Article pubs.acs.org/IECR
Filtration Efficiency of Submicrometer Filters Muralidhar Lalagiri,† Gajanan Bhat,‡ Vinitkumar Singh,† Siva Parameswaran,§ Ronald J. Kendall,† and Seshadri Ramkumar*,† †
Nonwovens and Advanced Materials Laboratory, Texas Tech University, Lubbock, Texas 79409, United States Nonwovens Research Laboratory, University of Tennessee, Knoxville, Tennessee 37996, United States § Department of Mechanical Engineering, Texas Tech University, Lubbock, Texas 79409, United States ‡
ABSTRACT: Meltblown technology has recently gained a lot of attention in the production of submicrometer fiber webs with a fiber size less than 1 μm. Single layer polypropylene meltblown webs with fiber diameters ranging from 520 to 2100 nm were produced using nano and micro dies. To our best knowledge, there are no substantial data in the public domain on the filtration characteristics of single layer stand alone submicrometer meltblown webs and their comparison with micrometer sized webs. This paper focuses on the influence of fiber diameter on the filtration characteristics of standalone single layer submicrometer and micrometer sized meltblown webs. Submicrometer single layer webs showed higher filtration efficiency and a higher quality factor, whereas webs with fiber diameters more than a micrometer showed lower filtration efficiency and a lower quality factor. of the webs used were within a narrow range of 520−590 nm.15 Furthermore, this study has not presented any comparative data with micrometer sized webs, which in the authors’ view is important to prove the improvement in the filtration efficiency of submicrometer filters. Hassan et al.3 have investigated the effect of basis weight of meltblown webs on their filtration efficiency, MPPS, and quality factor. The work focused on the effect of die designs and process variables in developing meltblown webs with small fiber diameters. The samples in their study had fiber diameters ranging from 300 to 1500 nm with wide variations in their basis weights.3 Our work focused on understanding the characteristics of single layer submicrometer filter media used as a barrier for particulate matter in the atmosphere and contaminated environments. Such applications are mostly single use, and understanding the performance over a period of time is outside the scope of this study. In our study, we have used webs with fiber size ranging from 520 to 2100 nm, representing a broader range in fiber diameters than the webs used by Uppal et al.15 Additionally, the variation in the basis weight of the webs used in our study was small, and the weights ranged between 24.8 and 30 g/m2, unlike the single layered webs used by Hassan et al.,3 which varied from 0.22 to 20 g/m2. The study with comparative data of webs with fiber diameter ranging from 520 to 2100 nm and with less variation in basis weight will be useful to understand the influence of fiber diameter on the filtration characteristics of single layer submicrometer meltblown webs. This helps with the refinement of the meltblown technology to develop submicrometer filter substrates. Our paper presents the results of the comparative study on the filtration characteristics of meltblown webs with fiber sizes ranging as low as 520 nm to as
1. INTRODUCTION Microparticles from air and water are removed using nonwoven filter media, which are mostly made up of microfibers.1 Efficient removal of submicrometer and micrometer size particulate matter, while maintaining a relatively low pressure drop, are important qualities for filter substrates. Meltblown nonwoven substrates have emerged as promising filter media exhibiting higher filtration efficiency and effective performance. However, it is well understood that particles less than 100 nm are not efficiently filtered by nonwoven filter substrates.2 This necessitates the need to develop nonwoven filter media whose fiber size is less than a micrometer to have higher filtration efficiency, a lower pressure drop, and a higher quality factor at a smaller most penetrating particle size (MPPS). Submicrometer fibers due to their unique properties such as low density, large surface area to mass, high pore volume, and narrow pore size distribution are suited for developing highly efficient filter media.3 Although electrospinning is a well researched technique to develop submicrometer fibers in the laboratory, its production threshold value limits its practical utility.4−8 Currently, a majority of the filter media are developed using meltblown technology, which can conveniently produce webs with fibers of 1 μm size and above. Meltblown technology, which is based on mechanical dispersion, is a one step process, wherein a molten polymer is subjected to hot and high-velocity air to produce a web consisting of microfibers.9 A review of the literature has revealed a large body of work on the process characteristics of meltblown technology and properties of regular micrometer sized meltblown webs.10−14 However, limited information is available on submicrometer meltblown webs.3,15 Recently, two research studies have focused on the filtration efficiency of webs with smaller fiber diameters. Uppal et al.15 have reported the effect of meltblown parameters such as dieto-collector distance and air pressure on the filtration properties. Although submicrometer webs investigated in this study have shown higher filtration efficiencies, fiber diameters © 2013 American Chemical Society
Received: Revised: Accepted: Published: 16513
September 18, 2013 October 18, 2013 October 22, 2013 October 22, 2013 dx.doi.org/10.1021/ie403093t | Ind. Eng. Chem. Res. 2013, 52, 16513−16518
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Table 1. Physical and Filtration Characteristics of Submicrometer and Micrometer Sized Meltblown Webs fiber diameter sample ID
nm
SD
A B C D E
520 590 1620 1960 2100
120 170 230 410 560
a
a
weight (g/m2)
average pore size (micrometers)
air permeability (cm3/sec/cm2)
filtration efficiency (%)
pressure drop (mmH2O)
quality factor (mmH2O)−1
increase in quality factor (%) with sample E as base reference
24.8 25 28.6 30 28.7
6.4 6.7 9.06 10.00 10.09
14.8 17.1 34.48 30.84 32.91
76.65 73.48 58.24 50.89 34.62
5.45 5.53 3.88 4.39 3.61
0.267 0.240 0.225 0.162 0.118
126 110 90.6 37.2 NA
SD = Standard Deviation.
