Thermal degradation of polyester–metal oxide mixtures: Fibrous morphology of carbonaceous compounds and pyrolysis products distribution

Journal of Analytical and Applied Pyrolysis 89 (2010) 183–190

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Thermal degradation of polyester–metal oxide mixtures: Fibrous morphology of carbonaceous compounds and pyrolysis products distribution O. Terakado ∗ , M. Ueda, M. Hirasawa Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, 464-8603 Nagoya, Japan

a r t i c l e

i n f o

Article history: Received 11 November 2009 Accepted 10 August 2010 Available online 17 August 2010 Keywords: Poly(butylene terephthalate) Poly(trimethylene terephthalate) Poly(ethylene naphthalene-2,6-dicarboxylate) Metal oxide Carbonaceous compound Morphology

a b s t r a c t The influence of the presence of metal oxide on thermal decomposition of various polyester, poly(butylene terephthalate), PBT, poly(trimethylene terephthalate), PTT, and poly(ethylene naphthalene-2,6-dicarboxylate), PEN has been studied. The addition of zinc oxide and trivalent rare earth oxide, Ln2 O3 , results in the formation of fibrous pyrolysis residue for PBT and PTT, terephthalic acid derivative polymers, while only the morphology of oxide agglomerates coated with carbonaceous pyrolysis residue was observed for PEN, a naphthalene dicarboxylate derivative polymer. Carbonaceous compounds obtained from acetone and acid treatment of pyrolysis residue have high BET surface area, exceeding 1000 m2 /g in the case of PBT–Ln2 O3 mixtures. Gaseous, liquid and some tar products have been analyzed with GC/MS. The addition of metal oxide reduces the amount of benzoic acid, and consequently the increase of benzene is observed. On the other hand, metal oxide has less influence on the suppression of the formation of naphthoic acid for the decomposition of PEN, with exception of the addition of ZnO, whereby the formation of naphthalene, i.e. decomposition product of naphthoic acid, is considerably enhanced. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The production of thermoplastic aromatic polyester resins has been ever increasing because of their peculiar properties in various fields, such as beverage, automobile and electronic industries. The development of recycling process of these polymers is, thus, of great importance. Pyrolysis treatment is one of the alternatives, and many studies have been carried out mainly for the most commodity polyester, poly(ethylene terephthalate), PET. In the view of the increasing production, studies on thermal degradation behaviour of other commercial polyesters, such as poly(butylene terephthalate), PBT, poly(trimethylene terephthalate), PTT, and poly(ethylene naphthalene-2,6-dicarboxylate), PEN are also reported in literature [1–6]. Moreover, the influence of the addition of metal oxide and hydroxide has been studied with respect to the selective recovery of benzene from PBT and PEN [1], and flame retardancy of the polyester resins [7]. In our previous paper, we reported the formation of unique morphology of pyrolysis residue of the mechanical mixture of PET and metal oxide (MO) powders [8]. The addition of zinc oxide and trivalent rare earth oxides gives the fibrous pyrolysis residues with the fibre diameter of 1 ␮m for ZnO and <100 nm for Ln2 O3 .

∗ Corresponding author. Tel.: +81 52 789 3250; fax: +81 52 789 3250. E-mail address: [email protected] (O. Terakado). 0165-2370/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2010.08.001

