We examine the mechanism of pyrolysis and charring of large (> 10,000 atom) phenol-formaldehyde resin structures produced using pseudo-reaction curing techniques with formaldehyde/phenol ratios of 1.0, 1.5 and 2.0. We utilise Reactive Molecular Dynamics (RMD) with a hydrocarbon oxidation parameter set to simulate the high-temperature thermal decomposition of these resins at 1500, 2500 and 3500 K. Our results demonstrate that the periodic removal of volatile pyrolysis gasses from the simulation box allows us to achieve near complete carbonisation after only 2 ns of simulation time. The RMD simulations show that ring openings play a significantly larger role in thermal decomposition than has previously been reported. We also identify the major phases of phenolic pyrolysis and elucidate some of the possible mechanisms of fragment formation and graphitisation from the RMD trajectories and compute the thermal and mechanical properties of the final pyrolysed structures. [GRAPHICS] .
The thermal decomposition of polyphenolic resins was studied by reactive molecular dynamics (RMD) simulation at elevated temperatures. Atomistic models of the polyphenolic resins to be used in the RMD were constructed using an automatic method which calls routines from the software package Materials Studio. In order to validate the models, simulated densities and heat capacities were compared with experimental values. The most suitable combination of force field and thermostat for this system was the Forcite force field with the Nosé–Hoover thermostat, which gave values of heat capacity closest to those of the experimental values. Simulated densities approached a final density of 1.05–1.08 g/cm3 which compared favorably with the experimental values of 1.16–1.21 g/cm3 for phenol-formaldehyde resins. The RMD calculations were run using LAMMPS software at temperatures of 1250 K and 3000 K using the ReaxFF force field and employing an in-house routine for removal of products of condensation. The species produced during RMD correlated with those found experimentally for polyphenolic systems and rearrangements to form cyclopropane moieties were observed. At the end of the RMD simulations a glassy carbon char resulted.