (3) Textile products containing aramid fibers as described in (1) or (2) above. [Effects of the Invention]

[0015] The present invention relates to porous aramid fibers. By having excellent tensile strength and appropriate elongation, the fibers are excellent in handling properties and can be easily made into fabrics such as woven fabrics and knitted fabrics. Further, by using such fabrics, it is possible to provide fiber products having excellent heat retention and heat insulation properties due to a so-called aerogel structure.

Brief Description of the Drawings

[0016] [Figure 1] FIG. 1 is a schematic view showing an example of the fiber cross section of the aramid fiber of the present invention. [Figure 2] FIG. 2 is a schematic view showing the measurement range of the orientation ratio of the aramid fiber of the present invention. [Figure 3] FIG. 3 is a schematic view showing each distance related to the orientation measurement range setting of FIG. 2.

Modes for Carrying Out the Invention

[0017] <Structure of the Fiber> The aramid fiber of the present invention is an aramid fiber having a porous structure.

[0018] The porous structure in the present invention is a three-dimensional network structure in which aramid fibrils form a network, and represents a so-called aerogel structure in which the micropores formed between the fibrils are filled with gas.

[0019] In the present invention, when the aramid fiber is two-dimensionally observed with a scanning electron microscope (hereinafter, SEM) or a transmission electron microscope (hereinafter, TEM), if a pore structure other than spherical pores can be confirmed, it is considered to have a porous structure. For example, by observing the fiber cross section at an observation magnification of 50,000 times or more using SEM and confirming the shape of the pores, it can be determined whether or not it has a porous structure.

[0020] Furthermore, it is particularly preferable that the average pore diameter of the micropores contained in the aramid fibers of the present invention is 70 nm or less. By setting the size to be less than or equal to the mean free path of air, heat transfer due to air movement and convection is suppressed, and dead air is retained in the micropores inside the fibers, resulting in excellent heat retention and insulation properties.

[0021] Furthermore, in the aramid fibers of the present invention, it is preferable that larger pores also exist. In particular, when a high spinning draft is applied, aggregation of fibrils present in a certain range progresses, and the voids around each fibril come together, which can lead to the formation of larger pores in the μm size. In such cases, the number of micropores decreases, so the fiber density tends to be high, but the tensile strength of the fiber can be further increased by the increased orientation of the fibrils.

[0022] The aramid fiber of the present invention requires that a non-porous layer be formed on at least 90% of the total surface area of ​​the fiber.

[0023] In this invention, the non-porous layer is a non-porous layer in which oriented fibrils aggregate and the pore structure substantially disappears. When observed two-dimensionally with SEM or TEM, it exhibits a structure in which no pore structure can be confirmed up to a depth of 10 nm from the fiber surface. This non-porous layer is also called the skin layer, and having such a non-porous layer makes it possible to exhibit high mechanical properties due to highly oriented fibrils. Figure 1 schematically shows the porous structure portion 1 and the non-porous layer (skin layer) 2 in the fiber cross-section of an aramid fiber.

[0024] Furthermore, the greater the proportion of the non-porous layer on the fiber surface, the less unevenness in tensile strength is in the fiber cross-sectional direction. In fabric formation processes such as weaving and knitting, the fiber cross-section is subjected to external stress from all directions due to bending and friction by various thread guides, which can cause process problems such as bending and thread breakage due to stress concentration. From the viewpoint of preventing such problems, it is preferable to reduce unevenness in tensile strength and create a fiber structure that is less prone to stress concentration, and the proportion of the non-porous layer on the fiber surface is more preferably 92% or more, and even more preferably 95% or more.

[0025] <Fiber diameter> The aramid fibers of the present invention must have a fiber diameter of 500 μm or less.

[0026] For use in woven or knitted fabrics, flexible fibers are preferable, and a fiber diameter of 200 μm or less is more preferable, particularly from the viewpoint of making the resulting textile product smooth to the touch. Furthermore, a diameter of 100 μm or less is even more preferable, as it further reduces discomfort such as stiffness when worn, resulting in a denser woven or knitted fabric.

[0027] In this invention, the fiber diameter is determined by observing the cross-section of the aramid fiber with a digital microscope or the like. If the fiber has a round cross-section, the diameter along the fiber surface is determined. If the fiber has an irregular cross-section, the fiber diameter is determined as the average of the diameters of the inscribed circle and the circumscribed circle of the fiber.

