Lube Oil: Additives
Below is a list of the most commonly found additives in lubricating oil.
Detergents serve to hold the acid-neutralising compounds in solution in the oil. They are usually alkaline and react with the strong acids (sulphuric and nitric) which form during the combustion of the fuel and which would cause corrosion to the engine internals if left unchecked. Neutral detergents are also used to impart anti-corrosion, anti-wear and even extreme pressure properties to an oil. Overbased detergents are salts of alkaline earth metals such as calcium and magnesium that contain more alkaline metal than is required for their manufacture. They thus have both a good detergent property and an excellent ability to neutralise strong acids.
Dispersants keep soot and combustion products in suspension in the body of the oil charge and therefore prevent deposition as sludge or lacquer. These dispersants become depleted with time; one important reason for regular oil changes in heavily contaminated systems.
Antioxidants delay or inhibit the processes of decomposition that occur naturally in lubricants as they 'age' or oxidise in the presence of air. These oxidation processes give rise to formation of gums, lacquers and sludge resulting in an increase in acidity and viscosity. Excessive oxidation is a common reason for condemning a lubricant, usually because acidity and/or viscosity have exceeded the permissible limits. Some antioxidants also function at temperatures above about 100°C by de-activating metal surfaces. ZDTP is an example of this.
Substances that prevent foaming. Air entrapment in lubricating oil can cause oil starvation due to the presence of air bubbles at the contacting surfaces. This may lead to catastrophic failure of moving components. It is of particular importance for gear box systems where airation is often severe during normal running conditions.
Pour Point Depressants
Mineral oils, especially the higher viscosity and less refined ones, contain paraffin waxes that start crystallising at low temperatures. This process rapidly increases the viscosity of the oil and leads to faster crystallisation as the temperature decreases further. Pour point depressants prevent this rapid viscosity increase, usually by preventing agglomeration of the initial wax crystals that form.
Anti-Wear and Extreme Pressure Additives
Both types reduce wear of contacting surfaces. This is attributed to reactions on the contacting surfaces in the presence of the additive. The most common anti-wear additives are zinc and phosphorus-based, although other chemicals have been tried. Extreme pressure (EP) additives perform their task in a similar manner and are commonly made from varying proportions of chemically bound sulphur and phosphorus. They bind with exposed metal surfaces to form low rupture strength films that limit damage from micro-seizure if the oil film ruptures. Sulphur in fuel provides a similar protective function for fuel injection components.
These additives are used if the viscosity characteristic of an oil at different temperatures needs to be altered. Multigrade oils, with few exceptions, contain polymers to thicken a monograde oil of a lighter grade and give it multigrade properties, i.e. they are able to function better at much lower temperatures whilst retaining their high temperature characteristics. Some authorities stipulate the use of multigrade oils in emergency equipment due to their excellent viscosity/temperature characteristics. This type of additive is also used in some hydraulic oils to adjust the viscosity/temperature characteristics.
Included to protect vulnerable metal surfaces from atmospheric corrosion, especially when machinery is idle or during overhaul. Also include alkaline materials to neutralise strong acids as they are formed during combustion. This group of materials also provide detergency.
Characteristics of Additives AdditiveNameCharacteristicsDetergency & cleaning actionPhenaltes, Sulphonates, NaphthenatesInteracts with varnish or sludge to neutralise and solubilise.DispersancyPBI (Polyisobutylene) SuccinimidesDispersants are soluble in the oil and have a polar end which attracts and binds to contaminants preventing settling and adhesion to metal surfaces.AntifoamingSilicone Polymers (very low concentrations)Not really necessary for diesel engines in properly designed systems, but provides anti-foam in gearbox and also at the refinery during blending.Pour PointPolymethylacrylateUsed in SAE 30 grades and below to ensure point criteria are met.Anti-wear load carryingZDTP (Zincdialkyldithiophosphate)ZDDP (Zincdiethlydithiophosphate)Chemicals react with surfaces forming films which have a slower shear strength than parent metal.VI ImproversPolymers of: Methacrylate Acrylate Olefin Styrene-ButadieneIncrease in relative viscosity more at high than low temperature.Rust and corrosion inhibitionSulphates, Thiourea type chemicalsChemically absorbed onto bare metal surfaces providing protection and neutralisation.
Comment: Those indicated on this page are the ones most commonly found in industrial and marine applications. Some companies use different additives and components but generally the list is correct.
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