Moving Beyond Electrospinning: Melt Nanofiber Production
Melt-spun nanofiber processes address challenges of productivity, profitability and environmental responsibility.
Bryce W. Davis, Ph.D., and Kial Gramley
However, it is clear that the commercialization of the total potential of nanofiber technology has yet to be realized. In 2010, BCC Research LLC, Wellesley, Mass., estimated that the entire market for nanofiber products would reach approximately $183 million in 2012. In comparison, microfiber production today is several billions of dollars annually and growing rapidly, according to International Nonwovens Consulting, Raleigh, N.C. However, Donaldson Filtration Co. Inc., Bloomington, Minn., began pioneering nanofiber filters in the mid- to late 1980s. This was not long after meltblown microfibers began to make their way to the end market and Reicofil spunbond-meltblown-spunbond (SMS) lines began to proliferate. In evaluating the driver behind this divergence, one must look at the differences between melt-based processes and solvent-intensive processes such as electrospinning. Indeed, the primary challenge in scaling electrospinning has been the high solvent-to-polymer concentrations needed to produce the nanofiber. More solvents lead to lower yields, dramatically increased operating expense and an environmental burden that is counter to the vast sustainability efforts under way in most nonwovens and textile companies. Indeed, research is underway to develop melt-based electrospinning. At a recent conference in Japan, several leading electrospinning companies reported on the work they have done in this area, but the data clearly demonstrated that significant challenges exist in converting electrospinning to a melt-based process.
In an effort to finally begin to transfer the full potential of nanofiber technology to end products, leading materials companies have addressed the challenges of productivity, profitability and environmental responsibility by acquiring melt-based processes as the platforms they will leverage into the future. This effort has initiated a new era beyond electrospinning that is focused on melt nanofiber production technology. This article presents two process platforms that exist today, the materials that are available in fiber form, the near-term markets for end applications and the endless possibilities that are created when there is an unlimited availability of nanofibers.
Melt Nanofiber Processes
Meltblown is a one-step process in which high-velocity heated air blows a molten thermoplastic resin from an extruder die tip onto a conveyor or take-up screen to create fibers. Originally developed in its commercial form at Exxon in the late 1970s, meltblown has proven very useful for making fibers with diameters in the range of tens of microns and higher, but the focus of most of the development of this technology has been toward higher line speeds and integration into spunbond lines. Recently, the effort has shifted away from higher-volume production and toward finer fiber for advanced air filtration performance. Research conducted in Germany and the United States has been focused almost solely on polypropylene (PP) and has shown gains in generating fibers in the submicron range. The challenges faced in driving fiber diameter lower include the importance of increasing the air-to-polymer mass flux ratio. This higher ratio leads to a need for very high air velocities and limited polymer feed rates that increase operating cost and limit productivity. Another challenge is the broad distribution of fiber diameters that can significantly impact air filtration performance.
Recently, work has expanded beyond PP with gains being made by a research group at The University of Tennessee in processing polylactic acid (PLA) nanofibers. PLA is coveted as an eco-friendly thermoplastic with beneficial material performance. As work in meltblown continues to expand into new materials, applications will increase as well.
Melt nanofiber technologies include meltblown technology (above), diagram courtesy of Atul Dahiya, “Meltblown Technology”; and Forcespinning® technology (below), diagram courtesy of FibeRio Technology Corp.
Forcespinning® technology uses centrifugal force to process materials into nanofiber. A spinneret cup is rotated at high speeds to initiate hydrostatic and shear forces that push a liquid material, either melt or solution, though orifices lining the outer wall of a spinneret. As the material enters the orifice chamber, molecules disentangle and then align directionally to form nanofibers. This unique technology has only recently been developed to commercial-level production by FibeRio Technology Corp., McAllen, Texas, and it has already gained significant momentum with reporting of PP nanofibers with average diameters of less than 500 nanometers (nm). Furthermore, a 100-fold increase in productivity rate has enabled the technology to be viewed for more than air filtration applications. Dramatic productivity gains have been demonstrated with both the melt-based process and solution spinning via higher polymer concentration levels than those for electrospinning.
Other materials that have been melt-spun include PLA, polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polyamides (PA), polyvinylidine fluoride, low-molecular-weight polyethylene and thermoplastic polyurethane. The applications thereof are incredibly broad, ranging from performance apparel to fuel filtration and more. However, the full potential of Forcespinning technology will emerge as FibeRio sets its sites on melt processing engineered plastics into nanofiber.
