formation of hydrogen bromide and organobrominated compounds in the thermal degradation of...

Formation of hydrogen bromide and organobrominated compounds

in the thermal degradation of electronic boards

Federica Barontini a,*, Valerio Cozzani b

a Gruppo Nazionale per la Difesa dai Rischi Chimico-Industriali ed Ecologici, Consiglio Nazionale delle Ricerche,

via Diotisalvi n.2, 56126 Pisa, Italyb Dipartimento di Ingegneria Chimica, Mineraria e delle Tecnologie Ambientali, Universita degli Studi di Bologna,

viale Risorgimento n.2, 40136 Bologna, Italy

Received 6 October 2005; accepted 16 January 2006

Available online 28 February 2006

Abstract

The influence of the heating rate and of the presence of oxygen on the decomposition pathways and on the decomposition product yields

obtained from the thermal degradation of electronic boards containing brominated flame retardants was investigated. TG-FTIR techniques and a

fixed bed batch reactor coupled to different analytical techniques were used to simulate different thermal degradation conditions. The qualitative

and quantitative yields of decomposition products at different heating rates and in different reaction environments were obtained and compared to

those from the decomposition of the unlinked resin and of tetrabromobisphenol A. Bromine was mainly evolved in the gaseous fraction for all the

materials and the experimental conditions considered, although significant quantities are present as well in the condensable fraction. An increase of

bromine evolved as gaseous hydrogen bromide was evidenced at higher heating rates. In the presence of oxygen, PBDD and PBDF were detected

among the decomposition products. The possible precursors and the likely radical reaction pathways leading to the formation of these compounds

were identified on the basis of the experimental data. The results obtained confirm that the formation of highly hazardous organobrominated

compounds may be expected from the combustion of electronic scrap and of materials containing brominated flame retardants.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Electronic wastes; Brominated flame retardants; Thermal degradation; Decomposition products; Thermal degradation pathways

www.elsevier.com/locate/jaap

J. Anal. Appl. Pyrolysis 77 (2006) 41–55

1. Introduction

Brominated compounds are widely used as flame retardants

in the industrial practice. Due to their high efficiency,

compatibility and small influence on mechanical properties,

brominated flame retardants (BFRs) have a broad application

area, mainly in the field of polymeric materials. The increasing

use of BFRs in the manufacture of epoxy resins for printed

circuit boards lead the production of brominated flame

retardants to increase from 145,000 t/year in 1990 to

310,000 t/year in 2000 [1]. Therefore, the increasing produc-

tion of BFRs is strictly connected to their use in widespread

electronic products, as personal computers and other domestic

electronic appliances.

* Corresponding author. Tel.: +39 050 511265; fax: +39 050 511266.

E-mail address: [email protected] (F. Barontini).

0165-2370/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jaap.2006.01.003

Despite the technical advantages of BFRs, their wide

diffusion is causing an increasing concern on the end-life

disposal of products containing these materials, due to the

possible formation of dangerous thermal decomposition

products. The potential hazard coming from BFRs was

considered so high that the European Community decided

strong limitations in their use [2], and strict regulations for the

end-life disposal of electronic waste [3].

Although several studies were dedicated to investigate the

products formed in the thermal degradation and combustion of

materials containing BFRs, the influence of heating rate and

thermal degradation conditions on product yields still needs to

be fully understood. Most of the former studies were dedicated

to shed some light on the possible formation of extremely

hazardous compounds, as polybrominated dibenzo-p-dioxins

(PBDD) and polybrominated dibenzofurans (PBDF), in the

oxidation of BFRs [4–19]. More recently, several investigations

were dedicated to the distribution of bromine among the

thermal degradation products of the more common BFRs, as

F. Barontini, V. Cozzani / J. Anal. Appl. Pyrolysis 77 (2006) 41–5542

tetrabromobisphenol A (TBBA) and hexabromocyclodode-

cane (HBCD) [20–29], and of materials containing BFRs

[26,30–45]. The results obtained evidenced that, apart the

formation of extremely dangerous compounds as PBDD and

PBDF, hazards may derive also from the formation of relevant

quantities of hydrogen bromide and high molecular weight

brominated compounds. However, the influence of the

conditions present during the thermal degradation on the

product yields as well as on the thermal decomposition

pathways of materials containing BFRs was not system-

atically investigated. Understanding the influence of para-

meters as the heating rate and the reaction environment on the

formation of high molecular weight organobrominated

compounds may be of fundamental importance in the

perspective of the correct design of end-life disposal

processes of these materials [15,46–49], as well as in the

prediction of the hazards that may derive from the thermal

stress of materials containing BFRs.

The present study was focused on the analysis of the

product yields obtained from the thermal degradation of BFR-

containing materials at different heating rates and in different

reaction environments. Electronic boards obtained from

cross-linked epoxy resins containing tetrabromobisphenol

A were selected as sample materials, due to their wide

diffusion in electronic products. TG-FTIR techniques and a

fixed bed batch reactor coupled to different analytical

techniques were used to simulate different thermal degrada-

tion conditions. To better understand the influence of

experimental conditions on the thermal decomposition path-

ways, the results obtained were compared to those achieved

for linear brominated epoxy resins and pure TBBA. The

influence of the experimental conditions on the bromine

distribution among the different product fractions was

determined, and qualitative and quantitative data were

obtained on the products formed.

