Further Information - Type 3 & 4 Protection
|Type 3 & 4 Differences and Applications / Implications of Permeation Testing|
|Types 3 & 4 : Differences and Applications|
- Type 3 is a strong jet of liquid such as that from a pressure sprayer
- Type 4 is a lighter spray such as that from a sprinkler system – substantial volume but lower pressure.
In EN 14605 certification testing this difference is reflected primarily in the finished garment spray test and not by any comparative mechanical property, chemical permeation or barrier assessment.
Both Types 3 & 4 MUST HAVE sealed seams; EN 14605 requires at least one chemical permeation test on a seam (as well as the fabric) with a minimum result of Class 1 (i.e. >10 minutes). No type of stitched seam could achieve such a result against any chemical. (Referenced in clause 4.2 - Seams, Joints and assemblages).
Type 3 & 4 certification also requires a permeation test on the fabric against AT LEAST ONE chemical with a result of at least Class 1 (i.e. >10 minutes). The standard does not specify any particular chemical for this, so theoretically a garment could be certified to Type 3 and 4 with a permeation test against, for example, water, and showing a normalized breakthrough of 11 minutes (though a fabric with such a poor barrier against water would be unlikely to pass the finished garment spray tests.
Thus certification to Type 3 & 4 DOES NOT per se indicate general suitability for protection against any specific chemical or general chemical protective application. An assessment of the permeation barrier against specific chemicals should be made as part of the risk assessment.
|The Effectiveness and Implications of Chemical Permeation Testing|
The implications of permeation testing are often misinterpreted.
Permeation is the process by which a chemical passes through a fabric at a molecular level by a process of absorption into the surface, diffusion through the fabric and desorption from the other surface. It is a factor of elements such as the relative size, shape, polarity and solubility of the chemical and barrier material. It is also affected by temperature and other parameters. Permeation is not the same as penetration, which is the process by which a liquid physically passes through holes or gaps in the fabric structure (such as between the fibres of an SMS fabric or the "wormholes" of a microporous film). Chemical barrier fabrics are made using solid structure films so penetration cannot normally take place (unless a chemical corodes the fabric) and permeation is the method by which any chemical might pass through.
A permeation tests involves two chambers seperated by the test fabric. The chemical is introduced to one side. The air in the second chamber is sampled and analysed to assess the rate at which the chemical is permeating through the fabric during the test period (normally a maximum of 480 minutes).
The “Normalised Breakthrough” quoted in permeation testing (eg >480 Minutes) is measured at THE TIME TO REACH A SPECIFIED PERMEATION RATE AND NOT THE POINT AT WHICH ANY BREAKTHROUGH IS IDENTIFIED. In Europe EN 6529 offers the options of rates of 1.0 OR 0.1 µg/min/cm2 (*see note below) as the threshold. Usually 1.0µg is used though often not clearly specified. The equivalent North American test ASTM739 specifies 0.1ug. So the ASTM test is a more stringent test than the EN test (assuming the EN test uses the 1.0µg threshold).
A "breakthrough" of >480 minutes therefore means only that the rate of permeation has not achieved 1.0 µg/min/cm2 after 480 minutes, and not, as it is commonly assumed, that "No Breakthrough" has occured.
Thus the “normalized breakthrough” of >480 minutes does not indicate the time taken for any detectable trace of a chemical to permeate through the fabric; the fact that the rate of permeation has remained below the threshold of rate in the 480 minutes recorded actually provides little information about HOW MUCH of the chemical has or has not permeated.
In addition further complications make permeation test results problematic in applying to any real world situation. Permeation rates for different chemicals may be dramatically affected by temperature for example - a higher temperature often means a higher permeation rate - and permeation testing is conducted at 25oC (despite the fact that body temperature is 37oC). Further, the rate is denoted as "per minute" and "per centimetre squared". Clearly in an actual application contamination of just 1cm2 for just 1 minute is unlikely and a much larger area may be contaminated and probably for much longer. This must be understood to interpret the real implications of permeation data and avoid making incorrect assumptions about what a quoted permeation test result indicates.
In theory a chemical could achieve a "steady state" (the maximum rate it achieves and maintains) permeation rate of 0.99µg / min / cm2 after 1 minute and then maintain that rate for the remaining 479 minutes - and indeed indefinitely. Thus even though a >480M breakthrough would then be quoted (because the rate has not achieved 1.0 µg/min/cm2), the actual volume permeated could be:-
479 minutes x 0.99µg (micrograms) x the area contaminated in cm2.
Assuming a splash area contaminated of 50cm x 50cm this means:-
479 minutes x 0.99µg x 2500cm2
= 1,185,525µg or 1.85 grams of the chemical (as 1µg is 1 millionth of a gram) may have permeated through the fabric, even though the "normalised breakthrough" time quoted for the chemical is >480minutes
Thus laboratory permeation tests are of only limited value in assessing actual protective performance in the real world and are really only suitable to be used for relative comparison of chemical barrier fabrics. Therefor any chemical suit selection should include a suitable comprising of a number of elements including:-
- Assessment of the hazards presented by the chemical itself such as hazard type, toxicity limits and duration and/or frequency of task
- Any permeation or penetration data available (see Lakeland's guide to Permeation and Penetration on Chemical Suits for the distinction)
- The specifics elements of the application or task
- Other relative properties of the garment such as mechanical properties and design
* Note: 0.1 µg/min/cm2
= 0.1 microgram per minute per cm2 ...a "microgram" is one millionth of a gram
How Long Have I Got..?
The key question for a user of chemical protective clothing is "how long am I safe for... in this chemical suit... against this chemical... and in this environment..?"
Permeation test data offers little information to effectively answer this question. See Lakeland's Permasure App for ChemMAX 3 for a solution...