Preparation and Characterization of Polyester Staple Yarns

Article pubs.acs.org/IECR

Preparation and Characterization of Polyester Staple Yarns Nanowrapped with Polysulfone Amide Fibers Xi Tong and Xin Bin-Jie* College of Fashion, Shanghai University of Engineering Science, Songjiang District, Shanghai 201620, P. R. China ABSTRACT: In this study, a novel approach and the related electrospinning equipment were developed to fabricate a new type of polyester staple yarn wrapped with poly(sulfone amide) (PSA) fibers at the nanoscale. As the number of tubes of spinnerets, electrospinning voltage, and rotation speed of the funnel collector have effects on the structure and properties of the nanowrapped yarns, three series of wrapped yarns were prepared using the self-developed electrospinning and wrapping equipment; these wrapped yarns at the nanoscale were characterized systematically by scanning electron microscopy (SEM), thermogravimetry (TG), and infiltration evaluation. The three-dimensional electric field of the electrospinning system used for the nanowrapped yarn was simulated to model the electric fields of different electrospinning spinnerets. Our experimental results showed that the mechanical, thermal, and hygroscopicity properties of the nanowrapped yarns fabricated in this work were clearly improved obviously in comparison with those of single polyester yarns. The nanowrapped yarn has a core−shell structure composed of both nanoscale PSA fibers and micron-scale poly(ethylene terephthalate) (PET) fibers, so that it can unite and inherit the excellent thermal properties of the PSA and the excellent mechanical properties of polyester yarns. The newly developed nanowrapped yarns have potential applications in the development of high-performance industrial textiles and apparel in the future.

1. INTRODUCTION It has been reported that a large number of yarn spinning methods can be used to prepare different kinds of yarns, such as ring spinning,1 air spinning,2 and jet spinning.3 These spinning technologies can be used to produce yarns with different properties to satisfy the diverse requirements of human activities through the use of new materials,4 the design of the yarn structure, the setting of the spinning parameters,5 and so on. However, the spinning technology of these traditional methods still remains on the micron scale to fabricate the yarns (fiber assemblies). In this case, the only solution for manufacturing functional yarns with good hygroscopicity, heat resistance, or other properties is the utilization of highperformance fiber materials. Very little research has been done to investigate the possibility of yarn spinning or assembling at the nanoscale on the basis of traditional spinning methods. Because of the high surface-to-volume ratio, special hygroscopicity, permeability, and filterability of nanofibers, these materials can be used in many kinds of functional applications.6,7 There are many reports about the preparation of nanofibers, such as stretching method, template synthesis, and electric spinning. Electrospinning is considered as the one of the most important and convenient methods for manufacturing fibers at the nanoscale, in comparison with traditional methods (such as extrusion, phase separation, or drawing). However, the mass production of nanofibers to satisfy requirements for industrial applications seems to be difficult, although some new matrix electrospinning methods have been reported. Usually, membranes of nanofibers are not strong and stretchable enough to be used as supporting fabrics. In this case, one possible solution is to spin the nanofibers onto the surface of conventional yarns to guarantee the mechanical properties of the nanowrapped yarns, combining electrostatic spinning © 2015 American Chemical Society

technology and traditional spinning technology to spin special yarns composed of nanofibers and microfibers together. Many researchers8−15 have attempted to design spinning devices to prepare yarns through the assembling of nanofibers. Hosseini Ravandi et al.16 prepared nylon 66 filament coated with nylon 66 nanofibers to research wicking phenomena. Their results showed that coating with nanofibers increases the equilibrium wicking height. Ding et al.17 reported a new approach for fabricating a superhydrophobic nanofibrous zinc oxide (ZnO) film surface through a simple electrospinning coating technology. Gogotsi and co-workers18 obtained titanium dioxide-coated nanofibers filters by electrospinning polyamide nanofibers onto the surface of a conventional filter followed by electro-spraying a suspension of nanocrystalline titaniumdioxide onto the electrospinning nanofibers. Dabirian et al.19 explored the feasibility of electrospinning nylon nanofibers onto nylon yarn, as well as poly(L-lactic acid) (PLLA) onto copper. Zhou et al.20 used single-needle electrospinning to coat nanofibers onto monofilament arrays and then twisted them into hybrid yarns to improve cohesion. Although much research has been done for the development of core-yarn nanofibers, very few works have been reported on the spinning of nanowrapped yarns composed of poly(sulfone amide) and polyester fibers. It is valuable to present a new electrospinning and wrapping method together with a related spinning system that can be used to manufacture the poly(sulfone amide) and polyester core−shell yarns. In this work, polyester, which has good tenacity but poor hygroReceived: Revised: Accepted: Published: 12303

