Process of making microporous phase inversion membranes

Details Process of making microporous phase inversion membranes Process of making microporous phase inversion membranes

1999-10-05

2001-07-31

Tentoni, Leo B. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Pore forming in situ

Reexamination Certificate

active

06267916

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to methods and systems for producing a plurality of different pore size microporous phase inversion membrane each having any one of a plurality of different pore sizes from a single master dope batch. More specifically, it relates to methods and systems for selectively essentially instantaneously thermally manipulating at least a portion of a master dope batch to a temperature within about −0.2 to about −0.15° C. of a predetermined temperature which has proven to yield microporous phase inversion membrane having about a specific pore size formed therein when processed. Most specifically, it relates to methods and systems for exacting essentially instantaneous thermal manipulation to a finer degree of control over a small portion of dope incrementally processed from a master batch such that a wider range of possible pore sizes can be selectively formed in microporous phase inversion membrane produced therefrom than was previously believed possible from a single master batch of dope and in a short time frame.
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 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. A slight pressure, generally in the range of about five (5) to about fifty (50) psig (pounds per square inch gauge) is used to force fluid through the microporous phase inversion membrane. 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. The liquid and particles smaller than the pores of the membrane pass through. Thus, a microporous phase inversion membrane prevents particles of a certain size or larger from passing through it, while at the same time permitting liquid and particles smaller than that certain size to pass through. Microporous phase inversion membranes have the ability to retain particles in the size range of from about 0.01 to about 10.0 microns.
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. 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. The bubble point values for microporous phase inversion membranes are generally in the range of about five (5) to about one hundred (100) psig, depending on the pore size and the wetting fluid.
Methods and Systems for preparing the dope used to produce microporous membrane are known in the art. There are numerous methods of preparing the dope. A number of the known prior methods of dope preparation are discussed in representative U.S. Pat. No. 3,876,738 issued Apr. 8, 1975, U.S. Pat. No. 4,340,480 issued Jul. 20, 1982, U.S. Pat. No. 4,770,777 issued Sep. 13, 1988, and U.S. Pat. No. 5,215,662 issued Jun. 1, 1993, the disclosure of each is herein incorporated by reference.
One specific method for the preparation of dope (U.S. Pat. No. 3,876,738) to produce a specific pore size when processed into microporous membrane was to batch formulate the dope by polymer to nonsolvent to solvent ratio as a predictive control of pore size. Batch formulation was conducted at an assumed maximum temperature. In practice, to maintain a single precise mixing temperature over a four (4) to six (6) hour period necessary to compete the mixing cycle is very difficult. Precision in formulation and precision in the uniformity of mixing (shear history and temperature history) are equally important to the successful commercialization of phase inversion membrane having specific and controlled pore size formed therein.
During the mixing of the dope ingredients, solvent and nonsolvent were mixed first and then the polymer was added to the mixture of nonsolvent with the temperature being controlled and assumed not to exceed a certain temperature. In this formulation process, the solvent, such as, for example, formic acid, was first placed in a vessel. Next, the nonsolvent, such as, for example, methanol, was added and the nonsolvent and solvent were allowed to react and reach equilibrium. After the solvent and nonsolvent mixture reached equilibrium, the polymer, such as, for example, nylon was added and blended with the solvent and nonsolvent mixture for a sufficient amount of time and under reasonably controlled conditions of temperature and solution agitation (shear) to effect the dissolving of the nylon polymer in the solvent
on-solvent mixture until the polymer/solvent
onsolvent mixture reached equilibrium.
It is known that processing relatively large bodies of dope, such as that used in the production of microporous phase inversion membranes, is accompanied by many difficulties such as the need to formulate separate dope batches for each size pore phase inversion membrane produced as well as the problems in controlling the temperature of the dope during the batching process.
Dope that has been formulated according to a particular formulation may, due to process variables, produce out of specification phase inversion membrane. In the past, in order to salvage out of specification batches, the out-of-specific batches were reprocessed by bulk heating to a higher temperature which produced a larger pore size when reprocessed. Dope reprocessing included elevating the dope temperature of large amounts of dope such as, for example, up to one hundred (100) gallons and even larger batches to a predetermined target temperature.
Dope reprocessing was needed because, during the batch formulation process, formulation errors were introduced into the batch such as incorrect amounts of ingredients, different nylon batches and other processing errors that occurred in the batch mixing process. Because of the differences in the resulting dope batches due to different lots of nylon, different types of reactants and etc., microporous phase inversion membrane having the same exact pore size was not always produced from different batches prepared according to the same recipe each and every time. In fact, a band of predictable pore sizes for each specific formulation was developed over a period of time.
If the pore size from a particular batch of a particular formulation turned out to be too open or have larger than the maximum pore size permitted by the specification for the end use, then that batch was scrapped, due to production schedules as retaining the dope for a future run at that pore size was impractical and because it was not possible to reprocess the batch to produce phase inversion membrane having a smaller pore size. If the formulation of a specific dope batch resu

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