Dielectric Response of Modified Epoxy/Clay ... - IEEE Xplore

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4 Nottingham Trent University, UK. Abstract: Thermoset polymers are frequently used as matrices for nanocomposite materials because nanofillers, such as clay,.
Dielectric Response of Modified Epoxy/Clay Nanocomposites I. Preda 1,2, H. Couderc 2,3, M. Fréchette 2, S. Savoie 2, F. Gao 4, R. Nigmatullin 4, S.Thompson 4, J. Castellon 1 1

Institut d’Électronique du Sud, Université Montpellier 2, France 2 Institut de Recherche d’Hydro-Québec (IREQ), Canada 3 École de Technologie Supérieure, Université du Québec, Canada 4 Nottingham Trent University, UK

gaps between them called interlayer space. As the forces that hold the stacks together are relatively weak van der Walls forces, the intercalation of small molecules into the gaps between the layers is possible. A partial exchange of the sodium ions with organic ions can be carried out in order to increase the interlayer spacing and to improve the compatibility between the clay and polymer. Therefore intercalated structures containing organophilic surfactants can be obtained [8, 9]. Moreover, in this paper organoclay conditioning was applied to control the quantities of water and extra surfactants in the clay. Since the surfactant cations are charged molecules, they may have influence on both dielectric performance of the polymer composites and the intercalation behavior. In addition to the extra cations, water may play an important role in the intercalation and the dielectric properties. Therefore, the different influence of dryness of clay prior to mixing into the epoxy matrix was investigated in this work. Thermogravimetric Analysis (TGA) was used in order to obtain the volatile content of the nanocomposites based on dried and non-dried organoclays. Since the moisture trapped at the clay/epoxy interface was supposed to have a plasticizing effect for the non-dried samples, the thermal stability of all the samples was investigated using Differential Scanning Calorimetry (DSC), with the glass transition temperature chosen as an indicator. Finally, the dielectric response of the materials was investigated at room temperature (273 K) using Broadband Dielectric Spectroscopy (BDS).

Abstract: Thermoset polymers are frequently used as matrices for nanocomposite materials because nanofillers, such as clay, can be easily incorporated into liquid polymer precursors. Since in the crystal structure of clay such as Montmorillonite the layers are physically linked by cations, replacing these cations with larger organophilic molecules changes the clay surface into hydrophobic and increases the interlayer space. This improves the compatability between montmorillonite and most of engineering polymers, making it suitable for nanocomposite application. This paper investigates the influence of surface treatment on calorimetric and dielectric properties of a commercially available organoclay with different extent of free surfactant and moisture level, using filled epoxy nanocomposites. The moisture content of the nanocomposites was measured using Thermogravimetric Analysis (TGA). Thermal stability was investigated using Differential Scanning Calorimetry (DSC) with the glass transition temperature chosen as an indicator. Finally, dielectric response of the materials was investigated at room temperature using Broadband Dielectric Spectroscopy (BDS).

I. INTRODUCTION Nowadays the use of nanotechnologies in the field of electrical insulation for power and high voltage engineering has attracted researchers because innovative materials such as epoxy nanocomposites show enhanced electrical, mechanical and thermal properties [1, 2]. However, in order to form a desired nanostructure, the nanofiller content cannot be high due to the decrease of interparticle distance which could lead to agglomeration [3]. A possible solution to obtaining similar promising results is the use of nanostructured microcomposites, containing both nano and micro fillers [4]. Amongst nanocomposite materials, clay and layered silicate filled polymers have been investigated intensively [5,6], probably because natural clay minerals are easily available and their intercalation chemistry being studied long time [7]. Montmorillonite (MMT) is a crystalline, layered structured clay mineral with a central alumina octahedral sheet sandwiched between two tetrahedral silica sheets. The electrostatic bonds between the sheets are achieved by sodium or magnesium cations. MMT is considered nanostructured microcomposite because the thickness of each individual silicate layer is around 1 nm and the lateral dimensions of these layers are in the 100-300 nm range. The layers organize themselves to form a stacked layered structure with regular

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II.

