Modeling the Pyrolysis of Polyurethane Foam for Fire Safety Applications - Dr Sarah Scott

Abstract: Polymer foam encapsulants provide mechanical, electrical, and thermal isolation in engineered systems. It can be advantageous to surround objects of interest, such as electronics, with foams in a hermetically sealed containers to protect the electronics from hostile environments, such as a crash that produces a fire. However, in fire environments, gas pressure from thermal decomposition of foams can cause mechanical failure of the sealed system. Understanding this phenomenon and being able to predict the heat transfer to components and the pressurization within the sealed system is of interest to Sandia National Laboratories. In this work, a detailed study of thermally decomposing polymeric methylene diisocyanate (PMDI)-polyether-polyol based polyurethane foam in a sealed container is presented. Both experimental and computational work is discussed. When the foams decompose, organic products are produced, and these products can be in the gas, liquid, or solid phase. Experiments in which foam was placed in a can and heated from one end show that the physics are orientation dependent: the inverted cans pressurize, and thus breach faster than the upright. There are many reasons for this, among them: buoyancy driven flows, the movement of liquid products to the heated surface, and erosive channeling. Three models will be presented, of increasing complexity, to model this problem: the No Flow model formulation has Arrhenius type reactions, derived from Thermogravimetric Analysis (TGA), control the reaction. A three-step reaction is used to decompose the PMDI RPU (rigid polyurethane foam) into CO2, organic gases, and char. Though gas is created in the reaction mechanism, it does not advect, rather, its properties are taken into account when calculating the material properties, such as the effective conductivity. A Porous Media model is then added to allow for the advection of gases through the foam region, using Darcy’s law to calculate the velocity. Continuity, species, and enthalpy equations are solved for the condensed and gas phases. The same reaction mechanism as in the No Flow model is used. This model, due to the advection of gases, produces gravity dependent results that compare well to experiment, after calibration. To reduce dependence on calibration, Vapor Liquid Equilibrium (VLE) equations are added to the Porous Media model. These equations predict the vapor/liquid split of the organic decomposition products based on temperature and pressure. The addition of the VLE improved temperature and pressure prediction, both qualitatively and quantitatively.

Bio: Dr. Sarah Scott has been a researcher at Sandia National Laboratories in Livermore, California for the past eight years. She currently works in the Thermal/Fluids Science and Engineering Department where she leads projects related to pyrolyzing polyurethane foam, composite material fires, wildland fires, solution verification, and uncertainty quantification. She received her masters and PhD in Mechanical Engineering from UC Berkeley, advised by Professor Carlos Fernandez-Pello