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EVOH blends
Nuclear Instruments and Methods in Physics Research B 274 (2012) 139–144
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
The effects of gamma-irradiation on the structure, thermal resistance and mechanical properties of the PLA/EVOH blends Meihua Liu a,b, Yuan Yin b, Zhipeng Fan e, Xiaowei Zheng b, Shirley Shen c, Pengyang Deng b,⇑, Chunbai Zheng b, Hong Teng d,⇑, Wanxi Zhang a a
Department of Materials Science and Engineering, Jilin University, 5988 Renmin Street, Changchun, Jilin 130025, People’s Republic of China Changchun Institute of Applied Chemistry, Chinese Academy of Science, 5625 Renmin Street, Changchun, Jilin 130022, People’s Republic of China CSIRO Materials Science and Engineering, 37 Graham Road, Highett, Vic. 3190, Australia d Department of Obstetrics and Gynecology, The Second Hospital of Jilin University, 218 Ziqiang Street, Changchun, Jilin 130041, People’s Republic of China e Zhengzhou Yutong Bus Co., Ltd., Yutong Industrial Park, Yutong Road, Zhengzhou, Henan 450000, People’s Republic of China b c
a r t i c l e
i n f o
Article history: Received 26 August 2011 Received in revised form 8 December 2011 Available online 27 December 2011 Keywords: Polylactic acid Poly(ethylene-co-vinyl alcohol) Irradiation HDT
a b s t r a c t Polylactic acid (PLA) is as a well-known biodegradable polymer with strong potential for extending its applications into the commodity industry, which is still limited by its relative low heat distortion temperature (HDT). In this paper, poly(ethylene-co-vinyl alcohol) (EVOH) was blended with PLA by melt blending and followed by a gamma-irradiation at various absorbed doses in the presence of a multifunctional monomer, triallyl isocyanurate (TAIC). The results show that the enhanced irradiation is an effective method for improving the mechanical properties and HDT of PLA/EVOH blends. Remarkably, the HDTs of the irradiated blends have dramatically increased to 140 °C, double the value of neat PLA (70 °C). The content of TAIC and the absorbed dose of irradiation were found to play important roles in the enhancement. Gel extraction, scanning electronic microscopy (SEM) and Fourier Transform Infrared (FTIR) spectroscopy analyses also demonstrate that new co-crosslinked networks have been formed in the irradiated PLA/EVOH blends. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent times, polylactic acid (PLA) has attracted increased attention as a biocompatible and environmentally benign polymeric material [1–4]. PLA is a linear aliphatic polyester derived from plant starches and can be ultimately degraded into environmentally manageable compounds. Due to its original high cost, PLA had been limited to biomedical applications, such as implants and drug delivery carriers for a long time. However, as increasing feedstock prices for petroleum-based polymers and a continuous reduction in price of PLA, it has been used increasingly instead of non-biodegradable plastics in commodity applications [5–11]. For example, PLA is becoming increasingly popular for food packaging because of the excellent barrier property to fats and oils [10]. Fibers made from PLA exhibited low odor retention and good moisture wicking properties [11]. However, its resistance to thermal deformation is quite poor, particularly at an elevated temperatures [12], which limits PLA for use in the commodity applications. Several conventional approaches were employed to improve the HDT of PLA, such as annealing [13,14] and blending [14–17]. ⇑ Corresponding authors. Tel./fax: +86 431 85262577 (P. Deng), tel.: +86 431 85262329; fax: +86 431 85262839 (H. Teng). E-mail addresses: [email protected] (P. Deng), [email protected] (H. Teng). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.12.020
For example, Suryanegara et al. [14] prepared composites of PLA and micro-fibrillated cellulose (MFC), and found that the storage modulus of crystallized composite with 20 wt.% MFC remained around 1 GPa at 120 °C. Crosslinking is an alternative method to enhance the thermal properties of PLA, resulted from the restrained motions of macromolecule chains by three-dimensional networks. Yang et al. [18] introduced a crosslinking structure into PLA by the initiation of dicumyl peroxide (DCP) in the presence of triallyl isocyanurate (TAIC), and observed that the onset degradation and the complete degradation temperatures could be improved to approximately 310 and 375 °C, respectively, compared with 280 and 350 °C for neat PLA. Irradiation can, and is well-known to, induce crosslinking to polymers. It is a very powerful technique that has been widely used in both product manufactures and research laboratory [19–24]. Our recent work on the effects of gamma-irradiation on the PET/HDPE [25] and PET/POE [26] blends has extended the understanding of irradiation induced crosslinking and enhanced compatibility at the inter-phase boundaries of the two phases in the blends. Nagasawa et al. [12] and Mitomo et al. [27] reported that PLLA gelation (crosslinked networks) could be obtained by irradiation of electron beam (EB) in the presence of a small amount of TAIC. Furthermore, the corresponding thermomechanical analysis (TMA) results indicated that PLLA blended with 3 wt.% TAIC
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M. Liu et al. / Nuclear Instruments and Methods in Physics Research B 274 (2012) 139–144
Fig. 1. Gel fractions of the blends with different compositions at various absorbed doses: (a) PLA/EVOH 80/20, (b) 60/40, (c) 50/50 with various amounts of TAIC.
processability and gas-barrier properties [28], which has been widely used for packaging in commodity field [29–31]. The blends were irradiated with gamma-rays in the presence of TAIC, a proved excellent crosslinking agent for both PLA [32] and EVOH [33]. The effect of irradiation on the structure, HDT and mechanical properties of the PLA/EVOH blends have been studied through gel extraction, Vicat temperature test, scanning electron microscopy (SEM), Fourier Transform Infrared (FTIR) spectroscopy and tensile test.
