Preparation of high-performance microfiber synthetic leather base using thermoplastic polyurethane/sulfonated polysulfone electrospun nanofibers

Abstract

Compared to natural leather, microfiber synthetic leather has many excellent qualities, such as chemical resistance and physical and mechanical properties. However, preparation of microfiber synthetic leather with a high water vapor transmission rate (WVT), moisture absorption and wearing comfort property is still a challenge. In this study, we prepared thermoplastic polyurethane (TPU)/sulfonated polysulfone (SPSf) electrospun nanofibers and applied them to a microfiber synthetic leather base (MSLB). The effects of TPU/SPSf nanofiber content on the structure and properties of the MSLB were investigated. The results indicated that the TPU/SPSf nanofibers with an average diameter of 0.12 µm were well distributed at all directions in the MSLB. Differential scanning calorimetry analysis showed four T_g peaks, further demonstrating the existence of TPU/SPSf nanofibers. With the increase of TPU/SPSf nanofiber content from 0 to 30 wt%, the contact angles decreased gradually from 111.64° to 67.07°, leading to 55.19% improvement in the WVT value (from 2868.96 to 4452.24 g/(m²•24 h)) and 26.25% improvement in the moisture absorption (from 628.70% to 793.75% mm/s). Simultaneously, when the nanofiber content was 30 wt%, the nanofibers tended to bundle and 6.79% decrement of air permeability was observed. Specifically, the softness of the MSLB was improved by 88.55%. Moreover, the thermal stability and the tear strength were also obviously enhanced. Consequently, this research provided a feasible and promising way to prepare a high-performance MSLB using TPU/SPSf nanofibers.

Keywords

microfiber synthetic leather base sulfonated polysulfone nanofiber water vapor transmission rate moisture absorption softness

Microfiber synthetic leather as the third generation artificial leather is a composite material composed of a microfiber synthetic leather base (MSLB) and polyurethane (PU) elastomer. The MSLB is mainly made up of microfiber nonwoven fabric and PU, and the microfiber is mostly made up of polyester (PET) or polyamide (PA6) through blend spinning. Due to the existence of microfiber in the MSLB, microfiber synthetic leather has many excellent qualities, such as chemical resistance and physical and mechanical properties, which makes it an ideal substitute for natural leather. However, it is also known that microfiber synthetic leather suffers from two major hurdles, limiting its further development: a low water vapor transmission rate (WVT), moisture absorption and wearing comfort property resulting from the hydrophobic nature of microfiber, and large fiber fineness decided by its spinning technology.¹

Up to now, many modifications, including acid hydrolysis,² enzyme hydrolysis,³ hydrophilic polymer blending⁴ and grafting,^1,5–8 have been applied to increase the active groups of microfiber in the MSLB. Among these methods, the effect of hydrolysis was proved to be the weakest and the WVT was improved by 32% compared with the untreated MSLB. What is more, the mechanical properties of the MSLB declined obviously. Hydrophilic polymer blending was an effective method put forward by Qiang.⁴ By filling a hyperbranched polymer with terminal hydroxyl in the MSLB, the WVT was greatly increased by 35%. In recent years, a method of grafting hydrophilic polymer on the PA6 microfiber of the MSLB has been proposed for improving the WVT and moisture absorption, and this has become a research hot topic in this field. Qiang et al.¹ modified the surface of a sulfuric acid pretreated a MSLB by collagen in which the organic phosphine FP was used as a cross-linking agent for the purpose of increasing the active groups and improving its properties. The results showed that the WVT of the modified base increased by 65% and the moisture absorption increased by 181%. All the researches above have proved that increasing the fiber active group in the MSLB can effectively improve the WVT of microfiber synthetic leather. However, all the methods have focused on the modification of the MSLB, and little attention has been paid to preparing the MSLB with high WVT through modifying the hydrophilicity of the spinning materials. Sulfonated polysulfone (SPSf) has excellent hydrophilicity due to the presence of sulfonic acid groups and is widely used for membrane materials.^9–11 However, there is still dearth of applications of SPSf in fabricating microfibers for the MSLB.

