This article lists the key test methods for protective properties such as flammability behaviour, heat penetration resistance and perspiration removal that are used in product standards for heat and flame protective clothing in the Personal Protective Clothing sector.
It covers the outcome of previous projects to improve some test methods and looks at examples of current or proposed projects.
The standardisation activities covered in this article are those of ISO, the International Organisation for Standardisation, and of CEN, the Committee for European Normalisation.
EN ISO 15025:2016 – Test for Limited Flame Spread (LFS)
This test subjects vertically oriented specimens of fabric to a 10 second exposure to a small flame (40mm height when vertical) from a gas fuelled burner. Although the flame size is akin to a lighted match, the temperature is some 800°C, so sufficient to ignite most textile fabrics which are either not made from inherently flame-retardant fibre or not treated with a flame-retardant finish. The flame from the test burner can be applied to either the surface of the fabric test specimen and/or to its bottom edge.
Product specifications for clothing for firefighters typically have the following requirements:
- No spread of flame to edges of test specimens (200mm x 160mm width)
- No hole formation, molten or flaming debris, continuation of flaming or glowing greater than or equal to two seconds after removal of the test burner flame
This test has a long history of use in fire protective clothing specifications, at least from the early 1990s. Consistency of results between test laboratories is not an issue of concern within the testing community, in part because fabrics offered to the fire protective clothing sector of the PPE market almost always do not display marginal compliance with typical product performance standards.
EN ISO 9151:2016 – Test for protection against the transmission of convective heat (flame).
This test subjects horizontally oriented test specimens (140mm x 140mm) representative of a single layer or multiple layer garment to the flame from a so-called Meker gas fuelled burner. A copper disc calorimeter mounted in a non-combustible backing board and incorporating a constantan thermocouple acts as the sensor for heat transmitted through the test specimen. The test burner power is set to provide what is defined as an Incident heat flux of 80 kW/m². In comparison to the gas burner used in EN ISO 15025 above, the flame from this burner is both larger and more powerful.
“although the flame size is akin to a lighted match, the temperature is some 800°C”
The exposure of the test specimens to this burner flame is continued until the heat sensor records a 24°C increase in temperature. This increase represents the onset of 2nd degree skin burns, the longer the time taken for test specimens to record this increase in temperature, the greater the protection of the wearer of clothing made from the test specimen material(s).
Product performance standards express their requirements as an HTI (Heat Transfer Index) which is the time in seconds for this 24°C increase in sensor temperature. A related measure is HTI for a 24°C increase compared to a 12°C increase, that is HTI 24 minus HTI 12, an indication of warning time so that the larger the difference in time in seconds between these two measurements, the better. Product standards for clothing for fighting fires in structures often set requirements for both these measures of HTI.
EN ISO 6942:2002 – Test for protection against transmission of radiant heat (akin to heat from an oven).
This test subjects vertically oriented test specimens (230mm x 80mm) representative of a single layer or multiple layer garment, as in EN ISO 9151 above, to the heat from electrically powered silicon carbide heating rods having a surface temperature of 1150 ± 50°C. As in EN ISO 9151 above, a copper calorimeter/copper constantan thermocouple heat sensor system is mounted in a non-combustible backing board. Unlike this other test which can only be performed at one heat flux value, by adjusting the distance between the heating rods and the test specimen, the incident heat flux can be set over a wide range. Typically, product standards use 20 or 40 kW/m² depending on the type of fire protective clothing being tested.
“real exposure situations are rarely steady state”
Exposure durations to the chosen heat flux are again sufficient to record times for 24 and 12°C heat sensor increases, expressed as RHTI 24, RHTI 12 as in EN ISO 9151. Product performance requirements again set times for these values and often for clothing for fighting fires in structures, they specify the difference required between RHTI 24 and RHTI 12.
A method to determine Transmission Factor (TF) is also included in this test method. It is a measure of steady state heat transfer expressed as the ratio of transmitted heat flux to incident heat flux. Real exposure situations are rarely steady state, particularly for firefighters, which seems to be the argument for infrequent use of this performance parameter in product standards.
EN ISO 12127-1:2015 – Test for protection against transmission of contact heat (akin to contact with hot surfaces).
The existence of this test method completes the trio of the means by which materials are exposed to heat – by flame, by radiation, by contact hot surfaces. It consists of lowering a heated metal cylinder at a constant rate between 100 and 500°C, as chosen by product standards, onto the test specimen representative of single or multilayer garments. The test specimen is in contact with a heat sensor consisting of an aluminium block/thermocouple. The time taken for the heat sensor to record an increase of 10°C is reported as the Threshold Time. The product standards’ set times are considered to be appropriate. This is again a small-scale test, the test specimen size being less than those for the two tests above.
A recent ballot outcome on the status of this standard resulted in Australia wanting a revision, so they will be asked to lead any project group in ISO. Like many test methods, the objective should be to improve the consistency of results between testing laboratories. BTTG will most likely take part on behalf of the UK in any project group.
“the lack of adequate consistency of test results is causing problems for PPE end users”
EN ISO 9185:2007 – Test for protection against transmission of heat from large splashes of molten metal.
