Effects of Processing Techniques on Morphology and Mechanical Properties of Epoxy-Clay Nanocomposites N. Merah 1,a and M.

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1 Advanced Materials Research Vols (2013) pp Online: (2013) Trans Tech Publications, Switzerland doi: / Effects of Processing Techniques on Morphology and Mechanical Properties of Epoxy-Clay Nanocomposites N. Merah 1,a and M. Al-Qadhi 1,b 1 King Fahd University of Petroleum and Minerals, Department of mechanical engineering, P.O. Box 1758, Dhahran 31261, Saudi Arabia a nesar@kfupm.edu.sa, b alqadhi@kfupm.edu.sa Keywords: high shear mixing, sonication, morphology, epoxy-clay nanocomposites. Abstract. Proper dispersion of nano thin layered structure of nanoclay in polymer matrix offers new and greatly improved properties over pristine polymers. The degree of nanoclay dispersion and hence the improvements in the physical and mechanical properties depend greatly on the technique used and processing parameters. In this work, 2 wt.% epoxy-clay nanocomposites were fabricated using different mixing techniques to study the effect of mixing methods on the nanoclay dispersion and thus on the enhancement of properties of resultant nanocomposites. Three mixing techniques were explored: high shear mixing (HSM), ultrasonication and their combination as well as hand mixing. The effect of mixing techniques on morphology and mechanical properties of the resultant nanocomposites was investigated using scanning electron microscope (SEM), X-ray diffraction (XRD), transmission electron microscope (TEM) and tensile testing. The results of XRD and TEM showed that both exfoliated and disordered intercalated morphology were developed for the nanocomposites synthesized by HSM, while ordered intercalated morphology was observed for samples prepared by sonication. The tensile test results show that among the mixing techniques considered in this study HSM results in the optimum mechanical properties as a whole while hand mixing resulted in the worst mechanical properties. Introduction The addition of organically modified montmorillonite (MMT) clay to reinforced epoxy matrix have recently attracted considerable attention since the dispersion of nano thin layered structure of nanoclay in polymer matrix offer new and greatly improved properties over neat epoxy. The exceptional structure of nanoclay layers, 1 nm thickness with high aspect ratio (up to 1000), enable nanoclay to improve polymer matrix properties at very low clay loadings [1]. Different mixing methods have been used to fabricate epoxy-clay nanocomposites using different type and loading of nanoclay [2-9]. Using nanoclay to reinforce polymers and to develop nanocomposites has been investigated since the early 1990 s, and improvements in strength, modulus, fracture toughness and reduction in moisture absorption of polymers have been seen for a variety of nanocomposite systems [3-7]. In addition to the type of polymer and nanoclay to start with the improvement in epoxy properties resulted from nanoclay addition depends mainly on mixing techniques used to disperse nanoclay into epoxy matrix [3,7,10]. Although a lot of work has been conducted, contradictory results were reported about the effect of clay addition on the mechanical and physical properties of epoxy matrix. Some studies reported enhancements in tensile strength, fracture toughness and glass transition temperature (T g ) [7,11-13] due to nanoclay addition, however, other studies reported either no effect or reduction in these properties [14-16] with the clay addition. In fact the properties of epoxy-clay nanocomposites are highly dependent on the resultant morphology, which illustrates the degree of clay dispersion into epoxy matrix [1,3]. Intercalated or exfoliated morphology of epoxy-clay nanocomposites can be obtained depending of mixing method and the type of epoxy and nanoclay used. Generally, exfoliated structure is reported to possess better properties than the intercalated ones [1,17], since more reinforcment elements are available that carry the applied load in the exfoliated morphology. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-09/10/15,07:29:09)

