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Ki Myoung Yuna, b, Adi Bagus Suryamasa, Ferry Iskandara, d, Li Baoc, Hitoshi Niinumac and Kikuo Okuyamaa, 
, 
Polymer 
nanofiber
mats with various morphology structures (nanofiber, beaded-nanofiber, and composite particle/nanofiber) were prepared by electrospinning for application to aerosol particle filtration. The filtration performance of these polymer nanofiber
mats was evaluated based on quality factors generated from measurement
of penetration through the filter by sodium chloride (NaCl) aerosol
particles ranging from 20 to 300 nm. These filtration test results
showed that the quality factors of beaded-nanofiber
filter mats were the best, even though the aerosol particle penetration
of them was the highest of the morphology structures. The results of
the present study show that morphology optimization of polymer nanofibers
is an effective method for improvement of filtration performance, and
it must be the future direction for development of new filtration media.
Polymer nanofiber mats with various morphology structures (nanofiber, beade-nanofiber, and composite particle/nanofiber)
were prepared by electrospinning for application to aerosol particle
filtration. The filtration performance was evaluated based on quality
factors generated from measurement of penetration through the filter by
sodium chloride (NaCl) aerosol particles ranging from 20 to 300 nm.
Polymer nanofiber mats were prepared by electrospinning. The morphology of nanofiber can be modified by changing the precursor concentration. Polymer nanofiber mats were used as filtration media of aerosol particles. The filtration performance was evalutated based on quality factors. The quality factor of beaded-nanofiber mats were the best.
Keywords: Filtration; Electrospinning; Nanofiber; Aerosol particle; Quality factor; Optimization
nanofibers
have received considerable attention as promising materials for aerosol
filtration media, due to unique properties such as a high specific
surface area, a good interconnectivity of pores, and a small pore size,
which makes them preferable as an aerosol filtration media [1], [2], [3], [4], [5], [6] and [7].
According to the filtration theory, a reduction in fiber size leads to
better filtration efficiency, and typically the mass and thickness of nanofiber filters are quite low compared to their micrometer-sized counterparts. However, the pressure drop across nanofiber filters is often substantially higher than it is across traditional filter media [2] and [3].Electrospinning is a viable route for the production of exceptionally long and uniform polymer nanofibers. Recent study has shown that the morphology of a polymer nanofiber
can be easily controlled by varying the polymer concentration of the
precursor solution. Variations in polymer concentration result in
differing morphologies: microspherical particles, beaded-nanofibers, and nanofibers [8].
While electrospinning can be used to generate fibers of varying
morphological types, only the filtration performance of pure fibers has
previously been examined. To the best of our knowledge, there is no
report on the filtration performance of a modified-morphology nanofiber.
In
the present study, we propose a new direction in the development of
filtration media. The purpose of this work is to examine the performance
of electrospun fibrous filters in collecting aerosol nanoparticles.
Unlike previous studies, the effect of fiber morphology on filtration
performance (penetration and pressure drop) was determined. Three types
of polymer nanofiber mats with different morphologies, i.e., nanofibers, beaded-nanofibers, and composite particle/nanofibers, were prepared and then the filtration performances of polymer nanofiber
mats were evaluated based on quality factors by measuring the
penetration throughout the filter of aerosol particles ranging in sizes
from 20 to 300 nm.
Polyacrylonirile (PAN, Mw = 150 kDa) and polymethylmethacrylate (PMMA, Mw = 90 kDa) were used in the experiment. PAN was used to prepare nanofiber mats and PMMA was used to prepare beaded-nanofiber mats, and polymer particles for composite particle/nanofiber mats. The PAN and the PMMA pellets were dissolved in N,N-dimethylformamide (DMF) to produce suitable concentrations of the polymer solution.
A schematic of the experimental system for the preparation of fibrous filter mats is shown in Fig. 1.
