Gas separation – Two or more separators – Plies or layers of different characteristics or orientation
Reexamination Certificate
1999-11-09
2001-10-16
Spitzer, Robert H. (Department: 1724)
Gas separation
Two or more separators
Plies or layers of different characteristics or orientation
C055S524000, C055S528000, C096S013000
Reexamination Certificate
active
06302934
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to filter media for filters with the use of porous polytetrafluoroethylene (hereinafter referred to as “PTFE” for simplicity) membranes. More particularly, it relates to filtering media for filters with the use of porous PTFE membranes which are appropriately used for collecting particles suspended in the atmosphere in clean rooms used in the fields of semiconductors, drugs, etc. It also relates to filtering media for filters (for example, filtering media for vent filters) with the use of porous PTFE membranes which are appropriately used for collecting dust invading into hard disks or dust formed in hard disks.
BACKGROUND OF THE INVENTION
As filter medium for filters, there have been frequently used those produced by blending glass fibers with binders and processing into a paper. However, these filter media suffer from some problems, for example, self-dusting due to fine fibers contained therein, self-dusting at bending process and dusting caused by deterioration due to contact with chemicals such as hydrofluoric acid.
In recent years, filter media with the use of porous PTFE membranes, which are clean materials and highly resistant to chemicals, have attracted attention in the field of air filter technology. A porous PTFE membrane can be produced by, for example, molding PTFE into a sheet and then stretching the sheet to thereby make it porous (see, for example, WO94/16802, JP-A-10-30031; the term “JP-A” as used herein means an “unexamined published Japanese patent application”). Because of having a low pressure drop and a high collection efficiency, a porous PTFE membrane produced via the pore-forming step as described above has an excellent collection performance. Therefore, filter media with the use of porous PTFE membranes are suitable particularly for achieving a highly clean environment required in clean rooms used in the fields of semiconductors, etc.
However, an adequate rigidity for filter media can be hardly established by using a porous PTFE membrane alone. Therefore, in many cases, a porous PTFE membrane is bonded to an air-permeable porous material or laminated thereon to give a filter medium for filters. As the air-permeable material serving as the reinforcing member, use is generally made of nonwoven fabrics.
An air filter unit obtained by pleating a laminate consisting of a nonwoven fabric and a porous PTFE membrane can actually establish a remarkably excellent performance, i.e., collecting particles (0.1 &mgr;m) at an efficiency of 99.999999% or above. Namely, filter media for filters with the use of porous PTFE membranes have characteristics fundamentally appropriate for improving a high environmental cleanness required in clean rooms, etc.
However, these conventional filter media for filters consisting of a nonwoven fabric and a porous PTFE membrane suffer from problems of pollution caused by self-dusting from the nonwoven fabric and pollution with organic gases evolved from the nonwoven fabric. In particular, it is pointed out that organic gases (for example, dioctyl phthalate, 2,6-di-tert-butyl-p-cresol and dibutyl phthalate, hereinafter referred to respectively as “DOP”, “BHT” and “DBR” for simplicity) evolved from the conventional filter media exert unfavorable effects on products in the field of manufacturing semiconductors.
SUMMARY OF THE INVENTION
An object of the present invention is to overcome the above-described problems by taking advantage of the characteristics inherent to porous PTFE membranes to thereby provide a filter medium for filters being capable of establishing improved environmental cleanness.
To achieve the above object, the filter medium of the present invention comprises a laminate comprising a porous ultrahigh-molecular weight polyolefin membrane and a porous polytetrafluoroethylene membrane. Because of having as a reinforcing member a porous ultrahigh-molecular weight polyolefin membrane as a substitute for the nonwoven fabrics employed in the conventional ones, the filter medium according to the present invention can provide improved environmental cleanness compared with the existing ones.
The term “ultrahigh molecular weight” as used herein means having an average molecular weight of at least 500,000, preferably from 500,000 to 16,000,000, when determined by the viscosity method (ASTM D4020).
It is preferable that the filter medium for filters according to the present invention shows a degree of dusting of 500 particles/cf or less in a tear crease-flex test. The present invention makes it possible to provide a high performance filter medium for filters wherein the degree of dusting has been reduced to the level as defined above.
It is also preferable that the filter medium for filters as described above contains 2 &mgr;g/g or less (2 ppm or less) of a compound selected from among DOP, BHT and DBP when determined by gas chromatography. The present invention makes it possible to depress the evolution of the organic gases as described above which are harmful particularly in the field of manufacturing semiconductors.
In the filter medium for filters as described above, it is preferable that the porous ultrahigh-molecular weight polyolefin film is a porous film of ultrahigh-molecular weight polyethylene (hereinafter referred to as “UHMWPE” for simplicity). It is still preferable that this UHMWPE has a viscosity-average molecular weight of from 500,000 to 16,000,000. However, use may be made of other ultrahigh-molecular weight polyolefins (for example, ultrahigh-molecular weight polypropylene) as the porous ultrahigh-molecular weight polyolefin membrane.
Also, it is preferable that, in the above-described filter medium for filters, ultrahigh-molecular weight polyolefin particles in the porous ultrahigh-molecular weight polyolefin membrane are bonded to each other so as to form a porous structure provided with the voids among these particles. Such a porous structure can be confirmed by, for example, observing under an electron microscope.
REFERENCES:
patent: 4816328 (1989-03-01), Saville et al.
patent: 4877433 (1989-10-01), Oshitari
patent: 5019140 (1991-05-01), Bowser et al.
patent: 5096473 (1992-03-01), Sassa et al.
patent: 5108474 (1992-04-01), Riedy et al.
patent: 5417743 (1995-05-01), Dauber
patent: 5507847 (1996-04-01), George et al.
patent: 5538545 (1996-07-01), Dauber et al.
patent: 5593482 (1997-01-01), Dauber et al.
patent: 5830261 (1998-11-01), Hamasaki et al.
patent: 5876487 (1999-03-01), Dahlgren et al.
patent: 5888275 (1999-03-01), Hamasaki et al.
patent: 6030428 (2000-02-01), Ishino et al.
patent: 6110249 (2000-08-01), Medcalf et al.
patent: 6149702 (2000-11-01), Kawano et al.
patent: 0 633 053 A1 (1995-01-01), None
patent: 0 808 648 A1 (1997-11-01), None
patent: 10-30031 (1998-02-01), None
patent: WO 94/16802 (1994-08-01), None
Patent Abstract of Japan, vol. 1995, No. 10, 07-171,318 (07/11/95).
Kawano Eizo
Maeoka Takuya
Nabata Norikane
Nagai Yozo
Nitto Denko Corporation
Spitzer Robert H.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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