Development of pyrolysis models of composite materials for fire safety engineering

Abstract

The one-dimensional pyrolysis computational tool ThermaKin was used to predict the thermal decomposition behaviour of widely used synthetic polymers (polypropylene (PP) and polyethylene (PE)) with and without additives, in order to investigate the suitability of ThermaKin for novel fire retarded samples, under different thermal and fire conditions. The thermal decomposition of materials was investigated using simultaneous thermal analysis technique (STA) coupled with Fourier Transform Infrared Spectrometry (FTIR) at different heating rates and atmospheres. The results show that thermal decomposition of PP follows single mass-loss step, without formation of residue in nitrogen. It was also found that the pyrolysis shifted towards higher temperature with increase of heating rate at different atmospheres. ThermaKin fitted the TGA curves very well. The thermal decomposition behaviour of polypropylene grafted with 5wt% of maleic anhydride (MA), and reinforced with 5wt% of closite 20A as nanoclay (PP-gMA/NC) was also investigated. The main conclusions from this data are that during the thermal decomposition in different atmospheres, TGA curves showed a single step of decomposition process for all samples. The effect of clay is more pronounced during thermal oxidation.
In N2 and air, a two-step reaction mechanism was fitted the experimental curves fairly well. The thermal decomposition of PE, pure and reinforced with different types of carbon fillers (single/multi wall carbon nanotubes, carbon fibres, carbon black and single/few layers of graphene nanosheets), at different loadings (0.1, 0.5 and 1 wt%) and atmospheres were investigated, to determine their suitability as potential fire retardant additives.
Results showed that thermal decomposition of PE and its composites/nanocomposites followed a single mass-loss step at a range of temperatures, with no residue formation in N2. The DTG curve in air showed two mass loss rate peaks. The experimental results showed that all loadings of these different additives made no improvement to the thermal stability of PE/MA.
In air, the compatibilising agent (MA) improved the thermal stability of pure PE, compared to these composites/nanocomposites at the selected loadings. Mechanisms of single or two-step reaction in N2, and three-step reaction in air for the thermal decomposition of PE with and without additives predicted fairly well the experimental curves.
Finally, the work was extended to investigate the performance of ThermaKin to establish a model that is able to predict cone calorimetry results. ThermaKin predicted the burning rate of PE/MA, as a good agreement between the experimental and simulated curves was achieved. Sensitivity analysis was performed to investigate the influence of the variation of the material properties on the modelling results. It was found that the heat of decomposition is the most important parameter of those investigated and needs to be determined most accurately. Heat capacity and thermal conductivity are somewhat important. The absorption coefficient and the reflectivity are of lesser importance.
In conclusion, this work shows that the combination of pyrolysis modelling, thermal and chemical analysis techniques provides a strong and powerful tool for generating a comprehensive understanding of the thermal decomposition of novel fire retardant materials. However, further work is needed to study the influence of the changes of the material properties in polymeric material while reinforced with different additives and how this will be reflected on the modelling parameters and mechanism.