Thermal decomposition of flame-retarded high-impact polystyrene

Thermal decomposition of flame-retarded high-impact polystyrene

E. Jakab *, Md.A. Uddin, T. Bhaskar, Y. Sakata

Faculty of Engineering, Department of Applied Chemistry, Okayama University, 3-1-1 Tsushima-Naka,

Okayama, Japan

Received 30 November 2002; accepted 18 February 2003

Abstract

The thermal decomposition of four high-impact polystyrene (HIPS) samples containing

brominated flame retardants has been studied. Decabromodiphenyl ether (Br10-DPE) and

decabromodibenzyl (Br10-DB) were used as flame retardants and two samples contained

antimony trioxide (Sb2O3) synergist besides the brominated additives. The thermal decom-

position of HIPS samples was studied by thermogravimetry/mass spectrometry (TG/MS),

pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and pyrolysis-mass spectro-

metry (Py-MS). It was established that the brominated additives themselves do not change the

decomposition temperature of polystyrene (PS). However, Sb2O3 reduces the thermal stability

of the samples indicating that Sb2O3 initiates the decomposition of the flame retardants and

PS. Water and styrene products were detected during the first stage of decomposition from

HIPS samples containing Sb2O3. Nevertheless, the majority of PS decomposes at a higher

temperature. The two brominated flame retardants decompose by different pathways. The

scission of C�/C bonds, resulting in the formation of bromotoluenes, is the most important

reaction of Br10-DB additives. In contrast, Br10-DPE decomposes by an intermolecular ring

closure pathway producing brominated dibenzofurans (DBF).

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: High-impact polystyrene; HIPS; Brominated flame retardant; Sb2O3; Thermal decomposition;

Pyrolysis; Thermogravimetry

* Corresponding author. Present address: Research Laboratory of Materials and Environmental

Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest,

Hungary. Tel.: �/36-1-325-7760x381; fax: �/36-1-325-7892.

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

J. Anal. Appl. Pyrolysis 68�/69 (2003) 83�/99

www.elsevier.com/locate/jaap

0165-2370/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0165-2370(03)00075-5

mailto:[email protected]

1. Introduction

High-impact polystyrene (HIPS) is widely used for electrical appliances, electronic

instruments and building materials. HIPS is a composite material consisting of a PS

phase and a dispersed polybutadiene (PB) rubber phase. Polybrominated com-

pounds and antimony trioxide (Sb2O3) are synergistic flame retardant combinations

that are frequently added to HIPS [1]. Polybrominated flame retardants are

somewhat thermally labile and release Br+ radicals that quench the radical chain

reactions of combustion and fire spreading processes. The synergistic effect of Sb2O3

on the flame-quenching efficiency of organic halides is well-known [2,3], since Sb2O3

increases the rate of halogen release from aromatic halides via the formation of

antimony halides and oxyhalides during combustion.

The oxidative degradation behavior of brominated flame retardants is of interest

since flame-retarded plastics may be disposed of in municipal waste incinerators or

may be involved in accidental fires. It is well known that polybrominated diphenyl

ethers (DPE) may produce brominated dioxins and dibenzofurans (DBF) during

combustion [4]. The formation of polybrominated DBFs from decabromodiphenyl

ether (Br10-DPE) in a polybutylene�/terephthalate�/Sb2O3 matrix has been studied in

air by Dumler et al. [5]. They observed that high-temperature degradation (500�/

700 8C) results in high yield of tetrabromodibenzofurans. During the combustion of

HIPS�/Br10-DPE�/Sb2O3 composite, various brominated benzene, toluene and

styrene derivatives have also been identified [6].It has been demonstrated that various analytical pyrolysis techniques are suitable

for the analysis of brominated organic products from flame-retarded polymers [7�/

10]. Thermodesorption-GC/MS was applied for the analysis of halogenated products

from flame-retarded polymers modeling the evolution of low-molecular weight

substances from TV-sets and computers [11]. The photodegradation study of flame-

retarded polystyrene (PS) has demonstrated that brominated flame retardants reduce

the photostability of PS [12].