2. Experimental

2.1. Materials

Three different samples of flame retarded electronic boards

base materials were obtained from local manufacturers. These

Table 1

Results of the proximate analysis performed on the electronic boards investi-

gated in the present study

Board

A

Board

B

Board

C

Organic fraction

Volatiles (wt.%) 36.2 19.8 29.0

Fixed carbon (wt.%) 8.4 3.5 3.1

Total (wt.%) 44.6 23.3 32.1

Inert fraction

Metallic inerts (wt.%) 0 44.6 16.0

Non-metallic inerts (wt.%) 55.4 32.1 51.9

Total (wt.%) 55.4 76.7 67.9

materials that will be denoted as boards A, B and C in the

following are constituted of a cross-linked organic matrix (a

brominated epoxy resin) on a support of glass fibres. Some

samples were supplied also with the metal layers used to

produce the printed boards. Table 1 reports the results of the

proximate analysis of these materials. If present, the metallic

layers were removed by treatment with a dilute aqueous

solution of hydrochloric acid and hydrogen peroxide, followed

by washing with deionized water and drying at 110 8C. The

metal-free materials were milled in liquid nitrogen. All the

experimental analyses performed in the present study were

carried out on the milled metal-free samples. The milled metal-

free samples were analyzed to determine the bromine content,

and the results obtained are reported in Table 2.

In order to better understand the pathways leading to the

formation of brominated decomposition products, experimental

runs were also carried out on pure tetrabromobisphenol A (the

flame retarding agent used for the manufacture of all the BFR

samples) and on a linear brominated epoxy resin sample (the

intermediate obtained before the impregnation and cross-

linking process for the production of the board).

TBBA was supplied by Aldrich (Milan, Italy). Linear

brominated epoxy resin was prepared by reaction of

diglycidyl ether of bisphenol A (DGEBA, supplied by Shell)

with tetrabromobisphenol A, using a 1.85 DGEBA/TBBA

molar ratio and following the procedures described else-

where [50].

2.2. Techniques

Simultaneous thermogravimetric (TG) and differential

scanning calorimetry (DSC) data were obtained using a

Netzsch STA 409/C thermoanalyzer. Sample weights

between 10 and 50 mg were employed in experimental

runs. FTIR measurements were carried out using a Bruker

Equinox 55 spectrometer. TG-FTIR simultaneous measure-

ments for the on-line analysis of volatile compounds formed

during TG runs were carried out coupling the FTIR

spectrometer to the Netzsch TG using a 2 mm internal

diameter teflon tube. The 800 mm long transfer line and the

head of the TG balance were heated at a constant temperature

of 200 8C to limit the condensation of volatile decomposition

products. FTIR measurements were carried out with a MCT

detector in a specifically developed low volume gas cell

(8.7 ml) with a 123 mm pathlength, heated at a constant

temperature of 250 8C. The gas flow from the TG outlet to

the IR gas cell was of 60 ml/min (at 25 8C and 101.3 kPa)

Table 2

Bromine content of the materials investigated in the present study

Sample Bromine content

(wt.%)

Tetrabromobisphenol A 58.8

Linear brominated epoxy resin 26.0

Electronic board A 6.9

Electronic board B 6.7

Electronic board C 6.0

F. Barontini, V. Cozzani / J. Anal. Appl. Pyrolysis 77 (2006) 41–55 43

Fig. 1. Simulation of high heating rate conditions in the experimental devices

used in the present study: comparison between the standard ASTM E119 curve

and the experimental temperature recorded in the TG and BR devices.

and a mean value of 30 s could be estimated for the residence

time of evolved gases in the transfer line. This value was

assumed as the time delay correction to be used for

the comparison of TG and IR results. During TG-FTIR runs,

spectra were collected at 4 cm�1 resolution, co-adding 16

scans per spectrum. This resulted in a time resolution

of 9.5 s. Qualitative gas evolution profiles and quantities of

gaseous decomposition products formed in TG-FTIR

runs were obtained from the experimental data follo-

wing the procedures described in previous publications

[22,51].

A laboratory-scale fixed bed tubular batch reactor (BR)

was used to carry out thermal decomposition runs. The

experiments were mainly aimed at the recovery and

characterization of the different fractions of decomposition

products. Typical sample weights in the experimental runs

were comprised between 150 and 400 mg. Experimental runs

were performed using a purge gas flow (80 ml/min) of pure

nitrogen or air to control the reaction environment and to

limit the extension of secondary gas-phase reactions. Volatile

products evolved during thermal degradation were trans-

ferred by the gas flow in a series of cold traps, maintained at

�20 8C by a sodium chloride brine/ice bath. Condensable

products were recovered at the end of the run from the cold

traps for chromatographic analysis. The traps were followed

by a gas sampling cell for on-line FTIR gas analysis. To

check for the formation of molecular bromine, during

experimental runs a fluorescein test was carried out on the

gas outflow from the reactor [29]. Moreover, in some

experimental runs two absorbers containing a sodium

hydroxide solution for the absorption of gas-phase acidic

compounds were connected after the cooling traps. At the

end of the runs, the solution in the alkaline scrubbers was

checked by iodometric titration [29]. The solid residue

remaining on the sample holder at the end of the run, if

present, was recovered, and its bromine content was

determined by elemental analysis [29]. Further details and

a scheme of the experimental apparatus are reported

elsewhere [22].

A Fisons MD 800 quadrupole mass spectrometer inter-

faced to a Fisons GC 8060 gas chromatograph was used for

gas chromatography/mass spectrometry (GC/MS) analysis. A

Mega SE30 fused silica capillary column (25 m length,

0.32 mm internal diameter, cross-bonded, 0.25 mm film

thickness) was employed for the chromatographic separation,

with helium as carrier gas. The column temperature

programme was the following: 5 min isothermal at 40 8C,

heating to 250 8C (6 K min�1), then 20 min isothermal.

