WEATHERING OF WOOD AND PROTECTION BY CHROMIUM STUDIED BY XPS

In order to understand better the protection mechanism of aqueous CrO3, the surface of untreated and treated wood, before and after weathering, was studied by XPS. For comparison, the reaction of CrO3 with filter paper, used as a model compound for cellulose, and with lignin powder was studied as well. Owing to differences in the O/C ratio and the shape of the C Is signal, cellulose and lignin can be distinguished from each other with the aid of XPS. The low O/C ratio and the high C1 (C-C, C-H) contribution found for the surface of wood show that the upper layer is enriched with lignin and extractives. Weathering of wood starts with the degradation of lignin, and the appearance of cellulose on the surface increases the O/C ratio and shifts the maximum of the C 1s signal to the C2 (C-O-C, C-OH) position. Aqueous CrO3 oxidizes lignin powder and possibly also cellulose. The effect on wood, as observed with XPS, is small. The protection, however, is very clear; the increase of the O/C ratio of CrO3-treated wood during weathering is much slower than the increase for untreated wood. After weathering, the C 1s signal still resembles the C 1s signal of lignin, so the CrO3 protects the lignin from degradation.