(584ax) Molecularly Based Empirical Models for Pyrolysis of Polypropylene and Polyethylene

Quantitative, molecularly based, lumped kinetic models are constructed and presented for the pyrolysis of polypropylene and polyethylene. Simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) are applied to measure mass-loss and heat-consumption rate. The influences of heating rate, sample size, and end groups are examined as a basis for modeling. This work is aimed toward establishing fundamental kinetics of polymer decomposition toward understanding solid rocket fuels and the key chemistry for thermal recycling.

Lumped kinetic models are constructed from a set of reactions grouped together by reaction family. These parameters are then fitted to mass-loss data. Fitted activation energies are then compared to activation energies obtained via quantum-chemical calculations. The lumped models are further refined to include main decomposition products observed in experimental data.

This comparison gives insight into the identities and rates of elementary reactions that are occurring. The first, lowest-temperature volatilization is an initiation step. That may occur by random scission of the chain or by scission of weakly bound end groups or side groups to make a small, volatile radical (that can abstract H and become volatile) and a polymeric radical. Any initiation by random chain scission would produce polymer fragments too large to be volatile, yet such reactions can be detected through their thermal signatures. Second, the polymeric radical can beta-scission repeatedly into volatile monomer units and shortened polymeric radicals. This kind of step can be rate-limiting at lower temperatures if many polymeric radicals have been formed but are not hot enough for beta-scission to occur. At higher temperatures, volatilization can be rate-limited by internal H-abstraction or by termination via exothermic combinations to make strongly bound residue.

This research was funded by the USDoD/ONR+AFOSR as a Multi-University Research Initiative: Combustion of Solid Fuels in High Enthalpy Flow, award N00014-23-1-2501.