Weathering of chemically modified wood surfaces - Natural weathering of Scots pine acetylated to different weight gains

Scots pine wood veneers were acetylated to weight gains of 5, 10, 15 or 20% and exposed to natural weathering. Veneers acetylated to low weight gains of 5 and 10% and exposed to the weather showed greater losses in mass and tensile strength due to increased delignification and depolymerisation of cellulose than similarly exposed, untreated controls. Acetylation to 20% weight gain restricted the loss of veneer mass and holocellulose during exposure. Initially it also slightly reduced the depolymerisation of cellulose, and this was reflected in lower losses in veneer zero-span tensile strength after 35 days exposure. However, the photoprotective effects of acetylation, with the exception of veneer mass, were lost with prolonged exposure of veneers to the weather. Acetylation of wood blocks to 20% weight gain caused a bulking of the wood cell wall, particularly the Sg layer of the secondary wall, and reductions in lumen size. Acetylated latewood cells maintained their shape and radial orientation during exterior exposure, but acetylation did not prevent erosion of the middle lamella. Mechanisms to explain how acetylation may affect the weathering resistance of wood are suggested.