These can improve polymer compatibility of metal salts, provide catalytic FR functions and absorb pyrolysis products. This review summarises FR effects of different types of metal-organic hybrids. In OMIFERs (organically modified inorganic FRs) metal salts are surface modified by organic molecules to improve polymer compatibility. In MOSs (metal-organic salts) a metal is combined with an organic phosphorus molecule: the connection of the metal ion to the organic molecule improves its FR effectiveness and can impart specific catalytic effects during combustion. In MOFs (metal-organic frameworks) the specific structure impacts various FR effects, which can include porous structures which absorb flammable pyrolysis products in fire. Consequently, FR mechanisms of metal-organic hybrids include improving char formation and stability, releasing flame-inhibiting phosphorus radicals, catalytic smoke suppression by metal compounds and reduced emission of flammable pyrolysis products during polymer decomposition. Organic molecules used in metal-organic hybrids can include organophosphorus compounds, bio-based molecules (e.g. alginate, phytate …), melamine … Inorganics used can include various metals (e.g. Co, Mn, Mo, Ti, Zn …) or clays. Data from over 200 references are compared. In a second paper, the authors further assess nine different MOFs (metal-organic frameworks) in polyester and epoxy. They conclude that cobalt containing MOFs are more effective than those containing aluminium, copper, iron or zinc. This second study confirms the effectiveness of appropriate MOFs as PIN FRs reducing peak heat release rate, smoke and carbon monoxide emissions.
“A review on metal-organic hybrids as flame retardants for enhancing fire safety of polymer composites”, Y. Hou et al., Composites Part B 221 (2021) 109014, DOI. “Which part of metal-organic frameworks affects polymers’ heat release, smoke emission and CO production behaviors more significantly, metallic component or organic ligand?”, Y. Hou et al., Composites Part B 223 (2021) 109131, DOI.