Semi-permeable membranes

Abstract

A semi-permeable membrane which comprises #2# having high permeability characteristics is disclosed. This may comprise either

(i) a copolymer of acrylonitrile and at least one ionic or ionizable monomer, or

(ii) a copolymer of acrylonitrile, at least one ionic or ionizable monomer, and at least one non-ionic and non-ionizable monomer, or

(iii) a mixture of at least two copolymers as defined under (i) and/or (ii), or

(iv) a mixture of at least one copolymer as defined under (i) and/or (ii), and at least one copolymer of acrylonitrile and at least one non-ionic and non-ionizable monomer, the ionic or ionizable monomer units representing from 1 to 80 mole % of the monomer units in one of said copolymers, and representing from 1 to 50 mole % of the monomer units in the membrane, said membrane having a degree of salt rejection of less than 1% and a permeability to water of more than 100 liters/day.m² at a relative pressure of 2 bars is provided having excellent properties for use in ultrafiltration and dialysis. A process for preparing a semi-permeable membrane is also provided which comprises treating a film containing at least one copolymer of acrylonitrile and an ionic or ionizable monomer, with water or an aqueous non-solvent mixture at a temperature between 60° and 250°C

Description

This is a orthogenicautogenous pressure); thereafter the heating was stopped and the vessel allowed to cool to ambient temperature whilst leaving the membrane immersed in the water in the vessel.

All the membranes thus prepared showed a zero degree of salt rejection. Their permeability to water was measured as follows:

12 cm² of membrane were placed on a porous sintered metal plate; water was placed in contact with the free face of the membrane and a pressure of 2 bars applied thereto. The flow of water across the membrane was measured; this flow, expressed in liters/m².day, is the permeability of the membrane to water.

Specific information relating to certain examples:

EXAMPLE 13

The polymer used was a mixture of two copolymers in a weight ratio of 43/57. The first was a copolymer of acrylonitrile and sodium methallylsulphonate in a weight ratio of 82/18; the second was a copolymer of acrylonitrile and styrene in a weight ratio of 85/15.

EXAMPLE 14

The polymer used was a mixture of equal weights of two copolymers; one was a copolymer of acrylonitrile and sodium methallylsulphonate in a weight ratio of 82/18, the other was a copolymer of acrylonitrile and butyl acrylate in a weight ratio of 85/15.

EXAMPLE 15

The polymer mixture used only differs from that of Example 14 in that the butyl acrylate was replaced by an equal weight of vinyl benzoate.

EXAMPLES 17 TO 37

The membranes prepared in Example 1 to 16 were used firstly in ultrafiltration processes (A) and secondly in dialysis processes (B).

(A) 12 cm² of membrane were placed on a porous sintered metal plate; an aqueous solution of a specified macromolecular product was placed in contact with the free face of the membrane and a pressure of 2 bars applied thereto; the flow of ultrafiltrate in liters/day.m², as well as the degree of rejection expressed as a percentage, namely ##EQU2## was measured.

Tables II and III give details of the solution subjected to ultrafiltration, as well as the flow of ultrafiltrate and the degree of rejection of the macromolecule.

(B) A dialysis cell was divided into two compartments by a membrane having 10 cm² surface area. 100 cm³ of an aqueous solution containing 9 g/l of NaCl, 1 g/l of thiourea and 10 g/l of cattle albumen (molecular weight: approx. 70,000) flowed in closed circuit through the first compartment: this solution, hereafter referred to as "synthetic blood", circulated in the compartment at a flow rate of 1l/hour. 100 cm³ of an aqueous solution containing 10 g of NaCl/l flowed in closed circuit through the second compartment, this solution being hereafter called the "dialysis bath"; it circulated in the compartment at a flow rate of 0.9 l/hour.

The rate of dialysis was measured by determining the time in minutes taken for the concentration of thiourea in the dialysis bath to reach 0.1 g/l; this time is indicated for certain membranes in Table (II).

