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Simultaneous enhancements in toughness and electrical conductivity of polypropylene/carbon nanotube Nanocomposites by incorporation of electrically inert calcium carbonate nanoparticles Xing-Hua Li, Yadong He, Xiaofeng Li, Fei An, Dongzhi Yang, and Zhong-Zhen Yu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00446 • Publication Date (Web): 22 Feb 2017 Downloaded from http://pubs.acs.org on February 25, 2017

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Industrial & Engineering Chemistry Research

Simultaneous enhancements in toughness and electrical conductivity of polypropylene/carbon nanotube nanocomposites by incorporation of electrically inert calcium carbonate nanoparticles Xing-Hua Lia, Yadong Hea, Xiaofeng Lia*, Fei Ana, Dongzhi Yanga, Zhong-Zhen Yua,b* a

Beijing Key Laboratory of Advanced Functional Polymer Composites, Beijing University of

Chemical Technology, Beijing 100029, China b

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing

University of Chemical Technology, Beijing 100029, China Corresponding Authors: E-mail: [email protected] (X. Li); [email protected] (Z.-Z. Yu) ABSTRACT: Although the presence of carbon nanotubes (CNTs) makes polypropylene (PP) electrically conductive, the resulting PP/CNT binary nanocomposites become brittle limiting their practical applications. To toughen PP/CNT nanocomposites, calcium carbonate (CaCO3) inorganic nanoparticles are melt-compounded with PP and CNTs components to fabricate electrically conductive and tough PP/CNT/CaCO3 ternary nanocomposites. The PP/CNT nanocomposites have a relatively large percolation threshold of 6.2 wt%, which reduces to 5.6 wt% by the addition of 30 wt% of pristine CaCO3, and further to 3.6 wt% in the presence of 30 wt% of modified CaCO3. Simultaneously, the electrically conductive PP/CNT nanocomposites are efficiently toughened by the CaCO3 nanoparticles, and the notched impact strength increases from 16.0 to 33.1 KJ/m2 by compounding 30 wt% of modified CaCO3 with PP/9 wt% CNT components. The dual roles of CaCO3 in volume-exclusion and 1 ACS Paragon Plus Environment


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toughening are well demonstrated. KEYWORDS: nanocomposites; electrical conductivity; toughness; carbon nanotubes; calcium carbonate nanoparticles 1. Introduction Carbon-based conducting fillers, including carbon fibers, graphite flakes, carbon black (CB) nanoparticles, graphene sheets, and carbon nanotubes (CNTs), have been widely used to make polymers electrically conductive.1-5 Among them, one-dimensional CNTs have their own advantages due to their high aspect ratio, exceptional electrical conductivity,6,7 excellent electromagnetic interference shielding and electrostatic discharge efficiencies,8,9 and satisfactory mechanical properties.10,11 However, the addition of CNTs often causes embrittlement of thermoplastic polymers, which seriously limits the practical application of the electrically conductive polymer nanocomposites.12 It is thus imperative to toughen the electrically conductive polymer/CNT nanocomposites. In addition to traditional toughening agents of elastomers13,14 and organic fillers,14,15 inorganic particles like calcium carbonate (CaCO3) nanoparticles are also effective in toughening polyolefin thermoplastics.16,17 For example, Chan et al.16 reported the incorporation of CaCO3 nanoparticles significantly increased the notched impact strength of polypropylene (PP) and confirmed that the nanoparticles acted as stress concentration sites to promote cavitation at the particle-PP boundaries during loading, release plastic constraints and trigger massive plastic deformation of the PP matrix, thus leading to improved toughness. Sun et al.18 used silane-treated SiO2 nanoparticles as the toughening agent of polyvinyl chloride 2 ACS Paragon Plus Environment

