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

Authors: Nam, Sunghyun; Easson, Michael; Condon, Brian; HILLYER, MATTHEW - Collaborator; SUN, LUYI - University Of Connecticut; XIA, ZHIYU - University Of Massachusetts; NAGARAJAN, RAMASWAMY - University Of Massachusetts

Submitted to: RSC Advances
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2019
Publication Date: 4/8/2019
Citation: Nam, S., Easson, M.W., Condon, B.D., Hillyer, M.B., Sun, L., Xia, Z., Nagarajan, R. 2019. A reinforced thermal barrier coat of a Na–tannic acid complex from the view of thermal kinetics. RSC Advances. 9(19):10914-10926. https://doi.org/10.1039/c9ra00763f.
DOI: https://doi.org/10.1039/c9ra00763f

Interpretive Summary: Intumescence is an effective strategy for imparting flame resistance to materials, but it requires formulating several active components and consolidating them with a binder. This study showed that tannic acid, a plant-based polyphenol, itself has an intumescent function. Despite its nearly symmetric structure, the thermal decomposition of tannic acid was disproportionate, producing intumescent char via a two-stepped reaction. Four galloyl units in its outer layer were first decomposed and transformed into the multicellular char, and the subsequent decomposition of one galloyl unit further expanded the char. The thermal kinetic analysis using the differential isoconversional method indicates that the intumescence of tannic acid can be improved by complexation with sodium ions. The introduction of low concentrations of sodium ions raised the activation energy for the decomposition of tannic acid releasing volatile species, thus facilitating the formation of char with enhanced thermal stability. The intumescent coating of the Na-tannic acid complex effectively protected cotton fiber against heat and volatiles during the pyrolysis of the fiber, as demonstrated by an 80% decrease in the heat release capacity of cotton. These results help advance our understanding of the intumescence process for tannic acid and suggest an easy way to improve its intumescence efficiency—simply by mixing it with NaOH in a water solution. It also opens the door for examination of possible effects of complexation with other metal ions. The cotton nonwovens coated with this fully renewable and environmentally benign intumescent could serve as a fire-resistant barrier for many consumer products such as mattresses and home furnishings.

Technical Abstract: 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 report on how a natural polyphenol from plants—tannic acid—interplayed with sodium ions to improve the heat resistance of cotton. The combination of thermal, spectral, and kinetic analyses revealed that the outer layer of galloyl units in tannic acid disproportionately decomposed in two steps, producing the multicellular char of crosslinked aromatic rings followed by the blowing of the carbonaceous cells into a further expanded structure. This intumescent function of tannic acid was found to be improved by complexing with sodium ions, which greatly increased the activation energy for the first-step reaction to promote the formation of char. The intumescent coating of the Na-tannic acid complex was created on the surface of cotton fiber below the decomposition temperature of cellulose and sustained throughout the decomposition. The enhanced efficiency of this thermal barrier performance was demonstrated by the reduced heat release capacity of cotton, the value of which was only about one-third of that of tannic acid itself.