JP2019115873A - Method for production of filter medium for air filter - Google Patents

Abstract

To provide a method for producing in a comparatively short time, a filter medium for air filter improving a filter performance, particularly particle collection performance by using a nano-cellulose.SOLUTION: A method for production of a filter medium for air filter includes an attachment process of attaching a dispersion liquid including nano-cellulose to a fluid permeable support, and a drying process of drying the support gone through the attachment process. The drying process includes carrying out vaporization of a dispersant in the dispersion liquid, lowering of a dispersant temperature by vaporization heat, freezing of the dispersant by temperature lowering and sublimation of the frozen dispersant by placing the support gone through the attachment process under the reduced pressure atmosphere dipping from ambient pressure to dry the support.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to a method of manufacturing a filter medium for an air filter. More specifically, the present invention provides a method for producing an air filter medium suitable for air purification applications such as semiconductors, liquid crystals, clean rooms or clean benches related to bio / food industry, building air conditioning, internal combustion engines or indoor spaces in a relatively short time. Do.

  In order to collect submicron to micron particles in the air, a filter medium for air filter is generally used. Filter materials for air filters are roughly classified into coarse dust filters, medium performance filters, HEPA (High Efficiency Particulate Air) filters, and ULPA (Ultra Low Penetration Air) filters according to their collection performance. As basic characteristics of these air filter filter media, in addition to the low particle permeability of fine dust particles, a low pressure loss is required in order to ventilate the filter.

  In recent years, the use of nanocellulose has attracted attention. In general, nanocellulose is also referred to as cellulose nanofiber, and refers to (1) fine cellulose fiber having a number average fiber diameter of 1 to 100 nm, or (2) fine cellulose fiber subjected to chemical treatment (modification). As the nanocellulose of (1), for example, microfib related cellulose obtained by shearing cellulose fiber under high pressure, fibrillated, nanofib related cellulose (hereinafter, referred to as MFC, NFC), or fine particles produced by microorganisms are used. Bacterial cellulose (hereinafter abbreviated as BC). As the modified nanocellulose of (2), for example, cellulose nanocrystals (hereinafter referred to as CNC) obtained by treating natural cellulose with 40% or more concentrated sulfuric acid (hereinafter referred to as CNC) or fibers constituting wood pulp It is an ultra-fine, uniform, fine cellulose fiber having a diameter of microfibrils, which is the smallest unit, isolated as an aqueous dispersion by mild chemical treatment at ordinary temperature and pressure and slight mechanical treatment (for example, Patent Document 1 reference.).

  These nanocelluloses are expected to be used as filter materials or porous bodies. For example, a method of manufacturing a leukocyte removal filter material comprising a porous element having an average pore diameter of 1.0 μm or more and less than 100 μm and a fiber structure having an average fiber diameter of 0.01 μm or more and less than 1.0 μm. Are shown (see, for example, Patent Document 2). In Patent Document 2, bacterial cellulose produced by acetic acid bacteria as a fiber structure, or rayon is treated with sulfuric acid to use cellulose fibers that have been refined with a homogenizer.

  Because of the high hydrophilicity of nanocellulose, the cohesion that works during drying is stronger than nanofibers derived from thermoplastic polymers, and it is difficult to obtain a breathable material such as an air filter. Therefore, as a method of obtaining a breathable material using nanocellulose, nanocellulose is dispersed in a mixture of water and an organic solvent dissolved in water, and the mixture is freeze-dried to remove the dispersion medium. A method of producing a porous body is shown (see, for example, Patent Document 3).

JP 2008-1728 A Patent No. 3941838 JP, 2013-253137, A

R. Daoussi, E, Bogdani, S. Vessot, J. et al. Andrieu, O. Monnier, "Drying Technology" Vol. 29 (2011), p. 1853 to 1867

  In Patent Document 2, bacterial cellulose produced by acetic acid bacteria is used as the thinnest fiber, and the average fiber diameter is 0.02 μm (= 20 nm). When bacterial cellulose is used, acetic acid bacteria are cultured in a porous element, but there are problems that the culture conditions are many restrictions, and a long culture time of 0.5 hours or more and less than 48 hours is required. Furthermore, since the amount of bacterial cellulose produced by culture is also small, there is a problem that it is not suitable for industrial mass production. Further, the fiber structure is formed only on the surface of the porous element, and the fiber structure is not formed inside the pores of the porous element. Therefore, the filter material of Patent Document 2 is a void as a filter material for air filter There was a problem that the rate was too low.

  In patent document 3, it is not examined about using a cellulose porous body as a filter medium for air filters. In patent document 3, the porous body of nanocellulose is formed in the nonwoven fabric which consists of glass fibers using freeze-drying. However, lyophilization requires a freezing step before drying, which requires time for freezing, and for drying after freezing to take a long time to form a dense nanocellulose network, production is required. Problem that it takes a long time to Furthermore, facilities, such as a refrigerator, are indispensable in a freezing process, and there existed a problem that operation | movement of the facility of the said refrigerator requires cost. However, no means for solving these problems have been shown so far.

  An object of the present disclosure is to provide a method for producing a filter material for an air filter, which has improved filter performance, particularly particle collection performance, using nanocellulose in a relatively short time.

  In the method for producing a filter medium for an air filter according to the present invention, an adhesion process of adhering a dispersion containing nanocellulose to a fluid-permeable support, and a drying process of drying the support after the adhesion process; In the method for producing a filter medium for an air filter, the drying step is carried out by vaporization of the dispersion medium of the dispersion liquid and heat of vaporization by placing the support subjected to the adhesion step under a reduced pressure atmosphere below atmospheric pressure. The process is characterized in that the temperature of the dispersion medium is lowered, the dispersion medium is frozen due to the temperature fall, and the frozen dispersion medium is sublimated to dry the support. The frozen dispersion medium sublimes in the later drying stage, and the drying is completed. Thereby, aggregation of nanocellulose can be suppressed, a nanocellulose network can be constructed, and it becomes possible to manufacture the filter material for air filters which improved filter performance in a relatively short time.

  In the method of manufacturing a filter medium for an air filter according to the present invention, the reduced pressure atmosphere preferably has a degree of vacuum of 1000 Pa or less.

  In the method for producing a filter medium for an air filter according to the present invention, it is preferable that the dispersion medium of the dispersion liquid is a liquid mixture of water and at least one or more organic solvents. When the dispersion medium is a mixture of water and an organic solvent that dissolves in water, crystallization of the dispersion medium is suppressed during freezing of the dispersion medium at the initial stage of reduced pressure drying, which contributes to the formation of a uniform network of nanocellulose.

  In the method for producing a filter medium for an air filter according to the present invention, in the adhesion step to the drying step, the dispersion medium of the dispersion liquid is subjected to freezing treatment using a refrigerator under atmospheric pressure or a reduced pressure atmosphere lower than atmospheric pressure. Preferably not. In the lyophilization, the dispersion medium is vaporized by reduced pressure drying after passing through the freezing step, but by omitting the freezing step, not only can manufacture in a shorter time can be enabled but also freezing equipment becomes unnecessary, Costs associated with installation and operation of equipment can be reduced.

