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Lubricants naturally deteriorate while in use as a result of factors like oxidation, thermal degradation, and contamination.
Oxidation is far and away the most significant cause with regards to lubricant deterioration. This is not just because lubricants frequently come into contact with oxygen, but also because most lubricants include a substantial amount of air.
Lubricant additives are organic or inorganic substances dissolved or suspended in oil that offset oxidation and other degrading forces. When using oil additives, more isn't necessarily better, and sometimes performance drops off as more additive is put into the oil.
Furthermore, increasing the ratio of a particular additive may enhance one property of oil while degrading a different quality. When the concentrations of additives become out of balance, oil quality can also be affected.
The following additives have a prolonging effect on lubricant lifetime.
Anti-oxidants
As most common degenerative force on a lubricant, oxidation occurs when oxygen attacks the weakest chemical aspects of a lubricant. It takes place at all temperatures but is increased at elevated temperatures, as well as by water, wear metals and other impurities.
Temperature has the most significant impact on oxidation. Oxidation reaction rates double or triple for every 10 °C rise in temperature. Therefore, a reaction rate of unity at 70 °C will raise between two and three at 80 °C, four and nine at 90 °C and between eight and twenty-seven at 100 °C.
Due to its role in oxidation, temperature has critical implications for lubricant lifetimes. For instance, a hydraulic oil may have a functional lifetime of four years if the operating temperature averages 70 °C, but a lifetime of two years or less if the temperature averages 80 °C. It decreases further to one year if the temperature is 90 °C and six months or less if it reaches 100 °C.
Oxidation eventually causes the development of corrosive acids and sludge, which produces surface deposits and higher viscosity. Oxidation inhibitors, as they are also known, are sacrificial additives, which are consumed while stalling the onset of oxidation, and therefore protecting the base oil. Anti-oxidants can be found in nearly every lubricating oil and grease.
Viscosity Index Improvers
Viscosity index improvers are large-polymer additives designed to keep oil from losing viscosity as temperature rises. These additives are used substantially when combining multi-grade engine oils like SAE 5W-30 or SAE 15W-40.
A viscosity improver can be thought of as a spring that stays coiled at low temperatures, and it has minimal impact on the oil viscosity. If the temperature rises, the 'spring' expands, with the bigger size preventing the oil from losing too much viscosity at high temperatures.
Anti-foaming Agents
Anti-foaming additives have low interfacial tension properties, which weaken the walls of oil bubbles, causing foam bubbles to pop more easily. They reduce oxidation indirectly by lowering the degree of air-oil contact.
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How Oil Additives Get Depleted
It is essential to note that most additives get used up and depleted by decomposition forces, adsorption onto metal, contaminants, and separation as a result of settling or filtration.
The adsorption and separation processes involve mass transfer or physical activity of the additive. For a lot of additives, the longer it is in service, the less efficient the residual additive dose is in protecting the equipment. When the additive package is weakened, viscosity rises, sludge starts to develop, corrosive acids begin to attack metal surfaces and wear starts to increase. If low-quality lubricants are used, the point in time at which these difficulties start will happen much sooner.
For these reasons, it is often worth investing in top-quality lubricants that meet the industry specifications, such as API engine service classifications.
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