Today's KNOWLEDGE Share : 100% purity in PCR polyolefins? In reality, rPP often means PP + a bit of HDPE.

Today's KNOWLEDGE Share

100% purity in PCR polyolefins? In reality, rPP often means PP + a bit of HDPE.

In post consumer recycled (PCR) streams, achieving truly single polymer purity for rPP or rHDPE is harder than many specs assume. Why?

1) PP and HDPE behave almost the same in classic swim sink separation

Both PP (~0.91 g/cm³) and HDPE (~0.95 g/cm³) swim in water based sink–sink systems, so you can’t “density separate” them from each other in that step.

Even if a plant has 10+ sorting stages, this specific pairing remains a bottleneck. Just NIR and AI manage the separation.

2) Fillers change the game (and can “fake” separation)

If PP or PE is filled with inorganic additives, the density increases and parts may sink.

Reality from the line:

In many commercial rPP / rHDPE grades, a small cross contamination is normal. In practice, pure rPP often still contains around 1% PE/HDPE. And that small amount normally doesn’t cause problems in further processing.

🔬 Measuring that contamination: DSC is underrated

A surprisingly robust way to quantify cross polyolefin contamination is DSC at second heating:

PP melting peak typically ~163°C

HDPE melting peak typically ~125°C

By integrating the melting enthalpies (peak areas) you can estimate the HDPE-in-PP (or PP-in-PE) content quite accurately—> often accurate enough for specifications, supplier qualification, and process troubleshooting.

To achieve that, I suggest to build a calibration curve using known blends like 0.5%, 1%, 2%, and 5% HDPE in PP and express the result as the fraction of each polymer relative to the total melting peak area.

source : Žiga Vidic

#recycling #polyolefins #rPP #rHDPE #PCR #DSC #qualitycontrol