Written by Fred Lybrand, Elmarco, Inc.
Informal nonwovens, textile, and other engineered fibers industries will often accept the term nanofibers to describe fibers with any diameter size smaller than 1,000 nm or 1 µm.
The International Standards Organization (ISO) Technical Committee 229, which is charged with defining standards in the field of nanotechnology, considers nanomaterials to be those materials that are typically but not exclusively below 100 nm in at least one dimension. The discussion presented here is applicable for fibers with diameters ranging from about 500 to 50 nm, although these fibers can sometimes have surface structural features with sizes even smaller than 50 nm.
Nanofibers are seldom, if ever, used as a single fiber. They are deposited as a sheet or mat, commonly called a web (from the nonwovens industry). Nanofibers can be produced from a broad array of polymers, including Polyvinyl Alcohol (PVA) and Nylon-6 (PA6). Sometimes referred to as polymer nanofibers, it is important to note that in this context, nanofibers are not carbon nanotubes (although there are applications that use carbon nanotubes and nanofibers together) and that they are not related to fiber optic applications.
Nanofiber production was first patented in the US in 1902 with J.F. Cooley’s "Apparatus for Electrically Dispersing Fluids" (US692,631), what is now known as electrospinning. With that, nanofibers—then called microfibers—have long been used in air filtration and many other areas. Liquid filtration applications followed shortly thereafter, enabling thick webs of nanofibers to function like membranes. As production prices have fallen and manufacturing robustness has grown, nanofibers have increasingly found their way into a broad range of nonwovens applications such as medical barriers, protective garments, face masks, cosmetics, hygiene, acoustic and thermal insulation, battery separators, tissue scaffolds, and even fashion.
The adoption of nanofiber technology for a particular application is driven by the materials properties of the nanofiber web, such as surface area and pore size. Cutting edge research on nanofibers continues in many industries, but particularly in medicine and energy. Just as falling prices and scale allowed microfibers to enable new products and industries, the same is expected for nanofibers.
Issues – Metrology
Both the individual nanofibers and the webs formed from the nanofibers require accurate measurement and description in terms of their diameter, uniformity, length, web thickness, web structure, web size, fiber packing density, and production speed.
Nanofiber diameter: Electrospinning routinely results in fibers with diameters ranging from 500 nm to 50 nm, though there is some difficulty in obtaining smaller sizes for some polymer systems. Larger diameter beads and other structures can be created as needed to add tertiary morphology to the nanofiber web.
Nanofiber uniformity: Uniformity is usually defined in terms of standard deviation of the fiber diameter across the nanofiber web. This can vary broadly by production methodology, but a fiber diameter standard deviation of less than 20% is achievable with many methods. In applications where uniformity of nanofiber coverage is important, nanofiber webs with larger fibers are intentionally created for a broad dispersion of fiber size and therefore a heterogeneous web.
Nanofiber length: Often expressed in the textile measure of deniers, which is the weight in grams of a fiber 9,000 meters in length. Denier measurements are often below 0.01 for nanofibers, but depend on the gravity of the specific polymer in use.
Nanofiber web thickness: Web thickness is expressed as grams per square meter, or gms. Often referred to as ‘basis weight, ’ this measure indicates the thickness of the nanofiber web. Typical nanofiber coatings range from 0.03 gsm to 1.0 gsm. Free standing nanofiber webs (which have membrane-like potential and are often used in liquid filtration) can have basis weights from 1 – 60 gsm. Thicker webs are possible. Some processes refer to nanofiber web thickness in ‘layers’.
Nanofiber web structures/tertiary web morphology: As mentioned above, nanofiber webs can include additional structures and controlled surface characteristics to improve the performance of the end applications. This can include beads, hollow nanofibers, or fractured nanofibers. These beads can be referred to as caps or shot. Often times, other nanoparticles such as carbon nanotubes are added during the manufacturing process to imbue desired properties for the nanofiber web’s end application.
Nanofiber web width and length: These characteristics of the web are defined more by the production process and become a significant concern as a nanofiber process is transitioned from a research to a production environment. Production equipment 1.6 meters wide is available commercially that can handle traditional rolled goods as a substrate to produce long, uniform nanofiber layers.