MATERIALS AND SAMPLE PREPARATION

The epoxy resin used in the study was diglycidyl ether of bisphenol A based resin, commercially named DDR332. The selected hardener was poly(propylene glycol) bis(2aminopropyl ether) (PPG-b-AP). CPTES was used as a coupling agent. The ratio of hardener to the epoxy resinwas of 32.4 g to 100 g. For this work, Cloisite 30B (C30B), a natural montmorillonite (MMT) modified with a quaternary ammonium ion (MT2etOH - methyl, tallow, bis-2hydroxyethyl, quaternary ammonium) was chosen and the clay content for all the composites was 2 wt%. In order to further improve the compatibility between the clay and the polymer matrix, surface treatment using (3-

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chloropropyl) triethoxysilane (CPTES) was applied. To treat organoclay with CPTES, 15 g of organoclay were dispersed in 600 ml of distilled water by mixing under magnetic stirring for 3 hours. Also, 13.5 g of CPTES were stirred in distilled water for 1.5 hours to initiate silane hydrolysis at room temperature. The solution was added into the clay suspension and the resulting mixture was stirred for 48 hours at room temperature to obtain surface modified clay. The modified clay was separated from the solution by filtration and finally dried using a freeze dryer. Clay used directly after silanisation was defined as unwashed organoclay. In order to investigate the role of extra surfactant on dielectric properties, some silane treated organoclays were washed using Soxhlet extraction for 8 h with a mixture of H2O/ethanol (25/75 by volume). Afterwards, the washed products were dried at 80°C for 24 h. The organoclay after Soxhlet extraction was defined as washed organoclay. For two samples, before compounding, organoclays were additionally dried at 120oC for 3 hours under vacuum. For composite processing, two glass plates were used as mould. Their surfaces were coated with a mould release agent and partially sealed with silicon adhesive and desired gap equal to the thickness of the sample. The resin was injected into the gap in the unsealed opening and vertically placed in an oven. Following this process, the samples were cured at 100-105°C for 4 hours. Except for transportation, the samples were kept in a controlled dry and UV proof environment, at room temperature. The studied materials, their specifications and the codes used in this paper are shown in Table 1.

isotherm for 10 minutes at 523 K in order to remove thermal history, followed by a cooling run down to 223 K and ending with a second heating run to 523 K. The glass transition temperature (Tg) was measured in the final heating cycle. C. Broadband Dielectric Spectroscopy Dielectric relaxation phenomena were investigated using BDS, for frequencies going from 10-1 Hz to 300 kHz. Samples were produced in disc shape, with thickness of approximately 1 mm and a diameter of around 20 mm. They were studied in a parallel plate configuration. In addition, 50 µm thick silver foil electrodes were used to ensure a good contact between the sample and brass electrodes of the instrument. A sinusoidal voltage of 1Vrms was applied and, for each point, the final values for the obtained parameters represent the average value of 10 consecutive measurements. All the measurements were made under nitrogen with a ±0.01 K temperature deviation. The following measurement protocol was used for all the available samples: an initial characterization was performed at 293 K, followed by 24 hours of post-thermal treatment at 373 K and finally a dielectric relaxation map of the sample was performed from 223 K to 373 K, with a 5 K step. In this paper, only the results obtained at room temperature after the heat treatment will be discussed. Using BDS measurements, the real and imaginary parts of the complex dielectric permittivity were obtained. The complex dielectric function that connects the two parts is given by the following equation:

 * ( f )   ( f )  i ( f )

TABLE I INVESTIGATED SAMPLES Sample Neat epoxy resin Epoxy + 2wt% C30B+3Chlorosilane +Soxhlet H2O washed, dried Epoxy + 2wt% C30B+3Chlorosilane, unwashed, dried Epoxy + 2wt% C30B+3Chlorosilane +Soxhlet H2O, washed, undreid Epoxy + 2wt% C30B+3Chlorosilane, unwashed, undreid

(3)

where f is the frequency of the applied voltage, ε' the real or conservative part and ε'' the imaginary or loss part.

