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Oil condition

In the OELCHECK laboratory reports, the oxidation value is usually indicated under the heading of “Oil condition“. But why is it not mentioned in some reports? And what is the significance of this value anyway?

The oxidation value has been the most important indicator for the ageing of an oil for many years. However, for some lubricants with base oils containing ester and/or additives dissolved in esters, it is barely relevant any more. The following products are particularly affected by this:

Biodegradable ester-based hydraulic oils

Synthetic gear oils with a polyalphaolefin (PAO ester) basis which are often used in the transmissions of wind turbines, for example

Engine oils with a relatively high FAME (biodiesel) content

Engine oils with vegetable oil content

Engine oils with an elevated soot content

Gas engine oils with a high ester content, such as those used in the operation of engines with biogas, sewage gas or landfill gas. For the gas engine oils designed in this way, we stopped providing the oxidation value for individual engine manufacturers some time ago.

Table of contents

  1. Lubricants and oxidation
  2. Determination by means of FT-IR spectroscopy
  3. Limitations of FT-IR spectroscopy
  4. Accurate assessment of the ageing of ester-containing lubricants

Lubricants and oxidation

Every lubricant, whether mineral oil-based or synthetic, ages with time. Important factors which accelerate ageing include stress due to high temperatures, air admission, and catalytic processes, as well as the type and amount of any impurities. As oil ages, oxygen attaches to the hydrocarbon chains of the molecule. This produces double carbon-oxygen bonds (carbonyl groups). These carbon-oxygen bonds can be used in the analysis of the oxidised oil to infer its ageing and thus its remaining usability.

 

Determination by means of FT-IR spectroscopy

Fourier-transform infrared (FT-IR) spectroscopy is used when considering the oxygen bonds or the oxidation of an oil. It provides information about changes to the oil, such as oxidation, as well as any impurities. FT-IR spectroscopy makes use of the fact that, due to their typical binding form, the molecules present in the lubricant absorb infrared light to different degrees at specific wavelengths. For mineral oils, the different degree of absorption resulting from the ageing of the oil is displayed as a “peak“ in the IR spectrum at a certain wave number. According to DIN 51453, a numerical value for the absorption of the IR radiation based on a one-centimetre thick oil layer can be given in A/cm for mineral oils by subtracting the fresh spectrum from the used oil spectrum.

Limitations of FT-IR spectroscopy

For a mineral oil-based lubricant, even a relatively small „peak“ which increases continuously in the used oil spectrum can be interpreted as a clear sign of increasing oil oxidation. However, when determining the oil oxidation of lubricants with ester-containing base oils and/or additives dissolved in esters, FT-IR spectroscopy reaches its limits. Carbonyl bonds are inherent to ester oils, which absorb infrared light at a wave number of approximately 1,740 cm-1, and thus in the same range as the carbonyl bonds which are created as a result of the oxidation of the oil. In the infrared spectrum of a fresh ester-containing lubricant, an oversized „peak“ in the wave number range of approximately 1,740 cm-1 is formed by its carbonyl compounds alone. In contrast, if the used oil is examined, a change in this dominant „peak“ due to possible oil oxidation can no longer be calculated. Although it is no longer meaningful to determine the oxidation value for lubricants with ester-containing base oils and/or additives dissolved in esters by means of FT-IR spectroscopy, its determination according to DIN 51453 is still specified in the fuel specifications of several OEMs. Up to now, only the engine manufacturer MWM has taken these facts into account. The MWM specification for gas-engine lubricating oils does, indeed, indicate a limit value of 20 A/cm for oxidation according to DIN 51453. But there is a note: „The determination of the oxidation does not apply to lubricants containing synthetic esters.“

Accurate assessment of the ageing of ester-containing lubricants

Although the oxidation value of ester-containing lubricants cannot be reliably determined with FT-IR spectroscopy, OELCHECK engineers can still provide more accurate information on the ageing of these products. For the most part, they use the following criteria:

  • Changes in viscosity
  • Any reduction of the additive content
  • The degradation of antioxidants
  • The AN (Acid Number)
  • The BN (Base Number) of the lubricant, which provides information regarding its alkaline reserve for the neutralisation of acids. The BN cannot, however, detect the neutralisation capacity of an oil for every type of acid that may enter the oil during the operation of gas engines with landfill or sewage gas. For these oils, the i-pH value provides crucial additional information regarding the degree to which a used oil is loaded with strong, corrosive acids.

OELCHECK engineers consider the interaction of these values and, based on their own experience and on data from the large OELCHECK database, can issue accurate statements regarding the condition and the remaining usability of any ester-containing lubricant.

Source:

OELCHECKER Winter 2017, page 8