Recycling of Carbon Fiber-Reinforced Polybenzoxazine Composites Based on Exchange Reaction between Mannich Base Bridges and Primary Amines

Abstract

Carbon fiber-reinforced thermosetting composites (CFRTCs) are widely utilized across various fields owing to their outstanding performance. However, the simultaneous high-quality recovery of carbon fibers (CFs) and upcycling of the resin matrix remain a formidable challenge due to the three-dimensional cross-linked structure of the thermosetting polymer matrix in CFRTCs. Here, we harness a dynamic exchange reaction between the intrinsic Mannich base bridges in polybenzoxazine (PBZ) and primary amine groups and propose a mild and efficient degradation strategy to overcome this challenge. Under catalyst- and solvent-free conditions, PBZ-based CFRTCs were treated with aniline at 150 °C for 8 h, resulting in sufficient degradation of the resin matrix into soluble oligomers [denoted as D(PBA-a)] that contain phenolic hydroxyl (Ph-OH) and secondary amine groups. This process allows for complete stripping and recovery of nearly undamaged CFs, with a tensile strength retention of up to 96% of the original value. Remarkably, the recovered D(PBA-a) acts as an efficient catalyst for the polymerization of benzoxazine monomers. With the addition of merely 5 wt % D(PBA-a), the curing enthalpy (ΔH) of the benzoxazine monomers decreases from 326.1 to 262.2 J/g. The apparent activation energy (Eα), determined by the Kissinger and Ozawa methods, decreases from 95.68 and 98.98 kJ/mol to 86.77 and 94.94 kJ/mol, respectively. Additionally, the complete ring-opening temperature is reduced from 180 to 120 °C. Furthermore, the resulting polymers demonstrate superior thermal properties. The glass transition temperature (Tg) increases to 197 °C for poly(BA-a) with 1 wt % D(PBA-a) and slightly decreases to 195 °C with 5 wt % D(PBA-a). Meanwhile, the corresponding 5% weight-loss temperature (Td5) reaches 314 and 337 °C, notably higher than those of neat PBZ (Tg = 185 °C, Td5 = 280 °C).