Lube-Tech
number of methods can be employed to determine the extent of degradation: spectroscopy, titrimetry, chromatography, etc. In this report, most focus is devoted to film vaporisation trends and its solubility by soaking in fresh oils, Figure 3.
Figure 3. Simplified scheme of the thin film degradation experiment by testing vaporisation and solubility of aged film in fresh oil. Photos of coupons with actual films at relatively low degrees of degradation are shown below.
It must be admitted that such procedure still relies heavily on the operator skills, soaking in fresh oil in particular. However, even the vaporisation results pay back for the invested experimental effort. Counter- intuitive disagreement with widely accepted flash points was readily evident just by testing conventional basestocks, Figure 4.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.116 page 3
and volatile losses were measured gravimetrically [4], major differences in long-term decomposition trends were observed. During the early stages (1-2 hrs) of heating the emissions from oil films seemed to follow their flash points to some extent, but this correlation disappeared quite soon. Decomposition reactions appeared to dictate long-term volatile losses and these emission tendencies could not be forecast from either molar mass or viscosity. In-depth knowledge of sample composition (amounts of low molar mass fractions, unsaturation, aromaticity, ether linkages, etc.) could provide more avenues for predicting decomposition trends, but such exercise would not be simple. For example, it would be hard to expect that over long term mineral oils release less volatiles than synthetic or bio-based basestocks. In this particular study a possible effect of natural antioxidants in mineral oil was not addressed, but even for other basestocks the long term vaporisation trends are far from intuitive. Another notable finding shows substantial portions of high-flash basestocks being eventually vaporised from films. Tests were performed at 130°C, which is not frequent in reservoirs or other surfaces of lubricated systems. Nevertheless, if degradation duration and temperature are combined into one parameter, such as Time and Temperature Superposition (TTS) [5], decomposition trends might be extended to lower temperatures. It would become apparent that in many cases a larger volume of oil films vaporises than remains liquid over long term in additive-free basestocks.
Vaporisation from thin films of fully formulated lubricants
Figure 4. Differences in oxidative decomposition intensity among several types of basestocks during thin film tests at initial 50 µm thickness
All tested samples had flash points (ASTM D92) reported within 150°C to 250°C. Such similarity might imply that their films should evaporate at similar rates. However, after their thin films were degraded
32 LUBE MAGAZINE NO.145 JUNE 2018
It is clear that lubricants lose lower molar mass components by evaporation during early stages of film degradation. However, in long-term the most emissions are generated by decomposition reactions. Testing their trends is even more important in fully formulated lubricants, than their basestocks, because various additives make it nearly impossible to predict the dominant course and cause of decomposition.
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