Polymeric Nanofibre Web In Filtration

Electro spun Nanofibre, with fiber diameters of 0.25 microns have been used in industrial, consumer and Defense filtration applications for more than twenty years. Electro spun Nanofibre have fibre diameters that are 5-10 times smaller than the smallest melt blown fibres available. Nanofibre provide dramatic increases in filtration efficiency at relatively small (and in some cases immeasurable) decreases in permeability. In many laboratory tests and actual operating environments, Nanofibre filter media also demonstrate improved filter life and more contaminant holding capacity. Nanofibre filter media have enabled new levels of filtration performance in several diverse applications with a broad range of environments and contaminant. The performance of Nanofibre media in a mining vehicle cabin air filter will be discussed

Key Words: Nanofibre, Filtration, Nonwoven web, Electro spun, spun bond, melt blown, air filter media.

1. INTRODUCTION:

The nonwoven industry generally considers nanofibers as having a diameter of less than one micron, although the National Science Foundation (NSF) defines nanofibers as having at least one dimension of 100 nanometer (nm) or less. The name derives from the nanometer, a scientific measurement unit representing a billionth of a meter, or three to four atoms wide.

Nanofibres are an exciting new class of material used for several value added applications such as medical, filtration, barrier, wipes, personal care, composite, garments, insulation, and energy storage. Special properties of nanofibres make them suitable for a wide range of applications from medical to consumer products and industrial to high-tech applications for aerospace, capacitors, transistors, drug delivery systems, battery separators, energy storage, fuel cells, and information technology.

Generally, polymeric nanofibres are produced by an electrospinning process (Figure 1). Electrospinning is a process that spins fibers of diameters ranging from 10nm to several hundred nanometers. This method has been known since 1934 when the first patent on electrospinning was filed. Fibre properties depend on field uniformity, polymer viscosity, electric field strength and DCD (distance between nozzle and collector). Advancements in microscopy such as scanning electron microscopy have enabled us to better understand the structure and morphology of nanofibres. At present the production rate of this process is low and measured in grams per hour.

This paper will discuss the electrospinning process for making nanofibres and nonwoven nanofibre webs from synthetic fiber-forming polymers. The resulting physical characteristics of the nanofibre webs will be discussed. In order to provide a useful context for the nonwovens industry, nanofibre webs will be compared to both melt blown and spun bond nonwovens. Then discuss the construction and performance of filter media using nanofibres.

2. ELECTROSPINNING PROCESS:

A schematic diagram of electrospinning is as shown in Figure 1. The process makes use of electrostatic and mechanical force to spin fibres from the tip of a fine orifice or spinneret. The spinneret is maintained at positive or negative charge by a DC power supply. When the electrostatic repelling force overcomes the surface tension force of the polymer solution, the liquid spills out of the spinneret and forms an extremely fine continuous filament. These filaments are collected onto a rotating or stationary collector with an electrode beneath of the opposite charge to that of the spinneret where they accumulate and bond together to form nanofibre fabric.

The distance between the spinneret nozzle and the collector generally varies from 15 30 cm. The process can be carried out at room temperature unless heat is required to keep the polymer in liquid state. The final fiber properties depend on polymer type and operating conditions. By choosing a suitable polymer and solvent system, nanofibres with diameters in the range of 40-2000 nm (0.04 2 microns) can be made. Fiber diameters can be varied and controlled. [12]

3. POLYMER-SOLVENTS USED IN ELECTROSPINNING:

The polymer is usually dissolved in suitable solvent and spun from solution. Nanofibres in the range of 10 to 2000 nm diameter can be achieved by choosing the appropriate polymer solvent system. Table 1 gives list of some of polymer solvent systems used in electro spinning.

4. PROPERTIES OF NANOFIBRES:

Nanofibres exhibit special properties mainly due to extremely high surface area to weight ratio compared to conventional nonwovens.Low density, large surface area to mass, high pore volume, and tight pore size make the nanofibre nonwoven appropriate for a wide range of filtration applications [4].

Figure 2 shows how much smaller nanofibres are compared to a human hair, which is 50-150 m and Figure 3 shows the size of a pollen particle compared to nanofibres. The elastic modulus of polymeric nanofibres of less than 350 nm is found to be 1.00.2 Gpa.

5. Examples of ELECTROSPUN NANOFIBRES WEB:

Figure 4 is a 10,000X magnification scanning electron micrograph (SEM) of electro spun nanofibres. The fibres are approximately 250 nanometers in diameter. As the fibers themselves have a small diameter, the thickness of the nanofibre web can likewise be quite small, with a thickness of four nanofibre diameters approaching only one micron. The thin web has limited mechanical properties that preclude the use of conventional web handling. As a result, nanofibre webs have been applied onto various substrates. Substrates are selected to provide appropriate mechanical properties and provide complementary functionality to the nanofibre web. In the case of nanofibre filter media, substrates have been selected for pleating, filter fabrication, durability in use, and filter cleaning.