  In the method for producing a filter medium for an air filter according to the present invention, the drying method performed in the drying step is preferably a reduced-pressure drying method with a degree of vacuum of 1000 Pa or less. When the degree of vacuum is low, the dispersion medium in the dispersion evaporates quickly, and the temperature of the dispersion drops due to evaporation in the early stage of drying, which causes the dispersion to freeze, so the filter medium is manufactured in a relatively short time. Is possible.

  According to the present disclosure, it is possible to provide a method of producing a filter medium for an air filter, which has improved filter performance, particularly particle collection performance, using a nanocellulose in a relatively short time.

It is the image (observation magnification 5000 times) which observed the air filter of Example 1 by SEM. It is the image (observation magnification 10000 times) which observed the air filter of Example 1 by SEM. It is the image (observation magnification 5000 times) which observed the air filter of the comparative example 1 by SEM. It is the image (observation magnification 10000 times) which observed the air filter of the comparative example 1 by SEM. It is the image (observation magnification 5000 times) which observed the air filter of the comparative example 2 by SEM. It is the image (observation magnification 10000 times) which observed the air filter of the comparative example 2 by SEM.

  EXAMPLES The present invention will next be described in detail by way of embodiments, which should not be construed as limiting the present invention. The embodiment may be variously modified as long as the effects of the present invention are exhibited.

  The filter material for an air filter according to the present embodiment includes an attaching step of attaching a dispersion containing nanocellulose to a fluid-permeable support, and a drying step of drying the support after the attaching step under reduced pressure. . Unlike lyophilization, reduced pressure drying does not require a freezing step prior to the drying step, and it is only necessary to place the support on which the dispersion containing nanocellulose is adhered under reduced pressure without separately using equipment such as a refrigerator. Drying can be performed. Thereby, the manufacturing time of the said air filter medium can be shortened significantly. In the present specification, placing a support under a reduced pressure atmosphere lower than the atmospheric pressure without performing a freezing process in advance is referred to as a “reduced pressure drying method”.

<Support>
The support is not particularly limited as long as it has fluid permeability, and, for example, a porous material such as non-woven fabric, woven fabric, paper or sponge can be used. Among these, non-woven fabrics are preferable, and in particular, non-woven fabrics for filter media mainly comprising fibers such as glass fibers and organic fibers are preferable. Having fibers such as glass fibers and organic fibers as the main component means that the mass of the fibers with respect to the total mass of the support is 50% by mass or more. More preferably, it is 80 mass% or more. When the support is a nonwoven fabric composed mainly of the fibers, the basis weight is preferably from 10 to 300 g / ^{m 2,} and more preferably 30 to 200 g / ^{m 2.} Fluid permeability means at least the property of being able to transmit gas, more preferably the property of being capable of transmitting gas and liquid.

  The glass fibers used for the support may be, for example, wool-like ultrafine glass fibers produced by a flame drawing method or a rotary method, or a bundle of glass fibers spun to a predetermined fiber diameter to a predetermined fiber length. Chopped strand glass fiber manufactured by cutting. Among these, those having various fiber diameters and fiber lengths are selected according to the required physical properties, and used alone or in combination. Low boron glass fibers or silica glass fibers can also be used for the purpose of preventing silicon contamination of silicon wafers in semiconductor manufacturing process applications. Although the average fiber diameter of glass fiber is not specifically limited, It is preferable that it is 0.05-20 micrometers. More preferably, it is 0.1 to 5 μm. The average fiber length of the glass fibers is not particularly limited, but is preferably 0.5 to 10000 μm. More preferably, it is 1 to 1000 μm. On the other hand, organic fibers are, for example, polypropylene fibers, acrylic fibers, vinylon fibers, cellulose fibers, polyester fibers or aramid fibers. The average fiber diameter of the organic fiber is not particularly limited, but preferably 0.05 to 100 μm. More preferably, it is 0.1 to 50 μm. The average fiber length of the organic fiber is not particularly limited, but in the case of a short fiber, it is preferably 0.5 to 10000 μm. More preferably, it is 10 to 5000 μm. The method for producing the non-woven fabric is not particularly limited, and is, for example, a dry method or a wet method.

  The shape of the support is not particularly limited, and may not have a sheet-like planar structure. For example, the material of the support may be three-dimensionally processed as in a pleating process to form a zigzag fold in which mountain folds and valley folds are repeated. By using a support that has been pleated in advance, it is possible to dry a long support within a dry area with a limited volume, and to obtain a filter material for an air filter to which nanocellulose is efficiently attached. Can. Furthermore, although freezing is likely to be uneven in filter media subjected to three-dimensional processing in a refrigerator etc., in vacuum drying there is also the advantage that it is less susceptible to three-dimensional processing because it utilizes the temperature drop due to vaporization. . Furthermore, three-dimensional processing can also be performed in the post process of the filter material for air filters obtained by the manufacturing method of the filter material for air filters which concerns on this embodiment.

  Moreover, it is preferable that the average hole diameter of a support body is 0.1-50 micrometers. More preferably, it is 0.5 to 10 μm. If it is less than 0.1 μm, the fluid permeability may be poor. When it exceeds 50 μm, it may be difficult for nanocellulose to form a network structure uniformly in the pores of the support. In the present embodiment, the dispersion containing nanocellulose can be deposited in the pores of the support and then dried to form an air filter, but this can be achieved by using a support having an appropriate average pore diameter. Since the cohesion generated with respect to the nanocellulose is easily dispersed during drying, the network structure can be easily maintained after drying. Here, the average pore size can be measured according to ASTM E 1294-89 "half dry method".

  Preferably, the support itself is a material that can be used as a filter material for an air filter. In the method of manufacturing the air filter according to the present embodiment, by using such a support, it is easy to obtain an air filter having higher particle collection performance than the conventional air filter filter medium (support itself). .

<Nanocellulose>
Nanocellulose (hereinafter sometimes referred to as nanofiber) includes chemically treated (modified) nanocellulose. In nanocellulose, cellulose molecular chains form a bundle of two or more. When the cellulose molecular chains form a bundle of two or more, it means that two or more cellulose molecular chains are assembled to form an aggregate called microfibrils. In this embodiment, in the cellulose molecular chain, a part or all of the C6 position hydroxyl group in the molecule is oxidized to an aldehyde group or a carboxyl group, or a part or all of the hydroxyl groups other than the C6 position is oxidized to a carbonyl group , Those in which some or all of the hydroxyl groups containing hydroxyl groups other than C6 position are esterified as nitrate ester, acetate ester or phosphate ester, or methyl ether, hydroxypropyl ether or carboxymethyl ether etc. And forms which are substituted with other functional groups such as those which are etherified. When the cellulose molecular chain is modified to contain a carboxyl group, the counter ion is substituted by a cationic surfactant, that substituted by various metal ions, or the metal ion is reduced. It includes those in particulate form.

