GB2035133A - A modified membrane for membrane filtration - Google Patents

Abstract

A modified membrane for membrane filtration is obtained by contacting a membrane surface of a non-thermally cured membrane with a solution containing a water insoluble copolymer or polymer and an organic polar solvent. Preferably the polar arganic solvent is methanol and the polymer a polyvinylacetate.

Description

SPECIFICATION A modified membrane for membrane filtration The invention relates to a modified membrane for membrane filtration, obtained by treating a membrane surface with a water insoluble hydrofilic polymer compound.

A membrane of this type is known. Cellulose acetate membranes for reverse osmosis, used to prepare potable water from seawater are, after a long use of said membranes, treated with an aqueous ammoniacal solution of a copolymer of vinylacetate with a maleicacid-ethylester, the membrane after having been immersed into the polymer solution, being subsequently treated with a diluted aqueous hydrochloric acid solution, having a pH of 4 and containing 1% by weight of zincchloride. Hereby the copolymer precipitates upon the membrane.

Said known method offers the disadvantage that the ammoniacal solution will hamper the capacity of the membranes, in particular that of cellulose acetate membranes, whilst on the other hand membranes are treated having initially undergone a certain thermal curing treatment so as to give the desired porosity to said membranes.

In the state of the art it is also known to treat cellulose acetate membranes with a solution consisting of a copolymer or poly-vinylacetate with 5% of crotonic acid, whereby said copolymer is dissolved in an ammoniacal aqueous solution, whilst said copolymer is subsequently made insoluble by acidification.

Said copolymer is likewise applied upon a membrane of cellulose acetate which has initially undergone a thermal treatment.

It has been found that, although cellulose acetate membranes having been subjected to a thermal curing treatment at lower temperatures, could be coated with a copolymer coating, said copolymer coating is easily removed during subsequent use of the membranes.

It is a further disadvantage of the two methods for coating a cellulose acetate membrane with a polymer coating, as mentioned hereinbefore, that only polymers can be used being soluble in water, so that a polyvinylacetate first has to be modified and converted into a copolymer being soluble in water. From the foregoing it is evident that the use of polyvinyl acetate as a polymer coating upon said cellulose acetate membranes, is impossible, owing to the insolubility of polyvinyl acetate in water.

Summarizing the above features, it is clearly evident that the methods as mentioned hereinbefore firstly require the use of initially thermally treated membranes, secondly the preparation of copolymers being soluble in water and the application of said aqueous copolymers by means of an ammoniacal solution, which is followed by a precipitation of the copolymer with an acid and metal ions.

Nothwithstanding these features, said membranes appear to have a less longer lifespan in practice, which is possibly connected with an insufficient adherence of the polymer coating upon said membranes, mostly being a cellulose acetate membrane.

The present invention aims to provide a modified membrane to be used in a membrane filtration, obtained by treating a membrane surface, with a solution of a water insoluble hydrofilic polymer compound, whereby membranes are obtained having a long lifespan in practice. A water insoluble polymer compound is in this case a polymer compound not being soluble in pure water.

This object is obtained in accordance with the invention in that the modified membrane as mentioned above, to be used in a membrane filtration is obtained by treating a membrane surface with a solution consisting of a water insoluble hydrofilic polymer compound, containing a polar organic solvent. Said treatment is preferably carried out with a solution of a vinyl acetate polymer or a copolymer, containing a polar organic solvent.

A membrane of this type, having been modified in accordance with the invention, is very inconvenient in that the vinyl acetate polymer or copolymer coating, optimally adheres to the surface of the respective membrane, in particular a cellulose acetate membrane, thus causing a longer lifespan of said treated membranes.

It is a further important advantage of the present invention that a thermal curing treatment of membranes, in particular of cellulose acetate membranes, can be omitted when a polymer coating in accordance with the invention is applied.

A further important advantage is in that a membrane need no longer be coated with a coating only consisting of a vinyl acetate copolymer, but that polyvinyl acetate can be used as well, the latter polymer being water insoluble, but soluble in a polar solvent such as a methanol, or mixtures of methanol and water.

