JP7702782B2 - Filter substrate - Google Patents

Description

The present invention relates to a filter substrate having a fiber layer that constitutes a filter that can be washed and reused.

Conventionally, air filters and masks have been required to have low pressure loss, excellent breathability, and excellent collection performance for atmospheric dust, PM2.5 dust, and pollen. In order to meet these demands, the following conventional technologies have been considered.

JP 2014-114521 A (Patent Document 1) discusses a nonwoven fabric substrate for air filters or masks that has a nanofiber nonwoven fabric layer with an average fiber diameter of 50 to 400 nm in order to efficiently capture minute dust particles in the air. As a specific embodiment, Examples 1-2 and Examples 6-9 disclose a nanofiber nonwoven fabric layer made of homopolymer polyvinylidene fluoride with an average fiber diameter of 110 nm or 126 nm.

JP 2017-166106 A (Patent Document 2) studies a fiber assembly such as a nonwoven fabric mainly made of fluororesin fibers that has a large surface area per unit volume in order to realize a filter that can capture fine particles and reduce pressure loss. It also discloses that polyvinylidene fluoride fibers can be used as the fluororesin fibers, and that the lower limit of the average fiber diameter of the fiber assembly is about 0.2 μm. It also discloses that the fiber assembly is made of fluororesin fibers that have excellent waterproof properties, and therefore can be sterilized by washing or steam or boiling.

JP 2014-114521 (claims, 0028-0033, 0043-0052, etc.) JP 2017-166106 (claims, 0039, 0045, 0068, 0072, etc.)

In recent years, in order to extend the lifespan of air filters installed in air conditioners and the like, there has been a demand for air filters that can be reused by removing dust that has adhered to them. In addition, the habit of wearing masks daily as a measure against infectious diseases has emerged, and there is a demand for masks that can be reused hygienically. In response to this demand, there is a demand for air filters and masks that can be washed, for example, so that adhered dust can be removed and they can be reused hygienically.

In order to provide air filters and masks that meet these demands, the applicant considered preparing air filters and masks using filter substrates equipped with a fiber layer according to the conventional technology described above. However, the prepared air filters and masks had a problem in that they had poor collection efficiency after washing.

The applicant's objective is to provide a filter substrate that can provide a filter with high collection efficiency even after cleaning.

The present invention relates to
"A filter substrate for constructing a filter that can be washed and reused, which is made by laminating a fiber layer and a spunbond nonwoven fabric having an oil agent applied thereto and made of a core-sheath type fiber,
The fiber layer is made up of only continuous fibers,
The fiber layer includes, as a constituent fiber, a continuous fiber containing a homopolymer of polyvinylidene fluoride,
The average fiber diameter of the continuous fibers constituting the fiber layer is greater than 130 nm and less than 450 nm;
The fiber layer and the spunbonded nonwoven fabric are melt-bonded by the sheath component of the core-sheath type fiber.
Filter substrates (excluding those in which the constituent fibers of the fiber layer contain an ionic surfactant)
It is.

In order to clarify the cause of the problem that air filters and masks prepared using filter substrates according to the conventional technology have poor collection efficiency after cleaning, the applicant of the present application checked the structure of the filter substrate before and after cleaning. As a result of the check, it was found that the fiber layer that constitutes the filter substrate had breaks and cracks in the filter substrate after cleaning, which was thought to be the cause of the decrease in collection efficiency.

As a result of continued research, the applicant has discovered that the problem can be solved by providing a filter substrate with a fiber layer that satisfies the configuration of the present invention.

The fiber layer of the filter substrate according to the present invention contains fibers that contain a homopolymer of polyvinylidene fluoride, which has excellent strength as a constituent fiber. Therefore, the fibers become the skeleton of the fiber layer, and the shape of the fiber layer is maintained during cleaning, making the constituent fibers less likely to be cut. In addition, since the average fiber diameter of the fiber layer is greater than 130 nm, the constituent fibers are even less likely to be cut. As a result, breaks and cracks are less likely to occur in the fiber layer even when cleaned.

Furthermore, because the average fiber diameter of the fiber layer is less than 450 nm, the fiber layer has a dense structure. Therefore, even if the fiber layer breaks or cracks during cleaning, causing a decrease in the collection efficiency of the filter substrate having the fiber layer, the filter substrate having the fiber layer still has a high collection efficiency.

As a result, the present invention can provide a filter substrate that can provide a filter with high collection efficiency even after cleaning.

