JP2009226321A - Flame-retardant electret filter medium and filter unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a flame-retardant electret filter medium with a low pressure loss and of a long life, and to provide a filter unit. <P>SOLUTION: The flame-retardant electret filter medium contains a flame-retardant fiber including a silicone based compound containing a unit shown by RSiO<SB>1.5</SB>(R is an organic group) and including a compound having an imide structure, in which fibers of Young's modulus of 70 cN/dtex or larger are contained, in which the fibers are fused and fixed, of which the apparent density is 0.05-0.15 g/cm<SP>3</SP>, and of which the specific strength at extending by 1% is 150 N*cm/g or larger. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a flame retardant electret filter medium and a filter unit using the same.

  The performance required for the dust collection filter unit and the deodorization filter unit includes flame retardancy and low pressure loss, in addition to basic performance as a filter medium such as trapping property and long life that dust is not easily clogged.

  For example, in particular, flame retardant properties are required for fine dust collecting filter media for air conditioning, automobile mounting, and filters mounted on equipment, and many such attempts have been made. Patent Document 2 discloses a filter medium in which a flame retardant chemical bond nonwoven fabric and an electret ultrafine melt blown nonwoven fabric are laminated as a flame retardant electret filter media. However, the chemical bond nonwoven fabric has a low collection efficiency, and most particles of several tens of μm or less are collected by a dense electret ultrafine melt blown nonwoven fabric layer on the most downstream side. Therefore, the electret ultrafine melt blown nonwoven fabric has a very short life. In addition, in order to obtain long life, the idea which comprises all the filter media layers with electret material is shown by patent document 3. FIG. The filter medium described in this document is a filter medium obtained by injecting and welding fibers having a fiber diameter of 5 μm or more to a melt blown nonwoven fabric prepared in advance by a melt blow method, and the fibers constituting the melt blown nonwoven fabric are molecules in the fiber. Polypropylene undrawn yarn with insufficient orientation is soft, and the pleated filter medium is deformed by wind pressure, resulting in structural pressure loss (pressure loss generated when air flows through the filter unit containing the pleated filter medium (unit pressure loss)) Therefore, a value obtained by subtracting the pressure loss due to the material of the filter medium is likely to occur, and the low pressure loss property cannot be obtained. Patent Document 2 also discloses that a flame retardant (guanidine sulfamate) is mixed in a resin for fixing between fibers in order to obtain a flame retardant chemical bond nonwoven fabric. As a feature of this method, it is described that it is possible to use fibers having different fiber thicknesses and lengths, and it is also possible to use fibers having a high Young's modulus. However, there is no disclosure what kind of fiber is specifically used to produce a low elongation and high strength filter medium with low pressure loss.

Patent Document 4 and Patent Document 5 disclose electret filter media using polyolefin fibers composed of heat-fusible fibers. However, the Young's modulus of the heat-fusible short fiber drawn by melt spinning of an olefin-based polymer and stretched at a normal magnification of 5 times or less is as low as 25 to 45 cN / dtex, and is apparently obtained by bonding between fibers. The nonwoven fabric having a fiber density of 0.05 to 0.15 g / cm ³ was very soft, had a large elongation of the filter medium, and did not have a strength enough to withstand wind pressure. That is, structural pressure loss is likely to occur, and low pressure loss cannot be obtained. In addition, the filter medium having a low apparent fiber density has a problem that a bulky fiber structure cannot be maintained due to a large thickness change when wound.

  Furthermore, Patent Document 5 and Patent Document 6 disclose a filter medium using a flame-retardant heat-fusible fiber in which a flame retardant is kneaded into a fiber polymer. However, since it is a filter medium using polyolefin-based fibers obtained by stretching at a normal magnification, the Young's modulus of such fibers is low, and the nonwoven fabric obtained by interfiber bonding is very soft. Therefore, it has been difficult to prepare a filter medium that does not stretch due to wind pressure or does not change the fiber bulk density.

  On the other hand, as a technique for obtaining low-pressure loss, conventionally, the fiber density of the filter medium is reduced, the fiber diameter is increased, the bulk is increased, and the fiber density is decreased. However, in this method, although the pressure loss of the filter medium itself is lowered, the fixing force between the fibers of the filter medium is weakened, so that the filter medium is stretched and the structural pressure loss is easily increased. In order to obtain a strength that is difficult to deform, Patent Document 7 discloses that a bag filter is formed using heat-resistant fibers such as polyphenylene sulfide having a Young's modulus of 20 cN / dtex or more. However, this filter medium has a woven fabric or web shape, and high collection performance is required. Filter media used for home air cleaner filters, air conditioning filters for buildings and factories, in-vehicle filters, etc. The filter media had a structure that was remarkably different in use, structure, basis weight, thickness, and usage.

Thus, a low-pressure loss and long-life flame-retardant electret filter medium that has flame retardancy and has a strength that is difficult to be deformed by wind pressure or pressing pressure has not been provided so far.
JP 2004-82109 A JP 2006-136809 A JP 7-76053 A JP 2002-339256 A JP 2007-146357 A JP 2007-23415 A International Publication No. 04/87293 Pamphlet

  An object of the present invention is to solve the above-mentioned problems, and to provide a filter unit having a low pressure loss due to a flame-retardant electret filter medium having a dust collecting property and a holding capacity, in which the entire filter medium layer is not easily crushed. There is.

  To achieve the above object, the present invention has one of the following configurations.

(1) A flame retardant electret filter medium including a flame retardant fiber containing a silicone compound containing a unit represented by RSiO _1.5 (R is an organic group) and a compound having an imide structure, Fibers having a Young's modulus of 70 cN / dtex or more are included, the fibers are fused and fixed, the apparent fiber density is 0.05 to 0.15 g / cm ³ , and the specific strength at 1% elongation is 150 N · cm / The flame-retardant electret filter medium which is more than g.

(2) The flame-retardant fiber is a fiber having a core-sheath structure, and has a silicone compound containing a unit represented by RSiO _1.5 (R is an organic group) in the core-side polymer and an imide structure. The flame-retardant electret filter medium according to (1), which is a heat-fusible fiber containing a compound.

  (3) The organic group R constituting the silicone compound includes a phenyl group, and the content of the phenyl group is 30 mol% or more in a molar ratio with respect to all the organic groups constituting the silicone compound. The flame-retardant electret filter medium according to (1) or (2).