  In the present embodiment, the number average fiber diameter of the nanocellulose is preferably 1 to 100 nm. In order to make an air filter compatible with high particle collection performance and low pressure loss, it is important to form in the support a uniform fiber network of extremely narrow nanocellulose fibers. When an ultrafine nanocellulose having a number average fiber diameter of 1 to 100 nm is used, the number of fibers per unit volume in the filter medium for air filter increases remarkably, and particles in gas become easy to be trapped, and high collection performance It is possible to obtain In addition, the slip flow effect makes the air flow resistance of single fibers extremely low, making it difficult for the pressure loss as an air filter to rise. 2-30 nm is preferable and, as for the number average fiber diameter of nanocellulose, 3-20 nm is more preferable. If the number average fiber diameter is less than 1 nm, the single fiber strength of the nanocellulose is weak, and it may be difficult to maintain the fiber network in the filter material for an air filter. If it exceeds 100 nm, the number of fibers per unit volume in the filter material for air filter may decrease, and it may not be possible to form an effective nanocellulose network for capturing particles. Here, the number average fiber diameter is calculated according to the following. The nanocellulose cast on the carbon film-coated grid is observed with an electron microscope image using a transmission electron microscope (TEM, Transmission Electron Microscope). With respect to the obtained observation image, a random axis is drawn for each sheet of image in a row and column, and the fiber diameter of the fiber intersecting the axis is read visually. At this time, observation is performed at a magnification of 5000, 10000 or 50000 depending on the size of the fibers to be configured. In addition, a sample or magnification is taken as the conditions which 20 or more fibers cross | intersect an axis. Thus, images of at least three non-overlapping surface portions are taken with an electron microscope, and the values of the fiber diameter of fibers intersecting each two axes are read. Therefore, at least 20 x 2 axes x 3 = 120 pieces of fiber information can be obtained. The number average fiber diameter was calculated from the data of the fiber diameter thus obtained. In addition, about the fiber which has branched, if the length of the branched part is 50 nm or more, it is integrated in calculation of a fiber diameter as one fiber. Further, the number average fiber diameter may be calculated according to the following. The nanocellulose present on or in the surface of the support is observed with an electron microscope image using a scanning electron microscope (SEM, Scanning Electron Microscope). With respect to the obtained observation image, a random axis is drawn for each sheet of image in a row and column, and the fiber diameter of the fiber intersecting the axis is read visually. At this time, observation is performed at any magnification of 5000 to 50000 times depending on the size of the fibers to be configured. Images of a plurality of non-overlapping surface portions are taken with an electron microscope, and the fiber diameter values of fibers intersecting each of the two axes are read. The number average fiber diameter is calculated from the fiber diameter data of at least 120 fibers. In addition, about the fiber which has branched, if the length of the branched part is 50 nm or more, it is integrated in calculation of a fiber diameter as one fiber. In order to obtain a distortion-free observation image, the sample is coated in advance with a conductive coating, but the influence of the coating thickness is also taken into consideration. For example, in the case of using ion sputtering (E-1045, manufactured by Hitachi High-Technologies Corporation), the coating film thickness is 12 nm with a discharge current of 15 mA, a sample-target distance of 30 mm, a vacuum degree of 6 Pa, and a coating time of 2 minutes. However, when measuring the fiber diameter, the coating film thickness is half of the assumed thickness because the deposition direction of the coating film is perpendicular to the expected direction. That is, in the case of coating under the above conditions, the coating film thickness of 12 nm (= 6 nm + 6 nm) for both ends is excluded from the fiber diameter obtained from the SEM.

  The number average fiber length of the nanocellulose is not particularly limited, but is preferably 0.01 to 20 μm. More preferably, it is 0.05 to 10 μm. More preferably, it is 0.08 to 1.0 μm. If the number average fiber length is less than 0.01 μm, the nanofibers become close to particles, and there is a possibility that a fiber network can not be formed in the air filter filter medium. When it exceeds 20 μm, the entanglement of nanofibers increases, and the fibers may aggregate to make it impossible to form a uniform network. The number average fiber length is calculated by observing nanocellulose with an electron microscope image using a scanning electron microscope (SEM, Scanning Electron Microscope). For the obtained observation image, 10 independent fibers are randomly selected per image, and the fiber length is read visually. At this time, according to the length of the fiber to comprise, it carries out by one of the magnification of 1000-30000. In addition, a sample or magnification makes object the thing in which the starting point and the end point of a fiber fit in the same image. Thus, an image of at least 12 non-overlapping surface portions is taken with an SEM to read the fiber length. Therefore, at least 10 × 12 sheets = 120 pieces of fiber information can be obtained. The number average fiber length can be calculated from the fiber diameter data thus obtained. In addition, about the fiber which has branched, let the length of the longest part of the fiber be a fiber length.

  In the method for producing a filter medium for an air filter according to this embodiment, the number average fiber length of the nanocellulose is preferably smaller than the average pore diameter of the support. By making the number average fiber length of the nanocellulose smaller than the average pore diameter of the support, the nanocellulose can penetrate to the inside of the support to form a network inside the support. As a result, even if the packing density of nanocellulose is relatively low, it is possible to provide a filter material for an air filter with higher particle collection performance. Nanocellulose has a very high ability to connect fibers to form a network, so it is sufficient as a filter material for air filter even if the number average fiber length is smaller than the average pore diameter of the support, and even if the packing density is relatively low. Network strength. The value of the number average fiber length of the nanocellulose is preferably 0.1 to 99%, more preferably 5 to 36%, relative to the value of the average pore diameter of the support. If the amount is less than 0.1%, the contact points between the nanocelluloses and between the nanocellulose and the support may be reduced, and the nanocellulose may be detached from the support. If it exceeds 99%, nanocellulose may not be able to penetrate into the inside of the support.

  The type of nanocellulose is not particularly limited, but, for example, nanocellulose obtained by oxidizing cellulose using an N-oxyl compound such as the aforementioned MFC, NFC, CNC, or TEMPO described in Patent Document 1 It is. MFC and NFC are characterized in that the distribution of fiber diameter is broad because cellulose fibers are sheared into nanofibers by mechanical processing. CNC has a relatively uniform fiber diameter, but is characterized by a short fiber length of 0.1 to 0.2 μm. The nanocellulose described in Patent Document 1 is cellulose single microfibrils. In natural cellulose, microfibrils are bundled to form a high-order solid structure. Here, microfibrils are strongly aggregated by hydrogen bonds derived from hydroxyl groups in cellulose molecules. Cellulose single microfibrils refer to isolated microfibrils by chemical treatment and slight mechanical treatment of natural cellulose. As described in Patent Document 1, a cellulose raw material is oxidized with an oxidizing agent in the presence of an N-oxyl compound, a bromide, an iodide or a mixture thereof, and the oxidized cellulose is further subjected to wet granulation. It is manufactured as an aqueous dispersion by processing, disentangling and nanofiberizing, and it is characterized by having a uniform fiber diameter. Among these, the fine cellulose described in Patent Document 1 is particularly preferable in that it requires less energy for production than other cellulose fibers and is high in productivity. In the nanocellulose described in Patent Document 1, a part of the hydroxyl groups of cellulose molecules is oxidized to at least one functional group selected from the group consisting of a carboxyl group and an aldehyde group, and has a cellulose type I crystal structure. The maximum fiber diameter is 1000 nm or less. The nanocellulose becomes a transparent liquid when dispersed in water.