Sample Code NE

IV. RESULTS AND DISCUSSION

WD UW-D

A. TGA and DSC Results Changes in mass losses as a function of temperature obtained from thermogravimetric measurements are shown in Figure 1.

W-UD UW-UD

III. EXPERIMENTAL METHODS A. Thermogravimetric Analysis TGA experiments were conducted first under inert nitrogen atmosphere, between 323 K and 773 K, using a heating rate of 10 K/min. After a 5-minute isotherm at 873 K, for measurements performed at temperatures going from 773K to 1073 K (with the same heating rate), dry air was used in order to oxidize the residual carbon.

b)

a)

B. Differential Scanning Calorimetry DSC measurements were conducted under inert nitrogen atmosphere at a heating/cooling rate of 10 K/min, with the following experimental procedure applied for all the samples: an initial heating run, from 223 K to 523K, continued with an

Figure 1. Thermogravimetric analysis curves (weight as temperature function) for all the samples

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investigated since better dispersed clay can cause a decrease of Tg) have significant impact on the vitreous transformation . While trying to compare what can be compared and contrasted, one can observe that for the samples having close filler ratio (WD and UW-D), an important difference in volatiles content was found (about 10%). In the same time, the glass transition temperature was almost the same (about 347 K). In this case, we can assume that the given difference in volatiles does not correspond to water and that the solvent used for washing the samples had a more significant impact on thermal stability, as He [10] has discussed it in his work. B. BDS Results Many studies have shown that the polymer-particle interface is a potential location for water [11-13]. Since dramatic increase of polymer/clay interface area follows clay intercalation or exfoliation, these materials are thus particularly vulnerable to water uptake [14]. Post thermo treatment was found to have a substantial effect on dielectric stability of the material. This is mainly associated with the contribution at the end of the curing reaction, while partially removing the moisture contained in the material [15,16]. As already presented in our previous paper [17], several tests were carried out using neat epoxy samples to find the best way to complete the crosslinking process and to remove the uptake water. Ultimately, the condition for the selected post-thermal treatment required 24 hours at 373 K. After performing this post-thermal treatment, all the samples were considered to be in a stable but aged dielectric state and their dielectric responses could be used in the characterization process. The evolution of real permittivity (not presented here) was measured at 293 K and it was found that the low permittivity values of the samples increased when frequency decreased. At low frequencies, all the composite samples had lower permittivity than the neat epoxy: ε′ at 0.1 Hz goes from 4.05 to 4.26 for the composite samples, while ε′ for neat epoxy was 4.28. Similar results have been found in the literature [18]. Dielectric losses evolution at 293 K after the post-heat treatment process, are shown in Figure 4.

Figure 2. DSC curves obtained during the Tg measurements TABLE II TGA AND DSC OBTAINED VALUES Sample

Volatiles (%)

Filler (%)

Tg (K)

NE WD UW-D W-UD UW-UD

2.75 2.21 2 2.14 2.16

/ 0.81 0.89 1.21 1.43

361.1 ± 0.5 347.6 ± 0.5 347.2 ± 0.5 337.9 ± 0.5 332.6 ± 0.5

A slight weight decrease was initially observed for all the samples at temperatures lower than 500 K. Although some other substances may evaporate below this temperature, the weight loss is mostly attributed to water content. Also, as shown in inset a) from Figure 1, the addition of clay into nanocomposites increases the water resistance of epoxy. As the temperature increases, the weight loss becomes greater. At 773 K, when the purge gas is changed from nitrogen to air, a sudden weight loss occurs, probably due to the combustion of the sample. The corresponding filler wt% values are presented in Table 2. Considering the resulting values, the pure resin had the highest water content. However, the water content decreases with the introduction of clays. Even though an impact of drying the samples and reducing the volatiles content was expected, the composite samples have close values, with the UW-D sample having the lowest volatile content. The filler ratio is slightly different, going from 0.8 to 1.43 %wt. The DSC thermograms obtained during the second heating run are presented in Figure 2 and the corresponding glass transition temperatures Tg are given in Table 2. We can observe that the neat sample has the highest glass transition temperature. The reason for this may be associated with the change of functionality in the epoxy system due to the interaction with the clay. This could be confirmed for all the filled samples since the glass transition decreases as the filler percentage increases. Also, for the W-UD and UW-UD samples, even though they have close volatile content, they show a 5 K difference in Tg which confirms that the filler content and, maybe the dispersion degree (which needs to be

high frequency wing of α relaxation

low frequency wing of β relaxation

α' relaxation

Figure 4. Imaginary permittivity of the epoxy nanocomposite samples measured at 293 K after 24h of post heat treatment

