WET-LAID NONWOVEN FABRIC SHEET

Abstract

 The present invention relates to a wet-laid nonwoven fabric sheet configured to contain at least three types of thermoplastic fibers having different fiber diameters, wherein the fiber diameter ratio (R/r) of the fiber diameter R of the fiber having the largest fiber diameter to the fiber diameter r of the fiber having the smallest fiber diameter is 30 ≤ R/r ≤ 150, the average pore size is 0.10-15 µm, and the maximum frequency in the pore size distribution is 70% or more.

Description

TECHNICAL FIELD

[0001] The present invention relates to a wet-laid nonwoven fabric sheet including at least three types of thermoplastic fibers having different fiber diameters.

BACKGROUND ART

[0002] With diversification of living modes in recent years, a demand for creativity of a comfortable space in life is increasing year by year, and more precise control of living environments such as temperature, light, air, and sound is required. Although there are various forms of materials used for such control, it is no exaggeration to say that textile products that can cope with diversified product forms are matters of the mainstream, and among them, the use of a nonwoven fabric sheet to which ultrafine fibers that can easily exhibit properties while saving a space are applied is studied as a material that can exhibit high functionality in a wide range of fields from a living environment to industrial materials.

[0003] Ultrafine fibers, in particular, nanofibers having an extremely narrow fiber diameter of 1,000 nm or less, can be processed into a nonwoven fabric sheet having a very dense structure by utilizing morphological features of thin and long fiber materials. Such a dense structure, for example, exhibits high filtration performance by subdividing a fluid flowing in the sheet, or easily exhibits functionality such as easy retention of a functional agent or the like contained therein for a long period of time. Further, each of the ultrafine fibers constituting the sheet may exhibit specific properties that cannot be obtained by general general-purpose fibers or microfibers, that is, excellent adsorption performance and the like due to a so-called nano-size effect and an effect of increasing a specific surface area which is a surface area per weight. Therefore, the nonwoven fabric sheet obtained by processing ultrafine fibers is expected as a highly functional nonwoven fabric sheet.

[0004] On the other hand, in general, as the fiber diameter decreases, the rigidity of the fiber extremely decreases. Therefore, a sheet product obtained from a single ultrafine fiber, in particular, a single nanofiber, cannot have rigidity that can withstand molding processing or practical use, and this point may be a restriction on application development. In order to solve this problem, it has been proposed to use a wet-laid nonwoven fabric sheet obtained by mixing short cut fibers having a large fiber diameter and ultrafine fibers for the purpose of imparting rigidity to a sheet.

[0005] In such a wet-laid nonwoven fabric sheet, fibers having a large fiber diameter substantially have mechanical properties as a skeleton of the sheet and ensure the handleability and the molding processability of the sheet, and the ultrafine fibers are present in a so-called crosslinked shape using other fibers having a large fiber diameter as a scaffold to play a role of forming a fine space. Thus, such a wet-laid nonwoven fabric sheet is expected to be developed for applications to a high-performance filter medium, a sound absorption material capable of controlling a sound absorption wavelength, a battery separator, and the like as a sheet satisfying both characteristics derived from ultrafine fibers and mechanical properties.

[0006] A fine space formed by such ultrafine fibers has more characteristic effect as the denseness and the homogeneity thereof are higher. Therefore, the presence of the fibers constituting the sheet, in particular, the ultrafine fibers, in a three-dimensionally excellent dispersion state is indispensable as a new material that appeals further performance.

[0007] In order to implement three-dimensional homogeneous dispersion in wet-laid paper making, it is the most important factor to use a fiber dispersion liquid in which fibers are homogeneously dispersed. However, it is generally considered difficult to ensure water dispersibility of the ultrafine fibers. That is, due to an increase in the specific surface area caused by a reduction in the fiber diameter, a cohesive force derived from an intermolecular force overwhelmingly increases, so that the ultrafine fibers are entangled with one another to form a fiber aggregate. Therefore, it is difficult to obtain a fiber dispersion liquid in which ultrafine fibers are uniformly dispersed. Among them, an overwhelmingly higher aspect ratio in the case of nanofibers than that of other fibers promotes the aggregation. Therefore, it is difficult to implement a wet-laid nonwoven fabric sheet in which ultrafine fibers are homogeneously dispersed.

[0008] In addition, in microfibers in the related art, the dispersibility is improved by applying a dispersant to a fiber surface, but it is difficult to obtain a sufficient dispersibility improvement effect by adding a small amount of the dispersant. On the other hand, it is possible to improve the dispersibility by adding a large amount of the dispersant, but the handleability such as foaming may be deteriorated in a wet-laid paper making step.

[0009] As an approach to such a problem, Patent Literature 1 proposes a wet-laid nonwoven fabric using liquid-crystalline polymer fibers at least partially fibrillated to a fiber diameter of 1 µm or less.

[0010] Patent Literature 2 proposes a wet-laid nonwoven fabric containing fibers having a fiber diameter of 3.0 µm or less obtained by using split conjugate fibers and splitting the fibers after wet-laid paper making.

[0011] Patent Literature 3 proposes a wet-laid nonwoven fabric which is made of two or more types of fibers including ultrafine fibers having a fiber length that is less likely to cause aggregation and which is suitable for a filter having excellent trapping efficiency.

CITATION LIST

PATENT LITERATURE

[0012] 

Patent Literature 1: JP2002-266281A

Patent Literature 2: JP2019-203216A

Patent Literature 3: WO2008/130019

SUMMARY OF INVENTION

TECHNICAL PROBLEM

[0013] In Patent Literature 1, it is a technical point that fibrillated fibers of 1 µm or less are generated in a dispersion liquid of liquid-crystalline polymer fibers to form a wet-laid nonwoven fabric, thereby forming a wet-laid nonwoven fabric having a dense structure due to entanglement among the fibrillated fibers or with other fibers without water-dispersing single ultrafine fibers.

[0014] Such a method is a technique that is also implemented in pulp fibers or the like, but in order to fibrillate the fibers, it is necessary to repeatedly perform a high shear treatment on the fiber dispersion liquid at a high pressure, and as a result, the entanglement of the fibrillated fibers may be unnecessarily promoted, and the denseness and the homogeneity of the fine space may not be controlled.

[0015] Patent Literature 2 discloses a technique relating to a wet-laid nonwoven fabric in which special split conjugate fibers are used to form a wet-laid nonwoven fabric, followed by being subjected to a heat treatment or a step of applying a physical impact, thereby generating ultrafine fibers by splitting the conjugate fibers and forming a dense structure.

[0016] In this case, since the conjugate fibers are surely present in the state of a fiber dispersion liquid, aggregation of the ultrafine fibers in an aqueous medium can be avoided. However, since the fibers present in the wet-laid nonwoven fabric are present in a complicatedly entangled state, it is difficult to evenly divide all of the split conjugate fibers, and as a result, the homogeneity of the fine space in the sheet may not be controlled.

[0017] In Patent Literature 3, it is a technical concept that as a fiber form in which aggregation of ultrafine fibers in water dispersion is unlikely to occur, ultrafine fibers in which a ratio (L/D) of a fiber length (L) to a fiber diameter (D) is reduced are applied to form a wet-laid nonwoven fabric. Therefore, it is an object to prevent aggregation due to unnecessary entanglement among the ultrafine fibers and to uniformize pores appearing on a surface of the wet-laid nonwoven fabric.

[0018] However, such a method of providing a limitation on the form of the ultrafine fibers may not be a fundamental solution to implement homogeneous dispersion of the ultrafine fibers, and may not be able to stably form a homogeneous fine space implemented by three-dimensionally and homogeneously dispersing the ultrafine fibers.

[0019] In view of the above, an object of the present invention is to provide a wet-laid nonwoven fabric sheet in which ultrafine fibers are arranged in a state of being homogeneously dispersed even on a surface of the sheet and in a cross-sectional direction thereof, thereby forming a three-dimensionally homogeneous fine space.

SOLUTION TO PROBLEM

[0020] The present invention includes the following 1 to 6.1. A wet-laid nonwoven fabric sheet, including: at least three types of thermoplastic fibers having different fiber diameters, in which the wet-laid nonwoven fabric sheet has a fiber diameter ratio (R/r) of a fiber diameter R of a fiber having a maximum fiber diameter to a fiber diameter r of a fiber having a minimum fiber diameter of 30 ≤ R/r ≤ 150, an average pore size of 0.10 µm to 15 µm, and a maximum frequency of a pore size distribution of 70% or more.2. The wet-laid nonwoven fabric sheet according to 1, in which the fiber diameter r is 0.10 µm to 1.0 µm.3. The wet-laid nonwoven fabric sheet according to 1 or 2, having a porosity of 70% or more.4. The wet-laid nonwoven fabric sheet according to any one of 1 to 3, having a basis weight of 10 g/m² to 500 g/m².5. The wet-laid nonwoven fabric sheet according to any one of 1 to 4, having a ratio (L/r) of a fiber length L to the fiber diameter r in the fiber having the minimum fiber diameter of 3,000 to 6,000.6. A textile product at least partially including the wet-laid nonwoven fabric sheet according to any one of 1 to 5.

