Highly engineerable Schiff base polymer matrix with facile fiber composite manufacturability and hydrothermal recyclability

Researchers have attempted to use Schiff base vitrimer as a matrix resin for preparation of intrinsically recyclable CFRP materials. Because most of the reported Schiff base vitrimer resin systems have high viscosity or are in solid state, solvent is often used during product fabrication to reduce the viscosity and slow down reactions during handling. In this work, a low viscosity Schiff base vitrimer resin is prepared from commercially available diamine, dialdehyde, and bisphenol A epoxy in a solvent-free one-pot synthesis. The obtained crosslinked polymer presents a tensile strength up to 59.6 MPa and an elongation at break up to 7.1%. Moreover, compared with those highly acid-sensitive Schiff base vitrimers in the literature, the vitrimer in this work has excellent chemical resistance toward various solvents. Due to the imine bond exchange reaction, the material exhibits fast stress relaxation at elevated temperatures (>150 °C). By the same thermally induced imine bond exchange reaction, the CFRP prepared from this vitrimer matrix shows unique welding, shape changing, and chemical recycling properties. The CFRP strips can be thermally transformed to a new permanent shape at 180 °C under force, while the welded CFPR strips present a lap shear strength of 8.33 MPa and elongation at break of 2.99%. Hydrolysis of imine linkages is achieved in mild acidic solutions (0.2 M HCl), and the recovered carbon fibers show little damage and are reused to prepare new CFRP. The findings from the study will help to bring Schiff base vitrimer resins from research stage to practical composite applications.