Abstract:

Objective Lyocell fiber is a novel eco-friendly fiber produced through solvent spinning techniques with excellent flexibility and mechanical strength. Due to the outstanding performance, Lyocell fiber is extensively utilized in the textile, household, and medical sectors, rendering it an ideal substrate for fabricating functional fibers. However, Lyocell fiber is composed entirely of cellulose, poses a significant flammability risk. Simultaneously, its inherent insulating properties also impede the advancement of Lyocell fiber in the realm of flexible electronics. Therefore, enhancing the flame retardancy and electrical conductivity of Lyocell fiber is imperative to expand their functional applications.

Method In order to address the issues of flammability and limited functionality in cellulose fiber, this study utilized Lyocell fiber as the primary research material and employed a typically etherification reaction strategy to modify. By introducing carboxyl groups and metal ions (Na⁺), flame retardancy and water absorption properties were imparted, resulted in the formation of an ionic conductive hydrogel fiber upon water absorption. The surface morphology of the modified fiber was characterized, and flame retardancy of the carboxymethylated fiber as well as the sensing performance of the hydrogel fiber were investigated.

Results The carboxymethylation modification of Lyocell fiber had excellent flame retardancy and water absorption properties. The morphology of modified fiber remains similar to original fiber, exhibited a smooth outer surface. In thermogravimetric analysis, due to the introduction of carboxyl and Na⁺, the residual carbon content of the modified fiber was significantly increased from 17.0% to 24.4%. The limiting oxygen index (LOI) of original Lyocell fiber was merely 17.8%. However, the LOI of fiber can be significantly enhanced to reach an impressive 35.3% through carboxymethylation modification, thereby ensuring its non-ignitability even over prolonged periods in fire. The presence of metal ions exerted a flame retardant effect, resulting in a significant reduction in the peak heat release rate (PHRR) of Lyocell-Na from 184.4 W/g to 55.2 W/g. Moreover, the total heat release (THR) and heat release capacity (HRC) also decreased by 49.4% and 40.7%, respectively. It is noteworthy that Lyocell-Na exhibited a characteristic double heat release peak. This phenomenon arose from the promotion of carbonization in the fiber matrix by Na⁺, resulting in the formation of a dense barrier carbon layer on the fiber surface during the initial stage of combustion. Once sufficient heat accumulated within this carbon layer, it eventually breaches, leading to the second heat release peak. Compared to pure Lyocell fiber, the tensile strength of the fiber slightly decreased after carboxymethylation, from 3.9 cN/dtex to 3.2 cN/dtex. This could be attributed to that the reaction was carried out in an alkaline environment, and NaOH would decrease the crystallinity of Lyocell fiber, consequently impacted its mechanical strength. The hydrogel fiber showed a sensitive cyclic response to changes in finger bending angle. When the hydrogel fiber was attached to the finger joint for bending cycle action, it underwent deformation to yield and exhibited varying rates of current change corresponding to different bending angles.

Conclusion Cellulose-based hydrogel fiber was successfully prepared from Lyocell fiber by etherification reaction. By introducing carboxyl groups and metal ions into the molecular chain, the flame retardancy and water absorption of Lyocell fibers were significantly improved. Moreover, the gelled fiber exhibits a certain level of ionic conductivity upon water absorption. By considering the flame retardant performance, different degrees of deformation can generate corresponding changes in current signals, enabling identification of the operational state. Therefore, this work holds promising prospects for advancement in the field of flexible sensing.

Key words: cellulose fiber, carboxymethylation, hydrogel fiber, flame retardancy, sensitivity