Liquid purification or separation – Filter – Material
Reexamination Certificate
2000-03-15
2003-09-23
Kim, John (Department: 1723)
Liquid purification or separation
Filter
Material
C210S500100, C210S500210, C096S010000, C264S041000, C264S176100, C264S177130, C264S177140, C264S177170, C264S177190, C264S17800F, C264S209100, C264S211120, C264S211130, C264S211140, C264S211170
Reexamination Certificate
active
06623639
ABSTRACT:
BACKGROUND OF THE INVENTION
Microporous flat sheet and hollow fiber membranes are well known in the art. See, for example, U.S. Pat. Nos. 4,230,463 and 4,772,391. Such membranes are typically made by a solution-casting process (flat sheets) or by a solution precipitation process (hollow fibers), wherein the polymer is precipitated from a polymer/solvent solution. Conventional polymers used for microporous membrane formation by solution precipitation are not resistant to the solvents used to form the polymer solution for the casting or spinning fabrication, or to solvents of similar strength since such solvents dissolve or swell the polymer. Thus, membranes made from conventional polymers cannot be used to treat feed streams containing solvents or other harsh chemicals.
The manufacture of solvent-resistant membranes from polyimides is well known in the art. See, for example, commonly assigned U.S. Pat. No. 5,753,008. This patent discloses a process for spinning a fiber from a precursor polymer, and then rendering the fiber solvent-resistant in a post-casting step. Such membranes are indeed solvent-resistant. However, polyimides are known to be susceptible to hydrolysis when exposed to water at elevated temperatures. As a result, these solvent-resistant microporous polyimide fibers are not suitable for applications where the stream to be treated is hot and contains water.
One polymer that has been shown to be stable to hot water is polybenzimidazole (PBI). PBI has also been shown to be chemically resistant after crosslinking. See, for example, U.S. Pat. Nos. 4,693,824, 4,020,142, 3,720,607, 3,737,042, 3,841,492, 3,441,640, 4,693,825, 4,512,894 and 4,448,687. In these patents, various processes for making membranes from PBI are disclosed. However, the resulting membranes are not microporous, but instead have a dense skin on at least one surface, resulting in low permeation rates. These patents also disclose a number of techniques for crosslinking the PBI membranes. However, these crosslinking procedures lead to a dramatic increase in the brittleness of the membrane, making them difficult to manufacture and use.
BRIEF SUMMARY OF THE INVENTION
There are several aspects of the present invention.
In a first aspect, the invention comprises a microporous solvent-resistant hollow fiber membrane formed from polybenzimidazole (PBI), the membrane being characterized by exceptional nitrogen permeance, high tensile strength and high elongation at break, making it particularly well-suited as a coatable support for fabricating composite permselective membranes.
In a second aspect, the invention comprises a method of making such a solvent-resistant PBI membrane.
In a third aspect, the invention comprises a countercurrent flow separation module incorporating a composite membrane wherein at least one selective coating is placed on a surface of such a solvent-resistant PBS membrane.
In a fourth aspect, the invention comprises a method of crosslinking a membrane (hollow fiber, flat sheet, or tubular; microporous, isoporous, non-porous, or asymmetric) formed from PBI using a multi-functional alkyl halide.
The membranes of the present invention are useful for a variety of applications, including ultrafiltration, microfiltration and membrane contactors; and as supports for composite membranes that are used in such applications as reverse osmosis, nanofiltration, pervaporation, vapor permeation and gas separations.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In contrast to the procedures of the prior art, it has now been found that microporous PBI membranes with exceptional performance and solvent resistance, can be made by proper selection of the procedures for making and crosslinking the membranes.
