Metal Soap Greases - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 70

FIGURE 7 Variation of grease NLGI hardness with weight% soybean oil in the reaction
mixture using 1:1 equivalent ratio of lithium to fatty acid in the thickener system [25].
The NLGI hardness decreases as the wt% of soybean oil increases as seen in
Figure 7. The metal soap in the thickener is capable of holding a certain amount
of base À uid within its network. This is a two-stage process. First, absorption and
adhesion of base oil in the structure results, and second, the soap structure swells
when the remaining oil is added to the reaction mixture. As long as the base oil
is con¿ ned within the soap ¿ ber network, NLGI No.2 grease with good oxidative
stability can be achieved. A higher metal to fatty acid ratio can result in greases
with better oxidative stability. Oxidative stability and other performance proper-
ties were deteriorated if oil is released from the grease matrix due to overloading
of soap with base oil [1].
It was observed that the length of fatty acid chain in lithium soap structure af-
fects grease hardness which subsequently inÀ uence important physical performance
properties such as viscosity, boundary lubrication, and rheological behavior. Metal
soaps prepared with short chain fatty acids resulted in softer grease. Grease con-
sistency increased with long chain fatty acids used for synthesis of lithium soap
thickener (Figure 8).