Existence state of water in lyocell fibres and effect on fibres microstructure and macroscopic mechanical properties

Lyocell fibres are regarded as one of the most promising cellulose materials. Their sensitivity to moisture variations significantly impacts their applicability potential across diverse fields. Previous research have primarily focused on the interaction between cellulose fibres and water at high moisture levels (> 50%), while the critical influence of lower moisture content (< 25%) on lyocell fibres structure and properties remains largely ignored. This study systematically investigates the effects of low water content on fibres characteristics through four complementary approaches: low field nuclear magnetic resonance (LF-NMR) for analyzing binding force between cellulose and water, differential scanning calorimetry (DSC) for analyzing water phase states, small-angle X-ray scattering for characterizing microporous structural changes, and mechanical property evaluation. LF-NMR analysis indicated that all the water within the lyocell fibres exist in a bound state when the moisture content of cellulose is below 25%. Furthermore, it was observed that as the water content increases, the binding force between water and cellulose diminishes. DSC analysis indicated that three different phases of water exist within lyocell fibres at varying moisture contents: 0–4%, 5–11%, and 11–25%. These phases were identified as “bound water”, “secondary-bound water” and “free-bound water”, respectively. The bound water shows no melting peak, while secondary-bound water exhibit characteristic melting peaks at − 32 °C, and free-bound has a wider melting peak from − 32–0 °C. Notably, bound water enhances fibres strength by 40% through micropore size reduction, whereas secondary-bound water increases fibres elongation but reduces fibres modulus through plasticization effect on fibres mechanical properties. These findings establish a foundation for comprehending the relationship between lyocell fibres and water, highlighting the crucial role of water in enhancing the strength of lyocell fibres. Furthermore, this research offers valuable insights for optimizing the application of lyocell fibres.