Integral asymmetric polyether-sulfone membrane, process for its production, and use for ultrafiltration and microfiltration

An integral polyether-sulfone membrane with a pore system open at the outer boundaries is in the form of flat films, tubular films or hollow fibers having a maximum pore diameter of 0.02 .mu.m to 2 .mu.m. The pore system is cellular throughout with cells having polyhedrally symmetrical boundaries. The cells are arranged asymmetrically from one surface of the membrane to the other, the asymmetry factor AF relative to the maximum pore diameter being 0.01 to 2.0. The ratio of the maximum mean free path length of the flow path to the diameter of the largest pore is greater than 3. The cell size changes steadily from one surface to the other surface. The membrane is produced by dissolving 12 to 35% by weight, relative to the total solution, of polyethersulfone in a mixture of 15 to 65% by weight of .epsilon.-caprolactam, 0 to 85% by weight of latent solvent, 0 to 15% by weight of thickener and 0 to 50% by weight of non-solvent, and if appropriate up to 1% by weight of auxiliaries, forming the solution into flat films, tubular films or hollow fibers (the latter preferably with the aid of an internal fluid), and transformation into the solid phase and removal of the mixture forming the solvent. The membrane may be used for ultrafiltration and microfiltration.

TECHNICAL FIELD 
The invention relates to an integral polyethersulfone membrane with a pore 
system open at the outer boundaries, in the form of flat films, tubular 
films or hollow fibers having a maximum pore diameter of 0.02 .mu.m to 2 
.mu.m, measured by the blow point method, as well as to a process for 
producing the membrane and to its use for ultrafiltration and 
microfiltration. 
BACKGROUND 
From EP No. 121,911-A1, a filter membrane is known which consists of a 
polysulfone in the form of a hollow fiber, having a network structure 
across the entire thickness from the inner to the outer surface and in 
which the pores have a maximum pore diameter from 0.1 to 5 .mu.m and the 
pore orifices at the inner surface have a maximum diameter from 0.01 to 10 
.mu.m and the orifices of the pores formed in the outer surface have a 
maximum diameter from 0.01 to 5 .mu.m. 
Even though the pore structure is described as a homogeneous network 
structure or sponge structure, a broad distribution of the diameters of 
the orifices in the region of the outer wall surface is shown in an 
enlarged sectional illustration of the hollow fiber. The known membrane 
does not contain any skin in which only narrower pores than in the sponge 
structure exist, but contains widely different orifices, broken open 
outwards, of the network or sponge structure. 
From EP No. 228,072-A2, a filter membrane is known wherein the polymer 
forming the membrane is as such hydrophobic and has a water absorption 
capacity of about 2 to 4%, and the membrane is hydrophillic, has a pore 
size from 0.02 .mu.m to 20 .mu.m and, at a given blow point, shows a high 
water flow velocity. Preferably, the polymer is polyether-sulfone and 
contains additions of polyethylene glycol or polyvinylpyrrolidone. 
In the process for producing shaped articles having pores according to 
German Pat. No. 3,327,638, a porous polyamide-6 hollow fiber has been 
produced from a mixture of polyamide-6, .epsilon.-caprolactam and 
polyethylene glycol 300. Forming took place at a nozzle temperature of 
210.degree. C. The spinning solution was homogeneous and of low viscosity 
and was therefore extruded into a U-shaped cooling tube, in which the 
mechanical loading, to which the polymer mixture is exposed up to the time 
of starting solidification, that is to say the start of dimensional 
stability, is kept small. 
The membranes known from EP No. 121,911-A1 and EP No. 228,072A1 are formed 
from solutions of the polymer in aprotic solvents by known membrane 
formation processes. Examples of aprotic solvents are dimethylacetamide, 
dimethylformamide and N-methylpyrrolidone. The polymer contents of the 
solutions are decidedly low. For this reason, the viscosities are also low 
and thin-walled and mechanically less stable membranes are produced 
preferentially. The known membranes are virtually symmetrical, which has 
the consequence that the membrane flow decreases markedly with the 
thickness of the membrane. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a temperature-resistant 
filtration membrane, which is chemically stable, in particular to 
oxidizing agents, with a pore system open at the outer boundaries, the 
special flow characteristics of which mean that, at the same structural 
build-up, it allows a flow of water which is largely independent of the 
membrane thickness. 
