TITLE: Hydrophilic membrane, process for producing a hydrophilic membrane.
European Patent Application EP0445853 A1

ABSTRACT:
A membrane for membrane filtration is disclosed, comprising a polymer
matrix formed by coagulation with a liquid coagulant from a casting
syrup, which polymer matrix is linked to hydrophilic groups belonging
to a compound which has a hydrophilic chain section, is not coagulated
by the liquid coagulant, is coupled on either side to a hydrophobic
chain, is anchored in the polymer matrix by non-chemical bonding, and
is composed of a compound which has a molecular weight of at least 2000
and can be used as a thickening agent.

INVENTORS:
Weyenberg, Dirk Christiaan (NL)
Zwijnenburg, Arie (NL)
Snijders, Gerrit Jan (NL)
Rekers, Casper Johannes Nicolaa (NL)

APPLICATION NUMBER: EP19910200240
PUBLICATION DATE: 09/11/1991
FILING DATE: 02/05/1991

ASSIGNEE: STORK FRIESLAND BV (NL)
INTERNATIONAL CLASSES: B01D69/02; B01D67/00; B01D71/52; C09K3/00; (IPC1-7): B01D67/00
EUROPEAN CLASSES: B01D67/00K14B; B01D67/00K14F; B01D69/04; B01D69/06

DOMESTIC PATENT REFERENCES:
EP0082433 Hydrophilic asymmetrical macroporous membrane of a synthetic
 polymerizate.
EP0228072 Filtration membrane and method of making the same.
EP0305787 A process for manufacturing permselective asymmetric
 membranes suitable for haemodialysis and the so prepared membranes.

FOREIGN REFERENCES:
FR2368289A
NL8104496A

CLAIMS:
1. Membrane for membrane filtration, comprising a polymer matrix formed
by coagulation with a liquid coagulant from a casting syrup, which
polymer matrix is linked to hydrophilic groups belonging to a compound
which has a hydrophilic chain section and is not coagulated by the
liquid coagulant and which is coupled on either side to a hydrophobic
chain, which compound is anchored in the polymer matrix by non-chemical
bonding, characterised in that the compound which is not coagulated by
the liquid coagulant is composed of a compound which has a molecular
weight of at least 2000 and can be used as a thickening agent.
2. Membrane according to claim 1, characterised in that the central
section of the compound which can be used as thickening agent is
composed of an optionally modified polyglycol.
3. Membrane according to claim 1 or 2, characterised in that the
compound which can be used as thickening agent is composed of a
non-ionogenic polyalkylene glycol or dialkyl polyglycol ether.
4. Process for producing a hydrophilic membrane by forming a casting
syrup of a suitable membrane-forming polymer, mixing the casting syrup
with a compound containing hydrophilic groups and composed of a
hydrophilic chain which is coupled on either side to a hydrophobic
chain, forming a layer and then subjecting said layer to coagulation
with a liquid coagulant to form a polymer matrix as membrane, in which
process a section of the hydrophilic chain containing compound being
anchored in the polymer matrix by non-chemical bonding, characterised
in that the casting syrup is mixed with a compound which has a
molecular weight of at least 2000 and can be used as a thickening
agent.
5. Process according to claim 4, characterised in that an optionally
modified polyglycol is used as the compound which can be used as
thickening agent.
6. Process according to claim 5, characterised in that a non-ionogenic
polyalkylene glycol or dialkyl polyglycol ether is used as the compound
which can be used as a thickening agent.
7. Process according to claims 4-6, characterised in that a casting
syrup is formed which contains up to 250 g/l of a compound which can be
used as a thickening agent.
8. Use of a compound which has a molecular weight of at least 2000, to
be used as a thickening agent, as a membrane-modifying agent which
generates hydrophilic groups.

