JP6721919B2 - Filter material for air filter - Google Patents

Description

The present invention relates to a filter material for an air filter, particularly an air filter for use in an air purification facility such as a semiconductor, a liquid crystal, a clean room or a clean bench related to the bio/food industry, an air filter for a building air conditioner, or an air filter for an air cleaner. The present invention relates to a filter material for an air filter used.

A filter medium for an air filter is generally used to collect particles in the submicron to micron units in the air. Filter materials for air filters are roughly classified into a coarse dust filter, a medium performance filter, a HEPA (High Efficiency Particulate Air) filter, and an ULPA (Ultra Low Penetration Air) filter according to their collection performance. The basic characteristics of these filter media for air filters are that they have a high collection efficiency for fine dust particles, and that they have a low pressure loss in order to reduce the energy cost for ventilating air through the filter. It has been demanded.

Recently, the use of cellulose nanofibers has attracted attention. Cellulose nanofibers generally refer to (1) fine cellulose nanofibers (cellulose fibers) having a number average fiber diameter of 1 to 100 nm or (2) chemically treated (modified) fine cellulose nanofibers. The cellulose nanofiber of (1) is produced by, for example, microfibrinated cellulose, nanofibrinated cellulose (hereinafter abbreviated as MFC, NFC) or a microorganism which is obtained by shearing and defibrating the cellulose fiber under high pressure. It is fine bacterial cellulose (hereinafter abbreviated as BC). The modified cellulose nanofibers of (2) include, for example, cellulose nanocrystals (hereinafter abbreviated as CNC) obtained by treating natural cellulose with 40% or more concentrated sulfuric acid, or fibers constituting wood pulp. Microfibrils, which is the smallest unit of the above, are isolated as an aqueous dispersion by mild chemical treatment at room temperature and normal pressure and slight mechanical treatment, and are ultrafine and uniform fine cellulose fibers (for example, Patent Document 1). See.).

These cellulose nanofibers are expected to be used as a filter material or a porous body. For example, a method for producing a leukocyte-removing 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 retained in the porous element and having an average fiber diameter of 0.01 μm or more and less than 1.0 μm. Are shown (for example, refer to Patent Document 2). In Patent Document 2, bacterial cellulose produced by acetic acid bacteria, or cellulose fiber obtained by treating rayon with sulfuric acid and finely treated with a homogenizer is used as the fiber structure.

Not only cellulose but also nanofiber dispersions generally tend to aggregate when dried. Therefore, it is difficult to make the material breathable. As a breathable material using nanofibers, a porous body in which nanofibers having a number average diameter of 1 to 500 nm have a three-dimensional network structure in the holes of the support is shown (for example, Patent Document 3). See.).

Since cellulose nanofibers are highly hydrophilic, the cohesive force that works during drying is stronger than that of nanofibers derived from thermoplastic polymers. As a method for obtaining a material having air permeability using cellulose nanofibers, cellulose nanofibers are dispersed in a mixed solution of water and an organic solvent that is soluble in water, and cellulose is removed by freeze-drying the mixed solution. A method for producing a porous body is shown (for example, refer to Patent Document 4).

JP, 2008-1728, A Japanese Patent No. 3941838 JP, 2008-101315, A JP, 2013-253137, A

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

In Patent Document 2, bacterial cellulose produced by acetic acid bacteria is used as the thinnest fiber, and its average fiber diameter is 0.02 μm (=20 nm). As the other fibers, cellulose fibers obtained by treating rayon with sulfuric acid and finely divided with a homogenizer were used, and the thinnest had an average fiber diameter of 0.19 μm (=190 nm). When bacterial cellulose is used, acetic acid bacteria are cultivated in the porous element, but there are many restrictions on the culturing conditions and the production amount is small, which is not suitable for industrial use. Further, the fibrous structure is formed only on the surface of the porous element, and the fibrous structure is not formed inside the holes of the porous element. Cellulose fibers obtained by refining rayon are submicron fibers rather than nanofibers, and conventional microglass fibers can be sufficiently substituted. Patent Document 2 does not describe the dispersibility of fine fibers used in a fiber structure in a dispersion medium. Further, the filter material of Patent Document 2 is a filter material for liquid, and there is a problem that the porosity is too low as a filter material for air filters.

In Patent Document 3, it is preferable to use water as the dispersion medium for the nanofibers, and in the examples, only water is used as the dispersion medium. If only water is used as the dispersion medium, micron-sized crystals are generated when water is frozen, and nanofibers are aggregated around the crystals. Therefore, a uniform network of nanofibers is not formed, and it becomes unsuitable as a filter medium for air filters. Further, in Patent Document 3, a thermoplastic polymer having a number average diameter of 60 nm or more is exemplified as the nanofibers, but the same nanofibers have a fiber diameter of 1 to 50 nm and a fiber diameter of 60 to 500 nm. The performance of the obtained filter media for air filters is greatly different, and the intended use is also different. When the micron-sized crystals of water described above are produced, a nanofiber network having a fiber diameter of 1 to 50 nm cannot provide an extremely uniform nanofiber network, and a high-performance air filter medium cannot be obtained.

Patent Document 4 discloses a porous body in which cellulose nanofibers are adhered to a non-woven fabric made of glass fibers, but the density of the nanofibers becomes high because the ratio of the adhered amount of cellulose nanofibers is high. Therefore, there is a problem that the pressure loss is relatively high as a filter medium for an air filter.

An object of the present invention is to provide a filter medium for an air filter that uses cellulose nanofibers and has improved particle collection performance.

The filter medium for an air filter according to the present invention is a filter medium for an air filter in which cellulose nanofibers are attached to a support having air permeability, and the number average fiber diameter of the cellulose nanofibers is 1 to 50 nm, and the support is The ratio of the adhered amount of cellulose nanofibers to the body is 0.001 to 0.200% by mass, the target particle size is 0.10 to 0.15 μm, and the surface wind velocity is 5.3 cm/sec. And a PF value of 11.4 or more.

In the filter material for an air filter according to the present invention, the packing density of cellulose nanofibers is preferably 0.01 to 0.25 mg/cm ³ . The filter material for an air filter can have high particle collecting performance even though the pressure loss is relatively low.

In the filter material for an air filter according to the present invention, the number average fiber length of cellulose nanofibers is preferably smaller than the average pore diameter of the support. Even when the packing density of the cellulose nanofibers is relatively low, the filter material for an air filter can have a higher particle collecting performance.

The filter medium for an air filter according to the present invention preferably does not have a film in which the cellulose nanofibers are entangled with each other or a film in which the cellulose nanofibers are aggregated in the pores of the support. It is possible to prevent the pressure loss from increasing and increase the PF value. It is preferable that the filter material for an air filter according to the present invention does not have a region where the cellulose nanofibers are oriented, aggregated, and dense.

According to the present invention, it is possible to provide a filter medium for an air filter that uses cellulose nanofibers and has improved particle collection performance.

FIG. 6 is a diagram showing an SEM observation image of the filter material for an air filter of Example 3 (Osmium coating carried out, observation magnification: 10,000 times). FIG. 6 is a diagram showing an SEM observation image of an air filter medium of Comparative Example 2 (osmium coating carried out, observation magnification: 10,000 times). FIG. 8 is a view showing an SEM observation image of a filter medium for an air filter of Comparative Example 5 (Ion sputter coating was carried out, and an observation magnification was 10,000 times). FIG. 8 is a view showing an SEM observation image of the filter material for an air filter of Example 8 (Ion sputter coating was carried out, observation magnification was 10,000 times). It is a figure which shows the observation image (Ion sputter coating execution, observation magnification 1500 times) of the filter material for air filters of Example 11 immediately after a moisture resistance test by SEM.

Next, the present invention will be described in detail with reference to embodiments, but the present invention is not construed as being limited to these descriptions. The embodiments may be variously modified as long as the effects of the present invention are exhibited.

