CA2015413C - Process for preparing a microporous membrane and such a membrane - Google Patents

Process for preparing a microporous membrane and such a membrane

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Publication number
CA2015413C
CA2015413C CA002015413A CA2015413A CA2015413C CA 2015413 C CA2015413 C CA 2015413C CA 002015413 A CA002015413 A CA 002015413A CA 2015413 A CA2015413 A CA 2015413A CA 2015413 C CA2015413 C CA 2015413C
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CA
Canada
Prior art keywords
membrane
hydrophilic polymer
membranes
polymer
process according
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
CA002015413A
Other languages
French (fr)
Other versions
CA2015413A1 (en
Inventor
Hendrik Dirk Willem Roesink
Cornelis Antonius Smolders
Marcellinus Hendrikus Vincentius Mulder
Dirk Marinus Koenhen
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X Flow BV
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X Flow BV
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Application filed by X Flow BV filed Critical X Flow BV
Publication of CA2015413A1 publication Critical patent/CA2015413A1/en
Application granted granted Critical
Publication of CA2015413C publication Critical patent/CA2015413C/en
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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0002—Organic membrane manufacture
    • B01D67/0009—Organic membrane manufacture by phase separation, sol-gel transition, evaporation or solvent quenching
    • B01D67/0016—Coagulation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0002—Organic membrane manufacture
    • B01D67/0009—Organic membrane manufacture by phase separation, sol-gel transition, evaporation or solvent quenching
    • B01D67/0011—Casting solutions therefor
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0002—Organic membrane manufacture
    • B01D67/0023—Organic membrane manufacture by inducing porosity into non porous precursor membranes
    • B01D67/003—Organic membrane manufacture by inducing porosity into non porous precursor membranes by selective elimination of components, e.g. by leaching
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/02—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor characterised by their properties
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/08—Hollow fibre membranes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2323/00—Details relating to membrane preparation
    • B01D2323/04—Hydrophobization
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2323/00—Details relating to membrane preparation
    • B01D2323/12—Specific ratios of components used
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2323/00—Details relating to membrane preparation
    • B01D2323/30—Cross-linking
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/36—Hydrophilic membranes

Abstract

The invention relates to a process for preparing a microporous membrane, starting from a mixture of at least a hydrophobic polymer and a hydrophilic polymer, by dissolving these polymers first in a suitable solvent or mixture of solvents and subsequently by coagulating in a coagulation bath. According to the invention a more or less hydrophilic membrane is prepared by removing the membrane so obtained from the coagulation bath and subsequently by leaching at least a part of the hydrophilic polymer from the matrix, alternatively followed by hydrophobisation. The leaching of the hydrophilic polymer occurs by treatment of the membrane with an oxidising/hydrolising agent. The invention furthermore comprises microporous membranes, flat or tubular or in the form of hollow fibers, having a good chemical resistance and good mechanical strength.

