Viscosity Index Improvers

An increase in the Viscosity Index (VI) of an oil blend can be brought about by the incorporation of a VI Improver. CRISTOL-VI Improvers are solutions of alkyl methacrylate polymer in solvent refined hydrocarbon oils that reduce the tendency of oil to change viscosity with temperature. Hydraulic fluids, which provide effective lubrication over a broad temperature range, usually contain these VI Improvers.

The viscosity index, commonly designated VI, is an arbitrary numbering scale that indicates the changes in oil viscosity with changes in temperature. Viscosity index can be classified as follows: low VI - below 35, medium VI ?35 to 80, high VI ?80 to 110, very high VI - above 110. A high viscosity index indicates small viscosity changes with temperature. A low viscosity index indicates high viscosity changes with temperature. Therefore, a fluid that has a high viscosity index can be expected to undergo very little change in viscosity with temperature extremes and is considered to have a stable viscosity. A fluid with a low viscosity index can be expected to undergo a significant change in viscosity as the temperature fluctuates. For a given temperature range, say ?18 to 370o C (0 - 100o F), the viscosity of one oil may change considerably more than another. Oil with a VI of 95 to 100 would change less than one with a VI of 80. Knowing the viscosity index of oil is crucial when selecting a lubricant for an application, and is especially critical in extremely hot or cold climates. Failure to use oil with the proper viscosity index when temperature extremes are expected may result in poor lubrication and equipment failure.