Abstract
As lubricants with a high thermo-oxidative stability such as synthetic esters are gaining more importance in the lubricant market, a detailed knowledge regarding their oxidative degradation behaviour is of high importance. In order to reveal their degradation products and processes, a novel approach combining artificial alteration, isotope labelling based on oxidation with 16O2 and 18O2, and mass spectrometry (MS), was applied to a bis(2-ethylhexyl) adipate base oil. The degradation products such as 2-ethylhexanol and its monoesters with short-chain fatty acids pinpointed the C–O ester bond as the site prone to oxidative attack, allowing the collection of information about the oxidation mechanisms. Furthermore, the influence of the antioxidant (AO) 4,4′-methylene-bis(2,6-di-tert-butylphenol) as an additive on the oxidation behaviour and resulting products was studied: blends containing AO showed a remarkably higher resistance against oxidation. However, similar degradation products were obtained after AO depletion and without AO. AO cleavage occurred at the carbon atom that bridges the phenols to give 2,6-di-tert-butyl-p-benzoquinone and 3,5-di-tert-butyl-4-hydroxybenzoic acid. By applying the isotope labelling approach, sites of preferential oxidative cleavage and hence differentiation of the origin of oxygen atoms—either from the atmosphere or from base oil components—can be unambiguously related in oxygen-containing base oils, as well as in blends with additives.
Highlights
As a consequence of the trend towards machinery with an increasing performance, while scaling down in size, the appropriate selection of the lubricant quality, composition, and, its performance profile, to fulfil the requirements of lubrication is becoming a more critical issue in the major fields of application
In order to identify the degradation products, samples obtained from artificial alteration and in fresh condition were analysed with a capillary GC–EI–mass spectrometry (MS) consisting of a TriPlus autosampler and a
The pressure curves of base oil without AO show an abrupt rise in the first few minutes, In the caseadjustment of the oxidation of ester-based base oils with immediately, AO, the pressure rise in the first of due to the to the elevated temperature
Summary
As a consequence of the trend towards machinery with an increasing performance, while scaling down in size, the appropriate selection of the lubricant quality, composition, and, its performance profile, to fulfil the requirements of lubrication is becoming a more critical issue in the major fields of application. Diverse fields of application ester‐based lubricants can be explained their an as well as a The highhighly flash point, low volatility, and goodofthermo-oxidative stability They by display unique set of properties: they offerand advantageous properties high temperature high stability advantageous solubility for additives an adequate inherentforlubricity. Thereby, a knowledge of is that of renewable biolubricants being employed as esters based on synthetic and vegetable oils, structure-stability relationships lays the foundations for the adjustment of Thereby, thermo-oxidative stability opening a wide field for eco‐friendly and biodegradable lubricants [5]. While thestability alcohol chainthe a complete the β-carbons is these acid considerations on the optimum base oil chemistries, an improvement of the particularlyBeyond favourable, chain stability follows the order. To the best of our knowledge, this is the first publication that reports on the artificial alterations of base oil containing a phenolic antioxidant using 18 O2 labelling in combination with capillary GC–EI–MS
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