Modified Tannic Acid - a Bioinspired Fire Resistant Char Forming Additive for Polyamide

Utilization of bioderived feedstock is emerging as a new approach in the development of environmental friendly flame retardant (FR) facilitating the reduction of the use of toxic FR. Tannic acid (TA) is a naturally occurring polyphenol that exhibits intumescence and low heat release characteristics. However, the low thermal stability of TA derivatives limits its use as a char forming additive in polymers with high processing temperatures such as polyamides.

This research demonstrated that the thermal stability of TA can be significantly improved by a single-step chemical modification, rendering it suitable to be a char-forming additive that can be compounded into polyamides. Modified TA was synthesized by reacting TA with diacid chloride through an interfacial polycondensation. The modification of TA improved the thermal stability to 230°C and char yield to over 36%. Upon thermal degradation, polyamide 6 blends containing 15% chemically modified TA exhibit consistently lower peak heat release rate (pHRR) (52% reduction) and heat release capacity (HRC) (36% reduction) combined with enhanced char yield (9%).Polyamide 66 fabric coated with a thin layer of modified tannic acid (less than 8% weight add-on) exhibited self-extinguishing behavior and char length less than 3 inches in a vertical flame test. This study indicates that chemical modification of natural polyphenols can significantly improve overall thermal properties thus opening new possibilities for bio-derived polyphenol-based fire resistant char forming additives.