As these highly paraffinic Group II and Group III base oils make their way into industrial lubricant formulations, it is important to pay attention to the many important chemical and physical differences exist between these base oil types. The viscosity range covered in Group I is far wider, providing much needed high viscosity to industrial gear oils, greases and engine oils, Table 1.

Table 1. Typical base oil yield in different viscosity grades.

The solvency offered by Group II and Group III, with rapidly increasing aniline points, and lower aromatic carbon type content, is far lower than that of Group I base oils. Thus, some negative effect on the blending of industrial lubricants based on Group II or Group III base oils with existing Group I based industrial product can be foreseen and have indeed been reported from the field of industrial turbine and hydraulic oils during the last decade.

Industrial lubricant blenders are facing new challenges with formulation compatibility, additive solubility and are struggling with extensive re-formulations. Could naphthenic-paraphinic blends of base oils provide a Group I replacement, which has the proven ability to fulfill the viscosity and solvency needs for industrial lubricants. In addition, we developed and tested model hydraulic fluids, metalworking fluids and lubricating greases based on these new oils [1]. One important property of these novel base oils is their response to added antioxidants, as this has wide-ranging consequences for the usefulness and customer acceptance.

For different reasons, most of the modern base oils have a low Sulphur content. All highly refined base


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oils end up with a low Sulphur content, compared to traditional solvent neutral Group I base oils, as a consequence of hydrotreatment reaction design. The ultra-low Sulphur level is a design feature of base oils for all modern engine oil uses, especially for all types of “Mid-SAPS” and “Low-SAPS” formulations (SAPS= Sulphated Ash, Phosphorus and Sulphur). In the case of naphthenic specialty products (NSP), the process step of deep hydrogenation is designed to convert aromatics to naphthenics, and to remove Sulphur and Nitrogen organic compounds. Some of these contribute to smell and colour instability and are thus targeted for removal by the refining and hydrotreatment processes. But some Sulphur-containing organic molecules are actually very beneficial, acting as natural antioxidants in un-inhibited oils (i.e. base oils without added antioxidant additives). These Sulphur species are actually crucial as primary and secondary antioxidants, working in conjuncture with added primary antioxidants (AO) like the commonly used alkyl-phenol (BHT-type) and aryl-naphthyl-alkyl amines. The ability of various types of Sulphur-organic molecules to function as primary and secondary antioxidants is well-known text book chemistry but remains a challenge to compensate for in many industrial lubricant formulations!

Interestingly, it is also well-know that the AO response is considered to be higher, or better, in highly refined, low-Sulphur base oils. Obviously, there are chemical reaction processes running in opposite directions, with opposing effects, synergies and antagonisms, benefits and liabilities. Finding the correct level and chemical nature (structure) of secondary antioxidants for base oil and formulated lubricants might appear as a game of chance, but fortunately there are some very helpful tools out there for the formulator.

Some ways of controlling the base oil Sulphur are only open to the base oil refiners themselves. For the lubricant formulator, many options still exist for the successful formulation of the finished lubricant


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