Oils are homogeneous mixtures that can be easily analysed directly or after prior dilution. Greases are rarely suitable for this due to their composition. In order for a grease to become a grease, a thickener is added. This ensures that the lubricant remains stationary and that it only releases the bound oil under the intended conditions.
Lithium, calcium, sodium or aluminium soaps are predominantly used as thickeners. These compounds with inorganic content can be an obstacle to the analysis as they cannot be dissolved in organic solvents. Organic thickeners, often known as polyureas, are also used. But these are also not readily soluble in organic media.
To overcome this hurdle, a method can be used that does not require dilution with an organic solvent, e.g. XRF or RDE-OES. They can be used to examine greases directly.
However, there are a few things to consider with these two methods.
- XRF is not suitable for detecting so-called “light” elements, such as lithium. This is a significant disadvantage if a grease is to be examined with a lithium soap or if a mixture with lithium soap is to be detected. In addition, accurate matrix compensation has to be performed in the case of XRF. It must therefore be known exactly how the grease is composed so that any associated influences can be subtracted out. Otherwise, the results might be too high or too low. Knowledge of the exact composition is extremely rare in the analysis of used grease. Although we have the data for most new greases, in the case of individual use, the thickener of the grease changes and with it also the dropping point and its flow behaviour.
- The RDE-OES works with a “spark wheel” and a graphite electrode. The grease is applied directly and heated to over 8,000°C in an electric arc ignited at approx. 40,000 volts. The energy added in the form of temperature stimulates the elements and causes each element present to emit light in a characteristic wavelength. However, the coating thickness of the sample applied to the spark wheel must be precisely adjusted using a gauge.
However, lubricating greases often contain large abrasion particles, which can cause very large amounts of light in the spark range of the RDE-OES, leading to higher than expected findings, which in turn also affects other elements. In addition, the burning off of the grease layer on the spark wheel changes the composition and the so-called blank value. This can also result in impairments of the findings.