D1-204 PROPERTIES OF EPOXY-LAYERED SILICATE NANOCOMPOSITES T. SHIMIZU*, T. OZAKI, Y. HIRANO, T. IMAI, T. YOSHIMITSU TOSHIBA CORPORATION.

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1 21, rue d'artois, F Paris D1-204 Session 2004 CIGRÉ PROPERTIES OF EPOXY-LAYERED SILICATE NANOCOMPOSITES T. SHIMIZU*, T. OZAKI, Y. HIRANO, T. IMAI, T. YOSHIMITSU TOSHIBA CORPORATION (Japan) 1. INTRODUCTION Nanotechnology has brought rapid progress in various fields. Polymer-nanocomposites have attracted special interest, since the Toyota research group developed nylon-layered silicate nanocomposites [1]. These materials are organic-inorganic hybrid materials and greatly improve some properties of resins. Most research has focused on thermoplastics. Application of these materials will be enhanced by applying this technology to thermosetting resins [2-4]. For example, thermosetting resins are used in heavy apparatuses such as generators, rotators and switchgears because of their excellent electrical, mechanical and thermal properties. Innovative apparatuses are expected with development of high performance thermosetting nanocomposites. This report describes layered silicates dispersed to epoxy resins used for heavy apparatuses, to enhance the thermal, mechanical and electrical insulation properties of the composites. 2. MORPHOLOGY AND GENERAL PROPERTIES OF NANOCOMPOSITES 2.1. Materials and preparation of test specimens Layered silicates modified by quaternary alkylammonium were dispersed in epoxy resins (diglycidyl ether of bisphenol-a, DGEBA or diglycidyl ether of bisphenol-f, DGEBF) by stirring with shearing. Two kinds of organically modified silicates were used in this study. They differed in the alkylammonium ions present between their layers. Silicate A had no hydroxyl group while silicate B had one in alkylammonium. The mixtures of epoxy and the silicates were cured with methylendo-methylene-tetra-hydro phthalic and an amine curing accelerator. All specimens were cured at 100 o C for 3 hours and then at 150 o C for 15 hours Morphology To improve the properties of nanocomposites, it is essential to expand and/or exfoliate the silicate layers. TEM micrographs of DGEBA containing silicate A are shown in Figure 1(a) and (b). These micrographs show some aggregations with dimensions from 0.2 to 1.0 µm, which consist of many parallel silicate layers. The distance between the dispersed silicate layers was expanded slightly compared with the original ones. *toshio4.shimizu@toshiba.co.jp 1

2 However, the micrographs of DGEBA containing silicate B show excellent dispersion of silicates in the epoxy resins. In some areas, each silicate was observed as a thin layer. This was obtained by exfoliating the silicate layers. Silicate B had a hydroxyl group at the end of the alkyl chain in alkylammonium, while silicate A had no hydroxyl group. The deference of affinity and reactivity to epoxy resins between hydroxyl groups and alkyl may influence dispersion of silicates with expansion and exfoliation of layers. Consequently, DGEBA with silicate B had a mixed morphology including both parallel and exfoliated layers in the epoxy resins [5]. The dispersion and exfoliation of layered silicates in epoxy resins depend on the type of alkylammonium between the layers. (a) (b) 0.5µm 10nm (c) (d) 0.5µm 10nm Figure 1 TEM micrographs of DGEBA/silicate A and DGEBA/silicate B. (a)(b):dgeba containing 6 wt% of silicate A (c)(d):dgeba containing 3 wt% of silicate B 2.3. Thermal and mechanical properties The temperature dependence of viscoelasticity was measured by a dynamic mechanical analyzer (DMA, three point bending, 1 Hz) and is shown in Figure 2 and Table 1. The storage modulus (E ) decreased dramatically above the glass transition temperature (Tg). The tanδ peaks correspond to the Tg of the composites, and were shifted to higher temperature by dispersing silicates. The sifts of Tg for DGEBA containing 3 wt% and 6 wt% of silicate A were 14 o C and 20 o C respectively. Addition of 3 wt % of silicate B shifted Tg by 13 o C. E was separated into two plateau areas. One was a rigid glassy state and the other was a soft rubbery state. In the rubbery state, the E of the nanocomposites increased. DGEBA with 3 wt% of silicate B increased by 35.8 MPa compared with pure epoxy resin at 200 o C. These changes of thermal-mechanical properties were caused by the silicate pinning effect. The relaxation mobility of polymer chains is restricted by rigid silicates in the epoxy network [6]. Silicate B dispersed more effectively in the epoxy network, and showed stronger restriction than silicate A. Storage modulus(e ) [Pa] Temperature [ o C] Figure 2 Dynamic mechanical properties of nanocomposites. (, ) (, ) DGEBA containing 3 wt% of silicate A (, ) DGEBA containing 6 wt% of silicate A (, ) DGEBA containing 3 wt% of silicate B Table 1. Sample Summary of DMA measurements. Contents [wr%] T g a [ o C] E [MPa] at 30 o C at 200 o C Epoxy rein DGEBA/silicateA DGEBA/silicateA DGEBA/silicateB T g a : Temperature of tanδ peak. tanδ 2

