Projectable tannin foams by mechanical and chemical expansion

A new formulation of quebracho tannin-based resin has been used to prepare bio-based rigid foams. The proposed foams were blown by using the simultaneous combination of two expansion methods by mechanical expansion as used for fire-fighting foams and by chemical expansion based on the release of water and other gases during the exothermal self-condensation of furfuryl alcohol. The combination of both methods allowed us to overcome certain limitations found in the preparation of foams exclusively based on mechanical expansion These were the resulting density (16% lower than that obtained by mechanical expansion) and the mechanical properties (compressive strength improved of about 11%). Resin chemorheology was characterized under stressed conditions using frequency and time sweeps as well as temperature ramps. The evolution of the linear viscoelastic functions during the reaction clearly showed the transition from viscous to strong gel-like behavior. The foaming and hardening processes were followed by the use of FOAMAT equipment and the resulting foams were characterized in terms of density (0.059-0.063 g/cm3), mechanical properties (compressive strength of 0.090-0.144 MPa) and thermal conductivity (0.046-0.048 W m−1 K−1).