Repeat performance
The team has devised a step-by-step plan for recycling dyes from PET bottles.
Bert Weckhuysen will spend the next five years researching how dyes from plastic bottles can be better recycled. In August, he received a grant for this from the Circular Plastics NL initiative of the Groeifonds. Weckhuysen is working with Jules Roelofs of Holland Colours, a dye manufacturer, and CuRe Technology, a company that recycles plastic.
In the supermarket you are surrounded by a sea of plastic bottles in all colours of the rainbow. Green, red, blue – these colours are added to plastics to make packaging more attractive. But what happens to all those dyes when the bottle is emptied and goes to be recycled? At present, usually nothing: the dyes disappear down the drain when the plastic is burned or perhaps gets a second life in a recycling process. That has to change, according to Bert Weckhuysen, University Professor of catalysis, energy and sustainability at Utrecht University.
Thanks to a new grant from Circular Plastics NL , Weckhuysen has five years to investigate how these dyes can be recycled. Together with Holland Colours and CuRe Technology, he and his research group want to develop technology to recover inorganic and organic dyes from polyesters, assess their quality and reuse the dyes in recycled materials.
The value of dyes
Dyes from plastics are often lost during the recycling process, which is a shame, Weckhuysen believes.
“The dyes and other additives give PET bottles the right properties and therefore their value,” he explains. “It is also often precisely these molecules that are more expensive to produce, and the production of which is also harmful to the environment. What if we find a way to capture these dyes and reuse them, just like the plastic itself?”
Dyes and other additives give plastic bottles their value.
“We hope to find a way to capture and reuse these substances,” Bert Weckhuysen, says.
Greener solvents
In order to recycle these dyes, the current recycling process needs to be overhauled.
“Right now, the focus is really only on recycling the polymers – the long chains of atoms that make up plastics,” Weckhuysen explains. In order to also be able to include the dyes, they first need to be filtered out of the plastic.
This requires solvents, but many of the solvents currently in use are harmful to the environment. Weckhuysen wants to change that: “We are opting for greener solvents, made from biowaste, so that the entire process becomes more sustainable.” Weckhuysen calls this approach double green – both the dyes and the plastic are given a second life in a more environmentally friendly way.
Faded T-shirt
Weckhuysen explains that the way dyes are attached to plastic is similar to the colour of clothing.
“Think of a red T-shirt that you’ve washed dozens of times,” he says. “The colour is not a fixed part of the fibres, but is attached to them, so to speak. Over time, the dye particles come loose and the shirt starts to fade – still perfect for cycling, but no longer for a party,” he laughs. Just like in textiles, dye particles in plastics are mixed with the long molecular chains of the material, and can come loose under certain conditions. This also makes it easier to remove the dyes from the plastic and reuse them.
Step by step
The team has already devised a step-by-step plan to filter dyes from the PET bottles. After dissolving the coloured PET bottles in bio-solvents, thus separating the dyes and polymers, they use light in the form of spectroscopy to determine which dyes are still usable. This technique makes it possible to distinguish damaged dye molecules from intact molecules, and to see which part of the dye can still be used for another round. The usable molecules are then filtered out of the mixture for reuse.
It’s a process of refining,” Weckhuysen explains. “Every usable dye that we recover is one step closer to a fully circular system.”
Sort
According to Weckhuysen, not every dye will be equally easy to recycle, and sorting dyes is therefore a challenge in the project – some dyes can be more complex than others, or more expensive to process.
“Dyes affect recycling in ways that you don’t always see,” adds Jules Roelofs, global innovation manager at Holland Colours. He says that different dyes require different approaches. This can cause problems during the recycling process, for example when detecting and separating coloured plastics from each other. “Sorting is crucial,” he emphasises. “We want to know what types of dye streams there are and from which ones we can extract the dyes, and from which ones we might not.”
“It would be fantastic if we could soon make products with recovered dyes,” Jules Roelofs, global innovation manager, Holland Colours.
Apply
If everything goes according to plan, Weckhuysen and Roelofs hope to scale up their project and actually put it into practice.
We ultimately want to use the knowledge gained to also apply this technology.” CuRe Technology will play a crucial role in this. CuRe Technology, a medium-sized company specialising in polyester recycling, has developed an innovative technology for the continuous recycling of polyester. They currently process a few kilos per hour, but want to move towards an installation that can recycle several kilotonnes annually.
“It would be fantastic if we could make products with recovered dyes,” Roelofs says enthusiastically. “Our ultimate goal is 100% reusability. By removing dyes from the plastic, we can not only reuse the dyes, but also maintain the quality of the plastic, so that the material can be reused more often.”