ADVANTAGEOUS EFFECTS OF INVENTION

[0021] In the wet-laid nonwoven fabric sheet according to the present invention, since the ultrafine fibers are arranged in a state of being homogeneously dispersed even on a surface of the sheet and in a cross-sectional direction thereof, it is possible to form a three-dimensionally homogeneous fine space.

[0022] According to the wet-laid nonwoven fabric sheet according to the present invention, the adsorption performance derived from the specific surface area of the ultrafine fibers and the like can be satisfactorily exhibited in addition to the high functionality due to the three-dimensional and homogeneous formation of the fine space. Such a wet-laid nonwoven fabric sheet is expected to be developed into a high-performance filter medium, a next generation sound absorption material, a battery separator, and the like.

BRIEF DESCRIPTION OF DRAWINGS

[0023] 

[FIG. 1] FIG. 1 is a schematic diagram of an example of a fiber diameter distribution of fibers constituting a wet-laid nonwoven fabric sheet according to an embodiment of the present invention.

[FIG. 2] FIG. 2 includes diagrams showing examples of a pore size distribution of the wet-laid nonwoven fabric sheet, (a) of FIG. 2 is a diagram showing an example of a pore size distribution of a sheet in which a fine space is homogeneously present, and (b) of FIG. 2 is a diagram showing an example of a pore size distribution when a fine space is heterogeneously formed.

DESCRIPTION OF EMBODIMENTS

[0024] Hereinafter, preferred embodiments of the present invention will be described together.

[0025] A wet-laid nonwoven fabric sheet according to an embodiment of the present invention is a wet-laid nonwoven fabric sheet including at least three types of thermoplastic fibers having different fiber diameters, in which it is required that a fiber diameter ratio (R/r) of a fiber diameter R of a fiber having a maximum fiber diameter to a fiber diameter r of a fiber having a minimum fiber diameter is 30 ≤ R/r ≤ 150, an average pore size is 0.10 µm to 15 µm, and a maximum frequency of a pore size distribution is 70% or more.

[0026] The expression "at least three types of thermoplastic fibers having different fiber diameters" in the present invention refers to a state in which fibers observed on a surface of the wet-laid nonwoven fabric sheet have three or more fiber diameter distributions in a graph in which a horizontal axis represents the fiber diameter and a vertical axis represents the number. Here, a group of fibers having a fiber diameter within a range of each distribution (distribution width) is regarded as one type, and presence of three or more fiber diameter distributions means that three or more types of fibers having different fiber diameters in the present invention are mixed. The distribution width of the fiber diameter as used herein means a range of ±30% of a peak value having the largest presence number in each fiber diameter distribution. However, in the case where the distribution widths overlap with one another even though the peak values are clearly different from one another, the fiber group may be distinguished by setting a range of ±10% of the peak value as the distribution width. In order to more effectively form a homogeneous fine space which is the object of the present invention, the fiber diameter distribution is preferably discontinuous and independent, as shown in FIG. 1. FIG. 1 is a diagram showing the case where three fiber diameter distributions are present. In FIG. 1, a fiber diameter distribution 1 indicates a fiber diameter distribution of a fiber (fiber having a fiber diameter R) having a maximum fiber diameter, a fiber diameter distribution 2 indicates a fiber diameter distribution of a fiber having an intermediate fiber diameter, and a fiber diameter distribution 3 indicates a fiber diameter distribution of a fiber (fiber having a fiber diameter r) having a minimum fiber diameter.

[0027] The fiber diameter is determined as follows. That is, an image of a surface of the wet-laid nonwoven fabric sheet is captured at a magnification at which 150 to 3,000 fibers can be observed with a scanning electron microscope (SEM). The fiber diameters of 150 fibers randomly extracted from the captured image are measured. For 150 fibers randomly extracted from each image, a fiber width in a direction perpendicular to a fiber axis is measured as the fiber diameter from the two-dimensionally captured image. Regarding a value of the fiber diameter, the measurement is performed up to a second decimal place in units of µm. The above operation is performed for 10 images captured in the same manner, and the number of the above-described fiber diameter distributions is specified from evaluation results of the 10 images. Then, a value obtained up to a first decimal place by rounding off a second decimal place of a simple number average value of the fiber diameters with respect to the fibers that fall within the distribution width of each fiber diameter distribution is set as the fiber diameter of the fiber in each fiber diameter distribution.

[0028] In the wet-laid nonwoven fabric sheet according to the embodiment of the present invention, the fiber (fiber having the fiber diameter R) having the maximum fiber diameter has mechanical properties as the skeleton of the sheet, and plays a role of ensuring the handleability and the molding processability of the sheet. On the other hand, fibers (fibers having the fiber diameter r) having the minimum fiber diameter, that is, fibers such as ultrafine fibers having extremely low rigidity, are arranged in a crosslinked manner using other fibers as a scaffold, form a fine space, and play a role of exhibiting functionality such as adsorption performance derived from the specific surface area. The term "other fibers" as used herein refers to the fibers having an intermediate fiber diameter other than the fibers having the maximum and minimum fiber diameters among at least three types of fibers constituting the present invention. The other fibers play a role of a scaffold to prevent the fibers having the fiber diameter r from falling off from the sheet, and cause the fibers having the fiber diameter r to be stably present inside the sheet. From the above viewpoints, it is essential that the wet-laid nonwoven fabric sheet according to the present invention includes at least three types of fibers having different fiber diameters.

[0029] From the viewpoint of being applicable to a wide range of applications, the fibers constituting the wet-laid nonwoven fabric sheet according to the embodiment of the present invention need to be fibers (thermoplastic fibers) using a thermoplastic polymer excellent in mechanical properties and dimensional stability. Specifically, the thermoplastic polymer may be selected from various polymers depending on the intended use, and for example, may be selected from polymers that may be melt molded such as polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polytrimethylene terephthalate, polypropylene, polyolefin, polycarbonate, polyacrylate, polyamide, polylactic acid, thermoplastic polyurethane, and polyphenylene sulfide, and copolymers thereof. For example, the thermoplastic polymer may be selected in consideration of compatibility with an applied environment, and mechanical properties, heat resistance, chemical resistance, and the like that are finally required. The polymers may contain inorganic materials such as titanium oxide, silica, and barium oxide, colorants such as carbon black, a dye, and a pigment, various additives such as a flame retardant, a fluorescence brightening agent, an antioxidant, and an ultraviolet absorbing agent as long as the object of the present invention is not impaired.

[0030] Among them, the fibers (hereinafter, also simply referred to as "ultrafine fibers") having the fiber diameter r for implementing the wet-laid nonwoven fabric sheet according to the embodiment of the present invention are particularly preferably polyester fibers among the polymers described above, in consideration of ensuring dispersibility in an aqueous medium, which is important for enabling three-dimensionally homogeneous presence in the sheet. The reason will be described in detail below.

[0031] The reason why the homogeneous dispersion of the ultrafine fibers in the aqueous medium is inhibited is an attraction force acting among the ultrafine fibers, and in the related art, a method of providing a limitation on the form of the ultrafine fibers has been adopted. However, such a technique may not be a fundamental solution to implement homogeneous dispersion of the ultrafine fibers. On the other hand, the ultrafine fibers have a certain amount or more of carboxyl groups, so that the ultrafine fibers are negatively charged in the aqueous medium, and an electric repulsive force acts. Therefore, the dispersibility and the dispersion stability of the ultrafine fibers in the medium may be dramatically improved.

[0032] In view of the above, the ultrafine fiber used in the wet-laid nonwoven fabric sheet according to the embodiment of the present invention preferably has a carboxyl terminal group amount of 40 eq/ton or more. Accordingly, it is easy to ensure extremely high dispersibility regardless of the specification such as the aspect ratio, which is greatly restricted in the related art. That is, in the aqueous medium, electrical repulsive forces derived from a carboxyl group act among an infinite number of ultrafine fibers and repel one another, thereby enabling the ultrafine fibers to continue to float in the aqueous medium without aggregating with one another and ensuring the dispersion stability for a long period of time.

[0033] Further, from the viewpoint of ensuring the dispersibility, the ultrafine fibers are preferably made of a polymer having a high elastic modulus, that is, excellent rigidity, and are preferably polyester from this viewpoint.

[0034] By using polyester fibers as the ultrafine fibers, plastic deformation when deformation due to an external force is applied can be reduced. Accordingly, in a manufacturing step and a textile processing process of the wet-laid nonwoven fabric sheet according to the embodiment of the present invention, an effect of reducing unnecessary entanglement between fibers is exerted, the sheet may be processed while maintaining the dispersibility of the fibers, and a sheet in which ultrafine fibers are three-dimensionally and homogeneously arranged can be obtained.

[0035] The term "polyester" as used herein is made of a polyester such as polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, or polytrimethylene terephthalate, or a copolymer thereof, and is exemplified as a preferable example of the polymer in the implementation of the present invention.

[0036] In view of the above, it is preferable that the fibers having the fiber diameter R and the fibers having the intermediate fiber diameter are also polyester fibers in order not to unnecessarily impair the dispersibility of the ultrafine fibers in a paper making stock solution.

[0037] In addition, in order to effectively exhibit the roles of the fibers having the fiber diameter R and the fibers having the fiber diameter r in the present invention, the fiber diameter ratio (R/r) between the fiber diameter R of the fiber having the maximum fiber diameter and the fiber diameter r of the fiber having the minimum fiber diameter is required to be in the range of 30 ≤ R/r ≤ 150.

