Liquid purification or separation – Filter – Material
Reexamination Certificate
2000-10-23
2002-12-17
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Filter
Material
C210S500380, C210S500370, C210S500270, C210S500360, C264S041000, C264S048000, C264S049000
Reexamination Certificate
active
06495043
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a membrane which can be used in membrane filtration processes including microfiltration ultrafiltration and reverse osmosis and which has a reduced tendency to fouling.
Membranes are used in 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, including 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 components 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 U.S. Pat. No. 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 can be relatively complex and costly and are limited in their effectiveness.
EP Patent 0407665A1 discloses a polyether sulphone membrane made from polyether sulphone and other polymeric components and a method of making such blends by dissolving the polymer components in n-methylpyrrolidone (NMP), dimethylformamide (DMF) or dimethylacetamide and co-precipitating the polymer blend by a phase inversion process using water.
Patent Application WO 96/27429 discloses a membrane which comprises a blend of a polysulphone or polyether sulphone and an ethylene oxide/propylene oxide copolymer.
SUMMARY OF THE INVENTION
We have now devised polysulphone/polyether sulphone blended membranes with improved properties and a method of making such membranes.
According to the invention there is provided a membrane which comprises a blend of (a) a polysulphone or a polyether sulphone, and (b) a diamine substituted with moieties comprising polyethylene oxide groups, moieties comprising polypropylene oxide groups, or moieties comprising polyethylene oxide groups and polypropylene oxide groups, wherein the diamine has the formula
where R
1
, R
2
, R
3
and R
4
are the same or different moieties of formula
where X, X1, Y and Y1 can be the same or different in each of R
1
, R
2
, R
3
and R
4
.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferably the compound (I) has a weight average molecular weight of 400 to 100,000 and more preferably from 1.650 to 50.000 and X, Y, X1, Y1 are chosen accordingly.
The compounds of formula (I) can be made by sequential addition of propylene oxide and ethylene oxide to ethylene diamine. Compounds (I) are sold under the Registered Trade Mark “Tetronics” by BASF or as “Poloxamers” or “Superonics” by ICI and are commercially available.
The preferred compounds (I) are those in which R
1
, R
2
, R
3
and R
4
are of formula (II) i.e. the terminal groups are polyoxyethylene.
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 standard polysulphones and polyether sulphones.
Polysulphones are described in U.S. Pat. No. 4.230.463. Polysulphones having aromatic hydrocarbyl-containing moieties generally have good thermal stability. Polysulphones and polyether sulphones suitable for making membranes have molecular weights in the range 20.000 to 80.000 and are sold under the trade names UDEL, P-1700 and P-3500 by Amoco. ASTREL 360 Plastic by the 3M Company and as Ultrasons such as Polysulphone Ultrason S and Polysulphone Ultrason E. by BASF.
The molar ratio of polysulphone to polyethylene oxide/polypropylene oxide substituted diamine in the dope is preferably from 25:1 to 1:40 and more preferably from 8:1 to 1:30.
The membranes of the present invention preferably have a structure such that the ethylene oxide/propylene oxide copolymer substituted diamine 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 can have a greater affinity with the dissolved polysulphone polymer and one part (the more hydrophilic part) can have a greater affinity to the non solvent phase. By a variation of the ethylene oxide/propylene oxide copolymer substituted diamine the properties of the final composition can be varied.
The ethylene oxide/propylene oxide copolymer chains preferably have 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.
The molar ratio of ethylene oxide to propylene oxide groups in the ethylene oxide/propylene oxide copolymer side chains is preferably from 1:10 to 10:1.
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-methylpyrrolidone, dimethylformamide, dimethylacetamide and similar compounds.
The membranes of the present invention preferably have a pore size with a molecular weight cut off of greater than 500 and preferably the membranes of the present invention have a pore size with a molecular weight cut off of greater than 1.000 and with a pore size up to 1 micron (1&mgr;).
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, and polyvinylpyrrolidones (PVP).
The polyethylene glycol is preferably present in an amount up to 80% of the polymer solution, 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 causing 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 (polymer blend 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 chain 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
Baker & Botts L.L.P.
Fortuna Ana
Kalsep Limited
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