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(PVC) and the nanocomposite exhibited an improved impact strength of 9.9 KJ/m2 with 4 wt% SiO2, 1.4 times that of neat PVC. In attention, the addition of 1 wt% layered montmorillonite increased the ductility of PVC by 19%.19 Since the damages in toughness and ductility of polymers become serious with increasing the loading of conducting fillers, many efforts have been made to promote the formation of interconnecting conductance networks at smaller filler loadings by pretreating the surface of fillers to improve their dispersion,20-24 by selective localization of fillers according to the volume-exclusion principle,25-27 and by combination of conducting fillers with different shapes and aspect ratios.28,29 Vankayala et al.,20 pretreated multiwalled CNTs with polyacrylic acid followed by coating with in situ polymerized polyaniline (PANI). 1 wt% of the PANI-coated CNTs made nylon 6 electrically conductive with an electrical conductivity of 7.3×10-3 S/m. Liu et al.30 compounded clay with epoxy/CNT components and the resulting ternary nanocomposites had a reduced percolation threshold from 0.05 to 0.01 wt% due to the improved dispersion of CNTs. The incorporation of 20 wt% ethylene-propylene-diene rubber (EPDM) not only decreased the percolation threshold of CB nanoparticles in PP matrix due to the volume-exclusion effect of EPDM particles,25 but also significantly improved the notched impact strength of the brittle PP/CB nanocomposites.28 For PP/EPDM/CB ternary nanocomposites, Yang et al.25 observed that the CB nanoparticles were accumulated around EPDM particles to form a conducting network, leading to the decrease of percolation threshold and the increases in both electrical conductivity and toughness. Hu et al.2 simultaneously improved the electrical conductivity and toughness of nylon 6 by 3 ACS Paragon Plus Environment


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melt-compounding with maleic anhydride-grafted polyethylene-octene rubber (POE-g-MA) and CB components. Compared to the binary nylon 6 nanocomposite with 15 wt% CB, the ternary nanocomposite with 15 wt% CB and 30 wt% POE-g-MA exhibited a higher electrical conductivity and a super-toughness. Zhi et al.31 introduced numerous pores into electrically conductive polycarbonate (PC)/CNT nanocomposites by supercritical CO2 fluid foaming technique and resulted in comparable or even higher electrical conductivities and excellent electromagnetic interference shielding performances. Simultaneously, the introduced numerous pores also greatly improved the notched impact strength of the PC/CNT nanocomposites. Although the addition of rubbers and the introduction of pores facilitate the formation of interconnecting networks of conducting fillers in polymer matrices, both rubbers and pores seriously reduce the modulus of the conductive polymer/filler nanocomposites due to their own low moduli. To avoid the shortcoming of rubber toughening, in the present work, CaCO3 inorganic nanoparticles are used to toughen brittle PP/CNT nanocomposites. It is expected that the presence of numerous CaCO3 nanoparticles not only facilitates the formation of conducting network of CNTs in PP matrix but also toughens the PP/CNT nanocomposites. The binary and ternary nanocomposites are fabricated by melt-compounding and investigated in terms of dispersions of CNTs and CaCO3, microstructure, electrical conductivity, notched impact strength and tensile properties. The dual roles of CaCO3 nanoparticles in volume-exclusion and toughening are well investigated.

2. Experimental 4 ACS Paragon Plus Environment

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2.1 Materials PP (K8303) with a melt flow index of 2.3 g/10 min was provided by Yanshan Petroleum Chemical Co. Ltd. (China). CaCO3 nanoparticles (CC, NPCC-201) with a mean size of 50-70 nm and their aluminate surface modifier was supplied by Xintai Nanomaterials (China). The surface modifier was in powder form (Figure S1). Multiwalled CNTs with a purity of 97% were purchased from Chengdu Organic Chemicals (China) and their outer diameter and length are 50-70 nm and 2-12 µm, respectively. 2.2 Preparation of PP based nanocomposites After drying at 90 oC for 12 h in an air-circulating oven, CaCO3 and CNTs were melt-compounded with PP on a Haake Rheomix 600 mixer at 180 oC and 80 rpm for 15 min. To ensure the uniform dispersion of CaCO3 nanoparticles, their surfaces were pretreated with the surface modifier using a Fuxin GRH-10 high-speed mixer (China). By using a Haake MiniJet (Germany), the compounds were injection-molded into specimens for tensile and notched Izod impact strength tests. The specimens for electrical conductivity measurements were prepared by compression-molding with a KT-0906 vacuum hot-press machine (China) at 200 oC and 10 MPa. The ternary nanocomposites were designated as PP/xCNT/yCCz, where the letters of x, y and z represent the mass fractions of CNTs, CaCO3 and the surface modifier, respectively. 2.3 Characterization CaCO3 nanoparticles, CNTs, and fracture surfaces of PP nanocomposites were observed by a Hitachi S4700 field-emission scanning electron microscope (SEM). The aluminate surface 5 ACS Paragon Plus Environment


