The Use of Numerical Methods to Interpret Polymer Decomposition Data

Abstract

Polymer decomposition is the key to understanding fire behaviour. It is a complex process involving heat transfer, breakdown of the polymer chain, volatile fuel formation and gasification occurring as a moving interface through the polymer bulk. Two techniques, chemical analysis using STA-FTIR, and pyrolysis modelling have been combined as a tool to better understand these processes.

This work covers the experimental investigation of polymer decomposition using the STA-FTIR technique. Several polymers including polyacrylonitrile (PAN), polypropylene (PP) and ethylene-vinyl acetate (EVA) alone, and as potential fire retardant composites have been studied in different conditions to optimise the methodology and analysis of results. Polyacrylonitrile was used to optimise the experimental technique. Polypropylene, containing nanoclay and ammonium phosphate, was decomposed and the composition of the decomposition products analysed in order to investigate the fire retardant effects of the additives on the thermal decomposition. Ethylene-vinyl acetate copolymer containing nanoclay and/or either aluminium hydroxide or magnesium hydroxide was decomposed, with vapour phase FTIR analysis showing a change in the initial decomposition pathway with a shift from acetic acid evolution, to acetone production.

In parallel, this experimental data has been used to perform early attempts towards validation of numerical models developed by the use of a 1-dimensional pyrolysis computational tool called ThermaKin. As ThermaKin is relatively new and still not widely used for fire modelling, a detailed description of its capabilities has been provided. A detailed study of heat transfer of cardboard, leading to thermal decomposition, accompanied by pyrolysis and char formation has been described. Several microscale kinetics models have been proposed with different levels of complexity. Not only do the numerical approximations reflect the experimental results of single compounds, describing the material’s behaviour (expressed in terms of mass loss) when exposed to external heat, but also predictive models of fire retardant mixtures have been developed for different atmospheres and heating rates.

In addition, the powerful combination of pyrolysis modelling and chemical analysis by STA-FTIR has provided new insights into the decomposition and burning behaviour of both PP protected with nanoclay and ammonium phosphate, but also the industrially important cable sheathing materials based on EVA. The novelty of this work stems from the first use of the pyrolysis models to study fire retardant behaviour; the first reported combination of STA-FTIR with ThermaKin pyrolysis model, and a deep understanding of the pre-ignition behaviour of cardboard.