In addition, it is well known that the fiber fineness of the MSLB has a significant effect on the performance of microfiber synthetic leather as well as the morphology. At present, three kinds of microfibers are usually used: direct spinning fibers (diameter 4.8 µm); sea-island fibers (diameter 0.2 µm); and segmented pie fibers (diameter 2.0 µm).^12,13 According to the literature, collagen fiber (diameter 200 µm and a length of several millimeters) of natural leather is composed of 30–300 monofilaments (diameter 5 µm), which are built from 200–1000 fibrils (diameter 0.1 µm).¹⁴ Benefiting from the smaller diameter close to the fibrils of natural leather, sea-island fibers have attracted much more attention and are widely used in the MSLB. However, in the postprocessing step for the MSLB, it is necessary to remove the sea component using toluene (unfigured sea-island fibers) or sodium hydroxide (figured sea-island fibers), which brings serious environmental pollution and waste of resources.¹⁵ In contrast to the sea-island fibers, segmented pie fibers are usually split by mechanical treatment (needle punching, hydroentangling, ultrasonic, stretching) and have several advantages, such as high mechanical properties, being non-polluting and being easy to process; however, their nonwoven fabric still suffers from low permeability, poor tear resistance and hard softness, since the split fibers have relatively large diameter, filament structure and wedge cross-section, tending to pack tightly. To date, with the development of spinning technology, electrostatic spinning is considered as the most effective method for fabricating nanofibers with a diameter of <100 nm. Many polymeric nanofibers have been prepared and used in many fields, such as polysulfone (PSU),^16,17 thermoplastic polyurethane (TPU),^18,19 polyacrylonitrile (PAN),^20,21 etc. Among these, TPU nanofibers also have good elastomeric properties, benefiting to increase the softness of nonwovens. Considering that the diameter of nanofibers is closest to the fibrils of natural leather, blending the nanofibers to segment the pie MSLB could be an effective strategy for achieving promising performance of the MSLB.

In order to circumvent the two drawbacks of microfiber synthetic leather, the aim of the present work is to prepare the MSLB with TPU/SPSf electrospun nanofibers, while the WVT and moisture absorption could be enhanced thorough increasing fiber active groups, and the wearing comfort property could be improved by decreasing the fiber diameter. To the best of our knowledge, this is the first example of nanofibers applied to the MSLB. The effect of TPU/SPSf nanofiber content on the resulting MSLB morphology and performance was systematically investigated.

Experimental details

Materials

PET (FC510) with a density of 1.38 g/cm³ was purchased from Yizheng Chemical Fiber Co. Ltd, China. PA6 (1013B) with a density of 1.15 g/cm³ was received from UBE Co. Ltd, Japan. TPU (Estane X-595A-11) with a density of 1.22 g/cm³ was obtained from Lubrizol Corporation, USA. SPSf (25% sulfonation degree and 55 ml/g the intrinsic viscosity) was supplied by Tianjin Yanjin Technology Co. Ltd, China. N,N-dimethylacetamide (DMAc) as a solvent was provided by Tianjin Kermel Chemical Reagents Co. Ltd, China.

Preparation of PET/PA6 MSLB with TPU/SPSf nanofibers

The PET/PA6 MSLB with TPU/SPSf nanofibers was prepared by a four-step process. Firstly, the PET/PA6 hollow segment pie bicomponent fibers were produced by spunbonding according to Zhang et al.²² and then cut to 50 mm. The polymer ratio of PET/PA6 was 70%/30%. Secondly, TPU/SPSf nanofibers were made by electrospinning. The spinning solution was prepared by dissolving TPU (15 wt%) and SPSf (5 wt%) in DMAc with vigorous stirring over 10 h and then transferring into a syringe. The solution was pumped to the needle by a syringe infusion pump at a flow rate of 0.8 ml/h. High voltages of 18.5 kV were applied to the needle during electrospinning. The outer diameter of the needle was 0.9 mm and the length was 13 mm. The elastomeric TPU/SPSf nanofibers were received on the PET/PA6 hollow segment pie bicomponent fibers at selected distances of 20 cm with continuous manual mixing. All experiments were carried out at ambient temperature with a relative humidity of approximately 40%. The mixed ratios can be seen in Table 1. Thirdly, the mixed fibers were combed into fiber wets with a specific weight of 160 g/m². Lastly, the fiber wets were split and bonded by hydroentanglement using a water jet. Sixteen jets were used and the water jet pressure was 10 MPa.
Table 1.
Mixed ratios of polyester (PET)/polyamide (PA6) microfiber and thermoplastic polyurethane (TPU)/sulfonated polysulfone (SPSf) nanofiber in the microfiber synthetic leather base (MSLB)

MSLB PET/PA6 microfiber (g) TPU/SPSf nanofiber (g)

M0 16.00 0

M5 15.20 0.80

M10 14.40 1.60

M20 12.80 3.20

M30 11.20 4.80

Characterization

Morphology

The morphologies of the MSLB were observed by tabletop scanning electron microscopy (SEM, TM 3030, Hitachi, Japan). In order to produce electric conductivity, the samples were sputtered with gold ahead of SEM measurement. The fiber diameter and distribution in the MSLB were measured by Adobe Photoshop CS6 image analysis software.

