The viscosity of lubricating oil decreases with the increase of temperature, and the Viscosity Index (VI) is an indicator to measure the degree of viscosity change with the temperature. The higher the VI of the oil, the smaller the degree of viscosity change with temperature. Generally, the viscosity index of mineral oil (paraffin base) is usually 96-120, to prepare Multigrade Internal Combustion Engine (ICE or IC engine) Oil with excellent Viscosity – temperature Characteristics and other high Viscosity Index Industrial Lubricants, it is necessary to add Viscosity Index Improver (VII) or use synthetic base oil.

Viscosity Index Improvers VII in Lubricant

Names of Viscosity Control Additives - How People Call This Chemical

Viscosity control additives are also known by several other names, including:

Viscosity Modifiers (VM)
Viscosity Index Improvers (VII)
Viscosity Improvers
Thickening Agents
Thickening Additives
Viscosity Stabilizers
Viscosity Enhancers

These terms are often used interchangeably to refer to additives that help to control the viscosity of lubricants under different operating conditions.

Chief History of Viscosity Improver

Earlier in the 1930s High Molecular Compound was used for hydraulic oil and cannon gear oil to improve the viscosity-temperature performance.
Polyisobutylene (PIB) is one of the first VIIs to be developed, In the 1930s, researchers at Standard Oil Company (now ExxonMobil) developed a process to produce high-molecular-weight PIB, which had better VI-improving properties.
Polymethacrylate, (PMA) was first developed in the 1950s as a synthetic rubber substitute. In the 1970s, researchers began investigating the use of PMAs as VIIs in lubricants. They found that PMAs had excellent shear stability, good low-temperature performance, and resistance to oxidation.
Ethylene Propylene Copolymer (EPC) or Olefin Copolymers (OCP) were first developed in the 1960s as a replacement for polyisobutylene (PIB), which was the dominant VII at the time. OCPs are copolymers of alpha-olefins and other monomers, such as ethylene and propylene. In the 1970s and 1980s, OCP became widely used in the production of engine oils and other lubricants. They were particularly popular in North America, where they were used to improve low-temperature performance.
20th century 70s: Shell Chemical developed Styrene-isoprene Polymers, which are known for their good shear stability and low-temperature performance, making them widely used in viscosity index improvers for automotive engine oils.

These polymer compounds are called Thickening Additives, or Viscosity Index Improvers.

e.g. The addition of PMA to mineral oil can effectively enhance its viscosity, particularly at high temperatures. This compound is commonly referred to as a Viscosity Improver due to its ability to influence the Viscosity-Temperature Performance of the oil. Specifically, it enhances the oil's viscosity index, which is why it's also known as a Viscosity Index Improver.

The market share for Viscosity Index Improvers is on the rise due to the rapid development of multi-grade lubricants. Multi-grade motor oils require an improved viscosity index to maintain their performance across a wide range of temperatures. As a result, viscosity Index Improvers have become essential in the lubrication industry. According to statistics, these VII make up around 22.5% of the total consumption of lubricant additives.

Chorus Lubricant Additives offers a range of viscosity thickeners including: OCP, MPA, HSD, PIB, please contact us to know more.

Why Viscosity Index Improvers are So Important? - The Value of Viscosity Index Improvers (VIIs) in Lubricants.

Improve the Viscosity-temperature Performance of Lubricants

One of the key reasons why Viscosity Index Improvers (VII) additives are highly valued is that they improve the viscosity-temperature performance of lubricants. Lubricants with VII additives, such as engine oils, gear oils, and hydraulic oils, exhibit good low-temperature performance and high-temperature lubrication, which is essential for ensuring smooth operation and protection of machinery. Additionally, lubricants with VII additives can meet the requirements of multiple viscosity grades, making them versatile and suitable for use in different applications and conditions throughout the year.

Reduce Energy Consumption, Minimize wear, and Reduce the Friction in Machinery

Viscosity index improvers play a crucial role in reducing energy consumption, minimizing wear, and lowering friction in machinery. Compared to single-grade lubricants, multi-grade lubricants formulated with viscosity index improvers consume less lubricant and fuel oil, resulting in significantly reduced mechanical wear. Multi-grade lubricants have smoother viscosity-temperature performance, with smaller viscosity variations over temperature changes than single-grade lubricants. This ensures adequate lubrication of moving parts at high temperatures, reducing wear and tear. At low temperatures, multi-grade lubricants have a lower viscosity than single-grade oils, making it easier to start the engine and saving energy. In comparison to single-grade oils of the same viscosity level, such as SAE 10W/30 versus SAE 30, multi-grade oils can save up to 2% to 3% on fuel consumption.

