A completely different system
Blackburn accepts that widespread adoption could be a challenge in a conservative industry but adds: “That’s why it’s important to engage with brands because you would work with that supply chain to make the necessary interventions.
“Supercritical CO2 is a completely different system for existing dyes, whereas our approach was to modify the dyes to allow to them to work in the existing system. We do a bit of chemistry on the dyes, that’s the innovation.”
Another benefit is that the reductive clearing process, which produces harmful chemicals in existing polyester dyeing, can be achieved using a mild acid such as citric acid.
Crompton adds: “But if we up the conditions, we can also remove some of the dye which has been absorbed into the centre of the polyester. So, there are two ways we can exploit the reversibility after dyeing.”
In tests, the team has been able to recycle the dye bath up to five iterations. At the end of each cycle, they analyse how much dye has been exhausted and then top it up and add a bit more CO2 to get back to the original levels.
As well as saving on dye, they also found that water loss was only around 10% over five cycles, plus the savings on the chemicals needed for disperse dyeing that are not required for their method.
With conventional dyeing, you can’t recycle the water because its full of chemicals which would not be economically viable to remove. Whereas with our system, there’s nothing left other than dye. And that’s reusable and recyclable.
Blackburn says: “With conventional dyeing, you can’t recycle the water because its full of chemicals which would not be economically viable to remove. Whereas with our system, there’s nothing left other than dye. And that’s reusable and recyclable.”
The process also uses less energy as the temperature can remain at 80C between dyeing cycles without the need to heat it up again from 20C.
John Lewis funding
The research was funded by UK retailer John Lewis and also supported by the Hubbub environmental charity, the Clothworkers’ Company and UK Research and Innovation. However, the team are looking for further partners to take their work forward.
As Blackburn says: “The long term ambition is that this replaces a significant proportion of the existing dispersed dyeing industry.
“We’re currently running pilot scale trials for an external collaborator to demonstrate that we can use this on an industrial scale. We’ve got a mini jet dyeing machine in the university so we have the capability to do that.
“But ultimately, we need to be able to work with potential investors – inside or outside of the industry – who believe in the technology and can see the real benefits in scaling this up.
“This should be of interest to anybody that works with dyeing polyester, which is pretty much the whole apparel and garment industry. I think we’ve taken this as far as we can take it from lab scale. It’s all about scaling it up now.
“That’s all further down the line. But that’s our ambition. if we don’t have that ambition, what’s the point? We’re not doing this for a little capsule collection, it has to be transformative for the industry.”
Textile dyes and chemicals expert Phil Patterson commented that the University of Leeds method did appear to offer an opportunity to reduce chemical consumption in dyeing – especially in the post-dye wash-off.
“Due to the sheer volumes of polyester dyed, even relatively small improvements can result in large cumulative benefits,” he said, adding that some chemicals, such as anti-creasing agents and anti-foams would most likely still be needed.
“Overall, the development of a drop-in solution will be much more likely to scale than super-expensive new kit and the holistic thinking around lower impact application with circularity in mind has to be roundly applauded,” concluded Patterson.
