Short fibers, fiber dispersed liquid and nonwoven fabric

Abstract

The present invention provides short fibers wherein the flatness, which is the value obtained by dividing the length of the major axis by the length of the minor axis of a cross-section of each fiber, is 5 or more, and the average of the lengths of the minor axes is 2,000 nm or less.　The present invention relates to short fibers which exhibit excellent dispersibility in a liquid medium; and the present invention provides short fibers which do not entwine with each other even under stirring conditions in a wide range, and are suitable for the achievement of a homogeneous fiber dispersed liquid.

Description

Short fibers, fiber dispersions and nonwovens

[0001] The present invention relates to short fibers, fiber dispersions, and nonwoven fabrics, and more specifically to short fibers suitable for fiber dispersions, fiber dispersions in which the short fibers are dispersed in an aqueous medium, and nonwoven fabrics made of the short fibers and having a special structure in which complex voids are formed.

[0002] Fibers have various characteristics derived from their thin and long shapes, and are therefore widely used not only for clothing but also for industrial materials. As people's lives have become more affluent in recent years, there has been a demand for textile products with diverse functions.

[0003] One of the characteristics of fiber morphology is its large specific surface area, which is the surface area per unit weight. Utilizing this specific surface area can provide high adsorption for target substances and high reinforcing effects when added as a filler, and research is underway to develop a wide variety of high-performance materials that take advantage of the specific surface area of ​​fibers.

[0004] One type of fiber product that takes advantage of the specific surface area of ​​fibers is a fiber dispersion, in which short fibers cut to a desired length are dispersed in a medium. This dispersion itself can be used not only as an adsorbent or filler for resin products, but also as a high-performance filter medium or separation membrane when the dispersion is separated using a wet papermaking method or other method and formed into a sheet. Therefore, research and technological development into fiber dispersions is being actively conducted.

[0005] The performance of this fiber dispersion is determined by the degree to which the fibers are uniformly dispersed in the dispersion and their specific surface area can be fully utilized. However, due to the specific surface area of ​​the mixed fibers, the fibers tend to aggregate, and the entanglement of thin and long fibers tends to make the dispersion state non-uniform. Therefore, in order to maintain high-quality products and their moldability, various technologies aimed at homogenizing the fiber dispersion have been proposed.

[0006] Patent Document 1 proposes a technology for producing ultrafine fibers that are advantageous for enhancing the specific surface area effect. With ultrafine fibers, the cohesive force resulting from intermolecular forces increases dramatically due to the increase in specific surface area, making it difficult to obtain a homogeneous dispersion. However, by shortening the fiber length to less than 1 mm, it is possible to achieve homogeneous dispersion without causing clumps of entangled fibers in the dispersion.

[0007] Patent Document 2 relates to a technology for improving fiber dispersibility by actively making electrical repulsive forces act between ultrafine fibers, and by making the electrical repulsive forces acting between fibers greater than the cohesive forces, it is possible to achieve uniform dispersion even when the fibers are long.

[0008] Patent Document 3 describes a technology in which short fibers are processed to have thickness variations in the longitudinal direction, and the morphological characteristics are used to achieve uniform dispersion in a dispersion liquid. By thickening a portion of the fibers in the longitudinal direction to make them less likely to bend, it is possible to suppress the occurrence of poor dispersion caused by entangled fibers forming clumps.

[0009] JP 2007-107160 A International Publication No. 2020-101002 JP 2018-84008 A

[0010] Patent Document 1 utilizes the fact that as the fiber diameter decreases, the fibers become soft and bendable, and the fibers become intricately entangled in the dispersion liquid, which makes it more likely that clumps of poor dispersion will occur. Therefore, by shortening the fiber length, the fibers become less likely to become entangled.

[0011] That is, by reducing the aspect ratio, which is the value obtained by dividing the fiber length by the fiber diameter, the fibers are made less likely to entangle in the dispersion, but while this method can obtain a homogeneous dispersed state when stirring at low speed, it is prone to poor dispersion where the fibers are entangled when stirring at high speed, which can sometimes pose a problem in dispersibility. In particular, this poor dispersion tends to be more pronounced with ultrafine fibers, and there have been cases where the processes that can be applied to obtain a dispersion of ultrafine fibers are limited.

[0012] In Patent Document 2, by utilizing the electrical repulsive force between fibers, a repulsive force acts between the fibers, resulting in homogeneous dispersion, making them less likely to settle, and maintaining the dispersed state for a long period of time. However, when a force greater than the electrical repulsive force acts due to high-speed stirring, the fibers may come into contact with each other and form fiber clumps, which may limit the processes that can be applied to make a dispersion.

[0013] In Patent Document 3, ultrafine fibers are torn off with a roll press to form ultrafine flat staple fibers having thickness variations in the longitudinal direction, and a portion of the staple fibers becomes thicker and less likely to bend, making the fibers less likely to entangle. However, in the process of torn off the ultrafine fibers with a roll press to form staple fibers, the difference in thickness in the longitudinal direction of a single staple fiber is limited to an upper limit of about two times, and as a result, during high-speed stirring, the fibers may entangle with each other and form fiber clumps, which may pose a problem in dispersibility.

[0014] As described above, while there are techniques for uniformly dispersing high-specific surface area short fibers intended for use in fiber dispersions when left to stand or stirred at low speeds, there are no techniques for uniformly dispersing such fibers even when stirred at high speeds, which limits the manufacturing processes for fiber dispersions. Therefore, in response to recent demands, there has been a demand for short fibers that are suitable for fiber dispersions with excellent dispersibility, that can be applied to a wide range of processes, and that can be deployed in a variety of fields of use.

[0015] In order to solve the above problems, the present invention has the following configurations. That is, (1) Short fibers having a flatness of 5 or more, which is the value obtained by dividing the length of the major axis of the fiber cross section by the length of the minor axis, and an average length of the minor axis of 2,000 nm or less. (2) Short fibers according to (1), having a variation in the length of the minor axis of the fiber cross section (CV value) of 10% or more. (3) Short fibers according to either (1) or (2), having a degree of irregularity of the fiber cross section of 20% or more. (4) Short fibers according to (1), having a crystallinity of 20% or less. (5) Short fibers according to (4), having a melting point of 180°C or more. (6) A fiber dispersion obtained by dispersing the short fibers according to (1) or (4) in an aqueous medium. (7) A nonwoven fabric at least partially comprising the short fibers according to (1) or (4). (8) A density of 0.4 g / cm ３(9) A nonwoven fabric according to (8), having an average pore size of 6 μm or less and a maximum frequency of pore size distribution of 30% or less. (10) A textile product at least partially comprising the nonwoven fabric according to (7) or (8), wherein the arithmetic mean roughness (Ra) of the surface is 5.00 μm or less.

[0016] The present invention relates to short fibers that exhibit excellent dispersibility in liquid media such as water due to the morphological characteristics of their highly flat fiber cross sections, and can provide a homogeneous fiber dispersion without entanglement of the short fibers under a wide range of stirring conditions. Furthermore, by utilizing the excellent water dispersibility of the short fibers to form the short fibers into a sheet, a nonwoven fabric can be obtained that has a special structure in which complex voids are formed while still being dense, and that exhibits excellent properties in absorbing low-frequency sounds related to everyday noise and road noise, and in filtering and separating specific components, and is expected to be widely used as an industrial material.

[0017] Fig. 1 is a schematic diagram of an example of the cross-sectional structure of the staple fiber of the present invention. Fig. 2 is a schematic diagram of a cross-sectional structure for explaining the unevenness of the staple fiber of the present invention. Fig. 3 is a schematic diagram of an example of the cross-sectional structure of a multi-layer laminated fiber used as a raw material for the staple fiber of the present invention. Fig. 4 is a cross-sectional view for explaining an example of a method for producing a multi-layer laminated fiber. Fig. 5 is a characteristic diagram showing a luminance histogram of a fiber dispersion comprising the staple fiber of the present invention, in which (a) is a schematic diagram of a luminance histogram of a fiber dispersion in which fibers are uniformly dispersed, and (b) is a schematic diagram of a luminance histogram of a fiber dispersion in which fiber aggregates have been formed.

[0018] The present invention will be described in detail below together with preferred embodiments.

[0019] The present invention relates to short fibers characterized by excellent dispersibility in a liquid medium such as water. The term "short fibers" as used herein refers to fibers having a fiber length of less than 100 mm that have been cut to a desired length along the longitudinal direction of the fibers.

[0020] The short fibers of the present invention utilize their high specific surface area to exhibit a high adsorption effect for target substances and a high reinforcing effect when added as a filler. When the short fibers of the present invention are used as a filter material or a separation membrane, the specific surface area of ​​the short fibers is set to 0.0010 nm or less from the viewpoint of improving their performance.－１ It is preferable that this is equal to or greater than this.

[0021] The specific surface area referred to here is determined as follows.

[0022] A fiber bundle made of the short fibers of the present invention is embedded in an embedding agent such as an epoxy resin, and the fiber cross section is cut and scraped out using a microtome equipped with a diamond knife. This cross section is then photographed using a scanning electron microscope (SEM) or the like at a magnification that allows the cross section to be identified.

[0023] For the cross section of a single fiber present in the photographed image, an arbitrary position on the outer periphery of the cross section is set as the measurement start point using image analysis software (WINROOF), and the length from the measurement start point to the outer periphery is measured using a series of images, returning to the measurement start point. This value is the outer periphery of a single fiber, and is expressed as an integer in nm (rounded to the nearest integer). The area of ​​the inner part surrounded by this outer periphery is measured using image analysis software (WINROOF), and this value is taken as the cross-sectional area of ​​a single fiber, and is expressed as nm. ２ Expressed as an integer (rounded to the nearest whole number). Using this perimeter and cross-sectional area, calculate the specific surface area of ​​one fiber using the following formula, rounded to the nearest five decimal places.

[0024] Specific surface area (nm －１ )=Perimeter length (nm) / Cross-sectional area (nm ２ The above measurement is carried out for 100 fibers, the specific surface area of ​​each fiber is calculated, and the arithmetic average of these is taken as the specific surface area.

[0025] Specific surface area is 0.0010 nm －１ If the specific surface area is equal to or greater than this, it is possible to achieve a specific surface area effect equivalent to that of ultrafine fibers having a fiber diameter of several μm, and to exhibit excellent adsorption performance and the like.

[0026] From this perspective, the effect of the present invention becomes more pronounced as the specific surface area increases. －１ If the specific surface area is 0.0080 nm or more, excellent effects equivalent to those of nanofibers with a fiber diameter of several hundred nm can be exhibited, and therefore, this can be cited as a more preferable range. －１The above range is a particularly preferred range in the present invention, and within this range, blending the short fibers of the present invention into a fiber product will exhibit excellent effects. By utilizing this increased specific surface area, even when the short fibers are molded into a mixed material with other aggregates, for example, the fibers exhibit excellent filtration, separation, adsorption, and other unique properties. For example, since they can capture ions and the like in liquids without surface treatment, they can be used to recover valuable materials from seawater, or to adsorb malodorous components in the air.

[0027] The staple fibers of the present invention are characterized by their morphological characteristics, which not only improve the performance of the above-mentioned fiber products but also have excellent dispersibility in liquid media such as water. An important requirement for achieving both a specific surface area and excellent dispersibility of the staple fibers, which has not been achieved by conventional techniques, is that the flatness, which is the value obtained by dividing the length of the major axis of the cross section of the fiber by the length of the minor axis, must be 5 or more, and this is the first requirement of the present invention.

[0028] The flatness of the fiber cross section is determined as follows (see also FIG. 1).

[0029] For the cross section of one fiber present in the image taken when determining the specific surface area, the maximum length of the cross section is measured using image analysis software (WINROOF), and this value is expressed as the length of the major axis of one fiber, rounded to the nearest integer in nm. Next, the length at the midpoint of this maximum length where a line segment perpendicular to the line segment of the maximum length intersects with the fiber cross section is measured, and this value is expressed as the length of the minor axis of one fiber, rounded to the nearest integer in nm. Using these major axis lengths and minor axis lengths, the flatness of one fiber is calculated using the following formula:

[0030] Flatness=Length of major axis (nm) / Length of minor axis (nm) The above measurement is carried out for 100 fibers to calculate the flatness of each fiber, and the arithmetic average of these is taken as the flatness.

[0031] The short fibers of the present invention must have a flatness of 5 or more. Within this range, the bending rigidity in the minor axis direction and the major axis direction will differ significantly due to the morphological characteristics of the cross section, i.e., the shape anisotropy of the cross section, and when the short fibers are dispersed in a fiber liquid, the bending direction of the short fibers will be restricted to the minor axis direction. Due to this restriction in the deformation direction, even when the short fibers come into contact with each other in the dispersion, the short fibers will not bend and become intricately entangled, resulting in poor dispersion in the form of clumps, and the short fibers will exhibit excellent dispersibility.

[0032] As described above, the short fibers of the present invention are less likely to entangle with each other in a dispersion by restricting the bending direction by utilizing the shape anisotropy of the cross section. Based on this technical concept, the higher the flatness of the cross section, the greater the difference in bending rigidity between the minor axis direction and the major axis direction, and when a fiber dispersion is prepared, the bending direction is tightly restricted to the minor axis direction. That is, if the flatness is 15 or more, the bending rigidity between the minor axis direction and the major axis direction of the cross section will differ by 200 times or more, and the bending direction of the short fibers in a dispersion will be substantially restricted to only the minor axis direction. Since the bending direction is tightly restricted to the minor axis direction in this way, the short fibers are less likely to entangle with each other in the dispersion even when stirred at high speed, and excellent dispersibility is exhibited, so the flatness is preferably 15 or more.

