Key Base Stock Properties

Viscosity is a key lube oil property and is a measure of the fluidity of the oil. There are two measures of viscosity commonly used kinematic and dynamic. The kinematic viscosity is flow due to gravity and ranges from approximately 3 to 20 cSt (centistokes) for solvent neutrals and about 30-34 cSt at 100°C for Bright stock. The dynamic viscosity is flow due to applied mechanical stress and is used to measure low temperature fluidity. Brookfield viscosity for automobile transmission fluids (ATF s) at -40°C and cold cranking simulator (CCS) viscosity for engine oils at -25°C are examples of dynamic viscosity measurements. 

Lube oil volatility is a measure of oil loss due to evaporation. Noack volatility measures the actual evaporative loss which is grade dependent, and a function of molecular composition and the efficiency of the distillation step. The volatility is generally lower for higher viscosity and higher VI base stocks. The gas chromatographic distillation (GCD) can be used to measure the front end of the boiling point curve and may be used as an indication of volatility, e.g. 10% off at 375°C. 

Viscosity index or VI is based on an arbitrary scale that is used to measure the change in viscosity as a function of temperature. The scale was first developed in 1928 and was based on the best and worst known lubes at the time. The best paraffinic lube was assigned a value of VI = 100 and the worst naphthenic was assigned a VI = 0. The quality of Base stock has been improved dramatically since 1928 with the VI of high quality Base stock in the 140+ range. 

Pour point is the temperature at which the fluid ceases to pour and is nearly a solid. Typically the pour point ranges from -6 to -24°C for heavy to light neutrals. 

The cloud point is the temperature at which wax crystals first appear. 

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