Polymers undergo physical, chemical and structural changes when exposed to heat and/or fire. Thermoplastics melt, decompose and burn; thermosets decompose, char and/or burn, depending on the temperature changes due to external incident heat flux.
Detailed in this thesis is a theoretical and numerical heat transfer study, which is undertaken to simulate and experimentally validated temperature variations during melting, decomposition, charring and ignition phases of polymers. For melting, thermoplastic polymers (polypropylene, polyester, polyamide 6, polymethyl methacrylate, polycarbonate and polystyrene) have been used, whereas for decomposition, charring and ignition glass fibre – reinforced epoxy composites have been chosen.
For each case a one-dimensional finite difference method, using Matlab as the operator has been developed to determine the transient temperature distributions within the different types of polymers materials. The convective and radiative heat transfer boundary conditions, at the exposed and unexposed sides of polymer samples, have also been taken into account accordingly. While some experimental results to validate the different numerical models built are from other researchers’ work at Bolton, in addition to these, other sets of experiments were specifically developed for this work.
The melting behaviour of thermoplastics has been modelled in two scenarios: (i) vertically oriented sample where melt dripping occurs and (ii) horizontally oriented sample within a contained holder in order that the mass will not escape from the containment region. In the the first scenario the sample was placed in a tube furnace, where the radiant heat is uniform on all sides of the sample. This is based on the experimental methodology developed at Bolton University in an earlier project which studied the melt dripping behaviour of polymers. The thermogravimetric and rheological analysis of molten drops had indicated that, depending upon the temperature of the furnace (external heat flux) and the structure of the polymer, in some cases it was pure melting whereas in others it was accompanied by a partial decomposition of the polymer. A one-dimensional finite difference method based on a moving boundary approach has been developed to model the temperatures of the molten drops polymers. The simulated results showed good agreement with the molten drops’ temperatures measured by experiments. In addition, using kinetic parameters, degrees of decomposition in drops obtained at different furnace temperatures were also simulated, which were validated with previous experimental results.
For the second scenario, in which the sample is placed horizontally in a container, experiments were conducted using a cone calorimeter with the heat applied only on the top surface, while the other sides of the polymer sample are insulated, A further one-dimensional finite difference method based on a Stefan approach involving phase changing material, has been developed to determine the melting point temperature and to estimate the temperature profile within the polymer slab, to simulate pure melting and melting plus partial decomposition which may or may not catch fire depending upon the degree of decomposition. The predicted results matched well with the experimental results.
Furthermore, the heat transfer model was modified to simulate the temperature profiles through the thickness of a glass fibre - reinforced composite exposed to different heat fluxes in a cone calorimeter. This involved incorporating a kinetic model for the decomposition process taking into consideration the varying thermophysical properties as a function of temperature. This is achieved by using the critical heat flux that is the minimum incident heat flux leading to ignition, in the equation defining the ignition temperature,
The simulated temperature profiles matched well with the experimental results obtained from previous works at the University of Bolton, giving a much better agreement than previously published models describing this condition. Ignition phenomenon is well described by the model showing a jumping step when the composite polymer ignites and burns.
The last part of the work was to simulate the heat transfer in Intumescent coated glass fibre reinforced epoxy composites exposed to heat in a cone calorimeter. On exposure to heat the intumescent coating expands to form a char, the thickness and the thermal conductivity of which, depends on the type of coating. It was not the purpose of this work to model expansion of the coating; rather the emphasis was to understand the thermal barrier efficiency of the expanded char. However, changes to the surface, expansion of the local thickness and char region when exposed to heat were incorporated into the model to gain better agreement with experiment values
