A Bio-derived Char Forming Flame Retardant Additive for Nylon 6 Based on Crosslinked Tannic Acid

Abstract

Nylon is one of the most important engineering polymers used in textile, electronics, and automotive applications. In addition to good mechanical properties, flame retardancy is desirable in these applications. Nylon is typically processed at high temperatures and very few non-toxic synthetic flame-retardant (FR) additives are stable at these temperatures. Tannic acid (TA) - a naturally occurring polyphenol, has low heat release capacity (HRC) and good char forming capabilities but is not thermally stable and starts to degrade at the processing temperature of thermoplastics such as Nylon. Here we report the use of chemically modified TA (TAT) as a more stable char forming FR additive for Nylon. Upon thermal degradation, Nylon 6 blends containing 15 wt% TAT exhibit consistently lower peak heat release rate (pHRR) (52% reduction) and HRC (36% reduction) combined with enhanced char yield (about 9%). Evolved gas analysis through TGA-FTIR reveals that, in the presence of TAT, some of the primary amide/amine and secondary amide species in Nylon 6 degrade into less flammable moieties like ammonia while aromatic moieties in TAT enhance char formation in the condensed phase, thus reducing flammability. In the lab-scale flame tests, Nylon 6 blends containing 15 wt% TAT exhibited reduced afterflame time and self-extinguishing behavior. The oxygen index of the Nylon 6 - TAT blends also increased from 22% to 25%. This research opens new possibilities for modifying natural polyphenols to create melt-processable FR additives that can enhance char formation and lower HRC of Nylon.