Metal Soap Greases - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 73

was used 2-4% as antioxidant additive. [24]. The Low-density polyethylene (LDPE)
is also an example of additives to grease. Recycled LDPE as additive may improve the
rheological properties of common lithium lubricating greases. Recycled LDPE act as a
filler in the soap matrix increasing particles interaction resulting with low mechanical
stability but high value of viscosity and viscoelastic modulus. [28]. Teflon is added to
some grease to improve their lubricating properties. Gear greases consist of rosin oil,
thickened with lime and mixed with mineral oil, with some percentage of water. Spe-
cial purpose greases contain glycerol and sorbitan esters. They are used, for example,
in low temperature conditions. Some grease is labeled “EP”, which indicates Extreme
pressure greases contain solid lubricants, usually graphite and/or molybdenum disul-
fide, to provide protection under heavy loadings. The solid lubricants bond to the
surface of the metal, and prevent metal-to-metal contact and the resulting friction and
wear when the lubricant film gets too thin. Copper is added to some grease for high
pressure applications, or where corrosion could prevent disassembly of components
later in their service life.
6.8.1 Tribological Investigation of CaF 2 Nanocrystals as Grease Additive
Since, heavy metals involve sulfur ion, they are potential threat to environment. Alkali
metals could be used since they have low shear strength and stable thermophysical
and thermochemical properties at elevated temperatures. Due to small scale effect,
nano-sized materials dramatically lowered melting points compared with their bulk
counterparts. Antifriction and wear properties of CaF 2 nano crystals prepared by ion
exchange method were evaluated on a four ball tester. The friction and wear tests were
conducted at a rotating speed of 196, 294, 392, 490, and 588 N for test duration of 30
min. The wear scar diameter on three lower balls were measured by using an optical
microscope and friction coefficients were recorded automatically with a strain gauge
equipped with four ball tester.
The TEM images showed that CaF 2 particles in hydroxystearate soap ¿ ber when
added in grease. From EDS and XPS analysis, disproportion of Ca and F in boundary
lubricating ¿ lm indicated that tribological reaction of CaF 2 nanocrystals occurred on
the worn steel surface at severe trbological conditions. Upon addition of 0.5%, friction
coef¿ cient and wear scar decreased. After addition of 1% friction coef¿ cient and wear
scar increased. Therefore, optimum concentration is about 1 wt% compared to lithium
grease alone and lithium grease containing 1% CaF 2 as a function of applied loads, 1%
CaF 2 is lower and stable from friction coef¿ cient and wear scar diameter tests [29].
6.9 CONCLUSION
A phase diagram of an aqueous soap shows that fiber structure property of soap can
be manipulated through processing. A typical composition of grease includes base oil,
metal soap, and additive. Soybean oil used as vegetable base oil is better than mineral
base oil for environmental concerns. Lithium soap is widely used among other types
of metal soaps and lithium alone and lithium soap complex mixed lithium soaps were
studied with respect to their rheology, chain length, oxidation stability, soap to oil ra-
tio, and saturation. Additive as antimony dithiocarbamide has good oxidative stability
and CaF 2 nanocrystal can lower melting points compared with their bulk counterparts.