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carbon nanotube composites
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 67 (2007) 3071–3077 www.elsevier.com/locate/compscitech
Mechanical properties of high density polyethylene/carbon nanotube composites S. Kanagaraj *, Fa´tima R. Varanda, Tatiana V. Zhil’tsova, Mo´nica S.A. Oliveira, Jose´ A.O. Simo˜es Department of Mechanical Engineering, University of Aveiro, 3810 193 Aveiro, Portugal Received 16 October 2006; received in revised form 23 April 2007; accepted 30 April 2007 Available online 13 May 2007
Abstract Carbon-nanotubes (CNTs) have been used with polymers from the date of their inception to make composites having remarkable properties. An attempt has been made in this direction, in order to enhance mechanical and tribological properties of the composite materials. The latter, were achieved through the injection molding of high density polyethylene (HDPE) reinforced with specific volume fraction of CNTs. A considerable improvement on mechanical properties of the material can be observed when the volume fraction of CNT is increased. The composite reinforcement shows a good load transfer effect and interface link between CNT and HDPE. The volumetric wear rate is calculated from the Wang’s model, Ratner’s correlation and reciprocal of toughness. The results obtained clearly show the linear relationship with CNT loading which supports the microscopic wear model. It is concluded that both Halpin–Tsai and modified series model can be used to predict Young’s modulus of CNT–HDPE composites. From thermal analysis study, it is found that melting point and oxidation temperature of the composites are not affected by the addition of CNTs, however its crystallinity seems to increase. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Particle-reinforced composites; B. Mechanical properties; D. Differential scanning calorimetry; E. Injection moulding
1. Introduction Since the landmark paper by Iijima in 1991 [1], CNTs have generated huge activity in most areas of science and engineering due to their remarkable properties. The latter also highlighted their potential in the production of advanced composites with great interest for several applications. The promising area of composite research involves an enhancement of mechanical properties of polymer using CNTs as a reinforcing material. Although several studies have been focused on producing nano-composites, many practical challenges concerning their fabrication still remain, compromising the full realization of their enormous potential. Dispersing nanotubes individually and uniformly into the polymer matrix seems to be fundamental *
Corresponding author. Tel.: +351 234370830; fax: +351 234370953. E-mail address: [email protected] (S. Kanagaraj).
0266-3538/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2007.04.024
when producing composites with enhanced and reproducible properties. Therefore, for this study, HDPE is chosen due to its known excellent chemical and mechanical properties. To utilize and enhance the properties of HDPE, especially to improve its stiffness, wear resistance and rigidity, CNT–HDPE composites are prepared and the effect of CNT loading on mechanical properties is investigated. As the modulus and toughness are the most influenced properties by the crystallinity, thermal analysis of composites is also carried out. 2. Review Because of the difficulties associated with the fabrication of CNT composites, the literature reports several techniques to achieve homogeneous distribution of CNTs into the polymer matrix. Those techniques include: constant stirring [2]; surfactant-assisted processing [3]; solution-
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evaporation methods with high-energy sonication [4]; and covalent functionalization of nanotubes with a polymer matrix [5]. Gong et al. [6] reported the role of surfactants as processing aid in CNT–polymer composites and found their enormous influence on mechanical and thermal properties of composites. Ji et al. [7] have discussed the difficulties obtained to get uniform dispersion of CNTs in a polymer matrix. Zou et al. [8] found that HDPE/CNT composites fabricated at higher screw speed give uniform dispersion of CNT in HDPE. Thostenson and Chou [9] studied mechanical properties of aligned and random oriented nano-composites using DMA and observed an improvement on Young’s modulus with an addition of CNTs. Moreover, in the aligned composites, Young’s modulus could be observed five times greater than that of randomly oriented composites. Gorga and Cohen [10] found that in addition to a good dispersion of CNTs in polymer, their orientation and an interface between CNT and polymer also play a critical role to improve mechanical properties of composites. Edidin and Kurtz [11] showed that toughness of the material is strongly correlated to the wear resistance of polymers. Cadek et al. [12] studied mechanical and thermal properties of amorphous and crystalline polymers with MWCNT and observed significant improvement in their properties. Tang et al. [13] used melt processing technique to make MWNT/HDPE composite films and observed an enhancement of mechanical properties. Gorga et al. [14] prepared MWCNT–PMMA composites using melt mixing and melt drawing method and studied their mechanical properties as a function of CNT concentration and surface treatment on CNTs. Coleman et al. [15] reviewed the progress made over composite reinforcement using CNTs. 3. Experimental techniques 3.1. Materials
Fig. 1. SEM image of as-received CNTs.
and also the enhancement of load transfer, are believed to occur. The chemical treatment on CNTs was done as suggested by Esumi et al. [16]. The required sample of CNTs was suspended in the mixture of concentrated nitric acid (65%) and sulphuric acid (95–97%) by the volume ratio of 1:3 and refluxed at 140 °C for 30 min. After washing the nanotubes with deionized water until the supernatant attained a pH around 7, the samples were dried at 100 °C. Thus, the chemically treated sample was obtained. The chemically treated CNTs were formed with different functional groups like carboxyl, carbonyl, hydroxyl and others. The SEM image of the treated tubes is shown in Fig. 2. 3.3. Preparation of nanofluids The chemically treated CNTs were added with deionized water and sonicated for 1 h to have a homogeneous and uniform dispersion of CNTs in water. The colloidal stabil-
The MWCNTs were purchased from M/s Shenzhen Nanotech Port Co., Ltd., China. Fig. 1 shows the SEM image of as-received CNTs. The specifications of MWCNTs are as follows: range of diameter 60–100 nm, length of the tubes 5–15 lm, and purity P95%. The density of MWCNT is 2.16 g/cm3. The HDPE pallets were obtained from M/s The Dow Chemical Company, Portugal and their grade is HDPE KT 10000. The melt index and density of the HDPE pallets are 8 g/10 min and 0.964 g/ cm3, respectively. 3.2. Chemical treatment on CNTs The effective utilization of nanotubes in composites depends on the ability to disperse CNTs homogeneously throughout the matrix without destroying their integrity. Also, prior to the mixing of CNTs with a matrix, it is required to perform a chemical treatment on CNTs, since a better interfacial bonding between CNT and polymer
Fig. 2. SEM image of treated CNTs.
