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.