  Although the cellulose raw material used as the raw material of nanocellulose is not specifically limited, It is preferable that it is plant-based pulp, especially wood-based pulp. Plant-based pulps include, for example, kraft pulps derived from various woods such as hardwood kraft pulp (LBKP) and softwood kraft pulp (NBKP); sulfite pulp; waste paper pulp such as deinked pulp (DIP); grand pulp (GP) Mechanical pulps such as pressurized groundwood pulp (PGW), refiner groundwood pulp (RMP), thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), chemimechanical pulp (CMP) or chemiground pulp (CGP); Powdered cellulose obtained by grinding them with a high-pressure homogenizer or mill, or microcrystalline cellulose powder obtained by purifying them by chemical treatment such as acid hydrolysis can be used. Also, non-wood pulp derived from plants such as kenaf, hemp, rice, bagasse, bamboo or cotton can be used. The present embodiment is not limited to the raw material and production method of nanofibers.

  The method for producing nanofibers is, for example, the production method described in Patent Document 1. According to Patent Document 1, a method of producing nanofibers is a reaction product, wherein natural cellulose is used as a raw material, an N-oxyl compound such as TEMPO is used as an oxidation catalyst in water and the co-oxidant acts to oxidize the natural cellulose. The process includes an oxidation reaction step of obtaining a fiber, a purification step of removing impurities to obtain a reactant fiber containing water, and a disintegration step of dispersing the reactant fiber containing water as a nanocellulose in a dispersion medium.

  In the filter material for an air filter obtained by the manufacturing method according to the present embodiment, nanocelluloses are intertwined in the pores of the support to form voids between the nanocelluloses, and a film or nanocellulose in which nanocelluloses are intertwined It is preferred not to have an agglutinated membrane. The pressure drop can be prevented from rising and the particle permeability can be lowered. A film in which nanocellulose is entangled or a film in which nanocellulose is aggregated is a film-like material in which nanocellulose is formed by physical entanglement or chemical aggregation and the like, and which blocks all or a part of pores of a support. .

  In the present embodiment, the ratio of the adhesion amount of nanocellulose to the support is preferably 0.001 to 0.500% by mass. Preferably it is 0.010-0.300 mass%, More preferably, it is 0.050-0.200 mass%. By setting such an amount of adhesion, an air filter having high particle collection performance and relatively low pressure loss can be obtained. If the adhesion amount of nanocellulose to the support is less than 0.001% by mass, the particle collection performance may be poor. Conversely, if it exceeds 0.500% by mass, the pressure loss may be too high. The adhesion amount of nanocellulose to the support can be mainly controlled by the concentration of nanocellulose in the dispersion and the adhesion amount of the dispersion to the support, and the concentration of nanocellulose in the dispersion is increased As the amount of dispersion liquid attached to the support increases, the amount of nanocellulose attached to the support increases.

  In this embodiment, an air filter can be obtained by depositing a dispersion containing nanocellulose on a support and drying under reduced pressure. A dispersion can be obtained by dispersing nanocellulose in a dispersion medium. The form of the nanocellulose in the dispersion is, for example, a form in which the nanocellulose is dispersed separately or in a form in which the nanocellulose is partially aggregated. Among these, the form of nanocellulose in the dispersion liquid is preferably a form in which nanocellulose is dispersed separately.

<Dispersion medium>
In this embodiment, a dispersion containing nanocellulose is attached to a support and then dried under reduced pressure to obtain a filter medium for an air filter, but the dispersion medium in reduced pressure drying is preferably water, and it is dissolved in water and water. It is more preferable to use a mixed dispersion medium which is mixed with an organic solvent to be used. Here, the organic solvent refers to an organic compound which is liquid at normal temperature and pressure. Further, “dissolving in water” means that in a mixed dispersion medium in which water and an organic solvent are mixed, the water and the organic solvent are mixed at a molecular level with each other at an arbitrary ratio, and phase separation does not occur. That the dispersion medium is a mixture of water and an organic solvent that dissolves in water suppresses crystallization of the dispersion medium at the time of freezing due to the temperature decrease of the dispersion medium at the initial stage of reduced pressure drying, and contributes to the uniform network formation of nanocellulose Do. In the present embodiment, the concentration of the organic solvent in the mixed dispersion medium is preferably 2 to 50% by mass. More preferably, it is 5-40 mass%, More preferably, it is 10-30 mass%. When the concentration of the organic solvent exceeds 50% by mass, it becomes a highly hydrophobic dispersion medium, and the nanocellulose having hydrophilicity may not be uniformly dispersed in the dispersion, which may impair the formation of the uniform network of the nanocellulose. . When the concentration of the organic solvent is less than 2% by mass, the formation of water crystals (ice crystals) is remarkable at the time of freezing due to the temperature decrease of the dispersion medium at the initial stage of reduced pressure drying, causing aggregation and structural destruction of nanocellulose, It may not be possible to form a uniform nanocellulose network on the support.

  The organic solvent preferably contains at least one of alcohols, carboxylic acids or carbonyl compounds. By including such an organic solvent, water crystals (ice crystals) generated upon freezing due to the temperature decrease of the dispersion medium at the initial stage of reduced pressure drying can be made smaller, and the network formation of nanocellulose in the support becomes more uniform. It can be Also, the organic solvent is (1) methanol, (2) ethanol, (3) 1-propanol or (4) t-butyl alcohol as alcohols, (5) acetic acid as carboxylic acids, (6) acetone as carbonyl compounds It is more preferable to include at least one of (1) to (6) from the viewpoint of compatibility with water. Among these, it is particularly preferable that the organic solvent is only t-butyl alcohol. The freezing point of the mixed dispersion medium in which water and t-butyl alcohol are mixed is at most about −10 ° C. and is higher than the mixed dispersion medium of other organic solvent and water, and it is possible to start drying under reduced pressure at the beginning It is easy to freeze. In a t-butyl alcohol aqueous solution, it is known that water and t-butyl alcohol become eutectic at a t-butyl alcohol concentration of around 20% by mass, and the crystal size at the time of freezing becomes small. When the organic solvent is only t-butyl alcohol, the concentration of t-butyl alcohol in the mixed dispersion medium is preferably 15 to 35% by mass, and more preferably 20 to 30% by mass.

