Advanced Gas-Phase Fire Retardants Project

There is high motivation in industry to replace the widely used brominated fire retardants (and their antimony synergist).  In order to aid in the development and application of new compounds that are likely to be used in fire-retarded high-volume thermoplastics, this project will determine the gas-phase mechanism of the most promising replacement moiety (phosphorus), as well as the existing bromine-antimony system.  The goal of the work is to understand how the properties of the flames over burning polymers interacts with the gas-phase inhibition, so that proper screening methods can be employed by industry, and appropriate standard test methods can be used which  represent  the relevant full-scale behavior.

Objective:   To provide, by the end of FY2015, a database and guidelines on the mechanism of action of fire retardants (FRs) acting in the gas-phase, which will allow industry to develop new, bromine- and antimony-free fire retardant formulations for application to high-volume polymers, and to ensure that the new understanding is properly accounted for in standard tests.

What is the new technical idea? There is impending movement, on the part of polymer companies and fire retardant manufactures, away from very effective, bromine-based fire retardants^[1]. Elimination of these fire retardants will have a negative effect on fire safety, unless alternatives are developed; hence, new approaches are being sought by industry^[2].  Most recent research on improving the fire performance of polymers involves retardants acting on the condensed phase material itself.  While new, nano-structured materials, which usually act in the condensed phase, have great promise, the typical high-volume polymers (e.g., polyethylene terephthalate, polystyrene, polypropylene, ABS) have such a large production volume, and the existing manufacturing infrastructure is so well established,^[2] that industry seeks new FR additives which can be easily blended with the range of existing high-volume polymers. These additives, principally the bromine compounds and antimony oxide (almost always added with bromine, and also under regulatory pressure) both work in the gas phase.

There has been very little--almost no--research done to understand how fire retardants which act in the gas phase actually retard ignition or reduce heat release^[3],[4] .  Most of the insight for gas-phase FRs has been drawn from studies of different compounds of the element added to purely gaseous flames in studies of flame inhibition (in the context of fire suppression by extinguishing agents)^[5].  Nonetheless, the effectiveness of compounds in gaseous flame inhibition is quite complex, depending upon the additive type as well as flame properties.   That is, the properties of the flame itself profoundly affect the effectiveness of the chemical additive, and are themselves changed by the additive, which further changes the additive effectiveness^[6].  Industrial chemists, while very good at understanding the condensed-phase chemistry related to forming stable FR additives and their interaction with the base polymer, have little insight into what they are aiming to achieve in the gas phase: i.e., what compounds actually need to make it to the gas phase, and what they need to do once they are there.  For example, recent anecdotal information indicates that very small changes to a gas-phase FR additive can change its effectiveness by more than an order of magnitude, but the industry researchers do not yet have a credible hypothesis.

For many years, industrial research labs have approached the problem in a very empirical way (the adage being “Blend it and burn it.”). The extent of their understanding of the FR mechanism has been to apportion the mode of action between the gas phase and the condensed phase.  Now that the problem has more constraints due to environmental concerns, however, there is the belief in industry that a more fundamental understanding of the physics involved will help guide them to a solution more effectively.  Consequently, there has been recent work at universities (in Germany and the US) and at government laboratories (in Russia), and starting in the US (FAA Tech Center) on the fundamental mechanisms of FRs in the gas phase, and effective test methods. Nonetheless, none of these studies is looking at the interaction of the gas-phase mechanism with the flame structure, which varies with flame scale, fuel-oxidizer mixing, fuel type, and the time available for flame development over the polymer.

The idea of the proposed work it to understand two distinct things: the gas-phase chemical mechanisms of moieties of interest as potential gas-phase FRs, and the physical properties of flames over burning polymers which can influence the gas-phase mechanism of the FRs.  Some progress has been made in understanding the gas-phase inhibition mechanism of some species of interest as gas-phase FRs, but others have not been studied, and there has not been systematic variation of the properties expected to influence efficiency of the FR moiety.  More important though, is that the flame structure properties (temperature, species concentrations, flow field, etc.) have not been studied for burning polymers.  Consequently, there is no understanding of how the gas-phase mechanisms interact with the unique properties of the flames over condensed phase materials to render the FRs more or less effective.  These interactions will be studied for model laboratory flames, screening tests, and standard test methods so that the relevant performance can be delineated for the different situations, and then extended to full scale fire performance. Moreover, as progress is made towards developing alternative compounds (which may have altered chemical mechanisms), there exists a need to ensure that the standard tests for polymer flammability accurately capture the full-scale fire behavior of real materials.  Hence, the parameters to be varied in the laboratory flame experiments will be those expected to be relevant in full scale fires and standard tests.

