Theoretical Models and Experimental Study on Mechanical Properties of Reinforced Polymer Matrix Using Different Kinds of

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1 Theoretical Models and Experimental Study on Mechanical Properties of Reinforced Polymer Matrix Using Different Kinds of Theoretical Models and Experimental Study on Mechanical Properties of Reinforced Polymer Matrix Using Different Kinds of N. Kordani *1, R. Adibipour 2, A. Sadough Vanini 2, A. Zare 3, and V. Gil 4 1 Department of Mechanical Engineering, Mazandaran University, Mazandaran, Iran 2 Department of Mechanical Engineering, Amirkabir University of Technology, Tehran, Iran 1 Department of Mechanical Engineering, Science and Technology University, Tehran, Iran 4 Department of Ocean Engineering, Imam Khomeini University, Noshahr, Iran Received: 30 January 2014, Accepted: 12 May 2014 SUMMARY This paper is a study of the role of functionalized multi-walled carbon nanotubes (MWCNT-COOH) and singlewalled carbon nanotubes (SWCNT) in improving the mechanical properties of epoxy resins. In nanotube-based polymeric composite it is anticipated that high elastic modulus can be achieved by taking advantage of the interfacial friction between the nanotubes and the polymer. In order to evaluate the CNT effect on polymer composites, different weight fractions of CNTs and two different kinds of CNTs were dispersed in the matrix. First, the damping characteristics of the samples with 0, 0.5, 1 and 1.5 wt.% nanotube contents were measured and then the tensile modulus was determined experimentally. The experiments showed an increase of the elastic modulus and the damping due to adding CNTs. Theoretical models were used to show the effect of added reinforcement to compare with experimental elastic modulus data. The results showed that a modified Rule of Mixtures model, incorporating the effect of fibre orientation in 3D was closest to the experimental results. Keywords: Carbon nanotubes, Epoxy, Damping ratio, Elastic modulus, Theoretical models 1. Introduction A nanocomposite is a mixture of a reinforcement and a matrix material, with notably different properties. Nano reinforcements in composites have attracted the attention of researchers because they are able to improve the mechanical and physical properties of the matrix 1-3. The research on nano reinforcement especially carbon nanotubes (CNTs) has opened an entirely new view to develop polymer matrix composites. CNTs, which have a number of unique properties, both mechanical and physical, have great potential for use as reinforcement 4. There are a lot of nanoparticles used for reinforcing in nanocomposites including MWCNTs, SWCNTs, carbon nanofibres (CNFs), montmorillonite (MMT) nanoclays, polyhedral oligomeric silsesquioxanes (POSS), Al 2 O 3, TiO 2 and nanosilica. Comparing between nano-fillers, CNTs and carbon nanofibres provide a number of advantages. CNTs with their notable properties such as low density, high aspect ratio, high strength and stiffness, excellent electrical and chemical resistance are a suitable candidate as reinforcement for matrix, especially polymeric materials. The stiffness, fracture toughness, and interlaminar shear strength of composites properties can be enhanced by addition of a small amount of CNTs or carbon nanofibres. SWNTs with one graphite cylindrical tube that are held together with relatively weak van der Waals forces, and MWNTs consist of many coaxial graphite cylindrical tube and The van der Waals forces between the graphite cylindrical tube cause sliding of the layers 5,7. New construction and purification methods have increased the production of CNTs, and by adding a small dose of CNTs as reinforcing phase, lightweight structural polymers with perfect mechanical properties can be produced 8,9. Qian et al. reported that, with the addition of only 1% CNTs, the elastic modulus of polystyrene has been increased about 36 42% 10. Experimental results have shown that the improvement of material properties is related to CNTs dispersion and to CNTs-matrix interfacial bonding * Corresponding Author: Naser Kordani, Department of Mechanical Engineering, Mazandaran University of Technology, Mazandaran, Iran. Naser.kordani@gmail.com Smithers Information Ltd., 2015 Little attention has been given to the damping mechanisms and ability of composites based CNTs, while most Polymers & Polymer Composites, Vol. 23, No. 4,

