| 摘要 |
PROBLEM TO BE SOLVED: To prepare a polyolefin microporous film having good characteristics from a polyolefin solution of a high concentration with low stress. SOLUTION: A solution comprising from 10 to 50 wt.% of (1) an ultra-high- molecular-weight polyolefin having a weight average molecular weight of 7×10<5> or larger and the ratio of the weight average molecular weight to the number average molecular weight of from 10 to 300 or (2) a polyolefin composition which contains 1 wt.% or more polyolefin component having a weight average molecular weight of 7×10<5> or larger and has the ratio of the weight average molecular weight to the number average molecular weight of from 10 to 300 and from 50 to 90 wt.% nonvolatile solvent, is prepared and the above-mentioned solution is extruded from a die. Then, the solution is cooled to form a gel- formed sheet and the above-mentioned gel-formed sheet is biaxially oriented at a temperature 10 deg.C higher than the melting point of the above-mentioned ultra-high-molecular-weight polyolefin (1) or the above-mentioned polyolefin composition (2) or lower. Then, the residual nonvolatile solvent is removed through extraction by an easily-volatilizable solvent. Here, the ratio (A/B) of the tensile break strength of the biaxially oriented sheet before (A) and after (B) extraction is 0.75 or smaller. |
