e-Beam treatment of post-consumer recycled polypropylene: The influence of PE fluctuations on the melt-viscosity of PCR-PP

A source for post-consumer recycled (PCR) polypropylene (PP) is household waste, which mainly consists of packaging waste made from injection moulding. 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. 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.%. 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.