What Are Lubricant Additives?
The additive content in lubricating oils ranges from just a few parts per million to several percentage points. Depending on the function that the additives develop they may be classed as:
- Substances intended to improve the intrinsic characteristics of the base oils (viscosity index modifiers and pour point improvers).
- Lubricant protective substances (antioxidants).
- Substances giving new properties and protecting the metal surfaces of engines (detergents, dispersants, friction modifiers, anti-wear/Extreme Pressure (EP) additives, rust and corrosion inhibitors).
The additives added to improve lubricating oils include:
Viscosity Index Modifiers
Viscosity is the main physical property of a lubricant and it is a measure of the intermolecular interactions of the oil and hence of its resistance to flow. As temperature increases the viscosity of the lubricant tends to diminish also causing a decrease in the thickness of the lubricating film between parts in relative motion. Viscosity Index Improvers (VII), or Viscosity Modifiers (VM) influence the viscosity-temperature relation.
VIIs are polymers with a variable molecular weight belonging to the following main categories:
- Hydrogenated ethylene-propylene copolymers (also called OCP, Olefin Co-Polymers).
- Hydrogenated polyisoprenes which may be linear, partly branched or star-shaped.
- Polymetacrylates (PMA) of long-chain alcohols variable from C12 to C18, linear and/or partly branched.
- Hydrogenated styrene-isoprene copolymers, which may be linear, partly branched or star-shaped.
- Polyisobutenes (PIB).
At a low temperature, these polymers have a closely-knit structure which minimizes interactions with the lubricant base; as temperature increases, the polymer increases its interactions with the base, extending its chains and expanding, countering the decrease in viscosity of the base. In the production of VMs, control of the molecular weight and its distribution represents a critical element as these parameters regulate two important characteristics of the polymer, i.e. its thickening power and its mechanical shear stability.
Pour point improvers
These additives (Pour Point Depressants, PPD) improve the pour-point characteristics of the lubricant at a low temperature. The main types of these are polymethacrylates, ethylene-vinyl acetate copolymers and polyfumarates. The effect of PPDs depends largely on the characteristics of the bases used and on their concentration. Normally, the action of these additives is more effective compared to fluid bases (SN 80, SN 150). Each class of PPD has a limit to its effectiveness; above a certain percentage the effect on the pour point ceases (and in certain cases it could worsen) and the thickening effect becomes noticeable. The typical treatment percentages vary between 0.1 and 1%.
Oxidation is the result of the interaction of the components of the lubricant with oxygen. It is the chief cause of the degradation of oil and leads to the formation of acid species which gradually increase in molecular weight, giving rise to dirt and sludges which lead to an increase in the viscosity of the lubricant and form deposits in the cooler areas of the engine. The oxidative degradation of the lubricant occurs due to a complex series of radical chain reactions, which can be acted upon by special antioxidant additives or oxidation inhibitors. These additives on the one hand interrupt the chemical reactions responsible for the processes mentioned, and on the other hand decompose the first degradation products preventing any further evolution towards more harmful species. The main types of this class of substances are alkylated aromatic amines, sterically hindered phenols, zinc dialkyl dithiophosphates, and derivatives of dialkyl dithiocarbamic acid. The amines and the hindered phenols act as radical scavengers, transforming the reactive peroxides into inactive species. The zinc dithiophosphates, besides acting with these mechanisms, decompose the hydroperoxides (ROOH) heterolithically and deactivate them.
Detergents and dispersants
These are two of the most important categories of additives to engine oils and their function is to keep the engine clean. This aim is pursued by reducing the formation of deposits and keeping the insoluble substances in suspension, hindering their further aggregation and adhesion to the hot/cold metal surfaces.
- Metallic detergents: These serve to neutralize the acid products of combustion (organic acids and sulphur oxides), to reduce sludges and deposits on the pistons and to prevent problems for the piston rings under severe temperature conditions. Generally, they consist of colloidal dispersions in carbonaceous lubricant bases of alkaline or alkaline-earth metals, stabilized by an adsorbed layer of surfactant molecules. The carbonaceous nucleus, typically amorphous, represents the base reserve necessary to neutralize the acid compounds, whereas the surfactant layer consists of oleophilous chain acid salts (soap) long enough to ensure the stability of the colloid. The main chemical classes of metal detergents are sulphonates, sulphophenates and salicylates. The value of the basicity number (BN) determines the neutralizing capacity of the additive while the soap content determines its effective detergent action. Detergents are classified as neutral (BN_25) or supra basic (BN_25) according to their neutralizing ability. For heavy-duty vehicles, detergents containing a base of alkaline-earth metals are used, above all those with a calcium base.
