Optimization of solution blown PVDF nanofibers for air filtration applications

Air purification has become an important issue worldwide as a result of the increasing population and industrial development. Melt-blown filters, spunbond filters, and glass fiber filters are the conventional air filters. Nanofibrous filters, as another and novel alternative for air filtration, offer higher efficiency, lower resistance or pressure drop, longer filter life, and flexibility compared to traditional filters. Mainly, fiber diameter and packing density (solidity) are the dominant factors on the nanofibers for the air filtration performance. This can be explained by the fact that filter efficiency increases when the diameter of fibers decreases. In this study, PVDF based nanofibrous webs were produced for particulate air filtration application. PVDF has extremely good mechanical, thermal and physical properties to adapt to various application areas such as air filters, membranes, supercapacitors, batteries, special textile products in industrial scale application. Its hydrophobic properties make it a perfect material for protective clothes application. In literature, there are lots of publications about PVDF nanofibers and their excellent properties in different application areas. Most of these studies are used electrospinning method to produce PVDF nanofibers. Electrospinning is the well-known method to produce polymeric nanofibers with old history. Unfortunately, due to its low production rate and electric potential, electrospinning method has not been adopted by the industry completely. Centrifugal spinning and solution blown spinning methods are more suitable to produce nanofibers in the industry due to their high production rate and safer components. In non-electro nanofiber production methods, solution blowing is one of the promising technique. Pneumatic forces are used to form the fiber from a polymer solution. In this technique, compressed air forwards the polymer solution towards a collector and fibers are collected as a nonwoven structure on the collector. In this thesis study, PVDF nanofibers are produced by using a semi-industrial solution blowing machine, which has 70 cm width and infinite length. It also has a multi-nozzle system with eight needles, and a hot air system to heat the compressed air to improve the evaporation of solvents. The aim of the study is the optimization of PVDF nanofibers production and to adopt the production of nanofiber webs into the industry for air filters. Firstly to prepare the solutions, DMSO was chosen as the main solvent since it is partially a less hazardous solvent. As a result of this study, homogenous fiber production could not be performed due to the fact that the solvent could not be completely evaporated, so the second set of experiments was conducted with commonly used solvent DMF to produce PVDF nanofibers. To form the design of the experiment, Taguchi method was used and in accordance with the results, optimum conditions were determined. The effect of parameters over the fiber diameter was analyzed. Furthermore, the analysis of variance (ANOVA) was applied to observe the effects of each parameter on fiber diameter. The results were found to be statistically significant. The concentration of polymer solution was determined the most dominant parameter with making contribution approximately 66,75% over the fiber diameter increment. Under optimum conditions, nanofibrous webs were produced and subsequently the air filtration efficiency of the webs was tested. The filtration properties of the nanofibrous webs comprised of several samples that have different average fiber diameters, which involves the optimum sample, were analyzed. With the optimum sample was obtained the maximum filtration efficiency, Knudsen number, and quality factor values. The results showed that the average fiber diameter was dominant over the air filtration performance. The results also showed that the filtration efficiency of the optimum sample had approximately EPA 11 level efficiency. After the filtration results, solution blow spinning with higher fiber production efficiency can be concluded as a feasible method of producing high-performance air filters.