- Email: [email protected]
Effect of processing conditions on the mechanical and thermal properties of high-impact polypropylene nanocomposites
Materials Science and Engineering A 528 (2011) 6715–6718
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of processing conditions on the mechanical and thermal properties of high-impact polypropylene nanocomposites L.G. Furlan a,∗ , C.I. Ferreira b , C. Dal Castel b , K.S. Santos b , A.C.E. Mello b , S.A. Liberman c , M.A.S. Oviedo c , R.S. Mauler b a
Federal Institute of Rio Grande do Sul, IFRS, Campus Restinga, Estrada João Antônio da Silveira, 351, Porto Alegre 91790-400, Brazil Chemistry Institute, Federal University of Rio Grande do Sul, UFRGS, Av. Bento Gonc¸alves, 9500, Porto Alegre 91501-970, Brazil c Braskem S.A., III Pólo Petroquímico, Via Oeste, Lote 5, Triunfo 95853-000, Brazil b
a r t i c l e
i n f o
Article history: Received 23 July 2010 Received in revised form 29 March 2011 Accepted 17 May 2011 Available online 26 May 2011 Keywords: Nanocomposite Polypropylene Processing, morphology, mechanical properties
a b s t r a c t Polypropylene montmorillonite (PP–MMT) nanocomposites have been prepared by using a co-rotating twin screw extruder. The effects of processing conditions at fixed clay content (5 wt%) on polymer properties were investigated by means of transmission electron microscopy (TEM), flexural modulus, izod impact, dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC). It was noticed that the morphology and the mechanical properties of polypropylene nanocomposites were affected by different screw shear configuration. The results showed that the higher enhancement on mechanical properties was obtained by medium shear intensity profile instead of high configuration. An exceptional increase (maximum of 282%) on impact resistance was observed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Polypropylene/montmorillonite (PP–MMT) is one of the most commonly used nanocomposite(s) to obtain property improvements [1–6] produced in different forms and applications by means of a range of manufacturing processes at a relatively lowcost. There is ample evidence that PP–MMT formed by melting processing is the most preferred method to produce nanocomposites for commercial use. The improvement on final properties usually depends on the degree of exfoliation, delamination, and clay dispersion. Consequently, the quantitative information on nanocomposite morphology is one area of particular importance for advancing the fundamental understanding of the performance relation of these complex systems with morphological aspects [7–13]. So far, the fine control of the interface morphology of the polymer nanocomposites is one of the most critical parameters to impart desired mechanical properties on such materials. An understanding of how the property changes as the particle dimension decreases to the nanoscale level is important to optimize the resultant nanocomposite [14]. Extensive studies on nanocomposites keep the focus on, for example, the clay modification [15], the clay content and different compatibilizer agents in order to increase the compatibility between clay and polymer [16]. On the
∗ Corresponding author. Tel.: +55 51 3308 6294; fax: +55 51 33167304. E-mail address: [email protected] (R.S. Mauler). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.05.044
other hand, the processing conditions are not extensively explored, probably because of the complex experimental factors and the difficult interaction structural control. Furthermore, the adjustment of processing parameters are fundamental to determinate the best performance of polymer/clay system [17–22] necessary to increase the dispersion of clay in the polymer matrix, in order to maximize the interfacial contact between the polymer/clay and macroscopic property enhancement. As can be seen on literature results, the changes on processing conditions [23] greatly affect the nanocomposites dynamicmechanical properties. Thus, the mechanical dispersing action depends on processing parameters such as compounding equipment [24], geometry and type of mixing elements [25] and rheological characteristics of the polymer. Additionally, the optimal dispersion of nanocomposites can be increased by adjusting with parameters as shear stress, mixing time and decreasing mixing temperature [26]. In this way, one of the key roles of nanocomposites morphology is the screw profile design. This can be done, for example, by decreasing the proportion of micron size agglomerates, undesirable for the best mechanical properties enhancements, and a preserved aspect ratio exfoliate montmorillonite platelets. Thus, the aim of this work is to study the different processing conditions (screw configurations with different shear intensity) and their effects on morphological, thermal, and mechanical properties of polypropylene–MMT nanocomposites obtained by melting process.
6716
L.G. Furlan et al. / Materials Science and Engineering A 528 (2011) 6715–6718
Fig. 1. Scheme of shear screw profiles with different shear intensity: T = transport, LH = left hand and KB = kneading blocks. (a) Low profile: 7T-5KB30-3KB60-4KB90-3,5T3KB30-3T-3KB30-3KB60-6KB90-3T; (b) medium profile: 5T-4KB60-1KB30-5T-4KB30-3KB60-3KB90-0,5T-2T-5KB30-2KB30-LH-3KB90-2KB60-LH-4T; (c) high profile: 5T2KB90-4KB60-6KB30-LH-4T-4KB60-4KB90-2T-5KB90-6KB30-LH-4,5T.
