MXPA97006645A - Best membrane - Google Patents

Abstract

A membrane with a hydrophilic surface formed of a polymer blend of a polysulfone and a copolymer and ethylene oxide / propylene oxide with a molecular weight cutoff of more than 20,000 can be made by adding pore modifying agents to the polymer mixture and / or agents to enlarge the pore to the cooling liquid used to mold the membrane of the polymer solution

Description

IMPROVED MEMBRANE The present invention relates to a membrane which can be used in filtration and inverted osmosis and which has a reduced tendency to fouling. Membranes are used in phase separation techniques such as filtration, microfiltration, inverted osmosis, etc., and for the recovery of solids. The membranes can be made of polymeric material and a particular class of polymers are the polysulfones, such as the polyether sulfones. Polysulfones have been widely used due to their chemical resistance and good physical properties.

"Polysulfone" is used as a generic name for a type of high molecular weight polymer that contains aromatic nuclei and sulfone groups in the main chain. A typical sulfone is formed as the condensation product of bisphenol "A" and dichlorodiphenylsulphone.

Also widely used are polyether sulfones, polyphenyl sulfones and polyaryl ether sulfones. However polysulfones have a surface which is hydrophobic and, in membranes using polysulfone, are subject to fouling, particularly when used to filter liquids containing organic material such as a proteinaceous material. This embedding gives as REF: 25585 results in the creation of a layer on the surface of the membrane which blocks the pores of the membrane and causes deterioration of its functioning. It is known how to treat the surface of hydrophobic membranes to form a more hydrophilic surfaces and a method is described in US Pat. No. 4,618,553. Another method for treating a membrane to make it more hydrophilic is described in International PCT Application WO 90/14149. However, the methods described above for modifying the hydrophobic membranes to produce a more hydrophilic surface are relatively complex and expensive. Patent EP 0407665A1 describes a mixture of a polyether and a polysulfone and a method for producing such mixtures by dissolving both components in n-methylpyrrolidone (NMP), dimethylformamide (DMF) or dimethylacetamide and coprecipitating the polymer mixture by a phase inversion process using water. These mixtures are described as precursor membranes to form an affinity membrane by the reaction of the hydroxyl groups on the precursor membrane with biological reactive compounds. However, the precursor membranes have a pore size with a nominal molecular weight cutoff of between 20,000 and 60,000. The nominal molecular weight cutoff is a very small pore size measurement of pore size filters and is measured by the ASTM E1343-90 Method Designation. These membranes are not suitable for use as filtration membranes in other applications that are not applications of less than 100 Angstroms (the so-called inverted osmosis and nanofiltration range), this due to the small pore size of the membranes. In addition, these membranes do not have permanent hydrophilicity and the polyether copolymer continues to be leached from the membranes. The precursor membrane mixtures are not dried but are reacted with a biologically active compound in a wet condition. The drying of these mixtures could cause a collapse of the structure of the membrane and thus limit its use. It has also been found that they have a low resistance to compaction under operating expressions (15psig) which results in a decrease in molecular weight cutoff with use. It is thought that these are the reasons why they have only been used with a modified surface as described in EP 0 407 665 A1. Now we have devised polyether / polysulfone mixtures with a large size, which make them capable of being used in filtration processes and a method to produce such membranes.

According to the invention there is provided a membrane which comprises a mixture of a polysulfone and a copolymer of ethylene oxide / propylene oxide which has a pore size with a molecular weight cutoff greater than 60., 000 Preferably the membranes of the present invention have a pore size with a molecular weight cutoff greater than 250,000 and up to 1 micron (1μ). The polysulfone can be any polysulfone that can be produced in the form of a film, membrane, hollow fiber, or other configuration that is used in a conventional manner and preferably the polysulfones are polyether sulfones. Polysulfones are described in U.S. Patent No. 4,230,463. Polysulfones having aromatic hydrocarbyl-containing portions generally have good thermal stability. Polysulfones and polyether sulfones are sold under the trade names UDEL, P-1700 and P-3500 by Union Carbide, ASTREL 360 Plástic by Company 3M by ICI foot, and as Ultrasons such as Polysulfone Ultrason S and Polysulfone Ultrason E. molar ratio of the polysulfone to the ethylene oxide / propylene oxide copolymer is preferably from 1:10 to 2: 1 and most preferably from 1: 5 to 1: 1.

