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Formation of C7-species pyrolysis products from ethylene–propylene heterosequences of poly(ethylene-co-propylene)
Journal of Analytical and Applied Pyrolysis 92 (2011) 384–391
Contents lists available at ScienceDirect
Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
Formation of C7-species pyrolysis products from ethylene–propylene heterosequences of poly(ethylene-co-propylene) Sung-Seen Choi ∗ , Yun-Ki Kim Department of Chemistry, Sejong University, 98 Gunja-dong, Gwangjin-gu, Seoul 143-747, Republic of Korea
a r t i c l e
i n f o
Article history: Received 28 February 2011 Accepted 26 July 2011 Available online 4 August 2011 Keywords: Ethylene–propylene heterosequences Pyrolysis EPM PE PP EPDM
a b s t r a c t Properties of ethylene–propylene copolymer (EPM) are determined by ethylene/propylene ratio and degree of block and random sequences. EPM was pyrolyzed and the pyrolysis products were analyzed using gas chromatography/mass spectrometry (GC/MS) to examine pyrolysis products formed from the ethylene–propylene heterosequences. Pyrolysis products formed from EPM were compared with those formed from polyethylene (PE) and polypropylene (PP) to determine the pyrolysis products formed from ethylene–propylene heterosequences of EPM. Principal pyrolysis products formed from ethylene–propylene heterosequences were 3-methyl-1-hexene, 4-methyl-1-hexene, 2methyl-1-hexene, and 2-heptene. Order of the relative intensity of the pyrolysis products was 2-methyl-1-hexene > 4-methyl-1-hexene > 3-methyl-1-hexene > 2-heptene. The relative abundances of the pyrolysis products decreased as the pyrolysis temperature increased. Relative abundances of the specific pyrolysis products formed from ethylene–propylene heterosequences may be used for determination of the relative degree of random sequences of EPM as well as ethylene–propylene–diene terpolymer (EPDM). © 2011 Elsevier B.V. All rights reserved.
1. Introduction Ethylene–propylene rubbers including ethylene–propylene copolymer (EPM) and ethylene–propylene–diene monomer (EPDM) continue to be one of the most widely used and fastest growing synthetic rubbers having both specialty and general purpose applications. Since EPM and EPDM have saturated carbon–carbon backbone, they possess excellent resistance to oxygen, ozone, heat, and UV light, whereas the non-polar structure endows them with excellent electrical resistivity and resistance to polar solvents. Hence, it has broad applications to electrical insulation, building construction, and automobile components. Due to their importance, a significant number of studies about EPM and EPDM in various fields have been performed [1–16]. Contents of ethylene and propylene in EPM and EPDM determine their grades. Pyrolysis-gas chromatography (pyrolysis-GC) has been applied to characterize EPMs and EPDMs [17–21]. Pyrolysis products formed from ethylene–propylene heterosequences as well as ethylene and propylene homosequences have been reported [19–21]. Yamada et al. [19] reported that pyrolysis products formed from ethylene–propylene heterosequences in EPDM
∗ Corresponding author. Tel.: +82 2 3408 3815; fax: +82 2 3408 4317. E-mail address: [email protected] (S.-S. Choi). 0165-2370/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2011.07.014
were C6, C7, and C8 species. Especially they discussed about the C7 species but detail mechanisms except 2-methyl-1-hexene were not discussed. Sojak et al. [21] studied pyrolysis products (larger than C5-species) formed from ethylene and propylene homosequences. Wampler et al. [20] studied pyrolysis products (larger than trimer) formed from ethylene and propylene homosequences, and proposed a method to determine ethylene and propylene content ratio of EPM. Properties of EPM are determined by ethylene/propylene ratio and degree of block and random sequences. Thus, analysis of ethylene–propylene heterosequences of EPM is very important. In the present work, pyrolysis products formed from ethylene homosequences, propylene homosequences, ethylene–propylene heterosequences of EPM were differentiated by comparing the pyrolysis products formed from polyethylene (PE), polypropylene (PP), and EPM. Chemical structures of the pyrolysis products were identified by interpreting the mass spectra using a mass spectrum library searching software. Specific pyrolysis products formed only from EPM not PE and PP were determined and their formation mechanisms were proposed. The pyrolysis products formed only from EPM come from ethylene–propylene heterosequences. In this study, formation of the C7 pyrolysis products from ethylene–propylene heterosequences was focused. Variations of the pyrolysis products formed from ethylene–propylene heterosequences with the pyrolysis temperature were also investigated. Kinds and relative abundances of pyrolysis products of polymers
S.-S. Choi, Y.-K. Kim / Journal of Analytical and Applied Pyrolysis 92 (2011) 384–391
385
(a) E2
Intensity
(b) P2
(c)
15.0
15.3
15.6
15.9
16.2
16.5
Retention time (min) Fig. 1. Pyrolysis-GC/MS TIC chromatograms of PE (a), PP (b), and EPM (c) at 800 ◦ C.
Fig. 2. Expanded pyrolysis-GC/MS TIC chromatograms of PE (a), PP (b), and EPM (c) for 1- and 2-pentenes.
vary with the pyrolysis temperature [22–24]. Pyrolysis temperature is a very important factor not only for proper and efficient analysis of polymeric materials but also for recycling of scrap polymeric materials.
42
(a) 55
2. Experimental
70
Intensity
Polyethylene (PE) and polypropylene (PP) were purchased from Sigma–Aldrich Co. (USA). Ethylene–propylene monomer (EPM) (KEP-070P) was obtained from Kumho Polychem Co. (Korea) and its ethylene content was 70.5 wt%. Pyrolysis-GC/MS was carried out by using a pyroprobe 2000 system with a CDS 1500 interface (Chemical Data System, Oxford, USA) coupled to an Agilent 6890 gas chromatograph equipped with a 5973 mass spectrometer of Agilent Technology Inc. (USA). The sample (about 0.2 mg) was preheated at 250 ◦ C for 15 s and pyrolyzed at 700–940 ◦ C for
(b)
Table 1 Major pyrolysis products. Peak no.
