Patent Publication Number: US-2012031063-A1

Title: Low-basis-weight filter media for air filters

Description:
TECHNICAL FIELD 
     The present invention relates to a filter medium for air filters, and specifically relates to a filter medium for semi-high performance air filters or filter medium for high performance air filters, which is used for filtering microparticles in gases, in air filters for use in air cleaning facilities such as clean rooms and clean benches relating to semiconductor, liquid crystal, bio and foodstuff industries, air filters for use in air conditioning of buildings air filters for use in air cleaners, or the like. 
     BACKGROUND ART 
     In order to efficiently collect particles of from submicron to micron scale in the air, a collecting technique by an air filter has been conventionally used. Air filters are roughly classified into rough filters, middle-performance filters, semi-high performance filters, high performance filters (HEPA filters, ULPA filters) and the like depending on the difference in the intended particle diameter and dust removal efficiency of the filters. 
     Among these, as the specification of semi-high performance filters and high performance filters, the European Standard EN 1822 is exemplified. EN 1822 is roughly classified into seven steps from U16 to H10 according to degrees of collection efficiency at the maximum permeation particle size (MPPS). Other examples of the standards of high performance filters include IEST-RP—CC001 in the United States, JIS Z 4812:1995, “High Performance Air Filters for Use in Radioactive Aerosols” in Japan, and the like. Furthermore, as filter media that are used in semi-high performance filters and high performance filters, those satisfy these specifications as air filters are used. As a material for a filter medium, filter media for use in air filters made of non-woven fabric-like glass fibers are frequently used, and as a main constitutional material, glass short fibers having an average fiber diameter of from submicrometers to about 3 micrometers are used. 
     Furthermore, filter media for use in air filters that are used in clean rooms are provided with water repellency as necessary. The water repellency in the present invention is one defined by the measurement method of MIL-STD-282. The purposes of imparting water repellency to a filter medium is, for example, to prevent penetration of a sealing agent, a hot-melt and the like that are used during processing of the filter medium, to make the filter medium possible to be used as it is even when moisture is condensed due to the change in temperature, and the like. Furthermore, it is considered that a filter medium having high water repellency is required for preventing deliquescence of salinity under an environment in which much particles of sea salt are present. 
     A filter medium for air filters is folded into a zigzag shape by using a pleat converter so as to increase a filtering surface area, and housed in a frame made of aluminum, wood, a resin or the like to give an air filter unit. Although a standard size of an air filter unit is vertical x horizontal of 610 mm×610 mm, the size varies depending on the purpose of the air filter unit. Furthermore, the depth of the air filter unit also varies. 
     Examples of prior arts relating to a filter medium for air filters include the following ones. As an improvement strategy for improving the strength of a filer medium for air filters composed of glass fibers, a filter medium composed of glass fibers having a specific fiber diameter, polypropylene fibers, polyolefin type composite fibers containing a thermoplastic component as a part thereof, and vinylon binder fibers that are a long chain synthetic polymer containing 65% by weight or more of vinyl alcohol units having a water dissolving temperature of from 50 to 100° C., wherein the incorporation percentage of the glass fibers in the filter medium of from 0.5 to 20% by weight and the incorporation percentage of the vinylon binder fiber is from 0.5 to 10% by weight, and a method for producing the same, were suggested (see, for example, Patent Document 1). Furthermore, a filter medium for air filters, which is composed of 60 to 97% by weight of ultrafine glass fibers each having a diameter of 4 μm or less, thin-denier polyvinyl alcohol type fibers of 0.05 to 0.5 denier and 0 to 7% by weight of a polyvinyl alcohol-based fibrous binder, and has a basis weight of from 25 to 150 g/m 2 , was suggested (for example, see Patent Document 2). 
     Furthermore, a filter medium for bag filters containing a filtering layer in which fine synthetic fibers of 5 μm or less and glass fibers of 1 μm or less have been mixed together with a fiber binder by a wet papermaking method, and a strength-maintaining layer that is attached to the filtering layer and supports and reinforces the filtering layer, and the filter medium for bag filters restores its filtration function by brushing off collected dust adhered on the surface by regurgitating air by blowing the air in the direction opposite to the general direction of passage when dust in air is filtered, was suggested (for example, see Patent Document 3). 
     In the past, the present inventors have provided a filter medium for air filters obtained by attaching an organic type binder, a polyisocyanate compound and a water repellent to glass fibers that constitute a filter medium, and a method for producing the same, in order to improve the strength and water repellency of the filter medium, and to decrease pressure loss and improving collection efficiency as compared to conventional filter media (for example, see Patent Document 4). 
     Furthermore, as conventional methods for imparting water repellency to filter media for air filters containing glass fibers as major fibers, use of a water repellent composed of a silicone type resin (for example, see Patent Document 5), or use of a water repellent composed of a fluorine type resin (for example, see Patent Document 6) has been performed. In the past, the present inventors also suggested a filter medium for air filters formed by attaching an alkylketene dimer to the surface of glass fibers (for example, see Patent Document 7), and a filter medium for air filters that generates little outgas, which is formed by attaching to the surface of the above-mentioned glass fibers a condensed product that is obtained by hydrolyzing an alkoxysilane having at least three or more alkoxy groups in a molecule and condensing the product, as a water repellent that does not generate organic outgas containing mainly a low-molecule cyclic siloxane (for example, see Patent Document 8). 
     PRIOR ART DOCUMENTS 
     Patent Document 
     
         
         Patent Document 1: Japanese Patent Application Laid-Open No. 8-206421 
         Patent Document 2: Japanese Examined Patent Publication No. 6-13082 
         Patent Document 3: Japanese Patent Application Laid-Open No. 8-243321 
         Patent Document 4: Japanese Patent Application Laid-Open No. 9-225226 
         Patent Document 5: Japanese Patent Application Laid-Open No. 2-41499 
         Patent Document 6: Japanese Patent Application Laid-Open No. 62-090395 
         Patent Document 7: Republished WO02/016005 
         Patent Document 8: Japanese Patent Application Laid-Open No. 2007-29916 
       