  (4) Any one of (1) to (3), wherein the silicone compound contains a silanol group, and the content of the silanol group is 2% or more and 10% or less by weight with respect to the silicone compound. The flame retardant electret filter medium described.

  (5) The flame-retardant electret filter medium according to any one of (1) to (4), wherein the compound having an imide structure is a polyetherimide.

(6) The filter medium has two or more fiber layers having different average fiber diameters, and at least one layer located on the most upstream side of the air flow is composed of a plurality of short fibers having different fiber diameters, and the average fiber diameter is 20 It is a fiber layer in the range of ˜70 μm, and at least one layer located on the most downstream side of the air flow is a fiber layer having a basis weight of 3 to 50 g / m ² and an average fiber diameter of 0.5 to 15 μm. The flame-retardant electret filter medium according to any one of (1) to (5), which is characterized.

  (7) A deodorizing filter medium obtained by laminating the flame retardant electret filter medium according to any one of (1) to (6) on a deodorizer.

  (8) The flame-retardant electret filter medium according to any one of (1) to (7), which is produced by an airlaid method.

  (9) A filter unit using the filter medium according to any one of (1) to (8).

The filter medium of the present invention is a flame retardant electret filter medium including a flame retardant fiber containing a silicone compound containing a unit represented by RSiO _1.5 (R is an organic group) and a compound having an imide structure. It is. In addition, the fiber having a Young's modulus of 70 cN / dtex or more is included, the apparent fiber density where the fibers are fused and fixed is 0.05 to 0.15 g / cm ³ , and the specific strength at 1% elongation is 150 N · It is characterized by being set to cm / g or more. For this reason, the performance of the air filter unit in which the flame retardant electret filter medium is pleated and accommodated is less likely to expand even with the wind pressure when air is passed through, so that structural pressure loss is small and low pressure loss is obtained. Moreover, since the electret fiber by which the flame retardant was kneaded in the whole filter medium is used, it is flame-retardant, its collection performance is high, and it is bulky, so it has a long life. Further, since the fiber having a high Young's modulus maintains a bulky structure, it is difficult to be crushed even by the winding pressure when it is wound, and low pressure loss and long life (high dust retention) can be maintained.

  As a result, the filter unit and the storage device can be reduced in size, processed with a large air volume, and extended in service life. In addition, since the pressure loss is low, wind noise generated by friction between the filter medium and air is reduced, contributing to low noise, and air can be sent by a low-power blower fan with low power consumption. It can also contribute to sex.

The present invention is a flame retardant electret filter medium including a flame retardant fiber containing a silicone compound containing a unit represented by RSiO _1.5 (R is an organic group) and a compound having an imide structure. . Further, the fiber containing Young's modulus of 70 cN / dtex or more is contained, the fiber density is fusion-fixed, the apparent fiber density is 0.05 to 0.15 g / cm ³ , and the specific strength at 1% elongation is 150 N. -It is a flame-retardant electret filter medium having a bulkiness, low elongation, and high strength excellent in high collection property, long life property, and low pressure loss property of cm / g or more.

  The flame retardant fiber will be described.

The flame retardant is a compound having a imide structure and a silicone compound containing a unit represented by RSiO _1.5 (R is an organic group), and is kneaded into a fiber polymer.

Silicone compounds are monofunctional R3SiO _0.5 (M units), bifunctional R2SiO _1.0 (D units), trifunctional RSiO _1.5 (T units), and tetrafunctional SiO. _2.0 (Q unit). The silicone-based compound referred to in the present invention contains at least trifunctional RSiO _1.5 . By including the unit represented by RSiO _1.5 (T unit) in the structure of the silicone compound, the heat resistance of the silicone compound is improved, and a flame-retardant fiber having high flame retardancy can be obtained. From the viewpoint of heat resistance and flame retardancy, the content of RSiO _1.5 is preferably 20% or more, more preferably 50% or more, in terms of molar ratio to the silicone compound.

  The silicone-based compound is preferably a compound having Si atoms in which three oxygen atoms and one organic group are bonded to Si atoms.

  Examples of the organic group herein include a hydrogen group, a hydroxyl group, a hydrocarbon group, and an aromatic hydrocarbon group.

  Specific examples of the hydrocarbon group include a methyl group, an ethyl group, a propyl group, a butyl group, a vinyl group, an allyl group, a methacryl group, and a cyclohexyl group. Examples of the aromatic hydrocarbon group include a phenyl group and a naphthyl group.

  In the present invention, the organic group constituting the silicone compound contains a phenyl group and the content of the phenyl group is 30% in a molar ratio with respect to the total organic groups constituting the silicone compound in order to suppress a decrease in fiber properties. It is preferable that it is above, and more preferably 60% or more.

  In addition, the silanol group content of the silicone compound is preferably 2% or more by weight with respect to the silicone compound in order to have sufficient flame retardancy, and is preferably 10% or less from the viewpoint of processability. More preferably, it is 3% or more and 8% or less. By setting the content of the silanol group within the above range, it is possible to efficiently form a fiber and a crosslinked structure at the time of combustion and improve flame retardancy.

  Further, the weight average molecular weight of the silicone compound is preferably 500 or more, because the melt viscosity is high when the silicone compound is melt-kneaded, and the dispersibility to fibers is good. From the viewpoint of sex. More preferably, it is 1000 or more and 100,000 or less, More preferably, it is 3000 or more and 50000 or less.

  The compound having an imide structure is a compound having a structure of —CO—NR—CO— (R is an organic group) in the molecular structure. The organic group here is a C1-C12 alkyl group, alkene group, alkyne group, aromatic ring (benzene ring, condensed benzene ring, non-benzene aromatic ring) or the like.

  By containing the compound having an imide structure and the above silicone compound in the fiber, it is possible for the silicone compound, the compound having the imide structure, and the fiber as the base material to efficiently form a carbonized layer during combustion. The flame retardancy can be remarkably improved as compared with the case where each of the silicone compound and the compound having an imide structure is contained alone.

  A compound having an imide structure preferably has thermoplasticity from the viewpoint of processability. That is, the glass transition temperature is preferably 130 ° C. or higher and 300 ° C. or lower, more preferably 130 ° C. or higher and 250 ° C. or lower.

  Specific examples of the compound having an imide structure include polyimide, polyamideimide, and polyetherimide. Among these, polyetherimide is preferable from the viewpoint of processability, but this is not limited as long as the molecular structure has an imide structure.

