Synergism and antagonism of phosphorus-containing epoxy resin combined with different metal hydroxides

Phosphorus-containing epoxy resin is an intrinsic flame-retardant thermoset polymer with P element chemically grafted on the backbone of the macromolecular chains. In this article, two inorganic metal hydroxide flame retardants, aluminum hydroxide and magnesium hydroxide possessing similar flame retardance mechanism, show completely opposite effects, that is, synergism and antagonism when combined with phosphorus-containing epoxy resin, respectively. The corresponding mechanisms were investigated by comparing their theoretical releasing water ratio, the difference of their decomposition temperature and that of phosphorus-containing epoxy resin, and the chemical properties of their metal oxides. It revealed that the different alkalinity of their metal oxides is the main factor leading to the opposite effects. As an alkaline metal oxide, the decomposition product MgO of magnesium hydroxide has higher acid-combination capacity than amphoteric oxide Al2O3 of aluminum hydroxide. The former easily dissolves and reacts with the produced phosphorus acids from phosphorus-containing epoxy resin and is finally converted into magnesium phosphate with chemical inertness, thus greatly consuming the charring catalyst H3PO4; hence, the amount and quality of the formed char decreased. In contrast, Al2O3 hardly reacts with the H3PO4, which keeps enough concentration of the acid to catalyze the resin into more dense and continuous char; meanwhile, the insoluble Al2O3 particles imbedded in the residues further increase the barrier properties of the char layer.