2. Experimental 2.1. Materials A commercial grade of polypropylene (Braskem S.A., Brazil) with melt flow index (MFI) 3.5 g/10 min (230 ◦ C/2.16 kg) was used. The commercial montmorillonite (MMT) Cloisite 15A, modified with a quaternary ammonium salt, with cation exchange capacity (CEC) of 125 meq/100 g from Southern Clay Products. 2.2. Nanocomposites preparation The nanocomposites were prepared by melting intercalation in co-rotation twin-screw extruder model Rheomex PTW 16/25 (L/D = 25) connected to Haake rheometer Polylab, and operating at 80 rpm and temperature profile of 170–190 ◦ C. The polypropylene and the Cloisite 15A (5 wt%) were mixed and processed, according to the method described elsewhere [10] using three screw configurations (Fig. 1) called low, medium or high intensity, as per to shear level of screw profile. 2.3. Characterization methods Samples for testing were prepared in a Battenfeld injection moulding or in compressed films. The flexural properties were performed in accordance with ASTM D790 using an Instron 4466. Izod impact tests were carried out at 23 ◦ C using a pendulum-type impact tester (Ceast, Resil impactor), according to ASTM D256. Thermal and crystallization behaviors of nanocomposites were determined by differential scanning calorimetry (DSC) using a 2100 Thermal Analyst Instruments Perkin Elmer, where linear heating and cooling experiments were obtained at 10 ◦ C min−1 under a constant flow of nitrogen and calibrated with indium standard. The transmission electron microscopy (TEM) were carried out using ultra thin cuts obtained from the compressed specimens using a JEOL JEM-120 EXII TEM microscope, operating at an accelerating voltage of 80 kV. The cuts were placed on 300 mesh Cu grids. Ultra thin sections (50 nm thickness) of the specimens, were cooled at −80 ◦ C. The particle analysis (particle number, length, thickness, aspect ratio and particle area) were performed by using
an image editor (PhotoDraw 2000, Microsoft Corporation), and the original gray scale images were converted into black and white image in order to provide the identification of particles by the image analysis program. Agglomerates, intercalated structures and isolated platelets were treated as single particles in this process. The image analysis program (ImageTool 3.0, UTHSCSA) created files with thickness and length of each particle. Dynamical mechanical analysis (DMA), heat deflection temperatures (HDT) and the glass transition temperature (Tg ) were performed in a dynamic-mechanical thermo-analyzer TA Instruments, model Q 800. The storage modulus was collected by heating the sample film from -30 and 130 ◦ C at 3 ◦ C min−1 , using the tension film mode. The heating distortion (HDT) was determined according to ASTM D648 standard with a heating rate of 3 ◦ C min−1 using injection-molded samples in 3-point bending mode. 3. Results and discussion 3.1. Morphological aspects The effect of processing conditions on PP–MMT nanocomposites morphology was studied by TEM analysis (Fig. 2) to evaluate the magnitude of the MMT dispersion. The processing conditions may affect the morphology of the resulting material. TEM micrographs of nanocomposites prepared with different shear intensity showed a uniform dispersion of the MMT in the PP matrix. It can be seen the distinct influence on dispersion degree of the clay in the polymer, which is related to the processing conditions used. One mixing of exfoliated and intercalated structures was present in all profiles. In particular, the nanocomposites obtained in medium shear intensity present larger structures. Using low profile, intercalated and exfoliated structures were less ordered than with medium. Using high shear intensity, more exfoliated structures was formed. Furthermore, it cleared the presence of shorter layers than in medium and low intensity. One important observation in morphological aspect was that the screw configuration could modify the original clay due to high shear force used in the processing mode. Because of the higher stresses that could be imposed on the clay particle, more exfoliated structures were formed.
Fig. 2. TEM micrographs of PP–MMT nanocomposites prepared in a screw using configurations with (a) low, (b) medium and (c) high shear intensity profile.
L.G. Furlan et al. / Materials Science and Engineering A 528 (2011) 6715–6718
6717
Table 1 Image analysis results of PP–MMT nanocomposites (5 wt.%) produced with different screw profiles. Screw profile
Total average particle number
Average length (nm)
Average thickness (nm)
Average aspect ratio
Average particle area (nm)2
Low Medium High
107 100 157
682 564 489
74 65 32
9.2 8.7 15.3
47,792 39,852 15,580
Table 2 Thermal properties of PP nanocomposites obtained with different screw profiles. Tm (◦ C)
Tc (◦ C)
c (%)
Pure PP PP-low PP-medium PP-high
164 163 164 162
113 113 113 117
48 54 62 49
Particle analysis showed similar results on particle dimensions (average particle number, length, area and aspect ratio) when low and medium profile were used (Table 1). On the other hand, different behavior was found by using high shear profile, with increase on the particle population and aspect ratio. It was also observed that the decrease of the nanocomposite particle dimensions was caused by the use of the high shear profile.
