Molecules 2023 , 28 , 7984
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particle surface. They bind with cationic patches‚ just like a bridge‚ agglomerating small particles into large flocs. (c) Microparticle retention-aid system As previously mentioned, the dual-microparticle retention-aid system is currently recognized as the most advanced and effective retention system. It can be used in high- speed spray forming machines and fully enclosed circulation systems. The Compzil system was developed and applied in papermaking by a Swedish com- pany. It is a control system composed of cationic starch and SiO 2 nanoparticles (3–5 nm) with high specific surface area and high negative charge density. The retention mecha- nism of filler is generally believed to be that fine fibers and filler particles are aggregated by cationic starch, thus generating flocculent. When SiO 2 nanoparticles are added, this flocculent may be torn apart into small flocs. For low-charge-density polyelectrolytes, the flocculation mechanism seems to be mainly the bridging effect [113]. As for polyelectrolytes with a high charge density, electrostatic attraction plays a major role. Obviously, due to the migration of SiO 2 nanoparticles into the adsorbed cationic starch, the pulp and silicon particles form a network floc that has a tendency to reaggregate after shearing. Therefore, the uniformity of paper will not deteriorate even under high retention rate. In addition, the structure of finished paper will become loose, resulting in increased porosity and improved water filtration performance. The Hydrocol system is a typical duplex control system which was developed in the United Kingdom. The working principle may be that relatively coarse flocs are formed when cationic electrolytes with a high molecular weight are added into the pulp. However, these flocs will be cut into small flocs when it is subjected to the action of a mixing pump and a screening machine. These tiny flocs, under the action of Heidelberg pigments, will reaggregate into a uniform network flocculent. So filler particles or fine fibers are retained in the paper pulp to achieve the purpose of retention. 5. Spherical Polyelectrolyte Brushes Polymer brushes [114] with unique properties such as a high charge density, high symmetry or quasi-symmetric structure, and low molecular weight provide great potential and limitless possibilities for its applications in biomedical applications [115–117], nanore- actors [118–120], durable protective clothing [121,122], etc. The brush conformation is influenced by the curvature of grafted surface. Figure 6 displays the structure of polymer brushes on different grafting surface. When the radius of colloidal particles is much greater than the thickness of the grafted brush layer, planar brushes form (see Figure 6a). The colloidal particle size is much less than the thickness of grafted brush layer, resulting in the formation of a star-polymer (see Figure 6c). If the polyelectrolyte chains affix to the sphere surface, then, e.g., spherical polyelectrolyte brushes (SPBs) result [25]. As shown in Figure 6b, the core radius R c denotes initiator-immobilized spheres surface, from which polyelectrolyte chains are grafted. L refers to the thickness of the brush layer, R h is the hydrodynamic radius, and ζ is the zeta potential. SPBs not only possess many advan- tages originating from their special molecular structure, but also has a simple synthesis process [123,124] and easy-to-control molecular weight [125,126]. Therefore, SPBs have recently attracted worldwide research interest [127–132].
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