[0033] Furthermore, if the flatness is 30 or more, even under high-speed stirring conditions in which a high shear force is applied that defibrates short fiber bundles aggregated by cohesive forces such as intermolecular forces, the defibrated short fibers are less likely to entangle with each other due to restrictions on the bending direction resulting from the anisotropy of the cross-sectional shape. From the viewpoint of obtaining excellent dispersibility under a wide range of stirring conditions, including defibration, the flatness is more preferably 30 or more.

[0034] Furthermore, when the flatness is 50 or more, the remarkable shape anisotropy makes it easy for the short fibers to flow in a uniform flow state in the short axis direction of their cross sections due to the difference in flow velocity in the dispersion when the fiber dispersion is stirred. In such a flow state, the short axes of the short fibers are uniform, so that the short fibers are less likely to come into contact with each other, and a uniformly dispersed state is easily achieved. Therefore, the flatness is particularly preferably 50 or more.

[0035] On the other hand, as the flatness of the cross section increases, cracks tend to occur more easily in the longitudinal direction of the cross section when an external force is applied during a stirring process or the like. However, if the flatness is less than 800, there is no problem in practical use, and the object of the present invention can be achieved.

[0036] As described above, due to the high shape anisotropy of the cross section of the short fibers of the present invention, the bending direction in the dispersion is limited from all directions of 360° to only the minor axis direction, compared to conventional fibers with a round cross section, and the short fibers are less likely to form clumps of poor dispersion in which they are intricately entangled with each other, and even short fibers with a high specific surface area can exhibit excellent dispersibility under a wide range of stirring conditions.

[0037] The dispersibility of the short fibers is affected not only by the flatness of the cross section but also by the fiber diameter, and the fiber diameter is also an important factor in ensuring sufficient improvement in dispersibility due to the cross-sectional shape under a wide range of stirring conditions from low speed to high speed. As an index of the fiber diameter, the short axis length of the cross section is a second factor for the short fibers of the present invention, and the average length of the short axis must be 2,000 nm or less.

[0038] The average minor axis length referred to here is determined by rounding off the arithmetic average of the minor axis lengths of 100 fibers measured above to an integer in nm.

[0039] In the short fibers of the present invention, if the average length of the minor axis is 2,000 nm or less, the settling speed of the fibers in the dispersion liquid becomes sufficiently slow, and the dispersed state of the fibers can be maintained uniformly.

[0040] Based on this technical concept, the shorter the minor axis length, the slower the settling rate of the fibers and the less likely the fibers are to settle over time. Therefore, the average minor axis length of the short fibers of the present invention is preferably 1,000 nm or less. If the average minor axis length is within this range, the fibers will not settle and a homogeneous dispersion state can be maintained even when stirred with a weak force.

[0041] Furthermore, even if no stirring force is applied for a short period of time during the infusion process of the fiber dispersion, the fibers will not settle if the average minor axis length is 500 nm or less, and this can be considered a more preferable range in the present invention.

[0042] Furthermore, it is particularly preferable that the average length of the minor axes is 250 nm or less. If the average length is within this range, the fibers are less likely to settle and a homogeneous dispersion state is maintained even when the fiber dispersion is stored and no stirring force is applied for a long period of time.

[0043] On the other hand, the average length of the short axis of the short fibers of the present invention is 20 nm or more, which is the substantial lower limit in the present invention, and is less likely to break when an external force is applied during a stirring process or the like.

[0044] The staple fibers of the present invention have an ultra-flat cross section in which the major axis is extremely long relative to the minor axis, which limits the bending direction of the staple fibers themselves, and when dispersed in a liquid medium such as water, the range of conditions under which a homogeneous dispersion state can be maintained is much wider than that of conventional techniques, such as a wide range of stirring conditions from high shear to low shear. In this characteristic fiber cross section, from the viewpoint of suppressing entanglement and adhesion between adjacent fibers and ensuring dispersibility over time, it is preferable that the shape of the fiber cross section has a distribution within a certain range, and it is preferable that the staple fibers of the present invention have variation in the length of the minor axis.

[0045] The variation in minor axis length (CV value) in the present invention refers to an integer (unit: %) obtained by calculating the arithmetic mean and standard deviation using the minor axis lengths of 100 fibers measured above, dividing the standard deviation by the arithmetic mean to obtain a coefficient of variation, and rounding off any fraction after the decimal point.

[0046] When the short axis lengths are present with a moderate distribution, not only do the cross-sectional shapes of the fibers not match, making them difficult to bond, but when an external force such as shear is applied, the short fibers behave differently, resulting in differences in buckling behavior when the short fibers come into contact with each other and bend in a liquid medium, preventing entanglement of the short fibers and allowing them to maintain a homogeneous state over time. From the above perspectives, the short fibers of the present invention preferably have a variation in short axis length of 10% or more. By setting the variation in this range, even in a dispersion containing a high concentration of short fibers that tend to come into contact with each other, the bending behavior and other characteristics of the individual fibers differ, preventing entanglement and ensuring a homogeneous dispersion state.

[0047] Furthermore, when the variation in the length of the minor axis is 20% or more, the short fibers are less likely to entangle even in a clay-like fiber dispersion in which the short fibers are contained at an extremely high concentration relative to the medium, and excellent dispersibility is regained by diluting with a liquid, etc., and therefore this can be considered a more preferable range in the present invention.

[0048] Furthermore, in the present invention, it is particularly preferable that the variation in the minor axis length is 30% or more. If the variation is within this range, even when short fibers are dispersed in a medium from an aggregated short fiber bundle or fiber aggregate, the individual short fibers behave differently and are dispersed by an external force, thereby eliminating the aggregated state and enabling the fibers to be easily dispersed by stirring for a short period of time.

[0049] From this perspective, the greater the variation in the minor axis length, the better the dispersibility in the liquid medium. However, when an external force is applied during a stirring process or the like, uneven dispersibility may occur, or the short fibers may break if the minor axis length is too short. Therefore, it is preferable that the variation in the minor axis length be 50% or less, which is the substantial upper limit in the present invention.

[0050] In the staple fibers of the present invention, in addition to the suppression of entanglement due to the difference in fiber cross section between the staple fibers, the unevenness on the outer periphery of the flat cross section can suppress adhesion and entanglement between the staple fibers due to steric hindrance, and it is preferable that the unevenness in the fiber cross section is 20% or more.

[0051] The irregularity referred to in the present invention is determined by measuring the lengths of the line segments perpendicular to the maximum line segment at ten equal points along the maximum length of the cross section, calculating the arithmetic mean and standard deviation of these 10 lengths, and then dividing the standard deviation by the arithmetic mean and rounding off the decimal point to the nearest percent to obtain the irregularity of a single fiber (see also Figure 2). Similar measurements are performed on ten fiber cross sections, and the arithmetic mean of the irregularities calculated for the ten fibers is defined as the irregularity referred to here. When this irregularity is 20% or more, short fibers can be easily dispersed from the minute voids between fibers as starting points, allowing for uniform dispersion in a short period of time.

[0052] A high degree of unevenness suppresses entanglement between short fibers, but the range in which a load is concentrated on one part of the cross section and cracks do not occur is a degree of unevenness of 50% or less, which can be cited as the substantial upper limit in the present invention.

[0053] The short fibers of the present invention have a high specific surface area effect due to their cross-section and excellent dispersibility that allows this effect to function effectively, and are advantageous when added to resins as a filler. When made into a sheet, they can be used as a high-performance nonwoven fabric having properties such as separation, filtration, and adsorption.

[0054] When made into a nonwoven fabric, the mechanical properties of the sheet are exhibited by a bridge structure formed by frictional forces between adjacent short fibers, etc. This bridge structure is a structure in which short fibers present in a medium interact with each other to transmit force, and therefore, when short fibers are present in a medium at the same concentration, the thinner the fiber diameter and the longer the fiber length, the more bridge structures are formed and the more promoted the transmission of force. In other words, the higher the ratio of fiber length to fiber diameter, the more promoted the formation of bridge structures, and as an indicator of this, it can be said that a high aspect ratio, which is the value obtained by dividing the fiber length by the length of the minor axis, is preferable for the short fibers of the present invention.

[0055] The aspect ratio referred to in the present invention is determined as follows.

[0056] An image of a fiber bundle made of the staple fibers of the present invention is taken with a microscope at a magnification that allows observation of at least 10 staple fibers whose total length can be measured. Ten staple fibers are randomly selected from the image of the staple fibers, and the fiber lengths of these 10 staple fibers are measured. The fiber length referred to here is the longitudinal length of a single fiber from a two-dimensionally captured image, measured in mm units using image analysis software (WINROOF) and rounded to the nearest tenth. The above procedure is repeated for 10 similarly captured images, and the arithmetic average of the fiber lengths of 100 fibers is taken as the fiber length of the present invention. Using this fiber length converted to nm and the average of the minor axis lengths determined above, the aspect ratio is calculated using the following formula, rounded to the nearest whole number:

[0057] Aspect ratio = fiber length (nm) / average minor axis length (nm) When the staple fibers of the present invention are formed into a sheet, the aspect ratio is preferably 3,000 or more. If the aspect ratio is within this range, a sufficient bridge structure is formed between the staple fibers, and the obtained nonwoven fabric exhibits mechanical properties at a level that does not pose any problems in practical use, even without reinforcement with a binder or the like.

[0058] In the present invention, the aspect ratio of the short fibers is more preferably 6,000 or more. If the aspect ratio is within this range, when the short fibers are made into a sheet, not only will they exhibit sufficient mechanical properties, but they will also have excellent processability, such as minimizing the dropout of short fibers during the sheet-making process.

[0059] On the other hand, the aspect ratio of the short fibers is preferably 50,000 or less, which is the substantial upper limit in the present invention, and is a range in which entanglement between the short fibers does not occur in the dispersion and good handleability can be ensured without any restrictions on stirring conditions, etc.

[0060] In consideration of the intended effects of the present invention and practical use as a fiber product, the polymer constituting the staple fiber of the present invention is preferably a polymer excellent in heat resistance and chemical resistance. That is, the polymer constituting the staple fiber is preferably at least one polymer selected from the group consisting of polyester, polyamide, polyphenylene sulfide, and polyolefin. In addition to the above-mentioned advantages, these polymers are thermoplastic, and therefore not only can the staple fiber of the present invention be produced by a highly productive melt spinning method, but also are suitable from the viewpoint of adjusting mechanical properties, for example, by highly oriented crystallization in the drawing process.

[0061] In particular, from the viewpoint of ensuring dispersibility, it is more preferable that the staple fibers of the present invention are made of a polymer with a high elastic modulus, such as polyester or polyphenylene sulfide, which can suppress bending of the fibers when an external force is applied, and can effectively suppress the occurrence of poor dispersion due to entanglement of the fibers during the dispersion step of the staple fibers.

[0062] Furthermore, by selecting a polymer having a functional group that exerts an electrical repulsive force, such as a carboxyl terminal group, including polyester, the repulsive force between fibers will not occur, causing aggregation, and a uniformly dispersed state will be more easily achieved.

[0063] The staple fibers of the present invention are preferably dispersed in a medium to form a fiber dispersion. In this case, from the viewpoint of dispersing the staple fibers using the electrical repulsive force of a dispersant or the like, the medium is more preferably an aqueous medium. By forming such a fiber dispersion, a material having unique properties such as adsorption and reinforcement can be obtained.

[0064] As described above, due to the cross-sectional shape anisotropy of the short fibers of the present invention, when dispersed in a liquid medium such as water, they can maintain a homogeneous dispersion state under a wide range of stirring conditions, from high shear to low shear. By utilizing this excellent water dispersibility, even in sheet-forming processes such as wet papermaking, which involve various levels of shear, the short fibers can be laid down in a homogeneously dispersed state and evenly distributed without uneven distribution, making it possible to obtain a nonwoven fabric with a unique structure that is dense yet has complex voids. Furthermore, during the process of laying down the fibers into the nonwoven fabric, the short axis direction of the cross section naturally aligns in the thickness direction of the nonwoven fabric due to the highly flat cross-sectional shape of the short fibers, resulting in a dense stacking. This, combined with the effect of the homogeneous dispersion, forms a unique structure not achieved by conventional techniques. For this reason, it is preferable to use the short fibers of the present invention in a nonwoven fabric.

[0065] The nonwoven fabric of the present invention is characterized by a special structure in which complex voids are formed inside while being dense, and the density of the nonwoven fabric is 0.4 g / cm ３ The above is the first requirement.

[0066] The density of the nonwoven fabric in the present invention is determined as follows.

[0067] That is, the basis weight is determined by weighing a nonwoven fabric cut into a 250 mm x 250 mm square and measuring the unit area (1 m ２ The thickness is measured in mm using a dial thickness gauge SM-114 (manufactured by TECLOCK Corporation, probe shape 10 mmφ, graduation 0.01 mm, measuring force 2.5 N or less). This is done at any five points per sample, and the average is rounded to two decimal places to obtain the thickness of the nonwoven fabric.

[0068] Using the basis weight and thickness of the nonwoven fabric obtained above, calculate the density of the nonwoven fabric according to the following formula: This is obtained for 10 samples, and the simple average value is rounded to two decimal places to obtain the density of the nonwoven fabric.

