The invention relates to a method of making an epoxysilicone coated membrane by coating a porous asymmetric membrane layer with a UV-curable controlled release epoxysilicone coating. A mixture of the epoxysilicone resin and an onium photocatalyst are applied to the porous asymmetric membrane layer in a dilute non-polar solution and cured by UV or electron beam radiation to produce a dry epoxysilicone coated membrane. The porous asymmetric membrane layer is comprised of an asymmetric cellulosic membrane or an asymmetric polymer membrane with a low selectivity. The epoxysilicone coating was found to provide the asymmetric membrane layer with improved selectivity which extends to separation temperatures below 70xc2x0 C. and provides stable flux rates. Membranes produced in this manner are useful for the separation of gases such as carbon dioxide from natural gas.

This invention relates to semipermeable asymmetric membranes useful in selective separations of gases and liquids
Gas permeation may be defined as a physical phenomenon in which certain components selectively pass through a substance such as a membrane. Basically, a gas permeation process involves introducing a gas into one side of a module which is separated into two compartments by a permeable membrane. The gas stream flows along the surface of the membrane and the more permeable components of the gas pass through the membrane barrier at a higher rate than those components of lower permeability. After contacting the membrane, the depleted feed gas residue stream is removed from contact with the membrane via a suitable outlet on the feed compartment side of the vessel. The other side of the membrane, the permeate side, is provided with a suitable outlet through which the permeated gaseous components can be removed from contact with the membrane. The purpose of a membrane in a gas permeation process is to act as a selective barrier, that is, to permit passage of some but not all components of a gaseous feed stream. Generally, in gaseous membrane separation processes, the separation is due to molecular interaction between gaseous components of the feed stream and the membrane. Because different components interact differently with the membrane, the transmission rates (permeation fluxes) are different for each component. Hence, separation of different components can be effected. U.S. Pat. No. 4,130,403, which is hereby incorporated by reference, discloses the use of cellulosic membranes in processes for the separation and removal of acid components from hydrocarbon gases.
Semipermeable asymmetric cellulosic xe2x80x9cskinnedxe2x80x9d separation membranes formed by phase inversion and solvent exchange methods are known (see U.S. Pat. No. 3,133,132 which is hereby incorporated by reference). Such membranes are characterized by a thin, dense, selectively semipermeable surface xe2x80x9cskinxe2x80x9d and a less dense void-containing, non-selective support region, with pore sizes ranging from large in the support region to very small proximate to the xe2x80x9cskinxe2x80x9d. Such membranes have a serious shortcoming in that, in operation, fluxes decrease substantially over time. This decrease has been attributed to a collapse of some of the pores near the skinned surface of the membrane, resulting in an undue densification of the surface skin. One attempt at overcoming this problem has been the development of thin film composite or xe2x80x9cTFCxe2x80x9d membranes, comprising a thin selective skin deposited on a resilient porous support. See, for example, xe2x80x9cThin-Film Composite Membrane for Single-Stage Seawater Desalination by Reverse Osmosisxe2x80x9d by R. L. Riley et al., Applied Polymer Symposium No. 22, pages 255-267 (1973). While TFC membranes are less susceptible to flux decline than phase inversion-type membranes, fabrication of TFC membranes that are free from leaks is difficult, and fabrication requires multiple steps and so is generally more complex and costly.
Asymmetric membranes may be formed from other polymers such as polysulfone, polyethersulfone, polyamide, polyimide, polyetherimlide, cellulose nitrate, polyurethane, polycarbonate, polystyrene, etc. are also susceptible to flux decline and fabrication of asymmetric membranes from such other polymeric materials which are free of leaks is similar in difficulty and expense to producing TFC membranes.
One approach to overcoming the problem of leaks in asymmetric membranes has been the fabrication of an asymmetric gas separation membrane comprising a relatively porous and substantial void-containing selective xe2x80x9cparentxe2x80x9d membrane such as polysulfone or cellulose acetate that would have permselectivity were it not porous, wherein the parent membrane is coated with a material such as a polysiloxane or a silicone rubber in occluding contact with the porous parent membrane, the coating filling surface pores and other imperfections comprising voids (see U.S. Pat. No. 4,230,463). However, the coatings of such coated membranes, especially where the coatings are polysiloxane, are subject to degradation by solvents inherently present in the gaseous feed streams of common acid gas separations, and otherwise tend to leach out, thus permitting either flux decline or low selectivity that is more characteristic of the uncoated parent membrane. For example, U.S. Pat. No. 4,877,528 attempts to overcome the flux decline characteristic of coated cellulosic membranes by grafting or bonding an asymmetric cellulosic semipermeable membrane and a polysiloxane. U.S. Pat. No. 4,877,528 discloses that the grafting of the polysiloxane can be accomplished by either a polycondensation reaction whereby a polysiloxane containing one or more hydroxyl-reactive functional groups is reacted with the hydroxyl groups of the cellulose polymer of the cellulosic membrane, or by a polyaddition reaction whereby a polysiloxane containing one or more vinyl alkynyl, or free radical-forming groups is reacted with the cellulosic substrate. In both cases, the result is the bonding of the grafting of the polysiloxane to the cellulosic membrane by non-crosslinked, covalent bonds. Examples presented indicate that the grafting did reduce some of the loss of the siloxane content and loss in performance during operation, but such grafted membrane still exhibited more than a 50 percent loss in siloxane content and a 50 percent loss in performance.
These coatings are usually applied to substrates as dispersions in a solvent system in order to reduce the viscosity sufficiently so that the coating composition is easily coatable. Such prior art post-treatments may provide gas separation membranes which exhibit improved selectivity; however, unless the solvent is highly inert toward the membrane polymer, the treating solution may also cause or induce change or damage to the morphology of the membrane, which may cause loss of trans-membrane flux. The presence of the solvent, either water or some suitable low boiling organic solvent, necessitates evaporation. Thus, heat is typically applied to substrates coated with silicones for the removal of solvent and to thermally induced curing. This heating step may result in damage to the underlying substrate.
Fluorosilicones are generally well suited to use as conformal coatings because of their resistance to solvent swelling and degradation. Photo-curable silicones that are intended for low modulus cured coatings typically consist of linear silicone molecules that have photo-reactive centers widely separated by non-functional polysiloxane segments so that a low cross-link density results when the silicone is cured. Because they consist almost entirely of polydimethylsiloxane segments, such photo-curable silicone polymers are incompatible with onium ionic photo-catalysts. This incompatibility results in an inefficient and/or slow photo-cure. The fluorosilicone polymers suffer from these same drawbacks.
Therefore, a membrane post-treatment is needed which improves selectivity but does not change or damage the membrane, or cause the membrane to lose performance with time. In addition, gas separation membranes desirably have a high permeability to gases. This means that the effective portion of the membrane should be as thin as possible. Making the membrane as thin as possible necessarily leads to the membrane containing imperfections or defects. These defects pass gases indiscriminately thus reducing the selectivity of the membrane. In the past, these membrane defects have been sealed or reduced to improve gas separation membrane performance.
Other methods of improving membrane performance have relied on the chemical treatment of the membrane substrate with a swelling agent which are believed to improve the selectivity of a membrane.
What is needed, therefore, is an asymmetric membrane that can be inexpensively made by conventional single casting techniques, yet has a high selectivity and a stable flux rate which essentially do not decline in use. These needs and others are met by the present invention, which is summarized and described in detail below.
The present invention is a process for preparing a fluid separation membrane comprising an asymmetric membrane substrate which is directly coated with a radiation curable silicone coating which provides improved stable performance over a wider range of temperature than previously known materials. The asymmetric membrane substrate includes cellulosic membranes and membranes formed from other polymers such as polysulfone, polyethersulfone, polyamide, polyimide, polyetherimide, cellulose nitrate, polyurethane, polycarbonate, polystyrene, etc. More specifically, the membranes of the present invention exhibit essentially no loss in selectivity or loss in flux rates. By the term xe2x80x9cstable flux ratexe2x80x9d, it is meant that the flux declines less than about 30 percent, and more particularly the flux rate declines less than 20 percent over a typical operating period. Furthermore, unlike silicone coated membranes of the prior art, the fluid separation membranes of the present invention are coated with a UV curable epoxysilicone coating which is not chemically bonded to the membrane substrate. Epoxysilicone coated porous cellulosic membranes produced according to the present invention exhibit essentially no loss in selectivity and essentially no loss in siloxane content even following hexane extraction.
