Nonwoven fabric, and fibrous product and sheet containing same

Abstract

This nonwoven fabric has a thickness of 30.0 μm or less, and a value of toughness calculated as a product of a specific tensile strength and a square root of tensile elongation is 0.50 or more. The present invention provides a thin nonwoven fabric that is thin and has excellent mechanical characteristics and excellent processability.

Description

Nonwoven fabrics, textile products and sheets containing the same

[0001] The present invention relates to a thin film nonwoven fabric having excellent mechanical properties.

[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 is a demand for textile products with more advanced functions.

[0003] One type of textile product that utilizes the shape of fibers is nonwoven fabric, which is made by layering fibers three-dimensionally and bonding the fibers together using mechanical, chemical, or thermal methods.This material is characterized by a porous structure with voids between the fibers that make up the fabric.

[0004] Applications that utilize this porous structure include membranes that filter and separate desired substances, and substrates that impregnate and support materials that cannot maintain their shape on their own. However, with increasing demand for smaller, lighter, and more powerful modules that use these materials, there is also a demand for thinner nonwoven fabrics, which are the components of these modules.

[0005] To make the nonwoven fabric thinner, the basis weight (g / m), which is the amount of constituent fibers per unit area, is ２ ) is an effective approach, but as the amount of constituent fibers decreases, the texture of the nonwoven fabric deteriorates, making the product more susceptible to defects. Therefore, active research and technological development is being conducted into making the constituent fibers ultra-fine as a method of obtaining nonwoven fabrics that are thin yet have excellent texture.

[0006] Patent Document 1 proposes a technology relating to a thin film nonwoven fabric made by wet-laid papermaking using ultrafine fibers as the main fibers. Microfibers with a fiber diameter of several μm have five times more constituent fibers than general fibers with a fiber diameter of a dozen μm, and the amount of constituent fibers is extremely small, at a few g / m, which is prone to defects due to poor formation. ２ Furthermore, the densification of the nonwoven fabric structure due to the ultra-fine fibers constituting the fabric, combined with the effect of reducing the amount of fiber due to the low basis weight, makes it possible to obtain a thin nonwoven fabric with a thickness of less than 20 μm.

[0007] Patent Document 2 proposes a technology relating to a thin film nonwoven fabric made by wet-laid papermaking using ultra-fine fibers, which are the main fibers and are made by further promoting the ultra-fineness of fibers. By using nanofibers with a fiber diameter reduced to the utmost limit, the number of fibers can be increased several hundred times compared to fibers with a fiber diameter of a dozen or so μm, and while achieving excellent texture, the densification of the structure due to the ultra-fineness allows for a density of 10 g / m or more. ２ This makes it possible to obtain a thin nonwoven fabric with a thickness of about 20 μm at a basis weight as low as about 100 μm.

[0008] Patent Document 3 proposes a technology relating to a thin film nonwoven fabric made by wet papermaking using ultrafine fibers as the main fibers and moist heat adhesive fibers as the binder fibers. In the thermal bonding process, the moist heat adhesive fibers melt without leaving any fiber skeleton, thereby firmly bonding the nanofibers that are the main fibers. As the large-diameter binder fiber skeleton disappears, the structural densification effect resulting from the ultrafine main fibers becomes more pronounced. ２ This has achieved an ultra-thin film with a thickness of approximately 10 μm and a basis weight of approximately 10 μm.

[0009] JP 60-38193 A International Publication No. 2008 / 130020 JP 2023-69101 A

[0010] Patent Document 1 aims to achieve both a thin nonwoven fabric and excellent texture through low basis weight wet papermaking by utilizing the increased number of constituent fibers and the densified structure resulting from the ultra-thinning of the main fibers. In this technology, even though the constituent fibers are ultra-thinned, the fiber diameter is only about half that of ordinary fibers, and the effect of increasing the number of fibers is limited, with the fiber density being only a few g / m. ２ At low basis weights, the mechanical properties are extremely poor, and thin nonwoven fabrics with a thickness of 30 μm or less may break due to tension fluctuations or contact with other components during the modularization process.

[0011] Patent Document 2 aims to emphasize the effects of increasing the number of constituent fibers and densifying the structure by using nanofibers with extremely small fiber diameters as the main fibers. However, to produce a wetlaid nonwoven fabric with excellent texture, the nanofibers must be uniformly dispersed in the papermaking solution, and the fiber length must be extremely short, less than 1 mm, to prevent entanglement between the nanofibers. Therefore, the extremely short fiber length of the main fibers that form the skeleton of the nonwoven fabric reduces the number of junctions between the fibers, preventing effective transmission of stress applied to the nonwoven fabric between the fibers, resulting in poor mechanical properties compared to general-purpose nonwoven fabrics. In addition to poor mechanical properties, fiber shedding can occur, which may require improvements in the material's handleability and processability.

[0012] Patent Document 3 aims to form a thin film nonwoven fabric with excellent mechanical properties by using a heat-and-moisture adhesive fiber as the binder fiber while using nanofibers as the main fiber, as in Patent Document 2. With this technology, the binder fiber dissolves without leaving the fiber skeleton behind to form an adhesive film, which firmly bonds the nanofibers together, thereby preventing breakage due to fiber slip-through. However, because the binder fiber closes the interfiber voids, the void ratio is extremely low at around 30%, and the functionality utilizing the porous structure of the nonwoven fabric may be impaired.

[0013] As described above, there are technologies that can obtain high-quality thin wet-laid nonwoven fabrics despite their low basis weight by using various ultrafine fibers as the constituent fibers, but the small amount of constituent fibers makes defects that can become breakage initiation points more likely to become apparent, and the mechanical properties tend to be low, which can limit the manufacturing processes and molding processes that can be applied.In addition, the low mechanical properties of thin-film nonwoven fabrics have been an obstacle to begin with, making it technically difficult to form nonwoven fabrics with extremely low basis weights that can achieve the ultra-thin films required for dramatically reducing the size and weight and improving the performance of various modules.

[0014] For this reason, there has been a demand for a nonwoven fabric with a low basis weight and an extremely small amount of constituent fibers that is both thin and has good mechanical properties.

[0015] The above-mentioned objects are achieved by the following means: (1) A nonwoven fabric having a thickness of 30.0 μm or less and a toughness value of 0.50 or more, calculated as the product of the specific tensile strength and the square root of the tensile elongation. (2) The nonwoven fabric according to (1) above, having a porosity of 50% or more. (3) The nonwoven fabric according to (1) or (2) above, characterized in that the adhesive area per cross-sectional area of ​​the fibers constituting the nonwoven fabric is 3.00 or more. (4) A textile product at least partially comprising the nonwoven fabric according to (1) above. (5) A sheet in which the nonwoven fabric according to (1) above is filled with a resin or particles. (6) A solid electrolyte sheet in which the nonwoven fabric according to (1) above is filled with a solid electrolyte.

[0016] The present invention relates to a thin nonwoven fabric that has excellent mechanical properties despite being thin, and can provide a thin nonwoven fabric that is excellent in processability.

[0017] 1 is a schematic diagram of the cross-sectional structure of a flat fiber, which is an example of a constituent fiber of the nonwoven fabric of the present invention; 2 is a schematic diagram of the cross-sectional structure of a multi-layer laminated fiber used as a raw material for the flat fiber; 3 is a schematic diagram showing a portion where subject fibers and binder fibers are bonded perpendicularly to each other.

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

[0019] The present invention relates to a nonwoven fabric characterized by having excellent mechanical properties despite being a thin film. In order to achieve the object of the present invention, the thickness of the nonwoven fabric must be a thin film of 30.0 μm or less, which is the first requirement of the present invention.

[0020] The thickness of the nonwoven fabric is determined as follows.

[0021] The thickness of the nonwoven fabric of the present invention is measured in mm using a dial thickness gauge (DGN-250B manufactured by PEACOCK, measuring element shape 25 mmφ, graduation 0.0001 mm, measuring force 1.2 N or less), and the value is converted to μm. This is done at any five points per sample, and the arithmetic average of these values ​​is rounded off to one decimal place, and the value obtained is the thickness of the nonwoven fabric of the present invention.

[0022] When the nonwoven fabric of the present invention has a thickness of 30.0 µm or less, when the nonwoven fabric is used as a diaphragm or a substrate, the nonwoven fabric does not occupy space within the module, and general demands for small size, light weight, and high performance can be satisfied.