<Dispersion liquid>
In the present embodiment, it is preferable to set the solid content concentration of the nanocellulose in the dispersion to 0.001 to 0.150 mass%. More preferably, it is 0.005-0.100 mass%, More preferably, it is 0.010-0.080 mass%. In forming a network of nanocellulose on a fluid-permeable support to form a filter material for an air filter, it is preferable that the nanocellulose network be uniformly distributed without having a certain directionality. If the nanocellulose network in the support is not twisted in a specific direction (oriented), it is easy to satisfy the particle trapping performance as a filter material for an air filter. When the solid concentration of nanocellulose in the dispersion exceeds 0.150% by mass, many domains in which fibers are oriented are easily formed. The fact that the fibers are oriented means that the fibers are lined up in the same direction to some extent, and the orientation is not preferable for the filter material for an air filter, in which the particle trapping performance becomes higher as the degree of dispersion of the fibers is higher. In addition, when the packing density of nanocellulose in the filter material for air filter becomes excessively high, the distance between the nanocelluloses distributed in the filter material for air filter can not be maintained appropriately, and the moisture in the air causes aggregation etc. There is a possibility that the formation of the network by nano cellulose suitable as a filter material for air filters may be inhibited. A dispersion of nanocellulose obtained by oxidizing cellulose using an N-oxyl compound such as TEMPO has high transparency in water and is uniformly dispersed, but when the concentration is high, the fibers are particularly likely to be oriented. In the present embodiment, by setting the solid content concentration of the nanocellulose in the dispersion to 0.150% by mass or less, the distance between the fibers in the dispersion is appropriately separated, and the nanocellulose is not oriented at all or hardly. Therefore, even when nanocellulose is incorporated into an air filter, a uniform fiber network can be formed without specific directionality, and it has the effect of significantly enhancing the particle collection performance as a filter material for an air filter. On the other hand, when the concentration of nanocellulose in the dispersion is less than 0.001% by mass, the entanglement of the nanocelluloses is reduced, and there is a possibility that the network structure can not be maintained.

<Preparation of Dispersion>
In the present embodiment, the method of preparing the dispersion is not particularly limited, and nanocellulose may be dispersed in the above-described dispersion medium to obtain a dispersion. In the case of using a mixed dispersion medium in which water and an organic solvent soluble in water are mixed, nanocellulose is dispersed in water to prepare a nanocellulose aqueous dispersion, and then an organic solvent or water is added to the nanocellulose aqueous dispersion. It is preferable to carry out by adding a mixed solvent with an organic solvent. When an aqueous dispersion of nanocellulose is added to an organic solvent, aggregates may be generated.

  In the present embodiment, various assistants may be blended in the dispersion. In the vacuum drying described later, since the dispersion is frozen due to the temperature drop due to the vaporization of the dispersion medium in the dispersion, in order to stabilize it, for example, sucrose, trehalose, L-arginine or L-histidine is added can do. Moreover, as a surface modification agent of nanocellulose, a cationic surfactant, an anionic surfactant, a nonionic surfactant, or an amphoteric surfactant can also be mix | blended, for example.

<Adhesive process>
The method for attaching the dispersion to the support is not particularly limited, and is, for example, an impregnation method, a coating method or a spraying method. The adhesion amount of the dispersion to the support is appropriately adjusted according to the thickness, material and average pore size of the support, but as described above, in the present embodiment, the adhesion amount of nanocellulose to the support Ratio of 0.001 to 0.500% by mass. More preferably, it is 0.010-0.300 mass%, More preferably, it is 0.050-0.200 mass%. By setting the ratio of the adhesion amount of nanocellulose to the support to 0.001 to 0.500% by mass, a highly efficient filter material for an air filter having improved particle collection performance can be produced. When the ratio of the adhesion amount of nanocellulose to the support is less than 0.001% by mass, the adhesion amount of nanocellulose to the support is insufficient, and it is difficult to form a uniform nanocellulose network. As a result, there is a possibility that the particle collection performance as a filter material for an air filter can not be sufficiently improved. On the other hand, if it exceeds 0.500% by mass, the nanocellulose packing density in the filter material for air filter may be excessively high, and the pressure loss may be excessively high. Further, in order to form a network of nanocellulose to obtain a high PF value, the ratio of the adhesion amount of nanocellulose to the support is preferably 0.001 to 0.500 mass%, More preferably, it is 0.300% by mass. In the present invention, the method of calculating the ratio of the adhesion amount of nanocellulose to the support is not particularly limited. For example, when the support is composed of only inorganic fibers, only the nanocellulose is burned and burned. It can be calculated from the weight loss ratio later. Moreover, when a support body contains an organic synthetic fiber, only a cellulose can be melt | dissolved, for example using a copper ethylenediamine solution, and it can also calculate from the melt | dissolution amount. In addition, the ratio of the adhesion amount of nanocellulose to the support may be determined from the wet adhesion amount. That is, the ratio (unit%) of the adhesion amount of nanocellulose to the support is {(wet adhesion amount × solid concentration of nanocellulose in dispersion) / mass of support before adhesion of dispersion} × 100 It is. Here, the wet adhesion amount is the difference between the mass of the support in the wet state where the dispersion is adhered and the mass of the support before the adhesion, and the dispersion adhered to the support at the start of the drying step. It means the mass of the liquid. Therefore, the wet adhesion amount is preferably a value measured immediately before the drying step, for example, preferably measured within 10 minutes before the start of the drying step, and more preferably measured within 5 minutes.

  As the impregnation method, for example, there is a method of completely immersing the support in the dispersion or a method of immersing only the surface of the support. The method of completely immersing the support in the dispersion can form a more uniform nanocellulose network because the dispersion can permeate the dispersion into the pores of the support efficiently and reliably. It is excellent in point. In addition, if the pressure is reduced while completely immersing the support in the dispersion, air in the support is likely to be released, which is more effective for causing the dispersion to permeate. In addition, it is preferable to squeeze out the dispersion liquid adhering to excess with a roll dehydrator etc., or to remove with water absorption members, such as water absorption felt or water absorption paper. In the method of immersing only the surface of the support, the density difference of the nanocellulose network structure in the pores in the thickness direction of the support (the presence of the nanocellulose network structure on one side and the other side of the support) It is effective in the case of providing different ratios.

  The application method is a method of applying the dispersion to the surface of the support with a known application machine or brush. The known coating machine is, for example, a curtain coater (die coater). The coating method is excellent in that the control of the amount of the dispersion attached to the support is easy.

  The spraying method is a method of spraying the dispersion onto the surface of the support using a known sprayer such as spraying or spraying. In the spraying method, for example, when it is desired to form a nanocellulose network structure only in the vicinity of the surface of the support among the pores of the support, or the support is contacted with a large amount of impregnation solution or rolls or bars of a coating machine. It is effective when you do not want to make it happen. In the coating method or the spraying method, the dispersion can be made to penetrate into the inside of the support by reducing the pressure on one side of the support to cause a flow of air and applying or spraying from the other side. In vacuum drying, the amount of liquid adhering to the support can be reduced by attaching nanocellulose by spraying in the adhesion step, and the dispersion tends to freeze due to vaporization of the dispersion medium at the beginning of drying. It is preferable to select the spraying method in the adhesion step.

