Comparison of intumescence mechanism and blowing-out effect in flame-retarded epoxy resins

Epoxy resins (EPs) have been flame-retarded by an APP-MMT nanocomposite (ammonium polyphosphate montmorillonite nanocomposite) and an OPS/DOPO (octaphenyl polyhedral oligomeric silsesquioxane/9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) mixture, respectively. The flame retardancies and efficiencies of these systems have been investigated by LOI, UL-94, and cone tests. The OPS/DOPO mixture (blowing-out effect) was found to be more efficient for improving LOI, UL-94, p-HRR, and THR. However, 10 wt.% of the APP-MMT nanocomposite (intumescence mechanism) showed good flame retardancy and higher efficiency in reducing TSR. The flame retardancy mechanisms of the APP-MMT nanocomposite and the OPS/DOPO mixture have been investigated by TGA-FTIR, PY-GC/MS, FTIR, and SEM analyses. The results indicate that the APP-MMT nanocomposite accelerated the decomposition of epoxy resins, with most of the pyrolytic products consisting of small molecules. Furthermore, the melt viscosities of the pyrolytic residues correspond to the rate of gas release, which allows the formation of an intumescent and firm char layer. The OPS/DOPO mixture caused the epoxy resin to decompose rapidly, giving complex pyrolytic products. Moreover, EP/OPS/DOPO rapidly produced -Si-O-C- or -Si-O-P(O)-C- cross-linked structures in the condensed phase under the action of heat, leading to formation of solid carbonaceous char. Because EP/OPS/DOPO decomposed rapidly, the hard char layer could not swell to accommodate the released gases, and consequently blowing-out could occur. The differences between intumescence and the blowing-out effect are caused by differences in the structures of the char layers and the rates of gas emission.