[0069] Nonwoven fabric density (g / cm ３) = basis weight / thickness The nonwoven fabric of the present invention has a nonwoven fabric density of 0.4 g / cm ３ By setting the density within this range, even if the thickness of the nonwoven fabric is sufficiently thin, the nonwoven fabric has a dense structure with few voids, so when used as a sound absorbing material, an improvement in sound absorption coefficient in the low frequency band can be expected. If the thickness of the nonwoven fabric is made even thinner and it is assumed that it will be placed in a limited space, such as for an exterior component of a vehicle, the nonwoven fabric density should be 0.6 g / cm or more. ３ From this viewpoint, the higher the density of the nonwoven fabric, the more preferable it is. ３ It is more preferable that the thickness is within this range, and a very thin sheet can be obtained that does not affect the design or aesthetics of the structure. On the other hand, in the case of the nonwoven fabric of the present invention, the presence of voids that penetrate the sheet, which are suitable for the sound absorption mechanism, prevents the sound that is to be absorbed from being reflected, thereby enabling the object of the present invention to be satisfactorily achieved.

[0070] The nonwoven fabric density described above can be achieved by appropriately adjusting the sheet thickness and basis weight, assuming that the fibers constituting the sheet are present in a dispersed state. In this case, by setting the basis weight of the sheet to a certain extent, it becomes possible to form microvoids of the desired size, and the nonwoven fabric will maintain the strength of a practical sheet. From the above viewpoints, the nonwoven fabric of the present invention has a basis weight of 3 to 500 g / m ２ Within this range, the desired effect of the present invention is not impaired and a sheet in which each fiber is uniformly and stably present can be obtained.

[0071] The second requirement for the special structure of the nonwoven fabric of the present invention is that the average pore size is 6 μm or less and the maximum frequency of the pore size distribution is 30% or less.

[0072] The pore size referred to here refers to a value calculated by the bubble point method. For example, the bubble point method can be performed using an automatic porous material pore measurement system, Perm-Porometer (manufactured by PMI). In measurements using this Perm-Porometer, a nonwoven fabric is immersed in a liquid with a known surface tension, and a gas pressure is supplied from above the sheet while increasing, and the pore size is measured from the relationship between this pressure and the surface tension of the liquid on the surface of the nonwoven fabric.

[0073] Specifically, the pore size is calculated using a porous material automatic pore size measurement system, Perm-Porometer (manufactured by PMI).

[0074] Three pieces of nonwoven fabric were cut out as measurement samples, and pore size distribution measurements were carried out for each piece using Galwick (surface tension: 16 mN / m) as a measurement liquid with a known surface tension. The mean flow diameter obtained by automatic calculation was used as the average pore size, and the value calculated to one decimal place by rounding the average value of each sample to one decimal place was used.

[0075] The frequency of the pore size distribution was calculated automatically and expressed as a percentage. The maximum frequency was defined as the value at which the frequency of the pore size distribution was greatest. The average of the maximum frequencies for each sample was calculated and rounded to one decimal place.

[0076] The nonwoven fabric of the present invention must have an average pore size of 6 μm or less and a maximum frequency of 30% or less in the pore size distribution. Within these ranges, the pore sizes contained within the nonwoven fabric are small and a variety of pore sizes are present. Therefore, when used as a sound-absorbing material, the complex pore structure enables sound absorption over a wide range of low frequencies.

[0077] Furthermore, in order to have a high sound absorption coefficient at low frequencies and over a wide frequency range, it is more preferable that the average pore size is 2 μm or less and the maximum frequency of the pore size distribution is 20% or less. The lower limits for substantially fully achieving the object of the present invention include an average pore size of 0.1 μm or more and a maximum frequency of the pore size distribution of 10% or more. Within these ranges, effective sound absorption performance can be achieved without obstructing the fluid.

[0078] The nonwoven fabric of the present invention exhibits excellent properties even when used alone as a sound-absorbing material, but when considering its use in sound-absorbing materials for electric vehicles, which have been rapidly developed recently, it is expected to be used as a laminate material with other materials. In this case, when producing a molded product, the nonwoven fabric will be bonded to other materials with a binder, so in terms of increasing the bonding strength with other materials, it is preferable that the arithmetic mean roughness (Ra) of the nonwoven fabric surface be 5.0 μm or less.

[0079] The arithmetic mean roughness (Ra) referred to here is determined as follows: the surface of the nonwoven fabric is observed using a laser microscope (VK-X200 manufactured by Keyence Corporation or a laser microscope with equivalent performance), and measurement is performed using analysis software (VK-H1XA manufactured by Keyence Corporation or analysis software with equivalent performance) in accordance with JIS B 0601. This is performed at any five locations per sample, and the average value is calculated by rounding off to two decimal places, and this value is taken as Ra.

[0080] In the present invention, an arithmetic mean roughness (Ra) value of the nonwoven fabric surface of 5.0 μm or less means that the nonwoven fabric has sufficient smoothness to exhibit practical properties, and since there are no large irregularities on the surface, it can be easily integrated with other materials by bonding, etc., and it also has excellent resistance to peeling over time. Furthermore, within this range, a favorable effect is also exhibited in terms of sound absorption; for example, when the nonwoven fabric of the present invention is used as a sound-absorbing material, diffuse reflection of sound waves on the surface can be suppressed, making it possible to effectively absorb sound. From this perspective, in order to further suppress diffuse reflection of sound waves on the surface, it is more preferable that the arithmetic mean roughness (Ra) of the nonwoven fabric surface is 2.0 μm or less.

[0081] As described above, by utilizing the special structure of the nonwoven fabric of the present invention, which is dense yet has complex voids formed inside, it is possible to achieve excellent sound absorption performance mainly in the low frequency band as a sound absorbing material, and by designing the sheet structure and combining it with other materials, it is possible to achieve a wide range of sound absorption performance. Furthermore, the special sheet structure formed in the nonwoven fabric of the present invention is excellent not only for general filtration but also for adsorption and filtration of valuable materials and harmful substances, and can be effectively used as a filter medium substrate.

[0082] Among various methods for producing nonwoven fabrics, in order to form the dense structure that is a characteristic of the nonwoven fabric of the present invention, a wet method that is good at producing highly dense nonwoven fabrics, i.e., a wet papermaking method, is preferably used, and this method makes it possible to stably produce the special sheet structure that is a characteristic of the present invention.

[0083] The special structure of the nonwoven fabric of the present invention, which is dense yet has fine voids, can be adjusted appropriately by adjusting the blending ratio of the various fibers constituting the nonwoven fabric or by changing the cross-sectional shape of the staple fibers of the present invention. From the perspective of the cross-sectional shape of the staple fibers, the higher the flatness of the fiber cross-section, the more pronounced the effect of the fibers being laid in a uniform cross-sectional direction, and the shorter the length of the minor axis of the fiber cross-section, the more flexibly the fiber bends in the minor axis direction and the better it blends with other materials, resulting in a nonwoven fabric with a denser and more complex void structure. Furthermore, a certain degree of variation in the minor axis length of the fiber cross-section and the degree of irregularity act as steric hindrance within the nonwoven fabric, generating fine voids between the staple fibers and accentuating the special structure that is a characteristic of the nonwoven fabric of the present invention.

[0084] As described above, the nonwoven fabric of the present invention has a high specific surface area, due to the highly flat cross-sectional shape of the staple fibers, and while the staple fibers are uniformly distributed without uneven distribution, they are densely packed with the flat cross-sectional direction aligned, resulting in a dramatically increased contact area between fibers within the nonwoven fabric. By utilizing this characteristic to bond the fibers that make up the nonwoven fabric, it is possible to obtain a thin nonwoven fabric with excellent mechanical properties. In other words, by using the staple fibers of the present invention that make up the nonwoven fabric as binder fibers that bond the fibers within the nonwoven fabric, excellent adhesion can be achieved even in a thin, low-basis-weight nonwoven fabric, and a thin nonwoven fabric with excellent mechanical properties can be obtained.

[0085] In the short fibers of the present invention, the crystallinity is preferably 20% or less from the viewpoint of exhibiting excellent thermal adhesiveness. Here, the thermal adhesiveness refers to the ability to soften and flow when heated, adhere to an adherend, and then cool and solidify to bond and bond the adherends.

[0086] When the staple fibers of the present invention are used to form a nonwoven fabric, the morphological characteristics of the fibers result in a large contact area between the fibers. In addition, the fibers exhibit thermal adhesiveness, which firmly bonds the fibers together, thereby increasing the strength of the nonwoven fabric. This thermal adhesiveness is also affected by the impregnation state of the staple fibers at each bonding point, and in order to fully exhibit thermal adhesiveness, the fluidity of the fibers is also an important requirement. As an indicator of this fluidity, the crystallinity of the staple fibers of the present invention is preferably 20% or less.

[0087] The degree of crystallinity referred to here is determined as follows.

[0088] Approximately 5 mg of short fibers are weighed out using an electronic balance, and then the fibers are set in a differential scanning calorimeter (DSC) and subjected to differential scanning calorimetry under nitrogen at a temperature rise rate of 16°C / min in a measurement temperature range of 50 to 320°C.

[0089] The crystallization heat of heat ΔHc (J / g) is calculated from the area of ​​the exothermic peak in the obtained measurement results (DSC curve), and the heat of crystal fusion ΔHm (J / g) is calculated from the area of ​​the endothermic peak. If multiple exothermic peaks or endothermic peaks are observed, ΔHc and ΔHm are calculated from the total area of ​​all peaks. For each level, measurements are performed three times at different measurement positions, and the arithmetic averages of ΔHc and ΔHm are calculated. After that, the crystallinity is calculated using the following formula, rounded to one decimal place.

[0090] Crystallinity (%) = (ΔHm-ΔHc) / ΔHm ０ × 100 Here, ΔHm ０ is the heat of melting of the complete crystal (J / g).

[0091] In the short fibers of the present invention, if the degree of crystallinity is 20% or less, when the short fibers are heated to a temperature equal to or higher than the glass transition temperature of the short fibers, the amorphous regions are less likely to be inhibited by the crystalline regions and the short fibers can sufficiently soften and flow. Therefore, when the short fibers are hot-pressed by calendering or the like, the softened and flowed short fibers can penetrate between the materials to be adhered, thereby exhibiting high thermal adhesion.

[0092] Based on this technical concept, a lower degree of crystallinity allows the thermoplastic fiber to soften and flow well during the thermal bonding process, and it is more preferable that the short fiber of the present invention has a degree of crystallinity of 15% or less.Within this range, the short fiber can easily fit into the complex unevenness of the bonded material without any gaps, and can exhibit excellent thermal bonding properties regardless of the shape of the bonded material.

[0093] Furthermore, it is more preferable that the crystallinity is 10% or less. If the crystallinity is within this range, even when the heat press pressure in the calendaring process is low, the softened and fluidized short fibers can easily enter between the material to be adhered, thereby reducing adhesion to the rollers during the heat press and reducing the process tension, thereby suppressing breakage during the manufacturing process of thin, low-basis-weight sheet-like materials.

[0094] Furthermore, it is particularly preferable that the crystallinity is 8% or less. If the crystallinity is within this range, even when heated in a non-contact manner such as in an oven without using a calendar process, the thermoplastic fibers soften and flow, penetrating into the adherend, and exhibiting excellent thermal adhesion.

[0095] As described above, the short fibers of the present invention not only have the effect of increasing the bonding area due to the morphological characteristics of the fibers, but also have good fluidity when heated due to their fiber structure, which allows the thermoplastic fibers to penetrate seamlessly into the bonded materials, thereby exhibiting excellent thermal adhesiveness. As a result, it is possible to achieve excellent mechanical properties in thin, low-basis-weight sheet-like materials, which was difficult to achieve with conventional technologies.

[0096] From the viewpoint of exhibiting excellent thermal adhesiveness due to this good fluidity during heating, it is preferable that the short fibers of the present invention penetrate not only into the macroscopic irregularities between the adherends but also into the minute irregularities at the bonding points without any gaps at the molecular level, and as an indicator of this, it is preferable that the peak value of tan δ of the short fibers of the present invention is 0.10 or more.

[0097] The tan δ referred to here is measured using an automatic dynamic viscoelasticity measuring device (Rheovibron) under conditions of clamping a sample with a chuck distance of 30 mm, applying a tension of 0.07 g / dtex, a temperature rise rate of 3°C / min, and a frequency of 110 Hz.

[0098] The peak value of tan δ corresponds to the amount of molecular chains that can move without being restrained at that temperature, and the larger this value, the easier the material is to flow at that temperature.A peak value of tan δ of 0.10 or more can be considered a preferable range, as the softened and flowed short fibers can easily enter between the adherends, thereby enabling strong bonding between the adherends.

[0099] Furthermore, a peak value of tan δ of 0.15 or more can be cited as a more preferable range, since the softened and fluidized short fibers can easily penetrate into the irregularities on the surface of the adherend, thereby suppressing peeling between the adhered short fibers and the adhered material.A peak value of tan δ of 0.20 or more can be cited as an even more preferable range, since the softened and fluidized short fibers can easily penetrate into the minute irregularities on the surface of the adherend at the molecular level, thereby significantly suppressing peeling between the adhered short fibers and the adhered material.

[0100] The polymer constituting the staple fiber of the present invention is preferably a polymer having excellent heat resistance in view of practical use as a sheet-like material or a fiber product, and the melting point of the polymer constituting the staple fiber of the present invention is preferably 180°C or higher.

[0101] The melting point of the polymer referred to here is determined as follows.

[0102] The polymer is adjusted to a moisture content of 200 ppm or less using a vacuum dryer, and approximately 5 mg is weighed out. Using a differential scanning calorimeter (DSC), the temperature is raised from 0°C to 320°C at a rate of 16°C / min, and then held at 320°C for 5 minutes for DSC measurement. The melting point is calculated from the melting peak observed during the temperature rise process. Three measurements are performed for each sample, and the arithmetic average is taken as the melting point of the polymer of the present invention. When multiple melting peaks are observed, the melting point is taken as the top of the melting peak with the highest temperature.

