Polymers 2022 , 14 , 3309
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Although some fillers can present a slightly cationic surface, most of the commercial fillers present a negative surface charge, due to the use of dispersants [6]. With the cellulose fibers also presenting a net negative charge (mainly as a result of the ionization of carboxylic and sometimes sulfonic acidic groups that are introduced onto the fiber surface during the chemical pulping and bleaching steps) [7], cationic synthetic polymers are applied in papermaking formulations as retention agents to minimize the losses of fillers and/or small fibrous fragments and to take advantage of the natural affinity between oppositely charged particles. Cationic polyacrylamides (CPAMs) with high molecular weight (Mw), medium to high charge density (CD) and linear or branched chains are frequently used as retention agents [8]. Although PAMs are considered non-toxic in their native polymeric form, their non-renewable origin and the existence in the final product of some residual acrylamide monomers [9] pose environmental and human health concerns since they exhibit a high degree of neurotoxicity [10]. These concerns, together with increasing environmental restrictions, pave the way for the development of new natural-based alternatives to be used as retention systems. In the paper industry, the use of cellulose as a renewable resource that can serve as the backbone for the development of new additives, such as retention additives, appears to offer a logical alternative. Over the last few decades, lignocellulosic materials have been functionalized and/or deconstructed into their hierarchical sub-structures to produce new cellulose-based products, such as micro- and nanofibrillated celluloses (MNFCs). Purer forms of cellulose can be synthesized by some bacterial species (referred to as bacterial cellulose) and are typically explored for biomedical applications [11]. Some of these new products have already been tested in papermaking formulations and have shown their positive potential as filler retention agents [12,13]. From the published literature, it is possible to observe that nonionic or anionic cellulose-based products are far more common than their cationic counterparts, as ob- served by other authors [14]. In most recent years, the graft of cationic groups (typically, quaternary ammonium) into cellulosic materials, including lignocellulosic residues, has been explored as a possible alternative for the production of bio-based retention agents for the paper industry. However, the studies published are still relatively scarce and, in some cases, inconclusive [15–19]. Diab et al. [17] studied the use of cationic MFC in single and dual systems with bentonite for the retention of GCC in sheets produced with softwood and bagasse pulps. No improvements were observed in the filler retention, the results being attributed to the low degree of cationization (0.27 mmol/g). On the contrary, Li et al. [19] reported improvements in GCC retention by modifying the filler surface with soluble cationic cellulose, with a degree of substitution (DS) of 0.52. For cellulose cationization, two main strategies are typically described in the literature. The first, the simplest and most applied strategy, comprises the direct reaction between cellulose and the reagent epoxy-propyltrimethylammonium chloride (EPTAC) [20–22]. Pedrosa et al. [23] instead used the precursor (3-chloro-2-hydroxypropyl) trimethylammo- nium chloride (CHPTAC) and converted it to the more reactive form EPTAC, via a reaction with sodium hydroxide. The second cationization method is based on a two-step reaction in which cellulose is initially oxidized with sodium metaperiodate (NaIO4), causing the conversion of the two vicinal hydroxyl groups at the C2 and C3 positions into aldehyde groups and the subsequent cleavage of the C2–C3 bond. The resulting dialdehyde cellulose (DAC) can then react with Girard’s reagent T to form a stable imine structure that includes the quaternary ammonium groups [23–26]. The polymeric retention additives work through a process of chemical flocculation by the destabilization of the particles (fillers) in suspension. Three distinct mechanisms are typically referred to as being responsible for particle aggregation. Charge neutralization (or coagulation) occurs when a salt or low Mw polymer neutralizes the surface charge of
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