Molecules 2023 , 28 , 7984
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by reversible addition–fragmentation chain transfer polymerization, which is a method of acrylamide and methacryloyloxyethyltrimethylammonium chloride copolymerizion. The retention rate of fillers and fine fibers was improved through bridging mechanism. For cationic microparticles, whether organic or inorganic particles, few conformational changes in cellulose flocculation occur because of their rigid structure [84–86]. Zegui Yan et al. [87] explored the retention performance of cationic SiO 2 particles and cationic organic polymer particles in unwashed bleached kraft softwood pulp/PCC and found a significant improvement in retention rate. Xiaohong Peng et al. [88] prepared copolymer particles of acrylamide, acyloyloxyethyltrimethyl ammonium chloride (DAC), and polyethylene oxide (PEO) through inverse emulsion polymerization, and studied the retention effect of DAC and PEO on the fiber. Results indicated that the increase in the ratio of DAC and the chain length of PEO is beneficial to bridging, thereby improving the retention rate. In summary, there are several theoretical models, namely “Electric neutralization”, “Patching”, and “Bridging”, that describe retention mechanism of retention aids. The neutralization effect is mainly caused by simple electrolytes and low-molecular-weight polyelectrolytes, such as Al 2 (SO 4 ) 3 , FeCl 3 , and PEI. The patching effect is mainly caused by cationic polyelectrolytes with high charge density and medium molecular weight (100,000 to 1 million) (e.g., PEI and PAM). For high-molecular-weight polyelectrolytes, bridging effects have higher possibility than that of low-molecular-weight ones. 2.2. Dual-Component Systems With the continuous development of papermaking, existing single-component sys- tems have been increasingly unable to meet the needs of retention aids under high shear conditions. In order to obtain flocs with higher shear resistance, great attention has been paid to develop dual-component systems. The process of dual-retention systems generally refer to the addition of cationic additives first, followed by the addition of anionic additives. In general, dual-component systems mainly involve two different systems: “dual polymeric retention system” and “microparticle retention-aid system”. The former is composed of fixatives with a low molecular weight and high charge density, and flocculants with a high molecular weight and low charge density. This system can achieve a relatively good retention effect, but the paper forming performance is limited by forming large and loose flocs. The latter system, the so-called “microparticle retention-aid system”, has developed into one of the most successfully commercially available products. Compared to traditional retention systems, it not only has excellent retention effects on wet-end chemical additives, fine fibers, and fillers, but also reduces the concentration of white water and increases the paper machine speed, thus generating environmental and economic benefits [89,90]. Interestingly, even if the adding order of binary components is reversed, there is almost no difference in its flocculation index. Classical microparticle retention aids are anionic particles and cationic particles. Typical anionic particles are composed of the Compzil system (starch-colloidal sil- icon) [91] and the Hydrocol system (CPAM-bentonite) [92]. B. Alince et al. [93] studied the deposition behavior of calcium carbonate (PCC) on paper fibers in CPAM-bentonite systems. By changing the dosage of CPAM, it was found that CPAM provided an anchor point for bentonite to function on the fibers and PCC. Bentonite played a bridging role between the fibers and PCC. Excessive dosage of CPAM would weaken the bridging effect of bentonite. The interaction force between fibers and PCC obtained through the CPAM- Bentonite system was stronger than that of the CPAM system. Norlito Cezar et al. [94] reported cationic particles by modifying the surface of SiO 2 with 2, 3-epoxypropyl H- methylammonium chloride. Results showed that it significantly improved the flocculation efficiency of clay. The optimal dosage ratio of cationic (SiO 2 ) to anionic (APAM) was in the range of 2:1 and 5:1. Moreover, the rigidity of SiO 2 improved the efficiency of bridging. Cationic magnesium aluminum hydroxide/APAM was used by Wang Songlin et al. to study its retention performance on talc powder/reed pulp. The retention effect improved with the decrease in colloidal particle size of magnesium aluminum hydroxide particles.
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