[0038] In a case where the fiber diameter ratio R/r as used herein is extremely small, the function of each fiber in accordance with the fiber diameter may be insufficient. For example, in the case where the fiber diameter R is small, the rigidity of the sheet is likely to be insufficient, which may cause a decrease in handleability and molding processability of the sheet, and in the case where the fiber diameter r is large, the specific performance derived from the ultrafine fiber may not be exhibited. Thus, a lower limit of the fiber diameter ratio R/r is set to 30. On the other hand, in the case where the fiber diameter ratio R/r is extremely large, the performance of each fiber in accordance with the fiber diameter is satisfactory, but a speed difference occurs in accumulation of the fibers on a water filtering surface at the time of filtering water in the wet-laid paper making step, and as a result, a heterogeneous sheet structure may be obtained. Therefore, an upper limit of the fiber diameter ratio R/r is set to 150. From the above viewpoints, it is necessary that the fiber diameter ratio R/r is within the above range in the present invention, but in view of the point of more satisfactorily implementing the objective effect of the present invention, the fiber diameter ratio R/r is more preferably 30 ≤ R/r ≤ 100. Within such a range, it acts more effectively on the three-dimensional homogeneity of the fine space formed by the ultrafine fibers.

[0039] The present invention is directed to a wet-laid nonwoven fabric sheet intended for a highly functional material that appeals a specific surface area produced by ultrafine fibers and filtration or adsorption utilizing a fine space in the sheet, and in the embodiment of the present invention, it is important that an average pore size is 0.10 µm to 15 µm and a maximum frequency of a pore size distribution is 70% or more.

[0040] The pore size as used herein refers to a value calculated by a bubble point method. As the bubble point method, for example, measurement by a porous material automatic pore measurement system Perm-Porometer (manufactured by PMI) can be used. In the measurement by the Perm-Porometer, the wet-laid nonwoven fabric sheet is immersed in a liquid having a known surface tension value and is supplied from an upper side of the sheet while increasing a pressure of a gas, and the pore size is measured based on a relationship between the pressure and the liquid surface tension on the surface of the wet-laid nonwoven fabric sheet.

[0041] Specifically, the pore size can be calculated under the following conditions using a porous material automatic pore measurement system Perm-Porometer (manufactured by PMI). An average flow rate diameter obtained by automatic calculation by pore size distribution measurement in which a measurement sample diameter is set to 25 mm and Galwick (surface tension: 16 mN/m) is used as a measurement solution having a known surface tension is set to an average pore size, and a value obtained by rounding off a second decimal place to a first decimal place is used. In addition, the pore size frequency is expressed in % by converting the value obtained by the automatic calculation into a percentage, and a value obtained by rounding off a second decimal place to a first decimal place is used.

[0042] (a) of FIG. 2 shows an example of a pore size distribution (vertical axis: frequency, horizontal axis: pore size) of a wet-laid nonwoven fabric sheet in which a homogeneous fine space is formed, and (b) of FIG. 2 shows an example of a pore size distribution when a heterogeneous fine space is formed. In this way, when the fine space formed in the sheet is homogeneous, the pore size distribution becomes sharp, and the frequency at a specific pore size becomes significantly large ((a) of FIG. 2). On the other hand, when the fine space is heterogeneous, the pore size distribution is broad ((b) of FIG. 2). Accordingly, the homogeneity of the fine space can be evaluated.

[0043] From the above, the average pore size in the embodiment of the present invention is an average size of through-holes formed in the wet-laid nonwoven fabric sheet, and serves as an index of the denseness of the fine space in the sheet. In addition, the maximum frequency of the pore size distribution is an index of the homogeneity of the fine space in the sheet. That is, in the case where the average pore size is relatively small and the maximum frequency of the pore size distribution is relatively large, it means that the sheet is a sheet in which a densified fine space is homogeneously present. In the case where the average pore size and the maximum frequency of the pore size distribution are within the above ranges, the fluid uniformly flows into the entire sheet without disturbing the flow of the fluid passing through the wet-laid nonwoven fabric sheet. Accordingly, a wet-laid nonwoven fabric sheet which is expected to effectively exhibit excellent performance such as filtration performance and sound absorption performance is obtained.

[0044] In the wet-laid nonwoven fabric sheet according to the embodiment of the present invention, the average pore size is set to 0.10 µm to 15 µm, so that the performance corresponding to the intended use can be exhibited without inhibiting the flow of the fluid. The inhibition of the flow of the fluid as used herein is caused by an extreme increase in pressure loss as the average pore size becomes smaller. Therefore, a lower limit of the average pore size is set to 0.10 µm from the viewpoint of ensuring a stable fluid flow. On the other hand, an upper limit of the average pore size is 15 µm from the viewpoint that the specific performance depending on the fine space effectively acts.

[0045] Further, in the wet-laid nonwoven fabric sheet according to the embodiment of the present invention, it is extremely important that the maximum frequency of the pore size distribution is within the above range. Such a sheet structure is implemented by forming a complicated space in which ultrafine fibers are uniformly present not only in a planar direction of the sheet but also in a thickness direction thereof. The fine space is homogeneously present in this way, so that the fluid uniformly flows into the entire sheet, and the filtration performance, the sound absorption performance, the adsorption performance, and the like can be satisfactorily exhibited. Therefore, the maximum frequency of the pore size distribution is 70% or more, preferably 80% or more, and more preferably 90% or more.

[0046] The wet-laid nonwoven fabric sheet according to the embodiment of the present invention, which satisfies the above requirements, is directed to a sheet in which a dense and homogeneous fine space is formed by the presence of ultrafine fibers having functionality in a good dispersion state. In addition to the functionality such as the filtration performance and the sound absorption performance produced by the characteristic sheet structure, the adsorption performance derived from the nano-size effect of the ultrafine fibers can be satisfactorily exhibited. Accordingly, it is expected to be developed into a high-performance filter medium, a next generation sound absorption material, a battery separator, and the like.

[0047] Next, in the wet-laid nonwoven fabric sheet according to the embodiment of the present invention, the fiber diameter r of the fiber having the minimum fiber diameter is preferably 0.10 µm to 1.0 µm.

[0048] The present invention is directed to a wet-laid nonwoven fabric sheet for implementing a highly functional material that appeals filtration, adsorption, and the like utilizing a specific surface area, in addition to a dense fine space due to the presence of ultrafine fibers. In order to play that role, the fiber diameter r is preferably 0.10 µm to 1.0 µm. Within such a range, densification of a fine space in the sheet is promoted, and a specific surface area effect produced by the ultrafine fibers can be predominantly exhibited, and exhibition of excellent performance can be expected.

[0049] Among them, from the viewpoint of increasing the specific surface area, as the fiber diameter becomes smaller, the properties become more conspicuous. On the other hand, in view of the handleability and the molding processability during processing of the nonwoven fabric, a substantial lower limit of the fiber diameter r is 0.10 µm in the embodiment of the present invention. In addition, in the present invention, an upper limit of the fiber diameter r is set to 1.0 µm as a range in which the specific surface area effect with respect to general fibers is predominantly exerted.

[0050] In view of the above, the fiber diameter R of the fiber having the maximum fiber diameter is preferably 3.0 µm to 50 µm from the viewpoint of ensuring the strength of the sheet, and more preferably 5.0 µm to 30 µm as a range in which the handleability and the molding processability of the sheet are favorably exhibited.

[0051] In addition, in the wet-laid nonwoven fabric sheet according to the embodiment of the present invention, the fiber diameter of the fiber having an intermediate fiber diameter is preferably from 1.0 µm to 20 µm. Within such a range, it is easy to effectively act as a scaffold of ultrafine fibers, and it is possible to form a three-dimensionally homogeneous fine space.

[0052] In the wet-laid nonwoven fabric sheet according to the embodiment of the present invention, a porosity is preferably 70% or more from the viewpoint of efficiently exhibiting the effect of a fine space.

[0053] The porosity as used herein is determined as follows. That is, a value obtained by rounding off a first decimal place of a value calculated by the following equation from a basis weight and a thickness of the wet-laid nonwoven fabric sheet to an integer value is defined as the porosity. A fiber density may be a density of the constituent fibers, and is calculated as 1.38 g/cm³ in the case of polyethylene terephthalate (PET).

[0054] At this time, a weight of a fiber sheet cut out into a 250 mm × 250 mm square is weighed out, and a first decimal place of a value converted into a weight per unit area (1 m²) is rounded off to an integer value, which is defined as the basis weight of the wet-laid nonwoven fabric sheet.

[0055] In addition, the thickness of the wet-laid nonwoven fabric sheet is measured in units of mm using a dial thickness gauge (SM-114 manufactured by TECLOCK Co., Ltd., probe shape: 10 mm diameter, scale interval: 0.01 mm, measuring force: 2.5 N or less). The measurement is performed at any five positions per one sample, and a value obtained by rounding off a third decimal point of an average thereof to a second decimal point is defined as the thickness of the wet-laid nonwoven fabric sheet.

