Thermal stability and degradation of aromatic polyamides. 1. Pyrolysis and hydrolysis pathways, kinetics and mechanisms of N-phenylbenzamide

A kinetic study of the reaction of N-phenylbenzamide neat and in the presence of added water was carried out to resolve the contributions of homolytic and hydrolytic degradation mechanisms. N-Phenylbenzamide was reacted at 425°C in the presence of added water in molar ratios of 0:1, 25:1 and 50:1. Two competing reactions occurred during the reaction of N-phenylbenzamide in water. Neat pyrolysis led to aniline and 1,2-diphenylbenzimidazole as major products, benzene, benzimidazole, benzoic acid and benzonitrile as minor products, and high molecular weight material. The product recovery index showed that the selectivity to high molecular weight material during neat pyrolysis was high. Hydrolysis led to benzoic acid, aniline, benzaldehyde, and benzene which were easily recoverable and quantifiable products. The product recovery index was essentially 1·0 when the molar ratio H₂O: NPB was 50:1. The conversion of the reactant dramatically increased with the addition of water as did the selectivity to aniline and benzoic acid. A superposition of free-radical and hydrolysis elementary steps rationalized both the temporal variations of the product yields and the variation of the product recovery index as a function of water loading.