  The content ratio of the silicone compound used in the flame-retardant fiber and the compound having an imide structure is in the range of 5:95 to 95: 5 in terms of a weight ratio of (silicone compound) :( compound having an imide structure). The range of 10:90 to 90:10 is more preferable.

  The composition ratio of the flame-retardant fiber of the present invention is 94: 1: 5 in terms of the weight ratio of (fiber as a base material) :( silicone compound) :( compound having an imide structure) from the viewpoint of flame retardancy. The range of 40:20:40 is preferable, and 90: 3: 7 to 70:10:20 is more preferable.

  Moreover, the polymer of the flame retardant fiber used in the present invention is one or two selected from polyester and polyolefin.

  Polyester is a polymer synthesized from a dicarboxylic acid such as polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate or the like and an ester-forming derivative thereof and a diol or an ester-forming derivative thereof, as well as poly-L-lactic acid and poly-D-lactic acid. It is a non-petroleum polyester compound.

  Polyolefin is polyethylene, polypropylene, or the like.

  Next, the flame retardant fiber may be a flame retardant fiber in which a flame retardant is uniformly kneaded into the fiber. However, the flame retardant fiber acts on a hindered amine agent kneaded as an electret additive to reduce electret properties. Because there is a case that bleeds out of the fiber, a core-sheath structure in which a flame retardant is concentrated and kneaded on the inner core side as a core-sheath composite fiber, and a hindered amine electret additive is concentrated on the sheath side. The flame retardant, electret, and heat fusible fibers can be achieved with a single fiber, and the sheath component has a lower melting point than that of the core. It is preferable because it is effective for stability of the property. For the same reason, a polymer with concentrated flame retardant and a polymer with concentrated hindered amine electret additive are bonded in the longitudinal direction of the fiber to form a side-by-side type composite fiber, and electret added. It is also preferable to lower the melting point of the polymer on which the agent is intensively kneaded than the polymer on the other side. The low melting point polymer is preferably a polymer having a melting point lower by 20 ° C. or more than the other polymer.

  As the polymer constituting the composite fiber having a core-sheath structure, low-melting polyester / high-melting polyester on the sheath side / core side, polyethylene / polyester, polyethylene / polypropylene, modified polyolefin / polypropylene, polypropylene / polyester, and the like can be used. Among them, polyolefin resins such as low melting point polyolefin / polypropylene, low melting point polyolefin / high modulus polypropylene, polypropylene / polyester, and modified polyolefin / polypropylene are suitable because the fibers constituting the sheath side (outside cross section) can be electretized. Furthermore, because modified polyolefin / polypropylene and modified polyolefin / high modulus polypropylene have high electret performance obtained by the corona discharge method, hydrocharge method, and tribocharging method of general electretization, and excellent charge stability. Among them, modified polypropylene / high modulus polypropylene is the optimum fiber because it has a high specific strength achievement effect.

  As the low melting point component containing the modified polyolefin, a modified polyolefin alone, a mixture of at least two modified polyolefins, a mixture of at least one modified polyolefin and another thermoplastic resin, or the like can be used. The modified polyolefin generally has a tendency to lower the polymer strength when compared with the unmodified polyolefin. Therefore, in order to maintain higher fiber strength, the modified polyolefin having a high modification rate and the unmodified polyolefin are used as a low melting point component. It is preferable to use a mixture with a modified polyolefin, and it is particularly preferable to use the same polymer as the backbone polymer of the modified polyolefin in terms of compatibility.

  Here, the modified polyolefin is an ethylene-propylene copolymer or ethylene-propylene-butene copolymer containing a resin (referred to as a modifier) containing a polymer composed of a vinyl monomer having a reactive functional group. is there.

  The modified polyolefin can be polymerized using the vinyl monomer having the reactive functional group, and any of a copolymer such as block, random and ladder, and a graft polymer can be used. As the vinyl monomer having a reactive functional group, at least one unsaturated carboxylic acid selected from maleic anhydride, maleic acid, acrylic acid, methacrylic acid, fumaric acid, itaconic acid, a derivative thereof, or an anhydride thereof is used. Containing vinyl monomers, styrenes such as styrene and α-methylstyrene, methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, 2-hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, or similar acrylic esters, etc. At least one vinyl monomer, glycidyl acrylate, glycidyl methacrylate, butenecarboxylic acid ester, allyl glycidyl ether, 3,4-epoxybutene, 5,6-epoxy-1-hexene, vinylcyclohexene monoxide, etc. And vinyl monomers containing one.

  Among the above modified polyolefins, a modified polyolefin which is a graft polymer can be used more preferably because of its high polymer strength and good fiber processability. With regard to the modification rate, the fiber processability and the effects of the present invention can be achieved. It is preferable to have a high denaturation rate as much as possible within the range not hindering.

As the backbone polymer of the modified polyolefin, polyethylene, polypropylene, polybutene-1, or the like is used. As the polyethylene, high density polyethylene, linear low density polyethylene, and low density polyethylene are used. These are polymers having a density of 0.90 to 0.97 g / cm ³ and a melting point of about 100 to 135 ° C. As the polypropylene, a propylene homopolymer and a copolymer of propylene and another α-olefin having propylene as a main component are used. These are polymers having a melting point of about 130 to 170 ° C. Polybutene-1 is a polymer having a melting point of about 110 to 130 ° C. Among these polymers, polyethylene is preferable in consideration of melting point, copolymerization, and ease of graft polymerization, and high-density polyethylene having high polymer strength is more preferable in order to improve the strength of the nonwoven fabric.

  The modifier is a resin having a reactive functional group, and examples of the reactive functional group include hydroxyl, amino, nitrile, nitrilo, amide, carbonyl, carboxyl, glycidyl and other functional groups.

  As a modifier, it is generally preferable to have a vinyl monomer having the reactive functional group at a modification rate of 0.05 to 2.0 mol / kg with respect to the total weight of the modifier. It is more preferable to use a modifier having a modification rate of 2 mol / kg.

  Since the modifier has a high adhesiveness to other materials such as inorganic substances when constituting the nonwoven fabric, and increases the chargeability of the flame retardant fiber, the vinyl monomer and the polyolefin comprising the unsaturated carboxylic acid or its derivative The modified polyolefin consisting of can be preferably used.