Moreover, the acid treatment of these pyrolysis residues gives the carbonaceous compounds with developed mesopores of 5–50 nm size, suggesting the application of the materials to appropriate adsorbent and catalysis supports. These results stimulate us to prepare various carbonaceous compounds by pyrolysis of mixture of polyester resin and metallic compounds. It is very important to find out the correlation of various process parameters with the morphology of the pyrolysis residue. The process parameters include combination of polymer and oxide, composition of the mixture, reaction temperature and reaction time. In the present paper we report the influence of metal oxide on pyrolysis behaviour PBT, PTT and PEN with emphasis on the carbonaceous compounds. 2. Experimental In the present study we have used PBT, (–OCOC6 H4 COO (CH2 )4 –)n , from Aldrich (average Mv ∼ 38000, melting point = 227 ◦ C), PTT, (–OCOC6 H4 COO(CH2 )3 –)n , from Solotex Co. Ltd. (Inherent viscosity: 0.9, melting point = 229 ◦ C), and PEN, (–OCOC10 H6 COO(CH2 )2 –)n , supplied from Teijin Co. Ltd. (Type TN8065S, Inherent viscosity: 0.68, melting point = 265 ◦ C). They were ground with milling cutter, and powders sieved under 250 ␮m were mixed mechanically with metal oxide powders for ca. 10 min in mortar in the cases of PBT and PEN. As for the PTT, the milling treatment resulted in the wool-like form, so that the

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Table 1 Results of elementary analysis of pyrolysis residue, and carbon yield. Oxide

C (mass%)

H (mass%)

Carbon yield (mass%)

(a) PBT–MO systems 8.2 Nd2 O3 8.9 La2 O3 ∼0 Fe2 O3 ZnO 40 None 89

0.42 0.36 0.15 0.43 1.2

9.9 11 ∼0 15 2.4

(b) PTT–MO systems 7.8 Nd2 O3 11 La2 O3 9. 6 Fe2 O3 ZnO 51 None 84

0.24 0.66 0.14 0.39 1.2

10 14 4.9 9.2 5.6

(c) PEN–MO systems Nd2 O3 26 La2 O3 25 Fe2 O3 49 ZnO 56 None 94

0.36 0.39 0.16 0.61 0.83

33 29 34 29 35

milled samples were mixed in mortar with metal oxide without sieving. The metal oxide had a particle size of 1–10 ␮m with typical BET surface area of 1 m2 /g [9]. The composition of the mixture was [structural unit of polymer]:[oxide] = 2:1 in molar ratio. The sample mixtures were pelletised with a load of ∼14 MPa into a disc of 8 mm in diameter. The pyrolysis experiments were conducted with horizontal quartz reactor. The details of the experimental set-up are described elsewhere [8]. The sample pellet was placed on an alumina boat which was inserted in a quartz tube of 700 mm in length and 26 mm in i.d. Pyrolysis was performed under helium atmosphere with the flow rate of 100 ml/min. The sample was quickly inserted into the hot zone of the quartz reaction tube placed in a horizontal electric furnace. The reaction time was 15 min. Evolved gas and volatile organic compounds were captured at a trap cooled at liquid nitrogen temperature. After the pyrolysis run the trap was brought to room temperature, and the gaseous products were transferred to a gas bag and the volatile compounds were collected with acetone. Both products were analyzed by as chromatography–mass spectrum (GC/MS) analyzer (GC/MS 6890/5973, Hewlett-Packard). The reaction products adhered on the reaction tube were carefully collected with acetone after the pyrolysis run, and analyzed by GC/MS. We have used HP-PLOTQ column for the analysis of gaseous products, benzene and toluene and DB-5 column for the analysis of the products adhered on the reaction tube. The typical experimental error of ∼15% was for the amount of pyrolysis products.

Fig. 1. TG curves of polyester–metal oxide mixtures under helium atmosphere: (a) PBT, (b) PTT, and (c) PEN systems. The heating rate is 15 ◦ C/min.

The pyrolysis residues were characterised by elementary analysis (MT-6 equipment, Yanaco), scanning electron microscopy, SEM, (JSM-6060 equipment, JEOL) and X-ray diffraction, XRD, (XRD-6100 apparatus with Cu K␣ radiation, Shimadzu). The experimental error of the elementary analysis was within 10% for the analysis of carbon. Moreover, thermogravimetry (TG) measurements have been conducted under the same condition of carrier gas by using TGA2050 apparatus (TA Instrument) in order to investigate the thermal decomposition behaviour. In order to study the influence of metal oxides especially on the pore structures of the carbonaceous compounds prepared from polyesters, all pyrolysis residues were treated with acetone to remove the light organic compounds, and then with hydrochloric acid (ca. 1 M) for 24 h in order to remove metallic compounds. During the latter treatment, CO2 gas was bubbled to avoid the oxi-

