The present invention relates tomaterials permeable to water vapour.

As is well known, the polymers of perfluoroethylene sulphonic acid that are available under the trade name Nafion of E. I.DuPont de Nemours, have the useful property of being freely permeable to the passage of water vapour but almost impermeable to nearly all other gases andvapours. Tubing and membranes made of this material are therefore used to add cr remove water vapour from gas mixtures or from flowing stream of gas mixtures.

One disadvantage of such a perfluoroethylene sulphonic acid polymer is that it is a very strong acid and somewhat unstable to heat. It can not, for example, be used at the temperature of boiling water 100°C. This temperature limitation limits the use of this material for separating gases and vapours from steam.

It is an object of the invention to provide a material that is more stable than the foregoing polymer but still water- vapour permeable.

According to the present invention, there is provided a water-vapour permeable material comprising a perfluorocarbon polymer incorporating lithium sulphate groups covalently bonded to the carbon skeleton of the polymer.

The lithiumpolymer is not only more stable to heat but also neutral and less likely to react with labile compounds or to catalyse their decomposition.

Although it has previously been proposed to neutralise perfluoroethylene sulphonic acid polymers by replacing the hydrogen ion of the sulphonic acid by sodium or potassium, the resulting material, while being more heat stable, has been very much less permeable to water vapour.

In a preferred embodiment of the invention the polymer concerned is a polymer of perfluoroethylene of the following form:

However, other perfluorocarbon polymers may be used, for example:

The method of neutralisation is to steep a perfluorocarbon sulphonic acid polymer material in lithium hydroxide solution for a few hours, followed by washing with water and drying.

It is therefore a straightforward matter to produce products made of the lithium-neutralised polymer, since products made of the un-neutralised polymer material (for example, tubes, membranes) can simply be taken and immersed in lithium hydroxide solution to produce improved- stability products.

Gas-drying apparatus embodying the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing which is a longitudinal cross-section through the apparatus.

5 Small diameter, thin-walled tubing obtained from E. I. DuPont de Nemours under their trade name " Nafion" was immersed in lithium hydroxide for several hours before being washed and dried. The sulphonic acid groups of the original tubing were converted to lithium sulphate groups.

10 The tubing was divided into lengths 11 which were used to construct the illustrated gas-drying apparatus. This apparatus comprises a cylinder 10 surrounding a bundle of the tubing lengths 11. The tubing lengths pass through end seals 12 in the cylinder 10 and terminate in connections 14,15 that

15 permit a stream of wet gas to be passed through the tubing lengths 11. The body of the cylinder is provided with input and output connectors 16, 17 to enable a drying agent to be passed over the outside of the tubing lengths 11.

In use, a wet gas such as nitrogen and steam is passed 2o through the tubing lengths 11 via the connectors 14, 15 in the direction of the arrows A. A dry gas (dry air) is passed over the outside of the tubing lengths 11 via the connectors 16, 17 in the direction of arrows B. The water vapour in the wet gas stream passes through the walls of the tubing lengths 25 11 into the dry gas; as a result, the wet gas is substantially dried by the time it exits the apparatus.

Apparatus of the illustrated form has been found to work well and survive at 100°C and to dry steam-saturated nitrogen or other gases at this temperature. 0