**1. Introduction**

Lubricants have been in use for hundreds of centuries and are essential to our survival. Natural lubricants such as saliva and synovial fluid lubricate the food for easy mastication and reduce wear and tear of our joints respectively. Cooking oils prevent sticking of food onto frying pans and baking trays at the same time as conducting heat. Ancient Egyptians used lubricants to slide large stone blocks for building the great pyramids while the Romans used lubricant on the axles of their chariots [1]. Ancient lubricants were plant and animal based natural oils. With the onset of industrial revolution and our reliance on metal-based machinery and engines, petroleum-based lubricants witnessed a growth.

Modern lubricants are far more complex and perform various other functions in addition to lubricating such as cleaning, cooling, and sealing. The primary function of most lubricants is to reduce friction and this property is known as lubricity. A lubricant can be used in solid form, semi-solid, liquid form or gaseous form. Examples of solid lubricants are graphite and Molybdenum disulphide (MoS2), semi-solid lubricants are greases, and liquid are automobile engine oil. Depending on the requirements of a said application, the physical state of lubricant is chosen. For example, in space environments where liquid lubrication is not feasible due to

vacuum, solid lubricants are chosen. Air bearing are preferred in applications in machine tool applications where precision is of primary importance such as cutting and finishing of optical lenses. Greases are used where a liquid oil would not remain in position due to its tendency to flow or when a sealing action is needed to prevent water-ingress in addition to lubrication. Today's lubricants are designed and packaged to meet specific requirements for specific applications by lubricant formulators. The lubricant for automobile transmission and drive train has different requirements to satisfy compared to lubricant for an internal combustion engine or turbines. Further depending upon the type of turbines viz. gas, steam or hydraulic, the lubricant needs to be designed.

### **2. Lubricant composition**

Typically, industrial lubricants contain 70-90% base oils and the rest is additives [2]. Base oils impart primary vital properties of the lubricant such as viscosity, viscosity stability, thermal stability, solvency, low temperature flow and volatility, oxidation stability. Additives have been used in lubricating oil since the 1920s and the demand for lubrication has resulted in continuous growth in the size of the market (USD 14.35 billion in 2015) with huge investments in research and development to design and formulate superior lubricants that meets present and future environmental regulations and consumer expectations. Despite this, lubricant formulation has mostly remained an art. This is because blending a new formulation for optimizing viscosity and obtaining optimum performance through performance tests for friction and emissions is much easier than testing for parameters, such as impact on engine wear, sludge build-up and piston cleanliness which require long duration engine tests. For example, during the development of Castrol's engine oil, 'Edge with Titanium Fluid Strength Technology', over 2400 unique formulations were engine-tested for an equivalent of 1.9 million miles.

To understand the need for additives, one must understand the implication of the Stribeck curve shown in **Figure 1**. Machine elements such as engine bearings work in hydrodynamic lubrication regime where the major function of the lubricant is to maintain its viscosity at all temperatures while ensuring a thick fluid film to keep the two contacting surfaces in relative motion separated at all loads and speeds. Rolling element bearings work on elasto-hydrodynamic lubrication (EHL) where the contacting surfaces deform elastically and there is a very thin film separation. Cams and tappets work in the boundary lubrication regime where there is substantial metal to metal contact. And in the reciprocating motion of engine piston rings, all four kinds of lubrication regime occur.

Classical lubrication theory assumes that a lubricating oil is a Newtonian fluid with a fixed viscosity and the contacting surfaces to be rigid. George Osborne Reynolds approached the fluid film hydrodynamic lubrication using mathematical and physical approach (1886) to predict friction, film thickness and load carrying capacity [3]. In the real world, oils undergo shear thinning and behave as a non-Newtonian fluid due to heat and pressure developed at the contact. Furthermore, real surfaces are rough and can undergo elastic as well as local plastic deformation under fluid pressure. The EHL theory was developed by H.M. Martin (in 1916) and later by Ertel (in 1939) and Grubin (in 1949), Petrusevich (in 1951), Dowson and Higginson (in 1959), Dowson and Hamrock (in 1977) for predicting traction, load carrying capacity and film thickness in heavily loaded contacts [4]. The non-Newtonian behavior of thin films under high pressure and lower rolling speeds has guided lubricant formulators to consider the shear-stress/shear-strain behavior, pressure-viscosity dependence of the lubricant as these are closely linked to the molecular properties of the oil composition. On the