TABLE I
__________________________________________________________________________
Example
Nature of polymer
1  2  3  4  5  6  7  8  9  10 11 12 13  14 15 16
__________________________________________________________________________
Molar percentages of comonomers
(made up to 100% with acrylonitrile)
Sodium methallyl-
2.094
3.32
3.71
5.74
6.35
7.16  2.60
3.33     2.58
-3.36
3.90
3.91
sulphonate
Potassium vinyloxy-             3.40
benzenesulphonate
Sodium vinylsulphonate                                      5.08
N-methyl-4-vinyl-                        4.28
6.21
pyridinium sulphate
Methyl methacrylate                5.79
Butyl acrylate                                        3.45
Vinyl benzoate                                           2.99
Vinylidene chloride                   4.51
Acrylamide                                     7.97
Styrene                                           4.75
Specific viscosity
0.812
1.039
0.743
0.980
1.026
0.962 1.192
0.62  0.374
0.926        0.63
(measured at 25°C, at
2 g/l in DMF)
Concentration of the
9  8  8.5
10 10 10 10 10 10 10 10 10 8   10 10 7
copolymer solution
(% by weight)
Duration of drying in
42 67 42 45 52 24 24 48 48 40 70 72 72  24 24 21
the oven, in hours
Maximum temperature of
180
147
146
113
104
92 146
140
140
146
146
147
145 145
145
120
the treatment water,
in °C.
Thickness of the mem-
45 65 55 50 80 45 60 55 45 60 55 90 80  70 70 30
brane, in microns
Permeability to water
230
350
520
650
300
480
800
600
800
570
8000
2900
1800
1500
2600
360
under 2 bars pressure,
in l/day · m2
Tensile strength of the
130               130
100         90
membrane in kg/cm2
(measured at 23°C
whilst wet)
Tensile elongation at
48                31 41          41
break, in %
__________________________________________________________________________

TABLE II
__________________________________________________________________________
Example
17   18   19  20  21   22   23
__________________________________________________________________________
Membrane used, prepared according to
2    3    3   3   4    5    6
Example
Ultrafiltration
Nature of the initial solution
subjected to ultrafiltration
Concentration of NaCl in g/l
9    9    5.85
0   9    9    9
Nature of the macromolecule
cattle
cattle
pepsin
dextran
cattle
cattle
cattle
albumen
albumen      albumen
albumen
albumen
Average molecular weight of the
70,000
70,000
36,000
10,000
70,000
70,000
70,000
macromolecule
Concentration of macromolecule in g/l
10   10   0.5 4   10   10   10
Results obtained
Flow of ultrafiltrate in liters/day · m2
350  520  520 520 720  300  480
Degree of rejection in %
100  100  70  50  98   98.5 100
Dialysis
Rate of dialysis in minutes
52
__________________________________________________________________________
Example
24   25  26   27   28  29  30
__________________________________________________________________________
Membrane used, prepared according to
6    6   7    8    8   8   9
Example
Ultrafiltration
Nature of the initial solution
subjected to ultrafiltration
Concentration of NaCl in g/l
5.85 0   9    9    0   0   9
Nature of the macromolecule
lysozyme
dextran
cattle
cattle
dextran
dextran
cattle
albumen
albumen      albumen
Average molecular weight of the
15,000
10,000
70,000
70,000
40,000
10,000
70,000
macromolecule
Concentration of macromolecule in g/l
0.5  4   10   10   4   4   1
Results obtained
Flow of ultrafiltrate in liters/day · m2
480  480 760  600  600 600 700
Degree of rejection in %
100  40  100  100  50  28  100
Dialysis
Rate of dialysis in minutes      58
__________________________________________________________________________

TABLE III
__________________________________________________________________________
Example
31   32   33   34   35   36   37
__________________________________________________________________________
Membrane used, prepared according to Example
10   11   12   13   14   15   16
Ultrafiltration
Nature of the initial solution
subjected to ultrafiltration
Concentration of NaCl in g/l
9    9    9    9    9    9    9
Nature of the macromolecule
cattle
cattle
cattle
cattle
cattle
cattle
cattle
albumen
albumen
albumen
albumen
albumen
albumen
albumen
Average molecular weight of the
70,000
70,000
70,000
70,000
70,000
70,000
70,000
macromoleule
Concentration of macromolecule in g/l
10   10   10   10   10   10   10
Results obtained
Flow of ultrafiltrate in liters/day · m2
480  720  1,000
1,000
1,000
1,330
360
Degree of rejection in %
99   90   90   96   93   90   100
__________________________________________________________________________

EXAMPLE 38

Milk was concentrated in an ultrafiltration cell with the two compartments separated by a membrane prepared as in Example 3, having a useful surface area of 465 cm². A relative pressure (relative to the second compartment) of 3 bars prevailed in the first compartment. The membrane was supported by a rigid plate of sintered polyethylene. Milk was circulated between this compartment and a storage reservoir; the rate of circulation at the membrane was 85 cm/second and the total weight of milk in circulation was initially 8.015 kg.

