FILTER ELEMENT AND ITS PRODUCTION

Abstract: Described is a filter element made wholly of fluorocarbon resin, comprising as essential elements a filter material pro­ duced by folding a sandwich-form sheet comprising a filter membrane made of a fluorocarbon resin and net supporters made of a thermoplastic fluorocarbon resin superimposed on the both surfaces thereof into a pleat form and liquid-tightly welding the edge parts of the both sides of the pleated sheet, sealed end parts produced by embedding the both end parts of said filter material except the central openings into a ther­ moplastic fluorocarbon resin to force the resin into the pleats, whereby the both end parts are each integrally and tightly welded, and fluorocarbon resin caps welded liquid-tightly with said sealed end parts, and the production thereof.

Description: The present invention relates to a filter element made wholly of fluorocarbon resin and to the production of corresponding filter elements. In recent years, super-clarification techniques for air, gases, water, chemicals, etc. based on the membrane separation techniques have made a remarkable progress and are widely used in various fields such as the fields of foods, semi-conductors and the like. Particularly, a chemicals clarification technique required for fine processing in the process for producing semi-conductor integrated circuits is behind in technical response,as compared with the clarification technique for air, gases, water, etc., and is recently attracting special attention. As one of the main reasons for that, while increasing the purity of chemicals is being required for attaining an improvement in the yield and quality of products, both semi-conductor integrated circuit manufacturers and chemicals suppliers are behind in their technical response, there is given delay in the development of filter products which show less metal leaching and good in resistance to chemicals. As filters which meet the above requirements at present, those made wholly of fluorocarbon resins are being required, and membrane filters of a multi-flat membrane system and a tubular form have been developed (cf. JP-A-58-98112). However, these filters have not yet come to satisfy the needs of the market, because both effective filter area and flow rate of filtration are so small that this causes problems in filtration of highly viscous chemicals and mass treatment. On the other hand, a pleat-form filter element with fluorocarbon resin membranes has been developed (cf. "Gelman Sciences" catalogue, 1981 New Product Edition, Filtration Catalogue Supplement). This filter element has a large effective filter area as well as a fairly good resistance to chemicals, but because of the adhesion of fluorocarbon resin being very difficult in itself, parts other than the filter membrane are made of materials other than fluorocarbon resin which, however are not sufficient in terms of resistance to chemicals, temperature resistance and metal leaching. Consequently, satisfactory products have not yet been obtained. Further, a filter employed in the foregoing process for producing semi-conductor integrated circuits requires pores as fine as 0.01 to 10 µm. Also, the filter membrane itself is very low in strength, so that severe conditions cannot be employed on filter production, and besides the parts of the filter element must be assembled not only by mere adhesion, but by strong adhesion in a completely sealed state. US-A-3 457 339 discloses the production of a tubular shape filter which has been rendered leakproof by applying a thermoplastic end cap the inner face whereof is fused to a filter sheet material formed into a tubular shape. However, in case that the end cap is a fluorocarbon resin, even if the inner face of the cap is fused at a temperature above the melting point, the fused resin cannot penetrate the pleats of filter because of its high viscosity. Consequently, US-A-3 457 339 cannot be applied to a filter element made wholly of fluorocarbon resin. EP-A-62867 discloses a tubular filter element comprising at least one layer of filter sheet material comprising glass fibers coated and bonded together by fluoroethylene polymer, having end portions of the tube sealed to end caps via a seal of thermoplastic fluoroethylene polymer material, and having the outer surfaces of the end portions of the filter sheet joined together through a self-supporting channel strip of thermoplastic fluoroethylene polymer material. According to the present invention, the materials for the filter element are made wholly of fluorocarbon resin so that the problems of chemical resistance, temperature resistance and metal leaching etc. are not caused, and the fluorocarbon resins which have so far been regarded as difficult to weld together, are in a liquid-tightly welded state due to their special constitution. Specific embodiments of the filter element as