S. Kanagaraj et al. / Composites Science and Technology 67 (2007) 3071–3077
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ity of nanofluid was tested using an UV spectrophotometer and a good stability over time was observed. 3.4. Preparation of CNT–HDPE pallets The nanofluid, which was prepared with required quantity of CNTs, was mixed with HDPE pallets. This mixture was heated and magnetically stirred continuously to have a uniform coating on the HDPE pallets. Once the fluid was evaporated, these pallets were kept in an oven for 24 h at 110 °C to evaporate additional moisture present on the CNT coated pallets. 3.5. Preparation of tensile specimen The CNT coated HDPE pallets were used as a raw material in an injection moulding machine. HDPE was melted at the plasticized unit of the injection moulding machine which was kept at 200 °C to induce sufficient softening of polymer to mix with CNTs and this mixture injected into a tensile specimen. The test samples were obtained for different volume fraction of CNT in the composites: 0.11%, 0.22%, 0.33% and 0.44%. The test specimen has the basic shape of a tensile dog bone, 100 mm long, with the centre section 10 mm wide by 2 mm thick and 50 mm long.
Fig. 3. Micrograph of neck section of the sample (0.44 vol%).
3.6. Thermal analysis of composites The literature highlights the crystallinity as having a relative influence on the mechanical properties of composites, therefore thermal analysis of the CNT–HDPE sample is carried out using DSC and TGA. 3.7. Tensile test of the samples
4. Results and discussion Fig. 5 shows the stress versus strain curves for CNT– HDPE composites for a range of CNT volume fractions. It can be observed that the composite sample tends to reach the upper yield point, i.e. ultimate stress and draws through its neck where the sample extends elastically and reaches the lower yield point and then draws till it breaks. Table 1 gives the overall view of mechanical properties of nanocomposites, i.e. elastic modulus, ultimate strength, toughness, strain at fracture, wear rate and their percentage of
Fig. 4. Micrograph of fractured surface of the sample (0.33 vol%).
110 100 90 80
Stress (MPa)
Tensile test of the specimens was carried out with a Shimadzu Universal Testing Machine model AG-50kNG (M/s Shimadzu, Kyoto, Japan). The elastic modulus, ultimate strength, toughness, strain at fracture, wear rate and their percentage of increment with an addition of CNT were obtained. The tests were carried out at a cross head speed of 50 mm/min at room temperature. The micrographs of neck portion and fractured surface of the tested samples are shown in Figs. 3 and 4, respectively.
70 60 50
0.44 vol% CNT-HDPE 0.33 vol% CNT-HDPE 0.22 vol% CNT-HDPE 0.11 vol% CNT-HDPE 0.00 vol% CNT-HDPE
40 30 20 10 0 0
100
200
300
400
500
600
700
800
900
Strain (%) Fig. 5. Stress versus strain curves of CNT–HDPE composites.
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Table 1 Mechanical properties of CNT–HDPE nano-composites Volume fraction CNT concentration (%)
Young’s modulus (E) (GPa)
% Increment of E
Ultimate stress (MPa)
Strain (e) at fracture (%)
% Increment of e
Toughness (J)
% Increment of toughness
Multiplication factor – wear rate Wang (107)
Ratner (106)
(Toughness)1 (103)
0.00 0.11 0.22 0.33 0.44
1.095 1.169 1.228 1.287 1.338
0.00 6.74 12.13 17.56 22.23
105.80 105.51 106.67 106.38 109.86
863.4 948.5 978.5 1020.4 1069
0.00 9.86 13.34 18.19 23.82
634.53 743.35 756.24 776.27 842.47
0.00 17.15 19.18 22.34 32.77
10.64 9.728 9.276 8.932 8.124
10.95 9.992 9.580 9.212 8.515
1.576 1.345 1.322 1.288 1.187
increment with an addition of CNT. In each case, minimum of three samples were tested and the tabulated values are the average of these results. It can also be observed that the mechanical properties show good enhancement with an increase of CNT concentration, which is believed to be due to good interface between polymer and CNT thus transferring load from polymer to CNT. The percentage increase of Young’s modulus starts from 6.7% to 22.2% with an increase of CNT concentration and the increment is estimated to reach 100% at the volume fraction of around 2% of CNT. The increment of ultimate strength of composite is within 4% compared with that of HDPE with no addition of CNT and it reaches within 2% of fracture strain. In case of strain at fracture, it increases linearly from 863% to 1069%. The percentage increase of strain starts from about 10% to 24% with an addition of CNT. Moreover, the neck formation of composite samples starts to occur between 2% and 3% of fracture strain. This is followed by a plateau region till its breaking point which is due to strain induced crystallization of the polymer resulting into material hardening. In case of toughness, it is calculated from the area under the load versus stroke curve. It is also increasing linearly with an addition of CNT. With an increase of CNT concentration in HDPE, stiffness of the composites increased appreciably as it can be depicted from Fig. 6. The latter is believed to occur due
to the fact that CNTs tend to have more surface area per unit volume which results in good load transfer to the nanotube network. It is found that a polynomial equation gives a good relation between stiffness (GPa) and volume fraction of CNTs (%) which is given by Eq. (1) E ¼ 1:09464 þ 0:67723V CNT 0:28276V 2CNT
ð1Þ
4.1. Theoretical prediction of elastic modulus Though a great deal of theoretical work has been carried out to predict mechanical properties of fibre reinforced composites [17], some of the models are quite sophisticated to use for nano-composites. Here, a modified form of rule of mixture and Halphin–Tsai equations are used to predict Young’s modulus of CNT–HDPE composites as they used both volume fraction and aspect ratio of CNTs to predict stiffness of the composites. In case of rule of mixture, it is assumed that both CNT and HDPE are very well-bonded, equally strained and CNTs are homogeneously distributed within HDPE. The Young’s modulus of CNT and HDPE is 910 GPa [18] and 1.0935 GPa, obtained from these studies, respectively. In case of CNT–HDPE composites, it is considered as randomly oriented short fibre arrangement. Based on the modified form of rule of mixtures for series model [15,19], the composite modulus is given by EC ¼ ðgl go ECNT EHDPE ÞV CNT þ EHDPE
ð2Þ Tanhðaðl=dÞÞ , aðl=dÞ
Young´s modulus (GPa)
1.40
where gl ¼ length efficiency factor ¼ 1 a¼ ffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3EHDPE and go = orientation efficiency factor = 0.2 2ECNT lnðV CNT Þ
modified series model
1.35 Exp.values
1.30 1.25
Halphin-Tsai model
1.20
for randomly oriented fibre. Another model was reviewed by Halpin and Kardos [20] which is based on self-consistent micromechanics method and it is given by EC 3 1 þ 2ðl=dÞCNT gL V CNT 5 1 þ 2gT V CNT ¼ þ ð3Þ 8 1 gT V CNT Em 8 1 gL V CNT h i h i CNT =E PE Þ1 CNT =E PE Þ1 where gL ¼ ðECNTðE=E and gT ¼ ðE . ðECNT =EPE Þþ2 PE Þþ2ðl=dÞ CNT
1.15 1.10 0.0
0.1
0.2
0.3
0.4
Volume fraction of CNT(%) Fig. 6. Comparison of experimental and theoretical values of Young’s modulus of CNT–HDPE composites.