[0023] From this perspective, the thinner the nonwoven fabric, the more pronounced the space-saving effect when applied to various modules. A thickness of 15.0 μm or less is about half the thickness of a general-purpose thin-film nonwoven fabric, and can satisfy the demand for smaller, lighter, and higher performance even for high-performance modules such as small precision electronic devices that require even higher levels of thinness, and can therefore be considered a preferred range.

[0024] Furthermore, a thickness of 8.0 μm or less is a more preferred range in the present invention, and this range not only further emphasizes the space-saving effect, but also produces new effects by making the nonwoven fabric itself extremely thin. That is, the bending rigidity of the nonwoven fabric is 1 / 50 or less of that of a general-purpose thin film product (thickness 30 μm), which makes it significantly softer. When the nonwoven fabric of the present invention is used as a substrate to be combined with other materials, it can flexibly conform to the surface shape of the other materials, thereby improving adhesion, and therefore this range can be cited as a more preferred range.

[0025] A thickness of 5.0 μm or less is a particularly preferred range in the present invention. Within this range, in addition to the significant space-saving effect, the bending rigidity of the nonwoven fabric is 1 / 200 or less of that of a general-purpose thin film product, allowing it to conform to and adhere to complex shapes, thereby improving performance when used as a substrate. Furthermore, when the nonwoven fabric of the present invention is used as a substrate and other materials are coated with it, even a highly viscous coating liquid can penetrate into the nonwoven fabric without pressure due to the ultra-thin film structure. In other words, this range can be cited as a particularly preferred range because it simplifies the process of combining with other materials.

[0026] On the other hand, as the thickness decreases, the nonwoven fabric becomes softer and more susceptible to wrinkling and sticking during the manufacturing process. However, if the thickness is 0.5 μm or more, there will be no problems with processability or practical use, and the object of the present invention can be achieved.

[0027] As described above, the nonwoven fabric of the present invention not only achieves space-saving as a diaphragm or substrate due to its ultrathin thickness, but also enhances adhesion to other materials due to the flexibility and permeability inherent in its ultrathin film, thereby contributing to the achievement of miniaturization, weight reduction, and high performance of various modules.

[0028] The nonwoven fabric of the present invention is characterized by its thin film properties, which are difficult to achieve with conventional techniques, and its ability to be stably produced without breaking during processing. Therefore, mechanical properties are an important requirement. As an index of these mechanical properties, the nonwoven fabric of the present invention has a second requirement of high toughness, which must be 0.50 or higher.

[0029] This toughness value is determined as follows.

[0030] The nonwoven fabric of the present invention is cut into a predetermined size (length 100 mm x width 10 mm), and the maximum point load is measured in N using a tensile tester (for example, Tensilon UTM-III-100 manufactured by Orientec Co., Ltd.) under predetermined conditions (initial length 50 mm, sample width 10 mm, tensile speed 100 mm / min). This measurement is performed five times for each sample cut out in the manufacturing direction (MD) and cross direction (CD) of the nonwoven fabric, and the arithmetic mean of these values ​​is rounded to two decimal places to obtain the tensile strength (N / cm) of the nonwoven fabric of the present invention.

[0031] The strain at the maximum point load is measured in %, and the arithmetic mean of these values ​​measured in MD and CD is rounded to one decimal place to obtain the tensile elongation (%) of the nonwoven fabric of the present invention.

[0032] A total of 10 sample pieces (length 100 mm x width 10 mm) cut out in each direction for the tensile test were weighed in advance to determine the unit area (m ２ The weight (g) per unit area was calculated, and the arithmetic mean was rounded off to one decimal place to determine the basis weight (g / m) of the nonwoven fabric of the present invention. ２ )

[0033] The specific tensile strength and toughness are calculated from the obtained tensile strength and elongation and basis weight using the following formula, and the result is rounded to two decimal places to determine the specific tensile strength and toughness of the nonwoven fabric. ２ / g cm) = tensile strength (N / cm) / basis weight (g / m ２ ) Toughness = specific tensile strength (N・m ２ / g cm) × tensile elongation １／２ (% １／２ In the present invention, toughness refers to the amount of tensile energy absorbed per unit amount of fiber constituting the nonwoven fabric. In the case of thin-film nonwoven fabrics, which have a small amount of constituent fibers and therefore tend to have defects that become the starting point of breakage, this toughness tends to be extremely low.

[0034] If the toughness is 0.50 or more, the thin film nonwoven fabric can absorb more than twice the tensile energy of a thin film nonwoven fabric formed using ultrafine fibers, and is less likely to break due to tension during processing or contact with other members, thereby providing good handleability.

[0035] From this perspective, as the toughness increases, the handleability of the nonwoven fabric improves, and if the toughness is 1.00 or more, the nonwoven fabric is less likely to break and can be processed stably even in a roll-to-roll process in which tension fluctuations and friction with other members are likely to occur, and this can be considered a preferable range.

[0036] Furthermore, a toughness of 1.20 or more is a more preferable range in the present invention. If the toughness is in this range, sufficient process tension can be applied in the roll-to-roll process, and wrinkles and end breakage during process passing can be reduced, so this range can be cited as a more preferable range.

[0037] A toughness of 1.50 or more is a particularly preferred range in the present invention. This range can be cited as a particularly preferred range because the toughness can sufficiently withstand large process tension and friction that are applied when the speed of a roll-to-roll process is increased.

[0038] As the toughness increases, it is possible to absorb a larger tensile energy, which is preferable, but due to the structure of the nonwoven fabric in which the fibers are bonded and fixed, the upper limit of the toughness is essentially 4.0 or less.

[0039] As described above, the nonwoven fabric of the present invention exhibits excellent mechanical properties despite being thin, and solves the problem of poor processability, such as tearing during processing, that has been an issue with thin nonwoven fabrics of the prior art.Furthermore, these excellent mechanical properties make it possible to form ultra-thin nonwoven fabrics with extremely low basis weights, which was difficult to achieve with prior art due to insufficient mechanical properties.

[0040] The reduction in thickness is greatly influenced by the basis weight, which is the amount of fiber that makes up the nonwoven fabric, and the porosity. From the viewpoint of fully utilizing the functions that utilize the porous structure while also expressing the effects of the thin film, a low basis weight is preferable.

[0041] Basis weight: 15.0 g / m ２ If the thickness is less than this, a thin nonwoven fabric having a thickness of 30.0 μm or less can be obtained while fully utilizing the porous structure of the nonwoven fabric, and this can be cited as a preferable range.

[0042] The basis weight is 5.0 g / m ２ If the thickness is less than this, an ultra-thin nonwoven fabric having a thickness of 10.0 μm or less can be formed by taking advantage of the porous structure, and this can be cited as a more preferable range.

[0043] Furthermore, the basis weight is 3.0 g / m ２ If the thickness is less than this, a highly porous structure can be obtained even in an ultra-thin film having a thickness of 5.0 μm or less, which is expected to have an effect of improving adhesion and permeability with other materials, and this can be cited as a particularly preferable range.

[0044] From the viewpoint of utilizing the porous structure of the nonwoven fabric to achieve high functionality, a high void ratio is preferable. The void ratio is calculated from the thickness and basis weight using the following formula, rounded to the nearest tenth. The true density is calculated by extrapolating the density of the material. Apparent density (g / cm ３ ) = Basis weight (g / m ２) / thickness (μm) porosity (%) = (1 - apparent density (g / cm ３ ) / true density (g / cm ３ )) x 100 If the porosity is 50% or more, more than half of the volume can be used for filtration / separation or impregnation with other materials, and by utilizing this large amount of inter-fiber voids, advanced functions that are difficult to achieve with other porous materials can be easily achieved, and this can be considered a preferable range.

[0045] Furthermore, the porosity is more preferably 55% or more. If the porosity is within this range, when particles, resin, etc. are impregnated into the nonwoven fabric, the high porosity allows a sufficient amount of particles or resin to be supported, and the performance of the impregnated components can be exhibited even when the nonwoven fabric is used as a base material.

[0046] Furthermore, when impregnation of particles, resin, etc. is envisaged, a porosity of 60% or more allows the particles or resin to easily reach every corner of the nonwoven fabric, and the performance of the impregnated components can be fully demonstrated even when the nonwoven fabric is processed by dipping without applying high pressure, so this range can be cited as being particularly preferable.

