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Electron beam-induced crosslinking of poly(butylene adipate-co-terephthalate)
Nuclear Instruments and Methods in Physics Research B 268 (2010) 3386–3389
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Electron beam-induced crosslinking of poly(butylene adipate-co-terephthalate) In-Tae Hwang, Chan-Hee Jung, In-Seol Kuk, Jae-Hak Choi *, Young-Chang Nho Radiation Research Division for Industry and Environment, Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeongeup-si, Jeollabuk-do 580-185, Republic of Korea
a r t i c l e
i n f o
Article history: Received 27 July 2010 Available online 21 August 2010 Keywords: Electron beam Crosslinking Poly(butylene adipate-co-terephthalate) Thermal and mechanical properties
a b s t r a c t Biodegradable poly(butylene adipate-co-terephthalate) (PBAT) was crosslinked by electron beam irradiation and their properties were investigated in this research. PBAT films prepared by a solution casting method were crosslinked by electron beam under various absorbed doses ranging 20–200 kGy and their properties were characterized by using a crosslinking degree measurement, a thermogravimetric analyzer (TGA), universal testing machine (UTM), dynamic mechanical analyzer (DMA), and thermal mechanical analyzer (TMA). The results of the crosslinking degree measurement revealed that the PBAT could be crosslinked by electron beam irradiation and its crosslinking degree was dependant on the absorbed dose. In addition, the results of the UTM, DMA, TMA, and TGA analyses revealed that the thermal and mechanical properties of the crosslinked PBS was much improved in comparison to those of the control PBAT. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The development of biodegradable polymers has been a subject of great interest in material science over the last few years owing to the increasing concern in the environmental pollution caused by the accumulation of non-biodegradable polymer wastes in daily use [1]. Various biodegradable polymers such as polylactide (PLA), poly(butylene succinate) (PBS), poly(e-caprolactone) (PCL), poly(3hydroxy butyrate) (PHB) and their derivatives have been investigated to replace non-biodegradable conventional polymers [2,3]. Among these biodegradable polymers, poly(butylene adipateco-butylene adipate) (PBAT) is a fully-biodegradable aliphatic–aromatic copolyester, which is commercially available like Ecoflex from BASF, Gs PLa from Mitsubishi, Enpol from Ire Chemical, and Bionolle from Showa HighPolymer [4–6]. PBAT exhibits the comparable property with those of low density polyethylene, which can be extruded for a packaging application. However, its use in various applications has been limited due to its low mechanical strength and low thermal stability [7]. Thus, the modification of PBAT has been explored by blending or composites to enhance the thermal and mechanical properties [7,8]. Radiation processing has been considered as one of promising techniques to modify biodegradable polymers because it offers several advantages over other modification methods such as an eco-friendly process, a precise controllability, no additives, a temperature-independence, and a low energy consumption [9]. Thus, * Corresponding author. Tel.: +82 63 570 3062; fax: +82 63 570 3090. E-mail address: [email protected] (J.-H. Choi). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.08.010
this radiation technique has been widely explored to modify various biodegradable polymers [10–12]. However, the crosslinking of PBAT by electron beam irradiation has been rarely explored to improve its thermal and mechanical properties. In this research, we report the radiation-induced crosslinking of biodegradable PBAT to improve its thermal and mechanical properties. PBAT films were crosslinked by electron beam irradiation under various absorbed doses and their properties were investigated. 2. Experiments Poly(butylene adipate-co-terephthalate) (PBAT, trade name: Enpol, Grade: G8060) and chloroform were purchased from Ire Chemical (Korea) and Showa Chemical (Japan), respectively. PBAT films were prepared by casting a 10 wt.% PBAT solution in chloroform on well-cleaned glass substrates followed by drying in air at room temperature for a slow evaporation. The dried films were further dried in a vacuum oven for 24 h to remove the remaining solvent. The thickness of the resulting PBAT films was around 130 lm. For electron beam irradiation, the prepared PBAT films were put into aluminum pouches and thermally sealed after purging with nitrogen gas. The sealed pouches were irradiated at room temperature by using an ELV-3 e-beam accelerator installed at EB-Tech (Daejeon, Korea). The energy and current density of the electron beams were 2 MeV and 1 mA/cm2, respectively. The total absorbed dose ranged from 20 to 200 kGy. After irradiation, the pouches were placed in a vacuum oven at 80 °C for 2 h to eliminate the remaining radicals.
