Research Polymeric Brominated Flame Retardants: Are They a Relevant Source of Emerging Brominated Aromatic Compounds in the Environment? BRUNO GOUTEUX,† MEHRAN ALAEE,† SCOTT A. MABURY,‡ GRAZINA PACEPAVICIUS,† AND D E R E K C . G . M U I R * ,† Aquatic Ecosystem Protection Research Division, Environment Canada, Burlington, ON, Canada, L7R 4A6 and Department of Chemistry, University of Toronto, Toronto, ON, Canada, M5S 3H6
Received August 8, 2008. Revised manuscript received October 15, 2008. Accepted October 16, 2008.
A purge and trap method was used to study the release of brominated organic compounds from polymeric brominated flame retardants (BFRs), a relatively unknown class of flame retardant materials. Among the volatile brominated organics released, pentabromotoluene (PBTo), pentabromoethylbenzene (PBEB), and hexabromobenzene (HBB) were of particular interestbecauseoftheirhighpotentialtopersistintheenvironment. The impact of a thermal stress on the release of these compounds was assessed by applying different constant temperatures for one hour to a polymeric BFR sample. Release rates ranged between 22 ( 2.1 ng g-1 h-1 for PBEB to 2480 ( 500 ng g-1 h-1 for PBTo at room temperature. These rates of release reached 65 ( 11 ng g-1 h-1 for PBEB and 42 400 ( 4700 ng g-1 h-1 for PBTo at 100 °C. This suggests that the compounding of thermoplastic polyesters done at high temperatures, up to 290 °C, could lead to the release of significant amounts of volatile brominated compounds in the environment when crude polymeric BFRs are used as flame retardants. To assess if this unsuspected source of volatile brominated compounds to the environment was relevant to support air concentrations in the Great Lakes area, air samples collected at Egbert (ON, Canada) were analyzed and PBTo, PBEB, and HBB were detected at low levels in some air samples (500 °C, which results in the breakdown of polymeric BFRs. The main goal of this study was to assess if polymeric BFRs could effectively be a source of emerging brominated organic compounds to the environment. To address this issue, the identification of brominated organic compounds emitted from different commercially available polymeric BFRs was performed using a purge and trap method coupled with gas chromatography-mass spectrometry analysis. The effect of temperature on the release of low molecular weight brominated compounds from these materials was also assessed at temperatures ranging from room temperature to 100 °C, which is relevant to the typical use of polymeric BFRs. Finally, a Level III fugacity model approach was used to evaluate the relative importance of polymeric BFRs as a source of volatile brominated compounds in regard to prevailing air concentrations of these compounds in the area of the Laurentian Great Lakes of North America.
Materials and Methods Thermal Experiments. Samples. Three BFR oligomer samples were investigated (Supporting Information, Figure S1). Firemaster PBS-64 HW (CAS 148993-99-1), produced by copolymerization of di- and tribromostyrene, was obtained from Great Lakes Chemical Corporation (GLCC), now known as Chemtura Corporation. This BFR oligomer, sold as pellets, has an average molecular weight of about 60 000 g/mol and a bromine content between 64 and 65% (8, 17). PBS-64 is VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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mainly used to flame retard engineering thermoplastics such as high temperature polyamides and polybutylene terephthalate (PBT) (6). Great Lakes BC-58 (CAS 71342-77-3) was also obtained from GLCC. It is a tribromophenoxy-terminated carbonate oligomer of tetrabromobisphenol A (TBBPA), which is sold in the form of a powder. BC-58 has a bromine content of about 58% (17). BC-58 is also used to flame retard thermoplastic polyesters, in particular PBT (7). Finally, FR1025 (CAS 59447-57-3), a pentabromobenzyl acrylate oligomer, was obtained from the Dead Sea Bromine Group, now part of the ICL Industrial Products group. FR-1025 has a minimum bromine content of 69% (18). FR-1025 is particularly recommended for use with FR polyamides (nylons) and thermoplastic polyesters (PBT or polyethylene terephthalate). Melting temperatures of PBS-64, BC-58, and FR-1025 ranged from 137-156 °C, 230-260 °C, and 190-220 °C, respectively (17). Experimental Setup. Experiments were done using a simple experimental apparatus adapted from Wolf et al. (19) and composed of a 0.5 L glass flask in which a flow of nitrogen (0.3 L/min) was maintained during the experiment (Figure S2). Between 1 and 8 g of each BFR sample was placed on a