Behaviors And Simulations Of Small-Scale Burning Of Polymers Impinged By Small Flames

Small-scale burning tests are widely used for assessing flame retardancy and fire hazards of polymers. However, studies on them are not enough and most research reports are focused on burning of polymers in the cone calorimeter test in which materials are exposed to radiation heat fluxes. Few studies involve the small-scale burning process of polymers impinged by small flames. However, studies on the small-scale burning of polymers impinged by small flames are not only helpful in recognition of traditional fire tests such as the UL94 test and efficiently developing flame retardant materials but also useful in fire investigations and large-scale fire modeling. Therefore, it is necessary to study and modeling the small-scale burning process of polymers impinged by small flames.In this dissertation, the UL94 vertical burning test is adopted to delegate the typical small-scale burning of polymers impinged by small flames. Three problems including the ignition of polymers impinged by small flames, the small-scale burning and upward flame spread of polymers, and the dripping behavior are studied.Researches on the ignition problem show that the convection is the dominant heat transfer mechanism when small size polymer specimens are impinged by small flames. The convetive heat flux is estimated theoretically and measured experimentally, and it is found that the the convetive heat flux is as high as 100kW/m2. Furthermore, it is revealed that the edge effect is important in the ignition process. The high heat flux and the edge effect are ascribed to short ignition times of pure polymers impinged by small flames. Based on the convective heating and the multi-dimensional heat conduction with regrard to the edge effect, an ignition model has. been developed. The model predictions agree well with ignition times which are measured through experimental procedures presented specially for the ignition of polymers impinged by small flames. Sensitivity analyses indicate that some parameters such as the ignition temperature, the flame temperature and the convective heat transfer coefficient are key important. Through theoretical analysis, an ignition index is presented for quickly ranking the ignition resistance of materials.An experimental setup has been founded to investigate the burning of polymers in the UL94 test conditions. It is found that the burning rate is slow and the peak mass loss rate of pure polymers is in the magnitude order of 0.001-0.01 g/s. The burning rate does not vary significantly with the thickness of specimens. Furthermore, the small-scale burning and upward flame spread process is modeled. Model parameters such as the convective heat transfer characteristics of polymer flames, material properties and decomposition kinetics are measured and it is found that great certainties exist, which would affect the accuracy of the model. Fortunately, simulatios of the model for charring and non-charring polymers present burning rates which are in the same magnitude order as experimental results, indicating the validity of the model. Simulation results also show that the edge effect is also important to the after-ignition process of polymers. Sensitivity analyses based on simulation results reveal that decomposition kinetics parameters including the activaton energy and the pre-exponential factor are crucial to the ignition time, the burning rate and the flame spread rate. Parameters with regard to decomposition reactions, involving the activaton energy, the pre-exponential factor, the heat of decomposition and the char yield, are of key importance to the peak and average burning rates.Experimental studies on the dripping behaviors show that two types of dripping, the large-size dripping and the small-size dripping, exist in the small-scale burning of polymers. It is also observed that extinction sometimes occurs immediately after dripping for some polymers. The large-size dripping occurs late and obviously leads to decrease of the flame height while the small-size dripping occurs early and leads to flickering flames. The droplet size distribution of the large-size dripping is wide and the dripping frequency is low. On the contrary, the droplet size of the small-size dripping seems to be uniform and the dripping frequency is high. The average droplet mass is in the magnitude order of 0.01g and 0.001g respectively for the large-size dripping and the small-size dripping. In addition, a polymer melt dripping model based on the force balance between gravity and melt strength is presented for large-size dripping. A method is proposed to obtain the model parameters concerning the melt strength. The predictions of the dripping model are demonstrated to be reasonable. Simulation results show that the burning rate would be significantly decreased by dripping. The temperature within the polymer specimen and the melt strength parameters are crucial to the first dripping time.