Nanofibers | Nanofibers in Nonwoven | Properties of Nanofibers | Application of Nanofibers | Electrospinning Process

, a scientific measurement unit representing a billionth of a meter, or three to four atoms wide.

Nanofibers 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 nanofibers 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 [1,2].

Generally, polymeric nanofibers 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. Fiber 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 has enabled us to better understand the structure and morphology of nanofibers. At present the production rate of this process is low and measured in grams per hour.

Another technique for producing nanofibers is spinning bi-component fibers such as Islands-In-The-Sea fibers in 1-3 denier filaments with from 240 to possibly as much as 1120 filaments surrounded by dissolvable polymer. Dissolving the polymer leaves the matrix of nanofibers, which can be further separated by stretching or mechanical agitation.

The most often used fibers in this technique are nylon, polystyrene, polyacrylonitrile, polycarbonate, PEO, PET and water-soluble polymers. The polymer ratio is generally 80% islands and 20% sea. The resulting nanofibers after dissolving the sea polymer component have a diameter of approximately 300 nm. Compared to electrospinning, nanofibers produced with this technique will have a very narrow diameter range but are coarser [3].

2. Electrospinning Process

Electrospinning is a relatively simple process to produce nanofibre from polymer solutions or melts. Its roots go back to the early 1930s when the process was patented by Formhals [1–3]. The advantage of the electrospinning process is its technical simplicity and its easy adaptability. It is based mainly on applying an electrical field, by using high voltage source, between the tip of a nozzle and a collector in order to generate sufficient electrostatic force to overcome the surface tension in a droplet of polymer solution at the nozzle tip. When the surface tension is overcome, the hemispherical surface of the fluid at the tip of the nozzle stretches to form a conical shape known as the Taylor cone [4]. Further increase of the electric field’s strength will deform the Taylor cone until a jet stream is extruded from the cone’s apex. During the process, and depending on the solution properties and operating conditions, the solvent evaporates as the jet moves toward the collector which decreases the jet radius and increases the polymer concentration and viscosity. When the solvent is fully evaporated, the jet stretching stops and results in producing fibre of highly reduced diameter which deposits on the grounded collector in the form of a random nonwoven structure. The process of the electrospinning is well described in many papers [5–8]. Nanofibres in the range of 10 to 1000 nm diameter can be achieved by choosing the appropriate parameters such as viscosity, concentration, applied voltage, distance between the two electrodes, and nozzle tip (needle) diameters. However, the instability, the whipping of the fibre, and the beads formation remain important problems in the electrospinning process. This paper aims to validate experimentally the functionality of the new upward electrospinning approach introduced by Abdel Hady [9, 10]. In this new approach, as the fibre formation is produced and a jet is directed upwards, the gravitational force and the surface tension will work against the electrostatic force, which introduces more stretching to the fibre.

A schematic diagram of electrospinning is as shown in Figure 1. The process makes use of electrostatic and mechanical force to spin fibers 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. It has the misleading appearance of forming multiple filaments from one spinneret nozzle, but current theory is that the filaments do not split.

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 nanofiber fabric.

Figure 1.   Schematic representation of electrospinning process [4].

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. Fiber fineness can be generally regulated from ten to a thousand nanometers in diameter [1,4].

2.1 Polymer-Solvents used in ELECTROSPINNING.

The polymer is usually dissolved in suitable solvent and spun from solution. Nanofibers in the range of 10-to 2000 nm diameter can be achieved by choosing the appropriate polymer solvent system [5]. Table 1 gives list of some of polymer solvent systems used in electrospinning.

 

Polymer	Solvents
Nylon 6 and nylon 66	Formic Acid
Polyacrylonitrile	Dimethyl formaldehyde
PET	Trifluoroacetic acid/Dimethyl chloride
PVA	Water
Polystyrene	DMF/Toluene
Nylon-6-co-polyamide	Formic acid
Polybenzimidazole	Dimethyl acetamide
Polyramide	Sulfuric acid
Polyimides	Phenol

 Table 1. Polymer solvent systems for electrospinning [6]

2.2 Nanofibers from splitting bicomponent fibers

Figure 2. Nanofibers from Bicomponent fibers [3].

As described above, nanofibers are also manufactured by splitting of bicomponent fibers; most often bicomponent fibers used in this technique are islands-in-a-sea, and segmented pie structures. Bicomponent fibers are split with the help of the high forces of air or water jets.

Figure 2 shows the bicomponent nanofiber before and after splitting. A pack of 198 filaments in single islands is divided into individual filaments of 0.9 μm. In this example, Hills Inc has succeeded in producing fibers with up to 1000 islands at normal spinning rates. Furthermore bi-component fibers of 600 islands have been divided into individual fibers of 300 nm [1,3].

3. Properties of Nanofibers

Nanofibers exhibit special properties mainly due to extremely high surface to weight ratio compared to conventional nonwovens.

Low density, large surface area to mass, high pore volume, and tight pore size make the nanofiber nonwoven appropriate for a wide range of filtration applications [9].

