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  Reverse osmosis membrane with branched poly(alkylene oxide) modified antifouling surfaceUSPTO Application #: 20070251883Title: Reverse osmosis membrane with branched poly(alkylene oxide) modified antifouling surface Abstract: in which R is hydrogen or a C1-20 aliphatic or aromatic group, V is any group containing a polymerizable site, each R′ is independently hydrogen or a short chain alkyl group, n is an integer of 1-6, and m is an integer of 1 to about 200. The α end group can be either polymerized or copolymerized.
RO—[(CHR′)n—O]m-V
Composite membranes that exhibit long-term resistance to biofouling comprise a porous support and a crosslinked polyamide discriminating layer having an external surface, the discriminating layer comprising a branched poly(alkylene oxide) (PAO) polymer attached to its external surface. The branched PAO polymer typically has the structure of a molecular comb or brush, and is made by polymerization of a PAO macromonomer of the following formula: (end of abstract)
Agent: Whyte Hirschboeck Dudek S.c. - Milwaukee, WI, US Inventor: Q. Jason Niu USPTO Applicaton #: 20070251883 - Class: 210653000 (USPTO) Related Patent Categories: Liquid Purification Or Separation, Processes, Liquid/liquid Solvent Or Colloidal Extraction Or Diffusing Or Passing Through Septum Selective As To Material Of A Component Of Liquid; Such Diffusing Or Passing Being Effected By Other Than Only An Ion Exchange Or Sorption Process, Diffusing Or Passing Through Septum Selective As To Material Of A Component Of Liquid, Filtering Through Membrane (e.g., Ultrafiltration), Hyperfiltration (e.g., Reverse Osmosis, Etc.), Utilizing Specified Membrane Material The Patent Description & Claims data below is from USPTO Patent Application 20070251883. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] This invention relates to membranes. In one aspect, the invention relates to reverse osmosis (RO) membranes while in another aspect, the invention relates to thin-film-composite (TFC)RO membranes. In still another aspect, the invention relates to TFC RO membranes comprising a porous support and a discriminating layer in which the exterior surface of the discriminating layer is chemically modified to reduce or prevent fouling of the membrane during operation. In yet other aspects, the invention is a method of modifying the exterior surface of the discriminating layer of the TFC RO, and a method of using the modified TFC RO. BACKGROUND OF THE INVENTION [0002] Aromatic polyamide TFC RO membranes are ubiquitous in our daily lives finding application in many industrial areas such as desalting of brine, ultra-pure water production, environmental pollution treatment, and the like. The trend for the next generation of such membranes is to require more sophisticated and specified functions of the polymeric materials from which they are constructed to provide for an overall enhanced performance of the membrane. This, in turn, drives the need for so-called "tailor fit" materials whose functions and properties are precisely tuned for the intended application of the membrane. [0003] Tailor fit materials for RO TFC membranes are available through either (i) design and synthesis of totally new polymers forming the thin film discriminating layer of the RO membranes, or (ii) the physical and/or chemical modification of the thin-film. The former approach has produced TFC RO membranes of enhanced water flux but with an accompanying considerable loss of salt rejection, or vice versa. The latter approach results from one of two routes that involve either (i) the post-treatment of the thin-film surface of the membrane with various chemicals, or (ii) the use of additives during the formation of the thin film. [0004] Regarding posttreatment, a number of RO membranes have been coated with either with polyvinyl alcohol (PVA) or a vinyl acetate homopolymer with self-crosslinking functionality (e.g., Vinac.TM. available from Air Products Polymers, L.P.). Regarding the use of additives, a number of membranes, particularly nanofiltration membranes, have been prepared with polymer additives that presumably have been incorporated in the membrane. Important improvements to the membrane resulting from modification of the exterior surface of the discriminating layer include stabilizing the discriminating layer during long-term operations, and balancing the improvement of rejection against the loss of flow due to the alteration of the membrane transport characteristics. [0005] FIG. 1 is a schematic representation of a cross-section of a commercially successful RO TFC membrane, e.g., an FT-30 TFC RO membrane by FilmTec Corporation of Edina, Minn. The first or top layer is an ultra-thin barrier or discriminating layer typically comprising a crosslinked polyamide of 10-100 nanometers (nm) in thickness. One method of preparing this layer is by the interfacial polymerization of m-phenylenediamine (MPD) in the aqueous phase and trimesoyl trichloride (TMC) in the organic phase. [0006] The second or middle layer typically comprises an engineering plastic, such as polysulfone, and it typically has a thickness of about 40 