3.3 The oxidation products analysis and oxidation mechanism study
Since the base oils without antioxidants show slight change of color number, this paper focus on the oil samples with antioxidants. The oxidation reactions carried out in the paper could divided as four series experiments (each series has two categories depends on Cu sheet is applies in experiment or not): the PAO and T501 (P501, the total ions chromatograms shown in Figure 3s), PAO and Tz516 (P516, the total ions chromatograms shown in Figure 4s), PAO T501 and Tz516 (P501+516, the total ions chromatograms shown in Figure 5s), DIOS and T531 (D531, the total ions chromatograms shown in Figure 6s). The total ions chromatograms shown in SI give a rough sense of the oxidation products.
After careful analysis, the evidences from Figure 3s-6s show the species of base oils, antioxidants and Cu sheet catalyst could significantly change the color number of the oxidized oil samples. There are demands for identifying the oxidation products’ species and antioxidants to determine the yield of oxidation products and consumption of antioxidants. The oxidation products are too many to list, otherwise a large amount of oxidation chemicals would be resulting in confusion. After extraction, separation and analysis, the typical color materials existing in the oil samples can be distinguished as two categories: color- related-oxidation-products (CORP) from antioxidants (CORPA) and CORP from base oil (CORPB).
According to the molecular analysis of the typical color related oxidation products with or without Cu sheet, the typical CORPA and CORPB have considerable amount (have larger peak area) and highly repeatedly shown in their total ions chromatograms could be concluded as follows (Table 1). The listed chemicals (Table 1) that shown as the typical color related oxidation products are the representative substances detected in the oil samples after extraction and separation. It’s reasonable that the content of the typical CORPA and CORPB might reflect the color variation of the oxidized oil samples. Generally, the CORPA mainly have the basic molecular structure that similar to the antioxidants and the CORPB have longer alkane chain.