Metal Soap Greases - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 69

• The oxidation stability and lubricity of grease decrease with increasing fatty
acid chain length (within the range of 12-18) in soap molecules.
• The thermo-oxidative stability and lubricity of soybean oil based greases can be
improved by using suitable additives [18].
In another test [25], lithium hydroxide monohydrate and lauric, myristic, palmitic,
stearic, oleic, and linoleic fatty acids with antimony dithiocarbamide and sulfurized
ole¿ n antioxidant additive were used in soybean oil. Penetration test was used to ¿ nd
NLGI Hardness with cone penetration (Table 2) and thermal analysis was conducted
at pressurized differential scanning calorimetry (PDSC) to ¿ nd onset temperature that
is the temperature when a rapid increase in the rate of oxidation is observed in the test
sample. Using 1:1 and 1:0.75 equivalent ratio of lithium to fatty acid in the thickener
system (Figure 6), the weight ratio of soybean oil to lithium soap is varied from 65:35
to 80:20 to obtain grease with NLGI No.2 hardness and the higher is metal to fatty acid
ratio: the higher was the thermal stability. (Figure 6) With the 6 wt% of additive and
equivalent weight ratio of 1:1 maximum observed onset temperature of oxidation was
achieved as 165°C at 3450 kPa constant air pressure.

TABLE 2

The NLGI grease classification.

ASTM Penetration (10 -4 m)

Grade (ASTM D 217)

000 445-475

00 400-430

0 355-385

1 310-340

2 265-295

3 220-250

4 175-205

5 130-160

6 85-115

FIGURE 6 Effect of lithium hydroxide to fatty acid ratio on the oxidative stability of soy
grease 1:3 wt% ratio of soap to base oil, NLGI no.2 and antioxidant level of 2 wt% [25].