Antioxidants are additives designed to prolong the life of a lubricant by increasing the oxidative resistance of the base oil. Antioxidants allow lubricants to operate at higher temperatures than would otherwise be possible without them. Many synthetic lubricants, especially hydrocarbon-based lubricating oils, are susceptible to degradation by oxygen. This oxidation process, which is initiated by the formation of reactive free radicals and peroxides, is the major cause of oil thickening and the formation of sludge and varnish in many applications.
What happens during the oxidation cycle?
Destructive oxidation of an oil can be described as a cyclical process involving initiation, propagation, branching and termination. Unless the cycle is broken or terminated, oxidation will continue to occur until the oil or grease is no longer usable.
As shown above, primary antioxidants (AOs) react here with the very reactive radicals slowing the degradation by breaking the self-propagating radical chains. Secondary AOs react here with reactive peroxides that are present to break the oxidation cycle.
There are two types of antioxidants: primary and secondary antioxidants. Primary antioxidants are typically comprised of aromatic amines and hindered phenolics. Secondary antioxidants are typically comprised of phosphites and certain sulfur-containing compounds, such as thioethers and thioesters. Each type of antioxidant performs a different function to inhibit oxidation.
The oxidation process begins with the initiation phase, when free radicals are formed. Primary antioxidants are “radical scavengers” that react quickly with the free radicals during the propagation phase, slowing down the degradation process by forming new radicals that are more stable. Secondary antioxidants react with peroxides which are often present as the lubricating oil reacts with oxygen. These antioxidants are responsible for breaking the cycle and preventing branching and further propagation. Most often, grease formulators will use a combination of primary and secondary antioxidants to maximise the protection of the oil against oxidative degradation.
How is oxidative resistance measured?
Most commonly a Pressure Differential Scanning Calorimeter (PDSC) is used to measure the efficiency of an antioxidant additive package. This test measures the ability of an oil or grease to resist oxidation in a very demanding high pressure oxygen-rich environment at elevated temperatures according to ASTM Method D-6186. Typically reported in “minutes”, the greater the time it takes for an exothermic reaction to occur, the better the oxidative stability.
Getting the right combination of antioxidants is critical to the grease’s performance in the conditions for your application.