Filter substrate

Abstract

To provide a filter base material capable of providing an air filter and a mask where breathing is easy by having low pressure loss and excellent air permeability and the reduction of collection efficiency after washing is low even when being reused after washing.SOLUTION: In a filter base material with a fiber layer, a non-fibrous material where the constitution fibers of the fiber layer are bonded each other exists on one main surface of the fiber layer. The breaking and cracking of the fiber layer are prevented since specular gloss on one main surface of the fiber layer is larger than 8.8 even when the average fiber diameter of the fiber layer of the filter base material is 400 nm or less. An air filter and a mask having low collection efficiency after washing can be provided. The air filter and the mask where breathing is easy by low pressure loss and excellent air permeability since specular gloss on one main surface of the fiber layer is smaller than 46.6 can be provided.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a filter substrate having a fibrous layer that can be washed to form a reusable filter. [Background technology]

[0002] Filters and masks have traditionally been required to have low pressure loss and excellent breathability, making them easy to breathe through, as well as excellent performance in capturing atmospheric dust, dust particles such as PM2.5, and pollen. In order to meet such demands, the applicant of the present application has proposed a method for producing an ultrafine fiber nonwoven fabric in which ultrafine fibers (in the examples, ultrafine fibers with an average fiber diameter of 0.4 μm) are bonded with non-fibrous binder particles, as described in JP 2008-285793 A (Patent Document 1). The ultrafine fiber nonwoven fabric can be used as a fiber layer constituting a filter substrate. Because the ultrafine fiber nonwoven fabric has the above-described structure, it has excellent shape retention and is difficult to crush, perhaps because it maintains its bulkiness, even when the amount of particles collected during use increases, and is therefore able to exhibit the desired performance. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2008-285793 (claims 0002, 0004, 0012, 0137, etc.)

[0004] In recent years, in order to extend the life of air filters installed in air conditioners and the like, there has been a demand for the ability to remove dust adhering to the air filter and reuse it. Furthermore, as the habit of wearing masks on a daily basis to prevent infectious diseases and other issues has emerged, there is a demand for masks that can be reused hygienically. To meet this demand, there is a demand for air filters and masks that can be washed, for example, so that adhering dust can be removed and they can be reused hygienically.

[0005] In order to realize an air filter or a mask that satisfies these demands, the applicant of the present application investigated an air filter or a mask that includes the filter substrate according to the above-mentioned conventional technology. However, the prepared air filter or mask sometimes has a significant decrease in filtering efficiency upon washing, and it has been difficult to realize an air filter or a mask that has high filtering efficiency even after washing.

[0006] To clarify the cause of this problem, the applicant examined the structural changes in the prepared filter substrate before and after washing. As a result of this examination, it was found that the constituent fibers of the fiber layer constituting the filter substrate after washing were sometimes cut, resulting in breaks and cracks, which were thought to cause a significant decrease in the collection efficiency. This problem tended to occur particularly noticeably in filter substrates having fiber layers made of thinner constituent fibers, specifically, in filter substrates having fiber layers with an average fiber diameter of 400 nm or less.

[0007] Therefore, there has been a demand for a filter substrate that can be used to realize air filters and masks that have low pressure loss and excellent breathability, making them easy to breathe through, and that also have little loss in collection efficiency even after washing. Summary of the Invention [Problem to be solved by the invention]

[0008] The applicant of the present application aims to realize a filter substrate that has a fibrous layer with non-fibrous material on one main surface, which has low pressure loss and excellent breathability, making it easy to breathe through, and which can be used to provide air filters and masks that show little decrease in collection efficiency after cleaning, even when washed and reused. [Means for solving the problem]

[0009] The first aspect of the present invention is "A filter substrate having a fiber layer with an average fiber diameter of 400 nm or less, The resin constituting the constituent fibers of the fiber layer is only a homopolymer of polyvinylidene fluoride, The fiber layer is made up of only continuous fibers, a non-fibrous material is present on one main surface of the fiber layer, bonding the constituent fibers of the fiber layer together; The specular gloss of one main surface of the fiber layer is 10.1 to 21.0 , Filter substrate. is. The second aspect of the present invention is "The filter substrate according to claim 1, which is used for a filter that can be washed and reused." is. [Effects of the Invention]

[0010] As a result of further investigations, the applicant of the present application has found that the problem can be solved even in a filter substrate having a fiber layer with an average fiber diameter of 400 nm or less by satisfying the configuration of the present invention.

[0011] In the filter substrate according to the present invention, a non-fibrous material is present on one main surface of the fiber layer constituting the filter substrate, bonding the constituent fibers of the fiber layer together. Therefore, even in a situation where the main surfaces of the fiber layers are likely to come into contact with each other after cleaning or the like, the presence of the non-cleaned material makes it difficult for the constituent fibers to come into contact with each other. Furthermore, because the constituent fibers are fixed to each other by the non-fibrous material, the constituent fibers of the fiber layer are difficult to cut, making them less likely to break or crack.

[0012] Furthermore, in the present invention, it has been discovered that by adjusting the configuration of the main surface of the fiber layer constituting the filter substrate on the side where the non-fibrous material is present, the constituent fibers of the fiber layer are less likely to be cut, thereby preventing the occurrence of breakage or cracks and also preventing an unintended increase in pressure loss.

[0013] Although the reason for this has not been fully clarified, it is believed that the above-mentioned effects are efficiently exhibited by adjusting the size and distribution of the non-fibrous materials present on the main surface or the state of the exposed surface of the non-fibrous materials, etc. In the present invention, it has been discovered that whether the state of the main surface is in a state in which the above-mentioned effects are efficiently exhibited can be determined by an evaluation method using specular gloss. In other words, a fiber layer having a main surface with a specular gloss value of greater than 8.8 is prevented from breaking or cracking, and a filter substrate having such a fiber layer can be used to provide air filters and masks that exhibit little decrease in collection efficiency after cleaning, even when washed and reused.

