Glyoxal as Single Crosslinker for Mechanically Blown, Condensed and Hydrolyzable Tannin Foams

Tannin foams are polymeric, porous materials produced from plant tannins, with good thermal insulation and fire-retardant properties. Although research has mainly concentrated on usage of condensed tannins (CTs), interest in a second group, hydrolyzable tannins (HTs), is growing. This study evaluated the usability of glyoxal as a single crosslinker for condensed and hydrolyzable tannins in foams created through mechanical agitation, using various ratios of chestnut (HT) and quebracho (CT) tannins. Glyoxal could react with chestnut tannin, but foams with only chestnut collapsed before hardening due to its slow reactivity, with 70% chestnut as the maximum viable content. Increasing the chestnut tannin amount reduced the foamability and compression strength, resulting in higher density and increased pore size. At a similar density (~210 kg m^-3), the 70%-HT foam reached only one-third the compressive strength of the pure CT foam (0.22 vs. 0.61 MPa), while the pure CT foam showed a smaller mean pore size (189 vs. 365 µm) despite its lower mean density (208 vs. 241 kg m^-3). The fire resistance and thermal conductivity appeared unaffected by the tannin type and instead depended on the foam density, with thermal conductivities ranging from 56 to 71 mW/(m·K). Leaching tests showed a slight increase in leaching for formulations with higher chestnut tannin contents, with 15% to 24% of acid recovered after the leaching cycle. The ¹³C-NMR analysis revealed the glyoxal crosslinks at the free position of the A-ring in CTs and at the free ortho ones of the gallic/ellagic moieties in HTs. Overall, this study demonstrated that tannin foams can be produced using glyoxal as a single crosslinker, allowing for up to 70% substitution of the condensed tannin component in the formulation.