How to choose the Viscosity Modifier for your lubricant formulation?

In modern lubricant formulation, Viscosity Modifier is the key component that enables multigrade oils to work "in all seasons and climates." So, what are the common types of viscosity modifiers on the market? How to select the right one?

 

In the market, there are 4 common types of viscosity modifiers: Olefin Copolymer (OCP), Polymethacrylate (PMA), Hydrogenated Styrene-Diene Copolymer (HSD), and Polyisobutylene (PIB).

All the above viscosity modifiers are high molecular weight polymers, but their performance varies with molecular structure.

 

Olefin Copolymer (OCP)

 

Olefin Copolymer (OCP) is currently the most widely used Viscosity Modifier in internal combustion engine oils. Its molecular structure is relatively simple, consisting of a linear copolymer of ethylene and propylene.
OCP offers three key advantages:

• Strong thickening efficiency
• Good thermal stability
• Moderate cost.

 

The vast majority of 10W-40 and 15W-40 diesel engine oils on the market use OCP as the viscosity modifier.
The main disadvantage of OCP is its relatively poor low-temperature pumpability. When formulating low-viscosity grades such as 0W or 5W, it needs to be used in combination with a pour point depressant.

 

Polymethyl methacrylate (PMA)

PMA viscosity modifier's molecular structure is characterized by the flexible adjustment of its side-chain ester groups-by changing the carbon chain length of the ester groups, low-temperature performance can be "customized."
PMA's biggest advantage is its excellent viscosity-temperature properities; its thickening ability at high temperatures is stronger than at low temperatures, which is exactly the characteristic required for multi-grade oils. 

Furthermore, some types of PMA can also be used as pour point depressants, offering a dual effect.
However, PMA also has significant drawbacks: its thickening ability is relatively weak, and its thickening effect is not as good as other types at the same dosage; its thermal oxidation stability is also relativly poor compared to other thickeners. Therefore, PMA is mainly used in gasoline engine oils, high-end gear oils, and high-end hydraulic oils, which have stringent low-temperature performance requirements.

 

Hydrogenated styrene-diene copolymer (HSD)

Hydrogenated styrene-diene copolymer (HSD), star-shaped HSD viscosity improver maintains both high thickening ability and excellent shear stability, making it particulary suitable for large-span multi-grade oils with a wide range of 0W-40 and 5W-50. 

 

Polyisobutylene (PIB)

Polyisobutylene (PIB) exhibits excellent shear and thermal stability, and good compatibility with various additives. However, PIB's viscosity-temperature low-temperature performance is very poor. This makes PIB primarily used in lubricants with shear stability requirements but low viscosity-temperature requirements. 

 

How to choose the proper Viscosity Modifier for your lubricant formulation?

 

Different types of lubricants have drastically different performance requirements for viscosity modifiers. The key is choosing the right type to balance performance and cost.

 

Internal combustion engine oils represent the largest application segment for viscosity modifiers. For conventional grades such as 10W-40 and 15W-40 diesel engine oils, OCP offers the best cost-performance ratio. For wide-span viscosity grades like 5W-40 and 0W-40, HSD is more advantageous due to its excellent shear stability. For gasoline engine oils where low-temperature performance is a priority, PMA is a viable option.

Gear oils operate under severe shear conditions; gear oils demand extremely high shear stability. Low molecular weight OCP has become the mainstream choice for gear oil viscosity modifiers due to its balanced shear stability and controllable cost.
PIB is rarely used in modern gear oils because of its poor low-temperature properites. 

Low-SSI PMA is also a good choice for gear oils that require low-temperature performance, although cost and thickening efficiency trade-offs must be carefully evaluated.

Hydraulic oils operate under relatively mild conditions, but impose high requirements on the viscosity-temperature character and seal compatibility. PMA remains the first choise for CNC hydraulic oils and automatic transmission fluids (ATFs), thanks to its superior viscosity-temperature characteristics. 

 

A Simple Selection Guide

No single viscosity modifier fits every application. The key is matching it to the application. 

 

• Cost-sensitive engine oils - OCP
• Shear-severe gear oils - Low molecular weight OCP
• High low-temperature performance requirements - PMA
• Large-span multigrade oils (e.g., 0W-40) - star-shaped HSD
• High-performance formulations requiring both low-temperature and shear stability - low-SSI PMA