pre-extracted aluminum foil on the bottom of the flask, which was heated using a heating mantle. Temperature was recorded (( 1 °C) at the bottom of the flask. In a first set of experiments, BFR samples were heated from room temperature to 100.0 °C in 20 min as illustrated in Figure S3, and then quickly removed from the apparatus. A low maximum temperature relative to the melting temperatures of polymeric BFRs was chosen to avoid their eventual degradation. Another set of experiments was made using only the FR-1025 BFR sample, which was heated during one hour at different constant temperatures, i.e., room temperature, 50, 75, and 100 °C. Once again, the temperatures were chosen to avoid the degradation of FR-1025. Volatile brominated organic compounds were sampled both on a precleaned polyurethane foam (PUF) cartridge (6.4 cm diameter × 7.6 cm length) and on glass walls of the apparatus. Procedural blank values were uniformly low indicating that procedural contamination was negligible. Chemical Analysis. PUF plugs were Soxhlet-extracted using a mixture of dichloromethane (DCM) and hexane (50/50 v/v) for about 20 h. In parallel, all glass parts of the sampling system were rinsed with DCM. Soxhlet and rinse extracts were then combined, transferred to iso-octane, and concentrated to a similar final volume (100 µL) prior to gas chromatography-mass spectrometry analysis. Brominated compounds were analyzed using a HewlettPackard model 5890 Series II gas chromatograph (GC) equipped with an HP5-MS capillary column (30 m × 0.25 mm id × 0.25 mm film thickness) coupled to a HewlettPackard model 5989AB mass spectrometer (MS) operated in electron capture negative ion (ECNI) mode. Detailed information about the GC and MS operating conditions is provided in the Supporting Information section. Environmental Measurements. Samples. Air sample extracts were kindly donated by L. Jantunen and T.F. Bidleman (Center for Atmospheric Research Experiments (CARE), Environment Canada, Egbert, Ontario, Canada). Atmospheric sampling was conducted on January 13 and 19 and March 22 and 24, 2005 at the Integrated Atmospheric Deposition Network (IADN) satellite station of Egbert, a rural/ suburban site located about 70 km north of Toronto, Ontario, Canada. Air was drawn through a mass flow high-volume sampler containing a precleaned PUF cartridge (6.4 cm diameter × 7.6 cm length) for a total air volume sampled of about 300-350 m3 per PUF cartridge. Two PUF cartridges were combined to get four air samples of 703, 704, 658, and 653 m3. 9040
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Chemical Analysis. The treatment of air samples prior to analysis was done at CARE and a description is provided in the Supporting Information. Brominated compounds of interest were determined using an HP 6890 GC coupled with a Micromass Autospec Ultima high-resolution MS operated in electron ionization (EI) mode at a resolving power of 10,000. Chromatographic separations were accomplished using a 30 m × 0.10 mm i.d. DB5-HT column. Detailed information about the GC and MS operating conditions is provided in the Supporting Information. Quantification of PBDE-47, pentabromotoluene (PBTo), pentabromoethylbenzene (PBEB), and hexabromobenzene (HBB) was done using external calibration. Quality Assurance/Quality Control. QA/QC included the analysis of procedural blanks which revealed that blank values for brominated compounds of interest were uniformly low, indicating that procedural contamination was lower than the sample detection limits, which varied between 0.005 and 0.01 pg/m3. The recovery efficiencies, determined by CARE, based on R-d6-HCH and Mirex air sample spikes were 90 ( 4%. Modeling. A Level III fugacity approach was used to numerically evaluate the likely environmental behavior of chemicals of interest (20). The model (Version 2.80), developed by the Canadian Environmental Modeling Centre (Trent University, Peterborough, ON, Canada), was adjusted to the area of North America and coastal waters (Table S1). Physicochemical properties of PBTo, PBEB, and HBB used in the model are provided in Supporting Information Table S1. Molecular weights, log Kow, and reaction half-lives for each environmental compartment used in the model were found in the literature or, for the most part, estimated using the quantitative structure property estimation Program EPI Suite (21) or PBT profiler (22) (Table S1). Finally, default values of the model for intercompartment exchanges were used (23). The model and its description can be found on the Web site of the Canadian Environmental Modeling Centre (23).
