Composites: Part B 44 (2013) 750–755
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Physical, mechanical and morphological properties of polymer composites manufactured from carbon nanotubes and wood ﬂour Hamed Younesi Kordkheili a,⇑, Mohammad Farsi a, Zahra Rezazadeh b a b
Department of Wood and Paper Science and Technology, Sari Branch, Islamic Azad University, P.O. Box 48161-19318, Sari, Iran Department of Wood and Paper Science and Technology, Sari Agricultural Sciences and Natural Resources University, Iran
a r t i c l e
i n f o
Article history: Received 31 January 2012 Received in revised form 14 March 2012 Accepted 13 April 2012 Available online 21 April 2012 Keywords: A. Nanocomposites C. SWCTN D. MAPE B. Physical properties B. Mechanical properties
a b s t r a c t The objective of this investigation was to evaluate physical, mechanical and morphological properties of experimental polymer type panels made from single-wall carbon nanotube (SWCNT) and wood ﬂour. The composites with different SWCNTs (0, 1, 2, 3 phc) and maleic anhydride grafted polyethylene (MAPE) (0 and 3 phc) contents were mixed by melt compounding in an internal mixer and then the composites manufactured by injection molding method. The mass ratio of the wood ﬂour to LDPE was 50/50 (w/ w) in all compounds. Water absorption, thickness swelling, bending characteristics, impact strength and morphological properties of the manufactured composites were evaluated. Based on the ﬁndings in this work the water absorption and thickness swelling of the nanocomposites decreased with increasing with amount of the SWCNTs (from 1 to 3 phc) and MAPE (3 phc) in the panels. The mechanical properties of LDPE/wood-ﬂour composites could be signiﬁcantly enhanced with increased percentage of MAPE and SWCNTs content. Panels having 2 phc SWCNTs and 3 phc MAPE exhibited the highest impact strength value. Also Scanning Electron Microscope (SEM) micrographs showed that carbon nanotubes can ﬁll the voids of wood plastic composites as well as addition of MAPE and SWCNTs enhanced interaction between the components. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, wood plastic composites (WPCs) are becoming more and more commonplace by the development of new production techniques and discovery of new modiﬁcation methods. Compared to the traditional synthetic composites (such as carbon or glass ﬁber–polymer composites), WPCs present lower density, lower cost and they are biodegradable. However, WPCs exhibited weaker physical (higher water absorption and thickness swelling), mechanical (less ﬂexural and tensile strength) as well as thermal properties compared with traditional synthetic composites. In recent years, there have been considerable efforts to decrease defects and develop natural ﬁber-reinforced polymer based composites for production of affordable structural units [1,2]. On the other hand, using of nano-materials such as nanoclay and carbon nanotubes is one of the newest methods to overcome negative effect of various composites. These improvements include high moduli, increased tensile strength and thermal stability, decrease in water absorbance and improve ﬂammability properties [3,4]. So far several researchers focused on effect of nanoclay as nanoﬁller on WPCs [3,5] whereas about the inﬂuence of other nanoﬁllers such ⇑ Corresponding author. E-mail addresses: [email protected]
(H.Y. Kordkheili), [email protected]
iausari.ac.ir (M. Farsi), [email protected]
(Z. Rezazadeh). 1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.04.023
as carbon nanotube (CNT) on various properties of WPCs is not sufﬁcient information. Today, because of CNTs unique functional properties such as high mechanical resistance, high water and chemical resistance, high electrical and thermal conductivity, they are widely being used as reinforcement in polymer, ceramic and cement based composites . CNTs have two kinds, single cylindrical wall (SWCNTs) and multiple walls (MWCNTs). SWCNTs can be considered as simply a graphite-type sheet folded into a cylinder (Fig. 1), that has a large aspect ratio, often of the order of 1000– 25,00,000 . Strengths of continuous SWCNTs are reported to be 100 times stronger than steel . Results from researches by Loos et al. , Noguchi et al. , and Younesi et al.  show that adding CNTs as reinforcement to epoxy, aluminum and cement based nanocomposites increases their physical and mechanical properties [8,9,4]. Particularly, the mechanical properties of polymer/CNT composites as functions of CNT type, content, and processing parameters have been evaluated extensively [10,11]. Manchado et al. , indicate that incorporation of SWCNTs to plastic matrix increased crystallization of polypropylene. There are few researches which was introduced application of CNT to WPCs. Tavasoli Farsheh et al.  reported that by the addition of MWCNTs to rigid PVC/wood ﬂour composite foams increased physical and mechanical properties of the composites. Currently there is no information on effect of single wall carbon nanotubes (SWCNTs) on physical and mechanical properties of polymer based panels
