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MWCNT composites
Composites Part B 90 (2016) 107e114
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Thermal properties and thermal stability of PP/MWCNT composites T.Y. Zhou a, Gary C.P. Tsui b, J.Z. Liang a, b, *, S.Y. Zou a, C.Y. Tang b, Vesna Miskovi c-Stankovi cc a Research Division of Green Function Materials and Equipment, School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, PR China b Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Kowloon, Hung Hom, Hong Kong, PR China c Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 August 2015 Received in revised form 28 September 2015 Accepted 25 December 2015 Available online 6 January 2016
The effects of the filler size and content on thermal properties and thermal stability of polypropylene (PP) composites filled with four different sizes of multi-walled carbon nanotubes (MWCNTs) were investigated through thermogravimetric analysis. The results showed that the values of the decomposition temperature increased with increasing weight fraction and lengthediameter ratio of the filler, while increased with decreasing filler diameter. The values of the residues increased approximately linearly with increasing filler weight fraction. The values of maximum mass loss rate decreased roughly with increasing filler weight fraction, while the influence of the filler diameter and lengthediameter ratio on the maximum mass loss rate was insignificant. The thermal stability improvement might be attributed to the barrier function of the MWCNTs. This study provides a basis for further development of MWCNTs reinforced polymer composites with desirable thermal properties for potential application as energy materials. © 2015 Elsevier Ltd. All rights reserved.
Keywords: A. Polymerematrix composites (PMCs) A. Thermoplastic resin B. Thermal properties D. Thermal analysis Degradation
1. Introduction Carbon nanotube (CNT) is a new type of carbon fillers, which is considered to be a tube formed with a rolled-up graphene layer. In addition to the special seamless nanometer tube structure, CNT possesses some properties of general nano-materials, including its excellent mechanical, electrical and thermal conductive properties due to its specific surface area and length to diameter ratio. The properties of polymeric materials can be improved or some new properties can be obtained if they are filled with CNT [1e3]. CNT can be divided into two categories: single wall carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). In general, the cost of MWCNTs is much lower than that of SWCNTs. Polypropylene (PP) is one of general thermoplastics. It has become a highly consumed thermoplastic resin due to some of its advantages, such as low cost, easy of processing and recycling, and good comprehensive properties. However, PP has some disadvantages including high flammability, low strength and high notch
* Corresponding author. Research Division of Green Function Materials and Equipment, School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, PR China. E-mail address: [email protected] (J.Z. Liang). http://dx.doi.org/10.1016/j.compositesb.2015.12.013 1359-8368/© 2015 Elsevier Ltd. All rights reserved.
sensitivity [4]. Thus, the studies on modification of PP resin have been made to further extend its application range to other fields such as electromagnetic field shielding and energy materials [5], as well as biocomposites [6]. The PP/CNTs composites have been prepared by means of various methods such as melt blending [7e9], shear blending [10], melt spinning [11], in-situ polymerization [12], and solution blending [13]. The research results showed that addition of only a tiny amount of CNTs could increase the thermal stability [14e16], and mechanical properties [17e19] of the PP composites. Bhattacharyya and his colleagues [9] prepared PP/ SWCNT composites using the melt blending method and investigated the crystallization properties of the composites. They found that the addition of the SWCNT could not change the crystal type of the PP, but could become as a nucleating agent in the matrix to increase the crystalline rate of the composites, in which the size of spherules became smaller and had narrower distribution. Among various methods, melt blending has been extensively used in polymer processing industry owing to its low cost, strong generality and environment friendliness, especially for thermoplastic resins [20.21]. Thermal properties and thermal stability are important performance of polymeric materials. Recently, some researchers studied the thermal stability of PP/CNTs composites. Although a number of
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2.3. Instruments and methodology
studies have been made on the interaction mechanisms between CNTs and polymers, there are still some controversies. So far, there have been relatively a few perfect and systematic theories to explain these mechanisms. Therefore, it is necessary to have an indepth study on the thermal stability of polymer/CNTs composites. In addition, how does the size of MWCNTs affect the thermal stability of polymer composites. However, there have been relatively a few comprehensive studies on the thermal stability mechanisms of the PP/MWCNT composites. The objectives of the present study are to investigate the effects of the content and size of MWCNTs on the thermal properties and thermal stability of the PP/MWCNTs composites, including starting decomposition temperature, temperature of the end of decomposition, maximum mass loss rate and the amount of residues, to provide useful data for development of MWCNTs reinforced polymer composites.
The thermal stability of the PP/MWCNTs composites was measured using the thermogravimetric analyzer (TGA) (Model TG2009) supplied by the NETZCH Company. The test temperature range was varied from 30 to 600 C, the preheat time was 10 min, and the experiments were conducted in the Al2O3 crucible environment with 30 mL/min, and the protection gas was nitrogen. The specimen quality range was from 5 to 10 mg. The multi-walled nanotubes were examined by means of a scanning electron microscope (SEM) with model LEO 1530 VP supplied by LEO Co. Ltd. (Oberkochen, Germany) to characterize the size, shape and morphology at powder state of the fillers.
2. Experimental
3.1. TGA curves
2.1. Raw materials
Thermogravimetric curve (TGA curve) describes the relationship between mass and temperature of a specimen during testing, and also reflects the variation of the mass with a rise in temperature. That is, it demonstrates the thermal decomposition behavior of materials in nitrogen atmosphere. Thus, TGA curve presents the thermal stability of materials, especially for polymeric materials. Fig. 2 shows the TGA curves of the four PP/MWCNTs composite systems, including the results for PP/TNIM2 in Fig. 2(a), PP/TNIM3 in Fig. 2(b), PP/TNIM4 in Fig. 2(c) and PP/TNIM8 in Fig. 2(d). It can be seen that the thermal decomposition process of the four composite systems in the nitrogen atmosphere is similar to that of the unfilled PP resin; namely, it is a single step decomposition process. In general, when the weight loss (i.e. mass loss) reaches up to 5% of the total weight of sample, the corresponding temperature is defined as the starting decomposition temperature (T5%) of material [21]. It can be observed from Fig. 2 that the starting decomposition temperature of the four composite systems increases roughly with increasing MWCNT weight fraction, and the value of the starting decomposition temperature is around 430 C. When the decomposition is completed, the corresponding temperature is defined as the temperature (Tf) of the decomposition end. In general, the temperature range from T5% to Tf is defined as a decomposition temperature range (DTdr). Thus, the parameters T5%, Tf and DTdr are the important parameters for characterizing the thermal properties and thermal stability of polymeric materials. It can also be found in Fig. 2 that the mass of the specimen decreases significantly in the decomposition temperature range. In addition, when temperature is higher than Tf, the weight loss tends to be constant. The remaining amount of inorganic filler particles in the composite after TGA tests is usually used to analyze the dispersion of the inclusions in the resin matrix.
