IMPROVED MEMBRANE

Abstract: A membrane with a hydrophilic surface formed from a polymer blend of a polysulphone and an ethylene oxide/propylene oxide copolymer with a molecular weight cut off of greater than 20,000 can be made by adding pore modifying agents to the polymer blend and/or pore enlarging agents to the quenching liquid used to cast the membrane from the polymer solution.

Description: Improved Membrane The present invention relates to a membrane which can be used in filtration and reverse osmosis and which has a reduced tendency to fouling. Membranes are used in phase separation techniques such as filtration, micro- filtration, reverse osmosis etc. and for the recovery of solids. The membranes can be made of polymeric material and a particular class of polymers are the polysulphones, such as polyether sulphones. Polysulphones have been widely used because of their chemical resistance and good physical properties. "Polysulphone" is used as a generic name for a type of high molecular weight polymer containing aromatic nuclei and sulphone groups in the main chain. A typical sulphone is formed as the condensation product of bisphenol 'A' and dichloro-diphenyl-sulphone. Also widely used are polyether sulphones, polyphenyl sulphones and polyarylether sulphones. However polysulphones have a surface which is hydrophobic and, in use polysulphone membranes are subject to fouling, particularly when being used to filter liquids containing organic material such as proteinaceous material. This fouling results in the build up of a layer on the surface of the membrane which blocks the pores of the membrane and causes deterioration in its performance. It is known to treat the surface of hydrophobic membranes to form a more hydrophilic surface and a method is disclosed in US Patent 4,618,553. Another method of treating a membrane to make it more hydrophilic is disclosed in International PCT Application WO 90/14149. However, previously disclosed methods of modifying hydrophobic membranes to produce a more hydrophilic surface are relatively complex and costly. EP Patent 0407665 Al discloses a blend of a polyether and a polysulphone and a method of making such blends by dissolving both components in n- methylpyrrolidone (NMP), dimethylformamide (DMF) or dimethylacetamide and co-precipitating the polymer blend by a phase inversion process using water. These blends are disclosed as precursor membranes for forming an affinity membrane by reaction of hydroxyl groups on the precursor membrane with biological reactive compounds. However the precursor membranes have a pore size with a nominal molecular weight cut off of between 20,000 and 60,000. Nominal molecular weight cut off is a measure of the pore size of very small pore size filters and is measured by ASTM Method Designation El 343-90 These membranes are not suitable for use as filtration membranes in applications other than in applications below 100 Angstroms (the so called reverse osmosis and nano-filtration range) this is because of the small pore size of the membranes. In addition these membranes do not have permanent hydrophilicity and the polyether copolymer continued to be leached from the membranes. The precursor membrane blends are not dried but are reacted with the biologically active compound in the wet condition. Drying of these blends would cause a collapse of the membrane structure and this limits their use. They also have been found to have a low resistance to compaction under operational pressures (15psig) which results in lowering the molecular weight cut off with use. It is thought that these are the reasons they have not been used other than with a modified surface as disclosed in EP 0 407 665 Al. We have now devised poyether/polysulphone blends with a larger pore size which enables them to be used in filtration processes and a method of making such membranes. According to the invention there is provided a membrane which comprises a blend of a polysulphone and an ethylene oxide/propylene oxide copolymer which has a pore size with a molecular weight cut off of greater than 60,000. Preferably the membranes of the present invention have a pore size with a molecular weight cut off of greater than 250,000 and up to 1 micron (lμ). The polysulphone can be any polysulphone which can be produced in the form of a film, membrane, hollow fibre or other configuration which is conventionally used and preferred polysulphones are polyether sulphones. Polysulphones are described in US Patent 4,230,463. Polysulphones having aromatic hydrocarbyl-containing moieties generally have good thermal stability . Polysulphones and polyether sulphones are sold under the trade names UDEL, P-1700 and P-3500 by Union Carbide, ASTREL 360 Plastic by the 3M Company by ICI pic, and as Ultrasons such as Polysulphone Ultrason S and Polysulphone Ultrason E. The molar ratio of polysulphone to ethylene oxide/propylene oxide copolymer is preferably from 1 : 10 to 2: 1 and more 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 be rendered more hydrophilic with little or no loss in the performance of the membrane. One part of the ethylene oxide/propylene oxide copolymer molecule is substantially miscible with the polysulphone polymer and one part (the more hydrophilic part) can be less miscible. By a variation of the 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 such that the copolymer is substantially water soluble whilst being compatible with the polysulphone in solvent solution. Suitable copolymers which can be used preferably have a weight average molecular weight of 2,000 to 20,000. The molar ratio of ethylene oxide to propylene oxide groups in the ethylene oxide/propylene oxide copolymer is preferably from 1 : 10 to 9: 10. The blends can be prepared by dissolving both polymer components in a solvent and co-precipitating the blend by a phase inversion process. The solvent for the polymers should be one which is inert to the polymers and will dissolve both polymers for example n-methlpyrolidone, dimethylformamide, dimethylacetamide and similar compounds. We have surprisingly found that the addition of pore