Microporous hollow fiber and process and apparatus for preparing such fiber

A microporous polyolefinic hollow fiber having:

• (a) an average wall thickness in the range of from about one micron to about 75 microns;
• (b) a coefficient of variation in wall thickness throughout a cross-section taken perpendicular to the axis of the fiber of less than about 20%;
• (c) an average inner diameter in the range of from about 5 microns to about 1500 microns;
• (d) a coefficient of variation in inner diameter throughout a cross-section taken perpendicular to the axis of the fiber of less than about 8%;
• (e) a substantially uniform porosity across the circumference of said fiber as determined by the average ratio of the minimum pore density to the maximum pore density around the circumference of said fiber of less than about 3:1;
• (f) an oxygen flux of at least 100 cc/cm.sup.2 -min. at 10 p.s.i.g.;
• and
• (g) an average pore size as measured by electron micrograph of from about 50 to about 2000 Angstroms in width and from 500 to 10,000 Angstroms in length.