Hindered phenolic antioxidant with high transamidation reactivity for the long-term thermo-oxidative stabilization of polyamide 6,6

A hindered phenolic antioxidant (AO) based on a 1,3,5-triazine-2,4,6-N amide structure was synthesized. The resonance energy (E_R) and a theoretical reactivity descriptor for intermolecular reactivity analysis, namely local hyper-softness (s⁽²⁾(r)), were calculated by computational methods to assess the transamidation reactivity of the AO. The E_R of the amide bond in the AO was determined to be only 6–7 kcal/mol, which can be attributed to the electron-withdrawing effect of the triazine ring. Owing to this characteristic, the AO exhibits high transamidation reactivity, which allowed us to effectively graft the hindered phenol functional groups to the polyamide 66 (PA66) molecular chain with a grafting rate of almost 60 % through a facile twin-screw extrusion process. The results of accelerated aging experiments demonstrated that the PA66/AO samples, which were grafted with 5.97 μmol/g of the hindered phenol functional groups, exhibited thermo-oxidative stability comparable to that of the PA66 samples containing 8.30 μmol/g of the commercial hindered phenolic antioxidant Irganox 1098. Furthermore, the grafting process effectively prevented the loss of the antioxidant, allowing PA66/AO to maintain good thermo-oxidative stability even after extraction with organic solvents such as ethanol or ethylene glycol. In contrast, PA66/1098 was susceptible to thermo-oxidative degradation due to the physical loss of Irganox 1098 during the extraction experiment.