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Improving Nanofiber Production and Application Performance by Electrospinning at Elevated Temperatures Guilong Yan, Haitao Niu, Xueting Zhao, Hao Shao, Hongxia Wang, Hua Zhou, and Tong Lin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02850 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017

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Industrial & Engineering Chemistry Research

Improving Nanofiber Production and Application Performance by Electrospinning at Elevated Temperatures Guilong Yan, Haitao Niu, Xueting Zhao, Hao Shao, Hongxia Wang, Hua Zhou and Tong Lin* Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia

ABSTRACT: Needleless electrospinning can produce nanofibers on large scales, promising for industrial manufacture. It is known that solution temperature is an important parameter for electrospinning. However, the influences of solution temperature on needleless electrospinning has little been reported in research literature, and neither the application performances of needleless electrospun nanofibers. In this study, using a curved slot as the needleless electrospinning spinneret and PVA as material model, we show that solution temperature has a considerable effect on fiber diameter, packing density of nanofiber mats, and fiber productivity. Increasing the solution temperature from 20 °C to 60 °C can lead to finer fibers with larger fibrous production rate. The nanofiber mats produced at higher solution temperature show increased air filtration efficiency. Increasing solution temperature may form a new strategy to improving nanofiber productivity and meanwhile increasing nanofiber quality.

ACS Paragon Plus Environment

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1. INTRODUCTION Polymer nanofibers produced by electrospinning are collected chiefly in the form of randomly orientated nanofiber mats, which have a high porosity and small pores with excellent pore interconnectivity. These unique features together with the functionalities from the polymer materials offer electrospun nanofiber nonwovens enormous potential for applications in diverse fields, including energy generation and storage, biomedical, healthcare, electronics, chemical engineering, and environment protection 1-6. An important application of electrospun nanofiber nonwovens is air filtration, because air flow changes its aerodynamic behaviour dramatically when the fibre diameter in fibrous filters reduces to less than 1 micron. Knudsen number, Kn, is a dimensionless parameter used to characterize the flow regime of the gas around the fibres. (1)

Kn = 2λ / d f

Here λ is the mean free path of air molecules (about 66 nm at ambient temperature and pressure) and df is fibre diameter. Air flow can be classified into four different regimes: continuum flow, slip flow, transition flow, and free molecular flow. Air flow in electrospun nanofibrous filters is typically in the regime of transition flow, with the Kn in the range of 0.25-10. In such a regime, the air filtration efficiency increases with increasing Kn number.7 Several researchers have reported the air filtration properties of electrospun nanofiber nonwovens

8-18

. Sundarrajan et al

12

indicated that nanofibrous filters had a high removal

efficiency for the particles of 0.1 - 0.5 µm, and the flow resistance is small. Podgorski et al

13

reported that the addition of nanofibers to commercial filters enhanced filtration


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efficiency against most of the penetrating aerosol particles in air. Lee et al

14

fabricated

nylon-6 nanofiber nonwovens with a filtration efficiency of 99.993% for particle size of 0.3 µm (pressure drop < 40 mmAq), superior to commercial high efficiency particulate air (HEPA) filters. Ding et al

15, 16

fabricated high flux and low fouling ultrafiltration

membranes by combining polyacrylonitrile nanofibers with melt-blown polyethylene terephthalate fibres. These filters showed a filtration efficiency above 99% and a flow resistance as low as 29.5 Pa. Cui et al

18

reported transparent nanofibrous filters with

particle removal efficiency >99.97% and pressure drop of 275 Pa. However, most of the reported nanofiber air filters were produced by conventional electrospinning techniques, which use needle like nozzles to prepare nanofibers. It is known that needle electrospinning has a low fibre production rate, which restricts its wide applications in practice. Recent years, needleless electrospinning has emerged as a promising electrospinning technique to produce nanofibers on large scales. Needleless electrospinning is featured as electrospinning polymeric fluids without using needle nozzle. Various needleless electrospinning spinnerets, e.g. cylinder, disc, coil, ball and wire, have been reported. 19-22 These needleless electrospinning apparatuses are ease of operation and do not need regular maintenance of the spinnerets during electrospinning. The nanofibers produced have small diameter with good fibre uniformity, showing great potential for industrial applications. Regardless of electrospinning types, many parameters (e.g. applied voltage, collecting distance, spinneret geometry, solution properties, environmental temperature, and humidity) can influence electrospinning process and the resulting fibre quality. Solution


