Fire resistant polyphenols based on chemical modification of bio-derived tannic acid

Abstract

Utilization of renewable materials for the development of safer and nontoxic flame retardants has been of interest from environmental safety and sustainability perspective. Tannic acid (TA) is an abundantly available bio-based polyphenol that exhibits good intumescence and char forming characteristics upon being subjected to heat. Intumescence and char formation are important prerequisites for certain types of effective flame retardant (FR) additives. However, the potential for utilizing TA as an FR has been limited by its poor thermal stability. A single step chemical modification process that overcomes the limitations of TA while allowing the utilization of its beneficial properties is reported here. TA was crosslinked using interfacial polycondensation with terephthaloyl chloride to yield tannic acid terephthalate (TAT). The complex structure of TAT necessitated the synthesis of several model compounds based on methyl gallate (MG) to facilitate the complete structural characterization of TAT using FTIR and 1H NMR. TAT is thermally stable up to 230 °C (less than 3% weight loss) and shows 30% higher char yield and extremely low heat release capacity (<80 J/g-K) as compared to that for TA. Detailed thermal degradation studies combined with gas phase spectroscopy using thermogravimetric analysis - Fourier-transform infrared spectroscopy (TGA-FTIR) and pyrolysis - gas chromatography - mass spectrometry (Py-GC-MS) provide an understanding of the degradation process. Crosslinked phenolic species enhance char formation in the condensed phase and allow for utilization of these compounds as flame retardant coatings for polymers such as Nylon 66. TAT coating on Nylon 66 fabric significantly impedes flame propagation in the fabric, resulting in quick self-extinguishing behavior and reduced char length in vertical flame tests. Morphological characterization, thermo-oxidation studies and microscale combustion calorimetry (MCC) (at high heating rates) of TAT-coated fabric reveal the beneficial effects of char formation and its direct impact on flame retardancy.