Pure PP Low Medium High
4500
Storage Modulus (MPa)
Sample
5000
4000 3500 3000 2500 2000 1500 1000 500 0 -20
0
20
40
60
80
100
120
o
Temperature ( C)
3.2. Thermal properties
3.3. Mechanical properties The nano-addition of clay in the PP matrix also promoted improvements by heat deflection temperature (HDT) by 10 and 15 ◦ C (Table 3) showing a greater mechanical stability of nanocomposites. The nanodispersion of MMT in a PP matrix has been found to promote a higher HDT when low and medium profiles were employed. Also smaller increase on HDT was observed when high shear rate was used. The increase of heat deflection temperature due to montmorillonite dispersion is a very important improvement for PP, because it is difficult to achieve similar HDT enhancements by chemical modification or reinforcement by other fillers [27]. In comparison with neat PP, the glass transition temperature (Tg ) values of nanocomposites decreased when high shear intensity was used. Based on such results, the short and less organized layers Table 3 Mechanical properties of PP–MMT nanocomposites obtained with different screw profiles. Sample
Storage modulus at 25 ◦ C (MPa)
Izod impact (J m−1 )
Pure PP PP-low PP-medium PP-high
378 1343 2101 1403
34 114 130 77
± ± ± ±
5 4 7 7
HDT (◦ C)
Tg (◦ C)
89 103 104 99
11 11 10 7
Fig. 3. Storage modulus dependence of PP-nanocomposites prepared with different screw profiles.
played a role as plasticizer for PP which facilitates thermal motion of PP molecules leading to the decrease of Tg , probably by poor interfacial interaction in absence of compatibilizing agents [28,29]. The enhancement of mechanical properties was also related to the processing conditions. Fig. 3 clearly shows that the storage modulus of the PP–clay nanocomposites was higher than that of unfilled PP, showing that the incorporation of clay into the PP matrix enhances its stiffness and has a good reinforcing effect in all profiles used. An increase on storage modulus at 25 ◦ C and izod impact promoted by shear intensity modification were obtained in PP–MMT nanocomposites. The greatest reinforcement effect was observed with the use of medium profile (increase of 5.6 times). Similar reinforcement was obtained for high and low configurations. As shown in Fig. 4, low and medium shear configurations demonstrated an extraordinary enhancement on izod impact in 300
250
impact gain (%)
To investigate the effect of different processing conditions on the thermal behavior, the results of DSC curves were analyzed (Table 2). Compared with unfilled polypropylene, no significant changes on the crystallization temperature (Tc ) and melting temperature (Tm ) were observed when the high shear intensity was used for PP–MMT nanocomposite preparation. On the other hand, low and medium profiles presented increased of 6 and 14% on the crystallinity degree (c ), respectively. This was probably due to the discrete nucleating effect promoted by clay in function of MMT dispersion, mainly when the medium shear profile was used. Furthermore, the high shear profile did not present changes in the c .
200
150
100
50
0
Low
Medium
High
Fig. 4. Izod impact gain (%) of nanocomposites in relation of neat PP prepared with different screw profiles.
6718
L.G. Furlan et al. / Materials Science and Engineering A 528 (2011) 6715–6718
relation to unfilled PP. Compared with unfilled PP it becomes clear that the dependence on impact increased in the order Medium > Low > High. It is also worth noting that the high gain in impact values above 120% in all profiles reached 282% in medium shear (3.8 times higher than unfilled PP). These results can be attributed to an experimental procedure [10]. This suggests that such increase of the nanocomposite micro-deformation processes were identified as energy dissipating mechanisms, include crazing, cavitation or debonding of minerals with consequent micro-void formation, deformation bands and fibrillation [30–34]. The exact effect of the micromechanical mechanisms on the impact resistance promoted by the experimental procedure [10] are still under investigation. The extent of the mechanical improvement will also depend directly on the particle dimensions and dispersion. This factor is associated with the poor PP–organoclay interaction in absence of compatibilizing agents. In our work, when the better nanocomposites dispersion was achieved (by using high shear configuration), the smaller particles did not generate stress concentrations leading to premature failure of the nanolayers and this caused smaller increase on mechanical measurements. In general, a screw design optimization is a crucial step to obtain nanocomposites because the extrusion process starts with large organo-clay agglomerates and improved the structure dispersion by viscous shear forces [35,36]. Thus, if the PP–organoclay interaction is not effective, the mechanical performance is reduced. 