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carbon black nanocomposites
Composites: Part B 40 (2009) 218–224
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Positive temperature coefficient characteristic and structure of graphite nanofibers reinforced high density polyethylene/carbon black nanocomposites Qi Li a, Siddaramaiah a,b, Nam Hoon Kim c, Gye-Hyoung Yoo d, Joong Hee Lee a,c,* a
BIN Fusion Research Team, Department of Polymer and Nano Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea Department of Polymer Science and Technology, Sri Jayachamarajendra College of Engineering, Mysore 570 006, India c Department of Hydrogen and Fuel Cell Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea d R&D Center, KCR Co. Ltd., Woanju, Jeonbuk, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 9 November 2008 Accepted 9 November 2008 Available online 17 November 2008 Keywords: A. Nano-structures A. Thermoplastic resin B. Electrical properties C. Electron microscopy D. Thermal analysis
a b s t r a c t Graphite nanofibers (GNF) and carbon black (CB) filled high density polyethylene (HDPE) hybrid composites were fabricated using a melt mixing method. The effects of the CB and GNF content on the room temperature resistivity and positive temperature coefficient (PTC) behavior of the nanocomposites were examined. The room temperature resistivity of the composites decreased significantly with increasing GNF content, but this was not always the case with the PTC intensity. The incorporation of a small amount of GNF into the HDPE/CB composites significantly improved the PTC intensity and reproducibility of the hybrid nanocomposites. The maximum PTC effect, whose log intensity was approximately 7.2, was observed in the HDPE/CB/GNF (80/20/0.25 wt%) nanocomposite with relatively low room temperature resistivity. The mechanism for the effects of GNF in HDPE/CB/GNF hybrid composites were examined using differential scanning calorimetry, transmission scanning electron microscopy and X-ray diffraction. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Conducting polymer composites (CPCs) can be obtained by blending an insulating polymer matrix with conducting fillers. CPCs have recently become the subject of intensive studies [1–3]. Semi-crystalline polymer composites with conducting particles usually exhibit two important insulator–conductor transitions [4,5]. The first is the dependence of the resistivity on the filler content, and a critical concentration of fillers is necessary to construct a continuous conducting network, which is referred to as the percolation threshold [6,7]. The second is the temperature dependence of the resistivity. A composite usually shows a sharp increase in resistivity with temperature around the melting temperature (Tm) of the crystalline polymer, known as the positive temperature coefficient (PTC). Because of the commercial significance of such temperature-activated switching features, polymerbased PTC materials can be used in a variety of fields, such as self-regulating heaters, current protection devices, microswitch sensors and other outdoor equipments [8,9]. Polymer-based PTC materials have advantages over the conventional inorganic PTC materials, such as excellent formability, flexibility and lightweight.
* Corresponding author. Address: BIN Fusion Research Team, Department of Polymer and Nano Engineering, Chonbuk National University, Jeonju, Jeonbuk 561756, Republic of Korea. Tel.: +82 63 270 2342; fax: +82 63 270 2341. E-mail address: [email protected] (J.H. Lee). 1359-8368/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2008.11.002
However, these materials show poor reproducibility in resistivity during a long period of exposure or when undergoing thermal cycles [10]. Recently, improvements in resistance and the thermal responsiveness for the elements that protect the circuits have been in demand as a result of miniaturization and energy conservation efforts in electronic devices. Several studies have addressed the ability to compensate these drawbacks to the application of PTC materials [8,10,11]. Although many studies have been carried out, their subjects can be classified into the following: increasing the PTC intensity, improving the reproducibility, removing the negative temperature coefficient (NTC) effect, providing theoretical models and optimization of the processing conditions. An increase in PTC intensity can be achieved in two ways: lowering the room temperature resistivity and/or increasing the resistivity near the polymer melt temperature. Fillers can be used in higher volume fractions to lower the room temperature resistivity. However, this method is accompanied with rheological and processing problems. On the other hand, it will reduce the height of the PTC peak in the resistivity– temperature curve, which means lower PTC intensity. The use of substantially more conducting fillers, such as carbon black (CB) and metallic powders, is another way of reducing the room temperature resistivity. However, metallic powders generally suffer from oxidation with the consequent deterioration of electrical properties of the composite [12]. In addition, a high conducting filler loading usually reduces the mechanical performance of the polymer matrix and causes processing difficulties. Therefore,
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2. Experiments 2.1. Materials HDPE with a melt flow index and density of 20 g/min and 0.95 g/cm3, respectively, was supplied by Honam Petrochemical Co., Korea. The CB (particle size 24 nm) used as the conducting filler (Hi-Black 420B) was purchased from Korea Carbon Black Co., Korea. The graphite nanofibers (diameter: 130–150 nm, length: 25–30 lm, purity: >95 vol.% and q 2.0 g/cc) were provided by Nanomirae Co. Ltd., Korea. 2.2. Preparation of nanocomposite specimens Prior to melt mixing, different weight percentages of the GNFs were mixed with HDPE, as the precursor master batch using a solution method with xylene as the solvent. The prepared master batches with various amounts of GNFs were melt-mixed with HDPE particles and CB in a Haake mixer (Rheocord 9000, Germany) at 180 °C, 60 rpm for 30 min to achieve a reasonably uniform dispersion. The blended mixtures were hot pressed at 180 °C into sheets with a thickness of 1.5 mm. For the resistivity measurements, disc samples, 10 mm in diameter, were punched from the composite specimens. 2.3. Measurements and observations The volume resistivity of the hybrid composites was measured as a function of temperature using a computerized system, consisting of a computer, multimeter and programmable oven. The specimen temperature was increased from ambient temperature (about 25 °C) to 200 °C at a heating rate of 2 °C/min. A silver paste was used to ensure good contact of the sample surface with the multimeter electrodes. In order to check the reproducibility of the PTC profiles, the electric resistivity measurements were conducted for three heating cycles. The thermal behavior of the sample was measured by differential scanning calorimetry (DSC) (TA 2910). Approximately 5 mg of the sample was heated from ambient temperature to 170 °C at a heating rate of 5 °C/min under a nitrogen atmosphere. X-ray diffraction (XRD) was carried out using a Rigaku D/max 2500 Diffractometer at 40 kV and 50 mA, with Cu Ka monochromatic radiation at a wavelength of 1.5406 Å. The samples cut from the composite specimen prepared at the same condition were scanned in the 2h range of 2–50° at a scanning speed of 4°/min. The morphology of the freeze fractured and a microtomed surface was examined using field-emission scanning electron microscopy (FE-SEM; Hitachi S-4700) at 10 kV. The microstructures of the nanocomposites (connectivity between MWNTs and CB particles) were observed under transmission electron microscope (TEM,
JEM 2010F) at 100 kV. The TEM specimens were microtomed to ultrathin sections with a thickness of 70 nm. 3. Results and discussion 3.1. Room temperature electrical resistivity PTC composite materials of HDPE with a CB filler loading level of 15, 20, and 25 wt% were fabricated. Fig. 1 shows the log resistivity as a function of the CB content of the HDPE/CB composites. There was a rapid decrease in the log resistivity of the composites with increasing CB content. This rapid decrease is characteristic of the loading level at which the CB particles begin to come into contact with each other to form a conducting network. The resistivity of the composite decreases drastically with increasing amount of conducting filler in the polymer matrix, and the composite experienced an insulator-to-conductor transition at a certain critical filler content. This sharp break, reflecting the aggregation of conducting particles to form networks, is defined as the percolation transition, and the critical weight or volume fraction of the filler is the threshold that divides the composite into either an insulator or conductor. The percolation transition threshold for the PTC samples was found to be approximately 20 wt% of CB content. 3.2. PTC behavior Fig. 2 shows a semi-log plot of the resistivity of 20 wt% CB-filled HDPE composites with various GNF contents as a function of temperature. The PTC intensity has a strong dependence on the GNF content. Resistivity peaks were observed at approximately 151 and 159 °C for HDPE/CB without and with the GNF composites, respectively, temperatures slightly higher than the melting point of HDPE (which is 130–133 °C, as determined by DSC). Table 1 shows the PTC intensities, IPTC, and other electrical properties including the resistivity at room temperature qRT, and maximum resistivity qmax at switching temperature Ts for the PTC composites. As shown in Table 1, the IPTC values ranged from 2.803 to 7.212. The HDPE/CB/GNF, 75/25/0.75 system had minimum PTC intensity while the HDPE/CB/GNF, 80/20/0.25 composite had the maximum. The other samples showed moderate IPTC values ranging from 2.982 to 6.246. It is interesting that the highest IPTC values shown in Table 1 were obtained for PTC samples prepared with 20 wt% CB.
