A reinforced thermal barrier coat of a Na–tannic acid complex from the view of thermal kinetics

The poor burning resistance of cotton necessitates the control of its pyrolytic reactions, but many approaches have relied on the use of synthetically engineered chemicals. Herein, we show how a natural polyphenol from plants—tannic acid—acts with sodium ions to create a robust thermal barrier coat on cotton, with a focus on thermal kinetics. The kinetic information, combined with thermal and spectral analyses, revealed that the outer layer of galloyl units in tannic acid decomposes via a two-step reaction, producing a multicellular char of crosslinked aromatic rings, followed by the blowing of carbonaceous cells into a further expanded structure. This intumescent function of tannic acid was found to be enhanced upon its complexation with sodium ions, which greatly increased the activation energy for the first step of the reaction of tannic acid, to promote the formation of a stable char. The resulting blown char coated the cotton fiber below the thermal decomposition temperature of cellulose and was sustained throughout the decomposition. The enhanced thermal barrier performance of the Na–tannic acid complex was demonstrated by the reduced heat release capacity of cotton, the value of which was only about one-third that of tannic acid itself, and the inhibition of flame generation on cotton.