Fire retardance of polyethylene: Study of a formulation involving phosphorus and polyol systems

The fire retardance of low-density polyethylene has been achieved using a formulation based on the use of phosphoric esters of various polyols in association with a formo-p-cresolic resin (PCR). Two methods of implementation have been studied. The first consists in synthesizing the fire retardant before its incorporation into the polymer, while the second consists in making P2O5 react directly with the polyol while processing the polymer. This second route results in better efficiency. The formulation has been optimized by systematic study of the influence of various parameters such as additive load, nature of the alcohol, P2O5/alcohol ratio, etc. The best formulation is based on pentaerythritol (PER) and shows an oxygen index of 29·5 with the composition: PE + 25%[(P2O5/PER, 1 : 1)/PCR, 23 : 13 Extensive spectroscopic study of the material results in the proposal that the fire retardant has the structure of a polyphosphoric ester of pentaerythritol which keeps free alcoholic functions. Analysis of chars obtained in air at 400°C from the flame-proofed polymer confirms that phosphorus acts mainly in the condensed phase, eventually accumulating at the surface, most likely in the form of polyphosphoric acid.