Splitless injection with the injector at 250 8C was used. Mass

spectrometric detection was performed in full scan condi-

tions (scan range, m/z 10–819) in electron impact ionization

mode.

Quantitative GC analysis was carried out using a Thermo-

Quest Trace GC 2000 gas chromatograph equipped with a

flame ionization detector (FID). The capillary column and the

experimental conditions were identical to those used for GC/

MS analysis, detector temperature was fixed at 280 8C. More

details on GC quantitative determinations are reported in

previous studies [28,29].

2.3. Experimental conditions used in thermal degradation

runs

Different heating rates and reaction environments were used

in both TG-FTIR and BR runs in order to investigate the

influence of experimental conditions on the thermal degrada-

tion behaviour of the materials examined in the present

study.

To control the reaction environment a specific purge gas flow

was used during experimental runs. A purge flow of 100%

nitrogen or of air was employed to reproduce pyrolytic or

oxidative conditions, respectively.

Thermal degradation runs in both inert and oxidizing

environments were carried out under low and high heating rate

conditions. A 10 K min�1 constant heating rate was chosen as

representative of low heating rate conditions. High heating rates

are typical of fire scenarios. Several investigations evidenced

that under standard conditions there is a specific relationship

between time and temperature of materials involved in a fire

taking place in a confined environment, leading to the

development of several standard time–temperature curves for

fire tests [52]. A widely used reference time–temperature curve

is reported by ASTM E119 standard [53] that was used in the

present study as a representative time–temperature profile to

investigate the products evolved under high heating rate

conditions, specifically in fire conditions. The time–tempera-

ture profile defined in the Standard Test Methods E119 was

reproduced in the thermal analyzer and in the laboratory-scale

reactor. Fig. 1 shows a comparison between the standard ASTM

E119 curve and the temperatures measured in the TG furnace

and in the reactor during standard experimental runs. A

sufficient correspondence is evidenced in the figure between the

recorded time–temperature profiles and that of the standard

curve.


Fig. 2. Results of TG-FTIR runs performed on the linear brominated epoxy

resin (nitrogen 100%; continuous lines: ASTM E119 conditions; dashed lines:

10 K min�1 constant heating rate conditions). (a) Weight loss data. (b) Weight

loss rate data. (c) Hydrogen bromide emission profiles.

All the experimental runs performed in the present study

were carried out from ambient temperature up to a final

temperature of 800 8C.

3. Results and discussion

3.1. Thermal degradation behaviour of materials

containing brominated flame retardants

Experimental TG-FTIR runs at different heating rates were

carried out on the brominated materials listed in Section 2. The

time–temperature profile reported in Fig. 1 (ASTM E119

conditions) and a 10 K min�1 constant heating rate were used

to reproduce high and low heating rate conditions, respec-

tively. Experimental runs were performed under both oxidative

and pyrolytic conditions. Fig. 2 shows the typical results

obtained in TG-FTIR runs. The figure reports the sample

weight loss, the differential thermogravimetric curve (dTG),

and the qualitative emission profile of hydrogen bromide

obtained from FTIR data following the procedures extensively

described elsewhere [22]. The data reported in Fig. 2 were

obtained for the linear brominated epoxy resin in inert

atmosphere. A qualitatively similar behaviour was obtained for

the other materials considered in the present study. Fig. 3

shows a comparison of the results obtained in TG-FTIR runs

for the three milled metal-free samples of electronic boards. As

evident from Fig. 3(b and c), the behaviour of the organic

fraction of the three samples is almost coincident, confirming

the common nature of this fraction in most commercial

electronic boards. The limited differences introduced in the

conditions used for the cross-linking process seem to have a

limited influence on the thermal degradation of these materials.

Since all the electronic board samples resulted in a similar

thermal degradation behaviour, the following figures and

tables report the results of experiments performed on sample

A. Similar results were obtained for the other samples and are

not reported for the sake of brevity.

The simultaneous FTIR analysis allowed us to monitor the

evolution of volatile products generated during the thermal

degradation runs performed. A complex mixture of products,

including hydrogen bromide evolved in correspondence of the

main weight loss step. As evident from the results reported in

Fig. 2, hydrogen bromide concentration in the gas outflow from

the TG analyzer shows a peak corresponding with the

maximum of sample weight loss rate, in both ASTM E119

and 10 K min�1 heating rate conditions.

However, the temperature–time profile clearly affects the

weight loss behaviour and the hydrogen bromide emission. This

is more clearly evidenced in Fig. 4, which reports the TG and

dTG curves as a function of the sample temperature for the

different heating rates used. The different temperature–time

profiles result in different thermal degradation temperature

ranges, different volatile losses and different weight loss rates.

The temperatures corresponding to the maximum of sample

weight loss rate, corresponding as well to the maximum of

hydrogen bromide emission, are summarized in Table 3. As

expected, increasing heating rates result in increasing decom-

position temperatures. Obviously, the different materials

examined in the present study exhibited different decomposi-

tion temperatures. In particular, a higher thermal stability was

observed for the linear brominated resin with respect to TBBA

and to the electronic board samples.

