The Immobilization Of Fluorine / Ureido-containing Siloxanes And Their Effect On The Tracking Resistance Of Addition-Cure Liquid Silicone Rubber

Silicone rubber is widely used in high or ultra-high voltage insulation because of its excellent properties such as hydrophobicity,electric insulation,high-and low-temperature resistance and weather resistance.However,due to the increasing transmission voltage level,tracking destruction will happen on the silicone rubber in the polluted or humid environment,leading to the insulation failure,which threatens the safety work of transmission network.At present,the tracking resistance of silicone rubber is mainly improved by incorporation of inorganic fillers.But the fillers are incompatible with silicone rubber and a large loading will damage the mechanical properties of silicone rubber.Our groups found that organic tracking resistance additives like ureido-or urethane-containing silanes could enhance the tracking resistance of silicone rubber without damaging the mechanical properties.However,the ureido or urethane group has relatively strong polarity and increases the surface energy of silicone rubber.It will damage the hydrophobicity when large amount of ureido or urethane groups are incorporated into silicone rubber.Thus,how to effectively improve the tracking resistance and hydrophobicity of silicone rubber,is one of the key problems to be resolved in the development and application of high performance silicone rubber insulating materials.In this work,perfluorooctyltriethoxysilane(FAS)or ureido-containing siloxanes were immobilized on the silica(SiO₂)and ferric oxide(Fe₂O₃)to investigate the influence on the tracking resistance,hydrophobicity,thermal stability and anti-plasma irradiation of addition-cure liquid silicone rubber(ALSR).The possible enhancement mechanism of SiO₂and Fe₂O₃immobilizing fluorine/ureido-containing siloxanes on the tracking resistance of ALSR was explored.The main contents and results are as follows.(1)Fluorosiloxane immobilized on ferric oxide(F-Fe₂O₃)was prepared by the grafting reaction of Fe₂O₃and FAS.The structure of F-Fe₂O₃was characterized by Fourier transform infrared spectroscopy(FTIR),thermogravimetry(TGA),particle size analysis and hysteresis loop test.The effect of F-Fe₂O₃on the tracking resistance,hydrophobicity,thermal stability and anti-corona ageing of ALSR was investigated.The possible mechanism of F-Fe₂O₃on the tracking resistance of ALSR was explored.It was found that F-Fe₂O₃could remarkably improve the tracking resistance of ALSR.When the content of F-Fe₂O₃was 0.15 phr,ALSR/F-Fe₂O₃reached 1A3.5 level and the erosion mass was only 0.38%.The hydrophobicity,thermal stability and anti-corona ageing of ALSR were enhanced.The surface potential decay test(SPD)showed that F-Fe₂O₃could dissipate the charges.During the arc discharge,F-Fe₂O₃could catch the free radicals generated from the oxidation of methyl groups in silicone rubber chains and suppress the oxidation degradation of ALSR.Meanwhile,the fluorine atoms in F-Fe₂O₃could dissipate the charges and reduce the charge accumulation,which could protect the matrix from further destruction.Therefore,the tracking resistance of ALSR was enhanced.(2)Fluorosiloxane immobilized on silica(F-SiO₂)was prepared by the grafting reaction of SiO₂and FAS.The structure of F-SiO₂was characterized by FTIR,²⁹Si nuclear magnetic resonance spectroscopy(²⁹Si-NMR),TGA and X-ray photoelectron spectroscopy(XPS).Inclined-plane test(IP),SPD,plasma irradiation,XPS and scanning electron microscopy(SEM)were applied to investigate the effect of F-SiO₂on the tracking resistance,hydrophobicity,mechanical properties and anti-plasma irradiation of ALSR.The mechanism of F-SiO₂on the tracking resistance of ALSR was explored.The results showed that F-SiO₂could remarkably enhance the tracking resistance of ALSR.When F-SiO₂content was 1.0 phr,ALSR/F-SiO₂reached 1A4.5 level and the erosion mass was only 0.22%.Moreover,the mechanical properties and hydrophobicity of ALSR were improved.SPD showed that F-SiO₂could dissipate charges.It was revealed that F-SiO₂could dissipate the injected charges and diminished the density of charges in ALSR during the arc discharge,which reduced the damage of charges to ALSR chains and postponed the occurrence of intensive arc discharge and decreased the arc discharge frequency.Thus,the tracking resistance of ALSR was improved.(3)Alkyl-disubstituted ureido siloxane immobilized on silica(U-SiO₂)was prepared by the dehydration condensation of hydrolysates of alkoxy groups in alkyl-disubstituted ureido siloxanes(US)and hydroxyl groups on the SiO₂surface.The U-SiO₂structure was characterized by FTIR and TGA.The effect of U-SiO₂on the tracking resistance of ALSR was investigated by the IP.The mechanism of U-SiO₂on the tracking resistance of ALSR was explored.It was found that U-SiO₂could effectively enhance the tracking resistance of ALSR.Especially,diisopropylureidopropyltriethoxysilane and diisobutylureidopropyltriethoxysilane with isomeric substitution immobilized on silica(DIPUPES-SiO₂and DIBUPES-SiO₂)performed the best.With less than 1.00 phr content,ALSR samples could reach 1A4.5 level and the erosion mass was only 0.22%and 0.45%,respectively.Moreover,ALSR/DIPUPES-SiO₂and ALSR/DIBUPES-SiO₂had excellent water resistance.They could still reach 1A4.5 level and the erosion mass was almost unchanged after ALSR/DIPUPES-SiO₂and ALSR/DIBUPES-SiO₂were immersed in the water for five days.The results of plasma irradiation and thermal treatment showed that U-SiO₂could promote the crosslinking of ALSR under the elevated temperature to form a compact ceramic protective layer.It could not only suppress the thermal degradation of ALSR,but also resist the plasma bombardment.Therefore,the tracking resistance of ALSR was remarkably enhanced.(4)F-SiO₂and DIPUPES-SiO₂were simultaneously incorporated into ALSR to investigate the influence on the tracking resistance,hydrophobicity,thermal stability and anti-plasma irradiation of ALSR.The possible synergistic mechanism of F-SiO₂and DIPUPES-SiO₂for the enhancement of the tracking resistance of ALSR was explored.The results showed that F-SiO₂and DIPUPES-SiO₂could synergistically enhance the tracking resistance of ALSR.When the contents of F-SiO₂and DIPUPES-SiO₂were both 0.75 phr,the ALSR sample reached 1A4.5 level and the erosion mass was only 0.20%.Moreover,the hydrophobicity,thermal stability and anti-plasma irradiation of ALSR were improved.It was revealed that F-SiO₂enhanced the hydrophobicity of ALSR,which could postpone the occurrence of leakage current.During the arc discharge,F-SiO₂could dissipate the charges and reduce the charge density in ALSR.Meanwhile,DIPUPES-SiO₂could impede the aggregation of platinum and enhance its catalytic efficiency to promote the crosslinking of ALSR under elevated temperature.The organic-inorganic conversion rate of ALSR was increased to form an inorganic ceramic protective layer,which could isolate the oxygen and heat and suppress the thermal degradation of ALSR.Hence,the tracking resistance of ALSR was effectively improved.