September 21, 2015 November 20, 2015 November 24, 2015 November 24, 2015 DOI: 10.1021/acs.iecr.5b03505 Ind. Eng. Chem. Res. 2015, 54, 12303−12312

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Industrial & Engineering Chemistry Research

positive nozzles, so that electrostatic induction is present as negative charges. Under these circumstances, nanofibers electrospun from the nozzles are deposited onto the funnel collector, which also functions as a twister to wind PSA nanofibers on the surface of polyester staple yarns. Three series of samples (nine total samples in this work) were prepared using different electrospinning settings. The settings of the number of spinnerets, the electrospinning voltage, and the rotation speed of the funnel collector are listed in Table 1. In addition, three different spinnerets with various

scopicity, were wrapped with poly(sulfone amide) (PSA), which exhibits excellent thermal performance. These two materials were combined to achieve the purpose of the yarn performance optimization. The poly(ethylene terephthalate) (PET) staple yarn was used rather than the filament yarn because the surface of filament yarn is relatively smooth, which means the binding force between the PET filaments and the PSA nanofibers is relatively smaller than that between the PET staple yarn and the PSA nanofibers. Another reason is that the PET staple yarn is much more popularly used for the development of yarn products. In this article, a new type of multinozzle jet electrospinning process was proposed to fabricate continuous PSA nanofibers wrapped and twisted on the surface of the core PET yarns. The effects of the number of spinnerets, electrospinning voltage, and rotation speed of funnel collector on the yarn performance were investigated systematically and are discussed herein.

Table 1. Experimental Parametersa Used for the Electrospinning of Samples

2. EXPERIMENTS AND SIMULATIONS 2.1. Material Preparation. Poly(sulfone amide) (PSA) with an intrinsic viscosity of 2.3 dL/g was used as the spinning solution, and N,N-dimethylacetamide (DMAc) was selected as the solvent. The two materials were supplied by Shanghai Tanlon Fiber Co., Ltd. (Shanghai, China). Polyester staple yarn (32s, 100%, wound packaged) was supplied by Xiangshui Xinmao Textile Co., Ltd (Xiangshui County, China). All of these materials were used without further purification. A certain amount of PSA was dissolved in DMAc using ultrasonic vibration for 60 min at 60 °C. In our experiments, the solid concentration of the PSA solution was set to be 10 wt % to satisfy the viscosity requirements for the electrospinning process. Spinning was performed at 20 °C and 40−60% relative humidity in air. 2.2. Experimental Setup and Electrospinning Process. The electrospinning and wrapping system (as illustrated in Figure 1) consists of a grounded purpose-made funnel collector

sample ID

P1

P2

P3

S1 S2 S3 X1 X2 X3 Y1 Y2 Y3

20 25 30 25 25 25 25 25 25

1 1 1 1 2 3 3 3 3

40 40 40 40 40 40 20 40 60

a

P1 represents voltage (kV), P2 represents number of spinnerets, and P3 represents rotation speed (rpm).

Figure 2. Three different spinnerets.

numbers of tubes are shown in Figure 2. The effects of the fiber diameter and quantity of nanofibers on the yarn performance were investigated by changing the electrospinning voltage and number of spinnerets. Variations in the fiber diameters could be implemented using the different settings of electrospinning voltage; usually fibers with small diameters could be spun at high voltage because of the high electric field force. The quantity of nanofibers could be adjusted by the numbers of tubes in the spinneret used for the spinning; it can be proportionally related to the spinnerets. The PSA nanofibers could be electrospun under the electric field force generated between the spinneret and the funnel collector. A cone-like membrane could be formed and wrapped on the surface of the PET staple yarn used as the core yarn in this experiment. Finally, a twisting process is executed during the wrapping process because of the rotating of the funnel collector. Therefore, the PSA nanofibers could be assembled with the PET staple yarn continuously. It was observed that the edge angle of the cone-like membrane relative to the central line of the core yarn is proportionally related to the rotation speed of the funnel collector, as depicted in Figure 3. 2.3. Characterization. The surface morphologies of the prepared core−shell yarns were investigated by scanning

Figure 1. Schematic of nanowrapping yarn preparation.