[0014] The applicant also discovered that a fiber layer having a main surface with a specular gloss value of 46.6 or higher unintentionally increases pressure loss, possibly due to the dense presence of non-fibrous materials. Therefore, in order to provide an air filter or mask that is easy to breathe due to low pressure loss and excellent breathability, the filter substrate needs to have a fiber layer with a specular gloss value of less than 46.6. As described above, the present invention makes it possible to realize a filter substrate that can provide air filters and masks that have low pressure loss and excellent breathability, making them easy to breathe through, and that show little decrease in collection efficiency even after cleaning and reuse. DETAILED DESCRIPTION OF THE INVENTION

[0015] In the present invention, various configurations can be appropriately selected, such as the following configurations. Note that, unless otherwise specified, the various measurements described in the present invention were performed under atmospheric conditions of normal pressure and 25°C. Furthermore, unless otherwise specified, the various measurement results described in the present invention were measured to a value one digit smaller than the desired value, and the desired value was calculated by rounding off the value one digit smaller. As a specific example, when the desired value is measured to one decimal place, the value was measured to two decimal places, and the obtained value was rounded off to one decimal place to calculate the desired value, and this value was used as the desired value. Furthermore, the upper and lower limits exemplified in the present invention can be combined in any combination.

[0016] The filter substrate of the present invention includes a fiber layer. The fiber layer serves to capture and retain particles in the filter substrate and also serves to form the framework of the filter substrate. The fiber layer refers to a layer composed of fibers, and can be, for example, a layer derived from a fiber web, a nonwoven fabric, or a sheet-like fabric such as a woven or knitted fabric. In particular, a fiber layer in which fibers are randomly entangled, such as a fiber layer derived from a fiber web or nonwoven fabric, is preferred because it allows the shape and size of the voids between the fibers to be uniform, thereby enabling the realization of a filter substrate that can provide a filter with excellent filtration performance.

[0017] In the present invention, non-fibrous materials are present on one main surface of the fiber layer, bonding the constituent fibers of the fiber layer together. The non-fibrous materials herein refer to objects other than fibers that have a diameter at least 10 times larger than the average fiber diameter of the fibers constituting the fiber layer. Specific examples include droplet- or film-like binder components, and objects (generally referred to as shot or beads) derived from the deposition of spinning solution that adheres to the collection surface in the form of droplets without forming fibers when direct spinning is used to prepare the fiber layer. The presence or absence of non-fibrous matter on one main surface of the fiber layer can be confirmed by the following method.

[0018] (Method for identifying non-fibrous materials) (1) Using an electron microscope, an electron microscope photograph (magnification: 500x, field of view: 200 μm vertically × 300 μm horizontally) of one main surface of the object to be measured (for example, a fiber layer provided in a filter substrate) is taken. (2) Randomly select 50 fibers from the fibers (constituent fibers of the object to be measured) shown in the electron microscope photograph, and calculate the average length (fiber diameter) of each selected fiber in the direction perpendicular to the longitudinal direction. The calculated value is the average fiber diameter. Note that if the fiber diameter is too small to measure, it can be measured based on an electron microscope photograph at a magnification of more than 5000 times. If the cross-sectional shape of the fiber is non-circular, the diameter of a circle with the same area as the cross-sectional area is considered to be the fiber diameter. (4) Assuming a perfect circle having a diameter 10 times the average fiber diameter calculated as described above, the electron microscope photograph is checked for the presence or absence of any object other than fibers that is larger than the perfect circle and contains multiple constituent fibers. If the above-mentioned object is present, it can be determined that a non-fibrous substance bonding the constituent fibers together is present on one main surface of the object to be measured.

[0019] The non-fibrous material may be present inside the fiber layer in addition to the surface including one of the main faces of the fiber layer.

[0020] The size of the non-fibrous material provided on one main surface of the fiber layer is adjusted to achieve a filter substrate with a fiber layer that is resistant to breakage and cracking, and can be a size equal to or larger than a perfect circle with a diameter five times the average fiber diameter, or a size equal to or larger than a perfect circle with a diameter ten times the average fiber diameter. The upper limit of the size can be adjusted as appropriate, but if the size is excessively large, there is a risk of unintentionally increasing the pressure loss of the filter substrate, so it is more realistic for the size of the non-fibrous material to be equal to or smaller than a perfect circle with a diameter 10,000 times the average fiber diameter.

[0021] Furthermore, in the present invention, one of the main surfaces of the fiber layer confirmed as described above (the main surface on which non-fibrous material that bonds the constituent fibers of the fiber layer to each other is present) is subjected to the following measurement method, and the specular gloss obtained is greater than 8.8 and less than 46.6.

[0022] (How to calculate specular gloss) (1) For one of the main surfaces of the fiber layer, the 75-degree specular gloss (Gs(75°)) is measured 10 times at any one location on the one main surface of the fiber layer using a portable glossmeter (model number: GMX-203, manufactured by Murakami Color Research Laboratory Co., Ltd.) in accordance with JIS Z8741 "Specular gloss - Measurement method." (2) Next, for any one of the locations, measure the 75-degree specular gloss (Gs(75°)) 10 times in a direction perpendicular to the direction in which the 75-degree specular gloss was measured when viewed from above the main surface. (3) Calculate the average value of the 20 measurements obtained. (4) The measurements (1) to (3) above are carried out at any 10 points on one main surface of the fiber layer, and the average values ​​of the measurements are calculated. (5) The average of the calculated average values ​​is taken as the specular gloss of one of the main surfaces of the fiber layer.

[0023] The specular gloss according to the present invention is preferably 9 to 40, more preferably 10 to 30, and even more preferably 10.1 to 21.0, so that the above-mentioned effects can be more effectively exhibited.