2 168 Advances in Materials and Materials Processing The present work investigates the effects of processing technique with optimum processing parameters on the morphology and mechanical properties of 2 % epoxy-clay nanocomposite. Materials and Processing Methods Materials. The epoxy used in this study is the diglycidyl ether of bisphenol A (DGEBA), supplied by JANA, KSA. The curing agent used is isophoronediamine (IPDA), product of HUNTSMAN, USA. The clay used in this study was Nanomer I.30 E, from Nanocor Inc, USA, which is montmorillonite clay that has been modified with primary octadecyl ammonium ion. Preparation of Epoxy-Clay Nanocomposite. Epoxy-clay nanocomposites contain 2 wt.% clay loading, which was found to be the optimum clay loading [10,18], were fabricated to determine the effect of mixing process on morphology and mechanical properties. The preparation of nanocomposites started with manually mixing of nanoclay with epoxy for 5 min to ensure good distribution of the clay particles in the epoxy monomers. High shear mixer (Model L4RT, Silverson, UK) was then used to disperse the nanoclay into epoxy matrix at 6000 rpm for 60 min, which were found by the proponents to be the optimum HSM parameters for this mixer [19]. During HSM the mixture temperature was maintained between o C by using a cold water bath. After that, the epoxy/clay mixture was fully degassed at 100 o C for 2 hours and then at 65 o C for 8 hours. Stoichiometric amount of the hardener was then added to the mixture (24 g of hardener for each 100 g of epoxy) and gently mixed. Finally, the mixture was poured into an aluminum mold and pre-cured at 100 o C for one hour followed by post-curing at 170 o C for another hour. These curing conditions were determined in earlier work [20] to be optimum for epoxy and hardener used in this study. The procedure followed during the fabrication of nanocomposites by sonication is similar to that used to synthesize nanocomposites employing HSM except that instead of HSM, the nanoclay was dispersed for 30 min using Sonic vc-33 high intensity ultrasonicator, which was found to be the optimum sonication time for the epoxy and clay used in this work [21]. For nanocomposites synthesized using both HSM and sonication; the mixture of epoxy and nanoclay was first mixed using HSM and then degassed at 100 o C for 5 hours. After that the mixture was sonicated for 30 minutes followed by fully degassing at 100 o C for the first 2 hours and then at 65 o C for 8 hours. For the sample prepared by hand mixing; the preparation process was similar to that used during the fabrication of nanocomposites with HSM except that instead of HSM the epoxy and clay have been hand mixed for 10 minutes using a stirring rod. Characterization of Nanocomposites. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to inspect nanoclay dispersion in the resultant nanocomposites. All scans for XRD were performed from 2θ = 2 to 10º using steps of 0.02º. TEM specimens were cut by a diamond knife into slices of 150 nm thickness that collected on cupper grids. Mechanical Properties. Tensile tests were performed for neat epoxy and nanocomposites according to ASTM-D638-94b Type V specification with a loading rate of 1 mm/min, using Instron universal machine. Samples were cut and machined into dumbbell-shaped tensile specimens with a dimension of 2.5 mm thickness, 63.5 mm overall length, 9.53 mm length of the reduced section, 9.53 mm overall width and 3.18 mm width of the narrow section. Six specimens for each sample were tested. Results and Discussion Microstructure Examinations. The morphology of the resultant nanocomposite depends on the mixing technique used to disperse clay into epoxy matrix. Fig. 1 illustrates the XRD results for clay powder, neat epoxy and nanocomposites prepared using different mixing methods. As can be seen in

