CA1101161A

CA1101161A - Semipermeable membranes of copolyamides

Info

Publication number: CA1101161A
Application number: CA267,138A
Authority: CA
Inventors: Klaus Elfert; Gerhard D. Wolf; Francis Bentz; Hans E. Kunzel
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1975-12-06
Filing date: 1976-12-03
Publication date: 1981-05-19
Also published as: ATA897276A; JPS5270989A; BE849099A; FR2333548B1; NL7613507A; DE2554932A1; AT364733B; FR2333548A1; IT1123942B; GB1539264A

Abstract

Semipermeable membranes of copolyamides Abstract of the Disclosure The invention relates to semipermeable membranes with a water absorption capacity of from 4.5 to 10 % by weight preferably from 4.5 to 8% by weight as measured on approximately 40 µ thick symmetrical films at room temperature and 65 % relative air humidity, with a throughflow of at least 80 1/m2d for a desalination level of at least 85 % and consisting of a fully aro-matic copolyamide with a relative viscosity of z1,4, as measured on a 0.5 % N-methyl pyrrolidone solution at a temperature of 20°C.

Le A 16 828-Ca

Description

. ,~, ' i~Ol~l This invention relates to semipermeable membranes oi -fully aromatic copolyamides which are particularly suitable for inverse osmosis and ultra~iltration, and to the production ; of these membranes.
Inverse osmosis and ultrafiltration are mass separation processes which are particularly economical by virtue of the low energy costs involved, because in such forms of mass . 10 separation, in contrast to separation by distillation, there is no phase change of the solvent, and temperatures around ambient temperature are normally applied.
~- ~ The principle of inverse osmosis has been known for some `~ time. In inverse osmosis, solvents migrate from a solutio~
`~ 15 of relatively high concentration through a semipermeable ~ membrane into a solution of lower concentration under the ~ ~ .
~` effect of an applied pressure which is above the osmotic pressure of the system. In this way, it is possible for example to separate dissolved substances from the solvent.
Examples of the technical application of this separation process are the desalination of sea water or brackish water, the purification of contami~ated water for the production of drinking water or industrial water, also the concentration, removal or recovery o~ a variety of dif~erent substances -from aqueous solutions, for example the concentration of food stuf~s or the separation or concentration oi biological or pharmaceutical products.
Although numèrous installations for the industrial application oi inverse osmosis and ultrafilitration are 30~ already in operatlon, the provision o~ suitable membranes is still one o~ the main problems of these processes.
Numerous polymers~have been tested for their suitabilit~

lr~ 7.~ A 16 828^1 as a membrane material. They have to satisfy certain re~uirements in regard to their permeability and their selectivity and, in addition, have to be chemically, thermally and mechanically stable. Membranes of cellulose acetate an~ o~ aromatic polyamides have hitherto mainly been used for commercial purposes. It was only as a result of the development of the asymmetrical cellulose acetate membranes by Loeb and Sourirajan (cf. US Patent Specification No.3,133,1~2) that it became possible to produce membranes with satisfactory properties, i.e. high throughflow rates coupled with a high separation capacity. Howeverj cellulose acetate membranes show certain disadvantages in regard to their chemical and thermal stability. They are readily hydrolysed under acid or alkaline conditions and are degraded by microorganisms. This means that their filtration properties gradually deteriorate which restricts the useful life and, hence, the general serviceability of cellulose acetate membranes.
In addition, the membrane is compressad under the effect of pressure applied, resulting in a reduction in the throughflow rate.
Aromatic polyamides, of the type described for example in German Offenlegungsschrift Nos.1,941,022 and 1,941,932 and in US Patent Specification No.3,567,632, are also suitable ior the production of asymmetrical semipermeable membranes In par$icular, they are superior to the cellulose acetate membranes in their resistance to chemical and thermal influences One of their disadvantages, however, is their lower permeability to water in comparison with cellulose acetate membranes.
The permeability to water of a plastics material is related to its water absorption capacity. Cellulose acetate which as a membrane material is characterised by high flow kf 2 6~