The above equation was derived for the randomly oriented composites. The comparison of experimental and theoretical values of stiffness is shown in Fig. 6, where a linear increase of modulus with volume fraction of CNTs is observed. The comparison between experimental and HT model gives a close agreement within 2% which indicates that the assumptions made to derive these equations were
S. Kanagaraj et al. / Composites Science and Technology 67 (2007) 3071–3077
closely agreeing with experimental conditions, like tensile loads were successfully transmitted to the nanotubes across the CNT–HDPE interface. In case of modified series equation, the experimental results are closely following the predicted values within the maximum relative error of 4%. However, the relative error increases with CNT concentration but it is well within agreeable limits. Thus, it is suggested that Halphin–Tsai model and modified series model can be used to predict Young’s modulus of CNT– HDPE composites. In both cases, the optimum value of aspect ratio used was 86, which was obtained from HT model. Though, the length of CNTs used in this study varies from 5 to 15 lm, and it is believed to be changed due to chemical treatment, ultrasonication and also during mixing with polymer. A linear increase of modulus with volume fraction, as shown in Fig. 6, suggests that a good measure of reinforcement is given by dEC/dVCNT [15] at low volume fraction. This takes into account both magnitude of the stiffness increase and the amount of CNTs required to achieve it, which can be extracted from the experimental data. The term dEC/dVCNT is used to represent the magnitude of the reinforcement effect. In case of rule of mixture, the reinforcement effect is given by dEC ¼ ðgl go ECNT EHDPE Þ dV CNT
ð4Þ
and its value is 75 GPa at (l/d) = 86. In case of Halphin– Tsai model, the reinforcement effect is given by dEC 3EHDPE gL ð1 þ 2l=dÞ 15EHDPE gT ¼ þ 2 dV CNT 8ð1 gT V CNT Þ2 8ð1 gL V CNT Þ
ð5Þ
and the measure of reinforcement is 60.87 GPa, 61.00 GPa, 61.10 GPa and 61.21 GPa for the CNT volume fraction of 0.11%, 0.22%, 0.33% and 0.44%, respectively at the aspect ratio of 86. The measure of reinforcement increases with an increase of CNT concentration, which indicates that there is a good load transfer effect and an interface link between CNTs and HDPE. From the results of Ruan et al. [21], the measure of reinforcement for CNT–HDPE composites at 1 wt% comes about 57 GPa which has very close agreement with the results obtained in this study.
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where DV – volumetric wear rate (mm3 per million cycle), ru – ultimate tensile strength (MPa) and eu – elongation at tensile fracture of test sample. In case of experimental variables to calculate volumetric wear rate, the Wang’s model and Ratner’s correlation both insist on the importance of ultimate strength and strain at tensile fracture. These properties increase with an addition of CNT in the polymer composites. Thus, the volumetric wear rate decreases with the addition of CNT. It can also be observed that wear volume rate and the reciprocal of toughness show the excellent linear relationship which supports the theoretical prediction of the microscopic wear model [22]. Table 1 shows the reciprocal of toughness and the variable multiplication factors which were calculated from the experimental results. The requirements for mechanical reinforcement in any nano-composites are the optimum aspect ratio, good dispersion, alignment of CNT and interfacial stress transfer between CNT and polymer. The effect of aspect ratio on Young’s modulus is dealt with the earlier section with reference to HT model and modified series model. If the aspect ratio is high, Young’s modulus will also be high. In case of homogenous dispersion, nanotubes must be uniformly dispersed to the level of isolated nanotubes individually coated with polymer which has an efficient load transfer to the CNT network and results in a more uniform stress distribution and minimise the stress concentration area. If any agglomeration of CNTs occurs in the polymer, it leads to decrease in strength and modulus of the composites. In case of alignment, the difference between random orientation and perfect alignment is a factor of five in composite modulus [9]. However, the composite becomes an anisotropic material which is not beneficial in case of bulk materials. In case of interfacial stress transfer, the external stresses applied to the composite are efficiently transferred to the nanotubes, allowing them to take a disproportionate share of the load resulting in high performance in composites. At some point of stress, either the matrix in the vicinity of the interface or the interface bond between CNT and matrix will rupture. This stress is termed as interfacial stress which governs the maximum stress transfer to the CNT. 4.3. Thermal analysis of composites
4.2. Theoretical prediction of volumetric wear rate From the following models, it is observed that toughness of the material is the most important property in order to determine the volumetric wear rate. The latter, can be observed to decrease with increasing of the product of ultimate tensile strength and breaking strain. Wang’s model [22]: DV / ru3=2 e1 u
ð6Þ
Ratner–Lancaster’s correlation: DV / ðru eu Þ
1
ð7Þ
As the crystallinity of the polymer composites is having influence on Young’s modulus and toughness, it was decided to study thermal analysis of the composites, i.e. DSC and TGA. In the case of DSC curves, which are shown in Fig. 7, the addition of CNT in HDPE increases the total enthalpy of crystallization. The melting point of composites occurred at about 137 °C indicating the degree of polymer crystallinity in all composites. The reinforcement of CNTs in HDPE is not affected the melting point of the composites. The enthalpy of crystallization is 105.12 J/g, 110.33 J/g, 113.69 J/g, and 167.38 J/g for 0.00%, 0.11%, 0.22% and 0.44% volume fraction of CNTs,
S. Kanagaraj et al. / Composites Science and Technology 67 (2007) 3071–3077 0.44%CNT
Heat flow (mW)
0.22wt%CNT
10
0
0.11%CNT
-10 HDPE
-20
0
100
200
300
400
Temperature (°C) Fig. 7. DSC curves of CNT–HDPE composites.