3 2.4. Mechanical strength The flexural strength obtained by three point bending test is summarized in Figure 3. All the nanocomposites had higher flexural strength than pure epoxy resin. The flexural strength of DGEBA increased with increasing amount of silicate A, and was 19 % higher with 6 wt % of silicate A than that of pure epoxy resin. Silicate B showed superior strength to silicate A for the same amount of silicates. Flexural strength was increased by 21 % by adding only 3 wt% of silicate B. This was enabled by the better dispersion of silicate B. However, adding silicates often decreased mechanical strength due to remaining large segmentation defects. To obtain sufficient strength, these aggregations should be decomposed through the mixing process. Flexural strength [MPa] Silicate A(3wt%) Silicate A(6wt%) Silicate B(3wt%) Figure 3 Flexural strength by three-point bending test. ( )DGEBA containing 0, 3, 6 wt% of silicate A. ( )DGEBA containing 3 wt% of silicate B. Each symbol shows the mean value. The upper lines show the highest value, and the lower lines show the lowest value Insulating and dielectric properties For heavy apparatuses, electrical properties are most important. Volume resistivities are compared in Table 2. The volume resistivity of DGEBF with silicate A was slightly higher than that of pure epoxy resin. Temperature dependencies of relative permittivity (ε r ) and dielectric loss (tanδ) are shown in Figure 4. The ε r of pure epoxy increased gradually with increasing temperature, while that of DGEBF with 6 wt% of silicate A was suppressed at lower values. Tanδ increased drastically above Tg in pure epoxy. This increase was not observed in the nanocomposites up to 150 o C. The increase of ε r and tanδ will be associated with mobility of polymer chains. As a result of dynamic mechanical analysis, restriction of polymer segments by silicates may improve the dielectric properties. Further study will clarify these characteristics and mechanisms. Table 2. A comparison of volume resistivity. Sample Volume Contents resistivity [wt%] [Ω cm] Epoxy rein 7.6 x DGEBF/silicateA x Relative permittivity(εr ) [ ] Dielectric loss(tanδ) [ ] (a) (b) Temperature [ o C] Figure 4 Temperature dependencies of relative permittivity and dielectric loss. (a) relative permittivity (ε r ), (b) dielectric loss (tanδ). ( ). ( )DGEBF containing 6 wt% of silicate A. 3

4 3. DIELECTRIC BREAKDOWN STRENGTH MEASUREMENT OF MANOCOMPOSITES 3.1. Materials and preparation of Specimens Since the epoxy resin filled with silicate A had lower viscosity and an advantage in degassing compared with silicate B, silicate A was employed in this experiment. Silicate A was dispersed in epoxy resin (DGEBA) by stirring with high shearing at room temperature. The mixture was poured into an aluminum mold and was degassed. Epoxy-silicate nanocomposite specimens (DGEBA/silicate A, Figure 5) were acquired by curing the mixture Needle electrode in the presence of a stoichiometric amount of methylhexahydrophthalic anhydride (MeHHPA). All specimens were cured at 100 o C for 3 hrs and 150 o C for 16 hrs to reach full cure. 30mm A steel rod 1mm in diameter and 60 mm long, with a tip angle of 30 o and a curvature radius of 5 µm, was used as a needle electrode. The 15mm needle was set with a 3 mm gap between needle tip and sample 15mm bottom. Figure 5 Geometry of specimen 3.2. Measurement method A conductive silver coating was painted on to make a plate electrode under the specimen. After setting the specimen on the grounded electrode as shown in Figure 6, an alternative voltage with 50 Hz was applied between the needle electrode and the plate electrode at a continuous raising speed (0.6 kv/sec) to measure its dielectric breakdown strength. The state around the tip of the needle electrode was observed by CCD camera through the breakdown test. CCD camera Gap length (3mm) Insulating fluid Insulating oil Needle electrode Conductive coating Grounded electrode Figure 6 Dielectric breakdown test method 3.3. Breakdown strength and progress of electrical treeing The dielectric breakdown strength is shown in Figure 7. That of DGEBA/silicate A nanocomposite was 9 % higher than that of the pure epoxy in which no silicate was dispersed. One specimen of DGEBA/silicate A, however, had low breakdown strength (12.4 kv/mm) in comparison with the other specimens of DGEBA/silicate A (The shaded symbol in Figure 7 indicates this irregular specimen). This specimen had a different electrical treeing pattern from the others. At first, large carbonization appeared as an electric discharge around the needle electrode tip, and then a thread-like treeing progressed quickly to both the needle electrode tip and the grounded plate electrode from the carbonization grain. Figure 8 shows the particular carbonized grain and Dielectric breakdown strength [kv /mm ] DGEBA/Silicate A Figure 7 Dielectric breakdown strength Means are indicated by lines. The mean of DGEBA/silicate A doesn t contain the shaded symbol. 4

the breakdown pass. This peculiar behavior is assumed to be caused by a defect. An aggregation of silicates or a void may have existed near the needle tip.

