Continuous dip coating of PVDF hollow fiber membranes with PVA for humidification

Composite hollow fiber membranes for gas-to-gas humidification were fabricated and their performance was tested for use in external humidifiers of polymer electrolyte fuel cells (PEMFC). Therefore porous poly(vinylidene fluoride) (PVDF) hollow fibers were manufactured by a wet spinning process, coated with thin layers of polyvinyl alcohols (PVA) via a continuous dip coating and crosslinked by glutaraldehyde. Coating parameters, such as surface tension and viscosity of the coating solution, were correlated with the layer thicknesses of the coatings. Influences of the layer thickness on crosslinking, water vapor transport and thermal stability of the membranes were studied. The layer thickness can be adjusted in the range of 0.3-4.29 µm by controlling the coating parameters. It was found that thicker coatings have a higher water vapor permeability, which may be caused by a lower degree of crosslinking. A layer thickness of 1.75 µm was formed when using, for example, a 5 wt% PVA 67000 g/mol aqueous solution. With this composite membrane, a water vapor permeability of 2890 Barrer and a selectivity over nitrogen of 289 was achieved at 25 °C. The potential of the designed fabrication process was verified by thermal stability tests, which almost all membranes withstood.