Nanocellulose from recycled indigo-dyed denim fabric and its application in composite films

•Acid hydrolysis of indigo-dyed denim yielded stable indigo-CNC blue suspension.•TEMPO oxidation process degraded indigo dyes and produced colorless CNFs.•Indigo-CNCs functioned as a colorant, reinforcement, and UV blocker in plastic.•Indigo-CNCs reinforced polyvinyl alcohol films with high clarity retained.•Colorless CNFs had excellent reinforcing effect on polyvinyl alcohol films. In this study, nanocellulose was extracted from indigo-dyed denim fabric and the resultant nanocellulose properties were evaluated in comparison with those derived from bleached cotton fabric and wood pulp in order to investigate the potential of recycling denim waste for nanocellulose production and application. Sulfuric acid hydrolysis and (2,2,6,6-tetramethylpiperidin-1-yl) oxyl (TEMPO)-oxidation were utilized to produce cellulose nanocrystals (CNC) and cellulose nanofibers (TOCN), respectively. A stable CNC suspension with blue color was obtained after acid hydrolysis and the TEMPO process yielded colorless TOCN. The denim-derived nanocellulose possessed similar yield, morphology, size, crystallinity, and thermal stability to those derived from bleached cotton but higher crystallinity and thermal stability compared to the nanocellulose from wood pulp. When used to reinforce polyvinyl alcohol film, the blue indigo-CNC not only enhanced mechanical properties of the film but also provided the film with outstanding UV blocking.