GB2037222A - Process for producing semipermeable membranes - Google Patents

Abstract

The flux and solute rejection of a semipermeable membrane produced by treating an acrylonitrile homopolymer or copolymer (containing at least 40 mole % acrylonitrile) with a plasma may be improved by immersing the wet membrane, after wet casting but before exposure to the plasma, in hot water, suitably at a temperature from 50 to 100 DEG C. The membranes are particularly useful in reverse osmosis and ultra filtration.

Description

SPECIFICATION Process for producing semipermeable membranes The present invention relates to a process for producing semipermeable membranes which are useful for a variety of diffusion processes, but which are especially valuable in reverse osmosis processes. In particular, the process of the invention enables a semipermeable membrane to be made from an acrylonitrile polymer which has substantially improved solute rejection or water permeability (hereinafter referred to as "flux") or both.

In recent years, reverse osmosis and ultrafiltration processes using semipermeable membranes made from a variety of polymeric materials, such as cellulose acetate or polyamides, have been widely used in various fields, including desalination of sea water, waste water treatment, control of the composition of electrodeposition baths, the food industry and the medical industry. In all of these separation processes using membranes, what is important is the selective permeability of the membranes, i.e. the solute rejection and flux.

To date, many attempts have been made to increase the solute rejection and flux of semipermeable membranes used for reverse osmosis or ultrafiltration processes. Many of these attempts have been directed towards adjusting the nature of the material used to produce the membrane and innumerable studies have been reported in patents and academic papers.

It is well known that acrylonitrile polymers are of considerable value as raw materials for the production of semipermeable membranes. However, despite many attempts to produce semipermeable membranes from acrylonitrile polymers, the membranes produced tend to have a low solute rejection, even though the flux may be high. Moreover, the solute rejection is particularly low when relatively low molecular weight electrolytes, such as sodium chloride, are to be removed. For this reason, semipermeable membranes made from acrylonitrile polymers have not, in practice, hitherto been used for reverse osmosis, although they do find practical application in ultrafiltration processes.

Because acrylonitrile polymers have good film-forming properties, thermal resistance, acid resistance and alkali resistance, we have previously attempted to produce from them semipermeable membranes capable of rejecting even low molecular weight electrolytes, such as sodium chloride, at a high rate. These attempts led to the development of semipermeable membranes made of acrylonitrile polymers but having better thermal, mechanical and chemical properties and a higher solute rejection, when used in reverse osmosis, than the conventional cellulose acetate membranes. The process employed for producing these semipermeable membranes, which is now regarded as a major innovation, consisted of treating porous membranes of acrylonitrile polymer with a plasma, as disclosed in U.S. Patent Specification No. 4,147,745.

This process represented a major advance, which could not have been attained by the prior art, in the field of acrylonitrile polymer semipermeable membranes.

Our further study of the problems has now led us to a method of producing membranes from plasma-treated acrylonitrile polymers which not only enables the flux to be substantially increased but which also allows salt and low molecular weight substance rejection to be much improved. As a result, although the membrane so produced can be used for ultrafiltration processes, it also finds particular value in reverse osmosis processes.

Thus, the present invention consists in a process for producing a semipermeable membrane in which a porous wet membrane produced by wet casting an acrylonitrile polymer containing from 40 to 100 mole per cent acrylonitrile is treated with hot water and subsequently exposed to a plasma.

The acrylonitrile polymer used in the process of the invention may be a polyacrylonitrile (acrylonitrile homopolymer) or one of various types of copolymer containing acrylonitrile as one monomer component.