2 N. Kordani, R. Adibipour, A. Sadough Vanini, A. Zare, and V. Gil of the research has focused on their elastic properties. Recently, Koratkar et al. conducted direct shear testing of epoxy matrix reinforced by CNTs, and reported strong visco-elastic behaviour with up to 1400 percent increase in the loss factor of the baseline epoxy resin. Without sacrificing the mechanical strength and stiffness of the polymer, and with a minimal weight penalty, the excellent improvement in damping was obtained 15,16. Previously, researchers made composites reinforced by CNTs by mixing CNT into polymers directly and then using casting and injection methods. In industrial applications, melt mixing is the most preferred method for preparation of polymer/ CNT nanocomposites 17. In the previous research the effects of nanoscale particle reinforcement on the damping properties of composites based polymer have been explored. For an example of elastomeric materials, as reported at 18,19, rod-like aggregates of roughly spherical carbon black particles increased the material damping in the strain range in which the breakdown and reformation of carbon black aggregates occurs. This strain-dependent damping enhancement in elastomers reinforced with particle is known as the Payne Effect. Analogous effects can be expected for composites reinforced by CNTs. Due to the small scale of CNTs, the surface area of CNT is very large. So, in composites reinforced by CNTs, it is predicted that high damping and tensile modulus can be obtained by taking advantage of the weak bonding and interfacial friction between individual CNTs and polymer matrix. Buldum and Lu 20 by MD methods concluded that at first, CNT sticks and then when the force exerted on it is large enough, CNT slips. 2. Experimental Details The low-viscosity epoxy resin (LY 564 from Huntsman) with a polyamide hardener (HY 560 from huntsman) was used as a matrix. 14% polyamide hardener in the epoxy resin has been used. Based on evaluation of the CNT effect on fibre-reinforced polymer composites, different types of CNTs were also dispersed in the matrix. The CNTs used in this study, were purchased from the Research Institute of Petroleum Industry. The outer and inner diameters of the MWCNT were 23 nm and 11 nm respectively; the SWCNTs diameters were between 1 and 4 nm and the maximum length was less than 10 mm. SWCNTs and MWCNT were functionalized by oxidation and ultraviolet ray techniques from RIPI. Carbon nanotube TEM images before and after functionalization are shown in Figures 1a, 1b; the dark points are metal particles and amorphous carbon materials. In order to remove impurities, the CNTs were treated, the closed tubes were opened and the length of the nanotubes was shortened 21. Strong van der Waals forces cause agglomeration of nanotubes. The diameters of the dark areas, which are estimated to vary from 5 to 30 nanometres, are shown in Figure 1. Neat resin specimens were prepared by combining LY564 epoxy and HY560 hardener completely. For preparation of nanocomposites, the same weight fraction each of MWCNT and SWCNT or functionalized MWCNT (MWCNT_ COOH) (0.5 wt.%) was dispersed into hardener (HY560) directly. The mixture was ultrasonicated for 10 min with 60% power. To achieve better dispersion in the hardener, a high shear mixer was used at 700 rpm for 30 min. Then, the epoxy resin was degassed first and then combined with the HY560/CNT mixture. As shown in Figure 2, the composite plate was left for 15 hours at 50 C under a vacuum bag after it was prepared with casting. Figure 1. Multi walled carbon nanotube TEM micrograph (a) Before functionalization, (b) After functionalization (a) Figure 2. Specimens prepared by cutting a nanocomposites plate (b) The main goal of this article was to explore the mechanical properties of nanocomposite based epoxy with different amounts and various types of CNT. 252 Polymers & Polymer Composites, Vol. 23, No. 4, 2015

