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Thermal decomposition of flame retarded polycarbonates
J. Anal. Appl. Pyrolysis 79 (2007) 337–345 www.elsevier.com/locate/jaap
Thermal decomposition of flame retarded polycarbonates J. Bozi, Zsuzsanna Cze´ge´ny *, E. Me´sza´ros, M. Blazso´ Institute of Materials and Environmental Chemistry, CRC, Hungarian Academy of Sciences, H-1525, P.O. Box 17, Budapest, Hungary Received 4 July 2006; accepted 6 January 2007 Available online 11 January 2007
Abstract The effect of tetrabromobisphenol A (TBBA), ammonium polyphosphate (APP) and tris(2,4-di-ter-dibuthylphenyl-phosphite) (TBP) flame retardants were examined on the thermal decomposition of polycarbonate by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and thermogravimetry/mass spectrometry (TG/MS) techniques. The maximal rate of polycarbonate decomposition shifted to a slightly lower temperature in the presence of TBBA. More than 20 bromine containing products were identified among the pyrolysis products of the polycarbonate sample containing TBBA flame retardant at 550 and 700 8C temperature. Significant decrease of the total evaluated volatile compounds was found at 550 8C from the APP blended polycarbonate and at 700 8C pyrolysis temperature in the presence of both phosphor-containing flame retardants investigated. The presence of the examined phosphorous flame retardants has no effect on the temperature regarding the radical decomposition of polycarbonate, while partial hydrolysis takes place at a lower temperature due to APP. The polycarbonate chain keenly decomposes under gradual heating in the temperature range of ammonia evolution from APP. The product analysis indicates that APP accelerates the formation of phenol and isopropylene phenol from the bisphenol A segments of polycarbonate. # 2007 Elsevier B.V. All rights reserved. Keywords: Polycarbonate; Decomposition mechanism; Flame retardant; Pyrolysis-GC/MS; TG–MS
1. Introduction Polycarbonate (PC) produced from bisphenol A is one of the fastest growing engineering polymers, the annual volume of PC exceed 1 million metric tonnes. Most of its applications require a high degree of flame retardancy [1]. In spite of the environmental hazard of the halogenated flame retardants, tetrabromobisphenol A is still used in polycarbonate, because it has minimal effect on the favorable properties of PC. Intumescent flame retardants have wide applications due to thermo-protective properties, which decrease heat flow on the polymer surface. The flame retardant content typically varies from 0.5 up to 40% by weight, thus, when trying to recover useful materials from the polycarbonate waste, we have to take into consideration the notable flame retardant content. The presence of flame retardant additives could modify the thermal decomposition of polymers [2,3]. The thermal decomposition of bisphenol A based polycarbonate has been previously studied 40 years ago by Lee [4], * Corresponding author. Tel.: +36 1 4381100x381; fax: +36 1 4381147. E-mail address: [email protected] (Z. Cze´ge´ny). 0165-2370/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2007.01.001
postulating a radical cleavage of the polymer chain at the carbonate bonds that describes well the commonly observed formation of bisphenol A, phenylisopropylphenol and carbon dioxide from polycarbonate [5]. Later several details and side reactions have been published, namely oligomeric ring formation under vacuum [6], elimination leading to ethers [7], and isomerisation producing xanthone structures [7,8]. Phenol formation can be enhanced by hydrolysis carrying out pyrolysis of PC mixed with calcium hydroxide in a steam atmosphere [9]. The scission of carbonate linkage is significantly reduced in the presence of aryl phosphate flame reatrdants, thus, considerably less bisphenol A was obtained but the relative amount of diarylcarbonate increased [3]. Thermal decomposition of tetrabromobisphenol A (TBBA) flame retarded polycarbonate was examined by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) [10]. Beside the numerous brominated phenol derivatives, not negligible amount of bromobenzene and bromo-4-alkylbenzenes was found due to the reaction of bromine atoms cleaved from TBBA and the isopropylphenyl radical terminated intermediary products of the thermally fragmented polycarbonate.
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Tetrabromobisphenol A is a widely applied flame retardant in the plastics of electronic industry, such as epoxy and polycarbonate resins, ABS, high impact polystyrene, phenolic resins and others. The thermal decomposition of this compound has been studied by thermogravimetry and in a tubular batch reactor [11], and the pyrolysis and combustion products were analysed by Fourier transform infrared spectrometry (FTIR), GC and GC/MS techniques [12,13]. The decomposition of ammonium polyphosphate (APP) has been studied by evolved gas analysis [14] and has been reported that it decomposes in three stages. It is suggested that the first two decomposition steps – between 200 and 400 8C – are due to evolution of ammonia and water accompanied by cross-linking and acidic hydroxyl group formation. During the final decomposition stage phosphate chain fragments are evolved and ammonia is completely eliminated resulting in a cross-linked ultraphosphate structure. Thermal degradation of a series of metal ion— APP combinations has been studied by thermogravimetry and differential thermal analysis [15] and a reduced thermal stability was found. The aim of our study, was to examine the effects of flame retardants of different types on the thermal decomposition of bisphenol A polycarbonate in order to observe changes in thermal stability, in the composition of pyrolysis oil, and also understanding the chemical reactions taking place during pyrolytic waste elimination procedures. 2. Experimental 2.1. Materials The composition of the polycarbonate samples and the molecular structures of flame retardant materials studied in this work are shown in Table 1. 4,40 -Isopropylidenebis(2,6dibromophenol) (97% purity) (TBBA) was obtained from Sigma–Aldrich, ammonium polyphosphate was obtained from Clarion (product name: Exolit AP 422) and tris(2,4-di-terbutylphenyl)phospite (TBP) was obtained from Ciba (product name: Irgafos 168). To prepare the flame retarded polycarbonate sample the flame retardant additive was mechanically
kneaded into polycarbonate (BHD Chemicals, UK) at 250 8C in a Brabender EH 50 device. 2.2. Thermogravimetry/mass spectrometry (TG/MS) The TG/MS instrument was built from a Perkin-Elmer TGS2 thermobalance and a Hiden HAL 3F/PIC mass spectrometer and controlled by a computer. For the examination of the thermal stability of flame retarded polycarbonates, 0.5–0.9 mg samples were placed into the platinum sample pan. The samples were heated at 10 8C min1 up to 900 8C in argon atmosphere. A portion of the volatile products was introduced into the ion source of the mass spectrometer through a glass lined metal capillary held at 300 8C. The quadrupole mass spectrometer was operated at 70 eV electron energy. The ion intensities were normalized to the sample mass and to the intensity of the 38Ar isotope of the carrier gas. 2.3. Pyrolysis -gas chromatography/mass spectrometry (Py-GC/MS) Py-GC/MS measurements were carried out in a Pyroprobe 2000 (Chemical Data System, USA) pyrolyser equipped with a platinum coil and quartz sample tube, coupled to Agilent 6890 GC–5973 MSD (Agilent Technologies, USA) instrument. The pyrolysis temperature in the Pyroprobe was calibrated previously by thermocouple inserted into the quartz tube. Flash pyrolysis method was applied at 550 and 700 8C. Approximately 0.25 mg polymer sample was pyrolyzed for 20 s, the platinum coil was heated up by 400 8C s1 heating rate. Helium carrier gas at a flow rate of 20 mL min1 purged the pyrolysis chamber held at 250 8C. The GC separation was performed on a HP-5MS capillary column (30 m 0.25 mm 0.25 mm) (Agilent Technologies, USA), the temperature after 1 min 50 8C isotherm period was programmed to 300 8C at 10 8C min1 heating rate and held at 300 8C for 4 min. The temperature of the transfer line of GC/MS and the source of the mass spectrometer were 280 and 200 8C, respectively. The mass spectrometer was operating in electron-impact mode (EI) at 70 eV, in the scan range m/z 14– 400 Da and 14–600 Da to analyze the bromine containing