4. Conclusion This work showed the high importance of screw profile design on the nanocomposites properties. The different shear intensity in the preparation of PP–MMT nanocomposites showed differences on morphology and mechanical properties, and demonstrated that the reinforcing effect was dependent on the MMT particles dispersed in PP matrix and the interaction. Exceptional improvements on the izod impact strength (greater than 282%) were obtained. In this research, the aggressive screw shear profile is not the best for the development of optimal nanocomposites properties. Acknowledgements The authors thank Braskem S.A., CNPq FAPERGS/Pronex, FAPERGS and Finep for financial and technical support.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
M. Okamoto, Mater. Sci. Tech. 22 (2006) 756–779. S. Ray, M. Okamoto, Prog. Polym. Sci. 28 (2003) 1539–1641. X. Liu, Q. Wu, Polymer 42 (2001) 10013–10019. ´ A. Leszczynska, J. Njuguna, K. Pielichowski, J. Banerjee, Therm. Acta 453 (2007) 75–96. M. Quintanilla, S. Valdés, L. Valle, R. Miranda, Polym. Bull. 57 (2006) 385–393. Q. Yuan, R. Misra, Polymer 47 (2006) 4421–4433. A. Pozsgay, T. Fráter, L. Százdi, P. Müller, L. Sajó, B. Pukánszky, Eur. Polym. J. 40 (2004) 27–36. L. Paiva, A. Morales, T. Guimarães, Mater. Sci. Eng. A 447 (2007) 261–265. L. Százdi, B. Pukánszky Jr., A. Pozsgay, Eur. Polym. J. 43 (2007) 345–359. S. Liberman, L. Da Silva, T. Pelegrini, R. Barbosa, R. Mauler, Patent Numbers: WO2007009200-A2; BR200503777-A. C.I.W. Calcagno, C.M. Mariani, S.R. Teixeira, R.S. Mauler, Compos. Sci. Technol. 68 (2008) 2193–2200. C. Dal Castel, O. Bianchi, M.A.S. Oviedo, S.A. Liberman, R.S. Mauler, R.V.B. Oliveira, Mater. Sci. Eng. C 29 (2009) 602–606. Y. Dong, D. Bhattacharyya, Polymer 51 (2010) 816–824. D.R. Paul, L.M. Robeson, Polymer 49 (2008) 3187–3204. M. Alexandre, P. Dubois, Mater. Sci. Eng. 28 (2000) 1–63. D. Garcıa-Lopez, O. Picazo, J. Merino, J. Pastor, Eur. Polym. J. 39 (2003) 945–950. S. Pavlidou, C.D. Papaspyrides, Prog. Polym. Sci. 33 (2008) 1119–1198. K. Ratinac, R. Gilbert, L. Ye, A. Jones, S. Ringer, Polymer 47 (2006) 6337–6361. K. Yang, R. Ozisik, Polymer 47 (2006) 2849–2855. X. Meng, Z. Wang, Z. Zhao, X. Du, W. Bi, T. Tang, Polymer 48 (2007) 2508–2519. M. Modesti, A. Lorenzetti, D. Bon, S. Besco, Polymer 46 (2005) 10237–10245. C. Davis, L. Mathias, L. Gilman, D. Schiraldi, J. Shields, P. Trulove, T. Sutto, H. Delong, J. Polym. Sci.: Part B: Polym. Phys. 40 (2002) 2661–2666. M. Modesti, A. Lorenzetti, D. Bon, S. Besco, Polym. Degr. Stab. 91 (2006) 672–680. S. Ryu, Y. Chang, Polym. Bull. 55 (2005) 385–392. A. Vermogen, K. Masenelli-Varlot, R. Segue, J. Duchet-Rumeau, S. Boucard, P. Prele, Macromolecules 38 (2005) 9661–9669. W. Lertwimolnun, B. Vergnes, Polymer 46 (2005) 3462–3471. E. Manias, A. Touny, L. Wu, K. Strawhecker, B. Lu, T.C. Chung, Chem. Mater. 13 (2001) 3516–3523. Z. Liang, J. Yin, H. Xu, Polymer 44 (2003) 1391–1399. J Jordan, K. Jacob, R. Tannenbaum, M. Sharaf, I. Jasiu, Mater. Sci. Eng. A 393 (2005) 1–11. M. Tanniru, Q. Yuan, R.D.K. Misra, Polymer 47 (2006) 2133–2146. Q. Zhang, Z. Yua, X. Xie, Y. Maia, Polymer 45 (2004) 5985–5994. Q. Yuan, R.D.K. Misra, Polymer 47 (2006) 4421–4433. C. Chan, J. Wu, J. Li, Y. Cheung, Polymer 43 (2002) 2981–2992. K. Santos, S. Liberman, M. Oviedo, R. Mauler, Compos. Part A: Appl. Sci. Man. 40 (2009) 1199–1209. H. Dennis, D. Hunter, D. Chang, S. Kim, J. White, W. Cho, Polymer 42 (2001) 9513–9522. H. Huang, Y. Huang, C. Wang, Mater. Sci. Lett. 22 (2003) 1547–1549.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of processing conditions on the mechanical and thermal properties of high-impact polypropylene nanocomposites L.G. Furlan a,∗ , C.I. Ferreira b , C. Dal Castel b , K.S. Santos b , A.C.E. Mello b , S.A. Liberman c , M.A.S. Oviedo c , R.S. Mauler b a
Federal Institute of Rio Grande do Sul, IFRS, Campus Restinga, Estrada João Antônio da Silveira, 351, Porto Alegre 91790-400, Brazil Chemistry Institute, Federal University of Rio Grande do Sul, UFRGS, Av. Bento Gonc¸alves, 9500, Porto Alegre 91501-970, Brazil c Braskem S.A., III Pólo Petroquímico, Via Oeste, Lote 5, Triunfo 95853-000, Brazil b
a r t i c l e
