| Nanofibers, due to their ultrahigh specific surface area, unique surface properties and excellent mechanical properties, have great potential in application of textile, filtration,energy, bio-medical, sensor and protection areas, and have become the hotspot of research.The development of smart or functional textiles with 3D application of nanofibers has become the developing trend. Electrospun nanofiber filaments, as the key raw material of producing 3D nanofiber products, have great significance in expanding the application of nanofibers. However, the low yield and weak mechanical properties have limited the development and application of nanofiber products. Thus, this paper aims to figure out the stable and continuous forming mechanism of nanofiber filaments using multi-needle liquid-bath electrospinning method, to develop a continuous production method of nanofiber filaments with high yield, to establish a strength-enhancement system of nanofiber filaments to realize the continuous production of high strength nanofiber yarns and process into nanofiber textiles, providing a theoretical and experimental background for the research and development of 3D nanofiber products.In this paper, polyamide 6(PA6) nanofiber filaments were prepared by multi-needle liquid-bath electrospinning device and the influence of needle arrangement and number on structures and properties of nanofiber filaments was discussed. Through research on the electrostatic field of multi-needle electrospinning, the needle arrangement and number could affect the electrostatic field interference and further affect structures and properties of nanofiber filaments. Electrostatic field interference theory and Ansoft Maxwell electrostatic field simulation software were used to quantify the electrostatic field interference among needles, which was used to quantificationally analyze the influence of needle arrangement and number on structures and properties of nanofiber filaments. The results showed that the average offset of electrostatic field could quantify the electrostatic field interference among needles, of which the larger offset caused larger electrostatic fieldinterference. With the same needle number, increasing the electrostatic field interference by changing the needle arrangement, the nanofiber diameter almost appeared a linear increasing trend while the alignment degree of nanofibers and the breaking stress of filaments almost appeared a linear decrease. With increasing the needle number, the electrostatic field interference increased, the average nanofiber diameter almost appeared a linear increase, and the alignment degree almost appeared a linear decrease, while breaking stress was not too much affected.High electrostatic field interference could affect the continuity and stability of nanofiber filament production. To decrease the electrostatic field interference, auxiliary electrode was added to the multi-needle liquid-bath electrospinning device and the relationship between auxiliary electrode parameters and alignment degree and yield of nanofiber filaments were studied. Meanwhile, though research on nanofiber forming and bundling process, the continuous and stable forming mechanism of nanofiber filaments using multi-needle liquid-bath electrospinning device was elucidated. The results showed that the auxiliary electrode could increase the alignment degree and yield of nanofiber filaments with the optimum auxiliary electrode parameters of a rounded rectangular shape,a size of 100×60 mm, a height of 17.5 mm, and a voltage of 22 kV. Compared with the filaments spun without an auxiliary electrode, the productivity of the filaments spun with the rounded rectangular auxiliary electrode increased by 14.3%. The nanofiber diameter decreased by 9.9%, while the alignment degree increased by 24.8%. The crystallinity, glass transition temperature and the melting temperature increased by 9.2%, 4.0% and 0.7%respectively. The breaking stress of nanofiber filaments and single nanofiber increased by33.3% and 114.7% respectively, and their initial modulus were nearly doubled, while the breaking elongation decreased by 12.5% and 22.1%, respectively.The stable and continuous forming mechanism of nanofiber filaments using multi-needle liquid-bath electrospinning could be explained as: polymer jet flows were formed in multiple needles under high-voltage electrostatic field; auxiliary electrode was used to decrease the electrostatic field interference in order to decrease the offset of jet flow in the motion and meanwhile decreased the instability of Taylor cone and jet flow. A good jet flow was maintained with less nanofiber loss, and eventually nanofibers with good formation were deposited in the liquid-bath. With further wet processing, the nanofibers possessed a skin-core structure