In one embodiment, the present invention is a method for the production of a fluid separation membrane comprising directly coating a porous asymmetric membrane, such as cellulose acetate, with a dilute solution of the epoxysilicone resin and an onium ionic photo-catalyst dissolved in a non-polar, organic solvent to uniformly disperse the epoxysilicone resin over the asymmetric membrane, evaporating the non-polar organic solvent, and crosslinking the epoxysilicone resin by subjecting the epoxysilicone resin to ultra-violet or electron beam radiation. According to the invention, the method comprises casting a porous asymmetric membrane layer; drying the porous asymmetric membrane layer to form a dry porous asymmetric membrane; uniformly coating the dry porous asymmetric membrane with a coating comprising a dilute epoxysilicone solution to form a coated membrane; and irradiating and drying the coated membrane to produce the epoxysilicone coated membrane.
In a further embodiment, the present invention is a process for the separation of a permeable component from a feed stream comprising the permeable component and natural gas. The process comprises passing the feed stream at effective conditions to a membrane separation zone containing a porous asymmetric membrane layer coated with a uniform UV-curable epoxysilicone coating and recovering a permeate stream enriched in the permeable component relative to the feed stream, and a non-permeate stream depleted in the permeable component relative to the feed stream.
Acidic components, hydrogen sulfide, and carbon dioxide may be removed from a gaseous light hydrocarbon or a mixture of light hydrocarbons such as natural gas by the use of a suitable gas permeable membrane. In addition, a gas permeable membrane may be used to produce a relatively pure carbon dioxide gas which may be utilized in enhanced oil recovery processes such as the miscible flooding of carbonate reservoirs. The light hydrocarbons from which CO2 and/or H2S can be removed by the method of the instant invention include lower aliphatic hydrocarbons such as methane, ethane, propane, butanes, pentanes, hexanes and to a lesser extent, aliphatic hydrocarbons having more than about eight carbon atoms and mixtures thereof. Some feed streams may contain small amounts of aromatic components such as benzene and toluene. Natural gas streams which can be treated in accordance with the instant invention typically contain at least about 2 percent CO2. Natural gas streams can comprise from about 1 ppm-vol to about 98 volume percent H2S. Some natural gases may contain up to about 98 percent CO2. Gases containing high concentrations of CO2 cannot be economically processed by conventional processes to produce a substantially CO2-rich stream such as one containing about 95 percent CO2. Thus, the use of membrane technology is particularly attractive when the CO2 concentration ranges between about 2 and about 70 volume percent in natural gas or similar light hydrocarbon stream.
Generally, organic or organic polymers mixed with inorganics are used to prepare the porous separation membrane. Typical polymers suitable for the porous separation membrane according to the invention can be substituted or unsubstituted polymers and may be selected from polysulfones; poly(styrenes), including styrene-containing copolymers such as acrylonitrilestyrene copolymers, styrene-butadiene copolymers and styrene-vinylbenzylhalide copolymers; polycarbonates; cellulosic polymers, such as cellulose acetate-butyrate, cellulose propionate, ethyl cellulose, methyl cellulose, nitrocellulose, etc.; polyamides and polyimides, including aryl polyamides and aryl polyimides; polyethers; poly(arylene oxides) such as poly(phenylene oxide) and poly(xylene oxide); poly(esteramide-diisocyanate); polyurethanes; polyesters (including polyarylates), such as poly(ethylene terephthalate), poly(alkyl methacrylates), poly(acrylates), poly(phenylene terephthalate), etc.; polysulfides; polymers from monomers having alpha-olefinic unsaturation other than mentioned above such as poly (ethylene), poly(propylene), poly(butene-1), poly(4-methyl pentene-1), polyvinyls, e.g., poly(vinyl chloride), poly(vinyl fluoride), poly(vinylidene chloride), poly(vinylidene fluoride), poly(vinyl alcohol), poly(vinyl esters) such as poly(vinyl acetate) and poly(vinyl propionate), poly(vinyl pyridines), poly(vinyl pyrrolidones), poly(vinyl ethers), poly(vinyl ketones), poly(vinyl aldehydes) such as poly(vinyl formal) and poly(vinyl butyral), poly(vinyl amides), poly(vinyl amines), poly(vinyl urethanes), poly(vinyl ureas), poly(vinyl phosphates), and poly(vinyl sulfates); polyallyls; poly(benzobenzimidazole); polyhydrazides; polyoxadiazoles; polytriazoles; poly (benzimidazole); polycarbodiimides; polyphosphazines; etc., and interpolymers, including block interpolymers containing repeating units from the above such as terpolymers of acrylonitrile-vinyl bromide-sodium salt of para-sulfophenylmethallyl ethers; and grafts and blends containing any of the foregoing. Typical substituents providing substituted polymers include halogens such as fluorine, chlorine and bromine; hydroxyl groups; lower alkyl groups; lower alkoxy groups; monocyclic aryl; lower acyl groups and the like.
Selection of the porous separation membrane for the present multi-component membrane for gas separations may be made on the basis of the heat resistance, solvent resistance, and mechanical strength of the porous separation membrane, as well as other factors dictated by the operating conditions for selective permeation, as long as the coating and porous separation membrane have the prerequisite relative separation factors in accordance with the invention for at least one pair of gases. The porous separation membrane is preferably at least partially self-supporting, and in some instances may be essentially self-supporting. The porous separation membrane may provide essentially all of the structural support for the membrane, or the multi-component membrane may include a structural support member which can provide little, if any, resistance to the passage of gases.
When permeable components are acid components selected from the group consisting of carbon dioxide, hydrogen sulfide, and mixtures thereof and are removed from a hydrocarbon mixture such as natural gas, one module, or at least two in parallel service, or a series of modules may be utilized to remove the acid components. For example, when one module is utilized, the pressure of the feed gas may vary from 275 kPa to about 2.6 MPa (25 to 4000 psig). The differential pressure across the membrane can be as low as about 0.7 bar or as high as 145 bar (about 10 psi or as high as about 2100 psi) depending on many factors such as the particular membrane used, the flow rate of the inlet stream and the availability of a compressor to compress the permeate stream if such compression is desired. Differential pressures greater than about 145 bar (2100 psi) may rupture the membrane. A differential pressure of at least 7 bar (100 psi) is preferred since lower differential pressures may require more modules, more time and compression of intermediate product streams. The operating temperature of the process may vary depending upon the temperature of the feed stream and upon ambient temperature conditions. Preferably, the effective operating temperature of the membranes of the present invention will range from about xe2x88x9250xc2x0 to about 80xc2x0 C. More preferably, the effective operating temperature of the membranes of the present invention will range from about xe2x88x9220xc2x0 to about 70xc2x0 C., and most preferably, the effective operating temperature of the membranes of the present invention will be less than about 70xc2x0 C. Some membranes such as plastics and resins exhibit better permeability characteristics at higher temperatures; however, in the preferred embodiment, a cellulose acetate membrane is utilized. It was discovered that the permeability characteristics of the membrane of the present invention showed a surprising improvement over conventionally produced cellulosic membranes at operating temperatures below about 50xc2x0 C. As operating temperature was reduced below about 50xc2x0 to about xe2x88x9210xc2x0 C., the membranes of the present invention exhibited an almost 20 percent improvement in selectivity over conventional cellulosic membranes. As a practical matter, the operating temperature may be maintained at ambient temperature. The upper limiting temperature is thought to be that temperature at which the membrane deteriorates and no longer serves as a useful gas separator.
The selectivity of a membrane (also called separation factor) in separating two gas species A and B from each other is defined as the ratio of their gas permeances in that membrane. The selectivity (A/B) can be obtained by measuring the gas permeance of each gas in pure gas state or in mixed gas state. The selectivity is a measure of how well the membrane can separate the gases while the permeance is a measure of how fast the membrane can let the gases pass through. In most applications, the membrane is made to maximize both permeance and selectivity (see U.S. Pat. No. 5,702,503, column 15, lines 21-67, hereby incorporated by reference).
Radiation curable protective coatings are useful for a variety of applications, such as topcoats over printed matter and similar uses. Such radiation curable compositions are typically highly functional reactive substances which cure to yield highly cross-linked, glossy, and hard coatings. Silicone coatings of this type frequently contain so-called Q functional siloxane groups (SiO4/2) in their structure. Such Q containing silicones are generally referred to as resins to distinguish them from silicone fluids which are usually linear, consisting largely of repeating D groups (R2 SiO2/2, where R usually is methyl). Highly reactive organofunctional silicone Q resins are typically very viscous semi-solids or friable solids at room temperature and thus not readily coatable because they are not liquid. Indeed, when functionalized with polar organic moieties that are reradiation cross-linkable, even linear silicone fluids can become highly viscous and therefore difficult to coat using standard coating techniques.
