Tieteestä & Tekniikasta
How Bulk Heterogeneity in Feedstocks Drives Performance Variability in Post-Consumer Recycled Polypropylene Plastic consumption is expected to rise in the coming years. However, regulatory frameworks target that by 2050, only 35% of total plastic use should rely on fossil-based materials, while up to 65% should come from circular sources.
Authors: Milad Kianpisheh 1 , Tuan Anh Tran 2 , Doris Machl 2 , Essi Sarlin 1 Figures: Milad Kianpisheh
Although plastic recycling rates in Europe have steadily increased, recycled materials just account for about 15% of plastic used in new products. One of the reasons for this conservative, slow substitution of virgin plastic with mechanically recycled material is the lower quality and the inconsistency of its properties. Since this drawback stems from the inherent variability in collected waste, especially in post-consumer recycled (PCR) feedstocks, better understanding the root causes and overcoming them is a necessity that needs to be addressed. Due to the important position of polyolefins, especially polypropylene (PP), among consumer plastics, understanding and addressing the roots of heterogeneity in post-consumer recycled polypropylene (PCR-PP) is a high priority in mechanical recycling development. Although it is commonly assumed that this problem is related to the presence of contamination and impurities, modern sorting, washing, and separation technologies effectively limit their impact. For example, PCR-PP feedstocks sorted based on the DSD 324 system are reported to have an average PP purity of 94%. However, property inconsistency is not completely eliminated. Therefore, identifying sources within the intrinsic
features of washed and sorted PCR-PP feedstocks remains a key question that we addressed in our recent work at the Plastics and Elastomer Technology Group at Tampere University. To further investigate this, 11 rigid PCR-PP feedstocks collected over one year from two waste collection centers in different regions were analyzed. In the first step, feedstock characterization focused on conventional indicators of PCR-PP. As depicted in the Figure 2 below, the feedstocks exhibited significant variation in melt flow rate (MFR = 13–23 g/10 min, measured at 230°C and 2.16 kg load) and ethylene content ( = 5–11%), whereas ash content (Ash% = 1.2–1.9%) remained relatively consistent across all samples. Therefore, this conventional framework reveals two important sources of property inconsistency. However, differences in mechanical properties were still observed among feedstocks with similar MFR and ethylene content, indicating that these parameters alone cannot explain the observed variability. This observation indicates the presence of other important but unknown source of inconsistency among the samples. A comparison of feedstocks with identical MFR
and ethylene content demonstrates that this additional factor effectively influences many properties, including tensile strength, impact strength, and even elongation at break, as shown in Figure 2. Extensive experimental investigations, including rheological studies, were conducted to identify these hidden sources of discrepancy. Combined morphological and rheological analyses finally revealed that the additional variability arises from differences in the interfacial
Figure 2: Representative tensile-test images illustrating differences in elongation at break behavior among rPP samples
22 MUOVIPLAST 4–5/2026
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