This and other objects are achieved by an integral polyethersulfone 
membrane, wherein the pore system is cellular throughout. The cells have 
polyhedrally symmetrical boundaries and are arranged asymmetrically within 
the wall from one boundary of the wall to the other. The asymmetry factor 
AF relative to the maximum pore diameter is 0.01 to 2.0. The ratio of the 
maximum mean free path length of the flow path to the diameter of the 
largest pores is greater than 3. The cell size changes steadily from one 
surface to the other surface.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The maximum pore diameter is determined by means of the blow point method 
(ASTM no. 128-61 and F 316-70), for which the method described in DE No. 
3,617,724-A1 is suitable, for example. In this case, d.sub.max results 
from the gas space pressure P.sub.B belonging to the blow point in 
accordance with the equation 
EQU d.sub.max =.sigma..sub.B /P.sub.B 
where .sigma..sub.B is a constant which is mainly characteristic of the 
wetting liquid. The values of .sigma..sub.B at 25.degree. C. are, for 
example, 0.611 .mu.m.times.bar for isopropanol and 2.07 .mu.m.times.bar 
for H.sub.2 O. 
In contrast to the symmetrical membrane from the state of the art, for 
example produced according to German Offenlegungsschrift No. 2,833,493, 
there is a substantially smaller dependence of the flow on the membrane 
thickness in membranes according to the invention. 
FIG. 1 shows a diagram in which the straight line 1 corresponds to the 
membrane known from German Offenlegungsschrift No. 2,833,493. The straight 
line 2 corresponds to the membrane according to the invention (produced 
according to Example 1). 
The asymmetry in the membrane cross-section is defined by the asymmetry 
factor AF relative to the maximum pore diameter, where 
##EQU1## 
LS is the mean free path length of the flow path in the membrane between 
the cell walls, and is determined from a transmission electron micrograph 
of the membrane cross-section at 4100-fold magnification using an image 
evaluation system, for example Quantiment 970, in each case at the defined 
distance S from the membrane wall. 
LS.sub.max is the corresponding maximum free path length in a membrane and 
the associated distance from the membrane wall is termed S.sub.max. The 
resulting quotient is related to the maximum pore diameter d.sub.max, 
which is established by blow point determination and which governs the 
trans-membrane flow. 
The transmission electron micrograph is taken on thin sections transversely 
to the wall at a section thickness of 80 nm. To make the sections, the 
membrane is embedded in methacrylate, and the latter is washed out with 
carbon tetrachloride after cutting. 
The mean free path lengths determined at various distances from the 
membrane wall are plotted graphically as a function of the associated 
distance from the outer wall. The measured values can be represented as a 
steady curve with a maximum. The asymmetry factor AF then results from the 
maximum mean free path length and the distance of the maximum from the 
wall and the maximum pore diameter. 
The values belonging to Example 1C are listed in the table below and 
graphically shown in FIG. 3. 
TABLE 1 
______________________________________ 
Distance from Mean free Standard 
the outer edge path length 
deviation 
in .mu.m in .mu.m (.+-.) 
______________________________________ 
3.09 0.32 0.26 
6.19 0.34 0.26 
10.47 0.39 0.30 
16.66 0.60 0.46 
20.94 0.67 0.52 
26.18 0.93 0.74 
30.94 1.46 1.32 
38.79 1.43 1.10 
48.31 2.04 1.62 
57.12 2.52 1.74 
85.68 4.64 3.43 
114.24 6.99 4.19 
145.18 6.53 5.14 
180.88 7.03 3.85 
209.44 7.35 3.92 
242.28 6.34 4.81 
248.71 1.74 1.88 
154.18 0.50 0.44 
______________________________________ 
By selection of different process parameters, membranes according to the 
invention can be produced, the asymmetry factor of which is between 0.01 
and 2.0. 
It is presupposed here that the ratio of the maximum mean free path length 
to the diameter of the largest pore is greater than 3, preferably between 
5 and 100, that is to say that a marked asymmetry of the pore system is 
noticeable. At the same time, it is necessary that the pore size does not 
change suddenly, but steadily. 