2. Experimental 2.1. Materials and PLA/EVOH blend preparation
Fig. 2. FTIR spectra: (a) PLA; (b) EVOH; (c) TAIC; (d) a gel sample of the PLA/EVOH/ TAIC blend of 50/50/3 after gamma-irradiated at 30 kGy.
showed low elongation even at 200 °C after EB-irradiated at 50 kGy. For the purpose of improving the comprehensive properties of PLA and enlarge its application area as a general material, the polyblends of PLA and poly(ethylene-co-vinyl alcohol), another green material with a characteristic of biodegradation were prepared, and the effect of gamma-irradiation on the blends was investigated in the present work. EVOH is a polymer with good
PLA (REVODE 101) with density of 1.25 g/cm3 was supplied by Zhejiang Hisun Biomaterials Co. Ltd. (China). EVOH granules (E171, ethylene content 44 mol%) with density of 1.14 g/cm3 and a melt flow index of 1.7 g/10 min (2.16 kg, 190 °C) were purchased from Kuraray Co. Ltd. (Japan). Crosslinking agent TAIC was obtained from Laiyu Chemistry Co. Ltd. (China). Hexafluoroisopropanol (HFIP) and ethanol used in gelation extraction were from Zeus Fluors Technology Shanghai Co. Ltd. (China) and Beijing Chemical Plant (China), respectively. PLA and EVOH were firstly dried under vacuum at 110 °C for 3 h and 80 °C for 4 h, respectively. Blending of PLA and EVOH was carried out in a twin screw extruder (RC 500P, HAAKE, Germany) at 175–190 °C with a screw speed of 100 rpm. The weight ratios of PLA to EVOH were 80/20, 60/40, and 50/50. 1, 3 or 5 wt.% of TAIC was added to each blend. A control blend without TAIC was also prepared for a comparison. The extruded blends were then thermal
M. Liu et al. / Nuclear Instruments and Methods in Physics Research B 274 (2012) 139–144
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room temperature in silicon oil medium under constant load of 1000 g with a heating rate of 12 °C/6 min according to GB/T 1633-2000 (China). The fracture surfaces of the blends were gold-coated under vacuum, and then the morphology structures were observed by a XL30 ESEM FEG (USA) at 20 kV. FTIR measurements of a 50/50/3 blend gel sample and of PLA, EVOH and TAIC, were carried out with a FTIR spectrometer (Bruker Vertex 70, Germany) after drying and grinding in liquid nitrogen. The spectra were recorded in the range of 400–4000 cm1, with the resolution of 2 cm1 and the number of sample scans of 32.
3. Results and discussion 3.1. Gel fraction of the blends
Fig. 3. SEM photos of fracture surfaces of the PLA/EVOH/TAIC blends of 60/40/5: (a) without irradiation; (b) after gamma-irradiated at 50 kGy.
molded to testing samples, such as dumb bell specimens for the tensile test, in an injection mould machine (CJ80M3V, Chende Plastics Machinery Co. Ltd., China) after drying at 80 °C for 12 h under vacuum. 2.2. Gamma-irradiation The samples were sealed in PE (polyethylene) bags and irradiated by a Co-60 source with a dose rate of 3.9 Gy/s under the protection of N2 at room temperature. The absorbed doses were 10, 30, 50, and 100 kGy, respectively. 2.3. Property measurements and structure characteristics Tensile strength was investigated using an Instron Universal Testing Instrument (INSTRON 1121, America) at a crosshead speed of 10 mm/min at room temperature according to GB/T 1040.22006 (China). Samples for gel extraction were cut from fractured tensile specimens. They were then covered with nickel-mesh and extracted with HFIP in a Sokhlet apparatus for 60 h at 85 °C, followed by washing with alcohol and then drying. The gel fraction was calculated using the equation below:
G¼
W1 100%; W0
where G is the gel fraction (wt.%), W0 and W1 represent the weights of dried sample before and after extraction, respectively. A heat deformation Vicat temperature testing machine WKW-300 (Institute of Intelligence testing machine, Changchun, China) was used to obtain the HDT of the blend samples with the size of 12 12 3.3 mm3. The samples were heated from
Gel fraction of blends with different compositions is shown in Fig. 1. It is clear that gelation hardly formed in the PLA/EVOH binary blends without any TAIC before the irradiation degradation of both PLA [34] and EVOH [35]. The addition of TAIC enabled the blends to crosslink. When 1% TAIC was added, the gel fraction increased with absorbed dose initially, and reached the peak of ca. 40 wt.% when absorbed dose was 50 kGy. The blends became more sensitive to irradiation when the amount of TAIC exceeded 3% as the higher gel fraction occurred at lower absorbed dose compared to the result of additional 1 wt.% TAIC. This was noted especially in the PLA/EVOH blends of 60/40 and 50/50, where the amount of networks reached 80 wt.% when TAIC was at 3–5 wt.%. Irradiation with low doses at 10–30 kGy obviously had a significant impact on the crosslinking of the blends, but higher irradiation doses did not further produce gel fraction. The co-crosslinked networks consisted of PLA and EVOH were obtained, indicated by the higher content of gel fraction than that of either PLA or EVOH in the blends when 3–5 wt.% TAIC presented. As seen obviously in Fig. 1b and c: The gel fraction over 70 wt.%, when absorbed dose excesses 30 kGy and TAIC amount over 3 wt.%, is greater than the maximum PLA fraction of 60 wt.% in the blends. 3.2. Fourier Transform Infrared spectrum analyses The FTIR spectra of PLA, EVOH, TAIC and a gel sample of the PLA/ EVOH/TAIC blend of 50/50/3 gamma-irradiated at 30 kGy are shown in Fig. 2. A new crosslinking structure in the PLA/EVOH/ TAIC blend has been found by the comparison of infrared spectrums between the gel and the pure components. The characteristic peaks, at 1687, 1761 and 3363 cm1, assigned to the cyanurate group of TAIC, carbonyl group of PLA [36] and hydroxyl group of EVOH [37], respectively, are found in the spectrum of the gel sample, indicating that PLA, EVOH and TAIC entered the gel networks, and a new crosslinking structure like PLA-g-TAIC-g-EVOH has been formed. 3.3. Morphology structure The irradiation also had an obvious effect on the morphology of the PLA/EVOH/TAIC blends, as seen from the SEM images in Fig. 3. The clear sea-island structure and a poor interfacial adhesion is illustrated by the distinct (very smooth) interfaces between the matrix (PLA) and the dispersed sphere phase (EVOH) for the blends without gamma-irradiation in Fig. 3a. The interfacial adhesion between PLA and EVOH phases was improved by gamma-irradiation as reflected by the indistinct interfaces in Fig. 3b. It is clear that the gamma-irradiation has enhanced the interphase boundary compatibility, which improved the interfacial adhesion accordingly.
