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Compatibility effect on the thermal degradation behaviour of polypropylene blends with polyamide 6, ethylene propylene diene copolymer and polyurethane
Polymer Degradation and Stability 90 (2005) 481e487 www.elsevier.com/locate/polydegstab
Compatibility effect on the thermal degradation behaviour of polypropylene blends with polyamide 6, ethylene propylene diene copolymer and polyurethane J. Roeder, R.V.B. Oliveira, D. Becker, M.W. Gonc¸alves, V. Soldi, A.T.N. Pires * Grupo de Estudo em Materiais Polime´ricos (POLIMAT), Departamento de Quı´mica, Universidade Federal de Santa Catarina, 88040-900 Floriano´polis-SC, Brazil Received 9 September 2004; received in revised form 4 March 2005; accepted 11 April 2005 Available online 26 May 2005
Abstract The morphology and thermal behaviour of polypropylene/polyamide 6 (PP/PA6), polypropylene/copolymer ethylene propylene diene (PP/PEBAX) and polypropylene/rigid polyurethane (PP/PUR) blends compatibilised with polypropylene-graft-maleic anhydride (PP-g-MA) were studied using scanning electron microscopy and thermogravimetric analyses. The study focuses on the influence of different blends obtained by mixing a thermoplastic, thermoplastic elastomer or thermoset with PP, compatibilised with PP-g-MA. The compatibilising effect of PP-g-MA in an immiscible PP/PA6 blend induces a homogeneous dispersion due to interfacial adhesion. For the PP/PEBAX and PP/PUR binary blends studied slight changes in the morphology were observed with a continuous phase but the PEBAX or PUR domains remained in the PP matrix. The deconvolution of the TGA curve permitted an evaluation of the decomposition stage of the undiluted and blend systems. Thermal stability is slightly influenced by the position of the maximum decomposition rate temperature of the first derivative thermogravimetric curve (DTG). However, the DTG curve profile remains consistent. The activation energy of undiluted PP was in the range of 162e169 kJ molÿ1 determined by the Ozawa method. The stabilized activation energy value for all blends studied above a 0.4 weight-loss fraction is discussed. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Polymer blends; Compatibiliser; Thermal decomposition
1. Introduction In the last few years, polymer blends have been studied and they present many advantages, like the possibility to change the mechanical properties and thermal stability. Recently, some work has been focused on thermal behaviour using different methods to characterize durability and operating conditions of polymer blends [1e3]. Some characteristics are important to an understanding of thermal degradation of
* Corresponding author. Tel.: C55 48 331 6845; fax: C55 48 331 9711. E-mail address: [email protected] (A.T.N. Pires). 0141-3910/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2005.04.008
polymer blends, such as miscibility, compatibilisation and the possibility to form cross-linked structures. The degradation stage of miscible systems like the PEO/ carbopol blend has been evaluated through FTIR spectra at different temperatures during thermal treatment. The activation energy increases with the carbopol content in the blend, showing that with increasing temperature anhydride formation occurs first, and this in turn is degraded, forming carbon dioxide and radicals as products [4]. The immiscible system natural rubber/ polystyrene (NR/PS) showed lower weight loss than the undiluted component at the same temperature, and the initial decomposition temperature was raised with the addition of compatibiliser agent, suggesting better thermal stability of the compatibilised blend [5]. Blends
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J. Roeder et al. / Polymer Degradation and Stability 90 (2005) 481e487
of PA6 with polypropylene grafted with diethylmaleate (PP-g-DEM/PA6) have shown a wide dispersion and smaller particle size, which induces a better interfacial adhesion, improving the thermal stability of these mixtures [6]. However, thermogravimetric analysis of PP/EPDM-g-MA (ethylene propylene diene elastomer grafted maleic anhydride) showed that the presence of EPDM-g-MA reduces the thermal stability of PP, but the presence of epoxy (PP/EPDM-g-MA/epoxy) could improve the thermal stability [7]. Zhang et al. [8] showed that the design and synthesis of a suitable reinforcing component, like polyamide 6 copolymer, improves the thermal stability, due to the elimination of unstable terminal groups. In previous studies carried out by our research group the gas products from a PP sample submitted to degradation in a tubular oven at 450 C were evaluated by infrared spectroscopy [9]. The absorption bands such as eCeH (2985 and 2850 cmÿ1), ]CeH (3050 cmÿ1) and eC]Ce stretching (1608 cmÿ1) indicate the production of aliphatic hydrocarbons and unsaturated compounds during the decomposition [10e12]. The effects of maleated ethylene propylene diene (EPDMg-MA) on the thermal stability of polyamide/EPDM were also studied by FTIR. Based on the activation energy and reaction products, it was suggested that the thermal degradation of undiluted polyamides occurred firstly by chain scission of the weakest CeN and eC(O)eNH bonds. The breaking of the strongest CeN bonds occurred at temperatures exceeding 400 C [13]. Other studies on PP have reported the effect of compatibiliser addition on mechanical properties and morphological characteristics, but less attention has been given to the evaluation of PP thermal degradation behaviour [14e17]. In this study, we focused on the thermal degradation behaviour of PP mixtures with thermoplastic, thermoplastic elastomer or thermosetting, in the presence of grafted maleic anhydride polypropylene [PP-g-MA] as a compatibiliser. The kinetics parameters were used to evaluate the effect of blending on thermal stability.