Table 2 BET and BJH surface areas and the mesopore ratio of the carbonaceous compounds prepared by the acid-treated pyrolysis residue (pyrolysis temperature = 800 ◦ C). Oxide

Nd2 O3

La2 O3

Fe2 O3

ZnO

None

PBT–MO systems SBET (m2 /g) SBJH (m2 /g) Mesopore ratio (%)

1402 1135 81

1516 1289 85

n.a.a

886 394 44

22 0 0

PTT–MO systems SBET (m2 /g) SBJH (m2 /g) Mesopore ratio (%)

1141 930 82

1143 1053 92

456 451 99

868 490 56

n.a.b

PEN–MO systems SBET (m2 /g) SBJH (m2 /g) Mesopore ratio (%)

527 144 27

526 213 40

363 229 63

215 36 17

n.a.b

a b

Carbonaceous compounds not available. Data not available because of the incomplete carbonization.

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Fig. 2. SEM image of pyrolysis residue of PBT–metal oxide mixtures (pyrolysis temperature = 800 ◦ C, reaction time = 10 min).

dation of carbonaceous compounds. The obtained samples were characterized by adsorption isotherm which has been determined at −196 ◦ C using nitrogen gas by Tristar 3000 equipment (Micrometrics). 3. Results and discussion 3.1. TG results Fig. 1 shows the TG curves of polymer–MO mixtures under the helium atmosphere (flow rate = 100 ml/min) with heating rate of 15 ◦ C/min. For pure polymer a large mass reduction stage is observed at temperature of 400–500 ◦ C. Almost no char is observed for PBT and PTT heated up to 850 ◦ C, while ca. 20 mass% of the initial sample remains as residue in the case of pyrolysis of PEN. The addition of oxide does not change the essential feature of the TG curve of polymers. The main degradation of each polymer

is observed at the same temperature of pure polymer. However, it is clearly seen that the addition of zinc oxide lowers the initial temperature of mass loss of PBT. The difference in temperature, at which the initial 5 mass% loss occurs, between PBT and PBT–ZnO is 15 ◦ C, which is statistically significant from the experimental error of temperature in TG measurements (within ±2 ◦ C). Similar result is observed for thermal decomposition of mixture of ZnO and PET [8]. Moreover, the formation of light organic compound, such as benzene, is observed for PET–ZnO at temperature below the main weight reduction of the pure PET [8]. Thus, we suppose that the present observation of the decrease of the initial mass loss for the PBT–ZnO is ascribed to the promotion of polymer degradation by the addition of ZnO taking place at temperature above the melting of polymer and below the temperature of main decomposition stage. On the other hand, main mass reduction stage is less influenced by the addition of ZnO in the case of PTT and PEN, as seen in the TG result. This indicates less interaction between the polymers and

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Fig. 3. SEM image of pyrolysis residue of PTT–metal oxide mixtures (pyrolysis temperature = 800 ◦ C, reaction time = 10 min).

the oxide than the cases of PBT and PET at the initial degradation stage. The result is especially surprising for PTT–ZnO system, because similar TG results to those of PET– and PBT–ZnO mixture, i.e. the lowering of the temperature of the initial mass reduction of ca. 15 ◦ C in comparison to that of pure polymers, would be expected. Therefore, clear explanation of the difference in the influence of the addition of zinc oxide on TG curve between PTT and other terephthalic acid derivative polyesters is currently not provided. Other factor, such as physical characteristics, can be responsible for the difference in the TG curves between PBT, PET and PTT. It should be noted that the initial polymer sample has different form: that of PBT and PET is typical powder, while the PTT sample after milling treatment exhibits the wool-like form. Further researches should be carried out focusing the initial decomposition of polyester–ZnO mixtures. Another interesting observation of the TG results of the oxideadded sample is the appearance of the weight reduction stage after the main polymer degradation stage. That can be related with the formation of stable complex specie arising from the polymer and

