Gas separation – Two or more separators – Plies or layers of different characteristics or orientation
Reexamination Certificate
2000-11-09
2004-01-27
Smith, Duane (Department: 1724)
Gas separation
Two or more separators
Plies or layers of different characteristics or orientation
C055S487000, C055S495000, C055S497000, C055S524000, C055SDIG005, C156S324000
Reexamination Certificate
active
06682576
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to air filter media. More specifically, the present invention relates to filter media for use in air cleaning operation of clean rooms, liquid crystals and/or semiconductor production devices. The present invention further relates to an air filter pack having the filter medium. The present invention still further relates to an air filter unit having the air filter pack.
2. Background Information
In a clean room or in a semiconductor production device, electronic parts such as semiconductors and liquid crystal display devices are produced in an environment in which the air is treated to capture floating particles. A porous film having polytetrafluoroethylene (hereinafter referred to as “PTFE” for simplicity) has been used as filter medium for high performance air filters used for cleaning air of such clean rooms, liquid crystals and/or semiconductor production devices. Such porous PTFE film is used usually in the form of a laminated film, with air-permeable supporting members interposing the porous PTFE film from both sides thereof in order to increase strength and easiness in handling.
Collection efficiency and pressure loss are generally known as properties that represent performance of an air filter medium. Specifically, collection efficiency shows the ability of a filter medium to capture floating fine particles in the air. Pressure loss indicates a degree of pressure loss of air that passes through the filter medium. Since porous PTFE film is made from finer fibers than fibers from which a conventional glass filter medium is made, PTFE porous film has high collection efficiency and small pressure loss.
However, when porous PTFE film is laminated by heat fusion with non-woven fabrics, or when the laminate is processed to be shaped into a predetermined shape, fiber structure of the porous PTFE film changes due to pressure from the non-woven fabric. As a result, defects may result in the filter medium, the pressure loss of the filter medium may increase, and/or the collection efficiency thereof may decrease.
In view of the above, there exists a need for filter medium which overcomes the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an air filter medium having high performance by reducing an increase in its pressure loss and a decrease in its collection efficiency.
Another object of the present invention is to provide a process of manufacturing the high performance air filter medium.
Still another object of the present invention is to provide an air filter pack having the high performance air filter medium.
Yet still another object of the present invention is to provide an air filter unit having the air filter pack.
According to a first aspect of the present invention, an air filter medium is used for air cleaning, and comprises a porous film and a non-woven fabric. The porous film is made of a polytetrafluoroethylene. The non-woven fabric is laminated on at least one side of the porous film. The non-woven fabric on at least one side of the porous film has an apparent density satisfying the following equation:
apparent density (g/cm
3
)<1.5×(basis weight (g/m
2
)/1000)+0.11.
It has been found through studies by the applicants of the present invention that hardness of the non-woven fabric used in the air filter medium contributes to the degree of damage that the porous PTFE film receives in laminating the film or other occasions. Further, the applicants have also found that an apparent density of the non-woven fabric is one of factors that determine the hardness of the non-woven fabric.
In view of the above findings, according to the first aspect of the present invention, a non-woven fabric having an apparent density of less than a predetermined value is laminated on at least one side of the porous PTFE film. More specifically, a non-woven fabric softer than the conventional one is laminated on the porous PTFE film. Owing to this structure, the degree of damage that the porous PTFE film receives from the non-woven fabric at the time of lamination is decreased. Also, the degree of increase in pressure loss of the air filter medium and the degree of decrease in collection efficiency thereof are suppressed. As a result, a high performance air filter medium can be obtained.
According to a second aspect of the present invention, in the air filter medium of the first aspect of the present invention, the non-woven fabric on at least one side of the porous film has a compressibility satisfying the following equations:
compressibility (%)<0.2×(basis weight (g/m
2
))+66, and
compressibility (%)=(
d
1000
(&mgr;m)/
d
20
(&mgr;m))×100,
where d
1000
(&mgr;m) is a thickness (&mgr;m) under a load of 98 (kPa), and d
20
(&mgr;m) is a thickness (&mgr;m) under a load of 1.96 (kPa).
In the conventional air filter medium, a relatively hard material has been used as a non-woven fabric for the supporting material. As a result, the damage that the porous P film receives was large. It has been found through the studies by the applicants of the present invention that compressibility of the non-woven fabric is one of factors that determine the hardness of the non-woven fabric.
Therefore, in the air filter medium according to the second aspect of the present invention, a non-woven fabric having a compressibility of less than a predetermined value is laminated on at least one side of the porous PTFE film. More specifically, a non-woven fabric softer than the conventional one is laminated on the porous PTFE film. Owing to this structure, the degree of damage that the porous PTFE film receives from the non-woven fabric at the time of lamination is decreased, and the degree of increase in pressure loss of the air filter medium and the degree of decrease in collection efficiency thereof are suppressed. As a result, a high performance air filter medium can be obtained.
According to a third aspect of the present invention, in the air filter medium of the first or second aspect of the present invention, the non-woven fabric on at least one side of the porous film is made of composite fibers having a core-shell structure having a core portion and a shell portion. The core portion is made of a first material with a first melting point. The shell portion is made of a second material with a second melting point. The first melting point is higher than the second melting point.
It has been found through the studies by the applicants of the present invention that, in heat laminating the non-woven fabric made of composite fibers having a core-shell structure on the porous PTFE film, the shell portion is adhered to the porous PTFE film at many fine points, whereby an adhesive layer which is difficult to peel is formed. Also, the air filter medium is prevented from clogging. The applicants have also found that almost no heat shrinkage takes place in the non-woven fabric made of core-shell composite fibers at the time of lamination.
In view of the above findings, according to the third aspect of the present invention, the air filter medium uses the non-woven fabric formed from core-shell composite fibers, and the non-woven fabric has the apparent density or the compressibility having a value less than the predetermined value.
In such an air filter medium, almost no heat shrinkage takes place in the non-woven fabric when the fabric is subjected to heat lamination, and as a result, the degree of damage that the porous PTFE film receives from the non-woven fabric at the time of lamination is suppressed effectively.
According to fourth to sixth aspects of the present invention, a process of producing an air filter medium is a production process of the air filter medium of the first, second and third aspects of the present invention. The process of producing an air f
Inoue Osamu
Inui Kunihiko
Kiyotani Hideyuki
Shibuya Yoshiyuki
Tanaka Osamu
Daikin Industries
Greene Jason M.
Shinjyu Global IP Counselors, LLP
Smith Duane
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