with a tight skin and soft core structure. Then under winding force and liquid resistance, flake-like fiber aggregates started to bundle, andformed thin wet nanofiber filament in the edge of liquid-bath reservoir through bundling triangular zone. With drying device, dry nanofiber filament was formed and finally winded onto the winding roller to form the nanofiber filament.Post-treatment methods like post-drawing, plying, twisting and twist-setting were used to increase the mechanical property of nanofiber filaments, their influence on structures and properties of nanofiber filaments were studied and an effective post-treatment system was developed. The results showed that the nanofiber diameter was decreased by post-drawing, bent and hooked fibers became less, while the alignment degree of the nanofiber filament increased, as well as the fiber crystallinity, glass transition temperature, melting temperature and enthalpy value. Thus, the initial modulus and breaking stress of the nanofiber filament increased while breaking elongation decreased.Twist-setting gave better effect with increasing time and temperature and the breaking stress increased as well. However, with twist-setting temperature higher than 80?C and time exceeding 30 min, the breaking stress of nanofiber yarns decreased. The results showed that the optimum twist-setting temperature and time were determined to be 90 ?C and 30 min, respectively. When the twist of the yarn was 2500 tpm, with more filaments plied into a yarn, the yarn diameter increased, and the yarn-diameter uniformity improved,as well as the alignment degree of nanofibers along the twist direction(ADT), but the nanofiber diameter decreased. The breaking stress and initial modulus increased when the number of plies increased from 1 to 4, but when more than four filaments were plied into a yarn, both of them decreased, while the breaking strain increased. When the ply of the yarn was 4, with increasing yarn twists, both nanofiber and yarn diameters decreased, but the ADT increased. The breaking stress and initial modulus initially increased as the twists increased to 2500 twists per meter before decreasing, while the breaking strain kept increasing. Therefore, nanofiber yarn processed with 2500 twists and plies of 4(4-2500yarn) gave the optimum breaking stress, which could reach 2.57 times of that of the as-spun nanofiber filament.Four 4-2500 yarns were plied and sized to increase the weavability of nanofiber yarns.Weaving and knitting methods were used to prepare nanofiber textiles, of which the structures and properties were studied. The results showed that the sizing treatment increased the initial modulus, breaking stress and breaking strain of nanofiber yarns, while the breaking asynchronism was improved. So sizing treatment significantly increased the abrasion resistance and weavability of nanofiber yarns.For woven nanofiber fabric(WNPA), of which the specific surface area was larger than that of the reference fabric; the breaking stress, work of rupture and specific work of rupture in warp direction were superior to those in weft direction, while the breaking elongation was an exception. Compared with reference fabrics, WNPA had higher initial moisture absorption rate, equilibrium moisture regain, initial wicking rate and wicking height, wetting rate of liquid droplets, adsorbing capacity and its initial increasing rate,while lower K/S value and liquid droplet spreading time.For knitted nanofiber fabric(KNPA), of which the specific surface area was significantly larger than that of the reference fabric; the breaking stress, work of rupture and specific work of rupture in horizontal and vertical direction didn’t differ much.Compared with reference fabrics, KNPA had higher initial moisture absorption rate,equilibrium moisture regain, initial wicking rate and wicking height, wetting rate of liquid droplets, adsorbing capacity and its initial increasing rate, while lower liquid droplet spreading time and K/S value.When comparing WNPA and KNPA, the specific surface area, specific work of rupture, initial moisture absorption rate, initial wicking rate and wicking height, wetting rate of liquid droplets, adsorbing capacity and its initial increasing rate of KNPA were all superior to those of WNPA. The BET value of KNPA was 8.151m2/g, the wicking height reached 109.0mm, the spreading rate of liquid droplets reached 20.202μL/s and the adsorbing capacity of KNPA could reach 15.55mg/g at 90°C. Therefore, nanofiber fabrics possess relatively larger specific surface area, excellent moisture absorption, wetting and wicking properties and remarkable adsorption capacity at the same time, laying the foundation of the afterward development of functional textile fabric. |
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