Rheological properties of cellulosic fibre suspensions flocculated by cationic polyacrylamides

Cellulosic fibres in suspension form three-dimensional networks which display shear strength as a result of the mechanical entanglement of flexible fibres. The effect of high molecular weight cationic polyacrylamides of different charge densities on the viscoelasticity of softwood kraft pulp suspensions was investigated by rheological measurements in oscillatory shear as a function of straining frequency, straining amplitude, added amount of cationic polyacrylamide and fibre concentration. The flocculants increased the fibre flocculation and this was shown as an increase in the shear modulus and in the critical strain. The critical strain marks the onset of structural breakdown of the fibre network. The threshold concentration of fibres c* for a measurable shear strength was lower for suspensions flocculated by cationic polyacrylamide. The shear modulus showed a scaling relationship as a function of the fibre concentration in excess of the threshold concentration, c-c*. The effect of a flocculant is suggested to be due primarily to an increase in the number of fibres that are active in the network. The flocculant probably also increases the bonding strength in the fibre-fibre contact points. An attempt is made to discuss the results according to an elastic site/bond percolation concept. (C) 1998 Elsevier Science B.V.