Both the homopolymers and copolymers can be produced by well known methods. The comonomer used with acrylonitrile to produce an acrylonitrile copolymer may be any of the ionic or non-ionic monomers copolymerizable with acrylonitrile. For example, non-ionic monomers include acrylamide, diacetone acrylamide, N-vinyl-2-pyrrolidone, hydroxyethyl methacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate or vinyl acetate. If an ionic monomer is employed, this may be an anionic or cationic monomer. Examples of anionic monomers include acrylic acid, ethylenesulphonic acid, methacrylic acid, methallylsulphonic acid, sulphopropyl methacrylate, vinylbenzenesulphonic acid and metallic salts of these acids.Examples of cationic monomers include: tertiary amines, such as 2-vinylpyridine, 4-vinylpyridine and dimethylaminoethyl methacrylate; and quaternary amine salts obtainable by alkylation of these tertiary amines. A single one or two or more comonomers may be employed with the acrylonitrile to produce an acrylonitrile copolymer.

The acrylonitrile polymer employed in the process of the invention must contain at least 40 mole per cent of acrylonitrile and no more than 60 mole per cent of one or more comonomers. Copolymers containing less than 40 mole per cent of acrylonitrile have significantly worse mechanical and other properties than do copolymers containing larger quantities of acrylonitrile. Consequently, preferred acrylonitrile polymers for use in the process of the present invention contain from 40 to 100 mole per cent, and preferably from 70 to 95 mole per cent, of acrylonitrile.

In order to achieve a membrane having the best mechanical strength, the molecular weight of the acrylonitrile polymer is preferably within the range from 5,000 to 5,000,000.

The wet casting process which is used to produce the membrane employed in the process of the invention is carried out by dissolving the appropriate acrylonitrile polymer in a solvent, either alone or with one or more of the additives described hereinafter, preferably to a concentration of from 5 to 30% by weight.

Preferred solvents are aqueous solutions of inorganic salts or organic polar solvents, such as dimethylacetamide, dimethylformamide or dimethyl sulphoxide. Where the casting solution contains an additive, this may be selected from those compounds well known for this purpose, particularly polyols, such as polyethylene glycol or polypropylene glycol. A polyethylene glycol having an average molecular weight of 100 to 2,000 is most preferred. The amount of additive employed is necessarily limited to that amount which would be compatible with the polymer solution and thus the maximum will vary according to the nature of the additive and of the solution. However, in general, the amount of additive is up to 20% of the weight of the polymer solution, and more preferably from 5to 20% by weight.

The polymer membrane may be, and preferably is, cast on a glass plate, but it may also be cast on any other article having a smooth surface, e.g. a film or sheet. It is also possible to cast the membrance directly onto a support, such as a woven or non-woven fabric or other porous article. The advantage of casting the membrane on such a support is that the membrane obtained is greatly reinforced.

The polymer solution is preferably cast on the substrate, such as a glass plate, using a doctor knife. The temperature of the casting solution will, as is well known, be so chosen as to facilitate casting and is preferably from 10 to 800C. The thickness of the solution cast on the substrate will, of course, affect the thickness of the semipermeable membrane. In general, we prefer that the thickness of the cast solution should be adjusted so as to form a semipermeable membrane having a thickness of from 20 to 500 microns.

After casting, the solution is immersed in a non-solvent for the polymer, in order to gel the polymer. This immersion may take place immediately after casting or, if desired, part of the solvent may be evaporated from the surface of the solution before gelation takes place. Where the solvent is evaporated, this is preferably carried out at a temperature of from 0 C to the boiling point of the solvent and preferably takes place over a period not exceeding 60 minutes. After partial evaporation, the solution may then be immersed in a non-solvent to gel the polymer, as is also the case where the solvent has not been subjected to partial evaporation. As the non-solvent, we prefer to use either water or a mixture of water and an organic liquid.

Various conditions during the preparation of the porous wet membrane to be used in the process of the invention have some effect on the performance of the plasma-treated membrane, which is the final product of the process, although none of these conditions has a decisive effect. Conditions which have such an effect include: polymer concentration in the casting solution; casting temperature; evaporation time of solvent; temperature of gelation bath; and gelation time. The way in which each of these factors affects the performance of the final product is well known to those skilled in the art, who will choose a correct balance of these factors to provide a product having the desired performance characteristics.

