Yellowing mechanism of PA56 during thermal oxidation process

The yellowing behavior of aliphatic polyamides at high temperatures considerably affects their performance and usability. Herein, this is the case for PA56, a typical bio-based odd–even polyamide with a structure similar to PA66. Furthermore, it exhibits the yellowing behavior during fiber- and film-thermal setting processes more severely than PA66. However, the mechanism behind this yellowing remains to be elucidated. The yellowing of aliphatic polyamides such as PA66 is usually considered to be caused by the oxidation of the N-vicinal methylene group and subsequent formation of chromophores such as pyrrole-type groups, α,β-unsaturated aldehydes, and conjugated azomethines. However, the above yellowing mechanisms were inconsistent with the experimental results observed during studies on the yellowing of PA56 during thermal oxidation. Herein, to shed some light on the yellowing mechanism of aliphatic polyamides, PA56 was oxidized at 180 °C and the resulting yellow substances were extracted with solvents and separated. The characterization of thermo-oxidized PA56 and generated yellow substances via ¹H NMR, ¹³C NMR, FTIR, UV–vis and fluorescence spectroscopies, and mass spectrometry revealed that conjugated N-acylamide was the chromophore responsible for the yellowing of PA56 during the thermal oxidation process, thereby proposing the mechanism behind the yellowing phenomenon. Although the same chromophore structure was deduced for PA56 and PA66 under identical conditions, conjugated N-acylamide produced via the thermal oxidation of PA56 was more stable and accumulated easily than PA66 because of the presence of hyper-conjugation, rendering PA56 more susceptible to yellowing during thermal oxidation. This study offers new insights into the thermo-oxidative transformation of aliphatic AB-type polyamides and their yellowing mechanism, thereby helping in the development of strategies that inhibit the yellowing of aliphatic polyamides.