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A study from The University of Manchester into the effects of the cross-contamination of polymers on the outcome of mechanically recycling plastics has found that the presence of contaminants radically alters degradation mechanisms.

When it is done well, in other words when your yogurt pots (polypropylene, PP) are sorted from your milk jugs (high density polyethylene), mechanical recycling is the most environmentally positive way to turn plastic waste into new products in a circular plastics economy, turning pots into pots and jugs into jugs.

However, poor sorting can lead to mixing polyolefins together, changing the way that the plastic degrades during the recycling process.

While well understood for polyethylene, the study, published in , focuses on what happens when PP is mixed with HDPE. Researchers found that contaminants act as stress aggregators, and that the phase separation caused by contamination weakens the material. While HDPE can tolerate a few percent contamination, even 1% of HDPE in PP alters the degradation mechanism.

The result of this contamination, even at this small scale, is a recyclate with radically altered physical properties, a change which impacts structure-property relationships and reduces the overall performance of the material, especially important in the context of their industrial reuse.

Researchers argue that while efforts have been made to normalise polymers based on processing history and end-use application, approaches like these only consider a limited number of material grades and do not represent the broad range of polyolefin grades and blends.

They suggest that cross-contamination needs better analytical tools that can serve as quality control in industrial processes, evidencing the need for better sortation in household and industrial recycling.

This paper was published in the journal: ChemCircularity

Full title: Measuring the impact of cross-contamination on quality in polyolefin blends

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