BY Donna M Kasper, Hollingsworth & Vose Co, and Chair, Working Group CC021
The Institute of Environmental Sciences and Technology (IEST) has published an updated recommended practice (RP): IEST-RP-CC021.2 Testing HEPA and ULPA Filter Media. HEPA and ULPA filters are critical in ensuring that the air supplied to a controlled space, flow, or cleanroom meets cleanliness specifications. Filter media are the key components for any filter. Without filter media that meet the design specs for the filter, acceptable performance of any filter is impossible. Therefore, it is a common and good practice to test and certify the performance of all filter media used in filter construction. Typically, higher-performance filters require more rigidly tested media.
Unlike filters, filter media are made in a “process” and supplied in roll form. Typically, several hundred square feet of media are used in each 24-inch wide filter. In contrast, the wet-laid process used to manufacture HEPA and ULPA media typically uses equipment over 60 inches wide. HEPA filter media consist of microfiberglass with diameters as small as 0.1 mm and a polymeric binder material to hold these fibers in place. For manufacturing these media, a well-mixed slurry of these fibers in water is fed onto the web former of the paper machine (see Fig. 1). Alternate formers, such as a drum former or rotoformer, are also in common use. The water in this slurry is first drained by gravity and vacuum before the wet fiber sheet is dried by passing it over heated drums. The drying not only removes the water in the media but also helps cure the binder so that the resulting sheet of media is suitable for use in filters.
 Figure 1. A well-mixed slurry of media fibers in water is fed onto the web former of the paper machine. A machine with a wire former is shown here. |
Filter media are usually made in minimum production lots of approximately 21,000 linear yards and supplied in slit widths to meet specific filter sizes. Thus, 100 percent testing of all the media in a roll is impractical. Furthermore, several of these tests are destructive. Samples of filter media are tested periodically during production, typically for each roll of media, to ensure that the entire production lot of the media meets specification. Depending on the application, some of these rolls of media may undergo further testing for performance-such as flow rate or particle size-at specific conditions. The methods followed for these tests are prescribed in IEST-CC-RP-021.2.
In addition to this RP, IEST has published several other recommended practices that address the testing and classification of HEPA and ULPA filters: IEST-RP-CC001.4, HEPA and ULPA Filters; IEST-RP-CC007.1, Testing ULPA Filters; and IEST-RP-CC034.2, HEPA and ULPA Filter Leak Tests. These RPs have been discussed in earlier issues of CleanRooms.1
History
IEST-RP-CC021 was first published in 1995 and is now in its second revision. The RP addresses the performance as well as the physical property test procedures for HEPA and ULPA media. This revision has added the testing of polytetrafluorethylene (PTFE) membrane media in addition to the traditional wet-laid microfiberglass media.
Currently, most filter media manufacturers follow these recommended procedures. Similar methods have also become part of other standards around the world. The goal of this revision, as with any revision of an IEST RP, is to make the methods compatible with current materials and technology. Further, since the four filter RPs complement each other, the definitions in IEST-RP-CC021.2 are now consistent with the other RPs.
What’s in the RP?
In the current version, definitions have been revised for thermally generated aerosol and mass median particle size of an aerosol to minimize the confusion in their interpretation. When we think of the size of an aerosol, we assume count mean diameter (CMD), the average particle size of the number distribution of an aerosol. Mass mean diameter (MMD), however, is the average particle size of the mass distribution of an aerosol. The mass mean of an aerosol tends to be larger than the count mean because the mass of a particle varies as the cube of its diameter. Because filter efficiency is size-dependent, it is important to note this difference. For example, the efficiency of a filter at 0.3 mm MMD will be much lower than at 0.3 mm CMD. It is this difference that contributes to the phenomenon of “bleed through,” which is often attributed to reduced media basis weight.
At the core of this RP are the prescribed methods for determining the efficiency of the media. The recommended method for testing HEPA filters continues to be based on the monodispersed aerosols generated by condensation of vapors and multiple-particle detection using a photometer (set to ASTM 2986).
For ULPA filters, the performance is determined at the most penetrating particle size (MPPS) using either polydispersed aerosols and a particle counter or monodispersed particles and a condensation nucleus counter (CNC). One may use either oil aerosols or polystyrene latex (PSL) spheres or salts.