FIG. 4 and FIG. 5 show, by way of example, typical transmission electron 
micrographs of a membrane according to the invention, such as are used for 
image evaluation in the determination of the mean free path length. 
Preferably, the membranes according to the invention are obtained in such a 
way that they are formed from a mixture containing at least 15% by weight 
of .epsilon.-caprolactam. 
.epsilon.-Caprolactam is a hygroscopic substance melting at about 
70.degree. C. and having a boiling point (under normal pressure) of 
268.5.degree. C. It is readily soluble in water and numerous organic 
solvents such as, for example, toluene, isopropanol, glycerol, 
polyethylene glycol, butyrolactone, propylene carbonate, ethyl acetate, 
methyl ethyl ketone or cyclohexane. It is produced industrially on a 
considerable scale and forms the monomer for polyamide 6 polymers, and is 
therefore available at low prices. With exclusion of oxygen, 
.epsilon.-caprolactam is thermally stable, disregarding the fact that, at 
temperatures of 260.degree.-270.degree. C. in the presence of water, it 
undergoes a poly-addition to form polyamide-6, with ring opening. 
From its use as the monomer for polyamide-6, the properties of 
.epsilon.-caprolactam are well known. The recovery of 
.epsilon.-caprolactam from aqueous solution is likewise well known. 
.epsilon.-Caprolactam is a substance of very low toxicity. It is the view 
that, in handling .epsilon.-caprolactam, apart from the nuisance due to 
the bitter taste and the possible irritation of mucosae by 
.epsilon.-caprolactam powder, there is no health risk even in the case of 
repeated exposure. Because of the high solubility, any residues can be 
completely removed from the membranes formed by means of 
.epsilon.-caprolactam. 
.epsilon.-Caprolactam differs from the aprotic solvents usually employed 
for polyether-sulfone membranes by the proton present on the nitrogen atom 
and by the fact that it confers a relatively high viscosity on the 
solutions. 
The polyether-sulfone membranes according to the invention can be designed 
to be hydrophobic or, in a special embodiment of the invention, to be 
hydrophillic. In this special embodiment, additives conferring a 
hydrophillic character can be added to the mixture during membrane 
formation. 
In another embodiment of the invention, leading to hydrophillic membranes, 
the polyether-sulfone used is at least partially sulfonated. 
The invention also relates to a process for producing the membrane 
according to the invention. 
The production of the membrane according to the invention is carried out by 
dissolving 12 to 35% by weight, relative to the total solution, of 
polyether-sulfone in a mixture of 15 to 65% by weight of 
.epsilon.-caprolactam, 0 to 85% by weight of latent solvent, 0 to 15% by 
weight of thickener and 0 to 50% by weight of non-solvent, and if 
appropriate up to 1% by weight of auxiliaries, each relative to the 
mixture, and forming the solution into flat films, tubular films or hollow 
fibers (the latter preferably with the aid of an internal fluid), and 
transformation into the solid phase and removal of the mixture forming the 
solvent. 
Polyether-sulfones which are used according to the invention are 
represented by the formula: 
##STR1## 
If appropriate, they can also be substituted. In particular, they can also 
be partially sulfonated. The polymer can also be a copolymer, in which 
case a co-condensation with polyethers is particularly suitable. 
Within the scope of the present invention, latent solvents are understood 
to be those substances which dissolve the membrane-forming polymer only 
sparingly or at elevated temperature. Examples of such latent solvents are 
butyrolactone and propylene carbonate. 
Thickeners are understood to be those substances which increase the 
viscosity of the solution. Examples of thickeners in the solutions under 
consideration here are polyvinylpyrrolidone, polyethylene glycol, 
polyacrylic acid and polyacrylates. 
Examples of non-solvents within the scope of the present invention are 
water, glycerol, triacetin, ethyl lactate and polyethylene glycol. 
Auxiliaries within the scope of the present invention include conventional 
stabilizers, nucleating agents, pigments and the like. 
The dissolution of the polymer is preferably carried out at temperatures of 
60.degree.-140.degree. C. 
