Method for producing electret filter

Abstract: A method for producing an electret filter, which methods comprises forming a film comprising a high molecular weight compound, optionally stretching the film, splitting the optionally stretched film to form split yarns, subjecting the split yarns to homo charging to obtain charged split yarns, and forming the charged split yarns into the electret filter.

Description: This invention relates to a method for producing an electret filter. More particularly, this invention relates to a highly efficient method for producing an electret filter which is capable of collecting fine particles with a high collection efficiency. In a known method, an electret filter is produced by forming a film of a starting resin material, subjecting the film formed to a homo charging either before or after stretching the film, splitting the charged film to produce split yarns, and collecting the split yarns to form them into a filter of a desired configuration. Such a method wherein the film splitting step is carried out subsequent to the homo charging of the film suffers from frequent film tearing since the film strength is decreased by the surface oxidation or minute surface irregularities caused by the homo charging. The homo charging of the film also results in an increased adhesion of the film to electrodes, which may also lead to film tearing. Once the film is torn, the torn film, which has been electrically charged, easily becomes twined, for example, on the roll, which causes various problems in the production process. The present invention seeks to provide a reliable, highly efficient, and quick method which is capable of producing an electret filter having various excellent performances including an excellent collection efficiency with no production problems. Accordingly the present invention provides a method for producing an electret filter, which method comprises forming a film comprising a high molecular weight compound, optionally stretching the film, splitting the optionally stretched film to form split yarns, subjecting the split yarns to a homo charging to obtain charged split yarns, and forming the charged split yarns into the electret filter. FIG. 1 is a schematic view of the apparatus used for measuring the collection efficiency of the electret filter produced in accordance with the method of the present invention. The method for producing an electret filter according to the present invention is now described in further detail. In the method of the present invention, a film is first formed from a starting resin material which may be a polar, non-polar, crystal or amorphous high molecular weight compound or a mixture thereof. Examples of the non-polar high molecular weight compounds include polyolefins such as polyethylene and polypropylene, polystyrene, polyethylene tetrafluoride and ethylene tetrafluoride-propylene hexafluoride copolymers. Polar high molecular weight compounds include those having within the molecule a polar group selected from, for example, a carboxyl group, ester group, amide group, hydroxyl group, ether group, nitrile group, carbonyl group, and a halogen atom such as a chlorine atom. Examples of polar high molecular weight compounds include polyesters such as polyethylene terephthalate and polytetramethylene terephthalate; polyamides such as nylon 6, nylon 66 and nylon 12; polycarbonates; polyacrylates such as polymethyl methacrylate and polyethyl acrylate; acrylonitrile-styrene copolymer resins (AS resins); acrylonitrile-butadiene-styrene copolymer resins (ABS resins); polyvinyl chloride; polyvinylydene chloride; polyethylene trifluoride; polyacetal; and polyacrylonitrile. A modified non-polar high molecular weight compound prepared by graft copolymerizing a monomer having a polar group therein with a non-polar high molecular weight compound may also be used as a polar high molecular weight compound. Examples of such modified non-polar high molecular weight compounds include those prepared by graft copolymerizing at least one compound selected from unsaturated carboxylic acids and their derivatives with a non-polar high molecular weight compound in the presence of a radical polymerization initiator such as an organic peroxide. Typical unsaturated carboxylic acids and their derivatives which may be used for the graft modification of the non-polar high molecular weight compounds, include acrylic acid; methacrylic acid; α-ethylacrylic acid; unsaturated dicarboxylic acids such as maleic acid, fumalic acid, itaconic acid, citraconic acid, tetrahydrophthalic acid, methyltetrahydrophthalic acid, endocis-bicyclo[2,2,1]hepto-5-en-2,3-dicarboxylic