Lubricant wihtout                 Lubricant degradation prevented
    antioxidants                         by antioxidants

When lubricants are exposed to heat, gases, or mechanical stress, the molecules can break down and form radicals, which react with oxygen to cause oil thickening, deposits, and acid build-up. Antioxidants extend the useful life of lubricants by eliminating these radicals and preventing thermo-oxidative breakdown.

Lubricants formulated without antioxidants may have to be replaced after a short time due to degradation as shown in the above picture.

The thermo-oxidative degradation process has been investigated and is well understood. Schematically it follows the same schemes as the thermal degradation of polymers.   

Primary antioxidants

Ciba® IRGANOX® L 57, Ciba® IRGANOX® L 135
These are the lubricant additives which eliminate radicals like R ; ROO.

Secondary antioxidants

Ciba® IRGANOX® L 64, , Ciba® IRGANOX® L 115, Ciba® IRGAFOS® OPH
These antioxidants eliminate hydroperoxides ROOH.