5 the breakdown pass. This peculiar behavior is assumed to be caused by a defect. An aggregation of silicates or a void may have existed near the needle tip. It was difficult to remove voids from the nanocomposites because the addition of silicates increased the viscosity of the resin. The other specimens showed similar electrical treeing patterns. Typical behaviors of the electrical Figure 8 Particular break down treeing are summarized in Table 3 for pure epoxy pattern in DGEBA/silicate A resin and Table 4 for the nanocomposites. This is the first report concerning the direct observation of electric treeing growth in epoxy/silicate nanocomposites. As shown in Table 3, one clear black-colored carbonized treeing appeared from the needle tip in pure epoxy. Although treeing propagated with several branches, the growth seemed to be comparatively straight and to form a dendrite shape as a whole, just before breakdown. It is considered that this treeing growth caused an electrical energy concentration round the electrode tip. Just before the breakdown, this treeing became black-colored and looked clear. However, in the DGEBA/silicate A nanocomposites, as shown in Table 4, several treeings appeared from the needle tip. Each had many branches during the propagation process and finally formed a bush-shaped pattern as a whole just before breakdown. To confirm the treeing patterns in actual long operation, a constant voltage with 1kHz, which was estimated at about half the breakdown voltage, was applied between the needle electrode and the plate electrode of new specimens. After applying this voltage for some duration and then stopping it, treeings in the specimens were observed by microscope. The treeing patterns of DGEBA/silicate A nanocomposites and pure epoxy are shown in Table 5. Unlike the pure epoxy resin, the nanocomposites had plural treeings from the tip at first. They propagated with many branches and some of them seemed to grow in the opposite direction from the counter electrode. The branched treeings showed a bush-shaped pattern. Table 3 Growth of electrical treeing in pure epoxy Actual picture Duplication by tracing Just after treeing appeared Middle Just before a breakdown 5

Table 4 Growth of electrical treeing in DGEBA/silicate A Actual picture Duplication by tracing Just after treeing appeared Middle Just before a breakdown Table 5 Progress of electrical trees in

to rise once, seem to be arrested or separated into several passes by nano-scale silicates dispersed in epoxy resins as shown in Figure 9.

6 Table 4 Growth of electrical treeing in DGEBA/silicate A Actual picture Duplication by tracing Just after treeing appeared Middle Just before a breakdown Table 5 Progress of electrical trees in DGEBA/silicate A 20kV-1kHz for 18 minutes DGEBA/silicate A nanocomposite 15kV-1kHz for 310 minutes Epoxy matrix Epoxy matrix Tip of electrode Tip of electrode The electrical treeings, which happened to rise once, seem to be arrested or separated into several passes by nano-scale silicates dispersed in epoxy resins as shown in Figure 9. It is considered that this branching of the treeings decreased the electrical field strength around the tip by shielding each other. As a result, this phenomenon results in a higher dielectric breakdown strength of nanocomposites than that of the pure epoxy. 20kV-1kHz for 18 minutes Silicate Electrical trees progress straight relatively. Electrical trees progress with making many branches. Nanocomposite Figure 9 Branches formed by the dispersed silicates 6

7 4. CONCLUSION Epoxy-layered silicate nanocomposites were prepared to investigate some properties for application to heavy apparatuses. The dispersion and exfoliation of silicates depended on the types of alkylammonium in the silicate layers. The nanocomposites improved thermal, mechanical, and dielectric properties by adding a small amount of silicates. Electrical treeing growth was observed directly first in the epoxy-layered silicate nanocomposites. It was found that the dispersed silicates suppressed the discharge damage growth in epoxy resins and increased the electric breakdown strength. 5. REFERENCES [1] Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, T. Kurauchi, O. Kamigaito, One-pot synthesis of nylon 6-clay hybrid, [J. Polm. Sci. Part A], vol. 31, pp , [2] Muh S. Wang and Thomas J. Pannavaia, Clay-polymer nanocomposites formed from acidic derivatives of montmorillonite and an epoxy resin, [ Chem. Mater.], vol. 6, pp , [3] Zen Wang and Thomas J. Pinnavaia, Hybrid organic-inorganic nanocomposites: Exfoliation of magadiite nanolayers in an elastomeric epoxy polymer, [Chem. Mater.], vol. 10, pp , [4] C. Zilg, R. Mülhaupt, J. Finter, Morphology and toughness/stiffness balance of nanocomposites based upon anhydride-cured epoxy resins and layered silicates, [Macromol. Chem. Phys.], vol. 200, pp. 661, [5] Tie Lan, Padmananda D. Kaviratna and Thomas J. Pinnavaia, Mechanism of clay tactoid exfoliation in epoxy-clay nanocomposites, [Chem. Mater.], vol. 7, pp , [6] Janis M. Brown, David Curlss and Richard A. Vaia, Thermoset-layered silicate nanocomposite. Quaternary ammonium montmorillonite with primary diamine cured epoxies, [Chem. Mater.], vol. 12, pp ,

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