    
     SUMMARY OF INVENTION 
     Problems to be Solved by the Invention 
     In recent years, an air filter unit of a type having a thin depth is desired for the purpose of weight saving and downsizing of an air filter unit; however, there was a problem that when one intends to house a pleated filter medium having the same filtering surface area into a unit, the pleated surfaces of the filter medium contact with each other due to the thickness of the filter medium to cause structural resistance, whereby the pressure loss of the air filter unit is increased significantly. 
     In order to solve the problem, there is an attempt to decrease the thickness, i.e., to decrease the basis weight of a filter medium for air filters. However, a filter medium made of glass fibers is brittle as compared to other filter media of organic materials. Therefore, the basis weight of a filter medium is generally adjusted to about from 60 to 80 g/m 2 ; however, there was a problem when one attempts to adjust it to 50 g/m 2  or less, the strength of the filter medium is decreased by doing so, whereby the filter medium becomes easy to be broken during pleat processing and practical use as an air filter. Furthermore, there was also a problem that the water repellency of the filter medium is also decreased due to the decrease in the basis weight of the filter medium. 
     According to the studies by the present inventors, it was found that the above-mentioned breaking of the filter medium is deeply affected by the internal tearing strength of paper as defined in JIS P 8116:2000 “Paper-Method for Testing Internal Tearing Strength-Elmendorf Tearing Tester Method”, and also found that breakage occurs easily since the internal tearing strength is decreased in accordance with the decreased in the basis weight. Furthermore, it was also found that the water repellency is decreased since the thickness of the filter medium is decreased and water penetrates more easily in accordance with the decrease in the basis weight. 
     In the past, there was the technique of Patent Document 1 or 2 as a strategy for improving the strength of a filter medium for air filters made of glass fibers. However, the technique had problems that, although an effect of improving tensile strength is observed in the Examples of said patent literature, the internal tearing strength of the filter medium is not improved, little effect is exhibited on breakage of the filter medium during pleat processing or practical use, and no effect of improving water repellency is exhibited. 
     Furthermore, the technique relating to a strength-maintaining layer in Patent Document 3 has problems that a special equipment is required for binding the strength-maintaining layer to the filtering layer, the binding surface between the filtering layer and the strength layer is readily peeled, and no improvement of water repellency is observed. 
     Furthermore, it was difficult to make up for the decrease in the strength of the filter medium and the decrease in the water repellency of the filter medium due to the decrease in the basis weight even the technique of Patent Document 4 that was provided by the present inventors was used; specifically, improvement of internal tearing strength could be expected little. 
     In addition, although there are the techniques of Patent Documents 5 to 8 for water repellency, they treated the surface of glass fibers by using a reagent having water repellency, and have problems not only that it is necessary to increase the use amount of the reagent significantly so as to make up for the decrease in water repellency in accordance with decrease of the basis weight and high costs are required, but also that the improving effect by an adhesion amount is plateaued. Furthermore, there is also a problem that improvement of internal tearing strength is not observed even the adhesion amount of the reagent is increased. 
     As mentioned above, the techniques of Patent Documents 1 to 8 were not drastic measures against the decrease in the basis weight of the filter medium. Therefore, the problem of the present invention is to improve a problem of the decrease in internal tearing strength and water repellency in accordance with the decrease in the basis weight of a filter medium for air filters, and to provide a filter medium for air filters that can also address demands for flame retardancy. 
     Means for Solving the Problems 
     The present inventors have done intensive studies and found that the above-mentioned problems can be solved by using glass short fibers and short fibers of hydrophobic chemical synthetic fibers each having a fiber diameter of 5 μm or less as raw material fibers of a filter medium, and fixing the fibers by a hydrophobic synthetic resin type binder, and completed the present invention. That is, a low-basis-weight filter medium for air filters according to the present invention is characterized by: glass short fibers (A) and short fibers of hydrophobic chemical synthetic fibers each having a fiber diameter of 5 μm or less (B) by a mass ratio (A/B) in the range of from 70/30 to 95/5, as raw material fibers of the filter medium; a hydrophobic synthetic resin-based binder is added by 3 to 10 parts by mass with respect to 100 parts by mass of the raw material fibers; and a basis weight of from 25 g/m 2  or more to 50 g/m 2  or less. 
     In the low-basis-weight filter medium for air filters according to the present invention, the mass ratio (A/B) is in the range of from 78/22 to 95/5, and the filter medium for air filters preferably satisfies the combustion classification class 3 of a combustion test that conforms with the Guidance of Combustion Test Methods on Filter Media for Air Cleaning Apparatuses JACA No. 11A-2003. Flame retardancy is imparted by adjusting the mass ratio (A/B) to the range from 78/22 to 95/5. 
     In the low-basis-weight filter medium according to the present invention, a water repellent is added to the filter medium for air filters. The water repellent can be used in combination, and water repellency is imparted by doing so. 
     Effect of the Invention 
     In the filter medium for air filters of the present invention, the problem of the decrease in internal tearing strength and the water repellency due to the decrease in the basis weight has been improved. Regarding internal tearing strength, an internal tearing strength physical property similar to that of a filter medium having a conventional basis weight is maintained. Furthermore, even short fibers of combustible chemical synthetic fibers are incorporated, flame retardancy that conforms with JACA No. 11A-2003 can be imparted to the filter medium. 
     MODES FOR CARRYING OUT THE INVENTION 
     Hereinafter the present invention is explained in detail with referring to the exemplary embodiments, but the present invention is not construed to be limited to these descriptions. The exemplary embodiment may be modified variously as long as it exhibits the effect of the present invention. 
     The low-basis-weight filter medium for air filters according to the present exemplary embodiment contains as raw material fibers of the filter medium, glass short fibers (A) and short fibers of hydrophobic chemical synthetic fibers each having a fiber diameter of 5 μm or less (B) by amass ratio (A/B) in the range of from 70/30 to 95/5; to which 3 to 10 parts by mass of a hydrophobic synthetic resin-based binder is added with respect to 100 parts by mass of the raw material fibers; and the filter medium has a basis weight of from 25 g/m 2  to 50 g/m 2 . 