Fiber packing density (fibers per given area/weight): Nanofiber packing density, or the number of fibers in a given area, is measured by meters of fiber per square cm. For certain applications, packing density can be a more important measure than thickness. Further, for certain applications, such as membranes, measurements like BET surface area, which shows the square meters of surface area for a given weight, can be very important as well.
Production speed: When incorporating nanofibers into a production environment outside of the lab, there are many ways to measure the production speed. Keeping other critical measures like gsm, fiber diameter, and fiber uniformity constant, nanofiber production capabilities often default to standard textile definitions and focus on square meters of material produced per year. For example, properly configured equipment can produce upwards of 40 million square meters of PA6 nanofibers per year with a basis weight of 0.03 gsm, fiber diameter of 200 nm, and a standard deviation of fiber size of 20%.
Issues – Production Methods
Needle: In the research laboratory, nanofibers typically come from a needle, nozzle, or spinneret apparatus configured at a lab bench. A polymer is placed into solution and through hydrostatic pressure is extruded from the tip while a voltage gradient is applied between the tip and a collector plate. A glass slide, nonwoven, textile, or paper can be used as a substrate to collect the nanofiber sheet as it is formed. The major variables in producing nanofibers on such an apparatus are the viscosity of the polymer, the conductivity of the polymer, the amount of pressure applied through the needle, the amount of voltage passed between the needle and the collector, the gap between the needle and the collector, and finally the conductivity of the substrate. Needles are well-suited for initial work in the lab environment; however, they show difficulties in scaling to production capacity because the mechanical complexity of such equipment leads to lower quality, greater downtime, and higher production costs.
Free surface: Alternately, free surface electrospinning methods do not use nozzles, needles, or spinnerets. Similar to how nozzles work, many free surface electrospinning methodologies can be done in a sealed container with a controlled atmosphere. Nanospider™, for example, is a commercially available free surface electrospinning technology that controls the viscosity of the polymer, the conductivity of the polymer, speed of the rotation of the electrode or roller in the polymer bath, the amount of voltage passed between the electrode and the collector, the gap between the electrode and the collector and finally the conductivity of the substrate. Free surface electrospinning has been shown to be straightforward in converting from lab to production scales without the pricing and production issues seen in many nozzle-based processes.
- Donaldson is a well known and established seller of media containing nanofibers.
- Nanostatics is a contract manufacturer which will take a third party substrate and coat it with nanofibers, they also sell nanofiber media.
- Elmarco is a seller of commercial nanofiber production equipment for both laboratory and production use.
- Hills makes and sells a modified traditional nonwoven process capable of producing nanofibers for production use.
Nanofibers continue to be introduced into traditional areas of nonwovens and textile manufacturing and, at the same time, are an area of persistent cutting edge research to incorporate their novel material and surface area properties into new applications. Nanofiber research is driven by the use of new and novel polymers for fiber production and, increasingly, by the incorporation of novel nanoparticles or other materials into the web to imbue it with desirable properties. Nanofiber webs serve as an effective method of containing nano-particles in a form that allows for the surface area benefits to be realized, while preventing their release into the environment. From a production standpoint, nanofibers will continue to be pushed for wider production widths, faster speeds and cheaper costs, all while demanding higher quality. In this way, they are no different than other areas of nonwovens, textiles, and engineered fibers. The major point of difference will be the new markets they enable, system level production cost reductions, and introduction of new capabilities to pursue new applications.
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Cooley, J.F. “Apparatus for Electrically Dispersing Fluids,” US Patent #692,631.
Filatov, Y; Budyka, A.; Kirichenko, V. “Electrospinning of Micro- and Nanofibers: Fundamentals in Separation and Filtration Processes,” Begell House, Inc.: Redding, CT. 2007.
Gogotsi, Yury, “Nanotubes and Nanofibers,” Taylor and Francis Group, LLC 2006.
Hutten, Irwin M. “Handbook of Nonwoven Filter Media,” Elsevier Science, 2007.
Ramakrishna, Seeram; Fujihara, Kazutoshi; Teo, Wee-Eong; Lim, Teik-Cheng; Ma, Zuwei; “An Introduction to Electrospinning and Nanofibers,” World Scientific Publishing Co., Ltd. 2005.
Ramkumar, Seshadri and Purshothaman, Arvind “What’s next in the Nanotextiles Wave?” Nonwovens and Advanced Materials Laboratory, Texas Tech University, Lubbock, TX, USA.