[0024] The specular gloss is thought to vary depending on the lamination state of the fibrous materials present on one main surface of the fiber layer, the size of each non-fibrous material, the CV value of the size of each non-fibrous material, the distribution state of each non-fibrous material, and the heat treatment method after lamination of the fiber layer. Therefore, when a spinning solution is supplied to an electrospinning device to form a fiber layer according to the present invention, it is affected by the spinning environment, such as the amount of spinning solution discharged from the discharge port and the spinning distance, and the type of collector (such as the unevenness of the fiber collection surface of the collector). In other words, even if fiber layers with the same basis weight and average fiber diameter can be prepared, the prepared fiber layers will not necessarily have the same specular gloss as long as they are prepared using different spinning environments or heat treatment methods.

[0025] Other detailed configurations will be described below.

[0026] Well-known organic resins can be used as the resins constituting the constituent fibers of the fiber layer according to the present invention (hereinafter sometimes abbreviated as constituent fibers). Examples include polyether resins (polyethylene glycol, polypropylene glycol, polyether ether ketone, etc.), phenolic resins, epoxy resins, polyimide resins, polyamideimide resins, polyamide resins (e.g., aromatic polyamide resins such as aramid resins, aromatic polyetheramide resins, nylon resins, etc.), urethane resins, epoxy resins, polysulfone resins (e.g., polysulfone), polyethersulfone resins (e.g., polyethersulfone, sulfonated polyethersulfone, etc.), fluorine-based resins (polytetrafluoroethylene, homopolymers of polyvinylidene fluoride, polyisopropyl methyl acrylate, etc.), and the like. The resin may be a known resin such as a copolymer of vinylidene difluoride, a perfluorosulfonic acid resin, etc.), a vinyl alcohol resin (polyvinyl alcohol, polyvinyl butyral, polyvinyl acetate, etc.), polycaprolactone, polyglycolic acid, polyvinylpyrrolidone, a polybenzimidazole resin, or an acrylic resin (for example, a polyacrylonitrile resin copolymerized with an acrylic acid ester or a methacrylic acid ester, or a modacrylic resin copolymerized with acrylonitrile and vinyl chloride or vinylidene chloride), and may be one type of resin or a mixture of multiple types of resins.

[0027] These resins may be either linear or branched polymers, or may be block or random copolymers, and may have any three-dimensional structure or may have any crystallinity.

[0028] However, in order to realize a filter substrate in which the fiber layer has a higher strength, thereby preventing deformation even during cleaning and making the fiber layer less susceptible to breakage or cracking, it is preferable that the resin constituting the fiber is a polyvinylidene fluoride homopolymer (hereinafter sometimes referred to as PVDF homopolymer), and more preferably, the resin constituting the fiber is solely PVDF homopolymer. Note that polyvinylidene fluoride here refers to a polymer containing a -(CHCF)- structure within its structure. Furthermore, PVDF homopolymer refers to a molecular structure in which this structure is continuously formed.

[0029] The molecular weight of the PVDF homopolymer can be selected appropriately. The molecular weight may be 300,000 or more, 380,000 or more, 570,000 or more, 750,000 or more, or 780,000 or more. The PVDF homopolymer fiber may also be a mixture of PVDF homopolymers with different molecular weights. The "molecular weight" referred to in the present invention is a value measured by gel permeation chromatography. When the molecular weight of the PVDF homopolymer used is described in a catalog or paper, that molecular weight can be used as the molecular weight of the PVDF homopolymer.

[0030] The strength of the PVDF homopolymer can be selected as appropriate. However, the higher the strength of the PVDF homopolymer, the more likely it is that a filter substrate can be realized that can provide a filter with high collection efficiency even after washing. Therefore, it is preferable to use a PVDF homopolymer that can be prepared into a film having a breaking strength of 30 MPa or more, as measured according to the standard method of ASTM D638 Plastic Tensile Testing Method, more preferably a PVDF homopolymer that can be prepared into a film having a breaking strength of 40 MPa or more, and even more preferably a PVDF homopolymer that can be prepared into a film having a breaking strength of 50 MPa or more.

[0031] A small average fiber diameter of the constituent fibers contributes to the realization of a filter substrate capable of providing a filter with excellent filtration performance. From this perspective, the average fiber diameter of the fiber layer constituting the filter substrate is 400 nm or less, preferably 350 nm or less, preferably 300 nm or less, 250 nm or less, and preferably 200 nm or less. On the other hand, if the average fiber diameter of the constituent fibers is too small, the constituent fibers of the fiber layer are easily cut during cleaning, causing many breaks and cracks, making it difficult to realize a filter substrate capable of providing a filter with excellent filtration performance. From this perspective, the average fiber diameter of the constituent fibers is preferably greater than 130 nm, preferably greater than 150 nm.

[0032] The average fiber diameter of the constituent fibers can be calculated from the arithmetic mean value of the fiber diameters of 50 fibers included in the measurement sample. Here, "fiber diameter" refers to the length in the direction perpendicular to the longitudinal direction of the fiber, measured based on an electron microscope photograph of the fiber at 5000x magnification. If the fiber diameter is too small to measure, it can be measured based on an electron microscope photograph at a magnification higher than 5000x. If the cross-sectional shape of the fiber is non-circular, the diameter of a circle with the same area as the cross-sectional area was considered to be the fiber diameter.

[0033] The fiber length of the constituent fibers is selected appropriately, but can be short or long fibers having a specific length, or continuous fibers having a fiber length that is practically difficult to measure. A small number of fiber ends in the fiber layer results in a smooth surface, a uniform thickness, and excellent physical properties such as mechanical strength, resulting in a filter substrate that can provide a filter with better filtration performance. Therefore, it is preferable for the constituent fibers to contain fibers having a continuous length, and more preferably, the constituent fibers are only continuous fibers. The "fiber length" referred to in the present invention can be measured based on a 5000x electron microscope photograph of the cross section or surface of the object to be measured. If the fiber length is too long to measure, it can be measured based on an electron microscope photograph at a magnification lower than 5000x.