The filter medium for an air filter according to the present embodiment is a filter medium for an air filter in which cellulose nanofibers are attached to a support having air permeability, and the number average fiber diameter of the cellulose nanofibers is 1 to 50 nm, The ratio of the attached amount of cellulose nanofibers to the support is 0.001 to 0.200 mass %, the target particle diameter is 0.10 to 0.15 μm, and the surface wind speed is 5.3 cm/sec. The defined PF value is 11.4 or more.

<Support>
The support is not particularly limited as long as it has air permeability, and for example, non-woven fabric, woven fabric, paper or sponge can be used. Among these, non-woven fabrics are preferable, and non-woven fabrics containing glass fiber as a main component are particularly preferable. The glass fiber as a main component means that the mass of the glass fiber is 50 mass% or more with respect to the total mass of the support. The non-woven fabric containing glass fiber as a main component is, for example, a non-woven fabric made of glass fiber or a non-woven fabric prepared by mixing glass fiber with organic fiber. When the support is a nonwoven fabric mainly composed of glass fibers, the basis weight is preferably from 10 to 300 g / ^{m 2,} and more preferably 30 to 150 g / ^{m 2.}

The glass fiber used for the support is, for example, a wool-like ultrafine glass fiber produced by a flame drawing method or a rotary method, or a bundle of glass fibers spun to have a predetermined fiber diameter into a predetermined fiber length. It is a chopped strand glass fiber produced by cutting. Among these, those having various fiber diameters and fiber lengths are selected according to the required physical properties, and they are used alone or as a mixture. Further, low boron glass fibers or silica glass fibers can be used for the purpose of preventing boron contamination of silicon wafers in semiconductor manufacturing process applications. The average fiber diameter of the glass fibers is not particularly limited, but is preferably 0.05 to 20 μm. The average fiber length of the glass fibers is not particularly limited, but is preferably 0.5 to 10000 μm. The organic fiber blended with the glass fiber is, for example, acrylic fiber, vinylon fiber, polyester fiber or aramid fiber. The average fiber diameter of the organic fibers is not particularly limited, but is preferably 0.05 to 20 μm. The average fiber length of the organic fibers is not particularly limited, but is preferably 0.5 to 10000 μm. The method for manufacturing the non-woven fabric is not particularly limited, and is, for example, a wet method.

The average pore size of the support is preferably 0.1 to 20 μm. 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 20 μm, it may be difficult for the cellulosic nanofibers to uniformly form the network structure in the pores of the support. In the present embodiment, a mixed solution containing cellulose nanofibers can be deposited in the pores of the support and then dried to obtain a filter medium for an air filter, but a support having an appropriate average pore size is used. Thus, the cohesive force generated on the cellulose nanofibers during this drying is easily dispersed, so that the network structure is easily maintained even after drying. Here, the average pore diameter can be measured according to ASTM E1294-89 "Half Dry Method".

The support itself is preferably a material that can be used as a filter material for an air filter. By using such a support as the filter medium for an air filter according to the present embodiment, it is easy to obtain a filter medium for an air filter having a higher particle collecting performance than a conventional filter medium for an air filter (support itself). Becomes

<Cellulose nanofiber>
Cellulose nanofibers include chemically treated (modified) cellulose nanofibers. In the cellulose nanofiber, the molecular chains of cellulose form two or more bundles. The term “two or more bundles of cellulose molecular chains” means a state in which two or more cellulose molecular chains are aggregated to form an aggregate called microfibril. In the present embodiment, in the cellulose molecular chain, some or all of the C6 position hydroxyl groups in the molecule are oxidized to aldehyde groups or carboxyl groups, and some or all of the hydroxyl groups other than C6 position are oxidized to carbonyl groups. A part or all of the hydroxyl groups including hydroxyl groups other than C6 position are esterified such as nitric acid ester, acetic acid ester or phosphoric acid ester, or methyl ether, hydroxypropyl ether or carboxymethyl ether. It includes forms substituted with other functional groups such as etherified products. When the cellulose molecular chain is modified to contain a carboxyl group, it also includes those in which the counter ion thereof is replaced with various metal ions and those in which the metal ions are reduced to form particles.

In this embodiment, the number average fiber diameter of the cellulose nanofibers is 1 to 50 nm. In order to provide a filter medium for an air filter that has both high particle collection performance and low pressure loss, it is important to form a uniform fiber network of cellulose nanofibers having a very small fiber diameter in a support. When ultrafine cellulose nanofibers having a number average fiber diameter of 1 to 50 nm are used, the number of fibers per unit volume in the filter medium for an air filter is significantly increased, particles in a gas are easily captured, and high collection is achieved. It is possible to obtain performance. Further, due to the slip flow effect, the air flow resistance of the single fiber becomes extremely low, and the pressure loss as the filter material for the air filter hardly increases. The number average fiber diameter of the cellulose nanofibers is preferably 2 to 30 nm, more preferably 3 to 20 nm. When the number average fiber diameter is less than 1 nm, the single fiber strength of the cellulose nanofiber is weak, and it becomes difficult to maintain the fiber network in the filter medium for air filters. If it exceeds 50 nm, the number of fibers per unit volume in the filter medium for an air filter decreases, and it becomes impossible to form a cellulose nanofiber network effective for trapping particles. Here, the number average fiber diameter is calculated according to the following. Cellulose nanofibers cast on the carbon film-covered grid are observed with an electron microscope image using a transmission electron microscope (TEM, Transmission Electron Microscope). For each of the observed images obtained, two vertical and horizontal axes are drawn per image, and the fiber diameters of the fibers intersecting the axes are visually read. At this time, observation is performed at any magnification of 5000 times, 10000 times, or 50000 times depending on the size of the constituent fibers. The sample or the magnification is set under the condition that 20 or more fibers intersect the axis. In this way, at least three images of the non-overlapping surface portions are photographed by an electron microscope, and the value of the fiber diameter of the fiber intersecting the two axes is read. Therefore, a minimum of 20 fiber×2×3=120 pieces of fiber information can be obtained. The number average fiber diameter was calculated from the fiber diameter data thus obtained. In addition, regarding the branched fiber, if the length of the branched portion is 50 nm or more, it is incorporated into the calculation of the fiber diameter as one fiber. The number average fiber diameter may be calculated according to the following. Cellulose nanofibers existing on the surface or inside the support are observed by an electron microscope image using a scanning electron microscope (SEM, Scanning Electron Microscope). For each of the observed images obtained, two vertical and horizontal axes are drawn per image, and the fiber diameters of the fibers intersecting the axes are visually read. At this time, the observation is performed at any magnification of 5000 to 50000 times depending on the size of the constituent fibers. An image of a plurality of non-overlapping surface portions is taken with an electron microscope, and the value of the fiber diameter of the fiber intersecting each of the two axes is read. The number average fiber diameter was calculated from the fiber diameter data of at least 120 fibers. In addition, regarding the branched fiber, if the length of the branched portion is 50 nm or more, it is incorporated into the calculation of the fiber diameter as one fiber. In addition, the sample is previously subjected to conductive coating in order to obtain an observation image without distortion, but the influence of the coating film thickness is also taken into consideration. For example, when using ion sputtering (E-1045, manufactured by Hitachi High-Technology), the coating film thickness is 12 nm when the discharge current is 15 mA, the sample-target distance is 30 mm, the vacuum degree is 6 Pa, and the coating time is 2 minutes. However, when measuring the fiber diameter, the coating film thickness should be half of the expected value because the deposition direction of the coating film is perpendicular to the expected direction. That is, when coating is performed under the above conditions, the coating film thickness of 12 nm (=6 nm+6 nm) at both ends is excluded from the fiber diameter obtained from SEM.