HY. Kordkheili et al. / Composites: Part B 44 (2013) 750–755
2.2. Composite preparation Before preparation of the composites, SWCNTs was mixed with acetone in an ultrasonic generator to make them uniformly dispersed suspension so that the size of aggregated SWCNTs was minimized. After the sonication was carried out for 4 h, the acetone was allowed to evaporate. Then LDPE, oven dried wood ﬂour and SWCNTs were weighed and bagged according to formulations given in Table 1. The mixing was carried out at 180 °C and 40 RPM for 10 min by a HAAKE internal mixer (HBI System 90, USA). The compounded materials were then ground using a pilot scale grinder (WIESER, WGLS 200/200 Model). From the compounds which had been granulated, specimens were injection molded by injection molder (Imen machine, Iran) at molding temperature of 180 °C, and the injection pressure was 3 MPa. The specimens were stored under controlled conditions (50% relative humidity and 23 °C) for at least 40 h prior to testing.
Fig. 1. Construction of single wall carbon nanotubes.
manufactured from SWCNTs and wood ﬂour. Therefore, the objective of this study was investigate effect of single-wall carbon nanotubes as well as coupling agent (MAPE) used in experimental low density polyethylene (LDPE)-wood ﬂour composites on their physical, mechanical and morphological properties.
2. Experimental 2.1. Materials Low Density Polyethylene (LDPE), (MFI = 0.51 g/10 min, density = 0.91 g/cm3) was supplied by Bandar Imam Petrochemical Company, Iran. Wood-ﬂour was obtained from sawdust of Fagus orientalis was used as natural ﬁller. The particle size of wood ﬂour was 80 meshes. MAPE (MFI = 0.4 gr/10 min) provided by Kimia Javid Sepahan Company with trade name of KJS 111 was used as coupling agent. SWCNTs (outer diameter: 1–2 nm, length: 10 lm) were purchased from Research Institute of Petroleum Industry (RIPI), Iran. The SWNTs were prepared using a chemical vapor deposition (CVD) process, via methane as a carbon source, with a cobalt and molybdenum catalyst system and reaction temperature in the range 800–1000 °C. Puriﬁcation of SWCNTs was performed by HCl and HNO3, respectively. The SWCNTs were washed out several times with deionized water until the pH value of the solution became neutral. The samples were then dried in oven. Raman spectra of the used SWCNTs are presented in Fig. 2.
2.3.1. Water absorption and thickness swelling Water absorption and thickness swelling tests of the nanocomposites were performed according to ASTM D-7031–04 standard. Five specimens from each combination were taken and dried in an oven for 24 h at 100 ± 3 °C. The weight and thickness of dried specimens were measured at an accuracy of 0.001 g and 0.001 mm, respectively. The specimens were then immersed in distilled water for one week and kept at a temperature of 22 ± 2 °C. Weight and thicknesses of the specimens were measured after excessive water was removed from their surface. The value of the water absorption in percentage was calculated using the following equation:
WðtÞ W 0 100 W0
where WA(t) is the water absorption (%) at time t, W0 is the oven dried weight and W (t) is the weight of specimen at a given immersion time t. Also the value of the thickness swelling in percentage was calculated using the following equation:
TðtÞ T 0 100 T0
where TS(t) is the thickness swelling (%) at time t, T0 is the initial thickness of specimens, and T(t) is the thickness at time t.
2.3.2. Mechanical tests The ﬂexural tests were measured according to the ASTM D790–03, using an Instron machine (Model 1186, England), The tests were performed at crosshead speeds of 5 mm/min. A Zwick impact tester (Model 5102, Germany) was used for the un-notched Izod impact test according to ASTM D256 standard. Five replications were tested for each treatment in both ﬂexural and impact strength measurements.
raman shift (cm-1) Fig. 2. Raman spectra of the used SWCNTs.
