1 Introduction
The color darkening process of lubrication oils has been making sense of consumers and researchers out of confusion when makes decisions that the deteriorated oils should be replaced. Especially in the application of aviation lubrication oils, there is very limited room for the inaccurate assessment of the oil quality due to the strict flying safety standards. Many researches of the oils color number have established many interesting assessment standards about our daily life1-4. In industrial field, the color index of transformer insulating oil has been considering the very critical in ensuring the safety as well as optimal performance of the equipment5. The color of oils could also lead to some important application6 which is vital towards our living environment.
To be the important component of effective lubrication system, the aviation lubrication oil is facing the challenge of color change which is usually related to the quality deterioration of the oils. Colorimeter has a long history for measuring the quality of the refined petroleum products since 19337-9, which also suggests the color is an important parameter of aviation lubrication oils. As reported by the previous study10-12, the molecular structure change and the properties deterioration of the oxidized oil samples are confirmed, which also result in gradually changing of the oxidized oil samples color. However, the intrinsic relationship between the oil molecules (oxidizing products and antioxidants) and the resulting color variation needs to be established which could develop a fast and low-cost strategy to rapidly estimate the oil quality.
On the other hand, the gas chromatography mass spectroscopy (GCMS) is powerful method to be widely applied to determine from the molecular structures13 and degradation reactions14, 15 to oil quality16 of the lubrication oils. Based on the conjoint analysis of GCMS and color number, the key performances of the oxidized oil samples including fully formulated aviation lubrication oils and base oils are measured to study the relationship between the color, molecular structure and performances of the oxidized oil samples. Towards a fully formulated aviation lubrication oil, the color number could be an easy-acquired oil quality signal to fast evaluate the residual life of the in-service oil after the relationship between the color, molecular structure and performances of the oxidized oil has been established.