i n f o
Article history: Received 23 July 2010 Received in revised form 29 March 2011 Accepted 17 May 2011 Available online 26 May 2011 Keywords: Nanocomposite Polypropylene Processing, morphology, mechanical properties
a b s t r a c t Polypropylene montmorillonite (PP–MMT) nanocomposites have been prepared by using a co-rotating twin screw extruder. The effects of processing conditions at fixed clay content (5 wt%) on polymer properties were investigated by means of transmission electron microscopy (TEM), flexural modulus, izod impact, dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC). It was noticed that the morphology and the mechanical properties of polypropylene nanocomposites were affected by different screw shear configuration. The results showed that the higher enhancement on mechanical properties was obtained by medium shear intensity profile instead of high configuration. An exceptional increase (maximum of 282%) on impact resistance was observed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Polypropylene/montmorillonite (PP–MMT) is one of the most commonly used nanocomposite(s) to obtain property improvements [1–6] produced in different forms and applications by means of a range of manufacturing processes at a relatively lowcost. There is ample evidence that PP–MMT formed by melting processing is the most preferred method to produce nanocomposites for commercial use. The improvement on final properties usually depends on the degree of exfoliation, delamination, and clay dispersion. Consequently, the quantitative information on nanocomposite morphology is one area of particular importance for advancing the fundamental understanding of the performance relation of these complex systems with morphological aspects [7–13]. So far, the fine control of the interface morphology of the polymer nanocomposites is one of the most critical parameters to impart desired mechanical properties on such materials. An understanding of how the property changes as the particle dimension decreases to the nanoscale level is important to optimize the resultant nanocomposite [14]. Extensive studies on nanocomposites keep the focus on, for example, the clay modification [15], the clay content and different compatibilizer agents in order to increase the compatibility between clay and polymer [16]. On the
∗ Corresponding author. Tel.: +55 51 3308 6294; fax: +55 51 33167304. E-mail address: [email protected] (R.S. Mauler). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.05.044
other hand, the processing conditions are not extensively explored, probably because of the complex experimental factors and the difficult interaction structural control. Furthermore, the adjustment of processing parameters are fundamental to determinate the best performance of polymer/clay system [17–22] necessary to increase the dispersion of clay in the polymer matrix, in order to maximize the interfacial contact between the polymer/clay and macroscopic property enhancement. As can be seen on literature results, the changes on processing conditions [23] greatly affect the nanocomposites dynamicmechanical properties. Thus, the mechanical dispersing action depends on processing parameters such as compounding equipment [24], geometry and type of mixing elements [25] and rheological characteristics of the polymer. Additionally, the optimal dispersion of nanocomposites can be increased by adjusting with parameters as shear stress, mixing time and decreasing mixing temperature [26]. In this way, one of the key roles of nanocomposites morphology is the screw profile design. This can be done, for example, by decreasing the proportion of micron size agglomerates, undesirable for the best mechanical properties enhancements, and a preserved aspect ratio exfoliate montmorillonite platelets. Thus, the aim of this work is to study the different processing conditions (screw configurations with different shear intensity) and their effects on morphological, thermal, and mechanical properties of polypropylene–MMT nanocomposites obtained by melting process.
6716
L.G. Furlan et al. / Materials Science and Engineering A 528 (2011) 6715–6718
Fig. 1. Scheme of shear screw profiles with different shear intensity: T = transport, LH = left hand and KB = kneading blocks. (a) Low profile: 7T-5KB30-3KB60-4KB90-3,5T3KB30-3T-3KB30-3KB60-6KB90-3T; (b) medium profile: 5T-4KB60-1KB30-5T-4KB30-3KB60-3KB90-0,5T-2T-5KB30-2KB30-LH-3KB90-2KB60-LH-4T; (c) high profile: 5T2KB90-4KB60-6KB30-LH-4T-4KB60-4KB90-2T-5KB90-6KB30-LH-4,5T.
