Polyurethane foam response to fire in practical geometries Michael L. Hobbs ) , Gordon H. Lemmon Engineering Sciences Center, PO Box 5800, MS-0836, Sandia National Laboratories, Albuquerque, NM 87185-0836, United States Received 22 August 2003; received in revised form 1 October 2003; accepted 28 October 2003 Abstract An efficient polymer mass loss and foam response model has been developed to predict the behavior of unconfined polyurethane foam exposed to fire-like heat fluxes. The mass loss model is based on a simple two-step mechanism using distributed reaction rates. The mass loss model was implemented into a multidimensional finite element heat conduction code that supports chemical kinetics and dynamic enclosure radiation. A discretization bias correction model was parameterized using elements with characteristic lengths ranging from 0.1 cm to 1 cm. Bias corrected solutions with these large elements gave essentially the same results as grid- independent solutions using 0.01-cm elements. Predictions were compared to measured decomposition front locations determined from real-time X-rays of 9-cm diameter, 15-cm tall cylinders of foam that were heated with lamps. The calculated and measured locations of the decomposition fronts were well within 1 cm of each other and in some cases the fronts coincided. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Polyurethane; Foam; Decomposition; Discretization bias correction 1. Introduction This paper describes a simple polyurethane foam (SPUF) response model [1,2] that was developed to predict the fire-induced response of unconfined foam filled systems, where components are restrained within unsealed metal enclosures. The enclosures may have cable openings that provide pathways for the decompo- sition gases to exit the system. Consequently, the response model does not predict pressurization or liquid formation associated with confinement of the decompo- sition gases. The decomposition gases are assumed to exit the confining enclosure. The SPUF model is the third model developed at Sandia National Laboratories to describe polyurethane foam response in unconfined systems. The previous two models [3,4] were referred to as the polyurethane foam (PUF) and the chemical-structure based polyurethane foam (CPUF) response models. The two predecessor models (PUF and CPUF) were based on complex bond breaking mechanisms, lattice statistics, and vapore liquid equilibrium. The hypothesis addressed in the current paper is that a simpler model, without the complexity of a lattice statistics model and a vapor- eliquid equilibrium model, is sufficient to describe unconfined decomposition at ambient pressure. The foam response is modeled with a finite element code that solves the heat diffusion equation with a source term for the endothermic decomposition chemistry. The chemistry model also provides the time-resolved mass fraction of the condensed-phase and gas-phase decom- position products within each element. Foam elements are removed from the computational domain when the condensed mass fraction in the foam element is close to zero. Element removal, referred to as element death, creates a space within the metal confinement causing radiation to be the dominant mode of heat transfer be- tween the surface of the remaining foam elements and the interior walls of the confining skin. The radiation boun- dary conditions are inherited by the underlying elements and viewfactors are recalculated whenever elements are removed. The decomposition model can be significantly simplified for unconfined decomposition since the foam response model does not require the composition of the ) Corresponding author. Fax: C1-505-844-8251. E-mail address: mlhobbs@sandia.gov (M.L. Hobbs). Polymer Degradation and Stability 84 (2004) 183e197 www.elsevier.com/locate/polydegstab 0141-3910/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2003.10.009