WO2006112563A2

WO2006112563A2 - Filter for air cleaning and its manufacturing method

Info

Publication number: WO2006112563A2
Application number: PCT/KR2005/001135
Authority: WO
Inventors: Suk-Won Chun; Sung-Ki Ahn
Original assignee: Nanophil Co., Ltd.
Priority date: 2005-04-20
Filing date: 2005-04-20
Publication date: 2006-10-26
Also published as: WO2006112563A3

Abstract

Disclosed are a filter medium capable of being used for air cleaning, and its manufacturing method. The filter medium for air cleaning according to the present invention provides a filter medium including a porous membrane composed of a nano fiber web by an electrospinning process; and an air-permeable supporting material laminated into at least one side of the porous membrane, wherein the porous membrane has a collection efficiency of at least 99.9 % for particles ranging from 0.1 D to 0.3 D in size, and an airflow resistance of 30 30 H2Oor less at 20 SCFH.

Description

FILTER FOR AIR CLEANING AND ITS MANUFACTURING

METHOD

Technical Field

[1] The present invention relates to a filter medium having nano fibers capable of being used for air cleaning, and its manufacturing method. Background Art

[2] Recently, there have been rapid developments of air cleaning industries as the concerns have been increasingly focused on the environmental pollutions along with development of the up-to-date industries. In the air cleaning industries, it is important to effectively filter dust and suspended micropollutants, and removal of such suspended micropollutants depends mainly on a filter medium constituting a filter.

[3] Especially, it is necessarily required to remove such suspended micropollutants due to the ultra-integration of memory semiconductors and the growth of Bio ventures and the nano-grade ultra precision industries. Also, there has been the need for a high- efficiency filter medium due to the increased desires of its consumers that hope to prevent pathogens such as SARS and maintain clean indoor environments. And, there has been also the need for a high-performance filter medium for respirators due to the increased risk of biochemical wars.

[4] The filter medium, which has a high-efficiency collection performance so as to remove suspended micropollutants, is generally used for HEPA (High efficiency particle Air) and ULPA (Ultra low penetration air) filters. Such a high-efficiency filter may collect low-density submicron particles at a significantly high collection efficiency (Military specifications and standards (U.S.): HEPA filter: 99.97 % for 0.3 D particle size; ULPA filter: at least 99.999 % for 0.1 D particle size).

[5] Also in the case of such a high-efficiency filter, airflow resistance acts as an important factor. If the airflow resistance is high, the expected life span of a filter may become shorter due to continuous pressure applied to the filter medium, the air conditioning cost may be increased due to the pressure loss, and it may be difficult to breathe when the respirator is put on. Accordingly, a filter medium having a high collection efficiency as well as a low airflow resistance remains to be developed.

[6] The high-efficiency filter medium has been manufactured by various methods known in the art, for example a manufacturing method using a glass microfiber, a manufacturing method using a high-molecular microfiber by a melt-blown spinning process, and a manufacturing method using a porous polytetrafluorethylene.

[7] The filter medium of the glass microfiber has been widely used for a semiconductor clean room. In the filter medium of the glass microfiber, its fiber diameter ranges from about 0.2 D to 2 D, and about at least 10 % by weight of boron oxide (B) ) is added to the glass materials so that its relatively lower viscosity/temperature relation can be promoted to micronize a glass fiber. However, the boron oxide reacts with a hydrogen fluoride (HF) vapor, used for washing the semiconductor wafer, to generate boron gas, which causes a serious defect in the semiconductor wafer. Also in the case of the glass microfiber filter medium, a binder was used for adhesion between the glass mi- crofibers, but the binder has a problem that organic pollutants were generated at an early stage, and therefore the airflow resistance of a filter medium was increased due to its chemically unstable properties. Also in the case of the filter medium constituting the glass microfiber, the filter medium was bent to reduce the airflow resistance by increasing a surface area of the filter. At this time, the filter might be damaged at an early stage since cracks appear in a bending region.

[8] In order to solve the problem of the filter medium in the glass microfiber by the boron gas, there have been proposed a low-boron glass microfiber filter and a boron- free glass microfiber filter medium.

[9] That is to say, U.S. Patent Nos. 5,789,329, 6,277,777 and 6,358,871 disclose a method for manufacturing a filter medium, wherein boron is used in a lower amount, or substituted with other materials in manufacturing a glass microfiber. However, the low-boron glass microfiber filter and the boron-free glass microfiber filter medium were difficult to be manufactured and very expensive, and therefore its use was rather limited up to the present.