Retention time (min)
Product
A1 A2 E1 E2 E3 E4 E5 E6 P1 P2 P3 M1 M2 M3 M4
2.04 6.17 5.51 15.80 34.96 37.50 47.80 48.47 5.40 15.74 33.50 45.80 46.40 47.32 48.23
Propylene n-Butane 1-Butene 1-Pentene 1-Hexene n-Hexane 1-Heptene n-Heptane iso-Butene 2-Pentene 2-Methyl-1-pentene 3-Methyl-1-hexene 4-Methyl-1-hexene 2-Methyl-1-hexene 2-Heptene
40
50
m/z
60
70
Fig. 3. Mass spectra of the E2 (a) and P2 (b) peaks in Fig. 1.
386
S.-S. Choi, Y.-K. Kim / Journal of Analytical and Applied Pyrolysis 92 (2011) 384–391
propylene-ethylene-ethylene E
CH2
CH2 CH3
P-E-E P
CH2
CH
H3 C P'
P'-E-E
CH
CH2
ethylene-propylene-ethylene
E-P-E
E-P'-E Scheme 1. Possible heterosequences of triads of two ethylene units and one propylene unit.
ethylene-propylene-propylene
propylene-ethylene-propylene
E-P-P
P-E-P
E-P-P'
P-E-P'
E-P'-P
P'-E-P
E-P'-P'
P'-E-P'
CH3 E
CH2
CH2
P
CH2
CH
H3 C P'
CH
CH2
Scheme 2. Possible heterosequences of triads of two propylene units and one ethylene unit.
S.-S. Choi, Y.-K. Kim / Journal of Analytical and Applied Pyrolysis 92 (2011) 384–391
387
CH2 H
Fig. 4. Mass spectra of the peaks at 15.57 (a), 15.78 (b), and 15.92 min (c) in Fig. 2(c).
Scheme 3. Mechanism for formation of 3-methyl-1-hexene ethylene–propylene–ethylene sequence of EPM by pyrolysis.
55
69
41
from
(a) 98
Intensity
(b)
(c)
(d)
40
50
60
70
80
90
100
m/z Fig. 6. Mass spectra of the peaks at 45.08 (a), 46.40 (b), 47.32 (c), and 48.23 min (d) in Fig. 5. Fig. 5. Expanded pyrolysis-GC/MS TIC chromatograms of PE (a), PP (b), and EPM (c) for the M-series products.
388
S.-S. Choi, Y.-K. Kim / Journal of Analytical and Applied Pyrolysis 92 (2011) 384–391
CH2 H
H
Scheme 4. Mechanism for formation of 4-methyl-1-hexene from ethylene–propylene–ethylene or ethylene–propylene–propylene sequence of EPM by pyrolysis.
5 s under helium (He) atmosphere. HP-PLOT/Q capillary column (0.32 mm × 15 m, 20 m film thickness) of Agilent Technology Inc. (USA) was used. Temperatures of the interface and injector were 250 ◦ C. The GC oven temperature program was as follows: 100 ◦ C (held for 2 min) to 130 ◦ C (held for 5 min) at 1 ◦ C/min and raised up again to 220 ◦ C (held for 5 min) at 5 ◦ C/min. The interface temperature of GC to MS was 250 ◦ C. The electron ionization (70 eV) was used to ionize the pyrolysis products. The MS source temperature was 230 ◦ C. Each experiment was performed at least three times. 3. Results and discussion PE, PP, and EPM were pyrolyzed and their pyrolysis products were analyzed. Fig. 1 shows pyrolysis-GC/MS TIC chromatograms of PE, PP, and EPM. Major pyrolysis products were marked in the chromatograms and listed in Table 1. Their chemical structures
were identified by interpreting the mass spectra using a mass spectrum library searching software. The A-series are pyrolysis products formed from all of PE, PP, and EPM. The E-series are pyrolysis products formed from PE and EPM, whereas the P-series are pyrolysis products formed from PP and EPM. The M-series are specific pyrolysis products formed only from EPM. Propylene and n-butane were observed in all of PE, PP, and EPM. 1-Butene, 1-pentene, 1hexene, n-hexane, 1-heptene, and heptane were not generated from PP, that is those pyrolysis products were formed from ethylene homosequences (ethylene block). iso-Butene, 2-pentene, and 2-methyl-1-pentene were not generated from PE, that is those pyrolysis products were formed from propylene homosequences (propylene block). Retention times of the E2 and P2 peaks are nearly the same, but the two pyrolysis products are not the same. Fig. 2 shows the expanded pyrolysis-GC/MS TIC chromatograms around E2 and P2 peaks. Maximum points around 15.4–16.2 min in the TIC chromatograms of PE, PP, and EPM were different each other. Mass
S.-S. Choi, Y.-K. Kim / Journal of Analytical and Applied Pyrolysis 92 (2011) 384–391
389
CH2 H
H
Scheme 5. Mechanism for formation of 2-methyl-1-hexene from propylene–ethylene–ethylene, propylene–ethylene–propylene, or propylene–propylene–ethylene sequence of EPM by pyrolysis.
spectra of the E2 and P2 were also different each other as shown in Fig. 3. The E2 and P2 mass spectra were assigned to 1-pentene and 2-pentene, respectively. Fig. 4 shows mass spectra at the retention times at 15.57, 15.78, and 15.92 min in the TIC chromatogram of EPM. The mass spectrum at 15.57 min is the same mass spectrum of the P2, while that at 15.92 min is the same mass spectrum of the E2. The mass spectrum at 15.78 min is a mixed mass spectrum of the E2 and P2. 3-Methyl-1-hexene, 4-methyl-1-hexene, 2-methyl-1-hexene, and 2-heptene were only observed in EPM, that is those pyrolysis products were formed from ethylene–propylene heterosequences. Fig. 5 is the expanded pyrolysis-GC/MS TIC chromatograms around 45–49 min to clearly show the pyrolysis products formed from ethylene–propylene heterosequences. Fig. 6 shows mass spectra of the four pyrolysis products at 45.08, 46.40, 47.32, and 48.23 min. All those pyrolysis products are C7 H14 . The specific pyrolysis products formed only from EPM not PE and PP indicate existence of alternating point between ethylene and propylene.
