Thermosets have superior dimensional stability and mechanical performance at high temperatures as compared to thermoplastics which render them ideal in many technological applications. However, once thermosets form 3D shapes, e.g., through UV radiation triggered formation of covalent bonds, they cannot be reshaped, repaired and recycled. This leads to material waste and potentially serious environmental impacts with the continuous increase in consumption of 3D printing materials.
Here we present a family of new 3D printable recyclable thermosets (3DPRTs) by developing a two-step polymerization strategy: i. photopolymerization; ii. Transesterification. The photopolymerization takes place during the 3D printing process, and it is triggered by UV radiation to covalently connect monomers and crosslinkers, which transforms the liquid polymer resin into desired 3D shapes. In the 3DPRTs where both ester functional groups and hydroxyl functional groups exist, the transesterification occurs at high temperatures (e.g. 200 °C) where the dynamic covalent bonds start the bond exchange reactions (BERs) to make the materials reshapable, repairable and recycle. This solution addresses the problem that the parts that are printed using traditional UV curable polymer resins can not be reshaped, repaired and recycled.
Among all three-dimensional (3D) printing materials, thermosetting photopolymers claim almost half of the market, and have been widely used in various fields owing to their superior mechanical stability at high temperatures, excellent chemical resistance as well as good compatibility with high-resolution 3D printing technologies. However, once these thermosetting photopolymers form 3D parts through photopolymerization, the covalent networks are permanent and cannot be reprocessed, i.e., reshaped, repaired, or recycled. The developed 3D printing reprocessable thermosets (3DPRTs) from a two-step polymerization strategy that allow users to reform a printed 3D structure into a new arbitrary shape, repair a broken part by simply 3D printing new material on the damaged site, and recycle unwanted printed parts so the material can be reused for other applications. These 3DPRTs provide a practical solution to address environmental challenges associated with the rapid increase in consumption of 3D printing materials.