Research On Oxidation-resistance And Mechanisms Of Synthetic Ester Oil And Additives

Synthetic ester oil has been widely used in the aerospace and automotive industries, as well as under extreme working conditions, due to its high thermal-oxidation stability,excellent viscosity index, good thermal conductivity, and superior tribological performance.Currently, the overload and high speed requirements of machinery and equipment result in harsh working conditions in mechanical lubrication systems and elevated working temperatures. Although synthetic ester oil has good thermal-oxidation stability, it will undergo thermal-oxidation inevitably, thereby altering its service performance, especially under conditions of sustained high temperature. However, because of its unique ester bonds,the existing free radical theory relevant to the thermal-oxidation behavior of hydrocarbon mineral oils cannot be generalized to synthetic ester oil. Additionally, the thermal-oxidation mechanism and the relationship between the change in properties and thermal-oxidation products of synthetic ester oil have not been elucidated. Therefore, it is important to investigate the thermal-oxidation mechanism of synthetic ester oil, and further to improve its oxidation-resistance.This paper was supported by the Major State Basic Research Development Program of China(973 Program) named “Lubricant additive anti-friction, oxidation-resistance performance and its effect law on synthetic lubricants' action” and aimed at improving the thermal-oxidation performance of synthetic ester oil. Oxidation-resistance test, reactive force field molecular dynamic simulation, instrumental analysis and tribology test were used to research their thermal-oxidation mechanism, performance change law and additives' modification technique for typical synthetic ester oil TMPTO and DOA. Based on a large amount of experiments, thermal-oxidation mechanism of TMPTO and DOA ester oils and effect law of additives were proposed from molecular scale to provide an important scientific basis and base data for improving synthetic ester oil's performance and selecting additives.The main research contents and results are as follow.(1) The correlation and differences of four oxidation-resistance test methods were researched. The main factors(test conditions, metal-catalyzed, oxygen pressure) resulting in oxidation-resistance differences by various test methods were analyzed. Therefore, as single factor oxidation-resistance test of Lab, the selected test method should be suitable to the working conditions and simultaneously need many test methods to complement each other to overcome the errors caused by the single factor test.As conventional oxidation-resistance test methods, the trend of results obtained by DSC and PDSC is coherent. PDSC method is a strong suit for the ester oil system which has poor thermal stability because it can decrease effects of volatilization on the test results. RBOT method is professional and suitable for some special working conditions, such as the turbine oil and transformer oil oxidation performance testing. The critical transition temperature for friction coefficient tested by the low speed four-ball tester has good accordance to the initial inflection point temperature of the oxidation peak of DSC curve. The experimental results have confirmed that the heating rate and non-isothermal dynamic calculation can contribute to characterize the oxidation-resistance change of the ester oil with and without additives during the whole thermal-oxidation process.(2) The accelerated thermal oxidation experiment by the incubator were combined with the acid number and viscosity measure to systematically research the main performance change of TMPTO and DOA with the thermal oxidation time at 200?. The reactive force field molecular dynamic simulation was used for researching the thermal oxidation process of TMPTO and DOA and the thermal oxidation model on molecular scale was proposed.GC-MS, gel chromatography, infrared spectrum and Rama spectrum were combined to verify the molecular dynamic simulation results and the relationship of oxidation product components and performance changing law were established. The oxidation products mainly contained thermal oxidative decomposition products and thermal oxidative polymerization products. During the thermal oxidative decomposition process, the common reaction of TMPTO and DOA is the alcohol C? O bond of ester group breaking. The reaction of unsaturation C=C bond of TMPTO and tertiary carbon atoms of DOA are their characteristic parts. The thermal oxidative polymerization reaction occurs mainly because of the free racial collision and dehydration condensation of intramolecular and intermolecular. The generation of many oxygen-containing groups deepens the appearance color. The generation of highly acid oxygen-containing group such as carboxyl leads to increase the acid number. The small molecule substance generated by the oxidative decomposition reaction leads to the viscosity index decrease. The changing of tribology performance attributes to the generation of hydroperoxide, oxygen polar group etc. during the oxidation process.(3) The DSC(PDSC) method and accelerated thermal-oxidation test by the incubator were used to systematically research the effect of antioxidant additives on the oxidation-resistance of TMPTO and DOA base oil under different temperature.At the medium-low temperature condition of 135?, the phenolic antioxidant of T502 and L115 can effectively improve the oxidation-resistance of TMPTO. T502 can restrain the increase of kinematic viscosity and total acid number of TMPTO well. L115 restrained the kinematic viscosity increase better than T502, but it leaded to the total acid number increase rapidly due to itself degradation.At the medium-high and high temperature condition of 175? and 250?, the effect of T502 was not good. The amine antioxidant of T558 can restrain the increase of kinematic viscosity and total acid number of TMPTO and DOA obviously. The complex formulation of T502 and T558 can obviously restrain the increase of kinematic viscosity and total acid number of TMPTO and DOA under high temperature, which even had better effect than single T558 additive. The synergistic effect is good.(4) Synthetic ester oil was taken as the research object of engine base oil. Their compatibility for the two kinds of ester base oil and the anti-wear and friction reducing additives, detergent and dispersant etc. and their tribology performance change were discussed.The multifunctional additives containing the element of Mo remixed with phenolic antioxidant of T502 and amine type antioxidant additives of T558 can obviously improve the anti-oxidation property and tribology properties of TMPTO under high temperature. At room temperature to 250?, the friction coefficient keeps at about 0.20. Added 3% detergent of T106 and 3% dispersant of T151 A has no obvious effect on oxidation-resistance and high temperature tribology performance of TMPTO and has good compatibility with antioxidant and multifunctional friction-reduction and antiwear additive.