[0028] <Orientation ratio of inner and outer layers of fiber> The aramid fiber of the present invention has a low density due to its aerogel structure, and to achieve both the high strength of aramid and a high density non-porous layer formed by the aggregation of oriented fibrils, it has a high density non-porous layer. Here, the density difference between the fiber surface layer and the fiber inner layer can also be confirmed by evaluating the orientation of each part and calculating the difference, and can be expressed as the orientation ratio R1 / R2 of the fiber inner and outer layers.

[0029] Therefore, in the aramid fiber of the present invention, it is preferable that the orientation ratio (R1 / R2) of the inner and outer layers of the fiber is greater than 1.0.

[0030] Here, the orientation ratio of the inner and outer layers of a fiber is the orientation ratio obtained by dividing R1 by R2, where R1 is the orientation ratio of the outer layer (skin layer) of the fiber and R2 is the orientation ratio of the inner layer of the fiber.

[0031] The orientation ratio of each of the aforementioned parts was determined using a micro-Raman spectrometer, and the result was 1610 cm², which is due to the C=C stretching vibration of the aromatic rings constituting the polymer backbone of the aramid fiber. -1 For nearby peaks, the Raman spectra are measured with polarization settings of 0° (parallel) and 90° (perpendicular) to the fiber axis, and the relative intensity ratio of the parallel / perpendicular settings is calculated to determine the intensity.

[0032] The measurement ranges for the orientation ratio R1 of the fiber surface and the orientation ratio R2 of the fiber inner layer are shown in Figures 2 and 3.

[0033] To set each measurement range, first, the circumscribed circle of the fiber cross-section and the fiber center are determined from that circumscribed circle. Next, the orientation ratio R1 of the fiber surface is determined by finding the distance L0 from the fiber center to the fiber surface, and measuring within a range of 5% from the fiber surface (within distance L1) relative to that distance.

[0034] Furthermore, the orientation ratio R2 of the inner fiber layer is measured at a distance L0 from the fiber center to the fiber surface layer, within a range of 1 / 3 or less from the fiber center (within a distance L2).

[0035] For both R1 and R2, measurements are taken at five or more points in a single cross-section, and the simple average value is calculated and designated as the R1 and R2 for that cross-section. Then, R1 and R2 are similarly calculated for five cross-sections for each fiber, and their simple average value is designated as the R1 and R2 for that fiber.

[0036] By increasing the orientation of the fiber surface layer, the tensile strength of the fiber is improved, so it is more preferable that the orientation ratio R1 / R2 of the inner and outer layers of the fiber is 1.1 or higher. It is even more preferable that it is 1.2 or higher, as this makes the fiber less prone to crushing and easier to handle in post-processing and sewing processes.

[0037] On the other hand, if the orientation ratio of the inner and outer layers of the fiber is increased too much, delamination between the outer and inner layers of the fiber becomes more likely, leading to a decrease in the tensile strength of the fiber. This can cause problems such as entanglement with guides or thread breakage during post-processing or sewing. Therefore, the practical upper limit for the orientation ratio R1 / R2 of the inner and outer layers of the fiber is 10.0 or less.

[0038] <Tensile strength> The aramid fibers of the present invention preferably have a tensile strength of 35 MPa or higher. A tensile strength of 35 MPa or higher makes it less likely for the fibers to break during drawing or due to friction when processing, resulting in fibers with excellent handling properties.

[0039] From this perspective, a tensile strength of 50 MPa or higher is more preferable. Furthermore, a tensile strength of 100 MPa or higher is even more preferable. Increasing the tensile strength results in a stronger porous structure, which reduces the limitations on the drying methods that can be applied when forming the fibers.

[0040] In particular, even when conventional drying is applied, it becomes easy to obtain an aerogel structure while maintaining a porous structure. This allows for the use of simpler equipment, resulting in lower costs and a process that is easy to mass-produce.

[0041] <Elongation> The aramid fibers of the present invention preferably have an elongation of 10% or more. The elongation of a fiber is one of the indicators of its flexibility, and flexibility helps to suppress problems such as thread breakage during the fabric formation process of woven and knitted fabrics. From this viewpoint, an elongation of 20% or more is more preferable, and 30% or more is even more preferable.

