Screening approaches for gas-phase activity of flame retardants

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Abstract

Molecular beam mass spectrometry (MBMS) and optical diagnostic techniques, two common combustion science diagnostic tools for studying the impact of material on flames, are evaluated as tools for estimating the gas-phase potential of polymer-flame retardant additives. The gas-phase activity of hexabromocyclododecane (HBCD), a widely used commercial flame retardant, was studied and compared via the two combustion diagnostic techniques. MBMS data for HBCD were reviewed and provided identification of gas-phase active species as well as quantitative information on the degree of effectiveness based upon reduction of OH in a premixed CH4/air/N2 flame. In contrast, optical chemiluminescence detection of OH and CH provided a simpler technique for monitoring the gas-phase potential of flame retardants. Studies of CH and OH levels after addition of pyrolyzed products from polystyrene/HBCD blends into a diffusion flame system are compared with MBMS experiments of flames doped with pure HBCD. Comparison of chemiluminescence data with similar data from a small-scale heat release test, the pyrolysis combustion flow calorimeter, indicated that CH and OH activity relate to the heat release rate for flaming combustion.

Introduction

Many plastic materials are inherently flammable in the pure form. In order to use these plastic materials in certain commercial areas (buildings, vehicles, electronic appliances, etc…), these materials must meet fire safety requirements and must pass certain regulatory fire performance tests [1]. Flame retardant (FR) additives are typically blended with polymers to provide improved fire resistance for many commercial applications. The mechanistic action of FRs can be classified in the general categories of either gas or condensed-phase activity. The principle gas-phase mechanism utilized by many FRs involves chemical inhibition via radical scavenging of key combustion radicals (OH, H, O) to effectively shut down the combustion process. Condensed-phase activity involves action in the solid or melt phase of the polymer to impact/reduce the burning process. An example of condensed activity is the formation of a char layer that can insulate or isolate the remaining bulk material from the combustion process. Bromine based FRs are the most widely used FRs in commercial applications for a variety of polymer systems and provide a high level of gas-phase activity through radical scavenging by HBr, formed from the degradation of the parent FR [2]. Environmental scrutiny for many halogen FR systems has led to efforts to identify alternative halogen-free FR systems suitable for commercial polymer applications.

Previous work on mechanistic studies of flame retardant activity has focused on the use of traditional analytical pyrolysis and evolved-gas techniques [3], [4], [5], [6]. These methods are limited to the detection of relatively stable degradation species and are unsuitable for detection of unstable radicals and other species. These species are key to the complete understanding of the active flame retardant mechanism. Combustion science diagnostic tools such as molecular beam mass spectrometry (MBMS) and optical diagnostic techniques have been utilized for fundamental studies in combustion science chemistry for many years. These techniques provide the analysis capability to study chemical inhibition and other combustion reactions directly in stable flame systems. In particular, these techniques allow analysis of unstable radical species that are a key part of combustion chemistry.

A few previous studies showed the utility of combustion diagnostic techniques for studying the combustion chemistry of flame retardant additives in flames. Studies by Cullum [7], [8] and Siow [9] dealt with the use of laser-induced fluorescence (LIF) to study FR effectiveness by measuring the OH radical response after addition of FR or an FR/polymer blend to a flame system. MBMS was used to look at the combustion chemistry associated with bromine and phosphorous flame retardants [10], [11].

In the present work, we review two techniques for assessing the potential gas-phase activity of flame retardants applied to polymer systems: MBMS and optical detection via chemiluminescence. The two techniques are reviewed as potential methods to screen FR candidates for application in polymer systems. The gas-phase activity of hexabromocyclododecane (C12H18Br6), HBCD, a widely used commercial flame retardant, was studied and compared via the two combustion diagnostic techniques.

Previous work with MBMS showed the potential utility of this technique to assess the gas-phase combustion chemistry of flame retardants which included quantitative measurements of FR gas-phase effectiveness based on OH radical depletion measurements [11]. In contrast to MBMS, optical detection by chemiluminescence is presented as an alternative approach that is less complex and can be based on relatively simple instrumentation.