[0047] As the void ratio increases, a larger amount of particles and resin can be supported, which is preferable. However, due to the structure of nonwoven fabrics, which maintain their structure by bonding between fibers, the practical upper limit for the void ratio is 95% or less.

[0048] The nonwoven fabric of the present invention exhibits excellent mechanical properties despite its thin film, and when the main fibers that form the framework of the nonwoven fabric are firmly and uniformly bonded to each other, deformation and destruction of the nonwoven fabric structure due to external forces can be suppressed, and excellent dimensional stability can be exhibited during processing. That is, from the viewpoint of more firmly bonding the fibers that constitute the nonwoven fabric and preventing dimensional changes of the nonwoven fabric due to deformation and destruction at the bonded joints, it is preferable that each main fiber is bonded over a large area, and as an indicator of this, it is preferable that the bonded area per fiber cross-sectional area of ​​the fibers that constitute the nonwoven fabric is 3.00 or more.

[0049] The adhesive area per fiber cross-sectional area is determined as follows.

[0050] 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) or the like at a magnification that allows observation of the cross section of the fiber (subject fiber) having a fiber skeleton. The constituent fibers of the nonwoven fabric include a mixture of subject fibers and binder fibers that serve to bond these fibers. However, since the binder fibers usually soften and flow during the manufacturing process, causing deformation and disappearance of the fiber skeleton, the subject fibers and binder fibers can be distinguished from each other based on the cross-sectional shape. For the cross section of one subject fiber present in the photographed image (see Figure 3), the outer periphery of the cross section is specified using image analysis software to determine the fiber cross-sectional area in μm. ２ This measurement is carried out for 50 fibers, and the arithmetic average of these is rounded off to two decimal places to obtain the fiber cross-sectional area of ​​the present invention.

[0051] Next, using the same method as described above, the area where the subject fiber and the binder fiber (the fiber skeleton may be lost) are bonded almost perpendicularly to each other is identified (see FIG. 3 ), and an image is taken at a magnification that allows observation of that area. At this time, the cross section of the fiber may be etched with an alkaline solution or the like to clarify the contrast at the interface between the subject fiber and the binder fiber and efficiently perform the measurement described below. The length of the area where one subject fiber is bonded to one binder fiber in the direction perpendicular to the fiber axis of the subject fiber is measured in μm units using image analysis software, and this value is taken as the bond length. In calculating the bond area referred to in the present invention, it is assumed that a bonded area of ​​the same length as the bond length in the direction perpendicular to the fiber axis of the subject fiber is also formed in the fiber axis direction of the subject fiber, and the aforementioned bond length is squared to obtain the bond area in μm. ２ This measurement is carried out for 50 fibers, and the arithmetic average is rounded to two decimal places to obtain the bonded area of ​​the present invention. The bonded area per fiber cross-sectional area of ​​the present invention is calculated using the following formula from the obtained fiber cross-sectional area and bonded area, and the value obtained by rounding to two decimal places is the bonded area per fiber cross-sectional area of ​​the present invention. Bonded area per fiber cross-sectional area = Bonded area (μm ２ ) / fiber cross-sectional area (μm ２If the bonded area per fiber cross-sectional area is 3.00 or more, the subject fibers that form the skeleton of the nonwoven fabric are firmly bonded over a wide range by the binder fibers, and even if an external force greater than expected is applied due to fluctuations in process tension, the nonwoven fabric structure will not deform and excellent dimensional stability can be exhibited, and this range can be considered to be a preferable range.

[0052] Furthermore, it is more preferable that the adhesive area per fiber cross-sectional area is 5.00 or more. If the adhesive area is within this range, even if an external force is concentrated on a part of the nonwoven fabric due to contact with another member, the adhesive points are less likely to break and the external force can be propagated to the entire nonwoven fabric, thereby exhibiting better dimensional stability.

[0053] Furthermore, by having a bonded area per fiber cross-sectional area of ​​10.00 or more, even if a sudden external force is applied, the external force can be uniformly distributed over the entire nonwoven fabric without deformation or destruction of the bonded points, and this range can be cited as a particularly preferable range.

[0054] The larger the bonded area per fiber cross-sectional area, the stronger the bonded points will be, which is preferable, but the practical upper limit of the bonded area per fiber cross-sectional area is 500.00 or less.

[0055] As described above, from the viewpoint of uniformly distributing external forces throughout the nonwoven fabric, in addition to strong adhesion resulting from a large bonding area, the uniformity of the nonwoven fabric structure, such as the density distribution of the constituent fibers, also has an effect, and it is preferable that there are no visible macroscopic density distributions of the fibers, such as pinholes.

[0056] In particular, in a thin film nonwoven fabric such as that of the present invention, due to the small amount of constituent fibers, micrometer-scale straight pores are formed, which are regions in the thickness direction of the nonwoven fabric where not a single fiber is present, and even variations in the size of these straight pores have a significant effect on stress concentration in the nonwoven fabric. For this reason, it is preferable that the size of the straight pores in the nonwoven fabric is uniform, and specifically, it is preferable that the range of the straight pores in the nonwoven fabric is 120 μm or less.

[0057] The range of the straight holes of the nonwoven fabric is determined as follows.

[0058] The surface of the nonwoven fabric of the present invention is photographed using a scanning electron microscope (SEM) or the like at a magnification that allows observation of 100 or more direct holes. The term "direct hole" as used herein refers to an area surrounded by fibers and in which no fibers are present in the thickness direction when observed from the surface of the nonwoven fabric. The equivalent circle diameters of 100 direct holes present in the photographed image are measured in integer units of μm using image analysis software WINROOF2015 manufactured by Mitani Shoji Co., Ltd. The same procedure is performed for five images, and the range of the direct holes of the present invention is determined by subtracting the minimum value from the maximum value of the equivalent circle diameters obtained. If no direct holes exist due to a large amount of constituent fibers, for example, the range of the direct holes is not calculated.

[0059] If the range of the straight holes of this nonwoven fabric is 120 μm or less, stress does not concentrate around the large straight holes when an external force is applied to the nonwoven fabric, and even microscopic deformation of the nonwoven fabric structure is suppressed, so this can be considered a preferable range.

[0060] Furthermore, it is more preferable that the range of the straight holes of the nonwoven fabric is 80 μm or less, and within this range, even if an external force is concentrated on a part of the nonwoven fabric due to contact with another member, the external force can be borne uniformly by the fibers around the large and small straight holes, thereby exhibiting better dimensional stability. Furthermore, if the range of the straight holes of the nonwoven fabric is 50 μm or less, even if an external force is suddenly applied, the external force can be uniformly distributed throughout the nonwoven fabric without deformation of the nonwoven fabric structure, and this can be cited as a particularly preferable range.

[0061] The smaller the range of the straight pores of the nonwoven fabric, the more uniform the size of the straight pores, which is preferable; however, due to the structure of the nonwoven fabric, which is formed by randomly laying fibers, the practical lower limit of the range of the straight pores of the nonwoven fabric is 1 μm or more.

[0062] Furthermore, from the viewpoint of emphasizing good processability and high adhesion to other materials, it is preferable that the surface of the nonwoven fabric is smooth, and as an indicator of this, it is preferable that the arithmetic mean roughness (Ra) of the nonwoven fabric surface is 7.00 μm or less.

[0063] The arithmetic mean roughness (Ra) referred to here is determined as follows: the surface of the nonwoven fabric is observed with a laser microscope (Keyence VK-X200), and measurement is performed in accordance with JIS B 0601:2013 using analysis software (Keyence VK-H1XA). This is performed at any five locations per sample, and the arithmetic mean roughness (Ra) is determined by rounding off the arithmetic mean to two decimal places.

[0064] The arithmetic mean roughness (Ra) of the nonwoven fabric of the present invention is preferably 7.00 μm or less. Within this range, there are no large irregularities on the surface, and the surface is free from snags, thereby exhibiting good processability and, further, when compounded with other materials, allowing for close adhesion without gaps and high-quality integration.

[0065] From this perspective, it is more preferable that the arithmetic mean roughness (Ra) of the nonwoven fabric surface is 5.00 μm or less, which will result in higher adhesion when combined with other materials as a substrate, and will also provide excellent resistance to peeling over time.

