Numerical Modeling of Effective Through-Thickness Thermal Conductivity in Intumescent Glass Fiber-Filled Polypropylene for Thermoplastic Battery Enclosures under Various External Fire Exposures

This study examines the behavior of a glass-filled, intumescent, flame-retardant polypropylene (FR-PP). It extends previous investigations by incorporating the contributions of both solid and gaseous phases into the through-thickness thermal conductivity of intumescent composite systems across multiple spatial scales and under varying orientations of external fire exposure. Thermal computational models were developed and validated against experimental measurements. Two different approaches are used for the thermal loads. Firstly, the pyrolysis behavior of the material, along with relevant test geometries and boundary conditions, was modeled using PyroSim, which utilizes the Fire Dynamics Simulator solver. The model captures the material’s multi-step degradation kinetics, including reactions in the condensed and gas phases and associated phase transitions. Material properties were experimentally characterized, and reaction parameters were extracted through multiple analytical techniques. Secondly, an inverse heat transfer approach was applied to a surrogate stainless-steel plate to estimate the transient spatial heat flux distribution, enabling the quantification of thermal loads across a broad spectrum of complex fire exposure scenarios without necessitating explicit consideration of combustion or pyrolysis, nor reliance on direct temperature measurements at the exposed, actively burning surface of specimens. The resulting heat fluxes were mapped onto an Abaqus finite element analysis model as transient, spatially varying thermal loads. This study establishes lower and upper bounds for temperature prediction by applying series and parallel multiphase approaches, respectively, under each fire scenario. It also presents the interpretation of thermal conductivity, not merely as a temperature-dependent property, but also as a function of the rate of thermal exposure.