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modifier was characterized using a Bruker Quantax SEM with energy dispersive X-ray spectrometer (EDX) and a ThermoFisher Escalab 250 X-ray photoelectron spectroscope (XPS) (Figures S1 and S2), indicating that it contains 90.6 wt% C, 6.6 wt% O, and 2.6 wt% Al. Volume conductivities of PP nanocomposites higher than 10-4 S/m were measured with a RTS-8 four-probe resistivity meter (China), whereas the conductivities lower than 10-4 S/m were measured by a ZC-90G resistivity meter from Shanghai Taiou Electronics (China). The measurements of notched Izod impact strength (KJ/m2) were conducted on a XC-22 impact tester (China). Rectangular bars of 80×10×4 mm3 were machined with a V-notch (2 mm-depth) before the impact tests. Tensile properties of neat PP and its binary and ternary nanocomposites were measured on an Instron 1185 testing machine (USA) at a crosshead speed of 50 mm/min. The values of melt flow index (MFI) of PP and its nanocomposites were measured on a XNR-400B melt flow indexer (China). All mechanical tests were performed at 25 °C and at least five specimens were tested for each composition.

3. Results and discussion 3.1 Microstructure, electrical and mechanical properties of PP/CNT nanocomposites Figure 1 shows plots of electrical conductivity and notched impact strength as a function of CNT content for PP/CNT binary nanocomposites. It is clear that, with the increase of CNT content, there is a transition from electrically insulating to conducting with a percolation threshold of 6.2 wt%, at which CNTs start to interconnect with each other to form an electrically conducting network. As the electrical transition of the nanocomposites is significant in the range of 6-11 wt% CNTs, 9 wt% of CNTs are chosen to investigate the 6 ACS Paragon Plus Environment

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positive contributions of CaCO3 nanoparticles on toughening of the PP/CNT nanocomposites and on prompting the formation of an interconnecting network of CNTs. It is also seen that the presence of CNTs increases the tensile strength and Young’s modulus of neat PP due to the reinforcement of the fibrous and rigid CNTs (Figure S3 and Table S1). However, the notched impact strength of the PP/CNT nanocomposites decreases with increasing the content of CNTs (Figure 1), indicating the embrittlement of CNTs.12

Figure 1. Plots of notched Izod impact strength and electrical conductivity versus CNT content for PP/CNT binary nanocomposites. Figure 2 shows SEM images of fracture surfaces of neat PP and its nanocomposite with 9 wt% CNTs. Neat PP presents a tough impact-fracture feature with plastic deformation at the notch root area occurred during the impact testing (Figure 2a and b), which is consistent with its relatively large impact strength of 29.2 KJ/m2. However, the PP/9CNT binary nanocomposite exhibits a smooth fracture surface and no plastic deformation is observed even 7 ACS Paragon Plus Environment


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at the notch root region (Figure 2c). The freeze-fracture surface of the binary nanocomposite shows the aggregation of CNTs (Figure 2d), which is in accordance with the relatively large percolation threshold (6.2 wt%) of the nanocomposites. The agglomeration of CNTs and no plastic deformation of the PP matrix are responsible for the low impact strength of the PP/9CNT nanocomposite (16.0 KJ/m2).

Figure 2. SEM micrographs of (a,b) impact-fractured surface at the notch root of neat PP at different magnifications, and (c) impact-fractured surface at the notch root and (d) freeze-fractured surface of the PP/9CNT binary nanocomposite. 3.2 Influences of CaCO3 and surface modifier contents on electrical conductivity and mechanical properties of ternary nanocomposites When CaCO3 nanoparticles are used to toughen PP/CNT nanocomposites, their dispersion in 8 ACS Paragon Plus Environment

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the PP matrix is improved by surface modification of CaCO3 nanoparticles with 5 wt% of the surface modifier (based on the mass of CaCO3) before melt-compounding with PP (Figure S4). At a given CaCO3 content of 30 wt%, the increase in the surface modifier content benefits the increase in notched impact strength of the nanocomposites (Figure S5). For example, with 7 wt% of the surface modifier, the notched impact strength of PP/30CC binary nanocomposite shows a high impact strength of 55.7 KJ/m2, much higher than that of the unmodified counterpart (38.4 KJ/m2).11,23 It is noted that when the surface modifier is more than 5 wt%, the increase in notched impact strength becomes less significant. Therefore, 5 wt% of the modifier is chosen to modify CaCO3 for the preparation of PP nanocomposites.32 Figure 3 compares the toughening efficiency of the unmodified and modified CaCO3 nanoparticles with 5 wt% of the surface modifier. Compared to unmodified CaCO3, the presence of modified CaCO3 greatly increases the toughness of PP and the notched impact strength reaches 54.3 KJ/m2 with 30 wt% of modified CaCO3. The difference in notched impact strength can be well explained by observing the impact-fractured surfaces of PP/30CC and PP/30CC5 nanocomposites (Figure 4). As shown in Figure S4, the dispersion quality of CaCO3 nanoparticles is greatly improved by 5 wt% of the surface modifier. As a result, the PP/30CC nanocomposite presents a tough feature with certain plastic deformation zone (Figure 4a), while its modified counterpart exhibits an extensive plastic deformation throughout the fracture surface (Figure 4b). The addition of CaCO3 nanoparticles also influences the tensile strength and Young’s modulus of PP (Table S2). The presence of CaCO3 in PP does not apparently affect the yield strength of PP, whereas the Young’s 9 ACS Paragon Plus Environment