The whole operation, which was carried out at a temperature of +4° C., was continued for 11 hours 30 minutes. 4.007 kg of ultrafiltrate containing essentially, water, inorganic salts, lactose, and non-protein nitrogen-containing components had been collected in the second compartment; this amount of ultrafiltrate corresponds to a flow rate of 180 kg/day.m² ; the milk obtained in the first compartment had thus been concentrated two-fold.

EXAMPLE 39

Example 38 was repeated with the following modifications: the rate of circulation of the milk in the first compartment was 1 m/second, the temperature was +45°C, and the relative pressure between the two compartments was 6 bars.

After 11 hours 30 minutes, 6.835 kg of ultrafiltrate had been collected, corresponding to a flow rate of 307 kg/day.m² ; the milk obtained in the first compartment had been concentrated about seven-fold.

EXAMPLE 40

The dialysis experiment of Example 18 was repeated, replacing the so-called "synthetic blood" solution by cattle blood to which was added 1 g/l of thiourea as well as 25% by volume (the percentage being expressed relative to the blood) of a solution consisting of: 1.66 g of citric acid, 0.6 g of sodium hydroxide, 3 g of glucose and water sufficient to give 100 cm³ of solution.

The rate of dialysis (expressed in minutes, in accordance with the definition given above) was 91 minutes.

EXAMPLE 41

The following were introduced into a glass reactor: 330 g of a copolymer of acrylonitrile and sodium methallylsulphonate in the relative weight ratio of 91/9; the specific viscosity of the copolymer (measured at 25° C. in a 2 g/l solution in DMF) being 1; and 1600 cm³ Of DMF.

The mixture was stirred for 1 hour at 20°C and then for 4 hours at 90°C The gases which may have been dissolved were then removed by placing the reactor under a pressure of 100 mm/Hg for 30 minutes; the viscosity of the solution at 23°C was about 200 poises.

This solution was continuously cast onto a 17 em wide endless stainless steel belt circulating at a speed of 50 cm/minute; a 0.2 mm thick liquid film was thus desposited. The strip then passed into a 1.2 m long ventilated oven heated to 150°C and was thereafter cooled to 20°C; the film of acrylonitrile copolymer was moisturised by passing it over a fabric wetted with water and detached from the steel belt on which it was carried; this film then passed for 10 cm, through a bath of water at 90°C; it issued therefrom at a speed of 140 cm/minute, corresponding to a degree of stretching of 180%. The stretched film then passed through a bath of water at 75°C over 2 m, without being subjected to any tension whatsover; it issued from this bath at a speed of 100 cm/minute and then passed through a bath of glycerine (a mixture of water and glycerine in a weight ratio of 20/80) over 2 m, after which it was dried by passing between two rollers so as to drive off the excess glycerine.

A 30μ thick membrane was thus obtained; it had a permeability to water, under 2 bars, of 860 liters/day.m² and can be stored dry. Its degree of rejection was 100% for cattle albumen (molecular weight: 70,000) and for ovalbumin (molecular weight: 45,000); it was only 53% for dextran (molecular weight: 40,000).

EXAMPLES 42 TO 44

Example 41 is repeated, varying the degree of stretching. The following results were obtained:

______________________________________
Example
42      43        44
______________________________________
Degree of stretching in %
165       135       100
Permeability to water under
505       360       216
2 bars (l/day · m2)
Degree of rejection in %
Cattle albumen,  100       100       100
molecular weight: 70,000
Ovalbumin,       100       100       100
molecular weight: 45,000
Pepsin,           >90%      >90%     100
molecular weight: 36,000
Lysozyme          >90%      >90%     100
molecular weight: 15,000
Dextran,          74        78        85
molecular weight: 40,000
______________________________________

EXAMPLE 45

A membrane was prepared as in Example 41, but with a degree of stretching of 185%.