[10] In order to overcome such a problem of the filter medium constituting the glass microfiber, there has been proposed a filter medium composed of polypropylene mi- crofibers using a melt-blown spinning process. The melt-blown spinning process is a method for manufacturing a nonwoven fabric by discharging a molten high molecule solution at a high pressure through a micronozzle and applying high-temperature and high-pressured air around the nozzle to extend a high-molecular fiber. That is, U.S. Patent Nos. 6,123,752, 4,824,451, 5,273,565 and Korean Utility Model Registration No. 0289693 disclose a high-efficiency filter medium using such a melt-blown spinning process. However, the method for manufacturing a nonwoven fabric using the melt-blown spinning process is difficult to be commercialized since it has a problem of increasing the cost of the process for manufacturing a fiber having a mean fiber diameter of 2 D or less. Therefore, if a filter having a diameter of at least 2 D is use, it contributes to enhance the collection efficiency by electrifying constant voltage with the filter medium because its collection efficiency is not high. However, it has a problem that the electrified charges disappear with the passage of time, and therefore its collection efficiency was deteriorated. Also, the melt-blown spinning process causes a problem on an aggregation rate between the microfibers upon their spinning, and therefore an effect of micronization of the fiber may be reduced, and local problems may appear in the aspect of collection efficiency due to ununiform distribution of the microfiber, and therefore its ununiform thickness distribution in manufacturing its membrane. Also, the microfiber membrane prepared by the melt-blown spinning process has problems that its strength is deteriorated due to a short length of the single fiber, and detached materials are increased with the passage of time when it is used as the filter medium.

[11] Also, a filter medium was bent to reduce the airflow resistance by increasing its surface area in the constant space upon manufacturing a filter. For the filter medium constituting the glass microfiber component and the filter medium prepared by the melt-blown spinning process, it is impossible to manufacture a filter having a value below the certain airflow resistance because the filter medium has a thickness of at least 380 D and also it may be bent in limited times within a certain space.

[12] Accordingly, in order to solve the problems, there has been proposed a fiber medium using porous polytetrafluorethylene. That is, U.S. Patent Nos. 5,507,847 and 6,302,934 disclose a method for manufacturing a filter medium by manufacturing a porous thin-film polytetrafluorethylene membrane, followed by laminating it with a polyolefin-based nonwoven fabric. The filter medium prepared by the process is considered to be the most ideal filter medium since it has a high collection efficiency, a low airflow resistance and a small thickness. However, the filter medium prepared by the process has a problem that it is difficult to use multiple species of the high molecules, and therefore the filter medium may not be used for various application, and also it is very expensive since it is made of only one kind of the high molecule polytetrafluorethylene.

[13] Meanwhile, there has been proposed a technique for improving airflow resistance.

JP. Patent No. S57-147412 discloses a method for manufacturing a filter medium with a low air resistance value whose fiber diameters are dually separated by mixing 0.1 D to 2 D of microfibers with fibers having 2 to 10 times thickness of the microfibers using a melt-blown spinning process, wherein the fiber with 2 to 10 times thickness of the microfibers is included at an amount of at most 30 %, more favorably 2 to 20 %, to prevent a fiber having a relatively thicker diameter from being aggregated between the microfibers. However, the patent also relates to a manufacturing method using the melt-blown spinning process, and the problems of the increased manufacturing cost due to micronization of the fiber and the ununiform distribution of the microfibers are not fundamentally solved in the manufacturing method. Disclosure of Invention Technical Problem

[14] Accordingly, the present invention is designed to solve the problems of the prior art, and therefore it is an object of the present invention to provide a filter medium having a high-efficiency collection performance as well as a low airflow resistance, and a method for manufacturing the filter medium.

[15] Also, it is another object of the present invention to provide a method for manufacturing a porous membrane constituting a filter medium using an electrospinning process, and a filter medium manufactured by the method.

[16] Also, it is still another object of the present invention to reduce a packing density to decrease an airflow resistance by dually separating a nano fiber diameter distribution of the porous membrane constituting the filter medium.

[17] These and other objects, and advantages of the present invention will be more fully described in the following detailed description, and be known by preferred embodiments of the present invention. Also, these and other objects, and advantages of the present invention may be realized by the means and combinations as described in the appended claims. Technical Solution

[18] Generally, a filter medium includes a porous membrane in which a fiber is cut ul- trafinely and stacked. For the fibrous filter medium, its collection efficiency and also its airflow resistance are increased as diameter of a component fiber and its membrane porosity are reduced and its thickness is increased. Therefore, the filter medium may maintain the high collection efficiency, as well as the low airflow resistance when optimal conditions for the fibrous porous membrane are set by adjusting diameter of the component fiber, its membrane porosity and its thickness.