Yamada et al. [19] reported that C7 pyrolysis products formed from ethylene–propylene heterosequences were 3-methyl-1-hexene, 4-methyl-1-hexene, 5-methyl-1-hexene, 2-methyl-1-hexene, 1heptene, heptane, and 2-heptene by comparing the retention data. According to our experimental results, 1-heptene and heptane were generated from PE as well as EPM. Hence, 1-heptene and heptane were not the pyrolysis products formed from ethylene–propylene heterosequences. The pyrolysis products of M1, M2, M3, and M4 can be formed from ethylene–propylene heterosequences. Possible heterosequences of triads of two ethylene units and one propylene unit were shown in Scheme 1. And possible heterosequences of triads of two propylene units and one ethylene unit were also shown in Scheme 2. 3-Methyl-1hexene can be formed from ethylene–propylene–ethylene or propylene–ethylene–ethylene sequence of EPM. Formation of 3-methyl-1-hexene may be started by C–C bond cleavage and hydrogen atom migrates in the 6-membered structure as
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CH2 H
H
Scheme 6. Mechanism for formation of 2-heptene from propylene–ethylene–ethylene or propylene–ethylene–propylene sequence of EPM by pyrolysis.
shown in Scheme 3. 4-Methyl-1-hexene can be formed from ethylene–propylene–ethylene or ethylene–propylene–propylene sequence of EPM. Formation of 4-methyl-1-hexene may be started by C–C bond cleavage and hydrogen atom migrates in the 5membered structure as shown in Scheme 4. Propylene–propylene sequence of ethylene–propylene–propylene sequence should be head-to-head sequence. Thus, the possibility to form from ethylene–propylene–propylene 4-methyl-1-hexene sequence may be low since the head-to-head sequence generally is not common. 2-Methyl-1-hexene can be formed from propylene–ethylene–ethylene, propylene–ethylene–propylene (head-to-head), or propylene–propylene–ethylene (tail-to-tail) sequence as shown in Scheme 5. Formation of 2-methyl-1hexene from propylene–ethylene–ethylene sequence may be started by C–C bond cleavage and hydrogen atom migrates in the 6-membered structure. For formation of 2-methyl-1hexene from propylene–ethylene–propylene (head-to-head) (tail-to-tail) sequence, or propylene–propylene–ethylene hydrogen atom migration occurs through 5-membered structure. Since the head-to-head or tail-to-tail sequence of propylene–propylene is not common, the possibility to form 2-methyl-1-hexene from the propylene–ethylene–propylene
or propylene–propylene–ethylene sequence may be low. 2Heptene can be formed from propylene–ethylene–ethylene or propylene–ethylene–propylene sequence as shown in Scheme 6. Formation of 2-heptene from propylene–ethylene–ethylene sequence may be started by C–C bond cleavage and hydrogen atom migrates in the 6-membered structure. For formation of 2-heptene from propylene–ethylene–propylene sequence, hydrogen atom migration occurs through the 5-membered structure. Formation of pyrolysis products by rearrangement through the 6-membered ring may be more favorable than that through the 5-membered one due to strain effect. In order to examine the pyrolysis temperature effect on formation of the C7-pyrolysis products generated from ethylene–propylene heterosequences of EPM, relative abundance ratios of the C7-pyrolysis products compared to n-butane were plotted as a function of the pyrolysis temperature (Fig. 7). n-Butane is one of major pyrolysis products formed from all of PE, PP, and EPM, and its peak was well separated from other peaks. All the peak intensity ratios linearly decreased after 800 ◦ C as the pyrolysis temperature increased. The peak intensity ratios of 3-methyl-1-hexene, 4-methyl-1-hexene, 2-methyl-1-hexene, and 2-heptene at 700 ◦ C
S.-S. Choi, Y.-K. Kim / Journal of Analytical and Applied Pyrolysis 92 (2011) 384–391
1.2
4. Conclusions 3-methyl-1-hexene 4-methyl-1-hexene 2-methyl-1-hexene 2-heptene
C7-species/n-butane
1.0 0.8 0.6 0.4 0.2 0.0
391
800
850
900
950 o
Pyrolysis temperature ( C) Fig. 7. Variations of the peak intensity ratios of the C7-pyrolysis products generated from ethylene–propylene heterosequences of EPM. The reference was n-butane. Squares, circles, up-triangles, and down-triangles indicate the peak intensity ratios of 3-methyl-1-hexene, 4-methyl-1-hexene, 2-methyl-1-hexene, and 2-heptene, respectively.
