Page 25 - Occupational Health & Safety, May 2019

can be assessed. This testing will determine whether there is bulk flow of a chemical through voids in the material. These voids can include gaps, pores, holes, and imperfections. The most common liquid penetration test for CPC is ASTM F903. In this method, a sample of the material is sealed inside a cell and one side of the cell is filled with a liquid chemical (gases cannot be tested). This test is most commonly performed for 60 minutes with pressure ap- plied for some of the test period. The back side of the material is monitored to note any visible test liquid penetration. If liquid was observed, then the material failed. ASTM F903 gives a visual pass/ fail assessment only; it is a qualitative rather than quantitative mea- surement of liquid barrier.
Barrier to particles also can be checked with penetration test- ing. In this case, particles of known size distribution, typically salt or latex spheres, are generated in a laboratory. Actual hazardous particles are not used. A material’s ability to block penetration of particles of different sizes is most relevant to its level of protec- tion—not the particle’s exact composition. Test results are reported as percent filtration efficiency for the specific particle size tested.
The most accurate way to assess chemical barrier is permeation testing. It measures chemical migration through a material at a mo- lecular level via a three-stage process: absorption into the material on the outside, diffusion through the material, and desorption or release from the material to the inside. Permeation is different from
penetration because it does not require a void/defect in the material. The most common permeation test is ASTM F739, which can be run using liquid or gas chemicals. The test material is sealed in a cell between two chambers. One chamber is filled with the test chemical, which stays in direct contact with the material for the test duration. The other chamber contains a media (typically water or nitrogen) to collect the test chemical. The collection side is pe- riodically sampled. Sophisticated analytical techniques are used to determine if, when, and how much test chemical passed through the material in a set time period. The reported value for F739 is the time lapse from initial contact to when the chemical reached a
permeation rate of 0.10 ug/cm2-min.
Permeation is a very sensitive test and provides quantitative re-
sults. Due to unique chemical/barrier material exposure responses, you should strive to gather permeation data for the CPC you plan to use against your actual workplace chemicals.
Permeation Tests—A Closer Look at Reported Results
For permeation testing, it is important to be aware that some CPC manufacturers might report permeation results using a perme- ation rate of 1.0 ug/cm2-min. That is 10X less stringent than ASTM F739, which has an endpoint permeation rate of 0.10 ug/cm2-min. This difference in reported value criteria is critical for proper CPC selection. Figure 2 represents a typical permeation curve for an
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