microns (.mu.m). This second layer provides a hard, smooth (relative to the third layer) surface for the top layer, and it enables the top layer to perform under high operating pressure, e.g., 10 to 2,000 psi. [0007] The third or bottom layer is typically nonwoven polyester, e.g., a polyethylene terephthalate (PET) web, with a thickness typically of about 120 .mu.m. This third or bottom layer is typically too porous and irregular to provide a proper, direct support for the top layer, and thus the need for the second or middle layer. [0008] The RO TFC membrane is usually employed in one or two different configurations, i.e., flat panel or spiral wound. The flat panel configuration is simply the membrane, or more typically a plurality of membranes separated from one another by a porous spacer sheet, stacked upon one another and disposed as a panel between a feed solution and a permeate discharge. The spiral wound configuration is shown schematically in FIG. 2, and it is simply a membrane/spacer stack coiled about a central feed tube. Both configurations are well known in the art. [0009] From the viewpoint of performance efficiency, TFC membranes are usually required to have dramatically enhanced water permeability without sacrificing salt separability. Such aromatic polyamide TFC membranes with excellent water flux and reasonable salt rejection characteristics are formed by the interfacial reaction of MPD/TMC that have been kinetically altered with organo-metals and non-metals to form complexes of the TMC as taught in U.S. Pat. No. 6,337,018. This (i) reduces the rate of the reaction of the TMC by reducing the diffusion coefficient and use steric hindrance to block MPD from the acid chloride sites, and (ii) complexes the TMC to block water from hydrolyzing the acid chlorides. [0010] Unlike the chemically analogous FT-30 membrane, the kinetically modified version based on MPD/TMC interfacial polymerization results in modification in surface morphology and variation in the polymer chain organization during formation of the thin film. The combined effect is to increase the rejection of the membrane and to allow the use of other process variables to influence the rate of reaction and therefore the membrane performance. This approach allows for an increase to the membrane flux by over 100% in certain products, e.g., FilmTec's XLE membranes, due to the reduced residual acid chloride after interfacial polymerization and improved swelling ability of the resulting thin film, and it gives the possibility of compensating for flow loss in future post-treatment of thin-film surfaces. [0011] Many applications using membrane processes could benefit from the availability of a wide range of polymer chemistries, e.g., they could exhibit better performance, more robustness and less fouling, and they could use less expense polymers. However, due to the uncertainty of new chemistry and the reluctance of companies to invest in the development of new polymers, alternate approaches such as the surface modification of widely used polymers have increased in importance. [0012] One of the goals of research and industry in the RO membrane field is to enhance, or at least maintain, water flux without sacrificing salt rejection over a long period of time in order to increase the efficiency and reduce the cost of the operation. Nevertheless, the main difficulty in accomplishing this goal is fouling that produces a serious flux decline over the operational time of the membrane. [0013] The principal types of fouling are crystalline fouling (mineral scaling, or deposit of minerals due to an excess in the solution product), organic fouling (deposition of dissolved humic acid, oil, grease, etc.), particle and colloid fouling (deposition of clay, silt, particulate humic substances, debris and silica), and microbial fouling (biofouling, adhesion and accumulation of microorganisms, and the formation of biofilms). Various approaches to reducing fouling have been used, and these usually involve pretreatment of the feed solution, modification of the membrane surface properties (e.g., the attachment of hydrophobic or hydrophilic, and/or electronegative or electropositive groups), optimization of module arrangement and process conditions, and periodic cleaning. However, these methods vary widely in applicability and efficiency and this, in turn, has required continuous, on-going efforts to solve these problems. [0014] For polyamide RO TFC membranes, fouling from the formation of biofilm on the surface caused by microorganisms has been regarded as of the uppermost importance. Microorganisms, such as bacteria and viruses, in the water to be filtered, as well as other microscopic material, e.g., protein, adhere to membrane surfaces and grow at the expense of nutrients accumulated from the water phase. The attached microorganisms excrete an extra-cellular polymeric substance (EPS), and this, in combination with the microorganism and protein, form a biofilm. Biofilm formation is believed related to the depletion of residual disinfectant concentration, and that biofilm is not formed from disinfectant-treated water, such as chlorinated water containing a residual