[0103] When the melting point of the polymer constituting the staple fibers of the present invention is 180°C or higher, the staple fibers are unlikely to be softened by heat during processing and lose their adhesive strength, even when a sheet-like material made of the staple fibers is processed in various ways for use, and excellent processability is exhibited, so this range can be mentioned as a preferred range.

[0104] Furthermore, when the melting point of the polymer constituting the short fibers is 200°C or higher, even when other thermoplastic resins with lower melting points are applied or coated, the mechanical properties of the sheet-like material are less likely to be impaired due to softening of the bonding points, and the material can pass through the process smoothly, so this range can be cited as a more preferable range.

[0105] When the melting point of the polymer constituting the short fibers is 220°C or higher, the bonded points can be maintained in a sufficiently solidified state even when high tension is applied in the processing steps in which heat is applied as described above, and various processes can be applied without impairing the mechanical properties, so this can be cited as a more preferable range.

[0106] The staple fibers of the present invention maintain an amorphous state before the thermal bonding step and are thermally crystallized after the thermal bonding step, thereby achieving strong thermal bonding in low-temperature heat treatment while exhibiting heat resistance sufficient for practical use. Polymers that meet these requirements include polyethylene terephthalate or its copolymers, polyethylene naphthalate, and polyphenylene sulfide. The undrawn yarns of these polymers after spinning tend to maintain an amorphous state in a room-temperature environment, and the materials to be bonded are firmly bonded together by softening and flowing due to glass transition, and then crystallized, thereby exhibiting excellent heat resistance and chemical resistance.

[0107] As described above, by using the short fibers of the present invention as binder fibers that bond fibers together in the nonwoven fabric, a structure in which aggregate fibers are firmly and uniformly bonded together in the nonwoven fabric is formed. As an index of this structure, the circumferential length P of the fiber cross section in the cross section of the nonwoven fabric is ｍ And, P ｍ The adhesive length P that is in contact with the adhesive part ｂ The adhesion rate P is the ratio of ｂ / P ｍ It is preferable that the variation is 80% or less.

[0108] The bonded portion referred to here refers to a portion where the binder fibers constituting the nonwoven fabric bond adjacent aggregate fibers together by softening and flowing when heated, and the binder fibers in this bonded portion may completely lose the fiber shape they had before softening and flowing, or may partly maintain the fiber shape. ｂ / P ｍ The variation is calculated as follows.

[0109] A cross section of the nonwoven fabric of the present invention is cut out with a razor or the like, and this cross section is photographed with a scanning electron microscope (SEM) at a magnification that allows observation of the entire thickness of the nonwoven fabric.

[0110] The length of the outer periphery of the cross section of one fiber (aggregate fiber) present in the photographed image is measured using image analysis software (WINROOF). This value is the outer periphery P of one fiber. ｍ１ and expressed in μm units by rounding off to the third decimal place. ｍ１ The length of the outer periphery of the fiber that is bonded to the binder fiber is measured using image analysis software (WINROOF), and this value is taken as the bond length P of one fiber. ｂ１ and is expressed in μm units, rounded to two decimal places. In the cross section of the nonwoven fabric, the binder fibers are present in a form impregnated between the aggregate fibers due to softening and flow during the thermal bonding process (although they may no longer retain their fibrous form, for convenience they may be referred to as binder fibers hereinafter), but the contrast between the aggregate fibers and the binder fibers impregnated therein due to the difference in unevenness can be obtained, making it possible to distinguish the interface between the aggregate fibers and the binder fibers. This perimeter P ｍｎ and adhesive length P ｂｎ Using the formula below, calculate the adhesion rate of one fiber P as an integer (rounded off to the nearest integer). ｂｎ / P ｍｎ Calculate.

[0111] P ｂｎ / P ｍｎ (%) = P ｂｎ (μm) / P ｍｎ (μm) × 100 The above measurement was carried out for 100 fibers to determine the adhesion rate P of each aggregate fiber. ｂｎ / Pｍｎ (n = 1 to 100) are calculated, and the arithmetic mean and standard deviation are calculated. The standard deviation is divided by the arithmetic mean to obtain a coefficient of variation. The resulting integer (unit: %) is rounded off to the nearest whole number, and is the adhesion ratio P ｂ / P ｍ The variation is

[0112] In order for the nonwoven fabric of the present invention to exhibit excellent mechanical properties, it is preferable that the aggregate fibers are bonded uniformly throughout the entire nonwoven fabric. ｂ / P ｍ It is preferable that the variation in is 80% or less. Within this range, the proportion of aggregate fibers not bonded by binder fibers is small, and the aggregate fibers are less likely to pass through, thereby enabling the material to exhibit excellent mechanical properties.

[0113] Based on this technical concept, the adhesion rate P ｂ / P ｍ The smaller the variation in the adhesion rate P, the more uniformly the aggregate fibers are bonded throughout the nonwoven fabric, resulting in excellent mechanical properties. ｂ / P ｍ It is more preferable that the variation is 70% or less. Within this range, a constant bond length is ensured for the aggregate fibers throughout the nonwoven fabric, making it less likely that the nonwoven fabric will be destroyed due to bond points with extremely short bond lengths.

[0114] Adhesion rate P ｂ / P ｍ It is more preferable that the variation in is 60% or less. If the variation is within this range, the aggregate fibers throughout the nonwoven fabric can bear stress uniformly, so that even a thin, low-basis-weight nonwoven fabric can exhibit excellent mechanical properties.

[0115] In terms of the adhesive strength between the aggregate fiber and the binder fiber at each adhesive point, the adhesive ratio P ｂ / P ｍ The higher the adhesion rate P ｂ / P ｍ If the binder fiber content is 25% or more, the aggregate fibers are firmly bonded by the binder fiber and do not easily peel off at the interface, and this can be cited as a preferable range.

[0116] Furthermore, the adhesion rate P ｂ / P ｍ If the ratio is 30% or more, the bonded points will be bonded so strongly that destruction of the sheet-like material will occur due to destruction of the aggregate fibers themselves, and the mechanical properties of the aggregate fibers can be fully exerted as the mechanical properties of the sheet-like material, so this can be considered a more preferable range.

[0117] An example of a method for producing the staple fibers and nonwoven fabric of the present invention will be described in detail below.

[0118] The staple fibers of the present invention can be produced by cutting long fibers having the cross-sectional shape characteristic of the present invention to the desired length. Specifically, the long fibers are produced by bundling tens to tens of thousands of long fibers into a tow and cutting the tow to the desired fiber length using a cutting machine such as a guillotine cutter, a slicer, or a cryostat. Regarding the long fibers referred to here, it is also possible to spin fibers made of a single polymer. However, spinning fibers having a minor axis length of 2,000 nm or less, which is a characteristic of the present invention, without operational problems using conventional techniques may be subject to limitations in spinning conditions. Therefore, a method of generating the short fibers from fibers having a flat fiber cross section, which can be produced by removing the easily soluble polymer from a multilayer laminated fiber (Figure 3) composed of a slightly soluble polymer and an easily soluble polymer, is preferably used.

[0119] The multilayer laminated fiber referred to in the present invention means a fiber having a fiber cross section with a multilayer laminated structure in which two or more types of polymers are laminated alternately, in the same order, or in random order. The form of this laminated structure may be not only one in which the poorly soluble polymer and the easily soluble polymer are alternately laminated in one direction, but also one in which they are laminated radially from the center of the fiber to the outer layer, one in which they are laminated irregularly in the fiber cross section, or a combination of these.

[0120] The method for producing the multi-layer laminated fiber of the present invention can be appropriately selected depending on the production process and the polymers used, but from the viewpoint of excellent productivity, it is preferable to adopt a melt spinning method.

[0121] Multilayer laminated fibers produced by melt spinning can be spun from melt-moldable polymers such as polyethylene terephthalate or its copolymers, polyethylene naphthalate, polybutylene terephthalate, polytrimethylene terephthalate, polypropylene, polyolefin, polycarbonate, polyacrylate, polyamide, polyphenylene sulfide, polylactic acid, and thermoplastic polyurethane. These polymers may also contain various additives, such as inorganic substances such as titanium oxide, silica, and barium oxide; colorants such as carbon black, dyes, and pigments; flame retardants, fluorescent brighteners, antioxidants, and ultraviolet absorbers. When polymers containing these additives are selected, unevenness will be generated in each layer of the multilayer laminated fiber depending on the particle size of the additive microparticles, and this can be used to impart any desired unevenness to the resulting fiber.

[0122] A multilayer laminated fiber is produced by selecting two or more polymers from the above-mentioned polymers and spinning them, and the combination of polymers is also important from the viewpoint of forming a stable laminated structure. That is, the smaller the difference in solubility parameters (SP values) of the polymers to be combined, the more likely it is that a good laminated structure will be formed in which there is no merging between layers, and it is preferable to select polymers such that the difference in solubility parameters of the two polymers forming the interface is 3.0 or less.

[0123] The solubility parameter (SP value) referred to here means a parameter that reflects the cohesive force of a substance, defined as (evaporation energy / molar volume) 1 / 2, and can be calculated from the values ​​described, for example, on page 189 of "Plastics Data Book," co-edited by Asahi Kasei Amidas Corporation and the Plastics Editorial Department, and the absolute value obtained by subtracting the solubility parameter of one component from the solubility parameter of the other component means the solubility parameter difference referred to in the present invention.

[0124] Furthermore, by using polymers with different solubilities, the easily soluble polymer in the multilayer laminated fiber can be removed, allowing short fibers made of a poorly soluble polymer to be efficiently produced. For example, if the polymers constituting the multilayer laminated fiber are an alkali-soluble polyester and an alkali-slightly soluble polyester, or an alkali-soluble polyester and polyphenylene sulfide (poorly soluble in alkali), or an alkali-soluble polyester and polyamide (poorly soluble in alkali), then alkali reduction treatment will effectively produce short fibers made of the alkali-slightly soluble polymer. In particular, it is preferable to use a polyester copolymerized with polyethylene glycol and sodium sulfoisophthalate, either alone or in combination, as the easily soluble polyester, from the viewpoints of spinnability and ease of solubility in low-concentration aqueous solvents. Examples of suitable polymer combinations for generating staple fibers from multilayer laminated fibers include, in terms of melting point, polyethylene terephthalate copolymerized with 5 mol % to 15 mol % of 5-sodium sulfoisophthalic acid as the readily soluble component, and polyethylene terephthalate copolymerized with the aforementioned 5-sodium sulfoisophthalic acid and 5 wt % to 15 wt % of polyethylene glycol having a weight average molecular weight of 500 to 3,000, and polyethylene terephthalate, polyphenylene sulfide, or polyamide-6 as the poorly soluble component.

[0125] The spinning temperature when spinning the multilayer laminated fiber is set to a temperature at which the two or more polymers, primarily the high-melting-point polymer or high-viscosity polymer, exhibit fluidity. The temperature at which this fluidity is exhibited varies depending on the molecular weight, but is preferably set between the melting point of the polymer and melting point + 60°C. A temperature below this range is preferable because the polymer does not thermally decompose in the spinning head or spin pack, preventing a decrease in molecular weight. Stable production of the multilayer laminated fiber can be achieved by setting the output rate when spinning the multilayer laminated fiber to 0.1 g / min / hole to 20.0 g / min / hole. Furthermore, the ratio of component A to component B can be selected from a range of 5 / 95 to 95 / 5 in terms of the weight ratio of component A / component B based on the output rate. When a poorly soluble polymer is used as the A component and a readily soluble polymer is used as the B component to generate staple fibers from multilayer laminated fibers, a higher ratio of the poorly soluble polymer is preferable from the viewpoint of productivity of the flat ultrafine fibers. When the A component / B component ratio is 50 / 50 to 90 / 10, multilayer laminated fibers can be stably obtained without any break in the laminate structure, and staple fibers can be obtained with high production efficiency.

[0126] The polymer melted at the above-mentioned spinning temperature and given fluidity is introduced into a spinneret to form a composite flow, making it possible to produce a fiber having a multilayer laminated cross section. Although the spinneret used here can be a conventionally known composite spinneret that combines two or more polymers, the composite spinneret described below is preferably used to stably form a special multilayer laminated cross section.

[0127] The composite spinneret preferably used in the present invention is, for example, a composite spinneret in which three types of members, a metering plate D, a composite plate E, and a discharge plate F, are laminated as shown in Fig. 4. Incidentally, Fig. 4 shows an example in which two types of polymers, i.e., component A and component B, are used, but three or more types of polymers may be used for spinning if necessary.

[0128] In this composite spinneret, the metering plate D measures the amount of polymer per hole in the composite plate E, the different types of polymer streams measured by the composite plate E are merged to form a composite stream, which is then divided and re-merged to double the number of layers constituting the composite stream, and the discharge plate F compresses and discharges the composite stream formed by the composite plate E. Note that the composite stream referred to here means a fluid whose cross section perpendicular to the flow direction is composed of two or more types of polymers.

[0129] The fine flow channels of composite plate E are configured to minimize turbulence in the flow channels, making it possible to produce multilayer laminated fibers. Incidentally, the fine flow channels described above can be said to have the same characteristics as conventional static mixers in that they merge or split fluids within the channels, but general static mixers have flow channels designed for mixing two types of polymers, which makes it easy for turbulence to occur at the interface of the laminated composite flow. To prevent this, the method of using the composite spinneret described above is preferably adopted.

[0130] Although not shown in the drawings to avoid complicating the explanation of the composite spinneret, the components stacked above the metering plate D may be components that form flow paths in accordance with the spinning machine and spin pack. By designing the metering plate D to match existing flow path components, it is possible to utilize existing spin packs and their components as they are. Therefore, there is no need to dedicate a spinning machine specifically to this spinneret.

