| This paper was supported by National Program on Key Basic Research Project “Lubricanting additives’ anti-friction and anti-oxidation performance and its effect law on synthetic lubricants”. The physicochemical properties, thermal oxidation stabilities and tribological properties of trimethylolpropane trioleate(TMPTO), tri-isodecyl trimellitate(TDTM) and di-2-ethylhexyl adipate(DEHA) were analysed. The influence of temperature, oxygen and metal catlysis on the structure of TMPTO was researched using infrared spectroscopy and Raman spectroscopy, and the relationship between the molecular structure changes and the degradations of physicochemical properties and thermal oxidation stablities was researched. The oxidation of the chemical structure of TMPTO was blocked by using the funcional antioxidants on the basis of the revelation of thermal oxidation failure mechanism of TMPTO base oil in the molecular level, and the suggestion for extending the life of TMPTO base oil during high temperature oxidation was proposed. The main research results and conclusions are as follow.During the thermal oxidation, the oxidation stability of TDTM base oil which has the stable kinematic viscosity and total acid number is superior, the total acid number of DEHA base oil and the kinematic viscosity of TMPTO base oil increase fast. As the oxidation time prolongs, the kinematic viscosity of TMPTO base oil tends to increase exponentially, and its total acid number tends to increase linearly. The viscosity index of TMPTO and DEHA base oils tends to decrease. The tribological property of TMPTO base oil is superior to DEHA and TDTM base oils.The result shows that through in-situ Raman spectroscopy analysis of the thermal oxidation of TMPTO base oil first time, as the temperature increases, the Raman intensities of the =C-H, C=C and-CH2- weaken gradually, and the Raman shift of the C=C stretching vibration tends to decrease linely. When the temperature cools to room temperature, the Raman intensities of the =C-H and C=C structures can recover partially, and the Raman intensity of the-CH2- recovers completely, the frequency shift of the C=C stretching vibration disappears entirely.As the oxidation time prolongs, the =C-H of TMPTO consumes completely, and the C=C structure consumes partially during the thermal oxidation. The chemical reaction of the C=C structure follows first-order kinetics, and the reaction rate constant is affected significantly by oxygen permeation in per unit volume of lubricant.The metal-catalyzed oxidation experiment shows that the catalysis of iron aggravates the physicochemical properties and thermal oxidation stability of TMPTO base oil significantly. The copper is second and the catalysis of aluminum is the smallest. Compared to glass containers, the catalysis of iron increases the first order reaction rate constant of the C=C structure obviously.Under the oxidation conditions of 95℃ and 135℃, appropriate amount of antioxidants L115 and T558 can suppress the growth of the kinematic viscosity and total acid number of TMPTO base oil. Under the oxidation conditions of 175℃ and 250℃, amine antioxidant T558 and the composite antioxidants of amines T557, T558 and phenol derivative T502 can inhibit the increasing of the kinematic viscosity and total acid number of TMPTO base oil availably.The research shows that the test principle difference between pressure differential scanning calorimetry(PDSC) and rotary bomb oxidation test(RBOT) has a significant impact on the characterization of the antioxidants’ improvement effect of oxidation stability of TMPTO base oil. The oxidation induction time(OIT) of PDSC and RBOT is opposite at the same temperature. In the PDSC test, the improvement effect of anmine antioxidant is superior to phenolic antioxidant, and the RBOT test has the opposite result. |
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