Hygrothermal aging of a filled epoxy resin - Schneider Electric

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Schneider Electric 2007 - Conferences publications mass of 6% measured for the unfilled samples, this value leads to a filler content of 58% which corresponds well with the “rated” filler content of 60%. To simplify the comparisons, the variations of the residual masses were neutralized in the results displayed in Figure 3. This shows the relative mass losses between the initial mass values of the samples and the residual masses. A mass loss between 50°C and 200°C can be observed, as in unfilled samples. In these cases, the variations correspond well with the water uptake measured before. The resin decomposition can be observed in the same temperature range as above. As shown in the zoom in Figure 3, decomposition started at lower temperatures for longer conditioning periods. This is clear if we compare the curves for 50 and 70 days, which fit the curve of 14 days until about 200°C and decrease faster between 200°C and 300°C. This indicates an evolution of the material produced by the hygrothermal conditioning after about 50 days. Since this effect was not encountered in samples without fillers, the epoxy/silica interface regions are necessarily involved. We have already mentioned that the mass measurements suggest an accumulation of matter close to these interfaces. We may consider that this evolution is due to an hydrolysis of the filler/matrix interface regions which creates a degraded inter-phase region. Since the filler content is large, this degraded region may also fill the whole gap between neighbouring fillers and constitute “weak” path between them. This hydrolysis occuring after about 50 days is probably correlated with the increase of the mass uptake observed at the same time. The possible explanation of this is as follows: free space is created by the hydrolytic reactions consisting mainly in the attack of the ester linkages and creation of acid groups [4]. This allows extra water molecules to accumulate between the filler and the degraded region. However, the extra quantity of water involved in this mechanism would be low in comparison with the total quantity of absorbed water, which was almost entirely lost between 50°C and 150°C. Further experiments are under way to confirm this hypothesis. DMA results The dynamic mechanical analysis was carried out on four filled and four unfilled samples after 0, 5, 14 and 50 days of conditioning. The evolution of the elastic moduli E’ is displayed in Figure 4a. The magnitudes for the filled samples were about three times larger in the glassy state and five times larger in the rubbery state. The elastic modulus decreased during conditioning, especially in the glassy state. In practice, water molecules break the hydrogen bonds established inside the network. The bound water increases also the mobility of the polymer chains and causes a decrease of the elastic modulus. This is the well-known waterinduced plasticization effect. In the rubbery state, we would expect the effect to be less marked since the chains are already mobile. Thus in this case, the modulus E’ depends theoretically on the density of cross-links in the epoxy network [14]. Nevertheless, the conditioning induced a decrease of the elastic moduli E’ for the filled samples. For unfilled samples, the measurements were conducted at the lower limit of sensitivity of the DMA instrument. Such a decrease of the rubbery modulus has already been observed in [15]. It is not due to the plasticization but may be attributed to a degradation of the polymer, which probably results from hydrolysis of the resin. (a) M(Minitial-Mfinal)% 100 90 80 70 60 50 40 30 20 10 0 REF 5 days 14 days 50 days 70 days REF 5 days 14 days 50 days 70 days 0 100 200 300 400 500 600 T(°C) Figure 3: Weight loss during a TGA dynamic test for filled samples before (•) and after conditioning at 80°C and 80%HR during 5 (+), 14 (), 50 () and 70 () days with a zoom in the temperature range 0-350°C between 92 and 100%. (b) Figure 4: Elastic modulus E’ (a) and loss modulus E’’ (b) vs temperature of unfilled (---) samples and filled ( ___ ) samples before (•) and after conditioning at 80°C, 80%HR during 5(+), 14() and 50 () days. 241
