On the mechanism of cationic-polyacrylamide-induced flocculation and re-dispersion of a pulp fiber dispersion

The adsorption of cationic polyacrylamide on pulp fibers and the induced fiber flocculation have been measured at contact times relevant to the practical application of retention aids. In accordance with the La Mer model for bridging flocculation, a maximum in flocculation was found around 50% surface coverage by the polymer and re-dispersion occured above 100% surface coverage. Furthermore, the amount of adsorbed charges at full surface coverage after a short contact time corresponds to only 2% of the total fiber charge, whereas after 30 minutes it corresponds to 15% of the total charge. This indicates that the adsorption saturation after a short adsorption time is not controlled simply by the charges. Instead, there is probably a non-equilibrium charge or steric barrier against adsorption at these short contact times. It is also shown that the reconformation kinetics has an influence on the flocculation behavior during these short contact times.