The polymer solution was placed into a hypodermic syringe that was
operated using a syringe pump (Harvard Apparatus PHD 2000). A stainless
steel needle attached to hypodermic syringe with an inner diameter of
700 μm was connected to a high voltage source (HERR 20R3, Matsusada)
with a suitable liquid flow rate. The highly charged polymer nanofibers
and particles from the capillary were deposited on the rotating
cylinder collector, which was positioned at 15 cm from the tip of the
capillary for all experiments. A high-speed video camera (HPV-1,
Shimadzu Ltd., Japan) was used to observe the electrospun fiber during
the experiment. The morphologies of the prepared nanofiber mats were observed using a field emission scanning electron microscope (FE-SEM, S-5000, Hitachi Ltd., Japan).
| Full-size image (33K) |
Schematic of the experimental system for the preparation of fibrous filter mats.
Fig. 2 shows the schematic of the experimental system for the measurement of nanoparticle penetration throughout the filter mats. The nitrogen gas (3.9 L min−1) was sent into a NaCl evaporation–condensation aerosol generator that produced polydispersed NaCl nanoparticles [7]. An Am-241 bipolar charger was used to charge-neutralize the NaCl particles, followed by particle-size selection using a nano-differential mobility analyzer (Long-DMA, TSI model 3081, Shoreview, MN, USA). Singly charged, monodispersed particles ranging from 20 to 300 nm exited the Long-DMA in concentrations of 104 cm−3 and were sent to the test filter. To charge-neutralize the particles, they were passed through an Am-241 bipolar charger. The test filter was held in a 20 mm diameter filter holder, and the flow rates at the DMA outlet varied from 0.3 to 2.0 L min−1, which corresponded to a filter-face velocity of 5.3 cm s−1. The concentration of particles at the filter outlet was measured using an ultrafine particle counter (CPC, TSI model 3025) operated at a flow rate of 0.3 L min−1. Particle penetration was determined by measuring the concentration of particles at the filter holder outlet with and without a filter installed, and then calculating the ratio of the concentration of particles with the filter installed to the concentration without the filter.
| Full-size image (45K) |
Schematic of the experimental system for the measurement of nanoparticle penetration throughout the filter mats.
In
the presence of an electric field, conducting polymer solutions form
electrically driven jets at the outlet of a capillary because of the
balance between surface tension and electrical forces. Variation of
polymer concentration resulted in differing morphologies: microspherical
particles, beaded-nanofibers, and nanofibers [8].
At sufficiently low polymer concentrations, the liquid jet of a polymer
solution breaks up into droplets and subsequent solvent evaporation
results in the formation of microspherical polymer particles [9], [10] and [11].
On the other hand, by increasing the polymer concentrations, the liquid
jet that is emitted from the cone will either form as a beaded-nanofiber or a nanofiber
after volatile solvents have evaporated. The evolution of
particle-to-fiber formation as a function of polymer concentration is
shown in Fig. 3.
| Full-size image (50K) |
The formation of particles (a), beaded fibers (b), and fibers (c) as a function of polymer concentration during electrospinning.
Three types of electrospun polymer nanofiber filter mats with differing morphologies were produced: nanofiber (NF), beaded-nanofiber (BF), and composite particle/nanofiber
(CF). The morphology modification was accomplished by selecting a
suitable precursor during electrospinning. Three precursor solutions,
dissolved in N,N-dimethylformamide (DMF), were used in the
present experiment. For NF filter mats, a PAN solution (10 wt.%) was
used as the precursor. For BF filter mats, a PMMA solution (25 wt.%) was
used as the precursor. For CF filter mats, a PMMA solution (20 wt.%)
and the PAN precursor (10 wt.%) solution was the precursor. The NF and
BF filter mats were produced via a one-step electrospinning process.
Whereas, the CF filter mats were produced by a cycle process of the NF
filter mat production followed by polymer particle production to
incorporate microspherical particles to the NF filter mats. The
experimental parameters of each sample are summarized in Table 1. The experimental parameters were adjusted to obtained a mean fiber diameter of around 400 nm (±5%) for all samples.