The number average fiber length of the cellulose nanofibers 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. When the number average fiber length is less than 0.01 μm, the nanofibers are close to particles and there is a possibility that a fiber network cannot be formed in the air filter medium. If it exceeds 20 μm, the entanglement of the nanofibers increases, and the fibers may aggregate with each other, failing to form a uniform network. The number average fiber length is calculated by observing the cellulose nanofibers with an electron microscope image using a scanning electron microscope (SEM, Scanning Electron Microscope). With respect to the obtained observed image, 10 independent fibers are randomly selected for each image, and the fiber length is visually read. At this time, it is performed at any magnification of 1000 to 30,000 times depending on the length of the constituent fibers. In addition, the sample or the magnification is for a fiber whose start point and end point are within the same image. In this way, at least 12 images of the non-overlapping surface portion are photographed by SEM, and the fiber length is read. Therefore, a minimum of 10 fiber×12 fiber=120 fiber information can be obtained. The number average fiber length can be calculated from the fiber diameter data thus obtained. In addition, regarding a branched fiber, the length of the longest part of the fiber is defined as the fiber length.

In the filter material for an air filter according to the present embodiment, the number average fiber length of cellulose nanofibers is preferably smaller than the average pore diameter of the support. By making the number average fiber length of the cellulose nanofibers smaller than the average pore diameter of the support, the cellulose nanofibers can penetrate into the support and form a network inside the support. As a result, it is possible to obtain a filter medium for an air filter having a higher particle trapping performance even when the packing density of cellulose nanofibers is relatively low. Cellulose nanofibers have very high ability to form a network by connecting fibers, so even if the number average fiber length is smaller than the average pore diameter of the support, or even if the packing density is relatively low, it can be used as an air filter medium. Has sufficient network strength. The value of the number average fiber length of the cellulose nanofibers is preferably 0.1 to 99%, and more preferably 5 to 36% with respect to the value of the average pore diameter of the support. If it is less than 0.1%, the number of contact points between the cellulose nanofibers or between the cellulose nanofibers and the support becomes small, and the cellulose nanofibers may fall off from the support. If it exceeds 99%, the cellulose nanofibers may not be able to enter the inside of the support.

The type of cellulose nanofibers is not particularly limited, but is, for example, cellulose obtained by oxidizing cellulose using the above-mentioned MFC, NFC, CNC, or N-oxyl compound such as TEMPO described in Patent Document 1. It is a nanofiber. MFC and NFC are characterized by a wide distribution of fiber diameters because the cellulose fibers are sheared by mechanical treatment to form nanofibers. The CNC has a relatively uniform fiber diameter, but is characterized by a short fiber length of 0.1 to 0.2 μm. The cellulose nanofiber described in Patent Document 1 is cellulose single microfibril. In natural cellulose, microfibrils are multibunched to construct a high-order individual structure. Here, the microfibrils are strongly aggregated by hydrogen bonds derived from the hydroxyl groups in the cellulose molecule. Cellulose single microfibrils are microfibrils isolated by subjecting natural cellulose to chemical treatment and slight mechanical treatment. 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 wet granulated. It is characterized in that it is produced as an aqueous dispersion by subjecting it to a fibrillation treatment to defibrate it and making it into nanofibers, and has a uniform fiber diameter. Among them, the fine cellulose described in Patent Document 1 is particularly preferable because it requires less energy than other cellulose fibers for production and has high productivity. The cellulose nanofiber described in Patent Document 1 has a part of the hydroxyl groups of the cellulose molecule 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. This cellulose nanofiber becomes a transparent liquid when dispersed in water.

The cellulose raw material as the raw material of the cellulose nanofibers is not particularly limited, but plant pulp, particularly wood pulp is preferable. The plant-based pulp is, for example, kraft pulp derived from various woods such as hardwood bleached kraft pulp (LBKP) or softwood bleached kraft pulp (NBKP); sulphite pulp; waste paper pulp such as deinking pulp (DIP); ), pressurized groundwood pulp (PGW), refiner groundwood pulp (RMP), thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), chemimechanical pulp (CMP) or chemigrand pulp (CGP); Powdery cellulose obtained by pulverizing them with a high-pressure homogenizer or a mill; or microcrystalline cellulose powder obtained by purifying them by a chemical treatment such as acid hydrolysis can be used. Further, plant-derived non-wood pulp such as kenaf, hemp, rice, bagasse, bamboo or cotton can also be used. The present embodiment is not limited to the nanofiber raw material and the manufacturing method.

The nanofiber manufacturing method is, for example, the manufacturing method described in Patent Document 1. According to Patent Document 1, a method for producing nanofibers is based on natural cellulose as a raw material, and an N-oxyl compound such as TEMPO in water is used as an oxidation catalyst, and the natural cellulose is oxidized by reacting a co-oxidizing agent to produce a reaction product. An oxidation reaction step for obtaining fibers, a purification step for removing impurities to obtain a reaction fiber containing water, and a defibration step for dispersing the reaction fiber containing water as cellulose nanofibers in a dispersion medium are included. ..

In the filter material for an air filter according to this embodiment, the ratio of the amount of cellulose nanofibers attached to the support is 0.001 to 0.200 mass %. It is preferably 0.010 to 0.150% by mass, and more preferably 0.050 to 0.100% by mass. With such an adhesion amount, it is possible to obtain a filter medium for an air filter which has high particle collecting performance and relatively low pressure loss. If the amount of the cellulose nanofibers attached to the support is less than 0.001% by mass, the particle collection performance will be poor. On the contrary, when it exceeds 0.200 mass %, the pressure loss becomes too high. The amount of cellulose nanofibers attached to the support can be controlled mainly by the concentration of the cellulose nanofibers in the mixed solution and the amount of the mixed solution attached to the support, and the concentration of the cellulose nanofibers in the mixed solution. The higher the value, and the greater the amount of the mixed solution attached to the support, the greater the amount of cellulose nanofibers attached to the support.

Further, the filter material for an air filter according to the present embodiment has a PF value defined by the formula of Formula 1 of 11.4 or more when the target particle diameter is 0.10 to 0.15 μm and the surface wind velocity is 5.3 cm/sec. The PF value is an index for evaluating the superiority or inferiority of the balance between the pressure loss and the particle collection performance, and is calculated using the formula shown in Formula 1. The higher the PF value, the higher the efficiency of collecting the target particles and the lower the pressure loss of the filter material for an air filter. When the air filter material is used as, for example, a HEPA filter material or a ULPA filter material, the PF value is more preferably 11.6 or more, and further preferably 12.0 or more.

The filter medium for an air filter according to the present embodiment preferably does not have a film in which cellulose nanofibers are entangled with each other or a film in which cellulose nanofibers are aggregated in the pores of the support. It is possible to prevent the pressure loss from increasing and increase the PF value. A film in which cellulose nanofibers are entangled or a film in which cellulose nanofibers are aggregated means a film-like film formed by physically entanglement or chemical aggregation of cellulose nanofibers, which blocks all or part of the pores of the support. Say something.

Next, an embodiment of a method for manufacturing an air filter medium according to this embodiment will be described. In the present embodiment, the filter medium for an air filter can be obtained by adhering the mixed liquid containing cellulose nanofibers to the support and then drying it. The mixed liquid can be obtained by dispersing cellulose nanofibers in a dispersion medium. The form of the cellulose nanofibers in the mixed solution is, for example, a form in which the cellulose nanofibers are dispersed dispersively, or a form in which they are partially aggregated. Of these, the form of the cellulose nanofibers in the mixed liquid is preferably a form in which the cellulose nanofibers are dispersed in a dispersed manner.

<Dispersion medium>
Water can be used as the dispersion medium. When water is used as the dispersion medium, a surfactant may be included. The surfactant has the effect of lowering the surface tension of the liquid, and the hydrophilic part of the surfactant is adsorbed on the cellulosic nanofibers, and the hydrophobic part is directed to the outside to make it hydrophobic. It is considered to have the function of weakening the aggregation of fibers. The surfactant is not particularly limited, and may be a cationic surfactant, an anionic surfactant, a nonionic surfactant or an amphoteric surfactant. It is preferable to have. Since the surface of the cellulosic nanofiber exhibits anionic property, the cationic surfactant is easily adsorbed, and the surface of the nanofiber is easily covered with the surfactant. As a result, the effect of weakening the aggregation of the cellulosic nanofibers during drying is further enhanced, which contributes to the formation of a uniform network of cellulose nanofibers. Cationic surfactants are, for example, quaternary ammonium salts, alkylamine salts, sulfonium salts or phosphonium salts. From the viewpoint of high water solubility, a quaternary ammonium salt is more preferable. The content of the surfactant in the mixed solution is not particularly limited, but it is preferably 0.10 to 100 mass% in terms of solid content concentration with respect to the dry mass of the nanofibers. More preferably, it is 0.50 to 50.0 mass %. Especially preferably, it is 1.00-40.0 mass %. If it is less than 0.10% by mass, the effect of adding a surfactant may not be obtained. If it exceeds 100% by mass, it may be difficult for the cellulosic nanofibers to maintain the fibrous state.

