# Molecular Science of Lubricant Additives +
## 1 Introduction
### Components in Liquid Lubricants  
So-called lubricant formulation is a process of mixing substances to prepare a lubricant. The substances should be properly selected to meet the requirements of machine operation. The components in liquid lubricants are classified in two groups--base fluids and additives. Base oils are the major contents of lubricating fluids. It has been empirically known from ancient times that viscous fluids provide lubrication performances. It was found, about a hundred years ago, that certain substances can improve lubrication performances if dissolved in base fluids [26]. Then many engineering knowledge was accumulated and additive technology for lubricant became a common technique later on. Today, lubricating fluids are manufactured from base oils and 5-20 mass% of different additives depending on the practical requirement (Graphic abstract). Base fluids are viscous liquids having proper viscosity for the application. API defines the category of base oil, as summarized in Table 1. API Groups I-III are made from crude oil through distillation and refinery processes [27]. They are so-called mineral oils and they constitute the majority of base fluids today because of their availability at a reasonable cost. In brief, mineral oils are composed of various hydrocarbons (normal and branched alkanes, cycloalkanes, aromatics, heterocyclic compounds, etc.). It seems difficult to show the chemical contents in mineral oils, because they vary with production area and refinery processes. Therefore, viscosity represents the properties of base oil. Conventionally, kinematic viscosity at 40 \({}^{\circ}\)C and at 100 \({}^{\circ}\)C is considered for lubricants. The refinery processes have two major purposes; they are purification and modification of molecular structure. Typically, organic sulfides and unsaturated hydrocarbons were extracted by solvents to produce Group I, or decomposed by a catalytic hydrogenation to produce Group II. The importance of these processes for additive technology is that these impurities often show antagonism to many types of lubricant additives. On the other hand, a catalytic isomerization of hydrocarbon molecules produces Group III oils. The differences between Groups I-II and Group III oils are their rheological properties. Here we introduce the key properties of base fluids; viscosity index: dependency of viscosity by temperature. Viscosity of hydrocarbons drops as temperature increases. Viscosity index (_VI_) represents the rate of changes by temperature [28]. \"High\" viscosity index is defined as fewer changes in viscosity by temperature change, as illustrated in Figure 3. Both fluid A and B pose similar viscosity at 40 \({}^{\circ}\)C, while those at 100 \({}^{\circ}\)C are different. Needless to say, higher _VI_ is desirable because many lubrication systems generate friction heat during machine operations. When considerable viscosity drop happens during machine operation, it causes a shift of the lubrication regime to boundary (see Figure 1). As a consequence, it raises the risk of high friction and wear. We also discuss this issue in Section 2.2.1. _VI_ could be calculated according to ASTM D2270, for those fluids of _VI_ < 100. It is recommended to use software on computers, since currently base fluids with _VI_ > 100 are commonly used [29]. Besides mineral oils being the major resources of base fluids, natural triglycerides (TG, also known as vegetable oils, plant oils, or animal tallow) were the only resources of lubricants before the petroleum age. Although they were used before petroleum products, TGs are currently classified in Group V because of the fewer demands compared to mineral oils. Technically, those additives for mineral oils do not always work well for TGs. One reason for this is that additives are manufactured by petroleum industries. Therefore, industries are interested in improving the performances of petroleum products. The relevant scientific discussions are given in Section 4.4.