This is the first of a series dedicated to highlighting materials science innovation facilitated by Royce funding and equipment.
Background
Recycling plays a critical role in reducing the quantity of plastics going into landfill. In the UK, 78% of all post-consumer plastic is recovered, either via recycling or used to create energy from waste, whilst the overall EU average is 42%.
The circular economy has emerged as a promising solution to contemporary environmental and economic challenges. This of course prioritises the minimisation of waste by emphasising the reuse, recycling, and repair of materials, and in turn extending the lifespan of products and their components. By reducing the reliance on excessive resource consumption to drive economic growth, the circular economy aims to create a more sustainable and efficient system.
Currently, quality control for post-consumer recycled plastics poses a challenge to implementing circular economies. This is particularly true for recycling high-density polyethylene (HDPE), commonly used in packaging, consumer goods, and industrial applications.
Extrusion-based experiments currently test recycled HDPE (whereby the plastic is melted, reprocessed, and tested for property changes like molecular weight and tensile strength), which is time-consuming, expensive, and requires large amounts of material. This makes routine quality control impractical, especially for industries that want to integrate higher amounts of recycled content into their products.
Professor Michael Shaver and his research team at the Royce Sustainable Materials Innovation Hub (SMIH), based in the Royce Hub Building, have been developing a new method for assessing the degradation of HDPE during recycling that is more efficient, accurate and economically viable. The potential knock-on effect allows industry and governments alike to reduce plastics going to landfill and thus getting closer to meeting environmental targets pertaining to waste reduction.
The Challenge
A critical barrier in recycling is the quality of post-consumer recyclate (PCR), consisting of previously used plastic packaging and bottles (including those made of HDPE) that are collected, sorted and mechanically recycled – often repeatedly. However, impurities in the PCR can remain, such as food residue, dyes, and other polymer contaminants that can adversely affect the quality, in comparison to that of virgin plastics.
Due to the impracticality of current extrusion-based methods of assessing PCR quality, Prof. Shaver’s research team, in their paper, “Defining quality by quantifying degradation in the mechanical recycling of polyethylene”, wished to explore an alternative approach:
“We wanted to develop a method that could simulate degradation in a more practical way, using smaller sample sizes and less time. The result was a rheological simulation that tracks the changes in the material’s viscoelastic properties during recycling.”
The Solution: A New Rheological Simulation Method
By using rheology – measuring flow and deformation of materials – the research team developed a simulation that mimicked the extrusion assessment process for the recycled HDPE but reduced the barriers (timeframe and quantity of materials) that made the current process so impractical. Rheology allowed the team to examine the behaviour of HDPE under extreme heat and stress, and therefore determine the quality of the product for end-use.
Using a rheometer, the team was able to simulate the degradation mechanisms in HDPE. Two key degradation pathways were observed during mechanical recycling:
Chain Scission: This occurs when the long polymer chains that make up HDPE break into shorter fragments due to thermo-mechanical forces. Chain scission typically occurs during the early stages of recycling and leads to a reduction in the polymer’s molecular weight.
Long-Chain Branching (LCB): This describes when new branches form along the polymer backbone. As the recycling process continues, particularly in the presence of oxygen, degradation shifts from chain scission to long-chain branching. This can solidify the material, making it resistant to deformation.
As both degradation pathways can occur in tandem during reprocessing, unpicking this interconnection was essential.
A New Metric for Plastics Recycling
By analysing these changes in molecular structure, the researchers were able to develop a new degradation metric called Vdeg.
Derived from Van Gurp-Palmen (vGP) plots on a graph of the measured rheological data, the Vdeg metric quantifies the extent of degradation that a polymer undergoes during recycling. Vdeg allows for a clear comparison between different grades of HDPE, as well as between virgin plastics and PCR.
“What’s particularly exciting about this research is its potential for real-time quality control,” said Prof. Shaver, “The Vdeg parameter allows us to quantify degradation quickly and accurately, providing industries with a powerful tool to assess whether a given batch of recyclate is suitable for use.”
Vdeg allows manufacturers to gauge PCR quality early in the assessment process, identifying batches that may degrade faster or contain impurities, and assessing whether to blend batches with virgin plastics or to add stabilisers to improve its performance.
Wider Implications
This research has far-reaching implications for a wide range of sectors. A few examples are below:
For Industry: This method can make recycling more economically viable. With more efficient and reliable quality control, manufacturers can confidently integrate higher proportions of PCR into their products, lowering costs and reducing their reliance on virgin plastics. The Vdeg method could be integrated into smart manufacturing systems for real-time, in-line quality control to minimise costly product failures and recalls.
For Government and Policy: This research could inform new standards and regulations for the recycling industry. Governments could adopt the Vdeg metric as part of recycling certification systems, ensuring that post-consumer plastics meet quality benchmarks before they are reintroduced into the economy. This would support circular economy initiatives and help meet sustainability targets. Industry could adopt the Vdeg metric to help meet new legislation demands such as the Plastic Packaging Tax or Extended Producer Responsibility.
For Environmental Sustainability: Efficient PCR assessment and, consequently, better quality control will help reduce the amount of plastic waste sent to landfills and incinerators. By improving the quality of recycled plastics, this method could significantly reduce the need for virgin plastic production, thus lowering the carbon footprint associated with plastic manufacturing.
A Pathway for a Circular Economy
As the circular economy gains traction globally, this research represents a practical solution to addressing a critical challenge in the plastics industry: achieving parity in performance between recycled plastics, particularly HDPE packaging, and virgin materials. By leveraging tools like Vdeg to enhance the cost-effectiveness, efficiency, and sustainability of recycling processes, manufacturers can truly contribute to a sustainable future – one where plastics transition from being consumer waste to becoming continuous resources within a closed-loop system.
Read Prof Michael Shaver’s research paper here.
Read more about research activities at Royce here.