Figure 5 is a photomicrograph showing a cross-section of nanofibres electro spun onto a polyester spun bond substrate. The substrate is chosen to provide mechanical properties, while the nanofibre web dominates filtration performance.

Figure 6 is a photomicrograph of commercially available nanofibres electro spun onto a cellulose substrate for air filtration applications. The nanofibre diameter is approximately 250 nanometers, as compared to the cellulose fiber structure, with diameters exceeding ten microns.

This composite filter media structure has been successfully pleated on high-speed rotary pleating equipment with minimal damage to the nanofibre layer. Controlling parameters of electro spinning allows the generation of nanofibre webs with different filtration characteristics. Different fibre sizes can be made, some as small as 40 nanometers. Fibers can be put on one side or on both sides of a substrate.

Figure 7 shows a cellulose media and a cellulose/nanofibre composite media (with nanofibres on the upstream side), both of which have been loaded to the same pressure drop (0.5 w.g.) with ISO Fine test dust. ISO Fine test dust is commonly used for testing engine air cleaners, and contains particles in the size range of 0.7 70 microns. The cellulose sample shows depth loading and relatively few submicron particles, while the composite sample shows loading at the nanofibre surface and a significant quantity of submicron particles captured. The composite sample is holding 2.5 times more mass of dust than the cellulose sample. The improvement in dirt holding capacity of the nanofibres is due to the small fiber diameter and correspondingly increased surface area of the fibers. These filtration benefits should similarly translate to wipes applications.

6. PHYSICAL CHARACTERISTICS OF COMMERCIAL NANOFIBRE

WEBS: COMPARISON TO MELTBLOWN and SPUNBONDED WEBS:

It is useful to compare electro spun nanofibre webs to other nonwoven processes that directly produce fibrous web structures. While there are many processes to make nonwoven web materials, it is most useful to compare electro spun webs to two other important products in the nonwovens industry: melt blown and spun bonded webs. Indeed, the three processes and resulting products share two notable characteristics: (a) the process begins with a liquid phase polymer and makes fibers and webs directly in a one-step process; (b) the resulting products consist essentially of polymeric fibers with no other binders, resins or additives. Many other processes exist to make fibrous nonwovens, but generally the fibers are made independently of the web. Additionally, other nonwovens processes generally employ a chemical (such as the resin binders used in wet laying) or a subsequent mechanical process step (such as carding or hydro entangling) to secure the fibers within the web to each other.

The simplest comparison between electro spun nanofibres, melt blown fibers and spun bonded fibres is fiber size (Table 2). The difference in fiber size leads to vast differences in basic web properties such as fiber surface area (Table 3), basis weights (Table 4), thicknesses, permeability, and strength. While the calculations of nanofibre web physical properties are often straightforward from fibre counts and fibre diameters in SEMs, the results, when expressed in the conventional units of the nonwovens industry, illustrate that electro spun nanofibre webs are categorically distinct from other nonwovens. Electro spun nanofibres have diameters that are 1 to 2 orders of magnitude smaller than melt blown fibers. This leads to a corresponding increase in fiber surface area and decrease in basis weight. While not discussed in detail here, the diameters of electro spun fibres allow the creation of webs with substantially more and smaller micro pores than melt blown and spun bond webs.

Denier Calculation based on fiber specific gravity = 1. Specific gravity values of common Fibre polymers range from 0.92 (PP) to 1.14 (PA66) to 1.38 (PET)

Specific Surface area calculations based on fibre specific gravity = 1.

7. THE EFFECT OF NANOFIBRES ON FUNDAMENTAL FILTRATION PROPERTIES:

It is well known that particle filtration occurs via multiple collection mechanisms Including sieving, direct interception, inertial impaction, diffusion and electrostatic Collection. For practical purposes, sieving is not an important mechanism in most air filtration applications. Commercially available nanofibres are electrically neutral. As a result, the remaining mechanisms of importance in mechanical filtration are direct interception, inertial impaction and diffusion. The mathematical description of filter media is complex. However, reasonable approximations of media performance have been made using single fiber filtration theory.

The single fiber efficiency for direct interception ER = (DP/DF)² / Ku

Where DP is the particle diameter, DF is the fiber diameter and Ku is the Kuwabara constant. As can be seen, filtration efficiency due to direct interception ER is inversely Proportional to the square of the fiber diameter.

The single fiber efficiency for inertial impaction EI a St / (2Ku²)

Where St (Stokes Number) = SD / DF and where SD is the Stopping Distance. As can be seen, filtration efficiency due to inertial impaction EI is inversely proportional to the fiber diameter.

The single fiber efficiency for diffusion ED is ED = 2.7 / (Pe)^2/3

Where Pe is the Peclet number and is defined as Pe = DF U / D and where U is velocity and D is the coefficient of diffusion. As with the other filtration mechanisms, decreasing fibre diameter increases filtration efficiency due to diffusion.