In order to obtain the desired porosity membranes need no longer be subjected to an initial thermal curing treatment with the method in accordance with the invention. The manufacture of a membrane so treated will therefore cause a considerable saving of labour expenditures.

The polymers or copolymers to be used for treating the membrane surface in accordance with the invention are in particular a vinylacetate homopolymer having a molecular weight between 2000 and 1 00,000, but a copo lymer containing at least 40% of vinyl acetate and for the rest a monomer, whereby the formed copolymer has naturally to be soluble in polar organic solvents, in particular an alcohol or an alcohol-water mixture such as a methanol or methanol-water-mixture, can be used as well. Suitable monomers for modifying are unsaturated carboxylic acids or esters thereof having the general formula X -CH = CY-COOZ wherein X represents a hydrogen atom, a lower alkyl- or carboxylic group, Y is a hydrogen atom, a lower alkylgroup or the group CH2COH, and Z represents a hydrogen atom or a lower alkyl group.

A lower alkyl group is in this case a group containing 1 to 3 carbon atoms.

Dicarboxylic acids can expediently be used such as e.g. maleic-acid, fumaric acid and itaconic acid.

The formation of copolymers by means of copolymerisation is a method known per se, which need not be elucidated. Of course, the carboxylic acids as mentioned hereinbefore, are not only the carboxylic acids proper, but also their anhydrides, since e.g. a maleic acid anhydride is very suitable and can easily be applied. Unsaturated tricarboxylic acids can also be used such as for example aconitic acid and the same holds for unsaturated monocarboxylic acids such as e.g. crotonic acids, acrylic acids and methacryl acids.

The concentration of the polymer compound in the solution ranges from 0,01 to 5,0% by weight and is preferably comprised between 0,5 and 1,5% by weight. When a similar solution is added to the substrate, the concentration of the polymer compound in the substrate is comprised between 0,1 to 100 ppm.

The solution containing the vinyl acetate copolymer or polymer, contains at least 5% of methanol, and although pure methanol (100% methanol) could also be used as a solvent, this is less preferable in view of the hampering influence thereof upon the membranes, in so far as this relates to cellulose acetate membranes.

For the alcohol concentration an upper limit of expediently 75% is preferred and preferably of 45 to 55%, the polar organic solvent being preferably an alcohol and particularly a lower alcohol. More preferably the alcohol is methanol. Instead of methanol, obviously also ethanol, propanol and butanol can be used.

The present invention will now be illustrated by some examples.

Example I A tubular cellulose acetate membrane is manufactured which is not subjected to a thermal curing treatment as is usual for giving membranes the desired porosity. A 1% solution of a copolymer of vinyl acetate, being modified with 5% crotonic acid is poured into the tubular membrane, the copolymer having a molecular weight of approximately 50 000.

The pH of the polymer solution amounts to 7.

The solution consists of even parts of water and methanol as a polar organic solvent.

This solution is removed after 10 seconds and the membrane rinsed with water (instead of water an aqueous methanol solution can also be used).

After having rinsed the membrane by means of water or an aqueous methanol solution, the membrane is tested with a saline solution containing 5.000 parts per million of sodium chloride at a pressure of 40 ato (4 mPa).

The flux, that is to say the passed quantity of water, amounts to 3,6 cm3/cm2 per hour, whereas the salt retention amounts to 96%.

An identical experiment performed with membranes not having been subjected to a thermal curing treatment, but having been rinsed with water, resulted in a flux of 30cm3/cm2/h and a retention of 3%.

The important advantages of the present invention need no further explanation.

Example II Example I is repeated, but the inner side of the cellulose acetate membrane is coated with a homopolymer having a molecular weight of approximately 30.000. In this case also a solution consisting of 50% water and of 50% methanol is used.

The results of a reverse osmosis performed at a pressure of 40 ato show a flux of 5,9 cm3/cm2/h and a salt retention of 85%.

Long term experiments have proved that the coating upon the membrane will adhere to said membrane for about one and a half years. During said perior no decrease in the properties of the coating was observed.