Simplify Lubricant Products

Viscosity Index Improvers play a critical role in simplifying lubricant products. For instance, universal tractor lubricants can serve as engine oil, gear oil, transmission fluid, and brake oil all at once, thanks to the use of viscosity index improvers. This simplifies the inventory of lubricant products needed, reducing the costs associated with storing and managing multiple lubricants. In addition, the use of universal lubricants can lead to less confusion and error in lubricant selection and application, ultimately contributing to improved equipment performance and reliability. The ability to simplify Lubricant products through the use of viscosity index improvers has made them a valuable component in the lubricant industry.

Increases the production of high-viscosity lubricating oils

Due to the limited availability of high-viscosity base oil resources, viscosity index improvers have become essential ingredients in lubricating oils. By adding viscosity index improvers to low-viscosity base oils, the resulting oil can replace high-viscosity base oils, which increases the production of high-viscosity lubricating oils and makes more rational use of resources.

In engine oils, viscosity index improvers are crucial in creating multi-grade engine oils that are less reliant on temperature. In fact, viscosity index improvers are a vital component of engine oils according to the ILSAC GF-5 passenger car engine oil specification. Multi-grade engine oils formulated with viscosity index improvers can maintain viscosity at high temperatures and high shear conditions while also ensuring lubricating oils can be pumped at low temperatures.

In powertrain fluids such as gear oils and automatic transmission fluids, viscosity index improvers serve to minimize changes in viscosity across the widest possible operating temperature range while maintaining good shear stability. The most commonly used viscosity index improver is in multi-grade engine oils, which account for approximately 60% of all lubricating oils. The amount of viscosity index improver used in engine oils can reach up to 15% (mass fraction). Among various types of additives, viscosity index improvers are used in the largest amounts, accounting for around 23% of all additive sales.

Currently, energy-saving lubricating oils with better fuel economy are multi-grade engine oils combined from low-viscosity base oils with friction modifiers, viscosity index improvers, and pour point depressants. Viscosity index improvers can reduce friction loss in mixed and hydrodynamic lubrication zones, while friction modifiers can reduce friction loss in the boundary and mixed lubrication zones. With the right formulation, viscosity index improvers and friction modifiers can work together to reduce friction and improve the energy efficiency of lubricating oils.

Performance of Viscosity Index Improver

1. Shear Stability

Shear Stability is the ability of a polymer to resist shear stress, which is a crucial performance of viscosity index improvers. When a viscosity index improver has poor shear stability, the main chain of the polymer will break under the action of shear stress, leading to a drop in viscosity. As a result, the blended lubricating oil cannot maintain its original viscosity grade, which causes increased wear and fuel consumption. For more, please click Shear Stability & Viscosity Index Improver in Lubricant.

2. Thickening Ability

Thickening Ability is a very important performance of viscosity index improver. The greater the thickening ability of the viscosity index improver, the smaller the dosage, and the lower the cost of the multigrade Lubricant.
The thickening ability of the polymer mainly depends on the relative molecular mass of the viscosity index improver, the carbon number of the main chain on the molecule (-[-CH2-]-), and the form in the base oil.
The order of thickening ability of commercially available viscosity index improvers is as follows:

HSD ≈ OCP > PIB > PMA

3. Thermal/Oxidative Stability

Thermal/Oxidative Stability is another important evaluation index of VIIs. Viscosity index improvers are subject to high-temperature oxidation and thermo-oxidative decomposition in actual use, and the decomposition will lead to a series of problems such as viscosity decrease, acid value increase, and ring groove carbon deposit increase. High molecular polymers generally do not undergo obvious thermo-oxidative decomposition below 60°C, and begin to thermo-oxidatively decompose at 100-200°C. The thermo-oxidative stability of the polymer is related to the structure of VII.

The order of oxidation stability of commercially available VII is:

PMA > PIB > OCP ≈ HSD

4. Low-Temperature Performance

The low-temperature viscosity of motor oils is a crucial rheological characteristic. To enable the vehicle to start in cold weather, the viscosity of the motor oils in the bearings must be below a critical value. This value is determined through low-temperature engine start ability experiments and is defined within SAE J300 for all "W" grades.

The viscosity Index Improver has an important influence on the low-temperature performance of multi-grade motor oils, and there are two critical values indicating the low-temperature performance index of multi-grade motor oils: low-temperature start-up and low-temperature pumpability.