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temperature influences on solution viscosity, surface tension, and conductivity, which affect the electrospinning behaviours and fibre properties. Rodoplu et al

23

reported that

fibres become bead-free and flat when polyvinyl alcohol (PVA) solution was electrospun at an elevated temperature, and explained that increasing solution viscosity led to declined solution viscosity and increased chain mobility. When polyvinylidene fluoride (PVDF) solution was electrospun at an elevated temperature, high beta-phase content nanofiber resulted

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. Liu et al.25 reported that increasing solution temperature could decrease the

critical voltage for jet initiation and air pressure of crater-like electrospinning. Huang et al.26 indicated that solution temperature had a great influence on the formation, morphology, diameter, structure, and crystallinity of PVDF nanofibers. Despite of the results, the influences of solution temperature on needleless electrospinning have little been reported in research literature, and neither the application performance of needleless electrospun nanofiber nonwovens. In our previous study

27

, we have reported a high performance needleless electrospinning

technology using a curved slot as spinneret. The curved slot spinneret showed advantages in maintaining stable electrospinning and mitigating solvent loss. In this study, we examine the effect of solution temperature on the curved slot needleless electrospinning and the air filtration properties of the spun nanofiber nonwovens. Polyvinyl alcohol (PVA) was used as model polymer, which has high spinning ability and is environmentally friendly. We show that solution temperature has a considerable effect on fiber diameter, packing density of nanofiber mats, and fiber productivity. Increasing the solution temperature from 20 °C to 60 °C can lead to finer fibers with larger fibrous production rate. The nanofiber mats produced at higher solution temperature show


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increased air filtration efficiency. Increasing solution temperature may form a new strategy to improving nanofiber productivity and meanwhile increasing nanofiber quality.

2. MATERIAL AND METHODS 2.1 Materials Polyvinyl alcohol (PVA, weight-average molecular weight = 146,000–186,000, 98–99% hydrolysed) was purchased from Sigma-Aldrich and used as received. PVA solutions were prepared by adding PVA power in distilled water followed by vigorous stirring at 90 ℃ for 6 h. 2.2 Needleless electrospinning A purpose-built needleless electrospinning apparatus was used for the experiments, as shown in Figure 1a. A curved slot spinneret was used as fibre generator. The polymer solution was fed into the container via a syringe pump. The solution was charged with a high-voltage power supply (ES100P, Gamma High Voltage Research) through inserting an electrode into the solution. Rotating metal drums covered with a polyethylene terephthalate nonwoven mat was used as the collector. During electrospinning, the solution temperature was controlled in the range of 20-60 ℃, whereas the electrospinning zone and collector were maintained at 50 ℃. The as-spun nanofibers were dried in vacuum at 60 ℃ for 24 hours. The detailed spinning conditions are listed in Table S1. 2.3 Characterizations The viscosity of PVA solutions was measured by a TA HR-3 Rheometer with a cone-plate of 40 mm radius and 2° angle. The conductivity of the PVA solutions was tested on a LF 330/SET conductivity meter. The surface tension of PVA solutions was measured by KRUSS K100 Force


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Tensiometer using a Pt ring. Nanofiber morphology was observed under a scanning electron microscope (SEM, SUPRA-55VP). The average fibre diameter was calculated using the image analysis software (ImagePro+ 4.5) based on SEM images. Digital images and videos were taken using a Canon D60 digital camera. The through-pore size and pore size distribution of the nanofiber nonwovens were measured using a Porometer 3G analyser. The packing density of nanofiber nonwovens was calculated by the equation:

β=

Wnfm ( ρ PVA ⋅ lT )

(2)

where β is the packing density of nanofiber mat, Wnfm is the areal density of nanofiber mat, ρPVA is the density of PVA, and lT is the thickness of PVA nanofiber mat. 2.4 Testing of air filtration performance A TSI Automated Filter Tester (Model 8130) was used to measure the aerosol filtration performances. TSI 8130 could generate neutralized monodisperse solid sodium chloride (NaCl) aerosols with a mass median meter of 260 nm and count median diameter of 75±20 nm. The geometric standard deviation was smaller than 1.83. The NaCl aerosols were flown through the filter with continuous airflow flux of 32 L/min, 45 L/min, 58 L/min, 71 L/min, and 85 L/min. The effective filtration testing area is 100 cm2 and the testing time of each sample was 4 s. To evaluate the capacity of filtration stability of nanofiber membrane, the testing period is from 0 to 15 min. The quality factor (QF) was calculated using the following equation: QF = − ln(1 − η )

∆p

(3)

where η and ∆p are the air filtration efficiency and pressure drop across the nonwoven samples.


