Lubricants - Chemically active additives

Despite continuous progress in petroleum refining and petrochemicals, pure mineral or synthetic base oils almost never have all the properties required for their main automotive, industrial, marine or aeronautical applications. It is therefore necessary to incorporate additives into the finished lubricants (oils and greases), the content of which can vary from less than 1% for certain industrial oils to more than 25% for the latest oils for diesel engines in heavy-duty industrial vehicles, or for certain metalworking lubricants.

Additives are designed either to enhance certain intrinsic properties of base oils, such as pour point, viscosity index, oxidation resistance, anti-wear and anti-friction properties, or rust protection, or to provide properties that they do not (or only to a limited extent) naturally possess, such as detergency, dispersive power, alkalinity to neutralize acidic compounds, protection against non-ferrous metal corrosion, or extreme pressure (EP) properties. In some cases, they can also combat defects introduced by other additives. For example, antifoam additives prevent foaming caused by the presence in the lubricant of surface-active additives such as detergents, dispersants, emulsifiers for aqueous fluids, or even lubricity additives and rust inhibitors.

The formulation of a lubricant containing numerous additives, sometimes as many as twenty, is generally a lengthy operation, requiring two to three years of research, a great deal of experience and multidisciplinary know-how, and is often extremely costly, as it involves numerous tests in the laboratory, on test benches or in service. For example, the cost of a single engine test ranges from €15,000 to €75,000, and a large number of tests are required to qualify an engine oil formula.

We mustn't assume that a given additive works under any condition. In fact, its action depends on four main sets of factors:

• its chemical nature, degree of purity and concentration in the oil (or fat);

• interactions with other additives in the formula. In some cases, two additives may be antagonistic to each other. For example, an anti-wear additive may lose its effectiveness in the presence of a more polar additive which, for example, forms a barrier film, solidly adsorbed on surfaces, physically opposing the adsorption of the anti-wear additive – as in the case of a detergent additive or an anti-rust additive – or, alternatively, will "sequester" it within the liquid, preventing it from acting on surfaces; this is what happens in the presence of dispersant additives. Naturally, these antagonisms must be avoided or limited. In other cases, on the contrary, there is synergy, i.e. the beneficial effects...