Experiment And Molecular Dynamics Simulation On The Thermal-mechanical Properties Of Epoxy/SiO₂

Epoxy resin and its composites are widely used in electrical power equipment,but the low thermal conductivity limits the application.Especially with the increase of voltage grade and size reduction,equipment heat dissipation problems have been more badly,and the use of life will be seriously affected,it is a hot research topic to improve the thermal conductivity of epoxy in recent years.The traditional method is conducting experiments to obtain the parameters of the sample characteristics,which costs a lot of time and materials.The development of molecular dynamics provides a new method and thought to study the properties of polymer and its composite materials,it showes a significant advantage to tudy micro-structure and properties of polymer composites at the molecular level.This paper summarized the relevant literature at home and abroad about high thermal conductive polymer material,analysed the thermal-conducting mechanism and influence factors of thermal conductivity.of composites,studied the thermal-mechanical properties of composite materials with epoxy(EP)as the matrix and silica(SiO2)as the filler.EP and EP/SiO2 molecular models were established to analysis the effect of different particle sizes and surface modification on the glass transition temperature(Tg),coefficient of thermal expansion(CTE),thermal conductivity and elasticity modulus,and the effect of temperature on thermal conductivity and elasticity modulus of the composites was analyzed also.The simulation results were compared with experimental results.The composite material EP/SiO2-1(without grafting silane coupling agent on the surface of silica)and EP/SiO2-KH550-1(with grafting silane coupling agent on the surface of silica)were prepared by melt-compounding method,and compare with EP(pure epoxy).The thermal-mechanical properties of three kind materials were tested,such as Glass transition temperature,thermal conductivity and elastic modulus.The results showed that the addition of SiO2 can improve the thermal-mechanical properties of epoxy,andit can further improve the thermal-mechanical properties by grafting silane coupling agent on the surface of silica.With molecular simulating on the model of EP,its glass transition temperature was calculated about 410.2K,compared with the experiment result 388K,further calculation showed that coefficient of thermal expansion were 307.493x 10-6/K under glassy state,502.865 X 10-6/K under rubbery state.The thermal conductivity of EP simulated was 0.21384W/(m k),higher than the results of experimental test 0.208 W/(m k)slightly.The elastic modulus of EP simulated was 5.624GPa,also higher than the results of experimental test 4.65GPa.In order to analyze size effect SiO2 on thermal-mechanical properties of EP/SiO2,a range of different size SiO2 addition composite models were built,the SiO2 radius were 10A,11.41 A,12.56A and 13.97A respectively.Simulation results showed that,when the mass fraction of SiO2 is 17%,the thermal-mechanical properties of the composite moedls increased compared to that of EP model,and the elastic modulus decreasesd with the increase of SiO2 size,which were closed to the experimental values.But glass transition temperature and thermal conductivity of composite models showed no obvious change trend with the increase of SiO2 size.Finally,the model of silica grafted silane coupling agent at the rate of 5%and 10%were established to analyze the effect of SiO2 surface modification on thermal-mechanical properties of EP/SiO2.The simulation results showed that it can further enhance the thermal-mechanical properties of composite model by grafting silane coupling agent on the surface of silica.The glass transition temperature of EP/SiO2-5%and EP/SiO2-10%were 426.3K and 438.2K,higher than the experiment result 402K.After modification,the thermal conductivity at 300K of the two composite models increased by 20.28%and 25.585%respectively.As the temperature rise,the thermal conductivity showed a grater increase,the elastic modulus and shear modulus of four kind models showed the trend of decline,and when the temperature reaches over 500K,the mechanical properties showed a small decline,the fastest decline range of EP was 350K to 450K,while that of the other three composite models were 400K to 450K.