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

The filter substrate of the present invention includes a fiber layer. The fiber layer serves to capture and hold particles in the filter substrate, and also serves to form the framework of the filter substrate.

The fiber layer in the present invention refers to a layer composed of fibers, and can be, for example, a layer derived from a fiber web or nonwoven fabric, or a sheet-like fabric such as a woven fabric 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 preferable because it allows the shape and size of the gaps between the fibers to be uniform, thereby realizing a filter substrate that can provide a filter with excellent filtering performance.

The filter substrate of the present invention contains fibers containing a polyvinylidene fluoride homopolymer as the constituent fibers of the fiber layer (hereinafter, sometimes abbreviated as constituent fibers). Polyvinylidene fluoride as used herein refers to a polymer having a -(CH ₂ CF ₂ )- structure in its structure. Moreover, polyvinylidene fluoride homopolymer refers to a molecular structure in which the structure is continuously formed.

The fiber layer of the filter substrate according to the present invention contains fibers containing a homopolymer of polyvinylidene fluoride, which has excellent strength as a constituent fiber. Among polyvinylidene fluorides, homopolymers are known to have higher strength than copolymers of polyvinylidene fluoride. Therefore, the fibers become the skeleton of the fiber layer, and the shape of the fiber layer is maintained even during cleaning, and the constituent fibers are less likely to be cut by cleaning.

Hereinafter, polyvinylidene fluoride homopolymer will be referred to as "PVDF homopolymer", and fibers containing polyvinylidene fluoride homopolymer will be abbreviated to "PVDF homopolymer fiber".

The molecular weight of the PVDF homopolymer can be appropriately selected. 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" in the present invention is a value measured based on gel permeation chromatography. In addition, when the molecular weight of the PVDF homopolymer used is described in a catalog or paper, the molecular weight can be the molecular weight of the PVDF homopolymer.

The strength of the PVDF homopolymer can be appropriately selected. 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. For this reason, it is preferable to use a PVDF homopolymer that can be used to prepare a film having a breaking strength of 30 MPa or more, as measured based on the standard method of ASTM D638 plastic tensile testing method, more preferably a PVDF homopolymer that can be used to prepare a film having a breaking strength of 40 MPa or more, and more preferably a PVDF homopolymer that can be used to prepare a film having a breaking strength of 50 MPa or more.

In addition to the PVDF homopolymer, the constituent fibers may contain other resins, which will be described below. The other resins can be selected as appropriate, but examples include polyether-based resins (polyethylene glycol, polypropylene glycol, polyether ether ketone, polyacetal, modified polyphenylene ether, aromatic polyether ketone, etc.), phenol-based resins, epoxy-based resins, polyimide-based resins, polyamideimide-based resins, polyamide-based resins (e.g., aromatic polyamide resins such as aramid resins, aromatic polyether amide resins, nylon resins, etc.), urethane-based resins, epoxy-based resins, polysulfone-based resins (polysulfone, etc.), polyethersulfone-based resins (polyethersulfone, sulfonated polyethersulfone, etc.), etc. ), fluorine-based resins (polytetrafluoroethylene, PVDF copolymers, perfluorosulfonic acid resins, etc.), vinyl alcohol-based resins (polyvinyl alcohol, polyvinyl acetate, etc.), polycaprolactone, polyglycolic acid, polyvinylpyrrolidone, polybenzimidazole resins, acrylic resins (for example, polyacrylonitrile-based resins copolymerized with acrylic acid esters or methacrylic acid esters, modacrylic resins copolymerized with acrylonitrile and vinyl chloride or vinylidene chloride, etc.), and other known resins may be used. They may be one type of resin or multiple types of resins, such as a mixed resin.

These resins may be either linear or branched polymers, and may be block or random copolymers. The resins may have any three-dimensional structure or may be crystalline or non-crystalline.

However, it is preferable that the polymer constituting the constituent fibers is only PVDF homopolymer, so that the strength of the constituent fibers is higher, which prevents the shape of the fiber layer from changing even during cleaning and realizes a filter substrate in which breaks and cracks are less likely to occur in the fiber layer.