2. Experimental 2.1. Materials A commercial grade of polypropylene (Braskem S.A., Brazil) with melt flow index (MFI) 3.5 g/10 min (230 ◦ C/2.16 kg) was used. The commercial montmorillonite (MMT) Cloisite 15A, modified with a quaternary ammonium salt, with cation exchange capacity (CEC) of 125 meq/100 g from Southern Clay Products. 2.2. Nanocomposites preparation The nanocomposites were prepared by melting intercalation in co-rotation twin-screw extruder model Rheomex PTW 16/25 (L/D = 25) connected to Haake rheometer Polylab, and operating at 80 rpm and temperature profile of 170–190 ◦ C. The polypropylene and the Cloisite 15A (5 wt%) were mixed and processed, according to the method described elsewhere [10] using three screw configurations (Fig. 1) called low, medium or high intensity, as per to shear level of screw profile. 2.3. Characterization methods Samples for testing were prepared in a Battenfeld injection moulding or in compressed films. The flexural properties were performed in accordance with ASTM D790 using an Instron 4466. Izod impact tests were carried out at 23 ◦ C using a pendulum-type impact tester (Ceast, Resil impactor), according to ASTM D256. Thermal and crystallization behaviors of nanocomposites were determined by differential scanning calorimetry (DSC) using a 2100 Thermal Analyst Instruments Perkin Elmer, where linear heating and cooling experiments were obtained at 10 ◦ C min−1 under a constant flow of nitrogen and calibrated with indium standard. The transmission electron microscopy (TEM) were carried out using ultra thin cuts obtained from the compressed specimens using a JEOL JEM-120 EXII TEM microscope, operating at an accelerating voltage of 80 kV. The cuts were placed on 300 mesh Cu grids. Ultra thin sections (50 nm thickness) of the specimens, were cooled at −80 ◦ C. The particle analysis (particle number, length, thickness, aspect ratio and particle area) were performed by using
an image editor (PhotoDraw 2000, Microsoft Corporation), and the original gray scale images were converted into black and white image in order to provide the identification of particles by the image analysis program. Agglomerates, intercalated structures and isolated platelets were treated as single particles in this process. The image analysis program (ImageTool 3.0, UTHSCSA) created files with thickness and length of each particle. Dynamical mechanical analysis (DMA), heat deflection temperatures (HDT) and the glass transition temperature (Tg ) were performed in a dynamic-mechanical thermo-analyzer TA Instruments, model Q 800. The storage modulus was collected by heating the sample film from -30 and 130 ◦ C at 3 ◦ C min−1 , using the tension film mode. The heating distortion (HDT) was determined according to ASTM D648 standard with a heating rate of 3 ◦ C min−1 using injection-molded samples in 3-point bending mode. 3. Results and discussion 3.1. Morphological aspects The effect of processing conditions on PP–MMT nanocomposites morphology was studied by TEM analysis (Fig. 2) to evaluate the magnitude of the MMT dispersion. The processing conditions may affect the morphology of the resulting material. TEM micrographs of nanocomposites prepared with different shear intensity showed a uniform dispersion of the MMT in the PP matrix. It can be seen the distinct influence on dispersion degree of the clay in the polymer, which is related to the processing conditions used. One mixing of exfoliated and intercalated structures was present in all profiles. In particular, the nanocomposites obtained in medium shear intensity present larger structures. Using low profile, intercalated and exfoliated structures were less ordered than with medium. Using high shear intensity, more exfoliated structures was formed. Furthermore, it cleared the presence of shorter layers than in medium and low intensity. One important observation in morphological aspect was that the screw configuration could modify the original clay due to high shear force used in the processing mode. Because of the higher stresses that could be imposed on the clay particle, more exfoliated structures were formed.
Fig. 2. TEM micrographs of PP–MMT nanocomposites prepared in a screw using configurations with (a) low, (b) medium and (c) high shear intensity profile.
L.G. Furlan et al. / Materials Science and Engineering A 528 (2011) 6715–6718
6717
Table 1 Image analysis results of PP–MMT nanocomposites (5 wt.%) produced with different screw profiles. Screw profile
Total average particle number
Average length (nm)
Average thickness (nm)
Average aspect ratio
Average particle area (nm)2
Low Medium High
107 100 157
682 564 489
74 65 32
9.2 8.7 15.3
47,792 39,852 15,580
Table 2 Thermal properties of PP nanocomposites obtained with different screw profiles. Tm (◦ C)
Tc (◦ C)
c (%)
Pure PP PP-low PP-medium PP-high
164 163 164 162
113 113 113 117
48 54 62 49
Particle analysis showed similar results on particle dimensions (average particle number, length, area and aspect ratio) when low and medium profile were used (Table 1). On the other hand, different behavior was found by using high shear profile, with increase on the particle population and aspect ratio. It was also observed that the decrease of the nanocomposite particle dimensions was caused by the use of the high shear profile.
