Preparation Of Ureido-containing MQ Silicone Resin And Its Suppression Effect On The Tracking And Erosion Of Addition-cure Liquid Silicone Rubber

With the advantages of excellent insulation property,hydrophobicity,heat resistance and low shrinkage after cure,addition-cure liquid silicone rubber(ALSR)becomes one of the most potential and promising materials in high voltage insulation.As the economy develops,the voltage level is rising and the air pollution is increasingly serious,ALSR is more likely to be subjected to tracking and erosion.Once the tracking happens,it will seriously threaten the security and stability of the high voltage and extra high voltage power transmission,even lead to huge economic loss.Therefore,it’s of great significance to improve the tracking resistance of ALSR.In this dissertation,a ureido-containing MQ silicone resin(U-MTQ)was synthesized by hydrolysis and co-condensation reaction of ureido silane(US),tetraethoxysilane(TEOS),hexamethyldisiloxane(MM)and divinyltetramethyldisiloxane(MviMvi).Then ALSR with good tracking resistance was prepared with U-MTQ as an anti-tracking agent.The effects of U-MTQ on tracking and erosion resistance,mechanical properties,thermal stability,dielectric property,light transmittance and contact angle of ALSR were investigated,and the suppression mechanism of U-MTQ on the tracking and erosion of ALSR was studied.The main contents and results are included in the following:(1)U-MTQ was synthesized by hydrolysis and co-condensation reaction of TEOS,US,MM,and MviMvi under the catalysis of hydrochloric acid.U-MTQ was characterized by fourier transform infrared spectrometry(FTIR),1H nuclear magnetic resonance spectrometry(1H-NMR),29Si nuclear magnetic resonance spectrometry(29Si-NMR),gel permeation chromatography(GPC)and thermogravimetric analysis(TGA).The influences of M/Q molar ratio and hydrochloric acid content on the yield and structure of U-MTQ were investigated.The results showed that the M/Q molar ratio of the product calculated from the 29Si-NMR spectrum was much smaller than that in the feed.When the M/Q molar ratio in the feed increased,the number average molecular weight and its distribution index of U-MTQ decreased,and Si-OH content of U-MTQ also decreased,but the structural regularity of U-MTQ was enhanced.When increasing the HCl content,the number average molecular weight and its distribution index of U-MTQ decreased,but the Si-OH content of U-MTQ and the structural regularity of U-MTQ increased.The yield first increased and then decreased with the increase of the HCl content,and the highest yield was up to 71.9%when HCl content was 2.10wt%of the monomer.Compared with the MQ,TGA results showed that the onset weight loss temperature(T0)of U-MTQ decreased from 237℃ to 158℃,and the residue of U-MTQ at 900℃ increased from 3.5%to 29.5%.(2)With U-MTQ as an anti-tracking agent,ALSR with good tracking resistance was prepared by hydrosilylation reaction under the catalysis of karstedt’s catalyst.The effects of U-MTQ on vulcanizing property,contact angle,light transmittance,dielectric property,thermal stability,mechanical properties,tracking and erosion resistance of ALSR were investigated.It was found that U-MTQ could effectively enhance the tracking and erosion resistance of ALSR.When the content of U-MTQ with vinyl content of 2.5wt%was 2phr,the ALSR samples could pass the 1A4.5kV level in the inclined plane tracking and erosion test(IPT),and the erosion rate of ALSR decreased from 6.10%to 0.13%.U-MTQ also improved the thermal stability and mechanical properties of ALSR.T0 increased from 350℃ to 358℃,the residue at 800℃ increased from 58.3%to 69.6%,tensile strength increased from 7.3MPa to 9.1MPa,and tear strength increased from 31.6kN/m to 35.0kN/m.U-MTQ had little effect on the vulcanizing property,contact angle and light transmittance of ALSR,and slightly improved the permittivity of ALSR.(3)The effects of karstedt’s catalyst(Pt)content on tracking resistance and thermal stability of ALSR were investigated by IPT,TGA and thermogravimetry-fourier transform infrared spectrometry(TG-FTIR).When the Pt content was lOppm or 15ppm,the ALSR had excellent tracking and erosion resistance,and all specimens could pass the 4.5kV level in the IPT,the erosion rate were only 0.13%and 0.15%,respectively.When Pt content was 15ppm,ALSR samples possessed the optimum thermal stability.TG-FTIR results showed that ALSR generated less carbonyl compounds and cyclic oligomers,and more methane when the Pt content was 15ppm.It demonstrated that there were lower level of the oxidation of methyl group and the main chain unzipping degradation of ALSR,and more homolysis of Si-CH3 during the degradation.The macromolecular radicals would further cross-link to form the firm Si-C ceramic barrier to protect ALSR samples under the catalysis of Pt and high temperature.(4)The effect of low temperature plasma on ALSR was investigated,the thermal decomposition behavior was researched by TG-FTIR,and the morphology and element content of ALSR after IPT were analyzed by scanning electron microscopy-energy dispersive X-ray spectroscopy(SEM-EDX)and laser Raman spectroscopy(LRS),respectively.The suppression mechanism of U-MTQ on the tracking and erosion of ALSR was put forward.With the effect of electric field and conductive solution,ALSR was damaged by arc discharge and finally formed electrical tracking and erosion.Arc is a high-temperature plasma.U-MTQ possessed good effect on anti-plasma irradiation of ALSR,and a protective layer was formed,so U-MTQ improved the anti-arcing of ALSR.U-MTQ suppressed the oxidation of methyl groups,and the ureido groups of U-MTQ were initially decomposed to isocyanic acid to react with hydroxyl groups arising from the oxidation of methyl groups,which suppressed the unzipping reaction.ALSR with U-MTQ produced more methane during the thermal degradation,which could bring out some carbon element and lead a decrease in carbon deposion,especially conductive carbon,so it could remarkably suppress the tracking.Under the catalysis of Pt and high temperature,the macromolecular radicals produced by homolysis of Si-CH3 would further crosslink to form firm Si-C ceramic barrier to protect ALSR from arcing,so U-MTQ could improve the tracking resistance of ALSR.