The Microscopic Mechanism Research Of Cellulose Insulating Paper Modified By Nano-SiO₂ Surface-modified Via Silane Coupling Agent

The stable operation of power transformers is of great significance for ensuring the safe and reliable operation of power systems.The internal insulation system of the transformer is composed of insulating paper and insulating oil,and its performance directly determines the service life of the transformer.Due to the internal structure of the transformer,the performance degradation of insulating oil can be solved by regular addition and replacement,but the insulating paper cannot be replaced.Therefore,improving the performance of the insulating paper has the practical significance of ensuring the insulation performance and prolonging the service life of the power transformer.In recent years,improving thermal stability,mechanical properties and aging resistance of polymer materials doped by nanoparticle has become a trend in the development of modern materials.Due to the agglomeration is easy to occur,directly doping often fails to improve the performance of the material,and on the contrary,it may reduce the performance of the material.Therefore,weakening the agglomeration effect and improving its dispersion in the matrix of nanoparticles by the surface modification have important engineering and academic value for improving the performance of polymer materials.The research is based on molecular dynamics simulation technology,which has been widely used in chemical,biological and material sciences in the paper.Firstly,the degree of polymerization?DP?and the optimal selection of force field of cellulose model under common force fields?PCFF and COMPASS?were studied systematically and comprehensively.Secondly,the effect of surface hydroxylation of nano-SiO₂ on the interface between cellulose and nano-SiO₂ was studied under the correct selection of cellulose polymerization degree and force field.Then,the effects of silane coupling agents?KH550,KH560,KH570,KH792?on the interface between cellulose and nano-SiO₂ were compared and analyzed,and the optimum type and density of silane coupling agents were determined.The main conclusions are as follows:?1?The mechanical properties and solubility of cellulose are relatively stable when the DP is not less than 10 under three common force fields?PCFF?,which is consistent with true mechanical and chemical properties of cellulose.Force field has little effect on the selection of degree of polymerization of cellulose.The steady-state density of cellulose model is closer to the true density of cellulose.Therefore,under the three common force fields?PCFF and COMPASS?,the cellulose model DP selection in the molecular dynamics simulation should be no less than 10,and the force field selection COMPASS is better.?2?The order of the surface energy of nano-SiO₂ crystal surface determined by molecular simulation is:?110?>?111?>?101?>?011?>?010?>?001?>?100?.Next,the?110?crystal plane with the lowest surface energy was selected to study the microscopic mechanism of surface hydroxylation on the cellulose/nano-SiO₂ interface.The results show that compared with the NC?non-hydroxylated?layer model,the interaction energy and interaction energy of the SHN-C?hydroxyl?layer model are increased by 2.3 times and 2.2 times,respectively,the overlapping regions and peaks of the relative concentration of the interface is increased by 1.62 times and 1.24 times,respectively.The jump temperature of the mean square displacement?MSD?of cellulose was reduced by 20 K,and a hydrogen bond with a density of 4.18/nm² was formed between the interfaces.Therefore,the hydroxylation of the surface of nano-SiO₂helps to enhance the interaction between the interfaces,the combination of nano-SiO₂and cellulose is tighter,which helps to strengthen the binding of cellulose chain.?3?The results show that when the nano-SiO₂ is modified by KH792,the binding energy and binding density of interface are the largest,the number of hydrogen bonds is the most between interfaces,the binding effect on cellulose chain is the most significant,and the interface overlap is the largest.Moreover,in order to better improve the interfacial effect between nano-SiO₂ and the polymer,firstly,silane-coupling agent containing hydrogen bond-forming group with a polymer should be preferred,and secondly,under the same conditions,the chain length is relatively short and small molecular weight of silane coupling agent should be choosed.In addition,when the nano-SiO₂ is modified by KH792,the interfacial bonding energy fluctuates within 5%of 336.8 kcal/mol,and the number of hydrogen bonds formed between the interfaces remains relatively stable when the grafting density reaches 1.570?1/nm²?.Therefore,the relative optimum graft density is 1.570?1/nm²?.