- Dispersants: These additives are fundamental for purposes of performance as they control the state of aggregation of sludge and, in diesel engines, of soot. In some lubricant, dispersants account for more than 50% of the number of additives. Dispersants consist of amphiphilic molecules in which the lipophilic portion usually consists of polyolefinic chains (generally polyisobutene) with a molecular weight that varies between 700 and 3,000, while the polar group is, in general, the derivative of a polyamine or of a polyol. The bond between these two parts of the final molecule is obtained by means of different chemical reactions. The most important classes of dispersants are: succinimides, succinic esters, alkylphenolamine (Mannich bases), and polymeric dispersants. Of them all, the succinimides are probably the most important class and the one produced in the largest volumes. They are prepared in two stages: the first consists of functionalizing the chain of an alkyl oligomer (polyolefin, preferably polyisobutene) with maleic anhydride to produce a polyisobutylene succinanhydride (PIBSA). In the second stage, the PIBSA is converted into the final polyisobutylene succinimide (PIBSI) causing it to react with an N-amino-polyalkylamine (for example hexaethylene hepta-amine, HEHA; tetraethylene penta-amine, TEPA, etc.). The succinic esters used as dispersants for motor transport vehicles lubricants are products formed by esterifying a succinic derivative of a polyolefin (analogous to those used for succinimides) with mono- or poly-alcohols (for example pentaerythritol). Alkylphenolamines are polyisobutylenic phenols (or polyalkyl-substitutes) made to react with polyalkyleneamine by means of formaldehyde (through the Mannich reaction).
These additives are chemical species able to influence the friction coefficient under boundary lubrication conditions. They may consist of very long amphiphilic organic molecules or of metal-organic compounds (generally with a molybdenum base). The reduction of the friction coefficient of the surfaces takes place by means of the formation of an extremely smooth film of molecules over them.
These additives are mainly used for reducing wear under boundary lubrication conditions. Under conditions of medium-to-high or extreme pressure (EP), these additives react with the metal surfaces forming protective tribo-chemical layers. In engine oils, these essentially perform an anti-wear function. The EP role, which is marginal, is left to metal detergents. The main class of anti-wear additives consists of zinc dialkyl dithiophosphates, whose introduction coincided with the start of the technology of using additives in lubricants. There are also wear-prevention additives with a molybdenum base (dialkyl dithiophosphates, dithiocarbamates), organic compounds and metal detergents. EP additives are commonly used in transmission oils. Among these, the main ones are: with sulphur (anti-wear/EP), sulphur-phosphorus compounds (anti-wear/EP) and chlorinated paraffins (EP). The anti-wear additive breaks down at the metal-metal interface at high temperature and it reacts with the contacting surfaces forming layers with a low friction coefficient. As stated, the zinc dithiophosphates also carry out a very effective antioxidant function. Currently, the use of zinc dithiophosphates is limited because the phosphorus contained in them.
These additives protect the metal surfaces of the engine against corrosion and against the aggressive agents generated during combustion (water, acid products, oxidants, etc.). They are also used as protection during the transport and storage of the lubricant. Rust-inhibitors act by creating a physical barrier on the metal surface. This barrier prevents the corrosive agents from attacking the metal surface. The main types of anti-corrosive additives are: etoxylate alcohols, long-chain carboxylic acids, phosphoric esters, amines, imidazoline and thioderivatives. These additives can compete against other classes of additives (for example, zinc dithiophosphates) for treating metal surfaces. Therefore, these can negatively influence other properties of the medium in which they operate. Because of the presence of various kinds of metal surfaces, a mix of various anticorrosion additives it is often required.
 GREW, W., CAMERON, A. Role of Austenite and Mineral Oil in Lubricant Failure. Nature 217, 481–482 (1968). https://doi.org/10.1038/217481a0
 HALTNER, A., OLIVER, C. Frictional Properties of Molybdenum Disulphide Films containing Inorganic Sulphide Additives. Nature 188, 308–309 (1960). https://doi.org/10.1038/188308a0
The article is written by Riya Veluri, an editorial team member of Industrial Lubricants. After her graduation, Riya works as a website developer & SEO specialist in Lubrication & Tribology Industry & writes technical articles on Lubricants, Lubrication, Reliability & sustainability.