Study on the Compatibility of C₆F₁₂O Gas Mixture with EPDM

Objective SF₆ is widely used in electrically insulated equipment due to its excellent insulating properties. However, its global warming potential is 23 500 times greater than that of CO₂, with an atmospheric lifespan of up to 3200 years. As the severity of global warming intensifies, finding a new environmentally friendly insulating gas to reduce the using of SF₆ has become a hot research topic in recent years. C₆F₁₂O has both environmental benign characteristics and insulating properties, which has attracted the attention of scholars. However, the liquefaction temperature of pure C₆F₁₂O gas is 49 ℃ at atmospheric pressure, which is difficult to use as an insulating medium in practical engineering scenarios. It must be mixed with low-boiling point buffer gases, including CO₂ and N₂. It has been shown that a gas mixture comprising 4% C₆F₁₂O/CO₂, when pressurized above 0.14 MPa, can be used as an insulating gas in 10 kV switchgear. For new eco-friendly insulation gas, its compatibility with the internal sealing materials of the equipment is also need to be considered.Method The experimental study on the compatibility between the C₆F₁₂O/CO₂ gas mixture and the sealing rubber EPDM was carried out by building a thermally accelerated compatibility experimental platform in this paper. The effect of trace O₂ on the compatibility of C₆F₁₂O gas mixture and EPDM was investigated by adding a small amount of O₂ into the gas mixture. The changes in the surface morphology, the elements of rubber samples, the gas mixture components, and the mechanical properties were analyzed before and after the experiments. The interaction between C₆F₁₂O/CO₂/O₂ gas mixture and rubber was also calculated based on the MD theory.Results and Discussions The results from scanning electron microscopy showed that the scaly structures appeared on the surface of EPDM when mixed with C₆F₁₂O/CO₂ at 110 °C. A flocculent structure appeared when EPDM was mixed with C₆F₁₂O/CO₂/O₂ at only 90 °C. This observation suggested that the inclusion of O₂ promotes the interaction between the C₆F₁₂O/CO₂ gas mixture and EPDM, which in turn leads to the corrosion of EPDM. The results of X-ray photoelectron spectroscopy showed that a decrease in the carbon content alongside increases in oxygen and fluorine within the C₆F₁₂O/CO₂ mixed EPDM with increasing temperature. This trend indicated that C₆F₁₂O/CO₂ reacts with EPDM. In group of C₆F₁₂O/CO₂/O₂, the changes were more significant, which indicate that O₂ promoted the reaction between the C₆F₁₂O/CO₂ mixture and EPDM. Post-experimental analyses identified new spectral peaks at 284.5 eV, 285.8 eV and 289.2 eV, corresponding to C—C, C—H and —COO— ester bonds, respectively. Three characteristic peaks of elemental F were detected on the EPDM surface for metal fluoride at 683 eV, organofluoride at 684.5 eV, and —CF₂CH₂— at 687 eV. The intensity of C characteristic peak was significantly lower, while the intensity of O and F were higher in the C₆F₁₂O/CO₂/O₂ gas mixture after testing. These findings indicated that O₂ actively participates in the reaction between the C₆F₁₂O/CO₂ mixture and the EPDM, fostering an intensified interaction. The results of tensile properties showed that the mechanical properties of EPDM decreased significantly with increasing temperature. The mechanical property of EPDM in C₆F₁₂O/CO₂ atmosphere was slightly worse than those of the original samples. The mechanical property of EPDM under C₆F₁₂O/CO₂/O₂ was worse than that of C₆F₁₂O/CO₂. The reaction between the C₆F₁₂O/CO₂ mixture and EPDM was more intensive after adding O₂, leading to a significant decrease in the mechanical properties of EPDM. At the end of the experiment, a small amount of the resultant gas was collected and subjected to Gas chromatography–mass spectrometry for further analysis. The small amount of H₂O remaining inside the gas tank was involved in the decomposition reaction of the C₆F₁₂O gas mixture. Throughout the reaction, <inline-formula><tex-math id="Z-20240912144036">${\rm H}^{\,\cdot} $</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912144036.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912144036.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="Z-20240912144046">${\rm OH}^{\,\cdot} $</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912144046.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912144046.png"/></alternatives></inline-formula> were generated from the decomposition of H₂O, which can interact with particles such as <inline-formula><tex-math id="Z-20240912144102">${\rm CF}_3^{\,\cdot} $</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912144102.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912144102.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="Z-20240912144122">${\rm C_{3}F}_7^{\,\cdot} $</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912144122.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912144122.png"/></alternatives></inline-formula>, and <inline-formula><tex-math id="Z-20240912144203">${\rm F}^{\,\cdot} $</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912144203.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912144203.png"/></alternatives></inline-formula> produced by C₆F₁₂O to produce decomposition products such as C₃F₆ and C₃F₇H. Meanwhile, the reaction rate of the C₆F₁₂O gas mixture with EPDM and the concentration of C₃F₆ and C₃F₇H were increased by both the increasing temperature and O₂ introduction. The diffusion coefficient <italic>D</italic> indicates that the diffusion speed of gas molecules in EPDM can reflect the physicochemical relationship between the gas and the solid material. The diffusion coefficients of both C₆F₁₂O and CO₂ in EPDM increased with rising temperatures. At 110 °C, the diffusion coefficients of C₆F₁₂O and CO₂ were increased by 1.71 and 2.49 times, respectively, following O₂ addition. It indicates that the internal energies of gas molecules were increased by adding O₂, and the C₆F₁₂O and CO₂ gas molecules could be easier to diffuse on the EPDM surface and interact with EPDM. The Flory–Huggins interaction parameter (<inline-formula><tex-math id="Z-20240912143141">$\chi $</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912143141.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912143141.png"/></alternatives></inline-formula>) quantifies alterations in molecular interaction energy during miscibility and aids in determining the solubility of binary mixtures. A value of <inline-formula><tex-math id="Z-20240912143235">$\chi $</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912143235.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912143235.png"/></alternatives></inline-formula> tending towards 0 indicates good compatibility between the mixed systems. Meanwhile, the parameter (<inline-formula><tex-math id="Z-20240912143253">$\chi $</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912143253.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912143253.png"/></alternatives></inline-formula>) tends to be linear with the mixing energy (Δ<italic>E</italic>_mix). Both <inline-formula><tex-math id="Z-20240912143312">$\chi $</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912143312.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912143312.png"/></alternatives></inline-formula> and Δ<italic>E</italic>_mix values of the EPDM mixing system decreased with increasing temperature. The <inline-formula><tex-math id="Z-20240912143325">$\chi $</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202300004_Z-20240912143325.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.