[0066] As the arithmetic mean roughness (Ra) of the nonwoven fabric surface decreases, adhesion when the nonwoven fabric is combined with other materials increases, which is preferable. However, due to the structure of the nonwoven fabric, which is formed by randomly laying fibers, the practical lower limit of the arithmetic mean roughness (Ra) of the nonwoven fabric surface is 0.50 μm or more.

[0067] The characteristics of the nonwoven fabric of the present invention, such as its ultra-thin film thickness and excellent mechanical properties, can be achieved by the effective action of the shape of the constituent fibers, and it is preferable that the fibers (main fibers) constituting the nonwoven fabric of the present invention contain at least one type of flat fiber having a flatness of 3 or more, which is the value obtained by dividing the major axis length of the fiber cross section by the minor axis length.

[0068] The term "flat fiber" as used herein refers to a fiber having a flat cross section, and "flat" refers to a shape such as a rectangle or ellipse, in which the major axis length and the minor axis length are different. The degree of flatness is defined as the value obtained by dividing the major axis length by the minor axis length.

[0069] The flatness referred to in the present invention is determined as follows (see also FIG. 1).

[0070] That is, a nonwoven fabric is embedded in an embedding agent such as epoxy resin, and a fiber cross section is cut 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. For the cross sections of the single fibers present in the photographed image, the maximum length of the cross section is measured using image analysis software, and this value is expressed as the length of the single fiber's major axis in μm units, rounded to the nearest hundredths. Next, the length of the line segment perpendicular to the maximum length at the midpoint of the maximum length that intersects with the fiber cross section is measured, and this value is expressed as the minor axis length of the single fiber, rounded to the nearest hundredths. Using these major axis length and minor axis length, the flatness of the single fiber is calculated using the following formula: Flatness = Major axis length (μm) / Minor axis length (μm). The above measurement is performed on 100 fibers to calculate the flatness of each fiber, and the arithmetic average of these values, rounded to the nearest hundredths, is the flatness of the present invention.

[0071] The nonwoven fabric of the present invention preferably contains at least one type of flat fiber having a flatness of 3 or more. When the fibers are deposited as a nonwoven fabric, the cross-sectional direction of the fibers naturally aligns due to the cross-sectional shape anisotropy of the fibers, and the minor axis direction of the fiber cross section preferentially faces the thickness direction of the nonwoven fabric, promoting thinning of the nonwoven fabric. Furthermore, when flat fibers overlap with each other with the same cross-sectional direction, the overlapping fibers contact each other as a "plane" over a large area, thereby exerting a strong binding force that is difficult to achieve with point contact of ordinary round cross-section fibers.

[0072] From this perspective, increasing the flatness of the flat fibers constituting the nonwoven fabric will result in more pronounced effects of selective orientation and increased contact area due to the cross-sectional shape. Therefore, by including one or more types of flat fibers with a flatness of 8 or more, the minor axis direction of the flat cross section will be highly aligned in the thickness direction of the nonwoven fabric, which is expected to significantly reduce the thickness and increase the contact area, and this can be cited as a more preferable range.

[0073] Furthermore, when one or more types of flat fibers with a flatness of 15 or more are included, the fibers laid out in the same direction have a long length relative to the surface direction of the nonwoven fabric, which strengthens the binding force between adjacent fibers and increases the strength of the nonwoven fabric in both the manufacturing direction and the width direction. Furthermore, when wet papermaking is selected as the manufacturing method for the nonwoven fabric, the anisotropy of the cross-sectional shape significantly suppresses inter-fiber entanglement during stirring of the papermaking dope, dramatically improving the operational stability of the papermaking process and maintaining high uniformity and quality of the nonwoven fabric. Therefore, a flatness of 15 or more can be cited as a particularly preferred range in the present invention.

[0074] Furthermore, as the flatness of the cross section increases, bending or cracking tends to occur more easily in the longitudinal direction of the cross section when an external force is applied during the processing step. However, if the flatness is 500 or less, there is no problem in practical use, and the object of the present invention can be achieved.

[0075] From the viewpoint of thinning the nonwoven fabric and improving adhesion to other materials, it is preferable that the short axis length in the cross section of the above-mentioned flat fibers is short, and it is preferable that the short axis length of the flat fibers constituting the nonwoven fabric of the present invention is 2.00 μm or less.

[0076] The shorter the short axis length of the flat fibers used in the nonwoven fabric of the present invention, the thinner the thickness occupied by each fiber when the flat cross section is selectively oriented, making it easier to form a thinner nonwoven fabric.In addition, the fibers themselves become more flexible in the short axis direction, and the synergistic effect resulting from the thinning of both the constituent fibers and the nonwoven fabric can further enhance adhesion to other materials.

[0077] If the short axis length of the flat fibers used in the nonwoven fabric of the present invention is 2.00 μm or less, the thickness occupied by each fiber is small, making it easy to form an extremely thin nonwoven fabric.Furthermore, the softening of the fibers themselves allows each fiber to flexibly deform, thereby improving adhesion when combined with other materials.

[0078] Furthermore, if the short axis length of the flat fibers used in the nonwoven fabric of the present invention is 1.00 μm or less, it is possible to easily form even an extremely thin nonwoven fabric with a thickness of less than 10 μm, which is difficult to achieve with ordinary fibers, and the softening of the fibers is remarkable, allowing them to follow the shape of other materials to be combined and adhere closely even locally to the fiber cross section, so this can be cited as a more preferred range in the present invention.

[0079] Furthermore, if the short axis length of the flat fibers used in the nonwoven fabric of the present invention is 0.70 μm or less, it is possible to easily form an ultrathin nonwoven fabric having a thickness of less than 5 μm.Furthermore, when the short axis length of the flat fibers is on the submicron scale, intermolecular interactions such as van der Waals forces come into play, allowing for high adhesion even to fine and complex shapes, and therefore this range can be cited as a particularly preferred range in the present invention.

[0080] On the other hand, if the short axis length of the flat fibers is extremely short, the fibers are more likely to break during the manufacturing process. However, if the short axis length of the flat fibers is 0.05 μm or more, there will be no problems in the manufacturing process or in the actual use of the nonwoven fabric, and this can be considered as the substantial lower limit in the present invention.

[0081] It is preferable to use flat fibers having the above-mentioned flatness and minor axis length as both the main fiber and the binder fiber for forming the nonwoven fabric of the present invention. By using flat fibers as both the main fiber and the binder fiber, both fibers are closely overlapped with their cross-sectional directions aligned, and the binder fiber softens and flows, forming a "plane" between the fibers and bonding them over a wide area, thereby achieving extremely strong bonding and fixation.

[0082] Furthermore, from the viewpoint of achieving high functionality by making the most of the porous structure of the nonwoven fabric of the present invention, it is preferable that the shape of the fiber cross section has a distribution within a certain range, and it is preferable that the flat fibers used in the nonwoven fabric of the present invention have variation in the minor axis length.

[0083] The variation in minor axis length in the present invention is determined 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, and rounding off the resulting coefficient of variation to an integer in percentage units.

[0084] When the minor axis lengths are distributed with a moderate degree, the cross-sectional shapes of the fibers do not match, and when the fibers are laid out to form a nonwoven fabric, the flat fibers move differently, resulting in more uniform dispersion. From the above viewpoints, it is preferable that the minor axis length variation of the flat fibers used in the nonwoven fabric of the present invention is 10% or more. By setting the variation in the minor axis length within this range, the flat fibers move differently during the sheet-forming process, making them less likely to be unevenly distributed, resulting in more uniform dispersion.

[0085] Furthermore, if the variation in minor axis length is 20% or more, the bending behavior of the fibers during the sheet-forming process will differ among the fibers, making it difficult for the flat fibers, which are the main fibers, to overlap with each other, resulting in uniform adhesion between the main fibers and the binder fibers, and achieving effective adhesion.

[0086] From this perspective, the greater the variation in minor axis length, the more uniformly the flat fibers are dispersed, but if an external force is applied during the manufacturing process, etc., the flat fibers tend to be more likely to break because the minor axis length is too short. Therefore, the variation in minor axis length is preferably 50% or less, which is the practical upper limit in the present invention.

