Metal Soap Greases - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 72

The ¿ lm thickness increases with speed and reaches a relatively high thickness
with a slope of 0.67, characteristic of a fully À ooded elastohydrodynamic contact.
Composition has effect on lubricating ability and determining parameter is soap-base
oil interaction. Li-mineral oil-additive and Al complex soap-ester base oil and addi-
tive have large capacity to form a lubricating ¿ lm. Rheology has effect on lubricating
ability with determining parameter is G'. Ester-base oil which has high solvency with
lithium soap and polymer molecules, gives a strongly interconnected microstructure
and thus a high value of G'. At the opposite, the mineral base oil that presents lower
solvency with lithium soap and polymers leads to an heterogeneous network and lower
G' (elastic modulus). All these factors have an inÀ uence on the grease's elasticity and
the elastic modulus seems to be a good indicator of grease tribological behavior [26].
Another rheological characterization, including continuous, transient, and dynam-
ic tests was carried on eleven different lubricants with Li greases, Li-complex greas-
es and Urea greases. The varying parameters were the soap nature (lithium, lithium
complex, diurea, and tetraurea), the soap concentration (7 and 14%), and the base oil
viscosity (30 and 200 mm 2 s -1 ). It should be noted that greases contained no additive.
Stress sweeps were imposed to determine the yield stress, ı y and the stationary
À ow properties (apparent viscosity ȝ ap ) at very short and very long time intervals.
The instantenous elastic modulus G i and the instantenous critical elastic strain Ȗ c were
deduced from the ¿ rst milliseconds after the stress was applied. Ȗ c was observed when
the creep stress was applied or removed. G m the mean elastic modulus was measured
during the sample recovery which typically takes 5-10 min. G m is de¿ ned as the ratio
of the creep test over the total recovered strain. With regard to yield stress determi-
nation, the stress sweep was chosen to measure ı y , It consisted of plotting the shear
stress/strain rate values in linear scale sand extrapolating linearly the measured stress-
es toward the stress axis: the intersection gave ı y yield stress. At similar concentrations
lithium soap showed a larger yield stress than lithium complex and tetraurea lubri-
cants.14% weight lubricants had higher ı y yield stresses than 7% weight lubricants
at low and medium strain rates. At higher strains the base oil viscosity effect appears.
Dynamic tests were obtained at 0.5 and 5 Hz frequencies. For G',dynamic elastic
modulus, the largest strain in the linear reached 1% and with respect to dynamic loss
modulus the maximum strain reached 5.8%. For given thickener nature and concen-
trations the samples made with 30 mm 2 s -1 base oil gave larger linear domains than
those made with 200 mm 2 s -1 base oil. From TEM observations greases made from 200
mm 2 s -1 base oil appear more fragile due to heterogeneities. It has also observed that
network for Lithium complex greases are more open and thus unable to sustain large
strains. Dynamic viscosities obtained at 0.5 Hz are typically six times higher than
those measured at 5 Hz: this con¿ rms the strong shear-thinning behavior already seen
during stress sweeps. The authors suggest using rheometry as an alternative technique
to standardized methods. Yield stress varies in the same way as penetration does, but
is further inÀ uenced by the sample microstructure [27].
6.8 ADDITIVES IN GREASES
Antioxidants and viscosity regulators are the main additives in greases. The improve-
ment of thermo-oxidation of soybean oil was obtained when antimony dithiocarbamide