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Fig. 4. The HDTs of the PLA/EVOH blends of (a) 80/20, (b) 60/40, (c) 50/50 at various absorbed doses.
3.4. Heat distortion temperatures of the bends The HDTs of the PLA/EVOH blends of various compositions at different gamma-irradiation absorbed doses are shown in Fig. 4. The irradiation played an important role in the significant improvement of HDT, particularly with the presence of the crosslinking agent, TAIC. The enhanced irradiation produced co-crosslinked networks composed of PLA and EVOH, which hinders the movements of polymer chains and therefore delays the distortion to higher temperature. It is noted that the PLA/EVOH blends without gamma-irradiation but with 3–5 wt.% TAIC had lower HDTs (55–65 °C) than either the neat PLA (ca. 70 °C, not shown in the figure) or the PLA/EVOH blends without TAIC nor irradiation (shown in Fig. 4b and c). This occurs because the TAIC, without entering the networks, may act as a solvent to the blends, softening them. In general, the enhancement of HDT basically consists with the changes of the gel fraction. Additional 1 wt.% TAIC did not lead to a prominent enhancement of HDT, as sufficient crosslinking network might not yet form. When 3–5 wt.% TAIC was added, the HDTs increased with absorbed doses and reached a maximum value, after which no significant increase of HDT could be observed with the increasing absorbed doses. The HDTs of the PLA/EVOH blends at both 60/40 and 50/50 with 3–5 wt.% TAIC significantly increased to approximately 140 °C after irradiated at an absorbed dose over 50 kGy, doubling the values of the samples without irradiation. For the PLA/EVOH blends at 80/20 with 3 and 5 wt.% TAIC, the HDTs increased dramatically after the irradiation absorbed dose reached 100 kGy, reaching 140 and 110 °C, respectively, which might further increase if the absorbed doses kept increasing.
However, within the studied cases, the maximum HDT for the PLA/EVOH blends with the presence of TAIC reached 140 °C in a few cases with different absorbed doses and TAIC amounts. To the best of our knowledge there is no similar data in literature to compare with. A relevant work, reported by Yang et al. [18], explored the thermal degradation temperature by thermal analyses and observed about less than 10% temperature rises of the onset degradation and the complete degradation temperatures compared to those of neat PLA. When the HDT changes are compared with the gel fraction changes, the high gel fraction is an essential factor for the improvement of HDT, however, it is not a simple linear relationship between gel fraction and HDT. For instance, when the PLA/EVOH/ TAIC blend of a ratio of 80/20/3 was irradiated at 30 kGy, the gel fraction exceeded 70 wt.%, but there was no evident enhancement of HDT. However, the HDT increased to 140 °C, when the same blend exposed to irradiation of 100 kGy in despite of a similar gel fraction. The same results can be observed in the blends of other ratios when TAIC being 3 or 5 wt.%: Low dose irradiation can result in high gel fraction, but not increased HDT. Once there is sufficient TAIC dispersed in both PLA and EVOH, a few crosslinking junctions in both phases or at the boundaries of the two phases can easily formed, therefore, a three-dimensional network forms in the blends. Theoretically, a non-dissolved and non-molten structure (characterized by high gel fraction) can be formed with only a few junctions. However, only when crosslinking junctions become sufficient, the movements of the polymer chains can be constrained, particularly at elevated temperature, and result in
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Fig. 5. Tensile Strengths of the blends with different compositions at various absorbed doses: (a) PLA/EVOH 80/20, (b) 60/40, (c) 50/50.
concomitant high distortion temperature. So the crosslinking density may play a more important role in the HDT of the blends than the gel fraction. Using the same above 80/20/3 blend as an example again, the number of the crosslinking junctions formed at irradiation dose of 30 kGy (or less) was adequate to generate high gel fraction, but may not enough to hinder most of the chain movements, so the HDT cannot be improved significantly. Along the same line, when TAIC is only 1 wt.%, only few crosslinking junctions can be formed. The gel fraction remains low as well as the HDT, even at high irradiation levels. 3.5. Mechanical properties Fig. 5 shows the tensile strengths of the PLA/EVOH blends with different TAIC additions at various absorbed doses of gamma-irradiation. Gamma-irradiation in the presence of TAIC had an evident effect on the mechanical properties of the blends. Firstly, the tensile strength of the blends without TAIC decreased with absorbed dose, resulted from the irradiation degradations, shown by the empty squares. Secondly, it is clear that with 5 wt.% TAIC but no irradiation, the tensile strength is the worst. This result matched the HDT was the lowest when 5 wt.% TAIC was added but no irradiation. It may be because of the solvent effect of unreacted TAIC to the blends, mentioned earlier. The unreacted TAIC molecules entered the spaces between macromolecular chains, and weakened the intermolecular forces, eased the motions of polymer chains. Namely, the unreacted TAIC acted as a plasticizer in the blends. As a result, the corresponding tensile strength decreased with TAIC addition. However, once irradiation (even at a very low level) was introduced, the blends with 5 wt.% TAIC possessed the greatest
