| Author: | Sun, Qiangqiang |
| Title: | Multilayer PVDF electret nanofiber filters for enhanced aerosol filtration |
| Advisors: | Leung, Wallace (ME) |
| Degree: | Ph.D. |
| Year: | 2020 |
| Subject: | Filters and filtration Air filters Air -- Purification Nanofibers Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xv, xvi, 229 pages : color illustrations |
| Language: | English |
| Abstract: | The pervasive existence of submicron and nano- aerosols emitted from human activities have created serious air pollution problems and raised a major health concern. Filtration using fibrous filters is one main method used for removing particles from gas streams. However, traditional mechanical filters are either ineffective due to large fiber diameters or inefficient because of high pressure drop. Electret fibrous filters, being able to improve aerosol removal without increasing pressure drop to the filters through electrostatic filtration mechanisms compared to mechanical nanofiber filters, have been of increasing interest for researchers. Specially, more attention is paid to dielectrophoretic filtration effect between charged polymeric fibers and neutral particles. To date, several polymers have been used to make fibrous filters and investigated for their chargeability and filtration performance. However, the studied materials for electret fibrous filters are still limited, and charge instability and nondurability remain problems for their long-term storage and use. Polyvinylidene fluoride (PVDF), contributed by its outstanding physicochemical properties, is a promising material of great chargeability and charge stability. Nonetheless, few studies focused on the electrostatic charging properties of PVDF fibrous filters and their filtration performance for aerosol particles. The objective of this study was to investigate both the short-term and long-term aerosol filtration properties of multilayer nanofiber PVDF electret fibrous filters as a potential candidate for face masks. In this study, defect-free PVDF nanofiber filters were fabricated by electrospinning. It was confirmed that PVDF filters had superior chargeability. The concept of "multilayering" was then first applied for electret filters in this study based on the potential benefits of lowering pressure drop and enhancing filtration efficiency. Through comparative tests with 1-layer counterparts, the excellent filtration performance of multilayer PVDF electret filters was then verified. Without changing the filter basis weight, by redistributing the charged fibers all packed in a 1-layer filter into a multilayer filter, the filtration efficiency was improved attributed to the weakened electrical interference among randomly oriented charged nanofibers. Meanwhile, the air flow resistance was significantly alleviated due to the much more porous structure compared with the 1-layer filters. Moreover, via the analyses of singe fiber efficiency, it was proved that dielectrophoretic filtration effect played the main role in aerosol capture enhancement using PVDF electret filters and multilayering could help maintain the single fiber efficiency of multilayer electret filters at higher levels than the 1-layer counterparts. In addition, based on the filter tests during a three-month period, the charge stability on the multilayer PVDF electret filters was proved. In conclusion, by combining the advantages of PVDF and "multilayering", we managed to fabricate electret filters with high filtration efficiency, low pressure drop and long-term storage stability. After confirming the merits of multilayer PVDF electret filters, it is essential to optimize the filtration performance by modifying the physical properties of the filters. Several modified charging methods have been adopted in literature to facilitate electrostatic filtration effects of electret filters. However, these methods either have the risk of damaging filters and charging devices or need complicated apparatus to carry out, which makes the preparation of electret filters less feasible. Compared to the enhancement in electrostatic effects, a more realistic method to boost aerosol capture while maintaining a low fiber amount is using thinner fibers through improving the mechanical filtration effects of Brownian diffusion and interception. Nonetheless, there exists a limit on fiber size reduction because air resistance is more sensitive to a filter with finer fibers. Besides, the contribution of electrostatic effect can be insignificant because of the already remarkable mechanical filtration efficiency. To obtain the ideal filtration performance, an approach was taken based on first optimizing the 1-layer electret filters and then using multiple of the optimized filter (module layer) to construct multilayer electret filters. It was confirmed that an optimal basis weight existed for a 1-layer PVDF electret filter to ensure sufficient deposited charges and to prevent electrical interference so that they could obtain the highest quality factors. And as long as the quality factors of the module 1-layer filter were high enough with moderate filtration efficiency, excellent filtration performance of multilayer electret filters could also be assured. Thus, comparative filtration tests were first conducted using varied filter basis weights and fiber diameters of 1-layer PVDF electret filters. From the tests, it was demonstrated that high dielectrophoretic filtration efficiency did not guarantee high filtration performance of the 1-layer PVDF electret filters. The relative importance of dielectrophoretic filtration effect over mechanical filtration effect was found to be the key factor for filter performance improvement. Therefore, a proper basis weight was needed for 1-layer filters to not only ensure enough electrostatic effect but also to avoid excessive mechanical effect. In this study, the filter 525-S-0.765-C with the highest quality factors was chosen as the module layer to compose multilayer PVDF electret filters. The 4-layer PVDF electret filter with a mean fiber diameter of 525 nm and a basis weight of 