3. Results and discussion
The polypropylene with trademark CJS-700 serving as the matrix material was supplied by Guangzhou Petrochemical Works in Guangdong province (Guangzhou, China), and its density in a solid state and melt flow rate were 910 kg/m3 and 10 g/10 min (230 C, 2.16 kg), respectively. Four types of multi-walled nanotubes (MWCNTs) were selected as the fillers to determine the influence of the size on the thermal stability of the composite systems. The MWCNTs were supplied by the Chengdu Organic Chemical Co., Ltd. of the Chinese Academy of Sciences (Chengdu, China), and were prepared by means of chemical vapor deposition method, with a purity of greater than 90 wt.%. Table 1 lists the main characteristics of the four MWCNTs. Fig. 1 shows the photographs of the four MWCNTs using scanning electronic microscopy (SEM). It can be observed that the size, shape and morphology at powder state, and the diameter of the four MWCNTs is arranged in the order of TNIM2 < TNIM3 < TNIM4 < TNIM8, while the length to diameter of the four MWCNTs follows the order of TNIM2 > TNIM3 > TNIM4 > TNIM8. 2.2. Composite preparation The PP was separately mixed with the four MWCNTs in a high speed compounding machine with model of GH-10 supplied by Beijing Plastics Machinery (Beijing, China). The PP/MWCNTs blends were then melt-blended in a twin-screw extruder (Model: SHJ-26) supplied by the Nanjing Chengmeng Machinery Ltd. Co. (Nanjing, China) at a screw speed of 100 rev/min and in a temperature range from 190 to 210 C for preparing the four PP/MWCNTs composite systems with designations of PP/TNIM2, PP/TNIM3, PP/TNIM4 and PP/TNIM8, where the weight fractions of the MWCNTs were 1, 2, 3, 4 and 5 wt.%, respectively. The screw diameter and the length to diameter ratio for the extruder were 26 mm and 40, respectively. The extrudate of the composites was granulated, and the granules were dried at 80 C for 5 h before testing.
3.2. DTG curves Thermogravimetry derivative curve (DTG curve) describes the correlation between the mass loss rate with time (t) and temperature (T) of a specimen during the TGA test of specimen. That is
Table 1 Characteristics of multi-walled carbon nanotubes. MWCNTs type
d (nm)
L (mm)
L/d
r (g/cm3)
Specific surface area (m2/g)
TNIM2 TNIM3 TNIM4 TNIM8
8e15 10e20 10e30 50
30e50 20e100 10e30 10e20
2000e6250 1000e10,000 333e3000 400
2.1 2.1 2.1 2.1
250e300 100e200 >140 >40
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Fig. 1. SEM photographs of four MWCNTs.
3.3. Decomposition temperature
dm ¼ f ðTÞ dt
(1)
It can also be observed from Fig. 2 that the decomposition temperature of the four composite systems is approximately close to each other. These differences in the decomposition temperature and the weight loss between the composites systems can be determined by means of the DTG curve. This is because the peak of the DTG curve corresponds to the inflexion point (i.e. the maximum value of mass loss rate) of the TGA curve, and the peak area of the DTG curve is proportional to the weight loss. Fig. 3 displays the DTG curves of the four PP/MWCNTs composite systems, including those for PP/TNIM2 in Fig. 3(a), PP/TNIM3 in Fig. 3(b), PP/TNIM4 in Fig. 3(c), and PP/TNIM8 in Fig. 3(d). It can be seen that the position and area of the peaks of the DTG curves of the four composite systems vary with the MWCNT content. For both PP/TNIM2 and PP/ TNIM3 composite systems, with increase of the MWCNT weight fraction, the values of the peak area increase approximately and the peak position moves to the right (see Fig. 3(a)). For the PP/TNIM4 and PP/TNIM8 composite systems, the values of the peak area also increase and the peak position moves to the right with increasing the MWCNT weight fraction (see Fig. 3(c) and (d)). This indicates that there are certain effects of the size and content of the MWCNTs on the thermal properties and thermal stability of the PP composites under the experimental conditions.
3.3.1. Dependence of decomposition temperature on MWCNT content As stated above, decomposition temperature is an important parameter for characterizing the thermal stability of materials. In general, the decomposition temperature can be determined from TGA curves, and it is divided into starting decomposition temperature, maximum decomposition temperature and temperature of the end of decomposition. Fig. 4 illustrates the dependence of the starting decomposition temperature (T5%) on the MWCNT weight fraction of the four PP/MWCNTs composite systems. It can be found that the starting decomposition temperature of the unfilled PP resin is 407.2 C, while the values of the starting decomposition temperature of the PP composite systems are higher than those of the unfilled PP resin. For the PP/TNIM2 and PP/TNIM3 systems, the values of the starting decomposition temperature increase obviously with the MWCNT weight fraction when the MWCNT weight fraction is lower than 1 wt.%, and then increase slightly at the filler range from 1 to 4 wt.%, but decrease slightly at the filler content higher than 4 wt.%. For the PP/TNIM4 and PP/TNIM8 systems, the values of the starting decomposition temperature increase nonlinearly with the MWCNT weight fraction. For comparison, when the weight loss (i.e. mass loss) is 10%, the corresponding temperature is defined as the decomposition temperature at 10% mass loss (T10%). Fig. 5 shows the dependence of the decomposition temperature at the 10% mass loss on the MWCNT weight fraction of the
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Fig. 2. TGA curves of PP/MWCNT composites.