The small average fiber diameter of the PVDF homopolymer fibers contained in the constituent fibers can contribute to the realization of a filter substrate capable of providing a filter with excellent filtering performance. From this viewpoint, the average fiber diameter of the PVDF homopolymer fibers contained in the constituent fibers is less than 450 nm, and preferably 400 nm or less. On the other hand, if the average fiber diameter of the PVDF homopolymer fibers contained in the constituent fibers is too small, the constituent fibers of the fiber layer are easily cut during cleaning, and many breaks and cracks occur, making it difficult to realize a filter substrate capable of providing a filter with excellent filtering performance. From this viewpoint, the average fiber diameter of the PVDF homopolymer fibers contained in the constituent fibers is preferably greater than 130 nm, and 150 nm or more. The "average fiber diameter" in the present invention refers to the arithmetic average value of each fiber diameter in 50 fibers measured based on an electron microscope photograph of a cross section or surface of the measurement object at a magnification of 5000 times. In addition, 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. In addition, 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.

The percentage of the mass of the PVDF homopolymer fiber in the mass of the constituent fibers can be adjusted as appropriate. The constituent fibers may contain fibers composed of other resins as described above in addition to the PVDF homopolymer fiber. However, the higher the percentage of the mass of the PVDF homopolymer fiber in the mass of the constituent fibers, the more likely it is that the shape of the fiber layer will be maintained even during cleaning, and the constituent fibers will be less likely to be cut by cleaning. Therefore, the percentage is preferably 10% by mass or more, more preferably 20% by mass or more, more preferably 30% by mass or more, more preferably 40% by mass or more, more preferably 50% by mass or more, more preferably 60% by mass or more, more preferably 70% by mass or more, more preferably 80% by mass or more, and more preferably 90% by mass or more, and it is most preferable that the constituent fibers of the fiber layer are only PVDF homopolymer fibers.

The fiber length of the constituent fibers is appropriately selected, but they can be short or long fibers having a specific length, or continuous fibers having a length that makes it difficult to actually measure the fiber length. Since the number of fiber ends in the fiber layer is small, the surface is smooth, the thickness is uniform, and various physical properties such as mechanical strength are excellent, and a filter substrate that can provide a filter with better filtering performance can be realized. Therefore, it is preferable that the constituent fibers contain fibers having a continuous length, and it is more preferable that the constituent fibers are only continuous fibers. The "fiber length" referred to in the present invention can be measured based on an electron microscope photograph of the cross section or surface of the measurement object at a magnification of 5000 times. If the fiber length is too long and measurement is difficult, it can be measured based on an electron microscope photograph at a magnification of less than 5000 times.

The constituent fibers may be single fibers, fibrillated fibers, or composite fibers. Composite fibers may be, for example, core-sheath, sea-island, side-by-side, orange, or 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 fiber cross section that is hollow, polygonal such as a triangular shape, alphabetic such as a Y-shape, irregular, multi-lobed, or symbolic such as an asterisk shape, or a shape in which multiple of these shapes are combined.

The method for preparing the constituent fibers can be appropriately selected, but known methods such as melt spinning, dry spinning, wet spinning, direct spinning (melt blowing, spunbonding, electrostatic spinning in which an electric field is applied to the spinning solution to spin, spinning using centrifugal force, spinning using an accompanying airflow as described in JP 2011-012372 A, neutralization spinning, which is a type of electrostatic spinning as described in JP 2005-264374 A, etc.), and methods for extracting fibers with a narrow fiber diameter by removing one or more resin components from composite fibers can be used.

The fibers prepared using the above-mentioned method can be subjected to, for example, a dry method or a wet method 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 a method of entangling the constituent fibers using a needle, a water flow, or a fluid flow such as steam/gas, and a method of subjecting the fiber web to a heat treatment to bond or melt the constituent fibers together using a binder or adhesive fiber. The heat treatment method can be appropriately selected, and examples of the method that can be used include a method of heating and pressurizing using a calendar roll, a method of heating using a hot air dryer, and a method of heating the polymer contained therein by irradiating infrared rays without pressure.

In addition to fiber webs, nonwoven fabrics may also be subjected to the above-mentioned methods of entangling and/or integrating the constituent fibers.

Furthermore, by collecting fibers spun using a direct spinning method (particularly an electrostatic spinning method), a fiber web or nonwoven fabric composed only of continuous fibers can be prepared. Note that, depending on the spinning conditions, a film-like or granular nonfibrous material called shot may be attached to the main surface of the fiber web or nonwoven fabric prepared using the direct spinning method. The presence or absence of nonfibrous material on the main surface of the fiber layer can be appropriately adjusted, and a filter substrate may be prepared with a fiber layer having nonfibrous material on its main surface, or a filter substrate may be prepared with a fiber layer having no nonfibrous material on its main surface.

