Nanofibers have a very special place in nanomaterials world. Among other things, most important character of nanofibers is their extremely high aspect ratio, or in other words, the ratio between the length of the fiber to diameter of the fiber. Scientists can now make nanofibers from number of different types of materials. They have number of applications including materials for filters, tissue engineering, drug delivery, photocatalysts, battery materials, smart textiles, etc.
However, our main interest today is on polymeric nanofibers and their application in air filters. Polymeric nanofibers are very interesting. They are in some ways similar to the fibers that we are familiar with. Like cotton and polyester fibers, they are extremely bendable and can be used to make continuous fibrous assemblies like textile fabrics. However, unlike our everyday textiles, they are extremely small. You would require a bundle of more than 10 million of them to match a single strand of human hair. This small size give nanofibers an extreme boost in surface area allowing it to make more interactions with the environment.
Air filters play an important role in our everyday life. We use them in our vehicles, masks, air conditioning units mainly to filter out solid particulates that can otherwise cause problems. Most of the conventional air filters are nonwoven assemblies of randomly oriented fibers having diameters in the range of micrometers. They are mainly designed to reject dust, pollen, mold and sometimes bacteria; particles typically having microscale dimensions.
Air quality parameters are getting more stringent by the day and one of the major requirement is to filter out submicron particles such as smoke, submicron oil droplets, VOC, aerosols and viruses. However, conventional air filters struggle to keep up with the demand primarily because this would require thicker and/or denser assemblies of fibers. The problem is with the filter pressure. When you have thick and dense filter membrane it’s difficult to push the air through with a small pressure. This demands more energy and overall efficiency of the system, therefore suffers. On the other hand, fibers having bigger diameters have low surface to volume ratio. This will limit the collection efficiency as higher surface area would help retain pollutants more.
Nanofibers application in air filtration
Fortunately, nanofibers can step up to the challenge. Nanofiber mats, have nanoscale pore sizes, typically in the range of 30 to 100 nm, restricting most of the submicron size particulates from getting through. In addition, submicron diameters of these fibers can lead to advantageous surface and interface phenomena that are not significant in conventional filter assemblies. Nanofiber based air filters also have high porosity, typically in the range of 60-90% (volume of void area to volume of fibers). This makes the filter less solid, improving media design possibilities which can lead to more efficient filter designs.
Nanofiber based air filters use polymeric nanofibers that are produced using electrospinning process. In this technique, a high voltage electric field is used to spin a polymer solution in to a mat of nanofibers. Recent advances in the electrospinning process has given scientists to produce nanofibers with good precision and good control of diameter. Nanofiber based air filters generally use fibers with diameters ranging from 50 to 250 nm which can produce narrow and uniform distribution of pore size across the filter cross section.
Although, having smaller diameters should theoretically increase the pressure drop across the filter membrane requiring more energy to push the air through the surface. However, higher surface area and high coverage of polymeric nanofibers has led to interesting surface and interface effects that more than composite for the pressure drop increase. Nanofibers, greatly increase interception and inertial impaction efficiencies of the filter membrane which leads to very high filtration performances at low thicknesses.
Also, nanofibers have small enough diameters so that the molecular movements of the air molecules becomes more prominent compared to the bigger fiber sizes used in the conventional air filters. It has been observed that, gas molecules simply slip off around the nanofiber instead of making a shear contact with them. Due to this slip at the fiber surface, drag force on nanofiber based air filters are much smaller than that in the case of non slip flow seen in bigger fibers which translate in to lower pressure drop. Slip flow phenomenon can also increase the portion of air molecules traveling near to the fiber surface which result in higher interception and inertial impaction efficiencies.
The most common approach is to add a layer of electrosupn nanofiber layer on traditional filteration membranes. This simple modification can increase the filtration performance in several folds. In fact, polymeric nanofiber based air filters is a not new. In fact there have been number of commercial air filtration applications based on nanofibers over the last 20 years, mainly in automobiles and respirators.
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