[0042] Furthermore, in the fabric formation process, it is preferable to set the elongation to such an extent that the fabric does not deform significantly when process tension is applied, in order to stabilize the dimensions of the fabric, and the upper limit of the elongation is 150%.

[0043] In this invention, the tensile strength and elongation of the fibers are determined by measuring them in accordance with JIS L1013:2021.

[0044] <Fiber density> The aramid fiber of the present invention has a fiber density of 0.2 g / cm³. 3 The following is preferable:

[0045] With the aforementioned fiber density, it is easy to form a structure with many voids inside the fibers, and by retaining a large amount of air, which has low thermal conductivity, it is possible to exhibit excellent heat retention and insulation properties. In the present invention, the fiber density is more preferably 0.15 g / cm³. 3 More preferably, 0.10 g / cm³ 3 As described below, in addition to improving heat retention and insulation, it is possible to further enhance the lightness of the textile product.

[0046] In this invention, fiber density can be determined by calculating the volume from the length of a fiber cut to a predetermined length and the diameter of the fiber, measuring the mass of the fiber, and then calculating the mass / volume.

[0047] <Textile products> The porous aramid fibers of the present invention may be used as short or long fibers, or they may be used after various yarn processing methods have been applied. They can also be used as padding in various forms, such as granular cotton, sheet-like cotton, and long-fiber cotton, or as fabrics such as woven fabrics, knitted fabrics, nets, or nonwoven fabrics.

[0048] The porous aramid fibers of the present invention possess an aerogel structure while also exhibiting high strength and moderate elongation, making them less prone to breakage and easy to handle. Therefore, they are preferable for use in woven or knitted fabrics, which were difficult to achieve with conventional porous aramid fibers.

[0049] Applications of the aforementioned fabric include, for example, firefighter suits, sportswear, protective work clothes, and heat-resistant electromagnetic shielding sheets, as its features such as heat retention and insulation can be effectively utilized. In particular, it is preferable to use it as clothing worn in harsh environments, and the lightweight nature of the aerogel structure further reduces the burden during activities, making it possible to provide textile products that can reduce fatigue for the wearer.

[0050] <Manufacturing method> Here, we present an example of a method for producing aramid fibers according to the present invention.

[0051] The aramid fibers of the present invention are manufactured by dissolving raw material aramid (usually in fibrous form, hereinafter referred to as raw material aramid fibers) in a solvent to defibrillate it into fibrils, and then re-aggregating it to form fibers.

[0052] The raw material aramid fibers used in the present invention include meta-aramid fibers (hereinafter referred to as m-aramid fibers) and para-aramid fibers (hereinafter referred to as p-aramid fibers). Examples of m-aramid fibers include meta-aromatic polyamide fibers such as polymetaphenylene isophthalamide fibers (e.g., DuPont, trade name "Nomex"). Examples of p-aramid fibers include para-aromatic polyamide fibers such as polyp-phenylene terephthalamide fibers (e.g., Toray DuPont, trade name "Kevlar®") and copolyp-phenylene-3,4'-diphenyl ether terephthalamide fibers (e.g., Teijin Limited, trade name "Technora"). Among these aramid fibers, p-aramid fibers are preferred because they have high strength and flexibility. Furthermore, in order to exhibit excellent mechanical properties when the porous structure of the present invention is used as an aramid fiber, it is preferable that the intermolecular interactions acting between the fibrils are strong, and poly(p-phenylene terephthalamide) fibers are particularly preferred.

[0053] Next, a dispersion is prepared by dispersing the raw aramid fibers in a dispersant.

[0054] As an example, we will explain using p-aramid fibers as the raw material aramid fibers. During preparation, the raw material aramid fibers are washed before use. First, the p-aramid fibers are cut into pieces about 5 mm apart, then immersed in ethanol as a washing solution and washed using an ultrasonic cleaner. Furthermore, they are washed with distilled water to remove the ethanol, and the washed fibers are dried.

[0055] Furthermore, the dispersant should be pre-anhydrous. Examples of solvents that can be used as dispersants include dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), n-methylpyrrolidone (NMP), tetrahydrofuran (THF), acetophenone, and butyl cellosolve. Of these, DMSO is preferred due to its stability during handling. In addition, by mixing these solvents with an alkali such as potassium hydroxide (KOH) or tetrabutylammonium fluoride (TBAF) (using them as a mixed solvent), fibrilization can be achieved. The solvent and the mixed solvent are mixed to form the dispersant.

