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chain, in the beta position. Neopolyol esters do not have any ß-hydrogen in their molecules, so they decompose at a much higher temperature (320 to 340°C), while diesters decompose at about 280°C. Due to their poorer thermal stability, diesters have been dropped and replaced by neopolyol esters. Neopolyol esters are the only base fluids that deliver the thermal and oxidative stability, lubricity, cleanliness and low temperature behavior required for jet engine lubrication.
Similarly, the manufacturing process is key to neopolyol ester performance. There are different ways of producing, purifying, neutralising and filtering esters. The process, when suitably engineered, should lead to a fully esterified compound, and will remove impurities possibly brought by raw materials or generated by the reaction that is carried out at high temperature (oxidation and thermal degradation by-products).
ß-elimination mechanism
Aviation lubrication requirements have indeed been met by designing optimised chemical structures, strictly selecting raw materials, following tight production specifications, working out the best process, and using the right additive systems – all in the most cost-effective way.
Neopolyol ester
Four neopolyols are commonly used to produce such esters: neopentyl glycol (NPG), trimethylol propane (TMP), monopentaerythritol (MPE), and dipentaerythritol (DPE). They are all synthetic polyols made from methanol, ethylene and propylene. The possible use of several polyols and acids blends during esterification allows chemists to design in a very flexible way the structure showing the desired properties in terms of rheology, volatility, lubricity, thermo-oxidative stability, cleanliness... The most commonly used acids have 5 to 10 carbon atoms.
Optimising and controlling the performance of neopolyol esters
Neopolyol esters components may be selected from a variety of neopolyols and acids (short chains, long chains, linear, branched) that will greatly impact rheology, volatility, thermo-oxidative stability, cleanliness, lubricity and elastomer compatibility – sometimes in a conflicting manner. Optimising performance and reaching the best compromise requires careful engineering work to design the chemical structure that will meet technical requirements.
However, not all esters are alike even though they may display chemically identical structures. In particular, carefully selecting raw materials is essential to warrant performance as they may contain traces of impurities, either organic or inorganic, that may affect thermo- oxidative stability and deposit formation, as well as hydrolytic stability, amongst other properties.
16 LUBE MAGAZINE NO.143 FEBRUARY 2018
Aromatic amines are the best type of anti-oxidants for the usual operating temperatures reached by aircraft turbine oils (alkylated diphenylamines, phenyl- alpha-naphthylamines). Oligomers of aromatic amines have also gained popularity for the latest generation of aviation turbine oils: their main benefit lies in the increased stability of the turbine oil at high temperatures (above 200°C) by reducing the amount of carbonaceous deposit and varnish on the bearings and gears of the hot section of the engines. This new generation of anti-oxidant systems appeared in the ‘80s with the MIL-PRF-7808 Grade 4 and the MIL-PRF- 23699 Grade HTS performance standard. Anti-wear additives are generally necessary to fortify the load carrying capability of turbine oils. They are generally made of a phosphorus containing component (almost exclusively tricresylphosphate), that will also passivate iron to keep it from catalyzing oxidation and elimination reactions. Copper corrosion inhibitors (benzotriazole and its derivatives) are widely used to protect yellow metal parts, as well as to passivate copper against catalytic promotion of oxidation and elimination reactions.
Verifying performance is another essential aspect of the production of turbine oils. Such products must be evaluated and compliance verified with a sequence of tests including:
• Evaporation test at 204°C • Oxidation and corrosion tests at 175°C, 204°C, 218°C and 274°C
• Dynamic coking test at 375°C • Gear load carrying ability on gear test rigs • Bearing deposits on bearing test rig (260°C)
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