14 12 Log Resistivity (ohm-cm)
the incorporation of multi-walled carbon nanotubes (MWNTs) and nanoparticles into a CB-filled polymer is expected to not only reduce the required CB loading but also improve the electrical properties of the composites due to their high conductivity, aspect ratio and network structure [13–17]. Recently, Lee et al. [18] reported remarkable improvements in PTC behavior by incorporating a small amount of MWNTs in high density polyethylene/carbon black (HDPE/CB) composites. CB-filled HDPE composites have been used extensively as thermistors and self-regulating heaters in industry for many years. Hence, a HDPE matrix was selected. In this study, PTC composite materials were fabricated by melt mixing of HDPE with CB and graphite nanofibers (GNF). The effects of the GNF content on the PTC characteristic and on its reproducibility of the nanocomposites were investigated.
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CB content (wt%) Fig. 1. The plot of log room temperature resistivity versus carbon black content for HDPE/CB composites.
Q. Li et al. / Composites: Part B 40 (2009) 218–224
Log Resistivity (ohm-cm)
10 GNFs content 0 wt% 0.25 wt % 0.5 wt % 0.75 wt % 1.0 wt %
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Temperature (°C) Fig. 2. The resistivity as a function of temperature for the HDPE/CB (80/20) with various GNFs contents.
Table 1 Electrical properties of the prepared PTC composites. Samples
IPTCa
log qRTb (X cm)
log qmaxc (X cm)
Tsd (°C)
80/20 80/20/0.25 80/20/0.5 80/20/0.75 80/20/1.0 75/25 75/25/0.25 75/25/0.5 75/25/0.75
5.229 7.212 6.246 6.062 5.999 4.210 4.781 2.982 2.803
3.465 2.367 2.278 1.986 1.847 1.902 1.496 1.294 1.035
8.693 9.578 8.524 8.047 7.846 6.113 6.278 4.276 3.838
151 159 158 158 158 159 162 161 160
a b c d
PTC intensity, defined as the logarithm ratio of qmax to qrt. Logarithm resistivity at room temperature. Logarithm resistivity at Ts. Switching temperature.
Among the PTC curves, the HDPE/CB composite with a 0.25 wt% GNFs content showed the highest PTC intensity of 7.212 and those without GNF showed the lowest PTC intensity of 5.229. The log resistivity of HDPE/CB at room temperature is approximately 8.693 X cm, which is much higher than that of the material containing GNFs. With increasing GNF content, the materials showed low resistivities at room temperature, and the PTC intensity of the corresponding materials deteriorated. These results may be due to the fact that the GNFs bridge the non-contacting CB particles and connect the short conducting paths, thereby forming a network structure. Therefore, the resistivity of the composites containing CB and GNFs is not as sensitive to changes in temperature as that of the composites containing single filler only before the transition temperature. The composites require sufficient volume expansion to break off the entire network at higher temperatures. The crystals begin to melt when the temperature increases to the vicinity of the Tm of a polymer, which leads to the formation of new amorphous regions. With the thermal expansion that resulted from the melting of the polymer crystals, the gap between the CB particles in the polymer matrix increases significantly. Moreover, the resistivity of the conducting composites increases rapidly with a high PTC intensity [19]. This phenomenon is caused by two factors, the interparticle distance and number of conducting paths. The large thermal expansion of the polymer matrix can increase the interparticle distance significantly and decrease the number of conducting paths. According to tunneling theory, the tunneling probability of an electron is related to the interparti-
cle distance. Therefore, the resistivity increases dramatically with increasing interparticle distance. The probability of electron tunneling will be very low for an interparticle distance greater than 10 nm. For a composite containing 20 wt% CB and 0.25 wt% GNF, a large number of the interparticle distances may be greater than 10 nm after thermal expansion. Hence, the resistivity of the composite will increase considerably due to the combined effect of larger interparticle distances and a much smaller number of conducting paths. For a composite with a high CB and GNF content, the CB particles and GNF can form a continuous conducting network or the interparticle distances can be very small. The increase in interparticle distance is likely to be the only factor that causes an increase in resistivity. The PTC effect in these materials becomes weaker with increasing CB and GNF content. These results agree well with published data on CB-filled single semi-crystalline polymer composites [20–24]. Several mechanisms and models on the theoretical aspects of PTC behavior have been proposed. Some authors believe in a widening of the gaps between conducting particles at switching temperature Ts, which hinders the process of electron tunneling [25]. Deagglomeration or breakage of the conducting network has also been proposed by Lee et al. [26]. Hindermann-Bischoff and Ehrburger-Dolle [27] examined the PTC phenomenon using the experimental evidence collected by analyzing the DC and AC conductivity as well as the influence of thermal expansion of the matrix in a series of CB-filled polyethylene composites. They concluded that in semi-crystalline polymeric matrix, the relative amount of amorphous and crystalline regions also depend on the morphology and volume fraction of CB. They highlighted the importance of the interpenetration of the CB aggregates on the electrical behavior of the composites, which is modified during the matrix melting induced PTC effect. This is characteristic of CB aggregates, which can interpenetrate each other owing to their fractal properties. Therefore, by considering all the theories mentioned above, the reason for the variation in IPTC between the PTC samples prepared by the filler and nanofiber mixtures in this study can be attributed to the filler characteristics. The effect on the conducting path, aggregate properties of the fillers and relative amount of crystalline and amorphous regions in the matrix finally affect the IPTC due to the incorporation of filler and nanofibers. The other HDPE composites containing 25 wt% CB with different GNFs contents showed similar trends (Fig. 3). Nevertheless, the resistivity at room temperature and PTC intensity were much
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Log Resistivity (ohm-cm)
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GNFs content 0 wt % 0.25 wt % 0.5 wt % 0.75 wt %
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Temperature (°C) Fig. 3. The resistivity as a function of temperature for the HDPE/CB (75/25) with various GNFs contents.
