Thermal behavior of brominated and polybrominated compounds I: closed vessel conditions

doi:10.1016/S0165-2370(01)00144-9

Journal of Analytical and Applied Pyrolysis

Volume 63, Issue 1, March 2002, Pages 105-128

https://doi.org/10.1016/S0165-2370(01)00144-9 Get rights and content

Abstract

Israel is one of the leading manufacturers of organic brominated compounds, hence the very intensive manufacturing of such compounds creates both, hazardous industrial refuge and large amounts of brominated byproducts. Environmentally the storage of these huge amounts of materials may lead to major hazards: (a) water reservoirs contamination; (b) land contamination. It is suggested to incinerate the accumulated and presently produced organic wastes by the existing organic hazardous wastes incineration plants, one should take into account the fact that polybrominated compounds are fire retardants! The thermal behavior of the CBr bond was scantly investigated up-to-date. In the present work and those to follow we report on our examinations of various brominated organics pyrolysis experiments e.g. bromohexane (BH) bromocyclohexane (BCH), bromonaphthalene, 4,4′-isopropylidene bis(2,6-dibromophenol) measurements were carried out only for the solid and polybrominated compounds whereas time and temperature experiments were conducted under closed vessel conditions (quartz ampoules). The analyses of the pyro-products were initially monitored by gas, extractables and solids using both selective detectors (FID, ECD), GC and GC/MS (for gases and liquids). Experiments were conducted between 400 and 1000°C, and pyrolysis exposure times from 10 s to 1 h. We present the results obtained for single compound and hydrogen donor mixed with compound. The conclusions drawn as presented in the concluding remarks are: (1) most of the released bromine is in HBr form and probably the reason for retardation; (2) At 1000°C the organic products are low molecular weight gases and char; (3) The amounts of dibenzodioxins and dibenzofurans are minute and thermally unstable; (4) the aliphatic brominated compounds are thermally less stable than the aromatics; (5) Under oxygen conditions the formation of large amounts of HBr is acting as retardant even in the presence of excess hydrogen donors; (6) the presence of hydrogen donors is conducive to the formation of HBr, increase of rate and quantity.

Introduction

Being one of the leading manufacturers of organic brominated compounds Israel produces hazardous industrial refuge containing large amounts of brominated byproducts. The incineration of these byproducts is one of the possible treatments, therefore the understanding and knowledge about this family of compounds is the base thing to perform prior to reaching to this kind of solution.

As a matter of fact, the incineration of organic chemical wastes is a very complicated task, taking into account all the safety and environmental aspects. There are two main concerns in the incineration of halogentated organics (mainly chlorinated and brominated): (1) the risk of dibenzodioxins and dibenzofurans production, being their halogenated moieties of even more concern; and (2) the HCl and HBr emissions through the incineration plant stack.

During the last 15 years it became clear that chloroorganic pollutants could be formed by combustion processes like waste incineration or by car engines [1].

Although it was at first believed that thermal formation processes demand suitable precursors (like chlorinated phenols, dioxins precursors, etc) [2] it was established later by Stieglitz and Vogg [3] that pyrolysis and chlorination of natural compounds can form C₂- units which in turn may generate polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) upon heating.

Two well-known reaction pathways might give possible models for the reaction pathway for the synthesis of PCDDs and PCDFs: (a) when phenol or pentachlorophenol are the precursors the formation of DBFs is possible at 150°C, with PbO as a catalyst; (b) the Ullmann reaction, which uses copper as a catalyst, may produce PCDDs and PCDFs [4].

It is suggested that in the gas phase, where combustion is taking place, the gasified organics and some fly metals such as copper could turn HCl into active chlorine. This active chlorine can then react with particles of carbon in the fly ash [4], and form organochlorine compounds, including dioxins and furans [3], [5], [6].

The reaction pathways for the formation of PCDDs and PCDFs during combustion processes were also studied by Ballschmiter and Swerev [7]. They propose a complicated mechanism in which the polychlorinated moieties of styrene, phenol, naphtalene and others are formed from monochlorinated and non-chlorinated compounds.

Oberg's work showed high yields of brominated aromatics from combustion, compared to their chlorinated analogs [8], whereas Eklund reported that brominated aromatics are formed from the combustion of propane and HCl contaminated with HBr [9].

It was also reported that polybrominated dibenzodioxins (PBDDs) and polybrominated dibenzofurans (PBDFs) are formed during the pyrolysis of brominated flame retardants [10], [11], [12], [13].These and others experiments have shown high yields of bromine containing aromatics such as bromobenzenes, bromophenols, bromochlorophenols, etc. [8], [11], [14].