Description

~0~5~
A process for preparing a microporous membrane and such a membrane.
-This invention relates to a process for preparing amicroporous membrane, starting from a hydrophobic polymer and a hydrophilic polymer, by dissolving these polymers first in a suitable solvent or a mixture of solvents and, subsequently 5 by coagulating the dissolved polymers in a coagulation bath, as well as to such microporous membranes.
Such a process has been disclosed in Dutch Patent /~ PU31-/S~Dftf'~ /8, /98~
Application 8,602,40~ The just-mentioned Dutch Patent Application describes a process for preparing hydrophilic 10 porous membranes based on a mixture of at least two polymers, i.e., a hydrophobic polymer (preferably a polyetherimide) and a hydrophilic polymer (preferably polyvinylpyrrolidone). An essential aspect of the known process is based on the fact that the hydrophilic polymer, still present after coagulation, 15 will be cross-linked in an unswollen state, in such a way that swelling will be limited to an acceptable minimum, while the thus obtained membranes can be used for filtration applications. The permeability of these membranes is in accordance with the pore size and the porosity which can 20 be observed by means of e.g., an electron microscope. The above-mentioned cross-linking of the hydrophilic polymer preferably takes place by means of a suitable heat treatment, but the cross-linking can also be effected chemically.
It is noted that the cross-linking of the membranes 25 is absolutely necessary, because otherwise the hydrophilic polymer present, preferably polyvinylpyrrolidone, swells too much, resulting in only very poor permeability values.
A special aspect of the invention described in Dutch Patent Application 8,602,402 is the fact that a 30 considerable amount of hydrophilic polymer, especially poly-vinylpyrrolidone, remains in or on the membrane matrix after coagulation in a water bath or generally in a bath comprising a solvent (but a non-solvent for the hydrophobic polymer) for the hydrophilic polymer. It also appeared that 35 the hydrophilic polymer present in the polymer solution partly remains in the matrix and is thus also partly found in the coagulation bath. This and that can be demonstrated by a - 2 - ~ 3 method, which has been especially developed for measuring the ~ concentration of the hydrophilic polymer in the coagulation bath.
Besides the advantages described in the Dutch Patent 5 Application 8,602,402, the known membranes have a number of disadvantages.
As an important disadvantage can be mentioned that cross-linking the remaining hydrophilic polymer may give rise to a relatively brittle membrane causing problems with the 10 incorporation of the membranes in modules.
Further, the known membranes are always hydrophilic, which cannot always be considered an advantage.
Finally, the amount of hydrophilic polymer in the matrix is hard to control.
The present invention now aims at providing a process, which solves the above-mentioned disadvantages in an effective way.
To this end the present invention provides a process for preparing a microporous membrane, starting from a mixture 20 of at least a hydrophobic polymer and a hydrophilic polymer, by dissolving these polymers first in a suitable solvent or mixture of solvents and, subsequently coagulating the dissolved polymers in a coagulation bath, characterized in that a more or less hydrophilic membrane is prepared by 25 removing the membrane so obtained from the coagulation bath and thereafter by leaching at least part of the hydrophilic polymer from the matrix, eventually followed by hydro-phobisation.
Surprisingly, according to the present invention, 30 it appeared to be possible to control the amount of h~dlo~hilic polymer in the membrane matrix, in such a manner, that the amount of hydrophilic polymer in the membranes finally obtained can have a value of about 0-35% by weight, based on the total dry weight of polymer in the membranes.
According to the invention the hydrophilic polymer from the matrix can be removed partly or substantially, so that the hydrophilicity respectively the hydrophobicity of the membrane can be regulated according to necessity. In case that about all of the hydrophilic polymer is removed, the present ~ 3 ~ 201~

invention provides the interesting possibility to prepare a very porous hydrophobic membrane matrix on the basis of a mixture of a hydrophilic and a hydrophobic polymer. It is noted that for obtaining a porous matrix it is necessary per 5 se that use is made of a hydrophilic polymer with the followed membrane preparation procedure. That is the application of the hydrophilic polymer is essential for obtaining a porous membrane structure, in which the pores are interconnected in an optimal manner, so that the membranes can be applied for 10 filtration purposes in a suitable way.
An important aspect of the present invention can be found in the fact that the desired porous structure in the membranes can only be obtained when the membranes are subjected to a suitable post-treatment.
It appeared that in a number of cases the complete or incomplete removal of the hydrophilic polymer can be accomplished in a water bath, which absorbs much time and has further the disadvantage that the hydrophobic polymer present, e.g. polyetherimide, is affected by hydrolysis.
According to the present invention the removal/
leaching of the hydrophilic polymer is realised by treatment of the membrane with an oxidising respectively hydrolysing agent. Examples of these are acidified potassium permanganate (KMnO4), hydrogen peroxide, sodium peroxide etc. etc 25 Advantageously a sodium hypochlorite solution can be applied.
By applying a sodium hypochlorite solution leaching occurs considerably faster than in a water bath, whereby furthermore the remaining hydrophobic matrix is not at all or hardly affected.
Further, the treatment with sodium hypochlorite solution has the advantage, that the membranes so treated are very suitable as filtration membranes, because the swelling of the hydrophilic polymer still present is reduced in such a manner, that no clogging of the pores occurs. Owing to this 35 the fluxes of the membranes so obtained are in accordance with the size of the pores and the porosity of the membranes, as can be observed by means of an electron microscope. It should be noted, that after the treatment of the membranes with sodium hypochlorite solution there is still sufficient _ 4 _ ~2~