  When mixing the modifier with other thermoplastic resins, it is preferable to use a modifier with a high modification rate of about 0.1 mol / kg or more. The amount of the modifier added is preferably 1.0 to 50.0% by weight, more preferably 2.0 to 20.0% by weight, based on the low melting point component. It is desirable. By using a modifier, the effect of improving the chargeability can be added.

  In the present invention, the flame retardant fiber capable of such electret processing is made into a non-woven fabric to form a flame retardant filter medium. However, when there is a large amount of fabric or when the apparent fiber density is high, a flame retardant is kneaded into the fiber. In some cases, sufficient flame retardancy may not be obtained by the amount alone. In such a case, a flame retardant standard FMVSS-302 is obtained by spraying a liquid (for example, a flame retardant ratio of 70% by weight) mixed with an emulsion resin and a polyester melamine phosphate to the surface of the filter medium by 10% or less of the fiber weight. Can be achieved.

  Next, the flame retardant electret filter medium for the purpose of collecting fine dust used in a pleated state by making the flame retardant fiber described so far into a nonwoven fabric will be described.

The flame-retardant electret filter medium of the present invention includes flame-retardant fibers, heat-fusible fibers, and high Young's modulus fibers having a Young's modulus of 70 cN / dtex or more, and the electret property may be retained by each fiber. However, it is preferable that the heat-fusible fiber is composed of a fiber having electret property, flame retardancy, and high Young's modulus property. More preferably, it is composed of a non-woven fabric having an apparent fiber density of 0.05 to 0.15 g / cm ³ in which the outside of the heat-fusible fiber is melted and fused to other fibers to fix the fibers.

  Moreover, in order to achieve low elongation and high strength and to reduce structural pressure loss, it is effective that the nonwoven fabric includes a plurality of heat-fusible fibers having different fiber diameters. The heat-fusible fiber is formed by fusing and fixing a high Young's modulus fiber at an inter-fiber intersection to increase the specific strength. In general, it is considered that a fiber with a large fiber diameter is preferable for increasing the strength. On the other hand, since the number of fixing points between fibers is reduced, it is better to use a mixture with a fiber with a small fiber diameter to reduce specific strength and low pressure loss Excellent in all aspects of dust retention.

  Further, the filter medium of the present invention can achieve high collection efficiency with low pressure loss and long life by electretization. The chemical bond nonwoven fabric in which the fibers are fixed with resin does not have an electric charge, but the filter medium of the present invention is charged with at least one kind of fibers constituting the filter medium, so that a predetermined collection efficiency is obtained. Therefore, it can be formed with low pressure loss. In addition, since the collection performance can be improved by electret charge, it is not necessary to use thin fibers as in the past, and a filter medium structure consisting of thick fibers with a low fiber density can be used, so that the filter medium performance with a large amount of dust retention is obtained due to low pressure loss. .

  Furthermore, the specific strength of the filter medium can be increased by using the high Young's modulus fiber as described above. As a result, it is possible to achieve a long-life and high-collecting filter with low pressure loss as a filter unit with little structural pressure loss, and the pleated filter medium is less likely to stretch due to wind pressure or deform the pleat mountain shape. In addition, by using a high Young's modulus fiber, even a filter medium with a low apparent fiber density and a large amount of voids hardly undergoes deformation of deformation or compression, low pressure loss, a large amount of dust retention, and a long life.

  The high Young's modulus fiber has a Young's modulus of 70 cN / dtex or more, and is desirably contained at a ratio of 20% or more of the total mass of the filter medium. The Young's modulus of the preferred fiber is 100 cN / dtex or more, more preferably 130 cN / dtex or more, and still more preferably 300 cN / dtex or more. On the other hand, the upper limit is preferably 3000 cN / dtex or less. This is because when a fiber having a Young's modulus exceeding 3000 cN / dtex is used, specifically, an aramid fiber (4400 cN / dtex or more), ultrahigh molecular weight polyethylene (8000 cN / dtex or more), glass fiber (326000 cN / dtex or more), etc. Depending on the amount used and the fineness, the tensile elongation at break of the nonwoven fabric is 1.0% or less and the specific strength is 150 N · cm / g or more, but the tear strength is reduced or the pleated mountain This is because the nonwoven fabric may break due to failure to withstand bending and elongation at the portion.

  For the above reasons, the preferred Young's modulus range of the fibers used is 70 to 3000 cN / dtex, more preferably 100 to 2000 cN / dtex. More preferably, it is in the range of 250 to 1500 cN / dtex, optimally 300 to 1000 cN / dtex, and in this way, the nonwoven fabric is cut and the tear strength is not reduced by pleating, and the filter medium is less deformed even by wind pressure.

  Polyester, polyethylene, polypropylene, vinylon, rayon, etc. can be used as the material of the high Young's modulus fiber contained in the flame retardant electret filter medium of the present invention, among which high modulus vinylon and polypropylene are suitable and optimally used. In the present invention, high modulus polypropylene capable of electret processing is the optimum fiber in the present invention. High modulus polypropylene is obtained by highly orienting and crystallizing molecules by super-drawing undrawn yarn at 5 times or more, and has a high Young's modulus. .

  It is preferable to use a mixture of high modulus polypropylene and high modulus vinylon as the high Young's modulus fiber, since electret properties and specific strength can be improved at the same time.

  In the present invention, the above fibers are preferably non-crimped fibers. In the case of non-crimped fibers, the fiber accumulation in the non-woven fabric becomes planar, so that the orientation of each single fiber is one-dimensional and has no looseness. Therefore, when an external force is applied to the nonwoven fabric, there is little elongation due to looseness, which is preferable in that a tensile resistance according to the properties of the single yarn is readily generated.

In the present invention, the high Young's modulus fiber may be the above-mentioned flame retardant fiber. The flame retardant electret filter medium of the present invention using the above high Young's modulus fiber has a specific strength at 1% elongation. Becomes 150 N · cm / g or more. The specific strength is more preferably 150 to 8000 N · cm / g, further preferably 200 to 5000 N · cm / g, and most preferably 300 to 3000 N · cm / g. If the specific strength is 150 N · cm / g or more, the flame-retardant electret filter medium is not easily deformed, so that the structural pressure loss can be reduced and a low-pressure loss filter can be produced.