Recently, the degradation of flame-retarded plastics got into the focus of interest

due to the increasing role of recycling of plastic waste. Dehydrohalogenation of

brominated flame retardants has been studied by thermal and catalytic degradation

[13,14]. Br10-DPE flame retardant might release potentially toxic polybrominated

DBFs and are, therefore, increasingly substituted by other brominated flame

retardants. Although some manufacturers have begun to reduce the production of

brominated DPEs in the last decade, they are still widely produced. Furthermore,

plastic waste has increasing amounts of HIPS containing brominated flame

retardants. Material recycling of flame-retarded polymers may result in the

formation of brominated dioxin and DBF derivatives [15]. On the other hand,

Hamm et al. [16] established that HIPS�/Sb2O3�/Br10-DPE blend can be mechani-

cally recycled at least five consecutive times without debromination of the flame

retardants. Polybrominated DPEs may be a significant environmental challenge in

the future because the levels of these compounds appear to be rising in human tissues

[17].

E. Jakab et al. / J. Anal. Appl. Pyrolysis 68�/69 (2003) 83�/9984

The pyrolysis of HIPS�/Br10-DPE�/Sb2O3 blend has been studied [7,18] and several

brominated products were identified [7]. Flammability tests and thermolysis

experiments revealed [18,19] that polymeric-free radicals generated during the

burning process lower the halogen release temperature in HIPS�/Br10-DPE�/Sb2O3

blend. The initiation of this condensed phase reaction has been attributed to PB

radicals, although the decomposition temperature of PS is lower than that of PB [20].

Price et al. [21] studied the thermal decomposition of PS compounded with Br10-DPE, Sb2O3 and Zn-borate by mass spectrometric evolved gas analysis. They

obtained negative activation energy for styrene evolution, which was explained in

terms of a condensed phase reaction between Br10-DPE and polymer radicals.

The thermal decomposition of HIPS containing Br10-DPE and Sb2O3 flame

retardant system has been studied previously, as discussed above [7,18]. However,

reference samples were not used; hence the decomposition mechanism was not

understood completely. Furthermore, the thermal decomposition of HIPS flame

retarded with decabromodibenzyl (Br10-DB) and Sb2O3 has not been studiedpreviously, although it is used recently in electrical appliances. Therefore, we studied

the decomposition of four samples with two kinds of brominated flame retardants

and two samples contained Sb2O3 besides the flame retardants. The objective was to

clarify the effect of flame retardant formulations on the thermal decomposition

mechanisms of HIPS blends. We studied the thermal decomposition by three

analytical techniques: thermogravimetry/mass spectrometry (TG/MS), pyrolysis-gas

chromatography/mass spectrometry and pyrolysis-mass spectrometry (Py-MS).

2. Experimental

2.1. Materials

Four HIPS samples were investigated. Decabromodiphenyl ether (Br10-DPE) and

1,2-bis(pentabromophenyl)ethane (decabromodibenzyl, Br10-DB) were used as flame

retardants. Two samples contained Sb2O3 synergist. All of the samples were

prepared for research purposes by extrusion using the same HIPS raw material

and the above additives. The composition of the samples is summarized in Table 1.

For comparison, pure PS (BASF AG) sample was applied.

2.2. Thermogravimetry/mass spectrometry

TG/MS experiments were carried out on a Shimadzu TGA-51 thermobalance

coupled to a Minilab (LM 80) quadrupole mass spectrometer through a capillarytransfer line heated to 150 8C. Samples of 6�/10 mg were heated in an alumina pan at

a 5 8C min�1 heating rate, while the furnace was flushed with nitrogen of 200 ml

min�1 flow rate. The mass spectrometer operated in electron impact mode at 40 eV

electron energy. The intensities of 12 selected ions were monitored by using electron

multiplier detection.