The wet, porous membrane obtained after the casting and gelation steps preferably has a water permeability of from 0.01 to 5,000 LMH (litres per square metre per hour) under a pressure of 10 kg/cm2 and a bubble point of at least 1 kg/cm2. Possession of a water permeability and bubble point as suggested above is an indication that the wet membrane is free from defects. The bubble point of a membrane is defined as being that air pressure at which, in a system comprising water and air separated from each other by the membrane, the air begins to enter the water through the membrane when the pressure on the air is slowly increased.

The heat treatment, which forms the essential characteristic of the present invention must be carried out in water prior to treatment of the membrane with a plasma. The conditions under which this water and heat treatment are carried out have a very significant effect on the performance of the final plasma-treated membrane. The temperature of the hot water used for the treatment is preferably from 50 to 1000C, more preferably from 70 to 95 C. The heat treatment may be carried out with pure water or with an aqueous solution containing, for example, a small amount of an inorganic salt, a surface active agent or a water-soluble polymer.

The hot water treatment can be carried out with or without fixing the size of the membrane. Any membrane treated according to this procedure has an improved solute rejection and flux, as compared with a plasma-treated membrane which has not been subjected to the hot water treatment.

The time during which the membrane is treated with the hot water is preferably at least 1 minute and more preferably from 5 to 20 minutes. Increasing the time of treatment any further gives very little improvement in the performance of the semipermeable membrane finally obtained after plasma-treatment.

After treating the membrane with hot water, it is preferably dried prior to exposure to the plasma. Drying may be carried out by natural evaporation, by hot-air drying or by evaporation under reduced pressure.

The membrane thus obtained may then be treated with a plasma as disclosed in U.S. Patent Specification No. 4,147,745.

The plasma employed may be generated by various known techniques, for example glow discharge. For example, a plasma can be generated by glow discharge by introducing a gas which is not polymerizable by the plasma (for example, hydrogen, helium, argon, nitrogen, oxygen, carbon monoxide, carbon dioxide or ammonia) into a vacuum vessel containing a pair of electrodes, to a pressure within the range from 0.01 to 10 Torr, and then applying an alternating or direct current at a voltage from 0.5 to 50 KV between the electrodes. The membrane may be exposed to the plasma for a period of from 30 seconds to 1 hour.

The semipermeable membranes subjected to the process of the invention may be in various forms. For example, they may be in the form of a flat membrane, a tube, a hollow fibre or a yarn or, as previously described, they may be used in the form of a composite with another porous support.

The mechanism whereby the hot water treatment employed in the process of the present invention improves the properties of the membrane is not yet clear, but it is apparently different from the mechanism whereby the solute rejection of a cellulose acetate semipermeable membrane produced by the wet casting method is improved by treatment with hot water. In the case of treatment of a cellulose acetate membrane, elevating the temperature of the hot water increases the solute rejection but decreases the flux. On the other hand, the behaviour of acrylonitrile polymer membranes subjected to hot water treatment as the temperature of the treating water is increased is quite different and this behaviour seems to be characteristic of acrylonitrile polymers. In fact, the behaviour of acrylonitrile polymers as the temperature of the treatment water is increased appears to be discontinuous.Specifically, as the temperature is first raised, the solute rejection is reduced whilst the flux is substantially increased; on the other hand, as the temperature rises further, the solute rejection is substantially improved whilst the flux is reduced. However, in the latter case, where the membrane is treated with hot water at a temperature above the range where the solute rejection is reduced and the flux is increased, notwithstanding the reduction in the flux which then takes place, the flux is nonetheless better than that of a corresponding plasma-treated membrane which has not been subjected to the hot water treatment.