3 Advanced Materials Research Vols the figure, a sharp peak is observed for clay powder at diffraction angle of 4.0 o resulting in a d-spacing of 2.2 nm. This value of d-spacing of the clay powder is similar to that reported in other studies for I.30E nanoclay [10, 22]. However, no peak is observed for neat epoxy confirming the amorphous nature of the material structure. Meanwhile, observable peaks with intensities lower than that for clay powder are present for the nanocomposites prepared by hand mixing and sonication. The angular positions of these peaks are shifted to the left indicating an increase in the d-spacing. For the sample prepared using hand mixing the peak is at about 3.56 o, hence the d-spacing is 2.48 nm. This small increase in the d-spacing for the sample fabricated using hand mixing can be mainly attributed to the diffusion of some epoxy molecules into the intergallery spacing between clay layers. Similarly, the nanocomposites prepared using sonication showed peaks at about 3.26 o leading to an increase in the d-spacing to 2.7 nm. This means that the resultant structures for the samples fabricated using hand mixing and sonication are dominated by intercalated morphology. However, no peak is present in the curves of samples fabricated using either HSM or HSM & sonication indicating exfoliated or disordered intercalated morphology. This outcome indicates that HSM is more effective than sonication in the dispersion of nanoclay in epoxy. For more detailed examination of mixing techniques and its effect on the morphology of the resultant nanocomposites at the nanoscale, TEM was used. Fig. 2 shows high magnified TEM micrographs for nanocomposites fabricated using different mixing methods. As illustrated in the figure, the nanocomposites fabricated with HSM and HSM & sonication showed disordered intercalated morphology with some exfoliation explaining the absence of peaks in the XRD spectra for these nanocomposites; since the ordered intercalated morphology is the one producing the peaks in the XRD spectra. This morphology is comparable to that obtained by Liu et al. [3] with high pressure mixing and Ngo et al. [8] using high speed mixing of similar nanocomposites. As indicated by the circles in Fig. 2 (c), some of clay layers seem to have been broken during the process for sample prepared by HSM & sonication. For samples synthesized by sonication and hand mixing, ordered intercalated morphology was observed as shown in Fig. 2 (b) & (d) supporting the outcome of the XRD results. Fig. 1. X-ray diffraction spectra for clay powder, neat epoxy and nanocomposites (NC) prepared with different mixing techniques. To obtain microscopic view of the effect of mixing techniques on the nanoclay dispersion, low magnified TEM images were used as shown in Fig. 3. As can be seen in the figure, the samples prepared with HSM and HSM & sonication have almost the same degree of clay dispersion which seems better than that for sample fabricated with sonication. However and as expected, the sample prepared by hand mixing shows very bad dispersion of clay within the epoxy matrix where all clay are agglomerated in large clusters which provide clear evidence that hand mixing is ineffective in dispersing nanoclay in epoxy matrix. In fact, hand mixing is unable to generate the forces needed to overcome the attraction forces between clay.

170 Advances in Materials and Materials Processing Comparison of TEM micrographs and XRD curves for nanocomposites prepared with HSM and samples prepared with HSM & sonication showed that the

4 170 Advances in Materials and Materials Processing Comparison of TEM micrographs and XRD curves for nanocomposites prepared with HSM and samples prepared with HSM & sonication showed that the enhancement in the nanoclay dispersion as a result of sonication after HSM is small. To further examine the effectiveness of the combined HSM & sonication mixing method the order of sequence of the two techniques was reversed. Another nanocomposite was fabricated starting with sonication followed by high shear mixing. The results of XRD and TEM analyses showed that the mixing sequence does not have much effect on the morphology of resultant nanocomposite. The above finding provides clear evidence that HSM is more effective in clay dispersion than sonication and the additional sonication after HSM has a negligible effect on the microstructure of the resultant nanocomposites. Exfoliated morphology Fig. 2. High magnified TEM micrographs for nanocomposites prepared using (a) HSM, (b) sonication, (c) HSM & sonication and (d) hand mixing. Mechanical Properties and Fractographic analysis. Tensile tests were curried out to determine the mechanical properties of neat epoxy and nanocomposites. Fig. 4(a) displays representative stress-strain curves for neat epoxy and nanocomposites prepared with different mixing techniques. The effects of mixing techniques on the tensile strength and modulus of elasticity are illustrated in Fig. 4(b) which shows that the tensile strength of the nanocomposite fabricated by HSM is almost the same as that of neat epoxy. However, noticeable reduction in strength for other samples is observed especially for the one prepared by hand mixing which showed 10% reduction in strength compared with pure epoxy. This decrease in the tensile strength for nanocomposites can be attributed to the existence of agglomerated clay clusters, which act as preferred sites for crack initiation that lead to premature failure. This was clear from the fractographic analysis of the tensile fracture surface, Fig. 5 which illustrates that the cracks were initiated at clay aggregates their size were about 25 and 300 µm for nanocomposite fabricated using sonication and hand mixing; respectively. The cracks are expected to initiate at clay cluster because clay has a much greater modulus than epoxy, hence stress concentrations may have existed at the interfaces between clay cluster and epoxy matrix. The decrease in the tensile strength as a result of clay addition has been reported in a number of studies [4,12,22]. It should also be mentioned that some other studies showed an increase in the tensile strength as a result of clay addition [11,13].

