Thermal Oxidation Mechanism Of Ester Lubricants And Preparation And Performance Of Their Antioxidants

Although synthetic ester oils have particularly excellent performance,they still cannot fully meet the working requirements of high-end equipment.The harsh working conditions promote the thermal oxidation of ester oils,which deteriorates their performance.Studying the thermal oxidation behavior of ester oils,designing and preparing high-efficiency antioxidants are the key to enhancing the anti-oxidation ability of ester oils and prolonging their service life.It is beneficial to improve the operational reliability of equipment and guide the development and application of synthetic ester oils.The main research objects are typical ester base oils including trimethylolpropane trioleate(TMPTO),diisooctyl adipate(DOA)and triisodecyl trimellitate(TDTM)in this paper.The thermal oxidation laws of DOA and TDTM were researched as well as their performance difference by means of experimental tests and molecular simulation calculations,and consequently revealing the thermal oxidation mechanisms of both oils on the atomic scale.The catalytic effect of Fe surface on the physico-chemical properties and chemical structures of TMPTO,DOA and TDTM was studied,and the law of typical antioxidants and metal deactivators in suppressing Fe catalytic oxidation was discussed.The high-efficiency multi-phenol antioxidants were designed and prepared to study their effects on the anti-oxidative and tribological properties of ester oils.The structural changes of lubricating oils caused by the oxidation affect the lubricating properties.Aim to online study the change in chemical structures during high-temperature friction,a test system combining a high-temperature friction tester with an infrared spectrometer was assembled to study the relationship between friction coefficient changes and chemical structure transformations of lubricating oils.Based on the above research,the main conclusions are as follows:(1)TDTM has better oxidation resistance than DOA,showing more stable kinematic viscosity,longer oxidation induction time and higher oxidation onset temperature.TDTM has a lower free volume fraction and oxygen diffusion coefficient,indicating that its dissolved oxygen capacity is weaker than that of DOA.The results of reactive molecular dynamics simulations indicate that C-O bonds from both the alcohol chain and ester group are susceptible sites of DOA,while the main site of TDTM is the C-O bond located at the alcohol chain.The oxidation of DOA produces more hydrogen bonds and degradation fragments than that of TDTM.The molecular fragments of DOA bond by means of ether linkages to form polymerized products,while TDTM has no polymerized products during the oxidation.(2)The catalytic effect of Fe surface significantly affects the thermal oxidation behavior of ester oils.The Fe surface promotes the increase in kinematic viscosity and total acid value of DOA,as well as the accumulation of alcohol and carbonyl products under thermal oxidation conditions.For the unsaturated TMPTO,the Fe surface induces a dramatical increase in kinematic viscosity,while it slightly increases the total acid value.The C=C and=C-H bonds are the active sites of TMPTO,and the catalysis of Fe accelerates their chemical changes to generate more carboxylic acid products.Different from DOA and TMPTO,the Fe surface basically has no catalytic effect during the oxidation of TDTM.The mixture of dinonyl diphenylamine(DNDA)antioxidant and metal deactivator N,N'-di-n-butyl-amino-methylene-benzenetriazole(BTA)exhibits a synergistic antioxidant effect,which can effectively suppress the catalytic effect of Fe surface and slow down the increase in kinematic viscosity and the accumulation of oxidative products.(3)On the basis of clarifying the law of thermal oxidation behavior of ester oils,efficient antioxidants including 2,4,6-tris(3,5-di-tert-butyl-4-hydroxyphenylthio)-1,3,5-triazine(THA)and phloroglucinol tris(3-(3,5-di-tert-butyl-4-hydroxyphenyl))propionate(PTP)were prepared to improve the oxidation resistance of ester oils,which contain multiple hindered phenols in a single molecule.Both THA and PTP antioxidants possess excellent thermal stability.PTP effectively improves the oxidation resistance of the three ester base oils.THA can enhance the oxidation resistance of DOA and TDTM,however,it has limited effectiveness in improving the antioxidation ability of TMPTO.Moreover,PTP strengthens the anti-wear property of DOA.In TDTM,THA exhibits a great synergistic antioxidation effect with DNDA.The THA mixed with DNDA and ZDDP additives can further enhance the oxidation resistance of TDTM,as well as its tribological performance.It suggests the excellent compatibility of the synthesized THA antioxidant with DNDA amine antioxidant and ZDDP anti-wear additive in the ester oil.(4)A test system was designed and developed to continuously characterize the structure changes of lubricating oils during friction based on a high-temperature and low-speed four-ball friction tester combined with an infrared spectrometer.The result shows that the change in infrared spectra of lubricants can accuratedly experess their structure changes and the effect of antioxidants during the friction at high temperature.The changes in friction coefficient are in correspondence with the structure changes of lubricating oils.The analysis of online infrared spectra shows that the DNDA antioxidant inhibits the oxidation of lubricating oils through the dehydrogenation of the N-H functional group,which reduces the formation of alcohol and carbonyl oxidative products.As a multi-phenol antioxidant,the synthetic PTP effectively improves the oxidation resistance and high temperature lubrication performance of TMPTO ester oil.The phenolic hydroxyl of PTP antioxidant is prone to dehydrogenation to suppress the oxidation of TMPTO.The addition of PTP antioxidant reduces the friction coefficient of TMPTO during the stable friction stage at high temperature.