Sample-form-induced transport effects in the micro-scale decomposition of PMMA

Kinetic models for thermal decomposition of combustible materials are fundamental for predicting fire growth and flame spread. Kinetic parameters are typically derived from micro-scale experiments, assuming data are free from transport phenomena (the 0D-assumption). Different specimen forms can yield distinct decomposition behaviours, as observed for polymethylmethacrylate (PMMA), yet origins and implications for fire spread modelling remain uninvestigated. To address this gap, powder specimens of varying particle sizes and piece specimens of black cast PMMA were analysed using simultaneous thermal analysis, evolved gas analysis, morphological characterisation, and gel permeation chromatography. Results indicate that specimen-form-dependent differences are attributable to mass transport restrictions rather than heat transfer limitations, material alterations, or changes in decomposition chemistry. This conclusion required investigations beyond standard guidelines, as mass down-scaling alone would have implied the absence of transport limitations. In the low-temperature region, diffusion-limited gas release through the solid matrix dominates, whilst during the main decomposition peak the regime shifts to convective bubble transport, dependent on sample mass. Kinetic analysis confirmed derived parameters are implicitly influenced by transport effects, with direct consequences for fire spread predictions. Preliminary observations for other materials suggest specimen-form-dependent influences are not unique to PMMA, underlining the broader relevance for the fire science community.