  The specific strength is an index relating to the strength at 1% elongation when the nonwoven fabric is pulled in one direction. However, since the deformation of the nonwoven fabric is caused by bending due to wind pressure, the bending resistance is increased in addition to the specific strength management. It is extremely important. Bending resistance can be increased by increasing the thickness of the non-woven fabric and increasing the basis weight, but it increases the air flow resistance and narrows the air inflow interval, which in turn increases the pressure loss and cannot be increased. Therefore, to increase the bending resistance with a thin nonwoven fabric, it is effective to create a skeleton of the nonwoven fabric using 20% or more of thick fibers having a fiber diameter of 5 μm or more of the total mass of the fibers. Since the skeleton also has the purpose of forming a space for holding coarse dust, a single fiber having a high Young's modulus and a thick fiber that is difficult to bend is suitable. For this purpose, as the skeleton-constituting fiber, a fiber having a high Young's modulus as described above and having a fiber diameter of 10 μm or more is suitable. On the other hand, if it is too thick, it may pierce the skin, so the preferred fiber diameter is in the range of 10 to 71 μm, preferably the fiber diameter is 20 to 67 μm, and the optimum fiber diameter is 36 to 67 μm.

  And it is preferable that the fiber of this Young's modulus and fiber diameter is contained in the ratio of 20%-70% of the fiber total mass. More preferably, it is in the range of 25% to 70%, more preferably 30% to 65%. When the blending ratio of fibers having such Young's modulus and fiber diameter is less than 20%, the fiber density becomes dense and pressure loss increases, which is not preferable. On the other hand, when the blending ratio exceeds 70%, the air permeability can be secured, but the bonding strength between fibers is lowered and the specific strength tends to be lowered.

  In order to obtain an excellent specific strength, the apparent fiber density of the flame-retardant electret filter medium is important. In a filter medium structure in which the apparent fiber density is too low, low extensibility and high strength cannot be obtained. On the other hand, if the apparent fiber density is too high, low extensibility and high strength can be obtained, but the filter medium becomes a high pressure loss, and the space necessary for dust holding is reduced, so that it is difficult to obtain long life.

Appropriate fiber apparent density varies depending on the use form of the flame-retardant electret filter medium. For example, when the filter medium is used after being pleated, an appropriate apparent fiber density is preferably 0.060 to 0.15 g / cm ³ , and more preferably 0.070, because a filter medium that is too bulky causes large structural pressure loss. ˜0.14 g / cm ³ . Optimally, the apparent fiber density is preferably 0.080 to 0.13 g / cm ³ and the specific strength is 300 N · cm / g or more. By setting in such a range, the number of individual fibers that are fused at the inter-fiber intersection is made appropriate, and the inter-fiber restraint force that makes it difficult for fiber movement to occur freely against tensile stress is exhibited. .

On the other hand, when used on a flat surface, bulkiness and low-pressure loss resistance that are not easily crushed are important. Therefore, the preferred apparent fiber density is preferably in the range of 0.050 to 0.09 g / cm ³ , more preferably 0.060. and ~0.090g / cm ³ range, had better be as specific strength 150 N · cm / g or more.

  Next, the fiber length suitable for use will be described. When the mutual fiber length is short, sufficient tensile strength cannot be obtained even if the fibers are fixed between the fibers. Conventionally, the length of fibers used in the airlaid method has been about several millimeters to 10 mm. The reason for this was to reduce the fiber entanglement, which is a feature of the production method, and to obtain a uniform fiber basis weight. However, if the fiber length is too short, even if a short fiber with a Young's modulus of 70 cN / dtex or more and a fiber diameter of more than 5 μm that is difficult to bend is fixed between the fibers, one fiber is fused with another fiber. If there are few fusion points, the binding force between fibers is weak and it is difficult to increase the specific strength. For this reason, in this invention, it is preferable to make the fiber length 5-50 mm, preferably 8-40 mm, and to increase the location where one fiber is fixed. By doing so, the specific strength can be further increased. In addition, in the case of 50 mm or more, in addition to the increase in the unevenness per unit area, the converging portion of the fibers increases and the specific strength decreases.

  Next, examples of the method for producing the flame-retardant electret filter medium of the present invention include a thermal bond method, an airlaid method, and a spunbond method.

  The thermal bond method is a method in which a short fiber including a heat-fusible short fiber having crimps is web-formed through a card machine, and then the fibers are heat-sealed to create a nonwoven fabric. The spunbond method is a method in which a polymer is drawn out from a small hole in a melted state and then solidified to form a non-woven fabric directly. The airlaid method uses a mixture of short crimped fibers and non-crimped fibers having a fiber length of several millimeters to 10 millimeters, or after laminating short fibers in a single configuration, and then bonding the low melting point portion of the heat-fusible fiber It is a method that can be melted and fixed between fibers or sprayed with resin. Of the above-described production methods, the airlaid method and the spunbond method, which can make a non-crimped fiber into a non-woven fabric, are most preferable in order to achieve specific strength. Hereinafter, the reason why the airlaid method is the optimum manufacturing method will be described in detail.

  Non-woven fabric of chemical bond manufacturing method and thermal bond manufacturing method using a card machine that is essential to use crimped short fibers as a web forming means is passed through a card machine to make a fiber web, and then processed by resin processing or thermal bonding Obtained by fixing the gap. In the nonwoven fabric structure, since the used single fiber is originally provided with three-dimensional crimps, the nonwoven fabric tends to be thick even if the fibers are fixed with resin or the like, and the three-dimensional looseness is likely to remain. As a result, when an external force is applied, the fiber entanglement is unwound from the portion where the looseness remains, and the nonwoven fabric filter medium stretches ahead of the deformation resistance strength, so that structural pressure loss is likely to occur.

  On the other hand, the airlaid manufacturing method in which crimped short fibers or non-crimped short fibers having a fiber length as short as several millimeters to 10 mm are dispersed in air, dropped with a sieve, air-conveyed and then made into a web and then fixed between the fibers is particularly non-crimped. When fibers are used, the fiber accumulation is planar, so that the orientation of each single fiber is one-dimensional and has a low looseness.

  Since the filter medium in which the single fibers of the fiber aggregate are fixed has little elongation due to looseness when an external force is applied, a tensile resistance according to the properties of the single yarn is easily generated immediately. In addition, since fibers having different fiber diameters, fiber lengths, and fiber materials can be mixed or laminated at an arbitrary ratio, the fiber density can be controlled and a multilayer structure can be formed. The conductive electret filter medium is preferably produced from a nonwoven fabric by airlaid.