<Drying process>
In the present embodiment, the dispersion is attached to the support as described above, and after the support is in a wet state, drying under reduced pressure is performed. The vaporization of the dispersion medium in the dispersion in the early stage of drying lowers the temperature of the dispersion to freeze the dispersion, and in the latter stage of drying, the drying is completed by sublimation of the dispersion medium. Thereby, since aggregation of nanocellulose can be suppressed and a nanocellulose network can be constructed | assembled, it becomes possible to manufacture the filter material for air filters which improved filter performance in a comparatively short time.

  In the present embodiment, the pressure of the drying step in the drying process is preferably 1000 Pa or less, more preferably 100 Pa or less. When the pressure exceeds 1000 Pa, the dispersion medium in the dispersion does not freeze in the initial stage of drying, and a filter medium for air filter with improved filter performance may not be obtained. The lower limit of the pressure is not particularly limited, but the lower the pressure, the more likely freezing occurs in the early stage of drying.

  The drying temperature is a temperature at which the support and the nanofibers do not undergo decomposition or deformation, and the temperature at which the dispersion is frozen by vaporization of the dispersion medium in the dispersion at an early stage of drying is selected. Although different depending on the type of dispersion medium, when using, for example, a mixed dispersion medium of water and t-butyl alcohol, the drying temperature is preferably 0 ° C to 50 ° C, and more preferably 20 ° C to 40 ° C. Under conditions where the drying temperature is too high, it is difficult to freeze the dispersion only by the temperature drop due to the vaporization of the dispersion medium. When the dispersion medium is a mixed dispersion medium of water and t-butyl alcohol, the melting point of the dispersion containing nanocellulose and the dispersion medium is equal to the melting point of the dispersion medium. According to Non-Patent Document 1, the melting point of the mixed dispersion medium of water and t-butyl alcohol is at most −8.2 ° C. Therefore, when the dispersion medium is a mixed dispersion medium of water and t-butyl alcohol, it is preferable to set the temperature of the sample to −8.2 ° C. or less due to the temperature decrease due to vaporization of the dispersion medium at the beginning of drying.

  In the present embodiment, the temperature of the dispersion liquid is lowered to freeze the dispersion liquid by evaporating the dispersion medium in the dispersion liquid in the early stage of drying. Here, the initial stage of drying refers to a period in which the sample is placed under reduced pressure and then the dispersion medium is vaporized to freeze the sample. The ambient temperature of the sample is usually at room temperature unless specifically controlled. In the case of freezing at a temperature at which the dispersion medium is frozen in advance using a refrigerator or the like under atmospheric pressure or under a pressure lower than atmospheric pressure, it is necessary to remove all or most of the dispersion medium by sublimation. It takes time to dry. In the present invention, since the dispersion medium is partially removed by vaporization, the drying time can be shortened, and since the nanocellulose is fixed by freezing of the dispersion medium caused by the initial stage of drying, the filter performance can be reduced in a short time. It becomes possible to manufacture the improved filter material for air filters. When the pressure exceeds 1000 Pa, the dispersion medium in the dispersion is almost vaporized without freezing, and in such a case, the nanocellulose does not form a three-dimensional network structure, and the performance of the air filter is improved. Does not contribute to

  At the late stage of drying, the drying proceeds by sublimation of the frozen dispersion medium. Here, the late drying period is a period during which the dispersion medium is removed by sublimation of the sample frozen in the early stage. The method for determining the end of drying is not particularly limited, but the temperature of the most difficult-to-dry portion of the sample is measured with a contact-type or non-contact-type thermometer, and it is determined from the temperature rise Or a method in which the pressure in the drying system is measured and judged from the point of sufficient vacuum.

It is preferable that the filter material for an air filter obtained by the manufacturing method according to the present embodiment has a high PF value defined by Equation 1 below. The PF value is an index for evaluating the superiority or inferiority of the balance between the pressure drop and the particle collection performance, and is calculated using the equation shown in equation 1. The higher the PF value, the lower the particle permeability of the target particles, and the lower the pressure drop of the filter material for air filter.

  In equation 1, the pressure drop is measured, for example, using a manometer. The particle permeability is a ratio of PAO particles to be transmitted through the air filter or the filter material for air filter when air containing polydispersed polyalphaolefin (PAO) particles generated by the Raskin nozzle is allowed to pass. Particle transmittance is measured, for example, using a laser particle counter.

  The PF value of the filter material for an air filter is influenced by the type and constitution of the support, but the packing density of nanocellulose or the degree of formation of a network of nanocellulose greatly influences it. In the filter material for an air filter obtained by the manufacturing method according to the present embodiment, the ratio of the adhesion amount of nanocellulose to the support is preferably 0.001 to 0.200% by mass, but such an adhesion amount is Even if, for example, the adhesion of nanocellulose is concentrated only near the surface layer of the support and the packing density of nanocellulose is partially excessively high, the pressure loss is excessively increased, resulting in a decrease in the PF value. Do. The filter medium for air filter preferably has a network of nanocellulose on the inside and / or on the surface of the support and does not have film-like aggregates of nanocellulose. More specifically, when a dispersion having a high concentration of nanocellulose is attached to a support, the adhesion of nanocellulose is concentrated on the surface of the support, and it is thought that the nanocelluloses aggregate on the surface of the support. Be As a result, in the surface layer of the support, a network of nanocellulose is not formed, and film-like aggregates may occur. When a filter medium for an air filter in which such film-like aggregates are partially formed is used, an increase in pressure loss and a decrease in particle collection performance (that is, a decrease in PF value) occur, and in some cases an air filter It becomes impossible to maintain breathability. Even if adhesion of nanocellulose is concentrated only in the vicinity of the surface layer of the support, if the nanocellulose network is appropriately formed (unless the packing density of nanocellulose is too high), the pressure loss will increase so much. Therefore, it is possible to obtain a PF value suitable as an air filter.

  EXAMPLES The present invention will next be described in more detail by way of examples, which should not be construed as limiting the invention thereto. Moreover, unless otherwise indicated, "part" and "%" in an example show a "mass part" and "mass%", respectively. In addition, the addition part number is a value of solid content conversion.