were 0.28, 0.57, 0.94, and 0.15, respectively. The linear curve fitting equations were y = −4.95 × 10−4 x + 0.655, y = −1.28 × 10−3 x + 1.63, y = −2.27 × 10−3 x + 2.72, and y = −1.51 × 10−4 x + 0.296 for 34-methyl-1-hexene, 2-methyl-1-hexene, methyl-1-hexene, and 2-heptene, respectively. Order of the relative abundance was 2-methyl-1-hexene > 4-methyl-1-hexene > 3-methyl-1hexene > 2-heptene. This can be explained by relative stability of radical intermediate and required sequences. Since 2-heptene can be formed from a long ethylene sequence (equal or more than n = 3 of –(CH2 CH2 )n –), its formation may be relatively less favorable than formations of the other pyrolysis products. A radical intermediate to generate 2-methyl-1-hexene has a tertiary structure, whereas radical intermediates to generate 3-methyl-1-hexene and 4-methyl-1-hexene do not have tertiary structures as shown in Schemes 1–3. Hence, formation of 2-methyl-1-hexene may be more favorable than those of the others. A radical intermediate to generate 4-methyl-1-hexene has ␣-methyl group, whereas a radical intermediate to form 3-methyl-1-hexene has -methyl group as shown in Schemes 3 and 4. Therefore, formation of 4-methyl1-hexene may be more favorable than that of 3-methyl-1-hexene since a radical intermediate of the former is stable than that of the latter. Order of the slope (sensitivity to pyrolysis temperature) was also 2-methyl-1-hexene > 4-methyl-1-hexene > 3-methyl-1hexene > 2-heptene. If a certain EPM or EPDM sample has alternating sequence of ethylene and propylene more than other samples, relative abundances of the four specific pyrolysis products (2-methyl-1-hexene, 4-methyl-1-hexene, 3-methyl-1-hexene, and 2-heptene) formed from ethylene–propylene heterosequences of the former will be larger than those of the latters. Therefore, by using the relationship between the alternating sequence and the C7 specific pyrolysis products, relative degree of the randomness of EPM or EPDM can be compared.
Propylene and n-butane were major pyrolysis products formed from all of PE, PP, and EPM. 1-Butene, 1-pentene, 1-hexene, nhexane, 1-heptene, and heptane were pyrolysis products formed from PE and EPM not from PP. iso-Butene, 2-pentene, and 2-methyl1-pentene were pyrolysis products formed from PP and EPM not from PE. Specific pyrolysis products generated only from EPM were 3-methyl-1-hexene, 4-methyl-1-hexene, 2-methyl-1-hexene, and 2-heptene. Formation mechanisms of the pyrolysis products generated only from the heterosequences were proposed. Order of the relative abundance was 2-methyl-1-hexene > 4-methyl-1hexene > 3-methyl-1-hexene > 2-heptene. Peak intensity ratios of the four C7 pyrolysis products (reference: n-butane) decreased as the pyrolysis temperature increased. Since 3-methyl-1-hexene, 4-methyl-1-hexene, 2-methyl-1-hexene, and 2-heptene are only formed from the ethylene–propylene heterosequences in EPM or EPDM, relative abundances of the four specific pyrolysis products can be used for determination of relative degree of the ethylene–propylene alternating sequences. Acknowledgements This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. References [1] H. Yu, G. Xu, X. Shen, X. Yan, C. Hu, Y. Wang, Electrochim. Acta 55 (2010) 1843. [2] S.-S. Choi, H.-S. Chung, Y.-T. Joo, K.-M. Yang, S.-H. Lee, Elast. Compos. 45 (2010) 100. [3] K.H. Seo, K.-S. Cho, I.-S. Yun, W.-H. Choi, B.-K. Hur, D.-G. Kang, Elast. Compos. 45 (2010) 212. [4] H. Zhang, R.N. Datta, A.G. Talma, J.W.M. Noordermeer, Eur. Polym. J. 46 (2010) 754. [5] J. Su, S. Chen, J. Zhang, Z. Xu, Polym. Test. 28 (2009) 235. [6] S. Mitra, M. Jørgensen, W.B. Pedersen, K. Almdal, D. Banerjee, J. Appl. Polym. Sci. 113 (2009) 2962. [7] Q. Zhao, X. Li, J. Gao, Polym. Degr. Stab. 94 (2009) 339. [8] Q. Zhao, X. Li, J. Gao, Z. Jia, Mater. Lett. 63 (2009) 1647. [9] B.K. Hwang, K.H. Hong, H.Y. Park, I.R. Jeon, K.H. Seo, Elast. Compos. 44 (2009) 299. [10] T. Lee, C. Yoon, D.-S. Bang, D.-Y. Ahn, H. Kye, K.-C. Shin, Elast. Compos. 44 (2009) 164. [11] J.S. Kim, J.H. Lee, W.S. Jung, J.W. Bae, H.C. Park, D.P. Kang, Elast. Compos. 44 (2009) 47. [12] F. Barroso-Bujans, R. Verdejo, A. Lozano, J.L.G. Fierro, M.A. Lopez-Manchado, Acta Mater. 56 (2008) 4780. [13] M.A.J. van der Mee, J.G.P. Goossens, M. van Duin, Polymer 49 (2008) 1239. [14] J. Silva, A.V. Machado, J. Maia, Rheol. Acta 46 (2007) 1091. [15] M.-B. Coltelli, E. Passaglia, F. Ciardelli, Polymer 47 (2006) 85. [16] E. Passaglia, M.-B. Coltelli, F. Ciardelli, Helv. Chim. Acta 89 (2006) 1596. [17] L. Michajlov, H.-J. Cantow, P. Zugenmaier, Polymer 12 (1971) 70. [18] S. Tsuge, Y. Sugimura, T. Nagaya, J. Anal. Appl. Pyrolysis 1 (1980) 221. [19] T. Yamada, T. Okumoto, H. Ohtani, S. Tsuge, Rubber Chem. Technol. 63 (1990) 191. [20] T. Wampler, C. Zawodny, L. Mancini, J. Wampler, J. Anal. Appl. Pyrolysis 68–69 (2003) 25. [21] L. Sojak, R. Kubinec, H. Jurdakova, E. Hajekova, M. Bajus, J. Anal. Appl. Pyrolysis 78 (2007) 387. [22] S.-S. Choi, Bull. Korean Chem. Soc. 20 (1999) 1348. [23] F. Chen, J. Qian, Fuel Proc. Technol. 67 (2000) 53. [24] S.-S. Choi, D.-H. Han, Macromol. Res. 14 (2006) 354.
