MIT engineers design recyclable elastic yarn

"Eighty percent of textiles on the U.S. market currently contain some amount of spandex, which makes them nonrecyclable," says Svetlana Boriskina, a research scientist in MIT's Department of Mechanical Engineering. "There's no widely adopted technology now that recycles textiles into textiles. With our new yarn, we hope to change that."

Boriskina and her colleagues have published the details of the new yarn in a study published in the journal ACS Materials Letters. MIT co-authors include first author SeongHyeon Kim, Duo Xu, Volodymyr Korolovych, Domingo Flores-Hernandez, Kaniz Moriam, and Daniel Braconnier.

The core of the problem

Spandex is a polyurethane-based synthetic fiber that is springy but not very strong. A thread of an elastic yarn is made from two parts: a spandex-based core, surrounded by a sheath of tough polyester or nylon. The combination of these materials gives elastic yarns their unique stretch and strength.

But this same material mixture makes elastic yarns nearly impossible to recycle. Yarns would first have to be chemically treated to separate the polyester sheath from the spandex core. The polyester-based sheath material could then be melted down and reused. But there is no way to recycle the yarn as a whole, without chemical separation.

"Even though chemical separation technologies exist, they add extra cost and complexity, and usually require toxic chemicals that are harmful to the environment," Boriskina says. "That's why most stretchy garments go to the dump."

In 2021, Boriskina's group developed a new type of yarn made from polyethylene. Polyethylene is the most common type of plastic in the world, used to make everything from grocery bags, water bottles, trash bins, and toys to industrial pipes and plastic sheeting. Polyethylene is a thermoplastic, meaning that it can be melted down and remade, and thus recycled.

And yet, polyethylene had never really been considered as a textile. In their previous work, Boriskina and her colleagues showed they could spin yarn out of polyethylene, which they then wove into various garments. In those experiments, they focused on the yarn's moisture wicking, stain-resisting, and cooling properties.

Spaghetti yarn

In their new study, the group aimed to tailor polyethylene yarn to mimic the strength and flexibility of spandex; they also sought to demonstrate the yarn's recyclability.

They first looked for formulations of stretchy, polyethylene-based copolymers that resemble a spandex elastic core. Separately, they engineered polyethylene yarns that can act as the sturdier sheath. Looking through the scientific literature and combing through industrial reports, the team evaluated many chemical variations of polyethylene.

"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," Boriskina explains. "How these chains are arranged can change the properties of the whole structure."

"Polyethylene can give us a wide range of properties, depending on how you make it," adds first author SeongHyeon Kim.

For the yarn's core, the team used one polyethylene-based resin that results in a more stretchy fiber. They chose a second, stiffer resin as the basis for the yarn's sheath. The researchers obtained pellets of each resin from a chemical manufacturer, and then put each type of pellet through a process of fiber fabrication, first pouring them into a hopper, then heating the pellets to about 350 degrees Fahrenheit, past their melting temperature. The melted polyethylene was then drawn through small extruders to make hair-thin fibers.

"You just melt it in a barrel with a heater, and then you extrude and spin it into fibers," Kim says. "It's like a spaghetti machine."

The team used an industrial yarn spinner to wind the sheath fibers around a core fiber to make the final, elastic yarn.

Because both the yarn's core and sheath come from the same chemical family of polyethylene, Boriskina says the materials do not have to be separated before recycling, in contrast to spandex-based elastic yarns. The new yarn can be melted as is, and reformed into new yarn or other plastic products.