The membranes of the present invention preferably have a structure such that the ethylene oxide / propylene oxide copolymer molecules are concentrated towards the surface of the membrane, so that the more hydrophilic copolymer molecules cause the surface of the material to become more hydrophilic with little or no loss of membrane performance. A part of the ethylene oxide / propylene oxide copolymer film is substantially miscible with the polysulfone polymer and a part (the more hydrophilic part) may be less miscible. By a variation of ethylene oxide / propylene oxide copolymer the properties of the final composition can be varied. The ethylene oxide / propylene oxide copolymer preferably has a ratio of ethylene oxide to propylene oxide groups so that the copolymer is substantially soluble in water although it remains compatible with the polysulfone in solvent solution. Suitable copolymers that can be used preferably have a weight average molecular weight of 2,000 to 20,000. The molar ratio of the ethylene oxide to propylene oxide groups in the ethylene oxide / propylene oxide copolymer is preferably from 1:10 to 9:10. The mixtures can be prepared by dissolving both polymeric components in a solvent and coprecipitating the mixture by a phase inversion process. The solvent for the polymers should be one which is inert to the polymers and can be dissolved in both polymers for example n-methylpyrrolidone, dimethylformamide, dimethylacetamide and the like. It has surprisingly been found that the addition of pore modifying agents to the solution of the polymers can produce membranes of increased pore size. The pore modifying agents that can be used are solvents such as water, alcohols such as n-butanol, polyethylene glycols (PEG), glycerols, polyvinylpyrrolidones (PVP). The polyethylene glycols which have been found particularly useful in the formation of polymer blends of the appropriate pore size are those with molecular weights of 200,000-800,000. The polyethylene glycol is preferably present in an amount of up to 80% of the liquid, the PVP up to 50%, the butanol up to 20%, the glycerols up to 20% and the water up to 15%. It is very surprising that the addition of these compounds to the polymer solution does not render the solution unstable and may cause an increase in pore size. This is particularly true in the case of polymers such as PVP and PEG. It has also been found that the use of such additives can result in a membrane with a more open pore structure which is known as a tortuous pore structure. The process that is used to precipitate the polymer mixture of the solution is the precipitation by the process of phase inversion of the solution of the components (polymer mixture solution) using a precipitation liquid. Preferably, the ethylene oxide / propylene oxide copolymer is enriched on the surface of the ligand membrane in the phase inversion process, due to the migration of the water soluble component to the colloidal interface. It is thought that the ethylene oxide / propylene oxide copolymer and the polysulfone co-precipitate from the solvent and, due to the more hydrophilic nature of the ethylene oxide / propylene oxide copolymer, the copolymer migrates to the solvent / precipitation liquid interface thus enriching the surface of the membrane. The ethylene oxide / propylene oxide copolymer molecules are thought to align themselves with their hydrophilic component aligned to the precipitation liquid and the non-hydrophilic part aligned to the hydrophobic polysulfone polymer matrix by enriching the surface of the membrane to make it more hydrophilic.

The incorporation of the ethylene oxide / propylene oxide copolymer with the polysulfone polymer matrix is indicated by the fact that the ethylene oxide / propylene oxide copolymer can not be removed by repeated washing and also gives a permanent change of Physical characteristics such as strength and lining size. In EP04076651A1 water is described as the precipitation liquid, but it has surprisingly been found that, if agents are added to enlarge the pore, membranes with a larger pore size are produced in the precipitation liquid. The pore size enlarging agents that can be used are low molecular weight alcohols such as methanol, ethanol, polyethylene glycols, glycerols, solvents such as NMP, DMF, dimethyl acetamide and the like. Polyethylene glycols which have been found particularly useful for forming polymer blends of appropriate pore size are those with molecular weights of 200,000-800,000. The amount of these pore enlargement agents present in the precipitation liquid can be up to 100% (ie until it is the only precipitation liquid) in the case of alcohols and glycerols up to 90% in the case of polyethylene glycols and up to 80% in the case of solvents.

The ethylene oxide / propylene oxide copolymer can be made by conventional methods. Optionally, after the formation of the composition comprising the polysulfone and the ethylene oxide / propylene oxide copolymer, the copolymer can be crosslinked. The crosslinking can be carried out using an appropriate crosslinking agent. Crosslinking agents which can be used are isocyanates, dicarboxylic acid halides, chlorinated epoxides such as epichlorohydrin, crosslinking can also be achieved by UV radiation, for example by the use of iso-butronitrile and the subsequent reaction with suitable species and variants. The degree of crosslinking can be controlled by the type and concentration of crosslinking agent, the duration of the treatment and the temperature. The more severe the crosslinking treatment, the higher the molecular weight of the final crosslinked product. After cross-linking, the membrane is preferably washed to remove the excess unreacted ethylene oxide / propylene oxide copolymer. The crosslinking agent is virtually eliminated by leaching the copolymer. The membranes of the invention can be of the conventional type, for example, in the form of sheets, tubes, hollow fibers, etc.

