Yarn Forming Mechanism And Experiments Of Electrospinning Nanofiber Yarns By Airflow Twisting

Nano-effect gives nanofibers excellent properties including high specific surface area,small size which makes them have potential applications used as high performance materials,such as filters and tissue engineering scaffold and so on. Orientated nanofibers possessexcellent optical and electrical properties which make them have potential applications usedas Lithium ion battery materials and biomedical materials. Additionally, nanofiber yarnsimprove the mechanical properties of nanofiber bundle which make them easily to befabricated using knitting and weaving method which contribute to integrating them into textilemarket more easily. This thought presents a significant meaning for upgrading thetechnological content of textile products and expanding the application fields of nanofibers.At present, problems exist in fabrication of electrospinning nanofiber yarns usually includes:(1) less nanofiber source leads to nanofibers difficult to be bundled continuously and stably inthe yarns fabrication process;(2) lower strength and larger surface force make nanofiberbundle difficult to be twisted. In this paper, a kind of easily assembled and closedelectrospinning unit which could improve nanofibers productivity was design on the principleof bubble electrospinning. Based on the mechanism of conjugated electrpspinning, a kind ofmulti-nozzle conjugated electrospinning device was designed for preparing continuous andbatches nanofiber bundle. In order to overcome difficulties in yarns twisting process,including low strength and large surface force of nanofiber bundle resulting adhesion withmachinery, an airflow twister was designed for preparing continuous nanofiber yarn andnanofiber core spun yarn. The main conclusions are as follows:(1) Influences of positive electrode structure of the spinning unit and processingparameters during the electrospinning on nanofibers fabrication were investigated. Usingsimulation software of Maxwell, the electric field distribution with different positive electrodestructure was simulated. Simulation results demonstrate that, with one sixth of the nozzlecharged, the electric field distribution is the most uniform which resulted in the most uniformdistribution of fibers diameter and the lowest fibers coefficient variation (CV%) of15%inthe experimental range. Therefore, this positive electrode is considered as the optimal positiveelectrode. Results show that the lowest fibers CV%achieved when the applied voltage is32kV, spinning distance is18cm, air flow rate is800mL/min, and solution flow rate is8mL/h.Therefore, these processing parameters are considered as the optimal parameters forelectrospinning with this spinning unit. Simulation results indicate that the electric strength onnozzles is stronger with the most uniform electric field distribution at nozzles distance of55mm. Nanofibers microstructure produced by double-nozzle electrospinning device at differentnozzles distance were investigated. Results show that nanofibers reach to the lowest CV%of18%when nozzles distance is55mm. Therefore, nozzles distance of55mm is considered asthe optimal nozzles distance for double-nozzle electrospinning.(2) According to the mechanism of conjugated electrospinning, a multi-nozzleconjugated electrpspinning device was designed to fabricate continuous and batches nanofiberbundle. Using simulation software of Maxwell, the electric field distribution in the electrospinning area of multi-nozzle conjugated electrpspinning was simulated. According tothe mechanism of electrostatic induction and conjugated electrospinning, the basic conditionsof multi-nozzle conjugated electrospinning were analyzed. Furthermore, simulation resultsdemonstrate that the optimal arrangement achieve for this multi-nozzle conjugatedelectrpspinning device when the horizontal angle of positive nozzle is45°, the horizontalangle of the negative nozzle is55°, the distance between the positive and negative nozzles is19cm, the distance between the nozzles and nanofibers collecting device is4cm. Influencesof processing parameters of multi-nozzle conjugated electrospinning on nanofibersmorphology, nanofiber bundle clustering parameters were investigated. Results indicate thatnanofibers diameter and fibers CV%reach to the lowest value at the applied voltage of35kV.At the applied voltage of34kV, the cluster rate and orientation