Chitosan and Its Derivatives as Sustainable Functional Additives in Papermaking: Properties, Applications, and Emerging Trends—A Critical Review

With increasing environmental concerns and regulatory pressures, the industry is shifting toward bio‐based alternatives to traditional synthetic chemicals. Chitosan, a biodegradable and biocompatible biopolymer, has gained attention due to its multifunctionality in improving paper properties. This paper explores the chemical modifications of chitosan, such as molecular weight (MW) adjustments and functional group modifications, which enhance paper strength, water resistance, and antimicrobial properties. The impact of these modifications on the performance of chitosan as a retention agent, strengthening agent, and barrier coating is discussed, along with its applications in sustainable papermaking. The review further examines how chitosan‐based additives contribute to the United Nations Sustainable Development Goals (SDGs) through waste valorization, circular production, safer materials, cleaner process water, and biodegradable paper products. Additionally, challenges such as cost‐effectiveness, scalability, and process compatibility are highlighted, along with emerging solutions and future research directions. Chitosan's potential to replace synthetic additives and contribute to environmentally responsible papermaking practices is emphasized, underscoring its role in the development of high‐performance, eco‐friendly paper products.