Figure 1. SEM micrographs of meltblown webs developed using different die spinnerets. A and B represent the samples produced using nano die, and C, D, and E represent samples produced using the micro die. 16514
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where P is the particle penetration and ΔP is the pressure drop across the filters, which are obtained from the filtration tester. Efficient filters have higher efficiency and lower pressure drop values, leading to higher quality factors.19
large as 2100 nm, while maintaining the smallest possible variation in the basis weight using 300 nm dioctyl phthalate (DOP) aerosol particles.
2. EXPERIMENTAL SECTION Metallocene based Achieve 6936G1 Resin of 1800g/10 min melt flow rate from Lyondell Basell Chemical Company was used to produce the meltblown webs. 2.1. Preparation of Meltblown Samples. Meltblown webs were developed using a 15 cm wide meltblowing pilot line at the Nonwovens Research Laboratory, University of Tennessee−Knoxville. Two types of dies (die A and die B) were used to develop meltblown webs with a range of fiber diameters. Normally, meltblown dies produce fibers with an average fiber diameter above 1 μm. Die A is a nano die developed by Arthur G. Russell Company, Bristol, Connecticut and is capable of producing fibers as small as 400 nm. Die A consists of 25 orifices per centimeter.16 Die B is a regular Exxon type die and is used to produce fiber diameters above 1 μm for various applications. Die B consists of a hole diameter of 250 μm and has 10 orifices per centimeter.17 2.2. Basis Weight. Basis weights were measured according to ASTM test method D 1117-97. The weights of the samples developed were between 24.8 and 30 g/m2. 2.3. Air Permeability. Air permeability is measured as the rate at which the air flows through a sample of 38 cm2 area at an air pressure differential of 125 Pa. Air permeability values of the meltblown webs were measured using a TexTest FX 3300 instrument according to ASTM Test Method D737. 2.4. Characterization of Meltblown Webs. Fiber diameters were measured using Etec Systems’ autoscan scanning electron microscope. The samples were coated with a gold layer and images were taken at 12 keV. Photomicrographs were taken at different positions on each web. Image Pro software from Media Cybernetics, Inc. was used to calculate the fiber diameters of the webs. Average fiber diameters were calculated by taking 50 to 100 readings. 2.5. Filtration Performance Testing. An automated particulate filtration tester TSI 3160 from TSI Inc., Shoreview, Minnesota, was used to measure filtration efficiency and pressure drop of the meltblown webs. The size of the DOP aerosols generated for testing the meltblown webs was 300 nm. Three different flow rates of aerosols: 13, 32, and 72 L/min were used to test the meltblown samples. The testing area of each meltblown sample was 100 cm2. 2.6. Filtration Studies. To evaluate the overall performance, the useful criterion is the Quality Factor (QF), which is also known as the figure of merit “Q”. In particulate filtration, not only the filtration efficiency of the filter is important but also the pressure drop across the filter. An efficient filter with very high resistance to airflow is not practically useable.18 The filtration efficiency was calculated using the following eq 1. E = 1 − (Cdown /Cup) = 1 − P
3. RESULTS AND DISCUSSION Table 1 shows the values of filtration efficiency, pressure drop, and quality factor in addition to physical properties such as fiber diameter, basis weight, air permeability, and pore size of five different meltblown webs used in the study. 3.1. Fiber Diameter versus Filtration Efficiency. As alluded to in the introductory section, the main goal of our work was to understand the relationship between particulate filtration efficiency of a single layer meltblown web and its fiber diameter. The understanding of the filtration characteristics of single layer webs is important in the filtration field as the development of single layer webs with high filtration efficiency can have greater technological impact. To our best knowledge, there are minimal data available in the public domain on the effect of fiber diameters on the filtration characteristics of single layer meltblown webs and more importantly submicrometer meltblown webs. As is evident from Figure 1A−E, there is a difference in the nature of webs produced by nano and micro dies. SEM images (Figure 1A−E) of the samples showed that average fiber diameters of the webs developed ranged between 520 and 2100 nm. Figure 2 shows the filtration efficiency of single layer meltblown webs versus fiber diameter produced by die A and
Figure 2. Filtration efficiencies versus fiber diameter at 300 nm particle size of DOP at a 32 L/min flow rate.
