Acetylation of Solid Wood. Wood Properties and Process Development
The influence of acetylation on selected wood properties has been determined for Scandinavian pine and spruce wood, in order to predict the behavior of acetylated wood when it is used in different applications. Only minor differences in both bending strength and the modulus of elasticity were obtained for acetylated wood when compared with unmodified wood, and can, for practical applications, be considered unaltered. The Brinell hardness was found to increase and the tendency of wood to deform under cycled relative humidity conditions was greatly reduced, owing to acetylation of the wood. Acetylated wood was also shown to give strong adhesive bonds, with conventional wood adhesives, and when the laminated wood was tested under wet conditions, the bond strength was higher for the acetylated laminated wood than for unmodified. High dimensional stability in the acetylated laminated wood, was reflected by the absence of radial cracks, after exposure to alternating water soaking and drying. The resistance of acetylated wood against common decay fungi was shown to be in the same order of magnitude when compared with that of CCA preservative treated wood in a field test. Testing of acetylated wood in marine environments showed almost no preserving effect against attack from marine borers. However, a slight decrease in attack with an increase in acetyl content could be observed. The stability of the acetyl groups under conditions of high or cycled humidity shows that acetylated wood can be used for products exposed to changes in humidity.
Microwave heating has been shown to be beneficial to the acetylation of solid wood. The penetration depth of microwaves into acetic anhydride impregnated wood is about 10 cm and the reaction temperature (120-130°C) can be rapidly reached. In samples acetylated in the microwave reactor, the acetyl groups were found to be evenly distributed over the wood volume. Microwave heating could also be used for the removal of excess acetic anhydride and by-product acetic acid under vacuum after completed acetylation reaction.
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