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The high frequency peak corresponds to the left wing of the β relaxation and represents the dipole movements of lateral chains of the epoxy polymer, associated with the hydroxyl ether groups. The low frequency part (inferior to 100 Hz) corresponds to the right wing of the α relaxation peak and is associated with the main polymer chain movement before glass transition occurs. This relaxation is not visible for the neat epoxy sample at this temperature. Since the difference between the lowest glass transition (UW-UD sample) and that of the neat sample is of 30 K, the α relaxation of neat epoxy resin is expected to be visible at higher temperatures. At medium frequencies (100 to 102 Hz), an interesting phenomenon can be observed: between the main α relaxation and the secondary β relaxation a third α' relaxation peak is visible. This additional relaxation peak could be a slower α relaxation which is assigned to polymer chains close to the polymer/filler interface whose mobility is restricted due to interactions with the filler surface [19]. Also, a water-induced local molecular motion can be the origin of this process [2022]. This phenomenon needs to be further investigated using dielectric relaxation maps and the evaluation of the specific activation energies since the location and activation energy of the low temperature water-related relaxation are typical of local molecular motions, knowing that water-induced absorption is characterized by high values of the activation parameters.

REFERENCES [1] S. Singha and M.J. Thomas, "Dielectric Properties of Epoxy Nanocomposites", IEEE Trans. Dielectr. Electr. Insul, vol. 15, nr. 1, pp. 373-385, 2008. [2] T. Imai, F. Sawa, T. Ozaki, T. Shimizu, R. Kido, M. Kozako, T. Tanaka, "Influence of Temperature on Mechanical and Insulation Properties of Epoxy-layered Silicate Nanocomposite", IEEE Trans. Dielectr. Electr. Insul, vol. 13, no. 2, pp. 445-452, 2011. [3] T.J. Lewis, "Nanometric Dielectrics", IEEE Trans. Dielectr. Electr. Insul, Vol.1, no. 5, pp. 812-825, 1994. [4] J. Castellon, H.N. Nguyen, S. Agnel, A. Toureille, M. Frechette, S. Savoie, A. Krivda, L.E. Schmidt, "Electrical Properties of Micro and Nanocomposite Epoxy Resin Materials", IEEE Trans. Dielectr. Electr. Insul, vol. 18, no. 3, pp. 651-658, 2011. [5] M Alexandre, P. Dubois, “Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials”, Mat. Sci. Eng. Research, Vol. 28, pp.1–63, 2000. [6] F. Bergaya, B.K.G. Theng and G. Lagaky , “Handbook of Clay Science”, Vol.1, Elsevier, Amsterdam, 2006 [7] F. Annabi-Bergaya, “Layered clay minerals. Basic research and innovative composite applications”, Micropor. Mesopor. Mat, Vol. 107, pp.141–148, 2008. [8] T. Tanaka, Y. Ohki, M. Ochi, M. Harada and T. Imai, “Enhanced partial discharge resistance of epoxy/clay nanocomposites prepared by newly developed organic modification and solubilization methods”, IEEE T. Dielect. El. In, vol.15, no. 1, pp.81-89, 2008. [9] S. Thomas, G.E. Zaikov, “ Polymer nanocomposite research advances”, Nova Science Publishers, New York, 2007. [10] H.He, J.Duchet, J.Galy, J-F.Gérard, “Influence of cationic surfactant removal on thermal stability of organoclays”, J. Col. Int. Sci., Vol. 295, pp. 202-208, 2006. [11] W. Liu, S.V. Hoa and M. Pugh, “Water uptake of epoxy-clay nanocomposites: Experiments and model validation”, Compos. Sci. Technol, vol. 68, pp. 2066-2072, 2008. [12] C Zou, J.C. Fothergill