Viscosity Index Improver (VII) is a commonly used additive in lubricating oils, used to improve the viscosity characteristics of lubricating oils. The main purpose of this additive is to maintain a stable viscosity of the lubricant in an environment with changing temperatures, thus improving the performance and service life of the lubricant.
Viscosity index improvers are primarily made of high molecular polymers whose solubility in lubricating oils changes with temperature. The higher molecular weight of these polymers can form a three-dimensional network structure in the lubricating oil, thereby increasing the viscosity of the lubricating oil. As the temperature increases, the solubility of the polymer increases, the network structure becomes looser, and the viscosity of the lubricating oil decreases; while as the temperature decreases, the solubility of the polymer decreases, and the network structure becomes tighter , the viscosity of the lubricating oil increases. This property allows viscosity index improvers to maintain stable viscosity of lubricants at different temperatures.
Viscosity index (VI) improvers are typically long-chain polymer molecules added to motor oil to reduce the change in oil viscosity across a wide range of temperatures. Here’s a look at the common materials used:
1. Polymethacrylates (PMA):
- These are synthetic polymers known for their excellent thickening ability at low temperatures and good shear stability, meaning they maintain their viscosity-modifying properties under high shearing forces within the engine.
- PMA VI improvers are commonly used in high-quality engine oils designed for extended drain intervals and demanding operating conditions.
2. Polyisobutylene (PIB):
- This synthetic rubber-like polymer offers good viscosity thickening at low temperatures and decent shear stability.
- PIB VI improvers are often used in cost-effective motor oils or blended with other VI improvers to achieve desired performance characteristics.
3. Olefin copolymers (OCP):
- These are synthetic polymers formed by combining different types of building blocks (monomers). They offer a wide range of performance characteristics, depending on the specific composition.
- OCP VI improvers can be tailored to provide excellent low-temperature thickening and high shear stability, making them suitable for demanding applications and high-performance engine oils.
4. Radial polyisoprene:
- This synthetic polymer derived from isoprene offers good low-temperature thickening and shear stability.
- It’s sometimes used in specific VI improver packages to complement other components and achieve desired thickening behavior.
5.Ethylene-propylene copolymers (EPM/EPDM):
- These elastomeric polymers are sometimes used as viscosity index improvers, especially in applications where high-temperature stability is required.
6.Hydrogenated polymers:
- Certain polymers undergo hydrogenation to enhance their thermal and oxidative stability, making them suitable for use as viscosity index improvers in high-performance lubricants.
Selection and Use:
The type and amount of VI improver used in a motor oil are carefully chosen by the oil manufacturer based on several factors, including:
- Desired viscosity characteristics: The oil needs to be thin enough for easy cranking at low temperatures but thick enough to provide adequate lubrication at high temperatures.
- Base oil type: Different base oil types (synthetic, conventional, etc.) may require different VI improvers for optimal compatibility and performance.
- Target application: The oil may be formulated for specific engine types, driving conditions, or drain intervals, influencing the VI improver selection.
Important Note:
- Overuse of VI improvers can be detrimental. Excessive thickening can lead to increased oil drag, reduced fuel efficiency, and potential engine wear.
- Always use high-quality engine oil recommended by your vehicle manufacturer and avoid adding any unauthorized additives to your engine oil.
By understanding the materials used in VI improvers, you gain insights into their functionality and importance in maintaining optimal engine performance across varying temperatures.
Generally speaking, viscosity index improvers are made of high molecular polymers that adjust the viscosity of the lubricating oil by changing their solubility in the lubricating oil, thereby maintaining stable performance at different temperatures. The use of this additive is of great significance in improving the performance and service life of lubricants.