In addition, a filter substrate having a laminated structure may be prepared by collecting fibers spun using a direct spinning method (particularly, electrostatic spinning method) on another substrate such as a spunbonded nonwoven fabric. In this case, if the other substrate (particularly, the spunbonded nonwoven fabric) contains an oil (particularly, a nonionic surfactant), a filter substrate having excellent collection efficiency in which the directly spun (particularly, electrostatically spun) fibers are uniformly dispersed can be prepared, which is preferable. In addition, for the other substrate having the oil used in this case, it is preferable to adjust the type of oil and the amount of oil present so that the surface resistance of the main surface is 5.0 x 10 ¹³ Ω or less. In addition, the surface resistance is preferably 5.0 x ^{10 13} Ω or less, preferably 1.0 x 10 13 Ω or less, and more preferably 5.0 x 10 ¹² Ω or less. By directly spinning (particularly, electrostatic spinning) on another substrate having such a surface resistance of the main surface, the spun fibers can be uniformly dispersed and deposited on the other substrate. As a result, a filter substrate with excellent collection efficiency can be prepared, which is preferable.

The fiber layer of the filter substrate of the present invention is characterized in that its average fiber diameter is greater than 130 nm and less than 450 nm.

Because the average fiber diameter of the fiber layer is greater than 130 nm, the constituent fibers are less likely to be cut by cleaning. As a result, the fiber layer is less likely to break or crack even after cleaning. The larger the average fiber diameter of the fiber layer, the more effectively the effect is exerted, so the average fiber diameter of the fiber layer is preferably 140 nm or more, and more preferably 150 nm or more.

The fiber layer has a dense structure because the average fiber diameter of the fiber layer is less than 450 nm. Therefore, even if the fiber layer breaks or cracks during cleaning, causing the collection efficiency of the filter substrate including the fiber layer to decrease, the filter substrate including the fiber layer still has a high collection efficiency. The smaller the average fiber diameter of the fiber layer, the more effectively the effect is exerted, so the average fiber diameter of the fiber layer is preferably 440 nm or less, preferably 430 nm or less, preferably 420 nm or less, preferably 410 nm or less, and preferably 400 nm or less.

Various physical properties such as the fiber layer's basis weight and thickness can be selected appropriately.

First, when a layer (such as a fiber layer) constituting the filter substrate can be easily obtained (for example, when the layer can be easily peeled off from the filter substrate), a layer made of fibers, such as a fiber web or nonwoven fabric layer, is selected from the obtained layers 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.
For example, the thickness can be 0.1 μm to 200 μm, 0.2 μm to 150 μm, 0.2 μm to 100 μm, or 0.2 μm to 50 μm. Note that the "thickness" 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).
For example, the basis weight can be 0.05 to 50 g/ ^m2 , 0.1 to 30 g/ ^m2 , 0.1 to 20 g/ ^m2 , or 0.1 to 10 g/ ^m2 . The "basis weight" in the present invention means a value measured on a 10 cm x 10 cm area in accordance with JIS L1085.

On the other hand, when each layer (such as a fiber layer) constituting the filter substrate cannot be easily obtained (for example, when the layer cannot be easily peeled off from the filter substrate), the thickness of the fiber layer can be determined by the following method.
(1) An electron microscope photograph (magnification: 2000 times) of a cross section of a filter substrate cut in the thickness direction is taken.
(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 a line segment connecting both main surfaces of the filter substrate in the electron microscope photograph to the intersection (B) of the line segment connecting one main surface of each layer with the other main surface.
(3) Among the layers, a layer composed 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 calculated using the method of step (2) described above, and this is defined as the thickness of the fiber layer.

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) An electron microscope photograph (magnification: 2000 times) of a cross section of a filter substrate cut in the thickness direction is taken.
(2) The electron microscope photograph is visually inspected to determine whether the filter substrate has a fiber layer.
(3) In the case where 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 constituent fibers together) and the components constituting the 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 layers) other than the fiber layer without dissolving the fiber layer (for example, polar solvents such as DMF, DMAc, and 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 the solvent is removed is taken as the basis weight of the fiber layer. In addition, if there is no solvent capable of dissolving layers (other layers) other than the fiber layer 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 the solvent is removed is subtracted from the basis weight of the filter substrate, and the calculated value is taken as the basis weight of the fiber layer.