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located decreases the mobility of the polymer chains in that region. This further inhibits the migration of CB particles, resulting in a small NTC intensity. Fig. 4 shows the effect of the heating rates viz., 2, 4 and 6 °C/min on the PTC profiles. The switching temperature for the PTC effects increased with increasing heating rate. This is due to the fact that the temperature at which the conducting paths of CB/fibers are cut increases with increasing heating rate. Indeed, the apparent melting point associated with volume expansion shifted to the high temperature side with increasing heating rate. Fig. 5a and b shows the resistivity as a function of temperature of the HDPE/CB composites without and with 0.25 wt% GNFs, respectively for three heating and cooling cycles. The composites without GNFs show very poor reproducibility. The first PTC curve is significantly different from the subsequent curves and the room temperature resistivity decreases with increasing number of heating cycles (Fig. 5a). The slight decrease in initial resistivity for the subsequent heating cycle curves may have been caused by the
lower than that of the corresponding hybrid composites containing 20 wt% CB. This shows that it is difficult for volume expansion to prevent the aggregation of all the CB and GNFs when the filler concentration is higher than the upper limit of the percolation region. At low CB content, the large PTC intensity is caused by the significant increase in interparticle distance at temperatures near Tm. Consequently, considerable migration of CB particles into the newly formed amorphous phase by the crystalline melt is possible. Such migration is further enhanced by the decrease in viscosity of the polymer matrix with increasing temperature. The NTC effect following the behavior of PTC can clearly be observed in both figures. Beyond the Tm, the penetration of CB particles into the softened polymer phase can cause the formation of a series of new conducting paths, resulting in a significant decrease in resistivity; hence, an NTC intensity is observed [28]. At high CB concentrations, the increase in interparticle distance is relatively small, even at Tm, as indicated by a very low PTC intensity. In addition, a high CB content in the amorphous region where the CB particles are
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Temperature (°C) Fig. 4. Resistivity of the HDPE/CB/GNF (80/20/0.25) composite as a function of temperature at different heating rates.
b
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HDPE/CB (80/20) 1st run 2nd run 3rd run
8 Log Resistivity (ohm-cm)
Fig. 6. XRD profiles of (a) HDPE, (b) HDPE/CB (80/20) and (c) HDPE/CB/GNFs (80/20/ 0.25).
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8 Log Resistivity (ohm-cm)
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Fig. 5. Resistivity as a function of temperature for the HDPE/CB (80/20) composites for three heating cycles; (a) without GNFs and (b) with 0.25 wt% GNFs.
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enhanced contact surface area during the repeated heating cycles. These results confirmed the hypothesis that the decrease in resistivity was due to the further intermixing between CB and the polymer matrix at higher temperatures. Therefore, minute changes in the CB distribution in the polymer matrix during each heating cycle (annihilation) causes vast changes in resistivity. Hence, the thermoelectric behavior changes significantly between the first and subsequent heating/cooling cycles. Many methods have been proposed and developed in an attempt to overcome the disadvantages of poor reproducibility in thermal cycling [18,24]. Compared with the composites without GNFs, the reproducibility of the composites with GNFs was improved because the incorporation of GNFs minimizes the migration of CB and the shape deformation of resin due to the formation of a network by GNFs (Fig. 5b). In order to characterize the crystal structure of the polymer composites, XRD analysis was used. Fig. 6 shows the XRD patterns of the HDPE/CB composites with and without GNFs. All the composites showed two characteristic peaks for the crystalline phase of the polymer matrix. There is an intense peak at 2h = 21.5° (d 4.13 Å) and another less intense peak at 2h = 23.5° (d 3.74 Å). The peak intensities decreased when CB were added to the pure HDPE, but recovered when the GNFs were added to the composites. The decrease in peak intensity after the incorporation of CB is due to a decrease in the amount of crystalline phase HDPE in the composites. However, the slight increase in crystalline peak intensity after the incorporation of GNF was observed. The intensities of the HDPE/CB (80/20) composite were 3210 and 1060 for the first and second peaks, respectively. For HDPE/CB/ GNF (80/20/0.25), the first and second peaks intensities were 4280 and 1420, respectively, which are higher than that of the HDPE/CB composite. DSC is used to characterize the polymer matrix. Fig. 7a and b shows the obtained DSC thermograms of all hybrid nanocomposites for both heating and cooling cycles, respectively. It was noticed that the DSC melting curves of semi-crystalline polymers were not identical to the corresponding crystallization curves. The rapid increase and decrease in the DSC profiles at the different temperatures suggest different thermal behavior during the heating and cooling processes. Fig. 8 shows the DSC thermograms for the
Endotherm Heat Flow Exotherm
a 80/20/1 132.8 80/20/0.75 133.3 80/20/0.5 132.1 80/20/0.25 131.5 80/20/0 131.1
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Endotherm Heat Flow Exotherm
117.6 80/20/0 118.6 80/20/0.25 118.4 80/20/0.5 117.6 80/20/0.75 118.2 80/20/1
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Fig. 7. DSC thermograms for (a) heating cycle and (b) cooling cycle of the HDPE/CB/ GNF nanocomposites.
b
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3rd run
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Fig. 8. DSC thermograms at different heating cycles for the HDPE/CB composites with (a) 0.25 and (b) 0.5 wt% GNF content.
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Q. Li et al. / Composites: Part B 40 (2009) 218–224 Table 2 Data obtained from DSC thermograms for the HDPE/CB/GNF nanocomposites. HDPE/CB/GNF (wt%)
Crystallinitya (Xc) (%)
Thermal properties First scan
(a) Properties for the heating cycles 80/20/0 80/20/0.25 80/20/0.5 80/20/0.75 80/20/1.0
Second scan
Third scan
Tm (°C)
DHm (J/g)
Tm (°C)
DHm (J/g)
Tm (°C)
DHm (J/g)
129.0 131.4 132.9 132.1 131.8
95.8 99.9 109.4 101.4 101.4
131.1 131.5 132.1 133.4 132.8
96.3 100.8 106.0 101.4 100.2
131.1 131.4 132.1 133.5 132.7
96.4 100.1 106.3 101.6 99.3
39.3 41.1 43.2 41.3 40.8
Thermal properties First scan Tc (°C) (b) Properties of the corresponding cooling cycles 80/20/0 117.6 80/20/0.25 118.5 80/20/0.5 118.5 80/20/0.75 117.5 80/20/1.0 118.2 a
Second scan
Third scan
DHc (J/g)
Tc (°C)
DHc (J/g)
Tc (°C)
DHc (J/g)
74.6 80.8 85.9 86.3 88.2
117.6 118.6 118.5 117.5 118.2
75.7 81.0 86.0 86.3 88.1
117.8 118.6 118.5 117.6 118.2
76.0 80.9 86.3 86.4 88.0
Calculated from the ratio of the measured DHm of the nanocomposites to DHm0 of a 100% crystalline polyethylene (245.3 J/g) [29].