These effects of increasing filtration efficiency can be observed experimentally by measuring filter efficiency as a function of particle size (Figure 8).

Combining the 3 dominant filtration mechanisms and plotting results in a graph of Efficiency versus Particle Size illustrate that there is a most penetrating particle size (MPPS). As Figure 8 illustrates, decreasing fiber size in a filter media (designed for the same pressure drop) provides for improvement in the MPPS situation. The use of nanofibre filter media shifts the MPPS to smaller sizes while simultaneously increasing the efficiency for all particle sizes. The increased filtration efficiency due to nanofibres has been shown from classical filtration equations and fractional efficiency testing. The filtration merits of nanofibres can also be shown visually using scanning electron microphotographs. The above microphotographs show conventional cellulose filter media and a nanofibre filter media with a cellulose substrate (Figure 7). Both media samples were loaded with the same amount of ISO fine test dust. (ISO fine test dust is commonly used for testing engine air cleaners, and contains particles in size ranges from 0.7 70 microns.)

8. NANOFIBER FILTER MEDIA IN CABIN FILTRATION OF MINING VEHICLES:

Airborne contamination in the personnel cabins of mining equipment is of concern to mining workers, mining companies and government agencies such as the Mine Safety and Health Association (MSHA). Recent work with mining equipment manufacturers and the MSHA has shown that nanofibre filter media can reduce cabin dust concentration compared to standard (cellulose) filter media.

Nanofibre media has proven beneficial when used in either or both the intake filter and recirculation filter. The use of nanofibre filter media allows for high efficiency filtration at high airflow rates (due to the low additional pressure drop) which makes recirculation of air in the vehicle cabin effective. Figure 9 illustrates the cabin clean-up rate using various combinations of filter media.

The boost in filtration efficiency was obtained with no measurable increase in pressure drop and no sacrifice in filter life as shown in Figure 10.

9. CONCLUSIONS:

Nanofibre filter media has been successfully used in a variety of filtration applications ranging from engine air cleaners to cabin filters for mining vehicles to self-cleaning filter systems for industrial applications and turbine vehicles. Due to the sub-half-micron fiber diameter of nanofibres and the thin nanoweb layer possible, significant boosts in filter efficiency is possible with minimal pressure drop increases.

In both static and self-cleaning filtration applications, nanofibre filter media has demonstrated longer filter life over several conventional filtering materials. Filter engineers are continually balancing the three major technical parameters of filter performance: filter efficiency, pressure drop, and filter life. An improvement in one category generally means a corresponding sacrifice in another category. However, the proper use of nanofibres can provide favorable improvements in both filtration efficiency and life, while having a minimal impact on pressure drop.

10. REFERENCES:

1. Textile World Nano Technology and Nonwoven. P52, November 2003

2.Gajanan Bhat and Youneung Lee, Recent advancements in Electrospun nanofibers. Proceedings of the twelfth international symposium of Processing and Fabrication of Advanced materials, Ed TS Srivatsan & RA Vain, TMS, 2003

3.Electrostatic spinning of Nanofibers spin Technologies, Chattanooga, TN
4.Peter P Tsai, Effect of Electro spinning Material and Conditions upon Residual Electrostatic Charge of Polymer nanofibers. Proceedings of the 11th Annual international TANDEC Nonwoven conference, Nov.6-8, 2001, Knoxville, TN, USA

5.Heidi Schreuder-Gibson, Phillip Gibson, Use of Electrospun Nanofibers for aerosol Filtration in textile structures. US Army Soldier Systems Center AMSSB-RSS-MS(N)Natick, Massachusetts

6.J.Doshi, MH Mainz and GS Bhat, Proceedings of the Tenth TANDEC Nonwoven Conference, Knoxville, TN (2000).

7.www.nanospin.com

8.www.donaldson.com

9.www.ecmjournal.org

10.www.zapmeta.com

11.H. Schreuder-Gibson and P Gibson, 23rd Army science conference, Florida, December 2002.

12.Kalayci, V.E., Electrostatic Solution Spinning, MS Thesis, University of Massachusetts, Dartmouth, July, 2002

13.Schaefer, J.W., and Olson, L.M., Air Filtration Media for Transportation Applications, Filtration & Separation, March 1998

About the author:

S. Ariharasudhan - Department of Textile Technology, PSG College of Technology, Coimbatore- 641004 Email: ariharasudhan@gmail.com

Gopalakrishnan I am doing PG Diploma in Home Textile Management.i did my Diploma in Textile Technology & B.Tech in Textile Technology from PSG College of Technology & Polytechnic College. After my diploma I worked as a Production & maintenance Supervisor in Cambodia Mills (NTC) Coimbatore, after three years of experience I came back to my B.Tech.I did 17 paper presented in various technical symposiums, national & international confrences in all over india and i participated in various technical workshops & innovative project works. I published several articles in journals,magazines.

Area of Interest: innovative textiles, Technical textiles

Coimbatore-641 004, Email: dgk_psgtech@yahoo.co.in