Epoxy-functional silicones such as taught in U.S. Pat. No. 5,650,453 and U.S. Pat. No. 4,279,717, which are hereby incorporated by reference, when combined with certain compatible iodonium cationic photo-catalysts, are known to be useful for producing UV curable epoxysilicone release coatings. Epoxysilicone release coatings allow high speed processing with minimal energy expenditures. If the viscosity of the coating composition exceeds 1,000 centistokes (cstk) at room temperature, the absence of solvent in the composition renders them difficult to apply, particularly if a thin coating on the order of 1 gm/m2 is desired. The viscosity constraint imposed by processing equipment thus imposes constraints on the molecular weight of the silicone composition and on linearly functionalized photo-curable silicone fluids such as epoxysilicones. Additional constraints are provided by the need for photo-catalyst miscibility or solubility, the need for a rapid photo-cure response, and good release performance. While a high epoxy content in an epoxysilicone, as epoxy functional groups on a linear silicone molecule, tend to promote onium photo-catalyst compatibility with the silicone and a rapid photo-cure, a low epoxy content is required for premium or low force release characteristics.
Controlled release is an additional aspect of photo-curable epoxysilicone release performance. Compositions containing both epoxy functional and phenol functional silicones as taught in U.S. Pat. No. 5,138,012, and Q resins containing epoxysilicones as taught in U.S. Pat. Nos. 5,360,833 and 5,369,205 provide a so-called controlled release. Controlled release refers to a controllable and predictable release force that may be varied from very easy to very tight depending on the desired application.
The present invention employs an ultra-violet or electron beam curable silicone coating composition comprising:
(a) an epoxy functional silicone selected from the group consisting of
MDxDEyQzTuDRfjDAkDPl(Dxe2x80x2(CH(R)CH2O)m)nDBpM,
MEDxDEyQzTuDRfjDAkDPl(Dxe2x80x2(CH(R)CH2O)m)nDBpME,
MEDxDEyQzTuDRfjDAkDPl(Dxe2x80x2(CH(R)CH2O)m)nDBpM,
xe2x80x83and mixtures thereof;
where
M=(CH3)3SiO1/2,
ME=(C6H9O(CH2)2)(CH3)2SiO1/2,
D=(CH3)2SiO2/2,
Dxe2x80x2=(CH3)3SiO2/2,
DE=(C6H9O(CH2)2)(CH3)SiO2/2,
DRf=(CF3CH2CH2)(CH3)SiO2/2,
DA=((HO)(C2H3)C6H9(CH2)2)(CH3)SiO2/2,
DP=((HO)(C6H4)(CH2)3)(CH3)SiO2/2,
DB=((C6H5COO)(HO)(C6H9)(CH2)2)(CH3)SiO2/2,
Q=SiO4/2,
T=(CH3)3SiO3/2,
where R is selected from the group consisting of hydrogen, methyl, and ethyl, and
where j, k, l, m, n, p, x, y, z, and u are positive integers and k, l, n, p, u and z may be zero and wherein said epoxy functional silicone has a viscosity ranging from about 100 to about 100,000 centistokes at 25xc2x0 C.; and
(b) an effective amount of a bis(dodecylphenyl)iodonium salt photocatalyst, the photocatalyst being selected from the salts of the group of acids consisting of hexafluoroantimonic acid, hexafluoroarsenic acid, hexafluorophosphoric acid, tetrafluoroboric acid, tetra(perfluorophenyl)boric acid and mixtures thereof. The subscripts on the various components of the epoxyfunctional silicones may be varied at will within the constraints already listed, i.e. either non-zero or optionally zero such that the viscosity is within the defined range.
It is preferred that the epoxysilicone resin be combined with an effective amount of the onium photocatalyst, wherein the effective amount of the onium photocatalyst in an epoxysilicone UV-curable mixture is a ratio ranging from about 100:1 to 100:10 by weight of the epoxysilicone resin to the onium photocatalyst, and more particularly that the epoxysilicone resin be combined with the onium photocatalyst in a ratio ranging from about 100:1 to 100:4 by weight of the epoxysilicone resin to the photocatalyst. It is preferred that the epoxysilicone UV-curable mixture be diluted in a non-polar organic solvent such as hexane or heptane and mixtures thereof in a concentration of from about 0.1 to about 10 weight percent provided an effective coating. The epoxysilicone/catalyst mixture is applied to the surface of the base cellulose membrane in a dilute solution of hexanes, heptanes and mixtures thereof in a concentration of the epoxysilicone/catalyst mixture ranging from about 0.1 to about 10 weight percent. The hexanes and heptanes can be any combination of normal and iso-paraffins and may be present in any ratio of hexanes to heptanes ranging from 1:10 to 10:1.
Epoxysilicones which are UV curable are commercially available from various sources, for example, GE Silicones of Waterford, N.Y. makes and sells a number of UV-photocurable epoxysilicones under the denominations UV9315, UV9430, and UV9400. An example of the onium photo-catalyst compatible with such epoxysilicones is made and sold by GE Silicones of Waterford, N.Y. under the denomination UV9380C. A preferred formulation for achieving a cellulosic membrane coating of the controlled release epoxysilicone mixture would include about 100 parts UV-photocurable epoxysilicone and 1-4 parts onium photo-catalyst.
The asymmetric membrane will be further described in relation to an asymmetric cellulosic membrane, although it is believed that the techniques for coating the asymmetric membrane with the UV-curable epoxysilicone coating and the benefits of the present invention will also apply to asymmetric membranes produced from other polymers such as polysulfone, polyethersulfone, polyamide, polyimide, polyetherimide, cellulose nitrate, polyurethane, polycarbonate, polystyrene, etc. The asymmetric cellulosic membrane is a thin porous membrane having a skin thickness of less than about 10,000 angstroms. Preferably, the asymmetric cellulosic membrane has a skin thickness between about 200 and about 1000 angstroms, and more preferably, the asymmetric cellulosic membrane has a skin thickness between about 300 and about 500 angstroms. The permeability constant of a particular gas through a particular membrane may be determined experimentally by contacting the gas with a membrane of known area and thickness, recording the differential pressure across the membrane and measuring the rate of permeation, or diffusion, of the gas through the membrane. After the permeability constant for several components has been determined, the relative separation of the two gases can be determined. Generally, in order to separate one gaseous component from another, the ratio of the permeability constants of the more permeable component to the other component should be at least five, wherein the selectivity (xcex1) is well-known to those skilled in the art and is herein defined as the relative permeance of an acid gas component such as CO2 to the permeance of a hydrocarbon component such as methane (CH4), where the permeance of a component is the gas flow of a mixture of CO2 and CH4, divided by the area of the membrane surface times the differential partial pressures of the component between the feed pressure and the permeate pressure.
The term xe2x80x9casymmetric cellulosic membranexe2x80x9d in the context of the instant invention includes cellulose ester membranes such as cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose cyanoethylate, cellulose methacrylate and mixtures thereof may be utilized. These membranes may be flat film or hollow fiber. A particularly preferred membrane comprises cellulose acetate. The base cellulosic membrane layer can be made to any degree of initial porosity as characterized by its initial selectivity, which may range from 0.1 to about 15. The base cellulosic membrane layer of the present invention is porous and is characterized as having an initial selectivity of less than about 5, and more preferably having a selectivity between about 0.5 and about 3, and most preferably having a selectivity between about 1 and about 2. It was found that in order to make a stable coated membrane, the concentration of the epoxysilicone/catalyst content in the dilute solvent was dependent upon the initial porosity of the base membrane layer. When the selectivity of the base cellulosic membrane was greater than about 5, the concentration of the epoxysilicone/catalyst in the solvent solution ranged from about 0.1 to about 5 weight percent, and more preferably when the selectivity of the base cellulosic membrane was greater than about 5, the concentration of the epoxysilicone/catalyst in the solvent solution ranged from about 0.1 to about 2 weight percent. When the basic cellulosic membrane layer was very porous, that is having a selectivity less than about 5, it was discovered that the concentration of the epoxysilicone resin plus the catalyst in the dilute solvent solution was critical to the production of an acceptable membrane for gas separation applications. When the selectivity of the base membrane was less than about 5, a concentration of epoxysilicone resin/catalyst in the dilute solvent less than about 1 weight percent proved to be insufficient to block the defects in the base cellulosic membrane. When the selectivity of the base membrane was less than about 5, a concentration of the epoxysilicone resin/catalyst in the dilute solvent greater than about 10 weight percent provided a coating which was too thick and which actually reduced gas permeance through the membrane. Therefore, when the porous asymmetric layer comprises a selectivity less than 5, the dilute epoxysilicone solution should comprise a ratio of epoxysilicone mixture to solvent ranging from about 2 to about 6 parts per 100 parts weight. When the porous asymmetric layer comprises a selectivity greater than 5, the dilute epoxysilicone solution should comprise a ratio of epoxysilicone mixture to solvent ranging from about 0.5 to about 3 parts per 100 parts weight.
The improvement of the epoxysilicone coated asymmetric cellulosic membranes of the present invention over conventional asymmetric cellulose membranes in maintaining selectivity is dramatic and exhibits essentially no loss of silicone and exhibits a highly stable flux rate. By the term xe2x80x9cflux ratexe2x80x9d, it is meant the rate at which a gaseous composite passes through the membrane. The membranes of the present invention exhibited essentially no loss of selectivity following extraction with a non-polar solvent, hexane. It is believed that following extraction with other non-polar solvents such as pentanes, hexanes, cyclohexanes, heptanes, and the like, the membranes of the present invention will exhibit essentially no loss in selectivity. The invention is further illustrated by the following examples which are illustrative of specific modes of practicing the invention and are not intended as limiting the scope of the invention defined in the claims.


