23rd International Colloquium Tribology - January 2022 113 The unexpected active behaviour of synthetic esters as cobase stocks on resistance to oxidation Siegfried Lucazeau NYCO, Paris, France 1. Introduction Synthetic esters are group V, performance base fluids recognised as being highly resistant to thermo-oxidation. In particular, neopolyol esters demonstrate outstanding performance in high temperature environments. As a result, neopolyol esters are used in high temperature applications such as jet engine oils, turbine oils, high pressure compressor oils, or high temperature chain oils, where they provide extended lifetimes, excellent stability in operation and added cleanliness for the equipment. Beside full ester formulations, esters may also be used as components or cobase fluids. Whilst they have long been used as components of PAO based formulations to ensure the solubility of additives and as seal swell agents, they may also prove to be useful as boosters of thermo-oxidative performance. 2. Esters as boosters The neopentyl structure found in neopolyol esters combines inherent resistance to thermal degradation, as well as steric hindrance protecting, to various extents depending on structure, hydrogen atoms from oxygen attack. Such a structure not only delivers resistance to thermo-oxidation but also favours degradation pathways resulting in reduced sludge and deposit formation. When introduced in ISO VG 32 and ISO VG 100, gr II and PAO based compressor oils formulations, neopolyol esters did improve resistance to oxidation and cleanliness, as illustrated by ASTM D4636 oxidation test, GFC- Lu-27-A-13 Micro-Coking Test and thermogravimetric analyses. Whilst it is somewhat expected that introducing an oxidatively stable base fluid in a formulation proportionally reduces the effects of oxidation, things may not be that intuitive. • Performance in oxidation test does not follow the treat rate of ester, lower treat rates seem to generate better results in some cases • Thermogravimetric curves do not always show different kinetics with various ester treat rates The above findings rule out any proportionality mechanism or dilution effect of esters. In addition, the detergency effect brought by esters may explain cleaner metal plates in the MCT test but does not shed light on the oxidation or thermogravimetric test results. It looks as if esters could stabilize the whole formulation against oxidation, just like an antioxidant would. The question therefore lies in the antioxidant/ ester system in the formulation: is there an interaction between antioxidants and esters that would extend the action of the antioxidant? Figure 1: Micro-Coking Test and ASTM D4636 test results 3. Interactions between esters and antioxidants Further thermogravimetric testing shows that in the absence of antioxidant, the ester does not improve much the ability of the formulation to resist oxidation. Furthermore, these curves do show an Oxidation Induction Time that seems to be dependent upon the treat rate - a behaviour that is observed on formulations using various treat rates of antioxidant (the higher the treat rate, the longer the OIT). 114 23rd International Colloquium Tribology - January 2022 The unexpected active behaviour of synthetic esters as cobase stocks on resistance to oxidation This goes to show that the ester/ antioxidant system behaves like a longer lasting antioxidant, just as though the antioxidant was protected by the ester. Figure 2: Thermogravimetric analysis - 175°C, oxygen ISO VG 32 formulations 4. Going further Since metals from additives (Zinc, Molybdenum, Calcium, Magnesium…) may act as anti or pro-oxidants, it would be interesting to extend that study to determine if some metals are able to further synergize with the antioxidant/ ester system or if the presence of some metals are detrimental to this mechanism. 5. Conclusion Esters may be used in high performance lubricants, as full base fluids. They may also be introduced as cobase fluids to improve stability at high temperature. They are able to act as antioxidation and cleanliness boosters for mineral oils or PAO based formulations, using cost effective treat rates as low as 5%. One hypothesis is that esters combine with antioxidants to make them deplete more slowly and be more stable. But this is only one of the benefits that may be expected from introducing synthetic neopolyol esters in mineral oil or PAO based formulations.