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).