claimed are shown in the drawings, in which: Fig. 1 is a typical view illustrating the filter element of the present invention (in the view, the filter material and the cap are shown separately). Figs. 2 to 4 are typical views illustrating ways of welding the edge portions of the both sides of the pleat. The present invention, as shown in Figs. 1 and 2, provides a filter element made wholly of fluorocarbon resin and comprising as essential elements: (a) a filter material (3) made of a sandwich-form sheet which has a pleated structure and is welded liquid-tight at the edge portions (9) of the two adjacent sides of said sheet, the sandwich-form sheet comprising a filter membrane (1) made of polytetrafluoroethylene and net supports (2) made of thermoplastic fluorocarbon resin and being present on both surfaces of said membrane (1); (b) sealed end parts (6) consisting of two end portions (4) of the filter material (3) which have central openings (5) and are integrally and tightly welded with a thermoplastic fluorocarbon resin; and (c) fluorocarbon resin caps (8) welded liquid-tight onto said sealed end parts (6). In Fig. 1, the sealed end parts and the cap are separately shown for facilitating understanding. Also provided is a method for producing a filter element made wholly of fluorocarbon resin,characterized by forming a sheet comprising a filter membrane made of a fluorocarbon resin and net supports made of thermoplastic fluorocarbon resin provided on the two surfaces thereof into a pleated structure, bending the pleated sheet into a cylindrical form, welding the edge portions of the two adjacent sides liquid-tight, prewelding the pleats by heating the two end portions of the cylindrical pleat-form filter to a temperature higher than the melting point of the net supports and then cooling said end portions, inserting the end portions having prewelded pleats into a molten thermoplastic fluorocarbon resin to force the resin into the pleats, whereby the end portions and the molten resin are integrally welded together, fitting fluorocarbon resin caps having a predetermined shape in said end parts and providing, if desired, a fluorocarbon resin core material for supporting the filter material inside the central opening of the resulting structure, and then integrally welding said two or three parts of the assembly thus obtained. The filter element of the present invention is characterized in that the materials for the element are exclusively fluorocarbon resins, so that problems of chemical resistance, temperature resistance and leaching etc. are not caused; besides that,fluorocarbon resins which have so far been regarded as difficult to weld together, are in a liquid-tightly welded state due to their special constitution. The filter membrane (1) used in the present invention is made of tetrafluoroethylene resin (hereinafter referred to as PTFE). It may be laminated with a porous reinforcing material made of a fluorocarbon resin. This membrane itself is well known as a filter membrane. Relating to the thickness of the membrane and the diameter of the pore, it suffices to optionally select them according to the kind of liquids to be treated and the object of filtration. Generally, however, membranes of 30 to 200 µm in thickness and 0.01 to 10 µm in pore diameter are used. This membrane is low in mechanical strength and easily deformed by filter pressure so that, in order to keep the pleat form, the membrane is used in the form of a sandwich in which fluorocarbon resin net supports are superimposed on the both surfaces of the membrane. Known filter membranes, for example membranes produced by welding a fluoroethylene polymer onto the surface of glass fibers, as described in JP-A-14919/1983, have an increased strength, but no fine pores. Besides, they are not sufficient in terms of resistance to chemicals, metal leaching etc., thus not meeting the object of the present invention. Such membranes, therefore, cannot be used. The filter membrane of a fluorocarbon resin, especially PTFE, is placed between thermoplastic fluorocarbon resin net supports (2) in a sandwich form. These supports are spacers for the filter membrane to act effectively over the whole area of filtration, and at the same time, they prevent the membrane from getting deformed by filter pressure. Besides, they play an important role in ensuring the welding of the edge portions of the both sides of the membrane. PTFE, the material for the filter membrane, is very difficult to be thermally welded and very low in membrane strength, so that it is difficult to liquid-tightly weld the two end portions (4). This may be considered to be one of the reasons that cylindrical filter elements made wholly of fluorocarbon resin have not been provided so far. As material for the net supports (2), there may be used thermoplastic fluorocarbon resins such as tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer