Investigating the effects of furan ring substitution and network position on char formation in intrinsically flame-retardant epoxy resins

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• Char formation was influenced more by the furan ring position than its substitution. • The char yield can be tuned by changing the furan ring’s position or substitution. • Furan-based epoxies had carbon foaming morphologies without intumescent additives. • Intrinsically flame-retardant epoxies are achievable by incorporating furan rings. Flame-retardant chemicals are often added to epoxy resins to decrease their flammability. However, there is an increasing interest in transitioning from these chemicals to intrinsically flame-retardant epoxies. One promising approach involves incorporating furan into polymers, which is known to enhance char formation. Nonetheless, the chemical reactions that allow furan to convert from five-membered rings to six-membered rings in the final carbon structure remain poorly understood. In this work, three furan-based epoxy resins were developed to explore how furan ring substitution and network positioning affect char formation: furan diepoxy (FDE), methyl furan diepoxy (MethylFDE), and tetraglycidyl amine of difuran diamine (TGDFDA). Thermogravimetric analysis (TGA) and evolved gas analysis showed that the network position of the furan ring has a greater impact on char formation than ring substitution. PolyTGDFDA, which has main-chain furan rings, had the highest char yield of 44.0 ± 0.6 % at 1000 °C in a nitrogen atmosphere. In contrast, PolyFDE, containing pendant, monosubstituted furan rings, displayed a char yield of 38.6 ± 0.7 %, while PolyMethyl-FDE, with pendant, disubstituted furan rings, yielded 22.4 ± 2.1 %. The influence of furan ring substitution and network position significantly impacted the char yield, but not the type of carbon formed, as shown by Raman spectroscopy. The enhanced char yields positively affected the flammability performance, as evaluated by micro-combustion calorimetry (MCC). These furan-based epoxies showed a path toward developing intrinsically flame-retardant polymers without needing flame-retardant chemicals. Furthermore, insights into the role of furan rings during char formation were developed.