This membrane was used for the ultrafiltration of river-water (the Rhone, downstream from the city of Lyon) containing fecal streptococci and "Escherichia coli" bacteria. The difference in pressure between the first and the second compartment of the ultrafiltration apparatus was 2 bars; in the first compartment the river-water circulated over the surface of the membrane at a speed of 115 cm/second; a permeate of potable water was collected at the rate of 1150 l/day.m² ; the potable character of this water was determined by bacteriological examinations, which showed the total absence of the bacteria mentioned above.

EXAMPLE 46

The membrane of Example 41 was used for the ultrafiltration of 3.5 liters of whey having a solids content of 65 g/l, a lactose concentration of 50 g/l and a protein concentration of 7.6 g/l.

The difference in pressure between the two ultrafiltration compartments was 2.5 bars; the speed of circulation of the whey in the first compartment was 50 cm/second at the surface of the membrane.

The ultrafiltration was continued until 3.250 1 of ultrafiltrate had been obtained; the latter contained 59 g of solids/l, 50 g of lactose/l, and 0.7 g of proteins/l; the concentrate (withdrawn from the first compartment) had a solids content of 168 g/l, a lactose concentration of 48 g/l and a protein concentration of 96 g/l.

EXAMPLE 47

Example 46 was repeated, using the membrane of Example 44.

An ultrafiltrate having a solids content of 56 g/l and a lactose concentration of 50 g/l, and containing no proteins, was obtained; the concentrate had a solids content of 155 g/l, a lactose concentration of 58 g/l and a protein concentration of 93 g/l.

EXAMPLE 48

Human blood was ultrafiltered at 37°C in an apparatus of the filter-press type with 8 compartments each possessing a membrane of 125 cm² surface area, prepared as in Example 41.

The blood circulated in the apparatus in contact with the membranes at a flow rate of 0.2 l/minute; the difference in pressure between the compartments on either side of the membrane was 300 mm/Hg. After running for 8 hours, the protein concentration of the ultrafiltrate was 0.27 g/l; the haematocrit reading of the circulating blood had not varied significantly; the flow rate of ultrafiltrate was 0.4 l/hour.

EXAMPLE 49

A membrane was prepared as in Example 41 with the following modifications. The reactor was charged initially with 71 grams of an acrylonitrile/sodium methallylsulphonate copolymer (weight ratio 91/9; having a specific viscosity of 0.979), 179 grams of an acrylonitrile/sodium methallylsulphonate copolymer (in a weight ratio of 89/11; having a specific viscosity of 1.123) and 1200 cm³ of dimethyl formamide.

The drawing ratio was 186%.

A dialysis cell, maintained at 37°C, was separated into two compartments by 625 cm² of the membrane prepared above. In the first compartment of this cell human blood was circulated, in a closed circuit with a flow of 12.5 cm³ /MM; the total volume of blood in circulation was 1.1 liters and the blood contained 1.5 g/l of urea and 0.15 g/l of creatine creatinine. In the second compartment was circulated a dialysis bath having the same saline composition as the blood.

A reduction to a half the original concentration in the blood was observed after 1 hour 40 mins for the urea and after 2 hours 25 mins for the creatine creatinine.

EXAMPLE 50

An ascitic liquid containing 20 g/l of protein was removed from the peritoneum of a patient suffering from cirrhosis of the liver. This ascitic liquid was introduced to an ultrafiltration apparatus comprising eleven elementary cells arranged in series having a total membrane area of 1450 cm², prepared as in Example 49 but with a stretching ratio of 182%. The pressure differential across these membranes was 200 mm Hg (the excess pressure being on the side containing the ascitic liquid). The ascitic liquid passed through the membranes where it was concentrated by ultrafiltration and was then reinjected into a vein of the patient. A flow rate of 0.6 l/h was observed for the ultra filtrate while the ascitic liquid which has been concentrated was reinjected into the patient at a flow rate of 0.3 l/h.