metal oxide. The similar weight reduction stage is observed for the PET–oxide mixture [8]. 3.2. Characterisation of pyrolysis residue Table 1 shows the result of elementary analysis of pyrolysis residue. Carbon yield is also included in the table, which is defined as 100 × (the amount of carbon in pyrolysis residue)/(initial amount of carbon in sample). Comparison of the result among different oxides is not straightforward, because different amount, mass of oxide is added in the initial sample mixture. However, it is clearly seen that the carbon yield increases significantly by the addition of metal oxide in the cases of PBT and PTT mixture. The exception is found in the case of the addition of Fe2 O3 , whereby the carbon yield decreases by the oxide addition. Because of the low carbon yield of PBT, that of PBT–Fe2 O3 becomes apparently zero. XRD measurements show that the pyrolysis residue of both PBT– and PTT–Fe2 O3 mixture contains metallic iron, as is also observed in the PET–iron oxide mixture [8]. In these cases, the reduction of iron oxide to

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187

Fig. 4. SEM image of pyrolysis residue of PEN–metal oxide mixtures (pyrolysis temperature = 800 ◦ C, reaction time = 10 min).

metallic iron is an important reaction in which the carbon originating from polymer is consumed. It should be here noted that the reduction of ZnO also takes place in the polymer–ZnO systems, as confirmed by the deposition of metallic zinc at the cold part of the reaction tube. The amount of the deposits is ca. 10% of the total Zn, and the rest remains in the form of oxide in pyrolysis residue. Thus, the consumption of carbon is not significant in the ZnO systems. The result of carbon yield suggests that the zinc oxide plays an important role on the increase of the carbon yield. In the case of the PEN system, the carbon yield does essentially not change by the addition of any oxides examined in the present study, the result indicating less interaction between polymer and oxide, as will be further discussed in the following sections. The SEM image of pyrolysis residue is presented in Figs. 2–4. For pure polymers no characteristic morphology is observed for the pyrolysis residues. On the other hand, the addition of metal oxide causes significant changes in the morphology for PBT and

PTT systems. The pyrolysis residue of PBT– and PTT–Fe2 O3 mixture are porous with the particle size of a few micrometers. Taking the particle size of the iron oxide into account, we anticipate that the carbonaceous compound covers the oxide particles in these mixtures. And the SEM image of pyrolysis residue of PBT–iron oxide mixture is essentially that of the Fe particle, since the carbon content of the pyrolysis residue of the PBT–Fe2 O3 mixture is ∼0 mass%. The addition of rare earth oxide and zinc oxide gives rise to the formation of fibrous carbonaceous compounds with the diameter less than 1 ␮m. Since similar results are observed for PET systems, the oxides interact with terephthalic acid derivative polyester to form the unique fibrous morphology. The fibrous morphology is more clearly observed in the case of the PBT system than the case of PTT system. As discussed in the next section, this difference reflects the tendency in the relative surface area of both the systems. As for the PEN system, an interesting observation is that fibrous carbonaceous compounds are essentially not formed by the addi-

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Table 3 Yield of pyrolysis products, normalized by the weight of initial PBT. Pure PBT

+ZnO

+Fe2 O3

+La2 O3

+Nd2 O3

Gaseous products Carbon monoxide Methane Carbon dioxide Ethylene Ethane Propene Propane

0.055 0.025 0.020 0.0071 5.7E−04 0.0015 3.9E−05

0.054 0.017 0.035 0.012 7.0E−04 0.0025 4.7E−04

0.14 0.017 0.043 0.011 7.3E−04 0.0024 5.9E−04

0.12 0.018 0.029 0.013 6.7E−04 0.0029 3.1E−04

0.099 0.020 0.028 0.014 8.1E−04 0.0027 6.2E−04

Liquid products Benzene Toluene

0.018 0.0047

0.035 0.010

0.034 0.0054

0.038 0.0071

0.058 0.0077

Tar products 2-Methyl naphthalene Biphenyl Phenanthrene Fluorene Benzophenone Benzoic acid Naphthalene