hydrophilic polymer left behind to give the membrane a hydro-philic character which still allows wetting the membranes with water.
However this hydrophilicity or water wettability of 5 the membranes according to the invention disappears when the membranes are subjected to treatment at elevated tempera-tures after the mentioned treatment with sodium hypochlorite solution. Therefore the membranes are heated to approximately 150~C for 2-30 hours. The thus heat-treated membranes can be 10 considered to be hydrophobic.
It is noted that, as hydrophobic polymer according to the invention preferably polyetherimide, polyether-sulphone, polysulphone, polyimide, etc. and as hydrophilic polymer polyvinylpyrrolidone, polyacryl acid, polyvinyl-15 alcohol, polyvinylacetate, polyethylene glycol etc. etc. areused.
As a solvent for the polymer according to the invention one uses in general N-methylpyrrolidone, dimethyl-formamide, dimethylacetamide, dimethylsulphoxide, chloroform, 20 methylenechloride etc. etc.. For the preparation of the microporous membranes according to the invention hydrophobic polymer / hydrophilic polymer / solvent ratios of 10-35% by weight / 15-35% by weight / 85-30% by weight are generally used, and preferably 25-20% by weight / 10-15% by weight /
25 75-65% by weight.
It is noted that according to the invention flat membranes, tubular ones, either on or off a support, as well as in the form of hollow fibers can be prepared.
Finally, the invention also relates to microfil-30 tration and/or ultrafiltration membranes, flat or tubularor in the form of hollow fibers, consisting essentially of a hydrophobic polymer and more or less of a hydrophilic polymer, which hydLO~hilic polymer has been cross-linked and fixated in or at the polymer matrix, the membrane having pores of 35 0.0001-5 ~um, a heat resistance up to 250C, a water permeability up to 8000 l/m2.h.bar, also having a good chemical resistance and good mechanical strength.
It appeared that the membranes according to the invention are very suitable for membrane separations, based ~ 5 ~ ~0 `on particle sizes, e.g. ultra- and microfiltration. It is - apparent that the present membranes are not to be restricted to the applications just cited. For example the hydrophobic embodiments of the present membranes can advantageously be 5 used as aeration medium, oxygenerator, bioreactor etc.. For special applications it is also possible to provide the capillary membranes with a hydrophobic (e.g. the inner surface) as well as a hydrophilic surface (e.g. the outer surface).
The degree of hydrophilicity is especially of interest in connection with protein adsorption or in a general sense in connection with the so called fouling of membranes.
Until today there is little agreement in the literature concerning a good relationship between the degree of hydro-15 philicity and the fouling properties of membranes, while the plurality of the knowledge in this field is based on experimental results. It appeared from research by Applicants that the adsorption of BSA (bovine serum albumine, a model protein, which is often used for fouling studies) on membrane 20 surfaces depends on the post-treatment of the membranes.
The amounts of adsorbed BSA to PEI/PVP membranes, which have been subjected to a heat treatment (see Example I), is approximately 2 mg/m2 membrane surface and is comparable with the amounts for cellulose type membranes presented in the 25 literature. This latter type of membrane is still considered to be the best example for non-fouling membranes. If the PEI/PVP membranes are subjected to a NaOCl treatment (see Example II) or a NaOCl treatment followed by a heat treatment (see Example III), then the amount of adsorbed BSA is 30 approximately 8 mg/m2 membrane surface. It is remarkable that hardly any difference can be detected in the amount of BSA
adsorbed between the membrane of Example II (hydrophilic) and the membrane of Example III (hydrophobic), which gives a good illustration of the fact that the degree of hydrophilicity 35 plays an important role in fouling. It should be noted that it is conceivable that, in contrast, the membrane of Example II shows good anti-fouling properties if other feed solutions are used.
It may be obvious that the leaching process with 40 e.g. sodium hypochlorite solution affords the possibility to - 6 - ~ 3 control the hydrophilic character of the membranes and to - adjust it to the desired conditions.
The invention will now be illustrated with the following not limitative examples.

EXAMPLE I
A solution was prepared from 17 parts by weight polyetherimide (Ulte~ 1000), 13 parts by weight PVP in 70 parts by weight N-methyl-2-pyrrolidone. The thus obtained 10 polymer solution was spun to a capillary membrane by coagu-lation in a water bath with a temperature of 20C-80C.
After removal from the coagulation bath the membranes were dewatered by subsequent treatment with ethanol and hexane. After deswelling the membranes were subjected to 15 heat treatment at 150C for 2-30 hours.
After this treatment the membranes still contained 20-25 parts by weight PVP based on the total membrane weight.
The water flux of these membranes was 500-2700 l/m2.h.bar, while the water absorption capacity of the membrane material 20 was only 5% by weight.

EXAMPLE II
Membranes according to Example I were spun and after removal from the coagulation bath the membranes were treated 25 with a NaOCl solution of 40-4000 ppm for at least 24 hours.
After this treatment the membranes were still hydrophilic (i.e. water wettable) and having a water flux of 50-4200 l/m2.h.bar. The membranes contained a maximum amount of PVP
of 0-10% by weight. The water absorption capacity was less 30 than 5% by weight.

EXAMPLE III
A membrane prepared according to Example II was subjected to heat treatment at 150~C for 2-30 hours, after 35 the NaOCl treatment. After this treatment the membranes were no longer wettable by water (hydrophobic). The water flux (after moistening with e.g. ethanol) was 50-4200 l/m2.h.bar. The water absorption capacity of these membranes was less than 1% by weight.