Contents lists available at ScienceDirect
Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
Formation of C7-species pyrolysis products from ethylene–propylene heterosequences of poly(ethylene-co-propylene) Sung-Seen Choi ∗ , Yun-Ki Kim Department of Chemistry, Sejong University, 98 Gunja-dong, Gwangjin-gu, Seoul 143-747, Republic of Korea
a r t i c l e
i n f o
Article history: Received 28 February 2011 Accepted 26 July 2011 Available online 4 August 2011 Keywords: Ethylene–propylene heterosequences Pyrolysis EPM PE PP EPDM
a b s t r a c t Properties of ethylene–propylene copolymer (EPM) are determined by ethylene/propylene ratio and degree of block and random sequences. EPM was pyrolyzed and the pyrolysis products were analyzed using gas chromatography/mass spectrometry (GC/MS) to examine pyrolysis products formed from the ethylene–propylene heterosequences. Pyrolysis products formed from EPM were compared with those formed from polyethylene (PE) and polypropylene (PP) to determine the pyrolysis products formed from ethylene–propylene heterosequences of EPM. Principal pyrolysis products formed from ethylene–propylene heterosequences were 3-methyl-1-hexene, 4-methyl-1-hexene, 2methyl-1-hexene, and 2-heptene. Order of the relative intensity of the pyrolysis products was 2-methyl-1-hexene > 4-methyl-1-hexene > 3-methyl-1-hexene > 2-heptene. The relative abundances of the pyrolysis products decreased as the pyrolysis temperature increased. Relative abundances of the specific pyrolysis products formed from ethylene–propylene heterosequences may be used for determination of the relative degree of random sequences of EPM as well as ethylene–propylene–diene terpolymer (EPDM). © 2011 Elsevier B.V. All rights reserved.
1. Introduction Ethylene–propylene rubbers including ethylene–propylene copolymer (EPM) and ethylene–propylene–diene monomer (EPDM) continue to be one of the most widely used and fastest growing synthetic rubbers having both specialty and general purpose applications. Since EPM and EPDM have saturated carbon–carbon backbone, they possess excellent resistance to oxygen, ozone, heat, and UV light, whereas the non-polar structure endows them with excellent electrical resistivity and resistance to polar solvents. Hence, it has broad applications to electrical insulation, building construction, and automobile components. Due to their importance, a significant number of studies about EPM and EPDM in various fields have been performed [1–16]. Contents of ethylene and propylene in EPM and EPDM determine their grades. Pyrolysis-gas chromatography (pyrolysis-GC) has been applied to characterize EPMs and EPDMs [17–21]. Pyrolysis products formed from ethylene–propylene heterosequences as well as ethylene and propylene homosequences have been reported [19–21]. Yamada et al. [19] reported that pyrolysis products formed from ethylene–propylene heterosequences in EPDM
∗ Corresponding author. Tel.: +82 2 3408 3815; fax: +82 2 3408 4317. E-mail address: [email protected] (S.-S. Choi). 0165-2370/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2011.07.014
were C6, C7, and C8 species. Especially they discussed about the C7 species but detail mechanisms except 2-methyl-1-hexene were not discussed. Sojak et al. [21] studied pyrolysis products (larger than C5-species) formed from ethylene and propylene homosequences. Wampler et al. [20] studied pyrolysis products (larger than trimer) formed from ethylene and propylene homosequences, and proposed a method to determine ethylene and propylene content ratio of EPM. Properties of EPM are determined by ethylene/propylene ratio and degree of block and random sequences. Thus, analysis of ethylene–propylene heterosequences of EPM is very important. In the present work, pyrolysis products formed from ethylene homosequences, propylene homosequences, ethylene–propylene heterosequences of EPM were differentiated by comparing the pyrolysis products formed from polyethylene (PE), polypropylene (PP), and EPM. Chemical structures of the pyrolysis products were identified by interpreting the mass spectra using a mass spectrum library searching software. Specific pyrolysis products formed only from EPM not PE and PP were determined and their formation mechanisms were proposed. The pyrolysis products formed only from EPM come from ethylene–propylene heterosequences. In this study, formation of the C7 pyrolysis products from ethylene–propylene heterosequences was focused. Variations of the pyrolysis products formed from ethylene–propylene heterosequences with the pyrolysis temperature were also investigated. Kinds and relative abundances of pyrolysis products of polymers
S.-S. Choi, Y.-K. Kim / Journal of Analytical and Applied Pyrolysis 92 (2011) 384–391
385
(a) E2
Intensity
(b) P2
(c)
15.0
15.3
15.6
15.9
16.2
16.5
Retention time (min) Fig. 1. Pyrolysis-GC/MS TIC chromatograms of PE (a), PP (b), and EPM (c) at 800 ◦ C.
Fig. 2. Expanded pyrolysis-GC/MS TIC chromatograms of PE (a), PP (b), and EPM (c) for 1- and 2-pentenes.
vary with the pyrolysis temperature [22–24]. Pyrolysis temperature is a very important factor not only for proper and efficient analysis of polymeric materials but also for recycling of scrap polymeric materials.
