Acid catalyzed cross-linking of polyvinyl alcohol for humidifier membranes
Polyvinyl alcohol (PVA) is a hydrophilic polymer well known for good film forming properties, high water vapor permeance JW, and low nitrogen permeance. However, depending on molar mass and temperature, PVA swells strongly in water until complete dissolution. This behavior affects the usability of PVA in aqueous environments and makes cross-linking necessary if higher structural integrity is envisaged. In this work, PVA networks are formed by thermal cross-linking in the presence of p-toluenesulfonic acid (TSA) and investigated in a design of experiments approach. Experimental parameters are the cross-linking period tc, temperature TH and the TSA mass fraction wTSA. Cross-linking is found to proceed via ether bond formation at all reaction conditions. Degradation is promoted especially by a combination of high wTSA, tc and TH. Thermal stability of the networks after preparation is strongly improved by neutralizing residual TSA. Humidification membranes with a JW of 6423 ± 63.0 gas permeation units (GPU) are fabricated by coating PVA on polyvinyliden fluoride hollow fibers and cross-linking with TSA. Summarizing, the present study contributes to a clearer insight into the cross-linking of PVA in presence of TSA, the thermal stability of the resulting networks and the applicability as selective membrane layers for water vapor transfer.