Ultrasound-assisted dilute sulfuric acid method: green and controllable preparation of waste cotton-based multi-scale nanocellulose.

The controlled preparation and efficient utilization of multi-scale nanocellulose (NC) from waste cotton fabrics (WCF) is a significant challenge that cannot be ignored. In this paper, a closed-loop process chain is proposed, ranging from WCF to cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs), and finally to composite materials. A sequential strategy involving alkali-assisted swelling and hydrolysis with diluted sulfate acid solution was employed to precisely liberate nanocellulose from WCF, with its final multi-scale morphology controlled via high-intensity ultrasonication. The results showed that the prepared CNCs and CNFs exhibited high aspect ratios (40-200), high stability (zeta potentials of -34.29 and - 34.55 mV, respectively), and ultra-high crystallinity (90.02 % and 79.69 %, respectively). In addition, the tensile strength of the polyvinyl alcohol (PVA)/CNCs nanofiber membrane prepared with CNCs as the reinforcing agent can reach up to 2.74 MPa. Meanwhile, the compressive strength of the PVA aerogel reinforced by CNFs reaches 21.14 MPa, which is twice that of the base material. This study achieved the closed-loop conversion of WCF into high-performance nano-cellulose reinforcements, providing a scalable technical paradigm for the green, high-value utilization of waste textiles and the precise preparation of multi-scale nanocellulose.