The role of dispersion of LDH in fire retardancy: The effect of different divalent metals in benzoic acid modified LDH on dispersion and fire retardant properties of polystyrene- and poly(methyl-methacrylate)-LDH-B nanocomposites

The influence of three different divalent metals, calcium, magnesium and zinc, using benzoate-modified layered double hydroxides (LDHs), on the dispersion and fire properties of polystyrene (PS) and poly(methyl-methacrylate) (PMMA) is investigated. PS- and PMMA-layered double hydroxide (PS/LDHs PMMA/LDHs) nanocomposites are prepared by in situ bulk polymerization in the presence of the benzoate-modified LDHs. The morphology of the nanocomposites is characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD results indicate that the characteristic layered structure of Ca-Al-LDH-B completely disappear in both polymers, whereas for Zn-Al-LDH-B and Mg-Al-LDH-B that is not the case. TEM confirms excellent dispersion and exceptional distribution of exfoliated Ca-Al-LDH-B within both polymer matrices, while for Zn-Al-LDH-B and Mg-Al-LDH-B dispersion is poorer with more tactoids present. Thermal stability and fire properties are investigated by thermogravimetric analysis (TGA) and cone calorimetry (CC). All PMMA nanocomposites show enhanced thermal stability compared to neat PMMA, while thermal stability of PS is enhanced only by the addition of Ca-Al-LDH-B. PS/Ca-Al-LDH-B and PS/Zn-Al-LDH-B systems show better fire properties compared to the PS/Mg-Al-LDH-B; peak heat release rate reduction is 42 and 41%, respectively, vs. only 15% for the PS/Mg-Al-LDH-B at 10% LDH loading. The largest peak heat release rate reduction (PHRR), 52%, is measured for the PMMA/Mg-Al-LDH-B nanocomposites at 10% LDH loading, although dispersion is very poor. PHRR reduction of the PMMA/Ca-Al-LDH-B nanocomposites at 10% LDH loading is 45%, while 30% reduction of the PHRR is measured for nanocomposites containing 10% Zn-Al-LDH-B.