Unsupported multizone microporous membrane

Details Unsupported multizone microporous membrane Unsupported multizone microporous membrane

2001-08-07

2004-03-16

Fortuna, Ana (Department: 1723)

Liquid purification or separation

Filter

Material

C210S490000, C210S500410, C210S500420, C264S041000, C427S244000, C427S245000

Reexamination Certificate

active

06706184

ABSTRACT:

BACKGROUND OF 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.
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.
Multizone membrane offers much greater life and flow than conventional membranes while still maintaining adequate control of the thickness and pore size of each zone to ensure reliable retention. A three-zoned membrane may contain a tight or relatively small pore size zone sandwiched between two open or relatively large pore size zones. The open or relatively large pore size zones would not restrict flow but would serve to protect the tight or relatively small pore size zone from abrasion or damage, allowing it to be much thinner and still maintain integrity. Materials such as polyvinylidene fluoride (PVDF) or polyether sulfone (PES) do not require reinforcement because of their inherent strength. Scrimless membranes however, require a coating surface, such as, for example, a belt or drum to support them when cast. The die or dope applying apparatus required to cast multiple microporous membrane zones must be practically located on the same side of the coating surface and designed to control the membrane properties. Potential design constraints are discussed below. The membrane pore size of each zone can be controlled through polymer content, solvent and nonsolvent amount and temperature history of the dissolved dope. Potential constraints on these variables will be discussed below.
There is an extensive body of knowledge concerning multiple ply films and slot die technology. 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 multizone films are listed in the table below:
Patent
Issued
Inventor(s)
Title
U.S. Pat. No.
2000
Khanna et. al.
Nylon 6 or 66 Based
6040392
Compositions and Films
Formed Therefrom Having
Reduced Curl.
U.S. Pat. No.
1999
Luigi et. al.
Method of Multizone Die
5962075
Coating Using Viscosity
Adjustment
U.S. Pat. No.
1998
Maier et. al.
Slide Die Coating Method
5741549
and Apparatus with Improved
Die Tip
U.S. Pat. No.
1993
Hayward
Apparatus and Method for
5256357
Cocasting Film Zones
U.S. Pat. No.
1989
Chino et. al.
Coating Apparatus
4854262
U.S. Pat. No.
1977
Dittman et. al.
Method of Multizone Coating
4001024
Other art involves the manufacture of microporous membranes by other techniques. Grandine provides the first practical disclosure of the manufacture of PVDF membrane. The Grandine patent (U.S. Pat. No. 4,203,847) discloses, although does not claim, that thermal manipulation of the dope will lead to a change in pore size of the resulting membrane. Surprisingly, given that nylon is a very different polymer that is dissolved in ionic organic acids rather than an organic ketone, it experiences a similar phenomenon. Grandine did not suggest a mechanism for this phenomenon to indicate that it might be general for polymers used to make membranes.
Subsequent patents relating to PVDF disclose methods for making asymmetric PVDF membrane. The Wang patent (U.S. Pat. No. 5,834,107) discloses a variety of methods to manufacture asymmetric membrane. Other patents that are related to asymmetric structure and which are cited in the Wang patent are Costar (WO 93/22034), Sasaki (U.S. Pat. No. 4,933,081), Wrasidlo (U.S. Pat. No. 4,629,563 & U.S. Pat. No. 4,774,039), and Zepf (U.S. Pat. Nos. 5,188,734 & 5,171,445).
Other prior art is the use of thermal manipulation to create distinct zones of controlled pore size with nylon membrane by Meyering et. al. (application WO99/47246, the disclosure of which is incorporated herein by reference to the extent not inconsistent with the present disclosure) applying two layers of dope against opposite sides of a support scrim after the scrim was filled with a first dope. In some applications, especially pleated cartridge filters, Nylon is an intrinsically weak material which requires the use of a scrim to function in particular applications effectively, but unreinforced or unsupported nylon is used in other applications. The presence of the reinforcing or supporting scrim requires multiple dies, one to provide dope within and to fill the scrim for the middle membrane zone and the other two dies to apply the

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