Melt-Blown (MB) Filters
Melt-blowing is a commercially successful, low-cost process for producing microfibers used in filtration. Melt-Blown (MB) filters are created through this melt-blowing process, which entails blowing out melted polypropylene filaments out of a small nozzle at a high temperature and speed. This process often creates an uneven flow of melt, which makes fiber thickness uneven and affects breathability and filterability. The resultant MB filter has a structure similar to a non-uniform fishing net, with MB fibers diameters ranging between 1 – 10 micrometers (µm) and pores between 1 – 3 µm. These pores in the filter are much larger than both bacteria (0.5 – 2 µm) and viruses (0.1 µm). Thus, the filtration efficacy for MB filters relies heavily on the electrostatic forces between the MB fibers.
Nanofiber (NF) Filters
Electrospinning is widely considered the most effective method for producing nanofibers (NF). Electrospinning uses electrostatic forces to draw charged threads from a polymer solution to create fibers. The diameter of these nanofibers range between 10 – 300 nanometers (nm) and the resulting nanofiber filter also has much smaller and more uniform pore diameters. Thanks to these thinner nanofibers (NF), NF filters have both more surface area for any potential pollutants to deposit onto and tighter pores that couple for better filtration.
Why Nanofiber Filters Are Superior
- Better Filtration Efficacy
To understand how NF filters have enhanced filtration performance, we need to have a general understanding on the various particle capturing mechanisms. Particles are blocked by the filter via the five mechanisms listed in the figure below (for a more thorough description, please refer to this study). Generally, more surface area equates to better filtration, since surface interaction is the dominant driving force in air filtration. More surface area and tighter pores in NF filters allow NF filters to use the multiple aforementioned filter mechanisms to block particles and hence are not affected by electrostatic deposition to the same degree as MB filters.
- Better Breathability
Contrary to popular belief, the smallest particles are generally not the most difficult to filter. The region of lower filtration efficiency for most filters lie somewhere between 0.1 – 0.5 µm – these particles are also known as the most penetrating particle size (MPPS). In order to capture MPPS particles, multiple layers of either NF or MB fibers are required. However, multilayered filters are often paired with poor breathability and high pressure drop, both undesirable features. Electrospinning allows for the control of the porosity, packing density, fiber diameter, and surface area of the nanofibers. Hence, through modification of these parameters, NF filters are able to filter higher amounts and a wider range of particles than MB filters, while minimizing the pressure drop/breathability.
Moreover, another factor to consider for breathability is moisture transportation. A recent study conducted breathability tests between N95 MB and NF masks to evaluate their water vapor transmission rates (WVTR). The results of the study showed that the WVTR of NF filters was superior because MB filters have sponge-like structures that hinders moisture from passing quickly through the filter. In addition, MB filters displayed poor CO2 emission compared to NF filters. The finer structure and more uniform morphology and pore diameter of nanofibers give NF filters the upper hand over MB filters.
- Better Reusability
Though N95 masks are manufactured as one-time use equipment, this current pandemic has created the need to reuse these masks while retaining their efficacy. A recent study illustrated that MB filters are only effective for single use, as their filtration efficiency dropped to ~64% after ethanol cleaning. This is because the electrostatic charge of the MB filter is lost during cleaning, and once MB filters lose this static electricity when exposed to moisture, the filtering effect drops to nearly half the original performance. In contrast, NF filters showed that they can successfully be reused multiple times after ethanol cleaning, with filtration efficiency remaining at around ~97-99%.