3.060 gsm was determined as the optimized filter. It had a filtration efficiency value of 95.1% and a quality factor of 0.164 Pa-1 for aerosols of 300 nm. Moreover, the pressure drop of the optimized filter was only 18.4 Pa, indicating the high potential for further improvement in filtration performance. The merits of multilayer PVDF electret filters have been clearly verified in the lab filtration tests. Yet, what is more important is the effectiveness of the filters in real applications. In most of the researches on nano-aerosol filtration, experiments were carried out in labs, where monodisperse aerosols with a single component and a specific charge state were applied. However, real environmental nano-aerosols consist of varied components with irregular shapes, a wide range of sizes and an unstable charge distribution. Therefore, the test results were too preliminary to directly equate with filter performance in real applications. The complexities of airborne nano-aerosols render filter tests under real environments necessary. Given the importance and wide availability of vehicles, traffic environment located near a road with busy traffic was chosen as an example of real conditions for filter field tests. Through the field filtration tests, high filtration efficiency and quality factors of the multilayer PVDF electret filters could still be achieved, although the oily aerosols and the humid air are normally detrimental to the electret filters. The efficiency of aerosols of 36.87-433.7 nm followed the same trend as that of the lab tests, where the efficiency increased with aerosol size attributed to the stronger dielectrophoretic filtration effect on large particles. Aerosols with lower sizes were largely removed due to the dominant diffusion effect which was independent of electrostatic effects. While the merits of multilayer electret PVDF filters have been well-proved through short-term filtration tests, it is the long-term performance of the filters that is more valued when health impact, financial cost or energy consumption is considered. There have been some researches on variations of aerosol penetration and air resistance with aerosol deposition amount on electret filters. As commonly reported, efficiency of electret filters initially decreased to a certain point before increasing with loading due to shielding of electrostatic effect and enhanced mechanical effect. And aerosol holding capacity could be apparently improved because of more uniform distribution of deposited aerosols. Nevertheless, without investigating the real morphology of particle deposition, previous researches merely drew conclusions based on the difference in pressure drop evolutions between electret and mechanical filters. Moreover, the electret filters in these researches usually had a large thickness within just one single layer, where "skin effect" was easily initiated on filter surface and downstream charged fibers were insufficiently used. Since pressure drop has been confirmed to reduce by redistributing fibers from a single layer to multiple layers, it was intuitively hypothesized that a multilayer filter with an identical basis weight to a 1-layer filter but a much lower packing fraction has a higher resistance to clogging and a larger aerosol holding capacity. Furthermore, for a multilayer electret filter, the charges on the fibers in deep layers are hopeful to be more effectively used, and on the level of individual fibers, the distribution of captured particles is expected to be more even. Through the filtration efficiency variations of uncharged/electret, single-/multi-layer filters with aerosol loading, it was observed that mechanical PVDF filters had continuously increasing filtration efficiency, while PVDF electret filters had initially decreasing and subsequently increasing filtration efficiency until reaching 100%. By combining the pressure drop evolution of different filters during aerosol loading and detailed SEM images of the loaded filters, it was demonstrated that multilayer PVDF filters, especially the electret ones, could significantly slow down the pace of filter clogging (skin effect) and elevate the aerosol holding capacity during depth filtration. Generally, the multilayer filters received the most aerosol deposit during depth filtration, whereas the 1-layer filters received the most deposit during cake filtration. The multilayer nanofiber filters had approximately 70% of aerosol deposit in the filter during depth filtration fully utilizing the full filter thickness, especially for the electret filters that had charged fibers, and only 30% of deposit in the cake. In contrary, the 1-layer uncharged/electret nanofiber filters were exactly the reverse due to persistency of the skin effect with only 30% of deposit in the filter, yet 70% of deposit in the cake. During depth filtration, the pressure drop per added mass deposit for the multilayer electret filter was very low at 11 Pa gsm-1, which was at least twice below any other nanofiber filters. This was all attributed to the uniform capture of aerosols by electrostatic effect across the entire filter depth from the upstream to downstream layers of the multilayer electret filter, as confirmed by the detailed SEM taken across the different filter layers for the multilayer filter configuration. Based on highest efficiency and capacity standpoint with maximum pressure drop imposed on the filtration operation, the multilayer electret filter was the best among all 4 filters, it had 52% more aerosol holding capacity than the 1-layer uncharged filter and 38% more capacity than the 1-layer electret and multilayer uncharged filters. To conclude, the multilayer PVDF electret filters possess excellent filtration performance for both short-term and long-term aerosol filtration, as well as high stability during storage. They have a great potential of applications in the fields of personal health care and environmental protection. |
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| Access: | open access |
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