[0034] The constituent fibers may be monofilaments, fibrillar fibers, or composite fibers. Examples of composite fibers include sheath-core, island-in-sea, side-by-side, orange, and bimetal fibers. The constituent fibers may have a cross-sectional shape that is not only approximately circular or elliptical, but also irregular. Examples of irregular cross-sectional fibers include fibers with a hollow cross section, a polygonal cross section such as a triangular cross section, an alphabetic shape such as a Y-shape, an irregular shape, a multi-lobed cross section, a symbolic shape such as an asterisk, or a cross section formed by combining multiple of these shapes.

[0035] The method for preparing the constituent fibers can be appropriately selected, and known methods can be used, such as melt spinning, dry spinning, wet spinning, direct spinning (melt-blowing, spunbonding, electrostatic spinning in which an electric field is applied to a spinning solution to spin the fibers), spinning using centrifugal force, spinning using an accompanying airflow as described in JP 2011-012372 A, and a method of extracting fibers with a small fiber diameter by removing one or more resin components from composite fibers.

[0036] Fibers prepared using the above-described methods can be subjected to, for example, a dry or wet process to prepare a fiber web, and the constituent fibers of the prepared fiber web can be entangled and / or integrated to prepare a nonwoven fabric. Examples of methods for entangling and / or integrating the constituent fibers include entanglement using needles or a fluid flow such as water or steam / gas, and heat treatment of the fiber web to bond or fuse the constituent fibers together using a binder or adhesive fiber. The heat treatment method can be selected as appropriate, and examples include heating and pressurizing using a calendar roll, heating using a hot air dryer, heating the polymer contained in the web by irradiating it with infrared rays without pressure, and heating along a heated roll.

[0037] In particular, the method of heat treatment along the heating rolls suppresses wrinkles that occur during heat treatment, reduces the influence of breakage of the fiber web caused by the calendar rolls, and makes the surface flat. In addition to the fiber web, a nonwoven fabric may also be subjected to the above-mentioned method of entangling and / or integrating the constituent fibers.

[0038] Furthermore, by collecting spun fibers on a collector using direct spinning (particularly electrospinning), fiber webs and nonwoven fabrics composed only of continuous fibers can be prepared. Furthermore, by using direct spinning (particularly electrospinning), fiber webs and nonwoven fabrics having a film-like or granular non-fibrous material called shot or beads attached to one main surface, which bonds the constituent fibers together, can be easily prepared. As a result, it is easy to realize a filter substrate having a fiber layer according to the present invention, which is preferable.

[0039] Then, fibers spun using a direct spinning method (particularly electrospinning) can be laminated onto another substrate such as a spunbond nonwoven fabric to prepare a laminate in which a fiber web or nonwoven fabric is directly formed on the other substrate, with non-fibrous material adhering the constituent fibers to each other. A filter substrate can also be prepared using the laminate prepared in this manner. In this case, if the other substrate (particularly the spunbond nonwoven fabric) contains an oil (particularly a nonionic surfactant), the directly spun (particularly electrospun) fibers are uniformly dispersed, which allows the preparation of a filter substrate with excellent collection efficiency. It is noted that the surface resistance of the main surface of the other substrate containing the oil used in this case should be 5.0 × 10. 14 It is preferable to adjust the type and amount of oil so that the surface resistance is 5.0×10 14 Less than 1.0×10 is preferable. 13 Ω is more preferred.

[0040] By directly spinning (particularly electrospinning) the fibers onto another substrate having such a surface resistance on the main surface, the spun fibers can be uniformly dispersed and deposited on the other substrate, which is preferable because it allows the preparation of a filter substrate with excellent collection efficiency.

[0041] The various physical properties of the fiber layer, such as basis weight and thickness, can be selected appropriately.

[0042] First, when each layer (such as a fiber layer) constituting the filter substrate can be easily obtained, a layer made of fibers, such as a fiber web or nonwoven fabric layer, is selected as the fiber layer, and the thickness and basis weight of the fiber layer are directly measured to determine the thickness and basis weight of the fiber layer constituting the filter substrate.

[0043] The thickness of the fiber layer may be 0.1 μm to 20 μm, 0.2 μm to 15 μm, or 1 μm to 10 μm. The "thickness" in the present invention refers to the average value of 10 randomly selected measurements taken using an outside micrometer (0 to 25 mm) as specified in JIS B7502:1994.

[0044] For example, the basis weight is 0.05 to 5 g / m 2 and the density can be 0.1 to 3 g / m 2 and the density can be 0.2 to 2 g / m 2 It can be.

[0045] The thickness of the base layer of the fiber layer can be, for example, 5 μm to 500 μm, 6 μm to 400 μm, 7 μm to 300 μm, or 8 μm to 200 μm. Note that the "thickness" referred to in the present invention refers to the average value of 10 randomly selected points measured using an outside micrometer (0 to 25 mm) specified in JIS B7502:1994 according to the measurement method of JIS C2111 5.1(1).

[0046] For example, the basis weight is 0.5 to 100 g / m 2 and the thickness can be 1.0 to 80 g / m 2 and 1.5 to 60 g / m 2 and 2.0 to 40 g / m 2 The "weight per unit area" in the present invention means a value measured in an area of ​​10 cm x 10 cm in accordance with JIS L1085.

[0047] On the other hand, when each layer (such as a fiber layer) constituting the filter substrate cannot be easily obtained, the thickness of the fiber layer can be determined by the following method. (1) Take an electron microscope photograph (for example, at a magnification of 200x) of a cross section of the filter substrate cut in the thickness direction. The magnification should be adjusted so that the entire thickness of the filter substrate can be seen. (2) The electron microscope photograph is visually inspected to measure the thickness of each layer constituting the filter substrate. The "thickness" here refers to the length of the shortest distance from the intersection (A) of one main surface of each layer with the line segment connecting the two main surfaces of the filter substrate in the electron microscope photograph to the intersection (B) of the other main surface with the line segment connecting the two main surfaces of the filter substrate in the shortest distance. (3) Among the layers, a layer made of fibers, such as a fiber web or nonwoven fabric layer, is defined as a fiber layer, and the length of the shortest distance between intersection point (A) and intersection point (B) of the fiber layer is determined using the method in step (2) described above, and this is defined as the thickness of the fiber layer.