     The basis weight in the present exemplary embodiment is from 25 to 50 g/m 2 . When the basis weight is lower than 25 g/m 2 , the internal tearing strength and water repellency are decreased to lower than the quality level of a filter medium having an existing basis weight. Furthermore, when the basis weight is more than 50 g/m 2 , an effect of decreasing the thickness of the filter medium is decreased. In the present invention, the existing basis weight is defined as from 60 to 80 g/m 2 . 
     The fibers used as major fibers in the present exemplary embodiment are those referred to as glass short fibers, and the glass short fibers can be freely selected from glass short fibers having various fiber diameters according to the required filtering performance and other physical properties. Specifically, the glass short fibers are wool-like glass fibers that are produced by a flame attenuation method or rotary method, and are essential components for keeping the pressure loss of the filter medium at a predetermined value to give a suitable collection efficiency. Since the collection efficiency increases as the fiber diameter decreases, it is necessary to incorporate ultrathin glass fibers having a fine average fiber diameter so as to obtain a high performance filter medium. However, since the pressure loss may sometimes increases excessively when the fiber diameter decreases, those having a suitable fiber diameter should be selected within this range. In addition, fibers having several kinds of fiber diameters may be blended and incorporated. Those having approximately 5 μm or less as a fiber diameter are used. As the glass composition, nearly all of applications for air filters are borosilicate glass, in which C glass having acid resistance and E glass having electrical insulating property (alkali-free glass) are also included. Furthermore, for the purpose of preventing boron contamination in semiconductor processes and the like, low-boron glass short fibers and silica glass short fibers can also be used. Meanwhile, the major fibers in the present exemplary embodiment refer to fibers that account for 70% by mass or more of the composition of the whole raw material fibers. In the case of the present exemplary embodiment, they mean glass short fibers. 
     The present inventors have done intensive studies on the countermeasures against the decrease in internal tearing strength and the decrease in water repellency in accordance with the decrease in basis weight with respect to a filter material for air filters containing the glass short fiber as a main component, and consequently found that internal tearing strength and water repellency are significantly improved by incorporating short fibers of hydrophobic chemical synthetic fibers each having a small diameter as a part of raw material fibers and by adding a hydrophobic synthetic resin binder to the filter medium. 
     The short fibers of hydrophobic chemical synthetic fibers are obtained by spinning a spinning liquid from a spinneret by a spinning method such as a wet spinning method, a dry spinning method, an emulsion spinning method and a melt-spinning method to give thread-like fibers, and cutting the fibers into a suitable length, and are used for various applications such as raw materials for nonwoven fabrics and cotton for futons. Of these, the short fibers of hydrophobic chemical synthetic fibers used in the present exemplary embodiment should have a small fiber diameter of 5 μm or less. When the fiber diameter exceeds 5 μm, an effect of improving internal tearing strength is exhibited little. The short fibers of hydrophobic chemical synthetic fibers each have a fiber length of, preferably from 1 mm or more to 20 mm or less, more preferably from 3 mm or more to 10 mm or less. When the fiber length is longer than 20 mm, the fibers readily cause undispersion in a step for dispersing fibers during the production of a sheet, and a finished filter medium sheet may become uneven and cause a decrease in collection performance. On the other hand, when the fiber length is shorter than 1 mm, an effect of improving internal tearing strength may be decreased. 
     The composition of the short fibers of hydrophobic chemical synthetic fibers may include polyester fibers, acrylic fibers, polyethylene fibers, polypropylene fibers, and the like, but is not specifically limited as long as the fibers are hydrophobic. As the composition, besides these compositions, sea-island-type fibers and split-type fibers whose diameters have been decreased by further improving the above-mentioned spinning methods, and pulp-like type chemical synthetic fibers obtained by further adding a mechanical treatment to chemical synthetic fibers can also be used. Furthermore, these fibers may be used appropriately in combination. On the other hand, vinylon fibers that are hydrophilic, rayon fibers and cellulose-based fibers that are semisynthetic fibers, and the like are not preferable since they do not have an effect of improving the internal tearing strength of the filter medium, and inhibit improvement of the water repellency. 
     Furthermore, since filter media for air filters containing mainly glass fibers are generally composed of a majority of inorganic materials, they have flame retardancy that conforms with the Guidance of Combustion Test Methods on Filter Media for Air Cleaning Apparatuses JACA No. 11A-2003. However, there is a fear that the flame retardancy in said test methods is not satisfied by incorporating short fibers of hydrophobic chemical synthetic fibers that are combustible into the constitution of the filter medium. Although flame retardancy is not necessarily considered to be a necessary property since the degree of requirement differs depending on the purpose of use of the filter medium, it is possible that the filter medium cannot accept the case when flame retardancy is required. One of methods may include a method of making the short fibers of chemical synthetic fibers themselves flame retardant, but it is not so preferable since it is necessary for imparting flame retardancy to add a halogen that is not preferable for environments and/or phosphorous that decreases an yield in processes for the production of semiconductors. Therefore, the present inventors have done intensive studies, and consequently found that the chemical synthetic fibers themselves do not necessarily need to have flame retardancy, by controlling the incorporation percentage of the short fibers of hydrophobic chemical synthetic fibers each having a small diameter. 
