Thesis (D.Sc.)--Chulalongkorn University, 2010

Synergistic and antagonistic effects have been observed with the four sulfur-, phosphorus- and heavy metal-free antioxidants, hindered bis-phenol (HP), octylated diphenylamine (ODPA), octylated phenyl-alpha-naphthylamine (OPANA) and tolutriazole derivative (TZ), on the oxidation stability, thermal and physicochemical properties of dioctyl sebacate base oil. The oxidation stability was evaluated using a rotating bomb oxidation tester (RBOT). The results from isothermal and catalytic oxidation tests suggested that octylated diphenylamine has an antagonistic effect on the oxidation induction time (OIT). The combination of hindered bis-phenol: octylated phenyl-alpha-naphthylamine: tolutriazole derivative exhibited the highest synergistic antioxidant activity. Furthermore, synergism was observed in terms of both oxidation and thermal stabilities through thermogravimetric analysis (TGA).The confirmation has been studied by means of electron spin resonance spectroscopy (ESR) to obtain information on the thermal oxidation mechanism of the synergistic antioxidant.