in applications involving interleaving membranes with separators , for bioreactors , dialyzers , and membrane filters and the like , shrinkage of the separator , preferably made of polypropylene , polyamides , or polyethylene terephthlate fiber web , is avoided , and the need for the use of exotic and expensive fluorinated hydrocarbon polymer materials for such separators is avoided by subjecting the separator to untensioned heating at temperatures of from about b 120 l degrees c ., up to a temperature less than the glass transition or melting temperature of the polymer web under wet steam at a pressure of from about 1 . 0 to about 2 . 0 atmospheres for a time of at least about 20 minutes .