Originally Posted By: JAG
Tom, I figured that if Group III’s signature can be identified, then diesters can be identified too, because I think that diesters typically have superior thermal stability. See below. It mentions PAO, not Group III, but I would expect similarity between the two. I could be off base since I know so little about the tests that emod posted results from.
https://link.springer.com/article/10.1134/S0965544118030179
That paper is not useful on many levels. First it was not a thermal stability test, but rather an oxidative stability test, and the use of the term pyrolysis is incorrect. They tested DOA as the diester, which is not used in any high temperature lubricant applications due to its low viscosity, low Viscosity Index, and high volatility. The link only loaded the first two pages, but it appears they simply heated the samples at various temperatures for two hours in a steel beaker and then analyzed the samples. Furthermore, the base oils were not additized. Simply stirring neat base oils in a beaker on a hot plate for a couple of hours is unrealistic and I would never attempt to correlate those results to any real world applications.
A proper well controlled oxidation test used to qualify jet engine oils holds the oil at various high temperatures for 72 hours while blowing air through the oil, and with five different metal specimens submerged in the oil to catalyze reactions. Analysis is for change in viscosity, total acid number, sludge formation, and metal corrosion. This test shows PAOs and adipate type diesters (most common) to be close in oxidative stability when properly additized with the diester have somewhat better results.
Another common test combines oxidative, thermal, and hydrolytic conditions by heating the additized oils to >280°C in a thin film flowing over a steel panel for 24 hours in a chamber sparged with water saturated air to measure coking tendencies. Here PAOs fail miserably due to their lack of polarity and inability to solubilize and disperse degradation by-products, thus laying down polymeric and carbonaceous deposits. Diesters and POEs are substantially clean in this test. In fact, even Group I based oils show cleaner panels than PAO because they have some polarity.
The term "thermal stability" is often misapplied. A true thermal stability test is run in the absence of oxygen to eliminate oxidative reactions and focus only on molecular decomposition caused by heat, usually with a metal specimen in the oil for catalysis. Saturated hydrocarbons such as mineral oils are very thermally stable, in spite of their relatively poor oxidative stability. PAOs and POEs are also very thermally stable, but diesters suffer from the presence of a hydrogen on the beta carbon which triggers a series of thermal decomposition reactions at high temperatures (over 200°C). POEs do not have a beta hydrogen which is why they can be used at much higher temperatures than diesters.
