Sustainable biomass-based flame retardants: recent advances in starch, phytic acid, and chitosan systems for polymeric materials

The increasing demand for flame-retardant polymeric materials in industrial applications has stimulated significant interest in sustainable fire protection strategies. Most polymers are intrinsically flammable, posing serious fire risks during use and processing. While conventional flame retardants, including halogenated compounds, phosphorus-based additives, metal hydroxides, and silicon-containing materials, have been widely used, growing environmental and health concerns necessitate the development of greener alternatives. Biomass-based flame retardants have emerged as promising candidates due to their renewability, inherent carbon-forming ability, and compatibility with polymer matrices. This review specifically focuses on starch, chitosan, and phytic acid because of their abundance, cost-effectiveness, and well-documented flame-retardant mechanisms. The evaluation methods for polymer flame retardancy, including thermal stability and flammability tests, as well as techniques for analyzing both gaseous and condensed-phase mechanisms, are systematically discussed. Furthermore, the latest advances in starch, chitosan, and phytic acid-based flame retardants are reviewed from the perspective of combination strategies and chemical modifications. The discussion highlights their synthesis, structural modifications, and processing techniques, along with their integration into polymeric matrices and the resulting flame-retardant performance. The review aims to provide a comprehensive foundation for the rational design of efficient, biomass-based flame-retardant systems with potential scalability for industrial applications.