Acetylation of solid wood using microwave heating Part 1: Studies of dielectric properties

The possibility of using microwave energy as the source of heat for the modification of wood by means of acetylation with acetic anhydride was investigated, with the aims of reducing reaction time, achieving an efficient removal of excess chemicals after reaction, and obtaining a uniform distribution of acetyl groups within the acetylated wood. When using microwave heating in the acetylation process, the heat is instantly generated within the whole wood object and the absorbed microwave energy depends on the polarizability of the chemicals. The parameters, relative permittivity (epsilon') and relative loss factor (epsilon") give information on how the microwaves can penetrate into the wood object and of possible thermal gradient effects on the wood during modification. This investigation shows that microwave energy effectively heats the acetic anhydride used for modification by acetylation and that the power penetration depth (d(p)) of the microwaves is about 10 cm in acetic anhydride impregnated wood. The relative permittivity and the relative loss factor were found to decrease with increasing temperature, which indicates that the energy absorption will preferably be directed to less heated parts of the wood object. This will promote a more uniform heat distribution, and, accordingly, a more uniform acetylation of the wood.