Melt-spun Nanofiber Applications
Medical Nonwovens: Global Industry Analysts Inc., San Jose, Calif., estimates that the global disposable medical nonwovens market will reach $20.9 billion by 2017. Applications in this area include surgical gowns, drapes and tray covers. Melt-spun PP nanofibers can be utilized to provide the breathable barrier performance required for these products.
In the past year, FibeRio reported significant water-barrier performance improvements with almost 60 millibar of hydrostatic head — a value compliant with Level 3 of the Association for the Advancement of Medical Instrumentation (AAMI) PB70 standard — with only 6 grams per square meter (gm2) of material compared to the traditional 15 gm2 of material from meltblown microfibers, making the fabric lightweight and breathable through the use of nanofiber media.
Filtration: Nanofibers have been used for quite some time in air filtration applications. However, the use of melt-based processes will expand their use in that market. Melt-spun PP nanofiber from both meltblown and Forcespinning technology have demonstrated high efficiency ratings with low pressure drop. The efficiency ratings are driven by the utilization of Brownian Diffusion, while the pressure drop performance can be attributed to slip flow at the surface of the fiber. These benefits create a significant opportunity for many filtration companies because regulatory bodies have recently moved to eliminate the historical practice of electrostatically charging the media to achieve high efficiency ratings as demonstrated by the new European standard EN779:2012.
The use of melt-based production processes will expand the use of nanofibers for air filtration applications.
However, as fiber diameters continue to decrease, new applications such as water filtration become available. Significant opportunities are created using melt-spun PP and PET nanofibers to achieve pore sizes comparable to those of membranes while providing the lower pressure drop of a fibrous media. Additionally, PBT nanofibers can be used in fuel filtration and blood separation applications.
Performance Apparel: Recently, nanofibers have found their way into end products such as socks, jackets and even golf gloves. Materials have varied from PA 6 to PET. It is believed that these processes were solvent-based, and it would stand to reason that the emergence of melt-spun PET nanofibers, with higher annual capacities and lower operating expenses, could substantially increase the use of nanofibers in these applications.
Wound Care: Management of moisture is driven by control over porosity, which is why nanofibers have emerged as a critical tool in the wound-care industry. Furthermore, materials must at the very least be biocompatible and preferably, in some cases, bioresorbable. In these instances, toxic residue from solvents are a nonstarter and limit commercial opportunities. Melt-spun PLA nanofibers and, in the future, melt-spun polycaprolactone fibers overcome this challenge while simultaneously increasing productivity.
Acoustic Insulation: Environmental awareness and rising costs of fuel have driven the term "lightweighting" to the forefront of the automotive and aerospace transportation lexicon. In this regard, nanofibers enable automotive suppliers to substantially reduce material weights while improving acoustic insulation performance. The challenge until now has been the cost sensitivity and high volumes of such an industry. However, melt-spun nanofiber overcomes that challenge, and PP and PET are resins that are already familiar to the industry.
As the range of materials broadens, another area being evaluated is the use of nanofibers in dispersions and as reinforcements in composites. Nanoscale fibers are capable of even distribution in a polymer matrix and hold great promise for improving tear strength, regulating viscosity and improving other properties. Staple nanofibers are just beginning to be considered by materials engineers and could have vast applications far beyond composite reinforcement. Cost-effective volume manufacturing will enable these applications to become commercially viable.
Other markets that will come online as melt nanofiber processing achieves productivity and line speed scale-up will be applications in hygiene nonwovens. As private label suppliers continue to take market share, branded product suppliers are searching for new paths to differentiation. This development has led to a search for technologies that provide thinner materials and enhance fluid management capability.
Efforts in meltblown and Forcespinning technology are removing productivity, operating expense and environmental barriers. As final hurdles to high-volume nanofiber production are overcome, the conversation will shift away from process challenges and toward new end-product development and intellectual property required to compete. The future of nanofibers is primed for any applications that are looking for ways to reduce weight while improving material performance. Melt processing also enables the use of new, greener materials for existing applications. Furthermore, novel capabilities are enabled through the use of new materials such as aromatics, sulfones, fluoropolymers and urethanes. Already, these materials have demonstrated great promise at the research level, and new developments will rapidly scale them to continuous industrial production.
Editor’s Note: Bryce W. Davis, Ph.D., is materials engineer, and Kial Gramley is vice president, marketing and business development, at FibeRio Technology Corp.
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