modifying agents to the solution of the polymers can produce membranes with an increased pore size. The pore modifying agents which can be used are non-solvents such as water, alcohols such as n-butanol, polyethylene glycols (PEG), glycerols, polyvinylpyrrolidones (PNP). The polyethylene glycols which have been found particularly useful in forming polymer blends of the appropriate pore size are those of molecular weight of 200,000 - 800,000. The polyethylene glycol is preferably present in an amount 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 solution of the polymers does not render the solution unstable and can cause an increase in the 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 give rise to a membrane with a more open pore structure which is referred to as a tortuous pore structure. The process which is used to precipitate the polymer blend from the solution is precipitation by the phase inversion process from the solution of the components (polyblend solution) using a precipitation liquid. Preferably the ethylene oxide/propylene oxide copolymer is enriched at the surface of the membrane linking in the phase inversion process, because of the migration of the water soluble component to the colloidal interface. It is thought that the ethylene oxide/propylene oxide copolymer and polysulphone are co-precipitated from the solvent and, because of 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 formed. It is thought that the ethylene oxide/propylene oxide copolymer molecules align themselves with their hydrophilic component aligned towards the precipitation liquid and the non-hydrophilic part aligned towards the hydrophobic polysulphone polymer matrix enriching the surface of the membrane to make it more hydrophilic. The incorporation of the ethylene oxide/propylene oxide copolymer within the polysulphone polymer matrix is indicated by the fact that the ethylene oxide/propylene oxide copolymer cannot be removed by repeated washing and also gives a permanent change of physical characteristics such as strength and pore size. In EP04076651A1 water is disclosed as the precipitation liquid, but we have surprisingly found that, if pore enlarging agents are added to the precipitation liquid membranes are produced with a larger pore size. The pore enlarging agents which 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. The polyethylene glycols which have been found particularly useful in forming polymer blends of the appropriate pore size are those of molecular weight of 200,000 - 800,000. The amount of these pore enlarging agents present in the precipitation liquid can be up to 100% (i.e. up to being the sole precipitation liquid) in the case of the alcohols and glycerols and up to 90% in the case of polyethylene glycols and up to 80 % in the case of the solvents. The ethylene oxide/propylene oxide copolymer can be made by conventional methods. Optionally, after the formation of the composition comprising the polysulphone and ethylene oxide/propylene oxide copolymer, the copolymer may be cross-linked. The cross-linking can be carried out using an appropriate cross-linking agent. Cross-linking agents which can be used are isocyanates, dicarboxlic acid halides, chlorinated epoxides such as epichlorohydrin, cross-linking can also be achieved by UN radiation, for example by use of iso-butronitrile and subsequent reaction with a suitable divalent species. The degree of cross-linking can be controlled by the type and concentration of the cross-linking agent, the duration of the treatment and the temperature. The more severe the cross-linking treatment, the higher the molecular weight of the final cross-linked product. After cross-linking, the membrane is preferably washed to remove excess unreacted ethylene oxide/propylene oxide copolymer. The cross linking virtually eliminated leaching of the copolymer. The membranes of the invention can be of conventional type e.g. in the form of sheets, tubes, hollow fibres etc. It is a feature of the membranes of the invention that the hydrophilicity of a polysulphone membrane can be permanently increased with little or no deleterious effect on its performance in filtration. This increase hydrophilicity will reduce the tendency of the membrane to foul. A further feature of the membranes of the invention is that they have advantages when they are used in microfiltration or ultrafiltration. In microfiltration and ultrafiltration it is important that the membranes are wetted before use i.e. the pores which are filled with air become filled with liquid. With polysulphones this is not possible as they have a low hydrophilicity and in use can involve difficult pre-wetting of the polysulphone membrane with a liquid with a low surface tension e.g. alcohol and trying to ensure that the membrane is completely pre- wetted before it can be used in aqueous filtration. The membranes of the present invention, because of the more hydrophilic nature of the membrane, can be wetted with water and so can be used in microfiltration and, in particular microfiltration membranes are wetted instantaneously on contact with water, and after repeated drying. Normally micro-filtration membranes are supplied dry and wetted for use, and it is a feature of the invention that it can produce micro-filtration membranes which can be dried and subjected to repeated drying without collapse of the structure, which happens with membranes produced by the prior art methods. The micro-filtration membranes of the present invention generally have a pore size of 0. lμ to 1 micron and are hydrophilic. Unlike the membranes made by the process disclosed in EP 0407665A1 the membranes of the present invention can be dried without loss of membrane structure. The process of the present invention can also produce membranes with a "tortuous" structure, this means that the membranes have a sponge like structure rather than a macrovoidal structure and enables greater filtration capacity to be obtained. In a tortuous structure there is an interconnecting of polymer strands which form a reticulated open cell matrix and so the membranes have a high