Preferably, 15 to 25% by weight of polyether-sulfone, relative to the total 
solution, is dissolved in a mixture containing 40 to 60% by weight of 
.epsilon.-caprolactam. 
To form the lumen of hollow fibers, internal fluids are usually applied. 
Depending on the choice of internal fluid, the latter can, in the present 
invention, also have an effect on the structure of the zone adjoining the 
inner wall. In this case, the processing temperature, the characteristics 
of the solvent/non-solvent property and the miscibility with the solvent 
mixture play an important part. 
It is possible in the present invention to transform the formed solution 
into the solid phase in a conditioning chamber charged with non-solvent 
vapors. 
In an embodiment of the invention, it is also possible, with adequate 
residence time, to effect the transformation into the solid phase by 
spontaneous crystallization of the solvent mixture after supercooling. 
The transformation into the solid phase is in general effected by 
coagulation of the formed solution in a non-solvent bath. In a further 
embodiment of the invention, the transformation into the solid phase is 
effected by coagulation in a non-solvent bath, the temperature of the 
solution and the temperature of the non-solvent bath being 
40.degree.-60.degree. C. 
To increase the chemical stability, it is advantageous to heat-treat the 
membrane for one or more hours at temperatures of 170.degree.-210.degree. 
C. 
The invention also relates to the use of the polyethersulfones according to 
the invention for ultrafiltration and microfiltration. Ultrafiltration and 
microfiltration concern pressure-driven membrane filtrations for 
separating off defined particle sizes. The particle size ranges given in 
the literature for ultrafiltration and microfiltration largely overlap. On 
page 3 of the book "Synthetic Polymeric Membranes" by Robert E. Kesting, 
1971, FIG. 1.2, the membrane separation processes with the respective 
particle sizes are illustrated. This illustration shows that the range for 
ultrafiltration can comprise particle sizes of about 0.003 .mu.m to 10 
.mu.m and that for microfiltration about 0.03 .mu.m up to about 20 .mu.m. 
The membrane according to the invention can be used in the foodstuffs 
sector without restriction, because it does not contain any toxically 
relevant substances. 
As a result of the pronounced asymmetry of the cellular pore system, high 
permeabilities are achieved, and the flow through the membrane is largely 
independent of the total thickness of the membrane. 
FIG. 1 shows that, in spite of widely differing wall thicknesses, the 
transmembrane flows differ only slightly. 
FIG. 2 shows the solidification temperatures of the solvent mixture as a 
function of various solution constituents. In detail, the solidification 
temperature is plotted on the ordinate, and the compositions of the 
solvent mixtures are plotted on the abscissa. 
FIG. 6 shows a scanning electron micrograph (magnifications 350:1 and 
5000:1) of a membrane produced according to Example 1. 
FIG. 7 shows likewise scanning electron micrographs (magnifications 390:1 
and 5000:1) of membranes of the invention according to Example 5. 
The invention is explained in more detail by reference to the following 
non-limiting examples. 
EXAMPLE 1 
A homogeneous viscous solution (about 28 PaS/20.degree. C.) was formed at 
about 110.degree. C. from 15 parts by weight of commercially available 
polyether-sulfone (type Victrex 5200 from ICI), 77.5 parts by weight of a 
mixture consisting of caprolactam/butyrolactone/glycerol in a weight ratio 
of 45.87:45.87:8.26, and 7.51 parts by weight of polyvinylpyrrolidone as 
thickener. 
After degassing and cooling to the spinning temperature of 40.degree. C., 
hollow fibers of varying wall thicknesses were formed from this solution 
by means of a hollow fiber jet, using a fluid internal filling, and 
immediately solidified in the water bath heated to 40.degree. C. After a 
residence time of about 10-15 seconds in the water bath, the hollow fibers 
had been stabilized. The extraction of the solvents was carried out by 
washing with warm water at 80.degree. C. Drying at about 50.degree. C. was 
preceded by an extraction pass with isopropanol. 
The microscopic evaluation of the various capillary dimensions showed, in 
all variants in the outer region of the membrane, an about 50-100 .mu.m 
thick, fine-pored, outwardly open structure which merged into an 
increasingly coarse-pored texture towards the middle of the membrane. 