Pure PP Low Medium High
4500
Storage Modulus (MPa)
Sample
5000
4000 3500 3000 2500 2000 1500 1000 500 0 -20
0
20
40
60
80
100
120
o
Temperature ( C)
3.2. Thermal properties
3.3. Mechanical properties The nano-addition of clay in the PP matrix also promoted improvements by heat deflection temperature (HDT) by 10 and 15 ◦ C (Table 3) showing a greater mechanical stability of nanocomposites. The nanodispersion of MMT in a PP matrix has been found to promote a higher HDT when low and medium profiles were employed. Also smaller increase on HDT was observed when high shear rate was used. The increase of heat deflection temperature due to montmorillonite dispersion is a very important improvement for PP, because it is difficult to achieve similar HDT enhancements by chemical modification or reinforcement by other fillers [27]. In comparison with neat PP, the glass transition temperature (Tg ) values of nanocomposites decreased when high shear intensity was used. Based on such results, the short and less organized layers Table 3 Mechanical properties of PP–MMT nanocomposites obtained with different screw profiles. Sample
Storage modulus at 25 ◦ C (MPa)
Izod impact (J m−1 )
Pure PP PP-low PP-medium PP-high
378 1343 2101 1403
34 114 130 77
± ± ± ±
5 4 7 7
HDT (◦ C)
Tg (◦ C)
89 103 104 99
11 11 10 7
Fig. 3. Storage modulus dependence of PP-nanocomposites prepared with different screw profiles.
played a role as plasticizer for PP which facilitates thermal motion of PP molecules leading to the decrease of Tg , probably by poor interfacial interaction in absence of compatibilizing agents [28,29]. The enhancement of mechanical properties was also related to the processing conditions. Fig. 3 clearly shows that the storage modulus of the PP–clay nanocomposites was higher than that of unfilled PP, showing that the incorporation of clay into the PP matrix enhances its stiffness and has a good reinforcing effect in all profiles used. An increase on storage modulus at 25 ◦ C and izod impact promoted by shear intensity modification were obtained in PP–MMT nanocomposites. The greatest reinforcement effect was observed with the use of medium profile (increase of 5.6 times). Similar reinforcement was obtained for high and low configurations. As shown in Fig. 4, low and medium shear configurations demonstrated an extraordinary enhancement on izod impact in 300
250
impact gain (%)
To investigate the effect of different processing conditions on the thermal behavior, the results of DSC curves were analyzed (Table 2). Compared with unfilled polypropylene, no significant changes on the crystallization temperature (Tc ) and melting temperature (Tm ) were observed when the high shear intensity was used for PP–MMT nanocomposite preparation. On the other hand, low and medium profiles presented increased of 6 and 14% on the crystallinity degree (c ), respectively. This was probably due to the discrete nucleating effect promoted by clay in function of MMT dispersion, mainly when the medium shear profile was used. Furthermore, the high shear profile did not present changes in the c .
200
150
100
50
0
Low
Medium
High
Fig. 4. Izod impact gain (%) of nanocomposites in relation of neat PP prepared with different screw profiles.
6718
L.G. Furlan et al. / Materials Science and Engineering A 528 (2011) 6715–6718
relation to unfilled PP. Compared with unfilled PP it becomes clear that the dependence on impact increased in the order Medium > Low > High. It is also worth noting that the high gain in impact values above 120% in all profiles reached 282% in medium shear (3.8 times higher than unfilled PP). These results can be attributed to an experimental procedure [10]. This suggests that such increase of the nanocomposite micro-deformation processes were identified as energy dissipating mechanisms, include crazing, cavitation or debonding of minerals with consequent micro-void formation, deformation bands and fibrillation [30–34]. The exact effect of the micromechanical mechanisms on the impact resistance promoted by the experimental procedure [10] are still under investigation. The extent of the mechanical improvement will also depend directly on the particle dimensions and dispersion. This