[Step of preparing nanocellulose aqueous dispersion]
An equivalent of 2.00 g dry weight NBKP (mainly consisting of fibers with a fiber diameter greater than 1000 nm) and 0.025 g TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyradical) After dispersing 0.25 g of sodium bromide in 150 ml of water, a 13% aqueous solution of sodium hypochlorite is added to 1.00 g of pulp (NBKP) in an amount of 5.00 mmol of sodium hypochlorite. The reaction was initiated by the addition of sodium hypochlorite as indicated. During the reaction, the pH was maintained at 10 by dropwise addition of 0.50 mol / l aqueous sodium hydroxide solution. After reacting for 2 hours, the reaction product was filtered and thoroughly washed with water to obtain an oxidized cellulose (TEMPO oxidized cellulose) slurry. A 0.5 mass% TEMPO oxidized cellulose slurry is defibrillated at 15,000 rpm for 5 minutes using a biomixer (BM-2, manufactured by Nippon Seiki Seisakusho Co., Ltd.) to dilute the solid content concentration to 0.2 mass% Thereafter, it was further subjected to disintegration treatment for 8 minutes with an ultrasonic dispersion machine (type US-300E, manufactured by Nippon Seiki Seisakusho Co., Ltd.). Thereafter, coarse fibers were removed by centrifugation to obtain a nanocellulose aqueous dispersion in which TEMPO oxidized nanocellulose was dispersed in water. The number average fiber diameter was 4 nm, as a result of analyzing this nanocellulose water dispersion liquid from the observation image observed by 50000 magnifications using TEM (JEM2000-EXII, JEOL Ltd. make). Further, as a result of analysis from an observation image observed at a magnification of 10000 using a SEM (SU8010, manufactured by Hitachi High-Technologies Corporation), the number average fiber length was 0.8 μm. To the nanocellulose aqueous dispersion, water and t-butyl alcohol were added, the container was covered, and stirred with a magnetic stirrer for 5 minutes to obtain a nanocellulose dispersion. The solid concentration of nanocellulose relative to the total mass of the dispersion was 0.02%. Further, the mixing ratio of water to t-butyl alcohol in the nanocellulose dispersion was 70:30 by mass.

Example 1
A glass fiber with a basis weight of 51 g / m ² and a pressure loss of about 70 Pa (22 g of ultrafine glass fibers having an average fiber diameter of 0.65 μm and an ultrafine glass having an average fiber diameter of 2.4 μm) It was made to adhere to a non-woven fabric (hereinafter referred to as "support") consisting of 63 parts of fiber and 15 parts of chopped glass fiber having an average fiber diameter of 6 μm, and the amount of wet adhesion was determined. The ratio of the adhesion amount of nanocellulose to the support as calculated from the wet adhesion amount was 0.13%. The "support" in a wet state is placed in a thermostatic bath set at 25 ° C, the pressure is reduced with a dryer (VD-250F TAITEC) connected by a vacuum tube, and drying is performed under reduced pressure to obtain a "support". The air filter medium to which the cellulose adhered was obtained. With the start of the operation of the vacuum dryer, the temperature reading on the thermocouple reached 25 ± 0.5 ° C, and the indication on the thermometer fluctuated within ± 0.5 ° C over 30 minutes. The time from the start point to the end point was taken as the preparation time, with the end point at which drying was sufficiently completed. The “support” was frozen after 8 minutes from the start point, and the degree of vacuum at that time was 20 Pa.

(Example 2)
A solution obtained by diluting the aqueous dispersion of nanocellulose with water is attached to the support without adding t-butyl alcohol, and the ratio of the amount of nanocellulose attached to the support converted to the amount of wet attachment is 0.10%, An air filter medium was obtained in the same manner as in Example 1 except that the temperature of the surface of the air filter medium was measured during drying under reduced pressure with a thermocouple of a mold and controlled so that the pressure gauge indicated 300 Pa. Note that with the vacuum drying operation as the starting point, the temperature reading on the thermocouple reached 25 ± 0.5 ° C, and the indication on the thermometer fluctuated within ± 0.5 ° C over a 30-minute period. The time from the start point to the end point was taken as the preparation time, as the end point where drying was sufficiently completed. Also, the "support" was frozen after 15 minutes from the start point.

(Example 3)
The ratio of the adhesion amount of nanocellulose to the support converted from the wet adhesion amount is 0.11%, the temperature of the air filter material surface is measured during vacuum drying with a contact type thermocouple, and the pressure gauge indicates 900 Pa An air filter medium was obtained in the same manner as in Example 1 except that control was performed so that Note that with the vacuum drying operation as the starting point, the temperature reading on the thermocouple reached 25 ± 0.5 ° C, and the indication on the thermometer fluctuated within ± 0.5 ° C over a 30-minute period. The time from the start point to the end point was taken as the preparation time, as the end point where drying was sufficiently completed. Also, the "support" was frozen 12 minutes after the start point.

(Example 4)
An air filter was prepared in the same manner as in Example 1 except that t-butyl alcohol was used as the organic solvent in the nanocellulose dispersion liquid to obtain 2-propanol, and the mixing ratio of water to 2-propanol was 85:15 in mass ratio. A filter medium was obtained. Note that with the vacuum drying operation as the starting point, the temperature reading on the thermocouple reached 25 ± 0.5 ° C, and the indication on the thermometer fluctuated within ± 0.5 ° C over a 30-minute period. The time from the start point to the end point was taken as the preparation time, as the end point where drying was sufficiently completed. Also, the "support" was frozen 12 minutes after the start point.

(Comparative example 1)
The “support” made of the glass fiber of Example 1 was used as it was as an air filter medium.

(Comparative example 2)
In the same manner as in Example 1, a mixture containing nanocellulose was attached to a "support" by means of a Han display. The wet samples were frozen in a -50 ° C freezer for 1 hour. Thereafter, the frozen “support” is placed in a freezer set at −20 ° C., the pressure is reduced by a freeze dryer (manufactured by VD-250F TAITEC) connected by a vacuum tube, and dispersion in a “support air filter” By subliming the medium, an air filter medium in which nanocellulose was attached to the "support" was obtained. Note that with the vacuum drying operation as the starting point, the temperature reading on the thermocouple reached 25 ± 0.5 ° C, and the indication on the thermometer fluctuated within ± 0.5 ° C over a 30-minute period. The time required for freezing and the time obtained by adding the time from the start point to the end point of reduced pressure drying as the end point when drying was sufficiently completed was used as the production time.

(Comparative example 3)
The ratio of the adhesion amount of nanocellulose to the support converted from the wet adhesion amount is 0.14%, the temperature of the air filter medium surface is measured during drying under reduced pressure with a contact type thermocouple, and the pressure gauge indicates 1200 Pa An air filter medium was obtained in the same manner as in Example 1 except that control was performed so that Note that with the vacuum drying operation as the starting point, the temperature reading on the thermocouple reached 25 ± 0.5 ° C, and the indication on the thermometer fluctuated within ± 0.5 ° C over a 30-minute period. The time from the start point to the end point was taken as the preparation time, as the end point where drying was sufficiently completed. Also, freezing of the "support" was not seen during the operation of the vacuum dryer.

  The preparation conditions and physical property values of the filter material for an air filter obtained in each Example and Comparative Example are shown in Table 1. Each physical property value was measured by the following method.

"Pressure loss"
The pressure loss was measured by using a manometer (Manostar WO81, manufactured by Yamamoto Denki Seisakusho Co., Ltd.) when passing an air at which the surface air velocity to the air filter is 5.3 cm / sec.

