*2.2.1.2 Viscosity index improvers*

Viscosity index improvers (VII) also known as viscosity modifiers are additives that prevent the oil from losing its viscosity at high temperatures which is a natural tendency of any liquid. These additives are available in all shapes and sizes and quality [8]. Polymethylmethacrylates, olefin copolymers, hydrogenated poly(styrene-co-butadiene or isoprene), esterified polystyrene-co-maleic anhydride are commonly used VIIs. The large oil soluble flexible polymer molecules uncoil and spread out as temperature increases thereby increasing the viscosity as shown in **Figure 2**. Furthermore, their numerous branches entangle with those of other neighboring molecules. By doing this, these macromolecular structures can trap and control smaller oil molecules, thus increasing the viscosity of the lubricant.

Permanent and temporary shear thinning of VII-thickened formulations can also occur depending upon the quality of the VII. In heavy duty application, due to the large compressive pressure between the two mating surfaces, VII polymer molecules, tend to align with each other and get "squashed" or even get chopped to small pieces under high shear conditions. When the polymer coils elongate and become aligned in the direction of the flow the viscosity temporarily drops resulting in reduced oil film thickness. After the lubricant leaves the contact between

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*Lubricant and Lubricant Additives*

*2.2.1.3 Anti-wear agent*

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

alternatives are currently very expensive.

*2.2.1.4 Antioxidants*

*2.2.1.5 Defoamants*

the mating parts, the polymer coils return to their original shape and the viscosity of the lubricant returns to normal. This phenomenon is referred to as temporary shear-thinning. However, under further high shear rates, the long and flexible polymer chains can be cut or ruptured or pulled and ripped apart into smaller chains by molecular scission. Unfortunately, once this has occurred, the broken polymer chains cannot re-form into the single large chain and this causes the oil to permanently lose viscosity leading to a reduction in oil film thickness, oil film failure and an increase in wear. This phenomenon is referred to as permanent shear-thinning.

Anti-wear additives are additives that prevent two-body wear of the metallic countersurfaces in the boundary lubrication regime where the film thickness is small and there is asperity - asperity contact. These additives are polar in nature which enables them to attach to the metallic surfaces followed by tribochemical or mechanochemical reactions to form an anti-wear film. This newly formed film undergoes wear and formation at the top layers thus protecting the underlying metallic surface. As these additives form films by chemical reactions, they get used up and the amount of antiwear additives present in the lubricant reduces with time. These are typically phosphorous compounds. Zinc dialkyldithiophosphate (ZDDP) is the most common, the most researched and has been used since the 1940s [9]. Its use has been reduced in passenger vehicles in the last decade due to zinc metal causing poisoning of the catalyst in the exhaust gas catalytic convertor. ZDDP also provide antioxidant and corrosion-inhibition properties to the lubricant. Owing to the multi functionality of ZDDP, finding its replacement has been challenging because molybdenum-based additive such MoDTC (molybdenum dithiocarbamate) or MoDDP (molybdenum dithiophosphate) molecules cannot work as antioxidant. On the other hand, ash-less antiwear additives such as hindered phenols and amines are very expensive and are required in larger quantities. Till date, ZDDP is considered as the most cost-effective antioxidant and antiwear additive available, and the

Antioxidants or oxidation inhibitors prevent the oxidation of the components of the base oil there by increasing the life of the lubricant. Oxidation of the lubricant molecules occur at all temperatures but at higher temperatures, it is accelerated. The presence of wear particles, water, and other contaminants also promote Oxidation of the lubricant molecules which then leads to formation of acids and sludge. The acids may further cause corrosion in the metallic parts while the sludge formation increases the viscosity of the lubricant. Almost every lubricating oil and grease contains antioxidants and examples include Zincdialkyldithiophosphates, hindered phenols, sulphurized phenols, and aromatic amines. These compounds decompose peroxides and terminate free-radical reactions that occur in the lubricant. These are

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

sacrificial in nature hence their quantity gets reduced with time.

**Figure 2.** *Mechanism of VII.*

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

the mating parts, the polymer coils return to their original shape and the viscosity of the lubricant returns to normal. This phenomenon is referred to as temporary shear-thinning. However, under further high shear rates, the long and flexible polymer chains can be cut or ruptured or pulled and ripped apart into smaller chains by molecular scission. Unfortunately, once this has occurred, the broken polymer chains cannot re-form into the single large chain and this causes the oil to permanently lose viscosity leading to a reduction in oil film thickness, oil film failure and an increase in wear. This phenomenon is referred to as permanent shear-thinning.