factor is associated with the poor PP–organoclay interaction in absence of compatibilizing agents. In our work, when the better nanocomposites dispersion was achieved (by using high shear configuration), the smaller particles did not generate stress concentrations leading to premature failure of the nanolayers and this caused smaller increase on mechanical measurements. In general, a screw design optimization is a crucial step to obtain nanocomposites because the extrusion process starts with large organo-clay agglomerates and improved the structure dispersion by viscous shear forces [35,36]. Thus, if the PP–organoclay interaction is not effective, the mechanical performance is reduced. 4. Conclusion This work showed the high importance of screw profile design on the nanocomposites properties. The different shear intensity in the preparation of PP–MMT nanocomposites showed differences on morphology and mechanical properties, and demonstrated that the reinforcing effect was dependent on the MMT particles dispersed in PP matrix and the interaction. Exceptional improvements on the izod impact strength (greater than 282%) were obtained. In this research, the aggressive screw shear profile is not the best for the development of optimal nanocomposites properties. Acknowledgements The authors thank Braskem S.A., CNPq FAPERGS/Pronex, FAPERGS and Finep for financial and technical support.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
M. Okamoto, Mater. Sci. Tech. 22 (2006) 756–779. S. Ray, M. Okamoto, Prog. Polym. Sci. 28 (2003) 1539–1641. X. Liu, Q. Wu, Polymer 42 (2001) 10013–10019. ´ A. Leszczynska, J. Njuguna, K. Pielichowski, J. Banerjee, Therm. Acta 453 (2007) 75–96. M. Quintanilla, S. Valdés, L. Valle, R. Miranda, Polym. Bull. 57 (2006) 385–393. Q. Yuan, R. Misra, Polymer 47 (2006) 4421–4433. A. Pozsgay, T. Fráter, L. Százdi, P. Müller, L. Sajó, B. Pukánszky, Eur. Polym. J. 40 (2004) 27–36. L. Paiva, A. Morales, T. Guimarães, Mater. Sci. Eng. A 447 (2007) 261–265. L. Százdi, B. Pukánszky Jr., A. Pozsgay, Eur. Polym. J. 43 (2007) 345–359. S. Liberman, L. Da Silva, T. Pelegrini, R. Barbosa, R. Mauler, Patent Numbers: WO2007009200-A2; BR200503777-A. C.I.W. Calcagno, C.M. Mariani, S.R. Teixeira, R.S. Mauler, Compos. Sci. Technol. 68 (2008) 2193–2200. C. Dal Castel, O. Bianchi, M.A.S. Oviedo, S.A. Liberman, R.S. Mauler, R.V.B. Oliveira, Mater. Sci. Eng. C 29 (2009) 602–606. Y. Dong, D. Bhattacharyya, Polymer 51 (2010) 816–824. D.R. Paul, L.M. Robeson, Polymer 49 (2008) 3187–3204. M. Alexandre, P. Dubois, Mater. Sci. Eng. 28 (2000) 1–63. D. Garcıa-Lopez, O. Picazo, J. Merino, J. Pastor, Eur. Polym. J. 39 (2003) 945–950. S. Pavlidou, C.D. Papaspyrides, Prog. Polym. Sci. 33 (2008) 1119–1198. K. Ratinac, R. Gilbert, L. Ye, A. Jones, S. Ringer, Polymer 47 (2006) 6337–6361. K. Yang, R. Ozisik, Polymer 47 (2006) 2849–2855. X. Meng, Z. Wang, Z. Zhao, X. Du, W. Bi, T. Tang, Polymer 48 (2007) 2508–2519. M. Modesti, A. Lorenzetti, D. Bon, S. Besco, Polymer 46 (2005) 10237–10245. C. Davis, L. Mathias, L. Gilman, D. Schiraldi, J. Shields, P. Trulove, T. Sutto, H. Delong, J. Polym. Sci.: Part B: Polym. Phys. 40 (2002) 2661–2666. M. Modesti, A. Lorenzetti, D. Bon, S. Besco, Polym. Degr. Stab. 91 (2006) 672–680. S. Ryu, Y. Chang, Polym. Bull. 55 (2005) 385–392. A. Vermogen, K. Masenelli-Varlot, R. Segue, J. Duchet-Rumeau, S. Boucard, P. Prele, Macromolecules 38 (2005) 9661–9669. W. Lertwimolnun, B. Vergnes, Polymer 46 (2005) 3462–3471. E. Manias, A. Touny, L. Wu, K. Strawhecker, B. Lu, T.C. Chung, Chem. Mater. 13 (2001) 3516–3523. Z. Liang, J. Yin, H. Xu, Polymer 44 (2003) 1391–1399. J Jordan, K. Jacob, R. Tannenbaum, M. Sharaf, I. Jasiu, Mater. Sci. Eng. A 393 (2005) 1–11. M. Tanniru, Q. Yuan, R.D.K. Misra, Polymer 47 (2006) 2133–2146. Q. Zhang, Z. Yua, X. Xie, Y. Maia, Polymer 45 (2004) 5985–5994. Q. Yuan, R.D.K. Misra, Polymer 47 (2006) 4421–4433. C. Chan, J. Wu, J. Li, Y. Cheung, Polymer 43 (2002) 2981–2992. K. Santos, S. Liberman, M. Oviedo, R. Mauler, Compos. Part A: Appl. Sci. Man. 40 (2009) 1199–1209. H. Dennis, D. Hunter, D. Chang, S. Kim, J. White, W. Cho, Polymer 42 (2001) 9513–9522. H. Huang, Y. Huang, C. Wang, Mater. Sci. Lett. 22 (2003) 1547–1549.