1995-02-01

Chemiluminescence in Evaluating the Thermal Oxidative Stability of Automotive Oils 951025

A chemiluminescence multi-cell computerized apparatus and method are described for determining automotive oils thermal oxidative stability by measuring the intensity of light emitted during oxidation. The method of the treatment of the experimental results utilizes both the well established hydrocarbon oxidation mechanism and the generally accepted chemiluminescence mechanism. A sigmoidal change in the chemiluminescence intensity with time of isothermal oxidation is justified and it is shown that both major parameters of oxidation - induction period and oxidation rate constant - can be evaluated from the experimental data. The knowledge of of the above two parameters allows one to calculate the durability of the material. The approach developed is applicable to both uninhibited and inhibited oxidations. In the latter case conclusions with regard to inhibitor reactivity can be made and it is shown that there is a distinctive difference between strong and moderate inhibitors. The former are expected to prolong the induction period leaving oxidation rate unchanged, while the latter diminish the rate of oxidation without changing its induction period.
The comparative evaluation of original and purified mineral oils showed that purificaiton improves durability by both lowering the concentration of oxidizable sites and by removing some of the preoxidized material. Both BHT and Vitamin E introduced to the purified mineral oil behaved as strong inhibitors, although the effectiveness of the latter was found to be significantly higher. The superior quality of synthetic oils as compared to purified mineral oils is primarily defined by lower concentration of oxidizable sites in the former.

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