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

Abstract

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.