The basic components of a typical test system for particle counters are shown in Figure 2. As seen in the figure, a single particle detection instrument can be used in sequence up- and downstream of the test sample, or two can be used simultaneously. The methods are similar to those used in filter testing. In fact, the guidelines for estimating upper and lower acceptance limits for the measured data are the same as for filter testing. As shown, the components provide a test system that operates above ambient pressure, which is the preferred method of operation. The risk of small leaks, which can introduce particles that may interfere with the measurements, is greater in test systems that operate below ambient pressure. Systems that operate above ambient pressure help insure that system leaks do not affect these measurements.
 Figure 2. The basic components of a typical test system for particle counters. |
The RP provides guidelines for determining the physical properties of the media as outlined below. In many cases, the recommended guidelines are, by reference, identical to other standards from ASTM or TAPPI, including ASTM-D-2986, ASTM-F328, ASTM-F649, ASTM-F-778 and TAPPI-T-410, TAPPI-T411, TAPPI-T-413, TAPPI-T494, TAPPI-T543.
■ Resistance to airflow is usually measured at 5.3 cm/sec. Since many filters currently operate at much lower velocities, resistance may also be determined by these methods at any desired velocity. Usually, commercial test machines can be adjusted to accommodate different test flows.
■ Basis weight of a media is measured in g/m2 of the media. Heavier media tends to have more fibers, a correspondingly higher resistance, and presumably better strength. Typical HEPA/ULPA media are 78 g/m2.
■ The thickness of the media is measured using a rill at a standard mass load to ensure consistent compression and results. Typical media thickness varies from around 0.014 to 0.02 inch.
■ Tensile strength is a key indicator of how well the media will withstand the processing. Since most filter manufacturing machines subject the media to harsh conditions, consistent strength is necessary to ensure that the media can be processed without tearing. Typical media tensile strength is 7 lbs. in the machine direction of the sample.
■ Elongation allows a brittle and rigid material like glass fiber to be made into a sheet that can be flexed and folded into the pleats of a filter without being torn apart.
■ Weight loss at elevated temperatures, or combustible fraction, is gravimetrically measured by weighing the media before and after baking it at an elevated temperature. The difference is a good indication of the polymeric content or binder content in the media. Binder is critical in holding the fibers in the proper structure in the media and in ensuring its performance. Typically, it is less than 7 percent of the weight of most HEPA media. Higher amounts tend to cause problems meeting flammability requirements.
■ Water repellency is typically measured by exposing both sides of a media sample to a hydrostatic head of water, increased at a measured rate. The point at which water first penetrates the sample determines its reppellency, and it affects how the adhesives and potting compounds interact with the media. For example, low water repellency may cause some potting compounds to wick into the filter, reducing the effective filter media area and adversely affecting its performance.
Good filter manufacturing will exploit media properties to design the best filter for the manufacturing process.
Summary
This RP has been in use among all major media makers since its initial publication. When followed consistently, it allows filter manufacturers to use media to consistently meet filter specifications. The methods outlined in the RP are robust enough that they may be used, in principle, for testing even lower-performance media. Although filter efficiency is often considered the key performance indicator of media, the physical properties discussed above are also vital in ensuring that the media can be successfully made into a filter without loss of performance. The use of standards such as this RP ensures not only that the customer has a consistent measure for comparing and selecting media, but that the manufacturer has a consistent method to control its process.
References
1. Winters, Phil. “IEST updates and improves its Recommended Practice for testing ULPA filters,” CleanRooms, June 2005. Vijayakumar, R. “IEST-RP-CC001.4: HEPA and ULPA filters,” CleanRooms, February 2006. Vijayakumar, R. “IEST-RP-CC034.2: HEPA- and ULPA-filter leak tests,” CleanRooms, March 2006.
Donna Kasper is a senior account manager for Hollingsworth and Vose Company, manufacturer of specialty papers broadly used in the filtration industry. She has worked in the filtration industry for the past thirteen years. She is active in IEST and is the director of the Filtration Standards and Practices Committee, overseeing the development and revision of recommended practices used by the filtration industry.