Further, as the dispersion medium, a mixed dispersion medium in which water and an organic solvent soluble in water are mixed can be used. As will be described later, in the present embodiment, after adhering the mixed solution containing cellulose nanofibers to the support, it is possible to obtain a filter medium for an air filter by freeze-drying, but when performing this freeze-drying It is preferable to use a mixed dispersion medium in which water and an organic solvent that dissolves in water are mixed. Here, the organic solvent refers to an organic compound that is liquid at room temperature and atmospheric pressure. In addition, “dissolving in water” means that in a mixed dispersion medium in which water and an organic solvent are mixed, the mixing mass ratio of water and the organic solvent is within the range of 98:2 to 50:50, and both are mixed with each other at the molecular level. It means that they do not phase separate. 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 freeze-drying, which contributes to formation of a uniform network of cellulose nanofibers. In this embodiment, the concentration of the organic solvent in the mixed dispersion medium is preferably 2 to 50% by mass. It is more preferably 5 to 40% by mass, and even more preferably 10 to 30% by mass. When the concentration of the organic solvent exceeds 50% by mass, it becomes a highly hydrophobic dispersion medium, and the hydrophilic cellulose nanofibers do not disperse uniformly in the mixed solution, which may impair the uniform network formation of the cellulose nanofibers. There is. Further, when the concentration of the organic solvent is less than 2% by mass, the formation of water crystals (ice crystals) during freezing of the dispersion medium is remarkable, causing aggregation and structural destruction of the cellulose nanofibers, which is uniform on the support. It may not be possible to form a network of cellulose nanofibers.

The organic solvent preferably contains at least one of alcohols, carboxylic acids and carbonyl compounds. By including such an organic solvent, water crystals (ice crystals) generated during freeze-drying can be made smaller, and the network formation of the cellulose nanofibers in the support can be made more uniform. The organic solvent is (1) methanol, (2) ethanol, (3) 1-propanol or (4) t-butyl alcohol as alcohols, (5) acetic acid as carboxylic acids, and (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. Of these, the organic solvent is particularly preferably t-butyl alcohol only. The freezing point of the mixed dispersion medium in which water and t-butyl alcohol are mixed is about -10°C at the lowest, which is higher than that of the mixed dispersion medium of other organic solvent and water, and it can be frozen. It's easy. It is known that in a t-butyl alcohol aqueous solution, when the t-butyl alcohol concentration is around 20 mass %, water and t-butyl alcohol form a eutectic and the crystal size upon freezing becomes the smallest. 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, more preferably 20 to 30% by mass.

<Mixed liquid>
In the present embodiment, it is preferable that the solid content concentration of the cellulose nanofibers in the mixed liquid be 0.001 to 0.150 mass %. It is more preferably 0.005 to 0.100 mass%, and even more preferably 0.010 to 0.080 mass%. When forming a network of cellulose nanofibers on a support having air permeability to form a filter medium for an air filter, it is preferable that the network of cellulose nanofibers is uniformly stretched without having a certain directionality. When the network of cellulose nanofibers in the support is not stretched in a specific direction (orientated), it is easier to satisfy the particle capturing performance as a filter medium for an air filter. When the solid content concentration of the cellulose nanofibers in the mixed solution exceeds 0.150 mass %, many domains in which the fibers are oriented are likely to be formed. The orientation of the fibers means that the fibers are aligned in the same direction to some extent, and the orientation is not preferable for the filter medium for an air filter in which the higher the degree of dispersion of the fibers, the higher the particle capturing performance. In addition, the packing density of the cellulose nanofibers in the filter material for an air filter tends to be excessively high partially. As a result, the distance between the cellulose nanofibers distributed in the filter material for the air filter cannot be properly maintained, and the water in the air causes aggregation and the like, which hinders the formation of a network by the cellulose nanofibers suitable as the filter material for the air filter. May occur. A dispersion liquid of cellulose nanofibers obtained by oxidizing cellulose with 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 easily oriented. .. In the present embodiment, by setting the solid content concentration of the cellulose nanofibers in the mixed solution to be 0.150% by mass or less, the distance between the fibers in the mixed solution is appropriately separated, and the cellulose nanofibers are almost or almost not aligned. do not do. Therefore, even when the cellulose nanofibers are incorporated into the filter material for the air filter, a uniform fiber network can be formed without having a specific directionality, and the effect of remarkably enhancing the particle collecting performance as the filter material for the air filter can be obtained. Hold. On the other hand, when the concentration of the cellulose nanofibers in the mixed solution is less than 0.001% by mass, the entanglement of the cellulose nanofibers is reduced and the network structure may not be maintained.

<Preparation of mixed solution>
In the present embodiment, the method for preparing the mixed solution is not particularly limited, and cellulose nanofibers may be dispersed in the above-mentioned dispersion medium to prepare a mixed solution. When using a mixed dispersion medium in which water and an organic solvent that dissolves in water are mixed, cellulose nanofibers are dispersed in water to prepare a cellulose nanofiber aqueous dispersion, and then the cellulose nanofiber aqueous dispersion is prepared with an organic solvent. Alternatively, it is preferable to add a mixed solvent of water and an organic solvent. When an aqueous dispersion of cellulose nanofibers is added to the organic solvent, aggregates may occur.

In this embodiment, various auxiliaries may be added to the mixed solution. When freeze-drying is used in the drying step, sucrose, trehalose, L-arginine or L-histidine can be added as a freeze-drying stabilizer. Moreover, as the surface modifier of the cellulose nanofiber, for example, a cationic surfactant, an anionic surfactant, a nonionic surfactant or an amphoteric surfactant can be blended.

<Adhesion process>
The method for adhering the mixed solution to the support is not particularly limited, and examples thereof include an impregnation method, a coating method, and a spraying method. The amount of the mixed solution attached to the support is appropriately adjusted according to the thickness, material and average pore diameter of the support, but as described above, in the present embodiment, the attachment of the cellulose nanofibers to the support is performed. The amount ratio is 0.001 to 0.200 mass %. The content is more preferably 0.010 to 0.150% by mass, and further preferably 0.050 to 0.100% by mass. By setting the ratio of the amount of cellulose nanofibers attached to the support to 0.001 to 0.200% by mass, not only the particle collection performance is improved, but also a highly efficient filter material for air filters with low pressure loss is produced. can do. When the ratio of the adhered amount of cellulose nanofibers to the support is less than 0.001 mass %, the adhered amount of cellulose nanofibers to the support becomes insufficient, and it is difficult to form a uniform network of cellulose nanofibers. As a result, there is a possibility that the particle collection performance as the filter material for the air filter cannot be sufficiently improved. On the other hand, when it exceeds 0.200 mass %, the packing density of cellulose nanofibers in the filter medium for air filters becomes excessively high, and it may not be possible to obtain a filter medium with low pressure loss. In addition, in order to form a network of cellulose nanofibers and obtain a high PF value, the ratio of the amount of cellulose nanofibers attached to the support is preferably set appropriately according to the drying method in the drying step to be performed later. .. For example, when the drying method is drying at room temperature or heat, the ratio of the amount of cellulose nanofibers attached to the support is preferably 0.001 to 0.100 mass %, and 0.010 to 0.050 mass. % Is more preferable. When the drying method is freeze-drying, the ratio of the amount of cellulose nanofibers attached to the support is preferably 0.001 to 0.200% by mass, and 0.010 to 0.150% by mass. Is more preferable. In the present invention, the method for calculating the ratio of the adhered amount of cellulose nanofibers to the support is not particularly limited, but, for example, when the support is composed of only inorganic fibers, only the cellulose nanofibers are burned. , It can be calculated from the weight reduction ratio after combustion. In addition, when the support contains organic synthetic fibers, it can be calculated from the amount of dissolution, for example, by dissolving only cellulose using a copper ethylenediamine solution. Further, the ratio of the adhered amount of the cellulose nanofibers to the support may be calculated from the wet adhered amount. That is, the ratio (unit: %) of the adhered amount of cellulose nanofibers to the support is {(wet adhered amount x solid content concentration of cellulose nanofibers in the mixed liquid)/mass of the support before adhering the mixed liquid} It is ×100. Here, the wet adhesion amount is the difference between the mass of the support in the wet state where the mixed liquid is adhered and the mass of the support before the adhesion, and the mixture adhering to the support at the start of the drying step. It means the mass of liquid. Therefore, the wet adhesion amount is preferably a value measured immediately before the drying step, for example, it is preferably measured within 10 minutes before the start of the drying step, and more preferably within 5 minutes.