(iron, diameter = 100 mm, angle = 30°, length = 70 mm), a high-voltage power supply [ES60P-20W/DDPM, Kansai Electronics (Suzhou) Co., Ltd., Jiangsu, China] , a syringe pump system (stainless steel spinnerets, inner diameter = 1.25 mm, length = 3 mm), and a step motor used to wind up the coated yarns. The two electrospinning nozzles were placed ahead of either side of the funnel collector, which has a squareshaped tunnel (5 mm) in the middle for polyester staple yarns to run through, and the winder system is located between the nozzles at a distance of 200 mm. The two needles of the syringes are connected to the positive electrodes of a dc power supply. The funnel collector is placed opposite to the two 12304

DOI: 10.1021/acs.iecr.5b03505 Ind. Eng. Chem. Res. 2015, 54, 12303−12312

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Figure 3. Formation of nanowrapped yarn at different rotation speeds.

Figure 4. Electric field simulation of (a1−a3) one needle, (b1−b3) two needles, and (c1−c3) three needles, showing the distributions of (a1−c1) electric field lines, (a2−c2) electric fields surrounding the needles, and (a3−c3) electric fields in the center plane.

3. RESULTS AND DISCUSSION 3.1. Electric Field Distribution. The 3D electric fields formed by the electrospinning equipment with different needles was simulated with the COMSOL Multiphysics Finite Element Analysis software. The simulations were conducted with a 20cm working distance and a 25-kV applied voltage, which were found to be workable in our previous experiments for a stable spinning process.21 Obviously, the different spinnerets have different effects on the formation of nanofibers, so that the electric fields of one-tube, two-tube, and three-tube needles were simulated, as depicted in series a−c, respectively, of Figure 4. The distributions of these electric fields are similar to each other; all of them look like inhomogeneous electric fields with an extremely high electric force region concentrated in the area surrounding the needles. It was found that two concentrated distribution areas of electric fields (marked with red lines) were generated by the needles placed on either side of the funnel collector. Both sides of the needles were combined with the positive voltage that was seriously repulsive in the middle, and the two electric field circles became separated with increasing

electron microscopy (SEM) (S-3400N, Hitachi, Tokyo, Japan) after the process of gold coating (coating time = 60 s). The mechanical behavior of electrospun core−shell yarns was evaluated with a single-yarn tensile tester (YGB021DL) at a crosshead speed of 20 mm/min at room temperature. The thermal properties of the coaxial fibers were determined by thermogravimetry (TG) and differential thermogravimetry (DTG). 2.4. Electric Field Simulation. The three-dimensional (3D) electric fields were simulated and analyzed with COMSOL Multiphysics software (COMSOL Inc., Stockholm, Sweden). The electric field intensities were calculated with COMSOL software. The physical geometries of the electrospinning setups (e.g., electrode, spinneret, and collector) and polymer solutions used for the simulation were consistent with the practical dimensions, locations, and relative permittivity used in our experiments. 12305


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Figure 5. Influence of spinning voltage on fiber morphology.

Figure 6. Influence of collector rotation speed on fiber morphology.

