Effect of post‐curing temperature on the retention of mechanical strength of phthalonitrile thermosets and composites after a long‐term thermal oxidative aging

Abstract This work concerns the thermal‐oxidative aging behavior of phthalonitrile thermosets and composites at 280–350°C. The easy‐to‐process resin containing bis(3‐[3,4‐dicyanophenoxy]phenyl)phenyl phosphate with APB as curing agent and the resin‐based composites post‐cured at 330°C, 350°C, and 375°C were studied. The phthalonitrile thermosets post‐cured at 330°C retained flexural strength of 77 MPa after 200 h of thermal aging at 280°C and 37 MPa (40%) after the same time at 300°C while the resins post‐cured at 350°C and 375°C lost over 80% of the flexural strength in these experiments. The same trend of faster oxidation of the samples post‐cured at higher temperatures was observed for the composites. For the first time, crosslinking reactions were observed during the aging, despite the aging temperatures being below the Tg of thermosets. Thus, the work shows a high long‐term thermal oxidative stability of the studied phthalonitrile resins at temperatures up to 300°C and fast destruction of the materials at 350°C despite the resin decomposition temperatures determined by dynamic TGA being over 500°C. The work has shown the negative effect of high‐temperature post‐curing on the operating properties of the phthalonitrile composites as constructive materials for long‐term application at elevated temperatures. Highlights Oxidation of phthalonitrile thermoset occurs at temperatures over 300°C in air. Increase in post‐curing temperature decreases thermal oxidation resistance. Thermosets retained up to 77% of flexural strength after 200 h aging at 300°C. Composites retained over 90% of ILSS after 200 h of aging at 300°C.