Current Applied Physics 6 (2006) 1030–1035 www.elsevier.com/locate/cap www.kps.or.kr
Development of high efficiency nanofilters made of nanofibers Y.C. Ahn a
a,b
, S.K. Park b, G.T. Kim b, Y.J. Hwang b, C.G. Lee b, H.S. Shin b, J.K. Lee
b,*
Research Institute of Mechanical Technology, Pusan National University, San 30, Jangjeon-dong, Keumjeong-ku, Busan 609-735, Republic of Korea b Department of Mechanical Engineering, Pusan National University, San 30, Jangjeon-dong, Keumjeong-ku, Busan 609-735, Republic of Korea Received 4 September 2004; received in revised form 19 March 2005 Available online 11 August 2005
Abstract Electrospinning is a fabrication process that uses an electric field to control the deposition of polymer fibers onto a target substrate. This electrostatic processing strategy can be used to fabricate fibrous polymer mats composed of fiber diameters ranging from several microns down to 100 nm or less. In this study, optimized conditions to produce nanofibers using Nylon 6 are investigated and the Nylon 6 nanofilters using nanofibers of 80–200 nm in diameter are designed and evaluated the filtration efficiency and pressure drop across the filter. When the Nylon 6 concentration is 15 wt.%, electrospun fibers have an average diameter of 80 nm, but there are many beads, and the concentration increases to 24 wt.%, the fiber diameter gradually thickens to 200 nm, but there are not any beads. When the spinning distance is small, the thinner nanofibers are produced and the more fibers are collected on the grounded electrode. The filtration efficiency of Nylon 6 nanofilters is 99.993% superior to the commercialized HEPA filter at the face velocity of 5 cm/s using 0.3 lm test particles. Even though the high pressure drops across the nanofilter, they show the potential to have the application of HEPA and ULPA grade high efficiency filter. Ó 2005 Elsevier B.V. All rights reserved. PACS: 81.05.L Keywords: Electrospinning; Nanofibers; Nanofilter; Nylon 6; HEPA
1. Introduction Since the beginning of the nineteenth century, polymers have replaced metals in various applications for their lightweight and flexibility. Commercially, most synthetic manufactured fibers are created by extrusion, which consists of forcing a thick, viscous liquid through the tiny holes of a device called a spinneret to form continuous filament of semi-solid polymer. Electrospinning is a process of electrostatic fiber formation by which uses electrical forces to produce polymer fibers from polymer solution, with nanometer*
Corresponding author. Tel.: +82 51 510 2455; fax: +82 51 582 6368. E-mail address:
[email protected] (J.K. Lee). 1567-1739/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2005.07.013
scale diameters. Fig. 1 shows the classification by the fiber diameter. Nanofibers have a large specific surface area and a small pore size in comparison with commercial textiles. So polymer nanofibers are being used or finding uses in filtration, protective clothing, biomedical applications including wound dressings and drug delivery systems, as structural elements in artificial organs, and in reinforced composites. Recently, there has been a renewed interesting in these commercially variable processes, making literature involving the quantitative technical and scientific information of the process and product characterization extremely limited. Polymer fibers encouraged the invention of technology for manufacturing nanofibers such as electrospinning [2,3]. This paper derives an optimum condition to form Nylon 6 nanofibers by electrospinning process and
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Table 1 Experimental conditions to produce Nylon 6 nanofibers by electrospinning
Fig. 1. Classification by the fiber diameters [1].
develops nanofilters. The important parameters in the forming of electrospun Nylon 6 nanofilters such as concentration, and electric field strength are investigated experimentally. Also the filtration characteristics of the Nylon 6 nanofilters such as collection efficiency and pressure drop are evaluated.
