Several new biodegradable model lubricating greases based on
NCO-chemically modified biopolymeric thickener agents were
tribologically characterized. Results were compared with those
obtained when using conventional lithium and calcium soapbased
greases with alike rheological properties as lubricants. Viscous
flow curves of all samples were very similar and can be
satisfactorily described by the traditional power-law model.
Greases containing NCO-functionalized chitin and methylcellulose
exhibit higher consistency indexes at 25 °C, whereas this
parameter is higher for lithium soap and chitin-based greases at
125 °C, which is indicative of microstructures with higher thermal
resistance. In all cases, fracture phenomenon clearly arises at
moderate shear rates, which limits the study to not very high
shear conditions.
The general progression from boundary to elastohydrodynamic
lubrication regimes were found in the sliding velocity sweep tests
when using the biopolymer-based greases as lubricants in the
steel–steel ball-on plates tribo-contact, at both 25 and 125 °C.
However, the elastohydrodynamic regime was not detected at
125 °C when using conventional greases. Only calcium soap and
methylcellulose-based samples display a simple tribological
behaviour which leads to a unique Stribeck master curve by
superimposing the frictional curves obtained at different temperatures
and normal loads, using the base oil viscosity in the
hydrodynamic Stribeck parameter. This fact suggests that the base
oil governs film formation in these two greases, whereas the
thickener seems to play a key role in the remaining samples. The
stationary friction coefficient always decreases with the normal
load applied at 25 °C. However, the opposite tendency was found
at 125 °C for calcium soap-, methylcellulose or, in lower extent,
cellulosic pulp-based greases as lubricants. These results depend
on the balance between the rheological resistance offered by the
grease and the shear and temperature dependence of grease
microstructure. In this sense, particularly calcium soap and
methylcellulose microstructures, are more strongly affected by
temperature giving rise to a significant reduction of the lubricant
film thickness which favours wear at 125 °C. This fact is especially
evident at high normal loads. However, excepting for such severe
conditions, wear is negligible when using chemically modified
chitin and methylcellulose-based greases as lubricants, whereas
cellulosic pulp-based grease provides wear scars comparable to
those found with the conventional lithium and calcium greases.