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3. RESULTS AND DISCUSSION Figure 1a illustrates the setup for the curved slot electrospinning. A syringe pump was used to feed the electrospinning solution into the spinneret. During electrospinning, a high voltage was applied to the polymer solution in the spinneret, and the liquid level of the solution was maintained just at the top line of the slot, without overflow. When the applied voltage was above 50 kV, jets were generated from the solution surface at the slot top (see the photo in Figure 1b). These jets distributed at a regularly interval distance along the slot, attributable to the electrostatic repulsion. After jet stretching and solvent evaporation, the filaments solidified and deposited on the collector. The applied voltage required for running slot spinning was in the range of 50-70 kV. Higher applied voltage (over 70 kV) would lead to corona discharge, affecting the electrospinning process. All the fibres electrospun were deposited on a PET fibrous substrate (dimension: 100 cm × 30 cm) (Figure 1d). The slot electrospinning was very efficient, 10 minutes of electrospinning allowed to produce nanofibers of the weight equivalent to that produced by conventional needle electrospinning approximately for 6 hours.


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Figure 1. (a) Schematic drawing of the curved slot electrospinning setup. (b)Photograph of curved slot electrospinning during spinning process. (c) Digital image of PVA nanofibers collected on the PET nonwoven mat.

Figure 2 shows the influences of applied voltage and polymer concentration on the slot electrospinning. At ambient temperature (20 °C), no electrospinning took place when the applied voltage was below 50 kV (bottom zone). However, corona discharge resulted when the applied voltage above the breakdown limit 70 kV (top zone). When the applied voltage was set in between, the solution could be spun into various morphologies, including beads (zone I), beaded fibres (zone II), and uniform fibres (zone III), depending on the polymer concentration in the electrospinning solution.


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When the PVA concentration was below 8 wt.%, only beads were obtained (Figure 2I). This is due to the Rayleigh instability caused by surface tension and inefficient macromolecular interaction.28 Beaded fibres (Figure 2II) appeared with increasing the PVA concentration. However, electrospinning failed to happen when the PVA concentration exceeded 12 wt.%, due to the solution was so viscous to block the slot spinneret. When PVA concentration was in the range of 8 - 12 wt.%, both beaded fibres and bead-free fibres could be produced, depending on the voltage applied. For the solution with a relatively low PVA concentration, uniform nanofibers were produced at a relatively low voltage (Figure 2III). With increasing the PVA concentration, the voltage required for producing uniform nanofibers increased because at higher PVA concentration the increased solution viscosity requires stronger force to stretch the solution into fine filaments.


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Figure 2. Morphological profiles of PVA nanofibers fabricated at different conditions. SEM images of PVA nanofibers with different morphologies: (I) Beads, (II) beaded fibres, (III) uniform fibres.

Changing solution temperature was expected to have an apparent influence on slot electrospinning process. When the solution temperature was increased to 60 °C, beads without fibres were produced from 8 wt.% PVA solution, whereas electrospinning of 13 wt.% PVA solution resulted in uniform nanofibers. However, the solution with a PVA concentration higher than 13 wt.% was not spinnable due to the high viscosity. Apparently, higher solution temperature benefits electrospinning the solutions of higher polymer concentration. In this work, the solution temperature up to 60 °C was examined because further increasing the solution temperature led to rapid evaporation of water from the solution. It was interesting to note that 10 wt.% PVA solution was almost suitable for electrospinning the polymer solution at various temperature, in the range of 20 °C - 60 °C. The increase in the solution temperature only led to the shift of voltage range for producing uniform nanofibers. When the solution changed from 20 °C to 60 °C, the applied voltage range changed from 53.7-67.2 kV to 50.1-60.3 kV. Because of this, 10 wt.% PVA solution was chosen for further experiments. Temperature effect on electrospinning comes from the effect of temperature on solution properties. The viscosity change with the solution temperature follows the Arrhenius equation 29,