Table 1 Composition of samples and the chemical structures of flame retardant materials examined Name
Chemical structure
Flame retardant amount (w/w, %) in the policarbonate samples
Tetrabromobisphenol A (TBBA)
4
Tris-dibutilphenil phosphite (TBP)
0.8
Ammonium polyphosphate (APP)
30
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samples. The standard deviation of the peak area of the GC–MS chromatograms was between 5 and 30%. 3. Results and discussion 3.1. Thermal decomposition of flame retarded polycarbonate during gradual heating To examine the changes in the overall thermal decomposition of flame retarded polycarbonate samples, TG/MS technique was used. The mass loss (TG) and the rate of mass change (DTG) of polycarbonate samples are shown in Fig. 1. The TG curve of TBBA blended polycarbonate shows 4% weight loss between 220 and 280 8C which is due to the evaporation of TBBA from the polymer sample. The DTG maxima of this sample also shifted to 15 8C lower temperature, indicating that TBBA slightly modificates the polycarbonate before it evaporates. During slow heating, the thermal decomposition of polycarbonate blended with TBP does not differ significantly from the decomposition of plain polycarbonate sample. The most characteristic changes were found in the case of the APP blended polycarbonate. Calculated and experimental TG and DTG curves of APP blended polycarbonate are shown in Fig. 2a. The calculated curves were obtained by the superposition of the experimental curves of the individual components in appropriate ratio (polycarbonate:APP = 0.7:0.3). The shape of the TG and DTG curves of APP blended polycarbonate differs to a great extent from that of the calculated curves. This indicates that the thermal decomposition of polycarbonate is strongly influenced by APP. Fig. 2b presents the evolution curve of ammonia originating from APP, that of phenol evolved from polycarbonate, and that of hydrogen indicating the char formation under gradual slow heating of APP blended polycarbonate. The ion profiles show that a huge amount of phenol evolves from polycarbonate in the temperature range of ammonia and water evolution from APP. The similar shape and position of ion curves suggest that the release of phenol and ammonia proceeds parallel between 300 and 450 8C. This observation can be explained as the evolved ammonia could assist the basis supported hydrolitic decomposition of carbonate groups. The DTG maxima at 514 8C
Fig. 2. Experimental and calculated TG and DTG curves (a) and evolution of selected ion profiles (b) of polycarbonate flame retarded by APP.
shows that the maximal rate of the radical thermal decomposition of the rest of the polycarbonate was shifted to only a few degrees lower temperature than that of the pure polycarbonate. Thus, the presence of APP does not induce remarkable changes in the radical decomposition of polycarbonate chain, nonetheless partial hydrolysis takes place at lower temperature. The evolution of hydrogen under the last flat DTG peak sign char formation up to 700 8C. We should mention that in contrast to the pure polycarbonate and the TBBA and TBP blended polycarbonate where the reprodubilicity of the TG and DTG curves were excellent, the TG measurements of polycarbonate blended with APP were not satisfactorily reproduced, especially in the temperature range at above 550 8C. The reason could be the inhomogenity of the blend or the different shape of the sample, or even the complex chemical and physical interactions taking place during char formation. 3.2. Effect of flame retardant additives on the yield of pyrolysis products of polycarbonate under fast pyrolysis
Fig. 1. TG and DTG curves of polycarbonate and flame retarded polycarbonate samples.
The influence of flame retardants on the product distribution of polycarbonate under fast pyrolysis was examined by Py-GC/ MS. Among the conditions of analytical pyrolysis applying a fast heating rate the thermal decomposition of polycarbonate and flame retardant takes place simultaneously, thus, we may expect different effects than during gradual heating. The PyGC/MS experiments were carried out at 550 and 700 8C pyrolysis temperature. The total ion chromatograms of the flame retarded and pure polycarbonate samples are shown in
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structure of the unknown peaks was made with the help of usual mass spectrometric identification principles and gas chromatographic retention relations. In Table 2 those compounds are labelled by an asterix which could be undoubtedly identified by MS library search. The identity of peaks 4 and 6 were confirmed by GC retention data similarity to those published for authentic standards [3]. The retention index (RI) of the compounds is evaluated using retention time values of n-alkane pyrolysis products of polyethylene at 500 8C analyzed on the same GC column among the same conditions as the PC products. RIx ¼ N a þ
Fig. 3. Py-GC/MS total ion chromatograms of flame retarded and pure polycarbonate samples at 550 and 700 8C pyrolysis temperature.
Fig. 3. The chemical name of the main identified decomposition products labeled by numbers are given in Table 2. 3.2.1. GC/MS identification of pyrolysis products of PC and its blends The mass spectra of the separated peaks in the pyrograms are often not existing in the mass spectral libraries and are not available in the literature, thus, elucidation of the chemical
Rtx Rta Rtaþ1 Rta
where Na is the carbon number of the n-alkane of lower retention neigbouring compound x in the retention time scale, Rta the retention time of this n-alkane, Rta+1 the retention time of the n-alkane of higher retention neigbouring compound x, and Rtx is the retention time of the compound x. The retention index difference of peak 7 and diphenyl ether (RI = 14.05) is similar to that of 4-methylethenylphenol and phenol, namely 3.36 and 3.28 methylene units, respectively, – corresponding to an additional 2-propenyl group at position 4 of a phenol ring – that is confirming the structure identified and displayed in Fig. 4. Again similar retention index increments confirm the analogous structure change (addition of a hydroxy at position 4) from 2-phenyl-2-(40 -hydroxyphenyl)-propane – peak #9 – to bisphenol A – peak #15 – (DRI = 3.36) and from 9-methylfluorene (RI = 16.35) to 3-hydroxy-9-methyl-fluorene – peak #8 – (DRI = 3.18), moreover, from peak #8 to 3,6-dihydroxy-9methyl-fluorene – peak #14 – (DRI = 3.36). The mass spectra of the fluorenic compounds of peaks #8 and #14 are shown in Fig. 5, together with that of peak #10. The mass spectra of these compounds have similar pattern, and the fragment ion at m/z 119 or 121 characteristic for 4-alkenyl or -isoalkyl phenols is hardly observable confirming the GC identification.
