Molecular simulation of flame retardancy: solid-phase effects of aluminium hydroxide

The key physical and chemical processes of fire retardancy are connected to the thermal decomposition of an organic polymer matrix and a fire retardant additive. Physical fire retardant compounds act through solid-phase mechanisms such as endothermic dissociation and thermal shielding, and gas-phase mechanisms such as dilution and radical quenching. Chemical fire retardants, on the other hand, modify the decomposition pathway to minimize the production of volatile fractions that would fuel flames. The bulk of our current understanding on fire retardant chemicals is qualitative. A wealth of empirical knowledge exists, but understanding on the level of modelling capability remains in its infancy.
We have carried out reactive molecular dynamics (RMD) simulations based on the ReaxFF reactive force field to demonstrate molecular simulation of the thermal decomposition of amorphous polyethylene (PE) and aluminium (tri)hydroxide (ATH). The simulations reproduce the well-known mechanisms of fire retardancy associated with ATH, namely endothermic decomposition to produce water and an alumina residue, and heat absorption effects due to the presence of the filler and its residue. In addition, the simulations reveal a chemical interaction in which hydrogen is abstracted from the polymer by ATH, resulting in enhanced water production and enhanced charring of the polymer.
Based on the results of this study, we consider RMD simulations a promising tool for investigating existing and emerging fire retardant concepts, and the pyrolysis chemistry of fire retardant polymer systems.