Real-time char layer resistance monitoring for the study of expanded char layer morphology formed by IFR/SBS during combustion processes

Intumescent flame retardants (IFR) are used to improve the flame retardancy of polymers through the formation of an expanded char layer. However, there is no method that can be used to detect the morphological evolution of the expanded char layer during combustion in real time. Flame ablation of the surface of IFR flame retardant polymers produces an expanded char layer containing monomeric carbon, which makes the char layer electrically conductive. The expansion of the char layer would lead to corresponding changes in its electrical resistance. Therefore, detecting the real-time resistance of the expanded char layer can provide feedback on the morphological changes of the char layer during the combustion process. Herein, a novel device was fabricated for detecting real-time changes in the surface resistance of the char layer during the burning process of IFR flame retardant styrene-butadiene-styrene block copolymer (SBS). The flame retardancy was investigated by UL-94, LOI and cone calorimetry tests. The flame retardant mechanism was investigated using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. The morphology of the char layer under different ablation times was investigated by scanning electron microscopy. The real-time resistance of the charcoal layer surface during combustion was tested for samples with different IFR and gas source contents. Combined with the flame retardant mechanism and the charcoal layer morphology, it was found that the real-time changes in the char layer resistance could feed back on the evolution of the char layer's morphology, including the continuity, the density, and rupture of charcoal layers due to excessive expansion. •Fabricating a novel device for detecting real-time resistance of the char layer during combustion.•Real-time resistance for feedback of morphological changes of expanded char layer during combustion.•Evolution of charcoal layer morphology during ablation for the interpretation of flame retardancy.