Permeation vs. Penetration

Safety managers are well-advised to evaluate how chemical protective garments react in these tests.

The selection of any protective garment is complicated and carries the potential for serious consequences should the decision prove to be incorrect or the selection in any way inadequate. For this reason, standards bodies, vendors, customers, and workers are engaged -- often in the pages of this very magazine -- in ongoing conversations to determine the appropriate levels of protection for a given task.

In the area of liquid chemical protection these conversations often center around two alternate methods for testing garments—permeation and penetration. Often these approaches to evaluating chemical barrier protection are treated as being equally appropriate. This approach, however, is overly simplistic and rarely considers the strengths and deficiencies of each test.

Fundamentally, there is much that is not known regarding how a wide variety of chemicals react against skin to accept a compromised testing approach.At their core, all safety personnel know that what is unknown is, by definition, unsafe. With that principle in mind, a thorough investigation of what penetration and permeation tests are, and what they can achieve, should prompt safety managers to re-evaluate how they source chemical protection garments.

In the science of chemical protective clothing, the terms permeation and penetration represent very different mechanisms of chemical protection.

Penetration Testing
Penetration, as it relates to chemical protective garments, refers to the passage of chemicals through clothing material without changing its properties. Penetration is achieved when a chemical finds and passes through pores and imperfections in the clothing material, closures, seams, and interfaces. Penetration testing, therefore, is a defect test—an evaluation of how (or whether), a substance flows through openings in a material.

On the surface, the concept of a penetration test sounds entirely appropriate and sensible. In practice, however, penetration testing is the source ofmuch confusion. This stems from the way penetration tests are actually conducted, how the results are reported, and the way the results are perceived.

There are several factors to consider when evaluating penetration test results: pressure and duration of the penetration test, surface energy, viscosity, and chemical interactions between garment and hazardous liquid.

Pressure: A penetration test infers a level of pressure is involved in an attempt to push or force a chemical through a protective garment—to drive it through the void. This makes abundant sense; a penetration test with no pressure involved is simply a repellency test, the sort of approach a raincoat manufacturer might take to evaluate its product.

As much sense as this makes, however, some manufacturers report penetration test results without specifying the pressure and duration of testing.Others fail to explain the implications of a given pressure. Small variations in pressure can mean the difference between a Pass or Fail in penetration testing (especially when chemical properties and viscosity are also taken into account). Penetration tests use a variety of pressures, and only by examining the method and with full reporting of the test conditions can you begin to relate the penetration results to the potential exposure.

Chemical of the hazardous liquid: When conducting penetration tests, the physical and chemical properties of the hazardous liquid are also of note.While it is best to use the actual chemical of concern in penetration testing, this is sometimes not practicable or is considered “too” expensive.As a result, manufacturers often use surrogate liquids and assume that a surrogate of equal surface tension and viscosity will be representative.

Much like pressure, small variations in viscosity and surface energy can have a large impact on the ability of liquid to penetrate a garment. Small amounts of soaps and other surfactants can alter surface tension (the ability to “wet-out”) significantly and, in turn, reduce pressure required to penetrate. Some liquids/fabric interactions do not follow simple penetration models based on pore size, viscosity, and surface tension. Chemical interactions between the liquid and fabric lead to behaviors different than predicted by surface energy, pore size, and viscosity. This makes selecting the right surrogate liquid especially important, if one must be used.

Is the penetration performance of the surrogate representative of the hazard? Failure to evaluate the physical and chemical equivalence of the surrogate is as equally troubling as a failure to explore the pressure and duration of the test. One successful penetration test, regardless of the pressure and duration, is not sufficient, in and of itself, to ensure protection against a different liquid chemical with different chemistry, viscosity, and surface energy.

Permeation Testing
As opposed to penetration testing, permeation testing evaluates how chemicals move through protective garments without passing through voids or imperfections. Permeation testing looks at how a given test chemical is absorbed into the contact surface of a garment, the diffusion of the chemical through the material, and desorption of the chemical from the opposite side.Permeation can occur in the absence of penetration.

Like penetration testing, there are some variations involved with permeation testing. The military, for instance, gauges permeation of field clothing by “spotting” a material with drops of a given chemical. The theory behind this is that armed forces in warfighting situations are far less likely to encounter pools of chemicals than dispersed droplets.

Some tests are also constructed around intermittent splashes of a given chemical. The chemical is splashed against a fabric at a given interval -- once every 10 minutes, for instance. The time sequence for intermittent splash permeation testing is as much a judgment call as the pressure/time sequence used in penetration testing.As will all other forms of permeation testing, the material is monitored for the presence of the chemical on its reverse side.

By far the most thorough permeation test -- and the most thorough liquid chemical barrier test in general -- is to ensure the entire surface of the material is covered by the chemical in question. In these tests, the material is used to create a division between two chambers. On one side is a large pool of liquid chemical; the other is empty. Over time, the unexposed side is tested for the presence of the chemical.

Testing for Safety
For some, the differences between penetration and permeation testing are no cause for concern. Garments that pass penetration tests are said to be “safe” for certain activities where minimal exposure is expected; the more stringent permeation tests are said to be for garments that will be used in extreme conditions. But who and what determine “minimal” and “extreme”?

This minimal-exposure argument is the justification some manufacturers have used to promote garments with higher levels of moisture transport for the purpose of reducing heat stress on workers. These garments are reported to pass some penetration tests and are, therefore, said to be safe for activities involving the potential for liquid chemical exposure.

This is where the element of the unknown comes into play.Are the penetration tests representative of the hazard? The chemical industry does not have a large body of reliable information on how chemicals interact with skin. It simply does not have good skin toxicity data and does not know the skin exposure limits for a large number of chemicals—either liquid or vapor. Furthermore, the industry doesn’t know how these chemicals will interact with the skin under different conditions: dry skin; sweat-saturated skin; in the presence of sweat-soaked clothing; when exposed to damaged skin, abrasions, or cuts. There are many unknown factors.

Furthermore, the benefits of limited-use materials— namely the ability to reduce heat stress—have not been well established. True, at low-levels of work activity, and in air with low moisture content, there is some evidence to support some lower heat stress burden. Beyond that, however, there is no data that demonstrate heat stress is eliminated at normal working conditions. Yet advertising still promotes that barrier should be sacrificed to comfort.

This total lack of reliable data -- relative to both heat stress and the interaction of chemicals with skin -- means safety managers are not equipped with the information necessary to be able to select a lower level of protection for their workers with any peace-of-mind. The solution, of course, is to stick to the principle mentioned above: unknown is unsafe. Especially considering that the solutions being proposed do not eliminate the heat stress issue—it must still be managed.

With this principle in mind, the clear choice of manufacturers and safety managers is to err on the side of caution. In this case, caution means focusing on more sensitive, more stringent, testing. While penetration tests look for holes and imperfections, permeation tests, by their nature, test both permeation and penetration— after all, any voids or imperfections will be just as apparent in this test as they are in a standard penetration test.As a result, safety managers are well advised to evaluate how chemical protective garments react in these tests.

This article originally appeared in the July 2008 issue of Occupational Health & Safety.

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