break down to release ASA in contact with the fi bers [9,10].
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Alum/rosin sizing is commonly used for acidic papermaking with a typical pH range from 4.0 to 5.5. In this pH range, alum forms the right aluminum species to either react with soap rosin or to retain dispersed rosin (DRS) for sizing development. AKD and ASA are used in alkaline papermaking processes, where ASA has a higher reactivity to cellulosic fiber compared to AKD. Moreover, ASA has also been reported to be applied in acidic paper-making conditions [4–8]. Due to the insolubility of ASA in water, ASA oil needs to be emulsified before adding to the wet end for homogenous distribution in pulp slurries. The emulsifier can be starch- based (cationic starch) or polymer-based (polyacrylamide). Typical ASA emulsion particle size is around 1 µ m. The ASA emulsion has a short shelf life, and thus the chemical is typically emulsified on-site in the mill and dosed to the wet end as soon as possible. The emulsion particles are retained in the forming paper sheet at the wet end and forming section. Sizing development mostly happens in the dryer section, where the particles break down to release ASA in contact with the fibers [9,10]. The proposed and widely cited ASA sizing mechanism is the formation of an ester or covalent bond between ASA molecules and fibers. Figure 1 illustrates the ester bond forma- tion between ASA and fiber (cellulose). However, during the last 60 years of papermaking research, this popular mechanism has been questioned. Several scientists are still uncertain about the binding mechanism between ASA and cellulose. In addition, most scientists agree that ASA hydrolyzes in water during the papermaking process (Figure 2) [7,11–13]. The proposed and widely cited ASA sizing mechanism is the formation of an e covalent bond between ASA molecules and fi bers. Figure 1 illustrates the ester bon mation between ASA and fi ber (cellulose). However, during the last 60 years of pape ing research, this popular mechanism has been questioned. Several scientists are s certain about the binding mechanism between ASA and cellulose. In addition, mo entists agree that ASA hydrolyzes in water during the papermaking process (Fig [7,11–13]. Figure 1. Scheme of ester bond formation between ASA and cellulose.
Figure 2. Scheme of the formation of hydrolyzed ASA. Figure2. Scheme of the formation of hydrolyzed ASA.
The covalent bonding question becomes a heated debate between scientists wh port the ester bond formation as the primary sizing mechanism and scientists who b ester bonding is almost nonexistent and does not play a major role in paper sizing. the confusion around the covalent bonding questions, many authors prefer to use pressions “widely accepted”, “generally accepted”, “generally understood”, “tra ally”, or “assume” in reference to the ASA-cellulose covalent bond formation [7,12,1 In this review, patents and research papers with experimental data were revie understand the ASA sizing mechanism since 1963. In addition, chemistry consider for be tt er application of ASA are brie fl y discussed. The covalent bonding question becomes a heated debate between scientists who sup- port the ester bond formation as the primary sizing mechanism and scientists who believe ester bonding is almost nonexistent and does not play a major role in paper sizing. Due to the confusion around the covalent bonding questions, many authors prefer to use the ex- pressions “widely accepted”, “generally accepted”, “generally understood”, “traditionally”, or “assume” in reference to the ASA-cellulose covalent bond formation [7,12,14–19]. In this review, patents and research papers with experimental data were reviewed to understand the ASA sizing mechanism since 1963. In addition, chemistry considerations for better application of ASA are briefly discussed. 2. Debate on ASA Sizing Mechanism in Papermaking Prior to the introduction of ASA to the papermaking process in the 1960s, the additive was used in the textile industry to impart water repellency to cellulosic fabrics. Patent 2,903,382 by Robert Berls [20] in 1959 provided details on how to make hydrophobic cellulosic fabric using ASA. Different types of ASA with chains from 19 to 35 carbon atoms were dissolved in solvent such as isopropanol, benzene, toluene, chloroform, carbon tetrachloride, ammonia, morpholine, and water emulsions. The fabric was dipped into the resulting solution then heat cured. The recommended ASA concentration was from 0.7%
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