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Thermal degradation of polystyrene composites. Part I. The effect of brominated polyepoxy and antimony oxide
Journal of Analytical and Applied Pyrolysis 105 (2014) 301–308
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Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
Thermal degradation of polystyrene composites. Part I. The effect of brominated polyepoxy and antimony oxide Hatice Kaya a,1 , Jale Hacaloglu a,b,∗ a b
Middle East Technical University, Department of Polymer Science & Technology, TR-06800 Ankara, Turkey Middle East Technical University, Department of Chemistry, TR-06800 Ankara, Turkey
a r t i c l e
i n f o
Article history: Received 12 July 2013 Accepted 20 November 2013 Available online 28 November 2013 Keywords: Polystyrene Brominated polyepoxy Antimony oxide Thermal degradation Direct pyrolysis mass spectrometry
a b s t r a c t Thermal degradation of polystyrene (PS) involving brominated polyepoxy (BA) and antimony oxide (PS/BE/Sb2 O3 ) was studied systematically via direct pyrolysis mass spectrometry. Thermal decomposition of brominated polyepoxy was started by loss of end groups. The relative yields of high mass thermal degradation products of PS and the product distribution of brominated polyepoxy and antimony oxide were changed noticeably during the pyrolysis of PS/BE/Sb2 O3 composite. Its thermal decomposition was initiated by the interactions of Sb2 O3 with BE, and shifted to lower temperatures. Interactions of flame retardants with PS caused degradation of PS at lower temperatures to a certain extent. Loss of HBr and H2 O, in the temperature region where main PS decomposition took place, confirmed chemical interactions between the flame retardants and PS. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Brominated flame retardants being compatible and having small influence on mechanical properties have been used in polymer industry widely to reduce fire damage and harm. The most common brominated flame retardants are polybrominated biphenyls, polybrominated diphenyl oxides/ethers, tetrabromo bisphenol-A and polybrominated epoxy resins. However, evolution of toxic brominated compounds such as hydrogen bromide, polybrominated benzofurans and dibenzo-p-dioxins, during recycling, incineration, or in fires of such materials, provoked strong limitations in their use. Several studies were focused on the thermal degradation products and the distribution of bromine among thermal degradation products of polystyrene and high-impact polystyrene involving brominated flame retardants or their combinations with antimony oxide [1–9]. The synergistic effect of Sb2 O3 on the flame-quenching efficiency of organic halides by increasing the releasing rate of halogens from aromatic halides via the formation of antimony halides
∗ Corresponding author at: Middle East Technical University, Department of Polymer Science & Technology, TR-06800 Ankara, Turkey. Tel.: +90 312 210 5148; fax: +90 312 210 3200. E-mail addresses: [email protected] (H. Kaya), [email protected] (J. Hacaloglu). 1 Tel.: +90 312 210 3198; fax: +90 312 210 3200. 0165-2370/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jaap.2013.11.017
and oxyhalides during combustion enhancing the effectiveness of the brominated flame retardant is well-known [1,6–15]. Luijk et al. studied the thermal stability and the thermal degradation products of high impact polystyrene/decabromodiphenylether/antimony oxide (HIPS FR) in situ with derivative thermogravimetry (DTG), temperature resolved pyrolysis-mass spectrometry (Py-MS) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and detected antimony (oxy)bromides and brominated higher styrene oligomers [1]. Recently, Grause et al. studied the thermal decomposition of flame retardant free high-impact polystyrene (HIPS) and HIPS samples containing brominated flame retardants and antimony trioxide (Sb2 O3 ) using TGA at different heating rates and determined that the addition of a flame retardant and Sb2 O3 reduced the activation energy [8]. It has been determined that high molecular weight brominated flame retardants are more favorable in terms of reduced toxicity, thermal stability, melt flow characteristics, and mechanical properties compared with conventional non-oligomeric counterparts [16]. In the present study, we focused on the analysis of the thermal degradation characteristics and products of polystyrenes containing tribromophenol end-capped brominated epoxy oligomers with or without Sb2 O3 . The influence of the presence of each component on thermal degradation behavior of the other was determined systematically for the first time in the literature via direct pyrolysis mass spectrometry. Our next study will focus on thermal degradation behavior of PS/organoclay nanocomposites
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containing tribromophenol end-capped oligomers with or without Sb2 O3 .