This characteristic behaviour varies somewhat depending upon the nature and proportion of the comonomers, if any, in the acrylonitrile polymer and thus the optimum temperature for the hot water treatment will vary depending upon these factors and upon the desired solute rejection and flux. However, in all cases, treatment at a temperature above that range where the solute rejection is reduced and the flux is increased can give a semipermeable membrane having improved flux as well as solute rejection. In addition, the water treatment gives other advantages, for example a membrane so treated hardly compacts during use and the heat stability of the membrane is improved.

Thus, although the hot water treatment of the present invention superficially resembles that which is sometimes applied to conventional cellulose acetate membranes, the effect obtained and presumably the mechanism whereby it acts are completely different.

Moreover, the hot water treatment which is a feature of the present invention provides the following advantages, which are not achievable by conventional methods: (1) Compared with plasma-treated semipermeable membranes which have not been subjected to hot water treatment, the membranes of the invention have a higher water permeability, superior solute rejection and a sharp cut-off ability.

(2) During drying, which is a step in the production of the membrane, shrinking and curling do not occur, this making the membrane easy to handle.

(3) By varying the temperature of the hot water treatment, the plasma-treated membrane can be provided with a wide range of different properties.

(4) Very little compaction of the membrane occurs during use and thus the life of the membrane is prolonged.

(5) The heat stability is improved.

The semipermeable membranes produced by the process of the invention have much improved solute rejection compared with conventional semipermeable membranes made of acrylonitrile polymers and, in addition, may have greatly increased flux. As a result, the membranes can be used with economy in various industries. For example, they can be used for reverse osmosis in various preparation and concentration processes, for example the preparation of fresh water from sea water, the treatment of waste water, the concentration of fruit juices and the separation of non-aqueous fluids or gases.

The invention is further illustrated by the following Examples, in which all parts are by weight and in which the solute rejection is defined by the following equation: Concentration of solute in permeated Solute rejection (%) = (1 -solution ) > ( 100 Concentration of solute in feed solution Example 1 20 parts of a copolymer comprising 89 mole per cent acrylonitrile and 11 mole per cent methyl acrylate were dissolved in a mixed solvent comprising 70 parts of dimethylformamide and 10 parts of formamide.

This solution was cast to a thickness of 250 U on a glass plate heated at 40 C. After allowing the solvent to evaporate for 1 minute, the glass plate was immersed in a water bath at 17 C to gel the polymer. After 2 hours, the membrane obtained was separated from the glass plate. The membrane, in the wet state, was immersed in hot water for 10 minutes at a temperature of 70, 80 or 90 C. The wet membrane before treatment with the hot water had a water permeability under a pressure of 10 kg/cm2 of 920 LMH and a bubble point of 5.5 kg/cm2.

After drying the membrane at room temperature for 24 hours, it was treated with plasma in a bell jar having a pair of electrodes, under the following conditions: Gas Helium Degree of vacuum 0.2 Torr Discharge voltage : 3.0 KV Discharge current 25 mA Treating time 30 minutes After washing the membrane with distilled water, it was mounted in a circulation-type reverse osmosis apparatus so that the effective area of the membrane was 13.0 cm2. 0.5% saline was supplied at25 C into the reverse osmosis cell at the rate of 650 ml per minute under a pressure of 50 kg/cm2 and the solute rejection and flux were measured 20 hours after the beginning of the test.The results are shown in the following table: TABLE Treatment Solute Flux temperature rejection (LMH) ( C) (%) 70 75.5 71 80 98.3 37 90 99.4 25 A similar membrane was prepared, except that the hot water treatment was omitted. In this case, the flux was 20 LMH and the solute rejection was 98.0%.