42
(a) 55
2. Experimental
70
Intensity
Polyethylene (PE) and polypropylene (PP) were purchased from Sigma–Aldrich Co. (USA). Ethylene–propylene monomer (EPM) (KEP-070P) was obtained from Kumho Polychem Co. (Korea) and its ethylene content was 70.5 wt%. Pyrolysis-GC/MS was carried out by using a pyroprobe 2000 system with a CDS 1500 interface (Chemical Data System, Oxford, USA) coupled to an Agilent 6890 gas chromatograph equipped with a 5973 mass spectrometer of Agilent Technology Inc. (USA). The sample (about 0.2 mg) was preheated at 250 ◦ C for 15 s and pyrolyzed at 700–940 ◦ C for
(b)
Table 1 Major pyrolysis products. Peak no.
Retention time (min)
Product
A1 A2 E1 E2 E3 E4 E5 E6 P1 P2 P3 M1 M2 M3 M4
2.04 6.17 5.51 15.80 34.96 37.50 47.80 48.47 5.40 15.74 33.50 45.80 46.40 47.32 48.23
Propylene n-Butane 1-Butene 1-Pentene 1-Hexene n-Hexane 1-Heptene n-Heptane iso-Butene 2-Pentene 2-Methyl-1-pentene 3-Methyl-1-hexene 4-Methyl-1-hexene 2-Methyl-1-hexene 2-Heptene
40
50
m/z
60
70
Fig. 3. Mass spectra of the E2 (a) and P2 (b) peaks in Fig. 1.
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propylene-ethylene-ethylene E
CH2
CH2 CH3
P-E-E P
CH2
CH
H3 C P'
P'-E-E
CH
CH2
ethylene-propylene-ethylene
E-P-E
E-P'-E Scheme 1. Possible heterosequences of triads of two ethylene units and one propylene unit.
ethylene-propylene-propylene
propylene-ethylene-propylene
E-P-P
P-E-P
E-P-P'
P-E-P'
E-P'-P
P'-E-P
E-P'-P'
P'-E-P'
CH3 E
CH2
CH2
P
CH2
CH
H3 C P'
CH
CH2
Scheme 2. Possible heterosequences of triads of two propylene units and one ethylene unit.
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387
CH2 H
Fig. 4. Mass spectra of the peaks at 15.57 (a), 15.78 (b), and 15.92 min (c) in Fig. 2(c).
Scheme 3. Mechanism for formation of 3-methyl-1-hexene ethylene–propylene–ethylene sequence of EPM by pyrolysis.
55
69
41
from
(a) 98
Intensity
(b)
(c)
(d)
40
50
60
70
80
90
100
m/z Fig. 6. Mass spectra of the peaks at 45.08 (a), 46.40 (b), 47.32 (c), and 48.23 min (d) in Fig. 5. Fig. 5. Expanded pyrolysis-GC/MS TIC chromatograms of PE (a), PP (b), and EPM (c) for the M-series products.
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CH2 H
H
Scheme 4. Mechanism for formation of 4-methyl-1-hexene from ethylene–propylene–ethylene or ethylene–propylene–propylene sequence of EPM by pyrolysis.
5 s under helium (He) atmosphere. HP-PLOT/Q capillary column (0.32 mm × 15 m, 20 m film thickness) of Agilent Technology Inc. (USA) was used. Temperatures of the interface and injector were 250 ◦ C. The GC oven temperature program was as follows: 100 ◦ C (held for 2 min) to 130 ◦ C (held for 5 min) at 1 ◦ C/min and raised up again to 220 ◦ C (held for 5 min) at 5 ◦ C/min. The interface temperature of GC to MS was 250 ◦ C. The electron ionization (70 eV) was used to ionize the pyrolysis products. The MS source temperature was 230 ◦ C. Each experiment was performed at least three times. 3. Results and discussion PE, PP, and EPM were pyrolyzed and their pyrolysis products were analyzed. Fig. 1 shows pyrolysis-GC/MS TIC chromatograms of PE, PP, and EPM. Major pyrolysis products were marked in the chromatograms and listed in Table 1. Their chemical structures
were identified by interpreting the mass spectra using a mass spectrum library searching software. The A-series are pyrolysis products formed from all of PE, PP, and EPM. The E-series are pyrolysis products formed from PE and EPM, whereas the P-series are pyrolysis products formed from PP and EPM. The M-series are specific pyrolysis products formed only from EPM. Propylene and n-butane were observed in all of PE, PP, and EPM. 1-Butene, 1-pentene, 1hexene, n-hexane, 1-heptene, and heptane were not generated from PP, that is those pyrolysis products were formed from ethylene homosequences (ethylene block). iso-Butene, 2-pentene, and 2-methyl-1-pentene were not generated from PE, that is those pyrolysis products were formed from propylene homosequences (propylene block). Retention times of the E2 and P2 peaks are nearly the same, but the two pyrolysis products are not the same. Fig. 2 shows the expanded pyrolysis-GC/MS TIC chromatograms around E2 and P2 peaks. Maximum points around 15.4–16.2 min in the TIC chromatograms of PE, PP, and EPM were different each other. Mass
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CH2 H
H
Scheme 5. Mechanism for formation of 2-methyl-1-hexene from propylene–ethylene–ethylene, propylene–ethylene–propylene, or propylene–propylene–ethylene sequence of EPM by pyrolysis.
spectra of the E2 and P2 were also different each other as shown in Fig. 3. The E2 and P2 mass spectra were assigned to 1-pentene and 2-pentene, respectively. Fig. 4 shows mass spectra at the retention times at 15.57, 15.78, and 15.92 min in the TIC chromatogram of EPM. The mass spectrum at 15.57 min is the same mass spectrum of the P2, while that at 15.92 min is the same mass spectrum of the E2. The mass spectrum at 15.78 min is a mixed mass spectrum of the E2 and P2. 3-Methyl-1-hexene, 4-methyl-1-hexene, 2-methyl-1-hexene, and 2-heptene were only observed in EPM, that is those pyrolysis products were formed from ethylene–propylene heterosequences. Fig. 5 is the expanded pyrolysis-GC/MS TIC chromatograms around 45–49 min to clearly show the pyrolysis products formed from ethylene–propylene heterosequences. Fig. 6 shows mass spectra of the four pyrolysis products at 45.08, 46.40, 47.32, and 48.23 min. All those pyrolysis products are C7 H14 . The specific pyrolysis products formed only from EPM not PE and PP indicate existence of alternating point between ethylene and propylene.