brominated
epoxy
2. Experimental 2.1. Materials PS (Mw ∼192,000 g/mol) was obtained from Aldrich (St. Louis, MO, USA). The brominated flame retardant was a tribromophenol end-capped oligomer, BE (Mw 15,000 g/mol, ∼53 wt% Br), obtained from ICL Industrial Products (Beersheba, Israel). Sb2 O3 (99.9%) was an industrial-grade product obtained from a Chinese source. Materials were prepared via an ultrasound-assisted solution intercalation technique described in detail in a previous work [16]. Both of the samples PS/BE and PS/BE/Sb2 O3 contained 20 wt% BE. 3 wt% of Sb2 O3 were added to PS/BE. 2.2. Characterization Direct pyrolysis mass spectrometry, DP-MS, analyses were performed by Waters Micromass Quattro Micro MS/MS/MS Mass Spectrometer with a mass range of 10–1500 coupled to a direct insertion probe. During the pyrolysis, the temperature was increased to 50 ◦ C at a rate of 5 ◦ C min−1 , then, was raised to 650 ◦ C with a rate of 10 ◦ C min−1 and kept at 650 ◦ C for 5 additional minutes. 0.01 mg samples were pyrolyzed in the flared quartz sample vials while recording 70 eV EI mass spectra at a mass scan rate of 1 scan s−1 . The analyses were repeated several times to ensure reproducibility. Each time, almost exactly the same trends were detected. 20 eV EI mass spectra were dominated with identical peaks showing increased relative intensities for high mass fragments, yet, reproducibility of the relative ion yields diminished significantly. Thus, for the analyses the reproducible 70 eV EI mass spectra were used. The interpretation of fragments involving Br and/or Sb atoms were done with the use of isotopic peaks, as the corresponding series of peaks follow the Pascal’s triangle rule. Collision induced dissociation experiments were also performed for a reliable interpretation. Taking into account further dissociation of pyrolysis products during ionization, the analysis of the spectral data was achieved, with the use of NIST MS library. In collision induced dissociation (CID) experiments, daughter ions were produced by the collision of the selected particular ion with argon in the collision cell, the second quadruple. The CID spectra were obtained by scanning of the third quadruple mass analyzer. 3. Results and discussions Direct pyrolysis mass spectra of polymers are usually very complex, as thermal degradation products further dissociate in the mass spectrometer during ionization and all fragments with the same mass to charge ratio make contributions to the intensities of the same peaks in the mass spectrum [17]. The pyrolysis mass spectra of polymer composites, especially those involving similar units, are even more complicated. In order to investigate thermal degradation characteristics of PS samples containing BE with or without Sb2 O3 , DP-MS analysis of each component was performed. The trends in the single ion evolution profiles of diagnostic thermal degradation products of each component are used to determine the source of the product and/or the mechanism of thermal degradation. All ions with identical evolution profiles are grouped and analyzed separately. Among these, the fragment with highest mass may be assumed to be generated during thermal degradation. On the other hand, the low mass fragments having similar evolution profiles may be generated either during thermal degradation or
Fig. 1. (a) The TIC curve, (b) the pyrolysis mass spectrum at the peak maximum and (c) the single ion evolution profiles of some selected products generated by pyrolysis and/or dissociative ionization of thermal degradation products of PS.
during ionization in the mass spectrometer. Thus, in some cases it is almost impossible to determine whether a given fragment is generated during pyrolysis or dissociative ionization process. However, as the main purpose of this study is to investigate the influence of the presence of each component on thermal degradation behavior of the other by comparing the differences generated in the pyrolysis mass spectra of PS in the presence of BE, Sb2 O3 , and BE/Sb2 O3 the generation of fragments during pyrolysis or ionization is not of basic interest. 3.1. Polystyrene PS degrades in a single step by a depolymerization reaction yielding mainly styrene [18–23]. In Fig. 1, the total ion current (TIC) curve, variation of total ion yield as a function of temperature, the mass spectrum at the peak maximum and the single ion evolution profiles of some characteristic fragments generated during pyrolysis and/or ionization such as C6 H6 (78), C7 H7 (91), C7 H8 (92), styrene CH2 CHC6 H5 (104), CH2 CH2 C6 H5 (105), C3 H2 C6 H5 (115), C4 H4 C6 H5 (129) and CH2 C(C6 H5 )CH2 CHC6 H5 (207) are presented. The evolution profiles of all these fragments followed identical paths showing maxima at 441 ◦ C. 3.2. Brominated epoxy oligomer, BE The TIC curve of brominated oligomer showed a single peak with a maximum at around 407 ◦ C (Fig. 2). The most intense peak in the pyrolysis mass spectrum recorded at 407 ◦ C is at 43 due to CH3 CO fragment. Tetrabromo bisphenol A and HBr were determined to be the major decomposition products in accordance with the literature results [2,3]. Strong evidences for the
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also detected at around 312 ◦ C. Thus, it can be concluded that thermal decomposition of the epoxy oligomer started by loss of end groups (Scheme 1a). The evolution of tetrabromobisphenol A and other fragments involving bisphenol A units occurred at slightly higher temperatures together with the fragments involving the epoxy linkages. Among the products involving bisphenol A, tetrabromobisphenol A was the major product. Its generation may be explained by the mechanism proposed by Blazso et al. as shown in Scheme 1b [2]. However, although, Blazso et al. detected the evolution of hydrogen bromide and tetrabromobisphenol in the same temperature region under the high vacuum conditions, in the present study, elimination of HBr was detected at slightly higher temperatures, together with minor amounts of bromomethane, bromoacetone and tribromobisphenol A, pointing out the attack of HBr to epoxy units even under high vacuum conditions (Scheme 1c). Actually, as HBr peak is not present in the EI mass spectrum of tetrabromobisphenol A and tribromophenol, it should necessarily be formed during the thermal degradation of the epoxy oligomers. Furthermore, detection of high mass fragments such as C3 H5 O2 (Br2 )C6 H2 CCH3 C6 H2 (Br2 )C3 H5 O2 (637–645) and C3 OH4 O(Br2 )C6 H2 C(CH3 )2 C6 H2 (Br2 )OC3 OH6 OC6 H2 Br2 (888–890) revealed that random cleavages along the main chain also took place as an opposing thermal degradation pathway. 3.3. Antimony oxide Sb2 O3 The TIC curve, the mass spectrum recorded at 495 ◦ C, at the peak maximum, and the evolution profiles of intense and/or characteristic products recorded during the heating of Sb2 O3 are given in Fig. 3. Several oxides were differentiated contrary to our expectations. Besides the starting compound Sb2 O2 , Sb2 O3 , Sb3 Ox , where x = 1–5, Sb4 Ox , where x = 3 or 7, Sb5 Ox , where x = 2 or 7 and Sb6 Ox where x = 3 or 9 and Sb oligomers, Sbx for x = 1–4, were also identified. The relative yield of Sb2 O3 (290–294) was quite weak. The base peak was due to SbO (290–294). Sb4 O6 (580–588), Sb3 O4 (427–433) and Sb2 O2 (274–278) were the other abundant products. All products followed identical evolution profiles and reached maximum yield at around 495 ◦ C. Evaporation of Sb4 O6 was in agreement with previous literature findings [3,24]. Yet, to our knowledge antimony oxides involving more than four Sb atoms was not detected before. 3.4. PS/BE
Fig. 2. (a) The TIC curve, (b) the pyrolysis mass spectrum at the peak maximum and (c) the single ion evolution profiles of some selected products generated by pyrolysis and/or dissociative ionization processes during the pyrolysis of BE.
