Whether the line indicating oxidation is printed in the laboratory report does not depend on the type of component from which the oil sample was taken, but on the base oil type used for the lubricant in question. If raw materials are used for the formulation of the oils, which render it impossible to determine the oil oxidation by means of the FT infrared technology, no value can be measured. For this reason, no oxidation values can be specified for ester-based synthetic oils, for example, which are used as the starting basis for more rapidly biodegradable products. However, the oxidation value may also be missing for modern gear oils, such as those used in wind turbines, if ester-based base oils or additives dissolved in ester oil are used in addition to the PAO (polyalphaolefin) based synthetic oil. The ester contained in PAO oils, which can be several percent, results in better sealing compatibility of the end product and promotes easier dissolution of the additives during oil production. However, esters render it impossible to determine the oxidation value.

During the aging of mineral and synthetic oils, aging products are formed depending on time, stress and temperature. During this process ("oxidation"), oxygen reacts with the hydrocarbon chains of which the oils are composed. So-called carbonyl compounds are formed, which now contain a carbon-oxygen double bond at certain points in the molecule. When irradiated with infrared light, the double bond reacts differently than the comparable carbon-hydrogen bonds as contained in the fresh oil. The deviating absorption resulting from oil aging is visible in the IR spectrum as a peak. By subtracting the used oil and fresh oil spectra, a numerical value for the absorption of IR radiation related to one centimeter of layer thickness can be specified for mineral oils in A/cm according to DIN 51453.

The oxidation-related changes absorb in the IR spectrum at the wavenumber 1,710 cm^-1. In the same wavenumber range, however, the carbonyl compounds also absorb, which are present in ester oils as typical constituents in pronounced form and concentration. Oxygen compounds are not usually present in fresh mineral oils. Here, a relatively small peak that rises continuously in the used oil spectrum can be interpreted as a clear sign of increasing oil oxidation. In the case of synthetic oils based on esters or with proportions of esters, however, such a large peak is already visible in the fresh oil that either the oil oxidation is superimposed or a change in this huge peak due to additional oil oxidation cannot be reliably calculated and thus cannot be interpreted.

However, if no indication of the oxidation of an oil in A/cm is possible from the IR spectrum, OELCHECK engineers use other criteria to provide reliable information about the remaining lifetime for the oil. For example, the degradation of antioxidants, which can also be measured for synthetic oils using IR technology, a change in viscosity or a reduction in additive content (determination using ICP) and, if necessary, determination of oil acidification (AN or NZ) allow additional conclusions to be drawn about the oxidation of the oil and thus its further usability.

OELCHECK also answers your questions about tribology and lubricant analysis. Send us an e-mail (info(at)oelcheck.de).