E. Jakab et al. / J. Anal. Appl. Pyrolysis 68�/69 (2003) 83�/99 85

2.3. Pyrolysis-gas chromatography/mass spectrometry

Pyrolysis experiments were carried out on a Pyroprobe 2000 pyrolyzer (Chemical

Data Systems). About 0.1 mg samples was pyrolyzed at 600 8C (calibrated

temperature) for 20 s in a quartz tube using helium as a carrier gas. Analysis of

the volatile products was accomplished on line with a GC/MS (Agilent Techn., Inc.,

6890 GC/5973 MSD) using HP-5MS capillary column (30 m�/0.25 mm i.d., 0.25 mm

film thickness). The pyrolysis interface and the GC injector were kept at 320 8C. The

GC oven was programmed to hold at 50 8C for 1 min and then increases to 300 8C ata rate of 10 8C min�1 (program 1). For the analysis of the high-molecular mass

brominated products, the following temperature program was used: hold at 50 8Cfor 1 min; heating rate 30 8C min�1 up to 200 8C and heated by 8 8C min�1 up to

320 8C (program 2). The mass spectrometer operated at 70 eV in the EI mode. The

mass range of 33�/800 Da was scanned.

2.4. Pyrolysis-mass spectrometry

About 0.01 mg samples was placed into the quartz tube and pyrolyzed in the mass

spectrometer in vacuum. The samples were heated at 40 8C min�1 up to 520 8C and

kept there for 10 min using the Pyroprobe 2000 instrument. Hewlett-Packard 5985

mass spectrometer was applied at 15 eV electron energy in the EI mode. The mass

range 10�/980 Da was scanned.

3. Results and discussion

3.1. Thermal decomposition of flame-retarded HIPS in vacuum

Figs. 1 and 2 illustrate the results of the temperature-resolved Py-MS experiments

of two HIPS samples (I and III) containing Sb2O3 and brominated flame retardants.

The Py-MS experiments were carried out at a 40 8C min�1 heating rate in vacuum

near to the ion source of the mass spectrometer. Figs. 1 and 2 show the evolution

profiles of selected ranges of ions as well as the mass spectrum belonging to the

Table 1

Composition of the HIPS samples

Sample Flame retardant Synergist

Compound Abbreviation Content (wt.%) Compound Content (wt.%)

I Decabromodiphenyl ether Br10-DPE 13 Sb2O3 5

II Decabromodiphenyl ether Br10-DPE 13 �/ �/

III Decabromodibenzyl Br10-DB 13 Sb2O3 5

IV Decabromodibenzyl Br10-DB 13 �/ �/


maximum of the evolution curve. The thermal behavior of the two samples is

essentially independent from the nature of the brominated additives (Br10-DPE or

Br10-DB). The total ion current curve as a function of temperature/time shows one

major decomposition step in both cases (Figs. 1a and 2a) that is dominated by the PS

decomposition products as the mass spectra (Figs. 1d and 2d) point out (e.g., m /z

104 styrene monomer). The other polymer component, PB, decomposes in a similar

temperature range as PS, but its decomposition products have very low intensity due

to its low concentration.

Fig. 1b illustrates the summed ion intensity curve of the m /z 600�/980 range from

the sample containing Br10-DPE flame retardant. The high-molecular mass products

are released from the sample at low temperature (�/200 8C). The mass spectrum

(Fig. 1e) demonstrates that the brominated flame retardant (m /z 960, Br10-DPE)

evaporates from the sample at low temperature. Brominated dibenzodioxins were

not formed from Br10-DPE by Py-MS.

Br10-diphenyl ethane additive also evaporates from the sample as the ion profile

curve of m /z 970�/980 range illustrated in Fig. 2b. The corresponding mass spectrum

(Fig. 2e) indicates minor molecular ion peaks at around m /z 972. Peaks at m /z 485

and 487 in the mass spectrum can be attributed to the fragment ions of Br10-DB.

They also could be attributed partially to Br5-toluene; however, this compound has

Fig. 1. Temperature-resolved Py-MS experiment of sample I. Panels (a), (b) and (c), evolution profiles of

selected ion ranges and panels (d), (e) and (f), the mass spectra corresponding to the maximum of the

evolution curve a, b and c, respectively.


the major peaks at m /z 486 and 488. Bromine has isotopes of 79 and 81 Da to a

similar ratio (50.5:49.5), thus the mass spectrum of highly brominated compounds

forms ion clusters to a well characteristic ratios. For example, molecules having odd

Br atoms produce two molecular ions at a similar intensity at m /z (M�/1) and (M�/