Advanced Materials Research Vols. 652-654 171 Fig. 3. TEM images for the nanocomposites prepared using (a) HSM, (b) sonication, (c) HSM & sonication and (d) hand mixing. As indicated in Fig.

5 Advanced Materials Research Vols Fig. 3. TEM images for the nanocomposites prepared using (a) HSM, (b) sonication, (c) HSM & sonication and (d) hand mixing. As indicated in Fig. 4(b), regardless of the mixing method the addition of nanoclay improves the stiffness of the material. The type of the mixing technique does not seem to affect the modulus of elasticity of the nanocomposite. Fig. 4(a) shows that a noticeable reduction in the fracture strain was brought about by the addition of clay to the epoxy; the lowest value was for the sample prepared by hand mixing which shows a reduction of 34%. The improvement in elastic modulus and reduction in fracture strain are typical for the epoxy-clay nanocomposites [7,11]. The improvement in elastic modulus can be attributed to the dispersion of nanoclay that possess high elastic modulus which restricts the mobility of polymer chains during loading as well as to the good interfacial adhesion between the clay particles and the epoxy matrix [23]. a b Fig. 4. (a) Representative stress-strain curves for neat epoxy and nanocomposites prepared using different mixing techniques, (b) Comparison of tensile strength and modulus of elasticity.

$SEM fractographs for nanocomposites prepared by (a) sonication and (b) hand mixing (Х300). Fig.$ $6 illustrates high magnified SEM micrographs comparing the fracture surface morphology of neat epoxy and nanocomposites$ $As clear from the figure, the fracture surfaces of the nanocomposites fabricated by HSM and sonication are rough while$ $This roughness in the fracture surfaces of nanocomposites can be related to the change in the crack path due to the$ $This roughness may also indicate an improvement in fracture toughness since higher roughness means higher plastic$ $6 the fracture surface roughness for HSM and HSM & sonication is almost the same (Fig.$ $Other than the presence of clay aggregates, the fracture surface of the sample prepared by hand mixing is similar to that$

6 172 Advances in Materials and Materials Processing a b Fig. 5. SEM fractographs for nanocomposites prepared by (a) sonication and (b) hand mixing (Х300). Fig. 6 illustrates high magnified SEM micrographs comparing the fracture surface morphology of neat epoxy and nanocomposites fabricated by different mixing techniques. As clear from the figure, the fracture surfaces of the nanocomposites fabricated by HSM and sonication are rough while smooth surface was observed for neat epoxy, similar results were observed by other studies [7, 22]. This roughness in the fracture surfaces of nanocomposites can be related to the change in the crack path due to the presence of clay in crack direction. This roughness may also indicate an improvement in fracture toughness since higher roughness means higher plastic deformation and hence higher energy absorbed during crack propagation. As indicated in Fig. 6 the fracture surface roughness for HSM and HSM & sonication is almost the same (Fig. 6-b&d) which is higher than that for sonication (Fig. 6-c) while low roughness is observed for the sample prepared by hand mixing (Fig. 6-e). Other than the presence of clay aggregates, the fracture surface of the sample prepared by hand mixing is similar to that of neat epoxy. This indicates that the enhancement in the fracture toughness of the nanocomposites synthesized by HSM and HSM & sonication is higher than that fabricated by sonication, whereas, the fracture toughness of the sample prepared by hand mixing is comparable to that of neat epoxy. a b c d e Fig. 6. SEM fractographs (Х2000) of (a) neat epoxy and nanocomposites synthesized by (b) HSM (c) sonication, (d) HSM & sonication and (e) hand mixing.

