It is not often appreciated just how much the base oils character contributes to the performance of the VM in a finished lubricant. This is primarily through the solvency power – an oils ability to dissolve additives and oxidation products – which can be indicated by the aniline point of the base oil, in other words the minimum temperature at which equal amounts of aniline and the oil become a homogenous mixture.

The effective solvency of a base oil is not fixed, as an aniline point might imply, but changes as a function of temperature depending on the chemistry types of the oil and the VM. Solvency changes will induce polymers to swell or contract as solvency increases or decreases respectively.

VMs operate to raise VI in a finished lubricant by contributing a disproportionately large boost to the average carbon number of the base oil solution. Indeed, if we look at base stocks themselves, the VI generally increases with the grade or carbon number at constant chemistry type, at least over a limited range.

But if we can take that large carbon number VM and make it an even bigger molecule – by swelling it through better solvency of the base oil for the polymer – then a greater lubricant thickening power is achieved. We see this systematically change across the API groups in terms of VM thickening at a given base stock grade, with the greatest swelling for API Group I and the least for Group IV.

The story does not end here, however, because the solvency of a base oil for a VM polymer varies with temperature, as does any polymer/solvent system.

Broadly speaking, there are two types of VM/temperature response. The first, which covers most hydrocarbon VMs, is that the polymer contracts with increasing temperature in base oil solution, due to base oil-polymer solvency loss. This can be demonstrated through measuring the so-called intrinsic viscosity (a measure of individual polymer molecular volume) of the VM in the base oil solution, as a function of temperature.

The second general type of behavior, which is demonstrated by VMs containing oxygenated groups such as poly-alkylmethacrylates, is that the polymer actually swells or increases in size with increasing temperature in base oil solution due to improved base oil-polymer solvency.

Returning to the first type of behavior, it is here that the facts run counter to the often-quoted but actually incorrect picture of how hydrocarbon VMs perform in crankcases. The impression is that hydrocarbon VMs expand as the engine warms up. It makes a nice marketing video, but it is not correct. It would be correct if poly-alkylmethacrylates were used more in the crankcase domain, but they are largely used in industrial lubes for shear stability reasons. Most crankcase applications make use of hydrocarbon VM chemistries, such as olefin copolymers (OCPs) of ethylene and propylene, hydrogenated isoprene or hydrogenated butadiene chemistries.

So how then do hydrocarbon VMs impart increased VI to the base oil solution at elevated temperatures? It is simply that they are much bigger molecules than hydrocarbon base stock molecules at all temperatures. Even if they contract with increasing temperature, they still remain bigger in effective size than any hydrocarbon base stock molecules they are dissolved in. In fact, contracting with temperature probably contributes to the relative shear stability indices of hydrocarbon compared with oxygenated VM polymers in crankcases. It is another story in industrial lubricants where largely different mechanical stress regimes predominate.

Other aspects of base stock-polymer solvency can be used to fine-tune VM multigrade lubricant performance. For example, within the OCP range of VMs we can get high-ethylene OCPs with long runs of polyethylene inside the VM molecule. These runs make the individual VM molecules associate just like waxes do when in a low-solvency base stock. This association, or clustering, boosts the effective size of the VM molecules and enhances their thickening power in the base oil, although it can give rise to low-temperature issues. There are also OCPs that do not have these polyethylene blocks, rather they have more polypropylene-type character, and they do not undergo base oil solvency mediated associations but they do have lower thickening power.

Similarly, the literature shows with styrene-hydrogenated isoprene VMs that we see their associative or micellar behavior in base oil decrease quite critically with increasing temperature. This is because of the change in the base oils solvency characteristics for individual polymer sub-units over that same temperature range, causing cluster dissociation.

Anything that contributes to the base oils solvency interaction with individual polymer sub-units, whether it be temperature or the aromatics-saturates ratio, will have an influence on the VM thickening power and also the low temperature performance of VM polymers in the base oil.

Thus, it is important not to accept generalizations about VM performance but rather to judge their specific performance in the context of the base oil type, temperature regimes and stress regimes they are likely to experience. This is the sort of thing that can only be assessed by experienced formulators in the laboratory when a new product development is underway.