rates coup]ed with high selectivityJ has a water content of from 10 to 15%. By contrast, poly-m-phenylene isophthalic acid amide has a water content of from 3.8 to 4%~ as measured on an approximately llO ~I thick symmetrical film, and accord-ingly has a ]ow throughflow rate.
Another of the nitrogen-containing polycondensates commonly used in membrane technology is poly-(2,2'-(m-phenylene)-5,5'-bis-benzimidazole) which has a water absorp-tion capacity of 11 to 13~ (as measured on filaments at room temperature/65~ relative air humidity) and, as expected, a high throughflow rate. In contrast to cellulose acetate, however, this condensate has only very limited selectivity.
In the case o~ N-containing condensates, therefore, the impression is formed that high through~low rates are coupled with low selectivities.
The object of the present invention is to obviate the ~isadvantages referred to above and to develop membranes with high throughflow rates coupled with high selectivity.
It has surprisingly been found that it is possible to produce from certain copolyamides with an increased water absorption capacity polymer membranes which thus show in-~
creased permeability to water coupled with very high selec-ti~ity.
Accordlngly, the pre~ent lnvention provide~ semiper~e-able membranes with a water ab30rption capacity oi ~rom 4.5 to 10% by weight, pre~erably from 4.5 to 8% by weight, as measured on approximately 40 ~ thick symmetrical ~ilmY
at room temperature and 65% relative air humidityJ with a throughflow of at least 80 1/m2d for a desalination level o~ at least 85C/o and consi~ting of a fully aromatic copolyamide with a relative viscosity of ~ 1.4, as measured on a 0.5yo N-methyl pyrrolidone ~olution at a ~emperature o~ 20C.

k~ 3 l61 In a preferred embodiment, the invention provides semi-permeable membranes of the copolyamide described above having a throughflow of from 85 to 200 1/m2d for a desalination level of 95 to 99.60/o. The throughflow values and desali-nation levels are measured in a pressure osmosis apparatus, in which a 3.5/g NaCl-solution is pumped in a circuit past the membrane surface at room temperature under a pressure of 110 bars at a rate of 15 l/h.
Particular].y preferred semipermeable membranes have a water absorption capacity of from 4.5 to 10% by weight, preferably from 4.5 to 8% by weight, as measured on approx-imately 40 p thick symmetrical films at room temperature and at 65% rélative air humidity, consisting of a fully aromatic copolyamide of A) 10 to 90, preferably 50 to 85 mole percent of units corresponding to the formula (I) . .
: R R' e o -HN ~NH OC -Ar- CO~

kf 4 :in wllich R, R' and R" represent hydrogen, C1 4-alkyl, preferably methyl, or l1alogen and Ar repres~ts an optionally alkyl or halogen-substitute~
bivalent aromatic radical, pre~rably phenylene, naphthylene, biphenylene or a radical of the formula ~ o ~

and of B) 10 to 90, preferably 15 to 50 mole % of units corresponding to formulaeII, III or IV below R R"

- HN ~ X ~ NHOC-Ar-CO ~ (II) : ~' R"' R~ RIV

- HN ~ X ~ - Y ~ NHOC-Ar CO - (III) R' R"' R

R R" r~IV ~VI

- HN ~ X ~ Y ~ - Z ~ NHOC-Ar-CO

(IV) 6l in which R, R', R", R"', R , R , R and R represent hydrogen, Cl 4-alkyl or halogen, Ar is as defined above and X, Y and Z represent a direct bond or bridge members corresponding to the formulae -CONH-, -NHOC-, -O-, -OCONH-, -NHCOO-, -CH2-, - C - , -S02-, -NHCONH-, -COO-, -OOC- or - ,C, - 3 with a relative viscosity of 2 1.4, as measured on a 0.5% N-methyl pyrrolidone solution at a temperature of 20C.
In another aspect, the invention provides a process for the production of a semipermeable membrane wherein a polymer casting solution comprising 5 to 35% by weight, based on the weight of copolyamide and , solvent, of said aromatic copolyamide as defined above is applied to a sub-strate in a layer thickness of from 150 to 500 u~, the film thus formed is treated at a temperature of from 40 to 150C for a period of from 2 to 60 ; minutes and, after a cooling phase of 10 minutes, the resulting film is immersed for 30 minutes at O to 50C in a coagulation bath which is miscible with the aprotic solvent and represents a non-solvent for the copolyamide.
In formulae ~I) to (IV), the substituents have the following, preferred meanings:
R represents hydrogen or methyl and R' and R" represent hydrogen, whereas Ar represents an _- or ~-phenylene radical. At the same time, R, R", RIV
; and RV represent hydrogen, methyl or chlorine and R', R"', RV and R
represent hydrogen.
X in formula ~II) is preferably -O-, -CONH- or -NHOC-.
X and Y in formula (III) are preferably - O -, -NHOC-, -CONH-, -NHCOO-, -COO-, -OOC-, -NHCONH- or combinations of these bridge members.
In formula (IV) X and Y are preferably -O-, -CONH- or -NHCO- and Z is S02 or CH3 -- C -., :

. ~ .