Differential scanning calorimetry

In order to determine the existence of TPU/SPSf nanofibers, the glass transition temperature (T_g) of the MSLB was analyzed with differential scanning calorimetry (DSC). Samples were cut into small pieces, weighed At 6 ± 0.5 mg and placed into a pre-weighed aluminum crucible. Then, the sample was heated from 25℃ to 300℃ with a heating rate of 10℃/min. The T_g of the sample was determined as the midpoint temperature of the transition region in the heating cycle.

Water contact angle

The hydrophilicity of the MSLB was characterized by water contact angle measurement using a contact angle goniometer (OCA20, Dataphysics, Germany) equipped with video capture. A piece of 5 cm × 2 cm sample was attached on a glass slide and mounted on the goniometer. For the static contact angle measurement, a total of 3 mL double distilled water was dropped on the airside surface of the MSLB, and the contact angle was measured after 5 s. At least eight measurements were averaged to obtain a reliable value. The measurement error was ± 3.

Water vapor transmission rate

The WVT of the MSLB was estimated by the gravimetric cup method using a fabric water vapor permeability measuring instrument (YG (B)216-II, Wenzhou Darong Textile Instrument Co., Ltd, China) in accordance with GB/T 12704.2-2009 at a constant temperature of 38 ± 2℃ and relative humidity of 50 ± 2%. After establishing equilibrium, the weight measurements were done after a certain time interval for 1 h. The WVT was calculated by Equation (1)
$WVT = 24 M / (A \cdot t)$
(1)
where WVT is the water vapor transmission (g/m²•24 h), M is the increased weight in the internal time (g), A is the area of permeation (m²) and t is the internal time when the weight of cup increased constantly (h).

Moisture absorption

The moisture absorption of the MSLB was determined in accordance with GB/T 4689.21-2008. The distilled water with a temperature of 20 ± 2℃ and volume of V₁ was placed into a kubelka apparatus. Then the samples were weighed, marking as m, and put into the kubelka apparatus for 1 h. After removing the samples, the residual volume of distilled water was recorded as V₂. The moisture absorption was calculated by Equation (2)
$moistureabsorption = \frac{V_{1} - V_{2}}{m} \times 100$
(2)
where V₁ and V₂ are the volume of distilled water before and after testing for 1 h (ml), respectively. m is the weight of the samples (g).

Air permeability

The air permeability of the MSLB was measured according to GB/T5453-1997 using an automatic air permeability tester (YG461H, Ningbo Textiles Instrument Co., Ltd, China) with a surface area of 20 cm² and pressure drop of 100 Pa. Ten specimens were measured per sample and the average value was reported.

Softness

The softness was analyzed using a softness tester (YN-L-051, Dongguan YP Testing Equipment Co., Ltd, China) according to the IUP 36 standard method.

Thermal properties

The thermal properties of the MSLB were tested by thermogravimetric analysis (TGA, SDT Q600, TA, USA) with a heating rate of 10℃/min under nitrogen from room temperature up to 600℃. The pyrolytic temperatures (T_onset and T_p) were defined from the TGA curves.

Mechanical properties

The tensile strength and elongation at break of the MSLB were determined using a tensile tester (Instron 3369, America Instron Co., Ltd, USA) in light of GB/T 3923.1-2013 with a cross-head speed of 100 mm/min. The tear strength of the MSLB was also performed by a tensile tester according to GB/T 3917.2-2009. Each sample was tested five times.

Results and discussion

Morphology

Figure 1 shows the SEM photographs of the MSLB with a three-dimensional network structure. As shown in Figures 1(a)–(e), the PET/PA6 hollow segment pie bicomponent fibers were completely split by hydroentanglement. It was also obviously observed that the TPU/SPSf nanofibers were well distributed in all directions in the MSLB. When the content of nanofibers was more than 20% (Figures 1(d) and (e)), the nanofibers adhered to the bundles. After magnification of the picture, as shown in Figure 1(f), the nanofiber bundles were clearer. Figures 1(g) and (h) show the average diameter of microfibers and nanofibers in M30. It was shown that the average diameters of fabricated microfibers and nanofibers were about 4.75 and 0.12 µm, respectively, which were similar to the monofilaments and fibrils of natural leather.
Figure 1.
Scanning electron microscopy photographs of the microfiber synthetic leather base: (a) M0; (b) M5; (c) M10; (d) M20; (e) M30; (f) enlarged nanofibers from M30, and average diameter of (g) polyester/polyamide microfibers and (h) thermoplastic polyurethane/sulfonated polysulfone nanofibers.

