Flow properties of cellulose fiber suspensions flocculated by cationic polyacrylamide

Retention aid systems are commonly used in the papermaking process to flocculate the fine matericals in cellulose fiber suspensions. The addition of polymeric flocculants such as cationic polyacrylamide (C-PAM) also increases the fiber flocculation and the shear strength of the fiber flocs. The break-up process of fiber flocs in highly turbulent flow is of great importance to produce paper with an even fiber distribution. In this study, phase flow encoded nuclear magnetic resonance (NMR) imaging was used to study the pipe flow behavior of cellulose fiber suspensions when cationic polyacrylamide had been added. Axial velocity profiles were measured at different mean bulk flow rates. The mean velocity profiles for turbulent flow were analyzed in terms of the power-law. It was demonstrated that at low shear rates the polymeric flocculant induces phase separation in a dilute fiber suspension, while at high shear rates, polymer induced flocculation reduces turbulent intensities of the flow. Flow encoded NMR images were recorded for the fully turbulent flow of a 0.5 wt.% hardwood fiber suspension at different times (0.5–9.0 min) after polymer addition. Immediately after addition a steady plug flow profile was observed. After 9 min the flow is fully turbulent. This indicates that the polymer macromolecules are disrupted quickly in turbulent flow.