Claims

#2# 1. A semi-permeable membrane material for use in an apparatus adapted for dialysis and ultrafiltration of blood, having the following characteristics:

(a) high ultrafiltration properties indicated by a permeability to water in an aqueous macromolecular (of a molecular weight greater than 500) solution of from 230 to 8000 l/day m² at a relative pressure of 2 bars;

(b) a thickness of less than 100 microns and greater than 0.5 microns;

(c) a degree of rejection of salt as defined by ##EQU3## of less than 1%; (d) a degree of rejection of macromolecules defined by ##EQU4## which is from 90% to 100% for cattle albumin of a molecular weight of 70,000, when an aqueous solution comprising 10 g/l of cattle albumin and 9 g/l of NaCl is subjected to a relative pressure of 2 bars;

(e) a high hemodialysis rate as represented by a reduction to a half the original concentration in 1.1 liters of human blood being reached after at most 1 hour 40 mn for urea and at most 2 hours 25 mn for creatinine, said blood circulated in a closed circuit at a rate of 12.5 cm3/mn over 625 cm2 of membrane, circulated dialysis bath having the same saline composition as the blood.

2. #2# In an apparatus adapted for dialysis and ultrafiltration of blood which comprises two compartments separated by a membrane, the improvement wherein the membrane is one of claim 45 1 which has been obtained by treating a film containing at least one copolymer of acrylonitrile and an ionic or ionisable olefinically unsaturated monomer by immersion in water or an aqueous non-solvent mixture at a temperature between 60° and 250°C

3. #2# Apparatus according to claim 2 which comprises means for circulating, or causing turbulence in, the blood to be dialyzed and ultrafiltered.

4. #2# Ultra-filtration apparatus according to claim 2 which comprises means for establishing a pressure difference between the two compartments.

5. #2# Ultra-filtration apparatus according to claim 4, which comprises a pressure-resistant porous support for the membrane.

6. #2# Apparatus according to claim 2 which comprises means for continuously feeding to, and withdrawing from, the compartment designed to contain the blood to be dialyzed and ultrafiltered.

7. #2# dialysis apparatus according to claim 6, which comprises means for circulating dialysis liquid in the other compartment.

8. #2# Apparatus according to claim 2 wherein the film consists essentially of either:

(i) a copolymer of acrylonitrile and at least one ionic or ionisable olefinically unsaturated monomer, or

(ii) a copolymer of acrylonitrile, at least one ionic or ionisable olefinically unsaturated monomer, and at least one non-ionic and non-ionisable olefinically unsaturated monomer, or

(iii) a mixture of at least two copolymers as defined under (1) and/or (ii), or

(iv) a mixture of at least one copolymer as defined under (1) and/or (ii), and at least one copolymer of acrylonitrile and at least on non-ionic and non-ionisable olefinically unsaturated monomer, the ionic or ionisable monomer units representing from 1 to 80% of the monomer units in one of said copolymers, and representing from 1 to 50% of the monomer units in the membrane.

9. #2# Apparatus according to claim 8, wherein the ionic or ionisable monomer is one having the formula: ##STR3## in which: r represents an optionally salified sulphonic acid or phosphonic acid group, or a quaternary ammonium group;

each of R₁, R₂ and R₃, which may be identical or different, represents a hydrogen atom or an alkyl radical of 1 to 4 carbon atoms; and

A represents:

(a) a divalent purely hydrocarbon group, the free valencies of which are carried by a purely aliphatic saturated or unsaturated, straight or branched chain, or by an aromatic nucleus, or by a mixed chain, one of the free valencies being carried by an aliphatic carbon atom and the other free valency being carried by an aromatic carbon atom,

or (b) a divalent chain consisting of aliphatic and/or aromatic hydrocarbon groups, bonded to one another by an oxygen or sulphur atom or by a carbonyloxy or 1-oxo-2-aza-ethylene group, the free valencies being carried by aliphatic and/or aromatic carbon atoms,

or (c) a --O--A'-- or --S--A'-- group, wherein A' represents a group as defined under (a) or (b),

or (d) a divalent group as defined under (a), (b) or (c) in which one or more of the carbon atoms are substituted,

or (e) a valency bond.

10. #2# Apparatus according to claim 9, wherein R₁ and R₂ represent hydrogen atoms and R₃ represents a hydrogen atom or a methyl radical.

11. #2# Apparatus according to claim 10, wherein A contains less than 12 carbon atoms.

12. #2# Apparatus according to claim 8, wherein the ionic or ionisable monomer is vinylsulphonic, vinyloxybenzenesulphonic or methallylsulphonic acid or a salt thereof or a EN-methylvinyl-pyridinium salt.