3 Theoretical Models and Experimental Study on Mechanical Properties of Reinforced Polymer Matrix Using Different Kinds of To repeat tensile test, three identical specimens were prepared. Scanning electron microscopy (SEM) was performed using a Cambridge (S360) instrument to examine the fracture surface morphology of the epoxy/cnt nanocomposites. The fracture surface was prepared by fracturing the epoxy/ CNT nanocomposites in liquid nitrogen. The sample was sputtered with gold prior to observation. SEM photos were taken to evaluate the nanotube dispersion in the resin. 3. Modal and Tensile Testing and Analysis All the specimens were mechanically polished in order to minimize the influence of surface flaws. A regular composite beam without reinforcement and nanocomposite beams with reinforcement were used as the specimens for the tensile test. Experiments to measure mechanical properties such as tensile modulus were carried out at 25 C. According to ASTM D638, the two ends of sample were pulled apart with a relative speed of 1 mm/min. Specimens were tested by using a Zwick/Amsler tensile and fatigue testing machine with computer data acquisition to record tensile modulus values. (Fracture & Fatigue Laboratory of the Mechanics Department of Iran University of Science & Tech.) For the damping test, a force transducer (BK8200) was attached on one side of the specimen to measure the input force. An accelerometer (A/123E) was attached on the other side of the specimen to detect the acceleration. The specimen was in a free-free boundary condition and was excited by a shaker (BK4808). Sweep sinusoidal signals were used as the excitation source for the shaker, and the frequency response function (FRF) was derived using an analyzer in a conventional modal testing procedure. Figures 4a-9a show the measured FRF 21 and Figures 4b-9b show the measured Stress-Strain data for samples with MWCNTs content (0, 0.5, 1, 1.5 wt.%) and for more comparison in 0.5 wt.%, FRF and Stress-Strain plot is added. The damping ratios for these samples were computed according to the theoretical procedure 21. Experimental investigations were performed to evaluate the elastic modulus of the neat specimens, and the elastic modulus characteristics of composites that had been reinforced using CNT. By comparing Figure 4 and Figures 5-9, it can be seen that characteristics of the beams were considerably different between samples with and without CNTs. It is shown that the damping and the elastic modulus can be increased by adding CNTs. The damping ratio and the elastic modulus of the 1 wt.%- samples are much higher than those of the 0.5 wt.% samples and the neat epoxy Figure 3. SEM micro photo of epoxy/ MWCNT nanocomposite with 0.5 wt.% MWCNT material Figure 4. a) Frequency response function for damping test; b) Tensile test for epoxy Polymers & Polymer Composites, Vol. 23, No. 4,

4 N. Kordani, R. Adibipour, A. Sadough Vanini, A. Zare, and V. Gil Figure 5. a) Frequency response function for damping test; b) Tensile test for 0.5 wt.% MWCNT nanocomposite Figure 6. a) Frequency response function for damping test; b) Tensile test for 0.5 wt.% MWCNT_COOH nanocomposite Figure. 7 a) Frequency response function for damping test; b) Tensile test for 0.5 wt.% SWCNT nanocomposite 254 Polymers & Polymer Composites, Vol. 23, No. 4, 2015

5 Theoretical Models and Experimental Study on Mechanical Properties of Reinforced Polymer Matrix Using Different Kinds of Figure 8. a) Frequency response function for damping test; b) Tensile test for 1 wt.% MWCNT_COOH nanocomposite Figure 9. a) Frequency response function for damping test; b) Tensile test for 1.5 wt.% MWCNT_COOH nanocomposite sample. Figures 10 and 11 show that the damping ratio and the elastic modulus of the 0.5 wt.% MWCNT-COOH sample is greater than that of the 0.5 wt.% MWCNT and 0.5 wt.% SWCNT because of surface modification. The peak value in the FRF represents resonance at a certain frequency. From the FRF, it can be clearly seen that the sharp peak of the first mode, second mode and third mode are significantly reduced for the nanocomposite beam, which indicates that the nanocomposite beam has improved the damping property. With the same manner the nanocomposite beam has improved the tensile modulus. To further demonstrate the improved damping of the nanocomposite beam, the frequency responses of the regular composite beam and the nanocomposite beam are compared in Table 1. This comparison shows that the damping ratio values at these three natural frequencies and the elastic modulus values are much greater than those of the neat polymer beam. In terms of damping ratio comparison, the damping ratio of the nanocomposite beam increased up to % at the 2nd mode and 3rd mode frequencies and in terms of elastic modulus comparison, the elastic modulus of the nanocomposite beam increased up to 17 49%. However, there was little change in the mode frequencies, which means that there was only a slight change in the stiffness of the composites. It is still a benefit of the nanocomposite over the neat polymer beam. Due to the small size of CNTs the interfacial area between the CNTs and the epoxy is very large, which will cause greater frictional force, structural damping and elastic modulus. Polymers & Polymer Composites, Vol. 23, No. 4,