In addition, when the materials used to prepare the filter substrate are known to include materials other than those used to form the fiber layers, the thickness and basis weight of the other materials are directly measured in the same manner as described above for "when each layer (such as the fiber layer) that constitutes the filter substrate can be easily peeled off and obtained," and the thickness and basis weight of the materials used to form the fiber layers are calculated by subtracting the measured values from the thickness and basis weight of the filter substrate. The calculated values may then be regarded as the thickness and basis weight of the fiber layers that constitute the filter substrate.

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

The method of laminating the fiber layer and the substrate can be selected as appropriate, but it is possible to simply overlap them, to partially melt-bond the components of the fiber layer and/or the substrate, or to laminate them together using a binder, or to prepare a laminate by accumulating spun fibers on the main surface of the substrate to form a fiber layer on the substrate.

In particular, it is preferable that the substrate contains core-sheath fibers, because the sheath component of the core-sheath fibers can strongly melt and bond the fiber layer to the substrate. In addition, the substrate is unlikely to fall off from the filter substrate that includes the fiber layer and substrate, so the fiber layer can be effectively protected by the substrate. As a result, it is preferable that the fiber layer can be effectively prevented from breaking or cracking, and a filter with high collection efficiency can be provided even after cleaning. Examples of such substrates include spunbond nonwoven fabrics and dry nonwoven fabrics made of core-sheath fibers, as well as wet nonwoven fabrics.

In addition, it is preferable that the strength of the substrate is high so that the fiber layer can be effectively protected by the substrate. The "strength" of the present invention can be measured by subjecting the fiber assembly to the method described below. Specifically, in order to improve washing resistance, the substrate preferably has a strength of 2N/50mm or more in both the MD and CD directions, preferably a substrate of 3N/50mm or more, preferably a substrate of 10N/50mm or more, and preferably a substrate of 15N/50mm or more. On the other hand, the upper limit can be adjusted as appropriate, but if the substrate has too high a strength, the breathability of the mask may decrease. Therefore, it is preferable to use a substrate with a strength of less than 500N/50mm.

The strength of the substrate in the MD and CD directions can be determined by the following method.
(Method of measuring strength)
(1) A rectangular sample (short side: 50 mm, long side: 200 mm) was taken from the object to be measured. In this case, when the production direction of the object to be measured was known, the sample was taken so that the production direction and the long side direction of the object to be measured were parallel. In addition, when the production direction of the object to be measured was not known, multiple samples were 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 were parallel, and each sample was subjected to the measurement of maximum stress described below. As a result, the direction of the object to be measured that was parallel to the long side of the sample that showed the highest maximum stress was determined to be the production direction of the object to be measured.
(2) Using a tensile tester (manufactured by Orientec Co., Ltd., product name: Tensilon (registered trademark), TM-111-100), the sample was pulled in the long side direction until it broke under conditions of a gripping distance of 100 mm and a pulling speed of 50 mm/min.
(3) The maximum stress measured until the sample broke was taken as the "strength." In this manner, the strength (N/50 mm) in the direction parallel to the long side direction of the sample (MD direction) was measured.
(4) A new rectangular sample (short side: 50 mm, long side: 200 mm) was taken from the measurement object so that the short side direction was parallel to the direction parallel to the long side direction of the sample in the measurement object. In this case, if the production direction of the measurement object was known, the sample was taken so that the production direction and the short side direction of the measurement object were parallel.
(5) The sample taken in the above-mentioned step (4) was subjected to steps (2) to (3) to measure the strength (N/50 mm) in the direction parallel to the long side direction of the sample (CD direction) in the measurement target.

The fiber layer or fiber layer laminate produced in the above manner may be subjected to a pressure treatment such as a reliant press treatment to adjust the thickness, a hydrophilic treatment such as a sulfonation treatment, plasma treatment or fluorine gas treatment, or various secondary processes such as punching or molding to a shape, depending on the application or mode of use.

Next, a method for producing a filter substrate according to the present invention will be described by way of an example, with the explanation being omitted for the same configuration as that described above.
(1) preparing a spinning solution prepared by dissolving a PVDF homopolymer in a solvent, or preparing a spinning solution prepared by dispersing a PVDF homopolymer in a dispersion medium;
(2) feeding the spinning solution to an electrostatic spinning apparatus, spinning the resulting fibers into fine fibers, and collecting the fibers to prepare a fiber web;
(3) removing the solvent or dispersion medium remaining in the fiber web to prepare a fiber layer containing PVDF homopolymer fibers and having an average fiber diameter of more than 130 nm and less than 450 nm;
It is possible to use a method for manufacturing a filter substrate comprising the steps of:

First, step (1) will be described.
The type of solvent or dispersion medium is appropriately selected, but examples thereof include N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, acetonitrile, dimethyl sulfoxide, 1,4-dioxane, pyridine, formic acid, toluene, benzene, cyclohexane, cyclohexanone, carbon tetrachloride, methylene chloride, chloroform, trichloroethane, ethylene carbonate, diethyl carbonate, propylene carbonate, etc. The solvent or dispersion medium may be one type, or a mixed solvent or mixed dispersion medium in which a plurality of types are mixed.