This test certainly looks impressive compared to the heat transfer tests set out above! It consists of pouring molten metal, typically iron or aluminium heated in an induction furnace, onto inclined test specimens. Beneath the test specimen (260mm x 100mm) is a sheet of embossed PVC film which acts as the heat sensor. Instead of measuring a sensor temperature increase like the above heat transfer tests, the end point of this test is defined as the mass of molten metal required to cause smoothing or modification to the embossing or pin holing which indicate that a burn injury threshold has been reached. Product standards typically set performance requirements in the range of 60 to 350g depending on the type of metal used.
A serious problem has arisen concerning the ability of testing laboratories to undertake this test. This is because the stock of PVC heat sensor material, held only by the UK Health & Safety Executive at a site in England, has been exhausted. This material is manufactured as one large batch sufficient to provide the world’s testing laboratories with several years of supply. The current situation is that another UK organisation is offering to purchase a new batch and act as worldwide distributors. They are in the process of seeking sufficient indications of future demand to justify their financial commitment.
There has been some work undertaken in the past to try to develop and validate a conventional copper plate/copper constantan thermocouple replacement but even the consistency of test data within one testing laboratory (BTTG), that is its repeatability, was inferior to the PVC material.
Some other ideas for replacement of the PVC material by another kind of heat sensor may now be investigated but will take some time – a couple of years to get to a revised test method at best. This is why the availability of a new stock of the PVC material is the only route to quickly enabling test houses to be able to undertake this important test as set out in EN ISO 11612:2015.
EN 348:1992/ISO 9150:1988 –Test for protection against transmission of heat from small splashes of molten metal.
This test method utilises a gas-fuelled welding torch to create small droplets of molten metal from a commercial welding rod to fall onto a small test specimen behind which is yet another design/specification of heat sensor/thermocouple. The main product standard using this test method is EN ISO 11611:2016 – protective clothing for welders – which sets performance requirements for the number of drops of molten metal required to raise the temperature of the heat sensor by 40 K (Kelvin scale).
A revision of this test method, led by Hohenstein Institute in Germany and to include UK BTTG participation, has been agreed but is yet to commence. This is likely to be a fundamental revision, replacing the outdated use of a welding rod as the source of the molten metal drops as the heat source. Any new test method is likely to require three or four years for its development and validation led by ISO.
EN ISO 13506-1:2017/ISO 13506-2:2017 – Manikin fire test for protective clothing.
This spectacular test method actually originated in the 1980s in USA/Canada, with ISO 13506:2008 being its first iteration as an ISO standard. It consists of exposing fire protective clothing when dressing a full-scale male form manikin to flames from an array of gas-fuelled burners so as to engulf the manikin. This event simulates a flashover as can occur in fires in structures. The manikin is fitted with heat sensors that enable the heat that penetrates through the test clothing to be expressed in terms of Burn Injury Prediction (BIP) or, as in the current EN ISO 13506-1, in terms of Transferred Energy. The regions of the manikin that record first, second or third degree BIP are identified on a computer printout of the back and the front of the manikin.
Improving test methods
There is increasing realisation that the lack of adequate consistency (expressed as reproducibility) of test results between testing laboratories for many test methods, such as those called up in product standards (that is performance specifications) for the heat and flame protective clothing sector, is causing problems for PPE end users. They may not be aware in responses to tenders that if they permit test data to be provided by more than one testing laboratory, they may not obtain the correct technical ranking of competing items of PPE offered by responders to a tender.
This is why BTTG usually argues for revision rather than confirmation when EN ISO or ISO test methods come up for their five yearly reviews.
The example set out below shows that improvement is not necessarily easily achieved.
Revision of EN ISO 6942 – “Protective clothing – Protection against heat and fire – Method of test: Evaluation of materials and material assemblies when exposed to a source of radiant heat”.
This test is specified in the industrial heat and flame sector and the firefighting sector of product standards – EN ISO 11612 (industrial), EN 407 (industrial -gloves), EN 469 (firefighting in structures), EN ISO 15384 (wildland firefighting), EN 13911 (firefighting – fire hoods), ISO 11999-3 (firefighting in structures) being some key examples.
Several different materials have been included in these trials which were led by BTTG – fabrics in single layers as used in industrial coveralls, fabrics/materials used as multilayers in clothing for fighting fires in structures and an aluminium coated fabric for high levels of protection against radiant heat.
This inter-laboratory testing programme has now been completed with sobering results!
Eleven test houses (nine European, one American, one Japanese) took part in this comparison between the existing test method and the proposed revised method. No changes to the apparatus were proposed but an option on setting the incident heat flux was deleted.
“this inter-laboratory testing programme has now been completed with sobering results”
The outcome was that both repeatability (the consistency between repeated test in one laboratory) and reproducibility of RHTI 12 and RHTI 24 were good for both the current method and the proposed revision, the latter only marginally improving the results. For residual tensile strength, the main reason for wanting to try to improve this test method was that the revised method slightly improved what was still poor reproducibility. The aluminised fabric sample was subjected to the flex/crumple procedure as specified in EN ISO 11612 for such materials, but again the revised method did not improve the reproducibility of RHTI results for this fabric. It was the only one subjected to this flex/crumple procedure, the poor consistency of results suggesting that the flex/crumple apparatus/procedure was the probable source of much of this poor reproducibility.
