The emergence and technological development of lubricant antioxidants can be traced back to the 1930s and 1940s, a time when the automotive industry and other sectors were rapidly developing. Metals used in engines and various equipment were prone to oxidative corrosion under increasingly harsh conditions, affecting equipment performance and reducing lifespan, which in turn spurred the research and development of antioxidant products. The function of antioxidants is to inhibit the initiation, propagation, and termination of oxidation through physical or chemical reactions, thereby slowing and reducing the formation of sludge and varnish, effectively extending the service life of the oil and protecting the equipment.

To date, antioxidant products for lubricating oil have developed over nearly a century, forming specialized categories with distinct targeted functions. For example, in terms of chemical composition and structure, there are sulphur-based, phosphorus-based, nitrogen-based, amine-based, phenol-based, and phenol ester-based types. According to different mechanisms of action, they can primarily be classified as free radical terminators, peroxide decomposers, and metal deactivators, among others, and they can also be categorised based on the differences in their actual effects. The occurrence of oil oxidation reactions requires oxidisable reactants, intermediates, oxygen, and environmental temperature, and also involves the catalytic effects of the metals used in equipment. The entire oxidation process is quite complex, and the mechanism of oxidation has gradually been clarified after long-term research by numerous scientists. Here, we focus on the mechanism of action, which helps to fundamentally understand why antioxidants can resist oxidation.

The oxidation mechanism theory suggests that organic chemical substances in oils, such as alkanes, cycloalkanes, and aromatics, can transform into substances known as peroxide radicals when in an easily oxidisable environment. These peroxide radicals are the key intermediates in oil oxidation and, through subsequent reactions, form organic acids, alcohols, ketones, aldehydes, and eventually oxidise into sludge and varnish. The role of radical terminators, in simple terms, is to react with peroxide radicals to form inactive substances, thereby halting the continuation of oxidation reactions, preventing or delaying the formation of varnish and sludge, and extending the service life of the oil. Another type of antioxidant closely related to radical terminators is called a metal deactivator. As suggested by the name, this product reduces the activity of metal ions to achieve an antioxidant effect. It is important to note the relationship between metal ions and oxidation reactions. Data indicates that the presence of certain metal ions can catalyse oil oxidation, making the oil more susceptible to oxidation. Mechanical equipment inevitably contains various metal elements, with copper and iron being the most common. To avoid or reduce the catalytic effects of metal elements on oxidation, it is necessary to add an appropriate amount of metal deactivator. Generally, metal deactivators need to be used in combination with different types of antioxidants to achieve better antioxidation results.

Another common type of antioxidant is known as a peroxide decomposer, which primarily works by decomposing peroxides and is also referred to in practice as an antioxidant and anticorrosive agent. A well-known product in this category is dialkyl dithiophosphate, which has been one of the most popular and widely used antioxidants and anticorrosive agents in recent decades. The most representative product of dialkyl dithiophosphate is the zinc salt, which contains zinc, sulphur, and phosphorus elements. Sulphur and phosphorus each have specific roles and characteristics in oil antioxidation and anticorrosion, and when combined, they enhance the performance of zinc dialkyl dithiophosphate in antioxidation and anticorrosion, even contributing to extreme pressure and anti-wear properties. The oxidation of oil is closely related to high-temperature environments, and to achieve greater power or better performance, the demands on metal components are increasing. For example, increasing the compression ratio of an internal combustion engine necessitates higher mechanical strength and resistance to high-temperature corrosion. The development of antioxidants and anticorrosive agents is closely linked to the need for equipment that can withstand oxidation and corrosion.

For a long time, the use of lubricating oil has mainly been divided into two major categories: engine oils (including oils used in vehicles, ships, and other internal combustion engines) and industrial oils (including turbine oils, industrial gear oils, transformer oils, hydraulic oils, etc.). Engine oils generally operate under high temperature and high pressure conditions, whereas the working environment of industrial oils is relatively milder in comparison. The high-temperature oxidation and corrosion resistance required for engine oils necessitate not only a certain degree of refined base oil but also the indispensable use of antioxidants, primarily peroxide decomposers and free radical terminators. Industrial oils, due to their variety, require different effects for each type, achieved by using one or several types of antioxidants in combination. Overall, based on relevant research data and actual market usage, determining the product's intended use and operating environment, and using products with more than one different antioxidant mechanism in combination, is necessary to achieve good oxidation resistance. Of course, when combining various types of antioxidants, attention must be paid to potential interactions between different types of antioxidants that could negatively affect oil quality. (Due to the author's limited expertise, there may be deficiencies or errors in the above content, and readers are welcome to provide corrections.)