| Morphology | Sample name | Material | Flow rate (μL min−1) | High voltage (kV) | Mean fiber diameter (nm) | Collection time in electrospinning (min) | Filter weight (g/m2) |
|---|---|---|---|---|---|---|---|
| Nanofiber | NF4 | PAN | 40 | 13 | 420 | 20 | 1.15 |
| NF5 | PAN | 40 | 13 | 420 | 40 | 1.80 | |
| Beaded-nanofiber | BF4 | PMMA | 40 | 9 | 390 | 20 | 0.5 |
| BF5 | PMMA | 40 | 9 | 390 | 40 | 0.70 | |
| Composite particle/nanofiber | CF4 | PAN | 40 | 13 | 420 | 20 | 2.45 |
| PMMA | 40 | 7 | 20 | ||||
| CF5 | PAN | 40 | 13 | 420 | 20 | 3.50 | |
| PMMA | 40 | 7 | 40 |
Fig. 4
shows the SEM images of NF, BF, and CF filter mats. The SEM images show
that all prepared filter mats are of a non-woven type, with different
morphologies: nanofiber, beaded-nanofiber and hybrid particle/nanofiber.
Furthermore, the images also confirm that a suitable precursor resulted
the different morphologies in the electrospinning process. The fiber
size distribution for all types of filter mats is shown in Fig. 5.
The fiber size distribution was determined by measuring more than 200
fibers for all types of filter mats. The bar on the figure shows the
measurement distribution, whereas the line is an approximation of the
distribution function based on a Gaussian distribution approximation.
The mean fiber diameter was calculated based on this approximation. As
shown in Fig. 5,
all prepared filter mats had a mean fiber diameter of around 400 nm.
This result shows that electrospinning can be used for the production of
a uniform nanofiber.
| Full-size image (71K) |
SEM images of nanofiber (a), beaded-nanofiber (b), and composite particle/nanofiber (c) filter mats.
| Full-size image (33K) |
Size distribution of fibers in nanofiber (a), beaded-nanofiber (b), and composite particle/nanofiber (c).
The
size distribution of the beads in BF filter mats and the polymer
particles in CF filter mats were also measured by the same fiber-size
method. The beads and particle size distribution are shown in Fig. 6.
Although the fibers were of uniform size, the sizes of the beads in the
BF filter mats were relatively polydispersed. The particle sizes in the
CF filter mats, however, were relatively monodispersed. The primary
determinants of formation of electrospun beaded nanofibers are as follows: solution viscosity, net charge density carried by the electrospinning jet, and surface tension of the solution [12].
Taking these factors into account, the formation of beaded fibers is
dependent on the stability of the electrically driven jet of the polymer
solution. Additionally, to produce a filtration media of uniform filter
thickness, the positions of both the capillary and collector shown in Fig. 1
were kept constant, which resulted in jet bending instability. Hence,
non-uniform beads were produced in the present study, as shown in Fig. 6.
During the electrospray process, however, uniform droplets could be
produced with a stable cone-jet of liquid at the outlet of the
capillary. Previous studies of polymer particle production via the
electrospray of various polymer solutions have produced particles that
were relatively monodispersed [9], [10] and [11].
| Full-size image (28K) |
Size distributions of beads in beaded-nanofiber filters (a) and polymer particles in composite filters (b).
Fig. 7
shows the pressure drop analysis of the tested filters as a function of
the face velocity. Pressure drop was a nearly linear function of
filtration velocity. The beaded fiber filter had a low initial pressure
drop as compared with PAN nanofibers, due to their low deposition rate on the collector (Table 1).
Presumably, the formation of beads within the electrospun jet enhanced
jet instability and that resulted in a low collection rate for the
beaded fibers. The composite filters were produced by controlling the
collection time of the PMMA polymer particles. There was only a small
change in the values of the initial pressure drop of nanofiber and composite filters, as shown in Fig. 7,
even though the composite filter contained PMMA polymer particles. This
result suggests that insertion of the polymer particles into nanofiber
filters has a small effect on the pressure drop across the filters.
However, clogging of the pores in the electrospun web could have
occurred, because the polymer particles tend to collect within the pores
of the less-charged areas on the electrospun web with residual
electrostatic charges. This would lead to an increase in pressure drop.
As a result of these phenomena, only small changes in the pressure drop
were observed for the composite filters.
| Full-size image (34K) |
Pressure drop versus velocity plot for three kinds of tested filters versus face velocity.
The penetration of charge-neutralized nanoparticles in the size range of 20–300 nm for each filter tested is shown in Fig. 8.