degree of nanofiber bundlereach to the highest level of77%and0.97. At the same applied voltage, the nanofiber bundlediameter and nanofibers productivity also reach to the highest level of360?m and2.9g/h.When nozzles total flow rate of solution is28mL/h, the fibers diameter and fibers CV%reachto the lowest level. When nozzles total flow rate of solution is32mL/h, the cluster rate ofnanofiber bundle reach to the highest level of82%. At the same total flow rate of solution, thenanofiber bundle diameter and nanofibers productivity also reach to the highest level of365?m and3.2g/h. When the air flow rate is1200mL/min, the fibers diameter and fibers CV%reach to the lowest level, and the cluster rate of the nanofiber bundle reach to the highest levelof70%. At the same air flow rate, the nanofiber bundle diameter and nanofibers productivityalso reach to the highest level of368?m and3.2g/h. At the air flow rate of1300mL/min, theorientation degree of nanofibers bundle reaches to the highest level of0.93.(3) Based on the mechanism of airflow twisting, an airflow twister (nozzle) was designedfor twisting nanofiber bundle. Based on establishing the fluid mechanics model of airflowtwisting, numerical simulations of the airflow movement in the nozzle was simulated usingsoftware of Fluent. Results indicate that the airflow movement in the nozzle conforms to themovement characteristics of turbulence airflow. Therefore, the principle of airflow twistingnanofiber yarn was proposed. According to the simulation results of air movementcharacteristics, the structural parameters of nozzle were optimized. Results demonstrate thatspray angle of45°and yarn path diameter of1mm is the best structural parameters. Influenceof different air pressure of the nozzle on microstructure and mechanical properties of PAN(Polyacrylonitrile) nanofiber yarns was also investigated. Results show that when the airpressure of nozzle is0.4Mpa, the axial velocity of airflow on the plane of spray holes outletin radial cross section of yarn path is the fastest leading to the highest spinning efficiency. Theradial velocity of airflow on the plane of spray holes outlet in radial cross section of yarn pathpresents nonaxisymmetrical distribution. At this air pressure of nozzle, the radial velocity ofairflow is suitable for twisting nanofiber bundle. When the air pressure of nozzle increasefrom0.2Map to0.4Map, the twisting angle of nanofiber yarns increase from51.5°to79.9°,the yarns diameter and yarns hairiness decrease at the same time. When the air pressure ofnozzle is0.4Mpa, the breaking stress and breaking elongation of nanofiber yarns reach to thehighest level of94.2Mpa and101.6%in the experimental range. These conclusions areidentified with the airflow simulation results. (4) The method for twisting nanofiber core spun yarn by airflow is proposed. Based onestablishing fluid mechanics model of airflow twisting, numerical simulations of the airflowmovement in the nozzle are simulated to understand the movement characteristics of vortexairflow. As a result, the mechanism of airflow twisting for preparing nanofiber core spun yarnis proposed. According to the simulation results of airflow movement characteristics, thestructural parameters of nozzle were optimized. Results demonstrate that spray angle of45°and yarn path diameter of2mm is the best structural parameters for the nozzle. Influences ofdifferent air pressure of nozzle on yarns microstructure, core spun yarns mechanicalproperties and coated mass ratio of nanofibers were investigated. Results indicate that thebreaking stress of core spun yarn (larger than150Mpa) is stronger than the breaking stress ofnanofiber bundle (10Mpa) and core yarn (120Mpa). When the air pressure of nozzle is lowerthan0.4Mpa, the twisting angle of nanofiber core spun yarn increase, and the yarns averagediameter decrease with the increasing of air pressure of nozzle. When the air pressure ofnozzle is0.4Mpa, the coated mass ratio of nanofibers reaches to the highest level of81.7%.With the air pressure of nozzle of0.4Mpa, the breaking stress and breaking elongation of thenanofiber core spun yarn increase respectively to the highest level of178.52Mpa and28.86%in the experimental range. These conclusions are identified with the airflow simulation results.When the air pressure of nozzle is0.5Mpa, the breaking stress and breaking elongation ofnanofiber core spun yarn begin to decrease to162.82Mpa and24.36%.