Due to its excellent electrical insulation and hydrophobicity, silicone rubber has been widely used in outdoor power transmission lines. The superhydrophobic silicone rubber surface can be etched by a femtosecond pulse laser, which makes it possess an excellent self-cleaning effect. However, in practical applications, the performance and aging resistance of the superhydrophobic silicone rubber surfaces are still unclear due to the long-term exposure to the natural environments such as sunshine, rain, and high and low temperatures. Therefore, it is necessary to further study the aging resistance of the surface. In this paper, an artificially accelerated aging chamber is used to study the hydrophobicity stability of the superhydrophobic silicone rubber processed by a femtosecond laser. In order to understand the aging phenomenon of the superhydrophobic silicone rubber surfaces, the physical and chemical changes of the sample surface are detected by the analytical techniques. A simple and effective heat treatment method is adopted to quickly recover the hydrophobicity of the silicone rubber surface. This study has good guiding significance for the preparation of anti-aging silicone rubber surfaces and explains the change of surface hydrophobicity of superhydrophobic silicone rubbers after aging.
The silicone rubber surface sample is etched with a femtosecond laser at wavelength of 1030 nm and pulse duration of 480 fs. To evaluate the long-term performance of the superhydrophobic silicone rubber in the outdoor environment, the samples are tested in an artificial accelerated aging chamber equipped with two xenon lamps. A vacuum drying oven at 200 ℃ is used for the heat treatment of aged samples. The wettability of the surface is characterized by measuring the contact angle and rolling-angle of the sample surface with a contact angle measurement system. The morphology of the sample surface is detected by an optical interferometer and the scanning electron microscope (SEM). The chemical compositions of the sample surfaces are investigated by the attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR).
The superhydrophobic silicone rubber surface can be obtained by the femtosecond laser treatment. After aging for 700 h, the contact angle decreases to ~150°, and the water droplets on the surface cannot roll. From the change of the contact angle, the silicone rubber sample with a 5.0 J·cm-2 laser has the best hydrophobicity after aging for 700 h (Fig. 5). The experimental results show that the surface microstructure of the sample aggregates and gradually deteriorates, which may be due to the decomposition of the polymer chain triggered by the experimental irradiation (Fig. 8). The formation of more photoinduced hydrophilic species on the surface increases the adhesion of water droplets, which is what triggers the surface transition from the Cassie state to the Wenzel state. The microstructure of the sample with a 5.0 J·cm-2 laser has little change and the surface deterioration is not obvious. As a result of rain and sun exposure, the carbonyl degradation products appear on the surface (Fig. 9). The loss of hydrophobicity of the silicone rubber surface is caused by the increase of —OH groups. The silicone rubber with a 5.0 J·cm-2 laser produces the least carbonyl degradation products, which is the reason for the best hydrophobicity of silicone rubber samples under this laser fluence after aging. The hydrophobicity of the aged silicone rubber surface is recovered after the heat treatment (Fig. 11). The reduction of hydrophilic —OH groups on the surface of the silicone rubber and the change of microstructure are the reasons for the recovery of hydrophobicity of the silicone rubber surface.
The superhydrophobic surface is obtained by etching the silicone rubber with a femtosecond laser, and the contact angle increases from 110° to 160°. After the accelerated aging test for 700 h, the surface contact angles of superhydrophobic samples decrease from 160° to 150°, indicating that it has excellent aging resistance. The wettability of the silicone rubber is mainly related to the surface microstructure and chemical element compositions. Therefore, in order to analyze the reasons for the decrease of hydrophobicity, the white light interferometer is used to find that the roughness of the silicone rubber sample increases at the initial stage. However, after the further accelerated aging, the surface roughness of samples decreases and becomes stable after 210 h. The analysis of the surface microstructural morphology shows that the microstructure of the sample surface aggregates and deteriorates gradually when the accelerated aging experimental time reaches 700 h. The polymer chain may have been broken down by experimental irradiation. Fourier transform attenuated total reflection infrared spectroscopy is used to identify the chemical changes on the surfaces of silicone rubber samples before and after aging. It is found that some —CH3 groups are destroyed under the action of the radiation energy. The chemical groups on the surface of aging samples change obviously, and the increase of hydrophilic —OH groups on the surface causes the loss of hydrophobicity of silicone rubber surfaces. At the same time, the hydrophobicity of the aged silicone rubber surface is recovered quickly after heat treatment. Therefore, this study has good guiding significance for the preparation of anti-aging performance of silicone rubber surfaces, and provides a simple and effective heat treatment method that can quickly improve the hydrophobicity of aged surfaces.