die B. As is evident in Figure 2, a negative correlation was observed between filtration efficiency and fiber diameter. Uppal et al.15 in their study have shown that as fiber diameters decreased from 590 to 520 nm, they observed an increase in filtration efficiency of the webs. These results corroborate with our findings in this study. Sample A with a smallest fiber diameter of 520 nm showed the highest filtration efficiency of 76.65% among all the samples, whereas sample E with the largest fiber diameter of 2100 nm showed the least efficiency of 34.62%. It is clearly seen that fiber diameter plays a critical role in influencing the filtration characteristics. The filtration efficiency of a nano web with 520 nm has more than doubled in comparison to a micro web with a fiber diameter of 2100 nm. This can also be attributed to the packing density of the web, as the fiber diameter increases the packing density decreases, which in turn decreases the filtration efficiency.20
(1)
where E is the particulate filtration efficiency, Cdown and Cup are the concentrations of aerosol particles in the downstream and upstream, and P is the penetration of particles. The quality factor of the webs was calculated using eq 2.
QF = ln(1/P)/ΔP
(2) 16515
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3.2. Air Permeability versus Filtration Efficiency. As discussed above, the filtration efficiency decreases with an increase in fiber diameter, and it is attributed to low packing density of the web. This low packing density can be related to the increase in the air permeability of the webs. Air permeability can be used to understand the packing density of the webs, which relates to filtration efficiency. Hassan et al.3 have reported that as the fiber diameter decreases, the pore size decreases and the packing density increases, which is inversely proportional to the air permeability of the webs. The findings in our study corroborate the results of Hassan et al.3 For all the samples used in the study that vary very little in their basis weights, it is observed that as fiber diameter increases from 520 to 2100 nm, the pore size increases from 6.4 to 10.7 μm, which results in the increase of air permeability values from 14.8 to 32.91 cm3/s/cm2 as shown in Figure 3.
thickness. In their study, the pressure drop increased drastically from 0.44 to 16.4 mm of H2O with the increase in thickness and basis weight of the webs.22 Although there was an increase in the filtration efficiency, it was shown that the quality factor of the webs decreased with an increase in basis weight. These findings corroborate with other results reported in the literature.3,15,23 In our study, the increase in filtration efficiency for samples A and B, which are submicrometer webs, were above 100% when compared with the filtration efficiency of sample E, whereas the increase in pressure drop when compared with sample E was only about 50%. The weights of the samples used in our study were in a close range while the fiber diameters ranged from 520 to 2100 nm. Because of the narrow range of basis weights, the increase in the pressure drop values with the decrease in the fiber size does not negatively affect the overall quality factor. The results indicate that fiber diameter has a greater influence on the filtration efficiency there by resulting in more than 100% increase in the quality factor values with the use of submicrometer fibers. 3.4. Fiber Diameter versus Most Penetrating Particle Size. MPPS determines the particle size at which the filter media is least efficient. Figure 5 shows particle penetration (%)
Figure 3. Air permeability versus fiber diameter.