respectively. Polypropylene grafted (PP-g-MA) (Tg Z ÿ19 C and Tmax Z 148 C) with 0.4% maleic anhydride was a product of Exxon Chemical. The undiluted polymers were dried under vacuum for 24 h at 80 C and used without further purification. 2.2. Blend preparation Blends of different systems and compositions were prepared by placing polymer mixtures into a CSI Max Extruder, Model CS-194 A, set to a length/diameter ratio of 4 and a screw rotation rate of 70 rpm, with two heating zones at 230 C. PP/PUR blends were prepared at 160 rpm and 200 C. The composition of PP/X blends was 70 wt.% of PP and 30 wt.% of disperse X component, where X is PA6, PEBAX or PUR. To ternary PP/X/PP-g-MA blends, 10 wt.% of compatibiliser was added, maintaining a 30 wt.% constant composition of the X-polymer phase. The process was repeated at least twice in order to obtain better component dispersion. 2.3. Thermal analysis The degradation of the blends was analysed with a Shimadzu 50 thermogravimetric analyser in nitrogen atmosphere. Non-isothermal analysis was performed in a temperature range of 25e700 C, at different heating rates (5, 10 and 20 C minÿ1) for each sample. Nitrogen flow was maintained at 50 cm3 minÿ1 and samples of ca. 12 mg were used for all measurements. The deconvolutions were performed by Origin 7.0 software and Gaussian bases. The thermogravimetric data were analysed through the Ozawa method [18,19] using the associated TGA-50 software to determine the kinetics parameters. 2.4. Scanning electronic microscopy (SEM) Polymer blend samples were fractured under liquid nitrogen and coated with gold to avoid charge by electron beams, and were analysed by scanning electron microscopy (SEM, Philips XL 30).
2. Experimental 2.1. Materials
3. Results and discussion
Isotatic polypropylene (PP) (Tg Z ÿ21 C and Tmax Z 169 C), polyamide 6 (PA6) (Tg Z 40 C and Tmax Z 220 C), poly(ether-b-amide) with 59 wt.% of ethylene oxide segment and 41 wt.% of amide 6 (PEBAX) (Tg Z ÿ5 C and Tmax Z 24 and 205 C), and rigid polyurethane foam (PUR) (Tg Z 230 C) were supplied by OPP Petroquı´ mica S.A., Petronyl, Elf Aquitaine and Multibra´s Eletrodome´sticos S.A.,
3.1. Morphology Polypropylene is one of the most widely used polyolefin polymers. Since it does not include any polar groups in its backbone, mixing PP with polar polymers to improve mechanical properties can result in some difficulties in obtaining a homogeneous system. Large PA6 spherical domains were observed in the PP/PA6
J. Roeder et al. / Polymer Degradation and Stability 90 (2005) 481e487
blend (Fig. 1a), contrasting to a co-continuous morphology of the compatibilised blend PP/PA6/PP-g-MA (Fig. 1b). This homogeneous dispersion is related with the compatibilising effect, due to interfacial adhesion of PP and PA6, as was observed by infrared spectroscopy in a previous study [15]. The PP/PEBAX blend (Fig. 1c) morphology indicated that the PEBAX phase domains preferentially dispersed as spheres in the continuous PP matrix. The presence of PP-g-MA in the binary blend slightly changed the morphology resulting in a cocontinuous phase, but PEBAX domains remained (Fig. 1d). Micrographs of PP/PUR and PP/PUR/PP-gMA blends are shown in Fig. 1e and f, respectively. Fig. 1e displays the micrograph of the non-compatibilised blend, with large domains of PUR in the PP matrix. The shape of these irregular domains, after the milling process, can be attributed to the rigid polyurethane structure. No adhesion was observed between
483
the domains in the matrix, probably due to the weak interfacial bonding. On the other hand, the addition of compatibiliser promotes interfacial adhesion between domains and matrix, as can be seen in Fig. 1f. The similarity of the domain sizes in the compatibilised and non-compatibilised blends is not in disagreement with literature data [20] because in this specific system the disperse phase is a thermosetting material. Interfacial adhesion of PUR/PP/PP-g-MA blends was characterized in a previous study [21]. The increase in interfacial adhesion and morphological homogeneity, observed in PP/PA6 systems, is probably due to a greater compatibiliser effectiveness in the PA6 system than in the PUR and PEBAX systems. The final properties of these blends, such as thermal stability, could be strongly affected by the volume fraction of the components, size and shape of domains, interfacial tension and adhesion between phases.
Fig. 1. SEM micrographs for (a) PP/PA6, (b) PP/PA6/PP-g-MA, (c) PP/PEBAX, (d) PP/PEBAX/PP-g-MA, (e) PP/PUR and (f) PP/PUR/PP-g-MA.