[0048] When each layer (such as a fiber layer) constituting the filter substrate cannot be easily obtained, the basis weight of the fiber layer can be determined by the following method. (1) Take an electron microscope photograph (for example, at a magnification of 200x) of a cross section of the filter substrate cut in the thickness direction. The magnification should be adjusted so that the entire thickness of the filter substrate can be seen. (2) The electron microscope photograph is visually inspected to determine whether the filter substrate has a fiber layer. (3) If the filter substrate has a fiber layer, the components constituting the fiber layer and other layers are confirmed. The components constituting the fiber layer (for example, the type of resin constituting the fibers constituting the fiber layer and the type of binder bonding the fibers together) and the components constituting other layers can be determined by subjecting the fiber layer and other layers sampled from the filter substrate to various known analytical devices and analytical methods, such as IR, DSC, NMR, MS, Raman spectroscopy, elemental analysis, and combustion tests. (4) A solvent capable of dissolving layers other than the fiber layer (other layers) without dissolving the fiber layer (for example, polar solvents such as DMF, DMAc, or NMP can be used) is selected, and the filter substrate is immersed in the solvent to dissolve the layers other than the fiber layer in the solvent. The residue is removed from the solvent, and the basis weight of the residue after solvent removal is taken as the basis weight of the fiber layer. Note that if there is no solvent that can dissolve layers other than the fiber layer (other layers) without dissolving the fiber layer, a solvent capable of dissolving only the fiber layer is selected, and the filter substrate is immersed in the solvent to dissolve only the fiber layer in the solvent. The residue is removed from the solvent, and the basis weight of the residue after solvent removal is subtracted from the basis weight of the filter substrate, and the calculated value is taken as the basis weight of the fiber layer.

[0049] In addition, when the materials used to prepare the filter substrate (for example, nonwoven fabrics or fiber webs used to form fiber layers) are known, the thickness and basis weight of each material can be directly measured in the same manner as described above in "when each layer (such as a fiber layer) constituting the filter substrate can be easily peeled off and obtained," and the measured values ​​can be regarded as the thickness and basis weight of the fiber layer constituting the filter substrate.

[0050] The fiber layer according to the present invention can be used alone as a filter substrate, but if necessary, a filter substrate may be prepared by laminating a separately prepared substrate (e.g., a sheet-like fabric such as a spunbond nonwoven fabric, a porous film, etc.) on the fiber layer.

[0051] The method for laminating the fiber layer and the substrate can be selected as appropriate, and examples of methods that can be used include simply overlapping the layers, partially melt-bonding the components of the fiber layer and / or the substrate, laminating them together using a binder, and accumulating spun fibers on the main surface of the substrate to form a fiber layer on the substrate to prepare a laminate.

[0052] In particular, when the substrate contains sheath-core fibers, the sheath component of the sheath-core fibers can firmly melt-bond the fiber layer to the substrate, thereby effectively protecting the fiber layer with the substrate. As a result, it is possible to effectively prevent breakage or cracking in the fiber layer, and to provide a filter substrate that can provide air filters and masks with little loss in filtering efficiency even when washed and reused. Examples of such substrates include spunbond nonwoven fabrics and dry-laid nonwoven fabrics made of sheath-core fibers, as well as wet-laid nonwoven fabrics.

[0053] Furthermore, the substrate preferably has high strength so that it can effectively protect the fiber layer. The "strength" of the present invention can be measured by subjecting a fiber assembly to the method described below. Specifically, to improve washing durability, the substrate preferably has a strength of 2 N / 50 mm or more in both the MD and CD directions, preferably 3 N / 50 mm or more, more preferably 10 N / 50 mm or more, and more preferably 15 N / 50 mm or more.

[0054] On the other hand, although the upper limit can be adjusted as appropriate, it is practical to use a substrate with a strength of less than 500 N / 50 mm. The strengths of the substrate in the MD and CD directions can be determined by the following method.

[0055] (Strength measurement method) (1) A rectangular sample (short side: 50 mm, long side: 200 mm) is taken from the object to be measured. If the production direction of the object to be measured is known, the sample is taken so that the production direction and the long side direction of the object to be measured are parallel. If the production direction of the object to be measured is unknown, multiple samples are taken from various directions of the object to be measured so that the main surface of the object to be measured and the long side of the sample are parallel, and each sample is subjected to the maximum stress measurement described below. The direction of the object to be measured that is parallel to the long side of the sample that showed the highest maximum stress is determined to be the production direction of the object to be measured. (2) Using a tensile tester (manufactured by Orientec Co., Ltd., trade name: Tensilon (registered trademark), TM-111-100), the sample is pulled in the long side direction at a gripping distance of 100 mm and a pulling speed of 50 mm / min until it breaks. (3) The maximum stress measured until the sample breaks is taken as the "strength." In this way, the strength (N / 50mm) in the direction parallel to the long side of the sample (MD direction) is measured. (4) A new rectangular sample (short side: 50 mm, long side: 200 mm) is taken from the measurement object so that the short side direction is parallel to the direction parallel to the long side direction of the sample on the measurement object. At this time, if the production direction of the measurement object is known, the sample is taken so that the production direction and the short side direction of the measurement object are parallel. (5) The sample collected in step (4) above is subjected to steps (2) and (3) to measure the strength (N / 50 mm) in the direction parallel to the long side direction of the sample (CD direction) in the fiber aggregate.