6i In the context of the invention, copolyamides are co-condensates of aromatic diamines and aromatic dicarboxylic acid dichlorides, the individual components also consisting of several aromatic rings which are attached to one another through - 6a -~3 ",~

llQi~61 single bonds or even through other bridge members in the form of amide structures. Accordingly, possible bridge members are inter alia ester, urea, urethane, ether, alkylene, carbonyl and -S02-structures.
The copolyamides suitable for use in accordance with the invention are produced from two or more diamines and one or more dicarboxylic acid chloride(s). The diamines used are compounds corresponding to generai formulae V to VIII belo~:
R R' R" (V) R R"

H2N ~ X - ~ NH2 (VI) Rl R"' R R" RIV

H2N j~X~ Y~ NH2 ( III) R R" RI~ RVI

U~N~ X~Y~ Z~NH2(vII ) in which R, R'~ R", R1", RIV~ RV RVI RVII X Y
as ~fined above.

Otller suitablc tliamines are, for example, the diamines lcscribct~ US Patcnt Specfications Nos.2,989,~195 (column 1l, lines 1 to 70), 3,351l,127 antl 3,31l9,062.
In atldition, the foIlowin~ diamines for example may be usetl with advantage:

2N~NHoC~3-- ~3_ NH2 C HOC~O ~_ NH2 H2N~3CoHN~NHoC~ NH2 .

. .

H2N~ coo~3_ooc~3_ 2

2 ~NIUONH_~ NHCONU _~_ NH2 116~

2 ~N~ICOO~ NH2 ~ ~3 C ~30 ~ 2 H2N~30~502~30~3NH2 - The above formulae are merely intended to indicate a nllm~)er of ~ossibilities without limiting the in~ention thereto.
The acid component consists of one or more aromatic dicarboxylic acid dihalide(s) corresponding to the general formula (IX) ~ . .
Hal - OC - Ar - CO - Hal (IX) .
: in which lal represents chlorine or bromine and ~r represents an optionally alkyl- or halogen-substituted ~; 10 bivalent radic~l, but prefer~bly m-phenylene, ~-phenylena, : biphenylene, naphthylene or a radical corresponding to the formula ' ~-C~

11~1161 Tho following compounds are mentioned as specific examples: isophthalic acid dichloride, terephthalic acid ~lichloride, biphenyl dicarboxylic acid-l~,4~-dichloride, a~htllalene dicarboxylic acid-1,5-dichloride, naphthalene licarboxylic acid-2,6-chloride, ben~ophenone dicarboxylic acid~ -di-chloride and the corresponding dibromides, a]so alkyl- and halogen-substitution products of the above-mentione~l acid dihalides.
Polycondensation of the described diamine and dicarboxylic acid dichloride components is carried out by methods known er se, such as interfacial polycondensation, but preferably by solution polycondensation in polar organic solvents, such as N,N-dialkyl acid amides, preferably N,N-dimethyl ace1;-amide or N-alkyl-substituted lactams, preferably N_methyl pyrrolidone, or in tetramethyl urea, hexamethyl phosphoric acid triamide or in mixtures of these polar aprotic solvents, in the absence of additional acid acceptors, but optionally in the presence of solution promoters, such as alkali metal or alkaline earth metal halidesJ where they are requiretl for keeping the copolyamides formed in solution. The condensation reaction is carried out at temperatures of from -30 to ~-150C
and preferably at temperatures of from -20 to +30~C. The reaction times may be between 1 and 30 hours. The Rolu1;ion has a solids content of from 5 to 40%, preferably from ]5 to 25C/o. In order to obtain reaction products with as high a molecular weight as possible, it is best to use the sum of the diamines and the dicarboxylic acid dichloride component in equimolar quantities, although basically the polyconden-sation reaction may also be carried out with either an excess or deficiency of the dicarboxylic acid dichloride. ~he kf 10 6~