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modulus of PP significantly increases with increasing the contents of modified and unmodified CaCO3 due to the higher modulus of CaCO3 than that of PP matrix.

Figure 3. Plots of notched Izod impact strength versus CaCO3 content for PP/CC and PP/CC5 binary nanocomposites.

Figure 4. SEM micrographs of impact-fracture surfaces at the notch roots of (a) PP/30CC and (b) PP/30CC5 nanocomposites. 10 ACS Paragon Plus Environment

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3.3 Effects of CaCO3 content on electrical conductivity and notched impact strength of PP/CNT/CC ternary nanocomposites To prepare electrically conductive and tough nanocomposites, 30 wt% CaCO3 nanoparticles are melt-compounded with PP and CNT components. Figure 5a shows plots of electrical conductivity as a function of CNT content for PP/CNT/CC ternary nanocomposites at a fixed CaCO3 content (30 wt%). Interestingly, although CaCO3 is electrically insulating, its addition apparently increases the conductivity of the PP/CNT nanocomposites with a smaller percolation threshold of 5.6 wt%, which is attributed to the volume-exclusion effect of CaCO3 nanoparticles.33,34 That is, the 30 wt% CaCO3 solid nanoparticles occupy a considerable matrix space and therefore the CNTs have to be dispersed and distributed within a reduced PP matrix space to form a denser interconnecting network and thus a higher electrical conductivity. Compared to the unmodified CaCO3, the modified counterpart is more efficient in improving the electrical conductivity of the nanocomposites with an even smaller percolation threshold of 3.6 wt%. For example, at a given CNT content (9 wt%), the electrical conductivity of PP/9CNT nanocomposite is 1.94×10-7 S/m, while they are 1.38×10-2 and 8.00×10-2 S/m for PP/9CNT/30CC and PP/9CNT/30CC5 nanocomposites, respectively. To further study the effect of surface modification, Figure S6 shows the plots of electrical conductivity versus effective content of CNTs for the ternary nanocomposites. The effective content of CNTs is defined as the volume fraction of CNTs (VCNT) to the total volume fractions of CNTs and PP (VCNT + VPP).26 Interestingly, the electrical conductivity curves of PP/CNT and PP/CNT/30CC nanocomposites (Figure 5) combine into one, indicating that the 11 ACS Paragon Plus Environment


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unmodified CaCO3 nanoparticles only play a role of volume-exclusion, and do not affect the dispersion quality of CNTs and their formation of an interconnected conducting network. However, the PP/CNT/30CC5 ternary nanocomposites exhibits a smaller percolation threshold than PP/CNT binary and PP/CNT/30CC ternary nanocomposites, which should be attributed to the improved dispersion of CNTs and CaCO3 nanoparticles by the surface modifier.

Figure 5. Plots of (a) electrical conductivity and (b) notched impact strength versus CNT content for PP nanocomposites. In addition to the improved electrical conductivity, CaCO3 nanoparticles are also efficient in toughening the PP/CNT nanocomposites (Figure 5b). At a given CaCO3 content of 30 wt%, the addition of CNTs does harm to the notched impact strength of PP/CaCO3 nanocomposites. However, for a given PP/CNT nanocomposite, the presence of 30 wt% CaCO3 improves its notched impact strength, especially when modified CaCO3 nanoparticles are used. To identify the dispersions of CNTs and modified CaCO3 in PP matrix, Figure 6a and b show SEM photographs of freeze-fractured surface of the PP/9CNT/30CC and PP/9CNT/30CC5 nanocomposite. Modified CaCO3 and CNTs nanoparticles exhibit uniform 12 ACS Paragon Plus Environment

