MIT engineers have designed a polyethylene-based elastic yarn that can be woven into stretchy, lightweight apparel and repeatedly respun.
The yarn is intended as an alternative to spandex-polyester and spandex-nylon blends, which are difficult to recycle together.
After 10 melt-and-respin cycles, the yarn remained as strong and flexible as conventional thread.
The average American throws out around 81 pounds of clothing each year, adding up to more than 11 million tons of textiles that end up in landfill or incinerators. There is no textile recycling process comparable to established systems for bottles and cans, beyond options such as donation centres, MIT said in a press release.
The yarn is being positioned as an alternative to elastic spandex-polyester and spandex-nylon yarns, which combine fibres that cannot be recycled together.
Svetlana Boriskina, research scientist in MIT’s Department of Mechanical Engineering said: “Eighty per cent of textiles on the US market currently contain some amount of spandex, which makes them nonrecyclable. There is no widely adopted technology now that recycles textiles into textiles. With our new yarn, we hope to change that.”
Boriskina and colleagues have published the yarn details in a study in ACS Materials Letters. MIT co-authors include first author SeongHyeon Kim, Duo Xu, Volodymyr Korolovych, Domingo Flores-Hernandez, Kaniz Moriam, and Daniel Braconnier.
According to the institute, Spandex is a polyurethane-based synthetic fibre that is springy but not very strong. A thread of elastic yarn typically comprises a spandex-based core surrounded by a tougher polyester or nylon sheath, a combination that provides stretch and strength but makes recycling difficult.
The yarns would need chemical treatment to separate the polyester sheath from the spandex core; the polyester-based sheath could then be melted and reused, but the yarn cannot be recycled as a whole without chemical separation.
Boriskina noted: “Even though chemical separation technologies exist, they add extra cost and complexity, and usually require toxic chemicals that are harmful to the environment. That’s why most stretchy garments go to the dump.”
In 2021, Boriskina’s group developed a yarn made from polyethylene, which MIT described as the most common type of plastic in the world and a thermoplastic that can be melted down and remade. The earlier work showed polyethylene could be spun into yarn and woven into garments, with the group then focusing on moisture wicking, stain resistance and cooling properties.
In the new study, the researchers aimed to tailor polyethylene yarn to mimic the strength and flexibility of spandex while demonstrating recyclability. They looked for stretchy polyethylene-based copolymers resembling a spandex elastic core and separately engineered polyethylene yarns for the sturdier sheath, evaluating chemical variations through scientific literature and industrial reports.
Boriskina added: “The chemical structure of polyethylene is like Christmas garland — a backbone of carbon, carbon, carbon, and also these dangling ‘decorations’ of hydrogen atoms or short branches with the same structure as a backbone. How these chains are arranged can change the properties of the whole structure.”
SeongHyeon Kim, first author stated: “Polyethylene can give us a wide range of properties, depending on how you make it.”
For the yarn core, MIT in its release said the team used a polyethylene-based resin that produces a stretchier fibre, while a second, stiffer resin formed the basis of the sheath. The researchers obtained pellets of each resin from a chemical manufacturer and processed them through fibre fabrication, heating the pellets to about 350 degrees Fahrenheit before drawing the melted polyethylene through small extruders to make hair-thin fibres.
Kim added: “You just melt it in a barrel with a heater, and then you extrude and spin it into fibers. It’s like a spaghetti machine.” The team used an industrial yarn spinner to wind the sheath fibres around a core fibre to make the final elastic yarn.
Because the yarn’s core and sheath both come from the same polyethylene chemical family, the materials do not need to be separated before recycling, unlike spandex-based elastic yarns. Boriskina said: “Because they are exactly the same chemistry, they play nicely together. That’s what makes this yarn very recyclable.”
To demonstrate recyclability, the team twisted an elastic core-sheath yarn, then melted it down and respun it 10 times. Each time, researchers tested mechanical properties by stretching a thread and measuring the pulling force at which it broke. The recycled versions were found to be as strong as the original sheath yarn and can be used to make new stretchy yarns by twisting them around a newly spun elastic core.
Boriskina said: “Now we have something that can be knitted and woven. That is the next stage.” The team said its polyethylene yarn recipe can be scaled up into industrial-sized spools, while noting that, as with conventional spandex fibres, kilometres of yarn are required to weave a single textile.
The researchers envision that a polyethylene garment could eventually be placed in a recycling bin and sent to a facility to be melted down and respun, supporting a more circular textile model.
Boriskina iterated: “Hopefully it will prevent the need for making more and more textile materials, because you can keep recycling a large portion of it.”
The work was supported in part by the DEVCOM Soldier Center through the US Army Research Office, the Office of Naval Research Global via Tecnologico de Monterrey, and the MIT Portugal Program, the release added.
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