Viscosity Index Improvers: Shear Stability; Treat Rate and the Specification That Hides Both

Two viscosity index improvers arrive with the same thickening effect in a laboratory blend, and one of them loses a third of it after twenty hours in a running engine. The datasheet showed neither number clearly enough to compare.

This note is for lubricant blenders and buyers specifying viscosity index improvers. It covers what the treat rate conceals, how shear stability and high-temperature behaviour decide whether a formulation holds, and the specification points worth writing into an order.

A viscosity index improver is bought on treat rate, and the number that decides its value is shear stability in service

The improver thickens a base oil so that a lighter, more efficient base stock can meet a viscosity grade. The treat rate is the visible cost.

What the buyer actually purchases is the ability to hold that grade after the oil has been through a pump, a bearing and a ring pack for thousands of kilometres.

Ask for shear stability data from a recognised test, and ask which test. A result without the method is not comparable with another supplier's result.

The chemistry families behave differently and are not interchangeable at equal cost

Olefin copolymers give strong thickening at low treat rate and moderate shear stability. Polymethacrylates trade thickening for a pour point benefit and better stability.

Hydrogenated styrenic products sit between them, with good stability and a different response to the base oil.

Choosing a family is a formulation decision, not a purchasing one. A buyer who substitutes on price across families changes the finished oil's behaviour and discovers it in a field complaint.

The same thickening logic appears in grease, where the thickener does the structural work

In grease, a thickener network holds the base oil in place instead of a polymer holding viscosity. The mechanical stability question is the same one, expressed differently.

Comparing the two categories is useful when a plant buys both and wants to understand why one supplier's material behaves consistently and another's does not.

The structure of the grease side is described in this overview of the lubricating grease market, which covers how thickener systems are positioned and where the supply sits.

Shear stability separates temporary from permanent viscosity loss

Oil thins while it is being sheared and recovers when the stress stops. That is temporary loss, and it is normal.

Permanent loss happens when the polymer chains are mechanically broken and cannot re-entangle. The grade the oil was blended to is then no longer the grade in the sump.

Test at the shear rates the application produces, and compare the result after the same duration. A short test at high shear flatters some chemistries and penalises others.

Kinematic viscosity alone says nothing about high-temperature behaviour

Kinematic viscosity is measured at two reference temperatures, and every product can be made to look acceptable at both by adjusting the treat rate.

The behaviour that matters in a hot engine or gearbox sits outside those points, where the polymer's contribution changes with temperature and shear together.

Ask for high-temperature high-shear data. A formulation that meets the grade on paper and fails HTHS is a formulation that will not be approved by the equipment builder.

HTHS is the number engine builders constrain, and it is not negotiable

High-temperature high-shear viscosity is specified by the OEM because bearing film thickness depends on it. Falling below the limit risks wear that no additive package can compensate.

Improvers differ in how efficiently they deliver HTHS per unit of kinematic viscosity. A more efficient product can allow a lighter base oil and a fuel economy claim.

Compare products on HTHS contribution at equal kinematic performance, not on treat rate alone. That comparison is what the price should be judged against.

Compatibility with the base oil group affects solubility and haze

Improvers are supplied as solutions in a carrier oil, and the carrier must be compatible with the base stock. Group I and Group III stocks behave differently.

Incompatibility shows up as haze, sediment or a drop in thickening efficiency after blending and storage, sometimes weeks later.

Run a blend and store it before approving a supplier. A laboratory blend measured immediately tells you very little about what the finished oil will look like in a warehouse.

Handling high-viscosity material needs heat and patience

These products are thick, and pumping them cold is slow and inaccurate. Dissolution into the base oil takes time and agitation.

Heating reduces viscosity but overheating degrades the polymer and costs exactly the shear stability the product was bought for.

Write a maximum handling temperature into the procedure and check the tank thermometer. The damage is invisible until the oil is tested after service.

The improver can be the source of deposits and oxidation products

Polymers degrade under thermal stress and contribute to varnish and sludge, particularly in high-temperature zones. The additive package is usually blamed first.

Ask for thermal stability data and for experience in the application, not only in a bench oxidation test.

When a field problem appears, include the improver in the investigation. Excluding it because it is "just a thickener" is how the same failure repeats across a product line.

What a purchase specification should contain

The chemistry family, the active content, the carrier oil, the shear stability method and result, and the HTHS contribution at a defined treat rate.

Add the handling temperature limit, the storage stability requirement and the batch-to-batch tolerance on thickening efficiency.

Those seven items let two suppliers be compared honestly. Without them, the comparison reduces to price per kilogram of a product whose active content and efficiency are unknown.

Where the category sits commercially

Improvers are a small volume, high value part of a lubricant formulation, and the supply is concentrated among a few chemical producers.

That means availability can tighten for reasons unrelated to lubricant demand, and the price follows the polymer intermediates rather than the oil market.

The supply picture and how the category is positioned within finished lubricants is set out in this analysis of the viscosity index improvers market, which is a reasonable starting point when a blender is qualifying a second source.

Extended drains change what the improver has to survive

A drain interval set for a conventional oil puts far less cumulative shear through the polymer than an extended interval, and the difference is not linear.

The oil spends longer at temperature, sees more cycles through the pump and accumulates more oxidation products that interact with the polymer.

Ask the supplier for data at your intended drain interval rather than a standard bench duration. A result from a short test describes a duty cycle the equipment may never see.

FAQ: questions buyers ask

Can I compare two improvers on treat rate alone?

No. Treat rate reflects thickening efficiency at the test temperature, not shear stability or HTHS contribution. Two products at the same treat rate can deliver very different performance after service, and the cheaper one is often the one that thins fastest.

Which shear stability test should I ask for?

The one that matches your application and is used by the equipment builder you are selling to. The important thing is that both suppliers report the same method for the same duration, otherwise the numbers cannot be placed side by side.

Why did my blend turn hazy after a few weeks?

Usually carrier oil incompatibility with the base stock, or an improver that was not fully dissolved. Store a sample before approving the supplier, because haze often appears well after the blend was measured and released.

Does a higher treat rate always mean better protection?

No. Protection at temperature is about HTHS, and beyond the point the formulation needs, extra polymer adds cost, contributes to deposits and can worsen shear stability rather than improve it.

How many suppliers should a blender qualify?

At least two, given how concentrated the supply is. Requalifying takes time because the shear and HTHS work has to be repeated, so the second source should be qualified before it is needed rather than during a shortage.