Liquid purification or separation – Filter – Material
Patent
1994-12-22
1998-06-30
Fortuna, Ana
Liquid purification or separation
Filter
Material
21050027, 210490, 2103155, 2642107, 2642888, 2642902, 2642884, B01D 3900
Patent
active
057728844
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a porous film comprising polytetrafluoroethylene (referred to as "PTFE" hereinafter) and to a process for preparing the same. More particularly, the present invention relates to a novel porous PTFE film which is excellent as an air filter medium, particularly an ULPA filter medium which is used for cleaning internal atmospheres in clean rooms for a semiconductor industry and in productive facilities of precision electronic instruments, medicines, biochemicals, etc., and as a liquid filter medium, and to a process for preparing the same.
DESCRIPTION OF THE PRIOR ART
Recent semiconductor, precision and biotechnology industries require completely cleaned air or highly cleaned chemicals in their manufacturing processes. Particularly, the internal atmosphere of the equipment in the semiconductor industry is required to be highly cleaned with the development to high integration of semiconductors. The cleaned air is now obtained by filtering a circulating air through an air filter. The chemicals used in the semiconductor industry are also required to be highly pure. Their cleaning is effected by circulating the chemicals through a filter.
Various filters used for such the purposes are hitherto proposed and practically used. An example of the air filters mainly used now is a filter medium prepared by making paper using a mixture of glass fiber and a binder. However such a filter medium has several disadvantages, for example, existence of microfibers attached thereto, production of self-dust during folding it in processing or occurrence of dust due to deterioration of the glass fiber and the binder by contact with certain chemicals such as hydrogen fluoride, etc.
An electret filter medium of a synthetic fiber is proposed to solve such problems (see Japanese Patent Kokai No. 53365/1979). However, it is shown that the electrostatic force generated in the electret filter medium is masked to lose its effect when it collects microparticles and its collection effect is thus lowered (see the 11th Air Cleaning and Contamination Control Research Congress, pages 153-156). Thus, in order to avoid the disadvantage, auxiliary use of a porous stretched PTFE film is proposed to obtain a cleaned atmosphere (see Japanese Patent Kokoku No. 10364/1992 and Japanese Patent Kokai No. 284614/1990).
In these proposals, porous PTFE films having pore sizes of not less than 1 .mu.m are used to prevent the increase of pressure drop. The reason why floating particles in air having smaller sizes than the pore size of the film can be captured by the film is found to be based on the below described theories.
The main mechanisms for removing particles in a fluid are as follows (see the catalogue of Domnick Hunter Filter Limited): micro fibers to be removed as if they were sieved. among microfibers, they cannot so rapidly change a course as a gas does, and, after all therefore collide against the microfibers to be captured thereby. intermolecular forces or electrostatic forces to make a rotary motion in a gas, so that they increase in apparent size so as to attach to the microfibers as in the inertia collision.
However, a particle having a particle size of not more than 1 .mu.m cannot be completely removed as shown in the data described in Japanese Patent Kokai No. 284614/1990.
In general, a microporous configuration having a small pore size is needed to capture microparticles to enhance collection efficiency. However, the microporous configuration inevitably increases pressure drop, resulting in a high operating cost. In order to prevent the increase of the pressure drop, it may be contemplated to reduce the thickness of the film.
There are generally two ways for preparing a stretched PTFE film having a small thickness: In one way a film as thin as possible is used prior to stretching, while the stretch ratio of a film is increased in the other way. Since the PTFE film prior to stretching is generally prepared by paste extrusion and subsequent rolling, the thickness of the film industrial
REFERENCES:
patent: 4187390 (1980-02-01), Gore
patent: 4604170 (1986-08-01), Miyake et al.
patent: 4902423 (1990-02-01), Bacino
patent: 5110527 (1992-05-01), Harada et al.
patent: 5328760 (1994-07-01), Gillberg-LaForce
Asano Jun
Chaen Shinichi
Inoue Osamu
Kusumi Toshio
Tamaru Shinji
Daikin Industries Ltd.
Fortuna Ana
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