four PP/MWCNTs composite systems. It can be seen that the values of the decomposition temperature at the 10% mass loss increase roughly with increasing MWCNT weight fraction except for individual data point, and the values of the decomposition temperature at the 10% mass loss of the four composite systems are higher than that of the unfilled PP resin. It is also known from Figs. 4 and 5 that the decomposition temperature at 10% mass loss is higher than that of the starting decomposition temperature, and the difference between T10% and T5% of the four PP/MWCNTs composite systems decreases with increasing MWCNT weight fraction under the given conditions. As discussed above, the peak of DTG curve represents the maximum value of the mass loss rate of the specimen. Thus, the temperature at the peak of DTG curve is defined as the maximum decomposition temperature (Tmax). Fig. 6 displays the dependence of the maximum decomposition temperature on the MWCNT weight fraction of the four PP/MWCNTs composite systems. Similarly, the values of the maximum decomposition temperature increase with increasing MWCNT weight fraction. The above results present that the introduction of the MWCNTs can significantly improve the thermal properties and thermal stability of the filled PP composites.
3.3.2. Effect of MWCNTs size on decomposition temperature In addition to the filler content and the dispersion of the filler particles in the resin matrix, the thermal stability of polymer composites is closely related to the filler shape and size. For MWCNTs, their size is governed by their diameter, length, and length to diameter ratio. It can also be observed from Figs. 4 and 5 that the values of the decomposition temperature for the composites decrease with increasing diameter of the MWCNTs when MWCNTs weight fractions are respectively 5 wt.% and 10 wt.%, while the values of the decomposition temperature increase with increasing length to diameter ratio of the MWCNTs. The improvement of the thermal stability of polymer composites can be attributed to the mass transport barrier effect of the hollow structure of MWCNTs. This is because that the tortuous path in the composites can be increased by increasing the length to diameter ratio of MWCNTs or reducing the diameter of MWCNTs. Hence, this tortuous path can block or delay the transfer of the decomposition products from a condensed phase in the composites to the surface or gas phase. In addition, the interaction between MWCNTs and macromolecular chains of the polymer and the absorption effect of MWCNTs on free radical produced in the decomposition is one of the important reasons accounting for the improvement in the thermal stability of the resulting polymer composites.
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Fig. 3. DTG curves of PP/MWCNT composites.
Fig. 4. Dependence of starting decomposition temperature on MWCNT weight fraction.
Fig. 5. Dependence of decomposition temperature at mass loss 10% on MWCNT weight fraction.
It can also be seen in Fig. 6 that the values of the maximum decomposition temperature decrease roughly with increasing diameter of the MWCNTs except for individual data points. Similarly, the values of the maximum decomposition temperature increase roughly with increasing length to diameter ratio of the MWCNTs except for individual data points. Moreover, it can be
observed in Figs. 4e6 that the values of T5%, T10% and Tmax of the PP/ TNIM4 and PP/TNIM8 systems are lower than those of the PP/ TNIM2 and PP/TNIM3 systems, and the increase of former systems after 4 wt.% of MWCNT is different from the latter systems, the reason should be that the length to diameter ratio and specific surface area of the TNIM4 and TNIM8 systems are smaller than
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lower than 4 wt.%, and then increase slightly at the filler content higher than 4 wt.%. Fig. 8 displays that the correlation between the maximum mass loss rate and the MWCNT weight fraction at 440 C. Similarly, the values of the mass loss rate decrease with increasing MWCNT weight fraction for the PP/TNIM4 system, while the values of the maximum mass loss rates of the other three PP/MWCNT composites decrease with increasing MWCNT weight fraction at the filler content lower than 4 wt.%, and then increase slightly at the filler content higher than 4 wt.%. This also indicates that the introduction of the MWCNTs is beneficial to the improvement of the thermal stability of the PP/MWCNT composites. Moreover, it can be observed from Figs. 7 and 8 that the sensitivity of the maximum mass loss rate to the MWCNT weight fraction at 440 C is stronger than that at 430 C.
Fig. 6. Dependence of maximum decomposition temperature on MWCNT weight fraction.
those of the TNIM2 and TNIM3 systems (see Table 1), while small length to diameter ratio and specific surface area of inclusions will weakened, to a certain extent, the barrier effect and the interaction effect between the filler and matrix, in addition to reduction of the tortuous paths. As a result, the thermal properties and the thermal stability of the composites will be reduced correspondingly.
3.4. Maximum mass loss rate 3.4.1. Dependence of maximum mass loss rate on MWCNT content Mass loss rate in the TGA test is another important parameter for reflecting the thermal stability of materials. The mass loss rate can also be determined from the TGA curves. The maximum loss rate is defined as the mass loss rate at the maximum curvature of the TGA curves, it just corresponds to the peak of the DTG curves (see Figs. 2 and 3). It can be seen in Figs. 2 and 3 that the maximum curvature of the TGA curve of the samples is generated in temperature range between 430 C and 440 C. Fig. 7 presents the relationship between the maximum mass loss rate and the MWCNT weight fraction at 430 C. It can be seen that the values of the maximum mass loss rate decrease with increasing MWCNT weight fraction for the PP/TNIM4 system, while the values of the maximum mass loss rates of the other three PP/MWCNT composites decrease with increasing the MWCNT weight fraction for the filler content
Fig. 7. Relationship between maximum mass loss rate and MWCNT weight fraction at 430 C.