respectively where 100% pure crystalline HDPE polymer gives an enthalpy of 293 J/g, from literature. The nucleation and growth of individual crystallites are so rapid that a very large number of crystallites are formed together thus the large amount of heat is liberated during the crystallization of HDPE by adding CNTs. It is observed that crystallinity of composite increases appreciably with an addition of CNTs. It shows that CNT act as nucleation sites for the crystallization of polymer which is also supported by Coleman et al. [23] and Assouline et al. [24]. An appreciable increase of crystallinity of polymer suggests that CNT surface has a crystalline polymer coating with an average thickness that is constant for each nanotube which shows that all samples used in this study have a similar polymer–nanotube interface. Nanotubes not only nucleate polymer crystallization, but can also be used to propagate the crystallization for a large distance from their surface. This shows that nanotubes interact with a crystalline polymer surface at the interface. An ordered interface will result in a virtually complete polymer coating and thus the total polymer–nanotube binding energy will be maximized which results in the maximization of interfacial stress transfer. Coleman et al. [23] suggested that the presence of interface crystallinity has a large bearing on the mechanical properties of nano-composites. It may be noted that a small difference in the measurement of reinforcement may be related to the presence of crystalline interface which indicates the possibility that stress transfer may be maximized by the presence of ordered interface as suggested by Frankland et al. [25]. They also suggested that load transfer and hence modulus of nanotube–polymer composites can be effectively increased by deliberately adding chemical bonding between nanotube and polymer during processing which is in part responsible for the enhanced stress transfer. Coleman et al. [15] concluded that chemically modified tubes show the best results, which is due to the fact that functionalization significantly improve both dispersion of CNTs in polymer and thus stress transfer.
In case of TGA curves which are shown in Fig. 8, thermal stabilization of CNT–HDPE composites and the fraction of volatile components are observed with an atmosphere of air. It is observed that the onset temperatures of these composites are 395 °C, 355 °C, 327 °C and 316 °C for pure HDPE, 0.11%, 0.22% and 0.44% CNT, respectively. It is observed that onset temperature decreases with an addition of chemically treated CNT due to amorphous carbon present in the CNTs and other carbonaceous impurities that oxidize at temperature lower than that of CNTs. The oxidation temperature of composites, thermal stability of the composites, is found by differentiating the percentage of weight loss curve with temperature which is shown in Fig. 9. The temperatures at which the maximum rate of oxidation is taking place are 483 °C, 482 °C, 482 °C and 481 °C for 0.44%, 0.22%, 0.11% and HDPE, respectively. It is observed that the oxidation temperature of the composite is not much affected by the addition of CNT, which may be due to very low fraction of CNT in the composites. The secondary additional peaks observed in Fig. 9 are due to the functional groups attached in the nanotubes. 10
100% HDPE
0 -10
Weight loss (%)
20
0.11%CNT
0.44%CNT
-20
0.22%CNT
-30 -40 -50 -60 -70 -80 -90 -100 0
100
200
300
400
500
600
700
Temperature (°C) Fig. 8. TGA curves of CNT–HDPE composites.
0.5 0.0 -0.5
d(weight loss)/dT
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-1.0
0.44% CNT 0.22% CNT 0.11% CNT
-1.5 -2.0 -2.5 -3.0 -3.5 -4.0
HDPE
-4.5 -5.0 100
200
300
400
500
600
Temperature (°C) Fig. 9. d(TGA)/dT curves of CNT–HDPE composites.
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5. Conclusions The following conclusions are derived from the above reported studies: 1. CNT–HDPE composites show a good enhancement of mechanical properties with an increase of CNT concentration. 2. Halpin–Tsai model and modified series model give a close agreement to calculate Young’s modulus of CNT–HDPE composites. 3. The measure of reinforcement increases with an increase of CNT because of good load transfer effect and interface link between CNT and polymer. 4. Volumetric wear rate and the reciprocal of toughness show the excellent linear relationship which supports the theoretical prediction of the microscopic wear model. 5. Melting point and oxidation temperature of CNT– HDPE composites are not affected with an addition of CNT but crystallinity of composites increases. Acknowledgements Authors gratefully acknowledge the FCT for funding through the individual grant SFRH/BPD/19213/2004 and the project POCI/EME/56040/2004. References [1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56–8. [2] Sandler J, Shaffer MSP, Prasse T, Bauhofer W, Schulte K, Windle AH. Development of a dispersion process for carbon nanotubes in an epoxy matrix and the resulting electrical properties. Polymer 1999;40:5967–71. [3] Shaffer MSP, Windle AH. Fabrication and characterization of carbon nanotube/poly (vinyl alcohol) composites. Adv Mater 1999;11(11):937–41. [4] Qian D, Dickey EC, Andrews R, Rantell T. Load transfer and deformation mechanisms in carbon nanotube–polystyrene composites. Appl Phys Lett 2000;76(20):2868–70. [5] Bratcher M, Gersten B, Ji H, Mays J. Study in the dispersion of carbon nanotubes. Mater Res Soc Proc 2001;706:Z9.29. [6] Gong X, Liu J, Baskaran S, Voise RD, Young JS. Surfactant-assisted processing of carbon nanotube/polymer composites. Chem Mater 2000;12(4):1049–52. [7] Ji XL, Jing JK, Jiang W, Jiang BZ. Tensile modulus of polymer nanocomposites. Polym Eng Sci 2002;42(5):983–93.