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dispersions in the PP matrix in Figure 6b. However, the dispersion of the unmodified CaCO3 nanoparticles is less uniform with certain agglomerations (pointed by arrows), and aggregated CNTs are circled (Figure 6a). Figures 6c and d present the impact-fractured surfaces of the PP/9CNT/30CC5 nanocomposite. Extensive plastic deformation of PP matrix is observed, which dissipates a large amount of impact energy and is thus responsible for the improved toughness.32

Figure 6. SEM photographs of (a) freeze-fractured surface of PP/9CNT/30CC, and (b) freeze-fractured and (c,d) impact-fractured surfaces of PP/9CNT/30CC5 nanocomposite. To highlight the dual roles of CaCO3 in simultaneously improving toughness and electrical conductivity of PP/CNT nanocomposites, Figure 7 shows plots of electrical conductivity and notched Izod impact strength of PP/9CNT/CC5 ternary nanocomposites as a function of CaCO3 content. In the absence of modified CaCO3, the PP/CNT binary 13 ACS Paragon Plus Environment


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nanocomposite has a low electrical conductivity of 1.94×10-7 S/m. With the increase of modified CaCO3, the electrical conductivity of the nanocomposites increases and it reaches 8×10-2 S/m for PP/9CNT/30CC5 ternary nanocomposite, indicating that the presence of modified CaCO3 not only benefits the reduction in electrical percolation threshold (Figure 5a), but also improves the electrical conductivity of the PP/CNT nanocomposites because of its volume-exclusion effect.34 The melt flow index results of PP and its nanocomposites indicate that the presence of the surface modifier decreases the viscosities of the PP nanocomposites (Table S3), which facilitates the dispersions of CNTs and CaCO3 nanoparticles and thus the formation of the interconnected conducting network within PP matrix.

Figure 7. Plots of notched impact strength and electrical conductivity versus CaCO3 content for PP/9CNT/CC5 nanocomposites.

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Moreover, although the addition of 9 wt% CNTs decreases the notched impact strength of PP from 29.2 to 16.0 KJ/m2, the presence of modified CaCO3 makes the brittle PP/CNT nanocomposite tough. With 30 wt% modified CaCO3 and 9 wt% CNTs, the ternary nanocomposite exhibits a notched impact strength as high as 33.1 KJ/m2. It is noted that further increasing the loading of modified CaCO3 decreases the impact strength, which may be caused by the agglomeration of the CaCO3 nanoparticles in the PP matrix.22,23 In addition, as shown in Figure 8 and Table S4, with increasing the loading of CaCO3, the Young’s modulus of the nanocomposites increases due to the presence of CNTs and CaCO3 rigid components, indicating the advantage of toughening of inorganic nanoparticles over rubber toughening.12

Figure 8. Plots of Young’s modulus versus CaCO3 content for PP based nanocomposites.



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4. Conclusion The incorporation of CNTs makes PP electrically conductive with enhanced tensile strength and Young’s modulus. However, the resultant PP/CNT binary nanocomposites become brittle. Surface modification improves the dispersion of the CaCO3 nanoparticles and benefits the toughening of PP/CNT nanocomposites by inducing extensive plastic deformation of PP matrix. By adding 30 wt% modified CaCO3, the notched impact strength of PP nanocomposite with 9 wt% CNTs is greatly enhanced from 16.0 to 33.1 KJ/m2. Although both unmodified and modified CaCO3 are electrically insulating, they are efficient in improving the electrical conductivity of PP/CNT nanocomposites because of their volume-exclusion effect. The percolation threshold of PP/CNT nanocomposites is reduced from 6.2 to 5.6 wt% in the presence of 30 wt% unmodified CaCO3 and to 3.6 wt% by the addition of 30 wt% modified CaCO3. It is confirmed that the CaCO3 nanoparticles play dual roles in simultaneously improving toughness and electrical conductivity of PP/CNT nanocomposites. The electrically conductive and tough PP nanocomposites with enhanced modulus make them greatly promising in many applications.

Acknowledgements Financial support from the National Natural Science Foundation of China (51125010, 51403016, 51533001, 51521062) and the Fundamental Research Funds for the Central Universities (YS201402) is gratefully acknowledged.

Supporting Information 16 ACS Paragon Plus Environment

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Digital and SEM images and EDX of the surface modifier. SEM images and notched impact strength of PP/CaCO3 nanocomposites. Tensile properties and melt flow index results of PP and its nanocomposites.

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