MD Characterisation of Flame Retardant Mechanisms

Chemical Engineering Journal 480, 148169, 2024. (link)

 

​Localised flame-retardant (FR) chemistry of polymer composites is complex, versatile and microscopic depending on the combination of polymer matrix and various FR systems. Particularly, the pyrolysis behaviour of the polymers can be varied and reinforced by effective microscopic synergistic effects between coating compositions. However, these atomistic reactions can be difficult to be characterised via experimental approaches. In this study, experimental studies and reactive molecular dynamic (ReaxFF-MD) simulation techniques were utilised to identify the thermal degradation behaviour of flame retarded high-density polyethylene (HDPE), coated with phosphoric acid (PA) and pentaerythritol (PER). The simulations tracked the pyrolysis product distribution under different heating temperatures and elucidated the pyrolysis hindrance offered by the FR system. The atomistic reaction pathways of phosphorus-based FR mechanism were also characterised, including secondary reactions and intermediate species, such as P-O-C and P-O-P cross-linking between FRs and radical capturing effects. These reactions were found to be increasing the C/H ratios of pyrolysed hydrocarbons and the aromaticity of residues. After applying the periodic removal techniques, the simulations can even predict the residue weight percentage of the polymer and exhibit similar degradation behaviour to the experimental results.

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Highlights:

• MD-ReaxFF and experiments were utilised to study the flame retardant(FR) mechanisms.

• The pyrolysis hindrance offered by the FRs is elucidated by increasing C/H ratio.

• FR chemistries such as dehydration, radical capture and charring were characterised.

• Novel removal techniques were employed to predict char residue of HDPE and HDPE/FR.