Analysis of antioxidant behaviour in lubricating oils
Lubricating oils subjected to the operating conditions in an automobile engine degrade by many routes. The most important of these routes is oxidation, which at room temperature and atmospheric pressure is almost immeasurably slow. The performance of a new oil formulation is ultimately assessed by the use of a standard engine test. Such engine tests are very expensive and time consuming, which makes the use of an initial screening test a necessity. Screening tests are used to eliminate any oils that would fail the full engine test, thereby saving considerable amounts of time and money. In the present research the concern has been to compare two screening tests for the analysis of antioxidants based on differential scanning calorimetry (DSQ and cyclic voltammetry (CV). The two techniques have been applied to the analysis the of the antioxidants, dioctyldiphenyl amine (DODPA) and Topanol '0' in three base oils. DSC is a commonly used technique in which the time to oxidation is measured when the sample is maintained at a constant temperature in a high pressure atmosphere of oxygen. CV has not been used widely to examine lubricating oils and the present work has included the development of reliable experimental techniques for the two types of antioxidants. The results obtained for both freshly prepared oil formulations and those partially oxidised clearly reveal the different nature of the tests. DSC is a non specific technique with respect to the antioxidant and leads to an indication of the overall oxidative stability of the oil blend, whereas the voltammogram obtained in CV is specific to certain groups of antioxidant. The value of the current flowing through the circuit reaches a maximum value at a potential specific to and dependant on the concentration of the additive being analysed. A kinetic analysis of an antioxidant by CV leads to information about the diffusion coefficient of the molecule and the rate of electron transfer at the electrode surface. It was found that the electrochemically oxidised species of the amine antioxidant underwent a chemical reaction to produce an electro-inactive species over the potential range studied. The kinetic data for this reaction was obtained but their relevance to the oxidation of an oil in an engine environment is doubtful, whereas the results obtained by DSC are probably more applicable. The kinetic data obtained from isothermal DSC curves were employed in a combined kinetic scheme to model the shapes of DSC curves.