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Application of nanofibrous membranes with antimicrobial agents.

INTRODUCTION

This paper will introduce nanofibrous membranes as applied in the HVAC industry and discuss their ability to incorporate antimicrobial agents to inhibit bacterial and fungal growth on the membrane. Nanofibrous membranes are filter media comprised of fibers whose diameters are smaller than 1[mu] m, resulting in a large amount of surface area per unit mass (Graham et al., 2002). Nanofibrous membranes are a fairly new type of filter media that offer great potential because the small diameter fibers and the small pores within the membrane, which allow small particulate to be removed from the air stream. Nanofibrous membranes offer the option of incorporating antimicrobial agents at the time of manufacturing to suppress microorganism growth on the filter.

Nanofibers have been incorporated into "protective clothing, biomedical applications including wound dressings ... [and] structural elements in artificial organs and in reinforced composites" (Ahn et al., 2006, p. 1030). Nanofibers have been recently introduced to the HVAC industry because they offer great potential to the air filtration industry, because of their larger surface area and smaller pore sizes compared to current commercial filter media (Ahn et al., 2006). Nanofibrous membranes are currently only produced by Donaldson Company Inc., Finetex Technology Co., and Freudenberg Nonwovens for the HVAC market, as dust collectors and air filters.

Nanofibrous membrane fiber range from 40-2500 nm in diameter, whereas traditional commercial filters' fibers range from 0.1-30 [mu] m, or larger, depending on the type of filter. Nanofibers' small diameters and long lengths have raised some initial concerns as they show similarities to asbestos fibers; however, research has not shown that nanofibers have carcinogenic affects, although further research is needed to confirm or disprove this concern. Nanofibrous membranes have great potential as filters because of their ability to remove small particulate from the air stream with a membrane thickness of 100 [mu] m or smaller compared to traditional commercial filters ranging in thickness from 1 inch to 30 inches (Edelman, 2008). Shin, Chase and Reneker (2005) note that nanofibrous media are advantageous in filtration applications because of their high efficiency and economical energy cost, given the generally lower pressure drop and large surface area per unit mass. While nanofibrous membranes have much smaller pores than traditional commercial filters, they are still highly permeable to airflow. Thus, a membrane with the same efficiency as a traditional filter may have a slightly smaller pressure drop because of the increased permeability or provide increased efficiency while maintaining the same pressure drop.

These membranes must be installed with a substrate, a low efficiency nonwoven mat, which acts as a support for the nanofibrous membrane and gives it greater physical strength, because of the membrane's weak and thin structure. If a nanofibrous membrane were installed in a system without a substrate, it would become damaged quickly from the impact of particles being filtered from the air. Podgorski, Balazy, & GradoE (2006) recommend that the substrate be relatively thin composed of "fibers ... dozens of micrometers in diameter" (p. 6814). A commercially available low efficiency disposable panel filter can be used as the substrate. Another possibility for installing nanofibers is to incorporate the fibers into other filter media when manufactured, to increase the filter's efficiency without drastically increasing the pressure drop across the filter. This paper will focus on the idea of a nanofibrous membrane applied to a substrate and acting as a filter on its "own" to show the potential to replace traditional commercial filters. Nanofibrous membranes and nanofibers are a fairly new concept, but have gained great interest in the filtration industry, because of their construction and the ability to control the membrane composition. Figure 1 is a Scanning Electron Micrograph (SEM) of a nanofibrous membrane applied to a substrate. Figure 1 shows the drastic difference in fiber diameter and pore size of the nanofibrous membrane and its substrate. The nanofibers are approximately 250 nm in diameter while the substrate's fibers are greater than 10 [mu] m (Graham et al, 2002).

[FIGURE 1 OMITTED]

MEMBRANE CONSTRUCTION

Nanofibers can be created through two processes: the electrospinning process and the melt blown process. The manufacturing processes will be discussed in the following subsections because the processes yield nanofibers with varying fiber diameters. The discussion of these processes is important as it will give a foundation for later subsections when addressing the ability to control the fiber diameters and the membranes' filtering capabilities.

