Melt-spun nanofiber processes address challenges of productivity, profitability and environmental responsibility.
Nanofibers have long shown great promise for advancing textile and material performance in applications ranging from air and liquid filtration to battery separators, to sound insulation and fluid barriers in hygiene products. For example, in filtration, nanofibers have been shown theoretically and empirically to demonstrate exceptional pressure drop ratings through the engagement of the slip flow effect that cannot be achieved with bulk fiber. They are also best suited for enabling the filtration mechanism of Brownian Diffusion, which can capture particulate matter of .5 micron and smaller. Nanofibers also mimic the extracellular matrix that enables the propagation of living cells for wound care and tissue engineering. These fibers, generally described in commercial settings as having diameters of less than 1 micron, have been the subject of intensive study for quite some time. Indeed, a literature review reveals an extensive field of research and the reporting of performance improvements. One is hard-pressed to find a university that does not have a makeshift electrospinning needle that is being used to demonstrate the advances in material performance that occur at the nanoscale. Academia has applied great diligence in characterizing and reporting these findings. Furthermore, a patent search quickly reveals that many of the largest nonwovens, filtration and textile companies in the world have not only performed extensive R&D in this area, but have also made significant investments in protecting their position.
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.
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.
Future Directions
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.
Ref: Bryce W. Davis, Ph.D., and Kial Gramley, vice president, marketing and business development, at FibeRio Technology Corp.