generation of tribromophenol, bromoacetone and acetone were obtained. Evolution profiles of degradation products showed slight differences indicating degradation through different reaction pathways (Fig. 2c). The single ion evolution profile of tribromophenol (328–334) showed two peaks with maxima at around 312 and 392 ◦ C. Products due to the decomposition of epoxy linkages such as CH3 CO or CH2 COH (43) CH3 COCH2 or CH2 COHCH2 (57) and CH3 COCH3 or CH2 COHCH3 (58) and tetrabromo bisphenol A (540–548) and all the fragments involving bisphenol A units reached maximum yield at around 398 ◦ C. On the other hand, the evolution of HBr (80–82), CH3 Br (94–96) and CH3 COCH2 Br (136–138) were maximized at slightly higher temperatures, at around 403 ◦ C. Besides tribromophenol, evolution of CH3 COCH2 or CH2 C(OH)CH2 (57) was
In a recent study, it has been determined that BE, as the dispersed phase in PS/BE blends, acts in a way to reduce the peak heat release rate by ∼35% and increase the peak mass loss rate by ∼31% with respect to neat PS [17]. The ratio of total heat evolved to total mass loss, the product of combustion efficiency and effective heat of combustion of volatiles, was decreased from 3.0 MJ m−2 g−1 to 1.6 MJ m−2 g−1 with the incorporation of BE into PS. In addition, the limiting oxygen index, LOI increased from 18% to 21%. It has been determined that BE alters the degradation and combustion behavior in a manner that accelerates dripping and subsequent self-extinguishment of PS, which provides a UL-94 V-2 rating. In the presence of brominated epoxy (20 wt%), pyrolysis of PS yielded a TIC curve with a maximum at around 448 ◦ C (Fig. 4a). The spectra recorded at around these temperatures were dominated with characteristic peaks of PS. Those of BE can also be detected upon expansion of the spectra recorded (Fig. 4b). Single ion evolution profiles of characteristic degradation products of PS followed identical trends (Fig. 5). However, the relative yields of high mass thermal degradation products increased noticeably. Furthermore, though thermal degradation of PS chains started almost at around the same temperature ranges, evolution profiles were broadened indicating that thermal degradation was continued at higher temperatures compared to neat PS.
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Scheme 1. Thermal degradation pathways of brominated epoxy oligomer.
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Fig. 4. (a) The TIC curve and (b) the pyrolysis mass spectrum at the peak maximum recorded during the pyrolysis of PS/BE.
entrapment in the gas phase, which suppresses flaming combustion and reduces heat release. Thus, the decrease in the yield of HBr may be associated with its reactions with PS. Weak peaks, due to brominated monomer (182–184) and dimer (286–288), were detected. In addition significant increase in the relative yields of H-deficient monomer peak (103) may be associated with loss of Br during the ionization of brominated styrene units. Furthermore, the slight increase in the char yield for PS/BE composite (∼3.3%) may be thought that as a consequence of increased thermal stability of BE chains, possibility of crosslinked structures involving both PS and BE units increased [15]. Evolution of H2 O was detected in the temperature region where PS degradation took place, at around 450 ◦ C. The high temperature evolution of H2 O was in accordance with the proposed mechanism (Scheme 1c). 3.5. PS/BE/Sb2 O3 Fig. 3. (a) The TIC curve, (b) the pyrolysis mass spectrum at the peak maximum and (c) the single ion evolution profiles of some selected products generated by pyrolysis and/or dissociative ionization processes during the pyrolysis of Sb2 O3 .
It is clear that thermal stability of BE chains increased in the presence of PS. As in the case of pure BE, slight differences were present in the evolution profiles of thermal degradation products. Tribromophenol generated by loss of end groups reached maximum yield at around 415 ◦ C, whereas the products due to the decomposition of epoxy linkages, such as CH3 CO or CH2 COH (43), and tetrabromo bisphenol A and all the fragments involving bisphenol A units, reached maximum yield at around 419 ◦ C. Again the evolution of HBr (80–82), CH3 Br (94–96) and CH3 COCH2 Br (136–138) were maximized at slightly higher temperatures, at around 429 ◦ C. Another point that should be noticed was the change in the relative intensities of the thermal degradation products of BE in the presence of PS. Among the thermal degradation products of BE, the relative yield of brominated fragments decreased noticeably compared to the major fragment with m/z 43. The decrease was more than 2-fold for HBr, 1.7-fold for tetrabromobisphenol A and other fragments generated by decomposition of the main chain. On the other hand, the decrease was about 2.3-fold for tribromophenol produced by elimination of the end-groups. Hydrogen bromide is known to form as the main decomposition product from brominated flame retardants [2,3] facilitating hot radical
It has been determined that the combination of BE and Sb2 O3 yielded further reduction of peak heat release rate, about 64% compared with neat PS and 44% compared with PS/BE blend [15]. The synergistic flame-retarding action of the BE/Sb2 O3 combination manifests itself by the production of SbBr3 , a more effective radical scavenger than HBr, via the reaction between Sb2 O3 and HBr. However, flaming drips could not be avoided, leading to a retained UL-94 V-2 rating for PS/BE/Sb2 O3 as in the case of PS/BE [16]. The TIC curve of the PS/BE/Sb2 O3 showed two overlapping peaks with maxima at 393 and 446 ◦ C (Fig. 6). Decomposition in two distinct regions in the presence of Sb2 O3 was detected also by Jakab et al. but because of the experimental technique used it was not possible to identify all the degradation products generated at the lower temperature region [3]. But they were able to determine the evolution of styrene and H2 O. In an earlier study, it was suggested that SbBr3 and brominated organic compounds are evolved during this stage of decomposition [2]. In the present study, two peaks were detected in the evolution profiles of all products (Fig. 7). Yet, the relative intensity of the low temperature peak was higher in the evolution profiles of the degradation products of flame retardants whereas it was lower in those of PS. Thermal decomposition products of PS showed two maxima at 393 and 446 ◦ C, the second being more intense. Again an increase in the yield of high mass products was noted. Yet, this increase was not
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Fig. 6. (a) The TIC curve and (b) the pyrolysis mass spectrum at the peak maximum recorded during the pyrolysis of PS/BE/Sb2 O3 .