Schneider Electric 2007 - Conferences publications The glass transition temperatures have been taken at the maxima of the curves of loss moduli E’’ (Figure 4b). For the reference samples, the Tg were 70°C and 72°C respectively for the unfilled and filled samples. These maxima shifted towards the low temperatures during conditioning, thus highlighting water-induced plasticization. This decrease occurred mainly within the first days of conditioning and is more important in the unfilled samples. After a 50 days conditioning, the shift of the Tg values were of 14K and 11K for the unfilled and filled samples respectively. Similar evolutions have been already mentioned [16]. A secondary peak, which would have indicated the heterogeneity of the material [6, 12, 17], was not visible here. CONCLUSION Evidence of aging of filled epoxy resin conditionned at 80°C and 80%RH were provided by weight measurements, thermogravimetric analysis (TGA) and dynamical mechanical analysis (DMA). Above an apparent saturation value of about 1.5% reached within a few days, a slight but significant mass uptake was observed in the filled resin. An accumulation of water at the epoxy/silica interface can be inferred, part of which may have chemically reacted with the polymer. Thermogravimetric analysis showed a degradation of filled samples after a conditioning period longer than 50 days, which does not occur in unfilled samples. The decrease of the elastic modulus E’ and of the glass transition temperature Tg underlined the plasticization of the samples during conditioning. For the filled samples, the elastic modulus in the rubbery state decreased with conditioning. These results suggest the creation of a degraded interphase region between the silica and the epoxy matrix due to the hydrolysis of the resin. REFERENCES [1] H.C. Kärner and M. Ieda, “Technical aspects of interfacial phenomena in solid insulating”, Proc. of the 3 rd IEEE Int. Conf. On Prop and Appl of Diel Mat, p. 592-597, 1991. [2] T. Kumazawa, M. Oishi and M. Todoki, “Highhumidity deterioration and internal structure change of epoxy resin for electrical insulation”, IEEE Transactions on dielectrics an Electrical Insulation, vol. 1, n°1, p. 133-138, 1994. [3] T. Kumazawa, M. Oishi, M. Todoki and T. Watanabe, “Physical and chemical structure change of filled epoxy due to water absorption”, 5 th IEEE int.Conf. on Prop and Appl of Diel Mat, p. 491-494, 1997. [4] M.K. Antoon and J.L. Koenig, “Irreversible effects of moisture on the epoxy matrix in glass-reinforced composites”, J.Polym. Sci. Phy. Ed., vol. 19, p. 197-212, 1981. [5] Janssen H., Seifert J.M. and Kärner H.C., Interfacial phenomena in comopsite high voltage insulation, IEEE Transactions on dielectrics an Electrical Insulation, vol. 6, 1999, p. 651-659. [6] D.H. Kaelble and P.J. Dynes, “Hygrothermal aging of composite materials”, J. Adhesion, vol. 8, p. 195- 212, 1977. [7] M. Ezoe, M. Nakanishi and J. ShouGou, “Effects of water absorption on interfacial phenomena in HV insulating materials”, IEEE High Voltage Engineering Symposium, 1999, 4.244-4.247, 1999. [8] A.C. May, Epoxy resins, New York, Dekker Edition, 1988. [9] J. Verdu, Action de l’eau sur les plastiques, Paris, Techniques de l’ingénieur, vol AM 3 165. [10] T. Nguyen, E. Byrd and D. Bentz, “In situ measurement of water at the organic coating/substrate interface”, Progress in organic coatings, vol. 27, p. 181-193, 1996. [11] K.A. Kasturiarachchi and G. Pritchard, Scanning electron microscopy of epoxy-glass exposed to humid condition, J. of Mat. SC., vol. 20, p. 2038- 2044, 1985. [12] M.J. Adamson, “Thermal expansion and swelling of cured epoxy resin used in graphite/epoxy composite materials”, J. Mater. Sci., vol. 15, p. 1736-1745, 1980. [13] S. Xu and D.A. Dillard, “Environmental aging effects on thermal and mechanical properties of electrically conductive adhesives”, J. Adhesion, vol. 79, p. 699-723, 2003. [14] T. Murayama and J.P. Bell, “Relation between the network structure and dynamic mechanical properties of a typical amine-cured epoxy polymer”, J. Polym. Sci., vol. 8, p. 437-445, 1970. [15] K.I. Ivanova, R.A. Pethrick and S. Affrossman, “Investigation of hydrothermal aging of a filled rubber toughened epoxy resin using dynamic mechanical thermal analysis and dielectric spectroscopy”, Polymer, vol. 41, p. 6787-6796, 2000. [16] A. Apicella, L. Egiziano, L. Nicolais and V. Tucci, “Environmental degradation of the electrical and thermal properties of organic insulating materials”, J. Mater. Sci., vol. 23, p. 729-735, 1988. [17] L. Wang, K. Wang, L. Chen, C. He, L. Wang et al, “Hygrothermal effects on the thermomechanical properties of high performance epoxy/clay nanocomposites”, Polymer Engineering and Science, p. 215-221, 2006. 242
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