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3.2. Thermal analysis Fig. 2 shows the first derivative TG curve [(d(TG)/ dT ) denoted by DTG] as a function of temperature, obtained from the thermogravimetric curve. The thermogravimetric behaviour of undiluted polymers is described by Gaussian deconvolution of the DTG curve (dashed line insert in Fig. 2). In the DTG curve for the pure PP there are two different peaks with an area ratio of 0.59, one of them related to the formation of active nuclei centres and the other to the propagation decomposition [22]. These peaks show a temperature of maximum decomposition rate (Tmax) at 434 C and 466 C, respectively. The DTG curve for pure polymers PA6 and PEBAX presented only one degradation peak, while PUR presented two, in agreement with the Gaussian deconvolution curve. Solid residues of all undiluted polymers were less than 4 wt.%. Table 1 shows the onset temperature of the TG decomposition curve (To), the maximum temperature of decomposition rate (Tmax), and peak areas of the DTG curve. The PP/PA6 blend exhibited a single weight-loss step and, with the asymmetric peak DTG, three deconvoluted peaks could be obtained, two of them (peaks I and III in Fig. 3) with a similar shape to pure PP and an area related to PP percentage in the mixture. Peak II corresponds to 32% of the total area assigned to PA6, in agreement with the quantity of this component in the blend. The Tmax of the first peak for the PP/PA6 blend presents the same value as that for the pure PP. This behaviour can be related to the morphological characteristics of these binary blends, where the second phase (domains) is large and heterogeneity in the domain dispersions is observed. The deconvoluted curve from the compatibilised system showed three peaks, the total area of which (73%) is related to the PP peaks, and the Tmax of the first peak decomposition was 448 C,
Table 1 Data obtained from TGA analysis of undiluted components and blends Systems
Toa ( C)
Stageb
Proportionc (%)
Tmaxd ( C)
PP
382
PA6 PEBAX PUR
375 365 272
PP/PA6
420
PP/PA6/PP-g-MA
430
PP/PEBAX
405
PP/PEBAX/PP-g-MA
404
PP/PUR
347
PP/PUR/PP-g-MA
350
I II I I I II IPP IIPA6 IIIPP IPP IIPA6 IIIPP IPEBAX IIPP IIIPP IPEBAX IIPP IIIPP IPUR IIPP IIIPUR IVPP IPUR IIPP IIIPUR IVPP
37 63 100 100 50 50 16 32 52 16 27 57 33 34 34 33 34 34 16 14 26 44 16 17 22 45
434 466 456 438 316 550 431 460 481 448 470 488 467 498 510 470 500 513 357 455 484 502 355 461 479 496
a
To: Temperature of onset decomposition. Stage: Obtained from derivative TG curve. c Proportion: Obtained from the total area of the deconvoluted DTG curve. d Tmax: Temperature on the deconvoluted peak. b
indicating that the compatibilisation had a slight influence on the thermal stability. The morphological characteristics are in agreement with these results, showing phase homogeneity due to the interfacial adhesion.
PP / PA6 / PP-g-MA
PP
I
I
II II III d(TG) / dT
d(TG) / dT
PA6 I
PEBAX
PP / PA6
I II
I
PU
III I
II 200
200
250
300
350
400
450
500
550
600
Temperature / ºC Fig. 2. DTG thermograms for undiluted components. The symbols in the figure indicate: (I) first and (II) second degradation stages.
250
300
350
400
450
500
550
600
Temperature / ºC Fig. 3. DTG thermograms for PP/PA6 and PP/PA6/PP-g-MA. The symbols in the figure indicate: (I) first, (II) second and (III) third degradation stages.
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PP / PEBAX / PP-g-MA I
d(TG) / dT
II
PP / PEBAX I II
200
III
250
300
350
400
450
500
III
550
600
Temperature / ºC Fig. 4. DTG thermograms for PP/PEBAX and PP/PEBAX/PP-g-MA. The symbols in the figure indicate: (I) first, (II) second and (III) third degradation stages.
PP / PU / PP-g-MA I
II
III IV
d(TG) / dT
The DTG curves for the PP/PEBAX and PP/ PEBAX/PP-g-MA blends have a shoulder, and after the deconvolution three weight-loss steps were observed, Fig. 4. The total area percentage corresponds to the component quantities in the blend. The compatibilised and non-compatibilised blends presented, in the first stage, the same To (404 G 1 C) and Tmax (467 G 3 C), with higher values than their pure polymers. As discussed previously, compatibilised and noncompatibilised mixtures presented the same size and domain distributions. With this characteristic the products of the domain degradations diffuse through the PP matrix at the same rate in both blends. Peak II could be related to PEBAX and peaks I and III to PP degradation, with similar areas. This fact suggests that the degradation rate increases in the first degradation stage, i.e. during the process which occurs at the lowest temperature. The deconvolution of the DTG curves for the PP/ PUR and PP/PUR/PP-g-MA blends shows four weightloss steps (Fig. 5). Peaks I and III are associated with the first and second degradation stages in the case of undiluted PUR, while peaks II and IV are related to PP (same area ratio as pure PP), evidencing that the degradation process of one pure component does not interfere with that of the other. The Tmax for compatibilised and non-compatibilised is the same (355 G 2 C) and lower than the value for the first decomposition stage of pure PP. The decomposition of the blends showed the same DTG curve profile as the pure PP with a decrease in the Tmax from the second decomposition stage (peak III in Fig. 5) to a temperature close to the PP decomposition temperature range. This effect is attributed to the influence of the PP matrix decomposition process.
