Pre-metered, unsupported multilayer microporous membrane

Details Pre-metered, unsupported multilayer microporous membrane Pre-metered, unsupported multilayer microporous membrane

2002-02-07

2004-05-18

Fortuna, Ana (Department: 1723)

Liquid purification or separation

Filter

Material

C210S490000, C210S500410, C210S500420, C264S041000, C427S244000, C427S245000

Reexamination Certificate

active

06736971

ABSTRACT:

BACKGROUND OF THE DISCLOSURE
The present disclosure relates to continuous, unsupported, microporous membranes having two or more distinct, but controlled pore sizes and to processes of making and using same, more particularly to unsupported microporous membranes made from a first dope and at least one additional dope being applied directly to one another prior to the at least two dopes being quenched and to apparatus for manufacturing and processes for making such membrane.
Microporous phase inversion membranes are well known in the art. Microporous phase inversion membranes are porous solids which contain microporous interconnecting passages that extend from one surface to the other. These passages provide tortuous tunnels or paths through which the liquid which is being filtered must pass. The particles contained in the liquid passing through a microporous phase inversion membrane become trapped on or in the membrane structure effecting filtration. The particles in the liquid that are larger than the pores are either prevented from entering the membrane or are trapped within the membrane pores and some particles that are smaller than the pores are also trapped or absorbed into the membrane pore structure within the pore tortuous path. The liquid and some particles smaller than the pores of the membrane pass through. Microporous phase inversion membranes have the ability to retain particles in the size range of from about 0.01 or smaller to about 10.0 microns or larger.
Many important micron and submicron size particles can be separated using microporous membranes. For example, red blood cells are about eight (8) microns in diameter, platelets are about two (2) microns in diameter and bacteria and yeast are about 0.5 microns or smaller in diameter. It is possible to remove bacteria from water by passing the water through a microporous membrane having a pore size smaller than the bacteria. Similarly, a microporous membrane can remove invisible suspended particles from water used in the manufacture of integrated circuits in the electronics industry.
Microporous membranes are characterized by bubble point tests, which involve measuring the pressure to force either the first air bubble out of a fully wetted phase inversion membrane (the initial Bubble Point, or “IBP”), and the higher pressure which forces air out of the majority of pores all over the phase inversion membrane (foam-all-over-point or “FAOP”). The procedures for conducting initial bubble point and FAOP tests are discussed in U.S. Pat. No. 4,645,602 issued Feb. 24, 1987, the disclosure of which is herein incorporated by reference to the extent not inconsistent with the present disclosure. The procedure for the initial bubble point test and the more common Mean Flow Pore tests are explained in detail, for example, in ASTM F316-70 and ANS/ASTM F316-70 (Reapproved 1976) which are incorporated herein by reference to the extent not inconsistent with the present disclosure. The bubble point values for microporous phase inversion membranes are generally in the range of about two (2) to about one hundred (100) psig, depending on the pore size and the wetting fluid.
An additional method which describes a pore measurement technique is ASTM E1294 89 which describes a method for determining pore size by clearing fluid from the pores of the membrane and measuring the resulting flow. This method is used to measure mean flow pore but is similar to the method of Forward Flow Bubble Point in that the wet portion of the test uses a similar protocol.
The Forward Flow Bubble Point (FFBP) test is described in U.S. Pat. No. 4,341,480 by Pall et. al., the disclosure of which is herein incorporated by reference to the extent not inconsistent with the present disclosure. This patent discloses how the FFBP can be used to distinguish a symmetric membrane from an asymmetric membrane. The FFBP curve is generated by saturating the membrane with fluid and subjecting one side to a rising air pressure while measuring air flow on the downstream side. For a single layer symmetric membrane with a well defined pore size, a plot of air flow versus pressure remains flat but slightly above zero due to diffusion through the liquid in the membrane. When the pressure reaches a point when it can overcome the surface tension of the fluid in the pore, the air will push the liquid out of the pore and air will flow through the pore in bulk (bulk flow). This pressure point is a function of the surface tension of the liquid and the radius of the pore as defined by the equation of Young and Laplace (see
Physical Chemistry of Surfaces
by Arthur Adamson, Wiley Press). When the pores all have essentially the same size, this event occurs simultaneously and is characterized by a transition of the flow versus pressure curve from horizontal (when diffusion flow is predominant) to vertical (where bulk flow is dominant), this type of FFBP characteristic is shown in FIG.
9
.
FIG. 9
also demonstrates that the FFBP characteristics for a single layer symmetric membrane are identical regardless of membrane orientation.
On the other hand, asymmetric membranes are characterized by a gradual change in pore size throughout the thickness and exhibit a different FFBP curve, when tested with the large pore size surface facing up stream against the applied air pressure. Since the pore size is gradually changing throughout the thickness depth, the pressure required to push fluid down the pores rises gradually and the resulting FFBP curve has a rising slope until the final bubble point is reached and bulk flow occurs. While an asymmetric membrane might be retentive, the above response is indistinguishable from an asymmetric membrane with defects, where certain pores are significantly larger than the remaining pores and exhibit bulk flow at lower pressure. The FFBP response of this type of membrane also exhibits a rising slope when flow versus pressure is plotted.
U.S. Pat. No. 3,876,738, the disclosure of which is herein incorporated by reference to the extent not inconsistent with the present disclosure, describes a process for preparing microporous membranes by quenching a solution of a film-forming polymer in a non-solvent system for the polymer. U.S. Pat. No. 4,340,479, the disclosure of which is herein incorporated by reference to the extent not inconsistent with the present disclosure, generally describes the preparation of skinless microporous polyamide membranes by casting a polyamide resin solution onto a substrate and quenching the resulting thin film of polyamide.
There is an extensive body of knowledge concerning the production of multiple layer films using pre-metered coating technology, such as, for example, slot dies, as taught by. This prior art deals with the extrusion of films that are essentially impermeable. This prior art also discusses manufacture of both photographic film and films used in the packaging industry (e.g. food packaging). Some examples of patents, each of which are herein incorporated by reference to the extent not inconsistent with the present disclosure, disclosing multilayer films are listed in the table below:
Patent
Issued
Inventor(s)
Title
US6040392
2000
Khanna et al.
Nylon 6 or 66 Based
Compositions and Films Formed
Therefrom Having Reduced
Curl.
US5962075
1999
Sartor et al.
Method of Multilayer Die
Coating Using Viscosity
Adjustment
US5741549
1998
Maier et al.
Slide Die Coating Method and
Apparatus with Improved Die
Tip
US5256357
1993
Hayward
Apparatus and Method for Co-
casting Film Zones
US4854262
1989
Chino et al.
Coating Apparatus
US4001024
1977
Dittman et al.
Method of Multilayer Coating
At least some of the above prior art teaches the use of pre-metered dies to apply coatings in the production of essentially non-porous films. Discussion of pre-metered dies can be found in two Troller Schwiezer Engineering (TSE) publications, “Concepts and Criteria for Die Design” and “Precision Coating: Pre-metered and Simultaneous Multilayer Technologies,” which are available from TSE upon request. Pre-metered coating methods comprise slot, extrus

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