resin (hereinafter referred to as PFA), tetrafluoroethylene/hexafluoropropylene copolymer resin (hereinafter referred to as FEP), ethylene/tetrafluoroethylene copolymer resin (hereinafter referred to as ETFE), trifluorochloroethylene resin (hereinafter referred to as PCTFE), ethylene/trifluorochloroethylene copolymer resin (hereinafter referred to as ECTFE), fluorovinylidene resin (hereinafter referred to as PVdF), fluorovinyl resin (hereinafter referred to as PVF), tetrafluoroethylene/hexafluoropropylene/perfluoroalkyl vinyl ether terpolymer resin (hereinafter referred to as EPE) and the like. Particularly preferred ones are PFA, FEP and EPE in terms of chemical resistance and temperature resistance. The net supports are present in the form of materials such as plain woven fabrics, welded non-woven fabrics, formed nets, knitting goods, punching sheets and the like of fluorocarbon resin fibers. Preferably, the supports have a flexibility such that they are not deformed easily by filter pressure. For this reason, supports of 0.1 to 1.0 mm in thickness and of about 0.1 to about 5 mm² in pore area are generally used. The filter membrane is places between the net supports in a sandwich form, folded in pleats and welded liquid-tightly at the edge portions of both sides into a pleat-form cylinder. The filter membrane and the net supports may form a multi-layer structure (for example, support - membrane - support - membrane - support). The edge parts (9) of both sides may be welded together, for example, by lengthening the edge part of one side of the sheet, externally covering the edge part of the other side with the lengthened edge part and integrally welding the two edge parts, as shown in Fig. 2; by placing a sealing tape (10) made of a thermoplastic fluorocarbon resin between said edge parts, followed by integrally welding, as shown in Fig. 3; or by covering the edge parts with a sealing cover (11) made of thermoplastic fluorocarbon resin, followed by integrally welding, as shown in Fig. 4. As said sealing tapes and sealing covers, PFA, FEP, EPE, etc. superior in chemicals resistance and weldability are preferred. Through the embodiments shown in Figs. 3 and 4, the pressure resistance of the seal is improved. The filter material thus obtained is thoroughly heated, at both of its end portions (4), to a temperature higher than the melting point of the resin constituting the net support in order to pre-weld the pleats. This treatment is important as pre-treatment for the subsequent formation of sealed end parts by forcing a fluorocarbon resin into the pleats at the two end parts of the pleat-form filter to effect tight welding. As described later, the welding of the two end parts of the pleat-form filter is carried out by pressing said both parts into a mold containing a molten fluorocarbon resin. In this case, if the foregoing pre-welding of the pleats has been omitted, the pleats, because of the high viscosity of the molten fluorocarbon resin, buckle and thus make it impossible for the resin to enter the pleats. In other words, the filter membrane does not become embedded in the fluorocarbon resin. Consequently, only an incomplete seal is obtained, and the pressure resistance is also insufficient. In the above pre-welding of the pleats, complete and integral welding of the whole pleats is not necessary, but it suffices to carry out the pre-welding to such a degree that the above problem occurring in the formation of sealed end parts can be avoided. The filter material (3) thus obtained is welded as its two end portions (4) by embedding said end portions, except for an opening (5) for the passage of liquid, in a thermoplastic fluorocarbon resin. The welding is carried out by filling a circular mold which can receive the end part of the filter material with a sealing thermoplastic fluorocarbon resin, melting the resin by heating, and pressing the end portion into the molten resin to force the resin into the pleats. In this case, the heating is continued until said fluorocarbon resin enters the pleats, and the end portion of the filter material made of fluorocarbon resin is melted at at least the surface and welded with said entering fluorocarbon resin to form an integrated mass. As the thermoplastic fluorocarbon resin tor sealing, any of PFA, FEP, ETFE, PCTFE, ECTFE, PVdF, PVF, EPE, but not PTFE, may be used. PFA, FEP and EPE are particularly preferred in terms of chemical resistance. PTFE is inferior in weldability as well as high in viscosity on melting, so that pressing the end portion of the filter material into PTFE is difficult, and air bubbles remain in the pleats to cause leaks. Therefore, PTFE is not used for said purpose. The sealed end part (6) is further welded with a cap (8) having a predetermined shape and, if desired, a porous hollow cylindrical core material (a passage for removing the filtrate) fit for