Fig. 5. Single ion evolution profiles of some selected products generated by pyrolysis and/or dissociative ionization processes during the pyrolysis PS/BE.
as significant as in case of PS/BE. The low temperature peaks may be associated with evolution of products produced by the interactions with products generated by the reactions of brominated epoxy and Sb2 O3 . Thermal decomposition products of BE showed maxima at 384 and 446 ◦ C, the first being more intense, indicating that degradation of BE chains shifted to lower temperatures compared to neat BE, in the presence of Sb2 O3 . Another point that should be noticed was the similarity in the evolution profiles of thermal degradation products of BE. All products reached maximum yield at the same temperature unlike what was observed for pure BE and PS/BE composite. The characteristic peaks present in the pyrolysis mass spectra of Sb2 O3 were either absent or significantly weak. Only, very weak peaks due to SbO, Sb2 O2 , Sb3 O4 and Sb4 O6 were identified. In spite, peaks due to oxybromides and bromides of antimony such as SbBr, SbOBr, SbO2 Br, SbOBr2 , SbBr2 , SbBr3 , Sb2 Br2 , Sb2 O2 Br and Sb2 OBr2 were noted. Their evolutions were observed at around 392 ◦ C, indicating that the evolution of antimony compounds occurred
at significantly lower temperatures than the temperature region where the pyrolysis products of pure Sb2 O3 were detected in the presence of brominated epoxy. Thus, it can be concluded that thermal degradation of BE was initiated by the reactions of antimony oxides with bromines of BE decreasing the stability of both. As a consequence of these interactions oxybromides and bromides of antimony were generated. Pyrolysis products of both BE and antimony compounds showed a weaker high temperature peak in their evolution profiles with a maximum at 446 ◦ C, at which decomposition products of PS reached maximum yield. Unlike the general trends observed in the evolutions of antimony compounds and the thermal degradation products of brominated epoxy polymer, HBr evolution was more pronounced at around 446 ◦ C as thermal degradation products of PS. In a recent study, it has been proposed that some of the flame retardant radicals reacted with the polymer matrix by radical recombinations and transfer reactions, were released at the final stages of degradation [9]. Thus, it may be suggested that the fragments decomposed at early stages of pyrolysis interacted with the host polymer, not only caused degradation of the polymer to a certain extent but also yielded segments with higher thermal stability. It was not possible to differentiate brominated styrene oligomers even if they were generated due to the presence of similar units in brominated poly epoxy and polystyrene. Yet, the higher yield of HBr in the temperature region where PS decomposition took place can be regarded as a strong evidence for bromination of PS. Evolution of water was also detected in two temperature regions. The yield of H2 O was increased about 1.8-fold. Again, contrary to diagnostic thermal degradation products of flame retardants, the high temperature peak was more intense as in case of HBr. Thus, it can further be suggested that not only bromination but also oxidation of polystyrene chains took place due to the reactions with decomposition products of flame retardants. These reactions seemed to be more effective in the presence of Sb2 O3 . The char yield of PS/BE/Sb2 O3 was determined to be 7.7% indicating that carbonization reactions took place due to the presence unsaturated polymer chain segments as a result of debromination and loss of water.
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flame retardants with the host polymer PS caused degradation of PS at lower temperatures to a certain extent. Strong evidences for oxidation/hydrolysis and bromination of PS were detected. Loss of HBr and H2 O from these PS segments yielded unsaturated units that can crosslink and increase the char yield.
Acknowledgements We acknowledged the supply of materials used in this work by Prof. Dr. C. Kaynak and Dr. A.N. Isitman.
References
Fig. 7. Single ion evolution profiles of some selected products generated by pyrolysis and/or dissociative ionization processes during the pyrolysis PS/BE/Sb2 O3 .