1), where M denotes the average molecular mass of the compound. On the other

hand, molecules having even number of Br atoms give a major molecular ion at M

and smaller ions at M9/2n (n�/1, 2, . . .).Figs. 1c and 2c shows the ion profile curves for the ion range m /z 580�/588, which

involves the major peaks of Sb4O6 [22]. Sb2O3 synergist does not react with the

brominated compounds in vacuum, but it evaporates from the sample as a dimer

(Sb4O6). It is in agreement with the findings of previous authors [3,22]. Sb4O6 is

released at somewhat higher temperature than the polymer matrix decomposes, but

the mass spectra are still dominated by the PS decomposition products (Figs. 1f and

2f). Nevertheless, the ion groups of Sb4O6 can be clearly identified with the major

peaks at around m /z 584. It is noted that antimony has two major isotopes, 121 and

123 with the ratios of 57.2:42.8.

The decomposition of the other HIPS samples lacking Sb2O3 (II and IV) occurs

similarly in vacuum as that of the above samples except for the absence of the

evaporation peak of the synergist.

Fig. 2. Temperature-resolved Py-MS experiment of sample III. Panels (a), (b) and (c), evolution profiles of

selected ion ranges and panels (d), (e) and (f), the mass spectra corresponding to the maximum of the

evolution curve a, b and c, respectively.


3.2. TG/MS analysis

From the temperature-resolved Py-MS experiments we concluded that in vacuum,

the evaporation of the brominated flame retardants take place at lower temperature

than the decomposition of PS. However, the thermal decomposition occurs

differently in argon under atmospheric pressure as the thermogravimetric curves

of the four samples demonstrate in Fig. 3. The decomposition of the polymer matrix

Fig. 3. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves of samples I and II (a)

as well as III and IV (b).


is only slightly influenced by the quality of the flame retardant (Br10-DPE or Br10-

DB), however, the presence of Sb2O3 significantly changes the decomposition

pattern of the HIPS samples. The flame-retarded HIPS samples without Sb2O3 (II

and IV) decompose in one major step between 380 and 520 8C with a small shoulder

at �/490 8C indicating that the brominated flame retardants evolve at similar

temperatures as the polymer matrix decompose.Fig. 4 shows the evolution profiles of a few products of polymer origin from

sample IV. Styrene monomer and methylstyrene are formed from PS in a single peak

and their evolution coincides with the main DTG peak. High-impact PS contains a

few percent PB. The decomposition products of PB are released at somewhat higher

temperature than styrene as the evolution profile of hexadiene shows. Thus, the

shoulder on the DTG curve at �/490 8C can be attributed to PB decomposition.

Unfortunately, we could not detect brominated products by TG/MS; they were likely

adsorbed in the capillary transfer line due to their less volatility caused by the high-

molecular mass and polarity.

The two HIPS samples containing Sb2O3 additive (I and III) decompose in a quite

different way (Fig. 3). The decomposition undergoes in two distinct regions; the

second temperature region coincides with the decomposition temperature of the

other two samples (II and IV). However, the decomposition of the samples

containing Sb2O3 starts at �/50 8C lower temperature producing a separated

DTG peak. This indicates that Sb2O3 initiates the degradation. It was suggested

previously [7] that SbBr3 and brominated organic compounds are evolved during this

stage of decomposition. Since we could not detect these products by TG/MS we

carried out an indirect experiment. We heated the samples I and III in the

thermobalance up to 420 8C at 5 8C min�1 to remove the components that evolve

in the first stage. Then, we accomplished temperature-resolved Py-MS experiments

Fig. 4. TG, DTG curves and the evolution profiles of a few products from sample IV.


on the heat-treated samples and we were able to detect neither brominated products

nor Sb4O6; PS and PB products were only detected. The char residue is about 5%

from samples II and IV and it is 6% from the samples containing Sb2O3. The small

difference between the char residues also indicates that Sb2O3 (5%) does not remain

in the solid residue, but it is released during the decomposition.

Samples I and III contain 18% flame retardant additives including Sb2O3.