A feature of the membranes of the invention is that the hydrophilicity of a polysulfone membrane can be permanently increased with little or no harmful effect on its filtration performance. This increased hydrophilicity will reduce the tendency of the membrane to undergo incrustation. An additional feature of the membrane of the invention is that they have advantages when used in microfiltration or ultrafiltration. In microfiltration and ultrafiltration it is important that the membranes are moistened before they are used, that is, that the pores are filled with air before being filled with liquid. With the polysulfones this is not possible since they have a low hydrophilicity and in use they can involve a difficult moistening of the polysulfone membrane with a liquid with a low surface tension, for example alcohol and try to ensure that the membrane is completely pre-wetted before that can be used in aqueous filtration. The membranes of the present invention, due to the more hydrophilic nature of the membrane, can be moistened with water and thus can be used in microfiltration and, in particular in the microfiltration membranes, they are moistened instantaneously on contact with water, and subsequently they dry repeatedly.

Normally microfiltration membranes are supplied dried and moistened for use, and a feature of the invention is that microfiltration membranes can be introduced which can be dried and subjected to repeated drying without collapsing the structure, which is expected with the membranes produced by the methods of the prior art. The microfiltration membranes of the present invention generally have a pore size of O.lμ to micron and are hydrophilic. In contrast to the membranes made by the process described in EP 0407665A1, the membranes of the present invention can be dried without loss of the structure of the membrane. The process of the present invention can also produce membranes with a "tortuous" structure, this means that the membranes have a structure similar to that of a sponge instead of a macro-ovoid structure and allow a greater filtering capacity to be obtained. In a tortuous structure there is an interconnection of polymer strands that form a reticulated open cell matrix and the membranes have a high vacuum species. In dead-end filtration systems, contaminants can penetrate into the membrane matrix. In this type of filtration the dust retention capacity of the membrane, which is associated with asymmetry and empty spaces, is the main determining factor that affects its operation. Therefore the tortuous structure is preferred and the interconnection of the matrix gives greater mechanical resistance to the membrane (bursting resistance) compared to the macro-ovoidal structure. The invention will now be described with reference to the following examples in which Example 1 is an example of a membrane prepared by the process of EP 0407 665A1.

Use 1 A polyether sulfone sold under the Trade Name of Ultrason E and a copolymer of ethylene oxide / propylene oxide of molecular weight of 9800 was dissolved.

(Pluronic F82) in n-methylpyrrolidone (NMP) and stirred until a clear solution was obtained. The solution was molded into a membrane by the phase inversion process by molding the solution onto a plate using water as a precipitating liquid. The composition by weight of the solution was 30% polyether sulfone (PES), 10% ethylene oxide / propylene oxide copolymer and 60% n-methylpyrrolidone. The formed membranes were left immersed in water for one hour after the formation.

This membrane was te and had a molecular weight cutoff of 18,000. The membrane was dried by blowing air on it and the structure collapsed and was unable to be used as a membrane. The structure of the membrane was examined under a microscope at an amplification of 300 and found to have a macro-ovoid structure. Repeated washing with pure water at 20 ° C led to the leaching of the surface copolymer.

Examples 2-12 The process of Example 1 was repeated with various pore modifying agents added to the polysulfone and NMP copolymer solution and using different precipitation liquid compositions. The results are shown in Table 1.

