THE PROBLEM

Today only a very small fraction of plastic packaging is actually recycled. One of the biggest hurdles to high volumes and quality of recycled plastics is the complexity of the packaging design. Countless packaging formats with many material combinations are in use while the sorting technology on the end-of-use side is capable of recognizing only a few properties (such as 3D vs. 2D shapes or different infrared absorption for different polymers and/or VIS for color recognition). There are several examples where the design of packaging disrupts the recycling, either by making an item ‘invisible’ (e.g. black PET trays cannot be seen by detectors) or leading to false positives/negatives (such as the negative sorting of a full-sleeve PET bottle because the sleeve is typically made of a different material which means the PET is not detected).

THE SOLUTION

One approach to improve automated detection and sorting that has received a lot of interest is to ‘tag’ an item with a unique ‘code’. There are several ways of doing this. The methods that have received the most interest involve chemical tracers and digital watermarks. Tagging is accomplished by applying chemical(s) to the label, sleeve or plastic object (chemical tracers) or by applying a ‘digital watermark’, which can be printed on the label (shrink sleeve, in-mould label, paper or other material) or physically incorporated as a subtle pattern embossed in the plastic itself.

The HolyGrail project was set up to investigate if such chemical tracers and digital watermarks could provide a reliable and efficient tagging system that could eventually be deployed on a large scale, and if so how they might contribute to a circular economy for plastics. 

Chemical tracers and digital watermarks tag items use the same basic principle – they introduce a machine-readable code or identifier into an item. Then, rather than relying only on the ability to distinguish one or several properties of collected items (e.g. shape, density, IR spectrum of the resin, visual identification), detecting and reading the code provides the sorting system with information unique to that item by pointing to a database where that information is stored. The information tells the system in which way to sort the item (e.g. food grade vs. non-foodgrade) Such information can be updated and expanded over time, e.g. to allow for changing sorting preferences if an items recyclability improves.

Participation in HolyGrail has grown significantly since its initiation and now includes stakeholders from the entire value chain (including firms and organizations from outside the New Plastics Economy participant group [see below]). This is key to determining how to use chemical tracer and digital watermarking technology in practice, since the whole value chain would need to agree on a number of principles as well as technical standards. The project group notably includes leading machine vendors Tomra and Pellenc, digital watermarking pioneer Digimarc and FiliGrade, a Digimarc licensee, as well as material producers, packaging manufacturers, brands, retailers and recyclers.

The HolyGrail project is led by Procter & Gamble and facilitated by the Ellen MacArthur Foundation.

Source: Ellen MacArthur Foundation

ALTERNATIVE SOLUTIONS

Current sorting technologies are capable of recognizing only a few properties (such as 3D vs. 2D shapes or different infrared absorption for different polymers and/or VIS for color recognition. Though with computer vision and machine learning models, these technologies are improving sorting capabilities and efficiencies.

CASE STUDY EXAMPLES

For chemical tracers, the UK-funded project called PRISM was started in 2016. PRISM has been led by Nextek which is also part of the HolyGrail participant group. It showed encouraging results for its pilot system to sort food grade plastics from non-food grade plastics. Sorting of full-sleeve PET bottles tagged with fluorescent tracers detected using a UV lamp resulted in high accuracy and yields at industrial speed. 

The PRISM proof of concept is an important benchmark for digital watermarks. However, digital watermark testing started later and is in an earlier stage of evaluation. The project group has conducted testing to understand applicability of sorting of different packaging formats (e.g. food vs. non-food grade, dark trays, multi-layer structures, full-body sleeved bottles, flexible plastics, etc.) using digital watermarks in the label or the mould itself. The preliminary tests, are so far encouraging, showing that digital watermark technology works in several different applications, in addition to sorting food-grade and non-food grade, and can be retrofitted into sorting facilities. A final round of tests within the initial scope of the project concluded in May 2019, where watermark-enhanced plastics items mixed with true waste was conducted.