HDPE/CB/GNF hybrid composites at the three heating and cooling cycles. Table 2a and b summarizes the thermal data obtained from the DSC curves for the three heating and cooling cycles for 20 wt% CB-filled nanocomposites, respectively. After the first thermal cycle, the endothermic peak of HDPE shifted from 129.0 to 131.1 °C. There was a slight change (only 1–2 °C) in melting temperature (Tm) for the different heating cycles. The heat of melting (DHm) was highest for HDPE (116 J/g), which decreased significantly after the incorporation of CB (95.8 J/g). This shows that the CB particles reduce the level of crystallinity. The heat of melting (DHm) and percentage crystallinity increased slightly after the
incorporation of GNF with the maximum value being observed for the 80/20/0.5 HDPE/CB/GNF composite. It is only possible to say that the conducting filler appears to promote spherulites with thicker lamellae. There was a slight increase in the crystallization temperature Tc. However, at a higher GNF content, the Tc did not changed significantly with GNF content. The reproducibility of the melting peaks from the DSC scans is almost identical to the corresponding PTC profiles. After the incorporation of GNFs into the HDPE/CB composites, the DSC curves showed similar melting curves, which indicate good reproducibility of the thermal behavior. Fig. 9a and b shows FE-SEM images of a freeze fractured cross section of the HDPE/CB composites without and with GNF, respectively. Fig. 9a shows the uniform distribution of CB particles along with some small voids. The existence of voids indicated incomplete adhesion or wetting between the polyethylene and CB particles. According to previous reports [30,31], the voids expanded at the first heating run. This reflects the different thermal expansion coefficients between the polyethylene and conducting fillers, which leads to fluctuations in the PTC curves. By adding GNF, the morphology of the fracture surface was greatly changed as shown in Fig. 9b. The morphology of the nanocomposites was also examined by TEM. Fig. 10 shows a TEM image of the HDPE/CB/GNF (80/20/ 0.5) hybrid composite. The CB particles were dispersed uniformly with slight agglomeration. The images in Figs. 9b and 10 clearly show that the CB particles are bridged with GNF nanofibers, which help make conducting tunnels between the CB-rich regions. These bridging nanofibers enhance the conductivity and their network structures improve the reproducibility of PTC phenomena of the nanocomposites. 4. Conclusions
Fig. 9. FE-SEM images of (a) HDPE/CB (80/20) composite and (b) HDPE/CB/GNF (80/ 20/1) hybrid composite.
The conducting filler loading in the filled PTC polymer composites has a significant effect on the electrical resistivity and PTC. Lower room temperature resistivity and high PTC intense peak was observed for the GNF-incorporated nanocomposites compared with HDPE/CB. Composites containing 20 wt% CB and 0.25 wt% GNF showed a maximum resistivity (9.578 X cm) at a switching temperature (Ts) along with a high PTC intensity (7.212). The thermal properties of the hybrid composites, such as the melting temperature (Tm), percentage crystallinity (Xc) and heat of melt (DHm), were higher than those observed for the HDPE/CB composites. Significant improvements in reproducibility and PTC intensity were
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Fig. 10. TEM image of the HDPE/CB/GNF (80/20/0.5) hybrid composite.
observed after the incorporation of a small quantity of graphite nanofibers into the HDPE/CB hybrid composite materials. This was attributed to the incorporation of GNFs minimizing the migration of conducting particles and the shape deformation of resin. Acknowledgements This study was supported by the National Space Lab(NSL) program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (S1 08A01003210). The authors also wish to acknowledge the financial support of the Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Regional Innovation. References [1] Weidenfeller B, Höfer M, Schilling FR. Thermal conductivity, thermal diffusivity, and specific heat capacity of particle filled polypropylene. Compos Part A – Appl Sci Manuf 2004;35:423–9. [2] Dang ZM, Nan CW, Xie D, Zhang YH, Tjong SC. Dielectric behavior and dependence of percolation threshold on the conductivity of fillers in polymer– semiconductor composites. Appl Phys Lett 2004;85:97–9. [3] Wang L, Dang ZM. Carbon nanotube composites with high dielectric constant at low percolation threshold. Appl Phys Lett 2005;87:042903. [4] Lei H, William GP, Lucas KM, Clifford KH. Resistivity measurements of carbon– polymer composites in chemical sensors: impact of carbon concentration and geometry. Sens Actuators B 2004;101:122–32. [5] Park SJ, Kim HC, Kim HYJ. Roles of work of adhesion between carbon blacks and thermoplastic polymers on electrical properties of composites. Colloid Interface Sci 2002;255:145–9. [6] Zribi K, Feller JF, Elleuch K, Bourmaud A, Elleuch B. Conductive polymer composites obtained from recycled poly(carbonate) and rubber blends for heating and sensing applications. Polym Adv Technol 2006;17:727–31. [7] Mamunya YP, Davydenko VV, Pissis P, Lebedev EV. Electrical and thermal conductivity of polymers filled with metal powders. Eur Polym J 2002;38:1887–97.
