Thermodynamic Study Of Modified Nano-SiO₂ On Epoxy Composites

Epoxy resin and its nanocomposites are widely used in basin insulators,dry-type transformer,dry-type bushing,aerospace equipment.However,its application and development are limited by its poor heat resistance and brittleness at high temperature.In recent years,with the increasing voltage level,it is necessary for power equipment to have better heat-resistant capacity to ensure the safe and stable operation of the equipment.Therefore,it is of great significance to consider how to effectively improve the thermal and mechanical properties of epoxy resin to ensure the normal operation of the equipments.In view of this,the effects of curing agent,crosslinking degree of epoxy resin and nano silica particles modified by silane coupling agent on the thermodynamic properties of epoxy resin were studied based on the experimental method and molecular dynamics technology.The bisphenol type A epoxy resin?DGEBA?was selected as the matrix material,and the m-phenylenediamine curing agent?mBD?and isoflurone diamine curing agent?IPD?were crosslinked with DGEBA respectively.The thermodynamic performance parameters were calculated when the degree of crosslinking was 26%,51%,66%and 84%after the curing of DGEBA/mBD and DGEBA/IPD,and the optimal curing agent and crosslinking degree were selected.Furthermore,nano-SiO₂ was modified by silane coupling agent and doped into epoxy resin.The effects of the length and grafting density of aminosilane coupling agent on the thermodynamic properties of epoxy resin were studied.The characteristic parameters such as loss tangent and elastic modulus of the samples of DGEBA/MBD and DGEBA/IPD under different crosslinking degrees were analyzed.And also loss tangent and elastic modulus of epoxy resin doped with nano-SiO₂ particles modified by aminosilane coupling agent were analyzed.The characteristic parameters,such as elastic modulus,shear modulus,bulk modulus,glass transition temperature?Tg?,free volume fraction,radial distribution function,mean square displacement,hydrogen bond and chain spacing were analyzed by the simulation.Finally,the silane coupling agent with better performance and the ralatively better grafting density were selected.The effect of silane coupling agent modified nano particles on the improvement of the thermodynamic properties of epoxy resin were explained from the microscopic point of view.The simulation results are compared with the test results.The main conclusions as follows:?1?The higher the crosslinking degree of DGEBA/mBD and DGEBA/IPD is,the better their thermodynamic properties are,but thermodynamic properties of the former is better than the latter.The test results show that with the increase of crosslinking degree,Tg of cured DGEBA/mBD and DGEBA/IPD also increased,while the peak value of loss tangent decreased slightly.When the crosslinking degree of DGEBA/mBD and DGEBA/IPD is 84%,the Tg increases by 6K and 23K respectively when compared to the crosslinking degree is 26%.And under the same conditions,the Tg and elastic modulus of DGEBA/m BD are better than DGEBA/IPD.The simulation results show that the higher the crosslinking degree is,the lower the free volume fraction of the two different curing agents after reacted with the epoxy resin,the lower the chain movement ability,and the better stacking ability between molecules.Under the same conditions,Tg of the cross-linking reaction of DGEBA/m BD is higher than DGEBA/IPD,and also the mechanical properties of the former is better than that of the latter.It can be seen that the simulation results are consistent with the test results.?2?Compared the thermodynamic properties of the epoxy resin doped with nano-Si O₂ grafted by 3-aminopropyltriethoxysilane?A1100?,N-?2-aminoethyl?-3-aminopropyltrimethoxysilane?A1120?and 3-[2-?2-aminoethylamino?ethylamino]propyl trimethoxysilane?A1130?,respectively.It was found that the thermodynamic properties of epoxy resin doped with nano-SiO₂ grafted by A1120 were relatively better.The experimental results show that the elastic modulus of the epoxy resin doped with nano-SiO₂ grafted with A1100,A1120 and A1130 were increased by 5.4%,18.36%and 12.2%respectively,and the glass transition temperature of nano particle grafted with A1120 is relatively better.The simulation results show that the number of hydrogen bond and toughness,mechanical properties and Tg of epoxy resin were increased,however,the free volume fraction was decreased.It was found that the effect of thermodynamic properties of nano particles grafted with A1120 doped into epoxy resin was relatively good.?3?As the graft density of the silane coupling agent on the surface of the nanoparticles increases,the mechanical properties and glass transition temperature of the epoxy resin composite materials tend to increase first and then decrease.When the graft density is 9%,the thermodynamic performance is relatively best,and the glass transition temperature is increased by 41K.Modification of nanoparticles with silane coupling agent to improve the thermodynamic properties of epoxy resin may be related to the OH·O hydrogen bonds formed between the nanoparticles and epoxy resin.?4?The higher the grafting density of the silane coupling agent on the surface of the nanoparticles,the lower the chain movement ability of the epoxy resin,and the higher the interaction energy between the epoxy resin and the nanoparticles modified by the silane coupling agent.And there is a negative correlation between interaction energy and temperature.