3.4.2. Effect of MWCNT size on maximum mass loss rate As stated above, the thermal stability of PP/MWCNT composites is closely related to the diameter and lengthediameter ratio of the MWCNTs, in addition to the content and the dispersion of the filler particles in the resin matrix. It can be observed from Figs. 7 and 8 that the values of the maximum mass loss rate change irregularly with the variation of the MWCNT weight fraction. It implies that the influence of the MWCNT size on the maximum mass loss rate of the PP composites is insignificant under the experimental conditions. 3.5. Residues 3.5.1. Dependence of residues on MWCNT content The amount of the residues in the composite after the TGA test can reflect both the thermal stability of the composites and the dispersion of the inclusions in the resin matrix. The amount of the residues in the composite after TGA test can be determined from the TGA curves (Fig. 2). For PP/MWCNT composites, the residues should be mainly the MWCNTs. That is
uR ¼ a þ b4f
(2)
where uR is the weight percentage of the residues; 4f is the filler weight fraction, a and b are the constants. If the residues are completely the MWCNTs from the PP/MWCNT composite specimen after the TGA test, then the parameters a and b in Equation (2) are found to be a ¼ 0, and b ¼ 1. In this case, Equation (2) can be rewritten as follows:
Fig. 8. Correlation between maximum mass loss rate and MWCNT weight fraction at 440 C.
T.Y. Zhou et al. / Composites Part B 90 (2016) 107e114
uR ¼ 4f
(3)
Fig. 9 illustrates the relationship between the residues of the MWCNTs in the four PP composites and the MWCNT weight fraction. It can be seen that the values of the residues increase almost linearly with increasing MWCNT weight fraction except for individual data points. Comparatively, the values of the residues deviated from the calculated values (i.e. theoretical curve of Equation (2)) for the PP/TNIM4 and PP/TNIM8 systems are higher than those for the PP/TNIM2 and PP/TNIM3 systems. 3.5.2. Effect of MWCNT size on residues The deviation of the values of the residues from the calculated values for the PP composites with larger diameter of MWCNTs is higher than that for the PP composites with smaller diameter of MWCNTs. In general, such deviation should be attributed to the following two aspects: (1) the uniformity of the MWCNTs in the PP matrix; (2) the purity of the MWCNTs and the PP resin. This is because that the number of the MWCNTs in the matrix decreases with increasing MWCNT diameter under the same filler volume fraction; thus the dependence of the values of the residues on the uniformity of the MWCNTs with larger diameter in the PP matrix is stronger than that of the MWCNTs with small diameter under the same conditions. It is extensively believed that the more uniform the dispersion of the filler in the matrix is, the smaller is the deviation of the measured data of the residues of the samples. As discussed above, the tortuous paths in composites reduce with decreasing the length to diameter ratio and specific surface area of inclusions, leading to weakened, to a certain extent, the barrier effect and the interaction effect between the filler and matrix. The sensitivity of the residues of the composites to the MWCNTs weight fraction can be enhanced correspondingly in this case. Consequently, the values of the residues deviated from the calculated values for the PP/TNIM4 and PP/TNIM8 systems are higher than those for the PP/TNIM2 and PP/TNIM3 systems. 3.6. Discussion In general, MWCNT network in the resin matrix will be generated with the filler concentration, and MWCNT is good thermal conductive meter, thus the thermal conductive properties of polymer composites will be improved in this case. However, although the formation of the MWCNT network in the resin matrix can increase the heat transfer rate of composite systems, the barrier effect
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of this hollow tube structure is enhanced at the same time due to the formation of the MWCNT network. In other words, there is a competition mechanism between the barrier effect and heat transfer effect in the PP/MWCNTs composite systems. As discussed above, the factors affecting the thermal properties and thermal stability are complicated for polymer composites. In addition to the thermal properties of the matrix resin and the filler, the thermal properties and thermal stability depend, to great extent, upon the filler content and the dispersion of the filler particles in the resin matrix [22,23]. When the dispersion of the MWCNTs in the resin matrix is uniform, the paths formed by the MWCNTs are tortuous. In this case, the thermal properties and thermal stability would be improved correspondingly, and the measured data fluctuation is relatively small. When the dispersion of the MWCNTs in the resin matrix is poor, the paths formed by the MWCNTs are simple; in this case, the thermal properties and thermal stability would be weakened correspondingly, and the measured data fluctuation is relatively obvious. However, as MWCNTs are nanometer particles, they tend to aggregate in the matrix at a high filler concentration. In this case, the thermal stability of the composites would be weakened. On the other hand, the tortuous degree of the paths formed by the MWCNTs in resin matrix can increase by increasing the lengthediameter ratio or decreasing the diameter of the MWCNTs under the same condition of the dispersion of the MWCNTs in the resin matrix. Therefore, the values of the decomposition temperature of the PP/MWCNT composites increase with decreasing filler diameter or with increasing the filler lengthediameter ratio (see Figs. 4 and 5). Lai and his colleagues [24] noted that there was a good blocking effect on the transmission of decomposition products of poly(hydroxybutyrate-co-hydroxyvalerate), when the dispersion of MWCNTs in the matrix was uniform, leading to improvement of the thermal stability of the composites. Yang and his co-workers [15] studied the thermal stability of PP/CNT composites, and obtained the similar results. Furthermore, as the specific surface area of the MWCNTs is larger and both the PP resin and MWCNTs are non-polar materials, there is a stronger physical adsorption effect on the surface between the PP matrix and the MWCNTs owing to the interaction between them. Moreover, the free radicals generated during thermal decomposition of the PP can be absorbed [25]. As a result, the thermal stability of the PP/ MWCNT composites can be improved. 4. Conclusions The effects of the size and content of the MWCNTs on the thermal stability of the filled PP composites were significant. It was found that the values of the decomposition temperature increased with increasing MWCNT weight fraction and filler lengthediameter ratio, while increased with decreasing filler diameter. The values of the residues increased approximately linearly with increasing MWCNT weight fraction. The values of maximum mass loss rate decreased roughly with increasing weight fraction of the MWCNTs, while the influence of the filler diameter and lengthediameter ratio on the maximum mass loss rate was insignificant. The improvement of the thermal stability of the PP/MWCNTs composites could be attributed to the barrier effect due to the formation of the tortuous paths of MWCNTs in the PP matrix. The findings in this study should be beneficial to the development of MWCNTs reinforced polymer composites. Acknowledgment
Fig. 9. Relationship between residues and MWCNT weight fraction.
