Fun Facts About Viscosity Index Improvers
Release time:
2021-06-17
Author:
Goncord Oil
Viscosity index improvers, as additives, enhance the viscosity–temperature characteristics of lubricating oils, enabling multigrade engine oils that deliver excellent low‑temperature start‑up performance while maintaining appropriate viscosity at elevated temperatures.
Viscosity index improvers, as additives, enhance the viscosity–temperature characteristics of lubricating oils, enabling multigrade engine oils that deliver excellent low‑temperature start‑up performance while maintaining appropriate viscosity at elevated temperatures.
Since most automotive lubricants are formulated with fuel economy and emissions in mind, it is essential to strike a balance between volatility and both low‑ and high‑temperature rheological properties. To meet the SAE multigrade oil requirements for CCS, at low temperatures it is necessary to optimize the trade‑off between low‑viscosity base oils and viscosity index improvers. The lower the base oil viscosity, the greater the VII content, the higher the CCS value, and the more significant the evaporation loss. At elevated temperatures, the selection of VII must not only satisfy the kinematic viscosity requirement at 100 °C but also comply with the SAE specification for minimum high‑temperature, high‑shear viscosity.
Viscosity index improvers from different chemical classes exert varying effects on a lubricant’s CCS low‑temperature viscosity, viscosity index, and high‑temperature high‑shear viscosity. Consequently, selecting viscosity index improvers with distinct chemistries and shear stability can significantly influence the formulation ratios of the lubricant’s components and substantially impact production costs.
Viscosity‑increasing capability is a key function of viscosity index improvers. The greater the viscosity‑increasing ability of a VI improver, the lower the required dosage and the lower the cost of multigrade oils. This capability depends primarily on the molecular weight of the VI improver, the number of —CH2— units in its backbone, and its dispersion state in the base oil. The relative viscosities‑increasing abilities of common VI improvers are as follows: ethylene–propylene copolymers > polyisobutylene > styrene–butadiene copolymers > polymethyl methacrylates; among these, the ethylene–propylene copolymer VI improver (VII) exhibits by far the strongest viscosity‑increasing performance.
Viscosity stability is an important property of gear oils during service. In particular, for wide-range viscosity grades such as 75W/90, selecting a viscosity‑index improver with excellent shear stability can significantly contribute to the reliable operation of a vehicle’s transmission system and to the effective protection of the gears.
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