A COMPREHENSIVE CHARACTERIZATION OF PYROLYSIS AND COMBUSTION OF INTUMESCENT AND CHARRING POLYMERS USING TWO-DIMENSIONAL MODELING: A RELATIONSHIP BETWEEN THERMAL TRANSPORT AND THE PHYSICAL STRUCTURE OF THE INTUMESCENT CHAR

Abstract

A quantitative understanding of an intumescent material’s reaction to fire remains largely an unsolved challenge. More specifically, the relationship between thermal transport and the resulting char structure is not well understood. Improved pyrolysis models for intumescent materials are necessary to advance the fields of fire modeling and material development. To aid in this endeavor, a systematic methodology to parameterize comprehensive pyrolysis models for charring and intumescent materials is presented. Rigid poly(vinyl chloride), flexible poly(vinyl chloride), Bisphenol A poly(carbonate), poly(ether ether ketone), and poly(vinylidene fluoride) were analyzed in this work. First, thermogravimetric analysis and differential scanning calorimetry were employed simultaneously to characterize the kinetics and thermodynamics of thermal decomposition. Microscale combustion calorimetry was utilized to parameterize the heats of complete combustion of gaseous pyrolyzates. ThermaKin, a numerical pyrolysis solver, was employed to inversely analyze all milligram-scale tests. A multi-step reaction mechanism, consisting of sequential steps, was constructed to capture all observed physical changes and chemical reactions. Gasification tests were conducted on 0.07 m diameter disk-shaped samples using the newly developed Controlled Atmosphere Pyrolysis Apparatus II to parameterize the thermal transport within the undecomposed material and developing char layer. A recently expanded version of ThermaKin, ThermaKin2Ds, was employed to inversely model the gasification experimental results. The model accounted for spatially non-uniform swelling of the sample and the ensuing changes within the thermal boundary conditions. The resulting two-dimensional models were shown to reproduce the experimental sample shape profiles, unexposed surface temperatures, and mass loss rates with excellent accuracy. An analysis of the char pore structure was also conducted to determine the pore size distribution and char porosity. Further analysis enabled the mean, median, and volume-weighted mean pore diameters to be computed from pore size distributions. Quantitative relationships were subsequently developed between relevant thermal transport quantities and the char’s physical structure. It was determined that the thermal insulating potential of the fully developed char was related to the number of pore walls positioned perpendicular to the direction of heat flow. Therefore, designing charring polymers capable of producing many small pores will aid in the development of intumescent materials with an enhanced thermal insulating potential.