PP / PU I
II
III IV
200
250
300
350
400
450
500
550
600
Temperature / ºC Fig. 5. DTG thermograms for PP/PUR and PP/PUR/PP-g-MA. The symbols in the figure indicate: (I) first, (II) second, (III) third and (IV) fourth degradation stages.
3.3. Kinetics parameters The activation energies (Ea) for the thermal degradation as a function of the weight-loss fraction (a), for the pure PP and the blends, determined by the Ozawa method, are shown in Fig. 6. According to these results, undiluted PP presents average Ea-values in the range of 162e169 kJ molÿ1. Values are in agreement with the range of values reported in the literature [6,18,23,24], slight differences can be attributed to the peculiarities of each method and polymer properties, such as impurities and molecular weight, as well as to the experimental techniques and operating conditions [25]. For all the compatibilised and non-compatibilised binary blends studied it is possible to identify different slopes up to approximately a 0.4 weight-loss fraction (Fig. 6). This may be associated with stages of decomposition at lower temperatures during the process of active centre formation and propagation of the decomposition process. It is interesting to highlight that the activation energy tends to stabilize above a 0.4 weight-loss fraction, for all systems, at 170 G 30 kJ molÿ1. This observation indicates that the decomposition stage of PP is not influenced by the second component in the binary blend, as also observed in the deconvoluted DTG curves, in which the last decomposition peak may represent the decomposition of the PP main chain. An activation energy value below a 0.4 weight-loss fraction has specific behaviour, depending on the compatibilisation efficiency. This is shown for PP/PA6/PP-g-MA by a lower variation in the activation energy in the first decomposition stage, due to the formation of a new copolymer at the interface between the domains and the matrix. On the other hand, the PP/PUR/PP-g-MA and PP/ PEBAX/PP-g-MA systems did not show analogous
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J. Roeder et al. / Polymer Degradation and Stability 90 (2005) 481e487 190
was changed to a co-continuous morphology in the compatibilised blend due to interfacial adhesion of PP and PA6. The addition of compatibiliser to PP/PEBAX and PP/PUR binary blends promotes the interfacial adhesion between the domains and the matrix, without domain reductions because in these specific systems the disperse phase is a copolymer and thermosetting material, respectively. The compatibilisation of PP/PA6 blends leads to an increase in the activation energy of the first decomposition stage, without influencing the decomposition of the main chain of the PP component. Compatibilised and non-compatibilised blends of PP/PEBAX showed analogous decomposition behaviour with temperature values (To) of 405 and 404 C, respectively. Deconvolution of DTG curves showed the same ratio for the stages of decomposition corresponding to each component in the blend. The weight-loss fraction down the 0.4 weight-loss fraction induces a broad range of Ea-values due to the juxtaposition of the deconvoluted peaks. For higher weight-loss fraction values, all systems studied showed activation energies close to the undiluted PP Ea-values.
(a)
180 170 160 150 140
PP PP/ PA6 PP/ PA6 / PP-g-MA
130
(b)
200
Ea (kJ mol-1)
190
180
170
160
Acknowledgements
PP/ PEBAX PP/ PEBAX / PP-g-MA
The authors would like to thank to CNPQ and CAPES for financial support.
(c) 200
References
180`
160
140
120
100 0,0
PP/ PUR PP/ PUR/ PP-g-MA 0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
Weight-Loss Fraction (α) Fig. 6. Plots of activation energies determined by the Ozawa method for: (a) PP, PP/PA6 and PP/PA6/PP-g-MA; (b) PP/PEBAX and PP/ PEBAX/PP-g-MA; (c) PP/PUR and PP/PUR/PP-g-MA systems.
phase stability which could be related to the activation energy.
4. Conclusions The spherical domains of the PA6 component observed in the SEM micrograph of the PP/PA6 blend
[1] Bate DM, Lehrle RS. Polymer Degradation and Stability 1998;62:57e66. [2] Chiantore O, Trossarelli L, Lazzari M. Polymer 1998;39:2777e81. [3] Wilkie CA. Polymer Degradation and Stability 1999;66:301e6. [4] Kanis LA, Viel FC, Crespo JS, Bertolino JR, Pires ATN, Soldi V. Polymer 2000;41:3303e9. [5] Asaletha R, Kumaran MG, Thomas S. Polymer Degradation and Stability 1998;61:431e9. [6] Albano C, de Freitas E. Polymer Degradation and Stability 1998;61:289e95. [7] Jiang X, Zhang Y, Zhang Y. Polymer Testing 2004;23:259e66. [8] Zhang Y, Cheng K, Xu J. Thermochimica Acta 2005;425:137e41. [9] Oliveira RVB, Pinheiro EA, Soldi V, Fredel MC, Pires ATN. Polı´ meros: Cieˆncia e Technologia 2004;14:149e54. [10] Lattimer RP. Journal of Analytical and Applied Pyrolysis 1993;26:65e92. [11] Chan JH, Balke ST. Polymer Degradation and Stability 1997;57(2):135e49. [12] Gao ZM, Kaneko T, Amasaki I, Nakada M. Polymer Degradation and Stability 2003;80(2):269e74. [13] Vieira I, Severgnini VLS, Mazera DJ, Soldi MS, Pinheiro EA, Pires ATN, et al. Polymer Degradation and Stability 2001; 74:151e7. [14] Ide F, Hasegawa A. Journal of Applied Polymer Science 1974;18:963e74. [15] Roeder J, Oliveira RVB, Goncalves MC, Soldi V, Pires ATN. Polymer Testing 2002;21:815e21.