acid (nadic acid) and methyl-endocis-bicyclo[2,2,1]hepto-5-en-2,3-dicarboxylic acid (methylnadic acid); and derivatives of the unsaturated dicarboxylic acids such as acid halides, amides, imides, acid anhydrides, esters of the unsaturated dicarboxylic acids. Illustrative examples of the unsaturated dicarboxylic acid derivatives include maleoyl chloride, maleimide, maleic anhydride, citraconic anhydride, monomethylmaleic acid and dimethylmaleic acid. Such unsaturated carboxylic acids and their derivatives may be used alone or in combination of two or more. Among these, preferred in the method of the present invention are unsaturated dicarboxylic acids and their anhydrides, and most preferred are maleic acid, nadic acid, and their anhydrides. The modified non-polar high molecular weight compound may generally contain from 0.05 to 15% by weight, and preferably from 0.5 to 3% by weight, of the unsaturated carboxylic acid or the derivative thereof. In other words, the modified non-polar high molecular weight compound may have been graft modified to such an extent. In the present invention, the film may be produced from either a single high molecular weight compound, or a mixture of two or more high molecular weight compounds as typified above. When a mixture of two or more high molecular weight compounds are used, it may comprise a mixture of polar high molecular weight compounds, a mixture of non-polar high molecular weight compounds, or a mixture of a polar and a non-polar high molecular weight compounds. The polar high molecular weight compound may partly comprise a modified non-polar high molecular weight compound. When a mixture of a non-polar and a polar high molecular weight compounds is used for the film production, they may be mixed in a weight ratio of the non-polar high molecular weight compound to the polar high molecular weight compound of 60 to 99 : 0.5 to 39.5, and preferably 80 to 95 : 1 to 10. When the polar high molecular weight compound partly comprises a modified non-polar high molecular weight compound, the modified non-polar high molecular weight compound may generally comprise about 0.5 to 20% by weight, and preferably about 4 to 10% by weight of the total composition. The high molecular weight compound used for the film production may optionally contain, for example, a heat stabilizer, a weathering stabilizer, an antistatic agent, a slip agent, an anti-blocking agent, a lubricant, an inorganic or organic filler, a dye or a pigment. In the method of the present invention, the high molecular weight compound as described above is formed into a film, which is subsequently split to produce net-like split yarns. The split yarns are then subjected to a homo charging to produce electretized split yarns, which are used to form the electret filter. The film formation from the above described high molecular weight compound may be carried out by any desired conventional process which is generally used to produce a film from such a resin, for example, a blown-film extrusion, slot die extrusion using a T-die or calendering. The film formed may have a thickness suitably selected to provide the electret filter product with a desired deposition weight and a pressure loss. The film may generally have a thickness of 10 to 100 µm, and preferably 20 to 50 µm, in order to enable subsequent optional stretching of the film to 6 to 10 folds in length. The film, after an optional stretching, is split with a splitting mean, for example, a wire fillet roll or a needle-mounted roll to produce the split yarns. When the film is stretched, the film may be stretched in machine direction at a temperature of not higher than the melting point and higher than the softening point of the high molecular weight compound used to form the film, for example, at a temperature of 130 to 150°C by using, for example, a roll, a hot plate or an oven. The film may be generally stretched to about 5 to 10 fold in length, preferably to 6 to 8 fold in length, for retaining fair stability after the stretching and for ease of the subsequent splitting of the stretched film. The resulting split yarns may be either directly fed to the next step without winding, or wound on a cardboard cylinder or a drum before feeding to the next step. Next, the split yarn is subjected to the homo charging so that the split yarn becomes an electret. The homo charging may be carried out by any conventional electretizing process. Such conventional processes include, for example, a thermal electretization wherein the split yarns are heated until the split yarns are softened, and the softened yarns are cooled with a direct high-voltage current