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Co-pyrolysis of brominated flame retardants with polymeric materials (polyethylene)

Ahmed, O.H., Altarawneh, M.ORCID: 0000-0002-2832-3886, Jiang, Z-T and Dlugogorski, B.Z. (2019) Co-pyrolysis of brominated flame retardants with polymeric materials (polyethylene). In: 10th International Conference on Materials for Advanced Technologies (ICMAT) 2019, 23 - 28 June 2019, Marina Bay Sands, Singapore.

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Thermal recycling is currently deployed as a main stream strategy in the safe disposal of materials laden with bromine, most notably the polymeric fraction in electronic and electrical waste (e-waste). Thus, it is essential to comprehend the combustion chemistry underpinning the reaction of brominated constituents with common polymeric entities. In this contribution, we systematically report reaction and activation energies for interaction of bromine atom, bromophenol and benzene with polyethylene (PE) as a model compound for polymeric materials in e-waste. Via periodic density functional theory (DFT) calculations, we map out potential energy surfaces for a large array of plausible reactions. HBr signifies the far most abundant bromine-bearing species from thermal decomposition of brominated flame retardants (BFRs) that exist in appreciable loads in e-waste. Herein, we illustrate that HBr could readily form via H abstraction from PE by a gas phase Br atom through a most activation barrier of 16 kcal/mol. Creation of an apparent radical site in the PE skeleton via this reaction frees up an active radical. Reaction of a 2-bromophenol molecule with the radical site in PE ensues in two competing channels, abstraction of the hydroxyl’s H or the aromatic Br. The latter reaction requires a lower activation barrier by about 5 kcal/mol. From another angle, we illustrate a plausible bromination mechanism that is initiated by the adsorption of a phenoxy radical over a brominated PE. Conversion of a phenoxy radical into a 4-bromophenol molecule demands a sizable activation barrier of 56 kcal/mol. Transfer of bromine atom bounded to PE to the phenoxy radical signifies a bottleneck for this bromination route. Alternatively, the radical site in a gas phase phenyl radical could abstract a Br-PE via a trivial barrier of 16 kcal/mol forming abromobenzene molecule. Formation of phenyl radicals could be brought by migration of an aromatic H from a benzene molecule to a radical site in the PE. Modelled reactions herein are useful to understand the chemistry of bromine transformation during combustion of brominated polymers in general.

Item Type: Conference Item
Murdoch Affiliation(s): School of Engineering and Information Technology
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