The concentration of the PVDF homopolymer and the type of PVDF homopolymer contained in the spinning solution are appropriately adjusted so as to prepare the filter substrate of the present invention.

The temperature and viscosity of the spinning solution are appropriately selected so as to prepare the desired fiber layer. 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" refers to the value measured using a viscosity measuring device at a temperature of 25°C and a shear rate of 100 s-1.

Next, the step (2) will be described.
The method of spinning by thinning the spinning solution is appropriately selected so as to prepare the desired fiber layer, and for example, a direct spinning method can be adopted. When electrostatic spinning is adopted, a voltage is applied to the spinning solution, and a voltage opposite to the voltage is applied to a counter electrode such as a metal plate provided at a distance from the discharge portion of the spinning solution, so that the spinning solution flies toward the counter electrode and is thinned. Then, the thinned spinning solution is collected in a collector to form a fiber web on the collector. Note that the counter electrode such as the above-mentioned metal plate may be used as the collector, but 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 substrate and fiber web laminate prepared in this way to the next step, a filter substrate consisting of a substrate and a fiber layer laminate can be easily prepared, which is preferable.

Next, step (3) will be described.
The method for removing the solvent or dispersion medium remaining in the fiber web can be appropriately selected, and as an example, a method of subjecting the fiber web to a heating device can be adopted. The type of heating device can be appropriately selected, and for example, a method using a device for heating or heating and pressing with a roll, an oven dryer, a far-infrared heater, a dry heat dryer, a hot air dryer, a device for heating by irradiating infrared rays, etc. can be adopted. The heating temperature by the heating device is appropriately selected, and is appropriately adjusted so that the remaining solvent or dispersion medium can be volatilized and removed, and the constituent components such as the constituent fibers do not unintentionally decompose or denature.

If an adhesive component or a crosslinkable resin is present in the constituent fibers of the fiber web, the fibers may be bonded by the adhesive component or the crosslinkable resin may be crosslinked by subjecting the web to a heating device.

In addition, in the above-mentioned method for manufacturing a filter substrate, the average fiber diameter of the fibers constituting the fiber layer can be adjusted by adjusting the molecular weight of the PVDF homopolymer used, the blending of the PVDF homopolymer (for example, using a spinning solution that is a mixture of a PVDF homopolymer with a high molecular weight and a PVDF homopolymer with a low molecular weight), the composition and viscosity of the spinning solution, the solids concentration of the PVDF homopolymer contained in the spinning solution, the spinning conditions, the method and conditions for removing the solvent or dispersion medium, etc., and a filter substrate that satisfies the configuration of the present invention can be manufactured.

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

The method of laminating the fiber layer and the substrate can be selected as appropriate, but it is possible to simply overlap them, to partially melt-bond the components of the fiber layer and/or the substrate, or to laminate them together using a binder, or to prepare a laminate by accumulating spun fibers on the main surface of the substrate to form a fiber layer on the substrate.

It is preferable to laminate substrates on both main surfaces of the fiber layer, as this allows the preparation of a filter substrate that can provide a filter with high collection efficiency even after cleaning.

The fiber layer or fiber layer laminate produced in the above manner may be subjected to a pressure treatment such as a reliant press treatment to adjust the thickness, a hydrophilic treatment such as a sulfonation treatment, plasma treatment or fluorine gas treatment, or various secondary processes such as punching or molding to a shape, depending on the application or mode of use.

The filter substrate manufactured in the above manner 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 in which the three-layer filter is used in the facepiece. Alternatively, an air filter can be prepared using a filter equipped with a filter substrate.

The following describes examples of the present invention, but the present invention is not limited to the following examples.