6 N. Kordani, R. Adibipour, A. Sadough Vanini, A. Zare, and V. Gil 4. Theoretical Models to Predict the Elastic Modulus of NanocompositeS Figure 10. Damping ratio of a) 0.5 wt.% MWCNT, SWCNT, MWCNT-COOH specimens b) the regular composite beam and the nanocomposite beams are compared for the first, second, and third natural frequencies Experimental results with the corresponding results from the theories are compared in Table 2. When CNTs dispersed randomly in the matrix, the Halpin-Tsai equation is a good choice to obtain the elastic modulus of nanocomposites reinforced CNTs. The Shear-Lag model use to investigate the effect of reinforcement dimensions on the elastic modulus of nanocomposites. To consider the effects of the reinforcement orientation and interaction between reinforcement and matrix, Curtis et al. modified the rule of mixtures into Equation (4). In the Voight-Reuss micromechanical model, the elastic modulus of the nano composite can be obtained from percent summation of longitudinal and transverse elastic modulus of nanocomposite. Volume fraction of reinforcement is given by following equation and it is related to reinforcement density, matrix density, and weight fraction of reinforcement 28 : Figure 11. Comparing between experimental results and theoretical models in Table 2 (6) Figure 11 is comparing between experimental elastic modulus of nano composite, Halpin-Tsai equation(h-t), modified Halpin-Tsai (MH-T)with Orientation factor, Shear-lag (SH-L), Modified Rule of Mixtures (MRM) and Voigt-Reuss (V-R). In this case, Modified Rule of Mixtures with The effect of fibre orientation in 3D is closer than the others. Based on experimental results, Thostenson and Chou plotted a straight line through the data that showed the relationship between the nanotube diameter and wall thickness (Figure 12). According to their study at smaller nano tube diameters the relationship between the nano tube diameter and wall thickness begins to deviate from the linear curve fit 29. To consider the effect of wall thickness of carbon nanotube, the following equation by Thostenson and Chou is used: (7) where T is the wall thickness, d is the nanotube diameter and R is the error of this curve fitting. To show the effect of wall thickness on the elastic modulus of the nano composites, Eq. (7) has been substituted into Modified Rule of Mixtures equation that was shown good aggregation with experimental result. 256 Polymers & Polymer Composites, Vol. 23, No. 4, 2015

7 Theoretical Models and Experimental Study on Mechanical Properties of Reinforced Polymer Matrix Using Different Kinds of Table 1. Damping ratio calculated by modified half-power method [21] 1st mode Frequency (Hz) 1st mode damping ratio 2nd mode Frequency (Hz) 2nd mode damping ratio 3rd mode Frequency (Hz) 3rd mode damping ratio Regular composite beam Nanocomposite beam Epoxy /SW/0.5 wt.% Nanocomposite beam Epoxy /MW/0.5 wt.% Nanocomposite beam Epoxy /MW-COOH/0.5 wt.% Nanocomposite beam Epoxy /MW- COOH /1 wt.% Nanocomposite beam Epoxy /MW-COOH/1.5 wt.% Table 2. Various theoretical models Model Equations Eq. No. Ref. name Halpin-Tsai 1 [22] more slippage and energy dissipation through friction. Due to the small size of CNTs, the interfacial area between the CNTs and the epoxy is very large, which will cause greater frictional force, structural damping and elastic modulus. Modified Halpin-Tsai 2 [23] So it can be concluded that the mixing of carbon CNTs could result in a marked increase in the structural damping and the elastic modulus of common fibre-reinforced composites. Shear-lag 3 [24] Modified Rule of Mixtures 4 [25] Voigt-Reuss 5 [26, 27] 5. Conclusions In this paper, it can be seen that the composites reinforced with MWCNTs had much higher damping ratios and elastic modulus than the neat polymer sample. Composites reinforced with CNTs had the largest interfacial contact region with the epoxy. CNTs with the highest stiffness among all the fillers caused high damping ratios and elastic moduli. As shown in Figure 13, decreasing the wall thickness of carbon nanotube caused an increased elastic modulus of nanocomposite. The damping ratio and the elastic modulus of the regular composite beam and the nanocomposite beam are compared in Figures 9b and 10. It is shown that the damping and the elastic modulus can be increased by adding CNTs. This comparison shows that the damping ratio at these three natural frequencies and the elastic modulus values of the CNTs-reinforced composite beams are much greater than those of the regular composite beam. Note that with greater stiffness, fillers have an increased capability to resist applied loading, which could lead to Nevertheless, it can be concluded that, by taking advantage of the large interfacial contact region between CNTs and resins, as well as the high stiffness and low density properties of CNTs, high performance in energy dissipation and structural damping and tensile modulus can be achieved by the proposed treatment. Increasing the MWCNT-COOH weight ratio up to 1 wt.% caused an increase in the damping ratio and elastic modulus. This is because of the fact that more Polymers & Polymer Composites, Vol. 23, No. 4,

