A novel interdigitated crystalline MMT–MCA shows outstanding fire retardancy owing to homogeneous dispersion of MMT and MCA in PA6.

A new, low surface charge density, anionically modified montmorillonite (MMT) has been prepared and its polyurethane nanocomposites are prepared. Their morphologies were studied using XRD, FTIR, SEM and EDS analysis, thermal stability by thermo-gravimetric analysis (TGA) and fire retardancy by cone calorimetry (CONE). Low surface charge density MMT forms due to conversion of [MgO₆]⁻ to [MgO₅OH] and [AlO₄]⁻ to [AlO₃OH] during the acidic treatment of MMT layers. Its anionic modification is related with pK_a of modifier and pH of water solution by using Henderson–Hasselbalch equation. The low surface charge density MMT gives exfoliated nano-dispersion in PU, indicating that the dispersion of the MMT layers is not only decided by the organophilic surface, but also enhanced with a low surface charge density and electrostatic interaction between layers and counter ions. Low surface charge density MMT can enhance thermal stability and flame retardancy of PU with a small amount of anionic modifier, due to exfoliated nano-dispersion, and catalytic dehydrogenation and char formation. The large reduction in the THR for the sodium stearate modified low surface charge density MMT, SST-t-MMT, means that this material could be a solution for both fire risk and fire hazard.