3.3. Fiber Diameter versus Quality Factor. Although filtration efficiency is an indicator of a filter’s performance, from a practical point of view, quality factor is considered to be the useful parameter for quantifying a filter’s capacity.15,20 The quality factor values of the meltblown webs are given in Table 1. As is evident from Figure 4, sample A, which has the smallest
Figure 5. Particle penetration (%) versus particle sizes (μm) at 32 L/ min flow rate.
of the samples from A to E for particle sizes ranging from 0.03 to 0.4 μm. Particle size with highest penetration percentage corresponds to the MPPS of the samples tested. From Figure 5, MPPS for samples A to E was determined using TSI 3160. It can be seen from Figure 6 that the MPPS value of the meltblown webs decreased from 0.22 (sample E) to 0.17 μm (sample A) with the decrease in fiber diameter and pore size. Sample A had a fiber diameter of 520 nm and a pore size of 6.4 μm, whereas sample E had a fiber diameter of 2100 nm and a pore size of 10.09 μm. It is clearly evident that as the fiber diameter of the webs decreased, the pore size of the webs decreased.3 Podgorski et al.1 have theoretically calculated MPPS values of meltblown webs and have confirmed that the theoretical MPPS values fall in the range of values obtained experimentally. In their calculations, for a web with a fiber diameter of 10 μm, the MPPS value was 0.3 μm, and for a web with a fiber diameter of 0.1 μm, the MPPS value decreased to 0.05 μm.1 In our study, a similar trend was seen as reported by Podgorski et al.,1 and it was evident that the MPPS shifted toward a smaller value with the decrease in fiber diameters.
Figure 4. Quality factor versus fiber diameter at a 300 nm particle size.
fiber diameter of 520 nm, has the highest quality factor among all samples, which also has the highest pressure drop. Generally, quality factor is inversely related to pressure drop of the web.3,20,21 Pressure drop increases due to various factors of the web such as smaller fiber size, increase in basis weight, high packing density, increase in thickness, decrease in pore size, etc. Leung et al.22 in their study have shown the impact of basis weight and thickness of electrospun nanowebs on pressure drop across the web. Electrospun webs used in that study had the same fiber diameters and varied in their basis weights and 16516
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Figure 6. MPPS (μm) versus fiber diameter (nm) at a 32 L/min flow rate.
Figure 8. Pressure drop at 300 nm particle size at different flow rates.
3.5. Flow Rate versus Filtration Efficiency and Pressure Drop. Meltblown webs were tested at three different flow rates of 13, 32, and 72 L/min to study the effect of flow rate on the filtration efficiency and pressure drop using 300 nm DOP aerosol. In our study, irrespective of the physical characteristics of webs, samples A−E have shown a decrease in the filtration efficiency with an increase in pressure drop as the flow rates increased. For sample A, which has the highest filtration efficiency, there was a decrease of 11% in the efficiency in comparison to a 36% decrease in sample E as the flow rate increased from 13 to 72 L/min. Results show that filtration efficiency decreased more for the samples with larger fiber diameters. As flow rate increases, particles with high kinetic energy flow through the filter media, which reduces the effect of particle capturing mechanisms such as inertial impaction, interception, and Brownian diffusion. This results in low filtration efficiency, as shown in Figure 7.
fiber diameter of the webs directly correlated with the filtration efficiency and quality factor of the webs. In comparing the filtration characteristics of single layer meltblown webs with different fiber diameters that are in close range of basis weights, it was clearly seen that the increase in filtration efficiency was much higher than the increase in pressure drop, which gave high quality factor values. MPPS decreased with a decrease in the fiber diameter, resulting in high filtration efficiency for submicrometer webs. The effect of particle flow rates on the filtration efficiency and pressure drop of the meltblown webs were investigated, and it was evident that webs with large fiber diameter had a high percentage decrease in efficiency compared to submicrometer webs.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: 806 742 4567. E-mail:
[email protected]. Author Contributions
All the authors contributed to the work. M.L., G.B., and S.R. designed the study. All the authors approve the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to acknowledge the help of R. Uppal, C. Eash, K. Akato, and H. Wanli of UTNRL-Knoxville in providing samples and mechanical test data used in this paper. The work was supported by The CH Foundation of Lubbock, Texas.
Figure 7. Filtration efficiencies at 300 nm particle size at different flow rates.
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REFERENCES
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On the other hand, pressure drop increased dramatically for sample A to sample E with an increase in flow rate from 13 L/ min to 72 L/min, as is evident in Figure 8. For sample A, the increase in pressure drop for flow rates from 13 to 72 L/min was 380%. As the flow rate increases, the pressure on the web increases in capturing the particle, which is in agreement with Darcy’s law.19
4. CONCLUSIONS Meltblown technology could serve as a high productive process to develop uniform submicrometer filters that could have high quality factors. A nano die was used in the meltblown process, and results showed that single layer submicrometer meltblown webs with good filtration efficiency could be produced. The 16517
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