GB2096941A

GB2096941A - Semi-permeable membrane

Info

Publication number: GB2096941A
Application number: GB8211378A
Authority: GB
Original assignee: ZLATAROV PROF DR VHTI
Current assignee: ZLATAROV PROF DR VHTI
Priority date: 1981-04-20
Filing date: 1982-04-20
Publication date: 1982-10-27
Also published as: DE3214538A1; CH658399A5; NL8201617A; BG33312A1; IT8248237A0; FR2504022A1; FR2504022B1; CS248272B1; GB2096941B; PL236036A1; HU189575B; DK162182A; IT1186692B; CS281482A1; PL134921B1

Abstract

A block terpolymer formed of 82 to 93% by weight acrylonitrile, 6 to 15% by weight methylmethacrylate and 1 to 3% by weight of the sodium salt of vinylsulphonic acid forms a semi-permeable membrane suitable for use in the concentration of solutions of proteins, having a long useful working life and being capable of storage in a dry state.

Description

SPECIFICATION Semi-permeable membrane and method for preparation thereof This invention relates to a semi-permeable membrane for use in the concentration of solutions of for example gamma globulin, albumin and milk proteins, and to a method for the production thereof.

Semi-permeable membranes made of a wide variety of materials have hitherto been proposed for use in the concentration of a variety of materials of biological origin, for example gamma globulin, albumin and milk proteins, one such membrane is a polysulphone membrane which is used as a supported membrane on a botting paper carrier (see "Pellican Cassette System", Catalogue, Millipore Corporation, Bedford, Mass., U.S.A. 01730). Without this carrier, the membrane could not withstand working pressures. This complicates the preparation of the membranes. Moreover, when used in, for example, the concentration of albumin, a reduction of from 0.029 to 0.017 m3/m2 hour in the membrane permeability occurs. This reduction is a result of the clogging of the membrane pores and on the one hand prolongs the process of membrane recovery, and on the other hand shortens its working life.

Another type of membrane is a cellulose acetate membrane produced by liquid forming, i.e. the so called "solution method". However, membranes of this type cannot be stored in a dry state. Once dry, their asymmetric structure is lost. Moreover, the permeability of such membranes varies over a wide range which amounts to from 0.028 to 0.002 m3/m2 hour with 95% retentivity with respect to albumins (see P. L. Kelashnikova, L. V. Andreevskaya, "lssledovanye Ultrafiltratoionnoi Obrabotki Podsirnoi Sivarotki" Molya Promishlenost-1 975, No. 11, p. 1 5-1 8 and F. E. McDonough, "Protein concentrate from cheese whey by ultrafiltration", "J. Dairy Sci." 1 971, vol. 54, No.10, p. 1 407- 1409).

Polyacrylonitrile copolymer membranes have also been proposed. Isobutene, ethylvinyl ether, vinylidene chloride, butadiene and methacrylonitrile have been used as co-monomers. The membranes so obtained exhibit a high permeability with respect to distilled water of 1.2 m3/m2h but they only retain substances of molecular weight above 45,000. The membranes are not capable of retaining molecules whose molecular weight is in the range of from 1 5,000 to 45,000 as is typical of milk proteins (see for example U.S. Patent Specification No. 4,181,694).

Another type of semi-permeable membrane which is known from U.S. Patent Specification No.

3,950,257 is based on an acrylonitrile-vinyl pyrolidone copolymer. The copolymers may be produced in either dimethylsuiphoxide or dimethylformamide solvents. When polymer-dimethylsulphoxide membranes were used in the ultrafiltration of whey they were found to possess an initial flow rate of 0.045 m3/m2h and 95% selectivity. The method proposed for the preparation of the membranes requires the heating of plates, onto which the film is poured, to a temperature of 500 C. This procedure causes some problems at the subsequent stage of coagulation at 200 C, and is responsible for reductions in the permeability and the selectivity. Whichever solvent is used, a membrane is obtained which cannot be stored in a dry state.

In a yet further method for the preparation of membranes described in GDR Patent Specification No. 1 34,448, use is made of the following copolymer composition: 93% by weight of acrylonitrile, 6% by weight of methyl ester of acrylic acid and 1% by weight of the sodium salt of ally sulphonic acid.

The coagulation process is carried out in a propyl alcohol medium or in an alcohol-water mixture.

However parameters of the process of preparation of such membranes have been varied, flow rates greater than 0.003 m3/m2 hour have not been obtained.

According to the present invention, there is provided a polyacrylonitrile terpolymer membrane for use in the concentration of protein-containing solutions, which terpolymer is a block terpolymer of from 82 to 93% by weight acrylonitrile, from 6 to 15% by weight methyl methacrylate and from 1 to 3% by weight of an alkali metal salt of vinylsulphonic acid.

