Filtration membrane with calcined .alpha.-alumina particles therein

A method for producing a membrane having a semipermeable asymmetric film supported on a flexible support having inner and outer surfaces, and inter-communicating macropores which place said inner and outer surfaces in fluid communication, said method comprising,

• coating said support's outer surface with a dope of a polymeric reaction product of
• (i) a complex of polyvinylidene difluoride (PVDF) with calcined .alpha.-alumina particles having a primary particle size in the range from about 0.1 .mu.m to about 5 .mu.m, and
• (ii) a hydrophilic polymer having reactive nucleophilic groups adapted to impart hydrophilicity to said membrane;
• wherein said complex is formed by dispersing less than 50 parts by weight of said particles per 100 parts by weight of said particles and PVDF in a solvent for said PVDF;
• wherein said reaction product is formed by the addition of said hydrophilic polymer to said complex in said solvent;
• said dope having a viscosity sufficient to be supported on said outer surface, so as to provide said support with a dope-coated surface;
• passing said dope-coated surface through a sizing means to provide a wall thickness of dope on said support sufficient to form a film less than 0.2 mm thick;
• and, coagulating said dope.