The impact of bromine- and phosphorous-based flame retardants on flame stability and heat feedback from laminar wall flames

To develop new, efficient flame retardants, it is critical to understand the controlling mechanisms by which they impact material flammability. Our research group has developed a systematic methodology to assess the flammability of polymeric materials through microscale combustion calorimetry (MCC) and bench-scale experimental measurement of flame heat feedback and material burning behavior during vertical burning and upward flame spread. In this work, that experimental framework is used to quantify the mechanisms of action of bromine- and phosphorous-based flame retardants. Here, a series of experiments was conducted in which flame heat transfer and sample mass loss rate were measured as a flame spread over 7 cm tall, 5 cm wide glass-fiber-reinforced poly (butylene terephthalate) (PBT) samples manufactured with increasing amounts (12, 16, and 24  wt %) of the brominated flame retardant, poly (pentabromobenzyl acrylate). In an additional series of tests, similar measurements were obtained for PBT samples manufactured with increasing amounts (8, 12, 16, and 20  wt %) of the phosphorous-based flame retardant, aluminum diethyl-phosphinate (trade name: Exolit OP 1230). These tests allow for the study of the impact of flame retardants on key features of the system including: peak flame heat flux, flame stability, and condensed phase decomposition behaviors (i.e., charring). Current measurements identify an optimal loading concentration of Exolit OP 1230 for the PBT samples tested in this work, identify each flame retardant's impacts on char yield and heat of combustion, and indicate that bromine- and phosphorous-based flame retardants (at least at the concentrations tested here) can affect wall flame stability and gas phase combustion efficiency but do not affect flame to surface heat transfer in the continuous region of wall flames. These measurements allow for greater understanding of tightly coupled condensed- and gas-phase processes that control flame spread and material burning behavior, thus providing a quantitative connection between material composition and the controlling mechanisms of fire growth.