CA2212526A1

CA2212526A1 - Improved membrane

Info

Publication number: CA2212526A1
Application number: CA002212526A
Authority: CA
Inventors: John Steven Wilkes
Original assignee: Individual
Current assignee: Kalsep Ltd
Priority date: 1995-03-03
Filing date: 1995-10-23
Publication date: 1996-09-12
Also published as: GB9504251D0; CN1177308A; AU3703995A; EP0814898A1; JPH11501251A; WO1996027429A1; MX9706645A

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

CA 02212~26 1997-08-07 Improved Mel.ll,l~le The present invention relates to a lllelllbl~le which can be used in filtration and reverse o.cmc si.c and which has a reduced tendency to fouling.

Membranes are used in phase sepa-~lion techniques such as filtration, micro-filtration, reverse osmosis etc. and for the recovery of solids. The lllt;lllbl~les 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 ~h~mic~l recict~n~e and good physical properties. "Polysulphone" is used as a generic name for a type ofhigh molecular weight polymer co--~ aromatic nuclei and sulphone groups in the main chain.

A typical sulphone is formed as the con~l~n.c~tion product of bisphenol 'A' and dichloro-dil,hellyl-slllphon~ Also widely used are polyether sulphones, polyphenyl sulphones and polyarylether sulphones. However polysulphones have a surface which is hydrophobic and, in use polyclllphnn~ membranes are subject to fouling, particularly when being used to filter liquids co..~ il-g organic material such as protçin~ceous material. This fouling results in the build up of a layer on the surface of the lll~llll,l~u~e which blocks the pores of the membrane and causes deterioration in its pelr~ ce.

It is known to treat the surface of hydrophobic lllelllbl~es to form a more hydrophilic surface and a method is disclosed in US Patent 4,618,553. Another method of treating a lllell~l~le to make it more hydlophilic is disclosed in Tnt~rn~tional PCT Application WO 90/14149.

However, previously disclosed methods of modify~ing hydluphobic mellll,l~les to produce a more hydrophilic surface are relatively colll~ and costly.
i CA 02212~26 1997-08-07 EP Patent 0407665A1 ~iier.loses a blend of a polyether and a polysulphone and a method of making such blends by dissolving both components in n-lllrLhyl~y~lulidone (NMP), dimt;Lllylrul",~ ide (DMF) or dimethyl~cet~mi~le and co-precipiLaLil~ the polymer blend by a phase inversion process using water.

These blends are disclosed as precursor ~ llLli~1es for forming an affinity lllt;lllbl~i1e by reaction of hydluxyl 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 beLwrrll 20,000 and 60,000.
Nominal molec~ r weight cut off is a measure of the pore size of very srnall pore size filters and is measured by ASTM Method D~eiPn~tion E1343-90.
These membranes are not suitable for use as filtration mt:lllblilnes 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 lll~lllbl~nes. In addition these ll~c;lll~l~les do not have prllnallrllL hydrophilicity and the polyether copolymer continued to be leached from the melllblililes. The precursor mt;~ ~le blends are not dried but are reacted with the biologically active compound in the wet condition. Dry-ing 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 lre;el~"l;e to compaction under operational pressures (15psig) which results in luwr-iilg the molecular weight cut offwith use.

It is thought that these are the reasons they have not been used other than witha modified surface as disclosed in EP 0 407 665 A1.

We have now devised poyether/polyslllrh~me blends with a larger pore size which enables them to be used in filtration processes and a method of making such melllbli~les.

According to the invention there is provided a lllelllbl~ile which colll~lises ablend of a polysulphone and an ethylene oxide/propylene oxide copolymer which has a pore size with a molecular weight cut offof greater than 60,000.

CA 02212~26 1997-08-07 Preferably the lllc;lllbl~les of the present invention have a pore size with a molecular weight cut off of greater than 250,000 and up to 1 micron (1~).

The polysulphone can be any polysulphone which can be produced in the form of a film, membrane, hollow fibre or other configuration which is conv~ntion~lly used and plc;rt;lled polysulphones are polyether s ~lphones Polys -lph~nes are described in US Patent 4,230,463. Polysulphones having aromatic hydrocarbyl-co~ g moieties generally have good thermal stability .
Polysulphones and polyether s -lphones are sold under the trade narnes UDEL, P-1700 and P-3500 by Union Carbide, ASTREL 360 Plastic by the 3M
Company by ICI plc., 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 plert:l~bly from 1:5 to 1:1.

The mellll,l ~ules of the present invention preferably have a structure such that the ethylene oxide/propylene oxide copolymer molecllles are conce~ dLed towards the surface of the l-lt;lllbl~le, 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 pelrc llllallce of the Ill~lllI,ldne.

One part of the ethylene oxide/propylene oxide copolymer molecule is subst~nti~lly 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 ~l~pelLies of the final composition can be varied.

