489: e-Beam treatment of post-consumer recycled polypropylene

A source for post-consumer recycled (PCR) polypropylene (PP) is household waste, which mainly consists of packaging waste made from injection moulding [1]. Its melt viscosity is low, with melt-flow rates (MFR, @ 230°C, 2.16 kg) above 10 g/10min. This makes these PCR-PPs perfect to again use them for injection moulding, but not for methods that require high melt viscosities such as extrusion (0-3 g/10min). Thus, the market for extrusion grade PCR-PP is scarce and recycling facilities need strategies to lower the MFR value of PCR-PP, to offer it in extrusion grade to their customers.
One strategy to reduce the MFR of PCR-PP is to irradiate it via e-beam radiation in presence of a cross-linking molecule [2]. Melt-rheology and gel content measurements show that this effect results from a change from a linear PP chain to a microgel structure, where cross-linked microparticles disperse in a PP matrix.
Contrary to their virgin counterparts, PCR-PPs often contain polyethylene (PE), which readily cross-links under irradiation. Due to fluctuations in consumer behavior, the amount of PE in pre-sorted waste bales of PCR-PP varies by up to 10 wt.% [1]. Thus, the amount of insoluble fraction in PCR-PP after irradiation varies too, which affects the change in MFR value significantly. This poster highlights three aspects: 1) accurate analytics for PE in PCR-PPs is necessary as well as 2) viable sorting machines to minimize fluctuations in pre-sorted waste bales and 3) e-beam irradiation of PCR-PP is a strategy to adapt its MFR to a different processing technique.

[1]    J. Geier, M. Bredács, A. Witschnigg, D. Vollprecht, G. Oreski, Analysis of different polypropylene waste bales: Evaluation of the source material for PP recycling, Waste Manag. Res. (2024) 734242X241227369. https://doi.org/10.1177/0734242X241227369.

[2]    J. Krämer, G.C. La Guedes de Cruz, W. Kern, J. Roitner, A. Witschnigg, F. Rittmannsberger, K. Schnetzinger, Increasing the melt viscosity of post-consumer recycled polypropylene via E-Beam techniques, Radiation Physics and Chemistry 222 (2024) 111846. https://doi.org/10.1016/j.radphyschem.2024.111846.