[0056] The fiber layer or the laminate of fiber layers produced as described above may be subjected to various secondary processes, such as a pressure treatment such as a reliant press treatment to adjust the thickness, a hydrophilization treatment such as a sulfonation treatment, a plasma treatment or a fluorine gas treatment, or punching or molding, depending on the intended use or mode of use.

[0057] Next, a method for producing a filter substrate according to the present invention will be described by way of an example of production, with the explanation of the same configuration as that described above being omitted. (1) A step of preparing a spinning solution prepared by dissolving a resin constituting a constituent fiber in a solvent, or a spinning solution prepared by dispersing the resin in a dispersion medium; (2) A step of feeding the spinning solution to an electrospinning device, spinning the resulting fibers by thinning the spinning solution, and collecting the resulting fibers, and at the same time, attaching the spinning solution in the form of droplets to the collected constituent fibers to prepare a fiber web; (3) removing the solvent or dispersion medium remaining in the fiber web to prepare a nonwoven fabric capable of constituting the fiber layer of the present invention; A method for manufacturing a filter substrate comprising the steps of:

[0058] First, step (1) will be described.

[0059] The type of solvent or dispersion medium is appropriately selected, and examples thereof include N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, acetonitrile, dimethyl sulfoxide, 1,4-dioxane, pyridine, formic acid, etc. The solvent or dispersion medium may be one type, or a mixed solvent or mixed dispersion medium containing a mixture of multiple types.

[0060] The concentration and type of resin contained in the spinning solution are adjusted as appropriate to prepare the filter substrate of the present invention. The spinning solution may contain a salt. The inclusion of a salt improves the conductivity of the spinning solution, preventing the formation of unintended non-fibrous materials when the solution is fed to an electrospinning device and spun. This allows for the production of a fiber web composed of thin fibers with uniform fiber diameters. The type of salt contained in the spinning solution can be selected as appropriate, but it is preferable to use a salt composed of an organic acid and a nitrogen compound that volatilizes when fed to step (3) described below so that salt is less likely to remain in the fiber web. Examples of such salts include salts composed of an organic acid and a nitrogen compound that volatilize upon heat treatment, such as ammonium acetate, ammonium formate, and ammonium oxalate. For example, it is preferable to use ammonium acetate, ammonium formate, and ammonium oxalate.

[0061] The mass of salt contained in the spinning solution (solids mass) is appropriately selected so as to prepare the desired fiber layer, but if the mass of salt contained in the spinning solution is too high, such as 30 mass% or more relative to the resin mass, the spinning solution may gel, making it impossible to prepare the desired fiber. Therefore, the mass of salt contained in the spinning solution is preferably 30 to 0.05 mass% relative to the resin mass, preferably 10 to 0.1 mass%, and more preferably 7.5 to 0.2 mass%.

[0062] The temperature and viscosity of the spinning solution are appropriately selected so that the desired fiber layer can be prepared. The temperature of the spinning solution can be 5 to 40°C, 10 to 35°C, or 15 to 30°C. The viscosity of the spinning solution can be 0.05 to 8 Pa·s, 0.1 to 6 Pa·s, or 0.2 to 5 Pa·s. This "viscosity" is measured using a viscosity measuring device at a temperature of 25°C with a shear rate of 100 s -1 The time value.

[0063] Next, step (2) will be described.

[0064] The method of spinning by reducing the diameter of the spinning solution is appropriately selected so as to prepare the desired fiber layer, and for example, direct spinning can be used. When electrostatic spinning is used as the direct spinning method, a voltage is applied to the spinning solution, and an opposite voltage is applied to a counter electrode such as a metal plate located at a distance from the discharge portion of the spinning solution, causing the spinning solution to fly toward the counter electrode and reduce the diameter. The reduced diameter spinning solution is then collected on a collector to form a fiber web on the collector.

[0065] Although the counter electrode such as the metal plate described above may be used as the collector, a laminate of the substrate and the fiber web can be prepared by forming a fiber web on a substrate such as a spunbond nonwoven fabric laid on the counter electrode. By subjecting the thus prepared laminate of the substrate and the fiber web to the next step, a filter substrate consisting of a laminate of the substrate and the fiber layer can be easily prepared, which is preferable.

[0066] Alternatively, a filter substrate consisting of a laminate of a substrate and a fiber layer may be prepared by peeling the fiber web formed on the collector from the collector and laminating it on a substrate. In this case, it is preferable to laminate the fiber web so that the main surface of the fiber web opposite to the main surface that was in contact with the collector faces the main surface of the substrate, so that a filter substrate consisting of a laminate of a substrate and a fiber layer having a smooth surface can be prepared.

[0067] In addition, a method for attaching the discharged spinning solution in the form of droplets to the collected constituent fibers can be appropriately selected. Examples include adjusting the distance between the spinning solution discharge part and the collector, adjusting the amount of spinning solution discharged from the discharge part, adjusting the voltage applied to the spinning solution, and adjusting the temperature and humidity of the spinning space. In addition to thinning the spinning solution to form fibers, the spinning solution can be attached in the form of droplets to the collected constituent fibers.

[0068] Next, step (3) will be explained.

[0069] The method for removing the solvent or dispersion medium remaining in the fiber web can be selected as appropriate, and one example is a method in which the fiber web is subjected to a heating device. The type of heating device can be selected as appropriate, and for example, a method using a device that heats or heats and pressurizes with a roll, an oven dryer, a far-infrared heater, a dry heat dryer, a hot air dryer, or a device that can heat by irradiating infrared rays can be used. The heating temperature of the heating device is selected as appropriate, and is appropriately adjusted so that the remaining solvent or dispersion medium can be volatilized and removed, and that the constituent components such as the constituent fibers do not unintentionally decompose or denature.

[0070] Among the above-mentioned heating methods, the method of heat-treating the fiber web along a heating roll is preferred because it allows the preparation of a filter substrate with a smooth surface. When an adhesive component or a crosslinkable resin is present in the constituent fibers of the fiber web, the fiber web may be subjected to a heating device to bond the fibers with the adhesive component or to crosslink the crosslinkable resin.