However, the mass loss is about 35% during the first stage of decomposition

suggesting that other products are also evolved besides the additives. Fig. 5 shows

the TG/MS plot of sample III that contains Br10-DB and Sb2O3. Water and styrene

formation was monitored during the low temperature decomposition centered

around 370 8C. Since this sample contained oxygen only in the Sb2O3, water must be

formed from Sb2O3. It is important that the O-atoms produce water and do not form

brominated dibenzodioxins with the brominated dibenzyl or DPE. The evolution of

water coincides with the low temperature DTG peak confirming the decomposition

of Sb2O3. The samples containing no Sb2O3 produce practically no water as Fig. 4

demonstrates. The TG/MS plot clearly proves that the PS decomposition commences

at �/330 8C and the rate of styrene evolution decreases above 370 8C. Later the

decomposition of PS continues producing another maximum on the evolution curve.

The double peak of PS decomposition indicates two kinds of mechanisms.

Scheme 1 summarizes the suggested reactions occurring during the first stage of

decomposition. The formation of SbBr3 and H2O is accompanied by H-abstraction

from the polymer chains and by the partial debromination of the flame retardants.

Other reactions of the flame retardants will be discussed later. H-abstraction may

take place mainly from the PS chains. The polymer radicals left behind undergo b-

Fig. 5. TG, DTG curves and the evolution profiles of a few products from sample III.


scission producing a polyene-ended macromolecule and a secondary radical. The

secondary radical reacts similarly as during the free radical chain reaction of PS

decomposition [23] leading to monomer and oligomer formation. The rate of this

type of PS decomposition decreases when less PS macroradical is formed by the

reaction (a). The chain cleavage of neutral PS molecules requires higher energy,

hence higher temperature. Thus, the two decomposition stages of PS are separatedby TG/MS. It should be noted that H-abstraction might occur from the PB, too. The

small alkadiene peak at low temperature might be an indication of the decomposi-

tion of PB (Fig. 5). Otherwise, PB decomposes at slightly higher temperature than PS

as indicated by the evolution curve of hexadiene.

3.3. Py-GC/MS study

Pyrolysis experiments were carried out at 600 8C for 20 s in order to ensure

relatively high degree of decomposition of the entire samples. Fig. 6 shows thepyrograms of the HIPS samples in comparison with the pyrogram of a pure PS

sample. As it is well-known [23], PS decomposes mostly by depolymerization

resulting in styrene monomer and smaller amounts of oligomers are formed by

radical transfer and b-scission reactions. The product distribution of the HIPS

samples is rather different from that of PS sample. Price et al. [21] observed earlier

Scheme 1.


Fig. 6. Pyrolysis total ion chromatograms of the HIPS samples in comparison with PS applying 600 8C, 20

s pyrolysis and GC temperature program 1. The pyrograms are normalized for the 1/4 height of the styrene

monomer.


that 10�/15% less monomer was evolved from PS�/Br10-DPE composite than from

PS, which was explained by the formation of a complex between polymer radical and

Br10-DPE in the condensed phase. As the pyrograms in Fig. 6 show the product

distribution of PS changed significantly in the HIPS samples. The intensity of

monomer, dimer and trimer is smaller, whereas the yield of toluene, ethyl benzene

and a-methyl styrene is significantly higher. Similar changes in the product

distribution of PS were observed in the presence of carbon black additive [24] andwood char [25]. Moreover, 2-phenylnaphthalene is evolved in similar amount as the

dimer. This compound may be derived from a dimer segment by cyclization and

strong dehydrogenation of the alkyl moiety. Similar mechanism might produce the

1,3,5-triphenylbenzene compound from a trimer segment. This compound has higher

intensity from the samples containing Sb2O3. As Scheme 1 illustrates, H-abstraction

from the polymer chains by Sb2O3 and the scission of polymer chains introduce

double bonds into the polymer. Debromination reactions of the flame retardants

also require H+ radicals that are only available on the polymer chains. Thus,unsaturated polymer chain segments are certainly prone to form condensed aromatic

and triphenylbenzene products. The formation of 5�/6% char residue also confirms

the occurrence of dehydrogenation and carbonization reactions. The pyrograms of

samples I and III (Fig. 6b and d) as well as that of samples II and IV (Fig. 6c and e)

resemble each other indicating that the quality of the brominated flame retardant

does not have much influence on the decomposition of the polymer component.

