Metal Soap Greases - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 68

Figure 5 shows the FTIR spectra of grease GR5 before the test and greases GR13
and GR25 after oxidizing in the oven at 100°C for 123 hr. The absorption at 1579 cm -1
caused by the thickener was not changed in addivated and unaddivated greases. This
shows that lithium stearate did not deteriorate during the test and is stable at 100°C.
The absorption band at 3473 cm -1 which is due to a hydrogen-bonded OH group, was
stronger in the unaddivated grease after the oven test (GR5-123 hr) compared to the
addivated greases after the oven test (GR13-123 hr and GR25-123 hr) and least in the
unaddivated grease before the oven test (GR5-0 hr). The oxidation of base oil present
in the grease occurs during the aging and results in the formation of organic acids as
byproducts. The presence of these organic acids in the oxidized grease samples (GR5-
123 hr) is visible from the broad hump in the region 3100-2500 cm -1 and also from
the peak shoulders at 1717 and 1700 cm -1 which is due to the carbonyl group of acid
byproducts. The presence of the additive in GR13 and GR25 delayed the oxidation
process as shown by lesser OH absorption and acid byproducts in these greases. It
maybe concluded that during the oven test thickeners in the grease remain unchanged
whereas the vegetable oil is oxidized and polymerized.
There was no major difference in coef¿ cient of friction as a result of change in
the fatty acid unsaturation in the Li soap structure. However, results indicate a small
increase in the friction coef¿ cient with increasing unsaturation.
A larger decrease in pressure drop was observed when the antioxidant concen-
tration was increased from 2 to 4% than when it was increased from 0 to 2%. The
important inferences can be drawn from the study on the thermo-oxidative stability of
vegetable oil based greases.
• A high soap to oil ratio (within the range of 1:2 to 1:4) results in increased oxida-
tion stability and lubricity of greases.
• A decrease in unsaturation in fatty acid chains of molecules results in increased
volatile loss as a result of the oxidative degradation of greases. Lubricity of
greases decreased with more unsaturation in fatty acid chains of soap molecules.
FIGURE 5 The FTIR spectra of grease GR5 before test (GR5í0 hr) and GR5, GR13, and
GR25 after 123 hr in oven test at 100°C [24].