Filter medium for air filters

Details Filter medium for air filters Filter medium for air filters

1999-06-30

2001-04-10

Smith, Duane S. (Department: 1724)

Gas separation: apparatus

Apparatus for selective diffusion of gases

Plural layers

C055S486000, C055S487000

Reexamination Certificate

active

06214093

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to filter media for air filters using a porous polytetrafluoroethylene (hereinafter referred to as “PTFE” for simplicity) membrane.
BACKGROUND OF THE INVENTION
As filter media for air 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 and dusting caused by deterioration due to chemicals such as hydrofluoric acid. In recent years, therefore, porous PTFE membranes, which are clean materials and highly resistant to chemicals, have been employed as filter media for air filters in the fields of semiconductor industry, etc. A porous PTFE membrane can be produced by molding PTFE into sheet and then stretching the sheet to thereby make it porous (see, for example, JP-A-7-196831 and JP-A-6-816802 (PCT); the term “JP-A” as used herein means an “unexamined published Japanese patent application”). Sole use of a porous PTFE membrane thus obtained lacks rigidity. Therefore, in many cases the porous PTFE membrane is used by bonding an air-permeable supporting member to one surface there. The filter medium thus reinforced by the air-permeable supporting member is pleated so as to give a continuous W-shaped structure (hereinafter referred to as “pleating”) and employed as a filter medium unit.
On the other hand, there are proposed various methods for producing laminates of porous PTFE membranes (see, for example, JP-A-57-131236 and JP-A-3-179038). JP-A-9-504737 (PCT) proposes a filter medium using such a laminate.
Namely, JP-A-9-504737 (PCT) discloses a filter medium showing a high collection efficiency which comprises a laminate comprising five layers of porous PTFE membranes having the same pressure drop, and air-permeable supporting members located at both sides of the laminate. In this filter medium, the air-permeable supporting members are loosely pleated together with the porous PTFE membrane laminate but not bonded thereto.
In a filter medium having an air-permeable supporting member bonded to one surface of a porous PTFE membrane, however, some of pores of the porous PTFE membrane are clogged by bonding and thus the pressure drop largely increases. On the other hand, a filter medium having air-permeable supporting members located at both sides of a laminate does not suffer from any decrease in the pressure drop, since the supporting members are not bonded to the laminate. In this case, however, the laminate and the supporting members are not integrated and thus are poor in handling characteristics. In this case, furthermore, the supporting members are not bonded to the laminate and thus sometimes give deviations in the step of the pleating, thereby causing undesirable bends in the laminate or supporting members.
SUMMARY OF THE INVENTION
The present invention has been made to overcome the above-described problems encountering in the prior art.
Accordingly, an object of the present invention is to provide a filter medium for air filters being capable of exerting the collection performance inherent to porous PTFE membranes even after processing (for example, pleating, bonding to air-permeable supporting members, etc.) and showing excellent processability and handling characteristics by improving the porous PTFE membrane laminate.
To achieve the above object, the present invention uses a porous membrane capable of sufficiently relieving influence of the processing as described above as the outermost layer of a laminate of porous PTFE membranes.
The filter medium for air filters according to the present invention comprises as a collection layer a laminate comprising at least three layers of porous PTFE membranes, wherein the laminate comprises a first porous PTFE membrane located as an outermost layer and a second porous PTFE membrane located as at least one intermediate layer excluding the outermost layer, and the first porous PTFE membrane has a pressure drop of ½ time or less a pressure drop of the second porous PTFE membrane.


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