Abstract
Di-tert-butyl-p-cresol(DBPC) is widely used as an antioxidant of lubricating oils, but its chemical change that is associated with degradation of lubricating oils is not clearly understood. So to elucidate this point, qualitative and quantitative analyses were made on the products derived from DBPC in turbine oils oxidized under various conditions which are indicated on Table 2. The products were analyzed by GLC and GLC/MS after extraction with methanol (methanol: oil=1:1).
Fig. 1 shows the chromatogram of the products derived from DBPC oxidized by method A. The main products were 4, 4'-di-hydroxy-3, 5, 3', 5'-tetra-t-butyldibenzyl and 3, 5-di-t-butyl-4-hydroxybenzaldehyde. Other compounds, 2, 6-di-t-butylmethylenequinone, 2, 6-di-t-butyl-4-hydroxyethylphenol, 3, 5-di-t-butyl-4-hydroxybenzylmethylether, 3 5-di-t-butyl-4-hydroxyacetophenone, 3, 5, 3', 5'-tetra-t-butylstilbenquinone in small amounts were identified by GLC/MS.
It was found that the composition of the products depended on oxidation conditions. Figs. 2-4 show the results of quantitative analyses of the products in turbine oils (FIX-150: Hydrofinished oil was used, TCX-150: Hydrotreated oil was used) oxidized by method B. While 3, 5-di-t-butyl-4-hydroxybenzaldehyde increased linearly with increase in reaction temperature, 4, 4'-dihydroxy-3, 5, 3'5'-tetra-t-butyldibenzyl increased only up to about 30wt% and then decreased. Fig. 5 shows the results oxidized by method B. 4, 4'-Di-hydroxy-3, 5, 3', 5'-tetra-t-butyldibenzyl decreased and 2, 6-di-t-butylmethylenequinone increased. 3, 5-Di-t-butyl-4-hydroxybenzaldehyde increased to about 45wt% and then decreased. It was considered that C oxidation conditions were severer than B conditions, Some mechanisms for the formation of these products were proposed.
3, 5-Di-t-butyl-4-hydroxybenzaldehyde and 3, 5, 3', 5'-tetra-t-butylstilbenquinone were synthesized1), 2) to investigate the influence of these compounds on the performance of used oils. Quinone type compounds have strong absorptions in the visible range (3, 5, 3', 5'-tetra-t-butylstilbenquinone; 420 and 446nm). These quinone type compounds may contribute to the degradation of oil color. Table 4 shows solubilities of these compounds in the base oils, whose properties are indicated in Table 1. The results indicate the possibility of these compounds separating from the oils as sludge. Table 5 shows the results of RBOT(ASTM D 2272), indicating that these compounds are not strong antioxidants.
