The effect of base stabilization on the recyclability of polypropylene as studied by multi-cell imaging chemiluminescence and microcalorimetrv

Increasing costs of landfill space and a growing interest in waste minimization have made recycling and reuse of plastic products a matter of considerable importance. The influence of the base stabilization on the recyclability of polypropylene (PP) was determined by subjecting two polypropylenes with different base stabilization systems to different sequences of oven ageing and reprocessing (only oven ageing, only reprocessing and alternating processing and oven ageing). The base stabilization of the PPs used was a combination of Irganox 1076 and Irgafos 168 (PP]) and a combination of Irganox 1010 and Irgafos 168 (PP2). The influence of oven ageing and reprocessing was determined using mechanical property, colour and oxidative stability measurements. The oxidative stability was determined using chemiluminescence (CL) techniques involving oxidation induction time (OIT) and total luminous intensity (TLI). For comparison, microcalorimetry was also used. It has been shown that during oven ageing in air PPI rapidly developed strong signs of heterogeneous degradation after more than 500 h at 130 DGC, while PP2 showed no indication of degradation at that time. Repeated reprocessing at 230 DGC showed a linear degradation rate and no convincing difference in drop in melt viscosity, elongation at break or yellowness change could be observed between PP1 and PP2. The combination of oven ageing and reprocessing gives an antagonistic effect. In this case, PPI and PP2 degrades more than during reprocessing and oven ageing performed separately. This shows the importance of combining processing and oven ageing for the assessment of the recyclability of polypropylene. From all techniques used, the OIT determined with CL and MC showed a clear difference in residual stability of differently treated materials and seem to be the most valuable analytical methods to determine the recyclability of PP. It was also shown that TLI is not a reliable technique when the stabilizer package is unknown. Although the stabilization system of PP1 seems to be good enough to prevent degradation during (re)processing and might be good enough as base stabilization system, it is not suitable for polypropylene that will be recycled.