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η = A⋅e

− Ea

R ⋅T

(4)

where η is solution viscosity, A is frequency factor, Ea is activation energy, R is gas constant, T is Kelvin temperature of the solution. For 10 wt.% PVA solution, the solution viscosity decreased from 1500 mPa s to 680 mPa s (Supporting Information Figure S1), and the surface tension decreased from 54.8 mN/m to 46.6 mN/m when the solution temperature increased from 20 ℃ to 60 ℃ (Supporting Information Figure S2), whereas the respective solution conductivity increase from 537 mS/cm to 605 mS/cm (Supporting Information Figure S3). These changes make the solution more favourable to jet initiation and fibre stretching. Figure 3a-e shows the SEM images of nanofibers electrospun from 10 wt.% PVA solution at different solution temperatures. All nanofibers look uniform without beads. Some fibres stuck to each other forming an interconnected fibrous structure, attributable to the insufficient water evaporation from the filaments. This happened especially when a large number of jets were generated simultaneously. Changing solution temperature in the range of 20 ℃ - 60 ℃ showed little influence on the fibre morphology. The nanofibers obtained in the range of 30 °C – 50 °C were finer with narrower diameter distribution than those from the solution at other temperature. With increasing the solution temperature from 20 °C to 50 °C, The average nanofiber diameter decreased from 410 nm to 355 nm. Further increasing the temperature led to an increase in average fibre diameter to 390 nm (at 60 °C) (Figure 3f). This is mainly due to the decreased surface tension and viscosity of PVA solution, making fibre stretching more easily. When the temperature above 50 °C, the solvent evaporation rate increased largely, leading to slightly increased solvent loss on the liquid surface and larger fibre diameter. Even if the fiber diameter


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increased when solution temperature increased from 50 °C to 60 °C during electrospinning, the mean diameter of the fibers electrospun at 60 °C was still smaller than that of those electrospun at room temperature. Figure 3g shows the effect of solution temperature on the fibre production rate. For a slot spinneret with the actual length of 12 cm, the fibre productivity changed from 1.60 g/h to 1.98 g/h, when the solution temperature increased from 20 ℃ to 60 ℃. This can be explained by the decreased surface tension and solution viscosity, making it easier to generate the jets. Figure 3h shows the packing density of the nanofiber mats. The packing density changed in a similar trend as the productivity. This is because finer fibres often give larger packing density in nonwoven. In our previous paper

27

, we reported a

comparison between our slot electrospinning with conventional electrospinning. The productivity for the curved slot electrospinning is 1.6 g/h, which is much larger than that of single needle electrospinning (< 0.3 g/h). The productivity per unit area for the curved slot is 0.615 g/(h cm2), which is comparable to the conventional needleless electrospinning, e.g. 0.748 g/(h cm2) for coil electrospinning. Figure 3i shows the pore size and pore size distribution of the nanofiber mats electrospun from 10 wt% PVA solution at different solution temperatures. With increasing the solution temperature from 20 ℃ - 60 ℃, the average pore size deceased from 45 µm to 24 µm, and the pore size distribution became narrower.


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Figure 3. SEM images of nanofibers electrospun from 10wt.% PVA solution at 60 kV applied voltage, 20 cm collecting distance and different solution temperatures: (a) 20 ℃, (b) 30 ℃, (c) 40 ℃, (d) 50 ℃, and (e) 60 ℃. Effect of solution temperature on (f) nanofiber diameter, (g) productivity, (h) nonwoven packing density, and (i) pore size in the nonwoven.