[0056] TBAF is preferred as the mixed solvent. The fluoride ions in TBAF inhibit intermolecular hydrogen bonding, and heating activates molecular motion, leading to fibril defibration. Defibration using TBAF suppresses the reduction of aramid molecular weight compared to conventional defibration using alkalis (such as KOH). As a result, the mechanical properties are easily enhanced, and as described later, the resulting spinning solution is less prone to yarn breakage even when a spinning draft is applied in the coagulation bath.

[0057] The aforementioned solvent is subjected to distillation to achieve anhydrousness, and the solvent after distillation is stored in an environment that prevents contamination with water.

[0058] Next, a dispersion of p-aramid fibers is prepared using raw aramid fibers and a dispersant.

[0059] First, prepare a solution by mixing DMSO and TBAF in a mass ratio of 11:1 as the solvent. To prevent water contamination of the dispersion, calcium hydride is used as a drying agent. Add calcium hydride to the total solution in a mass ratio of 30:1 and stir. Then, centrifuge is performed again using a centrifuge, and once it is confirmed that the supernatant is clear, the supernatant is collected in a separate container.

[0060] Next, the p-aramid fibers are added to the supernatant liquid (in this case, a DMSO / TBAF mixed solution) to a predetermined concentration, and the mixture is heated and stirred to prepare a dispersion in which the p-aramid fibers are dissolved. During this process, it is preferable to surround the container with aluminum foil to shield it from light.

[0061] Once the dispersion is prepared, it is spun into fibers. The aramid fibers of the present invention can be spun using a wet spinning method, in which acetone, DMSO, water, butyl cellosolve, etc., are used individually or in mixtures as a coagulation bath to coagulate the aramid fibrils dispersed in the dispersion and form fibers. The dispersion prepared as described above can be used as the spinning stock for obtaining the aramid fibers of the present invention.

[0062] The spinning solution is preferably supplied at a temperature of 40°C or higher, more preferably 60°C or higher, and even more preferably 80°C or higher, in order to prevent flow blockage caused by physical gel formation due to temperature decrease and to maintain fluidity from the discharge port to the discharge port.

[0063] Next, the spinning stock solution is discharged into the coagulation bath from a spinning nozzle equipped with discharge holes. From the viewpoint of orienting the aramid fibrils, it is preferable to align the directionality of the fibrils even immediately after discharge by applying shear during discharge. The diameter of the holes in the spinning nozzle is preferably 0.5 mm or less, more preferably 0.4 mm or less, and even more preferably 0.2 mm or less.

[0064] In this case, if the diameter of the spinning nozzle is too small, the pressure loss through the nozzle will be large, making it difficult to extrude the spinning solution. Therefore, it is preferable to set the lower limit of the spinning nozzle diameter to φ0.05 mm.

[0065] In the production of aramid fibers according to the present invention, a spinning draft is applied by actively taking up the fibers in the coagulation bath, thereby obtaining aramid fibers in which fibrils with promoted orientation of the fiber surface layer are aggregated. The spinning draft is influenced not only by the ratio of the discharge linear velocity to the take-up velocity, but also by the shrinkage rate of the fibers in the coagulation bath, and is applied by taking up the discharged polymer in the coagulation bath in a way that prevents it from becoming loose.

[0066] Here, if the spinning draft is too high, fibril orientation develops while the porous structure tends to decrease, making it difficult to obtain the effects of the present invention. Therefore, it is preferable to adjust the upper limit of the spinning draft as appropriate while checking the properties of the obtained aramid fibers.

[0067] Next, the obtained aramid fibers are subjected to solvent replacement in acetone. Preferred solvents for substitution include acetone, water, DMSO, DMF, DMAc, NMP, THF, acetophenone, and sorbitol. If the solubility parameter (hereinafter referred to as SP value) of the solvent is too far from that of aramid, the aramid will be more likely to aggregate. Conversely, if the SP value is close to that of aramid, the aramid will be less likely to aggregate, and the mechanical properties of the fibers, such as tensile strength, will tend to decrease. Therefore, it is more preferable to use acetone or sorbitol, which are solvents whose SP values ​​are appropriately close to those of aramid.

