Behavior investigation of phenolic hydroxyl groups during the pyrolysis of cured phenolic resin via molecular dynamics simulation

In this paper, the initial stage of the pyrolysis process of phenolic resin has been simulated by ReaxFF (reactive force field) molecular dynamics simulation under various temperatures (3000 K, 3250 K, and 3500 K), to study the behavior of phenolic hydroxyl groups and to investigate the correlation between the production of small oxygen-containing molecules (including H2O, CO and CO2) and the quantity of hydroxyl radicals at different temperatures. We observed that the highly active phenolic hydroxyl groups would be easily turned into phenoxyl radicals and hydroxyl radicals in primary evolution. And also, both the phenoxyl radicals and hydroxyl radicals had remarkable impact on the following process of pyrolysis in secondary evolution: the formation of phenoxyl radicals reduced the stability of the benzene ring and damaged backbone of phenolic resin accompanying with the release of CO molecules, while the hydroxyl radicals increased the amount of CO molecules and converted them into CO2 molecules, to reduce the char yield of resin. We found that the amounts of small oxygen-containing molecules increased with the rise in the number of phenolic hydroxyl groups or the rise in the temperature. We also found that both the two evolution modes of phenolic hydroxyl groups (I. phenolic hydroxyl groups – phenoxyl radicals – small oxygen-containing molecules; II. phenolic hydroxyl groups – hydroxyl radicals – small oxygen-containing molecules) can reduce the thermal stability of the backbone of resin. The negative effect of phenolic hydroxyl groups on the thermal stability of cured PR is valuable to the understanding of the pyrolytic process and char forming mechanism, and also the enhancement of the thermal stability of resin.