How Chemical Mixtures and Temperature Affect Permeation

The standard method used in North America for measuring the permeation of chemical protective apparel material is ASTM E739 – “Standard Test Method for Resistance of Protective Clothing Materials to Permeation by Liquids or Gases under Conditions of Continuous Contact.” In this test, the material is clamped between two chambers. One chamber is filled with the challenge chemical. The other chamber is checked for the presence of the challenge chemical. The amount of chemical in the sample chamber is measured over time to determine the breakthrough time and permeation rate.

Mixtures Each combination of barrier and chemical has a characteristic breakthrough time and permeation rate. This unique relationship is due to a combination of physical and chemical interactions. Mixtures often have chemical and physical characteristics that are different from those of their components. Take the classic mixture known as “aqua regia” (royal water), for example. Aqua regia is a mixture of two mineral acids. Individually, neither will dissolve gold; but in a specific ratio, they will.   Because mixtures have unique chemical and physical characteristics, it is reasonable to expect that the permeation of the mixtures will be different from the permeation of the components. Some differences can be explained. For example, some rubber-like barrier materials swell when exposed to certain solvents. This swelling is a change in the physical structure of the barrier. If there are other chemicals present, the permeation might be different because the physical structure of the barrier has changed.   The individual chemicals in a mixture may permeate at different rates. Thus, the mixture in the sampling chamber will be different from the mixture in the challenge chamber. Measuring total mass in the sample chamber will not provide information on the permeation of the individual chemicals. With mixtures, either certain assumptions must be used or each component must be measured.   There are a number of ways to measure chemicals that permeate. Few of these techniques “see” only one chemical. These techniques work well when measuring the permeation of individual chemicals. However, with mixtures, it is difficult to know whether you are seeing a small amount of an easy-to-measure chemical or a large amount of a hard-to-measure chemical.   Ideally, the analysis should be able to distinguish between the components in the mixture. The more chemicals present, the more sophisticated the separation technology. In addition, checks must be made to ensure that one chemical does not interfere with the detection of another.

How Temperature Affects Permeation In general, permeation is expected to increase with increasing temperatures. However, there are exceptions to this generality. The temperature response of permeation rates depends on the barrier material and the chemical. In general, permeation rate increases by a factor of 1.2 to 3.5 (depending on the material and the chemical) for each 5.60 C (10°F) increase in the surrounding temperature. For example, at room temperature, a 6°C increase in temperature will increase the permeation rate of simple gases through an oriented polyester film from 1.3 to 2.1 fold, depending on the gases. (“Diffusion in Polymers”, J. Crank and G.S. Park, Ed., Academic Press, 1968, PP 46-50). This illustrates the behavior of one film and a few chemicals within a narrow temperature range.   The effect of temperature on breakthrough times will diminish with temperature, but not with the same impact as seen on permeation rates.