dicarboxylic acid dichloride may be added to the solution or sllspension of the diamines in the solvent in several sm~ll portions over a prolonged period. In some cases, however, it is advisable to add all the dicarboxylic acid ~ichloride at once, preferably with cooling Most of the aromatic copolyamides used in accordance with the invention for the production of membranes are soluble in polar or~anic solvents, such as N,N-dimethyl formamide, N,N-dimethyl acetamide and N-methyl pyrrolidone, at least when a few percent of an alkali metal or alkaline earth metal salt, such as calcium chloride or lithium chloride, is added as solution promoter These copolyamides may readily be processed by known methods into asymmetrical membranes or hollow filaments.
The membranes prod~ced from the copolyamides used in accordance with the invention have an anisotropic or asymmetrical structure. Asymmetrical membranes according to Loeb and Sourirajan are characterised by the following structure:
a homogeneous and den~e membrane layer of minimal thickness (0,1 - 0.5 ~) changes substantially continuously into an underlayer with a porous structure which acts as carrier or supporting layer and has no influence upon the filtration properties. By contrast, the dense side of the membrane represents the actual selective ~eparation layer which allows economic throughflow rates by virtue of its minimal thickness.
The asymmetry of the strucutre is a result of the production process. In the production process, a casting solution of the polymer is prepared in a suitable solvent. A film is then cast from this solution and subjected to a heat treat-kf 11 11~116~

ment, durin~ W}liC}I the solvent partly evaporates and the asymmetrical structure is formed. Thereafter the polymer filln is coagulated in a non-solvent, the structure pre-rornle~ durin~ the heat treatment being consolidated.
The process by which the membranes are produced com-prises the following stages:
1. 5 to 35~ by weight of the polymer product, based on the weight of the polymer and solvent, are dissolved in a polar aprotic solvent in the presence of from 1 to 10~
]o by weight of an alkali metal or alkaline-earth metal salt, preferably LiCl, LiBr, LiN03, CaC12, CaBr~
Preferred solvents are dimethyl formamide, dimethyl acetamide, N-methyl pyrrolidone, dimethyl sulphoxiide, hexamethyl phosphoric acid triamide and mixtures 1;hereof.
Heat may optionally be applied to accelerate dissolution.
The solution is then filtered.
~ 2. The solution thus prepared is applied to a glass or metal ;~ substrate or to any other suitable substrate, ior example a moving belt or a drum, in a layer thickness o~ from 150 to 500 ~.

3. Thi~ film is then subjected to a heat treatment at a certain temperature for a-certain time. The film i~
preferably heat-tr~ated at a temperature of from 40 to 150C over h period of from 2 to 6G minutes.

4. After a cooling phase of 10 minutes, the ~ilm is immersed in a coagulation bath and left there for 30 minutes.
Suitable coagulation liquids are solvents of the type which are miscible with the organic solvent and, at the ~ame time9 are able to dissolve the salt, but which represent a non-solvent for the polymer. Suitable sol~ents of thi~ type are water, methanol, ethanol and kf 12 ~1~1161 i-propanol. I~ater is preferably used as the coa~ulation liquid. The tem~erature of the coagulation bath may ~e between 0 and 50C although it is preferably in the range from 0 to 25C.
~he invention also provides a process for the production of semipermeable membranes by heat treating a f;lm produced from a po].ymer casti~g solut-.on, the solvent being partly evaporated, and subsequently coagulating the polymer filrn in a non-solvent, characterised by the faot that from 5 to 35 %
by weight, based on the weight of copolyamide and solvent, of an aromatic copolyamide of A) 10 to 90, preferably 50 to 85 mole ~/~ of units corresponding to the formula R R' -UN ~ NUOC-Ar-CO-R"

in which :~ 15 R, R' and R" represent hydro~en, C1 4-alkyl, preferably methyl, or halogen and Ar represents an optionally alkyl- or halogen-substituted bivalent aromatic radical, preferably phenylene, naphthylene, biphenylene or a radical of the formula ,~-C-.
and of .. .. ..