4. Conclusions Direct pyrolysis mass spectrometry analysis revealed that thermal decomposition of brominated polyepoxy starts by loss of end groups. During the pyrolysis of Sb2 O3 , generation of antimony oxides and antimony oligomers was recorded under the high vacuum conditions. The relative yields of high mass thermal degradation products of PS increased noticeably and broadening in their evolution profiles was detected in the presence of BE. On the other hand, significant variations in the yield of BE based products was noted, in addition to a slight increase in thermal stability. For the PS/BE/Sb2 O3 composite, thermal decomposition was initiated by the interactions of Sb2 O3 with BE, and shifted to lower temperatures. Interactions of
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Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
Thermal degradation of polystyrene composites. Part I. The effect of brominated polyepoxy and antimony oxide Hatice Kaya a,1 , Jale Hacaloglu a,b,∗ a b
Middle East Technical University, Department of Polymer Science & Technology, TR-06800 Ankara, Turkey Middle East Technical University, Department of Chemistry, TR-06800 Ankara, Turkey
a r t i c l e
i n f o
Article history: Received 12 July 2013 Accepted 20 November 2013 Available online 28 November 2013 Keywords: Polystyrene Brominated polyepoxy Antimony oxide Thermal degradation Direct pyrolysis mass spectrometry
a b s t r a c t Thermal degradation of polystyrene (PS) involving brominated polyepoxy (BA) and antimony oxide (PS/BE/Sb2 O3 ) was studied systematically via direct pyrolysis mass spectrometry. Thermal decomposition of brominated polyepoxy was started by loss of end groups. The relative yields of high mass thermal degradation products of PS and the product distribution of brominated polyepoxy and antimony oxide were changed noticeably during the pyrolysis of PS/BE/Sb2 O3 composite. Its thermal decomposition was initiated by the interactions of Sb2 O3 with BE, and shifted to lower temperatures. Interactions of flame retardants with PS caused degradation of PS at lower temperatures to a certain extent. Loss of HBr and H2 O, in the temperature region where main PS decomposition took place, confirmed chemical interactions between the flame retardants and PS. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Brominated flame retardants being compatible and having small influence on mechanical properties have been used in polymer industry widely to reduce fire damage and harm. The most common brominated flame retardants are polybrominated biphenyls, polybrominated diphenyl oxides/ethers, tetrabromo bisphenol-A and polybrominated epoxy resins. However, evolution of toxic brominated compounds such as hydrogen bromide, polybrominated benzofurans and dibenzo-p-dioxins, during recycling, incineration, or in fires of such materials, provoked strong limitations in their use. Several studies were focused on the thermal degradation products and the distribution of bromine among thermal degradation products of polystyrene and high-impact polystyrene involving brominated flame retardants or their combinations with antimony oxide [1–9]. The synergistic effect of Sb2 O3 on the flame-quenching efficiency of organic halides by increasing the releasing rate of halogens from aromatic halides via the formation of antimony halides
∗ Corresponding author at: Middle East Technical University, Department of Polymer Science & Technology, TR-06800 Ankara, Turkey. Tel.: +90 312 210 5148; fax: +90 312 210 3200. E-mail addresses: [email protected] (H. Kaya), [email protected] (J. Hacaloglu). 1 Tel.: +90 312 210 3198; fax: +90 312 210 3200. 0165-2370/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jaap.2013.11.017
and oxyhalides during combustion enhancing the effectiveness of the brominated flame retardant is well-known [1,6–15]. Luijk et al. studied the thermal stability and the thermal degradation products of high impact polystyrene/decabromodiphenylether/antimony oxide (HIPS FR) in situ with derivative thermogravimetry (DTG), temperature resolved pyrolysis-mass spectrometry (Py-MS) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and detected antimony (oxy)bromides and brominated higher styrene oligomers [1]. Recently, Grause et al. studied the thermal decomposition of flame retardant free high-impact polystyrene (HIPS) and HIPS samples containing brominated flame retardants and antimony trioxide (Sb2 O3 ) using TGA at different heating rates and determined that the addition of a flame retardant and Sb2 O3 reduced the activation energy [8]. It has been determined that high molecular weight brominated flame retardants are more favorable in terms of reduced toxicity, thermal stability, melt flow characteristics, and mechanical properties compared with conventional non-oligomeric counterparts [16]. In the present study, we focused on the analysis of the thermal degradation characteristics and products of polystyrenes containing tribromophenol end-capped brominated epoxy oligomers with or without Sb2 O3 . The influence of the presence of each component on thermal degradation behavior of the other was determined systematically for the first time in the literature via direct pyrolysis mass spectrometry. Our next study will focus on thermal degradation behavior of PS/organoclay nanocomposites
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containing tribromophenol end-capped oligomers with or without Sb2 O3 .