[0068] Regarding the solvent substitution process, it is recommended to measure the conductivity and continue until the conductivity is less than 0.01 mS / m.

[0069] In the present invention, after solvent substitution, the internal solvent (acetone) is replaced with a gas by drying such as conventional drying or supercritical fluid drying, thereby obtaining porous aramid fibers having an aerogel structure that maintains a three-dimensional network structure. Supercritical fluid drying is preferred for drying.

[0070] The supercritical or subcritical fluid used in supercritical fluid drying is not particularly limited, and one or more known supercritical or subcritical fluids can be used in mixture form.

[0071] As the supercritical or subcritical fluid, it is preferable to use a supercritical or subcritical fluid mainly composed of one or more substances selected from the group consisting of carbon dioxide, nitrous oxide, trifluoromethane, hexafluoroethane, methane, ethane, and ethylene. The temperature of the supercritical or subcritical fluid is not particularly limited, but from the viewpoint of energy saving, reduction of equipment installation costs, simplification of equipment maintenance, and cost reduction, it is more preferable to use a supercritical or subcritical fluid with a temperature of 90°C or lower.

[0072] Furthermore, it is even more preferable to use a supercritical or subcritical fluid of carbon dioxide because it has excellent adsorption properties to fibrous materials, is safe as it is not flammable or explosive, and is readily available.

[0073] The porous aramid fibers of the present invention obtained by the above process may be used as is, or they can be made into fabrics such as woven or knitted fabrics using conventionally known equipment. Within the limits that do not impair the effects of the present invention, various functional agents such as surface smoothing agents and antistatic agents can be used as appropriate to further improve the handling when processing into textile products. [Examples]

[0074] The knitted fabric of the present invention and its effects will be specifically described below with reference to examples. The following evaluations were performed in the examples and comparative examples.

[0075] A. Confirmation of porous structure Observations were performed using a FE-SEM (Hitachi High-Tech Corporation's cold cathode field emission scanning electron microscope "Regulus 8220") at an acceleration voltage of 1.0 kV. For the observation samples, cross-sections were prepared using the BIB method for the fiber cross-section, followed by conductive treatment (OsO4 coating). Ten images were captured within a single fiber cross-section at a magnification of 50,000x. The presence or absence of a porous structure was confirmed by examining the captured images to identify pore structures other than spherical pores.

[0076] B. Percentage of non-porous layers The same method as in A. was used to obtain the results by observing the fiber surface of conductively treated fibers in two dimensions using a FE-SEM. Specifically, 20 images were taken of the fiber surface at a magnification of 50,000x, with the observation area changed so that it did not overlap. The captured images were binarized using image processing software, and the area ratio of the pores shown in black was calculated on the fiber surface (total surface area), which is shown in white, excluding areas where the surface is unclear, such as large grooves on the fiber surface. The percentage of the non-porous area was calculated by subtracting this percentage from the total area, which was set to 100%. The percentage of the non-porous layer (%) was obtained by rounding the simple average value of the 20 captured images to the first decimal place.

[0077] C. Fiber diameter Aramid fibers after supercritical drying were observed at 1000x magnification using a digital microscope (Keyence Corporation "VHX-6000"), and 25 images were taken by changing the position along the fiber axis. The fiber diameter was measured in measurement mode for each image, and the fiber diameter (μm) was defined as the simple average value of the 25 images, rounded to the first decimal place.

[0078] D. Tensile strength and elongation of fibers Using a tensile testing machine conforming to JIS L1013:2021, a sample length of 50 mm was used, and measurements were taken 10 times per sample at a tensile speed of 10 mm / min. The tensile strength was calculated by rounding the simple average of the breaking strength (N) detected in the 10 measurements to the first decimal place, and this value was then calculated from the fiber cross-sectional area (mm²) before the tensile test, which was determined from the fiber diameter measured in C. 2The tensile strength (MPa) was converted to units by dividing by (), and then rounded to the first decimal place to express it as an integer.

[0079] Furthermore, the elongation (%) was calculated by rounding the simple average of the elongation obtained from 10 measurements to the first decimal place, similar to the tensile strength mentioned above, and expressing it as an integer.