the central opening (5) of the two end parts of the filter material. For welding, various methods such as heat-welding, rotational welding, insert-injection, cylinder injection, etc. may be employed. In the case of heat-welding, the cap and core material having previously molded predetermined shapes, both of which are made of a fluorocarbon resin, are fitted to the sealed end part, and the assembly is inserted in a mold and heated to a temperature higher than the melting point of the fluorocarbon resin constituting the sealed end part, thereby liquid-tightly and completely welding the entire side surface and/or the entire bottom surface of the cap. Both of the caps to be welded with the two end parts (6) may have a central opening (7), or one of them may have a central opening and the other may be a cap having no central opening. When the sealed end part is not formed beforehand, welding between the cap and the filter material is incomplete and a completely sealed filter element cannot be obtained. The fluorocarbon resin used for the cap may be any of the foregoing fluorocarbon resins, but PTFE, FEP, PFA and EPE are preferred in terms of chemical resistance. More preferably, the same resins as used for sealing the end part are used in view of adhesiveness. Also in the case of the core material, any of the foregoing fluorocarbon resins may be used, but in terms of chemical resistance and adhesiveness, PTFE, FEP, PFA and EPE are preferred. Particularly, the same resins as used for sealing the end parts are preferred. With the filter element of the present invention, the outer surface of the filter material may be covered with a cylindrical protective jacket made of a fluorocarbon resin, if desired. This protective jacket may be made of a panel having a number of pores for a liquid to be treated to pass therethrough, and it does not always have to be integrally welded with the filter element. The jacket may be fitted to the inside of the cap, or to the outside of the cap. The filter element of the present invention is made wholly of fluorocarbon resin and, therefore, is superior in chemical resistance, temperature resistance and does not cause problems of metal leaching, etc. Besides, it is possible to filter therewith a liquid to be treated containing contaminants as fine as 0.1 µm and thus said filter element is particularly useful in clarifying chemicals, etc. used in a process for producing semi-conductor integrated circuits. In the following the present invention will be illustrated by an example. Example A sandwich-form sheet comprising a PTFE filter membrane (thickness 60 µm; average pore diameter 0.22 µm; surface area 260 x 2200 mm²) and EPE net supports (average thickness 0.3 mm; ratio of pore area, 50%, surface area 260 x 2200 mm²) superimposed above and below said filter membrane, is formed into a 78-pleated sheet. The edge portions of the two sides of the pleated sheet are placed one upon the other, and an EPE film (width 5 mm; length 260 mm; thickness 50 µm) is inserted between them (Fig. 3). The edge portions are then heat-welded by means of a commercially available impulse sealer to obtain a cylindrical pleat-form filter material (internal diameter 36 mm; largest external diameter 65 mm; length 260 mm). This heat-welded part has a completely liquid-tight seal. The two end portions of this cylindrical pleat-form filter material are inserted by 15 mm each into a heating furnace, heat-treated at 350°C for 30 minutes, taken out and then cooled. Separately from this, into a circular mold which fits the above heat-treated end potions there is inserted an EPE sheet (thickness 5 mm) of the same shape as that of the bottom of the mold, and the mold is heated to 320°C to melt the EPE sheet. Said heat-treated end portion is then inserted into the mold to force the molten resin into the pleats and is kept at 320°C for 15 minutes. Thereafter, the mold is cooled and the end part is taken out thereof. The sealed end part of the filter material obtained is in a state wherein the PTFE membrane is completely embedded in the EPE resin; moreover, the PTFE membrane and the EPE resin are integrally welded together, being resistant to high pressure. An EPE porous cylindrical core material (external diameter 35 mm; internal diameter 30 mm; length 253 mm; number of pores 36 x 176) is inserted from end to end into the central opening of the end part-sealed filter material. An EPE end cap, which is a set with the core material, is then fitted in one end part. This assembly is placed in a mold, heated to 320°C for 40 minutes, cooled, and then taken out thereof. The other end part is also treated in the same manner. The filter element thus obtained was liquid-tightly and strongly welded at the lateral edge portions of the pleat and the end parts as well as at the cap parts, there being no leaks, and, furthermore, it was resistant to high pressure.

