Modeling And Analysis Of Burning Process Of Typical Intumescent Fire-Retardant Polymer Materials

Traditional halogen-based fire retardant systems will release corrosive and toxic gases in fire and do harm to environment. The Intumescent technology is an alternative means to impart flame retardancy to polymer materials. Intumescent fire retardant materials generate intumescent char on the surface, which protects the underlying material from the insults of heating from fire. However, the mechanisms involved with the intumescence behavior, the thermal behavior and decomposition occurred in the burning process have not been fully understood yet. Therefore, it is important to study their burning process and to develop a prediction and simulation model.In this study, intumescent fire-retardant polypropylene (IFR-PP) and polycarbonate (PC) materials were selected to be typical intumescent fire-retardant polymer materials and their intumescent behaviors in fire were investigated firstly. The results from the analysis of TG/FTIR show that the IFR-PP composites are more thermally stable than PP because of the incorporation of intumescent flame additives at temperature higher than 290℃in air and 440℃in nitrogen. The fire retardancy level of IFR-PP materials can reach V-0 grade with UL-94 test at appropriate content levels of intuemscent fire reatardant. Simultaneously the heat release rates and mass loss rates will decrease remarkably. The analysis results of intumescent char formed from IFR-PP materials indicate that different formulation and contents will affect the densification and unity of the char. The formed char for IFR-PP materials tend to be denser and more uniform at lower incident heat flux than higher incident heat flux. The examination of IFR-PP materials by scanning electron microscope (SEM) shows that, compared to intumescent char layer after the complete combustion, the char formed after the material burnt for 10 s is more fine and compact, which can provide a better shield to inhibit the heat and oxygen penerating the char effectively. The burning behavior of PC is affected by many factors such as incident heat flux, sample thickness and sample mounting and so on. The intumescent velocity is nearly constant or tends to increase during burning.The heat transfer process for IFR-PP and PC materials were studied using the cone calorimeter. The results show that the surface temperatures for IFR-PP materials will increase faster in high incident heat flux than that in low incident heat flux. The surface temperatures for IFR-PP materials are also influenced by the change of intumescent fire retardant contents. According to the analysis results of temperature distributions inside the sample for IFR-PP materials, the temperature measured near the top surface is much higher than the temperature measured far from the top surface. The heat transfer process inside the IFR-PP materials is affected by difference in the formulations such as the contents change of APP and PER. The temperature distributions above the IFR-PP samples show that the temperature increases gradually with time before the intumescent char forms. The temperature values of the sample decrease sharply due to the formation of char cap and accumulating gases. The surface temperature distributions for PC materials indicate that the temperature will increase with time and then keep almost constant. A bending point in the temperature curves was observed for internal temperature distribution of PC sample.The thermal properties for IFR-PP materials were analyzed and studied. A method used to calculate thermal conductivity for polymer materials was proposed, which is used for the heating stage in cone calorimeter fire condition. At the same time, thermal conductivities for IFR-PP materials versus temperature in the range of 20℃~380℃were measured using DRX-I thermal conductivity test instrument. Specific heat capacities for IFR-PP materials were measured by differential scanning calorimetry NETZSCH DSC204. Two peak values appear in the specific heat capacity curve for IFR-PP materials due to melt and pyrolysis heat. The specific heat capacity values for the char formed from IFR-PP will decrease when the temperature beyond 127℃because of the oxidized pyrolysis heat. Thermal diffusivities of IFR-PP materials will decrease with the increase of the temperature. Thermal diffusivities of the char formed from IFR-PP will also decrease with increase of temperature but more gently. Surface emissivities of IFR-PP materials were measured using two different methods. One is to calculate surface emissivities with mathematical model and another is to measure it in experiment. The apparent densities of IFR-PP and its char were measured and the porosity of their char was calculated.A prediction model for the intumescence process in fire for intumescent flame retardant PP was developed. The model emphasizes the thermodynamic aspect of the intumescence process and a corresponding submodel is deduced. The intumescent fire-retardant polypropylene materials were selected to obtain experimental data such as MLR. The validation results showed that the temperatures, intumescent thicknesses and mass loss rates predicted by the model were in reasonably good agreement with the experimental results. The study shows that the present model can appropriately describe the intumescent behavior of polymer and numerically predict their intumescent thickness, temperature distribution and mass loss rates in fire.