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[8] Zou Y, Feng Y, Wang L, Liu X. Processing and properties of MWNT/HDPE composites. Carbon 2004;42:271–7. [9] Thostenson ET, Chou TW. Aligned multi-walled carbon nanotubereinforced composites: processing and mechanical characterization. J Phys D: Appl Phys 2002;35:L77–80. [10] Gorga RE, Cohen RE. Toughness enhancements in poly (methyl methacrylate) by addition of oriented multiwall carbon nanotubes. J Polym Sci, Part B: Polym Phys 2004;42:2690–702. [11] Edidin AA, Kurtz SM. Development of validation of the small punch test for UHMWPE used in total joint replacements. Funct Biomater 2001;198:1–40. [12] Cadek M, Coleman JN, Barron V, Hedicke K, Blau WJ. Morphological and mechanical properties of carbon-nanotube-reinforced semicrystalline and amorphous composites. Appl Phys Lett 2002;81(27):5123–5. [13] Tang W, Santare MH, Advani SG. Melt processing and mechanical property characterization of multi-walled carbon nanotube/high density polyethylene (MWNT/HDPE) composite films. Carbon 2003;41:2779–85. [14] Gorga RE, Lau KKS, Gleason KK, Cohen RE. The importance of interfacial design at the carbon nanotube/polymer composite interface. J Appl Polym Sci 2006;102:1413–8. [15] Coleman JN, Khan U, Blau WJ, Gun’ko YK. Small but strong: A review of the mechanical properties of carbon nanotube—polymer composites. Carbon 2006;44:1624–52. [16] Esumi K, Ishigami M, Nakajima A, Sawada K, Honda H. Chemical treatment of carbon nanotubes. Carbon 1996;34:279–81. [17] Tucker CL, Liang E. Stiffness predictions for unidirectional shortfiber composites: review and evaluation. Compos Sci Technol 1999;59(5):655–71. [18] Demczyk BG, Wang YM, Cumings J, Hetman M, Han W, Zettl A, et al. Direct mechanical measurement of the tensile strength and elastic modulus of multiwalled carbon nanotubes. Mater Sci Eng A 2002;334:173–8. [19] Callister WD. Materials science and engineering, An introduction. New York: Wiley; 2003. [20] Halpin JC, Kardos JL. The Halpin–Tsai equations: a review. Polym Eng Sci 1976;16:344–52. [21] Ruan SL, Gao P, Yang XG, Yu TX. Toughening high performance ultrahigh molecular weight polyethylene using multiwalled carbon nanotubes. Polymer 2003;44:5643–54. [22] Wang A, Sun DC, Stark C, Dumbleton JH. Wear mechanisms of UHMWPE in total joint replacements. Wear 1995;181– 183:241–9. [23] Coleman JN, Cadek M, Blake R, Nicolosi V, Ryan KP, Belton C, et al. High performance nanotube-reinforced plastics: Understanding the mechanism of strength increase. Adv Funct Mater 2004;14:791–8. [24] Assouline E, Lustiger A, Barber AH, Cooper CA, Klein E, Wachtel E, et al. Nucleation ability of multiwalled carbon nanotubes in polypropylene composites. J Polym Sci, Part B 2003;41:520–7. [25] Frankland SJV, Caglar A, Brenner DW, Griebel M. Molecular simulation of the influence of chemical cross-links on the shear strength of carbon nanotube–polymer interface. J Phys Chem B 2002;106:3046–8.
Mechanical properties of high density polyethylene/carbon nanotube composites S. Kanagaraj *, Fa´tima R. Varanda, Tatiana V. Zhil’tsova, Mo´nica S.A. Oliveira, Jose´ A.O. Simo˜es Department of Mechanical Engineering, University of Aveiro, 3810 193 Aveiro, Portugal Received 16 October 2006; received in revised form 23 April 2007; accepted 30 April 2007 Available online 13 May 2007
Abstract Carbon-nanotubes (CNTs) have been used with polymers from the date of their inception to make composites having remarkable properties. An attempt has been made in this direction, in order to enhance mechanical and tribological properties of the composite materials. The latter, were achieved through the injection molding of high density polyethylene (HDPE) reinforced with specific volume fraction of CNTs. A considerable improvement on mechanical properties of the material can be observed when the volume fraction of CNT is increased. The composite reinforcement shows a good load transfer effect and interface link between CNT and HDPE. The volumetric wear rate is calculated from the Wang’s model, Ratner’s correlation and reciprocal of toughness. The results obtained clearly show the linear relationship with CNT loading which supports the microscopic wear model. It is concluded that both Halpin–Tsai and modified series model can be used to predict Young’s modulus of CNT–HDPE composites. From thermal analysis study, it is found that melting point and oxidation temperature of the composites are not affected by the addition of CNTs, however its crystallinity seems to increase. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Particle-reinforced composites; B. Mechanical properties; D. Differential scanning calorimetry; E. Injection moulding
1. Introduction Since the landmark paper by Iijima in 1991 [1], CNTs have generated huge activity in most areas of science and engineering due to their remarkable properties. The latter also highlighted their potential in the production of advanced composites with great interest for several applications. The promising area of composite research involves an enhancement of mechanical properties of polymer using CNTs as a reinforcing material. Although several studies have been focused on producing nano-composites, many practical challenges concerning their fabrication still remain, compromising the full realization of their enormous potential. Dispersing nanotubes individually and uniformly into the polymer matrix seems to be fundamental *
Corresponding author. Tel.: +351 234370830; fax: +351 234370953. E-mail address: [email protected] (S. Kanagaraj).
0266-3538/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2007.04.024
when producing composites with enhanced and reproducible properties. Therefore, for this study, HDPE is chosen due to its known excellent chemical and mechanical properties. To utilize and enhance the properties of HDPE, especially to improve its stiffness, wear resistance and rigidity, CNT–HDPE composites are prepared and the effect of CNT loading on mechanical properties is investigated. As the modulus and toughness are the most influenced properties by the crystallinity, thermal analysis of composites is also carried out. 2. Review Because of the difficulties associated with the fabrication of CNT composites, the literature reports several techniques to achieve homogeneous distribution of CNTs into the polymer matrix. Those techniques include: constant stirring [2]; surfactant-assisted processing [3]; solution-
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evaporation methods with high-energy sonication [4]; and covalent functionalization of nanotubes with a polymer matrix [5]. Gong et al. [6] reported the role of surfactants as processing aid in CNT–polymer composites and found their enormous influence on mechanical and thermal properties of composites. Ji et al. [7] have discussed the difficulties obtained to get uniform dispersion of CNTs in a polymer matrix. Zou et al. [8] found that HDPE/CNT composites fabricated at higher screw speed give uniform dispersion of CNT in HDPE. Thostenson and Chou [9] studied mechanical properties of aligned and random oriented nano-composites using DMA and observed an improvement on Young’s modulus with an addition of CNTs. Moreover, in the aligned composites, Young’s modulus could be observed five times greater than that of randomly oriented composites. Gorga and Cohen [10] found that in addition to a good dispersion of CNTs in polymer, their orientation and an interface between CNT and polymer also play a critical role to improve mechanical properties of composites. Edidin and Kurtz [11] showed that toughness of the material is strongly correlated to the wear resistance of polymers. Cadek et al. [12] studied mechanical and thermal properties of amorphous and crystalline polymers with MWCNT and observed significant improvement in their properties. Tang et al. [13] used melt processing technique to make MWNT/HDPE composite films and observed an enhancement of mechanical properties. Gorga et al. [14] prepared MWCNT–PMMA composites using melt mixing and melt drawing method and studied their mechanical properties as a function of CNT concentration and surface treatment on CNTs. Coleman et al. [15] reviewed the progress made over composite reinforcement using CNTs. 3. Experimental techniques 3.1. Materials
Fig. 1. SEM image of as-received CNTs.
