Colloidal Characteristics of Cationic Polyacrylamide and Their Effects on Pre-flocculation of Ground Calcium Carbonate

Abstract

Pre-flocculation is a technique that aggregates filler particles by means of polyelectrolytes. The size of the flocculated fillers is a critical factor affecting the properties of paper. Process variables, such as the concentration of flocculating agent, the stirring speed, and the dilution of the suspension, can change the size of the aggregates. Ground calcium carbonate (GCC) and cationic polyacrylamide (C-PAM) were used to investigate the influence of these variables on the flocculation. The median particle size and the size distribution were examined, and the tensile and the optical properties influenced by the floc size and the ash levels were evaluated. Addition of C-PAM at higher concentrations increased the median size of aggregates. Stirring speeds at a range of 1,000-3,000 rpm were used to simulate various turbulent environments, and it was shown that higher stirring speeds drastically decreased the floc size. Diluting and low speed stirring the flocculated GCC suspension was effective in keeping the particle size intact without disruption before sheet forming. Larger GCC flocs in the handsheets resulted in better mechanical properties. Cationic polyacrylamides (C-PAMs) have been widely used as flocculating agents to aggregate particles carrying negative charges on the surfaces. However, C-PAMs are susceptible to chemical and thermal degradation. One of the objectives of this study was to examine the effects of hydrolysis and adsorption behavior of C-PAM on flocculation of negatively charged particles, especially GCC. Hydrolysis of C-PAM was examined by using polyelectrolyte titration and ion chromatography, while adsorption behavior of the polymer was monitored using quartz crystal microbalance with dissipation (QCM-D). Laser diffraction spectroscopy (LDS) was employed to measure the size of GCC aggregates under neutral and alkaline conditions. Charge density measurements and ion chromatograms indicated that the hydrolysis of C-PAM is accelerated with increasing pH and elevating temperature. It was revealed by QCM-D measurements that hydrolyzed C-PAMs adsorbed on a substrate with a flat and rigid structure. This indicates that a decrease in the size of aggregates under alkaline conditions is due to the reduction in ability which forms inter-particle bridging flocculation. Flocculation ability of polyelectrolytes is an important criterion in selecting adequate flocculating agents for filler pre-flocculation processes. Especially if the characteristics of a polyelectrolyte vary depending upon the chemical environment they are exposed, careful selection of the polyelectrolyte should be exercised to maximize the benefits of using it. The other purpose of this study was to evaluate the effect of the charge density and molecular weight of C-PAMs on the growth and structural characteristics of GCC aggregates under neutral and alkaline conditions. The structure of GCC aggregates were characterized by a three-dimensional mass fractal dimension analysis method. High charge C-PAM formed larger and denser flocs than low or medium charge C-PAMs both in neutral and alkaline conditions. Flocculation rate was also rapid when high charge C-PAM was used. Molecular weights did not make differences in GCC flocculation if it was greater than 8 Mg/mol. QCM-D data showed that high charge C-PAM adsorbed tightly while low charge one formed loose and fluffy structure after adsorption. The results suggested that C-PAMs with high charge density around 3.7 meq/g and molecular weight over 8 Mg/mol formed GCC aggregates good for pre-flocculation process in terms of the size and structure under both neutral and alkaline conditions.