[0056] In view of the subdivision of the fluid flowing into the sheet by the formation of the homogeneous fine space, which is the object of the present invention, a resistance received from the inside of the sheet is prevented from being excessively increased as the porosity inside the sheet is increased. Therefore, as a result, the fluid efficiently flows into the fine space, and the effect such as filtration performance is easily exhibited. Therefore, an aspect in which the porosity is preferably 70% or more is exemplified. Thus, an aspect in which the porosity of the wet-laid nonwoven fabric sheet according to the embodiment of the present invention is more preferably 80% or more is exemplified.

[0057] Such a porosity inside the sheet can be implemented by appropriately adjusting the thickness and the basis weight of the sheet on the premise that the fibers constituting the sheet are present in a dispersed state. At this time, in the case where the basis weight of the sheet is extremely decreased, it is difficult to form a fine space having an intended size, and a strength of the sheet becomes too low, which may result in an inappropriate sheet for practical use. On the other hand, in the case where the basis weight of the sheet is increased, it is preferable in that the through-holes formed by a three-dimensional fine space can be densified by accumulating more fibers, but in the case where the basis weight is extremely increased, the rigidity of the sheet is excessively increased, and the handleability and the molding processability of the sheet may be deteriorated.

[0058] In view of the above, the wet-laid nonwoven fabric sheet according to the embodiment of the present invention preferably has a basis weight of 10 g/m² to 500 g/m² because the fibers are stably and homogeneously present without impairing the objective effect of the present invention.

[0059] In the wet-laid nonwoven fabric sheet according to the embodiment of the present invention, the ratio (L/r) of the fiber length L of the fiber having the minimum fiber diameter to the fiber diameter r of the fiber having the minimum fiber diameter is preferably 3,000 to 6,000.

[0060] The fiber length L as used herein can be determined as follows. An image of the surface of the wet-laid nonwoven fabric sheet is captured with a microscope at a magnification at which 10 to 100 fibers having the fiber diameter r of which the entire length can be measured can be observed. The fiber lengths of 10 fibers having the fiber diameter r randomly extracted from each captured image are measured. The term "fiber length" as used herein refers to a length of a single fiber in a fiber longitudinal direction from an image captured two-dimensionally, which is measured up to a second decimal place in units of mm, and the decimal place is rounded off. The above operation is performed for 10 images captured in the same manner, and a simple number average value of an evaluation result of the 10 images is defined as the fiber length L.

[0061] In the present invention, the case where the ratio (L/r) is 3,000 to 6,000 is preferable from the viewpoint of exhibiting an excellent reinforcing effect because the number of contact points among the fibers can be increased, thereby reducing falling-off of the fibers and promoting formation of a crosslinking structure which is important for forming a fine space.

[0062] From the viewpoint of forming a crosslinking structure, as the fiber length is relatively larger, that is, the ratio is larger, the crosslinking structure is more easily formed, and the reinforcing effect can be enhanced. However, in the case where such a ratio is excessively increased, aggregation due to partial entanglement may occur, and the molding processing step may be complicated. Therefore, the upper limit is set to 6,000 as a range in which the reinforcing effect by the fiber length can be sufficiently exhibited, in addition to the specific surface area effect without the entanglement among the fibers.

[0063] In addition, in the present invention, as the ratio (L/r) is relatively smaller, it is more advantageous from the viewpoint of the handleability in the wet-laid paper making step. On the other hand, in the case where the ratio is excessively small, the specific effect exhibited as a sheet may be relatively small, and the lower limit is set to 3,000 as a range in which the fiber can pass through the molding step without causing any problem such as falling-off of the fiber.

[0064] Use of the ultrafine fibers having a fiber length within such a range is preferable in that the step passing property in molding processing or the like is remarkably improved because the fibers are appropriately entangled with one another to exhibit the reinforcing effect, thereby enhancing the sheet strength. Specifically, a specific tensile strength of the wet-laid nonwoven fabric sheet is preferably 5.0 Nm/g or more. In view of the wet-laid nonwoven fabric sheet having molding processability suitable for practical use, the specific tensile strength is preferably 15 Nm/g or less.

[0065] The specific tensile strength as used herein is determined as follows.

[0066] Five test pieces each having a width of 15 mm and a length of 50 mm were taken and subjected to a tensile test in accordance with JIS P8113:2006 using a tensile tester Tensilon UCT-100 manufactured by ORIENTEC CO., LTD. to measure the tensile strength of the wet-laid nonwoven fabric sheet. This operation is repeated five times, and a value obtained by rounding off a third decimal place of the simple average value of the obtained result is defined as the tensile strength of the wet-laid nonwoven fabric sheet, and a value divided by the basis weight is defined as the specific tensile strength.

[0067] A mixing ratio in a fiber weight of each of the fibers constituting the wet-laid nonwoven fabric sheet according to the embodiment of the present invention is not particularly limited, but it is preferable that the fiber having the fiber diameter r is 2.5 wt% to 30 wt% and the fiber having the fiber diameter R is 15 wt% to 85 wt% from the viewpoint of forming a stable fine space and ensuring the strength of the wet-laid nonwoven fabric sheet. The wet-laid nonwoven fabric sheet in which fibers are mixed within such a range is likely to be a sheet that exhibits good handleability and molding processability, and that is suitable for practical use.

[0068] On the other hand, binder fibers may be mixed as necessary for the purpose of improving the sheet strength and preventing the constituent fibers from falling off. Among them, by mixing heat-adhesive binder fibers, the fibers constituting the sheet may be physically bonded to one another, and the sheet strength can be improved. However, in the case where the binder fiber is excessively contained, the fine space may be blocked by melting or the fine space may be significantly reduced to inhibit the fluid flow. Further, a molding processing defect caused by the rigidity of the sheet that is increased more than necessary may occur. Thus, the mixing ratio of the binder fibers is preferably within the range of 5 wt% to 75 wt%. From the viewpoint of ensuring the adhesiveness among fibers in the sheet, a substantial lower limit of a blending ratio of the binder fibers is 5 wt%.

[0069] The binder fiber as used herein is not particularly limited, but is preferably, for example, a core-sheath conjugate fiber in which a polymer having a melting point of 150°C or lower is arranged in a sheath. After being formed, the wet-laid nonwoven fabric sheet is subjected to a drying step using a yankee dryer, an air-through dryer, or the like, or a heat treatment step using a calendar or the like, so that a sheath component on the surface of the binder fiber melts and adheres to other fibers, thereby increasing the rigidity of the fiber sheet. At the same time, fibers of a remaining core component can ensure the sheet strength as the fiber having the fiber diameter R in accordance with the fiber diameter and play a role of a scaffold as the fiber having an intermediate fiber diameter. In view of this point, the core-sheath conjugate fiber as described above is preferable. It is preferable that a melting point of the core component of the binder fiber is higher than a melting point of the sheath component and a difference in melting point therebetween is 20°C or higher from the viewpoint of implementing sufficient thermal adhesiveness and high rigidity because the sheath component on the surface of the binder fiber is likely to be sufficiently melted, and a decrease width in orientation of the core component is reduced.

[0070] Hereinafter, an example of a method for producing the wet-laid nonwoven fabric sheet according to the embodiment of the present invention will be described in detail.

[0071] A fiber having a maximum fiber diameter, a fiber having an intermediate fiber diameter, and a short fiber such as a thermally-fusible core-sheath conjugate fiber (binder fiber) of which a sheath component is made of a low-melting-point polymer are put into water, followed by being stirred in a disintegrator to prepare a fiber dispersion liquid in which the fibers are uniformly dispersed. At this time, in the core-sheath conjugate fiber serving as a binder, a core component remains in the sheet after thermal fusion, and thus the core-sheath conjugate fiber may be used as a fiber that plays a role of either the fiber having a maximum fiber diameter or the fiber having an intermediate fiber diameter. In this preparation step, it is possible to adjust the dispersibility by the charged amount of the fibers, the amount of the aqueous medium, the stirring time, and the like, and it is preferable that the short fibers are dispersed as uniformly as possible in the aqueous medium. In addition, a dispersant may be added in order to improve the dispersibility in water, but in the case where the wet-laid nonwoven fabric is subjected to post-processing, the addition amount of the dispersant is preferably limited to a necessary minimum limit so as not to affect the processability.

[0072] Next, an ultrafine fiber dispersion liquid in which ultrafine fibers are uniformly dispersed in an aqueous medium is prepared in accordance with a step to be described later. The ultrafine fiber dispersion liquid and the above-described fiber dispersion liquid are mixed to prepare a paper making stock solution, followed by being subjected to wet-laid paper making to thereby obtain a wet-laid nonwoven fabric sheet in which the ultrafine fibers are uniformly arranged.

[0073] The ultrafine fibers as used herein are preferably made of polyester having a carboxyl terminal group amount of 40 eq/ton or more from the viewpoint of ensuring the water dispersibility, and can be produced using a sea-island fiber made of two or more types of polymers having different dissolution rates in a solvent. The sea-island fiber refers to a fiber having a structure in which an island component made of a hardly soluble polymer is scattered in a sea component made of an easily soluble polymer.

[0074] As a method for producing the sea-island fiber, sea-island conjugate spinning by melt spinning is preferable from the viewpoint of improving productivity, and a method using a sea-island conjugate spinneret is preferable from the viewpoint of excellent control on the fiber diameter and the cross-sectional shape.

