INFLUENCE OF CARBON NANOFIBERS AND PIEZOELECTRIC PARTICLES ON THE THERMOMECHANICAL BEHAVIOR OF EPOXY MIXTURES

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1 Solid State Phenomena Vols (2007) pp Online: (2007) Trans Tech Publications, Switzerland doi: / INFLUENCE OF CARBON NANOFIBERS AND PIEZOELECTRIC PARTICLES ON THE THERMOMECHANICAL BEHAVIOR OF EPOXY MIXTURES S. Tsantzalis 1,a, P. Tsotra 1,b, P. Karapappas 1,c, A. Vavouliotis 1,d, N. Fanis 1,e, V. Kostopoulos 1,f, K. Friedrich 2,g 1 Applied Mechanics Laboratory, Department of Mechanical Engineering and Aeronautics, University of Patras, Patras 26500, Greece 2 Institut fuer Verbunwerkstoffe GmbH, Erwin-Schroedinger Str., 67663, Kaiserslautern, Germany a stavrost@mech.upatras.gr, b tsotra@mech.upatras.gr, c pkarapap@mech.upatras.gr, d vavoul@mech.upatras.gr, e nfanis@mech.upatras.gr, f kostopoulos@mech.upatras.gr, g klaus.friedrich@ivw.uni-kl.de Keywords: Epoxy, carbon nanofibers, piezoelectric particles, curing reaction Abtract. Vapor growth carbon nanofibers (CNFs), lead zirconate titanate piezoelectric (PZT) particles, as well as a combination of these two were added in an epoxy resin (EP), and their influence on the curing reaction was investigated. Moreover, the cured samples were characterised by dynamic scanning calorimetry and dynamic thermal mechanical analysis. The presence of the fillers had no significant effect of the curing reaction of the EP system and the glass transition temperature, T g. Introduction Carbon nanofibers (CNFs) are one of the most promising mechanical reinforcing materials for polymeric composites due to their high axial Young s modulus, high aspect ratio, large surface area and excellent thermal and electrical properties [1]. Nevertheless, to develop high performance CNF/polymer composites great attention should be paid in the homogeneous dispersion of CNFs in the polymeric matrix and the interface between the two components so as to achieve efficient load transfer from the matrix to the CNFs [1-4]. Piezoelectric (PZT) particles constitute another interesting type of filler materials. Based on the principles of the piezoelectric theory, PZTs can been used for the suppression of vibration and impact noices. Positive results towards this direction have been reported in the case where PZTs were spread between the interleaves of carbon fiber reinforced plastics (CFRPs) [5]. The goal of the present study was the investigation of the influence of CNFs and PZT particles on the curing of an epoxy matrix and the final thermomechanical 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-18/09/15,23:25:10)

2 1420 Nanoscience and Technology properties of the cured samples. Experimental CNFs, Pyrograf-III, with diameters from nm were obtained from Applied Sciences Inc. In the case of PZT fillers, lead zirconate titanate particles (PbZrO 3 -PbTiO 3 ) of ellipsoid shape were used, PE-60A by Fuji Titanium Industry Co., Ltd. The microstructure of the fillers, as observed by scanning electron microscopy (SEM), is presented in Fig. 1. The thermosetting matrix was an epoxy resin (EP), DER 331, Dow Chemicals, cured with an amine curing agent, HY 2954, Vantico. Isotropic samples with different fillers concentrations were prepared: 0.5, 1 and 1.5 vol.% CNF, and 4, 10 and 20 vol.% PZT, as well as the combination of 1 vol.% CNF and. The mixing of the epoxy with the fillers took place in a dissolver device (Dispermat AE, VMA-Getzmann GmbH), under vacuum in order to ensure the absence of air in the samples. Fig. 1: Scanning electron microscope images of a) CNFs and b) PZT particles. In order to investigate the influence of the fillers on the curing of the EP matrix, freshly mixed samples were put in a differential scanning calorimetric (DSC) device, DSC 821 from Mettler, and were heated up to 200 C with a heating rate of 10 C/min. Further on cured samples were heated up to 270 C, with a heating rate of 10 C/min, then cooled down and finally heated again. In this way the glass transition temperature (T g ) was estimated and a consensus was reached regarding their cured state. In addition, the viscoelastic response of the cured samples was studied by dynamic mechanical thermal analysis (DMTA). An Eplexor TM 150N (Gabo Qualimeter) device was employed to carry out the tests. The measurements were made under tension loading with a testing frequency of 10 Hz. Results and Discussion Fig. 2 shows the heat flow during the heating of EP mixtures with various contents of PZT particles and CNFs. A single exothermic peak is observed for the different concentration of CNFs and PZT particles in the EP matrix. The total heat of curing reaction was estimated by connecting the baseline before and after the peak with a straight line and by integrating the area under the peak. The heat of curing (ΔH) values, corresponding to each curve, are summarised in