Two filter mats for each type with different collection times were used
in the experiment. Filter thickness was controlled simply by adjusting
the collection time during electrospinning. In each test, the face
velocity was maintained at 5.3 cm s−1. The nanoparticle
penetration through the filters was decreased with increasing increments
of filter thickness. Nanoparticle collection typically is
diffusion-dominated. Therefore, the penetration increases as particle
diameter increases until it reaches a critical diameter size, which is
known as the most penetrating particle size (MPPS). For particle
diameters higher than MPPS, the penetration was slightly decreased for
each filter. High-efficiency filters for particle collection could be
easily produced by increasing the thickness of the filter; however, the
pressure drop across the filters increases linearly with filter
thickness, because the flow through electrospun filters obeys Darcy's
Law [13].
The penetration of the composite filters was decreased only slightly,
although increasing the collection time of the electrosprayed PMMA
particles dramatically increased the filter weight (Table 1). This observation indicates that insertion of the polymer particles into nanofiber filters has a small effect on nanoparticle penetration through the filters.
The quality factor is often used to evaluate filtration performance, which is defined by the following equation:where qf is the quality factor and Δp
is the pressure drop across the filter. The quality factor also is
known as a filter's figure-of-merit. This represents the ratio between
the efficiency and the pressure drops across a filter, and, thus, a
larger value indicates a better quality of filter. Fig. 9 shows the quality factor as a function of particle diameter for the tested filters in Fig. 8. The measured quality factor was the highest for the beaded-
nanofiber filters and was the lowest for the nanofiber
filter. A limited number of studies based on filtration theory have
investigated the quality factor effect for nanofibrous filters [6], [14] and [15].
On the basis of these studies, the quality factor is primarily a
function of the filter fiber diameter and the volume fraction. Both the
beaded fiber and composite filters enhanced the filtration performance
for particles in the MPPS range, although the polymer particles in the
composite filters had a very small effect on both pressure drop and
particle penetration. A possible reason for the enhancement of the
quality factor was the improvement of the volume fraction by the beads
and the PMMA particles in the nanofibers. Kalayci et al. [14] obtained similar results when they found that insertion of polymeric microspheres into nanofibers resulted in a physical separation of the nanofiber layers, which improved nanofibrous filter performance for particles 300 nm in diameter [14]. The authors concluded that use of a spacer to increase the distance between nanofibers
decreases the volume fraction of the structures; thus, filter
performance increases as air permeability increases. However, it seems
unlikely that the volume fraction is the only explanation for the
enhancement of filtration performance that results from use of either
beaded fiber filters or composite filters. Another possible explanation
is that electrospun nanofibers fuse to one another because of their extremely high aspect ratio and viscous-elastic properties. As a result, nanofibers might bundle together, forming pores in the web, which results in poor filtration performance [16]. Incorporation of microspheres into nanofibers
to form either beaded fiber filters or composite filters decreases the
volume fraction and increases the effective fiber surface area.
Nanofibrous
filters with mean fiber diameters around 400 nm were prepared by
electrospinning for use as a filtration media. Three types of filter
mats (nanofiber, beaded nanofiber, and composite particle/nanofiber)
were examined to investigate the effect of morphology modification on
filtration performance. The performance of electrospun filters was
evaluated by measuring the penetration of monodispersed NaCl aerosol
particles (less than 300 nm in size) through the filters. The
penetration of nanoparticles through the electrospun filter media was
reduced by increasing the filter thickness, which was controlled by
adjustment of the collection time during electrospinning. For an
electrospinning collection time of 40 min, the quality factor for
beaded-nanofiber, composite particle/nanofiber, and nanofibers ranged between 0.2351–0.3560 Pa−1, 0.0947–0.2068 Pa−1, and 0.0511–0.1740 Pa−1, respectively. Overall, this method, which modifies the internal structure of electrospun nanofibers
using microspheres, will allow the production of advanced fibrous
filter media. To enhance our understanding, controlling the size of
microspheres within nanofibrous filters will be the focus of our future
work.
This work was supported by Grant-in-Aid for Scientific Research (A) No. 22246099 from the Japan Society for the Promotion of Science (JSPS). We gratefully acknowledge the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan for the provision of doctoral scholarships (A.B.S.).
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