A sulfonated hyperbranched bio-based waterborne polyurethane sizing coating for the enhancement of mechanical properties and surface wettability of carbon fibres

Wet-weaving and cluster braiding in industrial production processes generate irreversible defects and grooves in carbon fibre (CF) filaments, which reduce the service life and mechanical properties of the fibres. Additionally, the fibre surface covered with numerous inactive carbon-containing groups and presents chemical inertness, which limits the prospects of CF applications. Hence, the hyperbranched bio-based waterborne polyurethanes (SWPU) possessing the synergism of sulfamate and carboxylate complex hydrophilic units were adopted as sizing coatings to improve the flexural and tensile strengths of CF, as well as fibre surface roughness and wettability. The SWPU is designed on dihydroxymethylbutyric acid and sulfamates (A95) as per- and post-chain extenders, respectively, trimethylolpropane (TMP) as an intramolecular cross-linker, and plant-extracts castor oil (CO) replacing petroleum-based petrochemicals as the soft-segments. The construction of CO-TMP hyperbranched cross-linking networks and the strengthened hydrogen-bonding effects of the polar sulfamates positively affect the intermolecular interactions and configuration stability of SWPU sizing coatings. SWPU revealed favourable thermo-mechanical properties achieving a T5% decomposition temperature and toughness of 297.83 degrees C and 35.24 MJ/m3. The coating effects of the bio-based sizing agents not only repaired defects and improved surface roughness on CF, but the polar groups in SWPU comprising SO3Na, carbamate and polyurea promoted the chemotactic activity and wettability of the fibres. The surface energy and tensile strength of the SWPU sized CF reached 43.75 mN/m and 5.68 GPa, respectively, which were 45.4% and 30.7% higher compared to original CF. The research contributes to the development and industrial production of high-performance, eco-friendly biobased sizing coatings.