Bioengineered chitosan derivatives: Design, multifunctionality and flame retardant efficacy in epoxy resin - a critical review

Epoxy resin (EP) is widely utilized for its excellent mechanical properties and thermal stability. However, its high flammability and the release of harmful substances during combustion restrict its use in safety-critical applications. Consequently, improving EP's flame-retardant qualities has emerged as a crucial research topic. Chitosan (CS), a natural and renewable polymer, has emerged as an ideal precursor for flame retardant modification of EP due to its rich functional groups and biodegradability. However, existing reviews typically address CS only within the broader context of bio-based flame retardants or mention it sporadically in relation to isolated performance enhancements, without providing a systematic and comprehensive analysis of its structure-function relationships, flame-retardant mechanisms, and multifunctional synergistic effects. This paper examines recent developments in the application of CS to enhance EP's flame retardant performance, with particular attention to the molecular makeup and functional modification of CS, its methods of flame retardance, and its synergistic effects with other flame retardant components. Various modification strategies have demonstrated that CS not only enhances the flame retardancy of EP but improves its mechanical properties, corrosion resistance, and thermal stability. However, the challenge still remains, such as the compatibility between CS's hydrophilic nature and EP's hydrophobicity, and achieving high flame retardancy with low loading. This review provides a comprehensive summary of the role of CS in flame retardant modification of EP and outlines potential research directions for future advancements in low-loading high-efficiency flame retardants, multifunctional optimization, and the development of green, sustainable materials.