[0080] E. Fiber density For fibers cut to 50 mm, the volume was calculated from the fiber diameter obtained in C. The mass of the fibers was also measured and calculated from mass / volume. Ten samples were evaluated, and the fiber density (g / cm³) was obtained by rounding the simple average value to the third decimal place. 3 )

[0081] F. Evaluation of tubular knitting Using a tubular knitting machine with a bobbin diameter of 3.5 inches (8.9 cm) and 27 g of yarn (NCR-BL, manufactured by Eiko Sangyo Co., Ltd.), tubular knitting was performed, and the presence or absence of yarn breakage, the feasibility of tubular knitting, and the tactile feel of the knitted fabric were evaluated on the following four-point scale. A: It allows for tubular knitting without yarn breakage, and has a flexible and excellent feel. B: Allows for tubular knitting without yarn breakage and has a good feel. C: It can be knitted in a tubular shape, but it has knitting defects, resulting in a stiff and foreign-textured feel. D: Tube knitting is not possible due to yarn breakage.

[0082] G. Thermal insulation A sample was prepared by cutting open the tubular knitted fabric of F., cutting one piece of knitted fabric to 5cm x 5cm, and preparing two pieces of glass fiber filter paper also cut to 5cm x 5cm. The knitted fabric was then sandwiched between the filter papers to create a sample, which was placed on a hot plate heated to 120°C at room temperature. A 3cm cube of ice (made at -18°C) was placed on top of the sample and heated on the hot plate. After heating for 40 seconds, samples with an ice height of 2cm or more were rated as ○ (good), and samples with ice melted to less than 2cm were rated as × (poor), and the presence or absence of heat insulation was evaluated.

[0083] H. Orientation ratio of inner and outer layers of fiber: R1 / R2 Using a micro-Raman spectrometer (HORIBA Jobin Yvon LabRAM HR-800), Raman spectra were measured for the inner and outer layers of the fiber at polarization settings of 0° (parallel to) and 90° (perpendicular to) the fiber axis.

[0084] The main measurement conditions for the micro-Raman spectrometer were as follows: Objective lens: 100x Beam diameter: 1 μm Light source: 633nm Slit: 100 μm Hole size: 200 μm

[0085] For the measurement samples, we used aramid fibers that had been embedded in epoxy and polished to prepare cross-sections of individual fibers (sections perpendicular to the fiber axis).

[0086] For the aforementioned sample, Raman spectra were measured for the inner and outer layers of the fiber within the measurement range shown in Figure 2, and the orientation ratio R1 of the fiber outer layer and the orientation ratio R2 of the inner layer of the fiber were determined.

[0087] For both R1 and R2, five or more measurements were taken in a single cross-section, the simple average value was calculated, and the value was rounded to two decimal places. These values ​​were then used as the R1 and R2 values ​​for the measured sample.

[0088] Next, using R1 and R2 from the measurement sample, R1 / R2 was calculated, and the value rounded to the second decimal place was taken as the orientation ratio of the inner and outer layers of the fiber, R1 / R2.

[0089] (Example 1) <Preparation of spinning solution> Poly(p-phenylene terephthalamide) fibers (Toray DuPont "Kevlar® 29") were cut into 5mm intervals and immersed in ethanol for 15 minutes using an ultrasonic cleaner. Further, the fibers were washed five times with distilled water to remove the ethanol. After washing, the fibers were dried at 120°C for 24 hours and then stored in a desiccator. Additionally, DMSO, used as a dispersant, was distilled and stored in a desiccator in the same bottle as molecular sieves to prevent water contamination after distillation.

[0090] Next, the DMSO and TBAF were mixed in a 100 ml screw-top bottle in a mass ratio of 11:1, and calcium hydride was added to the entire solution in a mass ratio of 30:1. A stirring bar was placed in the bottle, the bottle was covered with aluminum foil and further sealed with Parafilm, and the mixture was stirred for 48 hours at 310 rpm using a magnetic stirrer. After that, the mixture was centrifuged twice at a rotation speed of 4000 rpm for 10 minutes each time, and after confirming that the supernatant was clear, this supernatant was collected in a round-bottom flask. The aramid fibers removed from the desiccator during the centrifugation were dried at 100°C for 20 minutes.