0.0015 0.0070 0.0035 0.0010 4.6E−04 0.18 0.0077

4.2E−04 0.0033 0.0017 6.3E−04 0.0017 0.060 0.0013

5.4E−04 0.0027 0.0013 4.1E−04 0.0012 0.089 0.0032

2.2E−04 0.0030 8.8E−04 9.9E−04 0.0019 0.025 9.8E−04

6.6E−04 0.0045 0.0021 0.0011 0.0022 0.038 0.0017

tion of oxide. The SEM observation shows that the oxide particle is covered by the carbonaceous compounds originating from PEN. Thus, the interaction between PEN and oxide is not strong enough to form characteristic fibrous morphology. From the above results as well as our previous study on PET, it is concluded that the effect of metal oxide on the pyrolysis of polyesters is significantly different between terephthalic acid derivatives and those from naphthalene dicarboxylate. 3.3. Porous structure of carbonaceous compounds prepared from pyrolysis residue We have prepared carbonaceous compounds by washing pyrolysis residues with acetone and then with HCl aqueous solution. As mentioned above, the carbon content was essentially zero for the pyrolysis residue of PBT–Fe2 O3 mixture, from which it was not possible to obtain carbonaceous compound. For other systems we could prepare carbonaceous materials with the mean carbon content of 85 mass% after the washing treatment. Fig. 5 shows the adsorption and desorption isotherm of nitrogen at −196 ◦ C for the carbonaceous compounds. The isotherms for all the oxide added systems exhibits the large hysteresis at high relative pres-

sure range, which indicates the formation of mesopores with the pore size of 5–50 nm. For the discussion on porosity of the materials we have evaluated the BET and BJH surface areas which are measures of total surface area and that of mesopore contributions, respectively [8]. The latter surface area is, strictly speaking, valid only for the cylindrical pores whose adsorption isotherm is different from the present result [9]. However, quantitative understanding of the porosity of the obtained carbonaceous compounds is possible by the comparison of the BJH surface areas. The results are summarized in Table 2. The mesopore ratio is defined as 100 times SBJH /SBET . As for the carbonaceous compounds obtained from pure polymers, reasonable data were not available with exception of PBT. This is presumably due to the incomplete carbonization of the polymer. In this case, solid materials have very narrow micropores, so that the equilibrium adsorption data is not available because of very slow diffusion of nitrogen into the pores [10]. Taking the data of SBET data of PBT into account, we anticipate that the relative surface area of PTT and PEN is the order of 10 m2 /g. The addition of metal oxide leads to the development of mesopores for all the examined polymers and thus the BET and BJH calculation was possible. The influence of oxide on the surface

Table 4 Yield of pyrolysis products, normalized by the weight of initial PTT. Pure PTT

+ZnO

+Fe2 O3

+La2 O3

+Nd2 O3

Gaseous products Carbon monoxide Methane Carbon dioxide Ethylene Ethane Propene Propane

0.070 0.014 0.041 0.010 9.0E−04 0.0058 5.7E−04

0.13 0.014 0.044 0.012 9.9E−04 0.0063 7.6E−04

0.25 0.029 0.058 0.016 0.0011 0.0064 1.9E−04

0.17 0.018 0.043 0.016 0.0012 0.0065 7.0E−04

0.11 0.015 0.037 0.012 9.3E−04 0.0060 7.5E−04

Liquid products Benzene Toluene

0.036 0.011

0.053 0.0073

0.080 0.011

0.079 0.012

0.097 0.017

Tar products 2-Methyl naphthalene Biphenyl Phenanthrene Fluorene Benzophenone Benzoic acid Naphthalene

