Aromatic ethers, biphenyl π-conjugation and dynamically imine-bond enhanced waterborne polyurethanes: synergistic molecular design for high-performance carbon fiber sizing coatings

Waterborne polyurethanes (WPUs) have emerged as the preferred option for researchers aiming to replace solvent/emulsion-based resins in carbon fibers (CFs) coatings, owing to their superior adhesion, eco-friendliness, and cost-effectiveness. However, conventional polyether/polyester-based WPU coatings often exhibit insufficient mechanical properties, thermal stability, and weathering resistance. To address these limitations, a diol (PHBA-D1000) containing dynamically reversible imine bonds was synthesized via Schiff base formation between 4-hydroxybenzaldehyde (PHBA) and Jeffamine D1000. Ethoxylated bisphenol A (BPE) and 1,4-bis(2-hydroxyethoxy) benzene (HQEE) were copolymerized proportionally as co-monomers, using PHBA-D1000 as the soft-segment backbone to produce BPE/HQEE-modified WPUs (EWPUs). These EWPUs incorporate π-π conjugated benzene stacking, aromatic ether, biphenyl, carbamate, polyurea, imide, carboxylic acid, and carbonyl groups within their molecular structure. The synergistic effects of these co-monomers improved the overall properties of EWPUs, yielding coatings with better thermal stability (T5% at 269.5 °C), higher tensile strength (46.1 MPa), and greater adhesion (18.7 kPa), showcasing improvements of 26.7 %, 95.3 %, and 21.9 %, respectively. Utilizing EWPUs as sizing coatings significantly enhanced the reactivity, wettability, and mechanical properties of CFs. These coatings introduced functional groups and rigid chains onto the fiber surface, effectively repairing CF surface defects and increasing surface roughness. The surface energy and monofilament tensile strength of EWPU-coated CF reached 59.3 mN/m and 6.216 GPa, respectively, corresponding to improvements of 66.1 % and 24.5 % compared to unsized CF. EWPUs demonstrate outstanding overall performance and notable functional characteristics, highlighting their significant potential for diverse applications and further development.