Table 2 Main decomposition products of polycarbonate GC peak #
Compound name
[M]+
Retention time (min)
RIa (CH2 units)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Phenolb 4-Methylphenolb 4-Ethylphenol b 4-Ethenylphenol 4-Methylethylphenolb 4-Methylethenylphenol (4-(20 -Propenyl)-phenyl)-phenyl-ether 3-Hydroxy-9-methyl-fluorene 2-Phenyl-2-(40 -hydroxyphenyl)-propaneb 3-Hydroxy-9-dimethyl-fluorene 2-(4-Methylphenyl)-2-(4-hydroxyphenyl)-propane 4,40 -Methylenebisphenolb 1,1-Di-(4-hydroxyphenyl)-ethane 3,6-Dihydroxy-9-methyl-fluorene Bisphenol Ab 3,6-Dihydroxy-9-dimethyl-fluorene
94 108 122 120 136 134 210 196 212 210 226 200 214 212 228 226
5.98 7.44 8.82 9.56 9.71 10.78 16.25 17.31 17.51 18.41 18.51 19.47 20.02 20.55 20.61 21.05
9.68 10.70 11.61 12.11 12.21 12.96 17.44 18.44 18.60 19.53 19.61 20.65 21.22 21.80 21.85 22.39
a b
Retention index. Identified by MS library search.
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Fig. 4. MS spectrum and compound structure of Py-GC/MS peaks #6 and #7 of polycarbonate pyrolysed at 700 8C.
Fig. 6. MS spectrum and compound structure of Py-GC/MS peaks #11 and #13 of polycarbonate pyrolysed at 700 8C.
In Fig. 6 spectra of peaks #11 and #13 are given. The structures of the corresponding compounds can be derived from those of identified peaks #9 and #12, respectively, by the addition of a methylene group. A methyl group addition to the phenyl ring of compound #9 at position 4 should result in a peak of one methylene unit higher retention index. The retention index difference of peaks #9 and #11 is just 1.01 as read in Table 2. However, the index difference of peaks #12 and #13 is only 0.57, indicating that the additional methylene group to the molecule of compound #12 is at the methylene unit in compound #13.
3.2.2. Products of polycarbonate blended with TBBA Halogen-containing flame retardants act by interfering with the radical chain mechanism taking place in the gas phase during combustion, subsequently we should expect several bromine containing and so environmentally hazardous products: decomposition fragments of TBBA as well as brominated polycarbonate derivatives. Brominated compounds evolved from TBBA flame retarded polycarbonate at 500 8C pyrolysis temperature were reported previously [10]. In the present study some more brominated compounds were identified at elevated temperature. The integrated values of the selected ion peak areas related to the mass of TBBA in the sample are summarized in Table 3. The amounts of the main volatile compounds evolved from polycarbonate at 550 and 700 8C pyrolysis temperature are shown in Fig. 7. The decomposition of flame retarded polycarbonate is a little more extensive at 550 8C than by pyrolysing the pure polymer at that temperature. The hydrogen bromide evolved from TBBA could assist the decomposition of polycarbonate chain and effect an increased volatile compound formation. At 700 8C pyrolysis temperature, the amounts of evolved volatile products do not change significantly in the presence of TBBA, indicating that at higher temperature this factor became negligible as compared to unassisted thermal decomposition of polycarbonate chain. 3.2.3. Thermal decomposition of polycarbonate containing phosphorous flame retardants Phosphorous-containing flame retardants influence mainly the reactions taking place in the condensed phase. These intumescent type flame retardants assist the char formation by migrating to the polymer surface to form a thermo-protective solid layer. Thus, in the presence of phosphorous flame retardants we can expect decreased amount of evolved volatile products from the polycarbonate sample.
Fig. 5. MS spectrum and compound structure of Py-GC/MS peaks #8, #10 and #14 of polycarbonate pyrolysed at 700 8C.
3.2.3.1. Products of polycarbonate blended with TBP (Irgafos 168). Fig. 8 shows the amounts of the main volatile products
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Table 3 GC–MS selected ion peak areas of brominated pyrolysis products from TBBA blended polycarbonate at 550 and 700 8C [105(couts mg1)] Compound name Methyl bromide Bromobenzene 1-Bromo-4-methylbenzene 2-Bromophenol 1-Bromo-4-ethylbenzene 1-Bromo-4-ethenylbenzene 2-Bromo-4-methylphenol 1-Bromo-4-methylethylbenzene 1-Bromo-4-methylethenylbenzene 4-Bromophenol 2-Bromo-4-ethenylphenol 2-Bromo-4-methylethylphenol 2,4-Dibromophenol 2,6-Dibromophenol 2-Bromo-4-methylethenylphenol 2,6-Dibromo-4-methylethylphenol 2,4,6-Tribromophenol 2,6-Dibromo-4-methylethenylphenol 2-(Bromophenyl)-2-phenyl-propane Bromo-9,9-dimethyl-fluorene 2-(Bromophenyl)-2-phenyl-propane 2-(Bromophenyl)-2-hydroxyphenyl-propane 2-(Bromophenyl)-2-hydroxyphenyl-propane Monobromo bisphenol A Bromo-3,6-dihydroxy-9-methyl-fluorene Dibromo bisphenol A Dibromo bisphenol A Tribromo bisphenol A TBBA
Retention time (min) 1.84 5.27 7.04 7.42 8.52 8.83 9.07 9.46 10.19 10.53 10.84 11.26 11.47 11.89 12.05 15.08 15.15 15.71 17.4 18.19 20.38 20.64 21.64 21.71 22.709 22.78 24.23 25.16 27.28
Selected ion +
[M] [M]+ [M]+ [M + 2]+ [M]+ [M]+ [M]+ [M]+ [M]+ [M]+ [M]+ [M]+ [M + 2]+ [M + 2]+ [M]+ [M + 2]+ [M + 2]+ [M + 2]+ [M]+ [M]+ [M]+ [M]+ [M]+ [M 15]+ [M + 2]+ [(M + 2)15]+ [(M + 2)15]+ [(M +2 )15]+ [(M + 4)15]+
m/z
550 8C
700 8C
94 156 170 174 184 182 186 198 196 172 198 214 252 252 212 294 330 292 274 272 274 290 290 291 370 371 371 449 529
5.9 12.2 7.1 7.1 2.6 3.4 0.9 0.8 10.9 5.3 1.4 0.8 37.7 13.6 17.2 1.3 0.0 28.6 2.8 0.0 8.2 179.5 0.0 30.3 0.0 106.9 26.5 244.1 84.9
5.4 23.9 19.8 19.5 5.2 12.3 2.4 1.4 16.1 14.4 4.1 0.7 35.0 17.5 19.2 0.9 0.7 21.8 1.7 2.6 239.4 77.3 8.6 11.6 13.7 31.1 6.1 54.1 61.0
Fig. 7. GC–MS total ion peak areas of main pyrolysis products of polycarbonate and TBBA blended polycarbonate at 550 and 700 8C pyrolysis temperature [(couts/ mg) 107]. (Amounts of bisphenol A at 550 8C and 1,1-di-(4-hydroxyphenyl)-ethene at 700 8C are divided by 10 and 2, respectively.)