[0131] In practice, it is advisable to stack a plurality of flow path plates between the flow path and the metering plate D, or between the metering plate D and the composite plate E. This is intended to provide a flow path that efficiently transports the polymer in the cross-sectional direction of the spinneret and the cross-sectional direction of the single fiber, and to introduce the polymer into the composite plate E.

[0132] The composite stream discharged from the discharge plate F is cooled and solidified, and is then applied with an oil and taken up by rollers with a specified peripheral speed to form composite fibers. The take-up speed can be determined based on the discharge rate and the desired fiber diameter, but is preferably in the range of 100 to 7,000 m / min to stably produce the composite fibers used in the present invention. Furthermore, when intended for use as a binder fiber exhibiting thermal adhesiveness, it is more preferable to take up the composite fibers as undrawn yarns at a speed in the range of 100 to 3,000 m / min. From the viewpoint of highly orienting the multilayer laminated fiber and improving its mechanical properties, it is preferable to perform drawing. This drawing may be performed after the fiber has been taken up in the spinning process, or it may be performed immediately without taking up the fiber. As for the drawing conditions, for example, in a drawing machine consisting of one or more pairs of rollers, a fiber made of a polymer exhibiting thermoplasticity that can generally be melt-spun can be drawn reasonably in the fiber axis direction by a peripheral speed ratio between a first roller set at a temperature above the glass transition temperature and below the melting point and a second roller set at a temperature equivalent to the crystallization temperature, and the fiber is then heat-set and wound up to obtain a conjugate fiber having a conjugate cross section as shown in Figure 3. The upper limit of the temperature of the first roller is preferably a temperature at which the fiber path is not disturbed during the preheating process. For example, in the case of polyethylene terephthalate, whose glass transition temperature is around 70°C, the preheating temperature is usually set at about 80 to 95°C. Note that when use as a binder fiber is envisioned, the degree of crystallinity of the fiber can be maintained low by using the fiber as an undrawn yarn without drawing.

[0133] Although the above is an example in which the melt spinning method is employed, it goes without saying that the multilayer laminated fiber used in the present invention can also be produced by a spinning method that uses a solvent, such as solution spinning, if the above-mentioned composite spinneret is used. The obtained multilayer laminated fiber is made into a tow of several tens to several tens of thousands of fibers, which is then cut to the desired fiber length and the easily soluble polymer is removed, thereby obtaining the short fiber of the present invention.

[0134] To obtain a fiber dispersion containing the staple fibers of the present invention, the multilayer laminated fibers cut to the desired fiber length as described above may be immersed in a solvent capable of dissolving the readily soluble polymer to remove the readily soluble polymer. When the readily soluble polymer is copolymerized polyethylene terephthalate copolymerized with 5-sodium sulfoisophthalic acid or polyethylene glycol, an alkaline aqueous solution such as an aqueous sodium hydroxide solution can be used. In this case, the bath ratio of the multilayer laminated fibers to the alkaline aqueous solution (weight (g) of the multilayer laminated fibers / weight (g) of the alkaline aqueous solution) is preferably 1 / 10,000 to 1 / 5, and more preferably 1 / 5,000 to 1 / 10. By maintaining the ratio within this range, it is possible to prevent poor dispersion of clumps due to entanglement of the staple fibers when dissolving the readily soluble polymer.

[0135] In this case, the alkali concentration of the alkaline aqueous solution is preferably 0.1 to 10 wt %, and more preferably 0.5 to 5 wt %. By setting the alkali concentration within this range, the dissolution of the easily soluble polymer can be completed in a short time, and a fiber dispersion in which the short fibers of the present invention are uniformly dispersed can be obtained without unnecessarily degrading the poorly soluble polymer. The temperature of the alkaline aqueous solution is not particularly limited, but setting it to 50°C or higher can accelerate the dissolution of the easily soluble polymer. When the fiber is intended for use as a binder fiber, it is preferable to set the temperature of the alkaline aqueous solution to 40 to 70°C, since this can accelerate the progress of hydrolysis while maintaining a low degree of crystallinity of the fiber.

[0136] In such fiber dispersions, additives may be used as needed to suppress aggregation and sedimentation of short fibers over time or to increase the viscosity of the medium. Examples of additives include natural polymers, synthetic polymers, organic compounds, and inorganic compounds. For example, additives that suppress aggregation of fibers include cationic compounds, nonionic compounds, and anionic compounds. Such additives are preferably used in an amount of 0.001 to 10 equivalents relative to the weight of the short fibers. Within this range, sufficient functionality can be imparted.

[0137] The fiber dispersion obtained by the above-mentioned manufacturing method can be used as it is. Alternatively, the medium of these fiber dispersions may be neutralized with an acid such as hydrochloric acid or acetic acid, or the fiber dispersions may be diluted with water after the dehydration step before use. Neutralizing the medium in this way is preferable from the viewpoint of ease of handling.

[0138] As described above, the fiber dispersion in which the short fibers of the present invention are uniformly dispersed in a medium can be used as it is to develop high-performance adsorbents and reinforcing materials. Furthermore, when this dispersion is made into a fiber aggregate from which the medium is removed by a wet papermaking method or a spray method, the dispersion can be widely developed into high-performance filter media, separation membranes, sound-absorbing materials, and the like.

[0139] When a nonwoven fabric is formed by a wet papermaking method, for example, aggregate fibers that form the skeleton of the nonwoven fabric, binder fibers that bond the short fibers together by heat treatment, and functional short fibers that form a special structure are put into water, and after a defibrating step or a beating step as necessary, the mixture is stirred, and paper is made using a papermaking solution in which the various short fibers are uniformly dispersed. In the step of preparing the papermaking solution, the dispersibility can be adjusted by the amount of short fibers charged, the amount of aqueous medium, the stirring time, etc., and it is preferable to prepare the papermaking solution depending on the dispersion state of the various short fibers.

[0140] If necessary, a dispersant may be contained in order to suppress aggregation of the short fibers introduced into the water and to produce a nonwoven fabric having a stable dense structure.

[0141] Dispersants include natural polymers, synthetic polymers, organic compounds, and inorganic compounds. For example, additives that inhibit fiber aggregation include cationic compounds, nonionic compounds, and anionic compounds. Among these, anionic compounds are preferred for improving dispersibility, due to their electrical repulsive force in aqueous media. In the production of the present invention, the amount of dispersant added is preferably 0.001 to 10 equivalents relative to the weight of the fibers constituting the nonwoven fabric. This range allows for improved fiber dispersibility without impairing wetlaid papermaking processability, enabling the production of the nonwoven fabric of the present invention.

[0142] When wet papermaking is employed in the manufacturing method of nonwoven fabrics, it is preferable that the various short fibers used in the papermaking solution have a fiber length of 0.3 to 30.0 mm. If the fiber length is within this range, dispersibility in the papermaking solution is maintained and good wet papermaking is possible.

[0143] The fiber length of the short fibers can be adjusted appropriately depending on the application. Generally, as the fiber length increases, the degree of entanglement between the short fibers increases, and the mechanical properties and the dimensional stability of the nonwoven fabric tend to improve. However, since the short fibers tend to entangle and aggregate during stirring in the papermaking solution, it is advisable to adjust the fiber length according to the specifications of the short fibers to be mixed in the papermaking solution and the expected design.

[0144] From the viewpoint of stabilizing the shape of the nonwoven fabric during practical use and improving its mechanical properties, it is preferable to mix binder fibers at 5 to 70% by weight. This range fixes the dense structure of the nonwoven fabric, improves sheet strength, and increases the smoothness of the nonwoven fabric surface. Furthermore, aggregate fibers may be mixed as other fibers to provide the framework of the nonwoven fabric, and this can be appropriately adjusted to 10 to 95% by weight based on the total weight of the nonwoven fabric. When aggregate fibers are mixed as other fibers, the staple fibers of the present invention form a bridge structure with the aggregate fibers at both ends, and the staple fibers fill the spaces between the aggregate fibers, making it possible to produce a nonwoven fabric with complex microvoids.

[0145] The aggregate fibers and binder fibers referred to here can be appropriately selected in terms of fiber diameter and fiber cross section depending on the purpose. However, considering the production of a nonwoven fabric that is dense yet has a special structure in which complex voids are formed inside, which is a feature of the present invention, it is preferable to use fibers with a flat cross section in which the long axis length is longer than the short axis length for the aggregate fibers and binder fibers. In this case, a layer structure with the short fibers of the present invention is smoothly formed, making it possible to produce a nonwoven fabric with excellent density.

[0146] The papermaking solution containing the various short fibers is diluted to a certain concentration and then dehydrated on an inclined wire or cylinder to form a nonwoven fabric. Examples of equipment used for papermaking include a cylinder paper machine, a Fourdrinier paper machine, an inclined short-drain paper machine, or a combination of these. In the papermaking process, the papermaking speed, fiber amount, and aqueous medium amount are adjusted to match the desired basis weight in addition to the dispersibility of the fibers in the papermaking solution, thereby controlling the accumulation of fibers during drainage. The sheet-like product thus formed undergoes a drying process and a heat treatment process to remove moisture and bond the binder fibers, thereby producing the nonwoven fabric of the present invention. As the drying method, a method using hot air ventilation (air-through) or a method in which the sheet is brought into contact with a heated rotating roll (such as a heated calendar roll) is preferably used, as it allows for simultaneous drying of the sheet and thermal bonding of the binder fibers.

[0147] The short fibers of the present invention will be specifically described below with reference to examples.

[0148] The examples and comparative examples were evaluated as follows.

[0149] A. Melt viscosity of polymer Chip-shaped polymer is dried in a vacuum dryer to a moisture content of 200 ppm or less, and is then dried at a strain rate of 1,216 s using a Capillograph 1B manufactured by Toyo Seiki. －１ The melt viscosity of the mixture was measured at the same temperature as the spinning temperature. －１ The melt viscosity is shown in Table 1. The time from when the sample was placed in the heating furnace to when the measurement started was 5 minutes, and the measurement was carried out in a nitrogen atmosphere.

[0150] B. Melting Point of Polymer Chip-shaped polymer was adjusted to a moisture content of 200 ppm or less using a vacuum dryer, and approximately 5 mg was weighed out. Using a TA Instruments Q2000 differential scanning calorimeter (DSC), the temperature was raised from 0°C to 300°C at a heating rate of 16°C / min, and then held at 300°C for 5 minutes for DSC measurement. The melting point was calculated from the melting peak observed during the heating process. Measurements were performed three times per sample, and the average value was taken as the melting point. When multiple melting peaks were observed, the melting point was determined to be the top of the melting peak with the highest temperature.

[0151] C. Solubility Parameter Difference The solubility parameter (SP value) is a parameter that reflects the cohesive strength of a substance, defined as the square root of (evaporation energy / molar volume). It can be determined by immersing a polymer in various solvents and taking the value of (evaporation energy / molar volume) of the solvent at which the swelling pressure is maximized as the (evaporation energy / molar volume) of the polymer. The SP value determined in this manner is described, for example, in "Plastics Data Book," co-edited by Asahi Kasei Amidas Corporation and the Plastics Editorial Department, page 189, and this value was used. The solubility parameter difference between the combined polymers was calculated as the absolute value of (SP value of component A - SP value of component B).

[0152] D. Fineness The weight of 100 m of the composite fiber was measured, and the weight was multiplied by 100 to calculate the fineness (dtex). This measurement was repeated 10 times, and the average value was taken as the fineness (dtex). The value obtained by dividing the fineness by the number of filaments was taken as the single fiber fineness (dtex).

[0153] E. Specific surface area A fiber bundle consisting of short fibers was embedded in an embedding agent such as epoxy resin, frozen using a Reichert FC-4E cryosectioning system, and cut using a Reichert-Nissei Ultracut N (ultramicrotome) equipped with a diamond knife. The cut surface was then imaged using a Hitachi H-7100FA transmission electron microscope (TEM) at a magnification that allowed the cross section to be recognized. Using image analysis software (WINROOF), an arbitrary position on the outer periphery of the cross section was set as the measurement start point, and the outer periphery was traced from the measurement start point using a series of images, and the length until it returned to the measurement start point was measured. This value was taken as the periphery of one fiber and expressed as an integer in nm (rounded to the nearest integer). In addition, the area of ​​the inner portion surrounded by this periphery was measured using image analysis software (WINROOF), and this value was taken as the cross-sectional area of ​​one fiber. ２ The specific surface area of ​​one fiber was calculated using the perimeter and cross-sectional area by rounding off to the fifth decimal place according to the following formula:

[0154] Specific surface area (nm －１ )=Perimeter length (nm) / Cross-sectional area (nm ２The above measurement was carried out for 100 fibers to calculate the specific surface area of ​​each fiber, and the arithmetic average of these was taken as the specific surface area of ​​the present invention.

[0155] F. Flatness The maximum length of the cross section of the single fiber photographed above was measured using image analysis software (WINROOF), and this value was calculated as the length of the major axis of the single fiber, expressed as an integer in nm (rounded to the nearest integer). Next, the length of the line segment perpendicular to the line segment of the maximum length at the midpoint of the maximum length, intersecting the fiber cross section, was measured, and this value was calculated as the length of the minor axis of the single fiber, expressed as an integer in nm (rounded to the nearest integer). Using the lengths of the major axis and the minor axis, the flatness of the single fiber was calculated using the following formula.

[0156] Flatness = length in the major axis direction (nm) / length in the minor axis direction (nm) The above measurement was performed on 100 fibers to calculate the flatness of each fiber, and the arithmetic average of these values ​​was rounded off to the nearest integer to calculate the flatness.

[0157] G. Average Minor Axis Length The average minor axis length was calculated from the arithmetic average of the minor axis lengths of the 100 fibers measured above, expressed as an integer in nm (rounded to the nearest integer).