The present invention includes a method for stimulating increased PSA production by prostate cancer cells, comprising the steps of reducing the amount of iron available to the prostate cancer cells; and causing increased PSA production. In another aspects of the present invention, there are provided prostate cancer diagnostic kits comprising a first container containing an iron chelator; and second container containing a free-PSA-specific antibody, wherein the free-PSA-specific antibody is detectable. Also provided as part of the present invention are methods for diagnosing prostate cancer in a human, comprising reducing the amount of available iron in the human, and subsequently testing PSA levels.

In 1995, an estimated 41,000 deaths from metastatic prostate cancer occurred, despite some promising treatments. Some of these deaths would have been prevented had prostate specific antigen diagnostic tests been more reliable. The present invention improves the specificity of PSA screening tests, and will increase survival rates via early detection and early treatment.
Great strides have been made in detection of prostate malignancies since the identification of prostate specific antigen (PSA) by Wang et al. in 17 Invest Urol 159 (1979), the subject matter of which was claimed in U.S. Pat. No. 4,446,122 (reissued as Reissue No. 33,405 in 1990). Distinctions can now be made between patients whose PSA levels are elevated due to prostate cancer and those whose PSA levels are elevated due to benign prostate hypertrophy. For example, U.S. Pat. No. 5,614,372 and U.S. Pat. No. 5,599,677 describe some of the strategies used to distinguish the two forms. These three patents (U.S. Pat. No. 4,446,122, including Re. 33,405, U.S. Pat. No. 5,614,372 and U.S. Pat. No. 5,599,677) are hereby incorporated by reference in their entirety into this application. In particular, U.S. Pat. No. 5,599,677 at column 2, lines 1-13 discusses PCT patent application WO 92/01936. WO 92/01936 is incorporated by reference into U.S. Pat. No. 5,599,677 (see, colunmn 15, lines 51-54). Table 2a, found at page 12 of WO 92/01936 presents the results of the testing of patient samples for free, complexed and total PSA. In the table, BPH indicates benign prostatic hyperplasia, CAP indicates prostate cancer, G indicates the differentiation grade, and T indicates the grade. Table 2a is reproduced below:
Commercially-available PSA assays are commonly sold as kits, and the assays performed in regional or local laboratories. For example, PSA diagnostic kits are sold under the names of: PROS-CHECK PSA, from Yang Laboratories, Inc. Bellevue, Wash.; Hybritech Tandem-E and Hybritech Tandem-R, from Hybritech, Inc., La Jolla, Calif.; Abbott Imx PSA Assay, from Abbott Laboratories, Abbott Park, Ill.; and ACS PSA Assay, from Ciba-Corning Diagnostics Corporation, East Walpole, Mass.
These kits (and other assays, for example, those mentioned in U.S. Pat. Ser. No. 5,688,658) play a critical part of the current strategy for early detection of prostate cancer. A problem arises, however, when a modestly abnormal PSA value (4-10 ng/ml) is encountered in the context of a negative digital rectal exam (DRE). Only 20-30% of individuals with such findings will demonstrate carcinoma on biopsy (Kantoff and Talcott, 8(3) HematollOncol Clinics N Amer 555 (1994)). It has therefore been important to develop strategies that increase the positive predictive value of PSA testing. Such strategies now include establishing age-adjusted normal ranges, determining the free to total PSA ratio, correcting for prostate gland mass (density), and calculating the rate of change of PSA values (Kantoff and Talcott, 8(3) Hematol.Oncol Clinics N Amer 555 (1994) and Brawer, 45 CA-A Cancer J Clinicians 148 (1995)). While each of these strategies has made a contribution, considerable uncertainty nevertheless remains about how to proceed with a patient who is PSA positive and DRE negative.
The present invention provides hope for those patients who have modestly elevated PSA levels, in that additional procedures may be avoided (such as biopsy) or treatment may be initiated earlier in the disease progession. One aspect of the present invention is the effective reduction of available intracellular iron in prostate carcinoma cells prior to administration of a PSA diagnostic test. Most information describing the interaction between iron levels and cancer has been generated in the context of therapeutics, however, and not diagnostics.
Iron is a key nutrient in mammalian health; iron is necessary in proper hemoglobin structure and function (hemoglobin is the primary component of red blood cells), DNA synthesis, and energy transport. In the body, iron is absorbed from food or supplements in the intestine, and transported via the blood to cells by an appropriately-named protein called xe2x80x9ctransferrinxe2x80x9d. Receptors on the cell""s surface recognize and bind the iron-carrying transferrin and the iron is then made available to the cell""s interior. In certain situations, however, it has been shown to be therapeutically desirable to purposely cause a state of partial iron deficiency.
For example, three types of iron deprivation cancer treatment are currently under study. Deferoxamine (called xe2x80x9cDFOxe2x80x9d in this application, but also referred to as desferioxamine in the art) is the first treatment, and it is being investigated in the treatment of neuroblastoma (Donfrancesco et al., 4 Anti-Cancer Drugs 317 (1993)) and hepatoma (Kountouras et al., 42 Hepato-gastroenterol 31 (1995). The second treatment, gallium nitrate, is being investigated in transitional cell carcinoma (Seligman and Crawford, 83(21) J Nat Cancer Inst 1582 (1991) and lymphoma (Chitambar et al., 20(2) Am J Clin Oncol 173 (1997)). The third treatment involves the use of monoclonal antibodies against the transferrin receptor. These antibodies have shown promise against lymphomas in animal models (White et al., 50 Can Res 6295 (1990) and Kemp et al., 55 Can Res 3817 (1995)) and in a Phase I clinical trial (Brooks et al., 1 Clin Cancer Res 1259 (1995). It is now clear that all three forms of treatment, and iron deprivation per se, can induce apoptosis (Ul-Haq et al., 23 Exp Hematol 428 (1995); Hileti et al., 89 Br J of Haematol 181 (1995); Fukuchi et al., 350 FEBS Letters 139 (1994); Kovar et al., Abstract 38C13 in 65 Pathobiology 61 (1997).
In related research, iron appears to be a growth factor for tumor cells that metastasize to bone (Rossi and Zetter, 89 Proc Natl Acad Sci USA 6197 (1992). In addition, one effect of suramin (an agent that is active against prostate cancer) is to partially inhibit the binding of transferrin to its receptor (Forsbeck et al., 37 Scand JHaematol 429 (1986). Moreover, the present inventors found that combined iron deprivation/monoclonal antibody treatment produced significant inhibition of growth of prostate carcinoma cell lines in vitro (Kovar et al., 63 Pathobiology 65 (1995). These results then led to a Phase II trial of DFO as a single agent against disseminated prostate cancer (Dreicer et al., 15(4) Cancer Investigation 311 (1997). In that trial, prostate cancer patients were given consecutive eight hour intravenous infusions of DFO. The DFO treatment was associated with rising PSA levels in eight out of nine patients, even though their disease appeared to be stable by clinical criteria.
The result in Dreicer et al. was contrary to numerous studies which indicated that rising PSA levels indicate recurrent or residual disease. Brawer et al., 33 (5 supp) Urology 11 (1989); Siddal et al., 12 Eur Uro 1 (1986); Starney et al., 317 N Eng J Med 909 (1987); Lange et al., 141 J Urol 873 (1989); Starney et al., 141 J Urol 1076 (1989) Starney et al., 141 J Urol 1088 (1989) and Chan et al., 33 Clin Chem 1916 (1987).
Additionally, recent research has resulted in the production of a high molecular weight hydroxyethylstarch conjugate form of deferoxamine called HES-DFO (Hallaway et al., 86 Proc Natl Acad Sci USA 10108 (1989). HES-DFO has a better toxicity profile than the common mesylate salt form of DFO (ibid.). HES-DFO has been used in pre-clinical studies of combination iron deprivation treatment in a murine lymphoma model (Kemp et al., 55 Can Res 3817 (1995)).
Lastly, in a recent study of seminal plasma samples from patients with thalassemia, Carpino et al. noted that patients with high serum ferritin levels exhibited decreased PSA levels (Carpino et al., 105(3) Exp Clin Endocrin and Diabetes 169 (1997)).
Citation of the above documents is not intended as an admission that any of the foregoing is pertinent prior art. All statements as to the date or representation as to the contents of these documents is based on subjective characterization of information available to the applicant, and does not constitute any admission as to the accuracy of the dates or contents of these documents.
The present invention provides a strategy for increasing the specificity and predictive value of PSA testing. The use of this strategy will result in more prostate cancers being detected earlier and the number of unnecessary prostate biopsies being decreased.
It is therefore an object of the present invention to provide a method to stimulate prostate specific antigen (PSA) production by prostate carcinoma cells.
Specifically, it is an object to provide a method to stimulate prostate specific antigen (PSA) production by prostate carcinoma cells by exposing such tumor cells to an iron chelator.
It is a further object to provide a method for increasing PSA levels in a patient who has undiagnosed prostate cancer by administering an iron chelator and subsequently testing PSA levels.
Definitions (for the purposes of the present application, the following terms shall have the following meanings):
Bioavailable iron shall mean unchelated iron.
The present invention includes a method for stimulating increased PSA production by prostate cancer cells, comprising the steps of reducing the amount of bioavailable iron in prostate cancer cells; and thus causing increased PSA production. Preferably, the method is one wherein the prostate cancer cells are in situ or in vivo.
In one aspect of the present invention, the amount of bioavailable iron in the prostate cancer cells is reduced by means of an iron chelator. In this aspect, the preferable iron chelator is either DFO or HES-DFO. A method wherein the iron chelator is administered via a single bolus injection is most preferred.
In conjunction with the method for stimulating increased PSA production by prostate cancer cells, it is desirable to add the step of quantifying the increased PSA. Preferred means for quantification of PSA is through the use of a commercially-available PSA diagnostic kit, such as: PROS-CHECK PSA; Hybritech Tandem-E; Hybritech Tandem-R; Abbott Imx PSA Assay; and ACS PSA Assay.
Iron may be reduced by any conventional iron-reduction protocol, or by any protocols developed in the future. Essentially, the iron ought to be a) reduced in amount or b) reduced in activity. In other words, some iron needs to be eliminated from the system, or the iron that is in the system needs to be rendered inactive.
To reduce the available iron, one can administer a compound that binds to the iron cations and reduces their ability to affect other materials. Such a compound can temporarily bind the iron cations, and thereby reduce the amount of time the iron is available in the system, or permanently bind the iron cations, and render them unavailable for futher chemical reactions.
