Further Evidence and Mechanisms for Improvement in Tracking and Erosion Resistance of ATH-Free Silicone Rubbers by Direct Fluorination

Abstract

In order to provide further evidence for the improvement in tracking and erosion resistance of alumina trihydrate (ATH)-free silicone rubbers (SRs) by direct fluorination and to explore the improvement mechanisms, the ATH-free SR samples containing 15-parts per hundred parts of rubber by weight (phr) melamine cyanurate and 18-phr fumed silica were prepared, and then they were surface fluorinated using a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{F}_{{2}}/\text{N}_{{2}}$ </tex-math></inline-formula> mixture at 0.1 MPa and 85 °C for 120 min. Inclined plane tracking and erosion tests indicate that at 4.5-kV rms ac, all 15 fluorinated samples passed the test, while 7 of the 15 unfluorinated samples failed the test due to combustion. Analyses of erosion depth, length, and area show that the fluorinated sample, compared to the unfluorinated sample, has a larger average erosion depth but a smaller average erosion length or area. Attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray photoelectron spectroscopy (XPS) analyses indicate that both the SR matrix and the fillers of melamine cyanurate and fumed silica in the surface layer were fluorinated, forming the Si–F, C–F, and N–F bonds. Scanning electron microscopy (SEM) observations show that the fluorinated layer has a thickness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.15~\mu \text{m}$ </tex-math></inline-formula> and a rough surface, and the fillers are uniformly dispersed in the matrix. The resistance improvement by the fluorination is attributed to these changes of the surface layer in composition, structure, and interface of the SR matrix and the fillers and is also influenced by the change in surface morphology.