Hygrothermal aging of a filled epoxy resin

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1 Technical collection Hygrothermal aging of a filled epoxy resin Conferences publications E. Brun P. Rain G. Teissédre C. Guillermin S. Rowe

2 2007 InternationalConferenceon Solid Dielectrics, Winchester, UK, July 8-13, 2007 Hygrothermal aging of a filled epoxy resin. E. Brun 2, P. Rain 1*, G. Teissèdre 1, C. Guillermin 2, S. Rowe 2 1 Grenoble Electrical Engineering Lab (G2Elab), CNRS- Université de Grenoble, Grenoble - France 2 Schneider Electric, Grenoble, France * pascal.rain@grenoble.cnrs.fr Abstract: The hygrothermal conditioning of an epoxy resin at 80 C under 80% RH has been followed by weight measurements, thermogravimetric analysis (TGA) and dynamical mechanical analysis (DMA). The samples are either filled with 60% by weight of silica flour or are not filled. 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, especially after 50 days. The TGA showed an evolution of the filled samples with conditioning after 50 days as well, which was not observed on unfilled samples. For the filled samples, the elastic modulus in the rubbery state decreased with conditioning. These evolutions have been attributed to the formation of a degraded inter-phase region due to hydrolysis occurring after the debonding of the filler-matrix interface caused by the absorbed water. INTRODUCTION Filled epoxy resin has been used for many years in electrical engineering. The material is submitted to thermal and electrical stress and is in contact with the environment. The mechanisms leading to the occurrence of electrical breakdown are still not properly understood. A great variety of electrical, thermal, mechanical, chemical phenomena may be involved in the ageing of these polymeric insulations. Since the material may be exposed to a humid environment, the present work focuses on the specific influence of water on the material and its effects on the electrical rigidity. From an electrical point of view, the impact of water on the dielectric behaviour of filled epoxy resins has been extensively described. As concerns the electrical rigidity, the breakdown voltages of wet materials may fall by a factor of 5 to 10 in comparison with a dry material [1-3]. In composites, the interfaces between the matrix and the mineral fillers are known to be zones of weakness [4-6]. The shape of the fillers may also influence the breakdown voltage [7]. Furthermore, the deleterious influence of water on epoxy resin is well known [8]. The main mechanisms leading to physical and chemical degradation of epoxy resin have been illustrated or at least foreseen [9]. With the use of a FTIR spectrometer, water layers of a few hundred nm have been measured at an epoxy/glass interface [10]. This order of magnitude is in accordance with the MEB observations reported in [3]. Water may accumulate at the interfaces and lead to a filler/matrix debonding, which may be followed by mechanical cracks propagating in the bulk material [11]. For a better understanding of the overall mechanisms, the impact of the hygrothermal conditioning on the physical properties have thus been carried out first. Electrical characterisations will ensue. In the following, mass uptakes, thermogravimetric analyses and dynamic mechanical analyses are reported. EXPERIMENTAL PART Materials, Sampling and Conditioning The material used is a DGEBA based epoxy resin filled or not with silica flour and cured with an anhydrid acid. The filler content is 60% by weight. The fillers sizes range between a few 0.1 μm and 200 μm. The components were mixed, cast in a mould then maintained at 100 C during one hour. After crosslinking, samples were demoulded and then cured at 130 C during 16 hours. The sheets are 0.5 mm thick. Glass transition temperatures of the filled and unfilled samples are 72 C and 70 C respectively. Samples are cleaned with alcohol and dried in an oven at 50 C for 24 hours. After this preparation, their initial mass was measured. The samples were then conditioned in a climatic chamber at 80 C under 80%RH. Characterisations Moisture uptake measurements: Samples were periodically withdrawn from the climatic chamber. Before mass measurements were made, the temperature and hygrometry of the samples were stabilized. For this purpose, they were laid in a small quantity of water initially at 80 C. After 15 minutes, both water and sample were at ambient temperature. Samples were then dried with a suitable paper. The mass uptake was measured with an electronic Ohauss balance Explorer. Thermogravimetric analysis (TGA): The thermal stability of the materials has been evaluated throughout conditioning by TGA with a TA instruments The mass loss of the samples was measured during a temperature rise of 3 C/min between -40 C and 850 C. Experiments were carried under nitrogen to avoid added oxidation. A gas flow of 45 ml/min inside the oven allowed the extraction of the thermolysis by-products. The relative mass loss of samples with initial mass of /07/$ IEEE. 239

