|Bill Quesnel, B.Sc.
|Oxidation and Oxy-polymerization of Oils|
|In the presence of oxygen mineral oils will oxidize. For this reason all formulated oils contain an anti-oxidant additive.||Degradation
of oils in service
is an inherent part of
their use. The rate at which oils degrade is dependent on
several factors including the chemistry of the oil
basestock, the type and amount of various inhibitors, and
additives present in the oil, and the operating
conditions of the oil over its service life. The main
factors that attribute to the extended life of an oil in
service are its thermal and oxidative stability.
Thermal stability represents the oils ability to resist
chemical change with increasing temperature in the absence
of oxygen. Oxidation stability is the oils ability to
resist chemical change, typically with increasing
temperature, in the presence of oxygen.
Mineral based oils have good thermal stability. Under high temperatures in anaerobic conditions mineral oils resist chemical degradation. In the presence of oxygen mineral oils will oxidize. For this reason all formulated oils contain an anti-oxidant additive.
The oxidation of an oil occurs when atmospheric oxygen attacks the weak areas of hydrocarbon molecules. Basestocks are refined and hydrotreated to ensure that the hydrocarbons are, for the most part, completely saturated. Saturated hydrocarbons are carbon molecules that contain exclusively carbon-carbon bonds (C-C). Since no refinement process is completely 100% successful, the hydrocarbons will contain weak areas. These weak areas are typically carbon-carbon double bonds (C=C), or similarly reactive chemical species. Oxygen, in the presence of heat, is able to chemically alter the C=C in the hydrocarbon, to initiate the formation of peroxides (RCOOH). The newly formed peroxides degrade through a free radical process to form hydroperoxides. The formation of hydroperoxides is highly unfavourable as these chemical species will, through a condensation polymerization process, form high molecular molecules. The presence of metal salts and metal surfaces in the component acts as a catalyst to further increase the rate of this process.
After a significant degree of oxi-polymerization has occurred the oil will have a substantially increased viscosity. The increase in viscosity is a direct reflection of the physical size of the polymers themselves. Anti-oxidants present in the oil hinder the formation of these high molecular molecules by either inhibiting the formation of the initial peroxides, or by reducing the ability of peroxides to form hydroperoxides. Anti-oxidants are typically aromatic amines, hindered phenols, or alkyl sulfides which act as free radical scavengers in that they are capable of deactivating a free radical molecule like peroxide. Oxidation and oxi-polymerization will still occur in an oil that contains anti-oxidant, but to a far lesser degree. Anti-oxidants are sacrificial in that they are used up in the process of free radical scavenging. Once the anti-oxidant is depleted in an oil, the oxidation and oxi-polymerization process propagate unabated at a significantly rapid pace.
It is therefore important to perform regular oil changes, and to follow the recommended oil change out intervals. While mineral oils have been formulated to resist oxidation during service, they will thicken and form sludge and varnish if overextended. The problem becomes more serious when using biodegradable oils (e.g. vegetable-based oils) that are more reactive, and prone to peroxide formation, or when treating semi-synthetic PAO (polyalphaolephin) oils in the same fashion as fully synthetic oils. Although the PAO portion of the oil is more resistant to oxi-polymerization the remaining mineral oil portion is just as susceptible to oxidization. The formation of oxi-polymerized molecules in an oil will severely degrade the oils ability to lubricate the component, cause sludge, and varnish, and may even lead to the seizure of the component all together.
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