Example 2 21 parts of a copolymer comprising 90 mole per cent acrylonitrile and 10 mole per cent vinyl acetate was dissolved in a mixed solvent comprising 69 parts of dimethylformamide and 10 parts of formamide. This solution was cast to a thickness of 250 u on a glass plate maintained at 25 C. The solvent was allowed to evaporate off for 1 minute, after which the glass plate was immersed in a water bath at 170C to gel the solution. After 2 hours, the resulting membrane was separated from the glass plate and the membrane, in the wet state, was immersed in hot water at 85 C for 10 minutes. Prior to treatment with the hot water, the wet membrane had a water permeability of 1170 LMH under a pressure of 10 kg/cm2 and a bubble point of 6 kg/cm2.The membrane was dried and treated with a plasma under the same conditions as are described in Example 1. Evaluation of the performance of the semipermeable membrane under the conditions described in Example 1 showed it to possess a flux of 35 LMH and a solute rejection of 98.9%. On the other hand, a similar membrane which had been produced under identical conditions, except that the hot water treatment had been omitted had a flux of 21 LMH and a solute rejection of 98.1%.

Example 3 20 parts of the same acrylonitrile copolymer as was used in Example 2 were dissolved in a mixed solvent comprising 70 parts of dimethylformamide and 10 parts of polyethylene glycol 200. The solution was cast on taffeta made from polyethylene terephthalate at 250C to a thickness of 250 microns. After evaporating off the solvent for 1 minute, the cast solution with the taffeta was dipped in a water bath at 1 70C to gel the solution.

After 2 hours, the resulting membrane, reinforced with taffeta, was immersed in the wet state in hot water at 800for 10 minutes.

Prior to the hot water treatment, the wet membrane had a water permeability of 850 LMH under a pressure of 10 kg/cm2 and a bubble point of 5.5 kg/cm2. The membrane was then dried and treated with plasma as described in Example 1 and was then subjected to reverse osmosis as described in Example 1. The flux was 39 LMH and the solute rejection was 98.4%.

On the other hand, an otherwise identical membrane which had not been treated with hot water but which had been dried and treated with plasma under the same conditions as described above had a flux of 19 LMH and a solute rejection of 98.0%.

Claims (12)

1. A process for producing a semipermeable membrane in which a porous wet membrane produced by wet casting an acrylonitrile polymer containing from 40 to 100 mole % acrylonitrile is treated with hot water and subsequently exposed to a plasma.

2. A process according to Claim 1, in which said hot water is at a temperature of from 50 to 1000C.

3. A process according to Claim 2, in which said hot water is at a temperature of from 70 to 95 C.

4. A process according to any one of the preceding Claims, in which said acrylonitrile polymer is an acrylonitrile copolymer containing from 70 to 95 mole % acrylonitrile.

5. A process according to any one of the preceding Claims, in which said acrylonitrile polymer is a copolymer of acrylonitrile with one or more of the comonomers: acrylamide, diacetone acrylamide, N-vi nyl-2-pyrrol idone, hydroxyethyl methacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, vinyl acetate, acrylic acid, ethylenesulphonic acid, methacrylate acid, methallylsulphonic acid, sulphopropyl methacrylate, vinylbenzenesulphonic acid, a metallic salt of one of said acids, 2-vinylpyridine, 4-vinylpyridine, dimethylaminoethyl methacrylate, or a quarternary amine salt obtainable by alkylation of a tertiary amine.

6. A process according to any one of Claims 1,2 and 3, in which said acrylonitrile polymer is an acrylonitrile homopolymer.

7. A process according to any one of the preceding Claims, in which the treatment with water is carried out for at least 1 minute.

8. A process according to Claim 7, in which the treatment with hot water is carried out for from 5 to 20 minutes.

9. A process according to any one of the preceding Claims, in which the membrane is supported on a woven or non-woven fabric or on another porous support.

10. A process according to any one of the preceding Claims, in which said wet membrane has a flux of from 0.01 to 5,000 LMH under a pressure of 10 kg/cm2 and a bubble point of at leat 1 kg/cm2.

11. A process according to Claim 1, substantially as hereinbefore described with reference to any one of the foregoing Examples.

12. A semipermeable membrane when produced by a process according to any one of the preceding Claims.