*2.2.1.5 Defoamants*

Defoamants or antifoaming agents are additives that prevent the lubricant from forming a foam and speed up the collapse of the foam if it does form. Foaming occurs because of constant mixing of the oil with air or other gases leading to air entrapment. Foam disrupts cooling of parts as it is not a good conductor of heat. It reduces the load carrying capacity and the lubricant flow leading to excessive engine wear. Silicone

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.

### *2.2.1.6 Friction modifiers*

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 as water soluble or oil soluble lubricant additive.

### *2.2.1.7 Detergents*

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.

#### *2.2.1.8 Dispersants*

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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*Lubricant and Lubricant Additives*

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

polymeric and ashless dispersants are used today such as polymeric alkylthiophosphonates, alkylsuccinimides, succinic acid esters/amides, and their borated deriva-

Corrosion and rust inhibitors are additives that reduce or eliminate rust (corrosion of iron and steel) and corrosion by neutralizing acids and forming a protective film, either adsorbed or chemically bonded on the metal surfaces. Preferential adsorption of polar constituent on metal surface forms the protective film that prevents corrosive materials such as organic acids from reaching and attacking the metal. These are usually compounds having a high polar attraction towards metal surfaces such as succinates, alkyl earth sulfonates, metal phenolates, fatty acids, amines as well as zincdithiophosphates. Some of these inhibitors are specific to protecting certain metals. Hence, an oil may contain several types

Extreme Pressure additives are required to reduce friction, control wear and prevent severe surface damage in heavy duty application of gears and bearings at high temperatures and pressures. They are also known as antiscuffing additives. They react chemically with metallic surfaces to form a sacrificial surface film that prevents the welding and subsequent seizure of asperities at the metal-to-metal contact. Additionally, they contribute to smoothing of the surfaces as these are formed at contact asperities and the load is then distributed uniformly over a greater contact area, thus reducing the severity of wear and ensuring effective lubrication. Effectiveness of EP additives relies on their reactivity and their ability to readily form thick surface films at high loads and high contact temperatures that are created at the mechanical

tives as well as organic complexes containing nitrogen compounds.

*2.2.1.9 Corrosion and rust inhibitor*

*Adsorption of polar headgroups onto metallic surface.*

**Figure 3.**

of corrosion inhibitors.

*2.2.1.10 Extreme pressure (EP) additives*