1,1~116i 13) 10 to 90, preferably 15 to 50 mole /~, of units corresponding to formula II, III or IV below R R"

- Hll~X~--~NHOC-Ar-CO - (]:I) .. R~ R"~
R R" RIV

- HN--~X_~--Y~NHOC-Ar-CO ~ (III) R' . R"' R

: R R~ RIV Vl - HN~X~Y~Z~ NHOC-.4r-CO -R ~ . R~ ~ RV RVI I
( IV) in which ~ ~ R~ RIV RV RVI and RVII represent hydrogen, ~C1 4-alkyl or halogen, .
:Ar is as previously deiined, and X, Y and Z represcnt a direct bond or bridge members : corresponding to the formulae CE13 ~CONH-, -NE-IOC-, -O-, -OCONH-, -NHCOO-, -CH2-, - C - , -S02-, -NHCONH-, ~COO-, -OOC- or - C -O
with a relativc viscosity of ~ 1.4, as measured on a 0.5 %

solution in N-methyl pyrrolidone at a temperature of 20C, are dissolved, optionally under heat, in an aprotic so]vent, such as dimethyl formamide, dimethyl acetamide, N-methyl pyrroli(lone, dimethyl sulphoxide and hexamethyl phosphoric acid-tris~amide or mixtures thereof, optionally in the presence of ~rom 1 to 10% by weight Or LiCl, LiBr, LiN03, MgC12, CaC12 or CaBr2, or in the presence of an organic amine, such as triethylamine, tripropylamine, pyridine or ethanolamine7 the solution thus formed is optionally filtered and applied to a substrate in a layer thickness o~ from 150 to 500,u, the film thus formed is treated at a temperat;ure of from 40 to 150C for a period of from 2 to 60 minutes and, after a cooling phase of 10 minutes, the film is immersed for 30 minutes at 0 to 50C in a coagulation bath which is miscible with the aprotic solvent, which optionally contains added salt and which represents a non-solvent for the copolyamide.
The membranes according to the invention may be used in the form of flat membranes, in tubular form or even in the form of hollow fibres both for inverse o~mosis and for ultrafiltration. The techniques used for producing tubular structures or hollow fibres correspond accordingly to the process according to the invention.
The moisture absorption capacity of the polymers was determined on approximately 40 ~ thick symmetrical films.
To this end, the films were washed at 30C, dried and, for moisture absorption, were exposed for 24 hours to an atmosphere of 20C/650~ relative air humidity. The films or fibres were then dried in vacuo at 80C. The moisture absorption is expressed as the equilibrium absorption in % of the weight of the absolutely dry films or fibres.

k~ 15 In order to determine the e~fectiveness of the membranes, the finished membrane is applied to a porous sintered plate of metal, on which a piece of filter paper has been placed, and is introduced into a pressure osmosis apparatus in which S a 3.5 % NaCl-solution is pumped in a circuit past the surface of the membrane at room temperature and under a pressure of 110 bars. The pumping rate amounts to 15 l/h.
The throughput of water through the membrane is determined and the NaCl-content measured in the usual way.
EXA~LE
A copolyamide with a relative viscosity ~ rel of 1.43, as measured on a 0.5 % solution of the polyamide in N-methyl pyrrolidone at 20C (the viscosities in the following Examples were measured under the same conditions) was produced by solution polycondensation, with N,N-dimethyl acetamide as solvent, from 40.0 parts by weight of m-phenylene diamine, 10.4 parts by weight of a diamine with the following structure 2 ~ CONH ~ NHOC ~ NH2 and 81.2 parts by weight of isophthalic acid dichloride.
The moisture absorption capacity of this copolyamide amounted to 5.4 %.
A clear solution was prepared with stirring under heat ;- (60C) from 13.5 g of the polymer, 3.4 g of LiN03 and 50.6 g ; of N-methyl pyrrolidone. A casting solution ready for use was ~ .

, 11~1161 obtained after filtration and the remova]. of residua~air bubbles. A film was applied to a glass plate in a thickness of 250 ~ and then heated for 20 minutes at 80C on a heating plate. After a cooling phase of 10 minutes, the film was immersed in an ice/water bath and left there for 30 minutes, during which time the film detaches itself from the glass plate.., The film was 'stored in water at room temperature.
Under the conditions defined above, this membrane had a water throughflow of 197 l/m d and a salt retention capacity of 98.8 %.

A copolyamide with a relative viscosity ~ l of 1.66 was produced by the same method from 18.4 parts by weight of m-phenylene diamine, 9.9 parts by weight of diamine with the following structure .

