Differentiating Filters/Membranes by Capillary Flow Porometry

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Filters and membranes are devices which remove unwanted particulates such as dust, pollen, mold, bacteria and more from a continuous stream of either a gas or liquid. An important application for everyday use in air purification is notably built into building ventilation systems, engine air intakes, air compressors, gas turbines and vacuum cleaners. Importantly, cleanrooms that manufacture the latest electronics also require a high degree of clean air for material fabrication. The four main materials used in mechanical air filter media include paper, foam, synthetic polymers, and cotton. Some buildings, as well as aircraft and other man-made environments (e.g., satellites and space shuttles) use foam, pleated paper, or spun fiberglass filter elements to keep people or individual components protected from unwanted contaminants. In addition, some filters employ a static electric charge to attract dust particles to the filter.

Polyester and/or glass fibers are also commonly used to make air filters. Both materials have high temperature ratings near 120 °C, and are widely used in commercial, industrial and residential applications. In some cases polypropylene is used to enhance chemical resistance, but has a lower temperature tolerance. These materials can also be blended with cotton or other synthetic fibers to produce a wider range of performance characteristics. Tiny synthetic fibers known as micro fibers are also used in many types of HEPA (High Efficiency Particulate Air) filters. Filter efficiency is normally reported as Minimum Efficiency Reporting Value (MERV) – a measure of the efficiency with which particulate filters remove particles of a specified size from an air stream. The higher the MERV number, the better the removal efficiency, particularly of smaller particles. MERV levels 1 through 16 are determined using the American National Standards Institute/American Society of Heating, Refrigerating and Air Conditioning Engineers (ANSI/ASHRAE) Standard 52.2-2017 test method. However, this does not address HEPA filters or Ultra Low Penetration Air (ULPA) filters (MERV 17 – 20). Instead, HEPA/ULPA filters are assigned MERVs based on their performance in accordance with standards published by the Institute of Environmental Sciences and Technology (IEST). Both types of test are known as “challenge†tests which require standard dust particles. These tests do not truly measure pore size but their abilities to remove certain particulates. To truly know a material’s ability to filter particles over a given size range, its through pore size distribution must be determined, and this can be quickly and automatically done by capillary flow porometry experiments.