(Spinning solution)
The following polymers were dissolved in dimethylformamide (boiling point: 153° C.) to prepare spinning solutions A to B with a solid content of 14% by mass. The types of polymers contained in each spinning solution are as follows. Note that vinylidene fluoride-hexafluoropropylene copolymer is not a PVDF homopolymer.
Spinning solution A: vinylidene fluoride-hexafluoropropylene copolymer (a resin capable of preparing a film having a breaking strength of 25 MPa as measured according to the standard method of ASTM D638 plastic tensile testing method)
Spinning solution B: PVDF homopolymer (a resin capable of preparing a film having a breaking strength of 45 MPa as measured according to the standard method of ASTM D638 plastic tensile testing method)

(Comparative Example 1)
The spinning solution A was subjected to electrostatic spinning under the following spinning conditions, and collected on a substrate to prepare a laminated web in which a fiber web made of continuous fibers and the substrate were laminated.
The shape of the spinning solution discharge part of the metal nozzle (spinning solution discharge part): circular with an inner diameter of 0.44 mm. The distance between the tip of the metal nozzle and the collector (metal plate): 10 cm.
Voltage applied to spinning solution: 15 kV
Spinning solution discharged from a metal nozzle: 1 g/hour Electrostatic spinning environment: Temperature 25° C., humidity 30% RH
Substrate: Spunbond nonwoven fabric consisting of core-sheath fibers with a core of polyester (melting point: 260°C) and a sheath of polyethylene (melting point: 130°C) (basis weight: 15 g/ ^m2 , strength in MD: 38 N/50 mm, strength in CD: 16 N/50 mm, oil agent applied, surface resistance of the main surface on the fiber collecting side is 3.5 x ¹⁰¹² Ω)
The prepared laminated web was then brought into contact with a heating roll whose surface temperature was adjusted to 130°C to remove any solvent remaining in the laminated web and to 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.

Example 1
A nonwoven fabric was prepared in the same manner as in Comparative Example 1, except that spinning solution B was used instead of spinning solution A.

The nonwoven fabrics prepared were used as filter substrates, and their configurations, initial pressure loss before and after washing, and collection efficiency are summarized in Table 1. In the table, vinylidene fluoride-hexafluoropropylene copolymer is described as "copolymer" and PVDF homopolymer as "homo" for the type of polymer that constitutes the constituent fibers of the fiber web-derived portion of the filter substrate (hereinafter sometimes referred to as the fiber layer).

In addition, the initial pressure loss and collection efficiency before and after washing were determined using the following measurement method.

(Method of measuring collection efficiency and initial pressure loss before washing)
A test piece was taken from the filter substrate. The taken 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 the fiber layer side faced the upstream side.
First, the test flow rate was adjusted to 40 liters per minute ^per 44 cm2 effective filtration area of the test piece (for example, the test flow rate supplied to a test piece with an effective filtration area of 4.4 ^cm2 was 4 liters per minute), 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 a filter or mask can be provided.
Next, the test flow rate was adjusted to 30 liters per minute per 44 ^cm2 effective filtration area of the test piece (for example, the test flow rate supplied to a test piece with an effective filtration area of 4.4 ^cm2 was 3 liters per minute), and a test airflow containing sodium chloride particles (median particle size distribution: 0.06 to 0.10 μm, geometric standard deviation: 1.8 or less) at a concentration of 50 mg/ ^m3 or less (concentration fluctuation: ±15% or less) was supplied to the upstream side of the test piece. After supplying the test airflow for 1 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 by the test piece was calculated from both measured concentrations.
The percentage of the concentration of sodium chloride particles trapped by the test piece relative to the concentration of sodium chloride particles supplied to the upstream side 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.

(Method for measuring collection efficiency and initial pressure loss after washing)
Another test piece was taken from the filter substrate. The other test piece was sandwiched between polyester water-absorbing and quick-drying knits, and the periphery was welded to prepare a three-layer mask. The prepared mask was put into a home washing machine together with a commercially available laundry detergent, washed, and then dehydrated. In this way, the mask was washed 10 times, and the mask after dehydration was air-dried.
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 in the above manner were taken as the initial pressure loss and collection efficiency of the filter substrate after washing.

The filter substrate of Example 1 was superior to the filter substrate of Comparative Example 1 in terms of collection efficiency after washing.
In addition, in the filter substrate of Comparative Example 1, the collection efficiency was greatly reduced after washing. The reason for this was thought to be that the constituent fibers (fibers composed of vinylidene fluoride-hexafluoropropylene copolymer) were cut during washing, causing breaks and cracks in the fiber layer. In contrast, in the filter substrate of Example 1, the collection efficiency was not so reduced after washing. The reason for this was thought to be that the constituent fibers contained PVDF homopolymer fibers, which have excellent strength, became the skeleton of the fiber layer, and the shape of the fiber layer was maintained during washing, making it difficult for the constituent fibers to be cut, and preventing breaks and cracks from occurring in the fiber layer.

