Surface characterization of wood treated with acidic deep eutectic solvents

This study investigated the use of deep eutectic solvents (DESs) to improve the accelerated weathering performance of Scots pine (Pinus sylvestris L.) wood. In addition to being natural and easily obtained, the simple preparation of these solvents via heating and mixing has led to their use in a pretreatment process. The DES solutions were prepared by mixing, in different molar ratios, oxalic acid (OA) and acetic acid (AA) as hydrogen bond donors (HBDs) and choline chloride (ChCl) and betaine (BT) as hydrogen bond acceptors (HBAs). In this study, the novel DESs were prepared and characterized. Scots pine wood samples prepared in dimensions of 15 (radial) × 75 (tangential) × 150 (longitudinal) mm were treated with the DES solutions at 150 °C for 2 h. The weathering effects on the surface characteristics and morphology of the samples were examined by means of Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and measurement of color, gloss, and surface roughness. Weathering was carried out via cycles of 8-h UV-light irradiation and 15-min water spraying followed by 3.45-h conditioning in an accelerated weathering test cycle chamber. According to the FTIR results, the lignin ratio decreased significantly in the wood samples subjected to 2-h pretreatment at 150 °C. The results showed that the greatest color change occurred on the surfaces of the control samples, whereas the least color change was observed up to 168 h on the OA-BT samples. The glossiness values increased with the first 168 h of weathering. Thereafter, the gloss values of the samples treated with OA-BT and OA-ChCl remained stable. The surface roughness and degradation of surface components rose with the increase in the weathering exposure period. In accordance with the color and surface roughness measurements, the FTIR spectra proved that for all specimens, lignin continued to degrade on the exposed surfaces throughout 168 h of accelerating weathering.