H2N~NHOC--0_ O ~ NH2 and 40.6 parts by weight of isophthalic acid dichloride.
The m~sture absorption capacity amounted to 4-9'iO-A solution containing 15 g of the polymer, 3 g of CaCl2 and 72 g of N-mqthyl pyrrolidone was prepared. A
film cast in a thickness of 250 ~ was treated for 20 minutes at a temperature of 80C. The membrane thus produced ~as tested and produced a throughflow of 160 l/m2d and a salt rejection of 95.2 /0.
- A film 250 ~ thick was produced from a solution conta~ing 10 g of the same polymer, 2.5 g of LiN03 and 87.5 g of 1101~61 N-methyl pyrrolidone, an~ treated for 15 minutes at a temperature of 70C. Under the test conditions mentioned above, this membrane had a throughflow of 100 l~m2d and a salt rejection of 98.8 %.
EXA~LE 3 10 g of a copolyamide with a rel~tive viscosity ~ 1 of 1.79 and a moisture absorption capacity of 6~1 % (produced from 16.2 parts by weight of m-phenylene diamine, 16.7 parts by weight of a diamine with the following structure H2N~;~NHOC ~ ~ NH2 and 40.6 parts by weight of isophthalic acid dichloride) t 2.5 g of LiN03 and 87.5 g of N-methyl pyrrolidone were dissolved. A film 250 ~ thick was produced from this c~sting solution and treated for 20 minutes at a temperature of 70~ This membrane had a throughflow of 120 l/m2d and a salt rejection of 99.6 %.
EXA~LE 4 A copolyamide was produced from 504 parts by weight of m~phenylene diamine, 15.9 parts by weight of a diamine with t~e following structure H2N_~3CoNH~ ~

and 20.3 parts by weight of isophthalic acid dichloride.

The moisture absor~ion capacity amounted to 5.1 ~
A solution was prepared from 12.~ ~ of the polyamide, 3.1 g of LiN03 and 84.3 g of N-methyl pyrrolidone. A film cast in a thiclcness of 250 ~ was treated for 10 minutes at 90C. The membrane produced the following test results:
a throughflow of 85 1/m2d for a salt rejection of 99.~ %.
Comparison Example A poly-(m-phenylene isophthalic acid amide) with a relative viscosity of 2.02 was produced under standard conditions from 10.8 parts by weight of m-phenylene diamine and 20.3 parts by weight of isophthalic acid dichloride.
A moisture absorption capacity of 3.8 ~/0 was determined.
A film with a thickness of 250 ~ was cast from a casting solution of 10 g of the polymer, 2.5 g of LiCl and 47.5 g of N,N-dimethyl acetamide9 and heated for 20 minutes to 110C. A throughflow of 72 l/m d and a salt retention capacity of 95.8 /O were measured.
Compared with the other Examples, this result clearly - shows the importance of a certain water absorption capacity : 20 to the effectiveness of polymer membranes in terms of a high wster throughflow coupled witb a bigh seperation capacity.

; ~ '

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A semipermeable membrane having a water absorption capacity of from 4.5 to 10% by weight, measured on approximately 40 µ thick symmetrical films at room temperature and at 65% relative air humidity; a throughflow of at least 80 1/m2d for a desalination level of at least 85% and consisting of a fully aromatic copolyamide with a relative viscosity of >1.4, as measured on a 0.5% N-methyl pyrrolidone solution at 20°C, said fully aromatic copolyamide consisting of A) 10 to 90 mole % of units corresponding to the formula (I) (I) in which R, R' and R" which may be the same or different represent. hydrogen, C1 4-alkyl, or halogen, and AR represents an alkyl- or halogen-substituted bivalent aromatic radical or an unsubstituted aromatic-radical and of B) 10 to 90 mole % of units corresponding to formulae II, III or IV below (II) (III) (IV) in which R, R', R", RIV, RV, RVI And RVII which may be the same or different represent hydrogen, C1-4-alkyl or halogen, Ar is as just defined, and X, Y and Z which may be the same or different represent a direct bond or a bridge member corresponding to a member selected from the group consisting of kf Le A 16 828-A

-CONH-, -NHOC-, -0-, -OCONH-, -NHCOO-, -CH2-, , -SO2-, -NHCONH-, -COO-, -OOC- and - ? -.

2. A semipermeable membrane as claimed in claim 1, having a throughflow of 85 to 200 1/m2d for a desalination level of 95 to 99.6%, as measured in a pressure osmosis apparatus in which a 3.5% NaC1-solution is pumped in a circuit past the surface of the membrane at room temperature under a pressure of 110 bars at a rate of 15 l/h.

3. A semipermeable membrane as claimed in claim 1, consisting of 50 to 85 mole % of the units A and 15 to 50 mole % of the units B.

4. A semipermeable membrane as claimed in claim l wherein the alkyl group represented by one or more of R, R' and R" is methyl.