[0071] Furthermore, when a salt composed of an organic acid and a nitrogen compound that volatilizes upon heat treatment is present in the constituent fibers of the fiber web, the salt may be volatilized and removed by subjecting the fiber web to heat treatment. By volatilizing and removing the salt through heat treatment, a filter substrate having a fiber layer composed of fibers and non-fibrous materials that are substantially free of salt can be prepared.

[0072] In addition to the methods for preparing the fiber layer listed in the above-mentioned manufacturing method, a fiber web or nonwoven fabric having non-fibrous material on one main surface that bonds the constituent fibers of the fiber layer may be prepared by applying binder particles or a dispersion medium containing a binder to one main surface of a prepared fiber web and dissolving the binder. Alternatively, a binder solution may be applied to one main surface of a prepared fiber web and the solvent of the binder solution may be removed to prepare a fiber web or nonwoven fabric having non-fibrous material on one main surface that bonds the constituent fibers of the fiber layer.

[0073] Alternatively, a binder contained in the fiber web may be melted to prepare a fiber web or nonwoven fabric having non-fibrous material on one main surface thereof that bonds the constituent fibers of the fiber layer together.

[0074] The prepared fiber layer can be used alone as a filter substrate, but if necessary, a separately prepared substrate (e.g., a sheet-like fabric such as a spunbond nonwoven fabric, a porous film, etc.) may be laminated on the fiber layer to prepare a filter substrate.

[0075] The method for laminating the fiber layer and the substrate can be selected as appropriate, and examples of methods that can be used include simply overlapping the layers, partially melt-bonding the components of the fiber layer and / or the substrate, laminating them together using a binder, and accumulating spun fibers on the main surface of the substrate to form a fiber layer on the substrate to prepare a laminate.

[0076] The fiber layer or the laminate of fiber layers produced as described above may be subjected to various secondary processes, such as a pressure treatment such as a reliant press treatment to adjust the thickness, a hydrophilization treatment such as a sulfonation treatment, a plasma treatment or a fluorine gas treatment, or punching or molding, depending on the intended use or mode of use.

[0077] The filter substrate manufactured as described above can be used to prepare a filter that can be washed and reused. Specifically, a three-layer filter can be prepared in which a filter substrate is provided between woven or knitted fabrics, and a mask can be prepared using the three-layer filter in the facepiece. Alternatively, an air filter can be prepared using a filter equipped with a filter substrate. [Example]

[0078] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0079] Example 1 PVDF homopolymer (a resin capable of producing a film with a breaking strength of 45 MPa as measured according to the standard method of ASTM D638 plastic tensile testing method) was dissolved in dimethylformamide (boiling point: 153°C) to prepare a spinning solution with a solids concentration of 14% by mass. The spinning solution was then electrospun onto a grounded conveyor equipped with a substrate on the fiber collecting surface side, and collected on the substrate to prepare a laminated web in which a fiber web made of continuous fibers and a substrate were laminated. Base material: Spunbond nonwoven fabric (basis weight: 15 g / m2) made of core-sheath fibers with a core of polyester (melting point: 260°C) and a sheath of polyethylene (melting point: 130°C). 2 , MD direction strength: 38N / 50mm, CD direction strength: 16N / 50mm, oil agent is applied and the electrical resistance on the main surface of the fiber collection side is 3.5 × 10 12 Ω) The prepared laminated web was heat-treated by running it along a heated roll with a surface temperature adjusted to 160°C to remove any remaining solvent from the laminated web and melt the sheath component of the core-sheath fibers in the spunbonded nonwoven fabric, thereby bonding and integrating the fiber web and the substrate to prepare a nonwoven fabric.

[0080] (Comparative Example 1) A nonwoven fabric was prepared in the same manner as in Example 1, except that the prepared laminated web was naturally dried in an atmosphere at room temperature without using a heating roll.

[0081] Example 2 A fiber web consisting of continuous fibers was prepared by electrospinning on a grounded conveyor not equipped with a substrate and collecting the fibers on the conveyor. The prepared fiber web was then peeled off the conveyor, and the surface exposed on the conveyor was exposed and laminated on the above-mentioned substrate (spunbond nonwoven fabric). In this way, a laminated web consisting of a fiber web and a substrate was prepared. A nonwoven fabric was prepared in the same manner as in Example 1, except that the laminated web prepared in this manner was subjected to a heating roll.

[0082] Example 3 A nonwoven fabric was prepared in the same manner as in Example 1, except that the basis weight of the produced fiber web was increased.

[0083] In addition, on the main surface of each nonwoven fabric, where the fibers spun using the electrospinning method were deposited to form a fiber layer, non-fibrous materials (PVDF homopolymer non-fibrous materials) were present, bonding the constituent fibers of the fiber layer together.

[0084] The structure and properties of each of the prepared nonwoven fabrics used as filter substrates are summarized in Table 1. In the "Collector" column in the table, if the spun fiber was collected on a spunbond nonwoven fabric, it is recorded as "SB," and if it was collected on a conveyor, it is recorded as "Conveyor." The "Specular Gloss" column in the table shows the results of measuring the specular gloss of the main surface of the nonwoven fabric on which the fibers spun using electrospinning were deposited to form a fiber layer.

[0085] The initial pressure loss and collection efficiency before and after washing were determined using the following measurement methods.

