Conventionally, the air filter particle collection technology is used to efficiently collect particles on a submicron or micron unit. Depending on the targeted particle size and particle collection efficiency, air filters can be classified roughly into the following categories: Coarse particle filters; ASHRAE filters; sub high efficiency filters; and high efficiency filters (HEPA filters, ULPA filters). Among these filters, sub high efficiency filters and high efficiency filters have a European standard, namely EN1822. There are seven classes in EN1822 from U16 through H10 according to the collection efficiency level of the most penetrating particle size (MPPS). There are also other high efficiency filter standards such as IEST-RP-CC001 (USA), JIS Z 4812 (Japan), etc., and the materials used for sub high efficiency filters and high efficiency filters are those that satisfy these air filter standards. As to filter medium materials, nonwoven glass fibers are often used for the production of air filter media. The glass fibers, whose average fiber diameter is in the range of 100 nm (submicrons)˜several tens of microns and whose most penetration particle size (MPPS) mentioned above is between 0.1 μm and 0.2 μm, are the main constituent of the filter medium.
Key properties required for the air filter medium, besides collection efficiency, includes pressure drop, which is an indicator of a filter medium's air resistance. In order to increase the collection efficiency of the filter medium, the proportion of small diameter-glass fibers must be increased. However, this causes an increase in pressure drop of the filter medium at the same time. Since a large pressure drop increases the load on suction fan operation, the running costs of power are increased, which is a problem. In view of energy conservation, a reduction in pressure drop in filter media is required. Particularly in recent years, the trend in increasing volumetric airflow for air filters invited a growing demand for mitigating pressure drop and increasing collection efficiency to reduce running costs of fans used in cleanrooms, cleanbenches, etc.
As a means to overcome the problem, a method in which the surface tension of the binder added to a filter medium is reduced by containing silicone resin so as to cancel or reduce the webbed membrane of the binder has been proposed (Patent Documents 1 and 2). However, in recent years, particularly in the field of semiconductors, it was found that the diffusion of a small amount of low molecular siloxane contained in silicone resin into air in a cleanroom caused adverse effects on the yield in the manufacturing of large scale integrated circuits (LSI); this makes the use of silicone resin difficult in itself.
Previously, the present inventors proposed an air filter medium in which a binder and a fluorochemical surfactant, whose minimum surface tension is 20 dyne/cm or less when the surfactant was added to pure water at 25° C., are attached on glass fibers that constituted the filter medium (Patent Document 3). This invention had some effect on solving the above problem. Nevertheless, the attachment of a fluorochemical surfactant enhanced wettability of the binder resin surface, sometimes diminishing the water repellency characteristic of the filter medium. In order to improve this drawback, the present inventors proposed a filter medium on which a polymer dispersion having an average particle size of 100 nm or less and a fluorochemical surfactant having the minimum surface tension of 20 mN/m or less when the surfactant is added to pure water at 25° C. were attached (Patent Document 4). These proposals are limited to the binders to be attached on the filter media.
Moreover, for glass fiber base sheets proposed were a manufacturing method in which glass fibers were deflaked in neutral water, then neutral paper was made by using the water to which an N-alkyl betaine type amphoteric surfactant was added (Patent Document 5), and a manufacturing method in which glass fibers were deflaked in neutral water, then neutral paper was made by using the water to which a non-ionic surfactant of polyethylene glycol fatty acid esters was added (Patent Document 6). However, these methods resulted in low filtering medium strength due to a large amount of residual surfactant contained in the filter medium. Also proposed was another method for making glass fiber filter papers for ultra low penetration air filters comprising 5˜15% by weight of glass fibers having a fiber diameter in the range of 0.05˜0.2 μm and 95˜85% by weight of glass fibers having another diameter (Patent Document 7). Yet, the glass fibers having a diameter in the range of 0.05˜0.2 μm were too costly and could not be adopted for commercially acceptable filter media, which was another problem.
Nevertheless, these methods were proposed in view of binders, glass fiber sheet making, and glass fiber blending, while little investigation has been made into properties of glass fibers themselves, the main component of the filter medium.    Patent Document 1: JP 02041499A.    Patent Document 2: JP 02175997 A.    Patent Document 3: JP 10156116 A.    Patent Document 4: JP 2004160361 A.    Patent Document 5: JP 62021899 A.    Patent Document 6: JP 61266700 A.    Patent Document 7: JP 62004418 A.