The data reported in Fig. 4 also show a comparison of the

results obtained under different reaction environments. As

evident from the results reported in Fig. 4 and Table 3, the


Fig. 4. Results of TG-FTIR runs performed on the linear brominated epoxy

resin. (a) Weight loss data at 10 K min�1 constant heating rate (dashed line:

nitrogen 100%; dotted line: air). (b) Weight loss data in ASTM E119 conditions

(continuous line: nitrogen 100%; dash–dotted line: air). (c) Weight loss rate data

in 100% nitrogen atmosphere (continuous line: ASTM E119 conditions; dashed

line: 10 K min�1 constant heating rate conditions).

Fig. 3. Results of TG-FTIR runs performed on electronic boards at 10 K min�1

constant heating rate (continuous lines: board A; dotted lines: board B; dashed

lines: board C). (a) Weight loss data in 100% nitrogen. (b) Weight loss data in

inert atmosphere normalized with respect to the organic fraction present in the

samples. (c) Weight loss data in oxidizing atmosphere normalized with respect

to the organic fraction present in the samples.

presence of oxygen does not affect significantly the sample

decomposition temperature range. The analysis of the FTIR

results in oxidizing conditions confirmed the presence of a main

degradation step, during which a complex mixture of products

including hydrogen bromide was generated. Hydrogen bromide

emission profiles qualitatively similar to those obtained under

pyrolytic conditions were recorded, showing a peak in

correspondence of the maximum of the sample weight loss

rate. However, in the presence of oxygen a second weight loss

peak occurred at higher temperatures, probably due to the

oxidation of the solid residue formed in the first decomposition

stage. As a matter of fact, emissions of carbon monoxide and


Table 3

Maximum weight loss rate temperature (8C) recorded for the materials investigated in the present study

Sample Maximum weight loss rate temperature (8C)

10 K min�1 conditions ASTM E119 conditions

Inert

environment

Oxidizing

environment

Inert

environment

Oxidizing

environment

Tetrabromobisphenol A 337 337 408 416

Brominated epoxy resin 375 375 459 468

Electronic board A 315 315 392 392

carbon dioxide were mainly detected during this second

decomposition step. On the other hand, a progressive

degradation of the primary residue was experienced under

pyrolytic conditions.

3.2. Products formed in the thermal degradation of the

brominated materials

The TG-FTIR and BR experiments carried out allowed for

the characterization of products formed in the thermal

degradation of the brominated materials investigated in the

present study. The degradation process resulted in the

generation of three decomposition product fractions: a gaseous

fraction, a condensable fraction and a solid residue.

Detailed data about the nature of the gaseous compounds

generated and the relative emission temperatures were obtained

from TG-FTIR runs, and were confirmed by the results of BR

experiments. The FTIR on-line analysis of the gas flow entering

the sampling cell in BR runs allowed for the identification of

non-condensable products formed and, following the same

methodology used for the analysis of TG-FTIR data, emission

profiles for the compounds identified could be obtained as a

function of sample temperature.

As reported in the previous section, hydrogen bromide

formation was detected during the main decomposition step of

the materials investigated in all the experimental conditions

explored in the present work. Methane, carbon monoxide and

carbon dioxide emissions were also associated with the main

decomposition stage, while ammonia was generated only from

the electronic board samples. Both in pyrolytic and oxidizing

conditions, molecular bromine was not detected in the gaseous

products, either by the fluorescein test or by the iodometric

titration of the solution collected in the scrubbers at the end of

the experimental runs (see Section 2).

In addition to light gases, high molecular weight organic

compounds evolved in correspondence of the main weight loss

step in TG-FTIR runs. However, due to the simultaneous

evolution of several compounds, the identification of high

molecular weight products could not be achieved by FTIR data

analysis. The characterization of these products was performed

by the GC/MS analysis of the condensable fractions recovered

from BR experiments.

As stated in Section 2, experimental runs were carried out up

to a final temperature of 800 8C. Only gaseous compounds were

detected in the evolved products during the degradation of the

residue formed in the main thermal degradation step,

specifically carbon oxides, methane up to 700 8C (600 8C in

air), and in the case of the TBBA sample hydrogen bromide (up

to 550 8C in 10 K min�1 constant heating rate runs or 650 8C in

ASTM E119 conditions). In oxidizing conditions, the emission

peaks of CO and CO2 were observed in correspondence of the

weight loss step ascribed to the oxidation of the primary

residue.

No organic solid residue was present at the end of

experimental runs carried out in oxidative conditions. On the

other hand, a black solid residue, named char in the following,

was obtained in pyrolytic conditions.

The condensable fraction of the thermal decomposition

products was recovered from BR runs and analyzed by

chromatographic techniques. The GC/MS analyses performed

enabled the identification of the products present in the

fractions recovered. The identification was achieved by the

analysis of the mass spectra, by comparison with the best fits

found in the NIST spectral library, by comparison with

published MS data [26,29,37,54], or by the use of standards. For

some minor chromatographic peaks a complete structural

assignment was not possible, and only the molecular weight

and the number of bromine atoms present in the molecule could

be clearly identified.

Table 4 summarizes the products identified in the fractions

recovered from thermal degradation runs performed on TBBA

in the different experimental conditions explored in the present

work. The results obtained point out the presence of TBBA

among the products identified, confirming that TBBA decom-

position results in a competitive process with evaporation, as

previously reported [27–29,34,55].

A careful inspection of data reported in Table 4 reveals that

several chemical species (e.g. bromophenols, dibromophenols,

tribromophenol, brominated bisphenol A’s) were formed in all

the experimental conditions investigated. Likely pathways for

the formation of these compounds have been discussed

elsewhere [29]. However, important differences were found

in the experimental runs carried out in oxidative conditions.