Claims: A filter element made wholly of fluorocarbon resin and comprising as essential elements: (a) a filter material (3) made of a sandwich-form sheet which has a pleated structure and is welded liquid-tight at the edge portions (9) of the two adjacent sides of said sheet, the sandwich-form sheet comprising a filter membrane (1) made of polytetrafluoroethylene and net supports (2) made of thermoplastic fluorocarbon resin and being present on both surfaces of said membrane (1); (b) sealed end parts (6) consisting of two end portions (4) of the filter material (3) which have central openings (5) and are integrally and tightly welded with a thermoplastic fluorocarbon resin; and (c) fluorocarbon resin caps (8) welded liquid-tight onto said sealed end parts (6). A filter element as in claim 1, wherein the diameter of the fine pores of the filter membrane (1) is 0.01 to 10 µm. A filter element as in any of claims 1 and 2, wherein the net support (2) is made of tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer resin (PFA), tetrafluoroethylene/hexafluoropropylene copolymer resin (FEP) or tetrafluoroethylene/hexafluoropropylene/perfluoroalkyl vinyl ether terpolymer resin (EPE). A filter element as in any of claims 1-3, wherein the resin for sealing the end portions (4) is PFA, FEP or EPE. A filter element as in any of claims 1-4, wherein the cap (8) is made of PTFE, FEP, PFA or EPE. A filter element as in any of claims 1-5, wherein the element is provided with an additional fluorocarbon resin core material for supporting the filter material (3). A filter element as in any of claims 1-6, wherein the element is provided with an additional fluorocarbon resin protective jacket. A method for producing a filter element made wholly of fluorocarbon resin,characterized by forming a sheet comprising a filter membrane made of a fluorocarbon resin and net supports made of thermoplastic fluorocarbon resin provided on the two surfaces thereof into a pleated structure, bending the pleated sheet into a cylindrical form, welding the edge portions of the two adjacent sides liquid-tight, prewelding the pleats by heating the two end portions of the cylindrical pleat-form filter to a temperature higher than the melting point of the net supports and then cooling said end portions, inserting the end portions having prewelded pleats into a molten thermoplastic fluorocarbon resin to force the resin into the pleats, whereby the end portions and the molten resin are integrally welded together, fitting fluorocarbon resin caps having a predetermined shape in said end parts and providing, if desired, a fluorocarbon resin core material for supporting the filter material inside the central opening of the resulting structure, and then integrally welding said two or three parts of the assembly thus obtained. A method as in claim 8, wherein the edge portions of the two sides of the pleat-form filter are welded together by inserting a welded tape made of a thermoplastic fluorocarbon resin between said edge portions.

Inventors: FUJIMORI, SHOICHI, HAMADA, KATUSHI, HORI, YOSHIHIRO, INOUE, AKIHISA, MIYAGI, TOKUYA
Applicants: KURASHIKI BOSEKI KABUSHIKI KAISHA
Application year: 1984
Main classification: B01D27
IPC codes: B01D29, B01D63, B01D46, B01D39, B01D61