Filter material and process for producing same

Claim 1

A process of producing a porous polyurethane resin filter material comprising: a. providing a first solution containing 60 tp 80% by weight of an NCO-pre-adduct having 1.5 to 5% free isocyanate groups dissolved in a highly polar organic solvent; b. providing a second solution containing a highly polar organic solvent and at least one member selected from the group consisting of hydrazine hydrate, carbodihydrazide, piperazine and 1,4-diaminopiperazine and in which solution said member is present in an amount of from 0.02 to 0.05 mole percent; c. admixing said first solution into said second solution over an extended period of time during which the NCO pre-adduct and said member of said second solution react to form a highly viscous reaction product and continuing the admixing of said first solution into said second solution and monitoring the viscosity measured under reaction conditions until the viscosity of the admixture is such that the addition of even the minutest amount of said first solution would have resulted in instantaneous gelling of the admixture, and at which point the addition of said first solution is stopped immediately, the final admixture thus produced having a solids content of between 15 and 35% by weight; d. shaping the resulting high viscosity final solution into a sheet, e. introducing the sheet into a coagulating bath to provide a microporous sheet structure having two external non-porous skin stratum and a central stratum having pore channels aligned perpendicularly to the surfaces of the sheet structure, f. drying the resulting microporous sheet structure and g. abrading one or both sides of the resulting microporous sheet structure to provide a filter having pore channels perpendicular to the surfaces of the sheet structure.