     The incorporation ratio of the glass short fibers (A) and the short fibers of hydrophobic chemical synthetic fibers (B) is from 70/30 to 95/5 by a mass ratio (A/B). Furthermore, the incorporation ratio is preferably from 78/22 to 95/5 by amass ratio (A/B) so as to impart flame retardancy to the filter medium. When the incorporation percentage of the short fibers of hydrophobic chemical synthetic fibers (B) in the glass short fibers (A) and the short fibers of hydrophobic chemical synthetic fibers (B) is less than 5% by mass, the effect of improving the internal tearing strength and water repellency of the filter medium is decreased. When the incorporation percentage exceeds 30% by mass, the short fibers of hydrophobic chemical synthetic fibers disturb the network of the glass short fibers that constitute a sheet of the filter medium, whereby the collection performance of the filter medium is decreased. Furthermore, when the incorporation percentage of the short fibers of hydrophobic chemical synthetic fibers (B) in the glass short fibers (A) and the short fibers of hydrophobic chemical synthetic fibers (B) exceeds 22% by mass, the filter medium loses its flame retardancy and does not satisfy the classification class 3 of the measurement method of JACA No. 11A-2003. 
     Furthermore, unless the object of the present exemplary embodiment is obstructed, chopped glass fibers each having a fiber diameter of 5 μm or more that is larger than that of the glass short fibers, inorganic fibers, natural fibers each having a large diameter, short fibers of chemical synthetic fibers each having a large diameter, and the like can be incorporated as indirect materials by 15% by mass or less into the composition of the major fibers. 
     In order to exert the effect of the present exemplary embodiment, it is necessary to further add a hydrophobic synthetic resin type binder to the filter medium. These binders are added to the filter medium as those having a form of an aqueous solution or water type emulsion, namely, as binder liquids. Examples of the hydrophobic synthetic resin type binder may include acrylic ester type resins, urethane type resins, epoxy type resins, olefin type resins, and the like. On the other hand, starch, polyvinyl alcohol type resins and the like that are hydrophilic binders are not preferable since they do not have an effect of improving the internal tearing strength of the filter medium, and inhibit improvement of the water repellency. However, unless the object of the present exemplary embodiment is obstructed, a hydrophilic binder of a minimal amount can be added as an indirect material besides the hydrophobic synthetic resin type binder. Simultaneously, these binders also have an effect of attaching glass short fibers that have no self-adhesion ability on the fibers themselves to each other. The addition amount of the hydrophobic synthetic resin type binder is preferably from 3 to 10 parts by mass, more preferably from 4 to 7 parts by mass against 100 parts by mass of the raw material fibers. When the amount is less than 3 parts by mass, the effect of the present exemplary embodiment is not exerted, and when the amount exceeds 10 parts by mass, the resin film of the binder occludes the pores of the fiber network of the filter medium to cause problems that the pressure loss of the filter medium is increased, and the like. 
     Furthermore, in order to impart water repellency that is required for the filter medium in practical use, a water repellent is added in most cases. Generally, these water repellents are also mixed with a binder liquid or added solely to the filter medium as in the binder. As the water repellent, silicone type resins, fluorine type resins, waxes, alkylketene dimers, and the like are used. 
     Where necessary, other additives such as surfactants, anti-foaming agents, pH adjusting agents, wetting agents, water holding agents, thickening agents, crosslinking agents, mold-releasing agents, antiseptic agents, softening agents, antistatic agents, water resistant additives, plasticizers, fluorescent whitening agents, coloring pigments, coloring dyes, ultraviolet absorbers, antioxidants, fragrances and deodorants can be suitably selected and added to the binder liquid. 
     If either of the short fibers of hydrophobic chemical synthetic fibers and hydrophobic synthetic resin type binder is absent in the constitution of the present exemplary embodiment, the effect of the invention cannot be exerted. The reason is presumed that the short fibers of hydrophobic chemical synthetic fibers and hydrophobic synthetic resin type binder are both hydrophobic and thus the fibers and the resin type binder have good compatibility and adhere well, and the glass short fibers and the resin type binder (hydrophobic synthetic resin type binder) also adhere, whereby a great effect is exerted against internal tearing strength. In addition, since the short fibers of hydrophobic chemical synthetic fibers have a small diameter of a fiber diameter of 5 μm or less and have a large specific surface area, they promote tight adhesion among the glass short fibers—the short fibers of hydrophobic chemical synthetic fibers having a small diameter—the hydrophobic synthetic resin type binder. With respect to the effect of improving water repellency, it is presumed that the short fibers of chemical synthetic fibers strongly inhibit penetration of water since they have a small diameter and are hydrophobic. This is a phenomenon that is not observed in general chemical synthetic fibers. If the hydrophobic synthetic resin type binder is increased, a similar effect is exhibited; however, a problem is caused since the resin film of the binder occludes the pores of the fiber network in the filter medium as mentioned above. Since the short fibers of hydrophobic chemical synthetic fibers having a small diameter are fibrous, they do not cause such problem. 
     In the production process of the filter medium, a so-called wet papermaking process that contains dispersing fibers in water, spreading the fibers on a papermaking wire and dehydrating the fibers from under the wire to give a sheet is used. The kind of a paper machine used at this time is not limited in the present exemplary embodiment, and for example, a sheet-fed papermaking apparatus, or continuous papermaking apparatuses such as a fourdrinier paper machine, a cylinder paper machine, an inclined wire paper machine, a gap former and a delta former can be used, or a multi-ply paper machine containing one or more kinds of these paper machines may also be used. At this time, in order to obtain a filter medium for filters having higher performance, it is desirable to form a sheet uniformly with fine formation and high bulk as far as possible. 
     Next, a step for adding binder components to the sheeted fiber web is performed. The method for adding the binder is generally a method containing adding a binder liquid in which various components are diluted in a solvent such as water to the sheet by various methods such as impregnation, roll application, spraying and curtain coating, and removing the excess binder liquid by air of a negative pressure or positive pressure. Although the sheet before addition of the binder liquid may be in a moist state or dry state, the sheet is desirably in a moist state or semi-moist state so as to obtain a filter medium having higher performance. 
     The sheet to which the binder liquid has been added is dried in a drying zone. Although the drying method at this time is not specifically limited, hot air drying, drum drying, infrared ray drying and the like are preferably used. The drying temperature is desirably from 110 to 150° C. 
     The finished sheet is wound up by an online or offline, or cut by a cutter to become a product. 
    