One class of compounds that are particularly useful for reducing the amount of available iron are the iron chelators. These compounds are generally competitive binders of ferric iron (Fe3+) which is normally stored within the cell by ferritin.
Deferoxamine and HES-deferoxamine are examples of iron chelators. Other examples are: 1-2 dimethyl-3-hydroxypyrid-4-one (also known as L-1, deferiprone and CP20); N, N-bis(2-hydroxybenzyl)ethylenediamine-N,N-diacetic acid (HBED); desferrithiocin; phytic acid; pyridoxal isonicotinoyl hydrazone (PIH) or any analogues of the compounds in this list. Bacterial iron chelators such as parabactin and vibriobactin are also within the scope of the present invention. Information regarding these compounds is available through the National Institutes of Health""s PubMed database at www.ncbi.nlm.nih.gov/PubMed/.
For prostate cancer cells in situ, the levels of iron available to prostate cancer cells can be lowered so much that DNA synthesis is inhibited. These significantly-altered levels can be achieved using any known method or any method available in the future. Use of iron chelators is particularly effective. Iron chelators can be administered via any medically-acceptable method, although the oral, intravenous and subcutaneous routes are preferred.
The formulation of the iron chelators can be any iron reducing formulation. Any salt form is acceptable, so long as it meets the iron reducing requirement, and any buffer or conjugate is acceptable as well, so long as an ability to reduce available iron is present in the final form. Preferred salt forms are those which are pharmaceutically-acceptable, such as DFO mesylate. Preferred buffers are any which are pharmaceutically-acceptable, such as normal saline. Other preferred aspects of an iron chelator formulation are polymers which improve the function of known chelators, an example of which is HES-DFO.
DFO may be obtained through commercial sources or manufactured de novo, according to a process known in the art. The commercial source of DFO is Novartis (East Hanover, N.J.). DFO may further be altered for a certain purpose according to methods known in the art, especially with regard to pharmaceutical formulations.
The preferred dose of DFO is 50 mg/kg/day, parenterally, with determination of PSA increase on Day 6. However, any dosage and regimen which results in elevated PSA levels is within the scope of the present invention.
HES-DFO may be obtained through commercial sources or manufactured de novo, according to a process known in the art. The commercial source of HES-DFO is Biomedical Frontiers, Minneapolis, Minn. Moreover, HES-DFO can be manufactured de novo according to U.S. Pat. Ser. Nos. 4,863,964 and 5,217,998, which patents are hereby incorporated by reference into this application. HES-DFO may further be altered for a certain purpose according to methods known in the art, especially with regard to pharmaceutical formulations.
The preferred dose of HES-DFO is 25-150 mg/kg, via single bolus injection 96 hours prior to determination of PSA increase. A preferred dose is 50 mg/kg.
Despite the preferred embodiments described above, the iron chelator(s) can be administered at any time which allows increased PSA levels, in the event that prostate cancer is present in the subject. Preferrably, the iron chelators are administered approximately 96 to 120 hours before the time PSA levels are to be identified. More preferred is a method wherein the iron chelators are administered approximately 96 hours before the time PSA levels are to be identified. Of course, the dosage can be tailored to suit the timing of the PSA test. For instance, a higher dose can be given in anticipation of a shorter time between the dosing and the test. A lower dose can be given in anticipation of a longer time between the dosing and the test. Multiple dosings may also be given as needed. The PSA levels will be altered in response to the dosage and timing.
Quantification of PSA levels may be accomplished according to any known procedure, or any procedure developed in the future. In general terms, the known PSA tests share the following steps: first, a sample (generally a biological sample, such as blood, serum, plasma, prostatic fluid, seminal fluid, urine, lymph or spinal fluid) is taken; second, PSA (sometimes particular forms of PSA) amount is determined. The following assays are commercially-available, and more are mentioned at U.S. Pat. Ser. No. 5,599,677, which patent has been incorporated by reference: PROS-CHECK PSA, from Yang Laboratories, Inc. Bellevue, Wash.; Hybritech Tandem-E and Hybritech Tandem-R, from Hybritech, Inc., La Jolla, Calif.; Abbott Imx PSA Assay, from Abbott Laboratories, Abbott Park, Ill.; and ACS PSA Assay, from Ciba-Corning Diagnostics Corporation, East Walpole, Mass. These kits have instructions as part of the packaging, and, for the purposes of the present invention, no refinement of the kit instructions are necessary, even though those in the art are aware of certain modifications that can enhance the assays"" accuracy or precision.
In another aspects of the present invention, there are provided prostate cancer diagnostic kits comprising a first container containing an iron chelator; and second container containing a free-PSA-specific antibody, wherein the free-PSA-specific antibody is detectable. Preferrably, the free-PSA-specific antibody is detectable via a label or via a labeled antibody. In another embodiment, the kits of the present invention can comprise a third container containing a detectable antibody specific for any PSA, so as to allow comparison between free PSA levels and total levels.
Specifically, the preferred kits of the present invention comprise (a) a first container containing an iron chelator; and a commercial PSA assay kit selected from the group consisting of: PROS-CHECK PSA; Hybritech Tandem-E; Hybritech Tandem-R; Abbott Imx PSA Assay; and ACS PSA Assay.
The iron chelator in the present kits can be any iron chelator. Preferred kits contain DFO or HES-DFO in sterile solution, or a combination of both. However, a kit that contains powdered iron chelate with the appropriate amount of buffered sterile water in a separate container is also within the scope of the present invention. The kit can contain appliances needed to administer such iron chelator, for instance, a sterile syringe and needle. The amount in kit can be more than typically needed for the heaviest people, since all need not be used. For instance, a very heavy person of 300 pounds would typically require approximately 7 grams molar equivalent of iron chelator to produce the desired result. Therefore, the kits can contain this amount, and if the entire amount is not used, say for a lighter person, then the remainder can be thrown away. However, it is also possible to provide varying amounts of iron chelator, for varying people. If the iron chelator is provided in solution, then the container holding it is preferrably a puncture resistant one (with the exception of a needle-puncturable section for delivery into the syringe) and either rigid or flexible.
In the present kit, PSA can be detectable by any means, ie. enzyme immunoassay, radioimmunoassay (competitive or non-competitive, including MONO and MOLY sandwich assays), chemi- or bio-luminescent assay, fluroescent immunoassay and DNA based assays (ie. PCR amplification). Particular assays are: resolved immunofluorometry (as in U.S. Pat. Ser. No. 5,688,658, which patent is expressly incorporated by reference in its entirety into this application); chemiluminescence with acridinium esters as labels, enzumatically triggered chemiluminescence with alkaline phosphatase and by horseradish peroxidase; immunoassays using alkaline phosphatase and the fluorogentic substrate 4-methylumbelliferyl phosphate or p-nitrophenyl phosphate, immunoassay using horseradish peroxidase and substrates like ABTS and teramethylbenzidine and time-resolved immunoflurorometric assays with Bu3+. U.S. Pat. No. 5599677 describes immunoassays for prostate specific antigen and Kuriyama et al., 40 Cancer Res. 4658 (1980) describes an enzyme immunoassay which made it possible to detect low concentrations of PSA in the blood of patients with malignant and benign prostate disease.
The free-PSA-specific antibody can be alpha HE, According to U.S. Pat. No. 5,599,677 or others. CanAg Diagnostics AB, Gothenburg, Sweden sell at least 9 major antigenic determinants on the PSA molecule, and other PSA-specific antibodies are characterized in the patent applications which have been incorporated by reference into this application.
Optimally, the present kits include an iron chelator in a container and the materials from commercially-available kits, such as those materials found in PROS-CHECK PSA, from Yang Laboratories, Inc. Bellevue, Wash.; Hybritech Tandem-E and Hybritech Tandem-R, from Hybritech, Inc., La Jolla, Calif.; Abbott Imx PSA Assay, from Abbott Laboratories, Abbott Park, Ill.; and ACS PSA Assay, from Ciba-Corning Diagnostics Corporation, East Walpole, Mass.
Lastly, written instructions can also be included in kit of the present invention. Such instructions would included dosage and administration information, as well as contraindications. Information on quantification, such as protocols to follow, and interpretation of results would also be included.
Also provided as part of the present invention are methods for diagnosing prostate cancer in a human, comprising reducing the amount of available iron in the human, and subsequently testing PSA levels. A preferred method of this type is one wherein the amount of available iron is reduced by means of an iron chelator. In this embodiment, the iron chelator is preferrably DFO or HES-DFO. The additional step of comparing the level of free PSA to bound PSA is also contemplated as within the present method.
Most preferred is a method for diagnosing prostate cancer in a human, comprising reducing the amount of available iron in the human, and subsequently testing PSA levels via the instructions in a commercially available kit, such kit selected from the group consisting of: PROS-CHECK PSA; Hybritech Tandem-E; Hybritech Tandem-R; Abbott Imx PSA Assay; and ACS PSA Assay.
Although, for the most part, PSA assays are indicated for human males at risk of prostate cancer, the present method is not limited to human males. For example, U.S. Pat. Ser. No. 5,688,658 discloses the use of PSA as a marker for breast cancer, and the present invention therefore extends its usefulness to human females as well. Diagnosis of prostate or breast cancer in livestock or companion animals is also within the scope of the present invention. However, the best mode contemplated at the time of filing the present invention is for the use of iron chelators in human males prior to a PSA test.
DFO and HES-DFO may be obtained as described above and formulated according to known and described parameters. Moreover, administration of the iron chelators are according to any medically-acceptable method, especially in the dosage forms, amounts and time periods described above for the use of such chelators in a kit Quantification of PSA levels may be accomplished according to procedure described above as well.