Yamada et al. [19] reported that C7 pyrolysis products formed from ethylene–propylene heterosequences were 3-methyl-1-hexene, 4-methyl-1-hexene, 5-methyl-1-hexene, 2-methyl-1-hexene, 1heptene, heptane, and 2-heptene by comparing the retention data. According to our experimental results, 1-heptene and heptane were generated from PE as well as EPM. Hence, 1-heptene and heptane were not the pyrolysis products formed from ethylene–propylene heterosequences. The pyrolysis products of M1, M2, M3, and M4 can be formed from ethylene–propylene heterosequences. Possible heterosequences of triads of two ethylene units and one propylene unit were shown in Scheme 1. And possible heterosequences of triads of two propylene units and one ethylene unit were also shown in Scheme 2. 3-Methyl-1hexene can be formed from ethylene–propylene–ethylene or propylene–ethylene–ethylene sequence of EPM. Formation of 3-methyl-1-hexene may be started by C–C bond cleavage and hydrogen atom migrates in the 6-membered structure as
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CH2 H
H
Scheme 6. Mechanism for formation of 2-heptene from propylene–ethylene–ethylene or propylene–ethylene–propylene sequence of EPM by pyrolysis.
shown in Scheme 3. 4-Methyl-1-hexene can be formed from ethylene–propylene–ethylene or ethylene–propylene–propylene sequence of EPM. Formation of 4-methyl-1-hexene may be started by C–C bond cleavage and hydrogen atom migrates in the 5membered structure as shown in Scheme 4. Propylene–propylene sequence of ethylene–propylene–propylene sequence should be head-to-head sequence. Thus, the possibility to form from ethylene–propylene–propylene 4-methyl-1-hexene sequence may be low since the head-to-head sequence generally is not common. 2-Methyl-1-hexene can be formed from propylene–ethylene–ethylene, propylene–ethylene–propylene (head-to-head), or propylene–propylene–ethylene (tail-to-tail) sequence as shown in Scheme 5. Formation of 2-methyl-1hexene from propylene–ethylene–ethylene sequence may be started by C–C bond cleavage and hydrogen atom migrates in the 6-membered structure. For formation of 2-methyl-1hexene from propylene–ethylene–propylene (head-to-head) (tail-to-tail) sequence, or propylene–propylene–ethylene hydrogen atom migration occurs through 5-membered structure. Since the head-to-head or tail-to-tail sequence of propylene–propylene is not common, the possibility to form 2-methyl-1-hexene from the propylene–ethylene–propylene
or propylene–propylene–ethylene sequence may be low. 2Heptene can be formed from propylene–ethylene–ethylene or propylene–ethylene–propylene sequence as shown in Scheme 6. Formation of 2-heptene from propylene–ethylene–ethylene sequence may be started by C–C bond cleavage and hydrogen atom migrates in the 6-membered structure. For formation of 2-heptene from propylene–ethylene–propylene sequence, hydrogen atom migration occurs through the 5-membered structure. Formation of pyrolysis products by rearrangement through the 6-membered ring may be more favorable than that through the 5-membered one due to strain effect. In order to examine the pyrolysis temperature effect on formation of the C7-pyrolysis products generated from ethylene–propylene heterosequences of EPM, relative abundance ratios of the C7-pyrolysis products compared to n-butane were plotted as a function of the pyrolysis temperature (Fig. 7). n-Butane is one of major pyrolysis products formed from all of PE, PP, and EPM, and its peak was well separated from other peaks. All the peak intensity ratios linearly decreased after 800 ◦ C as the pyrolysis temperature increased. The peak intensity ratios of 3-methyl-1-hexene, 4-methyl-1-hexene, 2-methyl-1-hexene, and 2-heptene at 700 ◦ C
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1.2
4. Conclusions 3-methyl-1-hexene 4-methyl-1-hexene 2-methyl-1-hexene 2-heptene
C7-species/n-butane
1.0 0.8 0.6 0.4 0.2 0.0
391
800
850
900
950 o
Pyrolysis temperature ( C) Fig. 7. Variations of the peak intensity ratios of the C7-pyrolysis products generated from ethylene–propylene heterosequences of EPM. The reference was n-butane. Squares, circles, up-triangles, and down-triangles indicate the peak intensity ratios of 3-methyl-1-hexene, 4-methyl-1-hexene, 2-methyl-1-hexene, and 2-heptene, respectively.