The ethylene oxide/propylene oxide copolymer plt;~;l~ly has a ratio of ethylene oxide to propylene oxide groups such that the copolymer is subst~nti~lly water soluble whilst being compatible with the polyslllrhon~ in solvent sollltion CA 02212~26 1997-08-07 Suitable copolymers which can be used preferably have a weight averagemolecular 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 pl~rt;l~bly from 1:10 to 9:10.

The blends can be pl~;palt;d by dissolving both polymer components in a solvent and co-plt;~ g 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, dillleLllylr~ .dP, dimethyl~cet~mi(1e and similar compounds.

We have surprisingly found that the addition of pore modifying agents to the solution of the polymers can produce lll~lllI,l~nes 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, polyvhlyl~yllolidones (PVP). The polyethylene glycols which have been found particularly useful in r~,lll~ng polymer blends of the appropliate pore size arethose of molecular weight of 200,000 - 800,000.

The polyethylene glycol is pl~r~l~bly 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 poly}ners 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 preripit~te the polymer blend from the solution is pl ~ci~iL~Iion by the phase inversion process from the solution of the components (polyblend sollltif~n) using a pl~cipiL~tion liquid.

CA 02212~26 1997-08-07 WO 96127429 PCT/GB95/02~;00 Preferably the ethylene oxide/propylene oxide copolymer is Pn~ .hed at the surface of the ".e.l~l~.e linking in the phase inversion process, because of themigration of the water soluble component to the colloidal interface.

It is tho~lght that the ethylene oxide/propylene oxide copolymer and polysulphone are co-p~ecipiL~led from the solvent and, because of the more hydrophilic nature of the ethylene oxide/propylene oxide copolymer, the copolymer migrates to the solvent/pre~ it~tion liquid interface thus ell.iclf...g the surface of the ...e...l,.dne formed. It is thought that the ethylene oxide/propylene oxide copoly~mer molecules align thPm~elves with their hydrophilic component aligned towards the l..t;ci~iL~Lion liquid and the non-hydrophilic part aligned towards the hydl'uphobic 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 pe ---al-t; ~~ change of physical characteristics such as strength and pore size.

In EP04076651A1 water is disclosed as the ple~i~iLalion liquid, but we have surprisingly found that, if pore enlarging agents are added to the p-~ iL~Lion 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, di--~Lhyl ?~cet~mide and the like. The polyethylene glycols which have been found particularly useful in forming polymer blends of the app-up,iate pore size are those of molecular weight of 200,000 - 800,000. The a~nount of these pore enlarging agents present in the p.~ ;on liquid can be up to 100% (i.e. up to being the sole preGiri~tion 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.

CA 02212~26 1997-08-07 The ethylene oxide/propylene oxide copolymer can be made by convf~ntiQn~l methods. Optionally, after the formation of the composition cu~ hlg the polysulphone and ethylene oxide/propylene oxide copolymer, the copolymer may be cross-linked.

The cross-linking can be calTied out using an applopliaLe cross-linking agent.
Cross-linking agents which can be used are iso-;y~l~Les, dicarboxlic acid halides, chlorinated epoxides such as epichlorohydrin, cross-linking can also be achievedby W radiation, for example by use of iso-l~uLlollillile and subsequent reactionwith a suitable divalent species. The degree of cross-linking can be controlled by the type and co.~ ion of the cross-linking agent, the duration of the tre~tment and the telllpel~ re. The more severe the cross-linking trç~tmPnt the higher the molecular weight of the final cross-linked product. A~er cross-linking, the membrane is preferably washed to remove excess unreacted ethylene oxide/propylene oxide copolymer. The cross linking virtually ~limin~te(l le~r~hing 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 lllelnbl~les of the invention that the hydrophilicity of a polysulphone membrane can be perm~n~ntly increased with little or no deleterious effect on its pelr~ ce in filtration. This increase hydrophilicity will reduce the tendency of the lllelllbl ~le to foul.

A further feature of the mellll,l~u~es of the invention is that they have advantages when they are used in microfiltration or ultrafiltration. In microfiltration andultrafiltration it is illlpolL~lL that the membranes are wetted before use i.e. the pores which are filled with air become filled with liquid. With polyslllrh~ nes this is not possible as they have a low hydrophilicity and in use can involve difficult pre-wetting of the polyslllrhon~ lllelll~ ne with a liquid with a low surface e tension e.g. alcohol and trying to ensure that the lllellll)l~le is complet~ly pre-wetted before it can be used in aqueous filtration. The lllclllbl~es of the present invention, because of the more hydrophilic nature of the lllelllbl~le, can be wetted with water and so can be used in microfiltration and, in particular CA 02212~26 1997-08-07 WO 96/27429 PcrlGss5lo25 microfiltration mGllll)l~les are wetted inet~nt~neously on contact with water, and ' after repeated drying.