[0075] A reason of using a method according to the melt spinning is that the productivity is high and continuous production is possible. In the case of continuous production, it is preferable that a so-called sea-island conjugate cross section can be stably formed. From the viewpoint of the chronological stability of the cross section, it is important to consider a combination of polymers that forming the cross section. In the present invention, the polymers are preferably selected in a combination such that a melt viscosity ratio (ηB/ηA) of a melt viscosity ηB of a polymer B to a melt density ηA of a polymer A is within the range of 0.1 to 5.0.

[0076] The melt viscosity as used herein refers to a melt viscosity that can be measured by a capillary rheometer using a chip-shaped polymer having a moisture content of 200 ppm or less by a vacuum dryer, and refers to a melt viscosity at the same shear rate at a spinning temperature.

[0077] The easily soluble polymer of the sea-island fiber as used herein is selected from, for example, melt-formable polymers such as polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polytrimethylene terephthalate, polypropylene, polyolefin, polycarbonate, polyacrylate, polyamide, polylactic acid, thermoplastic polyurethane, and polyphenylene sulfide, and copolymers thereof. In particular, the sea component is preferably a copolymerized polyester, polylactic acid, polyvinyl alcohol, or the like that exhibits easy elution in an aqueous solvent, hot water, or the like from the viewpoint of simplifying an elution step of the sea component, and is particularly preferably polyester or polylactic acid in which polyethylene glycol or sodium sulfoisophthalic acid is copolymerized alone or in combination from the viewpoint of the handleability and easy solubility in an aqueous solvent having a low concentration.

[0078] The term "easily soluble" as used herein means that a dissolution rate ratio (easily soluble polymer/hardly soluble polymer) is 100 or more based on the hardly soluble polymer with respect to the solvent used for the dissolution treatment. In consideration of simplification and time reduction of the dissolution treatment in textile processing, the dissolution rate ratio is preferably large, and the dissolution rate ratio is preferably 1,000 or more, more preferably 10,000 or more. Within such a range, the dissolution treatment can be completed in a short period of time, and thus the ultrafine fiber suitable for the present invention can be obtained without unnecessarily deteriorating the hardly soluble component.

[0079] In addition, polylactic acid, polyester obtained by copolymerizing 3 mol% to 20 mol% of 5-sodium sulfoisophthalic acid, and polyester obtained by copolymerizing 5 wt% to 15 wt% of polyethylene glycol having a weight-average molecular weight of 500 to 3,000 in addition to the above-described 5-sodium sulfoisophthalic acid are particularly preferable from the viewpoint of the solubility in an aqueous solvent and simplification of waste liquid treatment generated during dissolution.

[0080] From the above viewpoints, examples of a suitable combination of polymers of the sea-island fiber include one or more selected from the group consisting of polylactic acid and polyester in which 3 mol% to 20 mol% of 5-sodium sulfoisophthalic acid is copolymerized and 5 wt% to 15 wt% of polyethylene glycol having a weight-average molecular weight of 500 to 3,000 is copolymerized, as sea components, and one or more selected from the group consisting of polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and copolymers thereof, as island components.

[0081] A spinning temperature of the sea-island fiber is preferably a temperature at which a polymer having a high melting point and a high viscosity mainly exhibits fluidity among the polymers to be used determined from the above-described viewpoints. The temperature at which the polymer exhibits fluidity depends on the properties of the polymer and the molecular weight thereof, but the melting point of the polymer serves as a criterion, and the temperature may be set to the melting point +60°C or lower. At this temperature, the polymer is not thermally decomposed in a spinning head or a spinning pack, a decrease in molecular weight is prevented, and the sea-island fiber can be favorably produced.

[0082] A melted and discharged yarn is cooled and solidified, is converged by applying an oil agent or the like, and is taken up by a roller whose peripheral speed is regulated. Here, the take-up speed is determined, for example, based on the discharge amount and the target fiber diameter. The take-up speed is preferably 100 m/min to 7,000 m/min from the viewpoint of stably producing the sea-island fiber. From the viewpoint of improving the thermal stability and the mechanical properties, the spun sea-island fibers are preferably drawn, and the spun multifilament may be drawn after being once wound, or may be drawn following spinning without being wound.

[0083] The sea-island fibers are preferably bundled in units of several tens to several millions and subjected to cutting processing to a desired fiber length using a cutting machine such as a guillotine cutter, a slicing machine, or a cryostat. The fiber length L at this time is preferably cut such that the ratio (L/r) with respect to an island component diameter (corresponding to the fiber diameter r) is within the range of 3,000 to 6,000. Within such a range, the number of contact points among fibers increases at the time of forming a wet-laid nonwoven fabric sheet, and formation of a crosslinking structure is promoted, and thus the reinforcing effect of the sheet can be enhanced.

[0084] The reason why such a range is preferable is that in the case where the ratio (L/r) is excessively increased, partial aggregation may occur in the aqueous medium, and the sheet may impair homogeneity, and on the other hand, in the case where the ratio is extremely decreased, falling-off during the wet-laid paper making step may occur.

[0085] The island component diameter as used herein is substantially equal to the fiber diameter of the ultrafine fibers, and is determined as follows.

[0086] The sea-island conjugate fiber is embedded in an embedding agent such as an epoxy resin, and an image of a cross section thereof is captured at a magnification at which 150 or more island components can be observed with a transmission electron microscope (TEM). In the case where 150 or more island components are not arranged in one filament, a cross section of fibers of several filaments may be captured to observe 150 or more island components in total. At this time, in the case where metal dyeing is performed, a contrast of the island components can be made clear. The island component diameter of 150 island components randomly extracted from each captured image of the fiber cross section is measured. The island component diameter as used herein refers to a diameter of a perfect circle circumscribing a cutting surface in a direction perpendicular to a fiber axis from a two-dimensionally captured image. A homogeneous dispersion liquid of ultrafine fibers can be produced by dissolving and removing the sea components from the sea-island fibers obtained as described above.

[0087] That is, in order to obtain an ultrafine fiber dispersion liquid suitable for the present invention, the sea-island fibers after the above-described cutting processing may be immersed in a solvent or the like capable of dissolving the easily soluble component (sea component) to remove the easily soluble component. In the case where the easily soluble component is one or more components selected from the group consisting of polylactic acid and copolymerized polyethylene terephthalate obtained by copolymerizing 5-sodium sulfoisophthalic acid, polyethylene glycol, or the like, an alkaline aqueous solution such as an aqueous solution of sodium hydroxide can be used. At this time, a bath ratio (sea-island fiber weight (g)/alkaline aqueous solution weight (g)) of the sea-island fiber to the alkaline aqueous solution is preferably 1/10,000 to 1/5, more preferably 1/5,000 to 1/10. Within this range, it is possible to prevent the ultrafine fibers from being unnecessarily entangled with one another at the time of dissolving the sea component.

[0088] At this time, an alkali concentration of the alkaline aqueous solution is preferably 0.1 wt% to 5 wt%, more preferably 0.5 wt% to 3 wt%. Within such a range, dissolution of the sea component can be completed in a short period of time, and a fiber dispersion liquid in which ultrafine fibers are homogeneously dispersed can be obtained without unnecessarily deteriorating the island component. In addition, a temperature of the alkaline aqueous solution is not particularly limited, but is preferably, for example, 50°C or higher because the progress of dissolution of the sea component can be accelerated.

[0089] In the present invention, a fiber in which an easily soluble component (sea component) is dissolved from a sea-island fiber may be used as it is, or ultrafine fibers may be separated once by filtration or the like, washed with water, freeze-dried, and then dispersed again in an aqueous medium to form a sheet. In addition, in consideration of textile processing to be used and handleability at that time, an acid or an alkali may be added, thereby adjusting the PH of the medium, or the medium may be diluted with water. For the purpose of stable sheet formation by preventing aggregation of the ultrafine fibers over time or increasing the viscosity of the medium, the ultrafine fiber dispersion liquid may contain a dispersant as necessary. Examples of the type of the dispersant include natural polymers, synthetic polymers, organic compounds, and inorganic compounds. Examples of an additive for preventing aggregation of fibers include a cationic compound, a nonionic compound, and an anionic compound. Among them, in the case where the purpose is to improve the dispersibility, it is preferable to use an anionic compound from the viewpoint of an electric repulsive force in an aqueous medium. In addition, the addition amount of the dispersant is preferably 0.001 equivalent to 10 equivalent with respect to the ultrafine fibers, and within such a range, the dispersibility of the ultrafine fibers is easily secured without impairing processability of wet-laid paper making.

[0090] The ultrafine fiber dispersion liquid prepared as described above is mixed with the fiber dispersion liquid prepared as described above, diluted and adjusted to a certain concentration, followed by being dehydrated with a tilted wire, a circular net, or the like to form a wet-laid nonwoven fabric sheet. Examples of the device used for paper making include a cylinder paper machine, a fourdrinier paper machine, an inclination type tanmo machine, and a paper machine in which the above machines are combined. In the paper making step, the paper making speed, the amount of the fibers, and the amount of the aqueous medium in addition to the dispersibility of the fibers in the paper making stock solution can be adjusted to control the accumulation of the fibers at the time of filtering , thereby producing a three-dimensionally homogeneous sheet. Here, from the viewpoint of sheet stable formation, the fiber length of the constituent fibers is preferably 30.0 mm or less. Within such a range, a wet-laid nonwoven fabric sheet having practical homogeneity can be formed as a highly functional sheet. In the case where the fiber length exceeds 30.0 mm, the fibers may be strongly entangled with one another during dispersion in the aqueous medium, a fiber mass may be formed, and it tends to be difficult to form a homogeneous sheet.

