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Abstract
Continual research, development, and advancement in air filtration technology are important to abate the ever-increasing health hazards of air pollution and global pandemics. The purpose of this review is to survey, categorize, and compare the mechanical and thermal characteristics of fibers to assess their potential applicability in air filter media. The history of high-efficiency particulate air (HEPA) filter development explains how we arrived at the current state-of-the-art nonwoven fibrous borosilicate glass filter paper. This review explores the history and practical uses of particular fiber types and explains fiber production methods in general terms. The thermal and mechanical properties of particular fibers are examined using the codes and standards produced by the American Society of Mechanical Engineers (ASME) to generalize the applicability of fiber categories for HEPA filter units within the nuclear air cleaning industry. This review discusses common measurements for specific strength and tenacity used by the textile and construction industries. Particular fibers are selectively compared for density, tensile strength, tensile stiffness, flexural rigidity, moisture regain, decomposition temperature, and thermal expansion. This review concludes with a subjective assessment of which types of fibers may be appropriate to study for HEPA filtration.
1 Introduction
Air filtration is vital to sustaining life. In 2016, the World Health Organization (WHO) estimated between seven and eight million people globally die from poor air quality and air pollution (United Nations Environment Programme 2021). Today more than 92% of the world's population lives in areas that exceed dangerous levels of particulate aerosol from both indoor and outdoor sources (United Nations Environment Programme 2021). Surgical masks and N95 masks for personal protection were commonplace throughout 3 years of the global SARS-COVID-19 pandemic. The United Nations Resolution 70/1, Transforming our world: the 2030 Agenda for Sustainable Development, set a goal for reducing the number of deaths worldwide caused by air pollution and contamination (General Assembly Resolution 2030). The 2019 UN Blue Skies Campaign clearly linked the negative effects of air pollution to human health, longevity, and climate change. General Assembly Resolution 74/212 of 2019 set September 7th as Clean Air Day and recognized, "Clean air is important for the health and day-to-day lives of people, while air pollution is the single greatest environmental risk to human health and one of the main avoidable causes of death and disease globally" (United Nations General Assembly Resolution 2020).
The development and advancement of high-efficiency particulate air (HEPA) filters demonstrate efforts toward sustaining clean breathable air. The U.S. Department of Energy (DOE) installs HEPA filters as a line of defense to contain hazardous particles in high-level waste (HLW) storage tanks. For many years Lawrence Livermore National Laboratory (LLNL) has conducted extensive research on better materials and methods to filter and clean the ventilation air for HLW storage tanks (Bergman and Sawyer 1988; Bergman et al. 1996; Mitchell et al. 2012; Adamson 1998). HEPA filters are common today, however, HEPA filter technology has seen only minor advancement over the past several decades. The most common filtration media in use today, glass fiber filter paper, is fragile and prone to fire and water damage, as demonstrated by the Rocky Flats fire of 1957 (Ackland 1999). While better than the original cellulose-asbestos fibrous media, glass fibrous media performs poorly in high-temperature applications and high-humidity environments. The Defense Nuclear Facilities Safety Board (DNFSB) highlighted the need for alternative nonflammable HEPA filters (Conway 1999).
Recent development of new materials and production techniques suggests better materials and designs may enable the advancement of air filtration technology. Currently, most personal protective masks are constructed with polypropylene fibers, while most industrial filter media are produced with pleated glass fiber paper. Alternative fiber types should be explored for inclusion in air filtration media, to continue the advancement of both the safety and performance of HEPA filters. The U.S. Customs and Border Protection agency maintains a list of 1200 fiber trade names that represent 43 generic fiber categories, acknowledging that the list is neither exhaustive nor complete (U.S. Customs 2021). The purpose of this review is to survey fiber types and compare mechanical, thermal, and chemical characteristics of particular fibers to advance air filtration technology and assist developers in the selection of fibers for air filter media. Special attention in this review is toward HEPA filter media intended for use by the nuclear air and gas industry.
2 Background Information
2.1 Brief History of HEPA Air Filtration
Lewis Haslett is credited with the first modern air filter respirator in 1849 with his "Lung Protector" invention and patent (Haslett 1849). Haslett developed his respirator with woolen fabric filter media. Ninety years later, at the outbreak of the Second World War, gas masks intended for smoke and dust filtration were still built with wool and cotton. The threat of chemical warfare brought new advancements in air filtration. The masks protecting German soldiers were produced from fine asbestos fibers and coarse esparto grass fibers and were noted for exceptionally high filtration efficiency and low airflow resistance to breathing (First 2022; Barnette and Smith 2003). British gasmask woolen filters suffered from clogging with oil-based smoke aerosols and did not perform as well as the asbestos fibrous media (Barnette and Smith 2003). The U.S. Army Chemical Warfare Services Command (CWS) in Edgewood Maryland tested the German filter materials and noted the superior performance of the asbestos fibers within the cellulose filter paper (Barnette and Smith 2003). From this experience, the CWS laboratory developed and produced "H-64" filtration paper which was later renamed "CWS type 6" paper and made from a combination of northern spruce sulfite, cotton, African esparto grass, and fine Bolivian Blue crocidolite asbestos fibers (First 2022; Barnette and Smith 2003). The laboratory noted the superior filtration performance of asbestos fibers cleaved to under 0.25 μm diameter (First 2022). The U.S. Army went on to produce "collective protector" filtration units that required more airflow than personal gasmasks, by deep-pleating CWS6 paper into square-framed canisters (Barnette and Smith 2003).
The Army's collective protector filter units were adopted by the U.S. Atomic Energy Commission (AEC) in the late 1940s to contain airborne radioactive particles in nuclear research facilities and ventilation exhaust systems of experimental nuclear reactors. To reduce dependency on foreign materials, the AEC replaced Bolivian crocidolite with Canadian asbestos. From 1944 to 1951, collective protector filter units were entirely made from all-natural materials. The AEC began considering alternatives to cellulose-asbestos filter media in 1949 to overcome the flammability hazard at moderate and high temperatures. This led to the development of combined glass-asbestos fibrous media and all-glass fibrous media in 1951. The Naval Research Laboratory (NRL) developed methods to reduce the diameter size of glass fibers to 250 μm suggesting glass could be a substitute for asbestos fibers. The AEC type 1 filters were also termed absolute, super-interception, super-efficiency, and nuclear filters (First 2022; Barnette and Smith 2003). Humphrey Gilbert coined the acronym "HEPA" from a 1961 AEC report, "High-Efficiency Particulate Air Filter Units, Inspection, Handling, Installation" (Barnette and Smith 2003). The term HEPA today describes filter materials that achieve a high efficiency of 99.97% of 0.3 μm particles while maintaining a low resistance to airflow, measured by pressure drop across the filter material. The pleated borosilicate glass nonwoven fibrous filter paper media produced in 1951 is essentially still in production and used as HEPA filter units today.
2.2 Fibers and Particulate Membranes
Air filters are commonly constructed with either fibrous media or porous particulate media (Hinds 1999). Porous membrane media is generally denser than fibrous filter media, which creates complex flow streamlines with high efficiency but also high airflow resistance. Porous media can be produced by a variety of methods with ceramics or sintered powdered metals. Composite air filter materials could be designed with both fibrous media and porous media, combining the advantages of both, for example gaining mechanical strength from a porous substrate layer while gaining high efficiency from a fibrous layer. A topic for further development is the optimization of composite filter media with porous membranes and fibrous layers, however, the remainder of this review will focus on fibrous media. See details.
Source: Online/NNA
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