[0087] In the flat fibers used in the nonwoven fabric of the present invention, in addition to the uniform dispersion effect due to the difference in fiber cross-section of each fiber, the steric hindrance effect due to the unevenness of the outer periphery allows the flat fibers to be well opened between each other before being subjected to the sheet-forming process, making it easier to achieve uniform dispersion of the fibers within the nonwoven fabric, so it is preferable that the unevenness in the fiber cross-section is 20% or more.

[0088] The irregularity referred to in the present invention is measured by the following method. First, using a photographed image of the fiber cross section, the lengths of the line segments perpendicular to the line segment of the maximum length at 10 equal points along the maximum length of the cross section are measured, and the arithmetic mean and standard deviation of these 10 lengths are calculated. The standard deviation is divided by the arithmetic mean and rounded to the nearest percent to obtain the irregularity of a single fiber. Similar measurements are performed on 10 fiber cross sections, and the calculated arithmetic mean of the irregularities of the 10 fibers is used as the irregularity of the present invention. When this irregularity is 20% or more, it becomes easier to open the flat fibers into individual fibers using the minute voids between the fibers as starting points, making it easier to achieve uniform dispersion in the sheet-forming process.

[0089] Furthermore, a high degree of unevenness allows the flat fibers to be opened well, but the range in which the 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.

[0090] The mechanical properties of the nonwoven fabric are exhibited by the bridge structure formed by the adhesive and frictional forces between adjacent fibers, and this bridge structure is a structure in which the fibers present in the nonwoven fabric interact with each other to transmit force, so that when the same amount of fibers are present in the nonwoven fabric, the thinner the fiber diameter and the longer the fiber length, the more bridge structures are formed and the more facilitated the transmission of force. In other words, the higher the ratio of fiber length to fiber diameter, the more facilitated 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 minor axis length, is preferable for the flat fibers used in the nonwoven fabric of the present invention.

[0091] The aspect ratio referred to in the present invention is determined as follows.

[0092] An image of the flat fibers is taken using a microscope or the like at a magnification that allows observation of at least 10 fibers whose total length can be measured. The fiber lengths of 10 fibers randomly selected from the image of the fibers are measured. The fiber length referred to here refers to the longitudinal length of a single fiber from a two-dimensionally captured image, measured in millimeters using image analysis software. The arithmetic average of these fiber lengths, rounded to one decimal place, is defined as the fiber length of the present invention. The aspect ratio of the present invention is determined by using this fiber length and the minor axis length calculated above, rounding to the nearest whole number using the following formula: Aspect ratio = Fiber length (μm) / Minor axis length (μm). The aspect ratio of the flat fibers used in the nonwoven fabric of the present invention is preferably 1000 or more. Within this range, a sufficient bridge structure is formed between the fibers, improving the strength of the nonwoven fabric.

[0093] Furthermore, it is more preferable that the aspect ratio of the flat fibers used in the nonwoven fabric of the present invention is 2000 or more.Within this range, the nonwoven fabric not only exhibits excellent mechanical properties, but also has excellent processability, such as minimizing fiber shedding during the manufacturing process.

[0094] As described above, the nonwoven fabric of the present invention is characterized by having excellent mechanical properties despite being a thin film, and applications (textile products) that take advantage of this characteristic include material applications such as filters, hazardous substance removal products, battery separators, and sound absorbing materials; daily applications such as cosmetics, cosmetic masks, and wiping cloths; medical applications such as scaffolding materials for cell culture, artificial blood vessels, and blood filters; and clothing applications such as jackets, skirts, pants, and underwear.

[0095] In particular, when the nonwoven fabric of the present invention is used as a membrane or substrate to form a sheet filled with a resin or particles, not only can the space required for the substrate be reduced, thereby enabling a smaller, lighter, and more powerful module, but the excellent mechanical properties of the substrate also enable the stable production of high-quality modules. Examples of this include pressure-sensitive adhesive tapes in which the nonwoven fabric of the present invention is combined with a pressure-sensitive adhesive, polymer electrolyte membranes composited with a polymer electrolyte, and functional membranes composited with functional particles to provide self-supporting properties and flexibility.

[0096] Furthermore, by forming a sheet using the nonwoven fabric of the present invention as a substrate and filling it with a solid electrolyte, even if the substrate is an ultrathin film that can suppress a decrease in ionic conductivity, the solid electrolyte can be imparted with self-supporting properties and flexibility that can withstand manufacturing, thereby enabling all-solid-state batteries to be made smaller, lighter, and have higher performance.

[0097] An example of a method for producing the nonwoven fabric of the present invention will be described in detail below.

[0098] The nonwoven fabric of the present invention is characterized by exhibiting excellent mechanical properties despite its thin film, and can be in the form of either a long fiber type or a short fiber type. The manufacturing method for these can be any of a spunbond method, a melt-blowing method, a dry method, a wet method, etc., and can be a combination of these methods as needed.

[0099] Among various methods for producing nonwoven fabrics, in order to form the thin film structure that is a characteristic of the nonwoven fabric of the present invention, a wet method, i.e., a wet papermaking method, which is good at producing thin film nonwoven fabrics with excellent texture, is preferably used, and this allows the thin film structure that is a characteristic of the present invention to be stably formed.

[0100] The wet papermaking method referred to here is a method of producing a nonwoven fabric by making a papermaking solution in which short-cut staple fibers are dispersed in an aqueous medium. That is, the main fibers that form the framework of the nonwoven fabric and binder fibers that bond the staple fibers together by heat treatment are put into water, and the fibers are loosened into individual fibers by defibration or beating as necessary, and paper is made using the papermaking solution in which various staple fibers are uniformly dispersed. In the process of preparing the papermaking solution, it is possible to adjust the dispersibility by changing the amount of staple fibers added, the amount of aqueous medium, the stirring time, etc., and it is preferable to adjust the dispersion state of various staple fibers.

[0101] In order to suppress aggregation of the short fibers introduced into the water and to produce a uniform thin film nonwoven fabric, a dispersant may be contained.

[0102] Types of dispersants include natural polymers, synthetic polymers, organic compounds, and inorganic compounds. For example, additives that suppress 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 fibers constituting the nonwoven fabric. Within this range, fiber dispersibility can be improved without impairing wetlaid papermaking processability, allowing the nonwoven fabric of the present invention to be produced.

[0103] The fibers constituting the nonwoven fabric of the present invention may be fibers produced by a conventionally known spinning method, but from the viewpoint of productivity, it is preferable to employ fibers produced by a technique utilizing melt spinning.

[0104] When producing fibers by melt spinning, a spinneret or composite spinneret having holes of a desired shape can be used to extrude a polymer with a desired cross-sectional shape to obtain the desired fiber.

[0105] The polymer used in this melt spinning can be a melt-moldable polymer such as polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polytrimethylene terephthalate, polypropylene, polyolefin, polycarbonate, polyacrylate, polyamide, polylactic acid, thermoplastic polyurethane, polyphenylene sulfide, liquid crystal polyester, or a copolymer thereof.

[0106] When used as an industrial material, it is preferable that the nonwoven fabric has a certain degree of heat resistance, and polycondensation polymers such as polyester and polyamide have high melting points and are suitable for use in the present invention. When used as a diaphragm or substrate, it is preferable to use a polyester with a high melting point, as the nonwoven fabric is heated to be composited with other materials. Furthermore, when used as a diaphragm or substrate in applications requiring chemical resistance, polyphenylene sulfide is preferably used because it has excellent heat resistance in practical use in addition to chemical resistance.

[0107] The polymer used in the present invention may 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.

[0108] When a wet papermaking method is adopted in the method for producing a nonwoven fabric of the present invention, the various short fibers used in the papermaking solution preferably have a fiber length of 0.3 to 30.0 mm. Within this range, dispersibility in the papermaking solution is maintained, and a nonwoven fabric with good texture can be obtained.

[0109] The length of the short fibers can be adjusted appropriately depending on the application. Generally, as the fiber length increases, the contact length between the short fibers increases, and the mechanical properties and dimensional stability of the nonwoven fabric tend to improve. However, the short fibers tend to become entangled and aggregate during stirring in the fiber papermaking solution. Therefore, it is advisable to adjust the length according to the specifications of the short fibers to be mixed in the papermaking solution and the expected design.

