Increasingly faster and more heavily loaded machines as well as longer oil life often lead to a higher temperature load on the lubricants. This in turn accelerates oil oxidation. The minimum requirements for many industrial lubricants are defined in DIN or ISO standards, which are regularly adapted. Lubricant manufacturers meet the increasing requirements by using saturated base oils (Group II or III), which are additionally improved with a higher proportion of newly formulated antioxidants.
This is particularly evident for circulating oils used for turbine lubrication. Until a few years ago, conventional turbine oils were based on API Group I base oils. They were predominantly mixed with phenolic antioxidants, combined with some amines, sulphides or phosphates. Modern turbine oils are manufactured from mineral API Group II (hydrotreated) or Group III (hydrocracked) base oils or even synthetic PAO oils (Group IV). Aminic antioxidants are mainly used as additives for these oils. Phenolic active ingredients are now only contained in low concentrations.
Both turbine oils have two characteristic peaks in the RULER diagram (see figure below). Each represents a type of antioxidant. In the left diagram, a relatively high content of phenolic antioxidants is visible with the second peak. The right diagram is typical for a turbine oil with a Group II base oil. While the signal for the phenolic antioxidants is only weak, the signal for the aminic antioxidants dominates. The RULER test provides very reliable statements about the degradation of aminic antioxidants in modern turbine oils. However, statements about the phenolic antioxidants are compromised due to their very low concentration.
Intensive investigations have shown that changes in phenolic antioxidants, especially at low concentrations, can be reliably determined with FT-IR spectroscopy.