Crosslinked poly(vinyl alcohol) membranes

The inherent hydrophilicity of poly(vinyl alcohol) (PVA) makes it an attractive polymer for water treatment applications based on membranes. Thermal and chemical resistance and a high anti-fouling potential are accompanied by high water permeability. The large swelling capacity requires that the PVA be adequately crosslinked to ensure that the contaminants in water can be retained, and that compaction under pressure can be minimised. There is a challenge to achieve this and still obtain economical permeate fluxes. The literature on crosslinking of PVA is reviewed. Many reagents have been explored. Glutaraldehyde is a more effective crosslinking agent than formaldehyde or glycidyl acrylate, which in turn gives a less swollen product than that obtained by increasing the crystallinity by heating. Toluene diisocyanate and acrolien give similar results in the preparation of reverse osmosis membranes, but at an extremely high applied pressure. Crosslinking with maleic anhydride/vinyl methyl ether copolymers gives as good a result, but at even higher pressure. Thus the high swelling of PVA can be overcome by crosslinking reactions, but with the consumption of some of the OH groups responsible for the hydrophilicity. What is really needed is network formation that provides a tight restraining without serious loss of hydrophilic behaviour. Similar membranes are used for the separation of organic compounds from one another or from water by pervaporation, where the vapor of one component is selectively transferred through the membrane on the basis of polarity differences. Here PVA membranes would be especially suited to dehydration procedures. The high swelling behaviour can be countered also by forming the active PVA component inside the pores of a microporous membrane. Crosslinked PVA inside such membranes has its swelling suppressed, and can function as salt removal membranes. By only coating the pore walls, leaving some porosity, microfiltration or ultrafiltration membranes can be prepared. In both situations a degree of grafting to the host membrane would be beneficial.