Influence Of NO₂ Exposure On Structure Of Silicone Rubber Used In Composite Insulator

In the long-term field running,due to the physical,chemical or electric ageing factors,ageing of a composite insulator can be inevitable.Among these ageing factors,N02 exposure is one of the most important chemical factors.When deformed electric field on the side of high voltage or improper installation of equalizing ring in the composite insulator,there often existed discharge,in which N2 and O2 in the air will generate NO2.In addition,exhaust emissions of the automobile and industrial-agricultural production during human activities also lead to much NO2 in the air.When surface of the composite insulator is exposed to N02 directly,the oxidation of NO2 can easily change its chemical structure and physical properties.Therefore,this paper used means of accelerated aging to simulate silicone rubber's aging in NO2 and carried out the following works:Using FTIR,TG-DSC and SEM to study chemical and physical structure change in silicone rubber with different SiO2 and ATH content and analyzed the impact of ATH and SiO2 content on reaction.Using FTIR,XPS,TG,DSC and SEM to study evolution law as to chemical and physical structure after different aging time on the surface of commercial silicone rubber and discuss the corresponding aging mechanism.By means of plasma treatment and GC-MS,the effects of NO2 aging on small molecule and hydrophobic recovery were discussed from the perspective of hydrophobic recovery and of small molecule and content's variation.The main conclusions were as follows:(1)FTIR,TG-DSC and SEM results showed that silicone rubber was affected by NO2 on a different degree because of different ATH content.For silicone rubber without ATH,N02 would lead to fracture of the main chain Si-O-Si in the organic components PDMS on the silicon rubber surface gradually,side chain Si-CH3 were gradually oxided generating a small amount of nitroso groups.Due to the increased inorganic Si-O links,roughness increases;After adding ATH,as the content of the ATH gradually increased,the aging caused decrease of PDMS which give birth to increase of irregular particles on the surface and the roughness.The existence of the ATH led to the emergence of new material and participates in the early process during thermal decomposition and the weight loss ratio increased as the aging time increases.This indicates that ATH not only affected the degradation of silicone rubber,but also accelerated the aging caused by NO2 as ATH content increased.(2)FTIR and TG,DSC and SEM results showed that whether or not to add SiO2 in silicone rubber did not affect the kinds of chemical groups on silicone rubber surface,which suggested that NO2 did not react with SiO2,but SiO2 would affect the reaction process between silicone rubber and NO2.When adding a certain content of SiO2,NO2 would also lead to fracture of the main chain Si-O-Si in organic components PDMS gradually,side chains Si-CH3 were gradually oxided,generating a small amount of nitroso group at the same time,the roughness on the surface increased due to the increased inorganic Si-O link and exposure of ATH;As the content of SiO2 increased gradually,decrease of organic chemical groups aggravated in the same aging time.Meanwhile,in the same aging time,PDMS decreased with the increase of Si02 content.SiO2 although did not react with NO2,but it was necessary for material system like silicone rubber,so the reasonable addition could enhance the overall performance of the silicone rubber,but,it may make structure system to produce more physical defects when addition of SiO2 exceed,thus accelerating the reaction of NO2 with silicone rubber.(3)Using FTIR,XPS,TG-DSC and SEM to explore the aging mechanism of commercial composite insulator silicone rubber in NO2.The results showed NO2 led to the rupture of Si-O-Si bond in PDMS network,and because of the oxidization of side chain in PDMS,a small amount of the-CH3 group changed into nitroso group during this NO2 exposure.Due to the decrease of the organic component PDMS in the reaction process,the exposed ATH further reacted with NO2 to generate Al(NO3)3,resulting in decrease of thermal stability.(4)The results of GC-MS,hydrophobicity and hydrophobicity recovery showed that,as the aging time increased,the relative content of D4-D7 increased,and correspondingly,the content of D12-D18 decreased.This was mainly due to the fact that NO2 caused the break-up of main chain in PDMS and further cross-link into small molecules.In addition,large circular molecules also decomposed and transformed into small circular molecules.NO2 aging caused the decrease of hydrophobicity to.But NO2 caused change of hydrophobicity to and the hydrophobicity increased with the aging time.However,the hydrophobicity of the aging sample did not recovered compared with the untreated samples.Using plasma treatment to make after silicone rubber surface hydrophilic completely,when aged less than 24 h,hydrophobic recovery was faster,this was due to the participation of increased D4-D7 during hydrophobic recovery.At the same time,the number of microchannel gradually increased and molecules were more likely to migrate to the surface in process of hydrophobic recovery,but when the aging time increased to 36 h,although the number of microchannel diminished,small molecules have loss a lot,and hydrophobic recovery kept at a same level with samples aged less than 24 h.