An atomistic study of the pyrolysis mechanism of polyurethane by ReaxFF molecular dynamics

• Atomistic characterisation of polyurethane pyrolysis mechanism using ReaxFF-MD for the first time. • Good agreements on activation energies (187.5 and 148.7 kJ/mol) and generated species spectrum between ReaxFF-MD and experimental data. • Identified the formation pathways for major (CO, CO 2 , C 2 H 4 ) and toxic (HCN, CH 2 O) species. • Revealed the distinct degradation mechanisms within the hard-, soft- segments and the urethane linkages. Despite its extensive engineering applications, polyurethane (PU) poses severe fire hazards and releases highly toxic asphyxiating gases during combustion. Consequently, understanding its fundamental pyrolysis mechanism is critical. This study employs Reactive Molecular Dynamics (ReaxFF-MD) to investigate the thermal degradation behaviour of PU across varying heating rates. Simulations revealed gaseous species comprising ∼ 37 ± 3% of the total mass and no residual char. Dominant gaseous products included CO, CO 2 , C 2 H 4 , and H 2 O, while trace chemically important species such as HCN, NH 3 , and CH 2 O were also detected. Mechanistic analysis established a sequential degradation: the initial mass-neutral C-N cleavage occurred at the interface between hard segments and urethane linkages, followed by C-O cleavage yielding C 2 H 4 and subsequent breakdown of urethane moieties led to the formation of CO 2 , which underwent secondary reactions to yield CO. Atomistic formation pathways for downstream trace HCN and CH 2 O were also elucidated. These findings demonstrate the capability of ReaxFF-MD in capturing PU multiscale decomposition chemistry, providing theoretical guidelines into molecular design optimisation of targeted flame-retardant materials.