DE1201055B - Process for the production of heterogeneous ion exchange membranes - Google Patents

Description

Process for the production of heterogeneous ion exchange membranes Es it is known to produce heterogeneous, selectively cation- or anion-permeable membranes, by mixing powdered cation or anion exchange material with thermoplastic Polymers such as polystyrene, polyvinyl chloride, polymethyl methacrylate or polyethylene intimately mixed and rolled out into foils, for example on a roller mill. Such Diaphragms are on the rise due to their relatively simple manufacturing technique Dimensions used e.g. in the electrodialytic desalination of sea and brackish water.

About the production and maintenance costs of electrodialysis systems Keeping as low as possible will be made to the ion exchange membranes in terms of their properties, such as permselectivity, electrical resistance, mechanical Resistance and flexibility, high demands made.

These properties are largely determined by the composition of the Membranes, especially due to the mixing ratio of ion exchange active Material intended for the thermoplastic polymer acting as a binder. With increasing Mixing proportion of ion-exchange-active material increases the permselectivity, the electrical resistance, mechanical resistance and flexibility of the membranes however strongly. So are striving to use heterogeneous ion exchange membranes the highest possible permselectivities and low electrical resistances to obtain an increase in the proportion of ion-exchange-active material, as a result the associated decreasing mechanical resistance and flexibility limits set.

It has been proposed to use heterogeneous ion exchange membranes with a content of up to 850/0 ion exchange active material, however only have themselves during continuous practical use in electrodialysis cells Membranes with a maximum of 75% ion exchange active material are used of polyethylene (high pressure polyethylene or low pressure polyethylene) as a binder. The use of all other thermoplastic polymers as binders, e.g. polystyrene, Polyvinyl chloride, rubber, also leads to lower contents of ion exchange active substances Material for membranes with insufficient flexibility for the technical requirements and mechanical resistance. In addition, the practice has gone over to due to the increased demands on mechanical resistance and flexibility of membranes when using modern electrodialysis machines these properties by further lowering of the ion exchange active material content at the expense of permselectivity and to improve electrical resistance.

It therefore represents a technical advance, a method to find that allows heterogeneous ion exchange membranes with improved mechanical strengths without deterioration of the permselectivity or the electrical To produce resistance.

It has now been found that heterogeneous, selectively cation or anion-permeable membranes by incorporating powdered cation or anion exchange resin can be produced in polyethylene at elevated temperatures and rolled into foils, if the polyethylene is a mixture of high-pressure polyethylene with a K value of 60 up to 70 and low-pressure polyethylene with a K value of 120 in a weight ratio 50:50 to 90:10 is used, the weight ratio being between the mixture made of high pressure polyethylene and low pressure polyethylene and the cation resp.

Anion exchange resin is 35:65 to 20:80.

As a cation exchange active powdery material is expediently a conventional strongly acidic cation exchange resin based on a sulfo group-containing, cross-linked polystyrene with an ion exchange capacity of 4.5> 4.5 meq / g dry substance and a grain size of <0.1 mm diameter is used.

As the anion exchange active material, a conventional one is expediently used strongly basic anion exchange resin based on an amino and / or ammonium group-containing, cross-linked polystyrene with an ion exchange capacity of 3 meq / g dry substance and a grain size of <0.1 mm diameter is used. The degree of networking of the dust-like ion exchange resins can contain 0.5 to 20s / o, preferably 4 to l20 / o, be.

The temperature range from 140 to 1600 C has proven to be the plasticizing temperature and 5 to 10 minutes has been found to be the appropriate plasticizing time. at higher temperatures and longer processing times cause a noticeable thermal effect Degradation of the ion-exchange-active component, associated with a sharp drop in the Permselectivity or a sharp increase in electrical resistance.

The heterogeneous ion exchange membranes produced according to the invention stand out in addition to a good permselectivity or a low electrical Resistance due to about three times greater mechanical resistance and flexibility compared to heterogeneous ion exchange membranes produced by known processes with comparable electrical resistances or permselectivities.

The mechanical resistance, strength and flexibility of ion exchange membranes can be determined by conventional methods, whereby the membrane, e.g. in a burst pressure tester, is subjected to high mechanical loads. 10 cm2 of the The diaphragm is exposed to increasing pressure and the value is given as the bursting pressure, at which the material bursts.

The following examples illustrate the invention.

For comparison purposes, tests using High pressure polyethylene and low pressure polyethylene are each given alone.

It is precisely from these comparative examples that the using the high-pressure polyethylene-low-pressure polyethylene mixture as a binder brought about an improvement in strength clearly without deterioration of the electrical resistance or the permselectivity recognize.

