| abstract |
A porous diaphragm of P.T.F.E. suitable for use in electrolytic cells is obtained by mixing a solid particulate additive substantially uniformly into an aqueous dispersion of polytetrafluoroethylene, coagulating the dispersion, forming the coagulum into a sheet and removing the additive. Additives which suitably have a particle size in the range approximately 5-30 m include for example starch or calcium carbonate, which are removed by soaking the sheet in hydrochloric acid, or a plastic polymer which may be dissolved out by an organic solvent or hydrolysed or depolymerized and vaporized by heating. A particulate inorganic filler or particle diameter not greater than 1 m may also be added to the aqueous dispersion before coagulation and this is not removed from the diaphragm sheet. Suitable fillers include barium sulphate, titanium dioxide and the amphibole and serpentine forms of asbestos and these are used in the range 40 to 70% calculated on the weight of polytetrafluoroethylene. In a preferred process the dispersion may be coagulated by adding electrolyte or a water-miscible organic solvent, e.g. acetone, the coagulum separated from supernatant liquor by filtration, the coagulum dried (hot air oven at 110 DEG C.), mixed to a stiff paste with a high-boiling hydrocarbon, e.g. petroleum ether b.p. 100-120 DEG C., made into a sheet by passing through rolls, additive removed and any filler retained. Reference is made to British Specification 865,288. |