Char Structure and Charring Mechanism of Phosphazene-Based Epoxy Resin during Combustion

The char structure and charring mechanism were important factors in determining the flame retardancy of the polymer. However, relevant research on phosphazene-based polymers, which have excellent fire retardancy, was limited. In our previous work, a phosphazene-based epoxy resin (EHEP-D230) was obtained with a honeycomb-like structure char after combustion. Herein, we further investigated gaseous phase degradation products of the resin through TGA-FTIR, while the condensed phase degradation products, the char, were studied using a variety of characterization techniques, including EDS mapping, FTIR, XPS, solid 31P-NMR and Raman. The study shows that nitrogen products of phosphazene were found in both the gaseous (e.g. NH3) and condensed phases (e.g. pyridine, pyrrole, and quaternary-nitrogen structure), whereas phosphorus products (e.g. phosphoric acid-like structures) were discovered in the condensed phase with an uneven distribution. This finding was confirmed by the degradation of the other two phosphazene epoxy resins (HGETP-D230 and HGETP-MNA). Additionally, the distance between two defects in the EHEP-D230 char (doped carbon structures) might range between 5.6 Å and 17.2 Å, indicating that the char of EHEP-D230 is highly disordered. Subsequently, we compared our research results with those of previous studies on phosphazene degradation in the field of epoxy, and offered a global chemical structure of EHEP-D230's char. Furthermore, a charring mechanism involving the aggregating-pressing process of phosphorous was proposed.