Example 1 17.5 parts by weight are obtained at 1400 ° C. on a roller mill High pressure polyethylene (K value 62), 7.5 parts by weight of low pressure polyethylene (K value 181) and 75 parts by weight of an 8 O / o crosslinked with divinylbenzene, dust-like, sulfonated polystyrene with an ion exchange capacity of 4.8 meq / g dry matter processed into a film in the Na + form within 10 minutes.

After swelling in water, a cation exchange membrane is made with obtain the following properties: Electrical resistance ...... 16 i2 / cm2 permselectivity 94 0 / o burst pressure. ............ 1.95 kp / cm2 Example 2 On a roller mill at 1500 C 24 parts by weight of high-pressure polyethylene (K value 64) with 6 parts by weight Low-pressure polyethylene (K value 190) and 70 parts by weight of a 40 / o with divinyl benzene cross-linked, dusty, ammonium-containing polystyrene with an ion exchange capacity of 3.5 meq / g dry substance in the C1 form processed within 8 minutes.

After swelling in water, an anion exchange membrane is made with obtain the following properties: Electrical resistance. 15 Q / cm2 permselectivity .. 91 0 / o burst pressure. 1.85 kp / cm2 Example 3 On a roller mill, at 1550 C 13 parts by weight of high pressure polyethylene (K value 64) with 7 parts by weight of low pressure polyethylene (K value 190) and 80 parts by weight of a 60 / omit divinylbenzene crosslinked, dusty, ammonium-containing polystyrene with an ion exchange capacity of 3.45 meq / g Dry matter in C1 form processed within 5 minutes. After swelling an anion exchange membrane with the following properties is obtained in water: Electrical resistance . 11 Q / cm2 permselectivity. 95 0 / o burst pressure. 1.8 kp / cm2 For comparison, the following experiments are cited: a) On a roller mill at 1400 C 25 parts by weight of high-pressure polyethylene (K value 62) and 75 parts by weight of a pulverulent, sulfonated polystyrene cross-linked to 8 8% with divinylbenzene with an ion exchange capacity of 4.8 meq / g dry substance in the Na + form incorporated within 10 minutes. The cation exchange membrane that forms has the following properties after swelling in water: Electrical resistance . 18 Q / cm2 permselectivity 900/0 burst pressure ..... 0.3 kp / cm2 b) On a roller mill 40 parts by weight of high-pressure polyethylene (K value 62) are given at 1400 C and 60 parts by weight of an 8 ovo cross-linked, pulverulent, sulfonated with divinylbenzene Polystyrene with an ion exchange capacity of 4.8 meq / g dry matter in the Na + form incorporated within 10 minutes. This is done after swelling obtain a cation exchange membrane in water with the following properties: Electrical resistance 130 Q / cm2 permselectivity. 880/0 burst pressure. 1.2 kp / cm2 c) 25 parts by weight of low-pressure polyethylene are placed on a roller mill at 1400 C (K value 181) and 75 parts by weight of an 8% crosslinked with divinylbenzene, powdery, sulfonated polystyrene with an ion exchange capacity of 4.8 meq / g dry substance treated in the Na + form for 10 minutes.

Even if the processing time is extended up to 1 1 hour due to the higher softening temperature of low-pressure polyethylene Foil.

In a corresponding test with a rolling temperature of 1800 C becomes a cation exchange membrane with the following properties obtained: electrical resistance. . 500 Q / cm2 permselectivity. 87% burst pressure . . 1.9 kp / cm2 d) 25 parts by weight of low-pressure polyethylene are placed on a roller frame at 1400 ° C. (K value 103) and 75 parts by weight of an 8% cross-linked with divinylbenzene, powdery, sulfonated polystyrene with an ion exchange capacity of 4.8 meq / g dry substance in the Na + form to a cation exchange membrane within 10 minutes rolled out the following characteristics: electrical resistance. 24 Q / cm2 permselectivity . . 89 o / o burst pressure. ...... 0.6 kp / cm2

Claims (1)

Claim: Process for the production of heterogeneous, selectively cationic or anion-permeable membranes by incorporating powdered cation resp. Anion exchange resin in polyethylene at elevated temperature and Rolling out into foils, characterized in that a mixture of polyethylene is used as the polyethylene High pressure polyethylene with a K value of 60 to 70 and low pressure polyethylene with a K value of 120 in a weight ratio of 50:50 to 90:10 is used, with the weight ratio between the mixture of high pressure polyethylene and low pressure polyethylene and the cation resp. Anion exchange resin is 33:65 to 20:80.