2. Experimental 2.1. Electrospinning to develop Nylon 6 nanofilters Fig. 2 shows a schematic diagram of the electrospinning process. It consists of the syringe feeder system, the collector system, and the high voltage power supply system. High electric field strength is generated between a
Parameters
Values
Polymer Solvent Concentration (wt.%) Electric voltage (kV) Tip to collector distance (cm) Collector Capillary diameter (mm) Fiber diameter range (nm) Temperature (°C) Relative humidity (%)
Nylon 6 Formic acid 15–24 25 5–14 Steel mesh 0.7 80–200 25 40
polymer fluid contained in a glass syringe and a metallic collector. The hemispherical shape of pendant droplet at the end of syringe tip is changed into a conical shape with increasing voltage, which is known as the Taylor cone. When the voltage reaches a critical value, the electric force overcomes the surface tension of the deformed drop of the suspended polymer solution formed on the tip of the syringe, and a jet is produced. A jet travels through the air, the solvent evaporates leaving behind polymer fibers to be collected on an electrically grounded target. This means that the electrospinning jet can be thought of as string of charge elements connected by a viscoelastic medium, with one end fixed at the point of origin and the other end free. The free end of the electrospinning jet follows a chaotic path as it travels toward the grounded collector [4–6]. This chaotic motion is the result of a complicated interaction of variables that involve viscosity, conductivity, surface tension, electrostatic force, air friction, gravity and ambient parameters [7–9]. Table 1 shows the experimental conditions to develop Nylon 6 nanofilters. Nylon 6 solutions are prepared from Nylon 6 pellet (Aldrich) and it is dissolved by formic acid in the ultrasonicator. The concentration of the Nylon 6 solution tested ranges from 15 to 24 wt.%. Spinning occurs from the droplet of solution protruding from the 0.7 mm internal diameter of the tip. A positive electric potential is applied to the polymer blend solution, by attaching the lead to the variable high voltage power supply (Chung-pa Co., EMT) directly to the copper wire, to the solution inside a syringe. The electric voltage is 25 kV. A drum collector covered with steel mesh is placed 5–14 cm vertically from the tip of the syringe as a grounded collector. The morphology and diameter of the electrospun fibers aggregates are determined with a scanning electron microscope (HITACHI, 4200) and image analyzer (IMT, IMT-2000). 2.2. Performance evaluation of the nanofilters
Fig. 2. Schematic diagram of the electrospinning set-up.
Fig. 3 is a schematic diagram of the filter testing apparatus used. The system is comprised of an atomizer,
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Y.C. Ahn et al. / Current Applied Physics 6 (2006) 1030–1035 Compressed Air HEPA Filter
Excess Aerosol Diffusion Dryer
Flow meter
HEPA Filter
ΔP HEPA Filter
Flow meter
Test Filte Atomizer HEPA Filter
Kr-85 Neutralizer Vacuum Pump
Dilution Air Condensation Particle Counter
Fig. 3. Schematic diagram for the performance evaluation of nanofilters.
a diffusion dryer, a charge neutralizer, a filter holder, and a condensation particle counter. In the atomizer, PSL (Polystyrene Latex) particles which have known sizes are dissolved in a distilled water and the solution is atomized. Much moisture are on the surfaces of the monodisperse particles, however, they are eliminated during passing through the diffusion dryer. In order to avoid unwanted electrostatic effects, a charge neutralizer (TSI, 3054) is used. Electrical charges on the aerosols are neutralized by exposing the aerosols to a cloud of bipolar ions produced by a radioactive source. The 10 mCi Kr85 source is placed inside an aluminum tube and the aerosol is passed through the tube. The particles are thus brought to a state of Boltzmann charge equilibrium. The filter to be tested is placed in a filter holder and the particle concentration upstream and downstream of the filter is measured using the condensation particle counter (TSI, 3010). The condensation particle counter can measure the number concentration of nano-sized particles to grow to micro-sized droplets in a supersaturated environment. Table 2 shows the experimental conditions for performance evaluation of nanofilters. The diameter of the test filters is 47 mm and the face velocity is varied from 3 to 10 cm/s. Nylon 6 nanofilters are prepared two samples according to the base weight of 5.75–10.75 g/m2 and filtration efficiency is evaluated using test particles of 0.085–2.0 lm in diameter.
