Polymers 2022 , 14 , 3309
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the suspended particles, reducing the repulsive forces that separate them. Patching occurs by the creation of patches on the surface of the suspended particles of opposite charge, creating a few oppositely charged zones that can interact with the non-patched areas of other particles, resulting in flocculation via electrostatic forces. This mechanism occurs preferentially by using low or medium Mw and high-charge polymers. Finally, in the bridging mechanism, polymers with high Mw and low charge density are preferably used. The polymers adsorb on the surface of the particles in an extended conformation, forming long tails and loops that can extend beyond the electrical double layer of the particles. The extended branches of the polymer can then adsorb into other particles, forming polymeric bridges between them [5,7,27–29]. The coagulation-flocculation mechanisms and the performance of the retention agents are highly dependent on the polymer type, the suspended particles to be flocculated and the conditions of the medium [30], this being the predominant mechanism that is mainly dictated by the Mw and CD of the polymer [31]. For example, Aguado et al. [15] tested the use of water-soluble cationic derivatives obtained from bleached Eucalyptus globulus kraft pulp (BEKP) to flocculate distinct fillers (kaolinite, GCC and PCC). With the dosages tested (10 or 20 mg/g), only the derivative with the highest degree of polymerization (DP) and CD (1703 and 5 mmol/g, respectively) showed promising results, but this was only for kaolin flocculation. When developing new retention additives, it is critical to understand the dominant mechanisms, the flocculation kinetics and the overall structure of the formed flocs. For papermaking tests, the flocculation performance of an additive is typically assessed via hydrodynamic techniques (the dynamic drainage jar test), by evaluating the drainage times and filler retention, and also by monitoring the zeta-potential (ZP) of the suspensions (which is most important for those mechanisms based on electrostatic interactions) [31,32]. More sophisticated alternative techniques based on light scattering, such as focused beam reflectance microscopy (FBRM) [33,34] and laser diffraction spectrometry (LDS) [31,32], have also been used, especially for their capability for real-time monitoring of the floc sizes. LDS not only allows the determination of the floc size distribution at each moment but also permits the extraction of information about the fractal dimension of the flocs that can be related to the compactness of the flocs formed [31]. LDS has already proven to be a very useful technique by which to assess the flocculation performance of polymers in real time, having already been used for the screening of synthetic polyelectrolytes [31,32,35], anionic MNFCs [12,36] and cationic cellulose polyelectrolytes [15,37,38]. In the present work, two cationization methods were applied to produce several cationic celluloses (CCs) with distinct DS and morphological properties (fibers, micro/nano- fibrillated celluloses and polyelectrolytes) (Figure 1). LDS was used to investigate the effect of the cationic cellulose characteristics, flocculant dosage and contact time on the flocculation of one of the most frequently used fillers in papermaking (PCC).
Figure1. Schematic representation of cationic celluloses with distinct morphologies.
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