improvements in tensile strength values. When the amount of TAIC was less than 5 wt.% (being 1 or 3 wt.%), the increases in tensile strength were mild. The trend of the tensile strength in relation to the irradiation doses seems more complicated than that of either the gel fractions or the HDTs, even though the tensile strength of the PLA/EVOH blends of all studied ratios was enhanced. PLA would become more capable in terms of physical performance, since the tensile strength reached close to 70 MPa when the appropriate amount of TAIC was added and suitable irradiation dosage was applied. The optimum amount of TAIC is 5% as seen in Fig. 5. However, the absorbed doses did not affect the tensile strengths sensitively, as after they surpassed the optimum level, which was in line with the gel fraction results. The trend of tensile strengths versus absorbed dose is similar to that of tensile strengths versus gel fractions. 4. Conclusion Gamma-irradiation has been shown to be an effective method for improving the heat resistance of PLA blended with EVOH in the presence of an appropriate polyfunctional crosslinker, namely TAIC. During gamma-irradiation, a new network structure of possible PLA-g-TAIC-g-EVOH was achieved at the interface boundaries of PLA and EVOH in the PLA/EVOH/TAIC blends. Therefore, the interfacial adhesions have been improved. Remarkably, the PLA/ EVOH/TAIC blends with such a crosslinked network have dramatically enhanced the HDT values, as high as 140 °C, which is double the value of neat PLA, 70 °C. Tensile strengths of the PLA/EVOH/ TAIC blends have also been remarkably increased by irradiations and the maximum value of tensile strengths reached 70 MPa,
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compared to 40–50 MPa of the control PLA/EVOH blends. The optimal dosage of the gamma-irradiation and the optimized amount of TAIC for the PLA/EVOH blends were found to be 50 kGy and 3–5% by weight, respectively in the studied scenarios. The effect of irradiation on the biodegradable nature of the PLA and EVOH or their blends would be very interesting, and further work will be carried out in this aspect. Acknowledgment Authors thank the National Natural Science Foundation of China (Project No. 2008AA03Z511) for financial support. References [1] M.S. Reeve, S.P. Mccathy, M.J. Downey, R.A. Gross, Macromolecules 27 (1994) 825. [2] H. Tsuji, S. Miyauchi, Polymer 42 (2001) 4463. [3] S. Jacobsen, H.G. Fritz, Polym. Eng. Sci. 39 (1999) 1303. [4] S. Jacobsen, P.H. Degee, H.G. Fritz, P.H. Dubois, R. Jerome, Polym. Eng. Sci. 39 (1999) 1311. [5] J. Lunt, Polym. Degrad. Stabil. 59 (1998) 145. [6] L. Yu, K. Dean, L. Li, Prog. Polym. Sci. 31 (2006) 576. [7] S. Singh, S.S. Ray, J. Nanosci. Nanotechnol. 7 (2007) 2596. [8] R.G. Sinclair, J. Macromol. Sci. A33 (1996) 585. [9] E.T.H. Vink, K.R. Rabago, D.A. Glassner, B. Springs, R.P. O’Connor, J. Kolstad, P.R. Gruber, Macromol. Biosci. 4 (2004) 551. [10] A.H. Tullo, Chem. Eng. News 80 (2002) 13. [11] A.H. Tullo, Chem. Eng. News 78 (2000) 11. [12] N. Nagasawa, A. Kaneda, S. Kanazawa, T. Yagi, H. Mitomo, F. Yoshii, M. Tamada, Nucl. Instrum. Methods Phys. Res. B 236 (2005) 611. [13] X.H. Ran, Z.Y. Jia, C.Y. Han, Y.M. Yang, L.S. Dong, J. Appl. Polym. Sci. 116 (2010) 2050.
[14] L. Suryanegara, A.N. Nakagaito, H. Yano, Compos. Sci. Technol. 69 (2009) 1187. [15] A. Bhatia, R.K. Gupta, S.N. Bhattacharya, H.J. Choi, Int. Polym. Proc. 25 (2010) 5. [16] T. Nishino, K. Hirao, M. Kotera, K. Nakamae, H. Inagaki, Compos. Sci. Technol. 63 (2003) 1281. [17] K. Oksman, M. Skrifvars, J.F. Selin, Compos. Sci. Technol. 63 (2003) 1317. [18] S.L. Yang, Z.H. Wu, W. Yang, M.B. Yang, Polym. Test. 27 (2008) 957. [19] Y.C. Zhu, X.H. Zhang, J.L. Qiao, G.S. Wei, Chin. J. Polym. Sci. 22 (2004) 147. [20] A. Oshima, S. Ikeda, T. Seguchi, Y. Tabata, Radiat. Phys. Chem. 49 (1997) 279. [21] I. Klier, J. Polym. Eng. 16 (1997) 311. [22] D.P. Kiryukhin, I.P. Kim, G.A. Kichigina, N.M. Perepelitsa, I.M. Barkalov, High Energy Chem. 30 (1996) 97. [23] D. Lopez, R. Esparza, G. Burillo, Radiat. Phys. Chem. 45 (1995) 637. [24] J. Gehring, Radiat. Phys. Chem. 57 (2000) 361. [25] Z.M. Xiang, H.R. Liu, P.Y. Deng, M.H. Liu, Y. Yin, X.W. Ge, Polym. Bull. 63 (2009) 587. [26] Y. Yin, M.H. Liu, X.W. Zheng, S. Shen, P.Y. Deng, J. Appl. Polym. Sci. (submitted for publication) [27] H. Mitomo, A. Kaneda, T.M. Quynh, N. Nagasawa, F. Yoshii, Polymer 46 (2005) 4695. [28] H.C. Silvis, Trends Polym. Sci. 5 (1997) 75. [29] G.W. Lohfink, M.R. Kamal, Polym. Eng. Sci. 33 (1993) 1404. [30] J.B. Faisant, A.A. Kadi, M. Bousmina, L. Deschenes, Polymer 39 (1998) 533. [31] C.K. Samios, N.K. Kalfoglou, Polymer 39 (1998) 3863. [32] F.Z. Jin, S.H. Hyon, H. Iwata, S. Tsutsumi, Macromol. Rapid Commun. 23 (2002) 909. [33] P.Y. Deng, M.H. Liu, W.X. Zhang, J.Z. Sun, Nucl. Instrum. Methods Phys. Res. B 258 (2007) 357. [34] P. Nugroho, H. Mitomo, F. Yoshii, T. Kume, Polym. Degrad. Stabil. 72 (2001) 337. [35] M.H. Liu, P.Y. Deng, G.E. Sun, W.X. Zhang, J.Z. Sun, L.S. Dong, J. Radiat. Res. Radiat. Process. 24 (2006) 337. [36] P.Y. Deng, M.H. Liu, X.W. Zheng, N.A. Liu, L.S. Dong, Chem. Res. Chinese U. 30 (2009) 1. [37] H.H. Li, Y. Yin, M.H. Liu, P.Y. Deng, W.X. Zhang, J.Z. Sun, Adv. Polym. Technol. 28 (2009) 192.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
The effects of gamma-irradiation on the structure, thermal resistance and mechanical properties of the PLA/EVOH blends Meihua Liu a,b, Yuan Yin b, Zhipeng Fan e, Xiaowei Zheng b, Shirley Shen c, Pengyang Deng b,⇑, Chunbai Zheng b, Hong Teng d,⇑, Wanxi Zhang a a
