M.C. Basso

Latest progresses in the preparation of tannin-based cellular solids

Tannins are non toxic, cheap and abundant polyphenolic oligomers extracted from tree barks. They can be easily used for preparing thermoset resins that are natural at the 70–95% level, depending on the cases. From these resins, highly porous monoliths can be obtained by different techniques such as classical physical or chemical foaming, mixed physical/chemical foaming, by less-conventional methods such as emulsion-templating and stabilisation and hardening of either aerated emulsions or liquid foams. The resultant materials are not only interesting per se, but are also key precursors of novel cellular carbons having a broad range of applications. The present paper briefly reviews the latest advances for producing tannin-based cellular materials and their related characteristics and properties.

Alkaline Tannin Rigid Foams

Condensed fl avonoid tannin foams were obtained under alkaline conditions. This entailed the elimination of furfuryl alcohol from these formulations, as under alkaline conditions, foam preparation could not rely on the heat generated by the self-condensation of furfuryl alcohol, which occurs only under strongly acidic conditions. The approach used to formulate the alkaline foam was: (i) the total elimination of furfuryl alcohol from the formulation, (ii) coupled with the use of an aldehyde hardener different from formaldehyde (for environmental reasons) and (iii) the application of moderate heat to allow foaming. These were not tannin/ furanic foams as their acid-curing counterparts, but tannin only foams. The open cell foams were evaluated for bulk density, compressive strength, thermal conductivity and fi re resistance. Their characteristics were similar to the acid-curing tannin/furanic foams.