[0091] The sheet formed by wet-laid paper making passes through a drying step in order to remove moisture. As the drying method, a method using hot air ventilation (air-through) or a method of bringing the sheet into contact with a thermal rotation roll (thermal calendar roll or the like) is preferable from the viewpoint of simultaneously performing the drying of the sheet and the thermal adhesion of the binder fibers.

[0092] The basis weight and the thickness of the wet-laid nonwoven fabric may be appropriately changed according to a supply amount of the paper making stock solution and the paper making speed in the wet-laid paper making step. The thickness of the wet-laid nonwoven fabric sheet according to the embodiment of the present invention is not particularly limited, but is preferably 0.050 mm to 2.50 mm. In particular, the thickness is preferably 0.10 mm or more from the viewpoint of being able to obtain excellent sheet molding processability.

[0093] The wet-laid nonwoven fabric sheet satisfying the above requirements can satisfactorily exhibit the adsorption performance and the like derived from a specific surface area of the ultrafine fibers, and in addition, can implement high filtration performance and the like by three-dimensionally and homogeneously forming a fine space because the respective fibers constituting the sheet are present in a homogeneously dispersed state. Therefore, the wet-laid nonwoven fabric sheet according to the present invention can be expected as a material that can be developed into a high-functional filter medium, a next generation sound absorption material, a battery separator, or the like. A textile product at least partially including the wet-laid nonwoven fabric sheet may be suitably used for such applications.

EXAMPLES

[0094] Hereinafter, a wet-laid nonwoven fabric sheet according to an embodiment of the present invention will be specifically described with reference to Examples.

A. Melt Viscosity of Polymer

[0095] A moisture content of a chip-shaped polymer was set to 200 ppm or less using a vacuum dryer, and melt viscosity was measured by changing a strain rate in a stepwise manner by Capillograph 1B manufactured by Toyo Seiki Seisakusho Co., Ltd. A measurement temperature was the same as a spinning temperature, and in Examples and Comparative Examples, a melt viscosity of 1,216 s^-1 is described. The measurement was performed under a nitrogen atmosphere setting the time when a sample was put into a heating furnace to the time when the measurement was started to 5 minutes.

B. Melting Point of Polymer

[0096] A moisture content of a chip-shaped polymer was set to 200 ppm or less using a vacuum dryer, about 5 mg of the chip-shaped polymer was weighed out, and DSC measurement was performed using a differential scanning calorimetry (DSC) Q2000 manufactured by TA Instruments at a heating rate of 16°C/min from 0°C to 300°C and then holding the temperature at 300°C for 5 minutes. A melting point was calculated from a melting peak observed during the heating process. The measurement was performed three times for each sample, and an average value thereof was defined as the melting point. In the case where a plurality of melting peaks were observed, a melting peak top on the highest temperature side was defined as the melting point.

C. Fiber Diameter

[0097] An image of a surface of the wet-laid nonwoven fabric sheet was captured with a scanning electron microscope (SEM) at a magnification at which 150 to 3,000 fibers can be observed, and a fiber diameter of 150 fibers randomly extracted from the captured image was measured. A fiber diameter was measured by using a fiber width in a direction perpendicular to a fiber axis from the two-dimensional image as the fiber diameter. A value of the fiber diameter was measured up to a second decimal place in units of µm. The above operation was performed for 10 images captured in the same manner, and the number of fiber diameter distributions was specified from evaluation results of the 10 images. Then, with respect to the fibers falling within the distribution width of each fiber diameter distribution, a value obtained by rounding off a second decimal place of a simple number average value of the fiber diameter to a first decimal place was defined as the fiber diameter of the fibers in each fiber diameter distribution.

D. Fiber Length

[0098] An image of a surface of the wet-laid nonwoven fabric sheet is captured with a microscope at a magnification at which 10 to 100 fibers of each fiber diameter at which the entire length can be measured can be observed. A fiber length of 10 fibers of each fiber diameter randomly extracted from each captured image was measured. The term "fiber length" as used herein refers to a length of a single fiber in a fiber longitudinal direction from the two-dimensionally captured image, and is measured up to a third decimal place in units of mm, and a second decimal place is rounded off. The above operation was performed for 10 images captured in the same manner, and a simple number average value of evaluation results of the 10 images was defined as the fiber length.

E. Average Pore Size and Maximum Frequency of Pore Size Distribution

[0099] A pore size was calculated by a bubble point method (based on ASTM F-316-86) using a porous material automatic pore measurement system Perm-Porometer (manufactured by PMI). An average flow rate diameter obtained by automatic calculation by fine pore size distribution measurement in which a measurement sample diameter was set to 25 mm and Galwick (surface tension: 16 mN/m) was used as a measurement solution having a known surface tension was set to an average pore size, and a value obtained by rounding off a second decimal place to a first decimal place was used. In addition, a pore size frequency was expressed in % by converting a value obtained by automatic calculation into a percentage, and a value obtained by rounding off a second decimal place to a first decimal place was used.

F. Basis Weight

[0100] A weight of a fiber sheet cut out into a 250 mm × 250 mm square was weighed out, and a first decimal place of a value converted into a weight per unit area (1 m²) was rounded off to an integer value, which was defined as a basis weight of the wet-laid nonwoven fabric sheet.

G. Thickness

[0101] A thickness of the wet-laid nonwoven fabric sheet was measured in units of mm using a dial thickness gauge (SM-114 manufactured by TECLOCK Co., Ltd., probe shape: 10 mm diameter, scale interval: 0.01 mm, measuring force: 2.5 N or less). The measurement was performed at random five points per one sample, and a value obtained by rounding off a third decimal place of an average thereof to a second decimal place was defined as a thickness of the wet-laid nonwoven fabric sheet.

H. Porosity

[0102] A value obtained by rounding off a first decimal place of a value calculated by the following equation based on the basis weight and the thickness of the wet-laid nonwoven fabric sheet to an integer value was defined as the porosity.

The fiber density may be a density of the constituent fibers, and was calculated as 1.38 g/cm³ in the case of PET.

I. Specific Tensile Strength

[0103] A specific tensile strength was determined as follows.

[0104] Five test pieces each having a width of 15 mm and a length of 50 mm were taken and subjected to a tensile test in accordance with JIS P8113:2006 using a tensile tester Tensilon UCT-100 manufactured by ORIENTEC CO., LTD. to measure a tensile strength of the wet-laid nonwoven fabric sheet. This operation was repeated five times, a value obtained by rounding off a third decimal place of a simple average value of the obtained results was defined as the tensile strength of the wet-laid nonwoven fabric sheet, and a value divided by the basis weight was defined as the specific tensile strength.

[Example 1]

[0105] As an island component, polyethylene terephthalate (PET1, melt viscosity: 160 Pa s, carboxyl terminal group amount: 40 eq/ton) was used, as a sea component, polyethylene terephthalate (copolymerized PET, melt viscosity: 121 Pa s) (melt viscosity ratio: 1.3, dissolution rate ratio: 30,000 or more) obtained by copolymerizing 8.0 mol% of 5-sodium sulfoisophthalic acid and 10 wt% of polyethylene glycol having a molecular weight of 1,000 was used, and a yarn melted and discharged at a sea component/island component composite ratio of 50/50 was cooled and solidified using a sea-island conjugate spinneret (number of islands: 2,000) in which a shape of the island component was a circle. Thereafter, an oil agent was applied and winding was performed at a spinning speed of 1,000 m/min, thereby obtaining an undrawn yarn (total discharge amount: 12 g/min). Further, the undrawn yarn was drawn 3.4 times (drawing speed: 800 m/min) between rollers heated to 85°C and 130°C to obtain a sea-island fiber.

[0106] The sea-island fiber had mechanical properties sufficient for cutting processing such as a strength of 2.4 cN/dtex and an elongation of 36%, and cutting processing was performed such that the fiber length was 0.6 mm. The sea-island fiber was treated with a 1 wt% aqueous solution of sodium hydroxide (bath ratio: 1/100) heated to 90°C, thereby obtaining an ultrafine fiber dispersion liquid.

[0107] Next, a fiber dispersion liquid was prepared by adjusting a mixing ratio of cut fibers of thermally fusible core-sheath conjugate fibers (fiber diameter of core component: 10 µm, fiber length: 5.0 mm) as a skeleton of the sheet and the binder fiber to 30 wt% and a mixing ratio of cut fibers of PET (fiber diameter: 4 µm, fiber length: 3.0 mm) serving as a scaffold of ultrafine fibers to 65 wt% and uniformly mixing and dispersing the cut fibers with water by a disintegrator. Here, in the core-sheath conjugate fiber, configurations of a core component and a sheath component were as follows.

Core component: PET

Sheath component: polyester (copolymerized polyester) having a melting point of 110°C obtained by copolymerizing 60 mol% of terephthalic acid, 40 mol% of isophthalic acid, 85 mol% of ethylene glycol, and 15 mol% of diethylene glycol.