Normally micro-filtration lllGlllbl~s are supplied dry and wetted for use, and it is a feature of the invention that it can produce micro-filtration lllenlbl~iles which can be dried and subjected to repeated dry-ing without collapse of the structure, which hal~pens with lll~;lllbl ~es produced by the prior art methods.

The micro-filtration mellll,l~nes of the present invention generally have a poresize of 0.1~1 to 1 micron and are hydlupllilic.

Unlike the lllGlllbl~ilGs made by the process disclosed in EP 0407665Al the c:lllbl~les of the present invention can be dried without loss of membrane structure.

The process of the present invention can also produce lll~ es with a "tortuous" structure, this means that the lll~;lllbl~es 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 hlLGrc~ l~nec~ of polymer strands which form a reticulated open cell matrix and so the mellll,l~eshave a high void space. In dead end filtration systems cQ..~ can penetrate into the membrane matrix. In this type of filtration the dirt holding capacity of the lllt;lllbl~ne, which is associated with assymetry and void space, is the principle c~ il-g factor affecting its pGll~..lAI~ce. The tortuous structure is therefore pl~rt;llGd and the hlLelcû...~ecting of the of the matrix gives ~nh~nced m~-~.h~nical strength to the membrane (burst ~Llc:llgLIl) colll~aled with the macrovoidal structure.

The Invention will now be described with lt;rGlGllce to the following ~",~ll~ules in which F~mple 1 is an example of a lllGlllI~l~ne plG~ed by the process of EP
0407 665Al CA 02212~26 1997-08-07 F,Y~mple 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 ~ ne by the phase inversion process by casting the sol~tion on to a plate using water as the p~ liquid. The weight composition of the solution was 30% polyether sulphone(PES), 10%
ethylene oxide/propylene oxide copolymer and 60% n-m~Lllyl~yl~lidone. The lll~n~ es 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 l,.e".l,l~ne was dried by blowing air over it and the structure collapsed and was unable to be used as a llle.lll,l~le. The structure of the membrane was ~Y~mined under a microscope at a m~ nific~tion 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 di~lwlL
precipitant liquid compositions. The results shown in Table 1 Tahle 1 . . .
Ex. PES Cop. NMP PIG~ LaIII Pore Mod TempPore Liquid Agent ~CSize %wt. %wt %wt %wt.

Water - -4 3-5K

2 25 10 65 50:50 H2O:ETOH - -5 lOK

3 20 15 40 Water 25 PEG 400 2025-30K

4 20 15 40 70:30 H20:NMP " 2030-50K
50:50 H20:NMP " 20150-200K
6 17.33.855.7 Water 10.6 Glycerol 20200-300K
7 18 10 22 50:50 H2O:NMP50 PEG 400 25500K
8 18 10 22 50:50 H20:NMP50 PEG 400 60 .1 9 16 4 64 ;0:50 H20:NMP9 Glycerol 50 0.2 16 4 64 50:50 H20:NMP9 Glycerol 70 0.5 I l 15 15 15 70:30 H2O:NMP55 PEG 400 70 0.8 The pore size in the Examples 1 to 7 was measured as the molecular weight cut off and measured by ASTM method E1343-90. The pore size in Examples to I l is the average pore size measured in nli~,lullS.

The telll~elal~re measured is the lellli)el~L~re ofthe qu~nchin~ liquid.

The InGIIIbl ~ GS of Examples 7 - I I were dried by blowing air over them and the nlbl ai~e structure r~ e(l intact enabling them to be handled in the dried state. Repeated wa:~hillg with pure water at 20 ~C did not leach any copolymer from the nIGIIIIJIane surface.

CA 02212~26 1997-08-07 Cros~linked ~ lllbl~le Cross-linking Agent Epichlorohydrin was solubilised in an aqueous n-llleLllyl~y,olidone blend comprising by weight 75% n-m~LllylL)y~ one, 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-mt;Lhyl~ylolidone and 25% water.

(c) Cross-linking A membrane p,e~ ed as in Example 3 with a moler,~ r weight cut off of 150,000 was ct nt~ctecl with the cross-linking agent of (b) for 12 hrs. at a ten~ Lule 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 reducti--n in flexibility of the lllc;llll,l~le and the fact that no more ethylene oxide/propylene oxide copolymer could be removed with l~pealed washing.

The cross-linked copolymer formed as in Example 1 was com~alt;d with the uncross-linked lllelllll~e for various fluxes of clean water at varying ples~LIles and the results shown in the ~cco...~ .yillg drawing. The cross-linked copolymer was colllp~d with the uncross-linked llwln~ e for various fluxes of clean water at varying pressures and the results shown in accc,lllp~lyillg drawing in which the flux of water in litres per square metre per hour is plotted against the time in ...;...-les As can be seen the cross-linking produces a wllll)l~le with an improved flux.

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 0.1µ.

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 ofpolysulphone 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.

12 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.