*Lubricant and Lubricant Additives DOI: http://dx.doi.org/10.5772/intechopen.93830*

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

*2.2.1.6 Friction modifiers*

*2.2.1.7 Detergents*

*2.2.1.8 Dispersants*

as water soluble or oil soluble lubricant additive.

polymers such as polymethylsiloxane at a few parts per million and organic copolymers such as alkoxy aliphatic acids, polyalkoxyamines, polyethylene glycols, and branched polyvinyl ethers, at higher concentration are widely used in mineral oils. The antifoaming agents are essentially insoluble in the lubricant hence they need to be finely dispersed in the lubricant. These droplets attach themselves to the entrapped air bubbles and aid in forming bigger bubbles (via coalesce). The larger bubbles rise readily to the surface followed by bursting to release the trapped air. Bursting occurs by thinning of the air bubble film as the additive spreads due to its low surface tension.

Friction modifiers are used in engine oil and transmission oil to alter the coefficient of friction that would be experienced between the sliding parts when only the base oil is present. Friction reduction results in improved fuel economy. Organic and sulfurised fatty acids, amines, amides, imides, high molecular weight organic phosphorus and phosphoric acid esters are added to the range between 0.1 and 1.5% in finished lubricants as friction modifiers. Glyceryl monooleates and Molybdenum compounds such as MoDTC and MoDTP also function as friction modifiers. They preferentially adsorb very strongly on to the metallic surface. The head of the friction modifier is attracted to the metal surface and the long tail with at least 10 carbon atoms remains solubilized in the oil as shown in **Figure 3** [10]. The chemical structure and the polarity of the molecules play a major role in the friction reduction. Ionic lubricants [11], a class of ionic liquids that are room-temperature molten salts consisting of cations and anions are also very good surface additives. The polarity of head group provides for strong surface adsorption. The physical, chemical and tribological properties of ionic liquids can be tailored to suit a wide variety of applications ranging from its use as polymer brushes in biological application, or

Detergents keep surfaces free of deposits and neutralize corrosive acids formed due to oxidation. These molecules are chemical bases consisting of a polar substrate and a metal oxide or hydroxide [12]. Metallo-organic compounds of calcium and magnesium phenolates, phosphates, salicylate and sulfonates are recommended. Overbased detergents are used in marine engine lubricants to neutralize large amounts of acidic components produced by fuel combustion or oil oxidation. Ash (burning of organometallic species) and soot particles (largely carbon with sulfur adsorbed) is formed by burning of the oil in internal combustion engines. Ash can then form unwanted residues at high temperatures or simply deposit on surfaces. The deposit precursors particles are insoluble in the oil and have greater affinity for detergent molecules. The additive molecules cling to the surface of the particle and envelop it thereby also acting as dispersants and prevent those particles to agglomerate and to later settle as deposits. A detergent additive is normally used in conjunction with a dispersant additive.

Dispersants are used mainly in engine oil along with detergents to keep engines surfaces clean and free of deposits [12]. Dispersants keep the insoluble soot particles and the precursors of deposits in the internal combustion engine finely dispersed or suspended in the lubricant even at high temperatures. These suspended particles are subsequently removed by oil filtration or oil change. Thus, dispersants minimize damage to engine surfaces and formation of high temperature deposits. Generally,

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**Figure 3.** *Adsorption of polar headgroups onto metallic surface.*

polymeric and ashless dispersants are used today such as polymeric alkylthiophosphonates, alkylsuccinimides, succinic acid esters/amides, and their borated derivatives as well as organic complexes containing nitrogen compounds.