DSC analysis

The measurement of the glass transition temperature (T_g) with blend composition is generally taken as a measure of the existence of components. To further confirm the existence of TPU/SPSf nanofibers in the MSLB, we tested the DSC. The DSC curves and the corresponding data of the T_g of the MSLB are shown in Figure 2. The T_g peak of pure MSLB (M0) was determined to be at 219.2℃ and 261.1℃, which were related to PA6 and PET, respectively. The MSLB with 20 wt% nanofibers (M30) exhibited four T_g peaks, namely 138.3℃, 219.3℃, 238.9℃ and 262.1℃. The 138.3℃ peak could correspond to TPU, and the 238.9℃ could correspond to SPSf. This further suggested that there are TPU/SPSf nanofibers in the MSLB.
Figure 2.
Differential scanning calorimetry (DSC) thermograms of M0 and M30.

Water contact angle

Generally, the hydrophilicity of materials is characterized by the water contact angle. In order to investigate the effect of SPSf on the hydrophilicity of the MSLB, the water contact angle was measured. The water contact angle values and corresponding images are shown in Figure 3. From the figure, the contact angle of the pristine MSLB (M0) shows a contact angle value of 111.64°. With the increase of TPU/SPSf nanofiber content from 5% (M5) to 30% (M30), its contact angles decreased gradually from 90.40° to 67.07°. This implies that the nanofibers have obvious influence on the hydrophilicity of the MSLB. This is because the increase of TPU/SPSf nanofiber content can enhance the number of active groups in the MSLB, causing more water molecules to be adsorbed on the fiber surface via physical or hydrogen-bond interactions.
Figure 3.
Water contact angle of the microfiber synthetic leather base.

Water vapor transmission rate and moisture absorption

Improving the WVT and moisture absorption of the MSLB has become the key research topic in the field of microfiber synthetic leather. Prior work has documented the effectiveness of increasing the fiber active groups by acid hydrolysis, enzyme hydrolysis and hydrophilic polymer blending and grafting in improving the WVT and moisture absorption of the MSLB. However, these studies have not focused on the hydrophilicity of spinning materials and the fiber fineness.

In this study we tested the WVT and moisture absorption of the MSLB with TPU/SPSf electrospun nanofibers. As shown in Figure 4, 55.19% improvement in the WVT value (from 2868.96 to 4452.24 g/(m²•24 h)) and 26.25% improvement in the moisture absorption (from 628.70% to 793.75% mm/s) were obtained when the nanofibers content extended from 0 to 30 wt%. There are two factors to explain the above results. On the one hand, sulfonic acid groups (-SO_3-) on the SPSf chains were hydrophilic because of the ability to combine with water, as presented in Figure 3. On the other hand, the fiber specific surface area was increased with the increase of nanofiber content, leading to the resultant MSLB with a high WVT and moisture absorption. Most notably, this is the first study to our knowledge to investigate the effectiveness of hydrophilic nanofibers in improving the WVT and moisture absorption of the MSLB. Our results provide compelling evidence for preparing the MSLB with electrospun nanofibers and suggest that this approach appears to be effective in enhancing the WVT and moisture absorption. However, some limitations are worth noting. Because of the difficulty of nanofiber combing, the content of nanofibers is limited. Meanwhile, the content of nanofibers is too high, which is not conducive to the mechanical properties of the MSLB.
Figure 4.
Water vapor transmission rate and moisture absorption of the microfiber synthetic leather base. WVT: water vapor transmission rate.

Air permeability

The sanitary properties are an important performance index of microfiber synthetic leather, mainly including WVT, moisture absorption and air permeability. Herein, the air permeability of the MSLB was inspected as demonstrated in Figure 5. It could be found in Figure 5 that the air permeability of the MSLB decreased from 145.23 (M0) to 135.37 mm/s (M30) with the increase of nanofiber content from 0 to 30 wt%. Actually, there is 6.79% decrement of air permeability, which is related to the nonwoven structure as mentioned in the Morphology section.
Figure 5.
Air permeability of the microfiber synthetic leather base.