3 to 20 mg is reported hereafter. Dynamic Mechanical Analysis (DMA): The evolution of mechanical properties and thermal transitions during conditioning were measured by DMA with a TA instruments 2980 using the 3-point bending mode. Taking into account the elastic domain of the material, the measurement conditions were the following: a dynamic magnitude of 50µm and a static force of 140% of the dynamic force. The samples were rectangular in shape mm 3. The frequency was 1Hz and the temperature rise was 3 C/min from -40 C to 140 C. RESULTS AND DISCUSSION Moisture uptake results Three different samples, either filled or not filled, were measured for each conditioning duration. Mean values and standard deviations are reported in Figure 1. For a better comparison of the two materials, mass variations of the filled samples were calculated taking into account the initial mass of unfilled resin, evaluated by the mean filling content of 60%. Water saturation of not filled samples is observed after about 5 days. The water uptake was then of 1.5%. The mass uptake of filled samples is lower at the beginning than the one of unfilled ones: the kinetics of water absorption is slower. The water diffusion is impeded by the silica grains, which slow down its propagation throughout the whole material. After 5 days conditioning, the mass variation was close to the one of unfilled samples but measurements tend to indicate that the mass uptake continues to progress after the quasi saturation has been reached. This specific behaviour of filled samples has already been reported in [12]. Furthermore, a slope increasing after about 50 days has been observed in a similar way to that shown in [13]. These measurements will be completed to confirm this tendency. This difference observed between the two materials can be attributed to phenomena taking place at the epoxy/silica interfaces. The following mechanism can be proposed. Firstly, water molecules break the physical bounds between the silica grains and the resin and form H-bounds with the polymer. Then, some water mass uptake (%) Unfilled samples Filled samples Conditioning time (days) Figure 1: Weight gain in unfilled ( ) and filled ( ) samples as a function of conditioning time. accumulates between the two phases, part of which possibly reacts with the resin. TGA results Thermogravimetric analysis was carried out for filled and not filled samples after until 70 days of conditioning. The thermal behaviour of unaged sample are mentioned as reference in the figures. The dots on the graphics are not measurements points but are added to improve visualisation of the data. The thermograms are displayed in Figure 2 for unfilled samples. The derivatives of the mass loss curves are also displayed to highlight the differences between the samples. A detail of the curves between 50 C and 200 C is enlarged. This shows a significant decreasing of the mass between 50 C and 120 C in aged samples, which is not observed in reference samples. This mass loss is slightly lower than the water uptake measured by gravimetry. It corresponds to the evaporation of absorbed water. Xu et al [13] observed an initial, weak, peak of the derivative weight in the same temperature range that they attributed to the evaporation of water contained only in the free volumes. They considered that the bound water was released at C where they observe a slight difference between as-cured and aged samples. Our results do not confirm this point. The main mass loss occurred between 300 and 450 C. For all aged unfilled samples investigated so far, the derivative weights display a maximum at about 398 C and a shoulder (maximum of the second derivative) at 364 C. This corresponds to resin decomposition. At 800 C, a residual mass of about 6% was measured which was not observed in similar experiment under oxygen flow. MEB analysis showed the presence of carbon with low quantities of oxygen. MEB photos showed blackish sheets. This residue was then mainly composed of carbon graphite. For filled samples, residual masses were scattered and not correlated with the duration of conditionning. The mean value was 64%. Taking into account the residual Figure 2: Weight loss during a TGA dynamic test for unfilled samples before ( ) and after conditioning at 80 C and 80%HR during 5 (+), 14 ( ) and 50 ( ) days. 240

4 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)% REF 5 days 14 days 50 days 70 days REF 5 days 14 days 50 days 70 days 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 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

5 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 , [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 , [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 , [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 , [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 [6] D.H. Kaelble and P.J. Dynes, Hygrothermal aging of composite materials, J. Adhesion, vol. 8, p , [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, , [8] A.C. May, Epoxy resins, New York, Dekker Edition, [9] J. Verdu, Action de l eau sur les plastiques, Paris, Techniques de l ingénieur, vol AM [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 , [11] K.A. Kasturiarachchi and G. Pritchard, Scanning electron microscopy of epoxy-glass exposed to humid condition, J. of Mat. SC., vol. 20, p , [12] M.J. Adamson, Thermal expansion and swelling of cured epoxy resin used in graphite/epoxy composite materials, J. Mater. Sci., vol. 15, p , [13] S. Xu and D.A. Dillard, Environmental aging effects on thermal and mechanical properties of electrically conductive adhesives, J. Adhesion, vol. 79, p , [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 , [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 , [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 , [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 ,

Characterization of water absorbed epoxy insulating coating material used in ZnO varistors by dielectric measurements

Materials Letters 60 (2006) 114 119 www.elsevier.com/locate/matlet Characterization of water absorbed epoxy insulating coating material used in ZnO varistors by dielectric measurements Shengtao Li a, Lei