13. #2# Apparatus according to claim 8, wherein the non-ionic and non-ionisable monomer is vinylidene chloride, acrylamide or styrene.

14. #2# Apparatus according to claim 2 wherein the ionic or ionisable monomer represents from 2 to 15 mole % of all the monomers in the membrane.

15. #2# Apparatus according to claim 2 wherein the membrane is reinforced with a support.

16. #2# Apparatus according to claim 2, wherein the temperature is between 80° and 190°C and the film is immersed in water or a liquid aqueous non-solvent mixture.

17. #2# Apparatus according to claim 12, wherein during the treatment the film is monoaxially or biaxially stretched between 20 and 1,000% measured linearly during said treatment.

18. #2# Apparatus according to claim 17, wherein the film is stretched between 50 and 500%, measured linearly.

19. #2# In a method of ultra-filtering and dialysing blood which comprises placing the blood in contact with a membrane, the improvement wherein the membrane is one of claim 1 which has been obtained by treating a film containing at least one copolymer of acrylonitrile and an ionic or ionisable olefinically unsaturated monomer by immersion in water or an aqueous non-solvent mixture at a temperature between 60° and 250°C

20. #2# A method according to claim 19 wherein the membrane is obtained from a film consisting essentially of either:

(i) a copolymer of acrylonitrile and at least one ionic or ionisable olefinically unsaturated monomer, or

(ii) a copolymer of acrylonitrile, at least one ionic or ionisable olefinically unsaturated monomer, and at least one non-ionic and non-ionisable olefinically unsaturated monomer, or

(iii) a mixture of at least two copolymers as defined under (i) and/or (ii), or

(iv) a mixture of at least one copolymer as defined under (i) and/or (ii), and at least one copolymer of acrylonitrile and at least one non-ionic and non-ionisable olefinically unsaturated monomer, the ionic or ionisable monomer units representing from 1 to 80 mole % of the monomer units in one of said copolymers, and representing from 1 to 50 mole % of the monomer units in the membrane.

21. #2# A semi-permeable membrane according to claim 1, which has been prepared by immersion of a film in an aqueous liquid both at a temperature from 80° to 190°C, the film consisting essentially of either:

(i) a copolymer of acrylonitrile and at least one ionic or ionisable monomer, or

(ii) a copolymer of acrylonitrile, at least one ionic or ionisable monomer and at least one non-ionic and non-ionisable monomer, or

(iii) a mixture of at least two copolymers as defined under (i) and/or (ii), or

(iv) a mixture of at least one copolymer as defined under (i) and/or (ii), and at least one copolymer of acrylonitrile and at least one non-ionic and non-ionisable monomer, said copolymer having a specific viscosity (measured at 25°C in a 2 g/l solution in dimethylformamide) of between 0.5 and 1.5, the ionic or ionisable monomer units representing from 1 to 80% of the monomer units in one of said copolymers and 2 to 15% of the monomer units present in the film, said ionic or ionisable monomer having the formula: ##STR4## in which: Y represents an optionally salified sulphonic acid or phosphonic acid group, or a quaternary ammonium group;

each of R₁, R₂ and R₃, which may be identical or different, represents a hydrogen atom or an alkyl radical of 1 to 4 carbon atoms; and

A represents:

(a) a divalent purely hydrocarbon group, the free valencies of which are carried by a purely aliphatic saturated or unsaturated, straight or branched chain, or by an aromatic nucleus, or by a mixed chain, one of the free valencies being carried by an aliphatic carbon atom and the other free valency being carried by an aromatic carbon atom,

or (b) a divalent chain consisting of aliphatic and/or aromatic hydrocarbon groups, bonded to one another by an oxygen or sulphur atom or by a carbonyloxy or 1-oxo-2-aza-ethylene group, the free valencies being carried by aliphatic and/or aromatic carbon atoms,

or (c) a --O--A'-- or --S--A'-- group. wherein A' represents a group as defined under (a) or (b),

or (d) a divalent group as defined under (a), (b) or (c) in which one or more of the carbon atoms are substituted,

or (e) a valency bond.

22. #2# A membrane according to claim 21 in which Y represents a sulphonic acid group.

23. #2# A membrane according to claim 22 in which the ionic or ionisable monomer is selected from the group consisting of vinyl sulphonic acid, vinyloxybenzene sulphonic acid and methallyl sulphonic acid and salts thereof and N-methylvinylpyridinium salts.

