Historically, API Group I light grades of around SN150 were the base stock of choice for some additive synthesis and viscosity modifier dilution. However, there has had to be some rethinking of base oil choices in additive manufacture. This has been brought about by the decline of Group I production over the years and, more importantly, with restrictions placed on sulfur in additive packages and the ever-increasing volatility constraints on finished crankcase lubes.

When making, for example, an ashless dispersant concentrate, base oil is used as the reaction solvent for the in-situ synthesis process, such as when maleic anhydride is linked to polybutene and also for subsequent aminations (when an amine group is introduced into the organic molecule).

In the production of over-based detergents, it is often the case that the synthesis to initially make the detergent and then to over-base that detergent is handled in some suitable volatile hydrocarbon solvent since it aids viscosity-limited reactions. But for final use, the over-based detergent is then solvent switched from the volatile hydrocarbon solvent to a concentrated over-based detergent in a base oil prior to the blending of the additive package. Other examples could be given for production of zinc dialkyldithiophosphate, a common antioxidant and anticorrosion additive.

But whether it is a dispersant or detergent, what we end up with usually is a very concentrated additive component in a base oil. This additive can then be combined later on with other components – also usually previously dissolved in a base oil – to make the final performance additive package.

Typical additive active ingredient concentrations in the case of ashless dispersants are greater than 40 percent by weight, so just less than 60 percent base oil. This means a good balance of properties must be struck in the choice of base oil for additive synthesis. For example, there must be sufficient solvency in there to dissolve the dispersant additive precursors and the final dispersant in the synthesis process at such high concentrations. However, there must not be too much solvency since this will result in the additive component becoming over-swollen, which in turn will result in making that additive component or package overly viscous and difficult to handle and pump.

It is important in blending plants that additive packages and components are not too viscous in order to allow easy and cost-effective blending without the need to apply excessive heat to thin them, or any heat at all in some cases. Additive companies usually have to work to a maximum total package viscosity to allow easy pumping and mixing in the blending plant. Hence the need to carefully choose the base oil or base oil blends at the additive component synthesis stage, since this can have a significant effect on final package viscosities.

Solvency is just one issue affecting the choice of base oil in additive production. Performance additive package treat rates can be up to around 20 percent for some finished lubes in top-tier crankcase formulations. Assuming that the package may reasonably contain around 60 percent base oil, then the performance additive package-derived base oil alone could be around 12 percent of all the base oil in the formulation.

More also comes from the diluent oil of the viscosity modifier concentrate. This is a high enough level of incidental base oil to have a significant influence on the Noack volatility (the evaporation loss at high operating temperatures) or even the cold crank viscosity of the finished lubricant.

In the past, when the choice was simply between Group I and Group II light grades, surprisingly Group I oils – at least those made in Europe – had a volatility advantage. Group II used to be largely the preserve of producers in the United States and the lighter grades were made only to meet APIs Noack requirements of a maximum evaporation loss of 15 percent. Whereas European-made Group I had to meet ACEAs Noack requirements of 13 percent.

Once the base oil choice is made at the additive component synthesis stage, we are stuck with it. To put another way, it is a shame if a base stock manufacturer goes to great lengths to provide a blender with low Noack Group III or polyalphaolefin light grades for the main base oil component only for the incidental base oil in the additive package or viscosity modifier concentrate to be the weakest link in the chain determining the finished lubricants physical properties. This is why additive companies pay a great deal of attention to the choice of base oil for additive production and their likely end use.

A simplistic response would be to just use Group III and a polyalphaolefin for additives manufacture. But as well as cost issues, there is also the issue of their solvency, which is worse, grade-for-grade, than Group I or Group II. This could result in limiting the levels of additive package active ingredients, which is counterproductive.

As the industry migrates to SAE 16, 12 and 8 type viscosity grades, it is going to become ever more important to ensure every volatile base stock blend component is as good as it can be in terms of its contribution to Noack and cold crank viscosity. That means the performance additive package oil, the main base oil itself and the viscosity modifier concentrate diluent oil are all optimized at the very earliest stages of additive manufacturing and blending.