Particle Permeability
The particle permeability is the upstream and downstream when passing air containing polydispersed polyalphaolefin (PAO) particles generated by the Raskin nozzle so that the surface air velocity to the air filter filter medium is 5.3 cm / sec. The particle transmittance from the ratio of the number of particles was measured using a laser particle counter (LASAIR-1001, manufactured by PMS). The target particle diameter is 0.10 to 0.15 μm. The lower the measured value, the higher the target particle collection efficiency.

"PF value"
The PF value was calculated from the measured values of pressure drop and particle permeability using the equation shown in equation 1. The higher the PF value, the lower the particle permeability of the target particles, and the lower the pressure loss of the air filter.

"Network observation"
The network was observed by observing the filter material for an air filter at a magnification of 5,000 to 10,000 using a scanning electron microscope (abbreviated as SEM, manufactured by Hitachi High-Technologies Corporation, SU 8010). Before observation, conductive coating was performed using ion sputtering (E-1045, manufactured by Hitachi High-Technologies Corporation) under conditions of a discharge current of 15 mA, a sample-target distance of 30 mm, a vacuum degree of 6 Pa, and a coating time of 2 minutes.

"Freeze by vaporization"
Freezing by vaporization was judged from the change in surface condition by visual observation. That is, it was determined that there was freezing due to vaporization when frost-like frozen material was generated on the surface of the support.

  Comparing the filter media for air filter of Examples 1 to 4 with the filter media for air filter of Comparative Example 1, the particle transmittance of the filter media for air filter of Examples 1 to 4 is greater than that of the filter media for air filter of Comparative Example 1 Also, both the pressure loss and the PF value of the filter media for air filter of Examples 1 to 4 are higher than the filter media for air filter of Comparative Example 1, and the filter media for air filters of Examples 1 to 4 have higher performance. It was shown. Moreover, although the observation image of the electron microscope of Example 1 and the comparative example 1 is shown in FIGS. 1-4 as an example, in the filter medium for air filters of the comparative example 1, although only the space | gap between the fibers which comprise a support body exists. On the other hand, in the filter medium for an air filter of Example 1, in addition to the voids between the fibers constituting the support, the nanocelluloses are intertwined to form a network, and the voids are further formed between the nanocelluloses. Was confirmed. In terms of final vacuum degree in reduced pressure drying of Example 1 and Example 3, the air filter medium of Example 1 having a final degree of vacuum of 5 Pa has a PF value of 5 Pa as compared with Example 3 having a final degree of vacuum of 900 Pa. it was high. In Example 2, t-butyl alcohol was not used, the dispersion medium was water, and the final ultimate vacuum was 300 Pa. However, although the PF value is inferior to that of the filter material for air filter of Example 3, Comparative Example 1 The PF value was higher than that of the filter.

  From the comparison of Example 1 and Comparative Example 2, the difference between the performance and the preparation time of the filter media produced by vacuum drying and freeze drying was examined. The preparation time of the filter medium was the sum of the time from the start of drying to the end of drying in vacuum drying and the time required for freezing and the time from the start of drying to the end in lyophilization. The air filter performance of the filter material for an air filter of Comparative Example 2 in which lyophilization was performed was a pressure loss of 150 Pa, a particle transmittance of 1.7% at a particle diameter of 0.10 to 0.15 μm, and a PF value of 13.2. The pressure loss of the filter material for air filter of Example 1 dried under reduced pressure was lower than that of the filter material for air filter of Comparative Example 2 that was freeze dried, and the particle transmittance was high, but the PF value was equivalent. The Although the observation image of the electron microscope of the comparative example 2 is shown to FIG. 5, 6, compared with Example 1 (FIG. 1, 2), it turns out that it has a three-dimensional network structure of nanocellulose more precise | minute. As a result, in Comparative Example 2, although the particle permeability of the particles is low, it becomes a filter medium having a high pressure loss, and the PF value becomes equivalent to that of Example 1. On the other hand, when the preparation time of the air filter material of Example 1 and Comparative Example 2 was compared, in Comparative Example 2, although preparation time of 17 hours was required, preparation time of Example 1 was 4 hours. From this result, when the nanocellulose attached filter medium is produced by vacuum drying, the nanocellulose attached filter medium can be produced in a short time as compared with the filter medium produced by freeze drying, and can be obtained by vacuum drying. It is understood that the PF value of the filter medium is equivalent to the PF value of the filter medium obtained by lyophilization.

  In Example 4, the organic solvent of the nanocellulose dispersion liquid is replaced with t-butyl alcohol, and 2-propanol is used, and the mixing ratio of water and 2-propanol is 85:15 in mass ratio to prepare a nanocellulose attached filter medium. did. When Example 4 and Comparative Example 1 are compared, Example 4 has a high pressure loss, particle permeability is low, and Example 4 has a large PF value. Moreover, when Example 4 and Example 1 were compared, PF value was substantially equivalent. From the above, it can be seen that the present invention can be practiced even if the type of organic solvent is changed.

  In Comparative Example 3, a nanocellulose attached filter medium was produced with a final vacuum degree of 1200 Pa. In Comparative Example 3, compared with Comparative Example 1, no significant change is observed in the pressure drop and the PF value. As in Comparative Example 3, freezing of the support is not observed under the condition of the final vacuum degree of 1000 Pa or more, and it is understood that the nanocellulose attached filter medium with improved filter performance can not be obtained under such conditions.

From the above results, it can be seen that the method of manufacturing the filter material for an air filter according to the present embodiment can provide a method of manufacturing a filter material for an air filter with improved filter performance in a relatively short time using nanocellulose.

Claims (5)

Attaching the dispersion containing nanocellulose to a fluid-permeable support;
And drying the support after the adhesion step, in the method of producing a filter medium for an air filter,
In the drying step, the support subjected to the adhesion step is placed in a reduced pressure atmosphere below atmospheric pressure, so that the dispersion medium is vaporized, the temperature of the dispersion medium is decreased due to the heat of vaporization, and the dispersion is caused due to the temperature decrease. A process for producing a filter medium for an air filter, comprising freezing the medium and sublimating the frozen dispersion medium to dry the support.   The method for manufacturing a filter medium for an air filter according to claim 1, wherein the reduced pressure atmosphere has a degree of vacuum of 1000 Pa or less.   The method for producing a filter material for an air filter according to claim 1 or 2, wherein the dispersion medium of the dispersion liquid is a mixed liquid of water and at least one or more organic solvents.   The freezing process of the dispersion medium of the said dispersion liquid using a freezer is not performed in the said adhesion process thru | or the said drying process under a reduced pressure atmosphere under atmospheric pressure or a pressure less than atmospheric pressure, The any one of the Claims 1-3 characterized by the above-mentioned. The manufacturing method of the filter material for air filters as described in any one.   The method for producing a filter medium for an air filter according to any one of claims 1 to 4, wherein the drying method performed in the drying step is a reduced pressure drying method with a degree of vacuum of 1000 Pa or less.