In one aspect, the invention comprises a microporous hollow fiber membrane formed from PBI, the membrane being fabricated by the following steps:
(a) providing a fiber-spinning polymer solution comprising 15 to 30 wt % PBI, 2 to 5 wt % high molecular weight pore-former with a molecular weight of >1000 daltons, 5 to 30 wt % low molecular weight pore-former, with a molecular weight of <100 daltons, and a solvent;
(b) forming a spun membrane by extruding the polymer solution through an orifice at a temperature of 25° to 60° C. while simultaneously injecting a coagulating fluid through a needle located in the orifice;
(c) providing a quench bath;
(d) passing the spun membrane through the quench bath at a temperature of 10° to 40° C. to form a microporous hollow fiber membrane; and
(e) rinsing the membrane.
Additional optional steps include drying and post-treating the membrane by crosslinking or annealing.
The microporous hollow fiber PBI membranes formed by this process have excellent properties for a wide variety of membrane separation processes. Generally, the membranes have a gas permeance of at least 5 m
3
/m
2
·hr·atm, preferably at least 10 m
3
/m
2
·hr·atm. In addition, the surface pores on the membrane (both inside and outside surfaces of the hollow fiber) are greater than about 0.05 &mgr;m, and less than about 1 &mgr;m. The fibers have a tensile strength of at least 100 g/filament, preferably at least 200 g/filament. The fibers also have an elongation at break of at least 10%, preferably at least 15%. The fibers can also be made with a wide range of diameters and wall thicknesses, depending on the application of use. Generally, the inside diameter of the fibers can range from about 200 &mgr;m to about 1000 &mgr;m, and the wall thickness of the fibers can range from about 30 &mgr;m to about 200 &mgr;m.
The invention can be used with virtually any PBI, and in particular with those described in U.S. Pat. Nos. 2,895,948, 5,410,012, and 5,554,715, the disclosures of which are incorporated herein by reference. These PBIs have the following general repeat structure:
where R is a tetravalent aromatic nucleus, typically symmetrically substituted, with the nitrogen atoms forming the benzimidazole rings being paired upon adjacent carbon atoms of the aromatic nucleus, and R′ is selected from (1) an aromatic ring, (2) an arylene group, (3) an alkylene group, (4) an arylene-ether group, and (5) a heterocyclic ring, such as a pyridine, pyrazine, furan, quinoline, thiophene, or pyran. A preferred PBI is poly(2,2′-[m-phenylene])-5,5′-bis-benzimidazole.
It has been found that to obtain a microporous fiber with high porosity and high gas permeance while maintaining excellent physical properties such as high tensile strength and elongation at break, a mixture of a high molecular weight pore-former and a low molecular weight pore-former should be used. The weight ratio of high molecular weight pore-former to low molecular weight pore-former should range from 0.05 to 0.5, preferably from 0.075 to 0.25.
The high molecular weight pore former should have a molecular weight of at least about 1000 daltons. It should also be soluble in the solvent used for the fiber-spinning polymer solution and in the materials used for the internal coagulation solution and the quench bath. Examples of suitable high molecular weight pore-formers include polyvinyl pyrollidinone (PVP), polyvinyl alcohol (PVA), polyvinyl acetate (PVAc), polyethylene glycol (PEG), and polypropylene glycol (PPG). A preferred high molecular weight pore-former is PVP.
The low molecular weight pore-former should have a molecular weight of no greater than about 100 daltons, and should be hydrophilic. It should also be soluble in the solvent used for the fiber-spinning polymer solution and in the materials used for the internal coagulation solution and the quench bath. In general, the class of useful low molecular weight pore-formers comprises (i) a lower alkanol, (ii) a polyfunctional alcohol, (iii) ester and ether derivatives of an alkanol, (iv) ester and ether derivatives of a polyfunctional alcohol, (v) mixtures of (i)-(iv), and (vi) mixtures of water and at least one of (i)-(v).
Examples of suitable low molecular weight pore-formers include monofunctional alcohols, such as methanol (MeOH), ethanol (EtOH), isopropyl alcohol
Barss Robert P.
Friesen Dwayne T.
McCray Scott B.
Pearson Kendall R.
Ray Roderick J.
Bend Research Inc.
Chernoff Vilhauer McClung & Stenzel LLP
Kim John
LandOfFree
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