EN 369 - CHEMICAL PERMEATION
"Resistance of Chemical Protective Clothing Materials to Permeation"
Explanation of Test Method:
Material is clamped between two chambers of a permeation cell acting as a barrier. The entry side of the cell is filled with chemical and the exit side is checked for the presence of diffused molecules of chemical using a gas/liquid collection and detection system. Once chemical is detected the rate of permeation through the material is measured over time.
The process by which a chemical moves through a material on a molecular level.
The three step process includes:
1) Absorption: Chemical is absorbed into the outer surface of a material.
2) Diffusion: Chemical then diffuses through the material on a molecular level.
3) Desorption: Chemical emerges as a vapor on the inside surface of the material.
Permeation Rate is expressed (i.e. µg/cm²/min).
NOTE: Permeation is typically NOT VISIBLE to the eye, even when permeation has occurred.
KEY WORDS RELATED TO PERMEATION
A measure of how fast a chemical is moving through the fabric. This is written as weight per surface area, per time. It is expressed as an amount of chemical passing through a given area of clothing per unit of time.
For example: a permeation rate could be written as 5 ug/cm²/min. (5 micrograms per square centimeter per minute)
The time after exposure when a chemical is detected by the measuring/detection instrument on the inside surface of the material.
System Detection Limit:
The sensitivity of the instrument being used to measure permeation. For example, some measuring instruments may be better than others. One instrument being used may be able to detect very small amounts of chemical breakthrough e.g. 0.001 ppm, and another instrument may only be able to detect breakthrough of 0.4 or 3.1 ppm.
This measurement is usually written in parts per million (ppm). The SDL must be determined for each individual test performed. This information is required in order to compare test data accurately.
EXAMPLES OF THE PERMEATION PROCESS:
Following are some specific examples that illustrate the concepts of chemical absorption in the permeation process:
- Mylar Balloon vs. Rubber Balloon If two balloons are filled with helium, one mylar film and one rubber, which will deflate the quickest? The rubber one. The helium permeates the rubber membrane much faster than the mylar film.
- Marking / Erasure on Thermoplastic/Elastomeric vs. Film Composite materials:If ink marks are made on a butyl rubber, PVC, Viton, and film composite materials and a short time has elapsed (approximately 5 minutes), differences in absorption can be observed.
The marks will rub off of the film composite fabrics very easily. With the elastomeric and thermoplastic fabrics, the marks will smear and be difficult, or impossible to remove.
This example illustrates permeation and the absorptive characteristics of elastomeric and thermoplastic materials.
Permeation Process of Chemical Contact, Molecular Diffusion, Desorption:
Permeation can be explained by considering a common garden hose and on/off valve. Assume that the on/off valve represents the exposure scenario, the water is the challenge chemical and the hose is the protective fabric. When the water is turned on, the hose begins to fill, representing initial contact of a fabric with a chemical and the beginnings of chemical diffusion through the fabric. As the hose fills, an initial shot of water will be expelled from the end of the hose (i.e. chemical breakthrough). After some time, the flow of water will become constant (i.e. steady state permeation rate). Contamination can be explained as the water inside the hose at any time.
When the valve is shut off the flow of water will continue, as does permeation for a period of time after the challenge chemical is removed. Even after the valve is turned off and the flow of water has stopped a certain amount of water remains in the line and unless encouraged by lifting one end of the hose will remain relatively stagnate until disturbed. This water represents residual contamination.
Permeation - Compared To A Sponge
Assuming the sponge is a protective fabric and water is the chemical. If water is poured on one side of the sponge (chemical contact), it will soak into the sponge (diffusion), and eventually start to come out the other side (breakthrough). When the water has stopped pouring onto the sponge, the sponge is still full of chemical. Even if the sponge is squeezed, it is still damp (residual chemical). Elastomeric material are much more absorbent than film laminate products.