Process for preparing membranes
Membranes and processes for preparing membranes having weakly acidic or weakly basic groups comprising the steps of: (i) applying a curable composition to a support; (ii) curing the composition for less than 30 seconds to form a membrane; and (iii) optionally removing the membrane from the support; wherein the curable composition comprises a crosslinking agent having at least two acrylic groups. The membranes are particularly useful for producing electricity by reverse electrodialysis.
1. A process for preparing a substantially non-porous membrane having weakly acidic or weakly basic groups comprising the steps of:
(i) applying a curable composition to a support;
(ii) curing the composition for less than 30 seconds to form a membrane; and
(iii) optionally removing the membrane from the support;
wherein the curable composition comprises
(i) 10 to 99 parts in total of crosslinking agent(s) comprising two acrylic groups;
(ii) 10 to 99 parts in total of curable compounds having one acrylic group, at least ten of which have one or more groups selected from weakly acidic groups, weakly basic groups and groups which are convertible to weakly acidic or weakly basic groups;
(iii) 0 to 50 parts in total of crosslinking agent(s) comprising more than two acrylic groups; and
(iv) 0 to 10 parts in total of compounds which are free from acrylic groups and comprise one or more methacrylic groups; and
(v) 0.01 to 5 parts in total of photoinitiator(s);
wherein the membrane is an anion exchange membrane or a cation exchange membrane and all parts are by weight and the number of parts of (i), (ii), (iii), (iv) and (v) add up to 100.
2. A process according to claim 1 wherein the curing is achieved by irradiating the composition for less than 10 seconds.
3. A process according to claim 1 wherein the said crosslinking agent(s) or the said curable compound or both the said crosslinking agent(s) and the said curable compound have one or more groups selected from weakly acidic groups, weakly basic groups and groups which are convertible to weakly acidic or weakly basic groups.
4. A process according to claim 1 which further comprises a further crosslinking agent or a further curable compound or both a further crosslinking agent and a further curable compound which in each case is free from weakly acidic groups, weakly basic groups and groups which are convertible to weakly acidic or weakly basic groups.
5. A process according to claim 1 wherein the said weakly acidic groups are selected from carboxy and/or phosphate groups and the said weakly basic groups are selected from secondary and tertiary amino groups.
6. A process according to claim 1 wherein the membrane has an ion exchange capacity of at least 0.3 meq/g based in the dry weight of the membrane.
7. A process according to claim 1 wherein the curable composition is applied to a support moving at a speed of over 15 m/min.
8. A process according to claim 1 wherein the curable composition is applied continuously to a moving support or a porous strengthening material by means of manufacturing unit comprising a curable composition application station, an irradiation source for curing the composition, a membrane collecting station and a means for moving the support porous a strengthening material from the curable composition application station to the irradiation source and to the membrane collecting station.
9. A process according to claim 1 wherein the curing is performed by irradiating the curable composition for less than 3 seconds, the weakly acidic groups are carboxy and/or phosphate groups, the weakly basic groups are secondary amine or tertiary amine groups, the composition is substantially free from methacrylates and methacrylamides and the membrane has an ion exchange capacity of at least 0.3 meq/g based on the dry weight of the membrane.
10. A process according to claim 1 wherein the curable composition is substantially free of water.
11. A polymeric membrane having weakly acidic or weakly basic groups and being suitable for use in exchange or reverse electrodialysis, the membrane having been obtained by a process comprising polymerisation of a curable composition of claim 1 .
12. A process according to claim 3 wherein the said weakly acidic groups are selected from carboxy and/or phosphate groups and the said weakly basic groups are selected from secondary and tertiary amino groups.
13. A process according to claim 8 wherein the curable composition is substantially free of water.
14. A process according to claim 12 wherein the curable composition is substantially free of water.
15. A process according to claim 7 wherein the curable composition is substantially free of water and the curing is achieved by irradiating the composition for less than 10 seconds.
16. A membrane according to claim 11 wherein the curable composition is substantially free of divinyl benzene.
17. A membrane according to claim 11 wherein the said weakly acidic groups are selected from carboxy and/or phosphate groups and the said weakly basic groups are selected from secondary and tertiary amino groups.
18. An anion exchange membrane or a cation exchange membrane according to claim 1 which has a charge density of at least 20 meq/m 2 .
19. An anion exchange membrane or a cation exchange membrane according to claim 1 wherein the curable composition is free from compounds having tetraalkyl-substituted quaternary ammonium groups and free from compounds having sulpho groups.