Molecules 2023 , 28 , 7984
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The retention rate was highest at the conditions of the amount of magnesium aluminum hydroxide 0.6% and the amount of APAM 0.06%. In addition, organic polymer particles not only have the three-dimensional nanostruc- ture of the ionized surface of inorganic particles, but also possess the controllable charge density and flexible chains of organic polymers, which is expected to be a novel retention aid [95–98]. Taking cationic organic polymer particles as an example, Xiao H et al. [99] pre- pared cationic organic particles by emulsifier-free emulsifier polymerization and studied their flocculation on fine clay. It was found that the dosage ratio of cationic particles and anions was 8:1 at the optimal flocculation point, and the dosage of anions accounted for 0.06 wt% of clay. These binary components played a synergistic role in the flocculation process. Ono Hiroshi et al. [100] synthesized cationic polymer particles (CPMP) with different charge densities and sizes by emulsion and micro emulsion methods, and studied the effect of CPMP on positively charged and negatively charged calcium carbonate. It was found that CPMP had no obvious effect on positively charged calcium carbonate. For negatively charged carbonate, however, the opposite is true. The addition of APAM could improve the flocculation effect. 3. Evaluation Methods of Retention Aids For single-component system and dual-component system, there are a variety of ways to measure the retention efficiency, such as fluorescence microscopy, field emission electron microscopy (FE-SEM), transmission (TEM), focused beam reflectance measuring instrument (FBRM), flocculation turbidity, zeta potential, and dynamic retention. The Dynamic Drainage Jar (DDJ) is a widely used reliable instrument for testing the retention efficiency of single or multiple retention aids. The first-pass retention ( FPR ) of pulp suspension and PCC can be calculated based on Equation (1):
C i − C 0
100%
(1)
FPR =
C i ×
where C i and C 0 are the concentrations of colloidal particles in slurry and in filtrate, respectively. 4. Basic Theory in Wet-End Papermaking Wet-end chemistry of papermaking mainly studies the interaction among various com- ponents of paper stock during retention, filtration, forming, and white water circulation, as well as their effects on the operational performance of paper machines and product quality. Interface chemistry is an important theory in wet-end chemistry of papermaking. Due to the complex composition of colloids in papermaking, although most fiber sizes are beyond the scope of colloids, the gaps in fibers belong to the colloid size. Therefore, papermaking process can be studied by the theories related to colloid and interface chemistry. 4.1. Main Forces The wet-end papermaking is a complex and diverse polydisperse system, in which there are various forces involved [101]. In terms of macroscopic components, it can be mainly summarized into following seven types: (l) adsorption of various additives on fibers, fine fibers, and fillers; (2) aggregation of fibers, fillers, and fine fibers; (3) aggre- gation among various additives; (4) interaction with water between fibers, fine fibers, and additives; (5) neutralization of surface charges of suspended and soluble anionic sub- stances; (6) formation and development of micelles that comprise surfactant molecules; (7) establishment of dynamic equilibrium between soluble inorganic salts and insoluble electrolytes. From the perspective of micro-force, the forces among main components mentioned above are manifested as van der Waals forces, hydrogen bonds, ionic bonds, and co- valent bonds. As for drying paper, the strength of the forces among fibers varies: the hydrogen bond is 8.81 KJ/mol, the ionic bond is 41.8–54.34 KJ/mol, the covalent bond is 292.6–351.12 KJ/mol [102], and the van der Waals strength is weaker than the others.
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