Advanced Gas-Phase Fire Retardants Project

Summary

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 that represent the relevant full-scale behavior.

Description

Objective: By 2016¹, to provide data on and an understanding of the role of particle formation in limiting the action of gas-phase active phosphorous-based fire retardants added to polymers.

What is the new technical idea? To effectively develop new fire retardants, industry needs an understanding of how the existing and most promising new gas phase FRs work, and how the properties of the fire (or standard test method) influence the gas-phase chemical inhibition mechanism of the FR. The goal of the present project is to provide that understanding for phosphorous fire retardants, so that efforts by industry can be more scientifically-based.

Polymer companies and fire retardant manufactures are motivated to develop halogen-free fire retardants², and hence, new approaches are being sought by industry³. Most recent research on improving the fire performance of polymers involves retardants acting on the condensed phase material itself. While new, nano-structured materials, that 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 established², that industry seeks new FR additives that can be easily blended with the range of existing high-volume polymers. The polymers are typically fire retarded in such consumer products as rigid insulation board, electronic enclosures, foams in soft furnishings, and others. The FR additives, principally the bromine compounds and antimony oxide (almost always added with bromine, and also under regulatory pressure) both are believed to work in the gas phase by interfering with the combustion chemistry.

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^4,5. 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)⁶.  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 influence the effectiveness of the chemical additive, and are themselves changed by the additive, which further changes the additive effectiveness⁷. 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 active phosphorus FR additive can change its effectiveness by more than an order of magnitude, but the industry researchers do not yet have a credible hypothesis as to why.

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 more effectively guide them to a solution. Consequently, there has been recent work at universities (University of Duisburg-Essen, Germany; University of Ulster; Northern Ireland; University of Central Lancashire, UK; Univ. of Michigan, USA; University of Maryland, USA) and at government laboratories (Institute of Chemical Kinetics and Combustion, Russia; FAA Technical Center, USA; Federal Institute for Materials Research and Testing, Germany) 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 (e.g., solid or liquid fuels) to render the FRs more or less effective (this is a focus of FY14). 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? Task 1: Determine the mechanism of gas-phase flame inhibition by phosphorus FR compounds which act in the gas phase, and how the effectiveness is changed in different flame types. There are two classes of compounds currently of promise and interest to industry⁸: 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.

Task 2: Determine if particle formation from phosphorus oxides limits the effectiveness of phosphorus FRs acting in the gas phase. It is possible that phosphorus loses its effectiveness in some flame types due to formation of condensed-phase particles, and this global property of the flame will be measured. The phosphorus compound to be used is dimethylmethylphosphonate (DMMP) (which is both an actual FR used in polymers, and a convenient way to add a representative phosphorus compound to either the fuel or air stream).

If particle formation limits the efficiency of phosphorus compounds, we can then start to formulate guidelines as to when this effect will be most important, and how this needs to be taken into account for burning materials in real fires, and in standard tests.

If particle formation does not limit the effectiveness of P, then the dilemma still remains:  why does the effectiveness of P vary so much?  We think it's due to particle formation.  Nonetheless, if it is not the reason, we can use our knowledge of gas-phase flame inhibition and flame structure to try and answer that question. Recently, the capability for detailed numerical modeling (time-dependent, 2-D, with full chemical kinetics) has become possible for diffusion flames (cup burner) similar in configuration to some standard tests (e.g., UL-94). We will perform these numerical simulations for a cup-burner flame (one situation in which P compounds were ineffective) and use the results, together with particle measurements, to try to unravel why P does not work in these flames (based on results in other flames, it should work in a cup burner flame, but it does not appear to be working).

This simulation provides a the time-dependent flame structure (the temperature, chemical species concentrations, and flow velocities) for a 2-D configuration. With post-processing, these solutions will provide very detailed data—the first ever—on the action of fire retardants acting in the gas phase of diffusion flames similar to fires. The computed properties of these flames can then be compared with the experimental measurements for such properties as: flame size, flicker rate, flame lift-off, flame extinction condition, and flame heat release. While the code does not currently model the condensation process or particle coagulation, the simulations will nonetheless answer the very important question: would the gas-phase active FR work if particle formation did not occur. This is a necessary first step in interpreting the action of any gas-phase active fire retardant; a comparison between the predicted gas-phase chemistry and the actual behavior of the inhibitor has been shown to provide great insight into the action of gas-phase chemical inhibitors.

In FY2013, we have constructed the diagnostic experiment for non-flickering flames with pure gaseous, non-sooting fuels, and will 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. The fuels to be used are methanol, heptane, and trioxane. Flames over these fuels will demonstrate the effects of heat feedback to the fuel surface, delineating the interaction between flame inhibition and flame liftoff, which will affect FR efficiency. 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 flame systems 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 test⁹; e.g. UL-94 and Steiner Tunnel tests.). While the detailed results from this project are specific to phosphorus, the principles determined will most certainly apply to other gas-phase active FRs as well.

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[1] The project is in the 2^nd year of phase 1. The long term vision is to enable the development of new gas phase fire retardants, for polymers. These new additives will be inexpensive, effective, and safe for the environment, humans, and animals.

• Phase 1: Understand mechanism of phosphorus FR systems.
• Phase 2: Understand the mechanism of and antimony/bromine FR systems. Determine how the structures of real fires and standard test methods influence performance of fire retardants active in the gas phase, so that appropriate test methods can be employed, and the actual real-scale performance of FRs can be predicted.
• Phase 3:  Based on Phase 1 and Phase 2, develop a screening method which accurately predicts the standard test performance, and the real-scale performance, so that industry can rapidly test new formulations with only small quantities of the new material.

[2] 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).

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

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

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

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

[7] 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.

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

[9] We will look at the properties (time-dependent flow-field, temperature, and chemical species concentrations) of reduced-scale flames which enhance particle formation, and try to estimate when those properties will exist in either standard tests or full-scale fires, so as to enhance particle formation there, and cause discrepancies between the desired and actual performance of a FR.

Major Accomplishments

Outcomes:

• New experimental database on the behavior of phosphorus in diffusion flames, and the role of particle formation in the effectiveness.
• Provide industry with information (data and its interpretation) on the role of particle formation on the effectiveness of phosphorus as a gas-phase FR so they can tailor their approaches accordingly.