J. Roeder et al. / Polymer Degradation and Stability 90 (2005) 481e487 [16] Gonzalez-Montiel A, Keskkula H, Paul DR. Polymer 1995; 36:4587e603. [17] Tedesco A, Barbosa RV, Nachtigall SMB, Mauler RS. Polymer Testing 2002;21:11e5. [18] Ozawa T. Bulletin of the Chemical Society of Japan 1965;38:1881. [19] Ozawa T. Journal of Thermal Analysis 1975;7:601e17. [20] Wilkinsom AN, Laugel L, Clemens ML, Harding VM, Marin M. Polymer 1999;40:4971.
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[21] Becker D, Roeder J, Oliveira RVB, Soldi V, Pires ATN. Polymer Testing 2003;22:225e30. [22] Gonzalez J, Albano C, Sciamanna R, Ichazo M, Rosales C, Martinez J, et al. Polymer Degradation and Stability 2000; 68:9e19. [23] Gambirozajukic M, Cunko R. Acta Polymerica 1992;43:258e60. [24] Ozawa T, Kato T. Journal of Thermal Analysis 1991;37:1299e307. [25] Carrasco F. Thermochimica Acta 1993;213e5.
Compatibility effect on the thermal degradation behaviour of polypropylene blends with polyamide 6, ethylene propylene diene copolymer and polyurethane J. Roeder, R.V.B. Oliveira, D. Becker, M.W. Gonc¸alves, V. Soldi, A.T.N. Pires * Grupo de Estudo em Materiais Polime´ricos (POLIMAT), Departamento de Quı´mica, Universidade Federal de Santa Catarina, 88040-900 Floriano´polis-SC, Brazil Received 9 September 2004; received in revised form 4 March 2005; accepted 11 April 2005 Available online 26 May 2005
Abstract The morphology and thermal behaviour of polypropylene/polyamide 6 (PP/PA6), polypropylene/copolymer ethylene propylene diene (PP/PEBAX) and polypropylene/rigid polyurethane (PP/PUR) blends compatibilised with polypropylene-graft-maleic anhydride (PP-g-MA) were studied using scanning electron microscopy and thermogravimetric analyses. The study focuses on the influence of different blends obtained by mixing a thermoplastic, thermoplastic elastomer or thermoset with PP, compatibilised with PP-g-MA. The compatibilising effect of PP-g-MA in an immiscible PP/PA6 blend induces a homogeneous dispersion due to interfacial adhesion. For the PP/PEBAX and PP/PUR binary blends studied slight changes in the morphology were observed with a continuous phase but the PEBAX or PUR domains remained in the PP matrix. The deconvolution of the TGA curve permitted an evaluation of the decomposition stage of the undiluted and blend systems. Thermal stability is slightly influenced by the position of the maximum decomposition rate temperature of the first derivative thermogravimetric curve (DTG). However, the DTG curve profile remains consistent. The activation energy of undiluted PP was in the range of 162e169 kJ molÿ1 determined by the Ozawa method. The stabilized activation energy value for all blends studied above a 0.4 weight-loss fraction is discussed. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Polymer blends; Compatibiliser; Thermal decomposition
1. Introduction In the last few years, polymer blends have been studied and they present many advantages, like the possibility to change the mechanical properties and thermal stability. Recently, some work has been focused on thermal behaviour using different methods to characterize durability and operating conditions of polymer blends [1e3]. Some characteristics are important to an understanding of thermal degradation of
* Corresponding author. Tel.: C55 48 331 6845; fax: C55 48 331 9711. E-mail address: [email protected] (A.T.N. Pires). 0141-3910/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2005.04.008
polymer blends, such as miscibility, compatibilisation and the possibility to form cross-linked structures. The degradation stage of miscible systems like the PEO/ carbopol blend has been evaluated through FTIR spectra at different temperatures during thermal treatment. The activation energy increases with the carbopol content in the blend, showing that with increasing temperature anhydride formation occurs first, and this in turn is degraded, forming carbon dioxide and radicals as products [4]. The immiscible system natural rubber/ polystyrene (NR/PS) showed lower weight loss than the undiluted component at the same temperature, and the initial decomposition temperature was raised with the addition of compatibiliser agent, suggesting better thermal stability of the compatibilised blend [5]. Blends
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J. Roeder et al. / Polymer Degradation and Stability 90 (2005) 481e487
of PA6 with polypropylene grafted with diethylmaleate (PP-g-DEM/PA6) have shown a wide dispersion and smaller particle size, which induces a better interfacial adhesion, improving the thermal stability of these mixtures [6]. However, thermogravimetric analysis of PP/EPDM-g-MA (ethylene propylene diene elastomer grafted maleic anhydride) showed that the presence of EPDM-g-MA reduces the thermal stability of PP, but the presence of epoxy (PP/EPDM-g-MA/epoxy) could improve the thermal stability [7]. Zhang et al. [8] showed that the design and synthesis of a suitable reinforcing component, like polyamide 6 copolymer, improves the thermal stability, due to the elimination of unstable terminal groups. In previous studies carried out by our research group the gas products from a PP sample submitted to degradation in a tubular oven at 450 C were evaluated by infrared spectroscopy [9]. The absorption bands such as eCeH (2985 and 2850 cmÿ1), ]CeH (3050 cmÿ1) and eC]Ce stretching (1608 cmÿ1) indicate the production of aliphatic hydrocarbons and unsaturated compounds during the decomposition [10e12]. The effects of maleated ethylene propylene diene (EPDMg-MA) on the thermal stability of polyamide/EPDM were also studied by FTIR. Based on the activation energy and reaction products, it was suggested that the thermal degradation of undiluted polyamides occurred firstly by chain scission of the weakest CeN and eC(O)eNH bonds. The breaking of the strongest CeN bonds occurred at temperatures exceeding 400 C [13]. Other studies on PP have reported the effect of compatibiliser addition on mechanical properties and morphological characteristics, but less attention has been given to the evaluation of PP thermal degradation behaviour [14e17]. In this study, we focused on the thermal degradation behaviour of PP mixtures with thermoplastic, thermoplastic elastomer or thermosetting, in the presence of grafted maleic anhydride polypropylene [PP-g-MA] as a compatibiliser. The kinetics parameters were used to evaluate the effect of blending on thermal stability.