Polybrominated benzenes, polybrominated dibenzo-p-dioxins

and polybrominated dibenzofurans were detected in the product

fractions recovered from BR runs performed in oxidizing

environment, both at low and high heating rates. Specifically,

mono- through tribrominated PBDD and PBDF were identified

in oxidative conditions. These species are likely to be generated

from the brominated phenols and/or brominated phenoxy

radicals formed during TBBA decomposition, which are known

to be specific precursors of PBDD/F.


Table 4

Products identified in the condensable fraction recovered from BR thermal degradation runs carried out on TBBA in the different experimental conditions explored in

the present work

Compound Pyrolytic conditions Oxidative conditions

10 K min�1 ASTM E119 10 K min�1 ASTM E119

Non-brominated compounds

Phenol U U U

Naphthalene U U

Biphenyl U U

Anthracene or phenanthrene U U

Dibenzofuran U U U U

Dibenzo-p-dioxin U U U U

Bisphenol A U U U U

Brominated benzenes

Dibromobenzene (2 isomers) U U

Tribromobenzene (3 isomers) U U

Tetrabromobenzene (2 isomers) U U

Brominated phenols

2-Bromophenol U U U U


2-Bromo-4-methylphenol U U

2-Bromo-4-(1-methylethenyl)phenol U U

2,4-Dibromophenol U U U U


2,6-Dibromo-4-methylphenol U U U U

2,6-Dibromo-4-(1-methylethenyl)phenol U U U U

2,6-Dibromo-4-(1-methylethyl)phenol U U

2,4,6-Tribromophenol U U U U

Other brominated compounds

Bromonaphthalene U U

2,4,6-Tribromo-1,3-benzendiol U U

Tetrabromoethylene U U

Brominated bisphenol A derivatives

Bromobisphenol A U U U U

Dibromobisphenol A (2 isomers) U U U U

Tribromobisphenol A U U U U

TBBA U U U U

PBDD/F

Bromodibenzofuran (2 isomers) U U U U

Bromodibenzo-p-dioxin (2 isomers) U U

Dibromodibenzofuran (2 isomers) U U

Dibromodibenzo-p-dioxin (4 isomers) U U

Tribromodibenzofuran (2 isomers) U U

Tribromodibenzo-p-dioxin (2 isomers) U U

In order to shed some light on the temperatures at which

PBDD/F are formed, further experimental BR runs in oxidizing

conditions were carried out up to a final temperature

corresponding to the end of the main decomposition step,

namely 450 8C at 10 K min�1 heating rate. The GC/MS

analyses performed on the recovered product fractions revealed

the presence of PBDD/F among the decomposition products.

This confirmed that PBDD/F are generated from specific

precursors in the main decomposition step.

On the basis of the fundamental mechanisms of dioxin

formation reported in literature [56–59], Scheme 1 (where X, Y,

Z and W represent either hydrogen or bromine) describes the

possible pathways to brominated dibenzo-p-dioxins from the

reaction of the different intermediates and products generated

during TBBA decomposition [29]. A radical–molecule reaction

pathway is shown in path a of Scheme 1: a phenoxy radical

ortho-bromo substituted reacts with an ortho-bromophenol

through displacement of bromine yielding a bromohydroxy

diphenyl ether, which, after hydrogen loss, leads to a radical (I).

Radical–radical pathways are proposed in paths b and c of

Scheme 1, which involve recombination reactions of phenoxy

radicals, resonance-stabilized species in which the radical

character is delocalized on the oxygen atom and on the ortho-

and para-positions of the aromatic system. In path b an oxygen-

centered phenoxy radical ortho-bromo substituted recombines

with a carbon-(bromine) centered ortho-bromophenoxy radical

to form a keto-ether which, after loss of bromine, yields radical

I. In path c an oxygen-centered phenoxy radical ortho-bromo

substituted recombines with a carbon-(hydrogen) centered

phenoxy radical to form a keto-ether which, after loss of


Scheme 1.

hydrogen, leads to intermediate I. Once I is formed, ring closure

occurs: the reaction proceeds through a five-member ring

intermediate (II, of the type involved in Smiles rearrangement),

from which displacement of bromine occurs, yielding two

dioxin isomers (III and IV).

The reaction pathways proposed may involve any of the

intermediates or products formed during the thermal decom-

position process of TBBA. The primary intermediates and

products consist of phenols (or phenoxy radicals) which may

bear bromine only in the ortho-positions [29], that is Z = W = H

in Scheme 1. This means that only dibenzo-p-dioxin, 1-

monobromodibenzo-p-dioxin, 1,6- and 1,9-dibromodibenzo-p-

dioxin may originate from TBBA primary degradation

products. Nevertheless, the experimental data collected in this

and in previous studies [26,29] show that these products are

involved in secondary radical bromination reactions that yield

para-bromo-substituted species (X, Y, Z, W = hydrogen or

bromine in Scheme 1). According to the mechanisms proposed,

several PBDD may thus be formed, specifically two mono-

bromodibenzodibenzo-p-dioxins (the 1- and the 2-congeners),

seven dibromodibenzodibenzo-p-dioxins (the 1,3-, 1,6-, 1,7-,

1,8-, 1,9-, 2,7- and 2,8-congeners), four tribromodibenzodi-

benzo-p-dioxins (the 1,3,6-, 1,3,7-, 1,3,8- and 1,3,9-congeners)

and two tetrabromodibenzodibenzo-p-dioxins (the 1,3,6,8- and

the 1,3,7,9-congeners). The identification of monobromodi-

benzodibenzo-p-dioxins (two isomers), dibromodibenzodi-

benzo-p-dioxins (four isomers) and tribromodibenzodibenzo-

p-dioxins (two isomers) in the condensable fraction of TBBA

thermal decomposition products (Table 4) suggests that PBDD

generation involve as well the species deriving from the

bromination of primary intermediates.

