University of Manchester Study Finds Plastic Contamination Weakens PP

Impact of HDPE on Polypropylene

The study, published in Chem Circularity, specifically examined the effects of mixing polypropylene (PP) with high density polyethylene (HDPE). While HDPE is capable of tolerating a few percent of contamination, the researchers discovered that PP is far more sensitive. Even a 1% presence of HDPE in PP is enough to alter the material’s degradation mechanism.

According to the findings, contaminants function as stress aggregators. This leads to phase separation, which ultimately weakens the material. This contamination results in a recyclate with radically altered physical properties, which impacts structure-property relationships and diminishes the overall performance of the plastic. These changes are particularly significant when the materials are intended for industrial reuse.

The Role of Mechanical Recycling

Mechanical recycling is described as the most environmentally positive method for converting plastic waste into new products within a circular plastics economy, provided it is executed correctly. Effective recycling requires strict sorting—for example, ensuring that PP yogurt pots are separated from HDPE milk jugs—to allow pots to be turned back into pots and jugs into jugs.

However, inadequate sorting leads to the mixing of polyolefins, which changes how the plastic degrades during the recycling process. While these degradation patterns are well understood for polyethylene, the University of Manchester study highlights the specific risks associated with PP contamination.

Need for Quality Control and Analytics

Because of these risks, researchers suggest that industrial processes require better analytical tools to serve as quality control. This highlights a broader need for improved sorting practices in both industrial and household recycling settings.

Professor Michael Shaver stated that developing the quality controls necessary to assess contamination in recycled products is an essential step toward a future where a circular plastics economy is essential for developing a sustainable relationship with plastic waste.

Broader Context of Plastic Waste

The challenges of mechanical recycling exist within a larger global pollution crisis. Projections from the OECD indicate that global plastic use could triple by 2060 relative to 2019 levels, reaching 1,231 Mt annually. During that same period, plastic waste generation is projected to rise to 1,014 Mt, with 34–55 Mt leaking into the environment each year.

Recycling rates remain low; in 2019, only 9% of global plastic waste was recycled, while nearly 50% was landfilled and 19% was incinerated. Although recycling rates are projected to increase to 17% by 2060, incineration and landfill are still expected to be the dominant waste management pathways, accounting for approximately 18% and 50% respectively.

Certain plastics, including PP, polyethylene (PE), and polystyrene (PS), possess stable covalent bonds in their backbones. This gives them an intrinsic resistance to enzymatic cleavage because they lack the reactive heteroatom-containing linkages that enzymes can target, making them more difficult to biodegrade compared to materials like polyethylene terephthalate (PET).