Modeling the ignition of poly(methyl methacrylate)/carbon nanotube nanocomposites

- A four-step mechanism is developed for PMMA decomposition in air or nitrogen.
- Decomposition kinetics is coupled with a comprehensive model to describe PMMA and PMMA/CNTs ignition.
- Quantitative agreement is obtained between predicted and measured ignition times.
- PMMA/CNTs is more resistant to ignition only for very mild heat flux intensities.
- Reaction environment and PMMA polymerization degree affect the predictions.

A transient one-dimensional model, including the description of the chief chemical and physical mechanisms of the thermo-oxidative decomposition of poly(methyl methacrylate)/carbon nanotube composite and the parent polymer, combined with the critical mass flux criterion, quantitatively predicts the ignition times measured in a cone calorimeter. At low heat fluxes, the ignition times are longer for the composite, owing to the flame retardancy action of the carbon nanotubes resulting in the formation of a surface charred barrier, prior the attainment of sufficiently high rates of volatile release. Instead, at high heat fluxes, surface (versus in depth) absorption of the thermal radiation locally enhances the decomposition rate leading to shorter ignition times for the composite. Of paramount importance for the quantitative prediction of the ignition time is not only the empirical criterion (critical surface temperature or volatile mass flux) but also accurate kinetics taking into account the polymer properties, in particular the polymerization degree, and the presence of oxygen during the initial transient stage.