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Posted on 15/08/2015 in News 32 2015
Do flame retardants improve fire safety?

A series of three papers by fire safety specialist M. Hirschler assess to what extent flame retardants, when correctly used, reduce heat release rate from polymers (such as plastics and textiles) and how important this is for fire safety. This includes a summary and assessment of available fire test data, analysis of why heat release rate is the key factor often defining whether a fire will lead to casualties and major damage and specific information concerning flame retardants’ impact on fire safety for 15 widely-used man-made and natural materials.

Overall, in (1), the author concludes that “the correct use of flame retardants (by using efficient systems, designed for the substrate, at sufficient levels) will decrease heat release rate and thus have a very positive effect on fire safety”.

Why heat release rate is critical?

Heat release rate (HRR) is frequently measured in material fire performance tests or specified in fire safety standards. Many assessed studies demonstrate that the widespread use of this parameter is justified, in particular this was highlighted in Babrauskas & Peacock 1992. This study shows that halving HRR of a burning chair considerably increases the time before conditions in a small room become lethal, from c. 3 to c. 10 minutes. On the other hand, making smoke emissions less toxic or making the chair less easily ignitable has virtually no effect. HRR is critical because it defines the magnitude of the fire and also, whether or not, and how rapidly, the first burning item will spread fire to other items.
Flame retardants can also, in many cases, prevent fires starting (reduce material ignitability) and impact smoke toxicity, but reducing heat release is the most critical factor for saving lives and preventing major fire damage. In particular, heat release rate is critical to “flashover”, when accumulation of heat and fire gases causes a whole room to ignite. Flashover is the key point at which fires undergo a massive and irreversible scale-change in hazard and impact, smoke emission and smoke toxicity. Once flashover has occurred, life in the room is no longer viable. NB: for an explanation of flashover and escape times, see http://pinfa2.org/index.php/flame-retardants/fire-safety

Smoke dangers

Studies presented, using both animals and human volunteers, show that both were able to escape even after exposure to massive concentrations of irritants, showing that irritants rarely cause incapacitation (they do not prevent escape). This, together with data demonstrating that human lethality is directly correlated to carbon monoxide (CO) levels in blood, confirms that the essential hazard in fire gases is carbon monoxide, emissions of which depend on the amount of material burning and on access to air during burning (which is reduced with increasing amounts of material burning). Carbon monoxide emissions are largely unrelated to the type of polymer material burning. This confirms the essential importance of heat release rate in fire hazard.
It should be noted, that the above addresses acute fire hazard (danger or lethality in the fire) and not possible long-term toxic effects of smoke exposure.

Relevance of fire tests

Assessment of 36 studies on materials fire testing as prescribed by heat release fire standards (e.g. calorimeter tests), concludes that these tests do give a good indication of heat release in real fire conditions, and are therefore a realistic and effective way of defining materials fire hazards or prescribing materials fire safety properties.
Also, 1988 NBS/NIST room scale fire tests of flame retarded and non-flame retarded items are reassessed. These studies have been recently questioned, but the author concludes that the conclusions of the NIST report are valid and fully relevant to the question of whether the use of flame retardants reduces fire dangers as used in household products: the study used “commonly used plastic products” with “commercial formulations” of flame retardants in the “most commonly used FR/polymer combinations” (of the time).
The items were tested both individually and in combination in a fully furnished room, comparing flame retarded with non flame retarded items: TV sets, computer housings, circuit boards, upholstered chairs and electric cables.
Key results were that the amount of material consumed in the full scale room fire with non flame retarded products was more than twice that in the room with flame retarded products, the heat release was four times higher and the carbon monoxide equivalent gas toxicity was two times higher. The study authors concluded that there was no evidence that the commercial use of flame retardants in these products had any negative effect on any aspect of fire hazard, including smoke. The most important result was that the estimated time to escape from the room with flame retarded items was fifteen times higher than in that from the room with non flame retarded items.

