The removal of epoxy resins from NdFeB magnets for recycling–A review

• Demand for electrical machines containing rare earth elements is increasing. • To recycle rare earth magnets, their polymer constituents must be considered. • Various epoxy resin degradation methods could be feasible. • Effects of epoxy resin degradation on magnetic material is yet to be determined. • This highlights an area for further scientific research. The growing importance of recycling rare earth permanent magnets is addressed in this paper, with a specific focus on the decarbonisation of transport. This has resulted from the growing demand for neodymium iron boron (NdFeB) magnets in the components of electric vehicle motors. While there is ever-improving technology focussed on the recycling of these magnetic materials, this paper highlights the challenges of recycling coated rare earth magnets. More specifically, due to the poor corrosion resistance of NdFeB, epoxy resins are applied to commercial grade magnets. These coatings are extremely stable and therefore act as a barrier to straightforward recycling. When it comes to epoxy resin degradation, various methods exist at different technology readiness levels. These methods include pyrolysis (thermal degradation), super-heated steam thermolysis (degradation with heat and steam), solvolysis (degradation with heat and solvents), micro-wave assisted heating, and electrochemical methods. Pyrolysis takes place at temperatures ranging from 350 °C to 750 °C, while thermolysis and solvolysis operate at 250 °C to 500 °C. In contrast, microwave heating with an amine-based catalyst enables a lower temperature of around 130 °C to be used since the introduction of amines facilitates the cleavage of C–N bonds more easily. Following an exploration of NdFeB magnets and their recycling methods, the chemistry and properties of epoxy resins are presented before each of the five degradation methods is outlined with examples. These methods are assessed with sustainability and closed-loop processing in mind, where closed-loop refers to the recycling of materials into the same application from which they were recovered. Pyrolysis has shown the greatest technology readiness level (TRL) of 8–9 and is used commercially, but the degradation products are lost and hence it cannot be a closed loop process. In contrast, solvolysis and electrochemical methods have much lower TRLs of 1–3 but show more promise in terms of product recovery for re-use. Despite this, it is important to consider both the operating conditions and the environmental impact of these processes, since solvent use can have detrimental effects. Solvents used in epoxy degradation include isopropanol, acetone, ethanol and water. While each of these methods has been researched to a range of extents for the degradation of epoxy resin in the recycling of carbon fibre reinforced polymers (CFRPs), there is limited research to suggests that these methods may be transferrable to magnet recycling. Therefore, various factors have been evaluated to assess the potential of each method for epoxy resin coating removal from end-of-life sintered NdFeB magnets.