Prostate cancer assays and related methods


The invention relates to a method for selectively restoring a sound signal delivered by an electric source in several parallel mounted transducers, each transducer being specialized for restoring a frequency range of the spectrum of the signal. The method consists more precisely in permanently fractionating the sound signal into frequencies so as to assign each component to the transducer optimized for the corresponding frequency range, without using any additional selection circuit such as filtering circuits.

The invention relates to a method for selectively restoring a sound signal delivered by an electric source in several parallel mounted transducers, each transducer being specialized for restoring a frequency range of the spectrum of the signal. The method consists more precisely in permanently fractionating the sound signal into frequencies so as to assign each component to the transducer optimized for the corresponding frequency range, without using any additional selection circuit such as filtering circuits or others.
The invention also relates to all the associated sets of transducers for implementing the method.

Method of fractionating the sound restoration of modulated signals in parallel mounted transducers, and sets of corresponding transducers


The invention relates to dielectric layers with a low dielectric constant, said layers being used to separate metallic interconnections especially during the production of integrated circuit boards (in the BEOL part of the circuit). According to the invention, the dielectric layer comprises SiC and/or SiOC, and is obtained from at least one precursor comprising at least one —Si—Cn—Si chain where n=1.

The present invention relates to a process for forming a dielectric film that can be used in the fabrication of semiconductors and to novel precursors for implementing this process.
During the fabrication of integrated circuits, for example integrated circuits for commercial applications or for microprocessors, a very large number of successive steps of selectively depositing under vacuum various layers of products are carried out, these successive steps themselves being separated by cleaning steps, for cleaning the silicon wafer (nowadays having a diameter of 300 mm) on which these successive layers are stacked.
In general, two types of successively deposited layer may be distinguished:                the first type of deposition or FEOL deposition essentially consists in depositing a multitude of successive layers in order to create active components of the field-effect transistor type (deposition of photosensitive layer, masking, exposure to UV radiation, cleaning, doping of the single-crystal layer depending on the type of transistor produced, then deposition of the electrodes, then deposition of the electrical contacts on the drain, source and gate zones of each transistor, etc.); and        the second type of deposition or BEOL deposition consists in creating a network of electrical interconnections between the various electrical contacts, especially semiconductors produced during the various steps of the first type of deposition, but also in creating, in particular in the case of the fabrication of random-access memories, the capacitors needed for recording information in digital form. In both these types of application, the aim is essentially to deposit metallic and/or electrically conducting layers, to deposit dielectric layers having a very low dielectric constant separating these conducting layers, and to deposit barrier layers, in particular for preventing diffusion into the lower layers or laterally when one or more lower layers, whether dielectric or conducting, are etched using liquid chemicals or gaseous products while it being desirable for the etching to be selective.
The present invention essentially relates to the production of dielectric layers in the case of the second (BEOL) type of deposition envisioned above.
To increase the integration density of the circuits and correspondingly their data processing power in a smaller and smaller volume, it is necessary to reduce the dimensions (thickness and length) of the interconnect lines, thereby enabling them to be brought closer together and thus saving in volume, provided however that the dielectric layers that separate, on the one hand, the electrical connections of two successive layers in a vertical plane of the circuit, which layers are also called ILD (interlayer dielectric) layers, and, on the other hand, the metal interconnects lying within any one horizontal plane of the circuit, which layers are also called IMD (intermetal dielectric) layers, have improved dielectric properties (even lower dielectric constant) in order to provide sufficient electrical insulation between two connection lines that are closer together, but above all to reduce the time constant of the interconnect circuits (the capacitive component of the time constant has a lower value the lower the dielectric constant of the dielectric, but on the contrary a higher value the closer together its electrodes).
The ever increasing constraints imposed on the electrical interconnect wiring in the upper or BEOL part of the integrated circuit (see above) has recently led to important technological progress being made.
Copper has replaced aluminum as interconnect metal owing to its better electrical conductivity. This has led to the adoption of a new metal deposition process using what is called a "dual-damascene" technique during which a trench is created by the selective etching of a photosensitive resin or photoresist, which trench is then filled with copper using an electrodeposition process.
Moreover, films of SiOC (i.e. carbon-doped silicon oxide) are now widely used as a replacement for the silicon dioxide used previously as dielectric in the interlayer dielectric and intermetal dielectric layers of an integrated circuit.
Finally, with the aim of lowering the dielectric constant of these layers further, dielectrics having a porous structure have been introduced. SiO2 dielectric films deposited by chemical vapor deposition (CVD), from a compound of silicon and oxygen, have a dielectric constant of about 4.0. The aim is to lower the dielectric constant of said layers so as to retain good insulating properties, while reducing their thickness. Films having a low dielectric constant, or low-k films, that is to say those made of materials having a dielectric constant of less than 4, may be produced in many ways, in particular by doping the silicon oxide with organic ligands so as to obtain films having a dielectric constant of between about 2.7 and 3.5. This approach, in which terminal organic ligands are used, that is to say those not connected to the network of the molecule of the final material, leads to the number of oxygen bonds (or "bridges") between two silicon atoms of the —Si—O—Si— type being reduced, which therefore reduces the cohesion of the chain structure and consequently leads to a reduction in the mechanical properties of this type of material compared with those of SiO2. However, these mechanical properties are critical for preventing the successive layers of the integrated circuit from delaminating during the polishing process carried out in a subsequent step in the fabrication of the circuit (chemical mechanical polishing or CMP). Another important parameter to be considered in producing the dielectric layers is the possible existence of internal tensile stresses in the structure of the material constituting the film: most of the electronic structure is created at a temperature of around 300 to 450° C. When the assembly is cooled, the materials, including the silicon substrate, the copper network and the dielectric films, contract differently owing to their different expansion coefficients. This creates a major source of mechanical stresses that degrade the most brittle layer, namely the low-k SiOC layer.
To deposit these low-k SiOC layers, there are presently two processes:                the chemical vapor deposition process, which uses precursors of the dimethyldimethoxysilane (DMDMOS), tetramethylsilane (4MS), tetramethylcyclo-tetrasiloxane (TMCTS) and octamethylcyclotetrasiloxane (OMCTS) type:
                the spin-on deposition process, which uses siloxanes to form films, such as methylsilsesquioxane (MSQ) or hydrogen silsesquioxane (HSQ) in a suitable solvent.
During deposition of the metal interconnect layers and of the SiOC layers that separate and isolate them, it is sometimes necessary to also deposit a thin layer of SiC (called an etch-stop layer) for stopping the passage of chemicals during a step of etching a layer deposited subsequently on top of said etch-stop layer. Silicon carbide is a refractory insulating material having a dielectric constant of about 5, which obviously increases the average dielectric constant of the dielectric layer thus separating two metal interconnect layers.
At the present time it is endeavored to reduce the dielectric constant of these etch-stop layers, especially SiC layers.
The object of the present invention is to improve particularly the mechanical properties of the dielectric layers, especially low-k layers, used in particular as interlayer dielectric layers or as intermetal dielectric layers for said BEOL electrical interconnection of the components of an integrated circuit. To solve the technical problem thus stated, the present invention consists in using precursors of layers of dielectric material containing at least one carbon atom and at least two silicon atoms, the carbon and silicon atoms forming a chain of the —Si—Cn—Si— type with at least one carbon atom linked to two silicon atoms, where n is greater than or equal to 1, preferably less than or equal to 5.
According to a first embodiment, it will be possible to use precursors of the alkoxysilylalkane (RiO)3(Si(—CRi2)n—Si(ORi)3 type, in which formula each radical Ri may be chosen from hydrogen or a carbon chain, for example of the alkyl, aryl, etc. type, with a number of carbon atoms between 1 and 5, preferably 1 or 2 carbon atoms, such as for example BTESE (bis(triethoxysilyl)ethane) of formula (EtO)3Si—CH2—CH2—Si(OEt)3, where Et=C2H5, the dielectric constant and the mechanical properties of which are substantially improved over the layers produced with the precursors of the prior art.
According to another embodiment, the invention consists in using cyclic molecules, which in particular make it possible to produce porous dielectric layers containing rings and chosen especially from one of the following families:

R, R1, R2 being chosen from hydrogen, linear and/or cyclic carbon chains, such as alkyl chains, aryl chains, etc., or alkoxides.
According to the invention it will be preferable to choose molecules such as "double-DMDMOS" (formula I in which R═H and R1=—O—CH3). More preferably still, R will be chosen from the following groups: H, —CH3 or in general an alkyl group, R1 being chosen from —O—CH3 or, more generally, an alkoxide or an amine. Other ligands for R1, such as Cl, Me or H, may also be chosen.
All these molecules contain at least one carbon atom between two silicon atoms. Preferably, these molecules form a ring or a cage (a three-dimensional structure, illustrated by the molecular structure IV) containing at least one Si—C—Si chain (with the possibility of several carbon atoms between two Si atoms). This type of three-dimensional molecule favors the formation of pores in the films obtained, thus ensuring both much greater structural cohesion than molecules with terminal alkyl groups and a very significant reduction in the dielectric constant.
These various precursors may for example be synthesized by reacting dichlorosilylene: —SiCl2 with hydrocarbons having π bonds, such as acetylene or butadiene. The dichlorosilylene source may be hexachlorodisilane or trichlorosilane. It may be formed either at high temperature by decomposition of hexachlorodisilane (HOD) or at low temperature from a solution containing for example a tertiary amine as catalyst (such as trimethylamine). The synthesis of the molecules of formula I will for example be carried out in two steps:                the first step is common to all the molecules according to the reaction:Si2Cl6(or a mixture of polychlorosilanes)+ethylene→Cl2Si—(CH═CH)2—SiCl2,acetylene, butadiene, benzene, etc. leading to this type of reaction according to the same principle: dichlorosilylene: —SiCl2 reacts with the n bond. In general, all unsaturated species are potential candidates for synthesizing this type of molecule;        the second step is specific to the type of molecule in question:Cl2Si—(CH═CH)2—SiCl2+MeOH→(MeO)2Si—(CH═CH)2—Si(OMe)2(for molecule I with R1═OMe);Cl2Si—(CH═CH)2SiCl2+LiAlH4 or NaBH4→H2Si—(CH═CH)2—SiH2(for molecule I with R1═H); andCl2Si—(CH═CH)2SiCl2+Speyer catalyst→Me2Si—(CH═CH)2—SiMe2(for molecule I with R1═Me).
It is also possible to obtain the fluorinated equivalent (for example F2Si—(CH—CH)2—SiF2) with this method, it being possible for this fluorinated equivalent to be used to form low-k films.
In general, a person skilled in the art will find all the information needed to synthesize these various products in, for example, the publication by Atwell and Weyenberg, J. Am. Chem. Soc., 3438, 90, 1968.
According to the invention, the processes and the compositions meet the need, for a person skilled in the art, of forming a thin insulating layer having excellent electrical and mechanical properties and also high uniformity. The films deposited according to the invention are produced by vaporizing a source of low-k precursors consisting of one or more precursors for generating a vapor source of said precursors and by delivering the vapors of these precursors into a deposition chamber in which the precursors are thermally decomposed and/or decomposed by the use of a plasma, forming a film of the desired composition. The film is formed on one or more substrates in a single formation step without a subsequent heat treatment necessarily being required. The low-k film that results therefrom will have the desired composition, so as to have a low leakage current. In general, the precursors used in the process for depositing films to the desired stoichiometry according to the invention will be in liquid phase, for example a liquid precursor or a liquid solution of a precursor in a solvent, such as a hydrocarbon. The precursor in liquid phase is injected into a vaporization system, the rate of vaporization of which is measured and controlled beforehand. This vaporized precursor is taken into the deposition chamber where the deposition is carried out at a pressure of the order of 1 torr (generally less than 200 pascals) and at a temperature generally between 0° C. and 450° C., preferably between 200° C. and 400° C. Optionally, the chamber is provided with a plasma for significantly increasing the deposition rate. In general, but not necessarily, a coreactant is also introduced into the deposition chamber so as to react with the precursor, this coreactant preferably being an oxidizing agent (oxygen, ozone, water vapor, hydrogen peroxide, alcohols, etc.).
According to the invention, the film is deposited by a CVD (chemical vapor deposition) technique, such as the PECVD technique or the thermal CVD technique, these being well known to those skilled in the art, starting from at least one precursor, by itself or in combination with a reactant, preferably an oxygen source. In general, a person skilled in the art will optimize the precursor source by lowering the dielectric constant (k) as much as possible while still maintaining an acceptable level of mechanical properties.
Preferably, the precursor is a molecule containing one or more carbon atoms between two silicon atoms. Even more preferably, this molecule is cyclic.
According to another aspect of the invention, the molecule is a disilane, which decomposes by thermal or plasma excitation and reacts with an unsaturated carbon chain (containing n bonds) to form intermediate species containing —Si—C bonds, preferably to form intermediate species containing a carbon or a carbon chain (whether linear or cyclic) linked to two silicon atoms, during the fabrication of the low-k film. Preferably, the carbons forming a carbon chain connecting the silicon atoms will have at least one double bond because the C═C double bond is stronger than a C—C single bond. This allows low-k films to be obtained that have mechanical properties improved over the films obtained from a chain of carbon atoms containing only single bonds.
Furthermore, the molecules described above that do not contain oxygen atoms have a molecular structure which is very suitable for depositing a thin layer of SiC used as etch stop in the BEOL part of the integrated circuit.
Preferably, the low-k SiC precursor is a molecule containing one or more carbon atoms linked to two silicon atoms, but not containing oxygen. Even more preferably, the molecule is cyclic.
Preferably, the carbon atoms forming a carbon chain connecting the silicon atoms have a double bond (in so far as a C═C double bond is more difficult to break than a C—C single bond), thereby making it possible to achieve high mechanical properties in the resulting films more easily.
According to another aspect of the present invention, the low-k SiC precursor will be a disilane not containing oxygen, which decomposes by thermal or plasma excitation and reacts with an unsaturated carbon chain (containing π (pi) bonds) to form intermediate species containing Si—C bonds, preferably to form intermediate species containing a carbon atom or a carbon chain linked to two silicon atoms, during the fabrication of the SiC film.
According to one other aspect, the invention relates to the dielectric layers formed from the abovementioned precursors and to the use of these precursors for producing low-k dielectric layers.
The invention will be more clearly understood with the aid of the following exemplary embodiments given by way of non-limiting example together with the FIGURE, which shows a sectional view of an integrated circuit with its various layers in its FEOL and BEOL parts.
In the FIGURE, the single-crystal silicon wafer 1 on which the various MOS transistors with their interconnects and their protective layer are produced represents the FEOL part of the integrated circuit, all of the upper layers beyond the stop layer 8 representing the BEOL part in which the electrical interconnects between the various circuits of the FEOL part are produced as explained above.
Produced on the single-crystal substrate 1 is an integrated circuit, represented for example by 2, on which the drain contact (3), the gate contact (4), the source contact (5) and the interconnects in the horizontal level of this circuit, which interconnects are represented by (6), have been shown schematically. A vertical interconnect (7) made of tungsten or copper connects, in the sectional view shown in the FIGURE, a connection (6) through the stop layer (8) to the upper interconnect level (9) in which the copper connection (10), seen in cross section, extends in fact perpendicular to the plane of the FIGURE and is itself connected to the connection (12) at the upper level, then to the connection (15) and then to the connections (15, 18, 28, 31, 34, 37) before terminating at the central connection (38) in the upper interconnect part of the integrated circuit.
Between the various interconnect levels 9, 13, 16, 19, 29, etc. are the respective stop layers 11, 14, 17, 20, 30, 33, 36, 41, etc. which separate copper metal connections in various horizontal planes from one another, by means of ILD dielectric layers such as 21, 22, 23, 24, 25, 26, 27. In a set of layers of any one level, such as for example the set lying between the two stop layers 30 and 33, there is, in the lower part, an ILD dielectric layer for providing isolation between two successive horizontal copper interconnect layers and between the copper connection sidewalls such as 44, 45, 46 and to have an IMD dielectric layer so as to electrically isolate the copper electrical connections such as 44 and 45.
In the example shown in the FIGURE, the ILD and IMD layers are produced for example using the same SiOC dielectric.
The stop layers are in general SiC or SiN layers.
The invention relates to the formation of these interlayer dielectric layers or ILDs and to the formation of intermetal dielectric layers or IMDs.
It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.

