Liquid purification or separation – Filter – Material
Reexamination Certificate
2001-05-02
2003-04-01
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Filter
Material
C210S500290, C210S500280, C210S500360, C210S500380, C210S500410, C210S500420, C210S500230, C210S484000
Reexamination Certificate
active
06540915
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to filters for the purification of liquids. In particular, the present invention relates to antimicrobial semi-permeable cast membranes such as cellulose acetate or composite polyamide, polysulfone and polyvinylidine fluoride membranes used in reverse osmosis, ultrafiltration
anofiltration and microfiltration.
BACKGROUND OF THE INVENTION
In recent years, the public has become increasingly aware of the deteriorating quality and quantity of our nation's and the world's fresh water supply. Pollutants, biological and toxic waste and other contaminants are being introduced into water supplies at an ever increasing rate, making such water supplies unfit for drinking and other necessary uses. For example, medical patients with low immunity are now being requested not to drink tap water, and disease and illnesses linked to poor quality drinking water have increased dramatically in recent years. This problem is especially significant outside of the United States where water quality has deteriorated to an all time low, with the major source of such contamination primarily being bacterial in nature.
In many areas of the world, potable water is not only contaminated but also scarce. In these areas, people must rely upon expensive purification systems to remove dissolved solids from seawater, brackish water, or well water. Reverse osmosis filtration systems are one of the most common solutions for improving water quality. Osmosis is the flow or diffusion that takes place through a semi-permeable membrane, such as in a living cell. The membrane typically separates either a solvent, such as water, from a solution, or a dilute solution from a concentrated solution. This membrane separation brings about conditions for equalizing the concentrations of the components on the two sides of the membrane because of the unequal rates of passage in the two directions until equilibrium is reached.
In reverse osmosis, pressure is deliberately applied to the more concentrated solution to cause the flow of solvent in the opposite direction through the membrane, for example, into the more dilute solution. In this way, the liquid can be separated from dissolved solids and thus increase the concentration of the dissolved solids in solution. Typically, the osmotic pressure of a solution containing 1000 ppm of dissolved salts is 10 psig. Most residential reverse osmosis units operate at less than 150 psig. Reverse osmosis units treating brackish waters operate at 150 to 450 psig, while those for seawater operate at 800 to 1000 psig.
The widespread use of reverse osmosis to produce potable water began in the early 1960's when Loeb and Sourirajan developed a cast thin-skin cellulose acetate membranes for use in reverse osmosis systems. These cellulose acetate membranes provided much higher salt rejection (approaching 95%) and solvent flow than previously known reverse osmosis methods. Cellulose acetate membranes are also relatively inexpensive and are very tolerant of chlorine, which is commonly used to eliminate bacteria in water. Since the 1960's, the use of reverse osmosis has grown dramatically in waste water applications and industrial desalinization plants to produce drinking water from brackish and sea waters. More recently, cast cellulose acetate membranes have been incorporated into consumer filtration systems to produce drinking water at the point of use. (Matsuura, T., Synthetic Membranes and Membrane Separation Processes, CRC Press, (1994)). Although cellulose acetate membranes greatly expanded the utilization of reverse osmosis treatment systems, such systems are still restricted by operational problems. For example, cellulose acetate membranes hydrolyze and biodegrade readily. Therefore, a need exists for alternative membranes for use in reverse osmosis systems.
Recently, a cast thin film composite polyamide membranes have been developed that offer better performance than cellulose acetate membranes. (See for example, U.S. Pat. Nos. 4,277,344, 4,520,044 and 4,606,943). Composite polyamide membranes have a bottom layer of reinforcing fabric usually made of polyester, on top of which is typically deposited a layer of polysulfone polymer. The layer of polysulfone polymer is typically 40 microns thick. A 0.2-micron ultrathin layer of polyamide is then cast on the top of the polysulfone layer. (Singh, R., “Membranes”, Ultrapure Water, March 1997). The porous polysulfone support is saturated with water-soluble amine solution, and acid chloride solution is then applied to bring about an in situ polymerization to the polyamide. For example, U.S. Pat. No. 3,551,331 describes a process for modifying the permeability of linear aliphatic polyamide membrane.
The polyamide layer enables the composite polyamide membrane to exhibit salt rejection rates greater than 99.5% at pressures much lower than the pressures used for cellulose acetate membranes. Additionally, polyamide membranes reject silica, nitrates, and organic materials much better than cellulose acetate membranes. Because of the high performance of composite polyamide membranes, these membranes are used in high purity or ultrahigh purity water systems in pharmaceutical and electronics industries. However, just as cellulose acetate membranes exhibit a limiting characteristic, for example, biodegradation, so do composite polyamide membranes. Composite polyamide membranes are also susceptible to damage from chlorine. To overcome some of these shortcomings, other types of cast membranes have been developed that use different types of polymers.
As the technology for manufacturing composite polyamide, cellulose acetate and other types of membranes has progressed, new fields of filtration, such as ultrafiltration, or nanofiltration, and microfiltration have been created. Many of these membranes utilize a support layer having a relatively high degree of porosity followed by an ultra-thin layer of another polymeric coating, such as the polyamide layer described above, that allows for a high salt rejection or rejection of various ranges of molecular weights of organic substances. Additionally, the support membranes, or structures, are either woven or nonwoven and are typically made from polyolefins, polyester, aromatic polysulfones, polyphenylenesulfones, aromatic polyether sulfone, bisphenol A, dichlorodiphenoxysulfone, aromatic polyether ketones, sulfonated polyether ketones, phenoxides made from epichlorohydrin and bisphenol A, polyvinylidene fluoride or sulfonated polyvinylidene fluoride, nylon, vinyl chloride homo- and co-polymers, polystyrene, polytetrafluorethylene, glass fiber, porous carbon, graphite, inorganic membranes based on alumina, and/or silica with coating of zirconium oxide. The support structure is either in the form of a flat sheet or a hollow fiber configuration depending on the desired characteristic nature of the final membrane.
U.S. Pat. No. 5,028,337 (“'337”) describes compositions of many types of cast membranes and methods of preparing the same. In particular, the '337 patent discloses an ultra-thin polymeric coating on a porous support which may be selected from the following polymers, which may be in turn halomethylated, quaternized and/or sulfonated, as desired or necessary prior to a coating step: aromatic oxide polymers, such as 2,6 dimethyl polyphenyleneoxides, aromatic polysulfones, aromatic polyethersulfones, aromatic polyether ketones, linear polyaromatic epoxides; aryl polymers, such as polystyrene and poly (vinyl toluene) polymers; and, sulfonated poly (haloalkylene) polymers, such as sulfonated polyvinyl chloride, polyvinyl fluoride, polyvinylidene fluoride or polyvinylidene fluoride/hexafluoropropylene.
Casting of polymeric membranes on the support structures described above that are made of polysulfones, polyether sulfones, polyether ketones, polyvinylidene fluoride, sulfonated polyvinylidene fluoride or polyacrylonitrile may be accomplished by any number of casting procedures extensively described in published patent and technical lite
Dougherty Clements & Hofer
Fortuna Ana
Microban Products Company
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