New recyclable yarn is as stretchy as spandex and could help tackle fashion's plastic waste problem

Your favorite stretchy workout clothes may be much easier to recycle one day, thanks to a new type of elastic yarn that is as strong and stretchy as today's spandex-containing fabrics.

The yarn, described in a July 25 study in the journal ACS Materials Letters, is built entirely from polyethylene, the plastic used in milk jugs and single-use grocery bags. By contrast, most of today's stretchy fabrics blend spandex, a bouncy-but-weak polyurethane fiber, with a tougher fiber like polyester or nylon.

That combination gives clothes their stretch and durability. But because the fibers are chemically different, it's difficult and costly to separate them for recycling, and chemicals used to break down the fabrics are toxic. The result is that almost none of this material is recycled. An estimated 80% of textiles sold in the U.S. contain spandex, and most end up incinerated or in a landfill, study co-author Svetlana Boriskina, a research scientist in the Department of Mechanical Engineering at MIT, said in a statement.

To solve that problem, the MIT team turned to polyethylene, one of the world's most widely recycled plastics. For the yarn's stretchy core, the researchers created a soft, springy version of polyethylene akin to a rubber band. Around that core, they wound a second, stiffer polyethylene fiber, more like a fishing line, coiling it tightly the way a spring or a twisted phone cord is coiled. Because both materials come from the same plastic family, they don't need to be separated before they're recycled.

In laboratory tests, the researchers melted a spool of the yarn and respun it into new yarn 10 separate times. Each time, they tested its strength, and the recycled yarn performed just as well as the original in every case. In head-to-head tests, the new yarn even outperformed commercial spandex-polyester yarn in maximum load, meaning it could bear more force before breaking, according to the study.

The team has already shown that these lightweight polyethylene fibers can be engineered for other perks, too ‪—‬ like cooling, moisture wicking and stain resistance ‪—‬ building on their earlier work spinning polyethylene into wearable fabric. Even buttons, zippers and buckles could eventually be made from compatible plastics, so a whole garment could be melted down and remade without being taken apart first.