I wanted to pass along a post by TomNJ regarding TAN and TBN that was really enlightening to me and may help others understand esters better. Background: In the past I had a redline UOA with both high TAN and high TBN and I have been really struggling to figure out how that could be. Normally I think of TBN as the buffering capacity of the oil and as TBN goes down TAN goes up. Typically we use the point when TAN = TBN as a condemnation point for oil. But a few facts had me questioning whether this "rule of thumb" applied to some of the more exotic ester-containing oils: 1) Terry Dyson's past statement about how a BStone TBN level of 0 was acceptable for redline 2) The redline oils start at about TAN of 2.5 rather than somewhere near 0 3) The UOA that showed both TAN and TBN to be high at the same time 4) Renewable lubricants Biosyn often shows decreasing TAN in a UOA vs. VOA Which is the opposite of what we would expect. I asked Tom if the same TAN=TBN rules should apply to ester oils and his post here really shed light on this issue for me! Thanks TOM!!!
Originally Posted By: Tom NJ
There probably should be a different set of rules for ester based motor oils. Pure synthetic ester base oils have a zero TAN and TBN, so the higher TAN in the formulated products must come from additives. Bio-based base oils may have a slight TAN (<1) to start with. Esters, whether synthetic or bio-based, are constructed from acids and alcohols, and are subject to hydrolysis (reaction with water). Unlike with hydrocarbon base oils, hydrolysis breaks the ester back to acids and alcohols, so a rise in TAN is more likely with esters than hydrocarbons. The acids liberated, however, are weak organic fatty acids and do not do damage unless present in high doses. This is why a high TBN can coexist with a high TAN in such oils. Indeed some of these fatty acids act as corrosion inhibitors! The acids liberated from the degradation of hydrocarbon base oils, however, are much stronger and corrosive. Hence I would expect a different set of rules for ester based oils to allow more TAN than with hydrocarbon based oils. There are different TAN rules for esters in other industries. In monitoring air compressor oils, for example, a TAN of only 0.2 is concerning for mineral oil formulations, while I have seen recommendations from some manufacturers allowing as high as 5 to 8 TAN before changing for ester oils. Without field data to the contrary, I would tend to ignore TANs in high ester based motor oils so long as other UOA data is normal. Tom