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Fig. 8. GC–MS total ion peak areas of main pyrolysis products of polycarbonate and TBP blended polycarbonate at 550 and 700 8C pyrolysis temperature [(couts mg1) 107]. (Amounts of bisphenol A at 550 8C and 1,1-di-(4-hydroxyphenyl)-ethene at 700 8C are divided by 10 and 2, respectively.)
Fig. 9. GC–MS total ion peak areas of main pyrolysis products of polycarbonate and APP blended polycarbonate at 550 and 700 8C pyrolysis temperature [(couts mg1) 107]. (Amounts of bisphenol A at 550 8C and 1,1-di-(4-hydroxyphenyl)-ethene at 700 8C are divided by 2.)
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Scheme 1.
evolved from the pure and TBP blended polycarbonate sample at 550 and 700 8C pyrolysis temperature. The composition of the pyrolysis oil evolved from TBP blended polycarbonate does not change significantly in comparison to the pure polycarbonate at 550 8C. At 700 8C pyrolysis temperature the main volatile products evolved in increased amount from the pure polycarbonate, however from the policarbonate blended with TBP the volatile product formation was even lower than at 550 8C. This observation indicates that the TBP flame retardant is effective at 700 8C, but not so at 550 8C among our conditions. The relative amounts of evolved products does not change significantly either at 550 or at 700 8C temperature, denoting that the presence of TBP does not affect the decomposition routes leading to volatile products. 3.2.3.2. Products of polycarbonate blended with APP (Exolit AP 422). Among the examined flame retarded polycarbonate blends, the most significant changes were observed in case of
polycarbonate sample containing APP. The amounts of the main volatile compounds evolved at 550 and 700 8C pyrolysis temperature are shown in Fig. 9. As it is demonstrated, the volatile compound formation is repressed at both temperatures, indicating the effectiveness of intumescent type APP flame retardant at these temperatures. Besides the overall decreased volatile product formation, significant changes were found in the composition of pyrolysis oil compared to pure polycarbonate. The product analysis indicates that APP accelerates the formation of phenol and isopropylene phenol from the bisphenol A moiety of polycarbonate resulting in an increased phenol and methylethenylphenol, and decreased bisphenol A ratio in the volatile product. This observation correlates with the results of decomposition at slow heating rate studied by TG–MS (see Section 3.1 above). The evolution of phenol at as low temperature as between 300 and 400 8C from the APP blended PC indicates that the reaction of decomposition is not a radical but an ionic one at this stage. An enhanced formation of phenol and 4-methylethenylphenol from bisphenol A segments is proposed explaining the experimental observations by the ionic interaction of ammonia evolving from APP by heating in Scheme 1. The cleavage of the polycarbonate chain at the C–C bond is facilitated through loosening this bonding in the phenolate ionic form, and results in an increased amount of the products of phenol and 4-methylethenylphenol. The proposed mechanism is shown in Scheme 1. In addition, not negligible carbonate group containing volatile products are also present in the corresponding pyrograms in Fig. 3 (among them the most important is the last unlabeled peak). The mass spectra of these compounds are shown in Fig. 10. 4. Conclusion
Fig. 10. MS spectrum and compound structure of Py-GC/MS peaks of carbonate products from APP blended polycarbonate pyrolysed at 700 8C.
Analytical pyrolysis experiments carried out under gradual and fast heating conditions revealed that the thermal decomposition of polycarbonate is affected by the presence of certain flame retardant additives. Under gradual heating in the temperature range of ammonia and water evolution from APP the polycarbonate chain decomposes by basis catalysed hydrolysis of carbonate groups. APP also accelerates the disproportionation of bisphenol A segment through the ionic cleavage of the C–C bond of ammonium 4-(2-propenyl phenolate) moiety. The presence of both phosphorous flame retardants investigated significantly decrease the quantity of the total evaluated
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volatile compounds at 700 8C pyrolysis temperature due to the intumescent flame retardants char forming feature by migrating to the polymer surface to form a thermo-protective solid layer. The thermal stability of polycarbonate slightly decreases in the presence of TBBA. Several bromine containing compound are evolving under pyrolysis from TBBA flame retarded polycarbonate. Acknowledgement This work was supported by the Hungarian National Research Fund (OTKA) contract No. T047377 and K61504. References [1] S.V. Levchik, E.D. Weil, Polym. Int. 54 (2005) 981. [2] E. Jakab, M.A. Uddin, T. Bhaskar, Y. Sakata, J. Anal. Appl. Pyrol. 68–69 (2003) 83.
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[3] B.N. Jang, C.A. Wilkie, Thermochim. Acta 433 (2005) 1. [4] L.H. Lee, J. Polym. Sci. 2 (1964) 2859. [5] M. Blazso´, in: J. Scheirs, W. Kamisky (Eds.), Feedstock Recycling and Pyrolysis of Waste Plastics, John Wiley & Sons, 2006,, p. 338. [6] C. Puglisi, L. Sturiale, G. Montaudo, Macromolecules 32 (1999) 2194. [7] G. Montaudo, S. Carroccio, C. Puglish, J. Anal. Appl. Pyrol. 64 (2002) 229. [8] A. Davis, J.H. Golden, Macromol. Chem. 110 (1967) 180. [9] T. Yoshioka, K. Sugawara, T. Mizoguchi, A. Okuwaki, Chem. Lett. 34 (2005) 282. [10] M. Blazso´, Z. Cze´ge´ny, J. Chromatogr. A 1130 (2006) 91. [11] F. Barontini, V. Cozzani, K. Marsanich, V. Raffa, L. Petarca, J. Anal. Appl. Pyrol. 72 (2004) 41. [12] F. Barontini, K. Marsanich, L. Petarca, V. Cozzani, Ind. Eng. Chem. Res. 43 (2004) 1952. [13] M.P. Luda, A.I. Balabanovich, A. Hornung, G. Camino, Polym. Adv. Technol. 14 (2003) 741. [14] G. Camino, L. Costa, L. Trossatelli, Polym. Degrad. Stab. 12 (1985) 203. [15] P.J. Davies, A.R. Horrocks, A. Alderson, Polym. Degrad. Stab. 88 (2005) 114.
