Antioxidant activity of p-phenylenediamines studied by DSC

The antioxidant activity of six N,N′-substituted p-phenylenediamines in polyisoprene rubber matrix has been studied by differential scanning calorimetry (DSC) under non-isothermal conditions. A method for obtaining the parameters from non-isothermal DSC measurements, based on the dependence of the onset temperature of the oxidation peak on heating rate, is presented. The parameters of an Arrhenius-like equation describing the temperature dependence of induction period have been obtained. The results show that the values of protection factors decrease with increasing temperature and decreasing concentration of the antioxidant. Two criteria characterizing the antioxidant effectiveness are proposed. Using both criteria, effectiveness of the antioxidants under study at 180 °C is in the order DPPD > SPPD > 6PPD > IPPD > MBPPD > CPPD. When DPPD is excluded, this order is inversely proportional to the dissociation energy of the C-H bond at the carbon atom in the neighbourhood of the nitrogen atom. Substitution of all hydrogen atoms at that carbon atom leads to a loss of antioxidant properties in the case of CPPD.