and also the enhancement of load transfer, are believed to occur. The chemical treatment on CNTs was done as suggested by Esumi et al. [16]. The required sample of CNTs was suspended in the mixture of concentrated nitric acid (65%) and sulphuric acid (95–97%) by the volume ratio of 1:3 and refluxed at 140 °C for 30 min. After washing the nanotubes with deionized water until the supernatant attained a pH around 7, the samples were dried at 100 °C. Thus, the chemically treated sample was obtained. The chemically treated CNTs were formed with different functional groups like carboxyl, carbonyl, hydroxyl and others. The SEM image of the treated tubes is shown in Fig. 2. 3.3. Preparation of nanofluids The chemically treated CNTs were added with deionized water and sonicated for 1 h to have a homogeneous and uniform dispersion of CNTs in water. The colloidal stabil-
The MWCNTs were purchased from M/s Shenzhen Nanotech Port Co., Ltd., China. Fig. 1 shows the SEM image of as-received CNTs. The specifications of MWCNTs are as follows: range of diameter 60–100 nm, length of the tubes 5–15 lm, and purity P95%. The density of MWCNT is 2.16 g/cm3. The HDPE pallets were obtained from M/s The Dow Chemical Company, Portugal and their grade is HDPE KT 10000. The melt index and density of the HDPE pallets are 8 g/10 min and 0.964 g/ cm3, respectively. 3.2. Chemical treatment on CNTs The effective utilization of nanotubes in composites depends on the ability to disperse CNTs homogeneously throughout the matrix without destroying their integrity. Also, prior to the mixing of CNTs with a matrix, it is required to perform a chemical treatment on CNTs, since a better interfacial bonding between CNT and polymer
Fig. 2. SEM image of treated CNTs.
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ity of nanofluid was tested using an UV spectrophotometer and a good stability over time was observed. 3.4. Preparation of CNT–HDPE pallets The nanofluid, which was prepared with required quantity of CNTs, was mixed with HDPE pallets. This mixture was heated and magnetically stirred continuously to have a uniform coating on the HDPE pallets. Once the fluid was evaporated, these pallets were kept in an oven for 24 h at 110 °C to evaporate additional moisture present on the CNT coated pallets. 3.5. Preparation of tensile specimen The CNT coated HDPE pallets were used as a raw material in an injection moulding machine. HDPE was melted at the plasticized unit of the injection moulding machine which was kept at 200 °C to induce sufficient softening of polymer to mix with CNTs and this mixture injected into a tensile specimen. The test samples were obtained for different volume fraction of CNT in the composites: 0.11%, 0.22%, 0.33% and 0.44%. The test specimen has the basic shape of a tensile dog bone, 100 mm long, with the centre section 10 mm wide by 2 mm thick and 50 mm long.
Fig. 3. Micrograph of neck section of the sample (0.44 vol%).
3.6. Thermal analysis of composites The literature highlights the crystallinity as having a relative influence on the mechanical properties of composites, therefore thermal analysis of the CNT–HDPE sample is carried out using DSC and TGA. 3.7. Tensile test of the samples
4. Results and discussion Fig. 5 shows the stress versus strain curves for CNT– HDPE composites for a range of CNT volume fractions. It can be observed that the composite sample tends to reach the upper yield point, i.e. ultimate stress and draws through its neck where the sample extends elastically and reaches the lower yield point and then draws till it breaks. Table 1 gives the overall view of mechanical properties of nanocomposites, i.e. elastic modulus, ultimate strength, toughness, strain at fracture, wear rate and their percentage of
Fig. 4. Micrograph of fractured surface of the sample (0.33 vol%).
110 100 90 80
Stress (MPa)
Tensile test of the specimens was carried out with a Shimadzu Universal Testing Machine model AG-50kNG (M/s Shimadzu, Kyoto, Japan). The elastic modulus, ultimate strength, toughness, strain at fracture, wear rate and their percentage of increment with an addition of CNT were obtained. The tests were carried out at a cross head speed of 50 mm/min at room temperature. The micrographs of neck portion and fractured surface of the tested samples are shown in Figs. 3 and 4, respectively.
70 60 50
0.44 vol% CNT-HDPE 0.33 vol% CNT-HDPE 0.22 vol% CNT-HDPE 0.11 vol% CNT-HDPE 0.00 vol% CNT-HDPE
40 30 20 10 0 0
100
200
300
400
500
600
700
800
900
Strain (%) Fig. 5. Stress versus strain curves of CNT–HDPE composites.
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Table 1 Mechanical properties of CNT–HDPE nano-composites Volume fraction CNT concentration (%)
Young’s modulus (E) (GPa)
% Increment of E
Ultimate stress (MPa)
Strain (e) at fracture (%)
% Increment of e
Toughness (J)
% Increment of toughness
Multiplication factor – wear rate Wang (107)
Ratner (106)
(Toughness)1 (103)
0.00 0.11 0.22 0.33 0.44
1.095 1.169 1.228 1.287 1.338
0.00 6.74 12.13 17.56 22.23
105.80 105.51 106.67 106.38 109.86
863.4 948.5 978.5 1020.4 1069
0.00 9.86 13.34 18.19 23.82
634.53 743.35 756.24 776.27 842.47
0.00 17.15 19.18 22.34 32.77
10.64 9.728 9.276 8.932 8.124
10.95 9.992 9.580 9.212 8.515
1.576 1.345 1.322 1.288 1.187
increment with an addition of CNT. In each case, minimum of three samples were tested and the tabulated values are the average of these results. It can also be observed that the mechanical properties show good enhancement with an increase of CNT concentration, which is believed to be due to good interface between polymer and CNT thus transferring load from polymer to CNT. The percentage increase of Young’s modulus starts from 6.7% to 22.2% with an increase of CNT concentration and the increment is estimated to reach 100% at the volume fraction of around 2% of CNT. The increment of ultimate strength of composite is within 4% compared with that of HDPE with no addition of CNT and it reaches within 2% of fracture strain. In case of strain at fracture, it increases linearly from 863% to 1069%. The percentage increase of strain starts from about 10% to 24% with an addition of CNT. Moreover, the neck formation of composite samples starts to occur between 2% and 3% of fracture strain. This is followed by a plateau region till its breaking point which is due to strain induced crystallization of the polymer resulting into material hardening. In case of toughness, it is calculated from the area under the load versus stroke curve. It is also increasing linearly with an addition of CNT. With an increase of CNT concentration in HDPE, stiffness of the composites increased appreciably as it can be depicted from Fig. 6. The latter is believed to occur due
to the fact that CNTs tend to have more surface area per unit volume which results in good load transfer to the nanotube network. It is found that a polynomial equation gives a good relation between stiffness (GPa) and volume fraction of CNTs (%) which is given by Eq. (1) E ¼ 1:09464 þ 0:67723V CNT 0:28276V 2CNT
ð1Þ
