Influence of the Extraction Medium of Tannins from Eucalyptus Bark on the Properties of Rigid Tannin–Furfuryl Alcohol Foams

Eucalyptus bark is an abundant forestry residue in Brazil and a promising renewable source of condensed tannins for advanced material applications. Among them, tannin-furfuryl alcohol foams stand out as sustainable alternatives to petroleum-based foams due to their intrinsic fire resistance, low density, and high porosity. The performance of such foams is strongly influenced by the chemical profile of the tannins, which depends on the extraction medium. This study investigated tannin extraction from eucalyptus bark using distilled water, sodium hydroxide (NaOH), and sodium bisulfite (NaHSO₃), and evaluated their impact on the structural, antioxidant, and functional properties of the extracts and on the performance of the resulting foams. Extraction yields were highest with NaOH (>50% for some clones), moderate with NaHSO₃, and lowest with water. However, aqueous extracts showed superior antioxidant activity, whereas NaOH extracts displayed enhanced ferric-reducing capacity despite lower phenolic content. FTIR analyses confirmed solvent-dependent differences in hydroxyl, carbonyl, and aromatic groups. The foams produced exhibited distinct physical and mechanical behaviors. NaOH-derived foams showed the highest compressive strength (∼280 kPa) and smaller, denser cells, while NaHSO₃-derived foams were lightweight and highly porous but structurally weaker. Aqueous extracts yielded intermediate strength but higher water uptake due to hydroxyl-rich compounds. All formulations maintained thermal stability above 400 °C and displayed self-extinguishing behavior under flame exposure. Nonetheless, incorporation of aqueous extracts slightly reduced fire resistance compared to control foams. These findings highlight the pivotal role of the extraction medium in determining tannin reactivity and, consequently, foam properties. Alkaline extraction favored mechanical performance, while aqueous extraction preserved phenolic-rich fractions with stronger antioxidant activity. This study links extraction chemistry directly to material performance. It contributes to the valorization of forestry residues into biobased foams, offering sustainable alternatives to synthetic polymeric materials.