5. A semipermeable membrane as claimed in claim 1, wherein Ar represents an alkyl- or halogen-substituted or unsubstituted divalent aromatic radical which is phenylene, naphthylene, biphenylene or a radical of the formula

6. A semipermeable membrane as claimed in claim 1, consisting of a copolyamide of A) 10 to 90 mole % of units corresponding to the formula (I) in which R represents hydrogen or methyl and R' and R" represent hydrogen and Ar is an m- or p-phenylene radical, and of B) 10 to 90 mole % of units corresponding to formulae (II), (III) and (IV) in which R, R", RIV and RVI which may be the same or different represent hydrogen, methyl or chlorine, R', R"', RV and RVII represent hydrogen and Ar represents the m- or p-phenylene radical.

7. A semipermeable membrane as claimed in Claim 6, consisting of 50 to 85 mole % of the units A and 15 to 50 mole % of the units B).

8. A semipermeable membrane as claimed in Claim 1, wherein X in formula (II) represents - O - , -CONH- or -NHOC-.

9. A semipermeable membrane as claimed in Claim 1, wherein X and Y in formula (III) which may be the same or different represent - O -, - NHOC -, - CONH -, -NHCOO-, - COO -, - OOC - or - NHCONH - or a combination of these radicals.

10. A semipermeable membrane as claimed in Claim 1, wherein X and Y in formula (IV) which may be the same or different represent - O -, - CONH -or - NHCO - and Z represents - SO2 - or .

11. A semipermeable membrane as claimed in Claim 1, consisting of a copolyamide of 10 to 90 mole % of units of the formula:

and 90 to 10 mole % of units of the formula:

12. A semipermeable membrane as claimed in Claim 1, consisting of a copolyamide of 10 to 90 mole % of units corresponding to the formula:

and 90 to 10 mole % of units corresponding to the formula:

13. A semipermeable membrane as claimed in Claim 1, consisting of 10 to 90 mole % of units corresponding to the formula:

and of 90 to 10 mole % of units corresponding to the formula:

kf Le A 16 828-A

14. A semipermeable membrane as claimed in Claim 1, in the form of a film.

15. A process for the production of a semipermeable membrane wherein a polymer casting solution comprising from 5 to 35 % by weight, based on the weight of copolyamide and solvent, of an aromatic copolyamide of A) 10 to 90 mole % of units corresponding to the formula (I) (I) in which R, R' and R" which may be the same or different represent hydrogen, C1-4-alkyl, or halogen, and Ar represents an alkyl- or halogen-substituted bivalent aromatic radical or an unsubstituted aromatic radical and of B) 10 to 90 mole % of units corresponding to formulae II, III or IV below (II) (III) (IV) in which R, R', R", R"', RIV, RVI and RVII which may be the same or different represent hydrogen, C1-4-alkyl or halogen, Ar is as just defined, and X, Y and Z which may be the same or different represent a direct bond or a bridge member corresponding to a member selected from the group consisting of Le A 16 828-A

-CONH-, -NHOC-, -O-, -NHCOO-, -CH2-, , -SO2-, -NHCONH-, -COO-, -OOC- and - ? -with a relative solution viscosity of ? 1.4, as measured in a 0.5 % N-methyl pyrrol-idone solution at a temperature of 20°C are dissolved in an aprotic solvent to form a solution which is applied to a substrate in a layer thickness of from 150 to 400 µ, the film thus formed is treated at a temperature of from 40 to 150°C for a period from 2 to 60 minutes and, after a cooling phase of 10 minutes, the resulting film is immersed for 30 minutes at 0 to 50°C in a coagulation bath which is miscible with the aprotic solvent and represents a non-solvent for the copolyamide.

16. A process as claimed in Claim 15, wherein the copolyamide consists of 50 to 85 mole % of the units A) and 15 to 50 mole % of the units B).

17. A process as claimed in Claim 15 wherein the casting solution also comprises from 1 to 10% by weight of LiC1, LiBr, LiNO3, MgC12, CaC12 or CaBr2 or also comprises an organic amine.

18. A process as claimed in Claim 15, wherein the solution is filtered prior to its application to a substrate.

19. A process as claimed in Claim 15, wherein the coagulation bath contains added salt.

20. A semipermeable membrane as claimed in Claim 1, in the form of tubing.

21. A semipermeable membrane as claimed in Claim 1, in the form of hollow fibres.