Liquid purification or separation – Filter – Material
Reexamination Certificate
1999-04-09
2001-02-06
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Filter
Material
C210S500350, C210S500270, C210S638000, C264S041000, C427S244000
Reexamination Certificate
active
06183640
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to membranes having internal anionic charges and processes to prepare such membranes. The anionically charged membranes are produced from membrane casting processes incorporating anionic components.
2. Background of the Technology
Anionically charge-modified membranes are useful in the removal of a variety of materials from solutions and also in certain biotechnological applications. For example, negatively charged membranes are useful in the removal of endotoxins from solutions. Endotoxins are toxic substances often derived from bacterial lysates. In addition, such membranes have found utility in the removal of positively charged species from feed-streams, such as in the preparation of ultrapure water for the semiconductor industry.
Ultrafiltration and microfiltration membranes utilized in industry, particularly in the food processing industry and in environmental applications, are typically hydrophobic membranes which may be surface-modified with a hydrophilic material to reduce fouling and to confer additional desirable properties to the membrane. Membranes may be isotropic or asymmetric (anisotropic) in their pore structure. Isotropic membranes have a uniform pore structure throughout the membrane. Asymmetric membranes do not have a uniform pore structure throughout the membrane. Asymmetric porous membranes are distinguished from isotropic, homogeneous membrane structures whose flow and retention properties are independent of flow direction. Asymmetric membranes are useful in microfiltration, ultrafiltration, and reverse osmosis processes.
Several different processes and reagents have been utilized to produce charge-modified, initially hydrophilic or hydrophobic membranes, and related membranes.
U.S. Pat. No. 4,012,324 to Gregor discloses casting formulations including a matrix polymer, a polyelectrolyte, a solvent, and a chemical cross-linking agent. Membranes are formed therefrom through a process of evaporating the solvent to form a membrane of uniform porosity and macroscopic homogeneity, having fixed anionic or cationic charges and a water content of from about 15 to about 75%. Membranes with substantial equilibrium water content are known as hydrogels and are subject to loss of water unless protected prior to use and additionally have limited application.
U.S. Pat. No. 4,673,504 to Ostreicher, et al., discloses cationic charge-modified microporous membranes that are produced from hydrophilic organic polymeric microporous membranes. These microporous membranes are hydrophilic and isotropic, with uniform pore structure throughout the membrane. However, anisotropic hydrophilic membranes are not disclosed in the Ostreicher patent.
U.S. Pat. No. 4,797,187 to Davis, et al., discloses a method to prepare ionically bonded coacervate layer membranes having improved selectivity. The Davis membranes are composite semi-permeable membranes of the type useful for reverse osmosis, gas separation, and ultrafiltration, and are post-treated to improve their selectivity.
U.S. Pat. No. 5,531,893 to Hu, et al., discloses a hydrophilic charge-modified microporous membrane having a crosslinked structure of an interpenetrating polymer network. The membrane comprises a homogeneous matrix of polyethersulfone (PES), polyfunctional glycidyl ether, and a polymeric amine such as polyethyleneimine (PEI) and like polyamines, and polyethylene glycol. A shortcoming of the '893 patent is that membranes heated for the stabilization of the network structure have a lower cationic charge density. This is stated to be due to gradual decomposition of crosslinked PEI adduct in the membrane structure.
Thus, while it can be seen that various different processes and reagents have been utilized to produce charge-modified membranes, each of the cited references has one or more undesirable features. None of the cited references produces stable, anionically charge-modified, isotropic or anisotropic, optionally non-hydrogel membranes in a simple casting process without chemical crosslinking agents. Accordingly, there remains a need for improved, stable, anionically charged membranes which possess a plurality of fixed formal negative charges that can be readily produced from polymer starting materials in a casting process without complication or expensive apparatus and which are not restricted to isotropic or hydrogel membrane types.
SUMMARY OF THE INVENTION
The present invention provides polymer membrane having permanent internal anionic charges, cast from a solution or suspension including a sulfone polymer, an anionic charge-modifying agent, a nonsolvent, and a solvent. The membrane has a first surface and a second surface, each surface having pores thereon; the membrane also has a porous supporting structure between the first and second surface, wherein the porous supporting structure includes a reticulated network of flow channels between the pores of the first surface and the second surface. The sulfone polymer may be, for example, polysulfone, polyethersulfone, or polyarylsulfone. The anionic charge-modifying agent may be 2-acrylamido-2-methylpropane sulfonic acid or 1-propanesulfonic acid 2-methyl-2-(1-oxy-2-propenyl amino). The nonsolvent may be, for example, low molecular weight organic acids, alcohols, ethers, surfactants, or water, including t-amyl alcohol, methoxyethanol, and propionic acid. The solvent may be N-methylpyrrolidone, and the casting solution or suspension may further include a cross-linking initiator.
The membrane of this aspect of the invention, may be an ultrafiltration membrane, and may have a molecular weight exclusion cutoff of about 10 kDa or about 100 kDa. Likewise, the membrane may be a microfilter, and may have a mean flow pore size of less than about 0.1 micron, about 0.2 micron, or about 0.3 to about 1.0 micron. The membrane may be asymmetric, and the pores of the first surface may be at least about 5 times smaller than the pores at the second surface. The flow channels of the porous supporting structure may gradually increase in diameter from the first surface to the second surface.
In another aspect, the invention provides a method of forming a polymer membrane having permanent internal anionic charges. The method includes: providing a casting solution or suspension including a sulfone polymer, an anionic charge-modifying agent, a nonsolvent, and a solvent; casting the solution or suspension to form a thin film; coagulating the film in a quench bath; and recovering a polymer membrane having permanent internal anionic charges, the membrane having a first surface and a second surface, each surface having pores thereon, the membrane also having a porous supporting structure between the first and second surface, wherein the porous supporting structure includes a reticulated network of flow channels between the pores of the first surface and the second surface. The sulfone polymer used in the method may be, for example, polysulfone, polyethersulfone, and polyarylsulfone. The anionic charge-modifying agent may be, for example, 2-acrylamido-2-methylpropane sulfonic acid or 1-propanesulfonic acid 2-methyl-2-(1-oxy-2-propenyl amino). The nonsolvent may include, for example, low molecular weight organic acids, alcohols, ethers, surfactants, or water, such as t-amyl alcohol, methoxyethanol, or propionic acid. The solvent may be N-methylpyrrolidone and the casting solution or suspension may further include a cross-linking initiator. The method of the invention may further include the additional step of heat crosslinking the membrane. The heat crosslinking step may occur at a temperature between 100° C. and 130° C.
The method of the invention may be used to produce an ultrafiltration membrane, which may have a molecular weight exclusion cutoff of about 10 kDa, or about 100 kDa. The membrane may be a microfilter, and may have a mean flow pore size of less than about 0.1 micron, about 0.2 micron, or about 0.3 to about 1.0 micron. The membrane produced by the method may be asymmetric, and the pores of the
Fortuna Ana
Knobbe Martens Olson and Bear LLP
USF Filtration and Separations Group Inc.
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