2.3.3. Scanning Electron Microscopy (SEM) The morphology of the composites was examined using a scanning electron microscope (XL 30) supplied by Philips Company Limited. The fracture surfaces of the specimens after impact test were sputter-coated with gold before analysis. All images were taken at an accelerating voltage of 17 kV.
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Table 1 Composition of evaluated formulations.
Wood ﬂour (wt.%)
Coupling agent (phc)
1 2 3 4 5 6 7 8
50 LDPE/50 WF/0 NT/0 M 50 LDPE/50 WF/0 NT/3 M 50 LDPE/50 WF/1 NT/0 M 50 LDPE/50 WF/1 NT/3 M 50 LDPE/50 WF/2 NT/0 M 50 LDPE/50 WF/2 NT/3 M 50 LDPE/50 WF/3 NT/0 M 50 LDPE/50 WF/3 NT/3 M
50 50 50 50 50 50 50 50
50 50 50 50 50 50 50 50
0 0 1 1 2 2 3 3
0 3 0 3 0 3 0 3
Per hundred compound (the weights of SWCNT and coupling agent were calculated based on 100% weight of compound).
10 0% MAPE
12 8 4 0
Thickness swelling (%)
water absorption (%)
6 4 2 0
SWCNT (phc) Fig. 3. Effect of SWCNTs and MAPE content on water absorption of WPCs. Fig. 4. Effect of SWCNTs and MAPE content on thickness swelling of WPCs.
3. Results and discussion
Figs. 3 and 4 show the water absorption and thickness swelling content of the composites after one week immersion in distilled water, respectively. Because of constant wood ﬂour content (50 wt.%) in all compounds, the different water absorption and thickness swelling values can be attributed to the role of MAPE and SWCNs. It can be seen that the composite without SWCNTs and MAPE exhibited the higher water absorption and thickness swelling values rather than those containing them. In constant level of SWCNTs content, the composites with 3 phc MAPE exhibited the least water absorption and thickness swelling values. This could be related to better adhesion between matrix and cellulosic materials by adding MAPE which caused decrease in the velocity of the diffusion processes (due to the existence of fewer gaps in the interfacial region and blocking hydroxyl groups by the coupling effect). Also chemically, coupling agent can form ester bonds between the anhydride carbonyl groups of MAPE and hydroxyl groups of the wood ﬂours . This hypothesis is conﬁrmed by previous studies  that show anhydride moieties of functionalized polyoleﬁn coupling agents entered into an esteriﬁcation reaction with the surface hydroxyl groups of wood. Upon esteriﬁcation, the exposed polyoleﬁn chains diffuse into the polymer matrix phase and entangle with polymer chains during manufacturing of the composites . Figs. 3 and 4 also indicated that in constant level of coupling agent, the composites containing SWCNs exhibited less water absorption and thickness swelling contents as compared with those made without them. According to Das et al.  results, initially, water saturates the cell wall of the ﬁber, and next water occupies void spaces. As the composite voids were ﬁlled with SWCNs, the penetration of water by the capillary action into the deeper parts of composite was prevented. This hypothesis conﬁrmed by SEM photomicrograph (Fig. 8). Another reason for less water absorption and thickness swelling could be explained by the hydrophilic nature of the CNTs surface, which tends to immobilize some of the moisture, which inhibits the water permeation
in the polymer matrix. The water absorption test results showed that the composite having 3 phc MAPE and 3 phc SWCNTs exhibited the least water absorption and thickness swelling contents. Also Tavasoli Farsheh et al.  indicated that by the addition of MWCNTs to rigid PVC/wood ﬂour composite foams decrease water absorption and thickness swelling of foamed samples. 3.2. Flexural behavior The ﬂexural modulus and strength of the composites containing different contents of SWCNTs and MAPE are presented in Figs. 5 and 6, respectively. Fig. 4 indicated that addition of SWCNTs and MAPE increase ﬂexural modulus of WPCs. As shown, maximum ﬂexural modulus of nanocomposite was 4500 MPa for nanocomposites with 3 phc SWCNTs and 3 phc MAPE, while minimum ﬂexural strength is approximately 1400 MPa for control samples (without MAPE and CNTs). In absence of MAPE ﬂexural modulus of the samples with 1, 2 and 3 phc SWCNTs were 81, 106 and
5000 0% MAPE
Flexural modulus (MPa)
3.1. Water absorption and thickness swelling
SWCNT (phc) Fig. 5. Flexural modulus of the LDPE/wood composites as a function of SWCNTs and MAPE.