The authors would like to thank for the support from The Research Committee of The Hong Kong Polytechnic University (Project code: G-UC81).
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Thermal properties and thermal stability of PP/MWCNT composites T.Y. Zhou a, Gary C.P. Tsui b, J.Z. Liang a, b, *, S.Y. Zou a, C.Y. Tang b, Vesna Miskovi c-Stankovi cc a Research Division of Green Function Materials and Equipment, School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, PR China b Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Kowloon, Hung Hom, Hong Kong, PR China c Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 August 2015 Received in revised form 28 September 2015 Accepted 25 December 2015 Available online 6 January 2016
The effects of the filler size and content on thermal properties and thermal stability of polypropylene (PP) composites filled with four different sizes of multi-walled carbon nanotubes (MWCNTs) were investigated through thermogravimetric analysis. The results showed that the values of the decomposition temperature increased with increasing weight fraction and lengthediameter ratio of the filler, while increased with decreasing filler diameter. The values of the residues increased approximately linearly with increasing filler weight fraction. The values of maximum mass loss rate decreased roughly with increasing filler weight fraction, while the influence of the filler diameter and lengthediameter ratio on the maximum mass loss rate was insignificant. The thermal stability improvement might be attributed to the barrier function of the MWCNTs. This study provides a basis for further development of MWCNTs reinforced polymer composites with desirable thermal properties for potential application as energy materials. © 2015 Elsevier Ltd. All rights reserved.
Keywords: A. Polymerematrix composites (PMCs) A. Thermoplastic resin B. Thermal properties D. Thermal analysis Degradation
1. Introduction Carbon nanotube (CNT) is a new type of carbon fillers, which is considered to be a tube formed with a rolled-up graphene layer. In addition to the special seamless nanometer tube structure, CNT possesses some properties of general nano-materials, including its excellent mechanical, electrical and thermal conductive properties due to its specific surface area and length to diameter ratio. The properties of polymeric materials can be improved or some new properties can be obtained if they are filled with CNT [1e3]. CNT can be divided into two categories: single wall carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). In general, the cost of MWCNTs is much lower than that of SWCNTs. Polypropylene (PP) is one of general thermoplastics. It has become a highly consumed thermoplastic resin due to some of its advantages, such as low cost, easy of processing and recycling, and good comprehensive properties. However, PP has some disadvantages including high flammability, low strength and high notch
* Corresponding author. Research Division of Green Function Materials and Equipment, School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, PR China. E-mail address: [email protected] (J.Z. Liang). http://dx.doi.org/10.1016/j.compositesb.2015.12.013 1359-8368/© 2015 Elsevier Ltd. All rights reserved.
sensitivity [4]. Thus, the studies on modification of PP resin have been made to further extend its application range to other fields such as electromagnetic field shielding and energy materials [5], as well as biocomposites [6]. The PP/CNTs composites have been prepared by means of various methods such as melt blending [7e9], shear blending [10], melt spinning [11], in-situ polymerization [12], and solution blending [13]. The research results showed that addition of only a tiny amount of CNTs could increase the thermal stability [14e16], and mechanical properties [17e19] of the PP composites. Bhattacharyya and his colleagues [9] prepared PP/ SWCNT composites using the melt blending method and investigated the crystallization properties of the composites. They found that the addition of the SWCNT could not change the crystal type of the PP, but could become as a nucleating agent in the matrix to increase the crystalline rate of the composites, in which the size of spherules became smaller and had narrower distribution. Among various methods, melt blending has been extensively used in polymer processing industry owing to its low cost, strong generality and environment friendliness, especially for thermoplastic resins [20.21]. Thermal properties and thermal stability are important performance of polymeric materials. Recently, some researchers studied the thermal stability of PP/CNTs composites. Although a number of
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2.3. Instruments and methodology
studies have been made on the interaction mechanisms between CNTs and polymers, there are still some controversies. So far, there have been relatively a few perfect and systematic theories to explain these mechanisms. Therefore, it is necessary to have an indepth study on the thermal stability of polymer/CNTs composites. In addition, how does the size of MWCNTs affect the thermal stability of polymer composites. However, there have been relatively a few comprehensive studies on the thermal stability mechanisms of the PP/MWCNT composites. The objectives of the present study are to investigate the effects of the content and size of MWCNTs on the thermal properties and thermal stability of the PP/MWCNTs composites, including starting decomposition temperature, temperature of the end of decomposition, maximum mass loss rate and the amount of residues, to provide useful data for development of MWCNTs reinforced polymer composites.
The thermal stability of the PP/MWCNTs composites was measured using the thermogravimetric analyzer (TGA) (Model TG2009) supplied by the NETZCH Company. The test temperature range was varied from 30 to 600 C, the preheat time was 10 min, and the experiments were conducted in the Al2O3 crucible environment with 30 mL/min, and the protection gas was nitrogen. The specimen quality range was from 5 to 10 mg. The multi-walled nanotubes were examined by means of a scanning electron microscope (SEM) with model LEO 1530 VP supplied by LEO Co. Ltd. (Oberkochen, Germany) to characterize the size, shape and morphology at powder state of the fillers.
2. Experimental
3.1. TGA curves
2.1. Raw materials
Thermogravimetric curve (TGA curve) describes the relationship between mass and temperature of a specimen during testing, and also reflects the variation of the mass with a rise in temperature. That is, it demonstrates the thermal decomposition behavior of materials in nitrogen atmosphere. Thus, TGA curve presents the thermal stability of materials, especially for polymeric materials. Fig. 2 shows the TGA curves of the four PP/MWCNTs composite systems, including the results for PP/TNIM2 in Fig. 2(a), PP/TNIM3 in Fig. 2(b), PP/TNIM4 in Fig. 2(c) and PP/TNIM8 in Fig. 2(d). It can be seen that the thermal decomposition process of the four composite systems in the nitrogen atmosphere is similar to that of the unfilled PP resin; namely, it is a single step decomposition process. In general, when the weight loss (i.e. mass loss) reaches up to 5% of the total weight of sample, the corresponding temperature is defined as the starting decomposition temperature (T5%) of material [21]. It can be observed from Fig. 2 that the starting decomposition temperature of the four composite systems increases roughly with increasing MWCNT weight fraction, and the value of the starting decomposition temperature is around 430 C. When the decomposition is completed, the corresponding temperature is defined as the temperature (Tf) of the decomposition end. In general, the temperature range from T5% to Tf is defined as a decomposition temperature range (DTdr). Thus, the parameters T5%, Tf and DTdr are the important parameters for characterizing the thermal properties and thermal stability of polymeric materials. It can also be found in Fig. 2 that the mass of the specimen decreases significantly in the decomposition temperature range. In addition, when temperature is higher than Tf, the weight loss tends to be constant. The remaining amount of inorganic filler particles in the composite after TGA tests is usually used to analyze the dispersion of the inclusions in the resin matrix.