Thermal decomposition of flame retarded polycarbonates J. Bozi, Zsuzsanna Cze´ge´ny *, E. Me´sza´ros, M. Blazso´ Institute of Materials and Environmental Chemistry, CRC, Hungarian Academy of Sciences, H-1525, P.O. Box 17, Budapest, Hungary Received 4 July 2006; accepted 6 January 2007 Available online 11 January 2007
Abstract The effect of tetrabromobisphenol A (TBBA), ammonium polyphosphate (APP) and tris(2,4-di-ter-dibuthylphenyl-phosphite) (TBP) flame retardants were examined on the thermal decomposition of polycarbonate by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and thermogravimetry/mass spectrometry (TG/MS) techniques. The maximal rate of polycarbonate decomposition shifted to a slightly lower temperature in the presence of TBBA. More than 20 bromine containing products were identified among the pyrolysis products of the polycarbonate sample containing TBBA flame retardant at 550 and 700 8C temperature. Significant decrease of the total evaluated volatile compounds was found at 550 8C from the APP blended polycarbonate and at 700 8C pyrolysis temperature in the presence of both phosphor-containing flame retardants investigated. The presence of the examined phosphorous flame retardants has no effect on the temperature regarding the radical decomposition of polycarbonate, while partial hydrolysis takes place at a lower temperature due to APP. The polycarbonate chain keenly decomposes under gradual heating in the temperature range of ammonia evolution from APP. The product analysis indicates that APP accelerates the formation of phenol and isopropylene phenol from the bisphenol A segments of polycarbonate. # 2007 Elsevier B.V. All rights reserved. Keywords: Polycarbonate; Decomposition mechanism; Flame retardant; Pyrolysis-GC/MS; TG–MS
1. Introduction Polycarbonate (PC) produced from bisphenol A is one of the fastest growing engineering polymers, the annual volume of PC exceed 1 million metric tonnes. Most of its applications require a high degree of flame retardancy [1]. In spite of the environmental hazard of the halogenated flame retardants, tetrabromobisphenol A is still used in polycarbonate, because it has minimal effect on the favorable properties of PC. Intumescent flame retardants have wide applications due to thermo-protective properties, which decrease heat flow on the polymer surface. The flame retardant content typically varies from 0.5 up to 40% by weight, thus, when trying to recover useful materials from the polycarbonate waste, we have to take into consideration the notable flame retardant content. The presence of flame retardant additives could modify the thermal decomposition of polymers [2,3]. The thermal decomposition of bisphenol A based polycarbonate has been previously studied 40 years ago by Lee [4], * Corresponding author. Tel.: +36 1 4381100x381; fax: +36 1 4381147. E-mail address: [email protected] (Z. Cze´ge´ny). 0165-2370/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2007.01.001
postulating a radical cleavage of the polymer chain at the carbonate bonds that describes well the commonly observed formation of bisphenol A, phenylisopropylphenol and carbon dioxide from polycarbonate [5]. Later several details and side reactions have been published, namely oligomeric ring formation under vacuum [6], elimination leading to ethers [7], and isomerisation producing xanthone structures [7,8]. Phenol formation can be enhanced by hydrolysis carrying out pyrolysis of PC mixed with calcium hydroxide in a steam atmosphere [9]. The scission of carbonate linkage is significantly reduced in the presence of aryl phosphate flame reatrdants, thus, considerably less bisphenol A was obtained but the relative amount of diarylcarbonate increased [3]. Thermal decomposition of tetrabromobisphenol A (TBBA) flame retarded polycarbonate was examined by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) [10]. Beside the numerous brominated phenol derivatives, not negligible amount of bromobenzene and bromo-4-alkylbenzenes was found due to the reaction of bromine atoms cleaved from TBBA and the isopropylphenyl radical terminated intermediary products of the thermally fragmented polycarbonate.
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Tetrabromobisphenol A is a widely applied flame retardant in the plastics of electronic industry, such as epoxy and polycarbonate resins, ABS, high impact polystyrene, phenolic resins and others. The thermal decomposition of this compound has been studied by thermogravimetry and in a tubular batch reactor [11], and the pyrolysis and combustion products were analysed by Fourier transform infrared spectrometry (FTIR), GC and GC/MS techniques [12,13]. The decomposition of ammonium polyphosphate (APP) has been studied by evolved gas analysis [14] and has been reported that it decomposes in three stages. It is suggested that the first two decomposition steps – between 200 and 400 8C – are due to evolution of ammonia and water accompanied by cross-linking and acidic hydroxyl group formation. During the final decomposition stage phosphate chain fragments are evolved and ammonia is completely eliminated resulting in a cross-linked ultraphosphate structure. Thermal degradation of a series of metal ion— APP combinations has been studied by thermogravimetry and differential thermal analysis [15] and a reduced thermal stability was found. The aim of our study, was to examine the effects of flame retardants of different types on the thermal decomposition of bisphenol A polycarbonate in order to observe changes in thermal stability, in the composition of pyrolysis oil, and also understanding the chemical reactions taking place during pyrolytic waste elimination procedures. 2. Experimental 2.1. Materials The composition of the polycarbonate samples and the molecular structures of flame retardant materials studied in this work are shown in Table 1. 4,40 -Isopropylidenebis(2,6dibromophenol) (97% purity) (TBBA) was obtained from Sigma–Aldrich, ammonium polyphosphate was obtained from Clarion (product name: Exolit AP 422) and tris(2,4-di-terbutylphenyl)phospite (TBP) was obtained from Ciba (product name: Irgafos 168). To prepare the flame retarded polycarbonate sample the flame retardant additive was mechanically
kneaded into polycarbonate (BHD Chemicals, UK) at 250 8C in a Brabender EH 50 device. 2.2. Thermogravimetry/mass spectrometry (TG/MS) The TG/MS instrument was built from a Perkin-Elmer TGS2 thermobalance and a Hiden HAL 3F/PIC mass spectrometer and controlled by a computer. For the examination of the thermal stability of flame retarded polycarbonates, 0.5–0.9 mg samples were placed into the platinum sample pan. The samples were heated at 10 8C min1 up to 900 8C in argon atmosphere. A portion of the volatile products was introduced into the ion source of the mass spectrometer through a glass lined metal capillary held at 300 8C. The quadrupole mass spectrometer was operated at 70 eV electron energy. The ion intensities were normalized to the sample mass and to the intensity of the 38Ar isotope of the carrier gas. 2.3. Pyrolysis -gas chromatography/mass spectrometry (Py-GC/MS) Py-GC/MS measurements were carried out in a Pyroprobe 2000 (Chemical Data System, USA) pyrolyser equipped with a platinum coil and quartz sample tube, coupled to Agilent 6890 GC–5973 MSD (Agilent Technologies, USA) instrument. The pyrolysis temperature in the Pyroprobe was calibrated previously by thermocouple inserted into the quartz tube. Flash pyrolysis method was applied at 550 and 700 8C. Approximately 0.25 mg polymer sample was pyrolyzed for 20 s, the platinum coil was heated up by 400 8C s1 heating rate. Helium carrier gas at a flow rate of 20 mL min1 purged the pyrolysis chamber held at 250 8C. The GC separation was performed on a HP-5MS capillary column (30 m 0.25 mm 0.25 mm) (Agilent Technologies, USA), the temperature after 1 min 50 8C isotherm period was programmed to 300 8C at 10 8C min1 heating rate and held at 300 8C for 4 min. The temperature of the transfer line of GC/MS and the source of the mass spectrometer were 280 and 200 8C, respectively. The mass spectrometer was operating in electron-impact mode (EI) at 70 eV, in the scan range m/z 14– 400 Da and 14–600 Da to analyze the bromine containing