(Comparative Example 2, Examples 2 to 3)
Each nonwoven fabric was prepared in the same manner as in Example 1, except that the solids concentration of the PVDF homopolymer in the spinning solution B was changed to 8 mass% in Comparative Example 2, 15 mass% in Example 2, and 17 mass% in Example 3.

(Comparative Examples 3 to 4)
Each nonwoven fabric was prepared in the same manner as in Example 1, except that the solid content concentration of the PVDF homopolymer in the spinning solution B was changed to 17.5 mass% in Comparative Examples 3 and 4, and the amount of fibers collected on the substrate was changed.

The nonwoven fabrics prepared were used as filter substrates, and their configurations, initial pressure loss before and after washing, and collection efficiency are summarized in Table 2. In the table, PVDF homopolymer is listed as "homo" for the type of polymer that constitutes the fibers of the fiber web-derived portion of the filter substrate (hereinafter sometimes referred to as the fiber layer).

The filter substrate of Comparative Example 3, in which the average fiber diameter of the fiber layer is 450 nm and the basis weight is 0.5 g/m ² , had a collection efficiency of 64.9% after washing. The filter substrate of Comparative Example 4, in which the average fiber diameter of the fiber layer is 450 nm and the basis weight is 0.9 g/m ² , had a collection efficiency of 72.0% after washing. From the results of Comparative Example 3 and Comparative Example 4, if a filter substrate with a fiber layer having an average fiber diameter of 450 nm and a basis weight of 0.8 g/m ² is prepared, it was considered that the collection efficiency after washing has a performance between 64.9% and 72.0%. In other words, it was presumed that the collection efficiency after washing could not exceed 72.0%. The reason for this was thought to be that the average fiber diameter of the fiber layer was as large as 450 nm, and the fiber layer could not have a dense structure.

In addition, in order to provide a filter substrate with excellent collection performance, it is desirable that the fiber layer has a dense structure. Therefore, it was thought that by reducing the average fiber diameter of the fiber layer constituting the filter substrate, a filter substrate with excellent collection efficiency after washing could be provided. However, the filter substrate of Comparative Example 2, which has a basis weight of 0.8 g/ ^m2 and an average fiber diameter of 130 nm, did not have a collection efficiency of more than 72.0% after washing. The reason for this was thought to be that the average fiber diameter of the fiber layer was as small as 130 nm, making it difficult for the constituent fibers to be cut.

On the other hand, the filter substrates of Examples 1 to 3, which had a basis weight of 0.8 g/ ^m2 and an average fiber diameter of more than 130 nm and less than 450 nm, all had a collection efficiency after washing of more than 72.0%.

From the above, it was found that a filter substrate containing PVDF homopolymer fibers with excellent strength as a constituent fiber and having a fiber layer with an average fiber diameter of more than 130 nm and less than 450 nm can provide a filter substrate that can provide a filter with high collection efficiency even after cleaning.

The present invention relates to a filter substrate having a fiber layer that constitutes a washable and reusable filter. By using the filter substrate, it is possible to provide masks that can be washed and reused (e.g., masks with a two-dimensional sheet shape, masks with a three-dimensional corrugated or pleated shape, masks with a folded shape, and masks with a cup shape), and air filters for air purifiers and air conditioners that can be washed and reused (e.g., filters with a two-dimensional sheet shape, filters with a three-dimensional corrugated or pleated shape, and depth filters). The filter substrate according to the present invention may also be used to prepare a liquid filter.

Claims (1)

A filter substrate for constructing a filter that can be washed and reused, the filter substrate being formed by laminating a fiber layer and a spunbond nonwoven fabric having a core-sheath type fiber and an oil agent applied thereto ,
The fiber layer is made up of only continuous fibers,
The fiber layer includes, as a constituent fiber, a continuous fiber containing a homopolymer of polyvinylidene fluoride,
The average fiber diameter of the continuous fibers constituting the fiber layer is greater than 130 nm and less than 450 nm;
The fiber layer and the spunbonded nonwoven fabric are melt-bonded by the sheath component of the core-sheath type fiber.
A filter substrate (excluding those in which the constituent fibers of the fiber layer contain an ionic surfactant).