respectively. Polypropylene grafted (PP-g-MA) (Tg Z ÿ19 C and Tmax Z 148 C) with 0.4% maleic anhydride was a product of Exxon Chemical. The undiluted polymers were dried under vacuum for 24 h at 80 C and used without further purification. 2.2. Blend preparation Blends of different systems and compositions were prepared by placing polymer mixtures into a CSI Max Extruder, Model CS-194 A, set to a length/diameter ratio of 4 and a screw rotation rate of 70 rpm, with two heating zones at 230 C. PP/PUR blends were prepared at 160 rpm and 200 C. The composition of PP/X blends was 70 wt.% of PP and 30 wt.% of disperse X component, where X is PA6, PEBAX or PUR. To ternary PP/X/PP-g-MA blends, 10 wt.% of compatibiliser was added, maintaining a 30 wt.% constant composition of the X-polymer phase. The process was repeated at least twice in order to obtain better component dispersion. 2.3. Thermal analysis The degradation of the blends was analysed with a Shimadzu 50 thermogravimetric analyser in nitrogen atmosphere. Non-isothermal analysis was performed in a temperature range of 25e700 C, at different heating rates (5, 10 and 20 C minÿ1) for each sample. Nitrogen flow was maintained at 50 cm3 minÿ1 and samples of ca. 12 mg were used for all measurements. The deconvolutions were performed by Origin 7.0 software and Gaussian bases. The thermogravimetric data were analysed through the Ozawa method [18,19] using the associated TGA-50 software to determine the kinetics parameters. 2.4. Scanning electronic microscopy (SEM) Polymer blend samples were fractured under liquid nitrogen and coated with gold to avoid charge by electron beams, and were analysed by scanning electron microscopy (SEM, Philips XL 30).
2. Experimental 2.1. Materials
3. Results and discussion
Isotatic polypropylene (PP) (Tg Z ÿ21 C and Tmax Z 169 C), polyamide 6 (PA6) (Tg Z 40 C and Tmax Z 220 C), poly(ether-b-amide) with 59 wt.% of ethylene oxide segment and 41 wt.% of amide 6 (PEBAX) (Tg Z ÿ5 C and Tmax Z 24 and 205 C), and rigid polyurethane foam (PUR) (Tg Z 230 C) were supplied by OPP Petroquı´ mica S.A., Petronyl, Elf Aquitaine and Multibra´s Eletrodome´sticos S.A.,
3.1. Morphology Polypropylene is one of the most widely used polyolefin polymers. Since it does not include any polar groups in its backbone, mixing PP with polar polymers to improve mechanical properties can result in some difficulties in obtaining a homogeneous system. Large PA6 spherical domains were observed in the PP/PA6
J. Roeder et al. / Polymer Degradation and Stability 90 (2005) 481e487
blend (Fig. 1a), contrasting to a co-continuous morphology of the compatibilised blend PP/PA6/PP-g-MA (Fig. 1b). This homogeneous dispersion is related with the compatibilising effect, due to interfacial adhesion of PP and PA6, as was observed by infrared spectroscopy in a previous study [15]. The PP/PEBAX blend (Fig. 1c) morphology indicated that the PEBAX phase domains preferentially dispersed as spheres in the continuous PP matrix. The presence of PP-g-MA in the binary blend slightly changed the morphology resulting in a cocontinuous phase, but PEBAX domains remained (Fig. 1d). Micrographs of PP/PUR and PP/PUR/PP-gMA blends are shown in Fig. 1e and f, respectively. Fig. 1e displays the micrograph of the non-compatibilised blend, with large domains of PUR in the PP matrix. The shape of these irregular domains, after the milling process, can be attributed to the rigid polyurethane structure. No adhesion was observed between
483
the domains in the matrix, probably due to the weak interfacial bonding. On the other hand, the addition of compatibiliser promotes interfacial adhesion between domains and matrix, as can be seen in Fig. 1f. The similarity of the domain sizes in the compatibilised and non-compatibilised blends is not in disagreement with literature data [20] because in this specific system the disperse phase is a thermosetting material. Interfacial adhesion of PUR/PP/PP-g-MA blends was characterized in a previous study [21]. The increase in interfacial adhesion and morphological homogeneity, observed in PP/PA6 systems, is probably due to a greater compatibiliser effectiveness in the PA6 system than in the PUR and PEBAX systems. The final properties of these blends, such as thermal stability, could be strongly affected by the volume fraction of the components, size and shape of domains, interfacial tension and adhesion between phases.
