A large number of the electrolytic cells now in existance and contemplated for future use in the production of chlorine and caustic by the electrolysis of brine are diaphragm type cells. Almost without exception, these diaphragms are formed by deposition directly on the foraminous cathode from a slurry of asbestos fibers. Such diaphragms have the serious disadvantage that under load the asbestos swells considerably, e.g., up to 800 percent, filling the anode-diaphragm gap and thus increasing cell voltage and subjecting the diaphragm itself to attrition by gas released at the now proximate anode surface.
The prior art has proposed the use of a composite asbestos-polymer sheet as a diaphragm material. However, because of the complex geometry of the majority of existing diaphragm type cells (i.e., Hooker, Diamond), such a sheet, of necessity formed exterior the cell, cannot be employed without significantly reducing the active diaphragm surface area. A filter press, or "sandwich", type cell design is required to successfully employ such sheets.
Another suggestion has been to impegnate a preformed asbestos diaphragm with a monomer or polymer solution, followed by in situ polymerization of the monomer or curing of the polymer. Such a technique, however, results in the formation of a continuous polymer coating on the surface of the asbestos fibers, thus eliminating the advantages of the ion exchange and water permeable properties of the asbestos fibers. Of course, attempts to impregnate a preformed diaphragm with a particulate polymer are not uniformly successful since the asbestos mat usually acts to filter out the polymer particles on the surface thereof to no appreciable advantage.