[0110] With regard to the subject fibers and binder fibers used in the nonwoven fabric of the present invention, the fiber length is more preferably 1.0 to 10.0 mm, since the aim is to produce a high-quality nonwoven fabric by making the papermaking dope homogeneous and free of aggregation, and also to achieve excellent mechanical properties as a nonwoven fabric and dimensional stability during processing. From this perspective, the fiber length is particularly preferably 2.0 to 5.0 mm. Within this range, the short fibers do not aggregate even in general papermaking equipment where shear forces are generated during processing, and the short fibers are less likely to become caught in the papermaking mesh. This maximizes the effect of improving strength by increasing the contact length between the short fibers, and makes processing more stable.

[0111] In particular, in the nonwoven fabric of the present invention, by using flat fibers, it is possible to achieve both extremely small fiber diameter and long fiber length, which were not possible with conventional materials, and as a result, it is possible to achieve very good processability even in wet papermaking with an extremely low basis weight, which was previously difficult to achieve.

[0112] The main fiber and binder fiber may each be one type or two or more types, and the blending ratio of the main fiber and binder fiber may be adjusted within a range of 5 to 95% by weight depending on the application and required properties of the nonwoven fabric. Note that, although softening and flowing the binder fiber firmly bonds the fibers and improves the mechanical properties, the softened and flowing binder fiber closes the inter-fiber voids, so from the viewpoint of achieving both mechanical properties and a porous structure, the blending ratio of the binder fiber is preferably 10 to 60% by weight.

[0113] The subject fibers preferably have a higher softening temperature than the binder fibers and are preferably fibers with a high degree of crystallinity where crystallization has progressed sufficiently. When binder fibers made of a polymer of the same composition are used in an amorphous state, the subject fibers do not soften and function as a strong skeleton of the sheet-like material, while only the binder fibers soften and flow to firmly bond the subject fibers together.

[0114] The diameter and cross-sectional shape of the main fiber and binder fiber can be appropriately selected depending on the purpose, but when both the main fiber and the binder fiber are flat fibers, a special nonwoven structure can be smoothly formed that can achieve both thin film and excellent mechanical properties. In particular, by using flat fibers with a flatness of 3 or more and a minor axis length of 2.00 μm or less for both, the flat fibers overlap with their cross-sectional direction aligned, making it possible to produce a nonwoven fabric that combines thin film and excellent mechanical properties.

[0115] The flat fibers having a flatness of 3 or more and a minor axis length of 2.00 μm or less referred to here may be difficult to stably produce by conventional melt spinning using a spinneret simply equipped with irregularly shaped holes, because the molten polymer discharged from the spinneret tends to become circular due to surface tension.

[0116] For this reason, in the present invention, a method is preferably employed in which a multilayer laminated fiber (Figure 2) in which a slightly soluble polymer and a readily soluble polymer are laminated in multiple layers on the fiber cross section is first spun, and then the readily soluble polymer is dissolved and removed from this multilayer laminated fiber. The multilayer laminated fiber referred to here means a fiber having a laminate structure in which two or more types of polymers are laminated alternately or in random order on the cross section, and the laminate structure may be not only one in which the lamination direction on the fiber cross section is the same, but also one in which the lamination direction is radial, concentric, or irregularly changing, or a combination of these.

[0117] In order to form a laminated structure for generating stable flat fibers in this multilayer laminated fiber, it is preferable to first form a cross section in which the hardly soluble polymer and the easily soluble polymer are stably laminated in multiple layers, and it is preferable that the polymers constituting the multilayer laminated fiber have high affinity.

[0118] From the above perspective, it is preferable that the combination of polymers constituting the multi-layer laminate fiber of the present invention be polyethylene terephthalate copolymerized with 5 mol % to 15 mol % of 5-sodium sulfoisophthalic acid as the readily soluble component, polyethylene terephthalate copolymerized with the above-mentioned 5-sodium sulfoisophthalic acid and 5 wt % to 15 wt % of polyethylene glycol having a weight average molecular weight of 500 to 3000 as the sparingly soluble component, and polyethylene terephthalate as the slightly soluble component.

[0119] In particular, the use of a polyester copolymerized with polyethylene glycol and sodium sulfoisophthalate, either alone or in combination, as the easily soluble component allows for stable production of flat fibers from the viewpoints of spinnability and ease of dissolution in low-concentration aqueous solvents, and from this viewpoint as well, the above-mentioned polymer combination can be cited as a more preferred embodiment for achieving the present invention.

[0120] In the present invention, the multilayer laminated fibers from which the soluble components have been dissolved can be used directly for wet papermaking, or the flat fibers can be separated by filtration or the like after dissolving the soluble components from the multilayer laminated fibers, and then washed with water, dried, etc., and dispersed again in an aqueous medium for use. Furthermore, taking into consideration the advanced processing to be used and the handling therein, it is also possible to adjust the pH of the medium by adding an acid or alkali, or to dilute the medium with water before use.

[0121] The flat fibers generated from the above-mentioned multilayer laminated fibers exhibit excellent dispersibility under a wide range of conditions, from low shear to high shear, due to the fiber cross section having a high degree of shape anisotropy, with a short minor axis length and a long major axis length, and are less likely to become entangled between the flat fibers, resulting in a papermaking solution with excellent uniformity.

[0122] 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. During the papermaking process, the papermaking speed, fiber content, and aqueous medium content are adjusted according to the desired basis weight, in addition to the fiber dispersibility in the papermaking solution, to control the accumulation of fibers during drainage. The wet paper thus formed undergoes a drying and heat treatment process to remove moisture and bond the binder fibers, resulting in the production of the nonwoven fabric of the present invention. For this drying method, a method using hot air ventilation (air-through) or a method in which the paper is brought into contact with a heated rotating roll (e.g., a Yankee dryer or a heated calendar roll) is preferably used, as it allows for simultaneous drying of the wet paper and thermal bonding of the binder fibers. The resulting nonwoven fabric undergoes a thermocompression process to more firmly bond the binder fibers, improving the mechanical properties of the nonwoven fabric. As the thermocompression bonding method, a method using a heat press or a heat calender roll (such as a super calender or soft calender) is preferably adopted from the viewpoint of enabling high-precision processing.

[0123] As a method for impregnating the obtained nonwoven fabric with a resin, particles, or a solid electrolyte to integrate them, various coating methods such as a dip coater, a die coater, or a spray coater can be applied depending on the viscosity of the coating liquid of these materials, the coating amount, and the coating speed.

[0124] The nonwoven fabric of the present invention will be specifically described below with reference to examples.

[0125] The examples and comparative examples were evaluated as follows.

[0126] A. Thickness The thickness of the nonwoven fabric was measured in mm using a dial thickness gauge DGN-250B (measuring point shape 25 mmφ, graduation 0.0001 mm, measuring force 1.2 N or less) manufactured by PEACOCK Corp. Measurements were taken at five randomly selected locations per sample, and the arithmetic average of these measurements was converted to μm units and rounded off to one decimal place to obtain the thickness of the nonwoven fabric.

[0127] B. Basis Weight Five sample pieces of 100 mm in length and 10 mm in width were cut out from each of the manufacturing direction (MD) and cross direction (CD) of the nonwoven fabric, and weighed using an electronic balance GR202 (scale interval 0.01 mg) manufactured by A&D Co., Ltd. to determine the weight per unit area (g / m ２ The arithmetic mean was rounded to one decimal place to give the basis weight (g / m ２ )did.

[0128] C. Porosity Using the thickness and basis weight of the nonwoven fabric calculated above, the value calculated by the following formula, rounded to one decimal place, was taken as the porosity. Apparent density (g / cm ３ ) = Basis weight (g / m ２ ) / thickness (μm) porosity (%) = (1 - apparent density (g / cm ３ ) / true density (g / cm ３ )) × 100. The true densities of polyethylene terephthalate and polyphenylene sulfide given in the examples are 1.38 (g / cm ３ ) was used.

