A Review of Recent Progress in Phosphorus-based Flame Retardants

Recent patent and technical works indicate a growing interest in halogen-free solutions with the predominance of the literature focusing on phosphorus-based flame retardants. Patents published on the flame retardancy of polycarbonate and its blends significantly exceed the number of patents on flame retardancy of any other polymer. Bridged aromatic diphenyl phosphates, especially resorcinol bis(diphenyl phosphate) and bisphenol A bis(diphenyl phosphate) have found broad application because of their good thermal stability, high efficiency, and low volatility. Another actively reported group of compounds are the metal salts of dialkylphosphonic acid as well as calcium hypophosphite, which have recently been found to be particularly effective in poly(butylene terephthalate) and polycarbonate. These products are synergistic with a number of phosphorus and nitrogen-containing compounds, such as melamine salts, which seem to be very efficient and commercially useful in nylons. Printed wiring boards comprise the largest market for flame-retardant polymeric materials. Recently, there has been a strong interest in halogen free solutions in East Asia and Europe. A recent halogen-free introduction is the 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, which can be reacted into epoxies. Another reactive product with some processing and property advantages is poly(m-phenylene methylphosphonate). Because of the banning of pentabromodiphenyl ether in Europe and voluntary withdrawal of this product from the market in the US, the polyurethane (PU) industry is searching for a more environmentally acceptable low-scorch alternative. Both halogenated and halogen-free solutions are being considered but the PU industry seems to have a preference for the halogen-free products, generally containing phosphorus.