I.-T. Hwang et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 3386–3389
The crosslinking degree of the irradiated PBAT films was quantified by measuring their insoluble part in the dried samples after extraction in chloroform. The crosslinking degree was calculated from the following equation:
Crosslinking degree ð%Þ ¼ W 2 =W 1 100; where W1 and W2 are the weights of the dried samples before and after extraction in chloroform, respectively. Tensile properties such as tensile strength and elongation-atbreak were measured by using an universal testing machine (UTM, Instron Model4210, Instron Engineering Co., USA) according to the ASTM Standard D638. Dynamic viscoelastic properties were investigated using a dynamic mechanical analyzer (DMA, Q800 model, TA Instrument) under a film-tension mode in a temperature range of 70–200 °C at a heating rate of 2 °C/min and a frequency of 1 Hz. Thermomechanical analysis (TMA) was performed on a TMA Q400 analysis system (TA Instrument, USA) using extension probe between 20 and 200 °C at a heating rate of 10 °C/min under nitrogen atmosphere. Thermogravimetric analysis (TGA) was carried out on a SDT Q600 analysis system (TA Instrument, USA) between 50 and 700 °C at a heating rate of 10 °C/min under nitrogen atmosphere. The decomposition temperature (Td) in this measurement was defined as the 5% mass loss temperature.
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The tensile properties of the control and irradiated PBAT films are shown in Fig. 2. As presented in Fig. 2a, the tensile strength of the irradiated PBAT films was gradually increased up to 23% with an absorbed dose in comparison to that of the control film. On the other hand, as shown in Fig. 2b, the elongation-at-break of the irradiated PBAT films was reduced up to 9% with an absorbed dose. The increase in the tensile strength and the reduction in the elongation-at-break could be ascribed to the formation of threedimensional network structures in the irradiated PBAT films that could make the PBAT more rigid than the control [14]. The dynamic viscoelastic properties of the control and irradiated PBAT films were investigated by using DMA. Fig. 3a shows the change in the storage modulus (E0 ) as a function of the temperature. The E0 of the control PBAT film fell down above the melting temperature, 120 °C. On the other hand, the irradiated PBAT films showed a steady decrease in the E0 from above the melting temperature up to 190 °C with an increasing absorbed dose. Moreover, as shown in Fig. 3b, in case of the control PBAT film, the temperature at the peak of the tan d curves that is associated with the glass transition temperature (Tg) of the PBAT appeared at 21 °C. On the other hand, in case of the irradiated PBAT films, the temperature at the peak of the tan d was gradually moved to a higher temperature with an increasing dose, implying that the Tg of the PBAT was increased with an absorbed dose. Therefore, these changes in the viscoelastic properties could be attributed to the formation of crosslinked structures into the PBAT during irradiation that could restrict the chain mobility [15].
3. Results and discussion Crosslinking and chain scission reactions in a polymer simultaneously occur during electron beam irradiation, which is mainly dependant on the inherent chemical structure of a polymer and the irradiation conditions. The insoluble network structure in a polymer could be formed if the crosslinking is predominant over the chain scission, which could have a considerable improvement of the thermal and mechanical properties of the polymer [13]. Thus, to examine the formation of network structures in the irradiated PBAT films, the measurement of crosslinking degree was performed and the results are shown in Fig. 1. The crosslinking degree of the PBAT was increased with an increasing absorbed dose. The irradiated PBAT films at 200 kGy showed the highest crosslinking degree, 52%, in this study. These results indicate that the network structure of the PBAT was successfully generated by electron beam irradiation.
