Anionic aqueous-based polyurethane (PU) dispersions were derived from a water dispersion process of carboxylic groups containing PU prepolymers together with a neutralization and a chain extension. These PU prepolymers were prepared from a conventional polyaddition of isophorone diisocyanate, polypropylene glycol-1000, and dimethylolpropanic acid. A covalently bonded phosphorus was introduced into PU by a phosphorus-containing curing agent. A series of phosphorus and multi-aziridinyl groups containing curing agents were synthesized for this purpose. Each of them served in a dual-function capacity as a postcuring agent for the anionic aqueous-based PU system. The resulting postcured PU exhibited improvements in its mechanical, physical, and thermal properties; furthermore, postcured PU synergistic flame inhibition was also observed due to the presence of nitrogen and phosphorus content in the process.