British scientists claim to have developed the world's first comprehensive system for separating and recovering useful materials from old footwear.
The newly developed recycling process is able to granulate and segregate leather, plastic foams and rubber so that they can be re-used in products ranging from rubber playground surfacing to new shoes, researchers said.
The system was developed and tested at Loughborough University's Innovative Manufacturing and Construction Research Centre (IMCRC).
“Footwear is incredibly difficult to recycle as it can contain up to forty different types of material, many of which are stitched or glued together,” said Professor Shahin Rahimifard, who led the project.
“In our process, the first, manual step is to pre-sort shoes into broad types, such as trainers, and to recover metals, such as eyelets. Next the shoes are automatically shredded and granulated, with the granules automatically separated into four waste streams: leather, foams, rubber and other material,” Rahimifard said.
The shoes are turned into 3-4 mm fragments using a granulator.
Low-cost air-based technologies developed by the project then separate the materials by exploiting their different sizes and weights.
An air-cascade separator first removes lighter textile particles and other fine leather and foam residues by blowing them away from heavier granules; then a series of vibrating air-tables separate rubber from foam and leather by stratifying the granulated materials, with lighter granules ending up on top of heavier ones.
For each recovered material stream, there are a variety of applications, researchers said.
Recovered leather fibres can be reformed to produce bonded leather sheets and reclaimed rubber can be used as a running track or playground surfacing product.
For some types of footwear rubbers, finely ground rubber can be put back into new shoe soles - achieving so-called 'closed loop' recycling while recycled foams can be used in underlay material for laminate floors and carpets.
A key use for mixed textiles and other lighter residues could be as insulation material for buildings, researchers said.
The team has also developed a computerised tool that advises footwear designers on materials selection and helps them explore whether particular combinations of materials would make recycling harder or easier.
The more similar two materials are in density, the harder it is to separate granules made of them, driving up the cost of recycling.