Fig. 1. SEM micrographs for (a) PP/PA6, (b) PP/PA6/PP-g-MA, (c) PP/PEBAX, (d) PP/PEBAX/PP-g-MA, (e) PP/PUR and (f) PP/PUR/PP-g-MA.
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3.2. Thermal analysis Fig. 2 shows the first derivative TG curve [(d(TG)/ dT ) denoted by DTG] as a function of temperature, obtained from the thermogravimetric curve. The thermogravimetric behaviour of undiluted polymers is described by Gaussian deconvolution of the DTG curve (dashed line insert in Fig. 2). In the DTG curve for the pure PP there are two different peaks with an area ratio of 0.59, one of them related to the formation of active nuclei centres and the other to the propagation decomposition [22]. These peaks show a temperature of maximum decomposition rate (Tmax) at 434 C and 466 C, respectively. The DTG curve for pure polymers PA6 and PEBAX presented only one degradation peak, while PUR presented two, in agreement with the Gaussian deconvolution curve. Solid residues of all undiluted polymers were less than 4 wt.%. Table 1 shows the onset temperature of the TG decomposition curve (To), the maximum temperature of decomposition rate (Tmax), and peak areas of the DTG curve. The PP/PA6 blend exhibited a single weight-loss step and, with the asymmetric peak DTG, three deconvoluted peaks could be obtained, two of them (peaks I and III in Fig. 3) with a similar shape to pure PP and an area related to PP percentage in the mixture. Peak II corresponds to 32% of the total area assigned to PA6, in agreement with the quantity of this component in the blend. The Tmax of the first peak for the PP/PA6 blend presents the same value as that for the pure PP. This behaviour can be related to the morphological characteristics of these binary blends, where the second phase (domains) is large and heterogeneity in the domain dispersions is observed. The deconvoluted curve from the compatibilised system showed three peaks, the total area of which (73%) is related to the PP peaks, and the Tmax of the first peak decomposition was 448 C,
Table 1 Data obtained from TGA analysis of undiluted components and blends Systems
Toa ( C)
Stageb
Proportionc (%)
Tmaxd ( C)
PP
382
PA6 PEBAX PUR
375 365 272
PP/PA6
420
PP/PA6/PP-g-MA
430
PP/PEBAX
405
PP/PEBAX/PP-g-MA
404
PP/PUR
347
PP/PUR/PP-g-MA
350
I II I I I II IPP IIPA6 IIIPP IPP IIPA6 IIIPP IPEBAX IIPP IIIPP IPEBAX IIPP IIIPP IPUR IIPP IIIPUR IVPP IPUR IIPP IIIPUR IVPP
37 63 100 100 50 50 16 32 52 16 27 57 33 34 34 33 34 34 16 14 26 44 16 17 22 45
434 466 456 438 316 550 431 460 481 448 470 488 467 498 510 470 500 513 357 455 484 502 355 461 479 496
a
To: Temperature of onset decomposition. Stage: Obtained from derivative TG curve. c Proportion: Obtained from the total area of the deconvoluted DTG curve. d Tmax: Temperature on the deconvoluted peak. b
indicating that the compatibilisation had a slight influence on the thermal stability. The morphological characteristics are in agreement with these results, showing phase homogeneity due to the interfacial adhesion.
PP / PA6 / PP-g-MA
PP
I
I
II II III d(TG) / dT
d(TG) / dT
PA6 I
PEBAX
PP / PA6
I II
I
PU
III I
II 200
200
250
300
350
400
450
500
550
600
Temperature / ºC Fig. 2. DTG thermograms for undiluted components. The symbols in the figure indicate: (I) first and (II) second degradation stages.
250
300
350
400
450
500
550
600
Temperature / ºC Fig. 3. DTG thermograms for PP/PA6 and PP/PA6/PP-g-MA. The symbols in the figure indicate: (I) first, (II) second and (III) third degradation stages.
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PP / PEBAX / PP-g-MA I
d(TG) / dT
II
PP / PEBAX I II
200
III
250
300
350
400
450
500
III
550
600
Temperature / ºC Fig. 4. DTG thermograms for PP/PEBAX and PP/PEBAX/PP-g-MA. The symbols in the figure indicate: (I) first, (II) second and (III) third degradation stages.