DESCRIPTION:
Hydrophilic membrane, process for producing a hydrophilic membrane.
The invention relates to a membrane for membrane filtration, comprising
a polymer matrix formed by coagulation with a liquid coagulant from a
casting syrup, which polymer matrix is linked to hydrophilic groups
belonging to a compound which has a hydrophilic chain section and is
not coagulated by the liquid coagulant and which is coupled on either
side to a hydrophobic chain, which compound is anchored in the polymer
matrix by non-chemical bonding.
Such a membrane is known from the Japanese patent specification
53.090367.
This known membrane contains hydrophilic groups composed of a
polyethylene glycol chain. This chain is coupled on either side to, for
example, an alkyl chain. The molecular weight of this known compound
which provides hydrophilic groups is, however, low. In addition, only a
matrix formed from cellulose ester is described as membrane matrix.
Such membranes are, however, themselves already hydrophilic to some
extent.
This known membrane matrix is, however, inherently already less
susceptible to contamination. The drawback of using such a membrane
consists in the limited chemical resistance of the membrane.
A membrane which does not have this drawback and, in addition, has good
chemical resistance when more hydrophobic polymers are used, has now
been found.
The membrane according to the invention is characterised in that the
compound not being coagulated by the liquid coagulant is composed of a
compound which has a molecular weight of at least 2000 and can be used
as a thickening agent.
Preferably, the central section of the compound which can be used as
thickening agent is composed of an optionally modified polyglycol.
Such a membrane enables a very high product flux as a result of the
hydrophilic groups present. Surprisingly, it has also been found that
this type of compounds also causes the number of pores to increase
considerably. The hydrophobic chains linked to the central section of
the compound which can be used as thickening agent are inserted in the
polymer matrix during the coagulation in the liquid coagulant in a
manner such that said groups are firmly anchored in said matrix.
More particularly, the compound which can be used as thickening agent
is composed of non-ionogenic polyalkylene glycol or dialkyl polyglycol
ether. Compounds which have been found to be particularly expedient are
those compounds which are commercially available through the AKZO
company under the trademark DAPRAL GT 282 and DAPRAL T 210 and T 212.
It is pointed out that Polymer Bulletin 4, pages 617-622, 1981
discloses a membrane which is formed by coagulation of a casting syrup
composed of a mixture of hydrophobic polymerisate and polyethylene
glycol. The membranes obtained after coagulation do not, however, have
the hydrophilic properties to a sufficient extent because the
hydrophilic compounds present are eluted from the coagulated membrane
during coagulation by the aqueous liquid which is used for the
flocculation.
Dutch Patent application 86.02402 furthermore discloses hydrophilic
membranes which are produced by coagulating a mixture of a hydrophilic
polymer and a hydrophobic polymer, dissolved in a suitable solvent, in
a coagulating medium and, after the coagulation, chemically
cross-linking the hydrophilic polymer present in or on the membrane
matrix by a treatment and fixing it in position in or on the polymer
matrix.
The compound used according to the invention and containing hydrophilic
groups is not, however, anchored in the polymer matrix by chemical
bonding, but purely by physical bonding, while the chemical linking
treatment used for the abovementioned known membrane is absent.
However, if one or more reactive groups are present, preferably in the
hydrophobic chains, an additional linking treatment is possible if
desired.
Finally, US Patent Specification 4,203,848 discloses a hydrophobic
membrane which is rendered hydrophilic using a surface-active agent
which is essentially insoluble in water and is composed of a
hydrophobic chain section which is coupled on either side to a
hydrophilic chain. Said membrane is rendered hydrophilic by means of a
posttreatment (coating). The disadvantage of this is that the coating
is anchored non-chemically or physically and is eluted after the
passage of time.
It is pointed out that the hydrophilic membrane according to the
invention is particularly suitable for microfiltration and/or
ultrafiltration purposes, for example in the filtration of milk and
dairy products and the filtration of foodstuffs.
Obviously, the use of the hydrophilic membrane obtained by the addition
is not limited to these fields.
Suitable polymers for the polymer matrices may be chosen from polyvinyl
chloride, polyamide, modacryl, cellulose acetate, polypropylene,
polyvinylidene fluoride, polytetrafluoroethylene, polyacrylonitril,
polyether imide, polyphenylene oxide, polysuphone, polyether sulphone,
etc.
The additions now found have, moreover, the important property that
they are resistant to a large number of chemicals.
For the purification of protein containing substrates, very good
results are obtained with a polymer matrix which is formed on the basis
of polyacrylonitril or polysulphone.
The solvent used for forming the casting syrup is preferably