brominated
epoxy
2. Experimental 2.1. Materials PS (Mw ∼192,000 g/mol) was obtained from Aldrich (St. Louis, MO, USA). The brominated flame retardant was a tribromophenol end-capped oligomer, BE (Mw 15,000 g/mol, ∼53 wt% Br), obtained from ICL Industrial Products (Beersheba, Israel). Sb2 O3 (99.9%) was an industrial-grade product obtained from a Chinese source. Materials were prepared via an ultrasound-assisted solution intercalation technique described in detail in a previous work [16]. Both of the samples PS/BE and PS/BE/Sb2 O3 contained 20 wt% BE. 3 wt% of Sb2 O3 were added to PS/BE. 2.2. Characterization Direct pyrolysis mass spectrometry, DP-MS, analyses were performed by Waters Micromass Quattro Micro MS/MS/MS Mass Spectrometer with a mass range of 10–1500 coupled to a direct insertion probe. During the pyrolysis, the temperature was increased to 50 ◦ C at a rate of 5 ◦ C min−1 , then, was raised to 650 ◦ C with a rate of 10 ◦ C min−1 and kept at 650 ◦ C for 5 additional minutes. 0.01 mg samples were pyrolyzed in the flared quartz sample vials while recording 70 eV EI mass spectra at a mass scan rate of 1 scan s−1 . The analyses were repeated several times to ensure reproducibility. Each time, almost exactly the same trends were detected. 20 eV EI mass spectra were dominated with identical peaks showing increased relative intensities for high mass fragments, yet, reproducibility of the relative ion yields diminished significantly. Thus, for the analyses the reproducible 70 eV EI mass spectra were used. The interpretation of fragments involving Br and/or Sb atoms were done with the use of isotopic peaks, as the corresponding series of peaks follow the Pascal’s triangle rule. Collision induced dissociation experiments were also performed for a reliable interpretation. Taking into account further dissociation of pyrolysis products during ionization, the analysis of the spectral data was achieved, with the use of NIST MS library. In collision induced dissociation (CID) experiments, daughter ions were produced by the collision of the selected particular ion with argon in the collision cell, the second quadruple. The CID spectra were obtained by scanning of the third quadruple mass analyzer. 3. Results and discussions Direct pyrolysis mass spectra of polymers are usually very complex, as thermal degradation products further dissociate in the mass spectrometer during ionization and all fragments with the same mass to charge ratio make contributions to the intensities of the same peaks in the mass spectrum [17]. The pyrolysis mass spectra of polymer composites, especially those involving similar units, are even more complicated. In order to investigate thermal degradation characteristics of PS samples containing BE with or without Sb2 O3 , DP-MS analysis of each component was performed. The trends in the single ion evolution profiles of diagnostic thermal degradation products of each component are used to determine the source of the product and/or the mechanism of thermal degradation. All ions with identical evolution profiles are grouped and analyzed separately. Among these, the fragment with highest mass may be assumed to be generated during thermal degradation. On the other hand, the low mass fragments having similar evolution profiles may be generated either during thermal degradation or
Fig. 1. (a) The TIC curve, (b) the pyrolysis mass spectrum at the peak maximum and (c) the single ion evolution profiles of some selected products generated by pyrolysis and/or dissociative ionization of thermal degradation products of PS.
during ionization in the mass spectrometer. Thus, in some cases it is almost impossible to determine whether a given fragment is generated during pyrolysis or dissociative ionization process. However, as the main purpose of this study is to investigate the influence of the presence of each component on thermal degradation behavior of the other by comparing the differences generated in the pyrolysis mass spectra of PS in the presence of BE, Sb2 O3 , and BE/Sb2 O3 the generation of fragments during pyrolysis or ionization is not of basic interest. 3.1. Polystyrene PS degrades in a single step by a depolymerization reaction yielding mainly styrene [18–23]. In Fig. 1, the total ion current (TIC) curve, variation of total ion yield as a function of temperature, the mass spectrum at the peak maximum and the single ion evolution profiles of some characteristic fragments generated during pyrolysis and/or ionization such as C6 H6 (78), C7 H7 (91), C7 H8 (92), styrene CH2 CHC6 H5 (104), CH2 CH2 C6 H5 (105), C3 H2 C6 H5 (115), C4 H4 C6 H5 (129) and CH2 C(C6 H5 )CH2 CHC6 H5 (207) are presented. The evolution profiles of all these fragments followed identical paths showing maxima at 441 ◦ C. 3.2. Brominated epoxy oligomer, BE The TIC curve of brominated oligomer showed a single peak with a maximum at around 407 ◦ C (Fig. 2). The most intense peak in the pyrolysis mass spectrum recorded at 407 ◦ C is at 43 due to CH3 CO fragment. Tetrabromo bisphenol A and HBr were determined to be the major decomposition products in accordance with the literature results [2,3]. Strong evidences for the
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also detected at around 312 ◦ C. Thus, it can be concluded that thermal decomposition of the epoxy oligomer started by loss of end groups (Scheme 1a). The evolution of tetrabromobisphenol A and other fragments involving bisphenol A units occurred at slightly higher temperatures together with the fragments involving the epoxy linkages. Among the products involving bisphenol A, tetrabromobisphenol A was the major product. Its generation may be explained by the mechanism proposed by Blazso et al. as shown in Scheme 1b [2]. However, although, Blazso et al. detected the evolution of hydrogen bromide and tetrabromobisphenol in the same temperature region under the high vacuum conditions, in the present study, elimination of HBr was detected at slightly higher temperatures, together with minor amounts of bromomethane, bromoacetone and tribromobisphenol A, pointing out the attack of HBr to epoxy units even under high vacuum conditions (Scheme 1c). Actually, as HBr peak is not present in the EI mass spectrum of tetrabromobisphenol A and tribromophenol, it should necessarily be formed during the thermal degradation of the epoxy oligomers. Furthermore, detection of high mass fragments such as C3 H5 O2 (Br2 )C6 H2 CCH3 C6 H2 (Br2 )C3 H5 O2 (637–645) and C3 OH4 O(Br2 )C6 H2 C(CH3 )2 C6 H2 (Br2 )OC3 OH6 OC6 H2 Br2 (888–890) revealed that random cleavages along the main chain also took place as an opposing thermal degradation pathway. 3.3. Antimony oxide Sb2 O3 The TIC curve, the mass spectrum recorded at 495 ◦ C, at the peak maximum, and the evolution profiles of intense and/or characteristic products recorded during the heating of Sb2 O3 are given in Fig. 3. Several oxides were differentiated contrary to our expectations. Besides the starting compound Sb2 O2 , Sb2 O3 , Sb3 Ox , where x = 1–5, Sb4 Ox , where x = 3 or 7, Sb5 Ox , where x = 2 or 7 and Sb6 Ox where x = 3 or 9 and Sb oligomers, Sbx for x = 1–4, were also identified. The relative yield of Sb2 O3 (290–294) was quite weak. The base peak was due to SbO (290–294). Sb4 O6 (580–588), Sb3 O4 (427–433) and Sb2 O2 (274–278) were the other abundant products. All products followed identical evolution profiles and reached maximum yield at around 495 ◦ C. Evaporation of Sb4 O6 was in agreement with previous literature findings [3,24]. Yet, to our knowledge antimony oxides involving more than four Sb atoms was not detected before. 3.4. PS/BE
Fig. 2. (a) The TIC curve, (b) the pyrolysis mass spectrum at the peak maximum and (c) the single ion evolution profiles of some selected products generated by pyrolysis and/or dissociative ionization processes during the pyrolysis of BE.
