The Thermal Stability Improvement Mechanism Of MPIA Insulating Paper Modified By Nano-SiO₂ Surface-grafted Via APTS

In recent years,with the continuous development of the high voltage transmission network in China and the increase of the voltage level of the power grid,higher requirements for the insulation performance of the transformer which is the core equipment of transmission and distribution are put forward.M-phenylene isophthalamide?MPIA?insulation paper,because of its good electrical insulation,thermal stability and corrosion resistance,has been used as the one of the main insulating materials for large transformers for recent years.The demand for aramid insulation paper in China is also increasing with the development of UHV construction.However,the performance of the MPIA insulating paper made in China can not hold a candle to that of large foreign companies,so that it is of great engineering and academic value to study the microcosmic mechanism of oil paper insulation aging and to improve its thermal stability.The research shows that,it is a feasible method to improve the performance by adding nano SiO₂ particles grafted and modified by3-aminopropyltriethyloxy silane?APTS??GNP?.This modification can not only increase the interaction between nano-particles and aramid fiber,but also weaken the agglomeration of nano-particles and enhance the dispersion of MPIA insulating paper.The study of this paper is based on molecular dynamics.Firstly,considering the actual operation of transformer,the thermal stability of crystal region and amorphous region of MPIA insulation paper is compared.The adsorption effect and mechanism of MPIA insulating paper on naphthenic mineral oil,water and gas?represented by CO₂and CH₄?and acid?represented by formic acid?are analyzed.Then the agglomeration phenomenon and its mechanism of nano SiO₂ are studied,and the surface of SiO₂ is grafted and modified by APTS.The relative optimum grafting density is determined by studying the interface interaction of GNP/GNP and GNP/MPIA fibers.Finally,on the basis of previous studies,GNP is added to MPIA insulation paper,and the promotion effect and mechanism of GNP on the thermal stability of MPIA insulation paper are studied.The main conclusions obtained in this paper are as follows:?1?The thermal stability of MPIA fibrous crystal region is stronger than that of the amorphous region,and the aging products such as mineral oil and water,acid and gas are easier to enter the amorphous region.Under the operation temperature of transformer,the modulus value of crystalline region is 2³ times higher than that of amorphous region.The incompressibility,rigidity and anti deformation ability of crystal region are much higher than that of amorphous region.The plasticity and toughness of the crystal region are higher than that of the amorphous region,while the ductility of the amorphous region is better than that of the crystal region.The stability of crystal structure leads to its thermal stability better than that of amorphous region.The order of the adsorption strength of aramid fiber to four kinds of aging products is as follows:formic acid>water>CO₂>CH₄.The free volume fraction and pore size of the amorphous region are larger than that of the crystalline region,which makes it easier for mineral oil and aging products such as water,acid and gas to enter the amorphous region.?2?The agglomeration of nano SiO₂ particles is related to particle spacing,size and content.When the distance is large,the motion of nano-particles is dominated by diffusion motion,and the small size nano-particles are faster than the large size nano-particles.When the particle distance is close,the interaction between the nano-particles shortens the time to close each other.The interaction between nano-particles,including hydrogen bonds,causes the aggregation of each other.The higher the content is,the greater the probability that the nano-particles will be close to each other,and the more easy they are to agglomerate.Different grafting density will affect the interaction between GNP and MPIA insulation paper and the aggregation between GNP,and the effect is the best when the grafting density is 5.02 1/?².On the one hand,the combination effect of nano SiO₂ particles and MPIA insulation paper is relatively the best,which is conducive to the modification of GNP to MPIA insulation paper,and on the other hand,it can reduce the occurrence of aggregation between GNP.?3?When GNP is added to MPIA insulation paper,the thermal stability of the amorphous zone of the modified insulator paper is improved,the hydrogen bonding structure in insulation paper is changed,and a new hydrogen bond network between GNP and MPIA fiber is formed.The addition of GNP on the one hand weakens the chain movement of MPIA fibers and increases the thermal stability of fibers.On the other hand,it fills the larger gap and increases cohesive energy density,playing a role similar to adhesive.The addition of GNP weakens the diffusion of water,acid and gas in MPIA fiber,weakens the effect of these substances on the thermal stability of aramid fiber,which improves the thermal stability of aramid insulation paper,and slows the aging process of aramid paper.