No higher brominated PBDD/F were identified, and no

PBDD were detected in the product fractions recovered from

runs performed in inert atmosphere. The data obtained in the

present study are in good accordance with the results of

previous investigations on the formation of PBDD/F that

showed that TBBA decomposition yields mainly mono-

through tribrominated PBDD and PBDF in the ppm range

[7,8,11,13,14,16–19].

Tables 5 and 6 list the compounds identified in the

condensable product fractions recovered from thermal degra-

dation runs carried out on linear brominated epoxy resin and

electronic board samples, respectively. As evident from a

comparison of results reported in Tables 5 and 6, a strong

correspondence is present between the decomposition products

of linear epoxy resins and electronic boards. Phenol,

alkylphenols, brominated phenols and brominated alkylphe-

nols, bisphenol A and brominated bisphenol A derivatives,

were identified. These are possibly originated from the

bisphenol A and tetrabromobisphenol A building blocks of

the resin. Also for these samples, PBDD/F were identified in the

product fractions recovered from degradation runs performed in


Table 5

Products identified in the condensable fraction recovered from BR thermal degradation runs carried out on the linear brominated epoxy resin in the different

experimental conditions explored in the present work




Phenol U U U U

Methylphenol (2 isomers) U U U U

Ethylphenol (2 isomers) U U U U

Propylphenol U U

4-(1-Methylethyl)phenol U U U U

Methyl-(1-methylethyl)phenol U U U

Naphthalene U U U U

Methylnaphthalene U U

Biphenyl U U U U

Anthracene or phenanthrene U U U U

2,2-Diphenylpropane U U

2-Methylbenzofuran U U U U

3,4-Dihydro-2H-1-benzopyran U U U

3,4-Dihydro-2H-1-benzopyran-3-ol U U U U

1-Phenoxy-2-propanone U U


Dibenzo-p-dioxin U U

p-Hydroxybiphenyl U U U U

4-(1-Methyl-1-phenylethyl)phenol U U U U

2,40-Isopropylidenediphenol U U U U

Bisphenol A U U U U

DGEBA U U U U

Brominated benzenes

Dibromodimethylbenzene U U

Tribromobenzene (3 isomers) U

Tetrabromobenzene (2 isomers) U

Brominated phenols



2-Bromo-4-(1-methylethenyl)phenol U U U U

2-Bromo-4-(1-methylethyl)phenol U U U U



2,6-Dibromo-4-methylphenol U U U U


2,6-Dibromo-4-(1-methylethyl)phenol U U U U



Bromonaphthalene U

2-Bromo-1,4-benzendiol U U

1-Bromo-3-phenoxy-2-propanol U U U U

1,3-Dibromo-2-propanol U U





TBBA U U U U

PBDD/F

Bromodibenzofuran (2 isomers) U U


Dibromodibenzofuran U U

Dibromodibenzo-p-dioxin U

Tribromodibenzo-p-dioxin U


Table 6

Products identified in the condensable fraction recovered from BR thermal degradation runs carried out on electronic board A in the different experimental conditions

explored in the present work




Styrene U U

Dimethylpyrazine U U

Phenol U U U U

Methylphenol (2 isomers) U U U U

Ethylphenol (2 isomers) U U U U

Propylphenol U U U U

4-(1-Methylethyl)phenol U U U U

Methyl-(1-methylethyl)phenol U U U U

Naphthalene U U U U

Methylnaphthalene U U

Biphenyl U U U U

Anthracene or phenanthrene U U U U

2-Methylbenzofuran U U U U

3,4-Dihydro-2H-1-benzopyran U U U

3,4-Dihydro-2H-1-benzopyran-3-ol U U U U


Dibenzo-p-dioxin U U

p-Hydroxybiphenyl U U U U

4-(1-Methyl-1-phenylethyl)phenol U U

2,40-Isopropylidenediphenol U U U U

Bisphenol A U U U U

Brominated benzenes

Tribromobenzene U U

Brominated phenols



2-Bromo-4-(1-methylethenyl)phenol U U U U

2-Bromo-4-(1-methylethyl)phenol U U U U




2,6-Dibromo-4-(1-methylethyl)phenol U U U U



2-Bromo-1,4-benzendiol U

1-Bromo-3-phenoxy-2-propanol U U U U

1,3-Dibromo-2-propanol U U U





TBBA U U U U

PBDD/F

Bromodibenzofuran (2 isomers) U U


Dibromodibenzofuran U U

Dibromodibenzo-p-dioxin U U

oxidizing environment, but not in pyrolytic conditions.

Experiments performed in oxidizing atmosphere up to a final

temperature of 450 8C at 10 K min�1 heating rate confirmed

that PBDD/F are generated during the main decomposition

step. However, mainly mono- and dibromodibenzo-p-dioxins

and dibenzofurans were detected, suggesting a reduced impo-

rtance of the secondary bromination reactions.