[0091] Next, the dried p-aramid fibers were added to the supernatant (DMSO / TBAF mixed solution) to a concentration of 1.0 wt%, and the mixture was heated and stirred for 1 hour at 80°C and a rotation speed of 400 rpm using a magnetic stirrer and an oil bath. During heating, the round-bottom flask was surrounded with aluminum foil to shield it from light. After confirming that the fibers had completely dissolved, the preparation of the dispersion was considered complete.

[0092] <Spinning> The prepared dispersion was quickly filled into a syringe of a spinning apparatus heated to 80°C using a temperature-controlled bath. The dispersion was then discharged into a coagulation bath (water depth 25 cm) containing a mixture of acetone and distilled water in a mass ratio of 85:15, from a nozzle with a diameter of φ0.10 mm at a discharge linear velocity of 12.73 m / min. The discharged polymer was then drawn up by a roll installed in the coagulation bath at a draw speed of 5.50 m / min, ensuring that there was no slack in the fibers in the bath and that a spinning draft was applied. The fibers were then stretched and wound up.

[0093] <Solvent replacement, drying> The wound fibers were subjected to solvent replacement in acetone until the conductivity was less than 0.01 mS / m. Then, they were placed in a high-temperature, high-pressure container at 80°C, and the container was filled with supercritical carbon dioxide to a pressure of 10.5 MPa. Supercritical carbon dioxide was then continuously flowed at a rate of 1.5 ml / min until the solvent of the sample was replaced with supercritical carbon dioxide. The container containing the sample was then filled with acetone so that the entire sample was immersed. Subsequently, the pressure was reduced by setting ΔP to 0.01 MPa for 3 hours to obtain porous aramid fibers in a continuous fibrous structure.

[0094] The porous aramid fiber of Example 1 had a fiber diameter of 35 μm, a strength of 120 MPa, and an elongation of 30%, exhibiting high strength and moderate elongation. Its density was 0.10 g / cm³. 3 In this example, 95% of the surface layer of the fiber cross-section had a non-porous layer, and the interior had a porous structure. Furthermore, the orientation ratio of the inner and outer layers of the fiber in Example 1 was 1.2, resulting in high tensile strength, no significant decrease in elongation, and excellent mechanical properties.

[0095] The porous aramid fiber of Example 1 did not experience any process problems such as yarn breakage during tubular knitting evaluation, and it was possible to obtain a tubular knitted fabric. The tubular knitted fabric was flexible and had a superior feel (Evaluation A).

[0096] Furthermore, a thermal insulation test was conducted using the aforementioned tubular knitted fabric. The ice placed on the sample did not melt and maintained a height of 3 cm even after 40 seconds of heating (rated ○), indicating that it possesses thermal insulation properties due to its porous structure. The results are shown in Table 1.

[0097] (Examples 2-4) The spinning solution was prepared in the same manner as in Example 1. The spinning conditions were changed as shown in Table 1, with the take-up speed when taking up the material with a roll installed in the coagulation bath altered. The other spinning conditions were the same as in Example 1, and the fibers were wound up.

[0098] Furthermore, solvent replacement and drying after winding were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0099] (Examples 5 and 6) The spinning solution was prepared in the same manner as in Example 1. The spinning conditions were as follows: the coagulation bath was 100% acetone, and the take-up speed when taking up the fibers with a roll installed in the coagulation bath was changed as shown in Table 1. The other spinning conditions were the same as in Example 1, and the fibers were wound up.

[0100] Furthermore, solvent replacement and drying after winding were carried out in the same manner as in Example 1. The results are shown in Table 2.

[0101] (Examples 7 and 8) The spinning solution was prepared in the same manner as in Example 1. The spinning conditions were as follows: the coagulation bath was 100% distilled water, and the draw speed when drawing the fibers with a roll placed in the coagulation bath was changed as shown in Table 1. The other spinning conditions were the same as in Example 1, and the fibers were wound up.

[0102] Furthermore, solvent replacement and drying after winding were carried out in the same manner as in Example 1. The results are shown in Table 2.

[0103] (Example 9) The spinning solution was prepared in the same manner as in Example 1. Under the spinning conditions, the solution was discharged into the coagulation bath from a nozzle with a hole diameter of φ0.25 mm at a discharge linear velocity of 2.04 m / min. The solution was then drawn up using a roll installed in the coagulation bath at a draw speed of 0.43 m / min. This ensured that there was no slack in the fibers in the bath and that a spinning draft was applied. The fibers were then stretched and wound up.

