INSIGHT
From jet engine oils to high temperature industrial lubricants:
High performance anti-oxidants for synthetic esters Siegfried Lucazeau, Marketing and Project Manager, NYCO, Paris
The formulation of latest generation jet engine lubricants has led to the development of specific anti-oxidant systems that, used in fully synthetic esters, deliver increased stability and longevity, and improved control over deposit formation and coking.
It is only relatively recently that such a technology was extended to industrial applications, the first of which certainly being high temperature chain oils – where temperatures of the same order of magnitude as in aviation turbines may now be encountered.
The specific thermo-oxidative stability of neopolyol esters
Neopolyol esters possess a unique set of performance features as base fluids for lubricants, including excellent rheology, low volatility, inherent lubricity, and innocuous environmental profile. But the one property that makes this class of synthetic fluids really remarkable is their specific behaviour towards oxidation and thermal degradation.
There are 3 essential factors that impact resistance to oxidation of esters: - The neopentyl structure of neopolyol esters, which suppresses any possibility of β-elimination reaction degrading the ester (figure 1). In addition, the specific steric hindrance introduced by such a structure protects neopolyol hydrogens from oxygen attack.
Figure 1
- The reactivity of hydrogen atoms towards oxygen, depending on whether they are attached to a primary, secondary or tertiary carbon (figure 2). Branching of acid hydrocarbon chains increases the amount of stable primary carbon hydrogens, whilst introducing steric hindrance protecting other less stable hydrogens.
Figure 2: bonding energy of C-H
- The additives present in the formulation, that play an important role. Metals may catalyse β-elimination and oxidation reactions, so the use of metal passivators, together with anti-oxidants delivering a strong response in neopolyol esters, is an essential feature of high temperature performance.
With regards to cleanliness, neopentyl structures also greatly favor clean degradation pathways into light and volatile fractions, thus preventing carbonaceous deposits and varnish from building up on metal surfaces.
Continued on page 28 LUBE MAGAZINE NO.154 DECEMBER 2019 27
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