[8] Park ES, Yun SJ, Kim GT, Park IJ, Choi WH, Jeong JW, et al. Preparation of positive-temperature coefficient heaters using platinum-catalyzed silicone rubber. J Appl Polym Sci 2004;94:1611–7. [9] Park ES, Jang LW, Yoon JS. Resistivity and thermal reproducibility of carbon black and metallic powder filled silicone rubber heaters. J Appl Polym Sci 2005;95:1122–8. [10] Pourabbas B, Peighambardoust SJ. PTC effect in HDPE filled with carbon blacks modified by Ni and Au metallic particles. J Appl Polym Sci 2007;105:1031–41. [11] Park JS, Kang PH, Nho YC, Suh DH. Effects of thermal ageing treatment and antioxidants on the positive temperature coefficient characteristics of carbon black/polyethylene conductive composites. J Appl Polym Sci 2003;89:2316–22. [12] Kalyon DM, Birinci E, Yazici R, Karuv B, Walsh S. Electrical properties of composites as affected by the degree of mixedness of the conductive filler in the polymer matrix. Polym Eng Sci 2002;42:1609–17. [13] Pötschke P, Bhattacharyya AR, Janke A. Carbon nanotube-filled polycarbonate composites produced by melt mixing and their use in blends with polyethylene. Carbon 2004;42:965–9. [14] Watts PCP, Hsu WK, Kotzeva V, Chen GZ. Fe-filled carbon nanotube– polystyrene: RCL composites. Chem Phys Lett 2002;366:42–50. [15] He XJ, Du JH, Ying Z, Cheng HM. Positive temperature coefficient effect in multiwalled carbon nanotube/high-density polyethylene composites. Appl Phys Lett 2005;86:062112. [16] Liang GD, Tjong SC. Electrical properties of low-density polyethylene/multiwalled carbon nanotube nanocomposites. Mater Chem Phys 2006;100:1–5. [17] Lisunova MO, Mamunya YP, Lebovka NI, Melezhyk AV. Percolation behaviour of ultrahigh molecular weight polyethylene/multi-walled carbon nanotubes composites. Eur Polym J 2007;43:949–58. [18] Lee JH, Kim SK, Kim NH. Effects of the addition of multi-walled carbon nanotubes on the positive temperature coefficient characteristics of carbonblack-filled high-density polyethylene nanocomposites. Scripta Mater 2006;55:1119–22. [19] Zhang JF, Zheng Q, Yang YQ, Yi XS. High-density polyethylene/carbon black conductive composites. II. Effect of electron beam irradiation on relationship between resistivity–temperature behavior and volume expansion. J Appl Polym Sci 2002;83:3117–22. [20] Bar H, Narkis M, Boiteux G. The electrical behavior of thermosetting polymer composites containing metal plated ceramic filler. Polym Compos 2005;26:12–9. [21] Chan CM, Cheng CL, Yuen MMF. Electrical properties of polymer composites prepared by sintering a mixture of carbon black and ultra-high molecular weight polyethylene powder. Polym Eng Sci 1997;87:1127–36. [22] Tang H, Piao JH, Chen XF, Lou YZ, Li SH. The positive temperature coefficient phenomenon of vinyl polymer/CB composites. J Appl Polym Sci 1993;48:1795–800. [23] Tang H, Liu ZY, Piao JH, Chen XF, Lou YZ, Li SH. Electrical behavior of carbon black-filled polymer composites: effect of interaction between filler and matrix. J Appl Polym Sci 1994;51:1159–64. [24] Kalappa P, Lee JH, Rashmi BJ, Venkatesha TV, Pai KV, Wang X. Effect of polyaniline functionalized carbon nanotubes addition on the positive temperature coefficient behavior of carbon/high-density polyethylene nanocomposites. IEEE Trans Nanotechnol 2008;7:223–8. [25] Yi XS, Shen L, Pan Y. Thermal volume expansion in polymeric PTC composites: a theoretical approach. Compos Sci Technol 2001;61:949–56. [26] Lee GJ, Suh KD, Im SS. Study of electrical phenomena in carbon black-filled HDPE composite. Polym Eng Sci 1998;38:471–7. [27] Hindermann-Bischoff M, Ehrburger-Dolle F. Electrical conductivity of carbon black–polyethylene composites: experimental evidence of the change of cluster connectivity in the PTC effect. Carbon 2001;39:375–82. [28] Di W, Zhang G, Xu J, Peng Y, Wang X, Xie Z. Positive-temperature-coefficient/ negative-temperature-coefficient effect of low-density polyethylene filled with a mixture of carbon black and carbon fiber. J Polym Sci Part B: Polym Phys 2003;41:3094–101. [29] Brandrup J, Immergut EH, Grulke EA. Polymer handbook. 4th ed. New York: Wiley; 1999. [30] Marr DWM, Wartenberg M, Schwartz KB, Agamalian MM, Wignall GD. Void morphology in polyethylene/carbon black composites. Macromolecules 1997;30:2120–4. [31] Oakey J, Marr DWM, Schwartz KB, Wartenberg M. Influence of polyethylene and carbon black morphology on void formation in conductive composite materials: a SANS study. Macromolecules 1999;32:5399–404.
Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Positive temperature coefficient characteristic and structure of graphite nanofibers reinforced high density polyethylene/carbon black nanocomposites Qi Li a, Siddaramaiah a,b, Nam Hoon Kim c, Gye-Hyoung Yoo d, Joong Hee Lee a,c,* a
BIN Fusion Research Team, Department of Polymer and Nano Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea Department of Polymer Science and Technology, Sri Jayachamarajendra College of Engineering, Mysore 570 006, India c Department of Hydrogen and Fuel Cell Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea d R&D Center, KCR Co. Ltd., Woanju, Jeonbuk, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 9 November 2008 Accepted 9 November 2008 Available online 17 November 2008 Keywords: A. Nano-structures A. Thermoplastic resin B. Electrical properties C. Electron microscopy D. Thermal analysis
a b s t r a c t Graphite nanofibers (GNF) and carbon black (CB) filled high density polyethylene (HDPE) hybrid composites were fabricated using a melt mixing method. The effects of the CB and GNF content on the room temperature resistivity and positive temperature coefficient (PTC) behavior of the nanocomposites were examined. The room temperature resistivity of the composites decreased significantly with increasing GNF content, but this was not always the case with the PTC intensity. The incorporation of a small amount of GNF into the HDPE/CB composites significantly improved the PTC intensity and reproducibility of the hybrid nanocomposites. The maximum PTC effect, whose log intensity was approximately 7.2, was observed in the HDPE/CB/GNF (80/20/0.25 wt%) nanocomposite with relatively low room temperature resistivity. The mechanism for the effects of GNF in HDPE/CB/GNF hybrid composites were examined using differential scanning calorimetry, transmission scanning electron microscopy and X-ray diffraction. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Conducting polymer composites (CPCs) can be obtained by blending an insulating polymer matrix with conducting fillers. CPCs have recently become the subject of intensive studies [1–3]. Semi-crystalline polymer composites with conducting particles usually exhibit two important insulator–conductor transitions [4,5]. The first is the dependence of the resistivity on the filler content, and a critical concentration of fillers is necessary to construct a continuous conducting network, which is referred to as the percolation threshold [6,7]. The second is the temperature dependence of the resistivity. A composite usually shows a sharp increase in resistivity with temperature around the melting temperature (Tm) of the crystalline polymer, known as the positive temperature coefficient (PTC). Because of the commercial significance of such temperature-activated switching features, polymerbased PTC materials can be used in a variety of fields, such as self-regulating heaters, current protection devices, microswitch sensors and other outdoor equipments [8,9]. Polymer-based PTC materials have advantages over the conventional inorganic PTC materials, such as excellent formability, flexibility and lightweight.
