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Correlating Retention and Drainage in Papermaking with Flocculation: Effect of Polyelectrolyte Branching

Elisabete Antunes, Fernando A.P. Garcia, Paulo Ferreira, and Maria da Graca Rasteiro. Chemical Eng., Coimbra University, Polo II, Pinhal de Marrocos, Coimbra, 3030-290, Portugal

In papermaking, chemical flocculation is fundamental for achieving high retention and high drainage rate simultaneously. However, the choice of the retention aids systems have to be made with caution since retention, drainage and sheet formation depend on the flocculation mechanisms involved and on flocs characteristics. Additionally, flocculation mechanisms depend on several factors namely on flocculants characteristics and dosage and on contact time, among others. Hence, the flocculation evaluation is of great importance to control the wet-end stage in paper production.

Some studies have shown that the branched polymers seem to exhibit better performance than the linear ones on the modern faster paper machine and, thus, have a significant potential as papermaking retention aids. However, only few fundamental studies have been presented so far and most of them have studied retention and drainage performance of these polymers in microparticulate systems. Therefore, it is of great interest to study these new polymers in single component systems to better understand the mechanisms involved.

In this context, the objective of the present study is to evaluate the effect of the degree of polymer branching on retention and drainage performance and simultaneously to correlate the results with flocculation kinetics and flocs structure. Additionally, the effects of flocculant concentration, flocculant charge density and flocculant contact time were investigated. Several new cationic polyacrylamides (C-PAM) of high molecular weight were used to flocculate the suspension. These C-PAMs differ on their charge density and number of branches. The dynamic drainage analyser (DDA) was used to evaluate retention and drainage of a flocculated kraft pulp fibre suspension containing precipitated calcium carbonate (PCC). Flocculation kinetics were assessed by using a light diffraction scattering (LDS) technique which gives also information about flocs size distribution and flocs structure. The flocs structure was quantified by both the mass fractal dimension and the scattering exponent parameters.

The results show that polymers of medium charge density are more adequate to be used as retention aids since lower drainage time and higher filler retention are obtained at low flocculant contact time and low flocculant dosage. The branched polymers of medium charge density exhibit better performance than the linear ones. Drainage performance is significantly improved because the highly branched polymer produces smaller flocs with a more open structure. The results also demonstrate that it is possible to correlate the flocculation process by LDS with the flocculant's performance in the drainage test.