When speaking about antioxidants, the first thing that comes to mind is oxidation. This is the primary reason that antioxidants exist: to reduce oxidation. But what is oxidation, and why should there be antioxidants?
Oxidation occurs in everything in life (not just finished lubricants). We see oxidation regularly when we leave certain fruits exposed to the atmosphere (think about cut pears or apples). After being in the elements for some time, they are no longer fresh and have degraded slightly.
A similar reaction occurs during the oxidation of finished lubricants. Greg Livingstone provides an excellent summary of the oxidation process in his article, “Varnish, Deposits in Bearings, Causes, Consequences, and Cures.” The oxidation degradation pathway begins with initiation, where free radicals are formed in the presence of heat, wear metals, water, and oxygen as shown in Figure 1.
Afterward, during propagation, the free radicals form hydroperoxides, which can create oxidation by-products (Alkoxy radicals), eventually leading to high molecular weight oxygenated by-products.
During this process, the free radicals can also react with primary antioxidants, or the hydroperoxides can react with secondary antioxidants to slow these reactions. However, they will still form the high molecular weight oxygenated by-products once depleted.
Next in the termination phase is polymerization and agglomeration, followed by the physical and chemical changes to the lubricant. It must be noted that there are various stages to oxidation, and typically, when we see sludge or varnish, oxidation has already occurred.
When oxidation occurs, the oil quickly loses its antioxidants; they can no longer protect the oil. As such, the oil begins to undergo physical changes where sludge and varnish appear, and viscosity usually increases. These oils also experience a rise in acid production after these reactions occur.
Now that we have a better understanding of oxidation, whereby the antioxidants are deployed to help reduce the oxidation rate, we can dive deeper into the world of antioxidants and how they can help fight against oxidation for the finished lubricant.
Want to read the entire article? Find it here in Precision Lubrication Magazine!
