A review on metal-organic hybrids as flame retardants for enhancing fire safety of polymer composites

Metal-organic flame retardants (MOFRs) have been constantly applied to enhance fire safety of polymers. Metallic compounds positively improve the MOFRs' performance in suppressing smoke and toxic gases emissions during combustion of polymers. While organics endow MOFRs with the possibility of high efficiency and excellent compatibility. Based on the combination types between organics and metallic compounds, MOFRs are systematically divided into three categories in this review, that is, organic modified inorganic flame retardants (OMIFRs), metal-organic salts (MOSs) and metal-organic frameworks (MOFs). Metallic components in OMIFRs are bonded with organics in three approaches: encapsulation, intercalation and chemical grafting. Metal ions in MOSs can be replaced via ion-exchange method and thus produce different organometallic salts. Based on sodium alginate, phytic acid and phosphate, various high-efficiency flame-retardant alginates, phytates and metalorganic phosphates are synthesized and applied. As burgeoning flame-retardant substance, MOFs are divided into different types based on types of ligands, that is 2-MIM, aromatic polyacids, and other synthesized materials. Under guidelines of molecular design and metal-organic synergistic flame-retardant mechanisms, MOFRs will be continuously updated and developed. This review summarizes combustion behaviors of polymer/MOFRs composites and presents short comments about upcoming research opportunities and challenges in this field.