Figure 5 The color evolution of PAO and DIOS oil samples with antioxidants (T501, T531 and Tz516) catalytically oxidized by the copper sheet at 175 oC
Due the different antioxidants, color darkening effect could be clearly seen from the oxidation of base oils with antioxidants (Figure 4). The Tz516 has the most obvious color darkening ability according to the color numbers shown inset the relevant oil samples. Without antioxidants, the base oils almost do not change their color numbers in same oxidation condition. The T531 also has similar effect on color change with T501, although in different base oils. Interestingly, the oxidized PAO oil sample which added 1% Tz516 and 1% T501 (P516/501) has lower color number (0-12) compared with the color number (11-14) of PAO oil sample with 1% Tz516 (P516). Especially under shorter oxidation time, the color number of P516/501 is 0 after oxidizing for half an hour while the color number of P516 is 11. Although the color number of P516/501 is sharply increased upto 12 when the oxidation reactions conducted for 8 hours, the color number is also lower than P516. The phenomena suggest that the addition of T501 could significantly reduce the color number of the oxidized oil samples without metal catalyst.
Under catalytical oxidation by Cu sheet, the color evolution of the base oil samples shown in Figure 5 is darker than the base oil samples oxidized without metal sheet (Figure 4). The phenomena also could be seen from the oxidation of fully formulated 928 and 50-1-4Φ aviation lubrication oil (Figure 2s shown the color change of 928 and 50-1-4Φ aviation lubrication oil after catalytic oxidized by Cu sheet from 200 oC to 290 oC). The PAO oil samples added Tz516 has largest color numbers (19-23) compared with other oil samples. Similarly, after T501 and Tz516 were added into PAO oil samples, the color darkening phenomena are inhibiting on a considering degree (color numbers down to 6-15), even the oxidation with the presence of Cu sheet.
Cu sheet catalyzed oxidation reactions of the base oil samples without any antioxidants also suggest that metal catalyst could significantly accelerate color darkening (Pure PAO,0-8; Pure DIOS, 0-7) compared with Figure 4 that the color number of Pure PAO and Pure DIOS without Cu sheet are 0-1. The PAO and DIOS without antioxidant would demonstrate a relative fast color darkening under Cu sheet catalytic oxidation reactions. The color numbers of DIOS with T531 also have sharply change oxidized for more than 4 hours which also suggest that Cu sheet catalytic oxidation has greater impact on the color darkening process of the oxidized oil samples.