Molecules 2023 , 28 , 7984
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Figure2. Structural diagrams of different polymer retention aids.
Based on charge characteristics, polyacrylamide (PAM) is divided into four types: cationic polyacrylamide (CPAM), anionic polyacrylamide (APAM), amphoteric polyacry- lamide (ACPAM), and nonionic polyacrylamide (NPAM). APAM and NPAM are mainly used in acidic papermaking process, so their application scope is restricted [59]. Due to the fact that paper pulps are generally negatively charged, CPAM has been applied in single-component retention systems [60–63]. The retention effect of CPAM is influenced by its charge density and molecular weight. ACPAM is widely used in papermaking owing to its anti-polyelectrolyte effect [64–67], which means its viscosity increases with the increase in salt ion concentration. Lu et al. [68] prepared ACPAM with different structures by inverse emulsion polymerization and found that favorable retention of fillers in the pulp was obtained when the anionic degree was 5%, the cationic degree was 20%, and the molecular weight was 200,000–300,000. Recently, hyperbranched polyacrylamide has attracted considerable interest because of its characteristics as a coagulant aid originating from its high cationic density and low molecular weight [69–73]. The formed flocs have the advantage of small volume, dense structure, and strong shear resistance. Further studies highlighted that branched polymers have higher retention and shear resistance than those of linear polymers [74–76]. Angeles Blanco et al. [77] studied the flocculation process of hyperbranched PAM on paper pulps using focused beam reflectance measurement (FBRM). It was noted that the charge density had a significant impact on the properties of the flocs. Low charge density had a negative impact on the growth of polyelectrolyte chain, thereby affecting the flocculation process, while high charge density may reduce the proportion of bridging mechanism and increase the proportion of patching mechanism. Polyacrylamide with a high molecular weight and hyperbranched structure has good application prospects in papermaking. Additionally, dendrimer or star polymers have gradually become research hotspots as retention aids in papermaking due to their highly branched three-dimensional morphol- ogy [78–81]. Compared with linear polymers, they usually exhibit high solubility, low solu- tion viscosity, small hydrodynamic radius, high shear resistance, small molecular volumes, and high-density functional groups [82]. Shan et al. [83] synthesized star-shaped CPAM
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