Low-Temperature Start Ability

There are many factors that affect the low-temperature start-ability, one of the most important indicators is the low-temperature viscosity, the smaller the low-temperature viscosity, the easier it is to start.
Generally, Cold Cranking Simulator (CCS) is used to measure the apparent viscosity of multi-grade oil at low temperatures. A cold start simulator is a rheometer operated at a high shear rate at a fixed ambient temperature to simulate the flow of lubricating oil into the engine bearings at start-up. After the engine is started, the oil must also be able to flow freely into the oil pump and be distributed to the various oil lines of the engine.
The low-temperature properties of different VIIs are quite different, and PMA shows lower viscosity in a wide range of shear rates, so PMA has the best CCS performance. The molecular chain of PIB is relatively rigid because it has many methyl side chains, and its viscosity increases rapidly at low temperatures, so the low-temperature performance of PIB is the worst.

Low-Temperature Pump-ability of Motor Oil

When the engine is started at a low temperature, the oil pressure of the lubricating oil system must be normalized in a short time to ensure that all parts of the engine are lubricated in time, otherwise it will cause wear and tear. The ability of engine oil to be pumped to various parts of the engine is called pumping capacity.

The pumpability of engine oil depends on the apparent viscosity under the pumping conditions. Tests have shown that the low-temperature pumping viscosity of multi-grade oil is not higher than 3Pa·s, which can ensure pumping oil supply, and this viscosity is called Critical Pumping Viscosity. The temperature at which the critical pumping viscosity is reached is called the Critical Pumping Temperature, which is measured with a Mini-Rotary Viscometer (MRV). The MRV is a low shear rate rheometer used to simulate the pumpability of a vehicle's multigrade motor oil after idling for two days in cold weather.

SAE J300 also specifies upper MRV viscosity limits for all "W" grade engine oils. MRV can measure both the flow limit of viscosity and the limit of air resistance. The average pumping temperature (BPT) predicted by MRV has a good correlation with the average limit pumping temperature of the engine.

Brookefield viscosity can determine the viscosity flow limit, and The following Table lists the effects of different types of viscosity index improvers on low-temperature pumping performance.

5. High temperature, high shear rate (HTHS)

High-Temperature High Shear (HTHS) is an indicator of the viscosity stability of engine oil under high temperature and high shear, which reflects the ability of engine oil to maintain lubrication under high temperature and high shear conditions. We can simply understand HTHS as the oil film strength.

Viscosity has a decisive significance on lubrication. The high-temperature viscosity of multi-grade motor oil is tested by a low-shear rate capillary viscometer to measure the kinematic viscosity at 100°C. For non-Newtonian fluids of multi-stage oil system, the viscosity measured by the low-shear capillary can not reflect the viscosity of the engine under the working conditions of high temperature (150°C) and high shear rate (106s^-1).

The research shows that the apparent viscosity measured at the temperature of 150°C and the shear rate of 10⁶s^-1 has a good correlation with the wear of the engine bearing. When SAE J300 was revised in 1995, and the minimum high temperature, and high shear (High temperature, high shear rate, HTHS) viscosity for each viscosity grade was added. High temperature and high shear viscosity are measured at a very high shear rate (10⁶ s^-1) and temperature (150°C), which is similar to the flow environment of operating crankcase bearings in a steady state.

Generally, A high-quality viscosity index improver not only needs to possess strong thickening abilities and good shear stability but also requires good performance at low temperatures and high thermal oxidation stability.

The chemical structure of viscosity index improvers is closely related to their performance. Hydrocarbon-based high polymers such as OCP have excellent viscosity-increasing effects, but their ability to improve the viscosity index (VI) is not as good.

On the other hand, polymers containing polar groups such as PMA are not as effective as OCP to thicken the lubricant, but they have excellent VI improvement capabilities and they can also reduce the pour point.

Typical Viscosity Index Improvers

Typical Viscosity Index Improvers	Abbreviation
Ethylene-propylene copolymer, olefin copolymer (ethylene and propylene)	OCP
Polymethacrylate	PMA
Hydrostyrene‐vinyl diene copolymer	HSD
Polyisobutylene	PIB

Star Polymer HSD Viscosity Index Improver

Viscosity Index Improver in Lubricant

Comparison of Properties of Several Commonly Used Viscosity Index Improvers (VII)

Performance VII	Thicken Ability	VI Improvement	Shear Stability	Decrease Pour Point	Low Temperature Viscosity		High Temperature Shear Viscosity	Oxidation Stability
Performance VII	Thicken Ability	VI Improvement	Shear Stability	Decrease Pour Point	CCS	Brookfield	High Temperature Shear Viscosity	Oxidation Stability
PMA	Good	Very good	not good - Very good	Very good	Very good	Very good	Good	Good
OCP	Very good	Good	not good - Good	Bad	Good	Bad	Good	Good
HSD	Very good	Good	Very good	Bad	Good	Bad	Good	Good
Mix of OCP&PMA	Good	Good	Good	Good	Very good	Good	Good	Good

Chorus Lubricant Additives offers a range of viscosity index improvers for the lubricant industry. Please contact our sales engineers to know more.

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