PP / PU / PP-g-MA I
II
III IV
d(TG) / dT
The DTG curves for the PP/PEBAX and PP/ PEBAX/PP-g-MA blends have a shoulder, and after the deconvolution three weight-loss steps were observed, Fig. 4. The total area percentage corresponds to the component quantities in the blend. The compatibilised and non-compatibilised blends presented, in the first stage, the same To (404 G 1 C) and Tmax (467 G 3 C), with higher values than their pure polymers. As discussed previously, compatibilised and noncompatibilised mixtures presented the same size and domain distributions. With this characteristic the products of the domain degradations diffuse through the PP matrix at the same rate in both blends. Peak II could be related to PEBAX and peaks I and III to PP degradation, with similar areas. This fact suggests that the degradation rate increases in the first degradation stage, i.e. during the process which occurs at the lowest temperature. The deconvolution of the DTG curves for the PP/ PUR and PP/PUR/PP-g-MA blends shows four weightloss steps (Fig. 5). Peaks I and III are associated with the first and second degradation stages in the case of undiluted PUR, while peaks II and IV are related to PP (same area ratio as pure PP), evidencing that the degradation process of one pure component does not interfere with that of the other. The Tmax for compatibilised and non-compatibilised is the same (355 G 2 C) and lower than the value for the first decomposition stage of pure PP. The decomposition of the blends showed the same DTG curve profile as the pure PP with a decrease in the Tmax from the second decomposition stage (peak III in Fig. 5) to a temperature close to the PP decomposition temperature range. This effect is attributed to the influence of the PP matrix decomposition process.
PP / PU I
II
III IV
200
250
300
350
400
450
500
550
600
Temperature / ºC Fig. 5. DTG thermograms for PP/PUR and PP/PUR/PP-g-MA. The symbols in the figure indicate: (I) first, (II) second, (III) third and (IV) fourth degradation stages.
3.3. Kinetics parameters The activation energies (Ea) for the thermal degradation as a function of the weight-loss fraction (a), for the pure PP and the blends, determined by the Ozawa method, are shown in Fig. 6. According to these results, undiluted PP presents average Ea-values in the range of 162e169 kJ molÿ1. Values are in agreement with the range of values reported in the literature [6,18,23,24], slight differences can be attributed to the peculiarities of each method and polymer properties, such as impurities and molecular weight, as well as to the experimental techniques and operating conditions [25]. For all the compatibilised and non-compatibilised binary blends studied it is possible to identify different slopes up to approximately a 0.4 weight-loss fraction (Fig. 6). This may be associated with stages of decomposition at lower temperatures during the process of active centre formation and propagation of the decomposition process. It is interesting to highlight that the activation energy tends to stabilize above a 0.4 weight-loss fraction, for all systems, at 170 G 30 kJ molÿ1. This observation indicates that the decomposition stage of PP is not influenced by the second component in the binary blend, as also observed in the deconvoluted DTG curves, in which the last decomposition peak may represent the decomposition of the PP main chain. An activation energy value below a 0.4 weight-loss fraction has specific behaviour, depending on the compatibilisation efficiency. This is shown for PP/PA6/PP-g-MA by a lower variation in the activation energy in the first decomposition stage, due to the formation of a new copolymer at the interface between the domains and the matrix. On the other hand, the PP/PUR/PP-g-MA and PP/ PEBAX/PP-g-MA systems did not show analogous
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J. Roeder et al. / Polymer Degradation and Stability 90 (2005) 481e487 190
was changed to a co-continuous morphology in the compatibilised blend due to interfacial adhesion of PP and PA6. The addition of compatibiliser to PP/PEBAX and PP/PUR binary blends promotes the interfacial adhesion between the domains and the matrix, without domain reductions because in these specific systems the disperse phase is a copolymer and thermosetting material, respectively. The compatibilisation of PP/PA6 blends leads to an increase in the activation energy of the first decomposition stage, without influencing the decomposition of the main chain of the PP component. Compatibilised and non-compatibilised blends of PP/PEBAX showed analogous decomposition behaviour with temperature values (To) of 405 and 404 C, respectively. Deconvolution of DTG curves showed the same ratio for the stages of decomposition corresponding to each component in the blend. The weight-loss fraction down the 0.4 weight-loss fraction induces a broad range of Ea-values due to the juxtaposition of the deconvoluted peaks. For higher weight-loss fraction values, all systems studied showed activation energies close to the undiluted PP Ea-values.
(a)
180 170 160 150 140
PP PP/ PA6 PP/ PA6 / PP-g-MA
130
(b)
200
Ea (kJ mol-1)
190
180
170
160
Acknowledgements
PP/ PEBAX PP/ PEBAX / PP-g-MA
The authors would like to thank to CNPQ and CAPES for financial support.
(c) 200
References
180`
160
140
120
100 0,0
PP/ PUR PP/ PUR/ PP-g-MA 0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
Weight-Loss Fraction (α) Fig. 6. Plots of activation energies determined by the Ozawa method for: (a) PP, PP/PA6 and PP/PA6/PP-g-MA; (b) PP/PEBAX and PP/ PEBAX/PP-g-MA; (c) PP/PUR and PP/PUR/PP-g-MA systems.
phase stability which could be related to the activation energy.
4. Conclusions The spherical domains of the PA6 component observed in the SEM micrograph of the PP/PA6 blend
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