void space. In dead end filtration systems contaminants can penetrate into the membrane matrix. In this type of filtration the dirt holding capacity of the membrane, which is associated with assymetry and void space, is the principle determining factor affecting its performance. The tortuous structure is therefore preferred and the interconnecting of the of the matrix gives enhanced mechanical strength to the membrane (burst strength) compared with the macrovoidal 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 Example 1 A polyether sulphone sold under the Trade Name Ultrason E and an ethylene oxide/propylene oxide copolymer of molecular weight 9800 (Pluronic F82) were dissolved in n-methylpyrolidone (NMP) and stirred until a clear solution was obtained. The solution was cast into a membrane by the phase inversion process by casting the solution on to a plate using water as the precipitant liquid. The weight composition of the solution was 30% polyether sulphone(PES), 10% ethylene oxide/propylene oxide copolymer and 60% n-methylpyrolidone. The membranes formed were left to soak in water for one hour after formation. This membrane was tested and it had a molecular weight cut off of 18,000. The membrane was dried by blowing air over 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 a magnification of 300 and it was found to have a macrovoidal structure. Repeated washing with pure water at 20°C led to leaching of copolymer from the surface. Examples 2 - 12 The process of Example 1 was repeated with various pore modifying agents added to the solution of polysulphone and copolymer in NMP and using different precipitant liquid compositions. The results shown in Table 1 Table 1 The pore size in the Examples 1 to 7 was measured as the molecular weight cut off and measured by ASTM method El 343-90. The pore size in Examples to 11 is the average pore size measured in microns. The temperature measured is the temperature of the quenching liquid. The membranes of Examples 7 - 1 1 were dried by blowing air over them and the membrane structure remained intact enabling them to be handled in the dried state. Repeated washing with pure water at 20 °C did not leach any copolymer from the membrane surface. Example 13 Crosslinked membrane Cross-linking Agent Epichlorohydrin was solubiUsed in an aqueous n-methylpyrolidone blend comprising by weight 75% n-methylpyrolidone, 25% water. The solution was rendered alkaline by the addition of sodium hydroxide until a pH of 13 was achieved. This solution had a composition by weight of 5% epichlorohydrin, 70% n-methylpyrolidone and 25% water. (c) Cross-linking A membrane prepared as in Example 3 with a molecular weight cut off of 150,000 was contacted with the cross-linking agent of (b) for 12 hrs. at a temperature of 20°C. The membrane was washed with water and it could be shown that cross-linking had taken place by the increase in density and reduction in flexibility of the membrane and the fact that no more ethylene oxide/propylene oxide copolymer could be removed with repeated washing. The cross-linked copolymer formed as in Example 1 was compared with the uncross-linked membrane for various fluxes of clean water at varying pressures and the results shown in the accompanying drawing. The cross-linked copolymer was compared with the uncross-linked membrane for various fluxes of clean water at varying pressures and the results shown in accompanying drawing in which the flux of water in litres per square metre per hour is plotted against the time in minutes. As can be seen the cross-linking produces a membrane with an improved flux.

Claims: Claims 1. A membrane comprising a blend of a polysulphone 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. A membrane as claimed in claim 1 in which the membrane has a molecular weight cut of greater than 250,000. 3. A membrane as claimed in claim 1 in which the membrane has a molecular weight cut off greater than 0. lμ. 4. A membrane as claimed in any one of claims 1 to 3 in which the polysulphone is a polyether sulphone. 5. A membrane as claimed in any one of claims 1 to 4 in which the molar ratio of polysulphone to ethylene oxide/propylene oxide copolymer is from 1 :10 to 2:1. 6. A membrane as claimed in any one of claims 1 to 5 in which ethylene oxide/propylene oxide copolymer has a weight average molecular weight of 2,000 to 20,000. 7. A membrane as claimed in claim 6 in which the molar ratio of ethylene oxide to propylene oxide groups in the ethylene oxide/propylene oxide copolymer is from 1:10 to 9: 10. 8. A method of making a membrane comprising a blend of a polysulphone and a copolymer of a polyethylene oxide and a propylene oxide, which method comprises dissolving the polysulphone and the copolymer of a polyethylene oxide and a propylene oxide in a polymer blend solvent which contains a pore modifying agent, to form a polymer blend solution casting the polymer blend solution by quenching with a precipitation liquid. 9. A method as claimed in claim 8 in which the polymer blend solvent is n- methylpyrrolidone, dimethylformamide or dimethylacetamide. 10 A method as claimed in claim 9 in which the pore modifying agent is one or more of water, an alcohol, a polyethylene glycol, glycerol or a polyvinylpyrrolidone. 11. A method as claimed in claim 8,9, or 10 in which the precipitation liquid contains a pore enlarging agent. 12. A method as claimed in claim 11 in which the pore enlarging agent is one or more of methanol, ethanol, a polyethylene glycol, n-methylpyrrolidone, dimethylformamide or dimethylacetamide. 13 A method as claimed in any one of claims 8-12 in which the temperature at which the polymer blend solution is quenched is above 15°C. 14 A membrane as claimed in any one of claims 1 to 7 made by the method of any one of claims 8 - 13. 15. A membrane as claimed in claim 1 made as hereinbefor described with reference to any one of the examples.

Inventors: WILKES JOHN STEVEN, WILKES, JOHN, STEVEN
Applicants: KALSEP LTD, WILKES JOHN STEVEN, KALSEP LIMITED, WILKES, JOHN, STEVEN
Application year: 1995
Main classification: B01D71
IPC codes: B01D67, B01D71, C08J9