Poly(methyl dihydroxybenzoate) modified waterborne polyurethane sizing coatings with chemical and hydrogen-bonded complex cross-linking structures for improving the surface wettability and mechanical properties of carbon fiber

High wettability and adhesion of sizing coatings on the carbon fiber (CF) surface were achieved by establishing chemical cross-linking structures in polyester-based polyurethane (WPU) molecules and introducing methyl 2,6dihydroxybenzoate as the chain extender. The chemical cross-linking and hydrogen-bonded interactions in the molecular configuration of modified polyurethane (DWPU) effectively improved the thermal stability and mechanical properties of coatings. The thermal weight loss temperature T5% of DWPU film was increased from 255.8 degrees C to 284.3 degrees C and Tmax was increased from 416.5 degrees C to 460.1 degrees C. The tensile strength and Young's modulus of DWPU reached 33.9 MPa and 12.2 MPa, which was 3.2 times higher compared to WPU. The benzene 7C-7C bonding could enhance the intermolecular interaction force, while the side methyl formate groups strengthened the hydrogen bonded effect with urethane/urea chain segments in WPU. The synergistic effect of chemical cross-linking and hydrogen-bonded physical cross-linking gave DWPU excellent strength and rigidity as the sizing coating on CF surface. The coating effect of DWPU effectively repaired and filled the defects and grooves on CF, which improved the surface roughness. The strong polar groups in DWPU reduced the original inertness of CF while enhancing the surface activity. The surface free energy and polar component of DWPU-CF increased 1.5- and 4.7-fold, respectively, compared to untreated CF. In addition, the tensile strength of fiber was increased by 19.5 % due to the repairing and reinforcing effect of the sizing coating. The DWPU coatings could optimize the application of CF in drive shafts of energy vehicles and vibration damping devices of high-speed trains, and contribute to the innovation of environmentally friendly materials.