<Note>Morphological Changes in Acetylated Wood Exposed to Weathering

Abstract

Resistance of acetylated wood to degradation caused by natural weathering was investigated at different levels of acetylation. The color difference caused by sunshine exposure was minimized at an acetylation level of 20% weight gain (WG) with a rapid decline from 10% WG, even though the exposure time was prolonged. Weathering led to early disintegration and enlargement of bordered pits, extensive separation of the latewood tracheid walls in the middle lamella and checkings oriented along the microfibril angle of the S_2 layer. These characteristic patterns of cell-wall erosion due to weathering were essentially the same in both untreated and acetylated wood. However, scanning electron microscope observations revealed that more extensive microscopic and macroscopic checks leading to distortion of the cell alignment appeared in untreated wood and wood which had been treated with a low-level of acetylation. For highly acetylated wood, severe intra- and inter- cell wall erosion was limited, and the original cell alignment was retained. However, after long-term exposure to natural weathering, the surface cell-layers of acetylated wood were defibrated separately and were attached to the unexposed cellular structure beneath them. This may explain the observation that acetylated wood has more severe surface-contamination than untreated wood, in which the new cell-layers appear successively because of the severe erosion of the exposed surfaces.