Acetylation of solid wood using microwave heating - Part 2. Experiments in laboratory scale

Microwave energy has been used as a heat source the fbr acetylation of wood with acetic anhydride. Process parameters for the proper acetylation of wood in timber dimensions were investigated using a lab-scale microwave reactor. A high degree of reproducibility and a uniform acetylation was obtained when spruce wood samples (10.5 x 9.7 x 16 cm; r x t x l) of the same origin were acetylated separately, but under the same conditions in the microwave reactor. The variation in acetyl content both within and between the samples was less than 2% units. In general, a somewhat higher acetyl content was obtained in the middle of the acetylated wood than in the outer part of it. Microwave energy was also shown to be efficient in the removal of excess acetic anhydride and by-product acetic acid by evaporation under vacuum. During the vacuum step, about 70% of the chemicals could be removed within the first 30 minutes. Pine and spruce wood samples acetylated for 2 hours at 130 degrees C followed by a vacuum step for two hours at 120 degrees C, obtained an acetyl content of about 17% and the content of residual chemicals was about 3% calculated on a basis of dry acetylated wood. The temperature in the wood samples could be maintained at about 130 degrees C even at very low contents of residual excess chemicals without any formation of hot spots.