Molecules 2023 , 28 , 7984
3of 23
2. Retention Systems The retention of fine colloidal particles (e.g., fine fibers and filler particles) on the mesh in the slurry without additives is caused by mechanical interception. Therefore, the retention rate is mainly determined by the size of the interstitial space in fibers. In practical industrial production, a large amount of filler and fine fibers are lost in white water through the mesh due to high turbulence and high shear forces, resulting in a relatively low retention rate. Therefore, it is often necessary to add retention aids to improve the retention rate of fine colloidal particles in the slurry on the mesh. Retention aids can change the aggregation behavior of hydrophobic colloidal. With the continuous development of wet-end papermaking systems, retention aids are also constantly improved. According to the composition of retention aids, commonly used retention systems are single-component systems and dual-component systems. 2.1. Single-Component Systems Classical single-component systems are inorganic and organic, among which organic retention aids can be further divided into two categories: natural polymers and synthetic polymers. Commonly used inorganic retention aids include Al 2 (SO 4 ) 3 [29,30], polyaluminium chloride (PAC) [31,32], and FeCl 3 [33,34]. Due to the existence of a negative charge on the surface of paper pulp, the electroneutralization of multivalent metal ions results in floccula- tion [35,36]. Aluminum salts in water have complex structures and ion characteristics with changes in pH values and changing conditions, resulting in different adsorption properties on the surface of paper pulp [37]. Masato Kato et al. [38] investigated the retention perfor- manceofAl 2 (SO 4 ) 3 in bleached wood pulp. It was found that oxalic acid or Mg 2+ reduced the content of aluminum, which was hardly affected by ionic strength. However, the shear resistance of the flocs in turbulence is limited, making it difficult for inorganic flocculants to meet the development needs of modern papermaking [39,40]. Natural polymers are widely used because of their characteristics of being cheap, renewable, and environmentally friendly. The commonly used natural polymers in pa- permaking are starch, chitosan, guar gum, and cellulose. Among these natural polymers, nature starch is by far the most used polymer due to its wide source and low price [41,42]. However, starch with low cationic degree ( ≤ 0.2) [43], which is available in many commer- cial areas, cannot provide sufficient attraction to stabilize adsorbed substances, resulting in its low retention [44]. So starch is usually modified physically or chemically to extend its usefulness. Svetlana Bratskaya et al. [45] reported a cationic starch with a cationic degree ranging from 0.25 to 1.54. They found that the cationic degree had a significant impact on retention performance, and the retention was significantly improved with increased cationic degree. Chitosan, as another type of natural polymer, has also been widely ap- plied in papermaking [46–49]. Li et al. [50] studied the retention of modified chitosan in reed pulp/CaCO 3 , indicating that the maximum retention rate of CaCO 3 could reach 80%. However, the low charge density and uncontrollable structure of natural polymers limit their application in papermaking. So efforts to overcome these drawbacks have led to a large amount of research attention focusing on the synthetic polymers. Currently, polyacrylamide (PAM) and polyethylene imine (PEI) are the most widely used synthetic polymers in papermaking, such as linear polyacrylamide [51], hyper- branched polyacrylamide [52], dendritic or star-shaped polyacrylamide [53], branched polyethylene imine [54], etc. The structure of polymer retention aids is shown in Figure 2. PEI is a multi-branched polymer with different weight and charge density. However, PEI unmodified by cationic activation is not suitable for alkaline papermaking [55–57]. It is because it loses charge when the pH is above 5.5. Carrasco et al. [58] studied the performance of cationic PEI in bleached wood pulp using zeta potential measurement and colloidal titration method. It was found that the presence of electrolytes had a negative effect on flocculation. Better retention effect of paper material was achieved when zeta potential fluctuated between − 5 mV and 5 mV, but the shear resistance of PEI was not improved.
Made with FlippingBook Annual report maker