Synthesis of polyhedral oligomeric silsesquioxane crosslinkers with different ratios of functional groups and their application in flame retardant and hydrophobic silicone rubber

A series of fluorine-containing polyhedral oligomeric silsesquioxane crosslinkers (FC-POSS) were obtained by one-step click reaction with (4-(trifluoromethyl)phenyl)methanethiol and 3-mercaptopropyltriethoxysilane. These crosslinkers not only improved the thermal stability and flame retardancy of room temperature vulcanized (RTV) silicone rubber, but also enhanced the hydrophobicity of silicone rubber. The chemical structure of FC-POSS was confirmed by using Fourier transform infrared (FTIR) spectroscopy, ¹H, ¹³C and ²⁹Si nuclear magnetic resonance spectroscopy (NMR) and X-ray diffraction (XRD). The FC-POSS with different group ratios were applied to polydimethylsiloxane (PDMS) and effectively improved its flame retardancy, thermal stability and hydrophobicity. After the addition of 10 phr 6F2C-POSS (one POSS reacted on average with six (4-(trifluoromethyl)phenyl)methanethiols and two 3-mercaptopropyltriethoxysilanes), the manufactured silicone rubber passed the V-1 rating in vertical combustion test (UL-94), with the limiting oxygen index (LOI) values up to 28.4 % and an improved water contact angles from 90.9° to 121.7° Importantly, the surface maintained excellent hydrophobicity after tests such as high loads of sandpaper abrasion or mechanical cutting. The simple and efficient one-step preparation process used in this study provides a useful guide for the design and preparation of POSS-modified crosslinkers with good flame retardancy and hydrophobic properties.