Porous aramid membranes and emulsions useful for the casting thereof

In the casting of membranes from fully aromatic polyamide resins, which method comprises preparing a casting dope by forming a solution of resin in an amount sufficient to produce a coherent membrane, casting the casting dope into a surface and precipitating the cast film by phase inversion by a quench bath,

• the improvement comprising forming integral skinned microporous membrane having skin pores having a mean flow diameter of from about 0.05 micrometers to about 3.0 micrometers, from a modified casting dope formed by the additional steps prior to casting of:
• adding a pore modifier to the solution of resin in an amount of from about 40 to about 150 weight percent of the said casting dope and in an amount sufficient to permit casting of such microporous membrane but less than the amount which forms visually aparent turbidity in said solution, the pore modifier being an organic liquid which is essentially miscible with the solution, but is a non-solvent for the said resin, and
• thereafter adding a precipitant which is substantially miscible with the solvents of the said solution, in an amount sufficient to form a visably turbid dispersion which will not separate at a moderate degree of mixing.