Flocculation of dilute titanium dioxide suspensions by graft cationic polyelectrolytes

The flocculation of a dilute titanium dioxide (TiO₂) suspension using homopolymers and graft copolymers of acrylamide (AM) and diallyldimethylammonium chloride (DADMAC) was investigated. The graft copolymers produced by γ-irradiating the mixtures of polyacrylamide (PAM) and polyDADMAC gave better flocculating performance than homopolymers, reflecting the higher fractions of large particles and bigger floc size. A kinetic delay in the onset of flocculation was observed after adding the copolymers in the dose range 5–30 mg polymer]/ g TiO₂]. Increasing dosage resulted in a longer delay period. No significant flocculation was observed when the dose was above 50 mg polymer]/g TiO₂]. This delay was interpreted in terms of the re-conformation of polymer chains driven by charge neutralization, between the positively charged polymer branches and the negative particle surface. Depending on the dosage used, the flocculation behavior of the graft copolymer has been suggested to be equilibrium and non-equilibrium flocculation. It was also observed that re-conformation is not affected by the ion strength of the media, but a strong shear force significantly reduces the chain reconformation time. Received: 9 April 1998 Accepted in revised form: 28 August 1998