Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE 2.2.2 Resistance to oxidation
The first step of the commonly accepted oxidation mechanisms is the abduction by oxygen of a hydrogen atom, thus producing a free radical. –CH2
- groups from alcohol chains are
expected to be highly reactive, however they are protected by steric hindrance from acid chains on neopolyol esters. As a result, –CH2
oxygen attack [4].
On the acid chain, hydrogen atoms exhibit different reactivities towards oxygen, depending on their positions. Hydrogen atoms bonded to tertiary and secondary carbons are the most likely to be oxidized, i.e. –CH- and CH2
related to the thermodynamic stability of the resulting free radical. Therefore, if the number of –CH3
with respect to the number of –CH2
- groups. This is directly - hydrogens increases
- and –CH- hydrogens,
the kinetics of the oxidation reactions will slow and oxidation stability will increase [3]. The direct consequence of this is: • the shorter the acid chains, the more stable the structure against oxidation (less –CH2
- );
• the more branched the acid chains, the more stable the structure against oxidation (more –CH3
from steric hindrance) (Figure 7). -, added protection
Figure 8. Micro-Coking Test, 230-280°C GFC-Lu-27-A-13
Cleanliness is not only a matter of resistance to oxidation: it is also linked to the ability of an oil to decompose cleanly. Highly branched neopolyol esters will strongly resist elevated temperatures and will eventually decompose cleanly, leaving little or no residue. A fully branched neopolyol ester, for instance, will start showing initial signs of degradation at roughly 210°C.
Figure 7. Oxidation and corrosion test ASTM D4636
Oxidation reactions, like thermal degradation reactions, are strongly catalyzed by transition metals (iron in particular).
2.2.3 Coking propensity Whatever their structure, esters (like any other compound) will eventually start to degrade at elevated temperatures. For ultra-high temperature applications (oven chains oils for instance), the question of what happens when the oil practically “burns” must be raised:
• it may polymerize, get viscous, and generate sludge and insoluble particulate matter;
• it may generate coke (hard, carbonaceous deposits on surfaces);
• it may also decompose and break down into light, volatile fractions, in a kind of pyrolytic mechanism.
Esters were reported to show gas evolution during oxidation process (CO2
such decomposition pathways that preserve cleanliness in , H2 32 LUBE MAGAZINE NO.131 FEBRUARY 2016 , and CO) [5], and some structures will favour
2.2.4 Additives Anti-oxidant response is excellent in esters in general. Preferred antioxidants are generally alkylated diphenylamines. Also, taking in consideration that metals (iron and copper in particular) do catalyze oxidation reactions, any additive capable of deactivating such a catalytic effect will have a positive impact on oxidation resistance. Phosphorus additives are useful for inhibiting catalytic effect of iron, whilst metal deactivators like heterocyclic compounds are used to passivate yellow metals. Additives play a major role in the high temperature performance of synthetic esters.
2.3 Biodegradability
The ester chemical function can be degraded by bacteria. It is believed that the initial step of this process is hydrolysis of ester. As a consequence, esters that are highly hydrolytically stable tend to show low biodegradability features, even though this is not verified for all esters.
The majority of esters do show high levels of biodegradability as measured according to OECD 301B (typically 70-80%, up to 100%). They generally show higher biodegradability levels than any other base stock. In addition, whilst oil soluble PAGs or PAOs for instance only demonstrate significant biodegradability for the lower viscosity grades, synthetic ester technology is not limited by viscosity : 79% biodegradability (OECD 301B) can be achieved with an ISO VG 1000 synthetic ester.
- groups from acid chains are the main sites of
No.102 page 3
operation over polymerization and coking mechanisms. It is believed that highly branched structures in particular will favour such chemical decomposition reactions (Figure 8).
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