[19] Meanwhile, the diameter of the component fiber, its membrane porosity and its thickness show the same effect in the collection efficiency of the fibrous filter medium since thethree factors are in proportion or in inverse proportion to each other. However, the most important factor is the membrane porosity, and the next is the fiber diameter and the thickness in order since the 3 factors do not show the same effect in the airflow resistance. That is, the airflow resistance is mainly affected by the membrane porosity. For the general fibrous filter medium, the porosity and the thickness are reduced due to aggregation between the stacked fibers. Reduction of such porosity functions as the more important factor than reduction of the thickness, which causes a relatively higher airflow resistance. On the contrary, the airflow resistance of the filter may be further reduced if its thickness medium is increased, but the porosity is not so decreased. That is, in order to solve the problems, a filter medium, which may reduce the airflow resistance while maintaining collection efficiency similar to that of the filter medium prepared previously, may be manufactured by adding component fibers having more thicker diameter than that of the microfiber to the microfibrous porous membrane. It is supposed that this results from the fact that the porosity is increased although its thickness is increased, and therefore its airflow resistance is reduced since a relatively thicker fiber functions to prevent aggregation between mi- crofibers and secure a space between the microfibers.

[20] In order to establish the optimal conditions of a diameter of the component fiber, its membrane porosity and its thickness, a fibrous porous membrane was manufactured using the electrospinning process in the present invention.

[21] A basic mechanism of the electrospinning process was known in various references

([J.M. Deitzel, J.D. Kleinmeyer, J.K. Hirvonen, N.C. Beck Tan, Polymer 42, 8163-8170(2001)], [J.M. Deitzel, J.D. Kleinmeyer, D. Harris, N.C.Beck Tan, Polymer 42, 261-272(2001)], [Y.M. Shin, M.M. Hohman, M.P. Brenner, G.C. Rutledge, Polymer 42, 9955-9967(2001)]).

[22] When the microfiber is manufactured using such an electrospinning process, fiber diameter distribution is varied according to a kind of high molecules, their weight ratio, a compositional ratio of solvents, discharge capacity of such a high molecule solution, and voltage applied to a nozzle. Accordingly, the parameters should be suitably adjusted to manufacture a microfiber having the desired physical properties.

[23] A filter medium according to the present invention includes air-permeable supporting materials and a porous membrane. At this time, the filter medium may be formed into a 2-layer structure in which the porous membrane is stacked on the air- permeable supporting materials; a 3 -layer structure in which porous membrane is interposed between the air-permeable supporting materials; or a multiple-layer structure in which the air-permeable supporting materials and the porous membrane are stacked alternately.

[24] The porous membrane is manufactured by an electrospinning apparatus as shown in

Fig. 1, and it is composed of nano fiber webs having a collection efficiency of at least 99.9 % for the particle ranging from 0.1 D to 0.3 D in size, and an airflow resistance of 30

[25] As described above, packing density of a porous membrane, diameter and thickness of a fiber should be suitably adjusted so as to manufacture a fiber having an excellent collection efficiency as well as a low airflow resistance. For this purpose, the inventors manufactured a web-shaped porous membrane having a uniform distribution by using a repulsive force between the fibers to prevent aggregation between the fibers by means of the electrospinning process.

[26] Especially, diameters of the nano fiber were separately distributed so as to reduce packing density of the porous membrane. [27] Membrane thickness of the porous membrane is preferably 5 D to 100 D, and more preferably 25 D to 100 D. The thickness of the porous membrane is suitably adjusted, considering the total thickness of the filter medium to be finally manufactured.

[28] Also, porosity (or, porous ratio) of the porous membrane is preferably 80 % to 95

%, and more preferably 85 % to 90 %. That is, packing density of the porous membrane is preferably 5 % to 20 %, and more preferably 10 % to 15 %.

[29] Accordingly, the porous membrane has a mean flow pore diameter of 0.5 to 2 D, and more preferably 0.9 to 1.5 D, and a maximum pore diameter of 1.5 to 10 D, and more preferably 2 to 6 D.

[30] At this time, the porous membrane preferably has the mean flow pore diameter or the maximum pore diameter, as described above, because its collection efficiency may be increased but its airflow resistance may be also increased if its pore diameter is extraordinarily small, while its collection efficiency may be reduced if its pore diameter is extraordinarily high.

[31] Also, the diameter of the nano fiber constituting the porous membrane may have a single-range distribution of 50 to 700 D, or dually separated distributions of 50 to 700 D and 700 to l,500 D.

[32] As described above, if the diameter of the nano fiber constituting the porous membrane has the dually separated distribution, the nano fiber having the diameter of 700 to 1,500 D desirably has 50 % by weight or less, based on the total weight of the web.

[33] In order to maintain the diameter distribution at a low level by reducing packing density of the porous membrane, it is suitable to dually separate the diameter distribution of the nano fiber, as described above. Dual separation of such a diameter distribution may be accomplished by spinning different high molecules or by controlling their discharge capacity or voltage in the electrospinning apparatus of Fig. 1.