were 0.28, 0.57, 0.94, and 0.15, respectively. The linear curve fitting equations were y = −4.95 × 10−4 x + 0.655, y = −1.28 × 10−3 x + 1.63, y = −2.27 × 10−3 x + 2.72, and y = −1.51 × 10−4 x + 0.296 for 34-methyl-1-hexene, 2-methyl-1-hexene, methyl-1-hexene, and 2-heptene, respectively. Order of the relative abundance was 2-methyl-1-hexene > 4-methyl-1-hexene > 3-methyl-1hexene > 2-heptene. This can be explained by relative stability of radical intermediate and required sequences. Since 2-heptene can be formed from a long ethylene sequence (equal or more than n = 3 of –(CH2 CH2 )n –), its formation may be relatively less favorable than formations of the other pyrolysis products. A radical intermediate to generate 2-methyl-1-hexene has a tertiary structure, whereas radical intermediates to generate 3-methyl-1-hexene and 4-methyl-1-hexene do not have tertiary structures as shown in Schemes 1–3. Hence, formation of 2-methyl-1-hexene may be more favorable than those of the others. A radical intermediate to generate 4-methyl-1-hexene has ␣-methyl group, whereas a radical intermediate to form 3-methyl-1-hexene has -methyl group as shown in Schemes 3 and 4. Therefore, formation of 4-methyl1-hexene may be more favorable than that of 3-methyl-1-hexene since a radical intermediate of the former is stable than that of the latter. Order of the slope (sensitivity to pyrolysis temperature) was also 2-methyl-1-hexene > 4-methyl-1-hexene > 3-methyl-1hexene > 2-heptene. If a certain EPM or EPDM sample has alternating sequence of ethylene and propylene more than other samples, relative abundances of the four specific pyrolysis products (2-methyl-1-hexene, 4-methyl-1-hexene, 3-methyl-1-hexene, and 2-heptene) formed from ethylene–propylene heterosequences of the former will be larger than those of the latters. Therefore, by using the relationship between the alternating sequence and the C7 specific pyrolysis products, relative degree of the randomness of EPM or EPDM can be compared.
Propylene and n-butane were major pyrolysis products formed from all of PE, PP, and EPM. 1-Butene, 1-pentene, 1-hexene, nhexane, 1-heptene, and heptane were pyrolysis products formed from PE and EPM not from PP. iso-Butene, 2-pentene, and 2-methyl1-pentene were pyrolysis products formed from PP and EPM not from PE. Specific pyrolysis products generated only from EPM were 3-methyl-1-hexene, 4-methyl-1-hexene, 2-methyl-1-hexene, and 2-heptene. Formation mechanisms of the pyrolysis products generated only from the heterosequences were proposed. Order of the relative abundance was 2-methyl-1-hexene > 4-methyl-1hexene > 3-methyl-1-hexene > 2-heptene. Peak intensity ratios of the four C7 pyrolysis products (reference: n-butane) decreased as the pyrolysis temperature increased. Since 3-methyl-1-hexene, 4-methyl-1-hexene, 2-methyl-1-hexene, and 2-heptene are only formed from the ethylene–propylene heterosequences in EPM or EPDM, relative abundances of the four specific pyrolysis products can be used for determination of relative degree of the ethylene–propylene alternating sequences. Acknowledgements This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. References [1] H. Yu, G. Xu, X. Shen, X. Yan, C. Hu, Y. Wang, Electrochim. Acta 55 (2010) 1843. [2] S.-S. Choi, H.-S. Chung, Y.-T. Joo, K.-M. Yang, S.-H. Lee, Elast. Compos. 45 (2010) 100. [3] K.H. Seo, K.-S. Cho, I.-S. Yun, W.-H. Choi, B.-K. Hur, D.-G. Kang, Elast. Compos. 45 (2010) 212. [4] H. Zhang, R.N. Datta, A.G. Talma, J.W.M. Noordermeer, Eur. Polym. J. 46 (2010) 754. [5] J. Su, S. Chen, J. Zhang, Z. Xu, Polym. Test. 28 (2009) 235. [6] S. Mitra, M. Jørgensen, W.B. Pedersen, K. Almdal, D. Banerjee, J. Appl. Polym. Sci. 113 (2009) 2962. [7] Q. Zhao, X. Li, J. Gao, Polym. Degr. Stab. 94 (2009) 339. [8] Q. Zhao, X. Li, J. Gao, Z. Jia, Mater. Lett. 63 (2009) 1647. [9] B.K. Hwang, K.H. Hong, H.Y. Park, I.R. Jeon, K.H. Seo, Elast. Compos. 44 (2009) 299. [10] T. Lee, C. Yoon, D.-S. Bang, D.-Y. Ahn, H. Kye, K.-C. Shin, Elast. Compos. 44 (2009) 164. [11] J.S. Kim, J.H. Lee, W.S. Jung, J.W. Bae, H.C. Park, D.P. Kang, Elast. Compos. 44 (2009) 47. [12] F. Barroso-Bujans, R. Verdejo, A. Lozano, J.L.G. Fierro, M.A. Lopez-Manchado, Acta Mater. 56 (2008) 4780. [13] M.A.J. van der Mee, J.G.P. Goossens, M. van Duin, Polymer 49 (2008) 1239. [14] J. Silva, A.V. Machado, J. Maia, Rheol. Acta 46 (2007) 1091. [15] M.-B. Coltelli, E. Passaglia, F. Ciardelli, Polymer 47 (2006) 85. [16] E. Passaglia, M.-B. Coltelli, F. Ciardelli, Helv. Chim. Acta 89 (2006) 1596. [17] L. Michajlov, H.-J. Cantow, P. Zugenmaier, Polymer 12 (1971) 70. [18] S. Tsuge, Y. Sugimura, T. Nagaya, J. Anal. Appl. Pyrolysis 1 (1980) 221. [19] T. Yamada, T. Okumoto, H. Ohtani, S. Tsuge, Rubber Chem. Technol. 63 (1990) 191. [20] T. Wampler, C. Zawodny, L. Mancini, J. Wampler, J. Anal. Appl. Pyrolysis 68–69 (2003) 25. [21] L. Sojak, R. Kubinec, H. Jurdakova, E. Hajekova, M. Bajus, J. Anal. Appl. Pyrolysis 78 (2007) 387. [22] S.-S. Choi, Bull. Korean Chem. Soc. 20 (1999) 1348. [23] F. Chen, J. Qian, Fuel Proc. Technol. 67 (2000) 53. [24] S.-S. Choi, D.-H. Han, Macromol. Res. 14 (2006) 354.