    
     EXAMPLES 
     Next, the present invention is explained more specifically with referring to Examples, but the present invention is not construed to be limited by these Examples. Furthermore, unless specifically mentioned, the “part(s)” and “%” in the Examples represent “part(s) by mass” and “% by mass”, respectively. 
     Example 1 
     An acidic water having a sulfuric acidic pH of 3.5 was added to 90% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less (Micro-Strand (registered trademark) Fiber Glass Micro-Fibers Type 475 manufactured by Johns-Manville Corporation) and 10% by mass of polyester short fibers each having a fiber diameter of 0.1 dtex (estimated fiber diameter: 3.0 μm) and a fiber length of 3 mm (TM04PN, manufactured by Teijin Fibers, Ltd.) to adjust the concentration to 0.5%, and these raw materials were defiberized in a mixer for foodstuffs (part number: MX-V200, manufactured by Matsushita Electric Industrial Co., Ltd.) for 1 minute. Then, the defiberized raw materials were diluted up to a concentration of 0.1% by using the same acidic water, and subjected to paper-making by using a hand-made papermaking apparatus to give a wet web. Next, a binder liquid containing a water repellent in which an acrylic type latex (trade name: Voncoat AN-155-E, manufacturer: DIC Corporation), a fluorine type water repellent (trade name: NK Guard NDN-9E, manufacturer: Nicca Chemical Co., Ltd.) had been mixed so that the solid content-mass ratio of the acrylic type latex/fluorine type water repellent became 100/8 was added to the wet web by impregnation, followed by drying by using a roll drier at 130° C. to give an HEPA filter medium having a basis weight of 40 g/m 2  and a binder addition percentage of 5.0% by mass against the filter medium. As used herein, the borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less refers to a suitable combination of borosilicate glass short fibers having various average fiber diameters of 2.5 μm or less so as to obtain an objective pressure loss (275 Pa in the Examples), and the same applies to the following Examples. 
     Example 2 
     An HEPA filter medium having a basis weight of 25 g/m 2  and a binder addition percentage of 5.0% by mass against the filter medium was obtained in a similar manner to Example 1, except that the composition of the fibers was 80% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less and 20% by mass of polyester short fibers each having a fiber diameter of 0.1 dtex (estimated fiber diameter: 3.0 μm) and a fiber length of 3 mm in Example 1. 
     Example 3 
     An HEPA filter medium having a basis weight of 25 g/m 2  and a binder addition percentage of 5.0% by mass against the filter medium was obtained in a similar manner to Example 1, except that the composition of the fibers was 75% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less and 25% by mass of polyester short fibers each having a fiber diameter of 0.1 dtex (estimated fiber diameter: 3.0 μm) and a fiber length of 3 mm in Example 1. 
     Example 4 
     An HEPA filter medium having a basis weight of 40 g/m 2  and a binder addition percentage of 5.0% by mass against the filter medium was obtained in a similar manner to Example 1, except that the composition of the fibers was 85% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less and 15% by mass of polyester short fibers each having a fiber diameter of 0.2 dtex (estimated fiber diameter: 4.3 μm) and a fiber length of 3 mm (TK08PN, manufactured by Teijin Fibers, Ltd.) in Example 1. 
     Example 5 
     An HEPA filter medium having a basis weight of 40 g/m 2  and a binder addition percentage of 5.0% by mass against the filter medium was obtained in a similar manner to Example 1, except that the composition of the fibers was 90% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less and 10% by mass of acrylic short fibers each having a fiber diameter of 0.1 dtex (estimated fiber diameter: 3.3 μm) and a fiber length of 3 mm (D122, manufactured by Mitsubishi Rayon Co., Ltd.) in Example 1. 
     Example 6 
     An HEPA filter medium having a basis weight of 40 g/m 2  and a binder addition percentage of 5.0% by mass against the filter medium was obtained in a similar manner to Example 1, except that the composition of the fibers was 93% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less and 7% by mass of acrylic short fibers each having a fiber diameter of 0.06 dtex (estimated fiber diameter: 2.6 μm) and a fiber length of 3 mm (a development article, manufactured by Mitsubishi Rayon Co., Ltd.) in Example 1. 
     Example 7 
     An HEPA filter medium having a basis weight of 50 g/m 2  and a binder addition percentage of 5.0% by mass against the filter medium was obtained in a similar manner to Example 1, except that the composition of the fibers was 95% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less and 5% by mass of acrylic short fibers each having a fiber diameter of 0.1 dtex (estimated fiber diameter: 3.3 μm) and a fiber length of 3 mm in Example 1. 
     Example 8 
     An HEPA filter medium having a basis weight of 40 g/m 2  and a binder addition percentage of 5.0% by mass against the filter medium was obtained in a similar manner to Example 1, except that the composition of the fibers was 70% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less and 30% by mass of polyester short fibers each having a fiber diameter of 0.1 dtex (estimated fiber diameter: 3.0 μm) and a fiber length of 3 mm in Example 1. 
     Example 9 
     An HEPA filter medium having a basis weight of 25 g/m 2  and a binder addition percentage of 5.0% by mass against the filter medium was obtained in a similar manner to Example 1, except that the composition of the fibers was 78% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less and 22% by mass of polyester short fibers each having a fiber diameter of 0.1 dtex (estimated fiber diameter: 3.0 μm) and a fiber length of 3 mm in Example 1. 
     Example 10 
     An HEPA filter medium having a basis weight of 40 g/m 2  was obtained in a similar manner to Example 4, except that the binder addition percentage was 3.0% by mass against the filter medium in Example 4. 
     Example 11 
     An HEPA filter medium having a basis weight of 40 g/m 2  was obtained in a similar manner to Example 4, except that the binder addition percentage was 10.0% by mass against the filter medium in Example 4. 
     Comparative Example 1 
     An HEPA filter medium having a basis weight of 80 g/m 2  and a binder addition percentage of 5.0% by mass against the filter medium was obtained in a similar manner to Example 1, except that the composition of the fibers was 100% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less in Example 1. 
     Comparative Example 2 
     An HEPA filter medium having a basis weight of 40 g/m 2  and a binder addition percentage of 5.0% by mass against the filter medium was obtained in a similar manner to Example 1, except that the composition of the fibers was 100% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less in Example 1. 
     Comparative Example 3 
     An HEPA filter medium having a basis weight of 40 g/m 2  and a binder addition percentage of 5.0% by mass against the filter medium was obtained in a similar manner to Example 1, except that the composition of the fibers was 90% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less and 10% by mass of polyester short fibers each having a fiber diameter of 1.7 dtex (estimated fiber diameter: 12.5 μm) and a fiber length of 3 mm (TTO4N, manufactured by Teijin Fibers, Ltd.) in Example 1. 
     Comparative Example 4 
     An HEPA filter medium having a basis weight of 40 g/m 2  and a binder addition percentage of 5.0% by mass against the filter medium was obtained in a similar manner to Example 1, except that the composition of the fibers was 90% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less and 10% by mass of polyester short fibers each having a fiber diameter of 0.6 dtex (estimated fiber diameter: 7.4 μm) and a fiber length of 5 mm (TAO4N, manufactured by Teijin Fibers, Ltd.) in Example 1. 
     Comparative Example 5 
     An HEPA filter medium having a basis weight of 50 g/m 2  and a binder addition percentage of 5.0% by mass against the filter medium was obtained in a similar manner to Example 1, except that the composition of the fibers was 97% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less and 3% by mass of acrylic short fibers each having a fiber diameter of 0.1 dtex (estimated fiber diameter: 3.3 μm) and a fiber length of 3 mm in Example 1. 
     Comparative Example 6 
     An HEPA filter medium having a basis weight of 40 g/m 2  and a binder addition percentage of 5.0% by mass against the filter medium was obtained in a similar manner to Example 1, except that the composition of the fibers was 65% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less and 35% by mass of acrylic short fibers each having a fiber diameter of 0.1 dtex (estimated fiber diameter: 3.3 μm) and a fiber length of 3 mm in Example 1. 
     Comparative Example 7 
     An acidic water having a sulfuric acidic pH of 3.5 was added to 90% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less, 10% by mass of acrylic short fibers each having a fiber diameter of 0.1 dtex (estimated fiber diameter: 3.3 μm) and a fiber length of 3 mm, and 4% by mass against the filter medium of PVA binder fibers (Fibribond, manufactured by Sansho Co., Ltd., No. 243) to adjust the concentration to 0.5%, and these raw materials were defiberized in a mixer for foodstuffs (part number: MX-V200, manufactured by Matsushita Electric Industrial Co., Ltd.) for 1 minute. Then, the defiberized raw materials were diluted up to a concentration of 0.1% by using the same acidic water, and subjected to paper-making by using a hand-made papermaking apparatus to give a wet web. Next, an impregnation liquid containing only a fluorine type water repellent (trade name: NK Guard NDN-9E, manufacturer: Nicca Chemical Co., Ltd.) was added to the wet web by impregnation so as to have an addition percentage against the filter medium of 0.4% by mass, followed by drying by using a roll drier at 130° C. to give an HEPA filter medium having a basis weight of 40 g/m 2  and a binder addition percentage of 4.4% by mass against the filter medium. 
     Comparative Example 8 
     An HEPA filter medium having a basis weight of 40 g/m 2  and a binder addition percentage of 4.4% by mass against the filter medium was obtained in a similar manner to Comparative Example 7, except that the composition of the fibers was 90% by mass of borosilicate glass short fibers having an average fiber diameter of 2.5 μm or less and 10% by mass of vinylon short fibers each having a fiber diameter of 0.6 dtex (estimated fiber diameter: 7.7 μm) and a fiber length of 3 mm (VPB053, manufactured by Kuraray Co., Ltd.) in Comparative Example 7. 
     Comparative Example 9 
     An HEPA filter medium having a basis weight of 40 g/m 2  was obtained in a similar manner to Example 4, except that the binder addition percentage was 2.0% by mass against the filter medium in Example 4. 
     Comparative Example 10 
     An HEPA filter medium having a basis weight of 40 g/m 2  was obtained in a similar manner to Example 4, except that the binder addition percentage was 11.0% by mass against the filter medium in Example 4. 
     The following tests were performed on the filter media of the Examples and Comparative Examples. 
     (1) Pressure Loss 
     Using a self-making apparatus, the pressure loss when air was passed through the filter medium having an effective surface area of 100 cm 2  at a surface wind velocity of 5.3 cm/sec was measured by using a differential pressure gauge. 
     (2) 0.3-0.4 μm DOP Penetration 
     Air containing polydispersed DOP particles that was generated at a Laskin nozzle was passed through a filter medium having an effective surface area of 100 cm 2  at a surface wind velocity of 5.3 cm/sec, and the penetration of DOP at that time was measured from the ratio of numbers of the particles at the upper stream and downstream by using a laser particle counter manufactured by Rion Co., Ltd. The objective particle size was from 0.3 to 0.4 μm. 
     (3) PF Value 
     The PF value used as an index of the filter performance of the filter medium was obtained by the following mathematical formula I by using the data in (1) and (2). The higher the PF value is, the lower the permeability is under the same pressure loss, or the lower the pressure loss is under the same permeability. 
     