[34] Mechanical strength of the porous membrane should be designed to have a tensile strength of at least 50 Df/D in a machine direction, and a tensile strength of at least 20 Df/D in a lateral direction. Especially, the porous membrane is preferably formed by the electrospinning process, and then passed through a pressure roll so as to give strength and conformational stability to the membrane. At this time, the pressure preferably ranges from 0.1 Df/D to 10 Df/D upon pressuring the membrane.

[35] The porous membrane is obtained by electrospinning a high molecule solution selected from the group consisting of polyvinylidene fluoride, polyvinylidene fluoride- hexafluorpropylene copolymer, polyacrylonitrile, polyvinylidene chloride- aery lonitrile copolymer, polyethyleneoxide, polyurethane, polymethylacrylate, polymethylmethacrylate, polyacrylamide, polyvinylchloride, polyvinylacetate, polyvinylpyrolidone, polytetraethylene glycol diacrylate, polyethyleneglycol dimethacrylate, cellulose, cellulose acetate, rayon, polyamide, polyethylene terephthalate, polytrimethyleneterephthalate, polybutyleneterephthalate, polyimide, polyphenylenesulfide, or mixtures thereof.

[36] The air-permeable supporting material is adhered to at least one side of the porous membrane and used to strengthen and protect the porous membrane and to filter coarse particles. Therefore, a handling property of the filter medium as well as its pro- cessability are improved if elements such as a filter medium unit are processed.

[37] Also, the air-permeable supporting material should have high impact strength, tensile and bursting strengths, and a low thermal shrinkage.

[38] Also, the air-permeable supporting material advisably has a lower pressure loss than that of the porous membrane. Generally, the air-permeable supporting materials are selected from the group consisting of a nonwoven fabric, a woven fabric, a mesh, a porous membrane, a knitting, etc., and a nonwoven fabric is especially preferred.

[39] For this purpose, the air-permeable supporting material is preferably a nonwoven fabric having a thickness of 80 to 120 D, and a porosity (or, porous ratio) of 10 to 40 %.

[40] The air-permeable supporting material preferably has a fiber diameter of 5 to 30 D, and a tensile strength of 100 to 200 Df/D in a machine direction and 50 to 100 Df/D in a lateral direction.

[41] Especially, polyethyleneterephthalate, polyolefin-based and cellulose-based nonwoven fabrics, or mixed nonwoven fabrics thereof is preferably used as the air- permeable supporting material according to the present invention.

[42] There have been various known methods for manufacturing a nonwoven fabric having the mechanical strength, the low airflow resistance and the thin thickness, as described above. Especially, a resin boding method, a needle punching method, a thermal bonding method, a spun bonding method, a spun-laced method, etc. have been known in the art, and the polyethyleneterephthalate nonwoven fabric prepared by the spun bonding method is desirably used for the air-permeable supporting materials in the present invention. Brief Description of the Drawings

[43] The accompanying drawing taken in the present invention will be illustrative of preferred embodiments of the present invention, and technical aspects of the present invention will be more fully described in combination with the following detailed description, so it should be understood that the description proposed herein is not intended to be limited referring to the accompanying drawings. In the drawings:

[44] Fig. 1 is a perspective view showing an electrospinning apparatus for manufacturing a porous membrane according to the present invention. Best Mode for Carrying Out the Invention [45] Hereinafter, a method for manufacturing a filter medium of the present invention will be described in detail referring to the accompanying drawings.

[46] An electrospinning apparatus 100 as shown in Fig. 1 will be described prior to describing a specific method for manufacturing a filter medium.

[47] Referring to Fig. 1, an electrospinning apparatus 100 according to the present invention includes a supplying unit 110 for supplying a melt high-molecular material for fiber materials; a spinning unit 120 including a plurality of spinning nozzles 122 for discharging, in the form of charged filaments, a high molecule solution supplied from the supplying unit 110; a collector 130 spaced apart by a predetermined gap from the spinning nozzles 122 to accumulate, in a predetermined thickness, the filaments spun from the spinning unit 120; a control unit 140 mounted at least to the both sides of the spinning unit 120; an induction unit 150 mounted between the control unit 140 and the collector 130 so as to surround a filament stream S; and an air conditioning unit 160 for injecting air into a space between the spinning unit 120 and the collector 130 and evaporating solvents in the space to release the evaporated solvents outside.

[48] The supplying unit 110 includes a reservoir 112 for storing a solution in which a high-molecular material used for a fiber material is dissolved; a pump 114 for pressuring the solution stored in the reservoir 112 to quantitatively supply the solution into the spinning unit 120; and a distributor 116 for distributing the solution into each nozzle.

