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while organosulfur and isothiazoline has almost no effect on sizing performance. More investigations are needed to determine the mechanism by which defoamers or biocides negatively affect ASA. 5. Conclusions Several experiment-result papers and patents about the ASA application were re- viewed from 1963 to 2020. • It was believed that ASA formed covalent bonding with cellulose during the paper- making process. Nevertheless, most recent scientific evidence supports that the major sizing material in a sized paper is hydrolyzed ASA and its salts. • The amount of covalent bonds between ASA and the fiber is very low and is not a requirement to sizing development. • Hydrolyzed ASA contributes to the sizing performance; however, the direct applica- tion of hydrolyzed ASA to the pulp or the application of hydrolyzed ASA to the sheet leads to flocculation and does not achieve sizing. • The pH, temperature, conductivity, and the ASA-emulsifier ratio are critical for ASA emulsion stability. • Alum or PAC, when properly used improve sizing performance. • The shelf life of the ASA emulsion is about 20 to 30 min. More work is needed to understand how the change in ASA concentration, ASA-starch ratio, temperature, and pH individually or collectively affects the ageing process. • The use of novel chemistries to stabilize or extend ASA emulsion shelf life should be investigated for large scale availability, health and environmental impacts, and repercussions on the papermaking process. Author Contributions: Writing—original draft preparation, Y.N. and L.X.; writing—review and editing, Y.N., L.X., S.B., Y.J., and P.W.H.; supervision, P.W.H. and Y.J.; project administration, P.W.H.; funding acquisition, Y.J. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by WestRock. The publication cost was provided by the Chemical Engineering Department of the University of North Dakota. Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article. Acknowledgments: The authors want to thank Nichole Kilgore, Data Scientist at WestRock, for her
constant help in providing needed articles for this review paper. Conflicts of Interest: The authors declare no conflict of interest.
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