Effect Of Graphene On Mechanical Properties Of NBR Composites After Thermal-oxidative Aging

Nitrile rubber(NBR)is widely used in the aerospace industry,military product research and development,automobile manufacturing and other fields because of its excellent heat resistance,chemical stability and corrosion resistance.However,the presence of unsaturated carbon-carbon double bonds in the NBR molecular chain makes the thermal oxygen aging phenomenon of NBR products in the manufacturing,use and storage process serious,and it is difficult to meet the NBR’s work under extreme conditions such as high temperature and high pressure.Therefore,the development and preparation of NBR composite materials with excellent aging resistance are of great practical significance.With the development of nanotechnologies,graphene(GNS)and graphene oxide(GO)are often used to improve the aging resistance of rubber composite materials due to their excellent physical properties,mechanical properties and thermal properties.In this thesis,the effects of GNS and GO on the mechanical properties and friction properties of NBR matrix are quantitatively studied at the atomic scale by molecular dynamics simulation(MD)method,and the migration characteristics and binding characteristics of NBR molecular chains and antioxidant molecular chains are analyzed.NBR composite materials were prepared by mechanical blending method for thermo-oxygen aging test,and the effects of GNS and GO on the mechanical properties,friction properties and surface morphology of NBR after thermo-oxygen aging were studied from the macroscopic performance level.Combining MD simulation results and experimental results to explore the mechanism of GNS and GO to enhance the mechanical properties and friction properties of NBR matrix after thermal oxygen aging,the specific research content is as follows:(1)The molecular models of three NBR composite systems,Pure NBR,GNS/NBR and GO/NBR,were established separately.The MD method was used to calculate the mechanical properties(Young’s modulus,shear modulus,volume modulus,Cauchy pressure)of the three NBR composite systems at temperatures of 298 K and 348 K.By extracting the binding energy,mean square displacement,free volume fraction and other microscopic information of the NBR composite system,the mechanism of action of GNS and GO on NBR thermal oxygen aging phenomenon is revealed.The study found that:compared with Pure NBR,the introduction of GNS and GO can not only significantly improve the elastic deformation resistance,volume deformation resistance and shear deformation resistance of the NBR composite system,but also effectively slow down the weakening effect of thermal oxygen aging on the mechanical properties of the NBR matrix.This is due to the strong interfacial adsorption capacity of GNS and GO,which can make the molecular chain pile up more tightly,effectively reduce the molecular chain migration capacity,so that the NBR composite system still maintains good stability when the temperature rises,and can effectively reduce the effect of thermal oxygen aging on the mechanical properties of rubber.In addition,compared with GNS,the surface of GO contains rich functional groups such as epoxy groups,which enhances the binding strength between the GO and NBR interface,so that the GO/NBR composite material exhibits better mechanical properties and thermal and oxygen aging resistance.(2)Pure NBR and GO/NBR composite materials were prepared by mechanical blending method,and the tensile strength,tear strength,100% constant tensile stress,elongation at break,hardness and other performance parameters of the two NBR composite samples before and after thermal oxygen aging were tested by electronic universal tester,Shore hardness tester and other instruments.The fracture morphology of NBR composite samples was observed through a scanning electron microscope(SEM),and the mechanical properties of pure NBR and GO/NBR composite system samples were compared and analyzed.Based on the above MD simulation results,the influence of GO on the mechanical properties of NBR composite materials after thermal oxygen aging is discussed from the perspective of combining macroscopic properties and microstructure.The study found that compared with pure NBR materials,GO/NBR composite materials can maintain good fracture toughness and good mechanical stability during thermal and oxygen aging.This is due to the large specific surface area and high elastic modulus of GO,which can significantly improve the mechanical properties of GO/NBR composite materials.(3)Taking the three composite systems of Pure NBR,GNS/NBR and GO/NBR as the intermediate layers,a friction paramolecular model composed of the metal iron(Fe)atomic layer was established separately.MD simulation was used to study the effects of GNS and GO on the friction properties of NBR composite materials after thermal oxygen aging,and the friction behavior between the three NBR composite systems and the Fe atoms was analyzed.At temperatures of 298 K and 348 K,the friction properties of the three composite systems are characterized by calculating the friction coefficients and wear rates.At the same time,the binding energy,mean square displacement,relative concentration,friction temperature and radial distribution functions of the NBR composite system are extracted to reveal the friction mechanism of GNS and GO on NBR composite materials.The study found that: the higher the aging temperature,the higher the coefficient of friction of the NBR composite system and the greater the wear rate,but the GNS/NBR and GO/NBR composite materials can still maintain good friction properties.This is due to the increase in temperature affecting the organizational structure of the NBR,causing some of its sub-chains to break,accelerating the thermal and oxygen aging of the NBR material,resulting in a significant decrease in friction properties.The addition of GNS and GO can hinder the movement of NBR molecular chains to the friction region,so that the atomic concentration at the friction interface decreases,thereby reducing the coefficient of friction and wear rate of NBR composite materials,so that NBR composite materials maintain good friction properties.Similarly,compared with GNS,GO exhibits better tribological properties and thermal oxygen aging resistance due to the presence of oxygen-containing functional groups on the surface.(4)Design the friction and wear test scheme of NBR composite materials,use a friction tester to test the friction coefficient and wear amount of pure NBR and GO/NBR composite samples before and after thermal oxygen aging,observe the wear surface morphology through SEM,combined with MD friction simulation results,compare and analyze GO to enhance the friction properties of NBR composite materials,and reveal the damage mechanism of friction and wear of NBR composite materials.The study found that GO/NBR composite materials can maintain a lower amount of wear during the thermal and oxygen aging process and have better resistance to wear.This is due to the rich oxygen-containing functional groups on the surface of GO,which enables it to interact strongly with NBR and improve its binding ability to the NBR matrix,thereby reducing the free movement space of the NBR molecular chain and inhibiting the movement of the NBR molecular chain.Thereby improving the shear resistance of NBR composite materials and significantly enhancing their friction properties.