Blog #11 - Permeation vs. Penetration Protective Gear
"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."
- http://ohsonline.com/Articles/2008/07/Permeation-vs-Penetration.aspx
"Penetration - is the flow of chemicals and micro-organisms through the porous material, seams, small holes or other small defects in a material.
Permeation - is the process where a chemical passes through material on a molecular level. Permeation means the following: a chemical’s molecules penetration through the outer material. Diffusion is the movement of molecules through the material. Desorption is the outward flow of molecules from the inside."
- http://www.guide.eu/en/info/EN/en374.html
When we talk about chemical protective clothing, penetration is the how a chemical passes through a pore or opening in the barrier material. Permeation is the how absorption and diffusion of a chemical occurs through the barrier material at the molecular level.
Factors that affect penetration:
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The size of the particle
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The size of the pores/openings in the fabric
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How open the fabric structure is, this makes it more likely a particle will be able to penetrate a given fabric.
Permeation tests, usually test for hazardous liquids and or vapors. Critical factors that influence permeation:
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The challenging chemical e.g. the concentration, the temperature, the surface tension, the size of the molecules.
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The makeup of the barrier material
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The exposure time
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Physical factors like the 'ambient' temperature as well as pressure
The Permeation rate is the rate which the chemical will move through the material. It is measured in a laboratory and is expressed in units like milligrams per square meter per second (or some other [weight of chemical] per [unit area of material] per [unit of time]). The higher the permeation rate, the faster the chemical will move through the material.
Permeation testing usually deals in very small quantities; the measured “Normalized Breakthrough” which is the time it takes to achieve a PERMEATION RATE (through a given fabric) of 0.1 or 1,0 * µg / minute / cm2. (1 µg or 1 Microgram is 1 millionth of a gram).
Penetration testing deals in larger amounts, the breakthrough being measured at the point at which a “visible” breakthrough of the chemical is seen.
The 'Breakthrough' time is time it takes a chemical to permeate completely through the material. This is determined by applying the chemical on the glove exterior then measuring the time it takes to identify the chemical on the inside surface. The sensitivity of the instruments used in these measurements determine when a chemical is first detected. The breakthrough time gives an estimate of how long a glove can be used before the chemical will permeate through the material.
'Degradation' is a measure of the physical deterioration of the material due to contact with a chemical. The material may get harder, stiffer, more brittle, softer, weaker or swell. Worst case scenario the material dissolves in the chemical.