[0104] Furthermore, solvent replacement and drying after winding were carried out in the same manner as in Example 1.

[0105] The porous aramid fiber of Example 9 had a fiber diameter of 140 μm. In the tubular knitting evaluation, it was possible to knit it in a tubular knit without breakage and it had a good tactile feel (Evaluation B). The results are shown in Table 2.

[0106] (Example 10) The spinning solution was prepared in the same manner as in Example 1. Under the spinning conditions, the solution was discharged into the coagulation bath from a nozzle with a hole diameter of φ0.42 mm at a discharge linear velocity of 0.72 m / min. The solution was then drawn up using a roll installed in the coagulation bath at a draw speed of 0.15 m / min. This ensured that there was no slack in the fibers in the bath and that a spinning draft was applied. The fibers were then stretched and wound up.

[0107] Furthermore, solvent replacement and drying after winding were carried out in the same manner as in Example 1.

[0108] The porous aramid fiber of Example 10 had a fiber diameter of 220 μm. In the tubular knitting evaluation, tubular knitting was possible, but there were partial misalignments in the stitches (knitting defects), resulting in a hard and foreign-feeling texture (evaluation C). The results are shown in Table 2.

[0109] (Example 11) The spinning solution was prepared in the same manner as in Example 1. The spinning conditions were changed as shown in Table 1, with the take-up speed when taking up the material with a roll installed in the coagulation bath altered. The other spinning conditions were the same as in Example 1, and the fibers were wound up.

[0110] Furthermore, solvent replacement and drying after winding were carried out in the same manner as in Example 1.

[0111] The porous aramid fiber of Example 11 had a high orientation ratio of 2.7 between the inner and outer layers of the fiber. In the tubular knitting evaluation, tubular knitting was possible, but partial splitting of the yarn was observed, confirming knitting defects. In addition, the high orientation of the fiber surface resulted in a hard and foreign-feeling texture (Evaluation C). The results are shown in Table 1.

[0112] (Comparative Examples 1-3) The spinning solution was prepared in the same manner as in Example 1, and the spinning conditions were changed as shown in Table 1, with the coagulation bath modified. Spinning was performed in the coagulation bath without taking up the fibers, and the other spinning conditions were the same as in Example 1, and the fibers were collected.

[0113] Furthermore, solvent replacement and drying after the fiber collection were carried out in the same manner as in Example 1. The porous aramid fibers of Comparative Examples 1 to 3 all had low tensile strengths of 30 MPa or less, making tubular knitting impossible due to yarn breakage. The results are shown in Tables 1 and 2.

[0114] (Comparative Example 4) In Comparative Example 4, the spinning solution was prepared using a solvent consisting of DMSO and KOH in a mass ratio of 11:1, except that the other preparations were the same as in Example 1. The spinning conditions were the same as in Comparative Example 2, and the fibers were collected.

[0115] Furthermore, solvent replacement and drying after the fiber collection were carried out in the same manner as in Example 1. The porous aramid fiber of Comparative Example 4 had a low tensile strength of 1 MPa (actual measured value before rounding was 0.5 MPa), and tubular knitting was not possible due to yarn breakage. The results are shown in Table 2.

[0116] [Table 1]

[0117] [Table 2] [Industrial applicability]

[0118] The porous aramid fibers according to the present invention can be widely used in textile products and the like that have excellent heat retention and insulation properties. [Explanation of Symbols]

[0119] 1: Porous structure 2: Non-porous layer (skin layer) 3: Circumscribed circle of a fiber 4: Fiber-centered 5: Measurement range of the orientation ratio R1 of the fiber surface layer 6: Measurement range of the orientation ratio R2 of the inner fiber layer 7: Distance L0 8: Distance L1 9: Distance L2

Claims

1. Aramid fiber having a porous structure, wherein a non-porous layer is formed on the surface of the fiber, and the non-porous layer occupies at least 90% of the total surface area of ​​the surface layer, and the diameter of the fiber is 500 μm or less.

2. The tensile strength of the fiber, as measured in accordance with JIS L1013:2021, is 35 MPa or higher, the elongation of the fiber is 10% or higher, and the density of the fiber is 0.2 g / cm³. ３ The aramid fiber according to claim 1, which is as follows:

3. A textile product comprising aramid fibers according to claim 1 or 2.

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