Evaluation of Alkali Metal Salts as Synergists for Arylamine Antioxidants in Synthetic Lubricants 2000-01-2922
Differential scanning calorimetry (DSC) and oxidation-corrosion tests were employed to evaluate the performances of ester lubricants containing alkali salts, e.g. lithium perfluorobutyrate, sodium stearate, sodium acetylacetonate, sodium perfluorobutyrate, sodium trifluoroacetylacetonate, potassium perfluorobutyrate, and potassium trifluoroacetylacetonate. In the isothermal and temperature programmed DSC oxidation test, alkali salts are not only good effective antioxidation synergists for arylamine compounds such as p,p′-dioctyldiphenylamine (DODPA), but also effective deposit inhibitors. When combined with arylamine antioxidants, the salts can also effectively reduce the increases in viscosity, acid number and deposit formation of the ester oils in oxidation-corrosion tests. The changes occurring to DODPA in the oxidation of esters, with and without C3F7COONa, were followed by mass (GC/MS) chromatography (MC). The proposed mechanism of the inhibition involves a synergy between alkali metal salts and arylamines. Although DODPA does not react directly with the alkali salt, the salt still functions within the antioxidation mechanism of DODPA. When DODPA reacts with free radicals to form alkyl and hydroxy substituted benzoquinonemonoimine compounds (with mass to charge ratio M/Z, 422), the alkali metal ion can participate and produce a new intermediate product having high antioxidant activity; this intermediate product serves as the main antioxidant, long after the depletion of DODPA. This process may involve a reaction between alkylphenylamino alkylphenol and the alkali metal ion. The intermediate product is believed to be the salt of alkylphenylamino alkylphenol.