Type: PhD / Textile Engineering
Thesis advisor: Assoc. Prof. Dr. Ali KILIÇ
Student's name/surname: Ali TOPTAŞ
Summary
Airborne particulate matter (PM), particles in the air with a size of 10 microns and smaller, poses serious health problems due to its density and dimensions, leading to various respiratory and cardiovascular diseases. The escalating industrial activities driven by increasing population worldwide have exacerbated air pollution, making PM a subject of extensive research from scientific, economic, ecological, and health perspectives. According to the World Health Organization (WHO), over 4 million people die annually due to prolonged exposure to polluted air containing particles smaller than 10 microns. Particles in the air are classified based on their size as PM10, PM2.5, and PM0.3. Among them, PM0.3 particles with a size of 300 nm pose the greatest danger as they are challenging to capture, remain suspended in the air for extended periods, and carry toxic properties.
To address this issue, air filters, especially High-Efficiency Particulate Air (HEPA) or Ultra-Low Particulate Air (ULPA) filters, are crucial, especially in environments like intensive care units, operating rooms, and places with individuals with health issues. These filters use micro and/or nano-porous nonwoven fabrics that are produced via spunbonding or melt-blowing processses.
Polyester (PET), polypropylene (PP), aramid, cellulose, and glass wool are among the materials used to create these fabrics. They are primarily utilized in deep filtration processes. Achieving high filtration performance in these microfiber fabrics often involves electrically charging the fabric. However, the filtration efficiency significantly diminishes when the fabric loses its electrical charge.
Nanofiber fabrics, with pore sizes in the nano range, can capture nano-sized particles without the need for electrical charging. Yet, the small size of the fibers can result in a tightly woven structure, increasing pressure drop in the filter material. Bimodal filter fabric structures, incorporating both nano- and microfibers, aim to filter nano-sized particles through nanofibers while allowing air passing through the fabric via the voids created by microfibers. Nanofibers generally have low mechanical resistance, and the presence of microfibers enhances the fabric's mechanical strength. Therefore, producing bimodal fabrics can yield durable filter elements with high filtration efficiency and low pressure drop.
An ideal air filter should provide high filtration efficiency and low pressure drop (ΔP) simultaneously. Traditional fibrous filter media, composed of micrometer-sized fibers, offer low ΔP but exhibit limited submicron particle capture efficiency due to their large pores. On the other hand, filters composed of nanofibers, with smaller pore sizes, offer high filtration efficiency but impede air flow, causing a high ΔP. Additionally, faster pore clogging during filtration leads to quicker cake formation and reduced filter lifespan. Researchers are exploring new approaches, such as bimodal fibrous filters and electrostatic modifications to enhance filtration performance and address varying particle sizes.
Bimodal (This term, in the thesis title, is a term that will be used widely throughout the entire study. It indicates that there are fibers with two different average thicknesses (diameters) on the filter surface.) fibers represent a novel approach achieved by using micro and nanofibers either in a single layer or in multilayer configurations. The integration of at least two different fiber size ranges allows the filter to efficiently capture a wide range of particles. While microfibers provide mechanical strength and stability to the filter, nanofibers offer a complex network with a high surface-to-volume ratio for effective particle capture. By optimizing the arrangement of the filter layers and distribution of fiber diameters, increased ղ, low ΔP, and enhanced dust-holding capacity can be achieved
In the study where PVDF-based electret (In the scope of this study, the term 'electret' is commonly used. This term refers to the charging of polymer surfaces with negative or positive charges, which occurs as a result of the orientation of free ends of polymer chains towards the polymer surface due to external factors applied to the polymer.) nanofiber mats were optimized with the electro-blowing technique, where the experimental parameters were systematically designed using the Taguchi three-level L9 orthogonal design and the results were then analyzed using ANOVA. In this context, it was determined that the most important factors affecting fiber diameters among the parameters examined (concentration, air pressure and electric field) were concentration and electric field strength. It was observed that the increase in air pressure had a negligible effect on fiber diameters but reduced the defects like droplet density. The optimal parameters yielding the thinnest fiber (124±71 nm) were determined as 9 wt.% concentration, 2 bar air pressure and 30 kV electrical voltage. Additionally, applying corona discharge treatment to the samples resulted in a remarkable increase in quality factors of over 70%. Additionally, the corona discharge process provided 78,7% improvement in QF, increasing the filtration performance to 98,97% in the best sample. This work highlights the potential of the electroblowing method in processing highly efficient PVDF nanofibers for air filtration.
In the second chapter where PVDF (polyvinylidene fluoride) based nanofiber nanogenerators were produced by the electroblowing method, the effects of the applied electric field and the air pressure used on the fiber morphology and piezoelectric properties were examined. was investigated. As expected, the resulting mats contained entangled fibers due to turbulent measurements of air, but the number of entangled fibers was minimized by applying voltage. Samples produced at 2 bar air pressure had lower avarage fiber diameters than samples produced at 3 bar pressure. In addition, 2 bar air pressure is more beneficial in terms of β phase formation than 3 bar air pressure. Additionally, it was observed that the β-phase content increased with the increase in the treatments applied. Among the samples produced with 2 bar air pressure, the S2 sample with 30 kV voltage showed the highest β-phase percentage. The maximum piezoelectric output was obtained from sample S2, where AFD was 224±60 nm, β phase was 88%, and Vmax was 1,92 V. Polarization effect at the strongest power to maximize the β phase and piezoelectric output of the produced PVDF fibers by electro-blowing method.