**171**

**2.1 Base oil**

**Figure 1.** *Stribeck curve.*

*Lubricant and Lubricant Additives*

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

Stribeck curve, when the speed is very low, the friction is governed by the chemistry of the lubricant molecules i.e. the molecular structure and orientation. It was W. B. Hardy (in 1920), who coined the term boundary lubrication and later with Ida Doubleday (in 1922) established the basic concepts of boundary lubrication theory [5]. The boundary lubrication properties such as friction and film thickness at the interface of two surfaces are affected by the force fields of molecules in relation to their structure and polarity. Hence, formulators add boundary film additives to reduce friction in this regime [6]. Additives must also reduce wear, as in this regime, the two surfaces in relative motion are in contact hence prone to significant wear. Furthermore, additives need to counter the side effects of continuous as well as intermittent use of the lubricant such as they need to control heat, deposit formation, prevent foam formation, prevent fouling and corrosion due to water ingress, prevent wear, increase film strength under concentrated contacts, control greenhouse gas emissions. Therefore, as high as 30% content of modern automotive lubricants are chemical additives

A typical petroleum-based oil with no additive is called a base oil or base stock [7]. The generation of base stock starts with the identification and selection of a good petroleum crude followed by atmospheric distillation, vacuum distillation and solvent processing. Solvent processing has two processes viz. solvent extraction, and solvent dewaxing where undesirable molecules are separated where as in hydroprocessing undesirable molecules are converted to desirable ones. Hydroprocessing a general term for conversion of less desirable crude fractions into good quality feedstock using catalyst and hydrogen at high temperature and pressure. This can be categorized as hydrofinishing, hydrotreating and hydrocracking according to increasing order of severity. Another process called hydrodewaxing is required to remove long chain linear paraffins, isomerize straight chains to branched chains which improves the pour point. Only 10 % of crudes are converted to base stocks for lubricants, hence the refinery owners call the shots regarding the choice of crudes to be converted to base oils while balancing the cost, yield and demand relative to all the other refinery products. However, to satisfy lubricant performance demands under severe operating conditions, high quality base stocks are needed. The lubricant manufactures (refinery owner can also be lube manufacturer) buy these base stocks and other chemical compounds and formulate their lubricant for example SAE15W40 or ILSAC GF-5, meeting standards set by Original Equipment Manufacturers (OEMs),

while some industrial oils may only contain 1% or less.

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

**Figure 1.** *Stribeck curve.*

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

were engine-tested for an equivalent of 1.9 million miles.

the lubricant needs to be designed.

**2. Lubricant composition**

of lubrication regime occur.

vacuum, solid lubricants are chosen. Air bearing are preferred in applications in machine tool applications where precision is of primary importance such as cutting and finishing of optical lenses. Greases are used where a liquid oil would not remain in position due to its tendency to flow or when a sealing action is needed to prevent water-ingress in addition to lubrication. Today's lubricants are designed and packaged to meet specific requirements for specific applications by lubricant formulators. The lubricant for automobile transmission and drive train has different requirements to satisfy compared to lubricant for an internal combustion engine or turbines. Further depending upon the type of turbines viz. gas, steam or hydraulic,

Typically, industrial lubricants contain 70-90% base oils and the rest is additives [2]. Base oils impart primary vital properties of the lubricant such as viscosity, viscosity stability, thermal stability, solvency, low temperature flow and volatility, oxidation stability. Additives have been used in lubricating oil since the 1920s and the demand for lubrication has resulted in continuous growth in the size of the market (USD 14.35 billion in 2015) with huge investments in research and development to design and formulate superior lubricants that meets present and future environmental regulations and consumer expectations. Despite this, lubricant formulation has mostly remained an art. This is because blending a new formulation for optimizing viscosity and obtaining optimum performance through performance tests for friction and emissions is much easier than testing for parameters, such as impact on engine wear, sludge build-up and piston cleanliness which require long duration engine tests. For example, during the development of Castrol's engine oil, 'Edge with Titanium Fluid Strength Technology', over 2400 unique formulations

To understand the need for additives, one must understand the implication of the Stribeck curve shown in **Figure 1**. Machine elements such as engine bearings work in hydrodynamic lubrication regime where the major function of the lubricant is to maintain its viscosity at all temperatures while ensuring a thick fluid film to keep the two contacting surfaces in relative motion separated at all loads and speeds. Rolling element bearings work on elasto-hydrodynamic lubrication (EHL) where the contacting surfaces deform elastically and there is a very thin film separation. Cams and tappets work in the boundary lubrication regime where there is substantial metal to metal contact. And in the reciprocating motion of engine piston rings, all four kinds