[0108] The above-described ultrafine fiber dispersion liquid was homogeneously mixed with the fiber dispersion liquid such that the mixing ratio of the ultrafine fibers was 5 wt%, thereby preparing a paper making stock solution. The paper making stock solution was made into paper using a square sheet machine (250 mm square) manufactured by Kumagai Riki Kogyo Co., Ltd, followed by being dried and thermally treated in a rotary dryer in which a roller temperature was set to 110°C to thereby obtain a wet-laid nonwoven fabric sheet.

[0109] The obtained wet-laid nonwoven fabric sheet was a sheet in which the ultrafine fibers were present in a crosslinked shape using other fibers having a large fiber diameter as a scaffold, and had a fiber diameter ratio R/r of 50, a basis weight of 25 g/m², a thickness of 0.09 mm, and a porosity of 79.9%. The obtained wet-laid nonwoven fabric sheet was a sheet in which an average pore size calculated by a bubble point method was 4.9 µm, a maximum frequency of the pore size distribution was 91.6%, and a fine dense space was formed very homogeneously. In addition, the specific tensile strength thereof was 6.7 Nm/g, and the handleability and the molding processability were favorable by the reinforcing effect due to the entanglement of the ultrafine fibers.

[Examples 2 to 5]

[0110] The procedure of Example 1 was performed except that the mixing ratio of the ultrafine fibers was changed in a stepwise manner to perform wet-laid paper making.

[0111] In Examples 2 to 5, in the case where the mixing ratio of the ultrafine fibers was increased, a fine space formed by the ultrafine fibers became dense, and in addition, the reinforcement effect was improved by promoting the entanglement, and the specific tensile strength was also improved. Further, since the paper making was possible without impairing the dispersibility in the aqueous medium, the wet-laid nonwoven fabric sheet was a sheet in which the maximum frequency of the pore size distribution was 80% or more and a very homogeneous fine space was formed.

[Example 6]

[0112] The procedure of Example 3 was performed except that the basis weight of the sheet was 150 g/m².

[0113] The sheet was a wet-laid nonwoven fabric sheet in which even when the basis weight of the sheet was increased, a three-dimensionally homogeneous sheet structure was formed, the average pore size was 0.8 µm, and a very dense fine space was stably formed.

[Example 7]

[0114] The procedure of Example 1 was performed except that as the fibers having an intermediate fiber diameter, cut fibers having a fiber diameter of 4 µm and a fiber length of 3.0 mm were mixed at a mixing ratio of 62.5 wt% and cut fibers of PET having a fiber diameter of 0.6 µm and a fiber length of 0.6 mm were mixed at a mixing ratio of 2.5 wt% to thereby form a sheet with four types of fibers having different fiber diameters.

[0115] The sheet was a sheet in which a homogeneous fine space was formed even when the sheet was made of four types of fibers having different fiber diameters.

[Examples 8 to 13]

[0116] Example 8 was carried out in accordance with Example 1 except that the fiber diameter of the ultrafine fibers was changed to 0.3 µm.

[0117] Example 9 was carried out in accordance with Example 8 except that the mixing ratio of the ultrafine fibers was changed to 10 wt%.

[0118] Examples 10 to 13 were carried out in accordance with Example 9 except that the basis weight of the sheet was changed to 12.5 g/m², 50 g/m², 100 g/m², and 300 g/m², respectively.

[0119] Even when the fiber diameter ratio R/r was decreased as compared with Example 1, formation of a fine space specific to the ultrafine fibers was implemented. Further, even when the basis weight of the sheet was changed in a stepwise manner, a stable and homogeneous fine space was formed without greatly impairing the dispersibility of each fiber.

[Examples 14 to 16]

[0120] Examples 14 to 16 were carried out in accordance with Example 8 except that the mixing ratio of the fiber having the fiber diameter R was changed to 15 wt%, 45 wt%, and 75 wt%, respectively.

[0121] Even when the mixing ratio of the fiber having the fiber diameter R was increased, the homogeneity of the fine space of the sheet was favorable, and the skeleton of the sheet was more strongly formed, thereby greatly improving the specific tensile strength.

[Examples 17 and 18]

[0122] Examples 17 and 18 were carried out in accordance with Example 1 except that the fiber diameter R was changed to 15 µm and 20 µm, respectively.

[0123] Even when the fiber diameter R was increased, the wet-laid nonwoven fabric sheet had a homogeneous fine space without inhibiting uniformly accumulation of the fibers in the wet-laid paper making step. In addition, since the fiber having the fiber diameter R was responsible for the mechanical properties of the sheet, the specific tensile strength of the obtained sheet was improved as compared with Example 1.

[Example 19]

[0124] The procedure of Example 1 was performed except that an ultrafine fiber was produced using polyethylene terephthalate (PET2, melt viscosity: 160 Pa s, carboxyl terminal group amount: 52 eq/ton) as the island component.

[0125] Since the dispersibility in the aqueous medium is further enhanced by increasing the amount of the carboxyl terminal group of the ultrafine fibers, a very homogeneous sheet structure was formed.

[Examples 20 and 21]

[0126] The procedure of Example 1 was performed except that the ultrafine fiber was cut to have a fiber diameter of 0.3 µm and fiber lengths of 1.2 mm and 1.8 mm, respectively.

[0127] Even when the ratio (L/r) of the fiber length to the fiber diameter of the ultrafine fiber was increased to 4,000 or 6,000 in comparison with Example 1, a fiber aggregate was easily formed in the aqueous medium, but the obtained sheet formed a homogeneous fine space. Further, the reinforcing effect due to the entanglement of the ultrafine fibers is exhibited, thereby improving the specific tensile strength as compared with Example 1.

[Comparative Example 1]

[0128] A wet-laid nonwoven fabric sheet was prepared in accordance with Example 1 except that an ultrafine fiber obtained using polyethylene terephthalate (PET3, melt viscosity: 120 Pa·s, carboxyl terminal group amount: 28 eq/ton) different from that of Example 1 as an island component was used.

[0129] Since in the obtained sheet, the water dispersibility of the ultrafine fibers was greatly impaired caused by insufficient electrical repulsive force derived from the carboxyl group, as a sheet structure in which the pore size distribution was board, the maximum frequency of the pore size distribution was small and a heterogeneous fine space was formed.

[Comparative Example 2 and 3]

[0130] Comparative Example 2 was carried out in accordance with Example 1 except that the fiber diameter of the ultrafine fibers was 0.6 µm.

[0131] Comparative Example 3 was carried out in accordance with Comparative Example 2 except that the mixing ratio of the ultrafine fibers was 20 wt%.

[0132] The obtained sheet was a sheet that hardly exhibited the effect specific to the ultrafine fibers due to the extremely small fiber diameter ratio R/r, and was inferior in specific tensile strength to Examples 1 and 5, and thus was a sheet in which it was difficult to implement both the sheet strength and the construction of a fine space.

[0133] Results of each example are shown in tables. In each table, a unit "%" of the mixing ratio of each fiber means "wt%".

[Table 1]

				Example 1	Example 2	Example 3
Constituent fiber	Fiber having fiber diameter R	Polymer	-	PET	PET	PET
		Fiber diameter R	µm	10.0	10.0	10.0
		Fiber length	mm	5.0	5.0	5.0
		Fiber length/fiber diameter	-	500	500	500
		Mixing ratio	%	30	30	30
	Fiber having fiber diameter r	Polymer	-	PET 1	PET 1	PET 1
		Fiber diameter r	µm	0.2	0.2	0.2
		Fiber length	mm	0.6	0.6	0.6
		Fiber length/fiber diameter	-	3,000	3,000	3,000
		Mixing ratio	%	5	2.5	7.5
	Other fibers	Polymer	-	PET	PET	PET
		Fiber diameter	µm	4.0	4.0	4.0
		Fiber length	mm	3.0	3.0	3.0
		Fiber length/fiber diameter	-	750	750	750
		Mixing ratio	%	65	67.5	62.5
Sheet	Fiber diameter ratio R/r		-	50.0	50.0	50.0
	Basis weight		g/cm2	25	25	25
	Thickness		mm	0.09	0.10	0.09
	Porosity		%	79.9	81.9	79.9
	Average pore size		µm	4.9	7.1	3.4
	Maximum frequency of pore size distribution		%	91.6	91.3	97.3
	Specific tensile strength		Nm/g	6.7	5.0	8.4

