These Boots Are Made for Recycling: Researchers Reclaim Materials from Worn-Out Shoes0

Rachel Nuwer | Fri Oct 26 2012

You know the moment will arrive, but dread it nonetheless: when you slide your favorite shoes on and realize they’re no longer fit for feet. The forlorn footwear then joins the more than 20 billion pairs of shoes each year that clog landfills. But what if shoes could be reborn as playgrounds, leather belts, running tracks or insulation?

To realize this dream, researchers are working on ways to break down beloved but busted shoes into their most basic components in order to reclaim and reinvent those materials.

“We’re trying to develop an economically sustainable, automated mechanical recycling system for shoes,” says Michael James Lee, a research engineer at the Sustainable Manufacturing and Reuse/Recycling Technologies Center at Loughborough University in England.

Automated shoe recycling system

Lee and his colleagues’ system provides an alternative for footwear disposal. While products such as cars and electronics contain pricey materials that make reclamation and salvage worth it, shoes lack high value components. They also contain a complex mixture of materials – ranging from textiles to foam to leather to rubber to metal – that complicates the recycling process.

“Just due to the nature of shoes, it becomes a real challenge to individually isolate materials,” Lee says.

First, grind the shoes then granulate them into tiny specks

Nike is one of only a few companies currently recycling post-consumer shoes through their ‘reuse-a-shoe’ scheme, which claims to have recycled around 25 millions pairs of shoes to date. But Nike’s operation only applies to sports shoes, not stilettos, sandals, boots or other footwear.

Lee and his colleagues designed an automated system that works for all types of shoes by first shredding them and then granulating them into tiny specks. The system then separates the material components by passing materials down a chute with a horizontally directed air supply, which blows the lighter textiles into a different compartment. Left with rubber, foam and leather, the system then uses a specially designed contraption called an air table that combines a mix of vibrations and air to separate the remaining materials, again according to weight.

Lee and his co-author provide a detailed description of their method in a paper published in the journal Resources, Conservation and Recycling.

Multiple uses

The researchers aim to recover the whole shoe, though the purity of each of the materials varies since separating out all of the individual types of plastic or foam is not feasible. As such, they must seek out useful applications for overall categories of materials – all rubber; all foam; all leather, for example – rather than specific types within those categories.

They found a number of uses for the reclaimed materials. Rubber, for instance, is useful for running tracks or playground surfaces. Foam can serve as an underlay product for flooring. Textiles can serve as insulation between housing wood blocks, and initial experiments showed the reclaimed textiles worked well in a variety of wall types for regulating temperature and acoustics. Finally, leather can be ground down into a powder then stuck back together again to make bonded leather, though the researchers are still struggling with attaining a high enough purity of the reclaimed product to realize this goal.

The goal is to reclaim 100 percent of the shoe

The system has a few shortfalls. It does not work well for shoes containing lightweight foamed soles in combination with leather, since both of these materials have a similar density when broken down and are difficult to separate. And, at the moment, the researchers must handpick out any metal components contained in shoes before granulating the shredded footwear. There are machines that could automatically do this if the technology ever reaches a commercial scale, Lee says, but that would raise the process cost.

One thing Lee and his colleagues have noticed, though, is that shoes contain a lot of seemingly superfluous materials that could eventually be eliminated or standardized. “We find metal in shoes that you really wouldn’t expect to be in there,” he says. “I wonder, why is it there? It really doesn’t serve any purpose.”

The team has begun initial conversations with shoe manufacturers about reducing their mix of different materials and making shoes more recycle-friendly, though Lee acknowledges, “if it’s not commercially viable for companies, they’re not going to do it.”

For example, vulcanized rubber resists melting, but if it could be replaced with thermoplastic-based rubbers – which perform just as well in shoes – that shift would increase a shoe’s recycle-friendliness.

Lee imagines a future in which shoes, after being worn down or reused a couple times, eventually find their way back to a recycling plant. “It’s very idealistic at the moment, but it would be nice one day to give all shoes a second life,” Lee says.

Top image: Worn-out shoes Courtesy/Flickr user erix!