Evaluation of the Durability of Engineered Wood Under Accelerated Artificial Weathering

Engineered wood composites were produced from wood waste fractions (hardwood/softwood) bonded with a rapeseed-oil-derived polyurethane. To improve durability, zinc oxide was used, or the wood particles were pre-impregnated with sodium silicate (water glass (Na2O(SiO2)·H2O) and dried at 50 °C before polyurethane bonding. Specimens were exposed to accelerated UV-water cycling using UVA-340 lamps (295–365 nm, peak at 340 nm) with intermittent water spray. Strength changes were interpreted using attenuated total reflectance Fourier-transform infrared microscopy and scanning electron microscopy. Weathering decreased O–H, aliphatic C–H, and lignin/aromatic bands, while enriching the contribution of silicates at 1050–1000 cm−1. These findings indicate lignin degradation at the surface, binder recession, and fragment leaching. In silicate-treated composites, the 1030 cm−1 Si-O band becomes significantly narrower. These changes reorganise the silicate network and often lead to coating cracking and salt crystallisation. These interphase defects cause significant mechanical loss: compressive stress at 10% strain decreased from 9.42 to 2.68 MPa, whereas the untreated reference decreased from 19.25 to 9.06 MPa. The addition of ZnO reduced the initial compressive stress at 10% strain and the flexural strength compared to the reference sample, while the initial tensile strength remained comparable. After weathering, the ZnO-containing composite showed a response dependent on the loading mode. Although its absolute post-weathering flexural strength remained lower (10.82 vs. 12.88 MPa), flexural strength retention was 97.1%, compared with 81.3% for the reference. Conversely, tensile strength retention was lower for the ZnO-containing composite (43.4% vs. 75.3%), while compressive stress retention was similar (47.9% vs. 47.1%). Therefore, ZnO did not uniformly improve mechanical weathering resistance under the applied conditions.