7 Advanced Materials Research Vols Conclusions Epoxy-clay nanocomposites containing 2% of I.30E clay as nano-reinforcement into diglycidyl ether of bisphenol A (DGEBA) epoxy matrix were synthesized using different mixing techniques. The effect of three mixing methods; namely: HSM, ultrasonication and hand mixing and their combination on the morphology and mechanical properties of the resultant nanocomposites were investigated. The results showed that the degree of nanoclay dispersion in the nanocomposites synthesized by HSM was better than that prepared by sonication and the additional sonication after HSM has negligible effect on the microstructure of the resultant nanocomposites. Both nanocomposites fabricated by HSM and HSM with sonication showed disordered intercalated and exfoliated morphologies while the nanocomposite prepared by sonication was dominated by intercalated morphology which indicates that HSM is more effective in clay dispersion than sonication. As expected, the sample prepared by hand mixing showed very bad dispersion of clay within the epoxy matrix and all clay platelets were agglomerated in large clusters. The results of the tensile tests showed 12% improvement in the modulus of elasticity due to clay addition and the tensile strength of the nanocomposite fabricated by HSM is almost the same as that of neat epoxy. However the strength of nanocomposites synthesized by other mixing techniques is lower than that of neat epoxy and the lowest strength is for the sample prepared by hand mixing. This outcome indicates that the nanocomposite synthesized by HSM has the optimum mechanical properties while hand mixing, as expected, led to the worst results. Acknowledgements The authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. 08-ADV6804 as part of the National Science, Technology and Innovation Plan. References [1] L.A. Utracki: Clay-Containing Polymeric Nanocomposites (First ed., Rapra Technology Limited, UK 2004). [2] K. Wang, S. Zhou, Z. Jiang and X. Zhu: Advanced Materials Research Vols (2012), p [3] W. Liu, S. V. Hoa, M. Pugh: Composites Science and Technology Vol. 65 (2005), p [4] M. J. Adinoyi, N. Merah, Z. Gasem and N.Al-Aqeeli: Key Engineering Materials Vols (2011), p [5] S. Ghafarloo and M. Kokabi: Advanced Materials Research Vols (2010), p [6] K. Dean, J. Krstina, W. Tian and R.J. Varley: Macromolecular Material and Engineering Vol. 292 (2007), p [7] L. Wang, K. Wang, L. Chen, Y. Zhang and C. He: Composites: Part A Vol. 37 (2006), p [8] T. D. Ngo, M. T. Ton-That, S. V. Hoa, K. C. Cole: Composites Science and Technology Vol. 69 (2009), p [9] J. Wang, X. Kong, L. Cheng and Y. He: Journal of University of Science and Technology Beijing Vol.15(3) (2008), p [10] S. C. Zunjarrao, R. Sriraman and R. P. Singh: Journal of Material Science Vol. 41 (2006), p