Department of Materials Science and Engineering, Jilin University, 5988 Renmin Street, Changchun, Jilin 130025, People’s Republic of China Changchun Institute of Applied Chemistry, Chinese Academy of Science, 5625 Renmin Street, Changchun, Jilin 130022, People’s Republic of China CSIRO Materials Science and Engineering, 37 Graham Road, Highett, Vic. 3190, Australia d Department of Obstetrics and Gynecology, The Second Hospital of Jilin University, 218 Ziqiang Street, Changchun, Jilin 130041, People’s Republic of China e Zhengzhou Yutong Bus Co., Ltd., Yutong Industrial Park, Yutong Road, Zhengzhou, Henan 450000, People’s Republic of China b c
a r t i c l e
i n f o
Article history: Received 26 August 2011 Received in revised form 8 December 2011 Available online 27 December 2011 Keywords: Polylactic acid Poly(ethylene-co-vinyl alcohol) Irradiation HDT
a b s t r a c t Polylactic acid (PLA) is as a well-known biodegradable polymer with strong potential for extending its applications into the commodity industry, which is still limited by its relative low heat distortion temperature (HDT). In this paper, poly(ethylene-co-vinyl alcohol) (EVOH) was blended with PLA by melt blending and followed by a gamma-irradiation at various absorbed doses in the presence of a multifunctional monomer, triallyl isocyanurate (TAIC). The results show that the enhanced irradiation is an effective method for improving the mechanical properties and HDT of PLA/EVOH blends. Remarkably, the HDTs of the irradiated blends have dramatically increased to 140 °C, double the value of neat PLA (70 °C). The content of TAIC and the absorbed dose of irradiation were found to play important roles in the enhancement. Gel extraction, scanning electronic microscopy (SEM) and Fourier Transform Infrared (FTIR) spectroscopy analyses also demonstrate that new co-crosslinked networks have been formed in the irradiated PLA/EVOH blends. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent times, polylactic acid (PLA) has attracted increased attention as a biocompatible and environmentally benign polymeric material [1–4]. PLA is a linear aliphatic polyester derived from plant starches and can be ultimately degraded into environmentally manageable compounds. Due to its original high cost, PLA had been limited to biomedical applications, such as implants and drug delivery carriers for a long time. However, as increasing feedstock prices for petroleum-based polymers and a continuous reduction in price of PLA, it has been used increasingly instead of non-biodegradable plastics in commodity applications [5–11]. For example, PLA is becoming increasingly popular for food packaging because of the excellent barrier property to fats and oils [10]. Fibers made from PLA exhibited low odor retention and good moisture wicking properties [11]. However, its resistance to thermal deformation is quite poor, particularly at an elevated temperatures [12], which limits PLA for use in the commodity applications. Several conventional approaches were employed to improve the HDT of PLA, such as annealing [13,14] and blending [14–17]. ⇑ Corresponding authors. Tel./fax: +86 431 85262577 (P. Deng), tel.: +86 431 85262329; fax: +86 431 85262839 (H. Teng). E-mail addresses: [email protected] (P. Deng), [email protected] (H. Teng). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.12.020
For example, Suryanegara et al. [14] prepared composites of PLA and micro-fibrillated cellulose (MFC), and found that the storage modulus of crystallized composite with 20 wt.% MFC remained around 1 GPa at 120 °C. Crosslinking is an alternative method to enhance the thermal properties of PLA, resulted from the restrained motions of macromolecule chains by three-dimensional networks. Yang et al. [18] introduced a crosslinking structure into PLA by the initiation of dicumyl peroxide (DCP) in the presence of triallyl isocyanurate (TAIC), and observed that the onset degradation and the complete degradation temperatures could be improved to approximately 310 and 375 °C, respectively, compared with 280 and 350 °C for neat PLA. Irradiation can, and is well-known to, induce crosslinking to polymers. It is a very powerful technique that has been widely used in both product manufactures and research laboratory [19–24]. Our recent work on the effects of gamma-irradiation on the PET/HDPE [25] and PET/POE [26] blends has extended the understanding of irradiation induced crosslinking and enhanced compatibility at the inter-phase boundaries of the two phases in the blends. Nagasawa et al. [12] and Mitomo et al. [27] reported that PLLA gelation (crosslinked networks) could be obtained by irradiation of electron beam (EB) in the presence of a small amount of TAIC. Furthermore, the corresponding thermomechanical analysis (TMA) results indicated that PLLA blended with 3 wt.% TAIC
140
M. Liu et al. / Nuclear Instruments and Methods in Physics Research B 274 (2012) 139–144
Fig. 1. Gel fractions of the blends with different compositions at various absorbed doses: (a) PLA/EVOH 80/20, (b) 60/40, (c) 50/50 with various amounts of TAIC.
processability and gas-barrier properties [28], which has been widely used for packaging in commodity field [29–31]. The blends were irradiated with gamma-rays in the presence of TAIC, a proved excellent crosslinking agent for both PLA [32] and EVOH [33]. The effect of irradiation on the structure, HDT and mechanical properties of the PLA/EVOH blends have been studied through gel extraction, Vicat temperature test, scanning electron microscopy (SEM), Fourier Transform Infrared (FTIR) spectroscopy and tensile test.
