Effect of shearing forces and flocculant overdose on filler flocculation mechanisms and floc properties

The increasing use of fillers makes filler retention a more critical issue. The effects of both shearing and flocculant overdose on filler flocculation kinetics and mechanisms and floc properties, in the absence of fibers, are presented. The flocculation by cationic polyacrylamide (CPAM) or poly(aluminum chloride) (PAC) was determined using a focused beam reflectance probe. The effect of shear forces on aggregation kinetics depends on particle size. PAC produces soft flocs. At 250 rpm, the number of counts decreases due to flocculation, while, at 750 rpm, flocculation of particles smaller than 0.5 μm increased the number of counts. Filler flocs induced by the optimal CPAM dosage, which was 3 times lower than the one that neutralized the ζ potential, were partially reversible, although they are generally believed to be irreversible; an excess of CPAM inhibited the reflocculation of the particles and decreased the flocculation rate, but a moderate excess improved floc strength and floc stability. These observations are consistent with the hypothesis of polymer flattening.