The impregnation method includes, for example, a method of completely immersing the support in the mixed solution or a method of immersing only the surface of the support. The method of completely immersing the support in the mixed solution allows the mixed solution to efficiently and surely penetrate deep into the pores of the support, thus forming a more uniform network of cellulose nanofibers. It excels in being able to do it. Further, when the pressure is reduced while the support is completely immersed in the mixed solution, the air in the support is easily released, which is more effective for permeating the mixed solution. It is preferable that the excessively adhered mixed liquid is squeezed out by a roll dehydrator or the like, or removed by a water absorbing member such as water absorbent felt or water absorbent paper. The method of immersing only the surface of the support is performed by varying the density of the network structure of the cellulose nanofibers in the pores in the thickness direction of the support (the network structure of the cellulose nanofibers on one surface side of the support and the other surface Are different).

The coating method is a method of coating the mixed liquid on the surface of the support with a known coating machine. Known coating machines are, for example, air knife coaters, roll coaters, bar coaters, comma coaters, blade coaters or curtain coaters (die coaters). The coating method is excellent in that it is easy to control the amount of the mixed solution attached to the support.

The spraying method is a method of spraying the mixed liquid on the surface of the support using a known sprayer such as a sprayer or a sprayer. The spraying method is, for example, when it is desired to form a network structure of cellulose nanofibers only in the vicinity of the surface of the support among the pores of the support, or a large amount of impregnating liquid, or a roll or bar of a coating machine is applied to the support. This is effective when you do not want to contact them.

<Drying process>
In the present embodiment, the mixed liquid is attached to the support as described above, the support is made wet, and then dried. The drying method is not particularly limited, and drying at room temperature or heat or freeze drying can be used. Moreover, you may dry by pressure reduction. In this embodiment, it is preferable to select the composition of the mixed liquid (dispersion medium) depending on the drying method. That is, when performing drying at room temperature or heat, it is preferable to use water (which may contain a surfactant) as the dispersion medium. Further, when freeze-drying is performed, it is preferable to use a mixed dispersion medium in which water and an organic solvent soluble in water are mixed.

The temperature for heat drying may be any temperature at which the support and the nanofibers are not decomposed or deformed. For example, a nonwoven fabric made of glass fibers is used as the support, and the nanofiber is disclosed in Patent Document 1. When the described nanofiber is used, the temperature may be 50 to 200°C. In the present embodiment, by using a porous support, the cohesive force generated on the nanofibers when the dispersion is dried is dispersed, and further, a large number of minute thin films are formed in each hole of the support. It is considered that, by drying, the nanofibers dispersed in the micro thin film remain in a network structure even when water is evaporated.

Freeze-drying is a technique in which the mixed solution is frozen together with the support (freezing step), and the dispersion medium is dried under reduced pressure to sublimate the dispersion medium (drying step). The method of freezing the mixed solution in the freezing step is not particularly limited, for example, a method in which the support to which the mixed solution is adhered is put in a refrigerant such as liquid nitrogen to be frozen, and the support to which the mixed solution is adhered is cooled. There are a method of placing the support on the top and freezing, a method of placing the support to which the mixed solution is attached in a low temperature atmosphere to freeze, or a method of placing the support to which the mixture is attached under reduced pressure to freeze. Preferably, it is a method in which the support to which the mixed liquid is attached is put in a refrigerant and frozen. The freezing temperature of the mixed liquid must be equal to or lower than the freezing point of the mixed dispersion medium in the mixed liquid, preferably −50° C. or lower, and more preferably −100° C. or lower. If the freezing temperature is high, that is, the freezing rate is slow, the crystals of the dispersion medium may become large even when using a mixed dispersion medium in which water and an organic solvent are mixed, and the cellulose nanofibers are condensed around the crystals. In some cases, aggregates may occur. On the other hand, by setting the freezing temperature sufficiently lower than the freezing point of the mixed dispersion medium, that is, by increasing the freezing rate, the dispersion medium can be frozen in a state close to amorphous.

In the present embodiment, it is preferable to set the ambient temperature of the sample to a temperature equal to or lower than the melting point of the mixed liquid in the drying step in the freeze-drying process. Here, the drying step in the freeze-drying process refers to a period during which the dispersion medium is sublimated from the frozen sample under reduced pressure. The ambient temperature of the sample will usually be room temperature unless otherwise controlled. As described above, when the ambient temperature of the sample exceeds the melting point of the mixed solution, a part of the frozen mixed solution may be melted and the homogeneity of the cellulose nanofiber network may be lost. For example, when the dispersion medium is a mixed dispersion medium of water and t-butyl alcohol and the concentration of t-butyl alcohol in the mixed dispersion medium is more than 0 and 50 mass% or less, the nanofibers and the dispersion medium are The melting point of the contained mixed liquid 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 least -8.2°C. Therefore, when the dispersion medium is a mixed dispersion medium of water and t-butyl alcohol and the concentration of t-butyl alcohol in the mixed dispersion medium is more than 0 and 50 mass% or less, the ambient temperature of the sample is set to −8. The temperature is preferably not higher than 2°C, more preferably not higher than -15°C, still more preferably not higher than -20°C. The lower limit of the ambient temperature of the sample varies depending on the type of the dispersion medium, but when a mixed dispersion medium of water and t-butyl alcohol is used, it is preferably −30° C. or higher.

The melting point of the mixed liquid can be judged from the first endothermic point that occurs when the mixed liquid changes from a solid to a liquid. The melting point of the mixed solution may be measured, for example, from the starting point of the first endothermic peak of the DSC curve obtained by a differential scanning calorimeter (DSC).

In the present embodiment, the pressure in the drying step in the freeze-drying process is preferably 200 Pa or less, more preferably 50 Pa or less. If the pressure exceeds 200 Pa, the dispersion medium in the frozen mixed liquid may melt.

<Filter material for air filter>
The filter material for air filters according to the present embodiment is, for example, a filter material for coarse dust filter, a filter material for medium performance filter, a filter material for HEPA filter or a filter material for ULPA filter. Further, the filter material for an air filter obtained by the manufacturing method according to the present embodiment can be applied as a dust mask or a mask for protecting pollutants such as pollen and virus.

As described above, the filter medium for an air filter according to the present embodiment has a PF value of 11.4 or more defined by the equation of Formula 1 in the target particle diameter of 0.10 to 0.15 μm and the surface wind velocity of 5.3 cm/sec. A filter material for an air filter.

In Formula 1, the pressure loss is a differential pressure when air is passed through the filter medium for an air filter having an effective area of 100 cm ^{2 at} a surface wind velocity of 5.3 cm/sec. The pressure loss is measured using, for example, a manometer. Further, the PAO permeability is obtained when air containing polydisperse polyalphaolefin (PAO) particles generated by a Ruskin nozzle is passed through a filter medium for an air filter having an effective area of 100 cm ² at a surface wind velocity of 5.3 cm/sec. The ratio of PAO particles having a target particle diameter of 0.10 to 0.15 μm permeates through the filter material for an air filter. The PAO transmittance is measured using, for example, a laser particle counter.