[0158] H. Variation in Minor Axis Length (CV Value) The arithmetic mean and standard deviation were calculated using the minor axis lengths of 100 fibers measured above, and the standard deviation was divided by the arithmetic mean to obtain a coefficient of variation, which was then rounded off to an integer in percentages to calculate the variation in minor axis length.

[0159] I. Irregularity Using the image of the fiber cross section taken above, the maximum length of the cross section was divided into 10 equal parts, and the lengths at which line segments perpendicular to the maximum length intersected the fiber cross section were measured. The arithmetic mean and standard deviation of these 10 lengths were calculated, and the standard deviation was divided by the arithmetic mean and rounded to the nearest percent to calculate the irregularity of a single fiber (see also Figure 2). Similar measurements were made on 10 fiber cross sections, and the arithmetic mean of the irregularities calculated for the 10 single fibers was calculated as the irregularity.

[0160] J. Aspect Ratio An image of a fiber bundle made of short fibers is taken with a microscope at a magnification that allows observation of at least 10 short fibers whose total length can be measured. The fiber lengths of 10 short fibers randomly extracted from the image of the short fibers are measured. The fiber length referred to here is the longitudinal length of a single fiber from a two-dimensionally captured image, measured in mm units using image analysis software and rounded to the nearest tenth place. The above operation is performed on 10 similarly captured images, and the arithmetic average of the fiber lengths of 100 fibers is defined as the fiber length of the present invention. Using this fiber length converted to nm and the average of the minor axis lengths calculated above, the aspect ratio is calculated using the following formula, rounded to the nearest whole number:

[0161] Aspect ratio = fiber length (nm) / average minor axis length (nm) K. Dispersion index with low-speed stirring A fiber dispersion was prepared by dispersing short fibers in an aqueous medium so that the concentration of short fibers was 0.01 wt% relative to the total fiber dispersion. The fiber dispersion was stirred at 100 rpm for 30 seconds using a stirrer and then placed in a 20 mL screw cap bottle manufactured by AS ONE Corporation. The screw cap bottle was photographed from the side under transmitted light using a digital camera. The resulting image was converted to a monochrome image using image processing software (WINROOF). A brightness histogram (vertical axis: frequency (number of pixels), horizontal axis: brightness; see also Figure 5) with a 256-series scale was then obtained, and the standard deviation of the brightness values ​​for one image was calculated. The same procedure was repeated for 10 images, and the standard deviation of the brightness values ​​for each image was arithmetically averaged. The value, rounded to one decimal place, was used as the dispersibility index with low-speed stirring.

[0162] L. Dispersion Index with High-Speed ​​Stirring A fiber dispersion was prepared by dispersing short fibers in an aqueous medium so that the concentration of short fibers was 0.01 wt % relative to the total amount of the fiber dispersion. The fiber dispersion was then stirred using a stirrer at 19,000 rpm for 30 seconds and then placed in a 20 mL screw cap bottle manufactured by AS ONE Corporation. The screw cap bottle was photographed from the side under transmitted light using a digital camera. The resulting image was converted into a monochrome image using image processing software (WINROOF). A luminance histogram (vertical axis: frequency (number of pixels), horizontal axis: luminance; see also Figure 5) with a series of 256 was then obtained, and the standard deviation of the luminance values ​​for one image was calculated. The same procedure was repeated for 10 images, and the standard deviation of the luminance values ​​for each image was arithmetically averaged. The value, rounded to one decimal place, was used as the dispersibility index with high-speed stirring.

[0163] M. Dispersibility Evaluation Using the dispersion index at low-speed agitation and the dispersion index at high-speed agitation measured as described above, dispersibility was evaluated according to the following criteria. Good: The dispersion index at low-speed agitation was less than 30, and the dispersion index at high-speed agitation was less than 30. Poor: Either the dispersion index at low-speed agitation or the dispersion index at high-speed agitation was 30 or greater. N. Average Pore Size and Maximum Frequency of Pore Size Distribution Using a Perm-Porometer (manufactured by PMI), an automatic porous material pore measurement system, the pore size was calculated according to the bubble point method (based on ASTM F-316-86). Three pieces of nonwoven fabric were cut out as measurement samples, and for each, pore size distribution was measured using Galwick (surface tension: 16 mN / m) as a measurement liquid with a known surface tension. The automatically calculated mean flow diameter was used as the average pore size, and the value calculated to one decimal place by rounding the average value for each sample to one decimal place was used.

[0164] The pore size distribution frequency was calculated by dividing the values ​​obtained by automatic calculation into classes with a width of 0.1 μm, and the maximum frequency was determined as the maximum frequency. The maximum frequency was then calculated as a percentage, and the average of the maximum frequencies for each sample was calculated. The value was rounded to one decimal place and used.

[0165] O. Basis weight A nonwoven fabric cut into a 250 mm x 250 mm square was weighed and the unit area (1 m ２The value converted into weight (g) per unit area was rounded off to the nearest whole number to obtain an integer, which was used as the basis weight of the nonwoven fabric.

[0166] P. Thickness The thickness of the nonwoven fabric was measured in mm using a dial thickness gauge SM-114 (manufactured by TECLOCK Corporation, probe shape 10 mmφ, graduation 0.01 mm, measuring force 2.5 N or less). Measurements were taken at five random locations per sample, and the average was rounded to two decimal places to determine the thickness of the nonwoven fabric.

[0167] Q. Nonwoven fabric density Calculate from the basis weight and thickness of the nonwoven fabric using formula (1). This was obtained for 10 samples, and the simple average value was rounded to two decimal places to obtain the nonwoven fabric density.

[0168] R. Arithmetic mean roughness (Ra) of nonwoven fabric surface The nonwoven fabric surface was observed using a laser microscope VK-X200 (manufactured by Keyence Corporation), and measurement was performed using analysis software VK-H1XA (manufactured by Keyence Corporation) in accordance with JIS B 0601. This was performed at any five locations per sample, and the average was rounded to two decimal places to obtain the average, which was taken as Ra.

[0169] S. Sound absorption coefficient: Measurements were made using an "automatic normal incidence sound absorption coefficient measuring instrument" manufactured by Denshi Sokki Co., Ltd. in accordance with JIS A 1405-1 (2007) "Method for measuring normal incidence sound absorption coefficient of building materials by the tube method." For each nonwoven fabric sample, a 20 mm thick solid cotton substrate made of PET fibers with a fiber diameter of 15 μm was prepared as a support, and each nonwoven fabric sample was attached to this support. The above measurement was performed on 10 samples, and the sound absorption coefficient at 1,000 Hz was calculated as a percentage to one decimal place. The simple average value was rounded to one decimal place to obtain the sound absorption coefficient.

[0170] T. Crystallinity Approximately 5 mg of thermoplastic fiber was weighed out using an electronic balance, and then the fiber was set in a differential scanning calorimeter (DSC) Model Q2000 manufactured by TA Instruments, and differential scanning calorimetry was performed under nitrogen at a heating rate of 16°C / min in a measurement temperature range of 50 to 320°C.

[0171] The crystallization heat of heat ΔHc (J / g) was calculated from the area of ​​the exothermic peak in the obtained measurement results (DSC curve), and the heat of crystal fusion ΔHm (J / g) was calculated from the area of ​​the endothermic peak. When multiple exothermic peaks or endothermic peaks were observed, ΔHc and ΔHm were calculated from the total area of ​​all peaks. Measurement was performed three times for each level, changing the measurement position, and the arithmetic averages were calculated to calculate ΔHc and ΔHm. The crystallinity was then calculated using the following formula, rounded to one decimal place.

[0172] Crystallinity (%) = (ΔHm-ΔHc) / ΔHm ０ × 100 Here, ΔHm ０ is the heat of melting of complete crystal (J / g), and 140.1 (J / g) was used for PET, and 146.2 (J / g) was used for PPS.

[0173] Dynamic viscoelasticity of the fiber was measured using a Rheovibron DOV-II-EP manufactured by Orientec Co., Ltd. The sample was clamped with a chuck distance of 30 mm, a tension of 0.07 g / dtex was applied, and the measurement was performed at a temperature rise rate of 3°C / min and a frequency of 110 Hz. The peak top temperature and peak value of tan δ were evaluated.

[0174] V. Adhesion rate and variation in adhesion rate A cross section of the nonwoven fabric was cut with a razor or the like, and this cross section was photographed with a scanning electron microscope S-5500 (SEM) manufactured by Hitachi High-Technologies Corporation at a magnification that allowed observation of the entire thickness direction of the sheet-like material. The length of the outer periphery of the cross section of one aggregate fiber present in the photographed image was measured using image analysis software (WINROOF). This value was taken as the outer periphery P of one fiber. ｍ１ The outer periphery length P ｍ１ The length of the outer periphery of the fiber that is bonded to the binder fiber is measured using image analysis software (WINROOF), and this value is taken as the bond length P of one fiber. ｂ１The perimeter was expressed in μm units, rounded to two decimal places. In the cross section of the nonwoven fabric, the binder fibers are softened and flowed during the thermal bonding process, and are present in a form that is impregnated between the aggregate fibers. However, the contrast between the aggregate fibers and the binder fibers impregnated therein due to the difference in unevenness was obtained, and the interface between the aggregate fibers and the binder fibers was identified. This perimeter P ｍｎ and adhesive length P ｂｎ Using the formula below, calculate the adhesion rate of one fiber P as an integer (rounded off to the nearest integer). ｂｎ / P ｍｎ was calculated.

[0175] P ｂｎ / P ｍｎ (%) = P ｂｎ (μm) / P ｍｎ (μm) × 100 The above measurement was carried out for 100 fibers to determine the adhesion rate P of each aggregate fiber. ｂｎ / P ｍｎ (n=1 to 100) and the arithmetic mean of these is the adhesion rate P ｂ / P ｍ The standard deviation was calculated in the same manner, and the coefficient of variation obtained by dividing the standard deviation by the arithmetic mean was expressed as an integer in percentage, with the decimal points rounded off to the nearest whole number, and this value was taken as the adhesion rate P ｂ / P ｍ The variation was as follows.

[0176] W. Uniformity The uniformity of a sheet cut into a 250 mm x 250 mm square was visually observed and evaluated on the following three-point scale: A: No fiber clumps in the sheet and good uniformity; B: Fiber clumps visible in the sheet; C: Fiber clumps visible in the sheet with tears and pinholes.

[0177] X. Strength Using a Tensilon UTM-III-100 manufactured by Orientec Co., Ltd., the maximum point load was measured under the conditions of a sample width of 15 mm, an initial length of 20 mm, and a tensile speed of 20 mm / min, and the strength of the sheet material (N / 15 mm) was calculated by simply averaging five measured values, and the calculated value was rounded to one decimal place.

[0178] [Example 1] As component A, polyethylene terephthalate (PET, melt viscosity: 120 Pa·s, melting point: 254°C, SP value: 21.4 MPa １／２ ) and, as component B, polyethylene terephthalate copolymerized with 8.0 mol% of 5-sodium sulfoisophthalic acid and 9 wt% of polyethylene glycol (SSIA-PEG copolymerized PET, melt viscosity: 95 Pa s, melting point: 233°C, SP value: 22.9 MPa １／２ The difference in solubility parameters between these polymers was 1.5 MPa. １／２ This becomes:

[0179] After melting components A and B separately at 290°C, the resulting composite polymer stream was introduced into a spinning pack incorporating a composite spinneret, as shown in Figure 4, with the composite ratio of components A / B set to 80 / 20. A composite polymer stream was then extruded from the extrusion holes. The composite plate had a microchannel G capable of laminating the two components alternately in 128 layers, and the composite stream was extruded to form a composite structure in which the two polymers were alternately laminated in multiple layers in one direction, as shown in Figure 3. The extruded composite polymer stream was cooled and solidified, then coated with an oil and wound at a spinning speed of 1,000 m / min, yielding an undrawn yarn of 200 dtex-24 filaments (total throughput of 20 g / min). The wound undrawn fiber was drawn 3.6 times between rollers heated to 90°C and 130°C, yielding a drawn fiber of 56 dtex-24 filaments.

[0180] The obtained multilayer laminated fiber was cut to a fiber length of 0.6 mm, and the cut multilayer laminated fiber was immersed in a 1 wt % aqueous sodium hydroxide solution (bath ratio 1 / 100) heated to 90°C for 30 minutes, thereby dissolving and removing 99% or more of the easily soluble polymer SSIA-PEG copolymerized PET, and thereby obtaining staple fibers having a flat cross section as shown in Figure 1.

[0181] The specific surface area of ​​the resulting short fibers was measured from a cross section, and was found to be 0.0108 nm －１The cross-sectional shape was ribbon-like with significantly different lengths of the major and minor axes, with a flatness of 80 and an average minor axis length of 188 nm. The variation in the minor axis length of the cross section was 36%, and the irregularity was 30%, indicating a moderate variation in the minor axis length and moderate irregularities on the surface. The crystallinity was 36%, indicating a sufficiently advanced state of crystallization, and the melting point was 254°C.

[0182] A fiber dispersion was prepared by dispersing these short fibers in an aqueous medium so that the short fiber concentration was 0.01 wt % relative to the total amount of the fiber dispersion. The dispersion state after stirring at various speeds was evaluated by image processing. If the fibers were uniformly dispersed, there would be no significant difference between light and dark, resulting in a small standard deviation of brightness (dispersion index). However, if the fibers were non-uniformly dispersed, there would be localized differences in light and dark, resulting in a large standard deviation of brightness (dispersion index). Evaluation of the dispersion index revealed a dispersion index of 7 at low speed and 8 at high speed, indicating uniform dispersion regardless of stirring speed, and both excellent short fiber specific surface area and excellent dispersibility were achieved. The results are shown in Table 1.

