Zn–Al LDH nanosheets exfoliated in a LiOH/urea solution in situ: an effective strategy for fabricating cellulosic fibres with enhanced mechanical properties

The alkali/urea system is an economical and environmentally friendly solvent system for dissolving cellulose. However, the mechanical performance of cellulose derived materials (such as fibres or films) treated with alkali/urea must be improved. The capacity of this system for cellulose dissolution is limited and the mechanical properties of the regenerated cellulose fibres are still unsatisfactory. Here, cotton linter pretreated with peracetic acid (PAA) was dissolved in a LiOH/urea aqueous suspension in which Zn–Al layered double hydroxide (Zn–Al LDH) nanosheets (NS thickness = 3 nm) were exfoliated in situ. After wet spinning, the strength and toughness of the regenerated cellulose fibres reached 290 ± 7.66 MPa and 23.5 ± 4.40 MJ/m³ respectively, which were 66.7% and 150% higher than those without the LDH NS treatment. It is interesting to note that the synergistic effect of Zn²⁺ and Al³⁺ distributed on the surface of the LDH host layer significantly improved the solubility of cellulose at low temperature and the interaction between the cellulose and LDH NSs also improved the mechanical performance. This work provides a strategy for improving both the solubility of cellulose in the alkali/urea system and the mechanical properties of regenerated cellulose fibres.