The use of lubricants is a fundamental element of any mechanical system, enabling it to function optimally over a long period of time. One aspect of lubricants that can significantly improve their properties is the use of performance-enhancing additives.
The importance of additives in lubricant formulation
The most important characteristic of a lubricant is its ability to prevent wear. This type of mechanical wear can take various forms within a lubricated system, including:
- Adhesive wear,
- Abrasive wear,
- Pitting,
- Spalling.
The importance of anti-wear additives, particularly in lubricants, cannot be overstated. By limiting wear, they become a cornerstone of operational efficiency and form an integral part of mechanical equipment. Today, they are an indispensable component of modern lubricant formulations, especially those operating under harsh conditions.
There are two types of such additives, used depending on the situation:
They act as a consumable material – they adhere to rubbing metal surfaces due to their polarisation and react with them under the influence of heat generated by physical contact. In this way, they form a protective layer that minimises wear, protects the base oil from oxidation and shields metals from the effects of corrosive acids.
How to determine the performance of an AW/EP additive?
The tribological properties of additives are determined using specialised test methods. One of these is the 4-ball test. This is a common technique that involves rotating a steel ball relative to three stationary, lubricated balls arranged in a cradle configuration. This test determines the properties that prevent lubricant wear under specific load, speed, temperature and time conditions, as specified in the ASTM D4172 (AW) or ASTM D2783 (EP) standards. Test conditions for AW properties are typically 1200 rpm and a load of 40 kg at 75°C for 60 minutes. EP properties are usually measured at room temperature, over a period of 10 seconds, under variable load. The results of the 4-ball wear test are presented as wear marks that appear on stationary balls, which are then measured in terms of size and averaged.
AW anti-wear additives
To reduce the rate of continuous and moderate degradation of mechanical systems, anti-wear (AW) additives are used in lubricating oil formulations.
- AW additives are suitable for lubricants operating under mild conditions, at low loads and high speeds,
- Their role is to reduce the coefficient of friction by protecting mating metal surfaces,
- They are activated by a rise in temperature caused by load and at low pressures,
- Their mechanism of action is based on the formation of a thin, protective tribological layer on the metal surface. They operate without chemical changes and require less activation energy. These processes (e.g. physical adsorption) are usually reversible.
- They are found in hydraulic oils, engine oils, gear oils, automatic transmission fluids and certain lubricants,
- Examples of AW additives include phosphoric acid esters, zinc dialkyldithiophosphates (ZDDP) and sulphur-phosphorus compounds,
- The PCC Group’s portfolio includes anti-wear additives, which are products from the Rokolub AD series (e.g. Rokolub AD 246 ultra).
EP anti-seize additives
To ensure lubrication under conditions where the oil film breaks down, anti-seize additives – Extreme Pressure (EP) – are used. They are designed to operate under so-called boundary lubrication conditions.
- EP additives are intended for use under higher loads, at high temperatures and low speeds. They activate at elevated pressures,
- They are intended for more intense metal-to-metal interactions, and therefore the coating is more durable and thicker than in the case of AW additives,
- Their mechanism of action is based on entering into an irreversible tribochemical reaction at the interface, resulting in the formation of a protective layer. This process requires a high activation energy,
- Chemically, they act more aggressively, characterised by a higher reaction rate with the metal, as well as faster formation of an anti-seize coating . In some cases, this high reactivity may cause certain EP additives to be corrosive to specific metals, which justifies their cautious use,
- They are suitable for more niche applications, which typically include gear oils and metalworking fluids,
- Typical EP additives are (often organic) compounds based on boron, chlorine, phosphorus or sulphur. These include chlorinated paraffins or aromatic compounds, sulphurised mineral oils, aryl(alkyl)phosphoric acid esters, chlorinated and/or sulphurised fatty acids or olefins, polyalkylene glycol, etc.,
- Anti-seize additives in the PCC Group include products from the EXOfos series (e.g. EXOfos PA-080S, EXOfos PB-184).