Figure 3 shows how much smaller nanofibers are compared to a human hair, which is 50-150 µm and Figure 4 shows the size of a pollen particle compared to nanofibers. The elastic modulus of polymeric nanofibers of less than 350 nm is found to be 1.0±0.2 Gpa.

Figure 3. Comparison between human hair and nanofiber web [4]. 

 Figure 4.  Entrapped pollen spore on nanofiber web [4].

4. Uses of Nanofibers

4.1. Filtration

Nanofibers have significant applications in the area of filtration since their surface area is substantially greater and have smaller micropores than melt blown (MB) webs. High porous structure with high surface area makes them ideally suited for many filtration applications. Nanofibers are ideally suited for filtering submicron particles from air or water.

Electrospun fibers have diameters three or more times smaller than that of MB fibers. This leads to a corresponding increase in surface area and decrease in basis weight. Table 2 shows the fiber surface area per mass of nanofiber material compared to MB and SB fibers [8].

   

Fiber Type	Fiber size, in Micrometer	Fiber surface area per mass of fiber material m2/g
Nanofibers	0.05	80
Spunbond fiber	20	0.2
Melt blown fiber	2.0	2

            Table 2. Fiber surface area per mass of fiber material for different fiber size [8].

Nanofiber combined with other nonwoven products have potential uses in a wide range of filtration applications such as aerosol filters, facemasks, and protective clothing. At present, military fabrics under development designed for chemical and biological protection have been enhanced by laminating a layer of nanofiber between the body side layer and the carbon fibers [10].

e-Spin Technologies, Inc has produced a prototype of activated carbon nanofiber web. PAN- based nanofibers were electrospun. Then these webs were stabilized, carbonized, and activated. These activated PAN nanofibers gave excellent results for both aerosol and chemical filtration [11,12].

Electrospun nanofiber webs are used for very specialized filtration applications. Donaldson is making and marketing filter media that incorporate electrospun nylon fibers for gas turbines, compressor and generators [13].

4.2. Medical Application
Nanofibers are also used in medical applications, which include, drug and gene delivery, artificial blood vessels, artificial organs, and medical facemasks. For example, carbon fiber hollow nano tubes, smaller than blood cells, have potential to carry drugs in to blood cells [14, 15].

 
Figure 5. Comparison of red blood cell with nanofibers web [4].

Nanofibers and webs are capable of delivering medicines directly to internal tissues. Anti-adhesion materials made of cellulose are already available from companies such as Johnson & Johnson and Genzyme Corporation [2]. Researchers have spun a fiber from a compound naturally present in blood. This nanofiber can be used as varieties of medical applications such as bandages or sutures that ultimately dissolve in to body. This nano fiber minimizes infection rate, blood lose and is also absorbed by the body [11].

To meet these varied requirements a layered composite structure is used. The bulk of the filter is generally made of one or multiple MB layers designed from coarse to fine filaments. This is then combined with a nanofiber web. The MB layer provides fluid resistance while the outer nanofiber layer improves smoothness for health, wear and comfort.

Nanofibers greatly enhance filtration efficiency (FE). Scientists at the U.S. Army Natick Soldier Center studied the effectiveness of nanofibers on filter substrates for aerosol filtration. They compared filtration and filter media deformation with and without a nanofiber coating of elastic MB and found that the coating of nanofiber on the substrate substantially increases FE [17].

With most of the nanofiber filter media, a substrate fabric such as SB or MB fabric is used to provide mechanical strength, stabilization, pleating, while nanofiber web component is used to increase filtration performance [2,18].

4.4 Nanofiber Composite Construction:

Nanofibers were applied to 0.6 ounces per square yard (osy) nylon SB material and to 1.0 osy nylon SB as shown in Figure 6 [8].

 
Figure 6.  Nanofiber impregnation to spunbond layers [1,8]

Then two such layers were laminated together. Figure 7 shows three different types of nanofiber composite fibers designed by altering the thickness and weight of base cloth.

Figure 7. Nanofiber composite fiber layer options [1,8]

The performance and the durability of the composite structure depends on the finished fabric architecture. The final nanofiber fabric architecture is as shown in Figure 8. The two type of constructions are;
1. The nanofiber/SB layer between outer shell layer fabric and chemical filtration layer.
2. Nanofiber /SB layer is impregnated over the shell fabric and free floats against chemical filtration layer.

Figure 8. Nanofiber composite fabric Designs [1,8]

Polymeric nanofiber composites can provide enhanced protection against chemical agent micro droplets, biological aerosols, radioactive ducts, etc.

5. Challenges in Nanofibers

The process of making nanofibers is quite expensive compared to conventional fibers due to low production rate and high cost of technology. In addition the vapors emitting from electrospinning solution while forming the web need to be recovered or disposed of in an environmental friendly manner. This involves additional equipment and cost. The fineness of fiber and evaporated vapor also raises much concern over possible health hazard due to inhalation of fibers. Thus the challenges faced can be summarized as:


· Economics
· Health hazards
· Solvent vapor
· Packaging shipping handling


Because of its exceptional qualities there is an ongoing effort to strike a balance between the advantages and the cost [19].


References


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