However, the presence of Sb2O3 involves more dehydrogenation reactions as

discussed above.Very small amount of aliphatic products, mainly alkadienes are released from the

PB component of the HIPS samples as indicated by the butadiene peak in Fig. 6. The

total intensity of the aliphatic products is only about 1%.

Unfortunately, Py-GC/MS turned out to be unsuitable for the detection of SbBr3

and HBr evolved from the HIPS samples probably due to adsorption in the interface

of the pyrolyzer. Among the small molecules, methyl bromide was detected in very

small amount from each sample. Nevertheless, the evolution of several brominated

aromatic compounds was monitored. Fig. 7 shows the mass spectra of threecharacteristic brominated products. Br6-DBF represents one of the most significant

brominated aromatic products from sample I containing Br10-DPE and Sb2O3 flame

retardant formulations. This compound is marked with A in the pyrogram (Fig. 6b).

The mass spectrum of Br6-DBF (Fig. 7a) presents very characteristic clusters of

peaks due to various isotope combinations of bromine atoms. The molecular ions

are the major peaks, and the other peak clusters can be deduced with bromine and

CO loss. It is noted that CO or HCO loss is characteristic for the mass spectrum of

DBF compounds [26]. Br10-DB containing samples (III and IV) produce relativelyhigh yield of brominated toluenes. Br5-toluene is formed from Br10-DB by scission of

the aliphatic C�/C bond and H-uptake. Br5-toluene is marked with B in the

pyrogram (Fig. 6e) and Fig. 7b shows its mass spectrum, which is, dominated by the

molecular ion cluster and the various debrominated ion species. Symbol C marks a

brominated compound in the pyrogram of sample IV that is produced in relatively

significant yield. The mass spectrum (Fig. 7c) indicates that it has four bromine


atoms. It is tentatively identified as tetrabromophenyl-1-phenylpropene. Its forma-

tion can be explained by the reaction of tetrabromotoluyl radicals with styrene

moieties. Its intensity represents about 0.8% in the total ion chromatogram.Br5-toluene represents about 1% (area% in TIC) from sample IV. As seen in the

selected ion chromatograms of brominated toluenes (Fig. 8), somewhat less Br4-

toluene is formed and the intensity of Br3-toluene is an order of magnitude smaller.

Br2- and Br-toluenes were not detected. It indicates that the degree of debromination

of Br10-DB in the absence of Sb2O3 is small. Smaller amounts of brominated

toluenes are released from the sample containing Sb2O3 together with Br10-DB;

apparently, the majority of the Br-content is released as SbBr3 in this case. Assuming

that all Sb2O3 forms SbBr3, the samples I and III have still about 30% excess of

bromine content. Br10-DB decomposes mainly by forming brominated toluenes;

besides traces of Br�/benzene were observed. It should be noted that evaporation of

Br10-DB is also possible; however, it is not GC amenable under the conditions

applied.

Fig. 7. Mass spectra of (a) hexabromodibenzofuran (compound A), (b) pentabromotoluene (compound

B) and (c) tetrabromophenyl-1-phenylpropene (compound C). A, B and C refer to peaks in the pyrograms

(Fig. 6).


The other flame retardant, Br10-DPE is also prone to evaporation [7], however, it

was not possible to monitor it with this instrument. Partially debrominated species of

DPE were found in the pyrogram as it is illustrated in the selected ion

chromatograms of sample II in Fig. 9. It is noted that the mass spectrometer is

able to detect ions up to 800 Da, thus fragment ion (m /z 721) of Br9-DPE was

depicted in Fig. 9. Furthermore, a different temperature program was applied for the

analysis of the brominated products; hence the retention times in the selected ion

chromatograms cannot be compared with that of the total ion chromatograms in

Fig. 6. Brominated DPEs containing nine and eight bromine atoms are the major

DPE compounds and small amount of Br7-DPE is also evolved. Less brominated

DPE products were not found. It should be noted that Br10-DPE may contain a few

percent nanobromo- and octabromo-DPE impurities, hence these compounds might

be evaporation products, too. Highly debrominated species form DBF derivatives as

the ion profiles demonstrate. Br6-DBF is the major DBF product followed by Br5-

DBF. Small amounts of Br7- and Br4-DBF are also released. Br8-DBF was not

formed that is in agreement with the results on Br10-DPE degradation in air [5].

