Poly (Vinyl alcohol) gel sublayers for reverse osmosis membranes. 1. insolubilization by acid-catalyzed dehydration
Both flat-sheet and tubular composite reverse osmosis (RO) membranes were prepared by depositing aqueous solutions of poly(vinyl alcohol) (PVA) and a dehydration catalyst on asymmetric poly (arylether sulfone) (PES) substrate membranes. The PVA coatings were insolubilized by heat treatment to create stable hydrophilic gel-layer membranes. The influence of variables such as PVA concentrations, catalyst concentration, curing time, and curing temperature was investigated. It was shown that a simple manipulation of one or two variables could lead to membranes with widely differing salt retention and water permeability characteristics. The insolubilized PVA coatings were intended to serve as hydrophilic gel sublayers on which ultrathin salt-retention barriers could ultimately be formed by interfacial polycondensation. For this purpose, high-flux gel layers were required, whereas salt-retention capabilities were not regarded as important. However, the promising salt retentions obtained at 2 MPa (up to 85% NaCl retention and 92% MgSO4 retention) showed that some of these PES-PVA composite membranes could function as medium-retention, medium-flux (RO membranes, even in the absence of an interfacially formed salt-retention barrier.