4.1. Theoretical prediction of elastic modulus Though a great deal of theoretical work has been carried out to predict mechanical properties of fibre reinforced composites [17], some of the models are quite sophisticated to use for nano-composites. Here, a modified form of rule of mixture and Halphin–Tsai equations are used to predict Young’s modulus of CNT–HDPE composites as they used both volume fraction and aspect ratio of CNTs to predict stiffness of the composites. In case of rule of mixture, it is assumed that both CNT and HDPE are very well-bonded, equally strained and CNTs are homogeneously distributed within HDPE. The Young’s modulus of CNT and HDPE is 910 GPa [18] and 1.0935 GPa, obtained from these studies, respectively. In case of CNT–HDPE composites, it is considered as randomly oriented short fibre arrangement. Based on the modified form of rule of mixtures for series model [15,19], the composite modulus is given by EC ¼ ðgl go ECNT EHDPE ÞV CNT þ EHDPE
ð2Þ Tanhðaðl=dÞÞ , aðl=dÞ
Young´s modulus (GPa)
1.40
where gl ¼ length efficiency factor ¼ 1 a¼ ffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3EHDPE and go = orientation efficiency factor = 0.2 2ECNT lnðV CNT Þ
modified series model
1.35 Exp.values
1.30 1.25
Halphin-Tsai model
1.20
for randomly oriented fibre. Another model was reviewed by Halpin and Kardos [20] which is based on self-consistent micromechanics method and it is given by EC 3 1 þ 2ðl=dÞCNT gL V CNT 5 1 þ 2gT V CNT ¼ þ ð3Þ 8 1 gT V CNT Em 8 1 gL V CNT h i h i CNT =E PE Þ1 CNT =E PE Þ1 where gL ¼ ðECNTðE=E and gT ¼ ðE . ðECNT =EPE Þþ2 PE Þþ2ðl=dÞ CNT
1.15 1.10 0.0
0.1
0.2
0.3
0.4
Volume fraction of CNT(%) Fig. 6. Comparison of experimental and theoretical values of Young’s modulus of CNT–HDPE composites.
The above equation was derived for the randomly oriented composites. The comparison of experimental and theoretical values of stiffness is shown in Fig. 6, where a linear increase of modulus with volume fraction of CNTs is observed. The comparison between experimental and HT model gives a close agreement within 2% which indicates that the assumptions made to derive these equations were
S. Kanagaraj et al. / Composites Science and Technology 67 (2007) 3071–3077
closely agreeing with experimental conditions, like tensile loads were successfully transmitted to the nanotubes across the CNT–HDPE interface. In case of modified series equation, the experimental results are closely following the predicted values within the maximum relative error of 4%. However, the relative error increases with CNT concentration but it is well within agreeable limits. Thus, it is suggested that Halphin–Tsai model and modified series model can be used to predict Young’s modulus of CNT– HDPE composites. In both cases, the optimum value of aspect ratio used was 86, which was obtained from HT model. Though, the length of CNTs used in this study varies from 5 to 15 lm, and it is believed to be changed due to chemical treatment, ultrasonication and also during mixing with polymer. A linear increase of modulus with volume fraction, as shown in Fig. 6, suggests that a good measure of reinforcement is given by dEC/dVCNT [15] at low volume fraction. This takes into account both magnitude of the stiffness increase and the amount of CNTs required to achieve it, which can be extracted from the experimental data. The term dEC/dVCNT is used to represent the magnitude of the reinforcement effect. In case of rule of mixture, the reinforcement effect is given by dEC ¼ ðgl go ECNT EHDPE Þ dV CNT
ð4Þ
and its value is 75 GPa at (l/d) = 86. In case of Halphin– Tsai model, the reinforcement effect is given by dEC 3EHDPE gL ð1 þ 2l=dÞ 15EHDPE gT ¼ þ 2 dV CNT 8ð1 gT V CNT Þ2 8ð1 gL V CNT Þ
ð5Þ
and the measure of reinforcement is 60.87 GPa, 61.00 GPa, 61.10 GPa and 61.21 GPa for the CNT volume fraction of 0.11%, 0.22%, 0.33% and 0.44%, respectively at the aspect ratio of 86. The measure of reinforcement increases with an increase of CNT concentration, which indicates that there is a good load transfer effect and an interface link between CNTs and HDPE. From the results of Ruan et al. [21], the measure of reinforcement for CNT–HDPE composites at 1 wt% comes about 57 GPa which has very close agreement with the results obtained in this study.
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where DV – volumetric wear rate (mm3 per million cycle), ru – ultimate tensile strength (MPa) and eu – elongation at tensile fracture of test sample. In case of experimental variables to calculate volumetric wear rate, the Wang’s model and Ratner’s correlation both insist on the importance of ultimate strength and strain at tensile fracture. These properties increase with an addition of CNT in the polymer composites. Thus, the volumetric wear rate decreases with the addition of CNT. It can also be observed that wear volume rate and the reciprocal of toughness show the excellent linear relationship which supports the theoretical prediction of the microscopic wear model [22]. Table 1 shows the reciprocal of toughness and the variable multiplication factors which were calculated from the experimental results. The requirements for mechanical reinforcement in any nano-composites are the optimum aspect ratio, good dispersion, alignment of CNT and interfacial stress transfer between CNT and polymer. The effect of aspect ratio on Young’s modulus is dealt with the earlier section with reference to HT model and modified series model. If the aspect ratio is high, Young’s modulus will also be high. In case of homogenous dispersion, nanotubes must be uniformly dispersed to the level of isolated nanotubes individually coated with polymer which has an efficient load transfer to the CNT network and results in a more uniform stress distribution and minimise the stress concentration area. If any agglomeration of CNTs occurs in the polymer, it leads to decrease in strength and modulus of the composites. In case of alignment, the difference between random orientation and perfect alignment is a factor of five in composite modulus [9]. However, the composite becomes an anisotropic material which is not beneficial in case of bulk materials. In case of interfacial stress transfer, the external stresses applied to the composite are efficiently transferred to the nanotubes, allowing them to take a disproportionate share of the load resulting in high performance in composites. At some point of stress, either the matrix in the vicinity of the interface or the interface bond between CNT and matrix will rupture. This stress is termed as interfacial stress which governs the maximum stress transfer to the CNT. 4.3. Thermal analysis of composites
4.2. Theoretical prediction of volumetric wear rate From the following models, it is observed that toughness of the material is the most important property in order to determine the volumetric wear rate. The latter, can be observed to decrease with increasing of the product of ultimate tensile strength and breaking strain. Wang’s model [22]: DV / ru3=2 e1 u
ð6Þ
Ratner–Lancaster’s correlation: DV / ðru eu Þ
1
ð7Þ
As the crystallinity of the polymer composites is having influence on Young’s modulus and toughness, it was decided to study thermal analysis of the composites, i.e. DSC and TGA. In the case of DSC curves, which are shown in Fig. 7, the addition of CNT in HDPE increases the total enthalpy of crystallization. The melting point of composites occurred at about 137 °C indicating the degree of polymer crystallinity in all composites. The reinforcement of CNTs in HDPE is not affected the melting point of the composites. The enthalpy of crystallization is 105.12 J/g, 110.33 J/g, 113.69 J/g, and 167.38 J/g for 0.00%, 0.11%, 0.22% and 0.44% volume fraction of CNTs,
S. Kanagaraj et al. / Composites Science and Technology 67 (2007) 3071–3077 0.44%CNT
Heat flow (mW)
0.22wt%CNT
10
0
0.11%CNT
-10 HDPE
-20
0
100
200
300
400
Temperature (°C) Fig. 7. DSC curves of CNT–HDPE composites.