This invention also provides a method for the preparation of a membrane for use in the concentration of protein-containing solutions, which comprises dissolving in an appropriate solvent a block terpolymer of from 82 to 93% by weight acrylonitrile, from 6 to 1 5% by weight methylmethacrylate and from 1 to 3% by weight of an alkali metal salt of vinylsulphonic acid, adding a sponging agent to the solution obtained, forming the solution into a solid film of said terpolymer and tempering and drying the film.

The polyacrylonitrile terpolymer-based semi-permeable membrane of this invention has been found to possess both stable and high permeability and selectivity rates, to have a stable structure and to be capable of storage in a dry state.

The method for producing the membrane will now be described in greater detail. The polymer, which has been obtained by a radical polymerisation, is dissolved in an appropriate solvent, such as dimethylformamide, preferably to a concentration of 12% to 18%, more preferably 1 5% by weight. A sponging agent, for example lithium nitrate, is added in an amount of up to 1% by weight to the solution so prepared. The polymer solution is filtered through a Buchner filter under pressure in order to remove both the solid impurities and undissolved particles. After storage for about 24 hours, the solution is completely free of air. A polymer film is obtained from this solution by spreading the solution onto glass plates using a frame. The film thickness is controlled by strips, which serve to cant the plates.The film is then stored in a chamber so as to keep its surface clean. A definite solvent vapour pressure is set up in this chamber and the storage time may be varied from 10 to 60 min, thereby controlling the process of solvent evaporation which takes place at the film surface. The polymer film, while still on the glass plate, is immersed in a coagulation bath. Water or a mixture of water and dimethylformamide may be used as coagulation agent. After storage for about 1 hour, the solidified polymer film is separated from the plate and is submitted to rinsing with an excess of distilled water.

All these procedures may be carried out under ambient conditions i.e. 20 to 250C and 70 to 80% humidity.

The membrane so obtained is usually attached to a metal frame and is tempered in water at a temperature of 800C for a time of from 10 to 15 min. Then, the membrane is dried while still on the frame at temperatures below 800 C. Finally, the membrane is separated from the frame and given the desired size.

The structure of the membranes so obtained has been examined by scanning electron microscopy.

The cross-section of the membrane given in the accompanying figure shows the asymmetry of the membrane structure. The investigations were performed by means of the apparatus Superprobe 733.

The principal characteristics of membranes produced in this way are summarised in the following Table 1.

Table 1 Parameters studied Units Results Notes Permeability m3/m2 hour 0.1 to 0.6 Results depend on the conditions under which membrane is prepared Electrical resistance Ohm.m 3x10-3 to 4X10-3 Strength parameters: ~tensile strength N/m2 40x 105 to 60x 105 ~pressure resistance N/m2 up to 3 x 105 Average pore size A 400 to 600 Bulk density m3/kg 2x10-3to 3.5x10-3 Membrane thickness m 1x10-4to 1.5x10-4 pH Stability stable at from 5.5 to 9.5 Moisture content of dry membrane % by wt. up to 5 Resistant to: benzene, toluene, brake fluids and other petroleum products Weather resistance good Attacked by sulphur dioxide, attacked by very hydrogen sulphide, aggressive media nitrogen oxides, nitric acid, sulphuric acid etc.

The results obtained show that semi-permeable ultrafiltration membranes having a pore size in the range of from 100 to 1000 A can be prepared by the method of this invention.

The membranes possess such advantages as a high tensile strength as well as pressure resistance while having a relatively low electric resistance. Their stability over a wide pH range makes them suitable for use in the food industry. The most important advantage of these membranes lies in their capability of storage in a dry state without undergoing any change.

The following non-limiting Examples illustrate this invention.

Example 1 A 15% by weight solution in dimethylformamide of a terpolymer of 93% by weight acrylonitrile, 6% by weight methylmethacrylate and 1% by weight of the sodium salt of vinylsulphonic acid was prepared. 0.5% by weight of lithium nitrate was added to the solution. Then, the solution was filtered through a Buchner filter in order to remove solid matter therein and after storage for 24 hours, was spread by a frame onto a glass plate to form a film. The thickness of the film was controlled by means of strips and was of the order of 1 xl 10-4 m. The film was stored for about 10 min. in a chamber under the ambient atmosphere of 70% humidity. The solvent evaporated off and the polymer was coagulated in distilled water at a temperature of 250C for 60 min. The film was then separated from the plate and was submitted to rinsing using an excess of distilled water. The membrane was then fixed to a metal frame and was tempered in water at a temperature of 800C for about 10 min. The membrane while still on the frame was dried at a temperature of 400C and at 5% relative humidity. Finally, the dry membrane so obtained was separated from the frame and was shaped to the desired size. The membranes so prepared possessed the following characteristics: a water permeability of 0.2 m3/m2h; a selectivity with respect to protein solutions of 99.5%; an average pore size of 400 A and a tensile strength of 45 x 105 N/m2.