The basis weight and thickness of the filter medium are not generally determined by the physical properties determined by the intended use of the filter medium, but in an air filter such as an air purifier or a cabin filter, the basis weight of the filter medium is 10 to 150 g / m ² , The thickness range is preferably 0.10 to 1.6 mm.

Next, the filter medium can have two or more fiber layers having different average fiber diameters. For example, at least one layer located on the most upstream side of the air flow is a fiber layer composed of a plurality of short fibers having different fiber diameters and having an average fiber diameter in the range of 20 to 70 μm, and is located on the most downstream side of the air flow. At least one layer to be formed is composed of a fiber layer having a basis weight of 3 to 50 g / m ² and an average fiber diameter of 0.5 to 15 μm. At this time, the electret fibers are mixed in the upstream layer and / or the downstream layer, whereby a longer life and higher collection can be achieved.

  Further, the fine dust collecting property can be further enhanced by laminating a melt blown nonwoven fabric having a higher density on the downstream side of the flame retardant electret filter medium of the present invention.

  Next, a deodorizing filter medium in which the flame retardant electret filter medium of the present invention is laminated on a deodorizer will be described.

Conventionally, many deodorizing filter media in which a deodorizing agent such as activated carbon is sandwiched between nonwoven fabrics are commercially available. Its content is polypropylene-based spunbonded nonwoven fabric (fiber-to-fiber fixing is embossed type, fiber diameter 20 μm, thickness 0.14 mm, basis weight 16 g / m ² , fiber density 0.11 g / cm ³ , collection efficiency 0.2%) 2 A granular activated carbon and a particulate low melting powder adhesive mixture are joined and integrated between the layers. The performance problem with this filter media configuration is that a thin nonwoven fabric with few fiber openings is used to prevent the activated carbon particles from falling off between the fibers of the nonwoven fabric, so that the dust is clogged on the nonwoven fabric surface and the life is short. It is. Moreover, there was a fault that dust collection efficiency was low (pollen particle collection rate was low).

In order to increase the amount of dust retained and increase the collection efficiency, it is only necessary to use bulky and thin fibers, but conventional bulky nonwoven fabrics composed of thin fibers with low Young's modulus are generated inside when the filter medium is wound. Since the material is crushed and thinned by the winding pressure, the bulkiness cannot be maintained, and the apparent fiber density is increased, so that the ventilation resistance is increased and the dust holding ability is also lowered. Therefore, there is no bulky filter medium that can be used on a flat surface with high collection property and long life. However, in place of the above-described polypropylene-based spunbonded nonwoven fabric, the flame retardant having an apparent fiber density of 0.05 to 0.15 g / cm ^{3 in} which the high Young's modulus fiber and the heat-fusible fiber are fused between the fibers of the present invention. If the electret filter medium is used, fibers with a high Young's modulus are fixed by bridging between the fibers, so it is difficult to deform even with a bulky structure with high collection efficiency using fibers with a thin fiber diameter, and electret properties Therefore, it is suitable for flat surface use with excellent dust holding ability.

  The combination of thick heat-fusible fibers that can be electret processed and high Young's modulus fibers have a strength that is difficult to deform, and is effective in increasing the collection and extending the life.

  As the deodorizing agent, a granular material having an average particle size of 0.1 to 1.0 mm such as coconut shell-based or wood-based granular activated carbon, zeolite, or porous silica can be suitably used. In addition, chemicals that react with malodorous gas components may be attached to each deodorant. It is also possible to use a combination of different kinds of deodorizing agents such as activated carbon and zeolite or porous silica.

  Hereinafter, the present invention will be described more specifically with reference to examples. In addition, the evaluation method of each characteristic of the filter medium in a present Example and the preparation method of a filter medium are described below.

<Young's modulus>
Evaluation is performed according to JIS-L1013 (1999). The apparent Young's modulus is obtained from the initial tensile resistance, and this value is taken as the Young's modulus. Further, since the fiber length is as short as several millimeters to several tens of millimeters, the pulling speed per minute is set to 100% of the fiber length. Further, the evaluation N number is at least 10 or more, and the arithmetic average is the Young's modulus in the present invention.

<Thickness>
JIS-L1085 measured density of the sample area 100 cm ² per one place in accordance with the test method (1977), to find the total 21 or more points in the thickness, and calculates the arithmetic mean value.

<Unit weight>
The sample is allowed to stand at room temperature of 24 ° C. and 60% RH for 8 hours or longer, and the mass of the evaluation sample is obtained. The mass per 1 m ² is corrected from the area and obtained as the basis weight of each evaluation sample. Minimum area sample ring shall be 0.01 m ² or more.

<Specific strength>
The tensile strength was determined according to JIS L1085 (1998), and the specific strength was determined from the tensile strength. Specifically, first, a 50 mm wide nonwoven fabric was evaluated with a constant speed tensile testing machine (Autograph SHIMADZU Co., Ltd., model AGS-J) at a length between chucks of 200 mm and a pulling speed of 100 mm / min. The generated strength (N) at 1% elongation is obtained from the curve, and the generated strength is divided by the cross-sectional area of the test piece (nonwoven fabric width 50 mm × nonwoven fabric thickness), and the tensile strength at 1% elongation (N / cm ² ). Asked. Subsequently, the tensile strength (N / cm ² ) was divided by the apparent fiber density (g / cm ³ ) of the nonwoven fabric obtained by the method described later to obtain the specific strength. In addition, let specific strength be the arithmetic mean value of the value which sampled and evaluated at least 5 or more 200mm length in the pleating direction (MD direction) of a nonwoven fabric.

<Pressure loss measurement of flame retardant electret filter media>
An evaluation sample is set in a testing machine according to JIS B9908 (2001) type 3 test method, and the wind speed passing through the evaluation sample is determined as 4.5 m / min. The number of evaluation N is 2 or more, and an arithmetic average value is used.

<Measurement of collection efficiency of flame retardant electret filter media>
An evaluation sample is set in a testing machine conforming to JIS B9908 (2001) type 3 test method, and the following formula is used.
Collection efficiency = (1− (C 2 _{O 3} / C _I )) × 100
C _O ; number of particles of 0.3 μm to 0.5 μm collected by the evaluation sample C _I ; number of particles of 0.3 μm to 0.5 μm supplied to the evaluation sample is atmospheric dust (particle size range: 0.3 to 0.5 μm), and the measurement wind speed is 4.5 m / min.
The number of evaluation N is 2 or more, and an arithmetic average value is used.