Towards the lumen side, the cells become more compact again and formed an 
open-pored inner surface. FIG. 6 represents the typical cross-section of 
this membrane. 
In Table 2 and FIG. 6, the trans-membrane flows of various hollow fibers of 
different wall thicknesses are compared. 
TABLE 2 
______________________________________ 
Hollow fibers of 1.0 mm internal diameter 
Dimension Wall max. pore Transmembrane 
d.sub.i 
d.sub.o 
thickness 
diameter 
AF flow water 
mm mm mm .mu.m [.mu.m.sup.-1 ] 
1/m.sup.2 .times. h 
______________________________________ 
.times. bar 
A 1.0 / 1.28 0.14 mm 
0.25 .mu.m 
0.25 5972 
B 1.0 / 1.43 0.215 mm 
0.25 .mu.m 
0.17 5532 
C 1.0 / 1.51 0.255 mm 
0.25 .mu.m 
0.14 4805 
D 1.0 / 1.56 0.28 mm 
0.25 .mu.m 
0.12 4362 
E 1.0 / 1.78 0.39 mm 
0.25 .mu.m 
0.09 4572 
F 1.0 / 1.90 0.45 mm 
0.25 .mu.m 
0.05 4000 
G 1.0 / 2.18 0.59 mm 
0.25 .mu.m 
0.08 4000 
H 1.0 / 2.34 0.67 mm 
0.25 .mu.m 
0.07 4452 
______________________________________ 
EXAMPLE 2 
A mixture of 11 25 parts by weight of the polyethersulfone used in Example 
1 and 3.75 parts by weight of a commercially available sulfonated 
polyether-sulfone was dissolved in caprolactam/butyrolactone/glycerol in a 
weight ratio of 48:48:6. The hollow fibers, produced in other respects by 
the method described in Example 1, were immediately wettable with water. 
They can be used without a hydrophillic treatment, for example with 
alcohol, for the filtration of aqueous or other hydrophillic media. 
EXAMPLE 3 
The polymer solution prepared according to Example 1 was spread at room 
temperature by means of a reverse-roll coater upon a carrier belt and 
immediately solidified in a warm water bath at 50.degree. C. The resulting 
flat membrane was washed in water and dried between 50.degree. and 
60.degree. C. 
The water-wetted flat membrane had the following test values: 
Membrane thickness: 0.15 mm 
Trans-membrane flow: 6.5 ml/cm.sup.2 .times.minute.times.bar measured with 
isopropanol 
Trans-membrane flow: about 8000 l/m.sup.2 .times.hour.times.bar measured 
with water 
EXAMPLE 4 
A commercially available polyether-sulfone was dissolved in the solvent 
mixture of Example 1 to give a 17% by weight solution and formed into a 
hollow fiber having an external diameter of 1.0 mm and a wall thickness of 
0.2 mm. 
The resulting, mechanically very stable hollow fiber had a trans-membrane 
flow with water of 4000 1/m.sup.2 .times.hour.times.bar at a maximum pore 
size of &lt;0.25 .mu.m. 
EXAMPLE 5 
A 15% by weight polyether-sulfone solution in 17.5 parts by weight of 
caprolactam and 82.5 parts by weight of propylene carbonate with an 
addition of 8.2% of thickener was spun into hollow fibers. The relatively 
small proportion of caprolactam caused a very slow stabilization of the 
fiber. Only after a residence time of about 1 minute was the hollow fiber 
solidified to such an extent that it was possible to extract it with 
water. 
This gave a water-permeable membrane of asymmetrical structure in the wall 
cross-section. The trans-membrane flow was 5000 1/m.sup.2 
.times.hour.times.bar. The resulting membrane is shown in the scanning 
electron micrographs of FIG. 7. 
EXAMPLE 6 
A warm solution at about 40.degree. C., consisting of 15 parts by weight of 
polyether-sulfone, dissolved in 66.75 parts by weight of caprolactam, 
21.25 parts by weight of butyrolactone and 11 parts by weight of glycerol, 
was spread on a cold glass plate. On cooling of the solution, the solvent 
crystallized and thus stabilized the membrane formed. After extraction by 
water, an open-pored, permeable membrane was formed.