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Description  |
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BACKGROUND OF THE INVENTION
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
nonchemical bonding.
DISCLOSURE OF THE PRIOR ART
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.
SUMMARY OF THE INVENTION
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, U.S. Pat. No. 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/1 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 quantities 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, to be used as
thickening agent, as membranemodifying agent which generates hydrophylic
groups in a process for producing a hydrophilic membrane according to the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Example I
Flat membranes have been prepared from two different casting syrups
according to the invention: composition of syrup A: 24% polysulphone +5%
DAPRAL (trademark of AKZO) in DMF, composition of syrup B: 24%
polysulphone in DMF state of the art.
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:
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CWF before test
mean flux CWF after test
membrane (1/m.sup.2 h .multidot. bar)
during test
(1/m.sup.2 h .multidot. bar)
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A 45.3 20.6 17.7
B 22.6 3.9 3.6
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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 (prior art) tubular polyacrylonitril membranes and
tubular hydrophilic polysulphone membranes prepared from syrup A of
example 1 (according to the invention) 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 1/m.sup.2 h.bar, while the hydrophilic polysulphone membranes
had a product flux of 20 1/m.sup.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 10.times. 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 (according to the prior art)
has been compared with a hydrophilic membrane prepared from a casting
syrup having a composition of 25% polysulphone +5% DAPRAL in NMP
(according to the invention).
The fluxes found (in 1/m.sup.2 h.bar) for the test with skimmed milk are
reported in table B:
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CWF CWF
before product flux
product flux
after
membrane test for cf = 1 for cf = 9
cleaning
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Standard 35.4 11.4 9.9 26.2
Hydrophilic
139.8 27.1 24.7 134.5
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Table C gives the results for the test on whey:
CWF CWF
before product flux
product flux
after
membrane test for cf = 1 for cf = 5
cleaning
______________________________________
Standard 31.9 9.2 7.9 29.1
Hydrophilic
154.6 25.7 28.9 156.3
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From both tests it is evident that the hydrophilic membranes according to
the invention 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 membrane 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% H.sub.2 O 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
1/m.sup.2 h.bar.
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