2. Experimental 2.1. Materials and PLA/EVOH blend preparation
Fig. 2. FTIR spectra: (a) PLA; (b) EVOH; (c) TAIC; (d) a gel sample of the PLA/EVOH/ TAIC blend of 50/50/3 after gamma-irradiated at 30 kGy.
showed low elongation even at 200 °C after EB-irradiated at 50 kGy. For the purpose of improving the comprehensive properties of PLA and enlarge its application area as a general material, the polyblends of PLA and poly(ethylene-co-vinyl alcohol), another green material with a characteristic of biodegradation were prepared, and the effect of gamma-irradiation on the blends was investigated in the present work. EVOH is a polymer with good
PLA (REVODE 101) with density of 1.25 g/cm3 was supplied by Zhejiang Hisun Biomaterials Co. Ltd. (China). EVOH granules (E171, ethylene content 44 mol%) with density of 1.14 g/cm3 and a melt flow index of 1.7 g/10 min (2.16 kg, 190 °C) were purchased from Kuraray Co. Ltd. (Japan). Crosslinking agent TAIC was obtained from Laiyu Chemistry Co. Ltd. (China). Hexafluoroisopropanol (HFIP) and ethanol used in gelation extraction were from Zeus Fluors Technology Shanghai Co. Ltd. (China) and Beijing Chemical Plant (China), respectively. PLA and EVOH were firstly dried under vacuum at 110 °C for 3 h and 80 °C for 4 h, respectively. Blending of PLA and EVOH was carried out in a twin screw extruder (RC 500P, HAAKE, Germany) at 175–190 °C with a screw speed of 100 rpm. The weight ratios of PLA to EVOH were 80/20, 60/40, and 50/50. 1, 3 or 5 wt.% of TAIC was added to each blend. A control blend without TAIC was also prepared for a comparison. The extruded blends were then thermal
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room temperature in silicon oil medium under constant load of 1000 g with a heating rate of 12 °C/6 min according to GB/T 1633-2000 (China). The fracture surfaces of the blends were gold-coated under vacuum, and then the morphology structures were observed by a XL30 ESEM FEG (USA) at 20 kV. FTIR measurements of a 50/50/3 blend gel sample and of PLA, EVOH and TAIC, were carried out with a FTIR spectrometer (Bruker Vertex 70, Germany) after drying and grinding in liquid nitrogen. The spectra were recorded in the range of 400–4000 cm1, with the resolution of 2 cm1 and the number of sample scans of 32.
3. Results and discussion 3.1. Gel fraction of the blends
Fig. 3. SEM photos of fracture surfaces of the PLA/EVOH/TAIC blends of 60/40/5: (a) without irradiation; (b) after gamma-irradiated at 50 kGy.
molded to testing samples, such as dumb bell specimens for the tensile test, in an injection mould machine (CJ80M3V, Chende Plastics Machinery Co. Ltd., China) after drying at 80 °C for 12 h under vacuum. 2.2. Gamma-irradiation The samples were sealed in PE (polyethylene) bags and irradiated by a Co-60 source with a dose rate of 3.9 Gy/s under the protection of N2 at room temperature. The absorbed doses were 10, 30, 50, and 100 kGy, respectively. 2.3. Property measurements and structure characteristics Tensile strength was investigated using an Instron Universal Testing Instrument (INSTRON 1121, America) at a crosshead speed of 10 mm/min at room temperature according to GB/T 1040.22006 (China). Samples for gel extraction were cut from fractured tensile specimens. They were then covered with nickel-mesh and extracted with HFIP in a Sokhlet apparatus for 60 h at 85 °C, followed by washing with alcohol and then drying. The gel fraction was calculated using the equation below:
G¼
W1 100%; W0
where G is the gel fraction (wt.%), W0 and W1 represent the weights of dried sample before and after extraction, respectively. A heat deformation Vicat temperature testing machine WKW-300 (Institute of Intelligence testing machine, Changchun, China) was used to obtain the HDT of the blend samples with the size of 12 12 3.3 mm3. The samples were heated from
Gel fraction of blends with different compositions is shown in Fig. 1. It is clear that gelation hardly formed in the PLA/EVOH binary blends without any TAIC before the irradiation degradation of both PLA [34] and EVOH [35]. The addition of TAIC enabled the blends to crosslink. When 1% TAIC was added, the gel fraction increased with absorbed dose initially, and reached the peak of ca. 40 wt.% when absorbed dose was 50 kGy. The blends became more sensitive to irradiation when the amount of TAIC exceeded 3% as the higher gel fraction occurred at lower absorbed dose compared to the result of additional 1 wt.% TAIC. This was noted especially in the PLA/EVOH blends of 60/40 and 50/50, where the amount of networks reached 80 wt.% when TAIC was at 3–5 wt.%. Irradiation with low doses at 10–30 kGy obviously had a significant impact on the crosslinking of the blends, but higher irradiation doses did not further produce gel fraction. The co-crosslinked networks consisted of PLA and EVOH were obtained, indicated by the higher content of gel fraction than that of either PLA or EVOH in the blends when 3–5 wt.% TAIC presented. As seen obviously in Fig. 1b and c: The gel fraction over 70 wt.%, when absorbed dose excesses 30 kGy and TAIC amount over 3 wt.%, is greater than the maximum PLA fraction of 60 wt.% in the blends. 3.2. Fourier Transform Infrared spectrum analyses The FTIR spectra of PLA, EVOH, TAIC and a gel sample of the PLA/ EVOH/TAIC blend of 50/50/3 gamma-irradiated at 30 kGy are shown in Fig. 2. A new crosslinking structure in the PLA/EVOH/ TAIC blend has been found by the comparison of infrared spectrums between the gel and the pure components. The characteristic peaks, at 1687, 1761 and 3363 cm1, assigned to the cyanurate group of TAIC, carbonyl group of PLA [36] and hydroxyl group of EVOH [37], respectively, are found in the spectrum of the gel sample, indicating that PLA, EVOH and TAIC entered the gel networks, and a new crosslinking structure like PLA-g-TAIC-g-EVOH has been formed. 3.3. Morphology structure The irradiation also had an obvious effect on the morphology of the PLA/EVOH/TAIC blends, as seen from the SEM images in Fig. 3. The clear sea-island structure and a poor interfacial adhesion is illustrated by the distinct (very smooth) interfaces between the matrix (PLA) and the dispersed sphere phase (EVOH) for the blends without gamma-irradiation in Fig. 3a. The interfacial adhesion between PLA and EVOH phases was improved by gamma-irradiation as reflected by the indistinct interfaces in Fig. 3b. It is clear that the gamma-irradiation has enhanced the interphase boundary compatibility, which improved the interfacial adhesion accordingly.
