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Lube-Tech


method, it is possible to estimate the oxidation stability of oil and grease. According to ASTM standard method (ASTM D7575), the oxidation stability is determined via oxygen consumption. In number of publications, e.g. [3], the method has been described. The procedure of the test is as follows: a breakpoint, at which there is a pressure drop of 10 percent below maximum pressure, is recorded as the induction time (IT) at a constant temperature of 140 ˚C. Maximum pressure is the sum of the applied oxygen pressure (700 kPa) and the vapor pressure of the sample.


Table 3: The measured induction time (IT) for the greases.


It has been reported that fully formulated mineral oil based lithium and lithium complex greases may have induction times ranging from 176 to 481 min. This means that the results for neat greases that are shown in Table 3 are good with respect to the fact that no antioxidant was used. Grease F shows a significantly longer induction time which can be explained by the fact that the oil was not exposed to high temperatures when bentonite grease was produced. In a previous study [3], it was shown that, e.g., the induction time for the fresh base oil is longer that the induction time for the same base oil that was used in the production of lithium grease.


Rheological behavior of the greases was studied by using a rotational rheometer in oscillating mode. The complex modulus of a lubricating grease is determined in a dynamic strain sweep measurement at constant temperature. The complex modulus (IG*I) is calculated from the viscous modulus (G’, the storage modulus) and the elastic modulus (G’, the loss modulus) and characterizes the nature of the lubricating grease, which is a viscoelastic material. In an oscillatory test, such as strain sweep at constant temperature and low deformation, G’ and G’’ are constant, which means that the grease structure is unchanged. This regime is also called the Linear Viscoelastic Region (LVR).


Figure 1: Elastic modulus, viscous modulus and complex modulus as functions of strain.


a) In Figure 2, the calculated complex modulus for the two lithium complex greases at 0.1% strain (within the LVR) may be interpreted as follow: at 25 and 40 °C, Grease C, (based on BO1) has higher complex modulus than Grease D (based on BO2). One possible explanation could be the higher thickener content of Grease C (about 13.5 wt.% higher). While at 100 and 150 °C, the larger complex modulus of Grease D may be justified by the higher viscosity of BO2 at these temperatures. One source of the deviation here is that we are assuming that the fibrous soap structures have been formed uniformly. Nevertheless, more investigation is needed in order to understand these behaviours.


PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE


No.125 page 4


The longer the LVR is a function of shear stress, the more shear stable is the grease. In order to carry out a temperature sweep test, e.g., at constant shear stress, it is essential that the chosen shear stress is in the LVR. Figure 1 illustrates the viscosity modulus, elasticity modulus and the complex modulus as functions of strain for Grease D.


Figure 2: Complex modulus at 0.1% Strain for the lithium complex greases at various temperatures.


LUBE MAGAZINE NO.154 DECEMBER 2019


35


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