To implement a system based on optical detection of radicals in a flame, a system was implemented based on a pyrolysis heating device that allowed controlled heating of the sample. The off-gases of the pyrolysis were then transferred via a heated transfer-line into a stable flame system with associated optical detection. The optical measurement system monitored the time-dependent response of a flame after the addition of an FR or an FR blend of polystyrene (via the pyrolysis). The prototype system utilized a thermogravimetric analysis for the pyrolysis step. A similar pyrolysis and sample transfer configuration was utilized in the original pyrolysis combustion flow calorimetry (PCFC) instrument developed by Walters and Lyon [12] which has since been improved with a standalone system with a pyrolysis furnace [13]. The PCFC is a small-scale polymer flammability test that measures the heat release rate via forced combustion of polymers after pyrolysis.

For optical detection in the flame, the use of a laser-induced fluorescence (LIF) measurement setup to measure OH radicals was found in preliminary experiments to be not feasible for the transient experiments due to the limited repetition rate of typical LIF systems (10–20 Hz) that did not allow averaging of a significant number of signals to achieve a signal/noise ratio good enough to detect small changes (∼10%) in OH concentration. However, recently high repetition rate LIF systems were developed for gas-phase measurements of biacetyl [14] and progress is expected in this field in the future. At this time, chemiluminescence detection was considered to perform time-resolved measurements. The detection of chemiluminescence signals is simple and can be accomplished with high temporal and spectral resolution. It has to be emphasized, however, that the signals that originate from chemically excited species, like OH and CH are not related to the concentration of ground state OH and CH but both signals have been linked with heat release and hence the strength of a flame [15], [16]. Therefore, the chemiluminescence signals from OH and CH are an attractive target to investigate the impact of FR on flames. The intention is to evaluate whether or not there is a correlation between the amount of FR that is added to a non-premixed flame and the recorded chemiluminescence signal. Such a relation could then also be explored to rank FR compounds in their effectiveness to suppress combustion via gas-phase FR activity. In addition, preliminary data for CH and OH flame measurements of pyrolyzed samples of HBCD/polystyrene blends are compared with similar measurements with a PCFC instrument. This comparison shows that the CH and OH measurements do track the relative heat release rate for HBCD/polystyrene blends from the PCFC heat release rate testing. The data also allow comparison between small-scale combustion testing based on flaming versus forced combustion (PCFC).

Both MBMS and optical chemiluminescence techniques are reviewed as potential screening tools to quickly assess the potential of a FR candidate for gas-phase FR activity. MBMS analysis provided a very complete qualitative and semi-quantitative analysis of the gas-phase combustion chemistry after addition of pure HBCD to a flame system. Although the MBMS technique provided a near complete analysis of the gas-phase flame chemistry associated with the FR HBCD, the technique is quite complex and not suitable for routine and rapid screening of FRs in a typical research and development laboratory. In contrast to MBMS, chemiluminescence detection provided limited information but appeared to be better suited as potential screening tool for gas-phase activity of FRs as it is simpler and requires instrumentation that is less complex.

Section snippets

Molecular beam mass spectrometry

MBMS studies and instrumentation have been described previously [11]. For flame testing of fire retardants, a premixed CH4/O2/N2 (9.18/15.54/75.28, ϕ = 1.18) flame stabilized on Mache–Hebra nozzle burner [17] was utilized at atmospheric pressure and at initial temperature of the combustible mixture of 368 K. The Mach–Hebra burner used a quartz tube, tapered at one end that results in a uniform distribution of flow velocity over the cross section of the burner outlet. The flame forms a regular

Review of molecular beam mass spectrometry measurements

MBMS measurements for HBCD have been described previously [11]. Figure 3 gives temperature profiles in the flame without additives and doped with 180 ± 10 ppm of HBCD. The post-flame temperature of the undoped flame was 1966 K, those of HBCD-doped were 1856 K to give an overall change of 110 K and an increase in the width of the combustion zone by a factor of ∼1.6–1.7. The calculated temperature profile for the undoped flame in Fig. 3 is in good agreement with the measured profile. Calculations show

Conclusions

Investigations on how to assess the activity level of flame retardants were performed in premixed and non-premixed flames. HBCD and its mixtures with polystyrene were used as an example substance class. The impact of the FR addition on OH radical concentrations and temperature profiles could be measured in a slightly rich premixed methane/air flame using a molecular beam mass spectrometry. Information obtained this way is very important for validation and development of detailed chemical

Acknowledgments

The assistance of Robert Opperman (Dow Chemical) with operation of the PCFC instrument is acknowledged as are technical discussions on the PCFC with Dr. Richard Lyon (Federal Aviation Administration Technical Center) and Dr. Stanislav Stoliarov (SRA International). Dr. M. Anne Leugers is acknowledged for technical assistance with optical spectroscopy.

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