[0183] Examples 2, 3, and 4 The same procedures as in Example 1 were repeated, except that the composite plate used was one with microchannels in which both components were alternately stacked in 64 layers (Example 2), 32 layers (Example 3), and 16 layers (Example 4). These multilayer laminated fibers were subjected to the same cutting and dissolution treatments as described above to obtain short fibers with a flat cross-sectional shape as shown in Figure 1. The evaluation results of these short fibers are shown in Table 1. All of the short fibers in Examples 2 to 4 had a specific surface area greater than that of typical ultrafine fibers, a highly flattened, extremely thin ribbon-like cross-sectional shape, and moderate minor axis length variation and irregularity. Compared to Example 1, the dispersion index at low and high speed stirring increased slightly with a decrease in flatness and an increase in average minor axis length, but both the specific surface area and excellent dispersibility of the short fibers were achieved. The crystallinity of the short fibers was 36%, and the melting point was 254°C.

[0184] [Example 5] The same procedure as in Example 1 was repeated, except that the composite plate used was a 512-ply composite plate with microchannels in which both components were alternately stacked (Example 5). The multilayer laminated fiber was subjected to the same cutting and dissolution treatment as described above to obtain short fibers with a flat cross-sectional shape. The evaluation results of these short fibers are shown in Table 1. They had an extremely large specific surface area, far exceeding that of nanofibers with fiber diameters of several hundred nanometers, and the cross-sectional shape was an extremely thin ribbon-like shape with extremely high flatness. The dispersion index was evaluated, revealing a dispersion index of 5 at low speed and 18 at high speed. Although the dispersion index increased with high-speed stirring, there was no significant clumping or poor dispersion. The short fibers exhibited both an extremely large specific surface area and excellent dispersibility. The crystallinity of the short fibers was 36%, and the melting point was 254°C.

[0185] Comparative Example 1 The same procedures as in Example 1 were repeated, except that a 15-filament spinning pack equipped with a 1,000-island-sea composite spinneret in which component A was an island component and component B was a sea component was used, the A / B component composite ratio was set to 50 / 50, and the extrusion rate was 42 g / min. The sea-island composite fibers were cut and dissolved in the same manner as above to obtain nanofibers with a round cross section (crystallinity: 36%, melting point: 254°C). The evaluation results of these nanofibers are shown in Table 1. Although the specific surface area was large, high-speed stirring caused clumps of entangled fibers to form, resulting in poor dispersion, and the dispersibility was evaluated as being extremely low.

[0186] Comparative Example 2 was carried out in the same manner as in Comparative Example 1, except that the island component fiber cross section was made flat. Nanofibers with a large specific surface area and a flat cross section (crystallinity: 36%, melting point: 254°C) were obtained, but the flatness was low, so the fibers were entangled with each other by high-speed stirring, similar to Comparative Example 1, and the dispersibility was extremely low.

[0187]

[0188] Examples 6 and 7 were carried out in the same manner as in Example 1, except that a composite plate with a different channel configuration from that of the microchannel G was used (Examples 6 and 7). The evaluation results of the resulting short fibers are shown in Table 2. While the fibers had a highly flat, ultrathin cross-sectional shape similar to that of Example 1, the change in the channel configuration of the composite plate resulted in a uniform structure with small variations in the minor axis length and small irregularities. When the dispersion index was evaluated, the dispersion index value was slightly higher than that of Example 1 due to the smaller variations in the minor axis length and small irregularities compared to Example 1. However, no noticeable clump-like poor dispersion was observed, indicating excellent dispersibility. The crystallinity of the short fibers was 36%, and the melting point was 254°C.

[0189] [Examples 8 and 9] The same procedure as in Example 1 was repeated, except that the fiber length was cut to 1.8 mm (Example 8) or 5.0 mm (Example 9). In Example 8, even when the aspect ratio was large, the fibers were prevented from entangling due to the dispersion effect derived from the cross-sectional shape, resulting in excellent dispersibility. In Example 9, the aspect ratio was even larger than in Example 8, which tended to slightly worsen dispersibility during high-speed stirring. However, no problematic clump-like poor dispersion was observed, and both the specific surface area of ​​the short fibers and excellent dispersibility were achieved. The crystallinity of the short fibers was 36%, and the melting point was 254°C.

[0190] Comparative Example 4 The same procedure as in Comparative Example 1 was carried out, except that the fiber length was cut to 5.0 mm. In Comparative Example 4, the aspect ratio was larger than in Comparative Example 1, and therefore the fibers were significantly entangled, even at low stirring speeds, resulting in clumps of poor dispersion and significantly poor dispersibility. The degree of crystallinity of the short fibers was 36%, and the melting point was 254°C.

[0191]

[0192] Example 10: A multilayer laminate fiber in which two types of polymers are alternately laminated in one direction as shown in FIG. 3, with component A being polyethylene terephthalate (PET, melting point: 254°C) and component B being polyethylene terephthalate copolymerized with 8.0 mol% 5-sodium sulfoisophthalic acid and 9 wt% polyethylene glycol (SSIA-PEG copolymerized PET), was cut to a fiber length of 0.6 mm, and component B was eluted from this multilayer laminate fiber in a 1 wt% aqueous sodium hydroxide solution (bath ratio 1 / 100) heated to 90°C, thereby obtaining a dispersion containing flat fibers (minor axis length: 1,000 nm) with a flatness of 20 as a functional fiber. The crystallinity of the obtained flat fibers was 36%, which was a sufficiently advanced state of crystallization, and the melting point was 254°C.

[0193] Next, cut fibers (core fiber diameter 10 μm, fiber length 5.0 mm) of heat-fusible core-sheath composite fibers (core component: PET, sheath component: polyester (copolymer polyester) having a melting point of 110°C copolymerized at a ratio of 60 mol % of terephthalic acid, 40 mol % of isophthalic acid, 85 mol % of ethylene glycol, and 15 mol % of diethylene glycol) were prepared as binder fibers at a mixing ratio of 30 wt %, and in a defibrating process and a beating process, the cut fibers were uniformly mixed and dispersed with water and a flat fiber dispersion adjusted to a mixing ratio of 70%, to prepare a papermaking solution.

[0194] This papermaking stock solution was mixed with a square sheet machine (250 mm square) manufactured by Kumagai Riki Kogyo Co., Ltd. to form a sheet having a basis weight of 50 g / m ２ The paper was dried and heat-treated in a rotary dryer with a roller temperature set at 110°C to obtain a nonwoven fabric.

[0195] The obtained nonwoven fabric had a uniform flat fiber orientation and a dense structure in the cross-sectional direction, and had a thickness of 105 μm and a nonwoven fabric density of 0.48 g / cm ３The average pore size calculated by the bubble point method was 0.5 μm, and the maximum frequency of the pore size distribution was 17.9%, indicating that the nonwoven fabric had dense spaces with various micropores. The Ra was 2.96 μm, and the nonwoven fabric had a smooth surface. The sound absorption coefficient at 1,000 Hz was 82%, indicating that the nonwoven fabric had excellent sound absorption properties in the low frequency band. The results are shown in Table 3.

[0196] [Examples 11 and 12] The basis weight of the nonwoven fabric was 25 g / m ２ (Example 11), 5 g / m ２ The procedure of Example 10 was repeated except for the changes made to (Example 12).

[0197] The obtained nonwoven fabric had a uniform orientation of the flat fibers (crystallinity: 36%, melting point: 254°C) and a dense structure in the cross-sectional direction, similar to Example 10. The evaluation results of these nonwoven fabrics are shown in Table 3. When the basis weight was reduced, the nonwoven fabric density decreased and the average pore size increased, resulting in a decrease in denseness compared to Example 10. However, all of the nonwoven fabrics had a sound absorption coefficient of 66% or more at 1,000 Hz, making them practically effective nonwoven fabrics with excellent sound absorption properties.

[0198] Examples 13 to 15 As shown in Table 3, the same procedures as in Example 10 were repeated except that the flatness and minor axis length of the functional fibers (flat fibers) constituting the nonwoven fabric were changed in various ways (Examples 13 to 15).

[0199] The resulting nonwoven fabrics had a uniform orientation of the flat fibers (crystallinity: 36%, melting point: 254°C) and a dense structure in the cross-sectional direction, similar to Examples 10 to 12. The evaluation results of these nonwoven fabrics are shown in Table 3. When the flatness was increased from that of Example 10, the nonwoven fabric density and average pore size increased, resulting in improved density compared to Example 10. Furthermore, the maximum frequency of the pore size distribution decreased, resulting in a nonwoven fabric with a more complex void structure. Furthermore, the shorter short axis of the flat fibers reduced Ra, resulting in a nonwoven fabric with lower surface roughness compared to Example 10.

[0200] Example 16 The procedure of Example 10 was repeated, except that the flatness of the flat fibers constituting the nonwoven fabric was changed to 10. The resulting nonwoven fabric had a uniform flat fiber orientation and a dense structure in the cross-sectional direction, similar to Examples 10 to 15. The evaluation results of this nonwoven fabric are shown in Table 3. When the flatness was reduced from that of Example 10, the nonwoven fabric density and average pore size increased, resulting in a slight decrease in denseness compared to Example 10. However, this did not pose any problems in practical use and satisfied the requirements for the nonwoven fabric of the present invention. The results are shown in Table 3. The crystallinity of the flat fibers was 36%, and the melting point was 254°C.

[0201] Example 17 The procedure of Example 10 was repeated except that the flat fibers (crystallinity: 36%, melting point: 254°C) used in Example 10 were used as the functional fibers constituting the nonwoven fabric, and further the binder fibers used in Example 10 and ultrafine PET fibers with a fiber diameter of 0.3 μm were used as other fibers, and a papermaking solution was prepared in a mixing ratio of functional fibers (flat fibers) / binder fibers / other fibers (ultrafine fibers) = 60 wt% / 30 wt% / 10 wt%.

[0202] The resulting nonwoven fabric had a dense structure in which the flat fibers were stacked so that their short axes were perpendicular to the thickness direction, similar to Examples 10 to 16, and the ultrafine fibers were bridged using the aggregate fibers as scaffolds. The evaluation results of this nonwoven fabric are shown in Table 3. Due to the presence of the ultrafine fibers in a bridging configuration, the nonwoven fabric had a denser structure than Example 10. Meanwhile, the ultrafine fibers homogenized the void structure, resulting in a higher maximum frequency in the pore size distribution than Example 10. In particular, Example 17 had a structure in which the flat fibers mixed as functional fibers not only densified the nonwoven fabric structure, but also contained ultrafine fibers mixed as other fibers, filling these voids, resulting in extremely excellent sound absorption properties. Furthermore, the pore size was small and the unique void structure was expected to demonstrate excellent properties in filtration, separation, and other performance.

[0203]

[0204] Comparative Example 4 The procedure of Example 10 was repeated, except that the fibers (aggregate fibers) constituting the nonwoven fabric were changed to ultrafine fibers (PET, crystallinity: 36%, melting point: 254° C.) having a fiber diameter of 3.0 μm.

[0205] The obtained nonwoven fabric had a thickness of 220 μm and a density of 0.23 g / cm ３ The average pore size calculated by the bubble point method was 14.3 μm, which was a less dense nonwoven fabric with larger voids than that of Example 10. The Ra was 13.62 μm, and the nonwoven fabric had a rough surface. The results are shown in Table 4.

[0206] Comparative Example 5 The procedure of Example 10 was repeated except that the fibers constituting the nonwoven fabric were ultrafine fibers with a fiber diameter of 3.0 μm as the aggregate fibers and the flat fibers (crystallinity: 36%, melting point: 254° C.) used in Example 10 as the functional fibers, and the paper was made in a ratio of aggregate fiber / binder fiber / flat fiber = 60 wt % / 30 wt % / 10 wt %.

[0207] The obtained nonwoven fabric had a thickness of 197 μm and a density of 0.25 g / cm due to the low mixing ratio of flat fibers. ３ The average pore size calculated by the bubble point method was 12.3 μm, and the nonwoven fabric had large voids and was low in density, similar to Comparative Example 4. The results are shown in Table 4.

[0208] Comparative Example 6 The procedure of Example 10 was repeated except that the flatness of the flat fibers constituting the nonwoven fabric was set to 2. The crystallinity of the flat fibers was 36% and the melting point was 254°C.

[0209] The obtained nonwoven fabric had a thickness of 264 μm and a nonwoven fabric density of 0.19 g / cm due to the small flatness of the flat fibers. ３ The average pore size calculated by the bubble point method was 29.6 μm, which was a less dense nonwoven fabric with even larger pores than those in Comparative Examples 4 and 5. The results are shown in Table 4.

[0210]

[0211] [Example 18] The procedure of Example 10 was repeated except that the aggregate fibers constituting the nonwoven fabric were ultrafine fibers having a fiber diameter of 3.0 µm, and the flat fibers (crystallinity: 36%, melting point: 254°C) and binder fibers used in Example 13 were used as the functional fibers and binder fibers, and paper was made in a ratio of functional fiber / binder fiber / other fibers = 40 wt% / 30 wt% / 30 wt%.

[0212] As a result, a nonwoven fabric was obtained in which the flat fibers bridged the ultrafine fibers that were the aggregate fibers, and a complex void structure was formed between the fibers, resulting in a nonwoven fabric with superior rigidity compared to Example 10. The results are shown in Table 5.