Electrospinning

Jet electrospinning to create nanofibrous membranes is the most commonly used method when creating nanofibrous membranes for testing. Figure 2 is a schematic of the equipment used to create nanofibers and nanofibrous membranes using the jet electrospinning process.

[FIGURE 2 OMITTED]

The equipment needed for the electrospinning process includes a voltage supply as large as 30 kV, a programmable supply syringe, a needle, and a grounded collector screen (Sawicka & Gouma, 2006). The electrical high voltage is applied to a polymer solution, and when the voltage "overcomes the surface tension of the solution" (Ahn et al., 2006, p. 1031), it causes the solution to deform and lengthen, forming a solution with nanofibers (Kalayci, Ouyang, & Graham, 2006). The solvent then evaporates out of the solution, leaving the nanofibers to collect randomly on a grounded screen (Shin et al., 2005; Sawicka & Gouma, 2006). The nanofibers can be electrospun directly onto a substrate in lieu of a screen, which would then become the filter media eliminating a step from the manufacturing process. The nanofibrous membranes can be formed on the screen or substrate in any size and shape to any desired thickness; size and thickness depend on the volume of solution that is electrospun and the amount of layering of the fibers. Kalayci et al., (2006) say that nanofibers created from the nozzle electrospinning process are capable of producing diameters as small as 40 nm and up to 500 nm.

The electrospinning process is advantageous because membranes can be customized to replace filters of various sizes and performance levels. The size of the membrane may be specific to the filter size and shape needed determined by either the space in which air is being distributed or by the size of the holding frame located in a piece of air distribution equipment. On the other hand, the membrane thickness will be defined by the desired efficiency of the membrane. As the membrane's thickness increases, the efficiency increases because a greater amount of particulate will be removed through depth filtration. While thicker filter media offer increased efficiency, the added thickness also results in additional pressure drop across the membrane. As with traditional filters, the added pressure drop across the membrane would directly affect the fan size and system energy consumption. Therefore, the membrane thickness depends on the membrane's application, the efficiency, and allowable pressure drop.

Through the jet electrospinning process, the production of a single membrane could take hours, depending on the desired size and thickness, as a single jet produces one continuous fiber to form the membrane. Clearly, to make jet electrospinning an economical means of producing nanofibers, multiple jets, possibly thousands, would need to be used at one time (Petrik & Maly, 2009). Accordingly, a nozzle-less jet electrospinning process has been developed to increase production of nanofibers without sacrificing the quality and consistency of the fibers. The nozzle-less process consists of a rotating electrode, which is dipped into the polymer solution, forming a small layer of the solution on the electrode (Petrik & Maly, 2009). As with the jet nozzle electrospinning process, a voltage is applied, but rather than a single jet, multiple jets are formed across the electrode.

Melt Blown

The melt blown process is the second method of constructing nanofibrous membranes. This process typically produces fiber diameters five to ten times larger than fibers created by electrospinning (Podgorski et al., 2006). Petrik & Maly (2009) state the melt blown process produces fibers with diameters of 800-2500 nm compared to the nozzle-less electrospun process, which produces fibers with diameters of 80-500 nm.

In the melt blown process, a polymer solution is placed in a container, which is transferred through the extruder and electric heater, then into the die with the flow rate being controlled by the motor (Podgorski et al., 2006). The polymer solution is then forced through a row of nozzles within the die, and hot air from the compressor transforms the solution into the desired fiber diameters (Podgorski et al., 2006). The fibers are finally collected on a rotating mandrel that moves back and forth to create a membrane of desired size and thickness (Podgorski et al., 2006). The melt blown process offers potential for commercial applications because of the ability to produce large quantities of nanofibrous membranes at a relatively low cost (Podgorski et al., 2006). Currently, research is geared at improving the melt blown process to produce fibers with smaller diameters (Barhate & Ramakrishna, 2007).

MEMBRANE PROPERTIES

Nanofibrous membranes can be formed from a variety of polymer and polymer blends, which offers variety in composition. Additionally, the membrane's fiber diameter, porosity, texture, and structure can be changed by using different polymer solutions (Burger, Hsian, & Chu, 2006). The ability to control construction offers opportunities to create a membrane with characteristics designed to match specific applications.