Table 1 Composition of samples and the chemical structures of flame retardant materials examined Name
Chemical structure
Flame retardant amount (w/w, %) in the policarbonate samples
Tetrabromobisphenol A (TBBA)
4
Tris-dibutilphenil phosphite (TBP)
0.8
Ammonium polyphosphate (APP)
30
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samples. The standard deviation of the peak area of the GC–MS chromatograms was between 5 and 30%. 3. Results and discussion 3.1. Thermal decomposition of flame retarded polycarbonate during gradual heating To examine the changes in the overall thermal decomposition of flame retarded polycarbonate samples, TG/MS technique was used. The mass loss (TG) and the rate of mass change (DTG) of polycarbonate samples are shown in Fig. 1. The TG curve of TBBA blended polycarbonate shows 4% weight loss between 220 and 280 8C which is due to the evaporation of TBBA from the polymer sample. The DTG maxima of this sample also shifted to 15 8C lower temperature, indicating that TBBA slightly modificates the polycarbonate before it evaporates. During slow heating, the thermal decomposition of polycarbonate blended with TBP does not differ significantly from the decomposition of plain polycarbonate sample. The most characteristic changes were found in the case of the APP blended polycarbonate. Calculated and experimental TG and DTG curves of APP blended polycarbonate are shown in Fig. 2a. The calculated curves were obtained by the superposition of the experimental curves of the individual components in appropriate ratio (polycarbonate:APP = 0.7:0.3). The shape of the TG and DTG curves of APP blended polycarbonate differs to a great extent from that of the calculated curves. This indicates that the thermal decomposition of polycarbonate is strongly influenced by APP. Fig. 2b presents the evolution curve of ammonia originating from APP, that of phenol evolved from polycarbonate, and that of hydrogen indicating the char formation under gradual slow heating of APP blended polycarbonate. The ion profiles show that a huge amount of phenol evolves from polycarbonate in the temperature range of ammonia and water evolution from APP. The similar shape and position of ion curves suggest that the release of phenol and ammonia proceeds parallel between 300 and 450 8C. This observation can be explained as the evolved ammonia could assist the basis supported hydrolitic decomposition of carbonate groups. The DTG maxima at 514 8C
Fig. 2. Experimental and calculated TG and DTG curves (a) and evolution of selected ion profiles (b) of polycarbonate flame retarded by APP.
shows that the maximal rate of the radical thermal decomposition of the rest of the polycarbonate was shifted to only a few degrees lower temperature than that of the pure polycarbonate. Thus, the presence of APP does not induce remarkable changes in the radical decomposition of polycarbonate chain, nonetheless partial hydrolysis takes place at lower temperature. The evolution of hydrogen under the last flat DTG peak sign char formation up to 700 8C. We should mention that in contrast to the pure polycarbonate and the TBBA and TBP blended polycarbonate where the reprodubilicity of the TG and DTG curves were excellent, the TG measurements of polycarbonate blended with APP were not satisfactorily reproduced, especially in the temperature range at above 550 8C. The reason could be the inhomogenity of the blend or the different shape of the sample, or even the complex chemical and physical interactions taking place during char formation. 3.2. Effect of flame retardant additives on the yield of pyrolysis products of polycarbonate under fast pyrolysis
Fig. 1. TG and DTG curves of polycarbonate and flame retarded polycarbonate samples.
The influence of flame retardants on the product distribution of polycarbonate under fast pyrolysis was examined by Py-GC/ MS. Among the conditions of analytical pyrolysis applying a fast heating rate the thermal decomposition of polycarbonate and flame retardant takes place simultaneously, thus, we may expect different effects than during gradual heating. The PyGC/MS experiments were carried out at 550 and 700 8C pyrolysis temperature. The total ion chromatograms of the flame retarded and pure polycarbonate samples are shown in
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structure of the unknown peaks was made with the help of usual mass spectrometric identification principles and gas chromatographic retention relations. In Table 2 those compounds are labelled by an asterix which could be undoubtedly identified by MS library search. The identity of peaks 4 and 6 were confirmed by GC retention data similarity to those published for authentic standards [3]. The retention index (RI) of the compounds is evaluated using retention time values of n-alkane pyrolysis products of polyethylene at 500 8C analyzed on the same GC column among the same conditions as the PC products. RIx ¼ N a þ
Fig. 3. Py-GC/MS total ion chromatograms of flame retarded and pure polycarbonate samples at 550 and 700 8C pyrolysis temperature.
Fig. 3. The chemical name of the main identified decomposition products labeled by numbers are given in Table 2. 3.2.1. GC/MS identification of pyrolysis products of PC and its blends The mass spectra of the separated peaks in the pyrograms are often not existing in the mass spectral libraries and are not available in the literature, thus, elucidation of the chemical
Rtx Rta Rtaþ1 Rta
where Na is the carbon number of the n-alkane of lower retention neigbouring compound x in the retention time scale, Rta the retention time of this n-alkane, Rta+1 the retention time of the n-alkane of higher retention neigbouring compound x, and Rtx is the retention time of the compound x. The retention index difference of peak 7 and diphenyl ether (RI = 14.05) is similar to that of 4-methylethenylphenol and phenol, namely 3.36 and 3.28 methylene units, respectively, – corresponding to an additional 2-propenyl group at position 4 of a phenol ring – that is confirming the structure identified and displayed in Fig. 4. Again similar retention index increments confirm the analogous structure change (addition of a hydroxy at position 4) from 2-phenyl-2-(40 -hydroxyphenyl)-propane – peak #9 – to bisphenol A – peak #15 – (DRI = 3.36) and from 9-methylfluorene (RI = 16.35) to 3-hydroxy-9-methyl-fluorene – peak #8 – (DRI = 3.18), moreover, from peak #8 to 3,6-dihydroxy-9methyl-fluorene – peak #14 – (DRI = 3.36). The mass spectra of the fluorenic compounds of peaks #8 and #14 are shown in Fig. 5, together with that of peak #10. The mass spectra of these compounds have similar pattern, and the fragment ion at m/z 119 or 121 characteristic for 4-alkenyl or -isoalkyl phenols is hardly observable confirming the GC identification.
