Method for preparing a uniform polyolefinic microporous hollow fiber

Claim 1

A process for preparing a uniform polyolefinic microporous hollow fiber comprising the steps of: (a) melt spinning hollow non-porous precursor fiber in a substantially vertically upward direction from a polyolefinic resin at a temperature of about 10 to 90° C. above its crystalline melting point; (b) spin-orienting said hollow non-porous precursor fiber by drawing said hollow non-porous precursor at a draw down ratio of at least about 10 while quenching said hollow non-porous precursor fiber with a substantially symmetrical flow of quenching medium; annealing the precursor fibers at a temperature between about 50° C. and less than 165° C. for a period of about 0.5 second to about 24 hours; (c) cold stretching the non-porous precursor hollow fibers in the direction of their length at a temperature greater than the glass transition temperature of the precursor fiber and not greater than about 100° C.; (d) Hot stretching the annealed cold stretched hollow fibers in the same direction of cold stretch at a temperature above the cold stretching temperature and below the melting point of the polymeric material to achieve a total degree of combined stretching of from about 80 to 200%, said cold and hot stretching having an extension ratio of from about 1:3 to about 1:20 and a strain rate of from about 10 to about 200%/minute; and heat setting the resulting hot stretched fibers of step (e) at a temperature within the range of from about 70° C. below the crystalline melting point of the polymer to less than the crystalline point of the polymer for at least about one second under conditions such that the fiber relaxes from about 0 to about 50% of its stretched length.