The effect of anion species in the interlayer of layered double hydroxides on the flame retardancy of composites

Layered double hydroxides (LDHs) are increasingly being used in polymer flame retardancy, but current developments often overlook the effects of various structural factors on flame suppression effectiveness. These factors are crucial for the design of high-performance flame retardants. To investigate the structure–function relationship between anion species and polymer flame retardancy, Mg[sbnd]Al LDHs of BAL, BPL and BSL, intercalated with anions of benzoic acid (BA), benzene hypophosphorous acid (BP) and benzene sulfinic acid (BS), respectively, were synthesised via co-precipitation. Their properties, including flame retardancy, were systemically investigated in this study. Despite their similar structures, with only differing acid species, BAL, BPL and BSL exhibited similar interlayer distances but varied in their intercalation capability, crystallisation, thermal decomposition and surface hydrophobicity. Compared to pure EP, the composites containing LDHs, particularly BSL, performed significantly better in limiting oxygen index, vertical burning and cone calorimeter tests. The improvement can be attributed to the carbonisation and altered decomposition pathways of the composites. All three LDHs seemed to have little impact on the tensile strength and dielectric properties of the composites. The results of this investigation indicate that the acid species can significantly affect both the properties of the LDHs and the composites. Furthermore, sulfinic acid, or the sulfur element in LDHs, may exhibit superior performance in polymer flame retardancy. This provides valuable insight into the structure–function relationship study of LDH-based flame retardants and lays a solid foundation for the design of novel, high-efficiency LDH flame retardants. © 2025 Elsevier B.V.