What are additives and which functions do they perform?
In the simplest terms, additives are oil-soluble agents or active substances that are dissolved or suspended as solids in oil. The organic or inorganic compounds that make up the additives are mixed into the base oil to modify its properties.
Additives perform three main functions:
- Surface Protection
Extends the base oil’s surface-protection properties by adding extreme pressure (EP) additives, detergents etc.
- Lubricant Protection
Improves base oil properties by means of antioxidants, anti-foam agents etc.
- Performance Enhancement
Suppresses undesirable base oil properties and improves the properties by means of pour-point depressants etc
Several additives are needed in modern oils, with each additive changing a specific property or combining with other additives to achieve a desired result. Engine oil has to work reliably at temperature extremes to protect the engine components, prevent the formation of rust and wear, maintain the health of gaskets and seals, keep the engine clean etc. In order to perform this multitude of tasks, all oils contain a finely tuned package of additives. The graphic shows what is typically contained in an engine oil.
Figure 1 is a representation of how an oil is typically made up. You can see an illustration of the different components under magnification.
Figure 2 shows how additives try to attach to the metal surface and that they interact and compete with each other.
Engine oil contains groups of additives that allow it to perform the functions required of it, such as protect surfaces inside the engine, maintain the stability and life-expectancy of the oil and enhance the lubricant’s performance.
Surface Protective Additives
Here, additives are used to build up extremely thin protective layers on the sliding surfaces, the shear strength of which are much lower than that of metals. The protective layer is solid under normal conditions, but slippery under wear conditions. This prevents excessive wear (seizing or welding). When required (metal/metal contact), the layers are constantly reformed by a chemical reaction.
- Anti-wear agents: reduce friction and wear and prevent scoring and seizing.
Extreme Pressure / Anti-Wear additives are surface-active substances and may contain the elements zinc, phosphorus and sulphur in various combinations. Zinc dithiophosphate (ZDDP) is the best-known additive of this type, ZDDP not only helps to prevent wear, it also acts as an oxidation inhibitor (anti-ageing) and corrosion inhibiting agent.
- Corrosion & rust inhibitors: prevent corrosion and rusting of metal components inside the engine
Corrosion is understood as a chemical or electrochemical attack on metal surfaces. For corrosion protection, it is preferrable to use surface-active additives. The additive attaches to metal surfaces and the alkyl radical forms dense, hydrophobic (water-hostile) barriers. Corrosion inhibitors compete with EP/AW additives, i.e., they can impair their effectiveness and thus care must be taken when choosing the amount to add to the oil.
- Friction modifiers: alter the coefficient of friction
Friction modifiers only act in areas where there is mixed friction. Friction modifiers form a film on surfaces that can separate metal surfaces from each other, lowering friction in the process.
- Detergents keep surfaces free of deposits and neutralize corrosive acids.
Detergents are surface-active washing substances that counteract the formation of deposits on thermally stressed components i.e., they keep the engine clean. In addition, they form the alkaline reserve in the engine oil. The alkaline reserve neutralises harmful acidic reaction products that are caused during combustion.
- Dispersants: keep contaminants dispersed in the lubricant.
Dispersants coat solid and liquid contaminants that are introduced into the oil during engine operation and keep them finely distributed in suspension. Trapping contaminants in suspension prevents deposits forming on components inside the engine.
Lubricant Protection Additives
- Anti-foaming agents: in the engine’s moving parts, such as crankshafts, connecting rods, camshafts etc. the oil mixes with air and has a tendency to foam. Foam formation considerably impairs the lubricating properties. Poor foaming behaviour can ultimately lead to significantly higher oil temperatures, wear and engine noise.
Polysilicones (silicone polymers), polyethylene glycol ethers, etc. reduce the foaming tendency of an oil. This is achieved by basically trapping fewer gases (air and combustion gases) in the oil. Anti-foaming agents also allow trapped gases to escape from the oil more quickly.
- Antioxidants: antioxidants are additives used to prolong the life of a lubricant by increasing the resistance of the base oil to degradation by oxygen.
Lubricating oils oxidise (age) under the influence of heat and oxygen. The decomposition process is accelerated by acidic reaction products from combustion and traces of metals that have a catalytic effect (abrasive or corrosive wear). Adding antioxidants results in significantly improved ageing protection, i.e., it is slowed down considerably.
- Metal deactivator: reduces the catalytic effect of metals on the oil’s oxidation rate.
Metal deactivators prevent corrosion by passivating metal surfaces. Passivation leaves a less chemically reactive layer on the metal surface. Correspondingly, metal deactivators render the metal surface less susceptible to corrosion.
- Pour Point Depressant: enables lubricant to flow at low temperatures.
The pour point refers to the low temperature in degrees Celsius at which the oil still (just) flows. The “solidification” of an oil is determined by the crystallisation of the paraffins present in the base oil at low temperatures. Adding pour point depressants, lowers temperature at which the crystallisation of the paraffins takes place and improves the low-temperature behaviour of the oil.
- Seal Swell Agent: added to swell elastomeric seals to restore them near original dimensions and improve elasticity.
Many base oils used in premium engine oils are known to cause seal shrinkage. Additives mixed with the oils can increase this effect, causing even greater damage to the seals. Seal shrinkage and loss of elasticity degrades sealing performance. Seal swelling agents are added to oils to counteract this effect.
- Viscosity Improver: reduce the rate of viscosity change with temperature.
VI-Improvers are very long, threadlike hydrocarbon molecules which roll up in a cold state and unroll when the temperature rises. The change in shape allows the oil to achieve the desired increase or decrease in viscosity, depending on the ambient temperature.
Finally, a thought on additive consumption. You might not realise it, but the additives contained in oil also have a service life. Under operating conditions, the additives in the oil are gradually consumed. The process is known as additive depletion and is designed into the oil during formulation. Processes that take place when the engine is in use e.g., ageing, build-up of acidity, depositing of additives onto components inside the engine etc, deplete the amount of additive contained in the oil or affect the additive’s effectiveness. The consumption/depletion of additives is designed into the oil during the formulation process and is just one of the reasons why oils have to be periodically replaced.