<Dust retention and dust collection efficiency>
A filter medium is set on an evaluation device according to JIS B9908 (2001) type 3 test method, and air is passed at a filter medium through-air speed of 4.5 m / min to determine an initial pressure loss (ΔP ₁ ) of the filter medium. Next, 15 types of dust described in JIS Z8901 (1974) are supplied until the final pressure loss reaches 150 Pa, and the amount of dust (W ₁ ) collected on the filter medium and the absolute value on the downstream side without being collected by the evaluation unit From the amount of dust (W ₂ ) collected by the filter, the dust collection rate is determined by the following formula.
Dust collection rate = [1- (W ₂ / (W ₁ + W ₂ ))] × 100
W ₁ = mass of JIS 15 type dust particles collected by the evaluation sample W ₂ = mass dust holding amount of JIS 15 type dust particles collected by the absolute filter is the value of W ₁ .

<Dust retention amount and dust collection rate of filter unit>
A filter unit is set on an evaluation device conforming to JIS B9908 (2001) type 3 test method, and air is flowed at an air volume of 1.05 m ³ / min to obtain an initial pressure loss (ΔP ₂ ) of the filter unit. Next, 15 types of dust described in JIS Z8901 (1974) are supplied until the final pressure loss reaches 200 Pa. The amount of dust (W ₃ ) collected in the filter unit and the absolute value downstream without being collected by the evaluation unit From the amount of dust (W ₄ ) collected by the filter, the dust collection rate in the filter unit is determined by the following formula.
Dust collection rate = [1- (W ₄ / (W ₃ + W ₄ ))] × 100
W ₃ = mass of JIS 15 type dust particles collected by the filter unit W ₄ = mass of JIS 15 type dust particles collected by the filter unit and the absolute filter The total dust holding amount is the value of W ₂ .

<Apparent fiber density>
JIS-L1085 in the measurement of one point per sample area 100 cm ² in accordance with the test method (1977) obtains a thickness of more than a total of 21 points, and calculates the arithmetic mean value. The filter medium weight is allowed to stand at room temperature of 24 ° C. and 60% RH for 8 hours or more to determine the mass of the nonwoven fabric, and the mass per 1 m ² is corrected from the area to obtain the weight per unit area. The sampling minimum area is 0.01 m ² or more, the number of samples to be evaluated is 2 or more, and the arithmetic average value thereof is used.

  From the thickness and basis weight of the nonwoven fabric obtained by the above method, the apparent density of the nonwoven fabric is obtained by the following formula.

Apparent fiber density (g / cm ³ ) = filter medium weight (g / cm ² ) / filter medium thickness (cm) × 10000.

<Flame retardance>
It carries out according to FMVSS-302.

<Average fiber diameter>
The average fiber diameter is obtained by photographing the nonwoven fabric surface with an electron microscope, measuring the fiber width of 100 or more fibers, and calculating the average fiber diameter.
<Ratio of phenyl group and methyl group>
The R portion described above was substituted with a phenyl group and a methyl group, respectively, and silicone compounds having different molar ratios of phenyl group and methyl group were prepared.
<Content of silanol group>
The reaction time at the time of condensing the silicone compound was adjusted, and silanol groups of the obtained silicone compound were measured by infrared spectroscopic analysis. When the silanol group content was 0%, R ₃ SiOCl was excessively added as a silanol group blocking agent to make the silanol group content 0%.

<Making flame retardant electret filter media>
(Manufacture of heat-fusible flame retardant fiber)
As RSiO _1.5 , a silicone compound having a molecular weight of MW 5590 was obtained, in which R was 50 mol% of a phenyl group, 50 mol% of a methyl group, and 5.5 mol% of a silanol group. Moreover, polyetherimide was used as an imide compound. A polypropylene chip having a mass ratio of polypropylene: RSiO _1.5 : polyetherimide = 55: 15: 30 was prepared, and a core side polymer was obtained as a flame retardant fiber. Further, as a modified polyolefin, a chip was prepared by kneading 1% by weight of hindered amine; “Kimasoap 944” (registered trademark) manufactured by Ciba-Gaigi Co., Ltd. in an ethylene-propylene-butene copolymer, and this was used as a sheath side polymer. Using these polymers, core-sheathed heat-fusible flame retardant fibers that can be electretized were produced with different fiber diameters.

(Manufacture of flame-retardant polypropylon fiber)
As RSiO _1.5 , a silicone compound having a molecular weight of MW 5590 was obtained, in which R was 50 mol% of a phenyl group, 50 mol% of a methyl group, and 5.5 mol% of a silanol group.
Moreover, polyetherimide was used as an imide compound. Polypropylene chips having a mass ratio of polypropylene: RSiO _1.5 : polyetherimide: hindered amine (“Kimasoap 944” manufactured by Ciba-Gaigi Co., Ltd.) = 84: 5: 10: 1 were prepared and manufactured by changing the fiber diameter. This fiber can be electretized.

(Manufacture of vinylon fiber)
Polyvinyl alcohol was subjected to gelation in a solvent after spinning using a solvent wet cooling gel spinning method, and then stretched at a high magnification to change the fineness and fiber length.

(Non-woven fabric)
Non-woven fabric was processed by electret after the filter medium was manufactured by the air raid method using the fiber composition shown in Table 1 of each Example. In the examples and comparative examples, the basis weight of the filter medium was 70 g / m ² unless otherwise specified. Electretization was performed by washing each nonwoven fabric with purified water and then sucking pure water into the nonwoven fabric.

(Examples 1-3)
Flame retardant electret filter media having different fiber densities and specific strengths with the fiber configurations shown in Table 1 were prepared. Polypropylene high Young's modulus fiber and heat-fusible fiber were included, and since it was electret processed, a product with high collection efficiency and high specific strength was obtained. When comparing the performance of a unit using a filter medium in which an electret meltblown nonwoven fabric is laminated on the flame retardant electret filter medium, the apparent fiber density and specific strength are suitable for Example 1 with a low apparent fiber density and a relatively low specific strength. It can be seen that the unit pressure loss is higher and the amount of dust adhering is smaller than in Examples 2 and 3. From these relationships, it is understood that the unit pressure loss is reduced and the dust adhesion amount can be increased within the range in which the apparent fiber density and specific strength affect the unit pressure loss and the dust adhesion amount and are appropriate. Various properties of the obtained filter medium were measured by the above-described methods. Table 1 shows the physical properties.

