Mark A. DeSorbo
WEST CONSHOHOCKEN, PA—The American Society of Testing and Materials (ASTM) is considering a proposed test method for determining the wiping efficiency, wet particle removal and contaminant contribution of nonwoven fabrics used in cleanrooms.
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DuPont's Oathout: “It [standard] measures the ability of a fabric to pick up liquid at a certain speed and pressure.”
Author J. Marshall Oathout, a senior research associate at DuPont Nonwoven Division (Old Hickory, TN) and a member of the Institute of Environmental Science and Technology (IEST; Mount Prospect, IL) Working Group 4 “Evaluating Wiping Materials Used in Cleanrooms and Other Controlled Environments,” says the cleanroom consumables are often selected based on cleanliness and not how well they clean.
If passed, the proposal will be yet another ASTM standard that deals with wiper testing. In September, the ASTM E21 committee passed a standard that joins existing wiper non-volatile residue tests from a particle standpoint and instead of chemical contaminants. [See “ASTM standard to keep wipers clean, CleanRooms, September 2000, p. 1]
Many tests, Oathout says, can assess the wiper suitability, functional characteristics, properties relating to cleanliness and absorption. None of the tests, however, addresses the ability of a wiper to remove liquid from a surface, namely under conditions that occur while manually wiping surfaces.
“When you wipe up a fluid, typically water, it is, of course, absorbed into the fabric. But at the same time, other forces counteract this process. Pressure exerted during wiping can slow or force liquid from the wiper,” says Oathout, adding that surface tension affects liquid distribution in the wiper.
The scope of the proposed standard includes a dynamic wiping efficiency, wet particle removal ability and fabric particle contribution. “It measures the ability of a fabric to pick up liquid, water typically, at a certain speed and pressure,” he says.
Oathout says part of the test involves pulling a quarter-folded swatch of wiper through a small known quantity of water. The percent of the liquid removed from the surface gravimetrically is determined as the dynamic wiping efficiency. The liquid, however, is “salted” with a known quantity and size of contaminants, and the number of contaminants left after wiping is calculated as the particle removal ability. Particles from the fabric are counted separately as the fabric particle contribution.