electric field distributions in the center between two needles are shown in images a3−c3 of Figure 4. Points A−C in Figure 4 are located in the same positions, but they exhibit various electric field intensities. 3.2. Effects of Spinning Process Parameters on Morphology. SEM images of the core−shell yarns prepared by different electrospinning voltage are shown in Figure 5. The cross sections of yarns are illustrated in the top row of images, and the surface morphologies are depicted in the bottom row of images in Figure 5. It can be clearly observed that the polyester staple yarns were wrapped closely by an outer layer comprising nanofibers of PSA. Partially magnified images of the spinneret, which are indicated by red arrows, are also illustrated. Because the diameter of the nanofibers is so small, a single fiber such as

numbers of tubes in the needles. However, all of the electric lines were linked with the funnel collector, which was used to collect PSA nanofibers surrounding the polyesters yarns. The two independent electric field circles ensure that the electrospinning process can be continuous and orderly, which is important for the wrapping of PSA fibers on the surface of polyester yarn. The distributions of electric fields near the needles are demonstrated in images a2−c2 of Figure 4. Green represents the lowest electric force, and red indicates the highest field of electric force, so it can be seen that more tubes inside the spinneret have a larger influence scope, shown in red. However, the highest forces of these electric fields are all equal to 2.5 kV, as illustrated using the magnified images. The cross sections of 12306


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Figure 7. Influence of number of spinnerets on fiber morphology.

electrospinning voltage was found not to affect the overall surface morphology, it has a significant influence on the fiber performance. 3.3. Effects of Electrospinning Parameters on Mechanical Properties. Mechanical properties are always considered to be among the most important properties for the characterization of textile materials. In this work, polyester yarns nanowrapped with PSA fibers were prepared with different sets of electrospinning parameters for the purpose of investigating the fracture mechanism and models suitable for the prepared nanowrapped yarns. It is well-known that electrospun fibers collected on a traditional plate receiver are distributed randomly or in a disorderly fashion corresponding to poor fiber alignment and orientation. To improve the mechanical strength of the nanowrapped assemblies, the bundles of PSA nanofibers were collected using a funnel receiver, so that large amounts of PSA nanofibers were clustered into one bundle or string, which was then twisted and wrapped on the surface of the polyester staple yarn. Therefore, it was possible to improve the mechanical properties of the yarn through the method of twisting and wrapping the PSA nanofibers around the polyester staple yarn. Detailed experimental data are discussed in the following sections for analysis of the results. Usually, the fiber diameter of traditional polyester yarn is in the micron range. When polyester yarn is wrapped with PSA nanofibers, the tenacity of the composite yarns can be greatly improved. In our opinion, there are two reasons for this phenomenon: (1) The combination between the polyester staple yarns and the PSA nanofibers contributes to the prevention of breakage, and the overall tenacity should be equal or close to the sum of the tenacities of the two components. Compared with micron-scale fibers, the diameter of the PSA nanofibers is much smaller; the nanoscale PSA fibers can thus fill the micropores inside or on the surfaces of polyester staple yarns, as illustrated by points A and B in Figure 8. (2) The quantity and degree of twisting of the PSA nanofibers have obvious effects on the tenacity of the

polyester can be impossible to distinguish. However, some micropores could be observed at the interface owing to the irregular shape of the core yarn. Moreover, the micropores surrounding polyester were filled by PSA nanofibers. The composite yarns prepared at the same rotation speed had similar twists, as can be seen from the surface images. SEM images of nanofiber core-spun yarns prepared at different funnel rotation speeds are shown in Figure 6. From the yarn cross sections, it can be seen that the amounts of external-layer nanofibers corresponding to the three types of yarn are different, which is because, in a certain time, more nanofibers could be wrapped at the same point at a higher rotation speed. The structure of the nanowrapped yarn is loose when there are few PSA nanofibers, which can be clearly seen from the partially magnified images. The aggregation and twisting of the nanofibers on the core-spun yarns relies on the grounded metal funnel; thus, the twist levels of the nanofiber core-spun yarns are dependent of the funnel rotation speed. It can be seen from the angles of the nanowrapped nanofibers that an increase in the rotation speed of the twisting unit can cause a reduction in the nanofiber axial alignment. No twist could be achieved at 20 rpm, whereas the twisting was obvious when the rotation speed was 60 rpm; this means that the twist level of the core-spun yarns increased with increasing funnel rotation speed. In addition, at lower rotational speeds, the coating appeared denser, as shown in Figure 6, which conforms with Dabirian et al.’s experiments.22 The effects of the number of tubes in the spinneret on the fiber morphology, which mainly affects the quantity of nanofibers, are shown in Figure 7, and the differences among the images can also been detected. The degree of compactness differs for different amounts of PSA nanofibers. More nanofibers can form a better morphology, as shown in Figure 7. When the nanofibers were prepared with a one-tube needle, a loose structure of PSA nanofibers was wrapped on the polyester staple yarns. From the preceding discussion, the quality of the yarn formed was better when it was prepared with a three-tube spinneret and at 60 rpm. Although the 12307