Although the PF value is also affected by the type and composition of the support, the packing density of cellulose nanofibers in the filter medium for air filters or the degree of network formation by cellulose nanofibers has a large effect. In the filter material for an air filter according to the present embodiment, the ratio of the adhered amount of cellulose nanofibers to the support is 0.001 to 0.200 mass %, but even with such an adhered amount, for example, the support If the adhesion of the cellulose nanofibers is concentrated only near the surface layer of (3) and the packing density of the cellulose nanofibers becomes excessively high partially, the pressure loss is excessively increased, and as a result, the PF value is lowered. Specifically, when a mixed solution having a high concentration of cellulose nanofibers is attached to a support, the adhesion of the cellulose nanofibers is concentrated on the surface of the support, and the cellulose nanofibers may aggregate on the surface of the support. Is possible. As a result, a network-like network of cellulose nanofibers is not formed on the surface layer of the support, and a film-like aggregate may occur. When such a film-like aggregate is partially generated, pressure loss is increased, particle collection performance is decreased (that is, PF value is decreased), and in some cases, air permeability as a filter medium for an air filter is increased. It becomes impossible to hold. Even if the adhesion of cellulose nanofibers is concentrated only near the surface layer of the support, if the network of cellulose nanofibers is appropriately formed (unless the packing density of cellulose nanofibers is too high), the pressure loss will be It is possible to obtain a PF value suitable for a filter medium for an air filter, which does not rise so much.

Therefore, in the filter material for an air filter according to the present embodiment, the packing density of cellulose nanofibers is preferably 0.01 to 0.25 mg/cm ³ . More preferably 0.05~0.20mg / cm ^3, particularly preferably 0.10~0.19mg / cm ^3. Here, the packing density of cellulose nanofibers is the amount of cellulose nanofibers attached per unit volume of the support. When the packing density of the cellulose nanofibers is in this range, the particle collection performance can be enhanced without excessively increasing the pressure loss, that is, the PF value of the air filter medium can be increased. The packing density of the cellulose nanofibers is also affected by the solid concentration of the cellulose nanofibers in the cellulose nanofiber dispersion, but the wetting and spreading when the cellulose nanofiber dispersion adheres to the support, that is, the cellulose nanofiber dispersion It is also affected by the affinity between the substrate and the support, or the diffusion of the pores in the support due to the capillary force.

On the other hand, the formation of a network of cellulose nanofibers is insufficient even if the cellulose nanofibers are uniformly attached to the support in all directions and the packing density of the cellulose nanofibers is not excessively high. In some cases, a high PF value may not be obtained. Specifically, this is the case where several fibers of cellulose nanofibers are aggregated among the fibers to form a large-diameter composite fiber, even if it is not formed into the above-mentioned film shape. In such a case, since the pore size of the mesh network for capturing particles is large, an excessive increase in pressure loss is not caused, but the particle collection performance is not improved, resulting in a high PF value. It may not be obtained.

As described above, in the air filter medium according to the embodiment, the adhesion amount of the cellulose nanofibers to the support and the packing density of the cellulose nanofibers are also important, but the most important thing is the cellulose nanofibers. A mesh network is properly formed in the support, which all appear in the PF value. Therefore, in the filter material for an air filter of the present invention, its configuration is defined by using the PF value.

Next, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples. Further, “parts” and “%” in the examples represent “parts by mass” and “mass %”, respectively, unless otherwise specified. The number of parts added is a value in terms of solid content.

[Preparation process of cellulose nanofiber aqueous dispersion A]
NBKP equivalent to 2.00 g of dry weight (mainly composed of fibers having a fiber diameter of more than 1000 nm), and 0.025 g of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxy radical) , 0.25 g of sodium bromide was dispersed in 150 ml of water, and a 13% aqueous solution of sodium hypochlorite was added to the pulp (NBKP) of 1.00 g so that the amount of sodium hypochlorite was 5.00 mmol. Sodium hypochlorite was added so that the reaction was started. During the reaction, the pH was maintained at 10 by dropping 0.50 mol/l sodium hydroxide aqueous solution. After reacting for 2 hours, the reaction product was filtered and sufficiently washed with water to obtain an oxidized cellulose (TEMPO oxidized cellulose) slurry. A 0.5 mass% TEMPO oxidized cellulose slurry was defibrated for 5 minutes at 15,000 rotations using a biomixer (BM-2, manufactured by Nippon Seiki Seisakusho Co., Ltd.) to dilute the solid content concentration to 0.2 mass %. After that, a further ultrasonic fiber disperser (model US-300E, manufactured by Nippon Seiki Seisakusho Ltd.) was used for defibration for 20 minutes. Then, coarse fibers were removed by centrifugation to obtain a cellulose nanofiber aqueous dispersion A in which TEMPO-oxidized cellulose nanofibers were dispersed in water. The dispersion A was analyzed from an observation image observed at a magnification of 50,000 using TEM (JEM2000-EXII, manufactured by JEOL Ltd.), and as a result, the number average fiber diameter was 4 nm. The number average fiber length was 0.8 μm as a result of analysis from an observation image observed with a SEM (SU8010, manufactured by Hitachi High-Technology) at a magnification of 10,000 times. The obtained cellulose nanofiber aqueous dispersion A was concentrated by a rotary evaporator until the solid content concentration became 0.5%, and used in the subsequent steps.

[Preparation process of cellulose nanofiber aqueous dispersion B]
As a raw material for the cellulose nanofiber aqueous dispersion B, commercially available cellulose nanofiber Vinfyth WMa-1002 (manufactured by Sugino Machine Limited) was used. Vinfyth WMa-10002 having a solid content concentration of 0.2% dispersed in water was disaggregated with a household mixer to obtain a cellulose nanofiber aqueous dispersion B. The number average fiber diameter was 35 nm as a result of analyzing the cellulose nanofibers obtained here from an observation image observed with a TEM at a magnification of 30,000. The number average fiber length was 5.1 μm as a result of analysis from an observation image observed at a magnification of 5000 to 20000 using an SEM.

[Preparation process of cellulose nanofiber aqueous dispersion C]
As a raw material for the cellulose nanofiber aqueous dispersion C, a commercially available cellulose nanofiber, Serish KY-100G (manufactured by Daicel Chemical Industries, Ltd.) was used. Cellish KY-100G having a solid content concentration of 0.2% dispersed in water was disintegrated with a household mixer, and then coarse fibers were removed by centrifugation to obtain a cellulose nanofiber aqueous dispersion C. The cellulose nanofibers obtained here were analyzed from an observation image observed with a TEM at a magnification of 20000 times, and as a result, the number average fiber diameter was 64 nm. The fiber length was more than 5 μm. The obtained cellulose nanofiber aqueous dispersion C was concentrated by a rotary evaporator until the solid content concentration became 0.2%, and used in the subsequent steps.

(Example 1)
Water and t-butyl alcohol were added to the cellulose nanofiber aqueous dispersion A, the container was covered and the mixture was stirred for 5 minutes with a magnetic stirrer to obtain a mixed solution. The solid content concentration of the cellulose nanofiber was 0.005% with respect to the total mass of the mixed solution. The mixing ratio of water and t-butyl alcohol in the mixed liquid was 70:30 by mass. A glass fiber having a basis weight of 64 g/m ² and a pressure loss of about 300 Pa (65 parts of ultrafine glass fibers having an average fiber diameter of 0.65 μm and ultrafine glass fibers having an average fiber diameter of 2.7 μm) was used as a support. 25 parts and 10 parts of chopped glass fibers having an average fiber diameter of 6 μm) were attached by hand spray to a non-woven fabric, and the mass of the support in a wet state (before drying) was measured. The wet adhesion amount was determined from the difference between the weight of the support in the wet state and the weight of the support before wetting. The ratio of the adhered amount of cellulose nanofibers to the support calculated from the wet adhered amount was 0.002%. The wet support was frozen with liquid nitrogen (-196°C), and the frozen support was placed in a freeze-dried bottle that had been previously cooled to -20°C. After that, the entire freeze-drying bottle was placed in a freezer set at -20°C, and the freeze-drying machine (VD-250F TAITEC Co., Ltd.) connected with a pressure-reducing tube reduced the pressure to sublimate the dispersion medium in the support. A filter material for an air filter was obtained. The pressure when reaching the vacuum was 50 Pa or less. The average pore diameter of the support was 2.7 μm.

