Semi-empirical lumped models of polymer pyrolysis for poly(methyl methacrylate) and polyoxymethylene

Quantitative lumped-kinetics models are constructed for pyrolysis of poly(methyl methacrylate) (PMMA) and polyoxymethylene (POM) by using known reaction classes to describe mass loss (volatiles loss) by purely 1st-order decomposition rates, averting the need for molar- or number-based concentrations. These semi-empirical models will aid in establishing fundamental kinetics of polymer decomposition for solid rocket fuels and thermal recycling. Simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) are applied to measure mass-loss and heat-consumption rates and the influences of heating rate, sample size, and end groups as a basis for modeling. This combination of rate data and products is useful for proposing pathways and establishing mechanisms. PMMA and POM homopolymers were selected for base-case pyrolysis studies due to their relatively simple structures and their tendencies to yield primarily monomer. For comparison, kinetics was also measured for POM copolymer, where -CH₂CH₂O- units are interspersed among the -CH₂O- units.

Two-, three-, and four-lumped-reaction parameterized models are presented for pyrolysis rates and yields from POM homopolymer, POM copolymer, and PMMA, respectively. The lumped reactions correspond to temperature regions that are dominated by a single type of first-order reaction, each with a mass fractional yield of volatiles, an Arrhenius pre-exponential factor, and an activation energy. The first, lowest-temperature lump may be pericyclic reactions to molecular intermediates, or the main chain or weakly bound end groups or side groups may homolytically scission. Polymer-radical fragments could be trapped by recombination and be too large to be volatile. If enough polymeric radicals are formed, beta-scission into monomers can be rate-limiting for volatiles formation, and at higher temperatures, homolytic scission would be rate-limiting. At highest temperatures, stages can be rate-limited by internal H-abstraction or termination via exothermic reactions to make strongly bound char residues.