Example 4
Non-crimped fibers with a small fiber diameter were frequently used as high Young's modulus fibers of polypropylene. As a result, it can be seen that the collection efficiency of the flame-retardant electret filter medium is improved, and the amount of dust attached to the unit is increased as compared with Examples 1 to 3. This is due to the effect that the amount of dust collected by the flame retardant electret filter medium layer and the amount collected by the dense electret meltblown nonwoven fabric is reduced. Various properties of the obtained filter medium were measured by the above-described methods. Table 1 shows the physical properties.

(Example 5)
A crimped fiber was used as the heat-fusible fiber. As a result, the specific strength was slightly lower than in Example 4, the unit pressure loss was increased, and the dust adhesion amount was reduced. It can be seen that the non-crimped fiber is also suitable for the heat-fusible fiber used. Various properties of the obtained filter medium were measured by the above-described methods. Table 1 shows the physical properties.

(Example 6)
Create a web A with a basis weight of 30 g / m ^{2 with} the fiber composition shown in the table, then create a web B with a basis weight of 50 g / m ² with the fiber composition shown in the table, and overlap the webs A and B. An electret-processed flame-retardant electret filter medium C was prepared. Subsequently, it was set as the three-layer filter medium which laminated the flame retardant electret filter medium C with the electret melt blown nonwoven fabric. When a unit was prepared with this three-layer filter medium and the amount of dust adhered was determined, the effect of 35 g / unit and all-electret dense structure filter medium lamination could be confirmed. Various properties of the obtained filter medium were measured by the above-described methods. Table 2 shows the physical properties.

(Example 7)
Using the fiber blend of Example 5, flame-retardant electret filter medium D having a basis weight of 18 g / m ^{2 and an} apparent fiber density of 0.06 g / cm ³ and a specific strength of 150 N · cm / g (collection efficiency 18%, dust holding amount is 180 g / m ² ). Polypropylene spunbond nonwoven fabric E (Fixed fiber is embossed, fiber diameter is 20 μm, thickness is 0.14 mm, basis weight is 16 g / m ² , fiber density is 0.11 g / cm ³ , collection efficiency is 0.2%, dust retention is 73 g. / M ² ) was prepared. Using these two nonwoven fabrics, deodorized filter medium level 1 and level 2 were prepared.

As Level 1, 30-60 # granular active 100 g / m ² and 30-60 # EVA low melting powder 40 g / m ² are uniformly distributed between the flame retardant electret filter medium D and the polypropylene spunbonded nonwoven fabric E. A three-layer filter medium which was mixed and heat-bonded was prepared. In addition, as a level 2, a filter medium having a three-layer structure in which the same amount of the deodorizer blended in the level 1 and the EVA low melting point powder was sandwiched between the polypropylene spunbond nonwoven fabric E2 layers was prepared. As a result of obtaining dust holding amount and dust collection efficiency after loading dust to Levels 1 and 2 (Level 1 is from the flame retardant electret filter medium side) and loading to a final pressure loss of 100 Pa, the dust holding amount is Level 1 : 54 g / m ² , Level 2: 21 g / m ² . Moreover, the dust collection efficiency was excellent at level 1: 97.0% and level 2: 89.1%, with level 1 having a large amount of dust retention and high collection performance.

Next, after winding these filter media to 300 m, the filter medium was wound up and the dust retention amount and dust collection efficiency were similarly determined. As a result, the dust retention amount was level 1:48 g / m ² and level 2:20 g / m ² . there were. The dust collection efficiency was level 1: 96.9% and level 2: 89.1%, and it was also confirmed that level 1 is superior even if there is a change in performance due to winding pressure.

(Comparative Examples 1 and 2)
As shown in Table 2, the filter medium was composed only of crimped heat-fusible fibers. It can be seen that since the specific strength is small, structural pressure loss is large and the amount of dust attached to the unit is small. Various properties of the obtained filter medium were measured by the above-described methods. Table 2 shows the physical properties.

  The filter medium and filter unit of the present invention can be suitably used for machines and devices such as a home air purifier filter, an air-conditioning filter for buildings and factories, an in-vehicle filter, and a plane-use filter medium.

Claims (9)

A flame retardant electret filter medium comprising a flame retardant fiber containing a silicone compound containing a unit represented by RSiO _1.5 (R is an organic group) and a compound having an imide structure, wherein the Young's modulus is Fibers of 70 cN / dtex or more are included, the fibers are fused and fixed, the apparent fiber density is 0.05 to 0.15 g / cm ³ , and the specific strength at 1% elongation is 150 N · cm / g or more. A flame retardant electret filter medium. The flame-retardant fiber is a fiber having a core-sheath structure, and includes a silicone compound containing a unit represented by RSiO _1.5 (R is an organic group) in the core-side polymer and a compound having an imide structure. The flame-retardant electret filter medium according to claim 1, which is a heat-fusible fiber. 2. The organic group R constituting the silicone compound contains a phenyl group, and the content of the phenyl group is 30 mol% or more in a molar ratio with respect to all organic groups constituting the silicone compound. Or the flame-retardant electret filter medium of 2. The flame retardant according to any one of claims 1 to 3, wherein the silicone compound contains a silanol group, and the content of the silanol group is 2% or more and 10% or less by weight with respect to the silicone compound. Electret filter medium. The flame retardant electret filter medium according to any one of claims 1 to 4, wherein the compound having an imide structure is a polyetherimide. The filter medium has two or more fiber layers having different average fiber diameters, and at least one layer located on the most upstream side of the air flow is composed of a plurality of short fibers having different fiber diameters and has an average fiber diameter in the range of 20 to 70 μm. The fiber layer and at least one layer located on the most downstream side of the air flow is a fiber layer having a basis weight of 3 to 50 g / m ² and an average fiber diameter of 0.5 to 15 μm. Claim | item 1-5 flame-retardant electret filter media in any one. The deodorizing filter material which laminated | stacked the flame-retardant electret filter material in any one of Claims 1-6 on the deodorizer.   The flame-retardant electret filter medium according to any one of claims 1 to 7, which is produced by an airlaid method.   A filter unit using the filter medium according to claim 1.