and S.W. Rowe, “A «Water Shell» model for the dielectric properties of hydrated silica-filled epoxy nano-composites”, IEEE Int. Conf. Solid Dielect, pp. 389-392, 2007. [13] T. Glaskova and A. Aniskevich, “Moisture absorption by epoxy/montmorillonite nanocomposite”, Comp. Sci. Tech, vol. 69, pp. 2711-2715, 2009. [14] S. Raetzke, Y. Ohki, T. Imai, T. Tanaka and J. Kindersberger, “Tree initiation characteristics of epoxy resin and epoxy/clay nanocomposites”, IEEE T. Dielect. El. In, vol.16, 6, pp.1473-1480, 2009. [15] H. Couderc, M.F.Fréchette, S.Savoie and E.David, “Nanofiller effect during post-heat treatment of micro-loaded epoxy”, IEEE Conf. Electr. Insul. Dielect. Phenom, pp. 608-611, 2010. [16] M.F. Fréchette, E. David, H.D. Martinez and S. Savoie, “Post-heat treatment effect on the dielectric response of epoxy samples”, IEEE Conf. Electr. Insul. Dielect. Phenom, pp. 705-709, 2009. [17] I. Preda, H. Couderc, M. Frechette, S. Savoie, F. Gao, R. Nigmatullin, S. Thompson and J. Castellon, “Dielectric response of various partially cured epoxy nanocomposites”, Proc. IEEE Conf. El. Insul. Diel. Phen, pp. 660-663, 2011. [18] R. Sarathi, R.K. Sahu, P. Rajeshkumar, “Understanding the Thermal, Mechanical and Electrical Properties of Epoxy Nanocomposites”, Mat. Sci. Eng. No. A 445–446, pp. 567–578, 2007. [19] D. Fragiadakisa, P. Pissis, L. Bokobzab, “ Glass transition and molecular dynamics in poly(dimethylsiloxane)/silica nanocomposites”, Polymer, Vol. 46, pp. 6001–6008, 2005. [20] P.D. Aldrich, S.K. Thurow, M.J. McKennon, “Dielectric relaxation due to absorbed water in various thermosets”, Polymer, Vol. 28, Issue 13, pp. 2289–2296, 1987 [21] A.S. Maxwell, L. Monnerie, I.M. Ward, “Secondary relaxation processes in polyethylene terephthalate–additive blends: 2. Dynamic mechanical and dielectric investigations”, Polymer, Vol. 39, Issue 26, pp. 6851– 6859, 1998. [22] G. Ceccorulli, M. Pizzoli, « Effect of water on the relaxation spectrum of poly(methylmethacrylate)”, Polymer Bulletin, Vol. 47, Issue 3, pp. 283289, 2001

V. CONCLUSIONS In this work, the effects of extra free surfactant in organoclay and of its dryness on nanocomposite thermal and dielectric behavior were investigated. Drying the clay was not found to have a significant impact on the volatiles content since, after preparation, epoxies can still absorb water from their environment. However, water content in nanocomposites depends on the surfactant content and the addition of clay into nanocomposites seems to increase water resistance of epoxy. The influence of filler content over the glass transition temperature was found to be more important than the presence of volatiles since samples having different volatiles content and the same filler content had almost the same glass transition temperatures. The Broadband Dielectric Spectroscopy was found to be an important tool in determining the plasticizing effect of water, an additional relaxation being found between the main α and the secondary β relaxation. This relaxation needs to be further investigated. ACKNOWLEDGMENT Financial support from the ANASTASIA European Project (Advanced NAno-Structured TApeS for electrotechnical high power Insulating Applications) is acknowledged. Also, the support from the Nanodielectric program at Hydro-Québec Research Institute (IREQ) is acknowledged. A.-F. Vaessen from Laborelec Belgium is thanked for TGA measurements.

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