Polymer transfer during fines detachment under turbulent flow: mechanism and implications

The transfer of polymer between papermaking surfaces was characterized for several commercial retention aids. Polymer transfer during particle detachment stabilizes the suspension, and leads to impaired polymer layers. A kinetic model, describing simultaneous particle deposition and polymer transfer, was developed and validated. In the experiments, the fines deposition on polymer-coated fibres was followed. The effect of polymer chemistry, salt concentration and shear rate on the kinetic parameters was investigated. Two main kinetic regimes can be distinguished, corresponding to flocculation mechanisms by charge reversal and bridging. Bridging polymers generate stronger bonds, but are more easily transferred. On the other hand, highly charged polymers, inducing aggregation by charge reversal or neutralization, produce weaker bonds but undergo less transfer. The observed kinetic constants were extrapolated to papermaking shear rates. Even with conservative estimates, fines are quickly and permanently detached in the presence of high shear. These results could explain the observed loss in retention on high-speed papermachines.