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fracture toughness, hardness and strength in polymer matrices can be developed by adding carbon nanotubes in the samples . Tavasoli Farsheh et al.  reported that compared to pure WPCs, addition of CNTs resulted in an increase of mechanical properties.
3.3. Un-notched impact strength
80 60 40 20 0
SWCNT (phc) Fig. 6. Flexural strength of the LDPE/wood composites as a function of SWCNTs and MAPE.
149% higher than control samples, respectively. Panels made with 3 phc MAPE with 1, 2 and 3 phc SWCNTs had ﬂexural modulus of 59, 111 and 145% higher than the panels made without SWCNTs. The ﬂexural modulus in composites is mainly function of the modulus of individual component . Flexural modulus of SWCNTs (1TPa) was considerably higher than wood ﬂour and LDPE, respectively. Hence it was expected that the composites with SWCNTs exhibited the higher ﬂexural modulus value. In addition to, increased ﬂexural properties for the composites with carbon nanotube can be attributed high aspect ratio of SWCNTs (typically higher than 1000:1 and as high as 2500,000:1) that transfer stress from polymer to the carbon nanotube . Also Fig. 5 indicate that compared to composites without MAPE, in the presence of MAPE, ﬂexural modulus was increased. So far several researches reported positive effect of compatibilizer on ﬂexural modulus of WPCs [17,18]. Also the positive effect of SWCNTs and MAPE on ﬂexural strength can be observed in Fig. 6. Greater ﬂexural strength was achieved in the composites when carbon nanotubes were used in the manufacture of the composites increase. The strength of the composites depended on the properties of constituents and the interfacial interaction [4,17]. One of the most important parameters in fabricating carbon nanotube composites is tube dispersion in the matrix. Tube aggregation is harmful to physical and mechanical properties of the resultant nanocomposites. Using of a solvent such as acetone is the best method to prevent the nanotubes from aggregating together and improve good dispersion of CNTs in nanocomposites. This can be explained by the fact that acetone, as a solvent, dilutes the polymer and reduces its viscosity. Reducing the viscosity leads to an enhanced efﬁciency of dispersion with tip sonication. Because of carbon nanotube size, aspect ratios and high mechanical properties, SWCNTs can be well distributed in the composites and improve adhesion between the elements. SEM results of such type of samples was evaluated by Gojny et al.  and it was found that a strong bonding between the polymer and the carbon nanotubes. Also the results indicate that adding of 3 phc MAPE enhances the interface adhesion between wood ﬂour and low density polyethylene. Compatibilizers can improve encapsulation of wood ﬂour by the plastic, which consequently results in higher ﬂexural strength. The samples containing 3 phc SWCNTs and 3 phc MAPE showed the highest ﬂexural strength (about 156 MPa) among the studied composites. SEM results of the studied nanocomposites showed that MAPE and SWCNTs improve the interaction between the elements and the gaps in WPCs can ﬁll with MAPE and SWCNTs (Fig. 8). Li et al.  indicated that carbon nanotubes had positive effect on ﬂexural modulus and strength of the carbon nanotube reinforced polymer composites. Also several researchers showed signiﬁcant improvements in
Fig. 7 shows the Izod impact strengths of the composites made with different content of SWCNTs and MAPE. In general, the energy required crack propagation was measured with un-notched impact strength. The impact strength test results show that in the absence of coupling agent, impact strength of composites containing 1, 2 and 3 phc of SWCNTs are respectively 74, 111 and 94% higher than control samples. In presence of 3 phc MAPE, impact strength of the nanocomposites with 1, 2 and 3 phc SWCNTs respectively 67, 93 and 71% is higher than control samples. These results demonstrated that adding 2 phc SWCNTs increase impact strengths of the composites. CNTs with bridge linkage mechanism prevent the spread of cracks and increase the impact strength of composites . Fig. 7 