3. Results and discussions 3.1. Effects of Nylon 6 concentration on fibers morphology Fig. 4 shows the SEM images of Nylon 6 nanofibers as a function of Nylon 6 concentration at the applied voltage of 25 kV. When the concentration is 15 wt.%, electrospun fibers have an average diameter of 80 nm, but as the concentration of Nylon 6 increases to 24 wt.%, the fiber diameter gradually thickens to 200 nm. The higher concentration of the polymer solution tries to form thicker fibers by preventing the stretching of fibers. Polymer concentration also has an effect on bead formation and bead density. Fibers produced from lower concentrated solution exhibit more beads while other parameters remain constant. A mixture of large beads and fibers is generated by electrospinning Nylon 6 solution at the concentrations below 18 wt.%. The reason is that at lower concentrations, electrospun fibers are harder to dry before they reach the collection drum. When solidification process is undergoing on the surface of the collection drum, surface tension makes bead spherical shape. At low concentration, surface tension becomes dominant influence over viscosity and form more beads. But as the viscosity increases in the solution, surface tension becomes dominated by viscosity and results in less number of beads, while the other variables are maintained constant. 3.2. Effects of spinning distance on fibers morphology
Table 2 Experimental conditions for evaluating the filtration efficiency of nanofilters Parameters
Values
Face velocity (cm/s) Particle size (lm) Test filter size (mm) Base weight (g/m2)
3–10 0.085–2.0 47 5.75 10.75
Sample 1 Sample 2
Fig. 5 shows SEM images of Nylon 6 nanofibers as a function of spinning distance at the Nylon 6 concentration of 24%. In general, the spinning distance performed a role on the fiber structure and morphology [3]. The electric field strength is increased when the spinning distance is decreased, and the polymer jet is discharged with a greater electrostatic repulsion that causes it to undergo higher levels of drawing stress and the jet velocity increases. The increasing electrostatic force reinforces
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Fig. 4. Scanning electron micrographs of Nylon 6 nanofibers as a function of concentration (applied voltage = 25 kV, spinning distance = 5 cm).
Fig. 5. Scanning electron micrographs of Nylon 6 nanofibers as a function of spinning distance (applied voltage = 25 kV, Nylon 6 concentration = 24%).
the stretch ability of a droplet. During the travel of the fibers towards the target, the higher electric field
strength has the opportunity to stretch the fibers more and results in thin fibers. The morphology of the
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nanofibers at the spinning distance of 5 cm shows thinner fibers and the collection efficiency to the grounded electrode is slightly high compared with the other cases. Therefore, the optimum conditions for mass production of nanofilters are selected as 25 kV of electric potential at 5 cm of spinning distance using solution of 24% Nylon 6 concentration.
100.00
Filtration Efficiency (%)
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99.98
99.96
99.94
Particle Size : 0.3 μ m HEPA Filter 2 Nano filter, Sample 1 (Base Weight: 5.75 g/m ) 2 Nano filter, Sample 2 (Base Weight: 10.75 g/m )
99.92
3.3. Performance evaluations of Nylon 6 nanofilters 99.90
Three samples of filters are used in this study. The one is HEPA(High Efficiency Particulate Air) filter to measure reference values and others are Nylon 6 nanofilters to evaluate the performances such as filtration efficiency and pressure drop. The HEPA filter is used as a commercialized one (Hollingsworth & Vose, HB5443), and Nylon 6 nanofilters are prepared by electrospinning. The homogeneity of the Nylon 6 nanofilters are confirmed as following methods. Several sample filters with the same nominal base weight and dimensions are prepared and the pressure drop across each filter is measured as a function of face velocity. The results show that the filters are the same. Fig. 6 shows the pressure drop of the HEPA filter and the Nylon 6 nanofilters as a function of face velocity. The normal criteria of the HEPA filter media show that the filtration efficiency is over 99.97% at 0.3 lm particles and the pressure drop is lower than 40 mmAq at 5 cm/s face velocity [10]. The test results show the pressure drop of the HEPA filter is in the normal range, however, the pressure drops of the Nylon 6 nanofilters are higher than that of HEPA filter. The pore size of the HEPA filter is 1.7 lm and the Nylon 6 nanofilters are 0.24 lm and the thickness of the HEPA filter is 500 lm and the Nylon 6 nanofilters are 50 lm (base weight: 5.75 g/m2) and 100 lm (base weight: 10.75 g/m2). When it is considered that the base weight of HEPA filter is 78.2 g/m2 and the Nylon 6 nanofilters are 5.75 and 10.75 g/m2, the Nylon 6 nanofilters are formed just like a thin film with very fine sized pores.
