Thickening of poly(vinyl alcohol) by borate

Viscosity and pH of aqueous borate–poly(vinyl alcohol) mixtures were determined under different conditions toward a clarification of the thickening mechanism. Boric acid has little effect on dilute poly(vinyl alcohol) (PVOH), but sodium borate causes sharp viscosity increases which are temperature sensitive and apparently stem from labile exothermic crosslinking of PVOH by borate ions. The pH data indicate that the complexing of borate with simple polyols such as mannitol and glycerol is a different process from borate crosslinking of PVOH. The PVOH thickening and gelling effect probably arises from the replacement of labile waters of hydration on borate ion by OH groups of PVOH rather than from borospirane formation.