being applied to the yarn to electretize the split yarns; an electo-electretization wherein the split yarns are treated on their surfaces with corona discharge or pulse high-voltage current to electretize the split yarns, or wherein the split yarns are held on their major surfaces between dielectrics to apply direct high-voltage current thereto to thereby electretize the split yarns; a radio-electretization wherein the split yarns are irradiated with γ rays or electron rays; a mechano-electretization wherein the split yarns are subjected to plastic deformation by application of a pressure to electretize the split yarns; and an auto-electretization wherein the split yarns are irradiated with light with a voltage being applied to electretize the split yarn. Among the processes as typified above, preferred are the processes wherein the heated or non-heated split yarns are intermittently treated with corona discharge, and the process wherein the split yarns are sandwiched between a pair of needle-shaped electrodes to effect the corona discharge treatment. In the method of the present invention, the thus charged split yarns are cut to a length of, for example, 90 mm, and fed to an opener to produce an electret raw cotton. The fiber or yarn constituting the electret raw cotton may have any desired size by, for example, preliminarily selecting the thickness of the film being split, extent to which the film is to be stretched or degree of the splitting. Next, the resulting electret fibers are collected to form them into a desired shape to thereby produce the electret filter. The electret fibers are formed into the electret filter by weaving, knitting, tufting, or forming into a non-woven fabric in accordance with a suitable conventional method. Typical methods include, for example, needle punching, thermal bonding and ultrasonic bonding. The electret filter produced in accordance with the method of the present invention is capable of collecting fine particles of a particle size of, for example, up to 1 µm at a high efficiency, and therefore, quite suitable for use as, for example, an air filter, a filter for an air cleaner, a vacuum cleaner or an air conditioner or a mask. The present invention is now described in further detail in the following Examples and Comparative Examples. EXAMPLES Example 1 A resin composition was prepared by mixing 9,000 g of polypropylene (HYPOL B200, manufactured by Mitsui Petrochemical Industries, Ltd., Melt flow rate: 0.5 g/10 min. measured by the method shown in ASTM D1238), 500 g of polycarbonate (Lexan 101, manufactured by General Electric), and 500 g of polypropylene modified with maleic anhydride (graft modification with maleic anhydride: 3% by weight). The resulting resin composition was fed to a blown film extrusion machine (manufactured by Toshiba Machine Co., Ltd.) to extrude a 30 µm thick film at 240°C. The film was then stretched in the machine direction to 6.6 fold in length at 135°C with a hot plate, and the stretched film was split with a wire fillet roll into net-like split yarns. The resulting split yarns were charged by feeding the split yarns between corona discharge electrodes disposed at an interval of 8 mm with a voltage applied of 9 kV (direct current) for a residential period of 0.5 s, and wound on a paperboard cylinder. The process comprising the step of blown film extrusion through to the step of winding on the paperboard cylinder was continued for a predetermined period (five hours). When the stretched film was split with the needle-mounted roll, the resulting split yarns were often torn because the yearns were fine. As a result, the torn split yearns were wound on the needle-mounted roll and then the film itself was torn by tension. The frequency of film tear caused by tension is shown in Table 1. The thus treated split yarns were cut into split fibers of 90 mm, and treated with an opener to produce a raw cotton of electret fibers. The electret fiber raw cotton was fed to a web forming machine to form a web, which was then needle punched to produce an electret filter having a deposition weight of 100 g/m 2 and a thickness of 2 mm. The frequency of film tear caused by tension during the production is shown in Table 1. The resulting electret filter was evaluated for its collection efficiency in accordance with the procedure as described below. The results are also shown in Table 1. Evaluation of collection efficiency The collection efficiency was evaluated with an evaluation apparatus schematically illustrated in FIG. 1. First, NaCl particles having a particle size of 0.3 µm produced in an aerosol generator 1 (manufactured by Japan Science Industries