In the third chapter nanostructured filters were produced using the electroblowing method from solutions containing polyvinylidene fluoride (PVDF) and polyethylene glycol (PEG) polymers at different rates. By increasing the water-soluble, low molecular weight PEG content and applying water bath treatment to the produced mat, fiber diameters were reduced and a more porous structure was obtained. In particular, the PVDF:PEG (3:7) sample with the highest PEG content exhibited clustered nanofiber/nanonet-like structures with average diameters of 170 nm and 50 nm. Removal of PEG after the water bath process enabled the formation of a nanonet structure, especially at the intersections of the fibers. Although this process resulted in a more porous structure and a slight decrease in filtration efficiency (-1,3%), the observed significant reduction in pressure drop led to a significant improvement in quality factors. Additionally, by exploiting the polarizability of PVDF under an electric field, the filtration efficiency of nanostructured PVDF filters was increased by 3,6% after corona discharge treatment, resulting in a 60% improvement in quality factor. As a result, the PVDF:PEG (3:7) sample offered an impressive filtration efficiency of 99,57%, a pressure drop (∆P) of 158 Pa, and a quality factor of 0,0345. This study has shown that nanostructured high performance filters can be obtained from PVDF-PEG mixtures by electro-blowing method.
In the fourth chapter, the filtration performances of nanofibrous mats obtained by combining layered and bimodal approaches were evaluated. Fibrous layers produced by melt-blowinng (MB) method were obtained with similar fiber diameters and different thicknesses with different feeding speeds. Bimodal structures obtained by fibers with an average diameter of 225 nanometers produced by the solution blowing (SB) method to fibers with an average diameter of around 800 nm obtained at 1, 5 and 10 rpm screw rotation/feed speeds had higher filtering performance than the samples without SB nanofibers. Then, among the 3 samples with an average basis weight of 15 gsm, only the MB sample (electro-blown nanofiber); The sample (L) containing 4 gsm EB nanofiber and the 4-layer sample (4L) containing 4 gsm EB nanofiber (138 nm) were compared. The 4L sample had the highest quality factor (0,0353) with a filtration efficiency of 96,01% and a pressure drop of 135 Pa. Although the filtration efficiency increased in all samples with the subsequent corona discharge, the highest value (99,34%) was obtained from the 4L sample.
In the fifth chapter the impact of the “bimodal” structural design was explored using fibrous mats composed of fibers with different diameters produced through MB, SB, and methods in various layer configurations and electrostatic charging While maintaining the basis weight of the filter samples as 30 gsm, using 4-layered filter (4L) resulted in structures in improved air permeability compared to single-layer samples. The 4L sample exhibited the highest performance at an airflow rate of 95 L/min, achieving 99,52% filtration efficiency at 148 Pa. Moreover, replacing MB layer in the 4L structure with bimodal mats (BM) obtained by homogeneously incorporating SB nanofibers into the MB layer increased the filtration efficiency 99,61%, keeping the ΔP nearly the same. The corona discharge treatment yielded the highest efficiency (99,99%) in the 4BML sample, produced by using PVDF:PP masterbatch and a bimodal approach. Even after one month the filtration efficiency was maintained at 99,90% which shows clear advantage of bimodal fiber distribution in electret electret filters.
In the sixth chapter, the changes in fiber morphology and filtration performance of PVDF nanofibers, which show high filter performance with their electret property, were examined with the additives added to the structure. In the study, where it was aimed for droplet-free, smooth and thin nanofibers to show an electrostatic effect for a long time, the thinnest fiber diameter and the smoothest nanofiber networks were obtained from solutions in which Al(NO3)3.9H2O, NaCl, LiCl, KCl additives were added to the structure at an amount of 1% of the PVDF solution weight. fiber morphology was obtained from the sample with Al(NO3)3.9H2O additive material. In addition, 99,95% filtration efficiency and 195 Pa pressure drop values were obtained after corona discharge. The highest values in the measured solution conductivity result belonged to this sample. As a result of all these measurements, the Al(NO3)3.9H2O sample, became the best sample with 99,95% filtration efficiency.
In the seventh chapter, layered studies were obtained from PVDF and PA-6 nanofibers in different triboelectric series. To examine the bimodal effect as well as the triboelectret effect, bimodal filter structures obtained from PA-6 nanofibers with an average diameter of 60 nm and PVDF nanofibers with a diameter of 176 nm reached a filtration efficiency of 99,997% and a pressure drop of 193 Pa after corona discharge. After 4 weeks of discharge with the IPA method, the samples subjected to filtration testing proved their triboelectric properties with both 0,26 kV surface potential and 99,829% filtration performance.