Production of Phenolic Foams Based on Tannins from Acacia mearnsii and Stryphnodendron adstringens Reinforced with Quercus suber Powder

Tannins are among the most abundant and versatile chemical compounds extracted from tree bark, with applications ranging from pharmaceuticals to wood-panel adhesives and, more recently, tannin-based phenolic foams. This study aimed to produce and characterize rigid phenolic foams based on Acacia mearnsii (De Wild) and Stryphnodendron adstringens (Mart.) Coville (Fabaceae) tannins, evaluating the influence of cork (Quercus suber) powder reinforcement and polysorbate 80 (Tween 80) on their properties. The latter species is an underexplored Brazilian source, here compared with the industrially established A. mearnsii under identical processing conditions. Eight formulations were prepared, containing each additive alone or in combination: AM1–AM4 (A. mearnsii ) and SA1–SA4 (S. adstringens ). The foams were characterized for bulk and real density, porosity, compressive strength, mechanical durability (friability), water absorption, contact angle, cell morphology (SEM), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The S. adstringens tannins exhibited a higher Stiasny index (81.1%) and condensed-tannin content (20.9%) than A. mearnsii (74.5% and 18.6%), yielding foams with higher compressive strength (up to 0.22 MPa) and elastic modulus (up to 2.27 MPa) than the corresponding A. mearnsii foams (0.13–0.05 MPa and 1.34–0.52 MPa). The combined use of cork powder and polysorbate 80 reduced friability (e.g., from 18.5% in SA1 to 8.4% in SA4) and produced smaller, more homogeneous cells (mean diameter reduced to 163.4 μm in SA4 and 205.5 μm in AM4). All foams showed good thermal stability, with significant char yield at 800 °C. These results demonstrate the potential of S. adstringens tannins as a renewable precursor for rigid phenolic foams and the beneficial role of cork powder and polysorbate 80 in improving their morphology and mechanical durability.