Fig. 1. The crosslinking degree of PBAT as a function of the absorbed dose.
Fig. 2. The tensile strength (a) and elongation-at-break (b) of the PBAT as a function of the absorbed dose.
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I.-T. Hwang et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 3386–3389
Fig. 5. The thermal decomposition temperatures (Td) of the PBAT as a function of the absorbed dose.
suggesting that the PBAT was successfully crosslinked during irradiation. To investigate the crosslinking effect on the thermal decomposition, the control and irradiated PBAT films were analyzed by TGA in nitrogen atmosphere and the results are shown in Fig. 5. The thermal degradation of all the samples underwent the one-stage weight loss. The Td of the control PBAT film was 371.5 °C. In case of the irradiated PBAT films, the Td was increased with an increasing absorbed dose in comparison to that of the control film. This increase in the Td of the irradiated PBAT films could be explained by the fact that large molecular chain networks were formed in the PBAT during irradiation, which could retard the thermal decomposition [12]. Fig. 3. The storage modulus (E0 ) (a) and tan d (b) of the control and irradiated PBAT films at different absorbed doses.
4. Conclusions In this work, PBAT was successfully crosslinked by electron beam irradiation, which could have a significant effect on the thermal and mechanical properties of the PBAT. The crosslinking degree of the PBAT was dependant on the absorbed dose. On the basis of the results of the UTM, DMA, TMA, and TGA, the formation of three-dimensional networks in the PBAT by electron beam improved the thermal and mechanical properties in comparison to those of control PBAT. The fabrication of new high-performance PBAT composites by electron beam-induced crosslinking process is under investigation. Acknowledgement This research was supported by the Nuclear R&D program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology, Korea. References
Fig. 4. The TMA thermograms of the control and irradiated PBAT films as a function of the temperature.
The thermomechanical analysis of the control and irradiated PBAT films is shown in Fig. 4. The control PBAT revealed the typical transition at the melting temperature of 120 °C. On the other hand, in case of the irradiated PBAT films, the melting temperature was gradually increased up to 132 °C with an increasing absorbed dose,
[1] [2] [3] [4] [5] [6] [7] [8] [9]
R.A. Gross, B. Kalra, Science 297 (2002) 803. S.S. Ray, M. Bousmina, Prog. Mater. Sci. 50 (2005) 962. I. Vroman, L. Tigher, Materials 2 (2009) 307. U. Witt, T. Einig, M. Yamamoto, I. Kleeberg, W.-D. Deckwer, R.-J. Müller, Chemosphere 44 (2001) 289. T.-Y. Liu, W.-C. Lin, M.-C. Yang, S.-Y. Chen, Polymer 46 (2005) 12586. J.M. Raquez, Y. Nabar, R. Narayan, P. Dubois, Macromol. Mater. Eng. 293 (2008) 310. Y. Iwakura, Y. Li, K. Nakayama, H. Shimizu, J. Appl. Polym. Sci. 109 (2008) 333. S. Mohanty, S.K. Nayak, Polym. Compos. 31 (2010) 1194. A.G. Chmielewski, M. Haji-Saeid, S. Ahmed, Nucl. Instrum. Methods Phys. Res. B 236 (2005) 44.
I.-T. Hwang et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 3386–3389 [10] M. Suhartini, H. Mitomo, N. Nagasawa, F. Yoshii, T. Kume, J. Appl. Polym. Sci. 88 (2003) 2238. [11] H. Mitomo, A. Kaneda, T.M. Quynh, N. Nagasawa, F. Yoshii, Polymer 46 (2005) 4695. [12] C. Han, X. Ran, K. Zhang, Y. Zhuang, L. Dong, J. Appl. Polym. Sci. 103 (2007) 2676.