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The average tenacity and its variance are illustrated in Figure 10. It was found that the variance of the tenacity of the prepared nanowrapped yarns increased with increasing funnel rotation speed; the reason for this trend might be structural inhomogeneities of the prepared nanowrapped yarns caused by instabilities in the spinning. As indicated in Table3, it was found that PET staple yarns wrapped with PSA nanofibers have high tenacities and large initial moduli; however, further research should be performed to investigate breaking elongation because the variation in breaking elongation with increasing tenacity is not obvious. The elongation was neither the longest nor the shortest for the nanowrapped yarn with the maximum tenacity. The breaking elongation was found to have no positive or negative correlation with the tenacity. 3.3.3. Number of Spinnerets. Spinnerets are very important for the electrospinning of nanofibers because spinnerets can affect the physical properties of nanofibers. Needles are usually used as spinnerets in the laboratory. One-, two-, and three-tube needles were used in this work to investigate the effects of the quantity of nanofibers on the mechanical properties of the nanowrapped yarn; all tubes inside the needles had the same diameter. It was found that the number of tubes inside the needle (spinneret) has a positive correlation with the tenacity of the nanowrapped yarn. The variance of the tenacity tended to increase as well, as shown Figure 11. However, the stress−strain curve in red in Figure 11 was not as smooth as the other three curves when the elongation was up to 7 mm, and breaking instability could be observed because of the asynchronous breaking of too many nanofibers. Compared with the effects of voltage and rotation speed, the effects of the spinneret appear to be obvious and significant. One reason is that a spinneret containing more tubes has the ability to produce more nanofibers; another is that more nanofibers can be wrapped onto the surface of the polyester staple yarn tightly. The elongations and breaking times are listed in Table 4. Neither of these parameters do not have exhibit significant changes compared with the data for pure PET staple yarns. 3.4. Effects of Spinning Parameters on Thermal Properties. Thermal properties are investigated widely for the purpose of developing functional textile products, such as flame-retardant yarns or fabrics. The characterization of a textile’s thermal properties is also necessary for quality control

Figure 8. Model of the wrapped yarn cross section.

nanowrapped yarns. Large quantities and high degrees of twisting can increase the overall tenacity without changing other parameters, and this is also correct regarding the reverse situation. 3.3.1. Voltage for Electrospinning. It was found that the voltage for electrospinning had no obvious effects on the tenacity of the nanowrapped yarn, as shown in Figure 9. It is well-known that the diameters of nanofibers can be adjusted by changing the voltage for electrospinning; specifically, fine fibers can be electrospun at high voltages. If the voltage for electrospinning has no obvious effects on the tenacity of the nanowrapped yarn, then the diameter variation of the nanofibers is not significant enough to affect its tenacity. The breaking elongations, initial moduli, and breaking times of composite yarns are listed in Table 2. These results indicate that there is no significant difference among the composite yarns electrospun at different voltages, whereas the differences between composite yarn and pure polyester yarn are obvious. 3.3.2. Rotation Speed of Funnel Collector. In this work, a funnel-shaped collector was used to collect PSA nanofibers and wrap them around the polyester. Therefore, the rotation speed of the funnel collector should have a significance influence on the twisting of the PSA nanofibers, which has the effect of improving the mechanical properties of the nanowrapped yarn. Typical stress−strain curves of nanowrapped yarns and pure PET yarn are presented in Figure 10. The experimental results indicate that the tenacity of the nanowrapped yarn increases with increasing number of twists at the same elongation. The circled region labeled A indicates the yielding points of nanowrapped yarns spun at different rotation speeds. It can be concluded from this figure that the stress and strain can be improved at a high number of twists. Wu and Qin reported in a similar work that the mechanical properties of yarn can be improved further by increasing the twisting of the yarn.23

Figure 9. (Left) Tensile curves and (right) average tenacities of nanowrapped yarns electrospun at different voltages. 12308