Method for forming a dielectric film and novel precursors for implementing said method

L'Air Liquide, Societe Anonyme pour l'Etude et l'Exploitation des Procedes Georges Claude


A method of conditioning an activated sludge in a membrane biological reactor comprising

This invention concerns the use of water soluble cationic polymers to condition biomass in membrane biological reactors and increase water flux through the membrane.
Biological treatment of wastewater for removal of dissolved organics is well known and is widely practiced in both municipal and industrial plants. This aerobic biological process is generally known as the xe2x80x9cactivated sludgexe2x80x9d process in which micro-organisms consume the organic compounds through their growth. The process necessarily includes sedimentation of the micro-organisms or xe2x80x9cbiomassxe2x80x9d to separate it from the water and complete the process of reducing Biological Oxygen Demand (BOD) and TSS (Total Suspended Solids) in the final effluent. The sedimentation step is typically done in a clarifier unit. Thus, the biological process is constrained by the need to produce biomass that has good settling properties. These conditions are especially difficult to maintain during intermittent periods of high organic loading and the appearance of contaminants that are toxic to the biomass.
Membranes coupled with biological reactors for the treatment of wastewater are well known, but are not widely practiced. In these systems, ultrafiltration (UF) or microfiltration (MF) membranes replace sedimentation of biomass for solids-liquid separation. The membrane can be installed in the bioreactor tank or in an adjacent tank where the biomass is continuously pumped from the bioreactor tank and back producing effluent with much lower total suspended solids (TSS), typically less than 5 mg/L, compared to 20 to 50 mg/L from a clarifier. More importantly, MBRs (membrane biological reactors) de-couple the biological process from the need to settle the biomass, since the membrane sieves the biomass from the water. This allows operation of the biological process at conditions that would be untenable in a conventional system including: 1) high MLSS (bacteria loading) of 10-30 g/L, 2) long sludge retention time, and 3) short hydraulic retention time. In a conventional system, such conditions could lead to sludge bulking and poor settleability.
The benefits of the MBR operation are low sludge production, complete solids removal from the effluent, effluent disinfection, combined COD, solids and nutrient removal in a single unit, high loading rate capability, no problems with sludge bulking, and small footprint. The disadvantages are aeration limitations, membrane fouling, and membrane costs.
Membrane costs are directly related to the membrane area needed for a given volumetric flow through the membrane, or xe2x80x9cflux.xe2x80x9d Flux is expressed as liters/hour/m2 (LMH) or gallons/day/ft2 (GFD). Typical flux rates vary from approximately 20 LMH to about 100 LMH. These relatively low flux rates, due largely to fouling of the membranes, has slowed the growth of MBR systems for wastewater treatment.
Membrane fouling can be attributed to two key components in the feed: proteins and colloidal/particulate material. The MBR membrane interfaces with so-called xe2x80x9cmixed liquor:xe2x80x9d water with aggregates of bacteria or xe2x80x9cflocsxe2x80x9d, free bacteria, protozoan, and various dissolved metabolites and cell components. In operation, the colloidal solids and dissolved organics deposit on the surface of the membrane. Colloidal particles form layers on the surface of the membrane, called a xe2x80x9ccake layer.xe2x80x9d Cake layer formation is especially problematic in MBRs operated in the xe2x80x9cdead endxe2x80x9d mode where there is no cross flow; i.e., flow tangential to the membrane. Depending on the porosity of the cake layer, hydraulic resistance increases and flux declines
In addition to cake formation on the membrane, small particles can plug the membrane pores, a fouling condition that may not be reversible. Compared to a conventional activated sludge process, floc (particle) size is reportedly much smaller in typical MBR units. Since MBR membrane pore size varies from about 0.04 to about 0.4 micrometers, particles smaller than this can cause pore plugging. Pore plugging increases resistance and decreases flux.
Therefore, there is an ongoing need to develop improved methods of conditioning the biomass in MBR units, particularly methods that reduce fouling of the membranes. This reduction in fouling, and the associated increase in membrane flux, permits the use of smaller systems, with a concomitant reduction in capital costs, or alternatively, increases treated wastewater volumetric flow from an existing system, with a corresponding reduction in cost of operation.
Polymeric water soluble coagulants and flocculants have not been used in MBR units, as it is generally understood that excess polymer fouls membrane surfaces, resulting in dramatic decreases in membrane flux. However, we have discovered that using water soluble cationic polymers in the MBR process can increase membrane flux by as much as 200 to 500 percent, while leaving virtually no excess polymer in the treated wastewater at the effective dose.
Accordingly, in its principal aspect, this invention is directed to a method of conditioning the biomass in a membrane biological reactor comprising
(i) adding to the biomass an effective amount of at least one water soluble cationic polymer to form a mixture of water and coagulated and flocculated suspended solids; and
(ii) separating the coagulated and flocculated suspended solids from the water by filtering through an ultrafiltration or microfiltration membrane.
As used herein, the following abbreviations and terms have the following meanings: AcAm for acrylamide; DMAEAxc2x7BCQ for dimethylaminoethylacrylate benzyl chloride quatemary salt; DMAEAxc2x7MCQ for dimethylaminoethylacrylate methyl chloride quaternary salt; Epi-DMA for epichlorohydrin-dimethylamine; DADMAC for diallyldimethylammonium chloride; pDADMAC for poly(diallyldimethylammonium chloride); and PEI for polyethyleneimine.
xe2x80x9cCationic polymerxe2x80x9d means a polymer having an overall positive charge. The cationic polymers of this invention are prepared by polymerizing one or more cationic monomers, by copolymerizing one or more nonionic monomers and one or more cationic monomers, by condensing epichlorohydrin and a diamine or polyamineor condensing ethylenedichloride and ammonia or formaldehyde and an amine salt.
xe2x80x9cCationic Monomerxe2x80x9d means a monomer which possesses a net positive charge. Representative cationic monomers include dialkylaminoalkyl acrylates and methacrylates and their quaternary or acid salts, including, but not limited to, dimethylaminoethyl acrylate methyl chloride quaternary salt, dimethylaminoethyl acrylate methyl sulfate quaternary salt, dimethyamninoethyl acrylate benzyl chloride quaternary salt, dimethylaminoethyl acrylate sulfuiric acid salt, dimethylaminoethyl acrylate hydrochloric acid salt, dimethylaminoethyl methacrylate methyl chloride quaternary salt, dimethylaminoethyl methacrylate methyl sulfate quatemary salt, dimethylaminoethyl methacrylate benzyl chloride quaternary salt, dimethylaminoethyl methacrylate sulfuric acid salt, dimethylaminoethyl methacrylate hydrochloric acid salt, dialkylaminoalkylactylamides or methacrylamides and their quaternary or acid salts such as acrylamidopropyltrimethylammonium chloride, dimethylaminopropyl acrylamide methyl sulfate quaternary salt, dimethylaminopropyl acrylamide sulfuric acid salt, dimethylaminopropyl acrylamide hydrochloric acid salt, methacrylamidopropyltrimethylammonium chloride, dimethylaminopropyl, methacrylamide methyl sulfate quaternary salt, dimethylaminopropyl methacrylamide sulfuric acid salt, dimethylaminopropyl methacrylamide hydrochloric acid salt, diethylaminoethylacrylate, diethylaminoethylmethacrylate, diallyldiethylammonium chloride and diallyldimethyl ammonium chloride. Alkyl groups are generally C1-4 alkyl.
xe2x80x9cNonionic monomerxe2x80x9d means a monomer which is electrically neutral. Representative nonionic monomers include acrylamide, methacrylamide, N-methylacrylamide, N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N-isopropyl(meth)acrylamide, N-t-butyl(meth)acrylamide, N-(2)-hydroxypropyl)methacrylamide, N-methylolacrylamide, N-vinylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide, poly(ethylene glycol)(meth)acrylate, poly(ethylene glycol) monomethyl ether mono(meth)acryate, N-vinyl-2-pyrrolidone, glycerol mono((meth)acrylate), 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, vinyl methylsulfone, vinyl acetate, glycidyl(meth)acrylate, and the like.
xe2x80x9cLatex polymerxe2x80x9d means an invertible water-in-oil polymer emulsion comprising a cationic polymer according to this invention in the aqueous phase, a hydrocarbon oil for the oil phase, a water-in-oil emulsifying agent and, potentially, an inverting surfactant. Inverse emulsion polymers are hydrocarbon continuous with the water-soluble polymers dispersed as micron sized particles within the hydrocarbon matrix. The latex polymers are then xe2x80x9cinvertedxe2x80x9d or activated for use by releasing the polymer from the particles using shear, dilution, and, generally, another surfactant, which may or may not be a component of the inverse emulsion.
Latex polymers are prepared by dissolving the desired monomers in the aqueous phase, dissolving the emulsifying agent(s) in the oil phase, emulsifying the water phase in the oil phase to prepare a water-in-oil emulsion, in some cases, homogenizing the water-in-oil emulsion, polymerizing the monomers dissolved in the water phase of the water-in-oil emulsion to obtain the polymer as a water-in-oil emulsion. If so desired, a self-inverting surfactant can be added after the polymerization is complete in order to obtain the water-in-oil self-inverting emulsion.
xe2x80x9cDispersion polymerxe2x80x9d means a water-soluble polymer dispersed in an aqueous continuous phase containing one or more inorganic/organic salts. Representative examples of low molecular weight cationic polymers prepared by dispersion polymerization of water-soluble polymers in an aqueous continuous phase are found in PCT/US01/09060.
xe2x80x9cSolution polymerxe2x80x9d means a water soluble polymer in a water continuous solution.