       
         
           
             
                 
             
              
             
               [ 
               
                 Mathematical 
                  
                 
                     
                 
                  
                 Formula 
                  
                 
                     
                 
                  
                 1 
               
               ] 
             
           
         
       
       
         
           
             
               P 
                
               
                   
               
                
               F 
                
               
                   
               
                
               value 
             
             = 
             
               
                 
                   
                     log 
                     10 
                   
                    
                   
                     ( 
                     
                       D 
                        
                       
                           
                       
                        
                       O 
                        
                       
                           
                       
                        
                       P 
                        
                       
                           
                       
                        
                       
                         
                           penetration 
                            
                           
                               
                           
                           [ 
                           % 
                           ] 
                         
                         / 
                         100 
                       
                     
                     ) 
                   
                 
                 
                   Pressure 
                    
                   
                       
                   
                    
                   
                     
                       loss 
                        
                       
                           
                       
                       [ 
                       Pa 
                       ] 
                     
                     / 
                     9.81 
                   
                 
               
               × 
               
                 ( 
                 
                   - 
                   100 
                 
                 ) 
               
             
           
         
       
     
     (4) Internal Tearing Strength 
     Test pieces each having a length of 63 mm and a width of 76 mm were collected from the filter medium, and five test pieces were superposed and measured by using an automatic digital-type Elmendorf tear tester (Kumagai Riki Kogyo Co., Ltd.). Furthermore, a tear index was obtained by dividing the internal tearing strength by the basis weight. 
     (5) Tensile Strength 
     A test piece that was cut into 1 inch in width×130 mm in length was collected from the filter medium, and measured at a span length of 100 mm and a tensile velocity of 15 ram/min by using a constant rate tensile tester (Toyo Seiki Seisaku-sho, Ltd.; Strograph M1). 
     (6) Water Repellency 
     Water repellency was measured in accordance with MIL-STD-282. 
     (7) Flame Retardancy 
     Flame retardancy was measured by using an inflammability tester complying with the Guidance of Combustion Test Methods on Filter Media for Air Cleaning Apparatuses JACA No. 11A-2003 (Suga Test Instruments Co., Ltd.; UL-94HBF). The evaluation was made as follows: those satisfied the combustion classification class 3 were evaluated as O, and others were evaluated as x. 
     The measurement results of these tests are shown in Table 1 and Table 2. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 Example1 
                 Example2 
                 Example3 
                 Example4 
                 Example5 
                 Example6 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Glass short fibers 
                 90% by mass 
                 80% by mass 
                 75% by mass 
                 85% by mass 
                 90% by mass 
                 93% by mass 
               
               
                 Short fibers of 
                 Polyester 
                 Polyester 
                 Polyester 
                 Polyester 
                 Acrylic 
                 Acrylic 
               
               
                 chemical synthetic 
                 fibers0.1dtex 
                 fibers0.1dtex 
                 fibers0.1dtex 
                 fibers0.2dtex 
                 fibers0.1dtex 
                 fibers0.06dtex 
               
               
                 fibers 
                 10% by mass 
                 20% by mass 
                 25% by mass 
                 15% by mass 
                 10% by mass 
                 7% by mass 
               
               
                 Binder 
                 Acrylic resin 
                 Acrylic resin 
                 Acrylic resin 
                 Acrylic resin 
                 Acrylic resin 
                 Acrylic resin 
               
               
                   
                 binder 5% by 
                 binder 5% by 
                 binder 5% by 
                 binder 5% by 
                 binder 5% by 
                 binder 5% by 
               