0.0013 0.0083 0.0038 0.0019 0.0039 0.16 0.0059

7.7E−04 0.0063 0.0039 0.0011 0.0021 0.015 8.2E−04

9.4E−04 0.015 0.0063 0.0019 0.0039 0.057 0.0016

3.3E−04 0.0033 0.0018 0.0012 0.0021 0.014 4.2E−04

0.0015 0.017 0.011 0.0041 0.0055 0.0075 0.0016

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Table 5 Yield of pyrolysis products, normalized by the weight of initial PEN. Pure PEN

+ZnO

+Fe2 O3

+La2 O3

+Nd2 O3

Gaseous products Carbon monoxide Methane Carbon dioxide Ethylene Ethane Propene Propane

0.15 0.0265 0.022 0.0030 5.1E−04 5.6E−04 5.4E−04

0.15 0.048 0.034 0.0055 7.9E−04 6.7E−04 8.1E−04

0.22 0.025 0.029 0.0041 5.6E−04 4.4E−04 7.4E−04

0.20 0.030 0.021 0.0023 3.9E−04 2.8E−04 3.6E−05

0.16 0.029 0.022 0.0027 4.5E−04 3.9E−04 2.7E−05

Tar products 2-Naphthoic acid Phenanthrene 2-Methyl naphthalene Naphthalene

0.048 0.0024 0.0074 0.13

∼0 0.0020 0.0035 0.24

0.030 8.0E−04 0.0028 0.12

0.058 0.0016 0.0078 0.21

0.11 0.0020 0.0085 0.18

areas is significant for the PBT system. SBET increases more than 40 times by the addition of rare earth oxide. This result can be reasonably interpreted by the morphology change by the addition of oxide where porous structures are observed. Significant amount of mesopores are formed by the addition of oxides, the mesopore ratio being over 80 percent in the case of rare earth oxide addition. The results for the PTT–MO system is comparable to those of the PBT–MO mixtures, though slightly lower values of BET sur-

face area may reflect the less fibrous morphology in the former systems. These results indicate the strong interaction between the terephthalic acid derivative polyesters and oxide in the formation of pores in the pyrolysis residue. Further systematic study can clarify the general principles of the interaction. In the viewpoint of application of the carbonaceous compounds derived from the mixture, high surface area of SBET > 1000 m2 /g for the PBT– and PTT–rare earth oxide mixtures is attractive as adsorbent or various catalysis supports. As for the PEN–MO systems, the SBET and SBJH values are lower than those of mixture of terephthalic acid derivative polymer and oxide. The results can be understood by the difference in morphology of the pyrolysis residues: porous structure of the carbonaceous compounds prepared from the PBT– and PTT–MO mixture can contribute to their higher surface area than those from PEN–MO mixtures.

3.4. Quantitative analysis of gaseous and liquid pyrolysis products

Fig. 5. Adsorption and desorption isotherm curves of nitrogen at −196 ◦ C for the carbonaceous compounds obtained from pyrolysis residues: (a) PBT, (b) PTT, and (c) PEN systems.

Tables 3–5 summarize the pyrolysis product yields of gaseous, liquid and tar products. Since complete mass balance could not be obtained mainly due to the products adhered to quartz reaction which have not been analyzed in the present study, we focus on the essential feature of the pyrolysis products of the polymer and the change of the product distribution by the addition of oxide. For the PBT and PTT systems benzoic acid is the main (more than 80%) tar product. The addition of oxide gives rise to the considerable reduction of benzoic acid. As for the liquid products, the formation of benzene and toluene was observed. The yield of benzene increased slightly by the addition of metal oxide. Moreover, the yield of carbon monoxide and dioxide increases under the presence of oxide. These results can be interpreted by the formation of complex specie between the pyrolysis products with carboxylic end group and metal oxide, and the consequent decarboxylation. It is considered that compounds with carboxylic end groups are formed at the initial stage of the thermal degradation of polyester by ␤CH hydrogen transfer reaction or intramolecular exchange reaction [2,11]. The presence of oxide can result in the formation of complex specie of carboxylic end and oxide. This complex formation gives rise to the reduction of the benzoic acid in the tar pyrolysis products. At higher temperature the complex specie further decomposes to benzene, CO or CO2 and oxide by decarboxylation reaction of the complex. At the current stage, the sufficient mass balance is not observed to prove this mechanism, so that further studies are needed to clarify the detailed role of the oxide. The pyrolysis products of the PEN–MO systems are rather different from those of the mixture of terephthalic acid derivative polyesters and metal oxide. The formation of benzene, toluene and