What is the research plan?  There are two classes of compounds currently of promise and interest to industry^[7]: phosphorus compounds and new metal synergists.  Phosphorus is of high interest to industry, is known to be very effective in the gas phase, and is the most likely candidate for Br replacement.  Phosphorus in known to exhibit outstanding gas-phase performance for some formulations, but for others, the effectiveness can be more than an order of magnitude less, and it is not known why.  This is very similar to the behavior of phosphorus as a flame inhibitor in pure gaseous flames: it is extraordinarily effective in some flame types, but essentially ineffective in others.  The goal of the present research is to understand the mechanism of phosphorus as a gas-phase FR to determine for what flame types and additive properties it can be effective7.   Phosphorus compounds will be added to the air and fuel stream of tractable flames for which the general properties are known, or can be calculated.  The effects on global properties (heat release, extinction, flame size, liftoff, etc.) will be measured and simulated.  For example, it is possible that phosphorus loses its effectiveness due to formation of condensed-phase particles, and this is the first global property of the flame to be measured.  In FY2013, we will construct the diagnostic experiment for non-flickering flames with pure gaseous, non-sooting fuels, and explore the range of conditions for additive effectiveness.  In FY2014, we will extend the experimental capabilities and investigations to examine flames over simple liquid fuels, with flicker.  Finally, in FY2015, flames over burning polymers, with and without additive chemicals will be examined.  In flames over the actual burning polymers, the longer-term goal is to understand the important complication of soot formation (which can affect the gas-phase FR mechanism).

The key is that for this model compound (phosphorus), we will understand the gas-phase inhibition mechanism, the properties of the flames over burning polymers which interact with the gas-phase mechanism, and how the overall performance is affected by this interaction.  Once this interaction is understood, we can use that insight to extrapolate to other compounds of use (or interest), providing guiding principles for ensuring that 1.) new gas-phase FRs are effective, 2.) FR screening tests are relevant, and 3.) standard tests properly account for the interactions between the gas-phase mechanism and the flame properties (which will vary with the type of fuel-scale fire and standard test).   

 

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[1] Polybrominated diphenyl ethers (PBDEs) and Hexabromocyclododecane (HBCD) are very widely used fire retardants, and are both the subject of future regulations.  The European Union has adopted the Restriction of Hazardous Substances Directive (RoHS) (and the related Waste Electrical and Electronic Equipment Directive (WEEE)), under which PBDEs are currently banned; similarly, HBCD is proposed to be banned under a pending RoHS revision.  Walmart and Washington State have recently banned products with PBDE. A global ban on HBCD is currently being considered under the framework of the Stockholm Convention on Persistent Organic Pollutants (an international environmental treaty, to which the US is a signatory).

[2] Comments by industrial participants at a recent workshop NIST Workshop on Gas-Phase Fire Retardants.

[3] Levchik, S.I. Comments at the NIST Workshop on Gas-Phase Fire Retardants, Feb. 24, 2012.

[4] Linteris, G.T., “Gas-Phase Mechanisms of Fire Retardants,” National Institute of Standards and Technology, NISTIR 6889, Gaithersburg MD, June, 2002, 32 p.

[5] Kirk-Othmer Encyclopedia of Chemical Technology,5th Ed. Vol. 10, John Wiley & Sons, Hoboken, NJ, p930-999, 2005.

[6] Takahashi, F., Linteris, G.T., and Katta, V.R., “Vortex-coupled oscillations of edge diffusion flames in coflowing air with dilution,”  Proceedings of the Combustion Institute 31 (2007) 1575–1582.

[7] This topic was of great interest to the industrial participants of the NIST Workshop on Gas-Phase Fire Retardants, Feb. 24, 2012.

 

Recent Results:

Outputs:

• Held a workshop NIST Workshop on Gas-Phase Fire Retardants with representatives from major FR companies, polymer manufactures, government, and universities.
• Preparing a chapter for the SFPE Handbook: Chemical Kinetics in Fire Applications (on schedule).

Standards and Codes: The relevant standards for polymer flammability appear in ASTM E5 (Fire Standards) and the NFPA Committee on Fire Tests.  The current EL representatives on these committees are Matt Bundy and Dick Gann. By determining the mode of action of widely used fire retardants which act in the gas phase, this project with provide a scientific basis for codes and standards related to material flammability.  Through the NIST representatives on the committees, the results of the project will lead to modifications to the codes and standards for polymer flammability to insure that they accurately represent the behavior of the materials in full-scale fires.  In addition, through presentations to and collaborations with industry researchers, the results of the project will influence the additives selected by industry to insure maximum effectiveness.