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What is meant by the term “oxidation stability” and why is it important?
Oxidation stability is a chemical reaction that occurs with a combination of the lubricating oil and oxygen. The rate of oxidation is accelerated by high temperatures, water, acids and catalysts such as copper. The rate of oxidation increases with time. The service life of a lubricant is also reduced with increases in temperature. Oxidation will lead to an increase in the oil's viscosity and deposits of varnish and sludge.
The rate of oxidation is dependent on the quality and type of base oil as well as the additive package used. Some synthetics, such as polyalphaolefins (PAO), have inherently better oxidation stability than do mineral oils. This improved oxidation stability accounts for the slightly higher operating temperatures that these synthetic oils can accommodate.
Generally, oxidation will reduce the service life of a lubricant by half, for every 10 degrees C (18 degrees F) increase in fluid temperature above 60degrees C (140 degrees F). This concept is based on the Arrhenius rate rule, which is named for the 19th-century Swedish chemist Svante Arrhenius.
There is a little controversy concerning the oxidation stability of natural mineral base oils as determined by the refining method. There is one school of thought that suggests that hydrotreated base stocks have superior oxidation resistance and thermal stability than does solvent-refined base oil.
This is based on the misconception that since hydrotreating removes all of the potentially undesirable compounds, the base oil tends to automatically reduce deposit-forming tendencies and thereby will better resist oxidation. However, removing all of the compounds considered undesirable can in fact be detrimental.
Solvent refining results in the production of base oils, which retain some sulfur compounds that are natural antioxidants. These base oils retain a natural ability to prevent oxidation, while hydrotreated base oils must be further fortified with antioxidants in order to maintain thermal and oxidation stability. Once the antioxidants are weakened or depleted, oxidation of some hydrotreated oils can occur very rapidly.
Severely hydrotreated base oils also have poor solubility characteristics. Without proper formulation, additives may not remain suspended, and some additive drop out could occur.
Several methods may be used to determine or evaluate the oxidation stability of an oil, which is usually regarded as the number of hours until a given increase in viscosity is noted or until there is a given increase in the acid number (AN).