[49] The spinning unit 120 functions to spin the fiber material solution, supplied from the supplying unit 110 and in a charged state, in a direction of the collector 130 to form a microfilament.

[50] The positive (+) voltage is excited with an output voltage of a high-voltage unit

170. The high- voltage unit 170 outputs a DC voltage ranging from 10 kV to 120 kV.

[51] The spinning unit 120 includes at least one spinning nozzle pack 126 in which a plurality of the spinning nozzles 122 are arranged. The number of the spinning nozzles 122 constituting the spinning nozzle pack 126, or the number of the spinning nozzle pack 126 constituting the spinning unit 120 is determined in total consideration of size or thickness of a web to be manufactured, a production rate, etc.

[52] The collector 130 may be earthed so that it can have a potential difference against a voltage applied to the spinning unit 120 (see Fig. 1), or be applied with a negative (-) voltage.

[53] The collector 130 may be configured to stack the charged filaments discharged from the spinning unit 120, for example to move continuously in a conveyor-belt manner by a locomotion means such as a roller 132.

[54] The control unit 140 functions to prevent filament streams S spun from each spinning nozzle 122 from being escaped from a path, for example to prevent the filament streams S from being spread by repulsing each other, and the control unit 140 is mounted at least to both sides of the spinning nozzle pack 126 in a longitudinal direction.

[55] A voltage having the same polarity as the control unit 140 is applied to the induction unit 150. The induction unit 150 is installed around a charged filament strea m S being extended to guide a traveling direction of the stream. The induction unit 150 is provided in the form of a conductor plate or a conductor rod. The induction unit 150 is induced so that filaments can be stacked in a certain region of an upper surface of the collector 130 by electrifying with the same polarity as the charged filaments.

[56] The air conditioning unit 160 functions to evaporate solvents, dissolved in the charged filaments, in the space between the spinning unit 120 and the collector 130 to release the evaporated gases outside, and it has, for example, a solvent- intake/exhaust means such as an intake fan and an exhaust fan, and a plurality of air-inflow slots 162.

[57] Various known air blowers may be used for the solvent-intake/exhaust means. For example, the intake fan, installed in an air intake passage, takes dry air from the outside of the apparatus to inject the dry air into the space between the spinning unit 120 and the collector 130 through the air-inflow slots 162 provided in an upper portion of the spinning nozzle pack 126. The introduced air evaporates solvents, dissolved in the charged filaments P spun from the spinning nozzle 122, and then is released outside through an air outlet passage in which the exhaust fan is mounted.

[58] Operation of the electrospinning apparatus of Fig. 1 having the aforementioned configuration is described in brief, as follows.

[59] If a material solution stored in the supplying unit 110 is quantitatively supplied into the spinning unit 120 through the pump 114 and a distributor 116, the solution is charged by a current-carrying unit in each spinning nozzle pack 126 of the spinning unit 120. Here, the current-carrying unit, with it being provided inside an body of the spinning nozzle pack 126, is installed so as to prevent direct electrical interaction with a collector 130.

[60] Then, the charged solution is discharged in the form of microfilament into the collector 130 while it is passed through a capillary tube of the spinning nozzle 122. Here, the filaments are extended and spun to have a nano-grade diameter by a strong electric field formed between the collector 130 and the charged filaments.

[61] In such a spinning process, a stream, which escapes from a traveling path to spread outside due to a repulsive force between the filaments, returns to an original position by the control unit 140, and therefore allowed to maintain a right traveling path.

[62] Meanwhile, the stream escaping from the path is induced into a stacking region in the collector 130 by means of the induction unit 150 since the induction unit 150 is installed to an upper side of the collector 130 to surround the discharged stream. [63] The filaments induced thus are continuously stacked on the collector 130 having a conveyor-belt or rotating-drum type, or stacked on an upper surface of an air- permeable supporting material 182, and therefore the filaments are manufactured in the form of a web-shaped porous membrane composed of nano fibers.

[64] By using the electrospinning apparatus having the aforementioned configuration, the porous membrane according to the present invention composed of the nano fiber web is manufactured by spinning the charged filaments on the collector 17 of Fig. 1. Otherwise, a nonwoven fabric may be installed on the collector 17 as an air-permeable supporting material according to the present invention, and then the charged filaments may be spun on the nonwoven fabric to directly form a porous membrane on the nonwoven fabric.

[65] As described above, the web-shaped porous membrane manufactured by the electrospinning process forms long fibers and has a relatively narrower fiber diameter distribution. Also, because the electrospun nano fiber itself is charged and spun, it may not be aggregated due to a repulsive force between fibers upon its spinning, and the web-shaped porous membrane may be manufactured to have a uniform thickness distribution.

[66] Strength-supporting nonwoven fabrics are stacked on one or both side(s) of the porous membrane prepared above, and then a filter medium is manufactured through the lamination process where a predetermined pressure and a predetermined temperature are applied.