Classical lubrication theory assumes that a lubricating oil is a Newtonian fluid with

a fixed viscosity and the contacting surfaces to be rigid. George Osborne Reynolds approached the fluid film hydrodynamic lubrication using mathematical and physical approach (1886) to predict friction, film thickness and load carrying capacity [3]. In the real world, oils undergo shear thinning and behave as a non-Newtonian fluid due to heat and pressure developed at the contact. Furthermore, real surfaces are rough and can undergo elastic as well as local plastic deformation under fluid pressure. The EHL theory was developed by H.M. Martin (in 1916) and later by Ertel (in 1939) and Grubin (in 1949), Petrusevich (in 1951), Dowson and Higginson (in 1959), Dowson and Hamrock (in 1977) for predicting traction, load carrying capacity and film thickness in heavily loaded contacts [4]. The non-Newtonian behavior of thin films under high pressure and lower rolling speeds has guided lubricant formulators to consider the shear-stress/shear-strain behavior, pressure-viscosity dependence of the lubricant as these are closely linked to the molecular properties of the oil composition. On the

**170**

Stribeck curve, when the speed is very low, the friction is governed by the chemistry of the lubricant molecules i.e. the molecular structure and orientation. It was W. B. Hardy (in 1920), who coined the term boundary lubrication and later with Ida Doubleday (in 1922) established the basic concepts of boundary lubrication theory [5]. The boundary lubrication properties such as friction and film thickness at the interface of two surfaces are affected by the force fields of molecules in relation to their structure and polarity. Hence, formulators add boundary film additives to reduce friction in this regime [6]. Additives must also reduce wear, as in this regime, the two surfaces in relative motion are in contact hence prone to significant wear. Furthermore, additives need to counter the side effects of continuous as well as intermittent use of the lubricant such as they need to control heat, deposit formation, prevent foam formation, prevent fouling and corrosion due to water ingress, prevent wear, increase film strength under concentrated contacts, control greenhouse gas emissions. Therefore, as high as 30% content of modern automotive lubricants are chemical additives while some industrial oils may only contain 1% or less.

### **2.1 Base oil**

A typical petroleum-based oil with no additive is called a base oil or base stock [7]. The generation of base stock starts with the identification and selection of a good petroleum crude followed by atmospheric distillation, vacuum distillation and solvent processing. Solvent processing has two processes viz. solvent extraction, and solvent dewaxing where undesirable molecules are separated where as in hydroprocessing undesirable molecules are converted to desirable ones. Hydroprocessing a general term for conversion of less desirable crude fractions into good quality feedstock using catalyst and hydrogen at high temperature and pressure. This can be categorized as hydrofinishing, hydrotreating and hydrocracking according to increasing order of severity. Another process called hydrodewaxing is required to remove long chain linear paraffins, isomerize straight chains to branched chains which improves the pour point. Only 10 % of crudes are converted to base stocks for lubricants, hence the refinery owners call the shots regarding the choice of crudes to be converted to base oils while balancing the cost, yield and demand relative to all the other refinery products. However, to satisfy lubricant performance demands under severe operating conditions, high quality base stocks are needed. The lubricant manufactures (refinery owner can also be lube manufacturer) buy these base stocks and other chemical compounds and formulate their lubricant for example SAE15W40 or ILSAC GF-5, meeting standards set by Original Equipment Manufacturers (OEMs),

professional bodies, and international institutions like American Petroleum Institute (API), International Lubricant Standardization and Approval Committee (ILSAC), Society of Automotive Engineers (SAE) to name a few.

Owing their origin to petroleum crudes, lubricant base stocks are also mixtures of long chain hydrocarbons containing three types of chemical groups. i.e. paraffins, naphthenes and aromatics. The paraffins can be further classified into branched or straight chains. The chain length and branching affects the melting point and crystallization temperature of the paraffins. During the production of lube base stock, most of the unsaturated bonds, paraffin wax and sulfur content are removed, however depending on the severity of hydroprocessing, some wax, unsaturates and sulfur may remain. Base Stocks have been classified into 5 categories by the API according to the presence of saturates, sulfur content and viscosity as shown in **Table 1**. Group I, II and III are derived from petroleum crude while Group IV is reserved for Polyalphaolefins (PAO) which are synthesized from gaseous hydrocarbons. Group V is for all other base stocks that are not included in other four groups such as mineral based napthenics, synthetic esters, polyglycols, silicones, polybutenes, phosphate esters etc. These oils are designed for severe performance requirements.