generally showed that composites containing 3 phc MAPE exhibited more impact strength compared to samples without it. Impact strength show the strength of material against breakage and start cracking in the weakest point of the composite, which is the connecting point between lingocellulose material and polymer. Increasing impact strength of composites with addition of MAPE, is done probably by the improvement of ﬁber–polymer connection level that cause tension concentration in WPC and as a result cause an increase in impact strength. Reduction of impact strength with increasing SWCNTs from 2 to 3% can be related to increasing the probability of SWCNTs agglomeration that creates regions of stress concentrations that require less energy to elongate the crack propagation . Generally the result of impact test showed that WPCs which contain 3 phc MAPE and 2 phc SWCNT have the more impact strength value. 3.4. Morphology characteristics SEM is an effective media for the morphological investigations of the composites. Through SEM study, the distribution and compatibility between the ﬁllers and the matrix could be observed. Fig. 8a corresponds to WPC without SWCNTs and MAPE. As can be seen, there is some evidence of ﬁber pull out from matrix. Therefore when stress is applied it causes the ﬁbers to be leave the matrix easily and cusses gaping holes. Fig. 8b is showing the position of nanoparticles in the composites with 1phc SWCNT and without MAPE at high magniﬁcation (30,000). It indicated that SWCNTs ﬁlled the voids of the wood plastic composites. The surface of nanocomposites containing 2 phc SWCNT and without MAPE analyzed by SEM is depicted in
2500 0% MAPE
Impact strength (J/m2)
Flexural strength (MPa)
1500 1000 500 0
SWCNT (phc) Fig. 7. Un-notched impact strength of the LDPE/wood composites as a function of SWCNTs and MAPE.
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Fig. 8. SEM photomicrographs of fractured samples of nanocomposites: 0 phc SWNCT/0 phc MAPE (a), 1 phc SWCNT/0 phc MAPE (b), 2 phc SWCNT/0 phc MAPE (c), 3 phcSWCNT/3 phc MAPE (d).
Fig. 8c. There are some cavities in WPC that can absorb water and/or reduce mechanical properties. This indicates that the level of interfacial bonding between the wood ﬂour and LDPE in the composites without coupling agent is weak and the extent of improvement in the physical and mechanical properties is more prominent with MAPE. Fig. 8d is showing composites with 3 phc SWCNT and 3 phc MAPE. As can be seen, there is no separation of the ﬁbers from the matrix and a very good interaction between the components can be inferred from the images. The strong adhesion that is observed at the interface has been already discussed in mechanical properties of the composites and is related to SWCN and MAPE, which encapsulated ﬁbers in the matrix and cusses strong bonding. The signiﬁcant decreasing in water absorption and thickness swelling of the blends including MAPE and SWCNTs were further supported by Fig. 8d, that when composite micro voids and the lumens of ﬁbers were ﬁlled with SWCNT and MAPE, there is smooth and monotonous matrix without any holes and penetration of water into the deeper holes and cavities of composite is prevented.
4. Conclusions This study investigated effect of SWCNTs (as reinforcing agent) as well as MAPE (as coupling agent) on physical, mechanical and morphological properties of wood ﬂour/LDPE composite. The physical and mechanical test results indicated that SWCNT loading and compatibilizing agent content signiﬁcantly inﬂuences on properties of WPCs. Because of high water resistance nature of CNTs and ﬁlling the voids of the composites by the nanoparticles, with increasing the amount of SWCNTs up to 3 phc, water absorption and thickness swelling of the composites decreased. In addition, adding of MAPE improves physical properties of nanocomposites. High aspect ratio and large surface area of SWCNTs were causes the enhancement of ﬂexural modulus and strength of the composites. The addition of MAPE had a positive effect on ﬂexural properties, because it improves the interfacial bonding between the ﬁber and the matrix polymer. Also mechanical test results indicated that the composites containing 3 phc MAPE and 2 phc SWCNTs exhibited the highest unnotched impact strength value. Morphological study also showed that there are distinct cavities between the LDPE and wood ﬂour, indicating poor adhesion, but a fewer pulled-out traces on the
