During lubricant operation, temperature, pressure and load are the main factors influencing degradation. To monitor this degradation process in lubricating oils, industry has defined specific test methods involving correct sampling and analytical techniques to provide efficient information on the correct oil change interval. Applying these condition monitoring practices to lubricating greases is much more complex, especially when they are ester-based biodegradable greases. This explains why industry relies on time-based intervals (preventive maintenance) for grease-lubricated system procedures.
In order to establish a correct maintenance protocol, it is important to understand how the degradation process occurs in biodegradable and mineral greases, and to identify adequate control parameters, limits and sampling frequency (or regreasing frequency).

Oxidation Process

To evaluate the different techniques, a mineral grease and a biodegradable grease were oxidized in a laboratory using a new oxidation method. One kilogram of each grease was oxidized in a bath reactor at 140 degrees C with air flow and constant stirring. Ten samples of each grease at different oxidation times were taken.

Figure 1. A bath reactor was used to oxidize the grease samples.

Degradation Detection Methods

Among the test methods used to detect early grease degradation were acid number (AN), Fourier transform infrared spectroscopy(FTIR), differential scanning calorimetry (DSC), RULER (remaining useful life) and rheology (yield stress). Some techniques were applied directly to the grease, while others were applied to oil extracted from the grease.

Acid Number

Both in new lubricants and in-service lubricants, acidic constituents will appear, either in the form of additives or as a result of lubricant oxidation. The acid number test method is a measurement of the quantity of those acidic constituents in the lubricant. For this case study, measurement was completed following ASTM D974-04 (a titration that estimates the amount of potassium hydroxide necessary to neutralize the acid compounds of the oil) and applied directly to the greases and also to the oil extracted from the greases.

Table 1. The results from the acid number measurements are listed in this table.
Figure 2 illustrates the acidity trend for the mineral grease with the corresponding oxidation time. The trend for both the grease and the oil extracted is very similar. There is a plateau area where the acidity remains nearly constant and then begins to increase (at 264 hours).

Figure 2. This chart shows the acid number trend for mineral grease.
The acidity trend for the biodegradable grease with the corresponding oxidation time can be seen in Figure 3. Again, the trend for both grease and the oil extracted is similar. There’s also a plateau area where the acidity remains constant and then begins to increase (at 392 hours).

Figure 3. The acid number trend for biodegradable grease is shown in this chart

RULER

The RULER (voltammetric) test was conducted to establish the consumption of antioxidants. This method is based on voltammetric analysis as an electro-analytical method in which a sample is mixed with an electrolyte and a solvent, and then placed in an electrolytic cell. With increased voltage to the sample in the cell, the various dissolved antioxidants oxidize electrochemically, resulting in an oxidation reaction that can be used to predict the remaining useful life of greases and oils.
These two greases were formulated with aromatic amines, which are primary antioxidants. They work by donating a hydrogen atom to the peroxy radical. The depletion of amine antioxidants for mineral and biodegradable grease as the oxidation time increases is shown in Figures 4 and 5.

Figure 4. This graph depicts the RULER test results for mineral grease.

Figure 5. The RULER test results for biodegradable grease are plotted in this graph.

Table 2. The remaining useful life (%) of mineral and biodegradable grease is compared.

Figure 6.  This graph of the RULER trend for mineral and biodegradable greases illustrates how the antioxidants decrease in the same way.

FTIR

FTIR is a well-known technique for analyzing the chemical properties of oil, including oxidation products and additives. In this study, FTIR was applied to grease and oil, but only the oil results will be discussed, as the infrared spectra of greases did not provide much information.

Figure 7. This infrared spectra shows the oils extracted from mineral greases at different oxidation times.
One area (3,500 to 3,350 cm^-1) of the mineral grease spectra was studied to determine the suitability of this band for the amine antioxidants quantification. The results obtained for this area are shown in the Table 3.

Table 3. The oxidation times for the measured area are listed in this table.
A second area (1,615 to 1,592 cm^-1) of the spectra was then examined to establish the suitability of this band for the aromatic amine antioxidants quantification. The results obtained for this area are shown in the Table 4.

Table 4. This table shows the results obtained for the measured area.
One of the most typical areas to control the mineral oil’s degradation was at the 1,700 cm^-1peak. In this case, this band was not quantified because there was an additive that appeared at the same wavelength, which makes it difficult to extract any conclusions.

Figure 8. This is an infrared spectra of the oils extracted from biodegradable greases at different oxidation times.
In the biodegradable grease spectra, one area (3,660 to 3,100 cm^-1) was quantified to determine the suitability of this band for degradation control. The results obtained for this area are shown in the Table 5.

Table 5. The oxidation times for the measured area are listed in this table.
Another area (1,100 to 1,200 cm^-1) was quantified to establish the suitability of this band for the biodegradable grease’s degradation control. The results obtained for this area are shown in the Table 6.

Table 6. This table shows the results obtained for the measured area.

Rheology (Yield Stress)

Yield stress value is the stress corresponding to the transition from elastic to plastic deformation. For this critical value, the internal network of the lubricating grease becomes unstable, and the deformation process within the material is suddenly transformed into flow.
In this case study, a rheometer was utilized to measure the yield stress along with a 25 mm parallel plate with a 2 mm gap. For biodegradable grease, the method to measure the yield stress was the Strain Sweep Test. The yield stress measurement is now being applied to mineral greases.

Table 7. The yield stress tends to decrease at the beginning of oxidation but later starts to increase again (the grease tends to harden).

DSC Variation

DSC is a thermal analysis technique that measures the heat flow associated with certain physical and chemical changes in a lubricant. The interest of this technique is the stability to oxidation, which shows up as the time delay to the onset of the oxidative exothermal reaction. The sample is placed in an oven heated to a set temperature, and oxygen is then passed over the two cells. When the antioxidant in the sample can no longer afford protection, oxidation of the lubricant takes place and is detected by a temperature rise in the cell containing the sample.

Figure 9. These thermograms were obtained for mineral grease at different oxidation degrees.
The DSC test was only applied to mineral grease and is being tested with biodegradable grease. It was observed that the mineral grease degraded earlier as the oxidation time increased.
For biodegradable greases, acid number, FTIR and yield stress measurement appear to be good techniques to detect early stages of oxidation.  

Figure 10. When the COO and OH group begins to increase, the plateau area of the biodegradable grease’s acidity ends.

Figure 11. The acid number and yield stress begin to increase at the same time.

CONCLUSION

The oxidation mechanism for biolubricants is different from that of mineral oils. Therefore, the change intervals to be established and the parameters are different for mineral and biodegradable lubricants.
It was determined that the RULER test is a suitable technique to monitor biodegradable greases when they are formulated with antioxidants that this test method can detect. Infrared spectroscopy is better suited for oils in order to quantify the biogrease degradation. By measuring the COO and OH area, early stages of oxidation appear to be detected.
Acid number can be applied directly to biodegradable grease and is a good technique to control the biogrease’s degradation. When the acid number begins to increase strongly, the grease must be changed.
More work is needed to determine if DSC is a suitable technique to control biodegradable greases during use.