				Example 4	Example 5	Example 6	Example 7
Constituent fiber	Fiber having fiber diameter R	Polymer	-	PET	PET	PET	PET
		Fiber diameter R	µm	10.0	10.0	10.0	10.0
		Fiber length	mm	5.0	5.0	5.0	5.0
		Fiber length/fiber diameter	-	500	500	500	500
		Mixing ratio	%	30	30	30	30
	Fiber having fiber diameter r	Polymer	-	PET 1	PET 1	PET 1	PET 1
		Fiber diameter r	µm	0.2	0.2	0.2	0.2
		Fiber length	mm	0.6	0.6	0.6	0.6
		Fiber length/fiber diameter	-	3,000	3,000	3,000	3,000
		Mixing ratio	%	10	20	7.5	5
	Other fibers	Polymer	-	PET	PET	PET	PET/PET
		Fiber diameter	µm	4.0	4.0	4.0	0.6/4.0
		Fiber length	mm	3.0	3.0	3.0	0.6/3.0
		Fiber length/fiber diameter	-	750	750	750	1,000/750
		Mixing ratio	%	60	50	62.5	2.5/62.5
Sheet	Fiber diameter ratio R/r		-	50.0	50.0	50.0	50.0
	Basis weight		g/cm2	25	25	150	25
	Thickness		mm	0.11	0.10	0.48	0.11
	Porosity		%	82.7	81.9	77.4	83.5
	Average pore size		µm	2.9	1.7	0.8	3.5
	Maximum frequency of pore size distribution		%	86.6	83.9	81.8	87.6
	Specific tensile strength		Nm/g	10	16.7	8.4	6.9

PET: polyethylene terephthalate

[Table 2]

				Example 8	Example 9	Example 10	Example 11
Constituent fiber	Fiber having fiber diameter R	Polymer	-	PET	PET	PET	PET
		Fiber diameter R	µm	10.0	10.0	10.0	10.0
		Fiber length	mm	5.0	5.0	5.0	5.0
		Fiber length/fiber diameter	-	500	500	500	500
		Mixing ratio	%	30	30	30	30
	Fiber having fiber diameter r	Polymer	-	PET 1	PET 1	PET 1	PET 1
		Fiber diameter r	µm	0.3	0.3	0.3	0.3
		Fiber length	mm	0.6	0.6	0.6	0.6
		Fiber length/fiber diameter	-	2,000	2,000	2,000	2,000
		Mixing ratio	%	5	10	10	10
	Other fibers	Polymer	-	PET	PET	PET	PET
		Fiber diameter	µm	4.0	4.0	4.0	4.0
		Fiber length	mm	3.0	3.0	3.0	3.0
		Fiber length/fiber diameter	-	750	750	750	750
		Mixing ratio	%	65	60	60	60
Sheet	Fiber diameter ratio R/r		-	33.3	33.3	33.3	33.3
	Basis weight		g/cm2	25	25	12.5	50
	Thickness		mm	0.11	0.11	0.05	0.19
	Porosity		%	83.5	83.5	81.9	80.9
	Average pore size		µm	6.9	3.2	6.0	2.8
	Maximum frequency of pore size distribution		%	92.3	94.9	93.4	88.2
	Specific tensile strength		Nm/g	6.1	7.2	7.2	7.2

				Example 12	Example 13	Example 14	Example 15	Example 16
Constituent fiber	Fiber having fiber diameter R	Polymer	-	PET	PET	PET	PET	PET
		Fiber diameter R	µm	10.0	10.0	10.0	10.0	10.0
		Fiber length	mm	5.0	5.0	5.0	5.0	5.0
		Fiber length/fiber diameter	-	500	500	500	500	500
		Mixing ratio	%	30	30	15	45	75
	Fiber having fiber diameter r	Polymer	-	PET 1	PET 1	PET 1	PET 1	PET 1
		Fiber diameter r	µm	0.3	0.3	0.3	0.3	0.3
		Fiber length	mm	0.6	0.6	0.6	0.6	0.6
		Fiber length/fiber diameter	-	2,000	2,000	2,000	2,000	2,000
		Mixing ratio	%	10	10	5	5	5
	Other fibers	Polymer	-	PET	PET	PET	PET	PET
		Fiber diameter	µm	4.0	4.0	4.0	4.0	4.0
		Fiber length	mm	3.0	3.0	3.0	3.0	3.0
		Fiber length/fiber diameter	-	750	750	750	750	750
		Mixing ratio	%	60	60	80	50	20
Sheet	Fiber diameter ratio R/r		-	33.3	33.3	33.3	33.3	33.3
	Basis weight		g/cm2	100	300	25	25	25
	Thickness		mm	0.30	0.88	0.11	0.10	0.07
	Porosity		%	75.8	75.3	83.5	81.9	74.1
	Average pore size		µm	2.2	1.0	5.7	5.6	5.7
	Maximum frequency of pore size distribution		%	78.2	74.6	93.6	91.0	88.4
	Specific tensile strength		Nm/g	7.2	7.2	2.6	11.9	20.7

PET: polyethylene terephthalate

[Table 3]

				Example 17	Example 18	Example 19	Example 20	Example 21
Constituent fiber	Fiber having fiber diameter R	Polymer	-	PET	PET	PET	PET	PET
		Fiber diameter R	µm	15.0	20.0	10.0	10.0	10.0
		Fiber length	mm	5.0	5.0	5.0	5.0	5.0
		Fiber length/fiber diameter	-	333	250	500	500	500
		Mixing ratio	%	30	30	30	30	30
	Fiber having fiber diameter r	Polymer	-	PET 1	PET 1	PET 2	PET 1	PET 1
		Fiber diameter r	µm	0.2	0.2	0.2	0.3	0.3
		Fiber length	mm	0.6	0.6	0.6	1.2	1.8
		Fiber length/fiber diameter	-	3,000	3,000	3,000	4,000	6,000
		Mixing ratio	%	5	5	5	5	5
	Other fibers	Polymer	-	PET	PET	PET	PET	PET
		Fiber diameter	µm	4.0	4.0	4.0	4.0	4.0
		Fiber length	mm	3.0	3.0	3.0	3.0	3.0
		Fiber length/fiber diameter	-	750	750	750	750	750
		Mixing ratio	%	65	65	65	65	65
Sheet	Fiber diameter ratio R/r		-	75.0	100.0	50.0	33.3	33.3
	Basis weight		g/cm2	25	25	25	25	25
	Thickness		mm	0.10	0.11	0.10	0.11	0.11
	Porosity		%	81.9	83.5	81.9	83.5	83.5
	Average pore size		µm	4.9	6.1	4.1	7.5	7.9
	Maximum frequency of pore size distribution		%	89.7	90.8	94.5	78.7	70.3
	Specific tensile strength		Nm/g	8.2	9.4	6.7	7.3	8.5

PET: polyethylene terephthalate

[Table 4]

				Comparative example 1	Comparative example 2	Comparative example 3
Constituent fiber	Fiber having fiber diameter R	Polymer	-	PET	PET	PET
		Fiber diameter R	µm	10.0	10.0	10.0
		Fiber length	mm	5.0	5.0	5.0
		Fiber length/fiber diameter	-	500	500	500
		Mixing ratio	%	30	30	30
	Fiber having fiber diameter r	Polymer	-	PET 3	PET 1	PET 1
		Fiber diameter r	µm	0.2	0.6	0.6
		Fiber length	mm	0.6	0.6	0.6
		Fiber length/fiber diameter	-	3,000	1,000	1,000
		Mixing ratio	%	5	5	20
	Other fibers	Polymer	-	PET	PET	PET
		Fiber diameter	µm	4.0	4.0	4.0
		Fiber length	mm	3.0	3.0	3.0
		Fiber length/fiber diameter	-	750	750	750
		Mixing ratio	%	65	65	50
Sheet	Fiber diameter ratio R/r		-	50.0	16.7	16.7
	Basis weight		g/cm2	25	25	25
	Thickness		mm	0.12	0.11	0.1
	Porosity		%	84.9	83.5	81.9
	Average pore size		µm	6.0	14.8	4.6
	Maximum frequency of pore size distribution		%	52.1	83.2	94.4
	Specific tensile strength		Nm/g	6.5	3.2	3.6

PET: polyethylene terephthalate

[0134] Although the present invention has been described in detail with reference to specific embodiments, it is apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and the scope of the present invention. The present application is based on the Japanese patent application (JP2021-008595) filed on January 22, 2021, and the contents thereof are incorporated herein by reference.

REFERENCE SIGNS LIST

[0135] 

1: fiber diameter distribution of fiber (fiber having fiber diameter R) having maximum fiber diameter

2: fiber diameter distribution of fiber having intermediate fiber diameter

3: fiber diameter distribution of fiber (fiber having fiber diameter r) having minimum fiber diameter

Claims

1. A wet-laid nonwoven fabric sheet, comprising:

at least three types of thermoplastic fibers having different fiber diameters, wherein

the wet-laid nonwoven fabric sheet has a fiber diameter ratio (R/r) of a fiber diameter R of a fiber having a maximum fiber diameter to a fiber diameter r of a fiber having a minimum fiber diameter of 30 ≤ R/r ≤ 150, an average pore size of 0.10 µm to 15 µm, and a maximum frequency of a pore size distribution of 70% or more.

2. The wet-laid nonwoven fabric sheet according to claim 1, wherein
the fiber diameter r is 0.10 µm to 1.0 µm.

3. The wet-laid nonwoven fabric sheet according to claim 1 or 2, having a porosity of 70% or more.

4. The wet-laid nonwoven fabric sheet according to any one of claims 1 to 3, having a basis weight of 10 g/m² to 500 g/m².

5. The wet-laid nonwoven fabric sheet according to any one of claims 1 to 4, having a ratio (L/r) of a fiber length L to the fiber diameter r in the fiber having the minimum fiber diameter of 3,000 to 6,000.

6. A textile product at least partially comprising the wet-laid nonwoven fabric sheet according to any one of claims 1 to 5.

Drawing