Method for making cellulose semipermeable hollow fibers

A process for making a semipermeable cellulose hollow fiber having an ultrafiltration coefficient K.sub.UFR in the range of about 2 to about 200 milliliters per hour per square meter per millimeter of mercury, a urea coefficient K.sub.UREA in the range of about 15.times.10.sup.-3 to about 45.times.10.sup.-3 centimeters per minute, and a wet intrinsic tensile strength in the range of about 2.times.10.sup.4 to about 11.times.10.sup.4 force per gram of cellulose polymer which comprises:

• (a) melt spinning a hollow fiber cellulose ester fiber through a nozzle from a melt spin composition consisting essentially of about 36 to less than 50 weight percent cellulose ester and the balance at least one polyol having an average molecular weight between about 106 and about 900;
• (b) hydrolyzing said cellulose ester fiber with an aqueous alkali solution free of glycerol, substantially into a cellulose hollow fiber;
• (c) replasticizing said cellulose fiber while said fiber is still wet with a water soluble, essentially non-volatile plasticizer;
• and
• (d) drying said plasticizer fiber.