Review of the Mechanistic Roles of Nanocellulose, Cellulosic Fibers, and Hydrophilic Cellulose Derivatives in Cellulose-Based Absorbents

Cellulose – either in solid form or as a highly hydrophilic chemical derivative of cellulose – can serve multiple and synergistic roles in the preparation of absorbent materials to meet the requirements of diverse absorbent products. Progress in the preparation of nanocellulose products, including nanocrystalline cellulose (CNC), nanofibrillated cellulose (NFC), and bacterial cellulose (BC), is opening up new possibilities for the reinforcement of hydrogels. Conventional cellulosic fibers, including kraft pulp fibers (e.g., fluff pulp), mechanically pulped lignocellulosic fibers, and recycled paper fibers can provide a structure to fine-tune the mechanical and drainage properties of products that can include superabsorbent materials. Carboxymethylcellulose (CMC) is an especially strong candidate for preparation of the swellable phase of a hydrogel. The high content of carboxylic acid groups in CMC gives rise to a strong swelling tendency, especially at neutral to alkaline pH values. The uptake of water can be understood based on concepts of osmotic pressure, in addition to any salinity in the fluid that is being absorbed. The swelling can be adjusted by the choice and amount of a cross-linking agent. Notably, some of the needed cross-linking effect can be optionally provided by nanocellulose or conventional cellulosic fibers. Combinations of solid cellulose entities and water-soluble cellulose-based polyelectrolytes can be used to prepare completely bio-based products that offer an alternative to the presently available disposable absorbents, which are based mainly on petroleum-based superabsorbent hydrogels. Chemical and physical aspects of cellulose and its derivatives also help determine what happens during drying of absorbent products; some swelling ability may be lost irreversibly due to highly organized hydrogen bonding and coalescence of the cellulose-based macromolecular chains. Since cellulose can be involved in both the structural and chemical aspects of highly absorbent products, there will be unique mechanistic roles governing water uptake, water holding, and even the environmental impacts of cellulose-based absorbent products.