Metal Soap Greases - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 59

Greases have gel structure as seen in Figure 1. If soap based grease is heated, its
penetration increases only very slowly until a certain critical temperature is reached.
At this point the gel structure breaks down, and the whole grease becomes liquid. This
critical temperature is called the DROP POINT.
6.4 GREASE TYPES
6.4.1 Powder Form Greases
Dry lubricants contain no liquid and are used where dripping or spilling cannot happen
or oil and grease are not recommended. They have a wide temperature range and may
act as both a lubricant and a sealant. A dry powder lubricant composition was formu-
lated comprising 48% by weight sodium acetate, 13% by weight lead acetate, 21% by
weight sodium oleate, 12% by weight powdered graphite, and 6% by weight sodium
chloride. This dry powder lubricant was sprayed on the hot cavity dies of a forging
press used to produce motor vehicle wheels. The temperature of the dies was about
370°C. The results, from a lubrication standpoint, were found to be equal to those
obtained using conventional oil base lubricants. However, the emission of smoke was
considerably reduced. Furthermore, no flame up was noted. Similar results can be
obtained using other 1-10 carbon fatty acid salts of the group IV-A metal as well as us-
ing other alkali metal soaps in place of the respective lead acetate salt and the sodium
oleate soap [17].
6.4.2 Single Soap Greases
Sodium-base Greases
Sodium-base greases are also general purpose greases. Because they have a higher
dropping point (approximately 148 to 176ºC), they are often used to lubricate machine
parts operating near heat. Sodium greases made with lighter oils are used for ball and
roller bearing lubrications, as are combinations (mixed base) of calcium and sodium
grease. Sodium soap greases have a spongy or fibrous texture and are yellow or green
in color. Because of their working stability and intermediate melting point, they are
used for lubricating wheel bearings (other than disc brakes) and for general purpose
industrial applications. Typical examples include rough, heavy bearings operating at
low speeds, as well as skids, track curves, heavy duty conveyors, universal joints,
and antifriction bearings. Sodium soaps with higher melting points (100 o C) were the
first generation of high temperature greases but their solubility limited their use in
many applications. Due to the large fiber size of traditional sodium soaps, they do
not contribute lubricity to the grease. Also, they suffer from hardening in the storage.
The sodium soap base is usually processed at high temperatures to drive off the water,
after which the mineral oil is worked in the base. Sodium soap greases are thus usually
anhydrous and water is not needed as a stabilizer. Sodium base greases are preferred
where operating temperatures are likely to be high and where wet operating conditions
are not involved [18].
The type of fat used to make the sodium soap base is an important factor in the ¿ -
ber structure or ¿ ber length of the ¿ nished grease. Low titre, unsaturated fatty oils and
acids produce ¿ nished soda greases of the long ¿ ber structure tending to be tough and
elastic. High titre, saturated fats, and acids tend to produce greases of the short ¿ ber