Hygrothermal aging of a filled epoxy resin - Schneider Electric

More documents

Recommendations

Info

Schneider Electric 2007 - Conferences publications 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 * E-mail : 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® 2050. 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 10 1-4244-0750-8/07/$20.00©2007 IEEE. 239
Schneider Electric 2007 - Conferences publications 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 40×10×0.5mm 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 (%) 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 Unfilled samples Filled samples 0 20 40 60 80 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 200-300°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
Page 1: Technical collection Hygrothermal a
Page 5: Schneider Electric 2007 - Conferenc

Hygrothermal aging of a filled epoxy resin - Schneider Electric

Create successful ePaper yourself

Delete template?

Save as template?