Utilization of Waste Cotton Fibers by Extracting Nanocellulose Crystals: A Study on Phosphoric Acid Method Compared with Sulfuric Acid Method and TEMPO Oxidation Method

Waste cotton fibers are an ideal raw material for extracting nanocellulose crystals (CNCs), benefitting from  their high cellulose content. In this study, the waste cotton fibers from the calendering finishing process were used to extract CNCs by sulfuric acid, TEMPO oxidation, and phosphoric acid methods, aiming to create a new way to reutilize the waste cotton fiber and also to verify the practicability that the phosphoric acid method can replace sulfuric acid and TEMPO oxidation methods. The CNCs obtained from the three methods are all in cellulose I state with an average length of 200-500nm and diameter of 15-20nm, indicating that the waste cotton fiber can extract CNCs. However, the CNCs from the phosphoric acid method showed the highest thermostability but the lowest crystallinity, while the ones from the sulfuric acid and TEMPO oxidation methods had higher crystallinity but lower thermal stability. Overall, the three methods are all acceptable for preparing CNCs, but the phosphoric acid method has more significant potential due to its low cost, environmental friendliness, and safety. Graphical abstract