(Example 2)
The solid content concentration of cellulose nanofibers relative to the total mass of the mixed solution was set to 0.010%, and the wet adhesion amount of the mixed solution to the support was adjusted to adjust the ratio of the adhesion amount of the cellulose nanofibers to the support to 0. A filter material for an air filter was obtained in the same manner as in Example 1 except that the content was 023%.

(Example 3)
A filter medium for air filters was obtained in the same manner as in Example 2 except that the ratio of the amount of cellulose nanofibers attached to the support was adjusted to 0.045% by adjusting the amount of wet attachment of the mixed liquid to the support. ..

(Example 4)
The cellulose nanofiber aqueous dispersion A was diluted so that the solid concentration of the cellulose nanofibers was 0.010% with respect to the total mass of the dispersion. Cetyltrimethylammonium bromide (manufactured by Wako Pure Chemical Industries, Ltd.) as a cationic surfactant was added to the dispersion so that the solid content concentration was 0.001% with respect to the total mass of the dispersion, and a magnetic stirrer was added. It was stirred at. The total solid content concentration of the dispersion was 0.011%. The support of Example 1 was dipped in the dispersion and taken out from the dispersion. Then, the dispersion adhering to the surface of the support was removed with a water absorbent paper, and the amount of the dispersion adhering was adjusted. The support to which the dispersion was attached was dried using a dryer under a drying condition of 105° C. and a drying time of 10 minutes to obtain a filter material for an air filter. The ratio of the adhered amount of cellulose nanofibers to the support calculated from the wet adhered amount was 0.015%.

(Comparative Example 1)
The support made of the glass fiber of Example 1 was used as a filter material for an air filter.

(Comparative example 2)
Adhesion of cellulose nanofibers to the support by adjusting the wet adhesion amount of the mixture to the support by adding only water to the aqueous dispersion of cellulose nanofibers A without adding t-butyl alcohol. A filter medium for an air filter was obtained in the same manner as in Example 2 except that the ratio was changed to 0.045%.

(Comparative example 3)
The solid content concentration of cellulose nanofibers relative to the total mass of the mixed solution was set to 0.001%, and the wet adhesion amount of the mixed solution to the support was adjusted so that the ratio of the adhesion amount of the cellulose nanofibers to the support was 0. A filter material for air filters was obtained in the same manner as in Example 1 except that the content was changed to 0008%.

(Comparative Example 4)
A filter material for an air filter was obtained in the same manner as in Example 2 except that the cellulose nanofiber dispersion C was used in place of the cellulose nanofiber aqueous dispersion A.

(Comparative example 5)
The solid content concentration of cellulose nanofibers relative to the total mass of the mixed solution is 0.050%, the solid concentration concentration of cetyltrimethylammonium bromide relative to the total mass of the mixed solution is 0.010%, and the wet adhesion of the mixed solution to the support is performed. A filter medium for air filters was obtained in the same manner as in Example 4 except that the amount of cellulose nanofibers attached to the support was adjusted to 0.120% by adjusting the amount to 154 g/m ² .

(Example 5)
The solid content concentration of cellulose nanofibers relative to the total mass of the mixed solution is 0.020%, and a nonwoven fabric having a basis weight of 70 g/m ² and a pressure loss of about 29 Pa is used as a support, and the cellulose nanofibers are used for the support. A filter material for an air filter was obtained in the same manner as in Example 1 except that the ratio of the amount of fibers attached was 0.041%. Here, the support comprises 7 parts of ultrafine glass fibers having an average fiber diameter of 0.65 μm, 48 parts of glass fibers having an average fiber diameter of 5.5 μm, 25 parts of chopped glass fibers having an average fiber diameter of 6 μm, and an average fiber diameter of 7 It was composed of 20 parts of an organic fiber (PET) fiber having a fiber length of 4 μm and a fiber length of 5 mm, and an acrylic latex binding these fibers, and the ratio of the mass of the glass fiber to the total mass of the support was 77%. The average pore size of the support was 14.3 μm.

(Example 6)
A filter medium for an air filter was obtained in the same manner as in Example 5 except that the ratio of the amount of cellulose nanofibers attached to the support was adjusted to 0.060% by adjusting the amount of wet adhesion of the mixed liquid to the support. ..

(Example 7)
A filter medium for air filters was obtained in the same manner as in Example 5 except that the ratio of the amount of cellulose nanofibers attached to the support was adjusted to 0.082% by adjusting the amount of wet adhesion of the mixed liquid to the support. ..

(Example 8)
A filter medium for an air filter was obtained in the same manner as in Example 5 except that the ratio of the amount of cellulose nanofibers attached to the support was 0.127% by adjusting the amount of wet adhesion of the mixed liquid to the support. ..

(Example 9)
The solid content concentration of cellulose nanofibers relative to the total mass of the mixed solution was set to 0.040%, and the wet adhesion amount of the mixed solution to the support was adjusted so that the ratio of the adhered amount of cellulose nanofibers to the support was 0. A filter material for an air filter was obtained in the same manner as in Example 5 except that the content was 190%.

(Example 10)
By using the cellulose nanofiber aqueous dispersion B in place of the cellulose nanofiber aqueous dispersion A and adjusting the wet adhesion amount of the mixed solution to the support, the ratio of the adhesion amount of the cellulose nanofibers to the support was adjusted to 0. A filter material for an air filter was obtained in the same manner as in Example 5 except that the content was 040%.

(Example 11)
A filter medium for an air filter was obtained in the same manner as in Example 5 except that the ratio of the amount of cellulose nanofibers adhered to the support was adjusted to 0.101% by adjusting the amount of wet adhesion of the mixed liquid to the support. ..

(Comparative example 6)
The support prepared by blending the glass fiber used in Example 5 with the organic fiber was used as a filter material for an air filter.

(Comparative Example 7)
The solid content concentration of cellulose nanofibers relative to the total mass of the mixed solution was set to 0.06%, and the wet adhesion amount of the mixed solution to the support was adjusted to adjust the ratio of the adhesion amount of the cellulose nanofibers to the support to 0. A filter material for an air filter was obtained in the same manner as in Example 5 except that the content was 220%.

(Comparative Example 8)
The solid content concentration of cellulose nanofibers relative to the total mass of the mixed liquid was 0.200%, and the wet adhesion amount of the mixed liquid to the support was adjusted to adjust the ratio of the adhesion amount of the cellulose nanofibers to the support to 0. A filter material for an air filter was obtained in the same manner as in Example 5 except that the content was changed to 470%.

Table 1 shows the constitution and physical property values of the air filter media obtained in the respective Examples and Comparative Examples. In addition, each physical property value was measured by the following method.

"Pressure loss"
The pressure loss was measured by using a manometer to measure a differential pressure when air was passed through an air filter medium having an effective area of 100 cm ^{2 at} a surface wind velocity of 5.3 cm/sec. The lower the measured value, the higher the air permeability of the filter material.

"Particle transmittance"
The particle transmissivity is the upstream and when air containing polydisperse polyalphaolefin (PAO) particles generated by the Ruskin nozzle is passed through the filter medium for an air filter having an effective area of 100 cm ^{2 at} a surface wind velocity of 5.3 cm/sec. The PAO transmittance from the downstream number ratio was measured using a laser particle counter (LASAIR-1001, manufactured by PMS). The target particle size was 0.10 to 0.15 μm. The lower the measured value, the higher the collection efficiency of the target particles.