K.K.) were fed to a chamber 3 together with a clean air which had passed through an air filter 2. When the NaCl particle concentration in the chamber 3 reached a constant level of from 2 to 6 x 10 6 (particles/ft 3 ), a blower 4 was actuated to start aspiration of the gas within the chamber 3 through a flow path 5 with the flow rate being regulated by a flow rate-regulating valve 6. In the flow path 5 was provided an electret filter 8 to be evaluated. The flow rate of the gas was measured with a flowmeter 7. When the flow rate reached a constant level of 0.5 m/s, the concentration C in and concentration C out of the NaCl particles in the upstream side and downstream side of the electret filter 8 were measured with particle counters 9a and 9b (KC-01A, Rion K.K.), respectively. The collection efficiency was calculated by the following formula: Collection efficiency = [1 - (C out /C in )] x 100 (%) Comparative Example 1 The procedure of Example 1 was repeated except that the film extruded and stretched as in Example 1 was subjected to the homo charging before the splitting of the film into the split yarns. The frequency of the film tearing as well as the collection efficiency of the resulting electret filter were evaluated as in Example 1. The results are shown in Table 1. Frequency of film tearing Collection efficiency, % Example 1 0 65.2 Comparative Example 1 5 67.0 In the method of the present invention, the homo charging is carried out after the splitting of the film. Therefore, the film does not suffer from decreased film strength due to the oxidation of the film surface or formation of minute surface irregularities caused by the homo charging as in the conventional process wherein the homo charging is carried out before the splitting of the film. Consequently, the film is less likely to be torn during the production leading to reduced production problems such as winding of the torn film onto rolls. Accordingly, the method of the present invention enables a reliable, highly efficient, and quick production of an electret filter having various excellent performances including the collection efficiency with no production problems as found in the prior art processes.

Claims: A method for producing an electret filter, which method comprises forming a film comprising a high molecular weight compound, optionally stretching the film, splitting the optionally stretched film to form split yarns, subjecting the split yarns to homo charging to obtain charged split yarns, and forming the charged split yarns into the electret filter. A method according to claim 1, wherein said film comprises at least one high molecular weight compound selected from non-polar compounds, polar compounds and modified non-polar compounds. A method according to claim 2, wherein the modified non-polar high molecular weight compound is a graft copolymer with at least one compound selected from unsaturated carboxylic acids and derivatives thereof. A method according to claim 3, wherein the modified non-polar high molecular weight compound contains from 0.05 to 15% by weight of the at least one compound selected from unsaturated carboxylic acids and derivatives thereof. A method according to claim 3 or 4, wherein said unsaturated carboxylic acids and derivatives thereof are unsaturated dicarboxylic acids and anhydrides thereof. A method according to claim 5, wherein said unsaturated dicarboxylic acids and anhydrides thereof are at least one compound selected from maleic acid, nadic acid, and anhydrides thereof. A method according to any one of the preceding claims, wherein said film comprises a mixture of a non-polar and a polar high molecular weight compound in a weight ratio of from 60 : 39.5 to 99 : 0.5. A method according to any one of claims 2 to 6 wherein said film comprises a polar high molecular weight compound and 0.5 to 20% by weight, based on the weight of the film, of a modified non-polar high molecular weight compound. A method according to any one of the preceding claims, wherein said film is from 10 to 100 µm thick. A method according to any one of the preceding claims, wherein said film is stretched by from 5 to 10 fold in length before the splitting step.

Inventors: MATSUURA SATOSHI, SHINAGAWA YOSHIO, YOKOYAMA AKIRA, MATSUURA, SATOSHI, SHINAGAWA, YOSHIO, YOKOYAMA, AKIRA, MATSUURA, SATOSHI, C/O MITSUI PETROCHEM. IND. LTD., SHINAGAWA, YOSHIO, C/O MITSUI PETROCHEM. IND. LTD., Yokoyama, Akira, c/o Mitsui Petrochem. Ind. Ltd.
Applicants: MITSUI PETROCHEMICAL IND, MITSUI PETROCHEMICAL INDUSTRIES, LTD.
Application year: 1992
Main classification: B03C3
IPC codes: B29C71, B03C3, B01D39, C07C51, B29C35, H01G7, B29C55