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Fig. 4. The HDTs of the PLA/EVOH blends of (a) 80/20, (b) 60/40, (c) 50/50 at various absorbed doses.
3.4. Heat distortion temperatures of the bends The HDTs of the PLA/EVOH blends of various compositions at different gamma-irradiation absorbed doses are shown in Fig. 4. The irradiation played an important role in the significant improvement of HDT, particularly with the presence of the crosslinking agent, TAIC. The enhanced irradiation produced co-crosslinked networks composed of PLA and EVOH, which hinders the movements of polymer chains and therefore delays the distortion to higher temperature. It is noted that the PLA/EVOH blends without gamma-irradiation but with 3–5 wt.% TAIC had lower HDTs (55–65 °C) than either the neat PLA (ca. 70 °C, not shown in the figure) or the PLA/EVOH blends without TAIC nor irradiation (shown in Fig. 4b and c). This occurs because the TAIC, without entering the networks, may act as a solvent to the blends, softening them. In general, the enhancement of HDT basically consists with the changes of the gel fraction. Additional 1 wt.% TAIC did not lead to a prominent enhancement of HDT, as sufficient crosslinking network might not yet form. When 3–5 wt.% TAIC was added, the HDTs increased with absorbed doses and reached a maximum value, after which no significant increase of HDT could be observed with the increasing absorbed doses. The HDTs of the PLA/EVOH blends at both 60/40 and 50/50 with 3–5 wt.% TAIC significantly increased to approximately 140 °C after irradiated at an absorbed dose over 50 kGy, doubling the values of the samples without irradiation. For the PLA/EVOH blends at 80/20 with 3 and 5 wt.% TAIC, the HDTs increased dramatically after the irradiation absorbed dose reached 100 kGy, reaching 140 and 110 °C, respectively, which might further increase if the absorbed doses kept increasing.
However, within the studied cases, the maximum HDT for the PLA/EVOH blends with the presence of TAIC reached 140 °C in a few cases with different absorbed doses and TAIC amounts. To the best of our knowledge there is no similar data in literature to compare with. A relevant work, reported by Yang et al. [18], explored the thermal degradation temperature by thermal analyses and observed about less than 10% temperature rises of the onset degradation and the complete degradation temperatures compared to those of neat PLA. When the HDT changes are compared with the gel fraction changes, the high gel fraction is an essential factor for the improvement of HDT, however, it is not a simple linear relationship between gel fraction and HDT. For instance, when the PLA/EVOH/ TAIC blend of a ratio of 80/20/3 was irradiated at 30 kGy, the gel fraction exceeded 70 wt.%, but there was no evident enhancement of HDT. However, the HDT increased to 140 °C, when the same blend exposed to irradiation of 100 kGy in despite of a similar gel fraction. The same results can be observed in the blends of other ratios when TAIC being 3 or 5 wt.%: Low dose irradiation can result in high gel fraction, but not increased HDT. Once there is sufficient TAIC dispersed in both PLA and EVOH, a few crosslinking junctions in both phases or at the boundaries of the two phases can easily formed, therefore, a three-dimensional network forms in the blends. Theoretically, a non-dissolved and non-molten structure (characterized by high gel fraction) can be formed with only a few junctions. However, only when crosslinking junctions become sufficient, the movements of the polymer chains can be constrained, particularly at elevated temperature, and result in
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Fig. 5. Tensile Strengths of the blends with different compositions at various absorbed doses: (a) PLA/EVOH 80/20, (b) 60/40, (c) 50/50.
concomitant high distortion temperature. So the crosslinking density may play a more important role in the HDT of the blends than the gel fraction. Using the same above 80/20/3 blend as an example again, the number of the crosslinking junctions formed at irradiation dose of 30 kGy (or less) was adequate to generate high gel fraction, but may not enough to hinder most of the chain movements, so the HDT cannot be improved significantly. Along the same line, when TAIC is only 1 wt.%, only few crosslinking junctions can be formed. The gel fraction remains low as well as the HDT, even at high irradiation levels. 3.5. Mechanical properties Fig. 5 shows the tensile strengths of the PLA/EVOH blends with different TAIC additions at various absorbed doses of gamma-irradiation. Gamma-irradiation in the presence of TAIC had an evident effect on the mechanical properties of the blends. Firstly, the tensile strength of the blends without TAIC decreased with absorbed dose, resulted from the irradiation degradations, shown by the empty squares. Secondly, it is clear that with 5 wt.% TAIC but no irradiation, the tensile strength is the worst. This result matched the HDT was the lowest when 5 wt.% TAIC was added but no irradiation. It may be because of the solvent effect of unreacted TAIC to the blends, mentioned earlier. The unreacted TAIC molecules entered the spaces between macromolecular chains, and weakened the intermolecular forces, eased the motions of polymer chains. Namely, the unreacted TAIC acted as a plasticizer in the blends. As a result, the corresponding tensile strength decreased with TAIC addition. However, once irradiation (even at a very low level) was introduced, the blends with 5 wt.% TAIC possessed the greatest
