Abstract
This study investigates the viability of using tannins extracted from pine bark─specifically Pinus ponderosa (PT), Pinus radiata (RT), and a commercial maritime pine bark tannin (CT)─as a substitute for phenol in phenol–resorcinol–formaldehyde (PRF) resins. Tannins were hot-water-extracted from bark and characterized by TGA, FTIR, ESI-MS, and NMR, revealing distinct differences in chemical structure, molar mass, and thermal stability. Resin systems were prepared using PRF, tannin, and tannin-PRF hybrids (1:1) and were (i) evaluated for adhesive properties by lap shear tests and (ii) incorporated into hot-pressed wood composites (50:50), followed by characterization of thermal, mechanical, rheological, and water absorption properties. The PRF and PT–PRF resins showed excellent adhesive properties. The result also revealed enhanced thermal stability and char formation in hybrid composites. Mechanical testing demonstrated that the PT–PRF hybrid composite had the highest flexural strength (37.0 MPa) and modulus (4.51 GPa), outperforming neat PRF composites. In addition, water absorption properties exhibited improved dimensional stability in PT–PRF composites, with lower uptake and diffusivity than neat tannin systems. Overall, ponderosa pine bark tannin unveiled promising reactivity, thermal robustness, and mechanical performance when hybridized with PRF, highlighting its potential for high-performance, biobased wood adhesives.