Fiber Diameter and Pore Size

Nanofiber diameters can vary depending on the specific polymer and solvent combination (Kalayci et al., 2006). An experiment performed by Yun et al. (2007) determined that with proper polymer concentration, the diameters of fibers created by the electrospinning process were more uniform than the fiber diameters in commercially produced filters. The nanofibers' diameters can also be changed by altering the concentration of the solution used in the electrospun or melt blown manufacturing process. Thus, a less concentrated solution creates a thinner fiber than does a more concentrated solution, but two concerns arise with lowering the concentration of the solution (Podgorski et al., 2006). The first concern is the increase in toxins and hazardous vapors released from the electrospinning process, which imposes additional costs and supplies to the process, to ensure these vapors are properly disposed of (Podgorski et al., 2006). The second issue with electrospinning a solution with too low of a concentration is the formation of small beads, rather than the desired continuous jet (Shin et al., 2005). The beads form because the solution has a lower viscosity, which results in extra polymer solution being released from the jet because the electric field is unable to stretch the solution properly (Patanaik, Jacobs, & Anandjiwala, 2010). Bead formation will directly affect the membrane's performance because it creates a large variation of fiber diameters throughout the membrane. This variation creates inconsistency throughout the membrane, which results in uneven loading, lower efficiency, and increased pressure drop. A study performed by Ahn et al. (2006) electrospun membranes with varying concentration of Nylon 6 and found that the membrane with 24% concentration has the largest fiber diameters at 200nm, compared to the 15% concentration fiber diameters of 80 nm. The increased concentration restricts the amount of stretching the fibers will undergo, therefore resulting in larger fiber diameters (Ahn et al., 2006).

The nanofibrous membranes have small pores, which allows for greater removal of small particulate through interception and straining compared to the more traditional filters used in the commercial building design industry. Straining is typically the only efficient means of filtration for large particulate, but as a membrane's pores become smaller, a greater amount of smaller particulate can be removed through straining. Additionally, these membranes are still very porous to the airstream. High porosity across a filter offers the potential for greater efficiency without increasing the pressure drop across the membrane because of the effects of slip flow.

Airflow Resistance and Efficiency

Generally, as fiber diameters decrease, the pressure drop related to a filter increases because of the smaller pores formed from the overlapping fibers. However, nanofibrous membranes have small pores without a dramatic increase in their pressure drop because of the concept of slip flow, whose effects are applicable because of small diameter fibers within nanofibrous membranes. Slip flow is the drag force that occurs on a small fiber, because "the molecular movements of the air molecules are significant in ... relation to the size of the fibers" (Graham et al., 2002, p. 3). The importance of considering slip flow can be determined by a fibers Knudsen number, Kn:

Kn = [lambda] /R (1)

where is the mean free path of air molecules, and R is the fiber's radius (Brown, 1993). The mean free path is "the dimension of the noncontinuous nature of the molecules" (Barhate & Ramakrishna, 2007, p. 5) of the air. When Kn is greater than 0.1, slip flow should be a factor considered for filtration, and when Kn is greater than 0.25, slip flow definitely needs to be considered (Graham et al., 2002). Barhate and Ramakrishna (2007) identify 0.066[mu]m as the mean free path for standard air conditions; therefore slip flow must be considered for fibers with diameters smaller than 0.5[mu] m, which encompass most nanofibers. Graham et al. (2002) state that "due to slip at the fiber surface, drag force on a fiber is smaller than that in the case of non-slip flow, which translates into lower pressure drop" ( p. 3).

Yun et al. (2007) found that a filter comprising electrospun polyacrylonitrile (PAN) fibers was capable of removing the same amount of nanoparticles as standard HEPA and ULPA filters, but the filter comprising nanofibers had a smaller pressure drop. Historically, a higher efficiency filter meant a higher pressure drop, but contrary to this belief, high efficiency filters or membranes can be installed without sacrificing pressure drop if they have nanofibrous membranes (Matela, 2006). Podgorski et al. (2006) state that nanofibrous membranes have a significantly greater efficiency of removing the most penetrating particle size (MPPS), 0.3[mu]m, compared to standard fibrous filter with only a slight increase in pressure drop. This would then mean a nanofibrous membrane with the same efficiency as a traditional filter would register a smaller pressure drop. Research is limited and offers differing results, but most studies note a decrease in pressure drop for nanofibrous membranes compared to that of a traditional filter with the same efficiency.