Therefore, the technical content of the existing 2002 edition of this standard will not change in the expected 2022 edition, the only changes being an annex setting out the precision of results from this trial of the existing test methods and also updated references to related standards.
Unfortunately, this is another example of the difficulties in improving test methods in this heat and flame testing sector, previous revision programmes for EN ISO 15025 and EN ISO 9151 also not proving that proposed changes improved the consistency of results. The other side of the coin is, however, that in these two examples the spread of test results does not seem to lead to reports of problems in the marketplace – passes and fails on the same products when tested in different laboratories. Also, in all three examples of attempted improvement, the consequence of no significant changes to test data means no need to reconsider the performance requirements in numerous product standards that cite these three key test methods.
Expanding on the distinction mentioned above between reproducibility and repeatability, for all test methods in the textiles and related sector the test data for the latter is always the more consistent, sometimes by a very considerable margin. This suggests that either the test procedure – mounting/preparation of test specimens for example – or the specification of the testing apparatus is inadequate and/or ambiguous, leading to construction differences.
Let’s hope that future revision programmes for test methods used in this heat and flame protective clothing sector can improve the consistency of results where this is really needed!
Revision of EN ISO 11092 – “Textiles – Physiological effects – Measurement of thermal and water vapour resistance under steady state conditions (sweating guarded-hotplate test).
This test method, now in its second edition dated 2014, is called up in many protective clothing product standards as well as in other textile sectors such as bedding. Recently it has become very obvious that the consistency of results between testing laboratories is not acceptable. This is because it is leading to results from a laboratory meeting a product performance requirement in a standard but another laboratory giving a fail result. The firefighting sector want the lowest possible water vapour resistance, that is the highest possible removal of perspiration, because it is essential that firefighters keep their body core temperature at a safe limit. Product standards express the result of this test as Pascal/Watt per m2. The lower this value is, the better is the breathability of the combination of materials used to create the protective clothing. For example, the Australian product standard, AS 4967:2019, has reduced this numerical value from 25 to 20 compared to its previous edition.
Inter-laboratory trails involving more than thirty testing laboratories worldwide plus additional test data generated by BTTG, and compared to some other laboratories, reinforces the need for this revision. It is expected that work will commence in the relevant ISO textiles committee in 2022.
Discussions between BTTG and Hohenstein Institute in Germany, mean that BTTG on behalf of UK will provide the Convenor (the title used in ISO and CEN) to oversee the work programme in the ISO textiles committee. The technical lead will be provided by Hohenstein and BTTG will be a member of this project group. Experience from testing laboratories such as BTTG indicates that the testing procedure and possibly the apparatus specification will need improvements. This revision will involve inter-laboratory trials of proposed changes to determine if they improve reproducibility and is expected to be at least a 36-month programme.
Revision of ISO 13506 – Manikin fire test for protective clothing – as Part 1 and 2.
This test method for determining burn injury prediction (via Part 2) or transferred energy (via Part 1) has the greatest need of being able to demonstrate much improved reproducibility in the heat and flame protective clothing sector. Burn injury prediction results from the last inter-laboratory trial of industrial coveralls and clothing for fighting fires in structures (EN 469 compliant) concluded that participants were at least not subjecting the test clothing to the same heat energy challenge (from a gas burner array) as demonstrated by the images of the tested items. Variation of this key parameter must be a major contributor to the very poor reproducibility.
A massive programme of work has just been completed by a group of 12 testing laboratories worldwide including participation by BTTG and its RALPH manikin fire test facility. Some 100 meetings of this group, almost all virtual ones due to Covid-19, have taken place over a five-year period. A non-confidential report has been submitted to the ISO committee that is responsible for the revision of the two parts of this standard together with a draft for a first stage ballot (CD stage) of the proposed revised Part 1 and Part 2.
The 99-page report on this work programme shows that improvements have been made in terms of the consistency of results now that there is a far greater understanding of the complexities inherent in this test method.
Three types of coveralls – Nomex, PBI, FR treated cotton – and one type of EN 469 compliant jacket/trouser ensemble were tested. The coveralls using 3s and 4s flame exposure durations and the firefighting clothing using the traditional 8s exposure duration.
The information that this work programme has generated is included as appropriate in the Part 1 and Part 2 drafts now about to be balloted for technical comments.
When there is a full appreciation by ISO and CEN standardisation committees of both the results of these trials and whatever changes to both Part 1 and Part 2 are eventually approved for publication, there can be discussion about the feasibility/desirability of putting performance requirements into some product standards. Examples are EN ISO 11612 and EN 469. Currently, because of poor reproducibility, this test is optional in both standards. However, it is the view of this author that, as with many tests, comparisons of competing items of protective clothing submitted to this very complex test will still be best undertaken in one laboratory of the client’s choosing.