Deterioration of Polypropylene Random Copolymer Formulations Based on Phenolic Antioxidants and Non‐Phenolic Stabilizers at Elevated Temperatures in Hot Air

Abstract

This article deals with the hot air aging behavior of polypropylene random copolymers additivated with phenolic antioxidants, hindered amine light stabilizers (HALS) and thiosynergists in a wide temperature range from 65°C to 135°C. Various stabilizer mixtures and their interactions were evaluated. To avoid diffusion‐limited oxidation effects, 100 μm thick specimens were prepared from injection molded sheets and examined by Fourier‐transform infrared spectroscopy in transmission mode and tensile testing. In addition to ultimate strain‐at‐break, the aging indicators phenol index (PI), nitroxide index (NI), and ester index (EI) were deduced. Monitoring the phenolic functional groups revealed the formation of splitting‐off derivatives which remained beyond full embrittlement of the micro‐specimen at a relative content of app. 20%. Oxidation of the phenolic stabilizer Irganox 1330 resulted in shifts of the IR‐band associated with single‐ and dual‐oxidized products. Gaussian fitting of the triple peak confirmed the formation of these splitting‐off derivatives. The combination of phenolic antioxidants with HALS led to a significantly faster decay of the phenolic groups. However, this effect was reduced by the addition of thiosynergists. Interestingly, a slower decrease of the phenol, nitroxide and ester indexes representing phenolic‐, amine‐ and thio‐stabilizers, respectively, was ascertained for the PP‐R formulation with the ternary additive mixture leading to higher endurance times by a factor of up to 2.5. The analytical aging indicators did not reflect the time‐to‐embrittlement, especially for complex additive formulations containing HALS.