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[13] A. Bhattacharya, Prog. Polym. Sci. 25 (2000) 371. [14] F. Jin, S.-H. Hyon, H. Iwata, S. Tsutsumi, Macromol. Rapid Commun. 23 (2002) 909. [15] P. Nugroho, H. Mitomo, F. Yoshii, T. Kume, K. Nishimura, Macromol. Mater. Eng. 286 (2001) 316.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Electron beam-induced crosslinking of poly(butylene adipate-co-terephthalate) In-Tae Hwang, Chan-Hee Jung, In-Seol Kuk, Jae-Hak Choi *, Young-Chang Nho Radiation Research Division for Industry and Environment, Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeongeup-si, Jeollabuk-do 580-185, Republic of Korea
a r t i c l e
i n f o
Article history: Received 27 July 2010 Available online 21 August 2010 Keywords: Electron beam Crosslinking Poly(butylene adipate-co-terephthalate) Thermal and mechanical properties
a b s t r a c t Biodegradable poly(butylene adipate-co-terephthalate) (PBAT) was crosslinked by electron beam irradiation and their properties were investigated in this research. PBAT films prepared by a solution casting method were crosslinked by electron beam under various absorbed doses ranging 20–200 kGy and their properties were characterized by using a crosslinking degree measurement, a thermogravimetric analyzer (TGA), universal testing machine (UTM), dynamic mechanical analyzer (DMA), and thermal mechanical analyzer (TMA). The results of the crosslinking degree measurement revealed that the PBAT could be crosslinked by electron beam irradiation and its crosslinking degree was dependant on the absorbed dose. In addition, the results of the UTM, DMA, TMA, and TGA analyses revealed that the thermal and mechanical properties of the crosslinked PBS was much improved in comparison to those of the control PBAT. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The development of biodegradable polymers has been a subject of great interest in material science over the last few years owing to the increasing concern in the environmental pollution caused by the accumulation of non-biodegradable polymer wastes in daily use [1]. Various biodegradable polymers such as polylactide (PLA), poly(butylene succinate) (PBS), poly(e-caprolactone) (PCL), poly(3hydroxy butyrate) (PHB) and their derivatives have been investigated to replace non-biodegradable conventional polymers [2,3]. Among these biodegradable polymers, poly(butylene adipateco-butylene adipate) (PBAT) is a fully-biodegradable aliphatic–aromatic copolyester, which is commercially available like Ecoflex from BASF, Gs PLa from Mitsubishi, Enpol from Ire Chemical, and Bionolle from Showa HighPolymer [4–6]. PBAT exhibits the comparable property with those of low density polyethylene, which can be extruded for a packaging application. However, its use in various applications has been limited due to its low mechanical strength and low thermal stability [7]. Thus, the modification of PBAT has been explored by blending or composites to enhance the thermal and mechanical properties [7,8]. Radiation processing has been considered as one of promising techniques to modify biodegradable polymers because it offers several advantages over other modification methods such as an eco-friendly process, a precise controllability, no additives, a temperature-independence, and a low energy consumption [9]. Thus, * Corresponding author. Tel.: +82 63 570 3062; fax: +82 63 570 3090. E-mail address: [email protected] (J.-H. Choi). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.08.010
this radiation technique has been widely explored to modify various biodegradable polymers [10–12]. However, the crosslinking of PBAT by electron beam irradiation has been rarely explored to improve its thermal and mechanical properties. In this research, we report the radiation-induced crosslinking of biodegradable PBAT to improve its thermal and mechanical properties. PBAT films were crosslinked by electron beam irradiation under various absorbed doses and their properties were investigated. 2. Experiments Poly(butylene adipate-co-terephthalate) (PBAT, trade name: Enpol, Grade: G8060) and chloroform were purchased from Ire Chemical (Korea) and Showa Chemical (Japan), respectively. PBAT films were prepared by casting a 10 wt.% PBAT solution in chloroform on well-cleaned glass substrates followed by drying in air at room temperature for a slow evaporation. The dried films were further dried in a vacuum oven for 24 h to remove the remaining solvent. The thickness of the resulting PBAT films was around 130 lm. For electron beam irradiation, the prepared PBAT films were put into aluminum pouches and thermally sealed after purging with nitrogen gas. The sealed pouches were irradiated at room temperature by using an ELV-3 e-beam accelerator installed at EB-Tech (Daejeon, Korea). The energy and current density of the electron beams were 2 MeV and 1 mA/cm2, respectively. The total absorbed dose ranged from 20 to 200 kGy. After irradiation, the pouches were placed in a vacuum oven at 80 °C for 2 h to eliminate the remaining radicals.