Results and Discussion Identification of Volatile Brominated Compounds Released by Polymeric BFRs. The ECNI GC/MS chromatograms obtained for the analysis of extracts collected after the three polymeric BFRs were heated from room temperature up to 100 °C revealed several important unknown brominecontaining GC peaks (Figure 1). The presence of bromine was revealed by the equal responses of bromide ions at m/z 79 and 81 in the selected ion monitoring results for all peaks, except one at a retention time of 11.03 min for BC-58 (Figure 1B). To identify these unknown brominated compounds, fullscan mass spectra, obtained in EI mode, were carefully analyzed taking into account information given by the available chemical structures of the three polymeric BFRs (Figure S1). For PBS-64, compounds (1), (2), (3) and compounds (4), (5), (6) (Figure 1A) were thought to be diand tribromostyrene monomers (Table S2), respectively. This was confirmed by the comparison of mass spectra obtained for peaks (1)-(3) (Figure S4) and peaks (4)-(6) (Figure S5) to the mass spectra of di- and tribromostyrene published by Blazso´ et al. (24). The mass spectrum obtained for compound (7) (Figure 1A), indicated, in addition to the successive losses of Br from M+, the loss of a CH2Br group from M+ and the successive losses of Br from the resulting [M - CH2Br]+ fragment (Figure S6). Furthermore, the molecular ion cluster starting at m/z 418 indicated the presence of four Br atoms in the molecule (Figure S6). On the basis of this information, this compound was believed to be an ethenyl benzene with three bromine atoms on the ring and one bromine atom on the ethenyl group (Table S2). However, no reference spectrum for this compound was found either in the NIST MS reference
FIGURE 1. ECNI GC/MS chromatograms of extracts obtained after a thermal stress (i.e., from room temperature up to about 100 °C) was applied to samples of (A) PBS-64, (B) BC-58, and (C) FR-1025 BFRs. An asterisk (*) indicates that a GC peak does not contain bromide ions (see text and Table S1 for the identification of numbered peaks). library or in the literature and there is no authentic material available to confirm this tentative identification. For the carbonate oligomer of tetrabromobisphenol A (BC58), some of the major brominated compounds released were identified. Compounds (8) and (9) (Figure 1B) were identified as di- and tribromophenol compounds (Table S2) by comparing their EI full scan mass spectra and GC retention times with those obtained for authentic materials (Figures S7 and S8). The fragmentation pattern of compound (10) (Figure 1B) led us to hypothesize that this compound was an ethyl tribromophenyl ether isomer, or tribromophenetole isomer (Table S2). The mass spectrum we obtained matched the only mass spectrum reported, to our knowledge, for this compound (25) (Figure S9). The full mass spectrum of compound (11) (Figure 1B) revealed a molecular ion cluster starting at m/z 386 and indicating the presence of three bromine atoms (Figure S10). Two ion clusters corresponding to the losses of OCH3-CO and CO2 from M+ were also present at m/z 327 and 342, respectively. These two ion clusters were more intense than the molecular ion cluster (Figure S10). Interestingly, this fragmentation pattern is similar to the pattern observed for the mass spectrum of the bromophenyl methyl carbonate found in the NIST MS library. On the basis of this information and on the chemical structure of BC-58, we believe that compound (11) is a tribromophenyl methyl carbonate isomer (Table S2). However, this compound was only tentatively identified. Compound (12) (Figure 1B) was identified as 2,3,4,5,6-pentabromotoluene (PBTo) (Table S2). This was confirmed by comparing the mass spectrum and the GC retention time of its peak to those of authentic PBTo (Figure S11). Finally, the last eluting identified compound (compound (13) in Figure 1B) was identified as 2,3,4,5,6-
pentabromoethylbenzene (PBEB) (Table S2) by comparing its EI full mass spectrum with the reference EI full mass spectrum reported in Hoh et al. (26) (Figure S12). In contrast to PBS-64, the monomer unit of BC-58, tetrabromobisphenol A, was not detected as shown in the ECNI GC/MS chromatogram where it should have been present at a retention time of 17.28 min (Figure 1B). However, tetrabromobisphenol A is a relatively polar compound for which a derivatization step would be helpful for GC detection. As this was not done, it is possible that this compound and similar ones were missed. The majority of brominated compounds released by FR1025 under thermal stress were the same as those released by BC-58. Hence, dibromophenol (compound (8)), tribromophenol (compound (9)), tribromophenetole (compound (10)), and tribromophenyl methyl carbonate (compound (11)) isomers were also identified (Figure 1C). More importantly, PBEB (compound (13)) and PBTo (compound (12)) were also detected and in high relative proportions (Figure 1C). EI full mass spectrum obtained for compound (14) (Figure 1C) was characterized by an intense molecular ion cluster starting at m/z 468, whose isotopic patterns revealed the presence