* Corresponding author. Address: BIN Fusion Research Team, Department of Polymer and Nano Engineering, Chonbuk National University, Jeonju, Jeonbuk 561756, Republic of Korea. Tel.: +82 63 270 2342; fax: +82 63 270 2341. E-mail address: [email protected] (J.H. Lee). 1359-8368/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2008.11.002
However, these materials show poor reproducibility in resistivity during a long period of exposure or when undergoing thermal cycles [10]. Recently, improvements in resistance and the thermal responsiveness for the elements that protect the circuits have been in demand as a result of miniaturization and energy conservation efforts in electronic devices. Several studies have addressed the ability to compensate these drawbacks to the application of PTC materials [8,10,11]. Although many studies have been carried out, their subjects can be classified into the following: increasing the PTC intensity, improving the reproducibility, removing the negative temperature coefficient (NTC) effect, providing theoretical models and optimization of the processing conditions. An increase in PTC intensity can be achieved in two ways: lowering the room temperature resistivity and/or increasing the resistivity near the polymer melt temperature. Fillers can be used in higher volume fractions to lower the room temperature resistivity. However, this method is accompanied with rheological and processing problems. On the other hand, it will reduce the height of the PTC peak in the resistivity– temperature curve, which means lower PTC intensity. The use of substantially more conducting fillers, such as carbon black (CB) and metallic powders, is another way of reducing the room temperature resistivity. However, metallic powders generally suffer from oxidation with the consequent deterioration of electrical properties of the composite [12]. In addition, a high conducting filler loading usually reduces the mechanical performance of the polymer matrix and causes processing difficulties. Therefore,
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2. Experiments 2.1. Materials HDPE with a melt flow index and density of 20 g/min and 0.95 g/cm3, respectively, was supplied by Honam Petrochemical Co., Korea. The CB (particle size 24 nm) used as the conducting filler (Hi-Black 420B) was purchased from Korea Carbon Black Co., Korea. The graphite nanofibers (diameter: 130–150 nm, length: 25–30 lm, purity: >95 vol.% and q 2.0 g/cc) were provided by Nanomirae Co. Ltd., Korea. 2.2. Preparation of nanocomposite specimens Prior to melt mixing, different weight percentages of the GNFs were mixed with HDPE, as the precursor master batch using a solution method with xylene as the solvent. The prepared master batches with various amounts of GNFs were melt-mixed with HDPE particles and CB in a Haake mixer (Rheocord 9000, Germany) at 180 °C, 60 rpm for 30 min to achieve a reasonably uniform dispersion. The blended mixtures were hot pressed at 180 °C into sheets with a thickness of 1.5 mm. For the resistivity measurements, disc samples, 10 mm in diameter, were punched from the composite specimens. 2.3. Measurements and observations The volume resistivity of the hybrid composites was measured as a function of temperature using a computerized system, consisting of a computer, multimeter and programmable oven. The specimen temperature was increased from ambient temperature (about 25 °C) to 200 °C at a heating rate of 2 °C/min. A silver paste was used to ensure good contact of the sample surface with the multimeter electrodes. In order to check the reproducibility of the PTC profiles, the electric resistivity measurements were conducted for three heating cycles. The thermal behavior of the sample was measured by differential scanning calorimetry (DSC) (TA 2910). Approximately 5 mg of the sample was heated from ambient temperature to 170 °C at a heating rate of 5 °C/min under a nitrogen atmosphere. X-ray diffraction (XRD) was carried out using a Rigaku D/max 2500 Diffractometer at 40 kV and 50 mA, with Cu Ka monochromatic radiation at a wavelength of 1.5406 Å. The samples cut from the composite specimen prepared at the same condition were scanned in the 2h range of 2–50° at a scanning speed of 4°/min. The morphology of the freeze fractured and a microtomed surface was examined using field-emission scanning electron microscopy (FE-SEM; Hitachi S-4700) at 10 kV. The microstructures of the nanocomposites (connectivity between MWNTs and CB particles) were observed under transmission electron microscope (TEM,
JEM 2010F) at 100 kV. The TEM specimens were microtomed to ultrathin sections with a thickness of 70 nm. 3. Results and discussion 3.1. Room temperature electrical resistivity PTC composite materials of HDPE with a CB filler loading level of 15, 20, and 25 wt% were fabricated. Fig. 1 shows the log resistivity as a function of the CB content of the HDPE/CB composites. There was a rapid decrease in the log resistivity of the composites with increasing CB content. This rapid decrease is characteristic of the loading level at which the CB particles begin to come into contact with each other to form a conducting network. The resistivity of the composite decreases drastically with increasing amount of conducting filler in the polymer matrix, and the composite experienced an insulator-to-conductor transition at a certain critical filler content. This sharp break, reflecting the aggregation of conducting particles to form networks, is defined as the percolation transition, and the critical weight or volume fraction of the filler is the threshold that divides the composite into either an insulator or conductor. The percolation transition threshold for the PTC samples was found to be approximately 20 wt% of CB content. 3.2. PTC behavior Fig. 2 shows a semi-log plot of the resistivity of 20 wt% CB-filled HDPE composites with various GNF contents as a function of temperature. The PTC intensity has a strong dependence on the GNF content. Resistivity peaks were observed at approximately 151 and 159 °C for HDPE/CB without and with the GNF composites, respectively, temperatures slightly higher than the melting point of HDPE (which is 130–133 °C, as determined by DSC). Table 1 shows the PTC intensities, IPTC, and other electrical properties including the resistivity at room temperature qRT, and maximum resistivity qmax at switching temperature Ts for the PTC composites. As shown in Table 1, the IPTC values ranged from 2.803 to 7.212. The HDPE/CB/GNF, 75/25/0.75 system had minimum PTC intensity while the HDPE/CB/GNF, 80/20/0.25 composite had the maximum. The other samples showed moderate IPTC values ranging from 2.982 to 6.246. It is interesting that the highest IPTC values shown in Table 1 were obtained for PTC samples prepared with 20 wt% CB.
14 12 Log Resistivity (ohm-cm)
the incorporation of multi-walled carbon nanotubes (MWNTs) and nanoparticles into a CB-filled polymer is expected to not only reduce the required CB loading but also improve the electrical properties of the composites due to their high conductivity, aspect ratio and network structure [13–17]. Recently, Lee et al. [18] reported remarkable improvements in PTC behavior by incorporating a small amount of MWNTs in high density polyethylene/carbon black (HDPE/CB) composites. CB-filled HDPE composites have been used extensively as thermistors and self-regulating heaters in industry for many years. Hence, a HDPE matrix was selected. In this study, PTC composite materials were fabricated by melt mixing of HDPE with CB and graphite nanofibers (GNF). The effects of the GNF content on the PTC characteristic and on its reproducibility of the nanocomposites were investigated.
10 8 6 4 2 14
16
18
20
22
24
26
28
30
32
CB content (wt%) Fig. 1. The plot of log room temperature resistivity versus carbon black content for HDPE/CB composites.
Q. Li et al. / Composites: Part B 40 (2009) 218–224
Log Resistivity (ohm-cm)
10 GNFs content 0 wt% 0.25 wt % 0.5 wt % 0.75 wt % 1.0 wt %
8
6
4
2
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40
60
80
100
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140
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Temperature (°C) Fig. 2. The resistivity as a function of temperature for the HDPE/CB (80/20) with various GNFs contents.
Table 1 Electrical properties of the prepared PTC composites. Samples
IPTCa
log qRTb (X cm)
log qmaxc (X cm)
Tsd (°C)
80/20 80/20/0.25 80/20/0.5 80/20/0.75 80/20/1.0 75/25 75/25/0.25 75/25/0.5 75/25/0.75
5.229 7.212 6.246 6.062 5.999 4.210 4.781 2.982 2.803
3.465 2.367 2.278 1.986 1.847 1.902 1.496 1.294 1.035
8.693 9.578 8.524 8.047 7.846 6.113 6.278 4.276 3.838
151 159 158 158 158 159 162 161 160
a b c d
PTC intensity, defined as the logarithm ratio of qmax to qrt. Logarithm resistivity at room temperature. Logarithm resistivity at Ts. Switching temperature.