of 0.04-0.05 milligrams per liter (mg/L) of free chlorine. However, chlorination, although effective for the destruction of microorganisms, generates harmful byproducts such as trihalomethanes and other carcinogens. [0015] Protein, cell and bacterial fouling of the membrane surface occur spontaneously upon exposure of the membrane surface, i.e., the external surface of the discriminating layer, to physiologic fluids and tissues. In many cases, biofouling is an adverse event that can impair the function of RO membranes. Common strategies for inhibiting biofouling include grafting antifouling polymers or self-assembled monolayers onto the membrane surfaces. Many synthetic polymers have been investigated as antifouling coatings, and these have met with variable success in antifouling tests. [0016] One common and prominent example of a material used to render a surface inert to nonspecific protein adsorption in medical devices is poly(ethylene oxide) (PEO), a linear, flexible, hydrophilic and water-soluble polyether. Self-assembled monolayers (SAMS) presenting oligo(ethylene glycol) (OEG) groups (as in HS(CH.sub.2).sub.11(EG).sub.nOH)) on a gold surface also prevent the adsorption of proteins, even if the number of ethylene glycol (EC) units present is as low as three. Anti-fouling membranes based on grafted, linear polyalkylene oxide oligomers are known, and they provide an improved resistance to fouling while offering excellent flux and salt passage performance (U.S. Pat. No. 6,280,853). SUMMARY OF THE INVENTION [0017] The present invention provides improved reduced fouling composite membranes and methods for their preparation. In one embodiment, the present invention develops and characterizes new branched poly(alkylene oxide) (PAO) modified TFC RO membranes capable of preventing nonspecific protein adsorption as a means of precluding the formation of biofilms and, hence, reduced fouling. These branched, particularly the highly branched, PAO-modified TFC RO membranes exhibit surprisingly improved stability in fouling, particularly biofouling, environments. Moreover, the membranes of this invention are more thoroughly cleaned under either basic or acidic conditions than linear PAO-modified TFC RO membranes. [0018] In another embodiment, the invention is a composite membrane comprising a porous support and a crosslinked polyamide discriminating layer having an external surface to which are attached crosslinked and branched poly(alkylene oxide) polymers of a relative weight average molecular weight (before crosslinking), as measured by size exclusion chromatography against a linear PEO standard, of at least about 5,000, preferably of at least about 10,000, more preferably between about 20,000 and about 1,000,000, and even more preferably between about 100,000 and about 500,000. [0019] In certain preferred embodiments of the invention, the branched PAO polymers used in the practice of this invention are made from the polymerization of macromonomers of the following formula: RO--[(CHR').sub.n--O].sub.m-V (1) in which V is the .alpha. end group, R is the .omega. end group, each R' is independently hydrogen or a short chain, e.g., C.sub.1-3, alkyl group, n is an integer of 1-6, and m is an integer of 1 to about 200. Polymerization of the macromonomer occurs through the .alpha. end group, and it can be either polymerized or copolymerized with comonomer processes through either V or other a end groups. R is typically a C.sub.1-20 aliphatic or aromatic group; V is a derivative of any compound containing a polymerizable site, e.g., a group containing a double bond such as a derivative of p- or m-vinyl benzene, or p- or m-vinyl benzoic acid, or methacryloyl chloride, or acryloyl chloride or isopropenyl oxazoline; R' is preferably hydrogen or methyl; m is an integer preferably of 2 or 3; and n is an integer preferably between about 3 and about 50, more preferably between about 7 and about 25. The macromonomers of formula I include both homo- and copolymers and if a copolymer, then random, block and mixed random/block polymers, e.g., PEO macromonomers, poly(propylene oxide) (PPO) macromonomers, and random and block macromonomers based on both ethylene oxide and propylene oxide units. As here used, "copolymer" means a polymer made from two or more monomers. [0020] The branched PAO polymers exhibit three prominent structural features that impart good protein resistance to the external surface of the discriminating layer, i.e., (i) a hydrophilic repeating unit, i.e., a unit that hydrogen bonds with water and is thus water-soluble (it swells in water), (ii) an oligomer side chain that is very flexible due to aliphatic ether bonds, and (iii) a branched, preferably a highly branched, architecture that forms a dense protective layer for the external surface. The "external surface of the discriminating layer" is the surface of the discriminating layer that is in contact with the material, e.g., solution, dispersion, etc., to be filtered and opposite the surface of the discriminating layer that is in contact with the porous support. BRIEF DESCRIPTION OF THE DRAWINGS Continue reading... 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