Mechanism of fire retardancy of polyurethanes using ammonium polyphosphate

In this work, we studied the mechanism of the fire retardancy of ammonium p olyphosphate (APP) in polyurethane (PU). Indeed, according to the limiting oxygen index test, the efficiency of APP in PU coating was proven. On the o ne hand, thermogravimetric analyses showed that the addition of APP to PU a ccelerates the decomposition of the matrix but leads to an increase in the amount of high-temperature residue, under an oxidative or inert atmosphere. This stabilized residue acts as a protective thermal barrier during the in tumescence-fire retardancy process. On the other hand, spectroscopic analys is of the charring materials using infrared spectroscopy, MAS NMR of the so lid state, and ESR enables better understanding of the carbonization proces s and, consequently, of the intumescence phenomenon. It has been shown that the char resulting from PU consists of an aromatic carbonaceous structure which condenses and oxidizes at high temperature. In the presence of APP, a reaction between the additive and the polymer occurs, which leads to the f ormation of a phosphocarbonaceous polyaromatic structure. Moreover, this ch ar is strongly paramagnetic. The presence of large radical species, such as a polyaromatic macromolecule trapping free radicals, was demonstrated. Bot h of these characteristics help to explain the fire-retardant performance o f PU/APP. (C) 2001 John Wiley & Sons, Inc.