Characterization of the influence of specimen thickness on the aging behavior of a polypropylene based model compound

Specimens with varying thickness of different polypropylene random copolymer formulations were oven aged in hot air at 135 °C. Besides macro-sized specimens die-punched from 2 mm thick extruded sheets, 50 μm, 200 μm and 500 μm thick micro-sized specimens, cut edgewise from the sheets by an automated planing procedure, were investigated. The formulations included a base stabilized reference compound and four formulations prepared with systematically varied additional contents of the primary antioxidants Naugard 445 and Irganox 1010 and the secondary antioxidants Irganox PS-800 and Irganox PS-802. Aging characterization was performed by tensile testing, differential scanning calorimetry and high performance liquid chromatography employing strain-at-break, oxidation onset temperature (OOT) and the content in phenolic antioxidants as aging indicators, respectively. Aging characterization revealed a systematic thickness dependence of the embrittlement time and the rate of the reduction in OOT as well as the reduction of phenolic antioxidants. The 50 μm thick specimens exhibited embrittlement times between 50% and 65% of the 2 mm thick macro-sized specimens. For low thicknesses, a high surface to volume ratio is presumably responsible for the fast physical loss of antioxidants, while for thick specimens, physical stabilizer loss is limited by the rate of stabilizer diffusion from the bulk to the surface. Hence, thicker specimens showed longer embrittlement times. Regarding stabilizer loss, Naugard 445 and Irganox PS-800 exhibited significantly higher loss rates than Irganox 1010 and Irganox PS-802.