Example Ill A cellulose acetate membrane is coated as described in Example I. After the usual treatments, the respective membrane is subjected to a membrane filtration at a pressure of 4 ato, 75 ml of a 1% solution of polymer according to Example I being added to the feed solution.

After a period of three hours, the flux amounted to 1,5 cm3/cm2/h and the retention to 85%. The experiment was continued for three days and a flux of 0,5 was obtained and a retention of 95%. The latter retention remains constant.

Uncoated thermally cured membranes will show a flux of 0,42 and a retention of 85% at a pressure of 4 ato. This unfavourable result clearly shows the advantages of a coating in accordance with the present invention.

Uncoated thermally cured membranes show a flux of approximately 4 and a retention of 96% at a pressure of 40 ato.

From the aforedescribed example it clearly appears that the present invention will cause a considerable flux and a high retention at very low pressures. This results in a considerable economy of energy during membrane filtration and thus to a saving in the overall expenditures.

Example IV Example I is repeated but instead of cellulose acetate membranes a polyacrylonitrile ("Orlon'' Registered Trade Mark) membrane is used. A treatment with a solution of 1% of a polyvinyl acetate, modified with 5% of crotonic acid (the polyvinyl acetate having a molecular weight of 50.000) in a solvent of 50/50 water/methanol, likewise results in a membrane having a very high retention and a high flux during a long period of time, even at low pressures of 4 ato. The properties obtained at this pressure, are approximately identical to this as described in Example Ill.

For completeness' sake it is observed that the inner side of the pre-formed membrane, contacting liquid to be subjected to a membrane filtration and being coated with a polymer coating, will only at that coated side be in contact with the polar solvent. The other side of said cellulose acetate membrane will not be in contact with said polar organic solvent, such as methanol.

Example V The surface of a tubular cellulose acetate membrane which will contact medium, to be subjected to a membrane filtration, is treated with a 1% solution of polyvinyl acetate in a mixture of methanol-water (50/50) as described hereinbefore in Example I.

Said membrane is also used for treatment with a 0,1% solution of sodium chloride in water.

At an operating pressure of 20 ato during a period of 1000 hours, the flux will remain 3,2 and the salt retention 95%.

The above mentioned results are also obtained with polysulfon membranes.

Claims (10)

1. A modified membrane for membrane filtration, obtained by treating a membrane surface with a solution of a water insoluble hydrofilic polymer compound, wherein the membrane surface is treated with a solution of a water insoluble hydrofilic polymer compound, containing a polar organic solvent.

2. A modified membrane as claimed in claim 1, wherein the water insoluble hydrofilic polymer compound is a vinyl acetate polymer or copolymer.

3. A membrane as claimed in claim 1 or 2, wherein the membrane consists of a cellulose-ester membrane or of a polyacrylonitrile membrane.

4. A membrane as claimed in claims 1 to 3, wherein the polar organig solvent is an alcohol.

5. A membrane as claimed in claims 1 to 4, wherein the polar organic solvent consists of methanol, ethanol, propanol and/or butanol, and more preferably of methanol.

6. A modified membrane as claimed in claims 1 to 5, wherein the solution contains 5 to 75% of polar organic solvent and the remainder is water, the solution preferably containing 45 to 55% of a polar organic solvent, and more preferably approximately 50% of a polar organic solvent.

7. A modified membrane as claimed in any or more of the preceding claims wherein a non-thermally cured membrane is, directly after its manufacture, treated with the solution containing the polymer compound, while preferably the polymer coating is washed with an aqueous liquid after having applied said coating.

8. A modified membrane as claimed in any or more of the preceding claims wherein solely the surface of the membrane which will contact the fluid to be subjected to a membrane filtration, has contacted the solution, containing the polymer compound.

9. A method for performing a membrane filtration while applying a modified membrane, said membrane being a modified membrane as claimed in any or more of the preceding claims.

10. A method for performing a membrane filtration as claimed in claim 9, wherein a saline solution is desalinated at a pressure of less than 20 ato and preferably at a pressure ranging between 3 and 7 ato.