3.3. Influence of heating rate and reaction environment on

the products yields in the thermal degradation process

Quantitative data on the products formed in the decom-

position of the brominated materials investigated under the

different experimental conditions explored were achieved from

both TG-FTIR and BR runs. TG-FTIR results were used to


Fig. 5. Products yields obtained in thermal degradation runs carried out on the

materials investigated in the present study. (a) Tetrabromobisphenol A. (b)

Linear brominated epoxy resin. (c) Electronic board A.

obtain data on the quantities of the gaseous compounds formed

in the decomposition process. The use of specific calibrations

for the quantitative analysis of TG-FTIR data, extensively

described elsewhere [51], allowed the evaluation of the overall

yields of gaseous products at the final temperature of the

experimental runs performed (namely 800 8C). Since the

weight of the solid residue at the end of experimental runs was

directly obtained from the TG data, a mass balance allowed the

estimation of the weight of the condensable decomposition

product fraction. The relative distribution of the condensable

decomposition products was obtained from the quantitative GC

analysis performed on the product fractions recovered from BR

runs. Based on the weight of the condensable fraction, the

absolute yield of each product could be evaluated. Fig. 5 shows

the overall yields of gases, condensables and solid residue

obtained in thermal degradation runs performed on the

materials investigated. As expected, a general trend towards

higher gas and char yields at higher heating rates was shown by

all these materials. Moreover, the results reported in Fig. 5 point

out that higher yields of gases were obtained in oxidative

conditions with respect to pyrolytic ones. No organic solid

residue remained at the end of experimental runs carried out in

the presence of oxygen (in the case of the electronic board

sample, the residue was composed of the glass fibre inert

support).

Coming to the yields of specific decomposition products,

Fig. 6(a and b) summarizes the results obtained for TBBA

decomposition under pyrolytic and oxidative conditions,

respectively. The data reported in Fig. 6 allow a direct

comparison of product yields in low and high heating rate runs.

The figure shows the yields of the main gaseous and

condensable products expressed on a weight basis with respect

to sample initial mass.

As reported above, TBBA decomposition results in a

competitive process with evaporation. The results obtained

indicate that the amount of TBBA evaporated accounts for

about 10–20% of TBBA initially present in the experimental

conditions used, and that the evaporative component increases

with increasing heating rate.

The analysis of data reported in Fig. 6 evidences that higher

heating rates result in an increase of the amount of HBr

produced. Quantities as high as 34.6 g/100 g of TBBA initial

sample weight, corresponding to 42 g/100 g of TBBA

decomposed, were formed in ASTM E119 conditions. On

the other hand, lower yields of brominated aromatic

compounds were formed. The influence of oxygen on

hydrogen bromide generation seems limited: similar HBr

yields were obtained in pyrolytic and oxidative conditions.

Appreciable quantities of carbon monoxide and carbon dioxide

were also formed in the decomposition process. However,

carbon oxides production was mainly detected during the

degradation of the primary residue formed in the TBBA main

decomposition stage and, as expected, it increased remarkably

in oxidative conditions.

In accordance with the decomposition mechanisms

previously reported [29], among the TBBA condensable

decomposition products, significant yields of 2- and 4-

bromophenol, 2,4- and 2,6-dibromophenol, 2,4,6-tribromo-

phenol and brominated bisphenol A species were obtained.

The results shown in Fig. 6 point out that higher heating rates

result in a decrease of 4-bromophenol, 2,4-dibromophenol

and 2,4,6-tribromophenol yields, particularly in pyrolytic

conditions, and in a simultaneous increase of the HBr yield.

The bromine radicals generated from brominated bisphenol A

species [29] may abstract hydrogen yielding hydrogen

bromide, as depicted in Scheme 2. However, these species

may also interact with the phenoxy radicals formed from the

scission of the bisphenol A unit [29], leading to para-bromo-


Scheme 3.

Fig. 6. Yields of main gaseous and condensable products formed in the thermal

degradation of tetrabromobisphenol A (wt.% with respect to sample initial

mass). (a) Pyrolytic conditions. (b) Oxidative conditions.

substituted phenols, namely 4-bromophenol, 2,4-dibromo-

phenol and 2,4,6-tribromophenol, as shown in Scheme 3. The

results of the present investigation seem to suggest that the

reaction pathway reported in Scheme 3 is favoured under low

heating rate conditions. It should be remarked that the

bromophenols formed by Scheme 3 are the more likely

precursors of PBDD, as shown in Scheme 1.

Specific data on the quantities of products formed in

different thermal degradation conditions were collected as well

for the other materials investigated in the present study. Figs. 7

Scheme 2.

and 8 show the results obtained for the brominated epoxy resin

and electronic board samples. As in the case of TBBA,

hydrogen bromide was the main gaseous product generated in

the decomposition process. A slight increase of HBr yield with

increasing heating rate was detected in the case of the electronic

board sample, while no influence of the heating rate on the yield

of this product was observed for the brominated epoxy resin

sample. A limited increase in the hydrogen bromide yields in

oxidative conditions was detected for both samples.

High yields of carbon monoxide and carbon dioxide were

produced in experimental runs performed in the presence of

oxygen, due to the oxidation of the primary residue. Carbon

oxides formation was also detected under inert conditions.


degradation of linear brominated epoxy resin (wt.% with respect to sample

initial mass). (a) Pyrolytic conditions. (b) Oxidative conditions.



degradation of electronic board A (wt.% with respect to sample initial mass). (a)

Pyrolytic conditions. (b) Oxidative conditions.

However, while a CO2/CO ratio >10 was observed for the

epoxy resin sample, a ratio <0.1 was measured for the

electronic board. Very low quantities of ammonia were

generated in the thermal decomposition of the electronic

board, due to the degradation of the cross-linking agent. Yields

lower than 0.05 wt.% with respect to sample initial weight were

obtained in all the experimental conditions explored, and these

data were not included in Fig. 8.