[0213] [Examples 19 and 20] Multilayer laminated fibers were obtained in which two types of polymers were alternately laminated in one direction, as shown in FIG. 3 , with the A component being polyethylene terephthalate (PET) and the B component being polyethylene terephthalate copolymerized with 8.0 mol% 5-sodium sulfoisophthalic acid and 9 wt% polyethylene glycol (SSIA-PEG copolymerized PET). These fibers were then cut to a fiber length of 1.0 mm (Example 19) or 5.0 mm (Example 20). The B component was then dissolved from each of these multilayer laminated fibers in the same manner as in Example 10, and a dispersion containing flat fibers (minor axis length: 500 nm) with different fiber lengths and a flatness of 40 was obtained. Utilizing this dispersion, a dispersion of 5.0 g / m was obtained in the same manner as in Example 12. ２ The paper was made so that the

[0214] The obtained nonwoven fabric had a uniform orientation of the flat fibers (crystallinity: 36%, melting point: 254°C) and a dense structure in the cross-sectional direction. Despite its very thin thickness, the rigidity of the sheet increased as the fiber length of the functional fiber increased, and the smoothness of the sheet surface also improved. The results are shown in Table 5.

[0215]

[0216] Reference Example 1 Polyethylene terephthalate (PET, melt viscosity: 120 Pa·s, melting point: 254°C, SP value: 21.4 MPa １／２) was melt-spun at 290°C. The discharged polymer stream was cooled and solidified, then an oil was applied, and the stream was wound at a spinning speed of 1,000 m / min to obtain an undrawn yarn of 200 dtex-72 filaments (total output rate: 20 g / min). The wound undrawn fiber was drawn 3.6 times between rollers heated to 90°C and 130°C to obtain a drawn fiber of 56 dtex-72 filaments. The drawn fiber was then cut with a cutter to obtain short fibers with an average fiber length of 5 mm.

[0217] [Reference Example 2] Polyphenylene sulfide consisting only of p-phenylene sulfide units (PPS, melt viscosity: 130 Pa s, melting point: 283°C, SP value: 25.8 MPa １／２ ) was melt-spun at 310°C. The discharged polymer stream was cooled and solidified, then an oil was applied, and the stream was wound up at a spinning speed of 1,000 m / min to obtain an undrawn yarn of 200 dtex-72 filaments (total output rate: 20 g / min). The wound undrawn fiber was drawn 3.6 times between rollers heated to 90°C and 200°C to obtain a drawn fiber of 56 dtex-72 filaments. The drawn fiber was then cut with a cutter to obtain short fibers with an average fiber length of 5 mm.

[0218] [Example 21] As component A, polyethylene terephthalate (PET, melt viscosity: 120 Pa s, melting point: 254°C, SP value: 21.4 MPa) １／２ ) and, as component B, polyethylene terephthalate copolymerized with 8.0 mol% of 5-sodium sulfoisophthalic acid and 9 wt% of polyethylene glycol (SSIA-PEG copolymerized PET, melt viscosity: 95 Pa s, melting point: 233°C, SP value: 22.9 MPa １／２ The difference in solubility parameters between these polymers was 1.5 MPa. １／２ This becomes:

[0219] After separately melting components A and B at 290°C, the components were fed into a spinning pack incorporating a composite spinneret as shown in Figure 4 at a component A / B ratio of 80 / 20, and a composite polymer stream was discharged from the discharge holes. A microchannel G capable of laminating both components alternately in 64 layers was used for the composite plate, and the composite was discharged in a composite form in which the two polymers were alternately laminated in multiple layers in one direction, as shown in Figure 3. The discharged composite polymer stream was cooled and solidified, and then an oil was applied. The stream was then wound at a spinning speed of 1,000 m / min, and an undrawn yarn of 100 dtex-24 filaments (total throughput of 10 g / min) was collected.

[0220] The obtained multilayer laminated fiber was cut to a fiber length of 5.0 mm, and the cut multilayer laminated fiber was immersed in a 1 wt % aqueous sodium hydroxide solution (bath ratio 1 / 100) heated to 70°C for 30 minutes to dissolve and remove 99% or more of the easily soluble polymer, SSIA-PEG copolymerized PET, and short fibers (melting point: 254°C) having a flat cross section as shown in Figure 1 were obtained.

[0221] The specific surface area of ​​the resulting short fibers was measured from a cross section, and was found to be 0.0041 nm －１ The cross-sectional shape was ribbon-like with significantly different major and minor axis lengths, with a flatness of 40, a minor axis length of 500 nm, and a major axis length of 20,000 nm. The variation in the minor axis length of the cross section was 32%, and the irregularity was 21%, indicating a moderate variation in the minor axis length and moderate irregularities on the surface. The crystallinity was 6%, indicating an amorphous state, and tan δ was 0.25, indicating high fluidity.

[0222] The short fibers described above were mixed as binder fibers at 50% by mass, and the short fibers obtained in Reference Example 1 were mixed as aggregate fibers for the adherend at 50% by mass in a papermaking dispersion to give a fiber concentration of 0.4% by mass. This papermaking solution was fed to a square sheet machine (250 mm square) manufactured by Kumagai Riki Kogyo Co., Ltd., and a basis weight of 5 g / m was obtained. ２ Further, using a flat plate heating press at 200°C, the mixture was subjected to thermocompression bonding at a pressure of 1.0 MPa for 1 minute to obtain a nonwoven fabric.

[0223] The obtained nonwoven fabric was free of agglomerates such as fiber clumps and had a good texture, and although it was thin and had a low basis weight, it had a good strength of 2.40 N / 15 mm. The results are shown in Table 6.

[0224] Examples 22 and 23 The same procedure as in Example 21 was repeated, except that the composite plates used were those with microchannels in which both components were laminated in 32 layers (Example 22) and 16 layers (Example 23). These multilayer laminated fibers were subjected to the same cutting and dissolution treatment as described above to obtain short fibers (melting point: 254°C) with a flat cross-sectional shape as shown in Figure 1. The evaluation results of these short fibers are shown in Table 6. Examples 22 and 23 had a large specific surface area and a very thin, ribbon-like cross-sectional shape with a high degree of flatness, and moderate minor axis length variation and irregularity. When evaluated as a sheet, compared to Example 21, the adhesion rate decreased slightly and the variation in adhesion rate increased slightly as the flatness decreased and the average minor axis length increased, but the sheet exhibited excellent strength despite its thinness and low basis weight.

[0225] Examples 24 and 25 The same procedures as in Example 22 were carried out, except that the temperature of the dissolution treatment for removing the easily soluble polymer was changed to 75°C (Example 24) or 80°C (Example 25). The evaluation results are shown in Table 6. Compared to Example 22, the higher dissolution treatment temperature increased the crystallinity of the short fibers (melting point: 254°C) and reduced tan δ. When made into a nonwoven fabric, the higher crystallinity and reduced fluidity resulted in a lower adhesion rate and increased variation in adhesion rate compared to Example 22, but excellent strength was maintained.

[0226] Examples 26 and 27 were carried out in the same manner as in Example 22, except that a composite plate with a different channel configuration from that of microchannel G was used. The results are shown in Table 6. Although the composite plate had a highly flat, ultrathin cross-sectional shape similar to that of Example 22, by changing the channel configuration of the composite plate, short fibers with a cross-section with little variation in the minor axis length (Example 26) and short fibers with a cross-section with little irregularity (Example 27) were obtained. When evaluated as a nonwoven fabric, the nonwoven fabric was homogeneous, with no fiber clumps. Compared to Example 22, the adhesion rate was slightly reduced and the adhesion rate variation increased, but the strength was excellent. The melting points of the short fibers in Examples 26 and 27 were 254°C.

[0227] Comparative Example 7 The same procedures as in Example 21 were carried out, except that the undrawn yarn (melting point: 254°C) described in Reference Example 1 was used as the binder fiber. The evaluation results are shown below. The fiber cross section was round and had a small specific surface area, and when this was used to form a nonwoven fabric, it was uneven with tears and pinholes in various places, had extremely large variations in the adhesion rate, and had extremely low strength. Note that because the nonwoven fabric was uneven, the average pore size and arithmetic mean roughness were not measured.

[0228] Comparative Example 8 The same procedures as in Example 22 were carried out, except that the multilayer laminated fibers were left in a hot air dryer heated to 120°C for 30 minutes to thermally crystallize them beforehand, and then subjected to a dissolution treatment at 90°C for 30 minutes. The evaluation results are shown in Table 6. The obtained staple fibers (melting point: 254°C) had a high crystallinity of 36%, and when they were made into a nonwoven fabric, the fabric was nonuniform with tears and pinholes in various places, had extremely large variations in the adhesion rate, and had extremely low strength.

[0229]

[0230] [Examples 28 and 29] In the method described in Examples 22 and 23, component A was replaced with polyphenylene sulfide (PPS, melt viscosity: 130 Pa s, melting point: 283°C, SP value: 25.8 MPa) consisting solely of p-phenylene sulfide units. １／２), and component B were extruded at 310°C as polyethylene terephthalate copolymerized with 5.0 mol% of 5-sodium sulfoisophthalic acid (SSIA copolymerized PET, melt viscosity: 130 Pa s, melting point: 245°C). The extruded composite polymer flow was cooled and solidified, and then an oil was applied thereto. The mixture was taken up at a spinning speed of 1,000 m / min, and an undrawn yarn of 100 dtex-24 filaments (total extrusion rate: 10 g / min) was collected.

[0231] The resulting undrawn yarn was immersed for 40 minutes in a 3 wt % aqueous sodium hydroxide solution (bath ratio 1 / 100) heated to 60°C for dissolution treatment, and 99% or more of the SSIA-copolymerized PET, which is an easily soluble polymer, was dissolved and removed, yielding staple fibers having a flat cross section as shown in Figure 1. DY-1125K (quaternary ammonium salt-type cationic surfactant) manufactured by Lion Specialty Chemicals Co., Ltd. was added as a dissolution promoter to the aqueous sodium hydroxide solution in an amount of 0.5 wt % based on the mass of the aqueous sodium hydroxide solution.

[0232] The thermoplastic fiber described above was used as the binder fiber in an amount of 50% by mass, and the short fiber obtained in Reference Example 2 was used as the aggregate fiber for the adherend in an amount of 50% by mass, and the mixture was blended in a papermaking dispersion to give a fiber concentration of 0.4% by mass. This papermaking solution was fed to a simple papermaking machine and a paper weight of 5 g / m was obtained. ２ Further, the nonwoven fabric was subjected to thermocompression bonding at a pressure of 1.0 MPa for 1 minute using a flat plate heating press at 220°C.

[0233] The evaluation results of the obtained short fibers (melting point: 283°C) are as shown in Table 7. Examples 28 and 29 had a large specific surface area, and the cross-sectional shape was an extremely thin ribbon with high flatness, and had moderate variation in the length of the minor axis and unevenness. When evaluated as a nonwoven fabric, the nonwoven fabric was uniform and free of fiber clumps, had a high bonding rate, low variation in the bonding rate, and excellent strength.

[0234] Example 30 The same procedures as in Example 28 were carried out, except that the temperature of the dissolution treatment for removing the easily soluble polymer was 90°C. The evaluation results are shown in Table 7. Compared to Example 28, the higher dissolution treatment temperature increased the crystallinity of the short fibers (melting point: 283°C) and reduced tan δ. When made into a nonwoven fabric, the higher crystallinity resulted in a lower adhesion rate and increased variation in adhesion rate compared to Example 28, but excellent strength was maintained.

[0235] Comparative Example 9 The same procedures as in Example 28 were carried out, except that the undrawn yarn (melting point: 283°C) described in Reference Example 2 was used as the binder fiber. The evaluation results are shown below. The fiber had a round cross section and a small specific surface area, and when this was used to make a nonwoven fabric, it was uneven with tears and pinholes in various places, had extremely large variations in the adhesion rate, and had extremely low strength.

[0236]

[0237] A: Short fiber B: Component A C: Component B D: Metering plate E: Composite plate F: Discharge plate G: Microchannel

[0238] The fiber dispersion in which the short fibers of the present invention are uniformly dispersed in a medium can be used as it is to develop high-performance adsorbents and reinforcing materials. Furthermore, when this dispersion is made into a sheet by removing the medium using a wet papermaking method or a spray method, it can be widely developed as an industrial material, such as a high-performance filter medium, a separation membrane, or a sound-absorbing material.

Claims

1. A staple fiber having a flatness of 5 or more, which is the value obtained by dividing the length of the major axis of the fiber cross section by the length of the minor axis, an average minor axis length of 2,000 nm or less, and an irregularity of the fiber cross section of 20% or more.

2. 2. The staple fiber according to claim 1, wherein the variation in the length of the minor axis of the fiber cross section (CV value) is 10% or more.

3. The short fiber described in claim 1, wherein the degree of unevenness is determined by measuring the length of a line segment perpendicular to the maximum length line segment intersecting the fiber cross section at nine points obtained by dividing the maximum length of the fiber cross section into ten equal parts for one fiber, dividing the standard deviation of these nine lengths by the arithmetic mean, and rounding off to the nearest decimal place to calculate the degree of unevenness of a single fiber in percent, and then calculating the arithmetic mean of the degrees of unevenness of the single fibers for 10 fibers.

4. 2. The staple fiber according to claim 1, having a crystallinity of 20% or less.

5. 5. The staple fiber according to claim 4, having a melting point of 180° C. or higher.

6. A fiber dispersion in which the short fibers according to claim 1 are dispersed in an aqueous medium.

7. A nonwoven fabric comprising at least a portion of the staple fiber according to claim 1.

8. Density is 0.4 g / cm ３ or more, and has an average pore size of 6 μm or less and a maximum frequency of pore size distribution of 30% or less.

9. 9. The nonwoven fabric according to claim 8, wherein the arithmetic mean roughness (Ra) of the surface is 5.00 μm or less.

10. A textile product comprising at least a part of the nonwoven fabric according to claim 7 or 8.