~ 7 ~ '2 EXAMPLE IV
-- A solution was prepared from 20 parts by weight polyethersulphone (VictrexO, 10 parts by weight polyvinyl-pyrrolidone, 5 parts by weight isopropylalcohol in 65 parts 5 by weight N-methylpyrrolidone. The thus obtained polymer solution was spun to a capillary membrane by coagulation in a water bath of 20-80C. After removal from the coagulation bath the membranes were dewatered by a subsequent treatment with ethanol and hexane. After deswelling the membranes were 10 subjected to heat treatment at 150~C for 2-30 hours.
After this treatment the membranes still contained 2-10 parts by weight PVP, based on the total membrane dry weight. The flux of the membranes was 500-5000 l/m2.h.bar, while the water absorption capacity of the membrane material 15 was only 3% by weight.

EXAMPLE V
Membranes according to Example V were spun and after removal from the coagulation bath the membranes were treated 20 with a sodium hypochlorite solution of 40-4000 ppm during at least 24 hours. After this treatment the membranes were still hydrophilic (capable of being moistened) and having a water flux of 500-7500 l/m2.h.bar. The membranes contained a r~x;rum amount of PVP of 0-5% by weight. The water absorption capacity 25 was less than 3% by weight.

Claims (10)

1. A process for preparing a microporous membrane, starting from a hydrophobic polymer and a hydrophilic polymer, by dissolving said polymers first in a suitable solvent or mixture of solvents and subsequently by coagulating in a coagulation bath, characterized in that, a hydrophilic membrane is prepared by removing the membrane so obtained from the coagulation bath and subsequently by leaching at least a part of the hydrophilic polymer from the matrix, alternatively followed by hydrophobisation.
2. A process according to claim 1, characterized in that, the hydrophilic polymer is leached by treatment of the membrane with an oxidising/hydrolising agent.
3. A process according to claim 2, characterized in that, a sodium hypochlorite solution is used as the oxidising/hydrolising agent.
4. A process according to claim 1, characterized in that, the hydrophobisation occurs by subjecting the membrane, from which the hydrophilic polymer has been leached, to heat treatment.
5. A process according to claim 4, characterized in that, the heat treatment is effected at approximately 150°C
for 2-30 hours.
6. A process according to any one of claims 1-5, characterized in that, polyetherimide is used as the hydrophobic polymer and polyvinylpyrrolidone is used as the hydrophilic polymer.
7. A process according to claim 6, characterized in that, N-methylpyrrolidone is used as the solvent for the polymers.
8. A process according to any one of claims 1, 5 and 7, characterized in that, the hydrophobic polymer /
hydrophilic polymer / solvent ratio is 10-35% by weight /
15-35% by weight / 85-30% by weight.
9. A process according to any one of claims 1, 5 and 7, characterized in that, a flat or tubular membrane, on or off a support, or in the form of hollow fibers is prepared.
10. Microporous membranes, flat or tubular or in the form of hollow fibers, essentially comprising at least a hydrophobic polymer and a hydrophilic polymer, which hydrophilic polymer has been cross-linked and has been fixated in or at the polymer matrix, the membrane having pores of 0.0001-5 /µm, a heat resistance up to 250°C, a water permeability up to 8000 1/m2.h.bar, also having a chemical resistance and mechanical strength.
CA002015413A 1989-04-28 1990-04-25 Process for preparing a microporous membrane and such a membrane Expired - Lifetime CA2015413C (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
NL8901090 1989-04-28
NL8901090A NL8901090A (en) 1989-04-28 1989-04-28 METHOD FOR MANUFACTURING A MICROPOROUS MEMBRANE AND SUCH MEMBRANE

Publications (2)

Publication Number Publication Date
CA2015413A1 CA2015413A1 (en) 1990-10-28
CA2015413C true CA2015413C (en) 1997-02-25

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Country Status (11)

Country Link
US (1) US5076925A (en)
EP (1) EP0395133B1 (en)
JP (1) JP3196029B2 (en)
AT (1) ATE117912T1 (en)
CA (1) CA2015413C (en)
DE (1) DE69016491T2 (en)
DK (1) DK0395133T3 (en)
ES (1) ES2070263T3 (en)
GR (1) GR3015885T3 (en)
HK (1) HK1007697A1 (en)
NL (1) NL8901090A (en)

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US5076925A (en) 1991-12-31
HK1007697A1 (en) 1999-04-23
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EP0395133A1 (en) 1990-10-31
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GR3015885T3 (en) 1995-07-31
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JPH02302449A (en) 1990-12-14
NL8901090A (en) 1990-11-16

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