3 Solid State Phenomena Vols Table 1. It can be observed that ΔH decreases with an increasing amount of PZT particles and CNFs in the mixtures. This is to be expected as the amount of EP in the mixtures is reduced. However, by normalising these ΔH values, i.e. dividing them by the heat of curing of the neat EP system and the weight fraction of EP in the blends (W EP ) no influence of the fillers on the curing process of the EP-system was evident. The extent of curing, that is the percentage of cured components, was stable with increasing amount of CNFs or PZT particles. The T g values as determined from the DSC thermograms of the cured samples are also shown in Table 1. The presence of PZT particles or CNFs had no significant influence on the T g compared to the sample of neat EP. This fact shows once more that the fillers do not disturb the formation of the crosslinked network. Heat Flow [mw/g] Exo 10 vol.% PZT 20 vol.% PZT EP Heat Flow [mw/g] EP 0.5 vol.% CNF Exo 1.5 vol.% CNF 1 vol% CNF + a) b) Temperature [ o C] Temperature [ o C] Fig. 2: DSC curves of uncured samples with various contents of (a) PZT particles and (b) CNFs and PZT particles (heating rate: 10 C/min). Table 1: The heat of curing, ΔH, of different EP/PZT-CNF mixtures, and T g values as determined from the DSC thermograms of the cured samples. CNF [vol.%] PZT [vol.%] CNF Epoxy PZT ΔH [J/g] T g [ o C] In Fig. 3a the influence of the PZT particles and CNFs on the loss factor, tanδ, is shown. The addition of has no significant effect on the glass transition temperature, T g, compared to the neat EP sample, while an increase in tanδ is observed, which implies enhanced damping behaviour. The higher PZT concentrations led to decrease in tanδ, probably due to the stiff nature of the ceramic particles, and therefore based on the DMTA the concentration of 4% PZT is the optimal one for enhancing the damping properties of EP without affecting the network quality. The presence of CNFs decreases the loss factor, which remained unchanged by further addition of the PZT particles. Therefore, the expectation of enhanced damping properties by the combination of the two fillers was not verified by this analysis. However, additional experiments are at the moment under process in order to study the damping properties by means of free vibration analysis. Regarding the storage modulus, E, the CNFs led to a significant increase in this value compared to the neat EP sample for temperatures under T g. This reflects the ability of the high

4 1422 Nanoscience and Technology aspect ratio fibers to stiffen the overall composite material. The addition of the PZT particles in EP resulted also in higher E values, however lower than that of CNFs filled samples, while their incorporation in the CNF/EP samples hindered the reinforcing effect of the CNFs. Loss Factor [1] & 4 vol.%pzt a) 3000 b) 0.4 Epoxy & 4 vol.%pzt 0.1 Epoxy Storage Modulus, E' [MPa] Temperature [ C] Temperature [ C] Fig. 3: Influence of PZT particles and CNFs on (a) the loss factor, tanδ, and (b) the storage modulus, E, of neat and filled EP samples. Summary CNFs and PZT particles were used as fillers for an EP matrix. The monitoring of the heat flow during the curing of the mixtures showed that the fillers had no effect on the curing reaction. The glass transition temperature of EP remained almost unchanged after the addition of the PZT particles or CNFs. Moreover, the DMTA showed and increase in tanδ by the addition of. The presence of CNFs decreased the damping properties of the EP system, which were not enhanced by the addition of the PZT particles. The storage modulus was considerably increased in the case of CNFs, indicating a strong reinforcing effect of this filler even for very low concentrations. Acknowledgements The financial support of German Academic Exchange Service (DAAD) and Greek State Scholarships Foundation (I.K.Y.) within the frame of IKYDA project is gratefully acknowledged. The authors are also thankful to Prof. T. Tanimoto for supplying the PZT particles and his scientific support. References [1] E. Hammel, X. Tang, M. Trampert, T. Schmitt, K. Mauthner, A. Eder and P. Prötschke: Carbon Vol. 42 (2004), p.1153 [2] O.S. Carneiro, J.A. Covas, C.A. Bernardo, G. Caldeira, F.W.J. Van Hattum, J.-M. Ting, R.L. Alig and M.L. Lake: Comp. Sci. Techn. Vol. 58 (1998), p.401. [3] R.D. Patton, C.U. Pittman Jr., L. Wang and J.R. Hill: Comp. Part A Appl. Sci. Vol. 30 (1999), p [4] J. Xua, J. P. Donohoeb, C. U. Pittman Jr., Comp. Part A Appl. Sci. Vol. 35, (2004) p.693. [5] T. Tanimoto: Tagungsband IVW-Kolloquium (Kaiserslautern, Germany, 2004).

5 Nanoscience and Technology / Influence of Carbon Nanofibers and Piezoelectric Particles on the Thermomechanical Behaviour of Epoxy Mixtures /