Chemical markers improve packaging waste sorting

Materials World magazine
6 Aug 2019

Chemical markers have been proven to quickly and accurately segregate food-grade plastics for recycling. Ceri Jones finds out more.

A polymer consortium has successfully demonstrated its European Commission-funded technology, Polymark, to improve recycling and reclaim more food-safe waste plastics.

‘A staggering 32% of plastic packaging escapes collection systems, generating significant economic costs. After a short first-use cycle, 95% of plastic packaging material value, or US$80–120bln annually, is lost to the economy,’ according to 2016 Ellen MacArthur Foundation report, The new plastics economy, rethinking the future of plastics.

This mountain of material waste is exacerbated by an EU regulation (EC/282/2008), which states that recycled packaging for food and drink products can only be manufactured from materials originally certified as food safe. Current processes do not help stakeholders in the materials value chain, as it is very difficult and time consuming to reclaim specific plastics from bales of mixed recycling. Therefore, an abundance of useful material resource is lost, and manufacturers use more virgin plastics, so are at the mercy of changeable prices of oil-based raw materials and carbon fees.

The Polymark chemical marker project gained attention in the MacArthur report and has progressed in the three years since. The programme proposes to impregnate polyethylene terephthalate (PET) food-grade plastics with a chemical that can be detected during waste sorting, to help segregate and salvage greater amounts of food contact materials. While PET is already widely recyclable, using markers would reduce the levels of virgin material used in food and drink products. Having run initial tests, the company claims this process has a 98% efficiency rate.

Bottle buy-in

While a seemingly simple system, Polymark would require cross-industry coordination to work effectively. The group has developed a two-part technology where the manufacturer uses a machine to coat or physically imprint the packaging or label, depending on the product type. Then a recycling facility uses a camera detection system to identify the marker at the end of its first life.

A range of chemicals were assessed to find a marker that met the criteria of being food safe, visible under UV even after being filled, distributed and bashed about, stable under heat and light, and water soluble. Of the 130 candidates, two top performers were found and one gained preference, 4,4’-bis(2benzoxazolyl)stilbene, called stilbene. This proved ideal for both compounding and coating techniques, and is detectable with inexpensive UV and visible (VIS) equipment.

In the second stage, the detection machine beams high-energy LED lights onto plastic packaging which excites the marker, causing it to emit fluorescent signals. UV light, VIS, or near-infrared can be used. A series of cameras with spatial resolution of 10mm ran up to 2,000 scans per second to identify marked PET bottles on a conveyor belt.

Recognised as safe

To test the equipment in a real-world environment, Polymark made a prototype sorting system to operate at practical speeds. Trials were run on fully and partially coated PET bottles, sorting on a belt at 3m/s, a working width of 1m and a throughput of two tonnes per hour. It averaged a 98% efficiency for fully coated bottles and a low of 94% when they were only partly coated. Overall, the system had just 5% false rejects.

The stilbene marker coating has received gained food-contact certification, is thermally stable, and does not affect the packaging appearance or product flavour. Importantly, it can be removed in the normal washing process, so would not accumulate or interfere with further processing. The process is now market-ready.   

Polymark was contacted for comment on how the technology would be rolled out, but was unable to respond in time. Please check the Materials World website for updates on this story.