[0086] (Method for measuring collection efficiency and initial pressure loss before washing) A test piece was collected from the filter substrate. The collected test piece was then attached to a measuring device "AP-9000" manufactured by Shibata Scientific Co., Ltd. The test piece was attached to the measuring device so that the main surface of the filter substrate on which the fibers spun using the electrospinning method were deposited and a fiber layer was formed faced the upstream side. First, the effective filtration area of ​​the test piece is 44 cm 2 Adjust the test flow rate to 40 liters per minute (for example, if the effective filtration area is 4.4 cm). 2 The test flow rate supplied to the test piece was 4 liters per minute, and the differential pressure between the upstream and downstream of the test piece was measured, and the initial pressure loss (unit: Pa) of the test piece was calculated from the measured differential pressure. Note that the lower the initial pressure loss value of a filter substrate, the more breathable the filter or mask that can be provided. Next, the effective filtration area of ​​the test piece is 44 cm2 Adjust the test flow rate to 30 liters per minute (for example, if the effective filtration area is 4.4 cm). 2 The test flow rate supplied to the test piece was 3 liters per minute, and sodium chloride particles (median particle size distribution: 0.06 to 0.10 μm, geometric standard deviation: 1.8 or less) were supplied at a concentration of 50 mg / m 3 A test airflow containing the following (concentration fluctuation: ±15% or less) was supplied upstream of the test piece. After supplying the test airflow for one minute, the concentrations of the sodium chloride particles present on the upstream and downstream sides of the test piece were measured using a light scattering dust concentration meter, and the concentration of sodium chloride particles captured on the test piece was calculated from the two measured concentrations. The percentage of the concentration of sodium chloride particles trapped on the test piece relative to the concentration of sodium chloride particles supplied upstream of the test piece was calculated, and this value was taken as the collection efficiency (unit: %) of the test piece. Note that the higher the collection efficiency value of a filter substrate, the more excellent the dust collection performance of a filter or mask that can be provided. The initial pressure loss and collection efficiency measured as described above were taken as the initial pressure loss and collection efficiency of the filter substrate before washing.

[0087] (Method for measuring collection efficiency and initial pressure loss after washing) Another test piece was taken from the filter substrate. The test piece was then sandwiched between moisture-wicking, quick-drying polyester knit fabrics and welded around the edges to create a three-layer mask. The prepared mask was then placed in a household washing machine with commercially available laundry detergent, washed, and then dehydrated. The mask was washed in this manner 10 times, and finally, the mask was allowed to air dry after dehydration. After air-drying, the other test piece was removed from the mask and subjected to the above-mentioned (method for measuring collection efficiency and initial pressure loss before washing) to similarly determine the initial pressure loss (unit: Pa) and collection efficiency (unit: %). The initial pressure loss and collection efficiency measured as described above were taken as the initial pressure loss and collection efficiency of the filter substrate after washing.

[0088] Breathability was evaluated using the following method. (1) A mask was created in which a filter substrate was used for the mask body and ear loops were attached to both the left and right ends of the mask body. (2) The masks were worn by a person and the ease of breathing was evaluated. The main surface of the filter substrate, on which the fibers spun using the electrospinning method and the fiber layer were deposited, faced the face when the mask was worn. (3) The ease of breathing when wearing a mask made using the filter substrate prepared in Comparative Example 2 was used as the standard and was evaluated as "X." Filter substrates that could be used to prepare masks that were easier to breathe through were evaluated as "Good."

[0089] Table 1 shows the physical properties of the filter substrates prepared as described above.

[0090] [Table 1]

[0091] The filter substrate of Comparative Example 1 (having a fiber layer with a main surface having a specular gloss value of 8.8) showed a decrease in the initial pressure drop after washing. This was thought to be because the constituent fibers were cut or dropped off during washing, causing breaks or cracks in the fiber layer.

[0092] On the other hand, the filter substrates of Examples 1 to 3 (which had a fiber layer with a main surface having a specular gloss value of greater than 8.8) did not show a decrease in the initial pressure loss value after washing. In other words, it was considered that Examples 1 to 3 were filter substrates with fiber layers that were prevented from breaking or cracking. Therefore, Examples 1 to 3 showed less decrease in collection efficiency after washing than the filter substrate of Comparative Example 1.

[0093] (Comparative Example 2) A nonwoven fabric was prepared in the same manner as in Example 2, except that the discharge amount was increased.

[0094] The physical properties of the filter substrates prepared as described above are summarized in Table 2. For ease of understanding, Table 2 also lists examples.

[0095] [Table 2]

[0096] The filter substrate of Comparative Example 2 (having a fibrous layer with a main surface having a specular gloss value of 46.6) was inferior in breathability. This was thought to be due to the dense presence of non-fibrous materials or the unintentional increase in initial pressure loss caused by the presence of non-fibrous materials.

[0097] On the other hand, the filter substrates of Examples 1 to 3 (which have a fiber layer with a main surface having a specular gloss value of less than 46.6) had low initial pressure loss and were excellent in breathability.

[0098] From the above, it has been found that a filter substrate satisfying the configuration of the present invention can provide air filters and masks that have low pressure loss and excellent breathability, making them easy to breathe through, and that, even when washed and reused, show little decrease in collection efficiency after washing. [Industrial Applicability]

[0099] The present invention relates to a filter substrate having a fiber layer that constitutes a washable and reusable filter. The use of this filter substrate can provide washable and reusable masks (e.g., two-dimensional sheet-shaped masks, three-dimensional corrugated or pleated masks, bifold masks, and cup-shaped masks), washable and reusable air filters for air purifiers and air conditioners (e.g., two-dimensional sheet-shaped filters, three-dimensional corrugated or pleated filters, and depth filters). Liquid filters can also be prepared using the filter substrate according to the present invention.

Claims

1. A filter substrate having a fiber layer with an average fiber diameter of 400 nm or less, The resin constituting the constituent fibers of the fiber layer is only a homopolymer of polyvinylidene fluoride, The fiber layer is made up of only continuous fibers, a non-fibrous material is present on one main surface of the fiber layer, bonding the constituent fibers of the fiber layer together; The specular gloss of one main surface of the fiber layer is 10.1 to 21.

0. Filter substrate.

2. The filter substrate according to claim 1, which is used in a filter that can be washed and reused.

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