Mechanism of Thermal Decomposition of Tetrabromobisphenol A (TBBA)

Abstract

This study presents a detailed investigation into the gas phase thermal decomposition of tetrabromobisphenol A (TBBA); i.e., the most widely used brominated flame retardant (BFR). Elimination of one of the methyl groups characterises the sole dominant channel in the self-decomposition of the TBBA molecule at all temperatures. A high-pressure rate constant for this reaction is fitted to IntroductionOwing to their significant potentials to reduce flammability tendency of polymeric materials, brominated flame retardants (BFRs) have been widely deployed during the last few decades in numerous consumers products. 1 Tetrabromobisphenol A (TBBA) currently constitutes nearly 60 % of the world's total production and usage of BFRs. TBBA is also deployed as a reactant for the formation of other commercial BFRs. Bromine radicals formed during the degradation of TBBA delay ignition of overheated electronic and electrical devices. 2 As thermal treatment represents a mainstream strategy for disposal of materials laden with TBBA, the pyrolytic decomposition of TBBA has attracted mounting scientific attention. 3Several experimental studies have been carried out to address the decomposition of either pure TBBA or TBBA immersed in polymeric matrices, under pyrolytic or oxidative conditions; typically in the range of 300 o C to 600 o C . [4][5][6][7][8][9][10][11] Major products from the pyrolysis of TBBA include HBr, brominated phenols and benzenes, in addition to char and a wide range of brominated aromatics. Almost half of the initial bromine in the parent TBBA was found to transform into HBr. 9 The main environmental burden of using TBBA stems from the fact that, its decomposition products, such as brominated phenols, serve as building blocks for the formation of polybrominated dibenzo-p-dioxins and polybrominated dibenzofurans (PBDD/Fs). Recent experimental studies on the pyrolysis of polymers containing TBBA have reported the formation of several congeners of PBDD/Fs. While the decomposition of TBBA mainly occurs in the condensed phase, gas phase reactions contribute significantly to the overall decomposition of TBBA. Guided by their results on the volatility of TBBA, Marsanich et al. 16 explained that, exposing TBBA to a temperature higher than its melting point (~450 K) results readily in the evaporation of TBBA. Barontini et al. 15 showed that, the evaporation of TBBA contributed significantly to the total weight loss of TBBA, at temperatures as low as 480 K. Available kinetic models were primarily formulated to describe decomposition in the condensed phase. Thus, they are not extendable to account for the decomposition chemistry of TBBA in the gas phase. A kinetic gas-phase model of the pyrolysis of TBBA differs fundamentally from a corresponding condensed-phase model. For example, bimolecular isomerisation reactions (prevailing reactions in condensed phase) are of negligible importance when compared with the unimolecularly-derived pathways in the gas phase. Furthermore, understanding the gas 6 phase decomposit...

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