               
                   
                 mass against 
                 mass against 
                 mass against 
                 mass against 
                 mass against 
                 mass against 
               
               
                   
                 filter medium 
                 filter medium 
                 filter medium 
                 filter medium 
                 filter medium 
                 filter medium 
               
               
                 Water repellant 
                 Used 
                 Used 
                 Used 
                 Used 
                 Used 
                 Used 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Basis weight 
                 g/m 2   
                 40 
                 25 
                 25 
                 40 
                 40 
                 40 
               
               
                 Internal 
                 mN 
                 490 
                 380 
                 400 
                 430 
                 450 
                 430 
               
               
                 tearing strength 
               
               
                 Tear index 
                 mN · m 2 /g 
                 12.3 
                 15.2 
                 16.0 
                 10.8 
                 11.3 
                 10.8 
               
               
                 Tensile strength 
                 kN/m 
                 0.58 
                 0.62 
                 0.65 
                 0.62 
                 0.58 
                 0.63 
               
               
                 Water repellency 
                 mm(H 2 O) 
                 640 
                 670 
                 730 
                 650 
                 630 
                 580 
               
               
                 Pressure loss 
                 Pa 
                 277 
                 277 
                 274 
                 276 
                 275 
                 278 
               
               
                 0.3-0.4 μm 
                 % 
                 0.022 
                 0.020 
                 0.026 
                 0.022 
                 0.023 
                 0.020 
               
               
                 DOP penetration 
               
               
                 PF value 
                   
                 12.95 
                 13.10 
                 12.84 
                 13.00 
                 12.98 
                 13.05 
               
               
                 Flame retardancy 
                   
                 ∘ 
                 ∘ 
                 x 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Example7 
                 Example8 
                 Example9 
                 Example10 
                 Example11 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Glass short fibers 
                 95% by mass 
                 70% by mass 
                 78% by mass 
                 85% by mass 
                 85% by mass 
               
               
                   
                 Short fibers of 
                 Acrylic 
                 Polyester 
                 Polyester 
                 Polyester 
                 Polyester 
               
               
                   
                 chemical synthetic 
                 fibers0.1dtex 
                 fibers0.1dtex 
                 fibers0.1dtex 
                 fibers0.2dtex 
                 fibers0.2dtex 
               
               
                   
                 fibers 
                 5% by mass 
                 30% by mass 
                 22% by mass 
                 15% by mass 
                 15% by mass 
               
               
                   
                 Binder 
                 Acrylic resin 
                 Acrylic resin 
                 Acrylic resin 
                 Acrylic resin 
                 Acrylic resin 
               
               
                   
                   
                 binder 5% by 
                 binder 5% by 
                 binder 5% by 
                 binder 3% by 
                 binder 10% by 
               
               
                   
                   
                 mass against 
                 mass against 
                 mass against 
                 mass against 
                 mass against 
               
               
                   
                   
                 filter medium 
                 filter medium 
                 filter medium 
                 filter medium 
                 filter medium 
               
               
                   
                 Water repellant 
                 Used 
                 Used 
                 Used 
                 Used 
                 Used 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Basis weight 
                 g/m 2   
                 50 
                 40 
                 25 
                 40 
                 40 
               
               
                   
                 Internal 
                 mN 
                 390 
                 660 
                 400 
                 380 
                 440 
               
               
                   
                 tearing strength 
               
               
                   
                 Tear index 
                 mN · m 2 /g 
                 7.8 
                 16.5 
                 16.0 
                 9.5 
                 11.0 
               
               
                   
                 Tensile strength 
                 kN/m 
                 0.55 
                 0.70 
                 0.64 
                 0.52 
                 0.70 
               
               
                   
                 Water repellency 
                 mm(H 2 O) 
                 560 
                 760 
                 690 
                 570 
                 690 
               
               
                   
                 Pressure loss 
                 Pa 
                 272 
                 274 
                 273 
                 275 
                 274 
               
               
                   
                 0.3-0.4 μm 
                 % 
                 0.024 
                 0.029 
                 0.021 
                 0.019 
                 0.029 
               
               
                   
                 DOP penetration 
               
               
                   
                 PF value 
                   
                 13.06 
                 12.67 
                 13.22 
                 13.27 
                 12.67 
               
               
                   
                 Flame retardancy 
                   
                 ∘ 
                 x 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
            
               
                   
                 Comparative 
                 Comparative 
                 Comparative 
                 Comparative 
                 Comparative 
                 Comparative 
               
               
                   
                 Example1 
                 Example2 
                 Example3 
                 Example4 
                 Example5 
                 Example6 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Glass short fibers 
                 100% by mass 
                 100% by mass 
                 90% by mass 
                 90% by mass 
                 97% by mass 
                 65% by mass 
               
               
                 Short fibers of 
                 Not used 
                 Not used 
                 Polyester 
                 Polyester 
                 Acrylic 
                 Acrylic 
               
               
                 chemical synthetic 
                   
                   
                 fibers1.7dtex 
                 fibers0.6dtex 
                 fibers0.1dtex 
                 fibers0.1dtex 
               
               
                 fibers 
                   
                   
                 10% by mass 
                 10% by mass 
                 3% by mass 
                 35% by mass 
               
               
                 Binder 
                 Acrylic resin 
                 Acrylic resin 
                 Acrylic resin 
                 Acrylic resin 
                 Acrylic resin 
                 Acrylic resin 
               
               
                   
                 binder 5% by 
                 binder 5% by 
                 binder 5% by 
                 binder 5% by 
                 binder 5% by 
                 binder 5% by 
               
               
                   
                 mass against 
                 mass against 
                 mass against 
                 mass against 
                 mass against 
                 mass against 
               
               
                   
                 filter medium 
                 filter medium 
                 filter medium 
                 filter medium 
                 filter medium 
                 filter medium 
               
               
                 Water repellant 
                 Used 
                 Used 
                 Used 
                 Used 
                 Used 
                 Used 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Basis weight 
                 g/m 2   
                 80 
                 40 
                 40 
                 40 
                 50 
                 40 
               
               
                 Internal 
                 mN 
                 420 
                 190 
                 230 
                 260 
                 310 
                 650 
               
               
                 tearing strength 
               
               
                 Tear index 
                 mN · m 2 /g 
                 5.3 
                 4.8 
                 5.8 
                 6.5 
                 6.2 
                 16.3 
               
