Viscosity index, noun
An arbitrary scale for lubricating oils that indicates the extent of variation in viscosity with variation of temperature.
A person or thing that improves.
Viscosity index (VI) is a commonly used method of measuring a fluid's change of viscosity in relation to temperature. The higher the VI, the smaller the relative change in viscosity with temperature. VI improvers (also known as viscosity modifiers) are additives that increase the viscosity of the fluid throughout its useful temperature range.
This article is meant to provide you with a better understanding of viscosity index improvers, what they are, what they do, and why they are important.
Viscosity modifiers are polymeric molecules that are sensitive to temperature. At low temperatures, the molecule chain contracts and does not impact the fluid viscosity. At high temperatures, the chain relaxes and an increase in viscosity occurs.
There are two ways to explain the characteristics of these polymer chains. The first is to compare the polymers to people. When a person is cold, his natural reaction is to hold his arms close to his body to retain warmth. Now imagine a crowd of cold people, arms drawn in, moving past one another in a congested hallway. Sure there is some congestion, but the people can still move freely.
Now imagine the opposite. When a person is hot, he tends to sprawl out. Picture a person holding his arms straight out from his sides. It would be much more difficult to navigate a congested hallway full of hot people, arms extended. Consider in this example, the flow of people is related to the viscosity of the crowd.
Another way to describe this chain is in comparison to a slinky, a lazy-spring coil-shaped toy.1 Working in a way similar to the people in a hallway analogy, the slinky contracts when cold and stretches out when hot. When contracted, the molecules flow past one another easily, but when extended, they get caught on one another and impede the flow of the fluid they occupy.
Keep in mind that as temperature increases, the viscosity decreases. The addition of modifiers will only slow down the rate at which the viscosity decreases.
Viscosity improvers are primarily used in multigrade engine oils, gear oils, automatic transmission fluids, power steering fluids, greases and various hydraulic fluids. Most of these uses involve an automobile, and this is because automobiles are subjected to tremendous temperature swings.
For instance, in the crankcase, an oil with a low viscosity at low temperature is needed so the oil pump can push the oil to the top of the engine during those cold morning starts. The oil also needs to be viscous enough to protect the engine when it reaches operating temperature. This is when the use of modifiers in multigrade oil is beneficial.
Figure 1 shows how an SAE 10W30 retains the low-temperature properties of an SAE10 (providing the low-temperature pumpability), while the additive gives it the characteristics of an SAE30 at higher temperatures (providing the protection of a thicker oil film). The SAE 10W30 is made by blending a viscosity modifier with an SAE10 base oil, and there is actually no SAE30 involved.
Unfortunately, viscosity index improvers do have some drawbacks. The primary disadvantage is they are susceptible to mechanical shearing. When referring to the slinky analogy, it is easy to imagine a stretched-out slinky cut in half by mechanical processes to produce two shorter slinkys.
As the additive is repeatedly sheared, it loses its ability to act as a more viscous fluid at higher temperatures. Higher molecular weight polymers make better thickeners but tend to have less resistance to mechanical shear. Lower molecular weight polymers are more shear-resistant, but do not improve viscosity as effectively at higher temperatures and, therefore, must be used in larger quantities.