Table 1 Eg PES Cop. NMP Liquid Agent Temp Precipitant Size Modifier ° C of Poro Poro % in% in% in% in weight weight in weight 1 30 10 60 Water - -4 3-5K 2 25 10 65 50:50 H20: ETOH - -5 10K 3 20 15 40 Water 25 PEG 400 20 25-30K 4 20 15 40 70:30 H20: NMP w 20 30-50K 20 15 40 50:50 H20: NMP II 20 150-200 6 17.3 3.8 55.7 Water 10.6 Glycerol 20 200-300K 7 18 10 22 50:50 H20: NMP 50 PEG 400 25 500K 18 10 22 50:50 H¿0: NMP 50 PEG 400 16 64 50:50 H20: NMP 9 Glycerol 7 RRP 16 64 50:50 H20: NMP 9 Glycerol 70 0.5μ 7 PVP 11 15 15 15 70:30 H, 0: NMP 55 PEG 400 70 0.8μ The pore size in Examples 1 to 7 was measured as the cutoff molecular weight and was measured by the method ASTM E1343-90. The pore size in Examples a 11 is the average pore size measured in microns. The measured temperature is the temperature of the cooling liquid. The membranes of Examples 7-11 were dried by blowing air over them and the structure of the membrane remained intact allowing them to be handled dry. Repeated washing with pure water at 20 ° C did not leach any copolymer from the surface of the membrane.

Example 13 Reticulated membrane Crosslinking Agent The epichlorohydrin was solubilized in an aqueous n-methylpyrrolidone mixture comprising 75% n-methylpyrrolidone, 25% water. The solution was made alkaline by the addition of sodium hydroxide until a pH of 13 was reached. This solution had a weight composition of 5% epichlorohydrin, 70% n-methylpyrrolidone and 25% water. (c) Crosslinking agent A membrane prepared as in Example 3 with a molecular weight cutoff of 150,000 was contacted with the crosslinking agent from (b) for 12 hours at a temperature of 20 ° C. The membrane was washed with water and it could be demonstrated that the crosslinking took place due to the increase in the density and reduction of the membrane flexibility and the fact that no more ethylene oxide / propylene oxide copolymer could be removed with the repeated washing . The crosslinked copolymer formed as in Example 1 was compared to the uncrosslinked membrane at several streams of cleaning water at various pressures and the results are shown in the accompanying drawings. The crosslinked copolymer was compared to the uncrosslinked membrane during several flows of clean water at various pressures and the results are shown in the accompanying drawings in which the water flow in liters per square meter per hour is plotted against the time in minutes. As can be seen, the crosslinking produces a membrane with improved flow.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (15)

1. A membrane, characterized in that it comprises a mixture of a polysulfone and a copolymer of a polyethylene oxide and a propylene oxide in which the membrane has a pore size with a molecular weight cut-off greater than 20,000.

2. The membrane according to claim 1, characterized in that the membrane has a molecular weight cut-off greater than 250,000.

3. The membrane according to claim 1, characterized in that the membrane has a molecular weight cut-off greater than O.lμ.

4. The membrane according to any of claims 1 to 3, characterized in that the polysulfone is a polyether sulfone.

5. The membrane according to any of claims 1 to 4, characterized in that the molar ratio of polysulfone to ethylene oxide / propylene oxide copolymer is from 1:10 to 2: 1.

6. The membrane according to any of claims 1 to 5, characterized in that the ethylene oxide / propylene oxide copolymer has a weight average molecular weight of 2,000 to 20,000.

7. The membrane according to claim 6, characterized in that the molar ratio of the ethylene oxide to propylene oxide groups in the ethylene oxide / propylene oxide copolymer is from 1:10 to 9:10.

8. A method for producing a membrane comprising a mixture of a polysulfone and a copolymer of a polyethylene oxide and a propylene oxide, method characterized in that it comprises dissolving the polysulfone and the copolymer of a polyethylene oxide and a propylene oxide in a solvent of polymeric mixture containing a pore mying agent, to form a polymeric mixture solution by molding the polymeric mixture solution by cooling with a precipitation liquid.

9. The method according to claim 8, characterized in that the polymer mixture solvent is n-methylpyrrolidone, dimethylformamide or dimethylacetamide.

10. The method of compliance of claim 9, characterized in that the pore mying agent is one or more of water, an alcohol, a polyethylene glycol, glycerol or a polyvinylpyrrolidone.

11. The method according to claim 8, 9 or 10, characterized in that the precipitation liquid contains an agent for increasing the pore size.

12. The method in accordance with the claim 11, characterized in that the agent for enlarging the pore size is one or more of methanol, ethanol, a polyethylene glycol, n-methylpyrrolidone, dimethylformamide or dimethylacetamide.

13. The method according to any of claims 8-12, characterized in that the temperature at which the polymer mixture solution is cooled is higher than 15 ° C.

14. The membrane according to any of claims 1 to 7, characterized in that it is made by the method according to any of claims 8-13.

15. The membrane according to claim 1, characterized in that it is made as described above with reference to any of the examples.