Chemical Modification of Lignocellulosics Using Microwave Technology

Abstract

The property enhancement of lignocellulosics by means of chemical modification has been studied for a long time. Most methods involve a reaction with wood polymer hydroxyl groups. One of the most promising methods is acetylation with acetic anhydride which gives a highly improved dimensional stability and improved biological resistance. The tendency of wood to absorb moisture is reduced when the hydroxyl groups of wood polymers react with acetic anhydride thus, forming covalently bonded acetyl groups. The equilibrium moisture content (EMC) decreases along with an increase in the degree of acetylation. In this study, the acetylation process was performed by using microwave energy as the heat source.

The use of microwave energy as a heat source in the acetylation process is likely to be most beneficial when solid wood of large radial and tangential dimensions is to be treated. In a conventional reactor, the wood to be acetylated is heated from the wood surface to the interior and the heat transfer is slow. The larger the wood dimensions, the more time consuming the process will be. In addition, heating the wood from the surface to the interior could lead to a lower acetylation level in the center of the wood being treated, due to temperature gradients within the wood.

Microwave energy effectively heats acetic anhydride and the microwaves have a penetration depth of about 10 cm in acetic anhydride impregnated wood, which means that wood samples up to dimensions of 20 by 20 cm (radial × tangential) can be evenly heated during acetylation.

When two different spruce samples, 10.5 × 9.5 × 16.0 cm; (r × t × l), were acetylated in separate experiments under the same conditions the variation in acetyl content both within and between samples were less than 2%. This result implies a high degree of reproducibility in the process. Generally, a somewhat higher acetyl content was obtained in the middle of a microwave acetylated wood sample than in the outer part of it. Further studies showed that the reaction temperature (130° C) could be reached in about 10 minutes without any formation of cracks in the wood due to the fast increase in temperature.

The potential for using microwave energy, not only for acetylation but also to get an efficient removal of excess chemicals by evaporation under vacuum, was investigated. During the vacuum step nearly 50% of the chemicals were removed after half an hour, and 70% after one hour. The remaining amount of excess chemicals requires a considerably longer vacuum period in order to be removed under the conditions applied. Pine wood samples acetylated for 3 hours at 130° C followed by a vacuum step for two hours at 130° C., showed an acetyl content of about 19% and the content of residual chemicals was about 5% calculated on basis of dry acetylated wood. The temperature in the wood samples could be maintained at 120 to 130° C even with a very low content of residual excess chemicals without any formation of hot spots, which otherwise could cause a pyrolysis reaction. The results obtained imply potential for further removal of excess chemicals from acetylated wood by means of microwave heating.