Eco-friendly functional two-component flame-retardant waterborne polyurethane coatings: a review

Green functional two-component flame-retardant waterborne polyurethane (2K-FWPU) coatings possess outstanding green traits, such as providing eco-friendly protection, having low toxicity, and generating no pollution. Based on various synthesis designs, such as the use of different flame-retardants and compositions, 2K-FWPU can exhibit various functionalities in addition to excellent flame retardance. This review introduces the components, performances, functionality, and mechanism of 2K-FWPU coatings, which are mainly based on different flame-retardant modifications, in three sections: (1) their components, which include flame-retardant hydroxyl components (two types of typical flame-retardants) and isocyanate components (three kinds of typical syntheses); (2) a comparison summarizing the effects of the flame-retardants and the synthesis method on the emulsion properties, mechanical performance, water resistance, thermal properties, and flame retardance; and (3) functionalization and mechanisms, particularly optical functionalization and thermal degradation/flame-retardance mechanisms. Current 2K-FWPU coatings not only show improved flame retardancy, but they also satisfy various functionality requirements. In future, the designs of 2K-FWPU coatings will significantly assist and guide the potential development of water-based coatings for engineering applications.