dimethylformamide or N-methylpyrrolidone, although other suitable
solvents may be used, such as dimethyl sulphoxide, dimethylacetamide,
etc.
The invention relates, furthermore, to a process for producing a
hydrophilic membrane by forming a casting syrup of a suitable
membrane-forming polymer, mixing the casting syrup with a compound
which contains hydrophilic groups and is composed of a hydrophilic
chain which is coupled on either side to a hydrophobic chain, forming a
layer and then subjecting said layer to coagulation with a liquid
coagulant to form a polymer matrix as membrane, in which process a
section of the hydrophilic chain containing compound is anchored in the
polymer matrix by non-chemical bonding.
It has been found that the use of such a compound which can be used as
a thickening agent, in the casting syrup results, on the one hand, in a
beneficial change in the viscosity of the casting syrup and, on the
other hand, makes possible a firm anchoring in the polymer matrix
because of the relatively high molecular weight, as a result of which
elution is prevented.
Preferably an optionally modified polyglycol is used for the compound,
which can be used as thickening agent.
Such a compound assures the formation of a hydrophilic membrane which
has a large number of hydrophilic groups.
The addition acts, moreover, as a pore former, as a result of which the
porosity of the membranes so obtained is increased.
Both effects promote the permeability to water and reduce the
susceptibility of the surface to contamination.
Expediently, a non-ionogenic polyalkylene glycol or a dialkyl
polyglycol ether is used as the compound which can be used as
thickening agent.
To obtain a hydrophilic membrane according to the invention, a casting
syrup is formed which preferably contains up to 250 g/l of a compound
which can be used as thickening agent.
The quantity to be used may therefore be varied to a wide extent, it
being possible to produce a hydrophilic membrane which has clean water
flux which is 5 to 6 times greater than is obtained with one of the
same membranes but without the hydrophilic groups, even if small
quantites of the compound which can be used as a thickening agent are
used.
There is, of course, no limitation at all with regard to the form in
which the membrane is provided. Depending on the use, it will possible
to choose, for example, a flat membrane, a membrane in the form of
hollow fibres, or tubular membranes.
The invention is explained in greater detail below in a few examples
which do not serve, however, to restrict the inventive idea.
The invention also comprises the use of a compound which can be used as
thickening agent as described in Claim 8. Example I
Flat membranes have been prepared from two different casting syrups:
composition of syrup A: 24% polysulphone + 5% DAPRAL (trademark of
AKZO) in DMF, composition of syrup B: 24% polysulphone in DMF.
After determining the clean water flux (CWF) of both membranes in an
Amicon test cell, filtration was carried out with a beta -lactoglobulin
solution (initial concentration 0.1%) in citric acid/phosphate buffer
(pH=4.7).
Table A gives a summary of the results obtained: EMI5.1
It appears clear that the flux decrease of the hydrophilic membrane A
as a result of protein contamination is appreciably less after
ultrafiltration of a protein solution than with the standard membrane
B. Example II
In a test, standard tubular polyacrylonitril membranes and tubular
hydrophilic polysulphone membranes prepared from syrup A of example 1
have been compared in a batch test for the separation of an oil
emulsion.
With a cf=17, the polyacrylonitril membranes where found to have a
product flux of 5 l/m<2>h.bar, while the hydrophilic polysulphone
membranes had a product flux of 20 l/m<2>h.bar.
At the end of the test it was found during inspection, in addition,
that the surface of the hydrophilic polysulphone membranes was
appreciably less contaminated. Example III
In a test on 10x diluted skimmed milk and centrifuged whey, the output
of a polysulphone membrane prepared from a casting syrup having a
composition of 24% polysulphone in 76% DMF has been compared with a
hydrophilic membrane prepared from a casting syrup having a composition
of 25% polysulphone + 5% DAPRAL in NMP.
The fluxes found (in l/m<2>h.bar) for the test with skimmed milk are
reported in table B: EMI6.1 Table C gives the results for the test on
whey: EMI6.2
From both tests it is evident that the hydrophilic membranes had much
higher product fluxes and were easy to clean after use. Example IV
In the abovementioned examples, the effect of the addition is based
both on making the mebrane matrix more hydrophilic and on increasing
the porosity of the obtained ultrafiltration membranes.
Adding the groups of substances mentioned to the casting syrup also
appears to result in a considerable increase in (product) flux in the
formation of microporous membranes.
Thus, although a flat PVDF membrane prepared by coagulating a film
spread from a casting syrup containing 18% PVDF in DMF in a bath
composed of 75% DMF/25% H2O gives a beautiful microporous cross
section, it gives no flux at 1 bar.
Addition of 7.5% DAPRAL, under otherwise identical circumstances, gives
a microporous structure having pores and a CWF of approximately 825
l/m<2>h.bar.