While tests have been conducted to determine the efficiencies of nanofibrous membranes, currently no set of criteria is established for testing nanofibrous membranes. Instead the efficiencies of these membranes have been determined using similar testing procedures that exist for standard filters, but such tests introduce inconsistency when evaluating the particle size and face velocity. For example, two separate studies determining the efficiency of nanofibrous membranes were performed by Patanaik et al. (2010) and Ahn et al. (2006). Each of the studies had different testing conditions, which resulted in efficiencies that varied greatly from one to another, making it difficult to directly compare membrane performance to establish an increased performance compared to traditional filters. The variation in test results show that nanofibrous membranes can be created with a variety of efficiencies, offering the potential for these membranes to be installed in a large range of applications. The study from Ahn et al. (2006) indicated that nanofibrous membranes are capable of competing with HEPA filters because the membrane exceeded the 99.97% efficiency minimum required by HEPA filters, although further research needs to occur because of the membrane's high pressure drop. The membrane studied by Patanaik et al. (2010) had an efficiency of 97.15% in removing particles 0.6-180 [mu]m. Although efficiency was less than required efficiencies of HEPA filters, the membrane could potentially replace HEPA filters depending on its efficiency with 0.3[mu]m because the membrane's pressure drop of 0.1" w.g. is much smaller than the pressure drop of a typical clean HEPA filter, which varies between 1.0" and 1.5", depending on the manufacturer. Clearly, for the nanofibrous membranes to be a competitive option in the HEPA filter industry the pressure drop would need to be equivalent to or lower than that of current filters to ensure system efficiencies. The varying results of the three tests reveal the need for the development of a set of testing standards to allow a more accurate comparison of different nanofibrous membranes. To date these membranes have primarily been tested in a laboratory setting. Additional testing is needed on membranes installed in a HVAC system to provide needed information of the membranes' performance in an actual application.

ANTIMIRCROBIAL AGENTS

Antimicrobial agents can be added to the filter media to inhibit bacterial and fungal growth. Bacterial and fungal growth can cause significant health problems as they may be transmitted through the air stream and "may cause a wide variety of illnesses when deposited in the respiratory tract" (Maus, Goppelsroder, & Umhauer, 2001, p. 105). The air conditions, filter characteristics, organisms of concern, and antimicrobial agents are all factors to consider when attempting to inhibit bacterial and fungal growth. Foarde, Hanley, and Veeck (2000) note that not all bacteria or fungus "are killed or suppressed equally by the same antimicrobial" (p. 52) agent. Therefore, an antimicrobial agent should be carefully selected based on the microorganism(s) intended to be suppressed on the filter media.

Nanofibrous membranes can incorporate antimicrobial agents into the polymer solution prior to the electrospinning process. This manufacturing application is an advantage of nanofibrous membranes because the agent composition maintains itself over the life of the membrane compared to the surface application used in traditional air filters. Yoon et al. (2008) note that the process can be done with ease. The effectiveness of antimicrobial agents has been established in numerous studies. Jeong, Yang, and Youk (2007) tested the effects of ammonium compounds added to a polyurenthane (PU) polymer to act as an antimicrobial agent. The polymer was electrospun into a nanofibrous membranes, and the filter experienced a 99.9% reduction in bacterial and fungal colonies after incubation for 24 hours (Jeong et al., 2007). Another study performed by Kim, Nam, Rhee, Park, and Park (2008) found that when benzyl triethylammonium chloride (BTEAC) was added to a polycarbonate (PC) solution, the number of bacteria colonies was reduced by 99.9% after 18 hours of incubation. A third test by Lala et al. (2007) tested the effectiveness of silver nanoparticles as an antimicrobial agent when incorporated into three different solutions prior to electrospinning. The three solutions were cellulose acetate (CA), polyacrylonitrile (PAN) and polyvinyl chloride (PVC). All three membranes saw a reduction of microorganisms (Lala et al., 2007).