24. #2# A membrane according to claim 21 in which the film is immersed in glycerine or a mixture of water and glycerine after said immersion of the film in an aqueous liquid bath.

25. #2# Process for preparing a semi-permeable membrane as defined in claim 21 which comprises immersing the film in an aqueous liquid bath at a temperature of 80° to 190°C

26. #2# Process according to claim 25 in which the ionic or ionisable monomer in said film contains a sulphonic acid group.

27. #2# Process according to claim 1 26 in which the ionic or ionisable monomer is selected from the group consisting of vinyl sulphonic acid, vinyloxybenzene sulphonic acid and methallylsulphonic acid and salts thereof and N-methylvinylpyridinium salts.

28. #2# Process according to claim 25 in which the film is immersed in glycerine or a mixture of water and glycerine after said immersion of the film in aqueous liquid bath.

29. #2# A semi-permeable membrane according to claim 1, prepared from a film consisting essentially of:

(i) a copolymer of acrylonitrile and at least one ionic or ionisable monomer, or

(ii) a copolymer of acrylonitrile, at least one ionic or ionisable monomer and at least one non-ionic and non-ionisable monomer, or

(iii) a mixture of at least two copolymers as defined under (i) and/or (ii), or

(iv) a mixture of at least one copolymer as defined under (i) and/or (ii), and at least one copolymer of acrylonitrile and at least one non-ionic and non-ionisable monomer, the ionic or ionisable monomer representing 1 to 80% of the monomer units in one of said copolymers and 1 to 50% of the monomer units in the film, said film being subjected to an aqueous thermal treatment by immersion in an aqueous non-solvent bath at 60° to 250°C, the film being monoaxially or biaxially stretched between 20 and 1,000%, measured linearly, during said treatment.

30. #2# A membrane according to claim 29 in which one or more of the acrylonitrilo copolymers has a specific viscosity (measured at 25° C. as a 2 g/l solution in dimethylformamide) between 0.1 and 3 and the temperature of the aqueous bath is between 80° and 190°C

31. #2# A membrane according to claim 29 in which the acrylonitrile copolymer has a specific viscosity between 0.5 and 1.5.

32. #2# A membrane according to claim 29 in which the film is stretched between 50 and 500% measured linearly.

33. #2# A membrane according to claim 29 in which the ionic or ionisable monomer has the formula: ##STR5## in which: Y represents an optionally salified sulphonic acid or phosphonic acid group, or a quaternary ammonium group;

each of R₁, R₂ and R₃, which may be identical or different, represents a hydrogen atom or an alkyl radical of 1 to 4 carbon atoms; and

A represents:

(a) a divalent purely hydrocarbon group, the free valencies of which are carried by a purely aliphatic saturated or unsaturated, straight or branched chain, or by an aromatic nucleus, or by a mixed chain, one of the free valencies being carried by an aliphatic carbon atom and the other free valency being carried by an aromatic carbon atom,

or (b) a divalent chain consisting of aliphatic and/or aromatic hydrocarbon groups, bonded to one another by an oxygen or sulphur atom or by a carbonyloxy or 1-oxo-2-aza-ethylene group, the free valencies being carried by aliphatic and/or aromatic carbon atoms,

or (c) a --O--A'-- or --S--A'-- group, wherein A' represents a group as defined under (a) or (b),

or (d) a divalent group as defined under (a), (b) or (c) in which one or more of the carbon atoms are substituted,

or (e) a valency bond.

34. #2# A membrane according to claim 33 in which Y represents a sulphonic acid group.

35. #2# A membrane according to claim 33 in which the ionic or ionisable monomer is selected from the group consisting of vinyl sulphonic acid, vinyloxybenzene sulphonic acid and methallyl sulphonic acid and salts thereof and N-methylvinylpyridinium salts.

36. #2# A membrane according to claim 29 in which the film is immersed in glycerine or a mixture of water and glycerine after said immersion in aqueous non-solvent bath.

37. #2# Process for preparing a semi-permeable membrane as defined in claim 29 which comprises immersing the film in an aqueous non-solvent bath at a temperature from 60° to 250°C, stretching said film monoaxially or biaxially by 20 to 1,000%, measured linearly, during the immersion.