3. Results and discussion
The polypropylene with trademark CJS-700 serving as the matrix material was supplied by Guangzhou Petrochemical Works in Guangdong province (Guangzhou, China), and its density in a solid state and melt flow rate were 910 kg/m3 and 10 g/10 min (230 C, 2.16 kg), respectively. Four types of multi-walled nanotubes (MWCNTs) were selected as the fillers to determine the influence of the size on the thermal stability of the composite systems. The MWCNTs were supplied by the Chengdu Organic Chemical Co., Ltd. of the Chinese Academy of Sciences (Chengdu, China), and were prepared by means of chemical vapor deposition method, with a purity of greater than 90 wt.%. Table 1 lists the main characteristics of the four MWCNTs. Fig. 1 shows the photographs of the four MWCNTs using scanning electronic microscopy (SEM). It can be observed that the size, shape and morphology at powder state, and the diameter of the four MWCNTs is arranged in the order of TNIM2 < TNIM3 < TNIM4 < TNIM8, while the length to diameter of the four MWCNTs follows the order of TNIM2 > TNIM3 > TNIM4 > TNIM8. 2.2. Composite preparation The PP was separately mixed with the four MWCNTs in a high speed compounding machine with model of GH-10 supplied by Beijing Plastics Machinery (Beijing, China). The PP/MWCNTs blends were then melt-blended in a twin-screw extruder (Model: SHJ-26) supplied by the Nanjing Chengmeng Machinery Ltd. Co. (Nanjing, China) at a screw speed of 100 rev/min and in a temperature range from 190 to 210 C for preparing the four PP/MWCNTs composite systems with designations of PP/TNIM2, PP/TNIM3, PP/TNIM4 and PP/TNIM8, where the weight fractions of the MWCNTs were 1, 2, 3, 4 and 5 wt.%, respectively. The screw diameter and the length to diameter ratio for the extruder were 26 mm and 40, respectively. The extrudate of the composites was granulated, and the granules were dried at 80 C for 5 h before testing.
3.2. DTG curves Thermogravimetry derivative curve (DTG curve) describes the correlation between the mass loss rate with time (t) and temperature (T) of a specimen during the TGA test of specimen. That is
Table 1 Characteristics of multi-walled carbon nanotubes. MWCNTs type
d (nm)
L (mm)
L/d
r (g/cm3)
Specific surface area (m2/g)
TNIM2 TNIM3 TNIM4 TNIM8
8e15 10e20 10e30 50
30e50 20e100 10e30 10e20
2000e6250 1000e10,000 333e3000 400
2.1 2.1 2.1 2.1
250e300 100e200 >140 >40
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Fig. 1. SEM photographs of four MWCNTs.
3.3. Decomposition temperature
dm ¼ f ðTÞ dt
(1)
It can also be observed from Fig. 2 that the decomposition temperature of the four composite systems is approximately close to each other. These differences in the decomposition temperature and the weight loss between the composites systems can be determined by means of the DTG curve. This is because the peak of the DTG curve corresponds to the inflexion point (i.e. the maximum value of mass loss rate) of the TGA curve, and the peak area of the DTG curve is proportional to the weight loss. Fig. 3 displays the DTG curves of the four PP/MWCNTs composite systems, including those for PP/TNIM2 in Fig. 3(a), PP/TNIM3 in Fig. 3(b), PP/TNIM4 in Fig. 3(c), and PP/TNIM8 in Fig. 3(d). It can be seen that the position and area of the peaks of the DTG curves of the four composite systems vary with the MWCNT content. For both PP/TNIM2 and PP/ TNIM3 composite systems, with increase of the MWCNT weight fraction, the values of the peak area increase approximately and the peak position moves to the right (see Fig. 3(a)). For the PP/TNIM4 and PP/TNIM8 composite systems, the values of the peak area also increase and the peak position moves to the right with increasing the MWCNT weight fraction (see Fig. 3(c) and (d)). This indicates that there are certain effects of the size and content of the MWCNTs on the thermal properties and thermal stability of the PP composites under the experimental conditions.
3.3.1. Dependence of decomposition temperature on MWCNT content As stated above, decomposition temperature is an important parameter for characterizing the thermal stability of materials. In general, the decomposition temperature can be determined from TGA curves, and it is divided into starting decomposition temperature, maximum decomposition temperature and temperature of the end of decomposition. Fig. 4 illustrates the dependence of the starting decomposition temperature (T5%) on the MWCNT weight fraction of the four PP/MWCNTs composite systems. It can be found that the starting decomposition temperature of the unfilled PP resin is 407.2 C, while the values of the starting decomposition temperature of the PP composite systems are higher than those of the unfilled PP resin. For the PP/TNIM2 and PP/TNIM3 systems, the values of the starting decomposition temperature increase obviously with the MWCNT weight fraction when the MWCNT weight fraction is lower than 1 wt.%, and then increase slightly at the filler range from 1 to 4 wt.%, but decrease slightly at the filler content higher than 4 wt.%. For the PP/TNIM4 and PP/TNIM8 systems, the values of the starting decomposition temperature increase nonlinearly with the MWCNT weight fraction. For comparison, when the weight loss (i.e. mass loss) is 10%, the corresponding temperature is defined as the decomposition temperature at 10% mass loss (T10%). Fig. 5 shows the dependence of the decomposition temperature at the 10% mass loss on the MWCNT weight fraction of the
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Fig. 2. TGA curves of PP/MWCNT composites.