generation of tribromophenol, bromoacetone and acetone were obtained. Evolution profiles of degradation products showed slight differences indicating degradation through different reaction pathways (Fig. 2c). The single ion evolution profile of tribromophenol (328–334) showed two peaks with maxima at around 312 and 392 ◦ C. Products due to the decomposition of epoxy linkages such as CH3 CO or CH2 COH (43) CH3 COCH2 or CH2 COHCH2 (57) and CH3 COCH3 or CH2 COHCH3 (58) and tetrabromo bisphenol A (540–548) and all the fragments involving bisphenol A units reached maximum yield at around 398 ◦ C. On the other hand, the evolution of HBr (80–82), CH3 Br (94–96) and CH3 COCH2 Br (136–138) were maximized at slightly higher temperatures, at around 403 ◦ C. Besides tribromophenol, evolution of CH3 COCH2 or CH2 C(OH)CH2 (57) was
In a recent study, it has been determined that BE, as the dispersed phase in PS/BE blends, acts in a way to reduce the peak heat release rate by ∼35% and increase the peak mass loss rate by ∼31% with respect to neat PS [17]. The ratio of total heat evolved to total mass loss, the product of combustion efficiency and effective heat of combustion of volatiles, was decreased from 3.0 MJ m−2 g−1 to 1.6 MJ m−2 g−1 with the incorporation of BE into PS. In addition, the limiting oxygen index, LOI increased from 18% to 21%. It has been determined that BE alters the degradation and combustion behavior in a manner that accelerates dripping and subsequent self-extinguishment of PS, which provides a UL-94 V-2 rating. In the presence of brominated epoxy (20 wt%), pyrolysis of PS yielded a TIC curve with a maximum at around 448 ◦ C (Fig. 4a). The spectra recorded at around these temperatures were dominated with characteristic peaks of PS. Those of BE can also be detected upon expansion of the spectra recorded (Fig. 4b). Single ion evolution profiles of characteristic degradation products of PS followed identical trends (Fig. 5). However, the relative yields of high mass thermal degradation products increased noticeably. Furthermore, though thermal degradation of PS chains started almost at around the same temperature ranges, evolution profiles were broadened indicating that thermal degradation was continued at higher temperatures compared to neat PS.
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Scheme 1. Thermal degradation pathways of brominated epoxy oligomer.
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Fig. 4. (a) The TIC curve and (b) the pyrolysis mass spectrum at the peak maximum recorded during the pyrolysis of PS/BE.
entrapment in the gas phase, which suppresses flaming combustion and reduces heat release. Thus, the decrease in the yield of HBr may be associated with its reactions with PS. Weak peaks, due to brominated monomer (182–184) and dimer (286–288), were detected. In addition significant increase in the relative yields of H-deficient monomer peak (103) may be associated with loss of Br during the ionization of brominated styrene units. Furthermore, the slight increase in the char yield for PS/BE composite (∼3.3%) may be thought that as a consequence of increased thermal stability of BE chains, possibility of crosslinked structures involving both PS and BE units increased [15]. Evolution of H2 O was detected in the temperature region where PS degradation took place, at around 450 ◦ C. The high temperature evolution of H2 O was in accordance with the proposed mechanism (Scheme 1c). 3.5. PS/BE/Sb2 O3 Fig. 3. (a) The TIC curve, (b) the pyrolysis mass spectrum at the peak maximum and (c) the single ion evolution profiles of some selected products generated by pyrolysis and/or dissociative ionization processes during the pyrolysis of Sb2 O3 .
It is clear that thermal stability of BE chains increased in the presence of PS. As in the case of pure BE, slight differences were present in the evolution profiles of thermal degradation products. Tribromophenol generated by loss of end groups reached maximum yield at around 415 ◦ C, whereas the products due to the decomposition of epoxy linkages, such as CH3 CO or CH2 COH (43), and tetrabromo bisphenol A and all the fragments involving bisphenol A units, reached maximum yield at around 419 ◦ C. Again the evolution of HBr (80–82), CH3 Br (94–96) and CH3 COCH2 Br (136–138) were maximized at slightly higher temperatures, at around 429 ◦ C. Another point that should be noticed was the change in the relative intensities of the thermal degradation products of BE in the presence of PS. Among the thermal degradation products of BE, the relative yield of brominated fragments decreased noticeably compared to the major fragment with m/z 43. The decrease was more than 2-fold for HBr, 1.7-fold for tetrabromobisphenol A and other fragments generated by decomposition of the main chain. On the other hand, the decrease was about 2.3-fold for tribromophenol produced by elimination of the end-groups. Hydrogen bromide is known to form as the main decomposition product from brominated flame retardants [2,3] facilitating hot radical
It has been determined that the combination of BE and Sb2 O3 yielded further reduction of peak heat release rate, about 64% compared with neat PS and 44% compared with PS/BE blend [15]. The synergistic flame-retarding action of the BE/Sb2 O3 combination manifests itself by the production of SbBr3 , a more effective radical scavenger than HBr, via the reaction between Sb2 O3 and HBr. However, flaming drips could not be avoided, leading to a retained UL-94 V-2 rating for PS/BE/Sb2 O3 as in the case of PS/BE [16]. The TIC curve of the PS/BE/Sb2 O3 showed two overlapping peaks with maxima at 393 and 446 ◦ C (Fig. 6). Decomposition in two distinct regions in the presence of Sb2 O3 was detected also by Jakab et al. but because of the experimental technique used it was not possible to identify all the degradation products generated at the lower temperature region [3]. But they were able to determine the evolution of styrene and H2 O. In an earlier study, it was suggested that SbBr3 and brominated organic compounds are evolved during this stage of decomposition [2]. In the present study, two peaks were detected in the evolution profiles of all products (Fig. 7). Yet, the relative intensity of the low temperature peak was higher in the evolution profiles of the degradation products of flame retardants whereas it was lower in those of PS. Thermal decomposition products of PS showed two maxima at 393 and 446 ◦ C, the second being more intense. Again an increase in the yield of high mass products was noted. Yet, this increase was not
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Fig. 6. (a) The TIC curve and (b) the pyrolysis mass spectrum at the peak maximum recorded during the pyrolysis of PS/BE/Sb2 O3 .
