Abstract:

Objective Strain sensing is one of the important functions of a smart fabric, which can transform the external stress (or strain) into visible electrical signals and monitor the physiological and motion characteristics of human body. At present, the flexible strain sensor has some problems, such as low sensitivity, small strain range and unstable performance after multiple stretching.

Method AgNO₃ was used as silver source material, NaCl and NaBr as nucleating agent, polyvinylpyrrolidone as ending agent, ethylene glycol as solvent and reducing agent in the reaction to prepare long silver nanowires. The commercial elastic core-spun yarn with single spandex fiber as inner layer, polyester fiber as sheath as flexible matrix, water-borne polyurethane (WPU) as dispersing agent and binder, and long silver nanowires (AgNWs) as conductive material was prepared by pre-stretch impregnation method with dual functions of strain sensing and electric heating.

Results AgNWs prepared by polyol method have uniform morphology, uniform dispersion, length up to 155 μm, diameter only 146 nm, and aspect ratio up to 1 000 (Fig. 1(b)). When the mass ratio of AgNWs/WPU is 2∶1 and the core-spun yarn is pre-stretched by 30%, AgNWs can adhere to a single cotton fiber to form a stable and dense AgNWs conductive network(Fig. 2). When the load of AgNWs was 15%, the percolation threshold is reached, and the conductivity value became 466 S/m(Fig. 3(a)). During stretching, the conductive network formed by AgNWs was deformed together with the inner spandex fiber, and the core-spun yarn exhibited a Gauge factor value of 12.7 at the highest within a wide strain range of 0%-70%. When the core-spun yarn was drawn, the stress gradually increases with the deformation. When the tensile length reached 25 mm, i.e., the elongation of 250%, the polyester fiber began to break. The cyclic tensile mechanical properties of core-spun yarns under 10% strain were further tested (Fig. 5(b)). The deformation of core-spun yarns could be quickly recovered after repeated stretching for at least 10 times, showing good mechanical stability. At 5 V voltage, when the static tensile range was increased from 0% to 50%, the conductive network structure formed by AgNWs is destroyed, resulting in a continuous decrease in its electrical conductivity. The maximum temperature range is 49.8-65.7 ℃, which reflects excellent electrothermal performance.

Conclusion In this paper, a strain sensing electrothermal core-spun yarn with elastic core-spun yarn as flexible substrate, WPU as dispersing agent and binder, and AgNWs as conductive material was prepared by the method of multiple impregnation of 30% of pre-drawn elastic core-spun yarn. The results show that when AgNWs/WPU (mass ratio of 2∶1) mixed emulsion is prepared, AgNWs can be uniformly dispersed and AgNWs/WPU film is formed only on the surface of a single cotton fiber. The cotton fiber is completely dispersed by using the 30% impregnation method of pre-drawn core-spun yarn. Dense AgNWs conductive network is formed on the single cotton fiber of the yarn sheath layer, and the electrical conductivity reaches the extreme value. When the load of AgNWs is 15%, the strain range of AgNWs core-spun yarn is wide (0%-70%), and the sensitivity is up to 12.8. After repeated stretching, the strain sensing and mechanical properties of AgNWs yarn are stable. At 5 V voltage, when the static tensile range is 0%-50%, the maximum temperature variation range is 49.8-65.7 ℃, which reflects the excellent electric heating performance. The AgNWs strain sensing electrothermal cored yarn made by pre-stretch impregnation is expected to be an ideal method for large-scale production of wearable smart devices.

Key words: core-spun yarn, pre-stretch, long silver nanowire, strain sensing, electrothermal property, smart wearable