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LUBRICANTS ARE USED TO REDUCE FRICTION AND WEAR

Dissipate heat from critical parts of equipment, remove and suspend deposits that may affect performance and protect metal surface damage from degradation and corrosion. They serve a diverse range of applications, each requiring a different combination of base oils and additives. With such a complex industry, there are many challenges to developing effective lubricants.

Base oils themselves perform most of the functions of lubricants. But they can only do part of the job. Additives are needed when a lubricant’s base oil doesn’t provide all the properties the application requires. They’re used to improve the good properties of the base oils and minimize the bad. They compensate for the deficiencies inherently present in basic base oil systems (see Examples of Engine and Non-Engine Lubricants). 

Typical lubricants are composed of a base oil, an additive package and, optionally, a viscosity index (VI) improver. Turbine, hydraulic and industrial gears demand much lower treat rates of additive packages compared to automotive gear, ATF and gasoline and diesel engines, which are the most demanding and require the most additives (see Figure 1). 

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Figure 1 | Composition of base oil, VI improver and additive packages in typical lubricants. 

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    Additives are combined into well- balanced, optimized packages allowing lubricants to meet specified performance criteria in a finished fluid. One example of an additive package is the Dispersant Inhibitor (DI) package used for engine oils, containing mainly dispersants, detergents, oxidation inhibitors, antiwear agents and friction modifiers. Additives may interact with each other and may have multi-functional properties. Each additive package is a unique blend whose combination pro- duces a complex chemistry with positive and negative interactions. (See Figure 2) 
 

Figure 2 | Example of Dispersant Inhibitor (DI) additive package for engine oils.

ADDITIVE USE AND TYPES

An additive’s effectiveness depends on where it’s used. For example, an additive may work well in one oil and not in another. Polar additives compete with each other for metal surfaces. Many additives exhibit either synergistic or antagonistic effects with other additives present in the system. Synergism gives the opportunity to use less overall additives. But antagonistic effects may prevent the use of certain types, and more may be needed to overcome the antagonistic effect.
The dosage or treat rate also de- pends on the system. Usually effective- ness diminishes as dosage increases. But some additives may need to reach a threshold dosage before becoming active. The dosage is affected by synergism or antagonism with other additives in the oil. Actual treat rates have to be based on specifications and performance requirements.
There are two types of additives: bulk oil additives and surface additives. Bulk additives affect the rheological, interfacial and chemical properties of the lubricant, and surface additives are active at metal surfaces (see Figure 3)

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Figure 3 | Lubricant additive types.

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OIL ADDITIVES 

There are different types of oils and they use many of the same types of ingredients. However, these are put together a little differently. Not all of these are found in every oil. Firstly, you have base oils, made from either crude oil at a refinery or man-made (synthetics). To achieve the functions required by finished lubricants, you must then put additives in the oil. These all do different things. 

DETERGENTS

Any oil with an API engine rating of SC (see page 16 for API Service Classifications), or above has a level of detergency. This detergency level is not necessarily related to all of the quoted API ratings of the oil, as some high detergent diesel oils may only meet lower petrol engine oil specifications. It is a balance. Detergents are usually metallic compounds and they control deposits and keep engines clean. 

DISPERSANTS

These are usually ashless (nonmetallic) organic chemicals. They keep contaminants and by-products dispersed in the oil helping to prevent deposits and sludge from forming. They are highly effective in controlling low temperature contaminants. They can keep them so fine in suspension, they pass through the oil filter with the oil additives! 

FRICTION MODIFIERS

Used to reduce internal engine friction and are common in low viscosity oils where fuel economy is important. 

OXIDATION INHIBITORS

Reduce oxygen attack or oxidation of the oil, helping to reduce oil thickening, especially at high temperatures. 

ANTI-WEAR AGENTS

Prevent wear due to seizure or scuffing of metal surfaces that would otherwise rub or contact each other. They are normally zinc and phosphorus or other organo-metallic based compounds.

RUST AND CORROSION INHIBITORS

Prevent rust and corrosion attack on metal surfaces from acids that can build up in oils. 

FOAM DEPRESSANTS/AIR RELEASE AGENTS

Prevent foam from forming, thereby maintaining a lubricating film based on oil not air bubbles, resulting in the ability of the oil to be pumped effectively at the required rate. 

POUR POINT DEPRESSANTS (PPD)

Reduce the oils tendency to crystallize at very low temperatures, i.e. Its ability to pour or flow. Most oils contain wax and, at very low temperatures, wax can crystallize. PPDs assist to lower the temperature at which this occurs. 

VISCOSITY INDEX IMPROVERS (VII)

These change the oils rate of thinning or its Viscosity Index (VI). The higher the VI, the lower the rate of thinning of the oil with increase in temperature. They are polymers that expand as temperature increases – think of them as like a slowly uncoiling spring. They also assist in making oils into multigrades.

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