Among condensable products, significant yields of phenol,

4-isopropylphenol, brominated phenols, brominated alkylphe-

nols, bisphenol A and brominated bisphenol A species were

produced in the decomposition of both linear epoxy resin and

electronic board. Specific trends in decomposition product

yields were observed with respect to reaction environment and

heating rate. The influence of experimental parameters on

decomposition product yields was found to be similar for epoxy

resin and electronic board.

For both materials a decrease in the yields of alkylphenols

(namely methylphenols, ethylphenols, isopropylphenol and

brominated isopropylphenols) was identified under oxidative

conditions with respect to pyrolytic ones. An oxidation of the

alkyl moieties might explain the observed decrease.

Furthermore, for both samples, while negligible amounts of

para-bromo-substituted phenols were formed in thermal

decomposition runs carried out in inert environment, appreci-

able quantities of 4-bromophenol and 2,4,6-tribromophenol

were detected in oxidative conditions. This might suggest that

oxygen promotes the radical bromination reactions leading to

the formation of such species (Scheme 3).

Increasing the heating rate caused a decrease in the yields of

1-bromo-3-phenoxy-2-propanol, which is likely generated by

the reaction of bromine radicals with ether, or epoxide, groups.

Lower yields of isopropylphenol and brominated isopropyl-

phenols were observed in ASTM E119 conditions. For the

electronic board sample, an overall decrease in the quantities of

phenol species and an overall increase in the quantities of

bisphenol A species could be detected under higher heating

rates, possibly suggesting a reduced importance of scission

reactions. In the thermal decomposition of the epoxy resin, the

yield of 4-(1-methyl-1-phenylethyl)phenol increased remark-

ably in ASTM E119 conditions.

3.4. Effects of the thermal degradation conditions on

bromine distribution among the different product fractions

The experimental data obtained allowed an estimation of

bromine distribution among the different fractions of thermal

decomposition products for the materials considered in the

present study. The amount of bromine initially present

evolved in the gaseous fraction was directly evaluated from

the quantitative TG-FTIR data on HBr generation, taking

into account that molecular bromine was never detected in

the experimental runs performed. Elemental analysis carried

out on the solid residues recovered at the end of BR runs

yielded the bromine content of the residue. A mass balance

on the recovered product fractions allowed the evaluation of

the distribution of bromine among the different fractions.

Fig. 9 summarizes the results obtained for the investigated

materials in the different experimental conditions explored.

The results reported in the figure clearly point out that

bromine is mainly evolved in the gaseous fraction for all the

materials and the experimental conditions considered. As a

matter of fact, quantities higher than 50% with respect to

bromine initially present in the samples were released as

gaseous hydrogen bromide. Significant quantities of bromine

were released as well in the condensable fraction of

decomposition products as high molecular weight organo-

brominated compounds. On the other hand, the bromine in

the solid residue accounts for less than 5% of bromine

initially present in the samples. For the TBBA and electronic

board samples, increasing heating rate resulted in an increase

of the fraction of bromine evolved in gaseous products and in

a decrease of the fraction of bromine present in conden-

sables, both in pyrolytic and oxidative conditions, while a

negligible influence of the heating rate was detected for the

linear brominated resin. The presence of oxygen caused an

increase in the amount of bromine released in the gaseous

product fraction, especially for the linear resin and electronic

board samples, with respect to pyrolytic ones.


Fig. 9. Bromine distribution in the different product fractions obtained in

thermal degradation runs carried out on the materials investigated in the present

study (wt.% with respect to bromine initially present in the sample). (a)

Tetrabromobisphenol A. (b) Linear brominated epoxy resin. (c) Electronic

board A.

4. Conclusions

The influence of the heating rate and of the presence of

oxygen on the thermal decomposition pathways of the organic

fraction of electronic boards obtained from cross-linked epoxy

resins containing tetrabromobisphenol A was investigated. The

influence of experimental conditions on product yields was

assessed and compared to the results obtained from the thermal

degradation of linear brominated resins and of pure TBBA.

Several organobrominated compounds were formed during the

thermal degradation of the cross-linked resins in pyrolytic as

well as in oxidizing conditions. Many of the brominated

decomposition products were formed as well in the thermal

degradation of the linear epoxy resin and of pure TBBA,

suggesting that the final product yields are mostly dependent on

the gas-phase radical reactions that follow the primary

decomposition of the substrate.

The experimental data on the quantitative yields of the

decomposition products also shed some light on the influence of

the heating rate on the compounds obtained from the

decomposition process. Bromine is mainly evolved in the

gaseous fraction for all the materials and the experimental

conditions considered, although significant quantities are present

as well in the condensable fraction. However, the results suggest

that higher heating rates cause an increase of bromine evolved as

gaseous hydrogen bromide and a corresponding decrease of that

contributing to the formation of high molecular weight

organobrominated compounds.

In the presence of oxygen, polybrominated dibenzo-p-

dioxins and polybrominated dibenzofurans were detected

among the decomposition products. The data on the decom-

position products formed also allowed the identification of the

possible precursors and suggested the likely radical reaction

pathways leading to the formation of these compounds. Thus,

the results obtained confirm that the formation of PBDD and

PBDF may be expected from the combustion of electronic scrap

and of materials containing BFRs.

Acknowledgement

Financial support from MIUR under the PRIN 2004 research

program titled ‘‘Sustainability of processes for the disposal of

materials containing brominated flame retardants’’ is gratefully

acknowledged.

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