"PF value"
The PF value was calculated from the measured values of pressure loss and PAO transmittance using the formula shown in Formula 1. The target particle size was 0.10 to 0.15 μm. The higher the PF value, the higher the efficiency of collecting the target particles and the lower the pressure loss of the filter material for an air filter.

"Network Evaluation"
The network was evaluated by observing the filter material for the air filter with a scanning electron microscope (abbreviated as SEM, SU8010, manufactured by Hitachi High Technology Co., Ltd.) at a magnification of 1,000 to 10,000 times. Before observation, conductive coating is performed with an osmium coater (Neoosmium coater, manufactured by Meiwa Forsys Co., coating thickness 2.5 nm) or ion sputtering (E-1045, manufactured by Hitachi High-Technology Co., coating thickness 12 nm). It was The evaluation criteria are as follows, and it was evaluated whether it was practical as a filter material for air filters.
1: Cellulose nanofibers did not have a specific orientation, and a uniform network of cellulose nanofibers was formed throughout the support without aggregation and compaction (practical level).
2: Cellulose nanofibers did not have a specific orientation, and a uniform network of cellulose nanofibers was partially formed without aggregation and aggregation (practical level).
3: Cellulose nanofibers were slightly dense and had a partially oriented region, but a relatively uniform network of cellulose nanofibers was formed (the lower limit of practical use).
4: The number of cellulose nanofibers per unit area was too small to form a uniform network of cellulose nanofibers (unsuitable level for practical use).
5: Cellulose nanofibers had regions where they were oriented, aggregated, and dense, and a uniform network of cellulose nanofibers was not formed (practical unsuitable level).

"Packing density of cellulose nanofibers"
The packing density of cellulose nanofibers was calculated from the thickness of the cellulose nanofiber layer and the amount of cellulose nanofibers attached per unit area of the filter medium for air filters. The thickness of the cellulose nanofiber layer was obtained as follows. First, the cross section of the filter material for an air filter was carved using an ion milling device (E-3500, manufactured by Hitachi High Technology Co., Ltd.), and the cross section was observed using SEM. The observation width is 80 μm, and the air filter medium is continuously observed from the upper end to the lower end. The observation magnification is adjusted so that the cellulose nanofibers can be confirmed, and the distance between the uppermost end and the lowermost end where the cellulose nanofibers are present is measured within the range of the observation width. The same photographing and measurement were carried out at 10 different observation locations, and the average value was taken as the thickness of the cellulose nanofiber layer.

From Examples 1 to 11, the number average fiber diameter of the cellulose nanofibers is 1 to 50 nm, the ratio of the amount of adhesion of the cellulose nanofibers to the support is 0.001 to 0.200% by mass, and the PF value is 11.4 or more. It was possible to obtain a high-performance filter material for an air filter. When the relationship between the ratio of the amount of cellulose nanofibers attached to the support and the PF value was examined, the PF values of Examples 6 to 8 and 11 were particularly large, and the PF value was about 0.082% in Example 7. It was found that there was an appropriate range for the ratio of the attached amount of cellulose nanofibers to the maximum. As an example, the observed images by SEM of Example 3 and Example 8 are shown in FIGS. 1 and 4, respectively. As shown in the figure, the filter media for air filters of Examples 3 and 8 have a network structure in which a network of cellulose nanofibers is evenly stretched without having a certain directionality, and a support structure is provided. A film in which cellulose nanofibers were entangled with each other or a film in which cellulose nanofibers were aggregated was not formed in the pores of the body.

Comparative Examples 1 and 6 did not include cellulose nanofibers in the air filter medium, and Comparative Example 2 had cellulose nanofibers aggregated to form a film-like material as shown in FIG. 2. In Comparative Example 3, the amount of cellulose nanofibers attached to the support was small, and a cellulose nanofiber network effective for capturing particles was not formed. Therefore, Comparative Example 3 was not a filter material for an air filter having a high PF value. It was In addition, the thickness of the cellulose nanofiber layer could not be measured because the adhesion amount was small. In Comparative Example 4, since the number average fiber diameter of the cellulose nanofibers was large and the number average fiber length was larger than the average pore diameter of the support, the cellulose nanofibers could not reach the inside of the support, and Packing density increased. Therefore, the filter material for an air filter having a high PF value was not obtained. Comparative Example 5 had a structure in which, as shown in FIG. 3, cellulose nanofibers were entangled with each other in the pores of the support to form a film, and the film had a hole. A part of the pores of the support was blocked by the film-like material formed by intertwining the cellulose nanofibers. As a result, the ratio of the total area of the non-occluded portions in the pores to the entire pores (open area ratio) was insufficient for the filter material for the air filter, and the pressure loss increased, resulting in a low PF value. That is, the network structure was not suitable as a filter material for air filters. In Comparative Example 7, since the ratio of the amount of cellulose nanofibers attached to the support was high, the packing density of cellulose nanofibers was also high, and the cellulose nanofibers were dense and oriented, and a uniform cellulose nanofiber network could not be formed. It was In Comparative Example 8, the ratio of the adhered amount of the cellulose nanofibers to the support was too high, so that the packing density of the cellulose nanofibers was also high, and it can be considered that the aggregation of the cellulose nanofibers due to the moisture in the air occurred. Therefore, the pressure loss did not increase so much despite the large amount of cellulose nanofibers attached. Further, since the porosity of the filter material for the air filter is lowered, the flow velocity in the pores is increased, the particle permeability is increased, and as a result, the PF value is greatly decreased.

Next, the effect of humidity will be verified. Since the air filter is used under various humidity environments, it is important to verify the influence of humidity on the filter material for the air filter. Using the filter media for air filters obtained in Example 11 and Comparative Example 6, a humidity resistance test was conducted by the following method.

"Moisture resistance test"
After leaving the filter material for air filter in an environment of temperature 30°C and humidity 70%RH for 24 hours, pressure loss, particle transmittance and PF value are measured, and then in an environment of temperature 30°C and humidity 90%RH. After standing for 24 hours, the pressure loss, particle transmittance and PF value were measured. Table 2 shows each physical property value before the test and after standing in each environment.

Since the air filter material obtained in Comparative Example 6 did not contain cellulose nanofibers, the physical properties did not change significantly even when exposed to high humidity. On the other hand, with the filter material for an air filter obtained in Example 11, the higher the humidity, the lower the pressure loss and the higher the particle transmittance. However, the PF value did not decrease significantly, and it was still a high value even when exposed to high humidity. FIG. 5 is an SEM observation image of the filter material for an air filter of Example 11 immediately after the humidity resistance test (immediately after standing in an environment of temperature 30° C. and humidity 90% RH for 24 hours in the humidity resistance test). Although the cellulose nanofiber network in the part contracted, no film-like substance was formed and the porous network shape was maintained. It is speculated that the contracted cellulose nanofiber network does not form a film and changes into a form that does not become a resistance to air because the pressure loss is reduced. From the above, it can be seen that the air filter medium of the present invention is an air filter medium that can be used even under high humidity.

Claims (5)

A support material having air permeability, which is a filter medium for an air filter having cellulose nanofibers attached,
The number average fiber diameter of the cellulose nanofibers is 1 to 50 nm,
The ratio of the amount of cellulose nanofibers attached to the support is 0.001 to 0.200% by mass,
A filter material for an air filter having a target particle diameter of 0.10 to 0.15 μm and a surface wind velocity of 5.3 cm/sec, and having a PF value defined by the formula of Formula 1 of 11.4 or more.
The filter material for an air filter according to claim 1, wherein the packing density of the cellulose nanofibers is 0.01 to 0.25 mg/cm ³ . The number average fiber length of the said cellulose nanofiber is smaller than the average pore diameter of the said support body, The filter material for air filters of Claim 1 or 2 characterized by the above-mentioned. The filter material for an air filter according to any one of claims 1 to 3, which does not have a film in which the cellulose nanofibers are entangled with each other or a film in which the cellulose nanofibers are aggregated in the pores of the support. .. The filter medium for an air filter according to any one of claims 1 to 4, wherein the filter medium for an air filter does not have a region where the cellulose nanofibers are oriented, aggregated, and dense.