Functions and Disadvantages of Viscosity Index Improvers

Function of viscosity index improver:

At low temperatures, lubricating oil should have good fluidity, so the viscosity should not be too high to prevent the lubricating oil from not flowing fully to the lubricating parts and causing dry friction. Dry friction during mechanical cold start is an important cause of machine wear. When the machine gradually warms up after startup, the fluidity of the lubricating oil is good, but if it becomes too thin, the thickness of the oil film formed at the lubricating parts is not enough, which will lead to poor lubrication, increased friction and wear. Therefore, good viscosity-temperature properties of lubricating oil (high viscosity index) help provide better lubrication protection and reduce the possibility of wear.

Lubricating oil is formulated by adding various additives to base oil. Additives can improve the performance of base oil. Viscosity index improver (VI improver) is an additive that improves the viscosity index of lubricating oil. In some places, it is also called a thickener. The viscosity index of lubricating oil is related to the type of base oil. Some base oils have a higher viscosity index (such as PAO, Class III oil, etc.) before using viscosity index improvers. Some base oils have a lower viscosity index and need to add more viscosity index improvers.

Disadvantages of viscosity index additives:

Viscosity additives are easily cut off due to shearing, which reduces the ability to improve viscosity. Shearing not only includes shearing during the movement of mechanical parts, but also high-temperature shearing. Under high temperature conditions, stretched molecules are more easily sheared by machinery. Therefore, attention should be paid to the thermal shear stability of viscosity index improvers. Generally speaking, polymers with large relative molecular weights have better viscosity-enhancing effects, but poor shear resistance. Polymers with small relative molecular weights are more resistant to shearing, but the viscosity-enhancing effect is limited, so the amount used is relatively large. Some lubricants experience a sudden drop in viscosity during use because the viscosity index improver molecules are sheared and lose their viscosity-enhancing effect. Lubricants differ in performance and stability due to the viscosity index of the base oil itself and the quality of the additives used.