Cryogenic and Cutting-Based Processing of TPU Footwear Waste for Sustainable Additive Manufacturing

Polyurethane (PU) waste represents a major challenge in polymer recycling due to its wide industrial use and the difficulty associated with its reprocessing. In the footwear industry, significant quantities of post-industrial PU waste are generated, much of which is currently landfilled or incinerated. Thermoplastic polyurethane (TPU) offers a promising alternative, as it is melt-processable and can potentially be mechanically recycled and reintroduced into additive manufacturing processes, such as fused filament fabrication (FFF) 3D printing.

This work aims to develop a viable procedure for recycling TPU waste from the footwear industry and converting it into feedstock for 3D printing. One of the main challenges is the elastomeric behavior of TPU at room temperature. Due to its segmented structure, composed of hard and soft segments, TPU remains highly flexible and rubber-like under ambient conditions, making conventional shredding or grinding processes inefficient.

To overcome this limitation, two complementary shredding strategies are being explored. The first is cryogenic-assisted shredding, in which liquid nitrogen is used to reduce the material temperature below its glass transition region, increasing rigidity and enabling more efficient fragmentation before extrusion and filament production. The second approach involves the development of a cutting-based shredding system, designed to process TPU waste using sharp blades. Unlike impact- or fracture-based methods, this system relies on slicing rather than brittle fracture, which may be more suitable for elastomeric materials that tend to deform instead of breaking.

Preliminary trials showed that cryogenic-assisted shredding significantly improves TPU fragmentation compared to conventional milling. Additionally, the blade-based system is being investigated as an alternative or complementary room-temperature processing route. The resulting material was successfully processed in a ThermoFisher Scientific Process 11 twin-screw extruder, demonstrating the feasibility of producing recycled TPU feedstock for additive manufacturing applications.