improvements in tensile strength values. When the amount of TAIC was less than 5 wt.% (being 1 or 3 wt.%), the increases in tensile strength were mild. The trend of the tensile strength in relation to the irradiation doses seems more complicated than that of either the gel fractions or the HDTs, even though the tensile strength of the PLA/EVOH blends of all studied ratios was enhanced. PLA would become more capable in terms of physical performance, since the tensile strength reached close to 70 MPa when the appropriate amount of TAIC was added and suitable irradiation dosage was applied. The optimum amount of TAIC is 5% as seen in Fig. 5. However, the absorbed doses did not affect the tensile strengths sensitively, as after they surpassed the optimum level, which was in line with the gel fraction results. The trend of tensile strengths versus absorbed dose is similar to that of tensile strengths versus gel fractions. 4. Conclusion Gamma-irradiation has been shown to be an effective method for improving the heat resistance of PLA blended with EVOH in the presence of an appropriate polyfunctional crosslinker, namely TAIC. During gamma-irradiation, a new network structure of possible PLA-g-TAIC-g-EVOH was achieved at the interface boundaries of PLA and EVOH in the PLA/EVOH/TAIC blends. Therefore, the interfacial adhesions have been improved. Remarkably, the PLA/ EVOH/TAIC blends with such a crosslinked network have dramatically enhanced the HDT values, as high as 140 °C, which is double the value of neat PLA, 70 °C. Tensile strengths of the PLA/EVOH/ TAIC blends have also been remarkably increased by irradiations and the maximum value of tensile strengths reached 70 MPa,
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compared to 40–50 MPa of the control PLA/EVOH blends. The optimal dosage of the gamma-irradiation and the optimized amount of TAIC for the PLA/EVOH blends were found to be 50 kGy and 3–5% by weight, respectively in the studied scenarios. The effect of irradiation on the biodegradable nature of the PLA and EVOH or their blends would be very interesting, and further work will be carried out in this aspect. Acknowledgment Authors thank the National Natural Science Foundation of China (Project No. 2008AA03Z511) for financial support. References [1] M.S. Reeve, S.P. Mccathy, M.J. Downey, R.A. Gross, Macromolecules 27 (1994) 825. [2] H. Tsuji, S. Miyauchi, Polymer 42 (2001) 4463. [3] S. Jacobsen, H.G. Fritz, Polym. Eng. Sci. 39 (1999) 1303. [4] S. Jacobsen, P.H. Degee, H.G. Fritz, P.H. Dubois, R. Jerome, Polym. Eng. Sci. 39 (1999) 1311. [5] J. Lunt, Polym. Degrad. Stabil. 59 (1998) 145. [6] L. Yu, K. Dean, L. Li, Prog. Polym. Sci. 31 (2006) 576. [7] S. Singh, S.S. Ray, J. Nanosci. Nanotechnol. 7 (2007) 2596. [8] R.G. Sinclair, J. Macromol. Sci. A33 (1996) 585. [9] E.T.H. Vink, K.R. Rabago, D.A. Glassner, B. Springs, R.P. O’Connor, J. Kolstad, P.R. Gruber, Macromol. Biosci. 4 (2004) 551. [10] A.H. Tullo, Chem. Eng. News 80 (2002) 13. [11] A.H. Tullo, Chem. Eng. News 78 (2000) 11. [12] N. Nagasawa, A. Kaneda, S. Kanazawa, T. Yagi, H. Mitomo, F. Yoshii, M. Tamada, Nucl. Instrum. Methods Phys. Res. B 236 (2005) 611. [13] X.H. Ran, Z.Y. Jia, C.Y. Han, Y.M. Yang, L.S. Dong, J. Appl. Polym. Sci. 116 (2010) 2050.
[14] L. Suryanegara, A.N. Nakagaito, H. Yano, Compos. Sci. Technol. 69 (2009) 1187. [15] A. Bhatia, R.K. Gupta, S.N. Bhattacharya, H.J. Choi, Int. Polym. Proc. 25 (2010) 5. [16] T. Nishino, K. Hirao, M. Kotera, K. Nakamae, H. Inagaki, Compos. Sci. Technol. 63 (2003) 1281. [17] K. Oksman, M. Skrifvars, J.F. Selin, Compos. Sci. Technol. 63 (2003) 1317. [18] S.L. Yang, Z.H. Wu, W. Yang, M.B. Yang, Polym. Test. 27 (2008) 957. [19] Y.C. Zhu, X.H. Zhang, J.L. Qiao, G.S. Wei, Chin. J. Polym. Sci. 22 (2004) 147. [20] A. Oshima, S. Ikeda, T. Seguchi, Y. Tabata, Radiat. Phys. Chem. 49 (1997) 279. [21] I. Klier, J. Polym. Eng. 16 (1997) 311. [22] D.P. Kiryukhin, I.P. Kim, G.A. Kichigina, N.M. Perepelitsa, I.M. Barkalov, High Energy Chem. 30 (1996) 97. [23] D. Lopez, R. Esparza, G. Burillo, Radiat. Phys. Chem. 45 (1995) 637. [24] J. Gehring, Radiat. Phys. Chem. 57 (2000) 361. [25] Z.M. Xiang, H.R. Liu, P.Y. Deng, M.H. Liu, Y. Yin, X.W. Ge, Polym. Bull. 63 (2009) 587. [26] Y. Yin, M.H. Liu, X.W. Zheng, S. Shen, P.Y. Deng, J. Appl. Polym. Sci. (submitted for publication) [27] H. Mitomo, A. Kaneda, T.M. Quynh, N. Nagasawa, F. Yoshii, Polymer 46 (2005) 4695. [28] H.C. Silvis, Trends Polym. Sci. 5 (1997) 75. [29] G.W. Lohfink, M.R. Kamal, Polym. Eng. Sci. 33 (1993) 1404. [30] J.B. Faisant, A.A. Kadi, M. Bousmina, L. Deschenes, Polymer 39 (1998) 533. [31] C.K. Samios, N.K. Kalfoglou, Polymer 39 (1998) 3863. [32] F.Z. Jin, S.H. Hyon, H. Iwata, S. Tsutsumi, Macromol. Rapid Commun. 23 (2002) 909. [33] P.Y. Deng, M.H. Liu, W.X. Zhang, J.Z. Sun, Nucl. Instrum. Methods Phys. Res. B 258 (2007) 357. [34] P. Nugroho, H. Mitomo, F. Yoshii, T. Kume, Polym. Degrad. Stabil. 72 (2001) 337. [35] M.H. Liu, P.Y. Deng, G.E. Sun, W.X. Zhang, J.Z. Sun, L.S. Dong, J. Radiat. Res. Radiat. Process. 24 (2006) 337. [36] P.Y. Deng, M.H. Liu, X.W. Zheng, N.A. Liu, L.S. Dong, Chem. Res. Chinese U. 30 (2009) 1. [37] H.H. Li, Y. Yin, M.H. Liu, P.Y. Deng, W.X. Zhang, J.Z. Sun, Adv. Polym. Technol. 28 (2009) 192.