38. #2# Process according to claim 37 in which the, or at least one of the, acrylonitrile copolymers has a specific viscosity (measured as a 2 g/l solution in dimethylformamide at 25°C) between 0.1 and 3 and the temperature of the aqueous bath is from 80° to 190°C

39. #2# Process according to claim 37 in which the, or at least one of the, acrylonitrile copolymers has a specific viscosity between 0.5 and 1.5.

40. #2# Process according to claim 37 in which the film is stretched from 50 to 500%, measured linearly.

41. #2# Process according to claim 37 in which the ionic or ionisable monomer has the formula: ##STR6## in which: Y represents an optionally salified sulphonic acid or phosphonic acid group, or a quaternary ammonium group;

each of R₁ , R₂ and R₃, which may be identical or different, represents a hydrogen atom or an alkyl radical of 1 to 4 carbon atoms; and

A represents:

(a) a divalent purely hydrocarbon group, the free valencies of which are carried by a purely aliphatic saturated or unsaturated, straight or branched chain, or by an aromatic nucleus, or by a mixed chain, one of the free valencies being carried by an aliphatic carbon atom and the other free valency being carried by an aromatic carbon atom,

or (b) a divalent chain consisting of aliphatic and/or aromatic hydrocarbon groups, bonded to one another by an oxygen or sulphur atom or by a carbonyloxy or 1-oxo-2-axa-ethylene group, the free valencies being carried by aliphatic and/or aromatic carbon atoms,

or (c) a --O--A'-- or --S--A'-- group, wherein A' represents a group as defined under (a) or (b),

or (d) a divalent group as defined under (a), (b) or (c) in which one or more of the carbon atoms are substituted,

or (e) a valency bond.

42. #2# Process according to claim 41 in which Y represents a sulphonic acid group.

43. #2# Process according to claim 42 in which the ionic or ionisable monomer is selected from the group consisting of vinyl sulphonic acid, vinyloxybenzene sulphonic acid and methallylsulphonic acid, and salts thereof, and N-methylvinylpyridinium salts.

44. #2# Process according to claim 37 in which the film is immersed in glycerine or a mixture of water and glycerine after said immersion in the aqueous non-solvent bath.

45. #2# A membrane material according to claim 1 in which the degree of rejection of macromolecules is from 50% to 85% for dextran of an average molecular weight of about 40,000, and is from 28% to 50% for dextran of an average molecular weight of about 10,000, when an aqueous solution comprising 4 g/l of the dextran is subjected to a relative pressure of 2 bars.

46. #2# An apparatus adapted for dialysis and ultrafiltration of blood, comprising membrane material of claim 1.

47. #2# A method of treating human blood to establish a lowered concentration of urea and creatinine in the human blood, which comprises the steps of circulating the human blood in the first compartment of a dialysis cell separated into two compartments by membrane material of claim 1, and circulating a dialysis liquid having the same saline composition at the blood in the other compartment.

48. #2# The membrane according to claim 1 wherein the film which is homogenous and symmetrical.

49. #2# The membrane according to claim 1 wherein the membrane has a tensile strength of from 90 to 130 kg/cm² at 23°C while wet, and a tensile elongation at break of from 31% to 48%. 50. A semi-permeable membrane material for use in an apparatus adapted for dialysis and ultrafiltration of blood, having the following characteristics:

(a) high ultrafiltration properties indicated by a permeability to water of from 230 to 8000 l/day m² at a relative pressure of 2 bars;

(b) a thickness of less than 100 microns and greater than 0.5 microns;

(c) a degree of rejection of salt as defined by ##EQU5## of less than 1%;

(d) a degree of rejection of macromolecules defined by ##EQU6## which is from 90% to 100% for cattle albumin of a molecular weight of 70,000, when an aqueous solution comprising 10 g/l of cattle albumin and 9 g/l of NaCl is subjected to a relative pressure of 2 bars;

(e) a high hemodialysis rate as represented by a reduction to a half the original concentration in 1.1 liters of human blood being reached after at most 1 hour 40 mn for urea, said blood circulated in a closed circuit at a rate of 12.5 cm3/mn over 625 cm2 of membrane, circulated dialysis bath

having the same saline composition as the blood. 51. The membrane according to claim 48 obtained by treatment of a homogeneous and symmetrical film.