Metal Soap Greases - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 56

zinc, and zirconium. Significant application areas for metal soaps include lubricants
and heat stabilizers in plastics as well as driers in paint, varnishes, and printing inks.
Other uses are as processing aids in rubber, fuel and lubricant additives, catalysts, gel
thickeners, emulsifiers, water repellents, and fungicides [2].
Metallic soap properties are determined by the nature of the organic acid, the
type of metal and its concentration, presence of solvent, additives, and the method of
production. Fatty acids which can be used to produce metallic soaps include higher
monocarboxylic acids having from about 12 to about 22 carbon atoms. Saturated or
unsaturated, substituted, or unsubstituted fatty acid is useful [3]. Metallic soaps are
prepared commercially by three general processes: Precipitation from aqueous solu-
tions of metal salts and alkali soaps, fusion of metal oxides, hydroxides or salts with
organic acids or esters and direct solutions of ¿ nely divided metals in heated organic
acids [4].
A lubricant is a substance (usually a liquid) introduced between two moving sur-
faces to reduce the friction and wear between them. A lubricant provides a protective
¿ lm which allows for two touching surfaces to be separated, thus lessening the friction
between them. Lubricants are an essential part of modern machinery. The two moving
surfaces could be metal-metal or polymer-metal. Metal soaps are used to regulate the
viscosity of lubricating oils and molten polymers.
From the view point of colloid chemistry, the greases can be considered as two
component system consisting of a dispersion medium and the dispersed phase. The
dispersed phase (5-25%) can consist of salts of high molecular weight carboxylic
acids known as soaps.
The dispersed phase forms in the course of grease production forms a three-dimen-
sional (3D) skeleton penetrating the dispersion medium throughout its volume. Skel-
eton elements (dispersed phase particles) have colloidal size in two (often more than
one) measuring directions. 60-80% of the dispersion medium is held in the 3D grease
skeleton by adsorption bonds, and the remaining part, mechanically. Soap thickeners
are polymorphic thickeners, whose interaction with dispersion medium strengthens as
they change over into high temperature mesomorphic phases that are thickeners that
do not contact with oils at ordinary temperatures and colloidally disperse at elevated
temperatures. The dispersion is additionally facilitated and accelerated by a vigorous
stirring or other physical means [5].
Solid ¿ lms on surfaces can reduce the coef¿ cient of friction and wear. Thick ¿ lm
formation was noted with saturated carboxylic acids such as stearic acid but not with
oleic acid. Copper oleate can generate boundary ¿ lms tens to hundreds of nanometers
thick in slow speed rolling contact. Free carboxylic acids are rarely used as organic
friction modi¿ ers: however, their salts such as metal oleates are all employed as ad-
ditives in lubricating oils. It is dif¿ cult to relate their structure to ¿ lm formation and
resultant ¿ lm friction performance since they are relatively impure therefore, it should
be synthesized to measure their boundary ¿ lm forming behavior. Oleates of metals
above iron in the electrochemical series form thin boundary layers whereas thick
boundary layer occurs only for metals lower than iron in the electrochemical series
and is due to a redox reaction involving from the steel surface and the metal oleate [6].