Similar brominated products are evolved from sample I containing Sb2O3 besides

Br10-DPE, however the intensity ratios are different. Lesser amounts of brominated

DPEs were released and more highly debrominated DBF compounds were detected.

All Brx-DBF were formed ranging from mono-DBF to hepta-DBF; Br6- and Br5-

DBF having the highest yield. Apparently, bromine radicals react with antimony

atoms leading to SbBr3 formation. The scission of the ether bond of Br10-DPE also

Fig. 8. Selected ion chromatograms of polybrominated toluenes evolved from sample IV. The monitored

ions represent one of the molecular ions of the compounds. (GC temperature program 1).


Fig. 9. Selected ion chromatograms of polybrominated diphenyl ethers (Brx -DPE) and polybrominated

DBFs (Bry -DBF) from sample II. The monitored ions represent one of the molecular ions of the

compounds with the exception of Br9-DPE where fragment ion m /z 721 was detected. GC temperature

program 2 was applied.


occurs; however, bromobenzenes of various degree of substitution are formed to a

lesser degree during thermal decomposition.

4. Conclusions

The thermal decomposition of HIPS samples is affected by the presence of flameretardant additives. The TG/MS study shows that the brominated flame retardants

(Br10-DPE and Br10-DB) decompose at the same temperature range as HIPS in the

absence of Sb2O3. However, the product distribution of HIPS is highly influenced by

the presence of additives: less styrene monomer, dimer and trimer are evolved and

the formation of various other products (e.g., 2-phenylnaphthalene and 1,3,5-

triphenylbenzene) is enhanced.

The presence of Sb2O3 synergist alters the thermal decomposition of HIPS

considerably indicating that Sb2O3 initiates the decomposition of brominated flameretardants and PS. The decomposition shifts to lower temperature by about 50 8Cand the DTG curves split into two major peaks. The brominated flame retardants

and Sb2O3 decompose during the first step and the partial decomposition of PS

occurs initiated by the H-abstraction by the additives. The decomposition of the

majority of PS takes place in a separate step at higher temperature because the

scission of the neutral polymer chains requires higher temperature than the cleavage

of the PS macroradicals. Water evolution was detected from Sb2O3 during the first

decomposition stage indicating that the oxygen of Sb2O3 does not react with theorganic molecules. It was established that the brominated additives and Sb2O3 could

be removed from the HIPS samples by heating up to 420 8C using 5 8C min�1

heating rate.

The two brominated flame retardants degrade differently during pyrolysis,

although partial debromination of both compounds occurs. The debromination

reactions are obviously more pronounced in the presence of Sb2O3 leading to SbBr3

formation. However, the excess of flame retardants undergoes similar reactions as in

the absence of Sb2O3. Br10-DB decomposes mainly by the cleavage of the aliphaticC�/C bonds accompanied by the evolution of Br5-toluene. Less brominated toluenes

are also released. Brominated benzyl radicals react with the styrene moieties and

significant amount of tetrabromophenyl-1-phenylpropene was detected by Py-GC/

MS.

The ether bond in B10-DPE is more stable than the aliphatic C�/C bridge in Br10-

DB. Only traces of bromobenzenes were measured during the decomposition of

samples I and II. Previous results on HIPS�/Br10-DPE�/Sb2O3 composite established

the formation of brominated DPEs and DBF derivatives [7], which is confirmed byour results. Partially debrominated DPEs were observed in the pyrogram. The

strongly debrominated species undergo ring closure and form DBF derivatives. Br6-

and Br5-DBFs are the major brominated aromatic compounds found in the

pyrolyzate of samples I and II at 600 8C pyrolysis temperature. However, sample I

(containing Sb2O3) produces less brominated DBFs, even monobromodibenzofuran.


Acknowledgements

The authors gratefully acknowledge Venture Business Laboratory of the

Okayama University for inviting E. Jakab to carry out this cooperative research

in Japan. Research conducted in Hungary was supported by the Hungarian National

Research Fund (OTKA No. T037704). The authors thank Dr. Marianne Blazso for

the valuable advice in pyrolysis experiments.

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Thermal decomposition of flame-retarded high-impact polystyrene

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