Example 2 The procedure of Example 1 was repeated using a polymer solution containing a terpolymer of the following composition: 82% by weight of acrylonitrile, 1 5% by weight of methylmethacrylate and 3% by weight of the sodium salt of vinylsulphonic acid in solution in dimethylformamide. It was established that a decrease in the percentage of acrylonitrile in the terpolymer results in an increase in the brittleness of the dry membranes as well as in a small decrease of the membrane permeability.

Therefore, it is preferred to use terpolymers having a relatively high percentage of acrylonitrile.

The membranes prepared in Example 1 were tested on an ultrafiltration apparatus of a type described in the aforementioned article by Kalashnikova and Andreevskaya in respect to solutions of gamma globulin and albumin. A comparison between the test results and those obtained with polysulphone membranes known in the art previously to be the most effective membranes for concentration of serum proteins, is shown in Table 2.

Table 2

Characteristics Permeability Selectivity System Rate of Membrane Test pressure concentration start end type protein 1Nlm2) (%) {m3/m2HJ (m3/m2HI (O/o) Polysul- albumin 1.5x105 30 0.029 0.017 above 95 phone gamma globulin 3x105 10 0.020 0.012 above 95 Poly- albumin i.5x105 30 0.035 0.030 99.5 acrylo nitrile ~ gamma globulin 3x105 10 0.021 0.019 99.5 The membranes prepared in Example 1 were also tested for ultrafiltration of whey using the same apparatus. A comparison between the test results and those obtained with other membranes is shown in Table 3.

Table 3

Characteristics Permeability lm3/m2H) Whey Selectivity Membrane /Membrane type sta#t end Water (O/oJ type wet Acrylonitrilevinylpyrrolidone 0.030 1 0.083 96 wet Celluloseacetate 0.028 0.002 - 95 wet 1 Terpolymer of the invention 0.015 0.013 0.120 99.5 dry Examination of these results shows that the acrylonitrile polymer membranes of this invention exhibit better permeability rates during the different stages of the process of concentration of the solutions. The variation in the membrane permeability which takes place from the beginning to the end of the process is negligible. Moreover, the membranes are more readily recoverable and possess a prolonged useful life. In addition, these membranes maintain a high selectivity at a sufficiently high rate of concentration. The asymmetric structure of the membranes according to the invention is stable and ensures the above indicated results. There is no need for carriers in use, so that the method of preparation of the membranes is simplified. Finally, in addition to being generally reliable in use, a membrane of this invention can be stored in a dry state.

Claims

1. A polyacrylonitrile terpolymer membrane for use in the concentration of protein-containing solutions, which terpolymer is a block terpolymer of from 82 to 93% by weight acrylonitrile, from 6 to 1 5% by weight methylmethacrylate and from 1 to 3% by weight of an alkali metal salt of vinylsulphonic acid.

2. A membrane as claimed in claim 1, wherein the alkali metal is sodium.

3. A polyacrylonitrile terpolymer membrane, substantially as described in either of the foregoing examples.

4. A method for the preparation of a membrane for use in the concentration of protein-containing solutions, which comprises dissolving in an appropriate solvent a block terpolymer of from 82 to 93% by weight acrylonitrile, from 6 to 1 5% by weight methylmethacrylate and from 1 to 3% by weight of an alkali metal salt of vinylsulphonic acid, adding a sponging agent to the solution obtained, forming the solution into a solid film of said terpolymer and tempering and drying the film.

5. A method as claimed in claim 4, wherein the alkali metal is sodium.

6. A method as claimed in claim 4 or 5, wherein the solvent is dimethylformamide.

7. A method as claimed in any one of claims 4 to 6, wherein said solution has a concentration of from 12 to 18% by weight.

8. A method as claimed in any one of claims 4 to 7, wherein the sponging agent is lithium nitrate.

9. A method as claimed in any one of claims 4 to 7, wherein the sponging agent is present in said solution in a concentration of up to 1% by weight.

10. A method as claimed in any one of claims 4 to 9, wherein said film is coagulated in a water or water/dimethylformamide bath.

11. A method as claimed in claim 4, for the preparation of a membrane for use in the concentration of protein-containing solutions, substantially as described in either of the foregoing Examples.