Kinetics of the oxidative aging of phthalonitrile resins and their effects on the mechanical properties of thermosets

•Thermal oxidation of phthalonitrile resins is a complex multi-step process.•Thermal stability of the phthalonitrile resins depends on post-curing temperature.•The long-term use of the phthalonitrile resins under air is viable up to 573 K. The process of thermal oxidation decomposition of phthalonitrile resins which had been post-cured at various temperatures (603 K, 623 K and 648 K) was studied using dynamic and isothermal measurement modes. It was shown that the degradation process is a complex branched four-stage process. We have concluded that the first stage is an nth order reaction with autocatalysis. The second stage is described with the expanded Prout-Tompkins equation which corresponds to an autocatalytic solid-state reaction caused by the formation of imperfections at the reaction surface. The third and fourth stages are also reactions of the nth order. The phthalonitrile resin post-cured at 603 K exhibited better thermal stability than the resins post-cured at 623 K and 648 K based on the activation energies of the 1st stage – 132 kJ/mol, 104 kJ/mol, and 106 kJ/mol respectively. These data were confirmed by comparable experimental activation energy values (111 kJ/mol, 87 kJ/mol, 86 kJ/mol respectively) and changes in mass loss and flexural strength values determined during long-term thermal aging in the temperature range of 553–623 K. It was shown that the phthalonitrile resin post-cured at 603 K with the initial flexural strength of 107 MPa is suitable for long-term application at temperatures up to 573 K.