# Surface Tracking of MgO/Epoxy Nanocomposites: Effect of Surface Hydrophobicity
## 2 Materials and Methods  
Commercial bisphenol-A epoxy resin (E51, Bluestar Wuxi Petrochemical Co., Ltd., China) and MgO nanoparticles (Beijing DK nano technology Co., Ltd., China) have been employed as the major raw materials. The nanoparticles were 50 nm, in average diameter, 50 m\({}^{2}\)/g in specific surface area, 0.34 g/cm\({}^{3}\) in apparent density, and 3.5 g/cm\({}^{3}\) in true density. Nanocomposites were prepared by dispersing the nano-MgO particles into the epoxy matrix, with the nanofiller contents of 0.1, 1, 2, 6, and 10 wt%, respectively. The procedure has been explained below, in detail [8]. First, the surface-treated nano particles were prepared by uniformly mixing the coupling agent (3-Glycidoxypropyltrimethoxysilane, KH-560, Shanghai Aladdin Biological Technology Co., Ltd., China), nano-MgO particles, and ethanol for 30 minutes, by a homogenization system (SPCH-18, China). Second, the epoxy matrix was added in and mixing was continued for another 30 minutes. The mixture was heated at 60 \({}^{\circ}\)C, until the ethanol was completely volatilized. Third, the curing agent (methyl tetrahydrophthalic anhydride, GH9303, Orient Chemical Factory Jia Xing, China) and accelerator (2.4.6-Tris(dimethylaminomethyl)phenol, DMP-30, Shanghai Aladdin Biological Technology Co., Ltd., China) were added in and the mixture was stirred, in vacuum, for an hour, until potential bubbles were sufficiently removed. Finally, the samples were prepared after pre-curing at 80 \({}^{\circ}\)C, for 2 h, heat preserving at 105 \({}^{\circ}\)C, for 2 h, and post-curing at 120 \({}^{\circ}\)C, for 8 h, in steel molds. Surface tracking was measured according to the IEC 60112; the experimental apparatus (KS-53B, China) is shown in Figure 1. Two wedge-shaped platinum electrodes were placed on the surface of the sample, with a distance of 4 mm. AC voltage was applied to the electrodes. Statistical electric field of the sample surface near the electrodes was applied to the right electrode when the left electrode was at 0 V and 100 V, as shown in the enlarged figure on the right. The solution had a resistivity of (1.70 \(\pm\) 0.05) \(\Omega\)-m at room temperature; it was prepared by dissolving 0.1% NH\({}_{4}\)Cl and 0.5% sodium-di-butyl naphthalene into deionized water, for a more severe tracking process, instead of a 0.1% NH\({}_{4}\)Cl solution [30]. The solution droplets were controlled to continuously drop 35 mm above the sample, at an interval of 30 s, when the AC voltage was applied to the sample surface. The sample was defined to fail the measurement when the discharge current was bigger than 0.5 A, for 2 s. The measurement was repeated at a voltage that was higher by 25 V, until the sample failed the text in the voltage range from 100 V to 600 V. The comparative tracking index (CTI), which was defined as the maximum voltage that the sample withstands, for 50 solution droplets, was obtained to characterize the tracking resistance. In addition, the surface ablation area was calculated, as another parameter, to precisely present tracking resistance, as well. Samples were set to withstand 50 droplets at a fixed voltage (425 V) and tracking pictures were taken on their surfaces. Surface ablation area was, thus, calculated by selecting the surface tracking regions and numbering the corresponding pixel numbers by an image processing software (Adobe Photoshop). Hydrophobicity greatly affects the distribution of liquid contamination, on the sample's surface. In this study, SCA was utilized to characterize surface hydrophobicity via a contact angle instrument (JY-PHa, Chengde vote Instrument Sales Co. Ltd, China). Deionized water was used as the experimental solution. To guarantee the accuracy of the measurement results, the samples should be placed in parallel, during the test, with each droplets of test solution ensured between 4-5 \(\mu\)L.  
Surface resistivity was measured by home-made devices. Two parallel rectangle electrodes (20 \(\times\) 8 mm) with the distance of 2 mm, were pasted on to the samples.