Fabric (woven – knitted – or nonwoven textile or cloth – etc.) – Nonwoven fabric – Including a foamed layer or component
Reexamination Certificate
1999-05-28
2001-07-17
Copenheaver, Blaine (Department: 1771)
Fabric (woven, knitted, or nonwoven textile or cloth, etc.)
Nonwoven fabric
Including a foamed layer or component
C055S521000, C210S493500, C210S500360, C428S182000, C428S315500
Reexamination Certificate
active
06261979
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a filter medium used for cleaning air in clean rooms, equipment for manufacturing liquid crystal or semiconductor, and the like, and to an air filter unit using the same.
TECHNICAL BACKGROUND
A high-performance air filter which has a folded filter medium made from glass fiber through a wet process provides clean air, and thus has contributed to the semiconductor industry. Also, high integration of LSI has been developed, and minimum pattern dimensions of a device have been reduced. Thus, the size of fine particles which should be removed has been decreasing. Accordingly, ULPA (Ultra Low Penetration Air) filters having greater performance than HEPA (High Efficiency Particulate Air) filters now are often used in clean rooms for semiconductor manufacture.
However, borosilicate glass fiber used in a filtration medium of a conventional ULPA filter is attacked by hydrofluoric acid, which is used in manufacturing semiconductor, and BF
3
gas is generated. The boron accumulates on a silicon wafer in the manufacturing process of semiconductor, thus causing abnormal characteristics. Thus, it has become an urgent matter to deal with this problem.
Recently, particular attention has been paid to a high-performance air filter using a filter medium made from polytetrafluoroethylene (hereinafter referred to as “PTFE”) which does not generate boron (e.g. Japanese Publication of Unexamined Patent Application (Tokkai) No. HEI 5-202217, or WO94/16802).
Such an air filter made from PTFE is free of the problem of off-gassing of boron etc. because the PTFE used in the filter medium is very clean and has excellent chemical resistance. Furthermore, the air filter made from PTFE also can achieve the same or higher collection efficiency of floating fine particles as the above-mentioned ULPA filter made from glass fiber. Moreover, because the PTFE air filter displays a considerably lower pressure loss than the glass fiber filter when they have the same collection efficiency (for example, the pressure loss of a ULPA filter made from PTFE is two third that of a ULPA filter made from glass fiber), the energy cost for the operation of the filter can be reduced. Because of these characteristics, the use of an air filter made from PTFE is spreading rapidly into various industries such as the semiconductor industry.
However, the requirement for cost reduction is severe in many industries. Therefore, it is desired to further develop a PTFE filter medium that satisfies the conditions “further improved collection efficiency and a lower pressure loss to reduce the cost for the filter operation” at the same time and an air filter unit using such a filter medium (not limited to ULPA type, but also including HEPA type and a medium-performance type).
However, an increase in collection efficiency and a reduction in pressure loss contradict each other (see, for example, Nitto Giho, Vol. 34, No. 1 (May, 1996)). Thus, it is not easy to solve the above-mentioned problems, and a satisfactory product has not yet been obtained.
Also, as is described in the above-mentioned Japanese Publication of Unexamined Patent Application (Tokkai) No. HEI 5-202217 or WO94/16802, for example, it is considered that when a conventional PTFE filter medium is used as a high-performance air filter, the aim of the filter cannot be attained unless its average pore diameter is in the range of 0.2 to 0.5 &mgr;m. However, it is not easy to control the average pore diameter within this very small and narrow range at all times.
DISCLOSURE OF THE INVENTION
Accordingly, it is an object of the present invention to provide a filter medium that has both a collection efficiency capable of attaining air cleanness required for each of medium-performance, HEPA, and ULPA type filters, and a pressure loss capable of achieving reduction of energy cost, which has not been possible to achieve in any of the above-mentioned filter types, and that also enables easy control of the average pore diameter, and an air filter unit using the same.
In order to accomplish the above object, the present invention provides a filter medium comprising a PTFE porous membrane, in which an average pore diameter of the PTFE porous membrane is more than 0.5 &mgr;m, and a pressure loss when air is passed through the medium at 5.3 cm/sec is from 2 to 50 mmH
2
O, and a PF value, which is calculated from the pressure loss and a collection efficiency using dioctylphthalate (hereinafter referred to as “DOP”) of a particle size of 0.10 to 0.12 &mgr;m in accordance with the following equation, is between 18 and 22:
(Equation 1)
PF
value
=
-
log
(
permeability
(
%
)
/
100
)
Pressure
Loss
×
100
Permeability (%)=100−Collection Efficiency (%)
Thus, the filter medium of the present invention displays excellent performance in both collection efficiency and pressure loss and also enables easy control of an average pore diameter, by having an average pore diameter, a pressure loss, and a PF value within the above specified ranges. It is particularly of note that the average pore diameter of the filter medium of the present invention is larger than the range of the average pore diameter that has been considered as a common size (0.2 to 0.5 &mgr;m). That is, the average pore diameter is more than 0.5 &mgr;m, so that the average pore diameter of the filter medium of the present invention can be controlled easily.
Furthermore, the PF value is an indicator showing a balance between the collection efficiency and the pressure loss. According to the PF value, for example, among two or more filter media having equal collection efficiency, the larger the PF value, the smaller the pressure loss.
In the filter medium of the present invention, it is preferable that the PTFE porous membrane is formed from intertwining PTFE fibers, and the PTFE fibers have an average fiber diameter in the range of 0.1 to 0.2 &mgr;m. This is because, when the average diameter of the PTFE fibers is within this range, the PF value falls in the above specified range sufficiently at the above-mentioned average pore diameter and pressure loss.
It is preferable that the filter medium of the present invention displays a pressure loss from 2 to 45 mmH
2
O when the average pore diameter of the PTFE porous membrane is from 0.58 to 5 &mgr;m and when air is passed through the medium at a flow velocity of 5.3 cm/sec.
It is preferable that the filter medium of the present invention displays a pressure loss from 2 to 45 mmH
2
O when the average pore diameter of the PTFE porous membrane is from 1.1 to 3 &mgr;m and when air is passed through the medium at a flow velocity of 5.3 cm/sec.
It is preferable that the filter medium of the present invention has a collection efficiency of at least 40% using DOP of a particle size of 0.10 to 0.12 &mgr;m. A collection efficiency in this range becomes equal to or higher than the collection efficiency of a medium-performance filer unit when the filter medium is incorporated in a filter unit.
Generally, a medium-performance filter unit is used as a prefilter, which roughly removes dust from an atmosphere directly taken therein at the first place, and is required to have a collection efficiency of at least 90% for DOP of a particle size of 0.3 &mgr;m (in conversion, a collection efficiency of at least 60% for DOP of a particle size of 0.10 to 0.12 &mgr;m). Moreover, the collection efficiency of a filter unit is measured according to a method mentioned below. To obtain a collection performance of a filter unit equal to or higher than this range, the filter medium of the present invention should have a collection efficiency within the above-mentioned range.
It is preferable that the filter medium of the present invention has at least 99.0% for the collection efficiency using DOP of a particle size of 0.10 to 0.12 &mgr;m. A collection efficiency in this range becomes equal to or higher than the collection efficiency of a glass fiber HEPA filter unit when the filter medium is incorporated in a filt
Kusumi Toshio
Shibuya Yoshiyuki
Tanaka Osamu
Tano Takeshi
Copenheaver Blaine
Daikin Industries Ltd.
Merchant & Gould P.C.
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