               
                 Tensile strength 
                 kN/m 
                 0.98 
                 0.53 
                 0.63 
                 0.68 
                 0.67 
                 0.70 
               
               
                 Water repellency 
                 mm(H 2 O) 
                 710 
                 430 
                 400 
                 470 
                 500 
                 660 
               
               
                 Pressure loss 
                 Pa 
                 276 
                 277 
                 280 
                 276 
                 276 
                 274 
               
               
                 0.3-0.4 μm 
                 % 
                 0.023 
                 0.021 
                 0.025 
                 0.021 
                 0.022 
                 0.043 
               
               
                 DOP penetration 
               
               
                 PF value 
                   
                 12.93 
                 13.02 
                 12.62 
                 13.07 
                 13.00 
                 12.05 
               
               
                 Flame retardancy 
                   
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Comparative 
                 Comparative 
                 Comparative 
                 Comparative 
               
               
                   
                 Example7 
                 Example8 
                 Example9 
                 Example10 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Glass short fibers 
                 90% by mass 
                 90% by mass 
                 85% by mass 
                 85% by mass 
               
               
                   
                 Short fibers of 
                 Acrylic 
                 Vinylon 
                 Polyester 
                 Polyester 
               
               
                   
                 chemical synthetic 
                 fibers0.1dtex 
                 fibers0.6dtex 
                 fibers0.2dtex 
                 fibers0.2dtex 
               
               
                   
                 fibers 
                 10% by mass 
                 10% by mass 
                 15% by mass 
                 15% by mass 
               
               
                   
                 Binder 
                 PVA 
                 PVA 
                 Acrylic resin 
                 Acrylic resin 
               
               
                   
                   
                 binder 4% by 
                 binder 4% by 
                 binder 2% by 
                 binder 11% by 
               
               
                   
                   
                 mass against 
                 mass against 
                 mass against 
                 mass against 
               
               
                   
                   
                 filter medium 
                 filter medium 
                 filter medium 
                 filter medium 
               
               
                   
                 Water repellant 
                 Used 
                 Used 
                 Used 
                 Used 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Basis weight 
                 g/m 2   
                 40 
                 40 
                 40 
                 40 
               
               
                   
                 Internal 
                 mN 
                 270 
                 190 
                 270 
                 430 
               
               
                   
                 tearing strength 
               
               
                   
                 Tear index 
                 mN · m 2 /g 
                 6.8 
                 4.8 
                 6.8 
                 10.8 
               
               
                   
                 Tensile strength 
                 kN/m 
                 0.68 
                 0.72 
                 0.40 
                 0.73 
               
               
                   
                 Water repellency 
                 mm(H 2 O) 
                 360 
                 310 
                 410 
                 700 
               
               
                   
                 Pressure loss 
                 Pa 
                 276 
                 276 
                 276 
                 275 
               
               
                   
                 0.3-0.4 μm 
                 % 
                 0.020 
                 0.020 
                 0.018 
                 0.044 
               
               
                   
                 DOP penetration 
               
               
                   
                 PF value 
                   
                 13.15 
                 13.15 
                 13.31 
                 11.97 
               
               
                   
                 Flame retardancy 
                   
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                   
                   
               
            
           
         
       
     
     Comparative Example 1 corresponds to a conventional example, and Comparative Example 2 is an example obtained by simply decreasing by half the basis weight of the Comparative Example; in Comparative Examples 1 and 2, as a result of decrease by half in the basis weight from 80 g/m 2  to 40 g/m 2 , the internal tearing strength, tensile strength and water repellency were decreased significantly. On the other hand, in Examples 1 and 5, as a result of incorporation of the polyester short fibers or acrylic short fibers of 0.1 dtex each by 10% by mass, internal tearing strength and water repellency of the degrees that were equal to or more than those under a basis weight of 80 g/m 2  in Comparative Example 1 could be achieved even under a basis weight of 40 g/m 2 . However, the effect of improving tensile strength is small, but no problem is caused in the processing property of the filter medium at this degree. In Examples 4 and 6, similar effects were observed by carrying out with varying the fiber diameter and incorporation percentage. On the other hand, in Comparative Examples 3 and 4 in which short fibers of chemical synthetic fibers each having a fiber diameter of more than 5 μm were incorporated, the internal tearing strength and water repellency were improved little. In Examples 2 and 3, as a result of increase in the incorporation of the 0.1 dtex polyester short fibers, the internal tearing strength and water repellency were not of problematic degrees even the basis weight was decreased to 25 g/m 2 . However, the flame retardancy was deteriorated when the incorporation percentage of the short fibers of chemical synthetic fibers exceeded 22% by mass as in Examples 3 and 8. As is apparent from Example 9, it was found that it was necessary to adjust the incorporation percentage of the short fibers of chemical synthetic fibers to 22% by mass or less in an application in which flame retardancy is required. Furthermore, as is apparent from Example 8, when the incorporation percentage was 30% by mass of the short fibers of chemical synthetic fibers, the DOP penetration and PF value were at acceptable degrees, whereas when the incorporation percentage exceeded 30% by mass of the short fibers of chemical synthetic fibers as in Comparative Example 6, increase in the DOP penetration and the decrease in the PF value were observed. Conversely, the effect was sufficient in Example 7 in which the incorporation percentage of the short fibers of chemical synthetic fibers was decreased to 5% by mass, whereas the internal tearing strength and water repellency were decreased in Comparative Example 5 in which the incorporation percentage was further decreased to 3% by mass. The internal tearing strength and water repellency were significantly decreased in Comparative Example 7 in which the binder was changed to hydrophilic polyvinyl alcohol (PVA) binder fibers, and the internal tearing strength and water repellency were further decreased in Comparative Example 8 by changing the short fibers of chemical synthetic fibers to hydrophilic 0.6 dtex vinylon fibers besides the binder. It is considered that the hydrophilic short fibers of chemical synthetic fibers and binder have little effect on improvement of the internal tearing strength but conversely decrease the water repellency. Comparative Example 9 in which the acrylic resin binder was 2% by mass against the filter medium was inferior to Example 10 in which the acrylic resin binder was 3% by mass against the filter medium in internal tearing strength, tensile strength and water repellency. Increase in the DOP penetration and the decrease in the PF value were observed in Comparative Example 10 in which the acrylic resin binder was 11% by mass against the filter medium, as compared to Example 11 in which the acrylic resin binder was 10% by mass against the filter medium.