8 174 Advances in Materials and Materials Processing [11] S.R. Ha, S.H. Ryu, S.J. Park and K.Y. Rhee: Materials Science and Engineering Part A Vol. 448 (2007), p [12] P I. Xidas and K. S. Triantafyllidis: European Polymer Journal Vol. 46 (2010), p [13] S. Zainuddin, M.V. Hosur, Y. Zhou, A. T. Narteh, A. Kumar and S. Jeelani, Materials Science and Engineering A Vol. 527 (2010), p [14] F. Hussain, J. Chen and M. Hujjati: Materials Science and Engineering A Vols (2007), p [15] B. Akbari and R. Bagheri: European Polymer Journal Vol. 43 (2007), p [16] S. R. Ha, K. Y. Rhee, H. C. Kim and J. T. Kim: Colloid and Surfaces A: Physicochem. Eng. Vols (2008), p [17] C. Chen, M. Khobaib and D. Curliss: Progress in Organic Coatings Vol. 47 (2003), p [18] B.C. Kim, S.W. Park and D.G. Lee, Composite Structure Vol. 86 (2008), p. 69. [19] M. Al-Qadhi, N. Merah, Z. Khan, K. Mezghani and J. Adinoyi: ECCM15 15th European Conference on Composite Materials, Venice, Italy, June 2012 [20] M. Al-Qadhi, N. Merah and K. Mezghani: Key Engineering Materials Vol (2011), p [21] M. J. Adinoyi, N. Merah, N. Al-Aqeeli and Z. Gasem: Key Engineering Materials Vols (2011), p [22] B. Qi, Q. X. Zhang, M. Bannister and Y. W. Mai: Composite Structures Vol. 75 (2006), p [23] H. Miyagawa, L. T. Drzal: Journal of Adhesion Science and Technology Vol. 18 (2004), p

9 Advances in Materials and Materials Processing / Effects of Processing Techniques on Morphology and Mechanical Properties of Epoxy-Clay Nanocomposites / DOI References [23] H. Miyagawa, L. T. Drzal: Journal of Adhesion Science and Technology Vol. 18 (2004), p [22] B. Qi, Q. X. Zhang, M. Bannister and Y. W. Mai: Composite Structures Vol. 75 (2006), p [21] M. J. Adinoyi, N. Merah, N. Al-Aqeeli and Z. Gasem: Key Engineering Materials Vols (2011), p [17] C. Chen, M. Khobaib and D. Curliss: Progress in Organic Coatings Vol. 47 (2003), p [16] S. R. Ha, K. Y. Rhee, H. C. Kim and J. T. Kim: Colloid and Surfaces A: Physicochem. Eng. Vols (2008), p [15] B. Akbari and R. Bagheri: European Polymer Journal Vol. 43 (2007), p [14] F. Hussain, J. Chen and M. Hujjati: Materials Science and Engineering A Vols (2007), p [12] P I. Xidas and K. S. Triantafyllidis: European Polymer Journal Vol. 46 (2010), p [11] S.R. Ha, S.H. Ryu, S.J. Park and K.Y. Rhee: Materials Science and Engineering Part A Vol. 448 (2007), p [9] J. Wang, X. Kong, L. Cheng and Y. He: Journal of University of Science and Technology Beijing Vol. 15(3) (2008), p [8] T. D. Ngo, M. T. Ton-That, S. V. Hoa, K. C. Cole: Composites Science and Technology Vol. 69 (2009), p [7] L. Wang, K. Wang, L. Chen, Y. Zhang and C. He: Composites: Part A Vol. 37 (2006), p [6] K. Dean, J. Krstina, W. Tian and R.J. Varley: Macromolecular Material and Engineering Vol. 292 (2007), p [5] S. Ghafarloo and M. Kokabi: Advanced Materials Research Vols (2010), p [4] M. J. Adinoyi, N. Merah, Z. Gasem and N. Al-Aqeeli: Key Engineering Materials Vols (2011), p

10 [3] W. Liu, S. V. Hoa, M. Pugh: Composites Science and Technology Vol. 65 (2005), p [2] K. Wang, S. Zhou, Z. Jiang and X. Zhu: Advanced Materials Research Vols (2012), p

EFFECT OF SHEARING ON DISPERSION, INTERCALATION/EXFOLIATION OF CLAY IN EPOXY

16 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS EFFECT OF SHEARING ON DISPERSION, INTERCALATION/EXFOLIATION OF CLAY IN EPOXY Tri-Dung Ngo*, Van-Suong Hoa*, Minh-Tan Ton-That** * Department of Mechanical