I.-T. Hwang et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 3386–3389
The crosslinking degree of the irradiated PBAT films was quantified by measuring their insoluble part in the dried samples after extraction in chloroform. The crosslinking degree was calculated from the following equation:
Crosslinking degree ð%Þ ¼ W 2 =W 1 100; where W1 and W2 are the weights of the dried samples before and after extraction in chloroform, respectively. Tensile properties such as tensile strength and elongation-atbreak were measured by using an universal testing machine (UTM, Instron Model4210, Instron Engineering Co., USA) according to the ASTM Standard D638. Dynamic viscoelastic properties were investigated using a dynamic mechanical analyzer (DMA, Q800 model, TA Instrument) under a film-tension mode in a temperature range of 70–200 °C at a heating rate of 2 °C/min and a frequency of 1 Hz. Thermomechanical analysis (TMA) was performed on a TMA Q400 analysis system (TA Instrument, USA) using extension probe between 20 and 200 °C at a heating rate of 10 °C/min under nitrogen atmosphere. Thermogravimetric analysis (TGA) was carried out on a SDT Q600 analysis system (TA Instrument, USA) between 50 and 700 °C at a heating rate of 10 °C/min under nitrogen atmosphere. The decomposition temperature (Td) in this measurement was defined as the 5% mass loss temperature.
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The tensile properties of the control and irradiated PBAT films are shown in Fig. 2. As presented in Fig. 2a, the tensile strength of the irradiated PBAT films was gradually increased up to 23% with an absorbed dose in comparison to that of the control film. On the other hand, as shown in Fig. 2b, the elongation-at-break of the irradiated PBAT films was reduced up to 9% with an absorbed dose. The increase in the tensile strength and the reduction in the elongation-at-break could be ascribed to the formation of threedimensional network structures in the irradiated PBAT films that could make the PBAT more rigid than the control [14]. The dynamic viscoelastic properties of the control and irradiated PBAT films were investigated by using DMA. Fig. 3a shows the change in the storage modulus (E0 ) as a function of the temperature. The E0 of the control PBAT film fell down above the melting temperature, 120 °C. On the other hand, the irradiated PBAT films showed a steady decrease in the E0 from above the melting temperature up to 190 °C with an increasing absorbed dose. Moreover, as shown in Fig. 3b, in case of the control PBAT film, the temperature at the peak of the tan d curves that is associated with the glass transition temperature (Tg) of the PBAT appeared at 21 °C. On the other hand, in case of the irradiated PBAT films, the temperature at the peak of the tan d was gradually moved to a higher temperature with an increasing dose, implying that the Tg of the PBAT was increased with an absorbed dose. Therefore, these changes in the viscoelastic properties could be attributed to the formation of crosslinked structures into the PBAT during irradiation that could restrict the chain mobility [15].
3. Results and discussion Crosslinking and chain scission reactions in a polymer simultaneously occur during electron beam irradiation, which is mainly dependant on the inherent chemical structure of a polymer and the irradiation conditions. The insoluble network structure in a polymer could be formed if the crosslinking is predominant over the chain scission, which could have a considerable improvement of the thermal and mechanical properties of the polymer [13]. Thus, to examine the formation of network structures in the irradiated PBAT films, the measurement of crosslinking degree was performed and the results are shown in Fig. 1. The crosslinking degree of the PBAT was increased with an increasing absorbed dose. The irradiated PBAT films at 200 kGy showed the highest crosslinking degree, 52%, in this study. These results indicate that the network structure of the PBAT was successfully generated by electron beam irradiation.
Fig. 1. The crosslinking degree of PBAT as a function of the absorbed dose.
