Microporous sheets and a process for making them

In a process for making microporous sheets which are permeable to water vapor comprising applying a polyurethane in a hygroscopic polar solvent to a substrate, coagulating the polyurethane with a non-solvent for the polyurethane, removing the solvent and non-solvent, and optionally, removing the substrate, the improvement which comprises coagulating a solution containing a polyurethane which has the following structural unit ##STR8## in which n .gtoreq. 2

• Z = an alkyl group containing 1 to 5 carbon atoms, an aryl radical containing 6 to 15 carbon atoms, a siloxyl or siloxanyl group or ##STR9## the groups Z being the same or different;
• R = an alkylene group and
• Y = --nh--, --o--, --s-- or NR.sub.1
• where
• R.sub.1 is an alkyl group containing 1 to 5 carbon atoms or a cyclohexyl radical
• and the total quantity of siloxane sequences in the microporous polyurethane sheet is about 0.1 to about 20% by weight.