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other benzene derivatives was not observed but the high yields of naphthalene derivatives. The result is reasonable because of the difference in chemical structure of the PEN and terephthalic acid derivative polymers. The influence of oxide addition on the pyrolysis products distribution is not significant for the PEN system. An interesting observation is that the formation of naphthoic acid is completely suppressed by the addition of zinc oxide. In this case, the yield of naphthalene increased, so that zinc oxide may play a role of complex agent, as discussed in the previous paragraph. 4. Conclusions In the present paper, we addressed the influence of metal oxide on the morphology of the pyrolysis residue and on the pyrolysis products distribution of various polyester resins. The addition of zinc oxide or trivalent rare earth oxide into the terephthalic acid derivative polymers, PBT and PTT, gives rise to the increase of the carbon yield of the pyrolysis residue with the fibrous morphology. The diameters of fibres are 1 ␮m for the former and several tens of nanometres for the latter oxides, respectively. Carbonaceous compounds prepared by treatment with acetone and HCl aqueous solution are rich in mesopores with the high BET surface area (up to 1500 m2 /g for the PBT–rare earth oxide mixture). However, no change in morphology was observed for the naphthoic acid derivative polymer, PEN. This significant difference is probably owing to the complex formation ability of carboxyl end group of the polymer decomposition product with metal oxide. By the addition of oxide, the formation of benzoic acid is considerably suppressed for the PBT and PTT systems, while the effect of oxide addition is not

significant for the PEN system with an exception of the case of ZnO addition. Acknowledgements The authors thank staff members of Chemical Instrumentation Facility, Research Center for Materials Science, Nagoya University for the elemental analysis of pyrolysis products. Financial supports by The Ogasawara Foundation for the Promotion of Science & Engineering and The General Sekiyu Foundation for the Promotion of Research are gratefully acknowledged. References [1] T. Yoshioka, G. Grause, S. Otani, A. Okuwaki, Polym. Degrad. Stabil. 91 (2006) 1002. [2] G. Montaudo, C. Puglisi, F. Samperi, Polym. Degrad. Stabil. 42 (1993) 13 (and references therein). [3] V. Passalacqua, F. Pilati, V. Zamboni, B. Fortunato, P. Manaresi, Polymer 17 (1976) 1044. [4] D.R. Kelsey, K.S. Kiibler, P.N. Tutunjian, Polymer 46 (2005) 8937. [5] L. Zhang, J. Ma, X. Zhu, B. Liang, J. Appl. Polym. Sci. 91 (2004) 3915. [6] T. Ueno, T. Kajiya, T. Ishikawa, K. Takeda, Kobunshi Ronbunshu 64 (2007) 575 (in Japanese). [7] T. Ishikawa, T. Ueno, Y. Watanabe, K. Mizuno, K. Takeda, J. Appl. Polym. Sci. 109 (2008) 910. [8] O. Terakado, M. Hirasawa, J. Anal. Appl. Pyrol. 73 (2005) 248 (and references therein). [9] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders and Porous Solids, Academic Press, London, 1999. [10] N. Cao, H. Darmstadt, C. Roy, Energy Fuels 15 (2001) 1263. [11] G. Botelho, A. Queiros, S. Liberal, P. Gijsman, Polym. Degrad. Stabil. 74 (2001) 39.