## 3 Results and Discussion  
### General Degradation Behaviour Determined by Conventional Analytical Methods  
In Figure 4, the oxygen pressure in the reaction vessel throughout the artificial alterations is displayed. The pressure curves of base oil without AO show an abrupt rise in the first few minutes, due to the adjustment to the elevated temperature. Almost immediately, a steep pressure drop is observed, induced by oxidation processes. After 2 to 3 h of alteration, the lower plateau of pressure is reached, characterised by a slow pressure decrease. In the case of the oxidation of ester-based base oils with AO, the pressure rise in the first hours of alteration is far higher, approximately twice that of blends without AO, and lasts longer, i.e., up to 5 h of alteration. After reaching the pressure maximum, a slow decrease begins, resulting in a drawn-out lower plateau with hardly any change in pressure. Generally, prolonged alteration durations until the beginning of the pressure decrease and the achievement of the stable lower pressure plateau designate the effectiveness of the applied AO. Thus, on the one hand, the start of the oxidative processes is retarded, and on the other hand, the base oil degradation itself is decelerated. Without the application of AO, oxidation begins immediately, whereas blends with AO only start to significantly deteriorate after approximately 5 h. The overall duration until reaching a stable degradation condition strongly depends on the utilisation of AO: around 72 h in the case of blends with AO, compared to around 24 h for ester oils without an AO dosage. Moreover, it has to be stated that the initial oxygen pressure in the vessel seems to possess little influence on the alteration behaviour, as a comparison of the alteration with 1 and 2 bar of the originally applied oxygen shows (Figure 4, dark blue curve versus dark green curve). Both alterations exhibit an identical pressure behaviour. In a first approach, the same degradation mechanisms, and hence the same reactions, can be assumed under both conditions. A comparison of the oxygen pressure curves of alterations with \({}^{16}\)O\({}_{2}\) and \({}^{18}\)O\({}_{2}\) suggests that the isotope-labelled atmosphere does not have an impact on either the degradation behaviour or on the degradation mechanisms, strongly indicated by the almost congruent curve shapes in both cases. The visual appearances of the fresh, as well as the artificially altered, ester-based oil samples are shown in Figure 5. It was found that all altered samples exhibit a bright yellow colour compared to colourless fresh blends. The only exception is the sample altered with 2 bar oxygen pressure with a dark brown colouring and significant turbidity. The latter can be attributed to an elevated water content, which was measured to be approximately five times higher than in all other investigated oil samples (see Figure 6). Figure 5: Visual appearances of ester-based oil samples with and without antioxidant (AO) before and after artificial alteration under the different oxygen atmospheres. Conventional condition monitoring was performed in terms of the NN and water content. Figure 6 compares the analytical data of the fresh oils with the respective results of the samples after artificial alteration. Fresh oil samples exhibited very low levels of water, as well as NN. On the contrary, a tremendous rise in both the water content and NN was detected in all samples after artificial alteration. Higher values for both parameters were found for oils without AO than for blends with AO, e.g., the NN for samples without AO was determined to be twice as high as the NN in samples with AO. The only outstanding exception is the oil sample after artificial alteration at a 2 bar oxygen pressure, which showed the highest values for both the NN and water content. In general, a good correlation was found between the detected water content and the NN in the oil samples after artificial alteration.