# Molecular Science of Lubricant Additives +
## 2 Individual Lubricant Additives and Their Working Mechanism
### Maintainers
#### 2.3.1. Antioxidants (AO)  
Autoxidation is the oxidative degradation of organic compounds that occurs spontaneously under aerobic conditions without any active treatments such as reagents, catalysts, or driving forces. Most organic materials experience this reaction as ageing processes, and lubricants are no exceptions. Therefore, they have been well studied both from engineering and scientific approaches [69; 70; 71; 72; 73]. The consensus reaction pathway is the radical chain reaction mechanism, which is initiated by the hemolytic dissociation of a C-H bond by heat or ultraviolet irradiation. Figure 12 explains the possible mechanism of radical formation under tribo-chemical conditions.  
* Thermal dissociation of a carbon-hydrogen bond to yield hydrogen radical and carbon radical.  
Figure 11: Working mechanism of pour point depressant.  
* Shear stress can dissociate a carbon-carbon bond though direct mechanical forces, yielding two carbon radical intermediates. Polymers can decompose through this mechanism.
* Wear of tribo-materials exposes nascent surfaces that may catalyze the dissociation of a carbon-hydrogen bond.  
Among the three initiation mechanisms, causes by shearing and nascent surfaces are unique in lubrication systems (see Section 1.4). The carbon radicals are ready to react with oxygen to yield peroxy-radical species that further react with another hydrocarbon molecule (Figure 13). The reaction results in regeneration of the carbon radical intermediate and oxidized molecule. This regeneration step is the characteristic to the radical chain reaction mechanism. Theoretically, the autoxidation can proceed again and again until the depletion of the hydrocarbon once the carbon radical species were generated. It should be noted that the radical intermediates can react with oxygen, even while the machine stops. Therefore, the radical intermediates should be deactivated as soon as they were generated.  
The radical scavengers can deactivate the active intermediates, thereby breaking the chain reaction. Phenol derivatives are commonly used for this role [74; 75; 76]. Even with these reagents, a small portion of radical species can react with oxygen and yield peroxide. In other words, the function of the radical scavengers is not always perfect in preventing the autoxidation reaction. Organic peroxides possess strong oxidative attack to other organic molecules and metallic materials. Aromatic amines [77; 78], alkyl sulfides [79], and ZnDTP [80] are frequently employed to decompose peroxy radicals or peroxides. Peroxide decomposers are post-active AO that deactivate the active intermediates after the reaction with oxygen. On the other hand, radical scavengers and metal deactivators are proactive AO that suppress the reaction prior to the oxidation step.  
It is empirically known that certain metal catalyzes the autoxidation of hydrocarbons. The wear process produces nascent surfaces that display catalytic activities. The surface reactions become significant when tiny wear particles are dispersed in the lubricant, because they have large surface area. Metal deactivators can interact with metal surfaces and block the active sites on the surfaces. From the mechanistic viewpoint, metal deactivators work at a solid-liquid interface and the others work in the liquid phase.  
Figure 12: Initiation step of autoxidation (generation of radical intermediates).  
Different types of AOs are frequently used together in a lubricant [81]. Since tribological processes usually induce different causes of autoxidation, different AO complement each other to prevent the ageing processes happening simultaneously.