[67] Otherwise, strength-supporting nonwoven fabrics are stacked on a porous membrane of the nonwoven fabric in which the porous membrane is manufactured by a direct spinning process, and then a filter medium is formed through the lamination process where a predetermined pressure and a predetermined temperature are applied. The lamination is preferably carried out under a linear pressure of 0.1 to 30 Df/D at 3 to 70 ⁰C lower temperature than a melting point of the component high molecule.

[68] If the temperature is high under the pressure, it is undesirable since a membrane is melt, and its packing density is increased if the pressure is too high. Mode for the Invention

[69] Hereinafter, the present invention has been described in detail with reference to the following preferred embodiments of the invention. However, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention. The preferred embodiments of the present invention will be provided with those skilled in the art from for the purpose of more detailed description of the present invention.

[70] <Electrospinning Apparatus> [71] An electrospinning apparatus of Fig. 1 used in this Embodiment has detailed specifications, as follows.

[72] A spinning solution was conveyed from a tank to a spinning nozzle pack using a precision conveyer provided with a metering pump. A capillary-type nozzle whose tip has an inner diameter of 0.2 D, an outer diameter of 0.4 D and a capillary length of 10 D was installed in the spinning nozzle pack, and a distance between the nozzle tips was 26 D. A distance between the spinning nozzle packs was set to 20 D. The spinning nozzle pack reciprocally moves right and left at a rate of 6 m/min.

[73] Each spinning nozzle pack was connected to a high- voltage generator unit [DEL

Global Technologies, Model Name: RLPS50-300P, Output Voltage: 50 kV, Output Current: 3 mA, (+) polarity] so that the spinning solution could be charged. At this time, an applied voltage was a (+) D. C. voltage of 20 to 40 kV, and the collector was earthed.

[74] <Embodiment 1>

[75] A web-shaped porous membrane was manufactured with a PVDF high molecule using the electrospinning apparatus as described above. That is, a high molecule solution, in which 15 % by weight of 100 % PVDF homopolymer [Elf Atochem North America, Inc., Product Name: Kynar 761] was dissolved in a mixed solvent including acetone and dimethylacetamide at a weight ratio of 5:5, was prepared, and then electrospun. The applied voltage was 28 kV, and its discharge capacity was 20 D/min per nozzle. Also, the membrane thickness was adjusted by increasing the time to be spun.

[76] Changes of collection efficiency and airflow resistance according to a membrane thickness are listed in Table 1. At this time, each sample has a porosity of 80 %. As shown in Table 1, the membrane having a high-efficiency collection performance was manufactured when its thickness was at least 25 D in the web having a fiber diameter distribution of 100 to 400 D. Also, it was revealed that its airflow resistance was increased as its membrane thickness was increased, indicating that Sample B had the optimal composition in the Table 1.

[77] Table 1

[78] A change of collection efficiency by the fiber diameter difference is listed in Table 2. For Sample B of Table 2, the discharge capacity was increased to 40 D/min to manufacture a web having a relatively higher fiber diameter. As seen in Table 2, it was revealed that a specific surface area of the membrane itself was reduced as the fiber diameter was increased, and therefore the collection efficiency of the membrane was reduced as a pore size of the membrane was increased.

[79] Table 2

Sample Thickness ( Diameter of Collection Airflow Resistance (mmH O)

□) Fibei (□) Efficiency (%) 20 SCFH 40 SCFH 60 SCFH

A 30 100 - - 400 99.9992 22 43 50<

B 30 300 - - 1,000 99.82 20 40 50<

[80] <Embodiment 2> [81] A web-shaped porous membrane was manufactured with a PVDF high molecule using the electrospinning apparatus as described above. That is, a high molecule solution, in which 15 % by weight of 100 % PVDF homopolymer [Elf Atochem North America, Inc., Product Name: Kynar 761] was dissolved in a mixed solvent including acetone and dimethylacetamide at a weight ratio of 5:5, and a high molecule solution, in which 13 % by weight of PVDF-HFP [(88 mol% : 12 mol%) [poly(vinylidenefluoride-co-hexafluoro propylene)]] copolymer [Elf Atochem North America, Inc., Product Name: Kynar Flex 2801: Number- Average Molecular Weight (Mn) = 120,000, Weight- Average Molecular Weight (Mw) = 380,000, Specific Gravity: 1.77 g/cc, Melting Point: 143 ⁰C] was dissolved in an acetone solvent, were used. The high molecule solutions were supplied into separate spinning nozzle packs and spun using the same electrospinning apparatus as in Embodiment 1. The high molecule solution including 100 % PVDF alone has a discharge capacity of 20 D/min per spinning nozzle, and the high molecule solution including 88 % PVDF copolymer has a discharge capacity of 20 D/min per spinning nozzle.