Table 2 Main decomposition products of polycarbonate GC peak #
Compound name
[M]+
Retention time (min)
RIa (CH2 units)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Phenolb 4-Methylphenolb 4-Ethylphenol b 4-Ethenylphenol 4-Methylethylphenolb 4-Methylethenylphenol (4-(20 -Propenyl)-phenyl)-phenyl-ether 3-Hydroxy-9-methyl-fluorene 2-Phenyl-2-(40 -hydroxyphenyl)-propaneb 3-Hydroxy-9-dimethyl-fluorene 2-(4-Methylphenyl)-2-(4-hydroxyphenyl)-propane 4,40 -Methylenebisphenolb 1,1-Di-(4-hydroxyphenyl)-ethane 3,6-Dihydroxy-9-methyl-fluorene Bisphenol Ab 3,6-Dihydroxy-9-dimethyl-fluorene
94 108 122 120 136 134 210 196 212 210 226 200 214 212 228 226
5.98 7.44 8.82 9.56 9.71 10.78 16.25 17.31 17.51 18.41 18.51 19.47 20.02 20.55 20.61 21.05
9.68 10.70 11.61 12.11 12.21 12.96 17.44 18.44 18.60 19.53 19.61 20.65 21.22 21.80 21.85 22.39
a b
Retention index. Identified by MS library search.
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Fig. 4. MS spectrum and compound structure of Py-GC/MS peaks #6 and #7 of polycarbonate pyrolysed at 700 8C.
Fig. 6. MS spectrum and compound structure of Py-GC/MS peaks #11 and #13 of polycarbonate pyrolysed at 700 8C.
In Fig. 6 spectra of peaks #11 and #13 are given. The structures of the corresponding compounds can be derived from those of identified peaks #9 and #12, respectively, by the addition of a methylene group. A methyl group addition to the phenyl ring of compound #9 at position 4 should result in a peak of one methylene unit higher retention index. The retention index difference of peaks #9 and #11 is just 1.01 as read in Table 2. However, the index difference of peaks #12 and #13 is only 0.57, indicating that the additional methylene group to the molecule of compound #12 is at the methylene unit in compound #13.
3.2.2. Products of polycarbonate blended with TBBA Halogen-containing flame retardants act by interfering with the radical chain mechanism taking place in the gas phase during combustion, subsequently we should expect several bromine containing and so environmentally hazardous products: decomposition fragments of TBBA as well as brominated polycarbonate derivatives. Brominated compounds evolved from TBBA flame retarded polycarbonate at 500 8C pyrolysis temperature were reported previously [10]. In the present study some more brominated compounds were identified at elevated temperature. The integrated values of the selected ion peak areas related to the mass of TBBA in the sample are summarized in Table 3. The amounts of the main volatile compounds evolved from polycarbonate at 550 and 700 8C pyrolysis temperature are shown in Fig. 7. The decomposition of flame retarded polycarbonate is a little more extensive at 550 8C than by pyrolysing the pure polymer at that temperature. The hydrogen bromide evolved from TBBA could assist the decomposition of polycarbonate chain and effect an increased volatile compound formation. At 700 8C pyrolysis temperature, the amounts of evolved volatile products do not change significantly in the presence of TBBA, indicating that at higher temperature this factor became negligible as compared to unassisted thermal decomposition of polycarbonate chain. 3.2.3. Thermal decomposition of polycarbonate containing phosphorous flame retardants Phosphorous-containing flame retardants influence mainly the reactions taking place in the condensed phase. These intumescent type flame retardants assist the char formation by migrating to the polymer surface to form a thermo-protective solid layer. Thus, in the presence of phosphorous flame retardants we can expect decreased amount of evolved volatile products from the polycarbonate sample.
Fig. 5. MS spectrum and compound structure of Py-GC/MS peaks #8, #10 and #14 of polycarbonate pyrolysed at 700 8C.
3.2.3.1. Products of polycarbonate blended with TBP (Irgafos 168). Fig. 8 shows the amounts of the main volatile products
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Table 3 GC–MS selected ion peak areas of brominated pyrolysis products from TBBA blended polycarbonate at 550 and 700 8C [105(couts mg1)] Compound name Methyl bromide Bromobenzene 1-Bromo-4-methylbenzene 2-Bromophenol 1-Bromo-4-ethylbenzene 1-Bromo-4-ethenylbenzene 2-Bromo-4-methylphenol 1-Bromo-4-methylethylbenzene 1-Bromo-4-methylethenylbenzene 4-Bromophenol 2-Bromo-4-ethenylphenol 2-Bromo-4-methylethylphenol 2,4-Dibromophenol 2,6-Dibromophenol 2-Bromo-4-methylethenylphenol 2,6-Dibromo-4-methylethylphenol 2,4,6-Tribromophenol 2,6-Dibromo-4-methylethenylphenol 2-(Bromophenyl)-2-phenyl-propane Bromo-9,9-dimethyl-fluorene 2-(Bromophenyl)-2-phenyl-propane 2-(Bromophenyl)-2-hydroxyphenyl-propane 2-(Bromophenyl)-2-hydroxyphenyl-propane Monobromo bisphenol A Bromo-3,6-dihydroxy-9-methyl-fluorene Dibromo bisphenol A Dibromo bisphenol A Tribromo bisphenol A TBBA
Retention time (min) 1.84 5.27 7.04 7.42 8.52 8.83 9.07 9.46 10.19 10.53 10.84 11.26 11.47 11.89 12.05 15.08 15.15 15.71 17.4 18.19 20.38 20.64 21.64 21.71 22.709 22.78 24.23 25.16 27.28
Selected ion +
[M] [M]+ [M]+ [M + 2]+ [M]+ [M]+ [M]+ [M]+ [M]+ [M]+ [M]+ [M]+ [M + 2]+ [M + 2]+ [M]+ [M + 2]+ [M + 2]+ [M + 2]+ [M]+ [M]+ [M]+ [M]+ [M]+ [M 15]+ [M + 2]+ [(M + 2)15]+ [(M + 2)15]+ [(M +2 )15]+ [(M + 4)15]+
m/z
550 8C
700 8C
94 156 170 174 184 182 186 198 196 172 198 214 252 252 212 294 330 292 274 272 274 290 290 291 370 371 371 449 529
5.9 12.2 7.1 7.1 2.6 3.4 0.9 0.8 10.9 5.3 1.4 0.8 37.7 13.6 17.2 1.3 0.0 28.6 2.8 0.0 8.2 179.5 0.0 30.3 0.0 106.9 26.5 244.1 84.9
5.4 23.9 19.8 19.5 5.2 12.3 2.4 1.4 16.1 14.4 4.1 0.7 35.0 17.5 19.2 0.9 0.7 21.8 1.7 2.6 239.4 77.3 8.6 11.6 13.7 31.1 6.1 54.1 61.0
Fig. 7. GC–MS total ion peak areas of main pyrolysis products of polycarbonate and TBBA blended polycarbonate at 550 and 700 8C pyrolysis temperature [(couts/ mg) 107]. (Amounts of bisphenol A at 550 8C and 1,1-di-(4-hydroxyphenyl)-ethene at 700 8C are divided by 10 and 2, respectively.)