Microbial growth on a filter's surface results in an increased pressure drop across the filter, and thus the filter would need to be changed more frequently. The antimicrobial agents' ability to suppress microbial growth offers the potential for a longer filter life as the membrane would not become loaded with bacteria and fungus. Although, further testing is needed to determine the filter life of a nanofibrous membranes, and the potential increased life when incorporated with antimicrobial agents. Similar testing conditions should be used with traditional filters so an accurate comparison can be made to commonly used commercial filters.

Along with the benefit of suppressing the growth of microorganisms on the filter media, adding antimicrobial agents to the polymer solution offers other advantages. Kim et al. (2008) found in their study that adding BTEAC to the PC solution resulted in a decrease in the fiber diameters along with greater uniformity among the fiber sizes. As addressed earlier, greater fiber uniformity results in a uniform pressure drop and enhanced efficiency across the membrane.

CONCLUSION

Nanofibrous membranes offer great potential for filtration, because the membrane is able to remove smaller particulate from the air stream without significantly increasing the pressure drop. The membrane's filter characteristics can be controlled depending on the composition of the polymer solution prior to the electrospinning process. The ability to control the membrane characteristics, by changing the concentration of the polymer, allows a membrane to be created for a specific application. Antimicrobial agents can also be incorporated into the polymer solution prior to the electrospinning process, and doing so inhibits fungal and bacterial growth on the membrane surface. While inhibiting the growth of microorganisms is beneficial in all applications, the increased initial investment may not be justifiable. Bacterial and fungal growth on filters could have a detrimental effect on all spaces, but some spaces carry a greater liability; in such instances, it is easier to justify the increased cost. Ultimately, nanofibrous membranes with antimicrobial agents offer the greatest opportunity for protection of spaces in which the process or occupant must be protected from airborne microorganisms.

The future use of nanofibrous membranes in the filtration industry by engineers and manufacturers depends on further research and testing of these membranes. Research must focus on the safety of the fibers used within the membranes to guarantee they do not have carcinogenic properties similar to those of asbestos fibers. Research should also be done to determine which antimicrobial agents are most effective in suppressing various bacteria and fungus. Many tests indicate that adding an antimicrobial agent is effective in killing or inhibiting the growth of microorganisms, but not all antimicrobial agents are effective against every strain of bacteria and fungus. It would be useful to know which antimicrobial agents should be incorporated with certain polymer solutions to suppress a specific microorganism.

Finally, a set of testing standards needs to be developed so these nanofibrous membranes can accurately be compared to other nanofibrous membranes and traditional commercial filters. While many research groups have created nanofibrous membranes and have tested their membranes' filtration characteristics under varying conditions, those different conditions make it difficult to provide a direct comparison of the various membranes' efficiencies and pressure drops. If a set of testing standards were to be developed, manufacturers and designers could directly compare nanofibrous membranes' performance. Such standards would also allow researchers to determine the most economical material and effective concentration of solution for creating nanofibrous membranes promoting their use in HVAC applications.

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Foarde, K. K., Hanley, J. T., & Veeck, A. C. (2000). Efficacy of antimicrobial filter treatments. ASHRAE Journal, 42(12), 52-58.

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Jeong, E. H., Yang, J. & Youk, J. H. (2007). Preparation of polyurethane cationomer nanofiber mats for use in antimicrobial nanofilter applications. Material Letters, 61, 3991-3994.

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Maus, R., Goppelsroder, A., & Umhauer, H. (2001). Survival of bacteria and mold spores in air filter media. Atmospheric Environment, 35, 105-113.

Patanaik, A., Jacobs, V., & Anandjiwala, R. (2010). Performance evaluation of electrospun nanofibrous membrane. Journal of Membrane Science, 352, 136-142.

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Andrea Gregg Associate Member ASHRAE

Julia Keen, P.E., HBDP, Ph.D. ASHRAE Member

Andrea Gregg is a Mechanical Designer at TME, Inc. Julia Keen is an associate professor in the Department of Architectural Engineering and Construction Science, Kansas State University, Manhattan, Kansas.
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Author:Gregg, Andrea; Keen, Julia
Publication:ASHRAE Transactions
Article Type:Report
Geographic Code:1USA
Date:Jul 1, 2011
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