[82] In Table 3, Sample A represents Sample A of Table 2, and Sample B is a membrane integrated into the collector using a combined spinning process, whose the component fibers have a diameter of 100 to 400 D and a diameter of 700 to 1,000 D, with theirs thin diameter and thick diameter mingled each other. 700 to 1,000 D of the fiber, which is a relatively larger fiber, had a weight ratio of 46 % (by weight), based on the total weight of the membrane. The manufactured membrane had a thickness of 30 D. For Sample C, its thickness was 50 D under the same condition as in Sample B. For Sample D, the weight ratio of the component fiber with a relatively larger diameter was increased to 60 % by weight by increasing the discharge capacity of the PVDF-HFP copolymer to 40 D/min under the same condition as in Sample C. As seen in Table 3, the optimal membrane was manufactured in the case of Sample C, and the web having the highest collection efficiency and the low airflow resistance might be manufactured if the ratio of the fiber with the relatively larger diameter was set to less than 50 % by weight.

[83] Table 3

[84] <Embodiment 3> [85] A web-shaped porous membrane was manufactured with a PAN high molecule using the electrospinning apparatus of Fig. 1. That is, a high molecule solution, prepared by dissolving 12 % by weight of 100 % PAN homopolymer [Polyscience, Inc., Product Name: Poly(acrylonitrile)] in a 100 % solvent of dimethylacetamide, was electrospun. At this time, a discharge capacity per spinning nozzle was 50 D/min. A distance of the tip and the collector was 20 D.

[86] The component fiber of the membrane integrated in the collector had a diameter of 100 to 700 D, and the manufactured membrane had a porosity of 80 % and a thickness of 30 D. Its collection efficiency was 99.993 % for 0.3 D particle size, and its airflow resistance value was 25 DH 2O at 20 SCFH.

[87] <Embodiment 4> [88] A web-shaped porous membrane was manufactured with a Nylon 6 high molecule using the aforementioned electrospinning apparatus. That is, a high molecule solution, prepared by dissolving 12 % by weight of 100 % Nylon 6 homopolymer in a solvent containing tetrafluoroacetic acid (TFA) and dichloromethane (DCM) at a weight ratio of 5:5, was electrospun. At this time, a discharge capacity per spinning nozzle was 50 D/min. A distance of the tip and the collector was 20 D.

[89] The component fiber of the membrane integrated in the collector had a diameter of 100 to 500 D, and the manufactured membrane had the porosity of 80 % and the thickness of 30 D.

[90] Its collection efficiency was 99.993 % for 0.3 D particle size, and its airflow resistance value was 25 DH 2O at 20 SCFH.

[91] <Experimental Embodiment

[92] Physical properties of the porous membranes manufactured in the Embodiments 1 to 4 were measured by the following methods.

[93] 1. Collection Efficiency

[94] The collection efficiency of a DOP particle (size: 0.3 D) was measured at a flow rate of 32 °C/min using the apparatus JAP AN-SIB ATA 6310K.

[95] 2. Airflow Resistance

[96] Airflow resistance was measured in the range of 20 to 200 SCFH (Standard cubic feet per hour). At this time, the measured area had a diameter of 81.6 D, and the airflow resistance value was expressed in a unit of DHO.

[97] It should be understood that the terms used in the specification and appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation.

[98] Accordingly, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. Industrial Applicability

[99] The filter medium for air cleaning according to the present invention has a web- shaped porous membrane composed of nano fibers integrated using the electrospinning apparatus. The web-shaped porous membrane composed of the nano fibers according to the present invention has a uniform fiber diameter distribution, a high-efficiency collection performance and a low airflow resistance. Also, performance of the filter may be improved since the airflow resistance may be more reduced by dually separating the fiber diameter distribution and further reduced by bending the fiber in a certain space due to its thin thickness to increase its surface area when the filter is manufactured. Also, the strength of the filter medium may be improved by coupling the strength-supporting nonwoven fabric with one or both side(s) of the web-shaped porous membrane composed of the nano fibers manufactured by the electrospinning process.

Claims

[ 1 ] A filter medium comprising: a porous membrane composed of a nano fiber web by an electrospinning process; and an air-permeable supporting material stacked on at least one side of the porous membrane, wherein the porous membrane has a collection efficiency of at least 99.9 % for particles ranging from 0.1 D to 0.3 D in size, and an airflow resistance of 30 IMor less at 20 SCFH.

[2] The filter medium according to claim 1, wherein the nano fiber constituting the porous membrane has a diameter distribution of 50 to 700 D, a membrane thickness of 25 to 100 D, and a porosity of 80 to 90 %.