of five bromine atoms in the molecule (Figure S13). The successive losses of Br from M+ leading to ion clusters of relatively low intensities was the main characteristic of the fragmentation pattern. On the basis of this information and the chemical structure of FR-1025 (Figure S1), compound (14) was tentatively identified as 1,2,3,4,5-pentabromobenzene (PBB) (Table S2). Compound (15) (Figure 1C) was identified as hexabromobenzene (HBB) (Table S2) by comparing its mass spectrum with the reference mass spectrum reported by Huber in 2002 (27) (Figure S14). Finally, compound (16) (Figure 1C) was identified as pentabromobenzyl acrylate (Table S2), the monomer unit of FR-1025, by comparing the mass spectrum and the GC retention time of its peak to those of authentic pentabromobenzyl acrylate (Figure S15). Interestingly, several of these brominated compounds were already reported as being decomposition products derived from the pyrolysis of materials similar to the three BFRs studied. For instance, di- and tribromostyrene isomers released by PBS-64 were identified as pyrolysates of poly(dibromostyrene) (PBDS-80 from GLCC) and brominated polystyrene (Pyrocheck PB68 from FERRO Corp.) (12). These compounds were also identified as pyrolysis products of an electronic junction made of phthalic polyester filled with ceramic fibers and flame retarded with brominated polystyrene (24). Di- and tribromophenol isomers were repeatedly identified during the pyrolysis of TBBPA (13, 14) or TBBPA derivatives such as diglycidyl ethers of bisphenol A (12, 14) and brominated polycarbonate (28). Di- and tribromophenols were also detected as pyrolysates of hardware components such as printed circuit boards or insertion sockets on extension circuit boards (16). Finally, the pyrolysis of a polymer flame retarded with pentabromobenzyl polyacrylate led to the formation of PBB and PBTo (12). The pyrolysis technique is characterized by the use of large amounts of thermal energy with temperatures applied usually superior to 500 °C. This results in the breakdown of polymeric chains to decomposition products such as monomers, oligomers, recombination products, and other fragments. In this study, the thermal stress imposed to polymeric BFRs was relatively low since temperatures used were between room temperature and 100 °C, i.e., below melting temperatures at which PBS64, BC-58, and FR-1025 underwent physical changes (cf. Material and Methods). This suggests that the brominated compounds identified in this study were more likely “free” monomer units resulting from the incomplete synthesis of polymeric BFR studied, or byproduct formed during their synthesis. They may also be impurities present in the starting VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Concentrations (in pg/m3) of PBTo, PBEB, HBB, and PBDE-47, the Preponderant PBDE Detected, in Air Samples from Egbert
FIGURE 2. Release rates ((standard deviation) of PBTo, PBEB, and HBB from FR-1025 in function of temperature. materials, rather than degradation products resulting from the decomposition of the starting material, due to the thermal energy used during the experiments. To confirm this hypothesis, polymeric BFRs were extracted with toluene and extracts were analyzed for brominated compounds. All brominated compounds released from polymeric BFRs were identified in these extracts suggesting that they were not formed during the heating of polymeric BFRs. Impact of the Thermal Stress on the Release of Volatile Brominated Compounds by Polymeric BFRs. The intensity of GC peaks obtained for the analysis of extracts collected during the thermal stress experiment on PBS-64 was shown to be dependent on the temperature applied to the experimental system. Hence, the thermal stress, from room temperature up to 100 °C, applied to PBS-64 for 20 min resulted in a drastic increase, as much as 7000 fold, of the GC-peaks associated with the brominated compounds released, compared to the same experiment but at a constant room temperature (Figure S16). Among volatile brominated organics released by the polymeric BFRs studied, PBTo, PBEB, and HBB were of particular interest because of their high potential to persist in the environment as shown by their physicochemical properties (Table S1). To assess the effect of temperature on the release of these particular volatile brominated compounds of interest from polymeric BFRs, experiments were done by applying different constant temperatures for one hour to the FR-1025 BFR sample. Results showed that, at room temperature, PBTo, PBEB, and HBB were released from FR-1025 at rates ranging from 22 ( 2.1 ng g-1 h-1 for PBEB to 2480 ( 500 ng g-1h-1 for PBTo (Figure 2). This indicates that polymeric BFRs could be considered as a source of volatile bromine compounds even when they are not submitted to a thermal stress. The release rates determined for PBTo and PBEB increased gradually with the thermal stress applied to FR-1025, reaching 42 400 ( 4700 ng g-1 h-1 for PBTo and 65 9042
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sample
sampling date
PBTo
PBEB
HBB
PBDE-47
A B C D
13/01/2005 19/01/2005 22/03/2005 24/03/2005