four PP/MWCNTs composite systems. It can be seen that the values of the decomposition temperature at the 10% mass loss increase roughly with increasing MWCNT weight fraction except for individual data point, and the values of the decomposition temperature at the 10% mass loss of the four composite systems are higher than that of the unfilled PP resin. It is also known from Figs. 4 and 5 that the decomposition temperature at 10% mass loss is higher than that of the starting decomposition temperature, and the difference between T10% and T5% of the four PP/MWCNTs composite systems decreases with increasing MWCNT weight fraction under the given conditions. As discussed above, the peak of DTG curve represents the maximum value of the mass loss rate of the specimen. Thus, the temperature at the peak of DTG curve is defined as the maximum decomposition temperature (Tmax). Fig. 6 displays the dependence of the maximum decomposition temperature on the MWCNT weight fraction of the four PP/MWCNTs composite systems. Similarly, the values of the maximum decomposition temperature increase with increasing MWCNT weight fraction. The above results present that the introduction of the MWCNTs can significantly improve the thermal properties and thermal stability of the filled PP composites.
3.3.2. Effect of MWCNTs size on decomposition temperature In addition to the filler content and the dispersion of the filler particles in the resin matrix, the thermal stability of polymer composites is closely related to the filler shape and size. For MWCNTs, their size is governed by their diameter, length, and length to diameter ratio. It can also be observed from Figs. 4 and 5 that the values of the decomposition temperature for the composites decrease with increasing diameter of the MWCNTs when MWCNTs weight fractions are respectively 5 wt.% and 10 wt.%, while the values of the decomposition temperature increase with increasing length to diameter ratio of the MWCNTs. The improvement of the thermal stability of polymer composites can be attributed to the mass transport barrier effect of the hollow structure of MWCNTs. This is because that the tortuous path in the composites can be increased by increasing the length to diameter ratio of MWCNTs or reducing the diameter of MWCNTs. Hence, this tortuous path can block or delay the transfer of the decomposition products from a condensed phase in the composites to the surface or gas phase. In addition, the interaction between MWCNTs and macromolecular chains of the polymer and the absorption effect of MWCNTs on free radical produced in the decomposition is one of the important reasons accounting for the improvement in the thermal stability of the resulting polymer composites.
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Fig. 3. DTG curves of PP/MWCNT composites.
Fig. 4. Dependence of starting decomposition temperature on MWCNT weight fraction.
Fig. 5. Dependence of decomposition temperature at mass loss 10% on MWCNT weight fraction.
It can also be seen in Fig. 6 that the values of the maximum decomposition temperature decrease roughly with increasing diameter of the MWCNTs except for individual data points. Similarly, the values of the maximum decomposition temperature increase roughly with increasing length to diameter ratio of the MWCNTs except for individual data points. Moreover, it can be
observed in Figs. 4e6 that the values of T5%, T10% and Tmax of the PP/ TNIM4 and PP/TNIM8 systems are lower than those of the PP/ TNIM2 and PP/TNIM3 systems, and the increase of former systems after 4 wt.% of MWCNT is different from the latter systems, the reason should be that the length to diameter ratio and specific surface area of the TNIM4 and TNIM8 systems are smaller than
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lower than 4 wt.%, and then increase slightly at the filler content higher than 4 wt.%. Fig. 8 displays that the correlation between the maximum mass loss rate and the MWCNT weight fraction at 440 C. Similarly, the values of the mass loss rate decrease with increasing MWCNT weight fraction for the PP/TNIM4 system, while the values of the maximum mass loss rates of the other three PP/MWCNT composites decrease with increasing MWCNT weight fraction at the filler content lower than 4 wt.%, and then increase slightly at the filler content higher than 4 wt.%. This also indicates that the introduction of the MWCNTs is beneficial to the improvement of the thermal stability of the PP/MWCNT composites. Moreover, it can be observed from Figs. 7 and 8 that the sensitivity of the maximum mass loss rate to the MWCNT weight fraction at 440 C is stronger than that at 430 C.
Fig. 6. Dependence of maximum decomposition temperature on MWCNT weight fraction.
those of the TNIM2 and TNIM3 systems (see Table 1), while small length to diameter ratio and specific surface area of inclusions will weakened, to a certain extent, the barrier effect and the interaction effect between the filler and matrix, in addition to reduction of the tortuous paths. As a result, the thermal properties and the thermal stability of the composites will be reduced correspondingly.
3.4. Maximum mass loss rate 3.4.1. Dependence of maximum mass loss rate on MWCNT content Mass loss rate in the TGA test is another important parameter for reflecting the thermal stability of materials. The mass loss rate can also be determined from the TGA curves. The maximum loss rate is defined as the mass loss rate at the maximum curvature of the TGA curves, it just corresponds to the peak of the DTG curves (see Figs. 2 and 3). It can be seen in Figs. 2 and 3 that the maximum curvature of the TGA curve of the samples is generated in temperature range between 430 C and 440 C. Fig. 7 presents the relationship between the maximum mass loss rate and the MWCNT weight fraction at 430 C. It can be seen that the values of the maximum mass loss rate decrease with increasing MWCNT weight fraction for the PP/TNIM4 system, while the values of the maximum mass loss rates of the other three PP/MWCNT composites decrease with increasing the MWCNT weight fraction for the filler content
Fig. 7. Relationship between maximum mass loss rate and MWCNT weight fraction at 430 C.