[0129] D. Toughness Using the sample pieces (length 100 mm x width 10 mm) cut out for the above basis weight calculation, a tensile test was carried out using a Tensilon UTM-III-100 tensile tester manufactured by Orientec Co., Ltd. under the conditions of an initial length of 50 mm, a sample width of 10 mm, and a tensile speed of 100 mm / min, and the value of the maximum point load was measured in N. This measurement was carried out on five sample pieces cut out in each of the MD and CD of the nonwoven fabric, and the value obtained by rounding off the two decimal places was taken as the tensile strength (N / cm) of the nonwoven fabric. In addition, the strain amount at the maximum point load was measured in %, and the value obtained by rounding off the two decimal places of the arithmetic average was taken as the tensile elongation (%) of the nonwoven fabric. Using the obtained tensile strength and basis weight, the value calculated by rounding off the two decimal places using the following formula was taken as the toughness of the nonwoven fabric. Specific tensile strength (N m ２ / g cm) = tensile strength (N / cm) / basis weight (g / m ２ ) Toughness = specific tensile strength (N・m ２ / g cm) × tensile elongation １／２ (% １／２) E. Adhesion area per fiber cross-sectional area A cross section of a nonwoven fabric was cut with a razor or the like and photographed at a magnification that allowed observation of the cross section of the fiber (main fiber) having a fiber skeleton using a scanning electron microscope (SEM) S-5500 manufactured by Hitachi High-Technologies Corporation. The cross section of one main fiber present in the photographed image was analyzed by specifying the outer periphery of the cross section using image analysis software WINROOF2015 manufactured by Mitani Shoji Co., Ltd. to determine the fiber cross-sectional area in μm ２ The fiber cross-sectional area was measured in μm units. This measurement was performed on 50 fibers, and the arithmetic average was rounded to two decimal places to determine the fiber cross-sectional area. Using the same method, the area where the main fiber and the binder fiber (in some cases the fiber skeleton was lost) were bonded almost perpendicularly was identified, and an image was taken at a magnification that allowed observation of the area. The length of the area where one main fiber was bonded to one binder fiber in the direction perpendicular to the fiber axis of the main fiber was measured in μm units using the same image analysis software, and this value was taken as the bond length. It was assumed that a bonded area of ​​the same length as the bond length in the direction perpendicular to the fiber axis of the main fiber was also formed in the fiber axis direction of the main fiber, and the square of the bond length was taken as the bond area in μm ２ The measurement was carried out for 50 fibers, and the value obtained by rounding off the two decimal places of the arithmetic average was taken as the adhesive area. Using the obtained fiber cross-sectional area and adhesive area, the value obtained by rounding off the two decimal places in the following formula was taken as the adhesive area per fiber cross-sectional area. The constituent fibers of the nonwoven fabric are a mixture of main fibers and binder fibers, and the main fibers were identified from the degree of deformation of the fiber skeleton. Adhesive area per fiber cross-sectional area = Adhesive area (μm ２ ) / fiber cross-sectional area (μm ２ ) F. Range of direct holes The surface of the nonwoven fabric was photographed using a scanning electron microscope (SEM) or the like at a magnification such that 100 or more direct holes could be observed. For the 100 direct holes present in the photographed image, the circular equivalent diameter was measured in integer units of μm using image analysis software WINROOF2015 manufactured by Mitani Shoji Co., Ltd. The same operation was performed on five images, and the range of direct holes was determined by subtracting the minimum value from the maximum value of the obtained circular equivalent diameter.

[0130] G. Arithmetic mean roughness (Ra) The surface of the nonwoven fabric was observed using a laser microscope VK-X200 manufactured by Keyence Corporation, and the arithmetic mean roughness was measured using analysis software VK-H1XA manufactured by the same company in accordance with JIS B 0601: 2013. The arithmetic mean roughness (Ra) was determined by rounding the arithmetic mean obtained at any five locations to two decimal places.

[0131] H. Flatness, Long Axis Length, and Short Axis Length of Flat Fibers A nonwoven fabric was embedded in an embedding agent such as epoxy resin, and a fiber cross section was cut using a microtome equipped with a diamond knife. This cross section was then photographed using a scanning electron microscope (SEM) or the like at a magnification that allowed for cross section identification. For the cross sections of single fibers present in the photographed images, the maximum length of the cross section of the single fiber was measured using image analysis software WINROOF 2015 manufactured by Mitani Shoji Co., Ltd., and this value was used as the long axis length of the single fiber, rounded to the nearest hundredths of a micrometer. The length of the line segment perpendicular to the line segment of the maximum length at the midpoint of the longest length intersecting the fiber cross section was measured, and this value was used as the short axis length of the single fiber, rounded to the nearest hundredths of a micrometer. The long axis length and short axis length were used to calculate the flatness of the single fiber using the following formula: Flatness = major axis length (μm) / minor axis length (μm) The above measurement was performed on 100 fibers to calculate the major axis length, minor axis length, and flatness of each fiber. The arithmetic mean of the major axis length and minor axis length of each fiber was rounded to two decimal places to obtain the major axis length and minor axis length, and the arithmetic mean of the flatness of each fiber was rounded to one decimal place to obtain the flatness.

[0132] I. Variation in Minor Axis Length and Irregularity The standard deviation was calculated from 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 the nearest whole number in percentages to calculate the variation in minor axis length.

[0133] Furthermore, using the image of the fiber cross section taken as above, the lengths of the straight lines perpendicular to the line segment of the maximum length at the points where the maximum length of the cross section was divided into 10 equal parts and intersecting with the fiber cross section were measured, and the arithmetic mean and standard deviation of these lengths at 10 points were calculated, and the standard deviation was divided by the average value and rounded to the nearest percent to calculate the irregularity of the single fiber. Similar measurements were made for 10 fiber cross sections, and the arithmetic mean of the irregularities of the calculated 10 single fibers was calculated as the irregularity.

[0134] J. Fiber Length and Aspect Ratio The surface of the nonwoven fabric was observed using a Keyence Corporation VHX-2000 microscope, and images were taken at a magnification sufficient to observe at least 10 fibers whose full lengths could be measured. The lengths of 10 fibers randomly selected from the photographed image were measured in mm in the fiber axis direction to the first decimal place. The arithmetic mean of the measured values ​​of the 10 images photographed and measured in the same manner was rounded to the first decimal place to obtain the fiber length. Using this fiber length and the minor axis length calculated above, the aspect ratio was calculated using the following formula, rounding off any decimal places: Aspect ratio = fiber length (μm) / minor axis length (μm) K. Uniformity The uniformity of a nonwoven fabric cut into a 250 mm x 250 mm piece was evaluated by visual observation using the following three-point scale. A: No fiber clumps or shading (not visible) and good uniformity. B: No fiber clumps are visible, but shading is visible. C: Shading and fiber clumps are visible.

[0135] L. Adhesion A nonwoven fabric sample piece cut to a length of 10 mm and a width of 10 mm was attached to the surface of the skin, and a drop of water was dropped on it with a dropper and visually observed. The adhesion to the skin was evaluated using the following three levels as an index of adhesion: A: The sample piece was in close contact with the wrinkles of the skin and could not be seen; B: The sample piece was floating in the wrinkles of the skin and could be seen; C: The sample piece was floating all over and could be clearly seen.

[0136] [Example 1] Using polyethylene terephthalate (PET) as component A and polyethylene terephthalate copolymerized with 8.0 mol% of 5-sodium sulfoisophthalic acid and 9 wt% of polyethylene glycol (SSIA-PEG copolymerized PET) as component B, the two polymers were extruded to form a composite structure in which they were alternately laminated in multiple layers in one direction as shown in Figure 2, and an undrawn yarn having a single fiber fineness of 2.8 dtex was collected. In addition, this undrawn yarn was drawn, and a drawn yarn having a single fiber fineness of 1.2 dtex was collected.

[0137] The unstretched and stretched yarns of the obtained multilayer laminated fiber were subjected to short-cut processing so that the fiber length was 3.0 mm, and the unstretched yarn was immersed in a 1 wt % aqueous sodium hydroxide solution (bath ratio 1 / 100) heated to 70°C for 60 minutes, and the stretched yarn 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 component B in each, thereby obtaining flat fibers with a flat cross-section as shown in Figure 1.

[0138] The crystallized flat fibers obtained from the drawn yarn were used as the main fibers, and the amorphous flat fibers obtained from the undrawn yarn were used as the binder fibers. The raw materials were added to water so that the mixing ratios of the main fibers and the binder fibers were 70% by weight and 30% by weight, respectively, to prepare a papermaking solution with a fiber concentration of 0.25% by weight.

[0139] This papermaking solution was used to make a sheet of paper with a basis weight of 3.0 g / m using a square sheet machine (250 mm square) manufactured by Kumagai Riki Kogyo Co., Ltd. ２ The paper was made so that the paper was as follows: the paper was dried and heat-treated in a rotary dryer manufactured by the same company set at 120°C, and then subjected to heat pressing for 1 minute in a Gonno heating press set at 200°C x 10 MPa to obtain a nonwoven fabric.