8 N. Kordani, R. Adibipour, A. Sadough Vanini, A. Zare, and V. Gil Figure 12. Linear relationship between wall thickness and nanotube diameter 29 Figure 13. Wall thickness effect of carbon nanotube on elastic modulus of nanocomposite in different CNT weight fraction energy would be dissipated with a larger frictional force. From the experimental results, the maximum damping ratio and elastic modulus were obtained at 1 wt.%. As shown in Figures 9 and 10, the composites with MWCNT-COOH had a much higher damping ratio and elastic modulus than the others. A comparison between the experimental elastic moduli of nanocomposites and theoretical models shows that the Modified Rule of Mixtures with the effect of 3D fibre orientation is closest to the experimental results. To show the influence of wall thickness of carbon nanotube on the elastic modulus, the above equation by Thostenson and Chou was used in a Modified Rule of Mixtures model. Decreasing the wall thickness of the carbon nanotube caused an increase of the elastic modulus of the nanocomposites. Of course these methods could be modified to be usable for elastic modulus studies of nanocomposites reinforced by CNTs at different temperatures. REFRENCES 1. Thostenson, E.T., Z. Ren, and T.W. Chou. Advances in the science and technology of carbon nanotubes and their composites: a review, Composites Science and Technology, 61(13), (2001) Lau, K.T., and D. Hui. The revolutionary creation of new advanced materials-carbon nanotube composites, Composites Part B: Engineering, 33(4), (2002) Lau, A.K.T., D. Bhattacharyya, and C.H.Y. Ling. Nanocomposites for Engineering Applications, Journal of Nanomaterials, (2009), Fereidoon, A., and M. Ghorbanzadeh Ahangari. Investigation of mechanical, thermal properties and non-isothermal crystallisation kinetic of polypropylene/single-walled carbon nanotube, International Journal of Nano and Biomaterials, 2(1-5) (2009) 339_ Gojny, F.H., M.H.G. Wichmann, U. K opke, B. Fiedler, and K. Schulte. Carbon nanotube-reinforced epoxycomposites: enhanced stiffness and fracture toughness at low nanotube content, Composites Science and Technology, 64(15), (2004) Qian, D., E.C. Dickey, R. Andrews, and T. Rantell. Load transfer and deformation mechanisms in carbon nanotube polystyrene composites, Applied Physics Letters, 76(20), (2000) Schadler, L.S., S.C. Giannaris, and P.M. Ajayan. Load transfer in carbon nanotube epoxy composites, Applied Physics Letters, 73(26), (1998) Wagner, H.D., O. Lourie, Y. Feldman, and R. Tenne. Stressinduced fragmentation of multiwall carbon nanotubes in a polymer Matrix, Applied Physics Letters, 72(2), (1998) Yu, M.F., O. Lourie, K. Moloni, M.J. Dyer, T.F. Kelly, and R.S. Ruoff. Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load, Science, 287(5453), (2000) Ru. C.Q. Effect of van der Waals forces on axial buckling of a doublewalled carbon nanotube, Journal 258 Polymers & Polymer Composites, Vol. 23, No. 4, 2015

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10 N. Kordani, R. Adibipour, A. Sadough Vanini, A. Zare, and V. Gil 260 Polymers & Polymer Composites, Vol. 23, No. 4, 2015