Among the PTC curves, the HDPE/CB composite with a 0.25 wt% GNFs content showed the highest PTC intensity of 7.212 and those without GNF showed the lowest PTC intensity of 5.229. The log resistivity of HDPE/CB at room temperature is approximately 8.693 X cm, which is much higher than that of the material containing GNFs. With increasing GNF content, the materials showed low resistivities at room temperature, and the PTC intensity of the corresponding materials deteriorated. These results may be due to the fact that the GNFs bridge the non-contacting CB particles and connect the short conducting paths, thereby forming a network structure. Therefore, the resistivity of the composites containing CB and GNFs is not as sensitive to changes in temperature as that of the composites containing single filler only before the transition temperature. The composites require sufficient volume expansion to break off the entire network at higher temperatures. The crystals begin to melt when the temperature increases to the vicinity of the Tm of a polymer, which leads to the formation of new amorphous regions. With the thermal expansion that resulted from the melting of the polymer crystals, the gap between the CB particles in the polymer matrix increases significantly. Moreover, the resistivity of the conducting composites increases rapidly with a high PTC intensity [19]. This phenomenon is caused by two factors, the interparticle distance and number of conducting paths. The large thermal expansion of the polymer matrix can increase the interparticle distance significantly and decrease the number of conducting paths. According to tunneling theory, the tunneling probability of an electron is related to the interparti-
cle distance. Therefore, the resistivity increases dramatically with increasing interparticle distance. The probability of electron tunneling will be very low for an interparticle distance greater than 10 nm. For a composite containing 20 wt% CB and 0.25 wt% GNF, a large number of the interparticle distances may be greater than 10 nm after thermal expansion. Hence, the resistivity of the composite will increase considerably due to the combined effect of larger interparticle distances and a much smaller number of conducting paths. For a composite with a high CB and GNF content, the CB particles and GNF can form a continuous conducting network or the interparticle distances can be very small. The increase in interparticle distance is likely to be the only factor that causes an increase in resistivity. The PTC effect in these materials becomes weaker with increasing CB and GNF content. These results agree well with published data on CB-filled single semi-crystalline polymer composites [20–24]. Several mechanisms and models on the theoretical aspects of PTC behavior have been proposed. Some authors believe in a widening of the gaps between conducting particles at switching temperature Ts, which hinders the process of electron tunneling [25]. Deagglomeration or breakage of the conducting network has also been proposed by Lee et al. [26]. Hindermann-Bischoff and Ehrburger-Dolle [27] examined the PTC phenomenon using the experimental evidence collected by analyzing the DC and AC conductivity as well as the influence of thermal expansion of the matrix in a series of CB-filled polyethylene composites. They concluded that in semi-crystalline polymeric matrix, the relative amount of amorphous and crystalline regions also depend on the morphology and volume fraction of CB. They highlighted the importance of the interpenetration of the CB aggregates on the electrical behavior of the composites, which is modified during the matrix melting induced PTC effect. This is characteristic of CB aggregates, which can interpenetrate each other owing to their fractal properties. Therefore, by considering all the theories mentioned above, the reason for the variation in IPTC between the PTC samples prepared by the filler and nanofiber mixtures in this study can be attributed to the filler characteristics. The effect on the conducting path, aggregate properties of the fillers and relative amount of crystalline and amorphous regions in the matrix finally affect the IPTC due to the incorporation of filler and nanofibers. The other HDPE composites containing 25 wt% CB with different GNFs contents showed similar trends (Fig. 3). Nevertheless, the resistivity at room temperature and PTC intensity were much
10
Log Resistivity (ohm-cm)
220
8
GNFs content 0 wt % 0.25 wt % 0.5 wt % 0.75 wt %
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Temperature (°C) Fig. 3. The resistivity as a function of temperature for the HDPE/CB (75/25) with various GNFs contents.
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located decreases the mobility of the polymer chains in that region. This further inhibits the migration of CB particles, resulting in a small NTC intensity. Fig. 4 shows the effect of the heating rates viz., 2, 4 and 6 °C/min on the PTC profiles. The switching temperature for the PTC effects increased with increasing heating rate. This is due to the fact that the temperature at which the conducting paths of CB/fibers are cut increases with increasing heating rate. Indeed, the apparent melting point associated with volume expansion shifted to the high temperature side with increasing heating rate. Fig. 5a and b shows the resistivity as a function of temperature of the HDPE/CB composites without and with 0.25 wt% GNFs, respectively for three heating and cooling cycles. The composites without GNFs show very poor reproducibility. The first PTC curve is significantly different from the subsequent curves and the room temperature resistivity decreases with increasing number of heating cycles (Fig. 5a). The slight decrease in initial resistivity for the subsequent heating cycle curves may have been caused by the
lower than that of the corresponding hybrid composites containing 20 wt% CB. This shows that it is difficult for volume expansion to prevent the aggregation of all the CB and GNFs when the filler concentration is higher than the upper limit of the percolation region. At low CB content, the large PTC intensity is caused by the significant increase in interparticle distance at temperatures near Tm. Consequently, considerable migration of CB particles into the newly formed amorphous phase by the crystalline melt is possible. Such migration is further enhanced by the decrease in viscosity of the polymer matrix with increasing temperature. The NTC effect following the behavior of PTC can clearly be observed in both figures. Beyond the Tm, the penetration of CB particles into the softened polymer phase can cause the formation of a series of new conducting paths, resulting in a significant decrease in resistivity; hence, an NTC intensity is observed [28]. At high CB concentrations, the increase in interparticle distance is relatively small, even at Tm, as indicated by a very low PTC intensity. In addition, a high CB content in the amorphous region where the CB particles are
8
2°C/min 4°C/min 6°C/min
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(c)
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(a)
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100 120 140 160 180 200 220
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Temperature (°C) Fig. 4. Resistivity of the HDPE/CB/GNF (80/20/0.25) composite as a function of temperature at different heating rates.
b
10
HDPE/CB (80/20) 1st run 2nd run 3rd run
8 Log Resistivity (ohm-cm)
Fig. 6. XRD profiles of (a) HDPE, (b) HDPE/CB (80/20) and (c) HDPE/CB/GNFs (80/20/ 0.25).
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8 Log Resistivity (ohm-cm)
a
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HDPE/CB/GNFs (80/20/0.25) 1st run
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Temperature (°C)
140
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Temperature (°C)
Fig. 5. Resistivity as a function of temperature for the HDPE/CB (80/20) composites for three heating cycles; (a) without GNFs and (b) with 0.25 wt% GNFs.