Softness

The softness of microfiber synthetic leather is one of the major concerns for its application. Since the softness is critical, the TPU nanofibers were prepared and the effect of nanofibers on the softness was investigated experimentally. It can be seen from Figure 6 that the softness obtained from M5 was 6.70 mm, which was higher than that obtained from M0 (4.54 mm). Furthermore, the softness of the MSLB increased from 6.70 (M5) to 8.56 mm (M30) with the nanofiber content increasing from 5 to 30 wt%. M30 had the best efficiency of softness and was improved by 88.55% compared to M0. Clearly, TPU/SPSf electrospun nanofibers would contribute to the high softness of the MSLB. According to the report by Gong and Nikoukhesal,²³ the bending rigidity of a fiber is proportional to the fourth power of the fiber diameter or to the square of the fiber linear density. Therefore, smaller fiber diameters are expected to make softer fabrics.
Figure 6.
Softness of the microfiber synthetic leather base.

Thermal analysis

To further gain an insight into the effect of TPU/SPSf nanofibers on the thermal stability of the MSLB, the thermal behaviors of the MSLB were analyzed with TGA. Figure 7 displays the TGA thermograms. It could be seen that the MSLB displayed a good thermal stability with an onset degradation temperature in the range of 367.62–349.80℃. The slight difference was caused by the TPU component. In addition, the peak degradation temperature and the char yields at 600℃ obtained from M0, M10 and M30 was 401.44℃, 400.08℃, 402.01℃ and 14.76%, 17.04% and 17.48%, respectively. The results indicated that the degradation rate of the MSLB decreased as TPU/SPSf nanofiber content increased.
Figure 7.
Thermogravimetric analysis thermogram of the microfiber synthetic leather base (MSLB).

Mechanical properties

As a key parameter of microfiber synthetic leather, the mechanical properties, including tensile strength, elongation at break and tear strength, are mainly provided by the MSLB. Figures 8 and 9 illustrate the effect of nanofiber content on the mechanical properties of the MSLB. As shown in Figure 8, with the increase of TPU/SPSf nanofiber content from 0 (M0) to 30 wt% (M30), the tensile strength and the elongation at break of the MSLB gradually decreased from 187.63 N and 121.34% to 101.58 N and 72.92%, respectively. This seems to be interesting and might be explained considering a dual effect with the inherent strength of the fiber and the network structure of the nonwoven fabrics. As we know, the smaller the average diameter of the fiber, the lower the strength of the fiber. It is clear from SEM analysis that the nanofibers have much lower diameter. On the other hand, higher nanofiber content could increases fiber entanglement in the hydroentanglement process due to lower bending rigidity, which consequently enhances the tensile strength. In this study, M30, with the highest nanofiber content, exhibited the lowest tensile strength and elongation at break, which indicated that the fiber diameter was preferred over fiber entanglement with TPU/SPSf nanofiber content increasing to 30 wt%. It can be also observed from Figure 9 that the tear strength obtained from M0, M5, M10, M20 and M30 was 8.72, 9.56, 12.86, 18.78 and 28.83 N, respectively. The tear strength of M30 (28.83 N) was 3.31 times higher than that of M0 (8.72 N). This could be attributed to the presence of TPU/SPSf nanofibers in favor of the entanglement between the fibers.
Figure 8.
Tensile strength and elongation at break of the microfiber synthetic leather base.

Figure 9.
Tear strength of the microfiber synthetic leather base.

Conclusions

In summary, we have successfully prepared the high-performance MSLB with TPU/SPSf electrospun nanofibers. We found that the morphology and performance of the MSLB were significantly affected by the addition of TPU/SPSf nanofibers. The average diameter of TPU/SPSf nanofibers was about 0.12 µm, and the nanofibers were well distributed in all directions in the MSLB. Simultaneously, DSC thermograms showed four T_g peaks at 138.3℃, 219.3℃, 238.9℃ and 262.1℃, which further demonstrated the existence of TPU/SPSf nanofibers. Moreover, with the increase of TPU/SPSf nanofiber content from 0 to 30 wt%, the contact angles of the MSLB decreased gradually from 111.64° to 67.07°, which implied that its hydrophilicity was enhanced. In addition, the MSLB M30 obtained from 30 wt% nanofiber content exhibited a high WVT (4452.24 g/(m²•24h) and moisture absorption (793.75%). Furthermore, M30 also presented the best softness, thermal stability and the tear strength.

MSLB	PET/PA6 microfiber (g)	TPU/SPSf nanofiber (g)
M0	16.00	0
M5	15.20	0.80
M10	14.40	1.60
M20	12.80	3.20
M30	11.20	4.80

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (Grant No. U1607117) and by the Tianjin Research Program of Application Foundation and Advanced Technology (Grant No. 15JCZDJC38500 and Grant No. 16JCZDJC36400).

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