300
HEPA Filter 2 Nano Filter, Sample 1 (Base Weight: 5.75 g/m ) 2 Nano Filter, Sample 2 (Base Weight: 10.75 g/m )
Pressure Drop (mmAq)
250
200
150
100
50
0 2
3
4
5
6
7
8
9
10
11
Face Velocity (cm/s)
Fig. 6. Pressure drop measurements across the HEPA filter and the Nylon 6 nanofilters as a function of face velocity.
2
3
4
5
6
7
8
9
10
11
Face Velocity (cm/s)
Fig. 7. Filtration efficiency of the HEPA filter and the Nylon 6 nanofilters as a function of face velocity.
Fig. 7 shows the filtration efficiency of the HEPA filter and the Nylon 6 nanofilters as a function of face velocity. The filtration efficiency of test particle can be calculated by comparing with the particle number concentration between upstream and downstream of the test filter medium for the size range of each particle. The nanofilter sample 1 shows a good filtration efficiency to satisfy the criterion of HEPA filter for the filtration efficiency of 99.97% at 0.3 lm particle size. At the condition of 10 cm/s of face velocity, the filtration efficiency of nanofilter sample 1 is decreased to 99.96% while the other filters still satisfy the criterion of HEPA filter. As the velocity is increased, collection by diffusion is reduced, but collection by impaction rises. In the high velocity conditions, some particles, acquired more momentum, pass through the filters because of extremely thin thickness of the nanofilter sample 1. On the other hand, the nanofilter sample 2, which has double thickness to the sample 1, shows better filtration efficiency than commercialized HEPA filter and 99.993% at 5 cm/s using 0.3 lm particle.
4. Conclusions Electrospinning is a process of electrostatic fiber formation by which uses electrical forces to produce polymer fibers from polymer solution, with nanometerscale diameters. Nanofibers have a large specific surface area and a small pore size in comparison with conventional commercial textiles. Therefore, polymer nanofibers are being used or finding uses in the industrial and the biomedical fields. Especially, in this study, the application for the filtration medium is evaluated using nanofilters made by electrospun Nylon 6 nanofibers. The concentration of the polymer solution has shown a great influence on the fiber diameter as well as on bead. When the Nylon 6 concentration is 15 wt.%, electrospun fibers have an average diameter of 80 nm, but there are many beads, and the concentration of Nylon 6 increases to 24 wt.%, the fiber diameter gradually thickens to 200 nm, but there are not any beads. When
Y.C. Ahn et al. / Current Applied Physics 6 (2006) 1030–1035
the spinning distance is small, the thinner nanofibers are produced and the more fibers are collected on the grounded electrode. The filtration efficiency of Nylon 6 nanofilters is 99.993% superior to the commercialized HEPA filter at the face velocity of 5 cm/s using 0.3 lm test particles. Even though the high pressure drops across the nanofilter, they show the potential to have the application of HEPA and ULPA grade high efficiency filter. Acknowledgements This study is supported financially by Korea Institute of Science and Technology Evaluation and Planning and Pusan National University in the program, PostDoc. 2004. The authors gratefully acknowledge the financial support.
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