High-strength, self-healable, transparent castor-oil-based waterborne polyurethane barrier coatings enabled by a dynamic acylhydrazone co-monomer

Abstract

Castor oil-based waterborne polyurethanes (CWPUs) are recognized as sustainable polymers sourced from renewable materials. However, these polymers often exhibit suboptimal mechanical properties and lack reprocessability due to their soft backbones and irreversible crosslinking structures. Herein, we synthesized a novel rigid diol (VSD) featuring dynamic acylhydrazone covalent bonds, which remain stable even in aqueous environments and can contribute additional hydrogen bonding sites, through the condensation reaction of commercially available succinohydrazide with lignin-derived vanillin. VSD was subsequently introduced into the CWPU system to form colorless and transparent CWPU-VSD films, which exhibited self-healable and reprocessable properties due to the dynamic nature of the acylhydrazone bond. By adjusting the ratio of VSD (the ‘hard’ section) to castor oil (the ‘soft’ section), the mechanical properties of CWPU-VSDs were finely tuned, achieving an optimal tensile strength of 33.9 MPa. Moreover, the application of this CWPU as a paper-based functional coating was explored. The coated paper exhibited excellent water and oil resistances, low water vapor permeability, good recyclability and biodegradability, suggesting a promising approach for the development of multifunctional and sustainable paper-based barrier coatings.