ABSTRACT
The large number of natural lubricating and specialty oils sold today are produced by blending a small number of lubricating oil base stocks and additives. The lube oil base stocks are prepared from selected crude oils by distillation and special processing to meet the desired qualifications. The additives are chemicals used to give the base stocks desirable characteristics which they lack or to enhance and improve existing properties. The properties considered important are:
1. Viscosity 2. Viscosity change with temperature (vicosity index) 3. Pour point 4. Oxidation resistance 5. Flash point 6. Boiling temperature 7. Acidity (neutralization number)
Viscosity. The viscosity of a fluid is a measure of its internal resistance to flow. The higher the viscosity, the thicker the oil and the thicker the film of the oil that clings to a surface. Depending upon the service for which it is used, the oil needs to be very thin and free-flowing or thick with a high resistance to flow. From a given crude oil, the higher the boiling point range of a fraction the greater the viscosity of the fraction. Therefore the viscosity of a blending stock can be selected by the distillation boiling range of the cut. Viscosity index. The rate of change of viscosity with temperature is expressed by the viscosity index (VI) of the oil. The higher the VI, the smaller its change in viscosity for a given change in temperature. The VIs of natural oils range from negative values for oils from naphthenic crudes to about 100 for paraffinic crudes. Specially processed oils and chemical additives can have VIs of 130 and higher. Additives, such as polyisobutylenes and polymethacrylic acid esters, are frequently mixed with lube blending stocks to improve the viscosity-temperature properties of the finished oils. Motor oils must be thin enough at low temperatures to permit easy starting and viscous enough at engine operating temperatures [180 to 250°F (80 to 120°C)] to reduce friction and wear by providing a continuous liquid film between metal surfaces. Pour Point. The lowest temperature at which an oil will flow under standardized test conditions is reported in 5°F or 3°C increments as the pour point of the oil. For motor
oils, a low pour point is very important to obtain ease of starting and proper start-up lubrication on cold days.