Fig. 5. Single ion evolution profiles of some selected products generated by pyrolysis and/or dissociative ionization processes during the pyrolysis PS/BE.
as significant as in case of PS/BE. The low temperature peaks may be associated with evolution of products produced by the interactions with products generated by the reactions of brominated epoxy and Sb2 O3 . Thermal decomposition products of BE showed maxima at 384 and 446 ◦ C, the first being more intense, indicating that degradation of BE chains shifted to lower temperatures compared to neat BE, in the presence of Sb2 O3 . Another point that should be noticed was the similarity in the evolution profiles of thermal degradation products of BE. All products reached maximum yield at the same temperature unlike what was observed for pure BE and PS/BE composite. The characteristic peaks present in the pyrolysis mass spectra of Sb2 O3 were either absent or significantly weak. Only, very weak peaks due to SbO, Sb2 O2 , Sb3 O4 and Sb4 O6 were identified. In spite, peaks due to oxybromides and bromides of antimony such as SbBr, SbOBr, SbO2 Br, SbOBr2 , SbBr2 , SbBr3 , Sb2 Br2 , Sb2 O2 Br and Sb2 OBr2 were noted. Their evolutions were observed at around 392 ◦ C, indicating that the evolution of antimony compounds occurred
at significantly lower temperatures than the temperature region where the pyrolysis products of pure Sb2 O3 were detected in the presence of brominated epoxy. Thus, it can be concluded that thermal degradation of BE was initiated by the reactions of antimony oxides with bromines of BE decreasing the stability of both. As a consequence of these interactions oxybromides and bromides of antimony were generated. Pyrolysis products of both BE and antimony compounds showed a weaker high temperature peak in their evolution profiles with a maximum at 446 ◦ C, at which decomposition products of PS reached maximum yield. Unlike the general trends observed in the evolutions of antimony compounds and the thermal degradation products of brominated epoxy polymer, HBr evolution was more pronounced at around 446 ◦ C as thermal degradation products of PS. In a recent study, it has been proposed that some of the flame retardant radicals reacted with the polymer matrix by radical recombinations and transfer reactions, were released at the final stages of degradation [9]. Thus, it may be suggested that the fragments decomposed at early stages of pyrolysis interacted with the host polymer, not only caused degradation of the polymer to a certain extent but also yielded segments with higher thermal stability. It was not possible to differentiate brominated styrene oligomers even if they were generated due to the presence of similar units in brominated poly epoxy and polystyrene. Yet, the higher yield of HBr in the temperature region where PS decomposition took place can be regarded as a strong evidence for bromination of PS. Evolution of water was also detected in two temperature regions. The yield of H2 O was increased about 1.8-fold. Again, contrary to diagnostic thermal degradation products of flame retardants, the high temperature peak was more intense as in case of HBr. Thus, it can further be suggested that not only bromination but also oxidation of polystyrene chains took place due to the reactions with decomposition products of flame retardants. These reactions seemed to be more effective in the presence of Sb2 O3 . The char yield of PS/BE/Sb2 O3 was determined to be 7.7% indicating that carbonization reactions took place due to the presence unsaturated polymer chain segments as a result of debromination and loss of water.
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flame retardants with the host polymer PS caused degradation of PS at lower temperatures to a certain extent. Strong evidences for oxidation/hydrolysis and bromination of PS were detected. Loss of HBr and H2 O from these PS segments yielded unsaturated units that can crosslink and increase the char yield.
Acknowledgements We acknowledged the supply of materials used in this work by Prof. Dr. C. Kaynak and Dr. A.N. Isitman.
References
Fig. 7. Single ion evolution profiles of some selected products generated by pyrolysis and/or dissociative ionization processes during the pyrolysis PS/BE/Sb2 O3 .
4. Conclusions Direct pyrolysis mass spectrometry analysis revealed that thermal decomposition of brominated polyepoxy starts by loss of end groups. During the pyrolysis of Sb2 O3 , generation of antimony oxides and antimony oligomers was recorded under the high vacuum conditions. The relative yields of high mass thermal degradation products of PS increased noticeably and broadening in their evolution profiles was detected in the presence of BE. On the other hand, significant variations in the yield of BE based products was noted, in addition to a slight increase in thermal stability. For the PS/BE/Sb2 O3 composite, thermal decomposition was initiated by the interactions of Sb2 O3 with BE, and shifted to lower temperatures. Interactions of
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