When determining oxidation using IR spectroscopy in accordance with DIN 51453, it is assumed that a clear peak can be seen in the IR spectrum of an oxidized oil at a wave number of 1,710 cm-1 , which is influenced by the oxygen double bonds. This wave number was therefore used as the basis for the oxidation determination of mineral oil-based used oils from engines and industrial lubricants.
In today’s lubricants, which use better refined or partly synthetic base oils or modern additive technologies, however, the peak-based approach no longer provides sufficiently interpretable findings. Even in trend analyses over several 10,000-hour periods, OELCHECK tribologists are increasingly confronted with oxidation values that hardly change, although changes in viscosity, acids or oxidation inhibitors indicate an increasing ageing of the lubricant.

Many of the newer lubricants contain synthetic base oils based on esters. In these oils, not only do the wear protection additives have a better effect, they are also more temperature-stable and age less. These synthetic long-term oils are particularly advantageous for oils in systems where oil changes are complex and expensive, or which run at high temperatures. In oil analyses, however, it is detrimental that oxidation can no longer be detected because the ester components in the IR spectrum overlap precisely in the area with the oxygen double bonds in which oxidation is calculated according to the DIN method. Oxidation also cannot be calculated for lubricants that contain viscosity index improvers (VI-improvers) or special additives that already have a clear peak area at 1,710 cm-1 in fresh oil. For modern lubricants, the specification of oxidation according to DIN 51453 is in many cases extremely inaccurate; for oils with ester components, oxidation according to DIN cannot be specified. To ensure that accurate information on oil ageing is also possible for lubricants for which DIN oxidation cannot be determined, OELCHECK tribologists have taken into account the increase in viscosity, the reduction in additive content, the degradation of antioxidants as well as the increase in AN (acid number) and, in the case of motor oils, the decrease in BN (base number).

Because the proportion of lubricants for which line-based IR oxidation determination was no longer reliable or impossible increased, OELCHECK tribologists provided the solution. After comparing various inspection and test procedures with differently aged oils from a wide range of applications, as well as after intensive discussions with lubricant and machine manufacturers, the solution is definite: IR spectroscopy is also used for the new oxidation index. But instead of only looking at the change in length defined in DIN at the peak oxidation of 1,710, the focus for the oxidation index is on the range of the IR spectrum between 1,500 and 1,700 cm-1 . With this area-based calculation method, the lateral limits can be defined depending on the oil type, so that they always reflect the change caused by oxidation. This means that any influences or overlays caused by lubricant components such as oxidation inhibitors, VI improvers or other additives can be eliminated.

Here is an example of this: The performance of the area-based oxidation index is exemplary when considering a synthetic gear oil from a wind turbine: Mineral gear oil was used until 2016. In July 2016, an oxidation of 3 A/cm was clearly determined for this. However, after switching to the synthetic product, the conventional method could no longer determine a meaningful oxidation value. In the IR spectrum from February 2021 (see bottom right), one peak overlaps the other at a wave number of 1,710 cm-1 . Any oxidation of the oil is not perceptible. However, if the last three IR spectra are subsequently considered area-based, the oxidation index provides a much more differentiated picture of oil oxidation.

The beginning ageing process of the oil would have already been discovered with the oxidation index in February 2019. Long before this, an even stronger PAI (Peak Area Increase) = 65 cm² increase in the IR spectrum was observed in February 2021 between the two wave number limits defined for this oil type by OELCHECK.