The role of quinoid derivatives in the UV-initiated synergistic interaction mechanism of HALS and phenolic antioxidants

Within this work, UV-initiated interaction mechanisms between the phenolic antioxidant Irganox 1330 and nine commercially used hindered amine light stabilizers (HALS) were successfully studied using high-performance liquid chromatography (HPLC) coupled to UV and high-resolution mass spectrometric detection (MS). An analytical evaluation of the stabilizer performances in the polypropylene-mimicking solvent squalane revealed that all investigated HALS exhibited a strong synergism when combined with the phenolic antioxidant. Up to now, the synergistic interaction was described as a result of the hydrogen transfer from a hydroxylamine derived from HALS to the oxidized form of a primary antioxidant, whereby the phenol is regenerated. Investigations on degradation products, however, indicated that the proposed interaction mechanism cannot be applied to a sterically hindered phenol such as Irganox 1330. Instead, a completely new stabilization mechanism of phenolic antioxidants in the presence of HALS, involving the formation of quinoid derivatives, was discovered and confirmed by using a model compound.