fracture surfaces of the test samples including 3 phc SWCNTs and 3 phc MAPE, due to the stronger interfacial bonding. These images are supported physical and mechanical results of the article. References  Behrooz R, Younesi KH, Kazemi NS. Physical properties of lignin added wood ﬂour–polypropylene composites: a comparison of direct and solvent mixing technique. Asian J Chem 2012;24(1):157–60.  Mohebby B, Younesi KH, Ghotbifar A, Kazemi NS. Water and moisture absorption and thickness swelling behavior in polypropylene/wood ﬂour/ glass ﬁber hybrid composites. J Reinf Plast Comp 2010;9(6):830–9.  Faruk O, Matuana L. Nanoclay reinforced HDPE as a matrix for wood–plastic composites. Compos Sci Technol 2008:2073–7.  Younesi KH, Hiziroglu S, Farsi M. Some of the physical and mechanical properties of cement composites manufactured from carbon nanotubes and bagasse ﬁber. Mater Design 2012;33:395–8.  Najaﬁ A, Kord B, Abdi A, Ranaee S. The impact of the nature of nanoclay on physical and mechanical properties of polypropylene/reed ﬂour nanocomposites. J Thermoplast Compos 2011. http://dx.doi.org/10.1177/089270571141281.  Salvetat JP, Bonard JM, Thomson NH, Kulik AJ, Forro L, Benoit W, et al. Mechanical properties of carbon nanotubes. Appl Phys A Mater 1999;69(3):255–60.  Schulter AD. Functional molecular nanostructures. Springer; ISSN: 0340-1022, doi: 10.1007/b79606.  Loos MR, Coelho LAF, Pezzin SH, Amico SC. Effect of carbon nanotubes addition on the mechanical and thermal properties of epoxy matrices. Mater Res 2008;11(3):347–52.  Noguchi T, Magario A, Fukuzawa S, Shimuzu S, Beppu J, Seki M. Carbon nanotube/aluminium composites with uniform dispersion. Mater Trans 2004;45(2):602–4.  Gojny FH, Wichmann MHG, Kopke U, Fiedler B, Schulte K. Carbon nanotubereinforced epoxy-composites: enhanced stiffness and fracture toughness at low nanotube content. Compos Sci Technol 2004;64:2363–71.  Xiao QL, Zhang LC, Zarudi I. Mechanical and rheological properties of carbon nanotube-reinforced polyethylene composites. Compos Sci Technol 2007;67:177–82.  Manchado MA, Valentini L, Biagiotti J, Kenny JM. Thermal and mechanical properties of single-walled carbon nanotubes–polypropylene composites prepared by melt processing. Carbon 2005;43(7):1499–505.  Tavasoli FA, Talaeipour M, Hemmasi AH, Khademieslam H, Ghasemi I. Investigation on the mechanical and Morphological properties of foamed nanocomposites based on wood ﬂour/PVC/Multi-walled carbon nanotubes. Bioresource 2011;6(1):841–52.  Das S, Sara AK, Choudhury PK, Basak RK, Mitra BC, Todd T, et al. SJ effect of steam pretreatment of jute ﬁber on dimensional stability of jute composite. J Appl Polym Sci 2000;76(11):1652–61.  Makar J, Margeson J, Luh J. Carbon nanotube/cement composites – early results and potential applications. In: 3rd international conference on construction materials: performance, innovations and structural implications. Canada: Vancouver; 2005. p. 1–10.
HY. Kordkheili et al. / Composites: Part B 44 (2013) 750–755  Li Q, Matuana L. Effectiveness of maleated and acrylic acid-functionalized polyoleﬁn coupling agents for HDPE–Wood-Flour composites. J Thermoplast Compos 2003;16:551–64.  Kazemi NS, Younesi KH. Effect of sea water on water absorption and ﬂexural properties of wood-polypropylene composites. Europ j wood wood produc 2011;69:553–6.  Kazemi NS, Bahra A, Abdouss M. Effect of oxidized polypropylene as a new compatibilizer on the water absorption and mechanical properties of wood ﬂour–polypropylene composites. J Appl Polym Sci 2011;119(1):438–42.
 Li GY, Wang PM, Zhao X. Pressure-sensitive properties and microstructure of carbon nanotube reinforced cement composites. Cement Concrete Comp 2007;29(5):377–82.  Park SH, Bandaru PR. Improved mechanical properties of carbon nanotube/ polymer composites through the use of carboxyl-epoxide functional group linkages. Polymer 2010;51:5071–7