The most common cause of ageing in oils and greases is oxidation. This process directly reduces the performance characteristics of the lubricant, causing, amongst other things, thickening, the formation of harmful deposits and the depletion of protective substances. Understanding the mechanism of oxidation and effectively managing this phenomenon are key challenges for modern industry.
Oxidative stability as a key factor in stability
The oxidation of a lubricant is a chemical process leading to irreversible changes in its structure, resulting in the loss of key physicochemical properties.
The consequences of oxidation include an increase in viscosity resulting from the polymerisation of oxidised molecules, the formation of organic acids, varnishes and deposits, as well as the depletion of most performance-enhancing additives, necessitating more frequent lubricant replacement. Its degraded form causes premature wear of mechanical components.
Oil degradation begins with the breakdown of the antioxidants it contains. Only after these have been completely depleted does the actual, rapid ageing process of the oil base take place. It is worth noting that certain synthetic bases naturally exhibit higher oxidative stability than mineral oils. However, it should be borne in mind that factors such as moisture, high temperature, the presence of metal particles (catalysts), UV radiation and constant exposure to oxygen significantly accelerate these adverse phenomena.
Read: how mineral oils differ from synthetic oils.
Oxidation of lubricants
Destructive oxidation in oils and greases proceeds in a cycle of initiation, propagation, branching and termination.
The cycle begins in the initiation phase, when a molecule loses one or more electrons and forms reactive free radicals and peroxides. This is caused by the influence of external energy, and the initiating factors are usually high temperature, UV radiation or mechanical shear.
During propagation, these radicals react with oxygen and multiply, causing further chemical changes in the grease.
As branching occurs, the number of reactive species increases, accelerating thickening and the formation of deposits until the lubricant is no longer able to protect the components. At this point, a rapid ageing process takes place.
Until the cycle is terminated, oxidation will continue until the oil or grease becomes virtually useless.
How can grease ageing be prevented?
The oxidation process can be controlled by intervening in its key chemical stages. Effective protection is based primarily on limiting initiation (isolation from oxygen and high temperatures) and interrupting propagation (blocking the chain reaction).
Oxidation inhibitors (antioxidants) are a key tool in combating the ageing of oils. These are additives that increase the base oil’s resistance to oxidation, thereby extending the lubricant’s service life.
Their action involves:
- Converting aggressive free radicals into stable forms – halting the oxidation chain reaction,
- Protecting the lubricant – antioxidants react much more quickly with oxygen than the base oil, thereby preserving its properties,
- Interacting with other components – in lubricant formulations, oxidation inhibitors work in conjunction with other additives (e.g. AW or EP).
What types of antioxidants are there?
To ensure comprehensive protection of the lubricant against ageing, the synergistic action of two types of oxidation inhibitors is utilised: primary and secondary antioxidants.
- Primary antioxidants
Primary antioxidants – often aromatic amines or phenols – act as radical scavengers. They capture free radicals during the propagation phase and neutralise them. They donate a hydrogen atom to the radical, converting it into a stable molecule. In doing so, they slow down the degradation process and help limit the chain reaction leading to the formation of deposits and varnishes.
- Secondary antioxidants
Secondary antioxidants (e.g. phosphites or sulphur compounds) play a supporting role and are crucial for the long-term stability of the formulation. They react with unstable peroxides, which are formed as by-products of the action of primary antioxidants. They interrupt the oxidation cycle and prevent the branching process, making them essential for the long-term stability of the formulation.
Importantly, in advanced synergistic systems, secondary antioxidants are able to ‘refresh’ primary antioxidant molecules, restoring their ability to scavenge radicals. Typically, in lubricant formulations, they work together throughout the entire oxidation cycle. Together, they increase the base oil’s resistance to oxidation, enabling lubricants to operate at higher temperatures and for longer periods than would be possible without them.
To support the stability of lubricants, the PCC Group offers the Rostabil series, e.g. Rostabil TDP. Organic phosphorus compounds play a key role in these products. Products from this series are a good solution not only for industrial lubricants, but also for plastics and coatings, where thermal stability is a critical parameter.
Testing the oxidation resistance of lubricants
Testing for oxidation resistance is an important part of assessing the durability of lubricants. One of the most recognised analytical methods in this field is the TOST (Turbine Oil Stability Test), conducted in accordance with ASTM D943.
An oil sample is subjected to extreme acceleration of ageing processes through exposure to pure oxygen, high temperature and the presence of metallic catalysts. During the test, the rate of increase in acid number is measured. This increase indicates the progressive chemical degradation of the oil’s components and the formation of acidic decomposition products resulting from oxidation.
This method allows us to determine how long a given lubricant will retain its protective properties.