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Industrial & Engineering Chemistry Research Table 2. Effects of Electrospinning Voltage on Yarn Mechanical Properties sample ID

tenacity (cN/dtex)

elongation (mm)

pure PET S1 (20 kV) S2 (25 kV) S3 (30 kV)

73790 ± 2005 100240 ± 5909 98130 ± 4663 97370 ± 5525

9.40 ± 0.71 9.55 ± 0.72 9.05 ± 0.69 10.25 ± 0.74

initial modulus (cN/dtex) 2980 3740 3520 3820

± ± ± ±

breaking time (s)

125 136 135 136

1.12 1.15 1.09 1.23

± ± ± ±

0.13 0.13 0.12 0.15

Figure 10. (Left) Tensile curves and (right) average tenacities of nanowrapped yarns electrospun at different rotation speeds.

Table 3. Effects of Funnel Rotation Speed on Yarn Mechanical Properties sample ID

tenacity (cN/dtex)

elongation (mm)

pure PET Y1 (20 rpm) Y2 (40 rpm) Y3 (60 rpm)

73790 ± 2005 92710 ± 2123 98850 ± 3124 116140 ± 13038

9.40 ± 0.71 9.70 ± 0.71 10.25 ± 0.74 9.40 ± 0.70


± ± ± ±

125 85 127 148

breaking time (s) 1.12 1.24 1.23 1.13

± ± ± ±

0.13 0.16 0.16 0.15

Figure 11. (Left) Tensile curves and (right) average tenacities of nanowrapped yarns electrospun with different numbers of tubes.

Table 4. Effects of Number of Tubes on Yarn Mechanical Properties sample ID

tenacity (cN/dtex)

pure PET X1 (one tube) X2 (two tubes) X3 (three tubes)

73790 ± 2005 84680 ± 2638 103240 ± 8030 115920 ± 13541

elongation (mm) 9.40 8.50 9.15 9.40

± ± ± ±

0.71 0.69 0.70 0.71

starting from fiber assembly to the final commercial products. Thermal analysis is one method for determining the relation-


± ± ± ±

125 126 134 134

breaking time (s) 1.12 1.02 1.15 1.13

± ± ± ±

0.13 0.12 0.13 0.13

ship between a material’s quality and temperature under programmed temperature control. In this work, the thermal 12309


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Figure 12. (Left) TG and (right) DTG curves at different voltages.

Figure 13. (Left) TG and (right) DTG curves for different numbers of spinnerets.

Figure 14. (Left) TG and (right) DTG curves at different speeds.

properties and that it can be used for the outer wrapping nanofibers to protect the core polyester yarns.24 There are two stages of weight loss in the burning of polyester yarns, as depicted in Figure 12. The temperature range of the first stage is 100−200 °C, and that of the second phase is 400−500 °C. It can be seen that there was no obvious

properties of the nanowrapped yarns (PET as core yarn, PSA as wrapped fibers) were investigated using the typical thermal analysis method; the pure PET staple yarn was also characterized for the comparison with the newly developed yarns. It was previously reported that PSA has excellent thermal 12310


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Figure 15. Height of the hygroscopic climb for yarns prepared at different voltages (in each panel, from left to right: pure PET, S1, S2, and S3).

Figure 16. Height of the hygroscopic climb for yarns prepared with different numbers of needles (in each panel, from left to right: pure PET, X1, X2, and X3).

Figure 17. Height of the hygroscopic climb for yarns prepared at different rotation speeds (in each panel, from left to right: pure PET, Y1, Y2, and Y3).