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enhanced contact surface area during the repeated heating cycles. These results confirmed the hypothesis that the decrease in resistivity was due to the further intermixing between CB and the polymer matrix at higher temperatures. Therefore, minute changes in the CB distribution in the polymer matrix during each heating cycle (annihilation) causes vast changes in resistivity. Hence, the thermoelectric behavior changes significantly between the first and subsequent heating/cooling cycles. Many methods have been proposed and developed in an attempt to overcome the disadvantages of poor reproducibility in thermal cycling [18,24]. Compared with the composites without GNFs, the reproducibility of the composites with GNFs was improved because the incorporation of GNFs minimizes the migration of CB and the shape deformation of resin due to the formation of a network by GNFs (Fig. 5b). In order to characterize the crystal structure of the polymer composites, XRD analysis was used. Fig. 6 shows the XRD patterns of the HDPE/CB composites with and without GNFs. All the composites showed two characteristic peaks for the crystalline phase of the polymer matrix. There is an intense peak at 2h = 21.5° (d 4.13 Å) and another less intense peak at 2h = 23.5° (d 3.74 Å). The peak intensities decreased when CB were added to the pure HDPE, but recovered when the GNFs were added to the composites. The decrease in peak intensity after the incorporation of CB is due to a decrease in the amount of crystalline phase HDPE in the composites. However, the slight increase in crystalline peak intensity after the incorporation of GNF was observed. The intensities of the HDPE/CB (80/20) composite were 3210 and 1060 for the first and second peaks, respectively. For HDPE/CB/ GNF (80/20/0.25), the first and second peaks intensities were 4280 and 1420, respectively, which are higher than that of the HDPE/CB composite. DSC is used to characterize the polymer matrix. Fig. 7a and b shows the obtained DSC thermograms of all hybrid nanocomposites for both heating and cooling cycles, respectively. It was noticed that the DSC melting curves of semi-crystalline polymers were not identical to the corresponding crystallization curves. The rapid increase and decrease in the DSC profiles at the different temperatures suggest different thermal behavior during the heating and cooling processes. Fig. 8 shows the DSC thermograms for the
Endotherm Heat Flow Exotherm
a 80/20/1 132.8 80/20/0.75 133.3 80/20/0.5 132.1 80/20/0.25 131.5 80/20/0 131.1
60
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100 120 Temperature (°C)
b
140
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Endotherm Heat Flow Exotherm
117.6 80/20/0 118.6 80/20/0.25 118.4 80/20/0.5 117.6 80/20/0.75 118.2 80/20/1
60
80
100 120 Temperature (°C)
140
160
Fig. 7. DSC thermograms for (a) heating cycle and (b) cooling cycle of the HDPE/CB/ GNF nanocomposites.
b
Endotherm Heat Flow Exotherm
3rd run
131.4
2nd run
131.5
1st run
3rd run
Endotherm Heat Flow Exotherm
a
132.1
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132.1 1st 1strun run
131.4
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Temperature (°C)
140
132.9
160
60
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Temperature (°C)
Fig. 8. DSC thermograms at different heating cycles for the HDPE/CB composites with (a) 0.25 and (b) 0.5 wt% GNF content.
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Q. Li et al. / Composites: Part B 40 (2009) 218–224 Table 2 Data obtained from DSC thermograms for the HDPE/CB/GNF nanocomposites. HDPE/CB/GNF (wt%)
Crystallinitya (Xc) (%)
Thermal properties First scan
(a) Properties for the heating cycles 80/20/0 80/20/0.25 80/20/0.5 80/20/0.75 80/20/1.0
Second scan
Third scan
Tm (°C)
DHm (J/g)
Tm (°C)
DHm (J/g)
Tm (°C)
DHm (J/g)
129.0 131.4 132.9 132.1 131.8
95.8 99.9 109.4 101.4 101.4
131.1 131.5 132.1 133.4 132.8
96.3 100.8 106.0 101.4 100.2
131.1 131.4 132.1 133.5 132.7
96.4 100.1 106.3 101.6 99.3
39.3 41.1 43.2 41.3 40.8
Thermal properties First scan Tc (°C) (b) Properties of the corresponding cooling cycles 80/20/0 117.6 80/20/0.25 118.5 80/20/0.5 118.5 80/20/0.75 117.5 80/20/1.0 118.2 a
Second scan
Third scan
DHc (J/g)
Tc (°C)
DHc (J/g)
Tc (°C)
DHc (J/g)
74.6 80.8 85.9 86.3 88.2
117.6 118.6 118.5 117.5 118.2
75.7 81.0 86.0 86.3 88.1
117.8 118.6 118.5 117.6 118.2
76.0 80.9 86.3 86.4 88.0
Calculated from the ratio of the measured DHm of the nanocomposites to DHm0 of a 100% crystalline polyethylene (245.3 J/g) [29].
HDPE/CB/GNF hybrid composites at the three heating and cooling cycles. Table 2a and b summarizes the thermal data obtained from the DSC curves for the three heating and cooling cycles for 20 wt% CB-filled nanocomposites, respectively. After the first thermal cycle, the endothermic peak of HDPE shifted from 129.0 to 131.1 °C. There was a slight change (only 1–2 °C) in melting temperature (Tm) for the different heating cycles. The heat of melting (DHm) was highest for HDPE (116 J/g), which decreased significantly after the incorporation of CB (95.8 J/g). This shows that the CB particles reduce the level of crystallinity. The heat of melting (DHm) and percentage crystallinity increased slightly after the
incorporation of GNF with the maximum value being observed for the 80/20/0.5 HDPE/CB/GNF composite. It is only possible to say that the conducting filler appears to promote spherulites with thicker lamellae. There was a slight increase in the crystallization temperature Tc. However, at a higher GNF content, the Tc did not changed significantly with GNF content. The reproducibility of the melting peaks from the DSC scans is almost identical to the corresponding PTC profiles. After the incorporation of GNFs into the HDPE/CB composites, the DSC curves showed similar melting curves, which indicate good reproducibility of the thermal behavior. Fig. 9a and b shows FE-SEM images of a freeze fractured cross section of the HDPE/CB composites without and with GNF, respectively. Fig. 9a shows the uniform distribution of CB particles along with some small voids. The existence of voids indicated incomplete adhesion or wetting between the polyethylene and CB particles. According to previous reports [30,31], the voids expanded at the first heating run. This reflects the different thermal expansion coefficients between the polyethylene and conducting fillers, which leads to fluctuations in the PTC curves. By adding GNF, the morphology of the fracture surface was greatly changed as shown in Fig. 9b. The morphology of the nanocomposites was also examined by TEM. Fig. 10 shows a TEM image of the HDPE/CB/GNF (80/20/ 0.5) hybrid composite. The CB particles were dispersed uniformly with slight agglomeration. The images in Figs. 9b and 10 clearly show that the CB particles are bridged with GNF nanofibers, which help make conducting tunnels between the CB-rich regions. These bridging nanofibers enhance the conductivity and their network structures improve the reproducibility of PTC phenomena of the nanocomposites. 4. Conclusions
Fig. 9. FE-SEM images of (a) HDPE/CB (80/20) composite and (b) HDPE/CB/GNF (80/ 20/1) hybrid composite.
The conducting filler loading in the filled PTC polymer composites has a significant effect on the electrical resistivity and PTC. Lower room temperature resistivity and high PTC intense peak was observed for the GNF-incorporated nanocomposites compared with HDPE/CB. Composites containing 20 wt% CB and 0.25 wt% GNF showed a maximum resistivity (9.578 X cm) at a switching temperature (Ts) along with a high PTC intensity (7.212). The thermal properties of the hybrid composites, such as the melting temperature (Tm), percentage crystallinity (Xc) and heat of melt (DHm), were higher than those observed for the HDPE/CB composites. Significant improvements in reproducibility and PTC intensity were
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Fig. 10. TEM image of the HDPE/CB/GNF (80/20/0.5) hybrid composite.
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