Importance of adequate fire safety standards, e.g. automobiles

The author underlines that flame retardants are only useful if used appropriately, and this depends on having adequate fire safety standards. A cited study of car seat padding foams shows that use of low levels of flame retardants to meet the (widely criticised and undemanding) FMSVSS 302 fire standard (automobiles) has no significant impact in reducing heat release, and so on real fire hazard.
Also, appropriate flame retardant use in combined materials in complete items is also essential. Other cited studies show that the use of flame retardants to meet the (now repealed) CA TB 117 in foams does not significantly reduce heat release, but when combined with a flame retarded covering textile (here, one meeting the NFPA 701 test) heat release reduction is very significant and the item extinguishes when the flame is removed.

Flame retardants and fire hazards of different materials

In (2), the author assesses the effectiveness of flame retardants in reducing fire hazard in 15 different materials, relevant because of their inherent fire hazard and their widespread use in consumer products, construction, etc: ABS / styrenics / HIPS, cellulose or cotton fabrics, engineering thermoplastics (including PC), epoxy resins, EVA / polyolefins, flexible PVC, LDPE (polyethylene), nylon / polyamides, polyesters / PET, polycarbonate, polypropylene, polystyrene, polyurethanes (foam and thermoplastic), rigid PVC, woods.
Data from over 100 studies for these different materials are summarised, showing to what extent appropriate flame retardant use has been demonstrated to reduce HRR (heat release rate).
The different flame retardant systems tested for each material and the HRR reduction achieved are detailed.
The author concludes that although studies available are variable in date and quality, “the breadth of the work covered and the similarity of the interpretation that can be obtained from the studies indicate that the conclusions that can be drawn are fully appropriate. In summary, this work demonstrates that flame retardants, when added as appropriately researched with the correct systems and in the proper amounts, will decrease the heat release rate for virtually all polymeric materials. Thus, the correct use of flame retardants will decrease heat release rate and lower fire hazard and, thus, have a very positive effect on fire safety”.

Importance of appropriate flame retardant use and of fire standards

In (3), the author assesses qualitatively the above cited data for the 15 different materials. Results for 15 different polymer systems, covering these materials, show that flame retardants reduce heat release rate (HRR) by 30 – 80%. This variability confirms that simply stating that a material is “flame retarded” does not mean that adequate fire safety improvement is achieved. “Flame retarded” can simply mean that some level of flame retardant has been added to the product, and in some cases this may not be appropriate or adequate. Also, some materials continue to show very high heat release rate, and so fire hazard, even with flame retardants. Therefore (as indicated above) even some fire performance standards are so inadequate that achieving them does not imply significantly improved fire safety.
The author underlines that flame retardant performances are polymer specific: flame retardant combinations effective in one material can be largely useless in another. However, appropriate flame retardant use (adequate and adapted) can reduce heat release rates by an order of magnitude, so considerably reducing the hazard of fires.

(1) “Flame retardants and heat release: review of traditional studies on products and on groups of polymers”, M. Hirschler, Fire & Materials, 39(3), 207-231, April 2015 http://onlinelibrary.wiley.com/doi/10.1002/fam.2243/abstract 
(2) “Flame retardants and heat release: review of data on individual polymers”, M. Hirschler, Fire & Materials, 39(3), 232–258, April 2015 http://onlinelibrary.wiley.com/doi/10.1002/fam.2242/abstract 
(3) “Fire safety and flame retardants”, M. Hirschler, 24th BCC Conference on Recent Advances in Flame Retardancy of Polymeric Materials, May 2013, Stamford, CT, 2013 www.bccresearch.com 
NOTE: M. Hirschler received financial support from the North American Flame Retardant Alliance of the American Chemical Council for the first two papers.
Presentation “Effect of flame retardants on polymer heat release rate”, M. Hirschler, Fire & Materials 2015 http://www.intersciencecomms.co.uk/html/conferences/fm/fm15/fm15prog.htm 
See also “Heat release rate: the single most important variable in fire hazard”, V. Babrauskas V, R. Peacock, Fire Safety Journal, 18:255–272, 1992.

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