decomposition peak between 100 and 200 °C and the curve variance was small. It can be assumed that macromolecules started to slip and crimp, and micromolecules started to break down, which means that polyester was damaged to a limited extent. Polyester usually start to decompose at 400 °C, and its maximum decomposition rate occurs at 450 °C. When PSA nanofibers were wrapped onto the polyester yarn, the residual rates of the nanowrapped yarns were larger than that of pure polyester, as shown in their TG curves. However, there were no significant differences among the nanowrapped yarns electrospun at different voltages, meaning that the diameter of the nanofibers had no obvious effects on the thermal properties of the nanowrapped yarns. The DTG curves of the nanowrapped yarns exhibited small vibrations between 100 and 200 °C, which supports the conclusion that PSA provides protection to the overall nanowrapped yarn and confirms that it is workable to use PSA as the wrapping fibers to protect PET core yarn. The starting point of thermal decomposition for PSA is about 450 °C, which is also the temperature of the highest decomposition rate of polyester, so the peak point of the combined yarns is not changed very much. Different numbers of spinnerets have obvious effects on the thermal properties of nanowrapped yarn, as illustrated in Figure 13. Large numbers of PSA nanofibers can be produced with increasing numbers of spinnerets, so the protection is much better in terms of the thermal properties. With increasing numbers of tubes, the residual rate of the nanowrapped yarn is higher. Point A in the DTG curve in Figure 13 indicates that the PSA macromolecule was broken. A shift of the peak point

corresponding to macromolecular decomposition was observed with increasing amount of PSA nanofibers. The trend of the TG curves in Figure 14 is similar to that of the curves in Figure13. The top lines of DTG curves in Figures 13 and 14 seem to be flatter than the bottom lines because more nanofibers were wrapped onto the polyester staple yarns when increasing numbers of tubes in the needle and increasing rotation speeds were used. PSA nanofibers can be used to prevent the PET staple yarn from burning more effectively during TG measurements. Therefore, the flatter lines indicate that the thermal decomposition rate has a decreasing trend. However, the lines in Figure 12 are very sharp because the quantity of PSA nanofibers was not sufficient when only the one-tube spinneret was used. However, there is no significant distinction between the curves for 40 and 60 rpm, meaning that the rotation speed can affect the twisting of the nanofibers only in a certain range. 3.5. Effects of Spinning Parameters on Hygroscopicity. Hygroscopicity is an important physical properties considered in our research; it can allow materials to be waterabsorbable, breathable, and antistatic. Nanofibers have excellent hygroscopicity because of their nanoscaled structure. Traditional polyester staple yarn has poor hygroscopicity; one solution to improve or optimize its hygroscopicity is to blend or wrap it with functional nanofibers with good hygroscopicity. The climbing height of water is a parameter used to evaluate the hygroscopic properties of textile materials. The climbing height of water was found to increase with increasing number of needles (spinnerets) and voltage, as shown in Figures 15 and 16. Increasing the number of spinnerets can be used to improve the quantity of nanofibers, as 12311


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discussed before; high voltage is selected to electrospin nanofibers with smaller diameters. However, the effect of twisting is too complex to be described in a positive or negative correlation model, which means that the hygroscopicity of yarns does not increase with more twisting. Hygroscopicity did not continuously increase with increasing twists, but reached a threshold value after which it started to decrease, as shown in Figure 17. When the rotation speed was 60 rpm, the hygroscopicity of yarns fell. It was found that excessive twisting reduces hygroscopicity as shown in Figure17.

4. CONCLUSIONS Polyester staple yarns nanowrapped with PSA fibers have been fabricated using different spinnerets, voltages, and rotation speeds on self-developed electrospinning and wrapping equipment. The morphology, mechanical and thermal properties, and hygroscopicity of the resulting composite yarns were characterized systematically. The three-dimensional (3D) electric field of the electrospinning system used for the nanowrapped yarn was simulated to model the electric fields of different electrospinning spinnerets. The electric field distribution produced by three-tube needles was wider and could be used to prepare more nanofibers wrapped onto the polyester staple yarn. As a result, more compact and functional yarns could be made using this method. Polyester yarns wrapped with PSA nanofibers provide an obvious improvement in terms of mechanical and thermal properties, because the PSA nanofibers can protect the inside polyester staple yarn from decomposing at high temperature. PSA nanofibers can also be used to improve the hygroscopicity of nanowrapped yarns. However, the twisting of the outer nanofibers is too high to reduce its hygroscopicity reversely. The thermal properties of PSA and the mechanical properties of polyester yarns can be combined through the method of electrospinning and wrapping. The resulting nanowrapped yarn has a core−shell structure composed of both nanoscale PSA fibers and micron-scale PET fibers.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Preparation and Characterization of Polyester Staple Yarns

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