A transparent epoxy vitrimer with outstanding flame retardancy, toughness, and recyclability enabled by a hyperbranched P/N-derived polyester

A transparent epoxy vitrimer with outstanding flame retardancy, toughness, and recyclability enabled by a hyperbranched P/N-derived polyester

Despite the tremendous advances in vitrimers, it is still a challenge to realize advanced vitrimers with high transparency, toughness, and fire safety. To achieve this performance portfolio, the closed-loop recyclable transesterification vitrimer (HPN1.80) with excellent transparency, toughness, and flame retardancy was prepared by incorporating a hydroxy-terminated hyperbranched polyester (HPN) into diglycidyl ether of bisphenol-A (DGEBA)/methyl tetrahydrophthalic anhydride (MeTHPA) system. HPN was easily obtained by esterification of triethanolamine (TEOA) and 9,10-dihydro-10-(2,3-dicarboxypropyl)-9-oxa-10-phosphaphenanthrene 10-oxide (DDP). Owing to the favorable compatibility of HPN with the matrix, the resulting HPN1.80 exhibited a high transmittance of 85.9% at 700 nm, a toughness of 2.3 MJ/m3, and an impact strength of 4.5 kJ/m2. The phosphorus-containing groups of HPN played an important role in the gas/condensed flame retardancy, allowing HPN1.80 to pass the vertical burning (UL-94) V-0 classification with a limiting oxygen index (LOI) of 32.6%. Due to the catalysis by hydroxyl and tertiary amines, the broken HPN1.80 can be recycled by hot-pressing at 200 °C for 1 h. The regenerated HPN1.80 achieved an LOI of 31.8% and a UL-94 V-0 rating even after two physical reprocessing cycles. For the carbon fiber-reinforced HPN1.80 (CF/HPN1.80) composites, the non-destructive recycling of carbon fibers (CFs) can be achieved due to the presence of a degradable ester group within HPN1.80. This work offers a rational strategy for creating advanced epoxy vitrimers with recyclability, transparency, toughening, and flame retardancy.