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Fig. 8. GC–MS total ion peak areas of main pyrolysis products of polycarbonate and TBP blended polycarbonate at 550 and 700 8C pyrolysis temperature [(couts mg1) 107]. (Amounts of bisphenol A at 550 8C and 1,1-di-(4-hydroxyphenyl)-ethene at 700 8C are divided by 10 and 2, respectively.)
Fig. 9. GC–MS total ion peak areas of main pyrolysis products of polycarbonate and APP blended polycarbonate at 550 and 700 8C pyrolysis temperature [(couts mg1) 107]. (Amounts of bisphenol A at 550 8C and 1,1-di-(4-hydroxyphenyl)-ethene at 700 8C are divided by 2.)
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Scheme 1.
evolved from the pure and TBP blended polycarbonate sample at 550 and 700 8C pyrolysis temperature. The composition of the pyrolysis oil evolved from TBP blended polycarbonate does not change significantly in comparison to the pure polycarbonate at 550 8C. At 700 8C pyrolysis temperature the main volatile products evolved in increased amount from the pure polycarbonate, however from the policarbonate blended with TBP the volatile product formation was even lower than at 550 8C. This observation indicates that the TBP flame retardant is effective at 700 8C, but not so at 550 8C among our conditions. The relative amounts of evolved products does not change significantly either at 550 or at 700 8C temperature, denoting that the presence of TBP does not affect the decomposition routes leading to volatile products. 3.2.3.2. Products of polycarbonate blended with APP (Exolit AP 422). Among the examined flame retarded polycarbonate blends, the most significant changes were observed in case of
polycarbonate sample containing APP. The amounts of the main volatile compounds evolved at 550 and 700 8C pyrolysis temperature are shown in Fig. 9. As it is demonstrated, the volatile compound formation is repressed at both temperatures, indicating the effectiveness of intumescent type APP flame retardant at these temperatures. Besides the overall decreased volatile product formation, significant changes were found in the composition of pyrolysis oil compared to pure polycarbonate. The product analysis indicates that APP accelerates the formation of phenol and isopropylene phenol from the bisphenol A moiety of polycarbonate resulting in an increased phenol and methylethenylphenol, and decreased bisphenol A ratio in the volatile product. This observation correlates with the results of decomposition at slow heating rate studied by TG–MS (see Section 3.1 above). The evolution of phenol at as low temperature as between 300 and 400 8C from the APP blended PC indicates that the reaction of decomposition is not a radical but an ionic one at this stage. An enhanced formation of phenol and 4-methylethenylphenol from bisphenol A segments is proposed explaining the experimental observations by the ionic interaction of ammonia evolving from APP by heating in Scheme 1. The cleavage of the polycarbonate chain at the C–C bond is facilitated through loosening this bonding in the phenolate ionic form, and results in an increased amount of the products of phenol and 4-methylethenylphenol. The proposed mechanism is shown in Scheme 1. In addition, not negligible carbonate group containing volatile products are also present in the corresponding pyrograms in Fig. 3 (among them the most important is the last unlabeled peak). The mass spectra of these compounds are shown in Fig. 10. 4. Conclusion
Fig. 10. MS spectrum and compound structure of Py-GC/MS peaks of carbonate products from APP blended polycarbonate pyrolysed at 700 8C.
Analytical pyrolysis experiments carried out under gradual and fast heating conditions revealed that the thermal decomposition of polycarbonate is affected by the presence of certain flame retardant additives. Under gradual heating in the temperature range of ammonia and water evolution from APP the polycarbonate chain decomposes by basis catalysed hydrolysis of carbonate groups. APP also accelerates the disproportionation of bisphenol A segment through the ionic cleavage of the C–C bond of ammonium 4-(2-propenyl phenolate) moiety. The presence of both phosphorous flame retardants investigated significantly decrease the quantity of the total evaluated
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volatile compounds at 700 8C pyrolysis temperature due to the intumescent flame retardants char forming feature by migrating to the polymer surface to form a thermo-protective solid layer. The thermal stability of polycarbonate slightly decreases in the presence of TBBA. Several bromine containing compound are evolving under pyrolysis from TBBA flame retarded polycarbonate. Acknowledgement This work was supported by the Hungarian National Research Fund (OTKA) contract No. T047377 and K61504. References [1] S.V. Levchik, E.D. Weil, Polym. Int. 54 (2005) 981. [2] E. Jakab, M.A. Uddin, T. Bhaskar, Y. Sakata, J. Anal. Appl. Pyrol. 68–69 (2003) 83.
345
[3] B.N. Jang, C.A. Wilkie, Thermochim. Acta 433 (2005) 1. [4] L.H. Lee, J. Polym. Sci. 2 (1964) 2859. [5] M. Blazso´, in: J. Scheirs, W. Kamisky (Eds.), Feedstock Recycling and Pyrolysis of Waste Plastics, John Wiley & Sons, 2006,, p. 338. [6] C. Puglisi, L. Sturiale, G. Montaudo, Macromolecules 32 (1999) 2194. [7] G. Montaudo, S. Carroccio, C. Puglish, J. Anal. Appl. Pyrol. 64 (2002) 229. [8] A. Davis, J.H. Golden, Macromol. Chem. 110 (1967) 180. [9] T. Yoshioka, K. Sugawara, T. Mizoguchi, A. Okuwaki, Chem. Lett. 34 (2005) 282. [10] M. Blazso´, Z. Cze´ge´ny, J. Chromatogr. A 1130 (2006) 91. [11] F. Barontini, V. Cozzani, K. Marsanich, V. Raffa, L. Petarca, J. Anal. Appl. Pyrol. 72 (2004) 41. [12] F. Barontini, K. Marsanich, L. Petarca, V. Cozzani, Ind. Eng. Chem. Res. 43 (2004) 1952. [13] M.P. Luda, A.I. Balabanovich, A. Hornung, G. Camino, Polym. Adv. Technol. 14 (2003) 741. [14] G. Camino, L. Costa, L. Trossatelli, Polym. Degrad. Stab. 12 (1985) 203. [15] P.J. Davies, A.R. Horrocks, A. Alderson, Polym. Degrad. Stab. 88 (2005) 114.