Tannic Acid as a Bio-Based Modifier of Epoxy/Anhydride Thermosets

Toughening an epoxy resin by bio-based modifiers without trade-offs in its modulus, mechanical strength, and other properties is still a big challenge. This paper presents an approach to modify epoxy resin with tannic acid (TA) as a bio-based feedstock. Carboxylic acid-modified tannic acid (TA⁻COOH) was first prepared through a simple esterification between TA and methylhexahydrophthalic anhydride, and then used as a modifier for the epoxy/anhydride curing system. Owing to the chemical modification, TA⁻COOH could easily disperse in epoxy resin and showed adequate interface interaction between TA⁻COOH and epoxy matrix, in avoid of phase separation. The use of TA⁻COOH in different proportions as modifier of epoxy/anhydride thermosets was studied. The results showed that TA⁻COOH could significantly improve the toughness with a great increase in impact strength under a low loading amount. Moreover, the addition of TA⁻COOH also simultaneously improved the tensile strength, elongation at break and glass transition temperature. The toughening and reinforcing mechanism was studied by scanning electron microscopy (SEM), dynamic mechanical analysis (DMA) and thermal mechanical analysis (TMA), which should be owned to the synergistic effect of good interface interaction, aromatic structure, decreasing of cross linking density and increasing of free volume. This approach allows us to utilize the renewable tannic acid as an effective modifier for epoxy resin with good mechanical and thermal properties.