respectively where 100% pure crystalline HDPE polymer gives an enthalpy of 293 J/g, from literature. The nucleation and growth of individual crystallites are so rapid that a very large number of crystallites are formed together thus the large amount of heat is liberated during the crystallization of HDPE by adding CNTs. It is observed that crystallinity of composite increases appreciably with an addition of CNTs. It shows that CNT act as nucleation sites for the crystallization of polymer which is also supported by Coleman et al. [23] and Assouline et al. [24]. An appreciable increase of crystallinity of polymer suggests that CNT surface has a crystalline polymer coating with an average thickness that is constant for each nanotube which shows that all samples used in this study have a similar polymer–nanotube interface. Nanotubes not only nucleate polymer crystallization, but can also be used to propagate the crystallization for a large distance from their surface. This shows that nanotubes interact with a crystalline polymer surface at the interface. An ordered interface will result in a virtually complete polymer coating and thus the total polymer–nanotube binding energy will be maximized which results in the maximization of interfacial stress transfer. Coleman et al. [23] suggested that the presence of interface crystallinity has a large bearing on the mechanical properties of nano-composites. It may be noted that a small difference in the measurement of reinforcement may be related to the presence of crystalline interface which indicates the possibility that stress transfer may be maximized by the presence of ordered interface as suggested by Frankland et al. [25]. They also suggested that load transfer and hence modulus of nanotube–polymer composites can be effectively increased by deliberately adding chemical bonding between nanotube and polymer during processing which is in part responsible for the enhanced stress transfer. Coleman et al. [15] concluded that chemically modified tubes show the best results, which is due to the fact that functionalization significantly improve both dispersion of CNTs in polymer and thus stress transfer.
In case of TGA curves which are shown in Fig. 8, thermal stabilization of CNT–HDPE composites and the fraction of volatile components are observed with an atmosphere of air. It is observed that the onset temperatures of these composites are 395 °C, 355 °C, 327 °C and 316 °C for pure HDPE, 0.11%, 0.22% and 0.44% CNT, respectively. It is observed that onset temperature decreases with an addition of chemically treated CNT due to amorphous carbon present in the CNTs and other carbonaceous impurities that oxidize at temperature lower than that of CNTs. The oxidation temperature of composites, thermal stability of the composites, is found by differentiating the percentage of weight loss curve with temperature which is shown in Fig. 9. The temperatures at which the maximum rate of oxidation is taking place are 483 °C, 482 °C, 482 °C and 481 °C for 0.44%, 0.22%, 0.11% and HDPE, respectively. It is observed that the oxidation temperature of the composite is not much affected by the addition of CNT, which may be due to very low fraction of CNT in the composites. The secondary additional peaks observed in Fig. 9 are due to the functional groups attached in the nanotubes. 10
100% HDPE
0 -10
Weight loss (%)
20
0.11%CNT
0.44%CNT
-20
0.22%CNT
-30 -40 -50 -60 -70 -80 -90 -100 0
100
200
300
400
500
600
700
Temperature (°C) Fig. 8. TGA curves of CNT–HDPE composites.
0.5 0.0 -0.5
d(weight loss)/dT
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-1.0
0.44% CNT 0.22% CNT 0.11% CNT
-1.5 -2.0 -2.5 -3.0 -3.5 -4.0
HDPE
-4.5 -5.0 100
200
300
400
500
600
Temperature (°C) Fig. 9. d(TGA)/dT curves of CNT–HDPE composites.
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5. Conclusions The following conclusions are derived from the above reported studies: 1. CNT–HDPE composites show a good enhancement of mechanical properties with an increase of CNT concentration. 2. Halpin–Tsai model and modified series model give a close agreement to calculate Young’s modulus of CNT–HDPE composites. 3. The measure of reinforcement increases with an increase of CNT because of good load transfer effect and interface link between CNT and polymer. 4. Volumetric wear rate and the reciprocal of toughness show the excellent linear relationship which supports the theoretical prediction of the microscopic wear model. 5. Melting point and oxidation temperature of CNT– HDPE composites are not affected with an addition of CNT but crystallinity of composites increases. Acknowledgements Authors gratefully acknowledge the FCT for funding through the individual grant SFRH/BPD/19213/2004 and the project POCI/EME/56040/2004. References [1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56–8. [2] Sandler J, Shaffer MSP, Prasse T, Bauhofer W, Schulte K, Windle AH. Development of a dispersion process for carbon nanotubes in an epoxy matrix and the resulting electrical properties. Polymer 1999;40:5967–71. [3] Shaffer MSP, Windle AH. Fabrication and characterization of carbon nanotube/poly (vinyl alcohol) composites. Adv Mater 1999;11(11):937–41. [4] Qian D, Dickey EC, Andrews R, Rantell T. Load transfer and deformation mechanisms in carbon nanotube–polystyrene composites. Appl Phys Lett 2000;76(20):2868–70. [5] Bratcher M, Gersten B, Ji H, Mays J. Study in the dispersion of carbon nanotubes. Mater Res Soc Proc 2001;706:Z9.29. [6] Gong X, Liu J, Baskaran S, Voise RD, Young JS. Surfactant-assisted processing of carbon nanotube/polymer composites. Chem Mater 2000;12(4):1049–52. [7] Ji XL, Jing JK, Jiang W, Jiang BZ. Tensile modulus of polymer nanocomposites. Polym Eng Sci 2002;42(5):983–93.
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