[0140] The obtained nonwoven fabric was an ultrathin film with a thickness of 5.3 μm, yet had a toughness of 2.09, making it easy to handle. Its porosity was 58.3%, indicating a sufficiently porous structure. The nonwoven fabric had no fiber clumps or shading, and was highly uniform. In the cross section of the nonwoven fabric, the cross-sectional direction of the flat fibers was highly aligned, resulting in "plane" contact between the fibers. The adhesive area per fiber cross-sectional area was 15.23 μm, indicating strong adhesion over a large area. The range of straight pores on the surface of the nonwoven fabric was 46 μm, and straight pores of uniform size were formed. Furthermore, the Ra was 4.43 μm, indicating an extremely smooth surface. When adhesion was evaluated, the nonwoven fabric adhered to the wrinkles on the skin, making them difficult to see, demonstrating excellent adhesion. The results are shown in Table 1.

[0141] [Examples 2 and 3] Nonwoven fabric basis weight: 2.0 g / m ２ (Example 2), 1.0 g / m ２ The procedure was carried out in the same manner as in Example 1 except for the changes made to (Example 3).

[0142] The obtained nonwoven fabrics had the same cross-sectional orientation of the flat fibers as in Example 1, with the fibers in contact with each other at a "plane" and firmly bonded over a wide area. The evaluation results of these nonwoven fabrics are shown in Table 1. As the basis weight decreased, the thickness decreased, achieving a dramatic ultra-thin film that could not be achieved with conventional technology. Furthermore, although the toughness tended to decrease as the basis weight decreased, the handleability of all the nonwoven fabrics was good.

[0143] [Examples 4 and 5] Nonwoven fabric basis weight: 10.0 g / m ２ (Example 4), 15.0 g / m ２ The procedure was carried out in the same manner as in Example 1 except for the change to (Example 5).

[0144] The obtained nonwoven fabrics had a highly aligned cross-sectional direction of the flat fibers, and the fibers were firmly bonded over a wide area, similar to Example 1. The evaluation results of these nonwoven fabrics are shown in Table 1, but as the basis weight increased, the thickness increased and the adhesion to other materials tended to deteriorate. In addition, the toughness was as high as in Example 1, and the handleability was excellent. The results are shown in Table 1.

[0145] [Examples 6 and 7] The same procedures as in Example 1 were carried out except that the mixing ratio of the main fiber to the binder fiber was changed to 50:50% by weight (Example 6) and 90:10% by weight (Example 7).

[0146] The obtained nonwoven fabrics had the same flat fiber cross-sectional orientation as in Example 1, and the fibers were strongly bonded over a wide area. The evaluation results of these nonwoven fabrics are shown in Table 1. As the binder fiber content increased, the thickness of the nonwoven fabric decreased and the porosity decreased, while the toughness tended to improve. Within the range of the examples, both a thin film and good mechanical properties were achieved, even when the binder fiber content was increased or decreased. The results are shown in Table 1.

[0147]

[0148] Example 8 The same procedure as in Example 1 was carried out, except that the pressure during hot pressing was changed to 1 MPa.

[0149] The obtained nonwoven fabric had the same flat fiber cross-sectional orientation as in Example 1, and the fibers were strongly bonded over a wide area, but the spacing between the fibers tended to be somewhat coarse. The evaluation results of these nonwoven fabrics are shown in Table 1. As a result of the decrease in the hot press pressure, the thickness of the nonwoven fabric increased, the porosity increased, and the toughness decreased due to the weakened adhesion between the fibers, but the handleability was sufficiently excellent. The results are shown in Table 2.

[0150] Comparative Example 1 The nonwoven fabric was produced in the same manner as in Example 1, except that a circular cross-section PET oriented yarn (fiber diameter 3.10 μm) was used as the main fiber and a circular cross-section PET unoriented yarn (fiber diameter 4.30 μm) was used as the binder fiber.

[0151] The evaluation results are shown in Table 2. The use of round cross-section fibers for both the main fiber and the binder fiber increased the thickness and tended to increase the void ratio. Furthermore, the toughness was extremely low at 0.18, and the nonwoven fabric was easily deformed or torn by even the slightest tension during handling, resulting in poor handleability. The nonwoven fabric surface also showed significant unevenness in shading, resulting in poor uniformity. Observation of the cross-section of the nonwoven fabric revealed that the fibers were in contact over small points, significantly reducing the adhesive area per fiber cross-sectional area. Furthermore, the Ra was large, resulting in poor adhesion.

[0152] [Comparative Example 2] As the fibers constituting the nonwoven fabric, a round cross-section PET sheath-core yarn (fiber diameter 10.50 μm) was used as the binder fiber, and the basis weight was 60 g / m ２ The same procedure as in Comparative Example 1 was carried out except that the paper was made so that the thickness was 100 mm and then heat-pressed at 200° C. and 10 MPa for 1 minute.

[0153] The evaluation results are shown in Table 2. Although the significant increase in the amount of constituent fibers made defects less likely to become apparent and the toughness was excellent at 1.27, the thickness was too large at 87.0 μm, and it was not possible to achieve both mechanical properties and a thin film. Because the nonwoven fabric was so thick, the adhesion was also extremely poor.

[0154] [Examples 9 and 10] The same procedures as in Example 1 were carried out except that the flatness and minor axis length of the flat fibers constituting the nonwoven fabric were variously changed (Examples 9 and 10). The flatness and minor axis length were changed by changing the number of layers of the multi-layer laminated fibers that were the raw material for the flat fibers.

[0155] The obtained nonwoven fabrics had the same cross-sectional direction of the flat fibers and were bonded over a wide area, but the uniformity of the cross-sectional direction was somewhat disrupted. The evaluation results of these nonwoven fabrics are shown in Table 2. As the flatness decreased and the minor axis length increased, the thickness of the nonwoven fabric tended to increase and the porosity tended to increase. On the other hand, as the flatness decreased and the minor axis length increased, the toughness decreased and the handling deteriorated, but this did not pose a serious problem, such as causing breakage. In the cross-section of the nonwoven fabric, as the flatness decreased and the minor axis length increased, the surface contact tended to weaken and become point contact, and the adhesive area per fiber cross-sectional area decreased. Furthermore, the Ra tended to increase, the surface smoothness tended to deteriorate slightly, and the adhesion also tended to decrease.

[0156] Examples 11 and 12 were carried out in the same manner as in Example 1, except that the variation in the minor axis length of the flat fibers constituting the nonwoven fabric (Example 11) and the degree of irregularity (Example 12) were changed. The variation in the minor axis length and the degree of irregularity were changed by changing the shape of the flow path in the spinneret used to produce the multilayer laminated fibers that are the raw material for the flat fibers.

[0157] The obtained nonwoven fabrics had the same cross-sectional direction of the flat fibers as in Example 1, and the fibers were firmly bonded over a wide area. The evaluation results of these nonwoven fabrics are shown in Table 2. However, due to the decrease in the variation in the minor axis length or the degree of unevenness, the nonwoven fabrics tended to have slightly more pronounced unevenness in shading when visually observed, and to have a worsening of uniformity. Although the bonded area per fiber cross-sectional area was similar to that in Example 1, the toughness tended to decrease due to the slightly worsening of uniformity.

[0158]

[0159] 1: Flat fiber 2: Minor axis length 3: Major axis length 4: Multi-layer laminated fiber 5: Component A 6: Component B 7: Main fiber 8: Binder fiber 9: Adhesion area 10: Fiber cross-sectional area

Claims

1. A nonwoven fabric having a thickness of 30.0 μm or less and a toughness value calculated as the product of the specific tensile strength and the square root of the tensile elongation of 0.50 or more.

2. The nonwoven fabric according to claim 1, which has a void ratio of 50% or more.

3. The nonwoven fabric according to claim 1 or 2, characterized in that the adhesive area per cross-sectional area of ​​the fibers constituting the nonwoven fabric is 3.00 or more.

4. A textile product comprising at least a portion of the nonwoven fabric according to claim 1.

5. A sheet in which the nonwoven fabric according to claim 1 is filled with resin or particles.

6. A solid electrolyte sheet comprising the nonwoven fabric according to claim 1 filled with a solid electrolyte.

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