What is the role of diphenylamine as a lubricant antioxidant?

1. Free radical scavenging effect

1.1 During the use of lubricants, due to the influence of high temperature, oxygen, metal catalysis and other factors, the hydrocarbon molecules in the lubricants are prone to oxidation reactions and produce free radicals. These free radicals will trigger a series of chain reactions, resulting in the degradation of lubricant performance. As an antioxidant, diphenylamine has the ability to scavenge free radicals.

1.2 In its molecular structure, the lone pair of electrons on the nitrogen atom enables it to react with free radicals. For example, when encountering peroxyl radicals (ROO·), diphenylamine can provide a hydrogen atom and transform itself into a relatively stable nitrogen-centered free radical (Ph2N·), thereby interrupting the chain oxidation reaction initiated by free radicals and playing a role in inhibiting lubricant oxidation. This reaction mechanism is like setting an "interception point" in the "chain transfer" link of a chemical reaction, which effectively prevents the further spread of the oxidation reaction.

2. Inhibition of the initial stage of oxidation reaction

2.1 In addition to scavenging the free radicals that have been generated, diphenylamine can also play a role in the initial stage of the oxidation reaction. It can interact with active components (such as unsaturated hydrocarbons) in lubricating oils, reducing the possibility of these components reacting with oxygen to generate initial free radicals.

2.2 Diphenylamine molecules can form relatively stable complexes with unsaturated hydrocarbon molecules, and such complexes have low reactivity to oxygen. For example, in ester-based lubricating oils containing a large amount of unsaturated hydrocarbons, diphenylamine can change the distribution of electron clouds around unsaturated hydrocarbon molecules by interacting with unsaturated bonds, thereby reducing their chances of contact with oxygen and inhibiting the occurrence of oxidation reactions from the source.

3. Delaying the formation of oxidation products

3.1 During the oxidation process of lubricating oils, various oxidation products such as aldehydes, ketones, acids, and gums will be produced. These oxidation products will cause problems such as increased viscosity of lubricating oils, increased acid value, sludge, and paint film formation, thereby affecting the normal operation of mechanical parts.

3.2 The presence of diphenylamine can significantly delay the formation of these oxidation products, inhibit the progress of oxidation reactions, reduce the accumulation of oxidation intermediates, and enable lubricating oils to remain relatively clean and stable for a long time. For example, adding diphenylamine as an antioxidant to the lubricating oil of automobile engines can effectively reduce the accumulation of sludge inside the engine, reduce the risk of engine wear, and extend the service life of the engine.

4. Metal deactivation

4.1 In the working environment of lubricating oil, metal surfaces (such as metal parts in the engine) often catalyze oxidation reactions. Metal ions can accelerate the generation of free radicals and speed up the oxidation reaction.

4.2 Diphenylamine can be adsorbed on the metal surface to form a protective film, which can prevent metal ions from directly contacting the active ingredients in the lubricating oil, thereby weakening the catalytic effect of metal on oxidation reactions. For example, in a lubrication system containing metal parts such as copper and iron, the interaction between diphenylamine and the metal surface can reduce the activity of metal ions and reduce the phenomenon of rapid oxidation of lubricating oil due to metal catalysis.