Recyclable tough thermosets with an imide-hexahydrotriazine structure

Developing sustainable thermosets is regarded as a major approach to cope with environmental pollution and resource waste, but it still remains a challenge to design recyclable thermosets with exceptional comprehensive performance. In this work, a kind of poly(imide-hexahydrotriazine) (PHT) thermoset with degradable and recyclable abilities is reported for the first time. To improve the overall properties, rigid-and-flexible cross-linked networks of the thermosets have been prepared by polycondensation between synthetic amino-capped polyimide oligomers and paraformaldehyde (PFA). Such thermosets, especially BP-PHT, possess excellent mechanical properties and toughness with the maximum tensile strength of 129 MPa and a breaking elongation ratio of 11.6%. Moreover, the prepared thermosets also exhibit exceptional thermal stability (T_g ∼ 211 °C), solvent resistance and ultraviolet (UV) stability. Most importantly, these thermosets could be completely degraded in mildly acidic solution within 6 h, which makes it available for efficient recycling of high-cost carbon fibers (CFs) and valuable starting materials (degraded products) from composites. In addition, the reprepared PHT thermosets using 80 wt% or 90 wt% degraded products still show high mechanical properties and the tensile strength of PHT90% remains up to ∼80 MPa. Motivated by the requirements of sustainable development, this study proposes a brand-new conception for the design and fabrication of excellent overall-performance thermoset materials with desired degradability and recyclability.