[3] The filter medium according to claim 1, wherein a diameter distribution of the nano fiber constituting the porous membrane is dually separated into 50 to 700 D and 700 to 1,500 D, and has a membrane thickness of 25 to 100 D, and a porosity of 80 to 90 %.

[4] The filter medium according to claim 3, wherein the nano fiber having the diameter of 700 to 1500 D accounts for 50 % or less by weight, based on the total weight of the web.

[5] The filter medium according to claim 1 or 3, wherein the nano fiber is composed of high molecules selected from the group consisting of polyvinylidene fluoride, polyvinylidene fluoride -hex- afluorpropylene copolymer, polyacrylonitrile, polyvinylidene chloride-acry- lonitrile copolymer, polyethyleneoxide, polyurethane, polymethylacrylate, polymethylmethacrylate, polyacrylamide, polyvinylchloride, polyvinylacetate, polyvinylpyrolidone, polytetraethylene glycol diacrylate, polyethyleneglycol dimethacrylate, cellulose, cellulose acetate, rayon, polyamide, polyethylene terephthalate, polytrimethyleneterephthalate, polybutyleneterephthalate, polyimide, polyphenylenesulfide, or mixtures thereof.

[6] The filter medium according to claim 2 or 3, wherein the air-permeable supporting material is a non woven fabric having a thickness of 80 to 120 D and a porosity of 10 to 40 %.

[7] The filter medium according to claim 6, wherein the air-permeable supporting material is selected from the group consisting of polyethyleneterephthalate, polyolefin-based, cellulose-based nonwoven fabrics, or mixtures thereof.

[8] The filter medium according to claim 2 or 3, wherein the porous membrane has a 2-layer structure to which porous materials are attached, in one side thereof.

[9] The filter medium according to any of claims 2 and 3, wherein the porous membrane has a 3 -layer structure to which porous materials are attached, in both sides thereof.

[10] A method for manufacturing a filter medium, the method comprising: preparing a non woven fabric having a thickness of 80 to 120 D and a porosity of 10 to 40 %; manufacturing a porous membrane composed of nano fibers having a collection efficiency of at least 99.9 % for particles ranging from 0.1 D to 0.3 D in size, and an airflow resistance of 30 DHO or less at 20 SCFH by means of an elec- trospinning process; and stacking the non woven fabric on at least one side of the porous membrane and laminating the nonwoven fabric at a predetermined temperature under a predetermined pressure.

[11] A method for manufacturing a filter medium, the method comprising: preparing a nonwoven fabric having a thickness of 80 to 120 D and a porosity of

10 to 40 %; and directly spinning a high molecule solution on the nonwoven fabric using an elec- trospinning process to stack, on the nonwoven fabric, a web-shaped porous membrane having a collection efficiency of at least 99.9 % for particles ranging from 0.1 D to 0.3 D in size, and an airflow resistance of 30 3Mor less at 20

SCFH.

[12] The method for manufacturing a filter medium according to claim 11, further comprising: stacking another nonwoven fabrics on the nonwoven fabric in which the porous membrane is stacked, and laminating the nonwoven fabrics at a predetermined temperature under a predetermined pressure.

[13] The method for manufacturing a filter medium according to any of claims 10 to

12, wherein the porous membrane is electrospun so that the nano fiber has a diameter distribution of 50 to 700 D, a membrane thickness of 25 to 100 D, and a porosity of 80 to 90 %.

[14] The method for manufacturing a filter medium according to any of claims 10 to

12, wherein the porous membrane is electrospun so that a diameter distribution of the nano fiber is dually separated into 50 to 700 D and 700 to 1,500 D, and the nano fiber has a membrane thickness of 25 to 100 D and a porosity of 80 to 90 %.

[15] The method for manufacturing a filter medium according to any of claims 10 to

12, wherein the nano fiber constituting the porous membrane is composed of high molecules selected from the group consisting of polyvinylidene fluoride, polyvinylidene fluoride-hexafluorpropylene copolymer, polyacrylonitrile, polyvinylidene chloride- aery lonitrile copolymer, polyethyleneoxide, polyurethane, polymethylacrylate, polymethylmethacrylate, polyacrylamide, polyvinylchloride, polyvinylacetate, polyvinylpyrolidone, polytetraethylene glycol diacrylate, polyethyleneglycol dimethacrylate, cellulose, cellulose acetate, rayon, polyamide, polyethylene terephthalate, polytrimethyleneterephthalate, polybutyleneterephthalate, polyimide, polypheny lenesulfide, or mixtures thereof.

[16] The method for manufacturing a filter medium according to any of claims 10 to

12, wherein the air-permeable supporting material is selected from the group consisting of polyethyleneterephthalate, polyolefin-based, cellulose-based nonwoven fabrics, or mixtures thereof.