xe2x80x9cDry polymerxe2x80x9d means a polymer prepared by gel polymerization. In a gel polymerization process, an aqueous solution of water-soluble monomers, generally 20-60 percent concentration by weight, along with any polymerization or process additives such as chain transfer agents, chelants, pH buffers, or surfactants, is placed in an insulated reaction vessel equipped with a nitrogen purging tube. A polymerization initiator is added, the solution is purged with nitrogen, and the temperature of the reaction is allowed to rise uncontrolled. When the polymerized mass is cooled, the resultant gel is removed from the reactor, shredded, dried, and ground to the desired particle size.
xe2x80x9cReduced Specific Viscosityxe2x80x9d (RSV) is an indication of polymer chain length and average molecular weight. The RSV is measured at a given polymer concentration and temperature and calculated as follows:   RSV  =            [                        (                      η                          η              o                                )                -        1            ]        c
wherein xcex7=viscosity of polymer solution;
xcex7o=viscosity of solvent at the same temperature; and
c=concentration of polymer in solution. As used herein, the units of concentration xe2x80x9ccxe2x80x9d are (grams/100 ml or g/deciliter). Therefore, the units of RSV are dl/g. The RSV is measured at 30xc2x0 C. The viscosities xcex7 and xcex7o are measured using a Cannon-Ubbelohde semimicro dilution viscometer, size 75. The viscometer is mounted in a perfectly vertical position in a constant temperature bath adjusted to 30xc2x10.02xc2x0 C. The error inherent in the calculation of RSV is about 2 dl/g. Similar RSVs measured for two linear polymers of identical or very similar composition is one indication that the polymers have similar molecular weights, provided that the polymer samples are treated identically and that the RSVs are measured under identical conditions.
IV stands for intrinsic viscosity, which is RSV in the limit of infinite polymer dilution (i.e. the polymer concentration is equal to zero). The IV, as used herein, is obtained from the y-intercept of the plot of RSV versus polymer concentration in the range of 0.015-0.045 wt % polymer.
The water soluble cationic polymers of this invention are added to the MBR unit to promote the incorporation of colloidal particles, such as cell fragments and single bacterium, into aggregate or floc structures and/or to increase the porosity of the cake layer. The water soluble cationic polymers may be solution polymers, latex polymers, dry polymers or dispersion polymers.
In a preferred aspect of this invention, the water soluble cationic polymer has a molecular weight of about 10,000 to about 2,000,000.
In another preferred aspect, the water soluble cationic polymer has a cationic charge of 100 mole percent
In another preferred aspect, the water soluble cationic polymer is selected from the group consisting of a polymer of epichlorhydrin-dimethylamine crosslinked with either ammonia or ethylenediamine; a linear polymer of epichlorohydrin and dimethylamine, a homopolymer of polyethyleneimine; polydiallydimethylammonium chloride; homopolymer of DMAEM.H2SO4; polymerized triethanolamine/methyl chloride quat, polymerized triethanolamnine and tall oil fatty acid/methyl chloride quat, polyethylenedichloride/ammonia, and modified polyethyleneimine.
In another preferred aspect, the water soluble cationic polymer has a cationic charge of at least 70 mole percent.
In another preferred aspect, the water soluble cationic polymer is a polymer of (meth)acrylamide and one or more cationic monomers selected from diallyldimethylammonium chloride, dimethylaminoethyacrylate methyl chloride quatemary salt, dimethylaminoethylmethacrylate methyl chloride quaternary salt and dimethylaminoethylacrylate benzyl chloride quaternary salt.
In another preferred aspect, the water soluble cationic polymer is diallyldimethylammonium chloride/acryamide copolymer.
The MBR unit combines two basic processes: biological degradation and membrane separation-into a single process where suspended solids and microorganisms responsible for biodegradation are separated from the treated water by a membrane filtration unit. See Water Treatment Membrane Processes, McGraw-Hill, 1996, p 17.2. The entire biomass is confined within the system, providing for both control of the residence time for the microorganisms in the reactor (sludge age) and the disinfection of the effluent.
In general, influent enters the bioreactor, where it is brought into contact with the biomass. The mixture is pumped from the bioreactor and then, under pressure, filtered through a membrane. The permeate is discharged from the system while the entire biomass is returned to the bioreactor. Excess sludge is pumped out in order to maintain a constant sludge age, and the membrane is regularly cleaned by backwashing, chemical washing, or both.
Membranes used in the MBR unit include ultra- and microfiltration, inner and outer skin, hollow fiber, tubular, and flat, organic, metallic, ceramic, and the like. Preferred membranes for commercial application include hollow fiber with an outer skin ultrafilter, flat sheet (in stacks) ultrafilter and hollow fiber with an outer skin microfilter.
The wastewater may be pretreated before entering the MBR. For example, a bar screen, grit chamber or rotary drum screen may be used to achieve coarse solids removal. Suspended solids that are not removed during course solids removal may be removed using a sedimentation tank or a clarifier, called a xe2x80x9cprimary clarifier.xe2x80x9d In some cases coagulants such as ferric chloride or Epi-DMA and an anionic flocculant, such as a co-polymer of acrylic acid and acrylamide, are used in the primary clarifier to remove additional solids. In industrial plants where synthetic oils are present in the untreated wastewater, such as an oil refinery, pretreatment to remove oil is accomplished in units such as the inclined plate separator and the induced air flotation unit (IAF). Often, a cationic flocculant, such as a co-polymer of DMAEM and AcAm, is used in the IAF unit to increase oil removal. Also, excess phosphate is sometimes precipitated in the bioreactor by the addition of metal salts such as ferric chloride, so that the phosphate does not pass through the membrane and into the final effluent.
Depending on the ultimate use of the water and the purity of the MBR permeate, the clarified wastewater may also be subjected to post treatment. For instance, in water reclamation where treated wastewater is ultimately recharged into an aquifer used as a source for drinking water, the permeate may be further treated with reverse osmosis (RO) to reduce the dissolved mineral content If the water is to be recycled into a process, then the requirements of that process may necessitate further treatment of the permeate for removal of recalcitrant organics not removed by the MBR. Processes such as nanofiltration or carbon adsorption might be used in these cases. Finally, all biologically treated wastewater must be disinfected prior to discharge into a receiving stream, generally by addition of sodium hypochlorite, although this is not required for discharge into a municipal sewer.
The cationic polymers are introduced into the aeration basin/bioreactor by various means, for example by dosing into the wastewater feed line ahead of the bioreactor or by dosing directly into the bioreactor.
In all cases, the polymer should be thoroughly mixed with the mixed liquor in the bioreactor to maximize adsorption. This may be accomplished by feeding the polymer into an area of the bioreactor where an aeration nozzle is located. So-called xe2x80x9cdeadxe2x80x9d zones in the bioreactor having little to no flow should be avoided. In some cases, a submerged propeller mixer may be needed to increase mixing in the basin, or the sludge can be re-circulated through a side arm loop.
Solution polymers can be dosed using a chemical metering pump such as the LMI Model 121 from Milton Roy (Acton, Mass.).
The recommended cationic polymer product dosage, based on mixed liquor in the bioreactor, is about 25 to about 1000 ppm, at MLSS (mixed liquor suspended solids) of approximately 1-2%. If the MLSS is lower than 1%, a proportionately lower dosage of polymer may be used. The polymer can be periodically pumped directly to the biorcactor mixed liquor or into the wastewater feed line. If polymer is fed continuously to the wastewater feed, then dosage would be considerably lower, about 0.25 to about 10 ppm. Polymer may be pumped intermittently (xe2x80x9cslug fedxe2x80x9d) or continuously to the wastewater.
Overdosing polymer may result in reduced biological activity and organics removal in the bioreactor. For this reason, a low polymer dosage should be used initially: for example about 25 to about 100 ppm in mixed liquor. Additional polymer can then be fed to increase flux while maintaining biological activity. Permeate TOC (total organic carbon), COD (chemical oxygen demand), or BOD (biological oxygen demand) can be monitored to ascertain biological activity.
Alternately, ajar test can be conducted with samples of mixed liquor. Using a four paddle mixer, the sample jars are dosed with sequentially higher amounts of polymer; one jar is left untreated. After mixing, the samples are allowed to sit forseveral hours, so that the solids can settle to the bottom of the jar. The turbidity of the water on top of the settled solids (supernatant) is measured to ascertain the effectiveness of the polymer dosage. A turbidimeter from Hach Company (Loveland, Colo.) could be used. A dosage that gives lower turbidity in the jar than the untreated sample will increase flux in the MBR.
In the event of a polymer overdose, dosing of cationic polymer should be halted until biological activity returns to normal levels. It may also be necessary to discharge more biosolids from the bioreactor to assist in recovery of bioactivity. Addition of bioaugmentation products containing appropriate bacteria may also be helpful in recovering activity after polymer overdose.
The foregoing may be better understood by reference to the following Examples, which are presented for purposes of illustration and are not intended to limit the scope of this invention.
Representative cationic polymers of this invention are listed in Table 1. Polymers B and C are from Ciba (Tarrytown, N.Y.); Polymers M and N are from BASF (Mount Olive, N.J.). All other polymers are from ONDEO Nalco Company, Naperville, Ill.

Method of using water soluble cationic polymers in membrane biological reactors

Abstract: (i) adding to the activated sludge an effective amount of at least one water soluble cationic polymer to form a mixture of water and coagulated and flocculated suspended solids; and