Tannic acid is found in tree bark and other plant materials and can be recovered from bio-based industry waste streams. It was tested as a flame retardant additive in DGEBA epoxy at 0.1 – 8% (and neat epoxy), by mixing into the epoxy before hardening and cur-ing. Loads of tannic acid <1% showed to reduce epoxy cross-linking, so reducing mechanical strength, whereas loads >1% increased cross-linking and strength. Mass loss calorimetry showed that the tannic acid did not reduce peak heat release rate, did not modify time to ignition and did not increase the mass of char produced, but did significantly increase char volume. It also reduced the propensity of the resin to crystallise before hardening, e.g. in storage. The authors conclude that tannic acid can be an effective, reactive intumescent in epoxy and that this depends on the tannic acid molecule’s functional external phenol and internal d-glucose and phenol groups.
“Tannic acid-based prepolymer systems for enhanced intumescence in epoxy thermosets”, M. Korey et al., Green Materials 2020 https://doi.org/10.1680/jgrma.19.00061