Chemistry Reference
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6.6.1 Thermo-oxidative Stability and National Lubricating Grease Institute
(NLGI) Hardness
The improvement of thermo-oxidation and tribological properties were evaluated in
the works of Sharma et al. [24]. The greases they prepared were constituted of 75-95%
lubricating oil, 5-20% thickener, and the rest (1-10%) additives as outlined in Figure
3. As thickener, lithium soaps of different acids having C
12
(lauric), C
14
(myristic), C
16
(palmitic), C
18
(stearic) carbon chains, and as additive antimony dithiocarbamide as
antioxidant were used. The ratio of lithium soap of different fatty acids and soybean
oil was 1:3 in all greases except GR11 with 1:2 and GR28 with 1:4. There were three
greases GR5, GR12, and GR28 in which soaps were prepared with lithium hydrox-
ide monohydrate and fatty acids with different degrees of unsaturation, 0 (stearic), 1
(oleic), and 2 (linoleic), respectively. The concentration of antimony dithiocarbamide
was between 2 and 4% to study the additive effect.
Weight loss in vegetable oil greases at 150
o
C for 24 hr was approximately 10%
which is lower compared to weight loss in mineral oil based greases (approximately
50%) under similar conditions. Figure 4(a)
showed that weight loss decreased with
increasing oil ratio or decreasing soap content. The rate of weight loss was constant
from 100 to 150°C and increased rapidly as the temperature was increased from 150 to
225°C (Figure 4(b)). This indicated the breakdown of molecules in soybean oil-based
greases as a result of oxidative degradation which is more rapid after 150
o
C. Increas-
ing the unsaturation to 2 yields a reduction in the weight loss (Figure 4(c)).The pres-
ence of more double bonds in the fatty acid structure of the soap are attractive sites for
reaction with primary oxidation products, which results in more polymerization (lead-
ing to grease hardening) and less volatile oxidation product formation. Figure 4(d)
showed that the oxidation onset temperature for addivate samples GR13 and GR25 is
much higher (150°C) compared to GR5 (<100°C) sample without any additive.
FIGURE 4
(Continued)
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