Oxidation Studies on Mineral Insulating Oil Using an Accelerated Method with Continuous Online Monitoring

In this study, an alternative method was applied to evaluate the accelerated oxidation of mineral insulating oils using continuous real time monitoring, with smaller samples and lower experimental times than the standard ASTM D2440 method. Using this method, the influence of temperature and presence of a metal catalyst on the oxidation behavior was evaluated. The activities of commercial metal passivators and antioxidants were also studied at different conditions. The addition of the metal passivators (BTA: 1,2,3-benzotriazole, and Irgamet-39: n-(1,2,3-benzotriazol-1-methyl)-n-alkylamine) at low concentrations (8 mg/kg) increased the induction period (IP) of doped samples. Furthermore synergetic effects on retarding the oxidation reactions when using both metal passivators and two commercial antioxidants (DBPC, 2,6-di-tert-butyl-p-cresol, and Irganox L115, 2,2'-thiodiethylen-bis(3,5-di-tert-butyl-4 hydroxyphenyl-proprionate) were observed, increasing the IP values by up to 25 times when compared to a fresh oil sample. Experimental IP values from this newly proposed technique (up to 10 h) may be used to estimate the Total Acid Number values obtained when using the ASTM D2440 method (164 h).