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The air filtration performances of the nanofiber mats were tested at a gas flux of 32 L/min using sodium chloride aerosol (size ~0.26 µm) as model particles. The substrate used for supporting the nanofibers was a spun bond nonwoven with an areal density of 18 g/m2. Without nanofibers, the nonwoven substrate had a very low particle filtration ability, with the filtration efficiency and pressure drop as low as 3.3 % and 0.7 Pa, respectively. When a thin layer of PVA nanofiber nonwoven (thickness is 15.6 µm) was deposited on the spun bond nonwoven substrate, the filtration performance was increased. Figure 4a shows the effect of solution temperature during electrospinning on the filtration performance of the nanofiber nonwoven (with PET nonwoven substrate). When the solution temperature increased from 20 ℃ to 60 ℃, the filtration efficiency increased monotonously from 62.5% to 90.1%. Meanwhile, the pressure drop of the nanofiber samples increased from 19.5 Pa to 61.5 Pa. The quality factor had a small change with the solution temperature and the largest value (0.042 Pa-1) was obtained from the sample prepared from 50 ℃ solution (Supporting Information Figure S4). The SEM imaging confirmed that substantial aerosol particles were intercepted by the nanofibers for the sample prepared from 60 ℃ PVA solution (Supporting Information Figure S5). In contrast, the nanofibers are partially covered with the particles for the nanofiber samples prepared from 20 ℃ PVA solution. This indicates that the fibres prepared at a higher solution temperature showed higher particle capture capability, attributing to the smaller fibre diameter and larger packing density of the nanofiber nonwoven. The results in Figure 3 and 4 indicate that electrospinning of 60 °C PVA solution leads to increased fiber diameter, fiber productivity, and fiber packing density, but reduced pore size. The prepared fibrous mat shows improved filtration efficiency.


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Figure 4b shows the effect of air flow rate on the filtration properties. Increasing the flow rate led to increase in both filtration efficiency and flow resistance. For the nanofiber mat (the thickness is 15.6 µm) prepared at the solution temperature of 50 ℃, when the air flow rate increased from 32 L/min to 85 L/min, the filtration efficiency increased from 90.6% to 93.7%, and the pressure drop increased from 120 Pa to 349 Pa. A similar trend was also reported by other researchers17, 30.

Figure 4. Effects of (a) solution temperature, (b) aerosol flow rate, and (c) nonwoven thickness on filtration efficiency and pressure. (d) Long period testing of filtration performance (nanofibers were prepared from 50 ℃ solution). All nanofiber membrane samples were prepared by needle electrospinning 10 wt.% PVA solution at 60 kV applied voltage, 20 cm collecting distance.


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Figure 4c showed the effect of nanofiber mat thickness on filtration efficiency and air flow resistance (solution temperature 50 ℃ for electrospinning, and air flow rate during filtration test 32 L/min). When the thickness of nanofiber nonwovens increased from 8.4 µm to 14.2 µm, a sharp increase in the filtration efficiency from 49.8% to 94.6% occurred, while the flow resistance increased from 20 Pa to 128 Pa. When the thickness increased from 14.2 µm to 20.5 µm, the filtration efficiency increased from 94.6% to 97.1%, and the flow resistance increased from 128 Pa to 249 Pa. The largely increased airflow resistance with increasing the nonwoven thickness was due to reduced free path for airflow. 15 Figure 4d shows the filtration performances of nanofiber nonwovens at different filtration time (solution temperature 50 ℃, thickness 13.6 µm). In 4 min, the filtration efficiency increased from 89.9% to 99.1%, while further increasing the testing time, the filtration efficiency had little change. The pressure drop increased from 120 Pa to 490.5 Pa. At the beginning sparsely distributed NaCl particles appeared on the surface of PVA nanofiber, and then some particles joined to form larger ones. With filtration test continued, NaCl particles accumulated between nanofibers so that the filtration efficiency and pressure drop became larger and larger, until NaCl particles covered the entire surface, which led to the high filtration efficiency (99.99%) and large pressure drop of 490.5 Pa. It is a typical dynamic filtration process from ‘depth filtration’ to ‘cake filtration’.31

4. CONCLUSIONS We have prepared nanofiber nonwovens using a curved slot as spinneret. The solution temperature shows substantial influence on solution viscosity, surface tension, and conductivity,


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which affects nanofiber productivity, nanofiber diameter, the porous structure of nanofiber nonwovens and the air filtration performances. When the solution temperature changed from 20 °C to 60 °C, the electrospinning productivity increased and nanofibers became thinner with higher fiber packing density. As a result, the filtration efficiency increased from 62.5% to 90.1%. Increasing solution temperature may form a new strategy to improving nanofiber productivity and meanwhile increasing nanofiber quality.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Table S1 and Figures S1−S5 reporting Supporting Information about slot electrospinning conditions, effect of temperature on PVA solution viscosity, effect of temperature on surface tension, effect of temperature on solution conductivity, and SEM images of nanofiber membranes after filtration test.

AUTHOR INFORMATION Corresponding Author *Tel: +61-3-52271245. E-mail: [email protected]. Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes


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The authors declare no competing financial interest.

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Table of Contents (TOC) graphic


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Improving Nanofiber Production and Application Performance by

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