Preparation and performance of CNT impregnating3D ethylene-propylene side by side (ES)/PET nonwoven-based flexible pressure sensors

With the rapid development of science and technology, smart wearable devices have become increasingly popular, which has led to an increased interest in sensor technology in this area. In order to overcome the challenges of flexible sensors in terms of lower sensitivity, complex production process and lack of breathability, a three-dimensional Ethylene-Propylene Side By Side (ES)/Polyester (PET) fiber nonwoven material has been created by needle-punching-hot-melt techniques. As a result of impregnating the material with carbon nanotubes (CNTs), a 3D ES/PET nonwoven-based pressure sensor with excellent performance was successfully developed with CNTs, and the effect of different CNT impregnation times and number of impregnations on the performance of the sensor was compared.

The needling-hot melt method was used to prepare three-dimensional ES/PET fiber nonwoven materials. Firstly, the fiber web mixed with ES and PET fibers (5:95 weight ratio) was repeatedly punctured by needling technology in order to create a nonwoven fabric with a certain strength and thickness by intertwining the fiber bundles within the fiber web. In the next step, the needled nonwoven is heated to 115-120℃, and the polyethylene in the ES fibers is partially melted by hot melt technology to reinforce the fiber network structure and produce a three-dimensional ES/PET nonwoven. By ultrasonic impregnating this 3D ES/PET nonwoven material with CNTs, ES/PET nonwoven materials that have been coated with CNTs have been obtained with different impregnation times and number of times, assembled into sensors, and examined in terms of their physical and electrical properties.

The 3D ES/PET nonwoven material offers excellent physical and mechanical properties, thin thickness, high strength and elongation, and excellent air permeability, making it an excellent substrate material for flexible wearable sensors. Meanwhile, the prepared flexible sensors have a thin thickness, excellent flexibility and elasticity, resulting in a more comfortable wearing experience. In a SEM analysis, it was found that the film-forming and integrity of CNT on the sensor fiber surface increased with an increase in CNT impregnation time, and the CNT distribution uniformized with an increase in CNT impregnation time. The sensor's sensitivity reached 0.127 kPa after 15 minutes of impregnation and the response range was 0-120.76 kPa. After 3 times of CNT impregnation for 15 minutes, the CNT film on the fiber surface was complete and homogeneous with vertical texture, when the sensor sensitivity was the highest (0.375 kPa, detection range 0-214.53 kPa). The agglomeration of the CNTs on the fiber surface increased after 4 impregnations, and the voids decreased, which resulted in a decrease in sensitivity (0.087 kPa, response range 0-189.29 kPa). ES/PET nonwoven-based pressure sensors demonstrate excellent response speed and sensing performance with a response time of 48 milliseconds at a pressure of 0.31 kPa and a recovery time of 122 milliseconds. As a result of the 2100 endurance cycle tests, excellent stability was maintained at constant pressures of 0.6 kPa without any signal shift. As compared to similar published sensors, the maximum sensitivity and response range of this sensor offer significant advantages. In joint flexion and daily activity monitoring, the sensor shows high levels of consistency and periodicity, resulting in rapid decreases in resistance when the finger is flexed and rapid recovery when it is straightened. The sensor is also capable of detecting signals such as gesture clicks, breathing, and swallowing within a very short period of time.Conclusions: By needle-punching-thermal melting technology, three-dimensional ES/PET nonwoven materials were prepared, and CNT surface modification was performed to create flexible pressure sensors with a three-dimensional fiber network structure. According to the SEM characterization, CNT can be uniformly attached to the surface of the 3D ES/PET nonwoven material, which significantly reduces the resistance of the fiber, thus enhancing the performance of the sensor. The sensitivity of the sensor can be improved by adjusting the impregnation time and the number of impregnations. The optimal conditions for impregnation were 15 minutes of CNT impregnation and three times of impregnation. It is found that the sensors prepared under this impregnation condition have both a high sensitivity and a wide detection range, with sensitivities of 0.375 kPa, 0.0034 kPa, and 0.0006 kPa for 0-0.6 kPa, 0.6-63.2 kPa, and 63.2-214.5 kPa, respectively. In addition, the sensors demonstrated fast response times (48 ms and 120 ms), as well as good resolution and long-lasting durability (>2100 cycles). As a result of the sensors, human joint flexion, finger pressure, and respiratory signals can be recognized and provided with accurate feedback.