3.4.2. Effect of MWCNT size on maximum mass loss rate As stated above, the thermal stability of PP/MWCNT composites is closely related to the diameter and lengthediameter ratio of the MWCNTs, in addition to the content and the dispersion of the filler particles in the resin matrix. It can be observed from Figs. 7 and 8 that the values of the maximum mass loss rate change irregularly with the variation of the MWCNT weight fraction. It implies that the influence of the MWCNT size on the maximum mass loss rate of the PP composites is insignificant under the experimental conditions. 3.5. Residues 3.5.1. Dependence of residues on MWCNT content The amount of the residues in the composite after the TGA test can reflect both the thermal stability of the composites and the dispersion of the inclusions in the resin matrix. The amount of the residues in the composite after TGA test can be determined from the TGA curves (Fig. 2). For PP/MWCNT composites, the residues should be mainly the MWCNTs. That is
uR ¼ a þ b4f
(2)
where uR is the weight percentage of the residues; 4f is the filler weight fraction, a and b are the constants. If the residues are completely the MWCNTs from the PP/MWCNT composite specimen after the TGA test, then the parameters a and b in Equation (2) are found to be a ¼ 0, and b ¼ 1. In this case, Equation (2) can be rewritten as follows:
Fig. 8. Correlation between maximum mass loss rate and MWCNT weight fraction at 440 C.
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uR ¼ 4f
(3)
Fig. 9 illustrates the relationship between the residues of the MWCNTs in the four PP composites and the MWCNT weight fraction. It can be seen that the values of the residues increase almost linearly with increasing MWCNT weight fraction except for individual data points. Comparatively, the values of the residues deviated from the calculated values (i.e. theoretical curve of Equation (2)) for the PP/TNIM4 and PP/TNIM8 systems are higher than those for the PP/TNIM2 and PP/TNIM3 systems. 3.5.2. Effect of MWCNT size on residues The deviation of the values of the residues from the calculated values for the PP composites with larger diameter of MWCNTs is higher than that for the PP composites with smaller diameter of MWCNTs. In general, such deviation should be attributed to the following two aspects: (1) the uniformity of the MWCNTs in the PP matrix; (2) the purity of the MWCNTs and the PP resin. This is because that the number of the MWCNTs in the matrix decreases with increasing MWCNT diameter under the same filler volume fraction; thus the dependence of the values of the residues on the uniformity of the MWCNTs with larger diameter in the PP matrix is stronger than that of the MWCNTs with small diameter under the same conditions. It is extensively believed that the more uniform the dispersion of the filler in the matrix is, the smaller is the deviation of the measured data of the residues of the samples. As discussed above, the tortuous paths in composites reduce with decreasing the length to diameter ratio and specific surface area of inclusions, leading to weakened, to a certain extent, the barrier effect and the interaction effect between the filler and matrix. The sensitivity of the residues of the composites to the MWCNTs weight fraction can be enhanced correspondingly in this case. Consequently, the values of the residues deviated from the calculated values for the PP/TNIM4 and PP/TNIM8 systems are higher than those for the PP/TNIM2 and PP/TNIM3 systems. 3.6. Discussion In general, MWCNT network in the resin matrix will be generated with the filler concentration, and MWCNT is good thermal conductive meter, thus the thermal conductive properties of polymer composites will be improved in this case. However, although the formation of the MWCNT network in the resin matrix can increase the heat transfer rate of composite systems, the barrier effect
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of this hollow tube structure is enhanced at the same time due to the formation of the MWCNT network. In other words, there is a competition mechanism between the barrier effect and heat transfer effect in the PP/MWCNTs composite systems. As discussed above, the factors affecting the thermal properties and thermal stability are complicated for polymer composites. In addition to the thermal properties of the matrix resin and the filler, the thermal properties and thermal stability depend, to great extent, upon the filler content and the dispersion of the filler particles in the resin matrix [22,23]. When the dispersion of the MWCNTs in the resin matrix is uniform, the paths formed by the MWCNTs are tortuous. In this case, the thermal properties and thermal stability would be improved correspondingly, and the measured data fluctuation is relatively small. When the dispersion of the MWCNTs in the resin matrix is poor, the paths formed by the MWCNTs are simple; in this case, the thermal properties and thermal stability would be weakened correspondingly, and the measured data fluctuation is relatively obvious. However, as MWCNTs are nanometer particles, they tend to aggregate in the matrix at a high filler concentration. In this case, the thermal stability of the composites would be weakened. On the other hand, the tortuous degree of the paths formed by the MWCNTs in resin matrix can increase by increasing the lengthediameter ratio or decreasing the diameter of the MWCNTs under the same condition of the dispersion of the MWCNTs in the resin matrix. Therefore, the values of the decomposition temperature of the PP/MWCNT composites increase with decreasing filler diameter or with increasing the filler lengthediameter ratio (see Figs. 4 and 5). Lai and his colleagues [24] noted that there was a good blocking effect on the transmission of decomposition products of poly(hydroxybutyrate-co-hydroxyvalerate), when the dispersion of MWCNTs in the matrix was uniform, leading to improvement of the thermal stability of the composites. Yang and his co-workers [15] studied the thermal stability of PP/CNT composites, and obtained the similar results. Furthermore, as the specific surface area of the MWCNTs is larger and both the PP resin and MWCNTs are non-polar materials, there is a stronger physical adsorption effect on the surface between the PP matrix and the MWCNTs owing to the interaction between them. Moreover, the free radicals generated during thermal decomposition of the PP can be absorbed [25]. As a result, the thermal stability of the PP/ MWCNT composites can be improved. 4. Conclusions The effects of the size and content of the MWCNTs on the thermal stability of the filled PP composites were significant. It was found that the values of the decomposition temperature increased with increasing MWCNT weight fraction and filler lengthediameter ratio, while increased with decreasing filler diameter. The values of the residues increased approximately linearly with increasing MWCNT weight fraction. The values of maximum mass loss rate decreased roughly with increasing weight fraction of the MWCNTs, while the influence of the filler diameter and lengthediameter ratio on the maximum mass loss rate was insignificant. The improvement of the thermal stability of the PP/MWCNTs composites could be attributed to the barrier effect due to the formation of the tortuous paths of MWCNTs in the PP matrix. The findings in this study should be beneficial to the development of MWCNTs reinforced polymer composites. Acknowledgment
Fig. 9. Relationship between residues and MWCNT weight fraction.
The authors would like to thank for the support from The Research Committee of The Hong Kong Polytechnic University (Project code: G-UC81).
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