Novel Application of Mechanochemistry in Waste Epoxy Recycling via Solid-State Shear Milling

Epoxy resin is one of the most popular thermosetting resins and has been widely used in many fields for its excellent performance. However, the stable crosslinking structure of waste epoxy (WEP) caused poor interfacial compatibility, which makes it very difficult to be recycled. This paper reported a novel method for recycling the waste epoxy through partial de-crosslinking and re-crosslinking processes via solid-state shear milling (S3M). The waste epoxy was effectively pulverized into an ultrafine powder about 17 μm and the crosslinking density decreased from 3.43 × 10–3 to 2.69 × 10–3 mol/mL. The C–N, C–C, and C–O–C bonds in the WEP were broken by the mechanochemical action and many hydroxyl and amino groups were formed on the surface of the WEP powder, which could react with the epoxy and curing agent to form the new crosslinking network. The interface strength between the WEP powder and epoxy (EP) resin matrix was improved greatly and there were no voids that could be observed by scanning electron microscopy (SEM) after the re-crosslinking process. The maximum tensile strength, flexural strength, and non-notched impact strength of WEP/EP composites reached up to 42.2 MPa, 73.2 MPa, and 8.8 kJ/m2, which means 62, 41, and 109% enhancement, respectively. This low-cost, high-efficiency, and environmentally friendly method shows great potential for value-added recycling of thermosetting resins.