It has been suggested that at higher temperatures of pyrolysis the proposed structure products, e.g. PBDDs or PBDFs, are thermally less stable than the non-brominated structures.

In order to control the above-mentioned pollutants, which may be produced during pyrolysis or combustion, a better knowledge and understanding of their thermal chemical behavior is needed. The thermal degradation and repolymerization in the presence of hydrogen donors, or in their absence, must be understood for the proper and less hazardous disposal of mono and polybrominated organic compounds.

The flame retardardation properties of this family of compounds causes another operational problem because of the difficulty that it presents for its ignition and the obvious necessity for mixing with big amounts of hydrogen donors.

The production of brominated organics is studied at present, in few countries, on big scale. As a result, the amount of information and experience compiled for the treatment of organic brominated industrial wastes are very scant.

The understanding and the knowledge of the thermal behavior of this important group, the pyrolytic characteristics of the mono and polybrominated moieties and the CBr bond patterns, may lead us to an optimal solution to the problem of the disposal and treatment of the brominated organic flame retardants refuges and their related compounds by incineration under environmentally safe conditions.

Comparison between the energies of activation (Ea) for carbon–halogen bond breakage (with hydrogen halide elimination) of the chlorinated and brominated alkyl monohalides and cycloalkyl monohalides show clearly that the Ea of the brominated moieties are lower in all cases [24], [25], [26]. Therefore it is expected that the brominated compounds will behave thermally quite differently in comparison to the chlorinated homologs. For review see [30].

However the work by Thoma and Hutzinger [29], conducted by an open system pyrojector, between 600 and 900°C, showed a qualitative appearance of bromobenzene, mono, di and tribromophenols, some brominated dibenzofurans and bromonaphthalenes. Whereas if 2,4,6-tribromophenol was used as the starting material, brominated benzenes and DBDs were the major products. This work did not even attempt any quantification because of the fast heating open system. Also the formation of HBr was not monitored.

The present study and the following work were designed to answer two fundamental questions: Does the production of HBr produce conditions of oxygen deactivation or the pyro-fragments concentration is too low to sustain combustion.

The approaches adapted for the present and future works involve: (1) the study of neat brominated flame retardants, 4,4′-isopropylidene bis(2,6-dibromophenol) (TBBA) and decabromodiphenyloxide (DECA) the chosen representatives; (2) the study of the relation between the structure and character of the studied compounds and the mode by which the bromine acts; (3) the selection of the system (we performed preliminary experiments in order to determine if either an open or closed system should be more appropiated for our target); (4) to study if the tendency of the brominated compounds to act as flame retardants is related to the production of bromine radicals or the production of HBr. Also to investigate any evidence of transbromination to form molecules of lower flamability, causing the desired effect of flame retardation; (5) analogically to the well known behavior of chlorinated organics that produce dioxins and chlorinated dioxins when partially oxidized, we examined to what extent the brominated and polybrominated compounds facilitate the production of brominated dioxins.

Section snippets

Thermogravimetry

A thermogravimetry apparatus Mettler, Model DSC 30, with option of Thermo Analyzer TMA 40, was used in order to determine the thermal behavior of the flame retardants in response to increasing temperatures, both under inert conditions and in the presence of oxygen.

Calorimetry

An isoperibolic calorimetric system from IKA, model C 7000, was used to determine calorific values of some of the compounds studied. Samples oxidation, employing oxygen under a pressure of 30 atmospheres oxygen, and absorption of the

Cyclic and acyclic aliphatic

Both monobrominated cyclic and acyclic aliphatic compounds, BH and BCH, show at the range of temperature studied (400–1000°C) approximately complete HBr elimination (as described in experimental section).

Table 1a presents the distribution of bromine after pyrolysis or combustion. The experiments conducted under the above conditions, show the semiquantitative distribution of bromine.

The produced unsaturated hydrocarbons thermal fate is highly dependent on its thermal stability at the temperature

Concluding remarks

The present work is unique in the sense that it is measuring under closed system the thermostability of brominated compounds and their pyroproducts.

While in the past flame retardants and other brominated compounds have been investigated either in open pyroprobes, pyroinjectors, fast heating devices; our investigation, although allows secondary reactions, gives us the opportunity to search semiquantitatively the formation of gases, liquid pyroproducts and the formation of polymeric char at the

Acknowledgements

This paper is part of Eitan J.C. Borojovich's Ph.D. thesis. We thank one of the referees for in depth reading and suggestions for the intensive revision of the paper, Professor Menachem Steinberg of H.U. for editing remarks and both Dr Marianne Blazso, Dr Robert Lattimer (Editors of JAAP) for the encouragement to rewrite the manuscript.

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