Fig. 2. The tensile strength (a) and elongation-at-break (b) of the PBAT as a function of the absorbed dose.
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I.-T. Hwang et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 3386–3389
Fig. 5. The thermal decomposition temperatures (Td) of the PBAT as a function of the absorbed dose.
suggesting that the PBAT was successfully crosslinked during irradiation. To investigate the crosslinking effect on the thermal decomposition, the control and irradiated PBAT films were analyzed by TGA in nitrogen atmosphere and the results are shown in Fig. 5. The thermal degradation of all the samples underwent the one-stage weight loss. The Td of the control PBAT film was 371.5 °C. In case of the irradiated PBAT films, the Td was increased with an increasing absorbed dose in comparison to that of the control film. This increase in the Td of the irradiated PBAT films could be explained by the fact that large molecular chain networks were formed in the PBAT during irradiation, which could retard the thermal decomposition [12]. Fig. 3. The storage modulus (E0 ) (a) and tan d (b) of the control and irradiated PBAT films at different absorbed doses.
4. Conclusions In this work, PBAT was successfully crosslinked by electron beam irradiation, which could have a significant effect on the thermal and mechanical properties of the PBAT. The crosslinking degree of the PBAT was dependant on the absorbed dose. On the basis of the results of the UTM, DMA, TMA, and TGA, the formation of three-dimensional networks in the PBAT by electron beam improved the thermal and mechanical properties in comparison to those of control PBAT. The fabrication of new high-performance PBAT composites by electron beam-induced crosslinking process is under investigation. Acknowledgement This research was supported by the Nuclear R&D program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology, Korea. References
Fig. 4. The TMA thermograms of the control and irradiated PBAT films as a function of the temperature.
The thermomechanical analysis of the control and irradiated PBAT films is shown in Fig. 4. The control PBAT revealed the typical transition at the melting temperature of 120 °C. On the other hand, in case of the irradiated PBAT films, the melting temperature was gradually increased up to 132 °C with an increasing absorbed dose,
[1] [2] [3] [4] [5] [6] [7] [8] [9]
R.A. Gross, B. Kalra, Science 297 (2002) 803. S.S. Ray, M. Bousmina, Prog. Mater. Sci. 50 (2005) 962. I. Vroman, L. Tigher, Materials 2 (2009) 307. U. Witt, T. Einig, M. Yamamoto, I. Kleeberg, W.-D. Deckwer, R.-J. Müller, Chemosphere 44 (2001) 289. T.-Y. Liu, W.-C. Lin, M.-C. Yang, S.-Y. Chen, Polymer 46 (2005) 12586. J.M. Raquez, Y. Nabar, R. Narayan, P. Dubois, Macromol. Mater. Eng. 293 (2008) 310. Y. Iwakura, Y. Li, K. Nakayama, H. Shimizu, J. Appl. Polym. Sci. 109 (2008) 333. S. Mohanty, S.K. Nayak, Polym. Compos. 31 (2010) 1194. A.G. Chmielewski, M. Haji-Saeid, S. Ahmed, Nucl. Instrum. Methods Phys. Res. B 236 (2005) 44.
I.-T. Hwang et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 3386–3389 [10] M. Suhartini, H. Mitomo, N. Nagasawa, F. Yoshii, T. Kume, J. Appl. Polym. Sci. 88 (2003) 2238. [11] H. Mitomo, A. Kaneda, T.M. Quynh, N. Nagasawa, F. Yoshii, Polymer 46 (2005) 4695. [12] C. Han, X. Ran, K. Zhang, Y. Zhuang, L. Dong, J. Appl. Polym. Sci. 103 (2007) 2676.
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[13] A. Bhattacharya, Prog. Polym. Sci. 25 (2000) 371. [14] F. Jin, S.-H. Hyon, H. Iwata, S. Tsutsumi, Macromol. Rapid Commun. 23 (2002) 909. [15] P. Nugroho, H. Mitomo, F. Yoshii, T. Kume, K. Nishimura, Macromol. Mater. Eng. 286 (2001) 316.